Ultrathin Two-Dimensional Pd-Based Nanorings as Catalysts for Hydrogenation with High Activity and Stability.

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Hollow Nanostructures

Ultrathin Two-Dimensional Pd-Based Nanorings as Catalysts for Hydrogenation with High Activity and Stability Yi Li, Wenxing Wang, Kaiyang Xia, Wenjun Zhang, Yingying Jiang, Yuewu Zeng, Hui Zhang,* Chuanhong Jin, Ze Zhang, and Deren Yang

Despite a few reports on the synthesis of ultrathin 2D nanosheets made of noble metals, it still remains a tremendous challenge to generate their ultrathin hollowed nanostructures, which are of particular interest in highly active catalysis due to their unique structural features. Here, the synthesis of ultrathin 2D Pd nanorings is reported with a hollow interior by selective epitaxial growth of Pd atoms on the periphery of the as-preformed Pd nanosheets in combination with oxidative etching. This approach can be extended to fabricate Pd-based bimetallic ultrathin nanorings such as Pd–Pt. The Pd nanorings exhibit substantially enhanced activity toward the hydrogenation of p-nitrophenol, which is 2.2 and 33.4 times higher than that of the Pd nanosheets and commercial Pd black, respectively. Significantly, the Pd nanorings are highly stable with only less than 11% loss in activity compared to 45.7% loss of the Pd nanosheets and 72.2% loss of the Pd black after ten cycles.

1. Introduction Ultrathin 2D nanosheets have attracted considerable attention in recent years due to their abundantly intriguing properties and widespread potential applications related to their unique structural features.[1–5] In these fascinating nanostructures with atomic thickness, most of the atoms are clearly distributed on the surface, leading to a remarkably high specific surface area with a large number of active sites.[6,7] Since many catalytic reactions are well known to be very sensitive to the surface structure of a catalyst, such atomically Y. Li, W. Wang, K. Xia, W. Zhang, Y. Jiang, Dr. Y. Zeng, Prof. H. Zhang, Prof. C. Jin, Prof. Z. Zhang, Prof. D. Yang State Key Laboratory of Silicon Materials School of Materials Science and Engineering and Cyrus Tang Center for Sensor Materials and Applications Zhejiang University Hangzhou, Zhejiang 310027, P. R. China E-mail: [email protected] DOI: 10.1002/smll.201500769 small 2015, 11, No. 36, 4745–4752

thin nanosheets hold tremendous potential in catalysis with excellent performances.[8,9] For example, ultrathin XS2 (X = Mo, W) nanosheets exhibited the substantially enhanced activities toward hydrogen evolution reaction relative to their bulk counterparts owing to a higher ratio of active edge sites preferentially exposed on the surface.[10–13] However, most of the successfully achieved ultrathin nanosheets are mainly limited to these materials with a lamellar structure that have the strong intralayer chemical bonding and weak interlayer interaction.[14] As important materials used in catalysis, noble metals such as Pd, Pt, and Rh have a close-packed structure in 3D, making it very difficult to spontaneously generate ultrathin nanosheets with few atomic layers.[15,16] Despite the large difficulty, there are a few successful examples for synthesizing ultrathin nanosheets made of noble metals in the presence of a strong capping agent.[17–19] To this end, Zheng et al. have demonstrated a facile synthesis of hexagonal-shaped Pd nanosheets of 1.8 nm in thickness with CO as a strong capping agent.[20,21] Li et al. have reported the fabrication of poly(vinylpyrrolidone)supported single-layered Rh nanosheets using a facile solvothermal method in the presence of formaldehyde.[22] All of these ultrathin nanosheets showed the superior activities in

