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. 2020 Aug 21;5(34):21338–21344. doi: 10.1021/acsomega.0c00505

Decomposition Study of Praseodymium Oxalate as a Precursor for Praseodymium Oxide in the Microwave Field

Peng Lv †, Liangjing Zhang †, Sivasankar Koppala ‡, Kaihua Chen †, Yuan He †, Shiwei Li †, Shaohua Yin †,*
PMCID: PMC7469113  PMID: 32905250

Abstract

graphic file with name ao0c00505_0005.jpg

Micron-sized praseodymium oxide powders are prepared successfully from the praseodymium oxalate in a microwave field at 750 °C for 2 h in the present study. X-ray diffraction (XRD) analysis demonstrates that the presence of cubic structured crystalline Pr6O11 and complete decomposition of the precursor are confirmed by Fourier transform infrared (FT-IR) analysis. The scanning electron microscopy (SEM) results show yield powders with the desired particle size and uniform morphologies. Particle size analysis demonstrates that the median diameter (D50) becomes stable at 750 °C. The D50, average surface area, pore diameter, and pore volume calculated by Brunauer −Emmett–Teller (BET) are 4.32 μm, 6.628 m2/g, 1.86 nm, and 0.026 cm3/g at 750 °C for 2 h, respectively. Moreover, loss on ignition (L.O.I.) analysis indicates that the L.O.I. is as low as 0.39%, meeting the enterprise requirement (<1%). In comparison, conventional calcination experiments are carried out in the electric furnace. Both XRD and FT-IR analyses are in consistence with thermogravimetry–differential scanning calorimetry, which indicates that the temperature required for the decomposition of praseodymium oxalate hydrate is higher than that of microwave heating. Furthermore, SEM, particle size distribution, and BET analysis indicate that agglomeration generates, particle size enlarges, and average surface area increases. In all, it is confirmed that preparing rare-earth oxides from rare-earth oxalates is feasible using microwave heating to replace conventional heating.

1. Introduction

Studies on rare-earth (lanthanide) oxides have received renewed attention in recent years because of their electronic, optical, and chemical properties. Praseodymium oxide has a special position within the series of the rare-earth oxides and has been largely used in the last decade in catalyst formulations for a variety of processes in the areas of both chemicals and environment.1 As a promoter, it has shown that Pr6O11 enhances the individual photocatalysis of the oxidation of oxalic acid over TiO2, ZnO, CuO, Bi2O3, and Nb2O5 catalysts.2 Otherwise, the addition of small amounts of Pr6O11 to cobalt–silica gel catalysts greatly enhances their activity and selectivity in the Fischer–Tropsch synthesis.3

In most of the domestic enterprises, rare-earth oxides are commonly obtained through firing precursors such as rare-earth oxalates or carbonates. The hard agglomeration is formed by diffusion and bonding among molecules on the powder surface during the heat treatment process. In these processes, resistance heating is usually used to heat materials, wherein the materials are heated from the outside to the inside by heat radiation when controlling the reaction temperature, resulting in a low heat utilization rate, high-energy consumption, uneven heating, and other shortcomings. Therefore, during conventional sintering, it is a real challenge to keep the nano-sized grains after the sintering stage. The goal of our work is first to reduce the energy consumption and second to avoid grain growth and particle aggregation as far as possible during the sintering cycle.

Grain growth is inevitable in the nanosintering process, and the major challenge today is to ensure product structures remaining on the nanoscale by preventing grain growth.4 Several studies have shed light on the behavior of nanosintering by microwave heating, such as the synthesis of nanometer yttria,5,6 microwave sintering of nanocrystalline zinc oxide,7,8 microwave-assisted hydrothermal synthesis of CeO2 nanowires,9 and alternative green microwave-assisted method, and developed metal oxide nanoparticles.10,11 Moreover, the nanoparticles obtained by microwave calcination have more uniform dispersion and a lower degree of agglomeration.

Based on the above analysis, microwave heating is proposed. Compared with conventional heating, microwave heating effectively avoids the problem of local sintering agglomeration12 in the calcination process to achieve the purpose of uniform dispersion.13 Although the thermal decomposition of the praseodymium oxalate has been widely investigated, the process in the microwave field has not been thoroughly studied. In order to obtain the praseodymium oxide, we carried out the thermal decomposition of the praseodymium oxalate in the microwave field. Also, the comparison between microwave and conventional methods has been studied, indicating the feasibility of preparing praseodymium oxide from praseodymium oxalate using microwave heating.

