Synthesis of Pt dendritic nanocubes with enhanced catalytic properties

Abstract

Pt dendritic nanocubes (DNCs) were successfully synthesized in oleylamine under a 1 bar H₂ atmosphere. The catalytic activity of Pt DNCs was much greater than conventional Pt nanoparticles (NPs) for the methanol electrooxidation reaction. Pt DNCs also showed excellent catalytic performance in the hydrogenation of unsaturated hydrocarbons and nitrobenzene.

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Noble metal nanomaterials have been extensively studied for catalytic and electrocatalytic applications recently.^1–3 In addition to large surface area-to-volume ratios, the morphology of nanomaterials has been widely pursued because the size and shape exercises a great influence on their surface area and exposed crystalline facets, and consequently their catalytic reactivity.^4–7 As one of the most important noble metals, Pt based nanomaterials are major catalysts for many industrial applications, including fuel-cell technologies, fine chemical synthesis and gas sensing etc.^8–12 A lot of studies have been devoted to the shape control of Pt nanomaterials such as nanoparticles, nanorods, nanowires, nanocubes, nanoframes, dendritic nanostructures, branched-NPs and multipods.^13–23 Among these morphologies, the dendritic nanostructures possess high surface area with open pores and rich edge/corner atoms, which make them highly favorable for low-cost catalytic applications.^24–27 However, the existing synthetic procedures are still requiring multiple steps or harsh conditions.^14,28 Therefore, single step methods for synthesizing Pt DNCs are highly desired. Herein, we report a facile approach for the synthesis of Pt DNCs under mild conditions. The formation mechanism was investigated and the electrocatalytic methanol oxidation activity of resulting Pt DNCs was much greater than conventional Pt NPs. Subsequently, Pt DNCs showed excellent catalytic performance in the hydrogenation of unsaturated hydrocarbons and nitrobenzene.

As illustrated in Scheme 1, the synthesis route was relatively straightforward: the Pt DNCs were synthesized via reduction of Pt(acac)₂ in oleylamine (OAm) under 1 bar hydrogen atmosphere. Typically, Pt(acac)₂ was dissolved in OAm in a three-necked round-bottom flask. The mixture was heated to 90 °C under 1 bar hydrogen atmosphere, maintained at this temperature for several hours, and then cooled to room temperature. The final product was precipitated out via centrifugation at 4000 rpm for 10 min. The products was dispersed in hexane for further use.

Scheme 1 The illustration of the formation of Pt DNCs.

Fig. 1A shows transmission electron microscopic (TEM) images of the as-synthesized Pt DNCs. The high-resolution TEM image (Fig. 1B) revealed that the as-prepared Pt DNCs were well crystallized. The interfringe distances are measured to be 0.19 nm and 0.23 nm, corresponding to (200) and (111) lattice spacing of the face-centered cubic Pt. The crystal structures were further determined by X-ray powder diffraction (XRD) analysis (Fig. S1†), where the pattern positions and relative intensities for (111), (200), (220), (311) and (222) matched well with standard patterns of face-centered cubic Pt.

Fig. 1 TEM and HRTEM images of Pt DNCs.

As shown in Fig. 2, the TEM images of the nanostructures were taken as a function of reaction time, which elucidate the formation process for the dendritic nanocubes. When the reaction temperature was raised to 90 °C for 1 min, the nanoseeds appeared (Fig. 2A), and first-generation Pt branches were grown on the surface of the seeds (Fig. 2B and C) in the first 10 minutes. After a further 20 minutes, second and third generation Pt branches grew on the arms of the first-generation metal branches to form the dendritic nanostructures (Fig. 2D). The nanostructure formed uniform dendritic nanocubes after 2 h reaction (Fig. 2E).

Fig. 2 Time dependent TEM images of Pt nanocrystals obtained at (A) 1 min, (B) 5 min, (C) 10 min, (D) 30 min, (E) 2 h and (F) 5 h.

According to the tracking analysis in the process of Pt DNCs formation, the dynamic composition of oleylamine is the key factor for Pt DNCs.^29–31 Interestingly, we found the growth manner is closely related to the transition of the solvent from oleylamine to octadecylamine under the Pt-catalyzed hydrogenation (Fig. 3). Such conclusion arise from the time-lapse detection on the solvent by ¹HNMR characterizations. Octadecylamine is detected after a very short reaction time and finally replace the original solvent of oleylamine at round 24 h. These data of molar ratio of octadecylamine is calculated of the chemical shift of hydrogen in different chemical environments from the NMR characterization (Fig. S2†) as well as the growth manner of Pt nanocrystals (Fig. 2 and S3†). We also conducted a series of experiments that solely utilizes pure octadecylamine, pure octadecene as solvents to grow the Pt nanocrystals, no nanobranches were formed after 2 hours (Fig. 4). These results reveal that the both the alkenyl and amino groups take crucial role in tuning the nanobranches, which could be degradation with reaction time due to the Pt-catalyzed hydrogenation.

