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of acetaldehyde, the photodegradation rate for KOH/WO3 is a little higher than that of CuBi2O4/WO3, while CO2 photogeneration is about 70% of the full amount ...

Catalysis Science & Technology PAPER

Cite this: Catal. Sci. Technol., 2014, 4, 657

Surface modification by loading alkaline hydroxides to enhance the photoactivity of WO3 M. Khajeh Aminian* and M. Hakimi Surface modification on WO3 was carried out by loading KOH, NaOH and CsOH to study the influence of surface interactions on photocatalytic activity of this material. Photodegradation of 2-propanol, methanol, ethanol, acetone and propane in gas phase under visible light irradiation shows loading of

Received 7th October 2013, Accepted 4th November 2013 DOI: 10.1039/c3cy00781b www.rsc.org/catalysis

KOH, NaOH and CsOH on WO3 enhanced the photodegradation rate about 4 and 3 times respectively. Loading of KOH on TiO2 were carried out for comparison and the results showed enhancement of photoactivity by KOH loading for WO3 which was much higher than that of TiO2. Increasing the rate of the photocatalytic reaction is attributed to compensation of the high tendency of WO3 for H+ donation and making multi-electron O2 reduction by loading of KOH on the surface.

1. Introduction Photocatalyst materials have been extensively investigated for decomposition of harmful organic substances under visible light irradiation.1–5 The photocatalytic reaction originates from a strong potential of photogenerated electrons and holes for a redox reaction. Several attempts have already been performed to study the influence of crystallinity, electronic band structure, surface area, particle size and morphology on the activity of a photocatalyst material.6–9 Surface interactions and band edges exert great influence on the photocatalytic reaction rate. Investigation of the adsorption influence on the photocatalytic reaction rate of TiO2 suspended in an acidified aqueous solution of silver salts shows the amount of silver ions (Ag+) adsorbed on the TiO2 surface, predominantly determining the reaction rate.10,11 The initial rate of the acetone formation, as well as that of the Ag deposition, followed a formal Langmuir equation as a function of 2-propanol concentration; the rate was proportional to the amount of surface adsorbed 2-propanol as a hole scavenger.12 Several researches reported the rate of the photocatalytic reaction in aqueous media strongly depends upon the pH of the suspension.11,13–15 It was found that the rate of the photocatalytic oxidation of Naphthol Blue Black dye on WO3 in alkaline aqueous solution is much slower than that in acidic solution.13 There are some reports show increasing the pH accelerates the photoreduction process10,11 and acetone formation.12 Also, it was observed that the oxygen evolution rate was enhanced by the pH up to a pH of 12.15 This

Physics Department, Yazd University, Yazd, Iran. E-mail: [email protected]

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indicated that there exists an optimum pH condition for oxygen evolution. The optimum pH seems to be determined by the competition between the influence of the concentration of OH− on the photo-oxidation and photoreduction processes.15,16 It is mainly because the pH of a solution controls the valence and conduction band potentials as well as the surface chemistry and surface interactions of a photocatalyst material. There are three basic phenomena influenced by the pH of the solution: (i) semiconductor flat-band potential, (ii) adsorption of the electroactive species (OH− ions), and (iii) photo-electrochemical oxidation of water and OH− ions competing with other reactants able to form powerful oxidants on irradiation.13 In fact, the pH of a solution can shift the valence band and the conduction band potentials and it influences the photo-electrochemical process rate. On the other hand, the pH of the solution controls the photo-electrochemical process by controlling the adsorption and desorption of the ions on the surface. It means interactions with cationic electron donors and electron acceptors will be favored for the photocatalytic activity at high pH under conditions in which the pH > pHzpc, while anionic electron donors and acceptors will be favored at low pH under conditions in which pH < pHzpc.17 It was proposed that the surface OH groups react with photogenerated holes to form the surface-bound OH radical (radical OHads),17 which then oxidizes the surface adsorbates. Several researchers also reported that the OHads radical is an important specie for the oxidation reaction because the diffusion of OHads radical from the TiO2 surface into the bulk solution is minimal.18,19 Nakato et al. investigated the photo-oxidation of water adsorbed on the TiO2 surface by in situ FT-IR absorption and photoluminescence

