Alloy catalysts ensemble effect

PtMo alloys are not as effective as PtRu for methanol, or ethanol, oxidation. As shown in Figure 29, the d band vacancy per Pt atom for the PtMo/C catalyst continues to increase until 0.6 V vs RHE, in contrast to the behavior of PtRu/C. ° The authors attribute this difference to the lack of removal of the Cl fragments from the particle surface by the oxy-hydroxides of Mo. However, the difference in the electrocatalytic activity of PtRu and PtMo catalysts may be attributed to ensemble effects as well as electronic effects. The former are not probed in the white line analysis presented by Mukerjee and co-workers. In the case of methanol oxidation, en-... 

The composition of an alloy surface is often very different than the alloy s bulk composition due to segregation effects. The overall activity of a catalyst is determined by the distribution of active sites. This distribution may be very heterogeneous both in terms of the local environments that define each site and their chemical reactivities. The reactivity of any specific active site can be affected by contributions from strain, ligand and ensemble effects. Computational methods are well suited to exploring these effects because one can simulate model systems where only one effect dominates as well as model systems where multiple effects are important. 

In conclusion by using rhenium as adsorbent instead of platinum, it is possible to achieve the ensemble control by sulfur passivation, at sulfur levels comparable to those applied for nickel and much lower than that which would have been required on a non-alloyed Pt-catalyst. A similar ensemble effect is achieved by alloying alone on Pt-Sn catalysts ... 

One of the questions which remains to be answered is whether the conclusion from the experiments with labeled carbon that CO dissociation is a fast step (cf. Section V,C) complies with the IR observation that CO js is abundantly present at the catalyst surface (cf. Section IV,B). A possible solution is to assume that CO storage and CO dissociation take place on different sites (48, 59). When FT catalysts such as Ni or Ru are alloyed with an inert metal such as copper, the activity decreases drastically. The alloy studies of Bond and Turnham (73) and Araki and Ponec (48) consistently indicate that the decrease in activity originates from a decrease in the preexponential rather than from an increase in the activation energy in the Arrhenius equation (48, 57, 73, 74). This is indicative of an ensemble effect... 

Boudart and co-workers [184] have shown a 50-fold increase in the rate of H2/O2 reaction to produce water over Pd-Au alloys. Such large effects cannot be explained by site-blocking ensemble effects. The new sites that are created by alloying have unique structure and bonding. In fact, a new catalyst is created with structural and... 

Structure promoters can act in various ways. In the aromatization of alkanes on Pt catalysts, nonselective dissociative reaction paths that lead to gas and coke formation can be suppressed by alloying with tin. This is attributed to the ensemble effect, which is also responsible for the action of alkali and alkaline earth metal hydroxides on Rh catalysts in the synthesis of methanol from CO/H2 and the hydroformylation of ethylene. It was found that by means of the ensemble effect the promoters block active sites and thus suppress the dissociation of CO. Both reactions require small surface ensembles. As a result, methanol production and insertion of CO into the al-kene are both positively influenced. 

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

---