Highly effective ferric hydroxide supported gold catalyst for selective oxidation of CO in the presence of H2

Article abstract of DOI:10.1039/b307181b

Ferric hydroxide supported Au prepared with co-precipitation without heat treatment could be a very effective catalyst for selective CO oxidation in the presence of H₂ at lower temperatures.

Full text of DOI:10.1039/b307181b

Highly effective ferric hydroxide supported gold catalyst for selective
oxidation of CO in the presence of H2†
Botao Qiao and Youquan Deng*
Centre for Green Chemistry and catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, 730000, China
Received (in Cambridge, UK) 24th June 2003, Accepted 15th July 2003
First published as an Advance Article on the web 28th July 2003
Ferric hydroxide supported Au prepared with co-precipita-
tion without heat treatment could be a very effective catalyst
for selective CO oxidation in the presence of H2 at lower
temperatures.
Catalytic activity measurements were carried out with a
micro fixed-bed reactor system, and 60 mg (60–80 mesh) of
catalyst sample was charged in each time. The feed gases were
1.0 vol% CO or 1.0 vol% H2 balanced with air, and 1.0 vol% H2
(or 50 vol% H2), 1.0 vol% CO and 4.0 vol% O2 balanced with
argon. The flow rate was 20 ml min21 (space velocity: 20 000
h21·ml g-cat21). The concentrations of CO and H2 and O2 in the
effluent gas were on-line analyzed by a gas chromatograph (HP
6890, molecular sieve 5 Å plot column).
Metal-oxide supported Au catalysts for CO oxidation at lower
temperatures have been studied extensively for many years,1–8
and several preparation methods such as conventional impreg-
nation,9 co-precipitation,10 deposition–precipitation,11 ion-ex-
change12 and chemical vapour deposition13 have been em-
ployed. The co-precipitation method has been particularly
utilized to obtain highly active supported gold catalysts, in
which chloroauric acid as gold precursor, including washing to
remove chloride, drying and then calcinations to form appro-
priate nano-Au metal particles on a support were generally
considered to be indispensable procedures for a successful
preparation of supported Au catalyst. Due to the peculiar
properties of gold as a catalytically active species, great
differences from the activities usually observed for CO
oxidation resulted from the co-precipitation method and various
preparation parameters could strongly affect the activity of the
supported gold catalysts. Recently, Gavriilidis et al.14 and Kiely
et al.15 reported respectively that the active supported Au
catalyst can be prepared by co-precipitation only by calcination
at 120 K and they possessed higher activity than those which
were calcined at elevated temperatures. This may imply that Au
as a catalytic material may exhibit much different behavior
during catalyst preparation in comparison with traditional
supported Pt and Pd catalysts which were generally prepared
through indispensable procedures, i.e. dryness, calcination in air
and even reduction in H2 at elevated temperatures.
In this communication, ferric hydroxide supported Au
catalysts have been prepared with co-precipitation by drying at
room temperature without any further calcining treatment at
elevated temperature. Oxidations of CO or H2 in air and
selective oxidation of CO in the presence of H2 were used as
probe reactions for testing their catalytic performances. The
experimental results showed that ferric hydroxide supported Au
could be a highly effective catalyst for selective oxidation of CO
in the presence of H2 in comparison with Au/Fe2O3 derived
from the ferric hydroxide supported Au which was then
calcined at 200 or 400 K.
Complete CO oxidations in air at ca. 7 °C, 22 °C and 87 °C
were observed over Au/Fe–U, Au/Fe-2 and Au/Fe-4 re-
spectively, Fig. 1, indicating that CO oxidation was sensitive to
heat treatment, and calcination at elevated temperatures were
detrimental to the catalytic performance. Therefore, Au/Fe–U
was the best catalyst for CO oxidation although its Au loading
was the lowest. As expected, the catalytic activity could be
markedly increased, i.e. complete CO oxidation in air at ca.
214 °C was achieved, if the Au loading was further increased
from 0.76 wt% to 2.52 wt%. Since more water was contained in
Au/Fe–U, the water gas shift reaction may favour to CO
transformation,18 however, H2 formation was not observed
during the reaction. In comparison with CO oxidation, oxida-
tions of H2 were much less sensitive to the heat treatment, and
occurred above ca. 25 °C over all three catalysts. Since the
chemical composition, structure and chemical state of Au/Fe–U
are completely different with that of Au/Fe-4, it could be
conjectured that CO oxidation was highly sensitive to such
changes, while the active species of Au for H2 oxidation was
already formed before any heat treatment and did almost not
change when they were further subjected to calcination at
elevated temperatures. It is worth noting that there is about a 20
°C gap between the temperatures of complete CO oxidation and
the occurrence of H2 oxidation for Au/Fe–U. This may imply
that Au/Fe–U could be an ideal catalyst for the selective CO
oxidation in the presence of H2 at lower temperatures.
When the catalysts mentioned above were exposed to a
mixture gas of 1.0 vol% H2, 1.0 vol% CO and 4.0 vol% O2,
different behaviors of catalytic oxidation were exhibited, Fig. 2.
The catalysts with varied Au loadings were prepared by co-
precipitation of an aqueous solution of HAuCl4 (0.1 g ml21) and
Fe(NO3)3 (1 mol L21) with an aqueous solution of Na(CO3)2 (1
mol L21) according to the procedures described in the
previously reported literatures.16,17 The resulting precipitates
without Cl2 were dried at room temperature (ca. 25–30 K) for
24 h to form ferric hydroxide supported Au catalyst (denoted as
Au/Fe–U), or the ferric hydroxide supported Au samples were
then calcined in air at 200 or 400 K for 6 h to form two different
supported Au catalysts denoted as Au/Fe-2 and Au/Fe-4
respectively. In order to obtain repeatable activity the prepara-
tion process must be conducted very carefully and precisely.
Fig. 1 Respective oxidations of CO or H2 in air. B G 0.76 wt% Au/Fe-U for
respective oxidations of CO or H2. C E 1.05 wt% Au/Fe-2 for respective
oxidations of CO or H2. D F 1.48 wt% Au/Fe-4 for respective oxidations of
CO or H2. A 2.52 wt% Au/Fe-U for CO oxidations
† This work was financially supported by The National Natural Science
Foundation of China (No. 20173068).
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CHEM. COMMUN., 2003, 2192–2193
This journal is © The Royal Society of Chemistry 2003

