J . Phys. Chem. 1990, 94, 4985-4990 of the tiny Ru clusters occurs after CO desorption following the decomposition of Ru(CO), species at high temperatures. Ru/ Alz03is the second example in which the structure of the metal clusters themselves is influenced by an adsorption of CO, which suggests that the disruption of tiny metal clusters is a ubiquitous
4985
phenomenon. Much more study is required to clarify the interaction between metal clusters and adsorbed species that eventually leads to reactions. Registry No. Ru, 7440-18-8; CO, 630-08-0; H2, 1333-74-0.
Potential Energy Hypersurface for Ammonia Adsorbing onto Nickel Oxide Stephen R. Cain,* Luis J . Matienzo, and F. Emmi IBM Systems Technology Division, 1701 North Street, Endicott, New York 13760 (Received: May 30, 1989; In Final Form: February 8, 1990)
The approach of an ammonia molecule to the surface of nickel oxide was probed theoretically by performing band calculations at the extended Huckel level on model systems. A hump in the energy vs Ni-NH, separation curve was attributed to the initial repulsion of the NH, lone-pair electrons by the electron-rich 3d orbitals of nickel. However, once the 3d,z orbital was pushed above the Fermi level, the antibonding component of the Ni 3d,z-NH3 lone-pair interaction was depopulated and the Ni-NH, interaction became strongly attractive. This was demonstrated to be completely analogous to the intended crossing of occupied and unoccupied orbitals in rearrangements of small molecules. A limited study of the potential energy surface was performed, the results of which suggested that NH3 prefers to attack directly at a surface Ni atom. From the principles derived in the NiO-NH3 study, qualitative predictions concerning the dynamics of NH, adsorbing onto Cr,O, and C u 2 0 were made. Only nickel-based substrates are expected to exhibit an activation energy toward adsorption of NH3. Further, predictions were made concerning the stability of the adsorbed NH, toward dissociation into NH2 and smaller fragments. More tightly bound NH3 should be more susceptible to dissociation. The decomposition is expected to follow the trend on Cr203> on NiO > on Cu20. Since the activation energy for adsorption has been observed experimentally to be a function of coverage, calculations were performed assuming both half-coverage and full coverage of the available adsorption sites (surface N i atoms). Our results indicated that most of the coverage effect could be attributed to NH,-NH3 steric repulsions.
Introduction In a recent theoretical study of ammonia adsorbing onto Cr, Ni, Cu, and their oxides,’ an energy barrier to adsorption was calculated for the NH3-nickel and NH3-nickel oxide systems, as shown in Figure 1 . In view of the present study, however, the preliminary estimate of the adsorption barrier reported in ref 1 was probably too high. Nonetheless, the activation energy of adsorption is not an artifact of the calculation since adsorption energy barriers have been reported for NH3-Ni, NH,-Pd, and NH3-W systems.z Further, there is evidence of adsorption barriers in other systems, for example, C O on metals., However, the theoretical study performed in ref 1 suggested that there is no energy barrier for ammonia adsorbing onto chromium- or copper-based substrates. The difference between the NiO-NH3 system and the Cr203-NH3 or Cu20-NH3 system was not fully discussed in ref 1 and is the topic of this paper. Since the chemical reactions are governed by the electronic structure of the reactants, it is instructive to take time to discuss the properties of NiO and NH,. Though ammonia has several valence orbitals, its chemistry is dominated by its “lone pair” of electrons. The lone pair of electrons occupies one the apex of a tetrahedral-like structure, while the hydrogens occupy the other three apexes. Ammonia has no low-lying virtual orbitals; it behaves strictly as a Lewis base. The electronic structure of NiO, on the other hand, is not so simple. As a zero-order approximation, a crystal of NiO may be viewed simply as alternating NiZ+ and 02-ions. This formal assignment of oxidation state gives Ni a d8 electronic configuration, with the electrons occupying the familiar octahedral eg and t2g orbitals. Though much insight can be gained through such a
treatment, understanding many of the interesting properties of NiO requires a more sophisticated theory. In one study,4 for example, photoelectron spectra and optical absorption spectra were modeled by performing molecular orbital calculations on Ni06’, the resulting ion after photoionization. Because the electrons are unpaired in the octahedral configuration, the electronic structure is complicated. Spin-localized band calculations show significant differences between the spin-up and spin-down density of states in N i 0 . 5 Also, differences in the density of states between antiferromagnetic NiO and paramagnetic NiO have been reported.6 Further, electron correlations are important and have been implicated in giving rise to a band gap that is substantially larger than would be predicted from a one-electron model.’ Fortunately, chemical properties are less subtle than the magnetic and electrical properties. Hence approximate treatments may indeed successfully describe chemical reactions, at least qualitatively. At this point, we note that surface defects may be important in the adsorption dynamics, but a full treatise is beyond the scope of this paper. Instead, we focus on behavior intrinsic to the material itself. Since our main interest is the dynamics of adsorbing NH3 onto a nickel oxide surface, it is necessary to understand the nature of an energy barrier to a chemical reaction. The most interesting cases are those in which the energy barrier results from intended crossing of occupied and unoccupied orbitals. Such orbital crossings have led to the celebrated Woodward-Hoffmann selection rules for pericyclic reactions.* The behavior of the NH3-nickel oxide system can be explained in terms of the same type of intended orbital crossings found during reactions of small
( I ) Cain, S. R.; Matienzo, L. J.; Emmi, F. In Merallized Plastics: Fundomental and Applied Aspects; Mittal, K. L., Susko, J. R., Eds.; Plenum Press: New York, in press. (2) (a) Logan, S.R.; Kemball, C. Trans. Faraday SOC.1960,56, 144. (b) Amene, A.; Taylor, H.S. J. Am. Chem. SOC.1954, 76, 4201. (3) Feng, X.-H.; Garfunkel, E. L. Langmuir 1987, 3, 353 and references therein.
