Characterization of cobalt oxides studied by FT-IR ...
八十三週年校慶基礎學術研討會
民國九十六年六月一日
Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG-MS Chih-Wei Tang1,2, Tsann-Yan Leu1,2, Wen-Yueh Yu2, Chen-Bin Wang1* and Shu-Hua Chien2,3* 1
Department of Applied Chemistry＆Materials Science, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan, 33509, Taiwan, R. O. C. 2 Institute of Chemistry, Academia Sinica, Taipei, 11529 Taiwan, R. O. C. 3 Department of Chemistry, National Taiwan University, Taipei, 10764, Taiwan, R. O.C. ABSTRACT A as-prepared cobalt oxide, CoOx, was prepared by precipitation-oxidation from cobalt nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by hydrogen peroxide. Further, other pure cobalt oxide species were refined from the high -valenced cobalt oxide by different calcinations temperature at 280, 440 and 950 ˚C, respectively. The as-prepared cobalt oxide obtain to the produce via calcinations process : CoOx → CoOOH → Co3O4 → CoO. Characterization by X-ray diffraction , FT-IR spectroscopy, Raman spectroscopy ,temperature -programmedreduction(TPR) and thermogravi metry -mass spectrometry (TG-MS). The results show the CoOOH decomposed to Co3O4 at 290 °C and CoO at 850 °C under N2 , respectively. Then the outlet gas analyzed to H2O and O2. The Co3O4 sample decomposed to the CoO at 820 °C under N2, and then the outlet gas belongs to O2. The CoO sample oxidized to Co3O4 between 200-900 °C under air. The CoOx →CoO decomposition reaction process were investigated, and found to be dependent upon the experimental conditions, namely and temperature. Keywords：CoOOH, Co3O4, CoO, TG-MS
1. INTRODUCTION Cobalt oxide has a wide range of applications in various fields of industry. For example, the rechargeable battery[1], catalyst for abatement of CO[2], magnetic materials[3] and CO sensor. The hydroxide of cobalt CoOOH have a hexagonal layered structure in which a divalent metal cation is located in an octahedral site generated by six
hydroxyl oxygen atoms. Nickel hydroxide is used as the electrochemically active material in positive electrodes of rechargeable alkaline batteries. Well-spread CoOOH is used as the conductive network to enhance the utiltzation of the hydroxide[5]. CoO is an antiferromagnetic transition metal oxide with a Ne´el temperature at 292 K (bulk), and many research works about its magnetic
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characters have been reported . CoO has an energy bandgap of 2.2–2.8 eV. Koshizaki et al. reported the gas-sensing properties of CoO-dispersed nanocomposite films, and it was also used as a precursor to produce rare-earth transition metal intermetallic nanoparticles.Co3O4 is a p-type semiconductor group, antiferromagnetic and has a spinel structure. Co3O4 has an energy bandgap of 1.4–1.8 eV. It is stable at temperatures below 800 °C, above which, it decomposes readily forming CoOwhich interacts with atmospheric oxygen giving cobaltic oxide. Cobalt oxides exhibited high catalytic activities in CO-oxidation by O2. CO-oxidation catalysts are useful for several applications including closed-cycle CO2 lasers and air purification.The cation of Co3O4 distribution of the stoichiometric spinel Co3O4 is shown to be 3+ 22+ Co [Co2 ]O4 , where cations inside the parenthesis are octahedrally and those outside are tetrahedral coordinated with oxygen ions. In this paper, we focus on the description of the single structure of the cobalt oxide obtained from the higher valence state cobalt oxide. Since our interest is the sample under the He and air to those of the reaction process, we engaged the thermogravimetry-mass spectrometry (TG-MS).Further, we have employed X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscopy and temperature-programmed reduction (TPR) for the characterization of a series of cobalt oxide. 2.EXPERIMENTAL 2.1 Preparation of a series of cobalt oxide
species The as-prepared cobalt oxide (assigned as CoOx)[2] with a high valence state of cobalt was synthesized by the precipitationoxidation method in an aqueous solution. The precipitation process was carried out at 50 ˚C with 50 ml of 0.6M Co(ON3)2⋅H2O solution added drop by drop to 100 ml of 3.2M NaOH solution; 100 ml of H2O2 (50 wt%) was then introduced drop by drop under constant stirring. Using the H2O2 as an oxidizing agent, instead of NaOCl, avoids possible chloride ion contamination. The precipitate was then filtered, washed with deionized distilled water and dried in an oven at 110 ˚C for 20 h. The dried product was ground and preserved in a desiccator as fresh samples, and then calcined to 280 ˚C, 450 ˚C and 950 ˚C for 4 hours under air. 2.2 Characterization of Techniques