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3.4.1 Proton Defects in Oxide Ceramics
ОглавлениеIn an environment containing hydrogen or water, protons are dissolved in the oxide lattice forming positively charged defects following Eq. (3.6) written in Kröger–Vink notation [78]. is a positively charge (2+) oxygen vacancy, represents an oxygen atom with a neutral charge placed on its original place in the crystal lattice, and is a proton defect (+1 charged).
Table 3.1 Hydrogen‐selective membrane types [70, 71].
Source: Adapted from Kluiters [70] and Al‐Mufachi et al. [71].
Dense polymer | Microporous ceramic | Dense metallic | Porous carbon | Dense ceramic | |
---|---|---|---|---|---|
Temperature range (°C) | <100 | 200–600 | 300–600 | 500–900 | 600–900 |
H2 selectivity | Low | 5–139 | >1000 | 4–20 | >1000 |
H2 flux (×10−3 mol m−2 s−1) DP = 100 kPa | Low | 60–300 | 60–300 | 10–200 | 6–80 |
Stability issues | Swelling, compaction, mechanical strength | Stability in H2O | Phase transition | Brittle, oxidizing | Stability in CO2 |
Poisoning issues | HCl, SO2, CO2 | H2S, HCl, CO | Strong adsorbing vapors, organics | H2S | |
Materials | Polymers | Silica, alumina, zirconia, titania, zeolites | Pd alloy | Carbon | Proton conducting ceramics (mainly SrCeO3, BaCeO3) |
Transport mechanism | Solution/diffusion | Molecular sieving | Solution/diffusion | Surface diffusion; molecular sieving | Solution/diffusion (proton conduction) |
Development status | Commercial by air products, Linde, BOC, and Air Liquide | Prototype tubular silica membranes available up to 90 cm. Other materials only small samples (cm2) | Commercial by Johnson Matthey; prototype membrane tubes available up to 60 cm | Small membrane modules commercial, mostly small samples (cm2) available for testing | Small samples available for testing |
The equilibrium constant of proton defect formation reaction in oxide ceramic materials (KOH·) is depicted in Eq. (3.7), where represents the proton defect concentration, is the oxygen vacancy concentration, represents the concentration of oxygen atoms with a neutral charge placed on its original place in the crystal lattice, and pH2O is the water vapor pressure.
(3.7)
For large bandgap oxide materials (e.g. Ce, Ti, and Zr), the formation of proton defects at moderate temperatures takes places through the dissociative absorption of water [80]. Water dissociates into a hydroxide ion and a proton, the hydrogen ion then occupies an oxide ion vacancy, and the proton forms a covalent bond with a lattice oxygen. The formation of proton defects implies a significant weight gain; hence, the concentration of such defects can be measured by thermogravimetric analysis (TGA) as a function of temperature and water partial pressure.