Remarkable Enhancement in Au Catalytic Utilization for Liquid Redox

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Remarkable Enhancement in Au Catalytic Utilization for Liquid Redox Reactions by Galvanic Deposition of Au on Cu Nanoparticles Dan Zhao,* Xiang Xiong, Chang-Lin Qu, and Ning Zhang* Department of Chemistry, Nanchang University, Nanchang, Jiangxi 330031,China ABSTRACT: Aum∧Cu nanostructured particles were prepared by galvanic deposition of Au on Cu nanoparticles (ca. 5.2 nm) and loaded on carbon as catalysts for liquid redox reactions, the reduction of 4nitrophenol and the epoxidation of styrene. By properly manipulating the Au/Cu ratio (m = 0.01) of the sample, it was demonstrated by UV− vis, HRTEM, and XRD characterizations that reduced Au could exist as unalloyed and highly dispersed depositions in monatomic or subnanoscale thickness on the surface of Cu nanoparticles in Aum∧Cu nanostructures. The mass-specific activity of Au in Aum∧Cu/C catalysts was enhanced by up to an order of magnitude for the reduction of p-nitrophenol and 40× for the epoxidation of styrene in comparison with pure Au catalyst, respectively, indicating a remarkable enhancement in Au catalytic utilization for liquid redox reactions on Aum∧Cu nanostructured catalyst. The finding implies that depositing of active precious metal component on other metal nanoparticles could be a novel and important way to prepare cost-saving precious metal nanostructures for thermal catalysis in many key technologies.

1. INTRODUCTION Gold nanostructures have been recognized as surprisingly active and extraordinary effective green catalysts for numerous novel and unprecedented reactions. Apart from the early discovery of the ultra low-temperature activity of Au nanoparticles (Au NPs) supported by metal oxides for oxidizing CO to CO2,1,2 the unique property of nanostructured Au catalyst has been also found in the frontier between homogeneous and heterogeneous catalysis such as the activation of dihydrogen, epoxides, alcohols, carbonyl compounds, alkynes, hydrosilanes, boron hydrides, as well as on CO2 fixation, C−C cross-coupling reactions, hydrogen transfer catalysis, and so on.3 These findings predict a high potential of Au nanostructures being applied in industry fields involving above catalytic processes, however, the low utilization of Au in the state-of-art nanostructured catalysts would be a basic obstacle for the practical application of Au catalysts. For example, the most popular Au catalyst was in a form of about 2.0−5.0 nm Au particles supported on metal oxides, carbon, or polymers,4−10 which means only 50−20% of Au atoms in such catalysts are present on the surface of particles,11,12 that is, at least half of Au atoms will be hidden under the surface and be inaccessible to reactant or be not used in reaction. Considering that gold is a precious metal resource and priced by weight, improving the catalytic efficiency, especially the catalytic utilization or the mass-specific activity of Au, should be focused as a top priority for the practical applications involving of Au catalyst. In fact, the utilization of precious metal in heterogeneous catalysis has been a long and hot topic in many important chemical systems depending on precious metal catalyst such as fuel cell electrocatalysis system based on platinum catalyst.11−24 In our previous work, a metal composite nanostructure with Pt deposition on other metal nanoparticles has been demonstrated © 2014 American Chemical Society

to be a novel electrocatalyst with higher Pt utilization and massspecific activity compared to traditional pure Pt particles.11−16 For instance, by reductive deposition of Pt on Au nanoparticles or galvanic deposition of Pt on Ag nanoparticles, it was found that the electrocatalytic utilization of Pt could be enhanced to 90−100% and lead to the dramatic enhancements in Pt massspecific activity for HCOOH or CH3OH electrooxidation in comparison with pure Pt catalysts.15,16 These works indicated that deliberately depositing of active precious metal component on the surface of other metal nanoparticles (labeled as APM∧M nanostructure) would be an effective way to enhance the catalytic utilization of precious metal in heterogeneous catalysis. However, up to our knowledge, the similar metal nanostructures have been seldom expanded into or developed for thermalcatalysis system. For eletrocatalysis system, the substrate metal particles for constructing of APM ∧ M nanostructure should be carefully chosen due to a well corruption-resistance against electrochemical conditions being demanded, unfortunately, only a few of but still expensive metals such as Ru, Au, and Ag can fulfill the demand,13−16 which limits the understanding and the application on such nanostructures. For thermalcatalysis system, obviously, such limits are not significant in contrast to eletrocatalysis system, implying that more and cheaper metals could be chosen as substrate metal to construct APM∧M nanostructures for thermal catalytic reactions, which could provide more chance to expand the knowledge on the nanostructure. More important, the property of metal−metal interface in APM∧M nanostructures and the consequent influence on their catalytic Received: January 26, 2014 Revised: July 25, 2014 Published: July 25, 2014 19007

