Direct oxidation of hydrogen to hydrogen peroxide over Pd-containing fluorinated or sulfated Al2O3, ZrO2, CeO2, ThO2, Y2O3 and Ga2O3 catalysts in stirred slurry reactor at ambient conditions

Article abstract of DOI:10.1016/S1381-1169(01)00359-4

Direct oxidation of H₂ to H₂O₂ over powdered PdO-containing fluorinated or sulfated Al₂O₃, ZrO₂, CeO₂, ThO₂, Y₂O₃, Ga₂O₃ catalysts with or without pre-reduction by hydrazine, using a pure water or 0.02 M H₂SO₄ as reaction medium and a gaseous feed consisting of 1.7 vol.% H₂ in O₂, in a magnetically stirred glass reactor at 22°C and atmospheric pressure has been investigated. The catalysts have been characterized for their surface area and also by CO chemisorption. The performance of the catalysts in the decomposition of H₂O₂ at 22°C has also been studied. The results showed a strong influence of catalyst reduction, fluorination (or chlorination) or sulfatation of the catalyst support and the acidity of reaction medium on both the conversion and H₂O₂ selectivity in the direct oxidation of H₂. An acidic medium is essential for achieving both the high conversion and high selectivity in the H₂ oxidation. The reduced catalysts showed much lower H₂O₂ selectivity in the H₂ oxidation process and much higher H₂O₂ decomposition activity. The water in the H₂ to H₂O₂ oxidation over the above catalysts is formed in parallel and/or consecutive reactions, depending upon the catalyst used.

Full text of DOI:10.1016/S1381-1169(01)00359-4

Journal of Molecular Catalysis A: Chemical 181 (2002) 143–149
Direct oxidation of hydrogen to hydrogen peroxide over
Pd-containing fluorinated or sulfated Al O , ZrO ,
2
3
2
CeO , ThO , Y O and Ga O catalysts in stirred
2
2
2
3
2
3
slurry reactor at ambient conditions
∗
A.G. Gaikwad, S.D. Sansare, V.R. Choudhary
Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India
Received 23 January 2001; received in revised form 8 June 2001; accepted 27 June 2001
Abstract
Direct oxidation of H2 to H2O2 over powdered PdO-containing fluorinated or sulfated Al2O3, ZrO2, CeO2, ThO2, Y2O3,
Ga2O3 catalysts with or without pre-reduction by hydrazine, using a pure water or 0.02 M H2SO4 as reaction medium and a
◦
gaseous feed consisting of 1.7 vol.% H2 in O2, in a magnetically stirred glass reactor at 22 C and atmospheric pressure has
been investigated. The catalysts have been characterized for their surface area and also by CO chemisorption. The performance
◦
of the catalysts in the decomposition of H2O2 at 22 C has also been studied. The results showed a strong influence of catalyst
reduction, fluorination (or chlorination) or sulfatation of the catalyst support and the acidity of reaction medium on both
the conversion and H2O2 selectivity in the direct oxidation of H2. An acidic medium is essential for achieving both the
high conversion and high selectivity in the H2 oxidation. The reduced catalysts showed much lower H2O2 selectivity in the
H2 oxidation process and much higher H2O2 decomposition activity. The water in the H2 to H2O2 oxidation over the above
catalysts is formed in parallel and/or consecutive reactions, depending upon the catalyst used. © 2002 Elsevier Science B.V.
All rights reserved.
Keywords: Oxidation of H2 to H2O2; Supported palladium catalysts; Fluorinated palladium catalyst; Sulfated palladium catalyst;
Decomposition of H2O2
1
. Introduction
presently practiced anthraquinone process [1] for the
production of H2O2 has several other limitations; the
cost of H2O2 produced by this process is not low
enough to use it for producing bulk chemicals and/or
for wastewater treatment. It is, therefore, of great
practical interest to produce H2O2 more economically
by a novel process, which is also environmentally
clean, such as that based on the direct oxidation of
H2. Direct oxidation of H2 to H2O2 over different
Pd-containing catalysts has been disclosed in several
patents [2,3]. But the information on this process in
the published papers is scarce [2–4]. Very recently,
Hydrogen peroxide is an environ-friendly strong
oxidizing agent. Hence, its demand for domes-
tic/industrial wastewater treatment, water disinfec-
tion, catalytic oxidation processes and other industrial
applications has been increasing day-by-day. The
∗
Corresponding author. Tel.: +91-20-589-3300x2163;
fax: +91-20-589-3041/3355.
E-mail addresses: vrc@che.ncl.res.in, vrc@ems.ncl.res.in (V.R.
Choudhary).
1
381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00359-4

