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  • 2590 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011

    Cite this: Phys. Chem. Chem. Phys., 2011, 13, 2590–2602

    Geometric and electronic effects on hydrogenation of cinnamaldehyde over unsupported Pt-based nanocrystalsw

    William O. Oduro,z Nick Cailuo, Kai Man K. Yu, Hongwei Yangy and Shik Chi Tsang*

    Received 16th September 2010, Accepted 16th December 2010

    DOI: 10.1039/c0cp01832e

    It is reported that catalytic hydrogenation of cinnamaldehyde to cinnamyl alcohol is a structural

    sensitive reaction dependent on size and type of metal doper of unsupported platinum

    nanocrystals used. Smaller sizes of platinum nanocrystals are found to give lower selectivity to

    cinnamyl alcohol, which suggests the high index Pt sites are undesirable for the terminal aldehyde

    hydrogenation. A plot of reaction selectivity across the first row of transition metals as dopers

    gives a typical volcano shape curve, the apex of which depicts that a small level of cobalt on

    platinum nanocrystals can greatly promote the reaction selectivity. The selectivity towards

    cinnamyl alcohol over the cobalt doped Pt nanocrystals can reach over 99.7%, following the

    optimization in reaction conditions such as temperature, pressure and substrate concentration.

    Detailed studies of XRD, CO chemisorption (for FTIR), TEM, SEM, AES and XPS of the

    nanostructure catalyst clearly reveal that the decorated cobalt atoms not only block the high index

    sites of Pt nanocrystals (sites for Co deposition) but also exert a strong electronic influence on

    reaction pathways. The d-band centre theory is invoked to explain the volcano plot of selectivity

    versus metal doper.

    Introduction

    Hydrogenation of organic compounds plays a very important

    role in chemical manufacturing processes. Among all

    the hydrogenation reactions reported, the hydrogenation of

    a,b-unsaturated aldehydes to their corresponding unsaturated alcohols draws the most attention as the hydrogenation of

    these compounds is of both fundamental and industrial

    importance.1,2 There has been much recent interest in

    synthesizing uniform metallic and bimetallic nanocrystals as

    new heterogeneous catalysts because of appropriate metal

    particle size and optimised geometric and/or electronic effects

    in metallic and bimetallic nanostructures which may allow the

    nanocrystals with tuneable catalytic properties 3,4 to overcome

    thermodynamic favourable CQC hydrogenation over CQO hydrogenation. Thus, this approach is especially important for

    nowadays’ catalyst development for a high performance catalyst

    material in terms of activity (to increase productivity), selectivity

    (to reduce the needs in product separation) and energy

    considerations (to reduce energy consumption).

    We have investigated hydrogenation of cinnamaldehyde to

    cinnamyl alcohol over unsupported Pt nanocrystals and its

    transition metal doped (bimetallic) nanoparticles. The substrate

    molecule contains three reducible groups (terminal aldehyde,

    double bond at a-b carbon position and benzene ring) as a chemical probe for this investigation. Cinnamaldehyde is one

    interesting model compound for hydrogenation because a

    number of partially hydrogenated products can be synthesized,

    depending on the selectivity of the hydrogenation reaction

    (see Scheme 1). In addition, the economic importance of

    selective hydrogenation of a,b-unsaturated aldehyde is particularly denoted,1,2 because the cinnamyl alcohol can be

    used as pharmaceuticals, fragrances, and perfumes.5 From the

    literature, selective hydrogenation of this compound is one of the

    most widely studied reactions. A wide range of catalysts,

    including promoted and unpromoted metals/alloys,6–8 metal

    oxides,9,10 microporous supports,11 and polymer fibre catalysts 12 in both liquid2,13–15 and vapour16,17 phases was systematically

    investigated. It has been empirically shown that the selectivity of

    the reaction can depend on some key parameters, including the

    nature of the metal and particle size,18 catalyst support,19–21 and

    type of promoters/additives21–23 used. There were postulations

    on the importance of structural and electronic properties of

    Wolfson Catalysis Centre, Department of Chemistry, University of Oxford, Oxford, UK OX1 3QR. E-mail: edman.tsang@chem.ox.ac.uk; Fax: +44 1865 272600; Tel: +44 1865 282610 w Electronic supplementary information (ESI) available. See DOI: 10.1039/c0cp01832e z Contact address: Institute of Industrial Research- CSIR. P. O. Box LG 576 Legon, Accra, Ghana. y Contact address: State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry of Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China.

