Periodic Density Functional Theory Study of Propane Dehydrogenation over Perfect Ga2O3(100)Surface

摘要

Propane dehydrogenation over perfect Ga2O3(100)was studied in detail by periodic density functional theory(DFT)calculations.It was found that the initial C-H bond activation mainly follows a radical mechanism that the two-coordinated surface oxygen site(0(2))abstracts a hydrogen atom from propane with the formation of propyl radical and hydroxyl group(0(2)H).Physically adsorbed propyl radical can easily form propoxide or propylgallium intermediate.Subsequently,propene is formed by a second H abstraction from propyl,propoxide,or propylgallium 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.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.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.