Promotional effects of Au in Pd-Au bimetallic catalysts supported on activated carbon cloth (ACC) for direct synthesis of H2O2 from H2 and O2

Article abstract of DOI:10.1016/j.cattod.2014.04.011

In this study, the promotional effects of gold in Pd-Au bimetallic catalysts supported on activated carbon cloth for direct synthesis of H₂O₂ has been investigated. The bimetallic catalysts were prepared by simultaneous co-impregnation approach. Direct synthesis was performed batch-wise in a stainless steel autoclave under high pressure (38 bar) at 0 °C and methanol (220 ml) was used as the reaction medium. A series of experiments were conducted to assess the effects of: (i) the surface chemistry of the support and (ii) the amount and ratio of palladium and gold on the surface morphology of the bimetallic catalysts and on their catalytic performances, and (iii) the oxidation state of the metallic components on the activity of the catalysts. Moreover, (iv) the effects of heat treatment of the catalysts in H₂ at 185 °C and in air at 185-275 °C on the surface chemistry of the support, surface morphology of the metallic component, and the performances of the final catalyst were investigated. Almost all of the Pd-Au bimetallic catalysts were more selective than the corresponding Pd monometallic catalysts. The results showed that the surface morphology of the Pd-Au bimetallic catalysts and their catalytic performance were highly influenced by the surface chemistry of the support and the amount and the ratio of Pd and Au in the catalyst. The presence of palladium oxide made the catalysts more selective and active than the corresponding zero-valent palladium (Pd⁰) catalysts.

Full text of DOI:10.1016/j.cattod.2014.04.011

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Promotional effects of Au in Pd–Au bimetallic catalysts supported on
activated carbon cloth (ACC) for direct synthesis of H O from
2
2
H and O2
2
a,∗
a
a
b
Davood Gudarzi , Warin Ratchananusorn , Ilkka Turunen , Markku Heinonen ,
Tapio Salmic
a
Department of Chemical Technology, Lappeenranta University of Technology, Lappeenranta FI-53851, Finland
Department of Physics and Astronomy, University of Turku, Turku FI-20014, Finland
Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Turku/Åbo FI-20500, Finland
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, the promotional effects of gold in Pd–Au bimetallic catalysts supported on activated car-
bon cloth for direct synthesis of H2O2 has been investigated. The bimetallic catalysts were prepared by
simultaneous co-impregnation approach. Direct synthesis was performed batch-wise in a stainless steel
autoclave under high pressure (38 bar) at 0 C and methanol (220 ml) was used as the reaction medium. A
series of experiments were conducted to assess the effects of: (i) the surface chemistry of the support and
Received 10 December 2013
Received in revised form 3 April 2014
Accepted 14 April 2014
Available online xxx
◦
(
ii) the amount and ratio of palladium and gold on the surface morphology of the bimetallic catalysts and
Keywords:
on their catalytic performances, and (iii) the oxidation state of the metallic components on the activity
of the catalysts. Moreover, (iv) the effects of heat treatment of the catalysts in H2 at 185 C and in air at
1
the performances of the final catalyst were investigated. Almost all of the Pd–Au bimetallic catalysts were
more selective than the corresponding Pd monometallic catalysts. The results showed that the surface
morphology of the Pd–Au bimetallic catalysts and their catalytic performance were highly influenced by
the surface chemistry of the support and the amount and the ratio of Pd and Au in the catalyst. The pres-
ence of palladium oxide made the catalysts more selective and active than the corresponding zero-valent
Pd–Au bimetallic catalysts
Activated carbon cloth
Hydrogen peroxide
Wet oxidation
◦
◦
85–275 C on the surface chemistry of the support, surface morphology of the metallic component, and
0
palladium (Pd ) catalysts.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
on-site production of dilute H O solutions in a green and econom-
2 2
ical manner. Moreover, the direct process may eliminate the need
of transportation with substantial safety risks involved [2].
A process for the direct synthesis of hydrogen peroxide from
hydrogen and oxygen represents considerable challenges in cataly-
sis and process design. Firstly, mixtures of hydrogen and oxygen are
explosive over a wide range of concentrations (4–94% H in O ) [3].
Secondly, the catalytic reaction of hydrogen and oxygen involves
several reaction pathways, most of them result in the production
of water [6] decreasing the selectivity.
Hydrogen peroxide (H2O2) is recognized as an important selec-
tive green oxidant since it contains a high fraction of active oxygen
and the only by-product from oxidation is water. It has a wide range
of applications in many large scale processes such as pulp bleaching,
waste water treatment, cosmetic and pharmaceutical industries. In
pulp and paper industry, hydrogen peroxide has partially replaced
chlorine containing bleaching agents to address environmental
issues. Industrially, H2O2 is produced by the cyclic sequential
hydrogenation and oxidation of alkyl anthraquinones. This process
is only economical at large scale and at high product concentra-
2
2
Pd is the most common catalyst for this process. However,
it suffers from low selectivity, as it catalyzes the decom-
tions [1]. However, H O2 is often required in smaller scales and
at lower concentrations. Direct synthesis would allow small scale
position/hydrogenation
of
H O
(H O → H O + (1/2)O ;
2
2
2
2
2
2
2
H O + H → 2H O) and the non-selective water synthesis
2
2
2
2
(
H + (1/2)O →H O). The performance of Pd catalysts is highly
2
2
2
affected by the support material. Acidic support like SiO2 results
in more active and selective catalysts than basic ones (e.g. Al O )
∗ Corresponding author. Tel.: +358 44 5177734.
E-mail addresses: davood.gudarzi@lut.fi, d.goudarzi@gmail.com (D. Gudarzi).
2
3
[
4,5]. It has been shown that the existence of the oxidized state of
http://dx.doi.org/10.1016/j.cattod.2014.04.011
0
920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: D. Gudarzi, et al., Promotional effects of Au in Pd–Au bimetallic catalysts supported on activated
carbon cloth (ACC) for direct synthesis of H O2 from H2 and O , Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.011
2
2

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Pd (PdO) makes Pd catalysts more selective and more active than
The Pd catalysts were prepared using impregnation method as
described previously [6]. The Pd–Au bimetallic catalysts on non-
oxidized and oxidized activated carbon cloth were prepared by
simultaneous co-impregnation of the supports with an aqueous
0
the corresponding zero-valent state of Pd (Pd ) [6–9]. Using of
−
−
+
halide ions (especially Cl and Br ) and protons (H ) as promoters
improve the selectivity of Pd catalysts in direct H O2 synthesis
2
[
7–23].
