Equilibrium and kinetics of NO and CO chemisorptions on nonstoichiometric nickel-copper manganites

Abstract

The equilibrium and kinetics of adsorption of NO and CO on nonstoichiometric nickel-copper manganites have been investigated through volumetric measurements. The adsorption isotherms were satisfactorily fitted to the Freundlich equation. The equilibrium coverages at 298 K were found to depend closely on the chemical composition of the oxide; thus, a decrease in the coverage beyond a maximum copper extent was observed. The adsorption isotherms of NO at various temperatures in the range from 298 to 473 K showed that the equilibrium coverage decreases with increasing temperature. This behavior enabled us to follow the logarithmic decrease of the heat of adsorption of NO on such surfaces. The adsorptions of NO and CO on surfaces preadsorbed with CO and NO, respectively, were also studied. These experiments showed the ability of NO to displace CO preadsorbed molecules whereas the contrary did not hold, suggesting the existence of common adsorption sites as well as some specific CO adsorption sites. Finally, some kinetic data are reported showing that the experimental adsorption results fit the Elovich equation (with t(0) approximate to 0), although two distinct rate processes could be identified.