A computerized infrared spectroscopic study of surface reactions on selected lanthanide oxides

Abstract

The natures of adsorption sites on La(,2)O(,3), Nd(,2)O(,3), and selected praseodymium oxides were investigated by examining surface reactions of probe molecules using computerized transmission infrared (IR) spectroscopy on unsupported samples. Additionally, the rehydration/dehydration behavior and crystallographic phase transitions of these oxides were examined in pretreatment temperature experiments involving rehydration of the sesquioxides to hydroxides by water exposure. Following rehydration of La(,2)O(,3) to La(OH)(,3), the effect of increasing vacuum pretreatment temperature (350 to 1000(DEGREES)C) is to gradually remove surface hydroxyl and carbonate entities (up to 650(DEGREES)C), and increase the degree of A-type crystallinity. Increasing crystallinity causes a concomitant decrease in surface oxide basicity. The removal of hydroxyl and carbonate species, as well as increases in oxide basicity, strongly correlated to increases in certain catalytic activities. At 50(DEGREES)C, NH(,3) weakly coordinates in Ln('+3) sites. CO(,2), on the other hand, chemisorbs at this temperature onto surface O('-2) ions to generate unidentate carbonate species which transform into bidentate structures at 350(DEGREES)C. Exposure to an NH(,3)/CO(,2) mixture at 50(DEGREES)C results in formation of carbamate species which dehydrate at 250(DEGREES)C into cyanate entities that are coordinated in Ln('+3) sites through their oxygen atoms. The cyanate bonding rearranges to produce bidentate coordinated species at 500(DEGREES)C. These resist vacuum heating up to 800(DEGREES)C, at which temperature they polymerize...