Formation of the metal-oxide nanocomposites during the partial reduction of Nd–Sr nickelates

A thermal reduction of Nd$_{2-x}$Sr$_x$NiO$_{4-\delta}$ ($x$ = 0–1.4) solid solutions in an Ar/H$_2$ gas mixture occurs in several separate stages. The initial stages of reduction presumably related to the reduction of Ni$^{3+}$ to Ni$^{2+}$ cause only minor changes in the initial K$_2$NiF$_4$ type structure and retain the morphology of the complex oxide particles. Occasionally, this process is accompanied by the appearance of Ni metal nanoparticles at the surface of oxide grains. As-obtained Ni/metal oxide nanocomposites similar to the products of redox exsolution demonstrate noticeable catalytic activity in the reaction of CO$_2$ hydrogenation at T = 350–400$^\circ$C.