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A general synthesis approach for supported bimetallic nanoparticles via surface inorganometallic chemistry

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Science  02 Nov 2018:
Vol. 362, Issue 6414, pp. 560-564
DOI: 10.1126/science.aau4414

More alloying on silica

Controlling the stoichiometry and achieving a high degree of alloying of metals at ultrasmall scales for catalysis can be difficult. Double complex salts, formed by a cation like Pd(NH3)42+ and an anion like IrCl62−, should be excellent precursors but are poorly soluble and difficult to adsorb directly on metal oxide surfaces. Ding et al. show that sequentially adsorbing the cations and anions from organic solvents onto a silica surface, followed by heating in hydrogen, creates well-mixed nanoparticles, most less than 3 nanometers in diameter, for a variety of alloys. These materials were then tested as catalysts for acetylene hydrogenation to ethylene.

Science, this issue p. 560

Abstract

The synthesis of ultrasmall supported bimetallic nanoparticles (between 1 and 3 nanometers in diameter) with well-defined stoichiometry and intimacy between constituent metals remains a substantial challenge. We synthesized 10 different supported bimetallic nanoparticles via surface inorganometallic chemistry by decomposing and reducing surface-adsorbed heterometallic double complex salts, which are readily obtained upon sequential adsorption of target cations and anions on a silica substrate. For example, adsorption of tetraamminepalladium(II) [Pd(NH3)42+] followed by adsorption of tetrachloroplatinate [PtCl42−] was used to form palladium-platinum (Pd-Pt) nanoparticles. These supported bimetallic nanoparticles show enhanced catalytic performance in acetylene selective hydrogenation, which clearly demonstrates a synergistic effect between constituent metals.

Heterogeneous catalysts that contain bimetallic nanoparticles (NPs) are used in many petrochemical processes, including reforming (1), selective hydrogenation (2, 3) and dehydrogenation (4), and acetoxylation (5). In recent years, bimetallic NPs have been used in biomass conversions (6), electrocatalysis (710), and many other catalytic processes (1113). The difference in catalytic properties of bimetallic NPs compared with their parent metals originates from their distinct geometric and electronic structures as well as the synergistic effects between the two metals (6, 1416). In particular, synthetic protocols have been demonstrated to be crucial to the structural, electronic, and, hence, catalytic performance of bimetallic NPs. The conventional impregnation method usually results in ill-defined bimetallic NPs with inhomogeneous particle sizes and compositions (17, 18). New strategies for the synthesis of bimetallic NPs include colloidal synthesis (19), surface organometallic chemistry (20), atomic layer deposition (21), coadsorption and coreduction of metal cations (18), and carbothermal shock synthesis (22). Nevertheless, the synthesis of ultrasmall (<3 nm in diameter) supported bimetallic NPs with well-defined stoichiometry and intimacy between constituent metals remains challenging.

We explored the preparation of supported bimetallic NPs by reducing heterometallic double complex salt (DCS) precursors (e.g., [Pd(NH3)4][PtCl4]), which differ from conventional precursors in that the stoichiometry and intimacy of two metals are already established. Although DCSs have received tremendous interest in the study of inorganometallic coordination compounds (23, 24), their poor solubility greatly hinders their applications in many fields (23, 2527). Limited success has been achieved in the synthesis of supported bimetallic catalysts using DCS precursors. Boellaard et al. (28) described a synthesis of supported Ni-Fe catalysts from DCSs by mixing two precursors in solution in the presence of a support. Owing to the lack of interaction between bulk DCSs and the catalyst support, the deposition of DCSs on the surface of the support is less controllable. Rather than depositing DCSs from solution, we show that heterometallic DCSs can be synthesized directly on a substrate by sequential adsorption of complex metal cations and anions (Fig. 1A). The supported DCSs with precisely paired metal cations and anions through electrostatic interactions can be converted to well-defined supported bimetallic NPs upon reduction. This approach eliminates the issue of competitive adsorption of different metal cations in the coadsorption and coreduction methods (18); on the other hand, it also overcomes the solubility issues of DCSs for the bimetallic catalyst synthesis. It can be applied to a large variety of bimetallic NPs, including several bulk immiscible systems. The bimetallic NPs are extremely small (1 to 3 nm in diameter) and have narrow size distributions (±25%), and their bimetallic nature was confirmed by single-NP elemental analysis. These bimetallic catalysts outperform their parent metals in selective hydrogenation of acetylene.

Fig. 1 Schematic illustration and electron microscopy analysis of the supported bimetallic NPs.

(A) A schematic illustration of the surface inorganometallic chemistry for the synthesis of supported bimetallic NPs. (B) HAADF-STEM images of 10 types of supported bimetallic NPs synthesized by this approach. All scale bars are 10 nm.

