Co3O4 Nanoparticles as Catalysts for Hydrogen Evolution from

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J. Phys. Chem. C 2010, 114, 16456–16462

Cu/Co3O4 Nanoparticles as Catalysts for Hydrogen Evolution from Ammonia Borane by Hydrolysis Yusuke Yamada,*,† Kentaro Yano,† Qiang Xu,‡ and Shunichi Fukuzumi*,†,§ Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan, National Institute of AdVanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan, and Department of Bioinspired Science, Ewha Womans UniVersity, Seoul 120-750, Korea ReceiVed: May 11, 2010; ReVised Manuscript ReceiVed: July 17, 2010

A series of nanosized Co3O4 particles in which Cu was loaded on the surface were examined as robust catalysts for hydrogen evolution by ammonia borane hydrolysis. Their catalytic activity was dependent on the shape and size of nanosized Co3O4. The shape of nanosized Co3O4 was cube, hexagonal sheet, or uncontrolled. Among these, the Co3O4 in the shape of hexagonal sheet showed the highest catalytic activity. To investigate the size dependence of the catalytic reactivity, Co3O4 particles with the controlled size of about 4, 20, or 500 nm were examined, and it was found that the one in the size about 4 nm showed the highest activity although the size dependence was not remarkable compared with the shape dependence. The robustness of the catalyst was assured by no significant activity loss after 10 times repetitive reactions. The structural characterizations of Cu/Co3O4 composite in the fresh and used conditions were performed by X-ray photoelectron spectroscopy, Auger spectroscopy, and powder X-ray diffraction spectroscopy. The X-ray diffraction patterns assigned to Co3O4 were observed for both fresh and used catalysts, indicating that the Co3O4 form was maintained at the core part of each particle after the reaction. On the other hand, the XPS peaks or Auger peak for Cu 2p, Cu L3M45M45, Co 2p, and O 1s of the used catalyst suggested that its surface was reduced or hydrolyzed to Cu2O, Co metals, and Co(OH)2 during the reaction. The observed Cu2O and Co metals are regarded as active species for ammonia borane hydrolysis. Introduction Hydrogen is regarded as a clean fuel of the next generation because its energy content per mass is higher than petroleum. Thus, hydrogen has been used for running a proton exchange membrane fuel cell (PEMFC) to increase the energy conversion efficiency over internal combustion engines and to decrease the formation of harmful chemicals or materials such as particulate matter.1-3 The drawback of hydrogen is obviously its poor energy content per volume.4,5 For transportation application, an energy carrier should have a high energy content in as small a volume as possible.4,5 Currently chemical hydrides attract a great deal of attention due to their high gravimetric and volumetric storage capacity. Among the chemical hydrides, the ammonia borane complex, NH3-BH3 (AB), is a quite promising candidate, because AB contains 19.6 wt % of hydrogen and, most importantly, this is stable in an aqueous solution at neutral pH as well as in a solid state.6 A large number of catalysts have been reported to evolve hydrogen from ammonia borane by both thermal decomposition7,8 and solvolysis or hydrolysis reactions.3,6,9-37 To accelerate the hydrolysis reaction, the N-B bond cleavage should be facilitated by a catalyst because the BH3 formed as an intermediate would react with a water molecule to evolve hydrogen.38 For the hydrolysis, noble metals such as Rh have been used as active * To whom correspondence should be addressed. Tel: +81-6-6879-7368. Fax: +81-6-6879-7370. E-mail: (S.F) [email protected]; (Y.Y.) [email protected]. † Osaka University. ‡ National Institute of Advanced Industrial Science and Technology (AIST). § Ewha Womans University.