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Table 1. Metal Composition and Kinetic Data of Catalysts rate constant (k/min−1)

metal loading (wt %) catalysts Cu/C Au0.01∧Cu/C Au0.06∧Cu/C Au0.13∧Cu/C Au0.1Cu/C Au/C Au-s/Cf

Cu 0.86 0.81 1.04 1.22 0.93

Au 0.02 0.20 0.48 0.29 0.50 0.22

Red-p-NPh 0.18 0.28 0.39 0.35 0.22 1.28 0.44

a

Epo-Sty. 1.56 8.32 6.77 3.62 6.41 4.55 3.92

× × × × × × ×

reaction ratec (r/mol·L−1 min−1) b

−3

10 10−3 10−3 10−3 10−3 10−3 10−3

Red-p-NPh 1.6 2.5 3.2 3.0 2.0 7.2 3.6

× × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4 10−4

Epo-Sty. 9.6 4.9 4.1 2.2 3.8 2.8 2.3

× × × × × × ×

−4

10 10−3 10−3 10−3 10−3 10−3 10−3

MSAAud (mol L−1 min−1 gAu−1) Red-p-NPh e

1.9 130.3 16.2 6.3 6.9 14.0 11.4

Epo-Sty. 0.74e 163.3 13.6 3.1 8.7 3.7 6.3

a

Reduction of p-nitrophenol to p-aminophenol was simplified as Red-p-NPh, 10 mg of carbon-supported catalyst was used for the reaction. Epoxidation of styrene was simplified as Epo-Sty., 150 mg of carbon-supported catalyst was used for the reaction. cReaction rates (r) were calculated from the data at reaction time t = 1 min for Red-p-NPh and t = 10 min for Epo-Sty. dMass-specific activity of Au (MSAAu = r/mAu), was reaction rate normalized by Au mass. eThe data were measured by Cu mass. fAu-s/C was a supplementary reference pure Au catalyst prepared according to ref 12, with a Au particle size of about 1.9 nm. b

deposition to get Cu colloid solution for the preparation of Au∧Cu nanostuctured samples. Aum∧Cu samples (m presents Au/Cu atomic ratio) were synthesized from galvanic deposition of Au onto the Cu nanoparticles (reaction 1) by directly adding the desired amount (0.5−2.0 mL) of 5.0 × 10−2 M HAuCl4 aqueous solution into as-prepared Cu colloid solution at 70 °C with stirring for 1 h under the atmosphere of purified nitrogen. A total of 2 mL of Aum∧Cu colloid solution was removed for UV− vis and TEM characterizations, then 1 g Vulcan XC-72 carbon black was added into solution to carry Aum∧Cu particles, the mixture was intensively stirred for 4 h under 70 °C. The result precipitation was filtered and intensively washed by deionized water to remove excessive chemicals and unsupported colloid, then dried by vacuum drier to obtain Aum∧Cu/C catalyst. The loadings of Au and Cu in Aum∧Cu/C catalysts were measured by ICP-AES, as listed in Table 1. According to relative loading of Au and Cu on carbon, the Au/Cu atomic ratio (m) of three Aum∧Cu/C catalysts were resolved as 0.01, 0.06, and 0.13, respectively. For reference, monometallic Au nanoparticles were prepared by NaBH4 reduction of HAuCl4 with citrate as protector at 30 °C for 30 min, the method is chosen for obtaining small Au particles; a coreduced Au0.1Cu sample was prepared from coreduction of Cu2+ and AuCl4− by hydrazine hydrate according to similar procedure of obtaining Cu colloid. Cu, Au, and Au0.1Cu particles were also loaded on Vulcan XC-72 carbon black to get corresponding carbon-supported catalysts. The metal loadings in these samples were also determined by ICP-AES and listed in Table 1. 2.2. Characterizations. UV−vis spectra of colloid samples were recorded on a Hitachi ultraviolet visible and near infrared spectrophotometer-U-4100, operated at a resolution of 0.2 nm. The sample solution was filled in a quartz cell of 1 cm light-path length, and the light absorption spectra was given in reference to deionized water. The morphology and size distribution of particles and carbon supported catalysts were characterized by trans-mission electron microscopy (TEM) using a JEOL JEM-2100 microscope operating at 200 kV. The samples were prepared by placing a drop of the colloidal solution or catalyst powder dispersion in an hydrous ethanol on a Formvar/carbon film coated Cu grid (3 mm, 400 mesh), followed by drying under ambient conditions. The actual loadings of metal components in catalysts were determined by ICP-AES on OPTIMA 5300DV (PerkinElmer).

