Effect of calcination temperature on Orange II photocatalytic degradation using Cu:Ni/TiO2 under visible light Nadia Riaz1, F.K.Chong2 , Binay K Dutta3, Zakaria B. Man1 M. Saqib Khan1 and Ela. Nurlaela1 1
Chemical Engineering Department Universiti Teknologi PETRONAS, 31750 Tronoh, Malaysia *
[email protected] Abstract—The decolorization of Orange II was studied under visible light using bimetallic Cu-Ni/TiO2 prepared via precipitation method. Cu:Ni mass composition of 9:1 was prepared and the effect of calcination temperature on the Orange II removal was investigated. The raw photocatalysts were activated by calcination at three different temperatures (180⁰C, 200⁰C and 300⁰C) for 1 hour duration. Photocatalysts were characterized using Thermogravimetric Analysis (TGA), Fourier-Transformed Infra red spectroscopy (FTIR), Powder XRay diffraction (XRD) and Field-Emission Scanning Electron Microscopy-Energy Dispersive X-ray (FESEM-EDX). The photocatalytic degradation of Orange II was conducted under the irradiation of visible light (500 W halogen lamp) at pH 6.8. The extent of degradation for Orange II with initial concentration of 50 ppm was monitored using UV-vis spectroscopy and TOC analysis was conducted at the end of the reaction. Results showed that complete Orange II removal was achieved for Cu:Ni/TiO2 photocatalysts calcined at 180⁰C, and 200⁰C Photocatalyst calcined at 180⁰C showed the best performance with 100the lowest TOC value of 16.1 ppm. Keywords- Cu-Ni Bimetallic photocatalysts, Orange II, TiO2 Photocatalysis, Azo dyes,metal doping
I.
INTRODUCTION
The main causes of surface water and groundwater contamination are industrial discharges [1]. The textile industry has a big pollution problem. The World Bank estimates that 17 to 20 percent of industrial water pollution comes from textile dyeing and treatment. Wastewaters generated by the textile industries are known to contain considerable amounts of non-fixed dyes, especially of azodyes, and a huge amount of inorganic salts. It has been estimated that more than 10% of the total dyestuff used in dyeing processes are released into the environment (Maguire, 1992) [2]. Azo dyes are the largest group of synthetic colorants used in textile industry [3] constituting 60–70% of all dyestuffs produced [4]. They have one or more azo groups (R1–N=N– R2) having aromatic rings mostly substituted by sulfonate groups (–OH, –SO3, etc.) [5, 6]. Azo-dyes like Orange II represent more than 15% of the world production of dyes used in the textile manufacturing industry. These dyes are for the most part non-biodegradable
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2
Fundamental & Applied Sciences Department Universiti Teknologi PETRONAS, 31750 Tronoh, Malaysia 3 Chemical Engineering Department, The Petroleum Institute, Abu Dhabi, United Arab Emirates and toxic and at present are abated by some common nondestructive processes [7, 8]. Orange II is an anionic monoazo textile dye of acid class. It is resistant to light degradation and the action of O2 and common acids and bases. Although its stability is useful in textile applications, this causes difficulty in managing its removal. Orange II does not undergo biological degradation in wastewater treatment plants [9, 10]. Several techniques have been used to abate the model azo-dye, Orange II, such as Fenton [11], photo-Fenton [12] and TiO2 photocatalysis [13]. The elimination of dyestuff from industrial wastewater is an important environmental target.
Figure 1. Chemical structure of Orange II
The removal of azo-dyes by advanced oxidation technologies (AOTs) has been the subject of several recent studies. The mechanism of dye destruction in AOPs is based on the formation of a very reactive hydroxyl radical (•OH), that, with an oxidation potential of 2.80 V [14, 15] can oxidize a broad range of organic compounds. A. Modification of TiO2 Semiconductor for Harvesting Visible Light TiO2 is a very suitable photocatalyst but its activity is mainly confined to the UV region of solar radiation. Thus doping with transition metals has been employed to extend the light absorption to the visible region [16]. In order to explore efficient visible light induced photocatalysts, much scientific effort has been conducted in recent years to reduce its band gap to make it suitable for utilizing solar energy with good efficiency [6]. These works include: doping TiO2 with various transition metals such as Pt, Au etc., [17, 18] non metal atoms (N, S etc.) [19, 20] and anchoring an organic dye sensitizer molecule on the surface of the photocatalyst [9]. The objective of this work is to investigate the effect of calcination temperature for Cu:Ni/TiO2 on the Orange II photodegradation under visible light irradiation.
II.
