Methane Nonoxidative Direct Conversion to C2Hydrogenations over CeO2-Supported Pt Catalysts: A Density Functional Theory Study

In this paper, we used the density functional theory calculations for analyzing the effect of the size of Pt clusters supported by CeO 2 (111) on the carbon deposition resistance during methane direct conversion in the absence of oxygen. We have adopted Pt cluster models with different coordination numbers, which are Pt 1 /Ce 1– x Pt x O 2−δ (111), Pt 3 /Ce 1– x Pt x O 2−δ (111), and Pt 10 /Ce 1– x Pt x O 2−δ (111). The reaction mechanism for the nonoxidative direct conversion of methane to C 2 hydrocarbons includes two major processes: methane decomposition without oxygen participation (CH 4 * → C* + 4H*) and C–C nonoxidative coupling reaction (CH x * + CH x * → C 2 H x *). On the one hand, the activation energy of CH* → C* + H* was the highest (1.94 eV) on the Pt 1 /Ce 1– x Pt x O 2−δ (111) compared with 1.61 eV on Pt 3 /Ce 1– x Pt x O 2−δ (111) and 1.17 eV on Pt 10 /Ce 1– x Pt x O 2−δ (111) illustrating that the trend for carbon deposition resistance is Pt 1 /Ce 1– x Pt x O 2−δ (111) > Pt 3 /Ce 1– x Pt x O 2−δ (111) > Pt 10 /Ce 1– x Pt x O 2−δ (111). On the other hand, microkinetics analysis is used for C 2 hydrocarbon selectivity at 1248 K, and ethylene is the most abundant species on Pt 1 /Ce 1– x Pt x O 2−δ (111) with the selectivity measured to be 92.65% followed by 6.77% of acetylene. On Pt 3 /Ce 1– x Pt x O 2−δ (111), ethylene is the dominant species (63.97%) followed by acetylene (20.01%) and ethane (16.02%), and 6.01% of carbon deposited on the surface. On Pt 10 /Ce 1– x Pt x O 2−δ (111), however, the converted methane almost all became coke (99.5%) that was deposited on the catalyst surface. So, with the kinetics and thermodynamics analysis, our results show that the methane nonoxidative direct conversion to C 2 hydrocarbons strongly depended on the metal size effect (i.e., structure-sensitivity), and the atomically dispersed Pt catalyst had both carbon deposition resistance and high selectivity.