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Applied Surface Science 254 (2008) 2436–2440

The effect of NO/O2 ratio in NO–CO–O2 reaction on a

catalytic surface: A computer simulation study

Waqar Ahmad a,b,*, E.V. Albano a

a Instituto de Investigaciones Fisicquimicas Teoricas y Aplicadas (INIFTA), Universidad Nacional de La Plata,

Sucursal 4, Casilla de Correo 16, (1900) La Plata, Argentinab Physics Division, PINSTECH, P.O. Nilore, Islamabad, Pakistan

Received 24 August 2007; received in revised form 19 September 2007; accepted 20 September 2007

Available online 29 September 2007

Abstract

The interaction among the reacting species in the NO–CO–O2 reaction on a metal catalytic surface that proceeds according to the Langmuir–

Hinshelwood mechanism is studied by means of Monte Carlo simulation. The study of this three-component system is essential for the

understanding of the influence of NO/O2 ratio on the catalytic reduction of NO into N & O and oxidation of CO to CO2. It is found that this complex

system, which has not been studied on these lines before, exhibits irreversible phase transitions between active states with sustained reaction and

poisoned states with the catalytic surface fully covered by the reactants. The phase diagrams of the surface coverage with CO, N or O and the steady

state production of CO2 are evaluated as a function of the partial pressure of CO in the gas phase. From this study, it is observed that with the

addition of NO in the CO–O2 reaction, the critical points in the phase diagram move towards lower values of CO partial pressure but the width of

reaction window remains almost the same. However, the maximum production rate of CO2 decreases continuously. On the other hand, the addition

of O2 in the NO–CO reaction shifts the critical points towards higher values of CO pressure. Moreover, the width of reaction window as well as the

production rate of CO2 increases with the increase in O2 concentration.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Catalytic surfaces; Adsorption; Phase transitions; Surface coverage; Monte Carlo simulation

1. Introduction

The importance of the CO–O2 and the NO–CO reactions is

mainly due to serious concern about the pollution of atmo-

sphere. The problem carries utmost attention in the developing

countries where the pollution control regulations are not very

effective. The reduction of NO on certain metal catalysts like

Pt, Pd and Rh plays a key role in the efforts to improve air

quality. It has gained considerable attention in automotive

exhaust emission. For this reason, the automobiles in the

industrial countries are bound to have catalytic reactors in their

exhaust systems, which greatly reduce NO emission. Similarly,

the oxidation of CO with O2 on these catalytic surfaces leads to

clean and safe environmental conditions.

* Corresponding author at: Physics Division, PINSTECH, P.O. Nilore, Isla-

mabad, Pakistan. Tel.: +92 51 2207722; fax: +92 51 9290275.

E-mail address: waqar@pinstech.org.pk (W. Ahmad).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.09.060

The CO–O2 and the NO–CO reactions have been extensively

studied experimentally, theoretically as well as by computer

simulation techniques [1–8]. Ziff et al. [9] introduced a lattice-

gas model commonly known as the ZGB model for the

heterogeneously catalyzed CO–O2 reaction in order to study

reactive processes using the Monte Carlo simulation. Their

model is based upon Langmuir–Hinshelwood (LH) mechanism

in which the reacting species are adsorbed on a surface before

the reaction takes place. The ZGB model exhibits two

irreversible phase transitions: one of 2nd order at low partial

pressure of CO and the other of 1st order at higher pressure. For

intermediate pressures between the two critical points, y1 and

y2, a steady reactive state (SRS) with continuous production of

CO2 is observed. Here y1 is the point where the system enters

from a saturated (poisoned) state with the surface fully covered

by O atoms into SRS and y2 is the point where it leaves SRS and

enters into another saturated state with the surface fully covered

by CO molecules.

On similar lines, Yaldram and Khan [10] presented a model

for the NO–CO reaction on a square lattice (YK model). They

W. Ahmad, E.V. Albano / Applied Surface Science 254 (2008) 2436–2440 2437

observe a saturated state for all values of CO feed concentration

(partial pressure). Under these conditions, the LH mechanism

in which, NO is adsorbed on the surface in dissociated form,

gained immense interest at the operating temperature of

catalytic converters (�700 K). So this simple mechanism has

been used by a number of authors to study different aspects of

this reaction scheme [11–13].

