The demand for high-efficiency, affordable, and environmentally friendly catalysts for producing hydrogen without consequent CO₂ production is a critical challenge for the emerging hydrogen economy. Catalytic pyrolysis is an appealing strategy to lower energy requirements for the conversion of available resources (natural gas, biomass, waste plastics) to hydrogen while retaining the initial carbon in a value added solid form. Essential to realizing the potential of this reaction is to ensure that catalysts are reusable and separable from the carbon product, and that catalyst deactivation is understood and controlled. In this contribution, we reveal for a series of promising base growth catalysts that we have developed (a key requirement for catalyst reuse), modeling and assessment of deactivation through carbon deposition vs. metal particle sintering. We reveal that in many instances, deactivation can be reversed, which we kinetically explore and model via introduction of a variety of co-reactants, (H₂, H₂O and CO₂), while further evaluating the intrinsic activity of the active sites that are uncovered immediately after the carbon removal. We illustrate how some of the most active sites bind carbon strongly and facilitate rapid rates in the presence of proper levels of co-reactants, but also accelerate deactivation via covering of carbon layers. We deconvolute the barriers and rates associated with the reverse reactions, revealing distinct differences among various metals with utility to activate methane (Fe, Co, and Ni), while also deconvoluting methane activation rates over the most active sites from others which are active at steady state in the absence of etchants. These results should shed much light on some of the conflicting reports associated with barriers and deactivation behavior during methane pyrolysis, particularly at varying levels of co-reactants.

Caleb Bavlnka

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