The thermal decomposition of methylamine on Ru(001) was studied with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction spectroscopy (TPRS). After the multilayer methylamine desorbs at 150 K, the RAIR spectra of the remaining monolayer methylamine undergo small changes due to structural rearrangements but do not indicate any chemical changes until 250 K. The results are in agreement with recent theoretical investigations indicating that CH3 dehydrogenation to produce HxCNH2 species occurs before N–H and C–N bond scission. Experimental spectra of 13C- and 15N-substituted methylamine combined with DFT calculations of HxCNHy fragments attached to a Ru19 cluster to simulate the RAIR spectra, including isotopic shifts, clarified the spectral assignments.

The adsorption and decomposition of decaborane (B10H14) on the Pt(111) surface was studied with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The molecule has a nido structure with 10 terminal B−H bonds and 4 bridging B−H−B bonds. Comparison of the experimental RAIR spectra with spectra calculated using density functional theory indicates that the molecule adsorbs at 85 K without dissociation and with an orientation in which the dipole moment is parallel to the surface. The TPD experiments show that the dissociation leads to H2 desorption up to ∼420 K but that no boron-containing species desorb. Dissociation occurs in stages via stable surface intermediates, as indicated by changes observed with RAIRS in the terminal B−H stretch region at ∼2600 cm−1. Boron, which is produced as a result of complete dissociation, is lost from the surface as indicated by XPS, most likely through dissolution into the bulk of the platinum crystal.

•Stoichiometric, amorphous V2O5 thin films were grown on Pd(111) at 300 K through physical vapor deposition by heating V2O5 powder in UHV.•The films were characterized with XPS, LEED, and RAIRS.•The V2O5 thin films reduce to an ordered form of VO2 below 800 K, and further reduce to an ordered form of V2O3 below 1000 K.A simple and efficient method has been used to grow V2O5 thin films on Pd(111) at a substrate temperature of 300 K through physical vapor deposition by heating a fine powder of V2O5 in a non-oxidative, UHV environment. X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS) and low energy electron diffraction (LEED) were used to characterize the thin films. When the as-grown films exceed a minimum thickness, characteristic features of V2O5 were revealed by XPS and RAIRS, which confirms the presence of stoichiometric V2O5. LEED indicates no long range order of the as-grown films at 300 K. Annealing to temperatures between 600 and 700 K causes a reduction of V2O5 to VO2 as identified by XPS and the formation of ordered structures as determined by LEED, and VO2 is predominant after annealing to 800 K. After further annealing to 1000 K, only an ordered form of V2O3 is present on Pd(111).Download high-res image (78KB)Download full-size image

Adsorption and thermal chemistry of propanal, 2-propenol, and 1-propanol on Ru(001) were studied using temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS). The results show that each molecule adsorbs molecularly at 90 K and displays the same spectral features as observed for the corresponding liquids after 1.0 L exposures. 2-Propenol was found to molecularly desorb at 200 K, dehydrate to yield propene around 130 K, isomerize to propanal at 180 K, and hydrogenate to 1-propanol at 220 K. Propanal, however, does not undergo isomerization on the surface but desorbs molecularly at 175 and 280 K. Similarly, 1-propanol also desorbs molecularly with two peaks centered at 227, and 298 K. Formaldehyde desorption was observed for each molecule. Furthermore, a reversible hydrogenation-dehydrogenation process was observed between propanal and 1-propanol in the range of 200 to 320 K. These results provided further insights into previous studies on hydrogenation pathways of acrolein on the Ru(001) surface and into the challenges of selectively increasing the yield of the unsaturated alcohol.

Temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) were used to study the adsorption and hydrogenation of acrolein on Ru(001). At low coverages, acrolein adsorbs on the surface at 90 K mostly via the C═O bond and completely decomposes to CO around 460 K. As the coverage increases, adsorption via the C═C bond predominates and most of the acrolein either desorbs molecularly or decomposes to CO and H2. However, a small amount of the acrolein also self-hydrogenates to yield all the possible hydrogenation products, propanal, 2-propenol, and 1-propanol, with TPRS peak temperatures of 180, 210, and 280 K respectively, with propanal having the highest yield. Co-adsorption with hydrogen enhances the adsorption via the C═C bond and the yield of all the hydrogenation products. The formation of propanal and 1-propanol was also confirmed by RAIRS to occur at approximately the same temperatures as observed with TPRS, with the intensity of the RAIRS peaks indicating that the extent of hydrogenation is significantly higher than the yields obtained from TPRS.

•Carbon is removed from Pt(111) by hydrogenation via an ethylidyne intermediate.•Acetylene decomposition at 750 K deposits mainly C2 molecules on Pt(111).•Hydrogenation of C2 on Pt(111) forms ethylidyne, which is detected with RAIRS.The hydrogenation of C2 molecules formed on the Pt(111) surface through acetylene exposure at 750 K was monitored in-situ with reflection absorption infrared spectroscopy (RAIRS) in the presence of up to 10 Torr of H2. The coverage of post-reaction surface carbon was measured with Auger electron spectroscopy. The RAIR spectra show that C2 is hydrogenated to an ethylidyne intermediate. The hydrogenation of ethylidyne was also monitored at 400 K for H2(g) pressures of 1.0 × 10− 2 to 10 Torr. At H2(g) pressures greater than 1.0 Torr, ethylidyne is completely hydrogenated. In an attempt to probe the nature of the C2 adsorption sites, RAIR spectra of coadsorbed CO were obtained. It is found that while C2 does not block CO adsorption, the spectra indicate that the surface carbon is free of hydrogen. In contrast, ethylidyne blocks CO adsorption sites. In the presence of coadsorbed CO, complete hydrogenation of ethylidyne occurs at 450 K versus 400 K in the absence of CO.

Acetylene hydrogenation was monitored at ambient pressure with polarization-dependent reflection absorption infrared spectroscopy (RAIRS), which permitted gas phase and surface species to be simultaneously monitored as C2H2(g) was converted first to C2H4(g) and then to C2H6(g). Experiments in which an acetylene-covered surface was hydrogenated with 1.0 × 10–2 Torr H2 between 120 and 300 K indicated that vinyl is the intermediate species to ethylene formation and that the addition of one H to acetylene is the rate-limiting step of the reaction. At a C2H2(g)/H2(g) ratio of 1:100, the reaction was monitored from 300 to 370 K and separately in a constant pressure and constant temperature reaction at 370 K. Ethylidyne and di-σ-ethylene were observed on the surface in both reactions and were found to be spectator species in the hydrogenation of ethylene to ethane. A minor hydrogenation pathway involves a third species, which is best assigned to an ethylidene intermediate. A small coverage of π-ethylene was also present during the annealing experiment at 350 K but disappeared at 370 K, indicating that it is also an intermediate in the reaction. In a separate experiment to compare acetylene hydrogenation with ethylene hydrogenation at 370 K, spectra were acquired with a C2H4(g)/H2(g) ratio of 1:10. Ethylene hydrogenation proceeds approximately three times faster when starting with ethylene as compared with hydrogenation of the ethylene produced by acetylene hydrogenation. This indicates that the surface is covered with different intermediates when it is first exposed to acetylene. The results presented here demonstrate a simple way to use polarization-dependent RAIRS to distinguish surface species from gas phase reactants and products. The method should be applicable to a wide range of catalytic reactions over metal surfaces and offers new opportunities for operando studies in catalysis.Keywords: acetylene; ambient pressure; ethylene; hydrogenation; polarization modulation; RAIRS

•We report infrared spectra of the products of vinyl iodide dissociation on Pt(111).•A series of species with distinct C–H stretch frequencies form as the temperature is increased.•The spectra imply that ethylidyne is formed from vinyl via ethylene and ethylidene intermediates.The thermal decomposition of vinyl iodide on Pt(111) was studied using reflection absorption infrared spectroscopy (RAIRS). Some of the vinyl iodide molecularly desorbs at 160 K and the remainder decomposes via scission of the C–I bond to form vinyl. In this way, the vibrational signature of vinyl on Pt(111) is directly determined by RAIRS. At 190 K, vinyl starts to convert to di-σ bonded ethylene. The ethylene undergoes further reaction at 230 K to hydrogenate to ethylidene, possibly by way of a vinyl intermediate. Upon annealing the surface to 300 K, ethylidene is converted to ethylidyne. Hydrogen pre-adsorption promotes the cleavage of the C–I bond of vinyl iodide and the formation of vinyl, which subsequently leads to an increase in the amount of di-σ bonded ethylene formed. The surface hydrogen enhances the formation of ethylidyne, possibly by removal of excess ethylidene by hydrogenation, as ethylidene was not observed when hydrogen was pre-adsorbed on the surface.

