Advanced Powder Technology 31 (2020) 2385–2393 Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt Original Research Paper Magnetic and catalytic properties of Cu-substituted SrFe12O19 synthesized by tartrate-gel method P.N. Anantharamaiah a,⇑, N. Sarath Chandra a, H.M. Shashanka a, R. Kumar b, B. Sahoo b,⇑ a b Department of Chemistry, Faculty of Mathematical and Physical Sciences, M. S. Ramaiah University of Applied Sciences, Bangalore 560058, India Materials Research Centre, Indian Institute of Science, Bangalore 560012, India a r t i c l e i n f o Article history: Received 9 January 2020 Received in revised form 19 March 2020 Accepted 3 April 2020 Available online 16 April 2020 Keywords: Strontium hexaferrite Cu-substitution Structure Magnetic properties Catalytic reduction of 4-nitrophenol a b s t r a c t Cu-substituted strontium-hexaferrites with chemical compositions SrFe12-xCuxO19 (0  x  0.8, x = 0, 0.2, 0.4, 0.6 and 0.8) were synthesized successfully by the tartrate-gel route, followed by calcination at 850 °C for 2 h. All the Cu-substituted compositions have single phase with hexagonal crystal structure and P63/mmc space group, but parent compound shows a small amount of hematite impurity phase. This suggests that introduction of Cu helps in reducing the calcination temperature to obtain a single phase compound. Furthermore, all the samples are nanocrystalline in nature with average crystallite size ranging from 40 to 50 nm. The variations in the magnetic parameters with increase in the copper content in the Sr-hexaferrite structure indicate the occupation of Cu at octahedral interstitial site. The above nanocrystalline-hexaferrite powders were employed as catalysts for the reduction of 4-nitrophenol to 4-aminophenol, in aqueous medium. The progress of the catalytic reaction was examined by UV-visible absorbance spectroscopy at different intervals of time. The results indicated that the parent compound (SrFe12O19) was catalytically inactive, while all other Cu-substituted samples exhibited good catalytic activities in transforming 4-nitrophenol into 4-aminophenol. Our results reveal that the catalytic property could be induced in Sr-hexaferrite samples by Cu-substitution at octahedral site. Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction Ever since the discovery of M-type Sr-hexaferrite (SrFe12O19) at the Philips research laboratory [1], it has received immense attention among the researches owing to its hard magnetic characterestics, chemcial stability and low cost of production [2]. Along with Sr-hexaferrite, the most commonly studied hard magnetic M-type hexaferrites are Ba-hexaferrite, Pb-hexaferrite and their combinations for various applications including microwave absorption [3–5]. Sr-hexaferrite belongs to hexagonal crystal structure wherein spinel (S ¼ ½Fe6 O8 Š2þ Þ and hexagonal (R ¼ ½SrFe6 O11 Š2 Þ layers have been stacked alternatively. The O2 ions form the closed-packed layers, with the Sr2+ substituting for an O2 in the hexagonal layer [6,7]. The Fe3+ ions are distributed in the five crystallographically distinguishable sites such as three octahedral (2a, 12 k and 4f2) one tetrahedral (4f1) and one trigonal bipyramidal (2b) interstitial sites [6,7]. The superexchange magnetic interactions between these sub-lattices occur via oxygen ⇑ Corresponding authors. E-mail addresses: anantharamaiah.cy.mp@msruas.ac.in (P.N. Anantharamaiah), bsahoo@iisc.ac.in (B. Sahoo). atoms leading to the ferrimagnetic ordering of the Fe magnetic moments [7]. Due to the low cost of production, high eletrical resistivity and high magnetic coercivity, the material can be exploited as active component to design high-performace recording medium, in telecommunication and for high-frequency microwave and optical devices [8–12]. Furthermore, due to the high coercivity (ranging from 500 to 6500 Oe) of Sr-ferrite emanating from the high magnetocrystalline anisotropy with easy magnetization axis along the c-direction, it has been projected as suitable alternative to the costlier neodymium alloy based permanent