ORIGINAL PAPER

Nano Design of Alumina Supported Monometallic Catalysts:A Promising Way to Improve the Selective Hydrogenationof Poly-Unsaturated Hydrocarbons

Cecile Thomazeau • Tivadar Cseri • Laure Bisson •

Julie Aguilhon • Doan Pham Minh • Cedric Boissiere •

Olivier Durupthy • Clement Sanchez

Published online: 2 August 2012

� Springer Science+Business Media, LLC 2012

Abstract In the field of catalysis by metals, a new insight

for the nanodesign of supported heterogeneous catalysts is

the tailoring of metallic nanoparticles. In this work, well-

faceted monometallic nanoparticles (Pd, Pt, Ni) exposing

mostly the {111} crystallographic facet are obtained in

aqueous solution and are deposited on an alumina support.

The involved mechanisms of nanoparticles formation are

determined and are evidenced to be different as a function

of the nature of the metal. In the case of palladium the

mechanism consists in an oriented attachment of palladium

nanoparticles leading to the energetically most favourable

stacking of nanoparticles, at the origin of the early differ-

entiation of the nanoparticles shapes and of the formation

of the well-faceted palladium nanoparticles. In the case of

platinum, the mechanism seems to be a combination of

aggregation of already reduced nuclei and direct reduction

depending on the experimental conditions. In the case of

the less reductible metal, nickel, well-faceted nanoparticles

are not obtained during the synthesis and only a thermal

activation under hydrogen can engender their formation.

The impact of the {111} crystallographic facet for platinum

and nickel is very important and induces a drastic increase

of selectivity towards olefins formation with a selectivity

close to the one of a palladium catalyst which is the most

selective metal for the selective hydrogenation of poly-

unsaturated hydrocarbons.

Keywords Metallic nanoparticles � Synthesis �Aqueous solution � Morphology � Catalysis �Selective hydrogenation � Selectivity

1 Introduction

In the field of hydrogenation reactions, selective hydroge-

nations are essential and widely used in refining (petroleum

cuts purification) and petrochemistry (production of inter-

mediates for chemical or polymers applications). The main

aim of these reactions is to convert the most unsaturated

hydrocarbons (alkynes, dienes), especially at high conver-

sion level, into the corresponding olefins (the high added

value desired product, with drastic specifications required),

thus avoiding a complete hydrogenation and formation of

the saturated hydrocarbons. The most active and selective

metals used for selective hydrogenations are palladium,

platinum and nickel. Industrial catalysts are usually sup-

ported on alumina with 0.1–0.5 wt% of metal for noble

metals, and with 5–18 wt % of metal in the case of nickel

[1].

C. Thomazeau (&) � T. Cseri � L. Bisson � J. Aguilhon �D. Pham Minh

IFP Energies nouvelles, Rond-point de l’echangeur de Solaize,

BP 3, 69360 Solaize, France

e-mail: cecile.thomazeau@ifpen.fr

L. Bisson � J. Aguilhon � C. Boissiere � O. Durupthy �C. Sanchez

Chimie de la Matiere Condensee de Paris, College de France,

UPMC, Universite Paris VI, UMR 7574, 11 Place Marcelin

Berthelot, 75231 Paris Cedex 05, France

L. Bisson � J. Aguilhon � C. Boissiere � O. Durupthy �C. Sanchez

Chimie de la Matiere Condensee de Paris, College de France,

CNRS, UMR 7574, 11 Place Marcelin Berthelot, 75231 Paris

Cedex 05, France

L. Bisson � J. Aguilhon � C. Boissiere � O. Durupthy �C. Sanchez

College de France, Chaire des Materiaux Hybrides, 11 place

Marcelin Berthelot, 75231 Paris Cedex 05, France

123

Top Catal (2012) 55:690–699

DOI 10.1007/s11244-012-9868-1

In the case of the supported metallic nanoparticles, a

high proportion of exposed atoms is often the key property

to improve catalytic activities. Consequently, catalysis

research efforts were until today essentially focused on

dispersion and particles size effects studies [2] for structure

sensitive reactions such as selective hydrogenation. Con-

ventional selective metallic catalysts used for selective

hydrogenations are made of supported nanoparticles with a

relatively isotropic shape usually represented by a trun-

cated cuboctahedron (Fig. 1e) [3]. These nanoparticles

present several different active sites: {111} and {100}

facets, corners and edges, each one with different catalytic

properties. Indeed, the strong dependence of reactants

adsorption, their activation, and products desorption as a

function of the metallic surface coordination was already

shown on single crystals [4–6], and is now extended to

supported nanoparticles [7]. The improvement of supported

metallic catalysts is now possible by the use of nanopar-

ticles with particular shapes (rods, cubes, tetrahedra…)

