Probing the hyperfine structure of Fe-based water-gas shift catalysts

Abstract

Hydrogen gas is an essential reagent in numerous industrial processes including ammonia synthesis. Ammonia is a key intermediate in the synthesis of nitrogen-based fertilisers, e.g. nitrates and urea. According to recent estimates (2008), approximately half of the world population is fed by nitrogen-based fertilisers of synthetic origin. Therefore, statistically speaking, every other person reading this sentence owes their existence to ammonia synthesis. Nowadays, most hydrogen gas is produced from natural gas via steam reforming followed by a dual stage water-gas shift reaction. The catalyst used in high-temperature water-gas shift (HTS) is chromium/copper promoted iron oxide. Chromium is known to stabilise the active iron-oxide phase magnetite (Fe3O4) from sintering and over-reduction to α-Fe and Fe-carbides, while copper enhances the activity by providing additional active sites. The chromium stabiliser has been used for over a century, because it provides excellent stability and its low cost. Chromium is added to the catalyst precursor via a co-precipitation/calcination route. An unintended side effect of calcination is that some of the chromium can oxidise to chromium-6, which is prone to strict handling and partial bans. The active magnetite phase has an inverse spinel structure composed of a 1:1:1 mixture of; tetrahedral Fe3+, octahedral Fe3+, and octahedral Fe2+, resulting in an octahedral Fe3+/Fe2+ redox couple. The active sites of the bulk magnetite catalyst are the surface octahedral Fe3+/Fe2+ redox couple. Rational design of catalysts with alternative dopants to chromium is severely hindered because of a poor understanding of chromium incorporation into the inverse spinel magnetite structure. Accordingly, the position of chromium and its effect on the magnetite structure and the octahedral Fe3+/Fe2+ redox couple was investigated in detail.....