Enhanced reducibility of Ce_1-xTi_xO₂ compared to that of CeO₂ and higher redox catalytic activity of Ce_1-x-yTi_xPt_yO_2-δ compared to that of Ce_1-xPt_xO_2-δ

Abstract

Nanocrystalline Ce_1-xTi_xO₂ (0 = x = 0.4) and Ce_1-x-yTi_xPt_yO_2-δ (x = 0.15, y = 0.01, 0.02) solid solutions crystallizing in fluorite structure have been prepared by a single step solution combustion method. Temperature programmed reduction and XPS study of Ce_1-xTi_xO₂ (x = 0.0-04) show complete reduction of Ti⁴⁺ to Ti³⁺ and reduction of 20% Ce⁴⁺ to Ce³⁺ state compared to 8% Ce⁴⁺ to Ce³⁺ in the case of pure CeO₂ below 675 °C. The substitution of Ti ions in CeO₂ enhances the reducibility of CeO₂. Ce_0.84Ti_0.15Pt_0.01O_2-δ crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. The H/Pt atomic ratio at 30 °C over Ce_0.84Ti_0.15Pt_0.01O_2-δ is 5 and that over Ce_0.99Pt_0.01O_2-δ is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce_1-x-yTi_xPt_yO₂ (x = 0.15, y = 0.01, 0.02) compared to Ce_1-xPt_xO₂ (x = 0.01, 0.02). Synergistic involvement of Pt²⁺/Pt^° and Ti⁴⁺/Ti³⁺ redox couples in addition to Ce⁴⁺/Ce³⁺ due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near E_F is shown to be responsible for improved redox property and higher catalytic activity.