Ethylene adsorption on Pt/Au/SiO2 catalysts

Ethylene adsorption on a Pt/Au/SiO₂ catalyst (2 wt% Pt; Au/Pt atomic ratio of 10) was studied using adsorption microcalorimetry and FTIR spectroscopy. Ethylene adsorption at 300 K on Pt/Au/SiO₂ produced π‐bonded, di‐σ‐bonded, and ethylidyne species with an initial heat of 140 kJ/mol, compared to a heat of 157 kJ/mol for Pt/SiO₂ on which only ethylidyne species formed. At 203 and 263 K, ethylene adsorbed on Pt as well as on Au surface atoms for the Pt/Au/SiO₂ catalyst. Quantum chemical, DFT calculations indicate that Au exerts a significantly smaller electronic effect on Pt than does addition of Sn to Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adsorbed NHx species on Pt(111) and Pt(100) surfaces studied by the semi‐empirical method of interacting bonds

The properties of the adsorbed NH_x species (x=0,1,2,3) on platinum(111) and (100)‐(1×1) single‐crystal planes are studied by the semi‐empirical method of interacting bonds. Both surfaces reveal similar features. The adsorbed species NH and NH₂ are stable on the surface, and stable NH_3(ads) species cannot form. The NH_2(ads) species is favourable in adsorbed hydrogen excess, but lack of the latter results in NH_ads becoming dominant. Both NH and NH₂ species are expected to diffuse easily over the surface due to the small difference between their bond strengths to various adsorption sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and characterisation of platinum- and gold-coated copper, iron, cobalt and nickel deposits on glassy carbon substrates

Pt- and Au-coated Cu, Fe, Co and Ni deposits have been formed on glassy carbon (GC) substrates by electrodeposition of controlled amounts of the core metal onto the substrate and its subsequent partial replacement by Pt or Au upon immersion into a chloroplatinic or chlorolauric solution. This process resulted in a thin Pt or Au shell over the bimetallic particle as electrochemical evidence suggests, while indicative sputter-etch Auger Electron Spectroscopy points to the coexistence of the two metals in the particle core. SEM/EDS characterisation of the deposits revealed the existence of Pt(M) and Au(M) extensively agglomerated nanoparticles (M: Cu, Fe, Co, Ni). The surface electrochemistry of the deposits (hydrogen adsorption/desorption on Pt and oxide formation/stripping on Au) proved the complete coverage of the bimetallic particles by Pt or Au and allowed an estimate of their electroactive surface area. The study of hydrogen evolution at these deposits points to a modification of the electronic properties of the Pt and Au surface layers by the core metal (due to strain effects and/or ligand-electronic interactions) and further confirmed that these layers form a very thin outer shell.     

Ultra‐selective epoxidation of styrene on pure Cu{111} and the effects of Cs promotion

Styrene undergoes efficient epoxidation to styrene epoxide on the Cu{111} surface. At the optimum condition (Θ_o = 0.03 ML) ∼20% of the styrene is converted to the epoxide with almost 100% selectivity. Comparison with Ag{111} shows that the epoxidation activity and selectivity of Cu greatly exceed those of Ag. Incipient oxidation of the Cu{111} surface does not suppress the adsorption of styrene, but the oxidised metal is catalytically inert. Submonolayer amounts of Cs enhance styrene uptake and increase conversion to the epoxide without adversely affecting epoxidation selectivity. This effect is due to inhibition of Cu oxidation by Cs. Our findings are discussed in the light of current understanding of Ag‐catalysed alkene epoxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

In/Au(111)和Ir/Au(111)面上巴豆醛选择加氢的机理研究及比较

基于巴豆醛在M/Au（111）合金表面（M=In,Ir）垂直吸附的最稳定吸附结构,采用密度泛函理论对其不完全加氢的反应机理进行探究。从不同加氢机理下各基元反应的活化能、反应热计算以及构型变化分析中可知,巴豆醛在M/Au（111）面上均优先对距离合金表面较近的C＝O进行加氢,且以C为活性中心优先进行加氢为最优机理,其中第1步加氢反应的活化能较高,是该机理的控速步骤。反应物巴豆醛的O原子与合金的掺杂原子M形成较强的化学吸附,提高了M/Au（111）面对C＝O加氢的选择性。巴豆醛按最优机理加氢的基元反应中在In/Au（111）面上最高反应能垒为0.969 eV,比在Ir/Au（111）面的最高反应能垒1.332 eV低,因此认为In/Au合金对其不完全加氢有更好的催化活性。     

