EFFECT OF THERMAL-TREATMENT ON THE KINETICS AND SINTERING CHARACTERISTICS OF NICKEL-HYDROXIDE AS A PRECURSOR FOR THE THERMAL GENESIS OF NICKEL-OXIDE CATALYST

The thermal events encountered throughout the calcination course of the parent nickel hydroxide precursor were investigated using a number of physicochemical techniques. The thermal decomposition products obtained by calcination of this precursor were investigated by infrared and X-ray techniques. Analysis of the surface excess oxygen and N2 adsorption data revealed the existence of a considerable disparity in their degrees of oxidation, surface area and pore structure. The variation of the surface area with time was correlated with the differences in the concentration of the created cationic vacancies during the calcination. The presence of such vacancies leads to an increase in the growth rate of the fine pores compared with the large ones. A reduction in the volume of the pores which are intercommunicating and which communicate with the surface of the catalyst, was observed at a certain moment during the sintering process of the products obtained. The thermal decomposition of the parent hydroxide was studied non-isothermally and isothermally. The kinetic analysis of the alpha-t data obtained under isothermal conditions in the temperature range 250-350-degrees-C was performed. From the complementary consideration of the sintering study and the kinetic evidence we can conclude that the thermal decomposition of the hydroxide occurs by a nucleation-and-growth reaction and that the main decomposition process obeys the Avrami-Erofe'ev equation. During the decomposition course, the nucleation reaction is followed by an advancing interface mechanism in the brucite-type structure of the reactant, and both proton and electron transfer steps are involved.