Abstract
Low temperature hydrocarbon emissions, such as during the cold-start of the vehicle, continue to pose a challenge to engine manufacturers today. Current diesel oxidation catalysts are unable to oxidize these emissions below the light-off temperature of the catalyst, often between 200 and 250 °C. One strategy to handle these emissions is to trap them at low temperatures and subsequently oxidize them once high enough temperatures are reached. This technology is known as the hydrocarbon trap. Hydrocarbon trapping materials are often zeolites with metal ion-exchange to add additional active sites for hydrocarbon storage. To serve as an effective hydrocarbon trap, the material must be active in the presence of a wide variety of pollutant molecules which exist in diesel exhaust. In this work, Pd/BEA hydrocarbon traps were studied for their effectiveness in dodecane and ethylene trapping in the presence of a multicomponent feed. Then, the effect of Si/Al ratio and catalyst aging was studied. Reactor studies and catalyst characterization tools were employed to evaluate the Pd speciation for each catalyst prior to and after aging.
A model hydrocarbon trap material consisting of palladium promoted zeolite beta (Pd/BEA) on a monolith core was studied with a multicomponent hydrocarbon feed. Dodecane and ethylene adsorption, followed by temperature programmed oxidation (TPO), was used in the presence of CO in wet and dry conditions to study the impact of various species on hydrocarbon storage, oxidation, and desorption. The low temperature adsorption experiments suggest a moderate impact of water on dodecane uptake as dry experiments adsorbed more dodecane overall than those under wet conditions. Additionally, the presence of CO and ethylene also impaired dodecane uptake. TPD studies indicated a coupled relationship between hydrocarbon oxidation light-off and desorption. By varying the composition of the feed, specifically via the inclusion of CO or ethylene, dodecane oxidation light-off was delayed, which increased the overall hydrocarbon trap efficiency.
Pd/BEA hydrocarbon trap monoliths were then synthesized in-house in order to study the effect of CO and H2O co-feed on hydrocarbon trap performance. Ethylene uptake was partially inhibited by CO and H2O when fed separately. When both were added, the loss in ethylene uptake was 90% relative to the condition with no H2O or CO. Dodecane uptake was unchanged under all conditions tested. During a temperature ramp, ethylene desorbed and was oxidized to CO2 and H2O over active Pd centers. Further, oxidation light-off of dodecane generated an exotherm which caused rapid desorption of the remaining hydrocarbon species from the zeolite. For both hydrocarbons, CO co-feed led to a decreased oxidation light-off temperature, and therefore lower desorption temperature. By pretreating the catalyst in CO and H2O at 80 °C, and even after removing CO from the feed, the enhanced oxidation light-off behavior was observed. DRIFTS characterization shows that some form of oxidized Pd was reducible to Pd0 by CO at 80 °C only in the presence of H2O. Further, this reduction appears reversible by high temperature oxygen treatment. We speculate that this reduced Pd phase serves as the active site for low temperature hydrocarbon oxidation.
Lastly, Pd/BEA catalysts if three Si/Al ratios were synthesized and subjected to hydrothermal aging (HTA) to simulated the deactivation modes of a Pd/BEA hydrocarbon trap. HTA led to decreased ethylene storage capacity and hydrocarbon oxidation activity. Characterization by x-ray absorption spectroscopy (XAS) and NOx storage indicated that the Pd2+ remained highly disperse after HTA. Temperature programmed desorption (TPD) of NOx revealed two desorption peaks, with a high temperature peak that increased in area with increased aging temperature. Using various reactor experiments and characterization tools, the high temperature NOx desorption peak was identified as NO stored on Pd+. The Pd+, however, formed in situ during NO adsorption, and existed originally as [Pd(OH)]+. This species was less active in ethylene storage and oxidation than the original Pd species, Pd2+. In contrast, [Pd(OH)]+ stored NOx to a higher temperature than Pd2+.
