Multi-Scale Modeling of CO Oxidation on Pt-Based Electrocatalysts

Abstract

A serious limitation of modern low-temperature fuel cells is the use of highly purified H2 as fuel [1, 2]. While using H2/CO mixtures from steam-reforming hydrocarbon fuels considerably improves fuel flexibility, trace amounts of CO in the feed stream poison electrode surface. One solution to this problem is the development of temperature-stable membranes that allow hightemperature (e.g. 200 C) fuel cell operations (for e.g. see Refs. [3, 4]), thereby reducing CO adsorption on electrode surfaces. An alternate approach is the development of CO-tolerant electrocatalysts. The search for new electrocatalysts can be performed using a purely combinatorial screening, where an alloy catalyst is chosen from a library of possible combinations by a rapid screening procedure [5]. Alternatively, the search can proceed in a materials-by-design approach, where new electrocatalysts can be tailor-made to overcome the CO poisoning problem. This process is guided by developing a working hypothesis on how best known CO electro-oxidation catalysts function. In this regard, there has been significant effort in the last few decades to understand electro-oxidation of CO on Pt-based electrodes. Surface X-ray scattering, scanning tunneling microscopy, IR spectroscopy, sum frequency generation, NMR, ex-situ low energy electron diffraction, voltammetry, and other experiments on clean single crystal surfaces have provided valuable information on structure, thermodynamics, and kinetics of the complex CO electro-oxidation phenomenon (for example see Refs. [6, 7, 8, 9, 10, 11, 12, 13, 14]. However, a complete molecular picture of the CO oxidation mechanism, and a deep understanding of the complex kinetics on electrode surfaces can only be obtained using an interplay of theory/simulations with experiment. Multi-scale modeling provides a hierarchical computational approach to describe macroscopic catalytic processes. In this approach, atomistic methods (first principle quantum chemistry calculations and classical molecular dynamics) are used which reveal microscopic insight into themechanisms and molecular-scale dynamics of reactions at electrode surfaces (for e.g. see [15,