Hollow Structured Ceria by Partial Thermal

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Design of Porous/Hollow Structured Ceria by Partial Thermal Decomposition of Ce-MOF and Selective Etching Guozhu Chen, Zeyi Guo, Wei Zhao, Daowei Gao, Cuncheng Li, Chen Ye, and Guoxin Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11916 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Applied Materials & Interfaces

Design of Porous/Hollow Structured Ceria by Partial Thermal Decomposition of Ce-MOF and Selective Etching Guozhu Chen,* † Zeyi Guo, † Wei Zhao, ‡ Daowei Gao, † Cuncheng Li, † Chen Ye† and Guoxin Sun† †

Key Laboratory of Interfacial Reaction & Sensing Analysis in Universities of Shandong,

Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong province, 255022, China. ‡

Shandong Institute and Laboratory of Geological Sciences, Jinan, Shandong province, 255013,

China. KEYWORDS Metal-organic-frameworks; Bimetal oxides; CO oxidation; CeO2; Hollow; Catalysis

ACS Paragon Plus Environment

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ABSTRACT

Metal-organic frameworks (MOFs) have been widely used to prepare corresponding porous metal oxides via thermal treatment. However, high temperature treatment always leads to the obtained metal oxides with large crystallite size, thus decreasing their specific surface area. Different from conventional complete thermal decomposition of MOFs, herein, using Ce-MOF as a demonstration, we choose partial thermal decomposition of MOF, followed by selective etching to prepare porous/hollow structured ceria because of the poor stability of Ce-MOF under acidic conditions. Compared with the ceria derived from complete thermal decomposition of CeMOF, the as-prepared ceria is demonstrated to be a good support for copper oxide species during the CO oxidation catalytic reaction. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and hydrogen temperature-programmed reduction (H2-TPR) analysis revealed that the asprepared ceria is favorable for strengthening the interaction between the ceria and loaded copper oxide species. This work is expected to open a new, simple avenue for the synthesis of metal oxides from MOFs via partial thermal decomposition.

INTRODUCTION In the past years, metal-organic frameworks (MOFs) have received great attention not only because of their high porosity and large surface area, but also their structural topology conversion into corresponding porous metal oxides.1-10 Small cavities and open channels within MOFs-derived metal oxides provide pathways for reactant species in- and out-diffusions, thus enabling these metal oxides to be good candidates for catalysts or catalyst supports. So far, porous and hollow structured Fe2O3 from thermal decomposition of Prussian blue and CuO from HKUST-1 have been prepared which indicates the broad generality in preparation of porous


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metal oxides from MOFs.11-16 During calcination process, the temperature is often set to high (>400 oC) in order to completely decompose MOFs, however, high temperature treatment is favorable for obtained metal oxides’ purity and crystallinity at the expense of their surface area. Since the catalysts with high surface area not only facilitate the dispersion of secondary components, but also favor the transportation of reactant molecules to the active sites more effectively, therefore, the development of novel approach to synthesize high surface area metal oxides derived from MOFs is of particular interest. Ceria, as an important rare earth oxide material, has been extensively investigated in various catalytic reactions due to the abundant oxygen vacancy defects, high oxygen storage capacity, and relatively easy shuttles between the Ce(III) and Ce(IV).17-23 Up to now, various strategies were well-established to construct ceria with high surface area. Recently, different Ce-MOFs were employed as sacrificial templates to prepare ceria via thermal decomposition. For example, Kim et al. developed a MOF-driven, self-templated route toward porous metal oxides, in which aliphatic ligand-based MOFs were thermally converted to nanoporous ceria at 500 oC.24 Sandipan et al. described a MOF derived synthesis route for carbon embedded ceria by varying the organic linker and using PVP as the structure directing agent. After calcination at 550 oC, CeO2@C composites with different morphologies were obtained.25 Besides thermal decomposition method, Zeng et al. synthesized porous CeO2 by means of interfacial reaction between Ce-BTC and 2 M KOH aqueous solution.26 Although great efforts have been made to synthesize porous CeO2 from different kinds of Ce-MOFs, either high temperature calcination step was required or strong alkali solution was needed, thus would inevitably raise energy consumption and environmental pollution. Therefore, combined with high surface area


