The tendency of supports to generate oxygen vacancies and the catalytic performance of Ni/ZrO2 and Ni/Mg(Al)O in CO2 methanation

Abstract

Ni/ZrO₂, Ni/Mg(Al)O, and Ni/SiO₂ catalysts were employed in CO₂ methanation. The catalysts were characterized by XPS, XRF, XRD (Rietveld refinement method), TPR, EPR, BET, CO₂ + H₂-TPSR, CO + H₂-TPSR, CO₂-TPD, CO-TPD, S/TEM-XEDS, and DFT calculations. The catalytic performance of these catalysts in CO₂ methanation was analyzed employing a conventional microreactor. CO₂ consumption and formation rates of the products were obtained under differential conditions. The Ni/ZrO₂ catalyst exhibited the highest activity and selectivity for the methanation of CO₂ compared to Ni/Mg(Al)O and Ni/SiO₂. The Mg-based catalyst reaches the Ni/ZrO₂ performance at high temperatures. Some authors have proposed that CO rupture or formate decomposition (the rate-limiting step of CO₂ methanation) occurs on pairs of oxygen vacancy–cus sites. DFT calculations showed that oxygen vacancies improve CO adsorption and dissociation over the Ni/ZrO₂ catalyst. For Ni/ZrO₂, the vacancies are generated by the insertion of Ni in the ZrO₂ lattice (DRX) and by the defects of ZrO₂ oxide (EPR), whereas Mg(Al)O shows intrinsic vacancies (EPR). Ni/ZrO₂ exhibits the same metallic area compared with Ni/Mg(Al)O. However, the former is more active and selective than Ni/Mg(Al)O at a large temperature range, showing that H₂ dissociation is not the rate-limiting step. Indeed, the Zr-based catalyst generates H₂O or eliminates oxygen, generating vacancies at much lower temperatures than Ni/Mg(Al)O (CO₂ + H₂-TPSR and CO + H₂-TPSR). However, at higher temperatures (>400 °C), the catalytic behavior of these two catalysts is similar. Indeed, the elimination of H₂O and oxygen vacancy formation are easier for both catalysts under these conditions. Thus, this work shows that the catalytic performance of Ni-based catalysts is associated with the support's facility to release O (reducibility).