Variability of particle configurations achievable by 2-nozzle flame syntheses of the Au-Pd-TiO2 system and their catalytic behaviors in the selective hydrogenation of acetylene

Article abstract of DOI:10.1016/j.apcata.2017.09.014

Catalysts with Au and Pd supported on TiO₂ (Au:Pd 1:1 wt/wt%) were prepared by 1- and 2-nozzle flame spray pyrolysis (FSP). The 2-nozzle configuration allowed to synthesize various particle configurations by separate or co-feeding of the metal precursor solutions to the two nozzles. For the Au-Pd/TiO₂ system, four different catalyst particle configurations were investigated: “TiO₂ + AuPd”, “Pd/TiO₂ +Au”, “Au/TiO₂ +Pd”, and “Pd/TiO₂ + Au/TiO_2”, where + separates the corresponding precursor solutions fed to the two nozzles. There were no significant differences in the specific surface areas and the average TiO₂ crystallite sizes of the catalysts (100 m²/g and 16–17 nm, respectively) with the exception of “Pd/TiO₂ +Au/TiO_2”, which exhibited larger surface area and smaller crystallite size (152 m²/g, 12 nm) due to halving of the Ti precursor concentration in each nozzle. As revealed by CO chemisorption, XPS, and STEM-EDX results, the catalyst properties varied largely in terms of bimetallic AuPd particle compositions, the interaction between metal–metal and metal-support, and the location of Pd (or AuPd) on the TiO₂. Among the catalysts studied, “TiO₂ + AuPd” prepared with the 2-nozzle system exhibited the highest conversion of acetylene (～50%) at 40 °C with high selectivity to ethylene ( > 95%). Co-feeding the noble metal precursors together with the Ti precursor afforded less active catalysts due to the formation of Ti-O species partially covering the most active bimetallic AuPd particles. Compared to the commercially available acetylene hydrogenation catalyst and the AuPd/TiO₂ prepared by conventional co-impregnation and deposition-precipitation, all the FSP-AuPd/TiO₂ catalysts showed superior performances under the reaction conditions used.

Full text of DOI:10.1016/j.apcata.2017.09.014

Accepted Manuscript
Title: Variability of particle configurations achievable by
2-nozzle flame syntheses of the Au-Pd-TiO2 system and their
catalytic behaviors in the selective hydrogenation of acetylene
Authors: Boontida Pongthawornsakun, Okorn
Mekasuwandumrong, Francisco J.Cadete Santos Aires, Robert
Bu¨chel, Alfons Baiker, Sotiris E. Pratsinis, Joongjai Panpranot
PII:
S0926-860X(17)30443-X
DOI:
Reference:
http://dx.doi.org/10.1016/j.apcata.2017.09.014
APCATA 16408
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10-9-2017
11-9-2017
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Variability of particle configurations achievable by 2-nozzle flame
syntheses of the Au-Pd-TiO2 system and their catalytic behaviors in the
selective hydrogenation of acetylene
Boontida Pongthawornsakuna, Okorn Mekasuwandumrongb, Francisco J. Cadete Santos
Airesc, Robert Bücheld, Alfons Baikere, Sotiris E. Pratsinisd*, and Joongjai Panpranota**
aCenter of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical
Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
bDepartment of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn
University, Nakorn Pathom 73000, Thailand
cInstitut de Recherches sur la Catalyse et l’Environnement de Lyon – IRCELYON (UMR 5256
CNRS/Univ. Lyon I), 2 Avenue Albert Einstein, 69626 – Villeurbanne cedex, France
dParticle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zurich,
Sonneggstrasse 3, CH-8092 Zurich, Switzerland
eInstitute of Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH
Zurich, Hönggerberg, HCI, CH-8093 Zurich, Switzerland
Date: September 10, 2017
Submitted to: Applied Catalysis A: General
* ,**
Corresponding authors.
Tel. 66-2218-6869 Fax. 66-2218-6877 E-mail: joongjai.p@chula.ac.th (J. Panpranot)
sotiris.pratsinis@ptl.mavt.ethz.ch (S. E. Pratsinis)
1

