Liquid phase hydrogenation of methyl-N-Boc-pyrrole-2-carboxylate over tailored Ru nanoparticles

Article abstract of DOI:10.1016/j.jcat.2012.02.017

Ru nanoparticles were prepared in ethylene glycol at different pH, temperature and RuCl₃·nH₂O concentration as well as with and without poly(N-vinyl-2-pyrrolidone) (PVP) as protective agent. They were characterized in the synthesis solution by small-angle X-ray scattering (SAXS) using Guinier analysis and indirect Fourier transformation (IFT) technique and by TEM, XRD, XPS and ATR-IR. From SAXS data evaluation, it was derived that size of the formed Ru nanoparticles depends mainly on the pH of the synthesis solution and only to a minor extent on Ru precursor concentration and reduction temperature. The impact of PVP on particle size was also only very low. The Ru nanoparticles were active for hydrogenation of methyl-N-Boc-pyrrole- 2-carboxylate in ethanol solution at 25 °C and p(H₂) = 5 bar. Particle size was found to be an important parameter determining hydrogenation activity. A maximum in the activity (TOF = 1000 h^-1) was obtained over Ru nanoparticles with a mean particle diameter of about 1.6 nm. However, differently prepared nanoparticles of nearly identical particle size diverge strongly in their catalytic activity. These differences might be due to different surface sites and a blocking of active metal surface sites by surface intermediates formed during nanoparticle formation. Both surface structure and the extent of such blocking strongly depend on the synthesis conditions. If the nanoparticles were protected by PVP, their hydrogenation activity clearly drops in comparison to similarly sized nanoparticles without PVP protection.

Full text of DOI:10.1016/j.jcat.2012.02.017

Journal of Catalysis 289 (2012) 249–258
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Liquid phase hydrogenation of methyl-N-Boc-pyrrole-2-carboxylate over tailored
Ru nanoparticles
⇑
Norbert Steinfeldt , Michael Sebek, Klaus Jähnisch
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Ru nanoparticles were prepared in ethylene glycol at different pH, temperature and RuCl ÁnH O concen-
3
2
Received 14 October 2011
Revised 6 January 2012
Accepted 23 February 2012
Available online 27 March 2012
tration as well as with and without poly(N-vinyl-2-pyrrolidone) (PVP) as protective agent. They were
characterized in the synthesis solution by small-angle X-ray scattering (SAXS) using Guinier analysis
and indirect Fourier transformation (IFT) technique and by TEM, XRD, XPS and ATR-IR. From SAXS data
evaluation, it was derived that size of the formed Ru nanoparticles depends mainly on the pH of the syn-
thesis solution and only to a minor extent on Ru precursor concentration and reduction temperature. The
impact of PVP on particle size was also only very low. The Ru nanoparticles were active for hydrogenation
Keywords:
Ru nanoparticles
Polyol process
SAXS
2
of methyl-N-Boc-pyrrole-2-carboxylate in ethanol solution at 25 °C and p(H ) = 5 bar. Particle size was
found to be an important parameter determining hydrogenation activity. A maximum in the activity
À1
(
TOF = 1000 h ) was obtained over Ru nanoparticles with a mean particle diameter of about 1.6 nm.
PVP
Particle size
Hydrogenation
Pyrroles
However, differently prepared nanoparticles of nearly identical particle size diverge strongly in their cat-
alytic activity. These differences might be due to different surface sites and a blocking of active metal sur-
face sites by surface intermediates formed during nanoparticle formation. Both surface structure and the
extent of such blocking strongly depend on the synthesis conditions. If the nanoparticles were protected
by PVP, their hydrogenation activity clearly drops in comparison to similarly sized nanoparticles without
PVP protection.
Ó 2012 Elsevier Inc. All rights reserved.
1
. Introduction
in different types of reactions e.g. ammonia synthesis [8] and oxi-
2
dation [9], NO [10] and N O [11] reduction and especially in hydro-
Metallic nanoparticles are considered as promising catalytic
genation of aromatics [12]. Previously, Ru nanoparticles with
tunable particle size and narrow size distribution have been pre-
pared by the polyol process using different Ru precursors (e.g.
material because their electronic and catalytic properties are
strongly influenced by their size and shape [1]. By controlling these
parameters, catalysts with high activity and selectivity can be de-
signed [2]. The polyol method [3] allows to prepare transition me-
tal nanoparticles with tunable particle size and narrow size
distribution. In this procedure, the polyol acts both as solvent
and reducing agent for the metal precursor. Stabilization of the
nanoparticles takes place by electrostatic or steric effects or com-
binations of both [4] and can be provided e.g. by oxidation prod-
ucts of the polyol [5], by addition of acetates ions [6] or more
protective stabilizers like PVP (poly(N-vinyl-2-pyrrolidone)),
which is one of the most frequently employed stabilizer in nano-
particle synthesis [4]. It is assumed that polymers like PVP protect
the nanoparticles from aggregating by steric stabilization and by
ligation of nanocluster surface atoms [7].
3 3
Ru(acac) , RuCl ) and polyols like ethylene glycol, propanediol
and butanediol often with PVP as surface-capping agent [13–18].
Most of these works were dealing only with nanoparticle prepara-
tion and characterization. For catalytic applications, such tailored
nanoparticles were deposited on metal oxide supports [15,16,18].
PVP–protected Ru nanoparticles prepared in aqueous solution
were also successful applied for the hydrogenation of arenes, ole-
fins and carbonyl compounds with turnover frequencies (TOF) lar-
À1
ger than 1000 h [19]. In order to achieve these high activities,
2
elevated temperature (80 °C) and high H pressure (4 MPa) have
to be applied. The Ru–PVP system was also applied for the selective
hydrogenation of o-chlorobenzene to o-chloroaniline using similar
conditions [20].
Among the noble metals, Ru is the cheapest and, therefore, of
particular interest for catalytic applications. Ruthenium is active
Particle size and size distribution of the Ru nanoparticles were
generally derived from TEM analysis. Small-angle X-ray scattering
(
SAXS) is another powerful technique to provide information about
colloidal structures within liquid phases without any pretreat-
ment. It was applied to follow the formation and growth of metal-
lic nanoparticles [21,22] and for determination of the particle size
⇑
Corresponding author. Fax: +49 (0)381 1281 50319.
E-mail address: norbert.steinfeldt@catalysis.de (N. Steinfeldt).
0
021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.02.017

2
50
N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
ethanol. For the hydrogenation experiments, an aliquot amount
O
Ru-NP catalyst
bar H2, EtOH, rt
O
of Ru was given to the reaction mixture.
