Research progress of monolithic catalysts for VOCs oxidation

Volatile organic compounds (VOCs) serve as primary precursors to fine particulate matter (PM2.5) and ozone (O-3), causing serious air pollution such as photochemical smog. The rapid urbanization and industrialization in China have led to a significant increase in VOCs emissions, resulting in severe negative impacts on the environment, economy, and society. Consequently, the prevention and control of VOCs pollution is highly valued by the government and various end-treatment technologies for VOCs have been developed. Among them, catalytic oxidation stands out due to its high treatment efficiency, broad applicability, and lack of secondary pollution. This method effectively removes VOCs at lower temperatures and is widely recognized as one of the most cost-effective VOCs treatment technologies. Operating on the principle of utilizing catalyst activation to reduce the reaction's activation energy, researchers have focused on designing and developing catalysts to enhance their performance. However, powder catalysts face limitations such as significant bed pressure drops and component loss. To better meet industrial needs, the preparation of monolithic catalysts has become a key research focus in VOC catalysis. Monolithic catalysts generally consist of a carrier and active components, which play the roles of support and catalysis, respectively, and sometimes a coating is introduced to increase the specific surface area of the carrier. The active component determines the upper limit of catalytic capacity, and the carrier affects the stability of the monolithic catalyst. In multiphase catalytic reactions, the reaction rates in the diffusion-controlled and chemical-controlled zones are primarily influenced by gas flow rate and intrinsic activity of the catalyst. For monolithic catalysts, these factors are associated with internal geometry and characteristics of active components, respectively. Therefore, the optimization of monolithic catalysts focus on the following points: (1) Enhancing mass and heat transfer capabilities of the carrier to facilitate swift reactions; (2) improving the dispersion of active phases on the catalyst surface and incorporating multiple metals for synergistic effects to maximize catalytic capacity; (3) increasing loading of active components, and ensuring strong adhesion to the carrier for enhanced activation potential. From the perspective of the composition and preparation of monolithic catalysts, the selection of suitable carriers, active components, and preparation methods emerges as a pivotal way to achieve high catalytic activity. In addition, in terms of driving the catalytic reaction, exploring the combination of multiple technologies and innovative energy supply approaches is a novel strategy to improve the catalytic performance of monolithic catalysts. Herein, this review focuses on discussing the carrier and active component types, preparation methods, and new coupling catalytic technologies of monolithic catalysts for VOCs oxidation. It provides a comprehensive introduction to the impact of different carriers, active component and preparation methods on catalyst activity, along with a brief overview of their respective advantages and disadvantages. Moreover, it highlights the advantages of new energy supply modes, and explores the synergistic effect between coupling technologies. Finally, this review presents an outlook on research prospects of monolithic catalysts for VOCs oxidation, aiming to provide a basis and reference for the design of integrated catalysts, the development of energy-efficient catalytic technologies, and their industrial applications.