Heterogeneous Photocatalysts Based on TiO 2 for Abatement of Hazardous Air Pollutants

A mixture of suspended solid particles and gases coming from car emissions, chemicals from factories, dust, and pollen in the air is called air pollution. Human exposure to common hazardous air pollutants such as NOx, SOx, CO, H 2 S, NH 3 , and VOCs is associated with chronic respiratory diseases and cancer. Moreover, exposure to one of the most dangerous forms of air pollution very fine particulates PM 2.5 can cause such deadly illnesses as lung cancer, stroke, and heart disease. Given all these facts, this paper discusses newly developed advanced smart building materials based on TiO 2 with self-cleaning and pollutant removal capability from the environment.


Introduction
Copyright © Amra Bratovcic RDMS.000850. 14(5).2021 They are a ubiquitous group of several hundred chemically related compounds, environmentally persistent with various structures and varied toxicity. PAHs are commonly detected in air, soil, and water [5].
One of the most dangerous forms of air pollution are very fine particulates capable of penetrating deep into the lungs and entering the bloodstream. Known as PM 2.5 , these particulates have an aerodynamic diameter of less than 2.5 microns -about onethirtieth the width of a human hair. Exposure to PM 2.5 can cause such deadly illnesses as lung cancer, stroke, and heart disease.
According to the 2019 World Air Quality Report [6], it is estimated to contribute toward 7 million premature deaths a year, while 92% of the world's population are estimated to breathe toxic air quality [7]. In less developed countries, 98% of children under five breathe toxic air. As a result, air pollution is the main cause of death for children under the age of 15, killing 600,000 every year [8]. In financial terms, premature deaths due to air pollution cost about $5 trillion in welfare losses worldwide [9]. Using a weighted population average, Bangladesh emerges as the most polluted country for PM 2.5 fine particulate matter) exposure, based on available data. Pakistan, Mongolia, Afghanistan and India follow behind respectively, deviating from one another by less than 10%. Bosnia and Herzegovina is the highest-ranking country in Europe for PM 2.5 pollution, featuring as the 14 th most polluted country globally, with only 4μg/m³ less than China's national PM 2.5 weighted average.
Air emissions from industry in Europe have decreased over recent years. Between 2007 and 2017, overall emissions of sulphur oxides (SO x ) declined by 54%, nitrogen oxides (NO x ) by more than one-third and greenhouse gases from industry, including power plants, by 12% [10].
Using data from Sentinel 5-P satellite shows that in lockdown areas, average NO 2 levels in 2020 for the period March 15 to April 30 were lower than levels in 2019. These results were expected as vehicular traffic, one of the main sources of NO 2 emissions was dramatically reduced during the lockdown [11]. Recently, has been discussed about the successful implementation of nanomaterials for the purification of air in automotive exhaust systems and petroleum refining systems with greater efficiency than conventional techniques [12].
Actually, nanotechnology is an interdisciplinary study that allows us to develop new materials with new, interesting, and useful properties. Especially, photocatalytic degradation techniques should be more used in air purification because with this technique it is possible to decrease pollutants to certain acceptable limits [13].
Photocatalysis is a promising technology for air purification because it can decompose gaseous pollutants (particularly volatile organic compounds (VOCs) directly into harmless CO 2 and H 2 O under ambient conditions. Photocatalysis can be particularly suitable for removing low concentration pollutants (sub-ppm levels) in indoor environments where conventional adsorption technologies are not very efficient. Although photocatalytic air purification has been extensively investigated, it still falls far short of satisfying the requirements for practical usage [14]. However, there are challenges with regards to photocatalytic efficiency improvements, lab to industrial scaling up, and commercial product production. Figure 1 schematically shows the problems of air pollution and possible photocatalytic solutions based on TiO 2 . In recent years, a number of products based on the photocatalytic properties of a thin layer of TiO 2 at the surface of the material (such as glass, pavement, etc.) or embedded in paints or concrete have been used as photocatalytic self-cleaning and "depolluting" materials as a remediation technology mainly for NOx and aromatic VOCs in urban areas. The use of TiO 2 photocatalysts

RDMS.000850. 14(5).2021
as an emerging air pollution control technology has been reported in many locations worldwide. However, up to now, the effectiveness measured in situ and the expected positive impact on air quality of this relatively new technology has only been demonstrated in a limited manner [15].