Adsorption mode of ethyl pyruvate on platinum: an in situ XANES study

The adsorption of ethyl pyruvate on Pt(111) has been studied by in situ XANES measurements in the presence and absence of hydrogen. Depending on the hydrogen and ethyl pyruvate pressure, the C and O K‐edge spectra exhibit distinctly different angular dependence. Without hydrogen ethyl pyruvate is oriented preferentially perpendicular to the surface, indicating bonding via the O lone pairs. In the presence of hydrogen the mean orientation is more tilted towards the surface. Likely, ethyl pyruvate also interacts with Pt via its π system under these conditions. The observed angle‐dependent shift of the energy of the π* and σ* resonances indicates the coexistence of differently adsorbed ethyl pyruvate species. The experimental findings demonstrate the importance of the in situ approach for unraveling the adsorption mode of ethyl pyruvate in the enantioselective hydrogenation over cinchona‐alkaloid‐modified Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization by CO/FTIR spectroscopy of Pd/silica catalysts and its correlation with syn‐gas conversion

Well‐dispersed Pd catalysts, supported on two morphologically different silicas (meso‐ and microporous Davison G59 and G03 grades, respectively) and used for syn‐gas activation at T=493–523 K and P=1–4 MPa (CO/H₂= 1/2.5), have been studied by CO chemisorption using FTIR spectroscopy. The long‐term exposure to 760 Torr CO(g) at 298 K produces deep changes on the surface structure of Pd particles on both supports. The Pd particles become rougher and/or show more open crystal planes. This phenomenon of surface restructuring seems to depend both on the exposed metal fraction (FE) of palladium and the morphology of the support. The rate of surface restructuring but not its extent, is a function of the superimposed CO(g) pressure. On the microporous G03 silica CO chemisorbs in multicoordinated or “hollow” sites (H band), but these signals are not shown by preparations of the supported metal of comparable dispersity on mesoporous G59 grade. Terminal (L band) and di‐coordinated CO_s (B band) appear in both types of catalysts. The high‐loading preparations on the microporous support showed a higher proportion of Pd(111) planes than those of low Pd loading, which seems to contribute to the high TOF_CH4 and high selectivity to methane in syn‐gas activation of these catalysts, while the remaining ones show excellent capability for methanol production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gold nanoparticles on ceria: importance of O vacancies in the activation of gold

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO₂ and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO₂ was investigated on Au/CeO₂(111) and AuO _x /CeO₂ surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO₂(111) was 4–7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO₂ on the Au/CeO₂(111) surfaces. The full decomposition of SO₂ was observed only after introducing O vacancies in the ceria support. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−x (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO₂(111) single crystal corroborate that cationic Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.     

The thermal activation of propyl groups on Pt(111)

The thermal chemistry of 1‐ and 2‐propyl moieties on Pt(111) was studied by using temperature‐programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). The propyl intermediates were prepared via thermal activation of the C–I bond of 1‐ and 2‐iodopropane adsorbed precursors, respectively. It was determined that the subsequent thermal activation of those propyl groups results in a competition between reductive elimination to propane, β‐hydride elimination to propene, and complete decomposition to propylidyne (and eventually to hydrogen and surface carbon). It was found that while the 2‐propyl intermediate favors propene production, 1‐propyl also yields significant amounts of propane. The formation of propene via β‐hydride elimination was identified by isotopic labeling TPD experiments, and directly about 200 K by RAIRS. Coadsorption experiments with hydrogen and deuterium were used to characterize hydrogenation and H–D reactions. All possible propene and propane isotopomers are formed from both 1‐ or 2‐iodopropane on the D/Pt(111) surface, indicating that exchange is likely to occur via a cyclic propyl–propene–propyl mechanism involving the formation of both 1‐ and 2‐propyl intermediates. Relative rates for 1‐ versus 2‐propyl conversion were estimated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Raman and infrared spectroscopies as in situ probes of catalytic adsorbate chemistry at electrochemical and related metal–gas interfaces: some perspectives and prospects

