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No. 448: Effect on air quality of photocatalytic coatings

Frederickson, L. B., Russell H. S., Hertel, O., Ellermann, T., Jensen S. S. 2021. Effekt for luftkvaliteten af fotokatalytiske belægninger. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 58 s. - Videnskabelig rapport nr. 448. http://dce2.au.dk/pub/SR448.pdf 

Summary

Background and purpose

Air pollution has significant negative effects on human health, environment, and well-being, having significant socio-economic consequences. The total number of cases of premature deaths due to air pollution is estimated at around 4,600 for in Denmark in 2019 (Ellermann et al., 2021). The health effects related to exposure to nitrogen dioxide (NO2) result in approximately 360 premature deaths, which makes up almost 8% of all air pollution-related premature deaths (Ellermann et al., 2021).

Limit values have been set for NO2, the annual average concentration limit was exceeded at measuring stations in busy streets in Copenhagen until 2016. Since 2016, no exceedances have been measured, and in 2019, the annual average value was about 20% below the limit value (Ellermann et al., 2021).

One possible solution to further improve urban air quality is photocatalytic reduction of nitrogen oxides (NOx). The active substance in photocatalytic reduction is titanium dioxide (TiO2), which, once embedded or applied as a coating to surfaces, can convert NOx to nitrate in a catalytic process. Nitrate is a solid which is deposited on the coating and leached by precipitation.

The use of photocatalytic surfaces is an example of a tool that reduces air pollution. It is therefore a tool that is not source-based, i.e. it does not reduce the actual emission from e.g. a car, but instead reduces the content of outdoor air afterwards (Jensen et al., 2020).

Nitrogen oxides consist of nitrogen monoxide (NO) and NO2, where NO2 constitutes the harmful component to health. In the outdoor air, NO is in an equilibrium with ozone (O3) and NO2 under the influence of solar radiation and temperature. NO can thus be converted to NO2 in reaction with O3 (Reaction 1), and NO2 can be divided via photolysis into NO and a free oxygen atom (O) (Reaction 2), after which O3 is also recovered via reaction between O and the oxygen in the air, O2 (Reaction 3) (Stockwell et al., 2012).

In a street with NOx emissions from traffic, about 10-15% of NOx emissions will be in the form of NO2 and the rest NO (Carslaw et al., 2016). The presence of O3 will convert part of the emitted NO to NO2. In relatively busy streets, this conversion to NO2 was previously limited by the presence of O3, so that much of the NO was not converted to NO2. In the past, it has been the case that in relation to reducing NO2 in the outdoor air in cities, the ability of photocatalytic coatings to reduce NO2 was therefore far more important than their ability to lead to a reduction of NO. When this no longer applies, it is because the NOx concentrations in the Danish cities have gradually been reduced sufficiently that there is usually enough O3 in the air to convert the emitted NO from the traffic in the city street to NO2. This means that reductions in NO will also lead to a reduction in NO2 in most cases.

NOx is converted over time in the atmosphere to nitric acid and ends up as nitrate in particulate form. This is done by nitric acid being taken up on the surface of existing particles in the atmosphere, or it is done by gas-phase reactions between nitric acid and ammonia. Nitrate is contained in a large proportion of air particles with a diameter below 2.5 µm (PM2.5), and PM2.5 makes the largest contribution to the overall health effects of air pollution.

As the conversion from gaseous air pollution to particles in the atmosphere takes time, the emission of NOx in e.g. a city lead to the formation of nitrate far from the city, and similarly, a reduction of NOx in the city will not lead to a reduction in health effects related to particles in the city itself, but rather far from the city where the emissions took place; it can e.g. be in other cities. On the other hand, a decrease in local NO2 naturally means a reduction in health effects related to NO2.

In relation to large-scale effects, seen on a large geographical scale - for example at European level, NOx also contributes to the formation of O3 in the atmosphere, and O3 also has health effects. However, it is relatively complex, as the emissions of NOx also reduce O3, in the city itself, where the emissions take place.

The potential effect of photocatalytic coatings is thus reduction of NOx in outdoor air in streets and cities, and reduction of PM2.5 on a larger geographical scale.

Many studies have been performed on the effect of photocatalytic coatings, but the results are contradictory and there are still many unanswered questions. This report provides a summary of the results of scientific articles that have examined the effect of photocatalytic coatings. This work has primarily focused on field studies, where the results have been used to assess the effect of photocatalytic coatings on air quality.

