Catalytic studies utilizing microfabricated reactors: Applications to improving diesel emissions reduction catalysts

In the field of heterogeneous catalysis, not all catalytic reaction processes are designed to run at steady state. Many require transient and/or cyclical operation and thus reactive surface species are transient in nature. Characterizing the mechanism of such catalytic reactions requires in-situ transient methods, which can be performed by most analytical and spectroscopic tools. However, many transient/cyclic catalytic processes are additionally operated using a monolith tube reactor in practice, and thus characterization of their mechanism and performance requires spatially resolved analytical tools to monitor the catalyst surface and reactive species as a function of position (in addition to time) along the reactor channel. The operation of the NOX Storage Reduction (NSR) process for NOX removal from lean burn diesel engines within monolith reactors is an example of a cyclic process requiring both temporal and spatial resolution for in-situ catalyst surface analysis. This work demonstrates the application of a novel spatially resolved infrared analysis system coupled with silicon microreactors or MicroElectroMechanical Systems (MEMS) to create an analytical tool capable of monitoring reaction species spatially and temporally to capture transient reaction phenomena. The design, fabrication and testing of a microreactor-FTIR imaging system is shown and used for the first time to demonstrate the ability to obtain in-situ transmission FTIR analysis of a model Pt/SiO2 catalyst during CO adsorption and oxidation with both spatial and temporal resolution. MEMS technologies were used to design and fabricate a microreactor with geometric and optical properties ideal for coupling with the high-throughput transmission FPA-FTIR system. Propagation of adsorbed species down the length of the microreactor was observed by investigating the amount of linearly bound CO to Pt and quantified from changes in the fractional coverage of such species during pulsed chemisorption experiments. The amount of adsorbed CO at each reactor postion was differentiated by pulse number (amount of CO introduced during chemisorption) and as a function of time during oxidation of the stored surface species. The demonstrated microreactor-FTIR imaging system will have important applications in the characterization of storage and release mechanisms occurring during cycling of NSR catalysts