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Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO₂ and HfO₂.

ABSTRACT: Structure and thermodynamics of pure cubic ZrO₂ and HfO₂ were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO₂ and 0.87 ± 0.08% for HfO₂), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO₂ and 61 ± 10 kJ/mol for HfO₂, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO₂ and HfO₂ are similar and reach (4 ± 1)·10^-5/K from experiment and (5 ± 1)·10^-5/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO₂ and ZrO₂, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO₂ is an excellent oxygen conductor, perhaps even better than YSZ.

SUBMITTER: Hong QJ

PROVIDER: S-EPMC6175917 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO2 and HfO2.

Hong Qi-Jun QJ Ushakov Sergey V SV Kapush Denys D Benmore Chris J CJ Weber Richard J K RJK van de Walle Axel A Navrotsky Alexandra A

Scientific reports 20181008 1

Structure and thermodynamics of pure cubic ZrO2 and HfO2 were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic ph ...[more]

PMID: 30297693

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Project description:The high ethylene selectivity exhibited on the zirconia-activated-carbon bi-support catalyst is investigated by experiment and density functional theory-based (DFT) analysis. This bi-support catalyst systems prepared by the physical mixing method for the tungsten catalyst show a significant increase in ethylene selectivity up to 90% compared to the zirconia single support system (~58%) during the ethanol dehydration reaction. Besides, the optimal percent weight ratio of zirconia to activated carbon, which results in the highest ethanol conversion is 50:50. The DFT-based analysis is used to investigate high ethylene selectivity in the bi-support system. It shows that the WO5/zirconia is the most stable model for the zirconia single-support tungsten catalyst represented by the zirconia (101) facet of the tetrahedral phase. The carbon atoms were added to the WO5/zirconia to model the tungsten catalyst on the bi-support system. The Bader charge analysis is carried out to determine the electron transfer in the catalyst. The bonding between ethylene and the WO5 active site on the catalyst is weakened when the system is bi-support, where the added carbon atoms on the catalyst in the ZrO2 region decrease the ethylene adsorption energy. Thus, the desorption and the selectivity of ethylene are promoted. The decrease in adsorption energy can be explained via the analysis of the projected density of states (PDOS) profiles of atom involving the adsorption. It was found that the added carbon in the ZrO2 region induces the electron transfer from the ethylene molecule to the surface, especially to the ZrO2 region. The depletion of the electron around the ethylene molecule weakens the bonds, thus, promote desorption. Hence, the advantages of using the bi-support system in the tungsten catalyst are that the catalyst exhibit (1) high conversion due to the zirconia support and (2) high ethylene selectivity due to the added carbon promoting the desorption of ethylene via the induction of electron from an ethylene molecule to surface.

Dataset Information

Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO₂ and HfO₂.

Publications

Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO<sub>2</sub> and HfO<sub>2</sub>.

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