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Photocatalytic concretes — The interface between photocatalysis and cement chemistry
Cement and Concrete Research 85 (2016) 48–54
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Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres
Photocatalytic concretes — The interface between photocatalysis and cement chemistry D.E. Macphee a,⁎, A. Folli b a b
Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, UK School of Chemistry, Cardiff University, Cardiff CF10 3AT, United Kingdom
a r t i c l e
i n f o
Article history: Received 26 October 2015 Accepted 28 March 2016 Available online xxxx Keywords: Dispersion (A) pH (A) Surface Area (B) Adsorption (C) Cement (D) Composite (E)
a b s t r a c t Concrete materials are ubiquitous in the developed world due to their versatility and cost-effectiveness as a construction material, but their great potential for increased functionality remains underdeveloped. As supports for photocatalysts, these structures offer viable solutions for the reduction of atmospheric pollutant concentrations, the source of which is often associated with urbanisation and the built infrastructure. This paper addresses (i) the photocatalytic mechanisms applicable to atmospheric depollution, (ii) the influence of doping, and (iii) the application of TiO2-based photocatalysts to concrete. Modifications to TiO2 will be discussed which can improve its activation in visible light and, in the treatment of NOx, improve catalytic selectivity towards nitrate rather than the more toxic NO2. The influence of the chemistry of cements on catalyst performance during both concrete placement and in service will also be addressed and some attention will be given to alternative strategies for introducing the photocatalyst to the concrete. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Increasing concerns over the economic and health impacts of deteriorating urban air quality and the introduction of pollutant reduction targets by 2020 in legislation [1] has provided an important focus for environmental remediation. This is made more acute as pollutant thresholds defined in legislative guidelines are being routinely exceeded around the world; a NOx concentration of 132 μg·m3 was recorded in London in March, 2015 (by way of comparison, the yearly average EU limit for NO2 is 40 μg·m3). One of the most sustainable technologies for atmospheric depollution is the use of photocatalysts in construction. Globally, concrete structures are ubiquitous in the built environment. Their surfaces are exposed to solar radiation at varying intensities, dependent on latitude, season, time of day, angle of exposure and local weather conditions and they represent significant surface areas which, particularly in urban settings, are typically exposed to the highest levels of air pollution. Photocatalysts, when activated by light, are capable of supporting chemical reactions which can degrade certain atmospheric pollutants, e.g. NOx and volatile organic compounds (VOCs), as well as non-volatile organic residues. This latter property is associated with the easy removal of dirt and gives the familiar ‘self-cleaning’, or more correctly ‘easy-cleaning’ property. Commercial photocatalytic products for concrete applications are already well established but industry–academic collaborations (e.g. PICADA [2],
⁎ Corresponding author. E-mail address: [email protected] (D.E. Macphee).
http://dx.doi.org/10.1016/j.cemconres.2016.03.007 0008-8846/© 2016 Elsevier Ltd. All rights reserved.
LIGHT2CAT [3]) continue to indicate the need to enhance performance through fundamental understanding of photocatalysis in the cement environment and across wider geographies. As an environmental remediation technology, semiconductor photocatalysis in general [4,5] and TiO2-based photocatalysts in particular [6–8] have been widely studied. This paper is also concerned with TiO2-based photocatalysis but addresses its application to concrete and considers aspects of catalysis and cement chemistry which combine to influence performance during both concrete placement and in service. In particular, catalyst dispersion in the alkaline and calcium-rich cement pastes is addressed, the role of surface texture is discussed and the impact of oxidation chemistry related to NOx degradation is presented. In addition, the importance of catalyst selectivity is addressed, its impact on air quality and how chemical modification of the catalyst can be manipulated to maximise performance is introduced. 2. Photocatalysis Anatase, the most commonly used photocatalyst in concrete, is capable, under natural sunlight, of degrading certain atmospheric pollutants, e.g. NOx, volatile organic compounds (VOCs) and non-volatile organic residues, due to charge transfer (redox) processes on the catalyst surface. Catalyst efficiency is influenced by: (i) energy and intensity of activating radiation received by the photocatalyst, (ii) the number and relative energy position of electronic states in the photocatalyst (Fig. 1), defined by the crystal structure and the redox potentials associated with the required oxidation and reduction processes (Figs. 1 and 2 [9]), (iii) charge carrier mobility in the semiconductor bands, (iv)
D.E. Macphee, A. Folli / Cement and Concrete Research 85 (2016) 48–54
A CB
e
e
The other charge carrier, the promoted electron may now participate in reducing reactions. Amongst the most common, is the formation of • the superoxide radical (O− 2 ),
A
EG D e
VB
49
h+
D+ Fig. 1. Photocatalyst (TiO2) schematic showing charge transfer between valence (VB) and conduction (CB) band positions and in possible surface redox processes.
kinetics of the charge transfer processes and (v) the accessible surface of the catalyst for pollutant species/oxygen/water adsorption. The band gap, Eg, defined as the energy separation between the valence (VB) and conduction (CB) bands of the photocatalyst, and characteristic of the bonding in the photocatalyst, is shown in Fig. 1. Absorption of electromagnetic radiation with energy matching Eg can promote an electron from the VB to the CB leading to the formation of a positive hole (h+) in the CB. The hole can aggressively oxidise species adsorbed on the catalyst, either directly, e.g.
