- Email: [email protected]
N2O adsorption and decomposition at a CaO(100) surface, studied by means of theory
,.... .A.....
:-x’-:::;.:::‘::::
..:.:i;:~~~.~~:~.~::_::~~~:.~:::.~:~:
;:j:..:? :::::;.:,,:, ,,: ,.,. >::;.::
:;‘i>i:gi.c~;t;
~:y:;::;;:::;~
Surface Science North-Holland
292 (1993) 317-324
):.
surface science .:::..::.::ii-j::..si:::..::... .F ,::: y::“. .. ..:i.: ,. ..A ..~:~j.:_::::::~ ::., 7) ,\,. ,,\_..:.-.~:..:~~ ,,, ,,,, .,,., ..:.>: “.‘:‘::.:jY.:. ...:.:.+:.::.:,:.:.:.: :i::>$:;?... “‘........ ...,.,.. ....,,:.:
N,O adsorption and decomposition studied by means of theory Anders
Snis, Dan Strijmberg
at a CaO( 100) surface,
and Itai Panas
Department of Inorganic Chemistry, University of Gcteborg, S-412 96 Giiteborg, Sweden Received
13 April
1993; accepted
for publication
4 May 1993
The adsorption and decomposition of an N,O molecule at different sites on a CaO(s) surface are investigated by means of ab initio quantum chemistry. The calcium, Ca2+, and oxygen, O:-, sites at a perfect (100) surface and at a corner position, Oz-, are considered. Adsorption energies at different sites are calculated and the largest value, 6 kcal/mol, is obtained for a corner site. The barrier for dissociation is calculated to 26 and 27 kcal/mol at the Oz- and Oz- sites, respectively. These values are some 10 kcal/mol lower than the experimental estimate, and the discrepancy is understood from methodological difficulties to describe the free N,O molecule. A mechanism for the dissociation over an 0 2- site is proposed, whereby the transfer of the 0 atom goes via a linear N-N . . 0 . 02- transition state.
1. Introduction
Detailed information regarding the complex gas-surface interactions in combustion chambers is of great environmental importance, and various surface-related processes are commonly employed as tools to control both the combustion conditions and the emission of flue gases. The SO, emission, in particular from boilers df fluidized bed type, is controlled by the addition of calcite to the fluidized bed whereby CaSO&s) is formed. Calcite is added in excess in order to get a good sulphur capture, and hence a CaO(s> by-product is obtained when CaCO,(s) is calcined. CaOW, in turn, is thought to influence the chemistry in the combustion chamber, both with regard to NO,(g) and SO,(g) formation. Hence, CaO(s> surfaces are believed to catalyse both the NO + CO reaction to form N, and CO,, and the decomposition of N,O [l-3]. The aim of the present study is to produce detailed theoretical information regarding the interactions between a CaO(100) surface and an N,O molecule. Adsorption energies for N,O are calculated at the Ca2+ and 02- surface sites, O:- and at an 02-corner site, O:-. The latter 0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
was included to investigate whether support for the proposed increased catalytic activities at similar sites, e.g. corners, kinks and steps, could be produced. Of central importance for the formulation of the present effort is the experimental evidence [2] of oxygen abstraction to constitute the first step in the decomposition of N,O. Consequently, the decomposition into a free N, molecule and a surface-bound 0 atom is considered at both the Ot- and Oz- sites. The binding energies of an 0 atom to different sites of the CaO(100) surface have been calculated previously [4]. The same computational approach as was used in ref. [4] is employed in the present study. Hence, an embedded cluster technique [51 is employed and the way it is applied in the present effort is thoroughly discussed in ref. [4] and outlined below.
2. Computational
details
An embedded cluster approach has been assumed, whereby an explicitly described surface site is embedded in an array of point charges. The point charges are optimized by a modified
B.V. All rights
reserved
318
A. Snti et al. / N,O adsorption and decomposition
Ewald technique [5-81 in order to improve on the sometimes slow conditional convergence of the Madelung potential, while the explicitly described region is treated by ab initio quantum chemical methods. In a previous study [4], the size and shape of the explicitly described surface site was investigated. Based on that study, the following clusters were used in the present investigation. In case of the reactive site being a CaZf ion, the cluster is minimal, i.e. a single explicitly described ion surrounded by point charges. If, on the other hand, the reactive site is chosen to be an 02- ion, the Pauli repulsions from the nearest neighbour Ca2+ ions have been shown to be of crucial importance ref. [4]. It was also shown in ref. 141,that projection operators on these Ca2+ ions describe these Pauli repulsion interactions well and that this procedure is computationally efficient. The applied projection operator technique is the same as that used in the effective core potential (ECP) approach [91. The CaO crystal has a NaCl structure with a nearest neighbour Ca-0 distance of 2.40 A [lo]. The structural relaxation effects at the (100) surface have been experimentally shown to be minor [11,12] and therefore the surface structure was not optimized in the present study. While any surface reconstruction is readily observed by experiment, little is experimentally known about these effects at a corner site. Since bonding to a corner 02- ion is considered in the present study, reconstruction tendencies were investigated at this site. Test calculations were performed but only small geometrical relaxation effects (0.2 A) were found. Such small relaxation effects could very well be artificial, and any difference in the chemistry for these two 02- geometries was deemed to be negligible. Hence, an unreconstructed corner site was assumed in all calculations. The chosen corner site was that of a cube as depicted in fig. 1. It was built from 2 + and 2 point charges in four layers and sixteen point charges in each layer. The cube was placed on a (100) surface (fig. 1) with optimized point charges, similar to what was done for an ordinary surface site.
at CaO(lO0)
O=o’
0 =cL?+ Fig. 1. A cube of point charges with an 02- ion at the corner, Oz-. The N,O molecule dissociates in the x.z plane to Oads -Ozand N,(g).
