Cerium (IV) oxide (CeO2) is extensively used as a catalyst because it has numerous advantages over conventional catalysts, like low cost, better poisoning resistance, high catalytic activity due to the facile Ce4+ / Ce3+ redox reac-tion and high oxygen storage capacity. CeO2 nanoparticles have higher specific surface area and better redox properties, and thus an increased catalytic activity in comparison to bulk materials. However, at elevated tem-peratures ceria nanoparticles are very prone to coarsening. In order to increase the thermal stability of CeO2 na-noparticles, but also improve its catalytic properties, metal ions are incorporated into the CeO2 crystal structure. The aim of this study was to compare coarsening kinetics of nanocrystalline CeO2 and nanocrystalline CeO2 doped with 10 mol. % of copper in order to determine the effect of doping on thermal stability. Samples were prepared by hydrothermal synthesis and thermally treated at different temperatures and processing times. The samples were analyzed by X-ray diffraction analysis and the crystallite sizes were calculated using the Scherrer’s equation. Based on the obtained crystallite sizes, kinetic parameters were determined and it was found that cop-per addition has a positive effect on the thermal stability of CeO2

Aleksandar Bajrović

Katarina Mužina

Martina Župančić

Stanislav Kurajica

English

Cerium (IV) oxide (CeO2) is extensively used as a catalyst because it has numerous advantages over conventional catalysts, like low cost, better poisoning resistance, high catalytic activity due to the facile Ce4+ / Ce3+ redox reaction and high oxygen storage capacity. CeO2 nanoparticles have higher specific surface area and better redox properties, and thus an increased catalytic activity in comparison to bulk materials. However, at elevated temperatures ceria nanoparticles are very prone to coarsening. In order to increase the thermal stability of CeO2 nanoparticles, but also improve its catalytic properties, metal ions are incorporated into the CeO2 crystal structure. The aim of this study was to compare coarsening kinetics of nanocrystalline CeO2 and nanocrystalline CeO2 doped with 10 mol. % of copper in order to determine the effect of doping on thermal stability. Samples were prepared by hydrothermal synthesis and thermally treated at different temperatures and processing times. The samples were analyzed by X-ray diffraction analysis and the crystallite sizes were calculated using the Scherrer’s equation. Based on the obtained crystallite sizes, kinetic parameters were determined and it was found that copper addition has a positive effect on the thermal stability of CeO2

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Effect of copper on thermal stability of nanocrystalline ceria