performance have been recognized as a unique catalytic feature of such nanostructure being different from that of metalnonmetal interface or alloyed metal interface in traditional metal catalysts,15,16,21−24 which could offer the possibility to innovate more efficient precious metal catalyst. 2AuCl4 − + 3Cu → 2Au + 3Cu 2 + + 8Cl−

(1)

In this work, Aum∧Cu nanostructures (m presents Au/Cu atomic ratio) were successfully prepared via the galvanic replacement reaction (demonstrated as reaction 1) between AuCl4− and the surface of Cu nanoparticles (Cu NPs), and corresponding carbon-supported samples were investigated as catalysts for two thermal chemical probe reactions, the reduction of p-nitrophenol and the epoxidation of styrene. It was demonstrated that the reaction rate normalized to Au mass, that is, the mass-specific activity of Au on Au0.01∧Cu nanostructures could be enhanced by up to an order of magnitude for the reduction of p-nitrophenol and 40× for the epoxidation of styrene in comparison with pure Au catalyst, respectively, suggesting the remarkable enhancement in Au catalytic utilization for liquid redox reactions could be achieved on Au∧Cu nanostructured catalyst, this finding may have important implication for designed preparation of cost-saving precious metal nanostructures for thermal catalysis in many key technologies.

2. EXPERIMENTAL SECTION 2.1. Preparation of Samples (Cu NPs, Aum∧Cu and Referred Samples). Aqueous solution of Cu colloids was prepared by reducing CuSO4 with hydrazine hydrate according to ref 25 with some modifications. Briefly, Polyvinylpyrrolidone (PVP, 0.09 × 10−3 mol, Mw = 24000) and cetyltrimethylammonium bromide (CTAB, 3.0 × 10−3 mol) were added to 40 mL of 2.0 × 10−2 M aqueous solution of CuSO4 in a threenecked, round-bottom flask, oxygen in the atmosphere of flask was excluded by a flow of purified nitrogen during the whole preparation of Cu nanoparticles and Au∧Cu nanostuctured samples. The pH of the solution was adjusted to 10 with ammonia (NH3·H2O) solution, and the solution was heated to 50 °C, being stirred for 2 h. Then, 20 mL of 3.0 × 10−1 M aqueous solution of hydrazine hydrate was dropped slowly into the solution and the mixed solution was heated to 70 °C. The resulting mixture was stirred at 70 °C for 2 h until a red solution was obtained. Then the red solution were centrifuged at 12000 rpm for 10 min to separate out and discard the 19008

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X-ray diffraction (XRD) patterns of catalysts were measured with on a Persee XD-3 X-ray diffractometer at a scan rate of 2° min−1 in the angle (2θ) range of 10−90°. The wavelength of the incident radiation was 1.5406 Å (Cu Kα). 2.3. Catalytic Measurements of Catalysts. The catalytic properties of the prepared catalysts were investigated by two liquid redox probe reactions, the reduction of p-nitrophenol (pNPh) (to p-aminophenol (p-APh)) and the epoxidation of styrene. For the reduction of p-NPh, NaBH4 was used as H resource and molar ratio of p-NPh:NaBH4 was kept at 1:100 for all catalytic measurements. Considering that a small amount of Au is sufficient for catalytic reduction of p-NPh,26 in our case, 10 mg of carbon-supported catalyst (the real mass of Au present in the reaction was determined as the product of 10 mg with respective Au loading of carbon-supported samples listed in Table 1) was dispersed in 30 mL of freshly prepared aqueous solution of NaBH4 (1.0 × 10−1 M) and p-nitrophenol (1.0 × 10−3 M), the solution was stirred at 30 °C. The reaction progress can be monitored by UV−vis signal change of reaction mixture with time since the extinction of p-nitrophenol in UV− vis spectra is proportional to its concentration in the reaction solution.26 In contrast to the reduction of p-NPh, the epoxidation of styrene over our catalysts was found to be a slow process, and a relatively large amount of 150 mg carbon-supported catalyst was used for the reaction. Typically, the reaction was carried out in a glass reactor by adding 150 mg of carbon-supported catalyst into 10 mL acetonitrile solution containing styrene (1.0 × 10−2 mol) and tert-butyl hydroperoxide (TBHP 70% in water, 2.6 × 10−2 mol). Also, the real mass of metal components present in the reaction can be determined by the product of 150 mg with the metal loading of catalyst listed in Table 1. Then the mixture was stirred under reflux at 80 °C for a desired period. After the reaction, the reaction mixture was cooled to room temperature and filtered to separate the solid catalyst. The filtered reaction solution was analyzed by a gas chromatograph (Agilent Technologies 7890B GC system) with a capillary column (TM-1). The quantities of styrene substrate consumed and products formed were determined from the results obtained from authentic standards. The structure of the products was further confirmed with a gas chromatography− mass spectrometer (Agilent Technologies 7890A GC/MSD system).

