www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2005) 793–800

Sensitivity of styrene oxidation reaction to

the catalyst structure of silver nanoparticles

R.J. Chimentao a, I. Kirm a, F. Medina a,*, X. Rodrıguez a,Y. Cesteros b, P. Salagre b, J.E. Sueiras a, J.L.G. Fierro c

a Departament d’Enginyeria Quımica, Universitat Rovira i Virgili, 43007 Tarragona, Spainb Departament de Quımica Inorganica, Universitat Rovira i Virgili, 43005 Tarragona, Spain

c Instituto de Catalisis y Petroleoquimica, CSIC, Cantoblanco, 28049 Madrid, Spain

Available online 16 March 2005

Abstract

This study shows how different morphologies of silver nanoparticles affect the selective oxidation of styrene in the gas phase

using oxygen as oxidant. Silver nanoparticles (nanowires and nanopolyhedra), prepared using the polyol process, were

supported on a-Al2O3. For comparison, a conventional catalyst obtained by wet impregnation was also prepared. Phenyla-

cetaldehyde (Phe) and styrene oxide (SO) were the main products for nanoparticles catalysts. The promotion effect on the

catalytic activity of potassium and cesium on the silver nanowires catalysts was also studied. At 573 K, the styrene conversion

and selectivity to styrene oxide with the silver nanowires catalyst were 57.6 and 42.5%, respectively. Silver nanopolyhedra

catalyst showed 57.5% conversion and 30.8% selectivity to styrene oxide. The promotion by cesium played an important role in

improving the epoxidation of styrene. The samples were structurally characterized using X-ray diffraction (XRD), ultraviolet–

visible spectroscopy (UV–vis), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray

photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) were applied to characterize the oxygen

species detected (Ob, Og) on the silver surface.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Silver nanoparticles; Nanowires; Nanopolyhedra; Styrene; Selective oxidation

1. Introduction

Metal nanoparticles have attracted considerable

attention because of their novel physical properties

* Corresponding author. Tel.: +34 9775 59787;

fax: +34 9775 59667.

E-mail address: fmedina@etse.urv.es (F. Medina).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.02.064

and their potential applications in areas, such as

catalysis [1]. Metal nanoparticles with shape control

can have structures and properties significantly

different from those conventional materials [2].

Recently, silver nanoparticles have been synthesized

by reducing silver nitrate with ethylene glycol in the

presence of poly(vinyl pyrrolidone) (PVP) via a polyol

process [3–7]. It is well known that the activity and

.

R.J. Chimentao et al. / Applied Surface Science 252 (2005) 793–800794

selectivity of the catalysts are strongly dependent on

their size, shape and surface structure, as well as on

their bulk and surface composition [8]. The shape-

controlled synthesis of metal nanoparticles can open

new opportunities for heterogeneous catalysis. This

approach may help to understand the effect of crystal

planes on chemical reactivity [9]. Oriented nanopar-

ticles could also be expanded to industrial application

to obtain many useful chemicals. In these regards,

catalysts obtained from silver nanoparticles seem to be

particularly interesting for studying the selective

oxidation of olefins with oxygen as oxidant because

it has been demonstrated that silver is a selective

catalyst for olefins epoxidation [10].

This study investigates how different morphologies

of silver nanoparticles and supports, such as a-Al2O3

and MgO affect the selective oxidation of styrene in

the gas phase using oxygen as oxidant. Besides, the

promotion effect of Cs and K on the catalytic activity

was investigated. The effect of the molar ratio

O2:styrene on the catalytic performance was also

studied. The samples were structurally characterized

using X-ray diffraction (XRD), temperature-pro-

grammed reduction (TPR), scanning electron micro-

scopy (SEM), transmission electron microscopy

(TEM), X-ray photoelectron spectroscopy (XPS)

and ultraviolet–visible (UV–vis) absorption spectro-

scopy with the aim to correlate the morphological

dependence of metal particles with their catalytic

behaviour.

2. Experimental

2.1. Preparation of the catalysts

The catalysts were prepared by two procedures.

