Synthesis of heterogeneous catalysts withwell shaped platinum
particles to controlreaction selectivityIlkeun Lee, Ricardo
Morales, Manuel A. Albiter, and Francisco Zaera*
Department of Chemistry, University of California, Riverside, CA
Edited by Gabor A. Somorjai, University of California, Berkeley,
CA, and approved August 18, 2008 (received for review June 12,
Colloidal and sol-gel procedures have been used to prepare
het-erogeneous catalysts consisting of platinum metal particles
withnarrow size distributions and well defined shapes dispersed
onhigh-surface-area silica supports. The overall procedure was
de-veloped in three stages. First, tetrahedral and cubic colloidal
metalparticles were prepared in solution by using a procedure
derivedfrom that reported by El-Sayed and coworkers [Ahmadi TS,
WangZL, Green TC, Henglein A, El-Sayed MA (1996) Science
272:1924–1926]. This method allowed size and shape to be
controlledindependently. Next, the colloidal particles were
dispersed ontohigh-surface-area solids. Three approaches were
attempted: (i) insitu reduction of the colloidal mixture in the
presence of thesupport, (ii) in situ sol-gel synthesis of the
support in the presenceof the colloidal particles, and (iii) direct
impregnation of theparticles onto the support. Finally, the
resulting catalysts wereactivated and tested for the promotion of
carbon–carbon double-bond cis-trans isomerization reactions in
olefins. Our results indi-cate that the selectivity of the reaction
may be controlled by usingsupported catalysts with appropriate
metal particle shapes.
colloids � nanoparticles � shape � olefin � isomerization
On the basis of their kinetic behavior, catalytic reactions
areoften classified as either mild or demanding (1–3). Demand-ing
reactions—such as the oxidation of CO, NO, or hydrocarbons;the
synthesis of ammonia; and most oil processing conversions—usually
require high temperatures and pressures, and involve
smallconcentrations of intermediates similar to those identified
undervacuum. The performance of these reactions often
dependsstrongly on the structure of the catalyst used (4, 5). In
contrast, mildreactions—in particular, hydrogenations and
isomerizations of un-saturated hydrocarbons—take place under
less-demanding temper-ature and pressure conditions. Mild reactions
have historically beenconsidered structure-insensitive (6–8), but
that conclusion has beendrawn from studies on reactivity vs. metal
dispersion that usedill-defined supported catalysts (9, 10) and has
been questioned bymore recent studies using better catalytic models
(11). For instance,both experimental (12–14) and theoretical (15)
studies on theselective catalytic hydrogenation of CAO bonds in
unsaturatedaldehydes have suggested that such reactions may be
promoted byclose-packed (111) surfaces. In another example, the
dehydroge-nation of cyclohexene was found to be faster on Pt (111)
than onPt (100) single-crystal surfaces (16). Our recent
surface-scienceinvestigations on the isomerization of unsaturated
olefins (17–19)strongly suggest that selectivity toward the
formation of the cisisomer may be favored by Pt (111) facets.
Additional surface-science reports on the conversion of alkyl and
alkene adsorbatesunder vacuum conditions (20–25), as well as
studies with morerealistic model systems (26, 27), point to a
potential structuresensitivity in the conversion of other olefins
These results not only suggest that mild reactions are
structure-sensitive, but also highlight the separate roles that
particle size andparticle shape may play in defining activity and
selectivity in thosecatalytic processes. Accordingly, the
independent control of particle
size and shape during catalyst manufacture may offer avenues
forthe design of highly selective catalysts. The self-assembly and
nanotechnologies developed during the last few years may now
placesuch catalysts within reach. Indeed, several colloidal (28–30)
anddendrimer (31) methodologies are already available for
makingmetal particles with well defined sizes in solution. The
control ofparticle shape is more challenging but has also been
achievedrecently, largely by controlling the concentration ratio of
thecapping polymer material to the metal cation used during
synthesis(32–35). Tetrahedral, cubic, irregular-prismatic,
icosahedral, andcubo-octahedral particles have been produced in
this way. Alter-native routes have also been proposed for
controlling both size andshape in metal colloidal particles
The next challenge is to extend the liquid-phase
chemistrymentioned above to the preparation of well defined
supportedcatalysts. This requires the resolution of a number of
issues,including the proper dispersion of the colloidal particles
throughoutthe pores of suitable high-surface-area supports and the
removal ofthe organic material used to stabilize the particles in
solution toexpose the clean metal surface. A particularly important
issue inconnection with the latter step is the need to avoid the
loss of thewell defined size and shape of the particles or the
occurrence ofparticle agglomeration (sintering). Once proper
particle dispersionand activation are accomplished, the performance
of the resultingcatalysts needs to be tested to identify clear
correlations betweenactivity and/or selectivity and the size and/or
shape of the particles.We in our laboratory have been working to
resolve these issues andhere report on key results indicating the
feasibility of this approach.Specifically, the selective promotion
of trans-to-cis carbon–carbondouble-bond isomerizations in olefins
was achieved by using cata-lysts with tetrahedral Pt particles
exposing high fractions of (111)facets. The loss of those facets
upon high-temperature treatmentleads to a reversal to the
thermodynamically expected preferencefor cis-to-trans
ResultsSynthesis of Colloidal Platinum Particles. Supported
platinum cata-lysts with well defined shapes were prepared and
tested by using thestrategy shown in Scheme 1. The initial step
focused on the synthesisof colloidal platinum particles with
different, well defined sizes andshapes. The starting point for
this synthetic effort was the protocoldeveloped by El-Sayed and
coworkers (32, 39, 40), who identifieda shape-controlled growth
mechanism based on differences be-tween the rate of the catalytic
reduction process of Pt2� on (111)vs. (100) facets, a competition
between the Pt2� reduction and the
Author contributions: F.Z. designed research; I.L., R.M., and
M.A.A. performed research; I.L.and F.Z. analyzed data; and F.Z.
wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
*To whom correspondence should be addressed. E-mail:
This article contains supporting information online at
© 2008 by The National Academy of Sciences of the USA
www.pnas.org�cgi�doi�10.1073�pnas.0805691105 PNAS � October 7,
2008 � vol. 105 � no. 40 � 15241–15246
capping process on the different nanoparticle surfaces, and
aconcentration-dependent buffer action of the polymer itself.
Theseauthors have reported the selective production of cubic,
tetrahedral,and truncated octahedral platinum nanoparticles by
changing theratio of the concentration of the capping polymer to
that of theplatinum cation in the solution used for the reductive
synthesis ofthe colloidal nanoparticles.
We conducted a series of experiments to determine the
optimumparameters to be used in preparing our tetrahedral and cubic
Ptparticles. The best tetrahedral Pt particles were obtained by
usingH2PtCl6 and poly(N-vinyl-2-pyrrolidone) (PVP, average Mw
�360,000), and the best cubic particles employed K2PtCl4 and
sodiumpolyacrylate (SPA, average Mw � 2,100). The specifics of
thissynthetic approach are provided in the supporting information
(SI)Materials and Methods. Our own experience in these studies
indi-cated that, indeed, the most critical factor in controlling
bothparticle shape and size is the concentration ratio between
thetransition metal precursor and the capping agent. Use of
lowconcentrations of the polymer led to narrower size
distributions,within 10% of the mean, but not necessarily to well
defined particleshapes. The concentration ratio is more critical
with H2PtCl6, butsmaller amounts of the polymer are required for
the preparation oftetrahedral Pt nanoparticles. Also, in the case
of the cubic particles,using an excess of the polymer leads instead
to the growth ofoctahedral particles. The lighter the capping
polymer, the higherthe concentration needed: 10 times higher
concentration ratios ofPVP of average Mw � 40,000 than of PVP of
average Mw � 360,000
are needed to prepare similar tetrahedral Pt nanoparticles. In
ourexperiments, the optimum ratios for making tetrahedral and
cubicparticles were determined to be [H2PtCl6]:[PVP, average Mw
�360,000] � 1:0.0005 and [K2PtCl4]:[SPA, average Mw � 2,100]
�1:0.75, respectively. These ratios work in only a limited range
ofprecursor concentrations—somewhere between 8 � 10�5 and 4 �10�4
M—but within that range the concentration can be used tocontrol the
size of the particles (between �3 and 10 nm). Otherfactors, such as
pH and reducing time, also must be carefullycontrolled to obtain
high-quality shapes and highly monodispersednanoparticles. Slower
reduction rates appear to lead to narrowersize distributions.
Fig. 1 shows representative transmission electron
microscopy(TEM) images of Pt colloidal particles with cubic,
tetrahedral, andstar-like (tetrapod) shapes obtained by this
colloidal syntheticprocedure. The average sizes of tetrahedral and
cubic Pt nanopar-ticles from TEM images were estimated at 4.8 � 0.3
and 5.0 � 0.3nm, respectively, and the purity of the shapes
(fraction of tetrahe-dral or cubic particles over total) at 83% and
87%, respectively. Amore detailed analysis is provided in Fig. S2.
The nature of theexposed surfaces—(111) and (100) facets for the
tetrahedral andcubic particles, respectively—was confirmed by
electron diffraction(see Fig. S3).
We placed particular emphasis on lowering the concentration
ofthe capping polymers as much as possible, to minimize the
problemsassociated with their removal after dispersion of the
particles on thehigh-surface-area solids (see below). The final
ratios of polymer to
Scheme 1. Methodology used for the preparation of supported
platinum catalysts with well defined particle shapes and sizes.