magnets. Hence, constant efforts were made to improve the magnetic parameters (Ms, Hc, Mr, [BH]max) of strontium ferrites via numerous routes that tune the crystallite size, morphology, microstructure, chemical substitution, self-composite etc., by adapting suitable synthesis and processing conditions [13–18]. A few authors demonstrated the catalytic properties of Sr-hexaferrites and related ceramic powders for the photodegradation of various organic pollutants. Mishra et al. were able to degrade the methylene blue dye solution in a single step by employing semiconducting SrFe12O19 as an efficient photocatalyst under visible light illumination [19]. Mohammadi et al. have synthesized SrFe12O19 nanoparticles through coprecipitation route https://doi.org/10.1016/j.apt.2020.04.004 0921-8831/Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 2386 P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393 and used them as photocatalysts for the degradation of methyl orange under visible light irradiation, and observed enhancement in the degradation [20]. Xie et al. have illustrated that the photocatalytic degradation of methylene blue can be enhanced by making a suitable composite between Bi2O3 with SrFe12O19 phases, it has been suggested that the formation of p-n heterojunction between p-type Bi2O3 and n-type SrFe12O19 is responsible for the enhancement [21]. Kaur et al. reported the photocatalytic degradation (photon-Fenton degradation) of three organic pollutants such as methyl orange, remazole deep red and p-nitrophenol using Mnsubstituted Sr-hexaferrites, SrMnxFe12-xO19 (0  x  5.0), as photocatalysts synthesized by the chemical co-precipitation route, wherein the authors have shown that Mn-doped Sr-hexaferrites are highly efficient in degrading the pollutants [22]. Spinel ferrites including magnesium, nickel, cobalt and copper ferrites have been used as catalysts to perform the reduction of hazardous para-nitrophenol using NaBH4 as the reducing agent, under mild reaction conditions [23–25]. The catalytic properties in the spinel ferrite systems are governed by various factors such as cation distribution, particle size, morphology, surface area, etc. Recently, it was shown that catalytic properties can be induced in the nickel ferrite system by replacing a small fraction of Fe by Cu [26]. The enhanced catalytic properties in the metal substituted spinel ferrite systems have been attributed to the presence of substituents at the octahedral sites, since the octahedral sites are exposed to the surface of the particles that facilitates the reduction at a faster rate. Earlier reports demonstrate that the hazardous 4nitrophenol can be reduced by using metallic nanoparticles such as Au and Ag-Cu as catalysts [27–28]. However, the major concern associated with these non-magnetic particles is their recovery for reuse after the catalytic application. Considering this aspect, herein, we demonstrated that Sr-hexaferrite system, which is rather chemcially inactive, can be made catalytically active for the 4-nitrophenol reduction, by empoying substitution of Cu in Sr-hexaferrite samples. Hence, the prime objective of the present work is to induce catalytic activity in strontium hexaferrite through the compositional modification by substituting Cu for Fe. Due to the strong preference of Cu to occupy the octahedral sites of the strontium hexaferrite lattice and the variable oxidation states of Cu, catalytic activity of the Cu-substituted strontium hexaferrites towards 4-nitrophenol reduction can be expected to be enhanced. Simultaneously, another important objective of the present study is to synthesize the single-phase samples of Cusubstituted strontium hexaferrites at low calcination temperature (Ta < 900 °C) by tartrate-gel method. In particular, in the present study, a series of Cu-substituted Sr-hexaferrites, SrFe12-xCuxO19 (0  x  0.8), have been synthesized by the tartrate-gel technique. We have shown that it is indeed possible to synthesize single phase Cu-substituted Sr-hexaferrites even at low calcination temperatures, without significantly affecting the magnetic parameters. 2. Experimental methods 2.1. Synthesis of nanocrystalline SrFe12-xCuxO19 (0  x  0.8) Cu-substituted Sr-hexaferrite samples with chemical compositions, SrFe12-xCuxO19 (0  x  0.8, x = 0, 0.2, 0.4, 0.6 and 0.8) were synthesized by following the tartrate-get route [29], using high purity Sr(NO3)2 (SD-fine chemicals, >99% purity), Fe(NO3)3
9H2O (Loba Chemie, >98% purity) and Cu(NO3)2 (SD-fine chemicals, > 99% purity) salts, L-tartaric acid (Spectrochem, >99% purity) as raw materials. Initially, a homogeneous solution of metal nitrates was made by dissolving stoichiometric amounts of metal nitrates in optimum amount of distilled water. Typically we have used 3.0 g of Fe(NO3)3
9H2O and correspondingly stoichiometric amounts of Sr(NO3)2 (0.129 g to 0.1396 g) and Cu(NO3)2 (0.0299 g to 0.1275 g) to make the solution. The solution of metal nitrates was then mixed with 70 ml of 1.5 M/metal ion, tartaric acid solution taken in a 200 ml crystallizing dish (tartaric acid used was between 1.7853 g and 1.9316 g). The reaction mixture was stirred at 50 °C for 1 h using magnetic stirrer. Subsequently, 2 ml of the polymerizing agent ethylene glycol was added to the reaction mixture and then the temperature was raised to 70 °C. The stirring was continued under the same conditions until the formation of brown thick viscous gel and the resultant gel was dried in an oven at 80 °C for few hours. As-dried gel was ground into fine powder form and calcined at Ta = 850 °C for two hour under the furnace atmosphere (air). 2.2. Characterization techniques The phase formation and structural details of the calcined powder samples were investigated using a powder X-ray diffraction (PAN anlytical X’pert PRO X-ray diffractometer equipped with Cu Ka radiation as source). Fourier-transform infrared spectroscopy (FTIR) spectra of the samples were recorded at ambient temperature using a FTIR spectrometer (Bruker, Alpha-P, Diamond ATR cell). Scanning electron microscopy (SEM, Tescan Vega 3LMU) technique was employed to extract the information on morphological features of the calcined powder samples. Magnetic properties of the samples were measured at ambient temperature using a vibrating sample magnetometer (EG&G PAR 4500). Timedependent UV-visible spectra of the 4-nitrophenol reduction reaction were recoreded at room temperature using UV-visible spectrometer (Shimadzu, 2600). Fig. 1. Powder XRD patterns of the calcined (Ta = 850 °C) Cu-substituted Srhexaferrite (SrFe12-xCuxO19) samples compared with the simulated pattern of SrFe12O19. Table 1 Values of lattice parameters (a, c) and the c/a ratios of Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) samples. x a = b (Å) c (Å) c/a 0 0.2 0.4 0.6 0.8 5.8445 5.8595 5.8696 5.8571 5.8570 22.9169 22.9662 23.0034 22.9662 22.9539 3.9211 3.9194 3.9190 3.9211 3.9190 P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393 2387 2.3. Catalytic reduction of 4-nitrophenol Fig. 2. Room temperature FTIR spectra of calcined Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) samples. To evaluate the catalytic performance of the synthesized Cu-substituted Sr-hexaferrites, the reduction of 4-nitrophenol to 4-aminophenol was employed as a model chemical reaction. In a typical procedure, 100 ml of 0.2 mM 4-nitrophenol solution was prepared in a 250 ml beaker. The crystals of mild reducing agent sodium borohydride (NaBH4), corresponding to the concentration of 0.2 M, were added to the 4-nitrophenol solution and stirred mechanically for 0.5 min. Soon after the addition of reducing agent, the 4-nitrophenol solution colour transformed from pale yellow to bright yellow revealing the formation of nitrophenolate anion. Subsequently, a known amount of synthesized hexaferrite catalyst (10 mg) was added to the above reaction mixture and stirred. At different intervals of time, a small portion of the solution (~3 ml) was pipetted out from the heterogeneous reaction mixture and the UV-visible spectra were recorded. Before taking the small portion of the solution for the UV-visible measurements, all the magnetic hexaferrite nanoparticles/catalysts were magnetically separated from the solution by an external permanent magnet. Fig. 3. SEM images of the calcined Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) samples, all the images are in same scale and magnification. 2388 P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393 and Fe3+ helps in improving the chemical activity of Cu-substituted Sr-hexaferrite samples. 3. Results and discussion 3.1. Powder X-ray diffraction (PXRD) analysis Fig. 1 shows the powder XRD patterns of Sr-hexaferrite (SrFe12O19) and Cu-substituted Sr-hexaferrite samples calcined at Ta = 850 °C. The observed XRD patterns of the samples are compared with indexed simulated XRD pattern of SrFe12O19. In the XRD pattern of SrFe12O19, all the reflections in the experimental pattern are in agreement with the peaks of the simulated pattern, except a reflection at 2h ~ 33.3° attributed to hematite (a-Fe2O3) phase and it is designated by * symbol (see Fig. 1). As documented in the literature, the phase formation temperature of SrFe12O19 is 1100 °C [16]. But in the present case, the SrFe12O19 sample was calcined at a lower temperature (850 °C) and therefore, the temperature is inadequate to form pure SrFe12O19 phase. Although the samples, including the parent compound, were calcined at the same temperature and conditions, unlike SrFe12O19, all the Cu-substituted strontium ferrite samples are phase pure with hexagonal crystal structure. Absence of a-Fe2O3 impurity phase in the Cu-substituted Sr-hexaferrite samples is due to the fact that, presence of Cu in the samples tends to reduce the phase formation temperature of SrFe12O19. This is in line with the reduction of sintering temperature in Sr-doped ZnO sample [30]. Hence. it is clear from this study that it is possible to synthesize single phase Cusubstituted SrFe12O19 samples, even at low calcination temepratures. Note further that, the polymerizing agent (ethylene glycol) helps to form a thick viscous gel by extending the chemical bonding with the complexing agent (tartaric acid/citric acid). As a result of that the metal ions are distributed homogeneously in polymeric networks. Once the dried-gel is calcined at the suitable temperature, it helps to achieve a better homogeneity in particle size distribution. The peaks in the XRD patterns of all the samples are considerably broad owing to the nanocrystalline nature of the samples. Average crystallite sizes (t) of the samples are calculated using 0:9k Scherrer’s equation, t ¼ bcosh ; wherein k is the wavelength of Xray beam in Å, h is the Bragg’s angle and b is full width at half maximum intensity of the reflection, in radian [29]. The obtained average crystallite sizes are 41, 43, 46, 46 and 47 nm for SrFe12O19, SrFe11.8Cu0.2O19, SrFe11.6Cu0.4O19, SrFe11.4Cu0.6O19 and SrFe11.2Cu0.8O19 samples, respectively. The structural parameters such as lattice parameters (‘a’ and ‘c’) of the samples are estimated using the relation [31] presented below: 1 2 dhkl 2 ¼ 2 4 h þ hk þ k 3 a2 ! 3.2. Fourier-transform infrared spectroscopy (FTIR) Fig. 2 illustrates the FTIR spectra of the calcined samples, recorded at room temperature. All the samples exhibited two major IR bands. The IR band at lower wavenumber are attributed to stretching of octahedral metal-oxygen bonds ([Fe+3]Octahedral-O), while that at higher wavenumber corresponds to the stretching of tetrahedral metal-oxygen bonds ([Fe+3]Tetrahedral-O) [33]. Furthermore, each band is associated with a shoulder at low frequencies, i.e., the lower frequency octahedral band situated at 422 cm 1 has a shoulder at 393 cm 1 and the higher frequency tetrahedral band situated at 583 cm 1 has a shoulder at 543 cm 1. The reason behind the observation of the two bands is associated with the position of Sr at the oxygen site [5], wherein the frequencies of vibrations of Fe-O and Fe-Sr are decided by the mass and the ionic size difference between these two atoms, O and Sr. The main reason to have higher stretching