Highly active and stable Au@Cu_xO core–shell nanoparticles supported on alumina for carbon monoxide oxidation at low temperature

Abstract

Au@Cu_xO (x = 1 or 2) core–shell heterostructure nanoparticles (NPs) are synthesized by a facile aqueous solution approach. γ-Alumina support Au@Cu_xO (Au@Cu_xO/γ-Al₂O₃) catalysts used for CO oxidation at low temperature are prepared by dispersing the core–shell NPs on the surface of γ-alumina. It proves that the formation of the core–shell structure has led to the interaction between Au and Cu_xO and the co-existence of Au^δ+ and Au⁰, which accounts for the improvement in catalytic activity. It can even catalyze CO oxidation at room temperature and the conversion of CO can reach to 38%. In the same case, Au/γ-Al₂O₃ does not have any catalytic activity. In particular, the embedding of Au NPs into Cu_xO shells has also improved the catalytic stability of Au based catalysts remarkably, which remains unchanged after 108 hours of reaction. To the best of our knowledge, Au@Cu_xO/γ-Al₂O₃ is really one of the most stable catalysts with adequate activity at room temperature.

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Introduction

More and more studies and applications that focus on Au NPs as catalysts have been reported since the earliest report on the outstanding catalytic activity for oxidizing CO at low temperature by Haruta in 1987.¹ CO oxidation on Au based catalysts at room temperature generally proceeds by following the size-dependent reaction pathways^2,3 and Au NPs tend to sinter into larger particles^4–6 or change to bicarbonate-like species under the reaction conditions^6–13 and thus leads to a substantial loss of activity,¹⁴ which is a major drawback of Au based catalysts and this undesirable characteristic has reduced their potential for industrial applications.^8,15

To date, several strategies have been proposed to solve the deactivation problem of Au based catalysts, which includes (1) the replacement of Au particles with alloy or bimetallic particles,^16–18 (2) the modification of the catalysts with other additives^19,20 and (3) the fabrication of core–shell type structure particles using Au as the core.^21–24 Actually, the first two methods can really improve the catalytic stability to some extent, but cannot completely avoid the deactivation caused by sintering or poisoning. Compared with the first 2 methods, the encapsulation of Au NPs as a core inside an oxide shell is more practical to maintain the size and shape of Au NPs as well as to protect them from poisoning, which leads to the stability and compatibility enhancement of Au based catalysts.²⁵

Among the various types of core–shell NPs, metal@semiconductor particles represent a significant class of core–shell structures. The semiconductor shell plays both the role of the masker to protect the metal particles from sintering and poisoning and the role of additives is to improve the physical and chemical activities of the metal particles by interacting with them within nanoscale.²⁶ It has been extensively demonstrated that the strong metal-support interaction (SMSI) plays a significant role in affecting catalytic performance.^27–34 However, the SMSI between Au and support cannot normally exist because Au has a lower work function and surface energy.^35,36 The encapsulation of Au NPs by metal oxides can strengthen the SMSI and thus lead to the electron transfer between Au and the support.³⁷

Cu₂O is an important p-type semiconductor with a direct band gap of 2.17 eV, which could find practical applications in many aspects such as solar energy conversion,³⁸ hydrogen production by photocatalytic decomposition of water,^39,40 and lithium-ion batteries as electrode materials.⁴¹ It is also a promising alternative to expensive noble metals as the catalyst for CO oxidation at moderate temperatures, owing to its higher natural abundance and lower price. It has been reported that Cu₂O layers can grow epitaxially on the surface of Au–Cu particles with small lattice mismatch and the CO oxidation reactions can occur on the Cu₂O–Au interfaces.⁴² CuO is also a good catalyst for CO oxidation.⁴³ Consequently, Cu_xO (x = 1 or 2) is a reasonable choice as the shell material to improve the catalytic performance of Au NPs.

Herein, we report an entirely new design of Au based catalysts for CO oxidation at low temperatures. Instead of pure Au particles, Au@Cu_xO core–shell NPs are fabricated and dispersed on the surface of γ-alumina and a new type of CO oxidation catalyst (Au@Cu_xO/γ-Al₂O₃) is synthesized. By coating Au NPs with Cu_xO, both the catalytic activity and stability of Au particles are improved. It can not only catalyze CO oxidation at room temperature with 38% CO conversion, but it can also maintain its activity. The stability is tested for 108 hours and after the 108 hours' of application, the catalytic activity did not decrease. As a comparison, the general Au based catalyst without Cu_xO has no catalytic activity at room temperature.