Figure 1. UV−vis spectra of colloid samples.

TEM images of samples in Figure 2 showed the morphologic feature of unsupported and carbon-supported metal particles. As-prepared Cu nanoparticles were in a size range of 5.2 ± 0.2 nm (Figure2a), and Au nanoparticles were in a size range of 2.6 ± 0.3 nm (Figure2e). The size ranges of three Aum∧Cu samples (5.1 ± 0.4, 5.3 ± 0.5, and 5.5 ± 0.3 nm in Figure2b−d) were all close to that of starting Cu nanoparticles, suggesting that the insignificant change of particle size present during the deposition of Au on Cu particles to form Au m ∧ Cu nanostructures. The phenomenon could mainly attribute to that the surface atoms of Cu nanaoparticles act as the reductant for reduction of AuCl4− (reaction 1). Although the size of the Au atom is 1.4× bigger than that of Cu atom,31 the mole of removed Cu2+ is 1.5× higher than that of deposited Au atoms according to the stoichiometric ratio of reaction 1; thus, the factor of decreasing size upon a relatively higher amount of removed Cu2+ would be near-equally compensated by the factor of increasing size from a relatively bigger size of deposited Au atom during the process of Cu particles being “sculptured” and simultaneously “pasted” by Au atoms to form Aum∧Cu particles. Moreover, the difference in the average size of Aum∧Cu samples with different m was not more than 0.4 nm, even the amount of Au being increased by up to an order of magnitude (m varies from 0.01 to 0.13), suggesting that the agglomeration of reduced Au atoms themselves in Aum∧Cu nanostructures could not be significant, which also implies that Au could be highly dispersed as depositions in monatomic thickness or subnanoscale on Cu substrate particles since only Cu atoms on the surface layer can present and be replaced by Au atoms from the reaction 1. The overlooking and the corresponding size distribution of the particles supported on carbon were given as Figure2f−j (left), the corresponding HRTEM images of representative single particle were given at right section. In overview, for Cu/ C (5.5 ± 0.5 nm in Figure2f), Au0.01∧Cu/C (5.3 ± 0.6 nm in Figure2g) and Au/C (2.7 ± 0.8 nm in Figure2j), metal particles keep the size as similar as their corresponding unsupported samples; however, the average size of metal particles in Au0.06∧Cu/C and Au0.13∧Cu/C (6.4 ± 0.7 nm in Figure2h and 8.1 ± 2.5 nm in Figure2i) increased obviously in comparison with the unsupported particles. As illuminated by

3. RESULTS AND DISCUSSION The metal composition of samples were determined by ICPAES and listed in Table 1. Three Aum∧Cu/C samples were measured with m = 0.01, 0.06, and 0.13, respectively. Figure 1 shows the UV−vis spectra of colloid samples. Because of the surface plasmon resonance effect, Cu colloid usually exhibits absorption bands at around 570 nm27,28 and Au colloid exhibits absorption bands at around 520 nm.29,30 The observations of absorption peak on pure Cu and pure Au samples prepared in this work are in agreement with those in literatures, indicating monometallic nanoparticles were successively prepared by the procedure employed in this work. For the samples prepared from the galvanic replacement reaction between Cu nanoparticles and AuCl4−, a continued weakening of the Cu plasmon adsorption at around 570 nm with increasing Au amount in the preparation was observed, indicating a gradual covering of Au deposits on the surface of Cu colloids, that is, the formation of Aum∧Cu nanostructures. 19009

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Figure 2. TEM images of prepared samples: (a) Cu, (b) Au0.01∧Cu, (c) Au0.06∧Cu, (d) Au0.13∧Cu, (e) Au, (f) Cu/C, (g) Au0.01∧Cu/C, (h) Au0.06∧Cu/C, (i) Au0.13∧Cu/C, (j) Au/C.

