The hydrogen electrode reaction and the electrooxidation of CO and H 2 / CO mixtures on well‐characterized Pt and Pt ‐bimetallic surfaces

Abstract

Abstract Interrelationships between the kinetics of fuel cell reactions and the structure/composition of anode catalysts are reviewed. Model systems like Pt single crystals and well characterized Pt(hkl)/Pd and Pt 3 Sn(hkl) bimetallic surfaces are used to establish a link between the macroscopic kinetic rates and thermodynamic properties of the reacting system and the microscopic level of understanding of the bonding and reactivity of the reaction intermediates and spectator species. The results show that the hydrogen reaction is a structure sensitive process, with Pt(110) being an order of magnitude more active than either of the atomically “flatter” (100) and (111) surfaces. The difference in the activity at the crystal face is attributed to the structure sensitive adsorption of hydrogen (H upd ) and hydroxyl anions (OH ad ) and the effect these species have on the formation of an active intermediate (H opd ). The significant differences of the hydrogen reaction in alkaline versus acid electrolytes are thought to arise due to the presence of OH ad , even close to the reversible potential of the hydrogen reaction in the alkaline electrolyte. The hydrogen reaction on Pt(hkl) modified by pseudomorphic Pd (sub)monolayers shows “volcano‐like” behavior, having the maximum rate on Pt(111) modified by 1 ml of Pd. The physical model based on differences in the adsorption energies of H upd and H opd on Pd vs. Pt is used to rationalize the catalytic activity of Pd atoms. For the Pt(111)/CO system, a remarkable difference in both activity and stability of the p (2 × 2)–3CO ordered structures is observed between alkaline and acid solutions. The pH‐effect is explained as the pH‐dependent adsorption of OH ad , which starts in the H upd potential region at defect sites. The oxidation of H 2 in the presence of CO occurs concurrently with CO oxidation on Pt and Pt bimetallic surfaces. The PtSn(111) system is used to demonstrate that both the bifunctional effect and the ligand effect contribute to the influence of Sn on the CO oxidation rate and for H 2 oxidation process in the presence of CO.