Diverse impacts of water on Pd/SiO2 catalysts for formaldehyde oxidation: Unraveling the crucial role of support defects

Usually, active hydroxyl groups play a crucial role in the catalytic oxidation of formaldehyde (HCHO), especially on supported noble metal catalyst systems. However, in present work, we observed an unexpected phenomenon where H 2 O exhibited an inhibitory effect on Pd/SiO 2 catalyst for HCHO oxidation. The activity of Pd/SiO 2 catalyst could be significantly improved by constructing Si and O double vacancy-defects (V Si V o , through acid-base etching) and oxygen vacancy defects (V o , through H 2 reduction). Interestingly, the inhibiting effect of H 2 O on HCHO oxidation was completely reversed to a promotion effect on the Pd/SiO 2 -(V Si V o ) catalyst, which was due to the fact that such catalyst had the ability to continuously activate H 2 O producing reactive hydroxyl species. Reaction mechanism study revealed that the HCHO oxidation on all Pd/SiO 2 catalysts followed two reaction pathways: (1) HCHO → HCOO → CO 2 ; (2) HCHO → CHO → CO → CO 2 , while the difference of active hydroxyl content on the catalysts resulted in varying degrees of formate accumulation on the catalyst surfaces. This study provides a promising strategy to enhance the H 2 O resistance of Pd catalysts and design high-performance noble metal catalysts for catalytic HCHO oxidation.