Comparative Study on the Formation and Reduction of Bulk and

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J. Phys. Chem. C 2009, 113, 7186–7199

Comparative Study on the Formation and Reduction of Bulk and Al2O3-Supported Cobalt Oxides by H2-TPR Technique Yuguo Ji, Zhen Zhao,* Aijun Duan, Guiyuan Jiang, and Jian Liu State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Beijing, 102249 China ReceiVed: December 05, 2008; ReVised Manuscript ReceiVed: February 26, 2009

A systematic and comparative study of the decomposition procedures of unsupported and Al2O3-supported Co(NO3)2 · 6H2O, as well as the reduction procedures of bulk Co3O4 and Al2O3-supported Co3O4 was carried out. A series of decomposition products of bulk and Al2O3-supported Co(NO3)2 · 6H2O, and bulk cobalt oxides (Co3O4 and CoOOH) and Al2O3-supported Co3O4 (Co/Al2O3 catalysts) were prepared by the methods of calcination, deposition-precipitation, and incipient-wetness impregnation. The bulk and supported samples were characterized by different techniques, including XRD, UV-vis-NIR, TG-DTA, gas analyzer, and H2TPR. The results show that the CoO species first appears in the calcination process of bulk Co(NO3)2 · 6H2O decomposition, and then CoO is oxidized to form Co3O4. CoO-Al2O3 composite oxide first appears in the calcination process of Al2O3-supported Co(NO3)2 · 6H2O decomposition. CoO disperses on the surface of Al2O3 support at first and then forms a multilayer with the decomposition of Co(NO3)2. Finally, CoO-Al2O3 composite oxide, which is very difficult to be reduced to metal Co, was oxidized to Co3O4. Particularly, it is thoroughly analyzed that the attribution of each reduction peak in the H2-TPR profiles of a series of Al2O3-supported Co-based catalyst. Moreover, it is a first semiquantitative study on the formation process of CoO in the course of calcination of bulk Co(NO3)2 · 6H2O and Al2O3-supported Co(NO3)2 · 6H2O, by introducing parameter R. In addition, the reduction mechanisms of bulk Co3O4 and Al2O3-supported Co3O4 were studied in detail, too. The reduction performance of Al2O3-supported Co3O4 is different from that of the bulk Co3O4, because of the strong interaction between Co3O4 and Al2O3 support. The activation energies for the two transformations from bulk Co3O4 to CoO and from CoO to Co are 80 and 53 kJ/mol, respectively. However, the activation energies for the two transformations from Al2O3-supported Co3O4 to CoO-Al2O3 and from CoO-Al2O3 to Co are 90 and 95 kJ/mol, respectively. So the H2-TPR profile of Al2O3-supported Co3O4 exhibits two apparent peaks, whereas the H2-TPR profile of bulk Co3O4 exhibits the incorporated trend of two peaks. 1. Introduction It is well-known that Al2O3-supported cobalt catalysts, which are very important in heterogeneous catalysts, show promising catalytic behaviors in the Fischer-Tropsch synthesis1-3 and in methane steam reforming.4,5 It is not only because cobalt is an interesting transition metal ion which has seven 3d electrons but also becuase Al2O3 has high surface area, favorable mechanical properties, and adjustable surface properties. Like other supported catalysts, the number of the active species is a key factor to determine the activity and the selectivity of the supported cobalt catalysts.1,6,7 So far, the Al2O3supported Co-based catalysts are often prepared by means of impregnating Al2O3 support with an aqueous solution containing cobalt nitrate hexahydrate, followed by drying and calcination to obtain the supported cobalt oxide species, including supported Co3O4 crystallites. And the cobalt oxides are reduced to obtain themetalliccobalt,whichistheactivephasefortheFischer-Tropsch synthesis1-3 and the methane steam reforming.4,5 Thus, the number of the active species is determined by the dispersion degree and reducibility of Al2O3-supported cobalt oxide species. Nevertheless, it is well-known that the Al2O3-supported Cobased catalysts have a limited reducibility due to a strong interaction between the support and the cobalt oxides. And the interaction may result in the formation of nonstoichiometric and * Corresponding author. [email protected].

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stoichiometric cobalt aluminate spinel, for example, CoO-Al2O3 and CoAl2O4, both are very difficult to be reduced to metal Co and are inactive for complete bezene oxidation. So it is important to clarify the formation and the properties of Al2O3-supported cobalt species in the procedure of preparation of the catalysts, including the process of the decomposition of cobalt nitrate hexahydrate on the surface of the Al2O3 support and the process of the reduction of cobalt oxide species at the atmosphere of H2, to maximize the number of the active species. Both procedures are described in some literature. Mansour8 claimed that cobalt nitrate hexahydrate decomposed in four steps: (a) melting at about 75 °C, (b) dehydration between 80 and 170 °C forming cobalt nitrate monohydrate, (c) dehydration and decomposition of the monohydrate into an unstable intermediate structure containing Co(NO3)2, CoO, Co2O3, and Co3O4, and finally (d) decomposition into Co3O4 at about 240 °C. Lapidus et al.9 studied the decomposition of cobalt nitrate hexahydrate and obtained a similar conclusion. They indicated that the exothermic enthalpy change due to the oxidation of Co2+ to Co3+ by nitrogen dioxide evolved in decomposition is covered up by the endothermic enthalpy change of the decomposition of cobalt nitrate to cobalt oxide. Loosdrecht et al.10 demonstrated that CoOOH formed during the decomposition of the cobalt nitrate hexahydrate, which is distinctly different from Co3O4. The presence of CoOOH in the calcined catalyst may lead to higher cobalt metal surface areas

10.1021/jp8107057 CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

Formation and Reduction of Cobalt Oxides in the reduced cobalt catalyst, thus increasing the Fischer-Tropsch synthesis performance of the catalyst. Jongsomjit et al.11,12 confirmed that Co3O4 was present in the calcined catalysts samples by Raman spectroscopy technique. And it is suggested that a surface Co compound species related to Co strongly interacting with the alumina as a Co “aluminate” appeared, after reduction. The identified Co aluminate is suggested to be different from CoAl2O4 (spinel) due to it being a nonstoichiometric surface Co aluminate compound. This highly dispersed Co aluminate (Co-AlxOy) may be formed, possibly, by Co atom migration into the alumina matrix. Chu et al.13 observed that cobalt reducibility was relatively low in monometallic cobalt alumina-supported catalysts and decreased as a function of catalyst calcination temperature. The effect was probably due to the formation of mixed surface compounds between Co3O4 and Al2O3 at higher calcination temperatures, which hindered cobalt reduction. Wang et al.14 concluded that the strong metal oxide-support interaction depends on metal loading and calcination temperature and attributed to cobalt ions in the tetrahedral and octahedral site of alumina which has a spinel crystal structure with a deficit of cations. The diffusion of cobalt ions during calcination into alumina lattice sites is limited to the first few outer layers of the support. However, the formation procedure of the mixed surface compounds between Co3O4 and Al2O3 (Co aluminate) has not been fully understood, because Co aluminate does not have long-range, three-dimensional order. Not only can H2-TPR technique be employed to study the nature and reducibility of cobalt oxides in the catalysts but also it can be used to calculate the quantity of cobalt oxides in the catalysts.15-17 Thus, it is possible for a quantitative analysis of the Al2O3-supported Co-based catalysts. Nevertheless, it is a challenge to illuminate the attribution of each reduction peak in the H2-TPR profiles of Co3O4, especially in the case of Al2O3supported Co-based catalysts. Bulk Co3O4 is reduced in two steps (Co3+ f Co2+ f Co0). However, the doublet is always overlapped because of the unsuitable TPR experiment conditions (especially the flow rate of the hydrogen-inert gas mixture, the heating rate and the sample mass).9 There are many reports concerning the reduction property of Al2O3-supported Co-based catalysts. However, there currently appears to be a lack of consensus on the attribution of each reduction peak in the H2TPR profiles. Some different hypotheses about the nature of reduction of cobalt/Al2O3 have been reported. For example, for a 15 wt % Co/catalyst calcined at 350 °C in flowing air, whose H2-TPR profile exhibit two peaks (a sharp peak, and a very broad peak): (a) Bechara et al.18 concluded that a two-step reduction operates with CoO as an intermediate oxide, via Co3O4 to CoO and CoO to Co0 transformations. This interpretation signifies that the second step is heavily dependent on support type, porosity, and resulting crystallite size. Jacobs et al.19,20 favored the explanation and provided information for a two-step reduction process over standard calcined catalysts. They observed and quantified over all catalysts exhibiting both weak interactions (e.g., Co/SiO2) and strong interactions (e.g., Co/ Al2O3) with the support, by employing the new TPR-XANES/ EXAFS method. (b) Others have developed a somewhat different perspective. Jongsomjit et al.11 suggested that the sharp peak at ∼300 °C was due to the reduction of Co3O4 f CoO, CoO f Co metal. Also the very broad high temperature peak ranging from 400 to 750 °C was assigned to a mixed oxide denoted CoXOY-Al2O3. Lapidus et al.9,21 reached a similar conclusion.