Bimetallic catalysts have attracted considerable attention
acidic solution of PdCl and HAuCl at room temperature for 6–7 h.
2 4
The impregnation was done in a glass reactor with stirring rate
of 400–500 rpm. The rate of the stirring was adjusted to get uni-
form distribution of the metal particles in the catalysts. The ratio of
the solution volume to catalyst mass was 500 ml/g. The amount of
the precursor (Pd + Au) between 7.2 and 17.3 mg per 200 mg of the
support in a 200 ml glass reactor gave metal loading (Pd + Au) in the
range of 1–5 wt%. After impregnation, the catalysts were dried at
because their properties often differ remarkably from the corre-
sponding monometallic ones. Among bimetallic catalysts, Pd–Au
has received a great deal of attention because of its superior activity
in a number of reactions, e.g. low-temperature CO oxidation, syn-
thesis of vinyl acetate monomer, hydrogenation of hydrocarbons,
cyclotrimerization of acetylene, and many others. Pure Au catalysts
◦
are not so active in direct H O2 synthesis [2,24]. The addition of a
room temperature for 3 h, and then at 60 C for 12 h. The reduction
2
second metal (e.g. Pt, Au, Ru, Rh) to Pd catalysts has been reported
to affect catalytic activity and selectivity in direct H O synthesis
process was done by treating the fresh dried catalysts with H2 at
185 C under 3.5 bar for about 12 h. A series of different calcined
◦
2
2
[
5,25–34]. Addition of Au or Pt, at their optimum concentrations, to
catalysts were prepared by oxidizing the fresh dried catalysts at
temperatures 185, 235, and 275 C in static air for 12 h. The calci-
nation temperatures were selected based on the TGA/DTA results
in order to be in the safe thermal treatment region of the supports
and their surface functional groups (in the case of oxidized ACC)
[6].
◦
a Pd catalyst was reported to improve the catalyst selectivity, while
the presence Rh or Ru exhibited an adverse effect [26].
The promotional effects of Au in Pd–Au bimetallic catalysts on
direct H O synthesis were firstly reported by G. J. Hutchings et al.
2
2
[
5,30,35,36]. It has also been found out that, unlike in the case of Pd
−
monometallic catalysts, the addition of promoters, such as Br ions
and H PO , were deleterious for the Au–Pd catalysts in this reaction
2.2. Catalysts characterization
3
4
[
31]. Supported Pd–Au catalysts can be prepared using traditional
impregnation or deposition–precipitation of Pd and Au salts, either
concurrently or sequentially, followed by calcination and reduc-
tion. The major drawback is the lack of homogeneity of the formed
catalysts in terms of particle size, composition and shape. For
example, it is not uncommon to observe Au and Pd monometallic
particles, and Pd–Au alloy particles with different sizes and compo-
sitions on the same support [5,30,35–37]. Au–Pd catalysts prepared
by impregnation have been shown to be very active for the direct
formation of hydrogen peroxide [33]. In contrast, the catalysts pre-
pared by co-precipitation or deposition–precipitation gave a very
low hydrogen peroxide production rate [33].
In our previous paper [6], we studied Pd catalysts supported on
activated carbon cloth for direct synthesis of hydrogen peroxide.
In this study, the promotional effects of gold in Pd–Au bimetallic
catalysts supported on activated carbon cloth were investigated.
The goal was to study the effects of the surface chemistry of the
support, the amount and ratio of palladium and gold, and the heat
treatment of the catalysts in either H2 or air on the dispersion and
morphology of metallic components, and on the performance of
the Pd–Au bimetallic catalysts.
The pore size distribution and specific surface area of
the supports and the catalysts were measured via nitrogen
adsorption/desorption isotherms at liquid nitrogen temperature
◦
(−350 C) using an Automated Gas Sorption System, (Sorptomatic
1900, Carlo Erba Instruments). All the samples were degassed at a
◦
temperature of 100 C for about 3 h prior to the measurements.
Metal (Au and Pd) contents of the catalysts were determined
using flame Atomic Absorption Spectroscopy (AAS). AAS tests
were performed with a Thermo Scientific ICE 3000 series atomic
absorption spectrometer using an air-acetylene flame. Samples for
analysis were prepared by dissolving 200 mg of the dried catalyst in
an aqua regia solution followed by the addition of 500 ml of deion-
ized water to dilute the solution. AAS was employed to determine
the weight % of the metal incorporated in the support after impreg-
nation. Moreover, it was used to determine the amount of Au or Pd
leached out into the reaction medium during the direct synthesis
test. The leached amount was calculated by comparing the Au and
Pd content of the used catalyst and fresh catalyst.
Temperature programmed desorption (TPD) was employed to
study different oxygen-containing surface functional groups in the
supports and catalysts. The TPD tests were run in a set-up made
of a U-shaped tubular reactor, placed inside an electrical furnace
2
. Experimental
coupled to a Blazers Omnistar mass spectrometer for gas analyses.
◦
2
00 mg of each sample was heated up to 1000 C with a constant
◦
2.1. Catalysts preparation
heating rate of 50 C/min. During the tests, He was passed through
the reactor with the flow rate of 60 ml/min. The amounts of CO
and CO2 desorbed from the samples were recorded by the mass
spectrometer.
A commercial activated carbon cloth ACC-5092-20 (Kynol
Europa GmbH), after cleaning and a chemical treatment, was
employed as a support. Palladium (II) chloride (PdCl ) and gold (III)
Ultra-high resolution field emission scanning electron
microscopy (UHRFESEM) and scanning transmission electron
microscopy (STEM) were hired to assess particle size, shape,
surface morphology, and to monitor the agglomeration tendency
of metal particles. The UHRFESEM, STEM, and energy dispersive
spectroscopy (EDS) analyses were done by a Hitachi S-4800 micro-
scope with an X-ray detector for micro-analytical X-ray mapping
and quantitative analysis. The active area of the X-ray detector for
2
chloride hydrate (HAuCl ) were used for making metal precursors.
