The polaron introduced by the oxygen vacancy (Vo) dominates many surface adsorption processes and chemical reactions on reduced oxide surfaces. Based on IR spectra and DFT calculations of NO and CO adsorption, we gave two scenarios of polaron-involved molecular adsorption on reduced TiO2(110) surfaces. For NO adsorption, the subsurface polaron electron transfers to a Ti:3d-NO:2p hybrid orbital mainly on NO, leading to the large redshifts of vibration frequencies of NO. For CO adsorption, the polaron only transfers to a Ti:3d state of the surface Ti5c cation underneath CO, and thus only a weak shift of vibration frequency of CO was observed. These scenarios are determined by the energy-level matching between the polaron state and the LUMO of adsorbed molecules, which plays a crucial role in polaron-adsorbate interaction and related catalytic reactions on reduced oxide surfaces.

CO2 adsorption and interaction on rutile TiO2(110) surfaces was studied by UHV-FTIRS combined with theoretical simulations. With increasing CO2 exposure, CO2 adsorbs in succession at the oxygen vacancy (Vo) sites, on the five-coordinated Ti cation (Ti5c) sites and the bridging oxygen (Obr) sites at low temperature. The coupling has occurred between neighboring CO2 adsorbed on Ti5c sites from rather low CO2 coverage (∼0.5 ML), leading the ν3(OCO) asymmetric stretching vibrations to split into two absorption bands in IR spectra. Two kinds of coupled geometries of adjacent CO2 on Ti5c sites are determined by theoretical simulations. For the higher CO2 coverage (∼1.5 ML), the horizontal adsorption configuration along the [11[combining macron]0] azimuth of CO2 adsorbed on Obr sites is identified for the first time using polarization- and azimuth-resolved RAIRS in experiments. The significant deviation of CO2 from the top of Obr sites demonstrates the strong coupling between CO2 adsorbed on Obr and Ti5c sites.

The adsorption and reaction of NO on both the oxidized and reduced single crystal rutile TiO2(110) surfaces were studied in a UHV-FTIRS system at low temperature. The monodentate adsorption configuration of the cis-(NO)2 dimer at bridge oxygen vacancy (Vo) sites was detected for the first time on reduced TiO2(110) surfaces. With the aid of (NO)2 dimer adsorption anisotropy, the bidentate configuration of the cis-(NO)2 dimer on fivefold coordinated Ti5c(4+) cation sites was clearly confirmed. The (NO)2 dimer converts to N2O on Ti5c(4+) cation sites at higher NO dosage on both oxidized and reduced surfaces, rather than at Vo sites. The (NO)2 → N2O conversion is independent of the presence of Vo on TiO2(110) surfaces. To explain the signs of absorption bands of the dimer monodentate configuration, the local optical constant at Vo sites was introduced.

Formation and evolution of active hydrogen species from H 2 on a metal oxide is of great importance in many hydrogenation reactions, while understanding the nature of surface hydrogen species remains a great challenge. Herein, high-vacuum-based transmission infrared spectroscopy coupled with temperature-programmed desorption has been applied to study surface hydrogen species over ZnO and ZnCrO x catalysts. We show that D 2 dissociates readily to form Zn−D and O−D on ZnO at 153 K, and simultaneous Zn−D and O−D desorption in the form of D 2 occurs at 223 K. This corroborates reversible dissociation and recombination of D 2 on ZnO. (Cr−)Zn−D and (Cr−)O−D species appear on ZnCr 2 O 4 in D 2 at 273 K, indicating a higher barrier for D 2 activation than that on ZnO. (Cr−)Zn−D can diffuse to form (Cr−)O−D at 573 K, and then desorption of surface O-D groups requires a higher temperature of 943 K. Theoretical studies demonstrate that surface geometry plays a crucial role in the evolution of the surface hydrogen species. The polar ZnCr 2 O 4 ( 111) surface can stabilize metal-H species due to the high barrier for its surface diffusion and the thermodynamic obstruction for its association with O−H, in contrast with the nonpolar ZnO(101̅ 0) surface. These results provide insights into the tuning of the stability of surface metal-H species on metal oxide catalysts, which will contribute to tailoring the hydrogenation processes in the H-involved catalytic reactions.