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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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b06491 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016
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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, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China ‡
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, USA
*
Address correspondence to
[email protected].
KEYWORDS. Palladium nanocrystals • etching • regrowth • nanoframes • formic acid oxidation
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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 the 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. Based on 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.
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Maximizing the activity of heterogeneous catalysts is mightily desired due to its economic benefits to a set of 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 contains the dramatically enhanced fraction of corner and edge sites with distinct reactivity, represents an important type of remarkably optimized catalysts that can show the highest activities.9,11,12 As a typical example, Peidong 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 metal nanocrystals into nanoframes. The main problem is that the synthetic approach to metal nanoframes is still limited, especially those monometallic nanocrystals. Palladium, an important member of platinum group metals, has been widely used as heterogeneous catalysts 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
One now is able to prepare the above mentioned Pd nanocrystals by
carefully manipulating the reaction conditions such as 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 has been reported to the successful
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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; (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
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Figure 1. Schematic illustrating the shape transformations of Pd nanocrystals through the delicate control over the etching and regrowth rates.
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 pre-synthesized Pd nanocrystals.40 Although the combination of the etching and regrowth process provide the possibility to the shape modification of pre-formed 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 the great potential in constructing rational nanostructures of metal nanocrystals for catalytic applications. Figure 1 shows our tactic for the synthesis of Pd nanoframes by excavating solid Pd nanocrystals. The key of this synthesis should rely on the 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
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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 of Rcorner,regrowth > Redge,regrowth > R face,regrowth, respectively.41,42 Imaging that the etching rate is kept constant, and then we 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 would be much smaller than the etching rate regardless of the types of surface sites (Rregrowth > Retching), the etching would be significantly hindered, leaving the solid nanocrystals unchanged (Figure 1, route 4).
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 , and zone axes, and the corresponding Fourier transform (FT) patterns, respectively. (d) A 3D model of a Pd octahedral nanoframe and its projections along , and zone axes.
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, ESI, shows a typical TEM image of the Pd octahedrons, with purity approaching 100%. The average length of these octahedrons was 37 nm. Then, these Pd octahedrons were washed and redispersed in N, N-dimethylforamide
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(DMF) in the presence of poly(vinylpyrrolidone), with O2/KI and formaldehyde (HCHO) were served 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