4
Nitric acid was used for acid pretreatment of the support. The acti-
◦
vated carbon cloth was first treated in an oven at 100 C for about
1
2 h. Then it was cleaned with 50% solution of methanol at room
temperature for 1 h, and finally washed with plenty of deionized
water. For wet oxidation, the cleaned ACC was treated with 20%
solution of nitric acid at room temperature for 40 h. After this oxida-
tive treatment the samples were washed with deionized water until
2
quantitative analysis was 10 mm .
◦
neutral conditions were reached, and then dried at 60 C for 24 h.
The oxidation states of the active components (Pd and Au) were
determined by X-ray photoelectron spectroscopy (XPS). The spec-
tra were obtained with a Perkin Elmer 5400 spectrometer, by using
the Mg Ka (1253.6 eV) radiation of twin anode in the constant ana-
lyzer energy mode with pass energy of 37.75 eV. The pressure of
This sample is referred as oxidized ACC (OACC). Acid pre-treatment
step was used to create oxygen-containing functional groups on the
surface of activated carbon and to modify the surface as suggested
by several authors [38–43].
Please cite this article in press as: D. Gudarzi, et al., Promotional effects of Au in Pd–Au bimetallic catalysts supported on activated
carbon cloth (ACC) for direct synthesis of H O2 from H2 and O , Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.011
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the analysis chamber was maintained at 2 × 10− Pa. The binding
7
Another parameter which can be used in order to take in to
account the metal content of the catalysts in their activities is pro-
ductivity:
energy scale was calibrated by setting the Au 4f7/2 transition to
4.0 eV. The spectra were analyzed by using Unifit peak-fitting pro-
8
gram.
moles of produced H O2 (mol)
reaction time (h) × the metal content of the catalyst (kg)
2
Productivity =
2.3. Direct synthesis reaction
(4)
The synthesis was carried out in a stainless steel autoclave (Parr
Instruments Ltd) with a nominal volume of 450 ml and a maxi-
mum working pressure of 140 bar. The autoclave was equipped
with an overhead stirrer (0–2000 rpm) and facilities to measure the
temperature and pressure. Hydrogen (18 mmol) was fed into the
reactor through a gas-holder with a volume of 33 ml and through
a flow controller. The gas-holder was used to measure the precise
amount of hydrogen. A circulation line equipped with a multi-way
valve was installed for sampling at different times (as shown in
the previous paper [6]). The sampling line was designed to cause
minimum pressure changes and gas losses during sampling. Typ-
ically, the reactor was charged with about 60 mg of catalyst and
successively with carbon dioxide up to 15.2 bar at room tempera-
ture. The pressure was thereafter elevated to 20.2 bar with oxygen
and further to 35.2 bar with carbon dioxide again at room temper-
ature. Catalysts were fixed in the reactor by means of a metal stand
as shown in the previous paper [6]. Methanol was employed as a
reaction medium. 175 g of methanol was pumped in and the reac-
3
.1. The effects of Au/Pd ratio and metal content in the catalytic
activity of Pd–Au bimetallic catalysts
3.1.1. Increasing the amount of gold with a constant amount of
palladium
The catalytic performance of Pd–Au bimetallic catalysts with
wt% Pd and 1–5 wt% Au supported on the oxidized ACC are pre-
1
sented in Fig. 1. For comparison, also the results with 1 wt% Pd
monometallic catalyst are shown. It can be observed that:
–
–
All Pd–Au bimetallic catalysts are more selective than the Pd
monometallic catalyst. Especially in the first 60 min of the
reaction time, almost all of the Pd–Au bimetallic catalysts are
considerably more selective than the Pd monometallic catalyst
(Fig. 1).
In the case of Pd–Au bimetallic catalysts, with increasing the
amount of gold, the selectivity drops, especially during the first
◦
tor was cooled down to about −1 C. Stirring (1250 rpm) was begun
after 2/3 of the methanol had been fed. When the desired tem-
perature was reached, the partial pressure of hydrogen was raised
up to 3.2 bar. The reaction time now began. The gas–liquid and
liquid–solid mass transfer effects were eliminated by adjusting the
stirring speed. This was done beforehand by carrying out a series of
tests varying the stirring rate. It was found out that 1250 rpm was
a sufficient stirring rate to eliminate the mass transfer limitations.
6
0 min. However, at reaction times above 1.5 h, the differences
between the selectivity of the different Pd–Au catalysts became
smaller (see the exact values in Table 1).
In the case of Pd–Au bimetallic catalysts, the activity (yield,
productivity and H2 conversion rates) was increased when
increasing the amount of gold (Fig. 1, Table 1).
The Pd–Au catalysts with less than 3 wt% Au are less active
in water production than the 1 wt% Pd catalyst. Especially the
catalysts with 1 and 2 wt% gold produced substantially lower
amount of water (more than 50% less) when compared to the
Pd monometallic catalyst (see the exact values in Table 1). How-
ever, increasing the Au content from 3 wt% to 4 wt% seemed to
cause a sudden increase in water production (see the exact values
in Table 1).
–
–
The concentration of H O2 which was formed in the reaction
2
was measured quantitatively by a volumetric iodometry titra-
tion using the following chemicals: KI (Merck), CH COOH (Purity
3
1
00%, Merck), K Cr O7 (purity 99.99%, Aldrich) and Na S O ·5H O
2
2
2
2
3
2
(
Merck). The produced water was determined by coulometric
Karl–Fischer titration using a Mettler DL35 Karl–Fischer titrator
and hydranal reagents (Fluka). The water content in the reac-
tion medium before catalyst addition was determined prior to
–
–
Indeed the 1% Pd 4% Au catalyst was substantially more active
each experiment. Palladium (II) chloride (PdCl , purity 99.999%,
2
than the other catalysts in both H O2 and water production.