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K. Mužina, A. Bajrović, M. Župančić, S. Kurajica, “Effect of copper on thermal stability of nanocrystalline ceria”, Technologica Acta, vol. 13, no. 1, pp. 15-19, 2020. 15 ISSN 1840-0426 (P); ISSN 2232-7588 (E) http://tf.untz.ba/technologica-acta EFFECT OF COPPER ON THERMAL STABILITY OF NANOCRYSTALLINE CERIA ORIGINAL SCIENTIFIC PAPER Katarina Mužina,1 Aleksandar Bajrović,1 Martina Župančić,2 Stanislav Kurajica1 DOI: 10.5281/zenodo.4059911  RECEIVED 2020-01-06 ACCEPTED 2020-05-11 1 University of Zagreb, Faculty of Chemical Engineering and Technology, Marulićev trg 19, 10000 Zagreb, Croatia 2 Leibniz-Institut für Kristallzüchtung, Max-Born-Straße 2, 12489 Berlin, Germany   kmuzina@fkit.hr  ABSTRACT: Cerium (IV) oxide (CeO2) is extensively used as a catalyst because it has numerous advantages over conventional catalysts, like low cost, better poisoning resistance, high catalytic activity due to the facile Ce4+ / Ce3+ redox reac-tion and high oxygen storage capacity. CeO2 nanoparticles have higher specific surface area and better redox properties, and thus an increased catalytic activity in comparison to bulk materials. However, at elevated tem-peratures ceria nanoparticles are very prone to coarsening. In order to increase the thermal stability of CeO2 na-noparticles, but also improve its catalytic properties, metal ions are incorporated into the CeO2 crystal structure. The aim of this study was to compare coarsening kinetics of nanocrystalline CeO2 and nanocrystalline CeO2 doped with 10 mol. % of copper in order to determine the effect of doping on thermal stability. Samples were prepared by hydrothermal synthesis and thermally treated at different temperatures and processing times. The samples were analyzed by X-ray diffraction analysis and the crystallite sizes were calculated using the Scherrer’s equation. Based on the obtained crystallite sizes, kinetic parameters were determined and it was found that cop-per addition has a positive effect on the thermal stability of CeO2.  KEYWORDS: cerium (IV) oxide; coarsening kinetics; hydrothermal synthesis; doping  INTRODUCTION  Cerium (IV) oxide (CeO2), also called ceria, is one of the most important and generally applied lan-thanide oxides. It possesses a fluorite crystal structure with space group Fm3m, which is stable from room temperature till the melting point (~2 400 °C). Ceri-um has two stable oxidation states, Ce3+ and Ce4+, and the special feature of cerium (IV) oxide is that the fluorite lattice can withstand significant reduction of cerium without alteration or collapse of the struc-ture, especially at elevated temperatures [1]. During the partial reduction of Ce4+ to Ce3+, oxygen mole-cules are released in order to maintain the electroneu-trality of the crystal lattice, which causes the for-mation of oxygen vacancies. These vacancies can freely move within the crystal lattice and can also re-adsorb oxygen molecules from the air. The good cat-alytic properties of cerium (IV) oxide are attributed to this easiness of transition between oxidation states, which allows the formation of oxygen vacancies and the good mobility and high oxygen storage capacity within the crystal lattice [2]. Ceria has many ad-vantages over traditional catalysts, like low price and availability, lower degree of thermal sensitivity, higher resistance to poisoning, while also allowing high conversions to be achieved for certain types of chemical processes. Cerium (IV) oxide is therefore widely used as a heterogeneous catalyst in three-way catalytic converters in car exhaust systems, for the conversion of water gas to produce hydrogen for var-ious industrial processes and for the removal of at-mospheric pollutants such as volatile organic com-pounds [3-5]. Modern catalyst design is aimed at preparation of nanoscale catalysts because they pos-sess superior properties in comparison to bulk mate-rials due to greater surface to bulk atoms ratio. The atoms on the surface have unsatisfied chemical bonds and therefore possess greater energy than bulk atoms. Increased energy influences the band gap, morpholo-gy, reactivity, and catalytic potential of a heterogene-ous nanocatalyst [6]. The problem with nanoparticles occurs at elevated temperatures since they are prone to aggregation and coarsening