Top Banner

Click here to load reader

13

Citethis:Phys. Chem. Chem. Phys.,2011,13 ,25902602 PAPERfulltext.calis.edu.cn/rsc/physical chemistry chemical physics/c0cp01832e.pdf · 2590 Phys. Chem. Chem. Phys., 2011, 13 , 25902602

Mar 21, 2020

ReportDownload

Documents

others

  • 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 cinnamaldehydeover unsupported Pt-based nanocrystalsw

    William O. Oduro,z Nick Cailuo, Kai Man K. Yu, Hongwei Yangy andShik 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 unsaturatedalcohols 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 CQOhydrogenation. 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 achemical 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 isparticularly 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 catalysts12 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 ofOxford, Oxford, UK OX1 3QR. E-mail: edman.tsang@chem.ox.ac.uk;Fax: +44 1865 272600; Tel: +44 1865 282610w Electronic supplementary information (ESI) available. See DOI:10.1039/c0cp01832ez Contact address: Institute of Industrial Research- CSIR. P. O. BoxLG 576 Legon, Accra, Ghana.y Contact address: State Key Laboratory of Physical Chemistry ofSolid Surfaces, National Engineering Laboratory for Green ChemicalProductions of Alcohols-Ethers-Esters, College of Chemistry ofChemical Engineering, Xiamen University, Xiamen 361005, Fujian,P. R. China.

    PCCP Dynamic Article Links

    www.rsc.org/pccp PAPER

    http://dx.doi.org/10.1039/c0cp01832ehttp://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 isassigned 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 tothe 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) wasrefluxed at 250 1C for 40 min in a three necked roundbottom 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 amounts

    of stabilisers (oleic acid and oleylamine) was investigated using

    Scheme 1 Reaction pathways of the cinnamaldehyde hydrogenation.

    Table 1 The quantities of metal precursors, surfactants and reducing agents used in the synthesis of unsupported Pt and bimetallic Pt basednanocrystals

    Bimetal system Quantity of second metal precursor salt usedQuantity of Pt precursor salt, oleic acid, oleylamine,and hexadecanediol used

    Pt Nil 0.30 g Pt(acac)2 ,100 ml, 100 ml and 0.2 gTiPt 0.22 g Ti(IV) isopropoxide (99.999%, Aldrich)MnPt 0.19 g Mn(acac)2 (Aldrich)FePt 0.13 g Fe(Ac)2 (95% Aldrich)CoPt 0.20 g Co(acac)2 (97% Aldrich)NiPt 0.20 g Ni(acac)2 (95% Aldrich)CuPt 0.20 g Cu(acac)2 AldrichZnPt 0.17 g Zn(Ac)2 dihydrate (98.5% BDH GPR)SnPt 0.18 g Sn(Ac)2 AldrichPbPt 0.29 g Pb(Ac)2 trihydrate (99.999%, Aldrich)

    NB: acac = acetyl acetonate and Ac = acetate.

  • 2592 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011

    the same process which resulted in a newmethod of controlling

    the Pt particle size in the range of 2.8–14 nm by the

    modification of the polyol process (see Table 3).4

    In the case of 4.2 nm Pt doped with other metallic element,

    i.e cobalt, a separate glass vial containing a solution of

    cobalt(II) acetyl acetonate, Co(acac)2, (min. 97%, Aldrich,

    0.20 g), 5.0 mL of dioctylether (98%, Aldrich), 64 mL of oleicacid (99+%, Aldrich) and 1,2-hexadecanediol (90%, Aldrich,

    0.50 g) was first pre-heated to 100 1C to remove water. Thistypical cobalt precursor solution was injected into the 4.2 nm

    platinum sol in the refluxing set-up using a glass Pasteur

    pipette at 200 1C. The nitrogen gas flow was increased asa precautionary measure to maintain the oxygen free

    environment during the injection process. The temperature

    was maintained at 250 1C for 20 min. The reaction mixturewas then allowed to cool to room temperature (20 1C) andrepeatedly washed in portions of an ethanol and hexane

    solvents before centrifugation. The collected particles after

    the centrifugation were then re-dispersed and stored in

    isopropanol (Fisher Analytical Reagent grade). The air dried

    particles was directly used for catalysis studies.

    The quantities of Pt(acac)2 for the Pt seed formation and

    the second metal precursor salt used in the preparation of

    the bimetallic systems are listed in Table 1 (metal mole ratio

    of ca. 1).

    Catalytic Testing of Pt nanostructures

    The hydrogenation experiments were carried out under liquid

    phase in a stainless steel autoclave with a glass liner fitted

    tightly to its inner wall. The autoclave, equipped with a

    magnetic stirrer, pressure gauge and a thermocouple, was

    heated and regulated by a Parr 4842 temperature controller.

    The size of reactor, weight of catalyst used and quantities of

    testing mixture were clearly stated in the experiments. Initially,

    for the comparison of selectivity, 37 mg of unsupported Pt

    nanocrystals, 5 mL of 12.5% vol/vol cinnamaldehyde in

    isopropanol (IPA), 20 bar H2, 100 1C for 2 h were placedin 25 mL stainless steel autoclave. The liquid sample was

    analyzed with GC-FID and GC-MS to identify and

    quantify all products, which showed a total consumption of

    cinnamaldehyde under the reaction conditions. Only two

    main products, the unsaturated alcohol (cinnamyl alcohol)

    and the saturated aldehyde, the 3-phenylpropionaldehyde

    (hydrocinnamaldehyde) were obtained with only trace of

    phenylpropanol detected. In order to verify the relationship

    between conversion and selectivity, the reaction was prevented

    from reaching a complete conversion. Thus, only 1.0 mg of

    unsupported metal nanocrystals was then weighed and placed

    in the autoclave together with a magnetic stirrer bar. 5.0 mL of

    3.0 vol% of trans-cinnamaldehyde (99% Aldrich) in IPA was

    then added. The autoclave was purged with pure hydrogen

    (99.99%, BOC) for a minute to remove any traces of oxygen. It

    was then charged to 20.0 bar of hydrogen (99.99%, BOC) at

    20 1C and heated to 100 1C. The reaction was allowed toprocess for 2 h. The autoclave was then cooled to room

    temperature, depressurised and the liquid content was

    analyzed by a Hewlett Packard 5890A GC - FID equipped

    with Supelco COWAX 10 polar capillary column. The

    parameters to be investigated i.e. reaction pressure, time and

    concentration of substrate were varied.

    TEM Characterisation of Pt nanostructures

    A few drops of the as-synthesised nanoparticles sol was

    dispersed in ethanol using an ultra-sonic bath for 15 min.

