Nonlinear dynamics of three-way catalyst with microkinetics and internal diffusion

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Nonlinear dynamics of three-way catalyst P. Kočí, V. Nevoral, M. Kubíček, M. Marek Center for Nonlinear Dynamics of Chemical and Biological Systems Prague Institute of Chemical Technology Technická 5, CZ-166 28 Praha 6, Czech Republic petr.koci@vscht.cz, milos.marek@vscht.cz Tel: +420-22435 3104, Fax: +420-23333 7335

Three-way catalytic monolith reactor (TWC) converted gas CO 2, N 2, H 2 O exhaust gas CO, HC, NO X monolith channels (~1 mm) support (wall) porous washcoat γ-al 2 O 3, CeO 2 (~10 µm) catalytic metals Pt, Rh, Pd (~10 nm)

CO oxidation on Pt/CeO 2 /γ-al 2 O 3 catalyst Harmsen J.M., Schouten J.C. & Hoebink J.H., 2000, 2001

C 2 H 2 oxidation on Pt/CeO 2 /γ-al 2 O 3 catalyst Harmsen J.M., Schouten J.C. & Hoebink J.H., 2000, 2001

C 2 H 4 oxidation on Pt/CeO 2 /γ-al 2 O 3 catalyst Harmsen J.M., Schouten J.C. & Hoebink J.H., 2000, 2001

NO X transformation and reduction on Pt/CeO 2 /γ-al 2 O 3 catalyst Harmsen J.M., Schouten J.C. & Hoebink J.H., 2000, 2001

Model mass balances Bulk gas Washcoat (Pt) (Ce) Pt/CeO 2 /γ-al 2 O 3 catalyst Surface sites (Al)

Model enthalpy balances Bulk gas Washcoat Model boundary conditions

Composition of model gas at the converter inlet Component CO C 2 H 2 C 2 H 4 NO O 2 CO 2 N 2 Mol. fraction 1.22 % 280 ppm 380 ppm 1130 ppm 0.5-0.9 % 12.2 % balance

CO oxidation evolution diagram in oxygen concentration stoichiometric Outlet CO concentration in dependence on inlet O 2 concentration. CO oxidation on Pt/γ-Al 2 O 3, δ=20 µm, D eff =6x10-7 m 2.s -1, L NM =80 mol.m -3, T=630 K (isothermal). Cf. Kočí, Marek & Kubíček, Proc. of ESCAPE 13, Lappeenranta, Finland, 2003

CO oxidation solution diagram obtained by continuation stoichiometric Outlet CO concentration in dependence on inlet O 2 concentration. CO oxidation on Pt/γ-Al 2 O 3, model with no diffusional resistance within the washcoat. δ=20 µm, L NM =80 mol.m -3, T=630 K (isothermal). Only stable periodic solutions are shown. Computations performed with continuation software CONT. Cf. Kočí, Marek & Kubíček, Proc. of ESCAPE 13, Lappeenranta, Finland, 2003 Cf. also poster of Nevoral, Kočí, Schreiber, Marek & Kubíček, ESF Reactor meeting, Budapest, 2003

TWC operation coexistence of stable steady state and oscillations Outlet HC concentrations - results for different initial conditions. TWC operation of Pt/CeO 2 /γ-al 2 O 3 catalyst, δ=10 µm, D eff =2x10-6 m 2.s -1, L NM =80 mol.m -3, L OSC =1000 mol.m -3, inlet y O2 =0.74 % (small excess), T=630 K (isothermal).

TWC operation 2-periodic oscillations gas washcoat layer wall Spatiotemporal concentration profile of atomic oxygen deposited on Pt sites (O*) in the washcoat layer. TWC operation of Pt/CeO 2 /γ-al 2 O 3 catalyst. δ=10 µm, D eff =2x10-6 m 2.s -1, L NM =80 mol.m -3, L OSC =1000 mol.m -3, inlet y O2 =0.74 % (small excess), T=630 K (isothermal).