which cause the in-crease of crystallite sizes and the reduction of specif-ic surface area. To solve this problem and to further improve the catalytic properties of ceria, transition metals having smaller radius and lower valence than cerium can be introduced into the ceria crystal lattice [7]. In this work, pure and Cu-doped ceria samples were prepared using a simple, affordable and eco-friendly hydrothermal method. The samples were characterized and then thermally treated at various temperatures and processing times. Coarsening kinet-ics analysis was performed in order to evaluate the 16  K. Mužina, A. Bajrović, M. Župančić, S. Kurajica, “Effect of copper on thermal stability of nanocrystalline ceria”, Technologica Acta, vol. 13, no. 1, pp. 15-19, 2020. ISSN 1840-0426 (P); ISSN 2232-7588 (E) http://tf.untz.ba/technologica-acta effect of the dopant on the thermal stability of cerium (IV) oxide.  EXPERIMENTAL  Samples of pure and copper doped ceria were prepared by hydrothermal method using the follow-ing procedure: The appropriate amounts of precur-sors, namely 0.8 mmol of Ce(SO4)24H2O for pure sample as well as 0.72 mmol of Ce(SO4)24H2O and 0.08 mmol of CuSO4x5H2O for Cu doped sample, were put in a Teflon-lined  stainless-steel autoclave with a capacity of 70 cm3, which was then filled with 56 cm3 of 8 mol dm-3 NaOH solution, sealed and placed in a temperature controlled oven at 120 °C for 16 hours. After the end of the reaction time, the auto-clave was cooled down, the NaOH solution was de-canted and the precipitate washed three times with demineralized water with help of sonification and centrifugation. The precipitate was then dried at 60 °C for 24 hours.   The pure and the doped sample were thermally treated at different temperatures and processing times (Table 1), after which X-ray powder diffraction (XRD) analysis was performed. The instrument used was Shimadzu XRD 6000 diffractometer with CuKα radiation operating at 40 kV and 30 mA in a step scan mode ranging from 5 to 105 °2θ, with steps of 0.02 °2θ and counting time of 0.6 s.   Table 1. Temperatures and processing times for the pure and Cu doped sample. 300 °C 350 °C 400 °C 450 °C 500 °C 15 min 15 min 15 min 15 min - 30 min 30 min 30 min 30 min - 1h 1h 1h 1h - 2h 2h 2h 2h 2h 4h 4h 4h 4h -    The as-prepared pure and Cu doped sample were additionally analyzed using high resolution transmis-sion electron microscopy (HRTEM) on transmission microscope JEOL ARM 200 CF. The samples were applied directly to a nickel grid and analyzed at ac-celerating voltage of 80 kV. ImageJ program was used for the analysis of the obtained micrographs [8]. In order to analyze the chemical composition of the samples, energy dispersive X-ray spectroscopy (EDS) analysis was performed on Tescan Vega Easy Probe 3 scanning electron microscope operating at 10 kV. Samples were fixed on a sample holder using double-sided carbon conductive tape.  RESULTS AND DISCUSSION  The obtained pure and Cu doped sample were first analyzed by XRD in order to gain insight into their phase composition and purity. The X-ray pat-terns are shown in figure 1. As can be seen, both the pure and the doped sample only contain peaks char-acteristic for cerium (IV) oxide (ICDD PDF No. 34-0394), which indicates that copper is incorporated in the ceria crystal lattice. Diffraction patterns can be used for the determination of crystallite sizes using the Scherrer’s method (Eq. 1).  𝑑 =𝐾𝜆𝐵 cos 𝜃                                                                    (1)  where d is the crystallite size, K is the shape factor which is 0.94 for spherical particles of cubic sym-metry, λ is the CuKα radiation wavelength, θ is the Bragg angle and B is the peak full width at half max-imum corrected for instrumental broadening.   Broad peaks usually point out to small crystallite sizes, which were proven by calculation, as seen in Table 2. The doped sample has a slightly lower crys-tallite size, but taking into account the limitations of the method, the difference between the pure and the doped sample is not significant.   Figure 1. Diffraction patterns of the pure and the Cu doped sample.   