Controllable Synthesis of Surface Pt-Rich Bimetallic AuPt Nanocatalysts for Selective Hydrogenation Reactions.

Bimetallic nanocatalysts, with efficient and controllable catalytic performance, have a promising application in chemical production. In this study, surface Pt-rich bimetallic AuPt nanoparticles with different Pt/Au ratios were prepared and tested in selective hydrogenation reactions of substituted nitroaromatics. Au nanoparticles were first prepared with n-butyllithium as a rapid reducer, which were further used as seeds in the slow growth process of Pt atoms. Because of the employed sequential reduction method and the following atom diffusion, surface Pt-rich bimetallic AuPt nanoparticles were obtained. Compared with the uniform AuPt alloy nanocatalysts synthesized by the co-reduction method with n-butyllithium as the reducer and monometallic Pt nanocatalysts, the obtained surface Pt-rich AuPt bimetallic nanocatalysts presented an enhanced catalytic selectivity or activity. The performance enhancement is assigned to the optimized Au/Pt interaction in the surface Pt-rich bimetallic nanostructures. This work demonstrates that the optimization of the stoichiometry and construction of bimetallic materials is a feasible method to synthesize controllable and efficient nanocatalysts.

Introduction

Metallic nanocatalysts have been playing a significant role in chemical industries; enhancing their catalytic performance is helpful for reducing the production cost.[1−4] With the unique characters of nanostructures, such as the quantum effect and high surface/volume ratio, metallic materials in nanoscale could present enhanced catalytic performance than that of bulk counterparts.[5] Bimetallic nanoparticles (NPs) have widespread applications, such as biosensors, biological medicines, wastewater treatments, and catalytic fields.[6−11] Through modulating the distribution of surface electrons and active sites, bimetallic nanocatalysts demonstrate a controllable performance.[12−14] Various architectures of bimetallic nanoclusters have been prepared, such as alloy NPs, core–shell NPs, and intermetallic compounds.[15−18] Accordingly, the catalytic activity or selectivity can be enhanced by optimizing the stoichiometry or construction of bimetallic structures.[19−21] Typical alloy nanomaterials in bimetallic structure, with a uniform distribution of metallic atoms, have been reported widely as efficient catalysts.[20] The nanoalloys displayed superior catalytic activities in certain reactions, but the catalytic stabilities of nanoalloys are lower than that of heterogeneous bimetallic nanostructures. The reasonable explanation is the metallic atoms tend to migrate and aggregate individually.[22,23] Heterogeneous bimetallic nanostructures refer to a nonuniform distribution of metallic elements, like core–shell NPs, where the core is formed by one kind of metal and the surrounding shell is formed by another kind of metal.[24,25]

In heterogeneous nanostructures, the most active metal atoms usually aggregate on the surface and the second metal forms the core or the second metal atoms intersperse on active metal core, which would moderate electronic effects of the active metallic atoms. For example, Pd–Pt core–shell NPs were synthesized by depositing a thin layer of Pt on the Pd seed surface. The catalytic performance of Pd–Pt nanocatalysts was superior than that of monometallic Pd or Pt nanocatalysts because of the interactions between the Pd cores and Pt shells.[26] Meanwhile, the density functional theory calculations also verified that the intermetallic effects in heterogeneous nanostructures could influence the catalytic performance.[27−29] There is a great amount of demand to design catalysts with high catalytic activity and selectivity for various reactions. Substituted nitroanilines, one kind of important and widely used chemicals, can be produced by the selective hydrogenation of the substituted nitroaromatics. Among the existing nanocatalysts, bimetallic nanocatalysts tend to achieve higher selectivity and activity than monometallic nanocatalysts.[30] One strategy is blending some kind of inert metal in overefficient noble metal to coordinate the catalytic activity and selectivity for designing high efficient bimetallic nanocatalysts. As reported, Au was inert in hydrogenation reactions, which could be used as an additive to improve the selectivity of Pt catalysts. Because of the synergetic effects between Au and Pt atoms, bimetallic AuPt nanocatalysts present enhanced catalytic selectivity.[31−33]