catalysts.6,9-11 However, more cost-effective and highly active catalysts are certainly required for any practical application. Among non-noble metals, cobalt, nickel, and iron metals can work as catalysts for the reaction although their activities are significantly lower than those of precious metal catalysts.6,12-14,36 A drawback of the transition metals is instability at their low valence states, which are active phases for the hydrogen evolution. These transition metals form the corresponding oxides under atmospheric conditions. Once such oxides form, it is difficult to be reduced by weak reductants such as AB. Thus, reductive pretreatment is necessary before use to attain an effective catalyst. For instance, it takes more than 60 min to start hydrogen evolution by AB hydrolysis over Co catalysts.13 Among the metal oxides, copper oxides, CuO and Cu2O, are exceptional catalysts working without reductive pretreatment in AB hydrolysis,19,20 although the agglomerates of Cu metals formed under reductive condition lessens its catalytic activity. The loading of Cu on a metal oxide may compensate the drawbacks of Cu although just modest activity was reported for Cu-loaded metal oxide catalysts to date.38 A cooperative synergetic effect can be expected between Cu species and an oxide support to facilitate the reaction. We report herein the superior catalytic reactivity of Cu-loaded Co3O4 nanoparticles in AB hydrolysis. Before Cu loading on Co3O4, the size and shape of Co3O4 nanoparticles were optimized to attain the highest catalytic reactivity after reductive treatment with NaBH4. Both fresh and used catalysts of Cu/ Co3O4, were structurally characterized and active species were identified by X-ray photoelectron spectroscopy, Auger spectroscopy and powder X-ray diffraction spectroscopy.

10.1021/jp104291s  2010 American Chemical Society Published on Web 09/14/2010

Cu/Co3O4 Nanoparticles as Catalysts Experimental Method Materials. All chemicals used for synthesizing nanosized Co3O4 or CuO were obtained from chemical companies and used without further purification. Cobalt nitrate hexahydrate was purchased from Nakalai Tesque. Cobalt acetate tetrahydrate and copper acetate monohydrate were obtained from Kishida Chemicals. Cobalt chloride, sodium dodecylsulfate, and sodium borohydride were received from Wako pure chemicals. Ammonia borane and oleylamine were received from Aldrich. Purified water was provided by a Millipore Milli-Q water purification system where the electronic conductance was 18.2 MΩ. Size and/or shape-controlled nanosized Co3O4 were synthesized by following reported methods.39-41 Size-Controlled Co3O4 Particles (Not Shape-Controlled).39 (4 nm) An aqueous solution of cobalt acetate (80 mM, 73 mL) was slowly added to an ammonia solution (25%, 7.3 mL) with vigorous stirring by a magnetic stirrer. After 20 min stirring, the obtained pale pink slurry was transferred to a Teflon cup with inner volume of 140 mL. The Teflon cup was sealed in a stainless steal jacket and heated to 150 °C in an oven for 3 h. The obtained particles were collected by filtration and washed with pure water three times and dried at 65 °C for several hours and calcined at 350 °C for 3 h. (20 nm) The similar synthetic procedure for 4 nm Co3O4 was applied with Co acetate (5.9 mmol), water (9 mL), and an ammonia solution (7.3 mL). (500 nm) The similar synthetic procedure for 4 nm Co3O4 was applied with Co acetate (23 mmol), water (51 mL), and an ammonia solution (29 mL). Co3O4 Cube.40 To an aqueous solution (56 mL) of cobalt chloride (2.8 mmol, 0.36 g) and sodium dodecyl sulfate (2.8 mmol, 0.81 g) was added sodium borohydride (1.4 mmol, 0.053 mg) with vigorous stirring. The black solution was transferred to a Teflon cup with 140 mL inner volume. The Teflon cup was sealed in a stainless steel jacket and heated to 140 °C for 12 h. The obtained particles were collected by centrifugation and washed with water and ethanol repeatedly. The obtained powder was dried at 65 °C for several hours and calcined at 350 °C for 3 h. Co3O4 Hexagonal Sheet.41 To an aqueous solution (70 mL) of cobalt nitrate hexahydrate (3.50 mmol, 1.02 g) was added oleylamine (7.0 mL) and ethanol (35 mL) while magnetically stirring for 30 min. During the stirring, the solution turned to blue-green color from pink color. The obtained solution was transferred into 140 mL Teflon cup with a stainless steel jacket at 180 °C for 12 h. The resulting product was filtered and washed with ethanol three times. The obtained powder was dried at 65 °C for several hours and then calcined at 350 °C for 3 h. The complete removal of a capping reagent was confirmed by thermogravimetric (TG)/differential thermal analysis (DTA) measurement. Size-controlled CuO (15 nm). Copper acetate monohydrate (0.83 mmol) was dissolved to absolute ethanol (16.7 mL). The solution was heated at 150 °C for 20 h in a Teflon cup (140 mL) with a stainless jacket. The obtained particles were separated by centrifugation (4000 rpm for 20 min) and washed with water and ethanol two times. Preparation of Cu/Co3O4 Catalysts. All Cu/Co3O4 composite catalysts were prepared by a conventional impregnation method. A typical procedure is as follows: To an aqueous solution of copper nitrate, nanosized Co3O4 was immersed. After sonication for 30 min, the resulting slurry was completely dried in an oven at 80 °C and calcined at 350 °C for 3 h. Catalysis Measurements. Cobalt oxide based catalytic particles (12 mg, 2.5 mM Co) were suspended in an aqueous

