Alloy catalysts ligand effect

CO oxidation on both the Ru and the Pt simultaneously. This is a somewhat unexpected result, and one that needs further study. The apparent greater effectiveness of the small amount of Ru on the Pt surfaces in the PtRu Mix catalyst vs. the PtRu Alloy catalyst, as exhibited in Fig. 15, is consistent with the effect of Ru island size as noted above for methanol. Small Ru islands, such as those in the PtRu Mix catalyts, cause the BF mechanism to be more effective at lower overpotentials, whereas larger islands promote the ligand effects. Consequently, more OH can be provided by the smaller islands on the PtRu Mix, before the CO mobility finally goes into synchronous mode and poisons these sites. 

At present, the Pt-Ru bimetallic system is recognized as the most promising CO-tolerant anode catalyst for the DMFCs. A large body of hterature exist demonstrating improvement of the CO oxidation on the Pt-Ru alloy and Ru-modified Pt catalysts. The superior CO tolerance of the Pt-Ru bimetallic catalysts compared with the monometallic Pt catalyst is frequently explained with concepts of bifunctional mechanism [17] and ligand effect [22, 23]. The former mechanism proposed by Watanabe and Motoo is widely accepted. They claimed that the Ru has higher reactivity with water than Pt and that formation of Ru-OH at a lower potential promotes the electrooxidation of the chemisorbed CO on the Pt (formulas (4) and (5)). 

Much work has been put into discovering the mechanism of the promoting effect of the alloying elements (such as Ru, Sn, W, etc.) on the activity of Pt catalysts towards the MOR. Although the nature of these binary alloys remains elusive, three major theories, i.e., the bi-functional effect, the electronic (ligand) effect, and the spillover effect, were established to explain the role of the second elements on the activity improvement of Pt binary alloys. 

In the case of PtSn catalysts, no evidence of a ligand effect was observed from an in situ FTIR study on Pt3Sn(l 10) bulk alloy and PtSn nanoparticles supported on carbon. It was proposed that the bifunctional mechaiusm was mainly involved in the oxidation process. The fact that the transition from positive to negative Stark shift of infrared v(CO) frequency during CO oxidation was much more pronoimced on a PtSn/C catalyst than on the Pt/C catalyst was interpreted in terms of the different ways in which OHads (necessary to oxidize CO) nucleates on each catalyst. 

A.3. Possible reasons for nonadditive properties of (Ru+Pd)/Si09 catalysts. According to the ideas developed by Ponec and Sachtler [22,23] in the theory of bimetallic catalysts containing alloy species, the analysis of catalytic properties should take into account, first, the effect of dilution of surface metal atoms by those of less active metals, which leads eventually to a decrease in concentration of multiple active centers for the reaction. In a number of cases, it is also necessary to consider the ligand effect, that is, the presence of the second metal in the nearest environment of a given atom, which may affect its electronic, chemisorptive, and, consequently, catalytic properties. For bimetallic particles of more than 10 A in size, the surface composition can differ from that of the bulk. In view of these ideas, we now consider the properties of Ru-Pd catalysts. 

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). 

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. 

These data further suggest a reason for the different efficacy of PtRu catalysts in reformate vs. methanol. Since the BF mechanism operates at lower anode potential than the direct ligand mechanisms, more homogeneously alloyed PtRu catalysts with smaller Ru islands should be more effective in reformate and less well-mixed alloys (with large Ru islands) better in methanol fed... 

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