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mentioned above, the creation of efficient porous/hollow structured ceria from Ce-MOFs under a mild condition is highly desirable. During the calcination process, MOFs convert to metal oxides gradually from the outer to inner part. Therefore, in principle, porous metal oxide@residual MOFs core@shell structure can be captured at the early stage of calcination. In view of poor stability of most MOFs under acidic conditions, the residual MOFs would be washed off by acid, resulting in porous hollow structured metal oxides. Based on the above considerations, herein, porous hollow structured ceria derived from partial decomposition of Ce-MOF was prepared with assistance of selective etching. Compared with the methods reported for the synthesis of ceria derived from Ce-MOFs, relatively low calcination temperature (300 oC) is used for MOF conversion to ceria. More importantly, lactic acid, one of green, environmentally friendly acid, is chosen to wash off residual Ce-MOF. Therefore, energy consumption and environmental pollution are largely prohibited in our preparation method. When they are employed to be supports for copper oxide species, the former ceria favors high dispersion of copper oxide and exhibits strong synergistic effect, resulting in excellent catalytic activity toward CO oxidation. EXPERIMENTAL SECTION Materials. All the reactants are of analytical grade and were used without further purification. Cerium nitrate (Ce(NO3)3·6H2O), sodium carbonate (Na2CO3), copper nitrate (Cu(NO3)2·3H2O) and ethanol (CH3CH2OH) were purchased from Sinopharm Chemical Reagent Co. Ltd. Trimesic acid (H3BTC) and lactic acid (C3H6O3) were purchased from Macklin. Synthesis of Ce-MOF. In a typical procedure,27 the Ce(NO3)3·6H2O (1 mmol) and H3BTC (1mmol) was mixed in ethanol-water solution (50 mL, v/v = 1:1) under vigorous stirring at room


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temperature. The reaction mixture was then refluxed at 90 oC for 2 h. Finally, the white precipitate was collected by centrifugation, washed several times with ethanol and water, and dried at 60 oC. Synthesis of Ce-MOF derived CeO2. The CeO2 was prepared from partial thermal decomposition of Ce-MOF, followed by selective etching with lactic acid. Specifically, the asobtained Ce-MOF was calcined at 300 °C for 2 h with a heating rate 5 °C·min−1. Then the sample was dispersed in diluted C3H6O3 aqueous solution for 18 h to completely remove residual Ce-MOF. Finally, the sample was collected by centrifugation, washed thoroughly with water, and dried at 60 oC. For comparison, CeO2 was also achieved by complete thermal decomposition of Ce-MOF at different temperature for 2 h, respectively. Synthesis of CeO2-CuO. The CeO2-CuO catalysts were prepared by a depositionprecipitation method. In a typical synthesis, 200 mg of CeO2 derived from Ce-MOF was suspended in 25 mL water under stirring. Meanwhile, Cu(NO3)2·3H2O (30.4 mg) was dissolved in 12.5 mL of water and then added into the above CeO2 suspension dropwisely. During the whole process, the pH value of the stock solution was controlled to ~9.0 by adding Na2CO3 aqueous solution (0.50 M). The samples were further aged and washed, followed by dry and calcination. 29 CHARACTERIZATION The as-prepared samples were characterized by X-ray powder diffraction (XRD) on a Japan Rigaku D/Max-γA rotating anode X-ray diffractometer equipped with Cu Kα radiation (λ=1.54178 Å). The morphology and structure of samples were studied by transmission electron microscopy (TEM) (JEM 2100) and high resolution TEM (JEOL 2100F), and scanning electron


