High-temperature water gas shift catalyst based on nanodisperse, metastable, partially hydrated iron oxide—two-line ferrihydrite

Abstract

Two-line ferrihydrite (2L-FH) is a metastable, heavily disordered, partially hydrated Fe(III) oxide. The catalyst prepared by heat treatment of 2L-FH promoted with chromium ions (∼9 at %) and copper ions (4–7 at %) is much more active in the water gas shift (WGS) reaction at low temperatures (<350°C) than the conventional Fe-containing catalysts. According to XAFS spectroscopy data, the copper cations in 2L-FH are in the Cu²⁺ state and are in a tetragonally distorted octahedral environment, while under the WGS conditions at <350°C, copper is in the reduced state, specifically, in the form of ultrafine (<2 nm) Cu⁰ particles. It is due to these particles that the catalyst is very active below 350°C. Above 400°C, the Cu⁰ particles are deactivated under the reaction conditions and the catalytic activity is only due to iron active sites, whose number is proportional to the specific surface area of the catalyst. The specific activity of the catalyst at these temperatures is close to the activity of the conventional (hematite-based) WGS catalysts. The high activity of the 2L-FH-based catalyst at <350°C makes it possible to reduce the starting temperature of the adiabatic high-temperature WGS reactor.