Thermodynamic activities, titanium

Acetic acid and other organic acids are thermodynamically unstable at sedimentary conditions and will eventually decarboxylate to CO2 and alkanes (24). Experimental studies of acetic acid decarboxylation show that the rate is extremely sensitive to temperature and the types of catalytic surfaces available (Table II 25-261. Extrapolated rate constants for acetic acid decarboxylation at 100 C differ by more than 14 orders of magnitude between experiments conducted in stainless steel and catalytically less active titanium (Table II 26). Inherent (uncatalyzed) decarboxylation rates are similar for acetic acid and acetate (26). However, in catalytic environments their rates of decarboxylation differ markedly (25-261. and therefore a pronounced pH effect on total decarboxylation rate is observed. 

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

Vre] Vrestal, J., Bechny, L., Cochnar, Z., Petrus, J., Pokama, A., Titanium Influence on the Thermodynamic Activity of Carbon in Iron (in Czech), Kovove Mater., 21(3), 217-222 (1983) (Experimental, Thermodyn., 7)... 

A theoretical analysis of combustion synthesis of refractory nitrides was presented by Munir and Holt in 1987.37 They predicted the existence of an activation energy due to chemical reaction or mass-transport. Glassman et al. in 1987,38 in their thermodynamic analysis of TiN formation, examined the possibility of creating TiN by a self-sustained reaction of the metal particles and nitrogen gas in a rocket motor. They reported that for the stoichiometric ratio of 0.5 mole N2/mole titanium, the reaction has... 

Carbodiimide Polymers An optically active carbodiimide, (/ )-152 ([a]365 +7.6°), gives a polymer by polymerization using a titanium (IV) isopropoxide catalyst (Scheme 11.9) [203], The polymer showed optical activity essentially identical to the monomer however, on heating, the polymer indicated mutarotation and specific rotation reached a plateau value of [a]363 -157.5°, which is considered to be based on excess helical sense of the main chain. The mutarotation has been ascribed to a conformational transition from a kinetically controlled one to a thermodynamically controlled one. Excess single-handed helical conformation can be induced for polyfdi-/ -hexyl carbodiimide) by protonating the polymer with chiral camphorsul-fonic acid. 

A very high stereoselectivity was observed in the reduction of 4-tert-butylcyclohexanone to the m-alcohol (> 95%), which is the industrially relevant product. The observed high selectivity to the thermodynamically unfavorable cis-alcohol was explained by a restricted transition-state for the formation of the trans-alcohol within the pores of the zeolites (Scheme 5). This reaction was found not only to be catalysed by Al-Beta, van der Waal et al. reported the catalytic activity of aluminum-free zeolite titanium beta (Ti-Beta) in the same reaction.74 Again, a very high selectivity to the cis-alcohol was observed indicating similar steric restrictions on the mechanism. Kinetically restricted product distributions were also reported for the 2-,3- and 4-methylcyclohexanone the cis, trans- and ds-isomers being the major products, respectively. In this case the tetrahedrally coordinated Ti-atom was assumed to behave as the Lewis acid metal center. Recent quantum-chemical calculations on zeolite TS-1 and Ti-Beta confirm the higher Lewis acidic nature of the latter one.75... 

For solutions with unit activities of ions, the standard reduction potentials for these reactions are 0.337 Vsh and -1.630 Vsh. respectively The reduction potentials in our case are shifted in the active direction however because the activities of the metal ions in solution are less than unity. Because the reduction potential of titanium is more active (more negative) than that of copper, the reduction of copper ions by the titanium metal and the concurrent oxidation of titanium metal by the copper ions will be thermodynamically favored. 

Zeolite titanium beta has been tested in the liquid- and gas-phase Meerwein-Ponndorf-Verley reduction of cyclohexanones and the Oppenauer oxidation of cyclohexanols. A high selectivity towards the thermodynamically unfavourable cis-alcohol was observed, which has been ascribed to transition-state selectivity in the pores of the zeolite. Under gas-phase conditions the dehydration of alcohols to cycloalkenes is observed as a side reaction. The catalyst was found to be active even in the presence of water and ammonia. 

The observed differences in scale thickness and microstructure between the oxide scales and subsurface zones at the various oxidation temperatures seem to be mainly attributed to the different diffusion rates at the respective temperatures. Since the oxidation products formed do not show any differences in the temperature range of 800°C to 1000°C it is concluded that no significant effect of the thermodynamic stability on the composition and structure of the oxidation products occurs. From the calculations of Rahmel and Spencer [21] it is known, however, that the activity of A1 and Ti in the system Ti-Al varies depending on the temperature. Thus it has to be taken into account that the temperature may have an influence on the expansion of the phase fields of some important phases in the system Ti-Al-N-O. Nevertheless it is evidently the temperature which mainly influences the kinetics because the structure of the metal/oxide interface, the formation of titanium nitrides, A127039N and an aluminium depleted metal phase is on principle always very similar. In this way the effect of different temperatures can, to a certain degree, be interpreted as that of a shift in the different stages of the oxidation process. 

The kinetic and thermodynamic selectivity for reactions of a titanium-imido complex with different types of C-H bonds has been determined. Reactions with substrates that possess primary and secondary C-H bonds occur selectively at the primary C-H bond. In addition, reactions with mixtures of alkanes and arenes occur selectively at the arene C-H bond. Like the stabilities of most low-valent, late metal complexes, the primary alkyl complex is thermodynamically more stable than the secondary alkyl complex, and the aryl complexes are more stable than the alkyl complexes. Activation of olefins at the ally-lie position occurs more slowly than reaction at the vinyl position, but when it does occur, the reaction generates a stable Ti -allyl complex. 

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