N
N
5
Boc
O
Boc
O
2.3. General procedure for the hydrogenation experiment
1
2
The hydrogenation was performed in a 100 mL autoclave (Büchi
Scheme 1. Hydrogenation of Methyl-N-(tert-butoxycarbonyl)-pyrrole-2-carboxyl-
ate 1.
miniclave) equipped with a magnetic stirrer. The substrate 1
À3
(
112 mg, 0.5 mmol) and ruthenium nanoparticles (1 Â 10
mmol Ru) solved in ethylene glycol (synthesis solution) or ethanol
after pretreatment) were added to a mixture of 20 mL ethanol
(
and size distribution [23]. Furthermore, it was demonstrated that
for nanoparticles in the sub-10-nm size range, SAXS analysis gives
comparable particle characteristics as the ones obtained by TEM or
XRD [24]. SAXS was also applied to study the spatial distribution of
metallic nanoparticles in colloidal dispersions containing poly-
meric stabilizers like PVP [25,26].
charged with 56 mg n-undecane as analytical standard. The auto-
clave was purged five times with hydrogen, and afterwards, the
reaction mixture was stirred (1100 rpm) at ambient temperature
and 5 bar hydrogen. To determine the reaction progress, samples
(
0.25 mL) were taken from the reaction mixture at regular intervals
and diluted with 0.75 mL ethanol.
Pyrrolidine derivates are valuable pharmaceutical intermedi-
ates and can be synthesized by hydrogenation of pyrroles derivates
using supported noble metals [27–29]. Until now, relationships be-
tween the structure of the active element and the catalytic perfor-
mance were not established. Therefore, in the present work,
tailored colloidal dispersed Ru nanoparticles synthesized in ethyl-
ene glycol at different conditions (pH, c(Ru), T) were employed for
hydrogenation of methyl-N-(tert-butoxycarbonyl)-pyrrole-2-car-
boxylate (butoxycarbonyl = Boc) in liquid phase at mild reaction
conditions (see Scheme 1).
SAXS data analysis is used to elucidate the influence of the syn-
thesis conditions on the particle structure and to identify parame-
ters that might influence the catalytic performance of the Ru
nanoparticles in the hydrogenation reaction. Additionally, the
influence of a pretreatment step on the catalytic activity of the
nanoparticles was studied. The obtained results could be useful
for the development of tailored catalytic material with improved
catalytic performance.
2
.4. Small-angle X-ray scattering (SAXS)
SAXS measurements were carried out with a Kratky-type
instrument (SAXSess, Anton Paar, Austria) operated at 40 kV and
0 mA in slit collimation using a two-dimensional CCD detector
T = À40 °C). The 2D scattering pattern was converted into a one-
dimensional scattering curve as a function of the magnitude of
the scattering vector q = (4 /k) sin (h/2) with SAXSQuant Software
Anton Paar). A Göbel mirror was used to convert the divergent
polychromatic X-ray beam into a collimated line-shaped beam of
Cu K radiation (k = 0.154 nm). Slit collimation of the primary
5
(
p
(
a
beam was applied in order to increase the flux and to improve
the signal quality. The liquid sample cell consisted of a quartz cap-
illary (internal diameter: 1 mm) stacked in a metal body with two
windows for the X-ray beam. Using this sample cell, an identical
volume of the solution was always irradiated. Scattering profiles
of Ru nanoparticles were obtained by subtraction of the detector
current and the scattering profile of the solvent from the scattering
profiles of the Ru nanoparticles containing solution.
2
. Experimental section
.1. Synthesis of ruthenium nanoparticles
In a typical experiment, RuCl O was dissolved in 10 mL
ÁnH
2
2
.5. ATR-IR measurements
3
2
ATR-IR spectra were acquired using a Bio-Rad FTIR spectrome-
ethylene glycol at room temperature with or without PVP (see Sup-
plementary Material). The pH was adjusted by dropwise addition of
NaOH solved in ethylene glycol (0.5 M) under stirring. Finally, the
solution was filled up to 20 mL with an appropriate amount of eth-
ylene glycol. Next, the solution was attached in a preheated oil bath
for 3 h under vigorous stirring (600 rpm) in a weak argon flow. At
the end, the dark brown solution was cooled down to room temper-
ature. For the seed-mediated growth of ruthenium particles, 2 mL of
the colloidal solution prepared at pH 8.3 and 150 °C (0.04 mmol Ru)
ter (FTS 3000MX) and a horizontal ATR accessory (Spectra Tech).
To separate the nanoparticle from the ethylene glycol bulk, the
colloidal solution was pretreated with a similar procedure as de-
scribed in Section 2.2. After this pretreatment, the nanoparticles
were again solved in ethanol and deposited on the ZnSe crystal sur-
face (5 Â 1 cm) as a thin film. After evaporation of the ethanol, the
nanoparticle deposition was repeated several times until a thin
brown film was visible on the crystal surface. The data collection
consisted of 128 scans per spectrum with a resolution of 2 cm
À1
.
were mixed with a solution of RuCl
3
ÁnH
2
O (0.04 mmol Ru) in
1
.75 mL ethylene glycol. The pH was adjusted to 8.3 with NaOH in
ethylene glycol (0.5 M), and 0.25 mL ethylene glycol was added.
The solution was stirred under an argon flow for 3 h (600 rpm) at
3. Results and discussion
1
50 °C and then cooled down to room temperature.
3.1. Ru nanoparticles prepared without PVP
2.2. Pretreatment of the ruthenium nanoparticles
3.1.1. Influence of synthesis condition on the particle structure
SAXS profiles (log I(q) vs log q, I – intensity) of Ru nanoparticles
synthesized at three different pH values but at constant tempera-
ture of 150 °C and Ru concentration of 20 mM are presented in
Fig. 1. A single scattering curve contains general information about
the particle size, shape and the surface per volume ratio of the
nanostructure [30]. To extract such parameters from the SAXS pro-
file, different methods of SAXS data analysis can be applied e.g.
Guinier analysis [31] and indirect Fourier transformation (IFT)
technique [32,33]. A thorough discussion of these methods is
provided in the Supplementary Material. The scattering profile of
A 0.5 mL of colloidal solution was mixed with 1.5 mL acetone.
After addition of 6 mL toluene, the mixture was stirred until a
brown precipitate occurred. Toluene was added to the acetone–
nanoparticle mixture to avoid irreversible particle aggregation in
the samples, which did not contain PVP as stabilizing agent. The
clear and colorless organic solvent was decanted, and the proce-
dure was repeated two times until a high viscous precipitate re-
mained. The viscous precipitate that contains the nanoparticles
and a rest of ethylene glycol was finally redispersed in 10 mL

N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
251
Fig. 1. Scattering profiles of Ru nanoparticles synthesized at different pH
T = 150 °C, c[Ru] = 20 mM, solid lines – fits using the IFT method).
