Silica-Supported Au–Ag Catalysts for the Selective Hydrogenation of Butadiene

Article abstract of DOI:10.1002/cctc.201700127

Gold and silver are miscible over the entire composition range, and form an attractive combination for fundamental studies on bimetallic catalysts. Au–Ag catalysts have shown synergistic effects for different oxidation and liquid-phase hydrogenation reactions, but have rarely been studied for gas-phase hydrogenation. In this study 3 nm particles of Au, Ag and Au–Ag supported on silica (SBA-15) were investigated as catalysts for selective hydrogenation of butadiene in an excess of propene. The Au catalyst was over an order of magnitude more active than the Ag catalyst at 120 °C. The initial activity of the Au–Ag catalysts scaled linearly with the Au-content, suggesting a direct correlation between the surface and overall compositions of the nanoparticles and the absence of synergistic effects. All Au-containing catalysts were highly selective to butenes (>99.9 %). The Au catalysts were stable, whereas the Au–Ag catalysts lost about half of their activity during 20 h run time at 200 °C, but the initial activity was restored by a consecutive oxidation-reduction treatment. Near ambient pressure x-ray photoelectron spectroscopy showed that exposure to H₂ at elevated temperatures led to a gradual enrichment of the surface of the Au–Ag nanoparticles by Ag. These observations highlight the importance of considering progressive atomic rearrangements in bimetallic nanocatalysts under reaction conditions.

Full text of DOI:10.1002/cctc.201700127

DOI: 10.1002/cctc.201700127
Full Papers
Silica-Supported Au–Ag Catalysts for the Selective
Hydrogenation of Butadiene
[
a]
[b]
[b]
[c]
Nazila Masoud, Laurent Delannoy, Christophe Calers, Jean-Jacques Gallet,
[c]
[a]
[b]
[a]
Fabrice Bournel, Krijn P. de Jong, Catherine Louis, and Petra E. de Jongh*
Gold and silver are miscible over the entire composition range,
and form an attractive combination for fundamental studies
on bimetallic catalysts. Au–Ag catalysts have shown synergistic
effects for different oxidation and liquid-phase hydrogenation
reactions, but have rarely been studied for gas-phase hydroge-
nation. In this study 3 nm particles of Au, Ag and Au–Ag sup-
ported on silica (SBA-15) were investigated as catalysts for se-
lective hydrogenation of butadiene in an excess of propene.
The Au catalyst was over an order of magnitude more active
than the Ag catalyst at 1208C. The initial activity of the Au–Ag
catalysts scaled linearly with the Au-content, suggest-
ing a direct correlation between the surface and overall
compositions of the nanoparticles and the absence of synergis-
tic effects. All Au-containing catalysts were highly selective to
butenes (>99.9%). The Au catalysts were stable, whereas the
Au–Ag catalysts lost about half of their activity during 20 h run
time at 2008C, but the initial activity was restored by a consec-
utive oxidation-reduction treatment. Near ambient pressure x-
ray photoelectron spectroscopy showed that exposure to H at
2
elevated temperatures led to a gradual enrichment of the sur-
face of the Au–Ag nanoparticles by Ag. These observations
highlight the importance of considering progressive atomic re-
arrangements in bimetallic nanocatalysts under reaction
conditions.
Introduction
Supported Au catalysts have been studied for a wide range of
oxidation and hydrogenation reactions since the mid-80’s
when Haruta discovered the high activity of Au nanoparticles
contain polyunsaturated impurities, such as acetylene, pro-
pyne, or butadiene. These impurities can poison polymeri-
zation catalysts, and thus must be selectively hydrogenated to
[
1]
in CO oxidation. Gold selectively catalyzes the hydrogenation
alkenes with residual concentrations lower than 10 ppm with-
[
2]
[3]
[5,6]
of unsaturated aldehydes, esters, acetylene in the presence
out over-hydrogenation to alkanes.
In industry, palladium-
[
4]
of an excess of ethylene, as well as butadiene in the presence
based catalysts are used to reduce the butadiene concentra-
tion from 1% to 100 ppm or less in a C4 alkene gas stream
[
5]
of an excess of alkenes. A potential application is the removal
of polyunsaturated (alkynes and alkadienes) impurities from
alkene feedstocks for polyolefin production. Alkenes streams
that are produced from the cracking of naphtha and gas oil
[7]
through selective hydrogenation.
The development of
a more selective catalyst, that reaches full conversion of buta-
diene without conversion of any alkenes, is desirable.
Gold catalysts are very selective for this reaction, but far less
[5]
active than Pd catalysts. For example, in the hydrogenation
[
a] N. Masoud, Prof. Dr. K. P. de Jong, Prof. Dr. P. E. de Jongh
Inorganic Chemistry and Catalysis
Debye Institute for Nanomaterials Science
Utrecht University
Universiteitweg 99, 3584 CG Utrecht (The Netherlands)
E-mail: P.E.deJongh@uu.nl
of acetylene at 308C, the activity of Au is about two orders of
[5]
magnitude lower than that of Pd. To increase the activity of
the catalyst for hydrogenation reactions, Au has been alloyed
[8]
[9]
[10]
with other metals, such as Pd, Pt, and Ni, but this led to
lower selectivities than for Au alone. For instance, for the hy-
drogenation of butadiene with an excess of propene, it is re-
ported that at the lowest temperature that yields full conver-
sion of butadiene, the propene conversion to propane is
[b] Dr. L. Delannoy, C. Calers, Dr. C. Louis
Laboratoire de Rꢀactivitꢀ de Surface
Sorbonne Universitꢀs, UPMC Univ Paris 06, UMR CNRS 7197
4
Place Jussieu, Case 178, F-75252, Paris (France)
[
c] Dr. J.-J. Gallet, Dr. F. Bournel
Laboratoire de Chimie Physique-Matiꢁre et Rayonnement
Sorbonne Universitꢀs, UPMC Univ Paris 06, CNRS
0
.03% if catalyzed by Au, whereas it is 0.3% if it is catalyzed
[11]
by Au-Pd with Au to Pd atomic ratio of 20.
Cu as well as Au-Cu nanoparticles have shown catalytic ac-
11 rue Pierre et Marie Curie, 75005 Paris (France)
[12]
and, Synchrotron-Soleil
tivity and selectivity for the hydrogenation of butadiene.
L’orme des Merisiers, Saint Aubin—BP48 91192,
Gif-sur-Yvette Cedex (France)
Silver catalysts selectively catalyze different hydrogenation re-
[13]
actions such as the hydrogenation of unsaturated aldehydes,
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cctc.201700428.
[14]
[15]
for instance, acrolein, crotonaldehyde, or citral, and hydro-
[
16]
[17]
genation of unsaturated esters, nitroaromatics, and acety-
[18]
lenic compounds. Grꢀnert et al. reported that for the hydro-
genation of unsaturated aldehydes, the only monometallic
This manuscript is part of a Special Issue on the “French Conference on
Catalysis”.
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[
19]
metals selective to allyl alcohol products are Ag and Au. Ac-
cording to a computational study by Nørskov et al., Ag is more
interested particularly in the stability of the different catalysts.
Furthermore near-ambient pressure x-ray photoelectron spec-
troscopy is used as an important tool to detect changes in the
atomic distribution of Au and Ag over the nanoparticles. We
will show that there is a correlation between surface composi-
tion and the activity and stability for these bimetallic Au–Ag
catalysts.
[
20]
active than Au in H dissociation, and Cu is more active than
2
Ag. Copper catalysts are indeed more active than Au catalysts
[
12,21]
in alkyne hydrogenation.
Au–Ag catalysts have been extensively studied for oxidation
[
22]
reactions, but have been less explored for hydrogenation re-
actions. The main studies on hydrogenation focus on liquid-
phase rather than gas-phase catalysis. For instance Au–Ag cat-
alysts showed much higher activity than monometallic Au and Results and Discussion
Ag catalysts for the selective hydrogenation of dimethyl oxa-
Structural characterization
late to methyl glycolate and ethylene glycol in the liquid-
[
16]
phase. The Au–Ag catalysts also showed activity for the se-
lective hydrogenation of acetylene in the presence of excess
Structural properties of the catalysts are summarized in
Table 1. Transmission electron microscopy (TEM) images
[
23]
ethylene. To the best of our knowledge, the catalytic proper-
ties of Au–Ag catalyst for the selective hydrogenation of buta-
diene have not been studied yet.
Table 1. Structural properties of the silica supported Au, Ag, and Au–Ag
catalysts.
Apart from activity and selectivity, catalyst stability is a crucial
[
24]
factor for industrial applications.
For bimetallic catalysts,
Catalyst Particle size
Au loading
[wt%]
Ag loading
[wt%]
Atomic Au:Ag
ratio
[
a]
[
nm]
many examples in the literature show higher stability than
[
25]
their monometallic equivalents. Zanella et al. demonstrated
Au
2.6Æ0.5
3.7
2.6
2.1
0
0
1:0
3:1
1.7:1
0:1
[
22a]
[26]
Au3Ag1 2.9Æ0.6
Au2Ag1 3.1Æ0.8
0.5
0.7
1.8
that titania supported AuÀAg
and AuÀCu catalysts show
a higher stability than Au catalysts in CO oxidation reactions.
