Nitrogen oxides (NOx) levels in urban air are monitored routinely in many cities around the world because of the significant effect of nitrogen dioxide (NO2) on human health . NOx affects the quality of urban air close to ground, where traffic is the main source of NOx . The EU has since 2010 prescribed a calendar yearly average urban air quality of max 40 µg/m3 NO2 . Nevertheless, streets with intense traffic in Europe often show significantly higher yearly values, in some cases above 80 µg NO2 per m3 . Even if the NO2 is kept 50% below the EU directive limit, severe health impacts are foreseen . For these reasons, vehicles are in many cases met with restrictions in terms of limited access to city centers or in terms of strict exhaust standards demanding the use of catalytic converters for controlling NOx emissions. New vehicles on the market are equipped with end-of-pipe catalytic converters , and even when biofuels are used in vehicles, converters would be required to control exhaust emission .
In recent years, the use of photocatalytic surface coatings   on pavements, roads and buildings has been introduced with the purpose of reducing NOx levels in urban air. This technology has, for example, been used in concrete and asphalt pavements, as well as paints  . The photocatalytic technology has been tested in several major cities around the world and has been shown to reduce NOx levels in urban air close to ground .
The catalytic converters used by vehicles and the photochemical coatings used on hard surfaces in cities are complementary NOx-reducing technologies, one regulating NOx emissions from vehicles, the other reducing NOx levels in urban air close to ground. However, the two technologies are very different in terms of the resources required for their production, operation and end-of-life (life-cycle stages), as well as their effect on other air pollutants. In terms of environmental sustainability, the two technologies may have an overall different environmental performance, taking into account environmental burdens for their production, operation and end-of-life compared to the benefits of air pollution control. The environmental profile of these technologies must be quantified for a balanced development of their roles and applications in reducing the levels of NOx in urban air.
Life Cycle Assessment (LCA   ) is a widely used standardized method for the quantification of potential environmental benefits and burdens connected to products and systems. LCA provides a stringent methodology for formulating and assessing the resources used for the life-cycle stages of products and services: production, operation and end-of-life. In this context, LCA can provide a systematic framework for the assessment of the environmental performance of NOx-reducing technologies contributing to NOx removal in urban air.
Few past LCA studies have assessed NOx reduction via photocatalytic surfaces or end-of-pipe catalytic converters. An LCA study of photocatalytic coating showed environmental benefits with respect to environmental indicators such as acidification, eutrophication, air pollution and smog formation exceeding the environmental impacts connected to the production phase . Similar findings are presented in another study on highway infrastructure with photocatalytic coating . A comparative LCA on the operation phase of concrete overlays for buildings with and without photocatalytic properties showed the benefits of adding photocatalytic coating with respect to traditional concrete overlays in all indicators considered in the study . The environmental impacts connected to the production, operation and end-of-life of vehicle catalytic converters have also been assessed by a few studies  . Belcastro  concluded that the majority of impacts were related to the use phase of the car converter, and in spite of additional fuel use, the converter reduced significantly the impacts on human health and ecosystems.
The results of the few LCA studies available in the literature cannot be generalized nor compared for assessing the environmental performance related to the function of NOx removal. Past LCA studies are often focusing on different products (e.g. concrete overlays or highway infrastructure where NOx removal is an additional function   ), on different life-cycle phases of the products (e.g. operational phase only  ), or simply on different data sources and life cycle modelling assumptions.
The aim of this study is to quantify the potential environmental impacts of reducing NOx levels in urban air close to ground with two complementary NOx-reducing technologies: A photocatalytic surface coating and a vehicle catalytic converter. The aim of the assessment is to assess the environmental profile of the two NOx-reducing technologies separately, in terms of resources required for production, operation and end-of-life of the technologies, opposed to the environmental benefits obtained during operation. This assessment aims to support a balanced development of both NOx-reducing technologies contributing to the reduction of NOx levels in urban air.
2. Materials and Methods
This study quantifies with LCA the potential environmental impacts connected to two different NOx-reducing technologies. The following paragraphs report the LCA methodology (2.1) and assessment (2.2). Principles and inventory of photocatalytic surface coatings and principles and inventories of vehicle catalytic converters are presented in the following sections (3.1 and 3.2).
