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

Diffuse reflectance FTIR spectra, X-ray diffraction patterns, and BET surface areas of La(NO3)3, La2(CO3)3, and La2O3 have been obtained after various stages of thermal decomposition in the presence and absence of O2. In situ DRIFTS provided information about the surface chemistry taking place during the adsorption of NO and CO2 on the La oxide surfaces obtained. Decomposition of La(NO3)3 under flowing Ar at 773 K resulted in a mixture of La2O3 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 O2, LaONO3 was the principal compound. NO adsorption on the oxide surface after decomposition enhanced the peak intensity of residual nitrate surface speciesno new peaks appeared. La2(CO3)3 was more stable and was only partly transformed into La2O2CO3 during thermal treatment at 773 K. The commercial La2O3 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 CO2-free medium must be used to guarantee transformation into principally La2O3, 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 La2O3 by adsorbing CO2 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, N2 and CO2. These results provide comprehensive references for the preparation and characterization of catalytic La2O3 surfaces.