Selective reduction of diolefins over the metal catalysts surface-modified by UHV treatment

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In this study, the influence of surface structure has been investigated on the activity and selectivity of hydrogenation of 1,3-butadiene. Metal and metal oxide catalysts were thermally treated under ultrahigh vacuum (UHV<10^<-7> Pa) condition (the UHV treatment). The hydrogenation activity of 1,3-butadiene was markedly enhanced over supported Pd catalysts by the UHV treatment. The selectivity of 1-butene from 1,3-butadiene was increased by the UHV treatment. Thermodynamically, 2-butenes are more stable than 1-butene. The higher selectivity of 1-butene from 1,3-butadiene over the UHV-treated Pd catalysts strongly suggests that 1,2-addition of 1,3-butadiene proceeded preferentially at the initial stage of the hydrogenation. For the formation of 2-butenes, 1,4-addition of 1,3-butadiene was requested. The adsorption intermediate for 1,4-addition requires a more flat surface of Pd, like (111) basal plane. On a roughened surface, much more 1,2-addition will be expected than 1,4-addition. The high selectivity of 1-butene on the UHV-treated Pd catalysts indicates that the surface of Pd was roughened by the UHV treatment. These results show that the UHV treatment can modified the surface structure of metals and influenced the selectivity of terminal olefins. The activity on ZnO catalysts was also significantly enhanced by the UHV treatment. It is clearly indicated that the UHV treatment was also effective for the surface modification of the metal oxide catalysts.