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IZUNIHICuciII, TAIKYUE IZEE AND HENRYEYRIKG [COXTRIBUTION FROM THE
Adsorption Kinetics.
DEPARTMENT O F CHEMISTRY,
\rol. 7D
UNIVERSITY O F UTAH]
11. Nature of the Adsorption Bond]
BY IZUNIHIGUCHI, TAIKYUE REE AND HENRYEYRING RECEIVED AUGUST 13, 1956 One may expect three types of chemisorptions: pure ionic, pure covalent and mixed types. Let us suppose the surface complex M-A, formed by chemisorption of an adsorbate A on an adsorbent M, forms a diatomic molecule. Then the ionic character (fraction) Ciz of the adsorption bond M-A is given by l/Ci2 = 1 (E - H i i ) / ( E - &).(eq. 1 ) . Here E is the bond energy of M-A, Hii and H , are the energies of the pure ionic and pure covalent bonds, respectively. Ci2 is calculated from 1.1’ = Ci’erMA (eq. 2), where p’ and Y X A are the dipole moment and the bond length of M-A. Since we can calculate f-lii and H,, semi-empirically, E is calculated from eq. 1 and 2. Thus, the desorption heat from a nearly bare surface is calculated. The calculated and observed desorption heats are compared for the systems Ba on W,Sr on W,and the diatomic gases (Hz, 0 2 , Nz, CO) OF metals (W, Ni, Fe, Ta, Rh, Cr, Cu, Pt). The agreement is satisfactory. From the results we conclude: (1) The adsorption bonds of the systems, Cs on W, Na on W, are purely ionic. (2) The bonds, Ba-W, Cs-\V, are ionic with small amount of covalent character ( Ci2 = 0.97 for Ba, Cj2 = 0.67 for Sr). (3) The bonds of the systems, gases on metals, are covalent with small amount of ionic character ( Ci2 = 0.02 to 0.09). The Decker-Zeldovich equation for deSr on \$‘, and oxygen on W. sorption, -dO/dt = a0 exp(b6) (eq. 3). holds for the desorption rates of the systems, Ra on \IT, I n eq. 3, 0 is the surface coverage, a and b are constants. The activation heat for desorption and heat of desorption (adsorption) decrease with 6 generally. This fact is explained as being due to the field of the adsorbed layer on the desorbing atom.
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I. Introduction Generally i t is understood that chemisorption accompanies electron transfer between adsorbates and adsorbents. Thus, the bonds between these two are chemical in nature. As there are ionic, homopolar and mixed-type bonds in molecules, one expects the presence of the corresponding three types of bonds in adsorption. We study in this paper the nature of the “adsorption bonds.” If we approximate the surface complex formed between a surface atom and an adatoni to a diatomic molecule, it is possible to calculate the percentage of ionic characters in the adsorption bond using the theories of Wallzaand Ree and MurovamaUzb Provided that the ionic percentage is known, the bond energy is calculated as Ree and 51uroyama2b did for diatomic molecules. Thus we can calculate the heat of desorption from a nearby bare surface, since the latter is closely connected to the energy of the adsorption bond. We also consider the changes of desorption heat arid of activation heat for desorption with surface coverages. Both changes are caused by the effect of an adsorbed layer on the desorbing atoms and molecules. Thus, the investigation of the change of desorption heat and activation heat with surface coverages elucidate the nature of the adsorption bond. 11. Theory of Chemisorption on Metallic Surfaces A. The Energy of the Adsorption Bond.During the last few decades, quantum mechanics has made great strides in the physics of solids. Thus, the physical properties, such as electric c ~ n d u c t i o n ,cohesive ~ force,3 l i a r d n e ~ s ,density* ~ and mechanical ~ t r e n g t h , are ~ successfully explained. Likewise, the studies of metallic sur( I ) Presented before t h e Division of Physical a n d Inorganic Chemist r y of the 130th American Chemical Society meeting a t Atlantic City, Ncw Jersey, September 19, 1956. ( 2 ) (a) 17 T. Wall, TIrrs J O U R N A L , 61, 1051 (1939); (b) T. Ree ( R i ) a n d N. 3 l u r o y a m a , P r o c . I m p . h a d . J a p a n , 20, 93 (1944); C. A , ,
43, 5240 (1949). (3) F. Seita, “ T h e Modern Theory of Solids,” McGraw-Hill Book Co., New York, E.Y., 1940. (4) I,. Pauling, Proc. R o y . SOC.(London), A196, 343 (1949). ( 5 ) W. Shockley, “Imperfections in Nearly Perfect Crystals,” John Wiley and Sons, Inc., New York, N Y., 1952.