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measurements and concluded that the oxygen photo evolution is initiated by a nucleophilic attack of a H2O molecule on a photogenerated hole at a surface lattice O site.20,21 Although the photo reactions which take place in gaseous media are similar to those in aqueous media in some characteristics, however they may be different in some other characteristics. In this paper the surface interactions and the surface band potentials during a photoreaction on WO3 in a gaseous media were studied. It was investigated that how surface modification of WO3 by alkaline hydroxides such as KOH, NaOH and CsOH can enhance the photoactivity of this material.

2. Experimental methods Different photocatalyst systems of alkaline hydroxide on WO3 and TiO2 were prepared by impregnation method. 100 mg of KOH (Wako corp.) were dissolved in 100 g distilled water separately. Five samples of 1.0 g commercial WO3-monoclinic (Wako corp., BET surface area 5.2 m2 g−1) were mixed with 0.50, 0.75, 1.0 and 1.5 ml KOH solution separately to load WO3 samples with KOH for 0.05, 0.075, 0.10 and 0.15 wt% of the material respectively. These samples were named 0.05 wt, 0.075 wt, 0.1 wt and 0.15 wt respectively. Loading of KOH on WO3 with 5 wt% of the material was carried out by mixing of 1.0 g WO3 with 1.0 g aqueous solution containing 50 mg KOH. It was named 5-wt. The mixtures were dried on hot plate and heated at 450 °C for 2 h. Loading of NaOH and CsOH on WO3 and loading of KOH on TiO2 (P25-Degussa Corp., BET surface area 50 m2 g−1) were performed using the same routine. The photocatalytic evaluation was carried out in the gas phase in a cylindrical air-filled static Pyrex glass vessel (500 ml of total volume) for degradation of methanol, ethanol, 2-propanol, acetone and propane to CO2. The photocatalyst powders (~0.6 g) were evenly dispersed on the bottom of a circular glass dish to have a uniform area of ~8 cm2 and the dish was mounted in the vessel. The vessel was filled by artificial pure air (O2–N2 = 1 : 9) to replace CO2-containing natural air. Some air enriched with 2-propanol or other mentioned organic material vapor was injected into the vessel which the final concentration of organic material in the vessel is about 130 ppm. After injection of organic material vapor and prior to light irradiation, the vessel was kept in the dark for one hour to get adsorption saturation of organic material vapor on the sample surface. All photocatalytic reactions were carried out at room temperature. Upon light irradiation, the gaseous sample (0.5 ml) was periodically extracted from the reaction vessel and measured on a gas chromatograph (GC-14B, Shimadzu) equipped with a flame ionization detector (FID) and a methanizer-FID for analysis of organic materials and CO2. The light source was a 300 W xenon arc lamp (ILC Technology, CERMAX LX-300). A cutoff filter (L42, Hoya Optics) was used to obtain visible light (λ > 420 nm). UV–visible diffuse reflectance spectrum was measured on a UV–visible spectrometer (UV-2500, Shimadzu) at room temperature.