Low-temperature scanning tunneling microscopy (LT-STM) was used to move hydrogen atoms and dissociate NH molecules on a Pt(111) surface covered with an ordered array of nitrogen atoms in a (2 × 2) structure. The N-covered Pt(111) surface was prepared by ammonia oxydehydrogenation, which was achieved by annealing an ammonia–oxygen overlayer to 400 K. Exposing the N-covered surface to H2(g) forms H atoms and NH molecules. The NH molecules occupy face-centered cubic hollow sites, while the H atoms occupy atop sites. The STM tip was used to dissociate NH and to induce hopping of H atoms. Action spectra consisting of the reaction yield versus applied bias voltage were recorded for both processes, which revealed that they are vibrationally mediated. The threshold voltages for NH dissociation and H hopping were found to be 430 and 272 meV, corresponding to the excitation energy of the N–H stretching and the Pt–H stretching modes, respectively. Substituting H with D results in an isotopic shift of −110 and −84 meV for the threshold voltages for ND dissociation and D hopping, respectively. This further supports the conclusion that these processes are vibrationally mediated.Keywords: action spectroscopy; H hopping; low-temperature scanning tunneling microscopy; NH dissociation; Pt(111);

The kinetics of aminocarbyne (CNH2) formation from coadsorbed CN and H on Pt(111) was studied in the temperature range of 220 to 235 K by monitoring the time-dependent increase in the δ(NH2) mode of CNH2 at 1566 cm–1 with reflection absorption infrared spectroscopy (RAIRS). The surface was exposed using a directed-gas doser to 0.05 L of HCN, which immediately dissociates in the temperature range studied to deposit CN and H on the surface. A peak observed at 3348 cm–1 is assigned to the CNH intermediate. A simple model with rate constants for CNH formation, CNH decomposition, and CNH2 formation was used to analyze the experimental time-dependent coverages of CNH2. A modified model in which hydrogen was assumed to diffuse rapidly over the surface to occupy all available sites as the reaction proceeded was also used to obtain the three rate constants. From the temperature dependence of these rate constants and fits to Arrhenius plots, activation energies of 0.26, 0.22, and 0.41 eV for CNH formation, CNH dissociation, and CNH2 formation, respectively, were obtained.

The formation of Pt and Rh nanoclusters on a graphene moiré pattern on Cu(111) was studied with ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Isolated graphene islands with different periodicities were successfully grown on the Cu surface. As a result of weak coupling to the Cu substrate, different graphene rotational domains were observed including one with a periodicity of 4 nm that has not been previously reported. Furthermore, our results have shown that Pt and Rh form dispersed nanoclusters on graphene/Cu(111) with Pt forming more regular structures trapped mostly at the hollow sites. This variation in growth behavior is mainly attributed to differences in Pt–carbon and Rh–carbon interaction energies. Thermal stability experiments were also performed, and Pt clusters were found to be structurally stable up to 700 K.

•A new interaction between CO and O2− on Pt(1 1 1) at 85 K is detected with RAIRS.•The CO–O2− interaction produces a perturbed form of CO on Pt(1 1 1).•The perturbation is due to a chemical effect rather than to vibrational coupling.•The perturbed CO species acts as a precursor to CO oxidation at low temperature.Reflection absorption infrared spectroscopy was used to study the reaction of adsorbed molecular oxygen with carbon monoxide to produce adsorbed carbon dioxide at 130 K. By simultaneously monitoring the O–O stretch of O2(ads), the C–O stretch of CO(ads), and the asymmetric O–C–O stretch of CO2(ads), it is found that there is an O2–CO interaction, suggesting that the transition state involves direct transfer of an O atom from O2(ads) to CO, rather than through reaction of CO(ads) with a hot surface O atom produced from O2 dissociation.

X-ray photoelectron spectroscopy (XPS) and reflection absorption infrared spectroscopy (RAIRS) have been used to study the structure of molecularly adsorbed ammonia on the ZrB2(0001) surface and its subsequent dissociation. Spectra were obtained as a function of ammonia exposure to the surface at 95 K and as a function of annealing temperature following exposures at 95 and 300 K. The infrared peak positions do not vary with exposure, from the lowest submonolayer coverages to thick multilayers and are at the same values as those of solid ammonia. This indicates that the NH3 molecules have high enough mobility at 95 K to aggregate into hydrogen-bonded clusters with the same structure as that of solid ammonia. The peak positions match those of solid ammonia much better than those calculated for an isolated ammonia molecule adsorbed on top of a Zr atom, which was found to be the most stable binding site. Although aggregation into clusters at low temperatures implies a weak interaction with the substrate, a minor dissociation channel may exist, even at 95 K, as indicated by XPS through the appearance of a small N 1s peak at 397.5 eV attributed to atomic nitrogen in addition to a N 1s peak at 401.9 eV due to molecularly adsorbed NH3. Ammonia completely dissociates to atomic nitrogen and hydrogen for adsorption temperatures of 300 K and above. The hydrogen product of dissociation desorbs between 535 and 555 K, as indicated by the disappearance of a Zr–H vibration at 999 cm–1.