    Three drops of the uniformly dispersed sol was then added

    drop-wise onto a carbon film on a cupper grid (Lacey carbon

    film, 300 Mesh Cu, Agar Scientific) and the solvent was dried

    off using a 60 watts lamp. The cupper grid was then placed in

    an oven at 100 1C to completely dry off the solvent. Thesamples were analyzed by JEOL JEM-3000F FEG TEM,

    JEOIL JEM-2010 (high resolution TEMs) and Philips CM

    20 (analytical TEM).

    SEM/EDX Characterisation of Pt nanostructures

    The nanoparticle sol was dried on a watch glass and the

    powder was then dispersed to cover the entire surface of the

    25 mm diameter carbon disc mounted on an aluminium SEM

    specimen stubs (Agar Scientific) by a double sided adhesive

    tape. The samples were then arranged in the multiple stub

    sample holder for analysis using a lower energy, o4 kVCambridge 360 Stereoscan, SEM which was equipped with

    an Oxford Instruments INCA X-ray analysis system.

    AES/XPS Characterisation of Pt nanostructures

    The AES/XPS analyses were carried out using the ESCA Lab

    II at Johnson Matthey technology centre, Sonning Common,

    Reading, UK. The spectra were measured using an Al Ka(1486.6eV) X-ray source and a hemispherical electron energy

    analyser. The acceleration voltage is 10 kV and the emission

    current is 12 mA for the X-ray source. The spectra were

    measured at around 90 degree emission angle. The electron

    energy analyser operates in FAT mode (Fixed Analyzer

    Transmission), with constant pass energy of 50 eV for survey

    (wide) scans and 25 eV for high resolution scans. The overall

    resolution of this XPS is around 1.1 eV. The peak fitting was

    conducted using CasaXPS, with a Shirley background

    subtraction, a lineshape of mixed Lorentzian (30%) and

    Gaussian (70%) character. The typical FWHM values

    obtained from the fitting are: C1s, O 1s, Pt 4f 7/2, Pt 4f 5/2,

    Co 2p3/2, Co 2p 1/2, etc. (the exact number varies from sample

    to sample but mostly close to these values).

    XRD Characterisation of Pt nanostructures

    A portion of the nanocrystals as unsupported nanocatalysts

    (powder) were packed into the groove of an aluminium sample

    holder and smoothened with a glass slid to ensure a very level

    sample with diameter ca. 23 mm. The sample was then loaded

    into a Philips DIFF 3, X-ray diffractometer with a Cu-anode

    source operating at 40 kV and 30 mA generating the Cu-KaX-ray radiation at wavelengths Cu-Ka1 = 1.5406 Å andCu-Ka2=1.54439 Å. The sample was rotated throughangular acquisition range in 2y of 31–701, step size 0.021, stepspeed 0.51 min�1 and at 1.25 s per step. The detector wasequipped with a graphite diffracted beam monochromator set

    for Cu-Ka NaI scintillator with pulse height analysis. The

  • This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2593

    phase search and match were carried out using X’Pert High

    Score software (version 1.0d).

    FTIR surface characterisation of CO pre-adsorbed Pt

    nanostructures

    Pure carbon monoxide (obtained from BOC) at 1 atmosphere

    was bubbled through the nanoparticles colloid in isopropanol

    for 15 min. The colloid sample was placed onto the crystal

    surface of the Smart Golden Gate for analysis. Excess solvent

    was evaporated by heating the top plate to 60 1C, andall spectra were obtained from an average of 512 scans using

    a Nicolet 6700 FTIR spectrometer equipped with an MCT

    detector at a resolution of 4 cm�1 over a range of wavenumber

    from 4000 cm�1 to 650 cm�1.

    Result and discussion

    TEM Imaging

    Typically, 4.2 nm Pt nanoparticles of a uniform size

    distribution were obtained by our chemical reduction process

    in solution, as shown in Fig. 1 (left). The high resolution TEM

    images suggested that many of the particles were found to be

    highly crystallized in cubooctahedron shape as nanocrystals

    under careful examination of the images (Fig. 2, left). We

    showed that the effect of varying the amounts of stabilisers

    (oleic acid and oleylamine) can also result in controlling the Pt

    particle size in the range of 2.8–14 nm (Table 3). Based on

    geometric models of size effect of nanocrystals from Boronin

    and Poltorak38 and Hardeveld and Hartog,39 smaller particle

    sizes (2–4 nm) possess more corners and edges than larger

    particle sizes, but the larger particles show more terrace faces

    than those smaller particles (Fig. 2, right). As a result, different

    particle sizes can give different catalytic properties if the

    catalytic process depends on nature of the active sites on the

    nanocrystal locations (faces, corners and edges). Experimental

    evidence was indeed obtained by Schimpf and co workers40 in

    1998, where their gold nanoparticles were characterized by

    transmission electron microscopy (TEM) with reference to

    catalytic activity. They showed that the increase in particle

    size resulted in an increase in the proportion of the surface

    atoms in the closely pack (111) plane with a dramatic decrease

    in the edge and corner atoms.

    Our 4.2 nm Pt nanoparticle sample was later decorated

    (doped) with a second metal using subsequent deposition via

    the same modified polyol process. It was reported that the

    pre-formed metal seed is able to catalyse the reduction of a

    second metal on the seed surface.41,42 Generally, surface atoms

    with the lower coordination numbers (such as adatom, kink,

    edge and step atoms) will be sites for deposition due to their

    high surface energy and instability. This process was aimed

    to modify the Pt nanocrystal surface with another metal in

    order to alter the electronic structure of the Pt. Transmission

    electron micrographs of the as-synthesized unsupported Pt

    nanocrystals (seed) and the second metal doped Pt showed

    no significant deviation in mean particle size within

    experimental error (ca. 4.25 � 1.0 nm for Pt seed particlesand 3.40 � 0.42 nm for doped Pt particles), as shown in Fig. 1(right). This implies that the extent of decoration was very

    small degree perhaps only in atomic levels.

