Passive in-line catalyzed hydrocarbon (HC) traps have been used by some manufacturers in the automotive industry to reduce regulated tailpipe (TP) emissions of non-methane organic gas (NMOG) during engine cold-start conditions. However, most NMOG molecules produced during gasoline combustion are only weakly adsorbed via physisorption onto the zeolites typically used in a HC trap. As a consequence, NMOG desorption occurs at low temperatures resulting in the use of very high platinum group metal (PGM) loadings in an effort to combust NMOG before it escapes from a HC trap. In the current study, a 2.0 L direct-injection (DI) Ford Focus running on gasoline fuel was evaluated with full useful life aftertreatment where the underbody converter was either a three-way catalyst (TWC) or a HC trap. A new HC trap technology developed by Ford and Umicore demonstrated reduced TP NMOG emissions of 50% over the TWC-only system without any increase in oxides of oxygen (NOx) emissions. Other HC trap technologies had at best a 25% NMOG emission reduction. Parallel laboratory reactor studies were conducted in an effort to understand the improved trapping and NMOG combustion features of the newly developed HC trap. Increased trapping efficiency of certain aromatics (toluene) and alkenes (2-methylpropene) was assigned to rapid and efficient polymerization of these species due to a combination of strong Brønsted acidity, precious metal (i.e., Pd), and base redox active metals. During the emissions desorption phase, the combustion of the adsorbed NMOG occurred without gas-phase oxygen due to the delayed desorption of the large NMOG molecules coupled with the high activity of the base redox active metal in the presence of steam. Besides acting as a source of oxygen during combustion, the ion-exchanged form of the base metal also stabilized Pd against sintering during the hot, four-mode aging process.
