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Reducing agent | reductant | reducer. the element or compound in an oxidation-reduction reaction that donates an electron to another species. Because the reducing agent is losing electrons, we say it has been oxidized.
Share This Page. Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system used in diesel engines. Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF).