Catalytic oxygen transfer between CO and CO2 on TiO2

Abstract

The rate of the exchange reaction: *CO2 + CO --> *CO + CO2 was measured at atmospheric pressure in a flow reactor containing different TiO2 catalysts. The variables investigated and there range were as follows: ratio (pCO2/pCO), 0.2 to 5.0; temperature, 450 [deg] to 575 [deg]C; flow rate, 10 to 50 cc min-1; additions of Fe, Cr, Nb, and P to TiO2. From the results, the influence of the (pCO2/pCO) ratio on the rate of reaction was determined and compared to that of pO2 on the electronic conductivity of TiO2 as gathered in previous investigations. From this information suggestions are advanced for the nature of the defect equilibrium reaction at the surface, the controlling step of the reaction rate, the nature of the adsorbed oxygen, and the influence of alterions upon the reactivity of TiO2. The results on TiO2 + Cr indicated that a space-charge layer at the surface governed the reactivity of TiO2. In this instance, the thermodynamic derivations were modified to take into account the influence of the surface space charge on the defect equilibrium. The values of the reaction rate constants were compared with those calculated with the help of the absolute rate theory and the experimental activation energies. Order of magnitude agreement was obtained assuming that the adsorption of CO2 was the controlling step of the isotope exchange reaction. On TiO2 with low impurity levels a mobile configuration of the adsorption complex is in best accord with the results. At higher impurity levels an immobile complex gives better agreement. The significance of the present results for the oxidation of CO on TiO2 is pointed out. Finally, the diverse yet complementary, roles of electron and ion defects in governing surface reactivity and catalytic activity of TiO2 and metal oxides in general is pointed out.