微孔和介孔材料中的热化学

这篇论文综述了美国加州大学戴维斯分校科学院院士Navrotsky课题组多年来在多孔材料上取得的一系列热化学研究结果。讨论了热化学对微孔、介孔材料的结构稳定性和合成过程的影响。借助多种测热手段对影响骨架结构的热焓、热熵和自由能进行了系统的测量和计算。研究数据表明一系列纯硅分子筛、介孔材料和磷酸铝多孔材料同相应的石英相和块磷铝矿相相比能量上最多只高出15 kJ·mol-1。一系列纯硅分子筛的熵值比石英相高出3.2—4.2 J·K-1·mol-1；在0—12.6 J·K-1·mol-1范围内相对应的自由能几乎没有差别。因此，对不同微孔、介孔材料，其骨架结构在能量上是几乎没有区别的。另外，本文通过介绍一种新型测热方法——原位测热，揭示了分子筛合成过程中的动力学和成核/结晶机理。     

Thermodynamic relations among olivine,spinel, and phenacite structures in silicates and germanates: I. Volume relations and the systems NiOMgOGeO2 and CoOMgOGeO2

An experimental approach is outlined to systematically obtain free energy differences among olivine, spinel, and phenacite forms of silicates and germanates from the thermodynamics of terminal solid solutions in ternary systems. This is applied to the ternary systems NiOMgOGeO₂ and CoOMgOGeO₂ at 1200°C in air and to the system NiOMgOGeO₂ at 800°C and 0.57 kbar water pressure. From the location of conjugation lines, activity-composition relations along each orthogermanate join are calculated. The free energies of transformation from the olivine to the spinel structure at 1200°C are estimated to be +1.6, ?3.5, and ?8.2 kcal/mole for Mg₂GeO₄, Co₂GeO₄, and Ni₂GeO₄, respectively.Volume changes for the spinel-olivine and olivine-phenacite transitions are estimated for the silicates and germanates of Mg, Mn, Fe, Co, Ni, and Zn.     

Formation enthalpies of rare earth titanate pyrochlores

High-temperature oxide melt solution calorimetry and Rietveld refinements of powder X-ray diffraction data were used to investigate the structure (Fd3m; Z=8) and energetics of a series of RE₂Ti₂O₇ (RE=Sm-Lu) compounds with the pyrochlore structure as well as La₂Ti₂O₇ with a layered perovskite-type structure. All of the RE-titanates were found to be stable in enthalpy with respect to their oxides. In the pyrochlore series, Lu₂Ti₂O₇ was least stable in enthalpy (ΔHf-ox at 298 K=−56.0±4.0 kJ/mol); the most stable materials were Gd-, Eu-, and Sm₂Ti₂O₇ with ΔHf-ox at 298 K=−113.4±2.7, −106.1±4.2, −115.4±4.2 kJ/mol, respectively. In general, as the radius ratio of the A- to B-site cations, R_A/R_B, decreases, the pyrochlore structure becomes less stable. The trend of ionic radius of the RE³⁺ cation vs. ΔHf-ox at 298 K is non-linear and approximately parallels the increasing “resistance” to ion-beam-induced amorphization as R_A/R_B decreases.     

Thermochemistry of Surfactant-Templating of USY Zeolite

With the aim of understanding the thermochemistry of the introduction of mesoporosity in zeolites by using surfactants, high temperature oxide melt solution calorimetry was used to determine the change in the enthalpy of formation of USY zeolite before and after the introduction of mesoporosity. Our results confirm that this process only slightly destabilizes the zeolite by the additional surface area. However, this can be overcome by the stabilizing effect of the interactions between the surfactant and the zeolite framework.     

Defect Chemistry of Singly and Doubly Doped Ceria: Correlation between Ion Transport and Energetics

Earlier studies have shown a strong correlation between the enthalpy of formation, ΔH_f,ox, and the ionic conductivity, σ_i, near room temperature in doped ceria systems, which are promising solid electrolytes for intermediate‐temperature solid oxide fuel cells (IT‐SOFCs). The present work demonstrates that this correlation holds at the operating temperature of IT‐SOFCs, 600–700 °C. Solid solutions of Ce_1?xNd_xO_2?0.5x, Ce_1?xSm_xO_2?0.5x, and Ce_1?xSm_0.5xNd_0.5xO_2?0.5x are studied. The ΔH_f,ox at 702 °C is determined by considering the excess heat content between 25 and 702 °C combined with the value of ΔH_f,ox at 25 °C. Both σ_i and ΔH_f,ox show maxima at x=0.15 and 0.20 for the singly and doubly doped ceria, respectively, suggesting that the number of mobile oxygen vacancies in these solid solutions reaches a maximum near those compositions. An increase in temperature results in a shift of the maximum in both ΔH_f,ox and σ_i towards higher concentrations. This shift results from a gradual increase in dissociation of the defect associates.     

