(560f) Pathways for Reaction in CO Oxidation Over Phosphate Supported Au Catalysts
Early work in catalysis by Au suggested that the use of reducible supports could lead to more active catalysts. Although the differences in activities for reducible supports have been noted and their role in activating oxygen have been suggested[2,3] there is little clear evidence that a Mars van Krevelen (redox) reaction mechanism plays a role in low temperature CO oxidation reactions. By proper synthesis, high activity can be achieved in Au catalysts supported on non-reducible supports. There are fewer investigations on non-oxide supported Au catalysts.
We have explored metal phosphates (LaPO4, FePO4, AlPO4, and others) as supports for Au catalysts. Au nanoparticles supported on certain metal phosphates showed not only high activity for CO oxidation and good thermal stability. The nature of Au species on an Au/FePO4 catalyst after oxidative and reductive pretreatments as well as their role in rt CO oxidation have been investigated using gas transient and operando DRIFTS-QMS and in situ Raman spectroscopy. Oxidative pretreatment (in O2) leads to cationic Au species that can be reduced by CO at rt. In situ reduction of the cationic Au during CO oxidation generates metallic Au that is active for CO oxidation. Reductive pretreatment (in H2) leads to metallic Au species that are more active for CO oxidation than those on an oxidatively pretreated catalyst.
CO oxidation results demonstrate that CO can be oxidized by structural O of FePO4 and that O2 can be activated by the thusly reduced support. Au assists in and is essential for both the utilization of structural oxygen and the activation of O2. Isotope studies using 18O2 demonstrate that this redox pathway occurs in competition with direct reaction (e.g. Langmuir-Hinshelwood or Eley-Rideal) catalyzed by metallic Au. To our knowledge, this is the first clear evidence of a redox or Mars-van-Krevelen pathway playing a role in CO oxidation by Au catalysts.
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(2) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. Journal of Catalysis 2001, 197, 113.
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(5) Ma, Z.; Yin, H.; Overbury, S. H.; Dai, S. Catalysis Letters 2008, 126, 20.
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