Effect of humidity on the photocatalytic degradation of gaseous hydrocarbons mixture

Materials Chemistry and Physics 197 (2017) 1e9

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Effect of humidity on the photocatalytic degradation of gaseous hydrocarbons mixture Mohamed S. Hamdy Chemistry Department, Science College, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The photocatalytic degradation of gaseous mixture contains five C1-C3 hydrocarbons was investigated.
The study was performed over four commercial photocatalysts under dry and humid condition.
TiO2(P25) was the most active photocatalyst in the dry conditions, then ZnO.
Hombikat TiO2(UV100) was the most resistant photocatalyst against humidity deactivation.
CeO2 and ZnO were the most negatively influenced catalysts by humidity.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 December 2016 Received in revised form 25 April 2017 Accepted 7 May 2017 Available online 9 May 2017

The effect of humidity on the photocatalytic performance of four semiconductors was investigated in the degradation of gaseous mixture contains five light C1-C3 hydrocarbons. The photocatalytic study was performed with/without 40000 ppm of water vapour at 40  C in order to simulate the real operating conditions of the hot climate areas with high humidity level. In dry environment, TiO2(P25) and ZnO exhibited the highest photocatalytic activity. However, in the presence of water vapour, the investigated photocatalysts were deactivated with different extend. ZnO and CeO2 were the most negatively influenced materials by humidity, while, TiO2(UV100) was the most resistant photocatalyst against the deactivation. © 2017 Elsevier B.V. All rights reserved.

Keywords: Photocatalysis Humidity Photocatalytic paint TiO2 ZnO C1-C3 hydrocarbons

1. Introduction As a result of increasing industrial activities, huge amounts of gases, liquids, and solids waste are realising every day in air, water streams, and soil which affect negatively on the environment [1,2].

E-mail address: [email protected]. http://dx.doi.org/10.1016/j.matchemphys.2017.05.013 0254-0584/© 2017 Elsevier B.V. All rights reserved.

Volatile short chain hydrocarbons (C1-C3) are releasing in air as a result of combustion processes such as of natural gas combustion in gas power plants and fuels combustion in vehicles [3,4]. Short chain hydrocarbons consider serious air pollutants because they have a potential contribution in the global warming [5] and create serval problems on human health [6]. Photocatalysis is an interesting alternative for the conventional methods which are used in indoor and outdoor air purification such

