Catalytic and Photocatalytic Degradation Activities of Nanoscale Mn-Doped ZnCr<sub>2</sub>O<sub>4</sub>

Sankudevan, P.; Sakthivel, R. V.; Gopal, Ramalingam; Raghupathi, C.; Ambika, S.; Mujahid Alam, Mohammed; Rangasamy, Baskaran

doi:https://doi.org/10.1155/2022/7056380

Advances in Materials Science and Engineering

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Abstract Introduction Results Conclusion Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2022 | Article ID 7056380 | https://doi.org/10.1155/2022/7056380

Catalytic and Photocatalytic Degradation Activities of Nanoscale Mn-Doped ZnCr₂O₄

P. Sankudevan,¹R. V. Sakthivel,¹Ramalingam Gopal,²C. Raghupathi,³S. Ambika,⁴Mohammed Mujahid Alam,⁵and Baskaran Rangasamy⁶

Academic Editor: Zhongchang Wang

Received24 Jul 2022

Revised16 Oct 2022

Accepted25 Nov 2022

Published20 Dec 2022

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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ 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 Zn_0.97Mn_0.03Cr₂O₄ catalyst with methylene blue has been obtained.

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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ (x = 0.01 to 0.03) compounds.

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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄, no specific change in the peak intensity is observed. The increase of Mn dopant concentration in Zn_1−xMn_xCr₂O₄ (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]: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 Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ 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

Figures 2(a)–2(d) shows HR-SEM images of pristine ZnCr₂O₄ and Zn_1−xMn_xCr₂O₄ (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.

(a)

(b)

(c)

(d)

(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].

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.

The catalytic performance of pristine ZnCr₂O₄ and Zn_1−xMn_xCr₂O₄ (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 Zn_1−xMn_xCr₂O₄ (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, Zn_1−xMn_xCr₂O₄ (x = 0.01 to 0.03), H₂O₂, and solvent are added [26, 27]. Among the three additives, Zn_0.97Mn_0.03Cr₂O₄ 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 Zn_0.97Mn_0.03Cr₂O₄ is excellent in the benzaldehyde formation reaction.

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 Zn_0.98Mn_0.02Cr₂O₄ catalyst, and upon further increase of Mn concentration in Zn_1−xMn_xCr₂O₄ catalyst, this led to a decrease in the yield of carbonyl compounds, as shown in Table 3. This decrement in the catalytic activity of Zn_0.97Mn_0.03Cr₂O₄ 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.

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 C_t/C₀, in which C_t and C₀ are the organic pollutant concentration at a specific time and starting concentrations at the adsorption/desorption equilibrium [29, 30].

Figure 4 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.

The 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.

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 Zn_0.97Mn_0.03Cr₂O₄ 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 Zn_0.97Mn_0.03Cr₂O₄ 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.

Data Availability

The data used to support the findings 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

The authors declare that they have no conflicts of interest.

Authors’ Contributions

Mr. P. Sankudevan conceptualized the study, performed sample synthesis, did characterizations, wrote the original draft, performed data interpretation, validated the study, performed data analysis, and reviewed and edited the article. Dr. R. V. Sakthivel conceptualized the study, wrote, reviewed, supervised, edited and developed the methodology. Dr. Ramalingam Gopal performed formal analysis and data curation, developed the methodology, and funded it. Conceptualization and data resources were done by Dr. C. Raghupathi. Conceptualization, data curation, and formal analysis were carried out by Dr. S. Ambika Data curation, writing, review and editing, and funding were done by Dr. Mohammed Mujahid Alam. Writing, data interpretation, data analysis, revising, visualization, review, validation, and editing were performed by Prof. Baskaran Rangasamy.

Acknowledgments

One of the co-authors, Dr. G. Ramalingam acknowledges the full financial support by the MHRD-SPARC-890(2019) and instrumentation facility utilized from RUSA 2.0 grant No. F.2451/2014-U, Policy (TN Multi Gen) Govt of India Projects. Dr. Mohammed Mujahid Alam, especially thanks the Deanship of Scientific Research, King Khalid University, Saudi Arabia, supported under grant number R.G.P.2/59/1443.

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Copyright © 2022 P. Sankudevan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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