In YK model, it is assumed that NO always decomposes into

N & O before adsorption on the surface i.e. two adjacent vacant

sites are needed for chemisorption. It is equally likely that one

of the sites is occupied by an N atom and other by an O atom.

This fact inevitably leads to poison the surface by the process of

‘checkerboarding’ of N & O atoms sitting one after the other

[14]. Meng et al. [15] have broken this checkerboarding process

by introducing a hypothetical reaction between CO and N

through the formation of CON radical. Khan et al. [16] have

controlled the checkerboarding by introducing the diffusion of

N atoms on the surface and obtained an SRS for CO2 and N2

gases.

The two-component reaction models (ZGB and YK) have

been studied separately and little efforts have been made to

investigate the three-component system due to complexity.

Although in real practice, the three gases are present all

together in automobile exhausts. So our model is based upon

both the CO–O2 reaction of ZGB model and the NO–CO

reaction of YK model.

The objective of this paper is to explore the effects of NO/O2

ratio on the phase diagram of the NO–CO–O2 reaction as well

as on the production rate of CO2 using the LH mechanism.

Similarly, the diffusion of the species on the surface has also

been taken into account. The paper is structured as follows: in

the next section, the reaction model and the simulation

procedures are elaborated. The results are presented and

discussed in Section 3. Finally, the conclusions are drawn in

Section 4.

2. Description of model and simulation procedure

The LH mechanism for the NO–CO–O2 reaction can be

written in the form of following steps:

COðgÞ þ S ! COc (1)

O2ðgÞ þ 2S ! 2Oc (2)

NOðgÞ þ 2S ! NcþOc (3)

COcþOc ! CO2ðgÞ þ 2S (4)

NcþNc ! N2ðgÞ þ 2S (5)

where ‘S’ is an empty surface site, (g) refer to the gas phase and

Xc represents chemisorbed X-species on the surface.

The simulation starts by assuming an empty surface in

contact with an infinite reservoir comprising CO, NO and O2

gas molecules with partial pressures yCO, yNO and yO2,

respectively. The surface is represented by means of a square

lattice of size L � L. Most simulations are performed by taking

L = 128 because it is observed that an increase in the lattice size

only causes slight changes of the critical pressures but the

overall qualitative nature of the phase diagrams and the

production rates are not affected [17]. During the simulation

process, periodic boundary conditions are applied in order to

avoid edge effects [18]. Since one has a three-component

system, it is convenient to keep the concentration of one of the

three reactants fixed while the ratio of the remaining two is

varied such that yCO þ yNO þ yO2¼ 1.

The steps involved in the simulation are as follows:

a. F

irst, some desired concentration of NO (yNO) is fixed and

the concentration of CO (yCO) is varied such that

yO2¼ 1� yNO � yCO.

b. A

surface site is picked up at random. If that site is occupied

by O or CO, the trial ends and a new cycle starts by selecting

another site at random. Otherwise, if the site is occupied by

an N atom, one proceeds according to (f) as described below.

On the other hand, if the site is empty, then CO, O2 or NO is

selected for chemisorption with probability yCO, yO2and

yNO, respectively.

c. I

f CO is selected, then it is adsorbed on the chosen vacant site

as in step (1). The nearest neighbors of this adsorbed COc

molecule are scanned for the presence of an adsorbed Oc

atom in order to complete reaction step (4). If an Oc atom is

found, then CO2 is formed which immediately desorbs from

the surface and trial ends.

d. I

f an O2 molecule is selected, then two neighboring sites are

required to be vacant for its adsorption in atomic form as in

reaction step (2). So if a randomly selected adjacent site of

the already selected vacant site is empty, then the two O

atoms adsorb on these sites. After adsorption, the first nearest

neighboring sites of the two adsorbed Oc atoms are scanned

for the presence of CO to produce CO2. If no reaction takes

place, the Oc atoms keep sitting on the selected sites and the

next Monte Carlo cycle starts again.

