On the role of bismuth as modifier in AuPdBi catalysts: Effects on liquid-phase oxidation and hydrogenation reactions

Article abstract of DOI:10.1016/j.catcom.2021.106340

There is much about the action of bismuth within heterogeneous catalysis that still require a deeper understanding. We observed that, when Bi was added to AuPd bimetallic nanoparticles (NPs) supported on activated carbon, Bi affected the activity and significantly alters the selectivity in two model liquid phase reactions, namely the oxidation of cinnamyl alcohol and the hydrogenation of cinnamaldehyde. A combination of transmission electron microscopy and X-ray absorption spectroscopy provided a detailed characterization of trimetallic AuPdBi systems. We propose that the introduction of bismuth on AuPd NPs results in a partial blockage of most active sites, limiting the occurrence of consecutive reactions.

Full text of DOI:10.1016/j.catcom.2021.106340

Catalysis Communications 158 (2021) 106340
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
On the role of bismuth as modifier in AuPdBi catalysts: Effects on
liquid-phase oxidation and hydrogenation reactions
a
,
*
a
a
a
b
Sebastiano Campisi , Sofia Capelli , Michele Ferri , Alberto Villa , Ellie Dann ,
c
d
,
e
,
f
g,**
Austin Wade , Peter P. Wells
, Nikolaos Dimitratos
a
Dipartimento di Chimica, Universit a` degli Studi di Milano, via Golgi 19, 20133 Milano, Italy
Department of Chemistry, University College London, London, UK
b
c
d
e
f
Thermo Fisher Scientific, 5350 NW Dawson Creek Drive, Hillsboro, OR, USA
School of Chemistry, University of Southampton, Southampton SO17 1BJ, UK
UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Oxon, Didcot OX11 0FA, UK
Diamond Light Source Ltd., Harwell Science and Innovation Campus, Chilton, Didcot OX11 0DE, UK
Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM Universit a` di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
g
A R T I C L E I N F O
A B S T R A C T
Keywords:
There is much about the action of bismuth within heterogeneous catalysis that still require a deeper under-
standing. We observed that, when Bi was added to AuPd bimetallic nanoparticles (NPs) supported on activated
carbon, Bi affected the activity and significantly alters the selectivity in two model liquid phase reactions, namely
the oxidation of cinnamyl alcohol and the hydrogenation of cinnamaldehyde. A combination of transmission
electron microscopy and X-ray absorption spectroscopy provided a detailed characterization of trimetallic
AuPdBi systems. We propose that the introduction of bismuth on AuPd NPs results in a partial blockage of most
active sites, limiting the occurrence of consecutive reactions.
Nanoalloys
EXAFS
Heterogeneous catalysis
Promoter
1
. Introduction
Being a non-precious and environmentally friendly metal, bismuth
exhibit unique selectivity and stability in alcohol oxidation processes.
Although numerous studies appeared investigating the nature of the
promoter role of Bi by advanced ex situ and in situ characterization
techniques [6,17–20], this subject still remains a matter of debate.
Various interpretations have been proposed for the promotional effects;
for example including (i) the geometric blocking of low-selectivity sites
(edges, corners, terraces) on the noble metal nanoparticles, (ii) modifi-
cation of the electronic properties of the vicinal noble metal atoms and
the subsequent effects on adsorption properties, (iii) the stabilization of
metal phases, and (iv) the ligand effects by the formation of complexes
among bismuth and noble metal atoms and the adsorbed reactants/
products. Further complicating matters, the nature and the role of bis-
muth strongly depends on the experimental procedure followed for its
deposition on the catalyst surface. Depending on the synthesis condi-
tions, different location and speciation of Bi can be obtained, alterna-
tively resulting either in the formation of a trimetallic ensemble or more
segregated systems [21,22].
has attracted increasing interest as a suitable alternative to replace more
expensive, corrosive, and toxic industrial catalysts. Bismuth has been
proposed as catalyst for several chemical transformations either in the
form of salts or organometallic compounds [1].