2
Sigma–Aldrich) and gold (III) chloride hydrate (HAuCl , purity
4
Fig. 1 demonstrates that the selectivity of catalysts (at least in
the case of Pd–Au bimetallic catalysts) could be changing with the
reaction time. This is because of the direct synthesis of H O has a
9
9.999%, Aldrich) were used as metal precursors. Nitric acid 65%
(
Merck) was used for acid pretreatment of the support. Research
2
2
grade oxygen, hydrogen and carbon dioxide were purchased from
AGA. Methanol with a purity of 99.9% was used as a solvent (Merck).
complex mechanism and could be influenced by different param-
eters. Therefore, in comparison of the performance of different
catalysts, it is needed to consider the results versus time.
3
. Results and discussions
In general and according to the results presented in Fig. 1 and
Catalytic performance was evaluated by the means of the
Table 1, the Pd–Au catalysts are more selective but less active (have
lower yield and productivity) than the Pd monometallic catalyst
even with lower total metal content. The only exception is the cata-
following parameters: the final concentration of the produced
hydrogen peroxide and water (wt%), selectivity (%), yield (%), and
total H conversion (%). The last three parameters are defined as
2
follows:
lyst with 4 wt% Au. This catalyst is substantially more active in H O2
2
moles of produced H O2
and water production than the other bimetallic and monometallic
Pd catalysts. Its selectivity is almost constant and does not change
with the reaction time. However, the selectivity of the other Pd–Au
catalysts is strongly dependent on reaction time, especially up to
2
Selectivity (%) =
× 100
moles of produced H O2 + moles of produced H O
2
2
(1)
1
h.
Yield (%) = moles
of produced H O2
moles of H2 fed
× 100
(2)
2
3.1.2. Increasing the amount of palladium with a constant
amount of gold
moles of produced H O2 + moles of produced H O
In order to study the effect(s) of increasing the amount of palla-
dium in Pd–Au bimetallic catalysts, a series of Pd–Au catalysts with
constant amount of gold (1 wt%) and maximum 4 wt% palladium
2
2
Conversion (%) =
× 100
moles of H2 fed
(3)
Please cite this article in press as: D. Gudarzi, et al., Promotional effects of Au in Pd–Au bimetallic catalysts supported on activated
carbon cloth (ACC) for direct synthesis of H O2 from H2 and O , Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.011
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◦
Fig. 1. The effect(s) of increasing the amount of gold on the catalytic activity of Pd Au bimetalic catalysts supported on oxidized ACC; mass of catalyst = 56 mg, Treaction = 0 C,
◦
and all catalysts calcined in air at 185 C.
were prepared. The results of the performance of the catalysts are
presented in Fig. 2. It can be observed that:
different Pd–Au catalysts became smaller (less than 15% in the
worst cases; see Table 1).
–
The Pd–Au bimetallic catalysts with 1 wt% gold and more than
1
wt% Pd almost demonstrated constant selectivity versus time
–
When increasing the amount of palladium from 1 wt% to 2,
, and 4 wt%, the selectivity of the Pd–Au bimetallic catalysts
(Fig. 2), otherwise than the corresponding catalysts presented in
Fig. 1.
3
dropped a lot, especially in the first hour of the reaction time.
It dropped more than 37% during the first 15 min. At the reaction
times above 1.5 h, the differences between the selectivity of the
– When increasing the amount of palladium in the Pd–Au cat-
alysts, the rate of H2O2 production, yield, and H2 conversion
were increased almost with similar trends (Fig. 2, Table 1). When
Table 1
The catalytic activity of the Pd and Au mono metallic, and Pd Au bimetallic catalysts supported on oxidized ACC after 3 h of the reaction time (in H2O2 direct synthesis). All
◦
catalysts were calcined in air at 185 C.
ꢀ
ꢁ
molH2
hreaction kgcat
O
2
Catalyst
H2O2 (mmol/L)
H2O (mmol/L)
Selectivity (%)
Yield (%)
Productivity
H2 conversion (%)
1
3
5
1
1
1
1
2
3
4
3
2
2
% Pd
% Pd
% Pd
12.47
24.45
19.72
6.33
11.96
15.03
32.56
22.85
35.28
39.51
26.88
21.98
30.41
13.04
28.13
24.75
3.77
48.88
46.49
44.34
62.63
63.00
54.40
57.08
51.22
56.58
47.70
50.81
56.41
56.34
14.20
27.63
24.87
7.67
12.66
15.81
37.51
25.68
44.62
45.34
28.52
24.36
32.45
1467
985
520
408
514
522
846
984
1141
1025
693
570
791
29.05
59.42
56.09
12.24
20.10
29.05
65.71
50.15
78.88
95.07
56.13
43.18
57.59
% Pd 1% Au
% Pd 2% Au
% Pd 3% Au
% Pd 4% Au
% Pd 1% Au
% Pd 1% Au
% Pd 1% Au
% Pd 2% Au
% Pd 3% Au
.5% Pd 2.5% Au
7.02
12.60
24.49
21.77
27.08
43.32
26.03
16.99
23.57
Please cite this article in press as: D. Gudarzi, et al., Promotional effects of Au in Pd–Au bimetallic catalysts supported on activated
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Fig. 2. The effect(s) of increasing the amount of palladium on the catalytic activity of Pd Au bimetalic catalysts supported on oxidized ACC; mass of catalyst = 56 mg,
◦
◦
Treaction = 0 C, and all catalysts calcined in air at 185 C.
increasing palladium from 1 to 2 wt% and from 2 to 3 wt%, the
– All Pd–Au catalysts with Au/Pd ≥ 1 were more selective when
compared to the 5 wt% Pd monometallic catalyst and the other
bimetallic catalysts with Au/Pd < 1 (see exact values in Table 1).
– All Pd–Au bimetallic catalysts were more selective than the 5 wt%
Pd monometallic catalyst when the reaction time was above 1.5 h.
– The bimetallic catalysts with Au/Pd < 1 were less selective when
compared to the 5 wt% Pd monometallic catalyst when the reac-
tion time was less than 1.5 h.
rate of H O production, yield, productivity, and H2 conver-
2
2
sion jumped a lot (see exact values in Table 1). However, when
increasing palladium from 3 to 4 wt%, these quantities did not
change considerably. Indeed, the catalyst with 3 and 4 wt%Pd
produced almost the same yields (see exact values in Table 1).
When increasing the amount of palladium, the production rate of
–
–
H O with the Pd–Au catalyst was also increased. However, when
2
increasing palladium from 2 to 3 wt%, the rate did not change so
much.
