Ce-Doped CuMgAl Oxide as a Redox Couple Mediated

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Ce-Doped CuMgAl Oxide as a Redox Couple Mediated Catalyst for Visible Light Aided Photooxidation of Organic Pollutants Karan Goswami, and Rajakumar Ananthakrishnan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01557 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 31, 2019

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ACS Applied Nano Materials

Ce-Doped CuMgAl Oxide as a Redox Couple Mediated Catalyst for Visible Light Aided Photooxidation of Organic Pollutants Karan Goswamia and Rajakumar Ananthakrishnana* a

Department of Chemistry, Green Environmental Materials & Analytical Chemistry Laboratory, Indian Institute of Technology Kharagpur 721302, India. E-mail: [email protected] KEYWORDS: Ce doped MMO, Redox couple, Charge separation, Organic pollutants, Oxidation mechanism.

Ce doped CuMgAl (10:40:50)-mixed metal oxides with different Ce contents (0.5 to 2.0 wt %) were synthesized by simple co-precipitation method followed by doping with cerium. Then their structural, optical and morphological properties were characterized. The results revealed that, the surface areas of the samples are very high (~ 138 m2/g), which is an important property of a photocatalyst. All the doped samples exhibited enhanced photocatalytic activity compared to the undoped sample for degradation of a model organic pollutant, 2,4-dichlorophenol (2,4-DCP). The amount of Ce doping in the material played an important role in the photocatalysis. Among the prepared samples, 1.0 wt% Ce doped sample exhibited highest photocatalytic activity due to lowest recombination of photogenerated electron-holes. The developed Ce4+/Ce3+ redox couple is a special feature of the catalytic design, which mainly facilitates separation of photogenerated electrons and holes. The catalytic steps on conduction band and valence band of the photocatalyst are favourable to form Ce4+/Ce3+ redox couples in continuous cycles and hence, accelerate the photocatalytic process effectively. Moreover, activity of the catalyst was investigated under various light irradiations, such as UV, visible, and UV-Vis light. Rate of the photodegradation is found to be 3.7 times and almost 5 times higher by using 1.0 wt% Ce doped sample compared to the undoped sample under visible light and UV-Vis light irradiation, respectively. A possible mechanism of photocatalysis and pathways of degradation of 2,4-DCP are also discussed. ABSTRACT:

INTRODUCTION Over the last decade, much research has been devoted to find innovative, economic and safe methods for complete destruction of chlorinated organic compounds. These compounds are highly toxic, particularly it affects human nervous system, and also have indirect or direct links to many diseases.1 Chlorophenols are very much stable, hardly biodegradable, and have been shown to accumulate in the environment.2, 3 Photocatalytic degradation using semiconductors is found to be an efficient, economic and environmentally favourable method for elimination of such organic compounds.4, 5 Recently, our group has successfully applied the photocatalysis technique for removal of a model chlorinated organic compound, 2,4-dichlorophenol (2,4-DCP) from the aqueous medium. 6, 7, 8

Layered double hydroxides (LDH) have widely been used as heterogeneous catalyst/ catalyst precursor/ catalyst support9 due to some of its special properties; like (1) metal cations can be uniformly distributed in the layer, (2) LDH has adjustable layer elements composition, (3) It has high dispersive property, which inhibits agglomeration of active species, and (4) they also possess high thermal stability.10 LDHs are made up of two dimensional brucite like layers with compact hexagonal packing. LDHs can be presented by the general formula, [MӀӀ1-xMӀӀӀx(OH)2]x+(An-)x/n . The positive charge in the layer is balanced by anions, located between the brucite like cation layers.10, 11 Calcination of LDH compounds yield mixed metal oxides (MMOs) with high surface area and they show remarkable property as photocatalyst with recycling capabilities.12

