Abstract: The decomposition of iridium acetylacetonate Ir(acac)(3) impregnated on amorphous silica-alumina (ASA) has been investigated by combined thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS) and by in situ X-ray diffraction (XRD). The resulting Ir/ASA hydrotreating catalysts have also been characterized by transmission electron microscopy (TEM). The effects of heating treatments under oxidative, reductive or inert gas flows are compared with each other and with similar experiments on ASA-supported acetylacetone (acacH). It is shown that Ir(acac)(3) undergoes exothermic combustion during calcination in air, leading to agglomerated IrO2 particles. Conversely, direct reduction involves hydrogenolysis of the acac followed by hydrogenation of the ligand residues to alkanes and water. These two processes are catalyzed by Ir clusters, the gradual growth of which is followed in situ by XRD. The resulting nanoparticles are highly and homogeneously dispersed.
Abstract: The oxidation of carbon monoxide in the presence of various concentrations of molecular hydrogen has been studied over a Au/TiO2 reference catalyst by combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. It is shown for the first time that H-2 enhances the CO oxidation rate on Au/TiO2 without leading to any major loss of selectivity. Increasing the H-2 pressure induces higher CO and H-2 oxidation rates. Under H-2-free conditions, the surface species detected are Au delta+-CO, Ti4+-CO, carbon dioxide and carbonates. Upon the addition of H-2, Au-0-CO, water and hydroxyl groups become the main surface species. The occurrence of a preferential CO oxidation mechanism involving HxOy species under the present experimental conditions is proposed. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Previous works have shown that palladium overlayers (Pd/Ni, Pd/Cu) are more active than pure Pd surfaces for alkene hydrogenation. These results have been ascribed to the specific nanostructure of the alloy surfaces. Here, we compare Pd(100), Pd(110) and Pd8Ni92(110) single-crystal surfaces toward 1,3-butadiene hydrogenation and hydrogen absorption, using a gas-phase static reactor We show that the. lower rate of butene formation on clean I'd surfaces can in fact be explained by the initial fast diffusion of hydrogen into the Pd crystal (conversely, hydrogen dissolution in Pd-Ni is negligible). However, the activity of Pd becomes higher at steady state, i.e. after several reaction cycles, due to the increase of the near-surface H concentration. Unlike the butane formation rate, the partial hydrogenation rate appears poorly affected by the I'd surface structure. These results suggest that, when hydrogen supply is rate-determining, hydrogen absorption effects can be more critical than structural effects for Pd-catalyzed hydrogenations. (C) 2009 Elsevier Inc. All rights reserved.
Abstract: The kinetics of CO oxidation, H-2 oxidation and preferential CO oxidation (PrOx) over Au/Al2O3 catalysts have been investigated. The catalysts with the smallest particles (similar to 2 nm) are the most active for all three reactions. As previously observed, the presence of H-2 greatly promotes CO oxidation, which becomes faster than CO-free H-2 oxidation at low temperature. From these results and on the basis of previous works, we propose a complete PrOx mechanism. The reaction involves Au-OOH, Au-OH and Au-H intermediates, also involved in H-2 oxidation, and benefits from the presence of low-coordination sites. (c) 2009 Elsevier Inc. All rights reserved.
Abstract: The low-temperature interaction of amorphous sulfur-rich sulfides MOS3 and MOSx (x approximate to 6) with hydrogen was studied under static and dynamic conditions using volumetric measurements, mass spectrometry, solid-state NMR spectroscopy, inelastic neutron scattering, temperature-programmed reduction (TPR), and X-ray absorption spectroscopy (XAS) at the S K and Mo K edges. It was observed that, at room temperature, hydrogen irreversibly interacts with the sulfur species of the amorphous sulfide. Interaction with hydrogen leads to opening of the S-S bonds within the structure. As a result, SH groups are formed, which are very labile and are easily transformed into molecular H2S under dynamic conditions (hydrogen flow). The total amount of absorbed hydrogen depends on the amount of S-S bonds in the amorphous sulfide, which can vary over a wide range.