•Ammonia dissociates on a zirconium diboride surface to deposit nitrogen.•Nitrogen on zirconium diboride is studied with X-ray photoelectron spectroscopy.•Density functional theory is used to interpret the experimental results.•This study is relevant to the epitaxial growth of GaN on ZrB2(0001).•Nitrogen desorbs from the surface between 950 and 1150 °C.Zirconium diboride has been proposed as a viable substrate for epitaxial growth of group III nitrides. In many methods of nitride growth on ZrB2 surfaces, ammonia gas is the nitrogen source. Here we use X-ray photoelectron spectroscopy at a series of fixed temperatures from room temperature to 535 °C and density functional theory to study the dissociative adsorption of ammonia on the ZrB2(0001) surface. A significant increase is observed between ~ 250 and ~ 400 °C for the deposition of nitrogen, which can be desorbed by annealing between 950 and 1150 °C. Two components of the N 1s peak are observed and are associated with bonding of nitrogen to boron or to zirconium. Comparison of spectra obtained at two different emission angles suggests that more N is bonded to B than to Zr at the surface and when boron is bonded to nitrogen, it migrates towards the surface. This may be a factor in limiting group III nitride epitaxial growth on the ZrB2(0001) surface.

The kinetics of NH and ND formation and dissociation reactions on Ru(001) were studied using time-dependent reflection absorption infrared spectroscopy (RAIRS). Our results indicate that NH and ND formation and dissociation on Ru(001) follow first-order kinetics. In our reaction temperature range (320–390 K for NH and 340–390 K for ND), the apparent activation energies for NH and ND formation were found to be 72.2 ± 1.9 and 87.1 ± 1.8 kJ/mol, respectively, while NH and ND dissociation reactions between 370 and 400 K have apparent activation barriers of 106.9 ± 4.1 and 101.8 ± 4.8 kJ/mol, respectively. The lower apparent activation energy for NH formation than that for ND as well as the comparison between experimentally measured isotope effects with theoretical results strongly indicates that tunneling already starts to play a role in this reaction at a temperature as high as 340 K.Keywords: activation energy; ammonia synthesis; Arrhenius plot; isotope effect; kinetics; reflection absorption infrared spectroscopy;

Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH3–O2 overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O2 molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to on-top sites of the Pt lattice that are surrounded by two or three O2 molecules. Different hole–hole distances are observed with 0.73 nm most common. These holes in the O2 network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH3–O2 complex that serves as a precursor to ammonia oxydehydrogenation.Keywords: ammonia oxidehygrogenation; low-temperature scanning tunneling microscopy; molecular structure; O2 network;

Reflection absorption infrared spectroscopy, temperature-programmed desorption, and density functional theory (DFT) have been used to study the surface chemistry and thermal decomposition of ethylamine (CH3CH2NH2) on Pt(111). Ethylamine adsorbs molecularly at 85 K, is stable up to 300 K, and is partially dehydrogenated at 330 K to form aminovinylidene (CCHNH2), a stable surface intermediate that partially desorbs as acetonitrile (CH3CN) at 340–360 K. DFT simulations using various surface models confirm the structure of aminovinylidene. Upon annealing to 420 K, undesorbed aminovinylidene undergoes further dehydrogenation that results in the scission of the remaining C–H bond and the formation of a second surface intermediate called aminoethynyl with the structure CCNH2, bonded to the surface through both C atoms. The assignment of this intermediate species is supported by comparison between experimental and simulated spectra of the isotopically labeled species. Further annealing to temperatures above 500 K shows that the C–N bond remains intact as the desorption of HCN is observed.

The growth of Pt nanoclusters on a graphene layer on Pt(111) was studied with ultra high vacuum scanning tunneling microscopy. Different periodicities in the moiré patterns of the graphene layer are observed corresponding to different orientations with respect to the Pt(111) lattice. Various graphene orientations are possible because of a relatively weak graphene–Pt interaction. Following Pt deposition onto the graphene-covered surface, small Pt nanoclusters are observed to preferentially form along the moiré domain boundaries. The weak interaction of graphene with Pt(111) leads to a weak corrugation in the superlattice compared to other transition metals, such as Ru, but it is found even this weak corrugation is sufficient to serve as a template for the formation of mono-dispersed one-dimensional Pt nanocluster chains. These Pt nanoclusters are relatively stable and only undergo agglomeration at annealing temperatures above 600 K.Highlights► Different graphene domains on Pt(111) are imaged with scanning tunneling microscopy. ► Pt nanoclusters preferentially nucleate in linear chains along the graphene domain boundaries. ► Pt nanoclusters formed on graphene on Pt(111) are relatively stable up to a temperature of 600 K.