(4) Fujimori, A,; Minami, F. Phys. Reo. E 1984, 30, 957. (5) Terakura, K.; Oguchi, T.; Williams, A. R.; Kubler, J . Phys. Rev. B 1984, 30,4734. (6) Oguchi, T.; Terakura, K.; Williams, A. R. Phys. Reo. E 1983.28, 6443. (7) Sawatzky, G. A,; Allen, J . W. Phys. Reo. Letf. 1984, 53, 2339. (8) Woodward, R . B.; Hoffmann, R. The Conservation of Orbital Symmetry; Verlag Chemie: Weinheim, FRG, 1971.
~~
0022-3654/90/2094-4985$02.50/0
0 1990 American Chemical Society
~~~
~~
4986
The Journal of Physical Chemistry, Vol. 94, No. 12, I990
Cain et al. TABLE 1: Parameters Used in the Tight Binding and Molecular Orbital Calculations orbital
-
H Is
N 2s N 2p 0 2s 0 2P Cr 4s Cr 4p Cr 3d“
I 4
3
Ni-N
2
Sep
1
(A)
Figure 1. Total energy (relative to that for a nickel-ammonia separation of 4 A) as NH3 approaches a nickel oxide surface. Through the course of the reaction, ammonia was assumed to approach the surface with its lone pair of electrons pointed directly a t a surface nickel atom.
Ni 4s Ni 4p Xi 3d“
cu 4s
cu 4p Cu 3d“
2
2.09
a
Figure 2. Unit cell of the NH,-nickel oxide. Note that only one of the two surface Ni atoms is attacked by NH3.
molecules; intended crossings in the following discussion result from geometric changes that perturb the density of states (DOS). Though the empty 4s and 4p orbitals of the metal play an important role in bonding N H 3 to the metal or metal oxide surface, the 3d orbitals primarily govern the dynamics of low-coverage adsorption since they are occupied to varying degrees. Thus, the Ni 3d orbitals will be a focal point for the arguments to follow. To better understand the adsorption process as well as demonstrate that tight binding band calculations may be interpreted in much the same manner as molecular orbital calculations, an in-depth study of the ammonia-nickel oxide system was performed. (For brevity’s sake, only the ammonia-nickel oxide system was investigated explicitly. Ammonia-metallic nickel also exhibits the adsorption energy barrier, but the effect is not as marked as in the ammonia-nickel oxide system.) Principles derived from the NiO-NH, study may be applied readily to NH, adsorption onto chromium-based and copper-based substrates. Technique for Performing the Calculations and Analyzing the output Our theoretical treatment employed tight binding calculations9 of the extended Huckel typelo using the orbital parameters given in Table 1. Slater-type functions were used for the atomic orbitals. A three-layered slab of nickel and oxygen atoms was used to simulate the nickel oxide (100) surface; a slab of three or four layers is sufficient to represent a semiinfinite surface.” A monomolecular layered slab was used for ammonia. The assumed unit cell geometry is shown i n Figure 2. In this portion of the study, only the Ni-NH, separation was varied; no other bond lengths or angles were relaxed along the reaction coordinate. As illustrated in Figure 2 , the entire surface can be generated by translations of the unit cell in the x-y plane. Reciprocal space was sampled by nine two-dimensional wave vectors,’* selected by permuting the components through the numbers 0.083,0.25, and 0.417.13 (These numbers are scaled by 2 ~ / a , where , a, is the (9) Band calculations were performed by using N E W B A N D I , a program developed by the Hoffmann group at Cornell University. (cf. Whangbo, M.-H.; Hoffmann, R. J . A m . Chem. SOC.1978, 100, 6093.) (IO) Hoffmann, R.; Lipscomb, W. N. J . Chem. Phys. 1962, 36, 2179. ( I I ) Saillard, J.-Y.; Hoffmann, R. J . Am. Chem. Soc. 1984, 106, 2006. (12) For a n introduction to the wave vector formulation, see: Kittel, C. Introduction to Solid State Physics, 5th ed.; Wiley: New York, 1976; pp 37 ff and 1 8 5 ff.
