Synthesis of Pd Nanoframes by Excavating Solid Nanocrystals for

Nov 28, 2016 - Due to the different regrowth rates at three typical types of surface sites (e.g., corners, edges, and faces), the removal of Pd atoms ...
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Synthesis of Pd Nanoframes by Excavating Solid Nanocrystals for Enhanced Catalytic Properties Zhenni Wang,† Huan Wang,† Zhaorui Zhang,† Guang Yang,‡ Tianou He,† Yadong Yin,§ and Mingshang Jin*,† †

Frontier Institute of Science and Technology and State Key Laboratory for Mechanical Behavior of Materials and ‡Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China § Department of Chemistry, University of California, Riverside, California 92521, United States S Supporting Information *

ABSTRACT: Synthesis of metal nanoframes has been of great interest for their open structures and high fractions of active surface sites, which gives rise to outstanding performance in catalysis. In this work, Pd nanoframes with well-defined structures have been successfully prepared by directly excavating solid nanocrystals. The success of this synthesis mainly relies on the fine control over the oxidative etching and regrowth rates. Due to the different regrowth rates at three typical types of surface sites (e.g., corners, edges, and faces), the removal of Pd atoms can be controlled at a certain site by carefully tuning the rates of the oxidative etching and regrowth. Without the presence of the reducing agent, etching dominates the process, resulting in the shape transformation of nanocrystals with well-defined shapes (e.g., octahedra) to cuboctahedra. In contrast, when a certain amount of the reducing agent (e.g., HCHO) is added, the regrowth rate at the corner and edge sites can be controlled to be equivalent to the etching rate, while the regrowth rate at the face sites is still smaller than the etching rate. In this case, the etching can only take place at the faces; thus, Pd nanoframes could be obtained. On the basis of this approach, solid Pd nanocrystals with different shapes, including cubes, cuboctahedra, octahedra, and concave cubes, have been successfully excavated to the corresponding nanoframes. These nanoframes can unambiguously exhibit much enhanced catalytic activity and improved durability toward formic acid oxidation reaction due to their three-dimensional (3D) open frameworks compared with solid Pd octahedra catalysts. KEYWORDS: palladium nanocrystals, etching, regrowth, nanoframes, formic acid oxidation

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metal nanocrystals into nanoframes. The main problem is that the synthetic approach to metal nanoframes is still limited, especially for monometallic nanocrystals. Palladium, an important member of platinum group metals, has been widely used as a heterogeneous catalyst in many industrial applications, including CO oxidation, alkene hydrogenation, Suzuki coupling reactions, and formic acid oxidation.13−26 Past decades have witnessed the successful synthesis of Pd nanocrystals with a set of different shapes, including cubes, octahedrons, concave nanostructures, plates, icosahedrons, pentagonal rods, and nanowires.13,20,27−33 It is now possible to prepare the above-mentioned Pd nanocrystals by carefully manipulating the reaction conditions such as

aximizing the activity of heterogeneous catalysts is greatly desired due to its economic benefits for important applications, including chemical, pharmaceutical, and petroleum industries.1−5 Tremendous studies have revealed that the catalytic activity of a metal catalyst is strongly dependent on the fraction of surface atoms located at the corners and edges.6−10 Upon this fundamental insight and exciting development, it was established that metal nanoframes, which contain the dramatically enhanced fraction of corner and edge sites with distinct reactivity, represent an important type of remarkably optimized catalysts that can show the highest activities.9,11,12 As a typical example, Yang and co-workers reported the successful synthesis of Pt3Ni nanoframes, which can achieve a factor of 36 enhancements in mass activity and a factor of 22 enhancements in specific activity, respectively, for oxygen reduction reaction.9 Therefore, it is an efficient way to enhance the catalytic activity of a metal catalyst by preparing © 2016 American Chemical Society

Received: September 26, 2016 Accepted: November 28, 2016 Published: November 28, 2016 163