1
44
A.G. Gaikwad et al. / Journal of Molecular Catalysis A: Chemical 181 (2002) 143–149
we have suggested the use of a novel composite
Pd-membrane catalyst for the non-hazardous direct
oxidation of H2 to H2O2 [2]. Earlier, Fu et al. [4] have
carried out liquid-phase oxidation of H2 to H2O2 in
an acidic medium over Pd/fluorinated carbon catalyst
also in the decomposition of H2O2, in a stirred slurry
reactor.
2. Experimental
◦
at high pressures (0.5–2.3 MPa) at −10 to 25 C; the
maximum H2O2 selectivity reported was 8.7% (at
2.1. Catalyst preparation
4
1% H2 conversion) even in the presence of a H2O2
stabilizer. They have observed that the H2O2 decom-
position to water and oxygen occurs in a consecutive
reaction on the same catalyst; the higher the H2O2
decomposition activity of the catalyst, the lower is
the H2O2 selectivity in the H2 oxidation process.
The flammability or explosive range for the con-
centration of H2 in O2 is very wide (4–94% H2 in O2).
Hence, the direct H2 oxidation process is highly haza-
rdous one. However, it can be made non-hazardous
by operating it with the H2 concentrations (in O2) far
away from the explosion range.
This work was undertaken with the objective of
studying the performance of different supported Pd
or PdO catalysts (such as Pd or PdO supported on
alumina, zirconia, ceria, thoria, yttria and gallia, with
or without their modification by fluorination, chlori-
nation or sulfatation) in the direct oxidation of H2
to H2O2 using a mixed H2/O2 feed (1.7 vol.% H2 in
O2, which is far away from the explosion range) and
The PdO-containing catalysts (Table 1) were prepa-
red by impregnating PdCl2 from its HCl solution on
␥-Al2O3, ZrO2, CeO2, Y2O3 or ThO2, fluorinated
␥-Al2O3, ZrO2, Ga2O3, Y2O3, ThO2 or CeO2, and
sulfated ZrO2 or Ga2O3 by the incipient wetness
impregnation technique. After impregnation, the wet
◦
catalyst mass was dried at 100 C for 4 h and then
◦
calcined under static air in muffle furnace at 500 C
for 3 h. The presence of PdO phase in the catalyst
was confirmed by XRD.
Fluorination (or chlorination) or sulfatation was
carried out by impregnating the respective support
with ammonium fluoride (or chloride) or H2SO4
(equivalent to 5 wt.% F, Cl or S on the support) from
its aqueous solution followed by drying on hot water
◦
bath and calcining in muffle furnace at 500 C for 3 h.
The PdO-containing catalysts were reduced by
treating them with an ammonical hydrazine solu-
tion on hot water bath for 2 h followed by washing,
Table 1
Surface properties of the catalyst
2
−1
◦
Catalysts
Surface area (m g
)
CO chemisorption (at 40 C) on
reduced catalystsa (␮mol g
−1
)
PdO/␥-Al2O3
PdO/fluorinated ␥-Al2O3
PdO/chlorinated ␥-Al2O3
174.0
203.0
171.0
4.1
18.8
5.0
PdO/ZrO2
PdO/sulfated ZrO2
PdO/fluorinated ZrO2
54.5
325.0
31.5
60.1
7.3
4.8
PdO/CeO2
PdO/fluorinated CeO2
12.4
4.7
6.1
17.2
PdO/Y2O3
8.1
6.1
PdO/fluorinated Y2O3
12.3
17.5
PdO/ThO2
PdO/fluorinated ThO2
0.7
0.7
3.7
4.3
PdO/fluorinated Ga2O3
PdO/sulfated Ga2O3
19.1
34.9
14.1
3.8
a
◦
Reduced by a H2–N2 mixture (10% H2) at 500 C.