Graphical abstract
Highlights
 Au-Pd-TiO2 system with various particle configurations were synthesized by 2-nozzle
FSP for the first time.
 Particle compositions, the interaction between metal-metal and metal-support, and the
location of Pd (or AuPd) on the TiO2 were varied.
 Fraction of bimetallic alloy particles on TiO2 was maximized by feeding Au and Pd
precursors to the same nozzle and the Ti precursor to the other one.
 The enhanced hydrogenation activity and high selectivity to ethylene (>95%) is
attributed to the presence of bimetallic AuPd alloy particles.
Abstract
Catalysts with Au and Pd supported on TiO2 (Au:Pd 1:1 wt/wt%) were prepared by 1-
and 2-nozzle flame spray pyrolysis (FSP). The 2-nozzle configuration allowed to synthesize
various particle configurations by separate or co-feeding of the metal precursor solutions to
the two nozzles. For the Au-Pd/TiO2 system, four different catalyst particle configurations
were investigated: "TiO2 + AuPd", "Pd/TiO2 +Au", "Au/TiO2 +Pd", and "Pd/TiO2+Au/TiO2",
where + separates the corresponding precursor solutions fed to the two nozzles. There were
no significant differences in the specific surface areas and the average TiO2 crystallite sizes
of the catalysts (100 m2/g and 16-17 nm, respectively) with the exception of "Pd/TiO2
2

+Au/TiO2", which exhibited larger surface area and smaller crystallite size (152 m2/g, 12 nm)
due to halving of the Ti precursor concentration in each nozzle. As revealed by CO
chemisorption, XPS, and STEM-EDX results, the catalyst properties varied largely in terms
of bimetallic AuPd particle compositions, the interaction between metal-metal and metal-
support, and the location of Pd (or AuPd) on the TiO2. Among the catalysts studied,
"TiO2+AuPd" prepared with the 2-nozzle system exhibited the highest conversion of
acetylene (~50%) at 40oC with high selectivity to ethylene (>95%). Co-feeding the noble
metal precursors together with the Ti precursor afforded less active catalysts due to the
formation of Ti-O species partially covering the most active bimetallic AuPd particles.
Compared to the commercially available acetylene hydrogenation catalyst and the AuPd/TiO2
prepared by conventional co-impregnation and deposition-precipitation, all the FSP-
AuPd/TiO2 catalysts showed superior performances under the reaction conditions used.
Keywords: flame spray pyrolysis; AuPd/TiO2; bimetallic AuPd alloy particles; 2-nozzle
system; acetylene hydrogenation
1. Introduction
During ethylene polymerization, the feed stream from thermal and/or steam cracking
contains small quantities of acetylene (< 2-3%) that poison the polymerization catalysts [1-3].
As a result, polyethylene products may not be properly vulcanized and become brittle [4]. In
addition, acetylene can form metal acetylides, which are explosive contaminants [5]. Thus,
the acetylene in ethylene feed should be eliminated or reduced to an acceptable level (<5
ppm) [4, 6]. The most commonly used method for acetylene removal is the selective
hydrogenation of acetylene using Al2O3 supported Pd-based catalysts [7-10]. However, at
high acetylene conversion (100%), the catalysts show low ethylene selectivity (11%) [11]. An
over-hydrogenation of ethylene to ethane can occur due to the strong adsorption of both
reactants and products on the Pd sites [12]. One of the attractive techniques to control the
3

contact of Pd with reactants and products is the addition of a second metal to form bimetallic
catalysts. Gold has been used as such metal for the selective hydrogenation of acetylene [3,
12-15].
The performance of AuPd catalysts in the selective acetylene hydrogenation was
found to depend largely on the AuPd structure. For examples, Sárkány et al. [14] reported an
activity decrease with increasing of Pd shell thickness of the Pdshell-Aucore/SiO2 catalysts.
Moreover, AuPd/SiO2 with AuPd alloy structure exhibited better catalytic activity and
selectivity than a core(Au)-shell(Pd) structure. Zhang et al. [3] synthesized AuPd/SiO2
catalysts by electroless deposition with various surface coverage of Au on Pd and observed
the formation of small ensembles of Pd sites at high Au surface coverage, which promoted
the adsorption of acetylene as π-bonded species favoring hydrogenation to ethylene, whereas
at low Au surface coverage larger contiguous Pd ensembles were formed resulting in ethane
formation. The ethylene selectivity was also decreased due to the strong adsorption of
acetylene as multi σ-bonded species [16]. Furthermore, alloying Pd with small Au
nanoparticles (~3 nm) on a SiO2 support with Pd/Au atomic ratios < 0.025 exhibited higher
catalytic performance than the monometallic Pd or Au [12]. Pei et al. [12] proposed that Pd
promoted acetylene conversion at lower reaction temperature, compared to Au, whereas the
Au addition helped to isolate the Pd atoms and suppressed the hydrogenation of ethylene to
ethane (improvement in selectivity). Additionally a weaker ethylene adsorption was observed
for the AuPd/SiO2 catalysts with Pd single-atom alloy structure in comparison to the
monometallic Pd/SiO2, thus leading to higher ethylene selectivity [12] . Zhang et al. [15]
synthesized AuPd/TiO2 catalyst (Au/Pd atomic ratio = 14) by deposition-precipitation of Au
followed by impregnation with Pd. Large amount of surface Pd sites (both of isolated Pd
atoms and contiguous Pd clusters) were formed, facilitating acetylene adsorption leading to
the improvement of acetylene conversion.
4

