Metal catalysts, particle sizes

Alkyl sulfonie acid-bearing polysiloxanes are also used as a support for powder-type preeious metal catalysts (particle size < 200 pm) that can be used in hydro-genolysis reactions. The conversion of 1-phenylethanol to ethylbenzene proceeds quantitatively (> 99 %) at the same rate as achieved with the combination of H2SO4 and a Pd/C catalyst [15]. 

Catalyst Active Metal Preparation Method Loading amount of metal (wt%) Particle Size (nm)... 

A variety of catalysts were used to examine the effect of changing pore size, metal crystallite size, and catalyst particle size. The catalyst characterisation is reported in Table 1. 

As mentioned above, the electronic properties of SWCNTs depend on their chirality and may be semiconducting or metallic. There is still no satisfying way to produce just one sort of SWCNTs, which would require the exact control of catalyst particle size at elevated temperature. Hence, the separation of semiconducting from metallic SWCNTs is of paramount importance for their application in, for example, electric devices, field emission and photovoltaics etc. 

Another way of investigating structure is through the classical method on metals of varying catalyst particle size. The key to this method is to measure active catalyst surface areas in order to determine changes in turnover rates with ensemble size. In recent years several chemisorption techniques have been developed to titrate surface metal centers on oxides (25). In this volume Rao and Narashimha and Reddy report on the use of oxygen chemisorption to characterize supported vanadium oxide. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Several length-scales have to be considered in a number of applications. For example, in a typical monolith reactor used as automobile exhaust catalytic converter the reactor length and diameter are on the order of decimeters, the monolith channel dimension is on the order of 1 mm, the thickness of the catalytic washcoat layer is on the order of tens of micrometers, the dimension of the pores in the washcoat is on the order of 1 pm, the diameter of active noble metal catalyst particles can be on the order of nanometers, and the reacting molecules are on the order of angstroms cf. Fig. 1. The modeling of such reactors is a typical multiscale problem (Hoebink and Marin, 1998). Electron microscopy accompanied by other techniques can provide information on particle size, shape, and chemical composition. Local composition and particle size of dispersed nanoparticles in the porous structure of the catalyst affect catalytic activity and selectivity (Bell, 2003). 

Chemical vapor deposition method is used as an alternative to circumvent the limitations of the other synthetic methods. In chemical vapor deposition method, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination of these materials. The diameter of the grown nanotubes is related to the size of the metal particles. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor a process gas (such as ammonia, nitrogen, hydrogen, etc.) and... 

A.A. Khassin, T.M. Yurieva, V.N. Parmon, Fischer Tropsch Synthesis over Cohalt Containing Catalysts in Slurry Reactor. Effect of the Metallic Co Particle Size on the Selectivity, React. Kinet. Catal. Lett. 64 (1998) 55 62. 

The activity and selectivity of heterogeneous catalysts generally depend on the state of metal dispersion (particle size), structure (shape and morphology), metal composition, and metal-support interactions. If the catalytic centers include many metallic atoms, the electronic and geometric distributions of the constituents elements may be strongly related to the chemical reactivities and catalytic performances of the bimetallic and alloy catalysts (5). 

Fig. 9.12. Effect of catalyst particle size and the strength of the metal support interaction on the metalrsupport contact area.

From the relative rates observed for separate particles and of coprecipitated metal (assumed to offer complete intimacy for the intermediate) we might estimate (see ref. 6) as of the order of magnitude 3 in Rylander and Cohn s observation, which leads for various catalyst particle sizes (carbon granules) to an estimate of concentration of intermediate as follows ... 

There is no agreement in the literature on the role of the catalyst particle size for methanol oxidation. Two main evidences have been reported one is related to the presence of a maximum in the mass activity as a function of particle size located, for some authors [40], at 3nm and at 2nm for others [41] moreover, for some authors there is no influence of the interparticle distance and particle size on specific activity. If these parameters are not controlling, an increase in mass activity should be observed as the metal surface area is increased [42]. In this context, the metal-... 

