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

Abstract

The adsorption of CO(2) over a set of gallium (III) oxide polymorphs with different crystallographic phases (alpha, beta, and gamma) and surface areas (12-105 m(2) g(-1)) was studied by in situ infrared spectroscopy. On the bare surface of the activated gallias (i.e., partially dehydroxylated under O(2) and D(2) (H(2)) 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 CO(2) at 323 K, a variety of (bi)carbonate species emerged. The more basic hydroxyl groups were able to react with CO(2), to yield two types of bicarbonate species: mono- (m-) and bidentate (b-) [nu(as)(CO(3)) = 1630 cm(-1); nu(s)(CO(3)) = 1431 or 1455 cm(-1) (for m- or b-); delta(OH) = 1225 cm(-1)]. Together with the bicarbonate groups, IR bands assigned to carboxylate [nu(as)(CO(2)) = 1750 cm(-1); nu(s)(CO(2)) = 1170 cm(-1)], bridge carbonate [nu(as)(CO(3)) = 1680 cm(-1); nu(s)(CO(3)) = 1280 cm(-1)], bidentate carbonate [nu(as)(CO(3)) = 1587 cm(-1); nu(s)(CO(3)) = 1325 cm(-1)], and polydentate carbonate [nu(as)(CO(3)) = 1460 cm(-1); nu(s)(CO(3)) = 1406 cm(-1)] species developed, up to approximately 600 Torr of CO(2). 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 CO(2), measured by volumetric adsorption (323 K), was approximately 2.0 micromol 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 CO(2) (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 CO(2) is made.

The role of Pd–Ga bimetallic particles in the bifunctional mechanism of selective methanol synthesis via CO2 hydrogenation on a Pd/Ga2O3 catalyst

“…Briefly, after reduction in pure hydrogen at 523 K for 1 h, the sample wafers were evacuated at the same temperature for 30 min and cooled to RT under vacuum. Then pure CO 2 was flowed (100 cm 3 min À1 ) at room temperature for 15 min, producing (bi)carbonate species on the gallium oxide surface [9]. Next, the flow was changed from pure CO 2 to a H 2 /CO 2 = 3 (100 cm 3 min À1 ) mixture.…”

“…The adsorption of CO 2 over this set of gallium oxide polymorphs was also examined via infrared spectroscopy. Two types of bicarbonate species (monoand bidentate), carboxylate, bridge carbonate, bidentate carbonate, and polydentate carbonate species were identified [9]. Interestingly, most of these carbonates were reversibly adsorbed on the gallia surface.…”

Photocatalytic performance of α-, β-, and γ-Ga2O3 for the destruction of volatile aromatic pollutants in air

“…Among these polymorphs, α-Ga 2 O 3 is rhombohedral, β-Ga 2 O 3 is monoclinic, and γ -Ga 2 O 3 is cubic. These different crystal structures significantly influence the surface properties of Ga 2 O 3 [19][20][21], accounting for the variation in catalytic activity among the polymorphs [19].…”

A new ATR-IR microreactor to study electric field-driven processes

“…CO 2 is also of interest from a chemical point of view, since, in the presence of surface oxides it may lead to carbonate species, with characteristic absorption [54,55,56,57,58] at the frequencies were we observe phonon absorption lines (see above CO 2 in the gas phase is a linear molecule with zero dipole moment, on which an electric field will not be effective. The E-field expected to have a significant effect on adsorbed CO 2 (or CO) is of the order of 2 to 6·10 7 V/cm [59,60] or according to others, of the order of 1 to 3·10 8 V/cm [61,62].…”