Flame spray pyrolysis (FSP) technology has a great potential for rapid, flexible, and
scalable synthesis of multi-component nanoparticles. The characteristics of such
nanoparticles can be tuned by the FSP operating conditions such as precursor type [17, 18],
concentration [19-21], flow rate [19, 21, 22], type of solvent [23, 24], dispersion gas type
[19], dispersion gas flow rate [19, 21], and other process conditions [20, 21]. By varying
these parameters, the combustion enthalpy change which affects the total net heating value of
the spray flame. The metal precursor and solvent should be selected not only by their
miscibility but also suitable combustion enthalpy for the FSP synthesis method.
The 2-nozzle FSP facilitates a better control of the mixing intensity of the individual
components of the two flames [25]. Up to date, only a few catalyst systems have been
reported using 2-nozzle FSP synthesis method (e.g., Al2O3 and CexZr1-xO2 supported Pt-Ba
catalysts [26, 27] for NOx storage, CoMo/Al2O3 [25] hydrotreating catalysts, Ba- and K-
promoted Rh/Al2O3 [28] for CO2 hydrogenation, and CO oxidation catalysts based on Pt-
FeOx/CeO2 [29], MnOx/Al2O3, and FeOx/Al2O3 [30]). An important feature of the 2-nozzle
FSP emerging from these previous studies is that the incorporation of the active phase into
the oxide support could be prevented.
The 2-nozzle FSP allows designing multi-component nanoparticles by controlling the
mixing of the individual components from the two flames. The present work reports the
characteristics as well as the catalytic properties of the bimetallic AuPd/TiO2 made by 2-
nozzle FSP with different noble metal mixing. The degree of AuPd alloy formation, the
interaction between metal-metal and metal-support, and the location of Pd (or AuPd) on the
TiO2 could be varied significantly. The catalysts were characterized by N2 physisorption, X-
ray diffraction (XRD), CO chemisorption, scanning transmission electron microscopy
(STEM), energy-dispersive X-ray spectroscopy (EDXS), and X-ray photoelectron
5

spectroscopy (XPS). Their catalytic performances were evaluated for low temperature (40
°C) gas-phase selective hydrogenation of acetylene in excess ethylene.
2. Experimental
2.1 Catalyst preparation
The bimetallic 1 wt.% Au-1 wt.% Pd/TiO2 catalysts were prepared by 1- and 2-nozzle
FSP using palladium (II) acetylacetonate, gold (III) chloride hydrate (Sigma-Aldrich), and
titanium isopropoxide (Sigma-Aldrich) as precursors. For the conventional 1-nozzle FSP, all
precursors were dissolved in xylene/acetonitrile (70/30 vol%) mixture solution of 0.5 M total
metal (Au, Pd, Ti) concentration. This precursor solution was fed through the FSP nozzle by
a syringe pump and dispersed with O2 to form a fine spray of droplet. The precursor flow
rate was 5 mL/min while the flow rate of O2 dispersion was 5 L/min. The supporting
premixed flame feed gases consisting of O2 (2 L/min) and CH4 (1 L/min) were provided
through a ring around the nozzle dispersion O2 outlet to ignite the spray resulting in a self-
sustaining spray flame. The produced particles were collected on a glass fiber filter with the
aid of a vacuum pump. With the 2-nozzle FSP and the specified liquid precursor solutions,
four different AuPd/TiO2 catalyst particle configurations were prepared differing in the
separation of TiO2, Pd and Au components. In the notation used here the catalysts produced
with 2-nozzle FSP are labeled with + symbol: Each side of the + indicates which metals were
produced in the same nozzle. As an example "TiO2+AuPd" indicates that the Ti precursor
was fed through one nozzle while the Au and Pd precursors were fed through the other
nozzle. The production with 1- and 2-nozzles is schematically shown in Fig. 1.
6