HRTEM analysis (Figure 2) shows aggregates of fine nanocrystallites with sizes corresponding to the sizes obtained by the Scherrer’s method (Table 3). The obtained nanoparticles have spherical morpholo-gy and there is no evident difference in morphology between the pure and the doped sample.The fringes distance of 2.7 Å for both samples matches the (200) crystal plane of cerium (IV) oxide (ICDD PDF No. K. Mužina, A. Bajrović, M. Župančić, S. Kurajica, “Effect of copper on thermal stability of nanocrystalline ceria”, Technologica Acta, vol. 13, no. 1, pp. 15-19, 2020. 17 ISSN 1840-0426 (P); ISSN 2232-7588 (E) http://tf.untz.ba/technologica-acta 34-0394). Since neither XRD nor HRTEM analysis show the presence of copper in the doped sample, it can be indicated that a solid solution of copper in the ceria crystal lattice is formed.                       Figure 2. HRTEM micrograph of the pure (a) and Cu doped (b) sample.  Table 2. Crystallite sizes obtained from diffraction patterns by Scherrer’s method and particle sizes and fringes distance ob-tained from micrographs using ImageJ program.  CeO2 10 mol. % Cu:CeO2 Crystallite size (XRD) 4.2 nm 4.0 nm Particle size (HRTEM) ~ 4 nm ~ 4 nm Fringes distance 2.703 Å 2.730 Å   To confirm the presence of copper in the doped sample, EDS analysis was performed. Results are shown in figure 3. As can be seen, both the pure and the doped sample show the presence of cerium and oxygen, as well as carbon because of the carbon tape used for the fixation of the sample onto the sample holder. Peaks characteristic for copper are found in the doped sample proving the doping successful.                        Figure 3. EDS spectra of the pure (a) and Cu doped (b) sample.  Both the pure and the doped sample were thermally treated at different temperatures and processing times, as shown in Table 2, analyzed by XRD, and the crystallite size values were obtained using the 18  K. Mužina, A. Bajrović, M. Župančić, S. Kurajica, “Effect of copper on thermal stability of nanocrystalline ceria”, Technologica Acta, vol. 13, no. 1, pp. 15-19, 2020. ISSN 1840-0426 (P); ISSN 2232-7588 (E) http://tf.untz.ba/technologica-acta Scherrer’s equation. The crystallite sizes were then used in the coarsening kinetics analysis based on the equations from the work of Lopez et al. [9], which are as follows:  𝑑𝑛 − 𝑑0𝑛 = 𝐾𝑡                                                                (2)  Where d is the average particle size, 𝑑0 is the initial particle size, 𝑛 is the particle growth exponent, 𝐾 is a constant dependent on temperature (𝐾 = 𝐾𝑜 exp(−𝑄/𝑅𝑇)) and 𝑡 is the time of thermal treatment.    This equation can be simplified to:  𝑑 = 𝐾′𝑡𝑚                                                                        (3)  Where m is equal to 1/n, and it can be derived from the logarithmic plot of 𝑑 versus 𝑡 as the value of the slope:  𝑙𝑛𝑑 = 𝑙𝑛𝐾′ + 𝑚𝑙𝑛𝑡                                                      (4)  While the intercept lnK’ is further used in the Arrhe-nius plot to obtain the activation energy, Ea:  𝑙𝑛𝐾′ = 𝑙𝑛𝐴 −  𝐸𝑎𝑅𝑇                                                        (5)  Where A is the pre-exponential factor, Ea activation energy, R universal gas constant and T is the tem-perature expressed in Kelvins [9].   Our chosen temperatures of thermal treatment were lower than the ones in the work of Lopez et al., because the targeted application of our ceria nanopar-ticles is in catalytic oxidation of volatile organic compounds and exhaust gases from automobiles which need to be accomplished at lower temperatures (less than 500 °C). Figure 4 shows the dependence of crystallite size on temperature for 2 hours duration of thermal treatment, including the values of initial crys-tallite size for both the pure and the doped sample. It can be seen that for both samples the crystal growth depends exponentially on the temperature. However, crystallites in the Cu doped sample grow slower at lower temperatures in comparison to the pure sample, although the difference is not drastic. Crystal growth accelerates significantly at 500 °C.  Figure 4. Initial size and crystallite sizes after thermal treatment for 2 hours for the pure and the doped sample.    The particle growth exponents (n) for the pure and the doped sample at various temperatures were determined from the logarithmic plots of d versus t (figure 5) and are listed in table 3.     Figure 5. Plots of lnd vs lnt for the pure (a) and the doped (b) sample.   