Catalytic application of shape-controlled Cu₂O particles protected by Co₃O₄ nanoparticles for hydrogen evolution from ammonia borane

Abstract

Cu₂O particles are active catalysts for hydrogen evolution from ammonia borane (AB) by hydrolysis, however, Cu₂O particles easily form agglomerates as a result of highly reduced conditions during the reaction. In order to suppress agglomerate formation, capping of Cu₂O with organic reagents or inorganic materials was performed and the catalytic reactivity in AB hydrolysis was examined. Among the examined methods, capping of Cu₂O particles with Co₃O₄ nanoparticles was the most effective to avoid agglomerate formation of Cu₂O particles. The finding enabled us to examine the shape effect of Cu₂O particles on the catalytic reactivity in AB hydrolysis in the presence of Co₃O₄ nanoparticles. Comparisons of turnover frequencies for hydrogen evolution of Cu₂O–Co₃O₄ composites, in which Cu₂O particles were in the shape of 50-facets, cube, octahedron or rhombicuboctahedron, indicated that the composite with Cu₂O with the shape of 50-facets showed more than 7-fold higher hydrogen evolution rate normalized by surface area than the composite with Cu₂O with the octahedral shape. The size and shape effects of Co₃O₄ nanoparticles were also investigated on their ability to protect Cu₂O from agglomeration. Comparisons of the catalytic reactivity of Cu₂O particles decorated with Co₃O₄ nanoparticles of different sizes and shapes in terms of amounts and rates of hydrogen evolved by AB hydrolysis indicated that the size of Co₃O₄ nanoparticles is more important than the shape to exhibit high catalytic reactivity.

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Broader context

Hydrogen is regarded as a potential clean fuel for the next generation together with PEM fuel cells. In order to transport hydrogen as a fuel for mobile uses, hydrogen storage is very critical because hydrogen has poor energy content per volume. Storage of hydrogen in ammonia borane could be a solution because ammonia borane contains relatively high hydrogen contents (19.6%) and easily releases hydrogen under aqueous conditions with an appropriate catalyst. Among non-precious metals, Cu₂O shows the high catalytic reactivity toward ammonia borane hydrolysis, however, severe deactivation has been observed due to agglomeration. Here we show a novel but conventional method to suppress the deactivation of Cu₂O by decorating Cu₂O particles partly with nanoparticles of Co₃O₄. The Co₃O₄ particles protect the Cu₂O particles from agglomeration, resulting in high catalytic reactivity of Cu₂O–Co₃O₄ composites.

1. Introduction

Hydrogen is regarded as a promising fuel in the future because of its high energy content per mass, which is higher than petroleum. Thus, hydrogen has been used for running a proton exchange membrane fuel cell, PEMFC, which increases energy efficiency over internal combustion engines and decreases formation of harmful chemicals such as SO_x, NO_x, CO₂ or particulate matters.^1–3 A critical drawback of hydrogen is obviously its poor volumetric energy density.^4,5Hydrogen evolution from liquid or solid chemicals on-site is suitable for application in transportation.⁶ Chemical hydrides are solid materials attracting a great deal of attention owing to their high gravimetric and volumetric storage capacity of hydrogen.⁷ Among the chemical hydrides, the ammonia borane complex, NH₃BH₃ (AB), is a promising candidate, because AB contains 19.6 wt% of hydrogen and more importantly this is stable in an aqueous solution at neutral pH as well as in the solid state.⁸

Various types of metals and metal complexes have been reported to catalyse hydrogen evolution by thermal decomposition^9,10 or solvo- or hydrolytic decomposition of ammonia borane.^11–27 For the hydrolytic decomposition, precious metals such as Pt and Rh have been employed as active catalysts.^8,28–30 However, industry demands more active catalysts made of abundant resources for a practical use. Among non-precious metals, copper catalysts such as Cu₂O have been reported to act as catalysts for hydrolytic decomposition of AB although their activities are moderate compared with precious metal catalysts.^8,31–34 For improvement of the catalytic behaviour of Cu₂O particles, size and shape effects of Cu₂O should be clarified. However, Cu₂O particles easily form agglomerates leading to the loss of the catalytic reactivity under reductive conditions. Protection of Cu₂O particles is necessary for evaluation of the shape effect of Cu₂O particles on catalysis under reductive conditions.^35,36

In general, covering surfaces of metallic particles with organic reagents or inorganic materials is a promising method to protect metallic particles from agglomeration. For example, an organic capping reagent of poly-N-vinyl-2-pyrrolidone (PVP) has been used for protecting Ni nanoparticles from agglomeration to improve durability even under reducing atmosphere.²² A drawback of organic capping reagents is the lack of the long-term stability under harsh reaction conditions. Additionally, the chemical interaction between organic molecules and catalyst surfaces may alter the surface properties of catalytic materials. Inorganic materials are durable enough to protect metal particles even under harsh reaction conditions with less interaction with metal surfaces. With inorganic materials, particles with core–shell structures, in which particles are covered with thermally and chemically stable porous materials, have been reported for catalytic application.³⁷ For example, Pt nanoparticles covered with silica were effectively protected under high temperature conditions such as 973 K for alkane oxidation.³⁸ A drawback of core–shell structures is the reduction of an available surface area of active core species. Even though the metal-oxide shell has a mesoporous structure, slow diffusion of a reactant reduces the apparent catalytic reactivity.

