Dispersion metal

Sodium hydride is manufactured by the reaction of hydrogen and molten sodium metal dispersed by vigorous agitation ia mineral oil (4). 

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 ... 

N. E. Cohen, 13th Symp (Int) Combust (Proc) (1970), 1019—29 CA 76, 61471 (1972) To analyze and explain the mechanism of combustion of powdered metals in contact with a solid oxidizer (AP) with the powdered metal dispersed in solid AP (I), the combustion of various compressed I-Al and I-Mg mixts in N2 under various conditions in a high-pressure window bomb was studied. The regression-rate laws of the mixts at high and low pressures, the pressure limits for deflagration, and the structures of the combustion zone and of the surface were detd. The burning rate of various I-Al mixts, as a function of pressure, I particle size, and mixt ratio was determined by cinematography. The combustion was difficult to achieve... 

Most of the above reactions are used for the cleavage of aryl sulphones. Recently, a note has appeared109 in which the use of potassium metal dispersed ultrasonically in toluene to cleave saturated cyclic sulphones is described. Addition of iodomethane permits the isolation of acyclic alkyl methyl sulphones (as outlined in equation (44)). 

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

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. 

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. 

Metal dispersions were obtained by the dynamic pulse method using either H2, CO or O2 chemisorption at 298 K (.J) ... 

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. 

A word should be said regarding the use of O2 chemisorption to measure Ru-Ir metal dispersions. The stoichlonetry of the CO adsorption on Ir (CO/Irjgj) was taken from the literature to be 0.5 (9-10). The measured CO/O2 chemisorption ratio on Ir was determined using the dynamic pulse method and found to be 1.55. These results give... 

Platinum was added to Nation before Incorporating CdS In order to avoid the reduction of CdS during the platlnlzatlon process. Nation (DuPont 117, 0.018 cm thick) films were soaked In Pt(NH2)2l2 (0.1 mM) solution for 4 hr. The amount of the Pt complex Incorporated was determined by measuring the optical absorption change In the liquid phase. The films were subsequently reduced with NaBH (0.1 M) solution for one day to produce Pt metal dispersed throughout the polymer film. The amount of Pt was found to be about 0.02 mg cm 2. 

Main physico-chemical characteristics of the solids weight percents of metals and additives, specific surface area, metallic dispersion. 

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]. 

Noble metal dispersions and surface areas Table 2 lists the apparent dispersions obtained from the CO methanation technique. No correlation is observed between dispersion and catalyst performance as measured by the CO/NOx crossover efficiencies. The C2 and C3 Pd-only TWCs, despite their extremely high CO/NOx crossover efficiencies, gave apparent dispersions of 3.5 and 3.0% after 75 and 120 h aging versus higher values of 5.9% for the Pd/Rh catalyst (E) and 4.3% for the Pt/Rh catalyst (G). both of which displayed low CO/NOx crossover efficiencies. Even between the two Pd/Rh catalysts, catalyst E h2is an apparent dispersion more than four times that of catalyst F, yet the two are nearly identical in their CO/NOx crossover efficiencies. 

Storage are the fresh and 75 h aged Pd ly TWCs (Cl and C2), and it is likely that rare earth oxides do contribute to oxygen up es in those catalysts. Interestingly, the C1 and C2 catalysts are the only pair which show a correlation between oxygen uptake and noble metal dispersion (i-e. the oxygen titrated by the first CO pulse drops from 35.5 to 27.2 /i-mol O/g-cat. as the dispersion drops from 10.8% (Cl) to 3.5% (C2)). 

Apparent noble metal dispersions of 75 and 120 h dynamometer-aged TWCs range from about 2 to 6%. 

---