Copper assisted intercoversion of NO to N2O: a quantum

chemical study

Yuan Zhanga, Yueming Suna,*, Ainian Caoa, Juzheng Liua, Gu Fanb

aDepartment of Chemistry, Southeast University, Nanjing, Jiangsu 210096,

People’s Republic of ChinabThermoenergy Engineering Research Institute, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China

Received 12 August 2002; accepted 11 October 2002

Abstract

Results of quantum density functional theory (B3LYP) calculations on the decomposition of NO to N2O in the presence of Cu

are reported. Three approaches of NO interacting with Cu and two decomposition channels via cis- and trans-(NO)2 dimers

have been identified. The configuration of NO approaching to the Cu atom with N is more stable than the other two

configurations, in which the NO interacts with Cu via oxygen. At low temperature, NO is in favor of decomposing to N2O via

the trans-intermediate Min6 in the presence of Cu atom, because the decomposition activation energy of this channel is only

61.1 kJ/mol at the B3LYP/Lanl2DZ level, and lower than the other decomposition channel’s. The potential energy surface

shows that the cis-intermediate Min8 is highly stabilized both thermodynamically and kinetically at low temperature, and the

calculation results also suggest that the N–O bond is more easily dissociated than the Cu–O bond in the Min8. Hence, it is very

different for the NO decomposition between in the gas phase and in the presence of Cu.

q 2003 Elsevier Science B.V. All rights reserved.

Keywords: Density functional theory; Nitrogen oxides; Dimer; Copper; Decomposition

1. Introduction

The nitric oxide (NO) and nitrous oxide (N2O) play

important roles in combustion and atmospheric

processes. NO is a free radical due to an unpaired

electron in the pp orbital, and it can use the unpaired

electron to form dimers [1,2]. The weakly bound

dimer of NO is a possible intermediate in the

reduction of NO in the gas phase, and in catalyzed

reactions on metal surfaces [3–5] and in zeolites [6].

However, many researchers have found that the

nitrous oxide is an unwanted byproduct in the

reduction of nitric oxide [7–9]. On the other hand,

N2O also plays a major role in the formation of NOx

pollutants during combustion [10]. For these reasons,

it is important to understand the relation between NO

and N2O.

The reaction of N2O with O atom can form NO in

both the combustion and the thermal decomposition

of N2O. Many experimental [11,12] and theoretical

[13,14] studies have been made of this interaction.

These works suggested that the activation energy

0166-1280/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.

PII: S0 16 6 -1 28 0 (0 2) 00 7 35 -2

Journal of Molecular Structure (Theochem) 623 (2003) 245–251

www.elsevier.com/locate/theochem

* Corresponding author. Tel.: þ86-25-379-2453; fax: þ86-25-

379-3171.

E-mail address: sun@seu.edu.cn (Y. Sun).

of the reaction is about 115.8 kJ/mol. Gonzalez et al.

[13,14] used ab initio and DFT methods to study the

reaction, and they confirmed that N2O decomposed to

NO via (NO)2 dimer. All of these researches show that

the N2O interacted with O can easily decompose to

NO in the gas phase.

On the other hand, NO can reduce to N2O via a

dimer intermediate on a variety of transition metal

surfaces [15–17]. The experimental results showed

that the N2O formation occurs via the reaction of two

intact NO molecules, namely a dimeric surface

intermediate [18]. Therefore, on the metal surfaces,

N2O can be formed from the adsorbed NO molecules.

Can NO reduce to N2O in the presence of a metal

atom in gas phase?

In the present work, the detailed chemical

computations were made to find the reaction paths

for NO intercoversion to N2O in the presence of the

copper atom, because Cu is of activity in NO

dissociation [19–21].

2. Computational details

The structures of the NO molecules interacted with

the copper atom have been predicted at the

B3LYP/Lanl2DZ level of theory. The transition states

(TS) were optimized by using synchronous transit-

guided quasi-Newton (STQN) methods included in

GAUSSIAN98 [22]. The STQN method, implemented

by Schlegel and co-workers [23,24], uses a quadratic

synchronous transit approach to get closer to the

quadratic region of the transition state and then uses a

quasi-Newton or eigenvector-following algorithm to

complete the optimization.

