Destructive Adsorption of CCl_4 over Lanthanum-Based Solids:Linking Activity to Acid-Base Properties

摘要

The relative activities of a low-surface crystalline and high-surface amorphous LaOCl,further denoted as S1 and S2,have been compared for the destructive adsorption of CC1_4.It was found that the intrinsic activity of S2 is higher than that of S1.Both samples were characterized with X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),N_2-physisorption,and Raman and infrared (IR) spectroscopy.IR was used in combination with CO_2,CO,and methanol as probe molecules.The CO_2 experiments showed that different carbonate species are formed on both materials.For S1,a high surface concentration of bidentate carbonate species and a lower concentration of monodentate carbonate were observed.In the case of S2,bulk carbonates were present together with bridged carbonates.CO adsorption shows that S2 and S1 have very similar Lewis acid sites.However,methanol adsorption experiments showed that S2 had a higher number of stronger Lewis acid sites than S1 and that twofold coordinated methoxy species were more strongly bound than threefold coordinated methoxy species.Because of the analogy between methanol dissociation and the removal of the first chlorine atom in the destructive adsorption of CCl_4,the sites enabling twofold coordination were likely to be the same Lewis acid sites actively involved in the destructive adsorption of CCl_4.La_2O_3 was less active than the two LaOCl materials,and therefore,the intrinsic activity of the catalyst increases as the strength of the Lewis acid sites increases.S2 contains more chlorine at the surface than S1,which is expressed by the higher number of sites enabling twofold coordination.Moreover,this explains the difference in destructive adsorption capacity for CCLt that was observed for the samples S1 and S2.Since LaCl_3,being the most acidic phase,is not active for the destructive adsorption of CC1_4,basic oxygen atoms,however,remain needed to stabilize the reaction intermediate CC1_3 as La-O-CCl_3.