The calculations have been performed by applying ab initio quantum chemical methods. Hence, electronic near degeneracy effects and dynamic correlation were described by applying the complete active space self-consistent field (CASSCF) method [13,14] and the multi-reference externally contracted configuration interaction method (MR-CCI) [15]. The active space in the CASSCF calculations was always chosen to include all bonding and anti-bonding orbitals included in the active space. All configurations with coefficients larger than 0.07 were taken as reference configurations in the MR-CC1 calculations. In the quantum chemical calculations, the calcium atom was described by the MIDI-4 basis set suggested by Sakai et al. [161 with an additional diffuse p function (0.07). The Dunning basis set, 9s3p contracted to 4s3p [17], augmented with one diffuse p and one diffuse d function, was employed for both nitrogen and oxygen. The expo-
A. Snk et al. / N,O
are displayed in fig. 2. The ways the N,O molecule has been allowed to bind to the CaO(100) surface are shown in figs. 2a-2e, while those for a corner site are depicted in figs. 2f-2g. The geometry of the free molecule was optimized (table 1) and kept fixed in all calculations, except for some exploratory investigations of bent molecular structures. The adsorption energies are displayed in table 2. The similar energetics obtained for the 2a-2f molecular arrangements indicate that these result mainly from van der Waals type interactions, while the electrostatic interactions cancel out. Indications of small contributions from dative
nents of the diffuse p functions were 0.05147 (N) and 0.06368 (0) and those of the diffuse d functions 0.9 (N) and 1.0 (0). Potential energy surfaces and bond distances were computed by using an ordinary grid technique.
3. Results and discussion 3.1. Adsorption of molecular N,O The different which adsorption
319
adsorption and decomposition at CaO(100)
molecular arrangements for energies have been computed 0
N
‘I
‘I
b)
a)
T
r
0 I
r
:
N-N-O
proj2+
proj2+
: I I
r
r 02+
proj2’
proj2+ ‘I
Fig. 2. The different
proj2+
proj2+
d)
proj2+
proj2+ ‘I
proj2’
‘I
2.
proj2+ proj2+
c)
.2+
f)
ProJ
sites and different locations of the N,O molecule that have been studied. The abbrevation point charge with projection operator representing a Ca*+ ion.
Proj’+
means
a
A. Sniset al. / N,O adsorption and decomposition at CaO(100)
320
Table 1 Bond distances in a N,O molecule and the dissociation energy for N,O(gl + Nz(g) + 3o(g) Method cc1
Experimental a)
N-O
D,
Figure ‘)
N
N
0
02-
(‘4)
6)
(kcal/mol)
1.137 1.126
1.189 1.188
21 40
2c 2e 2g Free N,O
7.04 7.08 7.00 7.03
6.51 6.50 6.51 6.52
8.49 8.46 8.52 8.45
9.96 9.96 9.97 10.00
N-N
a) Ref. [181.
a) See fig. 2 for the different sites and the location of the N,O molecule.
bonding can also be noted when comparing cases 2a and 2b with 2c and 2d. Hence, while some dative bonding is expected from the interaction between the incoming electron cloud of N,O and a Ca2+ site (2a and 2b), such a bond is not possible between N,O and a negative ion. In fact, correction for superposition error [19] makes the van der Waals minimum disappear for 2c and 2d. Geometry optimizations including superposition corrections at each point were not carried out but the molecule is expected to be very we&y bound at the resulting minima. Efforts were made to seek sites where the electrostatic interactions are expected to be very favourable, based on the Mulliken populations of free N,O (see table 3). Hence, an arrangement was tried with the positive central N-atom of the N,O molecule as close to the negative 02- ion as possible, and the negative ends pointing towards
Table 2 Dissociating energies, D,, and bond distances for a N,O molecule physisorbed to a (100) surface and to a Oz- site Site a) 2a 2b 2c 2d 2e d, 2f 2g
Table 3 Total Mulliken population when a NzO molecule physisorbs to different 02- sites
the positive Ca2+ ions (2e). While this arrangement binds poorly, a 6 kcal/mol binding is found for a similar arrangement at a corner site (2g). Detailed investigations were carried out to learn what causes the larger binding in case 2g as compared to 2e. Initially all atoms in the surface were treated as point charges (column I in table 4). Binding energies of 7 and 17 kcal/mol were calculated for cases 2e and 2g, respectively. Subsequently, projection operators were added on the nearest neighbour Ca2+ ions, and a Pauli repulsion estimate of 1.5 and 0 kcal/mol, respectively, was obtained (column II). These small effects are due to the long distances between the N,O molecule and the Ca2+ ions. SCF calculations were performed in order to make an estimate of the Pauli repulsion between the N,O molecule and the corresponding 02- site. This Table 4 Physisorption energy in kcal/mol for a N,O molecule adsorbed to an Oz- ion and to an Oz- ion (the atoms in the CaO crystal were treated as noted below)
D, (kcal/mol) b,
;a.“.)
Figure
I al
II b,
III c’
IV d,
V@
VI 0
1.6 1.7 2.0 (0.0) c, 1.2 (-0.8) 0.7 ( - 3.5) 2.8 (0.8) 10.2 (6.01
5.5 6.0 7.0 8.0 5.0 7.0 5.0
2e 2g
7 17
5.5 17
-2.9 4.4
- 1.1 6.8
0.7 10.2
-3.5 6.0
a) See fig. 2 for the different sites and the location of the N,O molecule. b, 2a and 2b were calculated with CASSCF and the rest with CCL ‘) Values inside brackets are corrected due to maximum error in superposition [18]. d, The distance in this case has not been optimised.
‘) All atoms in the crystal have been treated as point-charges. b, As in I, but with projection operators added to the closest Ca2+. ‘) A SCF calculation in which the orbitals on the 02- sites were kept frozen. The rest of the surface atoms were treated as in II. d, As in III, but without freezing the orbitals on the 02sites. e, A CC1 calculation. All atoms and orbitals were treated as in IV. o CC1 values when they have been corrected due to superposition error [18].
A. Snis et al. / N,O adsorption and decomposition
was done by introducing an explicitly described 02- ion at the surface and corner sites. If the ion orbitals were kept frozen, i.e. its charge distribution was not allowed to deform due to the incoming molecule, decreases in binding energies by 8.4 and 12.6 kcal/mol were obtained for cases 2e and 2g, respectively (column III). Thus, the 2e arrangement becomes unbound at this stage. Allowing the 02- ion to vary freely makes the binding energies increase somewhat (column IV). The results in columns V and VI are obtained when electron correlation and corrections for superposition errors are included. Hence, it can be concluded that the difference between arrangement 2e and 2g is mainly of electrostatic origin. The experimental value, which is difficult to determine, is 3.5-6.5 kcal/mol depending on the N,O partial pressure [21. Bent molecular structures were investigated for arrangements 2e and 2g. In both cases a linear molecule was found to be the more stable arrangement.
at CaO(100)
kcal/mol 30
20
10
0
0
40
Table 5 Net charges for an 0 atom bonded with a Mulliken population analysis Site
0 ads
(100) surface Corner
- 0.79 -0.85
to O*-
sites, calculated
“02- 1.21 - 1.15
>1
160
a
I
y?&_* x
proj2_t-
,_---
0
‘I
pro,i*+
proj2’
The bond between an 0 atom to an Oz- site is of “peroxy’‘-type, i.e. O:- (see table 5), and a binding energy of 57 kcal/mol compared to a free 3O atom is obtained at the equilibrium geometry (fig. 3). The equilibrium geometry is such that the 0 atom sits at a hollow site with the Ozion at one of the corners. This structure can be understood in terms of (i) the peroxy-bond, (ii) the electrostatic attractive Ca2+ ions and (iii) the 0 atom to Ca2+ Pauli repulsion. Hence, while the peroxy-bond determines the O-O:distance, the 115” angle results from the fine balance between contributions (ii) and (iii). This can bc understood from figs. 3 and 4.
120
80
Z
__--
3.2. Bonding of an 0 atom to an O,‘- site
321
r=
a=
l.S3A IIS-
Fig. 3. The potential energy curve as a function of the angle (Y for an 0 atom bonded to an Oz- ion.
The binding energy for the 0 atom to an Ozsite at the (100) surface was calculated to 50 kcal/mol in ref. [4]. The net 7 kcal/mol stabilization of the corner site as compared to the surface site can again be understood in terms of balancing electrostatic and Pauli repulsion effects. Hence, the potential acting on the surface ion is -0.74 a.u. while -0.61 a.u. is obtained for the corner ion. This makes the formation of the peroxy-bond more costly at a surface site, due to the electrons being stronger bound to this site than to a corner. An opposite effect is produced by considering the Pauli repulsion between the oxygen ion and the neighbouring CaZf ions. The reduction of ionic charge on a surface oxygen ion
A. Snis et al. / N,O adsorption and decomposition
322
at CaO(100)
Ca2+ ions. This large influence of the Pauli repulsion on the strength of the O--OS- peroxybond was demonstrated in ref. [4]. 3.3. The decomposition of N,O over a CaO(100) surface The dissociation of the N,O molecule has been studied both at an Oz- site and at an Oz- site. The potential energy surface for the dissociation of N,O over the Oz- site is presented in fig. 5. The N-N distance was optimized at each point along the whole reaction path but only small changes were found. Some exploratory calculations, where the molecule was allowed to bend, were also made without finding any route with a lower barrier. The barrier height was found to be 27 kcal/mol compared to free N,O. This value compares reasonably well with the experimentally obtained 34 kcal/mol. This qualitative agreement is somewhat misleading, since there is in fact a 19 kcal/mol discrepancy between the theoretical and the experimental values for the binding in N,O (see table 1). The reason for this error is not clear but similar problems have been reported for other
2 i ti 1
0
-1
X
-4
-3
-2
-1
0
1
a.u. Fig. 4. The potential energy surface in au. obtained when a probe charge (+ e) was moved in the xz plane near the corner of the cube. See fig. 1. The Oz- ion and the projection operators on the three closest Ca2+ ions have been omitted.
2
3
4
5
6
7
8
9
IO
Fig. 5. The potential energy surface in kcal/mol, obtained when a N,O molecule was dissociated on top of an 02ion in a (100) surface, O:-.
molecular oxides, e.g. ZnO [20] and HgO 1211. Hence, the dissociation of ZnO was calculated with large basis sets and accurate correlation methods, but still only about 40% of the experimental dissociation energy was obtained. No theoretical study on the dissociation energy of N,O has, to our knowledge, been published previously. How does the erroneous description of the N,O molecule affect the height of the dissociation barrier? While it is necessary to learn how to calculate the dissociation energy correctly in order to accurately determine this value, it is clear that an improved stabilization of the molecule at the 1.22 A N-O equilibrium geometry would not have the same impact at the saddle point CR,_, = 1.59 A>. Hence, a small increase in barrier height is expected. This is gratifying considering the above 7 kcal/mol underestimate. The potential energy surface for the dissociation on an Oz- site is almost identical to that obtained on the O$?- site, and a 26 kcal/mol barrier height was obtained. The angle (Y(see fig. 1) was optimized at several points along the reaction path and 115” was obtained for the transition state. This value agrees well with that obtained for an oxygen atom bound to an Oz- site. The mechanism for the N,O dissociation can be understood from the changes in the Mulliken
A. Snis et al. / N,O adsorption and decomposition Table 6 A Mulliken population analysis over the five different along the dissociation path given in fig. 5 Point
A) R, =m a.u. R, = 2.25 a.u.
