Degradation of gaseous formaldehyde via visible light photocatalysis using multi-element doped titania nanoparticles

Chemosphere 182 (2017) 174e182

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Degradation of gaseous formaldehyde via visible light photocatalysis using multi-element doped titania nanoparticles Maricris T. Laciste a, b, Mark Daniel G. de Luna c, Nolan C. Tolosa d, Ming-Chun Lu e, * a

Environmental Engineering Unit, College of Engineering, University of the Philippines, Diliman, Quezon City, 1101, Philippines Research and Development Division, Environmental Management Bureau, Department of Environment and Natural Resources, Quezon City, 1101, Philippines c Department of Chemical Engineering, University of the Philippines, Diliman, Quezon City, 1101, Philippines d Office for Research Promotion and Coordination, Malayan Colleges Laguna, Cabuyao, Laguna, 4025, Philippines e Department of Environmental Resources Management, Chia Nan University of Pharmacy and Science, Tainan, 71710, Taiwan b

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


Photoactivity of multi-element doped TiO2 releases reactive species (OH and O 2 ).
Hybridization of unoccupied Ti d(t2g) with O 2p exist in conduction bands of TiO2.
Intense peak on XPS verifies the incorporation of Ag and W in the lattice of TiO2.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 September 2016 Received in revised form 1 May 2017 Accepted 3 May 2017 Available online 5 May 2017

This study developed a modified titanium dioxide photocatalyst doped with multi-element synthesized via sol-gel process to productize a novel photocatalyst. The study includes degradation of gaseous formaldehyde under visible light using the synthesized novel titanium dioxide photocatalyst. Varying molar ratios from 0 to 2 percent (%mole in titanium dioxide) of ammonium fluoride, silver nitrate and sodium tungstate as dopant precursors for nitrogen, fluorine, silver and tungsten were used. Photodegradation of gaseous formaldehyde was examined on glass tubular reactors illuminated with blue light emitting diodes (LEDs) using immobilized photocatalyst. The photocatalytic yield is analyzed based on the photocatalyst surface chemical properties via X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared (FTIR) Spectrophotometry, Brunauer-Emmett-Teller (BET) and X-ray Diffraction (XRD) characterization results. The applied modifications enhanced the visible light capability of the catalyst in comparison to the undoped catalyst and commercially available Degussa P-25, such that it photocatalytically degrades 88.1% of formaldehyde in 120 min. Synthesized titanium dioxide photocatalyst exhibits a unique spin orbital at 532.07 eV and 533.27 eV that came from the hybridization of unoccupied Ti d(t2g) levels. © 2017 Elsevier Ltd. All rights reserved.

Handling Editor: Jun Huang Keywords: TiO2 doping Visible-light photocatalysis Gaseous formaldehyde Air purification Multi-element

1. Introduction * Corresponding author. E-mail address: [email protected] (M.-C. Lu). http://dx.doi.org/10.1016/j.chemosphere.2017.05.022 0045-6535/© 2017 Elsevier Ltd. All rights reserved.

Various organic and inorganic compounds mainly formaldehyde, benzene, toluene and other VOCs, NOx and SOx contaminate