Common deactivation mechanisms are particle growth, coke
Ag
2.9Æ0.8
[
7b]
i i i i
[a] Number-averaged diameter calculated as ꢀnd/ꢀn, in which d is the
individual particle diameter.
deposition, and catalyst leaching or poisoning. For bimetallic
catalysts, redistribution of atoms within the nanoparticles
owing to reaction conditions (elevated temperature, oxidative
or reductive atmosphere) can in addition play an important
role. For example, Fe segregates to the surface of FeÀPt, FeÀ (Figure 1a–d) show that particles with an average size of 2–
[
27]
Rh, and FeÀAu under O atmosphere at 2508C,
and Cu
4 nm are present inside the 8 nm pores of the ordered meso-
porous silica. Inductively coupled plasma-mass spectrometry
(ICP-MS) confirmed the intended Au and Ag loadings. Nitrogen
physisorption isotherms (Figure S.1) showed a loss of support
porosity upon Au deposition, but no significant change in the
pore size of the silica. Thermogravimetric analysis (TGA)
2
moves to the surface of CoÀCu if it is exposed to H and to
2
[
28]
the core if it is exposed to CO at 3708C.
Au–Ag nanoparticles are a very interesting model system
owing to similarity of Au and Ag in lattice spacing and hence
miscibility of these two metals over the entire composition
range. Ag has a lower surface tension and has a higher affinity
for oxygen than Au, hence in Au–Ag systems, Ag atoms tend
[22a,23a]
to segregate to the surface under oxidative conditions.
However, the surface composition of bimetallic Au–Ag nano-
particles under H atmosphere has not been reported yet.
2
In this paper, we discuss the selectivity, activity, and the sta-
bility of the Au–Ag as well as Au and Ag catalysts for the selec-
tive hydrogenation of butadiene in a gas stream containing an
excess of propene. As a support we chose an ordered mesopo-
[
29]
rous silica (SBA-15) with uniform hexagonally arranged pores
and a narrow pore size distribution. It has a high specific sur-
face area, facilitating the preparation of small particles and an
even distribution of metal particles over the support. Surface
functionalization of the silica support with aminopropyl groups
facilitates the preparation of small Au and Au–Ag nanoparticles
and a homogenous distribution over the support. The activities
of Au, Ag and Au–Ag catalysts with different compositions but
with similar, all 3 nm, particle sizes in gas-phase hydrogenation
are compared. An interesting question is how active Ag is for
this type of reaction, and whether synergistic effects, resulting
from either electronic or ensemble effects, on the activity are
observed in the case of the bimetallic catalysts. We are also
Figure 1. Bright field TEM images of the Au (a), Au3Ag1 (b), and Au2Ag1 (c)
catalysts, and high angle annular dark-field-scanning TEM images of the Ag
catalyst (d).
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(Figure S.2) showed that the removal of aminopropyl groups
started at 2508C, and that all aminopropyl had been removed
after calcination for 4 hours at 4508C. Hence, these 3 nm Au,
Ag and Au–Ag particles on SiO with 3 wt.% metal loading
2
and well-defined composition range form the basis for the
studies presented in this paper.
Ultraviolet-visible (UV/Vis) absorption spectra (Figure 2)
show clear surface plasmon resonance absorption for all sam-
ples after calcination. For the Ag nanoparticles the maximum
absorption occurs at 390 nm, whereas it occurs at 510 nm for
Figure 3. Butadiene conversions for the in situ reduced Au, Ag, Au3Ag1, and
Au2Ag1 catalysts for the hydrogenation of butadiene while heating from 50
À1
to 3008C with 18Cmin . Reaction mixture: 0.3% butadiene, 30% propene,
À1
2
0% H
2
, and He for balance at atmospheric pressure, flow rate 50 mLmin
,
À1
GHSV 11700 h
.
Table 2. Activities and Turnover Frequencies (TOF) of the Au, Ag, and
Au–Ag catalysts at 1208C for the hydrogenation of butadiene.
[
a]
[b]
[c]
Particle sizes Activity
TOF
[10
E
act
lnA
À1 AuÀ1
À13 À1
À1
[
nm]
[mmols
g
]
s ] [kJmol ]
Au
3.0
8.7
7.7
6.1
4.4
5.4
5.6
49.4Æ0.7 17.3Æ0.2
52.3Æ1.2 17.9Æ0.4
55.7Æ1.5 19.1Æ0.5
22.5Æ4.2 6.8Æ0.2
Figure 2. Ultraviolet-visible absorption spectra of the Au, Au3Ag1, Au2Ag1,
and Ag catalysts.
Au3Ag1 3.1
Au2Ag1 3.4
Ag
[
d]
[d]
3.4
0.6
0.2
3
2
the supported Au nanoparticles. The maximum absorption for
the Au–Ag catalyst lies in between these two values; at
[a] Surface-averaged, calculated as ꢀnd /ꢀnd , in which d is the particle
i
i
i
i
i
diameter, [b] Calculated based on Au surface atoms obtained from sur-
face-averaged particle size, considering that the fraction of Au surface
atoms corresponds to the overall Au fraction, [c] A is the pre-exponential
factor, [d] Based on Ag content.
500 nm and 485 nm for the Au3Ag1 and Au2Ag1 catalysts, re-
spectively. No remaining peaks at the Au or Ag absorption
wavelength are observed for the bimetallic samples suggesting
[
30]
alloy formation for the Au–Ag nanoparticles.