Mulliken
S
4. Concluding
population N
N
0
O:-
3.85 1.01 1.06 7.04
3.29 0.84 1.13 6.51
3.89 1.05 1.73 8.45
4.00 2.00 2.00 10.00
pot. en. = 30 kcal/mol B) R, = 4.5 a.u. R, = 2.5 a.u. pot. en. = 40 kcal/mol
PZ PXYp, tot
3.83 1.06 1.01 6.98
3.42 0.84 1.10 6.59
3.95 0.82 1.84 8.49
3.96 2.0 1.99 9.94
C) R, = 4.0 a.u. R, = 3.0 a.u. pot. en. = 57 kcal/mol
s PI px, p, tot
3.82 1.14 0.99 7.00
3.68 1.02 1.02 6.79
3.97 0.62 1.92 8.52
3.96 1.73 1.99 9.68
D) R, = 3.5 au. R, = 3.5 a.u. pot. en. = 40 kcal/mol
S
3.83 1.17 1.01 7.07
3.79 1.11 0.97 6.89
3.96 0.75 2.00 8.71
3.96 1.37 1.99 9.32
E) R, = 2.9 au. R, = m a.u. pot. en. = 0 kcal/mol
S
3.80 1.18 0.98 7.00
3.80 1.18 0.98 7.00
3.97 0.81 2.01 8.80
3.93 1.27 1.99 9.20
PZ Px, p, tot
PZ p.r, p, tot
323
remarks
points
PZ PX? py tot S
at CaO(100)
populations along the reaction coordinate (see table 6 and fig. 5). Hence, the dissociative channel is characterized by the molecule approaching the surface with its oxygen end towards an O*site. As the N-O distance increases, the oxygen p, (p, and p,) orbitals gradually become filled and the r-bond broken. This r-donation from N, to oxygen is accompanied by a u back-donation. The transition state is characterized by a linear N-N . . . 0 . . . 0:; arrangement, which can be understood in terms of a competition between the N, and 0:; (T lone-pairs for the binding to the empty 2p, orbital of the 0 atom. This competition results in the formation of a three-centre bond of c+-symmetry at the saddle point region. Hence, the linear N-N +. .O . **0:; arrangement can be understood as resulting from the symmetry of this bond. The barrier height is determined from the strained N,O molecule in conjunction with the stabilizing N,O . . . 0:; bond.
The results of the present study regarding the interactions and decomposition of molecular N,O at a CaO(100) surface can be summarized as follows: l The N,O molecule adsorbs preferably side-on to an 02- at a corner site and adsorption at a surface Ca2 + site is slightly stronger than at a surface 02- site. The binding energy estimates for the three sites are 6, 2 and 0 kcal/mol, respectively. l The binding energy for an 0 atom to O*at a corner site is calculated to 57 kcal/mol, which is 7 kcal/mol larger than that computed for a surface site previously [4]. l The barrier height for dissociation of the N,O molecule on top of an 02- is calculated to 27 and 26 kcal/mol at a surface and corner site, respectively. These values are lower than the experimentally obtained 34 kcal/mol activation energy. A source for this discrepancy can partly be found in the erroneous theoretical description of the N,O bond. Calculations underestimate the N, to 0 bond by 19 kcal/mol, and hence a stabilization by N 10 kcal/mol at the saddle point is not unreasonable. A 0.37 A intramolecular N-O bond elongation is calculated at the transition state. l A dissociation mechanism is proposed for N,O. The reaction is initiated by the molecule approaching an O*- surface ion with the oxygen end toward the surface. The 0 atom is transferred to the surface by a concerted mechanism whereby the N, to 0 r-bond is broken, the N, to 0 a-bond weakened while a 0-02a-bond is formed. The transition state is characterized by a linear arrangement with a N . . . 0 . . 02three-centre bond of (T symmetry.
Acknowledgements
The computing facilities at the Supercomputer Centre, University of Umel, Sweden are gratefully acknowledged. This work has been SUPported by NUTEK, Sweden.
324
A. Snis et al. / N,O adrorption and decomposition
References [l] P.F.B. Hansen, K. Dam-Johansen, J.E. Johnsson and T. Hulgaard, Chem. Eng. Sci. 47 (19921 2419. [2] E.R.S. Winter, J. Catal. 19 (1970) 32. [3] H. Miettinen, D. Stromberg and 0. Lindqvist, The Eleventh Int. Conf. on Fluidized Bed Combustion, April 1991, Montreal, Canada. [4] D. Stromberg, Surf. Sci. 275 (1992) 473. [5] K. BQrve and L.O.M. Pettersson, J. Phys. Chem. 95 (1991) 7401. [6] P.P. Ewald, Ann. Phys. 64 (1921) 253. [7] D.E. Parry, Surf. Sci. 49 (1975) 433. [S] D.E. Parry, Surf. Sci. 54 (1976) 195. [9] L.G.M. Pettersson, U. Wahlgren and 0. Gropen, Chem. Phys. 80 (1983) 7. [lo] W. Primak, H. Kaufman and R. Ward, J. Am. Chem. Sot. 70 (1948) 2043.
at CaO(lO0)
(111 V.E. Henrich, Rep. Prog. Phys. 48 (1985) 1481. [12] M. Prutton, J.A. Walker, M.R. Welton-Cook and R.C. Felton, Surf. Sci. 89 (1979) 95. [13] B. Roos, P. Taylor and P.E.M. Siegbahn, Chem. Phys. 48 (1980) 157. [14] P.E.M. Siegbahn, J. Almldf, A. Heiberg and B. Roos, J. Chem. Phys. 74 (1981) 2384. [15] P.E.M. Siegbahn, Int. J. Quantum Chem. 23 (1983) 1869. [16] Y. Sakai, H. Tatewaki and S. Huzinaga, J. Comput. Chem. 2 (1981) 100. [17] T.H. Dunning, J. Chem. Phys. 53 (1970) 2823. [18] R.C. Weast, Ed., Handbook of Chemistry and Physics, CRC Press, 60th ed. (CRC Press, Cleveland, 1979). [19] S.F. Boys and F. Bernadi, Mol. Phys. 19 (1970) 553. [20] C.W. Bauschlicher, Jr. and S.R. Langhoff, Chem. Phys. Lett. 126 (1986) 163. (211 D. Strdmberg, 0. Gropen and U. Wahlgren, Chem. Phys. 133 (1989) 207.
