Spectroscopic study of the adsorption of carbon monoxide on solid solutions of nickel oxide and magnesium oxide. Part 1.—Standard samples

Abstract

High surface-area NiO–MgO solid solutions have been used as model solids to investigate the effect of the coordination and agglomeration state on the reactivity of the substitutional surface Ni²⁺ ions.

The main results are as follows. (i) Isolated Ni²⁺ ions emerging on the {100} faces of the microcrystals do not react with CO at room temperature and form stable Ni²⁺—CO adducts only at 77 K. (ii) Ni²⁺ ions located on edges, steps and other defects are slowly reduced by CO at room temperature with formation of carbonate species and [(CO)₃Ni(CO₂)]^2– Mg²⁺ complexes, which can be interpreted as Ni(CO)₄ adsorbed on Mg²⁺O^2– pairs. (iii) Clustered Ni²⁺ ions are more readily reduced than isolated ones and give rise to dinuclear and polynuclear zerovalent carbonyls. (iv) The same dinuclear and polynuclear species are generated by CO abstraction at room temperature from adsorbed Ni(CO)₄: the process is totally reversible and the mononuclear species are readily regenerated upon exposure to CO.