Catalyst preparation with 2-nozzle FSP was similar to that with 1-nozzle FSP. For
each nozzle, the liquid precursor solution was fed at 5 mL/min and dispersed with O2 at 5
L/min forming a fine spray. The angle between the two nozzles was maintained at 160°,
resulting in a meeting distance of the center of both nozzles equal to 32 cm.
For comparison of the catalytic performance, bimetallic AuPd/TiO2 catalysts prepared
by conventional methods and a commercial acetylene hydrogenation catalyst were also
tested. The bimetallic 1 wt.% Au-1 wt.% Pd/TiO2 catalysts were prepared by conventional
co-impregnation (IMP) and deposition-precipitation (DP) methods according to the
procedures described in Ref. [31]. The commercial acetylene hydrogenation catalyst was a
bimetallic Pd-Ag/Al2O3 catalyst (Süd-Chemie).
2.2 Catalyst characterization
The XRD patterns were recorded from 20° to 70° by a Bruker AXS D8 Advance
diffractometer (40 kV, 40mA, λ = 0.154 nm) with Cu Kα radiation. The crystallite sizes were
calculated by Topas 3 software with the Rietveld refinement. The phase composition of TiO2
was calculated by [32]:
1
WR= (
) × 100
A
0. ( ) +1
R
where WR is the mass fraction of rutile, A and R are the peak areas of anatase (101) and rutile
(110), respectively. The number of 0.884 is the coefficient of scattering. The BET
measurement was performed by nitrogen adsorption at 77 K using a Micromeritics Tristar
instrument. The BET specific surface area (SSA) was measured by a 5-point nitrogen
adsorption isotherm after degassing of the sample with N2 at 200 °C for at least 1 h. The
7

actual metal loadings in the catalyst sample were analyzed by ICP-OES technique using a
Horiba Jobin Yvon Activa spectrometer. The chemisorbed CO on the samples was measured
by CO-pulse chemisorption using a Micromeritics Autochem II 2920 equipped with a thermal
conductivity detector (TCD) and mass spectrometer (MS). Before chemisorption, the samples
were reduced with 5% H2/Ar at 150 °C for 2 h. The CO was injected to the reduced catalysts
with 10 pulses at 40 °C or until the MS signal of effluent CO gas was constant. The HAADF
(high angle annular dark-field) imaging and the EDX analysis of the nanoparticles within the
samples was performed by an aberration-corrected STEM/SEM HD-2700CS HITACHI
microscope operated in the STEM mode equipped with an SDD energy-dispersive X-ray
spectrometer (EDX) and Genesis Spectrum (version 6.2) software from EDAX. The XPS was
performed on a Kratos AMICUS X-ray photoelectron spectrometer using MgKα as X-ray
radiation. The C 1s line (binding energy at 285.0 eV) was specified as internal standard.
2.3 Catalytic reaction study
The selective hydrogenation of acetylene in excess ethylene was performed in a
quartz reactor (inner tube diameter 4 mm) at 40 °C. Prior to the reaction, 50 mg of catalyst
was reduced with H2 (50 mL/min) at 150 °C for 2 h, and then cooled down to the reaction
temperature at 40 °C under He (30 ml/min). A gaseous mixture containing 1.5% C2H2, 2% H2
and the balance C2H4 was introduced into the reactor at a total flow rate of 100 mL/min. The
reaction temperature was kept constant for 1 h. The gas mixture at the outlet of the reactor
was analyzed by an on-line gas chromatograph equipped with a flame ionization detector
(FID) and a HP-PLOT Q column 30 m length, 0.32 mm in diameter and 0.2 µm film
thickness (Agilent Technologies).
8