[31]
reduced in situ, is in good agreement with literature. All re-
sults discussed from now on are on in situ reduced catalysts
unless stated otherwise.
Activity
The butadiene conversions in the temperature range of 50–
008C for the in situ reduced Au, Ag and bimetallic Au–Ag cat-
alysts are shown in Figure 3. An overview of the activities at
208C and apparent activation energies for these catalysts is
The Ag catalyst starts to hydrogenate butadiene at 1208C
and barely reaches 15% conversion of butadiene at 3008C. At
1208C, the Ag catalyst is an order of magnitude less active
than the Au catalyst. The Ag catalysts are reported to have
comparable or even higher activity than Au catalysts in liquid-
3
1
given in Table 2. The Au catalyst starts to hydrogenate buta-
diene at 608C and fully converts the butadiene at 2258C and
above. The in situ reduced Au catalyst is slightly more active
than the not reduced catalyst (Figure S.3a): the in situ reduc-
tion increases the activity of the catalyst at 1208C from 6.5 to
[16,18,33]
phase hydrogenation,
however, for gas-phase hydroge-
nation of m-dinitrobenzene, a carbon-supported Ag catalyst is
reported to be five times less active than a similar Au cata-
[34]
lyst. The activation energy for the gas-phase hydrogenation
À1
À1
À1
8
.7 mmols gAu . The apparent activation energy for in situ re-
of butadiene is 22.5 kJmol for the Ag catalyst (Table 2 and
duced Au catalyst for butadiene hydrogenation is 49.4Æ
Figure S.4). It is slightly different from the reported value of
À1
À1
0
.7 kJmol , as obtained from the Arrhenius plot shown in Fig-
38.4 kJmol (for this reaction but performed in different con-
ure S.4. This value is similar to the reported value of 50Æ
ditions and determined in the temperature range of 50–
À1
[13b]
2
kJmol
for Au/TiO2 prepared by deposition-precipitation
110 8C),
but less than half of that for the Au catalyst. This is
[
31]
with urea suggesting that the apparent activation energy
does not strongly depend on the support, in agreement with
in line with reports showing that Ag catalysts are in principle
[
20]
more active than Au in H dissociation, which is often the
2
[
5]
[31]
the literature. The turnover frequency (TOF) at 1508C (9.9ꢂ
rate limiting step in hydrogenation reactions for Au.
The
À3 À1
10
s ) is also in good agreement with the value reported by
lower activity of Ag catalyst, despite its low activation energy,
can be ascribed to the pre-exponential factor in the Arrhenius
model, which is three orders of magnitude lower than that for
Au (Table 2). In other words, apparently it is the low probability
Haruta et al. for the hydrogenation of butadiene at 1508C (6ꢂ
À3 À1
1
0
s , Au on silica catalyst prepared by gas-phase grafting,
[
32]
7
.0Æ3.0 nm).
Hence, the activity of our Au catalyst, if
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of the reactants to be adsorbed on relevant sites on the Ag
nanoparticles surface which causes the low conversion in this
gas-phase reaction, although this is less of a limiting factor for
reported selectivities for Au catalysts for the hydrogenation of
[31,39]
butadiene.
The butane concentration is below the detec-
tion limit for all the catalysts. To compare the selectivity of the
Au catalyst to that of the bimetallic Au–Ag catalysts, the pro-
pene conversions versus butadiene conversion were extracted
from the Figure S.5 and are plotted in Figure 4. For the Au cat-
alyst at full conversion of butadiene, the propene conversion is
only 0.1% indicating that the Au catalyst is very selective to-
wards hydrogenation of butadiene.
(low temperature) liquid-phase hydrogenation reactions. This
might be caused by inherently small heat of adsorption of the
butadiene on the Ag surface at these temperatures, or by com-
petitive adsorption of intermediates, products or other species
present in the reaction mixture. Ag is active for hydrogenation
reactions that involved oxygen containing substrates such as
[
14]
crotonaldehyde can support this hypothesis, as high affinity
of Ag for oxygen might lead to a higher heat of adsorption of
the substrates and therefore a higher activity.
The Au–Ag bimetallic catalysts show activities intermediate
between that of Au and that of Ag (Figure 3). Notably, at
1208C, the apparent activation energy (Table 2), deduced from
the corresponding Arrhenius plots (Figure S.4), is very similar
for all Au-containing catalysts. This strongly suggests, as might
be expected given the much lower activity of Ag, that the re-
action on the Au surface atoms dominates. Table 2 shows the
TOF of the catalysts based on the assumptions that only Au
surface atoms are active (because the activity of Ag was negli-
gible), and that the fraction of Au atoms of the surface is the
same as the overall Au fraction, assuming that an alloy had
formed. It seems a valid assumption, as we in situ reduced all
the catalysts at 4508C, and Au–Ag alloy formation upon high
temperature reduction has been already reported by Mou
Figure 4. Propene conversions at different butadiene conversions for Au,
Au2Ag1, and Au3Ag1 catalysts. Reaction mixture: 0.3% butadiene, 30%
À1
propene, 20% H , and He for balance, flow rate 50 mLmin
2
.