2.1. LCA Methodology
The environmental assessment of the two NOx-reducing technologies follows the principles of the LCA ISO standard procedure  . Moreover, the environmental assessment presented follows the recommendations of the European Union for Product Environmental Footprint (PEF) studies, aiming at increasing the reproducibility, consistency and comparability of environmental assessments of products and services   .
The two technologies assessed are a photocatalytic surface coating and a vehicle catalytic converter providing the service of reducing NOx in urban air. The environmental assessment of the two technologies is not a comparative assessment. The study does not aim to benchmark one technology against the other, as they belong to different service groups (manufacture of parts and accessories for motor vehicles and manufacture of chemical products  ). The intended application of the LCA is to support researchers and developers for both technologies for the development of technology applications for reducing the NOx levels in urban air.
This study assesses the two technologies in terms of their environmental performance with respect to a common functional unit (FU), which describes the common function provided by both technologies. The FU is complete reduction of NOx in urban air close to ground throughout the technology lifetime in Europe for the year 2021. The NOx reduction principles are reported in sections 3.1 and 3.2. The technology lifetime is considered to be 10 - 15 years for both the photocatalytic surface and the vehicle catalytic converter (refer to sections 3.1 and 3.2 for more details). The reference flow represents the amount to which all the resource flows in the environmental assessment are related to: reduction of 1 kg NOx in urban air close to ground.
Figure 1 provides a generic representation of the system boundaries of the LCA. The LCA takes into account the resources required for production, operation, and end-of-life of the technologies in a gate-to-grave perspective. Examples are materials and energy utilized for the production process. During operation, the system boundaries include removal of NOx, as well as removal or emissions of additional substances (e.g. pollutants). In the assessment of the two technologies, we pay attention to the following aspects, albeit their significance may vary considerable between the technologies:
● Production: Materials and energy used in producing and applying the technology.
● Operation: Materials and energy used in operating and maintaining the technology.
● End of life: Recycling and final disposal after use.
Figure 1. Representation of the system boundaries of the study and the product life-cycle phases considered.
● Removal of NOx and other air pollutants: Amount removed and end products of removed pollutants within product lifetime.
The life cycle inventory modelling (LCI, i.e. the representation of the technologies by means of the flows of resources required for their production, operation, and end-of-life) for both technologies is reported in detail in the following sections (3.1 and 3.2). The LCI modelling of the technologies was compiled based on literature data and personal communication with industry. The LCI modelling approach is attributional, representing flows of resources, and their environmental footprint with average data. The inventories for the resources utilized in the life cycle of the technologies were retrieved from the Ecoinvent database, version 3.7.1, allocation at the point of substitution . Multi functionality, which occurs in the model because the technologies also affect other air pollutants than NOx, is addressed with system expansion (e.g. further substances removed during operation, recovery of energy, and materials from end-of-life treatment).
2.2. LCA Results Interpretation
The LCIs for the two NOx-reducing technologies (Section 3) are assessed in terms of environmental impacts (life cycle impact assessment phase, LCIA). The study utilizes the European Commission’s recommended Environmental Footprint LCIA methods (EF.3), including 16 midpoint impact categories  . The impact categories included are climate change (CC), ozone depletion (OD), human toxicity (cancer, HTC and non-cancer effects, HTNC), particulate matter (PM), ionizing radiation (IR), photochemical ozone formation (POF), acidification (A), eutrophication (terrestrial, TE, aquatic freshwater, FE, and marine, ME), freshwater ecotoxicity (FET), land use (LU), water use (WU), and resource use (minerals and metals, RDM, and energy carriers, RDE). The results presented in this LCA study are generic midpoint impact potentials and do not predict impacts on category endpoints, threshold levels, safety margins or risk levels, nor impacts associated with any specific application. LCA results are further normalizes as person equivalents (PE) based on the global normalization factors of Sala et al. .
The time horizon of the inventory is the lifetime of the technologies (10 to 15 years), while the time horizon of the impact characterization is 100 years. We estimate that data collected and used for the LCIs of the two NOx-reducing technologies is representative of their current technological performance and valid for the next 10 years (in accordance to the lifetime of the technologies).
