The state of BEA zeolite supported nickel catalysts in CO2 methanation reaction

Abstract

Two model nickel catalysts for CO2 methanation reaction were prepared by the impregnation method using unmodified and dealuminated BEA-type zeolite supports, denoted NiHAlBEA and NiSiBEA. Transmission electron microscopy (TEM) and X-ray diffraction studies evidenced formation of metallic nickel crystallites of different size, partially located on the external surface of zeolite grains in of NiHAlBEA catalyst and small nickel nanoparticles uniformly distributed within the zeolite grains in the NiSiBEA catalyst. Higher CO2 conversion and selectivity towards methane at low reaction temperatures were observed for NiSiBEA catalyst. This catalyst showed an improved resistance towards sintering at high reaction temperatures. The decrease of CO2 conversion with the time on stream, accompanied by the drop in methane selectivity in NiHAlBEA catalyst was attributed to the increase in Ni crystallite size, resulted from the migration of small crystallites from the inner to external surface of the zeolite grains and subsequent agglomeration. Diffuse reflectance infrared Fourier transform, and quasi in-situ X-ray photoelectron spectroscopy studies evidenced the presence of metallic nickel crystallites in the catalysts and their partial reoxidation under CO2 methanation reaction conditions. High nickel dispersion and sintering resistance of NiSiBEA led to the retardation of the deactivation of catalyst by H2S.