(
Fig. 3. Guinier plots for Ru nanoparticles synthesized at different pH (T = 150 °C,
c(Ru) = 20 mM).
the Ru nanoparticles formed at the pH value of 8.3 differs from
those synthesized at the pH 12 and 10 as becomes evident from
a stronger decrease in the scattering intensity at lower q-values.
Scattering profiles of Ru nanoparticles formed at an initial pH of
that is represented by the maximum of the PDDF [36] again most
strongly depends on the pH value. For the pH value of 8.3, it is
0.81 nm in comparison to 0.64 nm for the pH of 10 and 0.67 nm
for the pH of 12. Guinier analysis supports this effect and provides
very similar mean radii of Ru nanoparticles of 0.87, 0.69 and
0.72 nm for the pH value of 8.3, 10 and 12, respectively. The
slightly higher values obtained from Guinier analysis might be
due to a stronger weighting of the larger particles in this method
[37].
Particle size of Ru nanoparticles obtained by SAXS data analysis is
in accordance with the result obtained by TEM (TEM images are pre-
sented in the Supplementary Material). As can be seen from the val-
ues in Table 1, differences in particle size between both techniques
were only in the sub-nanometre scale. TEM results also confirm the
narrow size distribution of the Ru nanoparticles derived earlier from
Guinier plots. If the nanoparticles prepared at pH = 8.3 were used as
seeds for formation of larger particles, then at the same pH, both the
particle size and the size distribution increase. However, increase in
size distribution (0.27 to 0.46 nm) is clearly larger than the increase
in particle diameter (1.69 to 1.99 nm).
The presented results reveal that Ru nanoparticle size is mostly
affected by the pH value of the synthesis solution and only to a
minor extent by the temperature and the Ru concentration. The
synthesis solution pH as a key factor that influences nanoparticle
size in ethylene glycol was already proposed for formation of PtRu
nanoparticles [5]. The role of the pH was explained by the forma-
tion of glycolate via oxidation of ethylene glycol, which acts as
stabilizer for the formed nanoparticles. The pH range where the
size of the nanoparticles could be varied was given between 6
and 2 [5]. Here, in agreement with literature [38], the smallest par-
ticles were obtained at pH 10.3 and 12 in strong alkaline solution
where particle formation proceeds at nearly constant pH and high
8
.3 and different temperature or Ru concentrations are shown in
Fig. 2. As can be seen, both parameters had approximately no influ-
ence on the shape of scattering profiles.
Fig. 3 shows Guinier plots from the scattering profiles of Ru
nanoparticles prepared at different pH. Similar plots were also ob-
tained for Ru nanoparticles synthesized at pH = 8.3, but at different
temperatures and Ru concentrations. Guinier analysis can be used
to estimate the particle size. Results of Guinier analysis are sum-
marized in Table 1. The linear correlation of the scattering intensity
À1
over a wide q range in the Guinier plots (0.1 < q < 3 nm ) indicates
that the Ru nanoparticles have a narrow size distribution. The
strong increase in scattering intensity at low q-values for the par-
ticles formed at the pH of 12 hints that at this high pH value beside
the main population of small nanoparticles, also a minor fraction of
Ru nanoparticles with larger particle size might be formed.
The scattering profiles of Figs. 1 and 2 were also transformed
into real space applying indirect Fourier transformation (IFT)
method using the program GIFT [34]. The result of this free-form
transformation is the pair distance distribution function (PDDF).
The PDDF represents a histogram of the distances inside the parti-
cle weighted with the electron density difference to the solvent
and contains information about the maximum particle size, shape
and internal structure of the particle in real space [35]. Fig. 4 shows
the PDDF of the Ru nanoparticles synthesized at different synthesis
conditions. The shape of the PDDF is very symmetrical indicating
that the particle shape is not far away from that of a sphere [30].
Some PDDF’s show a second maximum with very low intensity
at higher r-values, which might be due to the presence of a minor
fraction of larger Ru structures. The most probable particle radius
Fig. 2. Scattering profiles of Ru nanoparticles synthesized at pH 8.3 at different (a) temperature (c[Ru] = 20 mM) and (b) Ru concentration (T = 150 °C); Solid lines are fits
using the IFT method; in (a), the profiles were shifted by a factor of 2 for clarity.

2
52
N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
Table 1
Results of SAXS data evaluation (Guinier radius, PDDF) and TEM for Ru nanoparticles formed in ethylene glycol at different synthesis conditions.
À1
TEMa (nm)
c(Ru) (mol L
)
T (°C)
pH
8.3
R
G
(nm)
Max of PDDF (nm)
À4
À4
2
0
150
0.89 ± 6.59 Â 10
0.89 ± 4.59 Â 10
0.81
0.81
0.64
0.67
0.84
0.81
0.80
0.80
1.00
1.69 ± 0.27
1.78 ± 0.27
After pretreatment
À4
2
2
2
2
5
5
2
0
0
0
0
150
150
115
190
150
150
150
10.3
12
0.69 ± 3.2 Â 10
0.72 ± 3.3 Â 10
0.89 ± 1.6 Â 10
0.86 ± 1.6 Â 10
0.89 ± 4.0 Â 10
0.89 ± 1.6 Â 10
1.09 ± 3.8 Â 10
À4
1.25 ± 0.20
1.99 ± 0.46
À3
À3
À3
À3
À4
8.3
8.3
8.3
8.3
8.3
0
0 + 20b
a
Particle diameter.
b
Ru nanaparticles prepared at pH = 8.3, T = 150 °C and c(Ru) = 20 mM were used as seeds.
Fig. 4. PDDF of Ru nanoparticles synthesized at different (a) pH, (b) temperature and (c) Ru concentration obtained by IFT technique (see also Table 1).
glycolate concentration. At these conditions, nanoparticle forma-
tion might be dominated by fast nucleation reactions. The increas-
ing particle size at pH 8.3 indicates a larger influence of growth
reactions on particle formation compared to pH 10.3 and 12. The
low influence of temperature on the particle size of the Ru nano-
particles might be explained by the assumption that for the applied
pH (8.3), reduction of the metal precursor and nucleation already
take place at temperatures lower than 115 °C. This hypothesis is
supported by observation of the temperature range where the col-
or of the synthesis solution was changed during heating up. Color
change already begins at 90 °C and finished at 115 °C with a dark
brown color that indicates nanoparticle formation [9]. Assuming
that the main part of the metal precursor was consumed during
a short nucleation period between 90 and 115 °C, the final temper-
ature will influence the particle size only in low extent.
for catalytic experiments and analyzed by XPS using the Ru 3p3/2
binding energy. The XPS spectra (see Supplementary Material) show
a symmetrical peak at around 461 eV for both samples that indicate
that the nanoparticles prepared at pH 12 and pH 8.3 are in metallic
state even after pretreatment under ambient conditions [14].
