IR spectroscopic detection of Lewis acid sites on alumina using adsorbed carbon monoxide. Correlation with aluminum-hydroxyl group removal

Abstract

CO adsorption at low temperatures has been used to probe Al3+ Lewis acid sites created upon dehydroxylation of AI2O3, using transmission infrared spectroscopy. AI2O3 was dehydroxylated in the temperature range from 475 to 1200 K. There is an approximate linear correlation between the decreasing integrated absorbance of the Al-OH IR features and the increasing integrated absorbance of the adsorbed CO features throughout the full coverage range of hydroxyl groups. Two CO adsorption sites have been identified: the first (vco = 2195 cm'1) appears immediately upon mild dehydroxylation, while a second site (vco = 2213 cm-1) appears after an 800 K dehydroxylation treatment. CO adsorption on the Lewis acid sites formed by dehydroxylation of AI2O3 involves a binding energy of about 21 kJ/mol.

Heterogeneous oxidation of carbonyl sulfide on mineral oxides

“…The in situ DRIFTS spectra for SiO 2 at 298 K are shown in Figure 2 when exposed to 500 ppm OCS in air with 100 mL·min −1 . The consumption of surface hydroxyl at 3709 cm −1 [16,17] [18,19] . However, it is incredible for surface sulfate species at 1371 cm −1 [20][21][22] because of the low signal to noise ratio due to the strong absorbance of SiO 2 itself in the range of 1300-1400 cm −1 .…”

Surface hydration states of commercial high purity α-Al2O3powders evaluated by temperature programmed desorption mass spectrometry and diffuse reflectance infrared Fourier transform spectroscopy

“…Further heating causes the removal of the isolated hydroxyl groups, and two new bands could be resolved at 3798 and 3711 cm K1 after heating to 727 8C. Upon heating to 927 8C, no hydroxyl groups can be detected by infrared spectroscopy [25]. There are two high-frequency sharp bands, clearly ascribable to free surface hydroxyl groups, and a third band at 3691-3693 cm K1 which is also assigned to a free or nearly free OH species in view of the high frequency, moderate width, thermal resistance and the presence of a free OH species at nearly the same frequency on all transition aluminas [24].…”

FTIR study of CO adsorption on molybdena-alumina catalysts for surface characterization

“…The CO adsorption feature at 2237-2239 cm -1 is assigned to σ-bonded CO to tetrahedral Al 3+ ions with a slightly rise throughout the thermal behaviour of Al 2 O 3 due to the increase in the C-O stretching frequency above the free CO molecule (2143 cm -1 ) frequency of a strong Lewis acid character. The band at 2210-2213 cm -1 is assigned to a shoulder on 2190 cm -1 band following partial dehydroxylation at 773 K, which is created upon the interaction between tetrahedral Al 3+ ions and the adsorbed CO molecule, and this shoulder became more prominent at higher levels of dehydroxylation [5,[6][7][8][9][10]. The most intense band at 2190-2198 cm -1 is assigned to σ-bonded CO adsorbed on octahedrally coordinated Al 3+ sites from which more are present on the surface than from tetrahedral ones, having stronger Lewis acidity, since the lower wave number has been associated with increasing acid strength [14].…”

“…They observed two low temperature CO adsorption sites at 80 K on Al 2 O 3 and they attributed the first adsorption site (2140-2150 cm -1 ) to CO hydrogen-bonded to Al-OH groups and a physisorbed CO layer, while the second CO adsorption site (2195-2213 and 2238 cm -1 ) was assigned to tetrahedrally and octahedrally coordinated Al +3 sites [7,[8][9][10].…”