PROX

PROX is a technical term from the fuel cell technology. The acronym stands for preferential oxidation, that is, a preferred oxidation of a gaseous substance usually over a catalyst.

Example carbon monoxide

The catalyst, preferential oxidation of carbon monoxide (CO ) is carried out over a heterogeneous catalyst on a ceramic support to carbon dioxide (CO2). The catalysts used are precious metals such as platinum, platinum / iron, platinum / ruthenium, gold nanoparticles using novel and Kupfer-/Cer-Mischoxid-Katalysatoren. This reaction is the subject of current fuel cell research. In the preparation of hydrogen of carbon monoxide produced. CO contaminated hydrogen is in the reaction in the H2/O2-Brennstoffzelle efficiency decreased, as CO is adsorbed to the platinum anode. The PROX is used according to the water gas shift reaction of a concentration ranging from 0.5 to 1.5 % to lower the CO from the fuel gas to uncritical possible values ​​(PEMFC <10 ppm).

However, a selectivity of 100 % is never possible in chemical reactions. The selectivity S describes the extent of side reactions. The decisive competitive reaction is the oxidation of hydrogen

Disadvantage of this technology is the highly exothermic reaction, a very narrow temperature window for optimum operating conditions (about 50 kelvins ) and a hydrogen loss of one percentage point. It is an effective cooling required. To minimize the loss of hydrogen and a further dilution with air-nitrogen, the reaction generally is carried out in two stages with intermediate cooling. In the first reactor excess oxygen is added by a factor of 2 and implemented about 90 % of the CO. In the second stage, a much higher excess air is added to a factor of 4 lower <10 ppm for the remaining CO content to a concentration. To avoid even during load changes CO peaks in transient operation nor a downstream CO adsorption may be necessary. The apparatus and additional control circuitry is relatively large. The advantage over the selective methanation is a higher space velocity and thereby smaller reactors. In the case of a strong temperature rise can easily be interrupted the supply of air.

The technical origin of the CO is PrOx in ammonia synthesis. There, a far-reaching CO - free syngas is also absolutely necessary since CO is a strong catalyst poison for the iron-based catalysts available there.