Hydrogen Adsorption at the Au/TiO₂ Interface: Quantitative Determination and Spectroscopic Signature of the Reactive Interface Hydroxyl Groups at the Active Site

Infrared spectroscopy shows that H₂ adsorbs heterolytically at the metal–support interface (MSI) of Au/TiO₂ catalysts. This generates stable protonated MSI hydroxyls, which are chemically distinct from adsorbed water and free surface hydroxyls. IR spectra collected during H₂ adsorption revealed changes associated with the loss of unprotonated interface hydroxyls (_MSITiOH) and the appearance of protonated interface hydroxyls (_MSITiOH₂⁺). This allowed us to identify a spectroscopic signature associated with MSI hydroxyls interacting with Au nanoparticles and separate that signature from the unmodified hydroxyls that dominate the surface. Prior to H₂ adsorption, MSI hydroxyls are electron-rich relative to other surface hydroxyls on the catalyst. As a consequence, MSI hydroxyls are more basic, which likely contribute to their involvement in H₂ activation. The surface density of the _MSITiOH₂⁺ species was quantified with the broad-background absorbance (BBA) associated with electron injection into the support during H₂ adsorption. Quantifying these signals across a series of catalysts showed that each Au perimeter atom is associated with one reactive _MSITiOH group. This unexpected result indicates that Au modifies the local structural and electronic properties of the support. Thus, the synergism between Au and TiO₂ produces electron-deficient Au particles, which are stronger Lewis acids, and increases the number of electron-rich MSI hydroxyls, which are stronger Brønsted bases.