Journal of Catalysis 195, 406–411 (2000)
doi:10.1006/jcat.2000.2962, available online at http://www.idealibrary.com on
RESEARCH NOTE
Comparison of the Coverage of the Linear CO Species on Pt/Al
2
O
3
Measured under Adsorption Equilibrium Conditions
by Using FTIR and Mass Spectroscopy
Abdennour Bourane, Olivier Dulaurent, and Daniel Bianchi
1
Laboratoire d’Application de la Chimie
`
a l’Environnement (LACE), UMR 5634, Universit
´
e Claude Bernard, Lyon-I,
Bateille 308, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne, France
Received December 22, 1999; revised June 6, 2000; accepted June 6, 2000
There is controversy in the literature about the relationship be-
tween the IR band area (denoted by A) of the adsorbed CO species
(i.e., linear (denoted by L) and bridged (denoted by B) CO species)
on noble-metal-containing solids (supported metal catalysts and
single crystals) and the coverage (denoted by θ) of the surface by
CO. Usually, a straight line is observed at low coverages (θ<0.5)
but various profiles are observed at high coverages. This consti-
tutes a limitation on performing quantitative analysis using FTIR
spectroscopy. Several explanations for the various profiles for the
curves A =f(θ) (i.e., dipole–dipole coupling, nonadsorption equi-
librium) have been suggested. In the present study we show that
this also may be due to the fact that the coverage values involve the
amountsofseveraladsorbedCOspecies.Thecoverages(denotedby
θ
L
) of the L species adsorbed on a 2.9% Pt/Al
2
O
3
catalyst are deter-
mined at several adsorption temperatures (adsorption equilibrium,
T
a
range 300–740K, P
a
≈10
3
Pa )by using two analytical methods:
FTIR and mass spectroscopy. It is shown that at high adsorption
temperatures(T
a
>550K)thetwoanalyticalmethodsgivethesame
coverage values in the range θ =1–0.6. This leads to the conclusion
(a) that there is a proportionality between the IR band area of the
L species and its amount on the Pt surface and (b) that quantitative
studies involving the L species can be performed by using FTIR
spectroscopy (i.e., determination of the heats of adsorption of the L
species at several coverages).
c
2000 Academic Press
I. INTRODUCTION
The adsorption of CO at 300 K on supported platinum
catalysts exhibits a strong IR band at 2070 ±10 cm
−1
asso-
ciated with a weak and broad IR band at ≈1850 ±10 cm
−1
(1–3) ascribed to the linear (denoted by L) and bridged
(denoted by B) CO species, respectively. We have recently
reported (4) that the change in the IR band of the L species
1
To whom correspondence should be addressed. E-mail: daniel.
bianchi@univ-lyon1.fr.
canbestudiedduringCO adsorption (1%CO/He, 1 atm to-
talpressure,flow rate 200cm
3
/min)atseveral temperatures
T
a
, in the range 300–800 K. These FTIR spectra were used
to determine the changein the coverage θ
L
of the L species
with T
a
, at the fixed adsorption pressure P
a
=10
3
Pa. The
curve θ
L
=f(T
a
) provided the heats of adsorption of the
linear CO species at various coverages using an adsorption
modelthatassumed(a)immobile adsorbedCOspeciesand
(b) a linear decrease in the heat of adsorption with the in-
crease in the coverage (4). In another study (5), using sev-
eral CO partial pressures, we have shown that the heats of
adsorptionof thelinearCO species,measuredbythe above
procedure, were ingood agreement with the isosteric heats
of adsorption. This was considered proof that the various
assumptions included in the adsorption model were justi-
fied. However, to obtain these curves θ
L
=f(T
a
) at a given
adsorption pressure P
a
, it was assumed that the IR band
area of the L species (denoted by A
L
) was proportional to
its amount on the surface and that this proportionality was
not modified by a change in the adsorption temperature.
However, there is controversy in the literature concerning
these assumptions for the IR bands of the adsorbed CO
species on noble-metal-containing solids (denoted by NM-
CS or Pt-CS).
Forinstance, on a Pd/SiO
2
solid, Hicks et al. (6)observed
that the IR band area (denoted by A) of the adsorbed CO
species varied linearly with the amount of CO adsorbed
at the surface in the CO/Pd
s
ratio range 0–1.1. Cavanagh
and Yates observed a similar result for the gem dicarbonyl
CO species formed on Rh/Al
2
O
3
catalysts (7). Rasband
and Hecker (8) showed that the IR absorption coefficients
for both L and B species were not significantly modified
by the dispersion (100 to 22%) and the adsorption tem-
perature (range 323–473 K) on supported Rh/SiO
2
solids.
Srinivas and Chuang (9) determined (with an experimen-
tal uncertainty of 20%) the IR absorption coefficient for
0021-9517/00 $35.00
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c
2000 by Academic Press
All rights of reproduction in any form reserved.
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