:-x’-:::;.:::‘::::
..:.:i;:~~~.~~:~.~::_::~~~:.~:::.~:~:
;:j:..:? :::::;.:,,:, ,,: ,.,. >::;.::
:;‘i>i:gi.c~;t;
~:y:;::;;:::;~
Surface Science North-Holland
292 (1993) 317-324
):.
surface science .:::..::.::ii-j::..si:::..::... .F ,::: y::“. .. ..:i.: ,. ..A ..~:~j.:_::::::~ ::., 7) ,\,. ,,\_..:.-.~:..:~~ ,,, ,,,, .,,., ..:.>: “.‘:‘::.:jY.:. ...:.:.+:.::.:,:.:.:.: :i::>$:;?... “‘........ ...,.,.. ....,,:.:
N,O adsorption and decomposition studied by means of theory Anders
Snis, Dan Strijmberg
at a CaO( 100) surface,
and Itai Panas
Department of Inorganic Chemistry, University of Gcteborg, S-412 96 Giiteborg, Sweden Received
13 April
1993; accepted
for publication
4 May 1993
The adsorption and decomposition of an N,O molecule at different sites on a CaO(s) surface are investigated by means of ab initio quantum chemistry. The calcium, Ca2+, and oxygen, O:-, sites at a perfect (100) surface and at a corner position, Oz-, are considered. Adsorption energies at different sites are calculated and the largest value, 6 kcal/mol, is obtained for a corner site. The barrier for dissociation is calculated to 26 and 27 kcal/mol at the Oz- and Oz- sites, respectively. These values are some 10 kcal/mol lower than the experimental estimate, and the discrepancy is understood from methodological difficulties to describe the free N,O molecule. A mechanism for the dissociation over an 0 2- site is proposed, whereby the transfer of the 0 atom goes via a linear N-N . . 0 . 02- transition state.
1. Introduction
Detailed information regarding the complex gas-surface interactions in combustion chambers is of great environmental importance, and various surface-related processes are commonly employed as tools to control both the combustion conditions and the emission of flue gases. The SO, emission, in particular from boilers df fluidized bed type, is controlled by the addition of calcite to the fluidized bed whereby CaSO&s) is formed. Calcite is added in excess in order to get a good sulphur capture, and hence a CaO(s> by-product is obtained when CaCO,(s) is calcined. CaOW, in turn, is thought to influence the chemistry in the combustion chamber, both with regard to NO,(g) and SO,(g) formation. Hence, CaO(s> surfaces are believed to catalyse both the NO + CO reaction to form N, and CO,, and the decomposition of N,O [l-3]. The aim of the present study is to produce detailed theoretical information regarding the interactions between a CaO(100) surface and an N,O molecule. Adsorption energies for N,O are calculated at the Ca2+ and 02- surface sites, O:- and at an 02-corner site, O:-. The latter 0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
was included to investigate whether support for the proposed increased catalytic activities at similar sites, e.g. corners, kinks and steps, could be produced. Of central importance for the formulation of the present effort is the experimental evidence [2] of oxygen abstraction to constitute the first step in the decomposition of N,O. Consequently, the decomposition into a free N, molecule and a surface-bound 0 atom is considered at both the Ot- and Oz- sites. The binding energies of an 0 atom to different sites of the CaO(100) surface have been calculated previously [4]. The same computational approach as was used in ref. [4] is employed in the present study. Hence, an embedded cluster technique [51 is employed and the way it is applied in the present effort is thoroughly discussed in ref. [4] and outlined below.
2. Computational
details
An embedded cluster approach has been assumed, whereby an explicitly described surface site is embedded in an array of point charges. The point charges are optimized by a modified
B.V. All rights
reserved
318
A. Snti et al. / N,O adsorption and decomposition
Ewald technique [5-81 in order to improve on the sometimes slow conditional convergence of the Madelung potential, while the explicitly described region is treated by ab initio quantum chemical methods. In a previous study [4], the size and shape of the explicitly described surface site was investigated. Based on that study, the following clusters were used in the present investigation. In case of the reactive site being a CaZf ion, the cluster is minimal, i.e. a single explicitly described ion surrounded by point charges. If, on the other hand, the reactive site is chosen to be an 02- ion, the Pauli repulsions from the nearest neighbour Ca2+ ions have been shown to be of crucial importance ref. [4]. It was also shown in ref. 141,that projection operators on these Ca2+ ions describe these Pauli repulsion interactions well and that this procedure is computationally efficient. The applied projection operator technique is the same as that used in the effective core potential (ECP) approach [91. The CaO crystal has a NaCl structure with a nearest neighbour Ca-0 distance of 2.40 A [lo]. The structural relaxation effects at the (100) surface have been experimentally shown to be minor [11,12] and therefore the surface structure was not optimized in the present study. While any surface reconstruction is readily observed by experiment, little is experimentally known about these effects at a corner site. Since bonding to a corner 02- ion is considered in the present study, reconstruction tendencies were investigated at this site. Test calculations were performed but only small geometrical relaxation effects (0.2 A) were found. Such small relaxation effects could very well be artificial, and any difference in the chemistry for these two 02- geometries was deemed to be negligible. Hence, an unreconstructed corner site was assumed in all calculations. The chosen corner site was that of a cube as depicted in fig. 1. It was built from 2 + and 2 point charges in four layers and sixteen point charges in each layer. The cube was placed on a (100) surface (fig. 1) with optimized point charges, similar to what was done for an ordinary surface site.
at CaO(lO0)
O=o’
0 =cL?+ Fig. 1. A cube of point charges with an 02- ion at the corner, Oz-. The N,O molecule dissociates in the x.z plane to Oads -Ozand N,(g).