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3. Results and discussion The results of the photocatalytic evaluation of WO3 loaded with different amounts of KOH are depicted in Fig. 1. Fig. 1a shows the concentration of CO2 generated during photodegradation of 2-propanol on pure WO3, 0.05-wt, 0.075-wt, 0.1-wt and 5-wt which all samples were heated at 450 °C for 2 h. One observes loading of KOH on WO3 from zero up to 0.1 wt% of the material can enhance the photoactivity of it rather than pure WO3. Since the reaction between 2-propanol molecules and KOH is important and unknown in this experiment, it is useful to test a sample which was loaded with a high amount of KOH for this experiment. In this case, the sample 5-wt, which was loaded with 5 wt% of KOH, was tested for photodegradation of 2-propanol. Obviously the surface of 5-wt is mostly covered by potassium hydroxide or potassium oxide. The results of photodegradation of 2-propanol on the surface of 5-wt was shown in Fig. 1a. It shows photoactivity of 5-wt is less than that of pure WO3. Fig. 1a concludes that the sample 0.075-wt owns the highest activity which is more than 4 times higher than that of a pure one. Fig. 1b shows the result of the repeatability test of the photoactivity of 0.05-wt (450 °C) and pure WO3 (450 °C) sample. One observes the photoactivity of 0.05-wt (450 °C) and pure WO3 (450 °C) samples has not changed so much after using it for 3 times. Fig. 1c shows the results of photoreaction on pure WO3, 0.05-wt (450 °C) and unheated 0.05-wt together with CO2 generation from degradation of 2-propanol in the dark at 60 °C on 0.05-wt (450 °C). It was shown that the photoactivity of 0.05-wt (450 °C) and unheated 0.05-wt are close to each other. Since this idea may come to mind that CO2 generation is because of the heating effect of irradiation and not not photoexcitation, CO2 generation in the dark at 60 °C was investigated in the presence of 2-propanol vapour. The result in Fig. 1c shows the catalytic activity of KOH loaded samples is negligible and it confirms CO2 generation in the previous experiments is mainly because of photoexcitation. Photoactivity of 0.05-wt and pure WO3 was investigated in presence of other organic materials such as methanol, ethanol, acetone and propane. Fig. 1d shows CO2 generation rate from the photodegradation of methanol, ethanol, 2-propanol, acetone and propane in the same conditions. It can be observed in this figure that although the CO2 generation rates for different organic materials are not the same; it is well demonstrated that in all cases the photoactivity of 0.05-wt is much higher than that of pure WO3. To find the reason why loading of WO3 with KOH enhanced the photoactivity of WO3 and check if it works for other alkaline hydroxides, similar experiments were carried out for WO3 samples loaded with NaOH and CsOH. Fig. 2a and b shows the concentration of CO2 generated during the photodegradation of 2-propanol on WO3 samples loaded with NaOH and CsOH respectively. One can observe the photoactivity of the samples loaded with NaOH and CsOH enhanced up to 3 times than that of pure WO3. By comparing of Fig. 1a, 2a and b it is found loading of WO3

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Fig. 1 (a) Photodegradation results of 2-propanol on WO3 loaded with different amounts of KOH under visible light irradiation; (b) test of the repeatability of photodegradation experiments, (c) photodegradation rate comparison between 0.05-wt before and after heating at 450 °C together with CO2 generation in dark at 60 °C, (d) a comparison between CO2 generation rate in the photodegradation test of different organic materials such as methanol, ethanol, 2-propanol, acetone and propane on the surface of both pure WO3 and 0.05-wt.

with KOH, NaOH and CsOH has a similar effect on the photoactivity of WO3. Fig. 2c shows the highest activity samples loaded with KOH, NaOH and CsOH. It shows the samples loaded with KOH are more reactive than those loaded with NaOH or CsOH. The photoactivity of KOH/WO3 for photodegradation of 2-propanol is about 50% related to that of Pt/WO3.22 The apparent quantum efficiency of KOH/WO3 at 440 nm for 2-propanol and propane was measured as about 3 and 18% respectively. Also, the photoactivity of KOH/WO3 was compared to CuBi2O4/WO323 for degradation of acetaldehyde, the photodegradation rate for KOH/WO3 is a little higher than that of CuBi2O4/WO3, while CO2 photogeneration is about 70% of the full amount obtained using CuBi2O4/WO3. It seems that the enhancement of photoactivity is not only because of the chemical property, but because of photo-oxidation and photo-reduction processes on the surface. The SEM images of the samples are shown in Fig. 3. The surface of pure WO3 450 °C (Fig. 3a) and 0.05-wt 450 °C

