# Catalytic and Photocatalytic Degradation Activities of Nanoscale Mn-Doped ZnCr2O4

Authors: P. Sankudevan; R. V. Sakthivel; Ramalingam Gopal; C. Raghupathi; S. Ambika; Mohammed Mujahid Alam; Baskaran Rangasamy
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

In the present work, the effect of Mn doping in Zinc Chromite (ZnCr2O4) and particle size reduction on catalytic and photocatalytic degradation performance have been evaluated. The pristine Zn1−xMnxCr2O4 (x = 0 to 0.03) nanoscale samples are synthesized through a hydrothermal approach. The synthesized catalysts are characterized by XRD, HR-SEM, HR-TEM, catalytic, and photocatalytic degradation analyses. X-ray diffraction analysis results confirmed the formation of the ZnCr2O4 structure and its phase purity, crystallite size, and Mn dopant effect. The surface morphology and particle size of Zn1−xMnxCr2O4 samples are evaluated by SEM and TEM measurements. The textural properties of ZnCr2O4 samples are identified by the surface area analysis. The catalytic performance of Mn-doped ZnCr2O4 samples reveals superior catalytic performance compared to pristine ZnCr2O4 in benzaldehyde and carbonyl compound productions. Under UV irradiation, an excellent photocatalytic degradation efficiency of 89.66% for Zn0.97Mn0.03Cr2O4 catalyst with methylene blue has been obtained.


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## Body

## 1. Introduction

Among the spinel chromite, pure and Mn-doped Zinc Chromite (ZnCr2O4) is especially interesting due to their chemical stability, mechanical hardness, large magneto astrictive coefficient, high coercivity, moderate saturation magnetization, and large magneto crystalline anisotropy. One of the most difficult topics in the research of spinel ferrite materials is the cation distribution between the structure’s two interstitial sites and its effect on the various characteristics of chromite [1, 2].Furthermore, the utilization of heterogeneous catalysts has significant benefits 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 efficiency in the oxidation of alcohols. There are substantial research articles available for the most current developments in this topic [3–10]. Because of their simple synthesis technique, chemical as well as thermal stability, economic feasibility, and high catalytic efficiency, pure and Mn-doped ZnCr2O4 nanoparticles have captured a noteworthy interest for catalytic applications, primarily in organic reactions. The type and oxidation (catalytic reaction) of the metal ions present on the surface sites, size, and surface area of the pure and Mn-doped ZnCr2O4 are primarily connected with the catalytic performance of the pure and Mn-doped ZnCr2O4 [11].Metal oxides can be synthesized by various methods, for instance, solid-state reactions, nonaqueous routes, microwave-assisted synthesis, electrodeposition, solvothermal, sol-gel, combustion, microemulsions, coprecipitation, and hydrothermal methods [12–14]. 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]. The preparation of nanoalloy semiconductors under moderate circumstances is currently receiving a lot of interest.In the present study, pristine and Mn-doped ZnCr2O4 nanoscale powder samples have been synthesized by the hydrothermal technique. The pristine and Mn-doped Zn1−xMnxCr2O4 (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. The catalytic performance of Zn1−xMnxCr2O4 (x = 0 to 0.03) samples in conversion reaction of toluene oxidization with H2O2 to produce benzaldehyde and also in oxidization reactions of various primary alcohols with H2O2 oxidant to produce the corresponding carbonyl compounds are evaluated. In addition, the photocatalytic degradation of methylene blue with these Zn1−xMnxCr2O4 (x = 0 to 0.03) catalysts is estimated, and the results are discussed in detail.


## 2. Experimental Details

### 2.1. Chemicals and Synthesis Process

Pristine and Mn-doped ZnCr2O4 nanoparticles have been synthesized by the hydrothermal technique. Initially, the precursor materials such as zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (99%, Sigma Aldrich) and potassium dichromate (K2Cr2O7) (99.9%, Himedia) are dissolved in distilled water by magnetic stirring, and this homogeneous solution is used for the synthesis of ZnCr2O4. 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. The subsequent solution is transferred to a 50 ml Teflon beaker, which is placed in an airtight stainless-steel autoclave. The 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 ZnCr2O4 are collected. In a similar hydrothermal approach, pale white precipitates of Mn-doped ZnCr2O4 are also collected. The precipitates are cleaned with distilled water and ethanol several times to remove the impurities, followed by calcination at 400°C for 24 h. The nanoscale powder samples of ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) are obtained by using the above hydrothermal procedures.


