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A Molecular orbital investigation of the mechanism of the NO-NH3 reaction on vanadium oxide catalyst
315
A MOLE_ THE NO-N-E&
OREWIXL INVESTIGATION REAmON ON PANAJ3IU-M
AKIRA MTYAMOTO, MAEKOTO and YtIKXI MURAKAMI
INOMATA-.
~epzninenf of Sytititetic Chemie~, Facdty Furucho, Chikusu-ku, Nagoya 464 (Japar~J (Receivd
September
OF THE MECHANISM OXIDE CATALYST
ATSUSHI of
HATTORI,
Engineering,
ToStK4.KI
Nugoyci
OF
U-I
University,
25,1981)
‘The electronic nature of the ca+klysis in the NO-NH3 reaction on vanadiu_moxide cakaIyst has been investigated using the CND0/2 method. The caktitions
have shown that NH., is stab& chemiscrbed
on the l3rijnsted
acid site on the catalyst; whereas NO is hardIy chemisorbed, which is ti accordance with experiment. The charge distributions and bond energies of the system composed of NO and NH3 adsorbed on the catalyst have been c&uIated for various states on the reaction coordinate. The calculated resuks have indicated that elections on the adsorbed NH3 transfer to the antibonding orbitals of NO at the transition state tu dissociate the NO bond. This dissociation has been shown to Leadto the formation of N,, HzO, and V--OH species. Furthermore, the caku&ions have supported the validity of the EZey-Rideal mechanism experimentally proposed.
The reductions of nitric oxide with carbon monoxide, hydrogen, and ammonia are important reactions incorporated in the catalytic process for the control of nitric oxide from industrial effIuents_ Attention is especially focused OR the reaction of NO with NHs (the NO-NH3 reaction), since this reaction is acceIerated by the addition of oxygen, whereas the reductions of NO with CU and H2 (the NO-CO and NO-HZ reactions, respectively) are suppressed by the presence of oxygen [I - 61_ AIthough a mechanism of the NO-NJ& reaction has been proposed for various cataIysts, including supported precious metals [7 - II], mew oxides [I2 - 161 and ze0Lite.s[I7 201, electronic details of the mechanism have not yet been clarified.
*l?rese~t
Aidi
addrss:
Kinu-ua
Research
Department,
JGC Co.,
Sucosakidq
Hada.,
475, Japan.
o3#4-5~ozfe2/oooo-ooaJ/Soz.Ts
@ Elsevier Sequoia/Rinted
iu The NetherIan&
316
Nitric oxide is a peculiar molecule which has an unpaired electron in its antibonding X* orbital. This leads to the following property of NO: the bored strength of NO+ is smnger than that of NO, .EbiIe that of NO- is weaker than that of NO [2,21 - 23]_ Consequently, electrons should be given to the antibonding orbit& of NO in order to reduce NO, or, in other words, to dissociate the X0 bond. This property might easily explain the behavior of NO in the NO-CO and NO-H2 reactions on catalysts as follows: NO is considered to be converted to N,O or N, on a reduced metal ion on the catalyst-s in these reactiolns, and electrons on the reduced site are expected to Oansfer to NO to form NO-, followed by the production of NaO or N2 [24, 25]_ The presence cf oxygen would convert the reduced site to an oxidized site so as to prevent the reduction of NO. The mechanism of the NO-N& reaction, however, couid not be understood in terms of the above-mentioned discussion, since the NO-N& reaction, contrary to the NO-CO dr NO-HP reaction, proceeds more readily on the oxidized surface than on the reduced surface [S, 13,15,26]. A mechanism through the reaction between NHz(ad) and NOs(ad) may be one of the possible mechanisms of the NO-N& reaction in the presence of oxygen [ 14,271. The NO-N)& reaction on V205 in the presence of oxygen under dilute gas conditions, however, has been shown to proceed not by this mechanism [ 281, but 5y the mechanism shown in Fig. 1. In Fig. 1, V=O is the vanadiumqxygen double bond, and V,aH is the adsorption site of NHs, that is, the Brijnsted acid site. In Reaction (l), NHs is readily adsorbed on VS-OH on the catalyst (A) to form (B). Then in Reaction (2). NO attacks the adsorbed NHs, (B), to form N2, Ha0 and the reduced catalyst (D) through a transition state, (C). The reduced catalyst, (D), is oxidized by Oa to reproduce (A) by Reaction (3). When the concentration of O2 is high
-o-
8 x -o-&,-o(A)
/H NH3 FL51
-fg
-o-&o-3,.
o-
(8)
KExTl~U)
f-9
T
z~ci1cn~2~
NO
-o_&-o-:,(D) No +
Hz0
Fig. 1. Mechanism of the NO-NH3 of oxygen.
reaction on vanadium oxide catalyst in the presence
enough, it has been found that the V-H species is readily reoxidized to V=O by Reaction (3), whfie the Reaction (2) becomes the ratedetermining step. Furthermore, in Reaction (2) NO has been suggested to attack the adsorbed NHB, (B), from the gas phase by the Eley-Rideal mechanism. Neither gaseous NO= nor adsorbed NO, species pIays a roIe in the reaction under dilute gas conditions. Thus, it seems interesting to investigate the electronic natWe of the NO-NH8 reaction on V&, catalyst, especially the manner of dissociation of the NO bond in the reaction. In this study, a semi-empiricaI SCF MO method (the CXDO/Z method) has been applied to the analysis of the mechanism of the NO-NH3 reaction on V,O, in order to elucidate the etectronic nature of the NO-NH3 reaction and the rde of V=O species In the acceleration of the reaction, and to examine the validity of the EIey-Rideal mechanism experimentally proposed.
TABLE The
1
orbital
exponents,
ployed in the CNDO/S Atom
Type
<, bonding
parameters,
OP orbit4
K
1s
1.2
N
2s 2P
0 V
@A,
and aI+
A)
terms
for atoms
em-
calculations St:
$(l+A)
(eV)
(eV)
9.0
7.176
1.95 1.55
25.0 25.0
19.316 7.275
2s 2P
2.275 2.275
31.0 31.0
25.390 9.111
3d
1.792
16.5
4s
1.57 1.57
4P
3.5 3.5
4.455
3.822 0.777
Method of computation The calculations were carried out using the CNDO/B program of Kobayashi et LZ~.[293 with the parameters shown in Table 1. Parameters for H, N, and 0 a’dms were the same as those reported by Pople and Beveridge [30], and those for the V atom were given by Clack et (II. [31] _ Orbital exponents; (, for the V amm are smaller than those proposed by Clementi and Raimondi [32] but Iarger than those of SIater [SSI _ This was done since Blyholder [34]- has o&tied the orbital exponents for the cIuster of Ni atoms between those of SIater and those of Clementi and Raimondi. The difference in the orbi.taIexponents, however, was confirmed not to affect significantly the caIcuIated resuIts of the present system. Numerical calculations -were made-with a FACOM M-200 computer, Computer Center, Nagoya University.
318 -4237
Fig. 2. Tne model of V20~ catalyst employed in the cak~latiozts, hereafter denoted as ‘catalyst’. %%e sum of an individual atom is the numericzd order of the atom. The numbe: near a bond represenk the equilibrium length of the bond. Fig. 3. Electronic properties of the ‘catalyst’ c&Mated by the CNDO/2 chrge distributions on atoms; (b!: energies of the individual bonds.
method; (a):
Rsults and discussion Model
of V,O, cddyst Since all atoms in the V,Os
catiyst cannot be taken into account in the calculations, a simplif%d model of the catalyst was assumed as shown in Fig. 2. This type of catalyst model has been employed in quantum chemical investigations of the adsorptions on metal oxides 135 - 451. This ‘catiyst seems to be adequate for the model of the V,O, ca’alyst based on the following reasons: 1. The valence of V ion is 5. 2. A V=O species, that is, the active site for the NO-NH, reaction on V,O,, is taken into consideration in the ‘catalyst’. 3. A Brijnsted acid site, that is, the V,--OH specjes in Reaction (l), is involved in the ‘catalyst’. Furthermore, the calculated properties of the ‘catalyst’ support the adequacy of the model as follows: the geometry of the ‘catalyst’ in Fig. 2 was determined so as to give the minimum potential energy in the CNDO/B calculations of the ‘catalyst’. Although the bond lengths thus determined are different from the experimental ones, the differences are not significant. Figure 3 shows charge densities and bond energies in the ‘catalyst’. -The bond energies referred to hereafter are the diatimic contributions of the toal energy formulated by Pople and Beveridge [ 301. As shown in Fig. 3, the bond energy between VI and O2 is greater than that bekeen VI and 03, a& furthermore the equilibrium disknce between VI and O2 is shorter than that between V, and 0s. This means that the VI-O, bond is stronger than the VI-O, bond, and that the former has a double bond charackr. The charge density on Hs, H7, or Hs atom shows a fairly 1-e positive value, suggesting that H6, H,, and H, in the ‘catalyst’ play the BrGnskd acid site. As can be seen in Fig. 2, there is no Lewis acid site in the ‘catalyst’. This is in accordance with experiment, since NKs is adsorbed on the VsOs catalyst as NHf (ad) while other chemisorbed species have not been detected ir, the infrared spectra of adsorbed NH3 on V,O, [15]_
TABLE
2
TABLE
Total enew (I&& of NH3 adsorbed on the ‘catzlyst’ at arious J&-P*19 distances (rllp
3
Total enorgy (&g3 of NH3 adsorbed on the ‘cztaIy.st’ at mrious 03-&-Ns angles (8 )’
J%uta.l (eW
8
Etaml
(“1
(9
-2665.479 -2670.285 -2673.163 -2674.805 -2675.669 -2676.057 -2676.168 -2675.174
180 170 160 150 140 130
-2675.669 -2675.698 -2675.704 -2675.535 -2674.135 -2667.118
%I = L-2 A.