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indoor air quality and pose significant health effects (Huang et al., 2016a). The prevalence of symptoms associated with these organic and inorganic compounds will include skin irritation, chest tightness, headache, fatigue, dullness and dehydration (Xiao et al., 2013). Most of our indoor air quality contaminates by the fumes from various indoor activities such as cooking and smoking and the influx of outdoor pollutants through openings from doors or windows and the fresh air of air-conditioners (Yu et al., 2009). When the outdoor air environment pose more risk, air dilution by ventilation (i.e. opening of doors and windows) may not be considered as an option for improving indoor air quality (Zhong et al., 2012; Photong and Boonnamnuayvitaya, 2009). Volatile organic compounds, specifically formaldehyde (HCHO), is a simplest form of aldehydes that is commonly and naturally occur as major indoor air pollutants that can be found in some industrial resins such as the particle boards, coatings, adhesives and other wood-based materials (Fan et al., 2016; Huang et al., 2016e). Exposure of formaldehyde above acceptable level is hazardous to humans and animals which may cause serious pulmonary diseases (i.e. asthma, pneumonia, etc.), nasal sinus cancer and nasopharyngeal cancer (Fan et al., 2016; Huang et al., 2016a, 2016d). Reduction or elimination of the indoor HCHO emission has become the most vital aspect in the field of research (Fan et al., 2016). There are several air treatment technologies that are currently employed and investigated to address indoor air quality issues. But there are two main categories of conventional air purification technologies and this includes: (a) one mainly utilizes filters and sorbents to capture pollutants, and (b) the other applies heating at elevated temperatures and/or light illumination to decompose or oxidize contaminants into CO2 and H2O in the presence of photocatalysts (Coutts et al., 2011; Huang et al., 2015; Huang et al., 2016e; Huang et al., 2016a). Semiconductors on the other hand are one of the most popular photocatalyst that has been known and attracted much attention in the field of research because of its potential solutions in chronic environmental pollution (Huang et al., 2016b). Most of the photocatalyst utilizes UV light that is only about 3e5% of the solar energy with poor visible-light activity due to wider band gaps (Huang et al., 2014; De Luna et al., 2016a, 2016b). This can be modified by structural designing and modulation, heterojunction, noble metal decorating, material hybridization, and doping with metal or non-metal elements to make it more active under visible-light irradiation by reducing its band gap energy and increase photocatalytic activity (Huang et al., 2014, 2015, 2016b). Metal ions during excitation, produces electron-hole pairs, separates and traps which occupies a regular lattice sites which produces reactive species of hydroxyl ion and super oxide for redox reactions (Huang et al., 2016c). Hereafter, it inhibits or reduces the rate of recombination occurrence and widens the available wavelength range, as a result it increases the utilization rate of the visible light (Wilke and Breuer, 1999; Wen et al., 2015). Consequently, metal ions can suppress the recombination of photo-induced electron-hole pairs thereby increasing the photocatalytic efficiency (Zhang and Liu, 2008). Whereas, non-metal ion as dopants can be incorporated in the lattice structure of titanium dioxide that creates mid-gap states which also decreases the band gap and increasing the visible light photocatalytic response of the titanium dioxide (Zhang and Liu, 2008). Over the years, various visible-light active TiO2 photocatalysts have been synthesized including N-TiO2 (Nolan et al., 2012), C-TiO2 (Chen and Chu, 2012), I-TiO2 (He et al., 2012), Fe-TiO2 (Cui et al., 2009), W-TiO2 (Putta et al., 2011). In contrast to this, there were also study of co-doping performed by the researchers and this includes N:Ni-TiO2 (Zhang and Liu, 2008), Ce:Si-TiO2 (Chen et al., 2009), Zn:Fe-TiO2 (Srinivasan et al., 2006), K:Al:S-TiO2 (Tolosa et al., 2011), and K:Fe:C:N-TiO2 (Gotostos et al., 2014). However,

