Ru x Pd y Alloy Nanoparticles Uniformly Anchored on Reduced Graphene Oxide Nanosheets (Ru x Pd y @rGO): A Recyclable Catalyst.

In this work, Ru x Pd y alloy nanoparticles were uniformly decorated on a two-dimensional reduced graphene oxide (rGO) sheet by an in situ chemical co-reduction process. The resulting products were characterized by various physiochemical techniques such as X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic absorption spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Further, the synthesized Ru x Pd y @rGO nanocomposites have been employed as a heterogeneous catalyst for three different catalytic reactions: (1) dehydrogenation of aqueous ammonia borane (AB); (2) hydrogenation of aromatic nitro compounds using ammonia borane as the hydrogen source, and (3) for the synthesis of aromatic azo derivatives. The present work illustrates the sustainable anchoring of metal nanoparticles over the surface of rGO nanosheets, which could be used for multifarious catalytic reactions.

Introduction

In the last decade, metal nanoparticles (MNPs) have been extensively employed as catalysts in various organic transformations because of some distinct properties shown by them as compared to their bulk counterpart.[1−4] However, the major limitation associated with the MNPs has been their poor recyclability from the reaction mixture after the completion of the reaction.[5] This not only causes the contamination of the product but also results in the wastage of the catalyst (which reduces their cost-effectiveness). In the recent trend of nanocatalysis, a number of inorganic support materials have been used for incorporating metal and alloy nanoparticles. A number of different types of support materials have been employed for this purpose and these include Fe3O4 nanoparticles, SBA-15 silica, KCC-1 silica nanospheres, and various other nanocarbon materials.[6−9]

Graphene, a monolayer 2-D material having a sp2 hybridized carbon network has been extensively utilized for various applications.[10] This is due to its high conductivity, high mechanical strength and also due to its excellent chemical and thermal stability.[11] It is because of all these exceptional properties that graphene and its nanocomposites have been extensively utilized in a number of applications such as chemical sensing, biomedical application, drug delivery, adsorption, waste water treatment artificial photosynthesis, photocatalysis, and heterogeneous catalysis.[12−17] Generally, graphene oxide (GO) can be easily prepared from graphite flakes on a large scale via its oxidative exfoliation and then it can be converted chemically or thermally into reduced graphene oxide (rGO) sheets.[18] In the recent past, a number of different types of metal and metal oxide nanoparticles have been assembled on GO via simultaneous co-reduction of the metal salt along with GO in a single-step synthesis.[19−21] Because of oxygen-containing functionalities on the rGO sheet, metal, and metal oxide nanoparticles can be easily stabilized on the surface of rGO through the ligand-assisted approach. These graphene-decorated MNPs have shown improved properties because of the synergistic effect between the graphene sheet and MNPs.[22] Various reports also suggest that the agglomeration of graphene sheets can be minimized through a spacer, which could be either MNPs or some other organic molecules.[23]

Recently, alloy nanoparticles have been effectively utilized in the number of research areas because of their compositional dependent activity accompanied by unique magnetic, electronic, and catalytic properties.[24,25] Because of these unique properties, alloy nanoparticles have been extensively employed in diverse areas of research such as biomedicine, optics, electronics, and catalysis.[26,27] Recently, a number of reports have been published where alloy nanoparticles have been assembled on the surface of GO and then employed as heterogeneous catalysts in a numerous different organic reactions.[28,29] There are few reports where supported ruthenium–palladium (RuPd) alloy nanoparticles have been employed as an efficient heterogeneous catalyst.[30,31]

In the present era of the energy crisis, hydrogen is considered one of the alternative energy carriers because of high energy density, high abundance, and environmentally friendly nature.[32] Efficient storage of hydrogen and its catalytic controlled release from the various available storage materials is one of the major concerns for its wide application to address the energy as well as environmental concerns.[33] Among the number of available materials for chemical storage of hydrogen, ammonia–borane (AB) has been considered as the most promising and best suitable candidate. In the presence of a suitable catalyst, 1 mol of ammonia–borane (AB) can release 3 equiv H2 molecules at an ambient condition even without elevating the reaction temperature (eq ).