frequency for the tetrahedral metaloxygen bonds over the octahedral metal-oxygen bonds is due to their shorter bond length, because shorter bond length will have higher stretching frequency [34,35]. It is also interesting to know that even after replacing more and more Cu at octahedral Fe site in Sr-hexaferrite lattice, the positions of the bands are found to be approximately the same, revealing insignificant variation in the environments of Cu from that of Fe. Another important observation from the FTIR spectra is that the shape of the ~393 cm 1 band becomes sharper as the Cu content increased in the hexaferrite lattice, probably due to the effect of substitution of Cu only at the octahedral sites, since it has strong preference for the octahedral site over the tetrahedral site [36]. 3.3. Scanning electron microscopy (SEM) SEM images of the calcined hexaferrite compositions in SrFe12-xCuxO19 (0  x  0.8) have been presented in Fig. 3, and all the images are of similar magnification, which facilitates a better comparison. It is apparent from the images that majority of the particles of the parent compound (SrFe12O19) exhibit plate-like morphology and in fact the particles are heavily agglomerated 2 þ l c2 where dhkl is the interplannar spacing and h, k and l are the Miller indices of the plane corresponding to the particular XRD peak. The obtained structural parameters of the samples are listed in Table 1. It is clear from the table that even for a small amount of Cu substitution in Sr-hexaferrite structure, the lattice paremeters, both ‘a’ and ‘c’, suddenly increased (see these differences between x = 0 and x = 0.2). However, thereafter, i.e., from x = 0. 2 to x = 0.8, the lattice parameter changes are insignificant and within the error bar. These observations can be uderstood by considering the difference in the (1) ionic radii, and (2) oxidation states, of Cu2+ and Fe3+. Clearly, the higher ionic radii of Cu2+ (0.73 Å) than Fe3+ (0.645 Å), for octahedral coordination [32] leads to the increase in lattice parameters of Sr-hexaferrite due to Cu-substitution. Furthermore, on one-hand, the difference in oxidation states (i.e., 2+ for Cu and 3+ for Fe) leads to longer bond-length for CuAO. On the other-hand, this oxidation state difference induces oxygen vacancies in the Sr-hexaferrite structure due to Cu-substitution. As it will be discussed later that, this oxidation state difference between Cu2+ Fig. 4. Magnetic field dependent magnetization hysteresis loops of calcined Cusubstituted Sr-hexaferrite (SrFe12-xCuxO19) samples, recorded at room temperature. P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393 owing to the magnetic interaction between the neighbouring particles. As the Cu content in the Sr-hexaferrite samples increases, considerable change in the morphology as well as size of the hexaferrite particles is observed. The samples with x = 0.2 and 0.4 show smaller particles (larger surface area) without any noticeable plate-like morphology revealing that the Cu inclusion has significantly changed the morphology of the particles from the platelike structure to nearly spherical structure. With further increase in the Cu content, the size of the particles increases, and moreover, the particles are found to have irregular shape. The increase in the particle size for the sample with x > 0.4, can be attributed to decrease in the phase formation temperature of the samples as revealed from the XRD analysis. Table 2 Saturation magnetization MS, coercivity (HC) and remanence (Mr) of the calcined Cu-substituted Sr-hexaferrite samples. ‘x’ in SrFe12-xCuxO19 MS (emu/g) HC (Oe) Mr (emu/g) 0 0.2 0.4 0.6 0.8 56 60 58 59 56 5453 5257 5380 5654 5404 32 34 33 32 31 2389 3.4. Magnetic studies Magnetic field dependent hysteresis (M versus H) loops, recorded at room temperature in applied field range of 15 kOe  H  +15 kOe, are presented in Fig. 4. It is apparent from the magnetic data that all the samples are hard magnetic in nature as they exhibit large area under the loops. The magnetic parameters such as saturation