Mossbauer study of the impact of hydrogen fluoride on tin probe ions located on the surface of Cr2O3 microcrystals

Room-temperature exposure of tin-doped Cr2O3 surfaces to HF induces in the Sn-119 Mossbauer spectra changes that depend on the valence state of the tin probe and show differences between the Sn(II)- and Sn(IV)-containing Cr2O3 crystallite surfaces. In the Sn(II)-doped samples (Sn(II)/Cr2O3), the Sn and Cr surface cations are coordinatively unsaturated, and upon exposure to HF chemical corrosion occurs. Preliminary contact of Sn(II)/Cr2O3 with oxygen results in oxidation of Sn(II) to Sn(IV) and concomitant saturation of the coordination sphere of the surface cations; the apparent activity of HF then drastically decreases (passivation effect). Similar phenomena were previously observed for Sn(II)/ and Sn(IV)/Cr2O3 samples exposed to HCl or H2S, i.e., acid gases containing anions much larger than F-. Thus, the behavior of the dopant cations in the presence of acid gases should not be imputed to size incompatibility between oxygen vacancies and foreign anions coming onto the surface from its gaseous surrounding. (C) 2001 Elsevier Science.