(544bo) Dual CO Light-Off Effect on Pt/Al2O3, Pd/Al2O3, Pt/CeO2/Al2O3 and Pd/CeO2/Al2O3 in the Presence of C3H6

Introduction

The kinetics of CO and HC oxidation in automotive exhaust
gas on three-way and diesel oxidation catalysts (TWC, DOC) have been studied
for a couple of decades, however, there are still some phenomena affecting the
catalyst performance and pollutant conversion that need to be better understood
and controlled. The CO oxidation in the presence of hydrocarbons (represented
by propene) is studied in this paper. The light-off behavior is important
especially during the cold start period, in which the majority of CO and
unburned hydrocarbons (HC) is emitted. As the exhaust aftertreatment system
heats up, the rate of catalytic reactions exponentially increases so that almost
full conversion is achieved after reaching the light-off temperature. Two-step
CO light-off is an undesired effect that significantly delays the process of
reaching full CO conversion in the exhaust gas mixture – the CO light-off curve
can exhibit a plateau or shoulder where the CO conversion does not increase or temporarily
even decreases with temperature, though there is enough O₂ available
for both CO and HC oxidation. Hazlett et al. [1] provided a spatially resolved analysis of co-current CO and
C₃H₆ oxidation on Pt-Pd/Al₂O₃
and found that the onset of C₃H₆ oxidation may inhibit
the CO conversion due to blocking of active catalytic sites by the products of hydrocarbon
partial oxidation.

In this contribution we present the results of systematic
experimental study exploring the two-step CO light off phenomenon on Pt/γ-Al₂O₃,
Pd/γ-Al₂O₃, Pt/CeO₂/γ-Al₂O₃
and Pd/CeO₂/γ‑Al₂O₃
catalyst, and present a global kinetic model that can describe and
predict the observed phenomena [2].

Materials and Methods

The performance of catalyst
samples was examined using synthetic gas mixture simulating real automotive
exhaust gas. The measurements were performed with model catalysts coated on ceramic
monolith, provided by Johnson Matthey. Small monolith section (ca. 30 x 30 x 5
mm³) was placed in a heated lab reactor and temperature was
increased with the rate 6 K/min in the range 100–400 °C. The outlet
concentration of pollutants was measured on-line by FTIR and mass spectrometer.

Several light-off experiments
were performed with different inlet gas compositions in order to reveal the
effect of CO, C₃H₆, O₂ and NO on the studied
phenomenon. Finally, the whole experimental procedure was subsequently repeated
with Pt/γ-Al₂O₃, Pd/γ-Al₂O₃ and Pt/CeO₂/γ-Al₂O₃,
and Pd/CeO₂/γ‑Al₂O₃
catalyst and the results for different catalyst formulations were
compared.

Results and Discussion

a)
b)

Figure
1. Light-off experiment with a) Pt/γ-Al₂O₃,
b) Pd/γ-Al₂O₃ catalyst. Inlet
gas mixture: CO 1660 ppm, C₃H₆ 860 ppm, NO 600 ppm, O₂
4120 ppm, 7 % H₂O, 7 % CO₂, N₂ balance.

The presence of C₃H₆
was a necessary condition for occurrence of the two-step CO light-off phenomenon
(Figure 1a). The effect was further exaggerated by the presence of NO. An
excess of oxygen decreased the extent of the phenomenon. Two reaction pathways were
further investigated: (i) Accumulation of hydrocarbon
oxidation intermediates on the catalyst surface and subsequent inhibition of CO
oxidation, and (ii) formation of CO as a by-product of hydrocarbon
oxidation. The first pathway was confirmed by transient experiments of CO and C₃H₆
co-oxidation with isothermal period, during which CO conversion gradually decreased.
Only small traces of CO by-product were detected during C₃H₆
oxidation, suggesting that the second pathway played only a minor role. The
findings have been reflected in the developed kinetic model [2].

Pt/γ-Al₂O₃ catalyst exhibited the largest two-step CO
light-off effect, with a significant decrease of CO conversion (Figure 1a). Pd/γ-Al₂O₃ showed much smoother CO light-off curve,
though the inhibition of CO oxidation was still present (Figure 1b). Full CO
conversion was reached only after total C₃H₆ oxidation
and clean-up of the catalytic surface. The presence of CeO₂ in
catalyst formulation narrowed the temperature window for the two-step CO
light-off.

Conclusions

The experimental results reveal
differences in the CO light-off performance in full exhaust gas mixture on Pt/γ-Al₂O₃,
Pd/γ-Al₂O₃, Pt/CeO₂/γ-Al₂O₃,
and Pd/CeO₂/γ‑Al₂O₃catalysts. Pd-based
catalysts show less pronounced two-step CO light-off than the formulations with
Pt. The developed model can help in finding the optimum operating strategy for
suppressing the undesired effects.

Acknowledgements

Czech Science Foundation (project GA 17-26018S).

References

1.      Hazlett
M. J., Moses-Debusk M., Parks II J. E., Allard L.
F.,  Epling W.
S. Kinetic and mechanistic study of
bimetallic Pt-Pd/Al₂O₃
catalysts for CO and C₃H₆ oxidation. Applied
Catalysis B: Environmental, 2017,
202, 404–417.

2.      Buzkova Arvajova
A., Brezina J., Pecinka R.,
Koci P. Modeling of two-step CO oxidation light-off on
Pt/γ-Al₂O₃ in the presence of C₃H₆
and NO_x. Applied Catalysis
B: Environmental, 2018, 233,
167-174.