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as air filters [7,8]. Air decontamination by photocatalysis has several advantages such as it can be carried out under ambient conditions (i.e. room temperature and atmospheric pressure), it can utilize solar radiation and oxygen or water vapour from air as an oxidant, and more importantly, it is able to mineralize a wide range of organic and inorganic contaminants into harmless products such as CO2 and H2O [9]. Hence, the addition of photocatalysts to construction materials (i.e. paints [10], concretes [11], gypsum [12], and cements [13]) is a point of interest for academia and industry. The use of photocatalytic paints to purify air has been investigated. Tang and co-workers [10] investigated the use of TiO2 nanoparticles supported on Latex paints in the degradation of ammonia. Capucci and co-workers [14] reported the use of TiO2 Pigmentary to reduce NOx level around the coated area. In another study, Maggos and co-workers reported the NOx depollution in a closed parking area and he observed 20% reduction in NO and NO2  levels [15]. Aguia and co-workers studied the addition of several types of TiO2 to vinyl exterior paint [16,17], the photocatalytic activity of the prepared paints was evaluated in the NOx oxidation. Moreover, the disinfection of air by using photocatalytic paints was also investigated [18], for example, the paints contain ZnO exhibited high potential in microorganism inactivation [19]. Hence, the photocatalytic paints show promising results in the field of air purification, either in the photocatalytic degradation of contaminates or in disinfection. However, air purification by photocatalytic paints is facing several challenges [20,21] such as catalyst deactivation [22], change the paint colour [23], formation of undesirable toxic by-products [24,25], and paint chalking [26]. So, further investigations are required to improve the photocatalysts performance of the paints. Humidity is an important deactivation factor of photocatalytic paints [27]. Many countries are suffering from high humidity level such as Saudi Arabia, Egypt, Thailand, Mexico, and some cites in USA. In those countries, temperature can easily reach 40  C, if the humidity reaches 53%, this means 410000 ppm of water vapour molecules are present in the air. Therefore, in order to improve the performance of the photocatalytic paints, the effect of humidity on the photocatalytic process should be carefully investigated. In the current study, the photocatalytic oxidation of five short chain hydrocarbons present in a mixture with equal amounts, was investigated over four different commercially available photocatalysts (TiO2(P25), TiO2(UV100), ZnO, and CeO2) under dry and humid conditions. The study was carried out in order to establish the most efficient catalyst(s) which might be dedicated to paints industry [20] to be used in indoor and outdoor coatings at high humid areas. 2. Materials and methods Hombikat TiO2 (UV100) was obtained from Sachtleben©, Germany. TiO2 (P25) was obtained from Evonic©, Germany. CeO2 and ZnO were purchased from Sigma Aldrich. All the obtained materials were used directly without any further treatment/modification. A standard gas mixture of 1 vol% of CH4, C2H4, C2H6, C3H6, and C3H8 in Helium was used as reactants. XRD was performed by using Schimadzu 6000 DX instrument diffractometer equipped with a graphite monochromator using CuKa radiation (l ¼ 0.1541 nm). The diffuse reflectance spectra were converted into Kubelka-Munk function F(R) by using equation F(R) ¼ (1  R)2/2R. Moreover, the bandgap of the prepared composites was calculated from equation E ¼ h  C/l where h is Plank’s constant (6.626  1034 J/s), C is the speed of light (3.0  108 m/s), and l is the cutoff wavelength (nanometers). Nitrogen adsorption/ desorption isotherms were recorded on a QuantaChrome NOVA 2000e instrument. DR UVeVis spectra were collected at ambient conditions on a CaryWin 300 spectrometer in the wavelength range

of 200e800 nm by using BaSO4 as a reference material. Scanning electron microscopy (SEM) Jeol Model 6360 LVSEM, USA, was used to observe the pore structure of the synthesized sorbent materials. The photocatalytic oxidation of the hydrocarbons mixture was performed in a multi-cell home-made photocatalytic set-up as described earlier [28]. The set-up consists of 12 identical cylindrical reactors, the inner volume of each is 50 mL and the radius of the base is 1.5 cm which gives a total base area of 7 cm2. In a real reaction, 150 mg of the photocatalyst was spread in the bottom of the reactor to make a uniform film with a thickness of 1e2 mm. The reactors were evacuated down to 3 mbar and then a He stream containing 25 ppm of each component was introduced into the reactors over the catalyst films. Evacuation/filling cycle was also repeated at least three times before lamp ignition and starting the oxidation experiment. The effect of water was studied by introducing 4 vol% of water vapour (i.e. 400 000 ppm) with the gas feed into the reactors. The applied light source is a 120W high-pressure mercury lamp with a spectrum ranging from 280 to 650 nm. All the reactors were operated in batch mode and illuminated for 135 min. The concentration of hydrocarbons was monitored by a compact gas chromatograph equipped with TCD and FID detectors with an accuracy of ±0.5 ppm. Molsieve 5A (5 m) and a capillary Porabond Q column (10 m) were connected to the TCD detector while Porabond Q column (10 m) was coupled to an FID detector. The photoactivity profiles were fitted assuming first order kinetics: lnðC=C0 Þ ¼ Ktwhere C is the concentration of the hydrocarbons at time t, C0 is the initial concentration, and k is the observed rate constant. The activity change due to humidity was calculated from the following equation:

