Combined DFT, Microkinetic, and Experimental Study of Ethanol Steam Reforming on Pt

Density
functional theory (DFT) calculations for the thermal decomposition
and oxidative dehydrogenation of ethanol, mechanistic aspects of water–gas
shift reaction, and experimental kinetic data are integrated so as
to develop and assess a comprehensive DFT-based microkinetic model
of low temperature ethanol steam reforming on Pt catalysts. The DFT
calculations show (1) that the C–C scission should occur late
in the dehydrogenation sequence, (2) that the C–C scission
barriers in highly dehydrogenated intermediates are comparable to
early C–H abstraction barriers, and (3) that the oxidative
dehydrogenation reactions should not be important under steam reforming
conditions. The DFT-parametrized model shows good qualitative agreement
with experiments, with reasonable deviations attributed to modeling
only the metal chemistry (i.e., excluding support effects). Both the
model and the experiments show that the overall mechanism is simply
thermal decomposition of ethanol followed by incomplete water–gas
shift. The most abundant surface species in the model are the decomposition
products CO, H, and free sites, while the key reactive intermediates
are present in much lower amounts. Unlike findings of simplified previous
models, the rate determining step was identified as the initial dehydrogenation
of ethanol, while the selectivity to C1 products is controlled
by the C–C cracking of CHCO. Brønsted–Evans–Polanyi
(BEP) correlations for the oxidative dehydrogenation reactions are
developed and the effect of coadsorption on BEPs is discussed.