Density Functional Theory Study Of Selective Hydrogenation Of Acetylene On Ni-based And Pd-based Catalyst

Selective acetylene hydrogenation in the presence of ethylene is a very important process in polyethylene industry. First-principle calculations based on density functional theory have been performed to investigate the hydrogenation of acetylene on both clean and C-pre-adsorbed (both on the surface and in the subsurface) Ni(111), Pd(111), NiZn(110), and PdAg(111) surfaces. The effects of both promoters and the presence of C on the catalytic performance of different surfaces are examined. Furthermore, the effect of the promoters on the diffusion of C and H from the surface to the subsurface and the cleavage of C-C bonds are also investigated.At the same surface coverage, Pd(111) is found to have both higher activity and selectivity for the hydrogenation of acetylene than Ni(111). The hydrogenation of acetylene to ethane via the ethylene pathway and the hydrogenation of acetylene to ethane via the ethylidene pathway are kinetically more favorable on both Ni(111) and Pd(111). As a result, the deep hydrogenation of acetylene to ethane can not be hindered on both of these two surfaces. In this aspect, the selectivity of acetylene hydrogenation toward ethylene on both Ni(lll) and Pd(111) surfaces are very low. The selectivity toward ethylene together with the catalytic activity for acetylene hydrogenation was significantly increased by the introduction of Ag and Zn. The diffusion of H from the surface to the subsurface was inhibited by the introduction of the two promoters. The diffusion of C from the surface to the subsurface was inhibited by the introduction of the Ag into Pd. In contrast, the diffusion of C from from the surface to the subsurface was promoted by the presence of Zn. Furthermore, the activation barrier for C-C bond cleavage on the clean surfaces, ranked in descending order, is as follows:PdAg(111) NiZn(110)> Ni(111)> Pd(111).The selectivity toward ethylene on Ni(111), Pd(111), NiZn(110), and PdAg(111) is increased in the presence of C in the subsurface. The selectivities of the four surfaces are always increased in the order:PdAg(111)> NiZn(110)> Pd(111)> Ni(111), regardless of the presence of surface C and subsurface C. The activities of Ni(111) and NiZn(110) surfaces are decreased significantly in the presence of subsurface C. In contrast, the activities of Pd(111) and PdAg(111) surfaces are increased in the presence of subsurface C. The activities of the clean surfaces are in the descending order:PdAg(111)> NiZn(110) Pd(111)> Ni(111). whereas it changes to PdAg(111)> Pd(111)> Ni(111) NiZn(110) with the presence of subsurface C...