The ethylene selectivity was calculated from the measured ethane formation [33]. As
the hydrogenation of acetylene was investigated in the excess ethylene (96.5 vol%), a change
in the ethylene concentration could not have been detected accurately enough. The acetylene
conversion and ethylene selectivity are defined as follows [12, 33-35]:
C2H2(푖푛) - C2H2(표푢푡)
Acetylene conversion =
× 100%
C2H2(in)
( )
C2H6(out) - C2H6 in
Ethylene selectivity = (1 −
) × 100%
C2H2(in) - C2H2(표푢푡)
Ethylene and ethane are assumed to be the only products for selective hydrogenation
of acetylene, as no other GC signals were observed and due to the short contact time
oligomerization was negligible [12, 36, 37]. C2H6 (in) represents the C2H6 impurity
concentration in the C2H4 cylinder used.
3. Results and discussion
3.1 Catalyst characterization
With the 2-nozzle FSP system the mixing intensity of the noble metals and the
support was controlled [26, 38, 39]. A schematic diagram showing the structural features of
the AuPd/TiO2 prepared by different routes using 1- and 2-nozzle FSP systems is shown in
Fig. 1. These features emerged from the characterization results of the techniques used,
including XRD, N2 physisorption, CO chemisorption, XPS, and STEM-EDX. In Fig. 2 the
XRD patterns of all catalysts are shown. The major phase was anatase (TiO2). The average
TiO2 crystallite size, BET surface area, and CO chemisorption results of the catalysts are
summarized in Table 1. The average crystallite size of anatase was around 16 nm for
9

catalysts made by 2-nozzles and was similar to that prepared by the 1-nozzle (18 nm). An
exception is "Pd/TiO2+Au/TiO2" with a TiO2 crystallite size of 12 nm, which is attributed to
the division in halves of the Ti concentrations fed to the individual nozzles (total Ti
concentration fed to the flames was constant for all catalyst preparations). It has been shown
previously that the precursor concentration defines the growth rate of the particles [20]. The
smaller crystallite size of the anatase TiO2 of "Pd/TiO2+Au/TiO2" is in agreement with the
higher BET specific surface area (152 m2/g corresponding to a dBET of 10 nm). All other
samples had significantly lower surface areas in the range of 99-106 m2/g (dBET of 15 nm).
The actual Pd and Au metal loadings in the catalyst samples were analyzed by ICP-OES
technique as shown in Table 1. According to ICP results, it could confirm that almost of Pd
and Au metals were retained within the catalyst matrix.
The amount of CO adsorbed on the various catalysts listed in Table 1 was used to
calculate the Pd dispersion. The "Au/TiO2+Pd" catalyst showed the highest dispersion of
23%. Separating the Pd precursor into the 2nd nozzle FSP prevented the coverage of Pd
particles by Ti-O species, resulting in higher fraction of active Pd surface sites accessible for
CO chemisorption. Among the AuPd/TiO2 catalysts, the 1-nozzle FSP-AuPd/TiO2 and the
"TiO2+AuPd" showed relatively low Pd dispersion probably due to formation of AuPd
bimetallic particles with Au enriched on the surface due to lower surface free energy of Au
compared to Pd [40]. In addition, the synthesis of AuPd/TiO2 using 1-nozzle FSP can lead to
the formation of Ti-O species covering the AuPd alloy particles. As a result, CO could not
adsorb on Pd sites and hence a low CO uptake and low Pd dispersion was observed. This is in
line with the previous observation made by Mekasuwandumrong et al. [41], who reported
that the simultaneous condensation of Pd and TiO2 leads to partial coverage of Pd by TiO2.
The calculated Pd dispersion of the "Pd/TiO2+Au" and "Pd/TiO2+Au/TiO2" was 17 and 20%,
10

respectively, with a relatively high Pd dispersion, since Pd was sprayed separately from Au
and less (or even no) AuPd alloys could be formed.
The binding energy of the Pd 3d and Au 4f and the surface atomic compositions of the
metal species on the catalyst samples are presented in Table 2 and 3, respectively. The
typical binding energy of Pd 3d for Pd metal was reported to be in the range of 335.0-335.4
eV [42]. The XPS peaks for Pd 3d were shifted towards lower binding energy upon AuPd
alloy formation [43, 44] whereas shifting towards higher binding energy can be assigned to
electron-deficient Pd such as Pd oxide [45]. According to Table 2, the XPS Pd 3d spectra for
all samples were fitted with FWHM < 2 into 3 types indicating 3 components presented as Pd
in AuPd alloy, Pd metal, and Pd oxide. The binding energy of the Pd 3d peaks in AuPd alloy
for the "TiO2+AuPd" was lowest; in addition, the percentage of Pd forming AuPd alloy was
highest. It is suggested that the well-mixing between Au and Pd precursors in the nozzle
where only Pd and Au precursors were fed promoted the degree of AuPd alloy formation.
Compared to the "TiO2+AuPd", the "AuPd/TiO2" prepared by 1-nozzle FSP showed higher
binding energy of Pd 3d indicating weaker interaction between Au and Pd in bimetallic AuPd
particles and lower fraction of Pd being involved in alloy structure due to the presence of Ti
precursor together with Au and Pd precursors in the same nozzle. For the other catalysts
prepared by 2-nozzle FSP, the observed Pd forming bimetallic AuPd alloy structure showed
higher binding energy and lower fraction of Pd involved in AuPd alloy structure. These
results indicate that the separation of the noble metal precursors by feeding to different
nozzles leads to weak interaction between Au and Pd. In other words, their late mixing in the
flame strongly suppresses the formation of bimetallic AuPd structure with strong interaction
between Pd and Au. Furthermore, the fraction of Pd oxide was significantly higher for the
catalysts prepared by feeding Pd and Ti precursors together, as exemplified by
"Pd/TiO2+Au". The XPS Au 4f spectra for all the samples were fitted with one component
11

