J. Phys. Chem. 1992,96, 823-828
abstracted from adsorbed organic molecules, they cannot be regenerated by simply adding molecular oxygen (electron affinity -0.44 eV) at room temperature. This means that there would be no source of electrons for the formation of OF.The existence of such positive holes on calcined zeolites is, therefore, doubtful. In conclusion, aluminum Lewis acid sites are indispensable for the formation of radical cations from aniline adsorption on Hzeolites. Further work is needed to determine which aluminum species are responsible for the formation of the radical cations
823
and for the fixation of superoxide ions. Acknowledgment. This work is supported by DOE Grant DE-FG02-90 ER1430. F. R. Chen thanks Unocal (CA) for financial support. NSF Grant DIR-8719808 and NIH Grant RR 04095, which have partly supported the purchase of a GN-500 NMR instrument, are gratefully acknowledged. Registry No. 02,7782-44-7; Or,11062-77-4; aniline radical cation, 34197-49-4; aniline, 62-53-3.
Theoretical Study of the H2 Interaction with Feo, Fe', and Fe- Atoms M. Sbnchez,*Yt F. Ruette,t and A. J. Hernindezo Departamento de Quimica, Instituto Universitario de Tecnoloda, g r . Federico Rivero Palacio, Apartado 40437, Caracas, Venezuela, Insti!uto Venezohno de Investigaciones Cientificas, Apartado 21 827, Caracas, Venezuela, and Departamento de Quimica, Universidad Simbn Bolivar, Apartado 80659, Caracas, Venezuela (Received: May 29, 1991)
The present work deals with the applications of the semiempirical method MINDO/SR to study hydrogen interaction with single iron atoms, Feq (q = +1, 0, -1). Excited configurations for neutral and ionic species were also considered. Good correlation was found with respect to some experimental features reported for iron surfaces. Our results show the formation of a precursor state in the H2 dissociation with a dissociation barrier of about 35 kcallmol for the neutral Fe atom. The positively charged iron atom favors the formation of a dihydrogen complex instead of the dihydride one. On the other hand, a negative charge on the iron atom facilitates the Hz dissociation. In the case of the neutral atom, an excitation to the quintuplet state, with Fe(4s14p13d6)configuration state, allows the H2 dissociation without activation barrier. Hydrogen recombination from H-Fe-H can occur by photoexcitation from the PB1 to the 15Bl state.
I. Introduction The dissociative chemisorptionof hydrogen on iron plays a very important role in surface science because of its importance in catalytic technology.' Hence, it has been the subject of numerous theoretical and experimental investigations.*-I5 Industrial syntheses are generally performed with promoted metal catalysts. In the case of hydrogen adsorption on iron, potassium acts like a promotor,I2 increasing the binding energy of the hydrogen chemisorbed on the surface. Pure potassium is catalytically inactive, indicating that metallic iron is responsible for the catalytic activity. The role of potassium is related to an enhancement of electronic density on the iron surface by electron donation from the less electronegative atom. In contrast, chemisorption of carbon, oxygen, and sulfur on Fe( 100) single-crystal surface reduces the rate of reactionI4 because these more electronegative atoms withdraw electrons from the metallic surface. Thus, the preadsorption of electronegative or electropositive atoms on the metal surface induces local net charges" that affect the catalytic interaction with gas molecules. Recent studies of catalytic interaction of small molecules with a single transition metal center have been reported.I6 This single center can be a neutral, ionic, or excited metallic atom. In view of the general interest for catalytic reactions with one metallic center and particularly the importance of hydrogen chemisorption on metallic iron, in this work we present theoretical calculations for H2interactions (adsorption and dissociation) with a single iron atom. [FeH2]qsystems (q = 0, 1, -l), with different geometries, were selected to study those processes. These chosen systems can also be considered as preliminary models in the description of the chemisorption of a hydrogen molecule over an iron surface.4c A brief description of the theoretical method employed here, MINDO/SR, is given in the next section. Discussion of results for the H2interaction with neutral, excited, and charged iron atoms 'Instituto Universitario de Tecnologia. f Instituto Venezolano de Investigaciones Cientificas. 1 Universidad Simdn Bolivar.