2. Results and Discussion

2.1. Thermogravimetry–Differential Scanning Calorimetry Analysis of Praseodymium Oxalate Hydrate

The results of thermogravimetry (TG)–differential scanning calorimetry (DSC) analyses are shown in Figure 1. It can be seen that there are endothermic peaks at 49.5, 146.9, 396.7, and 992.2 °C and exothermic peaks at 440.4 °C within the DSC curve. The total weight loss is around 49.41% within the TG curve. The exothermic peak at 440.4 °C is due to the transformation from Pr2(C2O4)3 to Pr2CO5 in monoclinic, when heated to 650 °C, the Pr6O11 with cubic structure starts to form, and no obvious weight loss appears at 800 °C. TG–DSC analysis indicates that the entire calcination temperature needs to be at 800 °C. However, there is an endothermic peak at 49.5 °C, indicating that absorbed water can be found in the precursor. It is calculated by TG–DSC (Netzsch STA449F3) in air, which turned out to be Pr2(C2O4)3·(8H2O + 2.21H2O). The related reaction and weight loss are listed in Table S1.

Figure 1.

Figure 1

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TG–DSC of praseodymium oxalate hydrate.

2.2. Loss on Ignition

Loss on ignition (L.O.I.) is an important indicator in the rare-earth calcination process, the L.O.I. of rare-earth oxides measured by a gravimetric method is generally required to be less than 1% of the product in actual industrial production.14 According to GBT12690.2-2015,15 L.O.I. is calculated from the weight being lost after roasting the sample at 950 °C for 1 h, and all the results are obtained from three replicate measurements.

2.2. 1

where w is the mass fraction of L.O.I.; m0 is the weight of sample; m1 is the weight of empty crucible; and m2 is the total weight of the material and crucible after burning.

The results of the L.O.I. are listed in Table S2. It can be seen that the L.O.I. values at different heating conditions are all less than 1%, meeting the enterprise standard system. However, the L.O.I. value in the conventional heating system is higher than that in the microwave heating system, for example, 0.35 and 0.83% of L.O.I. values at 800 °C for 2 h are obtained from microwave and conventional heating, respectively. These results indicate that the residual impurity content in the product obtained from microwave heating is lower than that one. Thus, a better L.O.I. value is obtained from a lower temperature and shorter holding time in the microwave field; this may be due to the quick and internal heating, as well as the nonthermal effect with respect to microwave heating.

2.3. Powder Characterization

2.3.1. X-ray Diffraction Analysis

Figure 2 shows the X-ray diffraction (XRD) patterns of powders prepared at different temperatures holding for 2 h. For microwave calcination (Figure 2a), it is evident that heating the precursor to 450 °C is accompanied by a noticeable change in the relevant XRD patterns. Analysis shows that the main phase is identified to be Pr2CO5 [JCPDS: 25-0696], corresponding to the (002), (011), (013), (110), and (111) planes and the monoclinic structure for one-dimensional Pr2CO5 with a lattice constant of a = b = 4.019 and c = 13.310 Å. After heating to 600 °C, Pr2CO5 becomes a minor phase, and the other two phases Pr2O2CO3 [JCPDS: 37-0805] with a lattice constant of a = b = 4.011 Å and c = 15.689 Å and Pr6O11 [JCPDS: 42-1121] with a lattice constant of a = b = c = 5.468 Å appear. The high score plus analysis software shows that the integrated intensity of Pr6O11 at 600, 650, and 700 °C accounts for 3.5, 7.6, and 30.1% of the total, respectively. The samples heated at 750–800 °C illustrate the presence of Pr6O11 as the only phase with good crystallization. Thus, Pr6O11 completely crystallized at 750 °C in the microwave field.

Figure 2.

Figure 2

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XRD powder diffractograms obtained for praseodymium oxalate at different temperatures: (a) microwave heating and (b) conventional heating.

