Catalytic and Photocatalytic Degradation Activities of Nanoscale Mn-Doped ZnCr 2 O 4

In the present work


Introduction
Among the spinel chromite, pure and Mn-doped Zinc Chromite (ZnCr 2 O 4 ) is especially interesting due to their chemical stability, mechanical hardness, large magneto astrictive coefcient, high coercivity, moderate saturation magnetization, and large magneto crystalline anisotropy. One of the most difcult topics in the research of spinel ferrite materials is the cation distribution between the structure's two interstitial sites and its efect on the various characteristics of chromite [1,2].
Furthermore, the utilization of heterogeneous catalysts has signifcant benefts over homogeneous systems in terms of convenience of handling and catalyst recycling.
Supported platinum and palladium catalysts have long been recognized to have strong catalytic efciency in the oxidation of alcohols. Tere are substantial research articles available for the most current developments in this topic [3][4][5][6][7][8][9][10]. Because of their simple synthesis technique, chemical as well as thermal stability, economic feasibility, and high catalytic efciency, pure and Mn-doped ZnCr 2 O 4 nanoparticles have captured a noteworthy interest for catalytic applications, primarily in organic reactions. Te type and oxidation (catalytic reaction) of the metal ions present on the surface sites, size, and surface area of the pure and Mn-doped ZnCr 2 O 4 are primarily connected with the catalytic performance of the pure and Mn-doped ZnCr 2 O 4 [11].
Metal oxides can be synthesized by various methods, for instance, solid-state reactions, nonaqueous routes, microwaveassisted synthesis, electrodeposition, solvothermal, sol-gel, combustion, microemulsions, coprecipitation, and hydrothermal methods [12][13][14]. Due to the close association between these characteristics and their physical/chemical properties, there has been a lot of attention paid to manipulating the size, structure, and shape of nanostructured materials in recent years [15]. Te preparation of nanoalloy semiconductors under moderate circumstances is currently receiving a lot of interest.
In the present study, pristine and Mn-doped ZnCr 2 O 4 nanoscale powder samples have been synthesized by the hydrothermal technique. Te pristine and Mn-doped Zn 1−x Mn x Cr 2 O 4 (x � 0 to 0.03) samples are characterized with XRD, HR-SEM, and HR-TEM analyses to reveal the structure, morphology, and particle size evaluation. Te catalytic performance of Zn 1−x Mn x Cr 2 O 4 (x � 0 to 0.03) samples in conversion reaction of toluene oxidization with H 2 O 2 to produce benzaldehyde and also in oxidization reactions of various primary alcohols with H 2 O 2 oxidant to produce the corresponding carbonyl compounds are evaluated. In addition, the photocatalytic degradation of methylene blue with these Zn 1−x Mn x Cr 2 O 4 (x � 0 to 0.03) catalysts is estimated, and the results are discussed in detail. In the case of Mn doping, manganese (II) nitrate hexahydrate (99%, Sigma Aldrich) (0.5 molars) is dissolved in distilled water and then mixed with the above zinc nitrate hexahydrate and potassium dichromate solution. Furthermore, 6.0 ml of urea solution having a molar concentration of 0.6 is introduced in a drop-by-drop manner to the abovementioned solution, and then the solution is stirred until a homogeneous solution is obtained. Te subsequent solution is transferred to a 50 ml Tefon beaker, which is placed in an airtight stainlesssteel autoclave. Te whole setup is kept in an oven, and the temperature is slowly increased to 250°C. After 12 h at 250°C, the autoclave is cooled to room temperature, and then the white precipitates of ZnCr 2 O 4 are collected. In a similar hydrothermal approach, pale white precipitates of Mn-doped ZnCr 2 O 4 are also collected. Te precipitates are cleaned with distilled water and ethanol several times to remove the impurities, followed by calcination at 400°C for 24 h. Te nanoscale powder samples of ZnCr 2 O 4 and Zn 1−x Mn x Cr 2 O 4 (x = 0.01 to 0.03) are obtained by using the above hydrothermal procedures.