3.1.2. Catalytic results of Ru nanoparticles without PVP stabilization
Because of the absence of steric stabilizers in the synthesis solu-
tion, the Ru nanoparticles are stabilized electrostatically by surface
adsorption of organic molecules like glycolates. The pretreatment
step was applied to reduce both the amount of ethylene glycol
and adsorbed organics as far as possible without the nanoparticles
aggregate before using them for hydrogenation reaction. With this
+
À
step, also the Na and Cl ions concentration will be minimized.
SAXS data and TEM analysis (Table 1) support the assumption that
such pretreatment step has only a minor influence on the primary
structure of the Ru nanoparticle.
The Ru nanoparticles prepared at pH 12 and 8.3 at different tem-
peratures were also subjected to XRD (see Supplementary Material).
For all samples studied, the XRD pattern shows a broad peak with a
maximum at about 42° (2Theta) [17]. This maximum is between the
maximum peak of metallic Ru in face-centred cubic (ICDD-PDF-Nr
The synthesized Ru nanoparticles were applied for hydrogena-
tion of the methyl-Boc-pyrrol-carboxylate 1 in liquid phase at
ambient conditions. In preliminary experiments, the reaction solu-
tion was investigated by NMR in order to find out whether beside
the expected product methyl-N-(tert-butoxycarbonyl)-pyrroli-
dine-2-carboxylate 2 also other products were formed. Both with
NMR and GC–MS analysis, no further products were detected,
which indicated that 2 was the only product of the hydrogenation
reaction.
[
088-2333]) and hexagonal close-packed modification (ICDD-PDF-
Nr [070-0274]). None of the different diffractograms showed XRD
peaks that can be attributed to RuO . These results are in agreement
2
with literature data reporting that crystalline Ru nanoparticles in
metallic state are prepared with the ethylene glycol method
[
15,17]. From the HRTEM of a particle prepared at pH = 8.3 and
1
50 °C (c[Ru] = 20 mM), a lattice fringes distance of 0.233 nm was
Results of hydrogenation without and with pretreatment are
presented in Fig. 5 for nanoparticles synthesized at three different
pH values. Conversion over nanoparticles prepared at pH 8.3 is
much higher than conversion over nanoparticles prepared at
pH 10 and 12 independent of whether the particles were pre-
treated or not. While pretreatment of nanoparticles prepared at
pH of 8.3 did not influence their catalytic activity, it improved
clearly the activity of nanoparticles prepared at pH 10 and 12.
Without pretreatment, conversion of 1 over these particles after
3 h was lower than 10%. With pretreatment, conversion increased
determined, which fit those of the (1 0 0) Ru crystal plane (ICDD
PDF-Nr [070-0274]). This indicates that the Ru nanoparticles are
not only in metallic state, but also single crystalline. This assump-
tion is further supported by comparable values of the mean crystal-
lite size derived from the XRD line width and the mean particle size
determined by SAXS. To obtain information about the nanoparticle
surface state, Ru nanoparticles prepared at pH 8.3 (T = 150 °C,
c[Ru] = 20 mM) and at pH 12 (T = 150 °C, c[Ru] = 20 mM) were
deposited on alumina after a similar pretreatment procedure used

N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
253
from 5 to 1 bar. Otherwise, an increasing H
2
pressure from 5 to
1
0 bar gives the same catalytic activity as obtained for the experi-
ment at 5 bar. That means that for the applied conditions, H mass
2
transfer limitations are not the reason for the observed differences
in the catalytic activity between the Ru nanoparticles prepared at
different synthesis conditions.
Concentration of hydrogen in the reaction solution can be
approximated using Henry’s law constant for absorption of hydro-
3
gen in ethanol [39] (H = 29,380 Pa m /mol). Applying the relation
H = p/C, the hydrogen concentration in the liquid phase increases
À3
À2
from 3.4 Â 10 mol/L at 1 bar to 1.7 Â 10 mol/L at 5 bar. At
bar, the H concentration in the liquid phase is in the same range
as the concentration of methyl-N-Boc-pyrrole-2-carboxylate.
5
2
Fig. 5. Conversion of methyl-N-Boc-pyrrole-2-carboxylate 1 versus reaction time
3
.2. Ru nanoparticles protected by PVP
over Ru nanoparticles prepared at different pH without (open symbols) and with
À2
À5
pretreatment (closed symbols); (c(1) = 2.5 Â 10 mol/L, c(Ru) = 5 Â 10 mol/L,
T = 25 °C, pH = 5 bar, the line are drawn for clarity).
2
3.2.1. Influence of PVP on particle structure
To study the influence of PVP on particle structure and catalytic
activity, Ru nanoparticles with different PVP to Ru ratio were syn-
thesized at two different pH. The used Ru concentration (20 mM)
and temperature (150 °C) corresponds to the synthesis method,
which revealed Ru nanoparticles with the highest hydrogenation
activity at absence of PVP. Fig. 8 shows scattering profiles of Ru
nanoparticles prepared at pH = 8.3 employing different Ru to PVP
ratios and the corresponding PDDF obtained by the IFT method.
Scattering patterns of Ru nanoparticle in the central part of the
in this time interval to 30% for particles prepared at pH 10 and to
0% for particles prepared at pH 12. The conversion over the nano-
particles prepared at pH 8.3 by using seeds is only slightly higher
than conversion over particles prepared at pH = 12.
Fig. 6 compares the temporal evolution of conversion over Ru
nanoparticles prepared at constant pH (8.3), but different temper-
atures (a, c(Ru) = 20 mM) and Ru concentrations (b, T = 150 °C)
again with and without pretreatment. It can be seen that both
parameters (temperature and Ru concentration) affect the catalytic
performance of the Ru nanoparticles in the hydrogenation reaction.
Differences in conversion of 1 between nanoparticles formed at
4
À1
scattering curve (1 < q < 3 nm ) are similar for all samples. In con-
trast to the sample without PVP, scattering curves from Ru nano-
particles protected with PVP show some increase in scattering
intensity at low q-values (Fig. 8a), which is most pronounced for
the sample with the lowest molar PVP to Ru ratio (0.5:1). The PDDF
of the PVP-protected particles differ considerably from that pre-
pared without PVP. The PDDF is symmetrical only for r-values low-
er than 2 nm and shows for larger r a longer tailing with some
oscillations. This tailing increased with decreasing PVP to Ru ratio.
The appearance of the long tailing is connected with the occur-
rence of different slope in scattering intensity between the inner-
1
50 °C and 190 °C (X after 2.5 h > 90%) are low compared to the
conversion over particles formed at 115 °C (X after 2.5 h < 70%).