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microscopy (SEM-SU8010) equipped with an energy dispersive X-ray spectrometer (EDS). The valence states of Ce, Cu and O were characterized by X-ray photoelectron spectrometer (XPS) with Al Kα radiation. Hydrogen temperature-programmed reduction (H2-TPR) experiment was performed with a thermal conductivity detector on 50 mg sample in 80% (molar) argon and 20% (molar) hydrogen gas mixture, using a gas flow rate of 100 mL·min-1 with a temperature ramp rate of 10 K·min-1. Fourier transform infrared (FT-IR) spectroscopic analysis was carried out using pressed KBr disks in the region of 4000-400 cm-1. The nitrogen adsorption-desorption measurements were performed on a Micromeritics Tristar Ⅱ 3020 instrument at 77 K. All the samples were degassed at 200 °C under vacuum for over 6 h. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. Thermogravimetric analysis (TGA) of Ce-MOF was carried out at a heating rate of 5 °C·min−1 using a PYRIS Diamond TG-DTA (PekinElmer). Micro-Raman measurements were taken with an LabRAM HR800 equipped with an excitation laser of 633 nm. The Cu contents in CeO2-CuO samples were determined by inductively coupled plasma mass spectrometer (ICP-MS, Thermo Scientific XSeries-2). Catalyst Test. Catalytic activity was studied using a continuous flow fixed-bed micro-reactor at atmospheric pressure. In a typical experiment, the system was first purged with high purity N2 gas and then a gas mixture of CO/O2/N2 (1:10:89) was introduced into the reactor, which contained 50 mg of samples, at a flow rate of 50 mL·min−1, corresponding to a space velocity of 60,000 mLh-1g-1 of catalyst. Gas samples were analyzed with an online infrared gas analyzer (Gasboard-3100, China Wuhan Cubic CO.). RESULTS AND DISCUSSION


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Figure 1. TGA curves of the as-prepared Ce-MOF (a), and XRD patterns of Ce-MOF, CeO2 derived from partial decomposition of Ce-MOF and after acid etching (b). The thermal behavior of Ce-MOF was investigated using TG analysis. According to black curve (Figure 1a), the weight loss reaches ~64 % after complete thermal decomposition of CeMOF owing to the loss of the water and organic portion.28 When the Ce-MOF was calcined at 300 oC, it seems that this temperature is too low to activate MOF complete decomposition. However, there is a sharp rise in temperature at the early insulation stage, as shown in the plot of temperature verse time (blue curve), indicating the decomposition of Ce-MOF is exothermal. After treatment at 300 oC for 2 h, ~57 % weight loss was realized. If the Ce-MOF was calcined at lower temperature, e.g. 285 oC, only ~24 % weigh loss (data not shown here) was achieved, demonstrating the ceria cannot be derived at this temperature. To further study their difference under complete/partial decomposition conditions, the FT-IR spectra of the original Ce-MOF and commercial CeO2 are compared with the ones derived from partial decomposition and complete decomposition of Ce-MOF (Figure S1). The characteristic peaks at 1556 and 1434 cm-1 belong to the stretching vibrations Vassy (-COO-) and Vsym (-COO-) of the carboxylate ions vibration, respectively. After heat-treatment at 300 oC for 2 h, the characteristic peaks of carboxylate ions


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vibration still maintain, indicating the Ce-MOF is not completely decomposed. XRD was also used to examine the phase and crystal structure of the samples. As shown in Figure 1b, the main diffraction peaks of the Ce-MOF are located in the range of 10~30o, which are indexed to Ce(1,3,5-BTC)(H2O)6.28 After treatment at 300 oC, the diffraction peaks are all indexed as a facecentered cubic phase of ceria (JCPDS no.34-0394), and no diffraction peaks of residual Ce-MOF are obviously identified possibly due to its weak peak intensity and/or the low content of residual MOF in the sample. After selective etching by lactic acid, there are no changes in diffraction peak position and peak width, demonstrating the acid-treatment has insignificant effect on the ceria phase and crystallite size.

Figure 2. TEM images of Ce-MOF (a, b), partial decomposition of Ce-MOF (c, d), and the sample obtained from partial decomposition of Ce-MOF, followed by lactic acid etching (e). (f) is the HR-TEM image of marked part in (e).


