Surface segregation effects in electrocatalysis: Kinetics of oxygen reduction reaction on \npolycrystalline Pt3Ni alloy surfaces

Abstract

Effects of surface segregation on the oxygen reduction reaction (ORR) have been studied on a polycrystalline Pt 3 Ni alloy in acid electrolyte using ultra high vacuum (UHV) surface sensitive probes and the rotating ring disk electrode (RRDE) method.Preparation, modification and characterization of alloy surfaces were done in ultra high vacuum (UHV).Depending on the preparation method, two different surface compositions of the Pt 3 Ni alloy are produced: a sputtered surface with 75 % Pt and an annealed surface (950 K ) with 100 % Pt.The latter surface is designated as the "Pt-skin" structure, and is a consequence of surface segregation, i.e., replacement of Ni with Pt atoms in the first few atomic layers.Definitive surface compositions were established by low energy ion scattering spectroscopy (LEISS).The cyclic voltammetry of the "Pt-skin" surface as well as the pseudcapacitance in the hydrogen adsorption/desorption potential region is similar to a polycrystalline Pt electrode.Activities of ORR on Pt 3 Ni alloy surfaces were compared to polycrystalline Pt in 0.1M HClO 4 electrolyte for the observed temperature range of 293 < T < 333 K.The order of activities at 333 K was: "Pt-skin" > Pt 3 Ni (75% Pt) > Pt with the maximum catalytic enhancement obtained for the "Pt-skin" being 4 times 1 that for pure Pt.Catalytic improvement of the ORR on Pt 3 Ni and "Pt-skin" surfaces was assigned to the inhibition of Pt-OH ad formation (on Pt sites) versus polycrystalline Pt.Production of H 2 O 2 on both surfaces were similar compared to the pure Pt.Kinetic analyses of RRDE data confirmed that kinetic parameters for the ORR on the Pt 3 Ni and "Pt-skin" surfaces are the same as on pure Pt: reaction order, m=1, two identical Tafel slopes, activation energy, 21-25 kJ/mol.Therefore the reaction mechanism on both Pt 3 Ni and "Pt-skin" surfaces is the same as one proposed for pure Pt i.e. 4e -reduction pathway.