Some recent progress in the utilization of infrared and especially Raman spectroscopies for the in situ vibrational characterization of adsorbates at electrochemical interfaces having relevance to catalytic chemistry is briefly outlined, and illustrated by means of examples culled chiefly from our laboratory. The primary factors responsible for the differences as well as similarities in the experimental strategies pursued for metal–solution interfaces as compared with metal surfaces in gas‐phase and ultrahigh vacuum (UHV) environments are discussed, and the distinct virtues of surface‐enhanced Raman scattering (SERS) and infrared reflection‐absorption spectroscopy (IRAS) for scrutinizing the first interfacial type are assessed. The detailed influences of the electrochemical double layer on chemisorbate vibrational properties at ordered metal–solution interfaces as gleaned by in situ IRAS data in comparison with spectra for analogous “model electrochemical” interfaces in UHV are described, and briefly illustrated for carbon monoxide on Pt(111) and Ir(111). The significance of the surface potential φ in controlling chemisorbate properties on metal surfaces in gaseous and UHV as well as electrochemical environments is pointed out. Evidence for the occurrence of “redox pinning” of φ by gaseous species in ambient‐pressure systems is outlined, along with possible catalytic implications. The burgeoning prospects for utilizing SERS as a versatile as well as uniquely sensitive vibrational probe of catalytically significant, especially transition‐metal, interfaces in both electrochemical and gas‐phase environments are delineated. Emphasis is placed on the typically richer vibrational spectra attainable for SERS compared to IRAS, arising from differing surface selection rules along with the greater sensitivity and wider wavenumber ranges accessible to the former method, and exemplified by benzene adsorption on rhodium and palladium electrodes. The anticipated power of SERS for assessing the reactivity as well as identity of adsorbed intermediates in ambient catalytic systems by means of transient in situ spectral measurements is noted, and illustrated briefly for ambient‐pressure methanol oxidation on rhodium. This revised version was published online in August 2006 with corrections to the Cover Date.     

The influence of chlorine on the dispersion of Cu particles on Cu/ZnO(0001) model catalysts

One of the ways in which chlorine is thought to poison metal catalysts on oxide supports is by altering their dispersion. The effect of chlorine on Cu/ZnO(0001) model catalysts was studied by vapor‐depositing Cu onto Zn‐terminated ZnO(0001), both with and without preadsorbed Cl₂, using XPS, ion scattering spectroscopy (ISS), temperature‐programmed desorption (TPD), work function, and band bending measurements. A disordered, but nearly close‐packed overlayer of Cl adatoms forms at saturation with ∼0.30 Cl adatoms per Zn site. Without Cl, vapor‐deposited Cu grows in two‐dimensional islands that cover ∼33% of the ZnO, after which these islands thicken (i.e., as 3D Cu particles) while the clean ZnO between these Cu islands gets covered with Cu only very slowly. Preadsorbed Cl decreases the fraction of the surface that is covered by Cu islands by a factor of three, so Cl(a) either decreases the number of 2D Cu islands or their critical area before thickening. Both are consistent with weaker binding of Cu to the Cl‐covered surface than to the clean ZnO. The TPD features for formate decomposition after HCOOH adsorption onto Cu/ZnO(0001) were suppressed with preadsorbed Cl, but the CO₂ : CO selectivity increased. When Cu was deposited onto Cl‐presaturated ZnO, neither the Zn‐ nor Cu‐formate peaks were observed, showing that Cl covers both the Zn sites and the growing Cu islands, as suggested by ISS also. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of AFM and HRTEM to determine the metal particle morphology and loading of an Au/TiO2 catalyst

Atomic force microscopy (AFM) has been used to study the morphology of an ultrafine gold‐on‐titania catalyst. By using TappingMode^TM AFM (TMAFM) and SuperSharp silicon probes to minimize tip radius artifacts, we determined values for the average Au particle diameter and the gold loading in good agreement with high‐resolution transmission electron microscopy (HRTEM) results. These results demonstrate the ability of AFM to characterize real supported metal catalysts with small metal particles (<5 nm) and low metal loadings, achieving resolution comparable to HRTEM, but in the ambient environment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Theoretical study of CO adsorption on Au catalysts under environmental catalytic conditions

