Work Created by Shannon

Titanium Dioxide (TiO2) Photocatalysis in Concrete


1.0 INTRODUCTION


The US Environmental Protection Agency’s (EPA) National Ambient Air Quality Standard for nitrogen dioxide (NO2) was changed in 2010, adding a new one-hour 100 ppb standard in addition to the previously established 53 ppb annual requirement [1]. Historically, Utah has met the EPA standards for NO2 [2]. However, the smog inversions experienced in Utah’s valleys mainly consist of ozone smog which is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds [3]. Over the past decade, photocatalytic titanium dioxide (TiO2) has been proven to be able to absorb and oxidize certain harmful pollutants such as NOx in the presence of ultraviolet (UV) light. It is speculated that each square meter of high-performance photocatalytic material exposed to outdoor sunlight can remove NOx from 200 cubic meters of air per day [4].

Photocatalytic TiO2 in cementitious materials has seen basic development in the past 3- years and was patented for paving materials in the US in 2011 [5] [6]. A summary of previous work in this section will describe the reaction cycles and mechanisms which occur to oxidize the NOx pollutants as well as the NOx influence on the generation of ozone (O3) in the troposphere, lowest portion of the Earth’s atmosphere. A brief history of the development of TiO2 materials and the current availability of related construction materials will also be summarized. The state of knowledge and practice will be discussed and is broken up into two sections; the findings from laboratory studies, and the field studies and computer modeling.

2.0 REACTION CYCLES OF TIO2 PHOTOCATALYSTS


TiO2 is an n-type semiconductor photocatalyst with a band energy gap (Eg) of 3.2 electron volts (eV). An n-type semiconductor has a filled valance band and an empty conduction band; however when subjected to UV light energy greater than Eg, electrons from the valance band can be excited to a higher energy state and move from the valance to the conduction band. When the electrons leave the valance band a positively charged vacant site, or electron-hole (h+), remains. The electrons in the conduction band can carry a moderate current making it a semiconductor [7] [8] [9] [10]. The electron-hole in the valence band provides a site where adsorbed hydroxyl ions (OH-) and disassociated water (H2O) can lose an electron forming a hydroxyl radical (OH·). Hydroxyl radicals are electrically neutral but highly reactive. The excitation of valence band electrons to the conduction band allowing for the formation of hydroxyl radicals is what makes TiO2 a catalyst. TiO2 becomes a photocatalyst because the electron excitation is due to photons on UV light. The rate of formation and of recombination between a positive hole and a free electron are considered to be very rapid [11]. Figure 1 graphically demonstrates the mechanism of excitation and formation of hydroxyl radicals [7].

Electron_Bands.jpg
Figure 1. TiO2 electron valance to conduction band mechanism [7]

The symbolic chemistry and explanation for the formation of hydroxyl radicals follows and a diagram of the reaction can been seen in
Figure 2:

TiO2 + hv → e- + h+ (energy from a photon of light (hv) excite valance band electron and produce a conduction band electron and an electron-hole in the valence band)

H2O ↔ OH- + H+ (dissociation of water into a hydroxyl and hydrogen ions)

OH- + h+ → OH· (hydroxyl ion reacting with electron-hole forming 1 hydroxyl radical)

Airborne pollutant molecules can be absorbed into the TiO2 particle surface, where they react with hydroxyl radicals and are oxidized [4]. The symbolic chemistry and explanation of these reactions follow:

NO + 2OH· → NO2 + H2O (nitric oxide reacts with 2 hydroxyl radicals forming 1 nitrogen dioxide and 1 water)

NO2 + OH· → NO3- + H+ (nitrogen dioxide can then react with another hydroxyl radical forming 1 nitrate and a 1 hydrogen ion)

TiO2_system.jpg
Figure 2. NOx removal process schematic [7]

The product of the cycle is nitrate (NO3-) and hydrogen ions. Nitrate is soluble in water and becomes a weak nitric acid. The nitrate may need to be washed from the surfaces to free up active sites on the TiO2 surface. Under practical conditions, reaction products are flushed from the concrete surface by rain [12] [13].

NOx is the primary source for oxygen radicals (O·) which leads to the only significant means by which ozone is formed in the troposphere. Below are the chemical equations governing ozone formation. If the three reactions occur at equal rates, there will be no average change in the levels of any of the constituents over a long period. However, they often do not occur at the same rate and these reactions play a major role in governing the diurnal rise of ozone levels in the troposphere [14].

