EXAFS correlations

To be more confident on alloy formation, we performed EDS compositional analysis with an electron probe size of 1 nm FWHM focused on isolated clusters as expected for bimetallic clusters, Au and Fe signals are together present in the spectrum. The measured Au/Fe atomic ratio on the largest clusters is about 1.6 0.2, indicating an Au-enriched alloy. Moreover also preliminary EXAFS spectroscopy on the sample evidenced a clear Au-Fe correlation, unambiguously confirming alloy... 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The TPR-XAFS technique confirmed that doping Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) remarkably promotes the carburization rate relative to the undoped catalyst. The EXAFS results suggest that either the Hagg or e-carbides were formed during the reduction process over the cementite form. A correlation is observed between the a-value of the product distribution and the carburization rate. 

The interesting information is the correlation between first shell coordination numbers from EXAFS and H/M values from chemisorption, shown in Fig. 6.18. The correlation is as expected high dispersions correspond to low coordination numbers. Of course, what we really need is a relationship between particle size and H/M values. The right hand panel of Fig. 6.18 translates the experimentally determined H/M values of the catalysts into the diameter of particles with a half-spherical shape. Similar calibrations can be made for spherical particles or for particles of any other... 

Whereas H20 dissociation is advantageous for methanol oxidation, it is detrimental to oxygen reduction. Murthi et al. performed EXAFS and RRDE experiments in low water content electrolytes, and were able to correlate the formation of oxide with a decrease in ORR activity.127 The low water content electrolytes also isolated the source of the oxide as the product of water dissociation. Although alloying Ru with Pt improves the rate of methanol oxidation predominantly by... 

At this time diffraction data for ion-ion distributions in aqueous solutions of moderate concentration are beginning to become available. In aqueous NiCl2 solutions very refined neutron diffraction studies indicate that the Ni2+-Cl pair correlation function has a peak near 3.l8 under conditions in which the Cl does not penetrate the Ni(H20)g2+ unit. (J+2 ) It is reported that EXAFS studies give the same result. (1 3) While the information is most welcome it is puzzling because a geometrical calculation indicates that the closest center to center distance for the Ni2+ and a Cl that does not penetrate the hydration shell is closer to 3.98. (7)... 

It is important to remember that, despite common usage, EXAFS is at heart not a spectroscopic measurement but rather a scattering measurement. It is impossible to correlate specific EXAFS features with particular absorber-scatterer interactions, rather it is necessary to analyze the entire EXAFS region in order to determine the absorber environment... 

CP/MAS NMR study of CH4 activation on [(=SiO)2Ta(H)J shows the formation even at 150 °C of methyUdene and methyUdyne species by an a-H elimination process on several sites that should correspond to the tris-hydride on other sites a methyl group is transferred to the surface, leading to the formation of (=Si-Me) and of [(=SiO)3Ta]. Correlation with EXAFS suggests that the tris-hydride should exist on surface sites (=20%) quite distant from siloxy bridges whereas methyl transfer to the surface should happen on the specific sites (=80%) close to the siloxy bridges. The latter, which are formally 10 electrons species, exhibit a moderate to weak activity in methane C-H activation. To the best of our knowledge, this is the first observation of methyl group transfer on a surface (Scheme 2.18). 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

The case study of the tetranuclear manganese complex presented above and the specific examples of structure/spectroscopy correlations have established the validity of the proposed methods and set the stage for more ambitious applications. The first such application has been the recent evaluation of several structural models of the OEC in the S2 state ( 1 3) of the Kok cycle (107). Twelve structural models were considered, 10 of which were based on Mn405Ca core topologies derived by polarized EXAFS spectra. Figure 19 shows one of the models included in the set. 

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