Mechanistic Studies of Hydrogen Dissociation and Spillover on Au/TiO₂: IR Spectroscopy of Coadsorbed CO and H-Donated Electrons

Transmission Fourier transform infrared spectroscopy, coupled with CO adsorption, was used to study how the oxidation state of Au surface sites change during H₂ dissociation, migration, and electronic excitation of TiO₂ in a nanoparticulate Au/TiO₂ catalyst. Data reveals that atomic H, produced by H₂ dissociation on Au particles, readily hydroxylates the Au–O–Ti linkages around the periphery of the gold particles. Following passivation of peripheral sites, the H atoms diffuse into the bulk of the titania where they protonate the semiconductor, while donating an electron to shallow trapped (ST) states just below the conduction band (CB). The donated electrons effectively n-dope the semiconductor. By simultaneously monitoring changes in the infrared absorbance due to transitions involving ST electrons, free CB electrons, and the fundamental stretching mode of CO(a), we have found that the most active sites for hydrogen dissociation are Au⁰ sites at the free step edges or other defect sites in the Au nanoparticles that are located away from the Au–O–Ti interface.