The statistics of surface atoms and surface sites on metal crystals
Abstract
The atoms at the surface of a metal crystal can be differentiated according to the number (j) and arrangement of their nearest neighbours. We have determined how, in (different) crystals with f.c.c., b.c.c. and h.c.p. structures, the total number of surface atoms (Ns) and the proportions of the various types of surface atoms depend on the crystallite size.
In some cases we have also determined how the number of Bni,j,k,… sites varies with the crystallite size, a Bni,j,k,… site being defined as an ensemble of n surface metal atoms in a certain specified arrangement where the n surface metal atoms have i,j,k etc. nearest neighbours.
On the basis of the results we have considered the influence of crystallite size on the sorptive and catalytic behaviour of metal catalysts.
References (18)
- T.A. Dorling et al.
J. Catalysis
(1966) - D.J.C. Yates et al.
J. Catalysis
(1967) - R. van Hardeveld et al.
Surface Sci.
(1966) - A.J. Perry et al.
Surface Sci.
(1967) - A.J. Melmed
Surface Sci.
(1967) - V. Nikolajenko et al.
J. Catalysis
(1963) - H.E. Swift et al.
J. Catalysis
(1966) - K. Schrage et al.
J. Am. Chem. Soc.
(1966) - J.L. Carter et al.
J. Phys. Chem.
(1966)
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