Co-reporter:Xianchi Jin, Chao Li, Chenbiao Xu, Dawei Guan, Ajin Cheruvathur, Yi Wang, Jian Xu, Dong Wei, Hongwei Xiang, J.W. (Hans) Niemantsverdriet, Yongwang Li, Qing Guo, Zhibo Ma, Ren Su, Xueming Yang
Journal of Catalysis 2017 Volume 354(Volume 354) pp:
Publication Date(Web):1 October 2017
DOI:10.1016/j.jcat.2017.08.004
•A molecular picture of photocatalytic ethylene glycol (EG) dissociation on TiO2.•The CC bond cleavage of EG is the only pathway, forming formaldehyde and H2.•The rate determining step is the desorption of surface adsorbed H (Hads).•The metal nanoparticles promote the desorption of Hads into H2.•In-situ surface science studies are coupled with in-situ IR-mass spectrometry.Polyol conversion to value-added products is of great interest for the bio-diesel industry. Photocatalytic oxidation processes may offer a green approach for polyol conversion; however the lack of comprehensive mechanistic understanding from an interdisciplinary perspective limits or even misleads the design of highly selective and efficient photocatalysts for such process. Here we have studied the photocatalytic polyol conversion on pristine TiO2 and metal (Au, Pd, and Pt) nanoparticles (NPs) decorated TiO2 using ethylene glycol (EG) as the model compound. We have developed a mechanistic picture at molecular level by coupling in-situ surface science study on rutile (110) surface with in-situ vibrational-mass spectrometry study on TiO2 nanopowders. The CC bond cleavage was found to be the only pathway in EG photo-conversion under deaerated conditions, leading to the formation of formaldehyde and hydrogen. We rationalized that the desorption of the surface adsorbed H (Hads) to be the rate determining step (RDS), making pristine TiO2 a poor photocatalyst that only catalyze the EG conversion at very low surface coverages. The addition of metal NPs on TiO2 surface promotes the desorption of Hads significantly, thus leading to an enhanced CC bond cleavage performance at higher surface coverages that is more applicable.The CC bond cleavage is the only pathway in photo-conversion of ethylene glycol on TiO2 under deaerated conditions. The desorption of surface adsorbed H is the rate determining step and can be promoted by the addition of metal promoters (i.e., Au, Pd, Pt).Download high-res image (115KB)Download full-size image
Co-reporter:Wu Jiawei, Jun Chen, Qing Guo, Hai-Yan Su, Dongxu Dai, Xueming Yang
Surface Science 2017 Volume 663(Volume 663) pp:
Publication Date(Web):1 September 2017
DOI:10.1016/j.susc.2017.04.008
The co-adsorption of CO and H2O on a Co(0001) surface at 100 K has been systematically studied using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. While the TPD spectra of CO is almost not affected by the presence of H2O, the stabilization of H2O by co-adsorbed CO is found for the first time in a large coverage range (0.15 ML <θCO < 0.66 ML; 0.01 ML < θH2O < 0.6 ML). When the coverage of predosed CO is lower than 0.27 ML, the formerly single desorption peak of H2O is gradually separated into three peaks at 0.6 ML coverage. Those at lower and higher temperatures may be attributed to the repulsive interaction between H2O molecules and the attractive interaction between H2O and CO molecules, respectively. With increasing the coverage of predosed CO, not only the position of the high temperature peak shifts toward higher temperature (by about 15 K), but the intensity is greatly strengthened until a maximum is achieved when θCO = 0.36 ML. DFT calculations suggest that the attractive interaction between H2O and CO on Co(0001) originates from the formation of intermolecular hydrogen bonds. This work not only provides insights into water gas shift reactions with H2O and CO as reactants, but opens new avenues for a volume of catalytic process of technological importance.
Co-reporter:Yiqiang Liu;Fenghua Liu;Siyue Liu;Dongxu Dai;Wenrui Dong
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 31) pp:20786-20794
Publication Date(Web):2017/08/09
DOI:10.1039/C7CP04336H
The OH laser induced fluorescence method was used to study the kinetics of CH2OO reacting with SO2, (H2O)2, CH2I2 and I atoms. Decay of CH2OO is not strictly first-order since its self-reaction is rapid. With this consideration, we derived the rate coefficient of CH2OO + SO2/(H2O)2/CH2I2/I taking into account the contribution of the CH2OO self-reaction. For the CH2OO + SO2 reaction, the rate coefficient is measured to be (3.88 ± 0.13) × 10−11 cm3 molecule−1 s−1 at 10 Torr, which agrees very well with a previously reported value obtained by directly monitoring CH2OO using the UV absorption method with the CH2OO self-reaction considered. We did not observe obvious evidence for SO2 catalysed CH2OO isomerization or the intersystem crossing effect in this reaction. CH2OO + (H2O)2 is supposed to account for the major sink of CH2OO in the atmosphere, but previous rate coefficient measurements were not in good agreement. We have revisited this reaction including the self-reaction of CH2OO and obtained the rate coefficient to be (7.53 ± 0.38) × 10−12 cm3 molecule−1 s−1 at 60 Torr and 300 K. The rate coefficients of CH2OO + CH2I2 and CH2OO + I were measured to be (5.2 ± 2.6) × 10−14 and (2.2 ± 1.1) × 10−12 cm3 molecule−1 s−1 respectively.
Co-reporter:Zhigang He;Dongyuan Yang;Zhichao Chen;Kaijun Yuan;Dongxu Dai;Guorong Wu
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 44) pp:29795-29800
Publication Date(Web):2017/11/15
DOI:10.1039/C7CP06286A
The predissociation dynamics of water molecules in the electronic state were studied using the time-resolved photoelectron imaging method. Both vibrationless and vibrationally excited states in the electronic state were studied, with an emphasis on the vibrational excitation effects on the predissociation dynamics of the state. Besides the well-known rotationally and non-rotationally mediated predissociation pathways (Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 19148), an accidental resonance mediated predissociation pathway for the first bending mode excited state in the electronic state of H2O is revealed, providing an excellent example of competition between non-adiabatic decay pathways involving at least five electronic states.
Co-reporter:Dong Zhang;Jiayue Yang;Zhen Chen;Rongjun Chen;Bo Jiang;Dongxu Dai;Guorong Wu;Donghui Zhang
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 20) pp:13070-13074
Publication Date(Web):2017/05/24
DOI:10.1039/C7CP01428G
The effects of CH stretching excitation on the reactivity of the F + CHD3 → HF + CD3 reaction were studied experimentally using crossed-beam and time-sliced velocity map imaging techniques over the collision energy range of 1.21 to 9.00 kcal mol−1. The experimental results showed that the CH stretching excitation promoted its cleavage and enhanced the title reaction at low collision energies. This enhancement dropped with an increase of the collision energy. And at high collision energies, CH stretching excitation appeared to lower the reactivity of the above reaction, in contrast to the case at low collision energies. This decreasing trend in the enhancement of reactivity was in agreement with previous theoretical studies. The vibrationally excited reaction was further compared with the ground-state reaction at a same total reagent energy of 9.80 kcal mol−1, and similar reactivities were derived.
Co-reporter:Qing Guo, Chuanyao Zhou, Zhibo Ma, Zefeng Ren, Hongjun Fan and Xueming Yang
Chemical Society Reviews 2016 vol. 45(Issue 13) pp:3701-3730
Publication Date(Web):03 Sep 2015
DOI:10.1039/C5CS00448A
Photocatalytic hydrogen production and pollutant degradation provided both great opportunities and challenges in the field of sustainable energy and environmental science. Over the past few decades, we have witnessed fast growing interest and efforts in developing new photocatalysts, improving catalytic efficiency and exploring the reaction mechanism at the atomic and molecular levels. Owing to its relatively high efficiency, nontoxicity, low cost and high stability, TiO2 becomes one of the most extensively investigated metal oxides in semiconductor photocatalysis. Fundamental studies on well characterized single crystals using ultrahigh vacuum based surface science techniques could provide key microscopic insight into the underlying mechanism of photocatalysis. In this review, we have summarized recent progress in the photocatalytic chemistry of hydrogen, water, oxygen, carbon monoxide, alcohols, aldehydes, ketones and carboxylic acids on TiO2 surfaces. We focused this review mainly on the rutile TiO2(110) surface, but some results on the rutile TiO2(011), anatase TiO2(101) and (001) surfaces are also discussed. These studies provided fundamental insights into surface photocatalysis as well as stimulated new investigations in this exciting field. At the end of this review, we have discussed how these studies can help us to develop new photocatalysis models.
Co-reporter:Shu Su, Yvonne Dorenkamp, Shengrui Yu, Alec M. Wodtke, Dongxu Dai, Kaijun Yuan and Xueming Yang
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 22) pp:15399-15405
Publication Date(Web):06 May 2016
DOI:10.1039/C6CP01956K
Photodissociation dynamics of HBr at a series of photolysis wavelengths in the range of 123.90–125.90 nm and at around 137.0 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique. The branching fractions between the channels forming ground Br(2P3/2) and spin–orbit excited Br(2P1/2) atoms together with the angular distributions of the products corresponding to these two channels have been measured. The photolysis wavelengths in this work excited the HBr molecule from the ground state X 1Σ+ to various Rydberg states and the V 1Σ+ ion-pair valence state. Predissociation via these states displays rich behavior, indicating the influence of the nature of initially excited states and the coupling to other bound or repulsive states on the predissociation dynamics.
Co-reporter:Zhiqiang Wang, Qunqing Hao, Xinchun Mao, Chuanyao Zhou, Dongxu Dai and Xueming Yang
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 15) pp:10224-10231
Publication Date(Web):08 Mar 2016
DOI:10.1039/C6CP00556J
Photocatalytic chemistry of methanol on the reconstructed rutile TiO2(011)-(2 × 1) surface upon 266 nm and 400 nm light excitation has been investigated quantitatively using the post-irradiation temperature-programmed desorption (TPD) method. Photochemical products such as formaldehyde, methyl formate and water, which result from the recombination of surface bridging hydroxyls through the abstraction of lattice oxygen atoms, have been identified under both 266 nm and 400 nm light irradiation. However, ethylene is detected only under 266 nm light irradiation. Through an analogy experiment, ethylene production is attributed to the photochemistry and the following thermochemistry of formaldehyde. The absence of the ethylene signal under 400 nm light is consistent with the significantly lower conversion at this wavelength compared with 266 nm. The photocatalytic reaction rate of methanol is also wavelength dependent. Possible reasons for the photon energy dependent phenomena have been discussed. This work not only provides a detailed characterization of the photochemistry of methanol on the rutile TiO2(011)-(2 × 1) surface, but also indicates the importance of photon energy in the photochemistry on TiO2 surfaces.
Co-reporter:Daofu Yuan, Shengrui Yu, Wentao Cheng, Ting Xie, Xueming Yang, and Xingan Wang
The Journal of Physical Chemistry A 2016 Volume 120(Issue 27) pp:4966-4972
Publication Date(Web):February 9, 2016
DOI:10.1021/acs.jpca.5b12644
We report on an experimental study of the vacuum ultraviolet photodissociation dynamics of nitrous oxide as a function of photolysis wavelength. In this study, both the N(2DJ) + NO(X2Π) and N(2PJ) + NO(X2Π) product channels were investigated using the time-sliced velocity ion imaging technique. Images of the N(2DJ=5/2,3/2) and N(2PJ=3/2,1/2) products were measured at seven and ten, respectively, photolysis wavelengths between 124.44 and 133.20 nm. The vibrational states of the NO products were partially resolved in the acquired raw ion images. The total kinetic energy release and the branching ratios of different vibrational states of NO products were determined. The vibrational state distributions of NO were found to be inverted for the N(2DJ=5/2,3/2) and N(2PJ=3/2,1/2) product channels. This phenomenon indicates that the N–O bond is highly vibrational excited during the breaking of the N–N bond. Vibrational state resolved anisotropic parameters β in both the N(2DJ) and the N(2PJ) channels were acquired. The small β values (around 0.5) in the dissociation process suggest that transition states in a bent configuration play an important role in the formation of N + NO products.
