Sodium Enhances Ir/TiO2 Activity for Catalytic Oxidation of Formaldehyde at Ambient Temperature

The development of highly efficient catalysts for ambient formaldehyde (HCHO) destruction is of great interest for indoor air purification. Here, we show that a sodium (Na)-doped iridium (Ir) catalyst (Ir/TiO2) is a highly active catalyst for the catalytic oxidation of HCHO at room temperature. We observed that Na addition dramatically enhanced the activity of the Ir/TiO2-R catalyst, and 100% HCHO conversion was achieved over Na-Ir/TiO2-R catalyst at a gas hourly space velocity of 100,000 h(-1) and HCHO inlet concentration of 120 ppm at 25 degrees C. The Ir/TiO2 and Na-Ir/TiO2 catalysts were characterized using X-ray powder diffraction, Brunauer-Emmett-Teller surface area testing, high-angle annular dark-field scanning transmission electron microscopy, H-2 temperature-programmed reduction (TPR), X-ray absorption fine structure, X-ray photoelectron spectroscopy, and CO-TPR The characterization results show that the addition of Na species had no influence on Ir dispersion on the TiO2 surface but greatly promoted the activation of both chemisorbed oxygen and H2O. The reaction mechanism of HCHO oxidation was investigated by using in situ diffuse reflectance infrared transform spectroscopy. The results show that the reaction mechanisms on Ir/TiO2-R and NaIr/TiO2-R both followed the direct formate oxidation pathway (HCHO -> HCOO + OH -> CO2 + H2O), and the activated oxygen species mainly participated in the formate formation step while the activated OH groups were primarily responsible for the subsequent formate oxidation. Because of the improved capacities for the activation of oxygen and H2O induced by Na addition, the Na-Ir/TiO2 catalyst demonstrated much better performance than Ir/TiO2 for ambient HCHO oxidation.