Equation 2 implies that high values of n point out to sluggish crystallite growth due to restricted bound-ary mobility [9], which is confirmed in this experi-ment since the n value is indeed higher at lower tem-K. Mužina, A. Bajrović, M. Župančić, S. Kurajica, “Effect of copper on thermal stability of nanocrystalline ceria”, Technologica Acta, vol. 13, no. 1, pp. 15-19, 2020. 19 ISSN 1840-0426 (P); ISSN 2232-7588 (E) http://tf.untz.ba/technologica-acta peratures. The values for the doped sample are lower which would indicate faster grain growth, but the calculated crystallite sizes are lower at lower temper-atures than that of the pure sample, which would mean higher thermal stability.  Table 3. Particle growth exponent values for the pure and doped sample at various temperatures. T (°C) n (CeO2) n (Cu:CeO2) 300 73 38 350 43 21 400 29 13 450 17 12   Activation energy values for the pure and the doped samples were obtained from the Arrhenius plots and are shown in figure 6.  Figure 6. Arrhenius plot for the pure (a) and the doped (b) sample.   The obtained values are 9854 J/mol for the pure sample and 12208 J/mol for the Cu doped sample. The obtained values are much lower than in the liter-ature [9, 10] because our research was done only for lower coarsening temperatures for the beforehand mentioned reasons. At lower temperatures no signifi-cant changes in ceria occur, except coarsening, which is not an extremely energy consuming process. The activation energy is higher for the doped sample, which means that it is necessary to bring more energy to the system to induce nanoparticles coarsening and that the doped sample is indeed more thermally stable than the pure one. CONCLUSION  Nanoparticles of pure and Cu doped ceria were prepared by hydrothermal method. Samples were thermally treated at different temperatures and differ-ent processing times, and a coarsening kinetics analy-sis was conducted. Crystallites in both samples grow with increasing temperature and time. The addition of copper increases the thermal stability of ceria indi-cated by the higher activation energy of the doped sample. ACKNOWLEDGEMENT  This work has been fully supported by Croatian Science Foundation under the project IP-01-2018-2963. The sustenance of the University of Zagreb is gratefully acknowledged.  REFERENCES [1] L.F. Arenas, C. Ponce de León, F.C. Walsh, “Electrochemi-cal redox processes involving soluble cerium species,” Elec-trochim. Acta., vol. 205, pp. 226-247, 2016. [2] M. Mogensen, N. M. Sammes, G. A. Tompsett, “Physical, chemical and electrochemical properties of pure and doped ceria,” Solid State Ion., vol. 129, pp. 63-94, 2000. [3] G. Ramakrishnan, K. Naveen, “Emission and dynamic char-acteristics of three way catalytic converter by computational fluid dynamics,” Int. J. Eng. Sci., vol. 6, pp. 3503-3510, 2016.   [4] Z. Ren, F. Peng, J. Li, X. Liang, B. Chen, “Morphology-dependent properties of Cu/CeO2 catalysts for the water-gas shift reaction,” Catalysts, vol. 7 (2), pp. 48-60, 2017. [5] M. Duplančić, S. Kurajica, V. Tomašić, I. Minga, “Catalytic oxidation of toluene on hydrothermally prepared ceria nano-crystals,” Chem. Biochem. Eng. Q., vol. 31, pp. 375-383, 2017. [6] S. Kurajica, S. Lučić Blagojević, Introduction to Nanotechnology (In Croatian: Uvod u nanotehnologiju), HDKI, Zagreb, 2017. [7] L. Kundakovic, M. Flytzani-Stephanopoulos, “Cu- and Ag-Modified Cerium Oxide Catalysts for Methane Oxidation,” J. Catal., vol. 179, pp. 203–221, 1998. [8] C. A. Schneider, W. S. Rasband, K. W. Eliceiri, “NIH Im-age to ImageJ: 25 years of image analysis,” Nat. Methods, vol. 9, pp. 671-675, 2012. [9] H. F. Lopez, H. Mendoza, “Temperature Effects on the Crystallization and Coarsening of Nano-CeO2 Powders,” ISRN Nanomaterials, vol. 2013, Article ID 208614, 2013, https://doi.org/10.1155/2013/208614. V. K. Ivanov, O.S. Polezhaeva, G. P. Kopitsa, P. P. Fedorov, K. Pranzas, V. V. Runov, “Specifics of High-Temperature Coarsening of Ceria Nanoparticles,” Russ. J. Inorg. Chem., vol. 54, no. 11, pp. 1767–1775, 2009.20  Technologica Acta, vol. 13, no.1,  2020. ISSN 1840-0426 (P); ISSN 2232-7588 (E) http://tf.untz.ba/technologica-acta                        

Katarina  Mužina

Aleksandar  Bajrović

Martina  Župančić

Stanislav  Kurajica

Effect of copper on thermal stability of nanocrystalline ceria

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