The origin of rhodium promotion of Fe3O4–Cr2O3 catalysts for the high-temperature water–gas shift reaction

Rhodium promotion of iron–chromium oxide (Fe–Cr) catalysts for the water–gas shift (WGS) reaction was investigated through measurement of the rates of individual reduction and reoxidation steps. The temperature-programmed reduction of the starting Fe2O3 form by H2 is accelerated by rhodium, with the extent of oxygen removal at 450 °C corresponding to Fe3O4 formation. The corresponding reduction by CO proceeds further, in agreement with bulk thermodynamic expectations. However, rhodium has no effect on the oxygen removal rate, although it leads to substantially increased carbon deposition. The behavior with respect to reoxidation of reduced catalysts is somewhat similar. Rhodium greatly enhances H2 release during reoxidation by water, presumably by recombining hydrogen atoms transferred from oxide to metal by reverse spillover. The rate of H2 evolution from the Rh/Fe–Cr system at 280 °C is greater than that from Fe–Cr at 380 °C. Reoxidation by CO2 is not promoted by Rh and is much slower than that by H2O for the Rh-containing catalyst. Likewise, reoxidation by H2O/CO2 mixtures results in a much higher yield of H2 relative to CO for the promoted catalyst compared with the unpromoted one. The extent to which catalysts previously equilibrated in WGS feeds at 350 °C can then be reduced in pulses of CO or H2 depends on the composition of the WGS feed. Fe–Cr catalysts do not reduce in H2 after exposure to H2O/CO (2:1) or a reformate (H2O/CO/H2/CO2) feed. Hydrogen reduction is possible with Rh/Fe–Cr after exposure to H2O/CO but not after exposure to reformate, for which thermodynamic considerations suggest that the working oxidation state may be lower. The greater reducing power of CO leads to continuing reduction after treatment with both prior feeds. Overall, it is concluded that of the two steps that may restrict the rate of the WGS reaction over iron–chromium oxide catalysts (reduction by CO and H2 generation through reoxidation by water), rhodium acts primarily by accelerating the second.