The effect of impregnation sequence on the hydrogenation activity and selectivity of supported Pt/Ni bimetallic catalysts

The effect of impregnation sequence on the formation of Pt/Ni bimetallic nanoparticles supported on γ-Al₂O₃ was investigated for catalysts with Pt/Ni atomic ratios of 3/1 and 1/1. These bimetallic catalysts were prepared with a fixed Pt loading (5 wt.%) by incipient impregnation of one metal precursor and calcination, followed by the impregnation of the second metal precursor and a second calcinations step. The disproportionation activity of cyclohexene and the hydrogenation selectivity of acetylene in ethylene were used as probe reactions to compare the effect of the impregnation sequence. The bimetallic Pt/Ni catalysts showed significantly higher activity toward the disproportionation of cyclohexene than either Pt/γ-Al₂O₃ or Ni/γ-Al₂O₃. For catalysts with the Pt/Ni ratio of 3/1, the Pt-first catalyst, 1/3Ni-1Pt/γ-Al₂O₃, showed higher activity than the Ni-first catalyst, 1Pt-1/3Ni/γ-Al₂O₃. The effect of impregnation sequence was not as significant for catalysts with the Pt/Ni ratio of 1/1. In addition, kinetic analysis of the selective hydrogenation of acetylene in ethylene revealed an increase in the acetylene hydrogenation selectivity for 1/3Ni-1Pt/γ-Al₂O₃ as compared to both 1Pt-1/3Ni/γ-Al₂O₃ and monometallic 1Pt/γ-Al₂O₃. Extended X-ray absorption fine structure (EXAFS) measurements of the Pt L_III edge indicated that Pt{single bond}Ni bimetallic bonds were formed in 1/3Ni-1Pt/γ-Al₂O₃ but not in 1Pt-1/3Ni/γ-Al₂O₃. The Pt{single bond}Ni bonds were formed for the 1/1 ratio catalysts with both impregnation sequences. Overall, our results indicate that, at low Ni loadings, the impregnation sequence has a significant effect on the formation of Pt{single bond}Ni bimetallic bonds, which in turn lead to different catalytic behavior for both the disproportionation of cyclohexene and the selective hydrogenation of acetylene in ethylene.