### 2.2. Characterization

The X-ray diffraction patterns of Mn-doped ZnCr2O4 samples are examined by employing an X-ray diffractometer (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. The 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.


## 2.1. Chemicals and Synthesis Process

Pristine and Mn-doped ZnCr2O4 nanoparticles have been synthesized by the hydrothermal technique. Initially, the precursor materials such as zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (99%, Sigma Aldrich) and potassium dichromate (K2Cr2O7) (99.9%, Himedia) are dissolved in distilled water by magnetic stirring, and this homogeneous solution is used for the synthesis of ZnCr2O4. 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. The subsequent solution is transferred to a 50 ml Teflon beaker, which is placed in an airtight stainless-steel autoclave. The 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 ZnCr2O4 are collected. In a similar hydrothermal approach, pale white precipitates of Mn-doped ZnCr2O4 are also collected. The precipitates are cleaned with distilled water and ethanol several times to remove the impurities, followed by calcination at 400°C for 24 h. The nanoscale powder samples of ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) are obtained by using the above hydrothermal procedures.


## 2.2. Characterization

The X-ray diffraction patterns of Mn-doped ZnCr2O4 samples are examined by employing an X-ray diffractometer (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. The 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.


## 3. Results and Discussions

### 3.1. XRD Analysis

The phase identification of the pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) samples synthesized by the facile hydrothermal method has been carried out by X-ray diffraction (Figure 1 (a)–(d)) studies. The peaks obtained are referred to as the (220), (311), (400), (422), (511), and (440) planes of ZnCr2O4, which are very well matched with the JCPDS No. 73-1962. Based on the appearance of the planes, a cubic phase with a spinel structure is assigned to ZnCr2O4. Cheng and Gao reported a similar cubic-phased spinel structure for ZnCr2O4 nanoparticles synthesized by the hydrothermal route [16]. Yazdanbakhsh et al. obtained the cubic phase of spinel-type ZnCr2O4 nanoparticles by the sol-gel route at 700°C calcination temperature [17]. The XRD pattern for the pristine ZnCr2O4 nanoparticles has no additional impurity peaks, which indicate the phase purity of the synthesized ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) compounds.Figure 1
XRD pattern of (a) pristine ZnCr2O4 and (b–d) Zn1−xMnxCr2O4 (x = 0.01 to 0.03).The XRD pattern indicates that the calcination treatment, in addition to the hydrothermal approach, promotes a sharpening of the peaks in the pristine ZnCr2O4 sample. The sharpness and intensity of the X-ray diffraction peaks corresponding to ZnCr2O4 imply a relatively larger crystallite size and high crystallinity [18–20] of the as-synthesized sample. However, when the Mn dopant concentration is increased in the Zn1−xMnxCr2O4 (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 ZnCr2O4. At a lower concentration of Mn with x = 0.01 in Zn1−xMnxCr2O4, no specific change in the peak intensity is observed. The increase of Mn dopant concentration in Zn1−xMnxCr2O4 (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 ZnCr2O4 samples.The mean crystallite size from XRD peaks is calculated using the Debye–Scherrer equation [21]:(1)L=0.98λβcosθ,where θ, β (radians), λ, and L are the Bragg’s diffraction angle, full width at half maxima, incident X-ray light wavelength (1.54 Å), and mean crystallite size, respectively. The mean crystallite sizes of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) are determined to be about 22.80, 18.45, 17.34, and 16.68 nm, respectively.The sharp, intense peak observed for the pristine ZnCr2O4 sample gets slightly broadened when the Mn concentration increases, which reveals a gradual crystallite size reduction in the Zn1−xMnxCr2O4 sample from x = 0.01 to 0.03. However, the gradual decrease in peak intensity of ZnCr2O4 upon an increase of manganese dopant concentration indicates structural disorder in ZnCr2O4 host material with the presence of dopant. The XRD results reveal that any further increase in Mn dopant concentration may distort the structure of ZnCr2O4 abruptly.