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many catalytic reactions such as formic acid oxidation and hydrogenation. To further boost the catalytic activity of such ultrathin nanosheets toward a given reaction, a general and versatile strategy is to enhance their specific surface area and/ or maximize the number of coordinately unsaturated active sites on the surface.[23–26] Decreasing the thickness of the nanosheets provides a powerful route to improve their catalytic activity. However, these efforts may also bring the issues associated with stability during the reaction, in particular for such metals with a close-packed structure.[27] Hollowed nanostructures offer another promising approach to meet these goals with enhanced performance in catalysis.[28–30] Among them, metallic nanorings are of particular interest in highly active catalysis because of their outstanding features associated with the effective utilization of both exterior and interior catalytic surfaces and the convenient accessibility for the reactants.[31,32] Significantly, the interior part of the nanorings can have a lot of active sites, unsaturated bonds, and high-indexed facets exposed on the surface, which were of great importance for the catalytic reactions.[25] By taking these advantages, much less precious metal is needed to bring about the same catalytic activity compared to solid catalysts, which is essential to practical applications with increasing demand of low cost. Attempts to fabricate metallic nanorings have involved a rich variety of synthetic approaches including template-assisted method, lithography, and galvanic replacement.[33–37] Although significant advances have been achieved in the synthesis of metallic nanorings, it still remains huge challenge to generate ultrathin single-crystalline nanorings with few atomic layers.

With Pd as an example, here we have demonstrated a general and versatile strategy based on selective overgrowth plus chemical etching for synthesizing ultrathin nanorings. We chose Pd as our initial focus because nanocrystals made of this metal have received great interest as catalysts in a wide variety of reactions.[38] Significantly, this new approach can also be extended to generate Pd–Pt bimetallic ultrathin nanorings by replacing Pd source with Pt salt precursor fed in the synthesis. The Pd nanorings showed superior catalytic activity toward hydrogenation relative to the Pd nanosheets as well as the commercial Pd black due to the unique structural features with a higher ratio of active catalytic sites. More importantly, we found that the Pd nanorings are highly stable during the hydrogenation, while the Pd nanosheets gradually deactivate with cycle numbers due to the structural destruction.

2. Results and Discussion The ultrathin Pd nanorings were generated by selective epitaxial growth of Pd atoms on the periphery of the aspreformed ultrathin Pd nanosheets in combination with oxidative etching arising from Br−/O2 pairs (see the Experimental Section for the details). Figure 1 schematically illustrates the synthetic procedure and shape evolution of the ultrathin Pd nanorings. Pd hexagonal nanosheets with few atomic layers were employed as the seeds for the synthesis of the ultrathin nanorings. Owing to the selective adsorption of Br− ions on their side surface (i.e., {100} facets),[17] epitaxial Pd layer preferred to take a vertical growth on both top and bottom surfaces of Pd nanosheets. Interestingly, the newly formed Pd atoms were preferentially deposited on the

Figure 1. Schematic illustration of the synthetic procedure and shape evolution of the ultrathin Pd nanorings.

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periphery of the flat surface of Pd nanosheets through a selective growth. Since the reaction was conducted in the presence of Br− ions in air, the afore-mentioned epitaxial growth was usually accompanied by the oxidative etching.[39,40] As such, the interior part with relatively thin thickness was essentially removed by oxidative etching arising from Br−/O2 pairs, leading to the ultrathin Pd nanorings with a hollow structure. Ultrathin Pd hexagonal nanosheets with an average edge length of ≈15.7 nm and thickness of ≈1.1 nm were initially synthesized according to our previous method,[41] as shown in Figure S1 (Supporting Information). These Pd nanosheets were then used as the seeds for the synthesis of the Pd ultrathin nanorings. Figure 2 shows morphological and structural characterizations of the Pd nanorings that are prepared using the standard procedure. From the transmission electron microscopy (TEM) image (Figure 2a), most of the nanocrystals were observed to have a hexagonal profile with a hollow interior of ≈8 nm in size, implying the formation of ultrathin nanorings. The average edge length of the nanorings was measure to be ≈15.7 nm (Figure 2b), which was in agreement with that of the as-preformed nanosheets. This result indicates that no lateral growth was taken place in this synthesis. The typical high-resolution TEM image (HRTEM, see Figure 2c) of an individual Pd nanoring that lies flat on carbon film of the TEM grid clearly shows well-resolved, ordered fringes in the same orientation, indicating that the nanoring is a single crystal. The magnified HRTEM image and fast Fourier transform (FFT) analysis (insets of Figure 2c) indicate that the fringes with a lattice spacing of 2.4 Å can be indexed to the 1/3 {422} reflections of face-centered cubic (fcc) Pd. This

