Unexpected promotion of Au/TiO2 by nitrate for CO oxidation

Article abstract of DOI:10.1039/b500615e

The catalytic activity for Au/TiO₂ for CO oxidation can be significantly enhanced by the addition of nitrates and this may relate to the variable catalyst performance observed in many studies. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b500615e

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Unexpected promotion of Au/TiO2 by nitrate for CO oxidation
Benjamin Solsona,a Marco Conte,a Yu Cong,b Albert Carleya and Graham Hutchings*a
Received (in Cambridge, UK) 14th January 2005, Accepted 23rd February 2005
First published as an Advance Article on the web 14th March 2005
DOI: 10.1039/b500615e
this methodology two types of additives can be examined, viz
cations (Na+ or K+) or anions (NO32, CH3COO2, or C6H5O732),
and these were contrasted with a Au/TiO2 catalyst that was treated
with water alone in an analogous manner, to ensure all catalysts
had been treated the same way prior to use as catalysts for the
oxidation of CO.
The catalytic activity for Au/TiO2 for CO oxidation can be
significantly enhanced by the addition of nitrates and this may
relate to the variable catalyst performance observed in many
studies.
One of the most surprising recent observations is that gold, when
dispersed as nano-crystals on an oxide support, can be highly
effective for the oxidation of CO at very low temperatures.1,2 This
has led to a dramatic increase in the interest in gold as a catalyst,
not only for CO oxidation but for selective oxidation reactions
such as the epoxidation of propene,3 the selective oxidation of
alcohols4,5 and the selective oxidation of hydrogen to hydrogen
peroxide.6 Interest in the factors controlling the high reactivity is
showing no abatement: recently Corma and co-workers7 showed
that by using nano-crystalline CeO2 as a support the activity of CO
oxidation could be enhanced by a factor of two, and Haruta et al.
have shown8 that water plays a crucial part in the origin of the
high activity for this reaction. Most of the high activity catalysts
reported to date are prepared using precipitation methods, either
co-precipitation or deposition precipitation, often using nitrates as
reagents; furthermore, it is well known that the preparation
method employed can be of crucial significance.9
The catalysts were evaluated for the ambient temperature
oxidation of carbon monoxide using air.{ The Au/TiO2 catalyst
prepared by treatment with water alone gave a high catalytic
activity for CO oxidation, and we adjusted the flow conditions in
the reactor so that the CO conversion was typically ca. 70%. The
effect of time on line on CO conversion for Au/TiO2 catalysts
modified with sodium nitrate is shown in Fig. 1, and it is apparent
that the activity is stable for several hours after an initial
stabilisation period. Surprisingly, the addition of low concentra-
tions of sodium nitrate (0.00625 and 0.0125 wt.% of NaNO3) leads
to CO conversions of ca. 90%, a significant improvement in
catalytic activity when compared with the catalyst treated with
water (ca. 70%). However, higher concentrations of sodium nitrate
lead to much lower CO conversion, and for the catalyst containing
0.0375 wt.% Na the CO conversion was very close to zero under
our test conditions.
In this communication, we present the first observation that
nano-crystalline gold catalysts for CO oxidation at ambient
temperatures can be promoted by the addition of nitrate by a
simple impregnation method. To date the subject of the promotion
of the catalytic activity of gold by the addition of additional
cations and anions has received no attention. This is surprising
since there are numerous examples of the promotion of
heterogeneous catalysts, particularly the activity or selectivity of
supported metal catalysts, using this methodology.10,11 Perhaps the
most notable examples are the enhancement in activity of iron
catalysts by the addition of alkali metal cations for either ammonia
synthesis12 or the Fischer Tropsch reaction.13 Often, catalyst
formulations can only be used on a commercial basis with the
addition of these important components. It was this background
that prompted us to study the role of potential promoters in nano-
crystalline gold heterogeneous catalysts for the oxidation of carbon
monoxide at ambient temperature.
To determine if the observed promotion is due to the addition of
Na+ or NO32 (or both) a series of experiments were carried out in
which different anions and cations were added to the Au/TiO2
catalyst (Fig. 2). The addition of aqueous solutions of sodium
citrate or sodium acetate led to a dramatic decrease in the catalytic
performance for CO oxidation. In particular, negligible CO2
We have selected Au/TiO2 for investigation as this is known to
be a very active catalyst for this reaction.14,15 We prepared the
catalyst by deposition precipitation using a nitrate-free method.{
Subsequently, aqueous solutions with different concentrations of
dopants (sodium nitrate, potassium nitrate, sodium acetate or
sodium citrate) were added by impregnation. The catalysts were
dried at 120 uC and were not heat treated or washed after
impregnation to ensure that the anions were retained intact. Using
Fig. 1 Plot showing the evolution of the catalytic activity for CO
oxidation with the time on line, for sodium nitrate modified Au/TiO2
catalysts: 0 wt.% Na (&), 0.00625 wt.% Na ($), 0.0125 wt.% Na (m),
0.025 wt.% Na (.), 0.0375 wt.% Na (X). Reaction conditions are given in
the experimental section.
*hutch@cf.ac.uk
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2351–2353 | 2351