Furthermore, XRD patterns prepared for praseodymium oxalate and its conventional calcination (Figure 2b) products formed in the air at the 450–850 °C temperature ranges are also shown. The results shown that the main phase Pr2CO5 obtained at 450 °C is consistent with microwave calcination temperature and TG–DSC analysis. When heating to 600 °C, Pr2CO5 becomes a major phase, while it is a minor phase in the microwave heating system. Compared with microwave calcination, Pr2CO5 exists in the conventional high-temperature section for a short time, and it is easily decomposed to PrO1.83. So, another phase Pr2O2CO3 is not detected after heating to 600 °C. The samples are heated at 650 °C, the peak of Pr6O11 starts to form, with increasing calcination temperature, forming well crystallization at 800 °C, which is also consistent with TG–DSC analysis, indicating that the temperature required for Pr6O11 in the former system is lower than that in the conventional heating system. In this context, as calculated from the full width at half-maximum (fwhm) of the radiation peak (fwhm) using Scherrer equation,16 crystallite sizes of Pr6O11 for the samples heated at 750, 800, and 850 °C in the microwave field and conventional heating system are found to be about 7.57 (8.13), 8.63 (8.04), and (8.81) Å, respectively. Thus, a good crystallization phase of Pr6O11 can be obtained at 750 and 800 °C in the microwave and conventional heating system, respectively.

2.3.2. Fourier Transform Infrared Analysis

The Fourier transform infrared (FT-IR) analysis of the samples thermally treated by using different calcination routes is shown in Figure 3. In Figure 3a, the absorption band at 450 and 530 cm–1 could be attributed to the stretching mode of Pr–O at 650 °C.17 The peaks of the calcination products between 450 and 550 °C that appeared at 870, 2000, 2200, and 2300 cm–1 are assigned to the stretching mode of δs(COO–), νs(COO–), and νas(COO–), indicating the formation of Pr2CO5. As the temperature increased to 600–700 °C, the absorption band at 660 cm–1 would be attributed to the stretching mode of Pr–O.18 Both FT-IR and XRD analyses conclude that Pr6O11 is initially generated after microwave calcination at 600 °C. Furthermore, the absorption band appeared at 875 and 1350 cm–1, the peak disappeared at 2000 and 2300 cm–1, and the peak at 2200 cm–1 widened, which could be attributed to the stretching mode of νs(COO–), indicating the formation of Pr2O2CO3 in microwave calcination at 600 °C, thoroughly disappeared at 700 °C. At 700 °C, the absorption band at 875, 1350, and 2000 cm–1 disappeared absolutely, a new absorption peak appeared at 450 and 590 cm–1,19 illustrating the formation of purified Pr6O11.

Figure 3.

Figure 3

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FT-IR of praseodymium oxalate and products obtained for praseodymium oxalate at different temperatures: (a) microwave heating and (b) conventional heating.

As shown in Figure 3b, for the praseodymium oxalate, the peaks at 3450 and 1650 cm–1 are assigned to the stretching and bending vibrations of water molecules, respectively.20−22 The absorbance peak around 1350 cm–1 is ascribed to the frequency of the symmetric vibration (νs(COO–)), and the peak appeared at 490, 850, and 2280 cm–1 may be due to the deformation vibrations of the (νs(COO–)). The bands located at 490, 1350, 1650, 2280, and 3450 cm–1 disappeared23−27 by calcination, indicating that praseodymium oxalate has been decomposed, resulting in evaporation of water and production of carbon dioxide and oxygen. In addition, the peaks that appeared at 2000, 2200, and 2300 cm–1 and assigned to the stretching mode of (νs(COO–)) and (νas(COO–)) shifted to 1365, 1450, and 1560 cm–1, respectively, due to the blue shift. It might be attributed to the fact that microwave heating has a higher thermal radiation vibration frequency generating a shorter wavelength of the group (COO) than the conventionally heated, and there is also a possibility that the polar molecular water in the microwave field is rapidly evaporated, resulting in the group (COO) not having enough time for a secondary vibration, thus stretching vibration frequency of the group (COO) is changed. In addition, the peak at 2280 cm–1 disappeared after heating the precursor to 450 °C. However, this peak reappears again after heating to 700 °C and remains intact when subjected to higher temperatures (850 °C). From TG–DSC and XRD analyses, it can be seen that Pr2(C2O4)3 → Pr2CO5 + 5/2CO2 + O2 occurred after heating to 450 °C, and carbon dioxide was generated, indicating that the stretching mode (νs(COO–)) disappeared at this position, Pr2CO5 → 2PrO1.833 + CO2 occurred after heating to 700 °C, and pure praseodymium oxide was produced. According to the literature,27 pure praseodymium oxide absorbs water and carbon dioxide when exposed to air. Thus, it may be possible that carbon dioxide is absorbed when the peak appears again after heating to 700 °C. Therefore, pure praseodymium oxide must be stored in a sealed container.

