Application of Mesoporous Metal Oxide Immobilized Gold–Palladium Nanoalloys as Catalysts for Ethanol Oxidation

Article abstract of DOI:10.1007/s10562-018-2510-5

Gold–palladium nanoalloys (AuPd) were synthesized by a dendrimer templating method and the as-prepared nanoalloys were immobilized on several reducible mesoporous metal oxides (MMOs). The MMOs of MnO₂, Co₃O₄ and CeO₂ exhibited low catalytic activity in gas-phase oxidation of ethanol. Upon immobilization of the AuPd nanoalloys the activity increased significantly, with high acetaldehyde selectivity at 120?°C. However, this activity increase from pure MMOs to AuPd/MMOs was accompanied by decrease in selectivity to acetaldehyde. One other interesting observation lies on the amount of gold in the nanoalloy. Increasing the ratio of Au:Pd in the nanoalloy from 1:1 to 10:1 lowered the activity by a factor of six but had a positive effect on selectivity. From this, we postulate dissociation absorption of molecular oxygen to the reactive oxides occurs more effectively on the Pd metal surface. With higher Au loading, the acetaldehyde selectivity remained above 90% even at higher reaction temperatures of 160?°C. This led to a postulation of quick desorption of acetaldehyde from the Au surface more than it does on the Pd surface. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2510-5

Catalysis Letters
https://doi.org/10.1007/s10562-018-2510-5
Application of Mesoporous Metal Oxide Immobilized Gold–Palladium
Nanoalloys as Catalysts for Ethanol Oxidation
1
2
2
2
1
Ndzondelelo Bingwa · Nathan C. Antonels · Marc B. Williams · Marco Haumann · Reinout Meijboom
Received: 28 March 2018 / Accepted: 28 July 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Gold–palladium nanoalloys (AuPd) were synthesized by a dendrimer templating method and the as-prepared nanoalloys
were immobilized on several reducible mesoporous metal oxides (MMOs). The MMOs of MnO , Co O and CeO exhibited
2
3
4
2
low catalytic activity in gas-phase oxidation of ethanol. Upon immobilization of the AuPd nanoalloys the activity increased
significantly, with high acetaldehyde selectivity at 120 °C. However, this activity increase from pure MMOs to AuPd/MMOs
was accompanied by decrease in selectivity to acetaldehyde. One other interesting observation lies on the amount of gold in
the nanoalloy. Increasing the ratio of Au:Pd in the nanoalloy from 1:1 to 10:1 lowered the activity by a factor of six but had
a positive effect on selectivity. From this, we postulate dissociation absorption of molecular oxygen to the reactive oxides
occurs more effectively on the Pd metal surface. With higher Au loading, the acetaldehyde selectivity remained above 90%
even at higher reaction temperatures of 160 °C. This led to a postulation of quick desorption of acetaldehyde from the Au
surface more than it does on the Pd surface.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2510-5) contains
supplementary material, which is available to authorized users.
*
Reinout Meijboom
rmeijboom@uj.ac.za
1
2
Department of Chemistry, University of Johannesburg, PO
Box 524, Auckland Park 2006, Johannesburg, South Africa
Lehrstuhl für Chemische Reaktionstechnik (CRT), Frie
drich-Alexander-Universität Erlangen-Nürnberg (FAU),
Egerlandstrasse 3, 91058 Erlangen, Germany
Vol.:(0
1
123456789)
3

N. Bingwa et al.
Graphical Abstract
Keywords Ethanol oxidation · Mesoporous metal oxides · Gold–palladium nanoalloys
1
Introduction
acetates. Hence, global demand for acetaldehyde and acetic
acid exceeds million tons per year for each [8]. Currently,
acetaldehyde is produced via the Wacker–Hoechst process
which employs ethylene as a fossil feedstock and acetic acid
is produced via carbonylation of methanol [9, 10]. Both pro-
cesses are carried out using homogeneous transition metal
complexes and therefore require elaborate downstream
Oxygenated compounds are essential chemical precursors
in the pharmaceutical industry as well as fine chemicals
with production capacities ranging from kilogram to multi-
ton quantities. In particular, the oxidation of alcohols and
polyols is of great interest given the utilization of various
biological hydroxyl-derivatives [1, 2]. For example, the uti-
lization of glycerol from triglycerides such as sunflower oil
to value-added chemicals. Recent reviews have addressed
the oxidation of the C–OH group amongst others, utiliz-
ing catalytic systems based on Pt, Pd and Ru and further
extending into polymetallic systems [3–5]. The oxidation
of ethanol to industrially important reagents has attracted
interest both industrially and academically. The products
acetaldehyde and acetic acid are used as intermediates for
the synthesis of a wide range of organic substances such as
crotonaldehyde [6], pentaerythritol [7], acetic anhydride and
O
[
Au]
O2
[Au]
O2
OH
O
OH
O
OH
O
O
Scheme 1 Reaction network of ethanol oxidation using Au catalysts
1
3

