An experimental and theoretical study of furan decomposition on palladium(111) using scanning tunneling microscopy and density functional theory

Furan (C4H4O) has been investigated as a prototypical model species in heterogeneous catalyst studies, under both ultrahigh vacuum and industrial conditions. The adsorption and decomposition of furan on Pd(111) has been previously studied with thermal desorption and vibrational spectroscopies; these studies support intact adsorption of the heterocycle below 280 K, with dissociation to H, CO, and a C3H3 species for temperatures between 280 to 320 K. The hydrocarbon moiety is adsorbed through ∼400 K, whereupon complete dehydrogenation to a carbidic overlayer is onset.; Using scanning tunneling microscopy (STM), I have imaged the intactly-adsorbed molecules in UHV at temperatures below 280 K; these experiments have revealed strikingly different adsorption behaviors on narrow and wide terraces of vicinal Pd(111). For higher temperatures (up to 415 K), STM has been used to probe C4H4O dissociation to form the C3H3 species. These data exhibit structure-specific properties, most prominently a preferential adsorption of furan at upper step edge sites and an apparent depletion of such sites after the reaction has gone to completion. Such observations are consistent with previous experimental and theoretical studies of molecular adsorption, diffusion, and reaction on vicinal, low-Miller-index transition metal surfaces.; The structure and energetics of furan chemisorption on Pd(111) are explored with surface electronic calculations based on ab initio density functional theory (DFT). These calculations reveal two types of stable adsorption configurations, located in positions of high symmetry relative to hollow sites on the close-packed surface. The chemisorption strengths in these geometries are closely correlated to the relaxed structures of both the adsorbate and the substrate, and involve a partial loss of aromaticity in the furan molecule.; Calculated STM images of the stable furan configurations on the Pd(111) surface are also presented; these plots are generated from computational methods based on DFT and the extended Huckel molecular orbital theory (EHT). Calculations of the C3H3 species have also been performed using the EHT-based method. Overall, the theoretical results show a good qualitative correspondence to the experimental data. This investigation indicates that the EHT-based approach can reproduce the key symmetries of the adsorbate electronic structure in spite of its simplicity.