Sensing mechanism of high temperature hydrogen sulphide sensor based on sodium beta-alumina

A hydrogen sulphide galvanic probe for reducing gas mixtures (e.g., coal ga sification) has been developed. Sodium beta-alumina was selected as the ele ctrolyte. The reference consists of a sodium ferrite and ferric oxide powde r mixture open to the air. The equilibrium thus generated fixes a constant sodium reference activity. Both measuring and reference electrodes consist of Pt layers. Conditioning of the as-fabricated probes was performed by exp osure to a H2S/H-2 gas mixture at the operating temperature. The performanc e of the probes was evaluated in synthetic gas mixtures containing well def ined H2S/H-2 partial pressures as well as in an actual coal gasifier. In th is paper a possible sensor reaction mechanism is discussed in detail using (electro)chemical as well as thermodynamic considerations. The results are in agreement with the hypothesis that oxygen ions on Na2O sites in the cond uction planes at the surface of the electrolyte were in situ catalytically exchanged by sulphur ions. The activity of Na corresponds to NaxS within be ta-alumina rather than to pure NaxS. At the measuring electrode, sulphur pa rtial pressures are then automatically converted to sodium activities witho ut the need for a macroscopic auxiliary layer. At higher temperatures (e.g. , 860 K), it is postulated that Na2S is incorporated within the beta-alumin a structure, while at lower temperatures (e.g., 593 K), NaS appears to be t he preferred product in the ion-exchange reaction. (C) 1999 Elsevier Scienc e S.A. All rights reserved.