To identify the nature and the local structure of the surface of supported catalyst nanoparticles, we have performed a detailed comparative study of CO adsorption on two categories of oxide-supported Palladium catalysts: (1) polycrystalline MgO and gamma-Al2O3 supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal based Pd model catalysts prepared under ultrahigh vacuum (UHV) conditions. The assignment of the CO vibrational frequencies to different types of sites on these systems has allowed a detailed structural characterization. For the Pd model catalyst, at low CO coverage, the infrared (IR) reflection absorption spectra closely resemble the expected behavior for terminations by a majority of (111) facets and a minority of (100) facets. The spectral features are indicative of defect sites such as particle steps and edges. Occupation of the defect sites can be affected by surface contaminations such as atomic carbon. Thus the CO spectra at high coverage can be used as both a structural and chemical probe under reaction conditions, provided that complementary information on the particle morphology is available. For the MgO and gamma-Al2O3 supported Pd systems, two distinct narrow bands (v congruent to 2070 and congruent to 1970 cm(-1)) have been assigned to linearly bonded and bridge-bonded CO species, on Pd (100)/(111) edges or facets, in agreement with the previous results obtained on model catalysts. The broad character of the 2070 cm(-1) feature indicates the simultaneous presence of (100) and (111) faces, with edge and corner sites present at their intersection. The high intensity and the small half-width (fwhm) of the band at 1970 cm(-1) on a Pd/MgO sample treated at high temperature, assigned to bridge-bonded CO species, suggests that the metal particles expose faces with a high level of regularity. Further spectroscopic features (v congruent to 1920-1800 cm(-1)), are ascribed to the presence of different types of 3-fold hollow sites on (111) faces.

Employing molecular beam methods, we have studied the angular distribution of products desorbing from oxide-supported Pd nanoparticles during catalytic oxidation of CO. A large range of particle sizes was covered by combining preparation techniques based on (1) metal evaporation and growth and (2) electron beam lithography. Whereas the CO2 distribution is symmetric and independent of the reaction conditions on small Pd aggregates, a strong influence of the reactant incidence angles and fluxes was observed for large particles. For the first time, these observations experimentally verify the existence of local reaction rate variations on a catalyst nanoparticle as a consequence of limited adsorbate mobility. Furthermore, the results allow us to extract in situ information about surface diffusion rates.

Combining a multi-molecular-beam approach and in situ time-resolved IR reflection absorption spectroscopy (TR-IRAS), we investigate the kinetics of methanol oxidation on a well-defined supported Pd model catalyst. The model catalyst is prepared under ultra-high-vacuum (UHV) conditions by I'd deposition onto a well-ordered Al2O3 film grown on NiAl (110). In previous studies, this system has been characterized in detail with respect to its geometric and electronic structure and its adsorption properties. Crossing molecular beams of methanol and oxygen on the sample surface, we systematically probe the rate of total methanol oxidation to CO2 as a function of surface temperature and reactant fluxes. The results are compared with equivalent experiments for the related CO oxidation reaction. Pronounced differences are observed in the kinetics of the two processes, both under steady state and under transient conditions. The dissimilarities can be related to the dehydrogenation step of methanol, which is found to be strongly inhibited at high oxygen coverage. At low oxygen fluxes, CO is formed as the main product of methanol decomposition. Via a three-beam isotope-exchange experiment combined with TR-IRAS, the kinetics of CO formation is investigated as a function of reactant fluxes and surface temperature. Mean-field simulations of the kinetics are performed in a two-step procedure. First, the kinetics of CO oxidation is described, both under steady state and transient conditions. In a second step the microkinetic model is extended to include the formation of CO formed by methanol dehydrogenation. A comparison with the experimental data indicates that the transient kinetics cannot be fully described by a mean-field approach. (C) 2002 Elsevier Science (USA). All rights reserved.

Employing a multi-molecular-beam approach, we have measured the angular distribution of CO2 molecules formed during CO oxidation under steady-state conditions on well oriented and shaped nm-sized Pd crystallites grown on an ordered alumina film. The experiment is combined with kinetic Monte Carlo simulations based on a realistic structural model. The results obtained allow us (i) to differentiate between local reaction rates on the particle nanofacets and (ii) to conclude that oxygen diffusion on and between the (111) facets is rapid compared to reaction.