Since NH3 is adsorbed on V,O, as NH,‘(ad) [l&153, NH, is considered to interact with the BrSnskd acid site on the ‘catalyst’. Consequently, cakulations were made for the geometry shown in Fig. 4, where r, is the distance between M, and Ns, and 8 is defined as the Oa-Hs-hT9 angle. Similarly in the case of the ccataIyst’, the bond lengths in NH, have been determined so astogivethe minimum potential energy for NH,. Tables 2 and 3 show the cakrrlated resuks of the interaction between NH, and the ‘catalyst at various rf and 0, respectively. As shown in TabIe 2, the total energy (l&& of the adsorbed system is Iower than that of the separated one (rl = =) unless r, is shorter than 1.1 A. Tbis indicates that NH3 is stably adsorbed on the ‘catalyst’ in accordame with experiment. Table 3 shows that Etim exhibits its mjnimum at 6 = 160” but not at 8 = 180”, suggesting an attractive interaction-between V=O and NH,. This also agrees with experimental restits, since the adsorbed NHa on V& is considered to in&act with V=(3 species to prevent the rotation of NH8 on the surface of the catalyst [ 151_ The calculated vaIues of E,, in Table 2 indicate an extraordinarily Iarge
320
Fig. 5. Adsorption of NO on the ‘catalyst’; r2: O2-N13 angles are the same as those of the ‘catalyst’ and NO.
TABLE Total
0.9 1.0 1.1 1.2 1.3 1.4
1.5 1.6 1.7 m
length;other bond lengthsaQd
4 ene%y
(E,&
of NO adsorbed
on
the ‘catalyst’
at various
O2-N,3
distances
(9)
-3087.222 -3101.062 -3107.920 -3110.427 -3110.323 -3108.730 -3110.576 -3115.928 -3117.380 -3110.717
(rl > 1.4 A). Thismay be ascribed to the imperfections of the method in the calculation of iong-range interactions _ Calculations for the adsorption of NO on the ‘caLMy& were made for the geometry shown in Fig. 5, where r2 is the distance between O2 and &a. As shown in Table 4, Etiti of the adsorbed system is larger than that of the separated system (r - -) at any r, smaller than 1.5 A. Namely, in contrast to the adsorpt-ion of NH 3, the calculations show that NO is nof chemisorbed on the ‘catalyst’. This is in accordance with experiment, since NO is not adsorbed on the V20s catalyst under the experimental conditions of the NO-N& reaction on VsO,. Similarly ‘k the case of the-adsorption of NK3. E bLal shows a large interaction between NO and the ‘catalyst’ at r2 2 1.6 A. This may also be due to the imperfections of the method for the calculation of long-range interactions, since such a large interaction cannot be expected atsuchalarger2as1.7A. interaction outsidethechemisorptionrangeofr;
321
Although the CNDO/Z CdcrrEaticnstogether with the parameters in Table 1 and the catalyst model in Fig. 2 carrnot describe the Eong=rangeinteractions bekeen NH3 (or NO) and the V,O, catalyst, as mentioned above, they can express well the behv-ior of the chemisorptions of NH, and NO on V,Os. It is therefore considered that the model of V&s catalyst in Fig. 2 is, to a first approximation, an adequate one for an analysis of the mechanism of the NO-NH, reaction on V&l, cat&y& Reaction
of
NO
with NH3 aahrbed
OR
the %atalyst'
In order to determine the course of the reaction between N3 and. adsorbed NJ&, that is, the reaction coordinates, the CNDOIB calculations were
made for the system composed of ‘catalyst’, NJ&, and NO with various coordinates of atoms in the system. On the basis of a number of ‘trid and error’ caIcuIations, the 11 states shown in Fig. 6 were selected as states in the course of the reaction from the initial state (State I) to the final state (State XI). In the initial state (S’t.te I), the distance betieen NH3 adsorbed on the ‘catalyst’ and NO is infinite, and no interaction exists between both species. When NO approaches the adsorbed NH, (State II), the interaction bekeen both species causes a decrease in 19from 160” to x40”, that is, the access of NH3 to the V=O species in order to permit a -her progress of the reaction. Irr States III - VI, a further approach of NO to the adsorbed NH3 occurs whiie keeping the geometry of the adsorbed NH3 on the ‘catalyst constant. The elongation of the N13-Oi4 bond takes place in State WI accompanied with partial formation of HZi-Ok~~LP, N9-N13, and &-Hz0 bonds. In States VIII, IX, and X, in turn, the distance between N,, and 014 increases while the N,-I&, Hll-OZi, H12-O14, and 0,-H,, distances decrease. In the final state (State XI), N2, Hz0 and the reduced ‘catalyst’, that is, V(OH),, are separated infinitely from one another. The total energy and interatomic dtitances for the individual states in Fig. 6 were calculated in order to examine the qualification of the states in Fig. 6 for the points on the coordinate of the reaction of NO with NH3 adsorbed on the ‘catalyst’. This was done since the total energy and interatomic distances for the state on the reaction coordhte should exhibit continuous and smooth changes from the initial state to the final state. As shown in Fig. 7, the totztl energy (EtOm) of the system varies continuously from
State
I to St2t-e XI. l&,,
increases
monot~nicdly
from
Stat-e I to State
VI and attains its maz&num at State VI, which is considered to be the fmmsition state of the reaction_ From State VI to State X, Etotaldecreases monotonically to reach a local minimum at State X. Aithough Etiti of the f-Jlal state (State XI) is a Little higher than t-hat of State X, the difference is not significant. Table 5 shows interatomic distances of some bonds in States I XI. As can he seen in Table 5, any interatomic distances exhibit natur&-and continuous changes from State I to State XI, and do not show too short distances compared with those of covalent bonds. Consequently, it is considered thrit States I - XI are I&ated on the reaction coordinate and represent the course of the reaction of NO with- NH3 adsorbed on the.‘cataiyst’, from the initial state to the final state through the transition state (State VI).
322
In order to investigatz the electronic nature of the catalysis in the NO-N& reaction OR the ‘catalyst’, charge densities on atoms and bond energies of inditiual bonds were calctited for the states in Fig; 6; and the cakul&ed rtits axe indica+~ in Tables 6 and 7. respectively. Table 6 shesignificant changes ir. the cbwge densities before and after the fm~~ition
323
Fig. 6. Reaction of NO with adsorbed NH3 on the ‘catalyet’; (a}: State E (initial state); (6): At States LJI,III. EV, V, and VI, the respective vaIus_ of 2~0 me 3.50,32X, 3.10, 3.07, and 3.05 A ; Stati VI is the fmnsition state;(c): Skate VII; (d): Sfate Vm; (e): State UC; Cf): State X;(g): Stake _XE (ErtaI state). The geometry of the individual cfskes is alao shown iu TabIe 5.
TAlKI5 Interatomic diatonccsof vnriouopuirsof alams ut the Antes in Pig. 5 (A) -. -StatA?
No-N,a
I II III IV
i.127
1.692 1.7G3 1.726 I.707 1.292 1.260 1.200 1.146 1.1.46
v
VI VII
VIII IX X XI --
-
Q-%
. -
.I
.
1.160 1,150 l.lGO 1.160 1.150 1.160 2.066 2.391 3.107 4.224 OD
.
.
