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Cosmic rays, γ-rays, and X-ray-driven reactions on interstellar grains have been the main focus in understanding the extraterrestrial synthesis of complex organic molecules. Herein, using temperature-programmed desorption and X-ray photoelectron spectroscopy, we report for the first time X-ray-induced CO2 production from a CO-covered rutile TiO2(110) surface at 110 K, in addition to X-ray-induced CO desorption. The X-ray-induced CO2 production was found to follow a Mars and van Krevelen (MvK) mechanism via the reaction between adsorbed CO and surface lattice oxygen of TiO2. Defects of TiO2 suppress the X-ray-induced CO2 production but promote the X-ray-induced CO desorption. These findings suggest a novel X-ray-induced CO-to-CO2 reaction on oxide surfaces at cryogenic temperature likely occurring in the interstellar medium and also warn us to keep in mind the possibility of X-rays inducing chemical reactions during structural characterizations of oxides with X-ray-involved techniques.

The formation of HCOOCH3 initiated by photooxidation of CH3OH on TiO2 surfaces is interesting, but the mechanism remains ambiguous. Using thermal desorption spectroscopy and X-ray photoelectron spe...

Aligning the intrinsic photocatalytic activities of various types of TiO2 facets is of great importance and interest in TiO2 photocatalysis. Herein, we report a comparative study of the intrinsic p...

Photocatalytic water reduction to hydrogen over oxide semiconductors is one of the most extensively investigated artificial photocatalytic reactions, but the nature of the active species has not yet been elucidated. Here, we successfully prepared Pt/rutile TiO2(110) surfaces with hydrated proton species via co-adsorption of hydrogen and water and observed the photocatalytic reduction of hydrated protons to H2 upon UV light illumination. These results provide experimental evidence to prove hydrated protons as the active species for photocatalytic water reduction to hydrogen and demonstrate the occurrence of photocatalytic reduction of hydrated protons to H2 within the H-bonding network on the catalyst surface instead of directly on the catalyst surface. The Pt-TiO2 interface is capable of dissociating water to form hydroxyl groups that facilitate the formation of H-bonding network on the catalyst surface to enhance the photocatalytic H2 production. Our results greatly advance fundamental understanding of artificial photocatalytic water reduction.

Investigating the surface chemistry of formaldehyde on the surface of TiO2 is important in understanding the thermal-catalytic and photocatalytic reactions of formaldehyde on TiO2-involved catalysts. By combining thermal desorption spectroscopy and X-ray photoelectron spectroscopy, we studied the adsorption, thermo-induced surface reactions, and photo-induced surface reactions of formaldehyde on the rutile TiO2(011)-(2 × 1) surface. The dominant thermal-catalytic reaction is the formation of ethylene by a reductive carbon–carbon formation reaction of formaldehyde adsorbed at the oxygen vacancy sites, and the dominant photocatalytic reaction is the formation of formate, assisted by the bridge O2c sites, followed by carbon monoxide formation at elevated temperatures. The surface intermediates of formaldehyde reactions to ethylene and carbon monoxide on the rutile TiO2(011)-(2 × 1) surface were identified. The effect of the surface structure of the rutile TiO2(011)-(2 × 1) surface, particularly the oxygen va...

Surface chemistry and photochemistry of small molecules on the rutile TiO2(001) and TiO2(011)-(2 × 1) surfaces were studied by low energy electron diffraction, thermal desorption spectroscopy, and x-ray photoelectron spectroscopy. It was found that the TiO2(001) surface mainly exhibits the defects of Ti interstitials in the near-surface region, while the TiO2(011)-(2 × 1) surface mainly exhibits the defects of double-oxygen vacancies. The defect structures of TiO2 surfaces strongly affect their adsorption and thermal/photodesorption behaviors. On the TiO2(001) surface, CH3OH and H2O dissociatively adsorb at the surface Ti sites near Ti interstitials; O2 molecularly adsorbs at the surface Ti sites adjacent to Ti interstitials, forming photoactive O2 species that undergoes a hole-mediated photodesorption process; CO adsorbs at the nearest surface Ti sites close to the Ti interstitials, but CO2 does not, and the resulting CO species is photoactive; and both CO and CO2 species adsorbed at the normal Ti4+ sites are photoinactive. On the TiO2(011)-(2 × 1) surface, O2 adsorbs only at the double-oxygen vacancy sites, and the resulting O2 species dissociates to form two oxygen atoms to refill in the oxygen vacancies upon heating; CO2 adsorbs at the double-oxygen vacancy sites, but CO does not, and the resulting CO2 species is photoactive; and both CO and CO2 species adsorbed at the surface Ti4+ sites are photoinactive. These results broaden the fundamental understandings of the chemistry and photochemistry of TiO2 surfaces, and the established structure-reactivity relation of small molecules on TiO2 surfaces is useful in probing complex structures of TiO2 powder catalysts.

In the present study, the Ti-in-biotite geothermometer was revised using more than 300 natural rutile- or ilmenite-bearing metapelites collected worldwide. The formulation was empirically calibrated as \\( \\ln [T\\,(^\\circ {\\text{C}})] = 6.313 + 0.224\\ln (X_{\\text{Ti}} ) - 0.288\\ln (X_{\\text{Fe}} ) - 0.449\\ln (X_{\\text{Mg}} ) + 0.15P\\,({\\text{GPa}}) \\) , with \\( X_{j} = {{j}}/({\\text{Fe}} + {\\text{Mg}} + {\\text{Al}}^{\\text{VI}} + {\\text{Ti}}) \\) in biotite, assuming ferric iron content of 11.6 mol% of the total iron in biotite. This thermometer is consistent with the well-calibrated garnet-biotite thermometer within error of ±50 °C for most of the calibrant samples and can successfully distinguish systematic temperature changes of different metamorphic zones in both prograde and inverted metamorphic terranes as well as thermal contact aureoles. Thus, the thermometer truthfully reflects real geologic conditions and can be applied to TiO2-saturated metapelites metamorphosed at the crustal level within the calibration ranges (450-840 °C, 0.1-1.9 GPa, XTi = 0.02-0.14 in biotite).

A magnetic photocatalyst TiO2/NiFe2O4 (TN) with typical ferromagnetic hysteresis was prepared by a sol-gel method, which is easy to be separated from a slurry-type photoreactor under the application of an external magnetic field, being one of promising photocatalysts for wastewater treatment. The analysis of XRD indicated that the highly dispersed NiFe2O4 nanoparticles prevented the formation of rutile phase to some extent. A transmission electron microscope (TEM) was used to characterize the structure of the photocatalyst, indicating that the NiFe2O4 nanoparticles highly dispersed among TiO2 nanoparticles. The prepared photocatalyst showed high photocatalytic activity for the degradation of methyl orange in water. The degradation results revealed that the NiFe2O4 nanoparticles played the role of recombination centre of photogenerated electrons and holes for the TN photocatalyst, which gave rise to the decrease in photocatalytic activity. Moreover, the experiment on recycled use of TN demonstrated a good repeatability of the photocatalytic activity.