Studies of kinetics of HCOOH oxidation on Pt(100), Pt(110), Pt(111), Pt(510) and Pt(911) single crystal electrodes

The kinetics of HCOOH oxidation via a reactive intermediate on Pt(100), Pt( 110), Pt(111), Pt(510) and Pt(911) single crystal electrodes were studied q uantitatively. The difficulty due to the self-poisoning involved in HCOOH o xidation has been overcome successfully by designing a programmed potential step procedure, which is based on results of kinetic studies of dissociati ve adsorption of HCOOH. The data processing method of integration transform of j similar to t transient data was developed for extracting kinetic para meters. The rate constant (k(f)), the apparent activation energy (Delta H(n ot equal)degrees) and the transfer coefficient (beta) for HCOOH oxidation o n different Pt single crystal electrodes have been evaluated. The values of Delta H(not equal)degrees obtained on the 5 Pt single crystal electrodes v ary from 10.1 +/- 0.1 to 32.7 +/- 0.2 kJ mol(-1). The results demonstrated that the well-defined Pt(110) electrode possesses the lowest value of Delta H(not equal)degrees amongst the 5 Pt single crystal electrodes studied, si gnifying a higher electrocatalytic activity for HCOOH oxidation. According to the values of Delta H(not equal)degrees, the electrocatalytic activity o f the three basal planes of Pt single crystals can be arranged in the ascen ding order of Pt(110) > Pt(111) > Pt(100). The values of Delta H(not equal) degrees of the two stepped surface are close to that of Pt(100) (32.2 +/- 0 .5 kJ mol(-1)), which is in good accord with the configuration of surface s tructure of the two stepped surfaces since the majority surface sites are o f (100) symmetry. It has been found that the transfer coefficient beta does not vary with the reaction temperature, and manifests a similar variation as that of Delta H(not equal)degrees versus the orientation of the Pt singl e crystal electrode. The small values of beta (ranging from 0.102 +/- 0.004 to 0.251 +/- 0.008) may suggest a stepwise transfer of two electrons and i mply the unavoidable interaction of HCOOH with surfaces of Pt single crysta l electrodes. The variation of Delta H(not equal)degrees and beta versus th e orientation of the Pt single crystal electrode demonstrated, from a kinet ic point of view and for the first time, the effects of surface atomic arra ngement towards HCOOH oxidation. (C) 1999 Elsevier Science S.A. All rights reserved.