This study recycles titanium dioxide (TiO2) that is contained in waste selective non-catalytic reduction (SNCR) catalysts using acid or alkali. The waste SNCR is then filtered, baked, ground and calcined to form a photo-catalytic powder. The nano-TiO2 photo-catalysts that are obtained using both processes are then tested and compared. The two TiO2 photo-catalysts that are produced from waste SNCR catalysts have a diameter of 30–40 nm. Energy dispersive spectrometry (EDS) and inductively coupled plasma (ICP) are used to determine the elemental composition of TiO2 and X-ray diffraction (XRD) is used to determine the crystalline phase. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to determine the surface morphology, the structure and the particle size. The effect of placing porous TiO2 in a suspension is also determined. This study demonstrates the production of a photo-catalyst from an SNCR catalyst and its effect in advanced oxidation processes (AOP). When everdirect supra turquoise blue (FBL) dye wastewater is degraded in the presence of ultraviolet (UV) /TiO2, more than 90% of the total oxidizable carbon (TOC) is removed.
Selective non-catalytic reduction (SNCR) catalysts are used to decompose nitrogen oxides (NOx) that are contained in the air. Any device that generates waste gas with NOx compounds must have an SNCR catalyst installed at the exhaust to prevent the emission of NOx to the air. In waste SNCR catalysts, vanadium pentoxide (V2O5) and molybdenum trioxide (MoO3) are the active components of the catalyst and these are deposited on a titanium dioxide (TiO2) carrier. TiO2 is used as a dye in paint. It is also used in electronic ceramic material applications for the manufacture of barium titanate (BaTiO3), strontium titanate (SrTiO3) and lead zirconate titanate (ZrTiO3). Another application is as the carrier for a catalyst, for which application of its resistance to acid and alkaline corrosion and to high temperature is an advantage. In 2000, Zhong et al. used titanium isopropoxide as a precursor, which was added to a cation polystyrene micro ball to allow hydrolyzed TiO2 to cover a polystyrene (PS) micro ball (Zhong et al. 2000). In 2003, Yang et al. used titanium(IV) tert-butoxide as a precursor, and seed-emulsion polymerization was used to produce a PS micro ball with a particulate diameter of 275 nm (Yang et al. 2003). In 1972, Fujishima and Honda respectively used a photo-catalyst semiconductor titanium oxide single crystal and Pt as the electrode for a water decomposition reaction (Fujishima & Honda 1972). In 2002, Zhang and Gao used titanium isopropoxide as a precursor. Span 80, water and toluene were used to form a water/oil micro-emulsion. The diameter of the TiO2 particles that were formed was controlled by changing the ratio of water to Span 80 (Zhang & Gao 2002). In 2002, Jang and Lee used a surfactant that allows methanol to form micelles that are dispersed in water. A mixture solution of isooctane and styrene was then dropped in at 70 °C to produce a polystyrene hollow ball (Jang & Lee 2002). In 1998, Kasuga et al. used a temperature of 110 °C and a 5–10 mol·L−1 NaOH solution to treat anatase TiO2 powder. Treatment with HCl acid produced a TiO2 nano-piece. The Ti-OH bond at the end interacted to form a nano-tube (Kasuga et al. 1998). In 2002, Eiden and Maret used titanium ethoxide as a precursor. A PS micro ball carrying a negative charge was used as the carrier. The carrier was dispersed into absolute ethanol (Eiden & Maret 2002). There are many ways to produce polystyrene, such as emulsion polymerization, emulsifier-free emulsion polymerization, dispersion polymerization and suspension polymerization (Cho et al. 2016). There are many ways to produce TiO2, but the most extensively used methods are the sol-gel method (Su et al. 2006; Tseng et al. 2010; Angkaewa & Limsuwana 2012), chemical vapor deposition (CVD) (Ding et al. 2001; Loic et al. 2010; Tang et al. 2014), liquid phase deposition (LPD), the precipitation method (Lin et al. 2008; Kagomiya et al. 2010; Shipra & Manoj 2012; Liu et al. 2014; Muhamad et al. 2015), the microwave method and the sonochemical method (Saez & Mason 2009; Hasanpoor et al. 2015). The reuse of waste SNCR catalysts to produce nano-TiO2 photo-catalyst involves novel chemical technology. It has the potential to address the problem of pollution by waste catalysts and to turn waste into a resource. This study produces TiO2 nanoparticles with a pure anatase phase. A goal is to obtain hollow TiO2 nanoparticles.
