The role of substitution-induced micro-structural defects on the redox behavior and catalytic activity of LaxTh1−x(VO3−δ)4 mixed oxide catalysts

The catalytic properties and reduction behavior of mixed metal oxides with nominal compositions of La_xTh_1−x(VO_3−δ)₄, where 0.0 ≤ x ≤ 1.0, have been investigated as a function of the value of x. These compositions were synthesized by ceramic route and characterized using powder X-ray diffraction, differential thermal analysis, and temperature-programmed reduction/oxidation/desorption techniques. The substituted samples were comprised of two distinct phases, one corresponding to tetragonal thorium metavanadate with partial substitution at A-sites, i.e. La_xTh_1−x(VO_3−δ)₄, and the other a new phase identified as LaV₄O_11+δ. Both these phases exhibited considerable oxygen non-stoichiometry, depending upon the extent of substitution. Nevertheless, no significant change was observed in their respective crystal symmetry. Further, the substituted samples showed more reproducible redox behavior and also an improved catalytic activity for CO oxidation reaction. A definite correlation is found to exist between the micro-structural defects generated in the above-mentioned two phases because of the oxygen ion vacancies, resultant enhancement in thermal diffusitivity of lattice oxygen, and finally the catalytic activity of the substituted mixed oxides. It is envisaged that a kind of synergism between the micro-structural defects existing in the two inter-grown phases generated simultaneously during the synthesis may control the catalytic properties of composite metal oxide systems.