[
35]
et al. The TOF per Au surface atom is constant and inde-
pendent of the metal nanoparticle composition: there is appa-
rently no significant influence owing to nearby Ag atoms on
the activity of the individual Au surface atoms. Synergistic
effect for Au–Ag catalysts have been previously reported
As the activity of the Ag catalyst is low, measuring the selec-
tivity accurately is difficult (Figure S.5d). Concerning the bimet-
allic catalysts, for the Au3Ag1 catalyst at full conversion of bu-
tadiene, propene conversion is also close to 0.1%. Hence the
bimetallic Au–Ag catalysts display high selectivities at 100%
butadiene conversion similar to the Au catalyst, and the pres-
ence of Ag has no influence on the selectivity of the Au atoms
at the surface.
[
22a,b,36]
mainly for oxidation reactions.
For instance, Zheng et al.
reported synergistic effects for Au–Ag catalysts with atomic Au
[36b]
to Ag ratios of 4 in the dehydrogenation of benzyl alcohol.
For the selective hydrogenation of acetylene, Liu et al. report-
ed slightly higher activities for the Au–Ag catalysts than for the
Au catalysts. The catalysts were prepared by the same method
that is used in the present study, but they obtained larger Au
nanoparticles (4.9 nm). The higher activity of Au–Ag catalyst
might be attributed to its smaller particle size (2.9 nm) rather
Stability
The catalyst stability was assessed by monitoring the buta-
diene conversion at 2008C over 21 hours (Figure 5). The Au
catalyst shows 96% initial conversion and no significant de-
crease in conversion over 21 hours. We previously reported the
high stability of Au supported on silica for the selective hydro-
[
23b]
than to AuÀAg synergism.
For liquid-phase hydrogenation
reaction, Qiang Xu et al. reported a synergistic effect for Au–
[37]
Ag core–shell structure for reduction of p-nitrophenol and
showed that the synergistic effect is getting weaker if the Au
and Ag formed alloy. However, Yang et al reported the syner-
gistic effect for alloyed Au–Ag catalysts for the hydrogenation
[40]
genation of butadiene. In contrast, the Au–Ag catalysts grad-
ually loose activity. The Au3Ag1 catalyst initially converts 77%
of the butadiene, and only 47% after 21 hours on stream. The
main activity loss occurs within the first 5 hours. The Au2Ag1
catalyst shows a very similar profile.
[
38]
of crotonaldehyde. Thus, it appears that for gas-phase buta-
diene hydrogenation reaction there is no evidence of a syner-
gistic effect for Au–Ag catalysts.
Deactivation by deposition of carbonaceous species is a first
possibility and the main cause of deactivation for Pd catalysts
used for the selective hydrogenation of butadiene in indus-
Selectivity
[5,41]
Figure S.5 shows the evolution of the concentrations of all the
reactants and products for the catalysts in the temperature
range of 50–3008C. The main products of the hydrogenation
of butadiene in decreasing concentration are 1-butene,
cis-2-butene, and trans-2-butene. This is consistent with the
try.
Figure 6 compares the weight loss monitored during
TGA for the Au and Au3Ag1 catalysts, before and after 20
hours of catalysis. Although some carbonaceous species are
formed during reaction, the amounts deposited for both
catalysts seem very similar and hence the deposition of
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500 eV the measurements preferentially probe the surface, de-
spite an Au–Ag particle size of only 3.4 nm.
Photo-emission spectra of the Au2Ag1 catalyst at photon
energies of 700 and 500 eV are shown in Figure S.7. Quantifica-
tion of the areas of the Au 4 f and the Ag 3d peaks is given in
Table 3 (more detailed information in Table S.1). The higher Ag/
Au ratio at photon energy of 500 eV than the one at 700 eV
Table 3. Quantification on the Au 4 f and Ag 3d peaks obtained from the
photo-emission spectra of the Au2Ag1 catalyst.
