(467a) Oscillations and Hysteresis during Hydrocarbon Oxidation on a Diesel Oxidation Catalyst

Oscillations and Hysteresis during Hydrocarbon
Oxidation on a Diesel Oxidation Catalyst

Po-Yu Peng, Michael P. Harold*, Dan Luss**

Department of
Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204

*mharold@uh.edu, **dluss@uh.edu

We recently reported on the impact
of zeolite beta (BEA) during dodecane oxidation on a diesel oxidation catalyst
(DOC) comprising Pt-Pd/Al₂O₃. Oscillatory CO₂
formation was observed under stationary feed and transient conditions by the combined
spatio-temporal techniques of spatially-resolved mass spectrometry (SpaciMS)
and coherent optical frequency domain reflectrometry (c-OFDR) [1].
The
experimental findings reported in [1] suggested that the oscillatory behavior
is caused by the coupling of HC sorption and oxidation. In this study we
confirm the conjectured explanation through the development of a predictive
reactor model that the oscillations are the result of the coupling between
exothermic oxidation and sorption.

The CO₂ oscillations
are accompanied by local temperature fluctuations as shown as in Figure 1 (a). We proposed that
oscillatory CO₂ formation was caused by a kind of ‘tug-of-war’
between the hydrocarbon sorption and oxidation in Fig.1 (b). HC oxidation
occurs after the zeolite sites are occupied. Upon the commencement of HC
oxidation, the exotherm leads to a temperature rise, which triggers the HC
release from the zeolite sites. The sudden large HC release leads to reaction
along with CO₂ production and temperature rise. After the HC is
completely released from the sorption sites, the zeolite re-gains the capacity
to trap the hydrocarbon. This leads to hydrocarbon trapping which slows the
hydrocarbon oxidation. The sustained and localized switching between the two
processes results in a periodic CO₂ formation and temperature rise.
This proposed mechanism is well-explained by the oscillatory behavior and the
impact of various conditions.

The coupling between hydrocarbon
sorption and oxidation, proposed as the underlying mechanism for the
oscillatory behavior, is investigated through the development of a monolith
reactor model. Using independently measured dodecane oxidation and sorption
kinetics, the model predicts most of the data trends, and confirms the proposed
mechanism. This includes the existence and extent of the oscillatory behavior. To
illustrate, Fig. 2 compares the experimental data for C₁₂ conversion
and CO₂ yield as a function of the feed temperature. The CO₂
yield is the percentage of carbon fed to the reactor as dodecane that is
converted to CO₂. The vertical bars indicate the instantaneous CO₂
yield amplitude when an oscillatory CO₂ formation occurs. The
measured CO₂ yield amplitude increases after the catalyst light-off,
reaches a maximum at 155 ^oC, and then decreases for further increase
in the feed temperature. The corresponding model predictions are shown. Overall,
the model adequately predicts the CO₂ yield and the C₁₂
conversion , as well as the range of feed temperatures for which CO₂
oscillations occur. Further, the predicted oscillatory CO₂ yield
amplitude shows a similar trend with feed temperature although the absolute
magnitude of the amplitude is somewhat under-predicted .

To understand the oscillatory
behavior, the model predictions were examined in detail to understand the
coupling process. Figures 3 (a), (b), (c) and (d) are spatio-temporal maps of
the predicted rates of hydrocarbon oxidation and adsorption, along with the C₁₂
coverage on the zeolite sites during periodic oscillation of a stationary feed
of 160 ppm C₁₂, and 10% O₂ with balance Ar at a space
velocity of 12,630 h^-1 and 165 ^oC, the data point in Fig.
2. The high C₁₂ oxidation rate shown in Figure 3 (a) occurs in the
upstream region and quickly decreases in the downstream. The area surrounded by
the contour of 0.03 mol/s.m³ oxidation rate expands and contracts
periodically. It is reached at z = 12 mm at the time of 2, 14, and 22.5 min.
During these periods of time, lower adsorption rate are reached as shown in
Fig. 3 (b). The higher zeolite site coverage in Fig. 3 (c) indicates the higher
oxidation rate follows the adsorption. Soon after, an oxidation rate of 0.25 mol/s⸱m³
at z = 6 mm is reached at a time of 4, 15, and 26 min, This oxidation reaction
is dominant and generates the CO₂ and heat. In the meantime, the
downstream temperature increases because of the heat generated from the
exothermic reaction at z = 40 mm. This heat spreads both upstream and
downstream. The temperature increase triggers C₁₂ release from the
zeolite sites. The coverage decreases shortly after the increase of the
oxidation rate, as shown in Fig. 3 (c). A exuberant oxidation reaction during
the CO₂ peak formation occurs owing to the temperature rise and the
large amount of C₁₂ provided by the feed and released from the
zeolite site. After the release of a large amount of C₁₂, the
sorption rate increases in the region where the C₁₂ was just
released from the zeolite sites, which inhibits the oxidation reaction. Once
again, the zeolite site coverage increases due to the high sorption rate. When
the zeolite sites are occupied by C₁₂, the oxidation rate becomes
dominant again. The repetition of this scenario causes the oscillation of CO₂
formation and temperature.

In summary, the developed
mathematical model shows that the interesting periodic behavior during
hydrocarbon trapping and oxidation on a precious metal catalyst containing a
zeolite adsorbed is a result of their coupling. The competition between
sorption and oxidation leads to a significant localized variation in
temperature due to exothermic reaction which serves to release hydrocarbon from
the zeolite. The modeling results strongly supports the mechanism proposed in
our previous study and will be used to predict when the oscillation occurs.

Figure 1. (a) temperature as function of position and time. (Feed:
160 °C, 160 ppm C₁₂, 10% O₂ with Argon, S.V. = 10,420 h⁻¹).
(b) A schematic illustration of tug-of-war mechanism for the oscillatory
behavior.

Figure 2. Dependence of experimental and simulated C₁₂
conversion and CO₂ yield on feed temperature at a space velocity of
12,640 h^-1. The error bars specify the instantaneous CO₂
yield amplitude .

Figure 3. The rates of oxidation and sorption of C₁₂
oxidation on the Pt/Pd/BEA monolithic catalyst on thestationary feed condition
at a space velocity of 12,630 h^-1 and 165 ^oC feed
temperature.

References

1.            Peng, P.-Y., M.P. Harold, and D. Luss, Sustained concentration and
temperature oscillations in a diesel oxidation catalyst. Chemical
Engineering Journal, 2018. 336: p. 531-543.