Changing the -Band Center Ligand Effects

FIGURE 8.11 A schematic illustration of the connection between d-band center for a metal with a more than half-filled d-sheU and the bandwidth for an electronic band with a fixed number of d electrons. When the bandwidth becomes narrower, the only way of maintaining the number of d electrons fixed is to shift up the center of the band. 

Yet another way of changing the d-band center in a controlled way is by alloying. The formation of surface alloys can induce changes in the electronic structure, which can be understood in terms of the d-band model given that the electronic stracture changes are very localized and that the adsorption site remains unchanged. 

FIGURE 8.13 Effect of strain on the adsorption energy of CO and O and the dissociation harrier for CO on Ru(OOOl). Adapted from Mavrikakis et al. (1998). 

So-caUed near-surface alloys (NS As) provide a template system for studying such effects. NS As or skins have been extensively studied as oxygen reduction reaction (ORR) catalysts in PEM fuel cells. If one could reduce the contents of the expensive elemental metal Pt by forming a stable thin layer of the metal on top of a cheap and abundant metal host like Fe, Co, or Ni and improve or maintain its high activity and low overpotential for ORR, it would be a remarkable achievanent. 

By considering a Pt(lll) surface where a series of different 3d metals have been sandwiched between the first and second layers, the effect of the subsurface layer of atoms on the reactivity of a Pt(l 11) overlayer can be studied. The overall effect of the intercalated 3d metals is that the d-states of the snrface Pt atoms are shifted down 

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