Quantum state-resolved CH4 dissociation on Pt(111): coverage dependent barrier heights from experiment and density functional theory

Abstract

The dissociative chemisorption of CH₄ on Pt(111) was studied using quantum state-resolved methods at a surface temperature (T_s) of 150 K where the nascent reaction products CH₃(ads) and H(ads) are stable and accumulate on the surface. Most previous experimental studies of methane chemisorption on transition metal surfaces report only the initial sticking coefficients S₀ on a clean surface. Reflection absorption infrared spectroscopy (RAIRS), used here for state resolved reactivity measurements, enables us to monitor the CH₃(ads) uptake during molecular beam deposition as a function of incident translational energy (E_t) and vibrational state (ν₃ anti-symmetric C–H stretch of CH₄) to obtain the initial sticking probability S₀, the coverage dependence of the sticking probability S(θ) and the CH₃(ads) saturation coverage θ_sat. We observe that both S₀ and θ_sat increase with increasing E_t as well as upon ν₃ excitation of the incident CH₄ which indicates a coverage dependent dissociation barrier height for the dissociation of CH₄ on Pt(111) at low surface temperature. This interpretation is supported by density functional calculations of barrier heights for dissociation, using large supercells containing one or more H and/or methyl adsorbates. We find a significant increase in the activation energies with coverage. These energies are used to construct simple models that reasonably reproduce the uptake data and the observed saturation coverages.