Furthermore, similarly to other p-electron metals (e.g. Pb, Sn, Sb),
when added in low concentration, bismuth has demonstrated to act as
promoter and/or modifier of noble metal catalysts [2]. The modification
with Bi has been reported to affect the catalytic performances of Pd-, Pt-
and Au-based catalysts in several reactions, such as the selective
oxidation of biomass derived substrates (e.g. alcohols [2–6], glycerol
[
7,8], glucose [9], 5-hydroxymethylfurfural [10], glyoxal [11]), the
hydrochlorination of acetylene [12], the oxygen reduction reaction
ORR) [13], hydrogenation reactions [14], the electrooxidation of for-
mic acid [15] or NaBH [16] for hydrogen production, and many others.
Trimetallic Au-Pd-Bi catalysts have been already demonstrated to
(
4
In this work, we have investigated the role of Bi as modifier of
*
Corresponding author at: Dipartimento di Chimica, Universit a` degli Studi di Milano, via Golgi 19, 20133 Milano, Italy.
* Corresponding author at: Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM Universit `a di Bologna, Viale Risorgimento 4, 40136
Bologna, Italy.
E-mail addresses: sebastiano.campisi@unimi.it (S. Campisi), nikolaos.dimitratos@unibo.it (N. Dimitratos).
*
https://doi.org/10.1016/j.catcom.2021.106340
Received 2 June 2021; Received in revised form 15 July 2021; Accepted 16 July 2021
Available online 21 July 2021
1
566-7367/© 2021 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(
http://creativecommons.org/licenses/by-nc-nd/4.0/).

S. Campisi et al.
Catalysis Communications 158 (2021) 106340
bimetallic well-defined Au
–
Pd nanoparticles supported on carbon,
under vigorous magnetic stirring. Within a few minutes of their gener-
ation, the colloids (after pH regulation to pH = 2, by sulphuric acid)
were immobilized by dispersing the support in the vigorously stirring
colloidal solution. The amount of the support was controlled in order to
obtain a final metal loading of 1 wt% (on the basis of quantitative
loading of the metal on the support). The catalysts were filtered and
when used as catalysts for oxidation and hydrogenation reactions. Cin-
namyl alcohol and cinnamaldehyde were selected as model substrates
due to the fact we can study the preferential oxidation of the C-OH,
–
– –
C
–
O, and preferential reduction of C
According to Scheme S1, the cinnamyl alcohol oxidation can proceed
– –
C, C O functional groups.
◦
through a complex pathway leading to a large variety of products. The
dehydrogenation of the hydroxyl group produces cinnamaldehyde, an
interesting product with application in food industry as flavoring, in
agrochemistry as insecticide and in metallurgy as corrosion inhibitor.
Cinnamyl alcohol can also undergo hydrogenolysis resulting in the
production of trans-1-phenyl-1-propene or hydrogenation of the car-
bon–carbon double bond to form 3-phenyl-1-propanol. Both trans-1-
phenyl-1-propene and 3-phenyl-1-propanol can be further transformed
into 1-phenylpropane, used as solvent in printing ink industry and
textile industry.
washed several times and dried at 100 C for 2 h.
2.1.3. Trimetallic AuPdBi/AC and physical mixture preparation
Bismuth was added to a portion of AuPd/AC prepared with the
methodology reported above. BiO(NO)
100 mL of distilled water at pH = 2 (pH modified using concentrated
SO ). Bi amount was calculated in order to have a final loading of 1%.
3
(0.12 mmol) was dissolved in
H
2
4
AuPd/AC was added to the solution and left under stirring for 2 h. The
solid was re-dispersed in 100 mL of distilled water and a 0.1 M fresh
solution of NaBH
4
(metal/NaBH
4
= 1:1 mol/mol) was added under
Although the occurrence of hydrogenolysis and hydrogenation re-
action might sound implausible in oxidative conditions, it finds
adequate explanation in the reaction mechanism, involving formation of
metal-hydride species [23,24]. The latter can then transfer hydrogen to
the adsorbed reagent or product molecules. Indeed, also cinnamalde-
hyde can be hydrogenated to 3-phenylpropanal. In addition, cinna-
maldehyde can be decarbonylated to CO and styrene, while
carbon‑carbon bond cleavage leads to benzaldehyde.
stirring. The catalyst was filtered, thoroughly washed with distilled
◦
water and dried at 100 C for 2 h.