magnetization (MS), coercivity (HC) and remanence (Mr) are extracted from the magnetization data and their values are listed in Table 2. The saturation magnetization values are obtained by extrapolating the linear region of M vs 1/H curve to 1/H = 0. The Ms values of the samples vary in the range of 56–60 emu/g. Among all the studied samples, the sample with x = 0.2 has highest MS (60 emu/g) and lowest HC (5257 Oe) values. When the higher magnetic moment cation Fe3+ (5lB) is replaced by the lower magnetic moment cation Cu2+ (1 lB) from the lattice of Sr-hexaferrite, the magnetization of the substituted samples should be lower than the unsubstituted sample. But in the present case, the samples with x = 0, 0.2, 0.4 and 0.6 show higher magnetization than the parent counterpart and this could be interpreted based on the site preference of Cu, as given below. In the M-type hexaferrite, the magnetic moments of Fe located at the three octahedral (2a, 12 k and 4f2) sites are alinged parallel to one another and these moments are coupled antiparallelly to the magnetic moments of Fe located at the tetrahedral (4f1) and trigonal Fig. 5. Time-dependent UV-Visible spectra of 4-nitrophenol reduction using prepared Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) catalysts. 2390 P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393 bipyramidal (2b) sites, whereas the magnetic moments are parallel to each other within 4f1 and 2b sites [7]. The overall magnetization of the hexaferrite arises due to the difference in the net magnetization of the octahedral sites (2a, 12 k and 4f2) and the net magnetization of both the tetrahedral and trigonal bipyramidal sites (4f1 and 2b). If the Cu occupies one of the octahedral sites by replacing the Fe then the magnetization of the Cu-substituted hexaferrite should be higher than that of the parent counterpart, due to decrease in the net magnetization of the octahedral sites. As suggested in the literature [36], Cu preferably occupies octahedral site. Therefore, in this study we have considered that Cu occupies the 4f2 site by replacing an equivalent amount of Fe. Hence, the higher magnetization for the samples 0 < x  0.6, in SrFe12-xCuxO19 is observed. The decrease in the magnetization with further increase in the Cu content (x = 0.8) is likely to be due to lower magnetic moment of the Cu cation which leads the compound towards paramagnetic nature. Note that, for recovering the catalysts from the reaction medium, the catalysts should posses a minimum magnetic property (magnetization >10 emu/g). Hence, the magnetic nature of our catalysts helps in recovering them, after completion of the catalytic reaction, from the reactor by using a (small) permanent magnet. 3.5. UV-visible spectroscopy and catalytic activity of SrFe12-xCuxO19 (0  x  0.8) To assess the catalytic properties of most of the noble metal nanocatalysts such as Au, Pd, Pt, Ag, etc. [37–40], catalytic reduction of 4-nitrophenol to 4-aminophenol was employed as the model chemical reaction. Therefore, in this study we have chosen the same as the model chemical reaction to evaluate the catalytic performance of Cu-substituted Sr-hexaferrite samples using the NaBH4 as a reducing agent. The catalytic process could be systematically examined by means of UV-visible spectroscopy, since 4nitrophenol in the acidic medium, leads to the absorption band at ~317 nm, while the absorption band of 4-aminophenol appears at ~300 nm [41]. When NaBH4 is mixed with the solution of 4nitrophenol, the absorption band of 4-nitrophenol undergoes red shift from 317 nm to 400 nm owing to the formation of phenolate anion. Despite NaBH4 being one of the good reducing agents, it alone is incapable in reducing 4-nitrophenol to 4-aminophenol [26]. However, the reduction reaction can be driven by adding a suitable catalyst to the nitrophenol solution containing NaBH4. Furthermore, if the catalyst is chemically inactive then there won’t be any variation in the intensity of the ~400 nm absorption band with time. On the other hand, if the catalyst is active then