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(Fig. 3b) seems to be almost the same, while some nanorods have been formed on 5.0-wt 450 °C (Fig. 3c). It can be concluded that the surface change after loading of 0.05 wt% KOH is little; however a different phase of material has been formed on the surface after loading of 5 wt% KOH. The optical properties of 0.05-wt (450 °C) were investigated by UV–visible diffuse reflectance spectroscopy and the result is shown in Fig. 4. The spectrum of pure WO3 is also shown for comparison. One observes the absorption edges of both samples are at the same wavelength. The shape of the absorption spectrum of 0.05-wt (450 °C) is very similar to that of pure WO3; except that reflectance of WO3 above absorption edge was decreased by KOH loading which implies that some defects were formed in WO3 by KOH loading. It is concluded that loading of WO3 with KOH does not create a new electronic energy level for photo-excitation of the electrons. X-ray diffraction (XRD) spectra of pure WO3 and 0.05-wt sample show WO3 was crystallized in monoclinic structure. It can be observed there is no noticeable change in the structure after

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Fig. 2 Results of the photodegradation of 2-propanol on WO3 loaded with different amounts of (a) NaOH and (b) CsOH under visible light irradiation. It can be observed the sample loaded with NaOH or CsOH enhanced the photodegradation rate up to 3 times compared to pure WO3. (c) Comparison of the highest activity of the samples loaded with KOH, NaOH and CsOH. Samples loaded with KOH are more reactive than those loaded with NaOH or CsOH.

Fig. 3 SEM images of (a) pure WO3 450 °C, (b) 0.05-wt 450 °C and (c) 5.0-wt 450 °C. The SEM images show there is no difference between pure WO3 and 0.05-wt, while some nanorods have been formed on 5.0 wt.

loading of KOH on the surface, except the intensity of the peaks has slightly decreased after KOH loading, which proves a formation of some defects on the surface. To study the changes of the surface groups after loading of KOH, XPS experiment has been carried out on the samples. Fig. 5a and b show XPS spectra of oxygen and tungsten for the samples loaded by different amounts of KOH in high resolution. For both oxygen and tungsten, the intensity of the peaks is different and the position has shifted a little. The peaks of oxygen have been dissociated to two components 530.5 eV (belongs to WOx) and 531.3 eV (belongs to WOH). It is found that the ratio for the intensity of two components for all samples is approximately the same. The peaks of tungsten have been dissociated and it is found that they are composed of W+6 (35.8 and 37.9 eV) and W+5 (34.7 and 36.8 eV) with the same ratio for all samples. XPS spectra

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shows different peaks for oxygen and tungsten, but the ratios of the peak intensity are the same for all samples and it can be concluded the surface group has not changed significantly. Surface area measurement of pure WO3 and 0.05-wt using BET method yields 5.2 and 5.6 m2 g−1 respectively. It can be found from surface area measurement that immersion of WO3 in the solution of KOH makes some corrosion on the surface of WO3 which increases the surface area of material slightly. Also the gas adsorption experiment on the surface of KOH loaded WO3 was carried out in dark conditions using 2-propanol vapor in the glass vessel of the photoreaction. The results of the gas adsorption have been shown in Fig. 6. 2-Propanol molecules are adsorbed on the surface of the photocatalyst material and the concentration of 2-propanol has been reduced in time. The gas adsorption experiment results in the adsorption rate on the surface are almost the

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Fig. 4 (a) UV–visible diffuse reflectance spectra of pure WO3 and 0.05-wt sample. There is no noticeable change after loading except some defects which reduced the reflectance above absorption edge after loading. (b) Absorbance spectra of pure WO3 and 0.05-wt sample show the absorption edge has not changed after loading. (c) XRD spectra of pure WO3 and 0.05-wt sample show WO3 was crystallized in monoclinic structure and the crystal structure has not changed after loading of KOH on the surface.