exponent, A-’ 1 .30 I .95 1.95 2.28 2.28 1.70 1.70 4.95 (0.51) 1.80 (0.68) 2.10 2.10 5.75 (0.57) 2.0 (0.63) 2.20 2.20 5.95 (0.59) 2.30 (0.57)
H i i , eV -13.6 -26.0 -13.4 -32.3 - 1 4.8 -8.7 -5.2 -11.2
-9.2 -5.2 -13.5 -1 I . ?
--5.2 -14.0
“The 3d orbitals were represented by the sum of two Slater-type functions. The coefficients are given in parentheses next to the corresponding exponents. length of the unit cell in the ith direction.) Output from the calculations were interpreted by analyzing the DOS. DOS is the number of unit cell orbitals in the energy range E to E d E and roughly may be thought of as a degeneracy factor. From the total DOS and the number of electrons in the unit cell, the Fermi energy, e, was determined. Occupied bands lie below t in energy while unoccupied bands are higher in energy than t. Behavior of individual orbitals (e.g., the Ni 3d,z basis set function) was found by “projecting” out the contribution of the particular orbital of interest from the total DOS, as prescribed by
+
Dj(E) = 2[CCDOSj(E)CiJCkJSj,] J
k
(1)
D,(E) is the projected density of states of the ith basis set function; DOSj(E) is the contribution to the total DOS from thejth band; Cil and CkJare the coefficientsof the ith and kth basis set functions in the linear expansion of thejth unit cell orbital. Sikis the overlap between the ith and kth basis set functions, summed over the crystal lattice, weighted by the factor exp(ik.r). The actual occupations of the basis set functions were found by numerically integrating the projected DOS up to the Fermi level t. Another quantity of interest is the overlap population, a refined bond order that reflects the degree of ”bondedness” between atoms or individual orbitals. Differential overlap populations (the overlap population in the energy range E to E + dE) between basis set functions were determined by the formula oPik( E ) = 2 [EDOS,( E )CilckjS,] J
(2)
where OPikis the differential overlap population between the ith and kth basis set functions. The actual overlap population was found by integrating the differential overlap population up to e. Overlap populations between atoms were found by adding the appropriate OPik’s. Additional details are given e1~ewhere.I~ In order to complete the analogy between tight binding calculations and molecular orbital calculations, the approach of ammonia toward [Ni(OH),]* was investigated by using extended Huckel molecular orbital calculations.I0 ( [Ni(OH),],- was used to approximate the local environment as well as the electron count of Ni in a small piece of the nickel oxide surface. Though this approximation may not be particularly realistic, especially with a -3 charge on the complex, it is indeed useful for illustrative purposes.) Different pathways for the adsorption were probed. A limited energy surface was generated by tilting the NH, molecule to (13) Monkhorst, H. J.; Pack, J. D. Phys. Rec. B 1976, 13, 5188. (14) ( a ) Cain, S.R . Chem. Phys. Lett. 1988, 143, 361. (b) Cain, S. R.: Matienzo, L . J . ; Emmi, F. J . Phys. Chem. Solids 1989, 50, 8’7
Ammonia Adsorbing onto Nickel Oxide
The Journal of Physical Chemistry, Vol. 94, No. 12. 1990 4981
0 3.4
2.9
N i - N Sep. Figure 3. Geometry and valence orbitals of the [Ni(OH),13- complex.
The orbitals are ordered by energy. various degrees during the approach. Also, the effect of coverage was investigated by allowing a second monolayer of N H , to approach a surface already half-covered by NH,. In this phase of the study, one N H 3 molecule was fixed 2.0 A above a surface Ni atom with another NH, molecule approaching the other surface Ni atom (cf. Figure 2). At this point, it is important to note that extended Huckel calculations do not give very reliable results on an absolute scale. However, with thorough analysis of the output, the calculations are useful in determining trends. Thus, we do not use the calculations to match the numbers reported in the literature, but rather we use them to understand qualitatively surface chemistry.
