Hydrocarbon EXAFS

This work was extended to other unsaturated hydrocarbons (propene and toluene), other supports (MgO), and other clusters (Rh6) (Argo et al., 2006). As an example of the type of information that was learned in this work, Table 5 shows the EXAFS fit parameters characterizing the MgO-supported Ir4 clusters during toluene hydrogenation catalysis as a function of reaction temperature. 

Some preparations of iron exchanged into zeolite H-MFI by vapor-phase FeCL are known to be active and selective catalysts for the reduction of NO, with hydrocarbons or ammonia in the presence of excess oxygen and water vapor (45,46). The active centers in Fe/MFI are assumed to be binuclear, oxygen-bridged iron complexes, as follows from H2-TPR, CO-TPR, and ESR data (45,47) and EXAFS and XANES results (48,49). These complexes are structurally similar to the binuclear iron centers in methane monooxygenase enzymes that are employed by methanotrophic bacteria in utilization of methane as their primary energy source (50). It is believed that molecular oxygen reacts with these centers to form peroxide as the initial step in this chemistry (50). 

The above analyses of platinum in its reduced state show the advantages of EXAFS in providing in situ information under high temperature conditions and under reactive gases. This type of analysis enables a more realistic approach to the characterisation of this type of solid. It is also possible to envisage an analysis of catalysts under conditions similar to their use, in contact with hydrocarbon molecules. On contact of these catalysts with a hydrocarbon charge, it is possible to obser c the reversible formation of Pt -C bonds (Fig. 11.13), The number of carbon atoms in contact with a platinum atom (in this. study 1.4 on average) and their distance (1.96 A) can then be determined. In the case studied here, the structure of the metallic particle is not altered. 

Figure 11,13 During contact between a reduced catalyst and hydrocarbons, it i,s possible to obscr e the formation of Pt ( bonds. Stopping the flow of hydrocarbons causes the gradual disappearance of bonds (taken from N, Somoijai Guyot-Sionnest. Structural evolution of Pt-Al203 catalysts during reduction and hydrocarbon reaction a high temperature, high pressure EXAFS study. These, Universitc Paris-Sud, 1991).

Sinfelt has greatly contributed to the catalyses of bimetallic nanoparticles [18]. His group has thoroughly studied inorganic oxide-supported bimetallic nanoparticles for catalyses and analyzed their microstructures by an EXAFS technique [19-22]. Nuzzo and co-workers have also studied the structural characterization of carbon-supported Pt/Ru bimetallic nanoparticles by using physical techniques, such as EXAFS, XANES, STEM, and EDX [23-25]. These supported bimetallic nanoparticles have already been used as effective catalysts for the hydrogenation of olefins and carbon-skeleton rearrangement of hydrocarbons. The alloy structure can be carefully examined to understand their catalytic properties. Catalysis of supported nanoparticles has been studied for many years and is practically important but is not considered further here. 

The selective reduction of NO by hydrocarbon in an excess of oxygen was studied using a platinum catalyst doped or not with zinc. Successive impregnetion or co-impregnation of Pt and Zn on alumina were made. In some cases, in the presence of Zn, the NO conversion is increased in parallel with N2 formation. A better conversion of hydrocarbons was also observed. EXAFS experiments and N2O decomposition experiments have been carried out to explain these observations. 

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