(Fig. 1a–d). Indeed, their highly symmetric shapes induce a

precise control of their surface structure: type and amount

of exposed crystallographic planes, proportion between

atoms on corners, edges or facets.

This growing interest for nanostructured metallic

nanoparticles had induced important research efforts and it

is now possible to tune both their size and shape [8]. It has

also recently been established, for a set of reactions, that

both activity and selectivity are highly dependent on the

morphology of nanoparticles and reactions conditions [9].

This approach had already been investigated for reactions

such as water gas shift [10], hydrogenolysis [11], oxidation

[12], reduction [13], and, in our scope of interest, hydro-

genation [14–21] reactions.

Syntheses of well-faceted metallic nanoparticules occur

in aqueous or polyol media and involve the reduction of a

metallic salt precursor using a reducing agent, in the

presence of a surface stabilizer. Since the crystalline

growth rate varies exponentially with the surface energy,

the selective adsorption of such structure-directing agents

induces a preferential stabilization of specific crystallo-

graphic faces and/or inhibits the growth of particles along a

given crystallographic direction [22–30] leading to a

preferential growth along another specific direction.

Recent advances concerning size and shape control of

metallic nanoparticles over nucleation and growth pro-

cesses have been achieved [31–33]. Among the different

methods of synthesis developed in aqueous medium for the

control of nanoparticles shapes, the seed-mediated meth-

ods, based on the decoupling of nucleation and growth

kinetics, are now successfully used for the synthesis of

well-faceted metallic nanoparticles [34–36].

Gold, silver [24, 37], palladium [17, 38] and platinum

[39–42] well-faceted nanoparticles were thus obtained

either with single or mixed morphologies. However,

mechanisms of their formation still remain unclear.

Although the thermodynamic stabilization of given facets

by a preferential adsorption of the capping agent is the

more usually proposed driving force for the growth control,

other parameters could also be critical to achieve the shape

control of the nanoparticle. In the case of gold, the auto-

organization of CTAB molecules in a horizontal bilayer at

the surface of the growing particle is supposed to explain

the uniaxial growth of gold nanorods through an electric

assisted mechanism of molecular growth [43, 44]. A stable

Au(I)-CTAB complex would be formed in solution and

reduced to metallic gold directly on the CTAB-embedded

metallic surfaces [45]. For other metals such as palladium,

platinum and nickel, the involved mechanisms of formation

of metallic nanoparticles with particular shapes seem to be

quite different, as reviewed in this work. Indeed, their

rationalization is of paramount importance for controlling

the formation of one selected type of morphology. The

formation of particles with a unique type of morphology is

essential to establish clear relationships between the nature

of selectively exposed sites and catalytic properties, thus

giving the opportunity to tune the activity and/or the

selectivity of a catalytic system for a given reaction.

In this work, we focus on specific syntheses developed

to obtain well-faceted metallic nanoparticules (Pd, Pt, Ni)

in aqueous medium and deposited on an alumina support,

and on the establishment of the involved mechanisms of

nanoparticles formation. Our aim is, thanks to a real

comprehension of these mechanisms, to obtain one type of

morphology exposing selectively one type of active site.

Then, structure–activity relationships observed for

the supported catalysts would be really accessible and

explained for the selective hydrogenation reaction of

unsaturated hydrocarbons.