Accordingly, metallic precursors (Au3+ and Pt4+) can be reduced by sodium borohydride, polyol, and other reducers to synthesize bimetallic NPs by the wet chemistry method.[34−37] Co-reduction methods are exerted to synthesize homogeneous alloys, whereas successive reduction methods are used to synthesize heterogeneous bimetallic materials.[38,39] Recently, many ecofriendly synthesis approaches have been used in bimetallic NP preparation.[40−42] Wang and co-workers creatively used a facile theophylline-assisted green approach without any template or seed to synthesize AuPt nanospheres over graphene oxide.[43,44] However, synthesizing bimetallic NPs with controllable stoichiometry and construction is still being a challenge. In this work, a series of AuPt bimetallic NPs were synthesized containing homogeneous and heterogeneous structures with various Au/Pt atom ratios. The specific preparation process and catalytic application are illustrated in Scheme . During the process, n-butyllithium was chosen as the rapid reducer to alloying Au and Pt atoms in tiny nanostructures around 5 nm. Strategically, Au NPs (∼5 nm) were first reduced by n-butyllithium and then used as seeds in the slow growth process of Pt atoms. Because of the employed sequential reduction method by oleylamine and following the diffusion of atoms, surface Pt-rich bimetallic AuPt NPs were obtained. These as-synthesized bimetallic AuPt NPs were deposited on alumina (γ-Al2O3) to form AuPt NPs/γ-Al2O3 nanocatalysts. The bimetallic nanostructures were characterized by technologies such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible spectroscopy (UV–vis). The catalytic performance of the designed nanocatalysts for selective hydrogenations of various substituted nitroaromatics were tested, and the obtained surface Pt-rich AuPt bimetallic nanocatalysts presented enhanced catalytic selectivity or activity than uniform AuPt alloys and monometallic Pt nanocatalysts.

Scheme 1

Synthesis of Bimetallic AuPt NPs/γ-Al2O3 Nanocatalysts and Their Application in Selective Hydrogenations of Substituted Nitroaromatics

Results and Discussion

Characterization of a Series of Bimetallic AuPt Nanocatalysts

In this paper, homogeneous AuPt alloy NPs were synthesized by the one-pot reduction method with n-butyllithium as the reducer. For heterogeneous AuPt bimetallic NPs, n-butyllithium could rapidly reduce Au3+ to form Au NPs less than 5 nm. In the successive process, Pt4+ would be reduced gently by oleylamine to Pt atoms, which would then grow on the Au seed surfaces. Oleylamine was not only the reducer for Pt4+ but also the surfactant at the Au surface, which was significant to overcome their rejection. Herein, the Pt atoms could grow on the Au seeds rather than being aggregated alone. The detailed synthesis process and characterization method are presented in the Experimental Section and Supporting Information.

The XRD patterns of heterogeneous AuPt bimetallic NPs (the atom ratios of Au/Pt are 1/0.5, 1/1, 1/1.5 and 1/2) prepared by the successive reduction method are illustrated in Figure . Meanwhile, the pattern of the homogeneous Au/Pt alloy NPs by the co-reduction method is also displayed. Because the XRD patterns of Au and Pt are extremely near, the bimetallic AuPt NPs displayed XRD patterns between the pure Au and pure Pt diffractions. There was a meaningful consequence, where the peaks of XRD patterns shifted gradually from Au diffraction to Pt diffraction, with the Pt ratios improved. Such a result was associated with the TEM and UV–vis adsorption measurements, verifying that the Pt atoms tend to enrich on the Au surface.

Figure 1

XRD patterns of a series of bimetallic AuPt NPs; vertical lines indicate individual Au (JCPDS 04-0784) and Pt (JCPDS 04-0802).

