Oxygen electrocatalysis in alkaline electrolyte:: Pt(hkl), Au(hkl) and the effect of Pd-modification

The kinetics of the oxygen reduction reaction (ORR) was studied in alkaline electrolyte at 293-333 K on bare and Pd modified Pt(hkl) and Au(hkl) surfaces. The rotating ring-disk electrode technique was used to study the ORR with solution phase peroxide detected at the ring electrode. Pd modification was either by electrodeposition (Pt) or by vapor deposition in vacuum (An). The surface concentration of Pd was determined in vacuum using low energy ion scattering. In agreement to the structure sensitivity found at room temperature previously, on the bare Au(hkl) surfaces the ORR was found to be strongly structure sensitive in the temperature range from 293 to 333 K, with order of activity being (100) much greater than (110) > (111). The structure sensitivity for Pt(hkl) is much less and varies in the nearly the opposite order (111) > (100) > (110). The peroxide intermediate pathway is clearly operative on Au(hkl) surfaces. At elevated temperature, significantly smaller amounts of peroxide are formed. The kinetics of the ORR were significantly enhanced by modification of both Pt(hkl) and Au(hkl) surfaces with Pd. The catalytic effect is most pronounced on the surfaces that are less active surfaces in the unmodified state, with enhancement at least an order of magnitude faster kinetics. Pd modification of the Au(hkl) surfaces, therefore, significantly reduces the structure sensitivity of the ORR. Even on the highly active Pt(111) surface the kinetics can be improved by a factor of approximately two to four due to Pd modification. The catalytic enhancement can be achieved with as little as 18 at.% Pd in the Au(hkl) surface. (C) 2002 Published by Elsevier Science Ltd.