Physical Chemistry , 1st ed.



K^1

K

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1



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[



A

[

2

A

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2

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2

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 (22.33)

Now that we have a way to model the adsorption of gas species on a sur-

face, we should consider how adsorbed species interact with a surface. There

are two descriptions of molecule-surface interaction, differing mainly in terms

of degree of interaction. In physisorption,molecules interact with surfaces in a

weak and general way. It could be as simple as a van der Waals or dispersion

interaction that keeps a molecule on a surface, like molecules of methane

(CH 4 ) or diatomic nitrogen (N 2 ) on metal surfaces, or organic residues all over

the place. Or, it could be a dipole interaction with a surface atom, which is how

water molecules adsorb so easily on most surfaces.

In chemisorption,the strength of interaction between molecules and a sur-

face is high enough to be considered a bona fide chemical (covalent) bond. It

is not unusual for the strength of chemisorption to rival a true chemical bond.

Diatomic oxygen, for example, adsorbs on many metals with a chemisorption

strength of over 500 kJ/mol!

Chemisorption and physisorption are usually studied by measuring (di-

rectly or indirectly) the coverage of a surface versus the temperature. The lower

the temperature needed to reduce the coverage of a surface, the lower the en-

ergy of interaction between gas molecule and surface. Energies of interactions

are usually listed as heats of adsorption, or (^) adsH. They are typically listed as
positive numbers, although in all cases the process itself is exothermic.Ta b l e
22.3 lists some (^) adsHvalues for different gases and surfaces; keep in mind that
these are inexact numbers. Differentiation between physisorption and
chemisorption is inexact and is usually judged on the basis of the strength of
interaction as well as structural considerations. For example, the energy of in-
teraction between carbon monoxide, CO, and palladium is large enough to
consider it as chemisorbed, but in many cases the distribution of CO on a sur-
face is seemingly random with respect to the surface features, suggesting that
it is physisorbed.
Chemisorption does differ from physisorption in a significant way. It is not
unusual for chemisorbed molecules to break their chemical bonds and have
the resulting fragments bond directly to surface atoms. By making chemical
bonds to surface atoms, the fragments can satisfy their valence electron re-
quirements. Figure 22.24 shows the difference between physisorption and
chemisorption of a dihydrogen molecule. In Figure 22.24a, the hydrogen mol-
ecule is loosely bound to a surface point (and can have various orientations,
depending on the surface identity, the temperature, and the coverage). However,
in Figure 22.24b, the hydrogen atom’s bond has broken and the individual H
atoms are bonding directly to different surface atoms. The physisorption model
could apply to H 2 adsorbing on the Ta(110) surface, where the energy of ad-
sorption is about 40 kJ/mol, whereas the chemisorption model could describe
H 2 on a W(111) surface, in which the energy of adsorption is two to three
times higher.
The ability for surfaces to promote bond dissociation is a crucial part of
understanding why surfaces can catalyze reactions. Many gas-phase reactions
have some activation energy that must be overcome before reactants can
form products. However, when interacting with a surface, activation barriers
can be lowered significantly, speeding up the rate of (that is, catalyzing) the
reaction.
The steps occurring at a surface, and their respective changes in energies E,
can be simplified and generalized as follows:
22.6 Coverage and Catalysis 787
Table 22.3 Heat of adsorption for gases
on surfaces
Gas Solid surface (^) adsH(kJ/mol)
O 2 Cu(110) 205
O 2 Pd(110) 200–350
O 2 Pt(100) 187–290
H 2 Ni(111) 95
H 2 Pd(111) 87
H 2 Pt(100)  40
H 2 Pt(111) 75
H 2 W(211) 192
CO Cu(100) 64–48
CO Ni(110) 16–191
CO Ni(111) 98–111
CO Pd(100) 151
CO Pd(111) 125
CO Pt(100) 134
CO Pt(110) 105–133
Source:G. A. Somorjai,Chemistry in Two Dimensions:
Surfaces,Cornell University Press, Ithaca, N.Y., 1981.
H H
(a)
H H
(b)
Figure 22.24 There is a fundamental differ-
ence between physisorption and chemisorption,
shown here diagrammatically. (a) In physisorp-
tion, a molecule of hydrogen remains intact but
is attracted to a surface due to van der Waals
forces, London forces, or the like. (b) In chemi-
sorption, chemical species are virtually bonded
chemically to the surface. In the case of hydrogen,
it is necessary to break the H–H bond for it to be
chemisorbed. This is not always required: for car-
bon monoxide, the CO molecule might be as
strongly bonded to the surface but still retain the
C–O bond.