(294a) A Kinetic Analysis of the CO Electro-Oxidation Reaction on Bimetallics: Understanding the Interplay of Bifunctional and Electronic Effects

CO electro-oxidation (CO-ox) is one of the most widely studied reactions in electrocatalysis for both fundamental and technological reasons. CO* is a strongly-binding intermediate which poisons the most active oxidation catalysts (Pt,Pd) during conversion of many organic molecules (e.g. methanol, formic acid, and larger oxygenates such as molecules derived from biomass). For this reason, there has been much interest in developing CO-tolerant materials to mitigate this poisoning effect and improve electro-oxidation activity, in turn improving the efficiency of direct-organic fuel cells and/or oxidative electrosyntheses.

We recently published work in which we developed a descriptor-based microkinetic model for the CO-ox reaction, which predicted that the optimal material should bind CO* weaker and OH* stronger than Pd(111). Ag_xPd_1-x alloys are a class of materials which have been shown (using first-principles calculations) to exhibit these properties. It has been proposed that Ag_xPd_1-x alloys exhibit weaker CO* binding on Pd sites (relative to pure Pd) and stronger OH* binding on Ag sites (relative to pure Pd) due to a combination of electronic and geometric effects, making them promising materials for the CO-ox reaction.

In this work, we investigate a series of Ag_xPd_1-x/C alloy catalysts and compare their activities in both CO-ox and methanol electro-oxidation (Me-ox). It was found that each of the Ag_xPd_1-x alloys were more active than pure Pd for the CO-ox reaction at low overpotentials, while the optimal Ag:Pd stoichiometry for Me-OX differs from that for CO-ox. We discuss the reasons for these differences in light of insights from transient and steady state kinetic experiments, as well as X-ray absorption spectroscopy, and we further discuss the relative importance of electronic effects vs. bifunctional kinetics based on these studies.