is presented in section 111. Final remarks, with a summary of general trends from these calculations, are given in the last section. ( 1 ) (a) Ertl, G. Caral. Reu. Sci. Eng. 1980, 21. 201. (b) Huang, K. H.; Zeng, X. M.; Li, J. T. J. Caral. 1983,81, 259. (c) Bowker, M.; Parker, I.; Waugh, K. C. Sur- Sci. 1988,197, L223. (d) Stoltze, P.; Norskov, J. K. Surf. Sci. 1988,197, L230. (2) (a) Parks, E. K.; Weiller, B. H.; Bechthold, P. S.;Hoffman, W. F.; Nieman, G. C.; Pobo, L. G.; Riley, S.J. J. Chem. Phys. 1988.88, 1622. (b) Morse, M. E.; Geusic, M. G.; Heath, J. R.; Smalley, R. E. J . Chem. Phys. 1985,83, 2293. (c) Richtsmeier, S.C.; Parks, E. K.; Liu, K.; Pobo, L. G.; Riley, S. J. J . Chem. Phys. 1985,82, 3659. (d) Zakin, M. R.; Brickman, R. 0.;Cox, D. M.; Kaldor, A. J . Chem. Phys. 1988,88, 6605. (3) Anderson, A. B. J . Am. Chem. SOC.1977, 99, 696. (4) (a) Siegbahn, P. E. M.; Blomberg, M. R. A.; Bauschlicher, C. W., Jr. J . Chem. Phys. 1984,81,1373. (b) Schilling, J. B.; Gcddard, W. A,; Beauchamp, J. L. J . Am. Chem. Soc. 1986, 108, 582. (c) Siegbahn, P.; Blomberg, M.; Panas, I.; Wahlgren, U. Theor. Chim. Acra 1989, 75, 143. (5) Ozin, G. A.; McGraffrey, J. G. J. Phys. Chem. 1984,88, 645. (6) (a) Halle, L. F.; Klein, F. S.;Beauchamp, J. L. J . Am. Chem. Soc. 1984, 106, 2543. (b) Elkind, J. L.; Armentrout, P. B. J . Am. Chem. SOC. 1986, 108,2765. (c) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1986, 90, 5736. (d) Armentrout, P. B.; Beauchamp, J. L. Acc. Chem. Res. 1989, 22, 315. (7) (a) Shanabarger, M. R. Surf.Sci. 1985,150,451. (b) Shanabarger, M. R. J . Vac. Sci. Technol. 1986, A4, 623. (c) Nowicka, E.; Lisowski, W.; Dus,R. Surf.Sci. 1984, 137, L85. (8) Viiials, J.; Gunton, J. D. Surf.Sci. 1985, 157, 473. (9) Moritz, W.; Imbihl, R.; Behm, R. J.; Ertl, G.; Matsushima, T. J . Chem. Phys. 1985,83, 1959. (IO) Pasco, R. W.; Ficalora, P. Surf.Sci. 1983, 134, 476. (1 1) Lang, N . D.; Holloway, S.;Norskov, J. K. Surf.Sci. 1985, 150, 24. (12) (a) Weatherbee, G. D.; Rankin, J. L.; Bartholomew, C. H. Appl. Catal. 1984, 11, 73. (b) Rankin, J. L.; Bartholomew, C. H. J . Caral. 1986, 100, 533. (c) Ertl, G.; Lee, S.B. Chem. Phys. Lerr. 1979, 60, 391. (13) McCreery, J. H.; Markworth, A. J.; McCoy, J. K. Scr. Meroll. 1988, 22, 1557. (14) Benziger, J.; Madix, R. J. Surf.Sci. 1980, 94, 119. (15) Norskov, J. K.; Holloway, S.;Lang, N . D. Surf Sci. 1984, 137, 65. (16) (a) Burnier, R. C.; Byrd, G. D.; Freiser, B. S.J . Am. Chem. Soc. 1981,103,4360. (b) Jacobson, D. B.; Freiser, B. S.J. Am. Chem. Soc. 1983, 105, 5197. (c) Babinec, S.J.; Allison, J.; Beauchamp, J. L.; Illies, A. J.; van Koppen, P.; Bowers, M. T. J . Am. Chem. SOC.1988, 110, 1. (d) Brown, S. H.; Crabtree, R. H. J . Am. Chem. SOC.1989, 111, 2935. (e) Brown, S.H.; Crabtree, R. H. J . Am. Chem. SOC.1989, 111, 2946.