Co-reporter:Xiong Zhou
The Journal of Physical Chemistry C 2016 Volume 120(Issue 3) pp:1709-1715
Publication Date(Web):December 31, 2015
DOI:10.1021/acs.jpcc.5b11362
A series of model catalysts consisting of Pt single atoms and nanoclusters supported by monolayered CuO film grown at Cu(110) were successfully prepared, which could be stabilized well above room temperature and which exhibited a high performance in CO oxidation at temperatures as low as ∼360 K. Combined scanning tunneling microscopy and temperature-programmed desorption measurements directly evidenced that at the initial CO oxidation stage, oxygen vacancy in the CuO lattice was generated at the nearest neighbor of the Pt nanoclusters. The experimental measurements showed that the oxidation activity was inversely proportional to the Pt nanocluster size. In contrast, the Pt single atoms possessed no surface reactivity for the CO oxidation because of the early and complete desorption of CO before its oxidation on the model catalysts commenced.
Co-reporter:Zhenhua Geng, Xiao Chen, Wenshao Yang, Qing Guo, Chenbiao Xu, Dongxu Dai, and Xueming Yang
The Journal of Physical Chemistry C 2016 Volume 120(Issue 47) pp:26807-26813
Publication Date(Web):November 2, 2016
DOI:10.1021/acs.jpcc.6b07774
Photoinduced water dissociation on anatase-TiO2(101) has been investigated using a laser surface photolysis technique, in combination with temperature-programmed desorption and time-of-flight methods. Gaseous OH radicals have been clearly detected by the time-of-flight method during laser irradiation. Further result reveals that the water dissociation reaction occurs most likely via transferring a H atom to a two coordinated oxygen site nearby and ejecting an OH radical to the gas phase. As the water coverage increases, the yield of the water dissociation reaction is also enhanced, whereas the dissociation probability of water is nearly the same at different water coverages. In comparison with water dissociation on rutile-TiO2(110) where the dissociation probability of water is largely inhibited by the strong hydrogen bonds at high water coverage, the reaction on anatase-TiO2(101) is considerably more efficient at high water coverage, which is most likely due to the much weaker interaction between water molecules on the surface. This provides an important clue that strong hydrogen bond interaction should be avoided on a good photocatalyst for water dissociation.
Co-reporter:Zhenhua Geng
The Journal of Physical Chemistry C 2016 Volume 120(Issue 18) pp:9897-9903
Publication Date(Web):April 18, 2016
DOI:10.1021/acs.jpcc.6b02713
Carbonyl coupling of aldehydes is critical to many synthetic processes, including the development of alternative fuels and useful synthetic chemicals. We report a general photoinduced process for carbonyl coupling of aldehydes as a route to ketone synthesis. In contrast to thermal carbonyl coupling of aldehydes with O2 to ketones via carboxylate-type intermediates at high temperature, carbonyl coupling of aldehydes (CH3CHO, CH3CH2CHO, PhCHO) to produce ketones on anatase–TiO2(101) can be promoted by UV light below room temperature with high efficiency on the Ti5c sites, while aldehydes can also be decomposed during UV light irradiation. The mechanistic model constructed in this study elucidates the crucial role of TiO2 as a photocatalyst for synthetic reactions and provides a new pathway in conversion of aldehydes to ketones.
Co-reporter:Wenshao Yang; Dong Wei; Xianchi Jin; Chenbiao Xu; Zhenhua Geng; Qing Guo; Zhibo Ma; Dongxu Dai; Hongjun Fan
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 4) pp:603-608
Publication Date(Web):January 26, 2016
DOI:10.1021/acs.jpclett.6b00015
Photoinduced water dissociation on rutile-TiO2 was investigated using various methods. Experimental results reveal that the water dissociation occurs via transferring an H atom to a bridge bonded oxygen site and ejecting an OH radical to the gas phase during irradiation. The reaction is strongly suppressed as the water coverage increases. Further scanning tunneling microscopy study demonstrates that hydrogen bonds between water molecules have a dramatic effect on the reaction. Interestingly, a single hydrogen bond in water dimer enhances the water dissociation reaction, while one-dimensional hydrogen bonds in water chains inhibit the reaction. Density functional theory calculations indicate that the effect of hydrogen bonds on the OH dissociation energy is likely the origin of this remarkable behavior. The results suggest that avoiding a strong hydrogen bond network between water molecules is crucial for water splitting.
Co-reporter:Zhiqiang Wang; Bo Wen; Qunqing Hao; Li-Min Liu; Chuanyao Zhou; Xinchun Mao; Xiufeng Lang; Wen-Jin Yin; Dongxu Dai; Annabella Selloni
Journal of the American Chemical Society 2015 Volume 137(Issue 28) pp:9146-9152
Publication Date(Web):June 29, 2015
DOI:10.1021/jacs.5b04483
In reduced TiO2, electronic transitions originating from the Ti3+-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material’s blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti3+ species is identified at 2.5 ± 0.2 eV above the Fermi level (EF) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below EF, as they both result from the Jahn–Teller induced splitting of the 3d orbitals of Ti3+ ions in reduced TiO2. Localized excitation of Ti3+ ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti3+ self-doping. Our work reveals the physical origin of the Ti3+ related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.
Co-reporter:Shengrui Yu, Shu Su, Dongxu Dai, Kaijun Yuan and Xueming Yang
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 15) pp:9659-9665
Publication Date(Web):14 Aug 2014
DOI:10.1039/C4CP02734E
The state-to-state dynamics of high-n Rydberg H-atom scattering with para-H2 at the collision energies of 0.45 and 1.07 eV have been studied using the H-atom Rydberg tagging time-of-flight technique. Both the inelastic scattering and reactive scattering are observed in the experimental time-of-flight spectra. The products H2(v′, j′ = odd) come only from reactive scattering and present clearly forward–backward asymmetric angular distributions, which differ from those of the corresponding ion–molecule reaction. The products H2(v′, j′ = even), however, come from both reactive scattering and inelastic scattering. Simulating the rotational distribution from reactive scattering, we found that most of the H2(v′, j′ = even) products come from inelastic scattering. The angular distributions of the product H2(v′, j′ = even) are consistent with what is predicted by the conventional textbook mechanism of inelastic scattering, and are a little different from those of the corresponding ion–molecule inelastic scattering. These results suggest that the effect of Rydberg electron could not be neglected in describing the differential cross sections of H* + para-H2 scattering. From the simulation, the branching ratios of the inelastic scattering channel were determined to be 66% and 79% at the collision energies of 0.45 and 1.07 eV, respectively.
Co-reporter:Hongzhen Wang, Shengrui Yu, Shu Su, Dongxu Dai, Kaijun Yuan, and Xueming Yang
The Journal of Physical Chemistry A 2015 Volume 119(Issue 46) pp:11313-11319
Publication Date(Web):October 22, 2015
DOI:10.1021/acs.jpca.5b08865
The state-selective photodissociation of diacetylene (C4H2) was studied in the wavelength range of 127.5–164.4 nm by high-resolution Rydberg H atom time-of-flight spectroscopy measurements. In the wavelength region, two Rydberg series nR and nR′ were state-selectively excited using tunable vacuum-ultraviolet laser radiation. In all photolysis wavelengths, two decay channels with different dissociation dynamics were observed. In one channel, the characteristic and isotropic translational energy distributions with a peak around 1800 cm–1 can be found, suggesting statistical dissociation through internal conversion (IC) from the Rydberg state to the ground state and then dissociation on the ground-state surface. In contrast to this, in the second channel, nonstatistical and anisotropic translational energy distributions were observed, possibly through IC to the excited repulsive state. The vibrational progressions of C4H (A2Π) products have also been observed and assigned to the CCC bend and C≡C stretch progressions in the second channel at 3R Rydberg states.
Co-reporter:Shengrui Yu, Daofu Yuan, Wentao Chen, Xueming Yang, and Xingan Wang
The Journal of Physical Chemistry A 2015 Volume 119(Issue 29) pp:8090-8096
Publication Date(Web):June 24, 2015
DOI:10.1021/acs.jpca.5b04438
Vacuum ultraviolet photodissociation dynamics of nitrous oxide was investigated using the time-sliced velocity ion imaging technique. Images of the O(1SJ=0) and the O(3PJ=2,1,0) products were measured at nine photolysis wavelengths from 124.44 to 133.20 nm, respectively. Three main dissociation channels: O(1S0) + N2(X1Σg+), O(3PJ=2,1,0) + N2(A3Σu+), and O(3PJ=2,1,0) + N2(B3Πg) were observed and identified in the product images where vibrational states of N2 were fully resolved. Product total kinetic energy releases and angular distributions were acquired. In all product channels, the branching ratios of vibrational states of N2 products were determined. In addition, the O(3PJ=2,1,0) + N2(A3Σu+)/O(3PJ=2,1,0) + N2(B3Πg) branching ratios were determined. We found that in the O(3PJ=2,1,0) channels the O(3PJ=2,1,0) + N2(B3Πg) channel becomes dominant at long photolysis wavelength, indicating a strong coupling between the singlet D(1Σg+) state and the triplet 3Π state. For both O(1S0) and O(3PJ=2,1,0) products, the derived angular anisotropy parameters (β values) are very close to 2 at lower vibrational states of the correlated N2 electronic states and gradually decrease with the increasing vibrational quantum number. These behaviors suggest that the photodissociation processes are primarily governed by a fast dissociation in a linear geometry, while the N2 products at excited vibrational states are very likely produced via a more bent transition state.
Co-reporter:Tiangang Yang, Long Huang, Tao Wang, Chunlei Xiao, Yurun Xie, Zhigang Sun, Dongxu Dai, Maodu Chen, Donghui Zhang, and Xueming Yang
The Journal of Physical Chemistry A 2015 Volume 119(Issue 50) pp:12284-12290
Publication Date(Web):August 24, 2015
DOI:10.1021/acs.jpca.5b06395
The reaction of fluorine atom with vibrationally excited H2 at v = 1 has been studied using a high resolution crossed molecular beam apparatus at collision energies of 0.52 and 0.90 kcal/mol. Product HF rotational state-resolved differential cross sections (DCSs) were measured at v′ = 2, 3, 4 levels. The product angular distributions are predominantly backward scattered except for a small forward signal of HF(v′ = 4) at 0.90 kcal/mol. At the collision energy of 0.52 kcal/mol, the forward scattering peak of the HF(v′ = 2) product, which arises in F + H2(v = 0) reaction from the Feshbach resonances, disappears in F + H2(v = 1) reaction. Oscillatory structures do not appear in the backward direction of the scattering as the collision energy increases from 0.4 to 2.0 kcal/mol, indicating there are no explicit reaction resonances in the F + H2(v = 1, j = 0) → HF + H reaction in the studied energy range. Quantum dynamics calculations on a highly accurate potential energy surface are in good agreement with the experimental results and reveal that the reaction occurs via likely a direct abstraction mechanism, not via long-lived reactive resonances.
Co-reporter:Tiangang Yang;Jun Chen;Long Huang;Tao Wang;Chunlei Xiao;Zhigang Sun;Dongxu Dai;Dong H. Zhang
Science 2015 Volume 347(Issue 6217) pp:60-63
Publication Date(Web):02 Jan 2015
DOI:10.1126/science.1260527
A few very brief pauses in the action
Chemical reactions proceed by the cumulative effect of trillions upon trillions of collisions between atoms and molecules. Usually, a given collision bounces the participants right back out again, either in their original form or with the atoms shuffled around into distinct products. In certain cases, the reacting partners experience a brief lull, termed a resonance, before they rearrange. Yang et al. report the discovery of particularly short-lived resonances in certain reactive collisions of chlorine atoms with vibrationally excited hydrogen deuteride (HD). Their results suggest that similar, as yet overlooked, resonances may lurk in other reactions of vibrationally excited molecules.