### 3.2. Morphology Analysis

Figures2(a)–2(d) shows HR-SEM images of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) nanoparticles. The surface morphology of all the ZnCr2O4 samples indicates inseparable particle clusters. However, Figure 2(c) shows particle sizes less than 50 nm, which need to be visualized clearly at higher magnification. The SEM images reveal particle agglomeration in all the samples. This agglomeration is expected to be caused by the interface surface tension effect. Figures 3(a)–3(d) show the TEM morphology of pristine ZnCr2O4 and Mn-doped ZnCr2O4, which indicates a clear particulate structure. The pristine ZnCr2O4 at higher magnifications (Figures 3(a) and 3(b)) indicates uneven irregular tiled and spheroid particle distributions. The spheroidal shapes are ranging in 70–130 nm, and the irregular tiles are 20 nm in thickness and 80–120 nm in length.Figure 2
HR-SEM images of (a) pristine ZnCr2O4, (b) Zn0.99Mn0.01Cr2O4, (c) Zn0.98Mn0.02Cr2O4, and (d) Zn0.97Mn0.03Cr2O4.
(a)(b)(c)(d)Figure 3
HR-TEM images of (a, b) pristine ZnCr2O4 and (c, d) Zn0.97Mn0.03Cr2O4.
(a)(b)(c)(d)In the Mn-doped ZnCr2O4 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. The obtained pristine and doped ZnCr2O4 nanoparticle morphology is good for catalytic activity due to the huge surface-to-volume ratio [22]. The specific surface area and pore size properties estimated from SEM images are listed in Table 1, which indicates a low surface area for pristine ZnCr2O4 and, in the case of Mn-doped ZnCr2O4, a higher BET surface area. The high pore volume with reduced pore diameter values suggests that the Mn-doped ZnCr2O4 has a high specific surface area (cm3/g) related to deep pores. The 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. Thus, by the hydrothermal method, the BET surface area of pristine and Mn-doped ZnCr2O4 samples is increased along with the reduction in the average pore diameter. This 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 [23–25].Table 1
BET surface area, average pore diameter, pore volume, and their crystallite size (nm) of pristine and Mn-doped ZnCr2O4 samples.
Catalyst propertiesZnCr2O4Zn0.99Mn0.01Cr2O4Zn0.98Mn0.02Cr2O4Zn0.97Mn0.03Cr2O4SBET (m2/g)37.8744.8749.9995453.54Rp (Å)20.2016.3312.2315.89Vp (cm3/g)0.07310.08900.10980.0993Crystallite size (nm)29.6827.3422.4520.80


