Selox

Selox [Selective oxidation] A process for selectively oxidizing methane to syngas using a proprietary heterogeneous catalyst at temperatures up to 1,000°C. Developed on a laboratory scale by TRW, CA, partly financed by the U.S. Department of Energy in 1983. 

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). 

For the Selox reaction, the oxidation mechanism may involve oxygen species that are stored in/over the catalyst support, such the ceria-based materials used in three-ways catalytic exhaust systems [16]. Consequently, the oxygen storage capacity (OSC) can also be considered as an objective function, if one assumes or dem-... 

Pros The choice of targets or objective functions is rather wide, depending on the reaction and application envisioned. Possibly, one easily accessible objective function such as CO conversion can be replaced by another more intrinsic parameter like the OSC for the Selox reaction, subject to a preliminary mechanistic investigation that clearly demonstrates the equivalence of the two parameters for evaluating performance. For the WGS reaction, selectivity is not a problem, except for methane and methanol side formation. For the Selox reaction, selectivity towards CO oxidation rather than towards H2 oxidation becomes a priority objective function. 

This third strategy will be illustrated by HT experiments carried out under WGS, reverse WGS and Selox conditions, varying operating parameters such as temperature, partial pressures, space velocity, under stationary and non-stationary conditions [20]. 

Cons Perfectly isothermal conditions are not observed for simple prototypes such as described in Fig. 10.6, especially for highly exothermic reactions like partial or total oxidation, with observed temperature profiles and local hot spots (see results for the Selox reaction). However, these limitations do not prevent the observation of significant trends when large libraries are tested under various operating conditions. Commercial, improved systems adapted to the requirement of academia are now available on the market... 

As a second example using the DoE assistant tool for designing libraries, the first library of 48 diverse Selox catalysts (Tab. 10.2, Section 10.2.4.2) was tested in both the presence and absence of hydrogen to evaluate the impact of operating conditions on Selox performance [18]. From the set of data giving the conversion of CO (not reported here), an analysis of the effect induced by the four selected factors (alkali addition, type of support, nature of the transition metals and of the noble metal binaries) was carried out following different models [18]. 

Fig. 10.11 Quantitative effects on CO conversion in the Selox reaction, both in the absence and presence of hydrogen, generated by the presence of one specific element (or couple of elements for the noble metals) for each of the four predetermined classes of elements.

Cons Data quality for HT synthesis and testing (reproducibility, homogeneity, scale-up ability), as shown for this case of metals/mixed oxides WGS and Selox catalysts, remains a drawback for efficient data-mining. As such, even if the trends for new catalytic formulas may still be detected, even in the presence of poorly reproducible systems or outliers, the ultimate target of predicting the quantitative performance of a catalyst after teaching an ANN with experimental data was found to require new catalysts descriptors that would contain more information than the simple elemental composition of the materials. 

In addition, the contact time or space velocity was found to have a major effect on Selox conversion for a given catalyst (Fig. 10.17), which is largely explained by the... 

Fig. 10.17 Changes in CO conversion under Selox conditions as a function of temperature for two contact times (a) 170, (b) 48 ms. Thl thermodynamic equilibrium for Selox inlet composition (C0 02 H2 N2 = 1 2 20 77 vol%), Th2 thermodynamic equilibrium for similar Selox inlet composition but assuming that the water formed by CO/H2 oxidation is trapped by ceria.

Figure 2.1 Size-dependent selox activity of PVP-stabilized Au clusters in p-hydroxybenzal-dehyde production. (Reprinted with permission from Ref [60]. Copyright 2005, American Chemical Society.)...

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