Kinetics of low temperature CO oxidation was studied in hydrogen-rich streams using cobalt (1.25 wt%) and ceria (1.25 wt%) promoted 1.4 wt% Pt/Al2O3 catalyst prepared by incipient-to-wetness impregnation. Intrinsic kinetic data were obtained in the initial rate region in a microflow reactor operating in differential mode using eight different sets of CO and O2 concentrations in 60 percent H2 and He as balance. The experiments were carried out at 110 °C, both in the absence and the presence of 25 percent CO2 and 10 percent H2O, and at for two space times..
The plausible elementary reactions constituting the CO oxidation mechanism were determined and various alternative reaction paths based on those elementary reactions were constructed [1-3]. The mechanisms comprise of the single site monofunctional paths (proceeding on the platinum sites), and dual site bifunctional paths (proceeding on the platinum sites and the cobalt-ceria sites). Both the cobalt and the ceria sites are referred to by a single site for simplicity. H2, CO2 and H2O in the feed stream were not included in the reaction mechanisms, as their effect on the reaction rates were considered to be through the rate parameters, but not through the mechanism.
Model equations were derived for each reaction paths using appropriate assumptions about the rate determining steps, the equilibrium and the surface coverage, and the experimental data were fitted using Levenberg-Marquardt regression scheme. The model discrimination was performed by the positive sign of kinetic parameters and fitness of regression.
The three reaction paths given in Table 1 were found to be plausible. The reaction paths were further tested using Arrhenius plot, which is produced fairly straight line. Assuming the temperature dependence of equilibrium constant is weaker than the rate constants, and the inspecting the model equations for three paths, the paths B and C seems to be more plausible although no definitive distinction could be made.
Table 1. Reaction mechanisms considered
|
|
Reaction Path |
|
||
|
Elementary Step |
A |
B |
C |
Step Number |
|
|
aσA |
σC |
σD |
|
|
CO + * = CO* |
2 |
2 |
1 |
(1) |
|
CO + s = COs |
0 |
0 |
1 |
(2) |
|
O2 + * = O2* |
1 |
0 |
0 |
(3) |
|
O2* + * = 2O* |
1 |
0 |
0 |
(4) |
|
O2 + s = O2s |
0 |
1 |
1 |
(5) |
|
O2s + s = 2Os |
0 |
1 |
1 |
(6) |
|
CO + O* = OCO* |
0 |
0 |
0 |
(7) |
|
OCO* = CO2 + * |
0 |
0 |
0 |
(8) |
|
CO* + O* = CO2 + 2* |
2 |
0 |
0 |
(9) |
|
CO* + Os = CO2 + * + s |
0 |
2 |
1 |
(10) |
|
COs + Os = CO2 + 2s |
0 |
0 |
1 |
(11) |
*-Pt site, s-Co and Ce site
Acknowledgment
This work was partially supported by Bogaziçi University through project 06M104.
References [1] Nibbelke, R. H., M. A. J. Campman, J. H. B. J. Hoebink and G. B. Marin, 1997, “Kinetic Study of the CO Oxidation over Pt/γ-Al2O3 and Pt/Rh/CeO2/γ-Al2O3 in the Presence of H2O and CO2”, Journal of Catalysis, Vol. 171, pp. 358-373.
[2] Nibbelke, R. H., A. J. L. Nievergeld, J. H. B. J. Hoebink and G. B. Marin, 1998, “Development of a Transient Kinetic Model for the CO Oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 Three-Way Catalyst”, Applied Catalysis B: Environmental, Vol. 19, pp. 245-259.
[3] Rajasree, R., J. H. B. J. Hoebink and J. C. Schouten, 2004, “Transient Kinetics of Carbon Monoxide Oxidation by Oxygen over Supported Palladium/Ceria/Zirconia Three-Way Catalysts in the Absence and Presence of Water and Carbon Dioxide”, Journal of Catalysis, Vol. 223, pp. 36-43.