JCPDS Card #65−2870), respectively. The coexistence of Cu[111] and Au[111] surface on same particle of Au0.01∧Cu/C further demonstrated that the Cu atoms on the surface of starting Cu nanoparticles were partly replaced by Au atoms via the corrosion-deposition route employed in this work. For Au0.06∧Cu/C (Figure2h, right), besides Au[111] d-spacing

the HRTEM images, the observed d-spacing of 0.211 nm on Cu/C (Figure2f, right) was approach to the standard d-spacing value of Cu[111] (0.208 nm from JCPDS Card #65−9026); in contrast to Cu/C, two kinds of d-spacing values of 0.208 and 0.232 nm observed on Au0.01∧Cu/C (Figure2g, right) could be attributed to Cu[111] and Au[111] d-spacing (0.235 nm from 19010

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(0.235 nm), d-spacings of 0.202 and 0.142 nm were also found, which could be attributed to Au[200] and Au[220] (0.204 and 0.144 nm from JCPDS Card #65−2870). For Au0.13∧Cu/C (Figure2i, right), Au[111] (d-spacing: 0.236 nm) and Au[200] (d-spacing: 0.204 nm) facets were also observed. Interestingly, [200] and [220] were two kinds of high-index facets, to obtain such facets bounded metal particles, opening of the structure of popular metal nanoparticles (bounded by low-index facets such as [110], [100], and [111]) by an intensive corruption procedure had been demonstrated to be a key step to expose high-index metal crystalline out of the body of particles.32,33 In our case, the deep corrosion of low-index surface of Cu nanoparticles by high amount of AuCl4− and the following deposition of Au atoms during the preparation of Aum∧Cu samples with m ≥ 0.06 could be responsible for the formation of Au[200] and Au[220] surface on Cu nanoparticles. Meanwhile, the existence of Au[200] and Au[220] surface in Aum∧Cu samples with m ≥ 0.06 would also exclude the solitary formation of monometallic Au particles in preparation of Aum∧Cu samples since pure Au particles would only show lowindex surface such as Au[111] in Au/C (Figure2j, right, dspacing: 0.240 nm) sample prepared in this work. In contrast to above features of corrosion-deposition route for preparing Aum∧Cu samples, the negative effect of the route should also be considered. Obviously increased particle size on Au0.06∧Cu/C and Au0.13∧Cu/C compared to their corresponding unsupported samples indicated that the sever agglomeration of particles present in the loading process of Aum∧Cu on carbon support. The situation could be mainly attributed to that the surfactant-protector layer (PVP or CTAB) of Cu nanoparticles was destroyed or removed with the corrosion of Cu atoms from the surface of Cu nanoparticles by AuCl4−. The more Cu atoms (by the more of deposition of Au) that were replaced, the more protectors were simultaneously removed. Such protector-lack influence seemed to be intensified during Aum∧Cu particles were concentrated to load on carbon, which lead to an agglomeration of particles especially for Aum∧Cu/C samples with high m of 0.06 and 0.13. Therefore, properly manipulating the relative amount of Au to Cu is crucial to obtain samples with highly dispersed Au, in our case, m = 0.01 was testified as an acceptable Au/Cu atomic ratio for Aum∧Cu/C catalyst. In addition, from HRTEM images, d-spacing of Au[111] on Au0.01∧Cu/C samples (0.232 nm), Au0.06∧Cu/C (0.235 nm), and Au0.13∧Cu/C (0.236 nm) samples were found to be very close to or same as the standard value of Au[111] (0.235 nm from JCPDS Card #65−2870), which is different from an obviously quenched d-spacing such as 0.227−0.229 nm observed on Au−Cu alloy sample,34 thus, the surface alloy structure could not be predominated on Aum∧Cu/C samples. To further discern the structure of Aum∧Cu/C samples, XRD patterns of carbon-supported catalysts were measured, as shown in Figure 3. Cu/C exhibits diffraction peaks at 43.3 and 50.4° (Figure 3a), being consistent with the [1 1 1] and [2 0 0] planes associated with Cu crystalline.35,36 Besides of the diffraction of carbon support (44.2°), only a weak and board diffraction peak at 38.2° associated with Au [111] facet can be resolved on Au/C (Figure 3b), which could be correlated with that Au were highly dispersed as small particles (2.7 ± 0.8 nm seen in Figure 2j) in the sample. In contrast to pure Au/C sample, the diffraction peaks at 38.2, 44.5 (overlapped with diffraction of carbon), 64.6, 77.5, and 81.7° associated with [1 1 1], [2 0 0], [2 2 0], [3 1 1], and [2 2 2] planes of Au crystalline37,38 exhibited on Aum∧Cu/C samples (Figure 3c−e),

