Carbon dioxide-calcium oxide surface and bulk reactions: thermodynamic and kinetic approach

It is widely recognized that the dispersion (presumably related to the degree of catalyst-carbon contact) of a solid catalyst in a carbonaceous matrix plays an important role in explaining its catalytic activity in gasification reactions. CaO-CO{sub 2} interactions have been studied by thermogravimetry-differential thermal analysis (TG-DTA), temperature-programmed desorption (TPD), and temperature-programmed reaction (TPR). A less classical technique termed intermittent temperature-programmed desorption (ITPD) has also been used. A mass spectrometer has been employed as a gas flow analyzer in all the temperature-programmed techniques. The temperature at which CO{sub 2}-CaO interactions take place determines whether their nature is limited to CO{sub 2} chemisorption or involves bulk carbonate formation in CaO particles as well. While only chemisorption occurs at temperatures between 323 and 573 K, bulk carbonate formation is observed at higher temperatures; the higher the temperature, the deeper the carbonate formation of CaO particles. CO{sub 2} chemisorption is interpreted by applying models commonly used for heterogeneous surfaces, with variations in the heat of adsorption and in the desorption activation energy as functions of coverage. The interpretation of the thermodynamics and kinetics of the process in the temperature range 323-1,173 K led to the proposal of a mechanism for the first steps ofmore » the reaction of CO{sub 2} with CaO particles.« less