In the recent past, a number of nanostructure materials have been synthesized and studied for their catalytic application for hydrogen production.[34] Some of these reports suggest that alloy nanoparticles could be more efficient than their monometallic counterpart for the catalytic hydrolysis of ammonia borane because of the synergistic effects arising because of the interaction of two distinct metals.[35]

Aromatic azo compounds are considered as an important class of compounds having a very high commercial value. Aromatic azo compounds and their derivatives have many applications in wide areas of pharmaceutics such as organic dyes, pigments, food preservatives, indicators, and also as a precursor in chemical synthesis.[36−38] The conventional synthesis procedure for aromatic azo compounds involves the use of oxidants or nitrite salts (NaNO2) and the reaction proceeds via nitrosobenzene or diazonium intermediates.[39] These methods for the synthesis of azo compounds have disadvantages such as the usage of toxic nitrate salts. In the recent past, a number of catalytic procedures have been developed for the synthesis of aromatic azo compounds and these catalytic methods are more effective in terms of reaction time and selectivity. However, in most of the reported articles, either high-pressure or high-temperature conditions have been utilized. In some of the reported procedures, toxic and environmentally unfriendly bases have been used as an additive for the synthesis of azo compounds. Therefore, to overcome the limitations associated with these reported catalysts, the development of the heterogeneous catalytic system for the synthesis of aromatic azo compounds is highly desirable.

In continuation of our efforts to synthesize the heterogeneous catalytic system for organic transformation, herein, the RuPd alloy nanoparticles have been assembled on the surface of graphene sheets via the sonication-assisted co-reduction method. The uniformly dispersed RuPd nanoparticles on the graphene sheet have been utilized for the three different catalytic reactions, which includes hydrogen generation from aqueous ammonia borane, aqueous-phase reduction of aromatic nitro compounds (R–NO2) using ammonia borane as the hydrogen source, and the coupling of aniline derivatives to synthesize aromatic azo compounds. After the completion of the reaction, the catalyst could be easily recycled and reused for multiple catalytic cycles. Moreover, because of the higher surface to volume ratio, the supported alloy nanoparticles could provide an increased surface area, which results in an increment in the rate of all the three catalytic reactions.

Experimental Section

Materials

Ruthenium chloride, potassium tetrachloro palladate, natural flakes of graphite, and ammonia were purchased from Sigma-Aldrich. Sodium borohydride (NaBH4) was purchased from Spectrochem Pvt. Ltd, India. All the other chemicals were purchased from Cisco and Merck and were used as received without any further purification. All experiments were carried out by using deionized water.

Synthesis of Graphite Oxide Nanosheets

GO was synthesized by the already reported procedure.[18] Briefly, 180 mL of H2SO4 and 20 mL of H3PO4 were taken in a 500 mL round bottom flask and then 1.5 g of graphite flakes were added to it. Then after, 9 g of KMnO4 was slowly added in the mixture and heated it under stirring conditions at 50 °C for 12 h. Afterward, the reaction mixture was cooled to room temperature followed by the addition of 200 mL ice cold water. Subsequently, H2O2 (4 mL) was poured into the above mixture followed by centrifugation and the supernatant was decanted away. The obtained solid product was washed twice with water followed by HCl and then with ethanol. Finally, the clear brown solid (GO) was obtained.

Preparation of RuPd Alloy Nanoparticles Assembled on rGO Nanosheets (RuPd@rGO)

The typical procedure for the synthesis of Ru50Pd50 alloy nanoparticles assembled on the reduced graphene sheet is as follow:

In a 250 mL round bottom flask, 80 mg of GO was dispersed in 40 mL of deionized water via ultrasonicating for 30 min. After that, 0.15 mmol of RuCl3 and 0.15 mmol K2PdCl4 were dissolved in 15 mL of deionized water and then added it to the GO dispersion. The resulting mixture was sonicated for 15 min and then stirred for 1.5 h at room temperature. Then, a freshly prepared 8 mL of NaBH4 (0.5 M) solution was added slowly under ultrasonication conditions. The mixture was kept under ultrasound treatment for another 30 min. Finally, the mixture was centrifuged and the product was washed with water several times and dried in a vacuum oven at 60 °C. A similar synthesis procedure was applied for the synthesis of Ru60Pd40@rGO, and Ru40Pd60@rGO by adjusting the molar ratios of Ru/Pd salts accordingly. The stepwise synthesis of RuPd@rGO is depicted in Scheme . For comparison, Pd@rGO and Ru@rGO were synthesized by the similar procedure as discussed above.