Science, this issue p. 60
Co-reporter:Xinchun Mao
The Journal of Physical Chemistry C 2015 Volume 119(Issue 2) pp:1170-1174
Publication Date(Web):December 18, 2014
DOI:10.1021/jp511763w
Chemical changes on methanol/TiO2(110) interface after UV illumination have been investigated by two-photon photoemission spectroscopy (2PPE) and scanning tunneling microscope (STM). Spontaneous recombination of the photocatalytic products, i.e., formaldehyde and hydrogen atoms at the bridging oxygen sites, has been observed using 2PPE. In addition, tip induced recombination of formaldehyde and hydrogen atoms has also been imaged using STM, which further shows that recombination can occur only when the photocatalytic dissociation products stay in the original sites. The experimental observation that methanol can only be dissociated under external forces such as UV illumination while the reverse reaction can take place spontaneously at 130 K may provide an important system for the theoretical calculations of the energetics of adsorbates/TiO2(110).
Co-reporter:Xinchun Mao
The Journal of Physical Chemistry C 2015 Volume 119(Issue 11) pp:6121-6127
Publication Date(Web):March 3, 2015
DOI:10.1021/acs.jpcc.5b00503
Although it has been widely accepted that the crystal phase, morphology, and facet significantly influence the catalytic and photocatalytic activity of TiO2, establishing the correlation between structure and activity of heterogeneous reactions is very difficult because of the complexity of the structure. Utilizing ultrahigh vacuum (UHV) based temperature-programmed desorption (TPD) and density functional theory (DFT) calculations, we have successfully assessed the photoreactivity of two well characterized rutile surfaces ((011)-(2×1) and (110)-(1×1)) through examining the photocatalyzed oxidation of methanol. The photocatalytic products, such as formaldehyde and methyl formate, are the same on both surfaces under UV illumination. However, the reaction rate on (011)-(2×1) is only 42% of that on (110)-(1×1), which contradicts previous reports in aqueous environments where characterization of TiO2 structure is difficult. The discrepancy probably comes from the differences of the TiO2 structure in these studies. Our DFT calculations reveal that the rate-determining step of methanol dissociation on both surfaces is C–H scission,; however, the barrier of this elementary step on (011)-(2×1) is about 0.2 eV higher than that on (110)-(1×1) because of their distinct surface atomic configurations. The present work not only demonstrates the importance of surface structure in the photoreactivity of TiO2, but also provides an example for building the correlation between structure and activity using surface science techniques and DFT calculations.
Co-reporter:Dong Wei
The Journal of Physical Chemistry C 2015 Volume 119(Issue 31) pp:17748-17754
Publication Date(Web):July 10, 2015
DOI:10.1021/acs.jpcc.5b05074
Photocatalysis of a single methanol molecule on the TiO2(110) surface was investigated using a high-resolution scanning tunneling microscopy (STM) technique. Three different types of elementary methanol photocatalytic processes, methanol photodissociation, photoinduced migration of formaldehyde, and formaldehyde photodesorption, were clearly observed. Detailed chemical structures of the intermediates were obtained through careful comparisons between experimental STM images and theoretical simulations based on density functional theory (DFT) calculations. This work demonstrates that elementary photocatalytic processes of a single methanol molecule on the surface can be followed step by step using advanced STM imaging techniques. Such a study can provide unprecedented insights into the surface photocatalytic processes and will greatly help us to understand photocatalysis at the most fundamental level.
Co-reporter:Tao Wang, Tiangang Yang, Chunlei Xiao, Zhigang Sun, Long Huang, Dongxu Dai, Xueming Yang, and Dong H. Zhang
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 17) pp:3049-3055
Publication Date(Web):August 20, 2014
DOI:10.1021/jz501460k
An interesting trimodal structure in the HF (v′ = 2) rotational distribution produced by the F + HD (v = 0, j = 0) reaction, but monomodal structure in the HF (v′ = 2) rotational distribution produced by the F + H2 (v = 0, j = 0) reaction, were observed using a high-resolution crossed molecular beam apparatus. The rotational states of product HF (v′ = 2) are much hotter in the F + HD reaction. It is uncovered that the observations are due to the dominant role of the dynamical resonance states in these two isotopic reactions. The angular potential well in the region of the resonance state of the F + HD reaction is much deeper and supports wave function with high angular kinetic energy, which in turn comes from different H tunneling processes in the F + HD and F + H2 reaction.Keywords: Crossed Molecular Beam; F + H2 Reaction; Isotope Effects; Reaction Dynamics; Reactive Resonance; Rotational State Distribution;
Co-reporter:Jiayue Yang, Dong Zhang, Bo Jiang, Dongxu Dai, Guorong Wu, Donghui Zhang, and Xueming Yang
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 11) pp:1790-1794
Publication Date(Web):May 6, 2014
DOI:10.1021/jz5007252
The effects of CH stretching excitation on the F + CHD3 → HF + CD3 reaction are studied experimentally using crossed-beam and time-sliced velocity map imaging techniques at the collision energy of 9.0 kcal/mol. The fraction of the vibrationally excited CHD3 reagent in the crossed-beam region was determined accurately, allowing us to investigate quantitatively the effects of CH stretching excitation on the title reaction. Experimental data show that the vibrational energy in the excited CH bond of CHD3 is almost exclusively deposited into the HF product vibration, and hence, the HF products from the excited-state reaction are about one vibrational quantum hotter than those of the ground-state reaction, while the vibrational state distribution of the CD3 products is only slightly affected. The reaction is suppressed by the CH stretching excitation, and the overall reactivity of the vibrationally excited reaction is 74 ± 4% of that of the ground-state reaction for CD3(ν2 = 0, 1, 2, 3) product channels.Keywords: mode-selective chemistry; molecular reaction dynamics; polyatomic reaction; time-sliced imaging; vibrational excitation effect;
Co-reporter:Jiayue Yang, Kejie Shao, Dong Zhang, Quan Shuai, Bina Fu, Dong H. Zhang, and Xueming Yang
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 18) pp:3106-3111
Publication Date(Web):August 27, 2014
DOI:10.1021/jz5016923
Despite significant progress made in past decades, it is still challenging to elucidate dynamics mechanisms for polyatomic reactions, in particular, involving complex formation. The reaction of O(1D) with methane has long been regarded as a prototypical polyatomic system of direct insertion reaction in which the O(1D) atom can insert into the C–H bond of methane to form a “hot” methanol intermediate before decomposition. Here, we report a combined theoretical and experimental study on the O(1D) + CHD3 reaction, on which good agreement between theory and experiment is achieved. Our study revealed that this complex-forming reaction actually proceeds via a trapped abstraction mechanism, rather than an insertion mechanism as has long been thought. We anticipate that this reaction mechanism should also be responsible for the reaction of O(1D) with ethane and propane, as well as many other chemical reactions with deep wells in the interaction region.Keywords: complex-forming; crossed molecular beam ion imaging; global PES; polyatomic reactions; QCT; trapped abstraction;
Co-reporter:Huilin Pan, Jiayue Yang, Quan Shuai, Dong Zhang, Weiqing Zhang, Guorong Wu, Dongxu Dai, Bo Jiang, Donghui Zhang, and Xueming Yang
The Journal of Physical Chemistry A 2014 Volume 118(Issue 13) pp:2426-2430
Publication Date(Web):March 17, 2014
DOI:10.1021/jp501681h
Following our previous study on the H + CD4 → HD + CD3 reaction [ Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 12782], the reaction of H + CH4 → H2 + CH3 at collision energies ranging from 0.72 to 1.99 eV is studied using crossed-beam and time-sliced velocity map ion imaging techniques. The product angular and translational energy distributions at four different collision energies were derived from the measured images. The excitation function was also measured from these images together with a careful calibration of the H atom beam intensities at different collision energies. All of these results are compared with those of the H + CD4 reaction to investigate the isotope effects. The isotope effects are all observed in the product angular distributions, the translational energy distributions, and the excitation function and further confirm the reaction mechanism proposed in the previous study on the H + CD4 reaction.
Co-reporter:Zhiguo Zhang, Zhichao Chen, Cunshun Huang, Yang Chen, Dongxu Dai, David H. Parker, and Xueming Yang
The Journal of Physical Chemistry A 2014 Volume 118(Issue 13) pp:2413-2418
Publication Date(Web):March 10, 2014
DOI:10.1021/jp500625m
The NH(a1Δ) + CO(X1Σ+) product channel for the photodissociation of HNCO at 201 nm was investigated using the sliced velocity map ion imaging technique with the detection of NH(a1Δ) products via (2 + 1) resonance enhanced multiphoton ionization (REMPI). Images were measured for the NH(a1Δ) rotational states in the ground and vibrational excited states (v = 0 and 1). Correlation between the NH(a1Δ) and CO rovibrational state distributions were determined from these images. Experimental results show that the vibrational distribution of the CO fragment in the NH(a1Δ) + CO(X1Σ+) channel peaks at v = 1. The negative anisotropy parameter measured for the NH(a1Δ) (v = 0 and 1|j) products indicates a direct dissociation process for the N–C bond cleavage in the S1 state. A bimodal CO rotational distribution was observed, suggesting that HNCO dissociates in the S1 state in two distinctive pathways.
Co-reporter:Wenshao Yang ; Zhenhua Geng ; Chenbiao Xu ; Qing Guo ; Dongxu Dai
The Journal of Physical Chemistry C 2014 Volume 118(Issue 48) pp:27920-27924
Publication Date(Web):November 12, 2014
DOI:10.1021/jp509091a
Reductive C–C coupling of acetaldehyde (CH3CHO) on rutile TiO2(110) surface with controlled oxygen vacancy coverage was investigated by temperature-programmed desorption (TPD). Two main reaction products, 2-butanone and butene, have been detected. Experimental results suggest that the two products are produced via two distinctive vacancy-assisted C–C couplings on the surface. The 2-butanone product is produced via the reductive C–C coupling of CH3CHO on reduced rutile TiO2(110) in which one CH3CHO molecule adsorbed on the five coordinated Ti site (Ti5c) and the other on a nearby bridge-bonded oxygen (BBO) vacancy site; while butene is produced from the self-coupling of two CH3CHO molecules adsorbed on two adjacent BBO vacancy sites. The results of this work indicate that surface vacancy plays a key role in the C–C coupling reactions of CH3CHO.
Co-reporter:Chengbiao Xu ; Wenshao Yang ; Zefeng Ren ; Dongxu Dai ; Qing Guo ; Timothy K. Minton
Journal of the American Chemical Society 2013 Volume 135(Issue 50) pp:19039-19045
Publication Date(Web):December 3, 2013
DOI:10.1021/ja4114598
Photocatalytic dissociation of methanol (CH3OH) on a TiO2(110) surface has been studied by temperature programmed desorption (TPD) at 355 and 266 nm. Primary dissociation products, CH2O and H atoms, have been detected. The dependence of the reactant and product TPD signals on irradiation time has been measured, allowing the photocatalytic reaction rate of CH3OH at both wavelengths to be directly determined. The initial dissociation rate of CH3OH at 266 nm is nearly 2 orders of magnitude faster than that at 355 nm, suggesting that CH3OH photocatalysis is strongly dependent on photon energy. This experimental result raises doubt about the widely accepted photocatalysis model on TiO2, which assumes that the excess potential energy of charge carriers is lost to the lattice via strong coupling with phonon modes by very fast thermalization and the reaction of the adsorbate is thus only dependent on the number of electron–hole pairs created by photoexcitation.