ROD TETRAHEDRON CUBE

(100)(100)(111)

(b) (c)(a)[110]

(111)

(111)

POLYHEDRON

(d) (e)(111)

TRUNCATED CUBOCTAHEDRON

(100)

(111)

Fig. 1 Different morphologies and exposed facets observed for

metallic nanoparticles: a Rod, b Cube, c Tetrahedron, d Polyhedron,

e Truncated cuboctahedron

Top Catal (2012) 55:690–699 691

123

2 Experimental

2.1 Preparation of Supported Well-Faceted Metallic

Nanoparticles

Palladium nanoparticles were prepared according to the

following synthesis. First step: reduction of K2PdCl4(12.5 mL, 1 mM) by NaBH4 (3 mL, 0.01 M) in presence

of CTAB (cetyltrimethylammonium bromide, 25 mL,

0.15 M). Second step: stirring of K2PdCl4 (25 mL, 3 mM),

CTAB (25 mL, 0.24 M) and sodium ascorbate (1 mL,

0.08 M), and injection of 90 lL of the first step solution

under argon or hydrogen atmosphere.

Platinum nanoparticles were prepared according to the

following procedure. First step : formation of an aqueous

solution by mixing H2PtCl6 (18 mL, 3 mM) with CTAB

(37.5 mL, 7.5–90 mM) under argon or hydrogen at

323–343 K and one injection per minute of NaBH4

(0.3 mL, 12 mM). Second step: continuation of NaBH4

injections (0.3 mL, 12 mM), total injected volume:

4.5 mL.

Nickel nanoparticles were prepared according to the

following procedure. First step: formation of an aqueous

solution by mixing NiCl2 (40 mL, 16–32 mM) with CTAB

(250 mL, 45–150 mM) under argon atmosphere by a pro-

gressive addition of N2H4 (20 mL, 600–1,200 mM,

343 K). Second step: a reducing step performed at 683 K,

at ambient pressure, under H2 flux (1 l h-1 g-1) during

16 h of the impregnated alumina.

Alumina (d-Al2O3) was added to the different above

prepared solutions, and stirred during 2–10 h. After filtra-

tion, the solid was dried overnight at 303 K.

2.2 Transmission Electron Microscopy

Transmission electron microscopy experiments were per-

formed using a Philips TECNAI F20 apparatus operating at

an acceleration voltage of 200 kV. TEM images were

digitally recorded by a CCD camera. Solutions were

deposited on a perforated carbon film supported by a Cu

grid. The excess of surfactant was removed by rinsing the

grids with ethanol and consecutively drying them under an

IR light before analysis. The catalysts supported on

alumina were crushed in ethanol before deposition on

the grid.

The frequency distribution was defined by the percent-

age of each type of metallic nanoparticle of different

morphology (cubes, polyhedra, etc.) for a given synthesis

procedure. The yield of a type of morphology was calcu-

lated as follow:

Fið%Þ ¼ni

nT

with ni, the number of particles with a given morphology

and nT the number of particles used for the statistical

analysis (on at least two hundred nanoparticles).

2.3 Catalytic Measurements

The selectivity of the supported palladium and platinum

nanoparticles was evaluated for the selective hydrogena-

tion of buta-1,3-diene (model molecule used to represent

the selective hydrogenation of light hydrocarbons cuts such

as C3-propylene or C4-butenes cuts for which an

improvement of the catalysts selectivity is required for

petrochemical applications). With that model molecule,

formation of the intermediates butenes is expected selec-

tively without further hydrogenation towards butane

(Fig. 2). Buta-1,3-diene hydrogenation was performed in

liquid phase using a laboratory scale stainless-steel batch

reactor working under static conditions with variation of

the concentration of reactant and products over time.

Experimental conditions were previously optimized in

order to avoid mass transfer limitations. Buta-1,3-diene and

butenes are hydrogenated following zero order reaction

kinetics. One gram of palladium or platinum catalyst ini-

tially reduced during 2 h under H2 at 323 K was transferred

under Ar in a glove bag into the batch reactor filled with

n-heptane. The catalyst was then put into contact with buta-

1,3-diene at 293 K under 20 bars of H2 and high stirring

velocity. A pressure gauge before the batch reactor main-

tains the pressure constant inside the reactor at 20 bars of

H2. The course of the reaction was followed by the loss of

H2 pressure in the pressure gauge and by analysis of

samples by gas chromatography (plot alumina column

PONA, L = 50 m, split injector; FID detection). The

butenes selectivity is defined according to the following

equation.

