Effects of Sb adatoms on kinetics of electrocatalytic oxidation of HCOOH at Sb-modified Pt(100), Pt(111), Pt(110), Pt(320), and Pt(331) surfaces - An energetic modeling and quantitative analysis
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Electrocatalytic oxidation of formic acid on Pt(100),-Pt(110), Pt(1 11), Pt(320), and Pt(33 1) surfaces modified with Sb adatoms (Pt(hkl)/Sb) was investigated using cyclic voltammetry, potential step technique, and in-situ FTIR spectroscopy. Emphasis has been placed on investigations of the effects of Sb adatoms at Pt single-crystal surfaces on the kinetics of HCOOH oxidation. Quantitative data of kinetics of HCOOH oxidation via reactive intermediates were obtained by applying a potential step technique in combination with a data processing method of integration transform of j-t transients, Which has been described in our previous paper (Sun, S. G.; Yang, Y. Y. J. Electroanal. Chem. 1999, 467, 121). The current results illustrated that the kinetics of HCOOH oxidation is faster on bare surfaces of Pt(100), Pt(110), and Pt(320) than that on Sb-modified surfaces of these Pt single-crystal electrodes, but it is slower on bare surfaces of Pt(l 11) and Pt(33 1) than that on Sb-modified surfaces of the two Pt single-crystal electrodes. The peak potentials of HCOOH oxidation on Pt(hkl)/Sb were all converged in a narrow potential region between 0.20 and 0.30 V (SCE), implying that the adsorption of Sb on Pt(hkl) planes has altered the apparent activation energy of HCOOH oxidation. On the basis of quantitative results, we have proposed, for the first time, a rectifying factor of apparent activation energy (gamma/kJ mol(-1)) that describes the alteration by Sb adatoms of the apparent activation energy of HCOOH oxidation on Pt(hkl)/Sb surfaces. The values of gamma have been determined quantitatively for gamma(Pt(100)) 12.80 +/- 0.317, gamma(Pt(100)) = 18.74 +/- 0.393, gamma(Pt(320)) = 16.683 +/- 0.349, gamma(Pt(111)) = -7.886 +/- 0.288, and gamma(Pt(331)) = - 11.69 +/- 0.245. The transfer coefficient beta of HCOOH oxidation on the five Sb-modified Pt(hkl) electrodes was determined to be within the range of 0.12 +/- 0.03, which is less dependent on the orientation of a Pt single crystal when it is modified with Sb adatoms and signifies a stepwise transfer of two electrons involved in HCOOH direct oxidation.