We report herein a novel protection method of Cu₂O particles from agglomeration by partially covering with Co₃O₄ under highly reductive conditions. The Cu₂O particles partially covered with Co₃O₄ nanoparticles showed the high catalytic reactivity for hydrogen evolution by AB hydrolysis with high durability. The catalytic reactivity and durability of the Cu₂O–Co₃O₄ composites were compared with those of the Cu₂O particles protected by conventional methods, i.e., capping with organic reagents or inorganic materials. As organic capping reagents, PVP, hexadecyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used because of different electrical properties. As an inorganic capping and support material, mesoporous silica was used. Cu₂O particles partially covered with Co₃O₄, where the Cu₂O particles have the shape of 50-facets, cube, octahedron or rhombicuboctahedron, were also employed to clarify the shape effect of Cu₂O particles on the catalytic reactivity for AB hydrolysis. Also the shape and size effects of Co₃O₄ nanoparticles were investigated on protection of Cu₂O particles from agglomeration.

2. Experimental section

All chemicals used for synthesizing Cu₂O or nanosized Co₃O₄ particles were obtained from a chemical company and used without further purification. Copper acetate monohydrate and copper(II) nitrate trihydrate were purchased from Nakalai Tesque and Wako Pure Chemical Industries, Ltd, respectively. Cobalt precursors of cobalt acetate tetrahydrate and cobalt nitrate hexahydrate were received from Kishida Chemicals and Wako Pure Chemical Industries, Ltd, respectively. Sodium dodecyl sulfate (SDS), sodium borohydride, D-(+)-glucose, ammonia solution (28%) and sodium hydroxide were obtained from Wako Pure Chemical Industries, Ltd. Poly-N-vinyl-2-pyrrolidone (PVP, K-15, M_w = 10 000) and cetyltrimethylammonium bromide (CTAB) were obtained from Tokyo Chemical Industry. Ammonia borane, oleylamine, urea and 3-aminopropyltriethoxysilane (APTS) were received from Sigma-Aldrich Co. Tetraethoxysilane was received from Shin-Etsu Chemical Co., Ltd. Purified water was provided by a Millipore Milli-Q water purification system where the electronic conductance was 18.2 MΩ cm. Size and/or shape-controlled Cu₂O or nanosized Co₃O₄ were synthesized by following reported methods.^39–42

2.1. Synthesis of particles

Cu₂O particles (50-facets)³⁹. Copper acetate monohydrate (180 mg, 0.90 mmol) was dissolved in a mixed solution of water (84 mL) and absolute ethanol (6 mL) at 333 K. An aqueous solution of sodium hydroxide (8.5 M, 15 mL) was slowly added to form precipitates with vigorous stirring at 333 K. After 12 min stirring, an aqueous solution (45 mL) of D-(+)-glucose (0.90 g, 5.0 mmol) was slowly added to the solution with vigorous stirring at 333 K for 15 min. The obtained particles were separated by centrifugation, washed with water three times and dried in vacuo for several hours.

Cu₂O particles (cube, octahedron, rhombicuboctahedron)³⁹. Copper acetate monohydrate (180 mg, 0.90 mmol) was dissolved in a mixed solution (90 mL) of water and absolute ethanol [90 : 0 (v/v) for the octahedron and 9 : 81 for the rhombicuboctahedron (RCO) and the cube] at 333 K. An aqueous solution (15 mL) of sodium hydroxide (5.0 M for cube and 10 M for RCO and octahedron) was dropped into the mixture with vigorous stirring at 60 °C. After 12 min stirring, an aqueous solution (45 mL) of D-(+)-glucose (0.9 g, 5.0 mmol) was slowly added to the suspension with vigorous stirring at 333 K for 60 min for the cube and 15 min for the RCO and the octahedron. The obtained particles were separated by centrifugation, washed with water three times and dried in vacuo for several hours.

Co₃O₄ nanoparticles (sphere)⁴⁰. 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 an inner volume of 140 mL. The Teflon cup was sealed in a stainless steel jacket and heated to 423 K in an oven for 3 hours. The obtained particles were collected by centrifugation and washed with pure water three times and dried at 338 K for several hours.

