Water-assisted oxygen activation during selective oxidation reactions

Influence of solvent structure and hydrogen bonding on catalysis at solid-liquid interfaces

Solvent molecules interact with reactive species and alter the rates and selectivities of catalytic reactions by orders of magnitude. Specifically, solvent molecules can modify the free energies of liquid phase and surface species via solvation, participating directly as a reactant or co-catalyst, or competitively binding to active sites. These effects carry consequences for reactions relevant for the conversion of renewable or recyclable feedstocks, the development of distributed chemical manufacturing, and the utilization of renewable energy to drive chemical reactions. First, we describe the quantitative impact of these effects on steady-state catalytic turnover rates through a rate expression derived for a generic catalytic reaction (A → B), which illustrates the functional dependence of rates on each category of solvent interaction. Second, we connect these concepts to recent investigations of the effects of solvents on catalysis to show how interactions between solvent and reactant molecules at solid-liquid interfaces influence catalytic reactions. This discussion demonstrates that the design of effective liquid phase catalytic processes benefits from a clear understanding of these intermolecular interactions and their implications for rates and selectivities.

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

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.