Understanding the nature of the active site is critical for informed catalyst design. Characterizing the active sites for the chemistry occurring at the metal-support interface (MSI) of supported metal catalysts is particularly challenging because these multicomponent sites are in exceedingly low concentrations and are not easily observed. In-situ spectroscopy can be a particularly useful tool in identifying changes to these sites that occur during reactions, thus providing insight into their properties. We demonstrate H2 adsorption on Au/TiO2 catalysts results in electron transfer to the support, inducing shifts in the FTIR background we describe as a broad background absorbance (BBA) signal. IR spectra collected during H2 adsorption revealed the BBA signal correlates to changes associated with the loss of unprotonated interface hydroxyls (MSITiOH) and appearance of protonated interface hydroxyls (MSITiOH2+), indicating H2 adsorbs heterolytically at the MSI of Au/TiO2 catalysts. This also allowed us to identify a spectroscopic signature associated with MSI hydroxyls interacting directly with the Au nanoparticles and separate that signature from the unmodified hydroxyls that dominate the surface. The spectroscopic signature indicates MSI hydroxyls are chemically distinct from adsorbed water and free surface hydroxyls. Prior to H2 adsorption, the MSI hydroxyls are electron rich relative to other surface hydroxyls. As a consequence, the MSI hydroxyls are more basic, which likely contributes to their involvement in H2 activation. Correlations between the BBA signal spectroscopic signatures further allowed us to quantify the surface density of MSITiOH2+ species across a series of Au/TiO2 catalysts. Each Au perimeter atom was found to be associated with one reactive MSITiOH group, which is twice the expected ratio based on typical TiO2 surface terminations. This suggests Au plays an important role in modifying both the local structural and electronic properties of the support. Thus, the interactions 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. This synergism accounts for the considerably faster H2 activation kinetics over supported Au catalysts relative to extended Au surfaces.

Hydrogen adsorption at the Au/TiO2 interface: Quantitative determination and spectroscopic signature of the active site

ACS Spring 2022

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