The adsorption of metal cations was carried out on silica (18, 29) (see the supplementary materials). A porous silica support (fig. S1) was first dispersed in water, and the pH of the solution was adjusted to 9 to obtain a negatively charged silica surface. After the adsorption of complex metal cations, e.g., Pd(NH3)42+, silica was then separated from the mother solution, washed with copious water, and dried before the adsorption of metal anions. Our initial attempt for the subsequent anion adsorption using an aqueous solution led to the formation of either large aggregates (fig. S2) or monometallic complexes (fig. S3). This failure was likely caused by nonnegligible solubility of DCSs in water, which led to dissolution, ripening, or leaching of the DCSs. Alternatively, we adopted an aprotic solvent dichloromethane and introduced quaternary ammonium cations to assist dissolution (figs. S4 to S6). Complex metal anions in dichloromethane can precisely target the multivalvent metal cations preadsorbed on silica to form supported heterometallic DCSs. Further reducing these silica-supported DCSs under a flow of H2/N2 (v/v = 1/9) mixture at 400°C afforded the desired silica-supported bimetallic NPs.

Figure 1B shows high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of 10 types of supported bimetallic NPs synthesized by this approach. Large-area transmission electron microscopy and HAADF-STEM images and size distributions of these samples are shown in figs. S7 to S16. These NPs are mostly smaller than 3 nm in diameter and uniformly dispersed on the silica support. These NPs are smaller than most bimetallic NPs from colloidal synthesis (19) but slightly bigger than those synthesized by the coadsorption and coreduction method (18). The polydispersities are similar to the latter. The bimetallic nature of these NPs was confirmed by single-NP energy-dispersive x-ray (EDX) spectroscopy analysis. The composition of several individual NPs was also obtained for each sample, as well as large-area EDX spectroscopy analysis to obtain an average composition of each sample (quantification results are shown in tables S1 to S10). In all cases, the single-NP analysis confirmed that the NPs are bimetallic. Furthermore, the average composition from the single-NP analysis was generally consistent with the average composition provided by large-area EDX spectroscopy analysis, suggesting that the supported DCS precursors were evenly decomposed into bimetallic NPs.

The bulk phase diagrams of the Cu-Ir, Pd-Ir, and Pt-Ir systems indicate that these metal pair are immiscible below 800°C (30). Interestingly, these systems still form bimetallic NPs with metal ratios near 1. Aberration-corrected HAADF-STEM images of Cu-Ir/SiO2 and Pd-Ir/SiO2 show uneven contrast within single NPs (figs. S17 and S18), suggesting the occurrence of intraparticle phase segregation at a subnanometer level. This type of nanoscopic phase segregation has been reported in several bimetallic systems, including Cu-Ir (31), Ru-Pd (32), and Pt-Ni (33). The lack of macroscopic phase segregation in Cu-Ir, Pd-Ir, and Pt-Ir systems may be partially attributed to the extremely high melting point of Ir metal, which may prevent the nanoscopically phase segregated NPs from coarsening.

To validate our proposed surface inorganometallic chemistry for the bimetallic NPs synthesis, we used x-ray photoelectron spectroscopy (XPS) to study the structural evolution of the surface intermediates during the synthesis. The XPS results are shown in Fig. 2 and figs. S19 to S27. After the adsorption of complex metal cations [Pd(NH3)42+, Pt(NH3)42+, and Ru(NH3)62+], the XPS signals of metals and nitrogen were observed (Fig. 2 and figs. S19 and S23). Weak Cl 2p signals were also observed on these samples (fig. S25), indicating that the divalent complex metal cations require an extra Cl ion to balance the excess positive charge. After the adsorption of complex metal anions (PtCl42−, IrCl62−, and AuCl4), the intensity of the Cl 2p signal increased along with the appearance of Pt, Ir, and Au signals, confirming the successful adsorption of complex metal anions.

Fig. 2 XPS and UV-vis spectra of supported DCSs and reference compounds.

The structure of [Pd(NH3)4][PtCl4]/SiO2 is illustrated on the right. The absorption bands associated with the metallophilic interaction between Pd and Pt metal centers are marked by dashed lines.

In addition to the N 1s signal from the NH3 ligand (400 eV), a new N 1s signal was observed at 402 eV on the nonreduced [Pd(NH3)4][PtCl4]/SiO2, [Pd(NH3)4][IrCl6]/SiO2, [Pt(NH3)4][IrCl6]/SiO2, and [Ru(NH3)4][IrCl6]/SiO2 samples (Fig. 2 and fig. S23) from the N atom in the quaternary ammonium cation, which was used to assist the dissolution. The adsorption of quaternary ammonium cations was required to balance the excess negative charge of the divalent complex anions. No quaternary ammonium cation was needed when a monovalent anion, AuCl4, was used, and in this case, the N 1s signal at 402 eV was absent for the nonreduced [Pd(NH3)4][AuCl4]/SiO2 sample (Fig. 2).

