A review on photocatalytic asphalt pavement designed for degradation of vehicle exhausts

Abstract

This paper presents a comprehensive review on the most recent level of knowledge and developments achieved in utilizing photocatalytic asphalt pavement as a method to reduce vehicle pollutants. It included the mechanism of photocatalysis, available testing facilities, quantifying methods, widely studied techniques of applying TiO₂ particles onto asphalt pavements, influencing factors on pollutants degradation and future perspectives. Nano-TiO₂ crystal types, doping approach, ambient temperature, relative humidity, intensity and time of irradiation, and pollutant characteristics are the key factors that influence the efficiency and durability of photocatalytic asphalt pavement. The photocatalytic asphalt pavement can achieve a degradation efficiency of 55% for NOx pollutants and 20% for SO₂ pollutants at this moment, while suffering from short service life due to busy traffics and complex weather circumstances. In the future perspectives of photocatalytic asphalt pavement, practical costs, applying methods, durability, and negative effects are highly emphasized for the development of sustainable and durable photocatalytic roads.

Introduction

Controlling air pollution is a significant challenge that global society faces today. Both human health (Kampa and Castanas, 2008) and ecological balance (Greaver et al., 2012) are threatened by constantly deteriorating air condition. It was reported that appropriating 1.6 million victims of chronic obstructive pulmonary disease and 500 thousands deaths by lung cancer can be imputed to inferior atmospheric environment (Schraufnagel et al., 2019). Via tracking surveyed the data associated the healthy impacts of air pollution from 2001 to 2015, it is revealed that nitrogen dioxide from vehicle exhausts is a primary inducement to cause diabetes and cardiovascular diseases (Paul et al., 2020). Similar investigation also implied that long-term contacting with air pollutant (NO₂ and PM_2.5) increased the incidence of Parkinson’s disease (Kasdagli et al., 2019). That means the deteriorating air condition can worsen not only respiratory and endocrine systems, but also human nervous system.

Therefore, the increasing stakeholders have been emerging various measures on relieving the air pollution issues since Kyoto protocol signed (O'neill and Oppenheimer, 2002). Among which, automobile would be a dominant exhaust maker in recent decades. The main adverse componence from vehicle exhausts involves in nitrogen oxides, carbon monoxide, carbon dioxide and sulfur dioxide (Shafiq et al., 2019), generated 60–70 % environmental pollutants and undesirable smokes (Dwivedi and Tripathi, 2008, Harish, 2012). Specifically, motor vehicle consumption per kilogram of gasoline would emit 4–20 g of nitrogen oxide (NOx) to atmosphere (Zhang, 2000). However, the high pace of economic and technical development caused several barriers in vehicle exhausts controlling. For example, the volume of China's automobile production in 2000 alone have exceeded 2 million of units, reaching 6 million of units in 2005 (Tang, 2009), above 13 million of units in 2009, hitting 20 million of units in 2013, climbing 28 million of units in 2018 (Statista, 2019). Constantly increasing automobiles have released rising numbers of air pollutants and accelerated the pollution degree thereby.

Due to the alarming rise of harmful gases in the atmosphere, measures have been taken to limit air pollution levels by globally environmental legislation. The European Union has established a comprehensive legislative branch (2008/EC/50) to develop healthy standards and targeted for summarizing massive harmful substances in the atmosphere (UNION, 2008). India government implemented the National Clean Air Program (NCAP), aiming to reduce the concentration of pollutants in the atmosphere (Dey and Chowdhury, 2022). The US Environmental Protection Agency (EPA) proposed the professional Air Quality Index to evaluate the air pollutants (Perlmutt and Cromar, 2019), and conducted monitoring and censorship activities to enhance public health benefits (Furlow, 2022). The Chinese State Council launched the Three-year Action Plan to Win the Blue-sky Defense War in 2018, and overfulfilled the objectives set before in 2021(Jiang et al., 2021).