Co₃O₄ nanoplates⁴¹. To an aqueous solution (70 mL) of cobalt nitrate hexahydrate (1.0 g, 3.5 mmol) were added oleylamine (7.0 mL) and ethanol (35 mL) with magnetic stirring for 30 min. During the stirring, the solution turned into blue-green from pink. The obtained solution was transferred into a Teflon® cup (140 mL) with a stainless steel jacket at 453 K for 12 hours. The resulting product was filtered and washed with ethanol three times. The obtained powder was dried at 338 K for several hours and then calcined at 623 K for 3 hours.

Co₃O₄ rods⁴². Cobalt nitrate hexahydrate (0.58 g, 2.0 mmol) and urea (0.12 g, 2.0 mmol) were dissolved in water (80 mL) to form a pink homogeneous solution with stirring. This solution was transferred to a Teflon® cup with an inner volume of 100 mL. The Teflon® cup was sealed in a stainless steel jacket and heated to 413 K for 20 hours. The obtained particles were collected by centrifugation and washed with water and ethanol repeatedly. The obtained powder was dried at 338 K for several hours and calcined at 573 K for 3 hours.

Preparation of Cu₂O with organic capping reagents. Cu₂O particles (20 mg) in the solid state were added to an aqueous solution (5.0 mL) of a capping reagent (PVP, CTAB or SDS, 60 mg) and stirred at room temperature for 3 hours. The obtained particles were collected by filtration, washed with water two times and dried in vacuo for several hours.

Cu₂O@SiO₂. Cu₂O particles (35 mg) dispersed in water (162 mL) were slowly added to an aqueous solution of sodium hydroxide (0.050 M, 4.0 mL) with vigorous stirring to adjust the pH of the solution to around 10–11. To this basic solution, a controlled amount of 10 vol% tetraethoxysilane (169 mg) diluted with methanol was added and the mixture was stirred continuously for 12 hours at room temperature. The obtained particles were collected by filtration, washed with water two times and dried in vacuo for several hours.

Cu₂O/SiO₂ (SBA-15)⁴³. Mesoporous SiO₂ (SBA-15, 1.0 g) was immersed into an ethanol solution (63 mL) of 3-aminopropyltriethoxysilane (APTS, 2.5 g, 11 mmol) and the mixture was refluxed for 24 hours. The resulting powder was collected by centrifugation and washed with water three times. APTS-modified SiO₂ (SBA-15, 0.50 g) was immersed into an ethanol solution (30 mL) of copper nitrate (76 mg, 0.31 mmol). After magnetic stirring for 3 hours, the obtained powder was collected by centrifugation. The obtained powder was calcined at 773 K for 4 hours.

Cu₂O covered with Co₃O₄ nanoparticles. A mixture of Cu₂O particles and Co₃O₄ nanoparticles (12 mg in total, various ratios) dispersed in water (0.20 mL) were sonicated for 5 min. The resulting slurry was completely dried in an oven at 353 K.

2.2. Catalysts characterization

Transmittance electron microscope 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. Scanning electron microscope images of particles were observed by a FE-SEM (JSM-6320F or JSM-6701F) operating at 8.0 kV or 5.0 kV. Powder X-ray diffraction patterns were recorded by a Rigaku Ultima IV. Incident X-ray radiation was produced by a Cu X-ray tube, operating at 40 kV and 40 mA with CuK_α radiation of 1.54 Å. The scanning rate was 2° min⁻¹ from 20° to 80° in 2θ. X-Ray photon spectra were measured by a Kratos Axis 165× with a 165 mm hemispherical electron energy analyzer. The incident radiation was Mg K_α 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 the C 1s peak (248.6 eV) from residual carbon.

2.3. Catalysis measurements

Catalytic particles (12 mg) were suspended in an aqueous solution of AB (25 mM, 20 mL) at 293 K in a glass tube (50 mL). The suspension was vigorously stirred with a magnetic stirrer during the reaction. The hydrogen evolved during the reaction was trapped in a gas burette on a water bath, and each volume was recorded with the reaction time.

3. Results and discussion

3.1. Cu₂O particles capped with organic reagents and inorganic materials

The catalytic reactivity of Cu₂O particles capped with organic reagents was examined for hydrogen evolution from ammonia borane (AB) hydrolysis at 293 K. Prior to catalysis measurements, a capping reagent of D-(+)-glucose used for synthesis of Cu₂O particles was exchanged to one of the water soluble capping reagents chosen from poly-N-vinyl-2-pyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), which are electrically neutral, cationic and anionic surfactants, respectively. The Cu₂O particles were suspended in an aqueous solution of AB (0.5 mmol, 20 mL water) with vigorous stirring. Fig. 1a depicts the time course of hydrogen evolution by AB hydrolysis with the Cu₂O particles capped with different organic molecules. Hydrogen evolution was observed with all the Cu₂O particles, however, the yield of hydrogen was as low as 20% after the reaction time of 60 min. At the beginning of the reaction (within 1 min), fast hydrogen evolution, in which the reaction rate was 0.17 mmol s⁻¹g_cat⁻¹, was observed in all cases. This clearly indicates that the Cu₂O is very active for the reaction at the beginning, however, it is deactivated very soon after this period. Fig. 1b shows a typical SEM image of Cu₂O particles with an organic capping reagent after AB hydrolysis. This image clearly indicates agglomeration of Cu₂O particles after the reaction. Powder X-ray diffraction measurements of the Cu₂O particles after AB hydrolysis indicated that Cu₂O was completely reduced to Cu metal after the reaction (see Fig. S1 in the ESI†). The reduction of Cu₂O surfaces to Cu metal may weaken an ionic interaction between the organic capping reagents and the surface of catalytic particles. Thus, these organic capping reagents cannot protect Cu₂O particles effectively from agglomeration.

