Biomass-derived compounds are highly functionalized, making them difficult to selectively upgrade into a desired product. This is because the functional groups interact with the surface, resulting in multiple and often uncontrolled adsorption geometries. Previous studies on Pd indicated that adsorption geometry of these oxygenates dictates product selectivity, where flat lying configurations result in higher production of decarbonylation products, which are undesired, and upright conformations favor CâO bond activation, producing the desired hydrodeoxygenation (HDO) products. It was shown that on Pd(111), high surface coverages of furfuryl alcohol lead towards increased selectivity towards the HDO product, methylfuran. However, the mechanism is poorly understood, hampering efforts to design better catalysts. HDO can proceed through two pathways, 1) a low-barrier direct transfer of H atoms between alcohol adsorbates and 2) a high-barrier H transfer via metal-bound H atoms. Surface science techniques, such as temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS), using single crystals of Pd and Pt have provided a fundamental understanding of the low-barrier, direct H transfer mechanism.
Our recent studies have found that there appears to be key differences in the kinetics and mechanism for furfuryl alcohol hydrodeoxygenation. On Pd(111), the lowest-barrier reaction pathway was found to involve direct transfer hydrogenation between neighboring adsorbates, but this mechanism does not appear to be operable on Pt(111). However, on Pt(111), furfuryl alcohol can either proceed through high-barrier transfer to produce methylfuran or decarbonylation to furan, which also occurs on Pd(111). In the HDO pathway, furfuryl alcohol undergoes CâO bond scission to form methylfuran and water. Decarbonylation occurs through the dehydrogenation of furfuryl alcohol to make furfural followed by decarbonylation to produce furan, which undergoes further decomposition on Pt(111) to produce propylene and CO. Most furfuryl alcohol reaction and desorption pathways are similar on Pt(111) and Pd(111) and the relative barriers of the major products is the same. However, on Pt(111) benzene is also produced as a volatile product, and is likely formed via coupling of C3 surface intermediates.