REACTIONS OF CERIA SUPPORTED RHODIUM WITH HYDROGEN AND NITRIC-OXIDE STUDIED WITH TPR TPO AND XPS TECHNIQUES

The reactions of ceria supported rhodium with H-2 and NO have been inv estigated by temperature programmed reduction (TPR), temperature progr ammed oxidation (TPO) and X-ray photoelectron spectroscopy (XPS). In p recious metal/ceria systems, both the support and the precious metal m ay be subject to reduction and reoxidation processes. Since partly red uced ceria is an ion-conducting material, the electrical charging of t he whole sample during the XPS measurements depends on the oxidation s tate of the supporting ceria. Thus, the charging can be used as a sens itive indicator of the oxidation state of the support material which a llows distinction between reactions of the support and of the precious metal. Rhodium oxide precursors are reduced to Rh0 at temperatures be low 200-degrees-C, but a slight reduction of the ceria support in this temperature range was also evident from the XPS spectra. At higher te mperatures, the reduction of the ceria becomes more pronounced. Reoxid ation of the support by NO is very fast and complete even at room temp erature. Rh is oxidised to Rh2O3 at 200-300-degrees-C, i.e. below the normal working conditions of car exhaust catalysts.