Suspensions of platinum nanoparticles (PtNPs) were formed in molten LiCl–LiBr–KBr via thermal decomposition of H2PtCl6, and subsequently evaluated for thermal stability and CO oxidation activity.
Methane pyrolysis on solid catalysts begins with C–H bond dissociation. In the formation of solid carbon, the initial C–C bond formation is thought to occur on/in the surface of most transition-metal catalysts and not in the gas phase. This has been shown to be true for Ni surfaces. In CH4–D2 exchange reactions on polycrystalline Ni, CD4 is the major product observed, which is consistent with surface-mediated dehydrogenation. However, on Cu surfaces, CH3D is the dominant exchange product observed, suggesting that deep dehydrogenation is unfavorable. Further, in a methane flow cell reactor, the carbon density and crystal size of the graphene formed from pyrolysis on Cu surfaces depend on the distance downstream in the flow field, whereas no such dependence is observed for Ni. Density functional theory calculations support an entropically favorable pathway, whereby on Cu after dissociative chemisorption of methane, the surface adsorbed CH3° desorbs rather than undergoing further dehydrogenation; on Ni, complete dehydrogenation is favorable. Reactive methyl radicals from Cu surfaces would participate in gas-phase pyrolysis upon desorption, forming the initial C–C bond in the gas phase with subsequent re-adsorption and surface modification of gas-phase generated oligomers.
Current methods of hydrogen production from methane generate more than 5 kg of CO2 for every 1 kg of hydrogen. Methane pyrolysis on conventional solid heterogeneous catalysts produces hydrogen without CO2, but the carbon coproduct poisons the catalyst. This can be avoided by using a molten metal alloy catalyst. We present here a study of methane pyrolysis using mixtures of molten Cu–Bi alloys as the catalyst. We find that molten Cu–Bi is an active catalyst, even though pure molten Bi and Cu are not. Surface tension measurements and constant-temperature ab initio molecular dynamics simulations indicate that the surface is enriched in Bi and that the catalytic activity is correlated with the concentration of Bi at the surface. Bader charge analysis indicates that bismuth donates charge to copper. In the most stable configuration of dissociated methane on these liquid surfaces, CH3 binds to a bismuth surface atom and H to Cu. The energy barriers for the dissociative adsorption of methane, calculated using the nudged elastic band (NEB) method, are between 2.5 and 2.6 eV, depending on the binding site on the surface of the Cu45Bi55 alloy. The computed barriers are in rough agreement with the experimental apparent activation energy of 2.3 eV.
Author(s): Palmer, Clarke | Advisor(s): McFarland, Eric W | Abstract: 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.
The partial oxidation of methane to carbon and steam was investigated in molten salts for a process to produce CO2-free electrical power and solid carbon. Lithium iodide and lithium bromide catalysts were used in a bubble column where insoluble carbon accumulates on the melt surface and could be continuously removed. The salt acts as a heat transfer medium and reacts with oxygen to produce halogens and consume hydrogen halides in a chemical looping cycle. The halogens react with methane in gas-phase bubbles and form hydrogen halides and carbon. Hydrogen halides are then neutralized by an oxide and form steam and a halide salt. The halide salt reacts with oxygen, forming an oxide and closing both the halogen and the salt chemical looping cycles in a single vessel. Selectivities to carbon of 90% were measured for 56% methane conversion in a 12 cm bubble column reactor. The carbon was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and Raman spectroscopy. Iodide and bromide salts were investigated along with the behavior of iodine, bromine, methyl iodide, and methyl bromide intermediates.
While low-cost natural gas remains abundant, the energy content of this fuel can be utilized without greenhouse gas emissions through the production of molecular hydrogen and solid carbon via methane pyrolysis. In the absence of a carbon tax, methane pyrolysis is not economically competitive with current hydrogen production methods unless the carbon byproducts can be valorized. In this work, we assess the viability of the carbon byproduct produced from methane pyrolysis in molten salts as high-value-added anode or conductive additive for secondary Li-ion and Na-ion batteries. Raman characterization and electrochemical differential capacity analysis demonstrate that the use of molten salt mixtures with catalytically-active FeCl3- or MnCl2 result in more graphitic carbon co-products. These graphitic carbons exhibit the best electrochemical performance (up to 272 mAh/g of reversible capacity) when used as Li-ion anodes. For all carbon samples studied here, disordered carbon domains and retained salt species trapped and/or intercalated into the carbon structure were identified by X-ray photoelectron and multinuclear solid-state nuclear magnetic resonance spectroscopy. The latter lead to reduced electrochemical activity and reversibility, and poorer rate performance compared to commercial carbon anodes. The electronic conductivity of the pyrolyzed carbons is found to be highly dependent on their purity, with the purest carbon exhibiting an electronic conductivity nearly on par with that of commercial carbon additives. These findings suggest that more effective removal of the salt catalyst could enable applications of these carbons in secondary batteries, providing a financial incentive for the large-scale implementation of methane pyrolysis for low-carbon hydrogen production.
The catalytic decomposition of methane, propane, benzene, and crude petroleum was investigated between 900 and 1000 °C in molten metal bubble column reactors. The conversion to gas phase products and solid carbon was measured after introducing the gas phase reactants into a bubble column reactor containing a catalytic molten mixture of 27 mol % Ni and 73 mol % Bi. The conversions of propane, benzene, and crude oil are 100% at temperatures >950 °C at a reactor residence time of ∼1 s. Equilibrium selectivity of 100% H2 and carbon was not achieved in the short residence time, but can be achieved at longer residence times. The solid carbon products obtained from methane pyrolysis were more graphitic than those produced from the other, higher-molecular weight reactants; the latter were more amorphous, as measured by Raman spectroscopy and electron microscopy and resembled carbon black. A model is proposed for carbon formation in bubble column reactors, in which amorphous carbon products are derived from the gas-phase decomposition and graphitic carbon products are formed from dissolution and reprecipitation of carbon into and out of the molten metal.
While low-cost natural gas remains abundant, the energy content of this fuel can be utilized without greenhouse gas emissions through the production of molecular hydrogen and solid carbon via methane pyrolysis. In the absence of a carbon tax, methane pyrolysis is not economically competitive with current hydrogen production methods unless the carbon byproducts can be valorized. In this work, we assess the viability of the carbon byproduct produced from methane pyrolysis in molten salts as high-value-added anode or conductive additive for secondary Li-ion and Na-ion batteries. Raman characterization and electrochemical differential capacity analysis demonstrate that the use of molten salt mixtures with catalytically-active FeCl3-or MnCl2 result in more graphitic carbon co-products. These graphitic carbons exhibit the best electrochemical performance (up to 272 mAh/g of reversible capacity) when used as Li-ion anodes. For all carbon samples studied here, disordered carbon domains and retained salt species trapped and/or intercalated into the carbon structure were identified by X-ray photoelectron and multinuclear solid-state nuclear magnetic resonance spectroscopy. The latter lead to reduced electrochemical activity and reversibility, and poorer rate performance compared to commercial carbon anodes. The electronic conductivity of the pyrolyzed carbons is found to be highly dependent on their purity, with the purest carbon exhibiting an electronic conductivity nearly on par with that of commercial carbon additives. These findings suggest that more effective removal of the salt catalyst could enable applications of these carbons in secondary batteries, providing a financial incentive for the large-scale implementation of methane pyrolysis for "low-carbon" hydrogen production.
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