Materials and measurement tools
SNCR powder was provided by Trivision Technology, Taiwan Co., Ltd. The sulfuric acid (H2SO4), hydrochloric acid (HCl), sodium hydroxide (NaOH), monomer of styrene (C8H8), ammonium persulfate ((NH4)2S2O8), sodium dodecyl sulfate (CH3(CH2)11OSO3Na), everdirect supra turquoise blue (FBL), titanium dioxide (TiO2) and other reagents were all guaranteed reagent (G.R.) grade chemicals for synthesis. A computer-interface X-ray powder diffractometer (XRD) with Cu Kα radiation (Shimadzu XRD-600) was used to identify the nanoparticles. Images of the morphological features of the particles were obtained using a scanning electron microscope (SEM, JEOL JEM 200CX) with an accelerating voltage of 15 kV and a transmission electron microscope (TEM, TECNAI 20). The diameter of the particles in dispersion was measured using a Malvern Zetasizer Nano S-90 dynamic light scattering (DLS) instrument with 300 Has, 10 mw and a He-Ne laser source. A JSM-6700F field emission scanning electron microscope (FESEM) was used to characterize the surface structure of the distribution of the nano-TiO2 particles. The composite particles of SNCR were analyzed using inductively coupled plasma (ICP, 7500CE) and an atomic absorption spectrometer (GBC-908).
For the UVA/TiO2 experiment, dye and the treatment were placed into the reaction device. The reaction unit was a jacketed Pyrex heat-resistant glass unit that controls ultra violet (UV) light intensity, stirring speed and reaction temperature. The total volume of the reaction device is 2 l, as shown in Figure 1.
Experimental method and process
This study has three major parts. Initially, an alkaline process recycles the titanium part of the waste SNCR catalysts. The powder is then cleaned and calcined. After removing the PS micro ball, porous TiO2 powder is obtained. In the acid process, sulfuric acid solution and the waste catalyst are heated to form solid titanium oxysulfate. Titanium oxysulfate is then hydrolyzed to form TiO2 powder. A PS micro ball is added to produce porous TiO2, and the possibility of producing a suspension was also determined. The waste SNCR catalysts and an acid process are then used to recycle and produce TiO2 powder. The waste SNCR catalysts are then calcined at different temperatures to recycle the photo-catalyst. The degradation of FBL dye with wastewater is simulated. The details of the process and methods are described as follows.
Experiment to produce titanium tetrachloride solution
Initially, 5 g of Na2Ti3O7 was ground into powder to accelerate the reaction. The Na2Ti3O7 powder was then added to 125 ml HCl solutions of different concentrations. This then underwent magnetic agitation and was heated to 60 °C. It was agitated using a magnetic stone and sealed with preservative film. It was heated and agitated for 24 hours, and at the end of the heating process, it was placed in an exhaust cabinet and cooled to room temperature. The composition of the ICP was then tested.
Production of the porous photo-catalyst
A 1 g PS micro ball emulsion was added into 1,000 ml of water to form an aqueous solution and then 0.1% of sodium pyrophosphate was added. A volume of 5 ml of self-prepared titanium tetrachloride was then added into 500 ml of the PS micro ball aqueous solution. This was then agitated using a magnetic stone and sealed with a preservative film, and then heated and agitated for 24 hours. The precipitates were filtered using a vacuum filter and cleaned using de-ionized (DI) water. The filtering process was repeated five times. Alcohol was used to clean and filter twice. The product was then placed in a high temperature furnace and heated to 450 °C for two hours. A PS ball was immersed into concentrated sulfuric acid for sulfonation. After sulfonation, the PS ball acts as a carrier and disperses into a mixture solution of titanium(IV) tert-butoxide and alcohol so titanium(IV) tert-butoxide is adsorbed onto the PS ball. It was then centrifuged and dispersed into an aqueous alcohol solution for reaction. To prepare the PS/TO2 shell, N,N-dimethylformamide was used to dissolve the PS and the product was calcined to form a hollow TiO2 micro ball. The calcined and filtered residue was subjected to XRD analysis to determine its composition and its morphology was observed using SEM.
