A study of CO2 hydrogenation over Ni-MgAlOx catalysts derived from hydrotalcite precursors

Abstract

Ni/Mg/Al mixed oxides have been prepared by decomposing corresponding layered double hydroxides of the hydrotalcite family. XRD, FTIR and UV–vis–NIR analyses show that prepared materials are constituted by a rock-salt type Mg1−xNixO solid solution with Al3+ in tetrahedral interstices of the cube close packing of oxide anions. When activated at sufficiently high temperature, they convert into optimal size supported Ni metal catalysts which show very active and selective for CO2 methanation. Catalytic data show that these materials are more active at low temperature than Ni/γ-Al2O3 catalysts, with a comparable enhanced activity with respect to Ni/La2O3-Al2O3 ones. A role of basic oxides as activating components can be envisaged and related to the strength of the adsorption of CO2 on the “support” likely forming surface (bi)carbonates as active species. Activation energies and reactions orders have been calculated by flow reactor studies in differential reactor conditions. IR spectroscopy data show that carbon dioxide adsorbs on the reduced catalyst in the form of hydroxycarbonates, that convert at higher temperature in carbonates and strongly adsorbed linear and bridging carbonyl species on metallic nickel. The formation of surface CHx species is also evident.