the intensity of ~400 nm absorption band decreases gradually with simultaneous increase in the intensity of ~300 nm absorption band as the reaction time proceeds, indicating conversion of 4nitrophenol to 4-aminophenol [26]. The time-dependent UV-visible absorbance spectra for the catalytic reduction of 4-nitrophenol using SrFe12-xCuxO19 (0  x  0.8) catalysts are shown in Fig. 5. It is confirmed from the Fig. 5 that the parent compound (SrFe12O19) is found to be catalytically inactive for reduction of 4-nitrophenol, because there is no change in the intensity of 400 nm absorption band even after 13 min. But after the introduction of Cu2+ into the lattice of Sr-hexaferrite, the catalytic efficiency is remarkably enhanced. In the Cu-substituted samples, the intensity of 400 nm absorption band due to the phenolate anion decreases progressively, on the other hand the intensity of ~300 nm absorbance band due to the aminophenol increases gradually as the reaction time proceeds, signifying transformation of 4-nitrophenol to 4-aminophenol. It is worth mentioning here that regardless of the amount of copper content in the lattice of hexaferrite, catalytic performance of the Cu-substituted samples is observed to be approximately the same. It is reported for various metal-substituted spinel ferrite systems that the catalytic activity of the parent compound can be enhanced after replacing Fe by suitable metal ions such as Al, Ni, Co, Cu etc. [25,26]. In those reports, the authors have suggested that the presence of substituted metal Fig. 6. Proposed mechanistic route for the reduction of 4-nitrophenol (NP) into 4-aminophenol (AP), using the Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) catalysts with NaBH4 as the reducing agent. P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393 ions in the octahedral coordination environment is responsible for higher catalytic activities in the substituted spinel ferrites as the octahedral sites are exposed to the surface. In the present study, as the Cu2+ ions occupy the octahedral sites of the hexaferrite, much similar to that of the spinel ferrite systems, and these sites are exposed on the surface of the catalyst, the Cu-substituted Srhexaferrite samples show better catalytic performance. The mechanism of the catalytic reaction in reduction of 4nitrophenol to 4-aminophenol can be understood as follows. When the reducing agent NaBH4 is dissolved in an aqueous medium, there will be production of borohydride anion (BH4 ). These BH4 ions get adsorbed onto the surface of the Cu-substituted hexaferrite catalysts leading to the formation of metal-hydride complex (MAH complex). Since the Cu2+ ions are exposed to the surface, copper hydride (CuAH) complex will be expected to form. The prime reason to form M-H complex is due to the Lewis acidic nature of Cu2+. Note that the ability of the copper cations to have variable oxidation state drives the formation of MAH complex. 2391 Simultaneously, the 4-nitrophenolate ions also get adsorbed onto the surface of the CuAH complex. Once the adsorption process is completed, the hydrogen of the CuAH complex migrates to the nitroaromatic compound and thereby facilitates the reduction reaction. Reduction of 4-nitrophenol to the 4-aminophenol may take place either through the direct path or the condensation path [24]. The possible mechanism for the reduction of 4-nitrophenol to 4-aminophenol using NaBH4 as the reducing agent and the nano hexaferrite catalyst is shown in Fig. 6. Note here that, due to lower oxidation state of Cu2+ than Fe3+, oxygen vacancies are created in the samples which act as the active catalytic centres to attract the 4-nitrophenolate ions onto the particle surface to facilitate the reduction reaction. Furthermore, the variable oxidation state of Cu helps in adsorption and desorption of the 4-nitrophenolate ions and 4-aminophenol, to and from the surface of the catalyst particles (Cu-substituted Sr-hexaferrites). The reduction of nitrophenol reaction obeys pseudo first-order chemical kinetics since the NaBH4 concentration is taken in excess Fig. 7. Plots of ln(Co/Ct) versus time for the 4-nitrophenol reduction using the SrFe12-xCuxO19 catalysts. The data were fitted with linear function and slope of the linear line corresponds to rate constant. 