(551e) Creating Thermodynamically Consistent Microkinetic Mechanisms for Heterogeneous Chemistry Based Upon Gas-Phase Properties

Microkinetic models are useful tools in heterogeneous chemistry.  The parameters in these models are based upon physically meaningful quantities – activation energies and frequencies for elementary reactions, plus enthalpy and entropy for adsorbed species – that can be derived from first principles.  Unlike global mechanisms, which are postdictive, microkinetic models are predictive.  They are not limited to a narrow experimental range, but can be applied over a broad range of conditions, thereby allowing us to narrow the “pressure gap” between UHV surface science and high-pressure industrial application.

Creating microkinetic models, however, is a time-consuming and laborious task, beset with many difficulties.  In addition to obtaining all the necessary parameters, one of the challenges is ensuring that they are thermodynamically consistent.  Thermodynamic consistency is particularly critical for our intended application:  high-temperature and high-pressure adiabatic reactors in which the heterogeneous surface chemistry is coupled with homogeneous gas-phase chemistry.

We present a new method for maintaining thermodynamic consistency in homogeneous/heterogeneous mechanisms.  The method is based upon the assumption that the vibrational modes of the gas-phase molecule and the phonons of the active site are conserved upon adsorption.  The input parameters are molecular properties of the gas-phase molecule (e.g. vibrational frequencies and moments of inertia) and the heats of adsorptions.  These heats of adsorption are obtained either from experiment, Density Functional Theory (DFT), or scaling relations derived from other surfaces.  As the gas-phase molecule absorbs, the external translational and rotational degrees of freedom are converted into new surface vibrations.  These frequencies can be estimated with suitable accuracy from the input parameters.  The partition function of the adsorbed species is then computed using statistical mechanics.  Standard thermodynamic parameters, such as enthalpy, entropy, and heat capacity, are obtained.

The estimated vibrational modes for the adsorbed species are tested against detailed DFT calculations obtained from literature.  Equilibrium constants obtained from this method are compared against values obtained from other thermodynamically consistent mechanisms in the literature.  The results confirm the validity of the assumption of separability of normal modes when computing thermodynamic properties.