Development of high-performance solid acid catalysts for chemicals and materials production from bioresourced feedstock has become an important research topic in heterogeneous catalysis for renewable energy and green chemistry. We provide herein a comprehensive study on the catalytic performance of various K+-exchanged zeolites (KxNa1-xZ_y, x = 0.90–0.98) with similar molar K/Al ratios for acrylic acid (AA) production by gas-phase dehydration of lactic acid (LA) and discuss the effects of zeolite type (Z = ZSM-22, ZSM-35, MCM-22, ZSM-11, ZSM-5, ZSM-5/ZSM-11, and β) and SiO2/Al2O3 ratio (y). ZSM-5 and β are found more efficient than the other zeolites for this LA-to-AA reaction. Variation of y in the zeolite (β and ZSM-5) is shown to significantly affect the catalytic performance: not only higher AA selectivity and yield but also better catalytic stability is achieved by lowering y. A K0.97Na0.03ZSM-5_27 is then identified as the best-performing catalyst, offering very high AA selectivity (80–81 mol%) and yield (74–78 mol%) at 360 °C under high LA space velocity (WHSVLA = 2.1 h–1). This catalyst also shows a remarkable long-term stability, being capable to maintain a high AA selectivity (>70 mol%) and yield (>55 mol%) for longer than 80 h. Furthermore, an in situ calcination of the used catalyst with flowing air at 450 °C is shown to be efficient for complete catalyst regeneration. Correlating the catalyst performance with its surface acid–base property measured by NH3- and CO2-TPD clearly uncovers that balance between the surface acidity and basicity would be a key, besides Z and y of the zeolite, to the catalyst performance.Keywords: acid−base catalysis; acrylic acid; catalytic synthesis; ion-exchanged zeolites; lactic acid;

Attempts are made in this study to manipulate nanostructures of Pt-flecks on Ag nanoparticles (Ptm∧Ag) for advanced electrocatalysts by reflux citrate reduction of Pt from PtIICl42− or PtIVCl62− ions in solution containing Ag colloids at different atomic Pt/Ag ratio (m). Characterizations with UV−vis, SERS, XPS, and XRD showed a gradual Pt covering of the Ag colloids with increasing m when PtIICl42− was the precursor of Pt (Ptm∧Ag−A samples). However, due to an involvement of the galvanic replacement reaction between PtIVCl62− and the metallic Ag colloids during the citrate reduction of PtIVCl62− ions, a distinct alloying of Pt with the underlying Ag particles took place at the surface region of the colloidal Ag particles when PtIVCl62− was the precursor of Pt (Ptm∧Ag−B samples). Cyclic voltammetry (CV) measurement of the electrochemically active surface area (EAS) showed that the Pt utilization (UPt) in Ptm∧Ag−A increased with the decrease in m. The mass-specific activity (MSA) of Pt for the electrooxidation of either methanol or formic acid increased linearly with UPt in Ptm∧Ag−A, but was enhanced significantly with proper Pt−Ag alloying in Ptm∧Ag−B catalysts. Fine-tuning the extent of Pt−Ag alloying resulted in optimized Ptm∧Ag−B catalyst at 0.47 ≤ m ≤ 0.53, whose activity by MSA of Pt was 1 order of magnitude higher in methanol electrooxidation and six times higher in formic acid electrooxidation than its Ptm∧Ag−A counterpart of similar UPt.

Nanostructured Pt-on-Au electrocatalysts (coded as Ptm∧Au, m being the atomic Pt/Au ratio), prepared by Pt deposition on Au colloids in two size ranges (Au-I, 10.0 ± 1.2 nm; Au-II, 3.0 ± 0.6 nm) (Zhao and Xu, Phys. Chem. Chem. Phys. 2006, 8, 5106), were employed for the electrooxidation of formic acid (HCOOH) at concentrations of 0.2−3.2 M by cyclic voltammetry. The HCOOH electrooxidation over a Pt shell fully covering Au-I colloids (Ptm∧Au-I at m > 0.2, Pt dispersion < 20%) occurred mainly in the high potential range (0.6−1.0 V vs SCE). The lowering of m in Ptm∧Au-I samples resulted in a remarkable increase in the current of HCOOH electrooxidation in the lower potential range (−0.2 to 0.6 V) due to a continued enhancement in the Pt utilization associated with the changes in the Pt-dispersion state. The areal activity (intrinsic activity) of Pt flecks (Pt dispersion > 50%) was 5 times and their mass-specific activity 25 times higher than those of the conventional Pt/C and core@shell structured Pt∧Au-I catalysts. The use of Ptm∧Au-II for HCOOH electrooxidation produced qualitatively similar results, thus demonstrating a dramatic enhancement of the electrocatalytic activity of Ptm∧Au/C by reducing the domain size of Pt deposits on Au surfaces. Moreover, the intrinsic activity of Pt flecks in Ptm∧Au-II was found to be 4 times higher than those in Ptm∧Au-I, which uncovers that smaller Au particles can serve as a kind of “activity promoter” to their carrying Pt flecks. The current of HCOOH electrooxidation over the Ptm∧Au catalysts also varied significantly, according to the HCOOH concentration. The highest current was obtained only in an appropriate HCOOH concentration window for a given Pt∧Au/C catalyst.

•Method for the evaluation of surface acidity on working solid catalysts is provided.•Skeletal isomerization of 3,3-dimethylbut-1-ene and dehydration of 2-propanol are used as the probe reactions.•Typical Brønsted (H-ZSM-5), Lewis (γ-Al2O3), and water-resistant Brønsted and Lewis acidic (Nb2O5) solid acids are investigated.•Effects of water presence on the nature (Brønsted or Lewis), density and acidity strength of the surface acidic sites are reported.Two model reactions, namely the Brønsted-acidity-specific skeletal isomerization of 3,3-dimethylbut-1-ene for evaluating the Brønsted acidity and dehydration of 2-propanol for the overall acidity at the catalyst surface, were employed in this work to measure the effect of water on the surface acidic property of typical solid catalysts (HZSM-5, γ-Al2O3 and Nb2O5). For the isomerization reaction, water addition in the reaction feed produced little effect on the catalysis of HZSM-5 but significantly positive effect on the catalysis of Nb2O5; the typical Lewis acidic γ-Al2O3 showed no activity for this reaction in either the presence or absence of water. For the dehydration reaction, the effect of water was generally negative on the catalysis of every catalyst, demonstrating that water presence resulted in poisoning more or less of the Lewis acidity on the catalyst surface. Water poisoning of the Lewis acidity on Nb2O5 also resulted in generation of Brønsted acidity and the higher the H2O partial pressure (or H2O-to-substrate ratio) the higher the newly generated Brønsted acidity. However, generation of Brønsted acidity did not happen on γ-Al2O3 and HZSM-5, regardless of the H2O partial pressure. Three Nb2O5 samples with varying calcination temperature were involved, and their behavior clearly indicated that the water effects on Nb2O5 also depended sensitively on the sample calcination temperature. Moreover, water presence increased the activation energy and selectivity for di-isopropyl ether of the dehydration reaction over γ-Al2O3 and Nb2O5, and thus affected the acidity distribution at the catalyst surface during the reaction.Download high-res image (181KB)Download full-size image

