A brief survey of the practicality of using photocatalysis to purify the ambient air (indoors or outdoors) or air effluents

Abstract

This brief survey attempts to evaluate the possibilities of using heterogeneous photocatalysis to purify air in the three title domains. Regarding indoor air, the formation of degradation intermediate products, some of which can be toxic or irritating, is the key issue for obvious safety reasons. Accordingly, commercializing individual purifiers is questionable because their management cannot securely be left to the user’s responsibility. When the purifier is incorporated in air ducts, adequate control can possibly be achieved permanently via proper sensors automatically adjusting the air flow to always deliver healthy air; also, if the pressure drop enables it, an adsorbing filter before air admission in the occupied spaces could be added. The main issue would then be the cost, which depends on the photocatalyst efficacy and its gradual decrease essentially due to solid deposits (including SiOx formed from Si-containing compounds present in the air). Regarding outdoor air, the rates are inherently low because of the scanty percentage of UV radiation in sunlight. Above all, because of the obvious need for the pollutants to be transferred to the photocatalytic surfaces, a potential significant effect of photocatalysis will be limited to rare confined spaces. Additionally, the deposition of dirt and photocatalytically-formed inorganic salts - which can accumulate in rain-sheltered locations - will further decrease over time the already low efficacy. Hence, the interest of photocatalytic materials for purifying outdoor air seems moot, despite some claims. Regarding air effluents, the practicality of the photocatalytic treatment obviously depends on the effluent contents in pollutants and water vapor, the volume to be treated over a given time, and the requested quality of the treated air. If, after trials, the treatment is thought to be economically viable for the targeted effluent, the industrial environment should enable the necessary monitoring. Nevertheless, research aiming at improving the reactor design, the photocatalyst activity and lifetime, and, possibly, the sunlight use, can be needed, on a case-by-case basis, for competing more efficiently with well-established processes.