Synthesis of Pd/SiO2 catalyst with outstanding ambient-temperature formaldehyde decomposition via NH3 treatment of silica supports

Abstract

Formaldehyde (HCHO) is a significant indoor pollutant found in various sources and poses potential health risks to humans. Noble metal catalysts show efficient and stable catalytic activity for ambient-temperature HCHO oxidation, yet suffer from low metal utilization. Efforts focus on designing catalysts with enhanced intrinsic activity and reduced noble metal loading. In this study, we developed a simple pretreatment method using ammonia solution on SiO2 carrier to enhance the activity of the Pd/SiO2 catalyst for HCHO oxidation. After the carrier was pretreated with an ammonia solution, a significant promoting effect was observed on the Pd/SiO2(NH3·H2O)-R catalyst. It achieved almost complete oxidation of 150 ppmV of HCHO at 25 °C, much better than the Pd/SiO2-R (5% HCHO conversion rate). Multiple characterization results indicated that the ammonia solution pretreatment of the SiO2 carrier increased the surface defects, facilitating the anchoring of Pd nanoparticles and increasing their dispersion. The increase dispersion of Pd resulted in the generation of additional oxygen vacancies on the catalyst surfaces. The increase in oxygen vacancies on the catalyst was beneficial for enhancing the catalyst's ability to activate H2O to form surface hydroxyl groups, thereby accelerating the catalytic oxidation process of HCHO. The reaction mechanism of HCHO on the Pd/SiO2(NH3·H2O)-R catalyst mainly follows an efficient pathway: firstly, the HCHO being oxidized by surface active hydroxyl groups to formate; subsequently, the formate being oxidized by hydroxyl groups to H2O and CO2. This study provides a promising strategy for designing high-performance noble metal catalysts for HCHO catalytic oxidation.