18O Exchange in the System of Carbon Dioxide, Oxygen and Nickel Oxide

Abstract

Using various nickel oxide catalysts obtained by calcination at different temperatures, exchange of 180 was studied in the systems of carbon dioxide/catalyst and carbon dioxide/oxygen/catalyst. Reaction curves for carbon dioxide/catalyst system could be divided into two separate steps, i. e., the initial fast step and the final slow one (Fig.2). The exchangeable oxygen in catalyst decreases monotonously with increasing calcination temperature in the same way as the change in the surface area (Fig.3). On the other hand, the rate constants follow a peculiar curve having a minimum and a maximum as the calcination temperature is raised (Fig.4). This behavior is similar to that of the surface oxygen having a higher oxidation power.For the carbon dioxide/oxygen/nickel oxide system, analysis of the exchange curve leads to the reaction scheme that carbon dioxide exchanges oxygen instantaneously with catalyst, and the slow exchange of oxygen between oxygen and catalyst follows under the exchange equilibrium being always held between carbon dioxide and catalyst (Fig.6). The amounts of exchangeable oxygen agree well with those of the surface oxygen of one layer in this case (Table 2). The rate constant decreases monotonously but more quickly than the change in the surface area with increasing calcination temperature (Fig.8).These facts lead to the heterogeneous model of the catalyst surface, that is, exchange between gas and solid occurs on certain limited active sites, and further exchange proceeds through surface migration from these sites to the neighboring surface region defined as the exchangeable surface oxygen.