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Generally, an efficient photocatalyst should possess high surface area, high visible light absorption property, low recombination of photogenerated electron-hole pair, etc. MgAl-MMO has been widely used for their mesoporous nature and high surface area.13, 14 However, the MgAl-MMO is not capable of harvesting naturally available visible light owing to its high band gap energy. Recently, transition metals15, 16 or non-transition metals17, 18 are used as co-catalyst to improve light absorption of such photocatalysts in visible region by narrowing their band gap energy. Many researchers have been focused on Cu-containing MMOs because CuO is found to be highly active, able to absorb visible light and also very much cost effective. It is experimentally proved that Cu2+ along with another bivalent cation is able to form LDH structure at moderate temperature, otherwise high temperature is required.19, 20 There are very few reports on CuMgAl based materials, where the material had been used as catalysts or photocatalysts.21-25 Again, rare earth (RE) elements are known to reduce recombination of photogenerated charge carriers (i.e., electron-hole pairs) as it can easily trap the photogenerated electrons.26, 27 Moreover, the RE elements have high oxygen storage and release capacity due to presence of oxygen deficiency in the crystal lattice.28 Thus, RE elements are being used as catalyst support to enhance photocatalytic activity of a photocatalyst. Cerium has gained a special attention among the RE elements because it is relatively more abundant, less expensive and also exhibits excellent redox properties.28, 29 There always exists Ce3+ states along with Ce4+ in CeO2 due to presence of oxygen vacancies in the oxide, and the Ce4+/Ce3+ redox cycle plays an important role in photocatalysis.30 Joung et al. reported that the level of Ce3+ state increases on CeO2 with increasing oxygen vacancy, which ultimately results in enhanced photocatalysis.31 However, band gap energy of CeO2 is very high (~3.4 eV) and thus, the CeO2 alone cannot be used as an efficient visible light active photocatalyst.32 That is why, researchers are using CeO2 along with other semiconductors to produce better visible light active photocatalysts. Rajendran et al. reported ZnO/CeO2 composites, where they demonstrated that the composite showed high visible light photocatalysis compared to individual oxides due to the presence of Ce3+-ion and different energy states.33 Saravanan et al. also reported ZnO/CeO2 based nanocomposite for photocatalytic degradation of organic dyes.34 However, in these ZnO/CeO2 composites, surface area is very less (~ 13.0 m2/g) and rate of photodegradation is also very low (in the order of 10-4 min-1). There are some reports on Cu-Ce based catalysts, where the catalysts have been used in various fields such as, CO oxidation,35-37 Al air batteries,38 methane combustion,39 ferromagnetism study,40 etc. However, Cu-Ce based catalysts have rarely been explored for photocatalytic degradation of toxic compounds from aqueous medium.41 Based on these ideas, we have synthesized Ce doped CuMgAlmixed metal oxides (CMA-x% Ce) with different Ce content (0.5wt%, 1.0wt%, 1.5wt% and 2.0wt %) from their LDH precursors. The properties of the Ce doped samples were compared with undoped CuMgAl-mixed metal oxide (CMA). The photocatalytic performances of the prepared samples were

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tested for degradation of a model chlorophenolic compound, 2,4-dichlorophenol (2,4-DCP) under exposure of light (UV, visible and UV-Vis). A plausible mechanism of photodegradation of 2,4-DCP has been proposed here based on the identified intermediate products.

EXPERIMENTAL SECTION Preparation of CuMgAl-LDH The CuMgAl-layered double hydroxide (CMA-LDH) was prepared by co-precipitation method.42 Briefly, Cu(NO3)2.6H2O, Mg(NO3)2.6H2O and Al(NO3)3.9H2O were dissolved in 10:40:50 molar ratio in 50 mL distilled water. Subsequently, pH of the solution was adjusted to 9.0 by drop-wise addition of alkaline solution [NaOH (0.24 M) and Na2CO3 (0.1 M)] at room temperature under stirring condition and blue precipitate was obtained. It was aged at 80 °C for 18 h, then filtered and excess ions were eliminated by washing the precipitate with deionized water. The obtained CuMgAl-LDH was dried overnight at 60 °C in an oven and the dried sample was denoted as CMA-LDH.

Preparation of Ce-doped Samples The prepared CMA-LDH was calcined at 500 ̊C for 6h, and the obtained mixed metal oxide (MMO) was denoted as CMA. The freshly calcined sample (CMA) was immediately placed into aqueous solution of ammonium cerium (IV) sulphate, dihydrate. The solution was stirred vigorously for 2h, refluxed at 80 ̊C for 9h at pH ~ 8.0, then filtered and dried at 60 ̊C. In the aqueous solution, due to memory effect,43,44 the layered structure of CMA was regained and thus the obtained samples were denoted as CMA-x% Ce-LDH, where x% denotes the doping percentage of Ce. Here, we have prepared four different set of Ce doped CuMgAl-LDHs (0.5 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt% Ce doped CuMgAl-LDHs). The LDHs were then calcined at 500 ̊C for 6 h and finally Ce doped mixed metal oxides (MMOs) were obtained, which are referred as CMA-x% Ce. The whole preparation procedure is presented schematically in Scheme 1.

Photodegradation Experiment Photocatalytic efficiency of the prepared materials was investigated by degradation of a model aqueous organic pollutant, 2,4-dichlorophenol (2,4-DCP). The contaminant (20 mL, 10 mg/L) and catalyst (1 g/L) were taken in a quartz vial and ultrasonicated for 5 min. After dispersion by ultrasonication and before light irradiation, adsorption-desorption equilibrium of the contaminant was achieved on the catalytic surface by stirring the suspension in the dark. Afterwards, 3 mL solution was collected and centrifuged to remove the catalysts and denoted as 0 min. Then, the remaining solution was irradiated with light. Here, three types of light sources were used, 250 W MPMVL UV lamp, 250W tungsten visible lamp and UV-Vis lamp (250 W tungsten visible lamp + 250 W MPMVL UV lamp), the system was made by Lelesil Innovative System, India. At an interval of every 20 min, sample was collected (3 mL), and concentration of the pollutant was determined by HPLC-PDA. A reversed phased Acclaim Polar Advantage II column was used in HPLC (Thermo Fisher Dionex UltiMate@ 3000 SD) where


