Zinc oxide thermodynamics

Chaparro AM (2005) Thermodynamic analysis of the deposition of zinc oxide and chalco-genides from aqueous solutions. Chem Mater 17 4118-4124... 

Zinc oxide beds are limited to operation at temperatures below 430°C probably because of pore plugging during sulfur removal and sintering. Thermodynamics also favors lower temperatures. At the higher temperatures, the H2S cannot be reduced to levels low enough for shift catalyst or to reach fuel cell limits. 

The existence of the latter has been recognized for many years, since Wagner (20) applied his thermodynamic theory of defect oxides to the system zinc oxide-oxygen (21). According to this scheme, at sufficiently high temperature an equihbrium sets in between zinc oxide and oxygen in the gas phase, whereby excess zinc (Zn ) can be accommodated in interstitial positions of the lattice ... 

Bevan and Anderson (32) deduced that adsorbed oxygen controlled the resistance of sintered zinc oxide at temperatures between 500°C and 1000°C. This conclusion was based on the observations that (1) the oxygen pressure had a reversible controlling effect on the resistance down to 500°C, too low a temperature for thermodynamic equilibrium to be... 

Thus zinc oxide would clearly be (thermodynamically) unstable in contact with hot titanium, although the rate of such corrosion would be extremely slow, due to the extraordinarily tough oxide layer on zinc metal. Moreover, we see that the use of zinc as a reducing agent for Ti02 is quite out of the question, not only at 1000 K, but at any practicable temperature, because the titanium line remains below that of zinc, whatever the temperature. 

Dehydration of zinc hydroxide can then lead to formation of the thermodynamically more stable zinc oxide. 

Extrapolated thermodynamic measurements for the sulfur-poisoned Hu/Al2°3 catalyst show that, given a sufficiently long exposure time, feedstock sulfur levels of 1 ppb will be enough to reach this threshold of sulfur coverage. With sulfur contamination above this level (conventional zinc oxide guard beds typically reduce sulfur levels to 0,1 ppm)t steam/carbon ratios of greater than or equal to 3.D and/or more H2 arc needed to prevent CCD in the critical inlet portions of the catalyst bed. Virtually complete sulfur removal is required to avoid c t Ly t coking and deactivation ... 

Another form of surface coating is provided by galvanizing, the coating of an iron object with zinc. Because the latter s standard potential is —0.76 V, which is more negative than that of the iron couple, the corrosion of zinc is thermodynamically favored and the iron survives (the zinc survives because it is protected by a hydrated oxide layer). 

It may also be noted that there is some indeterminacy in the dependency, predicted by the Wagner thermodynamic approach, of conductivity on oxygen pressure. The predicted relationship depends on the model assumed. For example, for the classic case of zinc oxide, the relation is k Po2 if the zinc ions produced are assumed to be divalent and interstitial, as indicated by ZnO (lattice) = Zn++ (interstitial) + 2e + K O2 (g). The relation is k a Pot if the zinc ions are divalent and occupy normal lattice positions, or if they are monovalent and occupy either lattice or interstitial positions ... 

This evidence of reversibility in the acrolein-ethyl alcohol reaction at a temperature (396°, 1 atm.) where both allyl alcohol and ethyl alcohol are thermodynamically unstable with respect to the aldehydes and hydrogen indicated that the hydrogen transfer reaction was catalyzed by surfaces which were inactive for hydrogenation-dehydrogenation reactions. In order to explore the activity of magnesia and zinc oxide for hydrogenation, a number of these catalysts were tested for the direct hydrogenation of acrolein. 

The crystallinity of the formed particle is, in any case, determined by thermodynamic and kinetic parameters. Leite and coworkers [252,253] recently demonstrated that well-crystallized Sn02 nanocrystals could be produced at room temperature with no hydrothermal treatment. This process is based on the hydrolysis of SnCh in an ethanol solution, followed by dialysis to remove the Cl ions. The result of this dialysis is a transparent colloidal suspension formed by near-spheric particles, as illustrated in Fig. 9 later. Zinc Oxide (ZnO) nanocrystals have also been synthesized at room temperature. The process developed by Bahnemann et al. [67] consists of hydrolyzing zinc acetate dihydrate dissolved in 2-propanol by the addition of NaOH... 

The experimental program described in this paper confirmed the concept of the process and the thermodynamic analysis. That is, the sulfur in the feed materials can be fixed as a Fe-S-O matte using metallic iron or iron ore while both zinc oxide and sulfide materials can be simultaneously smelted in a single reaction medium. The combustion of carbon with oxygrai inside the matte has been proved to be feasible. 

At open circuit the interface with the silver electrode is In thermodynamic equilibrium. The interface with the zinc electrode has a mixed potential, which is defined by adding the zinc oxidation and proton reduction currents, with the latter reaction characterised as being very slow at the zinc electrode. The polarities of the electrodes in open-circuit conditions are defined by the experimentally measured potentials of each electrode vs a saturated calomel reference electrode. 

---