Fig. 1. TEM images of different platinum colloidal particles
obtained by tuning the conditions used for their assembly. Both
particle size and shape can becontrolled in this way. Note the
high-quality cubic (A), tetrahedral (B), and star-like (tetrapod)
(C) shapes obtained in the three cases reported here
[withpreferential exposed (100) and (111) facets in the first two
15242 � www.pnas.org�cgi�doi�10.1073�pnas.0805691105 Lee et
metal precursor used here are significantly lower than those
re-ported in the literature [i.e., 1:0.0005 vs. 1:0.035 for the
tetrahedralparticles (41)]. In further addressing this issue, it
will be importantto test other capping polymers.
Particle Dispersion on High-Surface-Area Supports. The
proceduresdeveloped for the deposition and activation of the
platinum nano-particles onto high-surface-area solid supports are
perhaps the mostcritical aspect of the preparation of our catalysts
because thestabilizing capping polymers must be removed with no
significantloss of particle size or shape. Three basic approaches
were explored,as indicated in Scheme 1. The first was to grow the
Pt particles insitu in the presence of a porous xerogel by carrying
the reductionstep after the addition of the high-surface-area
support. Unfortu-nately, all our attempts to prepare catalytic
samples by followingthis procedure failed because significant metal
particle aggregationwas observed and because particle penetration
inside the porousstructure of the support was quite limited (Fig.
Our second approach was to synthesize the solid support in
situin the presence of the colloidal particles, to encapsulate the
latterwithin the pores of the former during the growth process
(42). Theresults obtained from preliminary experiments using this
method-ology are encouraging. In one test, ammonia and
ammoniumfluoride were first mixed with a colloidal solution
obtained byleaving the metal precursor plus capping polymer mixture
in thedark for several hours and then adding an appropriate amount
oftetraethyl orthosilicate (TEOS) to initiate the gelling of a
silica solid(43, 44). The resulting xerogels were dried under
vacuum withoutaging until gray powders were obtained. A TEM image
of theresulting catalyst, with its tetrahedral particles embedded
in the insitu-grown sol-gel silica, is shown in Fig. 2B. Sol-gel
chemistry istypically carried out in ethanol; however, because
ethanol interfereswith the stability of the colloidal particles,
water was chosen hereas the solvent. We also learned that careful
control of the concen-trations and total amounts of all of the
chemicals is necessary tomatch the colloidal and gelling
chemistries. More studies areneeded in this area. A similar
approach has been used successfullyby Somorjai and coworkers (37,
45) to incorporate platinumnanoparticles synthesized by alcohol
reduction into mesoporousSBA-15 silica.
The main shortcoming of the second procedure is that
theresulting xerogels typically display inferior pore size and
shapedistributions. We estimate that in our synthesis �50% of the
porestructure shrunk during the drying process. Consequently,
weexplored a third synthetic route in which the Pt particles
weredispersed within the porous material by simple
impregnation,followed by either evaporation or filtration (46, 47).
The resultingdry catalysts displayed a gray color, indicating metal
deposition, andshowed good dispersion in the TEM images (Fig. 2C).
This ap-proach was also briefly tested for the dispersion of Pt
particles ona homemade SBA-15 mesoporous sample. That solid is
crystalline,with one-dimensional pores of �5–7 nm in diameter (as
by TEM), and only allows the incorporation of nanoparticles
withsizes of up to �5 nm; the larger particles were deposited only
on theoutside surfaces (see Fig. S4).
Catalyst Activation. After preparation, the catalysts were
washedwith either water or alcohol, dried, and annealed and/or
calcined inair for 1 h to produce the desired dispersed metal
catalysts. Asmentioned before, gentle calcination treatments are
needed tominimize the potential changes in size or shape of the Pt
particles.Interestingly, past TEM experiments with unsupported
platinumcolloidal particles have indicated that although heating to
550 K issufficient to remove the capping polymers, the metal
particlesretain their shape even after annealing at 625 K (48). In
the presentstudy, the thermal chemistry of Pt colloidal particles
deposited onxerogel supports was explored by using transmission IR
absorptionspectroscopy, X-ray photoelectron spectroscopy (XPS), and
TEM.Representative IR spectra are shown in Fig. 3. Those data,
whichcorrespond to cubic Pt particles dispersed on silica gel,
indicate thatmost (�95%) of the SPA capping polymer can be removed
byheating to �625 K, even after deposition of the colloidal
particleson the solid; the key vibrational peak for the
asymmetricOACOOO stretch, �a(OACOOO), of SPA at 1,558 cm�1
dis-appears completely by 675 K. However, some polymer
decompo-sition does occur, as manifested by the persistence of a
few of theCOH stretching bands in the IR traces (data not shown),
andsubsequent oxidation–reduction (OR) cycles are needed to
Fig. 3. Transmission IR spectra for 0.5 wt % cubic colloidal Pt
particles dispersedonaxerogel silica solidatdifferent
stagesofdepositionandactivation.Referencespectra for the SPA
capping polymer and the silica support are provided to aid inthe
peak assignment. (Left) Evidence for the deposition of the
colloidal particleson the silica; (Right) Evidence for the
successful removal of all hydrocarbonresidues after calcination and
reduction at 625 K.
