Use of hydrogen atoms for the low-temperature reduction of oxides

Abstract

The interaction of hydrogen atoms with the surface of supported (MoO₃ or V₂O₅ on SiO₂) or bulk oxides (TiO₂) kept in liquid nitrogen has been studied by e.p.r. after the oxides were pretreated in oxygen at high temperatures. It is shown that reduction occurs as evidenced by the appearance of e.p.r. signals assigned to (MoO)³⁺(g_⊥= 1.952, g_∥= 1.890), (VO)²⁺(g_⊥= 1.982, g_∥= 1.938, A_⊥= 64 G, A_∥= 190 G) and Ti³⁺(g_⊥= 1.976, g_∥= 1.950). These paramagnetic centres are found to be inert towards oxygen, showing that they are located below the surface, in contrast to the results obtained after thermal reduction in vacuo or in hydrogen. The presence of F centres (g_∥= 2.0015, g_⊥= 2.0000), observed in the supported oxides only, is related to the support, as shown by experiments performed with SiO₂ alone. The formation of reduced transition metal ions and of other centres (F centres, O^–₂, O₂H⁰) is discussed on the basis of the reaction of H⁰ atoms with O^2– oxide ions.