Studies of ethylene hydrogenation and of adsorbed C 2H 4 and H 2 on chromia and lanthana catalysts

Abstract

Adsorbed forms of C 2H 4 and H 2 (and hydrogenation of adsorbed C 2H 4 at −78 °C) on Cr 2O 3, Cr 2O 3 · 7Al 2O 3, La 2O 3, and Al 2O 3 pretreated at 400–900 °C, with increments of 100 °C, have been investigated by temperature-programmed desorption. Chemical desorption of the reactants by the catallytic poisons CO, CO 2, and H 2O at −78, −68, and 20 °C has been studied on La 2O 3. The hydrogenation activity of these oxides and the control of the poisoning of La 2O 3 in C 2H 4 hydrogenation have been researched by a circulation method at temperatures of −78, −68, and 20 °C with an equimolar mixture of C 2H 4 and H 2 at an initial pressure of 10 Torr. The results show that the catalytic activity, the number of adsorbed forms, and the adsorbed amounts of the reactants in the different forms depend on the pretreatment temperature. The distinctiveness of the adsorbed forms of the reactants originates from distinctions in the properties of adsorbed centers. The activity of the catalyst is due to the presence of at least two types of centers on the surface. Different reactivities have been found for different adsorbed C 2H 4 forms in their interaction with H 2 at −78 °C. It has been demonstrated that the poisoning effect of CO, CO 2, and H 2O is due to adsorption displacement of the adsorbed hydrogen and ethylene. The ratio between the amount of adsorbed poison and desorbed C 2H 4 and H 2 depends on the chemical nature of the poison.