    Elemental analysis

    The extent of coverage of the secondmetal on the surface of the

    Pt nanocrystals was determined using two analysis techniques,

    namely the lower energy, high grazing angle SEM-EDX

    (Fig. 3) and the XPS. The results summarized in Table 2

    confirm that the decoration was indeed limited to selected

    area on the Pt surface as both analyses still gave predominant

    Pt signals. The exception was the CuPt which might have

    formed bulk alloy under our reaction conditions. Also, in

    general, the composition of Pt-M particles obtained from

    SEM-EDX analysis is consistently lower than those of XPS

    (more closer to near surface analysis). This attributes to the

    difference in analytical depth of the two instruments (the

    analysis depth for XPS is of about o5 nm while SEM-EDXcan be up to 20 nm) and the presence of small amount of pure

    second metal nanoparticles from self-nucleation. It should be

    noted that estimation of the surface coverage of sub-10 nm

    particles by XPS is very challenging due to the similarity in

    particle dimensions and photoelectron escape depths. If the

    mean bimetallic nanoparticle dimensions arer4 nm then XPSwill sample the majority of the particles (likely to take place

    in this case), hence this technique is not as surface sensitive

    as when considering larger nanoparticles. The different

    sampling depths for each dopant bimetal could be estimated

    from associated inelastic mean free paths of the relevant

    Fig. 1 (left) A transmission electron micrograph (magnification of 300 000 X and 200 kV) shows the as-synthesised Pt nanocrystals with particle

    size distribution of 4.25� 1.0 nm; (right) TEM image of unsupported CoPt bimetallic nanoparticles before reaction with particle size distribution of3.4 � 0.42 nm.

  • 2594 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011

    photoelectron excitation monitored. On possible way to assess

    the degree of surface versus bulk dopant incorporation is to

    compare the atomic% of dopants observed using two different

    photon excitation energies, e.g. Mg vs. Al Ka sources. If the

    resulting compositions are identical then the dopant is likely

    uniformly distributed throughout these 4 nm NPs, while if the

    Mg-Ka yields a significantly higher value then surface

    segregation is proven and can be subsequently quantified.

    Such detailed analysis is yet to be collected. Nevertheless,

    taking the small degree of particle size variation and self-

    nucleation into account, the determined compositions by

    both techniques clearly suggest that the second metal content

    on Pt was much lower than the recipe value. Thus, there was

    only a small degree of deposition or decoration of the second

    Fig. 2 (L) High resolution TEM of typical platinum nanocrystals synthesised using the polyol process, and the corresponding modelled surface

    atoms ratio with particles size of cuboctahedal shaped nanocrystals.39

    Fig. 3 An SEM micrograph (insert) and EDX spectrum showing the chemical composition of Pt decorated with equal mole of (a) Ni, (b) Co,

    (c) Cu and (d) Zn in synthesis recipes.

    Table 2 Surface elemental composition analysis of the as-synthesised platinum bimetallic catalysts by SEM-EDX and XPS techniques

    Platinum bimetallic catalyst

    Atomic % of Pt Atomic % of 2nd metal

    SEM-EDX XPS SEM-EDX XPS

    FePt 83 88.5 17 11.5CoPt 85 87.2 15 12.8NiPt 85 n.d. 15 n.d.CuPt 63 n.d. 37 n.d.ZnPt 85 93.9 15 6.1TiPt n.d. 85.9 n.d 14.1

  • This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2595

    metal on specific locations on Pt surface (but no extensive

    encapsulation of Pt nanoparticle by the second metal). By

    taking the atom response factors into account, the doping

    metal contents were found to be below 14% with reference to

    the near surface Pt atoms. We believe that these second metal

    atoms on Pt nanocrystal surface play important geometric and

    electronic roles in affecting catalytic performance of the

    crystals.

    XRD analysis

    It is known that when a secondmetal is deposited (decorated) on

    another metal of different lattice parameters, a slight expansion

    or contraction of the lattice of top metal exerted by the

    underlying metal lattice can be resulted. This could cause a

    significant alteration in electronic properties of the metal due to

    the structural change (isomorphic effect).43 XRD was therefore

    carefully conducted (Fig. 4) in order to examine any possible Pt

    lattice alteration before and after the second metal doping.

    As seen from Fig. 4, we have only observed the typical Pt

    diffraction peaks with no detectable diffraction peak due to

    second metal (this may indicate the second metal was either

    in amorphous state or as ultra-thin layers where the severe

    peaks broadening rendered them indistinguishable from the

    background). There is also no observable shift in diffraction

    peaks to suggest any formation of M–Pt alloy or lattice

    expansion / contraction of the Pt nanocrystal after the

    modification of the second metal doper. On the other hand,

    owing to the poor sensitivity of XRD to surface change, we still

    cannot discount a small degree of geometrical disparity due to

    surface alloy formation or lattice parameter alteration of either

    the host or the gust metals at the interface. However, it is

    interesting to observe from the figure that the diffraction peaks

    of all second metal doped Pt samples becomes broadened,

    which implies the ensembles of Pt (nanocrystals) are broken up

    slightly by the presence of the metal doper.

    Catalytic testing and optimisations

    Table 3 summarises our earlier communication note4 that Pt

    nanocrystals of different sizes made by modified polyol process

    showed a very large difference in selectivity towards cinnamyl

    alcohol (2.8 nm gave 24.8% selectivity and 14.4 nm gave

    85.6%) at a complete cinnamaldehyde conversion, suggesting

    that this reaction is very structural sensitive. Clearly, the

    smaller Pt sizes with higher proportion of high index sites

    (undesirable sites) gave lower selectivity to cinnamyl alcohol

    (2.8 nm with 24.3% and 3.3 nm with 44.6%). The effect

    appears to be less significant at larger Pt sizes (6.0 nm with

    80.8% and 14.4 nm with 85.6%). It is interesting to note that

    the selectivity to unsaturated alcohol converged to around

    80–85%. Thus, it is consistent with the geometric models

    from Boronin and Poltorak38 and Hardeveld and Hartog39

    where the change in Pt coordination numbers is rather

    insensitive to particle size greater than 5 nm owing to the

    small variations. Thus, the supported Pt samples prepared by

    conventional means with a generally smaller size than the

    present unsupported particles should show greater

    attenuation in selectivity. However, with the accurate control

    in crystal size by the polyol method this work clearly underpins

    the size effect on selectivity of this hydrogenation, which can be

    separated from the effects of support and chemical promotion.

    Detailed analysis also showed that the selectivity depends on

    the coverage of the substrate molecule (the measured selectivity

    was higher when higher concentration of the substrate

    molecule was used. This effect implies that the higher degree

    of surface coverage at higher substrate concentration favours

    the terminal aldehyde hydrogenation by creating a steric

    effect that inhibits hydrogenation of the CQC bond ofcinnamaldehyde, see Fig. 6).

