CHEMICAL ENGINEERING TRANSACTIONS VOL. 76, 2019 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar S. Varbanov, Timothy G. Walmsley, Jiří J. Klemeš, Panos Seferlis Copyright © 2019, AIDIC Servizi S.r.l. ISBN 978-88-95608-73-0; ISSN 2283-9216 Ternary Ag/AgCl/BiOCl Synthesis and the Effects of its Constituents on Phenol Degradation Dorcas O. Adenuga*, Shepherd M. Tichapondwa, Evans M. N. Chirwa Water and Environmental Engineering Division, University of Pretoria, Pretoria 0002, South Africa dorcasadenuga@yahoo.co.uk Mineralisation of organic constituents in wastewaters emanating from petrochemical processing plants, coal powered energy generation, nuclear power and processing of algal infested waters could render the waste streams reusable for the purpose of reduction of water consumption and protection of the environment from harmful pollutants. Semiconductor photocatalysis, a particle physics based class of advanced oxidation processes (AOPs) has been tried as greener technology for removal of organic pollutants in gaseous phases (e.g. air and steam) and in aquatic phases. Pioneering investigators utilised titanium as a photocatalyst using UV light as the energy source resulting in an electron band-gap of 3.2 mV. The UV lamps consumed a lot of electricity which makes the technology operationally non-feasible. This study focussed on the synthesis and evaluation of an alternative photocalyst comprised of Ag/AgCl/BiOCl with the potential of achieving photocatalysis of organic compounds under solar irradiation. All degradation tests were carried out on synthetic phenol wastewater. The effect of the components that make up the composite was also investigated. The catalysts were characterised using X-ray diffraction (XRD) and Fourier-transform infrared (FTIR). The degradation efficiency of Ag/AgCl/BiOCl, AgCl/BiOCl and BiOCl under UV light were 60 %, 56 % and 55 %. The visible light irradiation achieved degradation of 52 %, 51 % and 15 % for the same catalysts after 4.5 h of irradiation. These results suggest a more practical and realistic application of photocatalysis in the industry through the development of a visible-light responsive catalysts. 1. Introduction Petrochemical pollutants such as phenol, chlorophenols, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), if discharged to the environment can cause lasting damage to natural ecosystems (Singh et al., 2018). These compounds are most known for their persistence and recalcitrance in the environment. The non-derivatised phenol was chosen in this study as a simplified model to demonstrate the degradation pathway under photocatalytic conditions. Photocatalysis makes use of a semiconductor catalyst suspended in aqueous medium which when illuminated with UV or visible light produces photogenerated electrons and holes on the surface of the catalyst. These photogenerated sites then act as sites for the oxidation of organic pollutants through the production of free radicals. Titanium dioxide is the most widely researched photocatalyst due to its high chemical stability and low cost. However, its commercial use has been limited by the need for UV light activation due to the wide band-gap of 3.20 mV (Magalhaes et al., 2017). This has led to concerted efforts in the development of photocatalysts with bandgap energies compatible with the energy of photons of visible light emitted by the sun (Ganeshraja et al., 2018). Two strategies have been used in this regard, sensitisation of TiO2 through the introduction of doping agents or the synthesis of new catalysts and catalysts combinations. The current study builds on previous work from our research group where a composite photocatalyst comprising of Ag/AgCl/BiOCl was synthesised and tested for its efficiency under UV and visible light irradiation (Adenuga et al., 2019). Bismuth-based oxides with 3D hierarchical structures such as Bi2O3, Bi2WO6, Bi2MoO6, BiPO4 and BiOX (X=Cl, Br and I) have received considerable attention as semi-conductor catalysts due to the nature of their valence bands which are hybridised by O 2p and Bi 6s and therefore reduce the potential for electron-hole recombination (Lin et al., 2016). Bismuth oxychloride (BiOCl) is a p-type semiconductor that is characterised by an internal structure of [Bi2O2] 2+ layers which are interleaved by double slabs of Cl- (Gao et al., 2015). DOI: 10.3303/CET1976022 Paper Received: 16/03/2019; Revised: 28/07/2019; Accepted: 30/07/2019 Please cite this article as: Adenuga D.O., Tichapondwa S.M., Chirwa E.M.N., 2019, Ternary Ag/AgCl/BiOCl Synthesis and the Effects of its Constituents on Phenol Degradation, Chemical Engineering Transactions, 76, 127-132 DOI:10.3303/CET1976022 127 BiOCl has a bandgap in the 3.2 - 3.4 eV range but electron flow is better due to its metal doped components (Sánchez-Rodríguez et al., 2018). BiOCl is an ideal