The result assessment phase involved quantifying the relative contribution of each process involved in the life-cycle of each technology, for each impact category assessed. The results focused on the benefits of NOx reduction versus the environmental burdens connected to provision of resources required to produce, operate, and dispose the technologies. Although characterized and normalized results are reported for all impact categories in the Supplementary Material (SM), the focus of the manuscript is placed on the impact categories of POF, PM, A, TE, FEM ME, and CC. The results obtained are discussed with respect to assumptions adopted for the study and with respect to previous literature findings.
The LCA was carried out with the LCA model EASETECH, developed at the Technical University of Denmark . Microsoft Excel was used for calculating the LCIs and for the interpretation and analysis of the results of the LCA model.
3. NOx Removal Principles and LCIs
The following sections summarize the principles of photocatalytic reduction for the two NOx-reducing technologies (3.1.1 and 3.2.1), the technologies and their life-cycle phases (3.1.2 and 3.2.2), and the LCIs utilized for the LCA (3.1.3 and 3.2.3).
3.1. Photocatalytic Surface Coating
The basic principle of photocatalysis was first discovered five decades ago by Fujishima and Honda , and the possibilities for using titanium dioxide (TiO2)-based photocatalysis for removal of various pollutants have since been the subject of various studies   . The photocatalytic properties of TiO2 are based on the semi-conducting properties of the crystalline form of the mineral. TiO2 nanoparticle exists in three polymorphs, anatase, rutile and brookite. However, the anatase form is the most widely used for photocatalytic purposes . The complex process of NOx absorption and degradation at the TiO2 surface is described in detail elsewhere  . Illuminated by UV light, the anatase in contact with air and water produces hydroxyl radicals OH∙, which oxidize NO2 to nitrate ( ):
The resulting is extremely soluble in water; hence it is easily washed away. Since the main catalytic property of the TiO2 surface is the effective formation of OH radicals, the levels of ozone (O3), volatile organic compounds (VOCs), and sulphur oxide (SOx) in the air may also be affected by the contact to the catalytic surface   . However, Mothes et al.  did not find photocatalytic degradation of the studied toluene and isoprene.
3.1.2. Photocatalytic Surface Technology and Life-Cycle Phases
We establish the background data for the LCI for the product NOxOFF supplied by Photocat (Denmark) and consider its application as part of a pre-fabricated concrete element used as pavement and as a sprayed coating on asphalt road.
The production of the NOxOFF products is based on commercial raw materials that are ground and mixed; a SiO2 product serves as a binder material and ammonia acts as a buffer to stabilize pH. Production of 1 kg of product is summarized in Table 1.
Table 1. Production data for 1 kg NOxOFF product for two applications (data from Photocat).
The product is transported to the application in black plastic containers (IBC) holding 1000 kg. The container consists of high density polyethylene (HDPE) and weighs 59 kg. The containers are used 5 - 10 times and afterwards sent to a municipal waste incinerator with energy recovery.
When used in concrete elements, the product is introduced during the manufacturing of the concrete product before the product is cured and the NOxOFF catalyst is integrated into the top 2 mm of the final product used for the final surfaces.
When used on asphalt roads and parking lots, the product is sprayed applied when the initial surface bitumen is worn off and the NOxOFF product can adhere and bind to the mineral granulates in the surface of the asphalt. The product is spray-applied on existing asphalt pavements using common equipment. We neglect any environmental burdens from the application process.
Modelling of operation of photocatalytic surfaces is based on full-scale experiences with the photocatalytic surface has been tested during the last 15 years  . Pedersen et al.  proposed a scheme for evaluation the different real-world studies including lifetime and durability. The lifetime not only depends on the quality of the coating but also on the quality of the material to which the coating adheres. We estimate that the lifetime of the photocatalytic concrete pavement elements is 15 years and in case of the asphalt road 10 years. We do not expect any maintenance during the lifetime and we do not expect that the photocatalytic surface coating affects other kinds of maintenance that may routinely take place on the surfaces. Faraldos et al.  and Bendix et al.  showed that the coating has a cleaning effect on the surface but we have not been able to quantify this additional effect into a format suitable for the LCA.
