Preparation of nanostructured ruthenium doped titania for the photocatalytic degradation of 2-chlorophenol under visible light

2.1 Preparation of the catalysts

For mesoporous TiO₂ materials (Huang et al., 2005), 3.6 g of tetrabutyl titanate was dissolved in absolute ethanol with a weight ratio of 1/7 under stirring. The resulting suspension was stirred for 3 h at room temperature, and then 5 ml of deionized water was added dropwise. The solution was stirred for an additional 2 h, followed by evaporation at 150 °C. The obtained solid product was exhaustively washed with deionized water and ethanol, dried at 80 °C overnight, and then calcined at 500 °C for 3 h.

Ru/TiO₂ samples with Ru loading ranging from 0.2% to 0.8% (w/w) using TiO₂ were prepared by the incipient wet impregnation method (Blackmond and Ko, 1984) with an aqueous solution of ruthenium trichloride trihydrate. An appropriate amount of the precursor, so as to obtain the desired Ru concentration in TiO₂, was mixed with TiO₂ in distilled water. The resulting slurry was subjected to continuous stirring at ambient temperature, and subsequently dried in an oven at 110 °C for 10 h. The solid residue was then crushed and calcined in air for 5 h.

2.2 Photolysis and photocatalysis experiments

500 cc of an aqueous solution containing 100 ppm of high purity 2-CP was subjected to UV irradiation using a 6 W lamp at a wavelength of 254 nm. All photodegradation experiments were conducted in a batch reactor at pH 6. The UV lamp was placed in a cooling silica jacket and placed in a jar containing polluted water. The catalyst was suspended in the solution with a magnetic stirrer at a controlled reaction temperature of 25 °C during the experimental period. At different irradiation time intervals, samples of the irradiated water were withdrawn for analysis using an HPLC chromatograph PerkinElmer (series 200) with photo-diode-array UV detector and a C18 column. The mobile phase was acetonitrile/water (60:40) injected in a rate of 1.0 ml min⁻¹.

The concentration of Cl⁻ and acetate ions produced in the solution during the photodegradation was determined by Ione Chromatography (IC Dionex-pac) attached with AS14 column of eluent consisted of 2:7 sodium carbonate/sodium bicarbonate mixture and the flow rate was 1.2 ml/min. In all cases, air was bubbled through the reaction mixture to ensure a constant dissolved O₂ concentration (3 × 10⁻⁴ mol dm⁻³).

2.3 Physical characterization of the catalysts

N₂ physisorption was utilized to study the effect of the preparation and pretreatment process on the bulk Ru/TiO₂ samples. Characteristics such as BET surface area, pore size, and pore volume were examined. N₂ physisorption studies were carried out in a NOVA2000 gas sorption analyzer (Quantachrome Corporation) system. For each measurement, the sample was degassed at 250 °C for 3–4 h, and then analyzed at 77 K (liquid N₂ temperature). The surface areas and pore volumes were determined using the BET (Brunauer–Emmett–Teller) method from the adsorption branch in the relative pressure range of 0.05–0.35. The total pore volume of the samples was calculated from the nitrogen uptake at P/P_o = 0.95, using the BJH (Barrett–Joyner–Halenda) method from the isothermal desorption data.

XRD analysis was performed to determine the effects of Ru doping on the rutile and anatase concentrations. X-ray powder diffraction (XRD) patterns were obtained with a PANalytical X’Pert PRO diffractometer in reflection mode using CuKα radiation over the scan range of 2θ between 20 and 80 at 295 K, in order to identify the phase present.

FTIR were performed using ATI Mattson model Genesis Series (USA) Infrared spectrophotometer adopting KBr technique. For all samples, the KBr technique was carried out approximately in a quantitative manner since the weight of sample and that of KBr, were always kept constant.