MATERIALS AND METHODS
A. Materials Copper nitrate trihydrate Cu(NO3)2.3H2O and nickel nitrate hexahydrate, Ni(NO3)2.6H2O (Acros brand >98% purity) were used as dopant metal salts. Titanium dioxide, TiO2 (Degussa P25 80% anatase, 20% rutile) was used as a support which also acts as the semiconductor in photocatalysis. Sodium hydroxide NaOH (Merck, 95% purity) was used as precipitating agent. Glycerol used was of 95% purity (Systerm). Orange II (Acros, pure) was used as the model azo dye for photocatalytic degradation study. All chemicals were used without further purification B. Preparation of bimetallic photocatalyst: 10 wt% bimetallic Cu-Ni/TiO2 photocatalysts with 9:1 copper to nickel mass composition was prepared using TiO2 as support with NaOH addition. Cu and Ni salts were weighed in appropriate amount and dissolved in 100 ml of distilled water followed by the addition of glycerol in 2:1 mol ratio of glycerol:total metals with continuous stirring. TiO2 powder was added to the solution and stirred for 1 h. pH of the solution was adjusted to pH 8.5 with 0.25M NaOH. The mixture was stirred at 10oC for 1 hour prior to filtering and drying the precipitates in an oven at 75 o C overnight. The dried photocatalyst was ground with a mortar and pestle, kept in air tight glass bottles as raw photocatalyst and stored in a desiccator at room temperature till further use. C. Pre-treatment of photocatalysts: In order to select suitable temperature range for calcination, TGA was conducted on the raw photocatalyst using Perkin Elmer (Pyris 1 TGA) instrument. Results from TGA were reported as thermogram which is a plot of the relative weight of the photocatalyst versus temperature. The raw photocatalyst was weighed using a built-in microbalance attached in the instrument which automatically read the weight of the sample in the range of 5-10 mg in a sample cup. The sample was heated from 30°C to 800°C at a ramp rate of 20°C/min using nitrogen as purge gas. D. Characterization: It is important to characterize the calcined photocatalysts in order to determine mainly their chemical and physical properties and then to relate these properties to their photocatalytic performance. In this study, photocatalysts were characterized using FTIR (Shimadzu FTIR-8400S), Powder X-ray Diffraction (XRD) and Field-emission Scanning Electron Microscopy-Energy Dispersive X-ray (FESEMEDX), (Supra55VP). E. Photocatalytic Degradation Study: Photocatalytic degradation of 50ppm Orange II was conducted to evaluate the photoactivity of Cu-Ni/TiO2 photocatalysts using halogen lamp (500W) as the visible light source at initial solution pH 6.8 at 25 oC. Photocatalysts with loading of 1mg/mL was added to 10 ml of distilled water and
sonicated for 10 min in an ultrasonic bath at 25oC followed by the addition of Orange II solution to give rise to a final volume of 30 ml. The suspension was stirred using a magnetic stirrer for 2 h in the dark and later this suspension was illuminated for 1 h using 500W halogen lamp as the visible light source at a distance of 25 cm (intensity 30856.66 lux) Reference experiments were also conducted without the addition of photocatalyst and also the addition of TiO2 P25 as the standard photocatalyst reference. F. Analytical Analyses The Orange II decolorization and concentration was monitored by measuring the solution absorbance from 400800nm using a Shimadzu UV-3101 UV/Visible spectrophotometer. TOC analyses were also conducted at the end of the reaction. Prior to absorbance measurement, the reaction samples were centrifuged twice to remove the suspended photocatalyst. The absorbance peak at 485.0 nm is used as the representative peak for Orange II concentration [21, 22]. Before the measurements, a calibration curve was obtained by using the standard Orange II solutions with known concentrations of 20, 30, 40 and 50ppm. A linear relationship between the absorbance of standard solutions at 485.0nm and Orange II ppm solution was obtained from 20 to 50 ppm. The photodecolorization efficiency (%) was calculated as follows: Efficiency (%) =
C₀ − C C₀
X 100
Where Co = initial concentration of Orange II (50 ppm) C = concentration of Orange II during reaction III.
RESULTS AND DISCUSSIONS
A. Pre-treatment of photocatalysts: To activate the raw photocatalysts thermal decomposition analysis of the raw photocatalyst was conducted using a thermal gravimetric analyzer (TGA). The thermogram of the raw photocatalyst is shown in Figure 2. The TG curve showed two decomposition steps between 30 oC to 180 oC and other at 180 oC to 500 oC onwards with total weight loss of 8.98%. The first step is attributed to evaporation of physically absorbed impurities and second decomposition step is attributed to decomposition of Cu(OH)2 and Ni(OH)2, respectively. Thus proposed decomposition steps for the fresh photocatalyst are as follow in Equation 1 and 2 [23]. Cu(OH)2 Ni(OH)2
CuO(s) + H2O(g) (1) NiO (s) + H2O(g) (2)
Based on the TGA results, calcination temperature at 180°C, 200°C and 300°C for 1 h duration were selected as research parameter.