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7187 (c) In addition, a small peak may appear between 180 and 260 °C in the H2-TPR profile, if the calcination temperature is lower than 300 °C. Borg et al.22 reported that the first part of the reduction process can be attributed to the reduction of supported cobalt nitrate remaining after calcination. Lapidus reported a similar result, too. However, Loosdrecht considered10 that the first peak in the H2-TPR profile is attributed to the reduction of CoOOH to Co3O4. So it is necessary to illuminate the attribution of each reduction peak in the H2-TPR profiles of Al2O3-supported Cobased catalysts. The kinetics of reduction of cobalt oxides by hydrogen has been studied in some literature. Bustnes et al.23 found that the activation energy of the reduction of CoO to Co obtained from TGA measurements in the range of 637-837 K is 54.3 kJ/mol. Chu et al.,13 reported that the activation energy of formation of cobalt metal phases for Co/Al2O3 is 130 kJ/ mol. However, there are few documents that explicated on the difference between kinetics of reduction of bulk Co3O4 and that of Al2O3-supported Co3O4. Thus, it is significant to clarify the formation of cobalt oxide or metallic cobalt species in the procedure of preparation of the catalysts, including the procedure of the decomposition of cobalt nitrate hexahydrate on the surface of the Al2O3 support and the process of the reduction of Al2O3-supported cobalt oxide species. For comparison, the decomposition process of bulk Co(NO3)2 · 6H2O and the reduction procedure of bulk Co3O4 and CoOOH, which are reference samples, are also studied. 2. Experimental Section 2.1. Preparation of Catalyst. Reference Samples. Cobalt nitrate hexahydrate (Co(NO3)2 · 6H2O, 99.9% of purity) purchased from Sinopharm Chemical Reagent Company, Ltd.S, was used for the preparation of the bulk cobalt oxides and the solution used in the preparation of the catalysts. Co3O4 was prepared by calcining Co(NO3)2 · 6H2O at 450 °C for 12 h. CoOOH was prepared from cobalt nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by hydrogen peroxide. An aqueous solution of NaOH (2 M) was added to a cobalt nitrate solution (0.6 M), with constant stirring for 2 h. To the brown precipitate obtained was added 200 mL of H2O2(50%), which is a strongly exothermic reaction. The precipitation was carried out at 70 °C under constant stirring. The resulting black precipitation was allowed to stay in the mother liquor for 10 h. Then it was filtered, washed with distilled water for 1 h, and dried at 110 °C for 6 h. Al2O3, which has a specific surface area of 293 m2/g and an average pore diameter of 6.65 nm, was used as the support of the catalysts. 5% and 15% Co/Al2O3 catalysts, with Co loadings of 5 and15 wt %, was prepared by incipient-wetness impregnation (IWI) method, with drying steps in air at 110 °C for 10 h and calcination steps in air at 450 °C for 10 h to get the Al2O3supported Co3O4. 30% and 40% Co/Al2O3 catalysts, with Co loadings of 30 and 40 wt %, were prepared by a multiple-step IWI method. Multiple impregnations of 10% Co were performed with drying steps in air at 110 °C for 5 h and calcination steps in air at 450 °C for 5 h. The last calcination step was in air at 450 °C for 10 h. Decomposition Products of Bulk Co(NO3)2 · 6H2O. The decomposition products of bulk Co(NO3)2 · 6H2O were prepared by calcining the sample, in which 50 g of Co(NO3)2 · 6H2O was calcined in air at a rate of 5 K/min from 20 to 450 °C. During the heating process, a 5 g sample was removed at 110, 170,

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200, and every temperature from 200 to 450 °C with a temperature interval of 50 °C, named as Co(110), Co(170), Co(200), and Co(250)sCo(450), respectively. Decomposition Products of Al2O3-Supported Co(NO3)2 · 6H2O. To study the decomposition process of Al2O3supported Co(NO3)2 · 6H2O, a series of decomposition products of Al2O3-supported Co(NO3)2 · 6H2O with different calcination temperatures were prepared. In this study, the Al2O3-supported CoOx species were prepared by IWI of Al2O3 support with suitable amount of aqueous solution of Co(NO3)2 · 6H2O. Cobalt nitrate hexahydrate was dissolved in deionized water and impregnated the support, using IWI to give a final reduced catalyst with 15 wt % of cobalt, named as Co/Al(wet). The sample was dried in air at 110 °C for 10 h, named as CoAl(dry). The sample Co/Al(dry) was calcined in air at a rate of 5 °C/ min from 110 to 450 °C. During the heating process, a 5 g sample was removed at 170, 200, and every temperature from 200 to 450 °C with a temperature interval of 50 °C, named as Co/Al(170),Co/Al(200),andCo/Al(250)sCo/Al(450),respectively. Mixture Samples. The Co3O4 and Al2O3, CoOOH and Al2O3, Co(NO3)2 · 6H2O and Al2O3, Co(200) and Al2O3, and Co(250) and Al2O3 mixture samples, with Co loading of 15%, were prepared by mechanical mixing with corresponding cobalt oxides and Al2O3, respectively. 2.2. Characterization. H2-TPR. T e reduction behaviors and the interactions between the active phase and the support of each sample were examined by using temperature-programmed reduction of H2 (H2-TPR) technique. Each sample was placed in a quartz tubular reactor fitted with a thermocouple for continuous temperature measurement. Prior to the H2-TPR measurement, to drive away water or impurities, 5% Co/Al2O3 catalyst (0.6 g), 15% Co/Al2O3 catalyst (0.2 g), 30% Co/Al2O3 catalyst (0.1 g), 40% Co/Al2O3 catalyst (0.075 g), Co3O4 (0.041 g), Co3O4 and Al2O3 (0.2 g), CoOOH and Al2O3 (0.2 g), Co(NO3)2 · 6H2O and Al2O3 (0.2 g), Co(200) and Al2O3 (0.2 g), and Co(250) and Al2O3 (0.2 g) were flushed with high purity argon at 200 °C for 1 h, respectively. The CoOOH sample (0.0486 g), bulk decomposition products (0.03 g), and Al2O3supported decomposition products (0.2 g) were flushed with high purity argon at 100 °C for 0.5 h and then cooled to 25 °C. Co(NO3)2 · 6H2O (0.05 g) was not pretreated with high purity argon. 10% H2/Ar was switched on and the flow rate through the reactor was controlled at 30 mL/min. The temperature was raised at a rate of 6 °C/min from 25 to 750 °C or at a rate of 10 °C/min from 25 to 950 °C and was held at the final temperature for 10 min. The H2 consumption (TCD signal) was recorded automatically by a PC. XRD. X-ray diffraction (XRD) measurements were performed using the Cu KR radiation of a BDX3200 powder diffractometer device. The XRD patterns were recorded from 2θ ) 10 to 90°. It worked under 40 kV, 20 mA, and 4°/min. Co3O4 crystallite diameters were calculated using the Scherrer equation24 (eq 1) from the most intense Co3O4 peak at 2θ ) 36.9°.

