Catalytic preferential oxidation

Present practice in the Haber-Bosch process for utilizing water-gas in ammonia synthesis results in the production of a gas containing 1 to 2 per cent carbon monoxide, a concentration too high to be removed practically in a catalytic preferential oxidation process because of reduction... 

A possible approach Natural gas can be converted at a high temperature into hydrogen, CO, C02 (syngas) in a steam reformer or partial-oxidation reactor, or autothermal reformer which is a combination of the first two. Most of the CO in the syngas is typically converted into carbon dioxide at a lower temperature in a water-gas shift reactor. The remaining small amount of CO must be removed to below 10 ppm level. This can be done using adsorption, or membrane separation, or catalytic preferential oxidation (at about 90°C with an air stream), or other practical means. Also, there are designs with membrane reformers in the literature. 

Gosavi, P.V. and Biniwale, R.B. (2013) Catalytic preferential oxidation of carbon monoxide over platinum supported on... 

Here again, as in the case of methanol steam reforming, the formation of methane under these conditions is prevented via the specificity of the shift-converter catalyst. For PEM fuel cells, further reduction in CO concentration is necessary (from about 0.5 percent down to less than 100 ppm). This is often carried out by way of catalytic preferential oxidation of CO in the presence of hydrogen with the addition of air at a flow rate that is a small multiple of the stoichiometric rate required for complete oxidation of the CO. 

The main unit is the catalytic primaiy process reactor for gross production, based on the ATR of biodiesel. After the primary step, secondary units for both the CO clean-up process and the simultaneous increase of the concentration are employed the content from the reformated gas can be increased through the water-gas shift (WGS) reaction by converting the CO with steam to CO and H. The high thermal shift (HTS) reactor is operating at 575-625 K followed by a low thermal shift (LTS) reactor operating at 475-535 K (Ruettinger et al., 2003). A preferential oxidation (PROX) step is required to completely remove the CO by oxidation to COj on a noble metal catalyst. The PROX reaction is assumed to take place in an isothermal bed reactor at 425 K after the last shift step (Rosso et al., 2004). 

Table 3.1 shows the catalytic performance of supported Au catalysts for the preferential oxidation (PROX) of CO in H2 together with the actual reaction conditions and targeted performances. Au/A1203 [61-63], Au/Mn203 [58], Au/ Fe203 [54, 60, 61, 64—66] and Au/Ce02 [54, 60-62, 67-70] have been reported to... 

Here, the chosen domain for our case study is on-board hydrogen production to supply pure H2 to a fuel cell in an electrical car. Among the sequential catalytic reactions that take place for H2 production, the hydrogen purification units are located downstream, after the primary reforming of hydrocarbons into a CO-H2 mixture or Syngas units. They consist of Reaction (1) the water-gas shift (WGS) reaction and Reaction (2), the selective or preferential oxidation of CO in the presence of hydrogen (Selox). 

Figure 2.81 Network of 27 catalyst beds (five for pre-reforming, nine for reforming, nine for catalytic combustion to supply the heat, two for water-gas shift and two for preferential oxidation) and of 20 heat exchangers [128]...

R 20] The fuel processing system consists of a fuel evaporator, a reformer, a reactor for the preferential oxidation of carbon monoxide and a catalytic burner (Figure 4.48) [95],... 

Although a suitable acceptor for the transfer dehydrogenation of benzylic alcohols has not yet been found, under the present conditions the low conversion of benzylic alcohols is only an apparent drawback. Indeed, it has a positive side as it allows us to fine-tune the system s selectivity. This makes the catalytic system unique among all the others known, operating both under aerobic and anaerobic conditions, that preferentially oxidize benzylic alcohols with respect to nonacti-vated secondary ones. 

However, the removal of carbon monoxide by water-gas shift to a low level still demands its selective oxidation to the minimum concentration possible. Much research and development has been conducted during the past decades to find a gold catalyst that can do this the target is usually described by the acronym PROX (preferential oxidation), but sometimes as SCO (selective catalytic oxidation). The task is somewhat simplified by the constraints that are externally imposed the preferred feed gas, often termed idealised reformate, has the composition 1.0% CO, 1.0% 02, 75.0% H2, balance nitrogen or other inert gas, and while of course variations to this composition can be made to explore the kinetics and mechanism, and the effects of the products water and carbon dioxide can be added to observe their effects, the successful catalyst must remove almost all the carbon monoxide (to <10 ppm) and less than 0.5% hydrogen. This requirement is expressed as a selectivity based on the percentage of the oxygen consumed that is taken by the carbon monoxide this should exceed 50%, under conditions where the conversion of carbon monoxide is above 99.5%.5... 

H. Tanaka, S. Ito, S. Kameoka, K. Tomishige, and K. Kunimori, Catalytic performance of K-promoted Rh/USY catalysts in preferential oxidation of CO in rich hydrogen, Appl. Catal. 

Besides the complex cases of mixed oxides, there exist more simple problems of oxide and scale formation in alloy production. The detrimental effect of oxide shells around metal particles preventing intermixing is well known. The compositional changes resulting from preferential oxidation of one component have also to be taken into account. Instability of the product and/or drastic changes in the thermochemical properties of the material after shell formation (such as massive increases in the required fusion temperature in noble metal eutectic mixtures) are common, in particular in small-scale preparations. These effects still set limits to the availability of catalytically desired alloys for practical purposes (e.g. for compounds with Zr, Si, alkali, Mg). 

The selective oxidation or preferential oxidation of CO in hydrogen-rich stream is another important object for ceria based catalysts. The gas mixture from steam reforming/partial oxidation of alcohols or hydrocarbons, followed by the WGS reaction contains mainly FI2, CO2 and a small portion of CO, H2O, and N2. When such gaseous stream would be taken as input for hydrogen fuel cells, the CO has to be removed to avoid poisoning of the anode electrocatalysts. Ceria based nanomaterials, such as ceria/gold, ceria/copper oxide catalysts exhibit suitable catalytic activities and selectivities for CO PROX process. 

Under normal operating conditions, in which the combustor is sufficiently warm and operated under fuel rich conditions, virtually no NOx is formed, although the formation of ammonia is possible. Most hydrocarbons are converted to carbon dioxide (or methane if the reaction is incomplete) however, trace levels of hydrocarbons can pass through the fuel processor and fuel cell. The shift reactors and the preferential oxidation (PrOx) reactor reduce CO in the product gas, with further reduction in the fuel cell. Thus, of the criteria pollutants (NOx, CO, and non-methane hydrocarbons [NMHC]), NOx CO levels are generally well below the most aggressive standards. NMOG concentrations, however, can exceed emission goals if these are not efficiently eliminated in the catalytic burner. 

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