result reveals the appearance of stacking faults parallel to the {111} planes, which is consistent with the previous report.[20] To measure the thickness of the Pd nanorings, carbon nanotubes were used as the supporter to allow the attachment of the nanorings vertically on the outer surface of the nanotubes. Figure 2d shows a typical HRTEM image of the side surface of an individual nanoring that vertically stands on the TEM grid. More TEM images can be found in Figure S2a,b (Supporting Information). The thickness of the nanorings was measured to be ≈2 nm, which was nearly two times greater than that of the origin nanosheets. Obviously, the vertical growth dominated the thickening of the nanorings. The fringes with a lattice spacing 2.3 Å can be indexed to the Pd{111}, indicating that the flat planes were covered by the {111} facets. Careful observation reveals that there were {111} stacking faults parallel to their flat faces in such nanorings, which is consistent with our previous report on the Pd nanosheets.[41] We also examined the Pd nanosheets with other different sizes as the seeds for the synthesis of the Pd nanorings. Figure S3 (Supporting Information) shows the TEM images of the Pd nanosheets with an average edge length of 6.4 and 24.5 nm, respectively, and their corresponding Pd nanorings. From these images, it is clear that high-quality Pd nanorings with roughly the same size as the starting Pd nanosheets were also obtained. The thickness of the Pd nanorings with different edge lengths was also measured to be ≈2 nm, further confirming that only vertical growth dominated the synthetic process. It should be noted that the hollow interior in the nanorings was gradually increased with the size of the

Figure 2. a,b) TEM images of the Pd nanorings that are obtained using the standard procedure. HRTEM images of an individual nanoring that c) lies flat and d) stands vertically on the TEM grid, respectively. The insets in (c) correspond to the HRTEM image at a higher magnification and FFT. small 2015, 11, No. 36, 4745–4752



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Figure 3. TEM images of Pd nanorings that are obtained using the standard procedure, except for different periods of the reaction: a) 5, b) 10, c) 30, and d) 60 min. The insets show the corresponding TEM images at a higher magnification.

nanosheets. Combined with the result obtained from the Pd nanosheets of 15.7 nm in size, we can conclude that the size of Pd nanorings could be controlled by simply using Pd nanosheets with different sizes as the seeds. To decipher the growth mechanism of the Pd nanorings, TEM images were taken from a series of samples obtained at different reaction periods. In the initial stage of the reaction (Figure 3a, t = 5 min), the morphology of the nanosheets seemed to almost remain intact. The magnified TEM image (inset of Figure 3a) clearly shows that some Pd atoms were preferentially deposited on the periphery of their flat surface covered by {111} facets (marked by white arrows), confirming the selectively epitaxial overgrowth along the vertical direction. Although the exact mechanism for the preferential growth of the Pd atoms on the periphery is not clear, we can understand this growth behavior through kinetic control.[42] For a Pd nanosheet with a hexagonal shape, the reactivity is supposed to decrease in the order of side face (i.e., {100} facets), edge, and flat surface (i.e., {111} facets). Owing to selective adsorption of Br− ions on the side face,[20] the newly formed Pd atoms were expected to preferentially diffuse onto the most active sites (12 edges on both top and bottom surfaces). The Pd atoms at edges could migrate to the flat surface through surface diffusion. Due to the fast reduction of the Pd precursor by ascorbic acid associated with the large redox potential of Pd2+/Pd (0.987 V), the Pd atoms around the periphery of the Pd nanosheets were always retained at a high supersaturation. As a result, these Pd atoms should have not enough time to migrate to the interior part of the


Pd nanosheets and stay to grow around the periphery of the Pd nanosheet. This preferential overgrowth resulted in the thicker periphery but the thinner interior part of the Pd nanosheets. As the reaction proceeded (Figure 3b, t = 10 min), more and more Pd atoms were selectively grown on the periphery of the nanosheets, leading to relatively thick hexagonal frame with dark TEM contrast. While the interior of the nanosheets had the thinner thickness due to the localized epitaxial growth, showing low TEM contrast (inset of Figure 3b). Since this reaction was conducted in the presence of Br− ions in air, it is inevitable to the involvement of oxidative etching caused by Br−/O2 pairs during the selective growth. This demonstration was confirmed by the existence of small pits at the center of the nanosheets from the start of the reaction (Figure 3a). In addition, the selective adsorption of Br− ions on {100} facets of the hexagonal Pd nanosheets could protect their side surface (i.e., {100} facets) from oxidation by O2, leading to the selective initiation of oxidative etching from the Pd{111} facets.[40] As such, the relatively thin interior part of the nanosheets was gradually removed by the oxidative etching. In this stage, some Pd nanorings with a hollow structure started to be generated. With extension of the reaction (Figure 3c, t = 30 min), the hollow interior was enlarged, and therefore regular Pd nanorings were essentially formed. After 1 h, most of the Pd nanosheets were transformed into the Pd nanorings by selectively epitaxial overgrowth in combination with oxidative etching (Figure 3d). Further extension to 24 h (Figure S4, Supporting Information) resulted in the formation of the irregularly broken


