Catalytic surface area

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... 

Two new technologies have reduced the cost of alkali fuel cells to the point where a European company markets taxis that use them. One is the use of CO2 scrubbers to purify the air supply, making it possible to use atmospheric O2 rather than purified oxygen. The other is the development of ultrathin films of platinum so that a tiny mass of this expensive metal can provide the catalytic surface area needed for efficient fuel-cell operation. 

GP 2] [R 3a] The performance of one micro reactor with three kinds of catalyst -construction material silver, sputtered silver (dense) on aluminum alloy (AlMg3), and sputtered silver on anodically oxidized (porous) aluminum alloy (AlMg3) -was compared with three fixed beds with the same catalysts [44]. The fixed beds were built up by hackled silver foils, aluminum wires (silver sputtered) and hack-led aluminum foils (anodically oxidized and silver sputtered), all having the same catalytic surface area as the micro channels. Results were compared at the same flow rate per unit surface area. 

Area (for example, catalytic surface) Area=Length x Width, and area has units of length squared (squcire meters, or m, for example). 

Two important ways in which heterogeneously catalyzed reactions differ from homogeneous counterparts are the definition of the rate constant k and the form of its dependence on temperature T. The heterogeneous rate equation relates the rate of decline of the concentration (or partial pressure) c of a reactant to the fraction / of the catalytic surface area that it covers when adsorbed. Thus, for a first-order reaction,... 

The residence times of SO2 and H2S04 in the troposphere are typically only a few days, but sulfuric acid aerosols reaching the stratosphere can be very persistent together with nitric acid, they provide the solid surfaces in polar stratospheric clouds on which reaction 8.9 and related processes occur heterogeneously. Indeed, studies suggest that NOx emissions of commercial supersonic aircraft in the lower stratosphere may pose less of a threat to the ozone layer than previously supposed however, the accompanying formation of sulfuric and nitric acid aerosols may exacerbate ozone loss by increasing the available catalytic surface area. 

From the foregoing dicussion it is apparent that residuum hydroconversion processes can be influenced adversely by pore diffusion limitations. Increasing the catalyst porosity can alleviate the problem although increased porosity is usually accompanied by a decrease in total catalytic surface area. Decreasing the catalyst particle size would ultimately eliminate the problem. However, a different type of reaction system would be required since the conventional fixed bed would experience excessive pressure drops if very fine particles were used. A fluidized system using small particles does not suffer from this limitation. However, staging of the fluidized reaction system is required to minimize the harmful effects that backmixing can have on reaction efficiency and selectivity. 

If one could disregard the complicated influence of poisons on mass transfer processes, it would be possible to state in a first approximation that catalyst activity for a selected reaction is a monotonic function of the surface area occupied by the active component. The problem that arises is the measurement of the catalytic surface area in the presence of a support material. In the case of Pt such a measurement is relatively simple, done by hydrogen chemisorption (56, 57) or titration (55), although even in this case there are uncertainties associated with surface stoichiometry (59, 60). These problems become more complicated when Pd, or other noble metals are incorporated at the same time, and still more so, when the catalysts have been contaminated (61). 

The internal structure, comprising pores and surface area, is important for making the active catalytic sites accessible to the reactant molecules. The location of the active species is important for minimizing diffusional resistance since reactants must diffuse within the particle to the active sites and products must diffuse away. Finally, high catalytic surface area and high dispersion of active species are advantageous for maximum reaction rate and utilization of the catalytic components. 

All of the internal properties such as pore size, surface area, catalytic species location, and catalytic surface area are important since the five fundamental steps mentioned in the opening paragraph are operative. 

Alternative techniques do exist, however, for obtaining information regarding the distribution and number of catalytic components dispersed within or on the support. Selective gas adsorption, referred to as chemisorption, can be used to measure the accessible catalytic component on the surface indirectly by noting the amount of gas adsorbed per unit weight of catalyst. The stoichiometry of the chemisorption process must be known in order to estimate the available catalytic surface area. One assumes that the catalytic surface area is proportional to the number of active sites and thus reaction rate. This technique has found use predominantly for supported metals. A gas that will selectively adsorb only onto the metal and not the support is used under predetermined conditions. Hydrogen and carbon monoxide are most commonly used as selective adsorbates for many supported metals. There are reports in the literature of instances in which gases such as NO and O2 have been used to measure catalytic areas of metal oxides however, due to difficulty in interpretation they are of limited use. 