Fig. 1 (a) Time course of hydrogen evolution with Cu₂O(50-facets) with D-(+)-glucose (black), PVP (red), CTAB (blue) and SDS (green) as capping reagents. (b) A typical SEM image of Cu₂O particles after AB hydrolysis. (c) Time course of hydrogen evolution with Cu₂O covered with SiO₂ shell. (d) Time course of hydrogen evolution with Cu₂O loaded on SiO₂ (SBA-15). The reaction was repeated 5 times under the same conditions.

The catalytic reactivity of Cu₂O particles covered with silica in a core–shell fashion (Cu₂O@SiO₂) was also examined for AB hydrolysis at 293 K. As indicated in Fig. 1c (red), a very small amount of hydrogen, which is less than 10% of the stoichiometric amount, was evolved. XPS spectra of Cu₂O@SiO₂ particles after the reaction shown in Fig. S2 (in the ESI†) indicate that silica coating is effective in avoiding agglomeration of Cu₂O particles, however, most part of Cu₂O-core surfaces could not be used for the catalytic reaction. When 5 wt% of Cu₂O particles were loaded on a support of mesoporous silica (SBA-15), a stoichiometric amount of hydrogen was evolved within 50 min with the hydrogen evolution rate of 0.048 mmol s⁻¹ g_cat⁻¹ as shown in Fig. 1d. The Cu₂O/SBA-15 catalyst showed high stability without the loss of the reactivity for hydrogen evolution during 5 times repetitive use. These results clearly indicate that Cu₂O particles act as catalysts for AB hydrolysis even after being contacted with SiO₂. Thus, the very low reactivity of Cu₂O@SiO₂ results from less permeability of AB into pores of SiO₂ shells.

3.2. Cu₂O particles partially covered with Co₃O₄ nanoparticles

In order to maintain the surfaces of Cu₂O particles accessible to AB, their surfaces were partially covered with metal oxide nanoparticles. Among metal-oxide nanoparticles active for AB hydrolysis, Co₃O₄ nanoplates can be expected to show the high catalytic reactivity by interacting with Cu₂O.³⁶ The Co₃O₄ plates were synthesized by the reported method and their sizes in between 100 nm and 500 nm and shapes of hexagonal plate were determined by TEM and SEM observations (vide infra, Fig. 7b). The calculated amount of Co₃O₄ plates and Cu₂O particles were mixed and dispersed in water with sonication and heated to dryness at 353 K. Formation of Cu₂O partially covered with Co₃O₄ (Cu₂O–Co₃O₄) was confirmed by SEM, as shown in Fig. 2a (more SEM images of Cu₂O–Co₃O₄ particles are displayed in Fig. S3 in the ESI†) with a schematic drawing of a Cu₂O–Co₃O₄ composite in Fig. 2b. Catalytic hydrogen evolution by AB hydrolysis with Cu₂O–Co₃O₄ (12 mg) was conducted in an aqueous solution of AB (0.50 mmol, 20 mL water) at 293 K. The time course of evolved hydrogen by AB hydrolysis with the Cu₂O–Co₃O₄ is depicted in Fig. 2c (black). The time course of hydrogen evolution by AB hydrolysis with Co₃O₄ (blue) or Cu₂O (red) is also shown as references. With Cu₂O–Co₃O₄, hydrogen evolution was observed with a hydrogen-evolution rate of 0.31 mmol s⁻¹ g_cat⁻¹ determined from the slope in between 1 and 4 min after starting the reaction. During the reaction, hydrogen of three times mol of AB should be evolved as given by eqn (1):

NH₃–BH₃ + 2H₂O → NH₄⁺ + BO₂⁻ + 3H₂ (1)

Fig. 2 (a) SEM image of Cu₂O(50-facets) decorated with Co₃O₄ nanoplates [Cu₂O/Co₃O₄ = 1/1 (w/w)]. (b) Schematic drawing of decorated particles. (c) Time course of hydrogen evolution from AB hydrolysis with Cu₂O decorated with Co₃O₄ nanoplates [black, 1 : 1(w/w)], Cu₂O and Co₃O₄ nanoplates separately reacted in an H-type cell [green, 1 : 1(w/w)], Co₃O₄ nanoplates pretreated by H₂ gas (blue) and Cu₂O with D-(+)-glucose (red) [NH₃BH₃, 0.5 mmol; catalyst, 12 mg; water, 20 mL].