ND-~
h-014
HIO-NM
B,J-ND
oz-ho
v1-02
No-ho
;,710 1.466 1,319 1.290 1.271 1,658 1.88G 2.360 3.116 cm
1.200 1.200 1.200 1.200 1.200 1,200 2.134 2.364 2.067 3,678 m
1.794 1.156 l.lGC 1.166 l.lG6 1.166 1.074 1.060 l.OG? 1.040 I .040
1.460 1.460 1.400 1.460 1.480 1.480 1.643 I.646 l.GG7 1.670 1.1370
1.060 1.060 1.060 1.060 1.060 1,060 1.261 1,209 1,546 2.237 DD
m
2m.067 1.864
1.824 1.6?9 1,660 1.631 1.600 1.700 2.204 3.329 m
1.647 1,522 1.497 1,4fJl 1.135 1.117 1.084 1.040 1.040
._-.-
Chnrgodcnoitieson variousatoms at thc,atnteain Fig, 6 .’
stat4 I II HI IV
v
VI VII VIII IX X XI
Vl 46,073 *0,118 *OJl4 +0.119 *0,117 *0,08&I, -Q.OSG ,-a.014 -0,007 -0.016 -0,016
Hll
-0,287 ‘. -0.336 -0.319 . -0.341 -0:32i ’ ‘. 4.339 -0~329 ‘: ,’ -0.386 ‘-0.330 -0.335 -0.312 .. -0.200 4.297 I’ -0.173 -0.211 -70.174 -0.194 ’ -0.176 -0.103 -0.172 -0Ml -0.172
*0.339 +0,304 +o.a[ia +0.339 +0.336 40.260 *o;ot30 *0.121 *0.142 *0.137 *0.140
-0.249 -0.369 -0.34G -0.304 -0.290 -0.196 -0.140 -0.061 -0.012 -0.004 0.0
*0.189 +0.323 +0.34G +0.364 *0.369 *0.429 *0.264 +G.194 +0.14a +0.130 *0.127
*0.124 +0.104 *0.117 +0.122 +0.121 *0.194 eO.264 +0.176 +0,128 *0.136 +0.136
*0,041 *0.043 *0.032 *0.017 *0.013 -0.299 -0.170 -0.116 -0.009 ~+o.oo? 0:o
-0.041 -0.0’13 -0.114 -0.162 -0.lG9 -0,137 -0.XB2 -0.198 -0.249 -0.269 -0.271
.
TABLE 7 Band caergieaof variouspairs of atome at the states in Pie. Ei(cV)
state
v1-02
%-ho
Oz--N~3
b-N13
No-h
HIO-NI3
I’
-40.723 -4G.231 -46.146 -40.085 -4B.074 -46.721 -39.629 -39.7 69 ~30.463 -37.008 -37.003
-9.917 -6.220 -6.229 -6.201 -6.188 -4.257 -13.6G7 -13.234 -14.963 -16.849 -16.964
0 -0,066 eO.018 +0.128 +0.1ti9 *On776 *0,660 eo.313 -0.006 -0.007 0
0 -0.964 -2.692 -4.792 -5.416 -6.281 -42.652 -50.926 -B8.969 -61.322 -61.377
0 *0.788 *1.317 *l.G20 *1.460 -G.280 -11,990 -G.O78 -0.203 +O.OOG 0
0 -1.062 -2.347 -3.604 -3.904 -6.897 -l..G97 -0.649 +0.004 *0.004 0
II 111 IV V VI VII, VIII IX .X, XI
H1rQ14
0 -0.140 -0.114 *OS01 +0.x92 -1.384 -17.413 -19.086 -20.008 -20.616 -20.636 -,
h--%4
-49.862 -48.939 -47.400 -46.387 -44.820 -34.108 +1.042 +0.415 +0.009, -0.006 0
K WI
I,,,,,,,,,,, I
I,
m
Iv
v
IAl
Ml
wr
lx
x
a
STATE
Fig. 7 _Changes in the total energy (E,& GF the system composed of NH3 adsorbed OE the ‘catalyst’ and NO; State I: initial state; State VI: transition state; State XI: final state.
state. The most striking change is observed in the charge density on Nrs, that is, the N atom of NO. Namely, the charge on N,, is positive in States I - V, while at the transition state (State VI), it changes to a large negative value. After the transition state, that is, States VII - IX, it remains negative, although its absolute value decreases in the course from State VII to State IX. Similarly, the value of the negative charge on Or4 of NO graduaI.lyincreases from the initial state to the final state. In cont_mst tc the change in the charge of NIB of NO, the fairly large negative charge of the Ne of NH3 before the transition state (States I - V) is abruptly reduced at the transition state (State VI) and decreases further after the transition state. The positive charge on IIll remarkably increases at States VI and VII, while it decreases again after these states. The charge density on the vanadium is positive before the transition state, while it shows a negative value after the transition state, indicating the reduction of the vanadium ion after the reaction. Bond energies in Table 7 aIso indicate significant changes in the electronic states of the system before and after the transition state. The absolute value of the energy of the NL3-0 rc bond is very large for States I - V, indicating that this bond is very strong before the transition,st.ate. At the transition state, the value signi&antIy decreases. At States VII - IX, it becomes positive, meaning that this bond becomes repulsive. Corresponding to the weakening of the NO bond, the negative value of the energy of the Oe--Hz0 bond increases abruptly after the transition state, indicating the gradual formation of the Hz0 molecuIe. Similarly, the strength of the Ns-N,s bond markedly increases after the transition stafk.The Vr-0, bond after the transition state becomes considerably weaker than that before the transition state, indicating a change of the V=O double bond to a single bond after the reaction. It
327
Fig. 8_ Zleckonic properties of the katalyst’ containing two vanadium atoms; (a): charge distributions on atoms; (b): energies of the individual bonds. The length of the V,-O, (or V2-0~) bond was determined to give the minimum potential energy es I.560 A The length of another bond was the same as that for the katalyst containing a single ~BLLEdium atom.
shodd be emphasized in Table 7 that the energy of the 02-N13 bond is positive at almost all states in Fig. 6. indicata the repulsive interaction between the 0 atom of V=O and the N atom of NO in States III - WEE. This means that direct interaction between NO and V=O is not important in the catatlysis of the reaction of NO with NE& adsorbed on the ‘catalyst’. Effects of the number of mrzadkmt a~mns in fhe %ataLyst' For the above-mentioned calcrrlations, the number of vanadium atoms in the ‘catalyst’ was one. This was done because much computation time was necessary for the ‘trial and error’ cakr.Iations to determine the state along the reaction coordinate_ In order to show the adequaq of the model of the ‘cat.aIyst’ in Fig. 2, calculations should also be made for the ‘catalyst containing two or more vanadium atoms. Figure 8 shows the caIculated results of the charge densities and bond energies for the ‘catalyst’ containing two vanadium atoms. The charge densities aad bond energies for the ‘catalyst’ containing two vanadium atoms are almost the same as those for the ‘catalyst’ containing a single vanadium atom. (Fig. 3). Similar results were also obtaked for the katalyst’ containing three vanadium atoms.
328
(a)
(b)
Fig. 9. Geometry of Aoms employed for the cdcuhtions. (a): NH3 edsorbed on the ‘catalyst’ containing tv~o vanadium atoms; (b): reeaction of NO with the adsorbed NH3 on the ‘catalyst containing two vanadium 2toms. For 2 detailed geomeky, see Table 8.
Calculations were aLso made for NH3 adsorbed on the ‘catalyst’ containing two vanadium atoms at various vaIues of r, and 8 (Fig. 9a). The tot-& energy of ‘;he adsorbed system was lower than that of the separated system (rl = -) unIess r, was shorkr than 1.2 A _ This indicates that NHs is stably adsorbed on the ‘cataIyst’ conteining two vanadium atoms within the chemisorption range of 1-r. The total energy of the adsorbed system exhibited a minimum at 8 = 130”. suggesting an attractive interaction between V=O and NHs. It was also found that NO is not stably adsorbed within the chemisorption range. These r=uIts are similar to those for the ‘catalyst’ containing a single vanadium atom. Some calculations were made for, the system composed of NO and NH3 adsorbed on the ‘c&taIyst containing two vanadium atoms (Fig. 9b). Cakulated resuks are shown in Tab!es 8.9 and 10. As shown in Tabks 5 and 8, the geometry of NO relative to the adsorbed PIHa in States I - VI for the ‘catalyst’ containing two vanadium atoms is almost the same as that in States I - VI for the ‘catalyst con+taining a single vanadium atom. This was done in order to decrease the number of ‘hial and error’ calculations far determining states along the reaction coordinate. As NO approaches the adsorbed NHs from State I to State VI, the electron density on the adsorbed NHs fNz4, II r5, and E&e atoms) decreases whiIe that on NO (Nrs and Or9 atoms) increases (Table 9). CorrespondingIy, the strength of the NO bond (N18-Olg bond) decreases as the reaction proceeds from State I to State VI (TabIe 10).
TABLE 8 Interatomic dM.anccs of various pairn of atoma at the states in Ipig,9b (Af’ 3tato
NlrNlt~
b+‘lo
Nw%-I
ki-%
h6-NlA
HI ah4
oa-EI16
v1-03
&I-h6
;.127 1.392 1.763 1.726 1639
1.160 1.160 1.1.50 1.160 1.160 l.lGO
;,007 1.824 1.679 l.GGO 1.612
Y.664 1,647 1.622 1,497 1.466
m 1.710 1.466 1,319 1,290 1.262
1.300 1,300 1.300 1.300 1.300 1.300
1.693 1,693 1.693 1.693 1.693 1.693
1.400 1.430 1.430 1.480 1.430 1.480
1.660 1.060 1.060 1.060 1,060 1.060
I
II III IV
“Il~e geometry of NO rclativa to the adsorbed NIIg in States I - V is the anma as thnt iu St.&e I - V for the ‘catalyst’ cont-ninlng n single vnnndium atom. In State VI for the ‘catrdyst’ containing two vnnndium atoms, the dintnnce between NO and NHa ieshorter by 0.02 A than thnt in State VI for tho ‘catnlyst’ containing n single vanadium ntom.
T@LE 9 Charge dorAtics on vnrlous atoms at the stetes in Big. 9b state
Vl
v2
Oa
hl
N14
Hl6
h_l
NIB
010
I ;:I b
+0.174 +0.173 co.177 +0.179 to.1ao +0.182
*0.124 wo.123 *0.123 to.124 ‘+0.124 *0.115
-0,270 -0.274 -0.278 -0.232 -9.283 -0,280
*0.360 to.366 to.347 eo.335 +0.332 *0.274
-0.246 -0.249 -0,230 -0.109 -0.176 -0.092
+0.163 *0.106 +0.137 +0,29G *0.210 to.303
+0.122 *0.13El +0,1G3 +0.160 +0.160 +0.21s
+0,041 +0.046 *0.040 *0.023 *0.017 -0.286
-9.041 -0.081 -9.13G -0.183 -0.198 -0.163
IV v ” VI
--.
I_..
..--_....
_,.,_
ti
331
These behaviors of charge densities and bond energies for the ‘catalyst’ containing two vanadium atoms.are almost the same as those for +Ae ‘cataIyst’ containing a single vanadium atom. The resuIts i&i&ate that the eIectronic s+..te of the sysLem composed of ‘catalyst’, NH3 and NO does not change significantly with the number of vanadium atoms in the ‘catalyst’. This supports the adequacy of the model of the ‘catalyst’ shown in Fig. 2. Electronic nechanissm of the NO-NH, reacfti~ OR the %ataLysf ’ On the basis of the calculated resuIts of the changes in the eiectionic
state of the system in the course of the reaction, the eIectionic details of the mechanism of the NO-NH3 reaction are discussed below. As mentioned. above (Table S). the electron density on the Nla atom of NO is markedly increased at the.transition state_ Similarly, the election density on the 014 atom of NO increases with the progress of the reaction. On the other hand, the electron densities on N,, TTI,~, and Hlz atoms of NH3 decrease significantly at the transition state. These results indicate that electrons on the adsorbed NH, transfer to NO at the transition state. Since the NO molecule has an unpaired electron in the antibondlng orbital, the bond strength of NO+ is greakr than that of NO, while that of NO- is weaker than that of NO. The transfer of electrons from the adsorbed NH3 to NO at the transition state should therefore lead to the weakening of the NO bond at this state. In fact, as shown in Table 7, the bond strength of NO is markedly weakened at the transition state. This weakening Ieads to the elongation of the NO bond after the transition state, which then causes a further decrease in the bond strength of NO. At State VU, the energy of the NO bond turns positive, meaning a repulsive interaction between N13 and 014 atoms of NO. The dissociation of the NO bond is follovzed by the formation of new bonds; the N13 atom of NO combines with the N9 atom of NH3 to form Nz; the O,, atom of NO links with two H atums of NH3 to produce HzO, and the residual H 2tum of NH3 binds to V=O tu form V-OH species (States X and XI)_ In conclusion, the transfer of electrons from the adsorbed NH3 ti NO plays LUIessential roIe in the NO-NH3 reaction on the ‘catalyst’. As shown above, the CNDOIB calculations together with the model of the ‘catalyst’ in Fig. 2 have eIectronicaUy substantiated the mechanism of the NO-NH3 reaction on V,O, proposed experimentally. Concerning the relation between the calculations and experiment, the following point should be emphasized: in previous work [151, it was proposed that the NO-N& reaction on V,O, is a reaction between adsorbed NH3 and gaseous NO by the EEey-Rideal mechanism. This inference was made with many experimental results, including those from the adsorptions of NH, and NO on V,O, by intied, pulse, and temperature-programmed desorption techniques, demed kinetics of the reaction, and of the reaction of the adsorbed NH3 with gaseous NO by in&ared and pulse techniques. In order to estabIish the validity of the Eley-Rideal mechanism, it should be proved that the direct interaction sf NO with the catalyst is tiuch weaker than the M-eraction-between NO and adsorbed NH, in the course of the reaction, especially at the
332
This is supported by the cah&ked results as follows: as shown in Table 7, the interaction between the O2 atom of V=O and the N,, atom of NO is repulsive throughout the course of the reaction except States II, IX, and X, where ‘he energy of the 02- NE3 bond is negligibly small. On the other hand, s shown in Table 7, the interaction between NO and adsorbed NH3 plays a significant role in the reaction. These r&u& support the
transition state.
validity
of the Eley-Rideal
mechsnism
inferred
experimentally.
Acknowledgements The authors thank Prof. K. K&o and Dr. H. Kobayashi for valuable discussions. This xork was partially supported by a Grant-in-Aid for Scientific Research from the Ministzy of Education, Japan (No. 57470055).
References 1 2 3 4 5 6 7 8 9 19 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
M. Markvart and V. L Pour. J_ Cat& 7 (1967) 279. M. She:ef and J. T. Kummer. Ciretn. Eng. Frog., Symp. Ser.. 67 (115:. 74 (1971). AM.Shelef, CaLal. Rev., 21 (1975) 1. G. L. Rauerle. S. C. Wu and K. Nobe, fnd. Eng. CRem. Prod. Res. Dew, I4 (2975) 268. N. Todo, M. Kurita. H. Hagiwara, H. Ueno and T. S&o. Preprints of Papers for the Japan-USA Seminar on Catalytic NO, Recciiorrs, 3 - I (1975). A. Miyamoto, Y. Yamazaki and Y. Murakami, Nippon Kctguk~; Kaiski, (1977) 619. K. Otto, M. Shelef and J. T. Kummer. J. Pfzys. Chem., 74 (1970) 2690. K_ Otto, M. Shelef and J. T. Kummer. J. Phys. Cfzem., 75 (1971) 875. K. Otto and M. Shelef, 2 Whys. Chem., 76 (1972) 37. K. Otto and M. Shelef, 2. Phys. Chem., 85 (1973) 308. J. L. Gland and V. N. Korchak, J_ CaraL. 55 (1978) 324. H. Niiyama, T. Ookawa and E. Echigoya, Nippon Kaguku Kutii, (1975) 1871. H. Niiyama, K. Murata and E. Echigoya. J. Catd. 46 (2977) 201. M. Takagi, T. Kawai, M. Soma, T. Or&hi and K. Tamaru, J- CdcL. 50 (1977) 441. M. Inomata. A. Miyamoto and Y. Mnrakami. J. CotaL, 62 (1980) 140. M. fmanari. Y. Watanabe, S. Mastuda and F. Nekajima. Proc. 7tfz InrrLCongr. CaIaEysis. Tokyo, 1980, p_ 841. T. Seiyama. T_ Arakaawa,T. Matsuda. Y. Takita and N. Yamazoe. J. Catal.. 43 (1977) 1. T. Arakawa. M. Mizumoto, Y. Takita, N. Yamazae and T. Seiyarna. Bull. Chem. Sac. Jpn. 60 (1977) 1431. M.. Mizumoto. N. Yamazoe and T. Seiyama, J. Ca~aaL.55 (1978) 119. W. E. Williamson and J. H. Lunsford, J. Phys. Chem.. 80 (1976) 2664. G. Slyholder and M. C. Allen. J. Phys. C~etn., 69 (1965) 3598. N- G. Connelly, fnorg. Ctu’m. Alc* 6 (1972) 47. J. H. Lunsford, in R. L. Klimicch and 9. G. Iarson (eds.), Tfze Catalyh’c Chemistry of ofNilrcgm Oxides. Plenum Tress. New York, 1975, pp. 3 - 18. H. NXyarna and E. Echig0ya.X CcfaL. 38 (1975) 238. H. Arai and H. Tominaggs, 5. CataL, 43 (1976) 131. A. Miyamoto an-d Y. Murakami. Nippurr Kagaku Kaiski (1978) 505. Y. Ochhi, K. Ohsato, K. Mori and K. Matsuaka, Nippurr Kugcrku Kai6ki. (1978) 16Oi. A. Miyamoto, M. Inomatz, Y. Yamazaki and Y. Murakarni, b. CafaL, 57 (1979) 526.
