Selectivity, catalytic performance

In summary, the organometallic approach is also efficient to prepare bimetallic nanoparticles. By precisely selecting the reaction conditions (precursor, stabilizer, reactant), we could access to ruthenium-based bimetallic nanoparticles displaying a controlled chemical order, i.e. alloy, core-shell, or even nanoparticles decorated with a second metal such as platinum, iron, or tin. These nanoparticles, which display different surface properties, can pave the way towards synergetic and selective catalytic performances. 

As already discussed in Chapter 1, a promoter is a substance which when added to a catalyst, usually on its surface, enhances its catalytic performance, i.e. it increases the rate, r, of a catalytic reaction or the selectivity to a desired product. 

CO2, N2 and N2O production as a function of the catalyst potential, UWR> obtained at 62IK for fixed inlet pressures of NO and CO. A sharp increase in reaction rate and product is observed as the catalyst potential is reduced below 0 V, i.e., upon Na supply to the Pt catalyst. The selectivity to N2, Sn2, is enhanced from 17% to 62%. This dramatic enhancement in catalytic performance is due to (a) enhanced NO vs CO chemisorption on Pt with decreasing potential and (b) Na-induced dissociation of chemisorbed NO. 

The aim of the newly introduced mesoporosity is to enhance the utilization of the microporous network by improved accessibihty of the active sites that are mostly present in the micropores. Although numerous papers have reported on the improved catalytic performance of desihcated zeolites in catalysis (details in Section 2.4.5), only few works are available that reaUy tackle the hierarchical nature of the desihcated zeohtes and demonstrate that selective sihcon removal leads to an enhanced physical transport in the zeohte crystals. 

As shown in Table 2.1, the improved catalytic performance of alkaline-treated zeolites compared to the parent purely microporous counterparts has been demonstrated decidedly by different groups active in academia and in industry. The positive effect is reflected in the enhanced activity, selectivity, and/or lifetime (coking resistance) of the hierarchical systems. The examples listed embrace not only a variety of zeohte topologies (MFl, MOR, MTW, BEA, and AST) but also reactions involving hghter hydrocarbons as well as bulky molecules. This illustrates the potential of the desihcation treatment, although more work is to be done in optimizing the catalytic system for the wide variety of applications. 

To improve selectivity towards phenol 0.5 wt% of Sn was added as a promoter while preparing 5.0Fe/AC catalyst. The catalytic performance of 5.0Fe-0.5Sn/AC catalyst was investigated under similar reaction conditions. The addition of Sn to Fe/AC catalyst seems to enhance phenol selectivity by 33% (Fig. 7). TOF and physical properties of iron loaded catalysts are shown in Table 1. 

The preparation of iron impregnated activated carbon as catalysts and the catalytic performance of these catalysts were studied in benzene hydroxylation with hydrogen peroxide as oxidant. 5.0Fe/AC catalyst containing 5.0 wt% iron on activated carbon yielded about 16% phenol. The addition of Sn on 5.0Fe/AC catalyst led to the enhancement of selectivity towards phenol. 

It is found that the CNF-HT has not catalytic activity for ODP. After oxidation, all the three samples show hi ly catalytic performances, which are shown in Fig.3. CNF-HL has the longest induction period among the three samples, and it has relatively low activity and propene selectivity at the beginning of the test. During the induction periods, the carbon balance exceeds 105% and then fall into 100 5%, which implies the CNF structure is stable and the surface chemistry of CNF reaches a dynamic equilibrium eventually. These results indicate that the catalytic activity of ODP can be attributed to the existence of surface oxygen complexes which are produced by oxidation. The highest propene yield(lS.96%) is achieve on CNF-HL at a 52.97% propane conversion. 

The catalytic activitira of synfliesized catalysts are given in Table 1. The TS-1 catalyst exhibited the highest epoxide yield and the best catalytic performance for the epoxidation of 1-hexene. The convasion of cyclohexene, however, is the lowest over TS-1. In case of TS-1/MCM-41-A and TS-1/MCM-41-B, the selectivity to epoxide is much hi er than that of Ti-MCM-41. Moreover, the conversion of 1-hexene as well as cyclohexene is found larger on the TS-l/MCM-41-Aand TS-1/MCM-41-B than on other catalysts. While the epoxide yield from 1-hexene is nearly equivalent to that of TS-1, the yield from cyclohexene is much larger than those of the otiier two catalysts. Th e results of olefins epoxidation demonstrate that the TS-l/MCM-41-Aand TS-1/MCM-41-B possess the surface properties of TS-1 and mesoporosity of a typical mesoporous material, which were evidently brou in by the DGC process. 

In industry, the emphasis is mainly on developing an active, selective, stable and mechanically robust catalyst. To accomplish this, tools are needed which identify those structural properties that discriminate efficient from less efficient catalysts. All information that helps to achieve this is welcome. Empirical relationships between those factors that govern catalyst composition (e.g. particle size and shape, and pore dimensions) and those that determine catalytic performance are extremely useful in catalyst development, although they do not always give fundamental insights into how the catalyst operates on the molecular level. 

Figure 26 shows that trimethylamine (TMA), a strong Lewis base with a pAia value of 9.9, introduced to the reactant gas stream at a concentration of 10-20 ppm, appreciably improves the catalytic performances in every aspect of catalytic performance, propylene conversion, PO selectivity, H2 utilization efficiency, and catalyst life [88]. It is worth noting that TMA makes used catalysts... 

The opposite behavior was observed after the treatment of the two catalysts in the steam-containing stream, at 380°C. The catalyst P/V 1.06 did not show any change of catalytic performance, whereas in the case of P/V 1.00 the treatment rendered the catalyst less active but more selective than the sample equilibrated in the reactive atmosphere at 380°C. This means that with P/V 1.00, the active layer is not fully hydrolyzed under reaction conditions, and that a hydrolyzed surface is more selective than the active surface of the equilibrated P/V 1.00 catalyst. On the contrary, the active surface of catalyst P/V 1.06 either was already hydrolyzed under... 

Koebel, M. and Elsener, M. (1998) Selective Catalytic Reduction of NO over Commercial DeNOx-Catalysts Comparison of the Measured and Calculated Performance, Ind. Eng. Chem. Res., 37, 327. 

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