The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(1 1 0) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400–500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300–500 K. For large and crystalline Pd particles, the system exhibits adsorption–desorption properties which are very similar to those of the Pd(1 1 1) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of ≈5–7 kJ mol−1 for 1 and 3–5 nm Pd particles studied.

Adsorption of CO on nanosize Pd particles was studied theoretically by density functional method and spectroscopically by means of infrared reflection absorption spectroscopy (IRAS) and sum frequency generation (SFG). A density functional approach was applied to three-dimensional crystallites of about 140 atoms. The model clusters were chosen as octahedral fragments of the face centered cubic (fcc) bulk, exhibiting (111) and (001) facets. Bare and adsorbate-decorated cluster models were calculated with Oh symmetry constraints. Various types of adsorption sites were inspected: 3-fold hollow, bridge, and on-top positions at (111) facets; 4-fold hollow and on-top sites at (001) facets; bridge positions at cluster edges; on-top positions at cluster corners; and on single Pd atoms deposited at regular (111) facets. Adsorption properties of the relatively small regular cluster facets (111) and (00 1) are calculated similar to those of corresponding ideal (infinite) Pd surfaces. However, the strongest CO bonding was calculated for the bridge positions at cluster edges. The energy of adsorption on-top of low-coordinated Pd centers (kinks) is also larger than that for on-top sites of (111) and (001) facets. To correlate the theoretical results with spectroscopic data, vibrational spectra of CO adsorbed on supported Pd nanocrystallites of different size and structure (well-faceted and defect-rich) were measured using IRAS and SFG. For CO adsorption under ultrahigh vacuum conditions, a characteristic absorption in the frequency region 1950-1970 cm(-1) was observed, which in agreement with the theoretical data was assigned to vibrations of bridge-bonded CO at particle edges and defects. SFG studies carried out at CO pressures up to 200 mbar showed that the edge-related species was still present under catalytic reaction conditions. By decomposition of methanol leading to the formation of carbon species, these sites can be selectively modified. As a result, CO occupies on-top positions at particle edges and defects. On the basis of the computational data, the experimentally observed differences in CO adsorption on alumina-supported Pd nanoparticles of different size and surface quality are interpreted. Differences between adsorption properties of Pd nanoparticles with a large fraction of (111) facets and adsorption properties of an ideal Pd(111) surface are also discussed.

Catalytically active nanoparticle sites: Different catalytic activity of different active sites coexisting on a supported nanoparticle has been directly observed using molecular-beam techniques and in situ surface vibrational spectroscopy. For methanol decomposition on a supported Pd model catalyst (see picture), it is shown that carbon–oxygen-bond breakage and dehydrogenation occur as competing reaction pathways at particle defects and on regular facets, respectively.

We have combined multi-molecular beam methods and in-situ time-resolved IR reflection absorption spectroscopy (IRAS) to explore the kinetics of methanol decomposition on a supported Pd model catalyst. The well-shaped Pd nanoparticles are prepared under ultra-high vacuum conditions on a well-ordered alumina film and have previously been characterized with respect to size, density, and morphology.

We have investigated the adsorption, decomposition and oxidation of methanol on a well-defined supported Pd model catalyst, utilizing a combination of molecular beam methods, reflection absorption IR spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The Pd model catalyst is prepared under ultrahigh-vacuum (UHV) conditions on a well-ordered Al2O3 film grown on NiAl(110). In previous studies, this model system has been characterized in detail with respect to its geometric and electronic structure. On the alumina support, two molecular adsorption states of methanol are distinguished by RAIRS and TPD. Moreover, we can differentiate between adsorption on the Pd particles and on the alumina support, enabling us to follow surface diffusion from the alumina film to the Pd particles during the adsorption process. Upon heating, methanol partially desorbs from the Pd particles and partially undergoes decomposition, with a reaction probability that is sensitively dependent on the initial methanol coverage. At 100 K, preadsorbed CO suppresses methanol adsorption on the Pd particles, whereas preadsorbed oxygen reduces the reaction probability. As a first intermediate, methoxy species are formed, which are stable up to temperatures of 200 K. Isotope exchange experiments indicate that a fast equilibrium is established between molecular methanol and methoxy species and that both species are rapidly exchanged with the gas phase. Further decomposition of methanol proceeds via two competing reaction pathways. The dominant pathway is dehydrogenation to CO, followed by CO2 formation in the presence of oxygen. Adsorbed oxygen has a pronounced inhibiting effect on the rate of decomposition. As a second pathway, we observe slow breakage of the carbon–oxygen bond, leading to formation of carbon and hydrocarbon species.

