High loading Ni/MgO catalysts. Surface characterization by IR spectra of adsorbed CO

The influence of calcination and reduction conditions on the surface properties of high loading Ni/MgO catalysts (18 wt% Ni) was studied. IR spectra of adsorbed CO show that the increase of the calcination temperature decreases the reducibilty of the Ni phase as diffusion of Ni²⁺ into the bulk of MgO particles is enhanced. CO adsorption (10 Torr) produces various carbonyls (mono- and poly-carbonyls, linear and bridged), the distribution of the different types depending on pretreatment conditions. Disruption of metal Ni particles by CO attack on edge and corner sites produces tetra-carbonyl species subsequently stabilized on Mg²⁺O^2- sites. Disproportionation of CO is also observed, monitored by the appearance of bands due to carbonate-like species as a consequence of the adsorption of CO₂ on the basic matrix. The activity for the Boudouard reaction decreases upon pretreatments at the highest temperature as the smoothing of metal particles reduces the concentration of Ni sites in the lowest coordination, which promote CO dissociation.