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fragment, further confirming the involvement of oxidative etching throughout the synthesis. To further understand their growth mechanism, we also systematically investigated the effect of other important experimental parameters on the morphology of the Pd nanorings. In the standard procedure except for the absence of CTAB (Figure S5a, Supporting Information), a Pd narrow slide was epitaxially deposited on the periphery of the nanosheet with a solid interior (marked by red cycles). This demonstration was clearly confirmed by using larger Pd nanosheets of 24.5 nm in size as the seeds (Figure S5b, Supporting Information). This result indicates that no oxidative etching was involved when CTAB was excluded from the reaction, which is consistent with the previous report.[39] In addition, some Pd nanosheets were enlarged through lateral overgrowth relative to the origin nanosheets, indicating that Br− ions derived from CTAB could promote the vertical overgrowth by selectively stabilizing the side surface of the nanosheets (marked by blue cycles). In the absence of Pd(acac)2 with all other parameters being the same as in the standard procedure, the irregularly broken Pd nanocrystals with a hollow structure were formed (Figure S5c,d, Supporting Information), indicating that oxidative etching was initiated from the {111} facets of the nanosheets. Due to the ultrathin thickness, the Pd nanosheets were easily broken during the dissolution caused by the oxidative etching in the absence of overgrowth. Taken together, the selectively epitaxial overgrowth in combination with the oxidative etching

are both indispensible processes for the formation of the Pd nanorings. Interestingly, the approach presented here can also be extended to generate Pd–Pt core-shell bimetallic nanorings by replacing Pd(acac)2 with Pt(acac)2 as the salt precursor. From the TEM images in Figure 4a,b, the hexagonal Pd–Pt nanorings of 15.4 nm in size with a hollow interior were successfully generated. The HRTEM image of an individual Pd–Pt nanoring that lies down on the TEM grid suggests that the nanoring is a single crystal in nature (Figure 4c). The internal structure and thickness of the Pd–Pt nanoring were characterized by taking HRTEM observation on such a vertically standing nanoring (Figure 4d). As can be seen, the thickness of the Pd–Pt nanoring was measured to be ≈2 nm relative to the starting Pd nanosheet with thickness of 1.1 nm. Careful observation reveals that there are {111} stacking faults parallel to their flat faces in such a nanostructure. The overall morphology of the nanorings was also supported by the high-angle annular dark-field scanning TEM (HAADF-STEM) image in Figure 4e. The corresponding energy-dispersive X-ray (EDX) mapping images (Figure 4f,g) show that the nanoring was made of a core-shell structure, with both Pd (green color) and Pt (red color) being homogeneously distributed throughout the nanoring. The atomic ratio of Pd and Pt in this core-shell structure was about 2.5:1 as quantitatively determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). We believe that this approach can be extended to synthesize other Pd-based

Figure 4. a,b) TEM images of the Pd–Pt nanorings. c,d) HRTEM images of an individual nanosheet that lies flat and stands vertically on the TEM grid, respectively. e) HAADF-STEM and f,g) EDX mapping of the Pd–Pt nanorings. The green and red colors in (f) and (g) correspond to Pd and Pt elements, respectively. small 2015, 11, No. 36, 4745–4752



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bimetallic nanorings, which might be fundamentally important to a rich variety of catalytic reactions. The reduction of p-nitrophenol into p-aminophenol by NaBH4 here was employed as a model reaction to quantitatively evaluate the catalytic property of the ultrathin Pd nanorings.[43,44] We benchmark their catalytic performance for hydrogenation against the commercial Pd black (99.95%, Sigma-Aldrich) as well as the as-prepared Pd nanosheets. Owing to the color change involved in the hydrogenation, we can easily monitor the kinetic process of this reduction reaction by recording the intensity change of the characteristic absorption peak at 400 nm associated with p-nitrophenol, as shown in Figure 5a. In addition, the concentration of NaBH4 (43.5 × 10−3 m) greatly exceeded that of p-nitrophenol (0.145 × 10−3 m) in our reaction system so that the reaction was first order in the concentration of p-nitrophenol. According to this pseudo-first-order kinetics,[45] we could calculate the apparent reaction rate (kapp) from the linear plot of −ln(Ct/C0) versus the reaction time (Figure S6, Supporting Information). To exclude the influence of the amount of the catalysts and show the intrinsic catalytic properties of the Pd nanorings, we determined k1 from the following equations by normalizing kapp with their mass (M) − dc t /dt = k app c t = K 1 Mc t