Relatively, high hydrogenation activity was also obtained for nano-
particles formed with Ru concentration of 20 and 50 mM in the
precursor solution (Fig. 6b). For catalytic active Ru nanoparticles,
pretreatment again had only a relatively small influence on conver-
sion of 1 as already observed for the nanoparticles prepared at dif-
ferent pH. The largest influence of pretreatment was obtained for
the nanoparticles prepared at low Ru concentration (5 mM). Here,
conversion obtained after 3 h increases from 20% (without pre-
treatment) to 70%. Generally, there seems to be a tendency that
the influence of pretreatment is higher the lower the catalytic
activity without pretreatment.
À1
most (q < 0.5 nm ) and the central part of the scattering curve
(see Table 2 and also Guinier plots in the Supplementary material).
A large-size tailing of the PDDF without oscillations was earlier ob-
served for polymer protected PbS nanoparticles and explained with
the assumption that one polymer coats several nanoparticles [40].
For the PVP to Ru ratio of 0.5:1, the size of the larger Ru/PVP struc-
tures is about 15 nm. However, the number of polymer chains that
contain more than one Ru nanoparticles will always be low. The
symmetrical shape of the sharp PDDF peak at r < 2 nm might indi-
cate a nearly spherical shape of the particles in agreement with
previous results [16]. The maximum position of the PDDF, which
In order to exclude that the observed differences in conversion
of 1 are dominated by mass transfer of hydrogen, the experiments
with the most active Ru nanoparticles were repeated using differ-
ent H
2
pressures. Results of these experiments are shown in Fig. 7.
pressure was reduced
Conversion of 1 decreased strongly if the H
2
Fig. 6. Conversion of methyl-N-Boc-pyrrole-2-carboxylate 1 versus reaction time over Ru nanoparticles prepared at pH = 8.3 and (a) – different temperature (c[Ru] = 20 mM)
À5
À2
and (b) – Ru concentration (T = 150 °C); without (open symbols) and with pretreatment (closed symbols); (c(Ru) = 5 Â 10 mol/L, c(1) = 2.5 Â 10 mol/L, T = 25 °C,
pH = 5 bar, the lines are drawn in for clarity).
2

2
54
N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
maximum particle radius is about 5 nm. For the molar PVP to Ru
ratio of 0.5:1, the form of PDDF is, however, quite asymmetric,
and a large particle tailing can be observed. This might be due to
a broader size distribution or as already observed for the particles
prepared at pH = 8.3 due to the presence of larger Ru/PVP struc-
tures that might consists of one polymer chain that coats several
particles with the latter being more probably. The most probable
particle radius was shifted to 1.44 nm. Guinier radius from the cen-
tral part of the scattering curve is again in correspondence with the
maximum of the PDDF. There is a general tendency that the value
À1
of the Guinier radius determined at low q (<0.4 nm ) increases if
the molar PVP to Ru ratio decreases (Table 2).
The low influence of the PVP concentration on the particle size
at pH = 8.3 might be explained by a high reduction and nucleation
rates and a weak interaction between nanoparticle surface atoms
and polymer functional groups. Increasing acidity of the synthesis
solution (pH = 4.3) should lead to lower reduction and/or nucle-
ation rates. Therefore, the amount of Ru that was used for nucle-
ation will be lower, and the growth processes are involved to a
larger extent on the particle formation. As a result, particle size in-
creases. Due to the slower nanoparticle formation at pH = 4.3,
interactions between surface metal atoms and polymer functional
groups might be stronger and influence the final particle size. Pre-
viously, it was reported that with lowering PVP/Ru molar ratio, lar-
ger Ru nanoparticles were formed [12]. The presented results
obtained in acidic media (pH = 4.3) confirm this observation. Both
the pH and PVP to Ru ratio can have an influence on the spatial dis-
tribution of the nanoparticles. At low molar PVP to Ru ratios, more
than one nanoparticle might be stabilized in neighborhood to each
other inside of a single PVP polymer chain. This leads to formation
of larger Ru/PVP structures that were detected by SAXS at low q-
values. The probability that more than one Ru nanoparticle inter-
acts with a single polymer chain decreases with increasing PVP
to Ru ratio, and therefore, the formation of larger structures is sup-
pressed. This finding is also in agreement with results from litera-
ture where Taylor dispersions measurements indicate that one
polymer chain can attach to multiple nanoparticles [41]. Otherw-
ize, MALDI-TOF experiments suggest that one nanoparticle might
Fig. 7. Influence of H
2
pressure on the conversion of methyl-N-Boc-pyrrole-2-
carboxylate over Ru nanoparticles prepared at pH = 8.3, 150 °C with c(Ru) = 20 mM
À2
À5
(
c(1) = 2.5 Â 10 mol/L, c(Ru) = 5 Â 10 mol/L, T = 25 °C).
corresponds to the most probable particle radius is nearly identical
0.75–0.84 nm) for the samples (see Table 2). Guinier analysis from
(
the central part of the scattering curve revealed mean particle radii
between 0.82 and 0.85 nm (Table 2). These radii are approximately
the same as found for the sample without PVP (0.81 nm, Table 1)
using identical synthesis conditions. This indicates that the size
of the primary formed nanoparticles is only influenced marginally
by the presence of PVP. TEM results of the sample with the PVP to
Ru ratio of 0.5:1 confirm the particle size derived from SAXS data
analysis. The difference in particle size obtained by SAXS data anal-
ysis and from the corresponding TEM image was again in sub-nm
range (Table 2).
Fig. 9 presents scattering curves and the corresponding PDDF
for Ru nanoparticles prepared at pH = 4.3 with two different PVP
to Ru ratios. At such low pH, protective stabilizers like PVP have
to be present in order to avoid a high degree of irreversible particle
agglomeration and precipitation. The obtained PDDF suggests that
Ru nanoparticles with nearly spherical shape and narrow size dis-
tribution were formed when preparing them with the PVP to Ru ra-
tio of 5:1. The most probable particle radius is 1.22 nm, and the
Fig. 8. (a) Scattering profiles of Ru nanoparticles synthesized with different molar Ru to PVP ratios and (b) the corresponding PDDF (pH = 8.3, T = 150 °C, c(Ru) = 20 mM, solid
lines – fits using the IFT technique; curves in (a) were shifted for clarity).
Table 2
Results of SAXS data evaluation (Guinier analysis, PDDF) and TEM for samples containing PVP (T = 150 °C, 20 mM Ru).
À1
À1
À1
À1
TEMa (nm)
1.56 ± 0.28
pH
PVP:Ru
R
G
(nm) (0.1 nm < q < 0.4 nm
)
R
G
(nm) (1 nm < q < 3 nm
)
Max of PDDF (nm)
À3
À3
8.3
8.3
8.3
4.3
4.3
5:1
1.64 ± 4.0 Â 10
2.85 ± 1.0 Â 10
3.85 ± 4.7 Â 10
1.64 ± 4.0 Â 10
3.29 ± 2.2 Â 10
0.85 ± 1.1 Â 10
0.85 ± 8.8 Â 10
0.82 ± 7.7 Â 10
1.33 ± 6.6 Â 10
1.54 ± 5.4 Â 10
0.75
0.75
0.84
1.21
1.44
À2
À3
À3
À3
À4
À4
À4
À4
2.5:1
0.5:1
5:1
0.5:1
a
Particle diameter.