First, wetness impregnation method was used to

impregnate a-Al2O3 and MgO supports with an

appropriate amount of an aqueous solution of silver

nitrate to obtain 15 and 40 wt% of silver, respectively.

The impregnated supports were dried in an oven at

393 K for 24 h and reduced in H2 at 623 K for 3 h

before the characterization and the activity tests. In the

second procedure, the silver nanoparticles were

synthesized via polyol process. All chemicals were

used without further purification. In a typical synthesis

of silver nanoparticles [4,5], 30 ml ethylene glycol

solution of AgNO3 (0.25 M, Aldrich) and 30 ml

ethylene glycol solution of PVP (0.375 M in repeating

unit weight-average molecular weight = 40,000,

Aldrich) were simultaneously added in 50 ml ethylene

glycol at 433 K under vigorous stirring. The reaction

mixture was then refluxed for 45 min at this

temperature. The nanoparticles obtained were diluted

with acetone (about 10 times by volume) and

separated from ethylene glycol by centrifugation at

4000 rpm for 20 min. Silver nanoparticles were also

prepared using a PVP/AgNO3 molar ratio of 3. The

silver nanoparticles (11 wt%) were dispersed on a-

Al2O3 with an acetone solution. The silver nanopar-

ticles catalysts were also dried in an oven at 393 K for

24 h and reduced in H2 at 623 K for 3 h before the

characterization and the activity tests.

2.2. Catalyst characterization

X-ray diffraction was performed on a Siemens

D5000 diffractometer using nickel filtered Cu Ka

radiation (l = 1.54056 A). For crystal phase identifi-

cation, the 2u range of scan was between 308 and 1208at a scan rate of 48 min�1 with a 0.058 data interval.

The UV–vis spectra of the silver nanoparticles were

recorded at ambient temperature using a HP8542

spectrophotometer by scanning wavelengths between

300 and 820 nm. The growth of silver nanoparticles

during the polyol process was monitored by sampling

small portions of the reaction mixture at various

reaction times (15, 20, 25, 30, 35 and 40 min) and

analyzed by UV–vis spectroscopy and TEM (JEOL

JEM-2000EX II transmission electron) operated at

80 kV. The morphologies of the catalysts were

observed by SEM with a JEOL JSM-35C scanning

microscope operated at an acceleration voltage of

15 kV. Temperature-programmed reduction (TPR)

experiments were performed in a TPDRO 1100

(Thermo Finnigan) equipped with TCD and mass

detectors. The samples were treated in O2 for 1 h at

different temperatures between 523 and 623 K before

TPR analysis. The TPR of silver catalysts was carried

out using 5% H2 in Ar flow as reducing agent, the gas

flow rate was 20 ml/min and the weight of sample was

1.0 g. The temperature was raised from 323 K up to

1073 K at a rate of b = 20 K/min. The XPS spectra

were acquired in a VG Escalab 200R electron

spectrometer equipped with a hemispherical electron

R.J. Chimentao et al. / Applied Surface Science 252 (2005) 793–800 795

analyzer, operating in a constant pass energy mode

and a non-monochromatic Mg Ka (hn = 1253.6 eV,

1 eV = 1.603 � 10�19 J). X-ray source operated at

10 mA and 12 kV. The background pressure in the

analysis chamber was kept below 7 � 10�19 mbar

during data acquisition. The binding energy (BE C

1s = 284.9 eV) of adventitious C1 was used as

reference. A Shirley background subtraction was

applied and Gaussian–Lorentzian product functions

were used to approximate the line shapes of the fitting

components.

2.3. Catalytic activity

The catalytic activity was carried out at steady-state

conditions using a stainless steel tubular down flow

reactor (10 mm internal diameter and 20 cm long)

with a temperature control system. The reactor was

filled with the catalyst (1.0 g), which had been

previously ground and sieved in the range of 75–

100 mesh. A mixture of O2–Ar was fed to the reactor

by independent mass flow controllers, using a total

flow rate between 100 and 300 ml/min. The styrene

was introduced into the reactor by a high-pressure

metering pump in a flow-rate range of 0.08–0.5 ml/h.

