What do we know about TiO2 and porous adsorbents?
After almost 40 years of research and study on photocatalysis, its practical application is still limited, mainly due to its slow kinetics and the challenges associated with reactor design. A possible solution to these issues could be combining photocatalysis with a second technology such as adsorption. This article reviews two interrelated areas of research: the first is the use of TiO2-supported adsorbent materials as enhanced heterogeneous photocatalysts and their application to various reactions for organic pollutant removal from air and water; the second is the combination of adsorbent materials with TiO2 photocatalysts which aims to efficiently regenerate adsorbent materials using illumination.
Various porous supports were considered including activated carbon (AC), alumina, zeolites and silica based materials. What is necessary for an effective supporting material is transparency, chemical inertness, stability on usage and recycling, and strong adsorption properties. An optimum system is pollutant and application dependent. For example, while the opaqueness of AC is an issue, its versatility as an adsorbent cannot be overlooked. Zeolites are the most promising supports given their good adsorption properties, high transparency and ease of regeneration. A limitation is their small pore size. Silica on the other hand offers a tunable pore size, high transparency and neutral framework which make it significantly attractive for application in both gas and aqueous-phase photocatalysis. Silica's adsorption properties are however less favourable than those of zeolites and AC. Finally, the mechanical integrity of TiO2/porous adsorbent composites is crucial for application, affecting recyclability and regeneration. Mechanical integrity can be optimised through careful TiO2/porous adsorbent synthesis.
A take home message is that each adsorbent has its own advantages for certain reactions and it is not possible to assign the appropriateness of one material for all pollutant types, however certain guidelines can be applied. As an overall vision, it can be concluded that applying a porous material as support for TiO2 could be a promising pathway for practical applications of photocatalytic TiO2 in pollutant removal either through one stage "adsorption – shuttle" or "adsorption – regeneration" approaches.
Read more on J. Hazardous Materials, 2018, 341, 404-423