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16457 solution of AB (25 mM, 20 mL) at 20 °C in a glass tube (50 mL). The suspension was vigorously stirred with a magnetic stirrer during the reaction. AB hydrolysis underwent by the following reaction: NH3-BH3 + 2H2O f NH4+ + BO2- + 3H2. Thus, the theoretical maximum amount of H2 evolved (1.5 mmol) was three times the moles of AB added. When NaBH4 was added in the solution, its concentration was adjusted to 7.5 mM before starting the reaction. NaBH4 hydrolysis proceeds by the following reaction: NaBH4 + 2H2O f Na+ + BO2- + 4H2. The maximum amount of H2 evolved (0.6 mmol) was four times the moles of NaBH4 added. Hydrogen evolved during the reaction was trapped in a gas buret on a water bath, and each volume was recorded with the reaction time. For durability tests, catalytic reactions were repeated 10 times on 2 wt % Cu/Co3O4 sheet. A concentrated AB solution was added for each reaction. Transmittance Electron Microscope. Transmittance electron microscope (TEM) images of nanoparticles, which were mounted on a copper microgrid coated with elastic carbon, were observed by a JEOL JEM 2100 operating at 200 keV. X-ray Diffraction. X-ray diffraction (XRD) patterns were recorded by a Rigaku Ultima IV. Incident X-ray radiation was produced by Cu X-ray tube, operating at 40 kV and 40 mA with Cu KR radiation of 1.54 Å. A scanning rate was 4°/min from 10 to 70° in 2θ. X-ray Photoelectron Spectra and Auger Spectra. X-ray photoelectron spectra (XPS) and X-ray induced Auger spectra were measured by a Kratos Axis 165x with a 165 mm hemispherical electron energy analyzer. The incident radiation was Mg KR X-rays (1253.6 eV) at 200 W. Each sample was attached on a stainless stage with a double-sided carbon scotch tape. The binding energy of each element was corrected by C 1s peak (248.6 eV) from residual carbon. TG/DTA. TG/DTA data was recorded on a SII TG/DTA 7200 instrument. The samples (∼10 mg) were heated from 25 to 500 °C with a ramp rate of 2 °C/min. A certain amount of γ-Al2O3 was used as a reference for DTA measurements. Results and Discussion Nanosized Co3O4 with Different Shape and Size. Shapeand/or size-controlled Co3O4 nanoparticles were hydrothermally synthesized from different precursors and capping reagents by following the reported methods with modifications (see Experimental Method).39 The shape-controlled particles were calcined at 350 °C for 3 h for the removal of capping agents before catalysis measurements. The TG/DTA data assured no phase change but a capping agent removal around the calcination temperature (Supporting Information Figure S1). Figure 1 displays TEM images of nanosized Co3O4 particles with different shapes and sizes. The average diameter of Co3O4 particles without shape-control was about 4, 20, and 500 nm as shown in Figure 1a-c, respectively. Nanosized Co3O4 with cube shape (Co3O4 cube) was nearly monodispersed with a uniform size of 20-30 nm in a diagonal direction (Figure 1d). The Co3O4 cube was single crystalline and enclosed by six (100) planes. Nanosized Co3O4 with hexagonal sheet shape (Co3O4 sheet) has the size of 0.5-1 mm in a diagonal direction (Figure 1e). The dominant plane of the Co3O4 sheet was previously indexed to (211) plane by Hu et al.39 Each nanosized Co3O4 is expected to demonstrate unique catalysis corresponding to the surface atom alignment (vide infra). Catalytic Behaviors of Size-Controlled CuO and Co3O4 for Ammonia Borane Hydrolysis. First, ammonia borane hydrolysis was examined on size-controlled CuO and Co3O4. The sizes of these particles were 15 nm for CuO and 4 nm for

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Figure 1. TEM images of nanosized Co3O4 particles. Size-controlled Co3O4 particles with the size of (a) 4, (b) 20, and (c) 500 nm and shapecontrolled Co3O4 with the shape of (d) cube and (e) hexagonal sheet.