Results and Discussion Approach of NH, to [Ni(OH)5]3-. In representing the nickel oxide surface by a relatively small molecular complex, we have employed what is commonly referred to as the "cluster" approach;I5 molecular orbital calculations were performed on the cluster. The assumed geometry and corresponding valence orbitals of [Ni(OH)J3- are shown in Figure 3. Because the valence orbitals in the nickel complex have a high degree of Ni 3d character, they are represented in the sketches by the 3d orbitals only. Ammonia was allowed to approach the uopenn nickel site, completing the pseudooctahedral arrangement around nickel. As shown in Figure 3, the Ni 3d,z orbital, properly oriented for interaction with the lone pair of electrons on NH,, is doubly occupied. (Other valence orbitals of the [Ni(OH)J3- complex are not well oriented for a strong interaction with NH,.) The NH3-[Ni(OH)J3- system has an electronic configuration that leads to a base-base repulsion early in the approach; both the Ni-NH, bonding and antibonding orbitals are doubly occupied. However, if the 3d,2 orbital is pushed high enough in energy, the electrons will be transferred from the Ni-NH, antibonding orbital to the Ni 3d,z_,,~orbital. The interactions described in the preceding paragraph are conveniently summarized by an orbital correlation diagram given in Figure 4. Strongly interacting Ni 3d,z and NH, lone-pair orbitals are sketched in the diagram. As the Ni-N separation decreases, the Ni ~ ~ F Nlone-pair H ~ interaction becomes stronger. As indicated by the orbital sketches on the right-hand side of the (15) Messmer, R. P. In The Naiure of the Surface Chemical Bond Rhodin, T.N., Ertl, G., Eds.; North-Holland: Amsterdam, 1979; p 5 1 and references therein.
2.4
1.9
(A)
Figure 4. Orbital correlation diagram with orbital occupation indicated by arrows (top), total energy relative to an infinite separation (middle), and Ni-N overlap population (bottom) for approach of NH, toward [Ni(OH),]'-.
correlation diagram, the NH, lone-pair orbital mixes the Ni 3d2z wave function in a bonding manner while the Ni 3d9 orbital mixes the N H 3 lone-pair wave function in an antibonding manner. Because both orbitals are doubly occupied, the net effect is a two-orbital four-electron type of repulsion. However, when the nickel-nitrogen separation is about 2.2 A, the antibonding 3 d , ~ lone-pair orbital tries to cross with the Ni 3d,2~~2orbital. (If the symmetry were rigorously C,,, the crossing would indeed take place.) Such an intended crossing has a dramatic effect on the total energ and the orbital occupations. For Ni-N separations below 2.2 , the lowest unoccupied orbital is the antibonding Ni 3 d 7 N H 3 lone-pair orbital and the highest occupied orbital is the Ni 3dzLY2orbital. With the antibonding component of the NiNH, interaction unoccupied, the approach of NH, toward the nickel complex becomes attractive. Consequences of the orbital occupations are reflected in the system energy and nickel-nitrogen overlap population curves, also shown in Figure 4. Initially, the energy increases with decreasing Ni-N separation but then reaches a maximum and finally decreases. The peak in the energy vs Ni-N separation curve coincides with the intended orbital crossing. Because the intended crossing implies electron transfer from a nickel-nitrogen antibonding-like orbital to a nickel-nitrogen nonbonding-like orbital (the Ni 3d,z_,~),the Ni-N overlap population experiences a sharp increase for a nickel-ammonia separation of 2.2 A. The small increase in the Ni-N overlap population during the initial stages of approach (Ni-N separation I2.5 A) may be traced to interaction of the NH, electron lone pair with empty Ni 4s and 4p, orbitals. Another aspect of the NH,-[Ni(OH)5]3- system that is not explicitly shown in Figure 4 involves changes in electronic occupation of NH3-like orbitals. The intended crossing implies transfer of electrons out of an orbital that has a substantial degree of N H 3 lone-pair character (cf. the correlation diagram in Figure 4). As a result, a buildup of positive charge on the ammonia fragment may be expected. Direct Approach of N H 3 to a Surface of Nickel Oxide. For this portion of the study, band calculations were performed with the use of the unit cell defined in Figure 2. There were two surface Ni atoms for every NH, molecule, and ammonia was assumed to approach the surface with its lone pair of electrons pointed directly at one of the surface Ni atoms. DOS analyses for NiO,
x
Total
d,2
dx2-y2
dxy
I
t
I
CT L -14
-16
-20
t
I ~
t
t
i
Figure 5. Total DOS for NiO and projected DOS for surface Ni 3d states. The Fermi level is indicated on the energy axis of each plot.