Experimental section

Materials preparation

Preparation of Au NPs. Au NPs were synthesized following the previously reported traditional Frens's citrate reduction synthesis method. Briefly, 150 mL of aqueous HAuCl₄ (0.25 mM) was prepared in a 250 mL beaker and brought to the boil in a 100 °C water bath under magnetic stirring. Then, 7.5 mL of 0.1 M sodium citrate solution was added rapidly. When the reaction mixture began to change color and finally reached a red wine color, it signalled the end of the reaction. The solution was stirred for 30 min, cooled to room temperature, and then Au colloids were collected by centrifugation, washed with anhydrous ethanol and water for 3 times, and finally redispersed in 7 mL of water.

Preparation of Au@Cu_xO core–shell NPs. Au@Cu_xO was prepared using the method reported by Zhang et al.²⁶ 1.0 g of polyvinylpyrrolidone (PVP, average M.W. 40 000, TCI Shanghai) was dissolved in 50 mL of 0.01 M Cu(NO₃)₂ (Sinopharm Chemical Reagent Co., Ltd.) aqueous solution under constant magnetic stirring and stirred for another 10 min to ensure that the PVP solution was uniform. Subsequently, 17 μL of N₂H₄·H₂O solution (85 wt%, Sinopharm Chemical Reagent Co., Ltd.) was immediately introduced into the reaction mixtures after 6 mL of fresh prepared Au colloid solution was added. The colloidal solution turned dark green within 10 s, which is the characteristic color of Au@Cu_xO core–shell NPs typically. The reaction liquid was kept stirring for 2 min, and then the Au@Cu_xO core–shell NPs were washed with anhydrous ethanol and water 3 times and finally redispersed in anhydrous ethanol and kept in a freezer at 6 °C.

Preparation of γ-Al₂O₃. The amorphous Al₂O₃ (Sinopharm Chemical Reagent Co., Ltd.) was calcined from room temperature to 550 °C with a heating rate of 5 °C min⁻¹ in air and maintained for 6 hours and then cooled to room temperature naturally.

Preparation of Au@Cu_xO/γ-Al₂O₃. Au@Cu_xO ethanol solution was taken in a beaker with a certain amount of γ-Al₂O₃ and stirred at room temperature until ethanol is entirely volatilized. Then, Au@Cu_xO/γ-Al₂O₃ was produced. Au contents were measured by the Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES).

Preparation of Au/γ-Al₂O₃. Au colloid was added to the beaker with a certain amount of γ-Al₂O₃ and stirred at room temperature until the solution was completely volatilized.

Preparation of Cu_xO/γ-Al₂O₃. Cu_xO solution was prepared using the method of Au@Cu_xO preparation wherein adding Au is omitted and the solution was added to the beaker with a certain amount of γ-Al₂O₃ and stirred at room temperature until the solution completely volatilized.

Catalytic activity test

For the CO activity measurement, about 0.1 g catalyst specimen (40–60 mesh) was placed into a stainless flow reactor (Φ 8 × 150 mm) with a fixed bed. Then, the gas mixture comprising 2 vol% CO and 98 vol% air was introduced into the reactor with a flow rate of 50 mL min⁻¹, and the GHSV (Gaseous Hourly Space Velocity) is 30 600 mL g⁻¹ h⁻¹. The activity tests of the samples were carried out in continuous flow reactors from room temperature to the temperature at which CO was completely converted into CO₂ with the heat-up period of 1 °C min⁻¹ and analyzed by gas chromatography (GC).

Characterization

Nitrogen adsorption–desorption isotherms using the Barrett–Emmett–Teller (BET) technique on a Micromeritics ASAP2020 V4.00 surface areas and porosity analyzer were used to detect the specific surface areas of the samples. X-ray diffraction (XRD) patterns were collected on a Dandong Haoyuan Instrument Co., Ltd. DX-2200V/PC diffractometer with Cu Kα radiation (λ = 0.154056 nm). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) were conducted on a JEM-2010F transmission electron microscope with an acceleration voltage of 200 kV. The surface morphology of the samples was observed with a ULTRA55-36-69 Zeiss field-emission scanning electron microscope (FESEM). Au content was measured by a Perkin-Elmer Optima 7300DV inductively coupled plasma-atomic emission spectrometer (ICP-AES). X-ray photoelectron spectroscopy (XPS) measurements are carried out on a Thermo ESCALAB 250 spectrometer using monochromatic Al Kα (hν = 1486.6 eV). All binding energies (BE) were calibrated by the C 1s signal (BE = 284.8 eV).

