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 C-1 products is controlled by the C-C cracking of CHCO. Bronsted-Evans-Polanyi (BEP) correlations for the oxidative dehydrogenation reactions are developed and the effect of coadsorption on BEPs is discussed.