Millisecond catalytic reactions on rhodium: Partial oxidation, steam reforming, and water-gas shift

Millisecond catalytic partial oxidation of methane and higher alkanes is of great interest due to the ease with which valuable products can be made without energy input. In particular, the catalytic partial oxidation of methane to produce syngas, a gaseous mixture of CO and H₂, over rhodium-coated monoliths at short contact times provides a way to produce feed streams for methanol production and Fischer-Tropsch synthesis of higher alkanes at conditions that are often more favorable than status quo industrial processes. Further, the partial oxidation of hydrocarbons to produce syngas enables the simple, quick, robust production of high purity hydrogen streams for fuel cell applications.; This thesis discusses research that seeks to understand, model, and control the reactions of hydrocarbons on rhodium at millisecond contact times. In this work, it was determined that the catalytic partial oxidation of methane on rhodium follows a two-stage reaction pathway in which direct partial oxidation and combustion reactions compete in an initial high temperature stage (1200°C) of the reactor followed by endothermic reforming reactions between unreacted methane and combustion products downstream. It was also discovered that steam reforming reactions, water-gas shift reactions, and methanation reactions could be promoted in millisecond rhodium reactors provided the proper reactor temperatures and steam contents exist. These experimental results are coupled with reactor modeling in which it was determined that only minor adjustments were required to allow previously developed elementary-step reaction mechanisms to describe both high temperature catalytic partial oxidation reactions (900°C–1200°C) and low temperature water-gas shift reactions (600°C–900°C). The most significant modifications to the model included the addition of CO surface coverage dependent rate expressions for two of the 38 elementary surface reactions. Finally, the knowledge gathered from these experimental and theoretical studies enabled the construction of two lab scale reactors that either maximized the production of syngas from methane (synfuels applications) or altered the outlet ratio of hydrogen and carbon monoxide from the system (fuel cell applications).