Gas-phase adsorption of Co2(CO)8 on totally dehydroxylated MgO occurs with the loss of CO and the formation of higher nuclearity clusters both neutral and negatively charged. Subsequent decarbonylation and reduction in hydrogen at 673 K produces finely divided cobalt particles. These particles on successive interaction with carbon monoxide (p ≤ 5 × 10-1 Torr) give the expected IR manifestations of CO adsorbed on finely divided cobalt (i.e., the appearance of a peak shifting from 2040 cm-1 at pco = 40 Torr to 2007 cm-1 at pco < 10-3 Torr). In addition to these features, other peaks appear at lower frequencies (particularly at pco ≥ 0.5 Torr) which are due to Co(CO)-4- and CO2-3-like species adsorbed on the matrix. Parallel experiments carried out on the Co2(CO)8 SiO2 system give only a single CO peak shifting from 2050 cm-1 (pco = 40 Torr) to 2015 cm-1 (pco < 10-3 Torr) which is assigned to the stretching of CO on Co particles supported on SiO2. The comparison of the two sets of results indicates that, on the Co MgO system, the metallic particles lose cobalt atoms in the form of Co(CO)-4 entities (which are stabilized on the basic ionic matrix) but that this effect is missing when the supporting oxide is SiO2. This new type of metal-support interaction is discussed in terms of a nucleophilic attack of the highly basic O2- ions of the MgO surface on the CoCO groups located at the border of the metallic particles. The analogy with the homogeneous reactions of cobalt carbonyls in basic media is also discussed.
IR investigation of CO adsorbed on Co particles obtained via Co2(CO)8 adsorbed on MgO and SiO2
SPOTO, Giuseppe;ZECCHINA, Adriano
1988-01-01
Abstract
Gas-phase adsorption of Co2(CO)8 on totally dehydroxylated MgO occurs with the loss of CO and the formation of higher nuclearity clusters both neutral and negatively charged. Subsequent decarbonylation and reduction in hydrogen at 673 K produces finely divided cobalt particles. These particles on successive interaction with carbon monoxide (p ≤ 5 × 10-1 Torr) give the expected IR manifestations of CO adsorbed on finely divided cobalt (i.e., the appearance of a peak shifting from 2040 cm-1 at pco = 40 Torr to 2007 cm-1 at pco < 10-3 Torr). In addition to these features, other peaks appear at lower frequencies (particularly at pco ≥ 0.5 Torr) which are due to Co(CO)-4- and CO2-3-like species adsorbed on the matrix. Parallel experiments carried out on the Co2(CO)8 SiO2 system give only a single CO peak shifting from 2050 cm-1 (pco = 40 Torr) to 2015 cm-1 (pco < 10-3 Torr) which is assigned to the stretching of CO on Co particles supported on SiO2. The comparison of the two sets of results indicates that, on the Co MgO system, the metallic particles lose cobalt atoms in the form of Co(CO)-4 entities (which are stabilized on the basic ionic matrix) but that this effect is missing when the supporting oxide is SiO2. This new type of metal-support interaction is discussed in terms of a nucleophilic attack of the highly basic O2- ions of the MgO surface on the CoCO groups located at the border of the metallic particles. The analogy with the homogeneous reactions of cobalt carbonyls in basic media is also discussed.
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