Electronic structure, reactivity and solid-state chemistry of La2 –xSrxNi1 –yFeyO4 +δ

Abstract

Ceramic samples of La_{2 –x}Sr_xNi_{1 –y}Fe_yO_{4 +δ}, designed to model the ICI Hydecat catalyst, have been prepared via precipitation followed by high temperature annealing. The samples are characterized by X-ray diffraction, EDAX and atomic absorption spectrometry. The small changes in electronic structure as a function of x and y have been probed using photoemission. This reveals a progressive shift of the valence-band edge towards the Fermi level as a function of Sr-doping level, consistent with the approach to a metal to non-metal transition. Resonant photoemission at the Fe and Ni 3p thresholds is used to identify the transition metal contributions to the valence band density of states, and confirms a valence state of 3.0 for Fe. High resolution ESCA is used to quantify the surface composition of the samples, revealing Sr segregation consistent with that found in related perovskites and layered perovskites. Adsorption of CO onto the surfaces of these materials is seen to give rise to the ready production of a carbonate species. Water adsorption studies, which test the potential stability of the materials for possible applications in aqueous solution, are also presented.