The calculations have been performed by applying ab initio quantum chemical methods. Hence, electronic near degeneracy effects and dynamic correlation were described by applying the complete active space self-consistent field (CASSCF) method [13,14] and the multi-reference externally contracted configuration interaction method (MR-CCI) [15]. The active space in the CASSCF calculations was always chosen to include all bonding and anti-bonding orbitals included in the active space. All configurations with coefficients larger than 0.07 were taken as reference configurations in the MR-CC1 calculations. In the quantum chemical calculations, the calcium atom was described by the MIDI-4 basis set suggested by Sakai et al. [161 with an additional diffuse p function (0.07). The Dunning basis set, 9s3p contracted to 4s3p [17], augmented with one diffuse p and one diffuse d function, was employed for both nitrogen and oxygen. The expo-
A. Snk et al. / N,O
are displayed in fig. 2. The ways the N,O molecule has been allowed to bind to the CaO(100) surface are shown in figs. 2a-2e, while those for a corner site are depicted in figs. 2f-2g. The geometry of the free molecule was optimized (table 1) and kept fixed in all calculations, except for some exploratory investigations of bent molecular structures. The adsorption energies are displayed in table 2. The similar energetics obtained for the 2a-2f molecular arrangements indicate that these result mainly from van der Waals type interactions, while the electrostatic interactions cancel out. Indications of small contributions from dative
nents of the diffuse p functions were 0.05147 (N) and 0.06368 (0) and those of the diffuse d functions 0.9 (N) and 1.0 (0). Potential energy surfaces and bond distances were computed by using an ordinary grid technique.
3. Results and discussion 3.1. Adsorption of molecular N,O The different which adsorption
319
adsorption and decomposition at CaO(100)
molecular arrangements for energies have been computed 0
N
‘I
‘I
b)
a)
T
r
0 I
r
:
N-N-O
proj2+
proj2+
: I I
r
r 02+
proj2’
proj2+ ‘I
Fig. 2. The different
proj2+
proj2+
d)
proj2+
proj2+ ‘I
proj2’
‘I
2.
proj2+ proj2+
c)
.2+
f)
ProJ
sites and different locations of the N,O molecule that have been studied. The abbrevation point charge with projection operator representing a Ca*+ ion.
Proj’+
means
a
A. Sniset al. / N,O adsorption and decomposition at CaO(100)
320
Table 1 Bond distances in a N,O molecule and the dissociation energy for N,O(gl + Nz(g) + 3o(g) Method cc1
Experimental a)
N-O
D,
Figure ‘)
N
N
0
02-
(‘4)
6)
(kcal/mol)
1.137 1.126
1.189 1.188
21 40
2c 2e 2g Free N,O
7.04 7.08 7.00 7.03
6.51 6.50 6.51 6.52
8.49 8.46 8.52 8.45
9.96 9.96 9.97 10.00
N-N
a) Ref. [181.
a) See fig. 2 for the different sites and the location of the N,O molecule.
bonding can also be noted when comparing cases 2a and 2b with 2c and 2d. Hence, while some dative bonding is expected from the interaction between the incoming electron cloud of N,O and a Ca2+ site (2a and 2b), such a bond is not possible between N,O and a negative ion. In fact, correction for superposition error [19] makes the van der Waals minimum disappear for 2c and 2d. Geometry optimizations including superposition corrections at each point were not carried out but the molecule is expected to be very we&y bound at the resulting minima. Efforts were made to seek sites where the electrostatic interactions are expected to be very favourable, based on the Mulliken populations of free N,O (see table 3). Hence, an arrangement was tried with the positive central N-atom of the N,O molecule as close to the negative 02- ion as possible, and the negative ends pointing towards
Table 2 Dissociating energies, D,, and bond distances for a N,O molecule physisorbed to a (100) surface and to a Oz- site Site a) 2a 2b 2c 2d 2e d, 2f 2g
Table 3 Total Mulliken population when a NzO molecule physisorbs to different 02- sites
the positive Ca2+ ions (2e). While this arrangement binds poorly, a 6 kcal/mol binding is found for a similar arrangement at a corner site (2g). Detailed investigations were carried out to learn what causes the larger binding in case 2g as compared to 2e. Initially all atoms in the surface were treated as point charges (column I in table 4). Binding energies of 7 and 17 kcal/mol were calculated for cases 2e and 2g, respectively. Subsequently, projection operators were added on the nearest neighbour Ca2+ ions, and a Pauli repulsion estimate of 1.5 and 0 kcal/mol, respectively, was obtained (column II). These small effects are due to the long distances between the N,O molecule and the Ca2+ ions. SCF calculations were performed in order to make an estimate of the Pauli repulsion between the N,O molecule and the corresponding 02- site. This Table 4 Physisorption energy in kcal/mol for a N,O molecule adsorbed to an Oz- ion and to an Oz- ion (the atoms in the CaO crystal were treated as noted below)
D, (kcal/mol) b,
;a.“.)
Figure
I al
II b,
III c’
IV d,
V@
VI 0
1.6 1.7 2.0 (0.0) c, 1.2 (-0.8) 0.7 ( - 3.5) 2.8 (0.8) 10.2 (6.01
5.5 6.0 7.0 8.0 5.0 7.0 5.0
2e 2g
7 17
5.5 17
-2.9 4.4
- 1.1 6.8
0.7 10.2
-3.5 6.0
a) See fig. 2 for the different sites and the location of the N,O molecule. b, 2a and 2b were calculated with CASSCF and the rest with CCL ‘) Values inside brackets are corrected due to maximum error in superposition [18]. d, The distance in this case has not been optimised.
‘) All atoms in the crystal have been treated as point-charges. b, As in I, but with projection operators added to the closest Ca2+. ‘) A SCF calculation in which the orbitals on the 02- sites were kept frozen. The rest of the surface atoms were treated as in II. d, As in III, but without freezing the orbitals on the 02sites. e, A CC1 calculation. All atoms and orbitals were treated as in IV. o CC1 values when they have been corrected due to superposition error [18].
