Understanding HCN heterogeneous adsorption on CaO(1 0 0) surface for the pyrolysis of sludge: A first-principles study and GCMC simulation

Abstract Pyrolysis and combustion of the sludge can eliminate the secondary pollution, and achieve the recovery of resource. However, it will produce large amount of nitrogen oxide. HCN is one of the major NOx precursors, and calcium oxide, the dehydrating agent and desulfurizer, have positive effect for HCN emission. Thus, the HCN on the perfect and defect CaO(1 0 0) surface heterogeneous adsorption is investigated through a first-principles calculation and Grand Canonical Monte Carlo simulation. Compared with single molecular model in our previous work, the CaO slab model with periodical boundary conditions has a better effect on describing the solid phase surface. The adsorption of HCN molecule on perfect CaO(1 0 0) surface proves to be chemisorption, and the most stable configuration is H-downward at O top site with the binding energy of −1.09 eV. Furthermore, HCN molecule adsorbs more tightly on to the O vacancy and Ca vacancy surface, and the binding energy increases to −2.76 eV and −2.79 eV, respectively. With the electron population and PDOS analysis, it reveals that the more electrons transfer from defect calcium oxide surface to HCN molecule. In the GCMC results, it is obvious that multiple HCN molecules easily adsorb on Ca vacancy surface. Meanwhile, HCN molecules are inclined to adsorb on the hollow site of the defect CaO(1 0 0) surface. From the isothermal adsorption curves, the adsorption ability decreases with the temperature increasing, and the fitted equation is coincide with the Freundlich isothermal adsorption equation. The variation of adsorption equilibrium constant K in the fitted equation indicates that the Ca vacancy surface is the most activity surface and the temperature increase is negative for the HCN adsorption, conforming to the adsorption rule calculated in the first-principle study. Thus, the point vacancy surface caused by uneven temperature can increase the adsorption ability of HCN on CaO(1 0 0) surface.