Powder X-ray diffraction data were recorded at room temperature on a Siemens D5000 diffractometer using Bragg-Brentano geometry withback-Monochromatized Cu-Ka radiation(λ=1:5405 Ǻ). The diffraction pattern was scanned in steps of 0.02° over the 2h range 10–80° and a counting time of 5 s per step. The peak positions in the powder pattern were determined by means of the peak search routine in the PC software package DIFFAC-AT.The crystallite sizes of cobaltic oxide and ceria were estimated using the Debye-Scherrer equation. Specific surface area measurements were carried out by using the Brunauer -Emmett -Teller (BET) method on a Micromeritics ASAP 2010 apparatus. Nitrogen adsorption isotherms at -196℃ were determined volumetrically. The catalysts were pre-
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outgassed at 5 x10-5 Torr for 3 h at 110 °C. The surface area was determined from the nitrogen adsorption isotherm. FTIR spectra of samples were obtained by a Bomen DA-8 spectrometer with a resolution of 4 cm−1, in the range of 500 to 4000 cm-1.One milligram of each powder sample was diluted with 200 mg of vacuum-dried IR-grade KBr powder and subjected to a pressure of 10 tons. The Raman spectrometer used in this study was a resonance Raman spectrum, equipped with a Coherent Innova 400 K3 Kr ions laser or Spectra Physics 2020 Ar ions laser, acting as the excitation source. A 5 mm exit slit offered a resolution of better than 1 cm−1. Combined to SPEX 1403 monochromator, RCA 31034 photomultiplier tube (PMT) and SPEX DPC2 photo-counting electronics measurement. Instrument parameter and obtain data analyzed with the computer. TPR of a series of cobalt oxides was performed using 10% H2 in Ar as the reducing gas. The gas flow rate was adjusted by mass flow controller under 25 ml min-1. The cell used for TPR was a quartz tube of inner diameter 8 mm, and 80 mg of the catalyst was mounted on quartz wool. The hydrogen consumption in the experiment was monitored by a thermal conductivity detector (TCD) on raising the sample temperature from RT to 500 °C at a constant rate of 5 °C min-1. In the experiments with linear heating at 10 °C/min the TG-MS spectra, and then outlet gas via mass mass spectrometric intensities plotted as a function of temperature showed two well separated regions of devolatilization. TG-MS measurements were performed simultaneously using the STA-409CD with Skimmer coupling from Netzsch, which is
equipped with a quadrupole mass spectrometer QMA 400 (max. 512 amu) from Balzers. 3. RESULTS AND DISCUSSION 3.1 XRD Figure 1 shows XRD patterns for species of cobalt oxide. The Figure 1(a) of results that the cobalt oxyhydroxide (CoOOH) samples are hexageonal structure. The XRD pattern of CoOOH is identical to the JCPDS (PDF-74-1057).Figure 1(b) show the Triobalt tetroxide Co3O4 sample obtained via calcined temperature to 440 °C under air. The XRD of Co3O4 converts into a spinel structure. JCPDS (PDF-76-1802) and Cobalt oxide (CoO) JCPDS (PDF-71-1178) has a faced-centered cubic structure. The Particle size of results shows the Table 1 of colume 3. The sample obtained have a particle size in the nanometer range (ca. 9.8 nm for CoOOH, ca. 10.4 nm for Co3O4 and ca. 15.5 nm for CoO) , as calculated from powder X-ray patterns using the Debye-Scherrer equation. Particle sizes are in accordance with the XRD measurement. Nitrogen adsorption on a series of cobalt oxide, were carried out at -196 °C. The obtained isotherms were similar to each other for investigated adsorbents. The obtained isotherms belong to typeⅡof Brunauer′s classification. The SBET data for a series of cobalt oxide calcined at higher temperatures enable the apparent sinter. The results shows the Table 1 of colume 4, exhibit the suface area order : CoOOH (58.8m2⋅g-1), Co3O4 (54.7m2⋅g-1) and CoO (10.5m2⋅g-1), respectively.
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elongated shape with a random orientation (Fig. 2) while the covered amorphous phase has been removed under the dehydration and reduction processes. As the calcined temperature reaches 280 ˚C, a marked change in the morphology is evident for the sample Co3O4. The most significant feature is the formation of overlayers of a hollow spheroidal shape (Fig. 3). The Co3O4 crystallites of a hollow spheroidal shape (the hollow inside diameter is 80–180 nm) are the main Co-containing phases. They are similar to the particles observed earlier by Potoczna-Petru et al. described as the torus shaped ones. No hollow spheroidal particles can be detected when the temperature raises to 450 ˚C sample CoO. The partially facetted CoO formed (Fig. 4) after the calcination of Co3O4.