*

But-1-ène *

But-2-ènes *

Butane *

k1

k2

k3

k6

k4 k5Buta-1,3-diène

H2

H2

H2

H2

buta-1,3-diene *

1-butene *

2-butenes *

butane **

But-1-ène *

But-2-ènes *

Butane *

k1

k2

k3

k6

k4 k5Buta-1,3-diène

H2

H2

H2

H2

buta-1,3-diene *

1-butene *

2-butenes *

butane *

* = adsorbed product

Fig. 2 Scheme of the selective hydrogenation of buta-1,3-diene

692 Top Catal (2012) 55:690–699

123

Butenes Selectivity ¼ %ðbutenesÞ%ðn� butaneÞ þ

P%ðbutenesÞ

Activity and selectivity of the supported nickel nano-

particles were evaluated for the selective styrene/isoprene

hydrogenation (mix of model molecules used to represent

the selective hydrogenation of a C5–C8 pyrolysis gasoline

cut for which an improvement of both activity and selec-

tivity is required for industrial applications). For this later

catalytic measurement, the interest is the selective hydro-

genation of isoprene towards the corresponding olefins, in

presence of aromatics for which the aim is the selective

hydrogenation of the exo-cyclic olefin, without further

hydrogenation of the aromatic ring. For selective styrene/

isoprene hydrogenation, the formation of olefins interme-

diates (methyl-1-butene, methyl-2-butenes), and ethyl-

benzene are expected without further hydrogenation to the

corresponding alkanes, and ethyl-cyclohexane (Fig. 3).

This reaction was also performed in liquid phase using a

laboratory scale stainless-steel and perfectly stirred batch

reactor. One gram of nickel catalyst initially reduced dur-

ing 16 h under H2 at 683 K was transferred under Ar in a

glove bag into the batch reactor filled with n-heptane. The

catalyst was then put into contact with a styrene/isoprene

solution at 343 K under 35 bars of H2. In this conditions

there is no hydrogenation of the aromatic ring. The rate of

the reaction was followed by the loss of H2 pressure in the

storage bottle. Activity and olefins selectivity are defined

according to the following equations :

Activity ðmol mn�1gðNiÞ�1Þ ¼ VolH2� DPH2

m�%wt Ni � 24

� �

With VolH2 is the H2 volume in the reactor (given in mL),

DPH2 corresponds to the H2 pressure variation over the

time unit (bar min-1), wt% Ni is the metal content of the

catalyst and m is the mass of catalyst (in grams).

Olefins Selectivity ¼ %ðolefinsÞ%ðalcanesÞ þ

P%ðolefinsÞ

3 Results and Discussion

The decoupling of nucleation (with the use of previously

prepared seeds) and growth steps, allows a better control of

each single step and thus leads to better defined morphol-

ogies. However, mechanisms that explain, for each metal,

the formation of a given nanoparticle morphology are far

from being completely known. The direct consequence is

that in most of the proposed syntheses in literature, a

mixture of morphologies (nanorods, nanocubes, nanopo-

lyhedra) is obtained. The study of the preparation of well-

faceted metallic nanoparticles of elements (Pd, Pt, Ni)

belonging to the same column of the periodic table may

give fruitful informations about the evolution of the reac-

tivity in aqueous solution and about the observed mecha-

nisms as function of the intrinsic parameters of these

elements.

3.1 Preparation of Well-Faceted Supported Metallic

Nanoparticles

3.1.1 Well-Faceted Palladium Nanoparticles [36]

During the first step, the fast chemical reduction of

K2PdCl4 palladium salt by sodium borohydride in the

presence of CTAB led to the formation of spherical

5–6 nm palladium seeds (when characterized after 2 h of

reaction). At the beginning of the second step, just after the

introduction of seeds in the growth solution, two distinct

populations of particles were observed: a 7–8 nm popula-

tion attributed to the growth of the initial seeds by

molecular reduction, and a 2–3 nm population assumed to

be nuclei resulting from a secondary homogeneous nucle-

ation process occurring when ascorbate was introduced.

