The quantitative study of methane adsorption on the Pt(997) step surface as the initial process for reforming reactions

Abstract

The adsorption states and thermal process of methane on a stepped Pt surface were investigated using temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) calculations including van der Waals interactions. By estimating the activation energies of methane desorption from the TPD data, it was revealed that the adsorption energy for methane on the step site (0.19 eV) was higher than that on the terrace site (0.15 eV). The IRAS spectrum after multilayer adsorption and subsequent 70 K heating showed two peaks at 3009 and 1305 cm−1, which were assigned to the antisymmetric stretching and the deformation modes for adsorbed methane on the step site, respectively. The present DFT calculations show that the most stable adsorption structure for methane on the step site is a “2H” structure, where two C-H bonds point toward Pt atoms. Based on the electronic structure analysis, we conclude that the stability of the 2H structure originates from the mixing between the methane molecular orbitals and the orbitals of step Pt atoms. As a result, the length of the C-H bond pointing toward the Pt step atom is elongated compared with that of other C-H bonds. Thus, the Pt step site already activates the adsorbed methane slightly.