For the NOxOFF product, three major real-world demonstrations were performed during 2012-2019 . From the study in the city of Roskilde in Denmark, it was calculated that the NOxOFF product removed 13.8 g of NOx/m2 per year. This was based on laboratory ISO test results, which were correlated to real-life results with good approximation. The tests showed a NOx removal efficiency of approximately 4.83 mg/m2 per hour. The UV influx in Denmark is around 50 kWh/m2 per year. is the oxidized product. This means that 10 m2 of coated surface during a 10 year lifetime removes 1.38 kg NOx and produces around 2.65 kg . We assume that the is dissolved into water and that the associated acidity production is only of marginal significance in the water that washes the surface during the lifetime of the product. We take into account the produced and, although the destination may vary, we assume that marine surface water will be ultimately affected.
In the LCA modelling we also considered effects on other air pollutants. Baral et al.  as well as Hernández-Alonso et al.  quantified the removal of SOx by photocatalytic surfaces. By scaling the removals of SOx and NOx observed over a 15 year life time , we include a removal of 0.033 kg SOx per kg NOx removed. The SOx is converted to sulphate that is washed off. The data on O3 removal is scarce  and not yet quantified relatively to the NOx removal, hence we did not include it.
The photocatalytic surface is not reused or recycled after its lifetime. We assume that the main product amended with surface coating after the end-of-life of the main product will enter into traditional material recycling and disposal and that the presence of the coating will not affect the environmental aspects of the material recycling and disposal. The photocatalytic effect is assumed lost when the material is being recycled. We assume that the final disposal of the photocatalytic coating occurs together with the concrete and asphalt pavement to which it is bound, and that the environmental profile of the recycling is not affected by the presence of the photocatalytic material.
3.1.3. LCI Inventory for Photocatalytic Surface Coating Removing 1 kg of NOx
Based on the above data compilation, the LCI for 1 kg NOx removed by photocatalytic surface coating has been developed as presented in Table2. Table2 summarizes resources in terms of materials and energy required for each life-cycle phase of the technology. The inventory differs with respect to photocatalytic surface coating applied to concrete or to asphalt pavement. The corresponding Ecoinvent processes utilized for the modelling of photocatalytic surface coating are reported in the SM, TableS1 and TableS2.
3.2. Catalytic Converters in Vehicles
The knowledge about the ability of certain surfaces to enhance chemical reactions is very old and has been used in catalytic converters applied as end-of-pipe technologies for vehicles since 1975 in US and since 1985 in Europe  . Today new cars are supplied with catalytic converters as emission control technologies. This applies to passenger cars, vans, and trucks as well as to gasoline and diesel driven vehicles. Several types of catalytic converters exist , but we have been able to build stringent LCI data only for the TWC (Three-Way Catalyst) which is often used on gasoline-driven passenger cars. Diesel cars and trucks use other types of after treatment systems, for example the Urea-Selective
Table 2. LCI inventory for 1 kg of NOx removed by photocatalytic surface coating. Positive values are uses and negative values are savings.
Catalytic Reduction (Urea-SCR) which uses urea additive to convert NOx to N2 and water . The TWC-converter creates reductive and oxidative reactions at elevated temperature that decrease/remove the content of air pollutants in the engine exhaust. Several reactions take place  and many air pollutants are affected. The dominating overall catalytic conversions are:
where THC is total hydrocarbons.
The TWC converter has first a reductive section where NOx is removed and a second oxidative section where CO and THC are removed. During the NOx reduction, oxygen is generated and hence additional oxidation reactions may take place in the reductive section. Some oxygen will feed into the oxidative section of the converter and contribute to the oxidation of CO and THC. The catalytic converter increases the fuel consumption because of the pressure loss in the converter.
3.2.2. Catalytic Converter and Life-Cycle Phases
Laboratory test as well as in-situ measurements on vehicles in operation have documented the effect of catalytic converters on vehicle emissions , but the studies have also demonstrated that the effect depends on many technical factors as well as temperature and mode of operation .
We establish the background data for the LCI for the TWC converter based on information in the literature. Due to the complexity of the reactions and the integrated function of the reductive and the oxidative sections of the converter, we establish two sets of data. A first data set for the whole converter considering the combined effect of the converter, and a second data set for the first part of the converter assuming that the reductive section only removes NOx. We aim at establishing robust LCIs of general value and, although we rely on specific data from Belcastro , we assume that the data can be generally representative of the performance of a generic catalytic converter, irrespective of vehicle, product or driving conditions.
