Composites with Catalytic Activity

Chemical reactions in general can be accelerated to go in a forward direction using catalysts which do not participate directly in the reaction. Ihe type of catalyst used depends on the nature of reactants in the reaction and the different materials used are Pd, Pt, Ag, Ni, TiO, ZnO and Fe-Oxides. The inherent catalytic property of these materials can be further enhanced by increasing their specific surface area available for reactions, i.e., by reducing the particle size to nanodimensions. However, agglomeration of the nanoscale materials in their innate state is a serious limitation which reduces the effective surface area available for reaction. The aggregates are easy to recover and recycle. These limitations can be overcome mainly in two separate ways (i) immobilization of the nanoparticles in a porous support or carrier, and (ii) synthesis of the catalytic material as a nanoporous network-like structiu e using different types of templates. Bacterial cellulose has been used extensively as a support material to host the catalytic nanoparticles, while in some cases it has also been used as a template to synthesize catalyst network structure. Some typical studies wherein BC has been used as a support to hold PdCu, Pd, TiO and CdS nanoparticles are discussed first, followed by template structure based composites. 

PdBC COATED ROTATING ACRYLIC DISCS MOUNTED ON 

5 to 6 hours. The white BC turns black due to palladization, indicating the formation of Pd nanoparticles in the BC network. This Pd-BC composite was investigated for its catalytic efficacy of dechlorination both as a batch process as well as a continuous process at atmospheric pressure and room temperature in the same reactor vessel. Dechlorination of PCP into phenol and HCl takes place due to reduction by active hydrogen present on the surface of Pd nanoparticles. The dechlorination reaction is written as  

Nanoparticles of TiO are known to be photocatalytic, and hence Sun et ah synthesized TiO nanoparticles supported into BC matrix [27]. These nanoparticles were 

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In this paper, the effect of sintering atmosphere and temperature on the activity of a typical three-way autoinotive exhaust catalyst system (Pt/Rh/y-Al O /CeO ) and the implications of correlating fundamental catalyst characterization data such as average noble metal crystallite size and composition with catalytic activity are discussed. 

Metal Composites with Catalytic Activity in Biomass Conversion... 

What is the relationship between carbide composition and catalytic activity Figure 5.9 (right) compares the two properties. The catalytic activity starts low, increases rapidly to a maximum and decreases slowly thereafter. The increase in activity occurs simultaneously with the carburization, but, without any direct correlation, the maximum in rate occurs when there is still metallic iron present. The interpretation given to the results in Fig. 5.9 is the following [22,24],... 

Many compositional variations give complexes with catalytic activity, some of which are shown in Table I. 

Qualitative differences in catalytic activities are referred to above. It is beyond the scope of this paper to present a detailed description of the interrelation of the differences in composition and preparative methods with catalytic activity. However, to give some perspective, results on the activation of one such catalyst by the combination of a coordinating agent and a salt is given in Table II. 

Figure 28 Calculated and experimental liquid compositions for experiments with catalytically active rings.

Hydrogenation of carbon dioxide to methanol was investigated over Cu/ZnO catalysts prepared by mechanical alloying(MA) method, which is suitable for excellent mixing of different materials to make alloys or composites. The catalytic activity increases with mechanical milling time, and methanol yield over the catalyst milled for 120 hour is about 1.5 times higher than that of conventional coprecipitated Cu/ZnO catalyst. The reason for increasing catalytic activity by MA method can be attributed to the preparation of well mixed structure of Cu and ZnO nanocrystals. 

Figure 10.10 demonstrates the simulated and measured concentration profiles for the pilot column with the reactive section filled with catalytically active rings. In the simulations, four components, namely, methanol, isobutene, MTBE and 1-butene, were chosen to represent the chemical system under consideration. Here, segment 1 corresponds to the reboiler. A satisfactory agreement between calculated and measured values can be clearly observed. In Fig. 10.11, the simulation results for the column packed with MULTIPAK are shown. Here, 16 components are considered, and, again, the liquid bulk composition profiles agree well with the experimental data. 

S. Lambert, B. Heinrichs, A. Brasseur, A. Ruhnont, and J.-P. E irard, Determination of Surface Composition of AUoy Nanoparticles and Relationships with Catalytic Activity in Pd-Cu/Si02 Cogelled Xerogel Catalysts, Appl. Catal. A, 270, pp. 201-08, 2004. 

Development of new, biodegradable, polymeric materials. The precise formulation of the LDH with catalytically active metal cations might permit their role as biocatalysts to favour degradation of the polymeric component. Polymer-nanoclay composites are currently very actively studied as potential candidates for oxo-biodegradable plastics [166]. 

Basic oxides of metals such as Co, Mn, Fe, and Cu catalyze the decomposition of chlorate by lowering the decomposition temperature. Consequendy, less fuel is needed and the reaction continues at a lower temperature. Cobalt metal, which forms the basic oxide in situ, lowers the decomposition of pure sodium chlorate from 478 to 280°C while serving as fuel (6,7). Composition of a cobalt-fueled system, compared with an iron-fueled system, is 90 wt % NaClO, 4 wt % Co, and 6 wt % glass fiber vs 86% NaClO, 4% Fe, 6% glass fiber, and 4% BaO. Initiation of the former is at 270°C, compared to 370°C for the iron-fueled candle. Cobalt hydroxide produces a more pronounced lowering of the decomposition temperature than the metal alone, although the water produced by decomposition of the hydroxide to form the oxide is thought to increase chlorine contaminate levels. Alkaline earths and transition-metal ferrates also have catalytic activity and improve chlorine retention (8). 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

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