Spatio-Temporal Features of the Hydrocarbon Trap: Spatial Nonuniformities and Sustained Oscillations

Michael  Harold, University of Houston

The function of the diesel oxidation catalyst (DOC) comprising a washcoat of Platinum (Pt) and Palladium (Pd) is to catalyze the oxidation of promoting mixture of CO and hydrocarbons (HCs). The DOC is designed to operate at very high CO and HC conversion (>90%) after a sufficiently operating temperature has been reached. However, during the “cold start” a fraction of the exhaust  hydrocarbons remain unreacted and are emitted to the environment. A large-pore zeolite such as Beta (BEA) is added to the DOC to trap hydrocarbons during the cold start. High 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 performance. In this work, we use coherent optical frequency domain reflectometer (c-OFDR) and spatially resolved mass spectrometry (Spaci-MS) to measure the transient behavior of HC trapping and oxidation during the cold start.  The objective is to gain a deeper understanding of the functioning of the HC trap to guide HC trap design and operating strategy.

In a typical experiment the catalyst is exposed to a feed-containing the HC (dodecane and/or propylene).   Subsequently the catalyst under a temperature ramp is exposed to gas feed containing oxygen. We compare the c-OFDR and Spaci-MS with the catalyst having BEA to understand the impact of the HC trapping on the oxidation.

he temperature ramp experiments reveal the existence of multiple temperature excursions along the length of the monolith. These excursions coincide with the oxidation of  the dodecane through the spatially-resolved measurement of the docane and product CO2 concentrations. The impact of the space velocity and temperature ramp rate are investigated to assess the generality of the findings. Steady-state experiments reveal the onset of sustained oscillations in the oxidation rate and monolith temperature. The mechanism for these features is the coupling between the HC trapping and oxidation. During an excursion local heating drives off the trapped HC. Upon its consumption the temperature drops and additional HC is trapped. The cycle repeats itself along the length of the monolith. A phenomenological monolith model predicts the main features.

This study is the first to provide insight into the spatio-temporal features of transient behavior of HC trapping and oxidation of BEA/DOC catalysts, and also the first discovery of multiple HC light-off phenomena. The results gained from this study will help to optimize the multi-functional and component catalyst formulation, reactor design and operating strategy for achieving high conversion of multiple hydrocarbon species.