Engineering Solutions for CO2 Conversion. Группа авторов

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Engineering Solutions for CO2 Conversion - Группа авторов


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of process intensification by shifting the thermodynamic equilibrium of a reaction [77–79].

      3.4.1 Proton Defects in Oxide Ceramics

      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 left-bracket normal upper O normal upper H Subscript normal upper O Superscript dot Baseline right-bracket represents the proton defect concentration,left-bracket upper V Subscript normal o Superscript dot dot Baseline right-bracket is the oxygen vacancy concentration, left-bracket normal upper O Subscript normal o Superscript x Baseline right-bracket 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.

      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.

      3.4.2 Proton Transport Membrane Fundamentals

      Understanding the mechanism of proton conduction is of utmost importance for the development of novel materials. It is generally accepted that proton diffusion in protonic conductors occur via the Grotthuss‐type mechanism assisted by water molecules [81–83]. Moreover, hydrogen separation is driven by the hydrogen partial pressure difference across the membrane.

      Proton conductivity has been observed in different types of materials. Perovskite‐type oxide ceramics are known to be proton conductors since the early 1980s. In general, perovskite structure with a general formula A2+B4+O3 (type II–IV), where A is Ba and B is Zr, Tb, Ce, or Th, exhibits the best proton conductivities being higher than 10−2 S cm−1, the lowest activation energies for proton transport, and high negative hydration enthalpies [78]. In particular, ceramic materials such SrCeO3, BaCeO3, or SrZrO3 are the most widely studied high‐temperature proton‐conducting perovskite‐type materials. Zirconate‐based materials are more interesting than cerates regarding their application in CO2 environments because of their higher stability under reducing atmospheres; however, they present an important grain boundary resistance and a high sintering


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