Fig. 5 XPS spectra of oxygen (a) and tungsten (b) on the surface of WO3 loaded by different amounts of KOH. The intensity of the peaks is different and the position has shifted a little. (a) For all samples, the peak of oxygen is composed of 530.5 eV (belongs to WOx) and 531.3 eV (belongs to WOH) with the same ratio. (b) The peaks of tungsten are composed of W+6 (35.8 and 37.9 eV) and W+5 (34.7 and 36.8 eV). Although the intensity of the spectra is different, but the ratios of the peaks for all samples are the same and it seems the surface group has not changed significantly.

same for pure WO3 and WO3 loaded with KOH and this confirms the results of surface area measurement. To do more investigation on this phenomenon, the influence of loading of KOH on photoactivity of TiO2 (P25) was investigated. Since the surface area of P25 is different from

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that of WO3, KOH loading was carried out in different concentrations. Fig. 7 shows the results of CO2 generation during photodegradation of 2-propanol on P25 loaded with 0.1, 0.3 and 0.75 wt% of KOH without a cutoff filter. It can be observed the activity of the sample loaded with 0.5 wt% of

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Fig. 6 2-Propanol molecules in the glass vessel are adsorbed on the surface of photocatalyst material and the concentration of 2-propanol has been reduced in time. The adsorption rate on the surface is almost the same for pure WO3 and WO3 loaded with KOH.

material is slightly higher than that of pure P25, while the other samples own the same activity as pure P25 approximately. It seems the influence of KOH loading on photoactivity of P25 is similar to that of WO3; except the enhancement of photoactivity for loading on WO3 is much higher than that of P25. Since the amount of KOH loaded on WO3 is a few percent, less than one monolayer, it can not change the morphology of the material, and it is expected the loaded material forms ions or clusters of K on the surface. The reaction between WO3 and KOH on the surface is the same as the one already reported on the surface of alumina:24 –WOH + KOH → –WOK + H2O

(1)

Sayama et al. suggested that the defects on the surface of the photocatalyst play important roles in the photocatalytic

Fig. 7 Photodegradation of 2-propanol TiO2 (P25) under UV light irradiation, loaded with different amounts of KOH. KOH loading on P25 enhanced the photodegradation rate slightly.

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reactions.23 In fact, some states on the surface which are formed by dangling bonds, interatomic spaces, surface hydroxyl groups, etc., can be traps of both electrons and holes and perform charge separation.25 To date, a great deal of research has been conducted on the adsorption and photocatalytic reactions of surface active ligands on TiO2,26–29 but their relationship is not yet well understood. Chelation of surface Ti atoms with electron-donating bidentate ligands, such as catechols, facilitates the charge transfer across the photocatalyst materials to the reactant molecules.30,31 The photogenerated holes in TiO2 are promptly scavenged by chemisorbed molecules on the surface. It seems that this charge transfer reaction is very rapid and competes with the charge recombination and trapping of the photogenerated holes by other surface defects.32 It has been reported that aliphatic alcohols, such as methanol, ethanol, and 1-propanol, adsorbed on TiO2 surface can be present in two forms: a hydrogen-bonded physisorbed species and a chemisorbed alkoxide species.33 Kinetic experiments indicate chemisorbed alkoxide species react much more rapidly.34 Also it was reported that photodegradation of 2-propanol on TiO2 could proceed through two parallel routes; the first route occurred through the formation of acetone from the H-bonded 2-propanol species. The second route occurred through relatively rapid oxidation of the 2-propoxide species to form CO2 directly.35 It was demonstrated that surface modification of Cs is a method to improve activity for photocatalytic O2 evolution and Fe+3 reduction over WO3 in the solution.36 It was considered that the reason for the increase in photo-oxidation activity was the partial ion-exchange of Fe+2 with Cs+ formed on the surface of H–Cs–WO3 during the photocatalytic reaction. The selective adsorption of Fe+3 on WO3 and H–Cs–WO3 and two-step photo excitation via Fe+3 were considered as the main causes of photoreduction improvement. However, the activity of H–Cs–WO3 for photocatalytic O2 evolution and silver reduction in the presence of Ag+ was generally lower than that of WO3.36 Also, it was reported that surface treatment using Cs+ and Fe+2 ions on WO3 can proceed VO2+ reduction and water oxidation.37 Since photoactivity was enhanced for Fe+3 and VO2+ but it was decreased for Ag+, it can be understood that the photo-oxidation–reduction process on WO3 and H–Cs–WO3 depends on a lot of factors and the reason for improvement is not trivial. One may think that photodegradation of organic materials on KOH loaded WO3 in the gas phase proceeds through a similar mechanism. Corresponding to two last reported papers,36,37 ion exchange and two-step photo excitation via ions are the main causes for promotion of photoactivity. Actually ion exchange and two-step photo excitation are impossible in gas phase, this experiment, and they are not promising to explain promotion of photoactivity. In addition to the mentioned reasons, there are some more problems. The first one is that KOH loading promotes photodegradation rate of various kinds of organic materials including alcohols, propane and acetone via different mechanisms. The other