e. I

f the selected molecule is NO, then the four nearest

neighbors of the selected site are scanned at random. If all the

sites are occupied, the trial ends. In case, an adjacent site is

vacant, NO dissociates into N & O which occupy the vacant

sites with equal probability as in step (3). The nearest

neighboring sites of adsorbed N and O atoms are then

examined for the presence of N and CO to complete reaction

steps (4) and (5), respectively.

f. S

ince SRS in the YK model on a square lattice is only

obtained by considering the diffusion of N atoms on the

surface [19], one has to incorporate this mechanism. For

this purpose, if the selected site is occupied by an N atom,

its four nearest neighbors are scanned for the presence of a

vacancy. The trial ends if a vacancy is not found in the

neighborhood. Otherwise, N moves to that vacant site,

vacating its original site. After diffusion, the possibility of

reaction steps (3) and (5) is examined in a similar way as

discussed above. In this simulation, the diffusion prob-

ability is set to unity. It has been observed that if N

diffusion is reduced to smaller values, then only the

production rate decreases and the width of reaction

window squeezes but the qualitative nature of the phase

Fig. 2. The phase diagram showing the surface coverage and production rate vs.

yCO in the YK model with the diffusion of N atoms on the surface. Here N is

represented by a half-filled circle. All other symbols are the same as in Fig. 1.

W. Ahmad, E.V. Albano / Applied Surface Science 254 (2008) 2436–24402438

diagram remains the same and the general trend of the

coverages on the surface persists [16].

To find the critical points, a run up to 50,000 Monte Carlo

(MC) cycles is carried out. If the run completes 50,000 cycles

without the lattice getting poisoned, the particular point is

considered to be within SRS. Then, in order to get the surface

coverages and production rates corresponding to SRS, the

initial 20,000 MC cycles are disregarded to allow the system to

reach a non-equilibrium stationary state. The relevant

observables are then computed by taking averages over the

subsequent 30,000 cycles [20]. The values of coverage/

production rate are obtained after 10 MC cycles to avoid

correlations, so that the final values are an average of 3000

configurations.

3. Results and discussion

The phase diagrams corresponding to both the CO–O2 (i.e.

the ZGB model [9]) and the NO–CO (i.e. the YK model [10])

reactions using the LH mechanism on a square lattice are well

known. For the CO–O2 reaction, an SRS bounded by two

irreversible phase transitions is observed with yCO close to the

first critical point at y1 = 0.389 and the second critical point at

y2 = 0.525, respectively as shown in Fig. 1. For the NO–CO

reaction, this SRS is only obtained by considering the diffusion

of N atoms on the surface. The two critical points are found

with feed concentration of CO close to y1 = 0.157 and

y2 = 0.281, respectively (Fig. 2). In both the cases, the phase

transition at y1 is of second-order (continuous) while the

transition at y2 is of first-order (discontinuous). As expected,

our three-component reaction system (NO–CO–O2) depicts

that behavior at the extreme ends. In fact, the system becomes

the YK model when the ratio NO/O2 is infinite and it becomes

the ZGB model whenever this ratio is zero.

In order to understand the influence of NO on the reaction

scheme based on the ZGB model, we recorded phase diagrams

Fig. 1. The phase diagram showing the surface coverage with O (open square)

and CO (open down triangle) as well as the production of CO2 (solid up triangle)

vs. yCO (partial pressure of CO) in the ZGB model.

for fixed values of yNO by varying yCO. A phase diagram

showing the coverage of adsorbed species and the rate of CO2

production at yNO = 0.2 is shown in Fig. 3. It is observed that

due to the presence of NO, the critical points shift towards

lower values of yCO. Furthermore, in contrast to the pure ZGB

model which exhibits unique poisoned states with one type of

atoms/molecules (O for yCO < y1 and CO for yCO > y2, as

shown in Fig. 1), here the poisoned states are non-unique. In

fact, one has mixtures of N + O and N + CO for yCO < y1 and

yCO > y2, respectively (Fig. 3). Nevertheless, the coverage of N

remains almost constant and it is independent of yCO.