Application of Mesoporous Metal Oxide Immobilized Gold–Palladium Nanoalloys as Catalysts…
separation techniques. Scheme 1 shows possible oxidation
products of ethanol oxidation.
oxidation reactions, but also a variety of reactions including
reduction of pollutants such as 4-nitrophenol [17, 19, 20].
Due to the high susceptibility to aggregation of these nano-
structures, a suitable stabilizing agent is required [21]. Sta-
bilizing nanomaterials is possible in different ways such as
use of ligating polymers or immobilizing them on catalytic
supports [22, 23]. Traditional supports such as titania and
silica have been widely used and have been reported to be
inactive [24, 25]. A report by Takei et al. mentioned the use
of reducible metal oxides as supports and reported different
product distributions on different metal oxides in ethanol
oxidation [26]. In addition, An et al. reported on the use of
different mesoporous oxides as catalysts and as supports for
Pt nanoparticles for use in oxidation of carbon monoxide
[24]. In their report, synergy at the interface of mesoporous
oxides and the Pt nanoparticles was suggested. We recently
reported the activity of AuPd nanoalloys immobilized on
various reducible mesoporous metal oxides as catalysts for
model reactions [17, 27]. We noted enhancement of the reac-
tion rates when the AuPd nanoalloys are immobilized on the
reducible metal oxides and this was attributed to variety of
factors such as the contribution of the catalytically active
support, the possibility of the combined Langmuir–Hinshel-
wood and Mars–van Krevelen mechanisms, and most impor-
tantly the electron relay mechanism between the nanoalloys
and the electron conducting supports.
Recently, gold has been shown to have a positive effect
in the field of catalytic oxidation reactions. Early work by
Haruta and Hutchings on catalytic oxidation by gold opened
doors for a range of catalytic oxidation research [11, 12].
Supported gold catalysts show great improvement in selec-
tivity in alcohol and carbohydrate oxidation as well as cata-
lyst stability. Various methods of preparing gold nanoparticle
catalysts can include impregnation, co-precipitation, depo-
sition–precipitation, metal vapor deposition and metal–sol
immobilization. Importantly, not all of these methods pro-
duce gold nanoparticle catalysts with small enough gold
nanoparticles. In most catalytic oxidations, nanoparticles
with a size of 2–6 nm are key to achieving good activity.
Moreover, this observation is of great importance for oxida-
tions utilizing cleaner oxidant sources such as oxygen where
the energy required dissociating molecular oxygen on a gold
nanoparticle surface increase exponentially for gold nano-
particles larger than 2 nm. Although particles smaller than
this can show even greater activity, their preparation and
stabilization can be challenging. One way of preparing small
nanoparticles while controlling the size and ensuring stabil-
ity is the dendrimer template method. Dendrimers are char-
acterized as well-defined hyper-branched macromolecular
polymers [13–16]. The size of the dendrimer is denoted by
their generation. Internal functional groups on the dendrimer
ensure coordination of the dendrimer to the nanoparticle
surface while the steric bulk of the dendrimer facilitates sta-
bilization with minimal passivation. This synthetic method
allows size control of the nanoparticle by controlling the
metal ion:dendrimer ratio while facile preparation of the
dendrimer-encapsulated nanoparticles (DENs) is achieved.
A schematic representation is given in Scheme 2 for the
synthesis of AuPd-DENs.
According to Grasselli, an efficient catalytic material for
oxidation reactions must be reducible to facilitate the oxi-
dation process using its lattice oxygen [28]. Furthermore,
while undergoing reduction during the reaction, the structure
of the oxide catalyst should be stable enough not to col-
lapse. It is on this basis that we decided to explore reducible
mesoporous metal oxides without and with AuPd nanoalloys
in catalytic oxidation of ethanol.
Herein, we report on the catalytic oxidation of ethanol
to industrially important acetaldehyde and acetic acid by a
greener and economical viable gas-phase oxidation route.
We also investigate the synergistic effect of the mesoporous
The combination of gold with another metal can signifi-
cantly improve the catalytic performance of such alloy mate-
rials [18]. Alloys of gold and palladium enhance not only
Scheme 2 General methodology for preparation of dendrimer-encapsulated gold–palladium nanoparticles [17]
1
3

