Water Poisons H₂ Activation at the Au–TiO₂ Interface by Slowing Proton and Electron Transfer between Au and Titania

Abstract

Understanding the dynamic changes at the active site during catalysis is a fundamental challenge that promises to improve catalytic properties. While performing Arrhenius studies during H2 oxidation over Au/TiO2 catalysts, we found different apparent activation energies (E app) depending on the feedwater pressure. This is partially attributed to changing numbers of metal–support interface (MSI) sites as water coverage changes with temperature. Constant water coverage studies showed two kinetic regimes: fast heterolytic H2 activation directly at the MSI (E app ∼ 25 kJ/mol) and significantly slower heterolytic H2 activation mediated by water (E app ∼ 45 kJ/mol). The two regimes had significantly different kinetics, suggesting a complicated mechanism of water poisoning. Density functional theory (DFT) showed water has minor effects on the reaction thermodynamics, primarily attributable to intrinsic differences in surface reactivity of different Au sites in the DFT model. The DFT model suggested significant surface restructuring of the TiO2 support during heterolytic H2 adsorption; evidence for this phenomenon was observed during in situ infrared spectroscopy experiments. A monolayer of water on the hydroxylated TiO2 surface increased the H2 dissociation activation barrier by ∼0.2 eV, in good agreement the difference in experimentally measured values. DFT calculations suggested H2 activation goes through a proton-coupled electron-transfer-like mechanism. During proton transfer to a basic support hydroxyl group, electron density is distributed through the gold nanorod and partially localized on the protonated support hydroxyl group. Water slows H2 activation by slowing this H+ transfer, forcing negative charge buildup on the Au and increasing the transition state energy.

Kinetics of H2 Adsorption at the Metal–Support Interface of Au/TiO2 Catalysts Probed by Broad Background IR Absorbance

“…These bands are assignable to ν OH and δ stretching and bending bands, respectively, and indicate formation of a water‐like species (WLS) as H 2 first adsorbs on the catalyst. This is consistent with our previous density functional theory (DFT) and experimental results, which indicate H 2 adsorption occurs via heterolytic dissociation at the MSI [9a,b] . The BBA signal is considerably more intense than the ν OH and δ bands, indicating the BBA signal offers a sensitive method for monitoring H 2 adsorption in these systems.…”

“…Since H 2 adsorption is dissociative, a kinetic isotope effect is expected, and indeed required if the BBA kinetics report on activated H 2 (D 2 ) adsorption. The small isotope effects are fully consistent with the conclusion the BBA arises from the elementary step of heterolytic H 2 dissociation at the MSI [9a] …”

“…Figure 1 D includes a time profile for D 2 adsorption under similar conditions. The BBA values after one hour of adsorption are essentially the same for H 2 and D 2 , consistent with the proposed structural/electronic origins of the BBA [9a, 22] . The D 2 adsorption profile also indicates somewhat slower D 2 adsorption kinetics.…”

“…The BBA signal intensity can be quantified by measuring the total area under the spectrum from 1800 to 1900 cm −1 , as reported previously [9a,b, 22] . This spectral range, shown with a blue bar in Figure 1, was selected due to the relatively large changes in BBA intensity and minimal interference from other features (e.g.…”

“…Since H 2 adsorption kinetics are difficult to measure, H 2 ‐D 2 equilibration and/or exchange reactions are commonly used as a proxy for H 2 activation kinetics [9a, 33, 35] . These reactions are widely understood to be dominated by H 2 activation, and are therefore appropriate in the absence of more direct methods.…”