Scheme 1

Schematic Representation of Stepwise Synthesis of RuPd@rGO

General Procedure for Hydrolytic Dehydrogenation of Ammonia Borane

In a typical reaction procedure, the synthesized catalyst was kept in a two necked reaction flask. Graduated glass tube filled with water was connected to one neck of the flask to measure the released amount of H2 gas. Ammonia borane was added from the other neck of the reaction flask. When 31.8 mg of AB was added into the reaction flask with vigorous stirring, the catalytic reaction started immediately. The volume of hydrogen gas was monitored through the displacement of water in the glass tube. The catalytic activity of Ru40Pd60@rGO, Ru50Pd50@rGO, and Ru60Pd40@rGO were studied by the abovementioned procedure at different conditions.

General Procedure for Tandem Catalytic Hydrogenation of Nitro-Aromatic Compounds

Typically, 1 mmol of nitro compounds was taken in a round bottom flask containing 10 mL of the water-methanol mixture followed by the addition (3 mmol) of ammonia borane. Subsequently, 10 mg of the highest active Ru40Pd60@rGO nanocatalyst was added into the reaction mixture. The reaction flask was sealed by a balloon. The reaction mixture was stirred until the completion of the reaction. The catalyst was separated from the reaction mixture through centrifugation and washed multiple times with water and then vacuum dried for further use.

General Synthesis Procedure for the Oxidative Coupling of Aromatic Amines

Typically, aniline (1.0 mmol) and potassium hydroxide (1.0 mmol) was added into 25 mL round bottom flask followed by the addition of 2.5 mL of dimethyl sulfoxide (DMSO) as a solvent. Subsequently, Ru40Pd60@rGO alloy nanoparticles (7 mg) were added into the reaction mixture. The reaction mixture was stirred at 60 °C in the open air for an appropriate time. The progress of the reaction was monitored through thin-layer chromatography (TLC). After the complete conversion (as indicated by TLC), the reaction mixture was extracted with diethyl ether and then purified by column chromatography. The catalyst was separated from the reaction mixture through centrifugation. The catalyst precipitated as a solid at the bottom of the centrifuge tube was washed with ethanol multiple times to remove any residual material. The separated catalyst could be further reused for multiple catalytic cycles.

Results and Discussion

Characterization

The prepared material, that is, RuPd@rGO has been characterized by X-ray diffraction (XRD) measurements for obtaining the information about the reduction of GO, phase purity, and composition of alloy nanoparticles anchored on graphene sheets (Figure ). From the XRD pattern of GO (Figure a), it could be observed that GO exhibited two characteristic peaks centered at 2θ values of 10 and 42° and this could be due to its (001) and (002) planes. Following the reduction of GO to reduced graphene oxide (rGO), these characteristic peaks of GO could not be observed in the XRD pattern, indicating that GO was completely reduced. Moreover in the XRD pattern of RuPd@rGO, a small hump cantered at a 2θ value of 24° could be observed, and this corresponded to the (002) reflection of reduced graphene nanosheets. The diffraction peaks observed in the XRD pattern of Ru50Pd50@rGO and Ru40Pd60@rGO are depicted in Figure a, the peak at 2θ values of 40.30, 46.16, and 68.26° could be attributed to the (111), (200), and (220) crystal planes of the face-centered cubic (fcc) alloy nanoparticles.[40]

Figure 1

(a) XRD patterns of rGO, Ru50Pd50@rGO, and Ru40Pd60@G. (b) FT-IR spectra of rGO and Ru40Pd60@rGO and (c,d) Raman spectra of GO and Ru40Pd60@rGO.

