Cobalt increases catalyst activity

Structural Promoters. The main functions of structural promoters are to influence the cobalt dispersion by governing the cobalt-support oxide interaction. A high Co dispersion results in a highly active Co metal surface and, therefore, in a high coverage by the reactants, and as a consequence an improved catalyst activity. Structural promotion may lead to an increased catalyst activity and stability, but in principle does not influence the product selectivity since it only increases the number of active sites in a catalyst material. This increase in active sites can be achieved by a stabilization of the... 

Indeed, eobalt and a promoter metal may form an integral metal particle deposited on the support oxide, altering the electronic properties of the surface cobalt metal atoms (Figure 4C). Depending on the promoter element added to the Co cluster, alloying might lead to an increased catalyst activity, selectivity, as well as stability. 

A synergistic effect leading to the increased catalyst activity and selectivity in selective catalytic reduction (SCR) of NO with methane or propane-butane mixtures was found when cobalt, calcium and lanthanum cations were introduced into the protic MFl-type zeolite. This non-additive increase of the zeolite activity is attributed to increased concentration of the Bronsted acid sites and their defined location as result of interaction between those and cations (Co, Ca, La). Activation of the hydrocarbon reductant occurs at these centers. Doping the H-forms of zeolites (pentasils and mordenites) with alkaline earth metal and Mg cations considerably increased the activity of these catalysts and their stability to sulfur oxides. 

Isolation of a cobalt phthalocyanine catalyst known to be active in autooxidation and to be deactivated by dimerization has been reported by Schutten (36). In this case, a polyvinylamine poly-dentate ligand was added to a dilute aqueous solution of the cobalt(II) phthalocyanine tetra(sodium sulfonate) in order to prepare a thiol oxidation catalyst. By employing dilute solutions, the polydentate polyamine polymer in effect isolated the cobalt(II) catalyst within an individual polyamine coil minimizing dimerization and significantly increasing catalyst activity. 

Ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [PBuJBr was reported by Knifton as early as in 1987 [2]. The author described a stabilization of the active ruthenium-carbonyl complex by the ionic medium. An increased catalyst lifetime at low synthesis gas pressures and higher temperatures was observed. 

A comparison of the initial rates obtained with various cobalt complexes (Table I) reveals that the chelate complexes of Co(II) are more efficient than the simple salts, the catalytic activity of Co(III) is lower than that of Co(II) and the reaction becomes slower by increasing the number of N atoms in the coordination spheres in both oxidation states. In general, the addition of amine derivatives increased the activity of the catalysts. 

Researchers at 3M have been able to increase catalytic activity with nanotextured membrane surfaces that employ tiny columns to increase the catalyst area. Other materials include nonprecious metal catalysts such as cobalt and chromium along with particles embedded in porous composite structures. 

Rhodium-phosphine catalysts are unable to hydroformylate internal olefins, so much that in a mixture of butenes only the terminal isomer is transformed into valeraldehydes (see 4.1.1.2). This is a field still for using cobalt-based catalysts. Indeed, [Co2(CO)6(TPPTS)2] -i-lO TPPTS catalyzed the hydroformylation of 2-pentenes in a two-phase reaction with good yields (up to 70%, but typically between 10 and 20 %). The major products were 1-hexanal and 2-methylpentanal, and n/i selectivity up to 75/25 was observed (Scheme 4.12). The catalyst was recycled in four mns with an increase in activity (from 13 to 19 %), while the selectivity remained constant (n/i = 64/36). 

Early in the nineties Ruiz et al. reported enhanced catalyst activities and increased selectivities to alkenes and higher hydrocarbons upon addition of V, Mg, and Ce oxides to Co-based F-T catalysts.These variations were attributed to electronic effects induced by the transition metal oxide. Similar results were obtained by Bessel et al. using a Cr promoter in Co/ZSM-5 catalysts.This group observed that the addition of Cr improved the catalyst activity, and shifted the selectivity from methane to higher, generally more olefinic, hydrocarbons. Based on H2 and CO chemisorption, as well as TPR and TPD results, they suggested that the promotion was caused by an interaction between the transition metal oxide and the cobalt oxide, which inhibits... 

Keyser et al. studied Mn-Co F-T catalysts and found that, under industrial relevant conditions, the WGS activity of the catalysts increases with increasing Mn content, but decreases with increasing pressure. A lower olefin yield was also observed at high pressures. It was stated that structural changes in the cobalt spinel occur over a long period of time and are responsible for the increased hydrogenation activity and increased WGS activity. Mn seems in this... 

The activity of this ruthenium system is comparable to, or somewhat greater than, that of cobalt catalysts under the same conditions of temperature and pressure. Rhodium catalysts provide substantially higher activity than either of these systems. As will be seen later, however, addition of ionic promoters can greatly increase the activity of ruthenium-based catalysts. 

These tetrahedral distorted cobalt atoms can be observed by NMR as a pure phase on carbon supports in the absence of molybdenum and are thus stable these probably correspond to the Co sites observed by Topspe s group using Mossbauer spectroscopy because Craje et al. (93) found a similar Mossbauer doublet for both cobalt in CoMo catalysts and pure cobalt sulfide on carbon support. They are also active for HDS and confirm the findings of Prins and co-workers (94) and Ledoux (96). These different structures are in full agreement with the XANES experiments performed by Prins and co-workers (95) and Ledoux (96). These structures also led Ledoux et al. to an incorrect interpretation of the synergy effect (64). On poorly dispersed catalysts supported on silica or in bulk form, their presence and activity are large enough to explain the increase in activity when cobalt is added to molybdenum, but on well-dispersed catalysts i.e., on alumina or carbon support this interpretation is shown to be incorrect if the activity is carefully measured. 

An iron-promoted cobalt molybdate catalyst (Fe0 03Co0.9 7MoO4) was studied by Maksimov et al. [195,196] with respect to the role of iron in the transfer of charge. Iron strongly enhances the catalytic activity and at the same time increases the conductivity by a factor of 100. Mossbauer spectroscopy reveals that 4% of the iron ions are present as Fe2+ impurity . This fraction is doubled at steady state reaction conditions, and indicates participation of iron in the charge transfer process. 

Y. Kamiya illustrates the influence on catalytic activity of the form of the catalyst. Thus, in the cobalt-catalyzed oxidation of hydrocarbons in acetic acid solution, introduction of bromide ions increases the activity of the catalyst, especially when the metal ion concentration is fairly high. The presence of bromides also results in a marked increase in the proportion of carbonyl compounds among the products and it is believed that these are formed as a result of a propagation step in which bromine-containing cobaltous ions react with alkylperoxy radicals. 

This phenomenon has also been observed for catalysts prepared using an aqueous route (182). Both the iron and cobalt promoters led to an increase in selectivity. The iron-promoted catalyst was characterized by an increase in activity, but the cobalt-promoted catalyst was characterized by a decrease in activity. The decrease in activity of the cobalt-doped catalyst was attributed to the formation of VOPO4 in the final catalyst. The VOPO4 is formed by the oxidation of V0HP04 1 H20 during the introduction of the promoters in the incipient wetness technique. A similar effect was reported for catalysts doped with indium and tetraethy-lorthosilicate (TEOS) (181). The improved performance was observed only with both promoters in the catalyst. It was proposed that the... 

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