Fig. 1 XRD profile of (a)CoOOH (b) Co3O4 (c)CoO 3.2 TEM The microstructures of cobalt oxide derivatives and the systematic changes accompanying the onset of dehydration and reduction of CoOx have been distinguished by comparative examination of the TEM. Figures 2–4 present the TEM images of sample under calcination. Some important observations are shown below. The CoOx sample possesses an elongated shape with a random orientation which is covered by the thickness of an amorphous phase. The TEM results of figures 2, the sample CoOOH also presents the distinct CH-21
Fig. 2 TEM micrograph for the
八十三週年校慶基礎學術研討會
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sample CoOOH.
distinctive bands originating from the stretching vibrations of the metal-oxygen bond, The first band ν1 at 570cm-1 is associated with the BOB3 vibrations in the spinel lattice, where B denotes the Co cations in an octahedral position , i.e Co3+ ions. The second bands ν2 at 661 cm-1 is attributed to the ABO3 vibrations, where A denotes the metal ions in a tetrahedral position. The band of CoO at 507cm-1 has been assigned to νCo-O vibrations. The band of CoOOH at 584cm-1 has been ascribed to νCo-OH vibrations. The IR spectrum of CoOOH and CoO(Fig. 5(a,c)) displays single band, the presence of the absorbency band 584 and 507 cm-1are likely to be due to
Fig. 3 TEM micrograph for the sample Co3O4.
Fig. 4 TEM micrograph for the sample CoO.
the presence of randomly oriented octahedral, i.e cobalt is situated in oxygen octahedral environment. Figure 6 displays the Raman spectra of a series of the cobalt oxide through the thermal decomposition process. Figure 6 (a) show the bands 367, 482, 599 and 809 cm-1 (assigned to CoOOH). Figure 3 (b) depicts the Raman spectrum showing bands at 469, 512, 607 and 674 cm-1(assigned to Co3O4). Figure 6 (c) can be seen that bands 672 and 468 cm-1 with low -intensities (assigned to CoO) . A previous studies have reported the Raman spectral bands of CoO. Seung Bin Kim et al reported that the Raman spectra exhibit four bands at ca. 672 (s), 605 (w), 510 (w), and 468 (s)cm-1, where s denotes relatively strong and w, weak, in the band intensity.
3.3 FTIR and Raman of spectroscopy Figure 5 shows the IR absorption spectra of CoOOH, Co3O4 and CoO. The IR spectrum of Co3O4(Fig. 5(b)) displays two CH-22
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Co3O4 (c)CoO 3.4 TPR The reduction of CoOx to Co metal has been study by our previous paper [2] using TPR method. Figure 7(a) shows the TPR profile of CoOOH. The reductive peaks total three singles that reduced temperature at 215, 260 and 350 °C, respectively. The CoOOH is initially reduced to Co3O4 [equation(1)], and then subsequently reduced to CoO and Co. According to the results, the following three successive steps are designated in TPR on raising the sample temperature. 6CoOOH + H2 → 2Co3O4 + 4H2O （1）
Fig. 5 FTIR spectra of (a)CoOOH (b) Co3O4 (c)CoO
Co3O4 + H2 → 3CoO + H2O
（2）
CoO + H2 → Co + H2O
（3）
Figure 7(b) shows the TPR profile of Co3O4 that reductive singles have two peaks. Sexton et al. found the reduction profile for Co3O4 consisting of a low-temperature peak below 300 °C and a high- temperature peak about 500 °C. According to the literature, the low-temperature peak can be ascribed to the reduction of Co3+ ions, present in the spinel structure, into Co2+ [equation(2)], with the subsequent structural change to CoO, which followed the higher-temperature peak and is due to the reduction of CoO to metallic cobalt[equation(3)].Figure 7(c) shows the TPR profile of CoO that reductive peak is single. The peak assigned to the reduction of Co2+ ions to metallic cobalt.
Fig. 6 Ranam spectra of (a)CoOOH (b) CH-23
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The final weight loss of 5% is the decomposition of Co3O4 into CoO according to equation (5) that is the theoretical value (4%). The outlet gas was oxygen at 280°C. Therefore, the CoOOH decomposed to Co3O4 and then CoO. Figure 9 show the TG-MS curves for the decomposition of Co3O4 in a Helium environment. Prior to 300 °C, the tardy weight loss should have come from the desorption of water on the Co3O4 surface in the heating process. Weight loss of 9% is mainly the decomposition of Co3O4 into CoO according to equation (6) (theoretical weight loss is 7%). 2Co3O4 → 6CoO + O2
Figure 10 show the TG-MS curves for th e oxidation of CoO in a air environment.
