HYDROGENATION CATALYSTS BASED ON NICKEL AND RARE-EARTH-OXIDES .2. XRD, ELECTRON-MICROSCOPY AND XPS STUDIES OF THE CERIUM NICKEL OXYGEN HYDROGEN SYSTEM

In a previous article we selected a preparation process of cerium and nickel oxides which permits catalysts to be obtained which contain as much hydrogen as the intermetallic compounds with the same composition , and which are more active in benzene hydrogenation. Moreover the cat alysts' behaviour varies with the Ni/Ce ratio (x) and at least two zon es can be distinguished. In the present paper we have found correlatio ns between catalytic activity, hydrogen content and some physical and chemical characteristics of the solids, both in the oxidized and reduc ed states. The techniques used were electron microscopy (TEM, SEM, EPM A), X-ray diffraction and X-ray photoelectron spectroscopy. Whatever t he method, the catalysts are classified into two families. For x less- than-or-equal-to 0.5 nickel is inserted in the ceria lattice to form a solid solution. Above 0.5, both crystallized nickel oxide and solid s olution coexist. In the reduced state anionic vacancies able to receiv e hydrogen, probably in an hydridic form, are created in the bulk and at the surface of the solid solution. The catalytic results can then b e explained by assuming the existence of three kinds of active sites w hich differ from each other in terms of the environment of nickel, and an explanation for the higher efficiency of the catalyst with x = 5 i s advanced. Finally the situation is shown to be almost identical in c eria-supported nickel catalysts.