Results and discussion

Characterization of Au@Cu_xO core–shell NPs and Au@Cu_xO/γ-Al₂O₃ catalysts

Au@Cu_xO core–shell structure NPs are fabricated and dispersed on γ-Al₂O₃. Herein, γ-Al₂O₃ is used as the catalyst support to anchor Au@Cu_xO particles so as to increase the surface area and decrease the Au use to reduce costs. Au content in the catalysts is measured by the ICP-AES. As reference, Au/γ-Al₂O₃ and Cu_xO/γ-Al₂O₃ are also prepared with the same method and same Au or Cu_xO content.

Fig. 1 shows the structural characterizations of Au@Cu_xO core–shell NPs, Au/γ-Al₂O₃ and Au@Cu_xO/γ-Al₂O₃ catalyst. As shown in Fig. 1a, Au@Cu_xO core–shell NPs with multiple Au cores are formed. According the report of Zhang et al.²⁶ reducing HAuCl₄·3H₂O with sodium citrate leads to the formation of quasi-spherical Au NPs and the subsequent reduction of Cu(NO₃)₂ with N₂H₄·H₂O results in the formation of Cu_xO shells via a self-assembly process, which deposits on the Au surface to form core–shell like particles with multiple cores. The Au particle size is mainly concentrated at 20 nm (Fig. 1d) and the distribution interval is narrow. The diameter of Au@Cu_xO nanospheres varies within the range of 60–120 nm and mainly concentrates at 100 nm (Fig. 1e). The average shell thickness is about 40 nm and the core–shell structure is not uniform. The greater details of the Au@Cu_xO core–shell structures can be observed by the HAADF-STEM (Fig. 1b) and HR-TEM (Fig. 1c) images. The HR-TEM image recorded on the Au core region indicates that the interplanar spacing is about 0.235 nm corresponding to the (111) planes of face-centered cubic Au.⁴⁴ The (111) planes of Cu₂O with a spacing of 0.248 nm are found to grow epitaxially on the facets of Au.⁴⁵ EDS line scan of an individual Au@Cu_xO core–shell heterostructure reveals strong Cu and O signals across the entire particle. The signal for Au appears only in the central region of the particle, where the Au core is located (Fig. 1b). It further confirms the formation of the core–shell structure.

Fig. 1 Structural characterization of Au@Cu_xO and the catalysts. (a) Bright-field TEM image of Au@Cu_xO core–shell NPs, (b) high-magnification HAADF-STEM image of an individual Au@Cu_xO core–shell NPs with spatial elemental distribution analysis by EDS line-scan, (c) HR-TEM image of the Au core and Cu_xO shell, (d) the particle size distribution of pure Au NPs, (e) the particle size distribution of Au@Cu_xO core–shell structure particles, (f) HAADF-STEM image of Au@Cu_xO/γ-Al₂O₃, (g) TEM image of Au/γ-Al₂O₃, (h) the particle size distribution of Au in Au/γ-Al₂O₃.

Fig. 1f and g are the HAADF-STEM image of Au@Cu_xO/γ-Al₂O₃ and TEM image of Au/γ-Al₂O₃ catalyst, respectively. It shows that the core shell structure of Au@Cu_xO and the diameter of Au particles and Au@Cu_xO nanospheres do not change after dispersing on γ-Al₂O₃. The particle size of Au is mainly concentrated at 20 nm (Fig. 1h).

The XRD patterns of Au@Cu_xO/γ-Al₂O₃, Au/γ-Al₂O₃, Cu_xO/γ-Al₂O₃ and pure γ-Al₂O₃ are shown in Fig. 2. The peaks at 37.6°, 45.8° and 67.0° of all the samples are due to the (311), (400), (440) reflections of γ-Al₂O₃. As the content of Au and Cu_xO in the samples is very low (0.11 and 0.088 wt%), their diffraction peaks could not be clearly made out, which suggests a well dispersed metal phase.⁴⁶

Fig. 2 XRD patterns of Au@Cu_xO/γ-Al₂O₃ with 0.11 wt% Au and 0.088 wt% Cu_xO, Au/γ-Al₂O₃ with 0.11 wt% Au, Cu_xO/γ-Al₂O₃ with 0.088 wt% Cu_xO and γ-Al₂O₃. The intensity scale for all the samples are the same.

