Improving the Photocatalytic Performance of Porous Ceria under Visible Light Illumination via Mn Incorporation

Abstract

Porous cerium oxide (ceria) nanoparticles were prepared with and without manganese (Mn) by using the flash combustion technique. Samples with different loadings (Ce/Mn ratio ranged from 100 to 10) were prepared by using a one-step process and water only as a solvent. Moreover, citric acid was utilized as a fuel in an aqueous medium, and the overall synthesis mixture was dried at 100 °C overnight and then calcinated at 550 °C for 3 h. The obtained final solid product was characterized by inductively coupled plasma (ICP), X-ray powder diffraction (XRD), diffuse reflectance ultraviolet-visible spectroscopy (DR-UV-Vis), and scanning electron microscopy (SEM), which was coupled with Energy Dispersive X-Ray Analysis (EDX), high resolution transmission electron microscopy (HR-TEM), and photoluminescence (PL) analysis. The characterization data showed that Mn ions were totally incorporated into the framework of ceria up to the applied loading. Under visible light illumination, the photocatalytic activity of the prepared samples was tested in the decolorization reaction of methyl green (MG) dye (wavelength greater than 425 nm). The results showed that increasing Mn content improved the photocatalytic activity of ceria. The sample with a Ce/Mn ratio of 10 performed 1.8 times better than bare porous ceria. Finally, the reusability of the best-performing sample was investigated in four consecutive runs without treatment, and slight deactivation was monitored after the fourth run.

1. Introduction

Water quality has recently deteriorated significantly due to the rapid pace of urban development and population growth. Various industrial processes discharged a large amount of toxic wastewater into the environment, resulting in severe environmental pollution [1,2]. Most conventional water treatment methods, e.g., coagulation and flocculation, are incapable of completely removing these harmful pollutants. These processes are slow and generate a large amount of sludge. Moreover, handling and disposal are also major issues associated with this process. On the other hand, adsorption is a successful technique, but regeneration was also a major issue. Membrane separations and ion exchange are also good techniques in water purification but have several disadvantages, such as the required high pressures and the overall operation costs. The advanced oxidation processes (AOP) are promising chemical treatment processes for wastewater pollutants because they have no sludge production, little or no chemical consumption, and high efficiency for recalcitrant dyes [3,4]. Advanced oxidation processes involve oxidation via hydroxyl radical reactions (^•OH) in the presence of homogeneous and heterogeneous photocatalysis [5]. Amongst (AOP), heterogeneous photocatalysis arises as an excellent process for the degradation of pollutants due to its effectiveness, eco-friendliness, and low cost [6,7]. Metal oxide nanoparticles have recently been seen as efficient, stable, and economical photocatalysts that effectively degrade toxic pollutants. A multitude of metal oxides such as CeO₂, ZnO, TiO₂, WO₃, and Fe₂O₃ are reported as photocatalysts to eradicate the contaminants from water bodies, thus establishing a reliable approach towards the degradation of organic matters [8,9,10].

Cerium oxide (CeO₂), a rare earth oxide, has shown promise in recent years for widespread use in catalysts [11,12]. It has been widely used in a variety of applications, including electrocatalysis, fuel cells, solar cells, and photocatalysis [13,14,15,16,17]. This is due to CeO₂’s superior properties, which include low toxicity, low cost, high chemical stability, and a stable Ce³⁺/Ce⁴⁺ redox couple, which may aid in the formation of abundant oxygen vacancies in CeO₂ [18,19]. Meanwhile, the abundance of oxygen vacancies available can reduce electron hole recombination and increase photocatalytic activity [20]. However, the performance of CeO₂ remains unsatisfactory due to some drawbacks, including a low specific surface area, a large bandgap (2.8–3.1 eV) with lower absorption in visible regions up to 400 nm, and a high electron–hole recombination. As a result, CeO₂ modification is required to address these shortcomings [21].

