Supported metals dispersion

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

In order to characterize the supported metal catalysts, various techniques have been applied. The infrared (IR) spectroscopic investigation of adsorbed CO is one such effective method and has been applied to various supported metal catalysts [2-5]. The states of adsorbed CO can be observed directly by IR examination, and reflect the state of supported metal dispersed over the support. Therefore, the information obtained from the examination also reflects the properties of the supports as the dispersion media for metal catalysts. 

FTIR spectroscopy data for the examined catalytic systems were compared with the data of XPS, diffuse reflectance electron spectroscopy, H2 and CO chemisorption for determination of supported metal dispersion, and temperature-programmed reduction as well as with the results of testing in the following catalytic reactions double-bond isomerization of 1-hexene, hydrogenation of benzene and isomerization of n-heptane and cyclohexane. 

Supported metal catalysts are reduced, for example, by treatment in hydrogen at temperatures in the range of 300—500°C. The reduction temperature may influence the stabiUty of the metal dispersion. 

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

As this field is very wide, we will discuss first the gases that can be used to study metal dispersion by selective chemisorption, and then some specific examples of their application. The choice of gases, is, of course, restricted to those that will strongly chemisorb on the metal, but will not physically adsorb on the support. Prior to determining the chemisorption isotherm, the metal must be reduced in flowing hydrogen details are given elsewhere. The isotherm measurement is identical to that used in physical adsorption. 

The use of CO is complicated by the fact that two forms of adsorption—linear and bridged—have been shown by infrared (IR) spectroscopy to occur on most metal surfaces. For both forms, the molecule usually remains intact (i.e., no dissociation occurs). In the linear form the carbon end is attached to one metal atom, while in the bridged form it is attached to two metal atoms. Hence, if independent IR studies on an identical catalyst, identically reduced, show that all of the CO is either in the linear or the bricked form, then the measurement of CO isotherms can be used to determine metal dispersions. A metal for which CO cannot be used is nickel, due to the rapid formation of nickel carbonyl on clean nickel surfaces. Although CO has a relatively low boiling point, at vet) low metal concentrations (e.g., 0.1% Rh) the amount of CO adsorbed on the support can be as much as 25% of that on the metal a procedure has been developed to accurately correct for this. Also, CO dissociates on some metal surfaces (e.g., W and Mo), on which the method cannot be used. 

As it was established by Geus et a/.[18, 19] the decrease of the rate of carbon deposition is a positive factor for the growth of fibres on metal catalysts. Si02 is an inhibitor of carbon condensation as was shown in Ref [20]. This support also provides possibilities for the stabilization of metal dispersion. Co and Fe, i.e. the metals that give the best results for the tubular condensation of carbon on graphite support, were introduced on the surface of siUca gel... 

The catalysts used in the process are essentially nickel metal dispersed on a support material consisting of various oxide mixtures such as alumina, silica, lime, magnesia, and compounds such as calcium aluminate cements. When the catalyst is made, the nickel is present as nickel oxide which is reduced in the plant converter with hydrogen, usually the 3 1 H2 N2 synthesis gas ... 

Bimolecular reactions are sometimes catalyzed using two different metals dispersed on a common support. A mechanism might be... 

As can be seen in table 1, with different preparation methods and active metals, the average size of the copper particle for the catalysts A and D were 20.3 nm and 50.0 nm. While those of the catalysts B and C were 51.3 nm and 45.4 run, respectively. CuO, non-supported metal oxide, made by impregnation is sintered and cluster whose particle size was 30 pm. The water-alcohol method provided more dispersed catalysts than the impregnation method. 

Fig. 4 shows the current density over the supported catalysts measured in 1 M methanol containing 0.5 M sulfuric acid. During forward sweep, the methanol electro-oxidation started to occur at 0.35 V for all catalysts, which is typical feature for monometallic Pt catalyst in methanol electro-oxidation [8]. The maximum current density was decreased in the order of Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It should be noted that the trend of maximum current density was identical to that of metal dispersion (Fig. 2 and Fig. 3). Therefore, it is concluded that the metal dispersion is a critical factor determining the catalytic performance in the methanol electro-oxidation.

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

For a supported metal catalyst, the BET method yields the total surface area of support and metal. If we perform our measurements in the chemisorption domain, for example with H2 or CO at room temperature, adsorption is limited to the metallic phase, providing a way to determine the dispersion of the supported phase. 

Metals Dispersion from LEISS. Since He scattering Is very selective to Che outermost surface layer, one should anticipate that LEISS would be a valuable Cool for studies of metals dispersion for supported catalysts. For low oietal concentrations on high area supports, the (oietal/support) LEISS Intensity ratio should be directly proportional to metals dispersion. Recent stiidles In our laboratory have confirmed that expectation. 

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

In ecent years the utility of extended X-ray absorption fine structure UXAFS) as a probe for the study of catalysts has been clearly demonstrated (1-17). Measurements of EXAFS are particularly valuable for very highly dispersed catalysts. Supported metal systems, in which small metal clusters or crystallites are commonly dispersed on a refractory oxide such as alumina or silica, are good examples of such catalysts. The ratio of surface atoms to total atoms in the metal clusters is generally high and may even approach unity in some cases. 

Surface Composition Measurements. The surface composition and metal dispersion for a series of silica (Cab-O-Sll) supported Ru-Rh bimetallic clusters are summarized In Table I. Surface enrichment In Rh, the element with the lower heat of sublimation, was not observed over the entire bimetallic composition range. In fact, to within the experimental limit of error of the measurements, surface compositions and catalyst compositions were nearly equal. A small local maximum In the dispersion was observed for the catalyst having a surface composition of 50% Rh. 

Metal dispersions were observed to decrease as the concentration of Ru was Increased. This same trend was observed for the Ru-Rh catalysts and was in marked contrast to observations on silica-supported Ft-Ru catalysts W. In this case a large Increase in dispersion was obtained as a result of bimetallic clustering in the cherry model configuration. 

Table III. Effect of Dilution on the Dispersion of Supported Metal Catalysts...

The dedicated STEM provides a means of obtaining mlcroanalytlcal Information of supported metals not readily obtained by other Instrumentation. The capability to observe and analyze some very highly dispersed metal particles on y-alumlna has been demonstrated and for the noble metals, verified by temperature prograouned hydrogen desorption. 

The present study was initiated to understand the causes of large differences in perfonnance of various catalyst formulations after accelerated thermal aging on an engine dynamometer. In particular, we wished to determine whether performance charaderistics were related to noble metal dispersion (i.e. noble metal surface area), as previous studies have suggested that the thermal durability of alumina-supported Pd catalysts is due to high-temperature spreading or re-dispersion of Pd particles [20-25]. 

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