Firstly, the structures of every stable configuration

were optimized at B3LYP/Lanl2DZ level. Then the

frequencies and zero-point energies (ZPE) of these

structures were calculated at the same level. Fre-

quency calculations were performed to distinguish

local minima from saddle points, meanwhile, they

were also used to confirm the reaction TS, which were

optimized using QST3, QST2 or TS function

implements in GAUSSIAN98. There was only one

imaginary frequency for transition state, whereas no

imaginary frequency for stable states. In addition, the

intrinsic reaction coordinate (IRC) method, which

examines the reaction paths leading down from

a transition structure on a potential energy surface,

was used to confirm the right transition structures.

And then, the activation energies of reactions was

gained from the energies (zero-point corrected) of the

reactants/products and the TS.

3. Results and discussion

We have considered three modes of one Cu atom

interaction with two NO molecules. The first one is

the two NO molecules approach to the Cu atom with

N and O atom, respectively. The second approach is

both NO molecules approach to the Cu atom with N

atoms, and the third is NO molecules only use the O

atoms to approach to Cu. The B3LYP/Lanl2DZ,

which has been proven [25,26] to be exceptionally

reliable computational approaches for the compu-

tation of both structural and energetic properties for

various chemical systems, relative energies are shown

in Fig. 1. Each minima structure in this figure is

labeled with Minn in order to facilitate the discussion,

and the transition structures are labeled with TSn.

Fig. 2 shows the structural parameters of each minima

and TS. The total energies (E ), ZPE, ZPE-corrected

relative energies (DE ) and Mulliken populations (q )

are listed in Table 1.

Firstly we discuss the first approach. In this

approach, the Cu atom interacts with the two NO

molecules to form the Min1 structure. The Cu–N and

Cu–O bond lengths are 1.984 and 1.999 A, respect-

ively, while the N–O bond lengths are 1.227 and

1.268 A for NO approach to the Cu atom with N and

O, respectively, (see Fig. 2). The process is highly

exothermic by 147.1 kJ/mol. And then the Min1 turn

to the trans-dimer intermediate Min4 via the TS1

transition structure by the exothermicity of 59.8 kJ/

mol, the barrier height at B3LYP/Lanl2DZ is only

12.8 kJ/mol. Therefore, the Min4 is more stable than

the Min1. In the intermediate Min4, the N–O

(1.304 A) bond length in Cu–O–N is elongated and

the N–N bond length (1.429 A) becomes shorter

compared with the Min1, but the N–O does not

change, it is still 1.227 A. It suggests that the

interaction between the two NO molecules is stronger

in the intermediate Min4 than in the Min1, and that the

formation of (NO)2 dimer is easier in the presence of

Cu than in the gas phase, because the barriers of

Y. Zhang et al. / Journal of Molecular Structure (Theochem) 623 (2003) 245–251246

the (NO)2 dimer formation from two NO molecules at

B3LYP/6-311G(2d) and CASSCF(18,14)/6-311G(2d)

level are high to 261.7 and 250.4 kJ/mol in the gas

phase, respectively, [13,14]. The Min4 further trans-

fers to the intermediates Min5 and then to the Min6,

but the TS have not been found in these processes at

B3LYP/Lanl2DZ level. The N–O bond length, which

connects to the Cu atom, is elongated from 1.304 A in

the Min4 to 1.381 A in the Min6, and the other N–O

bond changes from 1.227 to 1.284 A, while the Cu–O

bond is shortened to 1.894 A in the Min6. On the other

hand, the N–N bond length is only 1.290 A in the

Min6, shorter than any other configurations, while the

N–N–O angle increases to 130.18 in the Min6. It was

also estimated that the Min6 is 60.3 kJ/mol above the

Min4. Therefore, the Min6 intermediate in only

weakly bound with respect to the Min4 and the

Min5, but it can further dissociate into the products,

namely copper oxide (CuO) and nitrous oxide (N2O),

via the transition structure TS4. In TS4 structure, the

Cu–O and N–N bond lengths are shortened to 1.814

and 1.173 A, respectively, while the N–O (connected

to the Cu atom) is elongated to 1.860 A. The

calculated barrier height of the reaction Min6 !

