  We study the adsorption and ring-opening of lactide on the naturally chiral
metal surface Pt(321)$^S$. Lactide is a precursor for polylactic acid
ring-opening polymerization and Pt is a well known catalyst surface. We study
here the energetics of the ring-opening of lactide on a surface that has a high
density of kink atoms. These sites are expected to be present on a realistic Pt
surface and show enhanced catalytic activity. The use of a naturally chiral
surface also enables us to study potential chiral selectivity effects of the
reaction at the same time. Using Density Functional Theory (DFT) with a
functional that includes the van der Waals forces in a first-principles manner,
we find modest adsorption energies of around 1.4 eV for the pristine molecule
and different ring-opened states. The energy barrier to be overcome in the
ring-opening reaction is found to be very small at 0.32 eV and 0.30 eV for LL-
and its chiral partner DD-lactide, respectively. These energies are much
smaller than the activation energy for a dehydrogenation reaction of 0.78 eV.
Our results thus indicate that (a) ring-opening reactions of lactide on Pt(321)
can be expected already at very low temperatures and Pt might be a very
effective catalyst for this reaction; (b) the ring-opening reaction rate shows
noticeable enantioselectivity.
