Deactivation of Supported Pt Catalysts during Alcohol Oxidation Elucidated by Spectroscopic and Kinetic Analyses

The selective oxidation of 1,6-hexanediol with O2 to product aldehydes and acids occurs readily in water over supported Pt nanoparticles. The initial turnover frequency of 0.54 s–1 (at 343 K and 1 MPa O2) decreases significantly with reaction time because of product competitive adsorption and irreversible adsorption of unknown strongly bonded species. To identify the poisoning species, in situ surface-enhanced Raman spectroscopy (SERS) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy were applied in this work. In situ SERS during 1,6-hexanediol oxidation revealed an accumulation of di-σ-bonded olefinic species with features at ∼1150 and ∼1460 cm–1 on the poisoned Pt surface. Consistent with SERS, 13C NMR spectroscopy of a Pt catalyst deactivated by oxidation of 13C-labeled 1,4-butanediol revealed a C═C peak associated with ethylene. Molecules containing olefinic groups are 2 orders of magnitude more effective at competing for Pt surface sites in comparison to the aldehyde and acid products from alcohol oxidation. The poisoning olefinic species were generated by decarbonylation of product aldehyde (as revealed by head space analysis) and could be easily removed from the deactivated catalyst by mild treatment in H2.