Activity change % ¼

Cd  Ch  100 Cd

where Cd is the converted hydrocarbons in dry air (ppm), and Ch is the converted hydrocarbons in humid air (ppm). All the experiments were carried out at least for three times, the results presented here is the average of the three experiments with a standard deviation does not exceed than 3%. 3. Results Blank experiments were carried out to confirm the synergy between light and the applied catalysts in the oxidation of the gas mixture. In the first experiment, the gas mixture was introduced into the reactor cells as described in the experimental section but without catalyst. No change in the concentration of the gases was observed either in dark or under light illumination for 135 min. This is an indication for the non-leaking property for the used reactors and the high stability of the investigated gases against photolysis, respectively. In the second control reaction, the gas mixture was introduced into the reactor cells which contained the different catalysts. However, the reactions were carried out in the dark with/ without water vapour. Again, no change in the gases concentration was observed as an indication for the stability of the gas against catalytic decomposition at room temperature in the absence/ presence of water vapour. The change in the gases concentration was obtained only in the presence of light illumination and the catalytic materials, as an indication for the true photocatalytic reaction of the investigated gases. In the following section, the observed concentration change of the mixture components (C1-C3 hydrocarbons) is presented for each gas individually in three cases: i) dark-dry, ii) light-dry, and iii) light-wet conditions. Fig. 1 shows the concentration changes of C1-C3 mixture over

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Fig. 1. The concentration change (%) as a function of the photo-reaction time for a) ethane, b) ethene, c) propane, and d) propene over TiO2(P25).

TiO2(P25) photocatalyst. In the light-dry experiments, the concentration of the gases decrease as a function of illumination time as an indication for the photocatalytic oxidation of the gases over TiO2(P25). Methane exhibited high stability and its concentration did not change either in dry or wet condition. The high stability of methane towards the photocatalytic oxidation is in agreement with the previous studies [28,29]. The photocatalytic oxidation reaction of propene over TiO2(P25) was almost instantaneous and the oxidation rate was extremely high, the data points of the oxidation profile could not be measured. Total oxidation of propane was achieved after 30 min, while, 95% oxidation of ethene and ethane was obtained after 30 and 135 min, respectively. The photocatalytic oxidation rates can be arranged in the order: propene [ propane > ethene > ethane. Upon light-wet conditions, when water vapour was introduced with the gas mixture, the oxidation rates of propane, ethene and ethane decreased sharply. Total oxidation of propane and ethene was achieved after 105 min, while only 71% of ethane was oxidized after 135 min. These results clearly demonstrate the negative effect of the high humidity on the oxidation efficiency of TiO2. Over TiO2(UV100), the second investigated catalyst, the photocatalytic oxidation of C1-C3 gases followed the same order of TiO2(P25), i.e. propene [ propane > ethene > ethane. The high stability of methane and the very high activity of propene were also observed over TiO2(UV100). The total oxidation of propane and ethene was achieved after 30 and 90 min, respectively, while 58% oxidation of ethane was obtained after 135 min. The remarkable observation here is that water vapour does not seem to have high influence over UV100, the oxidation rates of light-dry and light-wet experiments for ethane and ethene are very close, while

considerable oxidation rate difference between light-dry and lightwet conditions was observed for propane (Fig. 2). By using ZnO and under light-dry conditions, the same photocatalytic oxidation behaviour of the investigated gases was obtained, however, the initial oxidation rates of the investigated gases were smaller compared to that of TiO2 (P25 and UV100). In propene, the oxidation reaction is not instantaneous, total oxidation was achieved after 45 min, while total oxidation of propane was obtained after 60 min. Surprisingly, ethane and ethene showed -almost- the same oxidation rate. In the presence of water vapour, serious deactivation was clearly observed in ZnO activity. The photocatalytic oxidation rate of all gases decreased sharply under light-wet conditions (Fig. 3). In CeO2 experiments (Fig. 4) and under light-dry conditions, ethane did not follow the observed rate order of TiO2 and ZnO, ethane did not oxidize over CeO2 as well as methane. Propene gas was the most active gas, a total conversion was achieved after 45 min. Moreover, total oxidation of propane and ethene was achieved after 60 min. In the presence of water vapour (light-wet conditions), CeO2 subjected to a very fast deactivation, the oxidation profiles of ethene and propane showed a catalyst deactivation after only 30 min (Fig. 4). In order to compare between the oxidation rates of the four investigated catalysts, the photocatalytic oxidation rate constant (K) of the gases over the investigated photocatalysts was calculated and plotted in Fig. 5. Generally speaking, the photocatalytic oxidation of ethane under light-dry condition follows the order TiO2(P25) >ZnO > TiO2(UV100) > CeO2, while the photocatalytic oxidation of ethene under dry condition follows the order TiO2(P25) > CeO2> TiO2(UV100) >ZnO. On the other hand, the