NO2 + hv → NO + O· (nitrogen dioxide reacts with photons of light forming nitric oxide and oxygen radicals)

O· + O2 → O3 (oxygen radicals react with oxygen forming ozone)

NO + O3 → NO2 + O2 (nitric oxide reacts with ozone forming nitrogen dioxide and oxygen)

3.0 TIO2 TYPES


TiO2 particles naturally crystallize in three forms: rutile, anatase, and brookite. Rutile is the most common, most stable, chemically inert, and can be excited by both visible and ultraviolet (UV) light (wavelengths smaller than 390 nanometers) [15] [16]. Anatase is only excited by UV light and can be transformed into rutile at high temperatures. Both rutile and anatase have a tetragonal ditetragonal dipyramidal crystal system but have different space group lattices. Brookite is not excited by UV light but its orthorhombic crystal system can be transformed into rutile with the application of heat. Figure 3 shows ball and stick models of the different tetragonal lattice systems for rutile, anatase and brookite [15] [17]. It should be noted that the volume for the three lattices are approximately 62, 136, and 257 x106 pm3, respectively. The titanium atoms are gray and the oxygen atoms are red. Figure 4 shows some examples of crystal TiO2.

TiO2_ball_and_stick.jpg
Figure 3. Crystal structures of rutile, anatase and brookite titanium dioxide [15] [17]


TiO2_Crystals.jpg
Figure 4. Crystal images of rutile, anatase, and brookite, titanium dioxide [18]

The TiO2 crystals are ground to micro or nano-particle sizes to be used as a white pigment to be used in paint, sunscreen, paper, plastics, food coloring, and a number of other applications. Two production forms are most commonly used to purify crude TiO2 into high purity material. The first method is the chloride process. The chloride process begins with the crude TiO2 ore (>70% TiO2) being reduced with carbon and oxidized with chlorine to produce titanium tetrachloride (TiCl4). The TiCl4 is then distilled and re-oxidized in a pure oxygen flame or plasma at high temperatures to produce pure TiO2 and chlorine. Unless aluminum chloride is added to the process to form rutile, anatase is formed [18]. The second method uses ilmenite (FeTiO3), a weakly magnetic titanium-iron oxide mineral, as its ore mineral. The ilmenite is then digested in sulfuric acid. This reaction produces iron (II) sulfate (FeSO4) and TiO2. The crystallized iron (II) sulfate is filtered off leaving only the TiO2 salt in the digestion solution. The TiO2 is then processed further to be purified [18]. After purification, the TiO2 is ground into a fine powder such as seen in Figure 5 to be used by many manufacturers.
TiO2_powder.jpg
Figure 5. Powder TiO2 [18]

4.0 TIO2 HISTORY AND CURRENT PRODUCTS


TiO2 powders have been commonly used as white pigments since ancient times because they are inexpensive, chemically stable, harmless, and have no absorption in the visible region, leading to their white color [16]. Over the past 40-years, each decade has developed a new use for TiO2.
In the 1970’s, a single crystal n-type TiO2 (rutile) semiconductor electrode was developed and used to investigate the photoelectrolysis of water. The developed electrode was very stable in in solution and could oxidize water to oxygen [16].

The “oil crisis” of the 1970’s drove the development of using powdered TiO2 to produce hydrogen gas (H2) using photocatalytics. However, the generation of H2 could occur with the use of electrodes, but could not be reproduced with TiO2 in powdered form. This is contributed to the production sites of the H2 and O2 gases are located close to each other and they can recombine back into water. Organic compounds were added to the system with promising results, but by the mid 1980’s, H2 production by TiO2 photocatalyst became unattractive as other semiconductors were better suited for future research and development [16].

In the 1990’s, the photocatalytic cleaning, antibacterial, and hydrophilic properties of TiO2 were investigated. Tiles coated with TiO2 can remain clean in high exhaust environments due to TiO2 capabilities of photocatalytic decomposition. It was also found that E.coli would completely disappear from a surface containing TiO2 after a week of UV irradiation. It was also found that the contact angle of water on a TiO2 containing surface would reach almost 0° when the surface was excited with UV light. This means the surface becomes non-water repellant, highly hydrophilic. The time to decrease the contact angle from 25-30° to 0° takes approximately 2-hours. For the contact angle to return from 0 to ~25° takes approximately 1500 hours. Therefore using TiO2 for hydrophilic properties is stable and semi-permanent. Figure 6 shows a graphical and practical example of this application [16].

hydrophilic.jpg
Figure 6. Changes in Contact Angle Leading to Hydrophilic Properties of Surfaces Containing TiO2, conventional mirror (left), TiO2 coated mirror (right) [16].