Co-reporter:Chenbiao Xu ; Wenshao Yang ; Qing Guo ; Dongxu Dai ; Maodu Chen
Journal of the American Chemical Society 2013 Volume 136(Issue 2) pp:602-605
Publication Date(Web):December 30, 2013
DOI:10.1021/ja411020t
Photocatalysis of methanol (CH3OH) on anatase (A)-TiO2(101) has been investigated using temperature programmed desorption (TPD) method with 266 nm light at low surface temperatures. Experimental results show that CH3OH adsorbs on the A-TiO2(101) surface predominantly in molecular form, with only a small amount of CH3OH in dissociated form. Photocatalytic products, formaldehyde (CH2O) and methyl formate (HCOOCH3), have been detected under 266 nm light irradiation. In addition to H2O formation, H2 product is also observed by TPD spectroscopy. Experimental results indicate that H2 product is formed via thermal recombination of H-atoms on the BBO sites from photocatalysis of CH3OH on the A-TiO2(101) surface, and H2 production on the A-TiO2(101) surface is significantly more efficient than that on the rutile (R)-TiO2(110) surface.
Co-reporter:Chenbiao Xu, Wenshao Yang, Qing Guo, Dongxu Dai, Maodu Chen, and Xueming Yang
Journal of the American Chemical Society 2013 Volume 135(Issue 28) pp:10206-10209
Publication Date(Web):July 2, 2013
DOI:10.1021/ja4030963
It is well established that adding methanol to water could significantly enhance H2 production by TiO2. Recently, we have found that methanol can be photocatalytically dissociated on TiO2(110) at 400 nm via a stepwise mechanism. However, how molecular hydrogen can be formed from the photocatalyzed methanol/TiO2(110) surface is still not clear. In this work, we have investigated deuterium formation from photocatalysis of the fully deuterated methanol (CD3OD) on TiO2(110) at 400 nm using a temperature programmed desorption (TPD) technique. Photocatalytic dissociation products formaldehyde (CD2O) and D-atoms on BBO sites (via D2O TPD product) have been detected. In addition to D2O formation by heating the photocatalyzed methanol/TiO2(110) surface, we have also observed D2 product formation. D2 is clearly formed via thermal recombination of the D-atoms on the BBO sites from photocatalysis of methanol. Experimental results indicate that D2O formation is more important than D2 formation and that D2 formation is clearly affected by the D2O formation process.
Co-reporter:Shengrui Yu, Shu Su, Dongxu Dai, Kaijun Yuan, and Xueming Yang
The Journal of Physical Chemistry A 2013 Volume 117(Issue 50) pp:13564-13571
Publication Date(Web):September 16, 2013
DOI:10.1021/jp407556k
Photodissociation dynamics of the H-atom channel from HNCO photolysis between 124 and 137 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. Two dissociation channels, H + NCO (X2Π) and H + NCO(A2Σ+), have been observed. The former channel involves two different dissociation pathways; one is a slow predissociation pathway through internal conversion from the excited state to the S0 state, and the other is a fast predissociation pathway through internal conversion from the excited state to the S1 state. The latter channel dominates by a prompt dissociation via coupling to the S2 state. As the photon energy increases, dissociation on the ground state S0 becomes dominant. Vibrational structures are observed in both the NCO(X) and NCO(A) channels, which can be assigned to the bending mode excitation with some stretching vibrational excitation.
Co-reporter:Shengrui Yu, Shu Su, Yvonne Dorenkamp, Alec M. Wodtke, Dongxu Dai, Kaijun Yuan, and Xueming Yang
The Journal of Physical Chemistry A 2013 Volume 117(Issue 46) pp:11673-11678
Publication Date(Web):March 15, 2013
DOI:10.1021/jp312793k
Photodissociation dynamics of HNCO at photolysis wavelengths between 200 and 240 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. At low photon energy excitation, the product translational energy distribution is nearly statistical and the angular distribution is isotropic, which is consistent with an indirect dissociation mechanism, i.e., internal conversion from S1 to S0 surface and dissociation on S0 surface. As the photon energy increases, a direct dissociation pathway on S1 surface opens up. The product translational energy distribution appears to be quite nonstatistical and the product angular distribution is anisotropic. The fraction of direct dissociation pathway is determined to be 36 ± 5% at 202.67 nm photolysis. Vibrational structures are observed in both direct and indirect dissociation pathways, which can be assigned to the NCO bending mode excitation with some stretching excitation.
Co-reporter:Chenbiao Xu, Wenshao Yang, Qing Guo, Dongxu Dai, Timothy K. Minton, and Xueming Yang
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 16) pp:2668-2673
Publication Date(Web):July 26, 2013
DOI:10.1021/jz401349q
We have investigated the photoinduced decomposition of formaldehyde (CH2O) on TiO2(110) at 400 nm using temperature-programmed desorption. Formate (HCOO), methyl radicals (CH3), and ethylene (C2H4) have been detected, while no evidence of polymerization of CH2O was found. The initial step in the decomposition of CH2O on TiO2(110) is the formation of a dioxymethylene intermediate in which the carbonyl O atom of CH2O is bound both to a Ti atom on the five-fold-coordinated lattice site (Ti5C) and to a nearby bridge-bonded oxygen (BBO) atom. During 400 nm irradiation, the dioxymethylene intermediate can transfer methylene to the bridging oxygen row and break the C–O bond, thus leaving the original carbonyl O atom on the Ti5C site. After this transfer of methylene, several pathways to products are available. Thus we have found that BBO atoms are intimately involved in the photoinduced decomposition of CH2O on TiO2(110).Keywords: bridge-bonded oxygen; dioxymethylene; formaldehyde; photoinduced decomposition; temperature-programmed desorption; titanium dioxide;
Co-reporter:Tao Wang, Tiangang Yang, Chunlei Xiao, Dongxu Dai, and Xueming Yang
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 3) pp:368-371
Publication Date(Web):January 9, 2013
DOI:10.1021/jz302103u
A primary prerequisite to study reactivity of vibrationally excited species is to efficiently prepare reacting species in a well-defined vibrational level. Efficient pumping of IR active vibrational modes in a molecule can be achieved by direct IR absorption. For vibrational modes that are only Raman active, however, efficient preparation of vibrationally excited states in those modes is not easily attainable. In this work, we have shown that highly efficient preparation of the HD(v = 1) state using the Stark-induced adiabatic Raman passage (SARP) scheme is feasible. As high as 91% population transfer from v = 0 to 1 of HD has been demonstrated in our experiment. This method provides new opportunities for future experimental studies on the dynamics of vibrational state molecules, especially H2, in both gas-phase and beam-surface reactions.Keywords: chemical dynamics; molecular beams; molecular dynamics; stark-induced adiabatic Raman passage; vibrational excitation;
Co-reporter:Xinchun Mao, Xiufeng Lang, Zhiqiang Wang, Qunqing Hao, Bo Wen, Zefeng Ren, Dongxu Dai, Chuanyao Zhou, Li-Min Liu, and Xueming Yang
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 22) pp:3839-3844
Publication Date(Web):October 30, 2013
DOI:10.1021/jz402053p
Many physical and chemical processes on TiO2 surface are linked to the excess electrons originated from band gap states. However, the sources (surface and/or subsurface defects) of these states are controversial. We present quantitative ultraviolet photoelectron spectroscopy (UPS) measurements on the band gap states of TiO2(110) with constant subsurface defect density and varied surface bridging hydroxyls (ObrH) prepared through photocatalyzed splitting of methanol, in combination with density functional theory (DFT) calculations. Our results clearly suggest both surface and subsurface defects contribute to the band gap states, whereas the contribution of subsurface defects corresponds to that of only 1.9% monolayer ObrH at the current bulk reduction level. As the surface defect concentration is usually much larger than 1.9% monolayer in real studies and applications, our work demonstrates the importance of surface defects in changing the electronic structure of TiO2, which dictates the surface chemistry.Keywords: band-gap states; electronic structure; subsurface defects; surface defects; TiO2;
Co-reporter:Qing Guo, Chenbiao Xu, Wenshao Yang, Zefeng Ren, Zhibo Ma, Dongxu Dai, Timothy K. Minton, and Xueming Yang
The Journal of Physical Chemistry C 2013 Volume 117(Issue 10) pp:5293-5300
Publication Date(Web):February 22, 2013
DOI:10.1021/jp401613s
Previous observations of methyl formate (HCOOCH3) during the photo-oxidation of methanol (CH3OH) on TiO2 catalysts suggested that photocatalysis on TiO2 could be used to build up complex molecules from a single precursor. We have investigated the mechanism of HCOOCH3 formation by irradiating a CH3OH-adsorbed TiO2(110) surface with 400 nm light at low surface temperatures. Through the detection of volatile products after irradiation by temperature programmed desorption, we have found, as previously reported [Phillips et al. J. Am. Chem. Soc.2013, 135, 574–577] that HCOOCH3 is formed by the cross-coupling reaction of CH3O and CH2O, which are products of the first and second dissociation steps, respectively, in the stepwise photocatalysis of CH3OH on TiO2(110). Unlike the previous study, we have observed the photocatalytic production of HCOOCH3 without preoxidation of the surface, and we have concluded that the final reaction step to produce HCOOCH3 (i.e., the cross-coupling reaction of CH2O with CH3O) does not involve a transient HCO intermediate.
Co-reporter:Zhibo Ma, Qing Guo, Xinchun Mao, Zefeng Ren, Xu Wang, Chenbiao Xu, Wenshao Yang, Dongxu Dai, Chuanyao Zhou, Hongjun Fan, and Xueming Yang
The Journal of Physical Chemistry C 2013 Volume 117(Issue 20) pp:10336-10344
Publication Date(Web):April 21, 2013
DOI:10.1021/jp309925x
Ethanol on TiO2(110) has been studied using the temperature-programmed desorption (TPD), femtosecond two-photon photoemission spectroscopy (2PPE), and density functional theory (DFT) calculations. The first layer of ethanol (binds to Ti5c) whose molecular state has been predicted to be more stable by DFT desorbs at 295 K. A photoinduced excited state that is associated with bridging hydroxyls has been detected at ∼2.4 eV above the Fermi level on ethanol/TiO2(110) interface using 2PPE. Detailed TPD studies show that ethanol on Ti5c can be photocatalytically converted to acetaldehyde by near-band-gap excitation with the hydrogen atoms transfer to bridging-bonded oxygen sites, which is consistent with the 2PPE results. TPD results also show a low-temperature water TPD peak that seems to bind to the Ti5c sites in addition to the ethylene TPD product. These results suggest that the Ti5c sites on TiO2(110) are the primary active sites for photocatalysis of ethanol on TiO2(110), while bridging-bonded oxygen sites also play an important role, as in the case of methanol. The kinetics of photocatalyzed ethanol dissociation on TiO2(110) has also been measured using the 2PPE technique, which is of heterogeneous nature.
Co-reporter:Tao Wang;Jun Chen;Tiangang Yang;Chunlei Xiao;Zhigang Sun;Long Huang;Dongxu Dai;Dong H. Zhang
Science 2013 Volume 342(Issue 6165) pp:1499-1502
Publication Date(Web):20 Dec 2013
DOI:10.1126/science.1246546
Access via Vibration
Molecular beam studies over the past decade have elucidated many subtle quantum mechanical factors governing the influence of vibrational excitation on the outcome of elementary chemical reactions. However, these studies have generally had to focus on reagents that can be easily made to vibrate by direct absorption in the infrared (IR). Wang et al. (p. 1499) show that a variation on stimulated Raman pumping can efficiently excite the IR-inactive stretch vibration in the diatomic molecule, hydrogen deuteride (HD). As a result, they can probe the influence of that vibration on the outcome of the HD + F reaction. Through a combined spectroscopic and theoretical investigation, they uncover Feshbach resonances along the reaction coordinate that are only accessible through vibrational preexcitation.
