Insights into the function of silver as an oxidation catalyst by ab initio atomistic thermodynamics

By Wei‐Xue Li, C. Stampfl, Matthias Scheffler. 2003

168 citations recorded by the source corpus.

To help understand the high activity of silver as an oxidation catalyst, e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of methanol to formaldehyde, the interaction and stability of many different oxygen species at the Ag(111) surface has been studied for a wide range of coverages. Through calculation of the free energy, as obtained from density-functional theory and taking into account the temperature and pressure via the oxygen chemical potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a thin surface-oxide structure is most stable for the temperature and pressure range of ethylene epoxidation and we propose it (and possibly other similar structures) contains the species actuating the catalysis. For higher temperatures, low coverages of chemisorbed oxygen are most stable, which could also play a role in oxidation reactions. For temperatures greater than about 775 K there are no stable oxygen species, except for the possibility of O atoms adsorbed at undercoordinated surface sites (i.e., imperfections, defects). At low temperatures $(\ensuremath{\lesssim}400\mathrm{K}$ at atmospheric pressure), provided kinetic limitations can be overcome, thicker oxidelike structures are predicted. Due to their low thermal stability, however, they can be ruled out as playing an important role in the heterogeneous reactions under technical conditions. Bulk dissolved oxygen and a molecular ozonelike species adsorbed at a surface vacancy, as have been proposed in the literature, are found to be energetically unfavorable. The employed theoretical approach for calculating free energies and predicting the lowest energy structures in contact with species in the environment (``ab initio, atomistic thermodynamics''), affords investigation of a system that seamlessly connects standard $(T=0\mathrm{K})$ density-functional theory results, characteristic of ``typical'' theoretical surface science studies, through to those valid for the conditions of catalysis. Though the error bar of the noted theoretical temperatures is noticeable $(\ifmmode\pm\else\textpm\fi{}\ensuremath{\approx}55\mathrm{K}),$ the identified trends and physical descriptions are useful.

Subfields: Atomic and Molecular Physics, and Optics, Catalysis, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/10d5f92722ecf370863a1bf0da342814c9441e49

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In Atomic and Molecular Physics, and Optics

In Catalysis

In Materials Chemistry

Citations in the corpus

AB Initio Atomistic Thermodynamics and Statistical Mechanics of Surface Properties and Functions (2004, background): “Most stable translated into the thermodynamic language meaning that the corresponding structure minimizes an appropriate thermodynamic function, which would in this case be the Gibbs free energy of adsorption � G ( Li, Stampfl, and Scheffler, 2003a, b).”
THE OXIDATION OF NIAL: What Can We Learn from Ab Initio Calculations? (2005, background): “Equations similar to Equation (10) can be obtained for all four types of point defects.”
The surface and materials science of tin oxide (2005, background): “For CO oxidation for instance, it has been shown that the oxides of Ag [1], Pd [2], and Ru [3] are the active phase under operating conditions.”