Selective oxidation of linear alcohols: the promotional effect of water and inhibiting effect of carboxylates over dilute PdAu catalysts

Abstract

Dilute PdAu alloys are promising catalysts for selective oxidation with high activity and high selectivity. The promotional effect of Pd for selective alcohol oxidation is strongly dependent on the alkyl chain length. Whereas Pd substantially promotes the selective oxidation of methanol, there is essentially no promotion of oxidation of longer-chain alcohols (C2–C4) for Pd₃Au₉₇ RCT-SiO₂ compared to pure Au RCT-SiO₂. The lack of promotion of selective oxidation of higher alcohols by Pd is attributed to their over-oxidation on the Pd₃Au₉₇ RCT-SiO₂ catalyst, yielding strongly bound carboxylates that poison the Pd sites. The longer-chain carboxylates are stable on the surface under reaction conditions whereas formate, which would form from over-oxidation of methanol, decomposes. The surface carboxylate was identified for 1-propanol oxidation as propionate using in situ Fourier transform infrared (FTIR) spectroscopy. Further evidence for poisoning by carboxylates is the production of CO₂ in temperature programmed oxidation following catalyst use. The poisoned catalyst can be regenerated in a flowing methanol/oxygen mixture, restoring the promotional effect of Pd for methanol oxidation. The residual activity of the Au and of the poisoned Pd/Au catalysts may be due to the migration of hydroxyl from the silica support to the nanoparticle. A reservoir of hydroxyl groups on the RCT-SiO₂ is identified using FTIR. Hydroxyls on Au are known to spontaneously disproportionate to adsorbed oxygen atoms and water. These results demonstrate the sensitivity of dilute alloy catalyst reactivity to molecular structure.