Green self-redox synthesis of Rh-PPy-RGO ternary nanocomposite with highly increased catalytic performances

1 Introduction

Owing to its incomparable merits like highly catalytic activity, stability, and excellent selection, noble metal nanoparticles have attracted increasing interests, and have been used as highly active catalysts in various fields, such as batteries, hydrolysis reactions and hydrogenation reactions [1–6]. Up to now, numbers of works have been reported with the focus on designing of functional noble metal nanoparticles with controllable sizes, shapes and compositions [7–14]. However, with the decreasing size of noble metal nanoparticles, the aggregation problem has become very serious, which may be caused by the high surface energy. In addition, the surface states of metals nanoparticles are also destroyed seriously. As a result, the original highly active noble metal nanoparticles may lose their catalytic properties heavily, which hinders its application in industrial use [15–19]. Considering these problems, an appropriate substrate material is needed to immobilize the noble nanoparticles in or on its surface and maintain the catalytic activity [20, 21].

Besides the catalytic ability of metal nanoparticles themselves, it has been proposed that the support material also plays a critical role in the catalytic performance of noble metal catalysts [22–25]. Reasonable design of the support material can largely increase the catalytic properties. Graphene, as a kind of layered sp²-hybridized carbon material, is most frequently used as an efficient support to host the growth of noble metal nanoparticles due to its huge surface area to bulk ratios, good thermal stability, high conductivity and low cost. More importantly, its surface is much easier to be modified with functional groups on their basal planes and edges, which could play an important role in depositing noble metal nanoparticles uniformly onto the graphene surface with tunable size. Upon this, graphene and its derivatives have provided a new class of path for utilizing two-dimensional or multidimensional carbon materials as support in catalytic area application [26–30].

Although many achievements of using reduced GO (RGO) as supports have been obtained, it remains a major challenge to address the weak anchoring affinity between graphene and noble metal nanoparticles. RGO nanosheet is charge-negative, however, the noble metals are also charge-negative, [31–34] as a result, the binding force between RGO and the noble atoms is relatively weak, and the noble metals are extremely easy to come off from the RGO support in the long term catalytic reactions [31]. How to enhance the binding force between noble metals and RGO is still an urgent issue to be solved. It is well known that polypyrrole (PPy) is also a kind of good support for growing and loading noble metal nanoparticles [31, 32]. On the basis of its good mechanical properties and environmental stability, PPy could be exploited in various potential technological applications. The construction of PPy-based nanocomposites is currently one of the most promising research areas in the field of polymer science and engineering. Moreover, the surface of PPy is charge-positive, that means if PPy component is successful hybridized in noble metal-RGO composite, and the binding force could be highly enhanced. This not only prevents the agglomeration of graphene layers but also reinforced their binding force, and also optimized the structure of the ternary hybrid materials [35–40].

Inspired by previous reports, the property of PPy has been noticed that the polymerization is an oxidation process. Pyrrole is unstable when oxidizing agent is added, such air and Fe^3 + ions. On the other hand, it was also proved that both of the noble metal salts and GO has strong oxidation ability. A redox relationship should exist among pyrrole, noble metal salt and GO component. In this work, triple-component nanocatalyst, noble metal-PPy-RGO has been successfully fabricated via a simple and green self-redox reaction. Experimentally, GO aqueous solution, noble metal salt and pyrrole are mixed together under Ar protection, followed by a heating treatment to trigger the redox reaction happen.

2 Results and discussion

The experimental process was showed as scheme 1, and the experiamental operations were showed in supporting information. The transmission electron microscope (TEM) images in Fig. 1 show that the as-obtained Rh-PPy nanocomposites are loaded onto the RGO surface uniformly, as shown in Fig. 1(a). A magnified image in Fig. 1(b) shows that the noble metal nanoparticles are randomly decorated on the surface of RGO, and the edges and folds of the graphene sheets are clearly visible. No scattered nanoparticles are found outside the RGO nanosheets. The diameter of Rh nanoparticle is about 4 nm in average. The HR-TEM image firmly confirms that the lattice spacing is 0.216 nm, which corresponds well with the characteristic (111) planes of Rh, Fig. 1(c). Furthermore, the typical lattice in the HR-TEM images illustrates the crystalline of Rh nanocrystals. Meantime, the Rh-PPy-RGO composites can be further confirmed from the energy-dispersive X-ray spectrum (EDX), as shown in Fig. 1(d), which revealed the existence of Rh, O, C and N elements.

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Scheme 1

Schematic illustration of the procedure used to fabricate Rh-PPy-RGO hybrids.

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Fig.1

(a, b) TEM images and (c) HR-TEM image of as-prepared Rh-PPy-RGO nanocomposite. (d) EDX spectrum of Rh-PPy-RGO nanocomposite.

Besides the synthesis of Rh-based nanocomposite, we also used other kinds of noble metal salts, such as K₂PtCl₄ and Na₂PdCl₄, to replace H₃RhCl₆ to react with pyrrole under the similar synthetic conditions. With the typical TEM images shown in Fig. 2, Pt-PPy-RGO and Pd-PPy-RGO hybrids are obtained. Interestingly, Pt-PPy-RGO nanocomposite has similar nanostructure with that of Rh-PPy-RGO nanocomposite, as shown in Fig. 2(a, b). But Pd-PPy-RGO has a total different nanostructure, in which several tiny Pd nanoparticles are assembling together to form a qusi-nanosphere shape, Fig. 2(c, d). The enlarge TEM image in Fig. 2(d) confirmed that the nanosphere is composed by Pd nanoparticles and PPy component. Such hybrid nanostructure is much similar with previously reported result.

