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First Principle Study on TiO2and Ni-Cu New Energy Materials

Author: JiaLiChao
Tutor: LiJian
School: Huazhong University of Science and Technology
Course: Materials Science
Keywords: First principle TiO2 two-elements-codoping Solid oxide fuel cell Surfaceadsorption Carbon deposition Sulfur poisoning
CLC: TB30
Type: PhD thesis
Year: 2012
Downloads: 113
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Abstract


Energy is an essential material basis for human survival and development.Development of new energy sources and new materials is a powerful impetus to theprogress of science and technology. Titania has been widely studied as a promising materialfor photochemical applications due to its excellent photocatalytic capability. Many studieshave been conducted to realize its catalytic activity by doping. Recent studies suggestedthat different ions codoping into TiO2can further extend the range of absorption to lightand improve the capability to visible light. Besides, for the study of carbon deposition andsulfur poisoning on the anode of Solid Oxide Fuel Cells (SOFCs), it’s generally believedthat C and S are adsorbed tightly on the Ni anode, cover the reaction active sites, leading tosignificant performance degradation. It has been demonstrated that carbon deposition andsulfur poisoning can be greatly suppressed in anodes where Ni is partly displaced by Cu.In this paper, the electronic and optical properties of (N, Fe)-,(N, S)-, and (N, C) twoelements-codoped anatase TiO2were calculated, respectively, based on the densityfunctional theory. Impurity concentration dependence of optical absorption for N and Cdoped TiO2was also evaluated. Meanwhile, we investigated the adsorption of C and S onNi(111), Cu(111) and alloyed Ni-Cu (111) surfaces, to explain the alleviation of carbondeposition and sulfur poisoning with Ni-Cu anode in direct hydrocarbon fuels. The majorresults obtained are described as follows:(1) Codoping with N and Fe leads to lattice distortion, which results in a change ofdipole moments and makes the separation of photoexcited electron–hole pairs easier. Theimpure states appear in the forbidden gap due to the synergistic effect of Fe and N. Theabsorption spectra show that a significant red-shift occurs and the efficiency ofphotocatalytic is promoted. Thus, N and Fe codoped TiO2will be a better photocatalystthan monodoped TiO2.(2) S substitution for Ti site is energetically favorable for S-doped and the codopedcases. The electronic structures show that doping of N and/or S into anatase titania will leadto the band gap narrowing. Compared to pure anatase TiO2, the absorption edge of all thedoped TiO2are shifted to the visible-light region. The hybridized states located in the bandgap in the codoped system lead to a better photocatalytic activity than that of monodoped systems. The (N, S)-codoped TiO2powders were prepared by a simple one-stephydrothermal method. The optical absorption spectra obtained by experiments reveals that(N, S)-codoped TiO2has a much stronger absorption of visible light than the undoped TiO2,which verifies the reliability of the calculation results.(3)C prefers to substitute Ti in the doped TiO2systems, present in cationic form. N, Cmonodoping and codoping of anatase TiO2can reduce the band gap significantly. N2pstates locate around the Valence Band Maximum (VBM), and the hybridization of C2p andTi3d states drops down the Conductive Band Minimum (CBM), the synergistic effectmakes the codoped system have a better photocatalytic activity than the monodopedsystems. At low impurity concentration, dopant concentration dependence of electronic andphoto absorption properties is not noticeable in N-doped TiO2system; when Cconcentration is2.08%in C-doped TiO2, the highest photocatalytic activity will beobtained.(4)C and S atoms preferentially adsorb on the hollow sites; The absolute value ofadsorption energy for adsorption of C and S atoms on Ni(111) surface is higher than that forthem on Cu(111) surface. And Cu alloying with Ni lowers the adsorption energy and in turnthe tendency of C or S adsorption on the Ni-Cu alloy surface, suggesting that Ni-Cu alloymay possibly be a better choice than Ni for SOFC anode in terms of carbon resistance and Stolerance. The C and S adsorption causes expansion of the first interlayer spacing of all thesurfaces. The degree of hybridization between C2p or S3p and metallic3d orbits is in theorder of Ni>Ni-Cu alloy>Cu, which explains the high absolute value of adsorption energyfor Ni and Cu alloying reduces the adsorption tendency.

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