Hydrogenation, XAFS spectroscopy

M.K. Oudenhuijzen, J.H. Bitter and D C. Koningsberger, The Nature of the Pt-H bonding for strongly and weakly bonded hydrogen on Platinum. A XAFS spectroscopy study of the Pt-H antibonding shaperesonance and Pt-H EXAFS , J. Phys. Chem. B, 105 (2001), 4616-4622. 

Surface complexes between As(V) or As(III) and B- and C-type hydroxyls are observed in IR studies but have not been verified by XAFS spectroscopy, probably because hydrogen bonding dominates the interaction between As and B- or C-type OH moieties (Sun and Doner, 1996) Fig. 8 d, e, and f). Hydrogen bonding is a long-range attractive force between atoms that does not involve direct bonding, and therefore would be classified as physisorption as opposed to chemisorption, which involves the formation of a chemical bond. 

The preparation of a successful supported bimetallic catalyst is quite a difficult proposition. The main problem is to ensure that the two components reside in the same particle in the finished catalyst, and to know that it is so. The main physical techniques to characterise bimetallic particles are hydrogen chemisorption, XRD, TEM, EDX, XPS, XAFS,197Au Mossbauer (Section 3.3) and CO chemisorption coupled by IR spectroscopy (Section 5.3). The characterisation of bimetallic catalysts is not always thoroughly done, and there is the further complication of structural changes (particularly of the surface) during use. In situ or post-operative characterisation would reveal them, but it is rarely done. 

As for all catalysts, well-characterized samples are necessary to be able to relate the catalytic performance to physico-chemical properties. Transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAFS) were used in this study to characterize the stabilized metal colloid. The necessity of such extensive characterization of particle size has been outlined by Harada et al. [6,7] showing that the formation of aggregates may be overlooked and misinterpreted as large metal particles when using TEM alone. The actual availability of the polymer stabilized surface has been probed by hydrogen/oxygen titration adopted from the description of Bernard et al. [8]. 

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