d)

0.89λ 180o B cos θ π

(1)

where d is the mean crystallite diameter, λ is the X-ray wavelength (1.54056 Å), and B is the full width half-maximum (fwhm) of the Co3O4 diffraction peak. UV-Vis-NIR Spectroscopy. The diffuse reflectance spectroscopy experiments of UV-vis-NIR were performed on Hitachi U-4100 UV-vis spectrophotometer with the integration sphere diffuse reflectance attachment. The powder samples were

Figure 1. XRD patterns of the samples: (a) Co3O4, (b) CoOOH, and (c) Co(NO3)2 · 6H2O.

loaded in a transparent quartz cell and were measured in the region of 200-800 nm, and the diffuse reflectance UV-vis-NIR spectra for catalysts and catalyst precursors were obtained under ambient conditions with the spectrophotometer, with BaSO4 as a reference. TG-DTA. Simultaneous differential scanning calorimetry and thermogravimetric analyses were carried out in a flow of N2, with a NETZSCH STA 409 PC/PG thermal analyzer. Typically, 30-35 mg of catalyst was put into the sample holder inside an aluminum crucible, using an empty crucible in the reference holder. The sample was submitted to a linear rise of temperature from room temperature to 1200 °C, with a heating rate of 10 °C/min. Analysis of the Concentrations for NO and NO2. The concentrations of NO and NO2 during the decomposition process of Co(NO3)2 · 6H2O were analyzed by the flue gas analyzer VarioPlus manufactured by MRU GmnH in D-74172 NSUObereisesheim. The Co(NO3)2 · 6H2O sample (0.05 g) was placed in a quartz tubular reactor fitted with a thermocouple for continuous temperature measurement. Then 99.999% Ar or 10% H2/Ar was switched on, and the flow rate through the reactor was controlled at 100 mL/min. The temperature was raised at a rate of 6 °C/min from 15 to 450 °C and was held at the final temperature for 10 min. The variations of the concentrations of NO and NO2 were recorded automatically by a PC. 3. Results 3.1. Reference Samples. 3.1.1. Bulk Co3O4. XRD pattern of Co3O4 is shown in Figure 1(a). The diffraction peaks at 31.3°, 36.9°, 45.0°, 59.5°, and 65.4° are assigned to those of Co3O4.11 The particle size of Co3O4 calculated using the Scherrer equation is 64 nm. The result shows that Co(NO3)2 · 6H2O can decompose to Co3O4 after calcination at higher temperature. The H2-TPR profile of bulk Co3O4 is illustrated in Figure 2. There are two peaks for the reduction of Co3O4. The H2-TPR spectrum of Co3O4 in this study is different from that of Co3O4, in which there is only one peak, reported in the literature.11,25 These peaks can be used to identify bulk Co3O4 species on cobalt oxides. Two peaks (at 333 and 384 °C) in the TPR profile (Figure 2) of bulk Co3O4 indicate two reduction steps of Co3O4. Figure 3, panels b and c, shows XRD patterns of the samples obtained by the reduction of 0.3 g Co3O4 at 333 and 384 °C for 10 min

Formation and Reduction of Cobalt Oxides

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Figure 4. TG-DTA profiles of the CoOOH sample. Figure 2. TPR profile of the Co3O4 sample and the peak deconvolution results.

Figure 5. XRD patterns of CoOOH samples: (a) CoOOH calcinated at 270 °C for 10 min in pure Ar atmosphere and (b) CoOOH prereduced at 190 °C for 5 min in 10% H2/Ar atmosphere. Figure 3. XRD patterns of the bulk Co3O4 before and after reductions: (a) before reduction, (b) after reduction at 333 °C for 10 min in 10% H2/Ar atmosphere, and (c) after reduction at 384 °C for 10 min in 10% H2/Ar atmosphere.

at 10% H2/Ar atmosphere. CoO (JCPDS: no. 43-1004) and metallic Co(JCPDS: no. 05-0727) were detected in Figure 3, panels b and c, respectively. So, bulk Co3O4 is transformed into CoO at 333 °C and then transformed into Co at 384 °C. The reduction process can be described by the following reactions (eqs 2 and 3):

Co3O4+H2 f 3CoO + H2O

(2)

CoO + H2 f Co + H2O

(3)

According to eqs 2 and 3, the theoretical ratio of peak area of the first peak to the second peak in the TPR profiles is 1:3, which is in good agreement with the experimental data (2500: 7350). 3.1.2. CoOOH. XRD pattern of CoOOH is shown in Figure 1b. The diffraction peaks, which are consistent with those of CoOOH, are detected at 20°, 38.8°, 50.6°, 65.3°, and 68.1°.26 Figure 4 exhibits the TG-DTA profiles of CoOOH in N2 atmosphere. The DTA profile of CoOOH showed three peaks, located at 160, 242, and 270 °C. Figure 5a shows an XRD pattern of the sample obtained by the calcination of 0.3 g of

CoOOH at 270 °C for 10 min in a pure Ar atmosphere, which is attributed to the Co3O4. Thus, Co3O4 can be formed by the decomposition of CoOOH at lower temperature. So, Co3O4 is more thermodynamically stable than CoOOH. The process of heating CoOOH can be described by the following reaction (eq 4).

12CoOOH f 4Co3O4+O2+6H2O

(4)

Thus, according to the results of TG-DTA, the sample lost weight due to the desorption of H2O and O2, as the temperature to 300 °C. Figure 4 shows that the weight loss in the reaction is 13.2%, according to the TG profile. The experiment result is close to the calculated value (12.7%) based on eq 4. There are three peaks (at 190, 270, and 350 °C) in the TPR profile of bulk CoOOH (Figure 6). Figure 5b shows an XRD pattern of the samples obtained by the reduction of 0.3 g of CoOOH at 190 °C for 5 min in 10% H2/Ar atmosphere. The XRD rersults indicate that the CoOOH is reduced to Co3O4 at 190 °C, whose particle size calculated by using Scherrer equation is 12 nm. So the first peak of the H2-TPR profile can be assigned to transformation from CoOOH to Co3O4, the second peak is due to Co3O4 to CoO, and the third peak is attributed to the transformation from CoO to Co0. It can be assumed that the reduction of CoOOH to Co metal is likely to


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Figure 8. TG-DTA profiles of Co(NO3)2 · 6H2O and Co/(dry) samples. Figure 6. TPR profile of CoOOH and the peak deconvolution results.

Figure 7. Concentration of NO and NO2 as a function of decomposition temperature of Co(NO3)2 · 6H2O in different atmosphere: (a) in pure Ar atmosphere and (b) in 10% H2/Ar atmosphere.

proceed in three steps (eqs 5, 2, and 3). Based on eqs 5, 2, and 3, the ratio of the hydrogen consumption during these three reduction steps is 1:2:6, which is in agreement with the experimental results (950:1880:5650).

