PROX
PROX is a technical term from fuel cell technology . The acronym stands for pr eferential ox idation, that is a preferred oxidizing a usually gaseous substance to a catalyst .
Example carbon monoxide
The catalytic, preferential oxidation of carbon monoxide (CO) takes place on a heterogeneous catalyst on a ceramic carrier to carbon dioxide (CO 2 ). The catalysts used are noble metals such as platinum , platinum / iron , platinum / ruthenium , gold nanoparticles use and novel copper / cerium mixed oxide catalysts. This reaction is the subject of current fuel cell research. The production of hydrogen produces carbon monoxide. Hydrogen contaminated with CO reduces the efficiency of the reaction in the H 2 / O 2 fuel cell , since CO is adsorbed on the platinum anode . The PROX is used to reduce the CO after the water gas shift reaction from a concentration in the range 0.5–1.5% in the fuel gas to values that are as uncritical as possible ( 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
The disadvantage of this technology is the very strong exothermic reaction, a very narrow temperature window for optimal operating conditions (approx. 50 Kelvin ) and a loss of hydrogen by one percentage point. Efficient cooling is required. In order to minimize the loss of hydrogen and further dilution with air-nitrogen, the reaction is usually carried out in two stages with intermediate cooling. In the first reactor, an excess of oxygen by a factor of 2 is added and around 90% of the CO is converted . In the second stage, a significantly higher excess of air by a factor of 4 is added in order to reduce the remaining CO content to a concentration of <10 ppm. In order to avoid CO peaks even during load changes , a downstream CO adsorption may be necessary in transient operation . The outlay in terms of apparatus and control technology is relatively large. The advantage over selective methanation is a higher space velocity and thus smaller reactors. In the event of a sharp rise in temperature, the supply of air can simply be interrupted.
The technical origin for the CO-PrOx lies in the ammonia synthesis . A largely CO-free synthesis gas is absolutely essential there, as CO is a strong catalyst poison for the iron-based catalysts commonly used there .