Fig. 7 TPR profiles of (a)CoOOH (b) Co3O4 (c)CoO 3.4 TG-MS In the experiments with linear heating at 10 °C/min the TG-MS spectra, and then mass spectrum analyzed the outlet gas of sample with decomposition from room temperature to 1100 °C in Helium or air. Figure 8 show the TG-MS curves for the decomposition of CoOOH in a Helium environment. The TG curve observed the 280 and 850 °C. The initial weight loss of 88% should be decomposed of CoOOH into Co3O4 according to equation(4)(theoretical weight loss is 87%).The outlet gas were H2O and O2 at 280°C. 12CoO(OH) → 4Co3O4 + O2 + 6H2O
(4)
2Co3O4 → 6CoO + O2
(5)
(6)
Weight increase of 7% is mainly the oxidation of CoO into Co3O4 from 250 to 900, and then conversion to CoO above at 900 °C. According to equation (7) (theoretical weight loss is 7%). 6CoO + O2 → 2Co3O4
(7)
The experimental and theoretical of TG-MS showed to the Table 2. In the results that understand the cobalt oxide sample thermal-decomposition behavior and outlet gas analyzed components clearly.
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4.CONCLUSIONS
Fig. 8 TG-MS of CoOOH decomposed to the outlet gases (-----)H2O (-⋅--⋅-)O2 at a n heating rate of 10 °C⋅min−1 under He.
TG-MS, Raman and FTIR analysis proved to be a useful tool for the characterization of a series of the cobalt oxide output of a process development unit. The results show the CoOOH decomposed to Co3O4 at 290 °C and CoO at 850 °C under N2 , respectively. Then the outlet gas analyzed to H2O and O2. The Co3O4 sample decomposed to the CoO at 820 °C under N2, and then the outlet gas belong to O2. The CoO sample oxidized to Co3O4 between 200-900 °C under air. The TPR measurement, the metal oxide (MOx), is redued under programmed temperature rise with the hydrogen. The metal oxide would be reduced to metal. We can defined the species of metal oxide, which correspond to the flowing process:
Fig. 9 TG-MS of Co3O4 decomposed to the outlet gas (-----)O2 at a heating rate of 10 °C⋅min−1 under He.
CoOx → CoOOH → Co3O4 → CoO → Co According to the results of XRD, FTIR, Raman, TPR and TG-MS. We can understanding a series of cobalt oxide via calcinations process of characterization. Further, these method could applied to material property of other metal oxide. 5.REFERENCES
Fig. 10 TG of CoO at a heating rate of 10 °C⋅min−1 under air.
[1] Wang, C. B., Tang, C. W., Gau, S. J. and Chien, S. H.; “Effect of the Surface Area of Cobaltic Oxide on Carbon Monoxide Oxidation”, Catalysis Letters, Vol. 101, 59-63. (2005). [2] Wang, C. B., Lin, H. K. and Tang, C. W., “Thermal characterization and microstructure change of cobalt oxides,” Catalysis Letters, Vol. 94, 69-74 (2004). [3] Grillo, F., Natile, M. M. and Glisenti, A., CH-25
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“Low temperature oxidation of carbon monoxide: the influence of water and oxygen on the reactivity of a Co3O4 powder surface,” Applied Catalysis B,
Vol. 48, pp.267-274, (2004).
氧化鈷之 FT-IR, Raman, TPR and TG-MS 鑑定 唐志偉 1,2 呂璨延 1,2 游文岳 2 汪成斌 1* 1
簡淑華 2,3*
國防大學理工學院應用化學及材料科學系 2 中央研究院化學研究所 3 台灣大學化學系 摘要
高價氧化鈷（CoOx）利用沉澱氧化法製備，前驅物為硝酸鈷、氫氧化鈉為沉澱劑和 過氫化物為氧化物，進一步分別藉由煅燒 280、450 和 950℃形成單一結構的氧化鈷物 種，高價氧化鈷經不同煅燒溫度會分解成單一結構氧化鈷物種，其分解過程如下： CoOx → CoOOH → Co3O4 → CoO。利用 X-ray 繞射光譜、紅外線光譜、拉曼光譜、程溫 還原和熱分析質譜等鑑定其特性。其結果顯示 CoOOH 在氮氣下進行熱分解在 290℃熱 分解為 Co3O4，850℃熱分解為 CoO，其相對溫度出口氣體為 H2O 和 O2。Co3O4 在氮氣 下溫度條件 820℃被分解為 CoO，然後出口氣體是氧氣。CoO 的樣品在空氣下溫度條件 為 200 至 900℃被氧化成 Co3O4。在 CoOx → CoO 熱分解反應中，研究變因依據實驗氣 體、物種名稱及反應溫度而改變。 關鍵字：CoOOH, Co3O4, CoO, TG-MS
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Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG-MS Chih-Wei Tang1,2, Tsann-Yan Leu1,2, Wen-Yueh Yu2, Chen-Bin Wang1* and Shu-Hua Chien2,3* 1
