Fourier-transform infrared study of the surface properties of cobalt oxides

Abstract

The surface properties of cobalt oxide after two different pretreatments have been investigated by means of Fourier-transform infrared (FTIR) spectroscopy of adsorbed carbon monoxide, methanol and ammonia. The surface of evacuated Co, O, exposes Co3+ cations that react rapidly with CO, producing Co2+ and probably Co', together with carbonate ions. This surface is extremely reactive, and decomposes at room temperature both ammonia and methanol. After mild reduction in hydrogen, COO is produced. After this treatment the surface exposes Co2+ and even more reduced sites that by contact with CO give small polycarbonyl clusters. This surface adsorbs methanol dissociatively and ammonia molecularly at room temperature.

Metal−Support Interactions in Co/Al2O3 Catalysts: A Comparative Study on Reactivity of Support

“…According to the literature, this band can be assigned to multicarbonyl ligation to Co + (or Co δ+ ), which show fewer electron-donor properties, or Co 0 sites. 5,15,19,20 The band at 2001 cm -1 is also quite large in IM2, compared to those IR spectra with similar CO exposure times (85 min in IM1 and 90 min in IM2). Overall IR absorptions at 2054-2001 cm -1 for the SG sample are very similar to those of IM2 except with much smaller intensities.…”

“…As shown in IR spectra of Figure 6, two large absorption bands at 2172 and 2117 cm -1 can be observed immediately after the CO gas was introduced. The band of 2172 cm -1 lies at a higher frequency than that of gaseous CO (2143 cm -1 ), 15,19 which suggests the CO molecule is absorbed in a relatively high oxidation state cation through donating its electrons in this σ-type coordination bond. 15,19 The band can be therefore be assigned to the CO adsorption in the coordinatively unsaturated Co 3+ sites.…”

“…The band of 2172 cm -1 lies at a higher frequency than that of gaseous CO (2143 cm -1 ), 15,19 which suggests the CO molecule is absorbed in a relatively high oxidation state cation through donating its electrons in this σ-type coordination bond. 15,19 The band can be therefore be assigned to the CO adsorption in the coordinatively unsaturated Co 3+ sites. The band centered at 2117 cm -1 , as reported previously in many cases in the literature, 5 can be ascribed to the CO on Co 2+ sites.…”

“…15,20 Unlike IM1, however, a small band at the low frequency side of 2054-2035 cm -1 appears within 10 min of CO adsorption. According to the literature, this band can be assigned to multicarbonyl ligation to Co + (or Co δ+ ), which show fewer electron-donor properties, or Co 0 sites.…”

“…According to the literature data, 5,15,19,20 both absorptions can be assigned to CO linearly ligated to zerovalent Co 0 sites (or more reduced sites). The highfrequency shoulder at 2026 cm -1 may be attributable to CO adsorption at higher coverage, as it appears only at a later stage of the adsorption in this sample.…”