Pseudomorfic overlayer

Figure 6.33. Trends in reactivity for an overlayer deposited pseudomorfically on a substrate. The diagonal gives the position of the center of the d band for the pure metals. The other numbers indicate the shift of the d band by formation of a pseudomorfic overlayer, irrespective ofw/hether it can be realized. Notice that in the low/er left-hand corner the d bands shift upw/ards, leading to higher...

Such calculations have also been performed for isolated impurities of late transition metals alloyed into the surface of other transition metals, and the trends are the same. The accuracy of the numbers in Fig. 6.33 is limited since many approximations had to be made to calculate them. Nevertheless, they reflect trends very well and give useful insight into reactivity trends that have actually been measured for a number of pseudomorfic overlayers [J.A. Rodriquez and D.W. Goodman, Science 257 (1992) 897]. 

Figure 6.41. Reactivity of a pseudomorfic overlayer of Ni deposited on Ru(OOOl) for the dissociative adsorption of methane. At zero coverage the measurements reveal the sticking of methane on pure Ru. When nickel atoms are deposited on the surface, the dissociation...

This expression indicates that the change in hybridization energy is opposite and proportional to the shift of the d band center. Thus, if the d band shifts upwards the hybridization energy increases and vice versa. Strain and the associated shift of the d band can be brought about by growing the desired metal pseudomorfically on another material with a different lattice constant. The term pseudomorfic means that the overlayer grows with the same lattice constant as the substrate. The overlayer may thereby be strained or compressed depending on the lattice constants of the two materials. 

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