Effects of Stress and Strain on Chemisorption

By differentiating expression Eq. (86) with respect to the center energy of the d band we obtain 

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. 

The strain or stress will either lead to narrower or broader d bands that are shifted up or down in energy, respectively. An upward shift leads to a stronger interaction with the 2jt orbital of adsorbed CO and thus to a stronger chemisorption bond. Stress has the opposite effect. 

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]. 

The same theory, i.e. Eqs. (86) and (87), allows us to understand why CO and similar molecules adsorb so much more strongly on under-coordinated sites, such as steps and defects on surfaces. Since the surface atoms on these sites are missing neighbors they have less overlap and their d band wUl be narrower. Consequently, the d band shifts upwards, leading to a stronger bonding. 

---