Periodic Density Functional Theory Study Of Propane Dehydrogenation Over Perfect Ga₂O₃ (100) Surface

Given the ever-increasing worldwide demand for olefins, alternative inexpensive ways to produce light olefins in industry are highly desired. Direct dehydrogenation of propane is an endothermic process which requires relatively high temperatures to obtain a high yield of propene. However, increasing temperatures would favor thermal cracking of light alkanes to carbon deposition and always lead to decrease of the product yield. Hence, the development of highly efficient catalyst has been of interest recently. Oxidative dehydrogenation of light alkanes has been a research topic of consistent interest from 1970s. The oxidants include O₂, CO₂, NO₂ and metal oxides, etc. In the presence or absence of CO₂, a series of metal-oxide catalysts have been studied in the dehydrogenation of alkanes, and among them Ga₂O₃ is a highly effective one. However, the activity of the Ga₂O₃ is greatly affected by the support oxides. In addition, the role of CO₂ can improve or suppress the activity of different catalyst. At the same time, the acidity or basicity of the supports also results in different catalytic activity and selectivity. Thus, study of the oxidative property and surface acidity of the catalysts has great effect on the activation and transformation mechanism of alkanes.Propane dehydrogenation over perfect Ga₂O₃(100) was studied in detail by periodic density functional theory (DFT) calculations. The Ga₂O₃(100) surface is composed of two inequivalent Ga sites and three inequivalent O sites, and they are the twofold coordinated O (2), threefold coordinated O(3), fourfold coordinated O (4), tetrahedral gallium Ga(t), and unsaturated gallium Ga(o) sites. It is expected that unsaturated sites are more active than saturated ones, and thus Ga(t) and 0(4) sites have little activity. Hence, we focus on the Ga(o) (named Ga without specific mentioning hereafter), O(2), and O(3) active sites on the Ga₂O₃(100) surface. We have firstly examined the molecular absorption of propane on various active sites. The potential energy surface for the adsorption is found to be quite flat. The most favorable adsorption site for propane is near the unsaturated Ga site. It was found that the initial C-H bond activation mainly follows a radical mechanism that O(2) abstracts a hydrogen atom from propane with the formation of propyl radical and hydroxyl group (O(2)H). Physically adsorbed propyl radical can easily form propoxide or propyl gallium intermediate. Subsequently, propene is formed by a subsequent H abstraction from propyl, propoxide or propyl gallium by surface oxygen and Ga sites. H abstraction by O(2) site always has low energy barrier. However, it is difficult for the hydrogen atoms in the hydroxyl groups to leave the surface in the form of either H₂ or H₂O. After oxygen sites are covered by H atoms, the activity of catalyst will decrease quickly, and this is in accordance with the experiment. In addition, propene formed through H abstraction by oxygen site has high adsorption energy and is prone to further dehydrogenation or oligomerization, leading to fast deactivation of the catalyst. Interestingly, there is a low-energy pathway for the formation of propene through H abstraction from physically adsorbed propyl by surface 0(2)H groups. However, the adsorbed H₂O is strongly bonded to the surface by two O-Ga bonds and is difficult to desorb from the surface. At high temperatures, however, it is possible for H₂O to desorb from the surface, creating oxygen vacancies and leading to the reduction of the surface. On the other hand, the formation of H₂ from GaH and hydroxyl group is much easier, although the formation of GaH has to overcome high energy barrier. Thus there is a shift of rate-determining step for propane dehydrogenation: At the initial stage of the reaction the rate-determining step is H abstraction by oxygen sites and then it shifts to H abstraction from various propyl sources by Ga sites to form gallium hydrides after the surface oxygen sites are consumed. Our results also indicate that dehydrogenation of propane mainly follows a direct dehydrogenation mechanism (DDH), whereas oxidative dehydrogenation (ODH) is energetically less feasible but cannot be ruled out in the presence of mild oxidant such as CO₂.In order to explain the growth mechanism of five-fold-twinned crystals, we calculated the surface-energies the (100) and (111) facets of noble metals, such as Ag, Au, Pd, Pt, before and after the adsorption of a-pyrrolidone. The results indicate that the surface energy difference between the (100) and (111) facets (Δ_(100)-(111)) plays an essential role in the aggressive growth of lateral (100) facets, overcoming the strain restriction. When enough PVP is adsorbed on the surfaces of Ag crystals,Δ_(100)-(111) will evidently decrease, and then the strain restriction on the lateral growth becomes dominant, resulting in pentagonal anisotropic structures.