333 39 30 _ 31 52 33 34 35 36
A. Kobayashi. H. K&o, K. Tarama and K. Fukui, J. Catal., 49 (1977) 294. P. A_ Pople and D. L. Eeveridge, Approrinzafe MoLecuh Urbifal TFreory, McGrawHill, New York, 1970. D. IV_ CTack. N. S. Hush and J.-R. YandIe, J- Chem. Phys, 57 (1972) 3503. E. CIernenti and D. L. Raimondi, 6. Chem. Phys., 38 (1963) 2666. J. C. Shteer. PAYS. Rev.. 36 (1930) 57. G. BIyhoIder, Sccrf. SC& 42 (LS74) 249. S. Yosh~da. S. Tai azd K_ T-a. J_ CataL. 45 (1976) 242. I. D. Mikheikin, I. A. Abronin, 6. M. Zhidomirov and V. B. Kazanski, J. MO!. Cctal., 3
37 38 39
(1978) 436. D_ R. Armstrong, R. Fortune and P. G. Perk+ J CafaL, 42 (1976) 435. 5. Dubse, S. Beran and V. Eosacek, L Mol. CataL. 6 (2379) 321. W. J. Mortier. P. Geerlings. C. Van Alsenoy and I-L P. Figeys. J. Phys. Chem..
40
(1979) 855. S. Bema and J. Dub&j?,
41 42 43 44 45
J. Phys. Chem., 83 (1979) 2538. W_ J. Mortier and P. Geedings, 6. Plrys- Chem. 84 (1980) 1982. W. Grabomski, M. Misono and Y. Yoneda, J. CataL. 61 (1980) 103. S. Beran, J. Ilfol. CuEaL, LO (1961) 177. A. P. Zeii and V. D. Sokolovskii. J_ MoL Cata!., 9 (1980) 399. J. Haber and M. Witko. Act. Chem. Res.. i4 (1981) 1.
83
A MOLE_ THE NO-N-E&
OREWIXL INVESTIGATION REAmON ON PANAJ3IU-M
AKIRA MTYAMOTO, MAEKOTO and YtIKXI MURAKAMI
INOMATA-.
~epzninenf of Sytititetic Chemie~, Facdty Furucho, Chikusu-ku, Nagoya 464 (Japar~J (Receivd
September
OF THE MECHANISM OXIDE CATALYST
ATSUSHI of
HATTORI,
Engineering,
ToStK4.KI
Nugoyci
OF
U-I
University,
25,1981)
‘The electronic nature of the ca+klysis in the NO-NH3 reaction on vanadiu_moxide cakaIyst has been investigated using the CND0/2 method. The caktitions
have shown that NH., is stab& chemiscrbed
on the l3rijnsted
acid site on the catalyst; whereas NO is hardIy chemisorbed, which is ti accordance with experiment. The charge distributions and bond energies of the system composed of NO and NH3 adsorbed on the catalyst have been c&uIated for various states on the reaction coordinate. The calculated resuks have indicated that elections on the adsorbed NH3 transfer to the antibonding orbitals of NO at the transition state tu dissociate the NO bond. This dissociation has been shown to Leadto the formation of N,, HzO, and V--OH species. Furthermore, the caku&ions have supported the validity of the EZey-Rideal mechanism experimentally proposed.
The reductions of nitric oxide with carbon monoxide, hydrogen, and ammonia are important reactions incorporated in the catalytic process for the control of nitric oxide from industrial effIuents_ Attention is especially focused OR the reaction of NO with NHs (the NO-NH3 reaction), since this reaction is acceIerated by the addition of oxygen, whereas the reductions of NO with CU and H2 (the NO-CO and NO-HZ reactions, respectively) are suppressed by the presence of oxygen [I - 61_ AIthough a mechanism of the NO-NJ& reaction has been proposed for various cataIysts, including supported precious metals [7 - II], mew oxides [I2 - 161 and ze0Lite.s[I7 201, electronic details of the mechanism have not yet been clarified.
*l?rese~t
Aidi
addrss:
Kinu-ua
Research
Department,
JGC Co.,
Sucosakidq
Hada.,
475, Japan.
o3#4-5~ozfe2/oooo-ooaJ/Soz.Ts
@ Elsevier Sequoia/Rinted
iu The NetherIan&
316
Nitric oxide is a peculiar molecule which has an unpaired electron in its antibonding X* orbital. This leads to the following property of NO: the bored strength of NO+ is smnger than that of NO, .EbiIe that of NO- is weaker than that of NO [2,21 - 23]_ Consequently, electrons should be given to the antibonding orbit& of NO in order to reduce NO, or, in other words, to dissociate the X0 bond. This property might easily explain the behavior of NO in the NO-CO and NO-H2 reactions on catalysts as follows: NO is considered to be converted to N,O or N, on a reduced metal ion on the catalyst-s in these reactiolns, and electrons on the reduced site are expected to Oansfer to NO to form NO-, followed by the production of NaO or N2 [24, 25]_ The presence cf oxygen would convert the reduced site to an oxidized site so as to prevent the reduction of NO. The mechanism of the NO-N& reaction, however, couid not be understood in terms of the above-mentioned discussion, since the NO-N& reaction, contrary to the NO-CO dr NO-HP reaction, proceeds more readily on the oxidized surface than on the reduced surface [S, 13,15,26]. A mechanism through the reaction between NHz(ad) and NOs(ad) may be one of the possible mechanisms of the NO-N& reaction in the presence of oxygen [ 14,271. The NO-N)& reaction on V205 in the presence of oxygen under dilute gas conditions, however, has been shown to proceed not by this mechanism [ 281, but 5y the mechanism shown in Fig. 1. In Fig. 1, V=O is the vanadiumqxygen double bond, and V,aH is the adsorption site of NHs, that is, the Brijnsted acid site. In Reaction (l), NHs is readily adsorbed on VS-OH on the catalyst (A) to form (B). Then in Reaction (2). NO attacks the adsorbed NHs, (B), to form N2, Ha0 and the reduced catalyst (D) through a transition state, (C). The reduced catalyst, (D), is oxidized by Oa to reproduce (A) by Reaction (3). When the concentration of O2 is high
-o-
8 x -o-&,-o(A)
/H NH3 FL51
-fg
-o-&o-3,.
o-
(8)
KExTl~U)
f-9
T
z~ci1cn~2~
NO
-o_&-o-:,(D) No +
Hz0
Fig. 1. Mechanism of the NO-NH3 of oxygen.
reaction on vanadium oxide catalyst in the presence
enough, it has been found that the V-H species is readily reoxidized to V=O by Reaction (3), whfie the Reaction (2) becomes the ratedetermining step. Furthermore, in Reaction (2) NO has been suggested to attack the adsorbed NHB, (B), from the gas phase by the Eley-Rideal mechanism. Neither gaseous NO= nor adsorbed NO, species pIays a roIe in the reaction under dilute gas conditions. Thus, it seems interesting to investigate the electronic natWe of the NO-NH8 reaction on V&, catalyst, especially the manner of dissociation of the NO bond in the reaction. In this study, a semi-empiricaI SCF MO method (the CXDO/Z method) has been applied to the analysis of the mechanism of the NO-NH3 reaction on V,O, in order to elucidate the etectronic nature of the NO-NH3 reaction and the rde of V=O species In the acceleration of the reaction, and to examine the validity of the EIey-Rideal mechanism experimentally proposed.
