Reactions of ceria supported rhodium with hydrogen and nitric oxide studied with TPR/TPO and XPS techniques

The reactions of ceria supported rhodium with H₂ and NO have been investigated by temperature programmed reduction (TPR), temperature programmed oxidation (TPO) and X-ray photoelectron spectroscopy (XPS). In precious metal/ceria systems, both the support and the precious metal may be subject to reduction and reoxidation processes. Since partly reduced ceria is an ion-conducting material, the electrical charging of the whole sample during the XPS measurements depends on the oxidation state of the supporting ceria. Thus, the charging can be used as a sensitive indicator of the oxidation state of the support material which allows distinction between reactions of the support and of the precious metal. Rhodium oxide precursors are reduced to Rh⁰ at temperatures below 200°C, but a slight reduction of the ceria support in this temperature range was also evident from the XPS spectra. At higher temperatures, the reduction of the ceria becomes more pronounced. Reoxidation of the support by NO is very fast and complete even at room temperature. Rh is oxidised to Rh₂O₃ at 200-300°C, i.e. below the normal working conditions of car exhaust catalysts.