Phase diagrams kinetic pathways

The enormous variety of possible surface reactions still reveals many open questions regarding exact reaction pathways, kinetics, and/or structural iirformation. The seemingly simple water formation reaction from H2 and O2 over a Pt-catalyst already shows many different reaction pathways. Regarding this reaction, we have performed periodic DFT calculations and thermodynamic considerations to evaluate the corresponding (a,7 ,A(zi)-surface phase diagram for Pt(lll) on contact with an aqueous electrolyte. Our work determined that below 0.95 V no stable and ordered oxygen overlayer forms, and that between 0.95 and 1.20 V the... 

Microporous framework solids are synthesised via solvent-mediated crystallisations from mixtures of reactive precursors. The reaction pathway is controlled by kinetic as well as thermodynamic considerations so that equilibrium phase diagrams, so relevant in the high-temperature preparation of ceramics, are not useful here. Rather, synthetic routes have been developed empirically via a major synthetic effort that continues today. The continuing industrial and academic interest in these materials provides a powerful incentive to understand the principles underlying their formation through the processes of gel formation and evolution, nucleation and crystal growth. 

The previous section highlighted the importance of the kinetic aspects since the development of co-crystals may be in competition with the crystallization of single component crystals. Knowledge of the start and end points in the phase diagram of a co-crystallization process is not sufficient in order to operate and optimize a co-crystallization pathway. Information about the kinetic pathway followed in the phase diagram during the operation remains essential. 

Undersaturated clear solutions of CBZ and NCT were obtained at 45 °C, and then cooled quickly to 25 °C. An isothermal plateau was then applied until equilibrium was reached. Crystal nucleation and growth occurred only during this isothermal step. Figure 9.5 displays three kinetic pathways in the phase diagram from representative experiments started in three domains where two solid phases may develop. 

The lesson to be taken from this study is that the development of a cocrystallization process first requires knowledge of a large part of the phase diagram with temperature and second that the possible kinetic pathways which may occur should be evaluated. Considerations based on thermodynamic... 

Experiments were performed with the CBZ/NCT/ethanol system in domains of the phase diagram that were always undersaturated with respect to NCT (see Figure 9.9). Crystallizations were started in domains 2a or 3. Once the equilibrium was reached at 25 °C, a known amount of NCT in dry form was added in order to shift the overall composition of the slurry in domain 4a in the phase diagram. The previously stable CBZ solid phase became metastable. Henceforth the stable phase was the CBZ/NCT co-crystal. A SMPT started with the primary nucleation of the co-crystals if they were absent in the slurry and continued with the dissolution of the CBZ crystals and with co-crystal growth, until the new equilibrium was reached. Three kinetic pathways are displayed in Figure 9.9. 

An-Chang Shi is a professor of physics at the McMaster Univereity, Hamilton, Ontario, Canada. He received his BSc in physics from Fudan Univereity in 1982 and obtained his PhD in physics from Univereity of Illinois at Urbana-Champaign in 1988. From 1988 to 1892 he was a Post-Doctoral Fellow and Research Associate at McMaster University. He joined Xerox Research Centre of Canada as a Member of Research Staff in 1992, and moved to the Department of Physics and Astronomy at McMaster Univereity in 1999. His main research area concerns the theory of phases and phase transitions in block copolymers. Over the last years, An-Chang Shi has worked on a number of problems in theoretical polymer physics, including the development of statistical mechanics theory for polymeric s) tem the investigation of phase diagrams of block copolymer melts, blends and solutions as well as block copolymers tmder confinement and the study of kinetic pathways of order-order transitions of block copolymer phases. 

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