Insights into High-Performance Monolith Catalysts of Co3O4 Nanowires Grown on Nickel Foam with Abundant Oxygen Vacancies for Formaldehyde Oxidation

Monolith catalysts of cobalt oxides grown on metal substrates exhibited good catalytic performance in the oxidation of volatile organic compounds (VOCs) and oxygen vacancies play a key role in activating. However, there is rarely a report about the increase in oxygen vacancies using a facile method in monolith catalysts. Besides, the corresponding mechanism of the promotional effects of oxygen vacancies over the monolith catalysts still remains elusive. In this work, Co3O4 nanowires with abundant oxygen vacancies in situ grown on the Ni foam (r-Co3O4 NW@Ni foam) were synthesized and used as high-performance monolith catalysts for catalytic oxidation of formaldehyde. The r-Co3O4 NW@Ni foam catalysts showed outstanding catalytic activity and stability. The T10 (temperature when HCHO conversion achieved 10%) of r-Co3O4 NW@Ni foam catalysts (75 °C) was lower than that of Co3O4 NW@Ni foam catalysts (100 °C) and Co3O4 NP catalysts (132 °C). More importantly, the r-Co3O4 NW@Ni foam catalysts have more active oxygen species because of the promotional effects of surface oxygen vacancies. In situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTs) results revealed that the formate species were reaction intermediates of HCHO catalytic oxidation reactions. However, the formate species on the surface of r-Co3O4 NW@Ni foam catalysts were more active thus could easily take part in catalytic reactions. Furthermore, combined with the calculation results, the abundant surface oxygen vacancies could weaken O2 adsorption energy and make r-Co3O4 NW@Ni foam catalysts adsorb and store more active oxygen species, thus promoting more reaction intermediates to generate with a higher rate in redox cycles. The present investigations in this work may lead to an alternative development of high-performance monolith catalysts for VOC catalytic oxidation.