Selective catalytic oxidation of CO: Effect of chloride on supported Au catalysts

Supported Au catalysts are generally prepared from chloride-containing Au precursors and their characteristics are highly sensitive to preparation procedures. To better understand and control the variables important in the preparation of these catalysts, the effect of chloride on the catalytic performances of highly active Au/Al₂O₃ catalysts in the selective catalytic oxidation of CO (SCO) was studied. The complex manner in which Cl- affected Au catalysis was deconvoluted in a series of complementary experiments where chloride was quantitatively removed and added. The residual chloride affected the activities in two different ways, i.e., it facilitated the agglomeration of Au particles during heat treatment and it inhibited the catalytic activity by poisoning the active site. Chloride poisoning was not only observed at the steady state, it was more pronounced in the transient toward steady state. To better assess the poisoning effect of Cl^-, phosphate was introduced to the support surface as a postsynthesis treatment before the addition of CI^- in some experiments. Activity suppression was observable at a Cl^-/Au molar ratio as low as 0.0006. Thus a very small fraction of gold was related to the active site. This implied that the activity per active site was extremely high.