For physical mixture preparation, solid Bi
2
O
3
and AuPd/AC were
was
physically mixed before adding to the reactor. The amount of Bi
2 3
O
calculated in order to have the same Bi loading as in the previous
catalyst 1%Bi.
2.2. Catalyst characterization
Similarly to cinnamyl alcohol oxidation reaction, the presence of
many functional groups in the substrate is responsible for the complex
reaction pathway of the cinnamaldehyde hydrogenation reaction
2.2.1. Electron microscopies
STEM/EDX measurements were performed on a FEI Talos 200×
scanning transmission electron microscope equipped with a Super-X
energy dispersive X-ray spectrometer at the University of Manchester.
AuPd/C particles were dry dispersed on lacey-C Cu grids with high-angle
annular dark-field (HAADF) imaging and EDS mapping performed with
an accelerating voltage of 200 kV. L-family X-rays were used for gen-
eration of Au and Pd background subtracted X-ray intensity maps. SEM
measurements were performed on a TESCAN MAIA 3 field emission gun
scanning electron microscope operating at 15 Kv. EDX maps were ac-
quired on an Oxford Instruments X-MAXN 80 detector.
(
Scheme S2). The hydrogenation of cinnamaldehyde can involve either
the alkenyl group or the carbonyl group. In the former case 3-phenylpro-
panal, a compound widely used as artificial flavoring additive in food
and cosmetics, is produced. The selective hydrogenation of carbonyl
group leads to cinnamyl alcohol, which can undergo hydrogenolysis to
trans-1-phenyl-1-propene. The hydrogenation of both carbonyl and
alkenyl group results in the formation of 3-phenyl-1-propanol. Both
trans-1-phenyl-1-propene and 3-phenyl-1-propanol can be converted
into 1-phenylpropane.
Herein, the comparison between two different Bi deposition experi-
mental procedures as well as the use of a combination of several char-
acterization techniques (transmission electron microscopy, X-rays
absorption spectroscopy) help in determining the structure-activity re-
lationships of trimetallic Au-Pd-Bi catalysts with respect to analogue
2.2.2. X-ray absorption spectroscopy
X-ray absorption spectroscopy measurements were carried out on the
B18 beamline, Diamond Light Source, UK. All measurements were car-
ried out in fluorescence mode using a 9-element solid state Ge detector.
A Si (111) double crystal monochromator (DCM) was used for mea-
surements of the L-edges of Au and Bi, with a Si (311) DCM was used for
the Pd K-edge. Each scan was ~30 min, with a minimum of 3 scans
required to achieve an appropriate signal-to-noise ratio. The merged
spectra were analysed using Athena and Artemis from the Demeter
IFEFFIT package [25,26].
Au
–
Pd catalysts.