The TEM images of monometallic Au NPs and AuPt alloy NPs are shown in Figure . A series of heterogeneous AuPt bimetallic NPs with Pt enriched on the surface (Au/Pt atom ratios of 1/0.5, 1/1, 1/1.5, and 1/2) are also illustrated in Figure b–e. As illustrated in Figure , the metallic nanomaterials synthesized by the wet chemistry reduction method mentioned before are uniform spheres with diameters around several nanometers. Figure presents the statistical results of these NPs, which came from nearly 100 randomly selected particles in the TEM images. The diameters of Au, AuPt-1/0.5, AuPt-1/1, AuPt-1/1.5, AuPt-1/2, and AuPt alloy-1/1 NPs are 4.16, 4.65, 4.92, 5.34, 6.02, and 3.89 nm, respectively. Therefore, the rapid reduction method by n-butyllithium could synthesize monometallic NPs and alloy NPs in a tiny size. The Au NPs with ∼4 nm diameter would be seeds in the following step, on which Pt4+ were reduced gently by oleylamine. As the Pt ratios increase, the size of heterogeneous AuPt bimetallic NPs accordingly increased from 4.65 nm (AuPt-1/0.5) to 6.02 nm (AuPt-1/2).

Figure 2

TEM images showing (a) Au NPs; (b) AuPt-1/0.5; (c) AuPt-1/1; (d) AuPt-1/1.5; (e) AuPt-1/2; and (f) AuPt alloy NPs.

Figure 3

Particle size analysis showing (a) Au NPs; (b) AuPt-1/0.5; (c) AuPt-1/1; (d) AuPt-1/1.5; (e) AuPt-1/2; and (f) AuPt alloy NPs.

The high-angle annular dark-field scanning TEM (HAADF-STEM) images and the relative atomic proportions of Au and Pt by energy-dispersive spectrometry (EDS) line scanning for bimetallic AuPt-1/1.5 NPs are shown in Figure . Figure a shows that one independent particle was selected, and the atomic ratio of Au/Pt is presented in Figure b. As demonstrated in the elemental analyzation, Au and Pt atoms distribute nonuniformly in AuPt-1/1.5 NPs; the AuPt NPs synthesized by the successive reduction method are heterogeneous and bimetallic structures. EDS elemental maps can demonstrate the surface Pt-rich bimetallic nanostructures visually. As displayed in Figure d,e, Au atoms tend to aggregate in the core. The distribution of Pt is nonuniform, whereas the Pt atoms on the surface are more than that on the core position. This surface Pt-rich heterogeneous nanostructure can also be proved by Figure S1, which displays several AuPt-1/1.5 NPs. Meanwhile, the surface Pt-rich heterogeneous nanostructures of AuPt-1/1 are also verified in Figure S2, as displayed in the HAADF-STEM image (Figure S2a) and EDS elemental maps of Au and Pt (Figure S2b,c).

Figure 4

(a) HAADF-STEM diagram of AuPt-1/1.5; (b) EDS elemental line scanning of Pt and Au of the chosen single particle in (a); (c) HAADF-STEM image of AuPt-1/1.5 and EDS elemental maps of Au (d) and Pt (e).

As XPS is a surface-sensitive technology, the XPS characterizations are expected to verify whether the bimetallic AuPt nanostructures are heterogeneous or homogeneous. A series of bimetallic AuPt NPs were measured, and the resulted XPS spectra are presented in Figure as an example showing heterogeneous AuPt bimetallic NPs with Pt enriched on the surface (the atom ratio of Au/Pt is 1/1.5). Accordingly, the binding energy at 71.2/74.6 and 71.9/75.1 eV can be assigned to 4f7/2/4f5/2 of Pt0 and Pt2+ species, respectively,[45] and the binding energy at 84.1/87.8 eV can be assigned to 4f7/2/4f5/2 of Au0 species.[46] Interestingly, an obvious portion of Pt is Pt2+ for heterogeneous AuPt bimetallic NPs. As the synthesis process was carried out under a N2 environment and without calcination in the following process, the existence of oxidized Pt did not originate from the oxidation in air. It is attributed to the transformation of electrons from the Pt shell to the Au core because of the intermetallic effects.[47] As mentioned in many papers, the enhanced Pt0 ratio is crucial to superior catalytic activity because the metallic state is more active than Pt oxide.[15,47−49] As the surface Pt-rich bimetallic AuPt NPs display higher Pt0 ratio than the AuPt alloy (Table ), they demonstrate superior catalytic activities for selective hydrogenation.