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there were no studies attempted to conduct research on the photocatalytic performance of quadruple-elemental doped TiO2. Such that in this study, a multi-element doping of TiO2 with W, Ag, N, and F was examined. According to Putta et al. (2011), a titanium dioxide synthesized using sol-gel method and doped with 0.5% W exhibited a 74% photodegradation of 2-chlorophenol under blue light irradiation is comparable to the same degree of reduction using commercial Degussa P-25 under UV irradiation (Putta et al., 2011). Whereas, according to the previous research by Barakat et al. (2011), the charge separation of electrons and holes of TiO2 with silver nanoparticles deposited on surface that acts as an electron acceptors was also enhanced (Barakat et al., 2011). Page et al. (2007) reported that the presence of the silver oxide Ag2O in conjunction with titania marked an increased in photocatalytic activity due to stabilization of photogenerated electronehole pairs at the surface of titania by localization of the photogenerated electron on the silver oxide (Page et al., 2007). Ananpattarachai et al. (2009) studied substitutional and interstitial N-doping of the TiO2 structure and reported that nitrogen dopants effectively extended the light absorption of TiO2 in the visible light range (Ananpattarachai et al., 2009; Bakar and Ribeiro, 2016). A narrowed band gap of the modified titania catalyst will facilitate more the excitation of electrons from the valence band to the conduction band even under visible light illumination, which results into higher photocatalytic activities. However, Li et al. (2005) reported that there are several advantages attributed to higher photocatalytic activity of F-doped titania in the degradation of gas-phase acetaldehyde which includes enhancement of surface acidity, creation of oxygen vacancies, and increased active sites (Li et al., 2005; Zhu et al., 2016). The high electronegativity of fluorine could stabilize the electron release upon oxygen depletion during calcination treatment. Although, the extrinsic absorption bands of the generated oxygen vacancies create free charge carriers that can take part in surface chemical reactions making a visible light photocatalytic excitation (Dozzi et al., 2011). One of the main reason of quadruple doping in TiO2 is to increase its photocatalytic activity in visible-light region. Doping with metals such as tungsten and silver will enhanced its adsorption capability. While doping with non-metal such as fluoride and nitrogen is to enhance TiO2 activity that is not vulnerable to thermal instability and increase in carrier-recombination centers. 2. Materials and methods 2.1. Reagents All chemicals used in preparation of catalysts and in photocatalytic activity were analytical grade. Tetra-n-butyl orthotitanate [C16H38O4Ti] MW ¼ 340.32 g mol1 (98%, Merck, KGaA, Darmstadt Germany) is the titanium dioxide precursor. Ethyl alcohol MW ¼ 46.068 g mol1 (99.5%, Merck) as solvent. Ammonium fluoride MW ¼ 37.037 g mol1 (99%, Ferak GMBH, West Berlin), silver nitrate MW ¼ 169.87 g mol1 (99.8%, Ferak GMBH, West Berlin) and, sodium tungstate dihydrate MW ¼ 329.86 g mol1 (99%, Ferak GMBH, West Berlin) were used as dopants. Deionized water with resistivity value of 18.2 MU cm was used the step by step process as diluent and washing. 2.2. Synthesis of the multi-element/quadruple-element photocatalyst The synthesis method used is based on a modified sol-gel method conducted by Tolosa et al. (2011). A 10 mL of tetra-nbutyl orthotitanate was added to 40 mL ethanol. The mixture was constantly stirred for 5 min at 400 rpm and 25  C. Then additional

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of 10 mL DI water was added in the mixture and continuously stirred for 1 h. At this stage a homogeneous white sol precipitates formed and an aqueous solutions of dopant was also added dropwise into the mixture. Varying amounts of the dopants from 0 to 2 %mole in TiO2 (0, 0.2, 0.35, 0.7, 1.4, and 2) were used during this synthesis. Another 10 mL of ethanol was also added while the mixture stirred continuously. After 1e2 h of complete mixtures, the resulting solutions were allowed to settle for 24 h for ageing and complete hydrolysis. 2.3. Photocatalytic activity of the multi-element/quadrupleelement photocatalyst The synthesized photocatalyst at a loading amount of 0.1 g L1

was immobilized inside glass test tubes using a Digisystem VM2000 Vortex mixer in a 5-min interval. The sample was dried at 100  C using Shin Shiang Tech RUD-45L Oven. The stock formaldehyde solution was prepared using 37 percent Formaldehyde (GMBH, West Berlin). Two micro-liter volume of the stock solution was placed inside the glass test tube reactors, 12 cm from the bottom of the tube. The test tubes are sealed using septum, made of polytetrafluoroethylene (PTFE) and parafilm as sealing tapes. The test tube reactors were heated at 40  C for 1 h to completely vaporize the formaldehyde solution. A 2-h adsorption equilibrium time is established at a controlled temperature of 30 ± 2  C. The tube reactors were then illuminated using blue LEDs with maximum light intensity measured at 25.7 W m2 for 2 h. Samples were taken at a 30-min interval (0, 30, 60, 90, 120 min). And then

Fig. 1. Normalized residual HCHO concentration plot a) adsorption only, b) photocatalysis, c) comparison between synthetic and commercial, d) synthesized photocatalysts with varying dopant molar concentrations. Experimental Conditions: 0.1 g L1 catalyst, 2 ppm initial formaldehyde concentration, 30  C, 25.7 W m2.