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Fig. 2. The concentration change (%) as a function of the photo-reaction time for a) ethane, b) ethene, c) propane, and d) propene over TiO2(UV100).

photocatalytic oxidation of propane under dry condition follows the order TiO2(UV100) > TiO2(P25) > CeO2> ZnO, and finally the oxidation of propene follows the order of TiO2(UV100) ¼ TiO2(P25) > CeO2>ZnO. Under wet condition, water vapour is negatively affect on the oxidation rate for all the investigated catalysts. The net results of the photocatalytic oxidation (if we exclude methane from the calculations) can be drawn as following: TiO2(P25) is the best catalyst, it oxidized 98.6 ppm of the investigated gases (100 ppm), then ZnO (95.3 ppm), followed by TiO2(UV100) with a total oxidation of 85.5 ppm, and finally CeO2 (75.9 ppm). More importantly, the most resistant catalyst towards the deactivation was TiO2(UV100). The activity change (the difference in activity between dry and wet conditions) is 0.3% only. Then, TiO2(P25) with a total activity reduction 5.8%, followed by CeO2 (40.9%), while ZnO was the most influenced catalyst by humidity, ZnO lost 74.9% of its original activity under dry condition (Fig. 6). 4. Discussion It has been established [30,31] that the photocatalytic process involves the following steps: i) the adsorption of photons by the photocatalyst, ii) the formation of electron/hole pairs, iii) the migration of charges to the surface iv) the formation of OH radicals through the reaction with OH group on the surface or with water molecules, and finally v) the action of the charges on the adsorbed molecule (here, the hydrocarbon gas molecules). Hence, the parameters of light adsorption, surface area, OH surface groups and morphological structure must be identified in order to understand

the photocatalytic behaviour of the four investigated material. Fig. 7 presents the characterization results of the investigated materials obtained from the techniques of XRD, N2 physisorption, and DR UVeVis spectroscopy. Moreover, Fig. 8 presents the SEM micrographs of the investigated materials. XRD pattern of TiO2(P25) shows the distinguished peaks of the rutile phase of TiO2 catalyst at 27, 36 , and 55 2q can be attributed to the 110, 101, and 211 crystalline structures of rutile (JCPDS card no. 76-1940), together with the distinguished peaks of the anatase phase of TiO2 at 26 38.5 and 48 2q can be attributed to the 101, 112, and 200 crystalline structures of anatase (JCPDS card no. 782486) [32]. However, XRD pattern of TiO2(UV100) is characterized by broadened reflections at 26 , 38.5 , and 48.6 2q, in agreement with an anatase phase of poor crystallinity [33]. For ZnO, the distinguished peaks at 31.7, 34.4 , 36.2 , 47.3 , 56.5 , 62.7, and 67.9 2q are clearly seen in the pattern. These peaks are corresponding to the crystal planes of (100), (002), (010), (102), (110), (103), and (112), respectively. The location and the intensity of the detected peaks are in a good agreement with the standard data of hexagonal ZnO of Wurtzite crystalline phase (JCPDS card no. 361451) [34]. Finally, The pattern of CeO2 shows the distinguished peaks at 28 , 33 , 48 , 57, 59 , 70 , 77, 79 2q, which are in a good agreement with the standard Fluorite crystalline phase (JCPDS card no. 43-1002) [35]. Fig. 7b shows the N2 physisorption of the four applied materials. For TiO2(UV100), the isotherm presents an intermediate shape between BJH types II and IV, with hysteresis loops of type H4. The enclosure of the hysteresis loop at P/Po ¼ 0.45 indicates the presence of a mesoporous model. For the other materials, i.e. TiO2(P25),

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Fig. 3. The concentration change (%) as a function of the photo-reaction time for a) ethane, b) ethene, c) propane, and d) propene over ZnO.