•NaOH is identified as a homogeneous catalyst for aerobic alcohol oxidation in water.•Glycerol (triol), ethylene glycol (diol), 2-propanol, and ethanol are included.•Product selectivity and relative reactivity of the different alcohols are provided.•Possible mechanisms for the NaOH-catalyzed oxidation reactions are proposed.Sodium hydroxide (NaOH) has been an indispensable additive in the selective aerobic oxidation of alcohols/polyols in water for ketone and carboxylic acid syntheses over supported metal catalysts, including Au, though the exact function of NaOH remains far from clear. We disclose here that NaOH alone can independently serve as a homogeneous catalyst for the oxidation reactions. This conclusion is supported by rigorous investigations of the effects of NaOH alone (without the presence of any metal catalyst) on the reactivity of aqueous glycerol, ethylene glycol, 2-propanol, and ethanol and their product distribution. Computational results based on density functional theory are also provided to explain the experimentally measured reactivity order of the various alcohols and to generate insight into the molecular mechanisms for the oxidation reactions. Besides challenging existing knowledge about the function of NaOH in alcohol oxidation reactions, this study opens a new avenue for catalytic alcohol/polyol upgrading.Download high-res image (96KB)Download full-size image

Transfer hydrogenation of α,β-unsaturated aldehyde is an important reaction (MPV reaction) for the synthesis of allyl alcohol because of its high selectivity for the hydrogenation of the carbonyl group. This reaction can proceed over many oxide catalysts but the role of acidic and basic sites on the oxide catalyst surface has remained an issue of debate. We report herein the catalytic data of serial Al2O3 and SiO2–Al2O3 catalysts for the MPV reaction of cinnamaldehyde at 150 °C using 2-propanol as the H-donor. The surface acidity of the catalysts is adjusted by changing the calcination temperature between 400 and 1000 °C and quantified to correlate with the catalyst performance. Surface Lewis acidic sites that are poisoned by pyridine but not by hindered pyridine (lutidine) under catalyst working conditions are identified as the catalytic sites for the MPV reaction. Lutidine-sensitive surface Brønsted acidic sites, which are among the spectator sites during the MPV reaction, are shown to serve as the catalytic sites for cascade cross-etherification reaction between cinnamyl alcohol and 2-propanol. The MPV reaction is also conducted under high-pressure CO2, the results of which exclude possible involvement of surface basic sites during the reaction.

Fabricating Pt-alloy and core–shell nanostructures with Au NPs in the cores are considered as two general approaches to improving the performance of Pt-based catalysts for the cathodic oxygen reduction reaction (ORR) in acidic electrolyte. These two approaches are combined herein to develop a heteroseed-mediated solvothermal method for synthesizing nearly monodisperse core–shell structured Au@NimPt2 nanoparticles (NPs) of 5.0–6.5 nm (with the atomic ratio of Ni/Pt/Au = m/2/1) as ORR catalysts. With respect to controlling the amount and relative concentrations of the metal precursors in the starting solution, this method enables not only facile manipulation of the shell composition and thickness but also fine-tuning of the core–shell interaction and surface electronic structures of the resultant Au@NimPt2 NPs, endowing the Au@NimPt2 NPs with improved Pt activity and durability for ORR. Subjecting the Au@NimPt2 NPs to an ex situ pretreatment in flowing 2%CO/Ar at 300 °C is shown to result in further improved Pt activity. Data are also presented to correlate the intrinsic Pt activity with experimentally determined CO adsorption property (CO-stripping peak potential) of Pt for the Au@NimPt2 samples, and to show the excellent electrochemical durability of the Au@NimPt2 NPs during 20 000 potential cycles between 0.6 and 1.1 V (vs RHE) in O2-saturated 0.1 M HClO4. Compared with the commercial E-TEK Pt/C catalyst, the most-active Au@Ni2Pt2 NPs exhibit 3–4- and 4–6-fold higher Pt activity at 0.9 V before and after the 20 000 potential cycles, respectively. Factors relevant to the activity and durability control of the Au@NimPt2 catalysts for ORR are discussed.Keywords: catalyst design; core−shell nanostructure; electrocatalyst; electrochemical treatment; multimetallic nanoparticles; oxygen reduction reaction; solvothermal synthesis

The direct redox reaction (galvanic displacement) between Pd2+ and substrate Si was used to deposit Pd on Si, and the Pd–Si catalysts enabled a chemoselective hydrogenation of para-chloronitrobenzene with the selectivity for para-chloroaniline higher than 99.9% at complete conversion of para-chloronitrobenzene.

Metal nanoparticles (NPs) from colloidal synthesis are advantageous in fundamental catalysis research because of their precisely controlled size and morphology but unfortunately are usually contaminated with residues from the organic stabilizer essentially required in the synthesis. These residues could modify the surface property and disturb the catalysis intrinsic to “clean” NPs. Herein, polyvinylpyrrolidone (PVP)-stabilized Au NPs (4.7 ± 1.0 nm) from colloidal synthesis were immobilized on SiO2 support and subjected to ultraviolet-ozone (UVO) treatment to remove the residues. Hydrogenation reactions of p-chloronitrobenzene (p-CNB) and cinnamaldehyde (CAL) were conducted to probe consequences of the stabilizer removal on the catalytic properties of Au NPs. Measurements by FTIR and XPS revealed a controlled removal and degradation of PVP according to the UVO-treatment duration. Careful HRTEM analysis disclosed that both the size and morphology of Au NPs remained unchanged after the UVO-treatment. Residual PVP significantly improved the activity of Au NPs for p-CNB hydrogenation but lowered the activity for CAL hydrogenation. Continued selectivity changes of CAL hydrogenation to favor the reaction at the C═C bond were observed on increasing the removal degree of the residues. The UVO-cleaned Au NPs were also “restabilized” with PVP and other stabilizers by adsorption in aqueous solution. Comparison of the catalytic properties of these Au NPs involving different stabilizers with those of the UVO-cleaned ones enabled a comprehension of the stabilizer impacts on the hydrogenation catalysis of Au NPs.Keywords: catalysis by gold; chloronitrobenzene; cinnamaldehyde; metal colloids; selective hydrogenation; stabilizer effect; ultraviolet-ozone

A big challenge in upgrading bio-oxygenate platform molecules is to develop catalysts for the selective oxidation of a nonterminal HO-bonded carbon atom in polyols. We report herein the first finding of a specific selectivity of oxide-supported nano-Au catalysts for dihydroxyacetone (DHA) production in glycerol oxidation in water without NaOH. Though the support nature (Al2O3, TiO2, ZrO2, NiO, and CuO) significantly affects the Au activity, a highly active Au/CuO catalyst offering DHA yields up to 80% at 40–50 °C has been identified. Rich data are provided to clarify that DHA is the only primary product of glycerol oxidation. This propensity of nano-Au for oxidizing the HO-bonded secondary (central) carbon is further verified by comparing the oxidation of propanediols and propanols. Molecular insight into the reactions is given on the basis of the kinetic isotopic effect study of deuterium on the oxidation of 2-propanol, uncovering an unanticipated chemistry of Au catalysis.Keywords: dihydroxyacetone; glycerol; heterogeneous catalysis; propane polyols; reaction mechanism; selective oxidation; supported gold catalyst

