RWGS reaction employing Ni/Mg(Al,Ni)O - The role of the O vacancies

The performance of Ni supported on Mg(Al)O catalysts was evaluated in the RWGS reaction. The catalysts were characterized by TPR, EPR, TPSR, XRD, XPS, cyclohexane dehydrogenation reaction, atomic absorption and N-2 physisorption. Two different catalytic systems were prepared by impregnation using water or ethanol as solvents of the Ni precursor. When ethanol was employed, Ni diffuses into the Mg(Al)O particles generating a homogeneous distribution of this metal through the catalyst particle leading to a Mg(Al,Ni)O solid solution, a very small amount of Ni and NiAl2O4. On the other hand, hydrotalcite is generated on the external layers of Mg(Al)O when water is the solvent. Nickel is trapped in the hydrotalcite structure. After calcination and reduction its concentration on the catalyst surface is higher than the one in the bulk. Both Mg(Al,Ni)O and Ni are observed on the catalyst surface. Oxygen vacancies were also observed for the two catalysts via EPR analyses due to the replacement of Mg by Al in the MgO lattice. The Mg(Al,Ni)O vacancies are active species for the reduction of CO2 to CO. These species and Ni are the catalytic sites of the RWGS reaction. Increasing the Ni concentration on the catalysts surface the activity of the catalysts increases as well. As far as we are concerned, this is the first time that Mg(Al)O or Mg(Al,Ni)O vacancies are associated with the performance of a catalytic system. This work exhibits that Ni/Mg(Al,Ni)O is a promising catalytic system for the RWGS reaction.