ZnO and CeO2, all the isotherms exhibited type II isotherm as an indication for the non-porous materials. The surface area of the applied materials are listed in Table 1. TiO2(UV100) exhibited the highest surface area; 242 m2/g, the TiO2(P25) which exhibits 52 m2/ g, moreover, CeO2 exhibited the poorest surface area, almost 3.1 m2/ g. Fig. 7c and d shows the diffuse reflectance UVeVis spectra of the four materials. The adsorption bands of the entire materials are located in the UV region. The cutoff wavelength of the four materials is almost identical. The calculated bandgap values of the four materials are very close, almost 3.2 ± 0.1 eV for the whole materials and the adsorption edge is 385e400 nm. This means that the four materials mainly can utilize light from UV region only. SEM micrograph of the four applied materials are introduced in Fig. 8. TiO2(P25) exhibited small spheres-like particles, while TiO2(UV100) exhibited bulky agglomerated particles (2e5 mm), however the high surface area of the material originates from its high porosity. The micrograph of ZnO shows the small spheres-like particles, moreover, CeO2 exhibited bulky sheet-like structure (10e15 mm), which is associates with the small surface area of the material. Hence, the four applied materials show big variation in the morphological structure and texture properties. These differences were summarised and listed in Table 1. In the following section, the effect of those properties on the photocatalytic activity of the applied materials is discussed. The high activity of TiO2(P25) can be explained by the mixed phase composition (77% anatase and 23% rutile). Hurum and coworkers [36] reported that the high activity of P25 can be explained by what so called ‘antenna theory’, in which the presence

of rutile crystals creates a more stable charge separation due to the rapid electron transfer from rutile to lower energy anatase lattice to prevent the fast electron/hole recombination. Despite its lower crystallinity, the high photocatalytic activity of TiO2(UV100) can be attributed to the high surface area of the active anatase phase [37], which means that many active sites are available for catalytic process. Furthermore, the high surface area enhances the adsorption capacity of contaminants and the capacity to generate more OH radicals. Moreover, the high porosity and the small size of UV100 crystals can lead to a better balance between surface and bulk photogenerated charge recombination. The low photocatalytic activity of CeO2 can be explained by imperfections in the crystal lattice manifested as electron and oxygen vacancies. Bennett and coworkers [38] reported that the migration of change carriers to the surface of CeO2 crystals are too slow which increase the chances for electron/hole recombination. In a recent study, it has been shown that CeO2 lattice contains two oxidation states for Ce (þ3 and þ4) [39], which means that CeO2 crystals can scavenge the formed free radicals from irradiation [40] due to the existence of oxygen vacancies in the lattice [41]. Several studied were reported to explain the effect of humidity on the photocatalytic performance of different catalysts [42e44]. Sebastiani reported the negative effect of 70% humidity on TiO2 in the photocatalytic oxidation of acetone [45]. Einaga and co-workers reported also the negative effect of high humidity level on TiO2 in the photooxidation of benzene, toluene, cyclohexene and cyclohexane [46]. In another study, Bouazza [47] suggested that humidity must be avoided in all levels during the photocatalytic oxidation of propene. Zhang reported the negative effect of humidity on the photocatalytic oxidation of toluene over TiO2 [48].

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Fig. 4. The concentration change (%) as a function of the photo-reaction time for a) ethane, b) ethene, c) propane, and d) propene over CeO2.

Fig. 5. A comparison between the first order rate constant (K) of the investigated gases over TiO2(P25), TiO2(UV100), ZnO, and CeO2 under light-dry and light-wet conditions.