Hydroxyapatites (HAPm-T) of varying molar Ca/P ratios m (1.58–1.69) and calcination temperatures T (360–700 °C) were prepared and comprehensively characterized by nitrogen adsorption, TG, XPS, XRD, CO2-TPD, and NH3-TPD and were employed to catalyze the gas-phase dehydration of lactic acid (LA) to produce acrylic acid (AA). While the texture and crystallinity of the HAPm-T sample were affected little by variation of m, its surface acidity decreased but basicity increased with the increase in m. The HAPm-T sample with a higher T showed a higher crystallinity but lower surface area, acidity, and basicity. The conversion of LA decreased with increasing either m or T of the HAPm-T catalyst; the selectivity for AA maximized at m = 1.62 but decreased steadily with the T increase. The HAP1.62-360 sample (m = 1.62, T = 360 °C) was identified as the most efficient catalyst, offering an AA yield as high as 50–62% for longer than 8 h (AA selectivity: 71–74 mol %) under optimized reaction conditions (360 °C, WHSVLA= 1.4–2.1 h–1). Correlating the catalyst performance with its surface acidity and basicity disclosed that the LA consumption rate increased linearly with the acidity/basicity ratio, but volcano-type dependence appeared between the AA production rate and the acidity/basicity ratio, which reveals a kind of cooperative acid–base catalysis for selective AA production. The HAPm-T catalysts became more or less deactivated after reaction, but the reacted ones could be fully regenerated by in situ treatment with flowing air.Keywords: acid−base catalysis; acrylic acid; catalytic dehydration; hydroxyapatites; lactic acid; sustainable chemistry

This work reveals an ordered mesoporous carbon material co-doped with nitrogen and iron (Fe–N–C) for ORR catalysis in an alkaline electrolyte, whose ORR performance surpasses most of the previous metal-free heteroatom-containing carbon materials and is comparable to that of conventional Pt/C in terms of half-wave potential, limiting current density and kinetic current density. A procedure for preparing this Fe–N–C catalyst is described.

Zirconia-supported tungsten oxide (WO3/ZrO2 or WZ) is known as an efficient catalyst for selective acrolein (AC) production from gas-phase dehydration of glycerol (GL). Two catalysts (WZ-CP and WZ-AN) were prepared herein using, respectively, a ZrO(OH)2 hydrogel (ZrO(OH)2-CP) and its derived alcogel (ZrO(OH)2-AN) for a precursor of ZrO2. To optimize the reaction variables and improve the catalyst performance, the WZ-CP catalyst was employed to show the effects of (A) reaction variables (temperature, partial pressures of GL and H2O, and co-feeding H2 or O2); (B) catalyst modification with alkali and alkali earth metal ions (Na+, K+ and Mg2+), or transition metals (Pt, Pd, Rh and Ni). The reaction at 315 °C always produced the highest AC selectivity, and this temperature was then used to investigate the effects of the other variables and catalyst modifications. Increasing the molar GL/H2O ratio led to lower AC selectivity and accelerated the catalyst deactivation. Introducing 4–8 kPa O2 to the reaction feed significantly reduced the catalyst deactivation rate but the AC selectivity was only slightly lowered. However, an addition of 4 kPa H2 produced almost no effect on the reaction. The modified catalysts performed no better during the reaction unless the modifier was Pt or Pd, whose catalytic stabilities in the O2-containing (4 kPa) feed were significantly higher and their selectivity for AC production slightly lowered. Working under the conditions optimized with WZ-CP, the WZ-AN catalyst offered a high AC yield (62–68%) for longer than 30 h, during which the GL conversion remained higher than 93%.

A series of tantalum oxide samples (Ta2O5-T) were prepared by varying the calcination temperature T in the range of 110–700 °C of a highly hydrated precursor (Ta2O5·2.1H2O), which was obtained from a hetero-phase reaction between tantalum pentachloride and aqueous ammonia. These Ta2O5-T samples were characterized by TG/DTA, XPS, nitrogen adsorption, XRD, and UV-Raman, and were employed to catalyze the gas-phase dehydration of glycerol (GL) to produce acrolein (AC) at around 315 °C. n-Butylamine titration using Hammett indicators, NH3- and CO2-TPD, and IR of adsorbed pyridine were measured to assess the surface acid–base properties of the catalysts. The Ta2O5-350 catalyst, which was in a hydrated amorphous state and showed no basicity but a maximum strong acidity at −8.2 < H0 ≤ −3.0 (H0 being the Hammett acidity function) prior to the reaction, was identified among all Ta2O5-T samples as the best performing catalyst in terms of AC selectivity and GL consumption rate. The AC selectivity increased with increasing the fractional strong acidity at −8.2 < H0 ≤ −3.0 but the catalytic GL consumption rate increased with increasing the number of strong acid sites on the catalyst prior to the reaction. The partly (Ta2O5-600) and fully (Ta2O5-700) crystallized samples showed almost no strong acidity but they exhibited some basicity in the CO2-TPD experiments; their much lower AC selectivity (<40 mol%) as compared with that on Ta2O5-350 (75 mol%) indicated that surface basicity was detrimental to the AC selectivity. The best performing Ta2O5-350 catalyst also showed after activation in the initial hours of the reaction a good catalytic stability in long term reactions, regardless of the reaction temperature (305–340 °C) and GHSVGL (80–400 h−1).

Au nanoparticles (NPs) carrying different stabilizers (PVP, PVA and CTAB) were prepared by stabilizer-exchange of freshly prepared citrate (Citr)-stabilized Au NPs (Au-Citr) with the substitute stabilizers, respectively. The stabilizer substitutions were monitored with UV-vis spectroscopy and the resultant Au particles were further characterized with FTIR and TEM after they were immobilized on a SiO2 support. Measurements of the catalytic performance of the immobilized Au NPs for the hydrogenation reactions of p-chloronitrobenzene and cinnamaldehyde enabled us to address the impact of stabilizer substitution on the hydrogenation catalysis of Au NPs.

