Exotemplated ceria catalysts with gold for CO oxidation Original Research Article

Cerium oxides were synthesized by an exotemplating procedure using both activated carbon and carbon xerogel as templates. The template effect and the preparation method were investigated. The surface area of the materials varied from 73 to 114 m2 g−1, being similar to samples prepared by standard precipitation and calcination techniques (150 and 98 m2 g−1, respectively), but much larger than a commercial ceria sample (20 m2 g−1), used for comparison purposes. Samples prepared using xerogel as template have, approximately, twice as much porosity than those prepared with activated carbon, and the mesopores of the first are roughly 4 times larger than those of the latter. Cerianite CeO2 was the only phase detected by XRD for all materials. Gold was loaded onto these materials by a double impregnation method. The catalytic performance of these ceria oxide materials alone and loaded with gold was tested in the oxidation of carbon monoxide. Xerogel prepared samples showed to be the most active in CO oxidation. Addition of Au increased the activity up to two orders of magnitude, at room temperature. With the unloaded samples, complete conversion was only achieved above 400 °C, but when Au was loaded, full CO conversion occurred already at 75 °C. Analysis of selected samples by HAADF and HRTEM showed that the gold particle size was 5–10 nm for the most active sample, and 8–13 nm for the least active. H2-TPR experiments proved that addition of gold significantly helped to improve the reducibility of the surface oxygen on CeO2. Both for ceria and Au/ceria materials, it was observed that the temperature at which the CO conversion started is related to the temperature of the first peak of the TPR spectrum, showing that the activity depends on the availability of surface oxygen. Experiments without oxygen in the feed, in the presence of ceria catalysts, suggest that the lattice oxygen can react with CO. It is shown that there is a significant correlation between the catalytic activity and the lattice oxygen donating ability. The TPR results suggest that gold is in its reduced state in the “fresh” Au/ceria samples.