A vehicle catalytic converter is contained in a steel canister and operates typically around 350˚C with a conversion efficiency above 90% . The exhaust gas is routed through two chambers with honeycomb structures with coated surfaces providing large catalytic surfaces sustaining the reactions. The catalytic elements are typically rhodium in the reductive section and platinum and palladium in the oxidative section. However, variations exist in both the construction, the catalyst, and their catalytic metal composition. Typical data are compiled in Table 3.
The cordierite is specifically produced as the honeycomb structure and no database offers an LCI for this product. We use data based on the compilation provided by Belcastro  as presented in Table 4 for a Cordierite honeycomb structure
Table 3. Production data for 1 TWC converter (compiled with data from various studies     ).
**Data are half the data from the TWC converter except the rare elements, which are distributed according to their function; **Coarse estimates based on data used by Belcastro .
Table 4. Production data for a 1400 gram cordierite honeycomb structure for a TWC converter (based on data reported in Belcastro  ).
at 1400 gram. The weight of input materials exceeds the final weight of the cordierite by approximately 10% because of the water content of the input materials (deducted from Belcastro  ).
The production of the canister from the steel requires energy, which we estimate as the energy needed with the energy required for the steel deep drawing process, as assumed by Belcastro .
Catalytic converters are typically manufactured based on components produced by specialized industries on commercial raw materials. The converters are in large quantities transported to the car assembly facilities. We do not have specific information on the packaging and transport of the individual components but we assume that the cardboard box for the converter weighs around 1 kg based on information in Belcastro .
As far as the operation phase is concerned, we assume that a converter has a lifetime corresponding to 160,000 km driven  . The converter is estimated to increase the fuel consumption due to lower air-to-fuel ratio and back pressure by 2% - 10% depending on the driving conditions . Assuming an emission of 150 - 220 g CO2 per km for a typical gasoline-driven passenger car , we estimate that the additional fuel consumption for the TWC converter amounts to approximately 2000 kg CO2. We assume that the converters require no maintenance during their lifetime.
Estimating the amount of NOx and other air pollutants removed during the lifetime of the converter requires data on average composition of the exhaust gas before and after the converter.
Several studies have addressed the removal of air pollutants by catalytic converters , but, since the engine technology as well as the catalytic converters constantly develop, only recent data should be applied. Fanick et al.  measured the exhaust gas composition before and after the converter on two Toyota models. The results are summarized in SI units in Table 5. As baseline for the LCA modelling over the TWC lifetime, we use a removal of 250 kg NOx, 1150 kg CO, 180 kg THC. In addition, the CO2 emissions increase with 2350 kg due to the conversion of THC and CO into CO2.
For the end-of-life phase of vehicle catalytic converters, we assumed that the majority of the converters are dismantled and properly managed. The energy
Table 5. Removal of NOx and other air pollutants by a TWC converter during a lifetime of 160,000 km (based on Fanick et al.  ).
and material requirements for the treatment of spent catalytic converters were taken from the Ecoinvent database. Platinum, rhodium and palladium may be recycled , and the remaining parts are landfilled. We considered two scenarios, one where the metals are not recycled, and one where the metals can be fully recycled, and later discuss how this affects in the LCA results for the catalytic converter.
3.2.3. LCI Inventory for a Vehicle Catalytic Converter Removing 1 kg of NOx
Based on the above data compilation, the LCI for 1 kg NOx removed by the catalytic car converter has been developed as presented in Table6. Table6 summarizes resources in terms of materials and emission involved in each life-cycle phase of the technology. The inventory column TWC represents a whole converter including both the first part where the NOx reduction take place and the second part where the reduction of THC and CO primarily takes place. The inventory column 1/2 TWC represents only the first part of the converter and is ascribed the NOx removal, the use of rhodium plus a proportional part of the materials for the honeycomb and the canister as well as the additional fuel use. This approach is used to bracket the environmental profile of the NOx removal, since the processes cannot be separated as stringently as done here. The corresponding Ecoinvent processes utilized for the modelling of catalytic car converter are reported in the SM, TableS3 and TableS4.
4. Results and Discussion
This section reports the main results of the LCA for NOx reduction via photocatalytic surface coating (4.1) and vehicle catalytic converter (4.2). Complete list of characterized and normalized results can be found in the SM (Tables S5-S8 for characterized, Tables S9-S12 for normalized).