Surface N i 3 d Z 2 COS
-18
OP
II
E
t
I
t
L.P.
dxz dyz
oFr-i--r-i---
Cain et al.
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990
4988
t
~
Figure 7. Projected DOS plots for the surface Ni 3ds orbital (left) and the NH3 lone-pair orbital (middle) and the total Ni-N differential overlap population curve (right). Peaks pointing to the right in the differential overlap population curve indicate bonding interactions, while peaks pointing to the left indicate antibonding interactions. The Fermi level is indicated on the energy axis of each plot.
-
% I L
-18
L
/ I
L
Figure 6. Projected DOS for surface Ni 3d,2 orbitals during the course of attack by NH,. Ni-N separations are 3.5, 3.0, 2.5, and 2.0 A going from left to right. Energy of the NH3 lone-pair orbital tracks one of the peaks in the Ni 3d,2 DOS (indicated by the small arrow). The Fermi level is indicated on the energy axis of each plot.
TABLE 11: OccuDetion Numbers for Surface 3d States of Nickel in Nickel Oxide ~
orbital 3dxLy2 3d,2
occuoation no.
orbital
0.8 1 1.75 1.99
34,
occupation no. 2.00
34,
2.00
~~
34,
Figure 5, show that the bands near t are comprised mainly of the 3d states of surface nickel atoms. Bands derived from oxygen 2p orbitals are at -17 to -15 eV; the 0 2s bands lie at even lower energy (2-30 eV) and are not shown. These results match photoelectron spectra7.l6and are similar to results of other band calculation^.^^^ I t should be noted, however, that the calculations reported in refs 5 and 6 were performed on bulk NiO, while ours are heavily weighted by the surface states. Because the bands in the immediate vicinity of the Fermi level are derived mainly from Ni 3d orbitals, the following discussion is based on the behavior of the Ni 3d component of the unit cell wave functions. Projected DOS plots for the surface Ni 3d orbitals indicate that the Ni 3d,2-,,2 orbital lies primarily above e . O n the other hand, c lies near the top of the 3d,2 band and at the top of the 3d,, 3d,,, and 3d,, bands. Hence, surface Ni atoms have the 3 d , ~ ~orbital 2 partially occupied, the 3d,2 orbital almost doubly occupied, and the 3d,, 3d,,, and 3d,, orbitals completely occupied. (Partial occupation of the 3 d ~ , 2orbital is a result of mixing with the lower energy oxygen 2p orbitals.) The orbital occupation numbers are given in Table 11. Electronically, a surface Ni atom in N i O is similar to the Ni atom in [Ni(OH),13-. Now consider the approach of N H , toward a surface Ni atom. Evolution of the 3d,2 orbital is summarized by the projected DOS plots in Figure 6. The main peak in the projected DOS "spectrum" is shifted to progressively higher energy as N H 3 approaches the Ni atom and finally is pushed above t (Ni-N separation of 2.0 A). Because the N H 3 lone pair interacts strongly with the Ni 3d,2 orbital, portions of the Ni 3 d s DOS and the N H , lone-pair DOS track with each other. The small arrows in Figure (16) (a) Lee, S.-B.;Boo, J.-H.; Ahn, W.-S. Bull. Korean Chem. SOC.1987, 8, 358. (b) McKay. J . M.: Henrich. V. E. Phys. Rea. Lett. 1984. 53,2343.
1
a 0 . - Q
10
a n >
0
. . . . .
.
..
* . . . . . . I
L A 0.1
4.0
I
3.5
Ni-N
.
I
.
3.0
Sep
2.5
2.0
(A)
Figure 8. Band correlation diagram (top), system energy relative to that for a 4.0-A Ni-N separation (middle), and total Ni-N overlap population (bottom) for approach of NH3 directly above a surface Ni atom in NiO.