Since Fujishima and Honda (Fujishima and Honda, 1972) published the research progress of titanium dioxide (TiO₂) being a photocatalyst in 1972, photocatalysis by TiO₂ particles has been widely recognized in the scientific literature. Actually, the earliest literature toward sunlight-induced chemical activities with TiO₂ was published by Keidel in 1929 (Keidel, 1929), and in 1938, Goodeve reported a photobleached dye with TiO₂ particles (Goodeve and Kitchener, 1938). Gradually, increasing fields applied TiO₂ photocatalysis in functional productions and techniques, especially environmental purification, such as water purification (Legrini et al., 1993), solar cells (Chen et al., 2013), energy transformation (Kiesgen de_Richter et al., 2013, Ni et al., 2007), electric appliances and lighting (Soma et al., 2001) and agricultures (Kuo, 2002). In view of purifying objects, existing references covered CO (Hwang et al., 2003), CO₂ (Adekoya et al., 2019), heavy metal (Murruni et al., 2008), NOx (Dalton et al., 2002), volatile organic compounds (VOCs) (Obee and Brown, 1995, Peral and Ollis, 1992), sulfur dioxide (Zhao et al., 2009) and so forth. Due to photocatalytic reactions using nanoscale TiO₂ being studied and claimed that Schottky barrier formation would be expired in nanoscopic dimensions (Hagfeldt and Graetzel, 1995), the continually developing NT (Nanotechnology) utilized well in many application fields is posing the researches on photocatalytic materials making much progress during the past decades (Dylla and Hassan, 2012b). Numerous associated patents, such as 11 patents listed in (Steyn et al., 2014) toward photocatalytic air purifications with TiO₂ signaled that outdoor air treatment has considerable potentials once fundamental technique barriers tackled (Paz, 2010). On the other hand, road construction is rapidly increasing due to dramatic modernization and urbanization all over the world. Hence, photocatalytic asphalt pavement becomes an increasing concerned aspect to be explored for reducing pollution levels, through degrading NOx in the atmosphere (Segundo et al., 2021).

In recent decades, photocatalytic asphalt pavement is recognized as one of the most promising functional properties imparted to infrastructures (Rocha Segundo et al., 2019). But little study comparatively explored the basic principle, photocatalytic efficiency and durability of applied TiO₂ methods in asphalt pavement in-depth, and lacking the systematical discussions for primary factors affecting the catalytic efficiency and future perspectives for photocatalytic pavements. According to aforementioned reviews, photocatalytic pavement by TiO₂ can degrade various of organic and inorganic pollutants under sufficient ultraviolet (Beeldens et al., 2011). Fig. 1 depicted the fundamental principles in exhaust purification process by photocatalytic pavement. As released exhausts (NOx, COx and HC) contact photocatalyst (coated on the surface layer of pavement) in the presence of sufficient ultraviolet, photocatalytic degradation would occur and pose the transformation from air pollutants to kinds of carbonates and nitrates. The transformants are then dissolved in water and washed away, preventing to be breathed into the organism (Hüsken et al., 2009, Jiang et al., 2018). Theoretically, there are four important aspects triggering the efficiency and feasibility of photocatalytic road in macro level. Firstly, the huge scale and length of pavement create vast applying market of photocatalysis in helping reduce air pollution levels. Modern road system has already covered 19-million-hectare surface space of earth (Strano et al., 2017), another at least 25 million kilometers roads anticipated to construct before 2050 (Laurance et al., 2014). Secondly, pavement locates near the exhaust point of vehicle emission, which makes it a workable option to handle vehicle exhaust promptly. Thirdly, ultraviolet irradiation is acknowledged as the essential catalytic for the period of photocatalysis. The fact that most of traffic asphalt pavements have access to sunshine offers the reaction platform for photocatalysis. Fourthly, most exhaust pollutants have higher density than oxygen which would remain in lower space close to the levels of traffic pavements, especially for the closed traffic environment such as underground parking lots and tunnels (Liu et al., 2015). Hence researchers all over the world have been working on various approaches to exploit this possibility (Carneiro et al., 2013, Chen and Chu, 2011, Mohammad et al., 2011), even for the utilization in traffic marking paint with photocatalysis (Taheri, 2017).

In relation to the proposed studies toward photocatalytic pavements, several on-site investigations were worldwide conducted. Joel summarized 12 places engaged TiO₂ to pavement surface with a NOx removal range of 10–60 %, involved in Belgium (Beeldens, 2007), France (Gignoux et al., 2010), United State (Hassan and Okeil, 2011), Japan (Ohama and Van Gemert, 2011), Italy (Crispino and Vismara, 2010) and Netherlands (Overman, 2009). Nevertheless, in the terms of feasibility of nano-TiO₂ based photocatalytic pavement, the applying methods, associated durability, degrading effectiveness, and evaluation of test methodologies remain important aspects to be further discussed. The present paper reviewed current state of art studies on using photocatalytic materials in asphalt pavement. The photocatalysis mechanism, available testing equipment and quantitative methods have been overviewed comprehensively. Moreover, the widely studied technologies of TiO₂ particles applied to asphalt pavement are comparatively discussed from the perspectives of basic principle, catalytic efficiency and durability. Additionally, the primary influencing factors on pollutants degradation by photocatalytic pavements were also highlighted. Meanwhile, the future prospective of photocatalytic asphalt pavement were outlooked for large-scale application in practice based on the integrated results. Fig. 2 demonstrated the basic structure and associated discussion points. This review would offer a valuable reference in photocatalytic asphalt pavement application in future.