Fig. 2. TEM of 0.5 wt % tetrahedral colloidal Pt particles
dispersed on porous silica supports, prepared by three different
procedures. (A) Precipitation in situ byreduction in the presence
of a high-surface-area silica xerogel powder. (B) Growing of the
silica xerogel in situ using a sol-gel method. (C) Impregnation of
a poroussilica xerogel powder with the Pt colloidal particles,
followed by filtration. Of the three methods, C appears to be the
best for obtaining well dispersed catalysts.
Lee et al. PNAS � October 7, 2008 � vol. 105 � no. 40 �
the residual carbonaceous deposits (1 h for each half cycle
under 1ml/s flow of either O2 or H2 at the same temperature of
calcination).This chemistry is confirmed by XPS, which also
indicates the extentto which the hydrogen treatment is able to
reduce the Pt particles(Fig. 4): the Pt 4f7/2 peak at 75.1-eV
binding energy due to theoxidized platinum seen in the fresh sample
is significantly dimin-ished (from 30% to 10% of the total signal)
after the reductiontreatment.
The evolution of the shape of the supported particles as
afunction of the severity of the cleaning procedure was also tested
byTEM. Fig. 5 shows a typical sequence of TEM images for 1.0 wt
%tetrahedral Pt nanoparticles deposited by impregnation on a
silicaxerogel, for the fresh sample and after calcination to
varioustemperatures between 375 and 575 K. Minor shape changes
alreadytake place after calcination at 375 K, but not until 575 K
do theparticles lose their original shape and become rounded; thus,
calcination temperature must be kept below 575 K to preserve
thedesired particle shape. Lowering the concentration of the
polymerused during the colloidal preparation step, as mentioned in
theprevious section, could be tested as a way to minimize the
severityof the conditions needed to clean the supported particles.
Displace-ment of the original capping polymers by other
easier-to-removeligands once the particles have been grown may also
help minimizethis contamination problem.
Catalytic Performance. The catalytic performance of the
materialsmade by using the procedures described above was
evaluated.Specifically, the selectivity for the isomerization of
olefins wasprobed by measuring the kinetics of isomerization of
both cis- andtrans-2-butene in the presence of a small amount of
hydrogen. Theability to control selectivity in these reactions may
have significantpractical implications because, for instance, the
trans fatty acids thatare produced during the partial hydrogenation
of natural oils toedible fats are believed to have serious negative
health effects (49,50). Although the conversion of alkenes by
transition metals is oneof the oldest and most studied systems in
catalysis (51–53), keyissues such as the dependence of activity on
structure remainunresolved (54). As mentioned in our Introduction,
results fromrecent surface-science work in our laboratory have
indicated that,at least on Pt (111) single-crystal surfaces, the
isomerization oftrans-2-butene to its cis isomer is easier than the
opposite cis-to-trans conversion (17–19). A kinetic study on the
catalytic conversionof butenes on single-crystal surfaces (55) has
also suggested differ-ent selectivities on Pt (111) vs. Pt (100)
Fig. 6 displays the ratios of the rates for trans-to-cis vs.
cis-to-transconversions obtained with both tetrahedral and cubic
supportedparticles as a function of the temperature used for
calcinationduring the catalyst pretreatment. For the cubic
particles, compa-rable rates are seen for both isomerizations—the
conversion ofcis-2-butene to its trans isomer and the reverse
trans-to-cis trans-formation—with all catalysts, regardless of the
calcination temper-ature used. With the tetrahedral particles, on
the other hand, notonly are the rates of reaction quite different,
but their relative valueschange dramatically as the calcination
temperature is increased. Onthe catalysts treated at low
temperatures (�500 K), the trans-to-cisconversion occurs
approximately twice as fast as the oppositecis-to-trans conversion.
In contrast, the trans-to-cis reaction domi-
Fig. 4. Pt 4f XPS data for samples prepared by deposition of 0.5
wt % tetra-hedral colloidal Pt particles on a silica xerogel. Some
Pt signal is lost uponcalcination (middle trace), but regained with
almost complete reduction to themetal state after reduction at 625
A B C
Fig. 5. TEM of 1.0 wt % tetrahedral colloidal Pt particles
dispersed on a porous silica xerogel, freshly prepared (A) and
after calcination to 375 K (B), 425 K (C), 475K (D), 525 K (E), and
575 K (F). The platinum particles retain their initial tetrahedral
shape after the low-temperature treatments but are clearly
transformed into largerand rounder shapes upon calcination at 575
K. These high temperatures are, therefore, to be avoided in the
catalyst activation stage.
