A Review of Photocatalytic Materials for Urban NOx Remediation

Hugo Savill Russell,Ole Hertel,Louise Bøge Frederickson,Steen Solvang Jensen,Thomas Ellermann

doi:10.3390/catal11060675

Abstract

NOx is a pervasive pollutant in urban environments. This review assesses the current state of the art of photocatalytic oxidation materials, designed for the abatement of nitrogen oxides (NOx) in the urban environment, and typically, but not exclusively based on titanium dioxide (TiO2). Field trials with existing commercial materials, such as paints, asphalt and concrete, in a range of environments including street canyons, car parks, tunnels, highways and open streets, are considered in-depth. Lab studies containing the most recent developments in the photocatalytic materials are also summarised, as well as studies investigating the impact of physical parameters on their efficiency. It is concluded that this technology may be useful as a part of the measures used to lower urban air pollution levels, yielding ∼2% NOx removal in the immediate area around the surface, for optimised TiO2, in some cases, but is not capable of the reported high NOx removal efficiencies >20% in outdoor urban environments, and can in some cases lower air quality by releasing hazardous by-products. However, research into new material is ongoing. The reason for the mixed results in the studies reviewed, and massive range of removal efficiencies reported (from negligible and up to >80%) is mainly the large range of testing practices used. Before deployment in individual environments site-specific testing should be performed, and new standards for lab and field testing should be developed. The longevity of the materials and their potential for producing hazardous by-products should also be considered.

Highlights

Semiconductors, in particular titanium dioxide (TiO2), have been used for photocatalytic oxidation (PCO) in a number of fields since at least the 1980s [1]
It is important to note the conditions that were used for obtaining the results as, for removal efficiency (RE), a relative measure, the final results will depend on external parameters as well as the actual material tested
The field studies in this area can be split into trials taking place on (1) streets, where the PC material is the actual pavement or road surface, (2) walls, where PC active paint is applied to surrounding buildings or model street canyons and the PC surface is predominantly vertical, and (3) enclosed spaces, this is mainly tunnels and car parks

Summary

Introduction

Semiconductors, in particular titanium dioxide (TiO2), have been used for photocatalytic oxidation (PCO) in a number of fields since at least the 1980s [1] These include self-cleaning or ‘easy-cleaning’ outdoor surfaces, pollutant remediation from water, and abatement of volatile organic compounds (VOCs) in indoor air, with varying degrees of success [2,3,4]. There is a great deal of interest in this use as a fast, easy and inexpensive solution to the issue of high levels of NOx present in the urban environment. Its efficacy in these environments is still unproven .

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Conclusion