A.G. Gaikwad et al. / Journal of Molecular Catalysis A: Chemical 181 (2002) 143–149
145
◦
3
filtration and drying at 100 C in an air oven. The
presence of metallic Pd phase in the reduced catalysts
was confirmed by XRD.
reactor containing 0.2 g catalyst and 150 cm acidic
(0.02 M sulfuric acid) solution containing 0.2% (w/v)
H2O2. The amount of O2 liberated in the decompo-
sition was measured as a function of time using a
constant pressure gas collector [6].
2
.2. Catalyst characterization
The surface area of the catalysts was measured
3
. Results and discussion
by the single-point BET method, using a Monosorb
Surface Area Analyser (Quantachrome, USA).
Chemisorption of CO on the catalyst, after its reduc-
3.1. Catalysts characterization
◦
◦
tion by H2 at 500 C, was measured at 40 C using a
GC pulse technique [5].
The catalysts have been characterized for their sur-
◦
face area, CO chemisorption (at 40 C) on reduced
2
.3. Catalytic reactions
catalyst, and relative acidity (measured in terms of the
amount of NaOH required to neutralize the acid sites
on the catalyst). The surface properties of the cata-
lysts are given in Table 1. The results show that the
surface area, CO chemisorption and the relative acid-
ity are strongly influenced by the fluorination, chlo-
rination or sulfatation of the supports (except ThO2)
used in the catalyst. In general, the acidity is increased
markedly due to the halogenation or the sulfatation of
the support.
The direct oxidation of hydrogen to hydrogen
peroxide reaction over the catalysts was carried out
in a magnetically stirred glass reactor (capacity =
3
3
2
50 cm ) containing 0.5 g catalyst and 150 cm
aqueous 0.02 M sulfuric acid solution as a reaction
medium by bubbling continuously a hydrogen and
oxygen mixture (1.7 vol.% H2) at a flow rate of
−3
−1
1
1.5 cm min through the liquid reaction medium
◦
under vigorous stirring at 22 C and atmospheric
pressure (0.95 atm) for a period of 3 h. The concentra-
tion of hydrogen in the effluent gases, after removing
3.2. Effect of reaction medium
◦
water vapors from them by condensation at 0 C, was
Results showing the effect of acidity of the aqueous
reaction medium on the performance of the different
supported PdO catalysts in the H to H O oxidation
measured by an online hydrogen analyzer. After the
reaction, the catalyst from the reaction mixture was
removed by filtration and the filtered reaction mixture
was analyzed for the hydrogen peroxide formed in
the reaction by iodometric titration method.
2
2
2
process are presented in Table 2.
The results show strong influence of the acidity
of the aqueous reaction medium on both the conver-
sion and selectivity in the process. All the catalysts
show much higher activity (H conversion) and H O
The decomposition of hydrogen peroxide over the
catalysts was carried out in the magnetically stirred
2
2
2
Table 2
◦
Effect of reaction medium on conversion and selectivity in the direct oxidation of H2 to H2O2 over the different catalysts (at 22 C)
Catalyst
Reaction medium
.02 M H2SO4
0
Pure water
Hydrogen
H2O2
Hydrogen
H2O2
conversion (%)
selectivity (%)
conversion (%)
selectivity (%)
PdO/fluorinated ␥-Al2O3
PdO/chlorinated ␥-Al2O3
PdO/fluorinated CeO2
PdO/fluorinated ZrO2
PdO/ThO2
41.8
43.1
34.4
43.0
28.2
21.0
30.3
28.6
36.4
26.5
47.3
56.1
2.6
4.6
2.6
18.5
13.9
9.2
15.0
8.3
15.0
19.3
4.1
PdO/CeO2
6.1

