Surface Science 66 (1977) 647-651 0 North-Holland Publishing Company

A GENERAL RULE FOR THE ADSORPTION OF GASES ON METALS

Received 1 April 1977

Since Taylor [ 1 ] emphasized the importance of chemisorption in heterogeneous catalysis, much effort has been directed at developing a full understanding of

1000 Ti

Ta Nb

*O:: 0 200 400 600 a 0

HEAT OF ADSORPTION OF CD (kJ/mole)

Fig. 1. Plot of the heat of adsorption of 02 against that of CO on various metals. Note that the data fall roughly on a straight line.

641

648 J.J. Burton / General rule for adsorption of gases on metals

800 Ti

Ta

200 400 600

HEAT OF ADSORPTION OF CO (kJ/mole)

800

Fig. 2. Plot of the heat of adsorption of CO2 against that of CO on various metals. Note that the data fall roughly on a straight line.

chemisorption. Interesting experimental correlations have been observed [2-4] and much effort has been directed at developing microscopic theories of chemisorption [5-121. In examining Anderson’s compilation of heats of adsorption of simple molecules on metal surfaces [ 131, we have observed a very striking experimental correlation which has, we believe, significant implications about the fundamental physics of chemisorption.

The heats of adsorption of 02, COa, C2Hq, N2, and Hz are plotted in figs. l-5 against the heat of adsorption of CO. The heats of adsorption data shown in figs. l-5 all fall on straight lines, within error. This correlation is in itself useful as it has predictive value. Knowing the heat of adsorption of one gas on a metal, we can predict the heats of adsorption of other gases. Thus, as the heat of adsorption of CO on Nb is 553 kJ/mole, we would predict from fig. 3 a heat of adsorption of C2H4 on Nb of about 650 Id/mole and, from fig. 4, a heat of adsorption of Na on Nb of about 565 kJ/mole.

The correlation of the heats of adsorption has another important implication

Or

o-

IO -

IO-

OL 0

J.J. Burton / General rule for adsorption of gases on metals 649

200 400 b

600

HEAT OF ADSORPTION OF CO (kJ/mole)

800

Fig. 3. Plot of the heat of adsorption of C2H4 against that of CO on various metals.

which pertains to the fundamental physics of chemisorption. Mathematically, from the results in figs. 1-5, we can write the heat of adsorption, Q(M, m) of any mole- cule, m, on any metal, M as

QW, m) = C(m, CO) cz(M, CO) , (1)

where Q(M, CO) is the heat of adsorption of CO on the metal, M, and Qm, CO) is the ratio of the heat of adsorption of the molecule, m, to that of CO. (qm, CO) is the slope of the straight lines in figs. I-5 and does not depend on the metal). Regarding now CO as a reference molecule, we can rewrite eq. (1) in the form

MM, m) = Co(m) Q’(M) , (2)

where e”(m) is a coefficient which depends only on the metal and Q”(M) depends only on the molecule.

The result which we have obtained in eq. (2) implies that the chemisorption of

650 J.J. Burton / General rule for adsorption of gases on metals

200 400 600

HEAT OF ADSORPTION OF CO (kJ/mole)

800

Fig. 4. Plot of the heat of adsorption of N2 against that of CO on various metals.

molecules in metals can be separated into a molecule dependent term and a metal dependent term. This result is surprising in that current thinking [S -121 would suggest that chemisorption depends strongly on the details of the relation of the electron states of the molecule to those of the metal. A first principles understand- ing of eq. [2], will, we believe, represent significant progress in understanding the fundamental physics of chemisorption.

J.J. BURTON

Exxon Research and Engineering Co., P. 0. Box 45, Linden, New Jersey 0 7036, USA

J.J. Burton / General rule for adsorption of gases on metals 651

/ w

MO

Ni

I I I

200 400 600 a 10

HEAT OF ADSORPTION OF CO (kJ/mole)

Fig. 5. Plot of the heat of adsorption of Hz against that of CO on various metals.

References

[l] H.S. Taylor, J. Am. Chem. Sot. 53 (1931) 578. (21 M.W. Roberts, Nature 188 (1960) 1020. [3] D. Brennan, D.O. Hayward and B.M.W. Trapnell, Proc. Roy. Sot. (London) A256 (1960)

[4] FTanaka and K. Tamaru, J. Catalysis 2 (1963) 366. [S] D.M. Newns, Phys. Rev. 178 (1968) 1123. [6] T.B. Grimley, J. Phys. C3 (1970) 1934. [7] T.L. Einstein and J.R. Schrieffer, Phys. Rev. B7 (1973) 3629. (81 R.P. Messmer, C.W. Tucker and K.H. Johnson, Surface Sci. 42 (1974) 341. [9] N.D. Lang and A.R. Williams, Phys. Rev. Letters 34 (1975) 531.

[lo] S.C. Ying, J.R. Smith and W. Kohn, Phys. Rev. Bll (1975) 1483. [ 1 l] E.A. Hyman, Phys. Rev. Bll (1975) 3739. [12] G. Doyen, Surface Sci. 59 (1976) 461. [13] J.R. Anderson, Structure of Metallic Catalysts (Academic Press, New York, 1975).