TWC operation complex oscillations Outlet CO concentrations - complex oscillatory behaviour. TWC operation of Pt/CeO 2 /γ-al 2 O 3 catalyst, δ=20 µm, D eff =2x10-6 m 2.s -1, L NM =80 mol.m -3, L OSC =100 mol.m -3, inlet y O2 =0.74 % (small excess), T=630 K (isothermal).

TWC operation forced inlet O 2 concentration X=38% X=75% X=77% Stabilized outlet NO X concentrations for periodic variation of inlet O2 concentration. Oscillation frequency f=1 Hz, amplitude A=10%. Pt/CeO 2 /γ-al 2 O 3 catalyst, δ=30 µm, D eff =1x10-6 m 2.s -1, L NM =30 mol.m -3, inlet <y O2 >=0.7375 % (stoichiometric), T in =573 K (adiabatic). Cf. Kočí, Kubíček & Marek, Proc. of IcheaP6, Pisa, Italy, 2003

TWC operation forced inlet O 2 concentration N 2 formation, NO X reduction CO 2 formation, CO and HC oxidation gas gas wall wall Spatiotemporal concentration profile of species deposited on Pt sites (N* and OCO*) in the washcoat layer. Inlet O 2 oscillation frequency f=1 Hz, amplitude A=10%. Pt/CeO 2 /γ-al 2 O 3 catalyst, D eff =1x10-6 m 2.s -1, L NM =30 mol.m -3, inlet <y O2 >=0.7375 % (stoichiometric). Cf. Kočí, Kubíček & Marek, Proc. of IcheaP6, Pisa, Italy, 2003

Conclusions Mathematical model of three-way catalytic converter (TWC) has been developed, including diffusion in the porous structure of catalytic washcoat layer and simultaneous reactions described by a complex microkinetic scheme. Multiple steady and oscillatory states (simple and doubly-periodic) and complex spatiotemporal patterns have been found for a certain range of operation parameters. The above effects arise from the used kinetic scheme, but they are influenced and modified by transport processes within the washcoat layer. Parametric continuation study of the respective system with no diffusional resistance in the washcoat has been performed. Thorough bifurcation analysis of the full model is desirable, but computationally complex (~ 10 3 stiff ODEs). TWC operation with periodic variation of inlet oxygen concentration has been simulated. It has been shown that such operation can significantly improve time-averaged outlet conversions. Acknowledgements This work has been supported by the Project MSM 223400007 of Czech Ministry of Education and ESF Reactor.

References Kočí P., Marek M. & Kubíček M., 2003, Oscillatory behaviour in mathematical model of TWC with microkinetics and internal diffusion, Proc. of 13 th Europ. Symp. on Comp. Aided Proc. Engng. (ESCAPE13), Lappeenranta, Finland, June 1-4 2003. Kočí P., Kubíček M. & Marek M., 2003, Dynamics of three-way catalyst with diffusion in the monolith washcoat, Proc. of 6 th Italian Conf. on Chem. And Proc. Engng. (ICHEAP6), Pisa, Italy, June 8-11 2003. Harmsen J., Hoebink J. & Schouten J., 2001, Acetylene and carbon monoxide oxidation over a Pt/Rh/CeO 2 /γ-al 2 O 3 automotive exhaust gas catalyst: kinetic modelling of transient experiments, Chem. Eng. Sci. 56, 2019-2035. Harmsen J., Hoebink J. & Schouten J., 2001, NO reduction by CO over automotive exhaust gas catalyst in the presence of O 2, Catal. Letters 71, 81-90. Harmsen J., Hoebink J. & Schouten J., 2001, Transient kinetic modeling of the ethylene and carbon monoxide oxidation over a commercial automotive exhaust gas catalyst, Ind. Eng. Chem. Res. 39, 599-609. Marek M., 1985, Periodic and aperiodic regimes in forced chemical oscillations, in Temporal Order, Eds. L. Rensing & N. Jaeger, Springer-Verlag, Berlin, p.105.

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