(449b) Predictive Microkinetic Modeling of Pt-Catalyzed Ethylene Glycol Steam Reforming

In the production
of fuels and chemicals from biomass, H₂ is an important co-reactant
for catalytic upgrading via processes such as hydrogenation,
hydrodeoxygenation, and others. Recent work on the catalytic steam reforming of
oxygenated hydrocarbons and sugars demonstrates the potential for producing H₂
renewably.¹ Full utilization
of these discoveries requires a detailed understanding of mechanistic pathways.

Microkinetic
modeling has emerged in the last two decades as a powerful tool for reproducing
experimentally-observed reaction kinetics in heterogeneous catalysis.^2-3 The recent integration of these
models with first principles-derived estimates of rate constants offers both a
theoretical basis for, and predictive capabilities of, model parameters.³ The application of such models to
oxygenated hydrocarbons has received some attention, mostly focused on thermal
decomposition chemistries.^4-5
Reforming co-reactants such as H₂O help to limit catalytic
deactivation and improve yields to H₂, but H₂O may also
catalyze oxygenate decomposition as shown in reactions (1) and (2):

                                                      H₂O* + * ↔ OH* +
H*                                                  
(1)

                                            C_xH_yO_z*
+ OH* ↔ C_xH_y-1O_z* + H₂O*                                        (2)

Microkinetic models that consider
reactions like (2) are scarce, despite demonstration of their relevance in
previous theoretical and experimental work.^6-7

We have developed a
detailed microkinetic model for ethylene glycol steam reforming over a Pt
catalyst that includes elementary steps for oxidative dehydrogenation via OH*. The model's predictive capabilities are established,
without parameter adjustment, by comparison to experimental data under
kinetically-controlled conditions.⁸The dominant reaction pathways are depicted in Figure
1. Sensitivity analysis indicates that early thermal dehydrogenation steps
control the reaction rate, while steps for oxidative dehydrogenation via
water-derived OH* are kinetically irrelevant. The model demonstrates that this
is due to low surface concentrations of OH*, rather than small rate constants.
Finally, we highlight a kinetic analogy between ethylene glycol steam reforming
and CH₄ steam
reforming on Pt catalysts.

Figure
1.  Principal reaction pathways (solid=major,
dotted=minor) in ethylene glycol steam reforming based on a feed of 5% wt
ethylene glycol in H₂O at 483 K and 1 bar. Flux percentages may not
sum to 100% due to contributions from minor pathways not shown.

References

1.         Huber GW, Shabaker JW,
Dumesic JA. Raney Ni-Sn Catalyst for H2 Production from Biomass-Derived
Hydrocarbons. Science. 2003;300(5628):2075-2077.

2.         Dumesic JA, Rudd DF,
Aparicio LM, Rekoske JE, Treviño AA. The
Microkinetics of Heterogeneous Catalysis. Washington, DC: American Chemical
Society; 1993.

3.         Salciccioli M, Stamatakis M,
Caratzoulas S, Vlachos DG. A review of multiscale modeling of metal-catalyzed
reactions: Mechanism development for complexity and emergent behavior. Chemical Engineering Science. 2011;66(19):4319-4355.

4.         Kandoi S, Greeley J,
Sanchez-Castillo M, et al. Prediction of Experimental Methanol Decomposition
Rates on Platinum from First Principles. Top.
Catal. 2006;37(1):17-28.

5.         Salciccioli M, Vlachos DG.
Kinetic Modeling of Pt Catalyzed and Computation-Driven Catalyst Discovery for
Ethylene Glycol Decomposition. ACS
Catalysis. 2011:1246-1256.

6.         Lin S, Johnson RS, Smith GK,
Xie D, Guo H. Pathways for methanol steam reforming involving adsorbed
formaldehyde and hydroxyl intermediates on Cu(111): density functional theory
studies. Physical Chemistry Chemical
Physics. 2011;13(20):9622-9631.

7.         Zope BN, Hibbitts DD,
Neurock M, Davis RJ. Reactivity of the Gold/Water Interface During Selective
Oxidation Catalysis. Science. 2010;330(6000):74-78.

8.         Kandoi S, Greeley J,
Simonetti D, Shabaker J, Dumesic JA, Mavrikakis M. Reaction Kinetics of
Ethylene Glycol Reforming over Platinum in the Vapor versus Aqueous Phases. J. Phys. Chem. C. 2011;115(4):961-971.