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Figure 1. Schematic illustrating the shape transformations of Pd nanocrystals through delicate control over the etching and regrowth rates.

of Pd nanoframes by excavating solid Pd nanocrystals. The key to this synthesis should rely on fine control of both the etching and regrowth rates. Theoretically, there are three types of active sites on the surface of a metal nanocrystal: corner, edge, and face sites. Since the environmental conditions around the atoms locate at these sites would be different, such as the surface energy and coordination number, the physical and chemical properties (e.g., growth rate on this site, chemical reactivity, and so on) of these atoms would thus be different. The growth rate of different sites would tend to follow the order Rcorner,regrowth > Redge,regrowth > Rface,regrowth, respectively.41,42 When the etching rate is kept constant, we then finely tune the regrowth rates of Pd at different sites by slowly increasing the concentration of reducing agent from zero. When the concentration of reducing agent is low, the regrowth rate is much smaller than the etching rate regardless of the type of surface sites (Rregrowth ≪ Retching). In this case, solid Pd nanocrystals, octahedrons, for example, will transform into cuboctahedra as shown in Figure 1, route 1. This result is consistent with the most reported works on the nanocrystals suffering from the etching.43,44 When the concentration of the reducing agent is increased, the regrowth rate will also increase. Considering the difference in the regrowth rate of surface sites on Pd nanocrystals, the regrowth rate of corners will hopefully approach the etching rate first. In this case, Rcorner,regrowth ≈ Retching, while the regrowth rates of edge sites and face sites still smaller than the corresponding etching rates (Redge,regrowth < Retching and Rface,regrowth < Retching). Obviously, the etching along the edges and faces would lead octahedral nanocrystals to slowly evolve into hexapods (Figure 1, route 2). Further increasing the regrowth rate can result in the case of Rcorner,regrowth ≈ Retching and Redge,regrowth ≈ Retching, while the growth rate of face sites still smaller than the etching rate (Rface,regrowth < Retching), implying that the etching would likely to take place only at the faces of octahedral nanocrystals. Not surprisingly, this etching habit can directly induce the shape transformation of octahedral nanocrystals to octahedral nanoframes, with all of the atoms located at the edge and corner sites (Figure 1, route 3). In case of a much higher growth rate relative to the etching rate (Rregrowth ≫ Retching), the etching would be significantly hindered, leaving the solid nanocrystals unchanged (Figure 1, route 4). In a typical synthesis, the solid Pd octahedrons to be used as seeds were prepared using a protocol previously reported in the literature.20 Figure S1 shows a typical transmission electron

temperature, reductants, capping agents, and concentrations of reagents and ionic species.13,20,27 However, although frame structure can significantly increase the utilization efficiency and even enhance the catalytic activity and durability of Pd catalysts, there is no effective approach that has been reported for the successful synthesis of Pd nanoframes by far. Pd nanoframes are of particular interest and importance for catalytic or electrocatalytic applications owing to the following attractive features: (i) they can offer much higher specific surface areas and thus improved activity relative to their solid counterparts; (ii) the presence of a hollow interior can help reduce the loading of Pd; and (iii) the frame structure can enhance the stability of Pd catalysts during catalytic processes.9,34−36 To this end, there is a strong motivation to develop an efficient approach for the preparation of Pd nanoframe for their catalytic applications. In this study, we demonstrate an approach to the fabrication of Pd nanoframes by excavating solid Pd nanocrystals. The success of this shape transformation from solid nanocrystals to nanoframes mainly relies on the delicate control over the rates of the oxidative etching and the regrowth of the corner, edge, and face sites. Through this approach, solid Pd nanocrystals with various shapes, including nanocubes, octahedrons, concave nanocubes, and cuboctahedrons, can be easily excavated to the corresponding frameworks. These Pd nanoframes with abundant of active sites exhibit substantially enhanced catalytic properties toward formic acid oxidation relative to original solid counterparts.

