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Enhanced photocatalytic activity of TiO2 rutile by coupling with fly ashes for the removal of NO gases Edith Luévano-Hipólito, and Azael Martinez-de la Cruz Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03302 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016
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Enhanced photocatalytic activity of TiO2 rutile by coupling with fly ashes for the removal of NO gases E. Luévano-Hipólito1,2,*, A. Martínez-de la Cruz1 1
CIIDIT, Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Ciudad Universitaria, C.P. 66451, San Nicolás de los Garza, N. L., México.
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Departamento de Ecomateriales y Energía, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León, Ciudad Universitaria, C.P. 66451, San Nicolás de los Garza, N. L., México.
ABSTRACT A composite of TiO2 rutile with fly ash was prepared by a coupling method assisted with ultrasound irradiation in order to enhance the photocatalytic activity of the semiconductor oxide. The effect of the type of the acid added (acetic, oxalic, citric and nitric) in the course of the coupling of TiO2 with the fly ash was investigated. The use of acids with carboxylic groups promoted a better integration of the fly ashes into the TiO2 surface, as was revealed from the photocatalytic experiments. Among the different acids tested, the acetic acid promoted the formation of the photocatalyst with the highest photocatalytic activity for the photo-oxidation reaction of nitric oxide (NO) in gaseous phase. For this reaction, the optimal load of fly ash on TiO2 rutile was determined in 1.0% of weight. The selectivity of the photo-oxidation reaction of nitric oxide until the formation of nitrate ions (NO3-) was also investigated. The addition of fly ash over TiO2 rutile had a positive effect in increasing the formation of nitrate ions in comparison with the bare TiO2 rutile. The composite made with affordable raw material represents a potential photocatalyst with low production cost. Keywords: TiO2 rutile, photocatalysis, NOx, fly-ash. __________________ Corresponding author Phone: +52 (81) 1442 4400 Ext. 5106 E-mail:
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1. INTRODUCTION The titanium dioxide (TiO2) is the semiconductor oxide most widely used as photocatalyst due to its high photocatalytic activity, photo-stability, high inertia to chemical corrosion, and low toxicity. The titanium dioxide is a solid of a marked ionic character due to the presence of Ti4+ and O2- ions in its crystalline structure. It has four polymorphs: anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO2 (B) (monoclinic)1. According to calorimetric data, rutile is the TiO2 polymorph thermodynamically stable in a wide temperature range and at pressures over 60 Kbar2. In the nature, the rutile polymorph is abundantly available. Synthetic rutile is produced employing as raw materials natural rutile, ilmenite mineral (FeTiO3), slags, and beach sands3. The TiO2 rutile produced is mainly used as pigment in paints (56%), plastics (25%), paper (9%), titanium sponge (4%), and for welding (6%)4. The commercial oxide used in the most of scientific papers in the field of heterogeneous photocatalysis is the Aeroxide TiO2 P-25 (Evonik), which is a mixture of the anatase (80% wt.) and rutile (20% wt.) polymorphs. There are limited reports that studied the photocatalytic activity of the rutile polymorph in liquid and gaseous phase. For example, Macphee et al. studied the photocatalytic activity of different TiO2 oxides including the rutile and brookite polymorphs5. They found that the commercial rutile had the lowest photocatalytic activity for NOx conversion. The high photocatalytic activity of the anatase compared with the rutile polymorph can be explained due to the higher electron mobility, low dielectric constant, higher Fermi level, lower ability to adsorb oxygen, and higher degree of hydroxylation of the anatase6,7. In addition, another explanation is based on the type of electronic transition of anatase (indirect) and rutile (direct)8. In general, semiconductors with indirect transitions produce charge carriers species with life times greater than the related with direct transitions. Different strategies have been proposed to increase the photocatalytic activity of TiO2. For NOx photo-oxidation reaction, several modifications of the photocatalyst have been proposed in order to increase the specific surface area, decrease the particle size, and reduce the recombination process of the charge carrier species generated in the photocatalyst9-11. Additionally the modification of the TiO2 by adding low amounts of other oxides or metals 2
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has been proposed in order to reduce the recombination rate of the charge carrier species1214
. By the other hand, different authors have proposed the addition of fly ashes (FA) into the
synthesis process of TiO2 anatase to increase its photocatalytic activity in different reactions. For example, Hak-Yong Kim et al. studied the addition of FA into the sol-gel synthesis of TiO2 anatase and its potential use as photocatalyst to remove organic pollutants from water and for antibacterial tests15. On the other hand, Yeon-Tae Yu used TiCl4, HCl, and fly ash to develop a TiO2 anatase photocatalyst with an ability to remove 67% of nitric oxide from air. They found that the transformation of Fe2O3 present in the fly ash from magnetite to hematite by heating the sample to 700°C plays an important role in increasing the photocatalytic activity16. Nevertheless, to reach these values of NO conversion degree was necessary to add 10% wt. of TiO2 anatase into the fly ash. Titanium dioxide can be incorporated as raw material in large applications such as cement, asphalt, ceramic tiles, glass, and paints. Due to these materials are usually applied over surfaces exposed to sun light irradiation, this technology can be considered for the development of new sustainable materials for the industry of construction. For this reason, an important factor to choose the commercial oxide is the cost involved. For example, the commercial cost of a rutile oxide (DuPont™ Ti-Pure® R-706 $2.95 USD/kg) is considerably lower than the traditional TiO2 used in photocatalytic applications (Evonik™ Aeroxide® P-25, $45.00 USD/kg). Over the basis of this parameter, and thinking in a possible incorporation of the TiO2 in a commercial material for the industry of the construction, the enhanced of the photocatalytic activity of the cheap rutile oxide is an interesting expectative. In the present work, the photocatalytic activity of the commercial TiO2 rutile (R-706) was enhanced by the addition of different amounts of fly ash into the matrix of the semiconductor oxide. The composite prepared incorporates the use of low cost raw materials by taking advantage of an industrial waste, i.e. the fly ash. The recollection of this type of industrial waste represents an additional step for air purification since its disposal requires large quantity of land, water, and energy. In addition, the fine particles present in the fly ash can volatilize in air and generate different issues in the environment. The
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photocatalytic activity of the materials prepared was tested in the photo-oxidation reaction of NO.
