Surface composition and electrical conductivity of La2CuO4 and CuO composite on exposure to ppm level H2S

A composite material containing La2CuO4 and CuO was directly synthesized by thermal decomposition of a heteronuclear CuLa(dhbaen)(NO3) nH(2)O complex, where dhbaen is N,N'-bis (3-hydroxysalicylidene)-ethylenediamine. The mean particle size was of the order of 200 nm for a sample decomposing at 800 degreesC. The exposure to 7.8 ppm H2S at 50 degreesC for 6 d (the exposure gas was alternately changed from air to 7.8 ppm H2S at 6 h intervals for 6 d) resulted in the formation of copper sulfide and lanthanum sulfate on the surface. At 100 degreesC, the surfaces were mostly covered with both sulfide and sulfate. Lanthanum sulfate and copper sulfide, without copper sulfate, were formed at temperatures below 200 degreesC, Copper sulfate was detected when the sample was exposed at 250 degreesC. For samples exposed at 100 degreesC, the atomic ratio determined by X-ray photoelectron spectroscopy (XPS) for [S (sulfate)]/[La] was 1.6 on the surface and decreased to 0.01 with increasing depth of Ar sputtering on the surface. The ratio of [S (sulfide]/[Cu] was 0.99 on the surface and decreased to about 0.45 upon Ar sputtering, However, [S(sulfate)]/[La] and [S(sulfide)]/[Cu] ratios on the surface were 0.03 and 0.39, respectively, for single-phase La2CuO4 prepared by the solid state reaction method after the exposure. It is concluded that the reactions of Cu and La in La2CuO4 with hydrogen sulfide are accelerated by the presence of well-dispersed CuO.