  The dissociative adsorption of methane on variously oxidized Pd, Pt and Pd-Pt
surfaces is investigated using density-functional theory, as a step towards
understanding the combustion of methane on these materials. For Pd-Pt alloys,
models of surface oxide structures are built on the basis of known oxides on Pd
and Pt. The methane adsorption energy presents large variations depending on
the oxide structure and composition. Adsorption is endothermic on the bare
Pd(111) metal surface as well as on stable thin layer oxide structures such as
the ($\sqrt{5}\times\sqrt{5}$) surface oxide on Pd(100) and the PtO$_2$-like
oxide on Pt(111). Instead, large adsorption energies are obtained for the (100)
surface of bulk PdO, for metastable mixed Pd$_{1-x}$Pt$_x$O$_{4/3}$ oxide
layers on Pt(100), and for Pd-Pt(111) surfaces covered with one oxygen
monolayer. In the latter case, we find a net thermodynamic preference for a
direct conversion of methane to methanol, which remains adsorbed on the
oxidized metal substrates via weak hydrogen-bond interactions.