6CoOOH + H2 f 2Co3O4 + 4H2O

(5)

3.1.3. Co(NO3)6 · H2O. Figure 7a displays the concentration profile of NO and NO2, when Co(NO3)2 · 6H2O was decomposed from 30 to 450 °C, in Ar atmosphere. It has been shown that a large amount of NO2 and little NO were desorbed between 170 and 330 °C (peak at 250 °C). Thus, the results show that Co(NO3)2 · 6H2O can decompose to Co3O4, NO2, and O2 (eq 6): 13

3Co(NO3)2 · 6H2O f Co3O4 + 6NO2 + O2 + 18H2O (6) Figure 8a shows the TG-DTA profiles of Co(NO3)2 · 6H2O in N2 atmosphere. The DTA profile of Co(NO3)2 · 6H2O exhibited four peaks located at 155, 190, 251, and 277 °C. According to the TG profile, the factual weight loss percentage in fact is 72% (between 70 and 300 °C). The experiment result is very close to the calculated value (75%), based on the above reaction (eq 6).

Figure 9. TCD signal curves of 0.05 g of Co(NO3)2 · 6H2O at a heating rate of 6 °C/min: (a) in 10% H2/Ar atmosphere and (b) in pure Ar atmosphere.

The H2-TPR profile for 0.05 g of Co(NO3)2 · 6H2O is shown in Figure 9a. The reduction occurred in five steps with the reduction peaks at 224, 240, 256, 277, and 302 °C. The most intense hydrogen consumption peak has a maximum at 240 °C. The H2-TPR profile for Co(NO3)2 · 6H2O shows that the reduction of Co(NO3)2 · 6H2O is a very complicated procedure. Figure 9b recorded the TCD signal intensity of O2, NO, and NO2 released, when 0.05 g of Co(NO3)2 · 6H2O was calcined in Ar from ambient temperature to 450 °C, at a heating rate of 6 °C/ min. Comparing the intensity of each curve, the signal intensity of O2, NO, and NO2 has less impact on the signal intensity of H2 consumption of reduction of Co(NO3)2 · 6H2O. Co(NO3)2 can be reduced to CoO and NO, in the atmosphere of H2 (eq 7):27

Co(NO3)2+3H2 f CoO + 2NO + 3H2O

(7)

According to the result of gas analysis (Figure 7) the concentration of NO increased, while that of NO2 decreased with the increasing of H2 concentration. So Co(NO3)2 · 6H2O decomposed into abounds of NO2 and then NO2 was reduced to NO in the atmosphere of 10% H2/Ar (Figure 7b). Figure 10 shows the XRD pattern of the sample obtained by the reduction of 0.5 g of Co(NO3)2 · 6H2O at 240 °C for 5 min in 10% H2/Ar atmosphere, which can be attributed to Co3O4



Figure 10. XRD pattern of the bulk Co(NO3)2 · 6H2O after reduction at 240 °C for 10 min in 10% H2/Ar atmosphere.

and CoO, according to Co3O4 (JCPDS: no. 65-3103) and CoO (JCPDS: no. 43-1004). So, some Co(NO3)2 · 6H2O can be decomposed into Co3O4 and NO2, in the reduction process of Co(NO3)2. Thus, the following reactions can be proceeded, in addition to the reactions (eq 6 and 7), in the atmosphere of 10% H2/Ar.

O2+2H2 f 2H2O

(8)

NO2+H2 f NO + H2O

(9)

Therefore, the first peak (at 224 °C) in Figure 9 is ascribed to the reduction of O2 and NO2 released from decomposed Co(NO3)2 · 6H2O in the reduction process. The second peak (at 240 °C) can be attributed to the reduction of Co(NO3)2. The third peak (at 256 °C) is attributed to the reduction of Co3O4. The fourth peak (at 277 °C) and the fifth peak (at 300 °C) can be ascribed to the reduction of different CoO species which has different physicochemical properties in some extent, because CoO species can come from the reductions of Co3O4 and Co(NO3)2. 3.2. Decomposition of Bulk Co(NO3)2 · 6H2O and the Reduction of Corresponding Products. XRD patterns of Co(NO3)2 · 6H2O and the various decomposition products of bulk Co(NO3)2 · 6H2O are shown in Figures 1c and 11. The XRD patterns of Co(110) and Co(170) are the same to that of Co(NO3)2 · 4H2O (JCPDS: no. 18-0425). The XRD patterns of Co(200) and Co(250) do not exhibit the distinct diffraction peaks of CoO, Co3O4, or CoOOH. The diffraction peaks of Co3O4, whose intensities are very small, are in the pattern of Co(250). The XRD patterns of Co(300), Co(350), Co(400), and Co(450) all show the distinct diffraction peaks of Co3O4. The particle sizes of Co3O4 calculated using Scherrer equation are listed in Figure 11. It can be seen that the particle sizes of Co3O4 do not have any noticeable increase with the increasing of calcination temperature. The DTA result (Figure 8) indicates that the decomposition of Co(NO3)2 · 6H2O can be mainly divided into three stages: from 70 to 200 °C, from 200 to 300 °C, and from 300 to 450 °C. In the temperature range from 70 to 200 °C, Co(NO3)2 · 6H2O melted and dehydrated to form Co(NO3)2 · 4H2O, which is seen clearly from the XRD patterns of Co(110) and Co(170). According to the XRD results of Co(200), most of Co(NO3)2 · xH2O began to dehydrate to anhydrous cobalt nitrate and decomposed. No distinct crystal phases of CoO,

Figure 11. XRD patterns of the samples obtained by the decomposition of bulk Co(NO3)2 · 6H2O at different temperatures.

CoOOH(Co2O3 · H2O), or Co3O4 were detected at Co(250). There are two possible reasons to cause this phenomenon. One is that no crystal phase of CoO, CoOOH(Co2O3 · H2O) or Co3O4 formed. The other is that some amorphous phases of CoO, CoOOH(Co2O3 · H2O) and Co3O4 may be formed. In the temperature range from 250 to 300 °C, residual Co(NO3)2 continued to decompose. A large amount of crystal Co3O4 could be formed at 300 °C, according to the XRD pattern of Co(300). In the temperature range from 300 to 450 °C, the particle size of Co3O4 has no obvious change, which was supported by the XRD pattern of Co(300), Co(350), Co(400), and Co(450). The H2-TPR profiles of bulk cobalt oxides and the peak deconvolution results are shown in Figure 12. The H2-TPR profiles of Co(110), Co(170), Co(200), and Co(250) all exhibit five main reduction peaks. The H2-TPR curves of Co(110), Co(170), Co(200), and Co(250) are similar to that of Co(NO3)2 · 6H2O. So, the attribution of each peak of every curve is the same as that of Co(NO3)2 · 6H2O. The intensity of the first and second peak decreases with the calcination temperature, according to the four H2-TPR curves. Thus, Co(NO3)2 · xH2O decomposes gradually with the increasing of calcination temperature. The area of the first peak is the approximately the same as that of the second peak in each profile, according to the peak deconvolution results of H2-TPR profiles of Co(110), Co(170), Co(200), and Co(250). So, the H2 consumption of Co(NO3)2 · xH2O is the approximately same with that of O2 and NO2 released from decomposed Co(NO3)2 · 6H2O in the reduction process. The TPR profiles of Co(300) and Co(350) all exhibit five reduction peaks. The first peaks in the profiles of Co(300) and Co(350) shift to lower temperatures, at 190 and 187 °C. According to the H2-TPR profile of CoOOH, the two first peaks can be attributed to the reduction of CoOOH. However, the XRD patterns of Co(300) and Co(350) did not exhibit the distinct diffraction peaks of CoOOH. Thus, some amounts of amorphous CoOOH were formed at about 300 °C in the calcination process. The second peak at 240 °C in the H2-TPR profile of Co(300) can be attributed to the reduction of Co(NO3)2. The third peak


Ji et al.

