The influence of the UV irradiation intensity on photocatalytic activity of ZnAl layered double hydroxides and derived mixed oxides

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ Chemical Industry & Chemical Engineering Quarterly 18 (2) 295−303 (2012) MILICA S. HADNAĐEV-KOSTIĆ TATJANA J. VULIĆ DMITAR B. ZORIĆ RADMILA P. MARINKOVIĆ-NEDUČIN University of Novi Sad, Faculty of Technology, Novi Sad, Serbia SCIENTIFIC PAPER UDC 66.085:661.14:661.847 DOI 10.2298/CICEQ110923006H CI&CEQ THE INFLUENCE OF THE UV IRRADIATION INTENSITY ON PHOTOCATALYTIC ACTIVITY OF ZNAL LAYERED DOUBLE HYDROXIDES AND DERIVED MIXED OXIDES Layered double hydroxides (LDHs) have been studied to a great extent as environmental-friendly complex materials that can be used as photocatalysts or photocatalyst supports. ZnAl layered double hydroxides and their derived mixed oxides were chosen for the investigation of photocatalytic performances in correlation with the UV intensities measured in the South Pannonia region. The low supersaturation coprecipitation method was used for the ZnAl LDH synthesis. For the characterization of LDH and thermal treated samples powder X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS) and nitrogen adsorption-desorption were used. The decomposition of azodye, methylene blue was chosen as photocatalytic test reaction. The study showed that the ZnAl mixed oxide obtained by thermal decomposition of ZnAl LDH has stable activity in the broader UV light irradiation range characterizing the selected region. Photocatalytic activity could be mainly attributed to the ZnO phase, detected both in LDH and thermally treated samples. The study showed that the ZnAl mixed oxide obtained by the calcination of ZnAl LDH has a stable activity within the measured UV light irradiation range, whereas the parent ZnAl LDH catalyst did not perform satisfactory when low UV irradiation intensity is implied. Keywords: ZnAl layered double hydroxides, ZnAl mixed oxides, UV irradiation, photocatalytic activity, methylene blue. Recently there has been an increasing demand for catalysts that can eliminate environmental pollution because the environmental deterioration has become a major obstacle. The most extensive solution for these ongoing problems has been the introduction of the photocatalytic phenomena where a semiconductor is activated by light irradiation leading to the photocatalytic reaction that produces compounds harmless to the environment [1]. Many semiconductors have been utilized in these reactions but the most common is TiO2 that has been very promising in the areas of environment and energy [2, 3]. However, this photocatalyst is activated by UV light (only up to 5% Correspondening author: M.S. Hadnađev-Kostić, University of Novi Sad, Faculty of Technology, Bulevar Cara Lazara 1, 21000 Novi Sad, Serbia. E-mail: Paper received: 23 September, 2011 Paper revised: 13 January, 2012 Paper accepted: 17 January, 2011 of the solar light) that limits the application to this excitation source. This disadvantage could be overcome by surface modification of TiO2 photocatalyst and catalyst support (texture, surface properties and composition) and therefore has been the topic of numerous studies [4,5]. For the past several years, layered double hydroxides (LDHs) have been studied to a great extent as environmental-friendly complex materials that can be used as photocatalysts or photocatalyst supports [6]. LDHs, also known as anionic clays and hydrotalcite-like materials, have a layered structure which enables specific properties of these materials and their thermally derived mixed oxides such as redox and acid-base properties, developed surface area, mesoporosity, ion exchange capability, etc. The LDHs layered structure is formed by isomorphic substitution of divalent cations in the brucite lattice with the trivalent cations, and the obtained positive charge is compensated by the intercalation of anions in the interlayer, the most common being CO32-. 295 M.S. HADNAĐEV-KOSTIĆ et al.: THE INFLUENCE OF THE UV IRRADIATION INTENSITY… In this study, ZnAl layered double hydroxides and their derived mixed oxides were chosen based on the literature data stating that among the semiconductors, ZnO (Eg ≈ 3.2 eV) has a photocatalytic potential assigned to its activation in the visible region of the solar spectra [1,7]. Layered double hydroxide composition and synthesis method were chosen based on our previous experience with LDH synthesis [8–10]. ZnAl layered double hydroxides were synthesized by low supersaturation coprecipitation method and thermally treated. Calcination of ZnAl LDH leads to the collapse of the layered structure and the formation of highly active mixed ZnAl oxides with high surface area and thermal stability. In selecting the test reaction and the measurement procedure the published literature data confirmed that there is good correlation in evaluating catalytic performance concerning the results obtained by spectroscopy and by TOC/ /COD [11,12]. This study is aimed to investigate photocatalytic activity of ZnAl layered double hydroxides and its derived mixed oxides at different UV irradiation intensities. The UV light irradiation intensities were chosen taking to consideration the intensities in the winter and summer periods measured in the South Pannonia region (Novi Sad, Serbia). These materials could further be applied as catalyst support with its involvement in catalytic act as the active phase, having also intrinsic activity shifted to the visible region of the solar light spectra. Additionally, this catalyst support provides a proper matrix for the optimal distribution of other possible active components (e.g., TiO2). EXPERIMENTAL For the synthesis of ZnAl LDH, the low supersaturation (LS) coprecipitation method at constant pH (9–9.5) was used. The precursors, Zn(NO3)2·6H2O and Al(NO3)3·9H2O (concentrations: [Zn(NO3)2] = 0.7 mol dm-3; [Al(NO3)3] = 0.3 mol dm-3) were added at constant flow (4 cm3min−1), whereas carbonate and hydroxide ions (0.67 M Na2CO3 and 2.25 M NaOH solution) were added with the flow adjusted to maintain constant pH value. The coprecipitation took place at constant temperature (40 °C) and under vigorous stirring. After the coprecipitation, the precipitates were aged for 15 h, washed with water until pH 7, dried for 24 h at 100 °C and calcined for 5 h, at 500 °C in air. The dried samples were denoted as ZnAl_LDH and calcined as C_ZnAl (the lower dash indicating that the sample was synthesized within the study). Scanning electron microscopy (SEM) was used to investigate the morphology of the samples and electron dispersive spectroscopy (EDS) for the surface 296 CI&CEQ 18 (2) 295−303 (2012) chemical analysis JEOL, JSM-6460LV instrument, at the working voltage of 20 kV and working distance of 10 mm. The crystalline phases of the dried and calcined samples were investigated and identified by X-ray powder diffraction (XRD) using a Siemens D500 X-ray diffractometer with filtered CuKα radiation in 2θ ran (...truncated)