EFFECTS OF THERMAL-TREATMENT ON THE PHYSICOCHEMICAL AND CATALYTIC PROPERTIES OF MICROPOROUS NIO CATALYST

Samples of NiO were prepared by the thermal dehydration of pure Ni(OH)2 at 200 and 250 °C under a reduced pressure of 10-6 Torr, and in air at 300, 400 and 500 °C. The stoichiometry, electrical properties, surface area, pore structure and catalytic activity towards the oxidation of CO at room temperature were studied for the range of samples. All the prepared oxides were found to be porous in structure, the solids prepared at 250 °C and 300 °C being entirely microporous and the other oxides being mesoporous. The catalysts prepared under vacuum were quasistoichiometric and yellowish-green in colour, but turned black on exposure to air at room temperature. Their electrical conductivity increased from 10-12 to 10-6 (ω cm)-1 owing to the adsorption of oxygen in ionic form. The electrical properties and surface ares of tjhe two oxides were very similar, but their catalytic activity was quite different. This difference is attributed to the evolution of the porous structure from mesoporous to microporous and to the departure of some oxygen from lattice positions to give rise to crystallites of nickel metal and anionic vacancies, thus increasing the concentration of lattice defects. The samples prepared in air were non-stoichiometric, containing an excess of oxygen. The electrical conductivity and the amount of excess oxygen ran in parallel. On prolonged outgassing in vacuum at 30 °C, the oxides exhibited no detectable catalytic at room temperature, owing to blocking of the activate sites by chemisorbed oxygen strongly bound to the surface. The determination of surface coverage with oxygen revealed that less than 10% of the active surface sites (Ni2+ ions) were accessible for oxygen adsorption. Outgassing of the non-stoichiometric oxides in vacuum at higher temperatures of 200-300 °C produced active catalysts, owing to the desorption of strongly bound oxygen which liberated active sites for adsorption and surfaces reactions. The evolution of different properties of NiO prepared at various temperatures revealed that its catalytic activity is governed by geometrical, textural and lattice defect parameters rather than by semiconducting properties. © 1979.