Co-reporter:Chuanyao Zhou, Zhibo Ma, Zefeng Ren, Alec M. Wodtke and Xueming Yang
Energy & Environmental Science 2012 vol. 5(Issue 5) pp:6833-6844
Publication Date(Web):07 Mar 2012
DOI:10.1039/C2EE21493H
Two-photon photoemission (2PPE) has been widely used in the study of electronic structure and dynamics of unoccupied electronic states on different types of surfaces and interfaces. Since 2PPE probes electronically excited states, it should be sensitive to surface excited electronic structure changes that accompany surface chemical reactions. Therefore, this method could potentially be used to study the kinetics and dynamics of surface chemical reactions as well as surface photocatalysis. In this article, we briefly review recent progress made in the study of surface photochemistry and photocatalysis using the time-dependent 2PPE (TD-2PPE) method. A few examples are given to demonstrate the application of this method in probing surface photochemistry and photocatalysis, particularly photocatalysis of methanol on TiO2 surfaces. Since many problems associated with surface photochemistry and surface photocatalysis are related to energy and environmental issues, the 2PPE technique could have important applications in the study of the fundamental problems in energy and environmental sciences.
Co-reporter:Qing Guo ; Chenbiao Xu ; Zefeng Ren ; Wenshao Yang ; Zhibo Ma ; Dongxu Dai ; Hongjun Fan ; Timothy K. Minton
Journal of the American Chemical Society 2012 Volume 134(Issue 32) pp:13366-13373
Publication Date(Web):July 13, 2012
DOI:10.1021/ja304049x
We have investigated the photocatalysis of partially deuterated methanol (CD3OH) and H2O on TiO2(110) at 400 nm using a newly developed photocatalysis apparatus in combination with theoretical calculations. Photocatalyzed products, CD2O on Ti5c sites, and H and D atoms on bridge-bonded oxygen (BBO) sites from CD3OH have been clearly detected, while no evidence of H2O photocatalysis was found. The experimental results show that dissociation of CD3OH on TiO2(110) occurs in a stepwise manner in which the O–H dissociation proceeds first and is then followed by C–D dissociation. Theoretical calculations indicate that the high reverse barrier to C–D recombination and the facile desorption of CD2O make photocatalytic methanol dissociation on TiO2(110) proceed efficiently. Theoretical results also reveal that the reverse reactions, i.e, O–H recombination after H2O photocatalytic dissociation on TiO2(110), may occur easily, thus inhibiting efficient photocatalytic water splitting.
Co-reporter:Shengrui Yu, Kaijun Yuan, Hui Song, Xin Xu, Dongxu Dai, Dong H. Zhang and Xueming Yang
Chemical Science 2012 vol. 3(Issue 9) pp:2839-2842
Publication Date(Web):25 Jun 2012
DOI:10.1039/C2SC20489D
Full quantum-state resolved differential cross-sections of the H*(n) + o-D2 → HD + D*(n′) reaction have been measured for the first time using the Rydberg H-atom time-of-flight method. Experimental results show that the angular distributions of HD product rotational states show a strong preference for forward scattering. This result is considerably different to that predicted by full quantum mechanical calculations on the corresponding ion–molecule reaction, suggesting that the ionic core and Rydberg electron coupling cannot be neglected in the Rydberg H-atom reactive scattering with D2 and, therefore, that the Fermi independent-collider model is not valid in describing the dynamics of Rydberg atom reactions with molecules.
Co-reporter:Shengrui Yu, Shu Su, Kaijun Yuan, Dongxu Dai, and Xueming Yang
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 17) pp:2420-2424
Publication Date(Web):August 15, 2012
DOI:10.1021/jz3010255
The state-resolved differential cross sections for the Rydberg-atom (RA) inelastic scattering process H*(n = 46) + O2(v = 0, j = 1,3) → H*(n′) + O2(v′, j′) have been measured by using the H-atom Rydberg tagging time-of-flight (HRTOF) technique. Extensive vibrational excitation of O2 products has been observed at the two collision energies of 0.64 and 1.55 eV. Experimental results show that the O2 products in the low vibrationally excited states are clearly forward-scattered, whereas those in the highly vibrationally excited states are mainly backward-scattered. Partially resolved rotational structures were also observed and assigned. The striking observation of extremely high energy transfer from translational to vibrational excitation at the backward direction could be explained involving charge transfer between proton and O2 molecule and possibly complex formation during the scattering process.Keywords: differential cross sections; inelastic scattering; Rydberg atom;
Co-reporter:Quan Shuai, Huilin Pan, Jiayue Yang, Dong Zhang, Bo Jiang, Dongxu Dai, and Xueming Yang
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 10) pp:1310-1314
Publication Date(Web):April 30, 2012
DOI:10.1021/jz300453f
The O(1D) + CD4 → OD + CD3 reaction was investigated using the crossed molecular beam technique with sliced velocity map imaging at four different collision energies: 1.6, 2.8, 4.6, and 6.8 kcal/mol. The vibrational ground state product CD3 was detected using a (2 + 1) resonance-enhanced multiphoton ionization (REMPI). Remarkably different features were found in the forward and backward scatterings, and gradually changed with the collision energy. These features were attributed to two distinctive reaction mechanisms—insertion and abstraction—that occur on the ground and excited state surfaces, respectively. Contributions from the two mechanisms were extracted from the experiment results, and a positive correlation was found between the abstraction proportion and the collision energy. The threshold for the abstraction pathway was determined and compared with results from calculations.Keywords: collision energy; crossed molecular beam; dual mechanisms; slice imaging; threshold;
Co-reporter:Kaijun Yuan, Richard N. Dixon, and Xueming Yang
Accounts of Chemical Research 2011 Volume 44(Issue 5) pp:369
Publication Date(Web):March 23, 2011
DOI:10.1021/ar100153g
Water and light are two common constituents of both the earth’s atmosphere and interstellar space. Consequently, water photodissociation is a central component of the chemistry of these environments. Electronically excited molecules can dissociate adiabatically (on a single potential energy surface, or PES) or nonadiabatically (with transfer between PESs), and water serves as a prototype for understanding these two processes in unimolecular dissociation. In recent years, extensive experimental and theoretical studies have been focused on water photolysis, particularly on the primary product of the dissociation, the OH radical. The use of the high-resolution H-atom Rydberg tagging technique, in combination with various vacuum ultraviolet (VUV) sources, has spurred significant advances in water photochemistry. As the excitation energy increases, different excited electronic states of water can be reached, and the mutual interactions between these states increase significantly. In this Account, we present the most recent developments in water photodissociation that have been derived from the study of the four lowest electronic excited states.The Ã1B1 state photodissociation of H2O has been studied at 157.6 nm and was found to be a fast and direct dissociation process on a single repulsive surface, with only vibrational excitation of the OH(X2Π) product. In contrast, the dissociation of the B̃1A1 state was found to proceed via two main routes: one adiabatic pathway leading to OH(A2Σ+) + H, and one nonadiabatic pathway to OH(X2Π) + H through conical intersections between the B̃ state and the ground state X̃1A1. An interesting quantum interference between two conical intersection pathways has also been observed. In addition, photodissociation of H2O between 128 and 133 nm has been studied with tunable VUV radiation. Experimental results illustrate that excitation to the different unstable resonances of the state has very different effects on the OH(X2Π) and OH(A2Σ+) product channels.The C̃1B1 state of H2O is a predissociative Rydberg state with fully resolved rotational structures. A striking variation in the OH product state distribution and its stereodynamics has been observed for different rotational states. There are two kinds of nonadiabatic dissociation routes on the C̃ state. The first involves Renner−Teller (electronic Coriolis) coupling to the B̃ state, leading to rotationally hot and vibrationally cold OH products. The second goes through a newly discovered homogeneous nonadiabatic coupling to the à state, leading to rotationally cold and vibrationally hot OH products. But the D̃1A1 state shows no rotational structure and leads to a fast, homogeneous, purely electronic predissociation to the B̃ state.These studies demonstrate the truly fascinating nature of water photochemistry, which is extremely variable because of the different electronic states and their interactions. The results also provide a rather complete picture of water photochemistry and should be helpful in the modeling of interstellar chemistry, with its abundant VUV radiation.
Co-reporter:Chuanyao Zhou, Zhibo Ma, Zefeng Ren, Xinchun Mao, Dongxu Dai and Xueming Yang
Chemical Science 2011 vol. 2(Issue 10) pp:1980-1983
Publication Date(Web):22 Jul 2011
DOI:10.1039/C1SC00249J
Photocatalytic dissociation of deuterated methanol (CD3OD) on both stoichiometric and reduced TiO2(110) surfaces was investigated using the time-dependent two-photon photoemission (2PPE) method, in order to understand the effect of defects on the kinetics of methanol dissociation on TiO2(110). By monitoring the time evolution of the photoinduced excited state on the methanol covered surface, the photocatalytic dissociation kinetics of methanol on the TiO2 surface were observed. The measured photodissociation rate on the reduced TiO2(110) surface is more than an order of magnitude faster than that on the stoichiometric surface. Since the reduced TiO2(110) surface has considerably more surface and subsurface defects than the stoichiometric surface, the experimental observation suggests that one or both of them could accelerate the photocatalysis process of methanol on the TiO2(110) surface in a significant way.
Co-reporter:Xueming Yang
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 18) pp:8112-8121
Publication Date(Web):09 Apr 2011
DOI:10.1039/C1CP00005E
In the last decade or so, the H-atom Rydberg tagging time-of-flight (HRTOF) technique has made a significant impact in the study of state-to-state reaction dynamics, and especially in the study of transition state dynamics of elementary chemical reactions and quantum state resolved dynamics of molecular photodissociation of important molecules. In this perspective, we will discuss mainly the state-to-state dynamics of three important elementary reactions: H + H2, O(1D) + H2 and F + H2 that have been studied in our laboratory in recent years using the HRTOF method. In addition, we will also mention briefly the experimental results of other reactive systems. In the end, we will also present a brief research outlook in the study of molecular reaction dynamics using this powerful experimental method.
Co-reporter:Zhichao Chen, Andre T. J. B. Eppink, Bo Jiang, Gerrit C. Groenenboom, Xueming Yang and David H. Parker
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 6) pp:2350-2355
Publication Date(Web):26 Nov 2010
DOI:10.1039/C0CP01794A
The OH + CH3 product channel for the photodissociation of CH3OH at 157 nm was investigated using the velocity map imaging technique with the detection of CH3 radical products via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images were measured for the CH3 formed in the ground and excited states (v2 = 0, 1, 2, and 3) of the umbrella vibrational mode and correlated OH vibrational state distributions were also determined. We find that the vibrational distribution of the OH fragment in the OH + CH3 channel is clearly inverted. Anisotropic distributions for the CH3 (v2 = 0, 1, 2, and 3) products were also determined, which is indicative of a fast dissociation process for the C–O bond cleavage. A slower CH3 product channel was also observed, that is assigned to a two-step photodissociation process, in which the first step is the production of a CH3O(X 2E) radical via the cleavage of the O–H bond in CH3OH, followed by probe laser photodissociation of the nascent CH3O radicals yielding CH3(X 2A1, v = 0) products.
Co-reporter:Zhichao Chen, Quan Shuai, André T. J. B. Eppink, Bo Jiang, Dongxu Dai, Xueming Yang and David H. Parker
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 18) pp:8531-8536
Publication Date(Web):30 Mar 2011
DOI:10.1039/C1CP00032B
The SH + CH3 product channel for the photodissociation of CH3SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH3 products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH3 formed in the ground and excited vibrational states (v2 = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH3 channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH3 (v2 = 0, 1, and 2) products is indicative of a fast dissociation process for C–S bond cleavage. Two kinds of slower CH3 products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH3S (X2E) radical via cleavage of the S–H bond in CH3SH, followed by probe laser photodissociation of nascent CH3S radicals yielding CH3(X2A1, v2 = 0–2) + S(3Pj/1D) products.
