New catalyst outshines platinum for producing hydrogen

Hydrogen, the most abundant element in the universe, packs a powerful punch. And because it contains no carbon, it produces only water when used as a fuel. But on Earth, hydrogen most often exists in combination with other ...

Breakthrough in harnessing the power of biological catalysts

The power of nature could soon be used to create day-to-day materials such as paints, cosmetics and pharmaceuticals in a much more environmentally friendly way, thanks to a new breakthrough from scientists.

Team solves decade-old mystery in chemical transformations

Researchers at Pacific Northwest National Laboratory (PNNL) have solved a mystery for a chemical reaction essential for fuel and fertilizer production. The so-called water-gas shift reaction forms hydrogen fuel and carbon ...

Nanocatalyst makes heavy work of formic acid

Hydrogen occurs in nature as H2 molecules; however when deuterium isotopes—so called "heavy hydrogen"—are introduced, the result can be deuterium hydride (HD) or deuterium gas (D2). These compounds are useful starting ...

Shocking heat waves stabilize single atoms

Single atoms work great as catalysts, but they usually don't stay single for long. Argonne scientists are part of a team that uses high-temperature shock waves to keep them in their place.

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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.

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