J. Phys. Chem. 1984,88, 5763-5764 be explained by comparing the highest u and a orbitals of the chalconides and the corresponding ions. Table VI11 shows the coefficients of the highest occupied u and a orbitals of SnO, SnO+ and PbSe, PbSe+ pairs. As one can see from that table, the orbital relaxation effects are much greater for SnO than PbSe. There is a significant increase in the s poplation of Sn (in the u orbital) accompanied by a decrease in the p population of Sn as well as 0. The a orbitals of both SnO and SnO+ are mainly on oxygen and are thus nonbonding. In the case of PbSe there is more a
5763
bonding. The breakdown of Koopman's theorem is thus explained on the basis of more relaxation of the orbitals of SnO than PbSe. At shorter distances the a orbitals are more localized on the chalconide atom, thus stabilizing the ua4configuration. Consequently, reverse ordering of and zllstates is observed at short distances for SnO+, PbO+, and PbSe+. Registry No. SnOt, 92456-21-8; PbO+, 92456-20-7; PbS+, 9245622-9; PbSe+, 92456-23-0; SnO, 21651-19-4; PbO, 1317-36-8; PbS, 1314-87-0; PbSe, 12069-00-0.
COMMENTS Surface Structure of Iron Catalysts for Ammonia Synthesis Sir: Ammonia synthesis catalysts consist of metallic iron with small amounts of unreduced oxides. A typical commercial catalyst, called KMI, made by Topsere A I S contains 3 wt % of AlzO3, 3 wt % of CaO, and 0.75 wt % of KzO. These so-called promoters represent 0.96 or 0.93 mole fraction of the surface as determined by Auger electron spectroscopy' or selective chemisorption,2 respectively. The rest of the surface is metallic iron. The purpose of this Comment is to point out that the structure of the iron seems to be that of the (1 11) face. The evidence is offered by data of Spencer et aL3 who report turnover rates on the three low index faces of iron single crystals with the result that the (1 11) face is much more active than the other two. Turnover rates ut at 748 K were measured at 10 atm and at four different partial pressures at 20 atmS3We note that these very low conversion data fit the rate equation of Temkin et aL4 which was shown to fit low conversion data on iron:
vt/s-' = 12.8 e ~ p ( - 9 7 7 0 / T ) ( f " ~ P ~ ~ ) ~ / ~ where Tis in kelvin, the value 9770 K corresponds to the activation energy reported by Spencer et al., and the partial pressures are in Pa. Our expression for ut assumes that each active site consists not of C4 atoms as assumed in ref 3 but consists of a pair of C7 atoms, as suggested earlier on the basis of Mossbauer effect data.s Surface atoms C7 with a coordination number of 7 are characteristic of (1 11) planes on bcc iron. With eq 1, we can extrapolate the data of Spencer et al. to the conditions of Topsere et namely, atmospheric pressure, reactants stoichiometric ratio, 673 K, and a conversion 15% of equilibrium. Because of this very low conversion the data of Topsoe et al. need not be corrected for inhibition of the rate by ammonia. Under these conditions, the value of ut from eq 1 is 0.3 s-'. This must be compared to a value of ut = 0.5 s-' reported by Topsere et aL6 for the KMI commercial catalyst discussed above. This value of ut was obtained by Topsere et al. by means of the classical titration of surface iron atoms by chemisorption of CO at 195 K, but with the assumption that each CO molecule adsorbed cor(1) D. C. Silverman and M. Boudart, J. Cutal., 77, 108 (1982). (2) A. Nielsen and H. Bohlbro, J . Am. Chem. SOC.,74, 953 (1952). (3) N. D. Spencer, R. C. Schoonmaker, and G.A. Somorjai, J . Catal., 74, 129 (1982). (4) M. I. Temkin, N. M. Morozov, and E. M. Shapatina, Kinet. Cutal., 4, 260 (1963). (5) J. A. Dumesic, H. Topsae, K. S. Khammouma, and M. Boudart, J . Catal., 37, 503 (1975). (6) H. Topsm, N. Topsae, H. Bohlbro, and J. A. Dumesic, "Proceedings of the Seventh International Congress on Catalysis", Part A, T. Seiyama and K. Tanabe, Eds., Kcdansha, Tokyo, 1981 p 247.
responds to two Fe atoms as may be the case on (1 1 1 ) planes.' Considering the bold extrapolation of the data of Spencer et aL3 to the conditions of Topsere et a1.,6 the close correspondence of the values of ut for (1 11) single crystals and the commercial catalyst suggests that the surface of the latter exhibits predominantly (1 11) faces, since the value of v, for the commercial catalyst was obtained by CO chemisorption that counts iron atoms as may be the case on (1 11) planes. Besides, the commercial catalyst possesses the same turnover rate as found on a clean single crystal, almost irrespective of the very large amount of surface promoters. Surely these tentative conclusions are of theoretical and practical significance. At any rate, as has happened many times in the past ten years, work on single crystals provides the standards by which work on commercial catalysts can be a ~ s e s s e d . ~ Acknowledgment. This work was supported by NSF Grant CPE 8219066. D.G.L., on leave from the University of Mar del Plata, thanks CONICET of Argentina. Registry No. Iron, 7439-89-6; ammonia, 7664-41-7. (7) M. Boudart and G. DjEga-Mariadassou, "Kinetics of Heterogeneous Catalytic Reactions", Princeton University Press, Princeton, NJ, 1984, p 157.
