Study on catalytic properties and applications of metal-doped NiTiO3 materials

Abstract

The development of human society and industry is inseparable from the applica-tion of catalysts. Current serious environmental problems and huge energy problem make us vulnerable to disasters and tragedies, now and in the future. As a promising oxidation technology, semiconductor catalysis has become a frontier subject that has attracted much attention. Among many materials, perovskite materials have attract-ed much attention due to their special electronic structure, unique photophysical and chemical properties. Through different preparation methods and several kinds of modification methods, such as transition metal loading or semiconductor com-pounding, this research hopes to conduct more research and explore its applications on perovskite materials. 1 Photocatalytic Decomposition of Toluene by NiTiO3 In this study, we prepared a nickel titanate (NTN) photocatalyst by a facile synthesis process with microwave method for photocatalytic decomposition of gas phase toluene. P25, a commercial photocatalyst, was also employed for the photo-catalytic decomposition to compare their photocatalytic performance. P25 showed better catalytic performance under UV irradiation, while NTN exhibited a higher reaction rate constant under visible light irradiation. Moreover, NTN generated more CO2 after the photocatalytic reaction than P25. From XRD patterns and Ra-man spectra, P25 consisted of anatase and rutile TiO2 structures and NTN existed pure nickel titanate structure. Based on UV-Vis spectra, the bandgaps of P25 and NTN were obtained at 3.2 and 2.47 eV, respectively, implying that NTN would be a visible light-responded photocatalyst. 2 Modification of NiTiO3 materials by transition metal dopants: The dopant size effect Metal doping is a common modification method. In this study, we investi-gated the changes of the structural and optical properties of NiTiO3 materials modi-fied by transition metal doping. Cobalt or tungsten-doped NiTiO3 materials were successfully prepared by a modified Pechini method via solvothermal treatment. Raman, FTIR, and XRD spectroscopic analyses showed that the Co2+ ions were se-lectively doped into Ni2+ sites in the NiTiO3 lattice while maintaining an ilmenite structure, resulting in a solid solution of triple transition metal oxides. The size similarity between Co and Ni induced the formation of a solid solution, CoxNi1-xTiO3, in the ilmenite structure. In contrast, W doping into the NiTiO3 ilmenite structure resulted in an irregularity of the materials due to the characteristics of the heavy transition metal dopant. Along with increasing the W content, the crystallite size in the ilmenite structure decreased from 90.2 to 74.5 nm and new Raman bands at 831 and 892 cm−1 for WOx appeared at high W contents. However, the PL emis-sion intensities gradually decreased with increasing doping content, implying that the recombination process was inhibited in the NiTiO3 materials by the dopants. 3 Modification of NiTiO3 by Nb doping and NbOx heterojunction: Effect of ox-ygen vacancy In this research, Nb-doped NiTiO3 and NbOx/NiTiO3 photocatalysts are pre-pared with various Nb amounts to enhance photocatalytic activity for dye photodeg-radation under visible light irradiation. Nb-doped NiTiO3 exhibits higher photocata-lytic activity than pure NiTiO3, whereas the inappropriate band structures of NbOx and NiTiO3 decrease photocatalytic activity. Incorporation of Nb into the NiTiO3 lattice induces Ti3+ sites and oxygen vacancies. Eventually, the Nb-doped NiTiO3 photocatalyst at the highest Nb content sample was transformed into a triple metal oxide phase with the highest surface area and oxygen vacancy. The abundant oxy-gen vacancy of NT-Nb-10 resulted in the lowest photoluminescence emission inten-sity, which resulted in the highest apparent photocatalytic reaction rate constant (kapp) of 14.1 min−1 owing to the suppression of the recombination process. The for-mation of the triple metal oxide phase in this study may allow a potential way to modify low cost visible light-driven NiTiO3 photocatalysts for sustainable photo-catalytic application. 4 Highly stable and selective CoxNiyTiO3 for CO2 methanation CO2 methanation with Sabatier reaction is an effective way to convert the greenhouse gases into valuable industrial product. In this work, we designed and de-veloped a stable and effective Co modified NiTiO3 catalyst. The best catalyst ex-hibits high CO2 conversion (86%) and high methane selectivity (nearly 100%), which can work continuously for 16 hours without significant deactivation at 350 ℃. As evidenced by various characterization methods, like XPS, H2-TPD and CO2-TPR, Co doping is found to influence the electron distribution of the catalyst, change the size of metal particle and the interaction of metal and support, which in turn has positive impact on methanation performance, while NiTiO3 support im-prove the ability to capture CO2 because of its abundant basic oxide and oxygen va-cancies after reduction. Cobalt modification proved to be an potential way to con-struct Ti-O-Ni interfacial structure, and increase the hydrogen activation site and carbon dioxide activation site simultaneously. This work may provide useful infor-mation for designing and optimizing multi-metal modified catalyst for CO2 methanation.