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mixture of water, acetonitrile and acetic acid (30:69:1 (v/v)) was used as a mobile phase. Scheme 1. Preparation of Ce doped CuMgAl-mixed metal oxides (CMA-X% Ce) by wet chemical steps.

characteristic peaks for a layered hydrotalcite like material. The crystallite sizes of the LDH materials were calculated using Debye-Scherrer’s formula: D = 0.9λ/ β cosθ (1)

Characterization Techniques Crystalline phases of the samples were identified using a X-ray diffractometer (Bruker Apex-2), where Cu-Kα radiation was used (λ = 1.54056 Å). Morphologies of the phases were obtained by FESEM (NOVA NANOSEM 450), Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM) (JEOL JEM2010 electron microscope, operating at 200 kV). Chemical compositions of samples were determined by X-ray energy dispersion spectrometer unit, which was equipped with the FESEM. Surface area of the samples were analysed from N2 gas adsorption/desorption isotherm by using BET analysis (Quantachrome® ASiQwin™ version 3.0 instrument). Chemical state of elements was determined by XPS analysis (PHI 5000 VersaProbe II). Their light absorbance property was recorded from UV-Visible diffuse reflectance spectroscopy (CARY 5000 UV-Vis-NIR spectrophotometer), and solidstate luminescence behaviour was recorded from photoluminescence spectroscopy (Horiba Fluorolog-3). Photoelectrochemical measurements were conducted in a conventional three electrode system using a CHI 760D electrochemical workstation (CH Instruments, Inc., USA). COD was tested by closed refluxed calorimetric method following standard procedure of American Public Health Association (APHA).

RESULTS AND DISCUSSION Structure, Morphology and Phase Analysis The formation of layered structure of the LDH precursors was first confirmed by powder XRD pattern. The XRD pattern of as synthesized LDHs are shown in Figure 1A. The layered structures of all the materials were confirmed, as the observed peaks at 2θ = 11.60° (003), 23.60° (006), 34.80° (009), 39.30° (012), 46.80° (018), 60.80° (110) and 62.15° (113) are the

Figure 1. PXRD pattern of (A) LDH precursors and (B) MMO samples, where in both the figures (a) undoped sample, (b) 0.5% Ce, (c) 1.0% Ce, (d) 1.5% Ce, (e) 2.0% Ce doped sample and (f) pure CeO2.

where, D is crystallite size, λ is wavelength of X-ray, β is full line width at half maximum height of the main intense peak, and θ is Bragg’s angle. The calculated crystallite size values are given in Table S1. Crystallite size of the CMA-LDH was found to be 23.47 nm, which gradually decreased with increasing amount of doped cerium, and the crystallite size of CMA-2.0% Ce-LDH was found to be 10.03 nm. The decrease in crystallite size is mainly attributed to the formation of Ce-O-M in the doped samples, which inhibited the growth of crystal grains.45 Similar effect has been reported for RE doped materials by other researchers.45, 46 Lattice parameters of the LDHs (a and c) were also calculated and are shown in Table S1. The lattice parameter values of all the Ce doped samples increased slightly compared to the undoped sample (CMA-LDH), which could be due to the incorporation of large Ce ion (ionic radius of Ce+4 = 97 pm and Cu2+ = 73 pm) in the octahedral coordination.45 Additionally, some new diffraction peaks appeared for Ce doped samples, corresponding to



Cu(OH)2 (2θ = 16.73°, 35.80° and 48.77°) (ICDD No. 00-0130420). The incorporation of Ce might results in separation of some Cu2+ from the brucite layer (substitution of cation by doping) to form Cu(OH)2, thus leading to a homogeneous distribution of cerium into the matrix.47 Powder XRD patterns of the calcined materials are shown in Figure 1B. The layered structure of LDHs collapsed after calcination and new crystallite phases appeared. The crystallite phase of CMA resembled with poorly crystalline MgO, however, the diffraction peaks shifted slightly to higher angle (2θ = 36.20°, 43.30° and 63.50°) compared to pure MgO (2θ = 36°, 43° and 63°) (JCPDS No. 78-0430). The peaks appeared due to formation of Mg(Cu, Al)O solid solution, where both Cu2+ and Al3+ cations got incorporated within the MgO structure. On the other hand, the peak shift to the higher angle was mainly due to the presence of Al3+, which has smaller ionic radii compared to Mg2+ (ionic radii 72 pm for Mg2+, 73 pm for Cu2+ and 54 pm for Al3+). There are several reports on this type of solid solution.47-49 Additionally, Ce doped samples showed diffraction peaks of CuO (2θ = 35.50°, 38.70°, 48.60° and 61.80°) (JCPDS No. 80-1917). XRD pattern of pure CeO2 was compared to the samples (Figure 1Bf) and no characteristic diffraction peak for the cerium oxide was noticed in our prepared Ce doped samples.