– The most selective one is the 2% Pd 3% Au bimetallic catalyst.
However, it is also the least active catalyst. The rate of water
Based on the results presented in Figs. 1–2 and Table 1, it can
be said that the bimetallic catalysts with 1 wt% Au and ≥2 wt%
Pd are more active (have higher yield and productivity) than the
corresponding bimetallic catalysts with 1 wt% Pd and ≥2 wt% Au
and Pd monometallic catalysts with same metal content.
productionand H conversionwith this catalystwas considerably
2
low when compared to the others (see exact values in Table 1).
– In the case of bimetallic catalysts, the 4% Pd 1% Au was substan-
tially more active than the others (see exact values in Table 1). The
rate of H O2 and water production and consequently yield and
2
H2 conversion rate with this catalyst were considerably higher
than with the others. However, it was the least selective one,
especially when the reaction time was below 1.5 h.
– The second active catalyst was the one with 1 wt% of Pd and 4 wt%
of Au. It was also one of the most selective ones. However, its
activity was considerably less than the most active catalyst (4%
Pd 1% Au).
In general, it can be concluded that increasing the amount of
palladium in the Pd–Au bimetallic catalysts resulted in better activ-
ity but worse selectivity. Moreover, it should be mentioned that
gold monometallic catalysts did not show any noticeable activity
in direct synthesis of H O .
2
2
–
The third active catalyst was the one with 2.5 wt% of Pd and
.5 wt% of Au. It was also one the most selective ones. Indeed,
its selectivity was a little smaller than with the most selective
catalyst (2% Pd 3% Au).
3
.1.3. Changing Au/Pd ratio with a constant total metal content
The effects of the Au/Pd ratio on the final performance of
2
the Pd–Au bimetallic catalysts with constant total metal content
Pd + Au = 5 wt%) are presented in Fig. 3. It can be observed that:
(
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Fig. 3. The effect(s) of the Pd/Au ratio on the catalytic activity of Pd Au bimetallic catalysts supported on oxidized ACC with constant amount of metal content (Pd + Au = 5 wt%);
◦
◦
mass of catalyst = 56 mg, Treaction = 0 C and all catalysts calcined in air at 185 C.
–
The rate of water production and H2 conversion of the all the
catalysts were almost the same, except with catalysts 4% Pd 1%
Au and 2% Pd 3% Au.
palladium and gold were in zero-valent state or in metallic form
0
+2
(Pd ). However, traces of palladium were in the ionic form (Pd ).
Ultra-High Resolution Field Emission Scanning Electron Micro-
scope (UHRFESEM) images of the bimetallic catalysts with 1 wt%
Pd and 1 wt% Au are presented in Fig. 4 (supplementary UHRFE-
SEM images of different bimetallic catalysts were presented in
the Supporting information (Figs. 3–5)). Based on the UHRFESEM
images, the bimetallic catalysts consist of mainly small metal par-
ticles (Fig. 4b) with the size mostly 5–15 nm and also relatively big
ones with different sizes in the order of 100 nm (Fig. 4a and c). All
of them (small and big particles) are well dispersed in the support.
Concerning the small metal particles, it can be said that:
3.1.4. Discussion
All of the catalysts presented in Figs. 1–3 were supported on
the same carrier (oxidized ACC). Therefore, surface chemistry of
the support is the same for all of them. However, different amount
of the loaded metal could slightly affect the free available surface
area of the support in the final catalyst (Table 2). Therefore, it can be
concluded that the surface chemistry of the support cannot be main
reason for the observed differences in the performance of different
catalysts in Figs. 1–3.
Oxidation state of palladium and gold in the catalysts were
determined by X-ray photo electron microscopy (XPS). XPS spectra
of the presented bimetallic catalysts revealed that almost all of the
–
–
In the case of catalyst with 1 wt% Pd and 1 wt% Au, the size of
small particles was mainly around 5 nm (Fig. 3a of Supporting
information).
When the amount of Pd was increased to 3 wt%, the size of the
small particles was also increased to 10–15 nm. However, some
particles with size 20–50 nm can be found (Fig. 4c and d of Sup-
porting information).
Table 2
The effect of impregnation of oxidized ACC with different amount of gold and pal-
ladium, on the specific surface area of the final bimetallic catalysts.
−1
)
Sample
SN2 (m2
g
–
The catalyst with 1 wt% Pd and 3 wt% Au consist of considerable
number of gold particles with different shape and the size mainly
Oxidized ACC (OACC)
1550
1521
1480
1468
1424
1397
(1% Pd 1% Au)/OACC
(1% Pd 2% Au)/OACC
(1% Pd 3% Au)/OACC
(3% Pd 1% Au)/OACC
(2.5% Pd 2.5% Au)/OACC
2
0–50 nm (Fig. 5c and d of Supporting information). However,
similar to the catalyst with 3 wt% Pd and 1 wt% Au, the small Pd
particles were mostly 10–15 nm (Fig. 5e of Supporting informa-
tion).
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Fig. 4. The ultra-high resolution field emission scanning electron microscopy (UHRFESEM) images (with different magnifications) of 1 wt% Pd 1 wt% Au catalyst supported
on oxidized ACC with different magnification.
–
–
Based on EDS analysis of UHRFESEM images, small particles con-
sist of monometallic palladium and gold along with Pd–Au alloy
particles.
In the case of catalyst with 3 wt% Pd and 1 wt% Au, small parti-
cles mostly consist of monometallic Pd particles. However, in the
case of catalyst with 1 wt% Pd and 3 wt% Au, the number of Au
monometallic particles was considerable.
Spectroscopy (EDS) analysis of STEM image of a Pd–Au particle
(Fig. 5) reveal that these relatively round and big particles con-
sist of a gold-rich core which was surrounded by a lot of small
palladium particles. As a result, the outer surface of activated
carbon cloth in 1% Pd 1% Au catalyst was changed to dark gray
which is slightly lighter than the black color of the ACC. This
pale gray color could possibly be assigned to the natural color of
palladium.
