Diana Vogel

Local Catalytic Ignition during CO Oxidation on Low-Index Pt and Pd Surfaces: A Combined PEEM, MS, and DFT Study

Shedding light on light-off : Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (“light-off”) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis.

Local Reaction Kinetics by Imaging: CO Oxidation on Polycrystalline Platinum

The surfaces of noble metal single crystals such as Pt(111) have served as successful model systems for heterogeneous catalysts. To account for the fact that supported Pt nanoparticles of technological catalysts typically exhibit different low Millerindex facets to the reactive gases, single-crystal studies were most frequently carried out for Pt(111), Pt(110) and Pt(100) terminations. Catalytic reaction properties were, for example, determined by temperature-programmed methods or molecular beam studies (both relying on mass spectroscopic product analysis) or, in case of atmospheric pressure conditions, by gas chromatography. In order to evaluate and compare the catalytic properties of the different crystallographic orientations, a relatively large parameter space of reactant gas pressure and reaction temperature must be investigated. Since this represents a significant body of work, there is hardly any catalytic system with a complete set of reported reactivity data. We present an alternative approach that allowed us to determine catalytic properties of different crystallographic terminations in a more efficient way, at least for specific surface reactions. Photoemission electron microscopy (PEEM) was employed to study the CO oxidation reaction on polycrystalline Pt foil, consisting of micrometer-sized domains of (100), (110) and (111) terminations. PEEM imaging was performed in situ, that is, during the reaction of CO and oxygen to give CO2, and the analysis of the local photoemission intensity of selected domains on the Pt foil enabled us to obtain locally resolved kinetic information for (100), (110) and (111) surfaces. It is important to note that at a given time the reactant gas pressure and temperature were identical for all terminations, allowing a direct simultaneous comparison of their catalytic properties. The kinetic behavior of the (100), (110) and (111) surfaces is described by “kinetic phase diagrams” (also known as bifurcation diagrams; for details see the Supporting Information), representing the domain-specific catalytic properties. It is shown that the (111)-, (100)and (110)-oriented grains behaved almost identical to the corresponding single crystals and that the superposition of the weighted contributions of the individual domains (as measured by PEEM) reproduced the global kinetics of CO oxidation on Pt foil (as measured by MS). Under the applied experimental conditions, the (100), (110) and (111) domains behaved independently to a large extent, indicating that diffusion and gas-phase coupling between the different facets were not sufficient to synchronize their kinetic transitions. The microscopic observation of reaction fronts that were confined within the domain boundaries corroborated the insufficient coupling. Implications of the current results on CO oxidation on supported Pt nanoparticles of technological catalysts are also discussed. Kinetic phase transitions on the Pt(111) single-crystal surface have been previously observed by PEEM (by averaging the whole image intensity, since the homogeneity of such a surface did not ask for spatially-resolved analysis) and PEEM images of CO oxidation on Pt foil have been reported (but without analysis of phase transitions). However, the present contribution shows for the first time how local kinetic phase diagrams of a catalytic reaction can be obtained for individual differently oriented domains of a polycrystalline material. The oxidation of CO on platinum surfaces is a seemingly simple reaction, but it exhibits several complex phenomena such as hysteresis (bistability), oscillations, dissipative structures and chaotic behaviour. Herein, we have concentrated on bistability and deliberately did not examine the parameter space (higher temperature and pressures) of oscillations which were extensively studied before. To illustrate the kinetic behavior of Pt foil, Figure 1 (right inset) shows the global (overall) reaction kinetics measured by mass spectroscopy. At a reaction temperature of 417 K, the Pt foil was exposed to a constant partial pressure of oxygen (1.3 10 5 mbar) while the CO partial pressure (pCO) was cycled and the CO2 production rate (RCO2 ) was monitored by mass spectroscopy. Upon exposing the oxygen-covered Pt surface to increasing CO pressure, RCO2 increased (high activity) until a kinetic transition point at pressure tA was reached. When pCO exceeded tA, the Pt surface switched from oxygen-covered to CO-covered, marked by the loss of catalytic activity due to CO self-poisoning (oxygen cannot adsorb on the CO-covered surface). When pCO was decreased, the catalyst remained in the poisoned (low reactivity) steady state until a second transition pressure tB was approached. Accordingly, at pCO tB<pCO<pCO , the catalyst can adopt one of two steady states, depending on the prehistory. The resulting hysteresis is characteristic for a bistable reaction behavior that originates from the asymmetric inhibition of dissociative oxygen adsorption by CO:oxygen needs two neighboring adsorption sites per molecule for dissociation and can not adsorb on a densely packed CO-covered [a] Prof. Dr. Y. Suchorski, C. Spiel, D. Vogel, Dr. W. Drachsel, Prof. Dr. G. Rupprechter Institute of Materials Chemistry Vienna University of Technology Veterin rplatz 1, 1210 Vienna (Austria) Fax: (+43)1-58801-16599 E-mail : grupp@imc.tuwien.ac.at [b] D. Vogel, Prof. Dr. R. Schlcgl Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg, 4–6, 14195 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201000599.

The Role of Defects in the Local Reaction Kinetics of CO Oxidation on Low-Index Pd Surfaces.

The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented μm-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10–5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.

Silicon Oxide Surface Segregation in CO Oxidation on Pd: An in situ PEEM, MS and XPS Study

The effect of silicon oxide surface segregation on the locally-resolved kinetics of the CO oxidation reaction on individual grains of a polycrystalline Pd foil was studied in situ by PEEM, MS and XPS. The silicon oxide formation induced by Si-impurity segregation at oxidizing conditions, was monitored by XPS and its impact on the global and local (spatially resolved) kinetics of the CO oxidation was determined by MS and PEEM. The results reveal a drastic inhibiting effect of silicon oxide on the Pd reactivity towards CO oxidation, manifested both in the collapse of the global CO2 formation rate and in the modified local reactive properties of individual Pd micrograins. The presence of adsorbed oxygen on the Pd surface effectively enhances the silicon segregation to the Pd surface.Graphical Abstract

Spatially coupled catalytic ignition of CO oxidation on Pt: mesoscopic versus nano-scale.

Spatial coupling during catalytic ignition of CO oxidation on μm-sized Pt(hkl) domains of a polycrystalline Pt foil has been studied in situ by PEEM (photoemission electron microscopy) in the 10−5 mbar pressure range. The same reaction has been examined under similar conditions by FIM (field ion microscopy) on nm-sized Pt(hkl) facets of a Pt nanotip. Proper orthogonal decomposition (POD) of the digitized FIM images has been employed to analyze spatiotemporal dynamics of catalytic ignition. The results show the essential role of the sample size and of the morphology of the domain (facet) boundary in the spatial coupling in CO oxidation.

The role of metal/oxide interfaces for long-range metal particle activation during CO oxidation

Electron microscopy and modelling are used to study CO oxidation on oxide-supported Pd. The perimeter of the metal/oxide interface is shown to affect CO tolerance of the entire particle, demonstrating a long-range effect over micrometre length scales.