RESULTS AND DISCUSSION Etching is a frequently observed phenomenon during the preparation of metal nanocrystals. Previously, Xia and some other researchers adopted it to tailor the population of singlecrystalline seeds of metal nanocrystals.37−39 The etching process can be further combined with regrowth so as to tailor the size and shape of presynthesized Pd nanocrystals.40 Although the combination of the etching and regrowth process provides the possibility of shape modification of preformed Pd nanocrystals, it is extremely difficult to excavate solid nanocrystals into nanoframes through etching by far. In this work, we propose a route to excavate solid Pd nanocrystals into nanoframes based on the combination of the etching and regrowth process, which will show great potential in constructing rational nanostructures of metal nanocrystals for catalytic applications. Figure 1 shows our tactic for the synthesis 164

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Figure 2. Pd octahedral nanoframes prepared by maneuvering the rates of oxidative etching and regrowth: (a) representative TEM, (b) HAADF−STEM images, and (c) HRTEM images of Pd octahedral nanoframe projected along ⟨110⟩, ⟨100⟩, and ⟨111⟩ zone axes and the corresponding Fourier transform (FT) patterns, respectively. (d) 3D model of a Pd octahedral nanoframe and its projections along ⟨110⟩, ⟨100⟩, and ⟨111⟩ zone axes.

unchanged (Figure S2d). As previously mentioned, there is no efficient approach that can directly excavate solid metal nanocrystals into nanoframes without changing the component by far. The above results clearly indicate that by taking the advantage of the fine control over the etching and regrowth rates at different surface sites solid Pd octahedra can be excavated into Pd octahedral nanoframes directly. Parts a and b of Figure 2 show TEM and the high-angle annular dark-field scanning TEM (HAADF-STEM) images of the high quality of octahedral Pd nanoframes. As can be seen, the obtained octahedral nanoframes consist of 12 edges (width ∼5 nm) of the parent octahedrons. High-resolution transmission electron microscopy (HRTEM) and the corresponding fast Fourier transform (FFT) patterns (Figure 2c) of the nanoframes along [110], [100], and [111] zone axes illustrate that the entire nanostructure is single crystalline, consistent with the corresponding 3D models of octahedral nanoframes viewed along these directions (Figure 2d). The X-ray diffraction (XRD) pattern of the nanoframes is shown in Figure S3, and the two peaks at 40.1° and 46.6° correspond to the {111} and {200} planes of face-centered cubic Pd (JCPDS no. 89-4897). In order to better understand the oxidative etching and regrowth process, a series of control experiments have been carried out. The corresponding products obtained with different controlled reaction parameters are summarized in Figure 3, where the upper row shows the influence of the reaction time, middle row the etching rate, and bottom row the regrowth rate. Clearly, in a typical formation process of Pd octahedral nanoframes, we can observe that the etching takes

microscopy (TEM) image of the Pd octahedrons, with purity approaching 100%. The average length of these octahedrons was 37 nm. These Pd octahedrons were then washed and redispersed in N,N-dimethylformamide (DMF) in the presence of poly(vinylpyrrolidone), and O2/potassium iodide (KI) and formaldehyde (HCHO) were used as the etchant and reducing agent, respectively. Compared with other etchants, such as Cl−/ O2, Br−/O2, HNO3, and Fe3+/Br−, the I−/O2 can provide a proper etching rate so as to manipulate the oxidative etching rate in a much more controllable way.45 In our experiments, both the rates of the etching and regrowth can be easily tuned by adjusting the concentrations of the etchant (the concentration of O2) and the reducing agent (HCHO). Figure S2 shows the TEM images of the shape transformation of Pd octahedrons suffering from different regrowth rates, while the etching rate was kept at a constant (the concentration of O2 and KI were kept at 10 mL and 1.5 mg, respectively). As can be seen in Figure S2a, without the addition of the reducing agent (HCHO), Pd octahedrons would tend to exhibit a cuboctahedra shape after the etching since Rregrowth ≪ Retching. When the addition of HCHO is 2 μL, the original Pd octahedrons would slowly evolve into Pd hexapods due to the selective etching along edges and faces (Figure S2b). When the regrowth process is accelerated by further increasing the addition volume of HCHO to 5 μL, the etching would likely to take place only at the faces of Pd octahedrons, resulting in the formation of Pd nanoframes (Figure S2c). However, when the volume of HCHO is increased to an extreme high value (50 μL), the etching will not take place, thus leaving Pd octahedrons 165