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Figure 2 shows N2 gas adsorption-desorption isotherms and pore size distribution curves (inset) of CMA and CMA-1.0% Ce. According to the IUPAC classification, both the isotherms are of type IV, corresponding to typical mesoporous solids.50 All the materials exhibited H3-type hysteresis loop, commonly found on aggregates or agglomerates of plate like particles, forming slit shaped pores.51 At high pressure, the desorption curve of CMA showed a lagging loop which appeared mainly due to pores having smaller diameters.52 The pore size distributions are very different among the samples. In 1.0% Ce doped sample, the pore size distributions are well defined with two maxima at 36 Å & 123 Å; whereas, CMA showed broad and poorly defined pore size distribution. The specific surface area of all the samples were found to be high, ~ 136 m2/g. In other reports, it is shown that the surface area of CeO2-CuO catalyst is 89 m2/g,35 and for MgAl-MMO is around 200 m2/g.13 Thus, the high surface area of Ce doped samples were obtained due to the presence of magnesium and aluminium oxides in the samples. The specific surface area of CMA-1.0% Ce (135 m2/g) has slightly decreased compared to CMA (139 m2/g). The decrease in surface area is associated with the decreased crystalinity of the doped samples, and the results are in agreement with literature, where RE elements were used as doping.51, 53 Other Ce doped samples also have shown properties similar to the CMA-1.0% Ce and the figures are shown in supporting information (Figure S2).

Figure 3. FESEM images of (a) CMA and (b) CMA-2.0% Ce. TEM images of (c) CMA and (d) CMA-2.0% Ce which shows that the Ce is homogeneously distributed onto the sample (marked by red arrow).

Figure 2. N2 adsorption-desorption isotherms and corresponding pore size distributions (inset), (a) CMA and (b) CMA-1.0% Ce.

Figure 3a & 3b shows the FESEM images of CMA and the highest doped sample, CMA-2.0% Ce, respectively. The images revealed round shaped flake type morphology of the samples. Doping with Ce has no effect on the morphology of CMA. The morphology of the prepared samples was further analyzed by TEM. Figure 3c & 3d shows the TEM images of CMA and CMA-2.0% Ce, respectively. It


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Figure 4. HRTEM images, fringe pattern and SAED of CMA-2.0% Ce: (a)-(c) on small round shaped particle, denoted as ‘1’, and (d)-(f) on place where the small particles are not present, denoted as ‘2’.

can be observed from the TEM image of CMA-2.0% Ce that, some small round shaped particles with size in the range of 2530 nm are distributed throughout the sample. The small particles could be due to the doped cerium, which was confirmed by high resolution TEM (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) analysis. The HRTEM, of CMA-2.0% Ce was recorded on two places; first, on a small round particle (denoted as ‘1’ in Figure 4a) and second, on other places, where the small particles are not present (denoted as ‘2’ in Figure 4d). Fringe width pattern from HRTEM, recorded on the place ‘2’ confirmed the presence of (200) plane of MgO and (111) plane of CuO, but no planes corresponding to CeO2 was observed (shown in Figure 4e). Whereas, the fringe width pattern recorded on place ‘1’ (Figure 4b) confirmed the (200) plane of MgO and (200) plane of CeO2 phases. The result indicated that the small particles appeared due to doped CeO2. This finding was also confirmed by SAED pattern, shown in Figure 4c and 4f. Moreover, EDX on place ‘2’ did not detect the presence of any Ce, but the Ce was detected in the place ‘1’, shown in Figure S3. All these analyses confirmed that the small particles, uniformly distributed on the CMA-2.0% Ce were nothing but the doped cerium particles. EDX mapping confirms the presence of different elements on CMA-2.0% Ce, which are shown

in Figure S4. The samples containing different amount of cerium (0 wt% - 2.0 wt%) are also shown in Figure S5. To analyze the oxidation states of the elements present in the samples, X-ray photoelectron spectra (XPS) were obtained for both the doped and undoped samples. Figure 5a gives Cu 2p XPS survey scans of CMA and CMA-1.0% Ce. For the undoped sample, the peaks at 933.8 eV (Cu 2p3/2) and 954.1 eV (Cu 2p1/2) are attributed to Cu2+ state. Two satellite peaks at 942.3 eV and 961.9 eV confirmed the Cu2+ state of copper present in the undoped sample.54 In addition to the above-mentioned peaks, the Ce-doped sample showed one additional peak at 931.7 eV, which could be attributed to Cu+ state. The Cu+ state appeared since cerium is known to increase the relative stability of Cu+ via electron transfer.53, 54 The O 1s XPS spectra of the sample (Figure 5b) was deconvoluted into two components at 529.8 eV and 531.2 eV, which are attributed to the lattice oxygen bound to metal cation and surface adsorbed oxygen, mainly of hydroxyl group, respectively.53 Furthermore, the Ce 3d XPS spectra of CMA-1.0% Ce are illustrated in Figure 5c. The peaks at around 881.6 eV (ν0), 888.9 eV (ν1), 897.7 eV (ν2) and 900.0 eV (νʹ0) are assigned to Ce4+ state, whereas, the peaks at around 883.8 eV (u0) and 903.9 eV (uʹ0) are attributed to Ce3+ state, suggesting a partial reduction of Ce4+ ions in the sample.55 Thus, the modified environment of copper in the Ce-doped materials confirmed