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the samples were analyzed for formaldehyde residual concentration using a PPM Formaldemeter with detection limit of 0e10 ppm. 2.4. Physical and structural properties characterization The physical and structural properties of the synthesized photocatalysts were checked using a Siemens Kristalloflex 760 X-ray Diffraction spectrophotometer. Surface compositions were determined using ULVAC- PHI 5000 Versa Probe X-ray Photoelectron Spectrometer and functional groups were analyzed using a JASCO FT-IR 4100 Fourier Transform Infra-red spectrophotometer. The specific surface area, average pore size and pore volume of commercial Degussa P-25 and the synthesized photocatalysts was performed using the ASAP 2020 V3.00 E Brunauer, Emmett and Teller e Nitrogen Gas adsorption. 3. Results and discussion

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3.2. X-ray diffraction analysis Comparison between adsorption (closed-dark room reaction) and photocatalytic reaction using the different synthesized photocatalysts after 120 min gives a trend similar to the XRD crystallinity trend observed in Fig. 2. Synthesized photocatalyst Wy35 gives the highest photocatalytic potential with 57% increase in gaseous formaldehyde degradation whilst Cy35 photocatalyst exhibited only about 47% increase. This is significant to the XRD plot in Fig. 2 which shows decreasing crystallinity in the order of Wy35>Cy35>Ay35>Ny35>Fy35. Increased crystallinity is associated with the increase in photocatalytic activity. Table 1 shows the comparison of crystallite sizes of the synthesized photocatalyst with different molar concentrations of silver, fluorine, nitrogen and tungsten. The data shows the mean crystallite size increases as the amount of dopant is increased. However the increased crystallite sizes values of the doped photocatalysts may indicate that there will be an increased amount of defects in the crystal lattice of TiO2.

3.1. Photocatalytic activity 3.3. X-ray photoelectron spectroscopy analysis Fig. 3 shows the O 1s peaks of each synthesized photocatalyst with individual dopant precursors. The O 1s peak shifts of individual photocatalysts, to decreasing binding energy values follows the trend of U35>Wy35>Ay35>Fy35>Cy35>Ny35 with the energy of 529.8563 eV > 529.7598 eV > 529.7322 eV > 529.6656 eV > 529.6333 eV > 529.5219 eV, respectively. This shift in binding energies from higher to lower values is a consequence of an increase in screening by additional electrons in the system suggesting a red shift due to the creation of mid band gap states. This also means that the shifting to lower binding energy of TiO2 with doping element induces surface band bending which changes the shape of the spectral line. As such, the Ti3þ ion is located in the oxygen vacancies that is also the preferred bond site for the doped element € thelid et al., 2011). The Fy35 synthesized photocatalyst uniquely (Go exhibits a spin orbital splitting with additional peaks at 532.0656 eV and 533.2656 eV. This is in agreement with a similar study on titanium dioxide-fluorine doped tin oxides of Kronawitter et al. (2012). They indicate that the unique peak come from the hybridization of unoccupied Ti d(t2g) levels with O 2p levels, which exist in the conduction bands of titanium oxides. The Ti 2p3/2 binding energies shown in Fig. 3 for the synthesized