2
. Experimental
2
.1. Catalyst preparation
2
.1.1. Monometallic Pd/AC preparation
Solid Na
mL of a PVA solution (1 wt%) were mixed in 0.1 L of H
wt/wt%), forming a yellow solution. After 3 min, a precise volume of
.1 M of NaBH solution (Pd/NaBH 1/8 mol/mol) was added in the
2
PdCl
4
(0.051 mmol) (Sigma-Aldrich, purity >99.9%) and 1
2.3. Catalytic tests
2
O (Pd:PVA 1/0.5
2.3.1. Cinnamyl alcohol oxidation
0
4
4
The reactions were carried out in a thermostat controlled stainless
steel reactor (30 mL) agitated with an electronically controlled magnetic
stirrer connected to a large reservoir (5000 mL) containing oxygen at 2
atm. The oxygen uptake was followed by a mass flow controller con-
nected to a PC through an A/D board. The oxidation experiments were
solution under magnetic stirring and a colloid was produced. Within a
few minutes of colloid generation the pH was regulated to the value of 2
by addition of sulphuric acid and the support was added to the vigor-
ously stirring colloidal solution. The amount of the support was accu-
rately weighed in order to obtain a final metal loading of 1 wt% (in the
assumption of quantitative loading of the metal on the support). The
carried out in xylene (0.3 M substrate, substrate/Pd = 1000 (mol/mol),
◦
80 C, pO
2
= 2 atm). The reaction was monitored by analysing period-
◦
catalysts were filtered and washed several times and dried at 100 C for
ically withdrawn samples. Mass balances, in the analysis, were always
2
h.
98% ± 3. Analyses were performed using a HP 7820A gas chromato-
graph equipped with a capillary column HP-5 30 m × 0.32 mm, 0.25
μm
2
.1.2. Bimetallic AuPd/AC preparation
Film, by Agilent Technologies. Authentic samples were analysed to
determine separation times. Identification of products was performed
using a Thermo Scientific Trace ISQ QD Single Quadrupole GC–MS
Solid NaAuCl
Au/Pd ratio 0.73/0.27 wt%) and 2 mL of a PVA solution (1 wt%) were
mixed in 0.2 L of H O (Au/PVA 1/0.5 wt/wt%). After 3 min, a 0.1 M of
NaBH solution (Au/NaBH 1/4 mol/mol) was added to the solution
4 2 2 4
. 2H O (0.062 mmol) and Na PdCl (0.040 mmol)
(
2
equipped with a capillary column HP-5 30 m × 0.32 mm, 0.25
μ
m Film,
4
4
by Agilent Technologies. Quantitative analyses with external standard
2

S. Campisi et al.
Catalysis Communications 158 (2021) 106340
Table 1
a
Cinnamyl alcohol oxidation.
b
ꢀ 1
Catalyst
TOF
h
Selectivity (at 90% conv)
cinnamaldehyde
3-phenyl propanol
3-phenyl propanal
Styrene
Benzaldehyde
Pd/AC
253
402
315
389
69
80
94
79
27
2
2
2
–
–
1
1
3
1
3
AuPd/AC
AuPdBi/AC
11
3
1
Bi
2
O
3
+ AuPd/AC
5
11
a
b
◦
Reaction conditions: 0.3 M cinnamyl alcohol (in xylene as solvent), Pd/substrate 1/1000 (molar ratio), 80 C, 2 bar O
Calculated as mol of converted reagent per mol of metal (considering all metal species) after 15 min.
2
.
method (n-octanol) was used.
Table 2
-phenyl propanol oxidation
.
a
3
2
.3.2. Cinnamaldehyde hydrogenation
b
ꢀ 1
Catalyst
TOF
h
Selectivity (at 30% conv)
3-phenylpropanal
The reactions were carried out in a thermostatted stainless steel
reactor (30 mL) agitated with an electronically controlled magnetic
Benzaldehyde
stirrer. The hydrogenation experiments were carried out in xylene (0.3
Pd/AC
10
89
75
83
90
96
98
95
9
4
2
4
◦
M substrate, substrate/Pd = 1000 (mol/mol), 25 C, pH
2
= 1 atm). The
AuPd/AC
AuPdBi/AC
reaction was monitored by analysing periodically withdrawn samples.
Mass balances, in the analysis, were always 98% ± 3. Analyses were
performed using a HP 7820A gas chromatograph equipped with a
Bi O
2 3
+ AuPd/AC
a
Reaction conditions: 0.3 M 3-phenyl propanol (in xylene as solvent), Pd/
◦
capillary column HP-5 30 m × 0.32 mm, 0.25
μ
m Film, by Agilent
substrate 1/1000 (molar ratio), 80 C, 2 bar O .