6 indicate a peak in the Ni 3d,2 DOS that follows the N H 3 lone-pair band. Note that as the 3d,2 band increases in energy, the N H 3 lone-pair band decreases in energy, characteristic of a two-orbital type of interaction. Though not shown, the projected DOS for the Ni 3d+,2, 3d,, 3d,,, and 3dy, orbitals were unaltered throughout the reaction. Comparison of the Ni 3d,2 projected DOS, the NH3 lone-pair projected DOS, and the Ni-N differential overlap population curves (Figure 7) shows that the Ni 3d,2 and NH, lone-pair orbitals interact to form a bonding-antibonding set of orbitals. (In this example, the Ni-N separation was 2.0 A.) Both projected DOS plots show peaks at -14.8 eV, which correspond to the "bonding" component of the interaction (Le., the N H 3 lone pair mixes the Ni 3d,2 in a stabilizing manner). Such a mixing gives rise to the prominent positive peak at -14.8 eV in the differential overlap population curve. Further, both projected DOS curves have peaks in the range of -1 1.8 to -9.8 eV. Here, the Ni 3d,2 orbital mixes the N H 3 lone pair in a destabilizing or antibonding manner, as reflected by the negative peak in the differential overlap population curve. Since this antibonding band lies above t, and hence is not occupied, the net Ni-NH, interaction is strongly attractive (strongly "bonding"). Other small peaks in the differential overlap population curve represent minor contributions from orbitals not considered in this discussion. With the aid of DOS analyses, diagrams similar to those in Figure 4 may be derived for the NH3-nickel oxide system. These are shown in Figure 8. Instead of single lines each representing
Tht? Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4989
Ammonia Adsorbing onto Nickel Oxide
rr NiO
2.5
FH. Lone P a i r
4.0
3.5
3.0
2.5
2.0
Sep ( A ) Figure 9. Occupation numbers for the Ni 3d,2, Ni 3d,2+ lone-pair orbitals as functions of the Ni-N separation.
Ni-N
and NH3
one orbital (Figure 4, top), 0.5-eV wide "bands" centered about the major peaks in the projected DOS curves are plotted. Only the strongly interacting Ni 3d,2 and NH, lone-pair bands are shown; other bands are unaltered during the adsorption. The lower band implies a nickel-NH, bonding (attractive) interaction; the upper band implies a nickel-NH, antibonding (repulsive) interaction. Though very approximate, such a treatment is illustrative. Band occupancy is indicated by the cross-hatched area. As the interaction becomes stronger, the lower band drops in energy and the upper band increases in energy. So long as the upper band remains nearly completely filled, this gives rise to a repulsion. As electrons are transferred to other partially filled bands, the upper band contributes less to the net Ni-N interaction. Finally, when the Ni-N separation is 2.0 8, or less, the upper band is completely unfilled and is inconsequential in the Ni-NH3 bonding; the interaction is strongly attractive. The Ni-NH, interactions and the electronic configuration of the system give rise to the total energy and Ni-N overlap populations plotted in the lower portions of Figure 8. An energy hump of roughly 0.1 eV occurs in the initial stages of the approach (Ni-N separation > 2.5 eV). The energy hump is lower for the band calculation than for the "cluster" calculation because too small a cluster was used to model the NiO surface. After the peak (Ni-N separation C 2.5 eV), the approach is favorable. A fairly sharp increase in the Ni-N overlap population also is consistent with the above discussion. In short, the curves in Figure 8 are qualitatively similar to those in Figure 4. Electron transfers among bands result in substantial changes in orbital occupations. Figure 9 shows the occupation numbers for Ni 3d,2 and 3 d + 2and the NH3 lone-pair orbitals as functions of the Ni-N separation. Notice the substantial drop in the Ni 3d9 and NH, lone-pair occupation during the later portion of the approach, which is consistent with electron transfer out of bands derived from the Ni ~ ~ T N Hlone-pair , antibonding interaction. However, the small increase in the Ni 3d,~,,zoccupation does not completely account for all of the electron transfer. The particular unit cell assumed in this study (Figure 2) has two surface nickel atoms for every ammonia atom. Hence, electrons are also transferred to adjacent Ni atoms. Charge transferred from the ammonia molecule is shared by the surface atoms. Finally, since the NH, molecule bears a partial positive charge as a result of the adsorption, the N-H bonds become weaker. This bond weakening does not result from a change in the bonding description (Le., N-H bonding and antibonding orbitals remain essentially unaltered during the adsorption), but results solely from the positive charge on the NH, molecule. It is analogous to the bond weakening associated from protonating NH,. (The hydrogens of NH4+ are more acidic than those of NH3.) This suggests that the more acidic the surface (the more tightly bound NH,), the more susceptible the adsorbed NH3 molecule is to dissociation into N H 2 and other fragments. Asymmetric and Off-Center Approaches. In order to decrease the repulsive interactions associated with the initial approach, ammonia may attack the nickel oxide surface in an asymmetric manner, e.g., with the electron lone pair pointed parallel to rather than directly at the surface. In so doing, the initial repulsion is either diminished or delayed until late in the reaction. If deviation from a symmetric approach allows increased interaction with
Figure 10. Projected DOS plots for surface 3d states of metal atoms in Cr203(left), NiO (middle), and Cu20(right). Solid lines show the total 3d projected DOS; shaded area shows the 3d,2 projected DOS. The Fermi levels are indicated on the energy axes.
