Influence of the partner oxide on the catalytic properties of Au/CexZr1−x highly loaded gold catalysts

Abstract

Since the ability to provide lattice oxygen should be an important factor dictating the catalytic activity of supported gold catalysts, we synthesized, simply by oxidation of the primary AuCe, AuZr, AuCeZr alloys, intimate mixtures of nanostructured Au with CeO2, ZrO2 or Ce–Zr mixed oxides which have unique redox properties and high mobility of lattice oxygen. This makes it possible to accurately study the influence of the support identity on the catalytic activity. A detailed investigation of the preferential oxidation of CO (PrOx) in the presence of H2 was undertaken also with the oxidation of CO and the oxidation of H2. For CO oxidation, it immediately appears that the nature of the oxide affects significantly the activity of gold, ceria being the best whereas for H2 oxidation no or only slight support effect has been observed. Catalytic performances in the PROX reaction were compared with those in the oxidation of CO. A change in the conversion of CO was observed in the presence of H2 at low temperature for all system, the effect depending on the support identity. A boost is clearly observed on Au/ZrO2 whereas the samples containing ceria present an opposite effect: when H2 is introduced in the reactive mixture, the CO conversion rate is drastically decreased. According to a Mars–van Krevelen mechanism, implying lattice oxygens of the cerium oxide it was suggested that the presence of H atoms in the vicinity of gold should reduce considerably the oxygen spillover activity. At higher temperature, hydrogen can desorb from ceria and the opposite trend is observed. Whereas the rate of CO’s oxidation decreases in a regular and fast way with the temperature for AuZr, it decreases more slowly when ceria is present. This could explain the better selectivity obtained with ceria containing gold catalysts as compared to gold–zirconia material in the high temperature range. Indeed when H2 is no longer present on the support, oxygen spillover activity should be recovered and CO oxidation favored. In any case we showed that Au/CeO2, in our conditions, is not active in the WGS reaction.