The unique properties can be enhanced by modifications of CeO₂, such as metal and non-metal doping, adding support materials, and coupling with another semiconductor [22,23]. In order to reduce the band gap and to enhance the photo-activity of CeO₂, transition metal ions of variable valency have been introduced into the CeO₂ lattice such as Fe, Cu, Ni, Mn, Co, and Ag. Amongst these transition metal ions, Mn ions are known to exhibit a multiple valence nature similar to that of Ce ions. The substitution of Mn ions with Ce ion or incorporation in the crystal lattice leads to the generation of new sub-energy levels between the valence, and conduction bands reduce the recombination and improve the rate of separation between the photogenerated electron and the hole, thus improving the photocatalytic properties of ceria significantly. The present study investigates the structural, morphological, electrical, and dielectric properties of Mn-doped CeO₂ compounds [24].

In the current study, a new one-step facile method of flash combustion was applied to produce Mn-doped porous ceria (Mn-CeO₂) with different Mn content to improve its photocatalytic performance. The photocatalytic performance of the prepared materials was investigated in the decolorization reaction of methyl green (MG) dye under visible light illumination. The kinetic and rate constant of each reaction were calculated, and the obtained results are reported and discussed herewith.

2. Results

2.1. Characterization Data

The ICP technique was applied to investigate quantitative elemental analysis.

Table 1. A comparison between Ce/Mn ratio in the synthesis gel and the final solid product as obtained from ICP elemental analysis.

Sample	Ce/Mn Ratio (Synthesis Gel)	Ce/Mn Ratio (Final Product)	Accuracy (%)
Porous Ce	-	-	-
Mn-CeO2(1%)	100	98.6	98.6%
Mn-CeO2(2.5%)	40	39.1	97.7%
Mn-CeO2(5%)	20	19.7	98.5%
Mn-CeO2(10%)	10	9.96	99.6%

shows the Ce/Mn ratio in the synthesis gel and the obtained final product. The obtained results were very close, and the synthesis accuracy was greater than 97% for the entire material. Due to the utilization of the one-pot synthesis technique, all the Mn ions added to the synthesis gel were found in the final solid product. Moreover, this can be an indication of the high predictability of the synthesis procedure.

The phase composition of the prepared materials was investigated by the XRD technique (Figure 1). Different peaks were observed at 29.51°, 33.48°, 48.02°, and 56.76° 2θ. These diffraction patterns are well indexed to the face-centered cubic-fluorite structure of ceria (JCPDS Card number 34-0394), with characteristic indexing planes of (111), (200), (220), and (311). Moreover, additional peaks related to the Mn oxide phase(s) were not observed. This can be indicated by the total incorporation of Mn ions in the ceria lattice. It is noteworthy that there is a slight decrease in the peak intensity of the Mn-doped porous ceria materials compared to that of bare ceria. This can be related to the effect of Mn ions on the integrity of the ceria framework. Furthermore, the lattice values were also determined and listed in

Table 2. Lattice constants were obtained theoretically for all Mn-CeO₂ samples.

Samples	d111 (Å)	d200 (Å)	a = b = c (Å)	V (Å)3
JCPDS#34-0394	3.12344	2.70564	5.4113	158.46
Porous CeO2	3.10853	1.90845	5.3841319	156.079933
Mn-CeO2(1%)	3.10007	1.90406	5.36947875	154.809063
Mn-CeO2(2.5%)	3.10836	1.90713	5.38383745	156.054328
Mn-CeO2(5%)	3.11704	1.91072	5.39887165	157.365312
Mn-CeO2(10%)	3.09375	1.90187	5.35853219	153.864182

