Kinetic Studies of the Catalytic Ortho-Parahydrogen Conversion and Hydrogen-Deuterium Equilibration on γ-Alumina

Abstract

Abstract Kinetic studies of the catalytic ortho-parahydrogen conversion and hydrogen-deuterium equilibration on highly purified γ-alumina were conducted in the range of temperature from 77 to 221°K and in the range of pressure from 10 to 120 mmHg. The rate equation obtained can be well explained by a mechanism which involves the reaction between an adsorbed atom and a molecule. The true activation energy, E, and the heat of adsorption of the reactive molecule, Q, were estimated as E=0.51–0.80 and Q=0.51–0.72 kcal/mol for the conversion (77–95°K), and E=4.6 and Q=1.9 kcal/mol for the equilibration (145–221°K). The conversion at temperatures below 90°K was found to proceed mostly via the magnetic mechanism. A thermal desorption study of adsorbed ethylene showed that there are three types of adsorption on γ-alumina with different activation energies of desorption, and that these species occupy a total of only a few hundredths of the surface. It was concluded that the most strongly-adsorbed ethylene blocks the reactions via the chemical mechanism, while the most weakly-adsorbed one retards the conversion via the magnetic mechanism. The nature of active sites for the reactions was discussed by taking the results of the ESR measurement into consideration.