After 5–10 min of reaction, the above described seeds

and nuclei tended to aggregate, leading to the formation of

intermediate particles. This early aggregation of nuclei and

seeds was oriented and stabilized by the adsorption of

CTAB. Considering the high CTAB concentration of the

solution, palladium nanoparticles were assumed to be

covered by a surfactant bilayer, allowing seeds and nuclei

rotation and circulation one on each other. A spontaneous

evolution of the aggregate shape is driven by the free

enthalpy minimization principle (which mechanism could

be described using a very simple geometrical model,

involving spheres stacking [36]). This oriented attachmentFig. 3 Scheme of the selective hydrogenation of isoprene and styrene

Top Catal (2012) 55:690–699 693

123

of seeds and nuclei was proposed to be responsible for the

early differentiation of the palladium nanoparticles shapes.

The different nanoparticles morphologies were found to be

triggered by the energetically most favourable stacking of

spheres. Indeed, the growth of these nanoparticles finally

led to the formation of secondary objects of non random

shapes (Fig. 4a) and to the well-faceted palladium nano-

particles (Fig. 4b) resulting from a dominant molecular

reduction when palladium concentration became too low to

promote homogeneous nucleation. At the end, a wide

variety of palladium nanoparticles with different mor-

phologies (rods, bipyramides, icosahedra, cubes) exposing

{100} and {111} facets was formed in the aqueous

solution.

Considering the known high interaction between CTAB

with the {100} crystallographic facet of palladium [27], it

was assumed that a destabilisation of this interaction could

indirectly have a strong impact on the morphology,

favouring the most stable palladium {111} surface forma-

tion in the absence of the {100} stabilisation by CTAB.

Thus, when argon was replaced by hydrogen bubbling, a

majority of different nanostructures was formed, all exposing

mainly the {111} crystallographic facet (octahedra, prims,

icosahedra). The proportion of {111} exposed crystallo-

graphic facet was above 90 %, as determined elsewhere by a

detailed MET characterization [46].

3.1.2 Well-Faceted Platinum Nanoparticles [68]

Concerning platinum nanoparticles, numerous syntheses

are reported in the literature, even in aqueous medium [13–

16], using strong reducing agents such as sodium borohy-

dride. Structured platinum nanoparticles such as cubes or

cuboctahedra [47–49] could be obtained. Nevertheless,

mechanisms of platinum nanoparticles formation are not

mentioned.

The method of synthesis developed for palladium was

extended to the case of platinum with slight modifications.

The starting aqueous solution prepared by mixing H2PtCl6with CTAB (7.5 mM) under argon at 50 �C contains a Pt

(?IV) precursor rather than a (?II) ion. Numerous

parameters were shown to be critical and had to be pre-

cisely controlled : (i) the nature of the surfactant, (ii) the

control of the hydroxylation of the platinum chloride pre-

cursor, (iii) the temperature and finally (iv) the control of

the reaction atmosphere (under argon or hydrogen).

For this optimized synthesis, during the first step, an

interaction between CTAB and the platinate precursor

could be supposed, leading to {Pt(?IV)-CTAB} moieties

formation, since CTAB probably forms micellar species

(its concentration in solution is higher than its critical

micellar concentration (0.8–1.1 mM) [50]). The Pt(?IV)

cation could thus be surrounded by different coordination

shells with chloride partially substituted by bromide and/or

hydroxide (the ageing of the solution is supposed to lead to

the partial or complete substitution of chloride ligands by

hydroxides ones [41, 42, 51]). The strong reducing agent,

NaBH4, was then injected. At the end of the first step,

spherical platinum nanoparticles of nearly 1–5 nm were

formed after 2 min of reducing agent injections. Theses

nanoparticles were either isolated one from each others or

formed aggregates with a size comprised between 10 and

20 nm.

During the second step, the injections of reducing agent

were maintained leading to the formation of well-faceted

platinum nanoparticles [almost 90 % of well-faceted plat-

inum nanoparticles were observed (Fig. 5)]. When higher

concentrations of CTAB were used (90 mM), platinum

nanoparticles were less faceted, and ill defined morpholo-

gies were preponderantly observed.

These observations led to a proposition of mechanism.

For low surfactant concentrations, well-faceted platinum

nanoparticles were obtained in a large proportion. A direct

molecular reduction of the platinum precursor onto the

growing nanoparticles and/or coalescence of the aggregatesFig. 4 Formation of well-faceted palladium nanoparticles by aggre-

gation after a reaction time of a: 20 min and b 3 h

694 Top Catal (2012) 55:690–699

123

(resulting from the previous aggregation step of the pri-

mary formed platinum nanoparticles) could be proposed as

preponderant. At higher CTAB concentrations, the direct

reduction of the platinum precursor onto the growing

platinum nanoparticles and/or the coalescence of the

aggregates should be disfavoured. As a result, the obtained

platinum nanoparticles were not well-faceted.

