Kinetic isotopic effect for methanol oxidation on Pt(111), Pt(100), Pt(775) and Pt(755) electrodes

Methanol oxidation is studied on well-defined Pt single-crystal surfaces to obtain information about the ratedetermining step of the mechanism. For that, a voltammetric analysis of the kinetic isotopic effect using CH3OH and CD3OH at two different concentrations (10-4 and 10-2 M) has been carried out. For both concentrations, a significant diminution of the currents is observed when CD3OH is used, implying that the break of the C-H bond is involved in the rate-determining step of the reaction. The quantification of the Kinetic Isotope Effect (KIE) using the voltammetric currents shows values between 2.5 and 3.5, depending on the potential and surface orientation. This complex behavior is a consequence of the existence of several paths in the oxidation mechanism, whose rates strongly depend on the interfacial conditions. The different values are discussed considering the mechanism. On the other hand, when signals leading to the formation of CO on the Pt(100) electrode are analyzed, a KIE factor of 4.7 is obtained. All these numbers imply that in the rate-determining step of the mechanism, a C-H bond is broken.