N. Bingwa et al.
metal oxides and AuPd nano-alloys. Furthermore, investiga-
tion into the effect of the ratio of Au to Pd in the nanoalloy
structure is also undertaken.
The mixture was then aged at 80 °C for 48 h without stirring.
The white powder was filtered and dried at 100 °C. The Plu-
ronic P-123 surfactant was removed by calcination for 2 h at
5
50 °C. Prior to calcination, the white powder was dispersed
in a mixture of ethanol and aqueous HCl (32%) (50/30 v/v).
Mesoporous metal oxides were prepared by the nano-cast-
ing method, often referred to as the hard-templating method.
The experimental procedure was adapted from the work of
An et al. [24]. Briefly, 16 mmol of Co, Mn, and Ce nitrate
salts were dissolved in 8 ml of de-ionized water. The metal
nitrate solutions were added to 300 ml beakers containing
4 g of KIT-6 in 50 ml toluene and stirred at 65 °C. After all
toluene evaporated, the resulting precipitate was dried at
100 °C and further calcined at 550 °C for 6 h. To remove
the template, the samples were washed with hot 2 M sodium
hydroxide, plenty of deionized water and dried thereafter.
2
Experimental
Generation 6 (5 wt% in methanol) PAMAM dendrimers with
hydroxyl terminal groups were purchased from Dendritech
Inc. Sodium hydroxide (NaOH) (99.8%) was purchased from
Merck. Hydrochloric acid (HCl 32%) was purchased from
Associated Chemical Enterprise (PTY) Ltd. Sodium boro-
hydride (NaBH ) (≤99%) was purchased from Fluka. Potas-
4
sium tetrachloropalladate (K PdCl ) (≤ 98%), chloroauric
2
4
acid (HAuCl ) (99.9%), pluronic P123 (EO PO EO ),
4
20
70
20
n-butanol (BuOH), cobalt nitrate hexahydrate (99%), cerium
nitrate (99%), manganese nitrate (99%) and tetraorthosili-
cate (TEOS) (≥99.0%), and toluene were all purchased from
Sigma-Aldrich. All chemicals were of analytical grade and
used as received. Milli-Q (18 MΩ cm) de-ionized water was
used in all experiments. All pH measurements were per-
formed using an ORION model 520A Schott pH blueline 25
electrode. All pH adjustments were performed using aque-
ous NaOH (0.1 M) and HCl (0.1 M).
2.3 Preparation of AuPd/MMO Catalysts
The AuPd-dendrimer encapsulated nanoalloys were immo-
bilized on mesoporous metal oxides and on silica. A 1.2 ml
colloidal solution of Pt nanoparticles was mixed with 0.5 g
of the support in deionized water, and sonicated for 3 h.
Thereafter, the solid catalyst and liquid medium were sep-
arated by centrifuging at higher rotations per minute and
dried at 80 °C overnight. After immobilization of AuPd-
DENs on MMOs and silica, the dendrimer template was
removed by calcination at 550 °C for 1 h. The metal loading
was determined by Spectro Acros inductive coupled plasma-
optical emission spectroscopy (ICP-OES) FSH 12 instru-
ment. Various concentrations of gold standard solutions used
to obtain the calibration curve were diluted in aqua-regia
2.1 Preparation of AuPd Nanoalloys
Gold and palladium bimetallic nanoparticles were synthe-
sized using the method adapted from literature [19, 20].
Briefly, 10 µM G6-OH aqueous dendrimer solution was
prepared by evaporating methanol from a predetermined
volume of the PAMAM dendrimer stock solution, then suf-
ficient amounts of deionized water were added. A 28 molar
solution (1:3 v/v of HNO :HCl). Approximately 15 mg of
3
2+
excess of Pd (0.1 M) was added. Thereafter, 28-fold molar
the gold containing catalysts were dissolved in 2.0 ml aqua-
3
+
excess of Au (0.1 M) was added and allowed to form a
complex with the dendrimer. Thirdly, tenfold excess of
regia solution and filled to 10 ml with deionized water.
NaBH (0.1 M) was added to reduce the metal ions to nano-
2.4 Catalytic Evaluation
4
particles. Finally, the nanoparticles were purified by dialysis
against 3 l of deionized water.
The catalytic oxidation experiments were carried out in
an in-house build continuous-flow fixed-bed reactor with
a stainless steel tube of 10 mm inner diameter (see Sup-
porting Information for details). An HPLC pump (Techlab)
was used to dose liquid ethanol solution in a pre-heater unit
set at 100 °C. The as-heated ethanol solution was mixed
with carrier gas nitrogen in an evaporator, followed by mix-
ing with air to adjust a molecular composition of EtOH/
O /N = 1/5.5/34 (molar ratio), resembling a stoichiomet-
2.2 Preparation of Mesoporous Metal Oxides
For the preparation of mesoporous metal oxides, the
SiO -based hard template, KIT-6, having a proportional
2
3-D structure, Ia3d symmetry, and large ordered domains of
bicontinuous mesostructures, was prepared using the method
previously described by Lebed et al. [29]. Briefly, 9.0 g of
Pluronic P-123 (EO20PO70EO20) was dissolved in 330 ml
of de-ionized water and 17.5 ml of HCl under vigorous stir-
ring. After Pluronic P-123 had dissolved, 9.0 ml of n-butanol
was added dropwise. The resulting mixture was stirred at
2
2
ric amount of molecular oxygen for complete oxidation of
ethanol. The reaction temperature was adjusted by means
of a heating jacket and the bed temperature was monitored.
Catalyst particles of 90 µm size were used and were placed
onto a guard-bed of silica gel. The effluent gases were ana-
lyzed by means of an online GC (Bruker).
3
5 °C for an hour. Tetraorthosilicate, TEOS, 19.4 ml was
added and the resulting mixture was stirred at 35 °C for 24 h.
1
3

