Synthesis of stable monodisperse AuPd, AuPt, and PdPt bimetallic clusters encapsulated within LTA-zeolites
Graphical abstract
Introduction
Bimetallic nanoparticles are useful as catalysts because of the unique electronic and structural features conferred by atomic mixing of two or more elements at the nanoscale. Such features, in turn, are consequential for turnover rates and selectivities in reactions as diverse as CO oxidation [1], alkane dehydrogenation [2], and NOx reduction [3]. These bimetallic synergies also bring ancillary benefits [4]; a second metal can assist the reduction of another one [5], inhibit sintering during thermal treatments [6], or weaken the effects of site blocking by S-atoms or other titrants [7]. These consequences may reflect ligand effects that cause one element to influence the electronic properties of another one [8] or ensemble effects caused by the dilution of monometallic domains [9]. The dissection of such effects into their causative components requires the synthesis of particles uniform in composition and size [9], an elusive objective because of the dearth of effective and general synthetic strategies.
Sequential adsorption and precipitation or co-impregnation of two metal salts onto mesoporous scaffolds [10] does not consistently place the component metals in atomic proximity [10], a challenge that can be addressed by sequential grafting of organometallic precursors onto supports [10]. Such grafting enforces metal-metal binding through covalent attachments between the first and second precursors deposited. The availability of precursors that prefer mutual interactions over those with the support limits the scope of such protocols, which often lead to the concurrent formation of monometallic clusters of the second precursor used in the sequence [10]. Galvanic displacement and electroless deposition, in contrast, selectively place a second metal into existing clusters of another metal via redox reactions [11]. Compositional uniformity in these methods requires seed clusters uniform in size and strategies to minimize homogeneous nucleation of the second component using solvents as reductants [9]. Colloidal synthesis methods involving the reduction of precursors in the presence of protecting polymers [11] can also form small clusters uniform in size and composition [9], [12]; such uniformity, however, is frequently compromised by thermal treatments essential to deprotect the metal surfaces, as required for their catalytic function [9], [11].
The nanometer-sized voids provided by crystalline zeolite frameworks can be used as containers for bimetallic clusters [11]. Their confinement within such voids allows the selection of certain reactants and transition states over others based on molecular size and the protection of active surfaces from large titrants and poisons by exploiting zeolite shape selectivity [13], [14]. Confinement is often achieved by the exchange of solvated cationic precursors into the anionic zeolite frameworks [4]. Reductive treatments then form monometallic clusters, and the subsequent exchange and reduction of a second metal can form, in some instances, confined bimetallic clusters that are less prone to sintering than their monometallic counterparts [4]. Inhomogeneous cluster compositions, however, are often observed and such exchange methods require zeolite channels that allow the diffusion of the solvated cationic precursors and their charge-balancing double layer [11], [15].
Here, we report an alternate route for the synthesis of small bimetallic clusters, uniform in size and composition, within LTA zeolite crystals, a framework with apertures too small to allow precursor exchange. We illustrate this general synthetic strategy for a range of AuPd, AuPt, and PdPt compositions. In doing so, we extend techniques that use protecting ligands to stabilize metal cation precursors against premature precipitation as colloidal metals of oxyhydroxides at the hydrothermal conditions required to crystallize zeolite frameworks [13], [14]. Hydrothermal LTA crystallization in the presence of ligated precursors of two different elements leads to the formation of nearly monodisperse bimetallic clusters (1–2 nm); these clusters expose surfaces free of synthetic debris after sequential thermal treatments in O2 and H2, without compromising LTA crystallinity. The bimetallic nature of the clusters was shown by X-ray absorption spectroscopy and confirmed by the infrared spectra of chemisorbed CO. The protecting 3-mercaptopropyl-trimethoxysilane ligands prevent precipitation, reduction, and coalescence of the metals before the formation of LTA frameworks. These ligands also form siloxane bridges with silicate oligomers to enforce confinement and uniform placement of precursors throughout zeolite crystals, thus ensuring bimetallic mixing and the nucleation of small confined clusters, even after thermal treatments that remove the ligands and their S-atoms. The retention of these clusters within zeolite crystals was demonstrated from ethanol oxidation rates on samples exposed to dibenzothiophene, which would irreversibly poison any unconfined clusters [13].
Section snippets
Reagents
HAuCl4·3H2O (99.999%, Sigma-Aldrich), Pd(NH3)4(NO3)2 (99.99%, Sigma-Aldrich), Pd(NH3)4Cl2 (99.99%, Sigma-Aldrich), H2PtCl6 (8% wt. in H2O, Sigma-Aldrich), 3-mercaptopropyl-trimethoxysilane (95%, Sigma-Aldrich), NaOH (99.99%, Sigma-Aldrich), Ludox AS-30 colloidal silica (30% wt. suspension in H2O, Sigma-Aldrich), NaAlO2 (53% Al2O3, 42.5% Na2O, Riedel-de Haën), mesoporous SiO2 (Davisil, grade 646, surface area: 294 m2 g−1), fumed SiO2 (Cab-O-Sil, HS-5, 310 m2 g−1), CaCl2·2H2O (EMD Millipore), acetone
Metal content and phase purity of metal-LTA samples
LTA-encapsulated metal nanoparticles were synthesized with Au-Pd compositions (AunPd100−nNaLTA), Au-Pt (AunPt100−nNaLTA), or Pd-Pt (PdnPt100−nNaLTA) and a broad range of atomic ratios and a 1% wt. metal content (nominal; based on amounts of reagents used). The measured elemental compositions reported in Table 1 confirm the essentially complete incorporation of the metal precursors into the final product. These data indicate that the ability of the ligands to bind to the metal cations through the
Conclusion
A general procedure was developed for the encapsulation of highly dispersed bimetallic clusters (1–2 nm), uniformly distributed in size and composition, within the voids of the LTA zeolite using a ligand-assisted hydrothermal synthesis technique. Samples with AuPd, AuPt, and PdPt clusters and a variety of metal compositions were synthesized to demonstrate the broad applicability of the technique. Metal encapsulation and alloying is conferred by introducing mercaptosilane-stabilized metal cation
Author contributions
T.O., S.I.Z., and E.I. conceived and developed the synthesis technique, and drafted most of the manuscript. J.M.R.-L, L.G., and F.G.R conducted XAS experiments, processed the XAS data with the IFEFFIT package, assisted in the interpretation of EXAFS data, and wrote the description of the XAS methods and data. T.O. performed all chemical syntheses and the other characterization experiments, including the catalytic experiments.
Notes
The authors declare the following competing financial interest(s): (1) The funding for a significant portion of this research was provided by the Chevron Energy Technology Co., and (2) Stacey I. Zones is an employee of this company and, more generally, also a stockholder of the Chevron Corp.
Acknowledgments
We gratefully acknowledge the generous financial support of the Chevron Energy Technology Co, as well as ancillary research support from CONICET (PIP No. 1035) and LNLS (Project XAFS1-18861) and an ARCS Foundation Fellowship (for TO). We thank Dr. Reena Zalpuri (Electron Microscope Lab) for support with TEM instrumentation and Dr. Antonio DiPasquale (X-Ray Facility) for assistance with the acquisition of diffraction data.
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