Abstract: Shape-controlled Pd nanoparticles supported on powder alumina are more efficient for selective butadiene hydrogenation to butene when they exhibit high fractions of (111) facets.
Abstract: The kinetics and mechanism of the preferential oxidation of carbon monoxide in the presence of hydrogen (PrOx) over an unsupported gold powder (mean particle size similar to 20 nm and free of silver) have been investigated using How fixed bed catalytic testing and diffuse reflectance infrared Fourier transform spectroscopy coupled to mass spectrometry (operando DRIFTS or DRIFTS-MS). It is shown that the presence of H-2 has a favourable effect on the oxidation of CO, either by strongly accelerating the reaction or by preventing the catalyst deactivation, depending on the conditions used. Variation of the hydrogen partial pressure has allowed us to determine partial reaction orders for both CO oxidation and H-2 oxidation under PrOx conditions. An infrared band at similar to 2113 cm(-1), corresponding to on-top CO adsorption on metallic gold, has been observed below 150 degrees C. In addition, adsorbed hydroxyl groups gradually develop simultaneously to gas-phase water in the course of the reaction at increasing temperatures. The promotional effect of hydrogen is ascribed to highly oxidative HxOy intermediates formed from the interaction between H-2 and O-2 on the gold surface. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: A free-radical mechanism has been evidenced in the liquid phase stereoselective epoxidation of trans-stilbene using methylcyclohexane (MCH) as solvent, limited amount of tert-butylhydroperoxide (TBHP), and supported gold catalysts. Trans-stilbene oxide is the major reaction product observed, with selectivities up to 88% when using the Au/TiO2 reference catalyst from the World Gold Council. However the selectivity decreases significantly when using Au/C instead of oxide-supported gold catalysts or H2O2 instead of TBHP. HPLC and GC-MS analyses indicate that a fraction of MCH is oxidized during the epoxidation process. It seems that TBHP is the radical source while MCH is propagating the active radical. On the other hand, hydroxyl radicals are responsible for the degradation of the molecule. XPS studies show the presence of Au-0 (90%) and Au+ (10%) on the Au/C catalyst and Au delta- (90%) and Au+ (10%) on the Au/TiO2 catalyst. Both gold and, to a minor extent, titania seem to be involved in the reaction cycle. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: The oxidation of CO on Pd(111) and Pd(70)AU(30)(111) has been studied under pressures upto 100 Torr. Gold is found to decrease the surface activity by inhibiting oxygen dissociation. For a sufficient conversion time depending on the CO coverage and the surface identity, a dramatic boost of activity occurs. This is ascribed to a switch from CO-induced inhibition of O-2, adsorption to a regime determined by CO adsorption. The other kinetic features are explained by oxidation of palladium and adsorption-induced restructuring of the surfaces.
Abstract: We have used spin-polarized density functional theory calculations to investigate the structure of the Pd8Ni92(110) alloy surface, which has been extensively studied for its excellent catalytic properties. Our calculations have corroborated that this alloy has a quite complex (Nx1, N similar to 6) structure, so- called "sawtooth" (ST) reconstruction. Moreover, we have accurately computed the metal-metal bond distances and the interlayer spacings. We have also proved that the inclusion of the spin-polarization effects is crucial for a good description of the energetics. Our results agree nicely with those arising from scanning tunneling microscopy and surface x-ray diffraction experiments. In addition, our model brings insights into the driving forces leading to the ST reconstruction.
