Abstract
The energetics of elementary surface processes relevant for CO oxidation, particularly CO and 02 adsorption, were investigated by a direct calorimetric method on model Pd nanoparticles and on the extended Pd(111) single crystal surface. The focus of this study lies on a detailed understanding of how a nanometer scale confinement of matter affects the binding strength of gaseous adsorbates. We report adsorption energies and sticking coefficients of CO and 02 measured as a function of the adsorbate surface coverage both on pristine and O-covered Pd surfaces. The reduced dimensions of the Pd substrate were found to affect the binding strength of the adsorbates in two principle ways: (i) via the change of the local adsorption environment that can result e.g. in stronger adsorbate bonding at the particle's low coordinated surface sites and (ii) via the contraction of the Pd lattice in small clusters and a concomitant weakening of chemisorptive interaction. Particularly for 02 adsorption, the change of the adsorption site from a three-fold hollow on Pd(111) to the edge site on Pd nanoparticles (approximately 4 nm sized on average) was found to result in a strong increase of the Pd-O bond strength. In contrast, CO adsorbs weaker on Pd nanoparticles as compared to the extended Pd(111) surface. In total, the binding energies of adsorbates on Pd and with this their surface coverages turn out to depend in a non-monotonic way on the particular structure of Pd surfaces, including the local structure of the adsorption site as well as the global properties of the small clusters arising e.g. from the lattice contraction.
Highlights
Confinement of matter at a nanoscopic scale is well known to result in new specific properties of a material
The model surface employed in this study consisted of Pd nanoparticles supported on a well-ordered thin Fe3O4 film grown on a Pt(111) single crystal (Fig. 1; for details of the preparation procedure and structural characterization by scanning tunneling microscopy (STM) see refs. 26 and 27)
We investigated the interaction of concentration of one reactant (CO) and O2 molecules with welldefined Pd nanoclusters and with the extended Pd(111) surface by a direct calorimetric method
Summary
Confinement of matter at a nanoscopic scale is well known to result in new specific properties of a material These new properties inherent to nanoparticles are important for a number of practical applications of heterogeneous catalysis, such as e.g. CO oxidation over transition metals.[1,2] This reaction has attracted much attention in recent decades both in surface science and ambient pressure studies and many phenomena have already been well understood.[3,4,5,6,7] Despite the comprehensive understanding and general agreement on the microscopic mechanisms of CO oxidation over many transition metals, less information is available on the thermodynamics of the underlying surface processes. The information would allow a deeper fundamental understanding of how the surface binds reactants and guides them through various elementary steps to the products, and provide a basis for a rational design of new catalytic and functional materials
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