Figure 5

XPS spectra showing heterogeneous AuPt bimetallic NPs with Pt enriched on the surface (the atom ratio of Au/Pt is 1/1.5).

Table 1

XPS and ICP–OES Measurements for a Series of Bimetallic AuPt NPs

bimetallic NPs	Au0/Aux+ (%)	Pt0/Pt2+ (%)	atom ratio Au/Pt (XPS)	atom ratio Au/Pt (ICP–OES)	Au/Pt wt % (ICP–OES)
AuPt-1/0.5	100/0	74/26	1/0.57	1/0.38	0.43/0.16
AuPt-1/1	100/0	61/39	1/1.38	1/1.10	0.44/0.48
AuPt-1/1.5	100/0	58/42	1/2.96	1/1.59	0.43/0.68
AuPt-1/2	100/0	60/40	1/4.29	1/1.94	0.50/0.96
alloy (1/1)	100/0	46/54	1/1.25	1/1.09	0.45/0.49

Table distributes the proportions of Au and Pt in bimetallic NPs calculated from the XPS spectra (Figure S3) by the XPS PEAK4.1 software. Meanwhile, the results from inductively coupled plasma–optical emission spectrometry (ICP–OES) are also displayed, which are associated with the ratios of Au and Pt precursors. According to the ICP–OES results, the heterogeneous AuPt bimetallic NPs with Pt enriched on the surface owned the atom ratios of Au/Pt near the feeding ratios of 1/0.5, 1/1, 1/1.5, and 1/2. The actual loadings are slightly lower than that of the calculations (Au-0.5 wt % on γ-Al2O3), which might originate from the loss caused by the loading treatment and cleaning process. The XPS results illustrate an Au-enriched core and Pt-enriched shell phenomenon, where the calculated ratios of Pt to Au are always bigger than the ratio resulted from the ICP–OES measurements. For example, the ratio of Au/Pt for AuPt-1/1.5 is 1/2.96 from XPS, whereas it is 0.43/0.68 obtained from ICP, which means the Pt atoms are almost on the surface and the bimetallic AuPt NPs are heterogeneous structures.

As reported, an Au surface plasmon resonance (SPR) band appears at around 520 nm in the UV–vis absorbance spectra when monometallic Au NPs are larger than 2 nm.[50,51]Figure presents the UV–vis absorption spectra of monometallic Au and heterogeneous AuPt bimetallic NPs (the atom ratios of Au/Pt were 1/0.5, 1/1, 1/1.5, and 1/2, respectively). Accordingly, the monometallic Au NPs display the clear SPR band at 522 nm, which is attributed to the Au response. With the atom ratio of Pt to Au increasing, the SPR band of Au decreases correspondingly. When the atom ratio of Au/Pt is 1/2, the SPR band of Au disappeared, which means Au NPs are covered by Pt atoms totally. The UV–vis adsorption measurements suggest that the bimetallic AuPt NPs synthesized by the successive method are heterogeneous structures, where Pt atoms are enriched on the surface of Au seeds.

Figure 6

UV–vis absorption spectra of monometallic Au and heterogeneous AuPt bimetallic NPs (the atom ratios of Au/Pt were 1/0.5, 1/1, 1/1.5, and 1/2, respectively).

Selective Hydrogenation Abilities of a Series of As-Synthesized Nanocatalysts

The catalytic performance of the as-synthesized AuPt bimetallic nanocatalysts with a metallic loading of 0.5 wt % on γ-Al2O3 was tested in selective hydrogenation reactions for various substituted nitroaromatics (as shown in Table ). p-Chloronitrobenzene, p-nitrophenol, p-nitroanisole (p-NAS), p-nitrotoluene, p-nitroacetophenone, and o-nitroacetophenone were chosen in this research.