Fig. 3 shows the SEM images of Au@Cu_xO/γ-Al₂O₃, Au/γ-Al₂O₃, Cu_xO/γ-Al₂O₃ and pure γ-Al₂O₃. The crystal surface of all the samples is rough and their shapes show a perceptible similarity to each other, which are stacked loosely by irregular alumina particles and we could not clearly observe the Au, Cu_xO or Au@Cu_xO core–shell NPs. Otherwise, the addition of a little amount of Au, Cu_xO or Au@Cu_xO has no effect on the morphology of alumina support.

Fig. 3 The SEM image of (a) Au@Cu_xO/γ-Al₂O₃ with 0.11 wt% Au and 0.088 wt% Cu_xO, (b) Au/γ-Al₂O₃ with 0.11 wt% Au, (c) Cu_xO/γ-Al₂O₃ with 0.088 wt% Cu_xO and (d) γ-Al₂O₃. All SEM images share the same scale-bar in panel (a).

To further study the physical properties of Au@Cu_xO/γ-Al₂O₃ catalyst, Brunauer–Emmett–Teller (BET) surface area and pore structure of the catalyst is measured by nitrogen adsorption at 77 K and is compared with that of pure γ-Al₂O₃. Fig. 4 displays a type of IV isotherms with a hysteresis loop at relative pressure (P/P₀) between 0.5 and 1, indicating the presence of mesopores. BET surface area is calculated to be 118 m² g⁻¹ for Au@Cu_xO/γ-Al₂O₃, which is almost same as that of pure γ-Al₂O₃. In the Barret–Joyner–Halenda (BJH) pore size distribution analysis, the sharp peak at about 4.6 nm and 4.9 nm for pure γ-Al₂O₃ and Au@Cu_xO/γ-Al₂O₃, as shown in the inset in Fig. 4, suggests a relatively narrow pore size distribution interval. It shows that Au@Cu_xO has little effect on the pore structure of the alumina support, whereas the relatively high surface area of alumina will provide abundant active sites for catalytic reactions.⁴⁷

Fig. 4 Nitrogen adsorption isotherms and pore size distribution of (a) pure γ-Al₂O₃ and (b) Au@Cu_xO/γ-Al₂O₃ with 0.11 wt% Au and 0.088 wt% Cu_xO.

CO oxidation on Au@Cu_xO/γ-Al₂O₃ catalysts

Fig. 5a shows the catalytic activity of Au@Cu_xO/γ-Al₂O₃, Au/γ-Al₂O₃, Cu_xO/γ-Al₂O₃ and pure γ-Al₂O₃. Here, 90% CO conversion temperature (T_90%) is used to estimate the catalytic activity of the catalysts. A low T_90% indicates a high activity of the catalyst. T_90% of pure γ-Al₂O₃ is about 216 °C, and the introduction of a little amount Au (0.11 wt%) decreases the T_90% to 203 °C, which indicates a small improvement in the catalytic activity. In general, the Au content in the Au based catalysts is equal to or greater than 1 wt%, and it even reaches 5 wt% in some catalysts;^48–50 the low catalytic activity of Au/γ-Al₂O₃ here can be ascribed to the low Au content. On the other hand, the large Au particle size (∼20 nm) is also a cause that leads to the low catalytic activity of Au/γ-Al₂O₃ because the CO oxidation on the simple supported Au catalyst at low temperatures generally follows the size-dependent reaction pathways.^2,3 For this type of catalysts, 3–5 nm Au particles are more active. The introduction of a little amount of Cu_xO (0.088 wt%) has no obvious effect on the catalytic activity of pure γ-Al₂O₃. T_90% of Cu_xO/γ-Al₂O₃ is about 220 °C, which is even a little higher than that of pure γ-Al₂O₃. In brief, the effect of a little amount of (0.11 wt%) pure Au or pure Cu_xO (0.088 wt%) alone on the catalytic activity of γ-Al₂O₃ is weak. However, the combination of these two types of material and the fabrication of Au@Cu_xO core–shell structure generate a material more suitable to be used as CO oxidation catalysts. Although the content and particles size of Au in Au@Cu_xO/γ-Al₂O₃ is the same as Au/γ-Al₂O₃, the catalytic activity has been greatly improved. T_90% of Au@Cu₂O/γ-Al₂O₃ is only 158 °C, which is 58 °C lower than that of pure γ-Al₂O₃. It indicates that Au@Cu_xO hybrid NPs exhibit good catalytic activity due to the combination of the properties from the individual components and nanoscale interactions between the disparate metal and semiconductor components.