Figure 2. Time course of hydrogen evolution from the hydrolysis of AB (25 mM, 20 mL) catalyzed by (a) CuO (15 nm) and (b) Co3O4 (4 nm). The reactions were repeated two times on the same catalyst (metal, 12 mg; Co/AB or Cu/AB ) 0.15). The theoretical maximum amount of hydrogen evolved in this system was 1.5 mmol.

Co3O4 determined by TEM observations (Supporting Information Figure S2 and Figure 1a). Each catalyst (12 mg) was poured in an aqueous solution of ammonia borane (25 mM, 20 mL) at room temperature. As shown in Figure 2a, the CuO particles in the size of 15 nm (CuO (15 nm)) showed high catalytic activity for first 10 min; however, a severe deactivation was observed with the formation of black precipitates. The H2 evolution ceased when its volume reached 60% of the theoretical maximum amount. When an additional AB solution was added to the used CuO catalyst, the H2 yield reached only 30% of the maximum amount. On the other hand, the Co3O4 particles with the size of 4 nm (Co3O4 (4 nm)) completely decomposed ammonia borane after a long induction period of 84 min as shown in Figure 2b. The turnover frequency of 1.7 mmol · g-cat-1 · min-1 for H2 evolution over the Co3O4 (4 nm) at steady state was somewhat smaller than that over the CuO (15 nm) (2.0 mmol · g-cat-1 · min-1). After the H2 evolution ceased, the same amount of AB solution for the first cycle was poured onto the used Co3O4 catalyst for the measurement of the second cycle. At the cycle, an immediate hydrogen evolution was observed where the H2 evolution rate was faster than that of the first cycle. These results clearly illustrated that the CuO (15 nm) is a highly active catalyst with low stability, and the Co3O4 (4 nm) is a robust catalyst although the long induction period is required before showing its activity under mildly reduced conditions. Hydrolysis of Ammonia Borane over Nanosized Co3O4 Treated with NaBH4. The addition of NaBH4 in the reaction solution dramatically shortened the induction period observed

for Co3O4 because NaBH4 reducibly activates the Co3O4 surfaces. The catalysis measurements of Co3O4 (4, 20, 500 nm), Co3O4 cube and Co3O4 sheet were carried out at room temperature in the presence of NaBH4 (Table S1 in Supporting Information). Each catalyst of 12 mg (2.5 mM Co) was put into an aqueous solution (20 mL) of AB (25 mM) and NaBH4 (7.5 mM). The time courses of H2 evolution over the catalyst of Co3O4 (20 nm), Co3O4 cube, or Co3O4 sheet, are depicted in Figure 3. Among these Co3O4 catalysts, the Co3O4 sheet exhibited the highest activity where the H2 evolution rate was 0.35 mmol/min in between 5 and 10 min, whereas Co3O4 (20 nm) showed the lowest H2 evolution rate of 0.09 mmol/min. The H2 evolution rate of 0.22 mmol/min obtained with Co3O4 cube was slower than the rate with Co3O4 sheet. The high activity of the Co3O4 sheet was rather unexpected because larger particles have only lower specific surface area where fewer numbers of active sites are available. The different catalytic properties of nanosized Co3O4 can be deduced by atom alignments on the surface. The surface of Co3O4 sheet predominantly consists of a (211) plane where the Co atoms are aligned with long separations. On the other hand, the surfaces of the Co3O4 cube consist of six (100) planes where atoms are aligned in a close-packed form. The higher activity of the Co3O4 sheet for AB hydrolysis is attributed to highly reducible property of a (211) plane.40 Although the Co3O4 sheet was the most active among the Co3O4 nanoparticles examined herein, the induction period of the Co3O4 sheet was as long as 52 min without the reductive pretreament by NaBH4.

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induction period (min)

H2 evolution rate (mmol/min)

0 0.5 1.0 2.0 4.0

52 6 2 1 0.7

0.02 0.42 0.46 0.50 0.63

a All data were taken at 20 °C using an aqueous solution containing 12 mg Co3O4 and 0.5 mmol NH3-BH3 in water (20 mL). Each H2 evolution rate was calculated based on the period between 1 and 95% volume levels of maximum H2 evolved.

