Adsorption properties of CO, H2 and CH4 over Pd/γ-Al2O3 catalyst: A density functional study

Density functional theory (DFT) calculations were employed to investigate the adsorption characteristics of carbon monoxide (CO), hydrogen (H2), and methane (CH4) on the surface of clean γ-Al2O3 and Pd supported γ-Al2O3, which is of significant for catalytic combustion. The adsorption intensities of the three gas molecules in pure γ-Al2O3 (1 1 0) and Pd/γ-Al2O3 (1 1 0) were in the order of CO > H2 > CH4. The corresponding adsorption energies on the Pd/γ-Al2O3 (1 1 0) surface were at least three times higher than those on γ-Al2O3 (1 1 0). Anlysis of Mulliken population and partial density of states (PDOS) showed that the adsorption mechanisms were as follow: (a) CO stably adsorbed on the bridge site of dimer Pd with two CPd bonds because of charges transfer from the surface to CO, and the triple bond (CO) was broken to a double bond (CO); (b) H2 was dissociated into hydrogen atoms on the dimer Pd and produced a stable planar configuration; and (c) the tetrahedral structure of CH4 was destroyed on the surface and formed a CH3 species bonded to the Pd atom, which contributes to the orbital hybridization between C and Pd atoms.