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The size and morphology of the products were examined by TEM measurement. As shown in Figure 2a, the Ce-MOF shows a rod-like shape with typical diameters in the range of 200~300 nm, and holds a smooth surface (Figure 2b). After thermal treatment at 300 oC, the contrast between the edge and center parts of these rods is characteristic of a core-shell structure (Figure 2c), and the surface of the rods becomes porous (Figure 2d). Upon lactic acid etching, the coreshell structure develops into hollow-like feature (Figure 2e). The interlayer spacing was measured to be ~0.31 nm that is indexed well to the {111} of CeO2, and the measured interplanar angle is around 90°, which could be {100} crystal planes of the cubic CeO2 structure (Figure 2f). During the heat-treatment of Ce-MOF, as expected, the Ce-MOF gradually decomposes with the time, resulting in the shell thickness change (Figure S2).

Figure 3. Nitrogen adsorption-desorption isotherms for the pristine Ce-MOF (a), CeO2 prepared from partial decomposition of Ce-MOF (b), followed by lactic acid treatment (c). The insets are the corresponding schemes. The porous and hollow structure derived from partial decomposition of Ce-MOF is expected to exhibit high surface area. As shown in Figure 3a, the measured BET surface area is only 22.3 m2·g-1 for pristine Ce-MOF. After calcination at 300 oC, the BET surface area increases to 65.7 m2·g-1 because of partial Ce-MOF conversion into porous ceria (Figure 3b). When the residual MOF is further washed off by lactic acid, the resultant hollow structure produces additional


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surface area to 86.7 m2·g-1 (Figure 3c). The gradually increase in BET surface area suggests the thermal treatment and selective etching play great roles in the creation of porous and hollow structure in the as-prepared ceria.

Figure 4. CO conversion as a function temperature for the CuO loaded CeO2 that are prepared from decomposition of Ce-MOF at different temperatures (a), and catalytic cycles of the 300 oCCeO2-CuO at 100 oC (b). It is well-known that the porous/hollow structure is highly desirable as supports in the catalysis fields because it can facilitate the dispersion of secondary species, and also enable substrate molecules to contact the active sites easily. Herein, we take the 300 oC-CeO2 sample as a demonstration, in comparison to the CeO2 prepared from thermal decomposition of Ce-MOF at 400, 500 and 600 oC, to load CuO, and study their catalytic activity using CO oxidation as a model reaction, since CeO2-CuO is one of the most widely studied catalysts for various catalytic reaction owing to its high activity and good selectivity as well as low price.30-37 The catalytic activity of pure CuO and CeO2 is also compared. The estimated CuO loading is around 5.89, 5.66, 5.55 and 5.56 wt % for 300, 400, 500 and 600 oC-CeO2-CuO samples, as determined by ICP-MS. As shown in Figure 4a, their light-off curves for CO oxidation show that the activity of


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300 oC-CeO2-CuO is higher than others. Specifically, 100% CO conversion is realized at 98 oC for the 300 oC-CeO2-CuO catalyst, in contrast to 105, 124, and 142 oC with 400, 500 and 600 oCCeO2-CuO catalysts, respectively. Impressively, there is no catalytic activity for pure CeO2 and less than 10% CO conversion for pure CuO at this temperature (Figure S3). It is noted that no obvious residual carbon or carbon-contained substance exists in the 300 oC-CeO2-CuO catalyst (Figure S4). To study the durability of the 300 oC-CeO2-CuO, the catalyst was continuously carried out, as shown in Figure 4b, nearly no catalytic deactivation was found even after 5 cycles, and the porous/hollow structure still remains (Figure S5), indicating the 300 oC-CeO2CuO catalyst has a good durability.

