Kinetic study of the reverse water-gas shift reaction over CuO/ZnO/Al2O3 catalysts

The kinetics of the reverse water-gas shift (RWGS) reaction over CuO/ZnO/Al2O3 catalysts was studied by use of CO2/H-2 cycles, hydrogen chemisorption and catalytic tests performed in both differential and integral plug flow reactors. The effect of the reactant composition on the reaction rate was specifically studied by changing the P-H2(0)/(PCO2)-C-0 ratio between 9.0 and 0.3. It was found that different reagents become rate limiting depending upon pressure. While in a H-2-rich region the rate increases strongly with CO2 partial pressure and is zero order in hydrogen, under low P-H2(0)/P-CO2(0) ratios the reaction is less active and is strongly positive order in hydrogen and low order in carbon dioxide. The experimental data were modeled by considering that the reaction proceeds through a surface redox mechanism, copper being the active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the dissociative CO2 adsorption (H-2-rich region) or both the CO2 dissociation and the water formation (H-2-lean region) determine the rate of the overall reaction. Based on previous studies performed on copper crystal surfaces, such a change in kinetics may be explained by assuming that under H-2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociation.