Background
Hydrogen (H2) is a versatile energy carrier and chemical feedstock with the potential to help the decarbonization of transportation, electricity generation, and manufacturing, which are the most energy intensive sectors of the economy. However, conventional H2 production is associated with significant greenhouse emissions, thereby diminishing its transformative potential. Conventional processes, such as the oxidative reforming of methane (CH4), require extreme temperatures (700-1000°C), pressures (30-50 bar), and large capital investments to produce H2. Extreme conditions and process scale require centralized site production coupled with H2 distribution networks.
The goal of our project was to develop a plasma-enabled catalytic process to convert CH4 into H2 and solid carbon (C(s)) at atmospheric pressure and room temperature. This process will eliminate CO2 emissions by sequestering carbon on the catalytic surface in a solid phase conducive to controlled harvesting. By co-designing the non-equilibrium excitation of CH4 and the catalyst, favorable reaction pathways are enabled that reduce thermodynamic operating constraints, break scaling laws for CH4 decomposition, and enable the design and usage of novel catalysts.
Approach
Non-Thermal Plasma Reactor: The most critical obstacle to the direct conversion of CH4 to H2 at low temperatures and pressures is that methane is a very stable molecule with high first C–H bond enthalpy (439 kJ/mol), making novel methods to activate CH4 highly desirable (CH4 conversion to H2 requires an energy input of 74.6 kJ/mol). Studies show that non-equilibrium reactants (i.e., vibrationally and electronically excited or radicals) have lower activation energies and break scaling relations to widen the catalyst design space. Non-thermal plasmas offer an efficient route to CH4 excitation at practical conditions using (green) electrical energy. Such plasmas are characterized by energetic electrons, which excite the internal modes of molecules and form ions and radicals. Thus, energy is used efficiently for activation at non-equilibrium temperatures (300 K) and pressures (1 atm).
A bench-scale dielectric breakdown discharge (DBD) annular flow-through reactor was specially configured for generating non-equilibrium plasmas of CH4/Ar mixtures and assessing the abundancies of excited species as a function of excitation parameters. Plasma excitation in the DBD reactor was provided by a 20 kV (at 20 kHz) RF power supply and was designed to operate at atmospheric pressure with continuous flow of gas reactant (~1000 sccm). The reactor was outfitted with in situ plasma diagnostic capabilities based on optical emission spectroscopy (OES) to enable real-time analysis of excited plasma species and reaction products in the plasma phase as the source gas (methane/argon mixtures) interacts with the catalyst bed.
Design of Tailorable Catalysts: Multi-principal element alloys (MPEAs), combining metals, metalloids, and non-metals, represent a vast, largely unexplored space for discovery of novel material systems and catalysts with potentially unprecedented properties. Data-driven computational algorithms developed under a previous IR&D program (18-R6233) were employed to narrow down the compositional space for MPEA catalysts that are predicted to be strong candidates for direct reformation of methane to hydrogen (and solid carbon).
Coking Inhibition Strategies: Strategies to mitigate catalyst degradation and fouling due to coke deposition were focused on passive methods by altering the surface energy of a catalyst either structurally in the context of presenting CH4 molecules to different crystallographic facets or through ion doping. The former strategy was investigated theoretically through first principles computations at the level of solid-state density functional theory (DFT). Periodic atomic slab models of nickel clusters supported on three different crystallographic planes of cerium oxide (CeO2), designated as Ni/CeO2, were built as a benchmark for methane conversion that is supported experimentally by the literature. The surface energy of each system was computed and compared with that of the native catalysts.
Accomplishments
We demonstrated that the generation of excited-state species of methane via non-thermal plasma under atmospheric conditions is a promising and energy-efficient mechanism of lowering the activation barriers (steric and electronic) that would otherwise impede the decomposition of methane on the surface of a solid catalyst to yield only hydrogen and carbon (Figure 1). OES measurements indicated that the excitation conditions employed to generate stable plasma from methane at high concentrations yielded exclusively the excited species of fully dissociated methane (C+ ion and C neutrals) at an average power of ~15 W.
![milled NiCoCuFeMn MPEA catalyst loaded (50 mg) onto high-purity quartz wool [A] and the catalyst substrate lightly packed into the annulus space of the DBD plasma reactor [B]. Successful ignition of a pure methane plasma in reactor [C] is shown through the OES window of the reactor [D].](https://www.swri.org/sites/default/files/186287-fig1-milled-nicocufemn-mpea-catalyst.jpg)
Figure 1: Images showing milled NiCoCuFeMn MPEA catalyst loaded (50 mg) onto high-purity quartz wool [A] and the catalyst substrate lightly packed into the annulus space of the DBD plasma reactor [B]. Successful ignition of a pure methane plasma in reactor [C] is shown through the OES window of the reactor [D].
A pentanary NiCoCuFeMn MPEA catalyst was selected via our data-driven computational algorithm and was successfully synthesized for further study using a commercially scalable method. Our DFT computations exploring an anti-coking strategy confirmed the notion that different crystallographic facets of a catalyst system, representing a surface structural alteration, exhibit marked differences in surface energy. These high surface energy facets theoretically equate to lower coking affinity. Although our analysis of the abundancies of plasma species was not possible when the DBD reactor was loaded with our catalyst, plasma-enabled dissociation of methane and generation of hydrogen should occur at even lower power levels than with plasma alone when activated by both the steric association between the catalyst surface and methane molecule and the generation of excited state surface (or near-surface) species of methane.