photocatalyst due to its high photo activity, chemical stability and its distinctive electrical, optical and magnetic properties. However, like TiO2, its wide bandgap permits activation mainly within the UV range (Cai, 2015). It has therefore been hypothesised that coupling the BiOCl with photoactive AgX (X = Cl, Br, I) may improve the response of the catalyst to visible light at various bandgaps (Ao et al., 2014). Previous work has illustrated the potential of the Ag/AgCl/BiOCl composite catalyst in the degradation of dyes such as methyl orange, rhodamine B and tetracycline (Zhao et al., 2018). Our recent work also highlighted the effectiveness of the composite catalyst in the degradation of phenol under both UV and visible light irradiation (Adenuga et al., 2019). Whilst the effectiveness of the ternary composite catalyst has been illustrated, the contribution of the individual compounds that make up the composite have not been reported. The current study investigates the influence of BiOCl and AgCl/BiOCl towards the photocatalytic degradation of phenol under UV and visible light irradiation and compares the results to the ternary composite. Based on the results obtained, a photocatalytic reaction mechanisim is proposed highlighting the interaction of the constituent compounds. 2. Experimental 2.1 Synthesis BiOCl was synthesised using a simple hydrolysis method at room temperature. The synthesis materials consisted of hexadecyltrimethylammonum chloride (CTAC) dissolved in water and bismuth (III) nitrate pentahydrate (Bi(NO3)3.5H2O) dissolved in a mixture of ultrapure water and glacial acetic acid. AgCl/BiOCl was synthesised by adding AgNO3 to the prepared BiOCl solution and stirring in the dark. For the Ag/AgCl/BiOCl synthesis step, the AgCl/BiOCL solution was irradiated under visible light for 1 h. All particles were collected, centrifuged and washed with ethanol, followed by water then dried at 80 °C for 8 h. The synthesis method is depicted in Figure 1. Figure 1: BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl synthesis 2.2 Characterisation The sample was analysed using a PANalytical X’Pert Pro powder diffractometer in θ–θ configuration with an X’Celerator detector and variable divergence and fixed receiving slits with Fe filtered Co-Kα radiation (λ = 1.789 Å). The phase identification was determined by selecting the best–fitting pattern from the ICSD database to the measured diffraction pattern, using X’Pert Highscore plus software. Infrared spectra (FTIR) were recorded using a Perkin Elmer Spectrometer with a MIRacle ATR with Zn/Se. The spectra were obtained in the wavenumber range of 4,000 cm -1 to 550 cm-1 with a resolution of 2 cm-1. 2.3 Photocatalytic activity The photocatalytic activities of BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl composite were evaluated by monitoring the degradation of phenol under UV and visible light irradiation. The experimental setup involved the use of a 36 W lamp for UV irradiation and six 36 W OSRAM Fluora lamps for visible light irradiation. For each test, 0.83 g/L concentration of catalyst power was added to a 10 mg/L phenol solution. The solution was stirred for 30 min in the dark to reach absorption-desorption equilibrium prior to irradiation. During the irradiation experiment, 2 mL samples were collected and centrifuged at 9,000 rpm for 10 min. The collected solution was further filtered, and the resultant solution was then analysed using a Waters High-Performance Liquid 128 Chromatography (HPLC) with Empower software. The mobile phase was a combination of 1 % acetic acid in water and 1 % acetic acid in acetonitrile. The efficiency of degradation was calculated by Eq(1): % Degradation 100/)( 00 ×−= CCC (1) where C0 is the initial concentration of phenol and C is the concentration of phenol at any given time, t. 3. Results and discussion 3.1 Characterisation Figure 2 shows the XRD patterns of BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl photocatalysts. The peaks are indexed from the ICSD database. The sharp peaks showed that the materials are of high purity and crystallinity with no impurities being detected. AgCl/BiOCl and Ag/AgCl/BiOCl had similar XRD patterns. This indicates that no Ag was detected. This is likely due to the low composition of Ag. This observation is consistent with work from Zhao et al. (2018). The weak peaks at 32.2˚, 57.5˚ observed coincide with AgCl patterns. Figure 2: XRD analysis for BiOCl and AgCl/BiOCl Figure 3 shows the FTIR analysis that was carried out in order to understand the functional groups present on the catalyst surface. A broad band noticed at the 3,400 – 3,500 cm-1 range could be as a result of interactions with water by hydrogen bonds (O – H) (Sánchez-Rodríguez et al., 2018). The bands at 2,923, 1,600 and 1,357 cm-1 correspond to carboxyl and alkanes groups which