faces have been greatly d e ~ e l o p e d . ~ -However, ~ the general theory of the behavior of surface atoms is still imperfectly understood. We may assume the presence of unpaired electrons for the surface atoms, M. Thus, the latter interact with an adsorbate, A, making a surface complex, M-A. The fraction, Ci2, of the ionic bond in the adsorption bond is given by2b
Here, E is the energy of the adsorption bond, and Hcc are the energies for ideal ionic and covalent bonds, respectively.1° To a good firstorder approximation, the ISii and Hc, are represented by the equations I3ii
ITa, = (E(h1-M)
+ E(A-.4)}/2
(3)
Here, -40and I are the electron affinity of ?rland t l i r s ionization potential of AJ respectively, if the electron transfer occurs from A to M. (When the direction of the electron transfer is reversed, is the electron affinity of A, while I is the ionization potential of M.) Y M A is the distance between the atoms, M and A. E ( X - M ) and E(A-A) are tlic bond energies of the single bonds, M-35 and A-A, respectively. e is the electronic charge 4.50 X e.s.u.” The bond moment, p ’ , is expressed by p’ = Ci2erMA
(4)
( 6 ) K. Huang and G. XVyllie, Disc. F a r a d a y Sor., 8 , 18 (19W) (7) C. A . Coulson and G. R . Baldock, {bid., 8 , 27 (1950). (8) R. Gomer and C . S . Smith, “Structure and Properties of Solid Surfaces,” T h e University of Chicago Press, Chicago, Ill., 1953. (9) W. F,. Garner, “Chemistry of t h e Solid State,” Academic Press, Inc., New York, N.Y., 1955. (IO) Equation 1 is obtained readily bv solving t h e wave equation, Ril. = E$. Here $ is a n eigenfunction of t h e adsorption bond, repreCoic0, where ibi and are t h e eigenfuoctions sented by ic = C i $ i of t h e ideal ionic a n d covalent bonds, respectively, a n d C i and Cc are constants. T h e quantities. Z i i i and H o e , are, Hii = S + i f I i c i d r ,
+
rice
= S ~ ~ , H W ~ . ( 1 1) T h e bonding energv.
LV, between a positive and negative ion separated a distance, I , is given by W = -e*/7 br-. At equilibrium distance, r ~ . b . , where the energy of the system I S minimum. t h e energy is represented by lV, = ( 8 / 9 ) r 2 / r ~ ~ . This is the last term on the right Equation ,? i5 Pmlinx’s equation (cf referenre 12. p 48). < , f eg 2
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March 20, 1957
THENATUREOF
Ree and Muroyama2b calculated bond moments applying eq. 1 to 4 for 16 diatomic molecules and 16 single bonds in polyatomic molecules with good agreement with experiment. Thus, the applicability of Equations 1 to 4 seems ascertained. According t o Pauling12 the following approximation holds E
=
Hco
+ 2 3 . 0 6 ( X ~- XA)’
(5)
THE
ADSORPTION BOND
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Here, B is the dielectric constant of the medium in the electric double layer formed by an adsorbed film; c m is the number of adsorption sites per unit area; CY is the polarizability of the adion; as already defined, do is the distance indicated in Fig. lA, where an ionic adsorption is shown. (B)
(A)
(C)
Here, XM and XAare the electronegativities of M and A, respectively. When X M - XAis small, the following approximation also applies E = H,,
+ 23.06(~‘)~
(6)
where p f is the bond moment of the bond, M-A. Next we consider the relation between the bond energy, E , and AHo, the desorption heat from a nearly bare surface. For the desorption of monoatomic gases, E is AH0 itself, i.e. E
AH0
(7)
If homonuclear diatomic gases adsorb dissociatively, but desorb as molecules, A H 0 is given by AH0 = 2E
- E(A-A)
(8)
B. Pure Ionic and Pure Homopolar Bonds.Equation 1 indicates that when the adsorbate is in an ionic state (ie., C i 2 = 1) then AH0 of a monoatomic gas is given by 2 . In our paper 1 , 1 3 we have shown that Cs and Na are adsorbed in the ionic state on tungsten, and that AH0 is represented by (9)