Moreover, as already discussed in the case of palladium,

the destabilisation of the CTAB interaction with the {100}

crystallographic plane could indirectly favour the most stable

platinum {111} surface. Thus, even for high CTAB con-

centrations (90 mM), replacing argon by hydrogen allowed

to obtain more than 96 % of well-faceted platinum nano-

particles with predominant {111} crystallographic facet.

3.1.3 Well-Faceted Nickel Nanoparticles

In the case of nickel, the formation of well-faceted nano-

particles is scarcely reported in aqueous solution probably

because of unwanted reoxidation reactions that are more

difficult to avoid with that element. That is why well

defined nickel metallic nanoparticles such as cubes, tetra-

hedra and rods are until now only obtained in organic

solvents [15, 52–56]. Nickel nanosheets could be prepared

either by direct reduction (hydrazine) of a nickel precursor

(nickel chloride) [57] or by an intermediate formation of

nickel hydroxide [58] and its further reduction. Nickel

hexagonal nanosheets exposing the {111} crystallographic

plane are synthesized, with conservation of the morphology

of the previously synthesized nickel hydroxide nanosheets.

Nickel nanoparticles with an urchin morphology are syn-

thesized by reduction of a nickel chloride precursor by

hydrazine in presence of a structure directing agent [59].

The method of synthesis developed in this work for

noble metals was thus finally extended to the case of nickel

with an impregnation of the solutions onto alumina. An

additional reduction at 683 K under H2 atmosphere was

mandatory to obtain nicely-faceted nanoparticles in

the 20–50 nm size range (Fig. 6). From the analysis

Fig. 5 2D-TEM characterization of each type of platinum nanoparticle in solution: a pseudo-cube; b cube; c polyhedron; d tetrahedron;

e cubooctahedron; f rod [68]

Fig. 6 Supported nickel nanoparticles on alumina

Top Catal (2012) 55:690–699 695

123

of frequencies and mean sizes of the different morpholo-

gies of nanoparticles observed by TEM (tetrahedra, ico-

sahedra, rods and decahedra), it appeared that all the

observed morphologies exposed mainly the {111} crys-

tallographic plane and that icosahedra were more often

observed.

The study of the mechanism of formation of these nickel

nanoparticles is still in progress but some elements of

comprehension could already be brought. First of all,

hydrazine (E(N2/N2H4) = -1.16 V/ENH [60]) was chosen

as it is a reductor stronger than NaBH4 (E(B(OH)3)/

BH4-) = -0.481 V/ENH) in order to reduce Ni(II) species

in solution (E(Ni2?/Ni) = -0.257 V/ENH) [61]. More-

over, the reduction had to be performed in basic medium

[62] and the temperature should be increased at 343 K to

kinetically activate the reduction [63]. The two other

drawbacks of NaBH4 are its propensity to decompose in

solution and its ability to form Ni2B species [64] for which

consequences onto catalytic properties are not well known.

Unfortunately, even hydrazine on nickel ratios up to 40 did

not permit to obtain metallic nickel nanoparticles. An

intermediate Ni(II) based structure with a strong interaction

with alumina was detected. This intermediate was charac-

terized as an hydrotalcite type layered structure involving

both Al(III) and Ni(II) cations. The well-faceted metallic

nickel nanoparticles were formed only during treatment

under hydrogen at 683 K, thus during a solid–gas process,

which generate the driving force at the origin of well-

faceted nickel nanoparticles formation.

3.1.4 Comparison of the Different Mechanisms of Well-

Faceted Nanoparticles Formation

When mechanisms of well-faceted metallic nanoparticles

formation are compared for gold, palladium, platinum and

nickel, it appears that they are different, depending on the

metal, and on its reductibility. Indeed, each of the studied

metals possess its own oxidation–reduction thermodynamic

potential [61]: E(Au?/Au) = 1.83 V/ENH, E(Pd2?/

Pd) = 0.83 V/ENH, E (Pt2?/Pt) = 1.2 V/ENH, E(Ni2?/

Ni) = -0.257 V/ENH. This will of course have conse-

quences on the choice of the reductor but no direct rela-

tionship between the oxidation–reduction thermodynamic

potential and mechanisms appears. This points out the fact

that kinetic factors must have to be taken into account and

are determinant for such kind of synthesis.

