Catalyst supports surface

The use of highly dispersed catalysts from soluble salts of molybdenum is another approach to the reduction of catalyst amount because of their excellent activity despite their higher price. Recently, metal carbonyl compounds, such as Fe(CO)5, Ru3(CO)i2, and Mo(CO)6 have been investigated as metal cluster catalysts. Preparation involved their deposition and decomposition on catalyst support surfaces (71-73). 

Catalyst/Support Surface area (m2 g ) Pore volume (m3 g I)... 

Another non-conventional preparative route to bimetallic catalysts has been developed where metal atoms (vapors) have been trapped at low temperature in solvating media. (A review has recently appeared).(17) By solvating two metals at the same time (eg. Co in toluene and Mn in toluene), followed by warming, bimetallic clusters/particles form. In the presence of a catalyst support, surface -OH groups can have a dramitic affect on the structure of the small bimetallic cluster produced. For example, with Co and Mn, a layered structure of MnOx covered by Co° in a particle of about 25 A was formed.(iS) With Fe and Co combinations, a layer of FeOx followed by Fe°Co° alloy and a surface rich in Co° was formed. (19)... 

Another level of surface chemical complexity results from catalytic metal-catalyst support surface interactions. Table 3, taken from Bell [7], shows the surface-specific activity of Rh for CO hydrogenation as a function of... 

Nature of the Catalyst Support. - Of equal importance to the ionic character of the impregnating solution is the ionic exchange type and capacity of the catalyst-support surface. This is, of course, directly related to the chemical structure of the support surface. To say that information in the catalyst literature on the chemical structure of support surfaces is sparse is almost an understatement. Evaluations of this important catalyst-preparation parameter are almost without exception entirely overlooked in the published literature on catalyst preparations. A very limited amount of information can be found in a review of 1970. This review is, however, primarily concerned with the physical rather than the chemical structure of catalyst supports. The chemical reactivity of an oxide support surface would appear to depend upon the extent of its hydroxylation. This in turn depends upon the chemical type of the support, the way it was made, and particularly upon its previous thermal history. A few generalizations can be made, as follows. 

The dramatic effect of catalyst support surfaces on solution pH was first demonstrated in the method called mass titration [22], a depiction of which is shown in Figure 13.4a. Starting at a pH value above the oxide s PZC, the pH lowers as successive pinches of oxide are placed into solution and protons are released by the oxide. If a sufficient amount of oxide is placed into solution, the pH arrives at the PZC of the oxide, at which point no driving force for proton transfer exists. 

Carbon corrosion through evolution especially of carbon dioxide and a corresponding reduction in catalyst support surface area can be detected with exhaust gas analysis. The increase in the cathode resistance can be observed with impedance spectroscopy. 

Catalytic properties are dependent on physical form, principally the exposed surface area which is a function of particle size. Industrial PGM catalysts are in the form of finely divided powder, wine, or gauze, or supported on substrates such as carbon or alumina (see Catalysis Catalysts, supported). 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

Transition aluminas are good catalyst supports because they are inexpensive and have good physical properties. They are mechanically stable, stable at relatively high temperatures even under hydrothermal conditions, ie, in the presence of steam, and easily formed in processes such as extmsion into shapes that have good physical strength such as cylinders. Transition aluminas can be prepared with a wide range of surface areas, pore volumes, and pore size distributions. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

M0S2 is one of the most active hydroprocessing catalysts, but it is expensive, and the economical way to apply it is as highly dispersed material on a support, y-Al202. The activity of the supported catalyst is increased by the presence of promoter ions, Co " or Ni ". The stmctures of the catalysts are fairly well understood the M0S2 is present in layers only a few atoms thick on the support surface, and the promoter ions are present at the edges of the M0S2 layers, where the catalytic sites are located (100,101). 

The primary determinant of catalyst surface area is the support surface area, except in the case of certain catalysts where extremely fine dispersions of active material are obtained. As a rule, catalysts intended for catalytic conversions utilizing hydrogen, eg, hydrogenation, hydrodesulfurization, and hydrodenitrogenation, can utilize high surface area supports, whereas those intended for selective oxidation, eg, olefin epoxidation, require low surface area supports to avoid troublesome side reactions. 

Some catalyst supports rely on a relatively low surface area stmctural member coated with a layer of a higher surface area support material. The automotive catalytic converter monolith support is an example of this technology. In this appHcation, a central core of multichanneled, low surface area, extmded ceramic about 10 cm in diameter is coated with high surface area partially hydrated alumina onto which are deposited small amounts of precious metals as the active catalytic species. 

Porosity and Pore Size. The support porosity is the volume of the support occupied by void space and usually is described in units of cm /g. This value represents the maximum amount of Hquid that may be absorbed into the pore stmcture, which is an especially important consideration for deposition of metal salts or other active materials on the support surface by Hquid impregnation techniques. The concentration of active material to be used in the impregnating solution is deterrnined by the support porosity and the desired level of active material loading on the catalyst. If the porosity is too low, inefficient use of the support material and reactor volume may result. If the porosity is too high, the support body may not contain sufficient soHd material to provide the strength necessary to survive catalyst manufacture and handling. 

Catalysts vary both in terms of compositional material and physical stmcture (18). The catalyst basically consists of the catalyst itself, which is a finely divided metal (14,17,19) a high surface area carrier and a support stmcture (see Catalysts, supported). Three types of conventional metal catalysts are used for oxidation reactions single- or mixed-metal oxides, noble (precious) metals, or a combination of the two (19). 

Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

Physical adsorption—surface areas of any stable solids, e.g., oxides used as catalyst supports and carbon black Chemisorption—measurements of particle sizes of metal powders, and of supported metals in catalysts... 

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