(84b) Spatial and Temporal Features of the Catalyzed Hydrocarbon Trap

Spatial and Temporal Features of the Catalyzed Hydrocarbon
Trap

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

The diesel oxidation catalyst
(DOC) system plays an important role in promoting CO and hydrocarbons (HCs)
oxidation by the precious group metals Platinum (Pt) and Palladium (Pd). The DOC
is designed to operate at very high HC conversion (>90%) after a sufficiently
operating temperature has been reached. However, during the “cold start”, most
of the engine-out hydrocarbons remain unreacted and are emitted to the
environment.[1,
2] A large-pore zeolite such as beta-zeolite (BEA) has been added to the DOC to
trap hydrocarbons during the cold start. In particular, large molecular weight hydrocarbons
are especially prone to trapping in BEA-modified DOCs. Although extensive research
has been conducted on the BEA storage capacity, there is a need to gain insight
on how BEA affects the performance of HC light-off (LO) and of the HC oxidation
transient behaviors. [3-7]

DOC and DOC/BEA (0.5 g/in³
BEA) monolith catalysts were used to study the transient behavior after the HCs
(Dodecane, C₁₂H₂₆) has been pre-stored on the catalyst, The
impact of BEA on the LO performance and HCs trapping were studied by flowing a
mixture of  10% O₂ in Ar during temperature-programed oxidation
(TPO). The transient behavior of the hydrocarbon trap was measured by both coherent
optical frequency domain reflectometer (c-OFDR) and capillary inlet mass
spectrometer (Spaci-MS) enabling the collection of transient temperature and
concentration profiles the complete spatio-temporal data during the TPO.   .

 Figure 1 reports the feed HC
concentration (150 ppm Dodecane, C12) and C12 effluent concentration.  The figure
reveals the completion of the HC pre-store and the removal of weakly-bonded HC
by the lean gas purge (10% O₂ balanced by Ar). The pre-stored HC
amount of 14.0 mg C12/g sample was determined by integrating the area between
the feed concentration and effluent concentration. During the TPO, the effluent
CO₂ concentration reveals the existence of multiple hydrocarbon LO
points where the CO₂ formation occurs sequentially. Specifically,
there are three major peaks that form during the TPO, indicating there are three
LO stages within the monolith catalyst.

Figure 2 reports and the 3-D temperature
dependence on position and time. The plot reveals the temperature rise at
different locations and different times in the monolith catalyst. The three
temperature hot zones occur due to the local HC combustion. Once the local HC
combustion is complete the temperature drops until the next local LO. This indicates
that the pre-stored HC is not release at once but occurs sequentially. We
conclude by analyzing the HC LO locations and timing that the multiple HC LO
phenomena is caused by a front of released HC which is trapped in the downstream
and lights-off later when the local temperature exceeds the LO value.

The multiple hydrocarbon LO
phenomena is strongly related to a tug-of-war between HC trapping and oxidation,
thus, the steady-state HC oxidation has been investigated. Figure 3 shows the
spatially-resolved C12, C₃H₆ and CO₂
concentrations at steady-state for a furnace temperature of 220 ^oC conditions
for which most of HCs are fully oxidized. The CO₂ concentration
fluctuated with a feed of 150 ppm C12 but was stationary under a feed of 0.2% C₃H₆.
The effluent CO₂ concentration exhibited periodic fluctuations. These
results suggest the existence of a “tug-of-war” between HC trapping and
oxidation. Therefore, this phenomena provided a good opportunity to design more
efficient catalyst.

Figure 1. Dodecane and CO₂ concentration measured during
adsorption and desorption followed by TPO with BEA-zeolite added Pt/Pd/Al₂O₃ monolith catalyst.
Condition during saturation: 150 ppm dodecane and 10% O₂ balanced
with Ar. Condition during desorption and TPO: 10% O₂ balanced with
Ar.

Figure 2. (a) Spatially-resolved temperature profile measured at
different time in the C12 pre-stored catalyst and (b) the 3-D plot of the
spatially-resolved temperature versus time in feed of 10% O₂
balanced with Ar. (Catalyst: D-05)

Figure 3. Spatially-resolved C12 and CO₂ concentration at
steady-state at 220 ^oC with the feed of (a) 150 ppm C12 and 10% O₂
balanced with Ar, and (c) 0.2% C₃H₆ and O₂
balanced with Ar. Spatially-resolved C12 and CO2 conversion and selectivity at
steady-state at 220 ^oC with the feed of (b) 150 ppm C12 and 10% O₂
balanced with Ar, and (d) 0.2% C₃H₆ and O₂
balanced with Ar.

References

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