Performance Studies of Multifunctional Pt/Pd Monolithic Catalysts for Hydrocarbon Oxidation Emission Control

The catalytic aftertreatment of combustion engine exhaust is critically important to their continued use from both environmental and economic standpoints. This dissertation investigates the performance of multifunctional Pt/Pd monolithic oxidation catalysts for hydrocarbon emission control. This is accomplished through several related yet distinct studies. First, addition of zeolite beta was investigated with aim of improving cold-start performance. Focus was placed on hydrocarbon (HC) storage and light-off behavior. Dodecane (C12) and carbon monoxide (CO) were used as representative pollutants. Individual reactant feeds, co-feeds, and pre-storage experiments provided insight into the effects of additional zeolite on catalyst performance. Increased zeolite loading subsequently increased the C12 storage capacity. The zeolite did not have a notable effect on the light-off behavior for single component feeds. Pre-storing HC had an inhibitory effect on the CO-light-off and the co-feed of C12 and CO increased the CO light-off temperatures even further. Next, the construction of a spatially resolved capillary-inlet sampling system for in-channel concentration measurements is described. A study on propylene oxidation was conducted and the newly built sampling technique was utilized. A monolith reactor model was used to simulate the propylene oxidation reaction system in terms of effluent conversions and propylene concentration profiles. The model and data show good agreement. Evaluation of third party proprietary catalysts (oxidation and NSCR) was performed utilizing a more complex feed stream. The performance of these metallic substrate catalysts was compared to the model Pt/Pd catalyst, followed by additional experiments to develop a more fundamental understanding. For NSCR testing, the optimum O2 feed to convert both NOx and HC/CO was dependent on temperature. Increased methane conversion at low O2 feed concentrations prompted further investigation.
Finally, methane oxidation by Pt/Pd monolith was examined over a wide range of temperatures and oxygen feed concentrations. A clockwise hysteresis loop exists in the rich regime consisting of two stable steady-states of methane conversion. The high steady-state conversion branch is eclipsed by multiple higher methane conversion branches that are transient in nature. The methane consumption observed under lean conditions is insensitive to the oxygen concentration and is classical lean methane oxidation on PdO. Spatially resolved capillary-inlet mass spectrometry (SpaciMS) measurements were utilized to reveal oxidation and partial oxidation reaction zones within the monolith catalyst. During operation in the rich regime, a front oxidative zone and downstream partial oxidation/reforming zone co-exist.