Application of Mesoporous Metal Oxide Immobilized Gold–Palladium Nanoalloys as Catalysts…
3
Results and Discussion
shown for Co3O4 in Fig. 2 (TEM images of MnO2 and CeO2
can be found in the SI).
3.1 Catalyst Characterization
In general, the 3-D structure of the metal oxides formed
via hard-templating with KIT-6 resembles the interpenetrat-
ing bicontinuous pores that are interconnected by the dis-
ordered micropores of the templating KIT-6 as reported in
literature [31].
The texture of the synthesized metal oxides was determined
by nitrogen sorption measurements. Appearance of hystere-
sis loops, see Fig. 1a, confirmed the mesoporous nature. The
order of the synthesized mesopores was analyzed by low-
angle powder X-ray diffraction (see Fig. S1 in SI). The low
intensity peaks observed below 2° 2θ for the as-synthesized
mesoporous metal oxides are indicative of meso-ordered
state or long range order [29, 30].
The use of dendrimers as a template and stabilizing agent
for AuPd nanoparticles formation yielded nanoalloys that
are spherical and with narrow size distribution (see Fig. 3).
The sizes of all the nanoalloys were 3.6 ± 0.8, 3.9 ± 0.9
and 2.4 ± 0.3 nm, for Au28Pd28, Au45Pd9 and Au50Pd5,
respectively.
The wide-angle measurements depicted (in Fig. 1b) sug-
gest a nanoparticulate nature for both mesoporous cobalt
and manganese oxides [30]. The low intensities of the peaks
observed in wide-angle measurements are indicative of a
breakdown of the interpenetrating bicontinuous structures
The reducibility of the mesoporous metal oxides was
investigated by hydrogen-temperature programmed reduc-
tion (H2-TPR). The H2-TPR profiles of the as-synthesized
mesoporous metal oxides and Au28Pd28 nanoalloys immo-
bilized on mesoporous metal oxides revealed change in
electronic properties of the mesoporous metal oxides upon
immobilization (see Fig. 4; Table 2). The complete set of
catalysts characterization using H2-TPR can be found in our
previous publication [17].
(
see Fig. 1b) of the mesoporous metal oxides to a certain
degree.
Furthermore, surface area, pore volume and pore size
were calculated from the sorption measurements and are
summarized in Table 1. The surface area of the synthesized
mesoporous metal oxides increase with increasing pore
volume. Manganese oxide has the smallest surface area
2
−1
(
18 m g ) and the largest pore size (31 nm). While on
Table 1 Surface properties of mesoporous metal oxides obtained
the other hand, cerium oxide has the highest surface area
2
−1
from nitrogen sorption analysis
(
130 m g ) and the smaller pore size (10 nm).
Entry Metal oxide SBET (m g−1)
2
Vpore (cm g−1)
3
dpore (nm)
TEM analysis of the hard-template and the formed
mesoporous metal oxides revealed that the ordered structure
of the template could be maintained in the metal oxides as
1
2
3
Co O
MnO2
CeO2
80
18
130
0.21
0.14
0.41
8
31
10
3
4
Fig. 1 a Nitrogen sorption studies of hard-template KIT-6 and resulting metal oxides and b wide angle powder XRD measurements of template
and metal oxides
1
3