5000 Wy35 4000 Ny35

Intensity

The normalized HCHO concentration plots as illustrated in Fig. 1 shows that in the photocatalytic activity of modified photocatalysts, the adsorption of gaseous formaldehyde is obviously minimal for samples Wy35, Fy35 and Ny35. Whereas, catalysts Ay35 and Cy35 adsorbs gaseous formaldehyde in 120 min of approximately 35 and 41%, respectively. During the photocatalytic activity, the synthesized TiO2 releases reactive species such as OH and O 2 ions that oxidizes gaseous formaldehyde contaminant. The figure also exemplifies that the adsorption and the photocatalytic reaction is significant to each other which means that the higher the adsorption of the catalysts results to higher degradation under visible light. This may attributed to the circumstances that molecule of contaminant which adsorbed in the surface of catalyst will be the first one to be oxidize. Since the catalyst upon activation using illumination releases hydroxyl and superoxide ions capable of oxidizing the molecule of gaseous formaldehyde adsorbs in the active surface of the photocatalyst. Photocatalyst Cy35 exhibits the highest adsorption of approximately 41% after 120 min in a closeddark room. Consequently, it gave the highest photodegradation of about 88.1% after 120 min of illumination. The plot also shows that the addition of silver nitrate has the highest photocatalytic effect on the degradation of gaseous formaldehyde among the single elemental doping. This might be influenced by the surface roughness and area of silver-doped TiO2 (Barakat et al., 2011). However, performance of the combined multi-element doped titanium dioxide (Cy35) showed the highest removal exhibiting approximately 12 percent edge compared with AgNO3-doped photocatalyst (Ay35). The photocatalytic activity performances of the individual dopant precursor increases in the order of N
3000

Fy35

2000

Ay35

1000 Cy35 0 20

30

40

50

60

70

80

2 theta Fig. 2. XRD spectra showing the peaks of anatase phase of the synthesized TiO2 photocatalysts.

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Table 1 Summary of crystallite size and specific surface area of the synthesized photocatalysts. Photocatalyst

Crystallite size (nm)

BET surface area (m2 g1)

Single point surface area (m2 g1)

Average pore size (nm)

Pore volume (m3 g1)

Degussa P-25 U0 U35 Cx35 Cy35 Cz35

e e 13.6 11.1 14.9 15.8

41.99 164.59 164.12 182.30 174.19 172.06

42.06 161.82 158.69 176.20 168.89 166.11

10.43 6.77 4.81 6.51 9.35 10.48

0.11 0.28 0.20 0.30 0.41 0.45

photocatalysts follow a trend of U35>Wy35>Fy35>Cy35>Ay35>Ny35 with energies of 458.66 eV > 458.56 eV > 458.47 eV > 458.43 eV > 458.33 eV > 458.3219 eV, respectively. This is significant to O 1s spectral of similar surface of synthesized catalysts wherein the energies shifted to lower binding energy. The shifting

of O 1s to the lower binding energy after recouping oxygen va€ thelid et al., 2011). On the other hand, the cancies with oxygen (Go intensity of the peaks is also related to the concentration of titanium within the sample. It is observed that the peak intensities vary according to the trend of Fy35
6000 U35

O1s

U35 Ay35

Cy35 Wy35

4000

Intensity (counts/sec)

Intensity (counts/sec)

5000

2p3/2

Ti2p

5000

Ay35

Ny35 Fy35

3000

2000

525

530

535

Wy35 Ny35 Fy35

3000

2p1/2

2000

f

1000

a e d c b a

1000

Cy35

4000

a

e d c b

0

540

450

455

Binding energy (eV)

460

465

470

475

Binding energy (eV)

(a)

(c)

6000 O1s

Intensity (counts/sec)

5000

Intensity (counts/sec)

5000

Cz35 Cy35 Cx35 U35

4000

3000

2000

1000 525

Cz35 Cy35 Cx35 U35

Ti2p

4000 3000 2000 1000 0

530

535

Binding energy (eV)

(b)

540

450

455

460

465

470

475

Binding energy (eV)

(d)

Fig. 3. O 1s XPS spectra of the synthesized photocatalysts doped with, a) different dopant and b) different dopant concentrations. Ti 2p XPS spectra of the synthesized photocatalyst doped with, c) different dopant d) different dopant concentrations.

