Catalytic reactions preferential oxidation

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). 

Although a variety of amines, particularly trimethylamine and n-butylamine have widely been used as poisons in catalytic reactions and for surface acidity determinations (20), comparably few spectroscopic data of adsorbed amines are available. As with ammonia, coordinatively adsorbed amines held by co-ordinatively unsaturated cations have preferentially been found on pure oxides (176, 193-196), whereas the protonated species were additionally observed on the surfaces of silica-aluminas and zeolites (196-199). However, protonated species have also been detected on n-butylamine adsorption on alumina (196) and trimethylamine adsorption on anatase (176) due to the high basicity of these aliphatic amines. In addition, there is some evidence for dissociative adsorption of n-butylamine (196) and trimethylamine (221) on silica-alumina. Some amines undergo chemical transformations at higher temperatures (195, 200) and aromatic amines, such as diphenylamine, have been shown to produce cation radicals on silica-alumina (201, 201a). 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

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. 

In 1979, Groves and co-workers published the first article on the use of a synthetic iron(III) porphyrin complex, Fe(TPP)Cl (TPP = weio-tetraphenylporphyrin), in catalytic olefin epoxida-tion and alkane hydroxylation reactions by iodosylbenzene (PhlO). Olefins were preferentially oxidized to the corresponding epoxides, and alcohols were obtained as major products in alkane hydroxylations (Equations (3) and (4)) ... 

Opportunities in Catalytic Reaction Engineering. Examples of Heterogeneous Catalysis in Water Remediation and Preferential CO Oxidation... 

The preferential oxidation of CO was studied in a silicon MSR consisting of two inlets, a premixer, a catalytic zone and a outlet cooling zone [60]. A Pt/alumina catalyst layer of 2-5 pm was obtained by successive sol-gel procedures. Using appropriate criteria, it was estimated that no heat- and mass-transfer limitations occurred below 220 °C, thus giving access to the intrinsic reaction kinetics. The authors pointed out the advantage of their M S R compared with packed-bed reactors that they were able to avoid any hot spots in the reactor, thus preventing the reverse WGS reaction from taking place. 

The left half of Figure 15.3, thus, depicts the catalytic processes that different fuels imdergo, such as reforming, high-temperature shift and low-temperatore shift, and preferential oxidation (PrOx), which are individually conducted at different temperatures so as to make the most of their thermodynamic limitations and kinetics. In direct fuel ceUs with relatively complex fuels, such as methanol, in fact, many of these reaction steps occur electrocatalylicaUy within the fuel cell, thus accounting for the large oveipotentials, e.g., in a DMFC. 

Formation of carbon monoxide over the catalyst by the reverse water-gas shift reaction (RWGS) in an oxygen-deficient atmosphere is frequently observed especially under conditions of partial load, because most catalysts for preferential oxidation of carbon monoxide have some activity for WGS and its reverse reaction. Therefore oversizing the reactor bears the danger of impaired conversion and the same applies for partial load of the reactor unfortunately. Because the concentration of carbon monoxide that is tolerated by low-temperature fuel cells is usually in the range below 100 ppm or less, even low catalytic activity for reverse shift becomes an issue. 

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