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.

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.

We demonstrate an unprecedented H2 generation activity in the hydrolytic dehydrogenation of ammonia borane over acid oxidation- and subsequent high temperature-treated CNT immobilized Pt nanocatalysts to combine the merits of defect-rich and oxygen group-deficient surfaces and unique textural properties of supports as well as optimum particle size of Pt.

Highly dispersed bimetallic Pd-In catalysts on Al 2 O 3 were prepared by a simple impregnation method. In comparison with the unsupported intermetallic catalyst, the supported Pd-In catalyst exhibited several magnitudes higher activity and similar selectivity for selective acetylene hydrogenation. Moreover, the activity, selectivity, and anticoking performance of the Pd-In catalyst were superior to those of the monometallic Pd catalyst. The electron transferred from indium weakened the adsorption of ethylene on the negatively charged Pd sites and hence improved the selectivity of Pd-In/Al 2 O 3 . The inhibited formation of hydride due to the presence of indium also contributed to the higher selectivity. The promoted activation of hydrogen, owing to the weak adsorption of acetylene on Pd-In/Al 2 O 3 , and decreased particle size jointly contributed to the enhanced activity of Pd-In/Al 2 O 3 . In addition, green oil formation on Pd-In/Al 2 O 3 was retarded by the presence of indium, contributing to the enhanced stability of the catalyst. The bimetallic Pd-In catalysts showed a strongly composition dependent performance, which resulted from the different extent of electronic and/or geometric modification of Pd active sites.

Au/TS-1 catalysts prepared by deposition-precipitation method are very promising for direct propylene epoxidation with H2 and O2. However, the catalysts usually suffer from rapid deactivation. In this work, calcined TS-1 with open micropores (TS-1-O) is first used to support Au catalysts, and then the used catalysts at different time-on-streams are characterized to understand the deactivation mechanism.The micropore blocking by carbonaceous deposits is found to be responsible for the deactivation. We therefore suggest a principle of catalyst design to improve the long term stability by depositing Au nanoparticles on the external surfaces of TS-1. For this purpose, uncalcined TS-1 with blocked micropores (TS-1-B) is used to support Au catalyst. As expected, the designed catalyst is not only very stable because of the elimination of pore blocking and the more accessible active sites, but also highly active with the PO formation rate of 125 gPOh -1 kgCat -1 for over 30 hours.

Oxygen evolution from water poses a significant challenge in solar fuel production because it requires an efficient catalyst to bridge the one-electron photon capture process with the four-electron oxygen evolution reaction (OER). Here, a new strategy was developed to synthesize nonsupported ultrasmall cobalt oxide nanocubanes through an in situ phase transformation mechanism using a layered Co(OH)(OCH3) precursor. Under sonication, the precursor was exfoliated and transformed into cobalt oxide nanocubanes in the presence of NaHCO3-Na2SiF6 buffer solution. The resulting cobalt catalyst with an average particle size less than 2 nm exhibited a turnover frequency of 0.023 per second per cobalt in photocatalytic water oxidation. X-ray absorption results suggested a unique nanocubane structure, where 13 cobalt atoms fully coordinated with oxygen in an octahedral arrangement to form 8 Co4O4 cubanes, which may be responsible for the exceptionally high OER activity.