(305b) Kinetics of Selective Co Oxidation in Hydrogen-Rich Streams over Pt-Co-Ce/Al2o3 Catalyst

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

                          
*-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.