The reaction temperatures were in the range of 523–

623 K. The products of the reaction were rapidly

cooled and analyzed using a Shimadzu GC 2010 gas-

chromatograph equipped with an Ultra 2 capillary

column and a flame ionization detector (FID). The

presence of combustion products was determined by

on-line TCD and mass spectrometer.

Fig. 1. Growth of silver nanowires monitored by TEM (a–d) and time evolu

polyol process (e).

3. Results and discussion

3.1. Synthesis and characterization of silver

nanoparticles

Previous studies showed that the morphologies of

the silver nanoparticles prepared by the polyol process

were found to depend heavily on the experimental

conditions, such as the molar ratio between PVP and

AgNO3 [11]. Different morphologies of silver

nanoparticles, such as nanowires (NW) and nanopo-

lyhedra (NP) can be obtained at PVP/AgNO3 molar

ratios of 1.5 and 3, respectively.

During the synthesis of silver nanowires, the

mixture solution changed from clear to a yellowish

color, red brown and finally to gray. Once the solutions

of AgNO3 and PVP had been introduced to the

reaction system, the bright yellow color gradually

appeared indicating the formation of silver nanopar-

ticles through the reduction of AgNO3 by ethylene

glycol. The growth of silver nanowires was monitored

by sampling aliquots from the refluxing solution at

different periods of time and analyzed by TEM. Fig. 1

indicates the evolution of the morphologies of the

silver nanoparticles as the reaction mixture was kept in

reflux. The initial particles (yellow solution) had sizes

at around 8–50 nm (Fig. 1a). As the reaction

proceeded the silver particles contact each other to

form a chain like network (Fig. 1b and c) and some

larger nanoparticles (brown red solution) started to

appear. The growth of silver nanowires was also

monitored by ultraviolet visible spectroscopy (UV–

tion of UV–vis spectra during the formation of silver nanowires in the

R.J. Chimentao et al. / Applied Surface Science 252 (2005) 793–800796

vis). Fig. 1e shows the UV–vis spectra of the silver

nanoparticles at different times under refluxing condi-

tion. The spectrum of the solution at 15 min (yellow

solution) shows a small plasmon band close to 410 nm,

which represents the formation of silver nanoparticles

by the reduction of Ag+ ions [12]. The appearance of the

plasmon band is caused by 4d ! 5s, p interband

transitions [13]. The absorption band of the silver

nanoparticles shifts to larger wavelengths with the

refluxing time. The nanoparticles obtained at 40 min

(gray solution) displayed a broad peak at around

430 nm. However, the peak at 410 nm probably still

remained even after the solution had been heated for

45 min or longer. This observation is supported because

the final product of our synthesis was a mixture of silver

nanowires and nanopolyhedra as is shown by TEM in

Fig. 1d. These nanowires were easily separated from the

nanopolyhedra through centrifugation obtaining a pure

sample of silver nanowires.

The X-ray diffraction of the nanowires and

nanopolyhedra synthesized using the polyol process

suggested that silver existed purely in the face-

centered cubic (fcc) structure (Fig. 2). The diffraction

did not suggest the presence of possible impurities,

such as Ag2O and AgNO3. The peaks detected for the

silver nanoparticles were assigned to diffraction from

the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0),

(3 1 1) and (4 2 0) planes of fcc silver, respectively.

The lattice constants calculated by XRD for the

nanowires and nanopolyhedra were 4.0839 and

4.0872 A, respectively, which are very close to the

report data (a = 4.0862 A, Joint Committee on Powder

Diffraction Standards file 04-0783). The ratio of

intensity between (1 1 1) and (2 0 0) peaks has values

Fig. 2. XRD pattern of silver nanowires (lower pattern) and nano-

polyhedra (upper pattern).

of 4.5 and 2.5 for nanowires and nanopolyhedra,

respectively. For the nanowires, this ratio is higher

than standard file (JCPDS) (4.5 versus 2.5) indicating

that the nanowires show preferred orientation in

(1 1 1) facets. Nanowires and nanopolyhedra tend to

grow as bicrystals twinned along the (1 1 1) planes,

showing (1 1 1) crystal faces at their surface [14].

