Catalysis Letters 56 (1998) 11–15 11
FTIR study of the photoinduced dissociation of CO
2
on
titania-supported noble metals
J. Rask
´
o
Reaction Kinetics Research Group of the Hungarian Academy of Sciences
∗
, PO Box 168, H-6701 Szeged, Hungary
Received 30 June 1998; accepted 30 September 1998
The effect of illumination on the activation and dissociation of CO
2
was investigated at 190 and 300 K on titania-supported noble
metals by means of Fourier transform infrared spectroscopy. The photoinduced dissociation of CO
2
(through the formation of CO
−
2(a)
)
resulting in CO
(a)
occurred on Pt/TiO
2
,Rh/TiO
2
and Ir/TiO
2
;noCO
(a)
formation, however, was observed on Pd/TiO
2
and Ru/TiO
2
.It
is assumed that the CO
2
on supported noble metals is bonded to the surface with both C (linked to a noble metal atom) and one of the
O atoms (linked to the oxygen vacancy of the supports), and an extended charge transfer induced by illumination leads to the cleavage
of a C–O bond.
Keywords: thermal and photoinduced dissociation of CO
2
,CO
2
adsorbed on oxide-supported noble metals, the effect of the nature of
noble metals and supports, FTIR spectroscopy
1. Introduction
In the conversionof CO
2
into more valuablecompounds,
the activation of rather inert and stable CO
2
plays a deci-
sive role. This activation very likely involves an electron
transfer to CO
2
and, hence, the transformation from linear
structure into a bent form in which the C–O bond is much
more reactive.
Attempts have been made to activate CO
2
on the single-
crystal surfaces of Pt-metals [1,2]; the electron transfer
process, however, is greatly limited by the high work func-
tion of these catalysts. The deposition of electropositive
alkali metals on the above surfaces lowers the work func-
tion of Pt-metals, consequently promoting the formation
of CO
−
2(a)
[2]. Illumination of a CO
2
+ K/Rh(111) sys-
tem further increased the surface concentration of CO
−
2(a)
species [3,4].
The activation of CO
2
on oxide-supported metals would
have more practical importance, since these catalysts
proved active in CO
2
hydrogenation into methane at rel-
atively high temperatures [5–7]. Graetzel et al. [8] have
found that the illumination of a Ru/TiO
2
catalyst leads to
the methanation of CO
2
even at room temperature.
Recently we have found by FTIR that the UV illumi-
nation of CO
2
/TiO
2
and CO
2
/Rh/TiO
2
systems enhances
the intensities of the bands at 1640 and 1219 cm
−1
due to
the formation of bent CO
2
[9]. Bent CO
2
having negative
charge (CO
−
2(a)
) is the activated form of the rather inert and
stable CO
2
on the catalyst surface. The enhanced reactivity
of CO
2
adsorbed in a bent form has been demonstrated [9]
during the illumination of the CO
2
/Rh/TiO
2
system, where
as a result of the photodissociation of CO
2
, CO bands at
∗
This laboratory is part of the Center for Catalysis, Surface and Material
Science at the University of Szeged.
2020–2040 cm
−1
developed at 190 K, far below the tem-
perature at which the thermal decomposition of CO
2
occurs
on this surface.
The present work is a continuation of the above study,
in which the research is extended to other noble metals
supported on titania. We also studied the effect of the nature
of supporting oxides on these processes in the case of Rh.
2. Experimental
The catalysts were prepared by incipient wetting of ti-
tania (P25 Degussa) with aqueous solutions of H
2
PtCl
6
(Reanal), H
2
IrCl
6
(Ventron), PdCl
2
(Johnson–Matthey) and
RuCl
3
(Ventron), respectively. The noble metal content
was 5 wt%. A1
2
O
3
(Degussa), SiO
2
(Cab-O-Sil) and
MgO (DAB6) were used similarly for the preparation of
supported Rh catalysts. In these cases RhCl
3
(Johnson–
Matthey) was the parent compound. For preparation, triply
distilled water was used. After impregnation, the samples
were dried in air at 373 K and pressed into self-supporting
wafers (30×10 mm, 10 mg/cm
2
). The pretreatment of sam-
ples was performed in a vacuum IR cell: the samples were
(a) heated (20 K/min) to 673 K under continuous evacua-
tion, (b) oxidized with 100 Torr O
2
for 30 min at 673 K,
(c) evacuated for 15 min, and (d) reduced in 100 Torr H
2
for 30 min at 673 K. This was followed by degassing for
30 min at this temperature and by cooling the samples to
the temperature of the experiment.
A mobile IR cell housed in a metal chamber was used for
illumination. The sample can be heated and cooled in situ.
The chamber can be evacuated to 10
−6
Torr. The sample
was irradiated in the presence of CO
2
by an LPS 220 Hg
arc lamp (PTI). The IR (heat) component of the lamp was
filtered out by using of a 15 cm long quartz tube filled
J.C. Baltzer AG, Science Publishers