Thus, the stoichiometric amount of hydrogen evolved in this system is 1.5 mmol. When the Cu₂O–Co₃O₄ was used as a catalyst, the volume of hydrogen evolved was 1.3 mmol, which corresponds to 87% of the stoichiometric amount of hydrogen. When Co₃O₄ alone was used as a catalyst, a long induction period, such as 60 min, was required to show the catalytic reactivity without a reductive pretreatment.^32,36 Even when Co₃O₄ was pretreated with hydrogen gas under the reaction conditions, no hydrogen evolution was observed in 10 min (Fig. 2c, blue). The catalytic reactivity was also examined for the reaction system with an H-type cell where Co₃O₄ and Cu₂O were separately put in two tubes of the H-type cell to evaluate the effect of interaction between Co₃O₄ and Cu₂O. As shown in Fig. 2c (green), only a small amount of hydrogen was evolved from the tube containing Cu₂O. Thus, the interaction between Cu₂O and Co₃O₄ is indispensable to achieve the high catalytic reactivity.

The repetitive catalytic tests with Cu₂O–Co₃O₄ showed no significant reactivity loss for three times use as shown in Fig. 3a. The hydrogen-evolution rates of the 2^nd and 3^rd cycles were 0.31 and 0.32 mmol s⁻¹g_cat⁻¹, respectively. For the 1^st cycle, a short induction period of 1 min was required before continuous hydrogen evolution, however, the induction period observed for the 1^st cycle was not observed for the 2^nd and 3^rd cycles. Also the amount of hydrogen evolved for the 2^nd and 3^rd cycles reached 1.45 mmol, which corresponds to 96% of the stoichiometric amount. These results indicate that a small amount of hydrogen evolved in the induction period was consumed for reducing the catalyst surfaces in the 1^st cycle. SEM observations of Cu₂O–Co₃O₄ particles after the reaction also assure no agglomeration of Cu₂O particles as shown in Fig. 3b. An SEM image of high magnification depicts the presence of Co₃O₄ particles on Cu₂O surfaces. The presence of Co₃O₄ nanoparticles effectively prevents the agglomeration of Cu₂O particles to improve durability of the catalysis.

Fig. 3 (a) Repetitive test (3 times) of hydrogen evolution by AB hydrolysis with Cu₂O–Co₃O₄ [Cu₂O/Co₃O₄ = 1/1 (w/w)]. (b) SEM image of Cu₂O(50-facets) decorated with Co₃O₄ nanoplates after reaction (NH₃BH₃, 0.5 mmol; catalyst, 12 mg; water, 20 mL).

The surface conditions of the Cu₂O–Co₃O₄ composite before and after AB hydrolysis were examined by X-ray photoelectron spectroscopy (XPS) as shown in Fig. 4. The measurements were performed for the binding energy fields of Cu 2p, Co 2p and O 1s. As shown in Fig. 4a, Cu 2p peaks of the fresh catalyst were complicated, because a part of the Cu(I) species were oxidized to Cu(II) during the composite preparation. The satellite peaks characteristic of Cu(II) species diminished in the Cu 2p peaks of the used catalyst (Fig. 4d), indicating low valent Cu species formed after the reaction.³⁶ For the Co 2p peaks of used catalysts (Fig. 4e), the strong satellite peaks were observed at 802.5 eV and 786.2 eV, which were very weakly observed in the fresh catalyst. These strong satellite peaks clearly indicated the presence of Co(II) species in the used catalyst.³⁶ Also, the O 1s peak of the used catalyst appeared at 531.2 eV (Fig. 4f), which was shifted from 530.0 eV of the fresh catalyst (Fig. 4c) corresponding to the formation of –OH species on the surface of used catalyst.³⁶ Thus, both Cu₂O and Co₃O₄ species have been reduced and formed corresponding hydroxide species during the reaction.

Fig. 4 X-Ray photoelectron spectra of the fresh (a–c) and used (d–f) catalysts of the Cu₂O–Co₃O₄ composite in the binding energy regions of Cu 2p (a and d), Co 2p (b and e) and O 1s (c and f) (* denotes satellite peak).

