Effect of chemical composition of isomorphous metavanadates on their catalytic activity towards carbon combustion

Metal vanadates of K, Rb, and Cs and their solid solutions were prepared by reaction between carbonates and vanadium(V) oxide, characterized by X-ray diffraction and tested as catalysts for carbon combustion. These vanadates are all orthorhombic but show different lattice parameters depending on the ionic radius of alkali metals. A complete solubility in the solid state was found to exist for the systems KVO3-RbVO3 and RbVO3-CsVO3, while only terminal solid solutions were found in the KVO3-CsVO3 system. The lattice parameters (mainly b0) of the orthorhombic cell increased with the increase of the ionic radius of the alkali metal. This increase was found to be closely linked to the substitution of a larger metal for a smaller one when a complete solubility occurs. The catalytic activity, investigated by temperature programmed oxidation (TPO) experiments, improves progressively along with the introduction of a more electropositive and larger alkaline metal in the vanadate crystal structure. The catalytic performance, however, does not seem to be dependent on the crystal structure because, due to polymorphic transformations, not all these vanadates keep the orthorhombic structure in the temperature range suitable for the catalytic carbon combustion (350-600 °C). The catalytic activity of these vanadates towards carbon combustion thus seems to be strictly related to their chemical composition only.