Deactivation of nickel during gasification of activated carbon, studied by X-ray photoelectron spectroscopy

Abstract

It is shown that a quantitative interpretation of XPS intensity ratios of nickel/carbon, carbon-bonded oxygen/carbon and nickel-bonded oxygen/nickel, absolute XPS carbon intensities and XPS Ni(2p 3 2 ) and O(1s) binding energy patterns reveal insight in phenomena occurring during heat treatment and gasification of finely dispersed nickel on activated carbon. In the preparation of high nickel dispersion by deposition-precipitation via urea thermolysis, the homogeneity of the nickel distribution in the carbon is poor and depends on the nickel content. Decomposition of the precipitate, reduction of the metal oxide by carbon, migration of nickel particles, particle growth and reoxidation during gasification determine the catalytic action of nickel in gasification reactions of activated carbon with steam or hydrogen. Reoxidation of pyroforic metal species occurs already at room temperature in air. The quantitative interpretation of X-ray photoelectron spectroscopic results appears to be fairly independent of the assumed shape for estimating crystallite sizes but strongly depends on sample inhomogeneity. The influence of the initial catalyst loading, heat treatment and gasification on the homogeneity, oxygen content, dispersion and chemical state of the catalyst is discussed in terms of deactivation during gasification. A model is proposed to explain this deactivation by a hindered carbon diffusion through the metallic phase due to nickel carbon contact loss. The contact loss is caused by particle mobility as a consequence of heat treatment, leading to sintering and surface enrichment, and as a consequence of gasification, that besides heat treatment effects, also features a preferential removal of carbon in the vicinity of metal particles. This model is supported by X-ray diffraction and electron microscopic results.