TABLE The
1
orbital
exponents,
ployed in the CNDO/S Atom
Type
<, bonding
parameters,
OP orbit4
K
1s
1.2
N
2s 2P
0 V
@A,
and aI+
A)
terms
for atoms
em-
calculations St:
$(l+A)
(eV)
(eV)
9.0
7.176
1.95 1.55
25.0 25.0
19.316 7.275
2s 2P
2.275 2.275
31.0 31.0
25.390 9.111
3d
1.792
16.5
4s
1.57 1.57
4P
3.5 3.5
4.455
3.822 0.777
Method of computation The calculations were carried out using the CNDO/B program of Kobayashi et LZ~.[293 with the parameters shown in Table 1. Parameters for H, N, and 0 a’dms were the same as those reported by Pople and Beveridge [30], and those for the V atom were given by Clack et (II. [31] _ Orbital exponents; (, for the V amm are smaller than those proposed by Clementi and Raimondi [32] but Iarger than those of SIater [SSI _ This was done since Blyholder [34]- has o&tied the orbital exponents for the cIuster of Ni atoms between those of SIater and those of Clementi and Raimondi. The difference in the orbi.taIexponents, however, was confirmed not to affect significantly the caIcuIated resuIts of the present system. Numerical calculations -were made-with a FACOM M-200 computer, Computer Center, Nagoya University.
318 -4237
Fig. 2. Tne model of V20~ catalyst employed in the cak~latiozts, hereafter denoted as ‘catalyst’. %%e sum of an individual atom is the numericzd order of the atom. The numbe: near a bond represenk the equilibrium length of the bond. Fig. 3. Electronic properties of the ‘catalyst’ c&Mated by the CNDO/2 chrge distributions on atoms; (b!: energies of the individual bonds.
method; (a):
Rsults and discussion Model
of V,O, cddyst Since all atoms in the V,Os
catiyst cannot be taken into account in the calculations, a simplif%d model of the catalyst was assumed as shown in Fig. 2. This type of catalyst model has been employed in quantum chemical investigations of the adsorptions on metal oxides 135 - 451. This ‘catiyst seems to be adequate for the model of the V,O, ca’alyst based on the following reasons: 1. The valence of V ion is 5. 2. A V=O species, that is, the active site for the NO-NH, reaction on V,O,, is taken into consideration in the ‘catalyst’. 3. A Brijnsted acid site, that is, the V,--OH specjes in Reaction (l), is involved in the ‘catalyst’. Furthermore, the calculated properties of the ‘catalyst’ support the adequacy of the model as follows: the geometry of the ‘catalyst’ in Fig. 2 was determined so as to give the minimum potential energy in the CNDO/B calculations of the ‘catalyst’. Although the bond lengths thus determined are different from the experimental ones, the differences are not significant. Figure 3 shows charge densities and bond energies in the ‘catalyst’. -The bond energies referred to hereafter are the diatimic contributions of the toal energy formulated by Pople and Beveridge [ 301. As shown in Fig. 3, the bond energy between VI and O2 is greater than that bekeen VI and 03, a& furthermore the equilibrium disknce between VI and O2 is shorter than that between V, and 0s. This means that the VI-O, bond is stronger than the VI-O, bond, and that the former has a double bond charackr. The charge density on Hs, H7, or Hs atom shows a fairly 1-e positive value, suggesting that H6, H,, and H, in the ‘catalyst’ play the BrGnskd acid site. As can be seen in Fig. 2, there is no Lewis acid site in the ‘catalyst’. This is in accordance with experiment, since NKs is adsorbed on the VsOs catalyst as NHf (ad) while other chemisorbed species have not been detected ir, the infrared spectra of adsorbed NH3 on V,O, [15]_
TABLE
2
TABLE
Total enew (I&& of NH3 adsorbed on the ‘catzlyst’ at arious J&-P*19 distances (rllp
3
Total enorgy (&g3 of NH3 adsorbed on the ‘cztaIy.st’ at mrious 03-&-Ns angles (8 )’
J%uta.l (eW
8
Etaml
(“1
(9
-2665.479 -2670.285 -2673.163 -2674.805 -2675.669 -2676.057 -2676.168 -2675.174
180 170 160 150 140 130
-2675.669 -2675.698 -2675.704 -2675.535 -2674.135 -2667.118
%I = L-2 A.
Since NH3 is adsorbed on V,O, as NH,‘(ad) [l&153, NH, is considered to interact with the BrSnskd acid site on the ‘catalyst’. Consequently, cakulations were made for the geometry shown in Fig. 4, where r, is the distance between M, and Ns, and 8 is defined as the Oa-Hs-hT9 angle. Similarly in the case of the ccataIyst’, the bond lengths in NH, have been determined so astogivethe minimum potential energy for NH,. Tables 2 and 3 show the cakrrlated resuks of the interaction between NH, and the ‘catalyst at various rf and 0, respectively. As shown in TabIe 2, the total energy (l&& of the adsorbed system is Iower than that of the separated one (rl = =) unless r, is shorter than 1.1 A. Tbis indicates that NH3 is stably adsorbed on the ‘catalyst’ in accordame with experiment. Table 3 shows that Etim exhibits its mjnimum at 6 = 160” but not at 8 = 180”, suggesting an attractive interaction-between V=O and NH,. This also agrees with experimental restits, since the adsorbed NHa on V& is considered to in&act with V=(3 species to prevent the rotation of NH8 on the surface of the catalyst [ 151_ The calculated vaIues of E,, in Table 2 indicate an extraordinarily Iarge
320
Fig. 5. Adsorption of NO on the ‘catalyst’; r2: O2-N13 angles are the same as those of the ‘catalyst’ and NO.
TABLE Total
0.9 1.0 1.1 1.2 1.3 1.4
1.5 1.6 1.7 m
length;other bond lengthsaQd
4 ene%y
(E,&
of NO adsorbed
on
the ‘catalyst’
at various
O2-N,3
distances
(9)
-3087.222 -3101.062 -3107.920 -3110.427 -3110.323 -3108.730 -3110.576 -3115.928 -3117.380 -3110.717
(rl > 1.4 A). Thismay be ascribed to the imperfections of the method in the calculation of iong-range interactions _ Calculations for the adsorption of NO on the ‘caLMy& were made for the geometry shown in Fig. 5, where r2 is the distance between O2 and &a. As shown in Table 4, Etiti of the adsorbed system is larger than that of the separated system (r - -) at any r, smaller than 1.5 A. Namely, in contrast to the adsorpt-ion of NH 3, the calculations show that NO is nof chemisorbed on the ‘catalyst’. This is in accordance with experiment, since NO is not adsorbed on the V20s catalyst under the experimental conditions of the NO-N& reaction on VsO,. Similarly ‘k the case of the-adsorption of NK3. E bLal shows a large interaction between NO and the ‘catalyst’ at r2 2 1.6 A. This may also be due to the imperfections of the method for the calculation of long-range interactions, since such a large interaction cannot be expected atsuchalarger2as1.7A. interaction outsidethechemisorptionrangeofr;
321
Although the CNDO/Z CdcrrEaticnstogether with the parameters in Table 1 and the catalyst model in Fig. 2 carrnot describe the Eong=rangeinteractions bekeen NH3 (or NO) and the V,O, catalyst, as mentioned above, they can express well the behv-ior of the chemisorptions of NH, and NO on V,Os. It is therefore considered that the model of V&s catalyst in Fig. 2 is, to a first approximation, an adequate one for an analysis of the mechanism of the NO-NH, reaction on V&l, cat&y& Reaction
of
NO
with NH3 aahrbed
OR
the %atalyst'
In order to determine the course of the reaction between N3 and. adsorbed NJ&, that is, the reaction coordinates, the CNDOIB calculations were
made for the system composed of ‘catalyst’, NJ&, and NO with various coordinates of atoms in the system. On the basis of a number of ‘trid and error’ caIcuIations, the 11 states shown in Fig. 6 were selected as states in the course of the reaction from the initial state (State I) to the final state (State XI). In the initial state (S’t.te I), the distance betieen NH3 adsorbed on the ‘catalyst’ and NO is infinite, and no interaction exists between both species. When NO approaches the adsorbed NH, (State II), the interaction bekeen both species causes a decrease in 19from 160” to x40”, that is, the access of NH3 to the V=O species in order to permit a -her progress of the reaction. Irr States III - VI, a further approach of NO to the adsorbed NH3 occurs whiie keeping the geometry of the adsorbed NH3 on the ‘catalyst constant. The elongation of the N13-Oi4 bond takes place in State WI accompanied with partial formation of HZi-Ok~~LP, N9-N13, and &-Hz0 bonds. In States VIII, IX, and X, in turn, the distance between N,, and 014 increases while the N,-I&, Hll-OZi, H12-O14, and 0,-H,, distances decrease. In the final state (State XI), N2, Hz0 and the reduced ‘catalyst’, that is, V(OH),, are separated infinitely from one another. The total energy and interatomic dtitances for the individual states in Fig. 6 were calculated in order to examine the qualification of the states in Fig. 6 for the points on the coordinate of the reaction of NO with NH3 adsorbed on the ‘catalyst’. This was done since the total energy and interatomic distances for the state on the reaction coordhte should exhibit continuous and smooth changes from the initial state to the final state. As shown in Fig. 7, the totztl energy (EtOm) of the system varies continuously from
State
I to St2t-e XI. l&,,
increases
monot~nicdly
from
Stat-e I to State
VI and attains its maz&num at State VI, which is considered to be the fmmsition state of the reaction_ From State VI to State X, Etotaldecreases monotonically to reach a local minimum at State X. Aithough Etiti of the f-Jlal state (State XI) is a Little higher than t-hat of State X, the difference is not significant. Table 5 shows interatomic distances of some bonds in States I XI. As can he seen in Table 5, any interatomic distances exhibit natur&-and continuous changes from State I to State XI, and do not show too short distances compared with those of covalent bonds. Consequently, it is considered thrit States I - XI are I&ated on the reaction coordinate and represent the course of the reaction of NO with- NH3 adsorbed on the.‘cataiyst’, from the initial state to the final state through the transition state (State VI).