    When the 4.2 nm Pt nanocrystal (100 mL oleic acid and100 mL oleylamine) was decorated with second metal acrossthe periodic table, it is interesting to note from Fig. 5 that a

    volcano response was observed for the selectivity towards

    cinnamyl alcohol (and also the yield) of this hydrogenation

    reaction. Only with the exceptionally lower selectivity of Ni

    (nickel is well known to be hydrogenation catalyst on its own)

    and higher selectivity of Sn (tin can form PtSn alloy) the volcano

    response across the transition elements in the periodic table

    clearly reflects the changes in electronic properties of Pt

    modified by the surface transition metal doper. This will be

    discussed in later section. It is worth noting that the selectivity

    with respect to the second metal doper reached the maximum

    value of >99% when cobalt element was doped on the

    Pt nanocrystal. As a result, Co doped Pt was selected

    for detailed investigation of experimental parameters on the

    reaction selectivity.

    Effect of substrate concentration on selectivity and conversion

    Fig. 6 shows clearly that the selectivity of the CoPt

    nanocatalyst towards the production of cinnamyl alcohol is

    linearly dependent on the concentration of cinnamaldehyde

    used. The preferential hydrogenation of cinnamaldehyde to

    cinnamyl alcohol increased gradually as the concentration

    of the substrate increased reaching 100% selectivity at a

    concentration of 0.983 mol L�1 (12% v/v cinnamaldehyde in

    isopropanol). The notable improvement in selectivity from

    94% to virtually 100% as substrate concentration increasing

    from 0.04 mol L�1 to 0.983 mol L�1 can be attributed to stericFig. 4 Plots of XRD patterns of the preformed Pt sample and samples

    obtained after doping with a second metal by polyol process.

  • 2596 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011

    effects as a result of the coverage of substrate molecules on

    catalyst surface.44,45 The high coverage would enable the

    cinnamaldehyde molecules to be aligned linearly for closer

    surface packing of the adsorbed molecules hence facilitating the

    hydrogenation of the terminal aldehyde. Thus, at low coverage,

    cinnamaldehyde would likely adsorb with the CQC and CQObonds co-planar with the metal surface (i.e. a flat-lying geometry

    that facilitates hydrogenation of unsaturated bonds), while

    at high coverage, a tilted geometry is likely adopted with the

    preferential CQO interaction with the surface as observed thosefor alkoxides on Pt. As the concentration of cinnamaldehyde

    increased, the conversion was also found to decrease (from ca.

    85% to 60%), as seen from Fig. 6.

    Effect of hydrogen pressure on selectivity and conversion

    Under this liquid phase batch process, reaction parameters

    such as 3.0 mg of unsupported CoPt catalyst, 5.0 mL of

    3% vol/vol cinnamaldehyde at 100 1C over a reaction time of2 h were kept constant, which enabled the effect of hydrogen

    pressure on conversion and selectivity to be studied (Fig. 7). It

    was found that the increasing the hydrogen pressure from

    Table 3 Size effect of Pt nanocrystals on reaction selectivity towards cinnmyl alcohol at complete cinnamaldehyde conversion, the main sideproduct is hydrocinnamaldehyde with only trace of phenylpropanol detected4

    Stabilizers

    Particle size (TEM) Cinnamyl alcohol selectivity (%)Oleic acid (mL) Oleylamine (mL)

    40 40 14.4 (XRD) 85.680 80 6.0 80.8120 120 4.8 49.1200 200 3.3 44.6300 300 3.1 30.2400 400 2.8 24.3

    Fig. 5 A volcano trend in catalytic performance (selectivity and yield) for 4.2 nm unsupported Pt decorated with second metal. Conditions:

    1.0 mg of unsupported Pt nanocrystals, 5.0 mL of 3% vol/vol cinnamaldehyde in isopropanol, 20 bar hydrogen, 100 1C and 2 h. Reactor volumeis 25 mL.

    Fig. 6 Effect of cinnamaldehyde concentration on selectivity and

    conversion. Conditions: 1.0 mg of unsupported CoPt nanocrystals,

    5.0 mL of cinnamaldehyde at various concentrations in isopropanol,

    100 1C, 20 bar of hydrogen, 2 h of reaction time. Reactor volumeis 25 mL.

  • This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2597

    1.5 bar to 10 bar resulted in an increase in the cinnamyl alcohol

    selectivity sharply from 70% to 96%. The selectivity was then

    levelled between 10 bar and 55 bar. Similarly, the conversion

    increased steadily from 21.6% at 1.5 bar to 83% at 10 bar

    reaching a plateau at about 20 bar, as shown in Fig. 7. This

    trend is similar to the generally proposed trend of activity

    trend in many hydrogenation reactions in liquid phase with

    respect to hydrogen pressure in the literature.4 The Langmuir

    adsorption model is generally adopted for catalytic

    hydrogenation, which requires a mass transfer of hydrogen

    gas to adsorbed hydrogen over active metal surfaces like Pt,

    Pd and Ru.46 Since hydrogen gas solubility is poor at low

    applied hydrogen gas pressure in the reactor, an increase in

    pressure would therefore favour the dissolution of hydrogen

    gas in isopropanol according to the Henry’s law from 1–10 bar

    of hydrogen until it reached saturation.47 The important

    information from this fit is to derive the saturation hydrogen

    pressure, more data point with vigorous statistical treatment is

    yet required in order to deduce the Langmuir equilibrium

    constant.

    Effect of temperature on selectivity and conversion

    Fig. 8 shows that the extent of hydrogenation is dependent on

    applied temperature while all other reaction conditions are

    kept constant. The conversion increases gradually from 13% at

    313 K to 19% at 373 K. A similar trend is observed for the

    selective towards the unsaturated alcohol (from 88% to 98%

    over the same temperature regime).

    Effect of reaction time on cinnamyl alcohol formation

    The concentration of cinnamyl alcohol increases almost

    linearly as the reaction progresses with initial experimental

    time from 0 to 4 h, as shown in Fig. 9. At the beginning of the

    reaction, there was simultaneous formation of both

    hydrocinnamaldehyde (i.e. the hydrogenation of the CQC)and cinnamyl alcohol so the initial selectivity at the first

    120 min was as low as 90% (not shown). However, further

    experimental time for this batch reaction led to higher

    selectivity to cinnamyl alcohol at the expense of the

    hydrocinnamaldehyde until the cinnamyl alcohol was almost

    exclusively formed after 90 min, a similar observation that was

    also made by Breen et al.49 It was believed that the initial

    catalyst was comprised of partially oxidised Pt nanoparticles

    which was subsequently reduced to active metallic phase over

    the 2 h under the reaction conditions.

    However, by pre-reducing the catalyst material at 100 1Cwith5 bar of hydrogen (99.99% BOC) for 1 h before injecting

    the substrate concentration (3% vol/vol cinnamaldehyde in

    isopropanol), the initial formation of hydrocinnamaldehyde

    can indeed be greatly suppressed, as shown in Fig. 10,

    giving high selectivity towards cinnamyl alcohol. It is therefore

    evident that Pt based catalysts were partially oxidised in air as

    previously shown.50–53 The pre-reduction step is important to

    restore the metallic surface (it took time for the reduction under

    the reaction conditions), which is essential for the production

    of cinnamyl alcohol through the terminal adsorption of the

    substrate. Partial oxidised Pt surface appears to give a greater

    extent of the initial CQC hydrogenation.