### 3.3. Catalytic Activity

In order to investigate the activity of spinel-type pristine and Mn-doped ZnCr2O4 sample nanostructures as a heterogeneous catalyst, the samples are dispersed in an acetonitrile medium. The oxidation of toluene is carried out by using H2O2 as an oxidant at a reaction temperature of 60°C for 6 h. A pure form of benzaldehyde is discovered as an oxidation product in the catalytic reaction mixture, as shown in Scheme 1.Scheme 1
Schematic representation for the oxidation of toluene with ZnCr2O4 catalyst.The catalytic performance of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) samples is examined by employing hydrogen peroxide (H2O2) as the oxidant in the selective oxidation of toluene to form benzaldehyde in an acetonitrile medium. Blank reactions are performed over pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) samples under the same reaction conditions, which show negligible yields of benzaldehyde. Only a 3.9% yield of benzaldehyde is obtained in the blank reaction when no oxidant or catalyst is employed. In order to increase the yield of benzaldehyde, several promoters, such as pristine ZnCr2O4, Zn1−xMnxCr2O4 (x = 0.01 to 0.03), H2O2, and solvent are added [26, 27]. Among the three additives, Zn0.97Mn0.03Cr2O4 with H2O2 is the most effective 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 Zn0.97Mn0.03Cr2O4 is excellent in the benzaldehyde formation reaction.Table 2
Catalytic activity of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) catalysts in the presence of H2O2 oxidant for benzyl alcohol to benzaldehyde oxidation.
CatalystOxidantYield (%)∗Without catalystWithout oxidant03.90With catalystWithout oxidant06.78Without catalystWith oxidant22.93ZnCr2O4H2O244.50Zn0.99Mn0.01Cr2O4H2O254.18Zn0.98Mn0.02Cr2O4H2O260.37Zn0.97Mn0.03Cr2O4H2O271.12∗Reaction conditions: 0.5 g catalyst; 5 mmol toluene; 5 mmol acetonitrile; 5 mmol H2O2; reaction temperature (60°C); and reaction time (6 h).Since pristine and Mn-doped ZnCr2O4 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 ZnCr2O4-based catalysts with H2O2 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 ZnCr2O4 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 Zn0.98Mn0.02Cr2O4 catalyst, and upon further increase of Mn concentration in Zn1−xMnxCr2O4 catalyst, this led to a decrease in the yield of carbonyl compounds, as shown in Table 3. This decrement in the catalytic activity of Zn0.97Mn0.03Cr2O4 might be due to the structural distortion, as discussed in the XRD results. The increase of Mn concentration to 3% affects the structure of ZnCr2O4 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. This 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.Table 3
Oxidation of various alcohols with ZnCr2O4-based catalysts in the presence of H2O2 oxidant.
AlcoholsYield (%)(Zn1−xMnxCr2O4 catalysts in the presence of H2O2)∗ZnCr2O4Zn0.99Mn0.01Cr2O4Zn0.98Mn0.02Cr2O4Zn0.97Mn0.03Cr2O419294734293155413442615439476852525869625667736560698069∗Reaction conditions: 0.5 g catalyst; 10 mmol alcohol; 10 mmol acetonitrile; 10 mmol H2O2; temperature, 80°C.In addition to catalytic activities in aldehydes and carbonyl compounds, the photocatalytic degradation effect of pristine and Mn-doped ZnCr2O4 on dye solutions has also been evaluated. The methylene blue (MB) dye solution, along with pristine or Mn-doped ZnCr2O4 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 ZnCr2O4. 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. The 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 efficient interfacial interaction between Mn-doped ZnCr2O4 nanoparticles and photon energy leading to charge separation is occurring, which leads to photocatalytic degradation [28]. The photocatalytic degradation efficiency 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 Ct/C0, in which Ct and C0 are the organic pollutant concentration at a specific time and starting concentrations at the adsorption/desorption equilibrium [29, 30].Figure 4
Degradation curves of methylene blue with (a) pristine ZnCr2O4 and (b–d) Zn1 −xMnxCr2O4 (x = 0.01 to 0.03).Figure4 shows various photocatalytic degradation curves of methylene blue with pristine and Mn-doped ZnCr2O4. The degradation efficiencies with MB under UV light after 60 minutes of irradiation of these pristine and Mn-doped ZnCr2O4 samples are 20.64, 70.78, 74.99, and 89.66%, respectively. Table 4 shows the degradation rate of all the ZnCr2O4 catalysts at different irradiation durations. The photocatalytic results indicate that the Mn doping in ZnCr2O4 effectively increases the catalytic activity and enhances the effective degradation of the dyes and organic compounds under UV light irradiation.Table 4
Photocatalytic degradation efficiency of ZnCr2O4-based catalysts with organic pollutants (MB).
CatalystsDegradation rate (%)After 10 minsAfter 60 minsZnCr2O42.0920.64Zn0.99Mn0.01Cr2O44.2170.78Zn0.98Mn0.02Cr2O46.4574.99Zn0.97Mn0.03Cr2O49.3289.66The Mn-doped ZnCr2O4 samples have high catalytic activity compared to pristine ZnCr2O4, which might be due to the synergetic dual effect of Mn doping [31] and particle size reduction. The transition metal ion (Mn2+) dopant and 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:(i)
The transition metal ion (Mn2+) doping plays a crucial role in catalytic and photocatalytic degradation. The Mn2+ played a role as a trapping center for both electrons and hole charge carriers. The effect of trapping charge carriers is higher for higher dopant concentrations, which may recombine by quantum tunneling. It is expected that the half-filled Mn2+ electronic structure could be acting as a 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)
The other reason for increased photocatalytic activity is due to the nanoscale particle size reduction upon Mn doping with ZnCr2O4, as evidenced by XRD and SEM measurements. The particle size reduction is an essential parameter that has a significant role in the properties of the catalysts [32, 33]. The reduced particle size to the nanometer scale played a vital role in increasing catalytic activity because of the increased surface area to volume ratio. The nanoparticles decrease the diffusion length of charge transfer kinetics across the reactants and the grain boundary resistances so that the catalytic activity in the Mn-doped ZnCr2O4 is increased.Scheme 2
Photocatalytic activity mechanism of Mn-doped ZnCr2O4.


