Ammonia synthesis over a multipromoted iron catalyst:: Extended set of activity measurements, microkinetic model, and hydrogen inhibition

The ammonia synthesis activity of a multipromoted iron catalyst (KM1R, Haldor Topsoe AIS) is reported for a wide range of conditions. The H-2:N-2 ratio is varied by a factor of 10, the total pressures are between 1 and 100 bar, and temperatures are in the range 320-440 degrees C. Data obtained here and literature data are compared. It is concluded that water poisoning is negligible in the present activity measurements and that hydrogen inhibition is important at the combined conditions of low temperature, low ammonia partial pressure, and high total pressure. From a fit to the activity data a microkinetic model is obtained, which has H* and N* as surface species. The equilibrium constant of hydrogen adsorption obtained from single crystal studies, 2.16 10(3) bar(0.5) exp(-48 kJ mol(-1) R-1 T-1), is used as input for the model. The obtained model parameters are the rate constant at zero coverage, 7.79 mmol g(-1) s(-1) bar(-1) exp(-6.6 kJ mol(-1) R-1 T-1 ) and the equilibrium constant for the equilibrium between ammonia, hydrogen, and adsorbed nitrogen, 0.027 bar(-0.5) exp(-27.1 kJ mol(-1) R-1 T-1). From these two parameters a rate of 1.43 x 10(12) s(-1) exp(-162.4 kJ mol(-1) R-1 T-1) for nitrogen desorption is inferred. The performance of the model is compared to three microkinetic models found in the literature. The model gives a quantitative account of the catalytic activity at all the measured degrees of conversions, pressures, and temperatures and is consistent with surface science data. (C) 1999 Academic Press.