Department of Applied Chemistry＆Materials Science, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan, 33509, Taiwan, R. O. C. 2 Institute of Chemistry, Academia Sinica, Taipei, 11529 Taiwan, R. O. C. 3 Department of Chemistry, National Taiwan University, Taipei, 10764, Taiwan, R. O.C. ABSTRACT A as-prepared cobalt oxide, CoOx, was prepared by precipitation-oxidation from cobalt nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by hydrogen peroxide. Further, other pure cobalt oxide species were refined from the high -valenced cobalt oxide by different calcinations temperature at 280, 440 and 950 ˚C, respectively. The as-prepared cobalt oxide obtain to the produce via calcinations process : CoOx → CoOOH → Co3O4 → CoO. Characterization by X-ray diffraction , FT-IR spectroscopy, Raman spectroscopy ,temperature -programmedreduction(TPR) and thermogravi metry -mass spectrometry (TG-MS). The results show the CoOOH decomposed to Co3O4 at 290 °C and CoO at 850 °C under N2 , respectively. Then the outlet gas analyzed to H2O and O2. The Co3O4 sample decomposed to the CoO at 820 °C under N2, and then the outlet gas belongs to O2. The CoO sample oxidized to Co3O4 between 200-900 °C under air. The CoOx →CoO decomposition reaction process were investigated, and found to be dependent upon the experimental conditions, namely and temperature. Keywords：CoOOH, Co3O4, CoO, TG-MS
1. INTRODUCTION Cobalt oxide has a wide range of applications in various fields of industry. For example, the rechargeable battery[1], catalyst for abatement of CO[2], magnetic materials[3] and CO sensor. The hydroxide of cobalt CoOOH have a hexagonal layered structure in which a divalent metal cation is located in an octahedral site generated by six
hydroxyl oxygen atoms. Nickel hydroxide is used as the electrochemically active material in positive electrodes of rechargeable alkaline batteries. Well-spread CoOOH is used as the conductive network to enhance the utiltzation of the hydroxide[5]. CoO is an antiferromagnetic transition metal oxide with a Ne´el temperature at 292 K (bulk), and many research works about its magnetic
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characters have been reported . CoO has an energy bandgap of 2.2–2.8 eV. Koshizaki et al. reported the gas-sensing properties of CoO-dispersed nanocomposite films, and it was also used as a precursor to produce rare-earth transition metal intermetallic nanoparticles.Co3O4 is a p-type semiconductor group, antiferromagnetic and has a spinel structure. Co3O4 has an energy bandgap of 1.4–1.8 eV. It is stable at temperatures below 800 °C, above which, it decomposes readily forming CoOwhich interacts with atmospheric oxygen giving cobaltic oxide. Cobalt oxides exhibited high catalytic activities in CO-oxidation by O2. CO-oxidation catalysts are useful for several applications including closed-cycle CO2 lasers and air purification.The cation of Co3O4 distribution of the stoichiometric spinel Co3O4 is shown to be 3+ 22+ Co [Co2 ]O4 , where cations inside the parenthesis are octahedrally and those outside are tetrahedral coordinated with oxygen ions. In this paper, we focus on the description of the single structure of the cobalt oxide obtained from the higher valence state cobalt oxide. Since our interest is the sample under the He and air to those of the reaction process, we engaged the thermogravimetry-mass spectrometry (TG-MS).Further, we have employed X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscopy and temperature-programmed reduction (TPR) for the characterization of a series of cobalt oxide. 2.EXPERIMENTAL 2.1 Preparation of a series of cobalt oxide
species The as-prepared cobalt oxide (assigned as CoOx)[2] with a high valence state of cobalt was synthesized by the precipitationoxidation method in an aqueous solution. The precipitation process was carried out at 50 ˚C with 50 ml of 0.6M Co(ON3)2⋅H2O solution added drop by drop to 100 ml of 3.2M NaOH solution; 100 ml of H2O2 (50 wt%) was then introduced drop by drop under constant stirring. Using the H2O2 as an oxidizing agent, instead of NaOCl, avoids possible chloride ion contamination. The precipitate was then filtered, washed with deionized distilled water and dried in an oven at 110 ˚C for 20 h. The dried product was ground and preserved in a desiccator as fresh samples, and then calcined to 280 ˚C, 450 ˚C and 950 ˚C for 4 hours under air. 2.2 Characterization of Techniques Powder X-ray diffraction data were recorded at room temperature on a Siemens D5000 diffractometer using Bragg-Brentano geometry withback-Monochromatized Cu-Ka radiation(λ=1:5405 Ǻ). The diffraction pattern was scanned in steps of 0.02° over the 2h range 10–80° and a counting time of 5 s per step. The peak positions in the powder pattern were determined by means of the peak search routine in the PC software package DIFFAC-AT.The crystallite sizes of cobaltic oxide and ceria were estimated using the Debye-Scherrer equation. Specific surface area measurements were carried out by using the Brunauer -Emmett -Teller (BET) method on a Micromeritics ASAP 2010 apparatus. Nitrogen adsorption isotherms at -196℃ were determined volumetrically. The catalysts were pre-