2
b
Technologies. Authentic samples were analysed to determine separation
times. Identification of products was performed using a Thermo Scien-
tific Trace ISQ QD Single Quadrupole GC–MS equipped with a capillary
Calculated as mol of converted reagent per mol of metal (considering all
metal species) after 15 min.
◦
column HP-5 30 m × 0.32 mm, 0.25
μm Film, by Agilent Technologies.
(solvent: toluene, pO = 3 bar and T = 120 C) close to those used in this
2
Quantitative analyses with external standard method (n-octanol) was
used.
work [32]. A higher selectivity to cinnamaldehyde was attained under
aerobic conditions compared to unaerobic conditions (3 bar N ), how-
2
ever also in this case hydrogenation/hydrogenolysis products were
3
. Results and discussion
formed and accounted for ca. 15%, in agreement to the results reported
in Table 2.
1
wt% AuPd/AC catalyst (Au:Pd molar ratio 6:4) was prepared by an
Alloying Au to Pd resulted in a catalyst 1.5 times more active than
ꢀ 1 ꢀ 1
established two step procedure, able to produce alloyed structures as we
have shown in previous publications [27,28]. The bimetallic catalyst has
been modified by subsequent Bi deposition to prepare a trimetallic
AuPdBi/AC catalyst (1 wt% Bi loading).
monometallic Pd (253 h vs. 402 h TOF for Pd and AuPd, respec-
–
tively) and limited the hydrogenation of the C C bond compared to Pd
–
(27% vs. 2% selectivity to 3-phenylpropanol, and 69% vs. 80% selec-
tivity to cinnamaldehyde on Pd and AuPd, respectively). In addition, the
3-phenylpropanol produced in more modest amount, was further
oxidized to the corresponding aldehyde (3-phenylpropanal). These re-
sults suggested that i) the addition of Au prevented the formation of
The bimetallic and trimetallic catalysts were tested in both oxidation
and hydrogenation model reactions. The comparison with monometallic
Pd/AC catalyst allowed to reveal the synergistic effects deriving from
the addition of Au and Bi in bimetallic and trimetallic catalysts,
respectively. Cinnamyl alcohol was selected as a model substrate to
evaluate the catalytic performances of as prepared materials in liquid
phase oxidation processes, using xylene as solvent. The main results are
summarized in Table 1.
Pd
–
H species, as reported in the literature [33,34], thus limiting the
–
–
hydrogenation of the C
C bond; ii) Pd is less prone to oxidize non
activated –OH group than AuPd. This evidence was further corroborated
by a further experiment consisting in 3-phenyl propanol oxidation (as an
intermediate of cinnamyl alcohol oxidation), which confirmed the
observed catalytic trend, being Pd about 7.5 times less active (TOF 10
The oxidation of cinnamyl alcohol on Pd/AC catalyst produced cin-
namaldehyde (69% selectivity at 90% conversion) and a large amount of
ꢀ
1
ꢀ 1
h
) than AuPd (TOF 75 h ) in the specific reaction (Table 2).
When trimetallic AuPdBi catalyst was tested in cinnamyl alcohol
3
-phenylpropanol (27%) as by-product deriving from the hydrogenation
–
of the C
–
C bond. As reported above, according to the literature the
oxidation, the catalytic activity was increased by a factor of 1.25
hydrogenation of the double bond is promoted by the formation of
Pd H species during the dehydrogenation process [29].
Although metal hydride species should be expected to react with
–
Table 3
a
oxygen to form water or hydrogen peroxide, actually substrate and
product molecules can compete with oxygen for these species. The
occurrence of hydrogenation and hydrogenolysis reactions even under
aerobic conditions has been reported by Keresszegi et al. [30], which
observed the formation of products from hydrogenation and hydro-
Cinnamaldehyde hydrogenation.
