Low-temperature water–gas shift reaction over gold deposited on TiO2
Hiroaki Sakurai,*† Atsushi Ueda, Tetsuhiko Kobayashi and Masatake Haruta
Osaka National Research Institute, AIST, Midorigaoka 1, Ikeda, Osaka 563, Japan
Gold exhibits very high catalytic activities, comparable with
that of a conventional Cu/ZnO/Al catalyst, for forward
and reverse water–gas shift reactions when supported on
titanium oxide by the deposition–precipitation method.
conversion (99.97%), therefore, the reaction rate, TOF, and
O
2 3
activation energy were calculated around this temperature. The
catalysts tested were stable during catalytic activity measure-
ments and the change of CO conversion during 2.5 h was less
than < 0.5%. In the calculation of TOF for supported metal
catalysts, generally, the number of exposed metal atoms is
The water–gas shift reaction is used as one of the important
industrial processes to adjust the CO/H
2
ratio for methanol
2
determined by titration using CO and H . In the case of
synthesis and to produce H for ammonia synthesis. For this
2
supported gold, the titration method is not applicable because of
the lack of irreversible stoichiometric adsorption of these
molecules on the surface of gold. The number of gold atoms
exposed to the surface was thus simply calculated from the
mean diameter of the gold particle. The shape of the gold
1
reaction, two kinds of catalysts are commercially used. One is
a high-temperature shift catalyst, which is based on iron oxide
structurally promoted with chromium oxide and is used in the
temperature range 583–723 K and at 2.5–3.5 MPa. Another is a
low-temperature shift catalyst composed of copper, zinc oxide
and alumina, and is used between 483 and 513 K. Recently,
Andreeva et al. reported that Au deposition enhances the
catalytic activity of iron oxide for the water–gas shift reac-
tion.2 The activity is comparable with that of a conventional
copper catalyst. Here, we report a novel and more active
2
particles deposited on the TiO surface was well defined and
hemispherical by TEM observation.4
Particle sizes of Au and Cu given in Table 1 were calculated
from the XRD data. The titania powder P25 is composed of both
anatase and rutile. Because the broad diffraction peaks for gold
overlapped with these diffraction lines, the diffraction profile of
,3
catalyst, Au deposited on TiO
nor iron oxide, but exhibits low-temperature activities both for
the forward and reverse water–gas shift reactions.
2
, which contains neither copper
TiO
Au/TiO
2
itself was mathematically subtracted from the profile of
, and then the Au particle size was calculated by using
2
ScherrerAs equation. It was found that Au was deposited on the
The catalysts were prepared by deposition–precipitation (DP)
oxide surface with diameters < 5 nm.
of gold hydroxide from HAuCl
4
on titanium dioxide powder
Among gold catalysts (Table 1), Au/TiO
reaction rates. While the reaction rate at 373 K increased with
increasing Au loading for a series of Au/TiO catalysts prepared
by the deposition–precipitation method, Au/TiO prepared by
coprecipitation showed a lower rate despite the higher metal
loading and the smallest gold particle sizes. This may be
attributed to differences in exposure of gold particles on the
2
showed the highest
(
P25, Japan Aerosil Co.).4 To obtain fine gold particles,
5,6
magnesium citrate solution was added during preparation.
2
The catalyst was finally calcined in air at 673 K. Gold loading
in the starting solutions is expressed as atom% [100Au/(Au+M)
for Au/M O ]. For comparison, other gold and copper catalysts
x y
were prepared by coprecipitation (Cop) and impregnation (Imp)
methods.
Table 1 shows the catalyst parameters and some kinetic data
under forward water–gas shift conditions. Other than Au
2
surface and the crystalline nature of TiO
preparation method. The activation energies listed in Table 1,
except for Au/Al and Au/ZnO, are close to the reported
values for Cu/Al
2
depending on the
2 3
O
2
1
catalysts, a commercial Cu/ZnO/Al
Chemicals Inc., Far East) was examined for comparison. The
reactant gas containing CO (1%), H O (2%) and He (balance)
O
2 3
catalyst (Catalysts and
2 3 2 3
O (55.6 kJ mol ) and Au/Al O (48.8
2
1
7
kJ mol ), both prepared by incipient wetness impregnation.