Dispersion of the metal particles could be related to surface
– When increasing the amount of palladium, like in 3 wt% Pd 1 wt%
Au bimetallic catalyst, the small Pd particles seemed to form a
thicker and denser layer around the golden core. Therefore the
Pd–Au particles on the outer surface of ACC fibers grew bigger
(Fig. 4a and b of Supporting information).
chemistry of the oxidized ACC and the electrostatic interaction
between the surface of ACC fibers and ionic gold and palladium in
the precursors. Ionic gold (Au3+) in chloroauric acid (HAuCl ) solu-
4
−
2+
tion is mainly in the form of (AuCl4 ) [44]. Ionic palladium (Pd ) in
acidic solution of palladium dichloride (PdCl ) appears mainly as
– When the amount of gold was increased, like in 1 wt% Pd 3 wt%
Au bimetallic catalyst, the round core–shell morphological Pd–Au
particles were formed over bigger thin spots of gold (Fig. 5a and
b of Supporting information). The round Pd–Au particles had
almost the same morphology as the particles in 1% Pd 1% Au cat-
alyst. The new morphology of Pd–Au particles was supported by
EDS analysis of UHRFESEM images (Fig. 6). However, the changes
of the color of the surface of the activated carbon cloth to pale yel-
low in the bimetallic catalysts with higher amount of gold can be
seen.
– The morphology of the Pd–Au particles on the outer surface of
ACC fibers in the 2.5 wt% Pd 2.5 wt% Au bimetallic catalyst looked
almost the same as in 1 wt% Pd 3 wt% Au catalyst.
– The big Pd–Au particles formed on the outer surface of ACC fibers
mainly consist of gold-rich core covered by small Pd particles
with the size mostly less than 10 nm (Figs. 4c and 3b of Supporting
information). Therefore, they could not be inactive. Indeed, it is
assumed that Au acts as an electronic promoter for the Pd-rich
shell [5,30,31].
2
tetrachloropalladic ion complex (PdCl42 ) [45]. Wet oxidation of
activated carbon cloth (ACC) with nitric acid leads to formation of
oxygen-containing functional groups on the surface of ACC fibers
−
(
see Section 3.3 for more detailed discussion). These acidic groups
charged negatively the surface of ACC fibers. The electrostatic inter-
−
2−
)
action (repulsion) between the metal precursors (AuCl4 , PdCl4
and the negatively charged surface of the oxidized ACC could result
in good dispersion of metal precursors on the support surface.
However, largely aggregated metallic particles (specially Au) were
obtained principally due to the redox properties of activated carbon
[
46,47] In fact, the higher variety of activated carbon functional-
ities than those present on oxides (like silica and alumina) can
promote other electrostatic interactions [46]. Concerning the big
metal particles, it can be observed that:
–
In the case of the catalyst with 1 wt% Pd and 1 wt% Au, the images
with higher magnification of a big metal particle (Figs. 4c and
3
b of Supporting information) and the Energy Dispersive X-ray
Fig. 5. STEM-EDS mapping and analysis of a separated Pd Au particle in the 1 wt% 1 wt% Au bimetallic catalyst; (a) STEM image, (b) Au map, and (c) Pd map.
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Fig. 6. STEM-EDS mapping and analysis of the 1 wt% 3 wt% Au bimetallic catalyst; (a) STEM image, (b) Au map, and (c) Pd map.
were in the form of Pd+2 originated from palladium dichloride
PdCl2.
Based on the discussion above, it can be concluded that the mor-
phology and size of the metal particles in the bimetallic catalysts
are determined by the amount of palladium and gold and their
ratio. Consequently, the different selectivities and activities of the
Pd–Au catalysts presented in Figs. 1–3 could mostly be related to
the different morphology and size of the metal particles.
◦
– When increasing the calcination temperature to 235 C almost
8 wt% of the total amount of the palladium was oxidized to pal-
ladium oxide (PdO).
◦
– In the catalyst calcined at 275 C, almost 63 wt% of the total
amount of palladium is in the form of palladium oxide. Therefore,
increasing the calcination temperature increases the amount of
palladium oxide.
3.2. The effects of the oxidation state of the loaded metal
–
In the case of 1% Pd-4% Au bimetallic catalyst, when increasing
the calcination temperature, the amount of PdO was substan-
tially low compare to the corresponding calcined catalysts with
In order to investigate the effects of the oxidation state of
the metal components (Pd and Au), the catalysts supported on
non-oxidized ACC were selected. In the case of oxidized ACC,
the presence of the oxygen-containing surface functional groups
also affected the final catalytic performance. Therefore, the results
would reflect combined effect of different factors (surface chem-
istry of the support, oxidation state of the metal part . . .). In the
case of the non-oxidized ACC, lack of the oxygen-containing sur-
face functional groups makes it possible to investigate merely the
effects of the oxidation state of the metallic part of the catalysts.
The results of the catalytic activities of the different 3% Pd 1% Au
bimetallic catalysts supported on non-oxidized ACC are presented
3
2
% Pd and 1% Au. Indeed, in the 1% Pd 4% Au catalyst calcined at
75 C, only 8% of the total amount of palladium was in the form
◦
of palladium oxide.
–
–
In general, in the case of Pd–Au bimetallic catalysts with
◦
Au/Pd > 1, calcination at 235–275 C resulted in smaller amount
of palladium oxide than with Au/Pd ≤ 1.
Gold in all calcined catalyst was still in metallic or zero-valent
state.
◦
in Table 3. One of the catalysts was calcined in air at 185 C and, the
other one at 275 C. It can be observed that:
Based on the results and discussions above, it could be con-
◦
cluded that the presence of palladium oxide (PdO) made the Pd–Au
bimetallic catalysts more selective and more active than the corre-
sponding catalysts with Pd .
–
–
–
The yield and H O production rates demonstrate that the cat-
0
2
2
◦
alyst calcined at 275 C is more active in H O production than
◦
2
2
Low amount of PdO in 1% Pd 4% Au catalyst calcined at 275 C
the other.
resulted in smaller differences in catalytic performance of the two
catalysts calcined at 185 C and 275 C when compared to the cor-
responding differences observed for the 3% Pd 1% Au catalysts
However, the H2 conversion and water production rates reveal
◦
◦
◦
that the catalyst calcined at 185 C is more active in water pro-
duction than the other.
Therefore, the catalyst calcined at 275 C is more selective (about
(Table 3).