The chemisorption of water (H2O and D2O) on a LaB6(100) surface was studied with reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS). The clean surface was exposed to H2O and D2O at temperatures from 90 K to room temperature, and spectra were acquired after heating to temperatures as high as 1200 K. It was found that water molecularly adsorbs on the surface at 90 K as a monomer at low coverages and as amorphous solid water at higher coverages. Water adsorbs dissociatively at room temperature to produce surface hydroxyl species as indicated by OH/OD stretch peaks at 3676/2701 cm−1. Room temperature adsorption also reveals low frequency loss features in HREEL spectra near 300 cm−1 that are quite similar to results obtained following the dissociative adsorption of O2. In the latter case, the loss features were attributed to the LaO stretch of O atoms bridge-bonded between two La atoms. In the case of dissociative adsorption of H2O, the low frequency loss features could be due to either the LaO vibrations of adsorbed O or of adsorbed OH.Highlights► Water dissociates on the LaB6(100) surface at room temperature to produce surface hydroxyl groups. ► At 90 K, water adsorbs molecularly. ► Water adsorbs as a monomeric species at low coverages at 90 K. ► At 300 K, spectra for H2O and O2 show similar peaks due to vibrations of the boron lattice.

Time-dependent reflection absorption infrared spectroscopy has been used to investigate the kinetics of HCN decomposition on the Pt(111) surface over the temperature range of 120 to 135 K. At these low temperatures, HCN bonds at an atop site with the HCN axis perpendicular to the surface, which gives rise to an intense C–H stretch at ∼3300 cm–1. Further support for this HCN adsorption geometry is obtained through HCN/CO coadsorption experiments in which both molecules are seen to compete for the atop sites. The disappearance of the C–H stretch peak of HCN at low temperatures is indicative of dissociation to produce adsorbed H and CN. When the decrease in HCN coverage is followed for a sufficiently long time, the data deviate from the expected first-order rate law, and the temperature dependence of the rate constant deviates from the Arrhenius form. Over a more restricted coverage range, simpler behavior is observed, and an activation energy for HCN dissociation of 0.33 eV is obtained.

Temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) were used to investigate the adsorption and reaction chemistry of styrene with clean and nitrogen covered Pt(111). The RAIR spectra indicate that the adsorption geometry of styrene is altered by both coverage effects and by the presence of nitrogen atoms on the surface. For an annealed monolayer of styrene on the clean Pt(111) surface, the most intense peaks correspond to out-of-plane bending vibrations indicating that the molecular plane is oriented parallel to the surface. For monolayer styrene on the nitrogen-precovered surface and for an unannealed monolayer of styrene, the in-plane and out-of-plane vibrations have comparable intensities indicating a more random orientation of the molecular plane. Upon heating, styrene reacts with the coadsorbed nitrogen atoms to form benzonitrile, which desorbs at 380 K. When benzonitrile is directly adsorbed on the Pt(111) surface, TPRS and RAIRS reveal that it desorbs at 350 K, indicating that its desorption at 380 K when formed from the reaction of styrene with nitrogen is reaction limited.

A low-temperature scanning tunneling microscope operated at 4.7 K was used to observe individual molecules produced from the thermal decomposition and hydrogenation reactions of acetylene on the Pt(111) surface. Acetylene molecules observed on the surface following adsorption at 50 K are seen to persist up to room temperature at the same time as two other moieties are observed to form. One moiety greatly increases in amount when the acetylene is coadsorbed with hydrogen and is attributed to the vinyl species, HCCH2, in agreement with a recent study using reflection absorption infrared spectroscopy. A third species observed at room temperature is identified as vinylidene, CCH2. The identification of these species is aided by using voltage pulses from the STM to further decompose them into species containing fewer atoms. Ethylidyne, CCH3, is identified after heating the surface to 400 K and was confirmed by comparison to results obtained following ethylene adsorption. Exposure of the surface held at 800 K to acetylene produced C2 molecules on the surface, which could be subsequently hydrogenated to ethylidyne. The hydrogenation of residual surface carbon also leads to the formation of ethylidyne, suggesting that the residual carbon was in the form of C2 molecules rather than carbon atoms.