2.3.3. SEM Analysis

Figure S1 shows the scanning electron microscopy (SEM) image of product powder prepared in the microwave and electric furnace at different temperatures. It could be seen that all samples prepared in the electric furnace present irregular morphologies and wide particle size distribution (PSD), while the samples calcined by the microwave possess uniform morphology, small particle size, and narrow size distribution. Because microwave heating can reduce the specific surface area and temperature gradient, the grain growth is more uniform in heat.27 On the one hand, from the XRD and FT-IR analyses, it is known that the temperature 600 °C is considered as a breakpoint for conventional calcination, whereas the product is a monoclinic crystal when the temperature is lower than 600 °C, otherwise, transformed to a cubic crystal. On the other hand, combining the TG–DSC analysis, it can be seen that at 450–600 °C, it is a monoclinic crystal, and over 600 °C, it turns into a cubic crystal. In comparison, the three crystal transformations in the microwave field happen at 450–550, 550–700, and beyond 700 °C, which correspond to the monoclinic crystal, hexagonal crystal, and cubic crystal, respectively.

2.3.4. PSD Analysis

The results of laser particle size analysis of products calcined at different temperatures with different calcination methods are shown in Figure S2. It can be seen that the volume average particle size (D50) shows a stable tendency at higher than 750 °C. Meanwhile, the D50 in the conventional calcination system is almost twice as large as that of microwave calcination. It is obvious that the involved calcination mechanisms are different from one process to the other which leads to the promotion of densification mechanisms to the detriment of grain coarsening in the case of microwave calcination.28 Furthermore, the PSD is another important index adopted in the industry. It is measured by particle classification precision

2.3.4. 2

where D25, D50, and D75 represent the maximum particle size within the cumulative PSD of 25, 50, and 75%, respectively, and S is the classification precision. The closer the S value is to 1, the narrower the PSD is and the higher the crystallinity is. The classification precision shows that the S ranges from 1.492 to 1.512 for microwave heated; however, it ranges from 1.787 to 1.963 for conventional calcination. Additionally, the frequency distribution of products is shown in Figure S3. For microwave (Figure S3a), the microwave radiation process is accompanied by the generation of water vapor that rapidly expands, bursts, and evenly disperses in the entire material system, turning into tiny monomers that grow independently. It can be seen from the XRD and SEM pictures that the material at 400 °C becomes a uniform and small monomer because of water loss, and the frequency distribution has a high value. A decomposition reaction occurs in the range of 400–750 °C, the PSD range has a tendency to decrease, and the frequency distribution is still a higher value; however, praseodymium oxide is completely generated at 750 °C, the PSD range reaches the minimum value, and the frequency distribution value remains high. The PSD range and frequency distribution value at 800 °C are almost the same as at 750 °C, which is due to the process of microwave heating, the materials are in a uniform thermal environment, which avoids the thermal gradient phenomenon in the heat conduction process. Therefore, the monomers produced can be kept away from agglomeration. For electric furnace heating (Figure S3b), the overall PSD range tends to decrease as the calcination temperature increases, however, the frequency distribution value decreases, which is due to the thermal gradient phenomenon caused by heat conduction in the traditional calcination process. Larger particles will grow at the expense of smaller particles. Thus, it is confirmed that preparing rare-earth oxides from rare-earth oxalates are feasible by microwave heating.

2.3.5. Surface Area and Porosity Analysis

The porosity of the praseodymium complex is studied by adsorption and desorption experiments of N2 gas at a low temperature (77 K). Nitrogen adsorption isotherms of praseodymium complexes for both microwave (Figure S4) and electric furnace (Figure S5) calcination are shown as type IV, which is a characteristic of mesoporous materials. During microwave calcination, as the temperature increases, the hysteresis loop with H3 type becomes significantly narrower, indicating that the pore size distribution becomes uniform. According to the relationship between the hysteresis loop and the pore type (Figure S6), it is determined as a slit-shaped hole. The changes in surface area, pore diameter, and pore volume calculated by the Barrett–Joyner–Halenda (BJH) method are shown in Figures S7–S9, respectively. It can be seen from Figure S7 that at 750 °C, the surface area value is the smallest, which is 6.628 m2/g, the pore diameter in Figure S8 is also the smallest, which is 1.86 nm, and the pore volume in Figure S9 is 0.026 cm3/g. During the electric furnace calcination process, as the temperature increases, the change in trend of the hysteresis loop is similar to that of the microwave, and it also shows an obvious narrowing, indicating that the pore size distribution is also becoming uniform, and it is also a slit-like pore. The changes in surface area, pore diameter, and pore volume calculated by the BJH method are shown in Figures S10–S12, respectively. It can be seen from Figure S10 that at 800 °C, the surface area value is 13.89 m2/g, the pore diameter value in Figure S11 is 1.71 nm, and the pore volume is 0.037 cm3/g in Figure S12. Compared with the two calcinations, the surface area and pore volume of particles for electric furnace calcination are larger than those under the condition of microwave calcination, while the pore diameter is smaller than that, which shows that the powder is more likely to agglomerate and overburned of electric furnace calcination.

