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/Z nO/Al2O3 catalysts was studied by use of CO2/H-2 cycles, hydrogen chem isorption and catalytic tests performed in both differential and integ ral 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 be come rate limiting depending upon pressure. While in a H-2-rich region the rate increases strongly with CO2 partial pressure and is zero ord er 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 ca rbon dioxide. The experimental data were modeled by considering that t he reaction proceeds through a surface redox mechanism, copper being t he active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the d issociative CO2 adsorption (H-2-rich region) or both the CO2 dissociat ion and the water formation (H-2-lean region) determine the rate of th e overall reaction. Based on previous studies performed on copper crys tal surfaces, such a change in kinetics may be explained by assuming t hat under H-2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociat ion.