The catalytic reactivity of Cu₂O–Co₃O₄ for AB hydrolysis was examined with different Cu₂O/Co₃O₄ ratios, 5/1 and 1/5 (w/w), under the same conditions for the Cu₂O–Co₃O₄ (1/1) catalyst. The catalytic hydrogen evolution by AB hydrolysis was conducted in an aqueous solution containing AB (0.5 mmol) with a catalyst (12 mg). When the catalyst contained a smaller amount of Co₃O₄ with a Cu₂O/Co₃O₄ ratio of 5/1 (5Cu₂O–Co₃O₄), a smaller amount of hydrogen of 1.1 mmol was evolved than Cu₂O–Co₃O₄ and the hydrogen-evolution rate was 0.13 mmol s⁻¹g_cat⁻¹, which is less than half of the hydrogen-evolution rate observed for Cu₂O–Co₃O₄ as shown in Fig. 5. When the catalyst with a Co₃O₄-rich composition of Cu₂O/Co₃O₄ = 1/5 (Cu₂O–5Co₃O₄) was employed, the hydrogen-evolution rate was slightly increased to 0.38 mmol s⁻¹ g_cat⁻¹. The successful suppression of Cu₂O-particles agglomeration by partial coverage with Co₃O₄ nanoparticles encouraged us to examine the effect of Cu₂O shapes on the catalytic reactivity.

Fig. 5 Time course of hydrogen evolution by AB hydrolysis with Cu₂O/Co₃O₄ catalysts with different Cu₂O/Co₃O₄ ratios [circle, 1/1 (w/w); square, 5/1; triangle, 1/5] (NH₃BH₃ = 0.5 mmol, catalyst = 12 mg, water = 20 mL).

3.3. Effect of Cu₂O shape on catalysis for ammonia borane hydrolysis

Fig. 6 shows SEM images of Cu₂O particles with different shapes prepared by the reported method.³⁹ The particles were prepared with D-(+)-glucose as a capping reagent. The size of Cu₂O with the shape of 50-facets [Cu₂O(50-facets)] is nearly 2 μm (Fig. 6a). As indicated in Fig. 6b, the size of cube-shaped Cu₂O [Cu₂O(cube)] was 500 nm (vertex–vertex). The Cu₂O particles with octahedron shape [Cu₂O(octahedron)] have 0.5–0.7 μm side length. The Cu₂O particles with rhombicuboctahedron shape [Cu₂O(RCO)] showed 0.8–1 μm. The crystal phase of these Cu₂O particles was determined by powder X-ray diffraction patterns. The diffraction peaks at 2θ = 29.6°, 36.5°, 42.3°, 61.4°, 73.6° and 77.4° are indexed to (110), (111), (200), (220), (311) and (222) planes, respectively, by comparison with reported values.³⁹

Fig. 6 SEM images of Cu₂O particles with the shape of (a) 50-facets, (b) cube, (c) octahedron and (d) rhombicuboctahedron. A typical powder X-ray diffraction pattern achieved for Cu₂O particles. The numbers in parenthesis are (hkl) indexes.

Cu₂O particles with different shapes of 50-facets, cube, octahedron and RCO were decorated with Co₃O₄ rods (vide infra)⁴⁵ with the ratio of Cu₂O/Co₃O₄ = 1/5. The hydrogen-evolution rates normalized by weights of Cu₂O–Co₃O₄ or surface areas of Cu₂O are summarized in Table 1. Specific surface areas were calculated from the average sizes and distributions of nanoparticles determined by SEM measurements. The fastest hydrogen-evolution rate normalized by catalyst weight was obtained for Cu₂O(cube)–5Co₃O₄ as 0.78 mmol s⁻¹ g_cat⁻¹. The hydrogen-evolution rate for Cu₂O(RCO)–5Co₃O₄ was 0.67 mmol s⁻¹g_cat⁻¹, which is higher than the rate for Cu₂O(50-facets)–5Co₃O₄ of 0.56 mmol s⁻¹ g_cat⁻¹. On the other hand, the hydrogen-evolution rate normalized per surface area of Cu₂O was the largest at Cu₂O(50-facets)–5Co₃O₄ as 9.1 mmol s⁻¹m_Cu2O⁻², which is more than two times larger than the rate of 3.6 mmol s⁻¹m_Cu2O⁻² with Cu₂O(RCO)–5Co₃O₄. The surface of Cu₂O(50-facets) consists of (100), (110), (111) and (311) planes. Each plane occupies 14.7%, 39.1%, 11.3% and 34.9% of the total surface area, respectively. The surface of Cu₂O(RCO) is composed of (100), (110) and (111) surfaces, where the (311) plane is excluded. Also, the surfaces of Cu₂O(cube) and Cu₂O(octahedron) are exclusively composed of the (100) surface and the (111) surface, respectively. Thus, the high catalytic reactivity of Cu₂O(50-facets) normalized by the surface area of Cu₂O resulted from the (311) plane exclusively included in the surface of the Cu₂O(50-facets). Such high reactivity of the (311) plane of Cu₂O has been reported for CO oxidation³⁹ and predicted from theoretical calculations.⁴⁴

Table 1 Hydrogen-evolution rates by AB hydrolysis with shape-controlled Cu₂O–5Co₃O₄ particles normalized by catalyst weight or surface area of Cu₂O^a