Abstract: The oxidation of CO was studied over a Au/TiO2 reference catalyst in a static reactor coupled to a thermodesorption device. The optimal vacuum activation/regeneration temperature is in the range 350-400 degrees C. The catalyst partly deactivates during the first runs, possibly through poisoning of the active sites by carbonate-like species, whose formation is clearly evidenced. Although a postreaction treatment under pure hydrogen is ineffective toward regeneration, the catalyst gradually recovers its initial activity for CO conversion in the presence of a low amount of hydrogen within the reactant mixture. Increasing the hydrogen pressure only promotes the oxidation of hydrogen, thereby decreasing selectivity in the preferential oxidation of CO. Consequently, we show that hydrogen-induced promotion of Au/TiO2 is a regeneration effect involving removal of carbonates rather than a boosting effect via new CO oxidation pathways. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: The kinetics of the H-2-O-2 reaction over Pd, Pd70Au30 and Au (1 1 1)-oriented surfaces has been studied in a static reactor near room temperature, using pressures in the torr range. The Pd-Au surface is highly enriched in gold (75 at.%) and unreconstructed, as shown by previous AES, LEIS and LEED investigations. While Au(1 1 1) is inert, Pd(1 1 1) and Pd-Au(1 1 1) are very active to catalyze the oxidation of hydrogen. The clean Pd surface is firstly more efficient to form water than the Pd Au surface, but deactivates upon exposure to the reactive mixture. A careful analysis of the reactant partial pressures allows deriving the amount of hydrogen absorbed in the crystal during each reaction cycle. This shows that Pd-Au absorbs much more hydrogen than Pd, and that bulk dissolution directly competes with surface oxidation of hydrogen. Gold-induced lowering of the oxygen sticking probability and absorption-induced structural modification of Pd(1 1 1), through formation of a surface hydride, are invoked to rationalize the results. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Pd70Au30 (1 1 1) and (1 1 0) surfaces have been characterized by low energy electron diffraction, Auger electron spectroscopy and low energy ion scattering spectroscopy. The reactivity of Pd-Au(1 1 1) towards the gas phase hydrogenation of 1,3-butadiene has been investigated in a static reactor, and compared to those of Pd-Au(1 1 0), Pd(1 1 1) and Au(1 1 1). Both Pd-Au surfaces are unreconstructed and exhibit strong surface segregation of gold. The gold concentrations in the top layer of Pd70Au30 (1 1 1) and (1 1 0) are found equal to 75 at.% and 85 at.%, respectively. The hydrogenation of butadiene has been performed at room temperature and 5 Torr in a large excess of hydrogen. While Au(1 1 1) is inactive.. Pd-Au(1 1 1) is initially less active than Pd-Au(1 1 0) and Pd(1 1 1) for the formation of butenes. However, a spectacular increase in the reaction rate is observed upon evacuation of the reacted gases and introduction of a fresh reactant mixture. Unlike Pd(1 1 1), Pd-Au(1 1 1) is weakly active for the conversion of butenes to butane. Its subsequently higher selectivity towards butenes could originate from an increased desorption of butenes from the Pd-Au surface. Moreover, a substantial absorption of hydrogen in the course of the reaction is evidenced, and its possible impact on the surface properties is discussed. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The model catalysts Au/Al2O3, Au/ZrO2, and Au/TiO2 were produced by laser vaporization of a metallic gold rod followed by deposition of the formed clusters onto the support powders. This technique allows to obtain a narrow size distribution of highly dispersed gold particles on the support and, most importantly, similar sizes whatever the support. This makes it possible to accurately study of the influence of the support identity on the catalytic reaction. A detailed investigation of the preferential oxidation of CO in the presence of H-2 was undertaken. Catalytic performances in the PROX reaction were compared with those in the oxidation of CO in the temperature range of 25-420 degrees C. A boost in the conversion of CO was observed in the presence of H-2 at low temperature the extent of this boost is dependent oil the support identity. Hence the reactivity order found for CO oxidation (Au/Al2O3 << Au/ZrO2 < Au/TiO2) was changed. In fact. in the presence of H-2. the reaction rates for the oxidation of CO become rather similar oil all three systems. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: Through an interplay between atomistic thermodynamics, isotherm models, and kinetics, all based on density functional theory calculations, we propose a mechanism for the removal of the (1x2) missing-row reconstruction on Au(110) under high CO pressure. Elevating the pressure from ultrahigh vacuum to one atmosphere causes the emergence of adsorption structures containing Au-CO entities. It is argued that the formation and the diffusion of these species may be the general phenomena which may control the morphology of metal surfaces under elevated CO pressure.