Density Functional Theory calculations with both standard GGA and hybrid functionals are performed on Au adatoms, steps, and low index surfaces with coordination numbers (CNs) varying from 3 to 9. The results are used to study adsorption thermodynamics and reactivity of CO on Au nanoparticles. We find that the hybrid functional improves calculated site preferences and predicts CO top site adsorption, regardless of the Au CN, in good agreement with experiments. The calculated adsorption energies vary monotonically with respect to Au CNs, and the results from the hybrid functional are around 20% smaller than the corresponding values from the GGA–PBE functional. A comparison with experimental adsorption energies suggests that these functionals may bound the true CO–Au interaction strength, and seven-coordinated Au atoms may be the active low-coordinated sites on many Au single crystal surfaces. However, thermodynamic analysis on Wulff-like Au particles at ambient temperatures shows that, even though the number of 6-coordinated corner Au atoms is much less than the number of 7-coordinated edge Au atoms and of higher-coordinated Au atoms, they are the dominant sites for CO adsorption on Au nanoparticles with sizes up to 10 nm. In addition, we find that CO adsorption is not influenced by the shape of Au nanoparticles, but the CO oxidation reaction may be.     

Gold nanoparticles: effect of treatment on structure and catalytic activity of Au/Fe2O3 catalyst prepared by co‐precipitation

1 wt% Au/Fe₂O₃ catalyst was prepared by a co‐precipitation method. The structure of the sample in the as prepared, oxidized and reduced states was investigated by means of X‐ray photoelectron spectroscopy (XPS), transition electron microscopy (TEM), electron diffraction (ED) and X‐ray diffraction (XRD). The structure of the samples after various treatments and their activity in the CO oxidation were compared. The results show the stability of the gold particle size during the treatments. However, after oxidation, a slight shift in the Au 4f binding energy towards lower values points to the formation of an electron‐rich state of the metallic gold particles compared to that revealed in the as prepared sample. Simultaneously, a goethite phase in the Fe₂O₃ support is present, which is not observed in the “as prepared” and reduced samples. In the reduced sample the presence of a crystalline maghemite‐c phase indicates a change in the support morphology. In the CO oxidation the oxidized sample shows the highest activity and it might be the result of the cooperative effect of goethite, FeO and the electron‐rich metallic gold nanoparticles. We suggest that a structural transformation occurs along the gold/support perimeter during the treatments and we propose a possible mechanism for the effect of the oxidation treatment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gold supported on well-ordered ceria films: nucleation, growth and morphology in CO oxidation reaction

Structure of gold nanoparticles formed by physical vapor deposition onto thin ceria films was studied by scanning tunneling microscopy (STM). Gold preferentially nucleates on point defects present on the terraces of the well-ordered, fully oxidized films to a low density. The nucleation expands to the terrace step edges, providing a large variety of low-coordinated sites. Only at high coverage, the Au particles grow homogeneously on the oxygen-terminated CeO₂(111) terraces. The morphology of Au particles was further examined by STM in situ and ex situ at elevated (up to 20 mbar) pressures of O₂, CO, and CO + O₂ at 300 K. The particles are found to be stable in O₂ ambient up to 10 mbar, meanwhile gold sintering emerges at CO pressures above ∼1 mbar. Sintering of the Au particles, which mainly proceeds along the step edges of the CeO₂(111) support, is observed in CO + O₂ (1:1) mixture at much lower pressure (∼10⁻³ mbar), thus indicating that the structural stability of the Au/ceria catalysts is intimately connected with its reactivity in the CO oxidation reaction.     

Catalysis in the nm‐regime: manufacturing of supported model catalysts and theoretical studies of the reaction kinetics

We briefly review the methods employed to fabricate model supported nm catalysts, including wetness impregnation, vacuum vapor deposition, electron‐beam lithography, spin‐coating, and vesicle‐mediated deposition. Recent simulations of the kinetics of heterogeneous reactions occurring on supported catalyst particles are discussed as well. The attention is focused on such effects as reactant supply via the support, interplay of the reaction kinetics on different facets and edges, and adsorbate‐induced reshaping of catalyst particles. This revised version was published online in June 2006 with corrections to the Cover Date.     

