SURFACE REACTIVITY OF IRON-OXIDE PIGMENTARY POWDERS TOWARD ATMOSPHERIC COMPONENTS - XPS, FESEM, AND GRAVIMETRY OF CO AND CO2 ADSORPTION

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.