    Reaction kinetics

    Considering the hydrogenation of cinnamaldehyde (CAL) to

    its products, the reaction is represented by the equation;

    CALþH2 �!k

    products

    The rate equation can be represented as

    R = k[CAL]m[H2]n (1)

    From Fig. 11, the concentration of cinnamaldehyde in the

    hydrogenation reaction decreases with the progress of the

    reaction where the rate constants, k, at different temperatures

    can be deduced.

    For a first order reaction, plotting ln[CAL] against time (t)

    should give a straight line.48,53 Fig. 12 depicts that the

    experimental data apparently fit to the first order reaction

    with respect to cinnamaldehyde concentration.

    Arrhenius equation below was used to estimate the reaction

    activation energy. Fig. 13 shows the activation energy

    Fig. 7 Langmuirian fit to experimentally determined data illustrating

    the effect of hydrogen pressure on selectivity and conversion.

    Conditions: 3.0 mg unsupported CoPt catalyst, 5.0 mL of 3% vol/vol

    cinnamaldehyde in isopropanol, 100 1C and reaction time 2 h. Reactorvolume is 25 mL.

    Fig. 8 A temperature effect on conversion and selectivity in the

    hydrogenation of cinnamaldehyde over CoPt; conditions: 3.0 mg

    unsupported CoPt, 5.0 mL of 3% vol/vol cinnamaldehyde in

    isopropanol, 20 bar of hydrogen and reaction time 2 h. Reactor

    volume is 25 mL.

  • 2598 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011

    Fig. 9 The effect of reaction time on the formation of cinnamyl alcohol catalysed by CoPt on carbon. Conditions: 30 mg supported catalyst, 60 mL

    of 3% vol/vol cinnamaldehyde in isopropanol, 100 1C and 20 bar hydrogen. Reactor volume is 300 mL.

    Fig. 10 Effect of time on cinnamyl alcohol selectivity and

    cinnamaldehyde conversion over the pre-reduced CoPt nanocrystals.

    Conditions: 30 mg supported catalyst, 60 mL of 3% vol/vol

    cinnamaldehyde in isopropanol, 100 1C and 20 bar hydrogen.Reactor volume is 300 mL.

    Fig. 11 A profile of the concentration of cinnamaldehyde dependence on time at various temperatures.

    Fig. 12 A fit of the experimental data assuming a first order reaction

    with respect to substrate concentration at 373.15 K and 20 bar

    hydrogen.

  • This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2599

    calculated from the slope of the graph.

    ln k ¼ lnA� EaRT

    ð2Þ

    The apparent activation energy of 17.26 kJ mol�1 was

    derived in our case. This value agreed with the finding of Li

    et al., who obtained their activation energy of 18 kJ mol�1 over

    cobalt boron amorphous alloy catalyst.52 Breen et al. estimated

    the activation energy to be 37 KJ/mol over iridium-carbon

    catalyst.49 All these low value of activation energies for

    this catalysed reaction suggest the rates could be somehow

    subjected from mass transport or diffusion limited.48

    In contrast, in our testing conditions, parameters were

    deliberately chosen so no mass or diffusion limitation could

    be encountered (used 20 atmosphere of H2, see Fig. 7; high

    substrate concentration, see Fig. 6, over 1 mg catalyst

    under optimum stirring) only except that in the kinetic rate

    law determinations where the considerably lower hydrogen

    pressures and lower substrate concentrations were used. They

    could affect the accurate measurements in activation energy. It

    would be interesting to apply a classical mathematically

    modelling to the system using Weisz equation54 in order to

    determine the valid experimental parameters that are not prone

    to mass limitations in combination with the use of a Rushton

    turbine with a hollow agitator for these fast reactions in the

    future.

    Geometric and electronic effects

    Fig. 14 (top) shows a collection of typical FTIR spectra after

    CO chemisorption on Pt nanocrystals with different sizes as

    compared with a spectrum collected from the cobalt decorated

    Pt (4.2 nm) sample. It demonstrates clearly that there is a sharp

    increase in multicarbonyl adsorption due to the increase in the

    number of highly uncoordinated Pt surface site upon

    decreasing the particle size. It is apparent that the cobalt

    deposition occurred principally on the highly uncoordinated

    Pt sites that blocked the formation of multicarbonyl band

    (M peak). As a consequence, the disappearance of the

    characteristic band at u(CO) of 1925 cm�1 was evidenced.Thus, the beneficial geometric modification of Pt nanocrystal

    by cobalt atoms is concluded. In addition, there was a red shift

    in the linear mode indicative of electronic promotion of Pt

    (back donation) by the surface cobalt atoms. Fig. 14 (bottom)

    shows a collection of FTIR spectra collected after CO

    chemisorption on surface modified 4.2 nm Pt nanocrystals

    where a range of transition metals were chosen as the dopant.

    As seen from the typical M–Pt spectra (NiPt and FePt), apart

    from linear CO adsorption mode of the Pt nanocrystals, they

    also show absorbance bands corresponding to linear modes of

    M–CO around 2050–1950 cm�1.27,32

    The trend observed is that the selective hydrogenation of

    CQO is favoured for catalysts in which the multicarbonyladsorption band at around 1925 cm�1 is suppressed by the

    second metal deposition. This phenomenon also occurred in all

    the bimetallic systems but to varying degree of efficiency with

    Co metal the most exemplary. The exception to this trend is the

    PbPt system (Fig. 14 bottom) in which the linear mode of CO

    adsorption almost disappeared. It was observed that there was

    a sharp decrease in the intensity for the linearly adsorbed CO

    on Pt mode (u(CO) between 2090–2040 cm�1) which suggestedthe poison effect of Pb on Pt. Pt decorated with Mn and Co

    metals showed similar u(CO) patterns and they both gave ahigh selectivity to cinnamyl alcohol above 90%.