Co-reporter:Lina Cheng, Kaijun Yuan, Yuan Cheng, Qing Guo, Tao Wang, Dongxu Dai, and Xueming Yang and Richard N. Dixon
The Journal of Physical Chemistry A 2011 Volume 115(Issue 9) pp:1500-1507
Publication Date(Web):January 19, 2011
DOI:10.1021/jp109169f
The dissociation dynamics of HOD via two-photon excitation to the C̃ state have been investigated using the H-atom Rydberg tagging time-of-flight (TOF) technique. The H-atom action spectrum for the C̃ ← X̃ transition shows resolved rotational structure. Product translational energy distributions and angular distributions have also been recorded for the H + OD channel for three excited levels each with ka′ = 2. From these distributions, quantum state distributions and angular anisotropy parameters (β2 and β4) for the OD product were determined. These results are consistent with the nonadiabatic predissociation picture illustrated in the one-photon dissociation process for H2O. The heterogeneous dissociation pathway via Coriolis coupling is the dominant dissociation process in the present study. A high proportion of the total available energy is deposited into the rotational energy of the OD product. The anisotropic recoil distributions reveal the distinctive contributions from the alignment of the excited states and the dissociation process. Comparisons are also made between the results for HOD and H2O via the equivalent rotational transitions. The OH bond energy, Do(H−OD), of the HOD molecule is also determined to be 41283.0 ± 5 cm−1.
Co-reporter:Wenrui Dong;Zhigang Sun;Xin Xu;Shu Liu;Dong H. Zhang;Chunlei Xiao;Tiangang Yang;Tao Wang;Dongxu Dai
Science 2011 Volume 333(Issue 6041) pp:440-442
Publication Date(Web):22 Jul 2011
DOI:10.1126/science.1205770
A theoretical analysis of a four-atom reaction has a level of detail and accuracy previously restricted to three-atom systems.
Co-reporter:Guorong Wu, Weiqing Zhang, Huilin Pan, Quan Shuai, Jiayue Yang, Bo Jiang, Dongxu Dai and Xueming Yang
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 32) pp:9469-9474
Publication Date(Web):25 Jun 2010
DOI:10.1039/B927455C
The dynamics of the Cl + SiH4 → HCl + SiH3 reaction has been studied using the crossed molecular beam technique with slice imaging. The ion images of the silyl radical (SiH3) product from the Cl + SiH4 reaction were recorded at different collision energies, using the (2+1) resonance enhanced multi-photon ionization (REMPI) technique. The product velocity and angular distributions of this reaction were determined from the recorded images. The vibrational state-to-state correlation of the relative population between the two reaction products, SiH3 and HCl, was also determined. Finally, the Cl + SiH4 results are compared with that of the F + SiH4 reaction.
Co-reporter:Yongwei Zhang, Kaijun Yuan, Shengrui Yu and Xueming Yang
The Journal of Physical Chemistry Letters 2010 Volume 1(Issue 2) pp:475-479
Publication Date(Web):December 22, 2009
DOI:10.1021/jz900303e
Photodissociation of CH4 has been studied using the high-resolution Rydberg tagging time-of-flight technique. The TOF spectra show an important single C−H bond fission channel with partially resolved sharp features. Careful simulations indicate that these sharp peaks are due to highly rotationally excited CH3 products, which are likely produced through a conical intersection dissociation pathway between the excited and ground potential energy surfaces. The energy-dependent anisotropy parameter of the CH3 product has also been determined at various photolysis wavelengths. The results of this work show that the conical intersection between the S1 and S0 surfaces plays an essential role in the photochemistry of CH4.Keywords (keywords): conical intersection; methane photochemistry; nonadiabatic dissociation;
Co-reporter:Chongfa Xiao, Guanlin Shen, Xiuyan Wang, Hongjun Fan and Xueming Yang
The Journal of Physical Chemistry A 2010 Volume 114(Issue 13) pp:4520-4523
Publication Date(Web):March 15, 2010
DOI:10.1021/jp100435q
The F-atom reaction with NH3 and ND3 has been studied using the universal crossed beams technique. Angular resolved time-of-flight spectra were measured for the HF and DF reaction products. Product angular distribution and product kinetic energy distribution in the center-of-mass frame were determined from the experimental TOF spectra. Experimental results show that the HF and DF products are largely forward-scattered relative to the F-atom beam direction with a considerable amount of product at sideway and backward scattering directions. High-level ab initio calculation on the reaction energy pathway suggests that the forward-scattered products are mainly produced via a direct abstraction mechanism at large impact parameters, whereas sideway- and backward-scattered products are likely due to a long-lived complex formation mechanism.
Co-reporter:Quan Shuai;Guorong Wu;Yong Zhou;Bo Jiang;Bina Fu;Yunpeng Lu;Huilin Pan;Weiqing Zhang;Liling Zhang;Dongxu Dai;Lan Liu;Shu Liu;Dong H. Zhang;Soo-Ying Lee;Michael A. Collins;Joel M. Bowman;Bastiaan J. Braams;Zhen Xie
PNAS 2010 Volume 107 (Issue 29 ) pp:12782-12785
Publication Date(Web):2010-07-20
DOI:10.1073/pnas.1006910107
Crossed molecular beam experiments and accurate quantum scattering calculations have been carried out for the polyatomic H + CD4 → HD + CD3 reaction. Unprecedented agreement has been achieved between theory and experiments on the energy dependence of the integral
cross section in a wide collision energy region that first rises and then falls considerably as the collision energy increases
far over the reaction barrier for this simple hydrogen abstraction reaction. Detailed theoretical analysis shows that at collision
energies far above the barrier the incoming H-atom moves so quickly that the heavier D-atom on CD4 cannot concertedly follow it to form the HD product, resulting in the decline of reactivity with the increase of collision
energy. We propose that this is also the very mechanism, operating in many abstraction reactions, which causes the differential
cross section in the backward direction to decrease substantially or even vanish at collision energies far above the barrier
height.
Co-reporter:Chunlei Xiao;Wenrui Dong;Dong H. Zhang;Tao Wang;Dongxu Dai
Science 2010 Volume 327(Issue 5972) pp:1501-1502
Publication Date(Web):19 Mar 2010
DOI:10.1126/science.1185694
Co-reporter:Zhichao Chen, Fuchun Liu, Bo Jiang, Xueming Yang and David H. Parker
The Journal of Physical Chemistry Letters 2010 Volume 1(Issue 12) pp:1861-1865
Publication Date(Web):June 3, 2010
DOI:10.1021/jz100356f
The spin-forbidden CO(ν) + O(3Pj) channel produced by 157 nm photodissociation of CO2 was investigated by velocity map imaging. O(3P) images were measured for the three 3Pj spin−orbit states (j = 0, 1, and 2), and the CO vibrational-state distributions that correlate to the O(3Pj=0,1,2) spin−orbit product were determined. Nearly all energetically allowed product CO(ν) levels (i.e., ν ≤ 8) are observed, with minor differences in the CO vibrational distributions for the three channels. The angular anisotropy, which also shows minor differences for the three j channels, is found to decrease with increasing CO vibrational excitation, suggesting that the lower vibrational states are produced by photodissociation from a more linear OCO geometry. Fits to the total kinetic energy distributions suggest that the CO rotational energy is relatively cold. The overall results further suggest that the three spin−orbit pathways are not mixed completely in the exit channel.Keywords (keywords): 157 nm; CO2; photodissociation; velocity map imaging;
Co-reporter:Fengyan Wang, Yongwei Zhang, Hua Wang, Jie Liu, Bo Jiang, Xiuyan Wang and Xueming Yang
Physical Chemistry Chemical Physics 2009 vol. 11(Issue 39) pp:8733-8740
Publication Date(Web):30 Jul 2009
DOI:10.1039/B903688A
The dissociation dynamics of HFCO+ ion has been studied using the velocity map ion imaging technique. The HFCO+ ion is prepared by one-photon resonant three-photon ionization in the region of 43100–43860 cm−1 excitation energy. The HFCO+ ions, produced by multiphoton ionization, have sufficient internal energy to dissociate into the F and HCO+ fragments without further absorption of another photon. Images of HCO+ have been recorded at various excitation energies. It is noticed that the angular distributions of HCO+ change dramatically from parallel distribution to perpendicular distribution and then back to parallel distribution in a very narrow excitation energy region of 43473–43500 cm−1. Analysis of anisotropy parameters of βn (n = 2, 4 and 6) reveals that the electronic states in the three-photon excitation of HFCO are mainly: HFCO(X1A′) → HFCO(A1A″) → HFCO(A′) → HFCO+(A2A″;B2A′). The purely perpendicular resonant transitions are likely responsible for the perpendicular angular distribution of the HCO+ ion fragment.
Co-reporter:Weiqing Zhang, Guorong Wu, Huilin Pan, Quan Shuai, Bo Jiang, Dongxu Dai and Xueming Yang
The Journal of Physical Chemistry A 2009 Volume 113(Issue 16) pp:4652-4657
Publication Date(Web):March 19, 2009
DOI:10.1021/jp8114429
The dynamics of the F + SiH4 → HF + SiH3 reaction has been studied using the crossed molecular beam technique with slice imaging at collision energies from 1.25 to 8.17 kcal/mol. The product silyl radical, SiH3(v2 = 0−5), was detected using the (2 + 1) resonance enhanced multiphoton ionization technique. The product velocity distributions and angular distributions of this reaction were obtained from the recorded images. Experimental results show that the silyl radical product is mainly forward scattered relative to the silane beam direction, and the majority of the available energy was partitioned into the vibration of the HF product. The state-to-state correlation between the two reaction products, SiH3 and HF, was also determined. In addition, we found that the reaction cross section goes down as the collision energy increases. From these results, we conclude that the F + SiH4 → HF + SiH3 reaction proceeds through a direct abstraction mechanism with little or no reaction barrier.
Co-reporter:Xingan Wang;Wenrui Dong;Chunlei Xiao;Li Che;Zefeng Ren;Dongxu Dai;Xiuyan Wang;Piergiorgio Casavecchia;Bin Jiang;Daiqian Xie;Zhigang Sun;Soo-Y. Lee;Dong H. Zhang;Hans-Joachim Werner;Millard H. Alexander
Science 2008 Vol 322(5901) pp:573-576
Publication Date(Web):24 Oct 2008
DOI:10.1126/science.1163195
Abstract
Elementary triatomic reactions offer a compelling test of our understanding of the extent of electron-nuclear coupling in chemical reactions, which is neglected in the widely applied Born-Oppenheimer (BO) approximation. The BO approximation predicts that in reactions between chlorine (Cl) atoms and molecular hydrogen, the excited spin-orbit state (Cl*) should not participate to a notable extent. We report molecular beam experiments, based on hydrogen-atom Rydberg tagging detection, that reveal only a minor role of Cl*. These results are in excellent agreement with fully quantum-reactive scattering calculations based on two sets of ab initio potential energy surfaces. This study resolves a previous disagreement between theory and experiment and confirms our ability to simulate accurately chemical reactions on multiple potential energy surfaces.