Application of scanning calorimetry to estimate soil organic matter loss after fires

The severe heating of soil during wildfires and prescribed burns may result in adverse effects on soil fertility due to organic matter loss. No rapid and reliable procedure exists to evaluate soil organic matter (SOM) losses due to heating. Enthalpy of SOM combustion correlates with organic matter content. Quartz is a ubiquitous mineral in soils and has a remarkably constant composition and reversible α–β phase transition at 575 °C. We suggest that SOM content in heated and unheated soils can be compared using the ratio of SOM combustion enthalpy on heating to the β–α quartz transition enthalpy measured on cooling of the same sample. This eliminates the need to dry and weigh the samples, making possible field applications of the proposed method. The feasibility of using the (ΔH _{comb SOM})/(ΔH _β–α Qz) ratio was established with experiments on soil samples heated in the laboratory and the method was then used for evaluation of SOM loss on two pile burn sites at UC Berkeley’s Blodgett Forest Research Station in Georgetown, California.     

Effect of structure and thermodynamic stability on the response of lanthanide stannate pyrochlores to ion beam irradiation

The lanthanide stannates, Ln2Sn2O7, Ln=La-Lu and Y, have the isometric pyrochlore structure, A2B2O7, and their structural properties have been refined by Rietveld analysis of powder neutron and synchrotron X-ray diffraction data. In this study, the enthalpies of formation of selected stannate pyrochlores, Ln=La, Nd, Sm, Eu, Dy, and Yb, were measured by high-temperature oxide melt solution calorimetry. Their radiation response was determined by 1 MeV Kr2+ ion irradiation combined with in situ TEM observation over the temperature range of 25 to 1000 K. The enthalpy of formation from binary oxides of stannate pyrochlores became more endothermic (from -145 to -40 kJ/mol) as the size of the lanthanide in the A-site decreases. A more exothermic trend of the enthalpy of formation was observed in stannate pyrochlores with larger lanthanide ions, particularly La, possibly as a result of increased covalency in the Sn-O bond. In contrast to lanthanide titanate pyrochlores, Ln2Ti2O7, that are generally susceptible to radiation-induced amorphization and zirconate pyrochlores, Ln2Zr2O7, that are generally resistant to radiation-induced amorphization, the lanthanide stannate pyrochlores show a much greater variation in their response to ion irradiation. La, Nd, and Gd stannates experience the radiation-induced transformation to the aperiodic state, and the critical amorphization temperatures are approximately 960, 700, and 350 K, respectively. Y and Er stannate pyrochlores cannot be amorphized by ion beam irradiation, even at 25 K, and instead disorder to a defect fluorite structure. Comparison of the calorimetric and ion irradiation data for titanate, zirconate, and stannate pyrochlores reveals a strong correlation among subtle changes in crystal structure with changing composition, the energetics of the disordering process, and the temperature above which the material can no longer be amorphized. In summary, as the structure approaches the ideal, ordered pyrochlore structure, radiation-induced amorphization is more easily attained. This is consistent with an increasingly exothermic trend in the enthalpies of formation of pyrochlores from the oxides, that is, the greater the thermochemical stability of the pyrochlore structure, the more likely it will be amorphized upon radiation damage rather than recover to a disordered fluorite structure.     

Thermodynamic Properties of Polymorphs of Fluorosulfate Based Cathode Materials with Exchangeable Potassium Ions

FeSO₄F‐based frameworks have recently emerged as attractive candidates for alkali insertion electrodes. Mainly owing to their rich crystal chemistry, they offer a variety of new host structures with different electrochemical performances and physical properties. In this paper we report the thermodynamic stability of two such K‐based “FeSO₄F” host structures based on direct solution calorimetric measurements. KFeSO₄F has been reported to crystallize in two different polymorphic modifications—monoclinic and orthorhombic. The obtained enthalpies of formation from binary components (KF plus FeSO₄) are negative for both polymorphs, indicating that they are thermodynamically stable at room temperature, which is very promising for the future exploration of sulfate based cathode materials. Our measurements show that the low‐temperature monoclinic polymorph is enthalpically more stable than the orthorhombic phase by ≈10 kJ mol^?1, which is consistent with the preferential formation of monoclinic KFeSO₄F at low temperature. Furthermore, observed phase transformations and difficulties in the synthesis process can be explained based on the obtained calorimetric results. The KMnSO₄F orthorhombic phase is more stable than both polymorphs of KFeSO₄F.