− − • 2O2 + 2e− → 2O− 2 ; (A + e → A )
which can ultimately lead to the formation of the hydroxyl radical via subsequent reactions. This rather oversimplification neglects to account for the complexities of charge transfer dynamics in and on the catalyst, much of which is mediated by trapping centres. Nevertheless, the general processes described are fundamentally responsible for the successful application of photocatalysts in a range of environments, e.g. industrial waste water remediation, and air quality. As with all catalysts, high active surface area is required so photocatalysts typically have a particle size in the range 10–20 nm. This not only provides surface sites for adsorption, a necessary step in the photocatalytic process, but also reduces recombination processes, where photogenerated charge carriers recombine, preventing their participation in the important surface reactions described above (Fig. 1). A further factor is that for oxidation and reduction reactions to take place at the catalyst surface, charge transfer processes must be energetically favourable. Fig. 3 illustrates successful charge transfer of an electron from a donor into a photogenerated hole in the VB. The redox potential for the oxidation of the donor is lower (higher in energy) than the VB edge; the corresponding successful transfer of the photogenerated electron from the CB is to a redox potential greater than the CB edge (lower energy). For sustainable photocatalysis, both conditions must be met [5,10].
H2O + h+ → HO• + H+; (D → D+ + e−)
2.1. Catalyst illumination and geography
or indirectly, where the reaction product becomes the oxidising agent, e.g.
It is quite clear from above why the stability and band structure of TiO2 has made it the focus of heterogeneous photocatalysis, despite its large Eg of 3.2 eV. Photosensitisation of anatase requires light of energy ≥3.2 eV (UV; 380–390 nm) which, at the earth's surface, is often quoted to be in the range 3–5% of the total solar irradiance [10]. By modelling solar irradiance as a function of location (Fig. 4a), it is shown that – even at the solar noon (i.e. best solar irradiation conditions) – this range is applicable only to latitudes below 35° [11]. Consequently, for a period of the year, the UV intensity may be considerably lower than 3%. The correlation between NO concentrations measured at a photocatalytic concrete test site in Copenhagen, (55.68°N) and modelled solar UV irradiance (Fig. 4b) shows that significant NO oxidation is only achieved when the UV intensity exceeds 600 kJ m−2 day−1 at this site (~2% of total solar irradiance at noon). This type of information can inform economic decisions on the placing of broadband (UV-activated) semiconductor photocatalysts at very high latitudes.
H2Oads + h+ → •OH + H+. In either case, the formation of the highly reactive radical species X• encourages ongoing reactions until complete mineralisation is achieved. Note that the hydroxyl radical •OH is amongst the most oxidising species known (E = +2.8 V).
3. Catalyst efficiency and cement chemistry The chemistry of the cement environment is quite different from the ambient conditions normally prevalent in environmental photocatalysis and this can influence the normal behaviour of the catalysts in concrete. In freshly mixed cement, the high pH, high ionic strength mix water containing multiply charged ions dramatically modifies the surface chemistry and behaviour of TiO2 suspensions, influencing dispersion behaviour and adsorption properties. In addition, the pH changes during early mixing and ageing influence band edge positions in the semiconductor photocatalyst and also the chemistry of the NOx oxidation pathway. The physical and photocatalytic stability of doped photocatalysts may also be pH dependent. 3.1. Surface chemistry — effect on dispersion
Fig. 2. Band positions of several semiconductors in contact with aqueous electrolyte at pH 1 (taken from Grätzel, M [9]).
The degree of photocatalyst dispersion can affect the accessible area for pollutant adsorption and consequently the efficiency of the photocatalyst. Dispersion is influenced by the magnitude of surface
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Fig. 3. Photocatalytic processes must be energetically favourable.
charge adopted by the particle and this is conditioned by pH and other solution characteristics but also by the surface composition and structure of the photocatalyst. The surface of TiO2 is uncharged at pH around 6.5 (pH of zero charge or PZC) [12–14]. In the presence of solely indifferent electrolytes (i.e. no specific adsorption of the electrolyte cations/anions on the TiO2 surface), this pH coincides with the isoelectric point (IEP), i.e. the pH at which the ζ-potential is zero. At much higher pH, TiOH groups are for the great majority deprotonated giving a negatively charged surface (TiO−) whilst at much lower pH, TiOH+ 2 groups make the surface positive. The greater the magnitude of charge, the more the particles will repel and disperse. Measuring the ζ-potential characteristics of a particulate suspension is a convenient way of characterising surface properties. Folli et al. [15] differentiated between the influence of pH in indifferent electrolytes from that in a cement-like electrolyte for two commercial TiO2s of different particle size distributions (n-TiO2; dmean ~ 170 nm, and m-TiO2; dmean ~ 17 nm). In the latter case, the electrolyte is no longer indifferent; Ca2+ associates strongly with the particulate surface (specific adsorption) inducing a charge reversal (a positive ζpotential, Fig. 5) [15] similar to that reported by Labbez for silica surfaces [16]. One of the important influences observed in this study was that of phosphorous in the m-TiO2. The acidic P5+ is expected to reduce the pH of the IEP. It can be expected that at cement pH and in the presence of Ca2+, this effect will render the surfaces more susceptible to charge compensation by Ca2+ and accounts for the higher ζ-potential (~40 mV) and a higher degree of particulate dispersion. In fact, this is observed (Fig. 6). Under cement conditions, n-TiO2 has a lower ζ-potential (b+20 mV) and the higher tendency for agglomeration is observed. Doping of TiO2 with tungsten and niobium gave similar effects [17,18] to that observed for P in m-TiO2. Of course, agglomerated photocatalysts may still be effective; in addition to their external surfaces, access to internal surfaces is as much a condition of the size of the adsorbate molecules as it is of the pore size in photocatalyst agglomerates. Folli et al. showed that NOx molecules (100–200 pm) are small enough to penetrate into n-TiO2 clusters whereas Rhodamine B molecules (1.6 nm) are not [15,19].
