Use of kinetic models to explore the role of base promoters on Ru/MgO ammonia synthesis catalysts

Abstract

The kinetics of ammonia synthesis over unpromoted and Cs-, Ba-, and La-promoted Ru/MgO (∼2 wt%) were studied in a tubular reactor at 20.7 atm. The reaction over all of the catalysts was nearly first order in N2 and nearly zero order in NH3. However, the order of reaction in H2 was largely dependent on the choice of promoter, with Ba and La significantly reducing the inhibition by dihydrogen seen on Cs-promoted Ru/MgO. The turnover frequencies on Ba- and La-promoted Ru/MgO under stoichiometric conditions and 673 K were almost an order of magnitude greater than that on Cs-promoted Ru/MgO. Two kinetic models with optimized parameters were used to describe the outlet ammonia pressures determined experimentally both close to and far from equilibrium. Although Cs promotion of Ru lowered the activation barrier for N2 dissociation, the enthalpy of dihydrogen adsorption and therefore the H atom surface coverage increased. Thus, promotion of Ru catalysts with bases cannot be attributed solely to an effect on dinitrogen dissociation, which is the rate-determining step. Base promotion is a trade-off between lowering the activation barrier for N2 dissociation and increasing the competitive adsorption of H2. The coverages of nitrogen-containing species determined by an optimized kinetic model matched well those determined experimentally by isotopic transient analysis over the same catalysts.