Infrared Spectroscopic Study of Carbon Dioxide Adsorption on the Surface of Ga2O3 Polymorphs

Abstract

The adsorption of CO2 over a set of gallium (III) oxide polymorphs with different crystallographic phases (alpha, beta, and gamma) and surface areas (12-105 m2 g-1) was studied by in situ infrared spectroscopy. On the bare surface of the activated gallias (i.e., partially dehydroxylated under O2 and D2 (H2) at 723 K), several IR signals of the O-D (O-H) stretching mode were assigned to mono-, di- and tricoordinated OD (OH) groups bonded to gallium cations in tetrahedral and/or octahedral positions. After exposing the surface of the polymorphs to CO2 at 323 K, a variety of (bi)carbonate species emerged. The more basic hydroxyl groups were able to react with CO2, to yield two types of bicarbonate species: mono- (m-) and bidentate (b-) [îas- (CO3) ) 1630 cm-1; îs(CO3) ) 1431 or 1455 cm-1 (for m- or b-); ä(OH) ) 1225 cm-1]. Together with the bicarbonate groups, IR bands assigned to carboxylate [îas(CO2) ) 1750 cm-1; îs(CO2) ) 1170 cm-1], bridge carbonate [îas(CO3) ) 1680 cm-1; îs(CO3) ) 1280 cm-1], bidentate carbonate [îas(CO3) ) 1587 cm-1; îs- (CO3) ) 1325 cm-1], and polydentate carbonate [îas(CO3) ) 1460 cm-1; îs(CO3) ) 1406 cm-1] species developed, up to approximately 600 Torr of CO2. However, only the bi- and polydentate carbonate groups still remained on the surface upon outgassing the samples at 323 K. The total amount of adsorbed CO2, measured by volumetric adsorption (323 K), was aprox 2.0 ímol m-2 over any of the polymorphs, congruent with an integrated absorbance of (bi)carbonate species proportional to the surface area of the materials. Upon heating under flowing CO2 (760 Torr), most of the (bi)carbonate species vanished a T > 550 K, but polydentate groups remained on the surface up to the highest temperature used (723 K). A thorough discussion of the more probable surface sites involved in the adsorption of CO2 is made.