3.2. Catalyst characterization

Fig. 3a and b shows the SEM images of the 11 wt%

silver nanowires (NW) and 11 wt% silver nanopoly-

hedra (NP) supported on a-Al2O3, respectively. The

nanowires have a mean diameter of 150 nm. When the

molar ratio between PVP/AgNO3 was increased from

1.5 to 3, nanopolyhedra was the major product. The

SEM image of the 40% Ag/MgO catalyst (Fig. 3c)

prepared by wetness impregnation shows the presence

of silver nanowires together with other silver particles

having irregular shapes with diameters between 100

and 500 nm. Irregularly shaped particles with dia-

meters between 200 and 1000 nm (Fig. 3d) were also

observed for the 15% Ag/a-Al2O3 catalyst prepared

by wetness impregnation.

Fig. 4 shows the TPR profiles of Cs (0.5 wt%

refereed to silver) promoted silver nanowires (11%

Ag(NW–0.5% Cs)/a-Al2O3) (A), 11% Ag(NW)/a-Al2O3

(B) and 15% Ag/a-Al2O3 (C), catalysts after treatment

in O2 flow for 1 h at 623 K. The profiles show two

broad peaks for the NW catalysts at around 633 K

Fig. 3. SEM images of silver catalysts.

R.J. Chimentao et al. / Applied Surface Science 252 (2005) 793–800 797

Fig. 4. TPR profiles of catalysts: 11% Ag(NW–0.5% Cs)/a-Al2O3 (A),

11% Ag(NW)/a-Al2O3 (B) and 15% Ag/a-Al2O3 (C).

Fig. 5. TPR profiles obtained after exposing the catalyst (11%

Ag(NW)/a-Al2O3) in oxygen flow at different pre-treatment tem-

peratures ((A) 523 K, (B) 573 K, (C) 623 K).

(most intense peak) and 873 K. Previous studies have

shown that the first peak is attributed to subsurface

oxygen (Ob) [15–20]. The second peak is attributed to

strongly chemisorbed surface atomic oxygen (Og)

[18,21–23]. The 15% Ag/a-Al2O3 catalyst has the

peaks shifted to higher temperatures (753 and 933 K,

respectively). The most intense peak for this catalyst is

the second one. The Ob is the main oxygen species for

the nanowires, whereas (Og) is the main species for the

silver impregnated catalyst. The results also indicated

that the oxidation treatment before the TPR analysis

oxidizes a little content of Ag to Ag2O. By measuring

the H2 consumption we found that the silver oxidation

content of the catalysts was 0.35 and 1.5%, for 15%

Ag/a-Al2O3 and nanowire catalyst, respectively. So,

much more O2 is retained in the nanowires catalyst

compared with the impregnated 15% Ag/a-Al2O3 one.

The amount of Ag2O for 40% Ag/MgO and

nanopolyhedra catalyst was 0.8 and 1.1%, respec-

tively. It is important to mention that the addition of

cesium in the silver nanowires produced a strong

increase in the amount of Ag2O (5.6%) where Ob was

the main species detected.

Furthermore, the apparent continued H2 consump-

tion at 973 K in the nanowires suggests that residual

oxygen was present in the silver nanowires even at

high temperatures in agreement with previous results

[16]. This residual oxygen could be related with hole

formation. High temperature reaction increases sur-

face defect concentration resulting the hole formation.

The effect of the temperature on the oxidation process

of silver nanowires were investigated using different

pretreatment temperatures (523, 573 and 623 K)

before the TPR analysis (Fig. 5). These oxygen

species probably did not differentiate in both their

location and bonding states with the different

pretreatment temperatures due to the fact that

negligible shift was observed for the peaks related

with Ob and Og species. This indicates absence of

morphological changes of the silver nanoparticles

during the pretreatment temperatures. However, the

peak signal of Ob species decreased when the

pretreatment temperature increased. What is even

more interesting is the slight increase observed for Og

peak in TPR analysis after pretreatment at 623 K,

which did not coincide with the decrease of the Ob

peak signal. This shows that Og species is mainly

formed at higher temperatures. Probably at 623 K the

majority of the Ob atoms jump from interstitial site to

interstitial site until subsequent desorption to the gas

phase without the formation of Og species [16].