Figure 3. Comparison of hydrogen evolution rates over nanosized Co3O4 with different shapes. Nanosized Co3O4 catalysts were activated by NaBH4 before catalysis measurements [Co3O4, 12 mg; NH3-BH3, 0.5 mmol, and NaBH4 0.15 mmol in water (20 mL)]. The theoretical maximum amount of hydrogen evolved (2.1 mmol) was the sum of three times of NH3-BH3 and four times of NaBH4.

Figure 5. Hydrogen evolution from the hydrolysis of ammonia borane catalyzed by 2% Cu/Co3O4 sheet (Co3O4, 12 mg; Co/AB ) 0.15). The catalyst was repeatedly used for the reactions a total of 10 times. The theoretical maximum amount of hydrogen evolved is three times of AB (1.5 mmol).

Figure 4. Hydrogen evolution from the hydrolysis of ammonia borane catalyzed by 2% Cu/Co3O4 nanoparticles [Cu/Co3O4, 12 mg; NH3-BH3, 0.5 mmol in water (20 mL)]. The theoretical maximum amount of hydrogen evolving was three times of NH3-BH3 (1.5 mmol).

Hydrolysis of Ammonia Borane over Cu-loaded Co3O4 Nanoparticles. Copper loading on nanosized Co3O4 particles was expected to improve the inherent catalytic property of Co3O4 because it is known that AB hydrolysis proceeds on CuO to evolve hydrogen, which can reduce and activate the Co3O4 surfaces. Figure 4 indicates the time course of hydrogen evolution over 2% Cu loaded Co3O4 (20 nm), Co3O4 cube, or Co3O4 sheet. Each catalyst (12 mg) was poured in an aqueous solution of ammonia borane (25 mM, 20 mL) at room temperature. Nearly 100% AB decomposition underwent over these Cu/Co3O4 catalysts and no agglomerate formation was observed after reactions. Even in the absence of NaBH4, the short induction period of less than 2 min was observed for these Cu/Co3O4 catalysts. In terms of the H2 evolution rate, 0.50, 0.49, and 0.25 mmol/min were recorded for 2% Cu/Co3O4 sheet, Cu/ Co3O4 cube, and Cu/Co3O4 (20 nm), respectively. These hydrogen evolution rates are higher than those of corresponding Co3O4 catalysts treated with NaBH4. There would be a synergetic effect between Cu and Co3O4 in terms of the H2 evolution. The effect of the loading amount of Cu on the catalytic performance of Co3O4 was examined in detail with Co3O4 sheet. A series of composite catalysts of Cu/Co3O4 sheet with different Cu loadings were prepared by a conventional impregnation method. The loading amount of Cu was changed in the range

from 0.5 to 4.0%. Cu loading induced no significant change in the shape and size of Co3O4 sheet observed by TEM (Supporting Information Figure S3). The catalytic properties of a series of Cu/Co3O4 sheet were summarized in Table 1 in terms of induction period and H2 evolution rate observed on each catalytic reaction. The small amount of Cu loading of 0.5% was even effective to shorten the induction period from 52 to 6 min without losing its high activity for H2 evolution. When the Cu loading amount increased to 4%, the induction period became as short as 0.7 min and the H2 evolution rate increased from 0.02 to 0.63 mmol/min. The Cu loading resulted in not only much shorter induction period but also much higher H2 evolution rate. In general, the severe deactivation is a serious drawback of a Cu-based catalyst owing to the easy sintering or agglomeration property of Cu metals under reduced conditions. The recyclability examinations were performed on 2% Cu/Co3O4 sheet and no significant activity loss was observed during 10 times repetitive use as shown in Figure 5. The similar improvement in the catalytic behavior was achieved in the size-controlled Co3O4 without shape control. When the Cu loading was increased to 2% on Co3O4 (4 nm), the induction period decreased from 85 to 3.3 min and the H2 evolution rate increased from 0.02 to 0.25 mmol/min as shown in Supporting Information Figure S4. The faster H2 evolution rates were observed after the repetitive use of these catalysts. For example, the H2 evolution rate of 1% Cu/Co3O4 (4 nm) at the second cycle is nearly double of that of the first cycle. This is ascribed to the activation of the Co3O4 surface under reductive

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TABLE 2: Catalytic Activity of Cu/Co3O4 (4, 20, and 500 nm) for Ammonia Borane Hydrolysis without Strong Reducing Reagenta H2 evolution rate (mmol/min) Cu loading Co3O4 size induction (nm) period (min) (wt %) 0 1.0 2.0 1.0 2.0 1.0 2.0

