A Systematic Search for the Adsorption Motif of All Stereoisomers of Propylene Glycol on a Palladium(111) Surface for Fuel Cell Applications.

Small organic molecules have been shown to produce sufficient power densities allowing them to be environmentally friendly renewable fuel sources and an important part of fuel cell research. Affiliated experimental work found propylene glycol, as a source of renewable fuel, produces viable power densities when utilized with an alkaline-acid fuel cell and a Pd(111) catalyst. There is limited theoretical work on propylene glycol's energy reaction pathway. Thus, the first step in understanding how propylene glycol reacts with the Pd(111) slab is understanding its adsorption. In this paper, we present the investigation of adsorption potential energies (APE) of propylene glycol stereoisomers ()-propane-1,2-diol (1,2PGS), ()-propane-1,2-diol (1,2PGR), and propane-1,3-diol (1,3PG) on Pd(111). The isomers are systematically scanned through different configurations to analyze the preferred stable orientation and positional motifs. Density functional theory (DFT) is used to optimize the molecular geometries and surface relaxations. The most stable configuration of the 1,2PG stereoisomers resulted in an APE of -0.97 eV. The most stable configuration of the 1,3PG resulted in an APE of -1.19 eV. Both the 1,2PG(S/R) and 1,3PG isomers favor a motif in which at least one hydroxyl oxygen atom interacts with the surface of the Pd(111) catalyst. The 1,2PG carbon backbone prefers to have the center carbon positioned away from the slab, while the 1,3PG prefers to have the center carbon positioned closer to the slab. The most stable 1,3PG differs from other reported 1,3PG and 1,2PG relaxed configurations in that both of the hydroxyl oxygen atoms interact with the Pd(111) surface. These results show more favorable APEs than previously reported calculations. This paper will discuss in detail the differences between the hydroxyl group motifs and their role in affecting adsorption.