The bi-functional mechanism of CH4 dry reforming over a Ni–CaO–ZrO2 catalyst: further evidence via the identification of the active sites and kinetic studies

A mesoporous Ni–CaO–ZrO2 catalyst which showed an excellent performance in the dry reforming of CH4 was thoroughly characterized by using a series of methods including N2 physical adsorption, temperature-programmed reduction (TPR), H2/CO chemisorptions, and so forth. Particularly, samples after different treatments such as calcination, reduction and different periods of reaction were subjected to X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis, by which changes in the phase structure and surface chemistry were followed. The results suggested that metallic Ni was gradually oxidized during the reaction, and a non-stoichiometric Ni–carbon compound was slowly formed. This latter species has a role as an important intermediate (or even active phase). Kinetic studies were then carried out based on these findings, according to which a Langmuir–Hinshelwood model was developed. Both the experimental results and the kinetic analysis provided novel evidence for the bi-functional mechanism of dry reforming over ZrO2-based catalysts.