, which are well-matched to the standard ones reported in JCPDS#34-0394. However, the peak intensity is remarkably affected by Mn doping content. The variations in the intensity of peaks signify that the crystallite size (L), dislocation density (δ), and other parameters will change. Therefore, these parameters were determined as mean for (111) and (200) peaks, and X-ray density (ρ_x-ray) and specific surface area (S) were estimated using the below rules [25]:

$$L=\frac{0.09\lambda }{\beta cos\theta }$$

(1)

$$\delta =\frac{1}{L^2}$$

(2)

$${\rho }_{x-ray}=\frac{ZM}{V_{cell }\times N_A}$$

(3)

$$S=\frac{6000}{{\rho }_{x-ray }{*}_L}$$

(4)

Here, N_A is the Avogadro constant, M is the molar mass, Z is the number of molecules per formula unit, and V_cell is the volume of unit cell [26], and the determined values of L, δ, ε, ρ_x-ray, and S are provided in

Table 3. Estimated values of size (L), dislocations (δ), strain (ε₂₀₀), and x-ray density (ρ_x-ray) and surface area (S) for all Mn-CeO₂ samples.

Samples	Scherrer Cal. Lave (nm)	δave (nm−2)	ρx-ray (g/cm3)	S (m2/g)	W–H Plot Data
					Lave (nm)	εave
Porous CeO2	34.52998388	8.565 × 10−4	7.324546474	23.72325	46.53	3.032 × 10−3
Mn-CeO2(1%)	10.22887532	9.607 × 10−3	7.384675683	79.43135	9.549	1.031 × 10−2
Mn-CeO2(2.5%)	8.440025906	1.410 × 10−2	7.325748305	97.04105	8.506	1.247 × 10−2
Mn-CeO2(5%)	7.909808453	1.604 × 10−2	7.264718686	104.4159	6.943	1.344 × 10−2
Mn-CeO2(10%)	6.258203389	2.56 × 10−2	7.430025071	129.0361	6.75	1.666 × 10−2

. It was clearly noticeable that the crystallite size values reduced from 34.5 nm to 6.25 nm as the Mn content increased from 0 to 10 wt% in a systematic way. Therefore, the dislocation density increased with doping. Additionally noticed was the reduction in crystallite size with Mn doping in CeO₂ prepared by using the flash combustion technique [27]. This implies that Mn doping in CeO₂ causes more defects and strain. The density of prepared nanostructures ranges between 7.32 and 7.43 g/cm³, which is close to the industry standard of 7.22 g/cm³ (JCPDS#34-0394). When the Mn content is increased to 10% wt, the specific surface area increases dramatically from 23.72 m²/g to 129 m²/g as the particle size is reduced from 34.5 nm to 6.25 nm on Mn doping; this effect of particle size on surface area has previously been reported [28]. However, for the surface area analysis, the BET/BJT is a more accurate technique, as discussed previously by Shi et al. [29]. Furthermore, we have applied the Williamson–Hall process to determine the values of L and ε [30]. Through XRD data, the peaks total broadening (β_τ) is because of the combinational impact of broadening due to crystallite size (β_D) and microstrain (β_ε), which is expressed as:

$${\beta }_{\tau }={\beta }_D+{\beta }_{\epsilon }$$

(5)

As per the Scherrer equation:

$$L=\frac{K\lambda }{{\beta }_{D}cos\theta }$$

$$\mathrm{Or} {\beta }_D=\frac{K\lambda }{Lcos\theta }$$

(6)

Similarly, the β_ε is expressed as:

$${\beta }_{\epsilon }=4\epsilon tan\theta$$

(7)

Using Equations (8) and (9) in (6), we will get:

$${\beta }_{\tau }=\frac{K\lambda }{Lcos\theta }+4\epsilon tan\theta$$

(8)

Or, after modification, we will get:

$${\beta }_{\tau }cos\theta =\epsilon \left(4sin\theta \right)+\frac{K\lambda }{L}$$

(9)

This equation signifies the strain line, in which the gradient/slope is ε and the y-intercept is Kλ/L and by considering the straight-line equation,

Y = mx + c

(10)

In this m is the slope and c is the intercept. Comparing Equations (9) and (10), we will have:

$$Y={\beta }_{\tau }cos\theta , m=\epsilon , x=4sin\theta and c=\frac{K\lambda }{L}$$

The estimated values of L and ε from the W–H method are listed in Table 3. The L values are in trend with the values obtained by the Scherrer equation and the ε value varies with Mn content (see Table 2).

