An inorganic material is described consisting of at least two elementary spherical particles, each of said spherical particles comprising metallic particles in the form of polyoxometalate of formula (XxMmOyHh)q- where H is hydrogen, O is oxygen, X is an element chosen from phosphorus, silicon, boron, nickel and cobalt and M is one or more elements chosen from vanadium, niobium, tantalum, molybdenum, tungsten, iron, copper, zinc, cobalt and nickel, x being equal to 0, 1, 2 or 4, m being equal to 5, 6, 7, 8, 9, 10, 11, 12 and 18, y being between 17 and 72, h being between 0 and 12 and q being between 1 and 20 (y and q being integers). Said metallic particles are present within a mesostructured matrix based on an oxide of an element Y chosen from the group consisting of silicon, aluminium, titanium, tungsten, zirconium, gallium, germanium, tin, antimony, lead, vanadium, iron, manganese, hafnium, niobium, tantalum, yttrium, cerium, gadolinium, europium and neodymium and the mixture of at least two of these elements. Said matrix has pores with a diameter between 1.5 and 50 nm and amorphous walls with a thickness between 1 and 30 nm, said elementary spherical particles having a maximum diameter of 200 microns.