Ring-Opening and Oxidation Pathways of Furanic Oxygenates on Oxygen-Precovered Pd(111)

The surface reactivity of furan, furfural, and furfuryl alcohol on oxygen-precovered Pd(111) (O/Pd(111)) was studied via temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to understand the oxidation processes of more complex alcohols and aldehydes. The primary reaction pathway explored was furfuryl alcohol dehydrogenation to furfural, followed by decarbonylation to furan and subsequent ring opening of furan through O–C^α scission; however, furfuryl alcohol and furfural also underwent other parallel reactions. At high coverages, deoxygenation of furfuryl alcohol to methylfuran was observed simultaneously with furfural production near 320 K, possibly through a disproportionation reaction of two alcohol adsorbates. Observation of furfural as a reaction product at this temperature was related to an increased prevalence of a weakly bound η¹(O) aldehyde state near 250 K on O/Pd(111) as compared to clean Pd(111). Furfural also reacted with surface O to produce furoate (C₄H₃OCOO−), which primarily underwent decarboxylation to furan and CO₂. The presence of surface oxygen resulted in a new, higher-temperature desorption channel for furan at 340 K; this channel was accompanied by the desorption of partial oxidation products 2­(5<i>H</i>)-furanone and maleic anhydride. TPD experiments on ¹⁸O/Pd­(111) showed incorporation of surface oxygen into furfural, H₂O, CO₂, CO, 2­(5<i>H</i>)-furanone, and maleic anhydride products, with the isotopic distributions suggesting that partial oxidation of furan occurred partly through a ring-opened carboxylate