Chemical information on the nature of silver–

oxygen bound located in the near-surface region of

approximately 3 nm of the silver nanowires (pre-

reduced and oxidized at 623 K) was obtained by X-

ray photoelectron spectroscopy (Fig. 6; Table 1). The

most intense Ag 3d5/2 peak shows a major component

at 367.5 eV and a minor one at 368.4 eV. In parallel,

the O 1s peak shows two components, with the major

one at 529.7 eV and a less intense one at 531.3 eV.

These O species have been ascribed to Og and Ob

species, respectively [23]. However, as we observed

from TPR the amount of Ob species is higher than

Og. The discrepancy between XPS and TPR results

arises from the fact that oxygen located deeper in the

silver bulk is not observable by XPS [17]. In addition,

R.J. Chimentao et al. / Applied Surface Science 252 (2005) 793–800798

Fig. 6. Ag 3d core-level spectra of Ag nanowires pre-reduced and

oxidized at 623 K.

Table 1

Binding energies (eV) of core-levels and surface atomic ratios of

silver nanowires

Catalyst

(NW)

Ag 3d5/2 O 1s Al 2p O/Ag

atomic

ratio

Pre-reduced Ag 367.5 (80) 529.7 (67) – 0.51

368.4 (20) 531.3 (33) –

Oxidized Ag 367.5 (72) 529.7 (70) – 0.60

368.4 (28) 531.3 (30) –

Table 2

Results for the selective oxidation of styrenea

Catalyst X (%) Selectivity (%)

Phe SO

15% Ag/a-Al2O3 4.9 53.2 15.6

11% Ag(NW)/a-Al2O3 57.6 57.5 42.5

11% Ag(NW)/a-Al2O3b 66.7 79.5 20.5

11% Ag(NW)/a-Al2O3b,c 93.2 82.7 17.3

11% Ag(NW)/a-Al2O3b,d 96.4 94.5 5.5

11% Ag(NW)/a-Al2O3e,g 41.0 20.5 79.5

11% Ag(NW)/a-Al2O3f,g 85.8 27.8 72.2

11% Ag(NP)/a-Al2O3 57.5 69.2 30.8

40% Ag(NW)/MgO 77.1 82.9 17.1

NW, nanowires; NP, nanopolyhedra; X, conversion of styrene.a Feed: 50 O2:1 styrene (mol%), reaction temperature at 573 K.b 400 ppm of KOH referred to Ag.c 65 O2:1 styrene (mol%).d 100 O2:1 styrene (mol%).e 503 K.f 533 K.g 500 ppm of Cs referred to Ag.

surface O/Ag ratios were determined after perform-

ing a Shirley background subtraction to the O 1s and

Ag 3d5/2 peaks. The O/Ag atomic ratios derived for

type Og species virtually coincides with the

stoichiometry of Ag2O and is slightly increased

(0.60) upon in situ oxidation at 623 K. This finding

suggests that near-surface silver atoms retain a

strongly chemisorbed oxygen species.

3.3. Catalytic activity

The selective oxidation of styrene at 573 K

(Table 2) at steady-state, over silver catalysts shows

phenylacetaldehyde (Phe) and styrene oxide (SO) as

the main products. The direct combustion route of

styrene was negligible for silver nanowires and

nanopolyhedra even at near total conversion. How-

ever, the 15% Ag/a-Al2O3 catalyst shows around

30% of total combustion products at this tempera-

ture, even at lower conversion (around 5%). Styrene

conversion of 57.6% with a styrene oxide selectivity

of 42.5% was obtained for 11% Ag(NW)/a-Al2O3

catalyst. The silver nanowires supported on MgO

give styrene conversion of about 77.1% and a SO

selectivity of about 17.1%. The conversion increases

when silver nanowires was promoted with KOH.

