Self-consistent periodic slab calculations based on gradient-corrected density functional theory (DFT-GGA) have been conducted to examine the reaction network of propane dehydrogenation over close-packed Pt(111) and stepped Pt(211) surfaces. Selective C-H or C-C bond cleaving is investigated to gain a better understanding of the catalyst site requirements for propane dehydrogenation. The energy barriers for the dehydrogenation of propane to form propylene are calculated to be in the region of 0.65-0.75 eV and 0.25-0.35 eV on flat and stepped surfaces, respectively. Likewise, the activation of the side reactions such as the deep dehydrogenation and cracking of C(3) derivatives depends strongly on the step density, arising from the much lower energy barriers on Pt(211). Taking the activation energy difference between propylene dehydrogenation and propylene desorption as the descriptor, we find that while step sites play a crucial role in the activation of propane dehydrogenation, the selectivity towards propylene is substantially lowered in the presence of the coordinatively unsaturated surface Pt atoms. As the sole C(3) derivative which prefers the cleavage of the C-C bond to the C-H bond breaking, propyne is suggested to be the starting point for the C-C bond breaking which eventually gives rise to the formation of ethane, methane and coke. These findings provide a rational interpretation of the recent experimental observations that smaller Pt particles containing more step sites are much more active but less selective than larger particles in propane dehydrogenation.

A comprehensive microkinetic model based on density functional theory (DFT) calculations is constructed to explore the reaction mechanism for dry methane reforming on Ni catalyst. Three low-index facets, namely, Ni(111), Ni(100), and Ni (211), are utilized to represent the contributions from the flat, open, and stepped surfaces. Adsorption energies of all the possible reaction intermediates as well as activation energies for the elementary reactions involved in dry reforming of methane on the three Ni surfaces are calculated through DFT. These results are further employed to estimate the rate constants for the elementary reactions under realistic temperatures and pressures within the framework of transition state theory and statistical mechanics treatments.The dominant reaction pathway is as CH 4 successive dissociation followed by carbon oxidation by atomic oxygen. The dependence of the rate-determining step on operating conditions is examined. At low CH 4 and CO 2 partial pressures, both CH 4 dissociative adsorption and carbon oxidation would jointly dominate the overall reaction rate, while at high pressures carbon oxidation is suggested as the rate-determining step for the DRM reaction. Our findings provide a rational interpretation of contradictory experimental observations.

Elemental silicon has a large impact on the economy of the modern world and is of fundamental importance in the technological field, particularly in solar cell industry. The great demand of society for new clean energy and the shortcomings of the current silicon solar cells are calling for new materials that can make full use of the solar power. In this paper, six metastable allotropes of silicon with direct or quasidirect band gaps of 0.39-1.25 eV are predicted by ab initio calculations at ambient pressure. Five of them possess band gaps within the optimal range for high converting efficiency from solar energy to electric power and also have better optical properties than the Si-I phase. These Si structures with different band gaps could be applied to multiple p-n junction photovoltaic modules.

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