CoO-MgO solid solutions

Fig. 4 Oxygen Is XPS spectra including curve-fitted components for (a) Catalyst I, (b) Catalyst I after reduction In Fig. 2, a marble-like pattern was observed, which is attributable to solid solution phase of CoO and MgO, because XRD measurement on Catalyst II showed the existence of CoO-MgO solid solution phase [7, 8]. On the other hand, for Catalyst I, no solid solution phase of CoO-MgO was observed. In addition, XRD pattern of Catalyst I indicated the existence of CoO or C03O4. These results suggest that in the case of Catalyst I, Co is loaded on the surface of MgO as CoO or C03O4 phase. Magnified TEM image of Catalyst I after reduction is shown in Fig. 3. In this figure, crystalline lattice image was observed. It is likely that the observed lattice corresponds to the metal phase of Co, because XRD measurement on Catalyst I after reduction showed the existence of Co metal phase [7, 8].

TEM-EDS and XPS analyses were conducted on Co/MgO catalysts. The results of surface analyses showed that Co metal is not supported on the MgO as particles, but covers MgO surface in the case of 12 wt.% Co/MgO calcined at 873 K followed by reduction. After the reduction of catalyst at 1173 K, both cobalt oxide and CoO-MgO solid solution are observed on the surface of catalyst. In the steam reforming of naphthalene, two types of coke deposited on the surface of catalyst are observed. These are assigned to film-like and graphite type carbon by TPO analysis. 

Recently, Ruckenstein and Wang (264-266) also successfully developed excellent CoO/MgO solid-solution catalysts for C02 reforming of methane. They reported that Co/MgO exhibited a good catalytic performance with a CO yield of 93% and a H2 yield of 90% at the high space velocity of 60,000 mL (g catalysts)-1 h-1 and 1163 K, which remained unchanged during 50 h of investigation (264). In contrast, Co/CaO, Co/SrO, and Co/BaO each provided low CO yields, and Co/CaO also had a low stability. The results indicate that the CoO/MgO catalysts are characterized by performances similar to those of NiO/MgO. 

In summary, the basicity and the strong NiO-MgO interactions in binary NiO/MgO solid solution catalysts, which inhibit carbon deposition and catalyst sintering, result in an excellent catalytic performance for C02 reforming. The characteristics of MgO play an important role in the performance of a highly efficient NiO/MgO solid-solution catalyst. Moreover, the NiO/MgO catalyst performance is sensitive to the NiO content a too-small amount of NiO in the solid solution leads to a low activity, and a too-high amount of NiO to a low stability. CoO/MgO solid solutions have catalytic performances similar to those of NiO/MgO solid solutions, but require higher reaction temperatures. So far, no experimental information is available regarding the use of a FeO/MgO solid solution for CH4 conversion to synthesis gas. 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

UV-VIS and IR spectroscopies was used in combination to investigate the electronic and vibrational transitions in CoO-MgO solid solutions in the entire composition range (0 < x < 1) (371). 

The adsorption of l502 and 1802 on a sintered CoO-MgO solid solution was reported (372) and later investigated by IR and EPR spectroscopies (373, 374). IR spectra of CO on CoO-MgO solid solutions were also reported (375-377). Experiments performed at liquid nitrogen temperature with Co-diluted samples indicate the formation of both a-bonded Mg2+ CO and a-tv -bonded Co2+ CO adducts on (001) faces (375). The latter are characterized by a significantly lower stretching frequency than CO gas(v(CO) = 2118 cm = 39 cm 1 at 0 - 0) and by a remarkable static shift as a function of 0 (Avstat = -18 cm-1), in line with the hypothesis that d-n overlap is significant (Fig. 13). 

CO molecules adsorbed on Mg2+ ions on (001) faces of sintered CoO-MgO solid solutions exhibit a dipole-dipole coupling similar to that found for CO on bare MgO, whereas CO molecular adsorbed on Co2+ are decoupled but interact with adjacent Mg2+ CO species via static effects (375). The intensity of the v(CO) peak of the Co2+ CO complex is greater than expected on the basis of the stoichiometry this result is in agreement with the fact that the peak intensity is proportional to av (141). 

The room-temperature chemistry of high-surface-area CoO-MgO solid solutions is dominated by the adsorption of CO on edges and steps. Co-ordinatively unsaturated Co2+ and O2 ions react primarily as 02 Co2+02 triplets with formation of [(CCh CoCO]2- species. In samples with high Co contents, the large amounts of clustered cobalt guest species are easily reduced by CO, even at room temperature, with formation of Co(CO)4 and carbonate-like species (377). The formation of polymeric radical anions of CO on high-surface-area CoO-MgO solid solution has also been reported (378). 

An IR and UV-VIS investigation of the interaction of NH3, ND3, and pyridine with CoO-MgO solid solutions with low Co contents has also been... 

Recently, Ruckenstein and Wang (264-266) also successfully developed excellent CoO/MgO solid-solution catalysts for CO2 reforming of methane. They reported that Co/MgO exhibited a good catalytic performance with a CO yield of 93% and a H2 yield of 90% at the high space velocity of... 

Let us consider first isovalent cations. MgO can dilute ions of similar size, as for instance Ni or Co forming NiO-MgO and CoO-MgO solid solutions with an infinite range of composition. The effect of progressively replacing Mg by Ni or similar cations (Co ", Cu ) on the surface properites has been investigated both experimentally [83,177,178] and theoretically [179,180]. The presence of Ni cations diluted in the MgO matrix results in an efficient catalyst for nitrous oxide, N2O, decomposition this has been attributed to the different bond strength of the Ni-0 and Mg-0 bonds at the surface [177]. Plane wave calculations on Ni-doped MgO have shown that the presence of Ni atoms on the... 

MgO can dilute ions of similar size, as for instance Ni or Co forming NiO-MgO and CoO-MgO solid solutions with an infinite range of composition. 

The lack of activity of Co/MgO is due to the non availability of cobalt metal due to the formation of a CoO/MgO solid solution during activation. In contrast, the relatively high dispersion of Co/zeolite suggests that cobalt metal is available on the catalyst surface. The sample is inactive possibly because the number of Co° atoms in each cluster is less than a lcwer limit required for dissociative CO chemisorption. Chemisorption with retention of integrity, as is measured in the dispersion experiment, is not subject to such a constraint. 

Infrared studies have helped to characterize the low-temperature adsorption of dioxygen onto CoO-MgO solid solutions as a superoxo... 

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