Co-reporter:Xueming Yang and Dong H. Zhang
Accounts of Chemical Research 2008 Volume 41(Issue 8) pp:981
Publication Date(Web):August 19, 2008
DOI:10.1021/ar700258g
The concept of transition state has played a crucial role in the field of chemical kinetics and reaction dynamics. Resonances in the transition state region are important in many chemical reactions at reaction energies near the thresholds. Detecting and characterizing isolated reaction resonances, however, have been a major challenge in both experiment and theory. In this Account, we review the most recent developments in the study of reaction resonances in the benchmark F + H2 → HF + H reaction. Crossed molecular beam scattering experiments on the F + H2 reaction have been carried out recently using the high-resolution, highly sensitive H-atom Rydberg tagging technique with HF rovibrational states almost fully resolved. Pronounced forward scattering for the HF (ν′ = 2) product has been observed at the collision energy of 0.52 kcal/mol in the F + H2 (j = 0) reaction. Quantum dynamical calculations based on two new potential energy surfaces, the Xu−Xie−Zhang (XXZ) surface and the Fu−Xu−Zhang (FXZ) surface, show that the observed forward scattering of HF (ν′ = 2) in the F + H2 reaction is caused by two Feshbach resonances (the ground resonance and first excited resonance). More interestingly, the pronounced forward scattering of HF (ν′ = 2) at 0.52 kcal/mol is enhanced considerably by the constructive interference between the two resonances. In order to probe the resonance potential more accurately, the isotope substituted F + HD → HF + D reaction has been studied using the D-atom Rydberg tagging technique. A remarkable and fast changing dynamical picture has been mapped out in the collision energy range of 0.3−1.2 kcal/mol for this reaction. Quantum dynamical calculations based on the XXZ surface suggest that the ground resonance on this potential is too high in comparison with the experimental results of the F + HD reaction. However, quantum scattering calculations on the FXZ surface can reproduce nearly quantitatively the resonance picture of the F + HD reaction observed in the experiment. It is clear that the dynamics of the F + HD reaction below the threshold was dominated by the ground resonance state. Furthermore, the forward scattering HF (ν′ = 3) channel from the F + H2 (j = 0) reaction was investigated and was attributed mainly to a slow-down mechanism over the centrifugal exit barrier, with small contributions from a shape resonance mechanism in a narrow collision energy range. A striking effect of the reagent rotational excitation on resonance was also observed in F + H2 (j = 1), in comparison with F + H2 (j = 0). From these concerted experimental and theoretical studies, a clear physical picture of the reaction resonances in this benchmark reaction has emerged, providing a textbook example of dynamical resonances in elementary chemical reactions.
Co-reporter:Zefeng Ren;Li Che;Minghui Qiu;Xingan Wang;Wenrui Dong;Dongxu Dai;Xiuyan Wang;Zhigang Sun;Bina Fu;Soo-Y. Lee;Xin Xu;Dong H. Zhang;
Proceedings of the National Academy of Sciences 2008 105(35) pp:12662-12666
Publication Date(Web):August 7, 2008
DOI:10.1073/pnas.0709974105
Reaction resonances are transiently trapped quantum states along the reaction coordinate in the transition state region of
a chemical reaction that could have profound effects on the dynamics of the reaction. Obtaining an accurate reaction potential
that holds these reaction resonance states and eventually modeling quantitatively the reaction resonance dynamics is still
a great challenge. Up to now, the only viable way to obtain a resonance potential is through high-level ab initio calculations. Through highly accurate crossed-beam reactive scattering studies on isotope-substituted reactions, the accuracy
of the resonance potential could be rigorously tested. Here we report a combined experimental and theoretical study on the
resonance-mediated F + HD → HF + D reaction at the full quantum state resolved level, to probe the resonance potential in
this benchmark system. The experimental result shows that isotope substitution has a dramatic effect on the resonance picture
of this important system. Theoretical analyses suggest that the full-dimensional FH2 ground potential surface, which was believed to be accurate in describing the resonance picture of the F + H2 reaction, is found to be insufficiently accurate in predicting quantitatively the resonance picture for the F + HD → HF +
D reaction. We constructed a global potential energy surface by using the CCSD(T) method that could predict the correct resonance
peak positions as well as the dynamics for both F + H2 → HF + H and F + HD → HF + D, providing an accurate resonance potential for this benchmark system with spectroscopic accuracy.
Co-reporter:Kaijun Yuan;Yuan Cheng;Lina Cheng;Qing Guo;Dongxu Dai;Xiuyan Wang;Richard N. Dixon
PNAS 2008 Volume 105 (Issue 49 ) pp:19148-19153
Publication Date(Web):2008-12-09
DOI:10.1073/pnas.0807719105
The photochemistry of H2O in the VUV region is important in interstellar chemistry. Whereas previous studies of the photodissociation used excitation
via unbound states, we have used a tunable VUV photolysis source to excite individual levels of the rotationally structured
C̃ state near 124 nm. The ensuing OH product state distributions were recorded by using the H-atom Rydberg tagging technique.
Experimental results indicate a dramatic variation in the OH product state distributions and its stereodynamics for different
resonant states. Photodissociation of H2O(C̃) in rotational states with k′a = 0 occurs exclusively through a newly discovered homogeneous coupling to the à state, leading to OH products that are vibrationally
hot (up to v = 13), but rotationally cold. In contrast, for H2O in rotationally excited states with k′a > 0, an additional pathway opens through Coriolis-type coupling to the B̃ state surface. This yields extremely rotationally hot and vibrationally cold ground state OH(X) and electronically excited
OH(A) products, through 2 different mechanisms. In the case of excitation via the 110 ← 000 transition the H atoms for these 2 product channels are ejected in completely different directions. Quantum dynamical models
for the C̃-state photodissociation clearly support this remarkable dynamical picture, providing a uniquely detailed illustration of
nonadiabatic dynamics involving at least 4 electronic surfaces.
Co-reporter:Kaijun Yuan, Yuan Cheng, Fengyan Wang and Xueming Yang
The Journal of Physical Chemistry A 2008 Volume 112(Issue 24) pp:5332-5337
Publication Date(Web):May 24, 2008
DOI:10.1021/jp800062t
Photodissociation dynamics of HN3 at 157.6 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been measured. From these distributions, three H-atom channels are observed. The vibrational structure in the fast-H channel could be assigned to a progression in the N3 symmetric stretching mode (ν100), together with a progression of the symmetric stretching mode with one quantum of bending motion (ν110). The broad translational energy distribution of the slow-H channel is energetically consistent with the cyclic-N3 formation process or a triple product dissociation channel. Photodissociation of DN3 was also investigated using the same technique. Isotope effect on the product translational energy distribution has been observed, in which the slow H-atom is clearly more pronounced.
Co-reporter:Xingan Wang;Wenrui Dong;Minghui Qiu;Zhigang Sun;Zefeng Ren;Soo-Y. Lee;Xin Xu;Dong H. Zhang;Xiuyan Wang;Dongxu Dai;Bina Fu;Li Che
PNAS 2008 Volume 105 (Issue 17 ) pp:6227-6231
Publication Date(Web):2008-04-29
DOI:10.1073/pnas.0710840105
Crossed molecular beam experiments and accurate quantum dynamics calculations have been carried out to address the long standing
and intriguing issue of the forward scattering observed in the F + H2 → HF(v′ = 3) + H reaction. Our study reveals that forward scattering in the reaction channel is not caused by Feshbach or dynamical
resonances as in the F + H2 → HF(v′ = 2) + H reaction. It is caused predominantly by the slow-down mechanism over the centrifugal barrier in the exit channel,
with some small contribution from the shape resonance mechanism in a very small collision energy regime slightly above the
HF(v′ = 3) threshold. Our analysis also shows that forward scattering caused by dynamical resonances can very likely be accompanied
by forward scattering in a different product vibrational state caused by a slow-down mechanism.
Co-reporter:Fengyan Wang, I-Chung Lu, Kaijun Yuan, Yuan Cheng, Malcom Wu, David H. Parker, Xueming Yang
Chemical Physics Letters 2007 Volume 449(1–3) pp:18-22
Publication Date(Web):26 November 2007
DOI:10.1016/j.cplett.2007.10.017
Abstract
Photodissociation dynamics of HI and DI have been studied at 157 nm using the H atom Rydberg tagging time-of-flight technique. The photofragment translational energy distribution spectra and angular distributions of H/D atom products have been measured. Both the I(2P3/2) and I(2P1/2) products come almost exclusively from a perpendicular transition at 157 nm dissociation process, in agreement with the prediction of LeRoy’s model [R.J. LeRoy, G.T. Kraemer, S. Manzhos, J. Chem. Phys. 117 (2002) 9353]. However, the branching ratios of I∗/I measured in the experiment suggests that weak coupling may take place between the potential energy curves, which is noticeably different from the photodissociation of HI in the UV region. The experimental result in this work also suggests that the repulsive state, which has little contribution to the A-band absorption (33 000–53 000 cm−1), plays a greater role in the dissociation of HI and DI at 157 nm.
Co-reporter:XueMing Yang;DaiQian Xie;DongHui Zhang
Science Bulletin 2007 Volume 52( Issue 8) pp:1009-1012
Publication Date(Web):2007 April
DOI:10.1007/s11434-007-0158-4
Reaction resonance is a frontier topic in chemical dynamics research, and it is also essential to the understanding of mechanisms of elementary chemical reactions. This short article describes an important development in the frontier of research. Experimental evidence of reaction resonance has been detected in a full quantum state resolved reactive scattering study of the F+H2 reaction. Highly accurate full quantum scattering theoretical modeling shows that the reaction resonance is caused by two Feshbach resonance states. Further studies show that quantum interference is present between the two resonance states for the forward scattering product. This study is a significant step forward in our understanding of chemical reaction resonance in the benchmark F+H2 system. Further experimental studies on the effect of H2 rotational excitation on dynamical resonance have been carried out. Dynamical resonance in the F+H2 (j = 1) reaction has also been observed.
Co-reporter:Li Che;Zefeng Ren;Xingan Wang;Wenrui Dong;Dongxu Dai;Xiuyan Wang;Dong H. Zhang;Liusi Sheng;Guoliang Li;Hans-Joachim Werner;François Lique;Millard H. Alexander
Science 2007 Volume 317(Issue 5841) pp:1061-1064
Publication Date(Web):24 Aug 2007
DOI:10.1126/science.1144984
Abstract
The reaction of F with H2 and its isotopomers is the paradigm for an exothermic triatomic abstraction reaction. In a crossed-beam scattering experiment, we determined relative integral and differential cross sections for reaction of the ground F(2P3/2) and excited F*(2P1/2) spin-orbit states with D2 for collision energies of 0.25 to 1.2 kilocalorie/mole. At the lowest collision energy, F* is ∼1.6 times more reactive than F, although reaction of F* is forbidden within the Born-Oppenheimer (BO) approximation. As the collision energy increases, the BO-allowed reaction rapidly dominates. We found excellent agreement between multistate, quantum reactive scattering calculations and both the measured energy dependence of the F*/F reactivity ratio and the differential cross sections. This agreement confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions.
Co-reporter:Minghui Qiu;Zefeng Ren;Li Che;Dongxu Dai;Steve A. Harich;Xiuyan Wang;Chuanxiu Xu;Daiqian Xie;Magnus Gustafsson;Rex T. Skodje;Zhigang Sun;Dong H. Zhang
Science 2006 Vol 311(5766) pp:1440-1443
Publication Date(Web):10 Mar 2006
DOI:10.1126/science.1123452
Abstract
Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H2 → HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v′ = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v′ = 3)-H′ vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances.
Co-reporter:Shengrui Yu, Kaijun Yuan, Hui Song, Xin Xu, Dongxu Dai, Dong H. Zhang and Xueming Yang
Chemical Science (2010-Present) 2012 - vol. 3(Issue 9) pp:NaN2842-2842
Publication Date(Web):2012/06/25
DOI:10.1039/C2SC20489D
Full quantum-state resolved differential cross-sections of the H*(n) + o-D2 → HD + D*(n′) reaction have been measured for the first time using the Rydberg H-atom time-of-flight method. Experimental results show that the angular distributions of HD product rotational states show a strong preference for forward scattering. This result is considerably different to that predicted by full quantum mechanical calculations on the corresponding ion–molecule reaction, suggesting that the ionic core and Rydberg electron coupling cannot be neglected in the Rydberg H-atom reactive scattering with D2 and, therefore, that the Fermi independent-collider model is not valid in describing the dynamics of Rydberg atom reactions with molecules.
