Relaxation processes during the selective oxidation of aqueous ethanol with oxygen on a platinum catalyst

The observed loss of activity at constant conditions during the selective oxidation of ethanol with oxygen in a continuous stirred-tank reactor with carbon-supported platinum can be described by a model considering reversible transformations between three oxidizing species on the catalyst. One of these species is much more reactive toward ethanol and can be considered as a reaction intermediate in the selective oxidation of the latter. The model is also able to simulate the relaxation of the catalyst potential when the reaction is performed with a platinum foil in an electrochemical cell. The loss of activity as well as the relaxation of the catalyst potential can be attributed to changes in the degree of coverage by the two less reactive forms of oxygen. The latter should be considered as an extrinsic relaxation in contrast to the establishment of the steady-state degree of coverage by the reaction intermediates in the selective oxidation of ethanol, i.e., the intrinsic relaxation

Selective oxidation of methyl α-D-glucopyranoside with oxygen over supported platinum : kinetic modeling in the presence of deactivation by overoxidation of the catalyst

A kinetic model is presented, which describes the platinum-catalyzed selective oxidation of-methyl alpha-D-glucopyranoside to sodium methyl alpha-D-glucuronate with molecular oxygen in the presence of deactivation by overoxidation. Overoxidation is completely reversible and most adequately described by a reversible transformation of oxygen adatoms into inactive subsurface oxygen. A clear distinction is made between the rapid establishment of the steady-state degree of coverage by the reaction intermediates at the platinum surface and the much slower reversible process of overoxidation. This clear distinction is reflected in the rate equation, which can be written as the product of an initial rate and a deactivation function. The deactivation function is given as a function of the degree of coverage by inactive subsurface oxygen. The rate-determining step in the selective oxidation consists of the reaction between dissociatively chemisorbed oxygen and physisorbed methyl alpha-D-glucopyranoside. The corresponding standard activation entropy and enthalpy amount to respectively -111 +/- 12 J mol(-1) K-1 and 51 +/- 4 kJ mol(-1). The standard reaction entropy for the transformation of oxygen atoms into subsurface oxygen amounts to -35 +/- 16 J mol(-1) K-1 and the standard reaction enthalpy to -36 +/- 15 kJ mol(-1)

Comparing the Catalytic Oxidation of Ethanol at the Solid−Gas and Solid−Liquid Interfaces over Size-Controlled Pt Nanoparticles: Striking Differences in Kinetics and Mechanism

Pt nanoparticles with controlled size (2, 4, and 6 nm) are synthesized and tested in ethanol oxidation by molecular oxygen at 60 ??C to acetaldehyde and carbon dioxide both in the gas and liquid phases. The turnover frequency of the reaction is 80 times faster, and the activation energy is 5 times higher at the gas-solid interface compared to the liquid-solid interface. The catalytic activity is highly dependent on the size of the Pt nanoparticles; however, the selectivity is not size sensitive. Acetaldehyde is the main product in both media, while twice as much carbon dioxide was observed in the gas phase compared to the liquid phase. Added water boosts the reaction in the liquid phase; however, it acts as an inhibitor in the gas phase. The more water vapor was added, the more carbon dioxide was formed in the gas phase, while the selectivity was not affected by the concentration of the water in the liquid phase. The differences in the reaction kinetics of the solid-gas and solid-liquid interfaces can be attributed to the molecular orientation deviation of the ethanol molecules on the Pt surface in the gas and liquid phases as evidenced by sum frequency generation vibrational spectroscopy. © 2014 American Chemical Society.close2