0022-3654/92/2096-823%03.00/0 0 1992 American Chemical Society
Stinchez et al.
824 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 TABLE I: Electrook and Booding Properties of Side-Boaded (FeH2)f Systems (q = 0, +1, -1) Re, A system IFeH,1°
state
configuration a: 4(al)bla2b2
7 \I /
H
I
0.77
H charge, e 0.077
0.73
0.14
-20.507
-33.8
0.86
-0.38
-20.624
-42.4
H-H
total energy, au -20.7290
binding energy,
kcal/mol -5.0
L
m
H-H
H-Fe
-+X
-Fe
Fe-H 2.01
-H+X
-20724I
b
a
C
Figure 1. FeHz geometries: (a) end-on; (b) side-on; (c) linear.
> (3
a
$
-20726-
w
11. Methodology
_I
The calculations were made with the semiempirical SCF method MINDO/SR. The details of this method have been reported previou~ly.'~The MINDO/SR method can handle transition metal atoms using s, p, and d valence shell orbitals. It is parametrized to reproduce experimental bond energies and bond lengths of diatomic molecules and bulk cohesive energies. Our MIND0 program also includes symmetry-adapted functions, selective orbital occupancy, and the generation of all singly and doubly excited c o d i t i o n s out of the Hiickel configuration used to obtain an initial density matrix in the SCF cycle. The electronic configuration for the most stable molecular geometry for a given symmetry is maintained along the dissociation reaction coordinate. We use the calculated Fock matrix at a given point as the initial guess for the next point.)* The parameters used in this work are those utilized in previous descriptions of H-Fe interaction processes.19 The FeCF(d7s')) state was used in these semiempirical calculations because it guarantees the transferab~tyof parameters from Fe-H and Fe-Fe diatomic molecules to (Fe)12and (Fe)12-H clusters.'9a That is, the jF(d7s') state is the most stable state of iron after adjustments of the Slatedondon parameters and Slater exponents to obtain better correlation between properties of diatomic molecules and properties of clusters.
III. Results and Discussion With the purpose of studying the interaction of Hz with a single iron atom, we start by searching for the most stable electronic configurationsof the sidebonded, end-bonded (FeH-H), and linear (H-Fe-H) systems, shown in Figure 1. All calculations were made using symmetry-adapted wave functions in C,, C,,, and Dmhsymmetries for bent, linear end-bonded (FeH-H), and linear (H-FeH) systems, respectively. Geometry optimization was camed out for the most stable electronic configuration of each geometry. The results of this search gave a quintuplet state (sBl) for the side-bonded geometry, a quintuplet state (?Z) for the end-bonded, and a quintuplet s& state for the linear system. Once we have obtained the most stable states for the above given geometries, the following transformation processes will be studied in the next sections: Fe-H-H
-
H
Fe' H '
H and 'eF
H'
-
H-Fe-H
1. Hydrogen Adsorption. To study in more detail Hz-Fe interaction, a minimum-energy distortion pathway was calculated from different geometric arrangements ranging from the end(17) Blyholder, G.; Head, J.; Ruette, F. Theor. Chim. Acta 1982,60,429. (18) Ruette, F.; Blyholder, G.; Head, J. J. Chem. Phys. 1984, 80, 2042. (19) (a) Blyholder, G.; Head, J.; Ruette, F. Surf. Sci. 1983,131,403. (b) Sgnchez, M.; Ruette, F.; Hernandez, A. J. In Lectures in Surface Science; Castro, G.R., Cardona, M., Eds.; Springer-Verlag: Berlin, 1987; p 124.
a c
-20728C
-207301 40
I
I
I
60
80
I00
I
120
1
I
160
140
I
180
DISTORSION ANGLE H-H-Fe(8)
Figure 2. Potential energy curve for transformation of side-on to end-on geometry.
