Study on Zinc Oxide-Chromium Oxide Catalyst. II. Variation of Catalytic Activity for Methanol Decomposition with Addition of Chromia and Structural Dependence of Activity

Abstract

Abstract From the results on the rates of methanol decomposition by the catalysts of different zinc oxide-chromium oxide compositions the rate (ν) of evolution of carbon monoxide per unit surface area of catalyst has appeared to be represented by the following equation v=ae^αE·e^-E/kT where a and α are constants applicable throughout the two series of catalysts produced by different methods. The activation energy (E) of the methanol decomposition attains to a minimum value when the catalyst has a certain chromia content. It is directly related with the activation energy of electrical conductivity of the catalyst in hydrogen at high temperature but neither with the concentration of n-type conductivity centres nor with that of p-type conductivity centres. At such temperatures, the adsorption of hydrogen on the oxygen atoms in the surface was considered to participate in the electrical conductivity. Moreover, the results on determinations of the surface area and the excess oxygen content, together with the information about the structure obtained from diffraction patterns, reveal that there is a similarity in tendency between the increase of concentration of zinc chromite spinels in the catalyst surface and the decrease of activation energy of the decomposition. This relation can be explained from the view of E. Cremer on the compensation effect. The quantity of chromia added in excess of the proper amount for combination with zinc oxide causes both the surface area and the activation energy to increase towards those of chromia alone. The addition of chromia has therefore a considerable influence upon the activation energy of methanol decomposition, which is quite contrary to what G. Natta described on the methanol synthesis.