N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
255
Fig. 9. (a) Scattering profiles of Ru nanoparticles protected by PVP and synthesized at pH = 4.3 and their fits (solid lines) and (b) the corresponding PDDF (T = 150 °C,
c[Ru] = 20 mM).
be stabilized by one, two or three polymer chains depending on its
size [42].
3.3. Activity-determining properties
The XRD pattern of the PVP-protected Ru nanoparticles show
also a single broad peak at about 42°, which indicates that the par-
ticles are crystalline and in metallic state (see Supplementary
Material). The pattern are identical with those XRD pattern ob-
tained from Ru nanoparticles prepared without PVP. HRTEM image
of a single Ru nanoparticle prepared at pH 8.3 and a PVP/Ru ratio of
In Fig. 11, the initial activity of the Ru nanoparticles prepared at
different synthesis parameters is plotted versus the mean particle
radius that was determined from SAXS data analysis (maximum of
the PDDF). The activity was obtained from the conversion versus
time plots using data at low conversion of 1 (<20%). It is generally
accepted that the catalytic properties of nanoparticles are strongly
influenced by their size [1,3]. In the presented work, nanoparticles
with a mean diameter of about 1.6 nm reveal the highest hydroge-
nation activity. The TOF (number of consumed mol pyrrole per mol
ruthenium per hour) of the most active Ru nanoparticles deter-
mined from the initial hydrogenation activity amounts to about
0
.5:1 reveals a lattice fringes distance of 0.226 nm, which fit those
of the (1 1 1) Ru crystal plane (ICDD PDF-Nr [088-2333]).
3.2.2. Catalytic performance of the PVP-protected Ru nanoparticles
À1
Fig. 10 compares the conversion of 1 over Ru nanoparticles
1000 h . If the particle size decreased, the catalytic activity com-
formed without PVP with those of nanoparticles formed at differ-
ent molar PVP to Ru ratios and pH. In these experiments, the Ru
nanoparticles were always pretreated with the acetone/toluene
mixture. Comparing temporal evolution of activity for the sample
without PVP with those of the samples containing PVP, it can be
clearly seen that the conversion of 1 decreases considerably if
the nanoparticles are protected by PVP independent on their size.
Without PVP protection, 100% conversion was obtained after 2 h.
In the same time interval, the conversion of the most active PVP-
protected Ru nanoparticles prepared at pH = 4.3 with a PVP to Ru
ratio of 5:1 was only 30%. Also differences exist in catalytic activity
between samples prepared at different PVP to Ru ratios. Ru nano-
particles prepared at pH = 8.3 with the PVP to Ru ratio of 2.5:1 and
pared to the most active nanoparticles drops down approximately
by a factor of about 4. An increase in particle size also leads to a
clear decrease in activity. While the lower activity of the larger par-
ticles (mean diameter 2 nm) might be explained by geometrical
factors like lower surface area and decreasing number of surface
atoms, there is no geometrical explanation for the loss in activity
with decreasing particle size (mean diameter 1.4 nm). XRD pattern
and XPS spectra of the nanoparticles prepared at pH 12 and 8.3
indicate that the Ru nanoparticles are crystalline and in metallic
state. Therefore, it is not assumed that a different degree of nano-
particle surface oxidation is responsible for the observed differ-
ences in hydrogenation activity.
During nanoparticle synthesis, ethylene glycol is partly oxidized
to aldehydes, carboxylic acids and CO [5]. Such processes involve
2
0
.5:1 show a higher activity than the sample prepared with a ratio
of 5:1, while this dependence is reversed when using the nanopar-
ticles prepared at pH = 4.3. Here the activity is higher for nanopar-
ticles prepared with a PVP to Ru ratio of 5:1 compared to that with
a ratio of 0.5:1.
the formation of surface intermediates that might be desorbed or
removed from the metal surface so long as particle formation is
not finished and an electron transfers occurs. At the end of the
formation process, some of these intermediates will stay on the
Fig. 10. Conversion of methyl-N-Boc-pyrrole-2-carboxylate versus reaction time over Ru nanoparticles prepared with different molar Ru to PVP ratios (150 °C, c(Ru) = 20 mM
À2
À5
at (a) pH = 8.3 and (b) pH = 4.3 (c(1) = 2.5 Â 10 mol/L, c(Ru) = 5 Â 10 mol/L, T = 25 °C, pH
2
= 5 bar).

2
56
N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
ethylene glycol might decompose to different oxygen containing
surface intermediates that stay adsorbed on the nanoparticle sur-
face. Furthermore, the ATR-IR results show that the applied pre-
treatment can not remove the adsorbed surface intermediates
completely from the nanoparticle surface. The differences in the
IR band intensities might be also an indication that the number
of the different structured surface sites depends on the applied
pH. Therefore, differences in catalytic activity between the differ-
ently sized nanoparticles might be attributed to (a) differences in
strength of interaction between the adsorbed intermediates and
the surface metal atoms leading to a different number of available
surface sites and (b) differences in the structure of the available
surface site. Volcano plots for the size-depending activity of nano-
particles in the sub-2-nm range were already observed for the aer-
obic oxidation of cyclohexane over Au nanoparticles and explained
with the unique electronic structure of a certain particle size [47].
Also, there exist clear differences in catalytic performance for
particles with the same mean particle size. The particles were syn-
thesized at the identical pH and temperature (or Ru concentration),
however, with different Ru concentration (or temperature). The
differences in their activity demonstrate that particle size is only
one important factor that influences the catalytic activity of the
Ru nanoparticles in the hydrogenation reaction. Other important
factors might be the particle shape [48] and the number of unsat-
urated surface atoms. SAXS data analysis using IFT technique has
shown that the shape of the PDDF is nearly identical for all nano-
particles prepared at pH 8.3 (Fig. 4). Therefore, it is assumed that
small differences in particle shape are not the main reason for
the strong differences in hydrogenation activity. An indication of
why particles with similar size and shape show clear differences
in activity can be obtained again from the comparison of catalytic
results with and without pretreatment (Figs. 5 and 6). While pre-
treatment again has no influence on the catalytic performance of
the most active Ru nanoparticles, the activity of pristine particles
showing low performance can be increased by the pretreatment
step. ATR-IR spectra from Ru nanoparticles formed at different
Fig. 11. Initial activity of methyl-N-Boc-pyrrole-2-carboxylate hydrogenation ver-
À5
sus particle radius (SAXS) for pretreated Ru nanoparticles (c(Ru) = 5 Â 10 mol/L);
(
number denotes different synthesis conditions – 1(115 °C, 20 mM Ru), 2 – (150 °C,
2
0 mM Ru), 3 – (190 °C, 20 mM Ru), 4 – (150 °C, 5 mM Ru), 5 – (150 °C, 50 mM Ru);
pH = 8.3).
