Decomposition Pathways of Glycerol via C-H, O-H, and C-C Bond Scission on Pt(111): A Density Functional Theory Study

Glycerol decomposition on Pt(111) via dehydrogenation or C-C bond scission is examined with periodic density functional theory (DFT) calculations. The thermochemistry of dehydrogenation intermediates is first estimated using an empirical correlation scheme with parameters fit to selected DFT calculations; the resulting estimates for the more stable intermediates are refined with full DFT calculations. Bronsted-Evans-Polanyi (BEP) relationships for dehydrogenation and C-C bond scission reactions are developed and used to estimate the kinetics of elementary dehydrogenation and C-C bond scission steps in the reaction network. The combined thermochemical and kinetic analysis implies that glycerol dehydrogenation products at intermediate levels of dehydrogenation are the most thermochemically stable. Additionally, although C-C bond scission transition state energies are high for glycerol and for intermediates at early stages of dehydrogenation, these energies decrease as the intermediates are successively dehydrogenated, reaching a minimum after the removal of several hydrogen atoms from glycerol. At these levels of dehydrogenation, the C-C scission transition state energies become comparable to those of O-H or C-H scission. These results suggest that C-C bonds are only broken after glycerol has been significantly dehydrogenated and demonstrate that DFT-based analyses, combined with simple correlation schemes, can be effective for elucidating general features of complex biomassic reaction networks.