Mossbauer spectroscopic study of the state of impurity Sn-119(II) ions at the surface of Cr2O3 grains upon adsorption of HCl

As probed by Sn-119 Mossbauer spectroscopy, impurity Sn(II) atoms located a t the surface of chromium(III) oxide grains react readily with gaseous HCl at room temperature. The interaction leads to the complete vanishing of spi n polarization of the Sn(II) electron shells and to an abrupt increase in t he mean-square amplitudes of Sn(II) thermal vibrations. Two types of the su rface [SnCl3](-) species are identified: One type of species contains a bri dging chloride ion, and the other type has only terminal chloride ions. Stu dying the thermal stability of these species indicates that the process is reversible: Annealing the Sn(II)/Cr2O3 samples after their contact with HCl in a hydrogen flow at temperatures above 500 degrees C completely restores the original local structure of the impurity sites. Heating the samples at lower temperatures (300-400 degrees C) restores the original local environ ment only of a fraction of the probe atoms, whereas the remaining tin atoms have a mixed oxygen-chloride environment.