unfilled orbitals, the energy barrier is lower along the asymmetric path." To test whether ammonia is likely to follow an asymmetric trajectory, consider the orbitals involved in the interactions. First, ammonia has no low-lying virtual orbitals and hence always acts as a base. Now, consider the empty bands on the NiO surface (cf. Figure 5). The only low-energy unoccupied bands are derived from the Ni 3d+2 orbitals, which are oriented in the plane of the surface. This is not a proper orientation for strong interaction with the ammonia lone pair. Any deviation from the symmetric approach will increase the interaction of the Ni 3d,, and 3d,, orbitals with the filled NH, orbitals. Since these orbitals are completely occupied, increasing their interaction increases the repulsion. Direct attack of a surface Ni atom by NH, should be the lowest energy path. Calculations were performed for two different asymmetric approaches: (1) ammonia approaching the surface 0.5 8, displaced from directly above a surface Ni atom and (2) ammonia approaching directly above a surface Ni atom but tilted so that the lone pair did not point directly at the Ni atom. In both cases, the energy barrier was found to be greater than that for the direct symmetric approach, confirming the previous arguments. Expected Adsorption Behavior of N H , on Cr203and Cu20. Because the Fermi level falls near the top of the Ni 3d,2 band, the nickel electrons repel the NH3 lone-pair electrons in the early stages of the adsorption. However, there are empty bands in NiO to which electrons can be transferred from the Ni-N antibonding orbitals; this occurs later in the approach. Thus, the electronic features that give rise to the behavior peculiar to NiO are (1) filled (or nearly filled) 3d,2 surface orbitals and (2) empty orbitals to which electrons can be transferred at some point in the reaction. With these two features in mind, the concepts discussed above may be extended to Cr203-NH3 and Cu20-NH, systems. TO do so, all that is needed is the projected DOS plots for the surface atoms and the Fermi energy of the different oxides. Computational results for Cr203(11 I ) , NiO( loo), and C u 2 0 (110) are summarized in Figure 10. In Cr20,, t falls in the middle of the Cr 3d band. However, the 3d,2 band (orbitals properly oriented for strong interaction with the N H 3 lone pair) is completely above t. Because the 3d,2 orbitals are empty, the antibonding component of the NH,-Cr 3d,2 interaction is unoccupied at all stages of the adsorption. Ammonia should approach the Cr20, surface directly with no activation energy. In C u 2 0 , t falls exactly at the top of the Cu 3d band. This implies that there are no empty bands to which Cu-N antibonding electrons can be transferred; the Cu 3d,rNH, interaction should be antibonding and repulsive throughout the course of the adsorption. Thus, any Cu-NH, attraction results from participation of the higher energy Cu 4s and 4pz orbitals. Since these are completely empty and there is no intended orbital crossing of the type described above, NH, should approach the Cu10 surface with (17) For a more thorough discussion of asymmetric reaction paths, see: Cain, S. R.; Hoffmann, R.; Grant, E. R. J . Phys. Chem. 1981, 85, 4046.
4990
The Journal of Physical Chemistry, Vol. 94, No. 12, I990 m
~
~
l, ,
l
.y,
. .~ .
L ,
,
,
~
~
tbl t Covered
0.0 2.5
3.0
3.5
4.0
U n i t Cell S i z e
4.5
(A)
Figure 11. Total energy of a monomolecular layer of ammonia as a function of the unit cell size. NH3-NH, separations of 4.18 A (corresponding to a "half-covered" NiO surface) and 2.95 A (corresponding to a "fully covered" NiO surface) are indicated on the curve.