3.2. Surface chemistry — effect on adsorption mode and degradation mechanism The mode of adsorption and strength of binding between an adsorbing molecule and a surface are conditioned by their relative charges. Therefore forcing a particular adsorption by tuning the electrokinetic properties of photocatalyst particles in cement becomes desirable for the design of efficient depolluting concretes. An insight into this topic is offered by Chen et al. [12] who studied Rhodamine B (RhB) degradation on two TiO2s engineered to have oppositely charged surfaces. RhB adsorbs on particles with negatively charged surfaces via its diethylamino groups whilst on positively charged surfaces it adsorbs via the carboxyl group. In the case of visible light irradiation, where TiO2 is not photoactivated, RhB degradation occurs exclusively by a dyesensitised pathway. Adsorption through the diethylamino groups triggers a selective stepwise deethylation process passing through various deethylated intermediates before the destruction of the chromophore structure. In the case of adsorption by the carboxyl group, no stepwise deethylation occurs. Chen et al. [12] showed that the dye-sensitised degradation occurring through the stepwise deethylation is more effective than the one occurring through adsorption via the carboxyl group, ultimately leading to much higher degradation rates.
3.3. Effect of pH The pH of a wet cement/concrete is typically in excess of 13. As discussed above, this has importance to the dispersibility of the photocatalyst in the concrete slurry but upon setting, the degree of dispersion is now fixed. The pH conditions can rapidly change at the surface as the concrete sets and the formwork is removed (Fig. 7a). The highly alkaline aqueous phase undergoes reaction with the atmospheric CO2 to produce carbonate ions, hence reducing pH, and the process can induce surface deposition of calcite, which can occlude deposited photocatalysts. Fig. 7b, c and d shows microscopic details of the surface
Fig. 4. (a) Modelled solar UV% at the solar noon as a function of latitude and time of year, and (b) correlation with photocatalytic oxidation of NO concentrations at 55.68°N [11].
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4. Catalyst doping, visible light activation and selectivity To address the geographical limitations of UV intensity discussed above, much research has focussed on visible light activation of TiO2. Nitrogen (N)-doped TiO2, was first reported in 1986 [22] with visible light absorption later being attributed, experimentally, to an interband gap state about 0.7 eV above the valence band; this is specifically attributed to interstitial N [23]. However, it has only been recently that increased levels of active N uptake by synergistic doping with transition metal ions has been observed [17,24] as shown by absorption at 450 nm (Fig. 9). 4.1. Nitrate selectivity
Fig. 5. m-TiO2 and n-TiO2 ζ-potentials vs [Ca2+].
calcite layer that can occlude TiO2 clusters present in the hardened matrix. Due to the Nernst relationship, the pH may also influence the redox processes taking place at the photocatalyst surface. Fig. 8 shows the valence and conduction band edges as well as some key redox potentials at pH = 0. These potentials are variable with pH. Whilst pH-induced changes may affect some redox couple potential and band edge positions equally, this is not always the case (see Fig. 8 [20]) due to differences in charge transfer characteristics or reactive species. In addition to the influence on the photocatalyst, the pH can also influence the target reaction. The important role of alkali (no catalyst) on NOx oxidation has been highlighted by Horgnies et al. [21] (see equations below): − 2NO2 + 2OH− ⇒ NO− 2 + NO3 + H2O
The interconversion between the major environmental NOx constituents can be summarised as in Fig. 10. Note the importance of the hydroxyl radical species, the photogeneration of which on TiO2 is referred to above. It is evident that redox reactions can be supported in both directions, the consequence being that the backward process reduces the efficiency of conversion from NO or NO2 to nitrate (HONO2 ads). The quantitative measure of conversion to nitrate is referred to as nitrate selectivity and would have a value of 100% if no other nitrogen species were formed in the process. As Fig. 11 shows, tungsten doping significantly improves nitrate selectivity but photonic efficiency is reduced. 4.2. The DeNOx index One of the attractions of photocatalytic concretes is their potential for improving air quality in urban environments. The high selectivity and the low photonic efficiency (activity) of the W-doped TiO2 require to be put in context. The photonic efficiency (ξ) is defined as the number of reactant molecules transformed or product molecules formed divided by the number of incident monochromatic light photons [5], and is directly linked to the concentration change of the species of interest ξ¼
_ ðCd −Ci ÞVp ΦART
NO2 + NO + 2OH− ⇒ 2NO− 2 + H2O
where Cd and Ci are the concentrations of the species of interest under dark and illuminated condition respectively, V_ is the volumetric flow
and must be considered an additional factor when assessing photocatalytic efficiencies of the deNOx process.
rate, p is the pressure, Φ is the photon flux impinging the photocatalyst surface, A is the irradiated area, R the universal gas constant and T is
Fig. 6. SEM-polished cross-section micrographs for cement specimens (1 day cured) prepared with: (a) m-TiO2, (b) n-TiO2.