Effects of the crystalline structure and surface property of the Al2O3 support on the NOx storage and reduction (NSR) performance of Pt–BaO/Al2O3 catalysts were studied using catalysts prepared from a series of Al2O3 samples obtained by varying the calcination temperature of an Al(OH)3 hydrogel in the range of 450–1000 °C (referred to as the pre-calcination temperature, PCT). The texture, crystalline structure and surface acidity of the Al2O3(PCT) supports were measured employing nitrogen adsorption–desorption, XRD, NH3-TPD and IR spectroscopy of adsorbed pyridine, respectively. The Al2O3(PCT) samples showed a gradual ordering of the γ-Al2O3 phase when increasing the PCT from 450 to 800 °C, and a phase transition from the γ- to θ-Al2O3 phase upon further increasing the PCT to 1000 °C. The surface density of Lewis acid sites of the Al2O3(PCT) samples exhibited a maximum at PCT in the range of 800–900 °C. The NSR performance of Pt–BaO/Al2O3 catalysts derived from the Al2O3(PCT) samples was studied under cyclic lean/rich conditions. The numbers of NOx stored and reduced on the Pt–BaO/Al2O3(PCT) catalysts showed similar volcano-type dependencies on PCT, peaking at PCT = 800 °C. The origins of the support PCT effect were discussed in the light of Pt particle size, the nature of BaO sites and Pt–BaO synergy. It was found that the increase in the crystallinity of the γ-Al2O3 phase and the surface acid site density of the supporting Al2O3 samples would result in improved proximity and synergy between Pt and BaO sites, leading to much more efficient NSR Pt–BaO/Al2O3 catalysts.

Controlled deposition of Pd on small Au NPs (3.2 ± 0.5 nm) is conducted to tune the Pd dispersion and electronic states in Pd-on-Au nanostructures. The as-prepared samples of high Pd utilization (>85%) are found to show stronger Pd–O interaction and intrinsically higher catalytic activity toward the electro-oxidation of formic acid.

This work investigates the catalytic performance of nanocomposite Ni/ZrO2-AN catalyst consisting of comparably sized Ni (10–15 nm) and ZrO2 (15–25 nm) particles for hydrogen production from the cyclic stepwise methane reforming reaction with either steam (H2O) or CO2 at 500–650 °C, in comparison with a conventional Ni/ZrO2-CP catalyst featuring Ni particles supported by large and widely sized ZrO2 particles (20–400 nm). Though both catalysts exhibited similar activity and stability during the reactions at 500 and 550 °C, they showed remarkably different catalytic stabilities at higher temperatures. The Ni/ZrO2-CP catalyst featured a significant deactivation even during the methane decomposition step in the first cycle of the reactions at ≥600 °C, but the Ni/ZrO2-AN catalyst showed a very stable activity during at least 17 consecutive cycles in the cyclic reaction with steam. Changes in the catalyst beds at varying stages of the reactions were characterized with TEM, XRD and TPO–DTG and were correlated with the amount and nature of the carbon deposits. The Ni particles in Ni/ZrO2-AN became stabilized at the sizes of around 20 nm but those in Ni/ZrO2-CP kept on growing in the methane decomposition steps of the cyclic reaction. The small and narrowly sized Ni particles in the nanocomposite Ni/ZrO2-AN catalyst led to a selective formation of filamentous carbons whereas the larger Ni particles in the Ni/ZrO2-CP catalyst a preferred formation of graphitic encapsulating carbons. The filamentous carbons were favorably volatilized in the steam treatment step but the CO2 treatment selectively volatilized the encapsulating carbons. These results identify that the nature but not the amount of carbon deposits is the key to the stability of Ni/ZrO2 catalyst and that the nanocomposite Ni/ZrO2-AN would be a promising catalyst for hydrogen production via cyclic stepwise methane reforming reactions.Highlights► Feature of nanocomposite Ni/ZrO2 for cyclic stepwise methane reforming reactions. ► Size and catalytic stability of Ni particles in conventional and nanocomposite Ni/ZrO2 catalysts. ► Correlation between the nature of carbon deposits and the catalyst nanostructure. ► The nature but not the amount of carbon deposits is the key to the catalyst stability.

Platinum is a widely used precious metal in many catalytic nanostructures. Engineering the surface electronic structure of Pt-containing bi- or multimetallic nanostructure to enhance both the intrinsic activity and dispersion of Pt has remained a challenge. By constructing Pt-on-Au (Pt∧Au) nanostructures using a series of monodisperse Au nanoparticles in the size range of 2–14 nm, we disclose herein a new approach to steadily change both properties of Pt in electrocatalysis with downsizing of the Au nanoparticles. A combined tuning of Pt dispersion and its surface electronic structure is shown as a consequence of the changes in the size and valence-band structure of Au, which leads to significantly enhanced Pt mass-activity on the small Au nanoparticles. Fully dispersed Pt entities on the smallest Au nanoparticles (2 nm) exhibit the highest mass-activity to date towards formic acid electrooxidation, being 2 orders of magnitude (75–300 folds) higher than conventional Pt/C catalyst. Fundamental relationships correlating the Pt intrinsic activity in Pt∧Au nanostructures with the experimentally determined surface electronic structures (d-band center energies) of the Pt entities and their underlying Au nanoparticles are established.Keywords: bimetallic nanostructure; formic acid electrooxidation; gold; oxygen reduction; platinum; size effect; surface electronic structure

Designed manipulation of the morphology of metallic nanostructures containing Pd and/or Pt represents a challenge in the search of highly efficient precious metal catalysts. We provide herein the first selective synthesis of narrowly sized small (21 nm) Au@Pd concave nanocubes enclosed with high-index Pd facets by deposition of Pd atoms on truncated-octahedral Au seeds. Moreover, further controlled deposition of Pt onto these Au@Pd concave nanocubes produces sub-30 nm trimetallic Pt-on-(Au@Pd) nanostructures having jagged Pt-rich surfaces. The as-prepared Au@Pd and Pt-on-(Au@Pd) nanostructures are found highly active and fairly stable when employed as anode catalysts for ethanol electrooxidation, their activity normalized to the mass of Pd or Pd plus Pt being 7–9 times higher than conventional Pd black catalyst. These data may echo the importance of innovative small multimetallic nanostructures for highly efficient catalysts that depend critically on use of precious metals, for applications in energy, environmental, and chemical technologies.

Supported Au nanoparticles (Au NPs) have been identified as highly selective catalysts for the chemoselective hydrogenation reaction potential for advanced and greener syntheses of many special and fine chemicals in organic chemistry, but their potential for applications has been hampered by their generally observed low activity arising from the intrinsic nobleness of gold to H2 activation. This work deals with a synergy between Au NPs and their carrying Pt entities in Pt-on-Au nanostructures (coded as Ptm∧Au, m denoting the atomic Pt/Au ratio) for hydrocinnamaldehyde production in the chemoselective hydrogenation of cinnamaldehyde. Ptm∧Au immobilized on a noninteracting SiO2 support (Ptm∧Au/SiO2) showed activity 1–2 orders of magnitude higher than that of monometallic Pt/SiO2 and Au/SiO2 catalysts. The high activity of Ptm∧Au nanostructures also remained distinct on interacting support materials such as Al2O3 and carbon and when varying the reaction temperature, H2 pressure, or both. Kinetic assessments suggest that the hydrogenation reaction could occur according to a Langmuir–Hinshelwood mechanism, in which cinnamaldehyde adsorbed on the Au surface was attacked by hydrogen atoms activated by Pt entities in the nanostructured Ptm∧Au catalysts. Pt dispersion or the size of the Pt entities and Pt–Au boundary, as well, strongly affected this synergic catalysis.Keywords: bimetallic PtAu catalyst; cinnamaldehyde; gold catalyst; kinetic modeling; Pt-on-Au nanostructure; selective hydrogenation; α,β-unsaturated aldehyde;

Adding a small amount of fully dispersed Pt entities onto the Au surface in Au/SiO2 catalyst is found to be an efficient approach to improve the catalytic activity of Au (up to 70-fold) for the hydrogenation of α,β-unsaturated carbonyl compounds, without alternating its selectivity towards CO or CC bond hydrogenation.