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orientations (Figure 4a3). Further reaction would result in the formation of nanoframes (Figure 4b1−b3). If we extend the reaction to 60 min, not surprisingly, the octahedral nanoframes with thin ridges can be obtained (Figure 4c1−c3). Overetching of Pd octahedrons will unambiguously cause the thoroughly dissolution of Pd nanocrystals to Pd2+ ions. In addition to the reaction time, the final morphology of the Pd nanocrystals is also determined by the ration of etchingrates/regrowth-rates, which are dependent on the concentrations of the etchant and reducing agent. As shown in Figure S2, changing the concentration of the reducing agent (HCHO) can lead to the shape transformation from octahedra to hexapods and nanoframes due to the differences in the etching and regrowth rates at different sites of the Pd octahedrons. Similar to the shape transformation induced by increasing the concentration of the reducing agent, O2, as the etchant, can also play an important role in this reaction while maintaining the growth rate a constant. In a standard experiment, 10 mL of O2 can hopefully lead to the formation of the Pd nanoframes. A higher concentration, for example, 15 mL, will cause a higher etching rate, which can subsequently yield the nanoframes with thinner ridges (width ∼2.6 nm). At even higher concentration of the etchant, more than 20 mL, the ridges on octahedral nanoframes may be broken due to overcorrosion. The atomscale evolution at different concentrations of the etchant is shown in Figure 3 (middle row), with their corresponding TEM images as shown in Figure 5. Because of the different capabilities of various solvents in dissolving O2, the type of the

Figure 3. Shape transformations of the Pd nanocrystals under different reaction conditions: top row, influence of the reaction time; middle row, etching rate; bottom row, regrowth rate.

place from the exposed {111} faces. When the reaction time is 10 min, the solid Pd octahedrons are found to transfer to concave octahedrons. As the reaction time prolonged, the ratio of the concave degree gradually increased, which finally resulted in the formation of nanoframes. This shape transformation process could be well monitored by characterizing the samples obtained at different reaction time. Figure 4 shows the representative TEM and STEM images of the samples prepared at 10, 30, and 60 min. As shown in Figure 4a1−a3, the etching initiates from the atoms on the faces of Pd octahedrons, resulting in the formation of concave octahedrons, which can be clearly observed in the TEM and STEM images, especially those images of TEM analysis along different

Figure 4. Pd nanocrystals obtained with different excavation times: (a) 10 min, (b) 30 min, and (c) 60 min. (a1−c1) TEM images. (a2−c2) HAADF-STEM images. (a3−c3) TEM images of individual Pd octahedral nanoframes oriented along the [111], [110], and [100] directions. 166

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Figure 5. TEM images of the Pd octahedrons prepared under different etching rates by controlling the amount of the etchant (O2): (a) 2 mL, (b) 10 mL, (c) 15 mL, and (d) 20 mL.