## 3.1. XRD Analysis

The phase identification of the pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) samples synthesized by the facile hydrothermal method has been carried out by X-ray diffraction (Figure 1 (a)–(d)) studies. The peaks obtained are referred to as the (220), (311), (400), (422), (511), and (440) planes of ZnCr2O4, which are very well matched with the JCPDS No. 73-1962. Based on the appearance of the planes, a cubic phase with a spinel structure is assigned to ZnCr2O4. Cheng and Gao reported a similar cubic-phased spinel structure for ZnCr2O4 nanoparticles synthesized by the hydrothermal route [16]. Yazdanbakhsh et al. obtained the cubic phase of spinel-type ZnCr2O4 nanoparticles by the sol-gel route at 700°C calcination temperature [17]. The XRD pattern for the pristine ZnCr2O4 nanoparticles has no additional impurity peaks, which indicate the phase purity of the synthesized ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) compounds.Figure 1
XRD pattern of (a) pristine ZnCr2O4 and (b–d) Zn1−xMnxCr2O4 (x = 0.01 to 0.03).The XRD pattern indicates that the calcination treatment, in addition to the hydrothermal approach, promotes a sharpening of the peaks in the pristine ZnCr2O4 sample. The sharpness and intensity of the X-ray diffraction peaks corresponding to ZnCr2O4 imply a relatively larger crystallite size and high crystallinity [18–20] of the as-synthesized sample. However, when the Mn dopant concentration is increased in the Zn1−xMnxCr2O4 (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 ZnCr2O4. At a lower concentration of Mn with x = 0.01 in Zn1−xMnxCr2O4, no specific change in the peak intensity is observed. The increase of Mn dopant concentration in Zn1−xMnxCr2O4 (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 ZnCr2O4 samples.The mean crystallite size from XRD peaks is calculated using the Debye–Scherrer equation [21]:(1)L=0.98λβcosθ,where θ, β (radians), λ, and L are the Bragg’s diffraction angle, full width at half maxima, incident X-ray light wavelength (1.54 Å), and mean crystallite size, respectively. The mean crystallite sizes of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) are determined to be about 22.80, 18.45, 17.34, and 16.68 nm, respectively.The sharp, intense peak observed for the pristine ZnCr2O4 sample gets slightly broadened when the Mn concentration increases, which reveals a gradual crystallite size reduction in the Zn1−xMnxCr2O4 sample from x = 0.01 to 0.03. However, the gradual decrease in peak intensity of ZnCr2O4 upon an increase of manganese dopant concentration indicates structural disorder in ZnCr2O4 host material with the presence of dopant. The XRD results reveal that any further increase in Mn dopant concentration may distort the structure of ZnCr2O4 abruptly.


## 3.2. Morphology Analysis

Figures2(a)–2(d) shows HR-SEM images of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) nanoparticles. The surface morphology of all the ZnCr2O4 samples indicates inseparable particle clusters. However, Figure 2(c) shows particle sizes less than 50 nm, which need to be visualized clearly at higher magnification. The SEM images reveal particle agglomeration in all the samples. This agglomeration is expected to be caused by the interface surface tension effect. Figures 3(a)–3(d) show the TEM morphology of pristine ZnCr2O4 and Mn-doped ZnCr2O4, which indicates a clear particulate structure. The pristine ZnCr2O4 at higher magnifications (Figures 3(a) and 3(b)) indicates uneven irregular tiled and spheroid particle distributions. The spheroidal shapes are ranging in 70–130 nm, and the irregular tiles are 20 nm in thickness and 80–120 nm in length.Figure 2
HR-SEM images of (a) pristine ZnCr2O4, (b) Zn0.99Mn0.01Cr2O4, (c) Zn0.98Mn0.02Cr2O4, and (d) Zn0.97Mn0.03Cr2O4.
(a)(b)(c)(d)Figure 3
HR-TEM images of (a, b) pristine ZnCr2O4 and (c, d) Zn0.97Mn0.03Cr2O4.
(a)(b)(c)(d)In the Mn-doped ZnCr2O4 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. The obtained pristine and doped ZnCr2O4 nanoparticle morphology is good for catalytic activity due to the huge surface-to-volume ratio [22]. The specific surface area and pore size properties estimated from SEM images are listed in Table 1, which indicates a low surface area for pristine ZnCr2O4 and, in the case of Mn-doped ZnCr2O4, a higher BET surface area. The high pore volume with reduced pore diameter values suggests that the Mn-doped ZnCr2O4 has a high specific surface area (cm3/g) related to deep pores. The 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. Thus, by the hydrothermal method, the BET surface area of pristine and Mn-doped ZnCr2O4 samples is increased along with the reduction in the average pore diameter. This 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 [23–25].Table 1
BET surface area, average pore diameter, pore volume, and their crystallite size (nm) of pristine and Mn-doped ZnCr2O4 samples.
Catalyst propertiesZnCr2O4Zn0.99Mn0.01Cr2O4Zn0.98Mn0.02Cr2O4Zn0.97Mn0.03Cr2O4SBET (m2/g)37.8744.8749.9995453.54Rp (Å)20.2016.3312.2315.89Vp (cm3/g)0.07310.08900.10980.0993Crystallite size (nm)29.6827.3422.4520.80