CuO þ N2O is 61.1 kJ/mol and the exothermicity is

7.3 kJ/mol. The low exothermicity of the above

reaction implies that the reverse reaction, namely

CuO þ N2O ! Min6, is also possible. However, the

dissociation of (NO)2 dimer into NO molecules is

very unlikely in the presence of the copper atom, due

to the maximum barrier is high to 147 kJ/mol in this

approach at B3LYP/Lanl2DZ level (see Fig. 1).

Hence, the above reaction is very different between

in the gas phase [13,14] and in the presence of Cu.

The Min2 structure is formed from two NO

molecules approach to the Cu only with the N

atoms. The exothermicity of this reaction is

152.6 kJ/mol. Hence, the Min2 is a slightly more

stable than the Min1. In the Min2, the N–O bond

length is 1.229 A, a little longer than the analogical

N–O bond in the Min1, but shorter than the other

N–O bond (connected to the Cu via O) in the Min1

(see Fig. 2). The Min2 can transform to another

intermediate Min3 via two TS, namely TS2 and TS3.

In this progress, the TS2 is directly connected with

TS3. The fact is that two TS can be directly connected

stems from the property of an IRC that its symmetry

must be conserved, as long as a stationary point is not

reached [13,27]. Hence, as the TS2 transition vector is

of A0 symmetry, a plane of symmetry is kept along the

IRC till to meet the TS3. In the Min3, the N–N is only

1.503 A, while it is 2.364 and 1.565 A in the TS2 and

TS3, respectively. The N–O bond length increases

from 1.223 A in the TS2 to 1.281 (connected to Cu)

and 1.253 A (the other N–O bond) in the Min3. The

barrier height is only 28.9 kJ/mol from the Min2 to the

Min3 via the transition structures TS2 and TS3 (see

Fig. 1). And then, the Min4 is formed from the Min3

Fig. 1. The potential energy surface of NO decomposition to N2O in the presence of Cu at B3LYP/Lanl2DZ.

Y. Zhang et al. / Journal of Molecular Structure (Theochem) 623 (2003) 245–251 247

Fig. 2. The optimized geometries at B3LYP/Lanl2DZ (bond: A; angle: degree).

Y. Zhang et al. / Journal of Molecular Structure (Theochem) 623 (2003) 245–251248

via the transition state TS5. The high barrier of this

reaction by 62.2 kJ/mol implies that the intermediate

Min4 is formed more difficultly from the Min3 than

from the Min1, because the activation energy of the

later reaction Min1 ! Min3 is only 12.8 kJ/mol. It

suggests that the Min1 may be the predominant

configuration to form the trans-dimer intermediate

Min4, although the Min2 is more stable than the

Min1.

The third approach is both NO molecules inter-

acted with Cu by O atoms to form the intermediate

Min7. The exothermicity of this reaction is

138.8 kJ/mol. Therefore, the Min7 is the slightly

unstable intermediate in the three approaches, but it

can further form the more stable structure Min8. In

this process, the N–N bond length is shortened, while

the N–O bond is elongated from 1.269 A in the Min7

to 1.348 A in the Min8 (see Fig. 2). The Min6 is

formed very difficultly from the Min8 via the

transition state TS6 (by breaking the Cu–O bond in

the Min8), because the barrier height of this reaction

comes to 125.4 kJ/mol. It indicates that the inter-

mediate Min8 is highly stabilized both thermodyna-

mically and kinetically. Hence, the formation of Min6

from the intermediate Min8 is very unlikely in

the presence of the Cu atom at low temperatures.