    It is very interesting to find that second metal atoms

    preferentially segregate at sites of low coordination at the

    surface of the Pt crystal, which can suppress the formation of

    the multi-carbonyl peak (1925 cm�1) despite their low surface

    coverage. Cyclic voltammetry can be used to analyse the

    surface of platinum crystal structures; the technique can

    provide structural information of the materials once they are

    impregnated on to an electrode. Using this technique it should

    be possible to see if the surface of platinum had been

    modification in terms of second metal decoration on the

    surface. Cyclic voltammograms for Pt single crystal surfaces

    are well known, and Pt/graphite catalysts show four features

    that correspond (by analogy with the single crystal data) to

    Pt{1 1 1} � {1 1 1} steps, Pt{1 0 0} � {1 1 1} steps, Pt{1 0 0}terraces and Pt{1 1 1} terraces. Changes in the populations

    of these surface features with catalyst treatment can be

    investigated and the effects on catalytic activity and

    selectivity determined. Wells and co-workers have shown

    that it is possible to work out the presence of different sites,

    and surface structures of different size crystals.55 Cyclic

    voltammetry was therefore conducted on our three samples

    CoPt (1 : 1), Commercial JM 5%Pt/C and Pt/C, which was

    made by the polyol process. The CoPt/C and Pt/polyol/C

    contained 2.4 and 6.1% m/m Pt respectively from ICP

    analysis. The JM 5%Pt/C was used as a reference sample in

    order to determine the crystal faces. Pt/polyol/C was also used

    as a reference sample as the platinum particles were synthesised

    by the same method but without the presence of the typical

    cobalt, this eliminated any effects of unwashed stabilisers on

    the surface that might have been present on the sample, which

    could have blocked certain crystal planes thus giving a

    false interpretation of the cobalt decoration on the surface

    of platinum.

    The cyclic voltammograms (Fig. 15) indicated the

    disappearance of Pt {111} � {111} and Pt {100} � {111}steps (distinctive features in CVs of Pt/graphite that

    correspond with single crystal data upon the decoration of

    Co atoms.55 It is clear that no stabiliser appeared to have

    blocked the crystals planes as all the distinctive features of

    platinum were visible from the spectra on the Pt/polyol/C

    Fig. 13 Arrhenius plot for cinnamaldehyde hydrogenation over CoPt

    nanocrystals.

  • 2600 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 This journal is c the Owner Societies 2011

    sample which was synthesised using the same stabilisers as the

    CoPt particles. Pulse CO chemisorption also confirmed site

    blocking (coverage) on platinum nanoparticle by Co atoms

    (Table 4). The CO: metal stoichiometry of 1 : 1 was assumed in

    these measurements. The addition of cobalt clearly resulted in

    a decrease of the platinum surface area. Thus, the preferential

    site-blocking by Co atoms reinforces the earlier postulation of

    catalytic deposition on low coordination, but highly reactive

    sites upon the subsequent reduction of Co2+ in the presence of

    Pt nanocrystals.

    Thus, the geometric (site) modification by doper is clearly

    demonstrated to play an important role in cinnamyl alcohol

    selectivity but the electronic modification on Pt nanocrystals

    exerted by metal dopant is not yet clear. On the other hand, the

    strength of surface bonding (chemisorption) with adsorbed

    atoms and molecules is generally known to depend strongly on

    electronic properties of underlying metal or bimetallic surface.

    In addition, it has been noted from the CO chemisorption

    spectra (Fig. 14) that there is a clear red shift in the linear mode

    of uCQO particularly upon the addition of cobalt doper.Bearing in mind that the Co atoms are specifically decorated

    on the low coordination sites at low surface coverage, the

    results therefore clearly imply that the exposed Pt atom bearing

    a CO ligand must have experienced an increasing degree of

    back-bonding of its d-electrons (increasing d- of Pt) to the p* ofthe CO when it is placed closer to the Co atoms on a smaller

    size of Pt crystals. X-ray Photoelectron Spectroscopy and

    Auger Spectroscopy (surface sensitive techniques) which can

    give information about the surface composition and electronic

    interactions of the metals, were conducted. Analyses of the

    surface compositions of the CoPt with different recipes’ (the

    number in () was the recipe Co/Pt atomic ratio) using XPS as

    shown in Table 5 confirm that the second metal did not

    completely encapsulate but merely decorate the platinum

    nanocrystals. Their surface percentages were in line with the

    results from the EDX analyses in Table 2. It is however noted

    from the XPS spectra that there was no detectable chemical shift

    in the binding energies (Co 2p3/2 and 2p1/2 and Pt 4f7/2 and 4f5/2)

    of both Pt and Co, presumably these innermost core electrons

    might be less sensitive to any change in nearby chemical

    environment. Interestingly, there were more significant and

    progressive shifts in Pt Auger peaks NNN and Co LMM peaks

    than their core peaks (Table 5). Co exerts a greater electronic

    influence to Pt on the outermost electrons and vice versa as

    reported in PtCo alloy in the literature,56 indicating the local

    electronic influence of Pt crystal by increasing Co atoms content.

    d-band Center analysis of cinnamaldehyde hydrogenation

    With the appreciation of the electronic influence of the

    Pt exerted by transition metal doper from the above

    characterisations one key question is how important this

    electronic effect to catalytic performance. The Density

    Functional Theory, DFT summarized by the d-band center

    model which has been successfully used in the literature to

    model various chemisorption systems over metals, in which the

    closer the d-band center is to the Femi level (towards zero), the

    higher adsorption energy for adsorbate obtained.57 Here,

    d-band center plot is used to examine the correlation

    Fig. 14 (top) Plots of ATR-FTIR spectra from CO chemisorption

    studies on Pt nanocrystals colloids with different particle sizes showing

    different characteristic peaks of CO linear mode u(2060 cm�1, (L)),multicarbonyl mode u(1925 cm�1, (M)) and bridging mode u(1830cm�1, (B)). The absence of multicarbonyl peak over 4.8 nm Pt

    nanocrystal decorated with cobalt is demonstrated;4 (bottom) Plots

    of ATR-FTIR spectra from CO chemisorption studies on Pt samples

    with a second metal decoration.

    Fig. 15 Cyclic voltammograms of 5% Pt (synthesised of polyol

    reduction) on graphite; JM commercial 5%Pt on graphite and 5%

    Co decorated Pt (synthesised of polyol reduction, 1 : 1 CoPt) on

    graphite using 0.5 M sulfuric acid as the electrolyte, voltage was

    swept between 0 and 0.8 V at a rate of 10 mV s�1.

  • This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2590–2602 2601

    between electronic effect and the catalytic performance of this

    reaction. As seen from Fig. 16, a volcano relationship between

    selectivity to cinnamyl alcohol with respect to the position of

    d-band center of the particular 1st row transition doper metal

    referenced to its Fermi level is obtained. The selectivity reaches

    the highest point at the CoPt, but starts to decrease at FePt and

    drops sharply at the NiPt catalyst on both side of the apex.