Because the adsorption of complex metal anions was carried out in an aprotic solvent, dichloromethane, ion pairs should mostly stay associated and form dipoles. Therefore, the adsorption of complex metal anions may not be solely driven by electrostatic interactions. Additionally, dipole-dipole and dipole-induced dipole interactions between the anionic complex and the surface may also play important roles. The surface chemistry described here works well for a silica surface but may not be simply extended to other types of support. For instance, alumina has a higher point of zero charge compared with silica (18). Thus, complex metal anions are normally adopted for the synthesis of alumina-supported noble metal catalysts. Extending our synthesis strategy to alumina support may require substantial efforts toward solvent engineering of metal precursors. Furthermore, chloride binds strongly to the alumina surface and may change the surface properties.

The metallophilic interaction between the adsorbed complex metal cations and anions was confirmed by ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, indicating the formation of supported DCSs. DCSs usually exhibit distinct colors originating from their metal-metal interactions; many DCSs are named after their colors, e.g., Magnus’ green salt [Pt(NH3)4][PtCl4] and Vauquelin’s pink salt [Pd(NH3)4][PdCl4] (23). Because of the relatively low loadings, the supported complex species obtained by our sequential adsorption approach do not exhibit noticeable colors. Nevertheless, the supported [Pd(NH3)4][PtCl4]/SiO2 synthesized by sequential adsorption shows identical UV-vis absorption bands (315, 390, and 535 nm) with bulk DCS [Pd(NH3)4][PtCl4], which exhibits a pink color (34, 35). By contrast, the UV-vis spectra of the impregnated Pd(NH3)4Cl2/SiO2 and K2PtCl4/SiO2 were different from the spectra of [Pd(NH3)4][PtCl4]/SiO2.

The amorphous nature of supported DCSs was confirmed by electron microscopy studies. No NPs were observed on the supported DCSs initially (Fig. 3A). However, within 2 min of electron beam exposure, many NPs in the 1- to 2-nm size range were observed, indicating the in situ decomposition of DCSs associated with the electron beam damage. Extended x-ray absorption fine structure (EXAFS) measurements of the Ir LIII edge were performed to study the structural evolution of coordination sphere of Ir atoms. As shown in Fig. 3B, the EXAFS spectra of nonreduced [Ru(NH3)6][IrCl6]/SiO2 and [Pd(NH3)4][IrCl6]/SiO2 were similar to that of the Na2IrCl6·6H2O reference. We associated the major peak between 1.5 and 2.5 Å with the Ir–Cl bond in IrCl62−. After the reduction of [Ru(NH3)6][IrCl6]/SiO2 at 400°C under a hydrogen atmosphere, a new peak appeared between 2.5 and 3 Å, corresponding to Ir–M (M = Ru or Ir) bonds. XPS analysis of the reduced DCSs/SiO2 samples confirmed the reduction of metal atoms into metallic states (figs. S20 and S22). Further, the disappearance of N 1s and Cl 2p signals indicated the successful removal of ligands (figs. S24, S26, and S27).

Fig. 3 Structural evolution of supported DCSs upon reduction.

(A) HAADF-STEM images of nonreduced [Ru(NH3)6][IrCl6]/SiO2 synthesized by sequential adsorption. E-beam, electron beam. (B) Fourier transforms of EXAFS spectra of various Ir-containing samples at the Ir LIII edge: Na2IrCl6·6H2O reference; nonreduced [Ru(NH3)6][IrCl6]/SiO2 and [Pd(NH3)4][IrCl6]/SiO2; and reduced Ru-Ir/SiO2 synthesized by sequential adsorption. χ(r), EXAFS function.

The catalytic performance of the supported bimetallic NPs was evaluated for acetylene hydrogenation under noncompetitive conditions (figs. S28 to S30). The bimetallic catalysts (Pd-Ir/SiO2, Pd-Pt/SiO2, Pd-Au/SiO2, Pt-Ir/SiO2, and Ru-Pt/SiO2) generally showed higher catalytic activity and lower alkane selectivity compared to their parent metals. Ru-Ir/SiO2 was an exception in that it showed the same negligible activity as its parent metals. Among the group VIII metals, Ru and Ir often showed the strongest binding strengths for many types of adsorbates (36), which might explain the low activity of Ru/SiO2, Ir/SiO2, and Ru-Ir/SiO2 in acetylene hydrogenation. The enhanced activity and suppressed ethane selectivity of bimetallic Pd-M/SiO2 and Pt-M/SiO2 compared to their parent metals, could be partly attributed to the relatively weaker binding strengths of acetylene and ethylene on these bimetallic NPs (2, 3).