However, on the other hand, Grasso reported a positive effect of high level of humidity (over 60%) on TiO2, Grasso observed the condensation of water and its presence as a liquid inside the pores

of TiO2 [49]. Generally speaking, the negative effect of high humidity level on the photocatalytic activity can be attributed to the competitive

M.S. Hamdy / Materials Chemistry and Physics 197 (2017) 1e9

Fig. 6. The activity change of the four applied photocatalysts as a result of high humidity presence.

adsorption of water molecules with contaminant gases for reactive sites on the surface of the catalyst [48]. From the data obtained in this study, CeO2 was the most deactivated catalyst in the case of ethene and propane, the photocatalyst did not even complete the first run in the presence of water vapour. Watkins and co-workers [50] reported that water molecules readily dissociate on the O vacancies of CeO2 surface to form hydroxyl groups, Chen and Chen [51] reported through computational calculations that the formed hydroxyl group facilitate the adsorption of O2 molecules on the surface, which should improve the photocatalytic activity of CeO2. Those results would be accepted at low

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level of humidity, however, the observed negative effect of high humidity level on the photocatalytic activity of CeO2 is -most likelydue to the stabilization of the formed products (carbon deposits) [46] by water molecules over the catalytic sites of CeO2. ZnO exhibited serious deactivation in the presence of water vapour. The deactivation of ZnO as a photocatalyst in aqueous media was discussed earlier [52]. In the presence of water, ZnO become unstable due to its reaction with water molecule to form 2 2þ ZnOHþ, Zn(OH)2, Zn(OH) and other products 3 , Zn(OH)4 , Zn which are not photocatalytically active [53]. TiO2(UV100) exhibited the minimum influence by the presence of water vapour, again, the high surface area is the key factor in the deactivation mechanism. Keller and co-workers reported the effect of the high surface area of TiO2(UV100) on the surface poisoning by sulphur [54]. By using the same hypothesis, the high surface area of TiO2(UV100) can store large amount of carbon residuals and facilitate the over oxidation reaction, this is strongly supported by the observed improvement in the deactivation resistance. However, further investigation is needed to prove this assumption. 5. Conclusions The photocatalytic oxidation of a short-chain hydrocarbon mixture consists of methane, ethane, ethene, propane, and propene by molecular oxygen was carried out over four commercially available photocatalysts; TiO2(P25), TiO2(UV100), ZnO and CeO2. In a dry environment, no photocatalytic oxidation for methane could be detected over the four photocatalysts, while propene was the most active gas, the reaction over TiO2 photocatalysts was instantaneous. The oxidation rate of the different gases can be arranges as in the order of propene [ propane  ethane > ethane. Under high

Fig. 7. Characterization of the four applied photocatalysts; a) XRD patterns, b) N2 physisorption isotherms, c) DR UVeVis spectra, and d) cutoff wavelength.

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Fig. 8. SEM micrographs of the investigated materials.

Table 1 The main morphological differences between the applied photocatalysts.

a

Phase composition Crystallinitya Surface areab (m2/g) Band gapc (eV) Surface eOHd (mmol/g) a b c d

TiO2 (P25)

TiO2 (UV100)

ZnO

CeO2

77% Anatase 23% Rutile Crystalline 52 3.2 0.4

100% Anatase Semi-crystalline 342 3.1 1.21

100% hexagonal Wurtzite Crystalline 22.1 3.2 0.12

100% cubic fluorite Crystalline 3.1 3.2 0.026

Investigated from XRD patterns. Calculated from N2 sorption measurements. Calculated from the diffuse reflectance spectra. Calculated according the reported procedure in Ref. [50].

humidity condition, a serious deactivation was observed in all photocatalysts, CeO2 and ZnO were the most influenced photocatalysts by humidity. TiO2(UV100) exhibited better resistance towards deactivation than TiO2(P25). Future work includes the use of other non-commercially available photocatalysts such as Fe3þdoped ZnS quantum dots [55,56] and Graphene quantum dots [57] to compare their activities with the commercial photocatalysts.