Table 2

Selective Hydrogenation Abilities of a Series of As-Synthesized Nanocatalysts

Bimetallic AuPt nanocatalysts were synthesized through a similar reduction method to verify that the intermetallic effects in bimetallic structures can modulate the catalytic performance. Under similar conditions, the heterogeneous bimetallic nanomaterial demonstrated superior selective hydrogenation ability than the alloy nanostructure for the six kinds of substituted nitroaromatics. AuPt-1/1 can obtain a yield of o-aminophenone at 80.9% after hydrogenation at 45 °C under atmospheric H2 for 2 h. As a reference, the AuPt alloy-1/1 catalysts just obtained 43.3% yield. The controlled monometallic Pt catalysts were prepared by the similar method with the same noble metal loading (Experimental Section and Supporting Information). As displayed in Table S1, the selective hydrogenation performance of Pt monometallic catalysts is inferior to the designed bimetallic catalysts, where a part of Pt atoms is replaced by Au atoms. As an example, Au/Pt-1/2 obtains a yield of o-aminophenone at 97.5%, whereas the monometallic Pt catalysts just obtained 66.8% under the same reaction conditions. Therefore, doping Au atoms can enhance the selective abilities of Pt for the substituted nitroaromatics. More importantly, the bimetallic nanostructures with Pt enriched on the surface are superior to the alloy nanostructures.

The catalytic stability of heterogeneous AuPt bimetallic nanocatalysts (the atom ratio of Au/Pt is 1/1.5) was tested in the p-NAS hydrogenation reaction. After each cycle, the nanocatalysts were centrifuged from the mixtures and washed for five times using a mixed solution of acetone and ethanol and then dried in an oven at 80 °C for 12 h. The weight of ethanol and p-NAS decreased according to the weight of the recycled nanocatalysts. According to the data in Figure , the catalytic activity and selectivity of the heterogeneous AuPt bimetallic nanocatalysts are stable, which means the designed structures are equipped with superior stability.

Figure 7

Cycle-to-cycle p-NAS hydrogenation over heterogeneous AuPt bimetallic nanocatalysts (the atom ratio of Au/Pt is 1/1.5).

Conclusions

In this work, a series of bimetallic AuPt NPs are designed and their selective hydrogenation abilities for various substituted nitrobenzenes are tested. The rapid co-reduction with n-butyllithium as the reducer can obtain uniform AuPt alloy NPs; a sequential reduction method with presynthesized Au NPs as seeds and the following atom diffusion process would result in surface Pt-rich AuPt bimetallic nanostructures. The surface Pt-rich bimetallic nanocatalysts with different Au/Pt atomic ratios demonstrate enhanced hydrogenation selectivity or activity than AuPt alloys and monometallic Pt nanomaterials. This enhancement originates from the optimization of Au/Pt interaction in surface Pt-rich bimetallic nanostructures. The strategy reported here could be exerted to prepare more kinds of bimetallic nanocatalysts with efficient and controllable catalytic performance.

Experimental Section

Synthesis of Homogeneous AuPt Alloy NPs

AuPt alloy NPs were synthesized by the co-reduction method. For obtaining AuPt alloy NPs with 1/1 Au/Pt atom ratio, 0.06 mmol of HAuCl4·xH2O and 0.06 mmol of H2PtCl6·xH2O were dissolved in 1 mL of decalin and 2 mL of oleylamine. In a 50 mL three-necked round-bottom flask, 20 mL of decalin was swept by a N2 flow at 110 °C for 2 h to remove water and then cooled down to 75 °C. The solution containing Au3+ and Pt4+ precursors was injected to the 20 mL of decalin, followed by an injection of 1 mL of n-butyllithium at 75 °C and N2 atmosphere. The mixture was heated to 140 °C quickly and maintained for 4 h under the N2 atmosphere. The black resultant mixture was centrifuged and washed with the mixed solution of toluene and acetone for four times to obtain homogeneous AuPt alloy NPs.