The topography and particle size of Ru/TiO₂ were measured using JEOL transmission electron microscopy (TEM) operating at an accelerating voltage of 120 kV. The structure resolution of microscope is 0.2 nm. Prior to the analysis, the catalyst sample was ground into powder (using mortar and pestle) and then ultrasonically dispersed in a solvent and placed on a carbon-coated copper grid. The sample was allowed to dry before TEM analysis.

To confirm the presence of RuO₂ attained on TiO₂, EDX analysis was carried out. Energy Dispersive X-ray Spectroscopy (EDX or EDS) is a chemical microanalysis technique used in conjunction with SEM. EDX analysis was used to characterize the elemental composition of TiO₂.

3.1 Characterization of Ru/TiO₂

The crystallinity of the prepared samples was examined by XRD analysis. XRD patterns of TiO₂-blank, and Ru doped (0.2, 0.4, 0.6 and 0.8 wt%) TiO₂ are shown in Fig. 1. TiO₂ is a mixture of anatase and rutile, The peaks at 2θ values of 25.3, 30.8, 37.8, 48.1, 53.9, 54.3, 56.6, 70.2 and 75.1, 82.7 were identified by comparison with literature data and confirm that the particles are polycrystalline with an anatase structure. Whereas; the peaks at 2θ values of 27.4, 36.1, 39.2, 41.3, 44.1, 62.8, 64.2, 69 were corresponding to rutile. The unsupported ruthenium oxide, which was produced from the decomposition of RuCl₃·xH₂O, shows a pattern that matches the RuO₂ pattern in the JCPDS card indicating that after calcinations the Ru precursor decomposed to produce RuO₂. Therefore, RuO₂ should be observed in the unreduced Ru/TiO₂. As can be seen in Fig. 1, TiO₂ is a mixture of anatase and rutile, and most of the RuO₂ peaks are overlapped with rutile TiO₂. This makes it difficult to identify RuO₂ from the TiO₂ support with XRD. This RuO₂ behavior is also observed by Venkatachalam et al. (2007).

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Figure 1

X- ray Diffraction spectra for (a) TiO₂, (b) 0.1 Ru/TiO₂, (c) 0.2 Ru/TiO₂, (d) 0.4 Ru/TiO₂, (e) 0.6 Ru/TiO₂, (f) 0.8 Ru/TiO₂.

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In Fig. 2, the percentages of rutile were calculated from X-ray diffraction intensities. The mass fraction of rutile, X_r, was determined by.

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Figure 2

Rutile fraction of TiO₂ in Ru/TiO₂ catalysts as function of Ru ratio.

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Figure 3

N₂ adsorption/desorption isotherms and pore size distribution plots of TiO₂(a), 0.2Ru/TiO₂(b), 0.4 Ru/TiO₂(c), 0.6 Ru/TiO₂(d), 0.8 Ru/TiO₂(e) samples.

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Fig. 4 shows a summary of Raman spectra of Ru/TiO₂ catalysts. Structure of TiO₂ is confirmed in the FTIR spectrum. The peaks of TiO₂ anatase at ca. 527 cm⁻¹ (Ti–O vibration), 633 cm⁻¹, and 450 cm⁻¹ corresponding to the TiO₂ rutile crystalline phase were observed in all samples. The two bands appearing at 1564 and 1615 cm⁻¹, correspond to the stretching vibration of hydroxyl groups (O–H) and hydrogen-bonded surface water molecules (Brownson et al., 2005) in coordinated water, Ti–OH groups, or other hydrated species. Bands at 3383, 3427 cm⁻¹, are characteristic of, associated hydroxyl groups. Additional stronger bands at 3623, 3676 cm⁻¹ are characteristic of tetrahedral coordinated vacancies, and designated Ti⁴⁺-OH. Two strong absorbances at 3744 and 3825 cm⁻¹ are due to octahedral vacancies and designated Ti³⁺-OH (Madhu Kumar et al., 2000). Ru doped TiO₂ with different ratios of anatase to rutile, show that; the amount of surface adsorbed water and hydroxyl groups has little relation with crystallite phases (Liu et al., 2009).