Characterization
. Te X-ray difraction patterns of Mndoped ZnCr 2 O 4 samples are examined by employing an X-ray difractometer (brand: Rigaku) utilizing a Cu Kα light source. Images of scanning electron microscopy with higher resolution (HR-SEM) are observed using a Philips XL30 FESEM microscope. Te energy-dispersive X-ray spectroscopy in the selected region is measured with a combined HR-SEM. Images of transmission electron microscopy with higher resolution (HR-TEM) are captured by utilizing a transmission electron microscope (make and model: Philips EM 208) having an accelerating voltage of 200 kV. Based on the appearance of the planes, a cubic phase with a spinel structure is assigned to ZnCr 2 O 4 . Cheng and Gao reported a similar cubic-phased spinel structure for ZnCr 2 O 4 nanoparticles synthesized by the hydrothermal route [16]. Yazdanbakhsh et al. obtained the cubic phase of spinel-type ZnCr 2 O 4 nanoparticles by the sol-gel route at 700°C calcination temperature [17]. Te XRD pattern for the pristine ZnCr 2 O 4 nanoparticles has no additional impurity peaks, which indicate the phase purity of the synthesized ZnCr 2 O 4 and Zn 1−x Mn x Cr 2 O 4 (x � 0.01 to 0.03) compounds.

