Methane Nonoxidative Direct Conversion to C-2 Hydrogenations 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 CeO2(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/Ce1-xPtxO2-delta(111), Pt-3/Ce1-xPtxO2-delta(111), and Pt-10/ Ce1-xPtxO2-delta(111). The reaction mechanism for the nonoxidative direct conversion of methane to C-2 hydrocarbons includes two major processes: methane decomposition without oxygen participation (CH4*-> C* + 4H*) and C-C nonoxidative coupling reaction (CHx* + CHx *-> C2H(x)*). On the one hand, the activation energy of CH*-> C* + H* was the highest (1.94 eV) on the Pt-1/Ce1-xPtxO2-delta(111) compared with 1.61 eV on Pt-3/Ce1-xPtxO2-delta(111) and 1.17 eV on Pt-10/Ce1-xPtxO2-delta(111) illustrating that the trend for carbon deposition resistance is Pt-1/Ce1-xPtxO2-delta(111) > Pt-3/Ce1-xPtxO2-delta(111) > Pt-10/Ce1-xPtxO2-delta(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/ Ce1-xPtxO2-delta(111) with the selectivity measured to be 92.65% followed by 6.77% of acetylene. On Pt-3/Ce1-xPtxO2-delta(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/Ce1-xPtxO2-delta(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.