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Figure 5. (a) XPS spectra of Cu 2p of CMA and CMA-1.0% Ce. The XPS peaks suggest that the Ce doped sample contains both the Cu2+ and Cu+ states. (b) XPS spectra of O 1s of CMA-1.0% Ce, and (c) XPS spectra of Ce 3d of CMA-1.0% Ce and the peaks indicate the presence of both Ce4+ and Ce3+ in the sample.

Figure 6. (a) DRS spectra of doped and undoped samples, and (b) Tauc plot of CMA.

the Cu-Ce interaction, which might occur during thermal treatment via a redox equilibrium,53 Ce3+ + Cu2+ = Ce4+ + Cu+ Moreover, +2 oxidation state of magnesium was confirmed from Mg 1s peak at 1305 eV, and +3 state of aluminium was confirmed from Al 2s and Al 2p peaks at 117.02 eV and 72.07 eV, respectively (Figure S6). Zhang et al. showed that presence of other oxides (e.g., Fe2O3, Co3O4 and NiO) with CeO2-CuO catalyst help in the formation of Ce3+, and promote the formation of reduced copper species in the catalyst.35 Likewise, it is expected that the presence of MgO and Al2O3 in Ce doped CMA samples helped to increase the formation of Cu+ sites, and made electronic transformation between Cu2+/Cu+ and Ce4+/Ce3+ more facile. The Cu-Ce interaction was expected to have positive influence on the catalytic activity of cerium doped samples, since Cu+ sites are more catalytically active relative to Cu2+ sites.56,57,53 Thus, it is arrived that there existed an interaction between Cu and Ce in the doped samples due to development of a redox equilibrium. The crystalinity of the doped samples decreased because of this interaction. The interaction also affected the BET surface area of the doped samples. However, doping had no influence on morphology of the samples.

Optical Property of the Samples The optical absorption property of the prepared samples was determined using UV-Vis diffuse reflectance spectroscopy (DRS). The CMA exhibited absorption over a wide region, starting from 200 nm to around 1000 nm (Figure 6a), which mainly appeared due to merging of various characteristic absorption bands of CuO.58 The characteristic absorption bands of CuO gradually became more intense with Ce doping, and the light absorption also increased significantly. All the bands appeared from 220 nm to 490 nm are due to charge transfer transitions (O2- to Cu2+) and from 560 nm to 1000 nm are due to dd transitions.58 Additionally, in the Ce doped samples a band appeared in the range of 400 nm to 480 nm, which is attributed to charge transfer transition involving Cu+ ion.58 Band gap energy of the samples was calculated using Tauc relation: 59 (αhv) = A(hv − Eg)n (2) where, α is the absorption coefficient, h is the Planks constant, A is a constant, hv is the photon energy, n is an index (value is 2 in our case) and Eg is the optical band gap energy. Since, the CMA had shown two band edges in the UV-Vis spectra, one in visible region and the other one in NIR region, we obtained two band gap energy for CMA, 2.20 eV (for visible region) and 1.32 eV (for NIR region), shown in Figure 6b. The band gap energy


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of the Ce doped samples gradually decreased with Ce doping (Figure S7), and the values are shown in Table 1. The decrease in band gap energy with Ce doping can be explained by the fact that, the cationic dopant might create a distinct impurity band lie below the conduction band of CuO by its 4f 5d electrons localized states. This impurity band overlapped with the bottom of the conduction band and lowered the conduction band position, which ultimately decreased the band gap.60 Furthermore, trapping level might be created by charge transfer between conduction band electrons of CuO with 4f5d electrons of Ce4+ ions, which also could reduce the band gap energy.61