322
In order to investigatz the electronic nature of the catalysis in the NO-N& reaction OR the ‘catalyst’, charge densities on atoms and bond energies of inditiual bonds were calctited for the states in Fig; 6; and the cakul&ed rtits axe indica+~ in Tables 6 and 7. respectively. Table 6 shesignificant changes ir. the cbwge densities before and after the fm~~ition
323
Fig. 6. Reaction of NO with adsorbed NH3 on the ‘catalyet’; (a}: State E (initial state); (6): At States LJI,III. EV, V, and VI, the respective vaIus_ of 2~0 me 3.50,32X, 3.10, 3.07, and 3.05 A ; Stati VI is the fmnsition state;(c): Skate VII; (d): Sfate Vm; (e): State UC; Cf): State X;(g): Stake _XE (ErtaI state). The geometry of the individual cfskes is alao shown iu TabIe 5.
TAlKI5 Interatomic diatonccsof vnriouopuirsof alams ut the Antes in Pig. 5 (A) -. -StatA?
No-N,a
I II III IV
i.127
1.692 1.7G3 1.726 I.707 1.292 1.260 1.200 1.146 1.1.46
v
VI VII
VIII IX X XI --
-
Q-%
. -
.I
.
1.160 1,150 l.lGO 1.160 1.150 1.160 2.066 2.391 3.107 4.224 OD
.
.
ND-~
h-014
HIO-NM
B,J-ND
oz-ho
v1-02
No-ho
;,710 1.466 1,319 1.290 1.271 1,658 1.88G 2.360 3.116 cm
1.200 1.200 1.200 1.200 1.200 1,200 2.134 2.364 2.067 3,678 m
1.794 1.156 l.lGC 1.166 l.lG6 1.166 1.074 1.060 l.OG? 1.040 I .040
1.460 1.460 1.400 1.460 1.480 1.480 1.643 I.646 l.GG7 1.670 1.1370
1.060 1.060 1.060 1.060 1.060 1,060 1.261 1,209 1,546 2.237 DD
m
2m.067 1.864
1.824 1.6?9 1,660 1.631 1.600 1.700 2.204 3.329 m
1.647 1,522 1.497 1,4fJl 1.135 1.117 1.084 1.040 1.040
._-.-
Chnrgodcnoitieson variousatoms at thc,atnteain Fig, 6 .’
stat4 I II HI IV
v
VI VII VIII IX X XI
Vl 46,073 *0,118 *OJl4 +0.119 *0,117 *0,08&I, -Q.OSG ,-a.014 -0,007 -0.016 -0,016
Hll
-0,287 ‘. -0.336 -0.319 . -0.341 -0:32i ’ ‘. 4.339 -0~329 ‘: ,’ -0.386 ‘-0.330 -0.335 -0.312 .. -0.200 4.297 I’ -0.173 -0.211 -70.174 -0.194 ’ -0.176 -0.103 -0.172 -0Ml -0.172
*0.339 +0,304 +o.a[ia +0.339 +0.336 40.260 *o;ot30 *0.121 *0.142 *0.137 *0.140
-0.249 -0.369 -0.34G -0.304 -0.290 -0.196 -0.140 -0.061 -0.012 -0.004 0.0
*0.189 +0.323 +0.34G +0.364 *0.369 *0.429 *0.264 +G.194 +0.14a +0.130 *0.127
*0.124 +0.104 *0.117 +0.122 +0.121 *0.194 eO.264 +0.176 +0,128 *0.136 +0.136
*0,041 *0.043 *0.032 *0.017 *0.013 -0.299 -0.170 -0.116 -0.009 ~+o.oo? 0:o
-0.041 -0.0’13 -0.114 -0.162 -0.lG9 -0,137 -0.XB2 -0.198 -0.249 -0.269 -0.271
.
TABLE 7 Band caergieaof variouspairs of atome at the states in Pie. Ei(cV)
state
v1-02
%-ho
Oz--N~3
b-N13
No-h
HIO-NI3
I’
-40.723 -4G.231 -46.146 -40.085 -4B.074 -46.721 -39.629 -39.7 69 ~30.463 -37.008 -37.003
-9.917 -6.220 -6.229 -6.201 -6.188 -4.257 -13.6G7 -13.234 -14.963 -16.849 -16.964
0 -0,066 eO.018 +0.128 +0.1ti9 *On776 *0,660 eo.313 -0.006 -0.007 0
0 -0.964 -2.692 -4.792 -5.416 -6.281 -42.652 -50.926 -B8.969 -61.322 -61.377
0 *0.788 *1.317 *l.G20 *1.460 -G.280 -11,990 -G.O78 -0.203 +O.OOG 0
0 -1.062 -2.347 -3.604 -3.904 -6.897 -l..G97 -0.649 +0.004 *0.004 0
II 111 IV V VI VII, VIII IX .X, XI
H1rQ14
0 -0.140 -0.114 *OS01 +0.x92 -1.384 -17.413 -19.086 -20.008 -20.616 -20.636 -,
h--%4
-49.862 -48.939 -47.400 -46.387 -44.820 -34.108 +1.042 +0.415 +0.009, -0.006 0
K WI
I,,,,,,,,,,, I
I,
m
Iv
v
IAl
Ml
wr
lx
x
a
STATE
Fig. 7 _Changes in the total energy (E,& GF the system composed of NH3 adsorbed OE the ‘catalyst’ and NO; State I: initial state; State VI: transition state; State XI: final state.
state. The most striking change is observed in the charge density on Nrs, that is, the N atom of NO. Namely, the charge on N,, is positive in States I - V, while at the transition state (State VI), it changes to a large negative value. After the transition state, that is, States VII - IX, it remains negative, although its absolute value decreases in the course from State VII to State IX. Similarly, the value of the negative charge on Or4 of NO graduaI.lyincreases from the initial state to the final state. In cont_mst tc the change in the charge of NIB of NO, the fairly large negative charge of the Ne of NH3 before the transition state (States I - V) is abruptly reduced at the transition state (State VI) and decreases further after the transition state. The positive charge on IIll remarkably increases at States VI and VII, while it decreases again after these states. The charge density on the vanadium is positive before the transition state, while it shows a negative value after the transition state, indicating the reduction of the vanadium ion after the reaction. Bond energies in Table 7 aIso indicate significant changes in the electronic states of the system before and after the transition state. The absolute value of the energy of the NL3-0 rc bond is very large for States I - V, indicating that this bond is very strong before the transition,st.ate. At the transition state, the value signi&antIy decreases. At States VII - IX, it becomes positive, meaning that this bond becomes repulsive. Corresponding to the weakening of the NO bond, the negative value of the energy of the Oe--Hz0 bond increases abruptly after the transition state, indicating the gradual formation of the Hz0 molecuIe. Similarly, the strength of the Ns-N,s bond markedly increases after the transition stafk.The Vr-0, bond after the transition state becomes considerably weaker than that before the transition state, indicating a change of the V=O double bond to a single bond after the reaction. It
327
Fig. 8_ Zleckonic properties of the katalyst’ containing two vanadium atoms; (a): charge distributions on atoms; (b): energies of the individual bonds. The length of the V,-O, (or V2-0~) bond was determined to give the minimum potential energy es I.560 A The length of another bond was the same as that for the katalyst containing a single ~BLLEdium atom.
shodd be emphasized in Table 7 that the energy of the 02-N13 bond is positive at almost all states in Fig. 6. indicata the repulsive interaction between the 0 atom of V=O and the N atom of NO in States III - WEE. This means that direct interaction between NO and V=O is not important in the catatlysis of the reaction of NO with NE& adsorbed on the ‘catalyst’. Effects of the number of mrzadkmt a~mns in fhe %ataLyst' For the above-mentioned calcrrlations, the number of vanadium atoms in the ‘catalyst’ was one. This was done because much computation time was necessary for the ‘trial and error’ cakr.Iations to determine the state along the reaction coordinate_ In order to show the adequaq of the model of the ‘cat.aIyst’ in Fig. 2, calculations should also be made for the ‘catalyst containing two or more vanadium atoms. Figure 8 shows the caIculated results of the charge densities and bond energies for the ‘catalyst’ containing two vanadium atoms. The charge densities aad bond energies for the ‘catalyst’ containing two vanadium atoms are almost the same as those for the ‘catalyst’ containing a single vanadium atom. (Fig. 3). Similar results were also obtaked for the katalyst’ containing three vanadium atoms.