for Au species (FWHM < 2 eV). The surface atomic ratio listed in Table 3 indicates that the
sample "AuPd/TiO2" made by 1-nozzle FSP has the lowest surface atomic composition of
both Au and Pd, proposing partial coverage of the metal particles with Ti-O species. The
"TiO2+AuPd" catalyst showed the highest Au and Pd atomic percentages, indicating the
presence of both Au and Pd on the catalyst surface. For the other particle configurations, the
separation of the flames fed with Au or Pd precursors from the one fed with Ti support
precursor resulted in poor mixing between noble metal and Ti precursors, leading to lower
coverage of the noble metals with Ti-O species.
The HAADF (Z-contrast) images of the bimetallic AuPd/TiO2 catalysts and selected
(typical) EDX spectra of individual AuPd nanoparticles within each sample prepared by the
2-nozzle FSP are shown in Fig. 3(a) and (b) respectively. In the HAADF images the
bimetallic nanoparticles appear as bright spots since the collected intensity is directly related
to the atomic number Z, the dependency being roughly proportional to Z2 [46, 47] . The
STEM-EDX analysis performed on several individual nanoparticles in each sample
confirmed the presence of both Au and Pd metals within each of them. This indicates the
bimetallic nature of the nanoparticles. Although all the catalysts showed the presence of both
Au and Pd metals in each analyzed nanoparticle, the fractions of Au and Pd within the
nanoparticles were different for each catalyst prepared by the 2-nozzle FSP. The
“TiO2+AuPd" catalyst showed a higher fraction of Pd (higher Pd/Au ratio) compared to the
others. Furthermore, in the "Au/TiO2+Pd" catalyst, small Pd particles dispersed on the TiO2
support were also detected in addition to the bimetallic ones. The former can be clearly
identified by EDX but also directly in the corresponding image HAADF in Fig. 3(a) since
they are much smaller and as such a much weaker contrast, due to smaller number of atoms
with lower Z, than the bimetallic ones.
12

3.2 Selective hydrogenation of acetylene over the AuPd/TiO2 catalysts
The performances of the AuPd/TiO2 catalysts prepared by 1- and 2-nozzle FSP were
examined in the selective hydrogenation of acetylene at 40 ºC and the results are shown in
Fig. 4. For the FSP-made catalysts, the conversion of acetylene followed the order:
"TiO2+AuPd" > "Au/TiO2+Pd" > "AuPd/TiO2 1-nozzle" > "Pd/TiO2+Au" ≈
"Pd/TiO2+Au/TiO2". Despite its lower CO uptake and lower Pd dispersion, the "TiO2+AuPd"
from the 2-nozzle FSP exhibited the highest acetylene conversion under the reaction
conditions used. The activity of the TiO2 supported AuPd bimetallic catalysts also did not
depend much on the physical properties the TiO2 but rather affected by the fraction and the
location of the bimetallic alloy particles. The results in this study suggest that the formation
of bimetallic AuPd particles enhanced the rate for acetylene hydrogenation. It has previously
been found that the presence of AuPd alloy also greatly enhanced the hydrogenation rate of
1-heptyne under mild reaction conditions [31, 48]. Moreover, the location of Pd or AuPd
seems to have a strong effect on the catalyst activity in the acetylene hydrogenation reaction.
The hydrogenation activity of the "TiO2+AuPd" and "Au/TiO2+Pd", in which most of the Pd
(or AuPd) particles were deposited on the surface of the TiO2 support (less coverage by Ti-O)
was higher than those where Pd and Ti precursor were mixed in a single flame (i.e.,
"AuPd/TiO2 1-nozzle", "Pd/TiO2+Au", and "Pd/TiO2+Au/TiO2"). In addition, the separation
of Pd and Au precursors into individual nozzles might result in weak interaction between Pd
and Au and partial coverage of the noble metal particles by the TiO2 support and
consequently the lowest acetylene conversion was obtained. Nevertheless, the selectivity to
ethylene was > 95% for all the flame-made catalysts. As acetylene adsorption is stronger than
that of ethylene, the latter may be desorbed before its complete hydrogenation to ethane [49].
The selectivity to ethylene has previously been shown to be significantly improved by adding
Au as the second metal to the Pd-based catalysts [3, 12, 13]. For the AuPd/SiO2 catalysts
13