Photocatalytic Activity of the Samples The effect of Ce doping on photocatalytic activity of the catalysts was explored by investigating photodegradation of 2,4DCP in aqueous solution under visible light irradiation over the doped and undoped samples. The photodegradation was monitored by HPLC. Comparison of the photocatalytic activities of all the catalysts are displayed in Figure 7a. Individual HPLC chromatograms of the photodegraded 2,4-DCP using CMA and Ce doped CMA samples are shown in Figure S8. All the Ce doped samples exhibited photocatalytic activity much higher than that of the undoped sample, and the enhanced activity was dependent on the percentage of doped cerium. Among the prepared samples, CMA-1.0% Ce exhibited highest photocatalytic activity. About 96% removal of 2,4-DCP was achieved in 60 min by adsorption (22%) followed by photodegradation (74%) using CMA-1.0% Ce. It was also observed that the photocatalytic activity of CMA increased with increasing amount of Ce doping up to 1.0 wt%, whereas, the activity started to decrease with further increase in the doping percentage of Ce. The phenomenon could be explained in terms of space charge thickness.62, 46 Yu et al.63 reported effective separation of photogenerated electron-hole pairs at an optimum concentration of dopant ions. At this optimum concentration, the thickness of space-charge layer is equal to the depth of the light penetration. When the doping concentration increases beyond the optimum concentration, the penetration depth of light into the sample exceeds the spacecharge layer; thereby the recombination of photo-generated electron-hole pairs becomes easier. Photoluminescence (PL) spectroscopy was used to investigate the separation efficiency of photogenerated electron-hole pairs of the prepared catalysts. Since, the PL emission results from recombination of free electrons-holes, hence, a lower recombination rate (or higher separation) can result in lower PL intensity.64 It can be observed from the Figure 7b that the Ce doping greatly influenced the separation of electrons-holes and the separation was maximum for CMA-1.0% Ce. In other words, 1.0 wt% was the optimal concentration of dopant and consequently, the CMA-1.0% Ce exhibits the highest photocatalytic activity among the prepared catalysts. As the doping concentration of Ce was increased further (˃ 1.0 wt%), the penetration depth of light into the sample exceeded the space-charge layer, thereby the recombination of photogenerated electron-hole pairs became easier, which led to a lower photocatalytic activity.

In order to provide additional support for enhanced separation and transportation of charge carriers in the CMA-1.0% Ce, transient photocurrent responses of CMA and CMA-1.0% Ce were recorded under the visible light. Detailed process is given in Supporting Information. Figure 7c indicates the fast and reversible photocurrent responses of both the samples. However, photocurrent density of CMA-1.0% Ce is 3 times higher than CMA, indicating that generation, separation and transportation of photogenerated electrons is higher in 1.0% Ce doped sample compared to the undoped sample, CMA. Rate constants of the degradation using all the catalysts are shown in Figure S9a. The photodegradation follows pseudofirst-order kinetics, which can be expressed by the equation: ln(C0/Ct) = kt (3) Where, C0 and Ct are the concentration of 2,4-DCP at t = 0 and t = t, respectively; k is the rate constant of the photocatalytic reaction. The apparent rate constant by CMA-1.0% Ce was found to be 0.0551 min-1, which is 3.7 times than the value by CMA (k = 0.0148 min-1). The percentages of degradation and rate constant values of the photodegradation using the prepared catalysts under visible light are shown in Table 1. For comparison, we have also checked the photodegradation of 2,4-DCP under the visible light by MgAl-MMO (photodegradation was not observed), 1.0% Ce doped MgAl-MMO (14%), CuO (27%) and CuO-1.0% Ce (52%) (Figure S9b). The results confirmed that both MgAl-MMO and Ce doping gave significant effects on enhancement of the photocatalytic activity. The MgAl-MMO helped in the photodegradation of CMA-1.0% Ce by enhancing surface area of the catalyst and also by increasing Cu+ state in the sample. Whereas, the doped Ce helped to separate the charge carriers for longer time and also played active role in the photocatalysis (see mechanism). Further, we had prepared a physical mixture of CMA and 1.0 wt% CeO2, and used it as a catalyst for the photodegradation reaction under the visible light. The physically mixed catalyst was not found to be much promising (Figure S9b), however, immobilization of Ce on the CMA by doping process demonstrate remarkable enhancement in photoactivity under the visible light condition. Again, mineralizing ability of the CMA-1.0% Ce was measured by monitoring the chemical oxygen demand (COD) of the reaction mixture. The COD was performed by closed reflux calorimetric method following a standard procedure mentioned in American Public Health Association (APHA). About 80% COD removed after 60 min of the photodegradation observed, while 98% COD was removed within 120 min of light irradiation. The CMA-1.0% Ce was also tested for reusability of the used catalyst after each photodegradation cycle at specific conditions, and it was noticed that the catalyst had retained its photoactivity even after reuse and sustained for recycling for number of cycles. The stability of the catalyst and its reusability in photocatalysis has been reported in Figure S10.



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Figure 7. (a) Comparison of photocatalytic activity of samples (with variable Ce %) under the visible light irradiation, (b) Photoluminescence spectra of the prepared samples, (c) Transient photocurrent responses of CMA and CMA-1.0% Ce, and (d) Photocatalytic activity of CMA1.0% Ce under different light conditions. [2,4-DCP = 10 ppm, and catalytic amount = 1 g/L, used in the degradation studies].