particle surface. Previously, it was reported that ethylene glycol on
Rh surfaces can be decomposed by dehydrogenation and CAC bond
breaking reactions [43]. Studies of the electrooxidation pathways
of ethylene glycol in alkaline aqueous solution on Pt electrodes
have shown that Pt converted ethylene glycol to carbonate through
sequences of chemisorbed intermediates [44]. Assuming similar
processes in strong alkaline medium on the surface of Ru during
particle formation, a larger number of active surface sites might
be blocked from relatively strong adsorbed species that can not
be removed during the applied pretreatment (Fig. 5). ATR-IR spec-
tra of Ru nanoparticles prepared at different pH and that of pure
ethylene glycol are shown in Fig. 12. Besides IR bands, which can
be assigned to residual ethylene glycol (two bands around
À1
2
900 cm ), the IR spectra of Ru nanoparticles contains three
À1
new bands located at about 2000 cm and one new band located
at about 1640 cm , which are assigned to adsorbed surface inter-
mediates. Three IR bands around 2000 cm were already observed
À1
Ru concentrations are shown in Fig. 13. In these spectra, intensity
À1
À1
differences of the three bands located at about 2000 cm
are
previously on PVP-protected Ru nanoparticles after CO preadsorp-
clearly visible. Therefore, it can be assumed that the applied syn-
thesis conditions (pH, temperature and Ru concentration) strongly
influence the Ru nanoparticle surface structure, the structure of the
formed intermediates and the strength of interaction between the
nanoparticle surface and the specific intermediate. Also, it can not
completely be excluded that differences in the degree of nanopar-
ticle surface oxidation might be involved in the observed differ-
ences in hydrogenation activity of equally sized nanoparticles.
Surface oxidation of Ru nanoparticles was often observed by XPS
À1
tion [45] and assigned to linearly bonded CO (2050 cm ), to bridg-
À1
ing carbonyls (1945 cm ) and to multiple individual CO molecules
adsorbed on low-coordinate Ru atoms (1975). The position of these
IR bands corresponds to frequencies that are attributed to CO ad-
sorbed on zerovalent Ru adsorption sites [46]. A broad IR band
À1
about 1600 cm was previously obtained after adsorption of for-
mic acid on the Ru nanoparticle surfaces and attributed to different
adsorbed formate species [45].
The nearly identical positions of the bands in Fig. 12 with that
given in literature indicate that during Ru nanoparticle formation,
[
14,16]. Presently, it is not clear whether this oxidation takes place
during nanoparticle formation or during sample preparation [16].
Fig. 12. ATR-IR spectra of pretreated Ru nanoparticles prepared at different pH
Fig. 13. ATR-IR spectra of Ru nanoparticles prepared at different Ru concentration
after pretreatment (pH = 8.3, T = 150 °C).
(T = 150 °C, c(Ru) = 20 mM).

N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
257
decrease of the catalytic activity that is probably caused by inter-
action between polymer functional groups and the nanoparticle
surface.
Acknowledgments
This work was supported by the Leibniz-Gemeinschaft. The
authors acknowledge Dr. M.-M. Pohl (TEM), Dr. M. Schneider
(
XRD) and Dr. J. Radnick (XPS) for the corresponding measure-
ments and for the helpful discussions.
Appendix A. Supplementary material
Fig. 14. ATR-IR spectra of Ru nanoparticles prepared at presence of PVP (pH = 8.3,
T = 150 °C, c(Ru) = 20 mM).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2012.02.017.
Otherwise, the results of ATR-IR give no indication of the presence
of oxidized Ru sites e.g. CO adsorbed on oxidized Ru sites shows IR
frequencies higher than 2070 cm [46].
À1
References
PVP-protected Ru nanoparticles compared with that of the most
active Ru nanoparticles show a clearly lower hydrogenation activ-
ity. The activity drop should be connected with a decreasing
amount of available active surface sites or a stronger interaction
between the PVP and the nanoparticle surface. PVP interact with
the nanoparticle surface by donor–acceptor interactions using
polymer functional groups (C@O, CAN) [7]. However, the number
of functional groups of a single polymer chain that interact with
the nanoparticle surface and the true conformation of the polymer
around the nanoparticle is presently unknown. Additionally, a
slower diffusion of substrate and product within the polymer in
which the nanoparticles are embedded might also reduce the ob-
served activity. Hydrogenation reactions with PVP-protected nano-
particles are often carried out at clearly higher hydrogen pressures
[
1] J.M. Campalo, D. Luna, R. Luque, J.M. Marinas, A.A. Romero, ChemSusChem 2
2009) 18–45.
[2] Y. Li, G.A. Somorjai, Nano Lett. 10 (2010) 2289–2295.
3] H. Bönnemann, K. S. Nagabhushana, in: B. Corain, G. Schmid, N. Toshima (Eds.),
Metal Nanoclusters in Catalysis and Material Science – The Issue of Size
Control, first ed., Elsevier B.V., Amsterdam, 2008.
(
[
[4] L.S. Ott, R.G. Finke, Coord. Chem. Rev. 251 (2007) 1075–1100.
[
5] C. Bock, C. Paquet, M. Couillard, G.A. Botton, B.R. Mac Dougall, J. Am. Chem. Soc.
26 (2004) 8028–8037.
6] J. Yang, T.C. Deivaraj, H.-P. Too, J.Y. Lee, Langmuir 20 (2004) 4241–4245.
1
[
[7] Y. Borodko, S.M. Humphrey, T.D. Tilley, H. Frei, G.A. Somorjai, J. Chem. Phys. C
11 (2007) 6288–6295.
[
1
8] R Schlögl, in: G. Ertl, H. Knözinger, F. Schüth, J. Wietkamp (Eds.), Handbook of
Heterogeneous Catalysis, second ed., Wiley-VCH, Weinheim, 2008, pp. 2501–
2575.
[
9] A. Miyazaki, L. Balint, K. Aika, Y. Nakano, J. Catal. 204 (2001) 364–371.
[
[
10] L. Balint, A. Miyazaki, K. Aika, J. Catal. 207 (2002) 66–75.
11] V.G. Komvokis, G.E. Marnellos, I.A. Vasalos, K.S. App, Catal. B 89 (2009) 627–
634.
[12] J.A. Widegren, R.G. Finke, J. Mol. Catal. A 191 (2003) 187–207.
13] G. Viau, R. Brayner, L. Poul, N. Chakroune, E. Lacaze, F. Fievet-Vincent, F. Fievet,
Chem. Mater. 15 (2003) 486–494.