(1)

Figure 5b shows a comparison of k1 for the five catalysts including Pd black, 15.7 nm Pd nanosheets, 6.4 nm, 15.7 nm, and 24.5 nm Pd nanorings. It is clear that the Pd nanorings exhibited much higher activity than the commercial Pd black

as well as the Pd nanosheets due to their unique hollow 2D structures. After calculating both the specific surface areas of a nanoring and nanosheet with an edge length of 15.7 nm (see Figure S7 in the Supporting information), we found that the specific surface area of a Pd nanoring is only slightly larger than that of a Pd nanosheet. As such, the enhanced catalytic properties of the Pd nanorings mainly arise from their active interior part with rough surface containing a large number of active sites, unsaturated bonds, and highindex facets. Obviously, the rough surface of the interior part was achieved by the oxidative etching, which was observed by the HRTEM analysis, as shown in Figure 2c. As the size of the nanorings was reduced, their k1 climbed up from 41.7 to 56.8 and 60.2 s−1 g−1 L, respectively. This catalytic reaction was repeated two times for five samples including Pd black, Pd nanosheets, and Pd nanorings with different sizes, which showed a good reproducibility. Among them, the 6.4 nm Pd nanorings showed the highest k1 (60.2 s−1 g−1 L) toward the reduction of p-nitrophenol, which was 2.2 and 33.4 times higher than those of the 15.7 nm Pd nanosheets and the commercial Pd black. In addition, we also evaluated the stability for the reduction of p-nitrophenol by choosing the 15.7 nm Pd nanorings, 15.7 nm Pd nanosheets, and commercial Pd black as the catalysts. Figure 5c shows the plots of k1 for these three catalysts against the cycle number of successive reduction reactions. As can be seen, k1 of the 15.7 nm Pd nanorings was kept at above 55 s−1 g−1 L in the first six cycles and then slightly dropped by ≈10.7% in the following four cycles. In comparison, k1 of the Pd black and 15.7 nm Pd nanosheets continuously decreased over the cycle numbers and finally got a drop

Figure 5. a) Extinction spectra taken at different reaction times with 15.7 nm Pd nanorings as the catalysts. b) The calculated k1 of the five catalysts for the first cycle. c) Plots of k1 against the number of successive reduction reactions. d) The calculated k1 of the different catalysts for the tenth cycle.



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of 72.2 and 45.7% in the tenth cycle of the reaction, respectively. Indeed, the 15.7 nm Pd nanorings remained ≈89.3% of the initial activity (50.7 s−1 g−1 L), which was 3.4 and 101.4 times higher than those of the 15.7 nm Pd nanosheets and commercial Pd black after stability test for ten cycles, respectively (Figure 5d). This result indicates that the Pd nanorings showed the substantially enhanced stability over the reduction of p-nitrophenol relative to the Pd nanosheets and commercial Pd black. TEM observation was employed to clarify the morphology evolution of the catalysts after stability test. It is indicated that the Pd nanosheets suffered from serious twist, deformation, and aggregation after ten cycles of the reactions, while the morphology of the Pd nanorings almost kept intact except for a slight aggregation (Figure S8, Supporting Information). It is clear that the Pd nanorings were much thicker than the Pd nanosheets (≈2 nm vs 1 nm), which was the main reason for the different stability toward hydrogenation. In addition, the Pd nanorings with a hollow interior could sustain the deformation better than the Pd nanosheets during the catalytic reaction since the hollow interior could release the deformation. As a result, the difference in structure stability of the Pd nanorings and nanosheets over the reduction of p-nitrophenol was responsible for the different stability.

