Mechanistic Study on NH3 Decomposition over Cu-Exchanged CHA Zeolites Using Automated Reaction Route Mapping Combined with Neural Network Potential and Density Functional Theory Calculations

The reaction mechanism for the decomposition of NH3 into N2 by Cu-exchanged CHA zeolite (Cu-CHA) was explored by using a combination of neural network potential (NNP) and automated reaction route mapping. According to the revealed reaction route, the formation of N2H4, which thermally decomposes into N2 and H2, is not assisted by both isolated Cu2+ cation and [Cu(OH)]+ species because of their high energy barriers (>300 kJ/mol). In contrast, [Cu(OH)]+ species easily generates [Cu(NH2)]+ species at a much lower energy barrier of 70 kJ/mol. Then, reaction route mapping was performed using two [Cu(NH2)]+ species at Al sites close to each other, resulting in a lower activation barrier than that obtained using only one [Cu(NH2)]+ species to generate N2H4 intermediates (Ea = 119 kJ/mol). In situ X-ray absorption spectroscopy (XAS) experimentally verified that the proximity of the Cu species in zeolite is crucial in suppressing the decomposition of NH3.