    This variation in selectivity is believed due to the difference in

    their d-band centre position from the Fermi level when the Pt

    nanocrystal surface was electronically modified by the

    transition metal. Amazingly, the same volcano response with

    the apex on cobalt was presented by Stamenkovic et al. who

    investigated the specific activity of bimetallic Pt electro-

    catalysts for oxygen reduction reaction (ORR) and the band

    center position of the added transition metal.58 They attributed

    their best CoPt activity to the proper balance of adsorption

    energy of O2 or reaction intermediates (O2�, O2

    2�, H2O2) for

    optimised surface electro-catalysis. They believed that for

    metal surfaces that binded oxygen or intermediates too

    strongly, as in the case of Pt, the d-band center was too close

    to the Fermi level and the rate of the ORR was limited by the

    availability of free Pt sites (poisoning effect). On the other

    hand, when the d-band center was too far from the Fermi level,

    as in their case of TiPt, the surface binded O2 and intermediates

    too weakly to significantly promote the ORR activity.

    Although our catalytic hydrogenation reaction was very

    different from the ORR in the cathode, the adsorption

    strengths of substrate or intermediates in the function of

    electronic properties of the underlying bimetallic surfaces

    must be closely related to this, giving the same volcano

    response. Since cinnamaldehyde adsorption on catalyst

    surface can take place with either adsorption of alkene or

    terminal carbonyl moieties followed by the formation di-smetal C–C or di-s metal C–O bonds, respectively. As d-bandcenter moves close to Fermi level, a stronger and more

    thermodynamic favourable di-s metal C–C bonds ispreferred over di-s metal C–O on NiPt and Pt surfaces,which reduce the selectivity in the hydrogenation of the

    CQO bond. As the d-band centre moves away from theFermi level, it is likely to expect the di-s metal C–C bindingenergy to decrease, thus leading to a more selective CQOhydrogenation pathway. Thus, the selectivity is maximized at

    around FePt and CoPt, which indicate that the value of the

    d-band centre on these surfaces correspond to the optimal

    binding energy of CQO in cinnamaldehyde. However, the shiftin the d-band centre will also reduce the di-s C–O bindingenergy and would likely make it too weak for hydrogenation to

    occur when the d-band centre is too far away from the Fermi

    level, such as that observed with the TiPt catalyst. As evident

    from the poor conversion shown in Fig. 5, the drop in cinnamyl

    alcohol selectivity at low conversion was due to rising

    significance of other side products. Therefore, d-band center

    indicates the need for a delicate balance between the strengths

    of these two bonding configurations which would lead to a

    volcano-type relationship for the production of cinnamyl

    alcohol, as illustrated in Fig. 16.

    Conclusion

    Bimetallic nanostructures are currently used as industrial

    catalysts for many important transformations. It has been

    postulated that the second metal can play several pivotal

    roles within the catalyst, including site blocking and

    electronic promotion on the primary metal. However, a poor

    understanding of these mechanisms hampers further

    development within this area. It is often difficult to disentangle

    and optimise these effects, especially using catalysts that have

    been prepared using traditional techniques, as invariably side

    reactions are promoted. In this paper, we have presented some

    new ways to approach these problems. By using simple nano-

    chemistry skills, unsupported Pt nanocrystals with tailored sizes

    can be decorated with another metal atom in a controlled

    Table 4 Pulse CO chemisorption analysis

    Catalyst (on graphite) Surface area/g metal (m2 g�1) Atom dispersion (%)

    Commercial Pt (JM) 4.8 1.8Pt (Polyol) 13.7 5.0CoPt (Polyol) 9.2 3.4

    Table 5 X-ray photoelectron spectroscopy and Auger emission data of Co decorated Pt nanocrystals

    Catalyst% surfaceatom ratio (Co/Pt)

    B.E Co0

    2P3/2B.E Co2+

    2P1/2B.E Pt4f7/2

    B.E Pt4f5/2

    K.E AugerPtNNN

    K.E AugerCoLMM2

    CoPt (0.24) 0.14 778.15 793.04 70.7 74.0 183.3 775.2CoPt (1.47) 0.22 778.21 793.26 70.8 74.1 184.5 774.7CoPt (5.33) — 778.21 796.74 70.8 74.7 185.2 772.2Co (Lit.) — 778.2 � 0.3 796.4 � 0.05 71.2 � 0.61 74.3 � 0.09 770.70

    Fig. 16 Selectivity towards cinnamyl alcohol on the hydrogenation of

    cinnamaldehyde as a function of the d-band center of transitionmetal doper.

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

    manner. Thus, we showed that the blockage of unselective low

    coordination metal sites and the optimisation in electronic

    influence of the decorated Pt surface, can be independently

    studied in the selective hydrogenation of a,b-unsaturatedaldehydes. In this paper, we also show that the terminal CQOhydrogenation can be achieved in high activity, whilst the

    undesirable hydrogenation of the CQC group can be greatlysuppressed by some second metal dopers. In particular, the Co

    decorated Pt nanocrystals display the best activity and

    selectivity for the formation of cinnamyl alcohol. Our work

    clearly demonstrates the advantage in engineering preformed

    nanoparticles via a bottom-up construction and illustrates that

    this route of catalyst design may lead to improved and greener

    manufacturing processes.

    Acknowledgements

    We thank Dr Richard Smith of Johnson Matthey technology

    centre, Sonning Common, Reading, UK for the XPS/AES

    studies.

    References

    1 P. Gallezot and D. Richard, Catal. Rev., 1998, 40, 81.2 P. Claus, Top. Catal., 1998, 5, 51.3 S. Bhattacharjee, D. M. Dotzauer and M. L. Bruening, J. Am.Chem. Soc., 2009, 131, 3601.

    4 S. C. Tsang, N. Cailuo, W. Oduro, A. T. S. Kong, L. Clifton,K. M. K. Yu, B. Thiebaut, J. Cookson and P. Bishop, ACS Nano,2008, 2, 2547.