In the 2000’s, the technology to use TiO2 as a photocatalyst to decompose air pollutants using building materials began to be studied. The difficulty facing research is the technology is attempting to affect a 3-dimensional space of air pollutants, not just the 2-dimensional surface of the material containing TiO2 [16].

Over the past decade commercial cement products have been introduced. One of the largest is TX Arca, a cement blend containing TiO2 produced by Italcementi of Italy. Italcementi is rapidly developing their commercial products with three new US patents granted in the last year [5] [6] [19]. Italcementi has partnered with HeidelbergCement, the parent company of Hanson Cement in the United Kingdom to develop TioCem. In a combined effort they are branding their pollution reducing cements at TX Active [20]. One of Italcementi’s most notable projects is the Jubilee church in Rome, shown in figure 7. It was constructed using white cement containing TiO2. This material was chosen to keep with building exterior white with the added advantage of reducing NOx pollution surrounding the building.

white_church.jpg
Figure 7. Jubilee Church of Rome, Exterior made from Italcementi Concrete Containing TiO2 [46]

Another commercial product is NOxer, produced by Eurovia in France. NOxer is a slurry made of a mineral filler, cement, fibers and TiO2 which is manually spread and used as an overlay [21]. The final type of product is a surface treatment such as PURETI. PURETI contains 99% water and 1% TiO2, and is applied by an electrostatic spray system which lasts up to three years [22].

5.0 PREVIOUS LABORATORY STUDIES


The most commonly used standard which laboratory testing are based on are the International Standard ISO 22197-1 “Fine ceramics (advanced ceramics, advanced technical ceramics) – Test method for air-purification performance of semiconducting photocatalytic materials” which was adapted from the Japanese Standard JIS TR Z 0018 “Photocatalytic materials – Air purification test procedures”. These tests standards call for using a chemilumiescent NOx analyzer and an ion chromatograph for analysis of nitrate concentrations [23] [24]. A typical NOx analyzer set up schematic can be seen in Figure 8.

Reactor_system.jpg
Figure 8. Example of Testing System Schematic

The compressed air is blended with the pollutant to the desired testing NO concentration. When the NO concentration, in the range of 0.4 ppm to 0.7 ppm, has become steady, the UV light source is turned on. The UV light excites the TiO2 particles allowing the NO to oxidize and dropping the NO concentration. After the testing time has completed, the UV lights are turned off, the TiO2 discontinues to be excited and the NO concentration returns to the same concentration it was before the UV lights were turned on. Figure 9 shows typical concentration data from a NOx analyzer when used with surfaces containing TiO2 photocatalyst.

Lab_data.jpg
Figure 9. Typical Data from NOx Analyzer for Surfaces Containing TiO2

Previous laboratory testing focuses either the characteristics of the TiO2 material, characteristics of the concrete material, or the environmental climate. When considering TiO2 materials choices, anatase is generally considered to have a higher photoactivity than rutile [25] [26]. However, there is evidence that rutile or blending rutile and anatase can be more effective than using anatase alone [27]. The higher photochemical activity of anatase, as compared to rutile, is believed to be the result of the number of electronic excitation lifetimes being an order of magnitude larger for anatase [28]. Smaller crystal sizes can cause an increase in photocatalytic oxidation compared to TiO2 materials with larger crystal systems [11]. The materials fineness can also play a role as higher fineness leads to higher photocatalytic oxidation. However, the higher fineness can cause problems regarding a homogeneous distribution of the powder when the powder is mixed dry with the cement [12].

Increases to total surface area can increase the pollution reduction potential of a construction material as it increases the surface area available to react with pollutants [11]. Simple methods such as texturing, roughening, or mixture design choices can aid in increasing the total surface area [27].
Surfaces containing TiO2 underwent weathered (loaded wheel test) and rotary abrasion. Testing showed that the abraded samples performed similarly to the original samples and the weathered samples actually had a higher NO removal efficiency due to more particles being accessible to the air pollution on the surface layer of the concrete material [26].