1
46
A.G. Gaikwad et al. / Journal of Molecular Catalysis A: Chemical 181 (2002) 143–149
selectivity when acidic medium (0.02 M H2SO4) used
in the process as compared to the pure water as the
reaction medium. This is consistent with our earlier
observation that the H2O2 decomposition activity of
Pd/carbon catalyst decreases markedly with increasing
the concentration of H2SO4 in the aqueous reaction
medium [7]. In the earlier studies using Pd supported
on phosphate viologen phosphonate catalyst using
aqueous H2SO4 as the reaction medium, there was no
significant production of H2O2 in the H2 oxidation,
even at much higher pressure [3]. The H2O2 selec-
tivity observed for the present catalysts using 0.02 M
H2SO4 or even water as the reaction medium is much
higher than that observed earlier for Pd/fluorinated
carbon catalyst using an acidic medium [4].
oxidation but much higher H2O2 decomposition
activity. The decrease in the H2O2 selectivity is be-
cause of the increased H2O2 decomposition activity
of the corresponding catalyst after its reduction. These
comparative results clearly show that the catalysts in
their oxidized form are more selective and even more
active in the oxidation of H2 to H2O2 but less active
in the H2O2 decomposition.
3.4. Comparison of supported PdO catalysts
The PdO-containing catalysts are compared in
Figs. 2 and 3 for their performance both in the H2 ox-
◦
idation and H2O2 decomposition reactions (at 22 C),
using 0.02 M H2SO4 as the reaction medium. From
the comparison, following important observations for
the catalyst with different supports can be made.
Al2O3-supported PdO catalysts. Fluorination (or
chlorination) has a small effect on the H2 conversion
activity but a very large effect on the H2O2 selectivity;
the later is increased drastically by the fluorination or
chlorination of the ␥-alumina. However, the H2O2 de-
composition (in 3 h) over these catalysts is very small
when compared to the H2 conversion. This shows that
the formation of H2O2 and water in the H2 oxidation
over these catalysts occurs essentially in the parallel
reactions:
3
.3. Effect of catalyst reduction
Results in Table 3 show the effect of reduction of
the supported PdO catalysts (by hydrazine) on their
performance in both the H2 to H2O2 oxidation and
◦
H2O2 decomposition reactions at 22 C. The kinetic
◦
data for the H2O2 decomposition (at 22 C) over the
catalysts, with or without pre-reduction (by hydrazine)
are presented in Fig. 1. The H2O2 decomposition
data could be fitted well to first-order rate law: ln[1/
(
1 − x)] = kat, where x is the fractional decomposi-
tion of H2O2, ka the first-order rate constant and t the
time.
0
The pre-reduced catalysts (Pd -containing cat-
alysts) showed much lower H2O2 selectivity and
also lower activity (except for Pd/CeO2) in the H2
(1)
Table 3
Effect of the pre-reduction (by hydrazine) of catalysts on their activity/selectivity in the oxidation of H2 to H2O2 and in the decomposition
of H2O2 reactions
◦
Catalyst
Oxidation of H2 to H2O2 (at 22 C)
First-order apparent rate constant for H2O2
decomposition (at 22 C), ka × 10 (min
◦
4
−1
)
H2 conversion (%)
H2O2 selectivity (%)
PdO/fluorinated ␥-Al2O3
Pd /fluorinated ␥-Al2O3
41.8
16.4
30.3
17.6
0.0
1.0
0
PdO/fluorinated ZrO2
Pd /fluorinated ZrO2
43.0
18.5
26.5
16.8
1.3
9.8
0
PdO/CeO2
Pd /CeO2
21.1
20.3
56.1
0.0
6.3
60.8
0
PdO/ThO2
Pd /ThO2
28.2
18.5
47.3
0.0
5.1
66.4
0

A.G. Gaikwad et al. / Journal of Molecular Catalysis A: Chemical 181 (2002) 143–149
147
◦
Fig. 1. Kinetic data for the decomposition of H2O2 (at 22 C) over the different catalysts.
Fig. 2. Performance of the alumina- and zirconia-supported (with or without modification) PdO catalysts in the direct oxidation of H2 to
◦
◦
H2O2 (at 22 C) and also in the decomposition of H2O2 (at 22 C).

1
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A.G. Gaikwad et al. / Journal of Molecular Catalysis A: Chemical 181 (2002) 143–149
Fig. 3. Performance of the ceria-, yttria-, thoria- and gallia-supported (with or without modification) PdO catalysts in the direct oxidation
◦
◦
of H2 to H2O2 (at 22 C) and also in the decomposition of H2O2 (at 22 C).
Because of the very poor activity of the catalysts for
H2O2 decomposition, the possibility of water forma-
tion in the H2 oxidation in the consecutive reaction
ZrO2-supported PdO catalysts also have high H2O2
decomposition activity, which is decreased after flu-
orination or sulfatation due to the increased acidity.
Hence, the formation of H2O2 and water in the H2
oxidation over these catalysts is expected also by the
consecutive reaction (reaction 2).
O2
−0.5O2
H2 → H2O2 → H2O
(2)
is small.
The higher H2O2 selectivity and the lower H2O2 de-
composition activity of the fluorinated or chlorinated
Al2O3-containing catalyst may be due to its increased
acidity (protonic acidity) resulting from the halogena-
tion of Al2O3 [8].
ZrO2-supported PdO catalysts. In this case also,
the H2O2 selectivity is increased drastically after
the fluorination or sulfatation of the support. How-
ever, unlike the alumina-supported PdO catalysts, the
CeO2-, Y2O3- and ThO2-supported PdO catalysts.
In these cases, the fluorination of support results in an
increase in the H2 conversion but a decrease in both the
H2O2 selectivity and the H2O2 decomposition activity
of the catalyst (Fig. 3). The H2O2 decomposition activ-
ity of PdO/fluorinated CeO2 (or ThO2) catalyst is very
low and hence the formation of H2O2 and H2O over
the catalyst is expected mainly by the parallel reac-
tions (reaction 1). However, the H2O2 decomposition