14] N. Chakroune, G. Viau, S. Ammar, L. Poul, D. Veautier, M.M. Chehimi, C.
Mangeney, F. Villain, F. Fievet, Langmuir 21 (2005) 6788–6796.
15] S.H. Joo, J.Y. Park, J.R. Renzas, D.R. Butcher, W. Huang, G.A. Somorjai, Nano Lett.
[
19]. ATR-IR spectra from PVP-protected Ru nanoparticles (Fig. 14)
À1
shows a intense IR band about 1657 cm , which is assigned to
[
[
[
[
C@O stretching vibration mode of PVP. Furthermore, IR bands of
À1
surface intermediates at about 2000 cm were also detected with
low intensity, which are not visible in the IR spectra of pure PVP
and ethylene glycol. The low intensity of these bands is attributed
to the presence of PVP, which lower the mass of nanoparticles on
10 (2010) 2709–2713.
16] M. Zawadzki, J. Okal, Mater. Res. Bull. 43 (2008) 3111–3121.
the ATR crystal. The differences in the IR band intensities around
[17] X. Yan, H. Liu, K.Y. Liew, J. Mater. Chem. 11 (2001) 3387–3391.
[18] J. Okal, Catal. Commun. 11 (2010) 508–512.
À1
2
000 cm indicate different surface structures also on PVP-pro-
[
[
19] F. Lu, J. Liu, J. Xu, J. Mol. Catal. A 271 (2007) 6–13.
20] M. Liu, W. Yu, H. Liu, J. Mol. Catal. A 138 (1999) 295–303.
tected nanoparticles if the preparation conditions are changed.
This might be also a reason for the observed difference in hydroge-
nation activity between the Ru particles protected by PVP, but dif-
ferent PVP to Ru ratios.
[21] B. Abecassis, F. Testard, O. Spalla, P. Barboux, Nano Lett. 7 (2007) 1723–1727.
[
[
[
[
[
22] H. Tanaka, S. Koizumi, T. Hashimoto, H. Itoh, M. Satoh, K. Naka, Y. Chujo,
Macromolecules 40 (2007) 4327–4337.
23] G.-W. Lee, K.S. Jin, J. Kim, J.-S. Bae, J.H. Yeum, M. Ree, W. Oh, Appl. Phys. A A91
(
2008) 657–661.
24] H. Borchert, E.V. Shevchenko, A. Robert, I. Mekis, A. Kornowski, G. Grübel, H.
Weller, Langmuir 21 (2005) 1931–1936.
25] T. Hashimoto, K. Saijo, M. Harada, N. Toshima, J. Chem. Phys. 109 (1998) 5627–
4
. Conclusions
5
638.
26] J.-M. Lin, T.-L. Lin, U.-S. Jeng, Y.-J. Zhong, C.T. Yeh, T.-Y. Chen, J. Appl. Cryst. 40
2007) 540–543.
[27] L. Hegedus, T. Mathe, A. Tungler, Appl. Catal. A 143 (1996) 309–316.
Small-angle X-ray scattering is a technique that can be used to
characterize the structure of catalytic active Ru nanoparticle pre-
pared in ethylene glycol in the sub-10-nm range. Results of SAXS
data analysis can be correlated with catalytic data to identify key
parameters that are relevant for high catalytic activity of the nano-
particles in liquid phase hydrogenation at mild reaction conditions.
The obtained results point out that particle size is only one impor-
tant factor that influences the catalytic activity of Ru nanoparticle
in hydrogenation reaction. It is believed that differences in cata-
lytic activity between similarly sized nanoparticles are caused by
different surface structure and by blocking of active sites though
adsorbed surface intermediates that are formed during nanoparti-
cle preparation. The surface structure of Ru nanoparticles and
number and bonding strength of the surface intermediates depend
on the applied synthesis parameters like pH, T and Ru salt concen-
tration. The protection of the nanoparticles by PVP leads to a clear
(
[
[
[
28] H.-P. Kaiser, J.M. Muchowski, J. Org. Chem. 49 (1984) 4203–4209.
29] C. Jiang, A. Frontier, J. Org. Lett. 9 (2007) 4939–4942.
30] O. Glatter, K. Kratky, Small Angle X-ray Scattering, Academic Press, New York,
1982.
[
31] A. Guinier, G. Fournet, Small-Angle Scattering of X-Rays, Wiley, New York,
1955.
[
32] O. Glatter, J. Appl. Crystallogr. 10 (1977) 415–421.
[33] PCG Software Package, Version 2.02.05, University Graz.
[34] G. Fritz, O. Glatter, J. Phys.: Condens. Matter 18 (2006) S2403–S2419.
[
[
35] O. Glatter, J. Appl. Crystallogr. 12 (1979) 166–175.
36] M.J. Hollamby, J. Eastoe, A. Chemelli, O. Glatter, S. Rogers, R.K. Heenan,
Langmuir 26 (2009) 6989–6994.
[37] D.R. Vollet, D.A. Donatti, A.I. Ruiz, J. Non-Cryst. Solids 306 (2002) 11–16.
[
[
[
38] Y. Wang, J. Ren, K. Deng, L. Gui, Y. Tang, Chem. Mater. 12 (2000) 1622–1627.
39] P. Lühring, A. Schumpe, J. Chem. Eng. Data 34 (1989) 250–252.
40] E. Koupanou, S. Ahualli, O. Glatter, A. Delgado, F. Krumeich, E. Leontidis,
Langmuir 26 (2010) 16909–16920.

2
58
N. Steinfeldt et al. / Journal of Catalysis 289 (2012) 249–258
[
41] N. Toshima, Y. Shiraishi, T. Teranishi, M. Miyake, T. Tominaga, H. Watanabe, W.
Brijoux, H. Bönnemann, G. Schmid, Appl. Organometal. Chem. 15 (2001) 178–
[44] S.-C. Chang, Y. Ho, M.J. Weaver, J. Am. Chem. Soc. 113 (1991) 9506–9513.
[45] K. Tedsree, A.T.S. Kong, S.C. Tsang, Angew. Chem. Int. Ed. 48 (2009)
1443–1446.
1
96.
42] J.K. Navin, M.E. Grass, G.A. Somorjai, A.L. Marsh, Anal. Chem. 81 (2009) 6295–
299.
43] M.M.M. Jansen, B.E. Nieuwenhuys, H. Niemantsverdriet, ChemSusChem 2
2009) 883–886.
[
[
[46] S.Y. Chin, C.T. Williams, M.D. Amiridis, J. Phys. Chem.
871–882.
B 110 (2006)
6
[47] Y. Liu, H. Tsunoyama, T. Akita, S. Xie, T. Tsukuda, ACS Catal. 1 (2011) 2–6.
[48] R. Narayanan, M.A. El-Sayed, Langmuir 21 (2005) 2017–2033.
(