The importance of gas purity in catalytic ammonia synthesis

Catalysts for ammonia synthesis/decomposition are very sensitive to poisoning by oxygen-impurities in the feed. Here we show that the ≤1 ppm O-species present in the purest commercially available gasses can lead to a one order of magnitude drop in ammonia synthesis rate for an industrial Fe catalyst. Density Functional Theory calculations show that adsorbed O creates repulsive interactions that repel and destabilize N in neighboring sites. This inhibits the rate limiting N₂ dissociation and leads to the lower rate. A K/Ru/C catalyst also suffers from significant O-poisoning although the activity drop is only half as strong as for iron catalysts, indicating that O-poisoning is a general phenomenon across different types of ammonia synthesis catalysts. An industrial Fe-based ammonia synthesis catalyst kept at room temperature can be used as a guard bed to clean the gas, but oxygen can start to penetrate the guard bed already at 4.4 % O-coverage, and the capacity of a given guard bed therefore needs to be validated before an experimental campaign. Due to the severity of this effect and the unexpectedly low adsorption capacity of guard beds we recommend that future studies of ammonia synthesis and decomposition start by evaluating the efficiency and life-time of the O-removal measures using the protocols established here.