A potential descriptor for the CO2 hydrogenation to CH4 over Al2O3 supported Ni and Ni-based alloy catalysts

Supported Ni,Cu, Fe, and Ni-M (M=Cu or Fe) catalysts of the same total metal loading and different Ni to M ratios are examined for the CO2 hydrogenation to CH4. Substituting Ni with Cu or Fe in a Ni/Al2O3 catalyst leads to smaller average particle size, sharper particle size distribution and alloy formation, which were determined from transmission electron microscopy (TEM) and X-ray diffraction (XRD). Surface properties of the Ni/Al2O3 and Ni-M/Al2O3 catalysts are found to be different as revealed by hydrogen temperature programmed desorption (H-2-TPD). Furthermore, the surface of the supported Ni-Cu catalyst was enriched with Cu. The catalytic activity increases for 75Ni25Fe/Al2O3 relative to Ni/Al2O3, but decreases for 75Ni25Cu/Al2O3. In both these catalysts, the Ni to M ratio was 3:1. The supported Ni and the two Ni3M alloy catalysts are modelled successfully as the shift in 20 between experimental and simulated XRD pattern were in accordance. Moreover, the trend in shifts in experimental and simulated optimized lattice constants relative to Ni was similar. Density functional theory (DFT) calculations revealed changes in the electronic properties. The electronic properties calculated based on the d-density of states (d-DOS) of surface Ni atoms are the d-band center (epsilon(d)) and number of d-DOS at Fermi level (N-EF). An excellent correlation between TOFCH4 and N-EF exists, whereas epsilon(d) did not satisfactorily capture the trends in TOFCH4. Thus, the N(EF)best describes the catalytic activity trend of the supported Ni and two Ni-based alloy catalysts. (C) 2017 Elsevier B.V. All rights reserved.