David Chester Upham

Methane pyrolysis over molten metal catalysts has gained attention as a promising route for CO2-free hydrogen production owing to the intrinsic resistance of liquid catalysts to carbon-induced deactivation. In this work, we investigate the role of intermediate carbon species on the surface composition and methane activation over molten In–Sn and In–Ni alloys using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. Free-energy calculations indicate that methane dissociation proceeds through the formation of carbon monomer and dimer intermediates, which are likely to serve as the initial building blocks for carbon nucleation and subsequent growth of larger poly-aromatic carbon structures. Carbon exhibits distinct segregation behavior in the two molten alloys, preferentially occupying the surface and subsurface regions in In–Sn, while migrating toward the subsurface and bulk in In–Ni. These contrasting distributions are largely independent of the carbon concentration and arise from the preferential interaction of carbon with Sn in In–Sn and Ni in In–Ni. Although carbon segregation modifies the local surface environment, methane activation barriers remain qualitatively unchanged, while the dissociated intermediates are preferentially stabilized on surface carbon species rather than on the molten alloy surface, highlighting the sustained catalytic activity of the molten catalysts.

Methane pyrolysis for hydrogen production is critical for decarbonization efforts, provided that the co-produced solid carbon can be separated affordably and valorized at scale. Studies demonstrate the potential of molten metal alloy catalysts operating in bubble column reactors, achieving industrial-relevant productivities (10⁻⁵ to 10⁻⁶ mol/s/cm³). Key catalytic mechanisms involve surface segregation and surface charge distribution, with electronic effects significantly influencing C–H and C–O bond activation rates. For instance, copper-indium alloys are shown to catalyze the formation of multi-walled carbon nanotubes (CNTs) from nanodroplets, with subsequent heat treatment enabling effective catalyst removal. Another approach, utilizing floating-catalyst gas-phase reactors with iron and sulfur precursors and gas recycling, has demonstrated net hydrogen production (84.5 vol% H₂) alongside CNTs. Significant advances in post-processing have led to CNT fibers with tensile strengths exceeding 8 GPa and recent reports of electrical conductivity surpassing copper and aluminum, making them competitive with high-end carbon fibers. Reactor productivity has increased 100-fold, though further optimization of catalyst selectivity and residence time is needed to prevent undesirable radial growth and ensure high-quality product. For industrial-scale application, a non-catalytic methane pyrolysis process aims for cost parity with steam methane reforming (SMR) coupled with carbon capture, utilization, and storage (CCUS). While initial Gen 1 reactors demonstrated hydrogen and carbon yields, achieving consistent commercial-grade carbon black requires improved reactor temperature and retention time control. New Gen 2 reactor architectures are being developed to increase hydrogen capacity and ensure consistent carbon quality. Overall, the economic viability and scalability of methane pyrolysis are contingent on developing robust carbon valorization pathways, with new markets beyond traditional carbon black, such as in construction materials, advanced conductors, and aerospace composites, being essential for accommodating the substantial volumes of carbon produced. Considerations for the long-term fate of carbon and the reduction of upstream fugitive methane emissions are also critical for the overall environmental impact. Welcome from the Chair Molten metal catalyzed carbon nanotube production Pyrolysis of Methane to Bulk CNT Materials: Manufacturing Carbons that Are Useful for Manufacturing EKONA: Our Journey to Develop Low-Cost, Clean Hydrogen from Methane Pyrolysis Discussion

The generation of multi‐walled carbon nanotubes (MWCNTs) typically utilizes solid catalyst nanoparticles. These particles often exhibit a liquid‐like nature during synthesis and remain encapsulated inside the final MWCNTs. Molten Cu–In has been recently reported to produce high amounts of MWCNT in bubble column reactors during methane pyrolysis for clean H ₂ generation. In the present work, nanodroplets are isolated and studied on supports. The droplets are observed to deform into a head and tail geometry and generate bamboo‐like MWCNTs. The compositions of 50–70 at.% Cu repeatedly generate dense bundles of MWCNTs, while higher or lower compositions yield little or no MWCNTs. The lower surface tension of the alloy at these compositions reduces the thermodynamic driving force for coalescence, stabilizing small droplets at high temperature. Graphitic structure with some defects is confirmed by transmission electron microscopy and Raman spectroscopy, showing 3.35 +/−0.08 Å interlayer spacing and an I _D /I _G ratio of 0.87 +/−0.13, respectively. Droplets distributed between 10 nm and 1 micron generate MWCNTs 10–400 nm in diameter, suggesting droplets above 400 nm do not generate MWCNTs at any catalyst composition. Bundles of MWCNT exceeding hundreds of microns are observed in reaction times over 1 h.

Molten metal catalysts for methane pyrolysis and dry reforming are becoming recognized for their potential in decarbonization efforts. Their use in bubble column reactors facilitates continuous operation by allowing the produced carbon to float to the surface for removal. While most reported molten metals produce low-value amorphous carbon or graphitic sheets containing some metals, our study introduces a Cu-In alloy that selectively produces high-purity carbon nanofibers. These nanofibers are tubular and have a smooth or bamboo-like segmented structure with a diameter of approximately 100 nm. We have identified a droplet-based pathway for the growth of these fibers and removal of the droplets, observed consistently across a bubble column reactor, a surface reactor, and both in the absence and presence of carbon dioxide for pyrolysis and dry reforming. The molten Cu-In system is shown to outperform other molten metal catalysts, producing fibers with a purity greater than 99.9% after heat treatment.

The commercial, CO₂-intensive, propylene generation process is challenged by a low equilibrium yield, costly separation processes, and severe carbon deposition. Oxidative dehydrogenation (ODH) of propane offers a promising alternative; however, its potential is hampered by the undesirable over-oxidation of propylene to carbon oxides. Chemical looping approaches have been used, where a metal oxide reacts with propane and/or hydrogen – instead of O₂ – which can avoid over-oxidation of the hydrocarbons to carbon oxides. Oxygen can then react with the reduced solid product to regenerate it to a stoichiometric oxide. However, solid chemical looping catalysts have a low oxygen carrying capacity, often less than 1% in order to avoid cyclic deactivation, arising from changes to the lattice structure in each cycle. Herein, we report the use of molten metals as chemical looping catalysts, where the catalyst completely reduces to metallic form and melts each half-cycle. A high oxygen capacity is possible, and melting reduces lattice strain upon reduction. Thermodynamic calculations indicate that 21 individual metal candidates could be viable, and the most promising 14 are experimentally compared. Bi–Sn had the highest conversion, and 50–50 mol% Bi–Sn was supported in a fixed-bed reactor. This resulted in 22% propylene yield at 600 °C, compared to a 20% yield from the reference using borosilicate beads at the same temperature. After 10 cycles with separate flows of O₂ followed by C₃H₈, totaling 37 hours on stream, no deactivation was observed. Enhanced propylene selectivity was observed using a 50–50 mol% Bi–Sn molten mixture, surpassing the performance of Bi or Sn alone, and leading to reduced carbon oxide formation. This improved selectivity occurred with both co-fed and pre-oxidized catalysts, suggesting the creation of a unique metal oxide selectively generating propylene while minimizing overoxidation to carbon oxides. Additionally, the oxygen conversion over the Bi–Sn alloy was lower than over Bi or Sn separately when co-feeding propane and oxygen. Selectivity to cracked products was high for most of the active melts tested, especially above 550 °C.

A new Sn–In alloy activates both CH ₄ and CO ₂ at similarly high rates.