A. Snis et al. / N,O adsorption and decomposition
was done by introducing an explicitly described 02- ion at the surface and corner sites. If the ion orbitals were kept frozen, i.e. its charge distribution was not allowed to deform due to the incoming molecule, decreases in binding energies by 8.4 and 12.6 kcal/mol were obtained for cases 2e and 2g, respectively (column III). Thus, the 2e arrangement becomes unbound at this stage. Allowing the 02- ion to vary freely makes the binding energies increase somewhat (column IV). The results in columns V and VI are obtained when electron correlation and corrections for superposition errors are included. Hence, it can be concluded that the difference between arrangement 2e and 2g is mainly of electrostatic origin. The experimental value, which is difficult to determine, is 3.5-6.5 kcal/mol depending on the N,O partial pressure [21. Bent molecular structures were investigated for arrangements 2e and 2g. In both cases a linear molecule was found to be the more stable arrangement.
at CaO(100)
kcal/mol 30
20
10
0
0
40
Table 5 Net charges for an 0 atom bonded with a Mulliken population analysis Site
0 ads
(100) surface Corner
- 0.79 -0.85
to O*-
sites, calculated
“02- 1.21 - 1.15
>1
160
a
I
y?&_* x
proj2_t-
,_---
0
‘I
pro,i*+
proj2’
The bond between an 0 atom to an Oz- site is of “peroxy’‘-type, i.e. O:- (see table 5), and a binding energy of 57 kcal/mol compared to a free 3O atom is obtained at the equilibrium geometry (fig. 3). The equilibrium geometry is such that the 0 atom sits at a hollow site with the Ozion at one of the corners. This structure can be understood in terms of (i) the peroxy-bond, (ii) the electrostatic attractive Ca2+ ions and (iii) the 0 atom to Ca2+ Pauli repulsion. Hence, while the peroxy-bond determines the O-O:distance, the 115” angle results from the fine balance between contributions (ii) and (iii). This can bc understood from figs. 3 and 4.
120
80
Z
__--
3.2. Bonding of an 0 atom to an O,‘- site
321
r=
a=
l.S3A IIS-
Fig. 3. The potential energy curve as a function of the angle (Y for an 0 atom bonded to an Oz- ion.
The binding energy for the 0 atom to an Ozsite at the (100) surface was calculated to 50 kcal/mol in ref. [4]. The net 7 kcal/mol stabilization of the corner site as compared to the surface site can again be understood in terms of balancing electrostatic and Pauli repulsion effects. Hence, the potential acting on the surface ion is -0.74 a.u. while -0.61 a.u. is obtained for the corner ion. This makes the formation of the peroxy-bond more costly at a surface site, due to the electrons being stronger bound to this site than to a corner. An opposite effect is produced by considering the Pauli repulsion between the oxygen ion and the neighbouring CaZf ions. The reduction of ionic charge on a surface oxygen ion
A. Snis et al. / N,O adsorption and decomposition
322
at CaO(100)
Ca2+ ions. This large influence of the Pauli repulsion on the strength of the O--OS- peroxybond was demonstrated in ref. [4]. 3.3. The decomposition of N,O over a CaO(100) surface The dissociation of the N,O molecule has been studied both at an Oz- site and at an Oz- site. The potential energy surface for the dissociation of N,O over the Oz- site is presented in fig. 5. The N-N distance was optimized at each point along the whole reaction path but only small changes were found. Some exploratory calculations, where the molecule was allowed to bend, were also made without finding any route with a lower barrier. The barrier height was found to be 27 kcal/mol compared to free N,O. This value compares reasonably well with the experimentally obtained 34 kcal/mol. This qualitative agreement is somewhat misleading, since there is in fact a 19 kcal/mol discrepancy between the theoretical and the experimental values for the binding in N,O (see table 1). The reason for this error is not clear but similar problems have been reported for other
2 i ti 1
0
-1
X
-4
-3
-2
-1
0
1
a.u. Fig. 4. The potential energy surface in au. obtained when a probe charge (+ e) was moved in the xz plane near the corner of the cube. See fig. 1. The Oz- ion and the projection operators on the three closest Ca2+ ions have been omitted.
2
3
4
5
6
7
8
9
IO
Fig. 5. The potential energy surface in kcal/mol, obtained when a N,O molecule was dissociated on top of an 02ion in a (100) surface, O:-.
molecular oxides, e.g. ZnO [20] and HgO 1211. Hence, the dissociation of ZnO was calculated with large basis sets and accurate correlation methods, but still only about 40% of the experimental dissociation energy was obtained. No theoretical study on the dissociation energy of N,O has, to our knowledge, been published previously. How does the erroneous description of the N,O molecule affect the height of the dissociation barrier? While it is necessary to learn how to calculate the dissociation energy correctly in order to accurately determine this value, it is clear that an improved stabilization of the molecule at the 1.22 A N-O equilibrium geometry would not have the same impact at the saddle point CR,_, = 1.59 A>. Hence, a small increase in barrier height is expected. This is gratifying considering the above 7 kcal/mol underestimate. The potential energy surface for the dissociation on an Oz- site is almost identical to that obtained on the O$?- site, and a 26 kcal/mol barrier height was obtained. The angle (Y(see fig. 1) was optimized at several points along the reaction path and 115” was obtained for the transition state. This value agrees well with that obtained for an oxygen atom bound to an Oz- site. The mechanism for the N,O dissociation can be understood from the changes in the Mulliken
A. Snis et al. / N,O adsorption and decomposition Table 6 A Mulliken population analysis over the five different along the dissociation path given in fig. 5 Point
A) R, =m a.u. R, = 2.25 a.u.