3. Conclusion In summary, we have demonstrated a facile and versatile approach for the synthesis of atomically ultrathin Pd nanorings with a tunable edge length. We found that the Pd nanorings were formed by selective epitaxial growth of Pd atoms on the periphery of the as-preformed Pd nanosheets in combination with oxidative etching arising from Br−/O2 pairs. This approach can also be extended to generate Pd-based bimetallic nanorings such as Pd–Pt by replacing Pd source with Pt salt precursor fed in the synthesis. The Pd nanorings exhibited substantially enhanced catalytic properties in terms of activity and stability toward the reduction of p-nitrophenol relative to the Pd nanosheets and commercial Pd black because of their unique structural features. This work not only provides a general and effective strategy to shape-controlled synthesis of ultrathin 2D Pd-based nanorings with a hollow structure, but also opens up a new opportunity to design advanced catalysts with enhanced performance for a rich variety of potential applications.

4. Experimental Section Chemicals and Materials: Palladium(II) acetylacetonate (Pd(acac)2, 99%), platinum(II) acetylacetonate (Pt(acac)2, 97%), palladium black (40–60 m2 g−1, 99.95%), L-ascorbic acid (AA, 99%), polyvinyl pyrrolidone (PVP, MW = 29 000), and tungsten hexacarbonyl (97%) were purchased from Sigma-Aldrich. Sodium borohydride (NaBH4, 98%) and p-nitrophenol (AR) were purchased from Aladdin Industrial, Inc. Citric acid (CA), hexadecyltrimethylammonium bromide (CTAB), N,N-dimethylformamide (DMF, AR), ethanol (AR), and acetone (AR) were purchased from Sinopharm small 2015, 11, No. 36, 4745–4752

Chemical Reagent Co., Ltd. All the chemicals and materials were used as received. Synthesis of Pd Nanosheets with Different Sizes: In a typical synthesis of 15.7 nm Pd nanosheets, 16 mg of Pd(acac)2, 90 mg of CA, 60 mg of CTAB, and 30 mg of PVP were dissolved into 10 mL of DMF and stirred for 1 h. The obtained homogeneous orange red solution was then transferred into a 25 mL flask and 100 mg of tungsten hexacarbonyl was added to the flask under Ar atmosphere. The flask was capped and heated at 80 °C for 1 h. After the reaction, Pd nanosheets were collected by centrifugation using a sufficient amount of acetone, and then redispersed in ethanol. This process was repeated three times. Pd nanosheets with an average edge length of 24.5 and 6.4 nm were synthesized by varying the amount of CA to 50 and 170 mg, respectively. Synthesis of Pd Nanorings with Different Sizes: In a standard procedure, Pd nanorings with an average edge length of 15.7 nm were synthesized by mixing 13.1 µmol of 15.7 nm Pd nanosheets, 4 mg of Pd(acac)2, 50 mg of AA, 60 mg of CTAB, and 50 mg of PVP into 10 mL of DMF in a 25 mL flask. Subsequently, the flask was capped and heated at 80 °C for 1 h. After the reaction, the Pd nanorings were collected by centrifugation using a sufficient amount of acetone, and then redispersed in ethanol. This process was repeated three times. The 24.5 and 6.4 nm Pd nanorings were also synthesized by using the as-preformed Pd nanosheets with the corresponding similar sizes as the seeds in the presence of 120 and 30 mg CTAB, respectively. Synthesis of Pd–Pt Nanorings: Pd–Pt nanorings with an average edge length of 15.7 nm were synthesized by mixing 13.1 µmol of 15.7 nm Pd nanosheets, 10.3 mg of Pt(acac)2, 100 mg of AA, 30 mg of CTAB, and 50 mg of PVP into 10 mL of DMF in a 25 mL flask. Subsequently, the flask was capped and heated at 140 °C for 2 h. After the reaction, the Pd–Pt nanorings were collected by centrifugation using a sufficient amount of acetone, and then redispersed in ethanol. This process was also repeated three times. Catalytic Reduction of p-Nitrophenol: The reduction of p-nitrophenol by NaBH4 was used as a model reaction to evaluate the catalytic properties of the Pd nanorings. First, the aqueous solution of p-nitrophenol (14.8 × 10−3 M) and NaBH4 (4.44 M) were freshly prepared. After that, 20 µL of p-nitrophenol and 20 µL of NaBH4 aqueous solution were both added to a quartz cuvette. Second, 2 mL of aqueous solution containing the Pd nanorings (0.5 µg mL−1) was injected into the cuvette to initiate the reaction. The intensity of the absorption peak at 400 nm in UV–vis spectroscopy was used to monitor the conversion of p-nitrophenol to p-aminophenol by NaBH4. Finally, another fresh 20 µL of p-nitrophenol (14.8 × 10−3 M) and 20 µL of NaBH4 (4.44 × 10−3 M) aqueous solution were further added to the quartz cuvette after each cycle of the reaction to investigate the stability of the catalysts. This step was repeated nine times. The Pd nanosheets and Pd black with the same concentration (0.5 µg mL−1) were also evaluated as the catalysts towards hydrogenation through the same procedure. Morphological, Structural, and Elemental Characterizations: TEM images were obtained using a Hitachi HT-7700 microscope operated at 100 kV. HRTEM was performed using an FEI Tecnai G2 F20 microscope operated at 200 kV. HAADF-STEM and EDX mapping analyses were taken on an FEI Titan ChemiSTEM equipped with a probe-corrector and a Super-X EDX detector system. This microscope was operated at 200 kV with a probe current of 50 pA