In order to have significant electrochemical promotion of metal-support interaction promotion of a catalytic reaction, t]p must be at least 0.2. Equation (66) and the corresponding Figure 44 show the range of Op and J values that allow for this to happen the Thiele modulus Op must be smaller than 5. This means small film thickness or catalyst particle size, small kinetic constant k for promoter destruction, and finite surface diffusivity, Dg, of the promoter. Also the dimensionless current J must be larger than 2, and this again dictates a small k value for promoter destruction, a finite current for electrochemical promotion, and a fast catalytic rate, r, for metal-support interactions. 

This topic was reviewed in detail by Che and Bennett [10]. In most of the supported metal catalysts, the size of the metal particles varies in the critical range between 1 to lOnm. The first point addresses the question as to below which particle size the metallic properties are lost how small is a metal [24]. The answer is not so simple and depends on the properties we are looking for. Bulk properties of metal, like the melting temperature, are not reached below 10 nm [24]. 

One further difference exists between HDS and HDM. Bridge [37] has shown, very clearly, that HDS is not limited by diffusion while HDM is. Using a nickel-molybdate based catalyst with a unimodal microporous size distribution, the demetalation of Arabian heavy atmospheric residuum was found to be affected by catalyst particle size, while HDS was not. As the diameter of the pore was decreased, the maximum in the metals deposition profile moved closer to the external surface of the pellet, agmn indicating difiusional limitations for FIDM. 

The majority of the catalysts used in the work reported here were based on MgO or ZnO and were promoted by Li or Ba ions. They were synthesised by wet impregnation using an appropriate alkali-metal salt. The preparation of other samples are reported elsewhere (refs. 5,6). All the samples were calcined in air at 850°C prior to being tested. The reaction system made use of quartz fixed-bed reactors and gas analysis was carried out with gas chromatography (ref. 5). The catalyst (particle-sizes from 0.3 - 0.6 mm) was diluted with the same weight of quartz particles of the same size. The process conditions used are given with the results. One set of data were obtained with a recycle reactor for this, a... 

Much of the art of the sensor is in the synthesis of the desired material. Characteristics such as homogeneity, grain size, and crystalline phase, which can be controlled to varying degrees during the synthesis process, greatly influence the ultimate sensor mechanisms [52]. Iterative experimentation with metal oxide particle size, prefired compositions, catalysts, and process variables is necessary to optimize porosity, resistance, sensitivity, and other sensor characteristics. 

Phenyl-1,2-propanedione (Aldrich, 99%) was hydrogenated in a pressurized reactor (Parr 4560, V=300 cm ) in the absence of external and internal mass transfer limitation (verified experimentally). The reactor was equipped with an propeller type stirrer (four blades, propeller diameter 35 mm) operating at stirring rate of 1950 rpm. The hydrogen (AGA, 99.999%) pressure was 6.5 bar and teii ierature was 15 - 35°C. Pt/Al203 (Strem Chemicals, 78-1660) was used as a catalyst. The catalyst mass and liquid volume were 0.15 g and 150 cm, respectively The metal content was 5 wt.%, BET specific surface area 95 m / g, the mean metal particle size 8.3 nm (XRD), dispersion 40% (H2 chemisorption), the mean catalyst particle size 18.2 pm (Malvern). Catalysts were activated under hydrogen flow (100 cm / min) for 2 h at 400°C prior to the reaction. 

Develop a method for the controlled incorporation of metal catalyst particles of discrete sizes and in specific locations so that SWNT hydrogen storage capacities may be reproducibly optimized. Optimize the purity, production rate, diameter, and chirality of the laser-generated SWNT samples to reliably achieve 6-7 wt% hydrogen storage. 

In recent years, substantial efforts have been made to synthesize Pt nanoparticles (2-10 nm) on different carbon supports. The size range of Pt nanoparticles is of particular interest from the viewpoint of their use in commercial applications because it allows to reduce the amount of precious noble metal in the catalysts. The controlled size of the metal catalyst particles can be achieved by stabilizing the particles in the liquid phase by using... 

---