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Cy25 Cy35 Cy45 Cy0

Ag3d5

280 240

Cy25

200 Cy0

160

Cy45

N1s

240

Intensity (counts/sec)

Intensity (counts/sec)

320

179

210

Cy25

180

Cy0

150

Cy45

Cy35

Cy35

120 360

365

370

375

120 390

380

395

400

(a)

(c) Cy25

F1s

W4f 520

Cy0

Cy25 Cy0

Cy35

Intensity (counts/sec)

Intensity (counts/sec)

410

Binding energy (eV)

Binding energy (eV)

1200

405

900

600

390

Cy35

260 Cy45

130 Cy45

300

0

680

685

690

695

Binding energy (eV)

(b)

25

30

35

40

45

Binding energy (eV)

(d)

Fig. 4. XPS spectra of synthesized photocatalyst calcined at different temperatures. (a) Ag 3d spectra, (b) F 1s spectra, (c) N 1s spectra, and (d) W 4f spectra.

Fig. 4 shows the O 1s and Ti 2p spectra of the synthesized photocatalysts doped with varying molar concentrations. The intensity of the XPS spectra is correlated with the concentration of oxygen and titanium of the sample. It is observed that for both spectra the concentration of oxygen and titanium found on the surface of the photocatalyst is least for Cy35. It is concluded that substitution of the titanium and oxygen atoms for the said photocatalyst are highest. The results of W 4f XPS plots in Fig. 4 show varying relative intensities for synthesized photocatalysts calcined at different temperature which verifies the incorporation of the dopants into titanium dioxide lattice. The results shows that there is absence or weak signal intensities of dopants in the photocatalysts at higher calcination temperature. The detection limit of the instrument may be too high to detect the small amount of dopants added into the titanium dioxide matrix. The XPS data verifies that the

incorporation of the tungsten into the lattice of titanium dioxide increases with decreasing calcination temperature. The trend of silver and nitrogen is similar to tungsten except for fluorine. The intense peak of silver and tungsten dopants may contribute to higher adsorption of target chemicals which is also significant to the photocatalytic performance of the photocatalyst shown in Fig. 1. 3.4. Fourier transform infrared spectroscopy analysis Fig. 5 shows the FTIR spectra of the synthesized photocatalysts with the individual dopant precursor. The spectrum has bands originating from hydroxyl group H-bonded OH stretch (around 3370-3420 cm1) originating from water adsorbed (Vasconcelos et al., 2011), OH-bending (1620-1630 cm1), TiO-stretching (450620 cm1) (Nishikiori et al., 2013). It is also observed that aside from the difference in relative intensities (Wy35
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the bands also shifted (Wy35>U35>Fy35>Ay35) towards decreasing wave number values for the bands around 3370-3420 cm1. On the other hand, the OH-bending associated with adsorbed water (16201630 cm1) shifted (Wy35
3.5. Brunauer, Emmett and Teller analysis The specific surface area of the synthesized and commercial (Degussa P-25) photocatalysts were analyzed by BET. Table 1 illustrates the physical characteristics of selected photocatalysts. The results shows that the specific surface area of the synthesized photocatalyst is dependent on the concentration of dopant. This observation indicates that the dopant concentration is inversely proportional to the specific surface area of the synthesized photocatalyst. The uncalcined photocatalyst (U0) yielded a specific surface area value of 164.5943 m2 g1 and no significant difference in the specific surface area value of 164.1226 m2 g1 after calcination (U35). This indicates that calcination at 300  C is significant and has minimal effect on the specific surface area of the synthesized photocatalysts. Comparing the results of three photocatalysts with different dopant concentration, the specific surface area of the multi-element doped titania nanoparticles decreases almost linear with the increasing amount of the dopant after calcination at 300  C in 5 h. In contrast to commercial Degussa P-25 and synthesized TiO2 by other researcher, the synthesized multi-element doped titania nanoparticle's specific surface area is approximately 2e4 times larger than the previous research produced (Wen et al., 2015; Bakar and Ribeiro, 2016; Zhu et al., 2016). However, the specific surface area of the synthesized TiO2 catalyst (Cx35) modified with aqueous multi-element solution gave the highest value of 182.30 m2 g-1. The high value of the specific surface area for the synthesized multielement doped titania nanoparticles was probably due to the