However, the intermediate Min8 can directly decom-

pose to CuO þ N2O via TS7. The decomposition

barrier of only 90.7 kJ/mol indicates that the Min8

inclines to directly decompose to N2O, and not to

decompose via the intermediate Min6 (see Fig. 1). It

implies that the N–O bond is more easily dissociated

than the Cu–O bond in the Min8. But the high

exothermicity (48.5 kJ/mol) of the reverse reaction

and the low barrier energy of 42.2 kJ/mol imply that

the reaction of CuO þ N2O ! Min8 is very likely at

low temperature. The structure analogical with the

Min8 was assumed as a crude model of an on-top site

adsorption for NO on the MgO surface [28,29].

Fig. 1 also shows that the Min4 and the Min8 are

the most stable intermediates in the process of NO

interacting with Cu. It suggests that the trans-(NO)2

dimer (Min4) is slightly more stable than the cis-

(NO)2 dimer (Min8) in the presence of the Cu atom,

whereas the cis-(NO)2 dimer is more stable than

trans-(NO)2 in the gas phase [30,31]. However,

according to the geometrical parameters shown in

the Fig. 2, we can see that the N–O bond (1.351 A) in

the Min8 is one of the longest bonds in all of the

minima structures, although the analogical N–O bond

Table 1

The zero-point energies, total energies (E ), relative energies (DE ) and Mulliken population (q ) at B3LYP/Lanl2DZ

Species qcu NO N0O0 E (hartrees) ZPE (hartrees) DE (kJ/mol)

qNO rNO (A) qN0O0 rN0O0 (A)

Cu þ 2NO – – – – – 2455.842135 0.008068 0.0

CuO þ N2O – – – – – 2455.903961 0.011285 2153.9

Min1 0.41 20.14 1.227 20.27 1.268 2455.900951 0.010853 2147.1

Min2 0.38 20.19 1.229 20.19 1.229 2455.904564 0.012374 2152.6

Min3 0.46 20.10 1.253 20.36 1.281 2455.910813 0.012009 170.0

Min4 0.43 20.13 1.304 20.30 1.227 2455.925467 0.012591 2206.9

Min5 0.43 20.25 1.315 20.17 1.280 2455.905044 0.011955 2155.0

Min6 0.46 20.33 1.381 20.13 1.284 2455.902118 0.012228 2146.6

Min7 0.52 20.26 1.269 20.26 1.269 2455.896517 0.009587 2138.8

Min8 0.54 20.27 1.348 20.27 1.348 2455.922598 0.011442 2202.4

TS1 0.39 20.13 1.214 20.26 1.266 2455.895186 0.009965 2134.3

TS2 0.32 20.16 1.223 20.16 1.223 2455.892263 0.011080 2123.7

TS3 0.41 20.21 1.253 20.21 1.253 2455.906084 0.011386 2159.2

TS4 0.41 20.33 1.860 20.08 1.254 2455.876782 0.010146 285.5

TS5 0.54 20.17 1.290 20.37 1.300 2455.886087 0.010955 2107.8

TS6 0.42 20.36 1.348 20.06 1.240 2455.873367 0.009974 277.0

TS7 0.49 20.36 2.046 20.13 1.293 2455.887501 0.010872 2111.7

Y. Zhang et al. / Journal of Molecular Structure (Theochem) 623 (2003) 245–251 249

length (connected to the Cu atom) is 1.381 A in the

Min6. Consequently, these N–O bonds can be easily

dissociated. Fig. 2 also shows that the N–O bonds

connected to Cu with O atom are weaker than with N

atom. All of these imply that when a NO molecule or a

(NO)2 dimer interacting with Cu via O atom, the N–O

bond is more easy dissociation than via N atom. It is

very agreement with our previous work [32].

According to the Blyholder model [33], bonding of

a NO molecule to a transition metal species can be

divided into s donation and p back donation. The

amount of electronic charge transferred from the

adsorbate to the substrate and vice verse can roughly

be estimated by a Mulliken population analysis. Table

1 lists the Mulliken populations in every configuration.