The results are also reported in a review article published in a peer-reviewed international journal (Russell et al., 2021).

Method

To assess the effect of photocatalytic coatings, an extensive literature search has been performed. Scientific articles have been identified in the Web of Science and Scopus from 2005 to 2020 by direct searches on the keywords: "photocatalytic surfaces", "photocatalytic materials", "ambient air NOx removal", and "TiO2". Furthermore, the publication lists of selected authors, who has worked in the field, was reviewed as well as the reference lists from a number of key articles. The articles examined are divided into two main categories; laboratory studies and field studies. Thereafter, the studies are divided according to their main focus within the following categories:

Laboratory studies examining:

·       Improvements to the photocatalytic materials

·       Impact of physical parameters on photocatalytic efficiency

·       The durability of the photocatalytic material and its effect

Field studies examining the effect on air quality of the following:

·       Photocatalytic coatings on horizontal surfaces (e.g. streets and pavements)

·       Photocatalytic coatings on vertical surfaces (e.g. walls and facades)

·       Photocatalytic coatings in semi-enclosed areas (e.g. tunnels and car parks)

In this report, the main focus is on the field studies, as the goal is to assess the effect of the photocatalytic coatings on air quality in the real world, in the outdoor environment. This is in contrast to the laboratory studies, which are mainly performed under conditions that are unrealistic for real outdoor environments.

The report is thus based on 115 scientific papers, which deal with laboratory studies, field studies, computational studies, review papers and studies that combine all of the aforementioned.

The effect of photocatalytic coatings for air purification

Photocatalytic materials and coatings are an active area of research where improvements in selectivity and activity continue to be explored. Field studies give very different results for the effect of the photocatalytic coatings on air quality. Many studies have observed insignificant reductions in NOx. In contrast, other but fewer studies observed reductions of up to 80% in the concentration of NOx in the air near the photocatalytic coating. Under realistic, standardized conditions, the available field studies show that photocatalytic materials used in a street space can be re-evaluated to have an upper limit of about 4% removal efficiency in daytime hours and 2% or less if diurnal averages are considered.

Studies have generally shown that the photocatalytic materials reduce NO more efficiently than NO2. When all factors have been considered, there are no convincing results for documentation of significant NO2 removal. The studies that report a reduction in NOx are most often driven by the reduction in NO and not the reduction in the more harmful NO2. In addition, it has been shown in many studies that the use of unmodified TiO2, for example, reference materials P25 (Evonik Degussa), will typically result in a production of NO2 when NO is introduced on the surface. This has also been confirmed in field studies, for example, in Folli et al. (2015), where the total NOx reduction was 30%, while insignificant changes were seen in the NO2 concentration.

A crucial point in assessing the effect of the photocatalytic materials is the durability of the photocatalytic effect. The durability of the effect has been studied, but many of the durability studies do not quantify clear changes in effect over time or exposure. Instead only some removal is shown after a longer exposure period. From the available studies, which make quantitative comparisons of performance before and after exposure to NOx in the laboratory or before and after installation in the field, it is clear that durability is a problem for the photocatalytic materials. It is shown that their lifespan is in months rather than years, and in some cases days. There are short-term losses of performance, which is mainly due to nitrate buildup, which is at least partially reversible, e.g. through nitrate removal by precipitation or road cleaning. There are also long-term, irreversible losses due to wear and tear and poisoning. Poisoning refers to chemical compounds causing permanent damage, thus preventing the photocatalytic material from functioning properly.

There is generally great uncertainty about the performance of photocatalytic materials across the field studies examined. This can be partly attributed to the lack of standardized protocols and the use of various photocatalytic materials and underlying materials. Based on the studies investigated, there is insufficient evidence that the photocatalytic materials can have a long-term effect on the improvement of air quality with regard to the reduction of NOx in outdoor environments. This is due to uncertainties regarding:

  • The surface must be sufficiently active under outdoor conditions to continuously reduce the concentration of NOx. The outdoor conditions may include high relative humidity, low concentration levels of the pollutant components, high wind speed and low light intensities, which are unfavorable conditions for photocatalytic reduction of NOx.
  • The durability of the surface is sufficient to withstand outdoor conditions without losing significant losses of photocatalytic activity.
  • The surface area to volume ratio is large enough so that enough air comes into contact with the surface, thus ensuring a significant NOx reduction.
  • The photocatalytic process does not form by-products that could potentially degrade air quality.

