The temperature-programmed desorption of N₂ from a Ru/MgO catalyst used for ammonia synthesis

The temperature-programmed desorption (TPD) of N₂ from a Ru/MgO catalyst used for ammonia synthesis was studied in a microreactor flow system operating at atmospheric pressure. Saturation with chemisorbed atomic nitrogen (N-*) was achieved by exposure to N₂ at 573 K for 14 h and subsequent cooling in N₂ to room temperature. With a heating rate of 5 K/min in He, a narrow and fairly symmetric N₂ TPD peak at about 640 K results. From experiments with varying heating rates a preexponential factor A_des = 1.5 × 10¹⁰ molecules/(site s) and an activation energy E_des = 158 kJ/mol was derived assuming second-order desorption. This rate constant of desorption is in good agreement with results obtained with a Ru(0001) single crystal surface in ultra-high vacuum (UHV). The rate of dissociative chemisorption was determined by varying the N₂ exposure conditions. Determination of the coverage of N-* was based on the integration of the subsequently recorded N₂ TPD traces yielding A_ads = 2 × 10^-6 (Pa s)^-1 and E_ads = 27 kJ/mol. The corresponding sticking coefficient of about 10^-14 at 300 K is in agreement with the inertness of Ru(0001) in UHV towards dissociative chemisorption of N₂. However, if the whole catalytic surface were in this state, then the resulting rate of N₂ dissociation would be several orders of magnitude lower than the observed rate of NH₃ formation. Hence only a small fraction of the total Ru metal surface area of Ru/MgO seems to be highly active dominating the rate of ammonia formation.