## 3.3. Catalytic Activity

In order to investigate the activity of spinel-type pristine and Mn-doped ZnCr2O4 sample nanostructures as a heterogeneous catalyst, the samples are dispersed in an acetonitrile medium. The oxidation of toluene is carried out by using H2O2 as an oxidant at a reaction temperature of 60°C for 6 h. A pure form of benzaldehyde is discovered as an oxidation product in the catalytic reaction mixture, as shown in Scheme 1.Scheme 1
Schematic representation for the oxidation of toluene with ZnCr2O4 catalyst.The catalytic performance of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) samples is examined by employing hydrogen peroxide (H2O2) as the oxidant in the selective oxidation of toluene to form benzaldehyde in an acetonitrile medium. Blank reactions are performed over pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) samples under the same reaction conditions, which show negligible yields of benzaldehyde. Only a 3.9% yield of benzaldehyde is obtained in the blank reaction when no oxidant or catalyst is employed. In order to increase the yield of benzaldehyde, several promoters, such as pristine ZnCr2O4, Zn1−xMnxCr2O4 (x = 0.01 to 0.03), H2O2, and solvent are added [26, 27]. Among the three additives, Zn0.97Mn0.03Cr2O4 with H2O2 is the most effective 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 Zn0.97Mn0.03Cr2O4 is excellent in the benzaldehyde formation reaction.Table 2
Catalytic activity of pristine ZnCr2O4 and Zn1−xMnxCr2O4 (x = 0.01 to 0.03) catalysts in the presence of H2O2 oxidant for benzyl alcohol to benzaldehyde oxidation.
CatalystOxidantYield (%)∗Without catalystWithout oxidant03.90With catalystWithout oxidant06.78Without catalystWith oxidant22.93ZnCr2O4H2O244.50Zn0.99Mn0.01Cr2O4H2O254.18Zn0.98Mn0.02Cr2O4H2O260.37Zn0.97Mn0.03Cr2O4H2O271.12∗Reaction conditions: 0.5 g catalyst; 5 mmol toluene; 5 mmol acetonitrile; 5 mmol H2O2; reaction temperature (60°C); and reaction time (6 h).Since pristine and Mn-doped ZnCr2O4 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 ZnCr2O4-based catalysts with H2O2 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 ZnCr2O4 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 Zn0.98Mn0.02Cr2O4 catalyst, and upon further increase of Mn concentration in Zn1−xMnxCr2O4 catalyst, this led to a decrease in the yield of carbonyl compounds, as shown in Table 3. This decrement in the catalytic activity of Zn0.97Mn0.03Cr2O4 might be due to the structural distortion, as discussed in the XRD results. The increase of Mn concentration to 3% affects the structure of ZnCr2O4 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. This 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.Table 3
Oxidation of various alcohols with ZnCr2O4-based catalysts in the presence of H2O2 oxidant.
AlcoholsYield (%)(Zn1−xMnxCr2O4 catalysts in the presence of H2O2)∗ZnCr2O4Zn0.99Mn0.01Cr2O4Zn0.98Mn0.02Cr2O4Zn0.97Mn0.03Cr2O419294734293155413442615439476852525869625667736560698069∗Reaction conditions: 0.5 g catalyst; 10 mmol alcohol; 10 mmol acetonitrile; 10 mmol H2O2; temperature, 80°C.In addition to catalytic activities in aldehydes and carbonyl compounds, the photocatalytic degradation effect of pristine and Mn-doped ZnCr2O4 on dye solutions has also been evaluated. The methylene blue (MB) dye solution, along with pristine or Mn-doped ZnCr2O4 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 ZnCr2O4. 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. The 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 efficient interfacial interaction between Mn-doped ZnCr2O4 nanoparticles and photon energy leading to charge separation is occurring, which leads to photocatalytic degradation [28]. The photocatalytic degradation efficiency 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 Ct/C0, in which Ct and C0 are the organic pollutant concentration at a specific time and starting concentrations at the adsorption/desorption equilibrium [29, 30].Figure 4