◦
5
0%) than the other.
3
.3. The effects of the surface chemistry of the ACC fibers on the
The X-ray photo electron microscopy spectra of the calcined 3%
Pd 1% Au catalysts (Fig. 7 of Supporting information) revealed that:
final catalytic performance and surface morphology of the Pd–Au
bimetallic catalysts
◦
–
Palladium in the catalyst calcined at 185 C was almost in zero-
The results of the catalytic performance of two bimetallic cat-
alysts with different amounts of palladium and gold supported on
0
valent or in metallic state (Pd ) and only traces of the palladium
Table 3
The catalytic activity of the Pd–Au bimetallic catalysts supported on non-oxidized ACC and ACC oxidized with nitric acid after 2 h of the reaction time (in H2O2 direct
◦
synthesis). The catalysts calcined in air at 185–275 C.
Catalyst
H2O2 (mmol/L)
H2O (mmol/L)
Selectivity (%)
Yield (%)
H2 Conversion (%)
Catalysts supported on non-oxidized ACC
◦
3% Pd 1% Au, Tcal = 185
3% Pd 1% Au, Tcal = 275
1% Pd 4% Au, Tcal = 185
1% Pd 4% Au, Tcal = 275
C
C
C
C
23.49
30.77
20.45
20.49
57.41
40.69
37.00
32.87
29.04
43.06
35.72
38.40
26.64
36.82
23.15
23.13
91.73
85.52
65.11
60.24
◦
◦
◦
Catalysts supported on oxidized ACC
◦
◦
3
1
% Pd 1% Au, Tcal = 185
% Pd 4% Au, Tcal = 185
C
C
29.09
24.19
19.31
17.03
60.10
58.69
36.79
33.30
61.22
47.47
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Table 4
the pHIEP, the surface is positively charged. During the impregna-
tion of non-oxidized ACC with gold and palladium precursor, the
pH of the precursor is considerably less than pHIEP of the surface
of the ACC fibers. Therefore, the electrostatic attraction between
the positively charged surface (pH < pHIEP) and the catalyst precur-
Total amount of CO and CO2 desorbed according to temperature programmed
desorption test.
Sample
CO (␮mol/g)
CO2 (␮mol/g)
Non-oxidized ACC
Oxidized ACC (OACC)
115.13
1147.52
1015.26
1390.23
1483.51
1569.35
82.73
1290.44
445.81
935.57
742.43
529.53
− 2−
sor anions (AuCl4 and PdCl4 ) facilitate the adsorption of ionic
◦
◦
◦
◦
3
3
3
3
%Pd 1% Au/OACC (Tred = 185 C)
%Pd 1% Au/OACC (Tcal = 185 C)
%Pd 1% Au/OACC (Tcal = 235 C)
%Pd 1% Au/OACC (Tcal = 270 C)
−
2−
gold and palladium species (AuCl4 and PdCl4 ) on the surface of
the ACC fibers. Moreover, carbon is essentially not hydrophilic in
nature and it has a very low affinity for solvents of polar character
such as water [40]. Therefore, the metal precursors will be mostly
exposed to the outer surface of the ACC fibers when using water.
This factor together with electrostatic attraction between the sur-
non-oxidized ACC and ACC oxidized with nitric acid are presented
in Table 3. It can be observed that:
−
face of non-oxidized ACC and the catalyst precursor anions (AuCl
4
and PdCl42 ) leads to a rapid massive deposition of Au and Pd on
−
–
The catalysts supported on oxidized ACC were substantially
more selective than the corresponding ones supported on non-
oxidized ACC. In the case of the catalyst with 3 wt% Pd and 1 wt%
Au, the catalyst supported on oxidized ACC was almost 2 times
more selective than the corresponding one on non-oxidized ACC.
According to the results of yield and rate of H O production, the
catalysts supported on oxidized ACC were also more active than
the corresponding catalysts on non-oxidized ACC.
H2 conversion results and the rate of water production demon-
strated that the catalysts supported on non-oxidized ACC were
considerably more active in water production than the corre-
sponding ones on the oxidized ACC.
Almost similar results were observed for the other bimetallic
catalysts presented in this paper. The only difference is that in
the case of bimetallic catalysts with Au/Pd ≥ 1, the differences
between performances of the catalysts supported on oxidized
ACC and non-oxidized ACC is smaller than the catalysts with
Au/Pd < 1. This can be explained as, in general, with increasing
the amount of Au, bimetallic catalysts become more selective
and less active.
the outer surface of ACC fibers.
Moreover, in the case of oxidized ACC, oxygen-containing
surface functional groups might also affect directly the final perfor-
mance of the catalysts. These acidic groups negatively charge the
surface of activated carbon cloth [6] and decrease the hydropho-
bicity of the surface [40]. Therefore, the concentration of water
on the surface of ACC’s fibers and pores is higher in the oxidized
ACC than in non-oxidized one. The affinity of the surface of oxi-
dized ACC holding water molecules might also affect the final
–
–
2
2
rate of water production by H O2 decomposition/hydrogenation
2
(H O → H O + (1/2)O ; H O + H → 2H O) and non-selective H O
2
2
2
2
2
2
2
2
2
synthesis (H + (1/2)O → H O), according to the Le Chatelier’s prin-
2
2
2
–
ciple.
3.4. The effects of heat treatment of the catalysts in H2 and air at
different temperatures
The results of the catalytic performance of the different 3% Pd 1%
Au catalysts supported on oxidized ACC are presented in Fig. 7. It
can be observed that:
◦
In our previous work [6], it was demonstrated that the oxida-
– The catalysts calcined in the air at 185–275 C were almost 3
◦
tion pretreatment of activated carbon cloth (ACC) with nitric acid
introduce considerable amounts of oxygen-containing functional
groups on the surface of ACC fibers. These functional groups can
be classified as strong acidic oxygen-containing groups (like car-
boxylic and anhydride groups) and weak acid ones (like lactones,
phenols and carbonyl groups) [38–43]. These acidic functional
groups affect a lot the activity and physicochemical properties
of final catalysts [6]. The oxygen-containing functional groups
can be characterized by thermal programmed desorption (TPD)
test [6,38–41]. Strong and weak acidic oxygen-containing groups
are responsible for CO2 and CO desorption in TPD, respectively
times more selective than the catalyst reduced by H2 at 185 C.