2. EXPERIMENTAL 2.1. Synthesis The titanium dioxide rutile used as pristine material was the oxide R-706 commercialized by DuPontTM, which according with the technical data has a chemical composition mainly of 93%wt. of TiO2, and the rest is Al2O3, SiO2, and ZnO oxides. The oxide R-706 is widely used as a pigment with an excellent dispersibility due to its alumina content. Three fly ashes (FA1, FA2 and FA3) with different compositions were supply from the local steel industry. The content of free carbon in each one was of 6.5%, 4.5% and 7.5% for FA1, FA2 and FA3, respectively. The chemical composition of the fly ashes was supplied by the provider and the data are shown in Figure 1. The preparation of the composite was carried out by a coupling method assisted with ultrasound irradiation. In a typical procedure, 1 g of TiO2 R-706 was added to a solution with 10% V/V of acetic acid (CH3CO2H, Aldrich, 99%) under vigorous stirring for 30 min. Afterwards, different amounts of fly ash (0.5, 1.0, 5.0 and 10%wt.) were added into the TiO2 dispersion under vigorous stirring for 30 min. Later, the dispersion of TiO2 and fly ash was placed in an ultrasonic bath for 1 h to promote the coupling of the ash in the particles of rutile. The dispersion obtained was kept in repose during one day and then it was washed three times with deionized water and ethanol. Finally, the powder was dried at 70°C for 12 h. Furthermore, the composites were prepared under different acidic conditions in order to carry out the activation of R-706 surface. In addition to the acetic acid (C2H4O2), the oxalic (C2H2O4), citric (C6H8O7), and one inorganic acid (HNO3) were chosen as surface activators. One additional experiment was performed using NaOH as activator due to its known properties to dissolve the silicates that can be present in the fly ashes.
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2.2. Characterization The structural characterization was carried out by X-ray powder diffraction using a Bruker D8 Advance diffractometer with Cu Kα radiation (40kV, 30 mA). A typical run was made with a 0.05° of step size and a dwell time of 0.5 s. The energy band gap of the samples was estimated from the UV-Vis diffuse reflectance absorption spectra obtained using an Agilent Technologies UV-Vis-NIR spectrophotometer model Cary 5000 series equipped with an integrating sphere. The reflectance spectra were transformed to the Kubelka Munk remission function considering the direct transition of TiO2 (rutile). The BET surface area measurements were carried out by N2 adsorption-desorption isotherms by means of a BelJapan Minisorp II surface area and pore size analyzer. The N2 adsorption-desorption isotherms were evaluated at -196°C after a pretreatment of the samples at 150°C for 24 h.
2.3. Photocatalytic experiments The removal of nitric oxide from air was determined at NTP conditions (T=293 K and P=1 atm) in a continuous flow reactor designed according with the ISO 22197-1 as was previously described17,18. The mass of the photocatalyst was 0.1 g and it was brush coated over an area of 0.08 m2 of a glass substrate using ethanol as agent dispersant. The concentration of inlet gas was 1 ppm of nitric oxide in air and was introduced into the reactor at 1 L·min-1. The irradiance in the center of the photocatalytic reactor was 8.2 W·m-2, and it was provided by two fluorescent black lamps (TecnoLite) of 20 W. The concentration of NO was continuously measured with a chemiluminescent NO analyzer (EcoPhysics CLD88p). In addition, the final products of the NO photo-oxidation reaction were also investigated. For this purpose, a dispersion of the photocatalyst used was sonicated for 30 minutes and then it was centrifuged to obtain a crystalline solution. The concentration of nitrates ions in the obtained solution was measured in a DR/890 Hach colorimeter trough the reduction of nitrate to nitrite using cadmium as catalyst. In the same way, a similar colorimetric method was used to the determination of nitrites. It was estimated from the reddish color of the
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solution, proportional to the concentration of nitrites, which was developed by a diazotization reaction in presence of sulfanilic and chromotropic acid.
3. RESULTS AND DISCUSSION 3.1. Characterization The commercial oxide R-706 and the composites obtained were characterized structurally by X-ray powder diffraction technique and their diffractograms are shown in Figure 2 and 3. In first instance, the main reflections of the diffractogram of the commercial TiO2 correspond with the crystalline structure of rutile according with the card JCPDS 01-1292. However, additional diffraction lines corresponding with the hexagonal ZnO phase (JCPDS 36-1451) were detected. Other compounds present in R-706 at low content (