The successful anchoring of RuPd alloy nanoparticles on the GO support and the subsequent reduction of GO to its reduced form (rGO) could be further explained by Fourier transform infrared (FT-IR) and Raman studies done on the prepared samples. The FT-IR spectrum of GO (Figure b) indicated the presence of numerous oxygen containing groups. The broad absorption band centered at 3389 cm–1 could be assigned to stretching vibrations of O–H groups present in GO. The other absorption bands centered at 1228.30 and 1752.18 cm–1 could be because of the stretching vibrations of C–O (epoxy) and C=O (carboxylic) groups, respectively. The absorption band at 1636 cm–1 could be observed because of the C=C bonds from the un-oxidized sp2 hybridized C–C bonds. As observed from the FT-IR spectrum of the Ru40Pd60@G (Figure b), a decrease in the absorption intensity could be seen for the broad peak at 3389 cm–1. Also, the absorption peaks present at 1228.30 and 1752 cm–1 were observed to be absent in the FT-IR spectra of Ru40Pd60@rGO. The FT-IR spectrum of Ru40Pd60@rGO indicated that GO had been converted to its reduced form (rGO) to a great extent. The prepared material was further characterized by Raman spectroscopy, which is considered as a powerful tool for structural analysis of materials containing carbon. As shown in Figure c, the Raman spectra of GO had two prominent peaks at 1368 and 1613 cm–1 because of D and G bands of carbon, respectively. Following immobilization of Ru40Pd60 alloy nanoparticles on the surface of graphene sheets, two main peaks, that is, 1347 and 1717 cm–1 could be observed in the Raman spectrum (Figure d). It has already been reported that the D band corresponds to the defects induced in the graphene nanosheets because of breathing mode of the A1g symmetry and the G band corresponds to the E2g symmetry associated with the sp2 hybridized carbon. The relative peak intensity ratio of D and G bands (ID/IG) for GO and Ru40Pd60@rGO was found to be 0.71 and 1.1, respectively. The higher value of (ID/IG) for Ru40Pd60@rGO indicated the re-establishment of the conjugated graphene network after reduction and formation of more sp3 defects in reduced graphene nanosheets.

The morphology, size, and crystal structure of prepared nanocomposites of RuPd@rGO were studied through SEM and transmission electron microscopy (TEM) analysis of the samples. The TEM analysis of GO clearly indicated the sheet-like structure without any aggregation (Figure a–c). The TEM analysis of the RuPd@rGO samples is shown in (Figure d–f. It could be observed that RuPd alloy nanoparticles were uniformly dispersed over the surface of graphene nanosheets without any aggregation or agglomeration. The alloy RuPd nanoparticles were not observed outside the graphene nanosheets in the TEM images and this indicated the effective interactions between the RuPd alloy nanoparticles and graphene nanosheets. The average size of Ru50Pd50, Ru40Pd60 and Ru60Pd40 alloy nanoparticles immobilized on the graphene support were found to be 18, 15, and 20 nm, respectively (Figure g–i). The decrease in size for Ru40Pd60 could be because of the replacement of the larger Ru atom by a smaller size Pd atom in the Ru matrix.

Figure 2

TEM images of GO at different magnification (a–c). TEM images of (D) Ru50Pd50@rGO, (e) Ru40Pd60@rGO, and (f) Ru60Pd40@rGO. Histograms showing the particle size distribution for synthesized (g) Ru50Pd50@rGO (h) Ru40Pd60@rGO, and (i) Ru60Pd40@rGO samples.

The energy-dispersive X-ray spectroscopy (EDS) elemental mapping of C, Ru, and Pd for Ru50Pd50@rGO is depicted in Figure a–d. It could be observed that both the Ru and Pd elements were uniformly dispersed over the surface of graphene sheets and the random intermixing of Ru and Pd could also be observed. The EDS elemental mapping strongly suggested that the uniformly dispersed Ru–Pd alloy nanoparticles had been successfully synthesized over the surface of the reduced graphene sheets.

Figure 3

EDS elemental mapping of Ru40Pd60@rGO showing (a) C, (b) Ru, (c) Pd, and (d) Ru, Pd, and C.

The elemental analysis and chemical composition of the prepared Ru50Pd50 alloy nanoparticles were obtained through EDS analysis of the samples, as shown in the Supporting Information (Figure S1: Supporting Information). Based on the EDS results, it could be confirmed that carbon, Ru and Pd were present in all the samples and the ratio of Ru/Pd obtained from EDS analysis could be seen to be almost consistent with an initial molar ratio of respective salt of Ru and Pd. The carbon element was detected in all samples of EDS analysis because of the presence of graphene nanosheets. The exact elemental composition and percentage loading of Ru and Pd present in the RuPd@rGO nanocomposites are further investigated by inductively coupled plasma atomic absorption spectroscopy (ICP-AES) study and the results are tabulated in Table .