Ptm⁁Ag nanostructures (m being the atomic Pt/Ag ratio, m = 0.1–0.6) were prepared by reflux citrate reduction of PtCl62− ions in aqueous solution containing colloidal Ag (6.3 ± 3.9 nm). A distinct alloying of Pt with Ag was detected due to an involvement of the galvanic replacement reaction between PtCl62− and metallic Ag colloids. The nanostructure transformed from a structure with an Ag-core and an alloyed PtAg-shell to a hollow PtAg alloy structure with the increase in m. Compared to a commercial E-TEK Pt/C catalyst, the catalytic performance of Pt in the Ptm⁁Ag/C samples for the cathode oxygen reduction reaction (ORR) strongly correlated with the electronic structure of Pt, as a consequence of varied Pt dispersion and Pt–Ag interaction. With either H2SO4 or KOH as an electrolyte, Pt in the Ptm⁁Ag nanostructures with a relatively high m (≥0.4) showed significantly enhanced intrinsic activity whereas Pt in those catalysts with low m (≤0.2) appeared less active than the Pt/C catalyst. These data are used to discuss the role of electronic structure and geometric effects of Pt toward ORR.

We show that the electrocatalytic properties of nearly monodispersed Au nanoparticles (NPs) and their derived Pt-on-Au (Pt^Au) nanostructures with similarly dispersed Pt entities are dependent on the nature of stabilizers involved in the colloidal syntheses of the Au particles. The effect of stabilizer on the activity for oxygen reduction reaction (ORR) of Au NPs significantly outweighed the Au nano-size effect and thus would raise an alert to those reported size-dependent properties of metal NPs carrying various stabilizers in earlier studies. It is also demonstrated that the stabilizer effect on the property of Au NPs can further induce changes in the catalytic properties of their carried Pt. These findings clearly suggest that a proper screening of the stabilizer in the colloidal synthesis of metal NPs would be important for innovative nanomaterials and catalysts.

Dealloyed PtAg/C nanostructures, prepared by selective electrochemical etching of Ag in 0.5 M H2SO4 from a series of alloyed PtmAg/C samples with atomic Pt/Ag ratio m = 0.1, 0.5, 1.0 and 1.5, were employed as cathode electrocatalysts for oxygen reduction reaction (ORR) in 0.5 M KOH. Compared with their as-prepared counterpart alloy catalysts, the dealloyed catalysts showed higher half-wave potentials (E1/2) and significantly higher Pt mass-specific activity (MSA) data. The intrinsic activity (IA) of Pt increased more or less after the dealloying treatment but was strongly dependent on the composition (m) of the alloyed sample. The Pt IA numbers were comparable for the dealloyed catalysts derived from PtmAg/C of m = 0.5, 1.0 and 1.5, which were nearly twice that for E-TEK Pt/C catalyst and 3 times that for the dealloyed catalyst derived from Pt0.1Ag/C.

Two series of Ni/MgO catalysts were prepared by reducing NiO/MgO samples of fixed Ni loading but different calcination temperatures and of varying Ni loadings but fixed calcination temperature. These catalysts were investigated in CO2 reforming of methane under atmospheric pressure and characterized with XRD, TPR and H2-TPD techniques. A complete incorporation of NiO into the MgO “support” to form NiO–MgO solid solution during the calcination stage of the catalyst preparation was identified essential for the formation of stable Ni/MgO catalysts, and the presence of readily reducible “free” NiO in the calcined (unreduced) NiO/MgO samples was shown to produce the deactivating Ni/MgO catalysts during the CO2/CH4 reaction. The reactivities of CO2/CH4 were found sensitive to the particle size (or dispersion) of metallic Ni; the catalytic activity by CH4 turnover frequency (TOF) decreased with increasing the Ni particle size. The reduced catalysts showed two H2-TPD peaks and the nickel sites corresponding to H2-TPD peak at higher temperature showed a higher activity than those associated with the peak at lower temperature. Our data demonstrate that the support in the stable catalysts was actually a kind of NixMg1−xO (x = 0.02–0.15) solid solution and the stable catalytic sites were associated with nanosized Ni particles (3–20 nm) in strong interaction with the solid solution support..Two series of Ni/MgO catalysts were prepared by reducing NiO/MgO precursors of different calcination temperatures or of different Ni loadings. These catalysts were investigated in CO2 reforming of methane and characterized with XRD, TPR and H2-TPD. The hard reduction of NiO in NiO–MgO solid solution is helpful for the stability of Ni/MgO and the readily reduction NiO in NiO/MgO is responsible for Ni/MgO deactivation.

Catalytic gas-phase dehydration of glycerol to produce acrolein was investigated at 315 °C over ZrO2- and SiO2-supported 12-tungstophosphoric acid (H3PW12O40, HPW) catalysts with different treatment temperatures. Characterizations with XRD and Raman spectroscopy of the prepared HPW/SiO2, HPW/ZrO2catalysts showed that the nature of support materials had a significant effect on the thermal stability of HPW; after thermal treatment at 650 °C, the Keggin structure of HPW remained intact on the surface of ZrO2 but was completely destroyed on SiO2. HPW on ZrO2 also showed higher dispersion than on SiO2. In comparison to HPW/SiO2, HPW/ZrO2catalysts exhibited significantly higher activity and selectivity for the formation of acrolein, as well as a slower deactivation rate during the dehydration reaction. The selectivity and yield for acrolein over the better HPW/ZrO2catalysts can be as high as 70 mol% and 54%, respectively, even after 10 h reaction.

The effect of Au3+ percentage in Au/TiO2 on its storage stability at room temperature was studied by varying the drying temperature and storage duration of a deposition–precipitation prepared Au/TiO2 sample. Carefully-designed room temperature storage in a desiccator, in the dark to exclude any interference of light irradiation, was referenced to the freezing storage (255 K) in a refrigerator. The samples were characterized by well-calibrated H2-TPR, TEM and TG measurements. Reduction of Au3+ ions and agglomeration of metallic Au particles were shown to be the main reasons for the deterioration of Au/TiO2 during desiccator-storage. Correlating the percentage of Au3+ ions, determined by H2-TPR, with the storage stability of Au/TiO2 for CO oxidation at 273 K revealed that Au/TiO2 samples with higher Au3+ percentages (>90%) were much more stable during the desiccator-storage than those with higher percentages of metallic Au. Residual water in fresh Au/TiO2 before storage showed a promotional effect on gold reduction and agglomeration during storage. By maximizing the percentage of Au3+ ions and minimizing the residual water in the fresh sample, the deterioration of the Au/TiO2 catalyst was successfully avoided during desiccator-storage of up to 150 days in dark. A possible mechanism of Au/TiO2 deterioration during the desiccator-storage was also discussed.