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outgassed at 5 x10-5 Torr for 3 h at 110 °C. The surface area was determined from the nitrogen adsorption isotherm. FTIR spectra of samples were obtained by a Bomen DA-8 spectrometer with a resolution of 4 cm−1, in the range of 500 to 4000 cm-1.One milligram of each powder sample was diluted with 200 mg of vacuum-dried IR-grade KBr powder and subjected to a pressure of 10 tons. The Raman spectrometer used in this study was a resonance Raman spectrum, equipped with a Coherent Innova 400 K3 Kr ions laser or Spectra Physics 2020 Ar ions laser, acting as the excitation source. A 5 mm exit slit offered a resolution of better than 1 cm−1. Combined to SPEX 1403 monochromator, RCA 31034 photomultiplier tube (PMT) and SPEX DPC2 photo-counting electronics measurement. Instrument parameter and obtain data analyzed with the computer. TPR of a series of cobalt oxides was performed using 10% H2 in Ar as the reducing gas. The gas flow rate was adjusted by mass flow controller under 25 ml min-1. The cell used for TPR was a quartz tube of inner diameter 8 mm, and 80 mg of the catalyst was mounted on quartz wool. The hydrogen consumption in the experiment was monitored by a thermal conductivity detector (TCD) on raising the sample temperature from RT to 500 °C at a constant rate of 5 °C min-1. In the experiments with linear heating at 10 °C/min the TG-MS spectra, and then outlet gas via mass mass spectrometric intensities plotted as a function of temperature showed two well separated regions of devolatilization. TG-MS measurements were performed simultaneously using the STA-409CD with Skimmer coupling from Netzsch, which is
equipped with a quadrupole mass spectrometer QMA 400 (max. 512 amu) from Balzers. 3. RESULTS AND DISCUSSION 3.1 XRD Figure 1 shows XRD patterns for species of cobalt oxide. The Figure 1(a) of results that the cobalt oxyhydroxide (CoOOH) samples are hexageonal structure. The XRD pattern of CoOOH is identical to the JCPDS (PDF-74-1057).Figure 1(b) show the Triobalt tetroxide Co3O4 sample obtained via calcined temperature to 440 °C under air. The XRD of Co3O4 converts into a spinel structure. JCPDS (PDF-76-1802) and Cobalt oxide (CoO) JCPDS (PDF-71-1178) has a faced-centered cubic structure. The Particle size of results shows the Table 1 of colume 3. The sample obtained have a particle size in the nanometer range (ca. 9.8 nm for CoOOH, ca. 10.4 nm for Co3O4 and ca. 15.5 nm for CoO) , as calculated from powder X-ray patterns using the Debye-Scherrer equation. Particle sizes are in accordance with the XRD measurement. Nitrogen adsorption on a series of cobalt oxide, were carried out at -196 °C. The obtained isotherms were similar to each other for investigated adsorbents. The obtained isotherms belong to typeⅡof Brunauer′s classification. The SBET data for a series of cobalt oxide calcined at higher temperatures enable the apparent sinter. The results shows the Table 1 of colume 4, exhibit the suface area order : CoOOH (58.8m2⋅g-1), Co3O4 (54.7m2⋅g-1) and CoO (10.5m2⋅g-1), respectively.
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elongated shape with a random orientation (Fig. 2) while the covered amorphous phase has been removed under the dehydration and reduction processes. As the calcined temperature reaches 280 ˚C, a marked change in the morphology is evident for the sample Co3O4. The most significant feature is the formation of overlayers of a hollow spheroidal shape (Fig. 3). The Co3O4 crystallites of a hollow spheroidal shape (the hollow inside diameter is 80–180 nm) are the main Co-containing phases. They are similar to the particles observed earlier by Potoczna-Petru et al. described as the torus shaped ones. No hollow spheroidal particles can be detected when the temperature raises to 450 ˚C sample CoO. The partially facetted CoO formed (Fig. 4) after the calcination of Co3O4.