Figure 13. UV-vis-NIR spectra of Co(NO3)2 · 6H2O and Al2O3supported cobalt oxides (Co/Al(wet), Co/Al(dry), Co/Al(170), and Co/ Al(200)).

Figure 12. TPR profiles of the cobalt samples obtained by the decomposition of bulk Co(NO3)2 · 6H2O at different temperatures.

can be ascribed to the reduction of Co3O4. The fourth and the fifth peaks can be attributed to the reduction of CoO. In the TPR profile of Co(350), the first peak (max at 187 °C) attributed to the reduction of CoOOH almost disappears. Thus, CoOOH can be decomposed at about 350 °C. The second and the third peak in the TPR profile of Co(350) can be attributed to the reduction of Co3O4, because Co3O4 species can come from the decompositions of CoOOH and Co(NO3)2. The fourth and the fifth peaks can be attributed to the reduction of CoO. The H2-TPR profiles of Co(400) and Co(450) exhibit four reduction peaks. In each H2-TPR profile, the first and the second peak can be assigned to the reduction of Co3O4, and the third and the fourth peak can be attributed to the reduction of CoO, according to the corresponding XRD patterns. In addition, almost all of the Co(NO3)2 · 6H2O decomposed below 300 °C according to the DTA profile of Co(NO3)2 · 6H2O in Figure 8. A big fraction of Co(NO3)2 began to decompose at about 200 °C. So there are a fraction of Co(NO3)2 in Co(250), and there is little Co(NO3)2 in Co(300). On the other hand, there is a big fraction of Co3O4 in Co(300) and a small fraction of Co3O4 of in Co(250) according to the XRD and H2-TPR results. Thus, the XRD pattern and H2-TPR profile of Co(300) is obviously different from that of Co(250). Therefore, the temperatures of 200 and 300 °C are very important borderlines in the calcination procedure of Co(NO3)2 · 6H2O. 3.3. Decomposition of Al2O3-Supported Co(NO3)2 · 6H2O and the Reduction of Corresponding Products. The UV-visNIR spectrum of Co(NO3)2 · 6H2O is shown in Figure 13a. The spectrum exhibits broad bands at 500 and 1200 nm, which are in agreement with the previous reports,28-30 and are attributed to the octahedral high-spin Co2+ complexes. The broadband at 500 nm can be assigned to the 4T1g(F) f 4T1g(P) d-d transition,28,29 and the broadband at 1200 nm can be assigned to the 4T1g f 4T2g d-d transition.30 A intense band of absorption at 296 nm is due to the NO3- n f π charge transfer

band.29 The absorption band at 1420 nm can be attributed to the vibration peak of H2O.31 UV-vis-NIR spectra of Al2O3-supported cobalt oxides of Co/Al(wet), Co/Al(dry), Co/Al(170), and Co/Al(200) are shown in Figure 13, panels b and c. The spectrum for Co/Al(wet) is similar to that of Co(NO3)2 · 6H2O. The bands centered at about 500 and 1200 nm, which were assigned to the octahedral highspin Co2+ complexes,28-30 still exist, but their intensities decrease with the increasing of calcination temperature. This suggests that cobalt predominantly exist as an octahedral coordination state and molecules of crystal water could be lost between 100 and 200 °C. In addition, the intensity of the absorption band at 296 nm, which is due to the NO3- n f π charge transfer band,29 decreases after drying and calcination. Thus, a fraction of cobalt nitrate in the catalyst decomposed. Meanwhile, a weak absorption band centered at 730 nm is observed in the spectra of Co/ Al(170) and Co/Al(200), which might be due to the presence of a low concentration Co3O4 phase. The distinct absorbance band centered at 1420 nm in the spectrum of Co/Al(wet), which can be attributed to the absorption band of H2O,31 disappears with the increasing of calcination temperature. XRD patterns of the various decomposition products of Al2O3-supported Co(NO3)2 · 6H2O are shown in Figure 14. The diffraction peaks at 31.3°, 36.9°, 45.0°, 59.5°, and 65.4° are assigned to Co3O4.11 The diffraction peaks at 45.66° and 66.6° are due to the Al2O3 support.24 The Co3O4 crystals began to appear in the sample Co/Al(170), when the calcination temperature was increased to 170 °C, according to the results of XRD patterns. The particle sizes of Co3O4 of Al2O3-supported cobalt oxides calculated using the Scherrer equation are listed in Figure 14. It can be seen that the particle sizes of Co3O4 do not have any noticeable increase with the increasing of calcination temperature and time. The TG-DTA profiles of Co/Al(dry) are show in Figure 8. In the DTA profile, there are two peaks at 206 and 285 °C. The large peak at 206 °C can be assigned to the dehydration and decomposition of supported Co(NO3)2 · xH2O, based on the UV-vis-NIR results of Co/Al(wet), Co/Al(dry), Co/Al(170), and Co/Al(200). The decomposition of Al2O3-supported Co(NO3)2 · xH2O lasted from 75 to 450 °C, according to the DTA profile. Thus, the small peak at 285 °C can be assigned to the decompostion of a small amount of residual Co(NO3)2. The temperature range of weight loss (from 75 to 450 °C) of



Figure 14. XRD patterns of the cobalt samples obtained by the decomposition of Al2O3-supported Co(NO3)2 · 6H2O at different temperatures.

CoAl(dry) is greater than that of weight loss (from 70 to 300 °C) of bulk Co(NO3)2 · 6H2O. These results indicate that the decomposition process of Al2O3-supported Co(NO3)2 · 6H2O lasts longer than that of bulk Co(NO3)2 · 6H2O. Therefore, the decomposition process of Al2O3-suppported Co(NO3)2 · 6H2O is more complicated and lasts longer than that of bulk Co(NO3)2 · 6H2O, due to the presence of Al2O3 support. The H2-TPR profiles of Al2O3-supported cobalt oxides obtained by the decomposition of Al2O3-supported Co(NO3)2 · 6H2O at different temperatures are shown in Figure 15. The H2-TPR profiles of Al2O3-supported cobalt oxides all exhibit five main reduction peaks. The peak deconvolution analysis results are also shown in the Figure 15. The H2-TPR curve of Co/Al(dry) has an apparent peak (at 215 °C), a peak (at 268 °C), and a very broad peak which can be divided into three peaks (at 520, 690, and 850 °C). Figure 16 shows XRD pattern of the sample obtained by the reduction of 0.3 g of Co/Al(dry) at 215 °C for 5 min, in 10% H2/Ar atmosphere. The XRD pattern exhibits the diffraction peaks of Co3O4, CoO, and Al2O3. So, the first peak can be ascribed to the reduction of Co(NO3)2 · xH2O and the reduction of O2 and NO2 released from the decomposition of Co(NO3)2 · xH2O during the reduction process. According to the peaking deconvolution results of H2-TPR profiles of Co(110), Co(170), Co(200), and Co(250) in Figure 12, the H2 consumption amount of Co(NO3)2 · xH2O is approximately the same as that of O2 and NO2 released from decomposed Co(NO3)2 · 6H2O in the reduction process. Thus, about a half of the area of the first peak of the H2-TPR profile of Co/Al(dry) can be attributed to the reduction from Co(NO3)2 to CoO. The second peak can be attributed to the reduction of Co3O4. The third, fourth, and fifth peaks can be ascribed to the reduction of CoO-Al2O3 composite oxides, which interacted with the support Al2O3 in gradually increased strength. Other TPR curves of Al2O3-supported cobalt oxides are similar to that of Co/Al(dry). So the distribution of each peak of every curve is the same as that of Co/Al(dry). The first peak in the profile of Co/Al(250) shifts from 210 to 200 °C. As described above, the reduction temperature of

Figure 15. H2-TPR curves of the cobalt samples obtained by the decomposition of Al2O3-supported Co(NO3)2 · 6H2O at different temperatures.