The results are presented as normalized environmental impacts in sixteen impact categories (Figure 2 and Figure 3). Burdens or loads are positive values, while benefits or savings are negative values. The results are divided according to the life-cycle phased described in Section 3: Production, operation, NOx removal, and end-of-life. The unit is mPE per kg of NOx removed. Net values are marked by diamonds.
For both technologies, the direct effect of the NOx removal is quantified as
Table 6. LCI inventory for 1 kg of NOx removed by a passenger car TWC-converter. The column “1/2 TWC converter” is a proxy representation of the first reductive section of the converter where the NOx removal takes place. The data is basically half the values of the TWC, except from differences in rare earth element content and pollutant conversion. Positive values are burdens and negative values are savings.
savings in the categories PM, POF, A, TE, and ME (see Figure 2 and Figure 3). The categories PM and POF are proxies for the health effect of NOx, acidification is due to the acidity produced when NOx is oxidized to , and TE and ME reflect that is a nutrient both in terrestrial and in marine environments. is also a nutrient in freshwaters, but we have chosen to allocate the emission to the marine environment; thus savings due to NOx removal is zero in the impact category FE.
Figure 2. Normalized environmental profile in mPE per kg of NOx removed for photocatlytic surface coating on concrete (con) and asphalt (asp). The 16 impact categories are: CC: Climate change; OD: Ozone depletion; HTc: Human toxicity, cancer effects; HTnc: Human toxicity, non-caner effects; PM: Particulate matter; IR: Ionizing radiation; POF: Photochemical ozone formation; A: Acidification; TE: Terrestrial eutrophication; FE: Freshwater eutrophication; ME: Marine eutrophication; FET: Freshwater eco toxicity; LU: Land use; WU: Water use; RDM: Resource use, minerals and metals; RDE: Resource use, energy carrier.
Figure 3. Normalized environmental profile in mPE per kg of NOx removed for catalytic car converter (TWC: whole converter; 1/2 TWC: NOx-reducing part of converter). The 16 impact categories are: CC: Climate change, OD: Ozone depletion, HTc: Human toxicity, cancer effects, HTnc: Human toxicity, non-caner effects; PM: Particulate matter; IR: Ionizing radiation; POF: Photochemical ozone formation; A: Acidification; TE: Terrestrial eutrophication; FE: Freshwater eutrophication; ME: Marine eutrophication; FET: Freshwater eco toxicity; LU: Land use; WU: Water use; RDM: Resource use, minerals and metals; RDE: Resource use, energy carrier.
4.1. Photocatalytic Surface Coating
Figure 2 shows the environmental profile of the concrete and asphalt coatings. The results are close to identical for the two applications and the presentation below applies to both applications.
4.1.1. Results: Photocatalytic Surface Coating
The direct effect of the NOx removal dominates the environmental profile of the photocatalytic coatings, showing significant savings in POF, A, and TE ranging approximately from −12 to −24 mPE per kg NOx. Some savings are also seen in PM (−2 mPE per kg NOx). The net value for ME is close to zero; this reflects that the NOx, with and without the photocatalytic surface, always ends up as .
The burdens from the production of the coatings are minor. Only a small burden of approximately 1 mPE per kg NOx is observed in FET due to the provision of TiO2.
The associated benefits of removal of SOx are marginal and do not provide a large contribution to the results (not visible contribution in Figure 3). No significant impacts are seen from the end-of-life phase.
4.1.2. Dicussion: Photocatalytic Surface Coating
The overall results obtained for the photocatalytic coatings showing that the benefits of removing the NOx far outbalances any environmental impacts from the production of the coating and is in accordance with what has been the main outcome of a few previous studies  . The main environmental impact connected to the production phase is due to the provision of TiO2. We include accurately the main materials and process in the production of the coating, and we follow standard procedures in quantifying the benefits of removing the NOx. Thus, we find the overall result robust, although we cannot judge their general representability due to the few data sets available. However, we may not fully cover the additional benefits of using the photocatalytic coating on surfaces. This is primarily due to lack of clear evidence and data. SOx removal seems of no importance for the results, but we neglect any effects related to removal of O3, oxidation of volatile hydrocarbons, effects related to the cleanness of the surface, and the potential increase in uptake of CO2 in the concrete due to the presence of TiO2. These aspects have been addressed in the literature    , but are all still very new and characterization factors and methods to account for these aspects in an LCA are still not available. However, although not possible to quantify at the moment, we believe that these additional functions (removal of O3, oxidation of VOCs, effects related to the cleanness of the surface, and the potential increase in uptake of CO2) all potentially would increase the benefits of reduction of NOx via catalytic surface coating.