Department of Chemical Engineering Stanford University Stanford, California 94305
M. Boudart* D. G. Loffler
Received: June 1 , 1984
Comment on "Equilibrium and Rate Constants for Ion Association in Liquid Ammonla" Sir: Stevenson et al.' calculate a value of 3.1 X for the association constant of the nitrobenzene anion radical with sodium cations. This value is surprisingly low and the paper tries to explain why ionic association is so low in liquid ammonia. Equilibrium constants for ionic association of many 1-1 salts in liquid ammonia at -33.5 "C have been calculated from conductance data (see Table 118, p 174, in ref 2); their values fall between lo2 and lo3. The same order of magnitude (1.3 X loz) is found for the association of solvated electrons, which are anion radicals, with sodium cation^.^ (1) Stevenson, G. R.; Reiter, R. C.; Ross, D. G.; Frye, D. G.J. Phys. Chem. 1984, 88, 1854-7. (2) Jander, J. "Anorganische und allgemeine Chemie in fliissigen Ammoniak"; Interscience: New York, 1966. (3) Evers, E. C.; Frank, P. W. J . Chem. Phys. 1959, 30, 61.
0022-365418412088-5763$01.50/0 0 1984 American Chemical Society
J. Phys. Chem. 1984, 88, 5764
5764
An association constant 3.1 X would be lo5 lower than usual. The authors suggest that the anomaly could be due to their lower temperature. But it would be quite anomalous to see an association constant increase with temperature and to have such a large temperature coefficient. It seems that there is simply a misinterpretation of eq 2. Its equilibrium constant K = (AN’ - &) [Na+]/(AN - AN’)
-
is not the association constant of reaction 1 PhN02-- Na+ PhN02-.,Nat
+
but its inverse, the dissociation constant. This can be easily verified by writing AN’ - A N [PhN02-*]
AN - AN’
-
[PhN02-.,Na+] The association constant is therefore the inverse of 3.1 X low3, i.e., 3.2 X lo2, without any anomaly. The conclusion is that N H 3 behaves as a conventional nonaqueous polar solvent, where ionic associations of dilute salts are mostly governed by the dielectric constant, and this is true as well for anion radicals as for usual ions. (However spin pairing between anion radicals must be taken into a c c o ~ n t . ~ ) Registry No. PhN02-.,Na+, 34480-34-7; NH3, 7664-41-7. (4) Schettler, P.D.; Lepoutre, G . J. Phys. Chem. 1975, 79, 2823.
Laboratoire d’Etude des Surfaces et Interfaces Hautes Etudes Industrielles (FLS, ISEN) CNRS, 59046 Lille, France
Gerard Lepoutre
Received: June 19, 1984
0022-3654/84/2088-5760$01.50/0
Reply to Comment on “Equilibrium and Rate Constants for Ion Association In Liquid Ammonia” Sir: As pointed out in the previous communication’ K , (the equilibrium constant for reaction 1 in liquid ammonia) previously PhN02-.
+ Na+
kd
PhNOp,Na+
(1)
reported by us2 is actually the value for Kd (the equilibrium constant for ion-pair dissociation). The values for K, and Kd are (3.2 f 0.3) X lo2 and (3.1 f 0.2) X respectively. This error does not affect the value reported for the rate constant for ion-pair formation k f = (1.1 f 0.2) X 1O’O L/(mol s). However, the value for kd is incorrect in ref 2. The correct value for kd is kd = k J K , = (3.4 f 0.8) X lo7 s-I. Since the rate of ion-pair dissociation is much smaller than we originally concluded, the last sentence in ref 2, stating that the lifetime of the ion pair is very short, is also erroneous. In fact, the nitrobenzene anion radical-sodium ion pair has a half-live of t I l 2 = 2.0 X lo-* s in liquid ammonia. This correction in kd is necessary now that Lepoutre has pointed out our error in K,. Registry No. PhN02-.,Na+, 34480-34-7; ammonia, 7664-41-7. (1) Lepoutre, G . J . Phys. Chem., preceding comment. (2) Stevenson, G . R.; Reiter, R. C.;Ross, D G.;Frye, D. G . J. Phys. Chem. 1984,88, 1854.
Department of Chemistry Illinois State University Normal, Illinois 61 761
Gerald R. Stevenson* Richard C. Reiter
Received: July 17, 1984
0 1984 American Chemical Society