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Figure 4

FTIR of (a) TiO₂, (b) 0.2Ru/TiO₂, (c) 0.4Ru/TiO₂, (d) 0.6Ru/TiO₂, (e) 0.8Ru/TiO₂ catalysts.

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The TEM images of undoped and Ru-doped TiO₂ are shown in Fig. 5. Blank TiO₂ and Ru-doped TiO₂ showed regular round shaped particles. There was no significant difference between the two particles. This was also previously confirmed from hysteresis loop of these samples. Small dark spots seen are assumed as Ru particles and seem to be non-uniformly distributed among the TiO₂ particles with a particle size of approximately 14 nm. Furthermore; Shen et al. (2008) reported that Ru particles varied in width from 1 to 14 nm with the majority of the particles having width between 1 and 6 nm giving an average Ru particle size of 4.1 nm. This leads to spreading or covering by the layer of TiO₂.

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Figure 5

TEM micrograph of (a) TiO₂, (b) 0.2 Ru/TiO₂, (c) 0.4 Ru/TiO₂, (d) 0.6Ru/TiO₂, (e) 0.8 Ru/TiO₂.

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A typical EDX pattern of the Ru/TiO₂ is shown in Fig. 6. The elemental composition of the samples was found to be 42.9% O, 56.9% Ti, 0.26% Ru for 0.2 Ru/TiO₂, and it was 56% O, 43.4% Ti, 0.45% Ru for 0.4 Ru/TiO₂ catalyst. However, it was confirmed in the presence of RuO₂ on TiO₂.

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Figure 6

EDX spectrum of 0.2, 0.4 Ru/TiO₂ catalysts.

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3.2 Direct photolysis

Photolysis of 2-CP by 254 nm reduced its concentration, up to 43% transformation after 60 min; as shown in Fig. 7. Previously, Ragaini et al. (2001) confirmed that 2-CP underwent reasonably fast photolysis under irradiation at 254 nm.

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Figure 7

Effect of Ru doping concentrations on the 2-CP photodegradation using UV irradiation, catalyst loading 2 mg l⁻¹.

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Fig. 8 shows the amount of chloride ions formed during the photodegradation process as a function of irradiation time. During photolysis; chloride ions reached 4 ppm after 60 min of irradiation time. This confirms that an efficient cleavage of the C–Cl bond occurs from the electronically excited state of 2-CP produced by direct light absorption at 254 nm. Furthermore, Barakata et al. (2005) reported the change in the pH is due to the liberation of HCl during the degradation reaction of 2-CP. The reaction can be represented mechanistically (Puma and Yue, 2002) and stoichiometrically (Rideh et al., 1997) as follows: $$2\text{CP}({\text{C}}_6{\text{H}}_4\text{OHCL})\stackrel{\mathit{hv}\text{,}H2O}{⟶}\mathit{Catechol}({\text{C}}_6{\text{H}}_4{(\text{OH})}_2)\stackrel{\mathit{hv}}{\to }\text{Intermediate}\stackrel{\mathit{hv}}{\to }2\text{HCl}+12{\text{CO}}_2+4{\text{H}}_2\text{O}$$ On the other hand, Fig. 9 shows the production of acetate ions under photolysis of 2CP. The higher acetate concentration at initial times decreases the pH value and encourages the formation of a positively charged TiO₂ (TiOH₂⁺) surface which cannot provide hydroxyl groups needed for hydroxyl radical formation and consequently degradation rate is decreased (Barakata et al., 2005). This was also provided in our previous study on photocatalytic degradation of 2,4,6-trichlorophenol using Ti-MCM-41 catalysts (Aboul-Gheit et al., 2011). In general, the rapid degradation of formed acetate ions may be the rate determining step in the photolysis of 2CP.

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Figure 8

Formation of chloride ions during the photodegradation of 2-CP using UV light, catalyst loading 2 mg l⁻¹.

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Figure 9

Formation of acetate ions during the photodegradation of 2-CP using UV light, catalyst loading 2 mg l⁻¹.

