Metal-support interface

I -Propylaminopropene is thus on a logical route to dipropylamine, which can form directly by hydrogenation of its C=C bond. This condensation-elimination reaction may indeed involve the support [2] or at least the metal-support interface. 

In conclusion, a "specific role" of these dual "Metal-Support" sites at the metal-support interface has to be considered in addition of the "intrinsic roles" of the metal and the support and a bifunctional mechanism can be reasonably proposed. 

The literature of metal-support interactions includes httle about the possible chemical bonding of metal clusters or particles to supports. Supported molecular metal clusters with carbonyl ligands removed have afforded opportunities to understand the metal-support interface in some detail, and the results provide insights into the bonding of clusters to supports that appear to be generalizable beyond the small clusters to the larger particles of conventional supported metal catalysts [6]. 

Structural information about the metal-support interface is provided by EXAFS spectroscopy. The EXAFS data provide average structural information and are most informative when the samples are most nearly uniform. 

Equations 16 and 17 imply that 02 adsorption is not dissociative, which is coherent with the kinetic data. However, 02 should be dissociated in further steps of the surface reaction. On ceria, new sites for 02 activation are created at the metal/support interface or in the vicinity of metal particles. As CO and 02 do not compete with the same sites, the rate equation becomes ... 

On reducible supports typically when Pd is deposited on LaCoOj then reduced in H2 at 450°C for obtaining Pd0/CoOx/La2O3, an alternative mechanism would likely occur, which accounts for steps involving the creation of active sites at the metal/support interface. These active sites would be composed of metallic Pd in interaction with anionic vacancies from the support potentially active for the dissociation of NO according to step (26) [54],... 

The argument of each sine contribution in (6-8) depends on k, which is known, r, which is to be determined, and the phase shift (f(k). The latter needs to be known before r can be determined. The phase shift is a characteristic property of the scattering atom in a certain environment, and is best derived from the EXAFS spectrum of a reference compound, for which all distances are known. For example, the phase shift for zero-valent rhodium atoms in the EXAFS spectrum of a supported rhodium catalyst is best determined from a spectrum of pure rhodium metal as in Fig. 6.13, while RI12O3 may provide a reference for the scattering contribution from oxygen neighbors in the metal support interface. 

A central question with respect to supported metal catalysts is that of the structure of the metal-support interface. Various possibilities have been proposed, varying from interfaces consisting of a mixed metal aluminate or silicate layer [17] or the presence of metal ions which serve as anchors between particle and support [18] to the attractive interaction between ions of the support and the dipoles that these ions induce in the metal particle [19]. EXAFS highlights the atomic surroundings of an atom in the catalyst, and if the supported metal particles are sufficiently small, oxygen atoms in the metal-support interface give a measurable contribution to the EXAFS spectrum. 

Figure 9.5 EXAFS of Rh/AKO, catalysts after reduction at 200 °C (left) and 400 °C (right) top the magnitude of the Fourier transform of the measured EXAFS signal, bottom the back transformed EXAFS corresponding to distances from Rh atoms of between 0.8 and 3.2 nm. The lower Fourier transform contains a dominant contribution from Rh nearest neighbors at 0.27 nm and a minor contribution from oxygen neighbors in the metal-support interface. After correction for the Rh-O phase shift, the oxygen ions are at a distance of 0.27 nm (from Koningsberger et at. 119]).

Figure 9.6 Possible configurations of Rh atoms (white circles) in the metal-support interface on y-AljOj (shaded circles), corresponding to Rh-Os coordination numbers between 2 and 4 (from van Zon el al. [20]).

Optimized catalysts. Samples should be suitable for investigating the particular aspect of the catalyst one is interested in. For example, meaningful information on the metal-support interface is only obtained if the supported... 

In bimetalUc systems PtSn-BM, it is possible to observe that a part of the Pt is alloyed with the metalUc Sn (PtSn, according to EXAFS results) and that the other part of the Pt is alloyed with metallic Pt atoms isolated from such an alloy. There also exists a Sn percentage of ionic nature (20-30%), probably placed in the metal-support interface. Scheme 6.1(b) gives an image of the catalytic surface for PtSn-BM. 

EM studies of chemical interactions at metal-support interfaces 175... 

As reviewed by Ponec,18 the formation of alcohols is observed when a metal is promoted by a transition metal oxide. Kiennemann et al,19 has associated the presence of anion vacancies at the metal-support interface with the capability to dissociate CO and allow CO insertion to produce higher alcohols. This model can be used to explain our results on tungsten carbides. 

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