Thermally induced deactivation of catalysts is a particularly difficult problem in high-temperature catalytic reactions. Thermal deactivation may result from one or a combination of the following (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse, (iii) reactions/transformations of catalytic phases to noncatalytic phases, and/or (iv) loss of active material by vaporization or volatilization. The first two processes are typically referred to as "sintering." Sintering, solid-state reactions, and vaporization processes generally take place at high reaction temperatures (e.g. > 500°C), and their rates depend upon temperature, reaction atmosphere, and catalyst formulation. While one of these processes may dominate under specific conditions in specified catalyst systems, more often than not, they occur simultaneously and are coupled processes. 

Nevertheless efforts to understand, treat and model sintering/thermal-deactivation phenomena are easily justified. Indeed deactivation considerations greatly influence research development, design and operation of commercial processes. While catalyst deactivation by sintering is inevitable for many processes, some of its immediate drastic consequences may be avoided or postponed. If sintering rates and mechanisms are known even approximately, it may be possible to find conditions or catalyst formulations that minimize thermal deactivation. Moreover it may be possible under selected circumstances to reverse the sintering process through redispersion (the increase in catalytic surface area due to crystallite division or vapor transport followed by redeposition). 

The process of chemisorption of reactants requires adsorption on the surface of the catalyst. Therefore to maximize the rate the catalytic surface area should also be maximized. This is achieved by dispersing the catalytic species onto a high surface area inorganic carrier. An ideal dispersion of Ni on A1203 is shown in Figure 7.1. 

For example, it is believed that defects in the crystal structure produce highly energetic and active sites for catalytic reactions. This may be true but the more crystalline the catalytic site the lower is the number of surface atoms and the lower is its catalytic surface area. All this being said there are reactions that favor certain catalyst crystalline sizes and are said to be structure sensitive. The above discussion points to the mystery of catalysis. The goal of finding a universal model describing the nature of the active catalytic site still eludes us today and will undoubtedly be the subject of fundamental research for years to come. 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

The usual way to obtain a large amount of catalytic surface area is to use a porous material with small pores. This can be understood assuming the pores are cylindrical and of the same radius. The relationship between the pore surface per unit particle volume S, the porosity ep, and equivalent pore radius r, is given by... 

A monolith reactor that might be particularly useful in fine chemicals manufacfure and biofechnology was developed at Delft Technical University (45,46). Monolithic structures in this reactor are moimted on the stirrer shaft, replacing conventional impeller blades (Figure 18). The monolithic stirrers can be mounted on a vertical or on a horizontal shaft, and more than one set of stirrers can be placed on the shaft. Compared to conventional stirrers, the monolith impellers have a much higher geometric catalytic surface area. 

The requirement for catalytic surface area may determine in what form the catalyst should be incorporated in the membrane reactor. If the underlying reaction calls for a very high catalytic surface area, the catalyst may need to be packed as pellets and contained inside the membrane tubes or channels rather than impregnated on the membrane surface or inside the membrane pores due to the limited available area or volume in the lauer case. 

A truly excellent discussion of the types and rates of adsorption together with techniques used in measuring catalytic surface areas is presented in... 

Effect of catalytic surface area on catalytic behavior... 

The catalytic surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600 m /g by the novel preparation method using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. 

The effects of the surface area on the catalytic behavior of silica-supported rhodium catalysts in CO2 hydrogenation were examined. It was found that the turnover frequencies for CO2 hydrogenation increased lineally with the catalytic surface area. 

Electrode surfaces can be modified with metal nanopartides and such surfaces have found numerous applications in the field of bioelectrochemistry, particularly in biosensors [7, 155]. Gold nanopartides are often utilized in such studies since they are known to retain the activity of the biomolecule with electrochemical activity intact as well [15]. It has also been observed that these nanopartides can act as conduction centers fadlitating the transfer of electrons. In addition, they provide large catalytic surface areas. 

One of the principal modes of catalyst deactivation for Fischer-Tropsch (FT) catalysts is loss of catalytic surface area due to the accumulation of carbonaceous species on metal/metal carbide surfaces and in the pores of the catalyst and/or formation of inactive carbide phases [1-3]. These carbon-containing species are probably products of the condensation/polymerization of atomic carbon or CH, reaction intermediates, formed during reaction by CO dissociation and subsequent partial hydrogenation of the atomic carbon [2]. 

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