Direct reduction of a stable Au(I)-CTAB complex on the

CTAB-embedded metallic gold surfaces was proposed in

literature in the case of gold. The mechanism evidenced in

the case of palladium was different from that proposed for

gold, and consisted in an oriented attachment of palladium

nanoparticles leading to the energetically most favourable

stacking of nanoparticles, at the origin of the early

differentiation of the nanoparticles shapes and of the for-

mation of the well-faceted palladium nanoparticles.

Moreover, in the case of platinum, the mechanism seems to

be a combination of aggregation of already reduced nuclei

and direct reduction depending of the experimental con-

ditions. In the case of the less reductible metal, nickel, only

a supplementary reduction step allows the formation of the

well-faceted nickel nanoparticles.

Nevertherless, once the mechanism is understood for a

given metal, it becomes possible to favour the formation of

a given morphology, or of one type of exposed cristallo-

graphic facet, through the control and adjustment of the

experimental parameters. Thus, in the case of palladium

and platinum in solution and even in the case of nickel in a

solid–gas process, the formation of the {111} cristallo-

graphic facet is favoured with hydrogen addition and could

be explained considering a ‘‘surface cleaning’’ by hydrogen

[65]. An adsorption competition of both molecules would

happen in particular on {100} facets, causing their desta-

bilization relatively to {111} facets thus favouring the

{111} facet formation.

We thus obtained metallic nanoparticles with selectively

one type of exposed active site, mainly the {111} crystal-

lographic facet. Consequently, structure–activity relation-

ships could be established for these supported metallic

nanoparticles for the selective hydrogenation reaction of

unsaturated hydrocarbons.

3.2 Catalytic Properties of the Well-Faceted Supported

Metallic Nanoparticules

The supported palladium and platinum nanoparticles

exposing mainly the {111} crystallographic facet were

prepared by deposition of the colloidal solutions on alu-

mina with conservation of the morphologies obtained in

solution. The well-faceted supported nickel nanoparticles

were obtained after the above mentioned supplementary

reduction step. The as prepared catalysts are labelled as

MCat-{111} (M = Pd, Pt, Ni). For each metal, a refer-

ence catalyst, labelled MCat-Ref (M = Pd, Pt, Ni) was

prepared, using conventional well-known methods of syn-

thesis. For the MCat-Ref catalysts, metallic nanoparticles

have a relatively isotropic shape usually represented by a

truncated cuboctahedron [3], thus presenting several dif-

ferent active sites: {111} and {100} facets, corners and

edges. Characterizations, proportions in each type of

crystallographic facets and catalytic properties are reported

Table 1.

Palladium is intrinsically the most selective metal for

selective buta-1,3-diene hydrogenation towards butenes

formation, so a high selectivity is observed for high buta-

1,3-diene conversions, even for PdCat-Ref (olefins selec-

tivity of 96 %, Fig. 7). Nevertheless, thanks to the higher

696 Top Catal (2012) 55:690–699

123

amount of exposed {111} crystallographic plane of PdCat-

{111}, an even higher selectivity of 99 % is reached. This

improvement of selectivity, due to the preferential expo-

sition of the {111} actives sites confirms improvements of

selectivities already observed in literature for the same

reaction for single crystals [67] or for supported well fac-

eted palladium nanoparticles [21] in different experimental

conditions.

In the case of platinum nanoparticles, the influence of

morphology on catalytic properties was already evidenced

for the selective hydrogenation of benzene with a higher

selectivity showed by cubic platinum nanoparticles, thus

exposing mainly the {100} crystallographic facet, in

comparison with cuboctahedric nanoparticles [17]. For

selective hydrogenation of buta-1,3-diene, platinum is

intrinsically less selective than palladium. This is visible

through an olefins selectivity of around 70 % observed for

PtCat-Ref at 85 % conversion of buta-1,3-diene, Fig. 7. A

drastic increase of selectivity towards olefins formation is

observed for PtCat-{111} with a selectivity[90 % at 85 %

conversion of buta-1,3-diene. This could again be associ-

ated with the exposition of the {111} crystallographic

facet. So depending on the type of hydrogenation consid-

ered (aromatic in the literature or dienes in our work), the

nature of the most selective site is different. Moreover, the

impact of platinum nanoparticles morphology on selectiv-

ity is evidenced with a behaviour of the PtCat-{111} cat-

alyst close to that of palladium which is the most selective

metal for such selective hydrogenation reactions.

