The effects of SO2 on the selective catalytic reduction of NO by NH3 over a Ce/TiO2 catalyst were studied. Conversion of NO remained above 90% in the presence of 100 ppm SO2 at 350 °C for 48 h. However, when 180 ppm SO2 was added at 300 °C, NO conversion only remained above 90% during the first 12 h and then gradually decreased with time. Characterizations of fresh and SO2-poisoned Ce/TiO2 catalysts were carried out using Brunauer−Emmett−Teller method, ion chromatography, X-ray photoelectron spectroscopy, and X-ray diffraction. The analytical results indicate that there was no obvious change in the crystal structure of the different samples; however, the specific area decreased with SO2 poisoning time. Sulfates were formed and preferentially diffused from the surface to a bulk phase during the poisoning process. Temperature-programmed desorption and diffuse reflectance infrared Fourier-transform spectroscopy results show that in the presence of O2, SO2 could react with NH3 to form NH4HSO4, which is deposited on the surface of the catalyst and blocked the active sites. Moreover, the main reason for the deactivation is that SO2 could react with the catalyst to form high thermally stable Ce(SO4)2 and Ce2(SO4)3, resulting in the disruption of the redox properties between Ce(IV) and Ce(III) and the inhibition of the formation and adsorption of nitrate species.

Samples of cerium supported on titania with different Ce loadings have been prepared by an impregnation method and tested for the selective catalytic reduction of NO by NH 3 in the presence of excess oxygen. The catalysts with 5% Ce and above had high activity in the temperature range 275-400°C at a space velocity of 50,000 h À1 . All the catalysts showed an excellent selectivity to N 2 and high tolerance to SO 2 and H 2 O under our test conditions.

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