Theoretical Investigation of the Formation Mechanism of NH3 and HCN during Pyrrole Pyrolysis: The Effect of H2O

Abstract

Coal is a major contributor to the global emission of nitrogen oxides (NOx). The NOx formation during coal utilization typically derives from the thermal decomposition of N-containing compounds (e.g., pyrrolic groups). NH3 and HCN are common precursors of NOx from the decomposition of N-containing compounds. The existence of H2O has significant influences on the pyrrole decomposition and NOx formation. In this study, the effects of H2O on pyrrole pyrolysis to form NOx precursors HCN and NH3 are investigated using the density functional theory (DFT) method. The calculation results indicate that the presence of H2O can lead to the formation of both NH3 and HCN during pyrrole pyrolysis, while only HCN is formed in the absence of H2O. The initial interaction between pyrrole and H2O determines the N products. NH3 will be formed when H2O attacks the C2 position of pyrrole with its hydroxyl group. On the contrary, HCN will be generated instead of NH3 when H2O attacks the C3 position of pyrrole with its hydroxyl group. In addition, the DFT calculations clearly indicate that the formation of NH3 will be promoted by H2O, whereas the formation of HCN is inhibited.

Effect of alkali metal ions on the formation mechanism of HCN during pyridine pyrolysis

“…All calculations were performed using Gaussian 16 (Frisch et al 2016). Many studies have used DFT calculations to shed light on the pyrolysis chemistry of N-containing compounds (e.g., pyrrole, indole, and pyridine) (Liu et al 2018a(Liu et al , b, 2020. The stable geometries of all reactants, intermediates, transition states, and products were obtained through an initial unconstrained optimization using the B3LYP/6-31G (d, p) method.…”

“…The stable geometries of all reactants, intermediates, transition states, and products were obtained through an initial unconstrained optimization using the B3LYP/6-31G (d, p) method. The geometries of the transition states were located using the TS (Berny) method, which has been confirmed to be accurate for the optimization of organic species (Liu et al 2018a(Liu et al , b, 2020. In addition, according to Glukhovtsev et al (1997), B3LYP calculations on iron-containing species gave reliable results comparable with experimental data.…”

Effect of Alkali Metal Ions on the Formation Mechanism of HCN During Pyridine Pyrolysis

“…Based on the electron density of the system, many studies have used DFT calculations to shed light on the pyrolysis chemistry of Ncontaining compounds (e.g., pyrrole, indole, and pyridine) (Liu et al 2018a(Liu et al , 2018b(Liu et al , 2020. All stable geometries of the reactants, intermediates, transition states, and products were obtained using the B3LYP/6-31G (d, p) method, which has been con rmed to be accurate to optimize organic species (Liu et al 2018a(Liu et al , 2018b(Liu et al , 2020. Besides, according to Glukhovtsev et al (1997), B3LYP calculations on ironcontaining species gave reliable results, which were comparable with experimental data.…”

“…Corresponding minima and rst-order saddle points were veri ed to be on the same potential energy surface by following intrinsic reaction coordinate (IRC) calculations. Enthalpies at 298.15 K and 1 atm were used for the energetics discussion (Liu et al 2018a). The activation energy was de ned as the enthalpy difference between the reactants and the concerted transition states (Glukhovtsev et al 1997).…”

Formation mechanism of HCN and NH3 during indole pyrolysis: A theoretical DFT study

“…Consequently, merely studying the mechanism of pyrrole pyrolysis [6] is not comprehensive to reveal the formation mechanism of NOx pollutant precursors. In fact, nitrogen has been found in coal and coal tar in the form of indole [7,8].…”