A low-temperature scanning tunneling microscope operated at 4.7 K was used to observe individual molecules of ethylene on the Pt(111) and Pd(110) surfaces following adsorption at 50 K. In both cases, two forms of the molecules were observed and were found to switch from one to the other under the influence of the tip. The form with the greater apparent height is attributed to π-bonded ethylene, and the other form is attributed to di-σ bonded ethylene. The images provide direct evidence that on both surfaces π-bonded ethylene occupies an on-top site and di-σ bonded ethylene occupies a symmetric bridging site. The results are compared with predictions based on DFT calculations that di-σ bonded ethylene should be the most stable form on both surfaces.Keywords: adsorption sites; ethylene; low-temperature scanning tunneling microscopy; single molecule imaging; tip-induced reaction;

The adsorption, thermal evolution, and electron irradiation of 2-butanol on Pt(111) were investigated with reflection absorption infrared spectroscopy (RAIRS). A simulated vibrational spectrum of a single 2-butanol molecule was calculated using density functional theory to facilitate vibrational assignments. Exposures of 0.2 Langmuir (L) and lower result in both isolated 2-butanol molecules with minimal lateral interactions and hydrogen-bonded clusters. The thermal evolution following a 4.0 L exposure shows that the hydrogen-bonded multilayer desorbs around 170 K, leaving a 2-butanol monolayer where hydrogen bonding still exists. At 190 K, a new feature at 1699 cm−1 is attributed to the formation of butanone. Irradiation with 750 or 100 eV electrons leads to 2-butanol desorption and partial conversion to butanone, as indicated by the appearance of a peak at 1709 cm−1.

Reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of oxygen on the (100) and (111) surfaces of lanthanum hexaboride. Exposure of the surface at temperatures of 95 K and above to O2 produces atomic oxygen on the surface and yields vibrational peaks in good agreement with those observed in previous HREELS studies. On the La-terminated (100) surface, RAIRS peaks correspond to vibrations of the boron lattice that gain intensity due to a decrease in screening of surface dipoles that accompanies oxygen adsorption. A sharp peak at ∼ 734 cm−1 in the HREEL spectrum shows isotopic splitting with RAIRS into two components at 717 and 740 cm−1 with full widths at half maxima of only 12 cm−1. The sharpness of this mode is consistent with its interpretation as a surface phonon that is well separated from both the bulk phonons and other surface phonons of LaB6. On the boron-terminated LaB6(111) surface, broad and weak features are assigned to both vibrations of the boron lattice and of boron oxide. On the (100) surface, oxygen blocks the adsorption sites for CO, and adsorbed CO prevents the dissociative adsorption of O2.

The adsorption of carbon monoxide on the LaB6(1 0 0) and LaB6(1 1 1) surfaces was studied experimentally with the techniques of reflection absorption infrared spectroscopy and X-ray photoelectron spectroscopy. The interaction of CO with the two surfaces was also studied with density functional theory. Both surfaces adsorb CO molecularly at low temperatures but in markedly different forms. On the LaB6(1 1 1) surface CO initially adsorbs at 90 K in a form that yields a CO stretching mode at 1502–1512 cm−1. With gentle annealing to 120 K, the CO switches to a bonding environment characterized by multiple CO stretch values from 1980 to 2080 cm−1, assigned to one, two, or three CO molecules terminally bonded to the B atoms of a triangular B3 unit at the (1 1 1) surface. In contrast, on the LaB6(1 0 0) surface only a single CO stretch is observed at 2094 cm−1, which is assigned to an atop CO molecule bonded to a La atom. The maximum intensity of the CO stretch vibration on the (1 0 0) surface is higher than on the (1 1 1) surface by a factor of 5. This difference is related to the different orientations of the CO molecules on the two surfaces and to reduced screening of the CO dynamic dipole moment on the (1 0 0) surface, where the bonding occurs further from the surface plane. On LaB6(1 0 0), XPS measurements indicate that CO dissociates on the surface at temperatures above 400 K.

The adsorption and thermal decomposition of N-methylaniline (NMA) on the Pt(1 1 1) surface has been studied with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). NMA adsorbs molecularly at 85 K through the nitrogen lone pair and is stable up to 300 K. At temperatures of 300–350 K it converts to two or more surface intermediates including the N-methyleneaniline (NMEA) species. This NMEA intermediate dissociates upon annealing to 450 K, and further annealing leads to the desorption of HCN and H2, leaving only C on the surface at 800 K.

Catalyst-assisted growth of single-crystal strontium hexaboride (SrB6) nanowires was achieved by pyrolysis of diborane (B2H6) over SrO powders at 760−800 °C and 400 mTorr in a quartz tube furnace. Raman spectra demonstrate that the nanowires are SrB6, and transmission electron microscopy along with selected area diffraction indicate that the nanowires consist of single crystals with a preferred [001] growth direction. Electron energy loss data combined with the TEM images indicate that the nanowires consist of crystalline SrB6 cores with a thin (1 to 2 nm) amorphous oxide shell. The nanowires have diameters of 10−50 nm and lengths of 1−10 μm.