CoOOH is lower than 200 °C. It can be concluded that a small amount of CoOOH was formed at about 250 °C in the calcination process. Thus, the first peak in the profile of Co/ Al(250) can be attributed to the reduction of CoOOH, besides the reduction of Co(NO3)2 · 6H2O and the reduction of O2 and NO2. The first peak in the profile of Co/Al(300) shifts from 200 to 217 °C. So, it is demonstrated that CoOOH almost decomposed into Co3O4 at about 300 °C.


Figure 16. XRD pattern of Co/Al(dry) reduced at 215 °C for 5 min, in 10%H2/Ar atmosphere.

The first peak in the profiles of Co/Al(300), Co/Al(350), Co/ Al(400), and Co/Al(450), whose intensity decreases with the increasing of calcination temperature, can be attributed to the decomposition of residual Co(NO3)2, according to the TG-DTA profiles of Co/Al(dry) in Figure 8. In addition, a big fraction of Co(NO3)2 · xH2O decomposed at about 200 °C, according to the DTA profile and XRD patterns of the various decomposition products of Al2O3-supported Co(NO3)2 · xH2O. · But the decomposition of Al2O3-supported Co(NO3)2 · xH2O lasted from 75 to 450 °C, according to the DTA profile, although no distinct crystal phases of Co(NO3)2 were detected in the XRD patterns of Co/Al(250)-Co/Al(450). There are two possible reasons that cause no detection of Co(NO3)2 by the XRD technique. One is that the amount of residual Co(NO3)2 is too small to be detected by XRD. The other is that the small amount of residual Co(NO3)2 is amorphous. So the decomposition of Al2O3-suppported Co(NO3)2 · 6H2O is more complicated than that of bulk Co(NO3)2 · 6H2O due to the presence of Al2O3 support. Thus the temperature range from 200 to 300 °C is very important in the calcination process of Al2O3-suppported Co(NO3)2 · 6H2O. XRD patterns of 5, 15, 30, and 40% Co/Al2O3 are shown in Figure 17. The diffraction peaks at 31.3°, 36.9°, 45.0°, 59.5°, and 65.4° are assigned to those of Co3O4.11 The diffraction peaks at 45.66° and 66.6° are due to the presence of Al2O3.19 The location of the diffraction peaks (Figure 17) confirms that Co3O4 was the only crystalline phase of Co. The particle sizes of Co3O4 of these samples with different cobalt contents calculated using Scherrer equation are 13, 19, 31, and 34 nm, respectively. Figure 18 shows the H2-TPR profiles of the 5, 15, 30, and 40% Co/Al2O3 catalysts. All the reduction temperatures of the first peaks of these catalysts are higher than that of bulk Co3O4 with the larger particle size (64 nm). So, it can be concluded that the strong interaction between Co3O4 and Al2O3 support impeded the reduction of Al2O3-supported Co3O4. All of the reduction temperatures of the first peak of these catalysts shift to the lower temperatures with the increasing of cobalt loading. Thus, it is implied that the interaction between Co3O4 and support Al2O3 decreased with the increasing of cobalt loading. There are two distinct reduction peaks located at about 400 °C, and between 500 and 900 °C (max. at about 600 °C) for the 5, 15, 30, and 40% Co/Al2O3 catalysts. The first peak shifts to low temperature, when the cobalt loading increases. A

Ji et al.

Figure 17. XRD patterns of 5, 15, 30, and 40% Co/Al2O3.

Figure 18. TPR profiles of the 5, 15, 30, and 40% Co/Al2O3 catalysts.

shoulder peak, whose intensity increases with the increasing of cobalt loading, appears in the high temperature range of the first peak of the TPR profiles of 30 and 40% Co/Al2O3 catalysts. Figure 19, panels a1 and a2, shows the XRD patterns of the samples obtained by the reduction of 0.3 g 15%Co/Al2O3 catalyst at 390 and 572 °C for 5 min in 10% H2/Ar atmosphere, whose diffraction peaks agree with those of CoO and Co (JCPDS: nos. 48-1719 and 15-0806), respectively. So, the results further show that the reduction process of Al2O3-supported Co3O4 is first transformed into CoO-Al2O3 and then into Co. Figure 19b shows the XRD pattern of the sample obtained by the reduction of 0.3 g 40% Co/Al2O3 at 362 °C for 5 min at 10% H2/Ar atmosphere. All of the diffraction peaks agree with those of CoO (JCPDS: no. 48-1719). So, the shoulder peak can be attributed to the reduction of CoO, which is easily reduced to Co. Thus, the results further prove that the different shoulders of the very broad second peak are likely due to varying degrees of the interaction between CoO and the support Al2O3. Figure 20 show the H2-TPR profiles of mixture samples of Co3O4 and Al2O3, CoOOH and Al2O3, Co(NO3)2 · 6H2O and Al2O3, Co(200) and Al2O3, and Co(250) and Al2O3. The H2-TPR profiles of the Co3O4 and Al2O3 and CoOOH and Al2O3 mixtures are similar to those of bulk Co3O4 and CoOOH samples, respectively. No profile exhibits the reduction


J. Phys. Chem. C, Vol. 113, No. 17, 2009 7195 comes from the decomposition of Co(NO3)2, can be described by the ratio R (eq 10). The ratio R of Co3O4 is 3.

1 AreaCoO - AreaCo(NO3)2 3 R) AreaCo3O4

Figure 19. XRD patterns of Al2O3-supported Co3O4: (a1) 15% Co/ Al2O3 catalyst reduced at 390 °C for 10 min in 10% H2/Ar atmosphere; (a2) 15% Co/Al2O3 catalyst reduced at 572 °C for 20 min in 10% H2/ Ar atmosphere; (b1) 40% Al2O3 catalyst reduced at 362 °C for 5 min in 10% H2/Ar atmosphere.

peaks between 450 and 750 °C, which can be attributed to the reduction of CoO-Al2O3 composite oxide. So the CoO-Al2O3 composite oxide does not generate from the mixture of Co3O4 and Al2O3 or the mixture of CoOOH and Al2O3. The H2-TPR profiles of Co(NO3)2 · 6H2O and Al2O3 and Co(200) and Al2O3 are similar to that of Co/Al(dry) and Co/ Al(200), respectively. All profiles exhibit the reduction peak attributed to CoO-Al2O3 composite oxide between 450 and 750 °C. The intensity decreases with a decrease in the content of Co(NO3)2 · 6H2O. The H2-TPR profile of Co(250) and Al2O3 is similar to the H2-TPR profile of Co(250), which contains a small amount of Co(NO3)2. So, the CoO-Al2O3 composite oxide can be generated from Co(NO3)2 interacted with the Al2O3 support during the calcination process. 4. Disscusion 4.1. Formation Mechanisms of Bulk and Al2O3-Supported Cobalt Oxides. The reduction process of bulk Co(NO3)2 · 6H2O and Al2O3-supported Co(NO3)2 · 6H2O, includes three parts: the reduction of Co(NO3)2, the reduction of Co3O4, and the reduction of CoO. So, in the reduction process of each sample, CoO has three sources: the decomposition of Co(NO3)2 in the calcination process, the reduction of Co(NO3)2, and the reduction of Co3O4. In the H2-TPR profile of each sample, the H2 consumption area of the reduction from Co(NO3)2 to CoO (named AreaCo(NO3)2), the area of the reduction from Co3O4 to CoO (named AreaCo3O4) and the H2 consumption area of the reduction from CoO to Co (named AreaCoO), can be gotten from the results of peak deconvolution of H2-TPR profiles in Figures 12 and 15. According to the eqs 2-9, the following relationships can be obtained: Co(NO3)2 + 3H2 f CoO + 2NO + 3H2O; Co3O4 + H2 f 3CoO + H2O; CoO + H2 f Co + H2O. The molar percentage of Co in the Co(NO3)2 · 6H2O, Co3O4, and CoOOH with the same mass decreases in this order: Co3O4 > CoOOH > Co(NO3)2. The reduction behaviors of bulk decompostion products with the same mass were examined by using the H2TPR technique. The percentage of Co(NO3)2, Co3O4, and CoOOH is different than each other from the samples. So the molar percentage of Co in the samples is different than each other from the samples. For the H2-TPR profile of bulk Co3O4, AreaCoO is three times as much as AreaCo3O4. The variety of the quantity of CoO, which