The use of an acid process to recycle waste catalyst that contains Ti to produce a TiO2 photo-catalyst
The 18N concentrated sulfuric acid solution was used to dissolve the waste SNCR catalysts and the solution was heated and agitated for 8 hours. The waste SNCR catalysts are converted into titanium oxysulfate and hydrolyzed into TiO2. A high temperature was used for the calcination process to generate activity.
DI water at 60 °C was used to clean and bake 5 g of waste SNCR catalyst. The 5 g of waste SNCR catalyst was ground into powder to facilitate dissolution. The waste SNCR catalyst powder was mixed with 125 ml of concentrated sulfuric acid solution and this was then placed into a magnetic stone agitator, which was heated to 180 °C. A preservative film was used to seal the product, which was then heated and agitated for 8 hours. A vacuum filtering machine was used to filter the mixture solution to produce a filtered solution and a filtered residue (titanium oxysulfate). The filtered residue was then baked in an oven. The composition of the filtered solution was determined using ICP. The composition of the baked and filtered residue was determined using XRD. The photo-catalyst powder was placed in a high temperature oven and heated to different temperatures (450 °C, 500 °C, 550 °C and 555 °C) and calcined for two hours. The experimental flow diagram for the production of the TiO2 photo-catalyst is shown in Figure 2.
Degradation of FBL dye wastewater using the photo-catalyst
An amount of 0.096 g of FBL dye was used to make 1 l of aqueous solution of chemical oxygen demand (COD) equal to 100 ppm. To degrade COD = 100 ppm FBL dye, 5 g of photo-catalyst was placed into the reactor. A UVA lamp tube was used to illuminate it for 8 hours (Iav = 1.324 mW/cm2) and then a sample was taken after different reaction times (0, 15, 30, 60, 90, 120, 180, 240, 360, and 480 min) for analysis. The sampled liquid was filtered using a 0.45 μm membrane filtration system (MFS) to determine its total oxidizable carbon (TOC) value.
RESULTS AND DISCUSSION
Both of these are strong oxidizers and can decompose organic substances (FBL). They convert organic substances into CO2.
XRD and ICP analysis of raw material waste SNCR catalysts
The specific diffraction angle of anatase was determined using the JCPDS standard database. The peak occurs at around 2θ = 25°. The JCPDS database shows that TiO2 from waste SNCR catalyst powder is of anatase crystalline phase, as shown in Figure 3. To accelerate the dissolution, waste SNCR catalyst particles were ground into powder in a mortar and compositional analysis used ICP. The weight percentage for each component is Na2O 0.48%, MgO 0.33%, Al2O3 14.81%, Fe2O3 3.02%, MoO3 9.14%, TiO2 60.82%, V2O5 2.6%, and SiO2 8.79%, so the waste SNCR catalysts have a high content of TiO2.
The DLS test and SEM analysis of the PS micro ball
An emulsion method was used to synthesize a PS micro ball. 100 ml of DI water was used to dissolve 2 g of sodium dodecyl sulfate by agitation. Fifty grams of styrene monomer was then added. An amount of 0.30 g of ammonium persulfate was slowly dissolved in 10 ml of DI water. This solution was agitated and heated to 85 °C and then agitated using a magnetic stone. It was then sealed with preservative film and heated and agitated for 24 hours. After dilution, the diameter of the particles was determined using DLS. It was then diluted and baked and SEM images were used to determine the surface morphology. The DLS data from the nano particulate diameter analyzer show that when the PS micro ball is uniformly dispersed in water a suspension solution is formed. The distribution chart for the diameter of the particles shows that the average particulate diameter is 72 nm, which is consistent with the result for SEM, as shown in Figure 4. The DLS result shows that the PS micro ball has good dispersive characteristics. Spherical PS particles with a narrow size distribution are formed in the mixed solution using a styrene monomer as the precursor. The SEM images of the morphology exhibit a regular spherical shape. The SEM images show that the diameter of the particles is about 60–80 nm.