Photon energy (eV)
00
500
500
500
500
Condition
T [8C]
Ag/Au ratio
7
UHV
UHV
100 Pa
100 Pa
100 Pa
RT
RT
RT
135
210
1.5
2.4
2.2
2.4
3.4
Figure 5. Evolution of the butadiene conversion for the Au (45 mg) and Au–
Ag catalysts: Au3Ag1 (68 mg) and Au2Ag1 (75 mg) at 2008C. Reaction mix-
ture: 0.3% butadiene, 30% propene, 20% H
2
, and He for balance, flow rate
À1
50 mLmin
.
carbonaceous species can likely not explain the difference in
stability. A second common reason for catalyst deactivation is
metal particle growth. Crystallite sizes of the Au3Ag1 catalyst
before and after catalysis are the same (3.8 and 3.9 nm, respec-
tively). Furthermore, the initial activity of the Au–Ag catalysts
was regenerated by in situ oxidation and reduction of the
spent catalyst (at 4508C, Figure S.6). As metal particle growth
is in general not reversible, it cannot be considered as the
reason for the loss of activity.
suggests that at room temperature and under vacuum, Ag
preferentially resides already near the surface. This might be
explained by the lower tabulated vacuum surface energy of Ag
[44]
in comparison to Au. Subsequently, the samples were ex-
posed to 100 Pa of H and gradually heated. The spectra show
2
(Figure S.7 and Table 3) no effect of the H exposure at room
2
temperature or 1358C, but upon heating to 2108C the fraction
of Ag atoms near the particle surface increases. As these meas-
urements are very time consuming, it was not possible to run
similar heating experiments under UHV to see if the Ag surface
enrichment also occurs in vacuum. However, Au and Ag are
miscible at all temperatures, and at higher temperature, entro-
py effect normally leads to alloy formation rather than the
phase segregation, it can safely be assumed that Ag surface
Specifically for bimetallic catalysts, an additional reason for
deactivation can be the rearrangement of the metal atoms
within the nanoparticles. Notably, the time scale of this rear-
rangement for particles of 3–4 nm at 2008C, using Fick’s first
law of diffusion and the bulk diffusion coefficient of Au metal
À22
2
À1 [42]
of 10 m s , corresponds to the deactivation time scale
which is around 5 h for both Au–Ag catalysts. The surface com-
position of the Au–Ag catalysts was assessed using near ambi-
enrichment is a result of H exposure and not just owing to
2
heating.
[35,45]
ent pressure (NAP) XPS under vacuum and H atmosphere at
It is known from literature,
that heating in an oxidizing
2
photon energies of 500 and 700 eV. This experiment could not
be performed in the presence of butadiene because hydrocar-
bons decompose in the beam, resulting in carbon contamina-
tion. The inelastic mean free path of both Au and Ag is short
atmosphere leads to Ag surface enrichment, and high temper-
ature reduction reverses it to form an Au–Ag alloy. However,
this is the first time that it is shown that at intermediate tem-
peratures H exposure induces Ag atoms segregation to the
2
[
43]
(0.7 nm at 500 eV and 1.0 nm at 700 eV). Hence, especially at
surface of Au–Ag nanoparticles. The driving force for such
À1
À1
Figure 6. Mass loss of the Au3Ag1 (left) and Au (right) samples during heating with 58Cmin under a 10 mLmin flow of oxygen.
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segregation can be a stronger AgÀH bonding than the AuÀH
bonding suggested by Hammer et al. Notably, in the NAP-
static air overnight. Then the precipitate was calcined at 5508C in
static air for 6 h to yield SBA-15.
[
20]
XPS experiments, H pressure is only 100 Pa which is far less
2
The silica support was functionalized using aminopropyl triethoxy-
[46]
than the 20 kPa partial pressure of H under reaction condi-
silane (APTES).
First, the support was dried at 1408C under
2
tions, under which the driving force for Ag segregation is
probably much higher. Hence one can assume that under reac-
tion conditions at 2008C a gradual enrichment of the surface
of these bimetallic particles by Ag also occurs. As Ag is less
active for the hydrogenation of butadiene than Au, this atomic
redistribution can explain why the bimetallic catalysts gradual-
ly deactivate whereas the monometallic Au catalyst remains
stable. The catalytic activity of the Au–Ag catalysts is revived
by high temperature oxidation/reduction (Figure S.6) and is re-
lated to reformation of the AuÀAg alloy, and hence confirms
the rearrangement of the metal atoms within the nanoparticles
as a major cause of bimetallic catalyst deactivation.
vacuum for 24 h. Then, dry toluene (50 mL) and APTES (1 g) were
added. The amount of APTES needed for covering the support sur-
face was calculated based on the BET surface area of the SBA-15,
2
[47]
considering three OH groups per nm for silica. The mixture was
refluxed for 24 h at 1108C in an N atmosphere. The functionalized
2
silica was recovered by centrifugation, washed with ethanol
(40 mL) twice at RT, and dried at 608C in static air
overnight.
[35a]
The metals were deposited following the method of Mou et al.
The functionalized silica (1 g) was dispersed in water (15 mL, dou-
bled distilled). To prepare the monometallic Au or Ag catalyst,
either a HAuCl .3H O aqueous solution (0.05m, 3 mL) or an AgNO
3
4
2
aqueous solution (0.05m, 3 mL) was added, and the mixture was
stirred at RT for 2 h. The powder was recovered by centrifugation
and washed twice with water at RT. Then, the powder was redis-
persed in water (15 mL), and the metal ions reduced by a rapid ad-
Conclusions
dition of NaBH solution (0.2m, 5 mL) under vigorous stirring at RT.
