Sol–gel auto-combustion synthesis of Ni–CexZr1−xO2 catalysts for carbon dioxide reforming of methane

Abstract

Carbon dioxide reforming of methane (methane dry reforming) over Ni–Ce_0.8Zr_0.2O₂ catalysts prepared by a sol–gel auto-combustion method and a conventional co-precipitation method were comparatively studied. We show that sol–gel auto-combustion is very promising for preparing thermal stable homogeneous mixed metal oxide catalysts. The auto-combustion synthesized catalyst exhibited higher initial activity and stability due to its smaller Ni crystalline size and intimate interaction between Ni and Ce_0.8Zr_0.2O₂. In contrast, the co-precipitated catalyst showed poor activity and deactivated rapidly. The rapid deactivation was caused by a higher graphitization degree of the deposited carbon over co-precipitated catalyst with larger Ni crystalline size. We also found that the physico-chemical properties and catalytic activity of sol–gel auto-combustion synthesized catalysts were closely related to the metal nitrate (MN)/citric acid (CA) ratio. High MN/CA ratio led to more violent combustion behaviour and an accordingly higher degree of crystallization of the synthesized catalyst. In contrast, a low MN/CA ratio resulted in more carbon species residues and poor catalytic performance. The Ce/Zr ratio also had a profound influence on the phase structure, reducibility, oxygen vacancies and catalytic performance of Ni–Ce_0.8Zr_0.2O₂ catalysts. Ni–Ce_0.8Zr_0.2O₂ catalyst with cubic phase exhibited the best catalytic performance because of high reducibility, high Ni dispersion and strong Ni-Ce_xZr_1−xO₂ interaction, and considerable amounts of oxygen vacancies.