Diffuse reflectance (DR) spectra of Mn-CeO₂ NPs are exhibited in (Figure 2). DR spectra have been used to estimate the optical band gaps of Mn-CeO₂ using Kubelka–Munk function F(R) [31].

$$F\left(R\right)=\frac{\alpha }{s}=\frac{{\left(1-R\right)}^2}{2R}$$

(11)

where α is an absorption coefficient and s is called the scattering factor, which is equal to one. The optical band gaps of undoped and Mn-CeO₂ were obtained by plotting the graph between [F(R)hv]² and E_g using Tauc’s relation [32] and exhibited in (Figure 3).

$${\left[F\left(R\right)hv\right]}^2=C\left(hv-E_g\right)$$

(12)

where C is the constant of proportionality. The resulting spectra (expressed by the Kubelka-Munk function) are plotted as a function of the measuring wavelength in Figure 2. The spectra showed a strong absorption band in the UV region (400 nm), which can be attributed to charge transfer during the O2p to Ce4f transition with Ce⁴⁺ and O²⁻ states. A small tail was found in the visible light region for the porous ceria sample with a clear cut-off wavelength of about 403 nm. However, in the Mn-doped ceria samples there was an obvious red-shift in the absorption spectra, and the location of the cut-off wavelength was found to be increased with the Mn content. Moreover, the bandgap (Eg) value calculation exhibited a clear decreasing trend with the increase of Mn content. The bandgap for porous ceria was calculated to be 3.1 eV, while for Mn-CeO₂ (1, 2.5, 5, and 10%) it was found to be 3.1, 3.09, 2.9, and 2.85 eV, respectively. The optical gap was reduced due to many-body effects on the conduction (CB) and valence band (VB) [33], the optical band gap narrowed due to spin exchange coupling interactions between p and d localized electrons of Mn and O ions [34], and the density of states in the CB increased. Furthermore, the optical band gap of as-prepared NPs is attributed to nanoparticle size and shape effects, dopant concentration, and microstructure features. The obtained optical band gaps were found to decrease with decreasing crystallite size and simultaneously increasing lattice strain, dislocation density, and surface area (Table 3). The higher surface area of the nanoparticles indicates more grain boundaries, which produces more structural defects/disorders across the nanoparticles. The inclusion of defects above and below the valence and conduction band reduces the optical band when increasing the Mn content in the CeO₂ structure.

The morphology of the prepared samples was investigated by SEM analysis. The obtained micrographs for the Mn-doped ceria samples are presented in Figure 4. From our previous study [35], it was observed that there is a huge difference between the morphology of commercial ceria and the porous ceria prepared by citric acid. The porous ceria sample has a rough surface with a sponge-like structure and there are obvious pores with different sizes on the surface, which were clearly seen by SEM analysis [35]. The same morphology was observed in the Mn-doped ceria samples. The rough surface with the voids and pores was also observed in the prepared samples, and more importantly, no separate crystals of Mn oxide(s) or other phases could be observed. This can be an indication of the total incorporation of Mn ions in the ceria lattice, which agrees with the XRD analysis.

EDX analysis was performed to investigate the purity of the prepared samples. The EDX analysis results are presented in Figure 5. Three main elements (Ce, Mn, and O) were obtained in the final product as an indication of the high purity of the prepared samples. Elemental carbon was also observed as a result of its presence in the carbon tape that was used during the analysis.

