Experimental Identification and Atomistic Simulations of Active Sites, Rates and Reaction Extent in Thermo-Catalytic Decomposition and Regeneration Towards Maintaining Autocatalytic Activity

Primary questions common to Thermo-Catalytic Decomposition (TCD)/regeneration and soot

growth/oxidation are its rate dependence on active sites, its rate relation to nanostructure and

the similarity of active sites in carbon deposition/regeneration to those associated with soot

growth/oxidation. The abundance of natural gas prompts these questions as natural gas is

increasingly used for power generation while it holds a high potential for transition to the H2

economy by decarbonization using TCD. This study combines experimental measurements of reaction

rates, active sites, nanostructure characterization with atomistic simulations for carbon surface

reactions to address knowledge gaps in soot models and TCD.

Reaction rates will be correlated to active sites during TCD and regeneration as a function of

reaction extent. Carbons with different nanostructures will serve as initial catalysts to establish

active site dependence (number and type) on nanostructure. Curvature in carbon lamellae formed

under TCD will be comparatively tested against “flat” lamellae for hypothesized increase in

regeneration rate with increasing reaction progress as signaling a corresponding increase in active

sites. Critically these active sites formed during regeneration will be tested for their

equivalency as active sites under TCD by comparative rate measurements. Temperatures and reactants

will include those relevant to soot formation by combustion of present-day fuels, since the active

sites, nanostructure and rates related to TCD are also central to understanding soot growth and

oxidation reactions. These same factors also define the connections between experiments and

modeling.

Experimental tools include an integrated fixed-bed reactor and coupled GC for reaction rates,

product speciation, reactant conversion and in situ temperature programmed oxidation (TPO) for

active site measurement to inform atomistic scale simulation. High resolution transmission electron

microscopy (HRTEM) coupled with custom image analysis algorithms will be applied for nanostructure.

Chemistry of the depositing carbon and post-regeneration action will be probed by electron energy

loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) to validate and inform

simulation predictions for carbon hybridization, sp2/sp3 and C/H ratio.

Atomistic simulations, using reactive molecular dynamics, will be compared to the experimental

metrics of active sites, reaction rates and apparent activation energies. Simulations will be used

to identify the nature of active sites and to test the hypothesized mechanisms whereby curvature

enables multiplication and regeneration of active sites. Unifying experimental carbon

characterization and reaction simulations will provide a predictive model for TCD activity based on

active sites and identified dependencies upon nanostructure and reaction conditions. The

cyclability of deposition and regeneration will be tested for congruity between rates, active sites

and control through nanostructure curvature. The role of active sites, loss during TCD, gain during

regeneration, their equivalency, rate dependence and relation to nanostructure will also allow us

to advance soot growth and oxidation models.

As goal, the outcome of this study will be predictive model for TCD grounded in active sites and

benchmarked against comparative nanostructures. Furthermore, definition of TCD-relevant parameters

and mechanistic insights by simulations will advance our understanding of soot growth and oxidation

towards

integrated soot models.