Low temperature carbon monoxide oxidation over gold supported on iron oxide catalysts.

Abstract

Catalysts consisting of ultra-fine gold particles supported on iron oxide have been synthesized by both coprecipitation and impregnation. Subsequent to preparation, each sample was calcined in air at temperatures ranging from 200 to $700\sp\circ\rm C.$ Results from steady state carbon monoxide oxidation over these catalysts show a significant variation in activity. Although certain coprecipitated gold/iron oxide catalysts exhibit a high initial activity for low temperature CO oxidation, their activity declined steadily upon extended exposure to an oxygen-rich reacting environment. The activity loss cannot be appreciably reversed by oxidation or humidification treatments. Several tests conducted to explore the deactivation mechanism have demonstrated that the activity loss does not appear to be due to such factors as carbonate formation, carbonaceous species buildup, or to a depletion of hydroxyl species on the iron oxide support. This prompted an examination of the structural and surface properties of both unreacted and reacted catalysts. Comparison of these catalysts before and after reaction, via X-ray photoelectron spectroscopy and X-ray diffraction, has revealed a depletion of gold on the surface as well as particle coarsening with reaction. A combination of various factors may have contributed to the activity loss. Proposed catalyst deactivation scenarios include particle coarsening (resulting in a loss of surface area), blocking of active sites by the formation of unreactive oxygen species, and/or a dislodgment of the active species through the production of significantly different molar volume species. Results from structural examination of these catalysts, employing transmission electron microscopy and X-ray diffraction, attest to the beneficial effect of a nanocrystalline material to enhanced CO oxidation.