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Fig.2

(a, b) Low- and high-magnification TEM images of Pt-PPy-RGO nanocomposites. (c, d) Low- and high-magnification TEM images of Pd-PPy-RGO nanocomposites.

To determine the oxidation state of carbon in Rh-PPy-RGO, XPS analysis was performed and the results are shown in Fig. 3. As shown in Fig. 3(b), the two peaks at 309 eV and 314 eV are corresponding well to Rh 3d5/2 and Rh 3d3/2, respectively. The peak associated with C–C (284.1 eV) becomes predominant, while the peaks related to the oxidized carbon species, such as C–OH (285.2 eV) and C = O (287.5 eV), are greatly weakened.

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Fig.3

XPS spectra of (a) Rh-PPy-RGO nanocomposite, (b) Rh 3d, (c) C 1 s and (d) N 1 s.

These findings indicate that GO was strongly deoxygenated and reduced to RGO. Otherwise, the nitrogen and oxygen signals are also clearly detected in the composite material. In addition, the N 1 s spectrum in Fig. 3(d) has shown that the two main contributions of binding energy at 401.3 eV and 399.2 eV. The peak at 399.2 eV is attributed to neutral nitrogen moieties (–NH–) in polypyrrole. The very low intensity peak at 401.3 eV is related to positively charged nitrogen species, which is consistent with a mostly neutral nature of polypyrrole in the composite. Based on the above analysis, it can be concluded that Rh-PPy-RGO hybrid materials are successfully produced. The possible reaction process is listed below:

Rh^3 + + GO + pyrrole ⟶ Rh-PPy-RGO

In order to further study the component content of Rh-PPy-RGO, the thermal gravimetric analysis was carried out. As shown in Fig. 4, a clear weight loss process appears between 200°C and 600°C, which could be attributed to the oxidation process. Thus the content of Rh could be calculated from the TGA result, as 4.2%.

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Fig.4

Thermal gravimetric analysis of Rh-PPy-RGO nanocomposite.

H₂ gas and NaBH₄ are used as the sources of hydrogen in most of traditional hydrogenation reactions. But there are two disadvantages seriously limited their further application for a H₂-based catalytic reaction. The first disadvantage is the storage problem, and the other one is the current hydrogenation reaction needs pressurized conditions. Under atmospheric pressure, the catalytic reaction is difficult to achieve using H₂, as the activation of H₂ on noble metal surfaces requires highly pressurized conditions, which are very dangerous. For a NaBH₄-based catalytic reaction, instability makes the reaction unsuitable for practical applications. As a hydrogen reservoir, Ammonia Borane (AB, NH₃BH₃) has drawn much attention due to its high hydrogen capacity of 19.6 wt %, strong stability, and nontoxicity. Thus, a new, safe hydrogenation reaction, using AB to replace NaBH₄ or H₂ gas in the catalytic liquid-phase reduction of p-nitrophenol (4-NP) has been developed; the catalytic properties of the as-prepared hybrid catalysts in this hydrogenation reaction are well-tested. It is well-known that the 4-NP solution exhibits a strong absorption peak at 317 nm under neutral or acidic conditions. As the alkalinity of the solution increases, 4-NP ions become the dominant species, producing a spectral shift to 400 nm. Therefore, in this work, we selected the classical catalytic reaction of chemical reduction of 4-NP ions by AB as the reaction model.

On the contrary, when the Pt-PPy-RGO is added, an obvious decrease of C/C₀ was found. The test shows that the activity of Pt-PPy-RGO is greatly enhanced, Fig. 5(a). Furthermore, when using Rh-PPy-RGO as the catalyst, the catalytic reaction can be realized completely in 600 s. More important, it is noted that ln(C/C₀) has a linear relationship with the change of time, which proves that the reduction reaction is a typical first order reaction. In addition, the effect of different amount of catalysts (Rh-PPy-RGO) on the catalytic reaction rate was also investigated. As shown in Fig. 5(b), with the increase of the amount of catalyst, the catalytic activity is more and more fast. When 60μL of catalyst is added, the whole catalytic reaction takes about 350 s to complete. Finally, we changed the amount of AB to investigate the effect of the catalytic activity. The changing the amount of AB does not change the catalytic rate or the catalytic type. It is still a typical first order reaction, Fig. 5(c).

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Fig.5

(a) The catalytic ability of Pt-, Pd- and Rh-PPy-RGO nanocomposite on 4-NP reduction. (b) The catalytic ability of different amount of Rh-PPy-RGO nanocomposite. (c) The catalytic ability of Rh-PPy-RGO nanocomposite on different amount of AB.

In conclusion, a redox reaction has been triggered among noble metal salts, pyrrole and graphene oxide. Herein, pyrrole plays the important role of reducing agent, which can efficiently react with noble metal salts and graphene oxide to form noble metal nanoparticles and RGO component. It should be noticed that such redox reaction is happened on the surface of each component, which could make sure the as-formed different components are coupled together strongly. Interestingly, no organic surfactant is needed in the whole synthesis, that could keep the active centers be with clean surface states. As a result, a series of noble metal-PPy-RGO hybrids are successfully fabricated, such as Pt-, Pd- and Rh-PPy-RGO nanocomposite. In catalytic test, the as-obtained Rh-based nanocatalyst exhibited the excellent catalytic activity and stability, which could be attributed to the unique hybrid nanostructure. It is believed that such clean synthesis has opened a new window for the design and fabrication of functional nanocomposite with high efficiency.