Four environmental climate factors have shown to affect the effectiveness of the photocatalyst: relative humidity (RH), temperature, irradiance level, and air flow rates. High relative humidity in the range of 80% is approximately 67% less as effective as compared to 25% RH [29] [30]. This reaction reduction is due to the role humidity plays in the pollutant adhesion to the materials surface [13]. Figure 10 shows sample data from four different specimens containing 3 or 5% TiO2 and their NO pollution reduction capabilities at different levels of relative humidity [29].

Relative_Humidity_Chart.jpg
Figure 10. Sample Data of the Effect of Relative Humidity on TiO2 Ability to Reduce NO Pollutant Concentrations [29]

As the temperature increases, the NOx oxidation rate accelerates [13]. The irradiance requirement to promote the electron from the valence band to the conduction band occurs around 100 µW/cm2. This level of irradiance occurs on the sunny side of a structure even on a cloudy winter day and potentially on most surfaces during a sunny summer day [31]. Finally, higher air flows lead to lower NO removal efficiencies; this is due to the decrease in residence time so there is less time for the pollutants to be absorbed by the photocatalytic compound [29]. In summary the best results were obtained by high temperature (> 25°C), low relative humidity, high light intensities, and long contact times. This environment is obtained on hot sunny days in arid climates when the winds are slow. Hot dry days are also when the risk of ozone and smog formation is highest, so the reduction of NOx concentrations would be most useful in reducing ozone formation [13].

Jayapalan et al. tested the early age effects of adding TiO2 to a mixture design. When TiO2 is used as a cement replacement, at upwards of 15%, it was found that the smaller particles of TiO2 accelerate the hydration reaction more than larger particles, and that the nucleation effect was more dominant than the cement dilution effect [32]. Mixtures containing TiO2 may exhibit decreased fluidity, an increase in water demand, and a decrease time to final set [14].

Zinc oxide (ZnO) is also a photocatalyst but exhibits less vigorous oxidation rates than TiO2 [33], unless under concentrated sunlight [34]. ZnO has a band gap of 3.37 eV [35] and the higher band gap energy than TiO2 requires more energy for the electron to be promoted to the conduction band [36]. This makes TiO2 a preferred material for ambient light photocatalyst applications.

6.0 FIELD STUDIES AND MODELING


In the past few years several real scale studies have been executed on improving air quality by employing photocatalytic materials in roads, with significant reductions in NOx levels [13]. In Antwerp, Belgium where a test section of 10,000 m2 of pavement blocks were placed on the parking lanes of a main road. The blocks used contained TiO2 in the weathering layer. They measured the concentration of NO3- deposited on the surface to determine the minimum amount of NO and NO2 that was oxidized. It was found that after 1 year, the air pollution reduction capabilities of the blocks had been reduced by 20%; however they have not reported the initial or 1-year NOx reduction rates [13].

Road_Pavers_project.jpg
Figure 11. Photos of the Brick/Overlays and the Road Built Using TiO2 Modified Bricks [13]

The first US highway pavement project using TiO2 was placed on October 24, 2011 in Missouri. The 1,500 linear feet two lane road section was a two lift placement which had a 2 inch overlay of Italcementi TX Active. Pollution motoring will be done with meteorological and solar radiation monitors which will be placed on the adjacent sound wall and with an ozone-titration method meter. As part of the study they will also be testing for changes in road runoff water quality [37].

Another large scale study found that artificial canyon streets lined with TiO2 panels saw NOx reductions of 36.7 to 82% [38].
Determining probable wide spread pollution reduction is difficult without computer modeling. Taking into account parameters such as 3D geometry, road traffic, effectiveness of pollution control, meteorology, and a surface of NOxer pavement, modeling showed that it is probable to have a pollution reduction of 30% at vehicle levels and an overall NOx pollution reduction of about 15% at a height of 20 m [39]. Using similar variables a 2D model created in the Netherlands determined that at 1.5 m a NOx reduction of 25% may occur [40].

NOxer_Model.jpg
Figure 12. 3D Model of NOx Reduction based on Height of air from Surface Containing TiO2 [39]

7.0 NOMENCLATURE


EPA – Environmental Protection Agency
eV – Electron Volt
h+ - Electron Hole
hv – Photon Energy
O· – Oxygen Radical
O3 – Ozone
OH· – Hydroxyl Radical
OH- – Hydroxyl Ion
NO – Nitric Oxide
NO2 – Nitrogen Dioxide
NOx – Mono-Nitrogen Oxides (NO and NO2)
NO3- – Nitrate
TiO2 – Titanium Dioxide
UV – Ultra Violet
VOC – Volatile Organic Compounds


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