The enthalpy of adsorption of CO2 on an environmentally friendly metal-organic framework, CD-MOF-2, has been determined directly for the first time using adsorption calorimetry at 25 °C. This calorimetric methodology provides a much more accurate and model-independent measurement of adsorption enthalpy than that obtained by calculation from the adsorption isotherms, especially for systems showing complex and strongly exothermic adsorption behavior. The differential enthalpy of CO2 adsorption shows enthalpy values in line with chemisorption behavior. At near-zero coverage, an irreversible binding event with an enthalpy of -113.5 kJ/mol CO2 is observed, which is followed by a reversible -65.4 kJ/mol binding event. These enthalpies are assigned to adsorption on more and less reactive hydroxyl groups, respectively. Further, a second plateau shows an enthalpy of -40.1 kJ/mol and is indicative of physisorbed CO2. The calorimetric data confirm the presence of at least two energetically distinct binding sites for chemisorbed CO2 on CD-MOF-2.

Sintering and nanostability (defined as the stability against sintering) are critical phenomena present in the processing and application of nanoparticles. With important implications in obtaining high-quality dense ceramics with fine grains or in enabling high surface areas in nanoparticles for catalytic applications, the control of these interrelated phenomena has been the focuses of several studies. From a thermodynamic perspective, it is recognized that surface energy is a fundamental parameter in both cases, since it is the main driving force for sintering and also the reason that nanoparticles are thermodynamically unstable and have the tendency to coarsen at elevated temperatures. The role of grain-boundary energies is less recognized as relevant, but is also connected to densification, grain growth, and nanoparticle stability. In this paper, we review the critical aspects of the role of interfacial energies in the microstructure evolution, in particular addressing them as parameters to allow better control in addition to more conventional kinetic parameters. The concept is based on the nonsingularity of interfacial energies in a given system, which varies with temperature, atmosphere, and most importantly, chemical composition-this last offering a method to induce particular microstructural evolutions. While the model assumes isotropic grain boundaries but consequences to anisotropy are also discussed. The paper presents examples of the role of dopants on interfacial energies, how this is quantitatively related to their segregation at the interfaces, and the impact in sintering and nanostability. Given the importance of interface energetics to these phenomena, we also present a short review on the current methods used to obtain reliable interface thermodynamic data.

Retaining large surface areas in alumina powders during high-temperature annealing is a major challenge in applications as catalyst supports and ceramic precursors. This is because the alumina surface area drastically decreases with transformation from the γ modification (defect spinel structure) into the α modification (corundum structure). The objective of this work is to show the thermodynamic basis of using additives, such as Zr and Mg, to control the γ-Al2O3 surface and bulk energetics and to manipulate the transformation temperature and surface area. These additives are observed to change the pattern of phase transformation and densification. Direct measurements of heats of solution in a lead borate melt of pure and doped alumina as a function of surface area enabled us to experimentally derive trends in the surface energies of hydroxylated surfaces. Accounting for heats of water adsorption measured on pure and doped alumina surfaces allowed us to delineate the thermodynamic effects of hydration on surface energies. Zr-doped γ-alumina showed a higher energy of the hydroxylated surface than did pure γ-alumina but showed a lower energy of the anhydrous surface. Mg addition does not change surface energies significantly but decreases the energetic instability of the bulk γ phase.