Table 1

ICP-AES Analysis of the Prepared RuPd@rGO Samples

sample	Ru wt %/Pd wt % (obtained through ICP-AES)
Ru50Pd50@rGO	3.8/4.6
Ru60Pd40@rGO	4.24/3.1
Ru40Pd60@rGO	2.82/4.2

Based on the ICP-AES results given in the Table , the ratios of Ru/Pd were found to be approximately near to the initial molar ratio of their respective salts during synthesis. Further, from the EDX and ICP-AES analysis of samples, it was confirmed that RuPd alloy nanoparticles were uniformly immobilized over the surface of the reduced graphene sheet.

X-ray photoelectron spectroscopy (XPS) analysis of the Ru40Pd60@rGO nanocomposite was adopted to know about the elemental composition and surface oxidation state (Figure ). The high-resolution XPS spectra showed the presence of C (carbon), O (oxygen), Pd (palladium), and Ru (ruthenium) elements in the corresponding regions of Ru40Pd60@rGO. In the C 1s spectra (Figure a), the peak corresponding to C–C and C–H at 284.5 eV had the highest intensity. The other peaks obtained in XPS spectra of C 1s at 285.9, 287.1, and 288.2 eV could be attributed to C–O, C=O, and O=C–O, respectively. The reduced intensities of C–O, C=O and O=C–O in C 1s spectra demonstrated the reduction of GO and restoration of the graphitic lattice in the Ru40Pd60@rGO nanocomposite. The Pd 3p spectrum was deconvoluted and the peaks centered at 336.4 and 341.2 eV could be attributed to the 3d5/2 and 3d3/2 electrons, respectively (Figure b).

Figure 4

XPS spectra of Ru40Pd60@rGO, showing peaks representing (a) carbon 1s, (b) palladium 3d, and (c) ruthenium 3p.

On further deconvolution, these two peaks, two pairs of doublets could be observed one in each 3d5/2 and 3d3/2 region. The peaks at binding energy 335.4 and 340.6 eV could be attributed to metallic Pd, while the other two peaks centered at binding energies 337.4 and 342.6 eV could be attributed to Pd(II) species. The high-resolution deconvoluted Ru 3p spectrum displayed two components consisting of 3p3/2 and 3p1/2 (Figure c). The peak cantered at 462.6 eV in the 3p3/2 region and another peak in the 3p1/2 region at 485.2 eV could be because of metallic Ru, as already reported in the literature.[41] The peaks centered at 465.1 and 487.5 eV in 3p3/2 and 3p1/2, respectively, could be because of Ru(IV) species. The XPS result of the prepared material indicated the successful synthesis of Ru40Pd60 alloy nanoparticles over the surface of rGO.

Catalytic Activity

Hydrolytic Dehydrogenation of Ammonia Borane

The prepared RuPd@rGO nanocomposites were evaluated for their catalytic activities in hydrolytic dehydrogenation of ammonia borane (AB) in the water-filled graduated burette system. Initially, different catalysts having a different ratio of the Ru/Pd amount immobilized over the surface of graphene nanosheets were investigated for their activity in the hydrolytic dehydrogenation of AB. As demonstrated in Figure a, the Ru40Pd60@rGO catalyst and took only 2.5 min for the complete hydrolytic dehydrogenation of AB as compared to its other 2 M ratios. For Ru50Pd50@rGO and Ru60Pd40@rGO, the reaction was completed in 4.5 and 5.4 min, respectively. When GO without any MNP was tested as a catalyst for the same reaction, no H2 generation was observed, and this concluded that MNPs were the real catalytically active species for this reaction. The effect of temperature was also investigated for the hydrolytic dehydrogenation of AB in the presence of most active Ru40Pd60@rGO.

Figure 5

Stoichiometric hydrogen evolution from aqueous AB solution catalyzed by (a) RuPd@rGO with different molar ratios of Ru and Pd at 25 °C; (b) Ru40Pd60@rGO at different temperatures; and (c) Ru40Pd60@rGO during the reusability test of the catalyst.

The reaction was performed at different temperatures, ranging from 20 to 40 °C using 3 mmol of AB, as depicted in Figure b. It could be observed that the rate of hydrolytic dehydrogenation of AB increased with the increase in temperature and the maximum rate of reaction was observed at 40 °C. The recyclability of Ru40Pd60@rGO was also checked for the hydrolytic dehydrogenation of AB, and it was observed that the catalytic activity gradually decreased after each catalytic cycle and in the fifth catalytic cycle Ru40Pd60@rGO retained only 55% of the initial catalytic activity (Figure c).