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and a convergent angle of 21.4 mrad for illumination. UV–vis spectrum was measured by Ocean Optics DH-2000 light source.

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Supporting Information

[21]

Supporting Information is available from the Wiley Online Library or from the author.

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Acknowledgements

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The work on electron microscopy was carried out in the Center for Electron Microscopy of Zhejiang University. The authors acknowledge ﬁnancial support by the National Science Foundation of China (51372222, 51222202, and 51472215), the National Basic Research Program of China (2014CB932500 and 2015CB921000), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037 and IRT13R54), and the Fundamental Research Funds for the Central Universities (2014FZA4007). We would like to thank Prof. Jun Yuan from University of York for HRTEM analysis.

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Received: March 19, 2015 Revised: June 1, 2015 Published online: July 6, 2015

small 2015, 11, No. 36, 4745–4752

Flower-like RuCu nanodendrites as catalysts for hydrogenation of p-nitrophenol with β-cyclodextrin as promoters.

TiO2 Catalysts for the Selective Hydrogenation of Butadiene.

Imine hydrogenation by alkylaluminum catalysts.

CNN pincer ruthenium catalysts for hydrogenation and transfer hydrogenation of ketones: experimental and computational studies.

Cobalt precursors for high-throughput discovery of base metal asymmetric alkene hydrogenation catalysts.

Optical high-throughput screening for activity and electrochemical stability of oxygen reducing electrode catalysts for fuel cell applications.

Rational development of iron catalysts for asymmetric transfer hydrogenation.

Iron-based catalysts for the hydrogenation of esters to alcohols.

Non-Pincer-Type Manganese Complexes as Efficient Catalysts for the Hydrogenation of Esters.

Half-sandwich rhodium(III) transfer hydrogenation catalysts: Reduction of NAD(+) and pyruvate, and antiproliferative activity.

Synthesis of carbon embedded MFe2O4 (M = Ni, Zn and Co) nanoparticles as efficient hydrogenation catalysts.

High mechanical and tribological stability of an elastic ultrathin overcoating layer for flexible silver nanowire films.

Preparation of silica-coated ultrathin gold nanowires with high morphological stability.

Easy To Synthesize, Robust Organo-osmium Asymmetric Transfer Hydrogenation Catalysts.

Exploring the decomposition pathways of iron asymmetric transfer hydrogenation catalysts.

Versatile iridicycle catalysts for highly efficient and chemoselective transfer hydrogenation of carbonyl compounds in water.

Amine(imine)diphosphine iron catalysts for asymmetric transfer hydrogenation of ketones and imines.

Highly active, low-valence molybdenum- and tungsten-amide catalysts for bifunctional imine-hydrogenation reactions.

High activity redox catalysts synthesized by chemical vapor impregnation.

Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation.

A polyphenylene support for Pd catalysts with exceptional catalytic activity.

Highly selective hydrogenation of arenes using nanostructured ruthenium catalysts modified with a carbon-nitrogen matrix.

Ultrasound-assisted selective hydrogenation of C-5 acetylene alcohols with Lindlar catalysts.

Design of ultrathin Pt-Mo-Ni nanowire catalysts for ethanol electrooxidation.