The particle size was further investigated by using HR-TEM, and the obtained micrographs are presented in Figure 6. The observed nanoparticles are mainly hexagonal in shape, and similar behavior was reported by Narayana et al. [36]. The measured inter-planner spacing of almost 0.31 nm corresponds to the (111) plane. Moreover, it was observed that the nanoparticle size of Mn-CeO₂(1%) was bigger than that of Mn-CeO₂(10)%, which is in good agreement with the particle size obtained from Scherrer’s calculation.

2.2. The Photocatalytic Study

At the beginning of the photocatalytic study, several blank reactions were carried out to confirm the photocatalytic decolorization of the dye. The blank reactions included the photolysis experiment, in which the dye solution was kept under visible light for 120 min. Moreover, the catalytic decolorization took place, in which the dye solution was stirred with the porous ceria sample in the dark. In the two blank experiments, neither reactions nor adsorption could be observed. Another set of experiments was performed to investigate the adsorption behavior of the prepared materials. The adsorption capacity experiments were performed under dark conditions for 60 min before the photocatalytic studies. Porous ceria did not show negligible affinity towards MG dye molecules. Moreover, it was found that Mn can slightly improve the adsorption of the dye molecules, but not more than 7% of the dye concentration was adsorbed on the surface of the catalyst. In general, cerium oxide has a weak adsorption affinity towards the applied dye.

The photocatalytic activity of the prepared materials was investigated in the decolorization reaction of methyl green dye under visible light illumination. The decolorization profiles of the color intensity of the investigated dye over the prepared materials are plotted in Figure 7. The porous ceria sample was able to catalyze the decolorization of 34.5% of the dye within 120 min. On the other hand, Mn-doped CeO₂ samples catalyzed the decolorization of 36.6, 38.4, 42.9, and 47.9% of the original MG color over 1, 2.5, 5, and 10% samples, respectively. Therefore, an obvious trend can be noticed, increasing the amount of Mn ions that lead to an increase in the photocatalytic activity of porous ceria. However, this was not feasible for higher Mn content. Because a sample with a Ce/Mn ratio of 5 was prepared, the photocatalytic activity of the sample was decreased sharply compared to the bare sample.

The kinetics of the reactions were estimated using a pseudo-first-order reaction model [37]. In this model, if Co is the initial concentration of the dye, and C is the concentration at time t, the plot of ln(Co/C) against the reaction time must produce a straight line, in which the slope will represent the rate constant of the reaction (k). The kinetics of the performed reactions are presented in Figure 8, in which the obtained decolorization data fit perfectly with the pseudo-first order model.

The calculated pseudo first-order rate constants of the prepared materials are compared in Figure 9. In general, there is an obvious increase in the rate constant as Mn content increases. The sample with 1% of Mn exhibited 1.2 high activity, while the sample with 10% Mn exhibited 1.8 times more activity than the neat sample.

The reusability study was performed using the most performed sample, Mn-CeO₂(10%), for four consecutive runs. The sample was applied to the reaction without any treatment. The obtained pseudo first-order rate constants were compared in Figure 10. The obtained results showed that the sample lost almost 12% of its original activity after four runs. This deactivation can be referred to as the accumulation of organic compounds over the surface of the sample. These results showed that there is a slight deactivation during the reusability of the sample and confirm that the prepared samples can be reused several times.

2.3. The Photoluminescence Study

Photoluminescence (PL) was used to investigate the possibility of electron–hole pair recombination in a photocatalyst. Because the PL emission spectra was reported from the recombination of excited electrons and holes, a higher PL intensity indicated a higher rate of electron–hole pair recombination under light irradiation [38].

