Determining Catalytically Relevant Surfaces through Coverage-Dependent Lattice Gas Models: Carbon Adsorption on Fe(100)

We have quantified the C-C lateral interactionson Fe(100)using a density functional theory (DFT)-parameterized lattice gascluster expansion (LG CE) model trained using 265 unique configurationsspanning a C coverage from 0 to 1 monolayer (ML). Our LG CE modelshows high predictive accuracy with a leave-multiple-out cross-validationscore of 10.2 and 16.6 meV/site for systems with and without the toptwo layers of Fe atoms fixed, respectively. Electronic ground-statestructures identified from the lattice gas model (including the structuresat 0 and 1 monolayers) were further used to generate ab initio phasediagrams under a range of temperatures and pressures. At low temperatures(<400 K), we found that the 1.0 monolayer structure is dominant,whereas at higher temperatures (>500 K), the 0.88 ML structureismost likely to form on the Fe surface. Interestingly, our model identifieda c (2 x 2) ordered structure at 1/2 ML, which correlates wellwith previous DFT studies for carbon adsorption on iron surfaces andmatches with the experimentally observed low-energy electron diffractionstructure. Overall, the DFT-parameterized energies for the C/Fe systemincluding effects of coverage and configurational space can furtherhelp in developing multiscale models for various heterogeneous reactionsinvolving C-C and C-Fe interactions.