4
The activity, selectivity, and stability of the Au, Ag, and Au–Ag
catalysts, as model bimetallic catalysts, for the gas-phase selec-
tive hydrogenation of butadiene in a propene gas stream were
investigated. The Au catalyst was more than an order of mag-
nitude more active than the Ag catalyst at 1208C. The low ac-
tivity of the Ag catalysts, despite lower activation energy for
the reaction than for Au, was ascribed to the low concentra-
tion of butadiene molecules adsorbed on the Ag surface under
these conditions. All Au-containing catalysts were selective,
and equally active if the activity is normalized per Au content;
hence the presence of Ag does not affect either initial activity
or selectivity of the Au in the bimetallic particles. These results
highlight the sometimes very different behavior of catalysts in
liquid and gas-phase oxidation reactions. Although the Au cat-
alyst was stable, the bimetallic catalysts showed a gradual ac-
tivity loss during the first 5 h on stream at 2008C, which was
explained by the segregation of the less active Ag atoms to
the surface under reaction conditions. The initial activity of the
bimetallic catalysts was restored after regeneration by oxida-
tion/reduction at 4508C. Furthermore, it is the first report of
After 20 min, the product was collected by centrifugation, washed
with water (80 mL) twice at RT and dried at 608C in static air
overnight.
Following this, to prepare Au–Ag catalysts, Ag was deposited on
the Au/SBA15 keeping the total mole percentage of metal con-
stant to ensure an atomic ratio (Au:Ag) of 3:1 and 2:1, which are
denoted as Au3Ag1 and Au2Ag1, respectively. To eliminate the
amine groups, the catalysts were calcined either at 5008C for Au
and Au–Ag catalysts, or at 4508C for the Ag catalyst in static air for
4
h. The Ag catalyst was calcined at lower temperatures to limit
particle growth.
Characterization
To obtain suitable samples for transmission electron microscopy
(TEM) characterization, the samples were cut into slices of 70 nm
thickness using an Ultracut E Reichert-Jung microtome (Leica). TEM
imaging was performed on a Tecnai 12 (FEI) microscope operated
at 120 kV. To assure Ag nanoparticles are imaged properly, high
angle annular dark-field scanning transmission electron microscopy
was performed on a Tecnai 20 (FEG) microscope operated at
200 kV. Particle sizes were determined from the TEM images by
measuring the sizes of typically 300 individual particles on different
areas of the sample. XRD was performed with a Bruker D2 phaser
with CoKa source. Elemental analysis was performed using induc-
tively coupled plasma-mass spectrometry at the Mikroanalytisches
Laboratorium Kolbe, Germany. UV/Vis analysis was performed on
a VARIAN 5000 spectrometer. Nitrogen physisorption measure-
ments were done at À1968C (77 K) (Micromeritics, TriStar 3000).
Thermogravimetric analysis (TGA) was performed on a PerkinElmer
Ag surface segregation under H , illustrating the sensitivity of
2
bimetallic Au–Ag catalysts to atomic rearrangement under
reaction conditions.
Experimental Section
Catalyst preparation
2
À1
The silica support (SBA-15, 1 g, BET surface area 800 m g ) was
(Pris 1) connected to a mass detector. The powder sample (approx-
[29]
prepared by the method of Sayari et al. In a typical preparation,
poly (ethylene oxide)-block-poly (propylene oxide)-block-poly (eth-
ylene oxide) triblock copolymer (4.0 g, EO PO EO , Pluronic P123,
imately 10 mg) heated for 30 min at 1508C and further heated to
À1
À1
8
008C (58Cmin ) under a flow of oxygen (10 mLmin ) over the
20
70
20
sample.
M =5800, Sigma Aldrich) was dissolved in mixture of aqueous
av
HCl (120 g, 2m) and water (30 g) at room temperature. After at
least 45 min at 358C, tetraethoxysilane (8.5 g) was added and the
solution was stirred for 5 min. After 20 h at 388C under static con-
ditions, the cloudy mixture was kept at 908C for 24 h. The precipi-
tate was filtered and washed at room temperature (RT) until all
chloride ions were removed and subsequently dried at 608C in
Catalytic tests
The hydrogenation of butadiene was performed at atmospheric
pressure in a pyrex plug flow reactor (internal diameter 4 mm). To
test the activity and the selectivity, 100 mg catalyst (sieve fraction,
&
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6
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ÝÝ These are not the final page numbers!

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1
50–212 mm) was exposed to a reaction mixture which consisted
The sample position was optimized by minimizing the copper in-
tensity. To obtain the surface composition, the photo-emission
spectra were collected under ultra-high vacuum (10 Pa) with
photon energy of 500 eV and 700 eV at RT. Au and Ag XPS regions
were obtained by adding up 40 individual spectrum, and Si and C
XPS regions were obtained from a single spectrum (step size of
of 0.3% butadiene, 30% propene, 20% H , and helium as balance
with a flow rate of 50 mLmin (normal temperature and pressure
conditions, gas hourly space velocity (GHSV)=11700 h ). The cat-
alyst was heated at a rate of 18Cmin , from RT to 3008C, while
monitoring the product flow every 15 min with online gas chroma-
tography (Perichrom PR 2100, column filled with sebaconitrile 25%
Chromosorb PAW 80/100 Mesh, FID (flame ionization detector).