anatase and rutile, respectively [23, 27, 28, 29]. No diffraction lines of Cu and Ni containing phases for photocatalysts were observed, which may indicate high metallic dispersion [23, 33, 34] onto the TiO2. Zhu and friends [32] reported that TiO2 with metal dopant concentration above 5 wt% could display small peaks attributable to a separate oxide phase in addition to the peaks for anatase TiO2. The absence of separate oxide phases clearly indicates that the dopants are well dispersed on TiO2. This might be due to the use of glycerol in photocatalyst preparation that favored the formation of small well dispersed metal particle as it prohibited the aggregation of metal particles [30]. No phase transition was observed for anatase into rutile for the calcined photocatalysts. Figure 2. Thermal decomposition of raw Cu-Ni/TiO2 photocatalyst
B. Characterization: 1) Fourier Transform Infrared Spectroscopy (FTIR) Figure 3 showed the FTIR spectra for the raw and calcined photocatalysts. Several absorption peaks were observed. The broad band around 3400 cm-1 was attributed to O–H stretching, and the peak near 1600 cm-1 to O–H bending [23], related to physically absorbed moisture. The IR band observed from 400 to 900 cm-1 corresponds to the Ti-O stretching vibrations [24]. The band at 1382.87 cm-1 attributed to nitrate (NO3-) group [25], is present in both raw and calcined photocatalysts even in that calcined at 300°C. Similar band at 1384 cm-1 was also observed by Li and Inui, [26], referred to nitrate which is always present when nitrate salts are used as precursors. However, in the present study, the presence of nitrate anions in photocatalysts did not affect the catalytic activity of the photocatalysts.
Figure 3. FTIR spectra of raw and calcined Cu-Ni/TiO2 photocatalysts
2) XRD The XRD patterns of the Cu-Ni/TiO2 photocatalysts calcined at 200°C and bare TiO2 are shown in Figure 4. Two phases were observed: Anatase and rutile. The peaks at 2θ = 25.34° and 2θ = 27.46° appeared for bare TiO2 as well as metal doped photocatalysts, corresponding to the main peak of
Figure 4. XRD patterns of a) bare TiO2, b) Cu-Ni/TiO2 calcined at 200°C (A= Anatase, R= Rutile)
3) FESEM- EDX: Morphology of the photocatalyst often plays important role in its catalytic property. The particle size has an important influence on the photocatalytic process, due to changes in transmission, dispersion and absorption of light, and in the catalyst surface availability. FESEM micrographs the photocatalysts (Figure 5) clearly depict uniform distribution with irregular sized spherical morphologies with slight agglomeration ranging from 11- 35nm. It can be clearly seen from the FESEM micrographs that the morphologies of all the photocatalysts are spherical and agglomerated. The degree of agglomeration was different for photocatalysts which may be caused by sintering during calcination process [34]. A few groups of researchers [23, 33, 34] also reported similar agglomerated morphology for Cu/TiO2. It was observed from EDX mapping of the photocatalysts that dopant metals are uniformly dispersed onto the support and present in the form of metal oxides (Cu and Ni oxides). This is in good agreement with the XRD results that doped metals are well dispersed onto the support.
The reaction using TiO2 standard displayed the lowest Orange II removal (21%) with the highest TOC value (55.7 ppm).
Figure 5. FESEM micrographs of 10wt%-9Cu:1Ni-200-1photocatalysts and bare TiO2 (150K).
C. Photocatalytic Degradation Study: The photocatalytic degradation of Orange II was conducted to evaluate the performance of synthesized photocatalysts under the irradiation of visible light at pH 6.8, as described in methodology section. The extent of degradation for Orange II with initial concentration of 50 ppm was monitored using UV-vis spectroscopy and TOC analysis was conducted at the end of the reaction. Results are presented in Figure 6 for the % Orange II removal as a function of time. Decolorization was the fastest for the CuNi/TiO2 calcined at 180ºC even at the initial stage of the photoreaction. 100% removal was observed after the 2 h dark reaction. Orange II % removal was 100% after 60 min of radiation for the photocatalyst calcined at 200ºC while for 300 ºC and bare TiO2 degradation rate was very low. It is evident that the percentage of decolorization and photodegradation increases with increase in irradiation time. The reaction rate decreases with irradiation time as it follows the pseudo first-order kinetics [35] and additionally a competition for degradation may occur between the reactant and the intermediate products. The slow kinetics of dye degradation after certain time limit is mainly attributed to: (a) the difficulty in converting the Natoms of dye into oxidized nitrogen compounds [36], (b) the slow reaction of short chain aliphatics with •OH radicals [37] and (c) the short life-time of photocatalyst because of active sites deactivation by strong by-products deposition (carbon etc.) [38]. 180-1
200-1
TiO2
300-1
100
% Removal
80 60 40 20 0 0
10
20
30
Time (min)
40
50
60
Figure 6. Photocatalytic degradation of Orange II with time
Complete removal of Orange II was observable using CuNi/TiO2 calcined at 180 oC and 200 oC giving TOC values of 16.1 and 43.2 ppm, respectively after 1 h (Table 1). At higher calcination temperature (300°C), lower performance was observed with only 57% dye removal with 45.2 ppm TOC.
TABLE 1 Removal of Orange II after 1 h radiation Cal. Temp Conc. % Removal TOC (⁰C) (ppm) 180 0 100 16.1 200 0 100 43.2 300 21.5 57 45.8 TiO2 39.5 21 55.7
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