no energy barrier. Bonding depends solely on the Cu 4s and 4p orbitals. At first, the results for the C u 2 0 may seem to contradict an earlier a b initio studyis in which the authors concluded that the 0-type bonding to copper is insignificant. In reality, we have said the same thing; covalent bonding with Cu 3d orbitals is essentially nonexistent owing to cancellation of the bonding and antibonding orbital pair. Any resulting bonding arises from polarization. Bonding to a surface has a profound effect on ammonia stability. For example, ammonia adsorbed onto NiO decomposes to NH2.I9 It is likely that this occurs via proton transfer to the surface oxygen atoms and is enhanced by the charge transfer from N H 3 to the metal resulting from binding. (Note: this is not a reductive elimination reaction typical of H 2 C 0 on metals but is rather analogous to weakening of N H bonds resulting from protonation.) In this case, more tightly bound N H 3 would be more susceptible to decomposition, which may be expected to occur most readily on Cr203and least readily on Cu20, with NiO being intermediate. Coverage Effects. In a report by AI-Shammeri and Saleh,20 an interesting coverage effect was reported. At low coverage, the activation energy for adsorption is less than 0.2 eV, but as the coverage increases, the activation energy quickly rises to a value of roughly 0.9 eV. This increase in activation energy is probably due to steric interactions between adsorbed N H 3 molecules. To probe the contribution of inter-ammonia interactions, band calculations were performed on a monomolecular layer of ammonia, with different NH,-NH3 separations. The total energy, as a function of the unit cell size (NH3-NH3 separation), is given in Figure 11. Note that there is very little interaction between the ammonia molecules with half of the available sites (surface Ni atoms) covered. However, with full coverage of the adsorption sites, the NH3-NH, repulsion is substantial and contributes about 1 eV to the total energy. In further calculations, performed to study explicitly the effect of coverage, a slab of NiO with NH, attached to one of the surface Ni atoms (Ni-N separation of 2 A) was approached by a second N H 3 monolayer. Results are summarized in Figure 12, which shows the plot of energy vs Ni-N separation for adsorption of the first NH, monolayer and that for adsorption of the second NH, monolayer. As can be seen by comparing Figure 12 to Figure 1 I , NH3-NH3 repulsion accounts for most of the adsorption barrier at high coverage. (18) Bagus, P. S.; Hermann, Klaus; Bauschlicher. Charles W. J . Chem. Phys. 1984, 81, 1966. (19) (a) Netzer, F. P.; Madey, T. E. Surf. Sci. 1982, 119, 422. (b) Bassignana, I . C.; Wagemann, K.; Kiippers, J.; Ertl, G. Surf, Sci. 1986, 175, 22. (20) AI-Shammeri, K . K.; Saleh, J . M. J. Phys. Chem. 1986, 90, 2906.
'
Cain et ai.
1.51 1 .. > W
-x 1 m
w C
AI
Socond Loysr
I.O[
0.51
o.o/,
-0.5
I
/
dT
,
f i r s t Layer
4.0
3.5
3.0
N i - N Sep
2.5
2.0
(A)
Figure 12. Energy versus Ni-N separation curves for adsorption on half of the available sites (first layer) and subsequent adsorption on the remaining half of the sites (second layer).
Conclusions We have performed a reasonably extensive theoretical investigation of N H 3 adsorption onto a surface of NiO. Ammonia probably approaches directly atop a nickel atom with its C3axis perpendicular to the surface. Because the Ni 3d band is almost completely occupied, the initial stages of N H 3 adsorption are repulsive. In later stages of the adsorption, electrons can be transferred out of Ni-NH, antibonding orbitals; the interaction becomes strongly attractive. Comparison of the band calculations with cluster calculations showed the two methods to give similar results. Further, the projected DOS of individual orbitals in the band calculations behaves very similarly to the corresponding orbitals in the cluster calculations. Concepts derived from the NiO-NH3 system were extended to describe the dynamics of NH, adsorption on Cr203and Cu20. Because the Cr 3d,z band is unoccupied, N H 3 should adsorb strongly via a direct (symmetric) approach with no energy barrier. On the other hand, the 3d band of Cu is completely filled, rendering the 3d orbitals a repulsive core throughout the adsorption. N H 3 should bind weakly via interaction with the empty Cu 4s and 4p orbitals; the 3d orbitals contribute nothing to the Cu-N bond. Because the Cu 4s and 4p orbitals are unoccupied, N H 3 should approach the surface in a symmetric manner with no energy barrier. These are the expected trends for low coverage. In the case of high coverage, the activation energy for adsorption is likely to be governed by the NH3-NH3 steric interactions. Such interactions are governed more by the geometry of the surface than by the electronic structure of the substrate. Thus, an activation energy to NH3 adsorption on metal oxides (and very likely clean metals) may be expected for high coverage in general. Because binding to a metal oxide surface involves sharing of the N H 3 lone pair, NH, may bear a substantial positive charge. The more acidic the surface (more tightly bound NH,), the greater the positive charge on NH, and the more susceptible NH, is to decomposition. The degree to which N H 3 decomposes on the surface is expected to follow the trend on C r 2 0 3> on NiO
> on C u 2 0
which follows our previously reported acidity trend.' This trend applies to the case of low coverage (NH, bound to less than half of the available surface metal atoms). These predicted trends need to be verified by experimental studies.