are as a result of remnants from the synthesis process such as CTAC and acetic acid and could also be as a result of adsorbed CO2 (Singh et al., 2018). The band at 550 cm-3 for BiOCl is attributed to the Bi – O bond which is an important characteristic for this compound. Figure 3: FTIR spectrum for BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl 129 3.2 Photocatalytic activity The photocatalytic potential of the prepared photocatalyst composite was investigated based on its ability for the degradation of phenol. Figure 4 shows the photocatalytic activity of BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl under UV irradiation. The results show that an average of 15 % phenol was adsorbed on the surface of the catalysts in the first 30 min of the experiment. After 4 h of UV irradiation, a total photocatalytic degradation efficiency of 55 %, 56 % and 60 % was attained by BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl. This shows that all three catalysts can be activated by UV light. Figure 4: BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl degradation of phenol under UV irradiation Figure 5 shows that phenol degradation efficiency of the catalysts under visible light irradiation was 15 %, 50.6 % and 52 % at 4 h for the different experiments with BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl. These results correlate with the work done by Wang et al. (2014) where the degradation efficiency of 4-nitrophenol using BiOCl was tested under both UV and visible light irradiation. They found that while 47.61 % degradation was obtained in UV light, only 5 % degradation was observed under visible light after 120 min. Figure 5 shows that BiOCl adsorbed 20 % of the phenol after contacting the catalyst with the simulated wastewater for 30 min in the dark. A slight change in concentration was noted after visible light irradiation, this suggests that the bandgap of the catalyst is too wide for it to be activated by visible light. Zhang et al. (2008) synthesised BiOX (X= Cl, Br, I) powders and investigated their activity in the degradaion of methyl orange (MO). They found the degradation of MO under UV-visible light using BiOCl to be 17 % and its degradation in 3 h under visible light to be 15 %. Figure 5: BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl degradation of phenol under visible light 130 AgCl/BiOCl and Ag/AgCl/BiOCl had similar photodegradation activity under visible light. Jiao et al. (2019) explains that while Ag nanoparticles advance the visible light absorption and photodegradation activity, its presence in excess on the surface of a catalyst may block incident light and reduce photocatalytic activity. These results show that coupling Ag/AgCl with BiOCl significantly improves the photocatalytic activity of BiOCl under visible light. This is as a result of efficient separation of photo-generated electron-hole pairs due to the modification in the spherical structures of the as-synthesised photocatalysts composites and the presence of the metallic Ag (Liu et al., 2017). The surface plasmon resonance (SPR) and dipolar effect of Ag traps visible light and the absorbed photon is efficiently separated into an electron and a hole thereby decreasing recombination (Ye et al., 2012). 3.3 Proposed photocatalytic mechanism Figure 6 shows a proposed mechanism for the photo-degradation of phenol in visible light based on the results obtained. During degradation of phenol in visible light, AgCl absorbs light and excites electron hole pairs by the photosensitive effect which are then transferred to the BiOCl (Zhao et al., 2019). The results show that BiOCl on its own is unable to absorb visible light but could absorb UV irradiation. The transferred electrons combine with O2 which generates the O2 - radical. It should be noted that the photogenerated hole in AgCl could also oxidise organic pollutants molecule (Chen et al., 2016). Hydroxy radicals (HO.) are formed in the presence of dissolved O2 which then reacts with phenol until mineralization is achieved (Chowdhury et al., 2017). Therefore, Cl, OH- and .O2 - are directly responsible for the oxidation of the organic pollutant. The use of AgCl and BiOCl as a composite material in photocatalysis ensures that the transportation of the photogenerated carriers is fast thereby reducing recombination (Zhao et al., 2019). Figure 6: Proposed photocatalytic mechanism 4. Conclusions BiOCl, AgCl/BiOCl and Ag/AgCl/BiOCl were successfully synthesised, and the main physical characteristics were determined using XRD and FTIR. All three catalysts showed significant photocatalytic activity under UV irradiation, with more than 50 % phenol being degraded by all the catalysts. The ternary and binary catalyst composites, Ag/AgCl/BiOCl and AgCl/BiOCl, exhibited substantial photocatalytic degradation efficiencies towards phenol under visible light. However, the BiOCl alone was unresponsive to visible light activation. This confirms that it is necessary to couple the BiOCl with AgCl in order to promote the synergistic interaction of the band gaps of the two materials for visible light activation. The positive results obtained under visible light irradiation indicate that natural sunlight can potentially be used to activate this composite catalyst, rendering the photocatalysis remediation process economically viable and sustainable. 