Air pollutants
Air pollutants can be classified according to their origin as primary or secondary pollutants. Primary pollutants are those that are emitted directly by the sources to the atmosphere, like carbon monoxide (CO), sulphur dioxide (SO 2 ), nitrogen oxides (NOx) and particles (PM 10 and PM 2.5 ). Secondary pollutants are formed from chemical reactions occurring in atmosphere, such as tropospheric ozone (O 3 ), nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ) [16]. Ozone (smog) is created when pollutants emitted by cars, power plants, industrial boilers, refineries, and other sources chemically react in the presence of sunlight. Chemical interactions between SO x and NO x can produce ground-level ozone, and this can happen in the upper atmosphere too, leading to photochemical smog and acid rain [17].
The major source of NOx in Europe is road transport, producing around 40% of the total emissions, comprising a mixture of nitric oxide (NO) and nitrogen dioxide (NO 2 ) [18]. Gaseous pollutants are mainly controlled or removed by a few significant techniques: absorption, adsorption, chemical oxidation, and incineration. These techniques may be employed either singly or in combination, which, in turn, can be decided based on the type of pollutant. Gas adsorption is a surface phenomenon where gas molecules are adsorbed or are attracted to the surface and held on the surface of the solid.

Heterogeneous photocatalysts for abatement of air pollutants
The photocatalytic process is usually performed at room temperature and atmospheric pressure, and the possibility of using natural resources as sunlight, the morphology modification of the photocatalyst, makes easier the separation and reuse steps of the catalyst. Photocatalytic nanoparticles are able to promote the degradation of pollutants in aqueous and gas matrices, with resulting products presenting a lower environmental impact [19].
Titanium dioxide considered as one of the most environmentally friendly active photocatalysts that can be used with building materials safely and effectively to react with nitric oxides. TiO 2 is frequently used to oxidize organic and inorganic compounds in air and water due to its strong oxidative ability and long-term photostability [20]. TiO 2 is also a very common, non-expensive, and nontoxic material [21].
When TiO 2 nanoparticles are stimulated by sunlight, they convert air pollutants such as nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), and ozone to more environmentally acceptable products such as calcium nitrate and carbon dioxide. Recently, Gopalan and co-workers summarized the prospective applications of TiO 2 -based building materials (cement, mortar, concretes, paints, coating, etc.) with relevance to the removal of outdoor/indoor NOx and volatile organic compounds, self-cleaning of the surfaces, etc. [22]. Already in 1999 Chiba of Japan [23] and Milan of Italy in 2002 [24] have applied the nano-TiO 2 in the construction of cement concrete pavement. Also, it has been reported the use of TiO 2 as a stone coating for selfcleaning and biocidal purposes for the protection of buildings and cultural heritage [25,26].

TiO 2 for NOx degradation
The mechanism for photocatalytic NOx removal involves the adsorption of NO 2 gas molecules onto hardened cement paste structures, the occurrence of hydrolysis of NO 2 into the pores, and the formation of soluble nitrates [27].
These experimental results demonstrated that cementitious materials could be tailored to decrease NO x levels through photocatalysis and showed their ability to bind NO x , in particular, NO 2 . Paved surfaces (e.g., highways, runways, parking lots, etc.) generally account for up to ~60% of developed urban areas and could be modified with a thin layer of TiO 2 -inclusive photocatalytic materials so that the topmost surface effectively clears the environment of the adverse effects of pollutants coupled through the influence of solar reflectance. In addition, urban temperatures could be controlled by simply coating wall surfaces with white photocatalytic materials. Such applications can be extended to parking lots and roads to reduce temperatures and to cool the environment, as well as to improve air quality Gopalan et al. [22].
Recently Huanan et al. 2020 evaluated nano-TiO 2 as a coating material for both road surface and roadside for NOx degradation. The nano-TiO 2 coating materials were prepared by using anatase nano-TiO 2 , activated carbon powder, silane coupling agent, and deionized water. The results show that the material has good photocatalytic degradation performance, and the proper amount of silane coupling agent can enhance the bonding performance of the material and asphalt mixture. For the roadside coating, sodium dodecylbenzene sulfonate was selected as the surfactant to carry out the photocatalytic degradation experiment of NO 2 with different dosages of surfactant. The results showed that when the mass ratio of nano-TiO 2 and surfactant was about 1:2, the catalytic degradation effect of the material was the best [28]. Hussein et al. [29] prepared two types of substrates coatings containing TiO 2 . First, mixing nanoparticles of TiO 2 with cement paste in 3% and 6% percent. Second, mortar substrates coated with nano TiO 2 aqueous solution. Two coating methods have been used dip and spray. They tested its efficiency in NO gas removal. Results showed the effectivity of coating building materials with titanium dioxide, the removal of gaseous pollutants like nitric oxide reached to 98.85% when spray and dip methods are used. Mixing nano titanium with a percent of 3% was also efficient in the removal of nitric oxides, the removal reached 97%. The research has shown that the spray method was more practical to be used.

