Cobalt catalysts characterization studies

Paolo Chini began his work in the late 1950s with the characterization of cobalt carbonyl species involved in the hydroformylation of olefins with cobalt catalysts, and in the course of these studies developed improved synthetic methods for the known cobalt carbonyls Co2(CO)8 and Co4(CO)12 [132]. His next steps were the preparation of the heterometallic hydrido complex HFe-Co3(CO)i2 (isoelectronic to Co4(CO)12) and the corresponding anion [FeCo3(CO)12], both a novelty at that time, and of the new hexanuclear cobalt clusters [Co6(CO)15]2, [Co6(CO)14]4, and Co6(CO)16 [133-139]. This work was followed by the synthesis of carbido carbonyl cluster anions [Co6(CO)i4C], [Co6(CO)15C]2 and [Co8(CO)i8C]2, containing an interstitial... 

The effect of cobalt content on the structure of cobalt species was studied using cobalt catalysts supported by S2 and S4 silicas. The characterization results are presented in Table 2. Specific area and total pore volumes decrease with increasing Co content in the catalysts. The specific area remains however, relatively high (> 90 mVg) even at higher cobalt loadings. Table 2 also shows that the average pore size diameter of mesoporous silicas is not affected by varying cobalt content in the catalysts. The XRD patterns (not... 

This study focused on carbon deposits on alununa- and zeolite-supported cobalt catalysts and their effects on CO hydrogenadon. A thermogravimetiic flow system and Auger electron spectrometer were employed to characterize the carbon deposits. 

On photolyzing CoziCOg in the matrix (20), a number of photoproducts could be observed. The results of these experiments are summarized in Scheme 4, which illustrates the various species formed. Of particular interest is the formation of Co2(CO)7 on irradiation of Co2(CO)g in CO (254 nm), as this species had not been characterized in the metal-atom study of Hanlan et al. (129). Passage of Co2(CO)g over an active, cobalt-metal surface before matrix isolation causes complete decomposition. On using a less active catalyst, the IR spectrum of Co(CO)4 could be observed. An absorption due to a second decomposition product, possibly Co2(CO)g, was also noted. 

In a recent work we were able to show that an electronic effect was detected between Bi2Mo30i2 and a mixed iron and cobalt molybdate with an enhancement of the electrical conductivity of the cobalt molybdate with the substitution of the cobaltous ions by the ferrous ions (7). However this effect alone cannot explain the synergy effect and we have investigated the influence of both the de ee of subtitution of the cobalt with the iron cations in the cobalt molybdate and the ratio of the two phases (for a given substituted cobalt molybdate) on the catalytic propert cs of the mixture.We have tried to characterize by XPS and EDX-STEM the catalysts before and after the catalytic reaction in order to detect a possible transformation of the solid. The results obtained are presented and discussed in this study. 

The characterization tools to investigate cobalt-based Fischer-Tropsch catalysts are mostly used to study the catalyst materials under conditions far from industrially relevant reaction conditions i.e., in the presence of CO and H2, as well as of the reaction products, including H2O at reaction temperatures and at high pressures. Since catalytic solids are dynamic materials undergoing major changes under reaction conditions it can be anticipated that the currently obtained information on the active site is at least incomplete. This holds also for the active state and location of the promoter element under reaction conditions. For example, an electronic elfect on the cobalt active phase induced by a promoter element can maybe exist only at high pressures and will remain -due to the lack of the appropriate instrumentation - unnoticed to the catalyst... 

In 1943, A. C. Byrns et al. (7), of Union Oil of California published the first study showing under semi-industrial conditions the relative activities of Mo03 and CoO and the mechanical mixture of these two oxides, which they compared to C0M0O4 supported on bentonite. These authors demonstrated that a mixture of molybdenum and cobalt in their oxidic state should be chemically associated in order to be very active, while the simple mechanical mixture only showed the additive activities of the individual oxides. However, these authors mainly emphasized the behavior of these catalysts under different industrial conditions and reduced their discussion of catalyst structure and characterization to a few lines of speculation. 

Along this line, the limitations of the technique used must be recognized. Some measure predominantly bulk properties, e.g., X-ray diffraction and magnetic susceptibility whereas, others are sensitive to surface composition, e.g., adsorption and ESCA. For example, in one reported study only cobalt in tetrahedral coordination was found on a catalyst by diffuse reflection spectroscopy, but magnetic measurements revealed that octahedral cobalt must also be present (10). Thus, it is dangerous to rely on any one method to characterize these catalysts. 

Molybdenum oxide - alumina systems have been studied in detail (4-8). Several authors have pointed out that a molybdate surface layer is formed, due to an interaction between molybdenum oxide and the alumina support (9-11). Richardson (12) studied the structural form of cobalt in several oxidic cobalt-molybdenum-alumina catalysts. The presence of an active cobalt-molybdate complex was concluded from magnetic susceptibility measurements. Moreover cobalt aluminate and cobalt oxide were found. Only the active cobalt molybdate complex would contribute to the activity and be characterized by octahedrally coordinated cobalt. Lipsch and Schuit (10) studied a commercial oxidic hydrodesulfurization catalyst, containing 12 wt% M0O3 and 4 wt% CoO. They concluded that a cobalt aluminate phase was present and could not find indications for an active cobalt molybdate complex. Recent magnetic susceptibility studies of the same type of catalyst (13) confirmed the conclusion of Lipsch and Schuit. 

For ferromagnetic cobalt particles in zeolite X, spin-echo ferromagnetic resonance has been used to obtain unique structural information (S6). In addition, study of the catalytic signature of metal/zeolite catalysts has been used by the groups of Jacobs (87), Lunsford (88), and Sachtler (47,73,89). Brpnsted acid protons are identified by their O—H vibration (90,91) in FTIR or indirectly, by using guest molecules such as pyridine or trimethylphosphine (92,93). An ingenious method to characterize acid sites in zeolites was introduced by Kazansky et al., who showed by diffuse reflection IR spectroscopy that physisorbed H2 clearly discerns different types of acid sites (94). Also, the weak adsorption of CO on Brpnsted and Lewis acid sites has been used for their identification by FTIR (95). The characterization of the acid sites was achieved also by proton NMR (96). 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

In this study, the CVD process is used to deposit cobalt oxide as planar model layers and as working monolithic catalysts to enable accurate characterization and test of performance. 

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