2.3.6. Effect of Different Holding Times on Praseodymium Oxide

2.3.6.1. XRD Analysis of Different Holding Times

XRD analysis of the product calcined at different temperatures by microwave shows that the crystallinity of the product is high when the calcination temperature is 750 °C compared with 800 °C. After comprehensive consideration, the calcination temperature is selected to be 750 °C to carry out the holding time experiments, and the results are shown in Figure S13. It is observed that the fwhm of the main characteristic peak at 28° of Pr6O11 is 0.231, 0.187, and 0.210, holding for 1, 2, and 3 h, respectively. The smaller the value of fwhm, the more complete the crystal growth, meaning a more mature product, that is, the higher the added value. Thus, the thermal insulation for 2 h is the best thermal insulation condition.

2.3.6.2. PSD Analysis of Different Holding Times

The comparative analysis results holding for different times are shown in Table S3. It is seen that with the increase in holding time, the volume average particle size (D50) and median particle size [(R = D90 – D10)/D50] both decrease first and then increase. Therefore, the D50 and R values are close to 1 and should be considered in 2 h of incubation, which indicates that large particles split into small particles under the microwave insulation route, thus improving the particle distribution of the product to some extent. The frequency distribution of products calcined at 750 °C with different holding times is shown in Figure S14. It is seen that with the increase of the holding time, the PSD range remains unchanged, while the frequency distribution is the smallest at 1 h and the largest at 2 and 3 h centered, indicating that during the heat preservation process, large agglomerates appeared with the increase of the heat preservation time, which reduced the frequency distribution value of the entire system. At the same time, combined with the analysis of surface area and SEM pictures, it is confirmed that the surface area of particles will decrease sharply and form large aggregates in the product after a long time of heat preservation. Hence, the best insulation calcination time is 2 h.

2.3.6.3. SEM Analysis of Different Holding Times

The SEM images of products obtained at different holding times by microwave calcination at 750 °C is shown in Figure S15. It is seen from the figures that after a short time of calcination, the sample particles are more dispersible. However, the powder agglomeration occurred after 3 h and generated a big group of particles. This observation indicates that long-term insulation will break the system’s thermal equilibrium, which triggers the aggregation of small particles to reduce the overall surface energy. Consequently, controlling the reasonable insulation time is essential to prepare Pr6O11 powders. In all cases, 2 h is considered to be the optimal time to yield powders with the desired particle size and uniform morphologies. The crystal transformation process of praseodymium oxalate with the increase of temperatures and holding times is shown in Figure S16. Praseodymium oxalate becomes praseodymium oxide through the following processes: dehydration, decomposition, crystal transformation, nucleation, and recrystallization (occurred with the increase of holding time). According to Wyckoff,29 the Ln2O3 rare-earth oxides exist in five-type polymorphic modifications, namely, A-, B-, C-, H-, and X-, and the lighter oxides (La-Pm) generally have the A-type hexagonal structure at room temperature. Additionally, the A-type cubic phase of various Ln2O3 species, including Pr6O11, exists at high pressures.30 The remaining two phases, the H-type hexagonal and X-type cubic phases are only observed at elevated temperatures, or in heavily irradiated samples.31

2.3.6.4. Surface Area and Porosity Analysis of Different Holding Times

The porosity of praseodymium oxide at different holding times is also studied by experiments of adsorption and desorption of N2 gas at a low temperature (77 K, Figure S17). With the increase of the holding time, the hysteresis loop of nitrogen adsorption isotherm of Pr6O11 gradually narrowed, indicating that the pore size distribution gradually became uniform. The changes in surface area, pore radius, and pore volume calculated by the BJH method are plotted in Figures S18–S20, respectively. It can be seen from Figure S18 that the surface area value increases in 1–2 h but decreases in 2–3 h. The pore diameter result of Figure S19 is consistent with the surface area result of Figure S18, and the pore volume of Figure S20 changes during the adsorption process is consistent with Figures S18 and S19 and is completely opposite during the desorption process. The above results indicate that the mesopores become larger within 1–2 h and shrink within 2–3 h. Therefore, the best holding time should be controlled at 2 h.

