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

Abstract

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 CuFe2O4-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, CO2, H2, and water, both the CuO and the CuFe2O4-phases are reduced to metallic Cu at 250 °C; Cr is present as Cr3+ and is slowly incorporated in the Fe3O4 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 CuFe2O4. 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.

Kinetics of the high temperature water–gas shift over Fe2O3/ZrO2, Rh/ZrO2 and Rh/Fe2O3/ZrO2

“…CO + H 2 O ⇄ CO 2 + H 2 ; ∆H°= −41 kJ mol −1 (1) In industry the WGS reaction is carried out in two stages: the high temperature shift (623-773 K) using Fe 2 O 3 /Cr 2 O 3 /Cu catalyst, reducing the concentration of CO to typically around 2% and the low temperature shift (473-513 K) using CuO/ZnO/Al 2 O 3 catalyst, reducing the concentration of CO to ppm levels (typically <3000 ppm) [2,3]. Recently, from industry [4] and academia [5,6] work is reported to develop chromium-free high temperature shift catalysts because of environmental and health hazards to handle chromium.…”

“…2) often describes data well in the limited range of process conditions [8,9]. 2 represents the approach to equilibrium, p i the partial pressure of each component, K e is the WGS equilibrium constant and k is the apparent rate constant. However, to have more meaningful representation of the kinetic data, micro-kinetic models based on different WGS mechanisms have also been used [11,12,18,20].…”

In Situ UV-Visible Assessment of Iron-Based High-Temperature Water-Gas Shift Catalysts Promoted with Lanthana: An Extent of Reduction Study

“…EOR is defined as the fraction of iron that has been reduced from Fe 2 O 3 to either Fe 3 O 4 (in the second column of Table 1) or to metallic iron (in the third and fourth columns of Table 1) as quantified by the integrated hydrogen consumption from the TPR peaks. The listed values have been corrected by subtracting the small amount of hydrogen expected to be consumed by the other reducible catalyst components, CuO to Cu and Cr 6+ to Cr 3+ , which are reported to reduce near the temperature range of the first peak [40][41][42]. Hydrogen consumption increases with La content for the first reduction phase and is approximately the same for the second reduction phase (within the ±5% error for this experiment).…”

“…Two peaks were observed during the reduction of the catalysts with H 2 , in which the first peak corresponds to the reduction of Fe 2 O 3 to Fe 3 O 4 , CuO to Cu and possibly Cr 6+ to Cr 3+ , while the second peak is attributed to the reduction of Fe 3 O 4 to FeO and metallic Fe [40][41][42]. The second peak is much larger than the first peak because it requires 8 times more H 2 to reduce Fe 3 O 4 to Fe than to reduce Fe 2 O 3 to Fe 3 O 4 .…”

Revisiting the synthesis of Au/TiO2 P25 catalyst and application in the low temperature water–gas shift under realistic conditions

“…In industry the WGS reaction is carried out in two stages: the high temperature shift (623-773 K) using Fe 2 O 3 /Cr 2 O 3 /Cu catalyst, reducing the concentration of CO to typically around 2% and the low temperature shift (473-513 K) using CuO/ZnO/Al 2 O 3 catalyst, reducing the concentration of CO to ppm levels (typically < 3000 ppm) [41,42]. Sakurai et al [43] reported Au (3.3-10 wt.%)/TiO 2 P25 as a highly active low temperature WGS better than the commercial low temperature WGS catalyst CuO/ZnO/Al 2 O 3 .…”