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Fig. 7. a. Schematic pH and light penetration profile in ageing concrete. b. Optical microscope image of the cross section of a concrete specimen containing TiO2 under X-polarised light. The surface exhibits a thin calcite layer that appears of a lighter colour. c. Optical microscope image of the cross section of a concrete specimen containing TiO2 under fluorescent light. The calcite layer appears dark green indicative of low porosity. d. Scanning Electron Microscope image (secondary electron mode) of the top surface of a concrete specimen containing TiO2 (same as in b and c). The surface consists of a layer of small, closely spaced, euhedral calcite crystals and no TiO2 clusters are exposed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
absolute temperature. The nitrate selectivity of the catalyst is given by [25]. S¼
ξNOx ξNO
Fig. 8. Effect of pH on redox potentials and band edges [20].
As Fig. 11 illustrates, W-doping of TiO2 has increased S but at the expense of the photonic efficiency. However, the consequences of a high S has a greater impact on air quality because high conversion rates to nitrate means removal of NO and the more toxic NO2 from the atmosphere. Conversely, a high photonic efficiency for NO conversion, if this is only to NO2, does not necessarily represent an effective catalytic process. The DeNOx index [25] has
Fig. 9. ‘Active’ N doping level in TiO2 as a function of nominal W content prepared at 600 °C [17]; inset — band structure diagram showing N — level (typical of an interstitial N doping) relative to valence (VB) and conduction (CB) band for a W loading of 4.8 at.% [17].
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Fig. 12. DeNOx index measured using W-doped TiO2 under broad band radiation simulating sunlight.
Fig. 10. Schematic conversion mechanisms between the NOx constituents.
therefore been introduced to accommodate these characteristics in a single qualifying parameter. ξDeNOx ¼ ξNO −3ξNO2 :
nitrate selectivity, i.e. it efficiently converts NO to NO2 but not effectively to nitrate. Both tungsten- and niobium-doped TiO2 clearly show a higher tendency towards nitrate and are better suited as remediation photocatalysts, both in powder form and as mortars. It should also be noted that the visible light response is also better for the doped TiO2s. 5. Conclusions In this paper we have attempted to show that the effective use of photocatalysts in cements relies on considering a number of key physico-chemical factors:
The index recognises the relative toxicity of NO2 and NO, conservatively identifying a multiplying factor of 3, and is positive if the catalyst lowers the total NOx toxicity. Fig. 12 summarises the results from several photocatalyst compositions, including samples prepared from different processes, and show a positive DeNOx index at nominal tungsten contents of N 4.2 at.%. A range of commercial TiO2 photocatalysts tested under comparable conditions gave DeNOx indices between 0 and −4000 indicating that these photocatalysts have poor nitrate selectivity and generate the more toxic NO2. These data are further supported by analyses on powder and mortar samples incorporating W-doped and Nb-doped TiO2 (Fig. 13). Photocatalytic performance data were undertaken using industry relevant testing (ISO/DIS 22197-1). It can be noted that the commercial, un-doped TiO2 (P25) has a net negative effect due to its high activity but low
• Photocatalysis is a surface phenomenon, influenced by the chemistry of the immediate environment. The concrete surface must be engineered to maximise photocatalyst accessibility (to reactants) and activation (illumination). • Pollutant degradation must be viable based on oxidation potentials relative to semiconductor band edge positions — these are pH dependent. • Geographical limitations of photocatalytic concrete will not be overcome by conventional TiO2 — more fundamental research is needed to optimise the photonic efficiencies of visible light photocatalysts. • Catalyst surface area must be maximised for the target application; care must be taken to ensure particle dispersion is optimised. Agglomeration can block access to internal surface, i.e. if pollutant molecule size is greater than pore entry diameter.
Fig. 11. Photonic efficiency of NOx removal and nitrate selectivity of W-modified TiO2 illuminated under broad band radiation [25].
Fig. 13. DeNOx data on powder and mortar samples containing W and Nb-doped TiO2 (broad band and visible light exposure).
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• Oxidative degradation requires the pollutant to be adsorbed on the surface. The adsorption mode, dependent on surface charge (hence pH again), can influence the oxidation pathway (efficiency). • Combining photocatalyst attributes (photonic efficiency and selectivity) in an environmental impact parameter such as the DeNOx index, provides a convenient means of screening candidate photocatalyst materials. • Conventional TiO2 is often very unselective and often exhibits large negative DeNOx index, a sign of an ineffective catalytic process (too much NO2 is released). Increasingly photocatalyst selectivity, not only activity, is required to reduce the emission of harmful by-products. • Thermodynamics (energetics) is (probably) the most valuable tool in understanding/designing selective photocatalytic processes. In doing this one must consider that the fastest process after e−–h+ pair recombination is charge trapping. Charge transfer from traps must be considered in the interpretation of photocatalytic processes and in the band structure engineering/design.