Fig. 3 The plot showing the molar ratio of octadecylamine varies with the reaction time. The ratio of each time point is calculated from the ¹HNMR data during the hydrogenation of oleylamine over Pt nanocrystals to octadecylamine in different reaction time.

Fig. 4 TEM image of Pt nanocrystals prepared in pure octadecylamine (A) and octadecene (B) in hydrogen atmosphere.

We further examined the electrocatalytic behaviors of the Pt DNCs and found the electrocatalytic activity could be enhanced by introducing the dendritic structures onto Pt nanocubes. Fig. 5A shows a typical cyclic voltammogram (CV) of Pt DNCs and Pt NPs (Fig. S4†) in an N₂-saturated solution of 0.5 M H₂SO₄. It is evident that the cathodic and anodic peaks between −0.25 V and 0.05 V (vs. SCE) were attributed to the adsorption and desorption of a monolayer of hydrogen in the acidic medium. The peaks at around 0.3–0.7 V were reduction peak of oxides. It is observed that three peaks of hydrogen desorption is characterized for Pt DNCs, which corresponds the facets of Pt (111), (110) and (100), respectively. In contrast, only one peak that corresponds to Pt (110) was observed for Pt NPs. Further, we utilize the electrochemically active surface area (ECSA) to characterize the catalytic activity of two samples, which can be calculated according to the peak area of hydrogen desorption:

where Q_H (mC cm⁻²) is the charge associated with hydrogen desorption, q_H (0.21 mC cm⁻²) is a monolayer of hydrogen on a Pt surface, L_metal is the metal loading (mg cm⁻²) on the GC electrode (3 mm in diameter). The calculated values of ECSA (Table S1†) shows that Pt DNCs (38.5 m² g⁻¹ Pt) possess more than two fold enhanced ECSA than that of Pt NPs (18.52 m² g⁻¹ Pt). This means that DNCs have more catalytic activity. Fig. 5B shows a typical cyclic voltammogram (CV) of Pt DNCs and Pt NPs in an N₂-saturated solution of 0.5 M H₂SO₄ and 0.5 M CH₃OH. Both the two catalytic reactions show two peaks, which corresponds to oxidation of methanol (∼0.62 V) and reduction of adsorbed OH or Pt (∼0.42 V). Besides, the forward scanning peak of Pt DNCs is about 5.3 fold higher than that of Pt NPs, showing a higher catalytic activity, which is also consistent to the calculated ECSA results.

Fig. 5 (A) CVs of Pt DNCs and Pt NPs in a N₂-saturated aqueous solution of 0.5 M H₂SO₄. Scan rate: 50 mV s⁻¹ and (B) CVs of the methanol oxidation catalyzed by Pt DNCs and Pt NPs in N₂-saturated 0.5 M H₂SO₄ + 0.5 M CH₃OH. Scan rate: 50 mV s⁻¹.

As Pt based materials is an excellent catalyst used in hydrogenation, the chemical catalytic activity of Pt dendritic nanocubes was investigated with alkenes, alkynes and nitrobenzene as substrates. As shown in Table 1, styrene, p-methyl styrene and p-methoxy styrene were converted completely to form C–C single bond in 1 h when 0.5 mol% of catalyst was carried out. In the view of the hydrogenation of alkynes, 1-ethynylbenzene, p-methyl and p-methoxy ethynylbenzene provided 100% conversion to corresponding ethylbenzene products, respectively. With Pt dendritic nanocubes catalysts, nitrobenzene, p-methyl nitrobenzene and p-methoxy nitrobenzene were converted to the corresponding aniline products in 3 h.

Table 1 Hydrogenation of olefin, alkynes and nitrobenzene over Pt DNCs^a

Enter	Substrate	Product	Yield^b (%)	Time (h)
a All reactions were carried out with 1 mg Pt dendritic nanocubes, 1.0 mmol substrates and 2 mL methanol at 50 °C in hydrogen balloon condition.b GC Yield.
1			>99	1
2			>99	1
3			>99	1
4			>99	1
5			>99	2
6			>99	3
7			>99	3
8			>99	3
9			>99	3

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In summary, we have successfully prepared Pt DNCs via a single step route under hydrogen atmosphere. The Pt DNCs exhibit higher electrocatalytic activity than Pt NPs and excellent catalytic performance in the hydrogenation of unsaturated hydrocarbons.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21373006, 51402203) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available: Details of general experimental procedures. See DOI: 10.1039/c4ra16920d

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