4.2. Vehicle Catalytic Converter
Figure 3 shows the environmental profile of the whole converter (TWC) and for the first part of the converter responsible for the NOx reduction (1/2 TWC). This is done in an attempt to identify the environmental profile solely of the NOx removal in spite of the fact that many interlinked processes are taking place in the converter including removal of THC and CO. In both cases, we assume removal of 1 kg of NOx and the environmental profile of the NOx removal is in between the two cases presented in Figure 3.
4.2.1. Results: Vehicle Catalytic Converter
The direct effect of the NOx removal dominates the environmental profile also of the catalytic converter. The effects in PM, POF, A, and TE are identical to the benefits also seen for removal of 1 kg NOx by the photocatalytic surface coating, but the converter also shows a significant saving in ME (−13 mPE per kg NOx). This is due to the fact that the catalytic converter produces N2 and not , and N2 has no environmental impacts. In addition, the TWC shows additional benefits in HTNC (10 mPE per kg NOx) and in POF (5 mPE per kg NOx). These additional benefits are not observed for the 1/2 TWC; they are caused by THC and CO reduction that primarily takes place in the second half of the converter, while NOx removal takes place in the first part of the converter.
The converter also reveals a minor burden in CC (2 mPE per kg NOx), due to the additional CO2 emissions from the conversion of THC and CO to CO2 and from the additional fuel use. Overall, the CC impacts connected to the provision of materials and energy for the NOx removal function of the converter is only 1/4 of the CC impact for the whole converter.
The burdens from the production of the converter are significant in the FET (TWC: 19 mPE per kg NOx; 1/2 TWC: 7 mPE per kg NOx). The provisions of the rare earth elements for the catalytic honeycomb coating are the cause. The NOx removal takes place in the first part of the converter where only rhodium is used (7 mPE per kg NOx), while platinum and palladium are used only in the second part of the converter (12 mPE per kg NOx). The other burdens for the production are very small (A and FE: <1 mPE per kg NOx). The end-of-life shows a small burden in RDM: around 1 mPE per kg NOx), due to the management of the spent converter.
Recycling of rare earth elements from the spent converter is not included in the main results because we have no reliable data on how frequently the converters are recycled nor on the efficiency of the recycling process regarding the actual recovery of the individual elements. Work is in progress in this area , and when data are established this impact can be reduced proportionally to actual recovery of each metal. A sensitivity analysis considering potential full recovery of metals shows that impacts connected to the provision of rhodium, palladium and platinum is counter-balanced by their full recovery as secondary raw materials, which reduces the impacts for the FET to <1 mPE.
4.2.2. Discussion: Vehicle Catalytic Converter
The overall results obtained for the vehicle catalytic converter showing that the benefits of removing the NOx well outbalance the environmental impacts from the production of the converter, is in accordance with what has been the main outcome of a few previous studies. Nevertheless, different approaches and assessments methods make direct comparison difficult   .
The fact that the benefits of the NOx removal have a relatively higher contribution to the overall results and the fact that the NOx removal even may claim some of the benefits from the removal of THC and CO (since the NOx removal provides oxygen for the second part of the converter), we believe that the overall results are robust, although several assumptions were made regarding the production of the converter and several proxies were use where datasets were missing.
The impacts from the production of the converter are related to the use of rare earth elements. The fact that we did not consider recovery of the rare earth elements, may mean that we overestimate the environmental burden from producing the converter. Recovery will reduce the impact proportional to the recovery efficiency because the burden is linked to the mining and refinement of virgin resources. Some of the rare earth elements are produced as by-products in other mining operations and the allocation of environmental burdens for virgin materials as well as the environmental savings form recovery depend on the development in the markets both in terms of supplies and prices, but it is likely that they are related, and thus recovery always will reduce the burden of producing the converter.
