The Cu Promoter in an Iron−Chromium−Oxide Based Water−Gas Shift Catalyst under Industrial Conditions Studied by in-Situ XAFS

The chemical state of the Cu promoter (1 wt %) in a Cu-promoted Cr-stabilized iron oxide based water−gas shift catalyst has been studied by in-situ X-ray absorption fine structure (XAFS) at 380 °C and elevated pressures, which is close to the conditions typically applied in industrial processes. The Cu promoter enhances the activity of the catalyst. In this study, the catalyst has been studied as fresh, after use for 2000 h, and after a further wash with an aqueous solution of ammonia. In all three cases, CuO and CuFe 2 O 4 -like phases are present; the used catalyst also contains some metallic Cu. X-ray absorption spectroscopy (XAS) shows that upon exposure to a typical feed gas for high-temperature shift containing CO, CO 2, H 2, and water, both the CuO and the CuFe 2 O 4 -phases are reduced to metallic Cu at 250 °C; Cr is present as Cr 3+ and is slowly incorporated in the Fe 3 O 4 matrix upon further heating to 380 °C and pressurizing to 12 or 24 bar. Washing a used catalyst with an aqueous solution of ammonia primarily removes the metallic Cu but also some CuO and CuFe 2 O 4 . Despite the fact that the Cu content in the catalyst after washing with ammonia varies, the activity is the same as that for the unwashed and fresh catalysts. This indicates that only a smaller part, estimated to <0.1 wt %, of the Cu is required to obtain the full promoter effect. These results indicate that the active sites for the water−gas shift reaction are provided by the magnetite surface and are promoted by metallic Cu.