131 Acknowledgements The research was funded through the Sedibeng Water Chair and the National Research Fund competitive programme for rated researchers (Grant No. CSUR180215313534) awarded to Prof EMN Chirwa. The second year of the student’s study program was supported through the National Research Foundation (NRF) Masters Innovation scholarship (SFH180526335365). References Adenuga D., Tichapondwa S., Chirwa E., 2019, Synthesis and characterisation of potential visible-light photocatalyst and its photocatalytic activity in the decomposition of phenol, Chemical Engineering Transactions, 74, 1087 - 1092. Ao Y., Tang H., Wang P., Wang C., 2014, Deposition of Ag@AgCl onto two dimensional square-like BiOCl nanoplates for high visible-light photocatalytic activity, Materials Letters, 131, 74-77. Cai L., 2015, Enhanced visible light photocatalytic activity of BiOCl by compositing with g-C3N4, Materials Research Innovations, 19, 392-396. Chen Y., Zhu G., Liu Y., Gao J., Wang C., Zhu R., Liu P., 2016, Preparation of hollow Ag/AgCl/BiOCl microspheres with enhanced photocatalytic activity for methyl orange under LED light irradiation, Journal of Materials Science: Materials in Electronics, 28, 2859-2866. Chowdhury P., Nag S., Ray A.K., 2017, Degradation of phenolic compounds through UV and visible- light- driven photocatalysis: technical and economic aspects, Phenolic Compounds - Natural Sources, Importance and Applications, 16, 395-417. Ganeshraja A. S., Zhu K., Nomura K., Wang J., 2018, Hierarchical assembly of AgCl@Sn-TiO2 microspheres with enhanced visible light photocatalytic performance, Applied Surface Science, 441, 678-687. Gao X., Zhang X., Wang Y., Peng S., Yue B., Fan C., 2015, Rapid synthesis of hierarchical BiOCl microspheres for efficient photocatalytic degradation of carbamazepine under simulated solar irradiation, Chemical Engineering Journal, 263, 419-426. Jiao Z., Zhang J., Liu Z., Ma Z., 2019, Ag/AgCl/Ag2MoO4 composites for visible-light-driven photocatalysis, Journal of Photochemistry and Photobiology A: Chemistry, 371, 67-75. Lin S., Liu L., Liang Y., Cui W., Zhang Z., 2016, Oil-in-water self-assembled synthesis of Ag@AgCl nano- particles on flower-like Bi(2)O(2)CO(3) with enhanced visible-light-driven photocatalytic activity, Materials (Basel), 9(6), 486-504. Liu Y., Xu J., Wang L., Zhang H., Xu P., Duan X., Sun H., Wang S., 2017, Three-dimensional BiOI/BiOX (X = Cl or Br) nanohybrids for enhanced visible-light photocatalytic activity, Nanomaterials (Basel), 7(3), 64 - 83. Magalhaes P., Andrade L., Nunes O.C., Mendes A., 2017, Titanium dioxide photocatalysis: fundamentals and application on photoinactivation, Reviews on Advanced Materials Sciences, 51, 91 - 129. Sánchez-Rodríguez D., Méndez M.G., Remita H., Escobar-Barrios V., 2018, Photocatalytic properties of BiOCl-TiO2 composites for phenol photodegradation, Journal of Environmental Chemical Engineering, 6, 1601-1612. Singh P., Sonu, Raizada P., Sudhaik A., Shandilya P., Thakur P., Agarwal S., Gupta V.K., 2018, Enhanced photocatalytic activity and stability of AgBr/BiOBr/graphene heterojunction for phenol degradation under visible light, Journal of Saudi Chemical Society, 1-14. Wang Q., Hui J., Huang Y., Ding Y., Cai Y., Yin S., Li Z., Su B., 2014, The preparation of BiOCl photocatalyst and its performance of photodegradation on dyes, Materials Science in Semiconductor Processing, 17, 87- 93. Ye L., Liu J., Gong C., Tian L., Peng T., Zan L., 2012, Two different roles of metallic Ag on Ag/AgX/BiOX (X = Cl, Br) visible light photocatalysts: surface plasmon resonance and z-scheme bridge, ACS Catalysis, 2, 1677-1683. Zhang X., Ai Z., Jia F., Zhang L., 2008, Generalized one-pot synthesis, characterization, and photocatalytic activity of hierarchical BiOX (X = Cl, Br, I) nanoplate microspheres, The Journal of Physical Chemistry C, 112, 747-753. Zhao M., Zhou W., Lu M., Guo Z., Li C., Wang W., 2019, Novel AgCl nanotubes/BiOCl nanosheets composite with improved adsorption capacity and photocatalytic performance, Journal of Alloys and Compounds, 773, 1146-1153. Zhao S., Zhang Y., Zhou Y., Qiu K., Zhang C., Fang J., Sheng X., 2018, Reactable polyelectrolyte-assisted preparation of flower-like Ag/AgCl/BiOCl composite with enhanced photocatalytic activity, Journal of Photochemistry and Photobiology A: Chemistry, 350, 94-102. 132