Mechanistic insight into the effects of alkali metal ions on the formation of NOx precursors during the pyrolysis of 2,5-diketopiperazine

The inherent metal ions in food wastes have significant influences on the production of NOx precursors to affect the release of NOx during the incineration process. However, the detailed catalytic mechanism of metal ions remains unclear. Herein, 2,5-diketopiperazine (DKP) was employed as the N-containing model to study the effects of Na+ and K+ on the production of NOx precursors in the pyrolysis process. Based on density functional theory calculations and wave function analyses, all the energy barriers of the initial reactions decrease with the catalysis of Na+, whereas K+ can only promote part of the reactions and with weaker effects than Na+. Because of the varied influence on distinct reactions, Na+/K+ can change the rate-determining steps of the production pathways of NOx precursors. As for the same pathway, Na+ usually results in a relatively lower energy barrier than K+. Elevated temperatures lead to higher rate constants of K+ than Na+ for the favorable production pathways of NOx precursors. With the catalysis of Na+ and K+, the formation of NH3 is always more competitive than HCN and HNCO based on the comparison of overall energy barriers and the rate constants. Particularly, the competitiveness for NH3 production is enhanced by the catalysis of Na+ and K+ according to the increased rate constant advantage over other pathways.