We have employed molecular beam techniques combined with in situ IR reflection absorption spectroscopy to study the CO oxidation kinetics on oxide-supported model catalysts. As model systems we utilize Pd particles of different size and morphology grown under ultrahigh vacuum conditions on a well-ordered alumina film on NiAl(110). Previously, these systems have been characterized in detail with respect to their geometric and electronic structure. Using a combination of two beam sources crossed on the sample surface, the transient behavior and the steady-state reaction rate are systematically probed over a wide range of CO and oxygen fluxes and at different sample temperatures. Comparing different particle sizes and structures, characteristic differences are observed with respect to both the steady-state reaction rate and the transient behavior upon switching off the CO beam source. The origin of these effects is discussed with respect to the different CO and oxygen adsorption properties of small Pd particles. To directly monitor changes in the CO adsorption during the reaction, we have performed in situ IR absorption spectroscopy under steady-state conditions. Whereas strong rearrangements of the CO adsorbate layer are found for large and well-ordered particles as a function of the applied CO and oxygen fluxes, no similar effects are observed on small Pd aggregates.

Utilizing a combination of molecular beam techniques and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions we have studied the interaction of oxygen with an alumina-supported Pd model catalyst as well as the influence of the oxygen pretreatment on the kinetics of the CO oxidation reaction. The Pd particles were deposited by metal evaporation in UHV onto a well-ordered alumina film prepared on a NiAl(110) single crystal. The particle density, morphology and structure are determined by STM both immediately after preparation and after oxygen adsorption and CO oxidation. The oxygen sticking coefficient and uptake in the temperature regime between 100 and 500 K and the kinetics of the CO oxidation reaction are quantitatively probed by molecular beam techniques. It is found that starting at temperatures below 300 K the Pd particles rapidly incorporate large amounts of oxygen, finally reaching stoichiometries of PdO>0.5. STM shows, that neither the overall particle shape nor the dispersion is affected by the oxygen and CO treatment. Only after saturation of the bulk oxygen reservoir are stable CO oxidation conditions obtained. In the low-temperature regime (<500 K), only the surface oxygen, but not the bulk and subsurface oxygen is susceptible to the CO oxidation. The activation energies for the Langmuir–Hinshelwood step of the CO oxidation reaction were determined both in the regime of high CO coverage and high surface oxygen coverage. A comparison shows that the values are consistent with previous Pd(111) single crystal results. Thus, we conclude that, at least for the particle size under consideration in this study (5.5 nm), the LH activation energies are neither affected by the reduced size nor by the oxygen pretreatment.

By 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 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 infrared 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 TR-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 suggest that the transient behavior is a consequence of the inhibition of the dissociative adsorption of O2 at high CO coverage. Additionally, the inherent heterogeneity of the supported particle system can enhance the observed effect.

We have employed multimolecular beam techniques to study the transient and steady-state kinetics of the CO oxidation on alumina-supported Pd model catalysts as a function of particle size and surface structure. The model systems were prepared under UHV conditions on a well-ordered alumina film on NiAl(110) and were previously characterized with respect to their geometric and electronic structure and their morphology. Crossing two molecular beams on the sample surface we have systematically probed the CO2 production rate over a wide range of reactant fluxes and at different sample temperatures. Characteristic differences as a function of particles size are observed in both the transient and steady-state regime. In order to relate these effects to the differences in structure and adsorption properties, we have performed microkinetic simulations of the entire series of transient experiments. Whereas it is found that the kinetics on large and ordered Pd particles can in general be described by a homogeneous surface model, significant deviations remain with respect to the kinetics on small and defect-rich particles. In order to semiquantitatively simulate these effects, we consider a heterogeneous surface model, which takes into account the simultaneous presence of different types of adsorption sites. Depending on their distribution, surface diffusion between these sites is included. It turns out that the differences observed for the small particles can be qualitatively understood by a simple model, where we add a small fraction of weakly CO binding sites to the regular adsorption properties. This type of modified adsorption behavior is in agreement with previous desorption studies.