– According to the results of yield and H O2 production rate, the
calcined catalysts were substantially more active in H O2 pro-
2
2
duction than the corresponding reduced one.
– The catalyst reduced by H was dramatically more active in water
2
production than the calcined catalysts.
◦
– The catalyst calcined at 235 C was considerably more active in
H O and also in water production than the other calcined cata-
2
2
lysts.
◦
– In the case of calcined catalysts, the catalyst calcined at 185 C
had the highest selectivity and, the one calcined at 235 C had
◦
(
Table 4). Different surface chemistry of the supports (Table 4) leads
the lowest selectivity. However, the selectivities of the different
calcined catalysts were quite close to each other.
– In the case of calcined catalysts, the catalyst calcined at 275 C
to different morphology in Pd–Au bimetallic catalysts supported
on non-oxidize and oxidized ACC. The morphology of Pd–Au parti-
cles in the catalysts supported on oxidized ACC has been discussed
in Section 3.1.4 In the case of non-oxidized ACC, palladium and
gold form really big and joined particles at the outer surface of ACC
fibers (Fig. 6 of Supporting information). In most areas, the metal
particles almost cover the outer surface of the ACC fibers. These
completely different surface morphologies result in different cat-
alytic performance. The considerable surface morphology changes
in the non-oxidized could be explained as follow.
◦
showed lower activity in H O2 and water production than the
2
other ones.
– Almost similar results were observed for the other bimetallic
catalysts supported on oxidized ACC.
The ultra-high resolution field emission scanning electron
microscope (UHRFESEM) images and EDS analysis confirmed that
◦
the heat treatment in H2 at 185 C and the calcination in air up
It was mention earlier (Section 3.1.4) that the ionic gold (Au3+)
to 275 C had no noticeable effects on the morphology of the
◦
2
+
and palladium (Pd ) in acidic solution of hydrogen tetrachloroau-
rate (HAuCl ) and palladium dichloride (PdCl ) appear mainly as
Pd–Au particles in the catalyst. However, the heat treatment of the
oxidized ACC damages the oxygen-containing surface functional
groups (Table 4). In the case of calcined catalysts, increasing the
calcination temperature increased the damage, especially to the
stronger acidic surface functional groups. These groups are respon-
sible for CO2 releasing in temperature programmed desorption
(TPD) test (Table 4). Heat treatment of Pd–Au catalysts supported
4
2
−
2−
ion complexes (AuCl4 and PdCl4 ). The pH of the precursors
during the impregnation of the ACC was in the range of 3.5–4.
According to isoelectric point measurements [6], the pHIEP of the
oxidized and non-oxidized ACC were 3.5 and 5, respectively. At pH
below the pHIEP the surface is negatively charged while at pH over
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0
◦
Fig. 7. The catalytic activity of the different 3 wt% Pd 1 wt% Au bimetallic catalysts on oxidized ACC; mass of catalyst = 56 mg, Treaction = 0 C.
◦
on oxidized ACC by H2 at 185 C causes maximum damage to the
stronger acidic oxygen functional groups (Table 4).
Wet oxidation of activated carbon cloth (ACC) resulted in for-
mation of oxygen-containing functional groups on the surfaces
of ACC’s fibers. These functional groups mainly control surface
morphology, and consequently the selectivity and activity of the
bimetallic catalysts in direct synthesis of H O . The bimetallic cat-
From the other side, increasing the calcination temperature
increases the amount of PdO in the final catalyst (see Section 3.2).
Based on the results and discussions in Sections 3.2 and 3.3, higher
amount of palladium oxide in the catalyst and fewer damages to the
surface functional groups (in the case of ACC oxidized with nitric
acid) are favored for direct catalytic synthesis of H O . Therefore,
increasing the calcination temperature has two opposite effects,
namely the damage to the oxygen-containing surface functional
groups (Table 4) and increasing the amount of palladium oxide, and
the resultant of these two effects mainly determine the different
activities observed in Fig. 7.
2
2
alysts supported on the oxidized ACC were substantially more
selective and also more active in H O2 production than the cor-
2
responding ones supported on non-oxidized ACC. However, the
catalysts supported on non-oxidized ACC had high H2 conversion
since they were considerably more active in water production than
the corresponding catalysts on oxidized ACC.
Almost all of the Pd–Au bimetallic catalysts were more selective
than Pd monometallic catalysts. In the case of bimetallic catalysts:
2
2
◦
In the case of the catalyst reduced by H2 at 185 C, it seems that
the lack of palladium oxide and maximum damages to the oxygen-
containing surface functional groups (Table 4) caused the catalyst
–
–
–
Increasing the amount of Au resulted in more selective but less
active catalysts.
Increasing the amount of Pd resulted in more active but less
selective catalysts.
Moreover, morphology of Pd–Au particles was also affected with
the amount and ratio of gold and palladium in the catalysts.
to be considerably less selective and less active in H O production
2
2
and substantially more active in water production than the calcined
catalysts.
4
. Conclusions
The results showed that the surface morphology of the Pd–Au
The presence of palladium oxide made the catalysts more
bimetallic catalysts and their catalytic performance in direct syn-
thesis of H O were highly influenced by the surface chemistry
selective and active in H O production but less active in water pro-
2
2
2
2
duction than the corresponding catalysts consisting of zero-valent
of the support and the amount and the ratio of Pd and Au in the
catalyst.
0
palladium (Pd ).
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Acknowledgement
[21] Y.-F. Han, J.H. Lunsford, J. Catal. 230 (2005) 313.
[
[
[
22] Q. Liu, J.H. Lunsford, J. Catal. 239 (2006) 237.
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Kolehmainen1, I. Turunen, D.Y. Murzin, T. Salmi, Chem. Eng. Trans. 21 (2010)
The authors thank Dr. Kari Eranen for helping with the TPD and
SSA measurements.
925.
[
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[
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Please cite this article in press as: D. Gudarzi, et al., Promotional effects of Au in Pd–Au bimetallic catalysts supported on activated
carbon cloth (ACC) for direct synthesis of H O2 from H2 and O , Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.04.011
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