15244 � www.pnas.org�cgi�doi�10.1073�pnas.0805691105 Lee et
nates after catalyst calcination at temperatures �500 K, being
asmuch as three to four times faster than the cis-to-trans
conversion.These changes correlate well with the changes seen in
the TEMimages in Fig. 5 and indicate that the preference for
trans-to-cisconversion is lost once the particles are calcined to
temperatureshigh enough to affect their shape and reduce the
fraction of (111)facets exposed. The inference is that it is the
(111) facets of theplatinum tetrahedral particles that promote the
nonthermodynamicisomerization of trans olefins to their cis
counterparts, as previouslyhypothesized.
Finally, the integrity of the catalysts after their exposure
tothe catalytic conditions was checked with TEM. It is wellknown
that exposure of supported metal particles to reactiveenvironments
can induce surface reconstruction (56) andcould in this case
reshape the original particles, reduce the highfraction of (111)
facets exposed, and remove their selectivity.However, this does not
seem to be the case here. The similarityof the TEM images obtained
before and after the catalyticreaction indicates minimal changes in
the shape of the particles(Fig. 7). It appears that, in this case,
the catalytic conditionsare sufficiently mild to avoid major
surface reconstruction onthe platinum-supported particles.
DiscussionIn this study, we have shown that colloidal chemistry
can beused to prepare catalysts consisting of dispersed metal
parti-cles with well defined sizes and shapes. Some
operationaloptimization in terms of catalyst deposition and
pretreatmentmay still be needed, but the results summarized above
clearlyprove the viability of this approach. Perhaps more
interest-ingly, it was also found that particles with different
shapes maypromote different reactions selectively. The example
providedhere refers to double-bond isomerizations, but similar
behav-ior is conceivable with other important catalytic reactions.
Theone caveat is that, because of the limited stability of
thedeposited particles, only mild reactions can be expected
tobenefit from the use of these shape-controlled
catalysts.Nevertheless, this includes a large number of processes
ofpotential interest to industry, including the hydrogenation
andisomerization of many unsaturated hydrocarbons
(olefins,aldehydes, ketones, organic acids, imines, etc.), as well
aselectrocatalysis such as that involved in fuel cells. It is
alsoworth mentioning that these colloidal synthetic methods,
inconjunction with other self-assembly procedures, offer anarray of
options for the preparation of specific catalysts. Forexample, the
use of crystalline mesoporous materials such asSBA-15 or MCM-21
could add shape selectivity, and bimetalliccolloidal particles
could be used to control the nature of theactive sites in these
The concept of using colloidal methods to control size and
shapein supported catalysts has been explored by others (57).
Theusefulness of the resulting catalysts has also been tested,
mostly forelectrocatalysis (35, 58–60), but in a few instances also
for gas-phasecatalytic conversions (61–64). In fact, recent designs
of shape-selective catalysts have been implemented industrially for
reform-ing and hydrogen peroxide synthesis (65). However, to the
best ofour knowledge, ours is the first systematic and
comprehensive studyof the evolution of the shape of the particles
throughout the variouspreparation and activation steps needed to
make and use thesesupported heterogeneous catalysts. It is also, we
believe, the firsttime that particle shape has been shown to
control selectivity in acatalytic process.
Materials and MethodsThe following chemicals were used in the
synthesis of the nanoparticles:dihydrogen hexachloroplatinate
hexahydrate (H2PtCl6�6H2O, 99.9% purity;Alfa Aesar); potassium
tetrachloroplatinate (K2PtCl4, 99.99% purity),
poly(N-vinyl-2-pyrrolidone) (PVP, [CH2CH(NC3H6CO)]n, average Mw �
40,000 and360,000), sodium polyacrylate (SPA, [CH2CH(CO2Na)]n,
average Mw � 2,100),2-methyl-2-propanethiol [(CH3)3CSH, 99%
purity], toluene (C6H5CH3, 99.5%purity), 1,3,5-trimethylbenzene
[(C6H3(CH3)3, 99.2% purity], all from Aldrich;Pluronic P123
triblock copolymer (BASF); and ammonium hydroxide (30%ammonia),
ammonium fluoride (NH4F, 98% purity), and tetraethyl orthosili-cate
(TEOS, 98% purity), all from Acros. In addition, the following
gases wereused: argon (ultrahigh purity), hydrogen (ultrahigh
purity), oxygen (ultrahighpurity), air (high purity), and nitrogen
(high purity), all from Airgas; andcis-2-butene (�95% purity) and
trans-2-butene (�95% purity), from MathsonTri-Gas. All compounds
were used as received.
UV-visible (UV-vis) absorption spectra between 190 and 1,000 nm
were re-corded by using a Varian Cary 50 UV-vis spectrometer.