Abstract: The (N x 1) reconstruction (N = 5-6) of the clean Pd8Ni92(1 1 0) surface has been investigated by surface X-ray diffraction. A good fit with experimental data collected under UHV conditions is obtained when introducing undulations in the outer dense rows, constituted mainly of Pd atoms which strongly segregate to the surface. This reconstruction can be regarded as a way to relax the strains induced on the bigger Pd surface atoms, at least partially. The modifications of the surface were studied under butadiene then hydrogen as well as during butadiene hydrogenation at elevated pressure at 300 K and the (N x 1) reconstruction was found to be still present. The main changes are observed under pure hydrogen and during butadiene hydrogenation. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The adsorption of carbon monoxide on the Au(111) single-crystal surface has been investigated in the [10(-3)-10(3) Torr] range at room temperature (RT). Using scanning tunneling microscopy (STM), a CO-induced modification of the surface morphology (step edge roughening) and terrace structure (22 x root3 --> 1 x 1 transition) is evidenced. Reflection absorption infrared spectroscopy (RAIRS) experiments suggest that CO molecules are linearly chemisorbed on top of gold atoms and that adsorption occurs only above similar to1 Torr CO pressure at RT. Density functional theory (DFT) calculations, through consideration of perfect, stepped or kinked surfaces support these results and give additional evidence of strong structure sensitivity for CO adsorption on gold. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: We have built an experimental set-up including a scanning tunneling microscope in order to study the behaviour of catalytic surfaces in environmental conditions close to that needed in catalysis (pressure of gas, temperature). We have studied gold surfaces under gas pressure since gold was recently found to be highly active in the low temperature catalytic oxidation of carbon monoxide without combustion of hydrogen. We have followed real-time morphological modifications of Au(Ill) surfaces under elevated pressures (I bar) of hydrogen and oxygen. We have also studied the adsorption of CO on Au(110)-(1x2) reconstructed surface and observed in-situ by STM the dramatic change of the morphology of the terraces under the influence of the CO pressure (up to 670 mbar) on the structure of the surface. Complementary XPS and IRRAS measurements were also performed and show CO dissociation the surface.
Abstract: CO chemisorption on Au(110) at 300 K is observed under pressures in the range 10(-1)-10(+2) Torr. This is identified by an infrared band at 2110 cm(-1), whose intensity depends on the pressure. Scanning tunneling microscopy reveals deep restructuring of the surface. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The morphology and atomic structure of a vicinal surface of Fe3Al(111) has been studied by thermal energy helium atom scattering and scanning tunneling microscopy, The second order B2 --> DO3 structural phase transition in the chemically ordered alloy induces a morphological transition at its surface: single-height steps, which are evenly distributed above the bulk critical temperature T-c (approximate to820 K), bUtich four by four below T-c. The latter morphology is related to bulk layer stacking and appears driven by minimization of terrace energy. Terraces are pure Al and exhibit a (root3 x root3)R30degrees surface reconstruction, corresponding to 2/3 Al atoms and 1/3 vacancies in the terminal layer. The first sub-surface layers are enriched in Al by surface segregation. Surface vicinality makes possible the emergence of bulk DO3 domain boundaries, where four-steps bunches separate in two pairs of steps, which allows following chemical ordering in the near-surface region. The results point out the strong influence of long range chemical order on surface morphology. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: In order to bridge the 'material gap' in heterogeneous catalysis it is shown that supported model catalysts can be used. They are prepared by epitaxial growth on an oxide single crystal. This work describes results obtained with Pd clusters supported on MgO(100) surfaces as a model catalyst for the reduction of NO by CO. The morphology of the Pd particles was analysed by transmission electron microscope. By using molecular beam techniques it was possible to determine the elementary steps of the reaction and the reaction mechanism. It is shown that the dissociation of NO is the key step for the reaction. After dissociation, two types of adsorbed nitrogen atom are obtained: a strongly bound one that does not desorb during the reaction and a loosely bound one that desorbs associatively, Oxygen atoms resulting from the NO dissociation react with adsorbed CO or diffuse in the lattice of the Pd particles. The reaction rate depends on both the particle size and the particle morphology. The reaction kinetics is also influenced by the support via the capture, by the metallic particles, of the reactants physisorbed on the support.