4 4 4 20 20 500 500

84 10 3.3 7 2 1.3 1.2

first cycle

second cycleb

0.02 0.15 0.25 0.12 0.25 0.11 0.13

0.31 0.36 0.18 0.31 0.13 0.16

a All data were taken at 20 °C on an aqueous solution containing 12 mg Cu/Co3O4 and 0.5 mmol NH3-BH3 in water (20 mL). b Twenty milliliters of an NH3-BH3 aqueous solution was added to the solution after reaction of first cycle.

atmosphere during the first cycle. The higher loading amount of Cu produced more hydrogen that can reduce and activate Co3O4 nanoparticle surfaces. The effect of Cu loading on the catalytic activity was commonly observed on Co3O4 particles as tabulated in Table 2. The remarkable effect on the induction period was also observed for the Co3O4 particles in all sizes. The induction periods observed for 1 and 2% Cu/Co3O4 (20 nm) were 7 and 2 min with the H2 evolution rates of 0.12 and 0.25 mmol/min, respectively. The effect of Cu loading was also noticeable on a series of catalysts with Co3O4 (500 nm). As small as 1% of the Cu addition decreased the induction period as short as 1.3 min although the H2 evolution rate is much slower than the others. The particles with a larger size usually have lower specific surface area, where the small amount of Cu is sufficient for covering the large part of Co3O4 surface. Thus, the high density of Cu on the Co3O4 surfaces at Cu/Co3O4 (500 nm) resulted in the shorter induction period. Structural Characterization of Cu/Co3O4 Sheet by XRD and XPS Measurements. After the reaction, the catalysts powder stuck on a magnetic stirrer bar. This fact indicates that some portions of nanosized Co3O4 were reduced to cobalt metal, which is well-known as a ferromagnetic material. The catalyst structures before and after reaction were investigated by the measurements of powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The X-ray diffraction patterns of both the fresh and used Cu/ Co3O4 sheet are shown in Figure 6a,b, respectively. All the peaks that appeared for the fresh catalyst (Figure 6b) were consistent with the peak positions of Co3O4 in JCPDS card No. 01-0710816. Even after the reaction, no significant change was observed in the diffraction patterns (Figure 6a), indicating that Co3O4 form is stable in a weakly reduced condition such as in an AB solution. No obvious peaks from other than Co3O4 appeared on the XRD spectrum of the used catalyst, so that the surface analyses of the Cu/Co3O4 in fresh and used conditions were performed by XPS and Auger spectroscopy. X-ray photoelectron spectra was applied to Cu/Co3O4 sheet before and after the reaction. The survey spectra for Co 2p, O 1s, and Cu 2p were displayed in Figure 7a-f. All the spectra were referenced to the residual carbon at the binding energy (BE) of 284.6 eV. The fresh catalyst showed typical peak sets for Co3O4 reported in a literature.42 The peak tops appeared at 779.6 eV for Co 2p3/2 and 794.6 eV for Co 2p1/2. These peaks of the used catalyst were increasingly shifted to 780.7 and 796.6 eV as shown in Figure 7a. The energy gap between Co 2p spin orbit doublets increased from 15.0 eV on the fresh catalyst to

Figure 6. X-ray diffraction patterns of (a) the used and (b) the fresh Cu/Co3O4 sheet. The numbers in parentheses are plane indexes of Co3O4.

