TEMPERATURE-DEPENDENT OXYGEN ELECTROCHEMISTRY ON PLATINUM LOW-INDEX SINGLE-CRYSTAL SURFACES IN ACID-SOLUTIONS

Using the rotating ring-disk technique (RRDPt(hkl)E), the oxygen-reduc tion reaction (ORR) was studied in sulfuric acid solution over the tem perature range 298-333 K. At the same temperature, the exchange curren t density increased in the order, i(111)(0)<i(100)(0)<i(110)(0) which gives the order of absolute kinetic activities of Pt(hkl) in 0.05 mol/ L H2SO4:Pt(111)<Pt(100)<Pt(110). We found that at high current densiti es every crystal face has an ideally temperature-dependent Tafel slope , i.e., -2 x 2.3(RT/F). The activation energy for the ORR is independe nt of the surface geometry, Delta H-hkl(#) = 42 kJ/mol. The insensitiv ity of the activation energy to surface structure implies that the rea ction pathway for the ORR is the same on all three single-crystal face s. A ''series'' pathway for the ORR was proposed, with the first elect ron transfer being the rate-determining step. The structure sensitivit y in the kinetics of the ORR on Pt(hkl) is attributed to the structure -sensitive adsorption of bisulfate and hydroxyl anions and a simple si te-blocking effect of these adsorbed anions on the rate of the ORR.