(10)

The ratio R values of the decomposition products of bulk Co(NO3)2 · 6H2O and Al2O3-supported Co(NO3)2 · 6H2O are shown in Tables 1;0?> and 2. Based on the data listed in Table 1, the analyzed value of ratio R of Co(110), Co(170), Co(200), and Co(250) increases with the increasing of calcination temperature. All of the values of ratio R are greater than 3. It can be concluded that some amounts of CoO were formed in the calciantion process from 110 to 250 °C. As described above, no crystal CoO was detected by XRD. So, Co(NO3)2 decomposed into amorphous CoO, between 110 and 250 °C. According to the XRD pattern of Co(250), there is a small fraction of spinel Co3O4 in the sample Co(250). That is to say that some amorphous CoO was oxidized to Co3O4, when calcination temperature increased to 250 °C. According to the H2-TPR profile of Co(300), there were some amounts of CoOOH in the sample Co(300). That indicates that some amounts of CoO were oxidized to CoOOH at 300 °C. The ratio of quantity of H2 consumption for CoOOH, Co3O4, and CoO is 475:1000:(950 + 2800) ) 1:2.1:7.89. Thus, a big fraction of amorphous CoO was oxidized to amorphous CoOOH and Co3O4, when the calcination temperature increased to 300 °C. The first peak (at 185 °C) of the H2-TPR profile of Co(350) can be attributed to the reduction of CoOOH, too. The decrease in the intensity of the first peak shows that CoOOH decomposed at 350 °C. The ratio R values of Co(350), Co(400), and Co(450) are similar to that of bulk Co3O4 (64 nm). This implies that Co(NO3)2 entirely transformed into Co3O4, when the calcination temperature is higher than 350 °C. According to the data listed in Table 2, the analyzed values of ratio R are all smaller than 3. That is to say, a fraction of CoO interacted with the Al2O3 support is too strong to be reduced above 900 °C in the 10% H2/Ar. For Al2O3-supported Co(NO)2 · xH2O system of samples, the ratio R values of Co/Al(dry) and Co/Al(170) are both close to 2. Based on the results above, it can be concluded that only a small amount of Co(NO3)2 decomposed in Co/Al(dry) and Co/ Al(170). The ratio R value gradually decreased with the increasing of calcination temperature form 170 to 350 °C, whereas it gradually increased with the increasing of calcination temperature from 350 to 450 °C. It can be concluded that CoO can disperse on the surface of Al2O3 support at first and then form a multilayer CoO or bulk CoO, in the decomposition process of Co(NO)2 · 6H2O. The inner layer CoO is more difficult to be reduced than that dispersed on the outer layer, because the interaction between the inner layer CoO and the support is stronger than that between the outer layer CoO and the support. In addition, both of the results that the particle sizes increased and the reduction peak temperature of CoO shifts to lower temperature direction demonstrated that the interaction between CoO and the support decreases with the increasing of the number of the layer, according to the XRD patterns and H2-TPR profiles of 5, 15, 30, and 40% Co/Al2O3 catalysts. It can be further proved that the inner layer CoO is more difficult to be reduced to Co than that on the outer layer. The stronger the interaction


Ji et al.

Figure 20. TPR profiles of the mixture samples: Co3O4 and Al2O3, CoOOH and Al2O3, Co(NO3)2 · 6H2O and Al2O3, Co(200) and Al2O3 and Co(250) and Al2O3.

Figure 21. TPR profiles of CoOOH reduced at a heating rate of 3, 6, and 12 °C/min.

between CoO and the Al2O3 support, the higher the initial temperature of reduction of Al2O3-supported CoO. The intensities of the second peak assigned to the reduction of Co3O4 in the profiles of Co/Al(250), Co/Al(300), Co/Al(350), Co/Al(400), and Co/Al(450), increase with the increasing of calcination temperature. The H2-TPR profile of 15% Co/Al (450 °C 10 h) exhibits two main reduction peaks, which are attributed to the transformations from Co3O4 to CoO-Al2O3 and then CoO-Al2O3 to Co. Based on the H2-TPR profiles, the reduction peaks of Co3O4 shift to higher temperatures as the calcination temperature and time increased. So, the interaction between Co3O4 and the Al2O3 support increased with the increasing of calcination temperature and time. Thus, the stronger the interaction between Co3O4 and the Al2O3 support, the higher the initial temperature of reduction of Al2O3-supported Co3O4. The decomposition procedures of bulk and Al2O3-supported Co(NO3)2 · 6H2O can be simply described. There is no need to

take the formation of CoOOH into account, because CoOOH is easily decomposed into Co3O4. Bulk Co(NO3)2 · 6H2O first melts, dehydrates, and then decomposes into CoO and Co3O4 and was oxidized to spinel Co3O4 finally. The decomposition procedure of Al2O3-supported Co(NO3)2 · 6H2O is similar to that of bulk Co(NO3)2 · 6H2O. However, the decomposition of Al2O3suppported Co(NO3)2 · 6H2O is more complicated and lasts longer than that of bulk Co(NO3)2 · 6H2O, due to the presence of Al2O3 support. Co(NO3)2 · 6H2O first interacts with the surface of Al2O3 support and dehydrates during calcination. Second, Co(NO3)2 decomposes into CoO and Co3O4 mixed oxides. CoO disperses on the surface of the Al2O3 support, and then it forms multilayer of CoO with the decomposition of Co(NO3)2. At last, CoO-Al2O3, which interacted with support in various degrees, was oxidized to Al2O3-supported Co3O4. The interaction between Co3O4 and the Al2O3 support increases with the increasing of calcination temperature and time.



TABLE 1: Analysis Results of Peak Deconvolution for the H2-TPR Curves of the Cobalt Samples Obtained by the Decomposition of Bulk Co(NO3)2 · 6H2O at Different Temperatures sample

AreaCo(NO3)2

AreaCo3O4

AreaCoO

ratio R

Co(110) Co(170) Co(200) Co(250) Co(300) Co(350) Co(400) Co(450)

780 766 730 724 200

540 457 314 450 1000 1804 1820 1900

2040 2322 1895 1933 3750 5105 5436 5308

3.3 4.5 5.2 3.8 3.6 2.8 62.9 2.8

TABLE 2: Analysis Results of Peak Deconvolution for the H2-TPR Curves of the Cobalt Samples Obtained by the Decomposition of Al2O3-Supported Co(NO3)2 · xH2O at Different Temperatures sample

AreaCo(NO3)2

AreaCo3O4

AreaCoO

ratio R

Co/Al(110) Co/Al(170) Co/Al(200) Co/Al(250) Co/Al(300) Co/Al(350) Co/Al(400) Co/Al(450)

1535 1290 1200 1380* 1039 675 295 100

1265 1210 900 1470 2054 2130 1900 1450

3147 3163 1840 2970 3400 3367 3511 3972

2.1 2.2 1.6 1.7 1.5 1.5 1.8 2.7

* The H2 consumption of CoOOH is assumed to be zero.