Figure 3. XRD patterns of prepared samples: (a) Cu/C, (b) Au/C, (c) Au0.01∧Cu/C, (d) Au0.06∧Cu/C, (e) Au0.13∧Cu/C, (f) Au0.1Cu/C.

and the intensity of these peaks increased with the increase of Au amount (m) in samples. We carefully compared the 2θ degree of diffraction peaks observed on Aum∧Cu/C samples with that of standard or popular diffraction peaks of Au crystalline, Cu crystalline and some typical AuxCuy alloy from JCPDS Card or references,34,37−40 there is no significant signal correlated to AuCu alloy such as the diffraction shift of 2θ degree relative to pure Au or Cu crystalline can be resolved on Aum∧Cu/C, suggesting the AuCu alloyed structure is not pronounced on as-prepared Aum∧Cu nanoparticles. One may question the supposed coreduction of Cu2+ and AuCl4− during the formation process of Aum∧Cu particles would lead to an alloy structure of samples. To clarify the possibility, a reference catalyst Au0.1Cu/C with metal particles from a coreduction process (the process being accepted to get alloyed metal particles34,41) of AuCl4− and Cu2+ in similar solution conditions was prepared and measured by XRD technique. Figure 3f showed the XRD patterns of Au0.1Cu/C, besides the diffraction of carbon support, no obvious diffraction can be resolved on the sample, suggesting that even if coreduction process of AuCl4− and Cu2+ could present, it could not lead to a formation of Au−Cu alloy structure for Aum∧Cu samples. Notably, the increasing intensity of Au diffraction peaks with increasing m and the absence of Cu diffraction peaks were observed on Aum∧Cu/C samples. The phenomenon could be attributed to the negative effect of corrosion-deposition route for obtaining Aum∧Cu samples. As illustrated by above TEM measurements, the percentage of agglomerated particles was obviously increased to 20−75% after the loading processes on carbon support for obtaining Aum∧Cu/C catalyst. Such severe agglomeration could lead to obvious aggregation of Au atoms from different particles, especially for Au0.06∧Cu/C and Au0.13∧Cu/C samples, Au atoms on high-index Au[200] and Au[220] surfaces would aggregate more easily during the agglomeration of particles due to the high surface energy of such surfaces. These factors could be responsible for the growth in size of Au crystalline observed on Aum∧Cu/C catalysts. In addition, the high-index Au[200] and Au[220] surfaces on Aum∧Cu/C samples also implied a deep corrosion to Cu substrate (as discussion on HRTEM images), the corrosion could destroy the continuality of small Cu crystalline and result in the absence of Cu diffractions on Aum∧Cu/C samples. 19011

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Figure 4. Time-dependent UV−vis spectral changes of the mixture of p-NPh, NaBH4, and catalysts.

The catalytic performance of Aum∧Cu/C catalysts were tested through two liquid redox reactions: the reduction of pnitrophenol (p-NPh) to p-aminophenol (p-APh) by NaBH4 and the epoxidation of styrene by TBHP. For reduction of pnitrophenol, because extinctions of p-NPh and p-APh in UV− vis spectra are proportional to their concentration in the solution,26 the reaction progress can be monitored by UV−vis signal change of reaction mixture with time, as shown in Figure 4. The p-NPh was inert to NaBH4 and the reaction did not occur in the absence of catalyst even for a period of 24 h,26 the dash line for every sample was the signal of reaction mixture without catalysts. After adding of catalysts, in general, the quick decrease of p-NPh extinction at around 400 nm and the concomitant appearance and increase of p-APh extinction at around 300 nm present,26,42,43 indicating an apparent catalytic effect on samples. Meanwhile, the change in the concentration of p-NPh could be readily derived from its extinction change. Since the concentration of NaBH4 largely exceeds the concentration of p-NPh, the reduction rate can be assumed to be independent of NaBH4 concentration, in which a pseudofirst-order rate kinetics with regard to the p-NPh concentration was proposed to evaluate the catalytic rate.43 Figure 5 correlated ln(Ct/C0) with reaction time on samples, it was observed that the linear relation could be resolved on all samples, proving a pseudo-first-order rate kinetic dominates the reaction process on our samples. For epoxidation of styrene, the change of composition of reactant mixture with time was monitored by a gas chromatograph. On all of Au-contained samples, during whole reaction period, only two products, styrene epoxide and byproduct benzaldehyde, were found and their yields changed similarly versus reaction time, suggesting

