Ethanol Decomposition on Pd–Au Alloy Catalysts

The mechanisms of ethanol (EtOH) decomposition via C–C or C–O bond cleavage on alloy surfaces are currently not well understood. In this study, we model EtOH decomposition on close-packed Pd–Au catalytic surfaces using density functional theory (DFT) calculations and derived Brønsted–Evans–Polanyi (BEP) relationships. Three characteristic Pd–Au surfaces are considered, Pd 1 Au 2 (111), Pd 2 Au 1 (111), and a Pd monolayer (ML), Pd ML (111), on a Au substrate. We show that, on close-packed Pd–Au surfaces, the C–C bond is easier to cleave than C–O, indicating that the formation of CH 4 and CO is favored as the products of EtOH decomposition. Interestingly, we find that, though the C–C and C–O activation barriers on Pd ML (111) are generally lower than those on the other two surfaces, it is less active for EtOH decomposition due to a slow release of H 2 and possible carbon coking. Pd 2 Au 1 (111), on the other hand, has a higher theoretical reaction rate due to facile H 2 evolution from the surface and less carbon coking. A comparison of the surface d-band with the activation energy barriers shows that there is a trade-off between the barriers for C–C bond cleavage and H 2 association, with Pd 2 Au 1 (111) having the best performance. Temperature-programmed desorption experiments of EtOH on Pd/Au surfaces show significant C–C bond cleavage and both CH 4 and CO production on surfaces with Pd–Au interface sites. Furthermore, neither Auger electron spectroscopy nor EtOH reflection–adsorption infrared spectroscopy provide evidence of C–O bond cleavage. Finally, the experimental reaction rate for methane production from C–C bond cleavage was higher on surfaces with more Au present due to minimal carbon contamination and the promotion of product desorption. This combined theoretical and experimental study shows that, though Au is catalytically inactive for EtOH decomposition, it can dramatically promote the surface activity for EtOH steam reforming due to the existence of active Pd–Au surface ensemble sites.