Shape	Hydrogen-evolution rate		Cu2O surface area/m^2 g^−1
	mmol s^−1gcat^−1	mmol s^−1mCu2O^−2
a Reaction conditions [Cu2O/Co3O4 = 1/5 (w/w); NH3BH3, 0.5 mmol; catalyst, 12 mg; water, 20 mL] (time course of hydrogen evolution is displayed in Fig. S4 in the ESI†). b Rhombicuboctahedron.
50-Facets	0.56	9.1	0.37
Cube	0.78	2.1	2.2
Octahedron	0.44	1.1	2.3
RCO ^b	0.67	3.6	1.1

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3.4. Effect of Co₃O₄ shape on catalysis for NH₃–BH₃ hydrolysis

The shape of Co₃O₄ nanoparticles can influence the catalytic reactivity of Cu₂O–Co₃O₄ composites by changing interaction between Cu₂O and Co₃O₄. Co₃O₄ nanoparticles in the shape of a sphere, a hexagonal plate and a porous rod were used for decorating Cu₂O(50-facets). These shape-controlled Co₃O₄ nanoparticles were synthesized by reported methods^40–42 and their sizes and shapes were determined by TEM observation as shown in Fig. 7a–c. As shown in Fig. 7a, the sizes of spherical particles of Co₃O₄ [Co₃O₄(sphere)] are in the range between 2 and 6 nm with an average size of 4 nm. Co₃O₄ nanoplates [Co₃O₄(plate)] have hexagonal shape and the sizes of the particles are in the range between 500 nm and 1 μm (vertex to vertex) with the thickness of 10–25 nm (Fig. 7b). Co₃O₄ nanoparticles with rod shape [Co₃O₄(rod)] have a porous structure and the size is 50–250 nm in width and 0.5–10 μm in length Fig. 7c.

Fig. 7 TEM images of Co₃O₄ nanoparticles with the shape of (a) sphere, (b) nanoplate and (c) rod. (d) SEM image of Cu₂O(50-facets)–Co₃O₄(rod).

Calculated amounts of the size- and/or shape-controlled Co₃O₄ nanoparticles and Cu₂O particles with the shape of 50-facets were mixed in water following sonication and dried at 353 K. The weight ratios of Cu₂O/Co₃O₄ were selected from 1/5, 1/1 and 5/1. A typical SEM image of a composite of Cu₂O(50-facets)–Co₃O₄(rod) is displayed in Fig. 7d. The image clearly indicates that the diameter of the rod as well as the thickness of the nanoplate (Fig. S3 in the ESI†) are critical parameters for the separation of Cu₂O particles. No clear SEM image was obtained for Cu₂O(50-facets)–Co₃O₄(sphere) because Co₃O₄(sphere) is too small (2–6 nm) to be observed by SEM. Fig. 8 indicates the time courses of hydrogen evolution with Cu₂O–Co₃O₄ composites. The catalytic tests were repeated three times for each catalyst. Fig. 8a indicates the time course of hydrogen evolution by AB (0.5 mmol) hydrolysis with 5Cu₂O–Co₃O₄ (12 mg) where the shapes of Co₃O₄ nanoparticles were sphere (black), nanoplate (red) or rod (blue). The volume of evolved hydrogen reached 1.44 mmol for the 2^nd and 3^rd cycles, which is 96% of the stoichiometric amount, when 5Cu₂O(50-facets)–Co₃O₄(plate) and 5Cu₂O(50-facets)–Co₃O₄(rod) were used as catalysts. The slightly lower yield of the 1^st cycle is due to the use of formed hydrogen for the reduction of surface of Cu₂O–Co₃O₄ composites. On the other hand, when 5Cu₂O(50-facets)–Co₃O₄(sphere) was used as a catalyst, the amount of evolved hydrogen was 1.2, 1.3 and 1.2 mmol for the 1^st, 2^nd and 3^rd cycle, respectively. The lower hydrogen yields, especially at the 3^rd cycle, indicate that Cu₂O(50-facets) starts to agglomerate because Co₃O₄(sphere) is too small to protect large Cu₂O particles. The large Co₃O₄(plate) and Co₃O₄(rod) particles effectively separate each Cu₂O particle not to form agglomerates, leading to high hydrogen yields even at the 2^nd and 3^rd cycles. When the ratio of Co₃O₄ increased to Cu₂O/Co₃O₄ = 1/5, the catalytic reactivity of Cu₂O(50-facets)–5Co₃O₄(sphere) was improved to the same level of Cu₂O(50-facets)–5Co₃O₄(plate) and Cu₂O(50-facets)–5Co₃O₄(rod) in terms of hydrogen-evolution rates and hydrogen yields as shown in Fig. 8b. By increasing the relative amount of Co₃O₄, even smaller particles of Co₃O₄(sphere) can protect Cu₂O particles from agglomeration.

