Influence of Pretreatment on Lanthanum Nitrate, Carbonate, and Oxide Powders

摘要

Diffuse reflectance FTIR spectra, X-ray diffraction patterns, and BET surface areas of La(NO_3)_3, La_2(CO_3)_3, and La_2O_3 have been obtained after various stages of thermal decomposition in the presence and absence of O_2. In situ DRIFTS provided information about the surface chemistry taking place during the adsorption of NO and CO_2 on the La oxide surfaces obtained. Decomposition of La(NO_3)_3 under flowing Ar at 773 K resulted in a mixture of La_2O_3 and a nitrate phase with ionic (noncoordinated) nitrate groups, i.e., those not directly coordinated to a La cation. However, when the decomposition was performed under flowing O_2, LaONO_3 was the principal compound. NO adsorption on the oxide surface after decomposition enhanced the peak intensity of residual nitrate surface species-no new peaks appeared. La_2(CO_3)_3 was more stable and was only partly transformed into La_2O_2-CO_3 during thermal treatment at 773 K. The commercial La_2O_3 samples contained mainly hydroxide if exposed to, or stored under, ambient air. The La(OH)_3 decomposes when heated to 773 K, but rehydroxylation occurs rapidly if the samples are exposed to ambient air. Both the temperature and the gaseous medium of the calcination pretreatment determine the final state of the material. High temperatures ( > 1173 K) and a CO_2-free medium must be used to guarantee transformation into principally La_2O_3, which still contains variable amounts of surface or subsurface carbonate groups. However, these high calcination temperatures result in considerable loss of surface area. Bidentate and unidentate carbonates were formed on La_2O_3 by adsorbing CO_2 at either 298 or 773 K, thus revealing the surface sites have medium Lewis basicity. NO adsorption at 298 and 773 K leads to an exchange reaction with carbonate ions to disproportionate NO and form nitrate and nitrite groups, N_2 and CO_2. These results provide comprehensive references for the preparation and characterization of catalytic La_2O_3 surfaces.