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.

Identifying the underlying nature of the structure sensitivity of oxygen reduction reaction (ORR) over carbon supported Pt catalysts is an important but challenging issue in electrochemical system. In this work, we combine experiments, density functional theory calculations with model calculations to clarify the size-dependent ORR activity and selectivity over differently sized Pt/CNT catalysts. HAADF-STEM, HRTEM and XPS measurements show that the Pt nanoparticles supported on CNT possess a well-defined truncated octahedron shape in [a] State Key

Competitive adsorption of prototype molecules such as (12)CO, (13)CO and CO2 at the two typical fivefold coordinated Ti5c(4+) cation sites of reduced rutile TiO2(110) surfaces was studied in a newly designed UHV-FTIR system. The measured binding energies of (12)CO, (13)CO or CO2 adsorbed at two kinds of Ti5c(4+) sites are different. The molecular occupying probability at these sites depends on the binding energy of the adsorbed molecules; while, the molecular exchanging probability at these sites depends on their binding energy difference due to the presence of competitive adsorption. A simple thermodynamic equilibrium model was proposed to qualitatively interpret the adsorption and competitive adsorption mechanisms. These results will contribute to the elucidation of the (photo)catalytic process on TiO2(110) surfaces.

CO adsorptions and configuration evolutions with temperatures on (2 × 1)­Ni–O row structures on Ni(110) surfaces were studied by using ultrahigh vacuum–Fourier transform infrared spectroscopy, thermal desorption spectroscopy, and density functional theory calculations. Two kinds of adsorption configurations were identified. In the top configuration at low temperature (90 K), the carbon atom of one CO binds to one Ni atom in the Ni–O rows with a tilt angle of about 53°, and the vibration frequency of CO is 2100–2119 cm–1 depending on CO coverage. In the bridge configuration near room temperature (280 K), the carbon atom of one CO binds to two Ni atoms from the neighboring Ni–O rows, and the vibration frequency is 2030–2039 cm–1. By annealing the sample prepared at 90 to 280 K, the top adsorption configuration gradually evolves into the ordered bridge adsorption configuration via a disordered state. In the disordered state, the top and bridge configurations are distributed in disorder, induce strong transverse distortion of the Ni–O rows along [11̅0] direction, and thus lead to the significant CO frequency shifts. The distortion disappears after the complete desorption of CO above 300 K. The high stability of Ni–O rows may be the key factor to prevent CO oxidation on such surfaces.

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