Catalytic Methane Chemistry in High-Temperature Molten Environments
At present there are few, if any, alternatives to fossil hydrocarbons that will provide continued growth in global economic prosperity while significantly reducing global CO2 emissions. Meanwhile, the continuous discovery of new natural gas reserves will likely provide abundant, low-cost methane in the United States (and elsewhere) for the next several decades. Methane pyrolysis (MP; CH4 ⇄ 2H2 + C(s)) could provide cost-competitive, CO2-free industrial hydrogen and serve as a ‘bridging’ solution until a long-term sustainable energy infrastructure is developed and deployed. Critically, there are two fundamental roadblocks to the widespread industrialization of MP: (1) finding a catalytic pathway that does not deactivate due to coking from the formation of solid carbon as traditional heterogeneous catalysts do, and; (2) a low-cost separations process that can separate the hydrogen and solid carbon continuously from the reactor. Although reactors can be “decoked” with oxygen or steam, this would result in the stoichiometric production of CO2. A promising route to overcome both roadblocks are molten environments (i.e., molten metals and molten salts) that have recently been demonstrated to both facilitate the separation of solid carbon while providing a continuously-renewed, catalytic, gas-liquid interface. Although the basic chemical transformation appears relatively simple, the atomic level mechanisms and microkinetics of the catalytic pathways are not known, and the role of inter-phase transport is not understood. The goal of this thesis project is to characterize the catalytic chemistry of methane pyrolysis (and other associated chemistries) in these high-temperature liquid environments and to leverage this understanding in the engineering of novel, multiphase chemical reactors. This dissertation presents work examining the catalytic activity of molten metal and molten salt surfaces, reaction pathways and mechanisms thereon, and carbon morphologies; formation routes; and separation strategies from residual molten media. Copper-bismuth (Cu-Bi) alloys are observed to have considerable activity for MP which is attributed to the surface metal compositions and electronic properties derived from intermetallic charge transfer. The pyrolysis of other hydrocarbons (e.g., propane, benzene, and crude oil) is explored in a molten Ni-Bi alloy in order to demonstrate that these liquid environments can accommodate any fossil fuel resource while producing CO2-free molecular hydrogen and solid carbon. This unique capability to continuously produce solid carbon is utilized in concert with dry reforming of methane (CH4 + CO2 ⇄ 2H2 + 2CO) to produce synthesis gas (syngas; H2 + CO) with variable H2:CO ratios. Work exploring similar catalytic activities and chemical transformation pathways in molten salt environments is also presented. Alkali-halide salts such as KCl and NaBr are found to possess little activity for MP, although are attractive mediums to utilize at commercial scale due to their low-cost, high thermal stability, and low toxicity. Hydrocarbon feed additives (such as ethane and propane) are shown to be effective at increasing the overall decomposition rate of methane by increasing the number of radical reactions in the gas phase. Molten salt surfaces with inherently higher catalytic activity such as mixtures of FeCl3-KCl-NaCl are also shown to considerably increase reactions rates. Overall, the understanding of methane transformations on and in molten media is furthered and key insights into the barriers for commercialization have been elucidated.