Hydrogenation of Aromatic Nitro Compounds

Inspired from the results obtained from the hydrolytic hydrogenation of AB, the prepared RuPd@rGO catalyst was also employed for the tandem hydrogenation of aromatic nitro compounds (R–NO2) using an ammonia borane complex (AB) as a hydrogen source (Scheme ).

Scheme 2

Schematic Representation of Application of RuPd@rGO for the Tandem Hydrogenation of Aromatic Nitro Compounds

To optimize the best suitable reaction conditions, nitrobenzene was used as a model substrate to demonstrate the catalytic activity of RuPd@rGO. Initially, different solvents were tried for the catalytic tandem hydrogenation of R–NO2 at room temperature. Before carrying out the reaction, the reaction mixture was tightly sealed to prevent the escape of the H2 gas into the atmosphere. As reported in the literature, mostly water, alcohols, and mixture of alcohol with water were used as a solvent for the tandem hydrogenation of R–NO2 using AB as the hydrogen source. Here, we have checked the effect of different solvents (including ethanol, methanol, water, and their mixture) on the catalytic efficiency of the prepared Ru40Pd60@rGO catalyst, and we observed that the most catalytic activity in terms of reaction time was observed in a mixed solvent of methanol/water (v/v = 7/3) (Table S1: Supporting Information). The activity of catalysts with different ratios of RuPd immobilized on graphene sheets was also checked. Among the three different synthesized catalysts (i.e., Ru40Pd60@rGO, Ru50Pd50@rGO, and Ru60Pd40@rGO); Ru40Pd60@rGO was found to be most active catalyst in terms of the product yield in the minimum reaction time (Table S2: Supporting Information). When graphene was checked for its catalytic activity in the same reaction, no yield of the product was observed, which indicated that the MNP was essential for both hydrogen generation from AB and the reduction of nitro compounds. For comparison, Pd@rGO and Ru@rGO were also checked for the catalytic activity, and we observed that the yield of the product was low as compared to Ru40Pd60@rGO (Table S2, entry 8–9: Supporting Information). Further, the amount of AB required for the hydrogenation of nitro compounds was optimized, and it was observed that the maximum yield of product in the shorter reaction time was obtained when 3 mmol of AB was used as the hydrogen source. When 1 and 2 mmol of AB were utilized as a hydrogen source, 40 and 75% yields of the product was obtained, respectively (Table S2, entry 6 and 7, Supporting Information), which showed that an excess amount of H2 was required for the conversion of −NO2 groups into −NH2 groups.

Based on the optimized results, the hydrogenation of various derivatives of the nitro compound (with electron withdrawing as well as donating groups) was studied using Ru40Pd60@rGO as a catalyst in the presence of 3 mmol of AB in the methanol/water mixture (v/v = 7/3) at room temperature (Table ). The results obtained from the tandem reactions of various nitro compounds catalyzed by Ru40Pd60@rGO into corresponding primary amines are tabulated in Table . All nitro compounds tested were converted into the corresponding primary amines at room temperature with good to excellent yields of the product. Halogenated nitro compounds were also converted into their respective primary amine with excellent yields of the product.

Table 2

Ru40Pd60@rGO Catalyzed Tandem Hydrogenation of Nitro Compoundsa

Reaction conditions: nitro substrate (1 mmol), H3NBH3 (3 mmol), Ru40Pd60@rGO (10 mg), and solvent: methanol/water (v/v = 7/3), (10 mL), room temperature.

The recyclability of the catalyst was also tested by using nitrobenzene as a model substrate. Upon the completion of one cycle of the reaction, the catalyst was separated from the reaction mixture via centrifugation and thoroughly washed with water and alcohol followed by drying in a vacuum oven. The catalyst could be reused for the next cycle and in the similar manner, the catalyst could be used up to six consecutive catalytic cycles with a small gradual decrease in its activity (Figure ).

Figure 6

Recyclability test of Ru40Pd60@rGO for the hydrogenation of nitrobenzene as a model substrate (yellow) and recyclability test of Ru40Pd60@rGO for oxidative aniline coupling at 60 °C in DMSO (cyan).