solvent can also play an important role in our synthesis. We have further carried out our synthetic process in different solvents, including water, dimethyl sulfoxide, and phenylamine. High-quality Pd nanoframes could be obtained when phenylamine was used as the solvent. However, when water or dimethylsufoxide was used as the solvent, only solid Pd nanocrystals were obtained, as shown in Figure S4. Theoretically, nearly all of the metal nanocrystals, whether regular- or irregular-shaped ones, possess the above-mentioned three types of surface sites: corner, edge, and face. Commonly, both the etching and growth rates on these sites can be finely tuned to achieve a frame structure starting with the corresponding solid ones. Therefore, our proposed synthetic strategy to the fabrication of Pd nanoframes should be a universal way to “excavate” solid Pd nanocrystals into frames. In order to confirm this consumption, Pd cuboctahedra, cubes, and concave nanocubes were further synthesized (Figure S5) by the methods reported previously13,20,27 and subjected to our standard experimental process. Not surprisingly, the corresponding frames can be easily obtained after the reaction (Figure 6), implying that our proposed synthetic approach is a universal way to the fabrication of Pd nanoframes with required shapes. This result indicates that our synthetic strategy could potentially be used to convert most metal solid nanomaterials with different shapes into corresponding nanoframes. Pd nanoframes with a large fraction of atoms at the corners and edges were then evaluated as electrocatalysts for electrooxidation of formic acid, benchmarked against the solid Pd

octahedra catalysts. Prior to the electrochemical measurement, Pd nanoframes were pretreated with O3 for 12 h to remove poly(vinylpyrrolidone) (PVP) that capped on the surface. TEM images show that the morphology of Pd nanoframes remains unchanged after the treatment of O3. The electrochemically active surface areas (ECSAs) of the Pd nanoframes and Pd octahedra were determined from the charges associated with the CO stripping on the surface of catalysts, as shown in Figure S6. Figure 7a shows cyclic voltammograms normalized against the ECSAs of these two catalysts for electro-oxidation of formic acid at room temperature. It is clear that the Pd nanoframes exhibited much higher electrocatalytic activity than the Pd octahedra catalyst, with nearly 7.5 times increase for the peak current. We believe that the large fraction of corner and edge atoms, as well as the 3D open structure, were responsible for the enhanced catalytic for formic acid oxidation. We further evaluated the long-term stability of the catalysts through an accelerated durability test (Figure 7b). After 1000 cycles, Pd nanoframes can also exhibit a catalytic activity 6.5 times higher than that of the pristine Pd octahedra, indicating the excellent durability of frames structures during catalytic reactions. Further TEM analyses of Pd nanoframes after 1000 cycles are shown in Figure S7. It is clear that the structure of the Pd nanoframes was well maintained after the durability test, implying the good stability of the obtained Pd nanoframes. 167

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Figure 6. TEM images of the Pd nanoframes obtained by excavating solid nanocrystals with different shapes: (a) octahedra, (b) cuboctahedra, (c) nanocubes, and (d) concave nanocubes.

Figure 7. (a) Electrochemical properties and (b) electrochemical durability of the Pd octahedral nanoframes and solid Pd octahedra catalysts. The current density was normalized against the corresponding ECSA.

CONCLUSION In summary, we demonstrate an approach to the synthesis of Pd nanoframes by excavating solid Pd nanocrystals. The success of this synthesis mainly relies on the fine control over the oxidative etching and regrowth rates. Due to the different regrowth rates at three typical types of surface sites (e.g., corners, edges, and faces), the removal of Pd atoms can be controlled at a certain site by carefully tuning the rates of the oxidative etching and regrowth. Without the presence of the reducing agent, etching dominates the process, resulting in the shape transformation of nanocrystals with well-defined shapes (e.g., octahedra) to cuboctahedra. In contrast, when a certain

amount of the reducing agent (e.g., HCHO) is added, the regrowth rate at the corner and edge sites can be controlled to be equivalent to the etching rate, while the regrowth rate at the face sites is still smaller than the etching rate. In this case, the etching can only take place at the faces, and thus, Pd nanoframes could be obtained. On the basis of this approach, solid Pd nanocrystals with different shapes, including cubes, cuboctahedra, octahedra, and concave cubes, have been successfully excavated to the corresponding nanoframes. The excavated Pd nanocrystals with frame structures show significantly enhanced activity and durability formic acid oxidation. We believe this strategy provides a simple and 168

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scope operated at 200 kV. XRD patterns was recorded using a diffractometer (X-ray diffractometer SmartLab(3), Rigaku) operated at 3 kW. Electrochemical measurements were conducted on an AutoLab (PGSTAT302N) electrochemical station.

highly effective method for creation of high fractions of active surface sites in metal nanocrystals.

