Elsevier

Journal of Catalysis

Volume 342, October 2016, Pages 125-137
Journal of Catalysis

Synthesis of stable monodisperse AuPd, AuPt, and PdPt bimetallic clusters encapsulated within LTA-zeolites

https://doi.org/10.1016/j.jcat.2016.07.017Get rights and content

Highlights

  • Synthesis of AuPd, AuPt, and PdPt bimetallic clusters encapsulated in LTA zeolite.

  • Small bimetallic clusters uniform in size and composition.

  • Thermal sinter stability through small-pore zeolite encapsulation.

  • Encapsulated metals are size selective and sulfur-tolerant catalysts.

Abstract

AuPd, AuPt, and PdPt bimetallic clusters uniform in size and composition were prepared using hydrothermal assembly of LTA crystals around cationic precursors stabilized by protecting mercaptosilane ligands. The sulfur moiety in these bifunctional ligands forms adducts that prevent premature reduction or precipitation of metal precursors during crystallization. The silane groups can form bridges with silicate oligomers as they form, thus enforcing homogeneous distributions of precursors throughout crystals and ensuring that subsequent reductive treatments lead to the two elements residing within small and nearly monodisperse clusters. Their confinement within LTA crystals, evident from microscopy and titrations with large poisons, renders them stable against sintering during thermal treatments at high temperatures (820–870 K). Infrared spectra of chemisorbed CO show that bimetallic surfaces are free of synthetic debris after thermal treatments; these spectra also indicate that intracluster segregation occurs upon CO chemisorption, a demonstration of the presence of the two elements within the same clusters. The number and type of atoms coordinated to a given absorber atom, determined from the fine structure in X-ray absorption spectra, are consistent with bimetallic structures of uniform composition. The rates of ethanol oxidative dehydrogenation on these bimetallic clusters were essentially unaffected by exposure to dibenzothiophene, a large poison that suppresses rates on unconfined clusters, indicating that bimetallic clusters are protected within the confines of LTA crystals. These synthetic protocols seem generally applicable to other bimetallic compositions and zeolites, for which the monometallic counterparts have been successfully encapsulated within several microporous frameworks using ligand-stabilized precursors and hydrothermal crystallization methods.

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.

References (55)

  • O. Rosseler et al.

    Structural and electronic effects in bimetallic PdPt nanoparticles on TiO2 for improved photocatalytic oxidation of CO in the presence of humidity

    Appl. Catal. B

    (2015)
  • Y. Barshad et al.

    Carbon monoxide oxidation under transient conditions: a fourier-transform infrared transmission spectroscopy study

    J. Catal.

    (1985)
  • D. Stacchiola et al.

    The adsorption and structure of carbon monoxide on ethylidyne-covered Pd(1 1 1)

    Surf. Sci.

    (2000)
  • F. Menegazzo et al.

    Quantitative determination of gold active sites by chemisorption and by infrared measurements of adsorbed CO

    J. Catal.

    (2006)
  • H. Toulhoat et al.

    Transition metals to sulfur binding energies relationship to catalytic activities in HDS: back to Sabatier with first principle calculations

    Catal. Today

    (1999)
  • C. Mottet et al.

    New magic numbers in metallic clusters: an unexpected metal dependence

    Surf. Sci.

    (1997)
  • J.M. Ramallo-López et al.

    Complementary methods for cluster size distribution measurements: supported platinum nanoclusters in methane reforming catalysts

    J. Mol. Catal.

    (2005)
  • C. Louis et al.

    Catalytic properties of silica-supported molybdenum catalysts in methanol oxidation: the influence of molybdenum dispersion

    J. Catal.

    (1988)
  • W.-Y. Yu et al.

    Oxygen activation and reaction on Pd-Au bimetallic surfaces

    J. Phys. Chem. C

    (2015)
  • R.M. Wolf et al.

    A comparative study of the behaviour of single-crystal surfaces and supported catalysts in NO reduction and CO oxidation over Pt-Rh alloys

    Faraday Discuss. Chem. Soc.

    (1989)
  • Z. Zhang et al.

    Proximity requirement for Pd enhanced reducibility of Co2+ in NaY

    Catal. Lett.

    (1989)
  • T. Rades et al.

    Characterization of bimetallic NaY-supported Pt-Pd catalyst by EXAFS, TEM and TPR

    Catal. Lett.

    (1994)
  • S. Kunz et al.

    Mechanistic evidence for sequential displacement-reduction routes in the synthesis of Pd-Au clusters with uniform size and clean surfaces

    J. Phys. Chem. C

    (2014)
  • O.S. Alexeev et al.

    Supported bimetallic cluster catalysts

    Ind. Eng. Chem. Res.

    (2003)
  • S. Alayoglu et al.

    Structural and architectural evaluation of bimetallic nanoparticles: a case study of Pt−Ru core−shell and alloy nanoparticles

    ACS Nano

    (2009)
  • M. Choi et al.

    Mercaptosilane-assisted synthesis of metal clusters within zeolites and catalytic consequences of encapsulation

    J. Am. Chem. Soc.

    (2010)
  • G. Bergeret et al.
  • Cited by (0)

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