bonded (Fe-H-H) to the side-bonded (Fe-(H2)) geometries. Calculations were carried out by starting with the most stable electronic state of the linear molecule (%) and closing the FeH-H angle until reaching the side-bonded molecule. Results are depicted in Figure 2. For each FeH-H angle, the bond lengths were optimized. The results show that the side-on geometry is more favorable than the end-on one. Both geometries lead to stable states with respect to infinitely separated Fe Hz species ( r e p resented by a dashed line in Figure 2). However, the end-on state can spontaneously transform in the side-bonded one, as can be seen in Figure 2. To study the H2-Fe interaction process on different electronic environments, we also perform a search for the most stable electronic configurationsof the charged molecules, (FeHz)- and (FeH2)+,in the side-bonded geometry with a H-FeH angle of 22*, corresponding to that obtained for the most stable neutral molecule. The calculated properties, electronic configuration, equilibrium bond distances, charge on one of the hydrogen atoms, total energy, and binding energy, for the more stable states of (FeH2)q (q = 0, +1, -1) systems are given in Table I. Here, binding energy corresponds to the energy difference between the sidebonded molecule (Fe(H2))q and the separated systems (Fe* + H2)in their calculated lowest energy states. A negative binding energy indicates the formation of a stable molecular dihydrogen complex. Table I shows for the most stable state of the neutral molecule (15B1)a small bond energy (-5 kcal/mol), a lon bond distance Fe-H (2.01 A), and a H-H bond length (0.77 ) close to that of the H2 molecule (0.74 A). These features indicate a weak iron-hydrogen interaction that has its equivalence in surface chemistry as a physisorbed state. A physisorbed molecular complex has previously been proposed as a precursor state of Hz dissociation on Fe and other metallic surfaces as in experimental and theoretical ~ 0 r k s . ~ ~ A3 ~small ~ 9 transfer ~~ of charge from
+
f
(20) Ertl, G . In Catalysis Science and Technolog& Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1982; p 209. (21) (a) Nowicka, E.; Dus, R. Surf. Sci. 1984, 144,665. (b) Nowicka, E.; Wolfram, Z.; Lisowski, W.; Dus, R. Surf. Sci. 1985, 151, L166. (c) Nowacki, P.; Lisowski, W.; Dus, R. React. Kinet. Catal. Lett. 1984,26,301. (d) Ruette, F.; Hernandez, A. J.; Ludeiia, E. V. Surf. Sci. 1985, 151, 103. (e) Martensson, A. S.;Nyberg, C.; Andersson, S.Phys. Rev. Lett. 1986,57, 2045.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 825
H2 Interaction with Fe
02 -Uu
1
-2)
(44.0~1
00
+40,
5
-2070
LT
w
'., QUINTUPLET
-2080 0
IC, H,
20
40
BO
60
100
I20
140
160
180
ANGLE H - F e - H ( e )
I'B1 [FeH,l'
'01 [Fe 43'
1
'A, [Fe Y1-
'F Fe
Figure 4. Potential energy curves for [FeH2I0with different multiplic-
ities.
Figure 3. Orbital energy diagrams for Fe, H2, and [FeH2]q(q = 0, +1,
-1).
hydrogen to the iron atom is observed such as in some dihydrogen organometallic c o m p l e x e ~ . ~ ~ ~ ~ ~ The most stable configuration for positively charged systems, with respect to Fe+ and H2, corresponds to the 4B1state. Here, due to the positive charge on the center site, there is a major electronic transfer from the dihydrogen molecule to the iron ion that leads to a H-H bond distance smaller than in the isolated molecule. The 4B1state correlates with the Fe'(4F) state, which was found to be the most reactive iron ion in the experimental gas-phase reaction of Fe+ with H2.6b In the FeH,- systems, the lowest binding energy corresponds to the 6A1state, which shows the major charge transfer from Feto the dihydrogen and the largest F e H and H-H bond distances. In these cases, the longer H-H bond distance (0.86 A) than in the H2 molecule and the large binding energy (about -42 kcal/mol) suggest that H2 has been activated by the interaction with Fe-. An analysis of the molecular orbitals can help us interpret the observed differences in the (FeH2)9 side-bonded systems. The energy levels together with their composition are shown in Figure 3 for the most stable systems. In this figure, the lal orbital is mainly the ugorbital of H2 with some participation of the 4s and 4pz atomic iron orbitals. The 2al
