A temperature programmed reduction study of Pt on Al2O3 and TiO2

The reducibility of platinum on γ-Al2O3- and TiO2 was studied with the aid of temperature programmed reduction. The reduction peak temperature was found to be dependent on the temperature of the primary oxidation after impregnation and drying. The higher the oxidation temperature the lower the TPR peak temperature and the higher the H2 consumption. During the oxidation small PtO2 particles were formed which were more easily reduced than the original isolated Pt2+ ions. For Pt/Al2O3 no decomposition of PtO2 was observed up to 750 K, while bulk PtO2 decomposed around 600 K. This demonstrated that there is a substantial interaction between PtO2 and Al2O3. For PtO2 supported on TiO2 and SiO2 this interaction is much weaker and on these supports PtO2 decomposed at lower temperatures than on Al2O3. Reduction of passivated catalysts, with H/Pt < 0.8 in the metallic state, took place even at 223 K. After passivation these catalysts consist of a metal core surrounded by a metal oxide skin. Due to the presence of the metallic core, H2 can be dissociatively chemisorbed at low temperatures and induce reduction of the oxide layer. The implications of this for the O2-H2 titration method are discussed. Reduction of Pt/TiO2 led to metal assisted reduction of the support. Below 500 K only a small part of the support is (reversibly) reduced in the near vicinity of the metal particles. Above 500 K further metal assisted reduction of the TiO2 support takes place, probably promoted by increased ion mobility.