Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (4): 1905021.doi: 10.3866/PKU.WHXB201905021

Special Issue: Solid-State Nuclear Magnetic Resonance

• Article • Previous Articles     Next Articles

NMR and EPR Studies of Partially Reduced TiO2

Yuhong Li1,2,Xin-Ping Wu3,Cong Liu1,Meng Wang1,Benteng Song2,Guiyun Yu4,Gang Yang1,Wenhua Hou2,Xue-Qing Gong3,Luming Peng2,*()   

  1. 1 Suzhou Key Laboratory of Functional Ceramic Materials, School of Chemistry and Material Engineering, Changshu Institute of Technology, Changshu 215500, Jiangsu Province, P. R. China
    2 Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China
    3 Key Laboratory for Advanced Materials, Centre for Computational Chemistry, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, P. R. China
    4 School of Chemical and Biological Engineering, Yancheng Institute of Technology, Yancheng 224051, Jiangsu Province, P. R. China
  • Received:2019-05-05 Accepted:2019-06-21 Published:2020-03-12
  • Contact: Luming Peng E-mail:luming@nju.edu.cn
  • Supported by:
    the Natural Science Foundation of Jiangsu Province, China(BK20170435);the National Natural Science Foundation of China(91745202);the National Natural Science Foundation of China(21573103)

Abstract:

Partially reduced TiO2 nanomaterials have attracted significant interest because of their visible-light activity for catalysis and photodegradation. Herein, we prepared a partially reduced anatase TiO2 (Re-A-TiO2) nanoparticle material using a fast combustion method, demonstrating good activity toward decomposing methyl orange under visible light irradiation. The surface structure of the prepared material, after being surface-selectively 17O-labeled with H217O (17O-enriched water), was studied via 17O and 1H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy, and the obtained results were compared to those of non-reduced anatase TiO2 (A-TiO2). The EPR results showed that the concentrations of paramagnetic species (i.e., oxygen vacancies (OV) and Ti3+) in Re-A-TiO2 were much higher than that in A-TiO2, while the former was associated with a higher OV/Ti3+ ratio. The intensities of the EPR signals were significantly affected by the adsorbed water, and this phenomenon was explored in combination with 1H NMR spectroscopy. The 1H species on Re-A-TiO2 appeared at larger chemical shifts, denoting the increased acidity of the sample, and these 1H species on Re-A-TiO2 were more difficult to remove than those on A-TiO2. On the other hand, different features were observed for the signals arising from the two-coordinated oxygen atoms (μ2-O) in 17O NMR, suggesting a typical anatase TiO2(101) surface on A-TiO2, but a more complex surface environment for Re-A-TiO2. Furthermore, a larger amount of hydroxyl groups (OH) were observed on Re-A-TiO2 compared to that on A-TiO2, indicating a larger proportion of exposed (001) facets on Re-A-TiO2. However, the μ2-O signals broadened and became similar when the drying temperature was increased to 100 ℃, indicating a non-faceted anatase TiO2 surface in such conditions. Based on the EPR and NMR results, a significant fraction of the OH species is believed to be formed from the reaction of the paramagnetic centers and adsorbed water molecules. The 1H→17O cross polarization (CP) MAS and two-dimensional heteronuclear correlation (2D HETCOR) NMR spectra were used to verify the spatial proximity of the hydrogen and oxygen species, confirming the spectral assignments of a strongly adsorbed water and one type of surface OH species. In particular, the 1H NMR signals at approximately 11 ppm were ascribed to the hydrogen species in the intramolecular hydrogen bond. In summary, this study investigated the paramagnetic species and surface structure of anatase TiO2 materials by combining EPR along with 1H and 17O solid-state NMR spectroscopy. The differences in the surface structures of Re-A-TiO2 and A-TiO2 should be closely related to their different properties toward the photodegradation of methyl orange.

Key words: Partially reduced TiO2, Solid-state NMR, 17O NMR spectrum, 1H NMR spectrum, EPR, Hydroxyl group, Hydrogen bond, Visible light degradation

MSC2000: 

  • O641