Synthesis of Monometallic Au NPs

HAuCl4·xH2O (0.06 mmol) was dissolved in a mixed solution with 1 mL of decalin and 0.5 mL of oleylamine. In a 50 mL three-necked round-bottom flask, 20 mL of decalin was swept by a N2 flow at 110 °C for 2 h to remove water and then cooled down to 75 °C. The Au3+ precursor solution was injected to the 20 mL of decalin, followed by an injection of 0.5 mL of n-butyllithium at 75 °C and N2 atmosphere. The mixture was stirred at 75 °C for 0.5 h and then heated to 110 °C quickly and maintained for 2.5 h. After the injection of n-butyllithium at 75 °C, the Au3+ precursors were reduced, with the clear solution changing to brown colloid. When the temperature increased to 110 °C and maintained for 2.5 h, the color of the colloid darkened to purple, indicating the growth of Au NPs. Once cooling down to room temperature, the colloid containing Au NPs would be used in the synthesis of AuPt bimetallic NPs with Pt enriched on the surface. The as-synthesized Au NPs served as seeds to anchor the Pt atom. Meanwhile, the monometallic Au NPs could be obtained from the colloid through centrifugation and washed by the mixed solution of toluene and acetone for four times.

Synthesis of Monometallic Pt NPs

Similar to the aforementioned monometallic Au NPs synthesis method, 0.06 mmol of H2PtCl6·xH2O was dissolved in a mixed solution with 1 mL of decalin and 0.5 mL of oleylamine. In a 50 mL three-necked round-bottom flask, 20 mL of decalin was swept by a N2 flow at 110 °C for 2 h to remove water and then cooled down to 75 °C. The Pt4+ precursor solution was injected to the 20 mL of decalin, followed by an injection of 0.5 mL of n-butyllithium at 75 °C and N2 atmosphere. The mixture was stirred at 75 °C for 0.5 h and then heated to 140 °C quickly and maintained for 4 h. After the injection of n-butyllithium, the Pt4+ precursors were reduced, with the clear solution changing to black colloid. Once cooling down to room temperature, the colloid containing Pt NPs was stored in a spiral glass bottle for subsequent washing and loading.

Synthesis of Heterogeneous AuPt Bimetallic NPs with Pt Enriched on the Surface in Different Atom Ratios

The bimetallic AuPt NPs with the surface enrichment of Pt were synthesized by the successive reduction method. For synthesizing AuPt bimetallic NPs in 1/1 Au/Pt atom ratio, 0.06 mmol of H2PtCl6·xH2O was dissolved in a mixed solution with 1 mL of decalin and 4 mL of oleylamine. In the reactions that followed, oleylamine would work as a solvent and as a slow reducer. The mixture of the Pt4+ precursor and solution was injected to the colloid containing Au NPs as seeds (the Au seeds were synthesized as mentioned before) at room temperature. After this, the mixture was heated to 140 °C and maintained for 4 h under a N2 atmosphere. The black resultant was centrifuged and washed by the mixed solution of toluene and acetone for four times. The product was collected and stored in a snail bottle for further characterization. Under the same synthesis conditions, the bimetallic AuPt NPs in 1/0.5, 1/1.5, and 1/2 of Au/Pt atom ratios were synthesized by the adjustment of the amount of the Pt precursors.

Synthesis of Supported Nanocatalysts

With 0.5 wt % of metallic loading, the calculated amount of metallic NPs and γ-Al2O3 powders was dissolved in 20 mL of cyclohexane. The mixture was placed in a 50 mL three-necked round-bottom flask and heated continuously in an oil bath at 55 °C. The left powders were washed for three times with a mixed solution of acetone and ethanol and then dried in an oven at 80 °C for 12 h to obtain nanocatalysts where metallic NPs were supported on γ-Al2O3.