Experimental and theoretical studies of ammonia generation: Reactions of H2 with neutral cobalt nitride clusters

Ammonia generation through reaction of H-2 with neutral cobalt nitride clusters in a fast flow reactor is investigated both experimentally and theoretically. Single photon ionization at 193 nm is used to detect neutral cluster distributions through time-of-flight mass spectrometry. ComNn clusters are generated through laser ablation of Co foil into N-2/He expansion gas. Mass peaks ComNH2 (m = 6, 10) and ComNH3 (m = 7, 8, 9) are observed for reactions of H-2 with the ComNn clusters. Observation of these products indicates that clusters ComN (m = 7, 8, 9) have high reactivity with H-2 for ammonia generation. Density functional theory (DFT) calculations are performed to explore the potential energy surface for the reaction Co7N + 3/2H(2) -> Co7NH3, and a barrierless, thermodynamically favorable pathway is obtained. An odd number of hydrogen atoms in ComNH3 (m = 7, 8, 9) probably come from the hydrogen molecule dissociation on two active cobalt nitride clusters based on the DFT calculations. Both experimental observations and theoretical calculations suggest that hydrogen dissociation on two active cobalt nitride clusters is the key step to form NH3 in a gas phase reaction. A catalytic cycle for ammonia generation from N-2 and H-2 on a cobalt metal catalyst surface is proposed based on our experimental and theoretical investigations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754158]