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Oxidation of Carbon Monoxide Over a RhAI2O3 Catalyst.

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Oxidation of Carbon Monoxide Over a
Rh/A1203 Catalyst
Gonul Glindilz
Chemical Engineering Department, Ege University, 35 100
Bornova - lzmir, Turkey
Rhodium has an important role in the three-way control of automobile exhaust
emissions. The kinetics of CO oxidation on polycrystalline and supported-Rh
catalysts has received considerable attention in recent years. The results on
supported- Rh catalysts indicate that the oxidation of CO proceeds via dissociative
oxidation by its own oxygen, as well as via direct oxidation by gas-phase oxygen
with a Langmuir-Hinshelwood type reaction.'-' The present work considers the
effect of product CO;! on the CO oxidation using an impregnation-type Rh catalyst
I
0.2
0.4
I
0.6
I
0.8
I
1.0
Figure 1 Conversion-space time curves for CO oxidation (10 mol % CO in air)
Developments in Chemical Engineering, Vol. 1, No. 1, Page 52
Oxidation of carbon monoxide
53
Table I Experimental data for the inhibition effect of C02.
X
YCO
0.0750
0.0750
0.0750
0.0750
0.0750
0.0750
0.0750
0.0458
0.0458
0.0458
0.0458
0.0458
0.0458
0.0458
0.0458
mk/vo a
1.o
1.o
0.4
0.4
0.2
0.2
0.2
1.o
.o
1
0.4
0.4
0.2
0.2
0.1
0.1
(YCO,>O
-
0.002 18
-
0.00098
-
0.00041
0.00055
-
0.00235
-
0.00088
-
0.00050
-
0.00025
(mol %)
3.67
2.76
2.08
1.72
1.20
1.07
0.67
11.11
8.89
6.30
4.93
3.78
1.56
2.1 1
0.96
r
(moVgcathx104)
4.12
3.10
5.84
4.83
6.74
5.99
3.74
7.49
5.99
10.67
8.30
12.73
5.24
14.23
6.44
mk/vo in g s cmp3.
mk = 0.5 g; T = 423 K).
a
Nomenclature
= amount of the catalyst (g)
mk
r
= rate of reaction (mol g-' cat. h-')
T
= reaction temperature (K)
= volumetric flow rate (cm3 s-l)
vo
mk/vo = space time (g s cm-3)
x
= conversion (mol%)
y c o = mole fraction of CO in the inlet stream
yco, = mol fraction of COz in the exit stream
(ycoJ0 = mol fraction of C 0 2 in the inlet stream
at atmospheric pressure. Details of catalyst preparation and the experimental
set-up are presented elsewhere. 9
Oxidation of C O with oxygen in air over 0.5 g of catalyst (mk) was studied for
CO-air mixtures containing 2.5, 4.58, 7.5 and 10 mol% CO. A temperature range
of 403-443 K and a space time (mk/vo) range of 0.07-1.00 g s cm-3 were used.
The conversions and rates of reaction (r) were calculated from the experimental
Goniil Giindiiz
54
T=423 K
20
16
t
n
V
12
9
X
f
di[8
\
9
0
E
.L
-
Qa .4
1
2
3
47
Mole Fraction of C02 in exit Strcam(y
co.,
x1
6)-
L
Figure 2 Dependence of reaction rate on mole fraction of C02 in exit stream.
data. Experimental problems arose because the catalyst showed continuous
activity changes, and a steady value for conversion was obtained after a long
reaction time. For example, a conversion-space time curve required more than
three days. The plot of conversion versus space time in Figure 1 (for 10% CO in
air) shows that the conversion to C 0 2 increases as the space time or temperature
increases. Figure 2 illustrates the dependence of reaction rate on the mole fraction
of product C 0 2 in the exit stream (yco,) for different CO mole fractions in the
inlet stream (yco), and for different volumetric flow rates (v,). It appears that on
the rhodium catalyst, reaction rate is inhibited not only by the reactant CO but also
by the product C02. Reaction rate increases as yco and yco, decrease.
In order to show this inhibition effect of C 0 2 on the oxidation rate of CO more
clearly, carbon dioxide was directly added to the feed. Oxidation experiments
were carried out by varying the fraction of C 0 2 in the feed stream (from zero to
Oxidation of carbon monoxide
55
a given value), while holding constant the fractions of CO and oxygen, the
temperature and the flow rate. Table 1 presents these experimental data for CO-air
mixtures containing 4.58 and 7.5 mol% CO, two runs were performed for each
mk/v, value. The first run is the oxidation of CO to C02 without C02 in the feed.
In the second run, the same experiment was repeated at the same conditions, but
with C02 in the feed. The comparison of these experiments for each set of mk/vo
values demonstrates the inhibiting effect of carbon dioxide on the oxidation rate
of CO. Although the data obtained without CO;? in the feed in Table 1 and the
results in Figure 2 are for identical conditions, different numerical values have
been obtained for the oxidation rates. This difference is due to the activity change
in the catalyst. As the two experiments in each set (with and without C02 in the
feed) in Table 1 were carried out consecutively, the decrease in rate observed
shows the relative inhibition effect of C02 on the oxidation rate of CO. However,
these are only preliminary results and further experimental work is required.
References
1 Campbell, C.T. and White, J.M. 1978. The adsorption, desorption and reactions of CO
and CO 2 on Rh. J. Catal, 54, 289-302.
2 Campbell, C.T., Shi, S.K.and White, J.M. 1979a. Kinetics of oxygen titration by carbon
monoxide on rhodium. J. Physical Chem., 83(17), 2255-2259.
3 Campbell, C.T., Shi, S.K. and White, J.M. 1979b. Carbon monoxide oxidation reaction
over Rh. J. Vac. Sci. Tech., 16(2), 609407.
4 Campbell, C.T., Shi, S.K. and White, J.M. 1979c. The Langmuir Hinshelwood reaction
between oxygen and CO on Rh. App. Sur. Sci.,2,382-396.
5 Matsushima, T. 1980. Tracer studies of the reaction paths of the CO oxidation over
polycrystailine palladium and rhodium. J. Catal., 64, 38-50.
6 Kim, Y.,Shi, S.K. and White, J.M. 1980. Oxygen inhibition of CO oxidation on
polycrystalline Rh. J. Catul., 61, 374-377.
7 Oh, S.H., Fisher, G.B., Carpenter, J.E. and Goodman, D.W. 1986. Comparative kinetic
studies of CO-02 and CO-NO reactions over single crystal and supported rhodium
catalyst. J. Cutul., 100, 350-376.
8 Cho, B.K. and Stock, C.J. 1989. Dissociation and oxidation of carbon monoxide over
Rh/Ai203 catalysts. J. Catal., 117, 202-217.
9 Gundilz (Tufan), 0.and Lintz, H.G. 1983. Oxidation of carbon monoxide by air over a
supported rhodium catalyst. Proc. Vth. Int. Symp. Heterogeneous Catalysis, Vurnu,
Part 11, pp.79-84.
Received: 23 September 1991; Accepted: 25 May 1992.
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