Conflict of interest The author has declared no conflict of interest.

Acknowledgement The author would like to express his gratitude to King Khalid University, Saudi Arabia for providing administrative and technical support. The experimental work of this manuscript was partially conducted at Photocatalytic Synthesis (PCS) Group, MESAþ Institute for Nanotechnology, University of Twente, The Netherlands. The author thanks Prof. Dr. Guido Mul, the head of the group for his support. Special thanks to Sachtleben© and Evonic©, Germany for the free TiO2 samples.

References [1] R.P. Pohanish, Sittig’s Handbook of Toxic and Hazardous Chemicals and Carcinogens, William Andrews Publishing, New York, 2002. [2] P.R. Gogate, A.B. Pandit, Adv. Environ. Res. 8 (2004) 501. [3] EPA, Total Exposure Assessment Methodology (TEAM) Study, Report 600/ 6e87/002a, Environmental Protection Agency, Washington DC, 1987. [4] US EPA, National Emission Standards for Hazardous Air Pollutants, 40 CFR, Part 63, 2006. [5] A.R. Moss, J.-P. Jouany, J. Newbod, Ann. Zootech. 49 (2000) 231. [6] Handbook of Compressed Gases, fourth ed., springer, 1999, p. 348. [7] J. Zhao, X. Yang, Build. Environ. 38 (2003) 645. [8] J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, R. Zhao, Atmos. Environ. 43 (2009) 2229. [9] C.H. Ao, S.C. Lee, Chem. Eng. Sci. 60 (2005) 103. [10] Q.-J. Geng, X.-K. Wang, S.-F. Tang, Biomed. Environ. Sci. 21 (2008) 118. [11] J. Chen, S.-C. Kou, C.-S. Poon, Build. Environ. 46 (2001) 1827. [12] Q.L. Yu, H.J.H. Brouwers, Adv. Mater. Res. 651 (2013) 751. [13] M. Lackhoff, X. Prieto, N. Nestle, F. Dehn, R. Niessner, Appl. Catal. B 43 (2003) 205. [14] C.L. Bianchi, C. Pirola, F. Galli, G. Cerrato, S. Morandi, V. Capucci, Chem. Eng. J. 261 (2015) 76. [15] T. Maggos, J.G. Bartzis, M. Lakou, C. Gobin, J. Hazard. Mater. 146 (2007) 668. [16] C. Aguia, J. Angelo, L.M. Madeira, A. Mendes, J. Environ. Mang. 92 (2011) 1724. [17] C. Aguia, J. Angelo, L.M. Madeira, A. Mendes, Polym. Degrad. Stabil. 96 (2001) 898. [18] L. Caballero, K.A. Whitehead, N.S. Allen, J. Verran, Dyes Pigm. 86 (2010) 56. [19] L. Hochmannova, J. Vytrasove, Prog. Org. Coat. 67 (2010) 1. [20] N. Rimawi, Al-Jazeera Paints Academy, Saudi Arabia, personal communication. [21] L. Zhong, F. Haghighat, Build. Environ. 91 (2015) 191. [22] J. Peral, D.F. Ollis, J. Mol. Catal. A 115 (1997) 347.

