Reduction and oxygen storage behavior of noble metals supported on silica-doped ceria

The redox features of Rh and other noble metals (Pt, Pd) supported on silica-doped ceria catalysts are investigated by temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO). Noble metal oxides are reduced to the corresponding metals at temperatures below 600 K, whereas at higher temperatures the reduction of the support takes place with formation of reduced ceria and a cerium silicate phase. After redox aging, reduction of the support in the bulk occurs at lower temperatures; this effect depends on the amount of silicon present. A more detailed reduction profile is obtained by monitoring water evolution during TPR in both ceria and ceria-silica-based catalysts. Differences are observed in H-2 consumption and water formation which are not always simultaneous: their positions depend on redox treatments, precursor salt used for preparation, and type of support. Oxidation of CO under oscillating feedstream conditions is used as an estimate of dynamic oxygen storage capacity. Redox treatment promotes oxygen storage on both the support and noble-metals-supported samples. This is related to the formation during reduction of a silicate phase, as evidenced by X-ray diffraction and HRTEM analysis, which on reoxidation gives small particles of nanocrystalline ceria.