Here, cp is the work function of tungsten, do is the distance of the adion from the W surface, the latter being assumed to be a smooth continuum. By comparing eq. 2 with 9, we see they are equivalent. Thus, in the case of ionic adsorption, the classical electrostatical treatment developed in paper I is valid. From eq. 1 one sees that E = Hccif C,2 E 0, i.e., the latter condition represents a homopolar adsorption. According to Mignolet,14 even in the case where van der Waals adsorption would prevail, the adsorbate shows a contact potential. This is explained by assuming a small amount of electron transfer or the polarization of the adsorbate due to a surface field. I n any case, a pure homopolar adsorption will not be so important in chemisorptions, since the latter accompany electron transfer more or less. C. Variation of Contact Potentials and of Desorption Heats with Surface Coverage 8.-In paper I, the contact potential, V , was given by the Helmholtz equation where
VO = 4sumedo
c
= 4rcrm/do
PO = e d o
(Ila) (1lb) (12)
(12) I,. Pauling, “Nature of t h e Chemical Bond,” Cornell University Press, Ithaca, N. Y . , 1948, p. AO. ( 1 3 ) I . Higuchi, T. Ree a n d 11. IIyring, THISJ O U R N A I . , 7 7 , 4060 (1055). (14) J . C. P. Mignolet, Disc.Faraday SOL.,8 , 105 (1950); Rec. I Y Q Y . chim., Pays-Bus, 1 4 , 685 (1955)
U W Fig. 1.-(A) ionic adsorption. The negative charges are t h e electrical mirror images of the positive charges of the adions. (B) covalent adsorption with ionic character. Here an example is shown, where adatorns are charged positively while the adsorbent atoms are negatively charged. (C) van der U‘aals adsorption. The adatotns (“admolecules”) are polarized by a surface field.
The Helmholtz equation holds also for the electric double layer formed by covalent chemisorption.15 Thus V in this case is given by
v = 2aomp’e
(13)
Here p f is the moment of the dipole shown in Fig. l B , and is given by 4. Equation 13 is readily obtained from 10 by substituting p f for 2p0 (the dipole moment in the ionic adsorption) and unity for E . Next, we consider the decrease of desorption heat, s(AH), with surface coverage 8. Here we neglect s(AI1) due to the surface heterogeneity of adsorbents. In paper I, we derived the equation
(The right-hand side of eq. 14a expresses also the decrease of adsorption heat, 6( - A n ) , with surface coverage, 8.) In eq. 14a, dl is the, distance between two neighboring adions at 8 = l , and ri, thc radius of the adion. The first term on the right of (14a) is the energy liberated when an adion desorbs from the surface with coverage 8. The (15) When dipoles with a small moment, p ’ , arrange vertically on a metallic surface, a s shown in Fig. l C , t h e contact potential is represented by V = 47rump’8. In t h e cases of A and B in Fig. 1, however, t h e contact potentials are both represented by 21romBp’ (cf. eq. 13). I n t h e literature, both equations V = 4aum0p’ and V = 2nomBp’, are used. T h e latter was proposed b y Langmuir,16a and used by Bosworth and Rideal,lGb and Eley.I7 However, Becker,I8 d e Boerlg and 4rrumOp’. I n his recent paper, Eleyzl Moore and Allisonzo use V uses t h e equation with t h e factor, 4 r . There is a n argument concernW e believe, however, t h a t ing which of t h e two equations is right.” t h e application of these equations is different according t o t h e mechanisms of adsorption. (16) (a) I. Langmuir, THISJ O U R N A L , 54, 2798 (1932); P h y s . Rev., 44, 423 (1933); (b) R. C. L . Bosworth a n d E. K. Rideal, Proc. Rov. SOL.( L o n d o n ) , 8162, 1, 32 (1937). (17) D. D. Eley, Disc. F a r a d a y Soc., 8 , 34 (1950). (18) J. Becker. “Advances in Catalysis,” Vol. 7, Academic Press, Inc., New York, N. Y., 1955. (19) J. H. d e Boer, “Electron Emission and Adsorption Phenomena,” T h e Macmiilan Co., New York, hT.Y..1933. ( 2 0 ) 0. E. Moore a n d €1. W. Allison, J . Chem. Plirs., 23, 1600 (195.5). (21) D. D. 131ey, “Catalysis an(] the Chemical Bond,” V u l . V I I . T h e P. C. Keilly Lectures in Chemistry, University of h-otre Dame. 1954.
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IZUMI HIGUCHI,TAIKYUE REE AND HENRYEYRING
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Vol. 79
second term is the extra-energy, Ui, t o overcome 0 0, whereas AH* is the activation heat a t the forces between the representative adion and the coverage 8. AH0 and AH are the desorption heats image charges of other adions. If Ca0