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one is why loading of KOH on WO3 enhanced the photoactivity several times while the enhancement of the photoactivity on TiO2 is slight. Considering all these experiments and reports, it can be assumed that promotion of photoactivity of WO3 in gas phase after KOH loading is due to catalytic property of OH groups on the surface and to multi-electron reduction. Actually pHzpc of WO3 is 1 and it shows WO3 has a high tendency to release H+ or donate H+ to other materials. Obviously loading of KOH can recompense the acidic property and shift the surface property from acidic to basic property. This phenomenon can facilitate the photocatalytic reaction of organic molecules on the surface. The high valence band energy, EVB, in WO3 makes the surface holes act as powerful oxidizing sites for generating radical oxidants in reactions with organic molecules. In our experiments the level of EVB is less important since the potential is high enough to oxidize the molecules, while the conduction band energy level, ECB, of WO3 (+0.35 V) is not negative enough for one-electron oxygen reduction (O2 + H+ + e− → HO2, −0.046 V). The other reason may consider for enhancement of photodegradation is multi-electron reduction. Abe has reported that two-electron oxygen reduction (O2 + 2H+ + 2e− → H2O2, +0.68 V and O2 + 4H+ + 4 e− → 2H2O, +1.23 V vs. NHE pH 0) proceeded over Pt/WO3.22 It seems reasonable to consider that such multi-electron O2 reductions more readily proceed on the surface of alkaline/WO3, compared to the bare surface of WO3. Unfortunately, we do not have any apparatus to check which mechanism is more probable for this experiment. The next question is why loading of KOH on TiO2 cannot promote the photocatalytic reaction as high as that on WO3. It is well known that the CB bottom of TiO2 lies at −0.16 V, slightly more negative than one-electron oxygen reduction potential, and single-electron reduction is generally considered the main pathway for electron consumption over TiO2.38,39 It is reasonable to suppose that enhancement of the photoactivity for KOH loaded TiO2 is considerably less than that for WO3 since it possessing sufficient CB level for the reduction of O2 by single electron. The other reason is that pHzpc of TiO2, 5.5, showing a low tendency for TiO2, in contrast to WO3, to release or donate H+, and KOH loading on TiO2 is not as highly effective as on WO3 for photocatalytic reactions.

4. Summary WO3 and TiO2 were loaded by different amounts of alkaline hydroxide and the influences of loading KOH on the photocatalytic activity of WO3 and TiO2 were investigated. Photocatalytic evaluation was carried out using photodegradation of 2-propanol, methanol, ethanol, acetone and propane in gas phase under visible light irradiation. It was shown KOH loading for 0.075 wt% of material enhanced the photodegradation rate of 2-propanol on WO3 more than 4 times, while NaOH and CsOH loading enhanced it up to 3 times; as well loading of KOH on TiO2 enhanced the photoactivity of this material

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slightly. The enhancement of photoactivity of WO3 was attributed to two phenomena. The first one is catalytic property of OH groups on the surface and compensation of the high tendency of WO3 for H+ donation. The second one is multielectron O2 reduction to which the high activity of KOH loaded WO3 is attributed, with regard to single electron O2 reduction which is considered for TiO2.

Acknowledgements We would like to express our sincere gratitude and thanks to the National Institute of Materials Science (NIMS), and especially to Prof. Jinhua Ye who provided the facilities for the experiments and study of the photocatalytic materials.

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