4.2. Reduction Kinetics of Bulk and Al2O3-Supported Cobalt Oxides. If the reduction of cobalt oxides is considered as a first-order reaction, the following equations will be gained.13,32

-

dR RA ) exp(-E/RT) dT β Eβ ) Ae-E/RTp RTp2

( )

ln

β AR E1 ) ln E R Tp Tp2

(11) (12) (13)

where R is the mole fraction of cobalt oxides, β is the heating rate, E is the activation energy, Tp is the temperature of the reduction peak of cobalt oxides, and R is universal gas constant. There is a linear relationship between ln(β/Tp2) and 1/Tp. By measuring the slope and intercept of the straight line, E can be obtained. The influence of the heating rate on the reduction performance of cobalt oxide is presented in Figure 21. The CoOOH sample that was calcined at 100 °C for 2 h (in Ar) was reduced at varied heating rates of 3, 6, and 12 °C/min. In each profile, the temperature of the reduction peak of cobalt oxides, CoOOH, Co3O4, and CoO, which are Tp(CoOOH), Tp(Co3O4), and Tp(CoO), can be obtained from Figure 21. From Figure 22, the E of reduction of cobalt oxides can be acquired at the atmosphere of 10% H2/Ar. They are 93.1, 80, and 53 kJ/mol for CoOOH, Co3O4, and CoO, respectively. Comparing the H2-TPR profiles of CoOOH and Co3O4 (64 nm), it is clear that Co3O4 with big particle size, for example, 64 nm, is more difficult to be reduced to CoO. When a bigger fraction of Co3O4 (64 nm) reduced to CoO at 333 °C, CoO can be quickly converted to Co metal at about 333 °C, because the E of CoO is smaller than that of Co3O4. So the TPR profile of

Figure 22. Dynamics profiles of CoOOH.

Co3O4 exhibits the incorporated trend of two peaks. Co3O4 with smaller particle size, for example, 12 nm, can be reduced at a lower temperature. The majority of Co3O4 (12 nm) had been reduced before CoO began to reduce. So the TPR profile of Co3O4 (12 nm) exhibits the separated two reduction peaks. The influence of the heating rate on the reduction performance of 15% Co/Al2O3 is presented in Figure 23. 15% Co/Al2O3 samples were reduced by using heating rates of 5, 10, and 20 °C/min. The temperature of the reduction peak of each cobalt oxide, Co3O4-Al2O3 and CoO-Al2O3, which are Tp(Co3O4/Al) and Tp(CoO/Al), can be obtained from Figure 23. The activation energies of the reduction reaction from Co3O4-Al2O3 to CoO-Al2O3 and the reaction from CoO-Al2O3 to Co0 calculated from the slop in the plot of ln(β/Tp2) vs 1/Tp (Figure 24) were 90 and 95 kJ/mol, respectively. It is clear that the activation energies of the reduction reaction from Al2O3supported Co3O4 to CoO-Al2O3 and from CoO-Al2O3 to Co are higher than those of the transformations from bulk Co3O4 to CoO and from bulk CoO to Co, respectively. So, it is clear that the support Al2O3 changed the reduction property of Co3O4 and CoO, because of the interaction between cobalt oxides species and the support Al2O3. The strong interaction between cobalt oxides and Al2O3-support can impede the reduction of Al2O3supported cobalt oxides. The activation energy of the reduction reaction from Al2O3-supported Co3O4 to CoO-Al2O3 is smaller than that of the reaction from CoO-Al2O3 to Co0. Thus, the H2TPR profile of Al2O3-supported Co3O4 exhibits two apparent peaks. According to the H2-TPR profiles of 5, 15, 30, and 40% Co/ Al2O3 catalysts, the starting temperature of the reduction of Al2O3-supported Co3O4 shifts to a lower temperature direction with the increase in the Co loading. It can be concluded that the interaction between Co3O4 and support Al2O3 decreased with the increase in the cobalt loading. When Co loading is the same, the starting temperature of reduction of Al2O3-supported Co3O4 shifts to a higher temperature with the increasing of calcination temperature and time, according to the H2-TPR profiles of Co/Al(250)-Co/Al(450) and 15% Co/Al. Thus, it is implied that the interaction between Co3O4 and support Al2O3 increased with the increase in the calcination temperature and time. Therefore, the calcination temperature can affect the reduction property of Al2O3-supported Co3O4. The activation energy of Co3O4 reduction on aluminasupported cobalt catalysts increases with calcination temperature.


Ji et al.

Figure 23. TPR curves of 15% Co/Al2O3 reduced at a heating rate of 5, 10, and 20 °C/min.

Figure 24. Dynamics profiles of 15% Co/Al2O3.

5. Conclusions (1) Bulk cobalt nitrate hexahydrate Co(NO3)2 · 6H2O decomposed in the following four steps: (i) melting and dehydration between 70 and 170 °C to form cobalt nitrate monohydrate; (ii) dehydration and decomposition of the majority of monohydrate into amorphous CoO and Co3O4 from 170 to 300 °C, and a big fraction of CoO is further oxidized to spinel Co3O4 at about 300 °C; (iii) some amorphous CoOOH is formed at about 300 °C and decomposed to Co3O4 from 300 to 350 °C; (iv) a large amount of Co3O4 is formed from 300 to 450 °C. (2) The decomposition of Al2O3-suppported Co(NO3)2 · 6H2O is more complicated and lasts longer than that of bulk Co(NO3)2 · 6H2O. Al2O3-suppported Co(NO3)2 · 6H2O decomposes in the following five steps: (i) melting and dehydration in the process of drying at 110 °C; (ii) dehydration and decomposition into CoO-Al2O3 mixed oxide from 170 to 400 °C, and CoO interacted with Al2O3

support in various degrees. CoO disperses on the surface of Al2O3 support at first, then form multilayer with the decomposition of Co(NO3)2. A large amount of CoO dispersed on the inner layer, which interacts with the support so strongly that can not be reduced above 900 °C in 10% H2/Ar, is formed in the temperature range from 200 to 400 °C. The inner layer CoO is more difficult to be reduced to Co than CoO dispersed on the outer layer; (iii) some CoO-Al2O3 is oxidized to spinel Co3O4 at 200 °C; (iv) some amounts of amorphous CoOOH are formed at about 250 °C and it decomposed at about 300 °C; (v) a large amount of Al2O3-supported Co3O4 is formed from 250 to 450 °C. (3) The bigger the particle size is, the higher the initial temperature of reduction of bulk Co3O4 is. Bulk Co3O4 is transformed into CoO and then transformed into Co. The activation energies for the two transformations from Co3O4 to CoO and from CoO to Co0, are 80 and 53 kJ/ mol, respectively. The two separate reduction peaks in TPR curves are easily combined to one peak, due to the big particle size and high H2 concentration. (4) The stronger the interaction between Co3O4 and the Al2O3 support is, the higher the initial temperature of reduction of Al2O3-supported Co3O4 is. The interaction between Co3O4 and the Al2O3 support decreases with the increase in the cobalt loading, and increases with the increase in the calcination temperature and time. The reduction process of Al2O3-supported Co3O4 is first transformed into CoO-Al2O3 and then into Co. The interaction between Co3O4 or CoO and support Al2O3 can affect the reduction property and activation energies of Co3O4 and CoO. The activation energies of Al2O3-supported Co3O4 for the two transformations from Co3O4 to CoO-Al2O3 and from CoO-Al2O3 and Co are 90 and 95 kJ/mol, respectively. The peak attributed to the transformation from CoOAl2O3 to Co in the TPR profile of Al2O3-supported Co3O4 exhibits a very broad peak because of various interactions between CoO and support Al2O3. A fraction of CoO could not be reduced even at 900 °C in an atmosphere of 10%

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JP8107057

Comparative Study on the Formation and Reduction of Bulk and

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