In this work we demonstrate that in the case of selective oxidation of CO cationic Au species on Au/CeO₂ catalysts are highly active. Catalysts having 0-1.3 Au wt. % were prepared by the deposition-precipitation method and were dried at 80^oC and calcined in air at 400^oC. Our results indicate that the activity depends on the Au content and on the reaction conditions used. The presence of Au as cationic species depends on the method of preparation and on the Au content. We observed only cationic species in the case of low Au content (≤ 0.4 wt %). Their presence correlated with the CeO₂ lattice expansion as observed by XRD and Raman spectroscopy. Catalysts having Au content around 0.8-1 wt% had both nanoparticles and cationic species. High Au content samples (≥ 1.3 wt %) had only nanoparticles and no CeO₂ lattice expansion was observed in this case. All our catalysts were highly active in the PROX reaction in the presence of 50% H₂ at 80-110^oC (GHSV = 12000 h^-1).

We found a correlation between specific rate and the lattice expansion of CeO₂ . The activity increased with lattice expansion. In particular, low-content catalysts were highly active and selective at 110^oC while high-content catalysts were less active and selective at 80^oC. Flytzani-Stephanopoulos et al. [1, 2] have reported that Au/CeO₂ having low Au content can be as effective as those with much higher loadings. They concluded that metallic Au nanoparticles are only spectators and that the minimum metal loading required for a desired activity may be determined from the ceria surface properties. However, the variation in the catalytic activity reported has been assigned to the lack of control in the generation of Au active sites [3]. We concluded that on CeO₂, catalysts having both cationic Au species and metallic nanoparticles are necessary to meet the desired PROX target. The effect of CO₂ and H₂O is also discussed.

References

[1]   Flytzani-Stephanopoulos et al Science 301 (2003) 935-938.

[2]   Q. Fu, W. Deng, H. Saltsburg, M. Flytzani-Stephanopoulus Appl. Cat. A 56(2005)57-68.

[3]   H.H. Kung, M.C. Kung, C.K. Costello, J. Catal. 216 (2003) 425.