In the case of nickel, morphologic effects [66] on catalytic

properties are also expected. Indeed, nickel nanoparticles

with specific morphologies such as whiskers, were shown to

display higher activities for benzene hydrogenation when

compared to spherical ones [14]. The evaluation of their

activity in the selective hydrogenation of a representative

Styrene/Isoprene mixture shows a decrease of the activity for

NiCat-{111} (0.15 mol min-1 gNi-1) compared to NiCat-

Ref (2.56 mol min-1 gNi-1), Table 1. However, only one

part of this diminution could be attributed to the higher size

of the NiCat-{111} nanoparticules compared to the reference

catalyst. The following order of activity {111} \{100} \ {110} was already reported in literature for single

crystals [19, 67]. So, the lower activity observed for the

NiCat-{111} catalyst could obviously be explained as a

combination of a diminution of the number of active sites

expressing a lower activity. The evaluation of the NiCat-

{111} catalyst selectivity towards olefins also shows an

Table 1 Characterization and catalytic properties of the palladium, platinum and nickel well-faceted supported nanoparticles

Catalyst wt% (M) % {111} facetsa A (mol min-1 gNi-1) Olefins selectivity (%)b

(M = Pd, Pt, Ni)

PdCat-Ref 0.20 70 – 96

PdCat-{111} 0.19 [90 – 99

PtCat-Ref 0.30 70 – 70

PtCat-{111} 0.16 [96 – 90

NiCat-Ref 15.7 70 2.56 93

NiCat-{111} 8 [85 0.15 96

a The proportion in {111} crystallographic facets is deduced from shape frequencies determined from MET and estimated according to

geometric models presented Fig. 1b The selectivity towards olefins is reported at 85 % buta-1,3-diene conversion for Pd and Pt catalysts, and at 85 % isoprene conversion for Ni

catalysts

6065707580859095

100

20 30 40 50 60 70 80 90 100

Buta-1,3-diene conversion (%)

But

enes

Sel

ectiv

ity (%

)

PtCat-RefPtCat-{111}PdCat-RefPdCat-{111}

Pt

Fig. 7 Butenes selectivity observed for the selective hydrogenation

of buta-1,3-diene

80

85

90

95

100

70 75 80 85 90 95 100

Isoprene Conversion (%)

Ole

fins

Sel

ecti

vity

(%

)

NiCat-Ref

NiCat-{111}

Fig. 8 Olefins selectivity observed for the selective hydrogenation of

a styrene/isoprene mixture

Top Catal (2012) 55:690–699 697

123

improvement due to the {111} crystallographic facet

(Fig. 8). For an isoprene conversion of 85 %, the selectivity

of NiCat-Ref is 93 % while this selectivity increases sig-

nificantly until 96 % for NiCat-{111}.

Moreover, in the case of the platinum and nickel metals,

the improvement of selectivity towards olefins becomes

more and more important for the highest conversions of the

unsaturated hydrocarbon, i.e. in the range of industrial

interest for such reactions.

4 Conclusion

The growing interest for the control of metallic nanoparti-

cules shapes with applications in catalysis, is well illustrated

through many examples of the literature. Well-faceted and

defined nanoparticles can now be obtained through different

syntheses pathways. In this work, different syntheses in

aqueous solution were successfully developed for palladium,

platinum and nickel. Supported catalysts obtained after

deposition of well-faceted nanoparticles on alumina showed

very high selectivity for the hydrogenation of poly-unsatu-

rated compounds. The tailoring of the morphology of these

nanoparticles represents thus an improvement for the nano-

metric scale design of new catalysts generations. The choice

of the appropriate type of active metallic site can then be

done for each targeted reaction in order to obtain the best

catalytic properties.

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