Transmission IR absorptionspectra of the catalysts (pressed into
pellets) were acquired with a Bruker Tensor27 and a deuterated
triglycine sulfate (DTGS) detector. For the colloidal samples,a
commercial ZnSe attenuated total reflection (ATR) accessory (PIKE
Technolo-gies) was used to minimize the contribution from
background water. A spectralresolution of 4 cm�1 was used in all
The TEM experiments were performed on a Philips TECNAI 12 TEM
samplesweredepositeddirectlyontoCugridscovered with a holey carbon
support film (Ted Pella), and the catalysts weredispersed in
Milli-Q water by ultrasonication and deposited from the
resultingsuspension onto the Cu grids.
The XPS data were collected in an ultrahigh vacuum chamber
equippedwith an Al K� (h� � 1,486.6 eV) x-ray excitation source and
a Leybold EA11
Fig. 6. Kinetic data for the conversion of cis- and
trans-2-butenes withhydrogen on catalysts prepared by impregnation
of 1.0 wt % Pt nanoparticleson silica xerogel as a function of
calcination temperature. Shown are the ratiosof the initial rates
for the conversion of the trans to the cis isomer vs. those ofthe
cis-to-trans isomerization for catalysts made with tetrahedral and
withcubic particles. The rates of both cis-to-trans and
trans-to-cis conversions arecomparable for the cubic particles over
the calcination temperature rangestudied here, but with the
tetrahedral particles, a dramatic switch in selectivityis seen at
Fig. 7. TEM images from a catalyst prepared by using tetrahedral
colloidalparticles, before and after being used for the catalytic
conversion of 2-butenes inexperiments similar to those reported in
Fig. 6. The initial catalyst corresponds toa 1.0 wt % load of Pt
nanoparticles on a silica xerogel after calcination to 475 K.The
similarity of the two images suggests minimal deterioration of the
shape ofthe supported Pt particles upon their exposure to the
Lee et al. PNAS � October 7, 2008 � vol. 105 � no. 40 �
electron energy analyzer with multichannel detection (66). A
constantbandpass energy of 100.8 eV was used, corresponding to a
spectral reso-lution of �2.0 eV.
The catalytic activity of the Pt-supported catalysts was
investigated by using a150-ml stainless-loop batch reactor (67,
68). The composition of the gas mixturewas determined periodically
by gas chromatography, using a 23% SP-1700 on
80/100 Chromosorb PAW column and a flame ionization detector.
Additionaldetails on the operation of the batch reactor are
provided in SI Materials andMethods.
ACKNOWLEDGMENTS. Funding for this work was provided by the
1. Boudart M (1969) Catalysis by supported metals. Adv Catal
20:153–166.2. Zaera F, Gellman AJ, Somorjai GA (1986) Surface
science studies of catalysis: Classifi-
cation of reactions. Acc Chem Res 19:24–31.3. Bond GC (2005)
Metal-Catalysed Reactions of Hydrocarbons (Springer, New York).4.
Sinfelt JH (1973) Specificity in catalytic hydrogenolysis by
metals. Adv Catal 23:91–119.5. Bennett CO, Che M (1989) Some
geometric aspects of structure sensitivity. J Catal
120:293–302.6. Schlatter JC, Boudart M (1972) Hydrogenation of
ethylene on supported platinum. J
Catal 24:482–492.7. Dautzenberg FM, Platteeuw JC (1972) On the
effect of metal particle size on the
isomerization of n-hexane over supported platinum catalysts. J
Catal 24:364–365.8. Gates BC, Katzer JR, Schuit GCA (1979)
Chemistry of Catalytic Processes (McGraw–Hill,
New York).9. Anderson JR (1985) Particle size effects in metal
catalysts. Sci Prog 69:461–484.
10. Che M, Bennett CO (1989) The influence of particle size on
the catalytic properties ofsupported metals. Adv Catal
11. Bond GC (1991) Supported metal catalysts: Some unsolved
problems. Chem Soc Rev20:441–475.
12. Oudar J (1996) Hydrogenation reactions on platinum single
crystals. Z Phys Chem197:125–136.
13. Englisch M, Jentys A, Lercher JA (1997) Structure
sensitivity of the hydrogenation ofcrotonaldehyde over Pt/SiO2 and
Pt/TiO2. J Catal 166:25–35.
14. Gallezot P, Richard D (1998) Selective hydrogenation of
�,�-unsaturated aldehydes.Catal Rev Sci Eng 40:81–126.
15. Loffreda D, Delbecq F, Vigne F, Sautet P (2006)
Chemo-regioselectivity in heteroge-neous catalysis: Competitive
routes for CAO and CAC hydrogenations from a theo-retical approach.
J Am Chem Soc 128:1316–1323.
16. Somorjai GA, Rupprechter G (1999) Molecular studies of
catalytic reactions on crystalsurfaces at high pressures and high
temperatures by infrared-visible sum frequencygeneration (SFG)
surface vibrational spectroscopy. J Phys Chem B 103:1623–1638.
