Mechanistic insight into the selective catalytic reduction of NO by NH3 over low-valent titanium-porphyrin: a DFT study

In this work, the reaction mechanism of ammonia selective catalytic reduction (NH3-SCR) of nitric oxide over a low-valance Ti-porphyrin catalyst was studied by density functional theory (DFT) calculations for both low- and high-spin states. The reaction proceeds via (i) NH3 complexation with the Ti-porphyrin complex, and its subsequent oxidation to NH2, with a large activation barrier of 32-34 kcal mol(-1) because of the difficulty of N-H bond dissociation. (ii) Bonding between NO and the NH2 ligand forms an NH2NO intermediate by an Eley-Rideal-type mechanism. The calculated activation energies for this step are 4.34 and 10.22 kcal mol(-1) for the low-and high-spin states, respectively. (iii) Formation of NHNOH by rearrangement of the NH2NO intermediate. Spin crossings in steps (ii) and (iii) play an important role in the overall reaction by providing a mechanism with a smaller activation energy of 17.05 kcal mol(-1), compared with 28.02 kcal mol(-1) for the un-catalyzed reaction. (iv) In the final step, the decomposition of NHNOH results in the formation of N-2 and H2O molecules, with a small energy barrier of approximately 7-9 kcal mol(-1). For pairwise pathway comparisons, Ti-porphyrin in the triplet state offers 8.43 kcal mol(-1) greater stability than the singlet does, and the reaction is more likely to proceed through a high-spin pathway because of its lower relative energies compared to the low spin. The obtained activation energies for NH3-SCR of NO are comparable with theoretical results for the reduction of NO over V2O5 and Fe-zeolite systems. Thus, Ti-porphyrin has potential as an alternative catalyst for NH3-SCR of nitric oxide.