The surface chemistry of 1,2-closo-dicarbadodecaborane (ortho-carborane), C2B10H12, on Pt(111), was studied with reflection–absorption infrared spectroscopy (RAIRS), temperature programmed desorption, and X-ray photoelectron spectroscopy (XPS). This molecule has a cage-like structure in which boron and carbon atoms occupy the vertices of a slightly distorted icosahedron with 2 C−H and 10 B−H radially directed bonds. At submonolayer coverages at 85 K, the RAIRS spectrum of carborane displays strong B−H stretching vibrations near 2600 cm−1 and a weak C−H stretch at 3090 cm−1 that are close in value to those of the isolated molecule. This indicates molecular adsorption at low temperature. At 85 K and low coverages, the B−H stretch peaks of carborane are unusually narrow with full widths at half-maxima as low as 4.4 cm−1. Comparison of calculated spectra for various assumed orientations reveals that the molecule is oriented with its permanent dipole moment and the C−C bond parallel to the surface. The molecule is stable on the surface up to 250 K, where it is transformed into a new intermediate with a strongly red-shifted B−H stretch vibration at 2507 cm−1. This intermediate is stable up to 400 K, above which no B−H stretch vibrations are observed. Hydrogen is released in stages as the carborane monolayer is heated from 85 to 800 K, indicating the formation of partially hydrogenated surface intermediates. From XPS measurements of the B 1s peak area as a function of annealing temperature, the boron coverage steadily decreases as the boron cage structure is disrupted and boron atoms diffuse into the bulk of the crystal.

The surface chemistry of isocyanate, NCO, and its precursor, isocyanic acid, HNCO, on Pt(111) was studied with the techniques of reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). Isocyanate is known to form on platinum-group metals in the course of the catalytic reduction of NO by carbon-containing molecules, such as CO or simple hydrocarbons. It is therefore important to establish its stability and reaction chemistry on well-characterized platinum surfaces. Exposure of the Pt(111) surface at 90 K to isocyanic acid leads to both molecularly adsorbed HNCO, as well as its dissociation products, H and NCO. At 90 K and low coverages, molecularly adsorbed HNCO exhibits peaks at 1338, 2227, and 3357 cm−1 due to the NCO symmetric stretch, the NCO asymmetric stretch, and the NH stretch, respectively. Isocyanate has a markedly lower asymmetric NCO stretch of 2113 cm−1 at low coverages, which shifts to higher values at higher coverages. The decomposition of HNCO is largely completed when the surface is heated to 150 K. As the temperature is increased from 150 to 300 K, a series of HNCO and NCO decomposition products are identified with RAIRS, including CO, NH, NH2, NH3, H2O, OH, and NO. These reaction products indicate that NCO dissociation can occur at both the N−C and C−O bonds. Dissociation at the C−O bond is also indicated by observation of HCN desorption with TPD.

The formation of a well-ordered p(2 × 2) overlayer of atomic nitrogen on the Pt(1 1 1) surface and its reaction with hydrogen were characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 × 2)-N overlayer is formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to 400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic, rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indicated by the N2 TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with RAIRS by an intense and narrow (FWHM ∼ 4 cm−1) peak at 3322 cm−1. The areas of the H2 TPD peak associated with NH dissociation and the XPS N 1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the methods used here.

Methylidyne (CH) was prepared on Pt(1 1 1) by three methods: thermal decomposition of diiodomethane (CH2I2), ethylene decomposition at temperatures above 450 K, and surface carbon hydrogenation. Methylidyne and its precursors are characterized by reflection absorption infrared spectroscopy (RAIRS). The C–I bond of diiodomethane breaks upon adsorption to produce methylene (CH2), which decomposes to methylidyne at temperatures above 130 K. Above 200 K, methylidyne is the only hydrocarbon species observed with RAIRS, although reaction channels for the formation of methane (CH4) and ethylene (C2H4) are indicated by temperature programmed desorption (TPD). As is well known from numerous previous studies, ethylene decomposes to ethylidyne (CCH3) upon exposure to Pt(1 1 1) at 410 K. Upon annealing to 450 K, ethylidyne dissociates through two reaction pathways, dehydrogenation to ethynyl (CCH) and C–C bond scission to methylidyne. Ethylene dehydrogenation on the surface at 750 K and under low ethylene exposures produces surface carbon that can be hydrogenated to methylidyne with C–H and C–D stretch frequencies of 2956 and 2206 cm−1, respectively. Hydrogen co-adsorption on the surface causes these frequencies to shift to higher values. Methylidyne is stable on Pt(1 1 1) to temperatures up to 500 K.

Reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been used to study the adsorption and decomposition of trimethylamine ((CH3)3N) on the Pt(1 1 1) surface. Trimethylamine (TMA) adsorbs molecularly at 85 K and is stable up to 250 K. Based on previous studies showing that methylaminocarbyne (CNHCH3) and aminocarbyne (CNH2) form from the partial dehydrogenation of dimethylamine and methylamine, respectively, dimethylaminocarbyne, CN(CH3)2 is expected from the analogous reaction of trimethylamine. Spectral changes that occur when adsorbed TMA is heated to 350 K, including the appearance of a CN stretch at 1507 cm−1, are attributed to formation of CN(CH3)2. Spectral assignments are supported by shifts observed following initial adsorption of the TMA isotopomers (CH3)3 15N and (CD3)3N and by density functional theory calculations based on a Pt2CN(CH3)2 model. The results provide further evidence that aminocarbyne species are common intermediates in the surface chemistry of molecules containing CN bonds.

Reflection absorption infrared spectroscopy and temperature programmed desorption have been used to study the reactive surface chemistry of dimethylamine (DMA) on the Pt(1 1 1) surface over the temperature range of 85–550 K. DMA adsorbs molecularly at 85 K through the nitrogen lone pair and is stable up to 350 K where it dehydrogenates to form methylaminocarbyne (CNHCH3), a species that was found in an earlier study to also form from the N-protonation of methyl isocyanide (CNCH3) on Pt(1 1 1). Upon annealing to 400 and 450 K, methylaminocarbyne decomposes to several different surface intermediates, including methyl isocyanide, as indicated by a ν(CN) peak at 2238 cm−1. Spectroscopic characterization of a surface intermediate that appears to retain a CNC unit and is stable up to 450 K is presented. This intermediate is formed from several precursors, including DMA, methyl isocyanide, and trimethylamine. At temperatures over 500 K, H2 and HCN desorb by the decomposition of the surface intermediates leaving CN and C on the surface.

Reflection absorption infrared spectroscopy has been used to detect formate (OOCH) following the adsorption of methanol on an oxygen precovered Cu(1 0 0) surface. Formate is identified from a weak peak at 1340 cm−1 due to the symmetric OCO stretch. This assignment is supported by a redshift of the 1340 cm−1 peak to 1317 cm−1 when the surface is precovered with . The amount of formate reaches a maximum for an intermediate oxygen coverage and is undetectable for both low and saturation oxygen coverages.

X-ray photoelectron spectroscopy was used to investigate the initial stages of oxygen adsorption on the (100) surface of a single crystal of lanthanum hexaboride. Numerous previous studies had not resolved the issue of whether oxygen adsorbs at lanthanum sites, boron sites, or both. We find that oxygen adsorption markedly alters the La 3d lineshapes, whereas the B 1s peak is unaffected. On the clean surface the La 3d3/2 peak is split into two components at binding energies of 854.7 and 851.8 eV, a splitting that is typical of rare-earth metals and their compounds. The two components are associated with two different final states. In one final state the 3d core hole is poorly screened (854.7 eV) and in the other it is well-screened (851.8 eV). The relative intensity of the two components is known to be very sensitive to the chemical environment of the rare earth atom and a 10 L O2 exposure at room temperature produces a large increase in the relative intensity of the well-screened component. Annealing the surface to 600°C and then to 700°C produces sharp c(2×2) and p(2×1) LEED patterns respectively. The La 3d peaks associated with the two LEED patterns are similar to those observed after the initial 300 K 10 L O2 exposure, indicating oxygen bonding to La in both overlayer structures. Thus while the XPS data clearly reveal oxygen adsorption at La sites, there is no indication of adsorption at boron sites for low O2 exposures. More extensive oxidation at higher temperatures shows formation of both boron and lanthanum oxides.

Adsorption and thermal chemistry of propanal, 2-propenol, and 1-propanol on Ru(001) were studied using temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS). The results show that each molecule adsorbs molecularly at 90 K and displays the same spectral features as observed for the corresponding liquids after 1.0 L exposures. 2-Propenol was found to molecularly desorb at 200 K, dehydrate to yield propene around 130 K, isomerize to propanal at 180 K, and hydrogenate to 1-propanol at 220 K. Propanal, however, does not undergo isomerization on the surface but desorbs molecularly at 175 and 280 K. Similarly, 1-propanol also desorbs molecularly with two peaks centered at 227, and 298 K. Formaldehyde desorption was observed for each molecule. Furthermore, a reversible hydrogenation-dehydrogenation process was observed between propanal and 1-propanol in the range of 200 to 320 K. These results provided further insights into previous studies on hydrogenation pathways of acrolein on the Ru(001) surface and into the challenges of selectively increasing the yield of the unsaturated alcohol.