Fig. 1 XRD profile of (a)CoOOH (b) Co3O4 (c)CoO 3.2 TEM The microstructures of cobalt oxide derivatives and the systematic changes accompanying the onset of dehydration and reduction of CoOx have been distinguished by comparative examination of the TEM. Figures 2–4 present the TEM images of sample under calcination. Some important observations are shown below. The CoOx sample possesses an elongated shape with a random orientation which is covered by the thickness of an amorphous phase. The TEM results of figures 2, the sample CoOOH also presents the distinct CH-21
Fig. 2 TEM micrograph for the
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sample CoOOH.
distinctive bands originating from the stretching vibrations of the metal-oxygen bond, The first band ν1 at 570cm-1 is associated with the BOB3 vibrations in the spinel lattice, where B denotes the Co cations in an octahedral position , i.e Co3+ ions. The second bands ν2 at 661 cm-1 is attributed to the ABO3 vibrations, where A denotes the metal ions in a tetrahedral position. The band of CoO at 507cm-1 has been assigned to νCo-O vibrations. The band of CoOOH at 584cm-1 has been ascribed to νCo-OH vibrations. The IR spectrum of CoOOH and CoO(Fig. 5(a,c)) displays single band, the presence of the absorbency band 584 and 507 cm-1are likely to be due to
Fig. 3 TEM micrograph for the sample Co3O4.
Fig. 4 TEM micrograph for the sample CoO.
the presence of randomly oriented octahedral, i.e cobalt is situated in oxygen octahedral environment. Figure 6 displays the Raman spectra of a series of the cobalt oxide through the thermal decomposition process. Figure 6 (a) show the bands 367, 482, 599 and 809 cm-1 (assigned to CoOOH). Figure 3 (b) depicts the Raman spectrum showing bands at 469, 512, 607 and 674 cm-1(assigned to Co3O4). Figure 6 (c) can be seen that bands 672 and 468 cm-1 with low -intensities (assigned to CoO) . A previous studies have reported the Raman spectral bands of CoO. Seung Bin Kim et al reported that the Raman spectra exhibit four bands at ca. 672 (s), 605 (w), 510 (w), and 468 (s)cm-1, where s denotes relatively strong and w, weak, in the band intensity.
3.3 FTIR and Raman of spectroscopy Figure 5 shows the IR absorption spectra of CoOOH, Co3O4 and CoO. The IR spectrum of Co3O4(Fig. 5(b)) displays two CH-22
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Co3O4 (c)CoO 3.4 TPR The reduction of CoOx to Co metal has been study by our previous paper [2] using TPR method. Figure 7(a) shows the TPR profile of CoOOH. The reductive peaks total three singles that reduced temperature at 215, 260 and 350 °C, respectively. The CoOOH is initially reduced to Co3O4 [equation(1)], and then subsequently reduced to CoO and Co. According to the results, the following three successive steps are designated in TPR on raising the sample temperature. 6CoOOH + H2 → 2Co3O4 + 4H2O （1）
Fig. 5 FTIR spectra of (a)CoOOH (b) Co3O4 (c)CoO
Co3O4 + H2 → 3CoO + H2O
（2）
CoO + H2 → Co + H2O
（3）
Figure 7(b) shows the TPR profile of Co3O4 that reductive singles have two peaks. Sexton et al. found the reduction profile for Co3O4 consisting of a low-temperature peak below 300 °C and a high- temperature peak about 500 °C. According to the literature, the low-temperature peak can be ascribed to the reduction of Co3+ ions, present in the spinel structure, into Co2+ [equation(2)], with the subsequent structural change to CoO, which followed the higher-temperature peak and is due to the reduction of CoO to metallic cobalt[equation(3)].Figure 7(c) shows the TPR profile of CoO that reductive peak is single. The peak assigned to the reduction of Co2+ ions to metallic cobalt.
Fig. 6 Ranam spectra of (a)CoOOH (b) CH-23
八十三週年校慶基礎學術研討會
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The final weight loss of 5% is the decomposition of Co3O4 into CoO according to equation (5) that is the theoretical value (4%). The outlet gas was oxygen at 280°C. Therefore, the CoOOH decomposed to Co3O4 and then CoO. Figure 9 show the TG-MS curves for the decomposition of Co3O4 in a Helium environment. Prior to 300 °C, the tardy weight loss should have come from the desorption of water on the Co3O4 surface in the heating process. Weight loss of 9% is mainly the decomposition of Co3O4 into CoO according to equation (6) (theoretical weight loss is 7%). 2Co3O4 → 6CoO + O2
Figure 10 show the TG-MS curves for th e oxidation of CoO in a air environment.
