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CO Oxidation Kinetics over Au/TiO2 and Au/Al2O3 Catalysts: Evidence for a Common Water-Assisted Mechanism
journal contribution
posted on 2018-02-25, 00:00 authored by Johnny Saavedra, Christopher J. Pursell, Bert D. ChandlerThe mechanism of
CO oxidation over supported gold catalysts has
long been debated, with two prevailing mechanisms dominating the discussion:
a water-assisted mechanism and a mechanism involving O-defect sites.
In this study, we directly address this debate through a kinetic and
mechanistic investigation of the role of water in CO oxidation over
Au/TiO2 and Au/Al2O3 catalysts; the
results clearly indicate a common water-assisted mechanism to be at
work. Water adsorption isotherms were determined with infrared spectroscopy;
the extracted equilibrium constant was essentially the same for both
catalysts. Added water decreases CO adsorption on Au/TiO2, likely by blocking CO binding sites at the metal–support
interface. Reaction kinetics (CO, O2, and H2O reaction orders) were essentially the same for both catalysts,
as were measured O–H(D) kinetic isotope effects. These data
indicate that the two catalysts operate by essentially the same mechanism
under the conditions of these experiments (ambient temperature, significant
amounts of water available). A reaction mechanism incorporating the
kinetic and thermodynamic data and accounting for different CO and
O2/COOH binding sites is proposed. The mechanism and kinetic
data are treated with an active site (Michaelis–Menten) approach.
This indicated that water adsorption does not significantly affect
reaction rate constants, only the number of active sites available
at a given water pressure. Extracted water and O2 binding
constants are similar on both catalysts and consistent with previous
DFT calculations. Water adsorption constants are also similar to independently
determined equilibrium constants measured by IR spectroscopy. The
likely roles of water, surface carbonates, and oxygen vacancies at
the metal–support interface are discussed. The results definitively
show that, at least in the presence of added water, O vacancies cannot
play an important role in the room-temperature catalysis, and that
the water-assisted mechanism is far more consistent with the preponderance
of the kinetic data.