Temperature-dependent anti-coking behaviors of highly stable Ni-CaO-ZrO2 nanocomposite catalysts for CO2 reforming of methane

Abstract

Coking is one of the main obstacles to the application of Ni-based catalysts for CO2 (dry) reforming of methane (DRM). In this work, the anti-coking behaviors of a highly stable Ni-CaO-ZrO2 nanocomposite catalyst were systemically studied by structure characterizations, activity tests and thermodynamic calculations under different temperature. The results revealed that the structures and anti-coking properties of the nanocomposite catalyst primarily depended on reaction temperature. The in-situ XRD, H2 chemisorption and TEM analysis of the catalysts indicated that only slightly sintering of Ni and ZrO2 took place at a lower reduction and reaction temperature (750°C). At a higher reduction and reaction temperature (850°C), severe sintering of ZrO2 and Ni was observed. However, it is interesting to find that the formed large nickel particles were almost covered by ZrO2 support after the high temperature reduction and only exposed very small nickel size. Coking analysis revealed that although a small Ni size could kinetically inhibit the carbon formation to some extent, a relatively higher amount of coking was formed at 750°C due to the strong thermodynamic tendency for coking. At 850°C, the amount of carbon deposition was much lower than that at 750°C, which was mainly attributed to the unfavorable coking thermodynamics and the small exposed size of Ni. Therefore, high reaction temperature resulted in both the thermodynamic and kinetic coking inhibition behaviors and contributed to the superior anti-coking performance of the nanocomposite catalyst. It is our belief that the results obtained in this work could provide valuable implications for optimizing catalyst structure and reaction condition to achieve coking-free DRM catalysts and processes.