Mulliken
S
4. Concluding
population N
N
0
O:-
3.85 1.01 1.06 7.04
3.29 0.84 1.13 6.51
3.89 1.05 1.73 8.45
4.00 2.00 2.00 10.00
pot. en. = 30 kcal/mol B) R, = 4.5 a.u. R, = 2.5 a.u. pot. en. = 40 kcal/mol
PZ PXYp, tot
3.83 1.06 1.01 6.98
3.42 0.84 1.10 6.59
3.95 0.82 1.84 8.49
3.96 2.0 1.99 9.94
C) R, = 4.0 a.u. R, = 3.0 a.u. pot. en. = 57 kcal/mol
s PI px, p, tot
3.82 1.14 0.99 7.00
3.68 1.02 1.02 6.79
3.97 0.62 1.92 8.52
3.96 1.73 1.99 9.68
D) R, = 3.5 au. R, = 3.5 a.u. pot. en. = 40 kcal/mol
S
3.83 1.17 1.01 7.07
3.79 1.11 0.97 6.89
3.96 0.75 2.00 8.71
3.96 1.37 1.99 9.32
E) R, = 2.9 au. R, = m a.u. pot. en. = 0 kcal/mol
S
3.80 1.18 0.98 7.00
3.80 1.18 0.98 7.00
3.97 0.81 2.01 8.80
3.93 1.27 1.99 9.20
PZ Px, p, tot
PZ p.r, p, tot
323
remarks
points
PZ PX? py tot S
at CaO(100)
populations along the reaction coordinate (see table 6 and fig. 5). Hence, the dissociative channel is characterized by the molecule approaching the surface with its oxygen end towards an O*site. As the N-O distance increases, the oxygen p, (p, and p,) orbitals gradually become filled and the r-bond broken. This r-donation from N, to oxygen is accompanied by a u back-donation. The transition state is characterized by a linear N-N . . . 0 . . . 0:; arrangement, which can be understood in terms of a competition between the N, and 0:; (T lone-pairs for the binding to the empty 2p, orbital of the 0 atom. This competition results in the formation of a three-centre bond of c+-symmetry at the saddle point region. Hence, the linear N-N +. .O . **0:; arrangement can be understood as resulting from the symmetry of this bond. The barrier height is determined from the strained N,O molecule in conjunction with the stabilizing N,O . . . 0:; bond.
The results of the present study regarding the interactions and decomposition of molecular N,O at a CaO(100) surface can be summarized as follows: l The N,O molecule adsorbs preferably side-on to an 02- at a corner site and adsorption at a surface Ca2 + site is slightly stronger than at a surface 02- site. The binding energy estimates for the three sites are 6, 2 and 0 kcal/mol, respectively. l The binding energy for an 0 atom to O*at a corner site is calculated to 57 kcal/mol, which is 7 kcal/mol larger than that computed for a surface site previously [4]. l The barrier height for dissociation of the N,O molecule on top of an 02- is calculated to 27 and 26 kcal/mol at a surface and corner site, respectively. These values are lower than the experimentally obtained 34 kcal/mol activation energy. A source for this discrepancy can partly be found in the erroneous theoretical description of the N,O bond. Calculations underestimate the N, to 0 bond by 19 kcal/mol, and hence a stabilization by N 10 kcal/mol at the saddle point is not unreasonable. A 0.37 A intramolecular N-O bond elongation is calculated at the transition state. l A dissociation mechanism is proposed for N,O. The reaction is initiated by the molecule approaching an O*- surface ion with the oxygen end toward the surface. The 0 atom is transferred to the surface by a concerted mechanism whereby the N, to 0 r-bond is broken, the N, to 0 a-bond weakened while a 0-02a-bond is formed. The transition state is characterized by a linear arrangement with a N . . . 0 . . 02three-centre bond of (T symmetry.
Acknowledgements
The computing facilities at the Supercomputer Centre, University of Umel, Sweden are gratefully acknowledged. This work has been SUPported by NUTEK, Sweden.
324
A. Snis et al. / N,O adrorption and decomposition
References [l] P.F.B. Hansen, K. Dam-Johansen, J.E. Johnsson and T. Hulgaard, Chem. Eng. Sci. 47 (19921 2419. [2] E.R.S. Winter, J. Catal. 19 (1970) 32. [3] H. Miettinen, D. Stromberg and 0. Lindqvist, The Eleventh Int. Conf. on Fluidized Bed Combustion, April 1991, Montreal, Canada. [4] D. Stromberg, Surf. Sci. 275 (1992) 473. [5] K. BQrve and L.O.M. Pettersson, J. Phys. Chem. 95 (1991) 7401. [6] P.P. Ewald, Ann. Phys. 64 (1921) 253. [7] D.E. Parry, Surf. Sci. 49 (1975) 433. [S] D.E. Parry, Surf. Sci. 54 (1976) 195. [9] L.G.M. Pettersson, U. Wahlgren and 0. Gropen, Chem. Phys. 80 (1983) 7. [lo] W. Primak, H. Kaufman and R. Ward, J. Am. Chem. Sot. 70 (1948) 2043.
at CaO(lO0)
(111 V.E. Henrich, Rep. Prog. Phys. 48 (1985) 1481. [12] M. Prutton, J.A. Walker, M.R. Welton-Cook and R.C. Felton, Surf. Sci. 89 (1979) 95. [13] B. Roos, P. Taylor and P.E.M. Siegbahn, Chem. Phys. 48 (1980) 157. [14] P.E.M. Siegbahn, J. Almldf, A. Heiberg and B. Roos, J. Chem. Phys. 74 (1981) 2384. [15] P.E.M. Siegbahn, Int. J. Quantum Chem. 23 (1983) 1869. [16] Y. Sakai, H. Tatewaki and S. Huzinaga, J. Comput. Chem. 2 (1981) 100. [17] T.H. Dunning, J. Chem. Phys. 53 (1970) 2823. [18] R.C. Weast, Ed., Handbook of Chemistry and Physics, CRC Press, 60th ed. (CRC Press, Cleveland, 1979). [19] S.F. Boys and F. Bernadi, Mol. Phys. 19 (1970) 553. [20] C.W. Bauschlicher, Jr. and S.R. Langhoff, Chem. Phys. Lett. 126 (1986) 163. (211 D. Strdmberg, 0. Gropen and U. Wahlgren, Chem. Phys. 133 (1989) 207.