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Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres
Photocatalytic concretes — The interface between photocatalysis and cement chemistry D.E. Macphee a,⁎, A. Folli b a b
Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, UK School of Chemistry, Cardiff University, Cardiff CF10 3AT, United Kingdom
a r t i c l e
i n f o
Article history: Received 26 October 2015 Accepted 28 March 2016 Available online xxxx Keywords: Dispersion (A) pH (A) Surface Area (B) Adsorption (C) Cement (D) Composite (E)
a b s t r a c t Concrete materials are ubiquitous in the developed world due to their versatility and cost-effectiveness as a construction material, but their great potential for increased functionality remains underdeveloped. As supports for photocatalysts, these structures offer viable solutions for the reduction of atmospheric pollutant concentrations, the source of which is often associated with urbanisation and the built infrastructure. This paper addresses (i) the photocatalytic mechanisms applicable to atmospheric depollution, (ii) the influence of doping, and (iii) the application of TiO2-based photocatalysts to concrete. Modifications to TiO2 will be discussed which can improve its activation in visible light and, in the treatment of NOx, improve catalytic selectivity towards nitrate rather than the more toxic NO2. The influence of the chemistry of cements on catalyst performance during both concrete placement and in service will also be addressed and some attention will be given to alternative strategies for introducing the photocatalyst to the concrete. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Increasing concerns over the economic and health impacts of deteriorating urban air quality and the introduction of pollutant reduction targets by 2020 in legislation [1] has provided an important focus for environmental remediation. This is made more acute as pollutant thresholds defined in legislative guidelines are being routinely exceeded around the world; a NOx concentration of 132 μg·m3 was recorded in London in March, 2015 (by way of comparison, the yearly average EU limit for NO2 is 40 μg·m3). One of the most sustainable technologies for atmospheric depollution is the use of photocatalysts in construction. Globally, concrete structures are ubiquitous in the built environment. Their surfaces are exposed to solar radiation at varying intensities, dependent on latitude, season, time of day, angle of exposure and local weather conditions and they represent significant surface areas which, particularly in urban settings, are typically exposed to the highest levels of air pollution. Photocatalysts, when activated by light, are capable of supporting chemical reactions which can degrade certain atmospheric pollutants, e.g. NOx and volatile organic compounds (VOCs), as well as non-volatile organic residues. This latter property is associated with the easy removal of dirt and gives the familiar ‘self-cleaning’, or more correctly ‘easy-cleaning’ property. Commercial photocatalytic products for concrete applications are already well established but industry–academic collaborations (e.g. PICADA [2],
⁎ Corresponding author. E-mail address: [email protected] (D.E. Macphee).
http://dx.doi.org/10.1016/j.cemconres.2016.03.007 0008-8846/© 2016 Elsevier Ltd. All rights reserved.
LIGHT2CAT [3]) continue to indicate the need to enhance performance through fundamental understanding of photocatalysis in the cement environment and across wider geographies. As an environmental remediation technology, semiconductor photocatalysis in general [4,5] and TiO2-based photocatalysts in particular [6–8] have been widely studied. This paper is also concerned with TiO2-based photocatalysis but addresses its application to concrete and considers aspects of catalysis and cement chemistry which combine to influence performance during both concrete placement and in service. In particular, catalyst dispersion in the alkaline and calcium-rich cement pastes is addressed, the role of surface texture is discussed and the impact of oxidation chemistry related to NOx degradation is presented. In addition, the importance of catalyst selectivity is addressed, its impact on air quality and how chemical modification of the catalyst can be manipulated to maximise performance is introduced. 2. Photocatalysis Anatase, the most commonly used photocatalyst in concrete, is capable, under natural sunlight, of degrading certain atmospheric pollutants, e.g. NOx, volatile organic compounds (VOCs) and non-volatile organic residues, due to charge transfer (redox) processes on the catalyst surface. Catalyst efficiency is influenced by: (i) energy and intensity of activating radiation received by the photocatalyst, (ii) the number and relative energy position of electronic states in the photocatalyst (Fig. 1), defined by the crystal structure and the redox potentials associated with the required oxidation and reduction processes (Figs. 1 and 2 [9]), (iii) charge carrier mobility in the semiconductor bands, (iv)
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A CB
e
e
The other charge carrier, the promoted electron may now participate in reducing reactions. Amongst the most common, is the formation of • the superoxide radical (O− 2 ),
A
EG D e
VB
49
h+
D+ Fig. 1. Photocatalyst (TiO2) schematic showing charge transfer between valence (VB) and conduction (CB) band positions and in possible surface redox processes.
kinetics of the charge transfer processes and (v) the accessible surface of the catalyst for pollutant species/oxygen/water adsorption. The band gap, Eg, defined as the energy separation between the valence (VB) and conduction (CB) bands of the photocatalyst, and characteristic of the bonding in the photocatalyst, is shown in Fig. 1. Absorption of electromagnetic radiation with energy matching Eg can promote an electron from the VB to the CB leading to the formation of a positive hole (h+) in the CB. The hole can aggressively oxidise species adsorbed on the catalyst, either directly, e.g.
