News tagged with catalyst
Related topics: fuel cell , chemical reactions , hydrogen , carbon dioxide , carbon monoxide
Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. The catalyst may participate in multiple chemical transformations. Catalysts that speed the reaction are called positive catalysts. Catalysts that slow down the reaction are called negative catalysts or inhibitors. Substances that increase the activity of catalysts are called promoters and substances that deactivate catalysts are called catalytic poisons. For instance, in the reduction of ethyne to ethene, the catalyst is palladium (Pd) partly "poisoned" with lead(II) acetate (Pb(CH3COO)2). Without the deactivation of the catalyst, the ethene produced will be further reduced to ethane.
The general feature of catalysis is that the catalytic reaction has a lower rate-limiting free energy change to the transition state than the corresponding uncatalyzed reaction, resulting in a larger reaction rate at the same temperature. However, the mechanistic origin of catalysis is complex. Catalysts may affect the reaction environment favorably, e.g. acid catalysts for reactions of carbonyl compounds, form specific intermediates that are not produced naturally, such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause lysis of reagents to reactive forms, such as atomic hydrogen in catalytic hydrogenation.
Kinetically, catalytic reactions behave like typical chemical reactions, i.e. the reaction rate depends on the frequency of contact of the reactants in the rate-determining step. Usually, the catalyst participates in this slow step, and rates are limited by amount of catalyst. In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts.
Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid-liquid system or evaporate in a solid-gas system.
This text uses material from Wikipedia and is available under the GNU Free Documentation License.
Enzyme hunters at UiO have discovered the function of an enzyme that is important in the spreading of cancer. Cancer researchers now hope to inhibit the enzyme.
Cancer Nov 15, 2012 | 5 / 5 (1) | 0 |
Researchers from the University of Alicante (Spain) have analysed ten brands of cigarettes and found that the concentrations of certain harmful and carcinogenic substances vary significantly from one brand ...
Health Oct 01, 2012 | not rated yet | 0
Researchers at the University of Copenhagen have designed, produced and patented a new chemical compound for the possible treatment of brain damage caused by stroke. The compound binds 1,000 times more effectively to the ...
Medical research Feb 21, 2012 | not rated yet | 0 |
Cancer Research UK scientists have found that blocking the pathway used by some kidney cancer cells to generate energy can kill the cancer cells, sparing the healthy ones. The research is published in Nature yesterday. ...
Cancer Aug 18, 2011 | not rated yet | 0 |