Fig. 8 Time course of hydrogen evolution by AB hydrolysis with Cu₂O particles (50-facets) partially covered with shape-controlled Co₃O₄ (black, sphere; red, plate; blue, rod) at room temperature (NH₃BH₃, 0.5 mmol; catalyst, 12 mg; water, 20 mL). The ratios of Cu₂O/Co₃O₄ (w/w) were (a) 5/1 and (b) 1/5. The data for Cu₂O/Co₃O₄ (1/1) are shown in Fig. S5 in the ESI†.

The reactivity data shown in Fig. 8 provided a clue to discuss active species where the same Cu₂O particles were used for the preparation of Cu₂O–Co₃O₄ composites. Co₃O₄(sphere) is as small as 4 nm, whereas Co₃O₄(plate) and Co₃O₄(rod) are much larger (see Fig. 2a and 7). If the actual active species for the reaction were Co species supported on Cu₂O, Cu₂O with smaller Co₃O₄(sphere) should have shown much higher reactivity than Cu₂O with larger Co₃O₄(plate or rod), because smaller Co₃O₄ particles have higher surface area and can more readily interact with Cu₂O particles. However, as indicated in Fig. 8a, 5Cu₂O–Co₃O₄(sphere, black) showed slower H₂ evolution rate than 5Cu₂O–Co₃O₄(rod, blue, and nanoplate, red). Additionally, as shown in Fig. 8b and S5†, Cu₂O–Co₃O₄ composites containing a higher amount of Co₃O₄ showed similar reactivity without Co₃O₄ shape dependence, which was observed for Co₃O₄ catalysts in AB hydrolysis with NaBH₄.³⁶ From Table 1 and Fig. S4†, the catalytic reactivity of Cu₂O–5Co₃O₄ composites depends on the shape of Cu₂O. Thus, Cu₂O particles play an important role to determine the catalytic performance of current Cu₂O–Co₃O₄ composites under the current reaction conditions although Co species may have a certain contribution to the catalytic activity of Cu₂O–Co₃O₄ composites.

The catalytic reactivity examined for a physical mixture of Cu₂O(50-facets) and Co₃O₄(rod) was similar to that of Cu₂O(50-facets)–Co₃O₄(rod) composites. This indicates that the particles of Cu₂O and Co₃O₄ form composites without any special operations under reaction conditions. Under neutral conditions, Cu₂O weakly interacted with Co₃O₄ by electrostatic interactions. Alternatively under reaction conditions, the interaction among Cu₂O particles and Co₃O₄ nanoparticles becomes stronger because some of the surface oxygen atoms of Cu₂O are reducibly removed to form oxygen vacant sites, which can interact with the surface oxygen of Co₃O₄ or Co(OH)₂ species.

Conclusions

Agglomeration of Cu₂O particles under reduced conditions was effectively suppressed by the partial coverage with Co₃O₄ nanoparticles to exhibit the high catalytic reactivity for hydrogen evolution by ammonia borane (AB) hydrolysis. The novel method to protect Cu₂O particles from agglomeration enables us to examine the shape effect of Cu₂O particles on their catalysis in AB hydrolysis under highly reducing conditions for the first time. Among four Cu₂O particles with the shape of 50-facets, cube, octahedron and rhombicuboctahedron, Cu₂O with the shape of 50-facets showed the highest specific reactivity per surface area. This may be ascribed to the higher surface energy of a high-index plane of (311) than those of low index surfaces. Additionally, the conditions to use small sized Co₃O₄ nanoparticles for protecting Cu₂O particles were clarified. When small Co₃O₄ particles were used for protecting large Cu₂O particles, a high Co₃O₄ ratio in the Cu₂O/Co₃O₄ composite was required to achieve sufficient effect for protecting Cu₂O particles from agglomeration. The method to protect catalytic particles with smaller nanoparticles disclosed here for the first time provides an opportunity for metal particles, which easily agglomerate under harsh reaction conditions, to be utilized as durable catalysts.

Acknowledgements

This work was supported by a Grant-in-Aid (no. 20108010), and a Global COE program, “the Global Education and Research Centre for Bio-Environmental Chemistry” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (to S.F.), NRF/MEST of Korea through WCU (R31-2008-000-10010-0) and GRL (2010-00353) Programs (to S.F.). Y.Y. acknowledges the financial support from the Ogasawara Foundation for the Promotion of Science and Engineering. We sincerely acknowledge the Research Center for Ultra-Precision Science & Technology for TEM measurements.

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45. Co₃O₄ with the rod shape was used for shape-controlled Cu₂O particles protection because the catalytic reactivity of Cu₂O–Co₃O₄ (rod) was slightly higher than Cu₂O–Co₃O₄ (plate) as shown in Fig. 8.

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Footnote

† Electronic supplementary information (ESI) available: Powder XRD pattern after the reaction (Fig. S1), XPS data of Cu₂O@SiO₂ (Fig. S2), SEM images of Cu₂O–Co₃O₄ composites (Fig. S3) and time course of hydrogen evolution (Fig. S4 and S5). See DOI: 10.1039/c1ee02639a

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