Results and Discussions
Te XRD pattern indicates that the calcination treatment, in addition to the hydrothermal approach, promotes a sharpening of the peaks in the pristine ZnCr 2 O 4 sample. Te sharpness and intensity of the X-ray difraction peaks corresponding to ZnCr 2 O 4 imply a relatively larger crystallite size and high crystallinity [18][19][20] of the as-synthesized sample. However, when the Mn dopant concentration is increased in the Zn 1−x Mn x Cr 2 O 4 (for x � 0.01 to 0.03) sample, the XRD peak intensity is found to decrease, which might be due to the increased dislocation of Zn in fewer sites caused by the replacement of doped Mn atoms in the cubic phase of spinel-structured ZnCr 2 O 4 . At a lower concentration of Mn with x � 0.01 in Zn 1−x Mn x Cr 2 O 4 , no specifc change in the peak intensity is observed. Te increase of Mn dopant concentration in Zn 1−x Mn x Cr 2 O 4 (x � 0.01 to 0.03) causes a slight shift in XRD peaks to higher angles, which implies development of lattice tensile stress in the crystal structure and hence the hindrance of crystallinity observed in the Mn-doped ZnCr 2 O 4 samples.
Te mean crystallite size from XRD peaks is calculated using the Debye-Scherrer equation [21]: where θ, β (radians), λ, and L are the Bragg's difraction angle, full width at half maxima, incident X-ray light wavelength (1.54Å), and mean crystallite size, respectively.  Figure 2(c) shows particle sizes less than 50 nm, which need to be visualized clearly at higher magnifcation. Te SEM images reveal particle agglomeration in all the samples. Tis agglomeration is expected to be caused by the interface surface tension efect. In the Mn-doped ZnCr 2 O 4 sample (Figures 3(c) and 3(d)), the spheroidal particle sizes are observed in the range of 50-200 nm, and in addition, as in pristine, some irregular tiled particle shapes are also found. Te obtained pristine and doped ZnCr 2 O 4 nanoparticle morphology is good for catalytic activity due to the huge surface-to-volume ratio [22]. Te specifc surface area and pore size properties estimated from SEM images are listed in Table 1, which indicates a low surface area for pristine ZnCr 2 O 4 and, in the case of Mn-doped ZnCr 2 O 4 , a higher BET surface area. Te high pore volume with reduced pore diameter values suggests that the Mn-doped ZnCr 2 O 4 has a high specifc surface area (cm 3 /g) related to deep pores. Te sample's mean pore diameter results from the creation of intergranular pores caused by the mixing of metal oxides and the occupancy of the Mn dopant. Tus, by the hydrothermal method, the BET surface area of pristine and Mn-doped ZnCr 2 O 4 samples is increased along with the reduction in the average pore diameter. Tis hydrothermal approach may be used to create catalysts with nanocrystal size distributions and large surface areas. It is expected that the higher BET surface area would exhibit better catalytic activity [23][24][25].  [26,27]. Among the three additives, Zn 0.97 Mn 0.03 Cr 2 O 4 with H 2 O 2 is the most efective promoter to achieve a higher yield of benzaldehyde, as indicated in Table 2. Despite a slight structural distortion observed in the XRD pattern, the catalytic performance of Zn 0.97 Mn 0.03 Cr 2 O 4 is excellent in the benzaldehyde formation reaction.
Since pristine and Mn-doped ZnCr 2 O 4 catalysts are found to be more active among the chromite catalysts, various alcohols, such as butanol, hexanol, heptanol, octanol, and 1-phenyl ethanol, are also oxidized with these ZnCr 2 O 4 -based catalysts with H 2 O 2 as the oxidant and acetonitrile as the reaction medium in order to obtain the corresponding carbonyl compounds, and the results are shown in Table 3. It is noteworthy that the yield percentage of the carbonyl compound is gradually increased when the pristine ZnCr 2 O 4 and Mn-doped catalysts are used, case by case. However, the yield of the corresponding carbonyl compound in each reaction reached an optimum level for the Zn 0.98 Mn 0.02 Cr 2 O 4 catalyst, and upon further increase of Mn concentration in Zn 1−x Mn x Cr 2 O 4 catalyst, this led to a decrease in the yield of carbonyl compounds, as shown in Table 3. Tis decrement in the catalytic activity of Zn 0.97 Mn 0.03 Cr 2 O 4 might be due to the structural distortion, as discussed in the XRD results. Te increase of Mn concentration to 3% afects the structure of ZnCr 2 O 4 up to a certain extent, which hinders the catalytic activity of the host material in the carbonyl compound formation reactions, whereas in the benzaldehyde reaction it is still active. Tis anomaly in catalytic activity beyond structural distortion might be related to the microscopic reaction kinetics, which is not clearly understood and need to be investigated further.
In addition to catalytic activities in aldehydes and carbonyl compounds, the photocatalytic degradation efect of pristine and Mn-doped ZnCr 2 O 4 on dye solutions has also been evaluated. Te methylene blue (MB) dye solution, along with pristine or Mn-doped ZnCr 2 O 4 catalyst, is subjected to UV irradiation by using a 40 W Xe lamp in order to determine catalytic performance at room temperature. In general, 100 ml of methylene blue aqueous solution (40 mg/l concentration) is mixed with 0.05 g of the pristine and Mn-doped ZnCr 2 O 4 . In order to reach adsorption equilibrium, the resulting mixture with methylene blue is stirred well and then held in a dark place for 30 minutes. Te solution is then placed before a xenon lamp, and the lamp is turned on to initiate the degradation reaction under continuous exposure (start time, t � 0). Under UV light exposure, the efcient interfacial interaction between Mn-doped ZnCr 2 O 4 nanoparticles and photon energy leading to charge separation is occurring, which leads to photocatalytic degradation [28]. Te photocatalytic degradation efciency is monitored for 10 to 60 minutes with 10-minute intervals during the UV irradiation, as shown in Figure 4. After the UV irradiation, the solution is then stirred and subjected to visible light for 2 hours at periodic intervals under atmospheric conditions in order to facilitate the transfer of the photogenerated charge carrier during the photocatalytic experiment conducted under atmospheric conditions. Using a Hitachi UV-Vis spectrophotometer, the organic pollutant degradation is calculated as C t /C 0 , in which C t and C 0 are the organic pollutant concentration at a specifc time and starting concentrations at the adsorption/desorption equilibrium [29,30].   the nanoscale particle size reduction together cause increased charge-transfer kinetics during catalytic and photocatalytic degradation reactions, which might be due to the following reasons described:    trapping center for the charge carriers, and hence there might be an increased photocatalytic activity with the increase of dopant, as demonstrated in the mechanism shown in Scheme 2.
(ii) Te other reason for increased photocatalytic activity is due to the nanoscale particle size reduction upon Mn doping with ZnCr 2 O 4 , as evidenced by XRD and SEM measurements. Te particle size reduction is an

Data Availability
Te data used to support the fndings of this study are available with one of the corresponding authors Dr. R.V. Sakthivel which can be shared upon reasonable request.

Conflicts of Interest
Te authors declare that they have no conficts of interest. Advances in Materials Science and Engineering 7