After titanium oxysulfate was hydrolyzed and after calcination at 450 °C, ICP was used to determine the composition of the TiO2 photo-catalyst. The result shows that the weight percentage of TiO2 is 81.76% and the percentages for other compounds are SiO2 7.16%, V2O5 1.22% and MoO3 5.52%. V2O5 and MoO3 increase the effectiveness of photo-catalysis. SiO2 has no effect on photo-catalysis and alkali is used to dissolve it. The method is very simple and allows the easy production of a nano-TiO2 photo-catalyst. To obtain porous TiO2, a homogenous distribution and a packed distribution of PS spheres is necessary (Wu et al. 2014; Zalfani et al. 2017). In this case, the PS is not homogenous so the TiO2 is not porous. The SEM result shows a TiO2 photo-catalyst powder of irregular shape that exhibits clustering, as shown in Figure 5(a). A TiO2 particle with a diameter of around 20 nm is shown in Figure 5(b).
Microscopic inspection and analysis of the porous TiO2 photo-catalyst
The SEM result shows that titanium tetrachloride solution hydrolyzed to form TiO2 particles and co-precipitated to form a PS micro ball. The powder was then cleaned and calcined. A porous photo-catalyst powder was obtained after the PS micro ball was removed. The porous photo-catalyst powder had an irregular fragmented shape and the coverage was not spherical, as expected. The particles precipitated and subsequently formed a powder with a non-uniform distribution. The SEM results showed that this was initially formed by many PS micro balls. The TiO2 particles are then covered externally, as shown in Figure 6(a). For the porous photo-catalyst, the TEM result shows that the photo-catalyst actually has a shell structure. However, because the PS micro ball acts as the core, after calcination, the pore that remains shows that during the co-precipitation process, there is clustering. The pore's inner diameter is about 40 nm, as shown in Figure 6(b). The TEM image shows that the PS micro balls are stacked during the process of covering TiO2.
Elemental analysis EDS shows that the porous photo-catalyst powder that is produced has very few impurities. When the photo-catalyst is produced using alkaline and acid methods, after the waste catalyst undergoes a melting reaction with NaOH, it is easily sintered in the container so it is very difficult to separate and collect. When producing the titanium tetrachloride solution, impurities can easily react with HCl to form a sol-gel state, so the separated titanium tetrachloride solution is consumed at a high rate. In the acid process, waste catalyst reacts with sulfuric acid solution to produce titanium oxysulfate. This reaction is very short and the reaction is more complete, with a high conversion rate. Therefore, the recycling rate is higher.
XRD and ICP analysis of the porous photo-catalyst
For a porous photo-catalyst that is produced using the alkaline process, the XRD diffraction pattern for a waste SNCR catalyst that is produced at 700 °C has the diffraction angle of meta-titanic acid, with the main peak at 2θ = 40°, as defined by the JCPDS standard database. The XRD diffraction pattern is for meta-titanic acid powder that is produced using the molten salt method. The reaction time is insufficient (8 h), so NaOH does not fully react with the waste SNCR catalyst. Part of the waste SNCR catalyst still remains in the anatase and rutile crystalline phases, as shown in Figure 7. If the reaction time is sufficient (12 h), the Na2Ti3O7 powder has a meta-titanic acid crystalline phase and there are no other impurities. The XRD diffraction pattern is for liquid titanium oxysulfate filtered residue that is produced using the sulfuric acid method. A comparison of the results and the JCPDS database shows that the filtered residue powder is solid titanium oxysulfate. Titanium oxysulfate is hydrolyzed by calcining at 450 °C. The X-ray diffraction pattern for the powder is compared with the JCPDS standard database and the diffraction angle for the photo-catalyst TiO2 powder exhibits a peak at around 2θ = 25°, as shown in Figure 7. The X-ray diffraction pattern shows the photo-catalyst powder that is produced using titanium oxysulfate; the JCPDS database shows that the photo-catalyst powder is anatase-TiO2, as shown in Figure 7.