b
Catalyst
TOF
Selectivity (at 90% conv)
ꢀ
1
h
3
-Phenyl
3-phenyl
propanol
1-phenyl
propane
Cinnamyl
alcohol
propanal
Pd/AC
AuPd/AC
AuPdBi/
AC
1274
1425
1209
65
70
93
28
26
5
1
–
–
3
2
2
2 3
genolysis reaction over Pd/Al O even in the presence of molecular
oxygen, which is more likely responsible for oxidative removal of
degradation products. In situ XAFS studies by Lee et al. demonstrated
that oxygen has an active role in minimizing the adsorbate-induced
catalyst reduction and restructuring phenomena on Pd/C catalyst, by
Bi
2
O
3
+
1410
72
24
2
2
AuPd/
AC
x
restoring surface PdO species which are responsible for high activity
a
Reaction conditions: 0.3 M cinnamaldehyde (in xylene as solvent), Pd/sub-
and selectivity in cinnamyl alcohol oxidation [31]. More recently,
Rucinska et al. investigated the role of oxygen in the cinnamyl alcohol
◦
strate 1/1000 (molar ratio), 25 C, 1 bar H
2
.
b
Calculated as mol of converted reagent per mol of metal (considering all
metal species) after 15 min.
2
oxidation over 1%AuPd/TiO catalysts under experimental conditions
3

S. Campisi et al.
Catalysis Communications 158 (2021) 106340
Fig. 1. STEM-EDX analysis of AuPd/AC catalyst particles displaying HAADF images (top), Au intensity maps (middle), and Pd intensity maps (bottom).
compared to Pd catalyst, even if ca. 20% decrease in activity (315 h 1)
ꢀ
Comparing Pd, AuPd and AuPdBi, in terms of activity the same trend
was observed as in the case of the oxidation reaction. Alloying Au to Pd
led to a higher catalytic activity by a factor of 1.12 than monometallic Pd
was observed compared to bimetallic AuPd catalyst (402 h 1). Inter-
estingly, for the trimetallic catalysts, the oxidation of cinnamyl alcohol
proceeded with a very high selectivity to cinnamaldehyde (90%). This
experimental evidence suggested that Bi significantly suppresses the
ꢀ
ꢀ 1
(1425 and 1274 h for AuPd and Pd respectively) whereas the addition
ꢀ 1
of Bi slightly decreased the activity (1209 h ). However, the differences
in the activity were not as remarkable as in the case of oxidation reac-
tion. More interestingly, concerning the selectivity, the addition of Bi
enhanced the selectivity to 3-phenylpropanal (from 70% with AuPd to
–
C C hydrogenation process in agreement with current literature [14].
–
Phenylpropanal and phenylpropanol were present just as traces, the
former deriving from the oxidation of the latter, as confirmed by 3-
phenyl propanol oxidation tests (Table 2).
–
93% in the case of AuPdBi), i.e. the C C hydrogenation product, while
–
To better understand the effect of Bi in the hydrogenation activity of
catalysts, interesting considerations could be drawn from the evaluation
of the catalytic performances in cinnamaldehyde hydrogenation reac-
it inhibited the formation of 3-phenylpropanol, which was present in
discrete amount in the case of Pd and AuPd catalysts and could derive
from the further hydrogenation of 3-phenylpropanal or from cinnamyl
alcohol hydrogenation pathway.
–
–
tion in terms of reduction of the C
–
C or C
–
O functional group
(
Table 3).
In order to understand the catalytic role of bismuth in relation with
4

S. Campisi et al.
Catalysis Communications 158 (2021) 106340
Table 4
Results of the EXAFS fitting procedure on bimetallic and trimetallic catalysts.
