Kinetic consequences of methane combustion on Pd, Pt and Pd–Pt catalysts

Abstract

The kinetic consequences of methane combustion on Pd_xPt_1−x oxide surfaces are investigated with kinetic data and density functional theory treatments. The catalytic activity of monometallic palladium drops significantly with time, and this loss is larger at higher temperatures. However, the addition of platinum to palladium catalysts significantly improves their stability. Besides, the Pd-rich bimetallic catalysts show higher activity for methane conversion. Turnover rates are independent of O₂ pressure (>10 kPa) but depend linearly on CH₄ pressure for all the catalysts. The calculated C–H bond activation energies are almost identical for Pd_1.0, Pd_0.75Pt_0.25 and Pd_0.5Pt_0.5. However, the Pt-rich catalysts (Pd_0.25Pt_0.75) show poor activity, and their activation energies and pre-exponential factors increase with increase in the platinum content of the catalyst. In these catalysts, methane dissociation is the only kinetically relevant step, but it occurs on different active sites. H₂O and CO₂ (>3–5 kPa) strongly inhibit methane combustion by titrating surface vacancies in a quasi-equilibrated adsorption–desorption step for Pd-containing catalysts. The DFT-derived C–H bond activation energies for O-saturated Pt surfaces are much larger than the values for PdO(101) surfaces, which is in agreement with the experimental results. Models of Pd–Pt oxide surfaces were built on the basis of XRD results and known structures of oxide Pd and Pt monometallic catalysts. Although the atom arrangement of the PdO(101)/Pt(100) outermost layer is similar to that of PdO(101), the C–H bond activation energy for PdO(101)/Pt(100) is much larger than that of PdO(101) because the surface Pd atoms on PdO(101)/Pt(100) are highly coordinated. However, when the Pd content is high enough (>25 mol%) in the Pd–Pt bimetallic catalysts, the low PdO(101) activation energy (61 kJ mol⁻¹) is retrieved for two monolayers of PdO(101) on Pt(100) (67 kJ mol⁻¹).