468482 Spatial and Temporal Features of the Catalyzed Hydrocarbon Trap

Monday, November 14, 2016: 8:20 AM
Union Square 14 (Hilton San Francisco Union Square)
Po-Yu Peng, Dan Luss and Michael P Harold, Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX

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

1.            Mukai, K., et al., Adsorption and desorption characteristics of the adsorber to control the HC emission from a gasoline engine. 2004, SAE Technical Paper.

2.            AL‐Harbi, M., et al., Competitive no, co and hydrocarbon oxidation reactions over a diesel oxidation catalyst. The Canadian Journal of Chemical Engineering, 2012. 90(6): p. 1527-1538.

3.            Li, H.-X., et al., Application of zeolites as hydrocarbon traps in automotive emission controls. Studies in Surface Science and Catalysis, 2005. 158: p. 1375-1382.

4.            Chang, H., et al., Gasoline Cold Start Concept (gCSC™) Technology for Low Temperature Emission Control. SAE Int. J. Fuels Lubr, 2014. 7(2): p. 480-488.

5.            Murakami, K., et al., Development of a high performance catalyzed hydrocarbon trap using Ag-zeolite. 2004, SAE Technical Paper.

6.            Elangovan, S., et al., A comparative study of zeolites SSZ-33 and MCM-68 for hydrocarbon trap applications. Microporous and mesoporous materials, 2006. 96(1): p. 210-215.

7.            Elangovan, S., et al., Silicoaluminophosphate molecular sieves as a hydrocarbon trap. Applied Catalysis B: Environmental, 2005. 57(1): p. 31-36.

 


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