We further studied Pd-Pt bimetallic NPs with different compositions in acetylene hydrogenation under competitive conditions (Fig. 4, A and B, and figs. S31 to S33). The compositions of supported Pd-Pt bimetallic NPs were adjusted by coadsorption of complex metal cations followed by adsorption of complex metal anions. For instance, equal molar Pd(NH3)42+ and Pt(NH3)42+ cations were coadsorbed on SiO2, followed by adsorption of PdCl42− or PtCl42− anions and subsequent reduction, yielding (Pd0.5Pt0.5)-Pd/SiO2 or (Pd0.5Pt0.5)-Pt/SiO2, respectively (fig. S34). Compared to the monometallic Pd/SiO2 and Pt/SiO2, the three bimetallic catalysts prepared by sequential adsorption method—(Pd0.5Pt0.5)-Pd/SiO2, (Pd0.5Pt0.5)-Pt/SiO2, and Pd-Pt/SiO2—all showed enhanced catalytic activity and suppressed alkane selectivity in acetylene hydrogenation in the presence of high-concentration propylene. Moreover, three bimetallic catalysts were prepared by coadsorption and coreduction of Pd(NH3)42+ and Pt(NH3)42+ cations (fig. S35) (18)—Pd0.25Pt0.75/SiO2, Pd0.5Pt0.5/SiO2, and Pd0.75Pt0.25/SiO2—which showed lower activity than a physical mixture of Pd/SiO2 and Pt/SiO2, and their alkane selectivity is greater than those prepared by sequential adsorption method.

Fig. 4 Catalysis and IR studies of the supported bimetallic NPs.

(A and B) Catalytic performance of supported monometallic and bimetallic NPs in acetylene hydrogenation. (C) IR spectra of CO molecule adsorbed on various catalysts. Pd-Pt/SiO2 and Pd0.5Pt0.5/SiO2 are synthesized by sequential adsorption (this work) and coadsorption (18) methods, respectively. All catalysts were reduced at 400°C. Experimental details are provided in the supplementary materials. a.u., arbitrary units.

Infrared (IR) spectroscopy with CO as a probe molecule was used to study the surface properties of these monometallic and bimetallic catalysts (Fig. 4C). We found that the bridge CO peak was very sensitive to the type of metals (1920 and 1790 cm−1 for Pd and Pt surfaces, respectively). Interestingly, the IR spectra of the Pd0.5Pt0.5/SiO2 catalyst prepared by the coadsorption method exhibits a Pt-like feature, whereas the Pd-Pt/SiO2 catalyst prepared by the sequential adsorption method exhibits a Pd-like feature. This suggests that the surfaces of the bimetallic NPs prepared by coadsorption and sequential adsorption methods are Pt- and Pd-rich, respectively. The superior intrinsic activity of Pd and the electronic modification by Pt underneath might be the origin of the improved catalytic performance of the Pd-Pt/SiO2 catalyst prepared by the sequential adsorption method.

Supplementary Materials

www.sciencemag.org/content/362/6414/560/suppl/DC1

Materials and Methods

Figs. S1 to S35

Tables S1 to S11

References (3739)

References and Notes

Acknowledgments: We thank J. Spivey for the access to the IR instrument and K. Dooley for the assistance in thermogravimetric and nitrogen sorption analysis. T. Blanchard is acknowledged for inductively coupled plasma analysis. Funding: K.D. acknowledges the startup funding from Louisiana State University. Electron microscopy and UV-vis spectroscopy were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. XPS was performed at the Shared Instrumentation Facility (SIF) at Louisiana State University. The Ketek seven-element detector array used in this study for x-ray measurements at CAMD was acquired with funds provided by The Louisiana Board of Regents [LEQSF(2016-17)-ENH-TR-07]. Author contributions: K.D. conceived the concept, designed the experiments, synthesized and characterized the nanoparticles, performed catalytic tests, and wrote the paper; D.A.C. performed HAADF-STEM and single-NP EDX spectroscopy characterization; L.Z. carried out IR, thermogravimetric analysis, x-ray diffraction, and nitrogen sorption measurements; Z.C. assisted with data analysis and manuscript preparation; A.D.R. carried out EXAFS measurements; I.N.I. carried out UV-vis measurements; and D.C. performed XPS characterization. Competing interests: K.D. has filed provisional patent application no. 62/721,690 regarding the synthesis of supported bimetallic nanoparticles via double complex salts. Data and materials availability: All data are available in the main text or the supplementary materials.

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