Nanoscale platinum and iron -cobalt catalysts deposited in microchannel microreactors for use in hydrogenation and dehydrogenation of cyclohexene, selective oxidation of carbon monoxide and Fischer -Tropsch process to higher alkanes
Date of Award
Doctor of Philosophy (PhD)
Micro and Nanoscale Systems
Chemical Process Miniaturization (CPM) has predominant advantages in heat and mass transfer limited unit operations, synthesis of hazardous materials, and as a process development tool. For years, engineers have been seeking ways to apply CPM to practical applications. Studies of catalysts and catalyst supports that can be applied to microreactors are important for a number of commercially desirable gas phase reactions. Parameters such as surface-to-volume ratio and the pore structure of catalyst supports influence the activity and selectivity of the catalysts.
In this study, platinum, iron and cobalt catalysts were fabricated by sputtering deposition and compared with catalysts deposited by chemical procedures. The chemical methods to fabricate silica-supported or alumina-supported Pt and alumina-supported Fe/Co catalysts were investigated using the sol-gel and ion impregnation techniques. A substantial increase in the reaction surface area was observed for the sol-gel supports; however, the sol-gel could not be uniformly applied in the smaller microchannels tested. The characterization of the catalysts and supports was performed using SEM, XPS, BET surface area measurement, EDX, and VSM. The support particles are approximately 80 nm in diameter, which results in a specific surface area of 400 m2/g and dramatically increases the surface area of the catalysts in a microreactor from 0.03 m2 to 7 m2.
The activity and efficiency of catalysts were evaluated in microreactors with 100 micron and 5 micron wide channels. Process optimization of the Inductive Coupled Plasma (ICP) etching was necessary to achieve the desired microchannel dimensions and uniformity. The ICP parameters' studies included cycle time of SF6 gas flow, bias power, and chamber pressure. The conversion of cyclohexene to cyclohexane and benzene is the model reaction for comparison of the various deposition methods of the catalysts and the supports.
In addition, screening studies were performed on two reactions of enormous commercial potential: Fischer-Tropsch (F-T) synthesis, and preferential oxidation of CO in fuel cell. An over 50% conversion of CO and 78% selectivity to propane in F-T synthesis has been achieved. Meanwhile, a 70% conversion of CO and 80% selectivity to CO2 in preferential oxidation is reached in the fuel cell feed gas reaction. Statistical modeling studies were done using a Central Composite Design (CCD) to achieve the optimal condition (temperature 158°C, CO: O2 ratio 1.77 and total flow rate 0.207 sccm) for preferential oxidation of CO in fuel cells.
Zhao, Shihuai, "" (2003). Dissertation. 654.