METHODS

ASSOCIATED CONTENT

Chemicals and Materials. Sodium tetrachloropalladate(II) (Na2PdCl4, 98%), PVP (Mw = 55000), L-ascorbic acid (AA), potassium bromide (KBr), HCHO soluion (37%), KI, dimethyl sulfoxide (DMSO), phenylamine, and DMF were all obtained from SigmaAldrich and used as received. Deionized water with a resistivity of 18.2 MΩ·cm was used for the preparation of all experiments. Synthesis of 18 nm Pd Nanocubes. The Pd nanocubes were synthesized by adding a Na2PdCl4 solution into a mixture of AA, PVP, and KBr according to our previous report.13 In a typical synthesis, 8.0 mL of an aqueous solution containing 105 mg of PVP, 600 mg of KBr, and 60 mg of AA was placed in a vial and preheated for 10 min at 80 °C under magnetic stirring. Subsequently, 3.0 mL of an aqueous solution of Na2PdCl4 (57 mg) was added with a pipet. After that, the reaction was allowed to continue at 80 °C for 3 h. The 18 nm Pd nanocubes was collected by centrifugation, washed three times with water, and redispersed in 11 mL water. Synthesis of Pd Octahedrons and Cuboctahedrons. These Pd nanocrystals were synthesized according to our previous report.20 In a typical synthesis, 8.0 mL of an aqueous solution containing 105 mg of PVP, 100 μL of HCHO, and 0.3 mL of an aqueous suspension of Pd cubic seeds 18 nm in edge length was capped in a vial and heated at 60 °C for 5 min under magnetic stirring. Subsequently, 3.0 mL of aqueous solutions containing 29 mg (octahedrons) and 8.7 mg (cuboctahedrons) of Na2PdCl4 were added with a pipet, respectively. Each reaction was allowed to proceed at 60 °C for 3 h, and the products were collected by centrifugation, washed two times with water, and redispersed in 20 mL of DMF for further use. Synthesis of Pd Concave Nanocubes. The Pd concave nanocubes were synthesized using the published procedure.27 In a standard procedure, 3.0 mL of an aqueous solution containing 14.5 mg of Na2PdCl4 was introduced into 7.7 mL of a mixture containing AA (60 mg), PVP (105 mg), KBr (300 mg), and 0.3 mL of the 18 nm Pd seeds, which had been heated at 60 °C for 5 min under magnetic stirring. The product was collected by centrifugation, washed two times with water, and redispersed in 20 mL of DMF for further use. Etching of Pd Nanocrystals. In a standard procedure, 1.5 mL of DMF suspension containing 20 mg of PVP, 1.5 mg of KI, 1.0 mL of the as-obtained Pd nanocrystals, and a certain amount of aqueous HCHO (10-fold dilution, 5 μL) was placed in a vial capped with a rubber plug. The reaction system was evacuated, subsequently, and a certain amount of O2 (10 mL) was injected with a syringe. The reaction proceeded at 100 °C for 1 h. After collection by centrifugation and washing three times with water, the final product was redispersed in water. Electro-oxidation of Formic Acid. The as-prepared Pd solution was treated with O3 for 12 h to remove the surface ligands on the Pd catalyst. The treated Pd frames and commercial Pd/C were dispersed in ethanol and treated by ultrasonication for 20 min. The working electrode was prepared by dropping suspensions of the catalysts onto precleaned glassy carbon electrodes. Upon drying at room temperature, the electrodes were capped with 5 μL of Nafion aqueous solution (0.5%) and dry in air at room temperature. A three-electrode cell was used with an Ag/AgCl electrode as reference electrode and a platinum plate as counter electrode, respectively. Electrochemical measurements were carried out in 0.5 M HCOOH + 0.5 M HClO4 solution at room temperature; before the cyclic voltammetry measurements, two cycles of potential sweeps between −0.2 and +1.2 V at a sweep rate of 250 mV/s were applied. For CO-stripping experiments, CO gas (99.99%) was bubbled for 20 min through a 0.5 M H2SO4 solution. The electrode was quickly moved to a fresh solution, and the stripping experiment was performed at a sweep rate at 20 mV/s. Characterizations. TEM images were performed using a Hitachi HT-7700 microscope operated at 100 kV. HRTEM and HAADFSTEM images were performed using a JEM-2100F (JEOL) micro-

