Despite intensive study over more than a century the precise role of oxygen in the oxidation of alcohols on metal catalysts in the liquid phase remains unclear. While it is well known that 'over-oxidation' of the metal surface results in catalyst poisoning, the detailed picture of surface oxidation of an active catalyst is unknown. The extent of oxygen coverage of thin film platinum metal catalysts was deduced using sum-frequency generation (SFG) vibrational spectroscopy as a surface-specific probe of the electronic density at the metal surface. Reaction rate studies reveal an induction period in the oxidation of ethanol to acetaldehyde lasting between 1 and 3 hours. Simultaneous non-resonant SFG studies show a corresponding decrease of the SFG signal from platinum at the end of the induction period taking place on a minute time scale. Control experiments demonstrate clearly that the SFG non-resonant signal is sensitive to the oxygen coverage of the platinum surface, and that the SFG decrease seen in situ is due to the transition from oxygen-absent (inactive) to oxygen-adsorbed (active) surface state. An inverse dependence of the induction time on oxygen concentration in solution corroborates this interpretation. We present a simple kinetic model which can explain the rapid switch between inactive and active states. The results of this study are the first example of non-resonant SFG being used to characterize the metal catalyst oxidation state during heterogeneously catalyzed liquid-phase catalysis and will be important to the detailed understanding of the mechanism of heterogeneous alcohol oxidation.

Monitoring metal oxygen coverage with sum-frequency generation: Studies of surface oxygen coverage of platinum in liquid-phase heterogeneously catalyzed ethanol oxidation

248th National Meeting (2014)

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