Fig. 5 Catalytic activity of the catalysts. (a) 90% CO conversion temperature of Au@Cu_xO/γ-Al₂O₃ with 0.11 wt% Au and 0.088 wt% Cu_xO, Au/γ-Al₂O₃ with 0.11 wt% Au, and Cu_xO/γ-Al₂O₃ with 0.088 wt% Cu_xO and γ-Al₂O₃, (b) the influence of Au content on 90% CO conversion temperature of Au@Cu_xO/γ-Al₂O₃, and (c) the CO conversion at different temperature on Au@Cu_xO/γ-Al₂O₃ with 1.24 wt% Au.

To improve the catalytic activity further, Au@Cu_xO/γ-Al₂O₃ catalysts with different Au content are prepared and the influence of Au content on the catalytic activity for CO oxidation is studied. Fig. 5b shows that T_90% decreases with Au content, which means that the catalytic activity can be improved by increasing the Au content. T_90% of Au@Cu₂O/γ-Al₂O₃ with 1.24 wt% Au is only 144 °C. The most important thing is that CO conversion on this type of catalyst can reach up to 18% at room temperature (Fig. 5c).

For CO oxidation at low temperatures, the catalytic activity of Au based catalyst is high enough but the stability is still lower. Wang et al.¹⁸ improved the catalytic stability of Au/Al₂O₃ by adding Rh into the catalyst and Lin et al. improved the catalytic stability of Au/Al₂O₃ by adding LaFeO₃ into alumina.⁵⁰ However, the stability of the catalyst is still limited. To study the catalytic performance of Au@Cu_xO/γ-Al₂O₃ deeply, Au@Cu_xO/γ-Al₂O₃ with 1.24 wt% Au is chosen as an example to study the catalytic stability at both 144 °C and room temperature. It can be seen from Fig. 6a that after 72 h reaction at 144 °C, the catalytic activity has not suffered any loss, which means that the high temperature stability of the catalyst is quite high.

Fig. 6 Catalytic stability of Au@Cu_xO/γ-Al₂O₃ with 1.24 wt% Au. (a) At 144 °C and (b) at room temperature.

The catalytic stability test of the catalyst at room temperature in Fig. 6b shows that the catalytic activity increases rather than decreases before 30 hours. CO conversion on this catalyst can reach up to 38% at room temperature after 30 hours' application and then hold the line to 108 h. To the best of our knowledge, this is one of the most stable Au based catalysts with high catalytic activity. The catalytic performance improvement of the catalyst may be ascribed to the reason that the interaction between Au and Cu_xO led to the high catalytic activity and the protection of Au by Cu_xO has led to the high stability.

CO oxidation mechanism on Au@Cu_xO/γ-Al₂O₃

In spite of a great number of contributions to reveal the origin of the high CO oxidation activity of Au catalysts, the reaction mechanism is still debated. The main debatable topics include the active Au species, the mechanism of activation of oxygen molecules, the role of the support and the Au-support interface, the interaction between supports and Au particles and the possible CO oxidation pathway. The difference between the catalytic systems and the reaction conditions may be the mandatory reason that leads to the controversies.

