Hydrocarbons as reducing agent

The catalytic reduction of nitrogen oxides, NO, that are produced by combustion of fuels at high temperatures (=1800°C during electric power generation) is one of the important environmental catalytic problems. Review the process that uses ammonia or small hydrocarbons as reducing agents, and list the catalysts that are employed [204, 205]. 

Nitrogen oxides (NO, i.e., NO, NOj, N2O) are considered as major air pollutants responsible for photochemical smog, acid rain, ozone depletion, as well as climate change. The conventional route to eliminate NO pollution is to reduce NO catalytically into N2 using NH3, urea, H, CO, and hydrocarbons as reducing agents. The decomposition of N2O is a kinetically limited reaction and inhibited by the oxygen molecule produced ... 

The reoxidation of the reduced copper phase was accomplished by temperature programmed oxydafion (TPO). All the studied catalysts could be almost completely reoxidized to CuO, as indicated by the values of oxidation-percent calculated (>60 %). Quite the same TPO profiles were observed on all the catalysts with two oxidation steps at. first, oxidation of surface copper species and at higher temperature, bulk oxidation. The effective redox properties of CuO allow it to find application in several reaction processes demanding cyclic reduction and reoxidation process with the reactants (i.e., selective catalytic reduction of NOx with hydrocarbons as reducing agent, [20-22]). 

The most widely employed thermal reduction process for preparing Mg metal uses PeSi as reducing agent. Mixtures of the substrate, usually calcined dolomite (i.e., MgO, CaO) and PeSi are fabricated into briquettes with a hydrocarbon binder and loaded into Ni-Cr steel (15/28) retorts. After evacuation the retort is subjected to a temperature gradient Mg distills from the hot mixture (at 1150°C) and high-purity Mg crystals collect at the water-cooled end of the retort ... 

Another way to work in transient conditions is to stop suddenly (or conversely to instantaneously introduce) one of the reactants, in order to destabilize the system and to enhance the concentration of labile species. With this method, for example, Poignant et al. studied the DeNO. reaction mechanism on a H—Cu-ZSM-5 catalyst, using propane or propene as reducing agents. The introduction of 2000 ppm of hydrocarbon in a flow of NO (2000 ppm) + 5% 02 allowed to evidence the formation of acrylonitrile, which behaved as an intermediate. Its reactivity with NO+ species constituted a fundamental point to describe a detailed SCR mechanism for NO removal on zeolitic compounds [137],... 

We have concentrated our efforts on reducing metal salts in ethereal or hydrocarbon solvents using alkali metals as reducing agents(l 6-54,77). 

As with the ELCD, post-column treatment of the effluent is involved. The RCD responds to compounds that serve as reducing agents, such as alcohols, aldehydes, oleflns, and carboxylic acids, and it is insensitive to many potentially interfering compounds such as water, oxygen, hydrocarbons and carbon dioxide ... 

At the outset, two major goals were established to achieve broad applicability for sulfur dioxide reduction in emission control. The first goal was to develop process capability encompassing the widest practical range of inlet sulfur dioxide concentrations while the second was to develop process modifications so that various gaseous and liquid hydrocarbons could be used as reducing agents. 

For catalytic reduction processes, in which hydrocarbons were used as reducing agents, copper exchanged zeolites showed the best performance, for example for propene containing feed compositions [11], since close to 100% NO conversion could be attained at as low as 623 K. An acidic form of iron-silicate showed high activity and stability for reduction of NO with propene, but this catalyst was very sensitive to the presence of SO2 in the feed [12]. Metallosilicates having ZSM-5 structure [13,14] seem to be a new class of catalysts for these reactions. 

Table IX, 7, Catiilytic oxidation of hydrocarbons by dioxygeii in the presence of solid metals as reducing agents.

Alternatively, hydrocarbons can also be used as reducing agents, by applying a technology closer to that of vehicles exhaust control [3]. 

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