Narrowly sized colloidal Au particles of varying average sizes (3–30 nm) were immobilized on an inert support (SiO2) to study the Au size effect on the aerobic oxidation of ethanol in aqueous solution. Au particles with an average diameter of 5 nm showed an areal activity that was about three times that of the smaller (3 nm), and 15 times that of larger (10–30 nm) Au particles. Investigation on the dependence of product yields on ethanol conversion over these differently sized Au particles clearly uncovered that the yield of acetic acid increased always with the ethanol conversion, while that of acetaldehyde passed a maximum at an ethanol conversion of 20–30%, therefore well demonstrating that acetaldehyde is the intermediate product in the oxidation of ethanol to acetic acid.

Synthesis of acrolein by catalytic gas-phase dehydration of biomass-derivate glycerol was studied over various solid catalysts with a wide range of acid–base properties. The catalyst acidity and basicity were measured, respectively, by n-butylamine and benzoic acid titration methods using Hammett indicators. The most effective acid strength for the selective dehydration of glycerol to form acrolein appeared between −8.2 ≤ H0 ≤ −3.0, with which acrolein was produced at a selectivity of 60–70 mol%. The catalysts having very strong acid sites (H0 ≤ −8.2) effected a lower acrolein selectivity (40–50 mol%) due to more severe coke deposition in the reaction. Solid acids holding medium strong and weak acid sites (−3.0 ≤ H0 ≤ +6.8) were found to be not selective for the acrolein production, the acrolein selectivity being less than 30 mol%. The mass specific catalytic rate for the acrolein production showed a general trend to increase with the fractional acidity at −8.2 ≤ H0 ≤ −3.0. The catalytic data also suggest that Brønsted acid sites were advantageous over Lewis acid sites in catalyzing the selective synthesis of acrolein from glycerol dehydration. Solid base catalysts were shown not to be effective for acrolein production.

Selective production of chloroanilines from chloronitrobenzenes without any dechlorination was found feasible by catalytic hydrogenation over zirconia-supported gold catalyst, which uncovers a clean synthetic approach for useful chloroanilines.

This work presents a detailed comparison between multi-walled (MWNT) and single-walled carbon nanotubes (SWNT) in an effort to understand which can be the better candidate of a future supporting carbon material for electrocatalyst in direct methanol fuel cells (DMFC). Pt particles were deposited via electrodeposition on MWNT/Nafion and SWNT/Nafion electrodes to investigate effects of the carbon materials on the physical and electrochemical properties of Pt catalyst. The crystalloid structure, texture (surface area, pore size distribution, and macroscopic morphology), and surface functional groups for MWNT and SWNT were studied using XRD, BET, SEM and XPS techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the electrochemically accessible surface area and charge transfer resistances of the MWNT/Nafion and SWNT/Nafion electrodes. CO stripping voltammograms showed that the onset and peak potentials on Pt-SWNT/Nafion were significantly lower that those on the Pt-MWNT/Nafion catalyst, revealing a higher tolerance to CO poisoning of Pt in Pt-SWNT/Nafion. In methanol electrooxidation reaction, Pt-SWNT/Nafion catalyst was characterized by a significantly higher current density, lower onset potentials and lower charge transfer resistances using CV and EIS analysis. Therefore, SWNT presents many advantages over MWNT and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts. The enhanced electrocatalytic properties were discussed based on the higher utilization and activation of Pt metal on SWNT/Nafion electrode. The remarkable benefits from SWNT were further explained by its higher electrochemically accessible area and easier charge transfer at the electrode/electrolyte interface due to SWNT's sound graphitic crystallinity, richness in oxygen-containing surface functional groups and highly mesoporous 3D structure.

This work attempts to enhance platinum utilization in a Pt-based electrocatalyst by the tuned covering of gold nanoparticles with small Pt entities. Reductive deposition of Pt on Au nanoparticles of two size ranges (Au-I: 10 ± 1.2 nm, Au-II: 3 ± 0.6 nm) up to different atomic Pt : Au ratios (m) was used to prepare two series of samples named Ptm⁁Au-I and Ptm⁁Au-II particles, respectively. The obtained Ptm⁁Au particles were characterized with TEM, XPS, UV-Vis and XRD techniques, and then loaded on conventional Vulcan XC-72 carbon to make Ptm⁁Au/C electrocatalysts. Cyclic voltammetry (CV) measurements showed that the electrochemical active surface area (EAS) and Pt utilization (UPt) in Ptm⁁Au/C were enhanced remarkably at m ≤ 0.2 for Ptm⁁Au-I/C or m ≤ 0.5 for Ptm⁁Au-II/C, in comparison to conventional Pt/C electrocatalyst. In particular, UPt was enhanced to nearly 100% in Ptm⁁Au-I/C catalysts at m ≤ 0.05 and in Ptm⁁Au-II/C at m ≤ 0.1. In the CV measurement of methanol electro-oxidation, the specific mass activity of Pt in Ptm⁁Au/C catalysts was found in proportional to UPt, confirming that the enhancement of Pt utilization is essential for the development of highly active Pt-based electrocatalysts. The highly dispersed Pt entities on Au nanoparticles proved to be stable during the electro-oxidation of methanol. Our study also showed that the use of smaller Au nanoparticles is advantageous for the generation of more active Pt catalyst at higher atomic Pt : Au ratios.

AbstractModification of conventional Pt/C electrocatalyst with silicomolybdic acid (SiMoA) was performed using electrochemical cyclic voltammetry method. The modified and unmodified catalysts were tested under identical conditions for electrooxidation of CO, methanol, and ethanol. In the CO-stripping experiments, the modified catalyst was characterized by significant shifts (80 and 60 mV) to lower onset potential and peak potential for CO electrooxidation, suggesting better CO-tolerant property of the modified catalyst. In the electrooxidation of methanol and ethanol, the modified catalyst was featured by significantly increased current densities due to reduced residence time of the reaction intermediates, showing significantly higher electrocatalytic activity for the electrooxidation of alcohols.