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3.3 Photocatalytic degradation in the presence of blank TiO₂

2-CP photocatalytic degradation was investigated under irradiation at 254 nm wavelength in the presence of mesoporous titania particles (UV + TiO₂). 2-CP photodegradation clearly proceeded faster in the absence of TiO₂ particles (Fig. 7). This may appear surprising at first sight. However, a similar behavior has been reported under irradiation either at 254 nm (Ilisz et al., 2002) or containing low wavelength components (Doong et al., 2001), while in general a rate increase was observed upon photocatalyst addition under irradiation at longer wavelengths (Ilisz et al., 2002). Of course, concomitant direct 2-CP photolysis and photocatalysis on TiO₂ occur under irradiation at 254 nm. The fact that, with increasing the fraction of light absorbed (and scattered) by the semiconductor particles, 2-CP underwent progressively slower degradation, indicates that the photocatalytic degradation path was slower than direct 2-CP photolysis. By contrast, photocatalytic conditions ensured faster mineralization. As shown in Fig. 7, a progressively higher percent mineralization was attained after 60 min irradiation in the presence of photocatalyst.

3.4 Effect of semiconductor content

In order to attain an optimum Ru dopant level, the target reaction was carried out on Ru doped TiO₂ with a dopant level of 0.2–0.8 wt% using UV light and the results are presented in Fig. 7. An interpretation of the reactivity order is difficult since it is probably the result of many factors: surface area, crystallinity, crystal size, etc. (Piera et al., 2003). An increase in the content of these metals leads to a decrease in the photocatalytic activity, while an increase in the anatase content in titanium dioxide leads to an increase in such activity (Kovalenko et al., 2003). The introduction of transition metals resulted in the change of the electronic environment of TiO₂. Based on the TEM results, Ru metals were adsorbed on the surface of TiO₂ as a little island of metal particles that act as the active sites of the photocatalytic reaction (Ozkan et al., 1998) and may favor separating charge carriers efficiently, inhibiting the recombination of electron–hole pairs, and ultimately causing enhancement of the reactivity. Barakata et al. (2005) suggested that by irradiation of the Co-doped TiO₂, Co(III) ions work as electron scavengers which may react with the superoxide species and prevent holes–electrons (h⁺/e⁻) recombination and consequently increase the efficiency of the photo-oxidation.

As shown in Fig. 7, the photodegradation efficiency of 2-CP increased with increasing Ru concentration, reaching a maximum value of 61% with sample containing 0.2 wt% Ru. A further increase in Ru concentration to 0.4 wt% resulted in a slight increase in the photodegradation efficiency to 63%.

On the contrary, if the Ru content is low in the case of 0.2 Ru/Ti wt%, there are fewer trapping sites available, thus reducing the activity. As a result; the activity of Ru/TiO₂ was dependent on the doping concentration and 0.4 wt% Ru/TiO₂ has turned out to be the optimum Ru doping level for photocatalytic reduction of 2-chlorophenol; because it has a high content of rutile as shown in Table 1. The enhanced light absorption activity of the mixed phases, suggested the transfer of photo generated electrons from a lower energy rutile to anatase electron site. This electron transfer would serve to reduce the recombination rate of anatase by increasing the separation between the electron and hole, resulting in greater catalytic reactivity (Bickley et al., 1991). Also, many catalytic “hot spots” may be located at the solid–solid interface of anatase and rutile, which is favorable for the improvement of photocatalytic activity (Li et al., 2007). However; Liu et al. (2009) studied the UV and visible-light photocatalytic activity of nitrogen doped TiO₂ with different ratios of anatase to rutile, and conclude that; nitrogen doped TiO₂ with a higher content of rutile can possess abundant surface water and hydroxyl groups, and a high photocatalysis efficiency. The highest chloride formation in the case of 0.4 Ru/TiO₂ catalyst (Fig. 8) indicates the rapid cleavage of C–Cl bond. However; a constant increase of acetate concentration (Fig. 9) provides this catalyst the ability to degrade aromatic rings during the photocatalytic process.