A.G. Gaikwad et al. / Journal of Molecular Catalysis A: Chemical 181 (2002) 143–149
149
activity of the PdO/ThO2, PdO/CeO2 and PdO/Y2O3
is quite high. Hence, over these catalysts, the forma-
tion of water in the H2 oxidation is expected mainly
by the consecutive reaction (reaction 2).
PdO/fluorinated (or sulfated) Ga2O3 catalysts.
These catalysts showed high H2 conversion activity
but very poor selectivity for H2O2 and also poor
H2O2 decomposition activity, indicating formation of
H2O2 and H2O in the H2 oxidation mainly by the
parallel reactions (reaction 1).
decrease in the selectivity and an increase in the H2
conversion activity of the catalyst in the H2 oxida-
tion. The PdO/ThO2, PdO/fluorinated/chlorinated
or sulfated Al2O3, ZrO2, and CeO2, and PdO/CeO2
are promising catalysts for the direct oxidation of
H2 to H2O2.
2. The water in the H2 oxidation over the supported
PdO catalysts is formed in the parallel and/or
consecutive reaction, depending upon the support
used in the catalyst. It is formed mostly by the
parallel reaction over the fluorinated (or chlo-
rinated) Al2O3, CeO2 and Ga2O3 and sulfated
Ga2O3-supported PdO catalysts but by the consec-
utive reaction over the ZrO2-, CeO2-, Y2O3- and
ThO2-supported PdO catalysts.
When compared for the H2O2 yield [conversion
(
%) × selectivity (%)/100] obtained in the H2 oxida-
tion process, the catalysts showed the following order
Figs. 2 and 3): PdO/ThO2 > PdO/fluorinated ␥-
Al2O3 > PdO/fluorinated CeO > PdO/chlorinated
(
2
␥
-Al2O3 > PdO/CeO2> PdO/fluorinated ZrO2 ≥
3. The catalysts after the reduction of their PdO to
0
PdO/sulfated ZrO2 PdO/fluorinated Y O3
>
>
Pd (by hydrazine) showed much lower selectivity
2
PdO/fluorinated ThO2> PdO/fluorinated Ga O3 ≥
in the oxidation of H2 to H2O2 and much higher
H2O2 decomposition activity.
2
PdO/sulfated Ga2O3 > PdO/Y O3 > PdO/␥-Al2O3
2
≥
PdO/ZrO2.
4. Both the activity and H2O2 selectivity (in the H2
oxidation) of the catalysts depend strongly on the
acidity of the aqueous reaction medium; the selec-
tivity is much higher when the reaction medium is
acidic.
The H2O2 yields obtained over the PdO supported
on ThO2 (13.3%), fluorinated ␥-Al2O3 (12.7%) or
CeO2 (12.5%), chlorinated ␥-Al2O3 (12.3%), CeO2
(11.8%), fluorinated or sulfated ZrO2 (11.4%), and flu-
orinated Y2O3 (7.4%) catalysts are much higher than
that (3.6%) reported earlier over the Pd/fluorinated
carbon catalyst.
References
[
1] G. Goor, W. Kunkel, O. Weiberg, in: B. Elver, S. Howkins,
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Weinheim, 1989, pp. 443–466.
4
. Conclusions
[
[
[
[
[
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From the above studies on the direct oxidation of
H2 to H2O2 and H2O2 decomposition reactions over
the different fluorinated or sulfated Al2O3-, ZrO2-,
CeO2-, Y2O3-, ThO2- and Ga2O3-supported PdO or
Pd catalysts, the following important conclusions have
been drawn.
(1992) 33.
5] V.R. Choudhary, L.K. Doraiswamy, Ind. Eng. Chem. Prod.
Res. Dev. 10 (1978) 218.
6] V.R. Choudhary, M.G. Parande, P.H. Brahame, Ind. Eng. Chem.
Fundam. 21 (1982) 472.
1
. The modification by fluorination or sulfatation
of Al2O3 or ZrO2 support causes a drastic in-
crease in the H2O2 selectivity of the catalyst
but that of CeO2, Y2O3 and ThO2 results in a
[
7] V.R. Choudhary, A.G. Gaikwad, unpublished work.
8] V.R. Choudhary, Ind. Eng. Chem. Prod. Res. Dev. 16 (1977)
12.
[