prepared by the electroless deposition of Au on Pd/SiO2, ethylene selectivity was improved
up to 75% compared to the monometallic Pd/SiO2 (55%) [3]. The Pd single atom alloy
structure in the AuPd/SiO2 catalysts also exhibited higher ethylene selectivity (85%)
compared to the monometallic Pd (50%) due to the weaker ethylene adsorption [12].
For comparison, the commercial acetylene hydrogenation catalyst and corresponding
AuPd/TiO2 catalysts synthesized by conventional co-impregnation (IMP) and deposition-
precipitation (DP) were tested under similar reaction conditions. The physicochemical and
catalytic properties of the IMP- and DP-AuPd/TiO2 catalysts compared to the "AuPd/TiO2 1-
nozzle" have been reported in a previous study for 1-heptyne hydrogenation [31]. The results
of the comparative catalytic tests summarized in Fig. 4 show that the FSP-synthesized
AuPd/TiO2 in this study exhibited superior catalytic performances compared to the
commercial catalyst (C2H2 conversion 17% and C2H4 selectivity 90%). The IMP- and DP-
AuPd/TiO2 catalysts also showed higher conversion of C2H2 than the commercial catalyst but
their selectivities to ethylene were lowest among the various catalysts studied. Unlike as with
the FSP-made AuPd/TiO2, non-uniform distribution of bimetallic AuPd particles was
observed on the catalysts prepared by DP or IMP, resulting in poor catalytic performances
[31]. As shown in this study, the properties of the 1-nozzle FSP bimetallic AuPd/TiO2 can be
further improved by using the 2-nozzle FSP, which also offers interesting possibilities for
large scale production of catalysts with unique and tailored properties for particular
applications.
4. Conclusions
Two-nozzle FSP facilitated the synthesis of various configurations of TiO2-supported
noble metal nanoparticles. Feeding all precursor components to a single nozzle results in
14

titania-supported bimetallic as well monometallic noble metal particles. Proper use of a 2-
nozzle system leads to significantly higher variability in the achievable particle
configurations. Bimetallic nanoparticle formation can be minimized by feeding the noble
metal precursors separately to the two nozzles, while co-feeding of the noble metal precursors
affords predominantly bimetallic particles. Also the support TiO2 could either be produced by
co-feeding either one or both noble metal precursors. For the selective hydrogenation of
acetylene in excess ethylene, highest conversion was achieved at 40oC for catalysts produced
by feeding Au and Pd precursors to the same nozzle and the Ti precursor to the other one
("TiO2+AuPd"), indicating that maximizing the fraction of bimetallic alloy particles is
beneficial for the selective hydrogenation of acetylene. The enhanced hydrogenation activity
at high selectivity to ethylene (>95%) is attributed to the presence of bimetallic AuPd
particles, as indicated by XPS and STEM-EDX results. The catalysts made by co-feeding of
one or both of the noble metals together with Ti ("Pd/TiO2+Au", "Pd/TiO2+Au/TiO2", and
"AuPd/TiO2 1-nozzle") led to partial coverage of Pd by Ti-O species, as indicated by CO
chemisorption measurements, and consequently lower acetylene conversions were obtained
on these three catalysts.
Acknowledgements
This research is supported by Ratchadaphiseksomphot Endowment Fund,
Chulalongkorn University for the Postdoctoral Fellowship (B.P.) and the International
Research Integration: Chula Research Scholar. We thank Dr. Frank Krumeich (ETH) for
STEM/EDXS investigation and the Electron Microscopy Center of ETH Zurich (EMEZ) for
providing the necessary infrastructure. The research leading to these results has also received
partial funding from the Thailand Research Fund (BRG5780010) and the European Research
15

