The High Temperature Water-Gas Shift Over CrO3/Fe2O3 Catalysts

Christopher Keturakis1, Israel E. Wachs1, and Marco Daturi2. (1) Department of Chemical Engineering, Lehigh University, 7 Asa Drive - Sinclair Laboratory, Bethlehem, PA 18015, (2) Laboratoire Catalyse et Spectrochimie, UMR 6506 CNRS/ENSICAEN et Université de Caen Basse-Normandie, 6, Bd du Marechal Juin, 14050 Caen Cedex, France

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, H2O, HCOOH and CH3OH 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 and CO2, and surface carbonates were not present. Analysis of the OH region revealed the formation of acidic surface OH groups in the CrOx monolayer. The presence of H2O at room temperature resulted in solvation of the surface CrOx species and most of the water desorbed above 150 °C. The surface chemistry was further probed with in situ IR studies of the more reactive CH3OH and HCOOH molecules. The IR analysis showed that surface CH3O* 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.