Figure 5. Plot of ln(Ct/C0) vs reacton time derived from the reduction of p-NPh on Aum∧Cu/C and reference catalysts.

there were no significant differences in reaction kinetic routes among our samples. The reaction data were collected in Figure 6. Figure 6A,B was the functions of styrene conversion and styrene epoxide yield on time, respectively; from the concentration change of styrene in reactant mixture with time, the relation of ln(Ct/C0) versus reaction time was derived and shown in Figure 6C, the near linear relation on every sample indicated a pseudo-first-order rate kinetic for epox19012

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Figure 6. Kinetic curves for the epoxidation of styrene on Aum∧Cu/C and reference catalysts. (A) Conversion of styrene as a function of time; (B) Yield of styrene epoxide as a function of time; (C) The relation of ln(Ct/C0) vs reaction time; (D) Conversion of styrene as a function of time derived from the reaction mixtures with (a) Au0.01∧Cu/C-present and (b) Au0.01∧Cu/C-absent (Au0.01∧Cu/C catalyst was separated out of reaction mixture after 60 min of reaction period).

result liquid mixture was analyzed by ICP-AES and there was insignificant amount of metal components found, therefore, the leaching of metal from Aum∧Cu/C catalysts in reaction could be ignored in this work. Further proof was also provided by the catalytic performance of reused Aum∧Cu/C catalyst. Figure 7 showed the conversion of styrene and yield of epoxide at 30 and 60 min on reused Au0.01∧Cu/C catalyst. Due to severe loss of the solid substance during the separating process, three catalytic runs based on collected catalysts were carried after the fresh catalyst was used. In comparison with fresh catalyst, the slight fluctuation or decrease in conversion of styrene and yield of epoxide were found in following catalytic runs on reused catalysts, suggesting that Aum∧Cu/C samples in solid state acted as catalysts and can be reused for the reaction. To clarify the catalytic utilization of Au of the samples, the rate constant, reaction rate and the mass-specific activity of Au (MSAAu) for two reactions on every sample were derived from the corresponding kinetic plots in Figures 5 and 6C, respectively, all of data were also collected in Table 1. In

idation of styrene was also present on catalysts prepared in the work. For metal-supported catalysts used for liquid reaction, the leaching of metal from the solid to the liquid phase or the leached metal ions acting as catalysts are important issues especially for oxidation reaction. Figure 6D compared the functions of styrene conversion on time between (a) Au0.01∧Cu/C being present in whole reaction period and (b) Au0.01∧Cu/C being absent (removed) after 60 min of reaction period. Two curves showed nearly overlapped change during 60 min of reaction period, however, in contrast to continuous increase in styrene conversion observed in Au0.01∧Cu/C-present mixture, near-unchanged styrene conversion after 60 min of reaction period was observed in Au0.01∧Cu/C-removed mixture. The result indicated that even if the leached metal species could present in reaction liquid mixture, such species would not act as the catalyst for the reaction. In addition, the reaction was repeated by employing fresh Au0.13∧Cu/C catalysts for several times and all the liquid mixtures after reaction had been collected together and concentrated by evaporation, then the 19013

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Figure 7. Catalytic performance of reused Au0.01∧Cu/C catalyst for the epoxidation of styrene. (A) Conversion of styrene; (B) Yield of styrene epoxide.