4.3. Development of NOx-Reducing Technologies
The strength of the present study resides in the novel, balanced and robust datasets obtained for the NOx-reducing technologies in focus. The environmental profiles obtained for the two NOx-reducing technologies are positive in the way that the environmental burdens of the production, operation and end-of-life phases is much less that the benefits obtained by fulfilling its purpose of removing NOx from the urban air at ground level. The environmental profile of the photocatalytic surface coating is characterized by having less environmental burdens connected to production and operation. However, we believe that they both technologies will be relevant in the years to come since they are complementary in obtaining low level of NOx in urban air.
The limitations of the study reside in the limited sources of data and studies from which the LCIs were built. In particular, limited data were available for the TWC. Even though built on scarce data, the LCIs obtained and their environmental profiles suggest areas to be considered for further improvement.
With respect to the photocatalytic surface coating, the highest burden derives from provision of TiO2. Furthermore, the potential benefits from reducing other pollutants should be better quantified and assessed in the future; in particular data are need for O3 degradation and THC removal.
With respect to the vehicle catalytic converter, the composition and concentration of coating liquids used should be further assessed, balancing removal efficiencies versus environmental burdens of producing the rare earth elements including their actual recovery. Our study has addressed one commonly used coating, but a variety of coatings are available, and they should all be assessed. The additional fuel use induced by the converter is significant but still minor compared to the benefit of the NOx removal and the other burdens and savings. However, the additional fuel use should always be kept low when possible.
Focusing on the removal of nitrogen oxides (NOx), which is a major urban air pollutant, we have compiled life-cycle-inventory (LCI) data for a photocatalytic surface coating containing TiO2 nanoparticles and for a vehicle catalytic converter employing the rare earth elements palladium, platinum, and rhodium. Although the two NOx-reducing technologies differ and are complementary in their application, we have addressed their life-cycle phases (production, operation, and end-of-life) with respect to the common function of NOx removal. We have obtained novel balanced datasets that we consider robust regarding the main contributions to the life cycle of these technologies, which we did not find in previous literature. However, we have identified potential areas where more and better data are in demand.
The environmental profiles of the two technologies assessed with the life cycle assessment (LCA) methodology with respect to sixteen impact categories show that the overall benefits of using the technologies in both cases are much higher than the burdens from their production, operation, and end-of-life. Thus, they are environmentally sound technologies for removing NOx, which would otherwise decrease the quality of urban air near the ground. The environmental profiles also indicate where further development should focus from an environmental point of view. While the burden from producing the photocatalytic surface coating is negligible, the use of rare earth elements in the vehicle catalytic converter is the main burden and, as also suggested by other studies, should be the focus in future developments.
The technical data on the photocatalytic surface coating has been provided by Dr. Lilja Dahl (employed by Photocat) and Dr. Henrik Jensen (CEO and co-founder). Dr. Lilja Dahl has furthermore contributed to the LCI data for the catalytic converters. Both have commented on the early drafts of the paper. Compiling of the LCIs, the LCA modelling, the result evaluation, the final content and conclusions of the study are the responsibility of the partners from DTU (Valentina Bisinella, Teis N. Mikkelsen, Thomas H. Christensen).
The Supplementary Material
Life Cycle Inventory (LCI) Modelling
Table S1. LCI modelling for the photocatalytic surface coating, concrete pavement element.
Table S2. LCI modelling for the photocatalytic surface coating, asphalt pavement.
Table S3. LCI modelling for the vehicle catalytic converter, three-way catalyst (TWC).
Table S4. LCI modelling for half the vehicle catalytic converter, 1/2 three-way catalyst (1/2 TWC).
Table S5. Characterized results for the photocatalytic surface coating, concrete pavement element.
Table S6. Characterized results for the photocatalytic surface coating, asphalt pavement.
Table S7. Characterized results for the vehicle catalytic converter, three-way catalyst (TWC).
Table S8. Characterized results for the ½ vehicle catalytic converter, three-way catalyst (1/2 TWC).
Table S9. Normalized results for the photocatalytic surface coating, concrete pavement element.
Table S10. Normalized results for the photocatalytic surface coating, asphalt pavement.
Table S11. Normalized results for the vehicle catalytic converter, three-way catalyst (TWC).
Table S12. Normalized results for the 1/2 vehicle catalytic converter, three-way catalyst (1/2 TWC).
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