Soft X-Ray Absorption Spectroscopy of Catalysts and Related Materials

An in situ investigation has been performed on a series of oxide based desulfurization absorbents and sulfur poisoned heterogeneous catalysts using the technique of sulfur K-edge Extended X-ray Absorption Fine Structure (EXAFS). The work has been complemented by the characterisation of standard sulfur containing materials using EXAFS, X-ray Photoelectron Spectroscopy (XPS) and Powder X-ray Diffraction (XRD). Using a model reactor, a zinc oxide based commercial absorbent (ICI 32-4) was treated with hydrogen sulfide at 370°C for several hours. Characterisation of these ex situ treated materials using powder XRD revealed two crystalline phases for each sample consisting of zinc oxide (wurtzite) and zinc sulfide (wurtzite) as well as a trace of calcium carbonate (aragonite). XPS analysis of the same series of samples confirmed the presence of sulfide. Analysis of the peak areas, after deconvolution of the oxygen 1s peak indicated a detectable sulfur profile which was maximum just after the midpoint of the reactor. Zinc K-edge EXAFS analysis of the materials yielded spectra similar to zinc oxide (wurtzite) but with additional peaks in the Fourier transform which were modelled to sulfur at 2.31 Å and 3.93 Å. Refinement of sulfur K-edge EXAFS confirmed the sulfide phase. In situ sulfur K-edge X-ray Absorption Spectroscopy (XAS) observations on the reaction of zinc oxide with hydrogen sulfide, using a specially constructed environmental cell, revealed a change in the local environment surrounding the sulfur atom as the reaction progressed. Analysis of the sulfur X-ray Absorption Near Edge Structure (XANES) region indicated a smooth transition occurred from chemisorbed hydrogen sulfide to zinc sulfide as the reaction progressed. Data obtained on the reaction was consistent with a mechanism of formation involving surface 2-dimensional islands of zinc sulfide which grow from a small number of zinc nucleating centres, to form 3-dimensional areas of bulk zinc sulfide. Nickel oxide and cobalt oxide were found to react with hydrogen sulfide by a similar mechanism forming the sulfides Ni3S2 and CoS, which was then further oxidized to cobalt sulfate.