ZnO/TiO2/SnO2 mixture was prepared by mixing its component solid oxides ZnO, TiO2 and SnO2 in the molar ratio of 4ː1ː1, followed by calcining the solid mixture at 200–1300 °C. The products and solid-state reaction process during the calcinations were characterized with powder X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and Brunauer–Emmett–Teller measurement of specific surface area. Neither solid-state reaction nor change of crystal phase composition took place among the ZnO, TiO2 and SnO2 powders on the calcinations up to 600 °C. However, formation of the inverse spinel Zn2TiO4 and Zn2SnO4 was detected at 700–900 and 1100–1200 °C, respectively. Further increase of the calcination temperature enabled the mixture to form a single-phase solid solution Zn2Ti0.5Sn0.5O4 with an inverse spinel structure in the space group of Oh7-Fd3m. The ZnO/TiO2/SnO2 mixture was photocatalytically active for the degradation of methyl orange in water; its photocatalytic mass activity was 16.4 times that of SnO2, 2.0 times that of TiO2, and 0.92 times that of ZnO after calcination at 500 °C for 2 h. But, the mass activity of the mixture decreased with increasing the calcination temperature at above 700 °C because of the formation of the photoinactive Zn2TiO4, Zn2SnO4 and Zn2Ti0.5Sn0.5O4. The sample became completely inert for the photocatalysis after prolonged calcination at 1300 °C (42 h), since all of the active component oxides were reacted to form the solid solution Zn2Ti0.5Sn0.5O4 with no photocatalytic activity.Comparison of the photocatalytic activities of ZnO, TiO2, SnO2 and ZnO/TiO2/SnO2 after the calcination at 500 °C for 2 h. Loading of the photocatalysts: 2.5 g/L; concentration of methyl orange: 20 mg/L .

Zn2TixSn1−xO4 (0⩽x⩽1) solid solutions with an inverse spinel structure (Fd3m) were synthesized by solid-state reactions at 1300°C of the stoichiometric mixtures of ZnO, TiO2 and SnO2. X-ray diffraction, thermogravimetric and differential thermal analyses, scanning electron microscopy, transmission electron microscopy and BET specific surface area measurements were used to gain insights into the solid-state reactions and phase transformation of the system. Optical absorption property of the Zn2TixSn1−xO4 (0⩽x⩽1) solid solutions was studied with the ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis DRS). The Zn2TixSn1−xO4 (0⩽x⩽1) solid solutions showed optical absorptions of the semiconductors in the near ultraviolet region; the adsorption band shifts with the composition of the solid solution.

Nanosized coupled ZnO/SnO2 photocatalysts with different Sn contents were prepared using the coprecipitation method, and characterized by X-ray diffraction, specific surface area and UV-Vis diffuse reflectance spectroscopy. The phases, mean grain sizes and band gap energy of the coupled ZnO/SnO2 photocatalysts varied with the Sn contents and the calcination temperatures. The photocatalytic activities of the coupled ZnO/SnO2 photocatalysts, evaluated using the photodegradation of methyl orange as a probe reaction, were also found to be related to the calcination temperatures and the Sn contents. The photocatalytic activities of the coupled ZnO/SnO2 photocatalysts decreased with the increasing calcination temperatures. The maximum photocatalytic activity of the coupled ZnO/SnO2 photocatalyst, which is about 1.3 times the photocatalytic activity of ZnO and 21.3 times that of SnO2, was observed with a Sn content of 33.3 mol% under calcination at 500 °C for 10 h. The enhancement of the photocatalytic activity might arise from the hetero-junctions ZnO/SnO2 in the coupled oxides. The photo-stability of the ZnO/SnO2 photocatalyst was also studied.

•Ni–Co/Si3N4 catalysts are prepared via reactions between metal halides and Si3N4.•Bimetallic Ni–Co nanoparticles encapsulated by a SiNx layer structures are formed.•The 4.0Ni–3.6Co/Si3N4 catalyst enables stable and coke-free CO2 reforming of CH4.A series of Ni–Co/Si3N4 catalysts with different Ni/Co ratios were prepared via reactions between commercial silicon nitride (Si3N4) and metal halides (i.e., NiCl2 and CoF3) at high temperature (930 °C). By using X-ray diffraction and electron microscopy, it was shown that this method of catalyst preparation leads to formation of bimetallic Ni–Co nanoparticles encapsulated by a SiNx layer (Ni–Co@SiNx) on supporting Si3N4 material. The 4.0Ni–3.6Co/Si3N4 catalyst was highlighted by showing highly stable catalysis for stoichiometric CO2 reforming of methane under widely varied reaction conditions, and was found completely free of coke formation after CRM reaction for 100 h.

Pt–(RuOxHy)m electrocatalysts (m being the atomic Ru/Pt ratio) supported on multi-walled carbon nanotubes, in which amorphous hydrous ruthenium oxide (RuOxHy) is the exclusive Ru-containing species, were prepared and comprehensively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, thermogravimetric analysis and transmission electron microscopy techniques. Cyclic voltammetry (CV) and chronoamperometry studies of CO stripping and methanol electro-oxidation indicated that the CO tolerance and catalytic activity of Pt improved remarkably by the co-presence of RuOxHy. Repeated CV pretreatments in 0.5 M H2SO4 up to potentials higher than 0.46 V (vs. SCE) induced significant dissolution of RuOxHy, which changed the RuOxHy content, Pt–RuOxHy proximity and surface structure of Pt, and consequently altered the electrocatalytic activity of Pt in the final electrode. However, RuOxHy dissolution was not identified when the pretreatment potentials was set no higher than 0.46 V. Discussion on the promotional function of RuOxHy was made based on the peculiarity of RuOxHy as a mixed electron/proton conductor.Amorphous hydrous ruthenium oxide (RuOxHy) alone is identified as an efficient promoter of Pt for the electro-oxidation of CO and methanol. However, its dissolution at potentials higher than 0.46 V (vs. SCE) would induce changes in Pt–RuOxHy proximity and even surface structure of Pt and thus alter the activity of Pt in electrocatalysis.Download high-res image (97KB)Download full-size image

Gas-phase dehydration of glycerol to produce acrolein was investigated at 315 °C over Nb2O5 catalysts calcined in the temperature range of 350–700 °C. The catalysts were characterized by nitrogen physisorption, TG-DTA, XRD, and n-butylamine titration using Hammett indicators to gain insight into the effect of calcination temperature on catalyst texture, crystal structure, and acidity. Calcination at 350 and 400 °C produced amorphous Nb2O5 catalysts that exhibit significantly higher fractions of strong acid sites at −8.2⩽H0⩽−3.0 (H0 being the Hammett acidity function) than the crystallized Nb2O5 samples obtained by calcination at or above 500 °C. Glycerol conversion and acrolein selectivity of the Nb2O5 catalysts were dependent of the fraction of strong acid sites (−8.2⩽H0⩽−3.0). The amorphous catalyst prepared by the calcination at 400 °C, having the highest fraction of acid sites at −8.2⩽H0⩽−3.0, showed the highest mass specific activity and acrolein selectivity (51 mol%). The other samples, having a higher fraction of either stronger (H0⩽−8.2) or weaker acid sites (−3.0⩽H0⩽6.8), were less effective for glycerol dehydration and formation of the desired acrolein.

