(112l) The High Temperature Water-Gas Shift Over CrO3/Fe2O3 Catalysts

The high temperature water-gas shift reaction is commercially conducted over Cr₂O₃*Fe₂O₃ (~10% CrO₃) catalysts. A series of model supported CrO₃/Fe₂O₃ catalysts were prepared by aqueous impregnation of Cr-nitrate onto a Fe₂O₃ support and subsequent calcination at 350 °C. The molecular structures and surface chemistry of the supported CrO₃/Fe₂O₃ catalysts (1, 2, 3, 5, 7 and 9% CrO₃) were investigated by combined in situ Raman and IR spectroscopy under dehydrated conditions. The in situ Raman and IR studies revealed that surface CrOx species are present as a two-dimensional monolayer on the Fe₂O₃ support. Crystalline Cr₂O₃ nanoparticles, however, are not formed above monolayer coverage (~3% CrO₃/Fe₂O₃) as is normally the case. It appears that the excess Cr₂O₃ formed a solid solution with the isostructural Fe₂O₃ support. The surface chemistry of these catalysts were chemically probed with IR spectroscopy of adsorbed CO, CO₂ and HCOOH monolayers. The surface HCOO* is frequently invoked as a reaction intermediate for the WGS reaction. The supported CrO₃/Fe₂O₃ catalysts were found to weakly interact with CO. Analysis of the OH region revealed the formation of acidic surface OH groups in the CrOx monolayer. The surface chemistry was further probed with in situ IR studies of the more reactive CO₂ and HCOOH molecules. The IR analysis showed that surface HCO₃-* and HCOO* are present and their reactivity was a function of the catalyst composition. The steady-state WGS shift reaction was investigated at high temperatures with operando IR spectroscopy (simultaneous spectroscopic and reaction conversion analysis). These fundamental experiments provide new insights into the WGS reaction mechanism over supported CrO₃/Fe₂O₃ catalysts that will be elaborated upon.