Density functional theory-based investigation of CaO/char catalyzing the transformation of NH3 to N2

During the pyrolysis of sludge, the heterogeneous reaction between char and NH3 can reduce the release of NH3. The low efficiency of this process can be increased by the addition of CaO. In this study, density functional theory (DFT) calculations are used to investigate the heterogeneous reaction mechanism of conversion of NH3 to N2 by char and char modified by adding CaO using 5-ring polycyclic aromatic hydrocarbon (PAH) as a char model. First, the adsorption characteristics of NH3 on char, CaO/char, and Ca/char were studied. The results indicate that among the six stable adsorption sites, the unsaturated sites in the char can provide adsorption sites for NH3, which can be adsorbed in the form of NH2(ad) and H(ad). Next, the activation process of NH3 in six adsorption configurations was studied. The results indicate that, on char, NH3 is stably adsorbed and cannot form NH2. radicals. The N-H fracture energy in NH2(ad) is lower than that in vacuum. Therefore, NH3 adsorbed to char can form NH(ad) to activate. In contrast, CaO/char and Ca/char can generate NH2. and H. free radicals through homolytic reactions and synergistically catalyze NH3. The activated NH3 continues to adsorb NH3 in the air to form N2. For char in the process of generating N2, the H atoms in N2Hx gradually transfer to the unsaturated C atoms and eventually lead to char inactivation; for CaO/char, the energy barrier of N2Hx to remove H atoms is significantly reduced, which promotes generation of N2. For Ca/char, the energy barrier in the process of removing H atoms from N2HX is higher than that of CaO/char, which is similar to that of char. However, the active sites of Ca/char are not occupied by H atoms and can permanently catalyze the generation of N2 from NH3. Therefore, adding CaO during the pyrolysis of sludge can effectively reduce the release of NH3. This more detailed understanding of the mechanism of catalysis of NH3 conversion to N2 by CaO/char during pyrolysis of sludge will provide guidance for future experiments.