However, the promotion of KOH decreased the

formation of epoxide favoring the formation of

phenylacetaldehyde. A conversion hi-gher than 95%

with total selectivity to phenylacetaldehyde and

R.J. Chimentao et al. / Applied Surface Science 252 (2005) 793–800 799

styrene oxide was obtained using the nanowires

catalyst promoted with KOH. Besides, for silver

nanowires catalyst promoted with KOH, the increase

in the O2:styrene molar ratio also improves the

conversion (>95%) favouring the formation of

phenylacetaldehyde at expenses of SO. The promo-

tion of the silver nanowires catalyst with Cs

increased the selectivity to styrene oxide. The

cesium promoted catalyst showed 85.8% of conver-

sion and 72.2% of selectivity to styrene oxide at

533 K.

Fig. 7 shows reaction rate versus temperature data

for selective oxidation of styrene over 11% Ag(NW)/a-

Al2O3 catalyst. The inset in the Fig. 7 shows the

corresponding Arrhenius plot of 11% Ag(NW)/a-

Al2O3. The average value obtained for the apparent

activation energy was 114 � 3 kJ mol�1 in good

agreement with the values previously reported [24].

The increase in the temperature from 523 to 573 K

increases dramatically the activity showing a conver-

sion of 0.5 and 57.6%, respectively, not detecting

combustion products. However, combustion products

were mainly obtained when the reaction temperature

increases up to 623 K. This change in the selectivity

can be explained taking into account the TPR results.

For the nanoparticles catalysts, the amount of the Ob

and Og species has a strong dependence with the

pretreatment temperature. The Ob species were the

most abundant at lower temperature (523–573 K),

while an important decrease is observed at 623 K.

However, a slight increase in Og species was observed

Fig. 7. Rate of styrene oxidation vs. temperature 11% Ag(NW)/a-

Al2O3 catalyst. Inset shows the corresponding Arrhenius plots.

at this temperature. We believe that the amount of Ob

species seems to be critical for this reaction.

Furthermore, they are responsible for the different

catalytic performance. The Ob preferentially leads to

high activity and selectivity while strongly chemi-

sorbed surface atomic oxygen (Og) shows lower

activity but high selectivity to combustion products.

This can also explain both the low activity and

selectivity observed for the 15% Ag/a-Al2O3 catalyst

since it could be expected that the Og species is more

strongly bound and harder to react.

4. Conclusion

We report the synthesis of Ag nanoparticles using

poly(vinyl pyrrolidone) as the template. Two types of

nanoparticles were obtained: silver nanowires and

nanopolyhedra. The different morphologies of the

silver nanoparticles were obtained using different

molar ration between PVP and AgNO3. The PVP/

AgNO3 molar ratio plays an important role in the

growth of the silver nanoparticles. The top crystal face

of these nanoparticles was (1 1 1), which may have a

beneficial effect on the selective oxidation of styrene.

The morphology and the chemical composition of the

silver catalyst determined the activity and selectivity

for the styrene oxidation reaction. Silver nanowires

and nanopolyhedra catalysts showed similar catalytic

behavior. It was found that the catalytic performance

of the Ag nanowires for the selective oxidation of

styrene was improved by increasing the basic

character of the catalyst, as well as the O2:styrene

molar ratio. The results present here show beneficial

properties of the cesium promoted silver nanowires

catalyst to the epoxidation of styrene. The promotion

of the silver nanowires by potassium improved the

activity but reduced the selectivity to styrene oxide.

The different catalytic behavior observed between the

impregnated 15% Ag/a-Al2O3 catalyst and silver

nanoparticles catalysts could also be explained taking

into account the presence of different oxygen species

in the silver catalyst detected by XPS and TPR

analysis. The increase in the catalytic activity to the

formation of selective oxidation products is accom-

panied by the increase in the intensity of H2

consumption of the first peak observed in the TPR

spectra, which is related with subsurface oxygen Ob.

R.J. Chimentao et al. / Applied Surface Science 252 (2005) 793–800800

The nature of silver-oxygen bond arises from the

different silver morphologies leading different cata-

lytic behavior. The controlled shape of silver

nanoparticles seemed to have potential applications

for the selective oxidation of olefins.

Acknowledgements

This work was supported by the Ministerio de

Ciencia y Tecnologia of Spain REN2002-04464-CO2-

01, PETRI 95-0801.OP and Destilaciones Bordas

S.A.

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