N. Bingwa et al.
Fig. 2 a Transmission electron microscopy of the hard-template KIT-6; b TEM of the prepared Co O metal oxide using KIT-6
3
4
Fig. 3 TEM analysis of a Au Pd , b Au Pd and c Au Pd . Respective size distribution histograms of d Au Pd , e Au Pd and f Au Pd
5
2
8
28
45
9
50
5
28
28
45
9
50
1
3

Application of Mesoporous Metal Oxide Immobilized Gold–Palladium Nanoalloys as Catalysts…
Fig. 4 Hydrogen temperature-
programmed reduction profiles
of a MnO and b AuPd/MnO
2
2
Table 2 Overview of the reduction temperatures of metal oxides
the second reduction temperatures almost remain constant
<10 °C). The first reduction temperature is assigned to the
determined by TPR
(
most electropositive and easily reducible cation and the sec-
ond is assigned to the less electropositive and least reduc-
ible cation, thus upon immobilization of gold–palladium
nanoalloys the shift of the second reduction temperature is
minimal. Amongst the metal oxides, MnO and Co O could
Entry
Metal oxide
1st reduction tem- 2nd reduction
perature (°C)
temperature (°C)
1
2
3
4
5
6
Co O4
375
331
360
311
520
510
620
610
390
386
700
689
3
Au Pd –Co O
28
28
3
4
2
3
4
MnO2
Au Pd –MnO
2
be easily reduced and CeO was the least reducible oxide.
2
28
28
CeO2
Au Pd –CeO
2
3
.1.1 Catalytic Ethanol Oxidation
28
28
The catalytic oxidation of ethanol was performed in a
continuous-flow fixed-bed reactor. The catalytic products
obtained were acetaldehyde, acetic acid and ethyl acetate
and can be seen in Scheme 1 in the “Introduction” section.
The catalytic results showed inactivity of silica (KIT-6)
The initial reduction temperatures of mesoporous metal
oxides shift to lower temperatures upon immobilization of
nanoalloys (e.g. compare Entry 1 and 2 in Table 2) while
Table 3 Ethanol oxidation
Entry Metal oxide Metal (s) Au (%) Temp. (°C) Pres- Conversion (%) Selectivity (%) Reference
sure
using mesoporous metal oxides
(
MMO) as support for Pd Au
28 28
(
bar)
nanoalloys at 120 ″C
1
2
3
4
5
6
7
8
9
1
1
1
SiO2
SiO2
–
–
120
120
120
120
120
120
120
120
350
200
350
140
3
3
3
3
3
3
3
3
5
–
5
–
<0.1
4.2
0.5
1.6
<0.1
1.1
<0.1
1.5
18
–
>73
–
>81
–
>92
–
>92
72
80
84
34
Pd Au 2.3
–
Pd Au 2.3
–
Pd Au
–
Pd Au 2.2
Au
28
28
Co O
–
3
4
Co O
3
4
28
28
MnO2
MnO2
CeO2
CeO2
SiO2
–
2
–
28
28
28
28
1
3.6
1
[32]
[33]
[32]
[26]
0
1
2
SiO2
AuCu
Au
Au
–
34
–
CeO2
Co O
11.4
3
4
1
3