    PCCP Dynamic Article Links

    www.rsc.org/pccp PAPER

    http://dx.doi.org/10.1039/c0cp01832e http://dx.doi.org/10.1039/c0cp01832e

  • This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2591

    metal catalysts as the main underlying factors.1,4,24,25 The studies

    of platinum particle size 1,4 and shape24 on activity and selectivity

    also gave circumstantial evidence that the particle geometry plays

    a key role in the hydrogenation reaction. However, a fundamental

    understanding in reaction selectivity by these structural

    parameters leading to tuneable properties has yet to be achieved.

    Fourier transmission Infrared Spectroscopy, FTIR, is

    commonly employed to study the surface chemistry of metal-

    substrate interaction in catalytic systems, which provides a

    means of observing the different types of adsorption emanating

    from the different geometric arrangement of atoms on

    the surface of a particle. Carbon monoxide-metal interaction

    is one of such matrices that can act as useful interrogating

    tools in elucidating surface properties because of the

    strong CO-metal bond and also the extensive background

    information in literature on the types of adsorbed

    species.26–34 Three types of adsorption modes of CO on Pt

    single crystal surfaces have been previously observed. A

    vibration frequency u(CO) between 2090–2040 cm�1 is assigned to linearly adsorbed CO mode on a Pt site and

    1860–1780 cm�1 to bridged mode adsorption26 whilst a

    strong vibration frequency u(CO) of ca. 1950–1925 cm�1

    prevalent in small size particles (o5.0 nm) is attributed to the stretching mode of multicarbonyl species.4,27 In additional,

    the degree of back bonding of adsorbed CO on Pt gives

    progressive red shift in adsorption frequencies that can

    reflect the electron density of the metal nanoparticles. Thus,

    in this paper, the technique is particularly employed to obtain

    the surface feature relationship with respect to catalytic

    performance and to provide a mechanistic understanding on

    the substrate interaction with metal surface.

    On the other hand, it should be noted that it is very challenging

    to disentangle the complex interplays between geometric,

    electronic, and steric effects in a working catalytic system (for

    example, supported bimetallic catalysts with wide heterogeneities

    in size, shape, composition and metal-support interfaces).

    In order to shed light on the geometric and electronic

    contributions on Pt and bimetallic nanoparticles in the

    selective hydrogenation of cinnamaldehyde to cinnamyl

    alcohol, we have employed unsupported Pt based nanocrystals

    to eliminate the support effect in this paper. In addition, a

    solution technique for controlled growth of metallic

    nanocrystals of defined size and surface feature by chemical

    reduction allowing tailoring of particle size has also been

    used. Apart from the using CO-chemisorption (FTIR),

    other spectroscopic techniques such X-ray Photoelectron

    Spectroscopy (XPS), Auger Electron Spectroscopy (AES)

    techniques have been employed to probe the electronic

    properties. X-ray Powder Diffraction (XRD), Transmission

    Electron Microscopy (TEM), Energy Dispersive X-ray analysis

    (EDX) and Scanning Electron Microscopy (SEM) were

    conducted in order to examine the structural and chemical

    changes for the Pd nanocrystals before and after modification

    with a second metal. Experimental parameters such as pressure

    of hydrogen, concentration of cinnamaldehyde, temperature,

    and reaction time were also studied.

    Experimental

    Synthesis of Pt nanostructures

    The Pt and its bimetallic nanocrystals were synthesized by a

    modified polyol process.35–37 Typically, a mixture of bis-(acetyl

    acetonato) platinum(II), Pt(acac)2, (Pt 49.49%min. Alfa Aesar,

    0.30 g), 1,2-hexadecanediol (90%, Aldrich, 0.20 g), oleic acid

    (99+%, Aldrich, 100 mL) and oleylamine (98%, Aldrich, 100 mL) in 6.0 mL of dioctylether (99%, Aldrich) was refluxed at 250 1C for 40 min in a three necked round bottom flask in an inert environment by bubbling nitrogen

    gas whilst ensuring continuous stirring with a magnetic stirrer.

    After 40 min the reaction mixture was cooled and 4.2 nm Pt

    nanocrystals were obtained. The effect of varying the amo

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