The reason for the mixed results is partly the lack of protocol or standardization of the field studies. Therefore, the studies are difficult to compare as many are performed with different methods and different materials with or without modifications. The main differences are:

  • The distance between the photocatalytic material and the area where the pollutant concentrations are measured.
  • Different photocatalytic materials and surfaces.
  • The length of the study and time resolution.
  • Surface area to volume ratio at the test site.
  • Meteorological conditions.
  • Errors in method, for example, by comparing an ‘active’ area with a control area where the areas are not directly comparable.

Recent laboratory studies are more directly comparable as many are performed using the same ISO protocols. Approximately half of the laboratory studies examined are performed under an ISO protocol, the remainder being performed under a wide variety of test and quantification methods, which makes them difficult to compare. However, it should be mentioned that the ISO protocol is not optimal for comparison with field studies, as it is based on an initial concentration of 1 ppm NO, which is unrealistic for outdoor conditions where the NO concentration is much lower (typically 10 to 50 ppb), i.e. a factor of 100 to 20 lower. If accurate, standardized field tests are performed with new and improved materials, then it is more likely that the implementation of photocatalytic surfaces in the urban environment will be possible based on a consensus on the effect the photocatalytic coatings can have on air quality.

It has been shown in both laboratory and field studies that physical parameters such as temperature, relative humidity, solar radiation and airflow/wind speed are of great importance for efficiency. This means that in colder, more humid and cloudy climates or areas with higher wind speeds, the photocatalytic materials will be less efficient. These are meteorological conditions that characterize Denmark.

The transition from documenting removal efficiency in laboratories to documenting efficacy for a specific area is a complicated process as a large number of known and unknown parameters are involved. Therefore, more studies are still needed that perform the experiments on a larger scale following improved standardized methods to demonstrate the effectiveness of the materials under realistic conditions, and assess the durability of the photocatalytic coatings over time.

Additional problems with the photocatalytic materials are related to deactivation, production of harmful by-products (such as nitric acid and O3), the release of TiO2 particles and conversion of NO to the more harmful NO2. However, many studies have developed and investigated modifications to the photocatalytic materials to improve performance, including selectivity, activity, longevity, and greater absorption in visible light. It has been shown that in specific cases, e.g. tunnels (Table 2.1), where the surface area to volume ratio is huge and the UV radiation can be controlled and increased, then there can be a reduction of NOx of 20% in the air in the tunnel compared to before the application. However, it has also been shown that heavily polluted environments (such as a heavily trafficked tunnel) can deactivate the photocatalytic materials relatively quickly.

Photocatalytic coatings as a tool for air purification

Although low NOx reductions are generally reported across field studies, photocatalytic coatings cannot be depreciated as they are relatively inexpensive to implement.

So far, however, there is a fundamental problem with photocatalytic coatings as a tool, the effect is very modest, very variable and, above all, uncertain. For the vast majority of other tools, there is not the same uncertainty as to how large the effect of the tool is.

Finally, there is great uncertainty about how durable a given NOx reducing effect is over a longer period of time, which has practical significance for using the instrument, and assessment of the significance for health effects and derived effect on the economy.

In connection with an assessment of the use of photocatalytic coatings as tools, the technology should be compared with other methods to improve the air quality in the urban environment. Some of the things that should be weighed are the following:

·       An advantage of photocatalytic coatings compared to many other instruments is that they can be implemented relatively quickly, where most other instruments take longer to implement. On the other hand, there are problems with the durability of the effect of the photocatalytic agents, which is not the case for most other types of agents.

·       Photocatalytic coatings have the potential to reduce NOx in a street space but do not reduce greenhouse gases such as CO2, whereas other means, such as electrification of road transport do. This may speak in favor of using other means rather than photocatalytic coatings if both air pollution and climate are to be addressed.

All in all, it is assessed that today there is no consistent knowledge base, which documents that catalytic coatings can be considered as an effective tool for improving the air quality of NOx in the outdoor environment.

Future research should focus on improving the efficiency of the photo-catalytic coatings against both NO and NO2, as well as the durability of the effect. Furthermore, the focus should be on developing laboratory methods that come as close to the conditions in the outdoor environment as possible, so that it becomes possible on the basis of these to give realistic assessments of the effect in the outdoor environment. In addition, models should be developed that can assess the effect of photocatalytic coatings in different outdoor environments.