    5 M. Ash and I. Ash, in Specialty Chemical Source Book, SynapseInformation Ressources Inc., ed., New York, 1997.

    6 B. Coq, F. Figueras, P. Geneste, C. Moreau, P. Moreau andM. G. Warawdekar, J. Mol. Catal., 1991, 78, 211.

    7 D. Richard, J. Ockleford, A. Girior-Fendler and P. Gallezot,Catal.Lett., 1989, 3, 53.

    8 L. Sordelli, R. Psaro, G. Vlaic, A. Cepparo, S. Recchia, C. Dossi,A. Fusi and R. Zanoni, J. Catal., 1999, 182, 186.

    9 B. Coq, P. S. Kumbhar, C. Moreau, P. Moreau andM. G. Warawdekar, J. Mol. Catal., 1993, 85, 215.

    10 R. Hubaut, J. P. Bonnelle and M. Daage, J. Mol. Catal., 1989, 55,170.

    11 P. Gallezot, A. Girior-Fendler and D. Richard, Catal. Lett., 1990,5, 169.

    12 J. Aumo, J. Lijla, P. Maki-Arvela, T. Salmi, M. Sundell, H. Vainioand D. Y. Murzin, Catal. Lett., 2002, 83, 219.

    13 R. Touroude, J. Catal., 1980, 65, 110.14 B. Sen and M. A. Vannice, J. Catal., 1989, 115, 65.15 T. B. L. W. Marinelli, S. Nabuurs and V. Ponec, J. Catal., 1995,

    151, 431.16 L. Mercadante, G. Neri, C. Milone, A. Donato and S. Galvagno,

    J. Mol. Catal. A: Chem., 1996, 105, 93.17 G. Neri, L. Mercadante, C. Milone, R. Pietropaolo and

    S. Galvagno, J. Mol. Catal. A: Chem., 1996, 108, 41.18 S. Galvagno, C. Milone, A. Donato, G. Neri and R. Pietropaolo,

    Catal. Lett., 1993, 18, 349.19 U. K. Singh and M. A. Vannice, J. Catal., 2001, 199, 73.20 R. Malathi and R. P. Viswanath, Appl. Catal., A, 2001, 208, 323.21 M. Consonni, D. Jokic, D. Y. Murzin and R. Touroude, J. Catal.,

    1999, 188, 165.

    22 Z. Poltarzewski, S. Galvagno, R. Pietropaolo and P. Staiti,J. Catal., 1986, 102, 190.

    23 M. A. Aramendia, V. Borau, C. Jiménez, J. M. Marinas, A. Porrasand F. J. Urbano, J. Catal., 1997, 172, 46.

    24 R. Narayanan and M. A. El-Sayed, Nano Lett., 2004, 4, 1343.25 P. Fouilloux, in Heterogeneous Catalysis and Fine Chemicals, ed.

    M. Guisnet, et al., Amsterdam, Elsevier, 2000, pp. 123–129.26 D. de Caro and J. S. Bradley, Langmuir, 1998, 14, 245.27 L. H. Little, in Infrared Spectra of Adsorbed Species, Academic

    Press, London, 1966.28 L. C. deMenorval, A. Chaqroune, B. Coq and F. Figueras,

    J. Chem. Soc., Faraday Trans., 1997, 93, 3715.29 O. Dulaurent, M. Nawdali, A. Bourane and D. Bianchi, Appl.

    Catal., A, 2000, 196, 271.30 P. Kubanek, H. W. Schmidt, B. Spliethoff and F. Scuth,

    Microporous Mesoporous Mater., 2005, 77, 89.31 T. I. Koranyi, J. Mihaly, E. Pfeifer, C. Nemeth, T. Yuzhakova and

    J. Mink, J. Phys. Chem. A, 2006, 110, 1817.32 M. Mihaylov, K. Hadjiivanov and H. Knozinger, Catal. Lett.,

    2001, 76, 59.33 K. Hadjiivanov, T. Venkov and H. Knozinger, Catal. Lett., 2001,

    75, 55.34 K. Tedsree, A. T. S. Kong and S. C. Tsang,Angew. Chem., Int. Ed.,

    2009, 121, 1471.35 S. H. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser,

    Science, 2000, 287, 1989.36 M. Chen, J. P. Liu and S. H. Sun, J. Am. Chem. Soc., 2004, 126, 8394.37 N. S. Sobal and M. Giersig, Aust. J. Chem., 2005, 58, 307.38 O. M. Boronin and V. S. Poltorak, Int. Chem. Eng., 1967, 7, 452.39 R. van Hardeveld and F. Hartog, Surf. Sci., 1969, 15, 189.40 S. Schimpf, M. Lucas, C. Mohr, U. Rodemerck, A. Bruckner,

    J. Radnik, H. Hofmeister and P. Claus, Catal. Today, 2002, 72, 63.41 H. L. Hu and K. Sugawara, Mater. Lett., 2009, 63, 940.42 J. L. Cheng, H. H. Hng, H. Y. Ng, P. C. Soon and Y. W. Lee,

    Mater. Res. Bull., 2009, 44, 95.43 M. Galeotti, A. Atrei, U. Bardi, B. Cortigiani, G. Rovida and

    M. Torrini, Surf. Sci., 1993, 297, 202.44 M. Bron, D. Teschner, A. Knop-Gericke, B. Steinhauer,

    A. Scheybal, M. Havecker, D. Wang, R. Fodisch, D. Honicke,A. Wootsch, R. Schlogl and P. Claus, J. Catal., 2005, 234, 37.

    45 S. Fujii, N. Osaka, M. Akita and K. Itoh, J. Phys. Chem., 1995, 99,6994.

    46 P. G. J. Koopman, A. P. G. Kieboom and H. Vanbekku, ColloidsSurf., 1981, 3, 1.

    47 V. Schettino and R. Bini, Chem. Soc. Rev., 2007, 36, 869.48 G. C. Bond, inHeterogeneous Catalysis Principles and Applications,

    Oxford Chemistry Series, Oxford University Press, Oxford, 1974.49 J. P. Breen, R. Burch andGomez-Lopez,Appl. Catal., A, 2004, 268,

    267.50 H. T. Stokes, C. D. Mokowka, P. K. Wang, S. L. Rudaz and

    L. P. Slichter, J. Mol. Catal., 1983, 20, 321.51 P. Ratnasam, A. J. Leonard, L. Rodrique and J. J. Fripiat,

    J. Catal., 1973, 29, 357.52 H. X. Li, X. F. Chen, M. H. Wang and Y. P. Xu, Appl. Catal., A,

    2002, 225, 117.53 J. E. House, in Principles of Chemical Kinetics, London, Academic

    Press, 2nd edn, 2007.54 P. B. Weisz, Ber. Bunsen Ges. Phys. Chem., 1975, 79, 798.55 G. A. Attard, K. G. Griffin, D. J. Jenkins, P. Johnston and

    P. B. Wells, Catal. Today, 2006, 114, 346.56 H. X. Li, H. Li and M. H. Wang, Appl. Catal., A, 2001, 207, 129.57 A. Ruban, B. Hammer, P. Stoltze, H. L. Skriver and J. K. Norskov,

    J. Mol. Catal. A: Chem., 1997, 115, 421.58 V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer,

    C. A. Lucas, G. Wang, P. N. Ross and N. M. Aovic, Nat. Mater.,2007, 6, 241.

Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.