which, the reduction of p-nitrophenol was simply labeled as Red-p-Nph and the epoxidation of styrene as Epo-Sty. For the reduction of p-nitrophenol, reaction rate was adopted as the instant rate at reaction time of 1 minitue; for epoxidation of styrene, reaction rate was the instant rate at reaction time of 10 minutes. For both of reactions, the data of MSAAu of Au containing samples were obtained by normalizing of the corresponding reaction rate to the mass of Au used in the reaction, for Cu/C sample the mass-specific activity was normalized to the mass of Cu as MSACu. It can be found that the MSACu of pure Cu/C (1.9 mol L−1 min−1 gCu−1 and 0.74 mol L−1 min−1 gCu−1) is not more than one-seventh and one-fifth of MSAAu of pure Au/C (14.0 mol L−1 min−1 gAu−1 and 3.7 mol L−1 min−1 gAu−1) for two reactions, respectively, indicating the activity of Cu was trivial relative to that of Au for both reactions. The result also suggested that the contribution on the catalytic performance from Cu could be ignored in comparison with that from Au in Aum∧Cu nanostructures. Obviously, MSAAu on Aum∧Cu/C catalysts gradually increased with decreasing m of samples, the enhancements in MSAAu compared to pure Au/C for both reactions were observed on Aum∧Cu/C catalysts with m ≤ 0.06, especially on Au0.01∧Cu/C, MSAAu was enhanced by nearly up to an order of magnitude for the reduction of p-nitrophenaol (130.3 mol L−1 min−1 gAu−1) and at least 40× for the epoxidation of styrene (163.3 mol L−1 min−1 gAu−1) in comparison with pure Au/C sample, respectively. As the advice of a referrer that the lower loading and the smaller size of supported Au catalyst would also govern its catalytic properties, the other one pure Au catalyst with Au particle size of about 1.9 nm and Au loading of 0.22 wt % was prepared according to ref 12 and employed as a supplementary reference catalyst (labeled as Au-s/C), the catalytic data of Aus/C for two reactions were also listed in the last row of Table 1. Although the size and the loading of Au-s/C are all smaller than those of Au/C, the similar magnitude of MSAAu for both reactions were observed on two pure Au catalysts, suggesting that the decrease in Au particle size or Au loading was not the key factor to remarkably enhance the activity of pure Au/C catalysts. Meanwhile, MSAAu of Au0.06∧Cu/C for either reaction was higher than that of Au-s/C although Au loadings in two

samples are in the same low-level. These results indicated that depositing of Au on Cu nanoparticles to prepare Aum∧Cu nanostructures could remarkably enhance the catalytic utilization of Au for these two liquid redox reactions. Some features of nanostructured Au catalysts such as the valence distribution of Au and interaction of Au with support were believed to play important role in a redox reaction such as CO oxidation and aerobic oxidation,3 however, these features could be excluded from Aum∧Cu nanostructures since Au was reductively deposited on the metallic surface of underlying Cu nanoparticles, and the dispersion state of Au could be an important clue to understand the catalytic performance of Au in such structures. As illustrated by TEM measurements, Au could be highly dispersed as depositions in monatomic or subnanoscale thickness on Cu particles in Au0.01∧Cu nanostructures, such high dispersion ensured that most of Au atoms present in the samples could be accessible to reactant, which results in a high catalytic utilization of Au. However, for Aum∧Cu samples with higher m of 0.06 and 0.13, particles unfortunately agglomerated severely after they were supported on carbon to make catalysts, resulting the obvious decrease in Au dispersion and relatively lower MSAAu compared to slightly agglomerated Au0.01∧Cu/C. One referrer mentioned that the MSAAu of Au0.13∧Cu/C was not better than that of pure Au/C catalysts for both reactions; also, the result could be attributed to the severe agglomeration of metal particles and low dispersion of Au in the sample. In addition, one might question that the interaction between Au and Cu could be a crucial factor for the enhanced MSAAu observed on Aum∧Cu/C samples. Here, we provided a reference bimetallic Au0.1Cu/C catalyst prepared from a coreduction process of metal ions, it is believed that the interaction between metal components in such bimetallic particles governed their catalytic properties.44,45 It can be found that MSAAu of Au0.1Cu/C was inferior to that of pure Au/C for the reduction of p-nitrophenol; although MSAAu of the sample for the epoxidation of styrene was two times higher than that of pure Au/C and Au0.13∧Cu/C, which was still far lower than MSAAu of Au0.06∧Cu/C and Au0.01∧Cu/C. These results indicated that the interaction between Au and Cu was not the domination factor to enhance the MSAAu for the two liquid 19014

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redox probe reactions. Moreover, XRD measurements proved that there was no obvious alloyed Au−Cu structure present in Aum∧Cu nanostructures; thus, the function of interaction between Au and Cu on the MSAAu of Aum∧Cu nanostructured catalysts was not prominent according to our present characterization results.

4. CONCLUSIONS Aum∧Cu nanostructures with highly dispersed Au were prepared from galvanic deposition of Au on Cu nanoparticles, by properly manipulating the Au/Cu ratio (m = 0.01) of the sample, the mass-specific activity of Au in AumCu nanostructures was enhanced by up to an order of magnitude for the reduction of p-nitrophenol and 40 times for the epoxidation of styrene in comparison with pure Au catalyst, respectively. Such remarkable enhancement in Au catalytic utilization for liquid redox reactions obtained on AumCu nanostructured catalysts could be an important clue for designed preparation of costsaving precious metal nanostructures for thermal catalysis in many key technologies.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +86-791-83969332. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (NSFC, No. 21003071) and Doctoral Fund of Ministry of Education of China (No. 20093601120007).

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