Sintering metal-support interaction effect

A wealth of experimental chemisorption data are presently available for NM/Ce(M)02-, catalysts. As reported in section 4.3.2.2, the increase of generally induces significant modifications on their chemical behaviour. In most of cases, partial rather than complete inhibition of their chemisorption capability is reported. In many cases, however, the techniques and/or experimental routines do not allow an unequivocal interpretation of the reported H(CO)/NM data. As noted in section 4.3.2.2, quite often, the role played by a number of very important side effects, like the metal or support sintering, the adsorption of the probe molecules (H2 and CO) onto the supports, the presence of chlorine in them, or the reversibility of the deactivation phenomena, has not been established. By contrast, there are a number of recent studies (97,117,163,235) from which meaningful conclusions may be drawn. Moreover, some of them (97,163) have provided some additional fine details about the nature of the metal/support interaction effects occurring in ceria-based catalysts. 

In situ ETEM permits direct probing of particle sintering mechanisms and the effect of gas environments on supported metal-particle catalysts under reaction conditions. Here we present some examples of metals supported on non-wetting or irreducible ceramic supports, such as alumina and silica. The experiments are important in understanding metal-support interactions on irreducibe ceramics. 

For the INi sample, the weaker metal-support interaction does not prevent a large and continuous sintering process to occur, leading to a drastic decrease of the metallic surface area (85 % after 75 h on stream). However, the particle growth involves a smoothing effect, so the simultaneous increase in intrinsic activity partially compensates the loss of surface, limiting the decrease of the overall catalytic activity to 25 % over the same ageing period. 

These effects are aU the more pronounced as the metal-support interactions of the parent catalyst are weaL In other words, when the metal-support interactions are prevailing, the metallic particles remain stable. Examples are reported in the literature for platinum [70, 72], palladium [38] and rhodium [41], showing that silica leads to a more severe sintering than alumina. Indeed, it appears that the... 

These results were interpreted in terms of a substantial surface enrichment in Cu, driven by Cu s lower heat of sublimation [23]. The reactivity of these catalysts for CO oxidation, and the clear spectroscopic evidence for surface Pt - CO species indicate that, at least for the heterogeneous systems, particle surface stoichiometries are very sensitive to metal-adsorbate interactions. Similar arguments were presented for the PtAu/silica system, in which monometallic Au particles severely sinter under dendrimer removal conditions. In this case, the retention of small bimetallic particles after activation was attributed to the strength of Pt-silica interactions, which effectively anchored the bimetallic nanoparticles to the support [24],... 

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