In addition, the degradation of 2,4-DCP using CMA-1.0% Ce was counted under UV and UV-Vis light irradiations (Figure 7d), and corresponding HPLC chromatograms are shown in Figure S11. Under UV light, the catalyst showed pretty low activity, implying that the CMA-1.0% Ce is not very much active under UV region, which is in agreement with the DRS profile. Under UV-Vis light, CMA-1.0% Ce showed complete degradation of 2,4-DCP in 45 min only with very high rate constant (k = 0.0796 min-1), which is almost 5 times higher than CMA (65% degradation of 2,4-DCP in 60 min, k = 0.01741 min-1), shown in Figure S12. The present work is compared with some of the recently reported materials by other groups on the degradation of 2,4DCP, keeping the concentration of 2,4-DCP and the catalyst amount similar. Our material is found to be superior for degradation of 2,4-DCP from aqueous medium (Table S2). Kumar et al. reported a Ce-Cu based photocatalyst for degradation

of MB dye, where the catalyst was able to degrade dye up to 97.83% in 300 min with a rate constant of 0.0143 min-1 under the visible light.65 In another report, Ekthammathat et al. reported only 31% degradation of MB dye in 180 min using a Ce-Cu based catalyst under the visible light.66 Thus for a comparative purpose, we also have checked the photocatalytic activity of CMA-1.0% Ce (1.0 g/L) on the degradation of MB dye (10 ppm) under the visible light, and the degradation was monitored by UV-Vis spectrophotometer, shown in Figure S13. The detailed experimental procedure of MB dye degradation is given in the Supporting Information, refer Figure S13. About 90% degradation of MB dye was achieved in 60 min along with a rate constant of 0.0362 min-1, and almost complete degradation of dye was achieved within 90 min. overwhelmingly, the result implies that our prepared hybrid material is much more efficient in photocatalysis of dye than the catalysts reported in the literature.


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Table 1. Band gap, surface area of the MMOs and photocatalytic efficiency of the prepared MMOs on the degradation of 2,4DCP under the visible light. Samples

Band gap

Surface area

Removal of 2,4-DCP

Rate constant

(eV)

(m2/g)

(%)

(min-1)

CMA

2.20 & 1.32

139

55

0.0148

CMA-0.5% Ce

2.03 & 1.17

136

78

0.0239

CMA-1.0% Ce

1.97 & 1.12

135

96

0.0551

CMA-1.5% Ce

1.95 & 1.09

135

89

0.0418

CMA-2.0% Ce

1.91 & 1.03

134

85

0.0335

Scheme 2. (a) A proposed mechanism using electron-hole involvement in the degradation of 2,4-DCP. The mechanism shows the role of doped Ce in the photocatalysis and the reactive oxygen species formed by catalyst under the visible light. (b) A plausible photodegradation pathways of 2,4-DCP based on the analysis of products from GC-MS analysis.

Mechanism of photocatalysis and degradation pathway of 2,4-DCP The enhanced photocatalytic activity of Ce doped catalysts was highly dependent on separation of charge carriers. Since, in this work photocatalysis was mainly done under visible light, 1.97 eV is considered as the band gap value of the CMA-1.0% Ce (as calculated from Tauc plot). The positions of valence band (VB) and conduction band (CB) of the semiconductor were calculated using a formula as described in our previous report7, and the values are shown in Scheme 2a. When the semiconductor is irradiated with light, VB electron goes to CB, leaving hole in the VB, shown in Scheme 2a. In general, the electrons-holes undergo recombination, resulting in decrease in the catalytic activity of semiconductors. However, the Ce doped photocatalysts contained Ce3+/Ce4+ redox couple as established from the XPS analysis (Figure 5c). Hence, the excited electrons could easily be trapped by Ce4+, present on the material. Due to the Lewis acidic nature of Ce4+ ions, it seems to be superior to the oxygen molecule (O2) in electron trapping ability.62, 67 Afterwards, the

electrons might be abstracted by the adsorbed O2 by oxidation process, producing O2•−, which facilities the formation of •OH.67-69 On the other hand, the holes are trapped by Ce3+, which helps the formation of •OH and regeneration of Ce4+ simultaneously. This synergistic process helps to prolong the existence of Ce3+/Ce4+ on the catalyst, which continuously generates the active radicals for enhanced photocatalysis. Therefore, the Ce 4f level plays an important role in interfacial charge transfer and inhibition of electron–hole recombination, which facilitates the increase of photocatalytic activity of the Ce doped photocatalysts. A trapping experiment was carried out to investigate the active species formed in the photocatalytic degradation of 2,4-DCP using CMA-1.0% Ce under the visible light. In this investigation, EDTA-2Na, 2-propanol and benzoquinone were used as h+ scavenger, •OH scavenger, and •O2− scavengers, respectively. As depicted in Figure S14, the degradation efficiency of 2,4DCP largely inhibited with addition of •OH scavenger, whereas, the degradation partially suppressed in presence of •O2− and h+



