Ammonia synthesis, proton-conducting

Proton conductors ammonia synthesis, 468 conductivity, 93 ethylene oxidation, 470 hydrogen oxidation, 457 list of electrochemically promoted reactions, 146... 

Basically, the solid-state membrane materials are classified by type of ions conducted. So far, there are two types of solid-state membrane that have been used for ammonia synthesis, proton and oxide ion conducting membranes. In those membrane reactors, the operating principles are slightly different, as depicted in Figure 18.2. 

Formally, ammonia synthesis is closely related to Fischer-Tropsch synthesis. Industrial operation involves the use of an iron catalyst promoted with calcium and potassium oxides. However, the reason we consider this process here is not directly in connection with alkali promotion of the catalyst. We are concerned with a remarkable achievement reported by Yiokari et al. [15], who use a ton-conducting electrolyte to achieve electrochemical promotion of a fully promoted ammonia synthesis catalyst operated at elevated pressure. Specifically, they make use of a fully promoted industrial catalyst that was interfaced with the proton conductor CaIno.iZro.903-a operated at 700K and 50 bar in a multipellet configuration. It was shown that under EP the catalytic rate could be increased by a factor of 13 when... 

Wang, B.H., Wang, J.D., Liu, R., Yie, Y.H., and Li, Z.J. 2007. Synthesis of ammonia from natural gas at atmospheric pressure with doped ceria -Ca/PO lj-KjPO composite electrolyte and its proton conductivity at intermediate temperature. Journal of Solid State Electrochemistry 11, 27-31. 

Zhang F, Yang Q, Pan B, Xu R, Wang H and Ma G (2007), Proton conduction in Lao,9Sro,iGao,8Mgo,203 ceramic prepared via microemulsion method and its application in ammonia synthesis at atmospheric pressure , Materials Lett, 61, 4144 148. 

As mentioned before, high ionic conductivity is an important requirement for electrolyte materials to be used for ammonia synthesis devices. The results from many experiments confirm that proton conductors with adequate ionic conductivity can be used as electrolytes for a membrane reactor. Some examples of materials investigated as a membrane reactor are described below. 

It is well known that, in general, BaCeOs oxides have higher protonic conductivity than SrCe03-based oxides. The increase of protonic conductivity in membrane material would be beneficial. A large number of cells with a barium cerate membrane were used in electrocatalytic ammonia synthesis. The results showed that there is no significant improvement between cells with SrCe03-based or BaCeOa.based oxides. It can be concluded that materials with protonic conductivity of 10 —10 S/cm would be adequate for ammonia synthesis. 

Typical operating temperatures of electrocatalytic ammonia synthesis devices via a ceU with a proton-conducting solid oxide electrolyte are around 400—700 °C corresponding to the working temperature of the proton conductor electrolyte. A volcano-shape dependence of ammonia formation rates on temperature is observed in most cases. The formation rate of ammonia increases with temperature until a maximum is reached due to the increase of the proton conductivity. When the temperature is further increased, the ammonia formalion rate then turns downhill, because the decomposition of ammonia becomes prominent. In addition, the water loss from the electrolyte membrane due to high temperatures may decrease proton conductivity of the electrolyte. 

Solid-state proton conductors such as perovskites, pyrochlores, fluorites, and polymers have been reviewed and discussed in detail, with particular emphasis on their application in ammonia synthesis. Ammonia synthesis using an oxide-ion electrolyte has also been presented. The possibility of ammonia synthesis from N2 and steam instead of molecular H2 using either proton or oxide-ion conducting electrolytes may offer many advantages. 

Chen, C., Ma, G. (2009). Proton conduction in BaCei.xGdx03.a at intermediate temperature and its application to synthesis of ammonia at atmospheric pressure. Journal of Alloys and Compounds, 485, 69—72. 

Litzelman SJ, Rothschild A, TuUer HL (2005) The electrical properties and stability of SrTi, jFe, jjOj 5 thin films for automotive oxygen sensor applications. Sens Actuators B 108 231-237 Liu P, Lee S-H, Tracy CE, Turner JA, Pitts JR, Deb SK (2003) Electrochromic and chemochromic performance of mesoporous thin-film vanadium oxide. Solid State Ionics 165 223-228 Liu RQ, Xie YH, Wang JD, Li ZJ, Wang BH (2006) Synthesis of ammonia at atmospheric pressure with Ce M Oj-delta (M=La, Y, Gd, Sm) and their proton conduction at intermediate temperature. Solid State Ionics 177 73-76 Logothetis EM (1980) Resistive-type exhaust gas sensors. Ceram Eng Sd Proc 2 281-301... 

Wang ID, Xie YH, Zhang ZF, Liu RQ, Li ZJ (2005) Protonic conduction in Ca -doped La M O (M = Ce, Zr) with its application to ammonia synthesis electrochemically. Mater Res Bull 40 1294-1302 Wang DY, Symons WT, Farhat RJ, Valdes CA, Briggs EM, Polikarpus KK, Kupe J (2006a) Ammonia gas sensors. US Patent 7,074,319 B2... 

The pressure of gas was 101.325 kPa. The ammonia synthesis rate was stable after passage of current for 2-6 min and this rate was at least three magnitudes higher than that of conventional catalytic reactor. The conversion of hydrogen was close to 100% which eliminated thermodynamic equilibrium limitation. The main problem of this method is that the conductivity of SCY ceramic is very poor at normal temperature. Even at 570°C, its conductivity is unsatisfied because the current density was smaller than 2 mA-cm and could not be further increased. This limited the efficiency of ammonia synthesis and theoretical research. The use of solid electrolyte with high proton conductivity at low temperature to replace the SCY ceramic may be favorable to decreasing the synthesis temperature and increasing the current density and production efficiency. 

Table 6.6 Ammonia synthesis in proton-conducting membrane reactors... 

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