Abstract: Combining molecular beam techniques and time-resolved infrared reflection absorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxidation reaction on an alumina-supported Pd model catalyst. The Pd particles are deposited by metal evaporation under ultrahigh vacuum (UHV) conditions onto a well-ordered alumina film, prepared on a NiAl(110) single crystal. Particle size, density and structure of the Pd deposits have been characterized in previous studies. In the low temperature region, transient and steady-state experiments have been performed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beams on the sample surface. We determine the steady-state CO2 production rate as a function of the CO fraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites under steady-state conditions is monitored by in situ IR spectroscopy. The origin of different types of CO2 transients is discussed. In particular we focus on the transient CO2 production after switching off the CO beam. For the model catalyst investigated, detailed reaction rate measurements in combination with time-resolved IRAS show that the origin of the particular transient behavior of the supported model system is not due to the presence of specific adsorption sites on small particles, as has been proposed previously. Instead, we show that the transient behavior can be semiquantitatively simulated on the basis of a simple kinetic model considering a homogeneous surface, and accounting for the inhibition of the dissociative adsorption of O-2 at high CO coverage. Moreover, it is discussed how the inherent heterogeneity of the supported particle system can additionally enhance the observed effect. (C) 2001 American Institute of Physics.
Abstract: The NO-CO reaction kinetics has been studied on Pd/MgO (1 0 0) model catalysts at low pressure by a pulsed molecular beam technique. The model catalysts have been prepared by epitaxial growth of metal clusters in UHV on cleaved MgO single crystals. Three samples with mean clusters sizes of 2.8, 6.9 and 15.6 nm have been studied. The reaction products are mainly N-2 and CO2. The steady-state reaction rate has been measured between 150 and 400 degreesC at low pressures (10(-9)-10(-6) Torr). The rate limiting step for the reaction is the dissociation of NO at low temperature and the adsorption of CO at high temperature. The reaction probability of NO has been accurately determined, taking into account the reverse spillover of NO from the MgO support towards the Pd particles. It is shown that in these conditions the turnover number is not an appropriate parameter to determine the intrinsic effect of particle size. The intrinsic activity depends not only on particle size but also on particle shape. The medium sized particles exhibiting mainly (1 1 1) facets are found to be more active. The largest particles, which exhibit principally (1 0 0) facets, are less active. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: The transient kinetics of reaction between carbon monoxide and an oxygen monolayer pre-adsorbed on palladium clusters supported on MgO(100), has been studied for various cluster sizes (4-15 nm), in the temperature range 120-400 degrees C, using molecular beam and mass spectrometry under ultrahigh vacuum. When the CO beam is opened, the CO2 production rate first increases instantaneously, and then increases slowly to its maximum, before decreasing to zero due to the lack of oxygen. The period of slow increase of the reaction rate, namely the induction period, appears at about 200 degrees C and becomes longer when temperature increases. Although it is not observed on Pd(111), this peculiar reaction kinetics does not depend on cluster size, and is attributed to a precursor state of CO chemisorption. The oxygen coverage at saturation is found equal to 0.4. At this high oxygen coverage, gaseous CO physisorbs above the oxygen adlayer. At high temperature, the precursor is more likely to desorb, which reduces its chemisorption probability, and thus the CO2 production rate. The temperature-dependant kinetics of CO adsorption and reaction with oxygen has been simulated thanks to a simple kinetic model accounting for the precursor mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: Palladium clusters epitaxially grown on MgO(100) have been used as model catalysts to study the oxidation of CO by NO. The size distribution and the shape of the Pd panicles are determined by transmission electron microscopy (TEM). Three samples corresponding to particle mean sizes of 3, 7 and 16 nm have been studied. The adsorption of ND and its reaction with CO have been studied, at low pressure, using a molecular beam of NO and an isotropic pressure of CO. NO dissociates on all the particles between 170 degrees C and 450 degrees C. During the reaction, CO2 and N-2 are produced with a little amount of N2O below 300 degrees C. The curve of the steady state reaction rate as a function of the temperature has a "volcano" shape. The maximum of activity is