The role of hydrogen partial pressure in the gas-phase hydrogenation of aromatics over supported nickel

The effect of varying the hydrogen partial pressure (from 0.19 to 0.96 atm) on turnover frequencies (TOFs) in the gas-phase hydrogenation of benzene, toluene, and o-, m-, and p-xylene over a Ni/SiO2 catalyst has been studied. Each system is characterized by well-defined reversible temperature relate d activity maxima (T-max) where T-max was shifted in the conversion of benz ene and toluene to lower values as the hydrogen partial pressure was reduce d but remained unaffected in reactions involving the isomers of xylene. The range of TOFs reported for each aromatic system and the occurrence of T-ma x are explained on the basis of interplay between the supply and reactivity of active surface species. The observed relationships between reaction tem perature and TOF are represented by means of a common extended power rate m odel. The nature of true and apparent reaction kinetics is discussed, and a compensation effect is established wherein the only reaction variable is t he hydrogen partial pressure. The experimentally determined activation ener gies do not represent the true catalytically relevant energetics because of the existence of preequilibria to the rate-determining step. The measured Arrhenius parameters are shown to be composite terms and are pressure depen dent. Catalytically significant heats of adsorption have been extracted fro m the reaction data, and the TOFs have been found to be inversely proportio nal to the magnitude of the heat of adsorption of hydrogen.