Figure 5. Nitrogen adsorption-desorption isotherms (a) and XRD patterns (b) for the CuO loaded CeO2 that are prepared from decomposition of Ce-MOF. Inset in (a) shows the BET surface area for these samples. Since surface area is one of important factors to influence catalyst performce, BET surface area values are first compared. As shown in Figure 5a, after loading CuO, the BET surface area of 300 oC-CeO2-CuO is 114.8 m2·g-1, indicating the CuO species are studded on the surface or embedded in the pore wall. In addition, the heat-treatment during the process of CuO loading is


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another possible factor responsible for the surface area increase. In contrast, the BET surface area of 400, 500, 600 oC-CeO2-CuO catalysts and pure CuO prepared from the same depositionprecipitation method is 76.5, 57.3, 42.8, 23.2 m2·g-1, respectively (Figure S6). The XRD patterns show that all diffraction peaks are well indexed as a face-centered cubic phase of ceria (JCPDS no.34-0394), and no characteristic diffraction peaks of copper oxide species are identified with these samples (Figure 5b), possibly due to the highly dispersed copper oxide species onto the ceria. The width of XRD diffraction peaks gets narrow with the increase of temperature, which indicates the crystalline size of CeO2 becomes larger at high calcination temperature.

Figure 6. TEM images and elemental-mapping of 300 oC-CeO2-CuO (a, b) and 600 oC-CeO2CuO (c, d). Insets in (a), (c) are their corresponding HRTEM images. To further study the structure-performance relationship for these catalysts, we choose 600 oCCeO2-CuO as a representative to compare with 300 oC-CeO2-CuO catalyst. After incorporation


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of copper oxide species, the obtained CeO2-CuO catalysts still remain the initial morphology of corresponding CeO2 without any obvious disturbance during deposition-precipitation process (Figure 6). HRTEM characterization reveals that the CeO2-CuO catalysts are mainly composed of small nanoparticles that are highly crystalline, randomly oriented with each other. The interlayer spacings are measured to be indexed well to crystal planes of the cubic CeO2 structure. No obvious copper oxide species crystallites are identified although the Ce and Cu elements coexist in these two samples. To further investigate the elemental distribution of Ce and Cu, SEM-EDS elemental-mapping was conducted, as shown in Figure 6 b, d. Ce, Cu and O are uniformly distributed in these two catalysts. Such uniform distribution of Ce and Cu is beneficial to strengthen the interaction between Ce and Cu oxides, and is advantageous in catalytic reactions, e.g. CO oxidation.

Figure 7. Raman spectra of the CeO2 prepared from decomposition of Ce-MOF, and CuO loaded CeO2 that are prepared from decomposition of Ce-MOF. Inset shows the magnified part denoted by dashed line.


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To finely determine the structure of the CeO2-CuO catalysts, the Vis-Raman spectra were recorded as shown in Figure 7, the spectra of the 300 oC-CeO2, 600 oC-CeO2, 300 oC-CeO2-CuO and 600 oC-CeO2-CuO catalysts exhibit a main band centered at 464 cm-1, corresponding to the triply degenerate F2g symmetric vibration (Ce-O-Ce stretching).20 In addition, compared with the pure CeO2 catalysts, the spectra of the CeO2-CuO catalysts exhibit a new band in the range of 550~650 cm-1, indicating the presence of oxygen vacancy defects in the CeO2 lattice (inset in Figure 7).20 The larger peak area with 300 oC-CeO2-CuO than that of 600 oC-CeO2-CuO shows that the former catalyst contains more defect sites. No separated CuO phase with Raman peaks located at 292 and 340 cm−1 was observed, further revealing that Cu species on ceria are highly dispersed and subnanometer sized.29

Figure 8. XPS spectra of the Ce 3d (a) and Cu 2p of the CuO loaded CeO2 that are prepared from decomposition of Ce-MOF. Since heterogeneous catalytic reactions occur on the catalyst surface, XPS analysis was further performed to understand the surface composition and elemental chemical states of the CeO2CuO. As shown in Figure 8a, the peaks labeled by v (882.29 eV), v// (888.73 eV), v/// (898.09 eV), u (900.67 eV), u// (907.67 eV), and u/// (916.68 eV), are characteristic of Ce(IV). The Cu


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2p3/2 spectra in both catalysts contain one peak at 933~934 eV (Figure 8b). However, the presence of shake-up peak in the range of 942~948 eV indicates that surface copper species mainly exist in Cu(II) in the 600 oC-CeO2-CuO catalyst, while lower binding energy and the absence of shake-up peaks are characteristic of Cu(I), which can be viewed as a good indication of enhanced synergetic interaction between surface copper and surface ceria.

