No abstract

We prepared Pt catalysts supported on various metal oxides, viz., ZrO 2 , CeO 2 , TiO 2 , yttria-stabilized zirconia (YSZ), SiO 2 , SiO 2 -Al 2 O 3 , and c-Al 2 O 3 , using an incipient wetness method and applied them to propane combustion. In the cases of ZrO 2 -, CeO 2 -, and TiO 2 -supported Pt catalysts, supports with different surface areas were also used. The Pt dispersion in Pt catalysts supported on metal oxides increased with increasing surface area of the support for the same metal oxide. Pt catalysts on supports with lower surface areas (ZrO 2 , CeO 2 , and TiO 2 ) showed higher catalytic activities for propane combustion than did Pt catalysts on supports with higher surface areas. The catalytic activity decreased in the following order: Pt/ZrO 2 (2) [ Pt/CeO 2 (9) [ Pt/TiO 2 (1) = Pt/SiO 2 (350) [ Pt/ZrO 2 (18) = Pt/YSZ [ Pt/ TiO 2 (330) [ Pt/SiO 2 -Al 2 O 3 (350) [ Pt/ZrO 2 (73) [ Pt/c-Al 2 O 3 (180) [ Pt/CeO 2 (160). The catalytic activity is inversely proportional to the amount of O 2 chemisorbed up to the reaction temperature. It can be concluded that metallic Pt is essential for propane combustion and is maintained for the Pt catalysts with large Pt metal particles, which can be prepared by using a support with a low surface area.

We prepared Pd catalysts supported on various metal oxides, viz. by an incipient wetness method and applied them to propane combustion. Several techniques: N 2 physisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), CO chemisorption, temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO) were employed to characterize the catalysts. Pd/SiO 2 -Al 2 O 3 showed the least catalytic activity at high temperatures among Pd catalysts supported on irreducible metal oxides, viz. was much superior for this reaction to Pd/a-Al 2 O 3 . The Pd catalyst supported on reducible metal oxides (CeO 2 and TiO 2 ) with a less specific surface area showed the higher catalytic activity compared with that supported on reducible metal oxides with a higher specific surface area, even though the former had a less Pd dispersion than the latter. In the case of Pd/SiO 2 -Al 2 O 3 , the initially reduced Pd catalyst was superior to the fully oxidized one. The oxidation of metallic Pd occurred in the presence of O 2 with increasing reaction temperature, which resulted in the change in the catalytic activity.