Microkinetic Modeling of Polyol Thermal Decomposition and Reforming On Platinum

There has been increasing interest in the utilization of biomass for renewable energy and chemical production.  Biomass, which is made up of oxygenated hydrocarbon building blocks, can be more efficiently utilized for chemical and energy production through the use of small-scale catalytic reforming technologies. Small-scale reactors can increase localization of synthesis gas production and increase reaction rates over current large-scale and enzymatic technologies.  Distributed synthesis gas will allow for local production of methanol, ammonia, liquid fuels (Fischer-Tropsch process) and hydrogen for fuel cell applications.  Recent technological successes in catalytic reforming of biomass derived oxygenates have shown high reaction rates and high product selectivity towards hydrogen for smaller oxygenates such as methanol, ethylene glycol and glycerol.  Future improvements and extensions of this technology to larger oxygenates will require rational design for reactor and catalyst optimization. 

Microkinetic models provide necessary insights into surface chemistry that are useful for reactor design and catalyst optimization. The focus of this study is the development of a thermodynamically consistent microkinetic model that describes the detailed mechanisms of thermal decomposition and reforming of methanol and ethylene glycol on a Pt  catalyst.   

The microkinetic model in this study was developed using the constraint of thermodynamic consistency^[1].  Thermochemical properties of reaction intermediates were estimated using G3B3 level^[2] quantum mechanical calculations and statistical mechanics for temperature dependent properties.  Enthalpy of formation for previously unstudied reaction intermediates were calculated using a methodology involving isodesmic reactions described by Wang and co-workers^[3]. Binding energies for reaction species and activation energies for individual reactions within this model were taken from various DFT studies on Pt(111) catalyst surface in literature, as well as from in-house DFT calculations and calculations using free energy Polanyi relationships.  Order of magnitude estimates of pre-exponential factors were tuned to published experimental data. 

The surface mechanism describing oxygenate chemistry on Pt includes over 100 reversible elementary reactions of the following classifications: adsorption/desorption, hydrogen extraction, carbon-carbon bond cleavage, hydrogen oxidation, carbon monoxide oxidation, hydrogen and carbon monoxide coupling reactions via the carboxyl intermediate (through an OH intermediate), and oxidative dehydrogenation. 

The microkinetic model accurately describes kinetically limited experimental data for polyol thermal decomposition at temperatures under 600 K with varying polyol inlet concentrations.  The major reaction pathway for methanol decomposition proceeds first through methanol adsorption followed by C-H bond cleavage, and then finally through the formyl radical to produce adsorbed CO and adsorbed hydrogen.  The reaction path analysis for ethylene glycol thermal decomposition indicates that there are four hydrogen extraction reactions before C-C bond cleavage.

This presentation will focus on key methodological aspects of building the microkinetic model, as well as an in depth look at model results and interpretation to reactor design applications and extensions of the model to other chemistry sets.  A key focus will be on the similarities and differences of reaction paths between steam reforming and thermal decomposition for polyols.

References

1.Mhadeshwar, A.B., H. Wang, and D.G. Vlachos, Thermodynamic consistency in microkinetic development of surface reaction mechanisms. Journal of Physical Chemistry B, 2003. 107(46): p. 12721-12733.

2.Baboul, A.G., et al., Gaussian-3 theory using density functional geometries and zero-point energies. Journal of Chemical Physics, 1999. 110(16): p. 7650-7657.

3.Wang, H. and K. Brezinsky, Computational study on the thermochemistry of cyclopentadiene derivatives and kinetics of cyclopentadienone thermal decomposition. Journal of Physical Chemistry A, 1998. 102(9): p. 1530-1541.