remaining alkoxy groups that was not utilized during the process of synthesis. This can be concluded that multi-elemental doping affects the specific surface area of synthesized TiO2 nanoparticles. The total porosity can be classified into three different categories according to its average pore diameter or size. This is recommended by IUPAC which includes macropores (d > 50 nm), mesopores (2e50 nm), and micropores (d < 2 nm). From which based in Table 1, all synthesized TiO2 photocatalysts including commercial Degussa P-25 are mesoporous materials. As stated earlier, the specific surface area of multi-element doped titania decreases as the dopant concentration increases. This will be attributed to the dopant molecules that blocks the pores of TiO2 and the sintering of the crystals that leads to the decrease of its specific surface area. However, in contrast to the average pore size of the photocatalysts, it increases with the increasing dopant concentration as shown in Table 1. This may be due to the presence of W and other materials in TiO2 catalyst that did not undergo full doping. This is obvious in the result of the XPS data of the synthesized photocatalyst as shown in Fig. 4. The BET results is also significant to the crystallite size of the photocatalyst. Wherein as the dopant concentration increases the specific surface area of the synthesized photocatalyst decreases which is significant to its crystallite size. This confirms that the defects on the crystal lattice of TiO2 affects its specific surface area and crystallite size. Nitrogen adsorption isotherm of the Degussa P-25 and multielement doped titania (Cy35) is presented in Fig. 6. The size and shape of the hysteresis loop in the figure can be used to identify the types of pores encountered during the sorption of N2. As shown in Fig. 6, the narrow hysteresis loop of both catalysts is indicative of narrow distribution of cylindrical or tubular pores (Anovitz and Cole, 2015). Based on the Brunauer-Deming-Deming-Teller (BDDT) classification, both samples classified as Type IV with H3 hysteresis loop. These indicates the presence of mesoporous materials in both catalysts which is significant to the porosity of the materials (between 2 and 50 nm) classification as recommended by the IUPAC. However, at higher relative pressures in the plateau region of the hysteresis loop reveals that there's a presence of a complex and interconnected pore network with narrow pore

Fig. 5. FTIR spectra of synthesized photocatalysts doped with, a) individual dopant precursors b) different dopant concentrations.

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Fig. 6. Nitrogen gas adsorption isotherm plots of Degussa P-25 and multi-element doped titania (Cy35) photocatalyst.

opening and assumed to have a pore-blocking phenomenon. Whereas, the narrow hysteresis loop extended to the plateau region of the lower relative pressure represents a typical slit-shaped pores formed in the aggregates of a materials (Anovitz and Cole, 2015; Akple et al., 2015). 4. Conclusions The gaseous formaldehyde degradation plots using the photocatalysts with dopant shows that the adsorption and photocatalytic activity increases with the increasing amount of dopant. As for the photocatalysts part, the photocatalytic activity performance for Cy35 and Cz35 is too close that is about 88% and 86%, respectively. However, the degradation of gaseous formaldehyde observed using Cy35 is the utmost at about 88.1%. To elucidate the results of the photocatalytic degradation experiments for the synthesized photocatalyst, XRD results were analyzed. Comparison of crystallite sizes of the synthesized photocatalyst with different molar concentrations of silver, fluorine, nitrogen and tungsten shows that the mean crystallite size increases as the amount of dopant is increased. However the increased crystallite sizes values of the doped photocatalysts may indicate that there will be an increased amount of defects in the crystal lattice of TiO2. Adsorption of gaseous formaldehyde follows the same trend as the crystallite size of the synthesized photocatalyst presumably because as the crystallite size increases, the number of available functional groups for the formaldehyde to attach increases as well. The hydroxyl group at 1630-1660 cm1 shows greatest wavenumber for the Cy35 synthesized photocatalyst. These wavenumbers suggest the strength of the bonds that hold the functional group on the surface. Since for all the variable synthesis parameters, Cy35 has the largest wavenumber and consequently the strongest hydroxyl bonded on the surface of the photocatalyst. It is presumed that this is one of the reason why Cy35 has the highest photocatalytic degradation of gaseous formaldehyde giving a removal of 88%. Acknowledgments The authors would like to thank the National Science Council, Taiwan (NSC 101-2923-E-041-001-MY2) and the Department of Science and Technology, Philippines for funding this research.

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