15.9 eV on the used catalyst. The intensities of satellite peaks appeared at 786.2 and 802.5 eV on the used catalyst were also increased. Such spectroscopic features clearly indicated Co(OH)2 formation on the surface of the used catalyst.42,43 The Co(OH)2 formation on the used catalyst was also confirmed by the appearance of O 1s peak. The O 1s peak of the fresh catalyst appeared at 529.6 eV, whereas it shifted to 531.2 eV for the used catalyst. The higher binding energy of O 1s was ascribed to the OH species formation.42 Additionally no CoB formation, which is often formed in the presence of NaBH4,12 was identified by the B 1s peak appearing at 191.7 eV as shown in Figure 7h, because the B 1s peak of CoB is known to appear around 188-189 eV.44 The XPS peaks for Cu 2p were observed at 934.0 and 953.8 eV with strong shaken-up satellite peaks at the fresh catalyst. The strong satellite peaks assured that the copper species of the fresh catalyst is Cu(II). Cu 2p peaks were negatively shifted to 933.1 and 952.9 eV for the used catalyst with losing satellite peaks. These features in the spectrum appearance were typically observed on reduced copper species of both Cu(I) or Cu(0). The Cu(I) species and Cu(0) species are often discriminated by Auger L3M45M45 peak appeared around 918 eV in kinetic energy.43,45-47 The Auger line appeared at 916 eV in Figure 7g clearly indicated the formation of Cu(I) species such as Cu2O after reaction. It has been reported that Cu2O shows higher activity for AB hydroloysis than Cu metal. The superior catalysis of Cu/Co3O4 is partly owing to the formation of Cu2O species.20 Co 2p peaks of the used catalyst were deconvoluted to find minor species, which would contribute to the superior catalytic activity of Cu/Co3O4. The spectrum of Co 2p showed spin-orbit splitting into 2p1/2 and 2p3/2 components, and both components contain the same qualitative information. Therefore, only the higher intensity Co 2p3/2 bands were curve-fitted in this study. Figure 8a shows the deconvolution peaks of Co 2p3/2 of the used catalyst. There are three peaks whose peak tops were 780.5, 782.4, and 786.2 eV. These values are good agreement with the reported values for Co(OH)2, indicating the absence of active Co species because we observed no catalytic activity for nanosized Co(OH)2 (data not shown).41 Then, the catalyst during the reaction was taken out from a reaction bottle with AB solution and was evacuated for XPS measurements to keep the reaction condition. Though the peak intensity was weaker at this condition, a new peak locating at the lower binding energy of 778.6 eV was clearly observed. It has been reported that a metallic Co species provides a Co 2p3/2 peak around 778 eV

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Figure 7. X-ray photoelectron spectra of the fresh (b,d,f) and used (a,c,e,h) catalysts of Cu/Co3O4 sheets and X-ray induced Auger line (Cu L3M45M45) of the used catalyst (g).

Figure 8. Deconvolution results of Co 2p2/3 peak observed for (a) the used catalyst and (b) the catalyst sampled with ammonia borane solution (catalyst: Cu/Co3O4 sheet).

lower than Co(II) and Co(III).12,43 The metallic Co species is known to exhibit a certain activity for AB hydrolysis.38 Thus, the metallic Co species found by the XPS measurement would contribute to the high catalytic activity of Cu/Co3O4. Conclusions A highly active and robust catalyst for the hydrogen evolution by AB hydrolysis was achieved by the Cu loading on nanosized Co3O4 particles. The catalytic activity of Cu/Co3O4 depends on the inherent activities of Co3O4 nanoparticles. The shape and size effects of Co3O4 nanoparticles were investigated, and it was found that the nanosized Co3O4 in hexagonal sheet shape showed better catalytic activity than the nanosized Co3O4 in cube or uncontrolled shape although their size effect was not significant. Loading of Cu on Co3O4 nanoparticles dramatically improved the catalytic activity of Co3O4 in terms of the induction period and the H2 evolution rate. Furthermore, no significant activity loss was observed on 2% Cu/Co3O4 sheet after 10 times repetitive reactions. The active species of the catalyst were formed only on the catalyst surface, and they were Cu(I) and Co metal with Co(OH)2 determined by XPS and XRD measurements.

Acknowledgment. This work was supported by a Grant-inAid from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (No. 20108010 to S.F.) and KOSEF/ MEST through WCU project (R31-2008-000-10010-0). Y.Y. thanks the financial support from the Ogasawara Foundation for the Promotion of Science and Engineering. We sincerely acknowledge Professors Norimitsu Tohnai and Nobuyuki Zettsu for their kind assistance on XRD and TEM measurements. Supporting Information Available: TG/DTA data of Cu/ Co3O4, TEM image of nanosized CuO, TEM image of Cu/Co3O4 sheet, time course of H2 evolution over Cu/Co3O4 (4 nm) with 0, 1, and 2% Cu loadings, table of pore size, BET surface area of Co3O4, and H2 evolution rate over Co3O4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hamilton, C. W.; Baker, R. T.; Staubitzc, A.; Manners, I. Chem. Soc. ReV. 2009, 38, 279. (2) Peng, B.; Chen, J. Energy EnViron. Sci. 2008, 1, 479. (3) Xu, Q.; Chandra, M. J. Alloys Compd. 2007, 446, 729. (4) Nocera, D. G. Chem. Soc. ReV. 2009, 38, 13.

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