− − • 2O2 + 2e− → 2O− 2 ; (A + e → A )
which can ultimately lead to the formation of the hydroxyl radical via subsequent reactions. This rather oversimplification neglects to account for the complexities of charge transfer dynamics in and on the catalyst, much of which is mediated by trapping centres. Nevertheless, the general processes described are fundamentally responsible for the successful application of photocatalysts in a range of environments, e.g. industrial waste water remediation, and air quality. As with all catalysts, high active surface area is required so photocatalysts typically have a particle size in the range 10–20 nm. This not only provides surface sites for adsorption, a necessary step in the photocatalytic process, but also reduces recombination processes, where photogenerated charge carriers recombine, preventing their participation in the important surface reactions described above (Fig. 1). A further factor is that for oxidation and reduction reactions to take place at the catalyst surface, charge transfer processes must be energetically favourable. Fig. 3 illustrates successful charge transfer of an electron from a donor into a photogenerated hole in the VB. The redox potential for the oxidation of the donor is lower (higher in energy) than the VB edge; the corresponding successful transfer of the photogenerated electron from the CB is to a redox potential greater than the CB edge (lower energy). For sustainable photocatalysis, both conditions must be met [5,10].
H2O + h+ → HO• + H+; (D → D+ + e−)
2.1. Catalyst illumination and geography
or indirectly, where the reaction product becomes the oxidising agent, e.g.
It is quite clear from above why the stability and band structure of TiO2 has made it the focus of heterogeneous photocatalysis, despite its large Eg of 3.2 eV. Photosensitisation of anatase requires light of energy ≥3.2 eV (UV; 380–390 nm) which, at the earth's surface, is often quoted to be in the range 3–5% of the total solar irradiance [10]. By modelling solar irradiance as a function of location (Fig. 4a), it is shown that – even at the solar noon (i.e. best solar irradiation conditions) – this range is applicable only to latitudes below 35° [11]. Consequently, for a period of the year, the UV intensity may be considerably lower than 3%. The correlation between NO concentrations measured at a photocatalytic concrete test site in Copenhagen, (55.68°N) and modelled solar UV irradiance (Fig. 4b) shows that significant NO oxidation is only achieved when the UV intensity exceeds 600 kJ m−2 day−1 at this site (~2% of total solar irradiance at noon). This type of information can inform economic decisions on the placing of broadband (UV-activated) semiconductor photocatalysts at very high latitudes.
H2Oads + h+ → •OH + H+. In either case, the formation of the highly reactive radical species X• encourages ongoing reactions until complete mineralisation is achieved. Note that the hydroxyl radical •OH is amongst the most oxidising species known (E = +2.8 V).
3. Catalyst efficiency and cement chemistry The chemistry of the cement environment is quite different from the ambient conditions normally prevalent in environmental photocatalysis and this can influence the normal behaviour of the catalysts in concrete. In freshly mixed cement, the high pH, high ionic strength mix water containing multiply charged ions dramatically modifies the surface chemistry and behaviour of TiO2 suspensions, influencing dispersion behaviour and adsorption properties. In addition, the pH changes during early mixing and ageing influence band edge positions in the semiconductor photocatalyst and also the chemistry of the NOx oxidation pathway. The physical and photocatalytic stability of doped photocatalysts may also be pH dependent. 3.1. Surface chemistry — effect on dispersion
Fig. 2. Band positions of several semiconductors in contact with aqueous electrolyte at pH 1 (taken from Grätzel, M [9]).
The degree of photocatalyst dispersion can affect the accessible area for pollutant adsorption and consequently the efficiency of the photocatalyst. Dispersion is influenced by the magnitude of surface
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Fig. 3. Photocatalytic processes must be energetically favourable.
charge adopted by the particle and this is conditioned by pH and other solution characteristics but also by the surface composition and structure of the photocatalyst. The surface of TiO2 is uncharged at pH around 6.5 (pH of zero charge or PZC) [12–14]. In the presence of solely indifferent electrolytes (i.e. no specific adsorption of the electrolyte cations/anions on the TiO2 surface), this pH coincides with the isoelectric point (IEP), i.e. the pH at which the ζ-potential is zero. At much higher pH, TiOH groups are for the great majority deprotonated giving a negatively charged surface (TiO−) whilst at much lower pH, TiOH+ 2 groups make the surface positive. The greater the magnitude of charge, the more the particles will repel and disperse. Measuring the ζ-potential characteristics of a particulate suspension is a convenient way of characterising surface properties. Folli et al. [15] differentiated between the influence of pH in indifferent electrolytes from that in a cement-like electrolyte for two commercial TiO2s of different particle size distributions (n-TiO2; dmean ~ 170 nm, and m-TiO2; dmean ~ 17 nm). In the latter case, the electrolyte is no longer indifferent; Ca2+ associates strongly with the particulate surface (specific adsorption) inducing a charge reversal (a positive ζpotential, Fig. 5) [15] similar to that reported by Labbez for silica surfaces [16]. One of the important influences observed in this study was that of phosphorous in the m-TiO2. The acidic P5+ is expected to reduce the pH of the IEP. It can be expected that at cement pH and in the presence of Ca2+, this effect will render the surfaces more susceptible to charge compensation by Ca2+ and accounts for the higher ζ-potential (~40 mV) and a higher degree of particulate dispersion. In fact, this is observed (Fig. 6). Under cement conditions, n-TiO2 has a lower ζ-potential (b+20 mV) and the higher tendency for agglomeration is observed. Doping of TiO2 with tungsten and niobium gave similar effects [17,18] to that observed for P in m-TiO2. Of course, agglomerated photocatalysts may still be effective; in addition to their external surfaces, access to internal surfaces is as much a condition of the size of the adsorbate molecules as it is of the pore size in photocatalyst agglomerates. Folli et al. showed that NOx molecules (100–200 pm) are small enough to penetrate into n-TiO2 clusters whereas Rhodamine B molecules (1.6 nm) are not [15,19].
