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First-Principles Studies on the Interaction Between Water Molecule and Ceria Based Catalysts

Author: WangQingGao
Tutor: YangZongXian；WeiShuYi
School: Henan Normal
Course: Condensed Matter Physics
Keywords: CeO2 CeO2 111 H2O Zr-doped First-principles The first principle of thermodynamics
CLC: O469
Type: Master's thesis
Year: 2010
Downloads: 31
Quote: 0
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Abstract


CeO 2 Ce ions with variable valence of (Ce 4? Ce 3 ), it has a high oxygen storage and release oxygen capacity, also showed good oxidation - reduction performance. Therefore, based on the CeO 2 catalyst in fuel steam reforming (water gas shift, CO preferential oxidation the CH 4 , CH3OH, and other organic matter decomposition and synthesis, etc.), automobile exhaust treatment has been widely used in the industrial production of fuel cell electrolyte and glass polishing grinding and chemical catalytic reaction process. In these processes, the H 2 O or as reactants present, or as a product exists; H 2 O in these processes has what role these chemical catalytic reaction what impact it is crucial. Doping can improve the activity of metal oxide catalysts, from the atomic and electronic level understanding of H 2 O and CeO 2 interactions and doping on this interaction affect help us rational design with higher catalytic activity of CeO 2 -based materials, in order to better service for the production and living. The first principles method based on density functional theory and supercell model system to study the interaction of H 2 O and CeO 2 (111) surface . And the oxidation state of CeO 2 surface interaction the H 2 O single hydrogen bond structure chemical adsorbed on the surface can not be dissociation; reduced state of CeO < sub> 2 substrate, H , 2 O no hydrogen bonding configuration adsorption at the slot top, form a more stable structure of the surface hydroxyl or dissociation adsorption. Once a hydrogen atom in the H 2 O in the absence of the hydrogen bonding structure towards the substrate, it will spontaneously dissociation forming structure of the surface hydroxyl groups. In the external environment (i.e., a certain temperature and pressure), H 2 O to what state exists? Combining our DFT theoretical calculation results and experimental data, to determine if each system during the Gibbs free energy change, can be obtained of the phase diagram of the system, thereby further analyzed in a certain temperature and under a pressure of H 2 O may exist state, the results show that H < the CeO 2 , this reaction is easy to occur in low-temperature sub> 2 O oxidation mild reduction (oxygen vacancy concentration) can be used to accompanied Ce 3 → Ce 4 changes; study also found that the higher concentration of oxygen vacancies, due to the distortion of the structure, of CeO , 2 To become Ce2O3-like structure, the H 2 O can not be oxidized high degree of reduction of CeO 2 . The study showed that, Zr-doped CeO 2 increase the CeO 2 in oxygen ion activity and thermal stability of the system; oxygen ion activity and it can restore the same a thing conflicting two aspects, in order to maintain a high catalytic activity, these two areas must be made to achieve an ideal state; Therefore, this system was studied H 2 O and Zr-doped The miscellaneous CeO 2 interaction. The results show that: in the oxidation state of the Zr-doped CeO 2 on H 2 O in oxygen (O *) and Ce bonding H 2 O chemisorption form a single hydrogen bond structure, O * and Zr bond, H 2 O to dissociation. Zr-doped CeO 2 surface there are two types of oxygen atoms (OI) with three Ce ions close neighbors, and the other (OII) with two Ce ions and Zr ions close neighbors, the corresponding exist in the surface of the two types of reduced state (VI, the surface loss of an OI; VII surface lose a OII). Reduced state Zr-doped CeO 2 on H 2 O in O * and Ce bond the H 2 O dissociation to form surface hydroxyl structure, when H 2 O in O * and Zr bond, H 2 O can not be decomposed; reduced state in the second category Zr doping CeO 2 on due to the distortion caused by the structural relaxation, H 2 O more complex behavior; occurrence of these processes depends on Zr and Ce electron affinity, and capacity differences and corresponding O activity.

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