In Figure 11, the photoluminescence spectra of the CeO₂ and Mn doped CeO₂ recorded an excited wavelength of 325 nm. As shown, the broad violet emissions with peak maxima in the range of 360 to 450 nm were observed, and these emissions were assigned to the electronic transition between the valence band and the Ce 4f level. In other words, the emission bands originate from the ligand-to-metal charge transfer (LMCT) states O → Ce⁺⁴ from ${\mathrm{O}}_2^{-2}$ ligand to Ce⁴⁺ ions via hopping of electrons above 3 eV. The defect levels being localized between the Ce 4f band and O 2p band are the basic reason for a broader emission peak. The analysis of the luminescent properties of CeO₂ also testifies to the presence of cerium ions in different valence states. The observed luminescence of the CeO₂ NPs due to radiative transitions in the Ce³⁺ ion. The final stage of the radiative process is described by the scheme (Ce³⁺)* → Ce³⁺ + hν. In addition, a weak band was observed in the PL spectrum, which can be associated with the emission of F₀ centers connected with the formation of oxygen vacancies [39]. Accordingly, undoped CeO₂ presented the highest intensity, which indicated that undoped CeO₂ provided the fastest recombination rate of electrons and holes. After the Mn ions were introduced to CeO₂, the intensity of the PL emission gradually decreased, which confirmed that the doping with Mn ions could delay the recombination rate of photogenerated charge carriers. The rate of the recombination between photogenerated holes and electrons might be reduced on increasing the amount of Mn, which leads to the complete quenching of the PL signal in ceria nanoparticles. The concentration quenching could be explained by two factors: excitation migration due to increased resonance between activators during doping concentration increase, and then the excitation energy reaches quenching centers, and activators were paired/coagulated and changed to quenching centers. These findings support the hypothesis that increasing the amount of Mn ions increases the photocatalytic activity of porous ceria.

3. Discussion

The obtained photocatalytic results are in good agreement with the previous research. Ma et al. [40] reported the incorporation of Mn in TiO₂, and the prepared materials were investigated in the decolorization of methylene blue. It has been reported that Mn content improves TiO₂ photocatalytic activity. In another study, Lin et al. [41] reported the incorporation of Mn ions in the ZnO lattice, and the produced material was applied to photodecompose 2,4-dichlorophenol. Again, it was reported that increasing the content of Mn in the ZnO lattice led to an increase in the photocatalytic activity of ZnO. Moreover, Narayana et al. [42,43,44] reported the incorporation of Mn ions in the CeO₂ lattice, and the prepared material was used in the degradation of malachite green dye. In that work, it was reported that Mn can improve the photocatalytic activity of ceria up to 20% Mn loading.

Based on the obtained results, the photocatalytic decolorization mechanism of MG dye can be proposed as follows: upon the visible light illumination, the photo-generated electron/hole pairs are formed. Electrons participate in the formation of superoxide anions (O₂^•−), while holes participate in the formation of hydroxyl radicals (HO^•). In the presence of Mn ions (Figure 12), which normally have different oxidation states [45] in the ceria lattice, the electrons are directed to reduce Mnⁿ⁺ first, which simply means increasing the separation time of electron/hole pairs and therefore increasing the chance to form the radicals which are required to decolorize the dye molecules. Hence, increasing the number of Mn ions means an obvious increase in the photocatalytic performance of ceria.

Another scenario can be proposed, in which Mn at high-loading samples can possibly form Mn₃O₄ nanoparticles (too small to be detected by XRD and SEM). In that case, a type-I heterojunction photocatalysis mechanism is expected [46], as seen in Figure 13. The photogenerated electrons can migrate from the conduction band of ceria to the corresponding conduction band of Mn₃O₄, while the photogenerated holes can migrate to the valance band of Mn₃O₄. This behavior minimizes the recombination of the photogenerated electron/hole pairs and increases the number of reactions on the photocatalyst surface. A similar mechanism was proposed earlier by Xingyu Pu et al. [47].

An activity comparison between Mn doped porous ceria and other reported photocatalysts under the same reaction conditions is listed in

Table 4. A comparison between the rate constant (k) of different photocatalysts in the decolorization of methyl green dye under the same reaction conditions.

