PROX is an acronym for PReferential OXidation, and refers to the preferential oxidation of a gas on a catalyst.
The catalyser preferentially oxidises carbon monoxide (CO) using a heterogeneous catalyst placed upon a ceramic support. Catalysing agents include metals such as platinum, platinum/iron, platinum/ruthenium, gold nanoparticles as well as novel copper oxide/ceramic conglomerate catalysts.
This reaction is a considerable subject area of research for fuel cell design. From the reaction of carbon monoxide with steam, carbon dioxide is evolved.
The presence of CO impedes the reaction of H2/O2 and reduces the fuel cell efficiency by the absorption of CO onto the platinum anode. The PROX process allows for the reaction of CO via the water gas shift reaction (WGS), reducing CO concentration from approximately 0.5-1.5% in the feed gas to less than 10 ppm.
However, a selectivity of 100% is impossible. The selectivity, S, is used in reference to the auxiliary reaction, which is the oxidation of hydrogen.
The disadvantage of this technology is its very strong exothermic nature, coupled with a very narrow optimal operation temperature window, and is best operated between 353 and 450 kelvins,[1] yielding a steam loss of around one percent. Effective cooling is therefore required. In order to minimise steam generation, excessive dilution with nitrogen is used. Additionally the reaction is interrupted with an intermediary cooler before proceeding to a second stage.
In the first reaction an excess of oxygen is provided, at around a factor of two, and about 90% of the CO is transformed. In the second step a substantially higher oxygen excess is used, at approximately a factor of 4, which is then processed with the remaining CO, in order to reduce the CO concentration to less than 10 ppm. To also avoid excess CO-fraction loading, the transient operation of a CO adsorber may be important.
The instrumentation and process control complexity requirements are relatively high. The advantage of this technique over selective methanation is the higher space velocity, which reduces the required reactors size. For the case of strong temperature rises, the feed of air can simply be broken.
The technical origins for CO-PrOx lies in the synthesis of ammonia (Haber process). Ammonia synthesis also has a strict requirement of CO-free syngas, as CO is a strong catalyser poison for the usual catalysts used in this process.