from first principles PP-I-1

Effect of Dopants upon the Acidic Properties of OxidesRudakova A.V.V.A. Fock Institute of Physics, St. Petersburg State University,St. Petersburg, 198504, Russiaarudakova@mail.ruPP-III-94Catalysts modified by various acidic dopants exhibit high activity and selectivity and arewidely used nowadays. To understand the mechanism of the promoting action is of a greatinterest for the creation of new catalytic systems.Co-adsorption of acids with basic test molecules leads to mutual enhancement of adsorption.This can be demonstrated, when in the presence of SO 2 , NO 2 , or H 2 S, protonation of suchbases as NH 3 , pyridine or 2,5-dimethylpyridine (DMP) occurs on silanol groups that nevermanifest any Brønsted acidity at normal conditions [1]. The effect suggests a new explanationof the promoting action of the above gases in the reactions catalyzed by Brønsted sites [2]. Onthe other hand, adsorption of bases increases the basicity of oxygen atoms, revealed by thespectra of adsorbed fluoroform [3].Lewis acidity of oxides can also be influenced by adsorbed acidic molecules. This effect isillustrated by IR spectroscopic study of low-temperature CO adsorption on the modified CaOsurface. The most high-frequency bands of CO adsorbed on CaO pre-exposed to CO 2 , CCl 4 ,SO 2 , SO 3 move from about 2157 up to almost 2190 cm -1 [4]. The dopants exist here in the formof surface CO 2- 3 , Cl - , SO 2- 3 , or SO 2- 4 ions, and one can consider such surface as a layer ofcorresponding salts, which exhibit higher electron accepting ability of surface cations, as2-compared with oxides. This effect is consistent with superacidity of oxides doped with SO 4and explains the extremely high Lewis acidity of cationic sites in zeolites as compared withthat of oxides of the same elements. The increase of catalytic activity of modified catalysts isillustrated by the reaction of 2-propanol decomposition over pure and sulfated CaO surfaces.References:[1] A.A. Tsyganenko, E.N. Storozheva, O.V. Manoilova, T. Lesage, M. Daturi, J.-C. Lavalley, Catal.Lett. 70 (2000) 159.[2] K. Otsuka, Y. Takizawa, A. Morikawa, J. Chem. Soc. Faraday Trans. 1 77 (1981) 81.[3] E.N. Storozheva, V.N. Sekushin, A.A. Tsyganenko, Catal. Lett. 107 (2006) 185.[4] A.A.Tsyganenko, K.V.Voronina, A.V. Rudakova. To be published.253