On the other hand; under irradiation at 254 nm, direct 2-CP photolysis appears to be faster than photocatalytic degradation using 0.6% and 0.8% Ru/TiO₂ catalysts. The reason attributed to the detrimental activity of Ru loading beyond 0.4 wt% is the increase in the recombination rate owing to the decrease in the distance between trapping sites in a particle with a number of dopants (as shown in TEM images of 0.6, 0.8 Ru/TiO₂catalysts). Also; this result is approved by Nahar et al. (2009) whereas; 0.5% iron doped sample in the mixed phase catalyst decreased UV light absorption as compared to the undoped sample using the same preparation method, due to the recombination sites of Fe₂O₃. Iwasaki et al. (2000) and Choi et al. (1994) also confirmed the retarding effect of increasing Co(III) ion doping ranging from 0.004 to 0.14 on the photocatalytic activity of TiO₂ under UV light irradiation. Moreover, the results of BET surface area could suggest that some photocatalytic active sites of the TiO₂ surface are blocked by Ru particles due to decreasing pore volume as doped metal ratio increases beyond 0.4%, resulting in lower catalytic activity.

3.5 Photocatalytic degradation using visible light

Effect of Ru doping concentrations on the 2-CP photodegradation under visible light is shown in Fig. 10. The photodegradation efficiency of 2-CP reaches a maximum value of 53% with sample containing 0.2 wt% Ru. A further increase in Ru concentration to 0.4 wt% resulted in decrease in the photodegradation efficiency to 34%. Besides, anatase and rutile ratio, the crystal size of catalyst particles and surface area are also important factors for the photocatalytic activity. For 0.4% Ru/TiO₂ catalyst, the crystal size of rutile is increased (Table 1). Its surface area is decreased and the pore volume is decreased. Thus, the 0.2% Ru/TiO₂ with a higher anatase ratio structure, larger surface area large pore volume and smaller crystal size has the best photocatalytic oxidation efficiency in the visible region. Pore volume is related to the diffusion of pollutants and samples during the photocatalytic reaction. The higher the pore volume of the samples, the diffusion between pollutants and samples becomes rapid and fast (Guo et al., 2009).

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Figure 10

Effect of Ru doping concentrations on the 2-CP photodegradation under visible light, catalyst loading 4 mg l⁻¹.

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Based on the observation of Ku et al. (1996) study, the differences between the global photocatalytic destruction rates of 2-CP by various forms of TiO₂ particles might be attributed to the difference of the surface area of TiO₂, not to the difference of the crystal properties in this study.

In addition, the activity has a direct correlation with the amount of ^•OH groups on the surface (Tsai and Cheng, 1997); as previously shown in FTIR spectrum, the 0.2 Ru/TiO₂ catalyst has a high amount of ^•OH groups on the surface than 0.4 Ru/TiO₂.

3.6 Effect of 0.2 Ru/TiO₂ loading

The photocatalytic degradation of 2-CP was carried out over a 0.2 Ru/TiO₂ catalyst at a different loading of 2–6 mg l⁻¹ in order to optimize catalyst loading during the visible irradiation process. A significant increase in the degradation of 2-CP as catalyst loading increases from 2 mg l⁻¹ to 4 mg l⁻¹; is illustrated in Fig. 11.

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Figure 11

Effect of catalyst weight on photocatalytic degradation of 2-chlorophenol using visible light.

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The decrease in the degradation of 2-CP above 4 mg l⁻¹ may be attributed to the scattering effect of light caused by the high concentration of photocatalyst. The photodecomposition rates of pollutants are influenced by the active site and the photoabsorption of the catalyst used. Adequate loading of the catalyst increases the generation rate of electron/hole pairs for enhancing the degradation of pollutants. However, addition of a high dose of the semiconductor decreases light penetration by the photocatalyst suspension and reduces degradation rate (Doongr and Chang, 1998).