View Article Online
XPS results indicate that there is no change in the oxidation
state of the gold due to the nitrate treatment. The surface
concentrations of nitrate are below the detection limits of our
spectrometer. Furthermore, there is no significant change in the
surface area of the catalysts following modification (Table 1).
Taking into account the preparation method, the different catalytic
behaviour observed can be due to the presence in the catalysts of
(i) water, (ii) alkali metal cations (sodium or potassium) and/or (iii)
anions. Since the presence of water must be equivalent for all the
samples, as we have specifically treated all catalysts in an identical
manner, we do not consider residual water resulting from low
temperature drying to be important in this case, or at least it will
affect all samples equally. Since the addition of sodium citrate and
sodium acetate both give a significant decrease in CO conversion
we conclude that Na+ and/or citrate/acetate acts as a poison for the
Au/TiO2 catalyst for CO oxidation, in agreement with previous
studies.9,17 As the promotional effect is observed with the addition
of low levels of sodium and potassium nitrate we propose that the
presence of nitrates, added via a simple impregnation step, exerts a
promotional effect on a Au/TiO2 catalyst, increasing the CO
oxidation rate. The origin of the effect is, as yet, unclear but a
possible explanation could involve site blocking, or the formation
of new sites at the periphery of the gold crystallites.
Fig. 2 Graph showing the influence of the alkali metal dopant (Na or K)
on the activity for CO oxidation. Alkali source: Na-nitrate (&), K-nitrate
($), Na-citrate (m), Na-acetate (.). Reaction conditions are given in the
experimental section; time on line 5 500 min.
formation was observed when sodium acetate was used. However,
the impregnation of the catalyst with potassium nitrate gave an
increase in the CO conversion similar to that observed when
sodium nitrate was used. Most importantly, the observed
promotion effect is not short lived and we have observed sustained
enhancement for the duration of our catalyst tests, i.e. 10 h (Fig. 1).
To determine if the effect can still be observed if air is replaced
by another oxidant we investigated the oxidation of CO using N2O
(Fig. 3). The only reported study16 using this oxidant found that it
was significantly less effective than dioxygen. In agreement with
that result, we found that for a given catalyst, the CO conversions
decreased (by a factor of ca. 15) when N2O was employed as
oxidant. However, it is significant that the promotional effect was
still observed when low concentrations of sodium nitrate and
potassium nitrate were added, and higher CO conversions were
obtained (Fig. 3). The degree of promotion achieved was similar
for both O2 and N2O as oxidants.
The observation of a significant enhancement in activity by the
addition of trace amounts of nitrate is significant for two reasons.
Firstly, this is the first unambiguous observation that the activity
of supported gold nano-crystals can be enhanced by the addition
of promoters, and it is likely that other cations and anions will
induce similar effects. In these initial studies we observe an increase
in activity by at least a factor of two for a nitrated treated catalyst
when compared with the catalyst treated with citrate, but
containing an equivalent concentration of Na+ (Fig. 2). This is
similar to the pronounced increase in activity observed recently by
Corma and co-workers via manipulation of the morphology of the
support.7 Secondly, many groups have commented on the distinct
problems associated with the reproducibility of the preparation of
very active gold catalysts.18 Our experiments help provide one
explanation of these observations, since many supported Au
catalysts involve precipitation methods using a nitrate as one of the
components. If the residual nitrate is not removed thoroughly, or
nitrate removal is variable, then it is a distinct possibility that
variable activity will result. Furthermore, we consider the
observation of this promotion effect may aid the design of
improved supported nano-crystalline gold catalysts.
Table 1 Surface areas of different catalysts after anion-doping
Amount of cation (Na or K)
added (wt.%)
Surface area
(m2 g21
)
Anion
—
—
45
45
47
43
39
46
44
45
47
46
48
2
NO32
0.00625
0.0125
0.025
0.0375
0.00625
0.0125
0.00625
0.0125
0.00625
0.0125
NO32
NO32
NO32
NO32
NO3
CH3COO2
CH3COO2
C6H5O7
Fig. 3 Graph showing the influence of the alkali (Na or K) content
added to the catalyst on the activity for CO oxidation using N2O as
oxidising agent. Alkali source: Na-nitrate (&), K-nitrate ($). Reaction
conditions are given in the experimental section; time on line 5 500 min.
32
32
C6H5O7
2352 | Chem. Commun., 2005, 2351–2353
This journal is ß The Royal Society of Chemistry 2005

