Comparative energy barriers for hydrogen activation by homogeneous and heterogeneous metal oxide catalysts

Abstract

Triethylene glycol solutions of alkali and alkaline earth metal hydroxide complexes are well-defined soluble oxide water-gas shift catalysts which equilibrate the reaction of carbon monoxide and water to yield hydrogen and carbon dioxide at temperatures ranging from 150 ° to 250 °C and carbon monoxide pressures of 1 to 300 atm. Significantly, catalysis proceeds cleanly, even in the complete absence of a metal center in the soluble oxide system. Thus, the rate of hydroxide ion catalyzed hydrogen evolution is highest in the presence of a noncoordinating organic cation: BuΔN+>Cs+>Na+>H+>Ca+2. Furthermore, the activation energy for the homogeneous sodium hydroxide catalyst in triethylene glycol solution, 26±1 kcal, is comparable to that exhibited by a commercially used heterogeneous iron oxide catalyst, 27±0.2 kcal. The alkali metal hydroxide system may be modified for metal cocatalysis. Thus, lead (II) oxide dissolves in the triethylene glycol solutions to yield a new species which exhibits a207Pb NMR resonance shifted 3350 ppm downfield from lead perchlorate. The activity of this lead modified system is improved by three orders of magnitude. Yet, the activation energy is unchanged, 26±1 kcal, suggesting that entropic factors may be important in these homogeneous metal oxide hydrogen evolution/activation systems.