(54b) 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.