Studies of kinetics of HCOOH oxidation on Pt(100), Pt(110), Pt(111), Pt(510) and Pt(911) single crystal electrodes
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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 quantitatively. The difficulty due to the self-poisoning involved in HCOOH oxidation has been overcome successfully by designing a programmed potential step procedure, which is based on results of kinetic studies of dissociative adsorption of HCOOH. The data processing method of integration transform of j similar to t transient data was developed for extracting kinetic parameters. The rate constant (k(f)), the apparent activation energy (Delta H(not equal)degrees) and the transfer coefficient (beta) for HCOOH oxidation on different Pt single crystal electrodes have been evaluated. The values of Delta H(not equal)degrees obtained on the 5 Pt single crystal electrodes vary 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, signifying a higher electrocatalytic activity for HCOOH oxidation. According to the values of Delta H(not equal)degrees, the electrocatalytic activity of the three basal planes of Pt single crystals can be arranged in the ascending 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 structure of the two stepped surfaces since the majority surface sites are of (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 single 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 imply the unavoidable interaction of HCOOH with surfaces of Pt single crystal electrodes. The variation of Delta H(not equal)degrees and beta versus the orientation of the Pt single crystal electrode demonstrated, from a kinetic point of view and for the first time, the effects of surface atomic arrangement towards HCOOH oxidation. (C) 1999 Elsevier Science S.A. All rights reserved.