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), La-2(CO3)(3), and La2O3 have been obtaine d after various stages of thermal decomposition in the presence and ab sence of O-2. In situ DRIFTS provided information about the surface ch emistry taking place during the adsorption of NO and CO2 on the La oxi de 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 (n oncoordinated) nitrate groups, i.e., those not directly coordinated to a La cation. However, when the decomposition was performed under flow ing O-2, LaONO3 was the principal compound. NO adsorption on the oxide surface after decomposition enhanced the peak intensity of residual n itrate surface species-no new peaks appeared. La-2(CO3)(3) was more st able and was only partly transformed into La2O2CO3 during thermal trea tment at 773 K. The commercial La2O3 samples contained mainly hydroxid e if exposed to, or stored under, ambient air. The La(OH)3 decomposes when heated to 773 K, but rehydroxylation occurs rapidly if the sample s are exposed to ambient air. Both the temperature and the gaseous med ium of the calcination pretreatment determine the final state of the m aterial. High temperatures (>1173 K) and a CO2-free medium must be use d to guarantee transformation into principally La2O3, which still cont ains variable amounts of surface or subsurface carbonate groups. Howev er, these high calcination temperatures result in considerable loss of surface area. Bidentate and unidentate carbonates were formed on La2O 3 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 an d form nitrate and nitrite groups, N-2 and CO2. These results provide comprehensive references for the preparation and characterization of c atalytic La2O3 surfaces.