Silicon-coordinated nitrogen-doped graphene as a promising metal-free catalyst for N2O reduction by CO: a theoretical study

Abstract

Metal-free catalysts for the transformation of N₂O and CO into green products under mild conditions have long been expected. The present work proposes using silicon-coordinated nitrogen-doped graphene (SiN₄G) as a catalyst for N₂O reduction and CO oxidation based on periodic DFT calculations. The reaction proceeds via two steps, which are N₂O reduction at the Si reaction center, producing Si-O*, which subsequently oxidizes CO to CO₂. The N₂O reduction occurs with an activation energy barrier of 0.34 eV, while the CO oxidation step requires an energy of 0.66 eV. The overall reaction is highly exothermic, with a reaction energy of -3.41 eV, mostly due to the N₂ generation step. Compared to other metal-free catalysts, SiN₄G shows the higher selectivity because it not only strongly prefers to adsorb N₂O over CO, but the produced N₂ and CO₂ are easily desorbed, which prevents the poisoning of the active catalytic sites. These results demonstrate that SiN₄G is a promising metal-free catalyst for N₂O reduction and CO oxidation under mild conditions, as the reaction is both thermodynamically and kinetically favorable.

Fig. 1. Structures of (a) SiN₄G, (b) SiN₄G/N₂O (N-bound), (c) SiN₄G/N₂O (O-bound), (d) SiN₄G/N₂, (e) SiN₄G/CO and (f) SiN₄G/CO₂. Selected bond lengths (Å) and E_ad values of adsorbed gas molecules on the catalyst are given.

Fig. 2. PDOS plots of selected atoms in (a) bare SiN₄G, (b) SiN₄G/N₂O, (c) SiN₄G/CO₂, (d) isolated N₂O gas and (e) isolated CO₂ gas. In (a) to (c), the PDOS peaks of Si, 4N, 8C and gas molecules are presented in the left panels, and the s- and p-states of selected Si and atoms of gas molecules are decomposed in the right panels. Positive and negative amplitudes of PDOS indicate the spin-up and spin-down states, respectively.

Fig. 3. Energy profiles of Path 1A (black line), Path 1B (red line) and Path 1C (blue line) with the corresponding structures.

Fig. 4. Energy profiles of Path 2A and 2B with the corresponding structures.

Fig. 5. Energy profile of the most favorable pathway of the N₂O reduction by CO on SiN₄G.