Trends in the Activation of Light Alkanes on Transition-Metal Surfaces

The first (oxidative) dehydrogenation step of light alkanes (ethane, propane, and n -butane) on transition-metal (closed-packed and stepped) surfaces is analyzed using density functional theory (DFT) calculations. It is shown that the transition-state energies (Δ E TS ) of the C–H bond activation scale linearly with the corresponding final-state energies (Δ E FS ), and all alkanes studied here share the same linear scaling relationships for the nonoxidative, oxygen-assisted, and hydroxyl-assisted reactions. Variations in Δ E TS between alkanes can be mainly attributed to differences in dispersion contributions determined by the carbon-chain length. As the carbon chain increases, the Δ E TS of the alkane C–H bond activation decreases. In addition, the Δ E TS of the first (O)DH step of propane and n -butane is linearly correlated with the Δ E TS of the first ethane (O)DH step. We also find that the oxygen and hydroxyl adsorption energies on the transition-metal surfaces (closed-packed and stepped) are dictating the promoting/poisoning effect of the C–H bond activation. Based on our extensive DFT calculations, we find that Pt has the lowest C–H bond transition-state energy for both the nonoxidative and oxidative pathways, and metals such as Au and Ag become active for C–H bond activation of alkanes only when oxygen and hydroxyl species are present on the metal surfaces. Finally, by establishing scaling relationships over a wide range of transition-metal surfaces, we have developed a simple and highly accurate model for the prediction of C–H bond activation barriers for the (oxidative) dehydrogenation of light alkanes.