17. Lee I, Zaera F (2005) Selectivity in platinum-catalyzed
cis-trans carbon–carbon double-bond isomerization. J Am Chem Soc
18. Lee I, Zaera F (2005) Thermal chemistry of C4 hydrocarbons
on Pt(111): Mechanism fordouble-bond isomerization. J Phys Chem B
19. Lee I, Zaera F (2007) Infrared spectroscopy characterization
of the chemistry of C4hydrocarbons on Pt(111) single-crystal
surfaces. J Phys Chem C 111:10062–10072.
20. Bent BE, Nuzzo RG, Zegarski BR, Dubois LH (1991) Thermal
decomposition of alkylhalides on aluminum. 1. Carbon halogen bond
cleavage and surface �-hydride elimi-nation reactions. J Am Chem
21. Tjandra S, Zaera F (1994) A study of the thermal reactions
of methyl iodide coadsorbedwith hydrogen on Ni(111) surfaces:
Hydrogenation of methyl species to methane. JCatal 147:598–600.
22. Tourillon G, Cassuto A, Jugnet Y, Massardier J, Bertolini JC
(1996) Buta-1,3-diene andbut-1-ene chemisorption on Pt(111),
Pd(111), Pd(110), and Pd50Cu50(111) as studied byUPS, NEXAFS, and
HRELS in relation to catalysis. J Chem Soc Faraday Trans
23. Yoon CH, Yang MX, Somorjai GA (1997) Hydrogenation of
1,3-butadiene on platinumsurfaces of different structures. Catal
24. Jenks CJ, Bent BE, Zaera F (2000) The chemistry of alkyl
iodides on copper surfaces. 2.Influence of surface structure on
reactivity. J Phys Chem B 104:3017–3027.
25. Zaera F (2005) The surface chemistry of heterogeneous
catalysis: Mechanisms, selec-tivity, and active sites. Chem Rec
26. Freund HJ (2005) Model studies on heterogeneous catalysts at
the atomic level. CatalToday 100:3–9.
27. Argo AM, et al. (2006) Catalysis by oxide-supported clusters
of iridium and rhodium:Hydrogenation of ethene, propene, and
toluene. J Phys Chem B 110:1775–1786.
28. Turkevich J, Stevenson PC, Hillier J (1951) The nucleation
and growth processes in thesynthesis of colloidal gold. Discuss
Faraday Soc 11:55–75.
29. Boutonnet M, Kizling J, Stenius P, Maire G (1982) The
preparation of monodispersecolloidal metal particles from
microemulsions. Colloids Surf 5:209–225.
30. Aiken JD, III, Finke RG (1999) A review of modern
transition-metal nanoclusters: Theirsynthesis, characterization,
and applications in catalysis. J Mol Catal A 145:1–44.
31. Scott RWJ, Wilson OM, Crooks RM (2005) Synthesis,
characterization, and applicationsof dendrimer-encapsulated
nanoparticles. J Phys Chem B 109:692–704.
32. Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA (1996)
Shape-controlledsynthesis of colloidal platinum nanoparticles.
33. Lisiecki I (2005) Size, shape, and structural control of
metallic nanocrystals. J Phys ChemB 109:12231–12244.
34. Tao AR, Habas S, Yang P (2008) Shape control of colloidal
metal nanocrystals. Small4:310–325.
35. Xiong Y, Wiley BJ, Xia Y (2007) Nanocrystals with
unconventional shapes—A class ofpromising catalysts. Angew Chem Int
36. Burda C, Chen X, Narayanan R, El-Sayed MA (2005) Chemistry
and properties ofnanocrystals of different shapes. Chem Rev
37. Rioux RM, et al. (2006) Monodisperse platinum nanoparticles
of well-defined shape:Synthesis, characterization, catalytic
properties, and future prospects. Top Catal39:167–174.
38. Xiong Y, et al. (2007) Synthesis and mechanistic study of
palladium nanobars andnanorods. J Am Chem Soc 129:3665–3675.
39. Wang ZL, Ahmad TS, El-Sayed MA (1997) Steps, ledges, and
kinks on the surfaces ofplatinum nanoparticles of different shapes.
Surf Sci 380:302–310.
40. Petroski JM, Wang ZL, Green TC, El-Sayed MA (1998)
Kinetically controlled growth andshape formation mechanism of
platinum nanoparticles. J Phys Chem B 102:3316–3320.
41. Narayanan R, El-Sayed MA (2004) Effect of nanocatalysis in
colloidal solution on thetetrahedral and cubic nanoparticle shape:
Electron-transfer reaction catalyzed byplatinum nanoparticles. J
Phys Chem B 108:5726–5733.
42. Beakley LW, Yost SE, Cheng R, Chandler BD (2005)
Nanocomposite catalysts: Dendrimerencapsulated nanoparticles
immobilized in sol-gel silica. Appl Catal A 292:124–129.
