Research on mechanism of catalytic reduction of CO₂ by alkali-heat treated SnO₂/g-C₃N₄

Electrocatalytic reduction of CO2 （eCO2RR） is a crucial pathway for China to achieve its "Carbon Peak and Carbon Neutrality" goals. Graphitic carbon nitride （g-C3N4） is widely employed as a catalyst or catalyst support in electrocatalysis due to its abundant raw materials， simple preparation process， and low cost. However， g-C3N4， when used as a catalyst support for eCO2RR， faces bottlenecks such as low specific surface area， weak alkalinity， and competitive reduction of low-concentration CO2 and O2 in actual flue gas. Therefore， this study proposed a modification strategy for SnO2/g-C3N4 composite catalysts through alkali-thermal treatment. Characterization techniques such as XPS and CO2-TPD revealed that the alkali-thermal treatment significantly increased the surface amino group content of the g-C3N4 support （from 3.7% to 7.0%）， enhancing the overall alkalinity and CO2 adsorption capacity of the catalyst （from 7.69 mmol/g to 48.80 mmol/g）. This treatment accelerated electron transfer from N atoms in g-C3N4 to Sn active sites， forming electron-rich Sn centers and generating more oxygen vacancies. These effects synergistically promoted the activation and reduction of CO2. Competitive adsorption experiments demonstrated that SnO2/CNOHHT exhibited a higher adsorption capacity for CO2 than for O2 in a CO2/O2 mixture （with a separation factor of 0.90）. This selectivity favors the formation of a CO2-enriched microenvironment at the reaction interface， thereby suppressing the competitive oxygen reduction reaction （ORR）. Electrocatalytic performance tests showed that under a pure CO2 atmosphere， SnO2/CNOHHT achieved a high Faradaic efficiency （FE） of 80.5% for formate production at a current density of 23.5 mA/cm². Remarkably， in a simulated flue gas atmosphere containing 4% O2， it still maintained a FE for formate of 59.4%， significantly outperforming both the unmodified and the alkali-treated-only catalysts. This study provides new insight for developing g-C3N4-based eCO2RR catalysts suitable for practical oxygen-containing flue gas conditions.