Hm. Ismail et al., SURFACE REACTIVITY OF IRON-OXIDE PIGMENTARY POWDERS TOWARD ATMOSPHERIC COMPONENTS - XPS, FESEM, AND GRAVIMETRY OF CO AND CO2 ADSORPTION, Journal of colloid and interface science, 194(2), 1997, pp. 482-488
The adsorption of carbon monoxide and carbon dioxide (CO and CO2) on a
number of specially prepared alpha-Fe2O3 samples was measured gravime
trically at 25 degrees C. The samples were prepared from a steel-pickl
ing waste (97 wt% FeSO4 . 7H(2)O) by roasting the original material at
700 degrees C for 5 h in air, oxygen, and nitrogen. Estimated surface
coverages by the adsorbed CO and CO2 were made on the basis of nitrog
en-adsorption-based surface areas, while the nature of the sample surf
aces was investigated by both X-ray photoelectron spectroscopy (XPS) a
nd field emission SEM (FESEM) techniques. In addition a depth profilin
g study utilizing a sputtering argon beam and XPS was undertaken. Morp
hological studies using FESEM showed that neither CO nor CO2 caused an
y significant structural changes. The nature of the resultant alpha-Fe
2O3 sample surfaces differed, with the degree of oxygenation decreasin
g in the order of preparatory gases: oxygen, (wet) air, nitrogen [IP(O
), IP(A), and IP(N)]. The amounts of both CO and CO2 adsorbed decrease
d in the sample order IP(A) > IP(O) > IP(N), though in the case of CO
adsorption, the amounts adsorbed on IP(A) and IP(O) were not greatly d
ifferent. In all cases the amounts adsorbed represented only fractiona
l coverage. Adsorption of the more acidic CO2 is thought to be favored
more by basic O-2(x-) than by O2- sites on both IP(O) and IP(A), but
with surface hydroxyl groups also playing a role (particularly on IP(A
)). The CO2 adsorption should result in the formation of mono-, di-, a
nd polydentate carbonate and bicarbonate species, with increasing dega
ssing temperatures favoring the polydentate species and the decomposit
ion of the bicarbonate and carbonate to form undissociated CO2. The ad
sorption of CO (a weak base) is postulated to take place on strong Lew
is acid, highly coordinated, metal sites to form metal carbonyl specie
s, on strong base sites (O2-) to form carbonite, oxalate, and ketenic
species, and, to a lesser degree, on surface hydroxyl groups to form f
ormyl and formate species. (C) 1997 Academic Press.