Specificity of the UPD of H to the structure of highly dispersed Pt on carbon support

Abstract

Home-made carbon cryogel synthesized by sol-gel polycondensation and freeze-drying is used as support for preparation of highly dispersed Pt catalyst that is made by a modified polyol synthesis method in an ethylene glycol (EG) solution. Specific surface area of carbon support and Pt/C catalyst is determined from nitrogen adsorption isotherm. The adsorption isotherm demonstrates a significant mesoporosity of carbon support. Specific surface area of the carbon support, calculated by the BET equation, is found to be 573 m(2) g(-1). X-ray diffraction (XRD) results demonstrate a successful reduction of the Pt precursor to its metallic form, and transmission electron microscopy (TEM) images show very uniform Pt particle size distribution with mean particle size of about 2.7 +/- 0.7 nm of the catalyst. Potentiodynamic studies of the underpotential deposition of hydrogen (H-upd) on Pt/C electrode in 0.5 mol dm(-3) HClO4 aqueous solution in the temperature range from 274 to 318 K are made, and thermodynamic state functions for the hydrogen adsorption are determined. The experimental results are analyzed assuming linear variation of the Gibbs energy of adsorption versus theta H-upd on the basis of the surface heterogeneity. The increase of Delta G(Hupd)(theta) with the surface coverage indicates the repulsive interactions between H-upd adatoms. From the temperature dependence of the Gibbs energy of adsorption, the enthalpy and the entropy of adsorption are calculated. The values of these functions are determined to be Delta H-Hupd(theta=0) = -5.6 kJ mol(-1) and Delta S-Hupd(theta=0) = 69.1 J mol(-1) K-1 The value of Delta H-Hupd(theta) allows determinations of the bond energy between electrode surface and H-upd that is found to be Ept-H = 223 kJ mol(-1) for theta = 0.