Hydrogenation of 1,3-butadiene over Au/ZrO2 catalysts with different gold loadings and calcination temperatures was reported. The catalysts were characterized in depth to understand their structure–property relationship. Gold oxidation states and surface hydroxyl groups, which were found to be sensitive to the gold loading, calcination temperature, and treatment with water, were shown to play vital roles in the hydrogenation activity of Au/ZrO2. Continued activity decrease was seen when the density of surface hydroxyl groups was lowered by elevating the pre-calcination temperature of ZrO2. Fully dehydroxylated Au/ZrO2 was essentially inactive, but became very active after partial regeneration of the hydroxyl groups by water treatment. Moreover, the activity of Au/ZrO2 increased with increasing Au3+/Au0 ratio. Isolated Au3+ ions at the support surface showed up to two orders of magnitude higher activity than Au0 atoms on Au particles. Several models are proposed to address the structural features of active sites for H2 activation in Au/ZrO2 catalysts.Graphical abstractActive Au/ZrO2 catalysts for 1,3-butadiene hydrogenation are composed of OH-carrying ZrO2, metallic Au0 nanoparticles and/or Au3+ ions. In the absence of isolated surface Au3+ ions, the activation of H2 in the reaction occurs on ensembles involving metallic Au0 atoms and OH-groups on ZrO2 or containing Au0 atoms and Au3+ ions.Download high-res image (145KB)Download full-size imageResearch highlights► Gold oxidation states and OH-groups in Au/ZrO2 catalysts for butadiene hydrogenation. ► Reaction sensitivities to the feed moisture and density of OH-groups on ZrO2. ► Effects of gold loading and Au3+/Au0 ratio on the activity of Au/ZrO2. ► Structural features of the active sites for H2 activation.

This work reveals an ordered mesoporous carbon material co-doped with nitrogen and iron (Fe–N–C) for ORR catalysis in an alkaline electrolyte, whose ORR performance surpasses most of the previous metal-free heteroatom-containing carbon materials and is comparable to that of conventional Pt/C in terms of half-wave potential, limiting current density and kinetic current density. A procedure for preparing this Fe–N–C catalyst is described.

Ptm⁁Ag nanostructures (m being the atomic Pt/Ag ratio, m = 0.1–0.6) were prepared by reflux citrate reduction of PtCl62− ions in aqueous solution containing colloidal Ag (6.3 ± 3.9 nm). A distinct alloying of Pt with Ag was detected due to an involvement of the galvanic replacement reaction between PtCl62− and metallic Ag colloids. The nanostructure transformed from a structure with an Ag-core and an alloyed PtAg-shell to a hollow PtAg alloy structure with the increase in m. Compared to a commercial E-TEK Pt/C catalyst, the catalytic performance of Pt in the Ptm⁁Ag/C samples for the cathode oxygen reduction reaction (ORR) strongly correlated with the electronic structure of Pt, as a consequence of varied Pt dispersion and Pt–Ag interaction. With either H2SO4 or KOH as an electrolyte, Pt in the Ptm⁁Ag nanostructures with a relatively high m (≥0.4) showed significantly enhanced intrinsic activity whereas Pt in those catalysts with low m (≤0.2) appeared less active than the Pt/C catalyst. These data are used to discuss the role of electronic structure and geometric effects of Pt toward ORR.

The effect of Au3+ percentage in Au/TiO2 on its storage stability at room temperature was studied by varying the drying temperature and storage duration of a deposition–precipitation prepared Au/TiO2 sample. Carefully-designed room temperature storage in a desiccator, in the dark to exclude any interference of light irradiation, was referenced to the freezing storage (255 K) in a refrigerator. The samples were characterized by well-calibrated H2-TPR, TEM and TG measurements. Reduction of Au3+ ions and agglomeration of metallic Au particles were shown to be the main reasons for the deterioration of Au/TiO2 during desiccator-storage. Correlating the percentage of Au3+ ions, determined by H2-TPR, with the storage stability of Au/TiO2 for CO oxidation at 273 K revealed that Au/TiO2 samples with higher Au3+ percentages (>90%) were much more stable during the desiccator-storage than those with higher percentages of metallic Au. Residual water in fresh Au/TiO2 before storage showed a promotional effect on gold reduction and agglomeration during storage. By maximizing the percentage of Au3+ ions and minimizing the residual water in the fresh sample, the deterioration of the Au/TiO2 catalyst was successfully avoided during desiccator-storage of up to 150 days in dark. A possible mechanism of Au/TiO2 deterioration during the desiccator-storage was also discussed.

Adding a small amount of fully dispersed Pt entities onto the Au surface in Au/SiO2 catalyst is found to be an efficient approach to improve the catalytic activity of Au (up to 70-fold) for the hydrogenation of α,β-unsaturated carbonyl compounds, without alternating its selectivity towards CO or CC bond hydrogenation.

Effects of the crystalline structure and surface property of the Al2O3 support on the NOx storage and reduction (NSR) performance of Pt–BaO/Al2O3 catalysts were studied using catalysts prepared from a series of Al2O3 samples obtained by varying the calcination temperature of an Al(OH)3 hydrogel in the range of 450–1000 °C (referred to as the pre-calcination temperature, PCT). The texture, crystalline structure and surface acidity of the Al2O3(PCT) supports were measured employing nitrogen adsorption–desorption, XRD, NH3-TPD and IR spectroscopy of adsorbed pyridine, respectively. The Al2O3(PCT) samples showed a gradual ordering of the γ-Al2O3 phase when increasing the PCT from 450 to 800 °C, and a phase transition from the γ- to θ-Al2O3 phase upon further increasing the PCT to 1000 °C. The surface density of Lewis acid sites of the Al2O3(PCT) samples exhibited a maximum at PCT in the range of 800–900 °C. The NSR performance of Pt–BaO/Al2O3 catalysts derived from the Al2O3(PCT) samples was studied under cyclic lean/rich conditions. The numbers of NOx stored and reduced on the Pt–BaO/Al2O3(PCT) catalysts showed similar volcano-type dependencies on PCT, peaking at PCT = 800 °C. The origins of the support PCT effect were discussed in the light of Pt particle size, the nature of BaO sites and Pt–BaO synergy. It was found that the increase in the crystallinity of the γ-Al2O3 phase and the surface acid site density of the supporting Al2O3 samples would result in improved proximity and synergy between Pt and BaO sites, leading to much more efficient NSR Pt–BaO/Al2O3 catalysts.

Au nanoparticles (NPs) carrying different stabilizers (PVP, PVA and CTAB) were prepared by stabilizer-exchange of freshly prepared citrate (Citr)-stabilized Au NPs (Au-Citr) with the substitute stabilizers, respectively. The stabilizer substitutions were monitored with UV-vis spectroscopy and the resultant Au particles were further characterized with FTIR and TEM after they were immobilized on a SiO2 support. Measurements of the catalytic performance of the immobilized Au NPs for the hydrogenation reactions of p-chloronitrobenzene and cinnamaldehyde enabled us to address the impact of stabilizer substitution on the hydrogenation catalysis of Au NPs.

The direct redox reaction (galvanic displacement) between Pd2+ and substrate Si was used to deposit Pd on Si, and the Pd–Si catalysts enabled a chemoselective hydrogenation of para-chloronitrobenzene with the selectivity for para-chloroaniline higher than 99.9% at complete conversion of para-chloronitrobenzene.