A Review of Photocatalytic Materials for Urban NOx Remediation

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.

Challenges surrounding nanosheets and their application to solar-driven photocatalytic water treatment

“…PCCs have gained significant attention for NOx abatement in urban environments, though there is debate over their effectiveness (sources vary wildly, but it is likely <2% reduction). 207 The coatings used vary in the use of suspension medium, porosity, and roughness that the actual effectiveness of the PC is obscured. What does stay consistent is the choice of commercial P25-degussa, a now lower performing photocatalyst in a rapidly evolving field.…”

Photocatalytic NOx Oxidation of BiOCl Nanostructure-Based Films Grown Using Aerosol-Assisted Chemical Vapor Deposition

“…It should be noted that for oxidizing photocatalysts, NO 3 – remains bound to the surface, which can inhibit their activity, and therefore typically requires rainfall or washing to restore their activity . A range of building materials have been incorporated with such photocatalysts to remedy NO x pollution, including paints, concretes, asphalts, and tiles for streets, walls, and tunnels. ,,, Nevertheless, what has been less well studied to date is the activity of photocatalytic coatings on low-surface-area glass substrates, where a commercial photocatalytic coating for general-purpose self-cleaning shows nominal activity toward NO gas …”

Ozone Formation during Photocatalytic Oxidation of Nitric Oxides under UV Irradiation with the Use of Commercial TiO2 Photocatalytic Powders

“…In their study [ 8 ], the degradation of NO during photocatalytic oxidation was measured using photocatalytic coatings applied to mortars and glass substrates. The impact of physical parameters on the efficiency of NO x removal has been widely analyzed [ 29 ] using different test setups [ 30 , 31 , 32 ].…”

“…However, NO x oxidation can lead to the formation of various by-products other than NO 2 , such as ozone (O 3 ) [ 29 ]. It has been observed that ozone formation is facilitated by high concentrations of airborne pollutants along with nitrogen oxides in natural conditions.…”