Table 1 Properties of the catalysts
Average
Au loading/ BET surface pore
wt%
B.E. of
B.E. of
area m2 g21 diameter/Å Au4f7/2/eV Fe2p3/2/eV
Au/Fe-U 2.52
Au/Fe-2 2.85
Au/Fe-4 3.32
328.4
194.7
20.13
22.1
39.0
106.6
84.1
84.1
83.6
711.6
711.2
711.0
(VG ESCALAB 210) indicated that the chemical states of Au
species on the Au/Fe-4 and Au/Fe–U were metallic (B.E.4f7/2
=
83.6 eV), and partially cationic (B.E.4f7/2 = 84.1 eV), and ferric
species with slightly more positive charge were observed on the
surface of Au/Fe–U in comparison with that of Au/Fe-4,
suggesting markedly different Au and Fe species on the catalyst
surfaces, and thus exhibiting markedly different catalytic
performances, between uncalcined and calcined catalysts
(Table 1).
Fig. 2 Oxidations of CO or H2 in CO + H2 mixture gas. A L 0.76 wt% Au/
Fe-u for oxidations of CO and H2(1%). B D 1.05 wt% Au/Fe-2 for
oxidations of CO and H2(1%). E G 1.48 wt%Au/Fe-4 for oxidations of CO
and H2(1%). C F 0.15 wt%Au/Fe-U for oxidations of CO and H2(1%). H K
1.05 wt%Au/Fe-2 for oxidations of CO and H2(50%). I J 1.48 wt %Au/Fe-4
for oxidations of CO and H2(50%)
In this work, a ferric hydroxide supported gold catalyst
prepared with co-precipitation without any heat treatment was
developed. It was found that such a catalyst was not only very
effective for selective CO oxidation in the presence of H2 at
lower temperatures, but also much better than the corresponding
catalyst calcined at elevated temperatures although the prepara-
tion method was not optimized. To the best of our knowledge,
ferric hydroxide supported gold catalyst is the most effective
catalyst reported so far for selective CO oxidation in the
presence of H2 at lower temperatures. Another important point
of this work is that traditional preparations of supported gold
catalysts was markedly innovated, which may be expanded into
other supported noble metals, such as Pd or Pt etc., catalyst
preparations.
Strong inhibition of CO oxidation by H2 was observed over Au/
Fe-4 and relatively high H2 conversion occurred before all CO
could be oxidized completely, indicating that highly selective
CO oxidation in the presence of H2 over Au/Fe-4 catalyst was
impossible. CO oxidations were also slightly affected with H2
over Au/Fe-2 and the temperature at which CO was completely
removed, i.e. ca. 30 °C, was high enough to cause detectable
oxidation by H2 (ca. 4–6% H2 conversion). As for Au/Fe–U,
conversion of CO oxidation as a function of temperature was
almost unchanged in comparison with the CO oxidation in the
absence of H2, i.e. the existence of H2 did not affect the CO
oxidation. CO could completely be oxidized at ca. 8 °C, while
oxidation of H2 was not detectable at this temperature, i.e. the
O2 selectivity for CO oxidation was approaching 100%.
The catalytic performances of these catalysts were also tested
with the mixture gas of 50 vol% H2, 1.0 vol% CO and 4.0 vol%
O2 in argon, Fig. 2. Conversion of CO oxidation as a function of
temperature was almost not affected over Au/Fe–U although the
concentration of H2 in feed gas was greatly increased. However,
CO oxidations were further inhibited over Au/Fe-2 and Au/Fe-4
in comparison with results obtained from a gas mixture of 1.0
vol% H2, 1.0 vol% CO and 4.0 vol% O2 in argon. At higher
temperatures, conversions of CO oxidations were turned down
due to competing O2 consumption by sufficient oxidation of H2.
This suggests that selective CO oxidation was also sensitive to
the catalyst heat treatments, and calcination at elevated
temperatures would cause CO oxidation to be sensitive to the
existence of H2. It is worthy of note that when a Au/Fe–U with
lower Au loading was employed, that the temperatures at which
CO was totally removed and that H2 oxidation was initiated was
increased simultaneously, suggesting that the temperature for
selective CO oxidation in the presence of H2 could be set by
adjusting the Au loadings without decreasing the selectivity of
CO oxidation. Furthermore, ca. 15 h and 28 h of stable catalytic
activities (ca. 100% conversion of CO oxidation) were
preliminarily achieved over 0.76 wt% Au/Fe–U when it was
exposed to the feed gases of 1.0 vol% H2, 1.0 vol% CO and 4.0
vol% O2 and 1.0 vol% CO in air, respectively.
The authors would like to thank Ms L. Gao for the XPS
invesigation, Ms Q. Wu for the Au loading analysis and Ms J.
Li for the measurement of BET adsorption.
Notes and references
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