TiO 2 for SOx degradation
Titanium dioxide (TiO 2 ) has been used in building materials to produce products that do not require major maintenance and contribute to improve air quality and extend the life of buildings. Thus, Fernandes and co-workers (2020) [30] developed mortars at a weight ratio of 1:3 (cement: sand), incorporating 2.5%, 5%, 7.5%, and 10% of TiO 2 relative to the cement weight. They studied prepared materials in process of degradation of sulphur dioxide (SO 2 ), as one of the largest atmospheric pollutants. The samples were exposed to an accelerated aging SO 2 (pollutant) chamber, then moistened and exposed to ultraviolet radiation. For this exposure of the samples, two Light Emitter Diodes (LEDs) with wavelengths covering the UV-A range were used: UV (380-420nm) and blue (420-493nm). The results have shown that the incorporation of TiO 2 improved the physical, mechanical and photocatalytic properties, and enabled the decontamination of mortars due to the action of the SO 2 pollutant.
Mostafa et al. [31] have prepared active photocatalytic nanostructures to harvest the abundant sunlight energy for clean energy production and environmental management. Different calcium carbonate-titania nanostructures were applied as novel photocatalysts for desulfurization of dibenzothiophene (DBT) and gas oil using different radiation sources at room temperature. The results have shown that 95% desulfurization of DBT was possible under 1h visible light irradiation with linear halogen lamp (LHL) at catalyst/DBT-solution = 10g/L, while ultra-clean diesel production (99% removal, 3.47ppm) could be obtained via normal sunlight photochemical desulfurization of diesel fuel by calcium carbonate titania photocatalyst in presence of H 2 O 2 and acetic acid as oxidizing agents and acetonitrile as a solvent. The prepared calcium titanate photocatalysts have bandgap energy (2.05eV), and stable photocatalytic activity with enhanced visible light removal of organosulfur compounds for economic ultra-clean fuel production, pollution control, and environmental management.

TiO 2 for VOC degradation
One of the major drawbacks of photocatalysis in the gaseous phase (the reactions occurring at the gas-solid interface) during VOC degradation is rapid loss of photocatalytic activity during prolonged irradiation and repeated uses because of either the intrinsic instability of the materials or the fouling of the photocatalyst surface [32].
During the Photocatalytic Degradation (PCD) of VOCs, degradation intermediates are formed on the surface of the photocatalyst and some of them are often more strongly complexed with the photocatalyst than the parent substrate molecules. The accumulation of the recalcitrant intermediates onto the catalyst surface not only blocks light to inhibit photon absorption but also limits the diffusion of dioxygen or other target compounds onto the active sites. As a result, the accumulation of recalcitrant carbon deposits on the surface eventually deactivates the photocatalysts [14].

Conclusion
Still, vast populations around the world lack access to air quality information. Often these areas are estimated to have some of the world's most severe air pollution, putting the health of huge populations at risk. More monitoring data is needed to bridge the information gap and better tackle air pollution globally.
The development of advanced building materials based on TiO 2 offers additional novel functionalities (such as self-cleaning, pollutant removal capability) with new technologies in both residential and commercial buildings, in the process of developing smarter, more energy-efficient, and more secure infrastructures that will address environmental and global social challenges. In this regard, scientific developments in modern material synthesis and nanotechnology provide an adequate platform for the design and creation of new and smart building materials with tailored properties.