View Article Online
either air or nitrous oxide as oxidants. The feed comprised either CO/air
(mol ratio 5 0.5/99.5) or CO/Ar/N2O (mol ratio 5 0.5/4.5/95). The
combined flow rate was maintained at 50 ml min21 and a constant catalyst
loading (15 mg) was employed. The catalyst temperature was maintained at
25 uC by immersing the reactor in a thermostatically controlled water bath.
The reaction products were analysed using on-line gas chromatography
(Porapack Q and Molecular Sieve 5A). X-ray photoelectron spectra were
recorded on a VG EscaLab 220i spectrometer, using a standard AlKa
X-ray source (300 W) and an analyser pass energy of 20 eV. Samples were
mounted using double-sided adhesive tape and binding energies are
referenced to the C(1s) binding energy of adventitious carbon contamina-
tion taken to be 284.7 eV.
This work is supported by the Auricat EU project (HPRN-CT-
2002-00174) and the National Natural Science Foundation of
China (program number 20303016).
Benjamin Solsona,a Marco Conte,a Yu Cong,b Albert Carleya and
Graham Hutchings*a
aSchool of Chemistry, Cardiff University, Cardiff, UK CF10 3TB.
E-mail: hutch@cf.ac.uk; Fax: +44 29 20 874075
bDalian Institute of Chemical Physics, Chinese Academy of Sciences,
457 Zhongshan Road, Dalian 116023, P. R. China
Notes and references
1 M. Haruta, N. Yamada, T. Kobayashi and S. Ijima, J. Catal., 1989,
115, 301.
{ The Au/TiO2 catalyst containing 1.4 wt.% Au was prepared by a
deposition precipitation method. The preparation procedure was as
follows: an aqueous solution of chloroauric acid (5 ml of a solution
prepared by dissolving of HAuCl4?3H2O (5.0 g) in distilled water (250 ml))
was added with stirring to a slurry of TiO2 (1.0 g, P25-Degussa,
SBET 5 50 m2 g21) in water (50 ml). The slurry was adjusted to pH 9.0
by the addition of aqueous sodium hydroxide (0.25 mol l21). The resulting
mixture was vigorously stirred at room temperature for 1 h, filtered, and
washed with distilled water several times to ensure that chloride was totally
removed. The solid was dried at 80 uC overnight. Modified Au/TiO2
catalysts were prepared by impregnating the dried Au/TiO2 catalyst with
aqueous solutions of sodium nitrate, potassium nitrate, sodium acetate or
sodium citrate (5 ml g21 catalyst) with a range of concentrations (0.0125,
0.025, 0.0375 and 0.050 mg of alkali per ml) to ensure that a series of
modified catalysts were obtained. The materials were dried in air (120 uC,
3 h) prior to use as a catalyst. A blank catalyst was prepared in an
analogous manner by treating the dried Au/TiO2 material with water
(5 ml g21 catalyst) without the addition of the sodium or the potassium
salt. This material was also dried in air (120 uC, 3 h) prior to use as a
catalyst. BET surface area measurements using nitrogen adsorption were
carried out using a Micromeritics ASAP 2000 instrument.The gold content
of the unmodified Au/TiO2 was determined by atomic absorption
spectroscopy. The surface areas of the impregnated catalysts were not
affected markedly by the impregnation process and were found to all be in
the range 40–50 m2 g21 using the BET method. The X-ray diffraction
patterns of the catalysts showed no characteristic reflections for metallic
gold and as expected showed the two crystalline phases of titania, i.e.
anatase and rutile.The catalytic activity was determined in a fixed bed
quartz micro reactor (3 mm id), operated at atmospheric pressure using
2 C. Bond and D. T. Thompson, Gold Bull., 2000, 33, 41.
3 A. K. Sinha, S. Seelan, S. Tsubota and M. Haruta, Angew. Chem., Int.
Ed., 2004, 43, 1546.
4 S. Carrettin, P. McMorn, P. Johnston, K. Griffin and G. J. Hutchings,
Chem. Commun., 2002, 696.
5 L. Prati and M. Rossi, J. Catal., 1998, 176, 552.
6 P. Landon, P. Collier, A. J. Papworth, C. J. Kiely and G. J. Hutchings,
Chem. Commun., 2002, 2058.
7 S. Carrettin, P. Concepcion, A. Corma, J. M. Lopez Nieto and
V. F. Puntes, Angew. Chem., Int. Ed., 2004, 43, 2538.
8 M. Date, M. Okumura, S. Tsubota and M. Haruta, Angew. Chem., Int.
Ed., 2004, 43, 2129.
9 G. C. Bond and D. T. Thompson, Catal. Rev. Sci. Eng., 1999, 41, 319.
10 W. D. Mross, Catal. Rev. Sci. Eng., 1983, 25, 591.
11 G. J. Hutchings, Catal. Lett., 2001, 75, 1.
12 L. A. Rudnitskii, M. G. Berengarten and A. M. Alekseev, J. Catal.,
1973, 30, 444.
13 W. M. H. Sachtler, D. F. Shriver, W. B. Hollenberg and A. F. Lang,
J. Catal., 1985, 92, 429.
14 S. Tsubota, T. Nakamura, K. Tanaka and M. Haruta, Catal. Lett.,
1998, 56, 131.
15 M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet and
B. Delmon, J. Catal., 1993, 144, 175.
16 N. W. Cant and N. J. Ossipoff, Catal. Today, 1997, 36, 125.
17 A. A. Mirzaei, H. R. Shaterian, R. W. Joyner, M. Stockenhuber,
S. H. Taylor and G. J. Hutchings, Catal. Commun., 2003, 4, 17.
18 M. Cortie, R. Holliday, A. Laguna, B. Nieuwenhuys and D. Thompson,
Gold Bull., 2003, 36, 144.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2351–2353 | 2353