No abstract

Density functional theory calculations have been performed to investigate the effect of Sn on the catalytic activity and selectivity of Pt catalyst in propane dehydrogenation. Five models with different Sn to Pt surface molar ratios are constructed to represent the PtSn surfaces. With the increase of the Sn content, the d-band of Pt is broadened, which gives rise to a downshift in the d-band center on the PtSn surfaces. Consequently, the bonding strength of propyl and propylene on the alloyed surfaces is lowered. With the decomposition of the adsorption energy, the change in the surface deformation energy is predicted to be the dominant factor that determines the variation in the adsorption energy on the surface alloys, while on the bulk alloys the change in the binding energy makes a major contribution. The introduction of Sn lowers the energy barrier for propylene desorption and simultaneously increases the activation energy for propylene dehydrogenation, which has a positive effect on the selectivity toward propylene production. Considering the compromise between the catalytic activity and selectivity, the Pt 3 Sn bulk alloy is the best candidate for propane dehydrogenation.

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.