3.2. Surface chemistry — effect on adsorption mode and degradation mechanism The mode of adsorption and strength of binding between an adsorbing molecule and a surface are conditioned by their relative charges. Therefore forcing a particular adsorption by tuning the electrokinetic properties of photocatalyst particles in cement becomes desirable for the design of efficient depolluting concretes. An insight into this topic is offered by Chen et al. [12] who studied Rhodamine B (RhB) degradation on two TiO2s engineered to have oppositely charged surfaces. RhB adsorbs on particles with negatively charged surfaces via its diethylamino groups whilst on positively charged surfaces it adsorbs via the carboxyl group. In the case of visible light irradiation, where TiO2 is not photoactivated, RhB degradation occurs exclusively by a dyesensitised pathway. Adsorption through the diethylamino groups triggers a selective stepwise deethylation process passing through various deethylated intermediates before the destruction of the chromophore structure. In the case of adsorption by the carboxyl group, no stepwise deethylation occurs. Chen et al. [12] showed that the dye-sensitised degradation occurring through the stepwise deethylation is more effective than the one occurring through adsorption via the carboxyl group, ultimately leading to much higher degradation rates.
3.3. Effect of pH The pH of a wet cement/concrete is typically in excess of 13. As discussed above, this has importance to the dispersibility of the photocatalyst in the concrete slurry but upon setting, the degree of dispersion is now fixed. The pH conditions can rapidly change at the surface as the concrete sets and the formwork is removed (Fig. 7a). The highly alkaline aqueous phase undergoes reaction with the atmospheric CO2 to produce carbonate ions, hence reducing pH, and the process can induce surface deposition of calcite, which can occlude deposited photocatalysts. Fig. 7b, c and d shows microscopic details of the surface
Fig. 4. (a) Modelled solar UV% at the solar noon as a function of latitude and time of year, and (b) correlation with photocatalytic oxidation of NO concentrations at 55.68°N [11].
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4. Catalyst doping, visible light activation and selectivity To address the geographical limitations of UV intensity discussed above, much research has focussed on visible light activation of TiO2. Nitrogen (N)-doped TiO2, was first reported in 1986 [22] with visible light absorption later being attributed, experimentally, to an interband gap state about 0.7 eV above the valence band; this is specifically attributed to interstitial N [23]. However, it has only been recently that increased levels of active N uptake by synergistic doping with transition metal ions has been observed [17,24] as shown by absorption at 450 nm (Fig. 9). 4.1. Nitrate selectivity
Fig. 5. m-TiO2 and n-TiO2 ζ-potentials vs [Ca2+].
calcite layer that can occlude TiO2 clusters present in the hardened matrix. Due to the Nernst relationship, the pH may also influence the redox processes taking place at the photocatalyst surface. Fig. 8 shows the valence and conduction band edges as well as some key redox potentials at pH = 0. These potentials are variable with pH. Whilst pH-induced changes may affect some redox couple potential and band edge positions equally, this is not always the case (see Fig. 8 [20]) due to differences in charge transfer characteristics or reactive species. In addition to the influence on the photocatalyst, the pH can also influence the target reaction. The important role of alkali (no catalyst) on NOx oxidation has been highlighted by Horgnies et al. [21] (see equations below): − 2NO2 + 2OH− ⇒ NO− 2 + NO3 + H2O
The interconversion between the major environmental NOx constituents can be summarised as in Fig. 10. Note the importance of the hydroxyl radical species, the photogeneration of which on TiO2 is referred to above. It is evident that redox reactions can be supported in both directions, the consequence being that the backward process reduces the efficiency of conversion from NO or NO2 to nitrate (HONO2 ads). The quantitative measure of conversion to nitrate is referred to as nitrate selectivity and would have a value of 100% if no other nitrogen species were formed in the process. As Fig. 11 shows, tungsten doping significantly improves nitrate selectivity but photonic efficiency is reduced. 4.2. The DeNOx index One of the attractions of photocatalytic concretes is their potential for improving air quality in urban environments. The high selectivity and the low photonic efficiency (activity) of the W-doped TiO2 require to be put in context. The photonic efficiency (ξ) is defined as the number of reactant molecules transformed or product molecules formed divided by the number of incident monochromatic light photons [5], and is directly linked to the concentration change of the species of interest ξ¼
_ ðCd −Ci ÞVp ΦART
NO2 + NO + 2OH− ⇒ 2NO− 2 + H2O
where Cd and Ci are the concentrations of the species of interest under dark and illuminated condition respectively, V_ is the volumetric flow
and must be considered an additional factor when assessing photocatalytic efficiencies of the deNOx process.
rate, p is the pressure, Φ is the photon flux impinging the photocatalyst surface, A is the irradiated area, R the universal gas constant and T is
Fig. 6. SEM-polished cross-section micrographs for cement specimens (1 day cured) prepared with: (a) m-TiO2, (b) n-TiO2.