In general, the size of the Au particles, the valence state of Au and the interaction between Au and support are the main factors that dramatically influence the activity of the catalysts, which are difficult to isolate because they are normally intertwined together.⁵¹ To study the possible reaction mechanism, XPS is employed to detect the surface elements and their valence states. The XPS profile of Au 4f region is shown in Fig. 7a. The main peaks near 83.8 eV and 87.5 eV corresponded to Au 4f7/2 and Au 4f5/2 of metal Au and the peaks at 84.3 eV and 88.0 eV corresponded to Au 4f7/2 and Au 4f5/2 of Au^δ+. The Au signal of Au@Cu_xO/γ-Al₂O₃ is weaker than that of Au/γ-Al₂O₃ due to the encapsulation of Au into Cu_xO. However, the catalytic activity of the former is higher. It may be ascribed to the reason that the Au species on the 2 samples are quite different. All the surface Au of Au/γ-Al₂O₃ are Au⁰, and there are 2 types of Au species (Au⁰ and Au^δ+) on the surface of Au@Cu_xO/γ-Al₂O₃. This indicates that the interaction between Au and Cu_xO led to the co-existence of Au^δ+ and Au⁰. The ratio of Au^δ+/Au⁰ is 1.1 and the co-existence of the 2 types of Au species makes the catalyst more active. Au cations⁵² and a pair of Au atoms and Au cations⁵³ have also been proposed as active sites by other researchers.

Fig. 7 The XPS spectra of different catalysts. (a) Au 4f, (b) Cu 3d and (c) O 1s.

Fig. 7b shows the XPS profile of Cu 3d region of Cu_xO/γ-Al₂O₃ and Au@Cu_xO/γ-Al₂O₃. The peaks at 932.3, 933.4 and 935.5 eV are ascribed to Cu⁺ of Cu₂O, Cu²⁺ of CuO and Cu²⁺ of CuCO₃ and Cu(OH)₂, respectively. As the catalytic activity of the sample without Cu_xO is lower, the function of Cu species may be ascribed to the interaction between Au and Cu_xO, which plays a crucial role in the electronic properties of Au particles and strongly affects the catalytic activity. The reducing nature of Cu_xO can donate^54–58 and withdraw^59–65 charges from Au and build up the possible charge at the metal/oxide interface.

As shown in Fig. 7c, the O 1s XPS spectra are fitted with two peaks contributions. One peak at BE = 531.8–531.7 eV can be ascribed to adsorbed oxygen. The other peak at BE = 530.7–530.8 eV may belong to lattice oxygen O²⁻ of Cu_xO. Li et al.⁶⁶ have shown unambiguously that with the promotion of Au NPs, the surface lattice oxygen of FeO_x participates directly in the CO oxidation via an Fe³⁺ ⇌ Fe²⁺ redox mechanism even when well below the room temperature, and this redox mechanism predominates in low temperature CO oxidation on Au/FeO_x. As Cu_xO is similar to FeO_x, the same reaction mechanism may also occur via a Cu²⁺ ⇌ Cu⁺ redox mechanism.

Fig. 8 is the diagrammatic sketch for CO oxidation mechanism on Au@Cu_xO/γ-Al₂O₃. To sum up, the pairs of Au atoms and Au cations are the active sites. Au clusters act as a reservoir of charge through the catalytic cycle facilitating both reduction processes involving the O₂ molecule and oxidation process such as the transformation of CO to CO₂. The detailed process is that Cu_xO donate and withdraw charge from Au and build up the possible charge at the metal/oxide interface. CO molecules quickly attach to the oxygen species bond at the interface. It has been proved that CO can react with the bonded oxygen and release CO₂ within 4 Ps.⁶⁷

Fig. 8 Schematic for CO oxidation mechanism on Au@Cu_xO/γ-Al₂O₃.

Conclusion

In summary, Au/γ-Al₂O₃ and Cu_xO/γ-Al₂O₃ do not have any catalytic activity at room temperature and their 90% CO conversion temperature (T_90%) is almost the same as pure γ-Al₂O₃. Au@Cu_xO/γ-Al₂O₃ can not only decrease T_90% but can also catalyze CO oxidation at room temperature and the stability is very high. The high catalytic activity of Au@Cu_xO/γ-Al₂O₃ can be ascribed to the reason that the formation of Au@Cu_xO core–shell structure led to the interaction between Au and Cu_xO, which generates the Au^δ+ and Au⁰ pairs and the co-existence of the 2 types of Au species makes the catalyst more active. The protective function of Cu_xO shell may be one reason that accounts for the catalytic stability improvement.

Acknowledgements

This research is supported by the National Natural Science Foundation of China (21103104). We also acknowledge the Instrumental Analysis and Research Center of Shanghai University for providing measurement services.

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Footnote

† These authors contributed equally to this work and should be considered co-first authors.

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