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 sootgrowth/oxidation are its rate dependence on active sites, its rate relation to nanostructure andthe similarity of active sites in carbon deposition/regeneration to those associated with sootgrowth/oxidation. The abundance of natural gas prompts these questions as natural gas isincreasingly used for power generation while it holds a high potential for transition to the H2economy by decarbonization using TCD. This study combines experimental measurements of reactionrates, active sites, nanostructure characterization with atomistic simulations for carbon surfacereactions to address knowledge gaps in soot models and TCD.Reaction rates will be correlated to active sites during TCD and regeneration as a function ofreaction extent. Carbons with different nanostructures will serve as initial catalysts to establishactive site dependence (number and type) on nanostructure. Curvature in carbon lamellae formedunder TCD will be comparatively tested against 'flat' lamellae for hypothesized increase inregeneration rate with increasing reaction progress as signaling a corresponding increase in activesites. Critically these active sites formed during regeneration will be tested for theirequivalency as active sites under TCD by comparative rate measurements. Temperatures and reactantswill include those relevant to soot formation by combustion of present-day fuels, since the activesites, nanostructure and rates related to TCD are also central to understanding soot growth andoxidation reactions. These same factors also define the connections between experiments andmodeling.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) foractive site measurement to inform atomistic scale simulation. High resolution transmission electronmicroscopy (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 energyloss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) to validate and informsimulation predictions for carbon hybridization, sp2/sp3 and C/H ratio.Atomistic simulations, using reactive molecular dynamics, will be compared to the experimentalmetrics of active sites, reaction rates and apparent activation energies. Simulations will be usedto identify the nature of active sites and to test the hypothesized mechanisms whereby curvatureenables multiplication and regeneration of active sites. Unifying experimental carboncharacterization and reaction simulations will provide a predictive model for TCD activity based onactive sites and identified dependencies upon nanostructure and reaction conditions. Thecyclability of deposition and regeneration will be tested for congruity between rates, active sitesand control through nanostructure curvature. The role of active sites, loss during TCD, gain duringregeneration, their equivalency, rate dependence and relation to nanostructure will also allow usto advance soot growth and oxidation models.As goal, the outcome of this study will be predictive model for TCD grounded in active sites andbenchmarked against comparative nanostructures. Furthermore, definition of TCD-relevant parametersand mechanistic insights by simulations will advance our understanding of soot growth and oxidationtowardsintegrated soot models.