328
(a)
(b)
Fig. 9. Geometry of Aoms employed for the cdcuhtions. (a): NH3 edsorbed on the ‘catalyst’ containing tv~o vanadium atoms; (b): reeaction of NO with the adsorbed NH3 on the ‘catalyst containing two vanadium 2toms. For 2 detailed geomeky, see Table 8.
Calculations were aLso made for NH3 adsorbed on the ‘catalyst’ containing two vanadium atoms at various vaIues of r, and 8 (Fig. 9a). The tot-& energy of ‘;he adsorbed system was lower than that of the separated system (rl = -) unIess r, was shorkr than 1.2 A _ This indicates that NHs is stably adsorbed on the ‘cataIyst’ conteining two vanadium atoms within the chemisorption range of 1-r. The total energy of the adsorbed system exhibited a minimum at 8 = 130”. suggesting an attractive interaction between V=O and NHs. It was also found that NO is not stably adsorbed within the chemisorption range. These r=uIts are similar to those for the ‘catalyst’ containing a single vanadium atom. Some calculations were made for, the system composed of NO and NH3 adsorbed on the ‘c&taIyst containing two vanadium atoms (Fig. 9b). Cakulated resuks are shown in Tab!es 8.9 and 10. As shown in Tabks 5 and 8, the geometry of NO relative to the adsorbed PIHa in States I - VI for the ‘catalyst’ containing two vanadium atoms is almost the same as that in States I - VI for the ‘catalyst con+taining a single vanadium atom. This was done in order to decrease the number of ‘hial and error’ calculations far determining states along the reaction coordinate. As NO approaches the adsorbed NHs from State I to State VI, the electron density on the adsorbed NHs fNz4, II r5, and E&e atoms) decreases whiIe that on NO (Nrs and Or9 atoms) increases (Table 9). CorrespondingIy, the strength of the NO bond (N18-Olg bond) decreases as the reaction proceeds from State I to State VI (TabIe 10).
TABLE 8 Interatomic dM.anccs of various pairn of atoma at the states in Ipig,9b (Af’ 3tato
NlrNlt~
b+‘lo
Nw%-I
ki-%
h6-NlA
HI ah4
oa-EI16
v1-03
&I-h6
;.127 1.392 1.763 1.726 1639
1.160 1.160 1.1.50 1.160 1.160 l.lGO
;,007 1.824 1.679 l.GGO 1.612
Y.664 1,647 1.622 1,497 1.466
m 1.710 1.466 1,319 1,290 1.262
1.300 1,300 1.300 1.300 1.300 1.300
1.693 1,693 1.693 1.693 1.693 1.693
1.400 1.430 1.430 1.480 1.430 1.480
1.660 1.060 1.060 1.060 1,060 1.060
I
II III IV
“Il~e geometry of NO rclativa to the adsorbed NIIg in States I - V is the anma as thnt iu St.&e I - V for the ‘catalyst’ cont-ninlng n single vnnndium atom. In State VI for the ‘catrdyst’ containing two vnnndium atoms, the dintnnce between NO and NHa ieshorter by 0.02 A than thnt in State VI for tho ‘catnlyst’ containing n single vanadium ntom.
T@LE 9 Charge dorAtics on vnrlous atoms at the stetes in Big. 9b state
Vl
v2
Oa
hl
N14
Hl6
h_l
NIB
010
I ;:I b
+0.174 +0.173 co.177 +0.179 to.1ao +0.182
*0.124 wo.123 *0.123 to.124 ‘+0.124 *0.115
-0,270 -0.274 -0.278 -0.232 -9.283 -0,280
*0.360 to.366 to.347 eo.335 +0.332 *0.274
-0.246 -0.249 -0,230 -0.109 -0.176 -0.092
+0.163 *0.106 +0.137 +0,29G *0.210 to.303
+0.122 *0.13El +0,1G3 +0.160 +0.160 +0.21s
+0,041 +0.046 *0.040 *0.023 *0.017 -0.286
-9.041 -0.081 -9.13G -0.183 -0.198 -0.163
IV v ” VI
--.
I_..
..--_....
_,.,_
ti
331
These behaviors of charge densities and bond energies for the ‘catalyst’ containing two vanadium atoms.are almost the same as those for +Ae ‘cataIyst’ containing a single vanadium atom. The resuIts i&i&ate that the eIectronic s+..te of the sysLem composed of ‘catalyst’, NH3 and NO does not change significantly with the number of vanadium atoms in the ‘catalyst’. This supports the adequacy of the model of the ‘catalyst’ shown in Fig. 2. Electronic nechanissm of the NO-NH, reacfti~ OR the %ataLysf ’ On the basis of the calculated resuIts of the changes in the eiectionic
state of the system in the course of the reaction, the eIectionic details of the mechanism of the NO-NH3 reaction are discussed below. As mentioned. above (Table S). the electron density on the Nla atom of NO is markedly increased at the.transition state_ Similarly, the election density on the 014 atom of NO increases with the progress of the reaction. On the other hand, the electron densities on N,, TTI,~, and Hlz atoms of NH3 decrease significantly at the transition state. These results indicate that electrons on the adsorbed NH, transfer to NO at the transition state. Since the NO molecule has an unpaired electron in the antibondlng orbital, the bond strength of NO+ is greakr than that of NO, while that of NO- is weaker than that of NO. The transfer of electrons from the adsorbed NH3 to NO at the transition state should therefore lead to the weakening of the NO bond at this state. In fact, as shown in Table 7, the bond strength of NO is markedly weakened at the transition state. This weakening Ieads to the elongation of the NO bond after the transition state, which then causes a further decrease in the bond strength of NO. At State VU, the energy of the NO bond turns positive, meaning a repulsive interaction between N13 and 014 atoms of NO. The dissociation of the NO bond is follovzed by the formation of new bonds; the N13 atom of NO combines with the N9 atom of NH3 to form Nz; the O,, atom of NO links with two H atums of NH3 to produce HzO, and the residual H 2tum of NH3 binds to V=O tu form V-OH species (States X and XI)_ In conclusion, the transfer of electrons from the adsorbed NH3 ti NO plays LUIessential roIe in the NO-NH3 reaction on the ‘catalyst’. As shown above, the CNDOIB calculations together with the model of the ‘catalyst’ in Fig. 2 have eIectronicaUy substantiated the mechanism of the NO-NH3 reaction on V,O, proposed experimentally. Concerning the relation between the calculations and experiment, the following point should be emphasized: in previous work [151, it was proposed that the NO-N& reaction on V,O, is a reaction between adsorbed NH3 and gaseous NO by the EEey-Rideal mechanism. This inference was made with many experimental results, including those from the adsorptions of NH, and NO on V,O, by intied, pulse, and temperature-programmed desorption techniques, demed kinetics of the reaction, and of the reaction of the adsorbed NH3 with gaseous NO by in&ared and pulse techniques. In order to estabIish the validity of the Eley-Rideal mechanism, it should be proved that the direct interaction sf NO with the catalyst is tiuch weaker than the M-eraction-between NO and adsorbed NH, in the course of the reaction, especially at the
332
This is supported by the cah&ked results as follows: as shown in Table 7, the interaction between the O2 atom of V=O and the N,, atom of NO is repulsive throughout the course of the reaction except States II, IX, and X, where ‘he energy of the 02- NE3 bond is negligibly small. On the other hand, s shown in Table 7, the interaction between NO and adsorbed NH3 plays a significant role in the reaction. These r&u& support the
transition state.
validity
of the Eley-Rideal
mechsnism
inferred
experimentally.
Acknowledgements The authors thank Prof. K. K&o and Dr. H. Kobayashi for valuable discussions. This xork was partially supported by a Grant-in-Aid for Scientific Research from the Ministzy of Education, Japan (No. 57470055).
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83