3. Conclusions

The praseodymium oxalate was successfully transformed into praseodymium oxide by microwave heating. XRD and FT-IR analyses indicate the complete decomposition of the precursor after microwave calcination at 750 °C for 2 h, giving cubic structure crystalline Pr6O11. SEM analysis indicates that 750 °C is the optimal calcination temperature for desired narrow PSD. L.O.I. analysis indicates that microwave calcination temperature approached to 750 °C, and the L.O.I. is as low as 0.39%. Particle size analysis demonstrated that the median diameter (D50) becomes stable when the calcination temperature is higher than 700 °C. Surface area and porosity analysis show that the relatively smaller surface area and the larger pore size can be obtained at 750 °C. Furthermore, the product is obtained by microwave calcination holding for different times at 750 °C, concluding that 2 h is the optimal condition. In comparison, praseodymium oxide powders are also prepared from praseodymium oxalate hydrate by conventional calcination in the electric furnace. TG–DSC, XRD, and FT-IR analyses demonstrate that the complete decomposition temperature of the precursor in the electric furnace should be above 800 °C, while that of in the microwave field could be lowered to 750 °C. The SEM, surface area, and porosity analysis depict that microwave heating has a more positive effect on the particle morphology and size distribution than conventional heating. All analyses draw conclusion that microwave-assisted calcination technique is feasible in the direct preparation of praseodymium oxide powders from praseodymium oxalate hydrate.

4. Experimental Section

4.1. Materials

The praseodymium oxalate precursor (purity>99.9%) was supplied by Ganzhou Zhanhai Industrial and Trade Co., Ltd., China, and the particle size is about 25 μm. The device of microwave heating was developed by the Key Laboratory of Unconventional Metallurgy, Kunming University of Science and Technology, China. A schematic diagram of the dielectric device is shown in Figusre S21. The raw materials determined by the thermal analyzer (Netzsch STA449F3, Germany) were calcined at various temperatures, which were analyzed by XRD (PANalytical Empyrean, England) with Cu Kα radiation at a scan time of 30 min in the range of 2θ = 5–90°. The FT-IR spectra were recorded using a Nicolet IS50 FT-IR spectrophotometer (Thermo Nicolet, USA) with KBr pellets. The morphology and microstructure of the powders were investigated by using a backscattering scanning electron microscope (Phenom ProX, China). Microparticle size and distribution of the powders were obtained by a laser PSD instrument (Sympatec Helos-Rodos, Sichuan, China) with water as a dispersing solvent. All other reagents are of analytical grade.

4.2. Methods

Praseodymium oxalate is calcined at different temperatures (450–800 °C) with various holding times (1–3 h) in microwave equipment consisting of four sections: the magnetron at the frequency of 2.45 GHz and 1.0 kW power which is cooled by cold air as microwave sources; two waveguides for transporting microwaves; a resonance cavity to manipulate microwaves for a specific purpose; and a control system to regulate the temperature and microwave power. The temperature is monitored by a thermocouple (connected to the computer system), which was placed at the closest proximity to the material. After heating, the sample is cooled to room temperature and then characterized. The effect of heating conditions on the product characteristics via microwave heating is compared to conventional heating in a muffle furnace (Shanghai Yifeng Electric Furnace Co., Ltd.) at a heating rate of 10 °C/min.

Acknowledgments

Financial aid from the following programs is gratefully acknowledged: the National Natural Science Foundational of China (grant numbers 51604135 and 51504116) and the Yunnan Ten Thousand Talents Plan Young & Elite Talents Project (grant number YNWR-QNBJ-2018-323).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00505.

  • Decomposition process of the praseodymium oxalate hydrate; comparison of calcination methods (microwave and electric furnace); characterization of praseodymium oxide; and crystal transformation process of praseodymium oxalate (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c00505_si_001.pdf (2.6MB, pdf)

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