Co-reporter:Qing Guo, Chuanyao Zhou, Zhibo Ma, Zefeng Ren, Hongjun Fan and Xueming Yang
Chemical Society Reviews 2016 - vol. 45(Issue 13) pp:NaN3730-3730
Publication Date(Web):2015/09/03
DOI:10.1039/C5CS00448A
Photocatalytic hydrogen production and pollutant degradation provided both great opportunities and challenges in the field of sustainable energy and environmental science. Over the past few decades, we have witnessed fast growing interest and efforts in developing new photocatalysts, improving catalytic efficiency and exploring the reaction mechanism at the atomic and molecular levels. Owing to its relatively high efficiency, nontoxicity, low cost and high stability, TiO2 becomes one of the most extensively investigated metal oxides in semiconductor photocatalysis. Fundamental studies on well characterized single crystals using ultrahigh vacuum based surface science techniques could provide key microscopic insight into the underlying mechanism of photocatalysis. In this review, we have summarized recent progress in the photocatalytic chemistry of hydrogen, water, oxygen, carbon monoxide, alcohols, aldehydes, ketones and carboxylic acids on TiO2 surfaces. We focused this review mainly on the rutile TiO2(110) surface, but some results on the rutile TiO2(011), anatase TiO2(101) and (001) surfaces are also discussed. These studies provided fundamental insights into surface photocatalysis as well as stimulated new investigations in this exciting field. At the end of this review, we have discussed how these studies can help us to develop new photocatalysis models.
Co-reporter:Shu Su, Yvonne Dorenkamp, Shengrui Yu, Alec M. Wodtke, Dongxu Dai, Kaijun Yuan and Xueming Yang
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 22) pp:NaN15405-15405
Publication Date(Web):2016/05/06
DOI:10.1039/C6CP01956K
Photodissociation dynamics of HBr at a series of photolysis wavelengths in the range of 123.90–125.90 nm and at around 137.0 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique. The branching fractions between the channels forming ground Br(2P3/2) and spin–orbit excited Br(2P1/2) atoms together with the angular distributions of the products corresponding to these two channels have been measured. The photolysis wavelengths in this work excited the HBr molecule from the ground state X 1Σ+ to various Rydberg states and the V 1Σ+ ion-pair valence state. Predissociation via these states displays rich behavior, indicating the influence of the nature of initially excited states and the coupling to other bound or repulsive states on the predissociation dynamics.
Co-reporter:Shengrui Yu, Shu Su, Dongxu Dai, Kaijun Yuan and Xueming Yang
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 15) pp:NaN9665-9665
Publication Date(Web):2014/08/14
DOI:10.1039/C4CP02734E
The state-to-state dynamics of high-n Rydberg H-atom scattering with para-H2 at the collision energies of 0.45 and 1.07 eV have been studied using the H-atom Rydberg tagging time-of-flight technique. Both the inelastic scattering and reactive scattering are observed in the experimental time-of-flight spectra. The products H2(v′, j′ = odd) come only from reactive scattering and present clearly forward–backward asymmetric angular distributions, which differ from those of the corresponding ion–molecule reaction. The products H2(v′, j′ = even), however, come from both reactive scattering and inelastic scattering. Simulating the rotational distribution from reactive scattering, we found that most of the H2(v′, j′ = even) products come from inelastic scattering. The angular distributions of the product H2(v′, j′ = even) are consistent with what is predicted by the conventional textbook mechanism of inelastic scattering, and are a little different from those of the corresponding ion–molecule inelastic scattering. These results suggest that the effect of Rydberg electron could not be neglected in describing the differential cross sections of H* + para-H2 scattering. From the simulation, the branching ratios of the inelastic scattering channel were determined to be 66% and 79% at the collision energies of 0.45 and 1.07 eV, respectively.
Co-reporter:Xueming Yang
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 18) pp:NaN8121-8121
Publication Date(Web):2011/04/09
DOI:10.1039/C1CP00005E
In the last decade or so, the H-atom Rydberg tagging time-of-flight (HRTOF) technique has made a significant impact in the study of state-to-state reaction dynamics, and especially in the study of transition state dynamics of elementary chemical reactions and quantum state resolved dynamics of molecular photodissociation of important molecules. In this perspective, we will discuss mainly the state-to-state dynamics of three important elementary reactions: H + H2, O(1D) + H2 and F + H2 that have been studied in our laboratory in recent years using the HRTOF method. In addition, we will also mention briefly the experimental results of other reactive systems. In the end, we will also present a brief research outlook in the study of molecular reaction dynamics using this powerful experimental method.
Co-reporter:Guorong Wu, Weiqing Zhang, Huilin Pan, Quan Shuai, Jiayue Yang, Bo Jiang, Dongxu Dai and Xueming Yang
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 32) pp:NaN9474-9474
Publication Date(Web):2010/06/25
DOI:10.1039/B927455C
The dynamics of the Cl + SiH4 → HCl + SiH3 reaction has been studied using the crossed molecular beam technique with slice imaging. The ion images of the silyl radical (SiH3) product from the Cl + SiH4 reaction were recorded at different collision energies, using the (2+1) resonance enhanced multi-photon ionization (REMPI) technique. The product velocity and angular distributions of this reaction were determined from the recorded images. The vibrational state-to-state correlation of the relative population between the two reaction products, SiH3 and HCl, was also determined. Finally, the Cl + SiH4 results are compared with that of the F + SiH4 reaction.
Co-reporter:Zhichao Chen, Quan Shuai, André T. J. B. Eppink, Bo Jiang, Dongxu Dai, Xueming Yang and David H. Parker
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 18) pp:NaN8536-8536
Publication Date(Web):2011/03/30
DOI:10.1039/C1CP00032B
The SH + CH3 product channel for the photodissociation of CH3SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH3 products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH3 formed in the ground and excited vibrational states (v2 = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH3 channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH3 (v2 = 0, 1, and 2) products is indicative of a fast dissociation process for C–S bond cleavage. Two kinds of slower CH3 products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH3S (X2E) radical via cleavage of the S–H bond in CH3SH, followed by probe laser photodissociation of nascent CH3S radicals yielding CH3(X2A1, v2 = 0–2) + S(3Pj/1D) products.
Co-reporter:Zhichao Chen, Andre T. J. B. Eppink, Bo Jiang, Gerrit C. Groenenboom, Xueming Yang and David H. Parker
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 6) pp:NaN2355-2355
Publication Date(Web):2010/11/26
DOI:10.1039/C0CP01794A
The OH + CH3 product channel for the photodissociation of CH3OH at 157 nm was investigated using the velocity map imaging technique with the detection of CH3 radical products via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images were measured for the CH3 formed in the ground and excited states (v2 = 0, 1, 2, and 3) of the umbrella vibrational mode and correlated OH vibrational state distributions were also determined. We find that the vibrational distribution of the OH fragment in the OH + CH3 channel is clearly inverted. Anisotropic distributions for the CH3 (v2 = 0, 1, 2, and 3) products were also determined, which is indicative of a fast dissociation process for the C–O bond cleavage. A slower CH3 product channel was also observed, that is assigned to a two-step photodissociation process, in which the first step is the production of a CH3O(X 2E) radical via the cleavage of the O–H bond in CH3OH, followed by probe laser photodissociation of the nascent CH3O radicals yielding CH3(X 2A1, v = 0) products.
Co-reporter:Zhiqiang Wang, Qunqing Hao, Xinchun Mao, Chuanyao Zhou, Dongxu Dai and Xueming Yang
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 15) pp:NaN10231-10231
Publication Date(Web):2016/03/08
DOI:10.1039/C6CP00556J
Photocatalytic chemistry of methanol on the reconstructed rutile TiO2(011)-(2 × 1) surface upon 266 nm and 400 nm light excitation has been investigated quantitatively using the post-irradiation temperature-programmed desorption (TPD) method. Photochemical products such as formaldehyde, methyl formate and water, which result from the recombination of surface bridging hydroxyls through the abstraction of lattice oxygen atoms, have been identified under both 266 nm and 400 nm light irradiation. However, ethylene is detected only under 266 nm light irradiation. Through an analogy experiment, ethylene production is attributed to the photochemistry and the following thermochemistry of formaldehyde. The absence of the ethylene signal under 400 nm light is consistent with the significantly lower conversion at this wavelength compared with 266 nm. The photocatalytic reaction rate of methanol is also wavelength dependent. Possible reasons for the photon energy dependent phenomena have been discussed. This work not only provides a detailed characterization of the photochemistry of methanol on the rutile TiO2(011)-(2 × 1) surface, but also indicates the importance of photon energy in the photochemistry on TiO2 surfaces.
Co-reporter:Chuanyao Zhou, Zhibo Ma, Zefeng Ren, Xinchun Mao, Dongxu Dai and Xueming Yang
Chemical Science (2010-Present) 2011 - vol. 2(Issue 10) pp:NaN1983-1983
Publication Date(Web):2011/07/22
DOI:10.1039/C1SC00249J
Photocatalytic dissociation of deuterated methanol (CD3OD) on both stoichiometric and reduced TiO2(110) surfaces was investigated using the time-dependent two-photon photoemission (2PPE) method, in order to understand the effect of defects on the kinetics of methanol dissociation on TiO2(110). By monitoring the time evolution of the photoinduced excited state on the methanol covered surface, the photocatalytic dissociation kinetics of methanol on the TiO2 surface were observed. The measured photodissociation rate on the reduced TiO2(110) surface is more than an order of magnitude faster than that on the stoichiometric surface. Since the reduced TiO2(110) surface has considerably more surface and subsurface defects than the stoichiometric surface, the experimental observation suggests that one or both of them could accelerate the photocatalysis process of methanol on the TiO2(110) surface in a significant way.
Co-reporter:Dong Zhang, Jiayue Yang, Zhen Chen, Rongjun Chen, Bo Jiang, Dongxu Dai, Guorong Wu, Donghui Zhang and Xueming Yang
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 20) pp:NaN13074-13074
Publication Date(Web):2017/04/27
DOI:10.1039/C7CP01428G
The effects of CH stretching excitation on the reactivity of the F + CHD3 → HF + CD3 reaction were studied experimentally using crossed-beam and time-sliced velocity map imaging techniques over the collision energy range of 1.21 to 9.00 kcal mol−1. The experimental results showed that the CH stretching excitation promoted its cleavage and enhanced the title reaction at low collision energies. This enhancement dropped with an increase of the collision energy. And at high collision energies, CH stretching excitation appeared to lower the reactivity of the above reaction, in contrast to the case at low collision energies. This decreasing trend in the enhancement of reactivity was in agreement with previous theoretical studies. The vibrationally excited reaction was further compared with the ground-state reaction at a same total reagent energy of 9.80 kcal mol−1, and similar reactivities were derived.
Co-reporter:Fengyan Wang, Yongwei Zhang, Hua Wang, Jie Liu, Bo Jiang, Xiuyan Wang and Xueming Yang
Physical Chemistry Chemical Physics 2009 - vol. 11(Issue 39) pp:NaN8740-8740
Publication Date(Web):2009/07/30
DOI:10.1039/B903688A
The dissociation dynamics of HFCO+ ion has been studied using the velocity map ion imaging technique. The HFCO+ ion is prepared by one-photon resonant three-photon ionization in the region of 43100–43860 cm−1 excitation energy. The HFCO+ ions, produced by multiphoton ionization, have sufficient internal energy to dissociate into the F and HCO+ fragments without further absorption of another photon. Images of HCO+ have been recorded at various excitation energies. It is noticed that the angular distributions of HCO+ change dramatically from parallel distribution to perpendicular distribution and then back to parallel distribution in a very narrow excitation energy region of 43473–43500 cm−1. Analysis of anisotropy parameters of βn (n = 2, 4 and 6) reveals that the electronic states in the three-photon excitation of HFCO are mainly: HFCO(X1A′) → HFCO(A1A″) → HFCO(A′) → HFCO+(A2A″;B2A′). The purely perpendicular resonant transitions are likely responsible for the perpendicular angular distribution of the HCO+ ion fragment.
