Kinetics of SO2 Adsorption on Photoexcited α-Fe2O3

SO 2 is an atmospheric pollutant whose oxidation leads to acid rain, while α-Fe 2 O 3 (hematite, a naturally occurring component of atmospheric aerosol particles) has a charge-transfer band gap (about 2.2 eV) smaller than the cutoff of solar radiation in the troposphere (about 4.3 eV) and is thus able to participate in photochemical reactions. The interaction of SO 2 with UV-irradiated, single-crystal α-Fe 2 O 3 was examined by using UPS, XPS, and Auger spectroscopies. Between 261 and 331 K, SO 2 adsorbs on the α-Fe 2 O 3 (0001) surface with a very low sticking coefficient in the absence of UV irradiation. The adsorbed species resembles SO 4 2-; its heat of adsorption is estimated to be 2.4 eV. UV irradiation of the α-Fe 2 O 3 substrate during SO 2 exposure leads to a significantly increased rate of adsorption. All of the cations in stoichiometric α-Fe 2 O 3 are Fe 3+; UV irradiation produces Fe 2+ cations through the creation of electron−hole pairs. The Fe 2+ sites are found to be much more reactive to SO 2 adsorption than are Fe 3+ sites. A model is proposed in which SO 2 adsorbs only at Fe 2+ sites. According to this model, the small number of adsorption sites which are present in thermal equilibrium are greatly enhanced by UV irradiation, leading to an increased rate of SO 2 adsorption.