The coverage of surface with reacting species at different

values of yNO, just before the start of SRS is shown in Fig. 4. It

can be seen that oxygen atoms are gradually replaced by N

atoms. Similarly, when O2 is added to the NO–CO reaction, the

Fig. 3. The phase diagram showing the surface coverage and the rate of CO2

production in the NO–CO–O2 reaction model at yNO = 0.2.

Fig. 4. The phase diagram of surface coverage vs. yNO at the critical point y1. Fig. 6. The critical points and reaction window width (w) vs. yNO.

W. Ahmad, E.V. Albano / Applied Surface Science 254 (2008) 2436–2440 2439

surface coverage before the start of SRS is shown in Fig. 5. Here

N atoms are replaced by O atoms with the increase of yO2.

In the first case, when NO is added into the CO–O2 reaction,

the dependence of the critical points y1 and y2 as well as the

width of reaction window (w) on the partial pressure of NO is

shown in Fig. 6. The monotonic shift of the critical points due to

the influence of NO is evident. However, w remains almost

unchanged till yNO is close to 0.72. Remarkably, at this point,

the reaction window becomes abruptly closed. This behavior is

consistent with the trend of CO2 production which decreases

very slightly when yNO is increased but abruptly stops when the

window width squeezes. This may be due to the fact that at this

point the role of O2 is diminished and the pressure of NO

becomes so high that the system switches over from ZGB to YK

model.

In the second case when O2 is added into the NO–CO

reaction, an abrupt transition is found at yO2¼ 0:475, after

which the surface is mostly covered by O atoms as in the ZGB

model. Moreover, the critical points move towards higher

Fig. 5. The phase diagram of surface coverage vs. yO2at the critical point y1.

values of yCO as in Fig. 7. In this case w increases from 0.13 to

0.21 and then closes at yO2¼ 0:475.

The production rates of CO2 versus CO feed concentration are

drawn for different values of yNO and yO2that are shown in

Fig. 8(a) and (b) respectively. Almost a linear dependence of the

maximum value of CO2 production is observed in both the cases

as the values of yNO and yO2are varied. It can be seen that the

maximum production decreases continuously with the addition

of NO in ZGB model. This is because after dissociation of NO

into N and O, nitrogen atoms start blocking the surface sites as

there is no reaction of N with CO. Therefore, it hinders the

reaction of CO with O and the production of CO2 is reduced.

Contrary to that, when O2 is added in the NO–CO reaction,

not only that w increases but the maximum production of CO2

also increases. It can be explained on the basis that enough

oxygen becomes available to increase CO2 production.

Therefore, side by side with the development of good catalysts

to increase the reaction rate in automotive exhausts, some

mechanism should also be developed to ensure sufficient

Fig. 7. The critical points and reaction window width (w) vs. yO2.

Fig. 8. (a) and (b) The production rate of CO2 vs. yCO for different values of yNO

and yO2, respectively.

W. Ahmad, E.V. Albano / Applied Surface Science 254 (2008) 2436–24402440

quantity of oxygen to enhance the burning of CO and NO

before escaping into the atmosphere.

4. Conclusion

The addition of NO in the CO–O2 and O2 in the NO–CO

reactions to study the three-component (NO–CO–O2) reaction

has produced some interesting results. The phase diagrams of

surface coverage with reacting species and the production of

CO2 reveals that with the addition of NO in the ZGB model, the

critical points shift towards lower values of yCO. Moreover, the

width of reaction window remains almost the same and

ultimately closes at yNO = 0.72 which corresponds to the

transformation of the ZGB model into the YK model. In the

other case, the addition of O2 in the YK model results in the

increase of CO2 production. The critical points shift towards

higher values of yCO and the window width increases.

Therefore, the addition of a small amount of oxygen may

leads to more burning of CO in the exhaust chamber of

automobiles which eventually results in clean environmental

conditions.

Acknowledgments

The authors are grateful to the Third World Academy of

Sciences, Trieste, Italy and the CONICET, Argentina for their

financial assistance under TWAS-UNESCO Associateship

Programme to carry out this research work at INIFTA, UNLP,

Argentina. The help and support of the Director INIFTA is

highly acknowledged especially for the access to computational

facilities on fast machines.

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