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Fig. 7. a. Schematic pH and light penetration profile in ageing concrete. b. Optical microscope image of the cross section of a concrete specimen containing TiO2 under X-polarised light. The surface exhibits a thin calcite layer that appears of a lighter colour. c. Optical microscope image of the cross section of a concrete specimen containing TiO2 under fluorescent light. The calcite layer appears dark green indicative of low porosity. d. Scanning Electron Microscope image (secondary electron mode) of the top surface of a concrete specimen containing TiO2 (same as in b and c). The surface consists of a layer of small, closely spaced, euhedral calcite crystals and no TiO2 clusters are exposed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
absolute temperature. The nitrate selectivity of the catalyst is given by [25]. S¼
ξNOx ξNO
Fig. 8. Effect of pH on redox potentials and band edges [20].
As Fig. 11 illustrates, W-doping of TiO2 has increased S but at the expense of the photonic efficiency. However, the consequences of a high S has a greater impact on air quality because high conversion rates to nitrate means removal of NO and the more toxic NO2 from the atmosphere. Conversely, a high photonic efficiency for NO conversion, if this is only to NO2, does not necessarily represent an effective catalytic process. The DeNOx index [25] has
Fig. 9. ‘Active’ N doping level in TiO2 as a function of nominal W content prepared at 600 °C [17]; inset — band structure diagram showing N — level (typical of an interstitial N doping) relative to valence (VB) and conduction (CB) band for a W loading of 4.8 at.% [17].
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Fig. 12. DeNOx index measured using W-doped TiO2 under broad band radiation simulating sunlight.
Fig. 10. Schematic conversion mechanisms between the NOx constituents.
therefore been introduced to accommodate these characteristics in a single qualifying parameter. ξDeNOx ¼ ξNO −3ξNO2 :
nitrate selectivity, i.e. it efficiently converts NO to NO2 but not effectively to nitrate. Both tungsten- and niobium-doped TiO2 clearly show a higher tendency towards nitrate and are better suited as remediation photocatalysts, both in powder form and as mortars. It should also be noted that the visible light response is also better for the doped TiO2s. 5. Conclusions In this paper we have attempted to show that the effective use of photocatalysts in cements relies on considering a number of key physico-chemical factors:
The index recognises the relative toxicity of NO2 and NO, conservatively identifying a multiplying factor of 3, and is positive if the catalyst lowers the total NOx toxicity. Fig. 12 summarises the results from several photocatalyst compositions, including samples prepared from different processes, and show a positive DeNOx index at nominal tungsten contents of N 4.2 at.%. A range of commercial TiO2 photocatalysts tested under comparable conditions gave DeNOx indices between 0 and −4000 indicating that these photocatalysts have poor nitrate selectivity and generate the more toxic NO2. These data are further supported by analyses on powder and mortar samples incorporating W-doped and Nb-doped TiO2 (Fig. 13). Photocatalytic performance data were undertaken using industry relevant testing (ISO/DIS 22197-1). It can be noted that the commercial, un-doped TiO2 (P25) has a net negative effect due to its high activity but low
• Photocatalysis is a surface phenomenon, influenced by the chemistry of the immediate environment. The concrete surface must be engineered to maximise photocatalyst accessibility (to reactants) and activation (illumination). • Pollutant degradation must be viable based on oxidation potentials relative to semiconductor band edge positions — these are pH dependent. • Geographical limitations of photocatalytic concrete will not be overcome by conventional TiO2 — more fundamental research is needed to optimise the photonic efficiencies of visible light photocatalysts. • Catalyst surface area must be maximised for the target application; care must be taken to ensure particle dispersion is optimised. Agglomeration can block access to internal surface, i.e. if pollutant molecule size is greater than pore entry diameter.
Fig. 11. Photonic efficiency of NOx removal and nitrate selectivity of W-modified TiO2 illuminated under broad band radiation [25].
Fig. 13. DeNOx data on powder and mortar samples containing W and Nb-doped TiO2 (broad band and visible light exposure).
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• Oxidative degradation requires the pollutant to be adsorbed on the surface. The adsorption mode, dependent on surface charge (hence pH again), can influence the oxidation pathway (efficiency). • Combining photocatalyst attributes (photonic efficiency and selectivity) in an environmental impact parameter such as the DeNOx index, provides a convenient means of screening candidate photocatalyst materials. • Conventional TiO2 is often very unselective and often exhibits large negative DeNOx index, a sign of an ineffective catalytic process (too much NO2 is released). Increasingly photocatalyst selectivity, not only activity, is required to reduce the emission of harmful by-products. • Thermodynamics (energetics) is (probably) the most valuable tool in understanding/designing selective photocatalytic processes. In doing this one must consider that the fastest process after e−–h+ pair recombination is charge trapping. Charge transfer from traps must be considered in the interpretation of photocatalytic processes and in the band structure engineering/design.
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