Modelling alkali promotion in heterogeneous catalysis: in situ electrochemical control of catalytic reactions

Electron spectroscopic data and reactor measurements show that electrochemical promotion (EP) of thin film catalysts deposited on solid electrolyte supports is the result of spillover phenomena at the three‐phase boundary between the electrolyte, the catalyst and the gas phase. Ions from the electrolyte are discharged at the electrode/electrolyte interface and migrate to cover the catalyst surface whose properties are thereby strongly altered. The EP effect and the phenomena that underlie it are illustrated here by reference to the Na‐promoted catalytic reduction of NO by CO over copper. Electro‐pumping of Na from a β″‐alumina solid electrolyte to the catalyst surface results in large improvements in both activity and selectivity of the latter. Under reaction conditions, the alkali promoter is present as submonolayer amounts of NaNO₃ on an oxidised Cu surface. The results indicate that Cu⁰ sites are not of significance and that the catalytically active surface is dominated by Cu⁺ and Cu²⁺ sites. They also show that Cu⁺ is the critically important site for NO adsorption and that EP is due to Na‐induced enhancement of the adsorption and dissociation of NO at Cu⁺ sites. This revised version was published online in June 2006 with corrections to the Cover Date.     

Why Au and Cu Are More Selective Than Pt for Preferential Oxidation of CO at Low Temperature

Self-consistent, periodic density functional theory (DFT) calculations and micro-kinetic modeling are used to compare selectivity for the preferential oxidation of CO (PROX) with respect to H₂ based on studies of elementary reaction steps on the (111) facet of Au, Cu and Pt. The first step of H oxidation (OH formation) has a higher activation barrier than the second step (H₂O formation) on all three metal surfaces, indicating that OH formation competes with CO oxidation for the removal of trace amounts of CO from a typical reformate gas. The activation energy barrier for CO oxidation is found to be 0.18eV on Au(111), 0.82eV on Cu(111) and 0.96eV on Pt(111), whereas the barrier for OH formation is 0.90, 1.28 and 0.83eV respectively. A micro-kinetic model based on the DFT results shows that trends in the selectivity of these metals at different temperatures is due to (i) differences in the rate constants of the competitive CO and H oxidation reactions, and (ii) differences in the CO and H surface coverages. Our results explain why Au and Cu are more selective PROX catalysts compared to Pt at low temperatures. At higher temperatures, Pt and Cu lose some of their selectivity to CO oxidation, whereas the selectivity on Au decreases substantially primarily because of the significantly weaker CO adsorption.     

On the reaction pathway for the formation of benzene from acetylene catalyzed by palladium

The reaction between gas‐phase acetylene and alumina‐supported palladium saturated with ¹³C‐labelled vinylidene is studied using both one‐pulse, ¹³C magic‐angle spinning, nuclear magnetic resonance (NMR) spectroscopy and by mass spectroscopic analysis of the reaction products to probe the reaction pathway. The presence of vinylidene on alumina‐supported palladium is confirmed by comparing the infrared spectra of the species formed on the supported sample with those found on a Pd(111) single crystal. It is shown using NMR that a high pressure (∼350 Torr) of gas‐phase acetylene reacts with adsorbed vinylidene at the same rate at which benzene is formed catalytically on the same sample. The resulting benzene incorporates two ¹³C atoms. This indicates that benzene is formed by a slow reaction between gas‐phase (¹²C‐labelled) acetylene and adsorbed vinylidene (¹³CH₂=¹³C=) to form a C₄ intermediate which reacts rapidly with further acetylene to yield benzene. There are precedents for such reactions in homogeneous phase. The proposed reaction pathway differs from that elucidated previously from ultrahigh vacuum studies on clean Pd(111), where it was found that benzene synthesis also proceeds via a C₄ intermediate, in this case formed from two adsorbed acetylenes. This revised version was published online in July 2006 with corrections to the Cover Date.