Council under the European Union’s Seventh Framework Program (FP7/2007-2013) / ERC
grant agreement n° 247283.
16

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19

Figure captions
Figure 1. Schematic of the 1-nozzle and 2-nozzle FSP for the preparation of AuPd supported
on TiO2.
Figure 2. XRD patterns of AuPd/TiO2 catalysts prepared by different routes (a) AuPd/TiO2 1-
nozzle, (b) Pd/TiO2+Au, (c) Au/TiO2+Pd, (d) TiO2+AuPd (e) Pd/TiO2+Au/TiO2.
Figure 3. (a) STEM-HAADF (Z-contrast) images and (b) STEM-EDX of individual
bimetallic AuPd/TiO2 catalysts.
Figure 4. Catalytic performance at 40 °C of AuPd/TiO2 catalysts prepared by different routes
in selective acetylene hydrogenation in excess ethylene ( : AuPd/TiO2 prepared by one
nozzle FSP,
: TiO2+AuPd,
: Au/TiO2+Pd, : Pd/TiO2+Au, : Pd/TiO2+Au/TiO2,
: commercial PdAg/TiO2 catalyst,
: AuPd/TiO2 prepared by co-impregnation [31],
:
AuPd/TiO2 prepared by co-deposition precipitation [31]).
20

Figure 1
21

Figure 2
22

Figure 3(a)
23

Figure 3(b)
24

Figure 4
25

Table 1. Characteristics of the bimetallic AuPd/TiO2 catalysts prepared by 1- and 2-nozzle
FSP
Metal loadings
Pd Au
(wt.%) (wt.%)
TiO2
BET
Pd
CO uptake
(μmole CO/g cat.
Catalysts
crystallite surface
dispersiona
(%)
)
size (nm) area (m2/g)
TiO2 + AuPd
17
16
16
12
104
104
99
9.3
13
26
19
22
11
0.94
0.91
0.82
0.83
Au/TiO2 + Pd
19.1
13.8
16.3
7.9
Pd/TiO2 + Au
Pd/TiO2 + Au/TiO2
0.89
0.90
0.92
0.87
0.88
0.88
152
106
AuPd/TiO2 1-nozzle 18
FSP
IMP-AuPd/TiO2
b
25
28
60
65
35
1.00
0.98
0.03
1.12
0.61
0.18
32.2
28.6
0.5
38
34
19
b
DP-AuPd/TiO2
Commercial catalyst n.d.
PdAg/Al2O3
aPd dispersion has been estimated from the actual metal loading and CO uptake
measurements assuming a stoichiometric factor CO:Pd = 1:1
bResults as cited in [31]
26

Table 2. Binding energy of Pd 3d and Au 4f of the bimetallic AuPd/TiO2 catalysts prepared by 1- and 2-nozzle FSP
Pd alloy
Pd metal
Pd oxide
Au 4f
Catalyst
B.E.
(eV)
B.E.
(eV)
B.E.
(eV)
B.E.
(eV)
FWHM Area (%)
FWHM Area (%)
FWHM Area (%)
FWHM Area (%)
TiO2+AuPd
333.30 2.00
334.16 1.88
334.04 1.94
333.90 1.66
28
21
20
22
25
335.20 1.99
335.20 2.00
335.35 1.98
353.30 2.00
335.05 1.86
51
56
40
48
46
336.81 1.77
337.21 1.88
336.97 2.00
336.91 2.00
336.37 2.00
21
23
40
30
29
83.00 1.56
82.65 1.47
82.62 1.40
82.96 1.69
83.02 1.46
100
100
100
100
100
Au/TiO2+Pd
Pd/TiO2+Au
Pd/TiO2+Au/TiO2
AuPd/TiO2 1-nozzle 333.52 2.00
27

Table 3. Surface atomic composition of the bimetallic AuPd/TiO2 catalysts prepared by 1-
and 2-nozzle FSP
Catalysts
Atomic
Atomic ratio
concentration
Pd (%) Au (%)
Pd/Ti
0.023
0.015
0.013
0.012
0.012
Au/Ti
(Pd+Au)/Ti
0.031
TiO2 + AuPd
0.501
0.280
0.249
0.259
0.225
0.165
0.106
0.139
0.127
0.085
0.008
0.006
0.008
0.006
0.004
Au/TiO2 + Pd
0.021
Pd/TiO2 + Au
0.021
Pd/TiO2 + Au/TiO2
AuPd/TiO2 1-nozzle
0.019
0.016
28