In the three approaches, the data of qCu are 0.37, 0.41

and 0.52e for the intermediates Min2, Min1 and Min7,

respectively. It suggests that the amount of electronic

charge transferred from the Cu to the NO molecule in

the models of NO interacting with Cu via nitrogen is

less than via oxygen. Therefore, the N–O bond lengths

in the configurations of NO interacting with Cu via

nitrogen are shorter than via oxygen. This is because

that there is one electron in the NO 2pp anti-molecular

orbits. Hence, the NO bond will be weakened and the

N–O distance will be elongated when the electronic

charge transferred from the Cu to the NO molecule.

The results are also supported by the structural

parameters in Fig. 2. On the other hand, according to

the relative energies, we can see that the configurations

of NO interacting with Cu via nitrogen are more stable

than via oxygen. It indicates NO molecules are in favor

of interacting with Cu atom via nitrogen. In the dimer

configurations, however, the relationship between the

charges transferred from Cu and the N–O bond lengths

is not ascertainment not only due to the complex

interactions between Cu and (NO)2 dimers, but also

due to the formation of N–N0 bond between the two

NO molecules in these configurations (see Table 1).

The Mulliken populations also show that the bonds

between the NO molecule or (NO)2 dimer and Cu are of

high ionicity [19,32].

In order to comprehend the bond property between

the NO molecules or dimers and the Cu atom, we

calculate the HOMO and LUMO orbitals of these

intermediates and TS. Fig. 3 shows some relevant

molecular orbitals. The HOMO character of the Min3

and Min4 are very similar. In the intermediates Min3

and Min4, the HOMOs, which are primarily con-

tributed by the (NO)2 dimers, are the p-orbital with

the N–O anti-bonding and N–N bonding character.

And according to the energies and the structural

parameters, we know that the N–N bond in the

intermediate Min4 is stronger than in the Min3.

Hence, the Min4 is the important intermediate for the

NO decomposition to N2O. On the other hand, the

HOMO of the Min7 shows that both the Cu–O bond

and the N–O bonds are of anti-bonding character, and

that the N–N bond does not formed between the two

NO molecules, which connect to the copper atom with

O in a plane. Although the N–O bond in the HOMO

of Min8 is still of anti-bonding character like that in

the Min7, the Cu–O and N–N in the HOMO of Min8

are of some bonding character due to the tortuosity of

ONN0O0. Hence, the HOMO of Min8 shows that the

Cu–O bonds are more difficultly dissociated than the

N–O bonds in the intermediate Min8.

4. Conclusions

We have shown that the NO molecules decompose

to N2O via the dimer intermediates in the presence of

Fig. 3. The HOMO orbital pictures of some intermediates at

B3LYP/Lanl2DZ geometries.

Y. Zhang et al. / Journal of Molecular Structure (Theochem) 623 (2003) 245–251250

Cu at B3LYP/Lanl2DZ level. Three modes of two NO

molecules interacting with Cu have been mentioned.

The configuration of the NO molecules approach-

ing to Cu only with N is slightly more stable than only

with O or with N and O. The intermediates Min4 and

Min8, which are representative of trans- and cis-

dimer interacting with Cu, respectively, are the most

stable configurations. And the relative energies show

that the trans-dimer is slightly more stable than the

cis-dimer in the presence of Cu, while the cis-dimer is

more stable in the gas phase. The cis-intermediate

Min8 not only can directly decompose to the N2O, for

which the activation energy is 90.7 kJ/mol, but also

can dissociate the Cu–O bond to form the intermedi-

ate Min6 by the energy barrier of 125.4 kJ/mol. But

the high endothermicity and the activation energy of

the Min8 decomposition show that the Min8 is highly

stabilized both thermodynamically and kinetically at

low temperature. On the other hand, the energy barrier

of the trans-intermediate Min6 decomposition to N2O

is only 61.1 kJ/mol. Consequently the NO dimer is in

favor of the decomposition to N2O via the trans-

intermediates in the presence of Cu at low

temperature.

Acknowledgements

This research is supported by the Special Funds

for Major State Basis Research Projects (No.

G1999022209).

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