Evolution of reaction mechanism in the catalytic combustion of ammonia on copper-cerium mixed oxide

Abstract

Carbon-free hydrogen-rich ammonia (NH3) is a potential next energy generation source, where a high ignition point and high nitric oxide contents, thereby limiting further development. This study aims to investigate the mechanism evolution during the catalytic ignition process of high NH3 concentration over copper-cerium catalyst to address the demerits of flame combustion. The phase composition, elemental valence, and active species of the copper-cerium oxide (CuO-CeO2) catalyst are investigated using various characterization techniques. The results indicated that Cu species are mainly present in copper-cerium solid solutions and highly dispersed CuO clusters, providing sufficient Cu sites to adsorb NH3 and abundant reactive lattice oxygen. The catalytic ignition triggered a kinetic transition from low-rate to high-rate steady-state. During the induction process, the predominance of the L-H mechanism is implied by the kinetic modeling and transient experiments (low-rate induced phase). Combined with IR spectroscopy and isotope (18O2)-transient response study, the results indicated that adsorbed NH3 reacted with surface adsorbed oxygen, simultaneously pulling out lattice oxygen to react with adsorbed NH3, confirming the coexistence of L-H and M−K mechanism over CuO-CeO2. The L-H mechanism (18O, contribution of adsorbed oxygen) plays a relatively dominant role in the induction stage, while the involvement of the M−K mechanism (16O, contribution of lattice oxygen) is significantly increased in the sustained combustion stage. At both stages, NH and NH2 served as critical species for N2 generation (i-SCR), respectively, playing an important role in N2 selectivity. Moreover, in the self-sustained combustion stage for CuO-CeO2 (14%NH3), the rapid gas diffusion and mass transfer facilitated the adsorption and activation of NH3. These results are envisaged to provide theoretical support for the handling and application of high concentrations of NH3.