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The reduction reaction is a type of redox reaction, which is the capture of electrons or a decrease in oxidation state. Selective catalytic reduction (SCR) is a common catalytic reduction reaction, which refers to the use of reducing agents (such as NH3, liquid ammonia, urea) to "selectively" react with NOx in flue gas and produce non-polluting non-toxic N2 and H2O under the action of a catalyst.


A common SCR catalyst consists of a carrier and an active component. The support is mainly various ceramic materials such as titanium oxide, and the active catalytic component is usually an oxide of a base metal such as vanadium, molybdenum and tungsten, a zeolite or various noble metals. Current activated carbon based catalysts are a novel SCR catalyst suitable for the removal of NOx at low temperatures.

Reducing agent:

Substances that are capable of reducing other substances to acquire electrons are called reducing agents. The reducing agent oxidizes itself after transferring the electrons to another substance, which is also called an electron donor, because it "donates" electrons. The electron donor can also form a charge transfer complex with the electron acceptor.

There are many types of reducing agents in chemistry. Metal elements that exhibit electropositivity, such as lithium, sodium, magnesium, iron, zinc and aluminum, are good reducing agents that give electrons easily. Hydride transfer reagents widely used in organic chemistry, such as NaBH4 and LiAlH4, mainly function to reduce a carbonyl compound to an alcohol. Another reduction method uses hydrogen (H2) and a palladium, platinum or nickel catalyst. These catalytic reductions are mainly adopted to reduce carbon-carbon double or triple bonds.


The reduction reaction can be seen everywhere in human society and even in nature. Its application is also very extensive, mainly in the three aspects of industry, biology and geology.

1) SCR denitrification in industry:

In the SCR denitration process, NOx can be converted into nitrogen (N2) and water (H2O) naturally contained in the air by adding ammonia. The main chemical reactions are as follows:

The illustration of selective catalytic reduction
Figure 1. The illustration of selective catalytic reduction

2) Biotechnology applications: A reduction reaction occurs every moment in the living body to meet the specific physiological activities of the organism. The most common are cellular breathing and energy storage and release.

  • Cellular breathing. Cellular respiration is the oxidation of glucose (C6H12O6) to CO2 and the reduction of oxygen to water.
  • Energy storage and release. For example, photosynthesis involves the reduction of carbon dioxide to sugar and the oxidation of water to the constituent oxygen.


  1. Bockris, John O'M (1970). 'Reddy, Amulya K. N. Modern Electrochemistry'. Plenum Press. 352-355.
  2. Hudlický, Miloš (1996). 'Reductions in Organic Chemistry'. Washington, D.C.: American Chemical Society.429-429.
  3. Robertson, William (2010). 'More Chemistry Basics'. National Science Teachers Association.82-82.
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