The characteristics of the TiO2 photo-catalyst that is prepared using the acid process are determined by ICP analysis of the filtered solution. Five grams of waste catalyst from the acid process that contains Ti was heated and reacted with 125 ml of concentrated sulfuric acid solution. ICP analysis shows the composition of the filtered solution. The results are listed in Table 1. This table shows that the filtered solution contains Ti at a weight percentage of 18.21% and elements such as V, Mo, Fe and Al are also dissolved. However, the dissolved Ti content is too low so the filtered solution from the acid process cannot be used to synthesize the photo-catalyst powder. The ICP result for the composition of the filtered residue shows that it contains Ti at a weight percentage of 91.47%. Although V, Mo, Fe and Al are also dissolved in small amounts, the solution is mostly Ti. Therefore, the filtered residue from the acid process can be used as a raw material to synthesize photo-catalyst powder.
The degradation of dye wastewater using a TiO2 photo-catalyst
The specific surface area analyzer (BET, Brunauer–Emmett–Teller) data show that the specific surface areas are TiO2 (7 nm) sold: 238.5 m2/g, photo-catalyst TiO2 (450 °C): 112.3 m2/g, photo-catalyst TiO2(550 °C): 87.9 m2/g and TiO2 (20 nm) sold: 11 m2/g. The dosage (5 g/l) of photo-catalyst TiO2 (450–555 °C) was maintained at a constant value to degrade FBL dye (concentration COD = 100 ppm), as shown in Figure 8. The photo-catalysts that are produced from waste SNCR catalysts exhibit better activity at 450 °C. Calcination at 550 °C produces photo-catalysts with a lower performance. The λmax values for the dyes are 606 nm (everdirect supra turquoise blue, FBL) (Gallo et al. 2012) and 329 nm (C.I. code: direct blue 199) and the structure is:
It is seen that calcination at 450 °C removes the most TOC, so this is the optimum temperature for FBL degradation. When the FBL COD = 100 ppm, 91.14% of TOC is removed. When the calcination temperature is increased, less TOC is removed. When the calcination temperature is 555 °C, about 64.39% of TOC is removed. Particles with a different diameter are formed at high temperature, so there is a reduction in the surface area of TiO2. To determine how effectively photo-catalysts with different specific surface areas degrade FBL dye wastewater, the dye (FBL) concentration was fixed at COD = 100 ppm. The results for photo-catalysts for different specific areas are shown in Figure 9. A dosage of photo-catalyst of 5 g, 7 nm and 20 nm and TiO2 photo-catalysts that are produced at 550 °C and obtained commercially were used for the degradation experiment. The results for degradation show that the catalyst is stable in the photo-catalytic process. The result also shows that the holes play the most important role in the photo-catalytic activity. The UVA lamp intensity (Iav) was 1,324 mW/cm2.
This study uses an alkaline/acid process to recycle waste SNCR catalysts and produce a porous photo-catalyst powder. Analysis of the particle size using a distribution analyzer (DLS) shows that the average diameter of the particles is about 72 nm. The results for the experiment to degrade FBL dye wastewater using photo-catalytic TiO2 powder that is produced using the acid process show that at 450 °C, 500 °C and 550 °C the TiO2 photo-catalyst does not remove much TOC from wastewater at a concentration of COD at 100 ppm. However, at 555 °C, the effect is reduced because MoO3 enhances the photo-catalysis effect. At 550 °C, the presence of MoO3 causes sublimation, which reduces the photo-catalytic effect. Photo-catalysts with different specific surface areas were used to degrade FBL dye wastewater and it was found that a commercially available 7 nm TiO2 gives better results at 30 min than 450 °C or 550 °C TiO2 photo-catalyst because commercial 7 nm TiO2 has a greater specific surface area than the 450 °C or 550 °C TiO2 photo-catalyst, so there is greater absorption. However, the photo-catalytic efficiency is worse than that for 450 °C or 550 °C TiO2 photo-catalyst, and this phenomenon is more significant if the COD in wastewater has a concentration of 200 ppm.
The authors of this work are grateful to the Ministry of Science and Technology (MOST) of Taiwan for financial support of this research under contract No. MOST 104-2221-E-274-003 and MOST 105-2221-E-274-003.