However, the reaction rate and turnover frequency were very
different depending on the preparation method. The TOF
2
2
1
21
was fed at a space velocity of 12 600 h ml (g cat) and under
a pressure of 0.1 MPa, and the introduction of H O was carried
2
reported for Au/Al
2
O
3
(Imp) was only 1/342 that of the value for
(Imp). On the other hand, Au/Al (Cop) was more
active and had half of the TOF of the Cu/ZnO/Al catalyst.
The activities of Au/TiO (DP) were much more striking. The
rates of reaction at 373 K were comparable and the TOF was
more than 4 times higher than that of Cu/ZnO/Al . Although
7
out using a micro-feeder or by bubbling. At 373 K, CO
conversion was < 20% and much lower than the equilibrium
Cu/Al O
2 3
2 3
O
2 3
O
2
Table 1 Catalytic activity of supported gold catalysts for the water–gas shift
a
reaction
2 3
O
this comparison is not made under industrial reaction conditions
Rate at
and the lifetime (the commercial Cu catalyst has a lifetime of ca.
c
D
metal
373 K/mol
TOF at
373 K/s
a
E /kJ
mol
1
2
–4 years by adding the alumina component ) has not yet been
Catalystb
/nm
21
21
21
21
s
(g cat)
examined, the results clearly show that gold is potentially as
active as copper, when highly dispersed on suitable support
oxide.
Fig. 1 shows the activities of gold catalysts under the reverse
water–gas shift conditions. The reactant gas mixture
d
27
24
Au/TiO
Au/TiO
Au/TiO
Au/TiO
2
2
2
2
(DP, 3.4%)
(DP, 5%)
(DP, 10%)
4.4
—
1.0 3 10
1.4 3 10
3.0 3 10
6.4 3 10
2.2 3 10
3.4 3 10
9.0 3 10
1.2 3 10
7.9 3 10
—
46
45
31
47
52
24
34
53
d
27
27
28
d
24
25
25
24
25
24
4.4
2.8
3.3
3.7
4.9
13.4
9.2 3 10
5.6 3 10
9.1 3 10
1.1 3 10
5.7 3 10
2.0 3 10
d
(Cop, 33%)
2
2
2
8
8
9
Au/Fe
Au/Al
2
O
O
3
(Cop, 5%)
(Cop, 5%)
2 2
CO (23%)– H (67%)–Ar(balance) was passed through the
21
21
2
3
catalyst bed at a space velocity of 3000 h ml (g cat) and
under a pressure of 0.1 or 5 MPa. Catalytic activities are
Au/ZnO(Cop, 5%)
Cu/ZnO/Al O
2 3
e
27
expressed by k defined as [CO][H
2 2 2
O]/[CO ][H ]. The concen-
tration of each component was calculated from analytical data
a
21
2
Reactant gas CO(1%)–H O(2%)–He(balance); SV = 12 600 h ml
8
21
b
and the material balance equations. Au on TiO
showed higher activities than Au/Fe , and the most active
Au/TiO reached thermodynamic equilibrium at a temperature
as low as 423 K at 5 MPa. However, Au/TiO
2 2 3
and Al O
(g cat) ; pressure 0.1 MPa.
DP, deposition–precipitation; Cop,
c
2 3
O
coprecipitation, Au loading in atom%. Calculated from XRD by using
d
Scherrer’s equation. Addition of magnesium citrate during preparation.
2
e
Commercial catalyst, with Cu content of 42 atom% by XRF analysis.
2
and Cu/TiO
2
Chem. Commun., 1997
271
Sakurai, Hiroaki