2
À1
À7
À1
À1
0.1 eV and step time of 0.1 s). To investigate the effect of H on the
2
surface composition, the analyzer chamber was pressurized with
1
00 Pa of H . The sample was heated using a button heater (fila-
2
To verify that the reaction was not mass transfer limited, Au cata-
lysts with different sieve fractions (75–150, 150–212 and 212–
ment in Al O ceramic) and the temperature measured with a K-
type thermocouple. The temperature was controlled by increasing
manually but gradually the heater output to have a heating rate of
2
3
4
25 mm) were tested. Half the amount of catalyst with the same
GHSV was tested as well. The activity was similar in all these tests
within the range of 6% of conversion of butadiene at 1208C,
showing that there were no internal or external mass transfer
limitations.
À1
around 58Cmin . The series of spectra were collected at around
1
35 and 2108CÆ108C (dwell time of 30 min at each step before
collecting spectra) with photon energy of 500 eV. Background cor-
rection was performed by using a Shirley background and the
spectra were calibrated with respect to the Si2p peak. The spectra
were fitted using Casa XPS v.2.3.16 software (Casa Software Ldt,
UK) with voigt profile applying a Gaussian/Lorentzian ratio equal
to 70/30. For semi-quantitative analysis, the Au:Ag ratio was calcu-
lated from the measured Au 4 f5/2 and Ag 3d3/2 intensities weighted
by their respective photoionization cross section.
After cooling down to RT, the same catalyst was reduced in situ
À1
À1
under pure H (100 mLmin ) from RT to 4508C (ramp 28Cmin )
2
and kept at 4508C for 180 min to test the effect of reduction on
the activity. The activity and selectivity of the same catalyst was
subsequently tested again.
Turnover frequencies (TOF) were calculated from the activityꢂMAu/
D, in which MAu is Au molecular weight, and D is 6(n /a )/d .
m
m
VA
Here, am is the area occupied by a surface atom, vm is volume occu-
Acknowledgements
pied by an atom in bulk metal, and d is the surface-averaged par-
VA
[48]
ticle size. The apparent activation energy was calculated from
the Arrhenius plots (ln (activity) versus 1/T) in the range of 75–
We gratefully acknowledge Hans Meeldijk for TEM imaging,
Marjan Versluijs-Helder for TGA, Marcel van Asselen for designing
and building the XPS sample holder, and the scientific staff of the
TEMPO beamline, Soleil synchrotron radiation facility in Paris: Dr
Ahmed Naitabdi, Anthony Boucly, and Georgia Olivieri. We are
grateful to Utrecht University for a short stay PhD fellowship and
NWO-Vici (16.130.344) for overall funding of the project.
1
208C, as in this temperature range the reaction is zero order with
[31]
respect to butadiene. The pre-exponential factor was calculated
from the intercept.
For the stability tests, either Au catalyst (45 mg) or Au3Ag1
(
68 mg) catalyst or Au2Ag1 (75 mg) catalyst, which contain the
same amount of Au in mole, were loaded into the reactor. After
in situ reduction, the reactor was cooled and maintained at 2008C,
and the catalytic reaction was performed for 21h.
Conflict of interest
Regeneration was performed after a stability test on Au2Ag1 cata-
À1
lyst in situ by consecutive oxidation (air flow, 100 mLmin ) and re-
À1
The authors declare no conflict of interest.
duction (H2 flow, 100 mLmin ) both for 180 min at 4508C fol-
lowed by another stability test.
Keywords: atomic rearrangements · Au–Ag · heterogeneous
catalysis · hydrogenation · photoelectron spectroscopy
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Accepted manuscript online: && &&, 0000
Version of record online: && &&, 0000
&
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ÝÝ These are not the final page numbers!

FULL PAPERS
Better than Gold? The activity, selectivi-
ty and stability of Au, Ag and Au–Ag
catalysts in gas-phase hydrogenation
and possible synergistic effects for the
Au–Ag are investigated. Near ambient
pressure x -ray photoelectron spectros-
copy is used to detect atomic rear-
rangements and its correlation to the
stability of bimetallic Au–Ag nano–cata-
lysts under hydrogenation reaction con-
dition.
N. Masoud, L. Delannoy, C. Calers,
J.-J. Gallet, F. Bournel, K. P. de Jong,
C. Louis, P. E. de Jongh*
&& – &&
Silica-Supported Au–Ag Catalysts for
the Selective Hydrogenation of
Butadiene
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These are not the final page numbers! ÞÞ