M.S. Hamdy / Materials Chemistry and Physics 197 (2017) 1e9 [23] N.S. Allen, M. Edge, G. Sandoval, J. Verran, J. Stratton, J. Maltby, Photochem. Photobiol. 81 (2005) 279. [24] E. Uhde, T. Salthammer, Atmos. Environ. 41 (2007) 3111. [25] O. Giess, C. Cacho, J.B. Moreno, D. Kotzias, Build. Environ. 48 (2012) 107. [26] R. Benedix, F. Dehn, J. Quaas, M. Orgass, Lacer 5 (2000) 157. [27] T. Guo, Z. Bai, C. Wu, T. Zhu, Appl. Catal. B 79 (2008) 171. [28] M.S. Hamdy, G. Mul, Appl. Catal. B 174 (2015) 413. [29] M.S. Hamdy, R. Amrollahi, I. Sinev, B. Mei, G. Mul, J. Am. Chem. Soc. 136 (2014) 594. [30] T.M. Twesme, D.T. Tompkins, M.A. Anderson, T.W. Root, Appl. Cata. B 64 (2006) 153. [31] T.N. Obee, R.T. Brown, Environ. Sci. Technol. 29 (1995) 1223. [32] W.H. Saputera, G. Mul, M.S. Hamdy, Catal. Today 246 (2015) 60. [33] M.S. Hamdy, W.H. Saputera, E.J. Groenen, G. Mul, J. Catal. 310 (2014) 75. [34] P. Fageria, S. Gangopadhyay, S. Pande, RSC Adv. 4 (2014) 24962. [35] P. Periyat, F. Laffir, S.A.M. Tofail, E. Magner, RSC Adv. 1 (2011) 1794. [36] D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajh, M.C. Thurnauer, J. Phys. Chem. B 107 (2003) 4545. [37] A.A. Tellez, R. Masson, D. Robert, N. Keller, V. Keller, J. Photochem. Photobiol. A 250 (2012) 58. [38] S.W. Bennett, A.A. Keller, Appl. Catal. B 102 (2011) 600. [39] H. Yin, T. Tsuzuki, K.R. Millington, P.S. Casey, J. Nanopart. Res. 16 (2014) 1. [40] R.W. Tarnuzzer, J. Colon, S. Patil, S. Seal, Nano. Lett. 5 (2005) 2573. [41] S.M. Hirst, A.S. Karakoti, R.D. Tyler, N. Sriranganathan, S. Seal, C.M. Reilly,

9

Small 5 (2009) 2848. [42] M.L. Sauer, D.F. Ollis, J. Catal. 158 (1996) 570. [43] M.L. Bechec, N. Kinadjian, D. Ollis, R. Backov, S. Lacombe, Appl. Catal. B 179 (2015) 78. [44] M.E. Zorn, S.O. Hay, M.A. Anderson, Appl. Catal. B 99 (2010) 420. [45] M. Bettoni, P. Candori, S. Falcinelli, F. Marmottini, S. Meniconi, C. Rol, G. Vittorio, G.V. Sebastiani, J. Photochem. Photobiol. A 268 (2013) 1. [46] H. Einaga, S. Futamura, T. Ibusuki, Appl. Catal. B 38 (2002) 215. [47] N. Bouazza, M.A. Lillo-Rodenas, A. Linares-Solano, Appl. Catal. B 84 (2008) 691. [48] J. Mo, Y. Zhang, Q. Xu, Appl. Catal. B 132e133 (2013) 212. [49] R. Li, M. Faustini, C. Boissiere, D. Grosso, J. Phys. Chem. C 118 (2014) 17710. [50] M.B. Watkins, A.S. Foster, A.L. Shluger, J. Phys. Chem. C 111 (2007) 15337. [51] H.-L. Chen, H.-T. Chen, Chem. Phys. Lett. 493 (2010) 269. [52] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. Cells 77 (2003) 65. [53] B. Neppolian, H.C. Choi, S. Sakthivel, B. Arabindoo, V. Murugesan, J. Hazard. Mater. 89 (2002) 303. [54] J.A.R. Van Veen, F.T.G. Veltmaat, G. Jonkers, J. Chem. Soc. Chem. Commun. (1985) 1656. [55] H.R. Rajabi, O. Khani, M. Shamsipur, V. Vatanpour, J. Hazard. Mater. 250e251 (2013) 370. [56] M. Shamsipur, H.R. Rajabi, O. Khani, Mater. Mat. Sci. Semicon. Proc. 16 (2013) 1154. [57] M. Roushani, M. Mavaei, H.R. Rajabi, J. Mol. Catal. A 409 (2015) 102.