Photocatalyst	Rate Constant (min−1)	Reference
CeO2	1 × 10−3	[35]
Porous ceria	3.38 × 10−3	[35]
ZnO	9 × 10−4	[35]
TiO2	2.3 × 10−3	[35]
Fe-ZnO (5%)	5.7 × 10−3	[48]
Co-ZnO	2 × 10−3	[49]
Mn-CeO2 (10%)	6.27 × 10−3	This work

. The obtained results clearly show that Mn doped porous ceria is a promising photocatalyst under visible light illumination, and further investigation must be performed to use this sample with different contaminants either in water or in air.

4. Experimental

4.1. Synthesis

The synthesis of Mn-doped ceria nanostructures was performed by using a one-step synthesis procedure. In a typical synthesis method, 5 g of cerium nitrate hexahydrate (Ce(NO₃)₃⋅6H₂O) and 1g of citric acid (C₆H₈O₇) were placed in cleaned porcelain crucibles with a stoichiometric amount of manganese nitrate hexahydrate (Mn(NO₃)₃⋅6H₂O). The solid content was dissolved in 5–7 g of demi-water under vigorous stirring until complete dissolution. It was then dried at 90 °C for 24 h, and finally the formed solid was calcined in a muffle furnace at 550 °C for 180 min by using a heating ramp of 18 °C/min.

4.2. Characterization

The prepared samples were characterized by several physical and chemical characterization techniques in order to clarify their properties. ICP elemental analysis (Thermo scientific, ICAP 7000 series, component No: 1340910, Qtegra Software) was applied to determine the exact Ce/Mn ratio. For phase confirmation, all the prepared powders were analyzed by X-ray diffractometer (XRD) by using a Shimadzu LabX-6000 diffractometer with CuKα radiation (λ = 1.54056 Å), operated at 40/30 kV/mA, at 2°/m between 20°–70°. A JEOL JSM 6310-SEM coupled with an EDX system operating at 20 kV was used to capture the e-mapping, elemental composition, and morphology. A Shimadzu UV-3600 diffused reflectance spectrophotometer (DRS) setup was employed to investigate the optical properties and the energy gap of the prepared samples. A JEOL-2100F transmission electron microscope was employed to capture high resolution transmission electron microscopy (HR-TEM) micrographs. Photoluminescence (PL) emission spectra were obtained at λ_exc of 325 nm on a Lumina-fluorescence spectrophotometer over the 350–600 nm wavelength under ambient conditions.

4.3. Photocatalysis

Under visible light illumination, the photocatalytic performance of the prepared materials was investigated in the decolorization reaction of methylene green (MG) aqueous solution. In a typical experiment, 0.1 g of catalyst is dispersed into 50 mL of 0.02 g/L dye solution, and the overall mixture is stirred for 1 h to achieve uniform catalytic material dispersion. Later, the suspension was kept in a photocatalytic chamber containing six Phillips light bulbs (18 W power), and the moment the light turned ON was considered as the initial time of the reaction. The samples were taken periodically at equal time intervals and the absorption spectra were recorded to measure the catalytic activity parameters. After the experiments were completed, the catalyst material was filtered and tested for reusability for up to four sequential runs.

5. Conclusions

In the current study, Mn was doped into porous ceria nanoparticles for the first time. Four samples with a different Ce/Mn ratio were prepared by using citric acid as a fuel. The characterization data showed a total incorporation of Mn ions in the ceria lattice without clear evidence for the formation of Mn oxide nanoparticles. It is interesting to mention that no change in morphological structure was observed in the Mn-doped ceria sample. However, the bandgap of the prepared materials exhibited clear red-shift absorption as a result of Mn incorporation. A remarkable photocatalytic activity was observed in the Mn-doped porous ceria samples as compared to the bare porous ceria. The PL study confirmed the quenching of photogenerated electron/hole pair recombination when Mn ions are present in a porous cerium lattice.