Purpose of a Catalyst

Increasing temperature to speed up a reaction is often a bad idea. If I have a plant that is desulferizing diesel oil with hydrogen, so as to convert the sulfur in the diesel to H S, I can achieve the required percent of sulfur removal in two ways  

At 850°F, desulferization wiU proceed quickly, but the diesel oil will also thermally crack to coke and gas and be useless as a fuel for trucks. What is wanted is a way to accelerate the conversion of the sulfur molecules attached to the diesel to H S at a moderate temperature (about 700°F). That s what a catalyst does. It does not change the equilibrium, it just makes the reaction go faster without having to increase the temperature. 

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As pointed out clearly by Weisz [1980], the unique role and purpose of a catalyst are to provide selectivity to direct the chemical transformation along a very specific, desired path. The selective feature of catalysis, whereby reactions can be turned onto desired pathways, greatly... 

Rubber blends with cure rate mismatch is a burning issue for elastomer sandwich products. For example, in a conveyor belt composite structure there is always a combination of two to three special purpose rubbers and, depending on the rubber composition, the curatives are different. Hence, those composite rubber formulations need special processing and formulation to avoid a gross dissimilarity in their cure rate. Recent research in this area indicated that the modification of one or more rubbers with the same cure sites would be a possible solution. Thus, chlorosulfonated polyethylene (CSP) rubber was modified in laboratory scale with 10 wt% of 93% active meta-phenylene bismaleimide (BMI) and 0.5 wt% of dimethyl-di-(/ r/-butyl-peroxy) hexane (catalyst). Mixing was carried out in an oil heated Banbury-type mixer at 150-160°C. The addition of a catalyst was very critical. After 2 min high-shear dispersive melt mix-... 

It is not the aldehyde or ketone itself that is halogenated, but the corresponding enol or enolate ion. The purpose of the catalyst is to provide a small amount of enol or enolate. The reaction is often done without addition of acid or base, but traces of acid or base are always present, and these are enough to catalyze formation of the enol or enolate. With acid catalysis the mechanism is... 

The actual product of the reaction is thus the ketene, which then reacts with water (15-3), an alcohol (15-5), or ammonia or an amine (15-8). Particularly stable ketenes (e.g., Ph2C=C=0) have been isolated and others have been trapped in other ways (e.g., as P-lactams, 16-64). The purpose of the catalyst is not well understood, though many suggestions have been made. This mechanism is strictly analogous to that of the Curtius rearrangement (18-14). Although the mechanism as shown above involves a free carbene and there is much evidence to support this, it is also possible that at least in some cases the two steps are concerted and a free carbene is absent. 

Noble metal ions can be easily reduced to the corresponding zero-valent metal atoms. Therefore, bimetallic nanoparticles consisting of two different noble metals have been extensively investigated for purpose of novel catalysts and optical materials. A simultaneous reduction of two noble metal ions with alcohol is a simple and useful technique to prepare bimetallic nanoparticles. The alcohol reduction of metal ions M + is followed by Equation (1). 

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

The practical application of a catalyst not only depends on its catalytic activity but also on its stability. Therefore, it was of interest to study the stability of the three catalysts during three successive acetophenone hydrogenation reactions. Tests carried out for this purpose consisted in hydrogenating acetophenone until reaching 100% conversion. The catalyst was then washed with isopropyl alcohol and allowed to act again, so that catalysts were tested in a series of three hydrogenation cycles. 

Hydroperoxides may be determined by measuring at 290 nm (e = 44100 M cm ) or 360 nm (e = 28000 cm ) the concentration of 13 formed in the presence of a large excess of ions. The reaction may be too slow for practical purposes, unless a catalyst is present. For example, an assay for lipid hydroperoxides conducted without a catalyst may require several measurements every 6 min until the absorbance reaches a maximum. Exclusion of air from the sample cuvette is important. The method is about 1000-fold more sensitive than thiosulfate titration The iodometric method with UVD at 360 was adopted for detecting the presence of hydroperoxides derived from protein, peptide or amino acid substrates subjected to y-radiation, after destroying the generated H2O2 with catalase. ... 

The above-mentioned study was followed by a report of the groups of Ji, Loh and coworkers, who reported the application of a catalyst based on cuprous iodide and Tol-BINAP for the same purpose . Noteworthy is that the effective use of a C2-symmetric ligand in this reaction marks the end of the aforementioned metal-differentiating coordination concept. It was shown that a variety of Grignard reagents could be used for the... 

The purpose of a reforming catalyst is, after all, to provide as favorable a yield-octane number relationship as possible, and all the foregoing can be sumnted up by the yield-... 

To complete the discussion of factors involved in the design of gas-solid heterogeneous catalytic reactors we will examine several aspects of the kinetics of chemical reactions occurring in the presence of a catalyst surface. We consider, for heuristic purposes, the equilibrium reaction ... 

The selection of a catalyst or the design of a new catalyst for a particular purpose will depend on activity, selectivity, and stability requirements. These requirements are generally related but not necessarily compatible in... 

The identification of the regions of high and low activity on the surface of a catalyst is of paramount importance. The purpose of this chapter is to show that for many reactions the different crystal faces have markedly different activities, the relative order of activity among faces being dependent on the conditions of the reaction. The identification of the active regions of a catalyst has long remained unnecessarily mysterious. There are several possible reasons for this. 

Alcohols normally require the use of a catalyst 11RF is frequently employed for that purpose. 

Up to this point, we have primarily studied the descriptive aspects of chemical reactions. That is, we ve discussed what s occurring during reactions without much detail about the driving forces behind the reactions. The purpose of the next two chapters is to provide such detail. In this chapter, we will look at the major models that have been developed to explain the mechanisms by which reactions occur. The focus of this chapter is kinetics, an area of chemistry that explores the rates at which chemical reactions occur. In the first portion of the chapter, we will focus on a few techniques we use to describe or define the rate of a reaction. Once that is established, we will move our attention to the factors that affect the rate of a reaction concentration, temperature, surface area, and the presence of a catalyst. 

Oxygen is readily evolved at the ordinary temperature on adding water to a mixture of bleaching powder and an alkali or alkaline earth peroxide in the presence of a catalyst such ns ferrous or copper sulphate. If the solid mixture is pressed into small lumps or cubes, it may be used in a Kipp s apparatus and thus afford a convenient method of preparing the gas for lecture or laboratory purposes.4... 

Cellulose acetate is usually produced by the so-called solution process with exception of the fully acetylated end product (triacetate). In the solution process the pulp is first pretreated with acetic acid in the presence of a catalyst, usually sulfuric acid. The purpose of this activation step is to swell the fibers and increase their reactivity as well as to decrease the DP to a suitable level. Acetylation is then performed after addition of acetic anhydride and catalytic amounts of sulfuric acid in the presence of acetic acid. After full acetylation the final triacetate obtained is dissolved. This "primary" acetate is usually partially deacetylated in aqueous acetic acid solution to obtain a "secondary" acetate with a lower DS of about 2 to 2.5. 

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