N. Bingwa et al.
as a pure support. The activities of the native mesoporous
metal oxides were very low, with a maximum conversion
of 0.5% shown by Co O , and increased in the following
From the temperature variations apparent activation ener-
gies, E
have been calculated (see Fig. S5 in SI). For
A,app,
native Co O and Au Pd /Co O the values were almost
3
4
3
4
28
28
3
4
−
1
−1
order, MnO <CeO <Co O as summarized in Table 3. The
identical with 74 kJ mol and 70 kJ mol , respectively. In
Table 4 the apparent activation energies are summarized.
Mass transport limitations due to pore diffusion of ethanol
and oxygen do not seem to limit the overall reaction rate.
This is not surprising since powdered catalysts with <90 µm
particle size have been used.
2
2
3
4
mesoporous Co O displayed higher activity than MnO and
3
4
2
CeO because of its relatively high surface area and lower
2
reduction temperature. The difference between activities of
MnO and CeO is small. The positive effect of low reduc-
2
2
tion temperature of MnO is probably cancelled by the low
2
surface area it possesses. While on the other hand, the posi-
The apparent activation energies are almost the same in
case of Co O and MnO (see entries 4 and 5 in Table 4).
tive effect that results from high surface area of CeO is
2
3
4
2
cancelled by a high reduction temperature which renders
Note that those two materials exhibited the 1st reduction
temperatures at 331 and 311 °C, respectively (see Table 2).
In contrast, the CeO support and the Au Pd nanoalloy
CeO less active for oxidation.
2
A significant increase in ethanol percentage conversion
was achieved upon immobilization of AuPd nanoalloy onto
the MMOs. The AuPd on Co O improved the activity by a
2
28
28
supported onto it required significantly higher reduction
temperatures, 520 and 510 °C respectively. Therefore, the
reducibility of Co O and MnO is faster at lower tempera-
3
4
factor of three (see entries 3 and 4) and the activity of MnO
2
3
4
2
and CeO based catalysts was improved by almost 20 (see
tures, especially in the regime between 120 and 160 °C.
The low activity of cerium oxide is based on its less reduc-
ibility nature as depicted by high reduction temperature
and the few oxygen vacancies in the cerium oxide structure
2
entries 5–8). The strongest improvement was observed for
AuPd supported on silica, where the final conversion level
was approximately 50 times higher compared to the native
support (see entries 1 and 2). However, this catalyst suffered
significantly towards acetaldehyde selectivity at higher tem-
peratures compared to AuPd/MMO catalysts. As a result,
further studies were performed using MMO supported cat-
alysts only. At 120 °C, formation of possible by-products
acetic acid and ethyl acetate could not be observed for the
native MMO materials due to low conversions. The obtained
catalytic results compare fairly well with reported literature.
At the same temperature, and a pressure of 3 bar and less Au
loading they show better selectivity towards acetaldehyde.
The AuPd/MMO catalyst showed high selectivity toward
acetaldehyde at 120 °C, however, the selectivity decreased at
higher temperatures as shown for Au Pd /Co O in Fig. 5.
Table 4 Apparent activation energies for native mesoporous metal
oxides and Au Pd /MMO catalysts
2
8
28
Entry
Metal oxide
Apparent activa-
tion energy E
A,app
(kJ mol−
1
)
1
2
3
4
5
6
Co O
MnO2
CeO2
74
68
74
70
70
58
3
4
Au Pd /Co O
2
8
28
3
4
Au Pd /MnO
2
2
8
28
Au Pd /CeO
2
2
8
28
2
8
28
3
4
Reaction condition: 3 bar, 0.2 l/min and 200 mg of the catalyst
Fig. 5 Conversion (top) and selectivity (bottom) versus temperature for ethanol oxidation using Au Pd /Co O catalysts
2
8
28
3
4
1
3

Application of Mesoporous Metal Oxide Immobilized Gold–Palladium Nanoalloys as Catalysts…
Table 5 Ethanol oxidation using different AuPd nanoalloys supported
4
Conclusion
on MnO2
Catalyst
Au% Temp. (°C) P (bar) XEtOH (%) SAA (%)
In this work, gold–palladium nanoalloys were successfully
templated in the interior voids of commercially available
PAMAM dendrimer. This synthetic route yielded nanoalloy
particles with a narrow size distribution. The as-prepared
AuPd nanoalloys immobilized on silica and mesoporous
metal oxides exhibited better catalytic selectivity towards
acetaldehyde (80–99%) compared to similar catalyst. The
alloying of the metallic nanoparticles showed selective
adsorption of the reactants on different components of the
alloy. In this study, activation of the reactants takes place
more on the Pd surface than the Au surface. A more detailed
study on the selective adsorption of substrates on different
atoms of the nanoalloy is necessary for development of cata-
lysts with minimum amounts of precious metal.
Au Pd /MnO
2
2
2
2
2.3
2
2
2.7
2
120
140
160
120
140
160
120
140
160
1
1
1
1
1
1
1
1
1
1.1
5.7
7.8
0.2
0.8
1.6
0.1
0.3
0.8
96.2
86.7
81.9
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
2
8
28
Au Pd /MnO
4
5
9
2
.3
.3
Au Pd /MnO
5
0
5
2
.7
.7
2
Reaction conditions: p=1 bar, volume flow=0.2 L min−1, 200 mg of
catalyst
at the temperature at which the reaction was conducted.
The highest concentration of lattice vacancies occur at
higher temperatures for ceria and these oxygen vacancies
enhance the reducibility of cerium oxide [34].
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(
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