Oxidative Coupling of Aromatic Amines

The findings on the RuPd@rGO alloy nanoparticles catalyzed coupling of aniline derivatives for the synthesis of azo compounds at 60 °C in DMSO has been highlighted in this chapter. It may be postulated that because of the synergistic effect developed in alloy nanoparticles, they could help in accelerating the coupling of aniline for the synthesis of aromatic azo compounds (Scheme ). The higher surface to volume ratio of alloy nanoparticles could provide the increased surface area for the substrates to react and this could result in the increased rate of the reaction.

Scheme 3

Schematic Representation of Synthesis of Aromatic Azo Compounds Catalyzed by RuPd@rGO

The RuPd@rGO alloy nanoparticles were checked for their catalytic activity in the coupling of aniline for the synthesis of azo compounds. In order to ascertain the role of the solvents on the reaction rate, various organic solvents were tested using aniline as a model substrate and Ru40Pd60@rGO as a catalyst (Table S3: Supporting Information). It was observed that the maximum yield of the product was obtained when DMSO was utilized as a solvent. Furthermore, the amount of the catalyst and type of the catalyst are also optimized and the results are tabulated in Table S4: Supporting Information. Based on the results entered in Table S4, it can be seen that Ru40Pd60@rGO is the most active catalyst among the various catalysts prepared with different ratios of ruthenium and palladium. Furthermore, the amount of the most active catalyst was optimized, and it could be seen that 7 mg of Ru40Pd60@rGO was the ideal amount of the catalyst with which the maximum yield of the product was obtained. Under a nitrogen atmosphere, the reaction did not proceed suggesting that the reaction occurs only under aerobic conditions.

Based on these findings, it could be confirmed that Ru40Pd60@rGO gave the maximum yield of the product and this could be because of the synergistic effect arising from the alloying of two metals having slightly different electronegativities. The effect of the reaction temperature was also studied, and it was observed that the best results in terms of the product yield were obtained when the reaction was performed at 60 °C. A variety of aniline derivatives were tested with the optimized reaction conditions, and it was observed that the maximum yield of the product with the minimum reaction time was reported when aniline was used as a reactant. Aniline derivatives with electron donating as well as withdrawing groups furnished aromatic azo coupling products in good to excellent yields (Table ).

Table 3

Synthesis of Aromatic Azo Compounds from Aromatic Amines Using Ru40Pd60@rGO as a Catalysta

Reaction conditions: amine (1 mmol), Ru40Pd60@rGO (7 mg), and KOH (1 mmol), DMSO (3 mL), temp: 60 °C.

Isolated yield.

The recyclability of the prepared, that is, Ru40Pd60@rGO catalyst could also be tested for five consecutive catalytic cycles using aniline as a model substrate for the synthesis of the azo product (Figure ). Following the completion of the individual catalytic cycle, the catalyst could be easily separated from the reaction mixture through centrifugation and then thoroughly washing it and drying it under vacuum at 50 °C. The catalyst could be then used in the next catalytic cycle, and it was observed that the catalyst could be reused up to five catalytic cycles without any noticeable loss in catalytic activity. After the fifth catalytic cycle, the catalyst was characterized with TEM analysis and we observed some slight agglomerations of RuPd alloy NPs, which could be because of their continuous use resulting in a slight decrease in the catalytic activity (Figure S2).

The activity of the prepared catalyst was compared with some of the reported heterogeneous catalysts for the same reaction, and the results are incorporated in Table S5: Supporting Information. Comparing the catalytic activity with that of the reported catalysts, it could be concluded that the prepared catalyst (i.e., Ru40Pd60@rGO) has better catalytic activity in terms of the yield of the product and reaction time.

Conclusions

The synthesis of RuPd alloy nanoparticles supported on reduced graphene nanosheets has been delineated in the present work. The devised material has been characterized by various physiochemical techniques. The ratio of Ru/Pd could be easily tuned during the synthetic procedure by adjusting the molar ratio of Ru and Pd salt accordingly. The prepared material was successfully employed as the recyclable catalyst for hydrolytic hydrogenation of ammonia borane and also for the hydrogenation of aromatic nitro compounds. Amid the three nanoparticles prepared, that is, Ru40Pd60@rGO, Ru50Pd50@rGO, and Ru60Pd40@rGO, the first one has corroborated itself as the most active catalyst for both the abovementioned reactions. A simple procedure for the synthesis of aromatic azo compounds has also been reported using recyclable Ru40Pd60@rGO nanocomposite as the catalyst. This method has been developed for the synthesis of aromatic azo compounds without the use of any hazardous additives or elevated temperatures.