Deoxygenation of methyl laurate as a model compound on Ni-Zn alloy and intermetallic compound catalysts: Geometric and electronic effects of oxophilic Zn

Abstract

A series of Ni-Zn bimetallic catalysts with different Ni/Zn atomic ratios were prepared from layered double hydroxides (LDHs) with an atomic ratio of 3 between divalent (Ni2+ and Zn2+) and trivalent ions (Al3+). After the mixed oxides derived from calcined LDHs were reduced with H2 at 650°C, Ni-rich alloy with a fcc structure was synthesized at Ni/Zn ratios≥2, while an Ni-Zn intermetallic compound (IMC) with a tetragonal L10 structure was generated at Ni/Zn atomic ratios of 1 and 1/2, and an Ni-Zn IMC was generated with a cubic structure at Ni/Zn ratio of 1/8. HAADF-STEM-EDS, H2 chemisorption and magnetic measurements showed that Ni and Zn atoms were uniformly distributed in catalysts, and the ensembles of Ni atoms decreased with decreasing Ni/Zn ratios. XPS and CO-TPD characterizations revealed a charge transfer from Ni to Zn. In the deoxygenation of methyl laurate as a model compound to diesel-like hydrocarbons, although the addition of Zn reduced the conversion of methyl laurate, mostly due to the decrease in Ni content, a synergetic effect between Ni and Zn was suggested to enhance turnover frequency (TOF), and higher TOFs were obtained on the catalysts with Ni/Zn ratios of 1/1 and 1/2. Because of the high oxophilicity of Zn, this synergetic effect also promoted the hydrodeoxygenation pathway. With decreasing Ni/Zn ratios, the molar ratio between C11 hydrocarbons and both C12 hydrocarbons and oxygenates decreased. Particularly, it was smaller than 0.2 on Ni-Zn IMC when the reaction temperature was 330°C, and much lower than that of 60.5 on the metallic Ni catalyst. Compared with metallic Ni, Ni-rich alloys and IMCs (especially IMCs) showed lower reactivity for CC bond hydrogenolysis and CO/CO2 methanation, which was more remarkable with decreasing Ni/Zn atomic ratios and increasing reaction temperatures. Particularly, when the conversion of methyl laurate was close to 100% at 400°C, the metallic Ni catalyst dominatingly gave methane, which was derived from CC bond hydrogenolysis as well as complete methanation of CO/CO2, and the total selectivity to C11 and C12 (i.e., SC11+C12) was only 1.1%. However, when the Ni/Zn atomic ratio was≤2/1, SC11+C12 reached as high as 96.2%, and the molar ratio between (CO+CO2) and C11 hydrocarbons was still larger than 1.0, i.e., CC bond hydrogenolysis and methanation were remarkably suppressed on Ni-Zn IMC catalysts. We suggest that the role of Zn is ascribed to its geometric and electronic modification of Ni in the alloy and IMC.