Fig. 7 TPR profiles of (a)CoOOH (b) Co3O4 (c)CoO 3.4 TG-MS In the experiments with linear heating at 10 °C/min the TG-MS spectra, and then mass spectrum analyzed the outlet gas of sample with decomposition from room temperature to 1100 °C in Helium or air. Figure 8 show the TG-MS curves for the decomposition of CoOOH in a Helium environment. The TG curve observed the 280 and 850 °C. The initial weight loss of 88% should be decomposed of CoOOH into Co3O4 according to equation(4)(theoretical weight loss is 87%).The outlet gas were H2O and O2 at 280°C. 12CoO(OH) → 4Co3O4 + O2 + 6H2O
(4)
2Co3O4 → 6CoO + O2
(5)
(6)
Weight increase of 7% is mainly the oxidation of CoO into Co3O4 from 250 to 900, and then conversion to CoO above at 900 °C. According to equation (7) (theoretical weight loss is 7%). 6CoO + O2 → 2Co3O4
(7)
The experimental and theoretical of TG-MS showed to the Table 2. In the results that understand the cobalt oxide sample thermal-decomposition behavior and outlet gas analyzed components clearly.
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4.CONCLUSIONS
Fig. 8 TG-MS of CoOOH decomposed to the outlet gases (-----)H2O (-⋅--⋅-)O2 at a n heating rate of 10 °C⋅min−1 under He.
TG-MS, Raman and FTIR analysis proved to be a useful tool for the characterization of a series of the cobalt oxide output of a process development unit. The results show the CoOOH decomposed to Co3O4 at 290 °C and CoO at 850 °C under N2 , respectively. Then the outlet gas analyzed to H2O and O2. The Co3O4 sample decomposed to the CoO at 820 °C under N2, and then the outlet gas belong to O2. The CoO sample oxidized to Co3O4 between 200-900 °C under air. The TPR measurement, the metal oxide (MOx), is redued under programmed temperature rise with the hydrogen. The metal oxide would be reduced to metal. We can defined the species of metal oxide, which correspond to the flowing process:
Fig. 9 TG-MS of Co3O4 decomposed to the outlet gas (-----)O2 at a heating rate of 10 °C⋅min−1 under He.
CoOx → CoOOH → Co3O4 → CoO → Co According to the results of XRD, FTIR, Raman, TPR and TG-MS. We can understanding a series of cobalt oxide via calcinations process of characterization. Further, these method could applied to material property of other metal oxide. 5.REFERENCES
Fig. 10 TG of CoO at a heating rate of 10 °C⋅min−1 under air.
[1] Wang, C. B., Tang, C. W., Gau, S. J. and Chien, S. H.; “Effect of the Surface Area of Cobaltic Oxide on Carbon Monoxide Oxidation”, Catalysis Letters, Vol. 101, 59-63. (2005). [2] Wang, C. B., Lin, H. K. and Tang, C. W., “Thermal characterization and microstructure change of cobalt oxides,” Catalysis Letters, Vol. 94, 69-74 (2004). [3] Grillo, F., Natile, M. M. and Glisenti, A., CH-25
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“Low temperature oxidation of carbon monoxide: the influence of water and oxygen on the reactivity of a Co3O4 powder surface,” Applied Catalysis B,
Vol. 48, pp.267-274, (2004).
氧化鈷之 FT-IR, Raman, TPR and TG-MS 鑑定 唐志偉 1,2 呂璨延 1,2 游文岳 2 汪成斌 1* 1
簡淑華 2,3*
國防大學理工學院應用化學及材料科學系 2 中央研究院化學研究所 3 台灣大學化學系 摘要
高價氧化鈷（CoOx）利用沉澱氧化法製備，前驅物為硝酸鈷、氫氧化鈉為沉澱劑和 過氫化物為氧化物，進一步分別藉由煅燒 280、450 和 950℃形成單一結構的氧化鈷物 種，高價氧化鈷經不同煅燒溫度會分解成單一結構氧化鈷物種，其分解過程如下： CoOx → CoOOH → Co3O4 → CoO。利用 X-ray 繞射光譜、紅外線光譜、拉曼光譜、程溫 還原和熱分析質譜等鑑定其特性。其結果顯示 CoOOH 在氮氣下進行熱分解在 290℃熱 分解為 Co3O4，850℃熱分解為 CoO，其相對溫度出口氣體為 H2O 和 O2。Co3O4 在氮氣 下溫度條件 820℃被分解為 CoO，然後出口氣體是氧氣。CoO 的樣品在空氣下溫度條件 為 200 至 900℃被氧化成 Co3O4。在 CoOx → CoO 熱分解反應中，研究變因依據實驗氣 體、物種名稱及反應溫度而改變。 關鍵字：CoOOH, Co3O4, CoO, TG-MS
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