Catalytic Chemistry

The basic steps of the hierarchical multiscale approach include:

• Quantum mechanical density functional theory (DFT) simulations on noble metal slabs and clusters to obtain binding sites and binding energies, vibrational frequencies, transition states, and activation energies.
• An advanced semi-empirical techniques toolbox, which includes the bond-order conservation in fully compatible form with Surface CHEMKIN.
• A hierarchical toolbox of surface simulators ranging from mean field to kinetic Monte Carlo methods to coarse-grained molecular models to mesoscopic simulators.
• Molecular and statistical mechanics tools, such as transition state theory, statistical mechanics, and molecular dynamics for estimation of activation energies and pre-exponentials from quantum mechanical simulations and sticking coefficients.
• Microkinetic models with coverage-dependent reaction parameters that interfaces surface CHEMKIN with semi-empirical and quantum mechanical techniques.
• A parameter-refinement toolbox consisting of state-of-the art surface response methods and optimization techniques.

Comparison of a full microkinetic model comprising 46 reactions, a reduced 1-step rate expression, and the experimental data (Xue et al.) for the water-gas shift (WGS) reaction on Pt. The experimental data is well captured by both models. For additional details, see Mhadeshwar and Vlachos, J. Phys. Chem. B. 108, 15246-15258 (2004) and Mhadeshwar and Vlachos, Cat. Today (2005).

• A thermodynamic consistency toolbox that checks for thermodynamic consistency and ensures consistency during parameter estimation from experiments.
• A reactor toolbox consisting of simplified reactor models and computational fluid dynamics reactor simulations.
• An analysis toolbox including sensitivity analysis, principal component analysis, reaction path analysis, computer-aided partial equilibrium and quasi-steady state criteria, and proper orthogonal decomposition.
• A model reduction toolbox for transport and/or chemistry.
• A nonlinear analysis (bifurcation) methodology for predicting regimes of instabilities, such as ignitions, extinctions, oscillations, chaos, and pattern formation.

Hierarchical modeling means that we employ simple and inexpensive tools first (first pass) to identify the 'key' steps of a process and then apply higher level, more expensive tools only for the few key steps.

Comparison of microkinetic model predictions for CO conversion and CO2 selectivity against the experimental data of Kahlich et al. for preferential oxidation (PROX) of CO on Pt. The experimental data is reasonably well captured by the model. For additional details, see Mhadeshwar and Vlachos, J. Phys. Chem. B. 108, 15246-15258, (2004).

Typical catalytic reaction systems studied include:

• Catalytic partial oxidation (CPOX) of methane and larger hydrocarbons to syngas in short contact time reactors
• Oxidative dehydrogenation of ethane to ethylene and larger hydrocarbons to oxygenates in short contact time reactors
• Catalytic combustion of fuels on noble metals
• Ammonia decomposition for hydrogen production
• Fuel processing for fuel cells including the water gas shift (WGS) reaction and the selective or preferential oxidation (PROX) of CO for 'CO-free' fuels, etc.