256367 Predictive Microkinetic Modeling of Pt-Catalyzed Ethylene Glycol Steam Reforming

Wednesday, October 31, 2012: 8:50 AM
320 (Convention Center )
Matthew A. Christiansen1,2 and Dionisios G. Vlachos1,2,3, (1)Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE, (2)Center for Catalytic Science and Technology, University of Delaware, Newark, DE, (3)Catalysis Center for Energy Innovation

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)

                                            CxHyOz* + OH* ↔ CxHy-1Oz* + 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.8 The 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_01.png

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, Treviņo 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.


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