Degradation curves of methylene blue with (a) pristine ZnCr2O4 and (b–d) Zn1 −xMnxCr2O4 (x = 0.01 to 0.03).Figure4 shows various photocatalytic degradation curves of methylene blue with pristine and Mn-doped ZnCr2O4. The degradation efficiencies with MB under UV light after 60 minutes of irradiation of these pristine and Mn-doped ZnCr2O4 samples are 20.64, 70.78, 74.99, and 89.66%, respectively. Table 4 shows the degradation rate of all the ZnCr2O4 catalysts at different irradiation durations. The photocatalytic results indicate that the Mn doping in ZnCr2O4 effectively increases the catalytic activity and enhances the effective degradation of the dyes and organic compounds under UV light irradiation.Table 4
Photocatalytic degradation efficiency of ZnCr2O4-based catalysts with organic pollutants (MB).
CatalystsDegradation rate (%)After 10 minsAfter 60 minsZnCr2O42.0920.64Zn0.99Mn0.01Cr2O44.2170.78Zn0.98Mn0.02Cr2O46.4574.99Zn0.97Mn0.03Cr2O49.3289.66The Mn-doped ZnCr2O4 samples have high catalytic activity compared to pristine ZnCr2O4, which might be due to the synergetic dual effect of Mn doping [31] and particle size reduction. The transition metal ion (Mn2+) dopant and 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:(i)
The transition metal ion (Mn2+) doping plays a crucial role in catalytic and photocatalytic degradation. The Mn2+ played a role as a trapping center for both electrons and hole charge carriers. The effect of trapping charge carriers is higher for higher dopant concentrations, which may recombine by quantum tunneling. It is expected that the half-filled Mn2+ electronic structure could be acting as a 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)
The other reason for increased photocatalytic activity is due to the nanoscale particle size reduction upon Mn doping with ZnCr2O4, as evidenced by XRD and SEM measurements. The particle size reduction is an essential parameter that has a significant role in the properties of the catalysts [32, 33]. The reduced particle size to the nanometer scale played a vital role in increasing catalytic activity because of the increased surface area to volume ratio. The nanoparticles decrease the diffusion length of charge transfer kinetics across the reactants and the grain boundary resistances so that the catalytic activity in the Mn-doped ZnCr2O4 is increased.Scheme 2
Photocatalytic activity mechanism of Mn-doped ZnCr2O4.


## 4. Conclusion

The nanoscale pristine and Mn-doped ZnCr2O4 nanoparticles have been synthesized by hydrothermal techniques. XRD results indicate spinel-structured ZnCr2O4 compound formation in which the relative crystallinity is found to be slightly distorted in Mn-doped ZnCr2O4. HR-SEM images reveal inseparable particle clusters with a minimum crystallite size of 20.8 nm. The TEM morphology at higher magnifications indicate irregular, uneven spheroid particle distributions. The catalytic activity of Zn0.97Mn0.03Cr2O4 in toluene with H2O2 as an oxidant indicates a higher yield of benzaldehyde. Various alcohols, such as butanol, hexanol, heptanol, octanol, and 1-phenyl ethanol, are also oxidized along with Mn-doped ZnCr2O4 catalysts, resulting in a high yield of corresponding carbonyl compounds. Under UV irradiation, the photocatalytic degradation efficiency of methylene blue is found to be 89.66% due to the photocatalytic effect of the Zn0.97Mn0.03Cr2O4 catalyst. In a nutshell, the nanoscale Mn-doped ZnCr2O4 catalysts reveal significant catalytic activity and photocatalytic degradation efficiency due to the synergetic effect of Mn doping and particle size reduction.



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