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06491. TEM images of Pd nanocrystals with different morphologies including octahedra, cube, cuboctahedra, and concave nanocube; XRD of Pd octahedral nanoframes; CO stripping curves of catalysts (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yadong Yin: 0000-0003-0218-3042 Mingshang Jin: 0000-0001-9708-1959 Author Contributions

M.J. and Y.Y. designed the project. Z.W., H.W., Z.Z., and T.H. performed the catalyst preparation, catalytic testing, and characterization and wrote parts of the paper. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS M.J. acknowledges financial support from the National Natural Science Foundation of China (NSFC, Nos. 21403160 and 21471123) and the “start-up fund” provided by Xi’an Jiaotong University. Y.Y. thanks the U.S. National Science Foundation (CHE-1308587) and the Department of Energy (DESC0002247) for partial support. REFERENCES (1) Anderson, J.; Boudart, M. Principles and Practice of Heterogeneous Catalysis. Angew. Chem., Int. Ed. Engl. 1997, 36, 2689−2690. (2) Armor, J. New Catalytic Technology Commercialized in the USA during the 1990s. Appl. Catal., A 2001, 222, 407−426. (3) Chorkendorff, I.; Niemantsverdriet, J. W. Concepts of Modern Catalysis and Kinetics; Wiley-VCH: Weinheim, 2003; pp 301−400. (4) Farrauto, R.; Bartholomew, C. Fundamentals of Industrial Catalytic Processes; John Wiley & Sons: New York, 2011; pp 342−398. (5) Kleboom, A. P.; Moulijn, J.; van Leeuwen, P. W. N. M.; van Santen, R. A. Industrial Catalysis. In Catalysis: An Integrated Approach, 2nd ed.; van Santen, R. A., van Leeuwen, P., Mooulijn, J., Averill, B. A., Eds.; Elsevier: Eindhoven, 2000; pp 4−17. (6) Zhu, W.; Zhang, Y.; Zhang, H.; Lv, H.; Li, Q.; Michalsky, R.; Peterson, A.; Sun, S. Active and Selective Conversion of CO2 to CO on Ultrathin Au Nanowires. J. Am. Chem. Soc. 2014, 136, 16132− 16135. (7) Hvolbæk, B.; Janssens, T.; Clausen, B.; Falsig, H.; Christensen, C.; Nørskov, J. Catalytic Activity of Au Nanoparticles. Nano Today 2007, 2, 14−18. (8) Narayanan, R.; El-Sayed, M. Effect of Nanocatalysis in Colloidal Solution on the Tetrahedral and Cubic Nanoparticle SHAPE: Electron-Transfer Reaction Catalyzed by Platinum Nanoparticles. J. Phys. Chem. B 2004, 108, 5726−5733. (9) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H.; Snyder, J.; Li, D.; Herron, J.; Mavrikakis, M.; et al. Highly Crystalline 169

DOI: 10.1021/acsnano.6b06491 ACS Nano 2017, 11, 163−170

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DOI: 10.1021/acsnano.6b06491 ACS Nano 2017, 11, 163−170