Catalytic reduction of N₂O by CO molecules using transition metal-phosphomolybdic acid (TM₁/PMA) single-atom catalysts: A theoretical perspective

The efficient catalytic conversion of hazardous gases (e.g., N₂O and CO) into non-harmful by products is important due to the severe environmental and health burdens posed by these gases. Here, quantum chemical studies have been carried out to investigate the catalytic reduction of N₂O by CO over a phosphomolybdic acid (PMA) cluster anchored with 3d transition metal atoms under mild reaction conditions. For N₂O reduction, all 3d-TM₁ adatoms on the PMA cluster have been systematically screened as single-atom catalysts (SACs). TM₁/PMA systems possess significant adsorption energies towards N₂O and CO, and co-adsorption energies of N₂O+CO are a necessary prerequisite for the start of the catalytic cycle. The results indicate that N₂O is decomposed first on the TM₁/PMA surface, forming N₂ and O-TM₁/PMA intermediate. The Ti₁-(0.53 eV), V₁-(0.40 eV), Cr₁-(0.86 eV), and Fe₁/PMA (0.94 eV) have low activation energy barriers that are comparable to those of the other catalysts that were chosen, making them active and selective catalysts for the N₂O decomposition. However, the remaining O atom on the TM₁/PMA was an active species for the oxidation of CO molecules. The Fe₁/PMA catalyst has an activation energy barrier of 0.43 eV and is a promising catalyst for the oxidation of CO. CO occupying the TM₁ site with stronger adsorption energy than N₂O will restrict the reaction’s effectiveness. A lower temperature can hinder the generation of the side products N₂ and O₂ generated due to the disproportionation of N₂O molecules. A concerted reaction mechanism can also initiate the reaction by first adhering CO molecules to the TM₁/PMA surface. The computed activation energy barriers of the rate-limiting step are (Ti₁=0.52 eV, V₁=0.76 eV, and Fe₁=0.88 eV), respectively. Thus, 3d TM₁/PMA is predicted to be an efficient catalyst for converting toxic gases N₂O and CO into non-hazardous N₂ and CO₂ under normal conditions.