2392 P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393 Table 3 A list of rate constants for 4-nitrophenol reduction reaction measured using different catalysts. y 1 Catalysts Amount of Catalyst (mg) Conc. of 4-NP (mM) Volume of 4-NP (ml) Amount of NaBH4 Rate Constant (min ) Ref. Co3O4/BiFeO3 CuFeCN 0.5 0.1 90 mg/L 3.6 15 mg 0.1 mg 0.774 0.8432 [42] [43] CuFe2O4 Co3O4/CoFe2O4 CuFe2O4 NiFe2O4 MnFe2O4@SiO2_NH2@Au CoMn0.2Fe1.8O4 SrFe12O19 SrFe11.8Cu0.2O19 SrFe11.2Cu0.8O19 0.01 mg/ml 10 5 mol% 5 mol% 3 5 mol% 10 mg 10 mg 10 mg 0.16 0.2 3.6 3.6 0.05 0.72 0.2 0.2 0.2 20 0.122 (122 lL) 2 16 20 20 3 20 100 100 100 0.1 mol 15 mg 1.36 g 1.36 g 5.67 g 1.36 g 0.75 g 0.75 g 0.75 g 0.002 0.1 0.344 0.873 0.126 2.1 0.0014 0.18 0.2 [44] [45] [46] [47] [48] [49] y y y Present work. quantity. Hence, the rate constant can be estimated using firstorder rate law equation [Ct = Co exp ( kt)], where, k is the pseudo first-order rate constant, Co is the initial concentration and Ct is the instantaneous concentration at time t. The plots of ln(Co/Ct) versus time for the 4-nitrophenol reduction using Cu-substituted Srhexaferrite particles as catalysts are shown in Fig. 7. In all the curves, the data points are fitted with the linear function and the slopes of the fitted straight lines of the ln(Co/Ct) versus time curves correspond to first order rate constants (k1). The obtained rate constants are 0.0014, 0.1808, 0.175, 0.1673 and 0.202 min 1, respectively, for samples with x = 0, 0.2, 0.4, 0.6 and 0.8. Higher the value of rate constant better will be the catalyst, and therefore, all the substituted samples, in the present study, exhibit better catalytic performance over the unsubstituted sample. Among the Cusubstituted samples, the highest value of rate constant is found for the SrFe11.2Cu0.8O19 sample. A list of rate constant values for many reported ferrite materials [42–49] are provided in Table 3. Our results clearly demonstrate the usefulness of our Cu-doped Srferrite samples. 4. Conclusions We have demonstrated that tartrate-gel route is a suitable synthesis technique for synthesis of phase pure Cu-substituted Srhexaferrite nanoceramics (SrFe12-xCuxO19, x = 0, 0.2, 0.4, 0.6 and 0.8), wherein the final calcination temperature can be reduced to about 850 °C than that is required for SrFe12O19. The results of the synthesized samples show that Cu helps in reducing the required calcination temperature. Absence of impurity peaks in the XRD patterns of Cu-substituted Sr-hexaferrite samples confirms the successful synthesis of single phase nanocrystalline ceramics. The FTIR results indicate that Cu goes to the tetrahedral site and Sr occupies the oxygen site. The variation in the saturation magnetisation of the samples with Cu content in the Sr-hexaferrite structure also indicates the occupation of Cu at the 4f2 octahedral interstitial site of Sr-hexaferrite. Furthermore, we have successfully illustrated that catalytic activity, for the 4-nitrophenol reduction using NaBH4, can be induced in the hard magnetic Sr-hexaferrite by replacing Fe by Cu, without affecting the magnetic parameters of the parent compound. According to our results, irrespective of amount of Cu substitution the catalytic activities of all the Cu-substituted samples were found to be similar. The induced catalytic activity in the strontium ferrite is mainly attributed to the presence of Cu2+ cations at the octahedral (4f2) site. Overall, our results demonstrate that, the presence of more octahedral sites on the surface of the nanocrystalline particles, in addition to the formation of oxygen vacancies due to lower oxidation state of Cu than Fe and the variable oxidation state of Cu, act together to facilitate the enhanced catalytic activity of Cu-substituted Sr-hexaferrite samples. 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