Effects of nitrogen vacancy sites of oxynitride support on the catalytic activity for ammonia decomposition

Nitrogen-containing compounds such as imides and amides have been reported as efficient materials that promote ammonia decomposition over nonnoble metal catalysts. However, these compounds decompose in an air atmosphere and become inactive, which leads to difficulty in handling. Here, we focused on perovskite oxynitrides as air-stable and efficient supports for ammonia decomposition catalysts. Ni-loaded oxynitrides exhibited 2.5-18 times greater catalytic activity than did the corresponding oxide-supported Ni catalysts, even without noticeable differences in the Ni particle size and surface area of the supports. The catalytic performance of the Ni-loaded oxynitrides is well correlated with the nitrogen desorption temperature during N2 temperature-programmed desorption, which suggests that the lattice nitrogen in the oxynitride support rather than the Ni surface is the active site for ammonia decomposition. Furthermore, NH3 temperature-programmed surface reactions and density functional theory (DFT) calculations revealed that NH3 molecules are preferentially adsorbed on the nitrogen vacancy sites on the support surface rather than on the Ni surface. Thus, the ammonia decomposition reaction is facilitated by a vacancy-mediated reaction mechanism. Oxynitride-supported Ni catalysts exhibit much higher activity than oxide-supported Ni catalysts for ammonia decomposition reaction. Ammonia is activated at nitrogen vacancy sites on the surface of oxynitride in close vicinity to the supported Ni nanoparticles rather than on the Ni surface, and therefore the catalytic performance is dominated by ease of nitrogen vacancy formation on the catalyst surface.