shifted toward higher temperature when the CO pressure increases. The turnover rare for CO2 production increases when particle size decreases. However, as the reaction is limited at high temperature by NO and CO adsorption, it is necessary to correct the reaction rate by the fact that a large part of the reactant molecules adsorbed on the Pd clusters comes from the capture of molecules physisorbed on the support. After this correction, which depends on panicle size, the medium-sized particles appear to be the more active. A possible explanation based on the particle shape is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: The adsorption of nitric oxide (NO) on the Pd/MgO(100) model catalyst has been studied ill the temperature range 160-430 degrees C, for various cluster sizes (d = 2.8 to 45 nm), using a pulsed molecular beam. From angular distribution measurements. the physisorption probability of NO on MgO is derived: alpha = 0.56 +/- 0.03. The physisorbed molecules call diffuse towards the clusters and become chemisorbed. This phenomenon, which increases drastically the NO adsorption rate on the particles, has been quantified as a function of the surface temperature and of the particle size. Once chemisorbed, NO molecules either dissociate to form nitrogen and oxygen adatoms, or desorb. The Frequency factor and the activation energy for desorption have been measured on large particles (d > 14 nm): v(des) =10(13) s(-1) and E-des = 32 kcal/mol. The equilibrium coverage of molecular NO is found much higher on small particles (3.1 nm) than on large particles (d > 6 nm). After an initial stage of intense dissociation on the fresh Pd catalyst, the dissociation capacity of the surface is lowered due to oxygen poisoning, but remains high and stable. The dissociation efficiency is between 40 and 80% of the molecules adsorbing on Pd, depending on the surface temperature and particle size. The dissociation of NO on palladium leads to the formation of N-2 (by association of two nitrogen adatoms), without production of N2O. From the transient kinetics of N-2 desorption, it is concluded that strongly bound nitrogen adatoms coexist, on the cluster surface, with loosely bound nitrogen species. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: The CO oxidation on Pd is generally considered to be structure insensitive and is not expected to depend on the size of the Pd particles. However, several size effects have already been evidenced for this reaction. Near the temperature where the steady-state reaction rate is maximum, the reactivity per Pd surface atom (turnover number (TON)) increases for small particles. At low temperature, in transient molecular beam experiments, a second peak of CO2 appears after the CO beam is closed, while the oxygen is still supplied to the sample. This peak shifts, and its shape changes with temperature and oxygen pressure. This peak is explained by the presence of CO strongly bound to the particle edges. This strongly bound CO reacts well after the CO adsorbed on the facets has desorbed. A kinetic model is presented which explains the evolution of this peak with temperature and oxygen pressure.
Abstract: Nanometer-sized metal clusters show singular chemical properties comparatively to bulk materials, and are used as models for studying heterogeneous catalysis. The reactivity of palladium clusters, epitaxially grown on MgO(100) and characterised by transmission electron microscopy, has been investigated. In addition to NO adsorption, CO+NO and CO+O2 reactions experiments have been performed under ultrahigh vacuum, using pulsed molecular beam methods. Nitric oxide molecules can diffuse on the MgO support before being chemisorbed on the Pd clusters. We have measured the probability of NO physisorption on MgO, and analysed this diffusion-capture phenomenon in a quantitative way. Once adsorbed on palladium, NO either dissociates, producing nitrogen, or desorbs. The activation energy for NO desorption is 32 kcal/mol. An influence of the cluster temperature and morphology on the NO adsorption and dissociation capacity has been evidenced. The NO reduction by CO produces mostly CO2 and N2. At equal pressures of reactants, the catalytic activity reaches a maximum at about 260°C. The rate-limiting steps of the reaction are, in the low temperature regime, the dissociation of NO and, in the high temperature regime, the adsorption of CO. Considering the diffusion of the reactants on the support, we have been able to accurately determine the effect of the cluster size on the catalytic activity, revealing a correlation with NO adsorption experiments. Finally, the kinetics of the CO oxidation reaction has been analysed in the transient regime, with the help of simple simulations. Below about 200°C, the reaction rate is limited by the inhibition of oxygen adsorption by CO strongly bound to the cluster edges. Above 200°C, at high oxygen coverage, a precursor state of CO chemisorption induces a lowering of the CO2 production rate.
