Effect of crystal facet on birnessite for high efficiency decomposition of formaldehyde: Understanding facet-dependent reactivity by experiment and DFT calculations

Crystal facets in the crystalloid own interior properties, which offer significant functionalities in the catalysis area. Birnessite is a promising catalyst for HCHO oxidation, but it still faces challenges in catalytic efficiency. In this work, we synthesized a novel nano-sphere-like birnessite (NSL-Birnessite) exposed massive (002) crystal facets through substrate-induced oriental nucleation method. Compared with the known bulk-like birnessite exposed only (001) crystal facets (B-Birnessite) and nanowire-like birnessite exposed few (002) crystal facets (NWL-Birnessite), the HCHO conversion was 57% in 80 ppm HCHO simulated air with a gas velocity of 60 L/g h at ambient temperature using NSL-Birnessite, while it was 39% and 26%, respectively, using NWL-Birnessite and B-Birnessite. Based on the DFT calculation and in-situ DRIFTS results, the decomposition of intermediates including DOM and formates were the main obstacle to overcome via transition state search method. The H2-TPR and O2-TPD tests confirmed that the NSL-Birnessite mineralized these intermediates faster than NWL-Birnessite and B-Birnessite, due to the abundant oxygen vacancies caused by (002) crystal facet, which weakened the Mn-O band and offered more active oxygen species. Moreover, the oxygen vacancy formation energy on (002) facet is lower than other facets. Therefore, the contents of (002) facets in birnessite can greatly improve the catalytic activity.