43. Gonzalez RD, Lopez T, Gomez R (1997) Sol-gel preparation of
supported metal cata-lysts. Catal Today 35:293–317.
44. Frenzer G, Maier WF (2006) Amorphous porous mixed oxides:
Sol-gel ways to a highlyversatile class of materials and catalysts.
Annu Rev Mater Res 36:281–331.
45. Song H, et al. (2006) Hydrothermal growth of mesoporous
SBA-15 silica in the presenceof PVP-stabilized Pt nanoparticles:
Synthesis, characterization, and catalytic properties.J Am Chem Soc
46. Bönnemann H, Richards RM (2001) Nanoscopic metal
particles—Synthetic methodsand potential applications. Eur J Inorg
47. Barkhuizen D, et al. (2006) Experimental approaches to the
preparation of supportedmetal nanoparticles. Pure Appl Chem
48. Wang ZL, Petroski JM, Green TC, El-Sayed MA (1998) Shape
transformation and surfacemelting of cubic and tetrahedral platinum
nanocrystals. J Phys Chem B 102:6145–6151.
49. Simopoulos AP (1996) Trans fatty acids. Handbook of Lipids
in Human Nutrition, edSpiller GA (CRC, Boca Raton, FL), pp
50. Scarbrough FE (1997) Some Food and Drug Administration
perspectives of fat and fattyacids. Am J Clin Nutr
51. Bond GC, Wells PB (1964) The Mechanism of the hydrogenation
of unsaturatedhydrocarbons on transition metal catalysts. Adv Catal
52. Anderson JR, Baker BG (1971) Catalytic reactions on metal
films. Chemisorption andReactions on Metallic Films, ed Anderson JR
(Academic, London), Vol 2, pp 63–210.
53. Zaera F (1996) On the mechanism for the hydrogenation of
olefins on transition-metalsurfaces: The chemistry of ethylene on
Pt(111). Langmuir 12:88–94.
54. Smith GV (1999) Does trans addition occur after all? J Catal
181:302–304.55. Yoon CH, Yang MX, Somorjai GA (1998) Reactions of
1-butene and cis-2-butene on
platinum surfaces: Structure sensitivity of cis-2-butene
isomerization. J Catal 176:35–41.
56. Somorjai GA (1996) The flexible surface: New techniques for
molecular level studies oftime dependent changes in metal surface
structure and adsorbate structure duringcatalytic reactions. J Mol
Catal A: Chem 107:39–53.
57. Narayanan R, El-Sayed MA (2005) Catalysis with transition
metal nanoparticles incolloidal solution: Nanoparticle shape
dependence and stability. J Phys Chem B109:12663–12676.
58. Narayanan R, El-Sayed MA (2004) Changing catalytic activity
during colloidal platinumnanocatalysis due to shape changes:
Electron-transfer reaction. J Am Chem Soc126:7194–7195.
59. Wang C, Daimon H, Lee Y, Kim J, Sun S (2007) Synthesis of
monodisperse Pt nanocubesand their enhanced catalysis for oxygen
reduction. J Am Chem Soc 129:6974–6975.
60. Tian N, Zhou ZY, Sun SG, Ding Y, Zhong LW (2007) Synthesis
of tetrahexahedralplatinum nanocrystals with high-index facets and
high electro-oxidation activity.Science 316:732–735.
61. Yoo JW, Hathcock DJ, El-Sayed MA (2003) Propene
hydrogenation over truncatedoctahedral Pt nanoparticles supported
on alumina. J Catal 214:1–7.
62. Miyazaki A, Balint I, Nakano Y (2003) Morphology control of
platinum nanoparticlesand their catalytic properties. J
Nanoparticle Res 5:69–80.
63. Konya Z, et al. (2007) Pre-prepared platinum nanoparticles
supported on SBA-15—preparation, pretreatment conditions, and
catalytic properties. Catal Lett 113:19–28.
64. Serrano-Ruiz JC, et al. (2008) Hydrogenation of �,�
unsaturated aldehydes overpolycrystalline, (111) and (100)
preferentially oriented Pt nanoparticles supported oncarbon. J
65. DegussaHeadwaters (2004) NxCat: Next Generation Catalyst
66. Tiznado H, Zaera F (2006) Surface chemistry in the atomic
layer deposition of TiN filmsfrom TiCl4 and ammonia. J Phys Chem B
67. Loaiza A, Xu M, Zaera F (1996) On the mechanism of the H-D
exchange reaction inethane over platinum catalysts. J Catal
68. Loaiza A, Zaera F (2004) Regiospecificity in deuterium
labeling determined by massspectrometry. J Am Soc Mass Spectrom
15246 � www.pnas.org�cgi�doi�10.1073�pnas.0805691105 Lee et