Figure 9. H2-TPR profiles of the CuO loaded CeO2 that are prepared from decomposition of CeMOF. The CO oxidation reaction is largely related to the redox property of catalysts. Herein, H2-TPR analysis was performed to gain the information about the reducibility and the chemical state of copper species of the as-prepared CeO2-CuO catalysts. As shown in Figure 9, two obvious reduction peaks exist in each TPR profile below 200 oC. The low temperature reduction peak (α) is mainly ascribed to the highly dispersed copper oxide species which strongly interacts with ceria on the surface of the catalysts, while the high temperature peak (β) could be related to the reduction of Cu2+ species incorporated into the ceria lattice or/and the reduction of bulk copper oxide.34,35 Compared with the 600 oC-CeO2-CuO catalyst, the peak α and β of the 300 oC-CeO2-


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CuO catalysts have obvious shifts toward low temperature. The easier reduction of 300 oC-CeO2CuO indicates there is a strong interaction between CeO2 and CuO, which can enhance oxygen mobility and decrease the reduction temperature of catalysts. It is widely acknowledged that various factors, including the surface area, the nature of active species, and the interaction between components of catalysts, comprehensively influence the catalytic performance. In this study, the specific surface area should be taken into account for comparing catalytic activity. The higher surface area of 300 oC-CeO2-CuO than that of 600 oCCeO2-CuO (114.8 vs 42.8 m2·g-1) favors diffusion and transportation of reactant molecules to the CeO2 and CuO more effectively. In addition, as demonstrated by Raman, H2-TPR, and XPS results, there is a strong synergistic effect between CeO2 and CuO. The incorporation of CuO species into CeO2 could largely promote the creation of oxygen vacancies at or around the CeO2/CuO interface, especially in the 300 oC-CeO2-CuO catalyst. In turn, the oxygen vacancies also strongly bind reactants and assist in their dissociation, thus facilitating the CO oxidation.37 CONCLUSIONS In summary, porous/hollow structured ceria was successfully fabricated by partial thermal decomposition of Ce-MOF, followed by a selective acid etching step. Besides the obtained ceria keeps the porous structure due to the decomposition of Ce-MOF, the ceria holds interior space after the removal of residual Ce-MOF by lactic acid, which is different from conventional complete thermal decomposition. The combination of hollow and porous structure enables the as-prepared ceria to possess high surface area, which was demonstrated to be a good support for copper oxide species. Based on the Raman, XPS, and H2-TPR results, we found there is a strong synergistic effect between the as-prepared ceria and copper oxide species. The developed


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synthetic route is simple and efficient and can be potentially extended to the synthesis of other metal oxide porous/hollow structured materials when suitable MOFs templates are chosen. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2015BM008) and Special Fund for Local Science and Technology Development Guided by Central Government (Grant No. Z135050009017). Supporting Information

The Supporting Information is available: Additional information including the FT-IR spectra of the original Ce-MOF and the one derived from partial decomposition of Ce-MOF , TEM images of the samples achieved from calcination of Ce-MOF at 300 oC for different time and samples after catalytic reaction, CO conversion as a function of temperature for 300 oC-CeO2-CuO and pure CeO2 and CuO, CO2 concentration as a function of temperature/time for samples under switching CO off or on in N2/O2 gas mixture, and Nitrogen adsorption-desorption isotherm for the pure CuO.


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TOC

Porous/hollow structured ceria was successfully fabricated by partial thermal decomposition of Ce-MOF, followed by a selective lactic acid etching step. Compared with the ceria derived from complete thermal decomposition of Ce-MOF, the as-prepared ceria holds high surface area and is demonstrated to be a good support for copper oxide species during the CO oxidation catalytic reaction.


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Hollow Structured Ceria by Partial Thermal

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