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 (S)-propane-1,2-diol (1,2PGS), (R)-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.