2
2
M-M Bond
N
R / Å
Δ
σ
ΔE
χ
R Factor
0.004
a
Au L
3
Edge
AuPd/C
Au-Au
Au-Pd
Au-Au
Au-Pd
Pd-Pd
Pd-Au
Pd-Pd
Pd-Au
Bond
7.2(5)
3.1(5)
6.6(7)
3.1(5)
2.8(8)
8(1)
2.815(5)
2.794 (6)
2.813(7)
2.789(9)
2.78(2)
2.78(1)
2.78(2)
2.79(1)
R / Å
0.0092(6)
0.0080(7)
0.0085(7)
0.008(1)
0.009(2)
0.010(2)
0.008(2)
4.1(4)
104
a
AuPdBi/C
3.9(6)
259
204
113
0.011
0.016
0.015
a
Pd K Edge
AuPd/C
ꢀ 9 (1)
ꢀ 8.5(9)
b
c
AuPdBi/C
AuPdBi/C
2.9(7)
7(1)
0.008(1)
2
2
N
Δ
σ
ΔE
ꢀ 4.1(5)
χ
R Factor
0.004
Bi L
2
Edge
Bi-Bi
5(set)
1.3(1)
1.5(5)
1.0(4)
0.6(2)
3.71(2)
2.24(3)
3.31(2)
2.12(3)
2.39(3)
0.024(3)
0.001(3)
0.007(5)
0.001(3)
0.001(3)
20.86
Bi-O1
Bi-O2
Bi-O3.1
Bi-O3.2
a
Fit Range 3 < k < 14, 1.25 < R < 3, Number of Independent Points 11.
Fit Range 3 < k < 14, 1.2 < R < 3, Number of Independent Points 12.
Fit range 2 < k < 10, 1 < R < 4, Number of Independent Points 15.
b
c
Fig. 2. SEM-EDX analysis of AuPdBi/AC catalyst.
its insertion in the preformed AuPd structure, morphological and
structural characterization of bimetallic and trimetallic catalysts was
performed.
prepared by this specific sol-immobilisation procedure.
Scanning transmission electron microscopy (STEM) coupled with
elemental mapping by energy dispersive X-ray (EDX) spectroscopy
confirmed the presence of random-alloyed structure of bimetallic AuPd
nanoparticles in AuPd/AC catalyst (Fig. 1).
Particle size distributions from TEM analyses (Fig. S1) demonstrated
that both monometallic (Pd/AC), bimetallic (AuPd/AC) had similar
mean particle sizes (3.2–3.9 nm), as typically observed in catalysts
3
X-ray absorption spectra (EXAFS) at the Pd K-edge and Au L -edge
5

S. Campisi et al.
Catalysis Communications 158 (2021) 106340
(
Fig. S2) and values of coordination numbers (N), nearest-neighbours
Declaration of Competing Interest
2
distances (R) and Debye-Waller factors (
confirmed the formation of an AuPd alloy. Actually, the presence of
Au Pd bonds was clearly observed and Pd Pd and Au Au distances
were comparable to the Pd Au (Tables 4). The alloyed structure of
AuPd was maintained also in the trimetallic catalysts, while concerning
bismuth EXAFS data analysis indicates the presence of only a Bi O and
a Bi Bi coordination.
σ
) reported in Table 4
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
–
–
–
–
–
Acknowledgment
–
It seems, then, that Bi is not alloyed with Au and Pd rather it is
present as a segregated bismuth oxide phase. Even if an alloyed structure
between Au, Pd and and Bi cannot be supposed on the basis of EXAFS,
we might guess that the bismuth oxide phase is in the close proximity of
AuPd NPs. In fact, SEM-EDX analysis revealed the co-presence of tri-
metallic systems with homogeneous Au, Pd, Bi distribution in close
contact with a segregated bismuth phase as is shown in Fig. 2. In any
case, the occurrence of moderate segregation did not alter the mean
particle size (Fig. S1).
Authors thank Diamond Light Source for access to facilities SP8071.
We would like to thank Prof. Grace Burke at the University of Man-
chester for providing access to the Scanning Transmission Electron
Microscope.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.catcom.2021.106340.
The modification of activity and selectivity observed for trimetallic
AuPdBi catalysts could be ascribed to the presence of bismuth oxide
phase highly dispersed and in intimate contact with Au and Pd sites.
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Sebastiano Campisi: Conceptualization, Methodology, Writing-
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Sofia Capelli: Visualisation, Investigation, Validation. Michele Ferri:
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