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scavengers. The results clearly indicate that, although photogenerated •OH, •O2− and h+ has participated in the photodegradation of 2,4-DCP, •OH is the most reactive species. Additionally, we carried out the reaction in a non-aqueous medium (100% CH3CN) without any scavenger, where formation of •OH was not possible, however, •O2− and h+ could have been generated in the medium. In the non-aqueous medium, there was no photodegradation observed, which proved that •OH radicals are the main active oxidants responsible for the degradation of 2,4-DCP in our catalytic systems. In the aqueous medium, photogenerated h+ produces •OH, and also the •O2− can react with protons to form •OH. Finally, the rapid formation of •OH accelerates the degradation of the organic pollutants. Further, investigation was carried out using GC-MS to establish the degradation products, and a plausible degradation pathway of 2,4-DCP is proposed. From the GC-MS study (Figure S15), we could confirm the degradation products/intermediates of 2,4-DCP decomposition by radical route. The main intermediates were found to be hydroquinone, acetic acid, formic acid, etc., which were responsible for the 20% COD in the mixture obtained after 60 min of photodegradation. From the occurrence of hydroxylated intermediate, we ascertain that •OH plays significant in the degradation of 2,4-DCP. The intermediates were finally mineralized through opening of the aromatic ring followed by the formation of the smaller intermediates (like acetic acid and formic acid), and finally produced CO2 and H2O. Based on the GC-MS analysis and literature directions,70 a plausible mechanism for the degradation of 2,4DCP is arrived in Scheme 2b.

doping (CMA-1.0% Ce). Since, the photocatalytic reactions over CB and VB are favourable to maintain the existence of Ce3+/Ce4+, a chain of processes (enhanced quantum efficiency of the reaction) goes on to produce radical species. The radical trapping experiment confirmed that the •OH was the main active species in the photocatalysis in the aqueous condition. The intermediates, formed during the photodegradation were analyzed by using GC-MS and a plausible degradation mechanism of 2,4-DCP has been arrived.

CONCLUSION

ACKNOWLEDGEMENT

In summary, CMA has been successfully prepared by co-precipitation method followed by a controlled calcination. Cerium in different ratios (0.5 wt% to 2.0 wt%) has been homogeneously doped over the CMA. The characterization results revealed that light absorption property of the sample increased with an increase in the amount of Ce doping, which further significantly influenced the photocatalytic activity of samples. The CMA with 1.0% Ce doping exhibited the best photocatalytic performance among the prepared series of catalysts. Rate constant of the degradation by using the CMA1.0% Ce (k = 0.0551 min-1) was 3.7 times higher than the undoped sample (k = 0.0148 min-1) under visible light. The degradation of 2,4-DCP increased with an increase in Ce doping upto an optimum doping concentration (1.0 % of Ce). Beyond that the increase of cerium proportion decreased the activity of the catalysts. Moreover, 80% COD removal was noticed in 60 min by CMA-1.0% Ce, while almost complete mineralization was observed after 120 min. The activity of the photocatalyst was also investigated under different light conditions, and found that the photocatalyst was significantly active under the UV-Vis light (simulated solar light), and the rate of the reaction (k = 0.0796 min-1) was almost 5 times higher compared to the undoped photocatalyst, CMA (k = 0.01741 min-1). The Ce4+/Ce3+ redox couple, found at heterojunction is expected to aid long-lived electrons and holes, which play a crucial role in the enhancement of photocatalytic activity upon Ce

Authors acknowledge MHRD-New Delhi and IIT Kharagpur for the research fellowship, and various instrumental facilities and DST, New Delhi for FESEM (special grant) and PXRD (FIST Scheme). RA thanks SERB, New Delhi (Project No. SR/FT/CS-146 for funding HPLC-PDA). Authors acknowledge Prof. D. Pradhan (Dept. of Materials Science Centre, IIT Kharagpur) for the photocurrent measurement and Mr. U. Ghosh (Dept. of Civil Engineering, IIT Kharagpur) for helping us in COD measurement. Also we are thankful to Prof. Ned Bowden, Editor of the journal for constructive comments and suggestions.

SUPPORTING INFORMATION Supporting information includes FT-IR spectra of all the samples (Figure S1), N2 adsorption-desorption isotherms of CMAx% Ce (x = 0.5, 1.5 and 2.0) (Figure S2), SAED pattern (Figure S3), EDX analysis (Figure S4) and mapping (Figure S5) of CMA-2.0% Ce, EDX analysis of all the samples (Figure S6), XPS spectra of Mg and Al (Figure S7), Tauc plot of CMA-x% Ce (x = 1.0, 1.5) (Figure S8), HPLC chromatograms of degradation of 2,4-DCP over all the prepared samples under visible light (Figure S9), Comparison of photocatalytic activity of CuO, CuO-1.0% Ce and CMA-1.0% Ce (Figure S10), reusability experiments of CMA-1.0% Ce (Figure S11), HPLC chromatograms of degradation of 2,4-DCP over CMA-1.0% Ce under UV and UV-Vis light irradiation (Figure S12), Comparison of photocatalytic activity of CMA and CMA-1.0% Ce under UVVis light irradiation (Figure S13), UV-Vis spectra of degradation of MB dye by CMA-1.0% Ce under visible light (Figure S14), Gas chromatogram (GC) of 2,4-DCP (Figure S15) Table S1 and Table S2. A detailed process of photocurrent measurement is also provided below Figure S8.

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