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An illustrative example is the effect of catalysts to speed the decomposition of hydrogen peroxide into water and oxygen: . 2 H 2 O 2 → 2 H 2 O + O 2. This reaction proceeds because the reaction products are more stable than the starting compound, but this decomposition is so slow that hydrogen peroxide solutions are commercially available.
The first step in the WGS reaction is the high temperature shift which is carried out at temperatures between 320 °C and 450 °C. As mentioned before, the catalyst is a composition of iron-oxide, Fe 2 O 3 (90-95%), and chromium oxides Cr 2 O 3 (5-10%) which have an ideal activity and selectivity at these temperatures.
In chemistry, a catalytic cycle is a multistep reaction mechanism that involves a catalyst. [1] The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, bioinorganic chemistry, materials science, etc.
These catalysts initiate radical chain reactions, autoxidation that produce organic radicals that combine with oxygen to give hydroperoxide intermediates. Generally the selectivity of oxidation is determined by bond energies. For example, benzylic C-H bonds are replaced by oxygen faster than aromatic C-H bonds. [2]
Often, substances are intentionally added to the reaction feed or on the catalyst to influence catalytic activity, selectivity, and/or stability. These compounds are called promoters. For example, alumina (Al 2 O 3) is added during ammonia synthesis to providing greater stability by slowing sintering processes on the Fe-catalyst. [2]
In these reactions, the conjugate acid of the carbonyl group is a better electrophile than the neutral carbonyl group itself. Depending on the chemical species that act as the acid or base, catalytic mechanisms can be classified as either specific catalysis and general catalysis. Many enzymes operate by general catalysis.
Examples include the Friedel-Crafts reaction, the aldol reaction, and various pericyclic processes that proceed slowly at room temperature, such as the Diels-Alder reaction and the ene reaction. In addition to accelerating the reactions, Lewis acid catalysts are able to impose regioselectivity and stereoselectivity in many cases.
Autocatalytic cycle of formose reaction showing how glyceraldehyde can be both the catalyst and the product of one portion of this complex reaction type. An early example of autocatalysis is the formose reaction , in which formaldehyde and base produce sugars and related polyols.