Carbon 42 (2004) 1507–1515

www.elsevier.com/locate/carbon

Experimental characterization and molecular simulationof nitrogen complexes formed upon NO–char reaction

at 270 �C in the presence of H2O and O2

Pilar Garc�ıa a, Juan F. Espinal a, Concepci�on Salinas Mart�ınez de Lecea b,*,Fanor Mondrag�on a,*

a Institute of Chemistry, University of Antioquia, A.A. 1226 Medell�ın, Colombiab Department of Inorganic Chemistry, University of Alicante, A.A. 99-E-03080 Alicante, Spain

Received 27 September 2003; accepted 26 January 2004

Available online 16 March 2004

Abstract

Surface nitrogen complexes formation upon reaction of coal char with NO at 270 �C in the presence or absence of O2 and/or H2O

was studied by XPS. Complexes such as pyrrolic-N type, pyridine-N-oxide, pyridinic-N and –NO2 were observed. The major

increment was in pyridinic-N. Water had a partial inhibition effect while oxygen promotes the formation of the nitrogen complexes

probably due to char gasification. The gasification level in mixtures with O2 varied according to the presence or not of H2O. TPD

was used to quantify reversible NO adsorption and oxygen surface groups formed on the char during reaction. Molecular modeling

was applied to determine the thermodynamics of the reactions. Mechanisms are proposed to explain the formation of pyridinic and

–NO2 complexes at low temperatures.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: C. Chemisorption, Computational chemistry, X-ray photoelectron spectroscopy; D. Catalytic properties, Surface oxygen complexes

1. Introduction

Acid rain, the depletion of stratospheric ozone layer

and photochemical smog caused by nitrogen oxidesemissions have resulted in strict regulations to control

the release of these polluting gases. Nitrogen oxides are

formed in several industrial processes such as power

generation units, waste incinerators, nitric acid manu-

facture as well as in mobile sources. Improvement of

these industrial processes and engine designs are neces-

sary to mitigate emissions of nitrogen oxides [1] like

nitric oxide, one of the polluting gases produced inconventional fossil fuel combustion systems.

Many catalytic processes have been extensively stud-

ied to reduce nitric oxide to N2 [2,3]. Moreover, NO

reduction using carbon materials has been widely

investigated with important findings. At temperatures

*Corresponding authors. Tel.: +57-421-056-59; fax: +57-426-382-

82.

E-mail addresses: c.salinas@ua.es (C. Salinas Mart�ınez de Lecea),

fmondra@catios.udea.edu.co (F. Mondrag�on).

0008-6223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2004.01.065

higher than 400 �C, NO is chemisorbed on the solid

carbon surface forming C(N) and C(O) complexes. Then,

the C(N) complexes react with NO forming N2 [4–6].

Carbon gasification at these temperatures creates newactive sites which promote NO chemisorption. Since

molecular oxygen increases the carbon gasification, its

presence enhances NO reduction [7,8]. The role of sur-

face nitrogen complexes in NO reduction has been

investigated with the aid of computational chemistry [9–

11]. However, some of the complexes suggested by this

method are different from those identified by XPS

[1,7,12,13]. Fig. 1 presents the most important nitrogencomplexes observed on char surfaces after reaction with

NO at temperatures higher than 400 �C [7]. They are (1)

pyridinic complex (N-6, Fig. 1(A)) where the N1s elec-

tron binding energy used for its characterization by XPS

is 398.7 ± 0.3 eV, (2) pyrrolic complexes (N-5, Fig. 1(B)),

binding energy at 400.2 ± 0.3 eV [7,13], (3) pyridone (Fig.

1(C)) and nitroso complexes (–NO, Fig. 1(D)) which

have XPS binding energies equal to that of N-5 [13–15],(4) complexes found in minor quantities such as qua-

ternary-N (N-Q) in the ‘‘centre’’ position at 399.2 eV

H

ON

O-

HO

N+

O

N

NN

NN

NN N

OH

O

(B)(C)(A) N-6

(D)

N-X

+

(H)

N-Q(F)(E) (G)

-NO2(I)

N-5

Fig. 1. Schematic representation of the most commonly found carbon

surface nitrogen complexes.

1508 P. Garc�ıa et al. / Carbon 42 (2004) 1507–1515

(Fig. 1(E)) and in the ‘‘valley’’ position at 401.7 eV (Fig.

1(F)) [13]. Another type of N-Q complexes assigned to

the signal at 401.7 eV are those of pyridinic-N with a

positive formal charge due to strong interaction with

oxygen functionalities [14–17] or with the hydrogen of

OH groups (Fig. 1(G)) [7,13,15], (5) pyridinic-N-oxide

complexes at 403.1 ± 0.3 eV (N-X, Fig. 1(H)), and lastly,(6) nitro type complexes (–NO2), identified at 406.1 eV

(Fig. 1(I)) [7,13,17].

In the NO–carbon reaction, the identification of

intermediates is an important step towards under-

standing the reduction mechanism. However, this is a

difficult task to carry out at high temperatures due to the

fast transformations that the intermediate structures can

undergo. To overcome this problem, reactions at lowtemperatures can be used to obtain information on some

of these complexes. By doing this, it is assumed that

some of the active sites capable of forming nitrogen

complexes at high temperatures are similar to those

found at low temperatures, as has been proposed by

Suuberg et al. [8,18]. In a recent study on the reaction of

NO–char at 100 �C [19], it was observed that –NO2 and

N-6 surface complexes were the ones predominantlyformed.

Although in the reduction of NO by carbon, the

formation of surface nitrogen complexes could be af-

fected by all of the other components in the combustion

effluents, our interest in the present research was to

study only the effects of O2 and H2O on the interaction

of NO with the carbon surface. It is known that at high

temperatures the O2 enhances the rate of NO reductionby the char and that the N-complexes formed on the

solid surface (mainly pyridinic and pyrrolic) are similar

to those formed in an oxygen free atmosphere [7,12]. In

addition, the presence of O2 decreases the NO reduction

temperature [12]. Studies on the effect of water on the

reaction system are rather scarce [8,20,21] and therefore

there is little information on how water interferes or

promotes the formation of nitrogen complexes.

Our objective in this research was to investigate the

reaction of NO with carbon surfaces at 270 �C in

the presence of water and molecular oxygen. After the

reaction, the surface was characterized by XPS and bytemperature programmed desorption of the complexes.

A complementary molecular modeling study was carried

out in order to obtain thermodynamic data and to ex-

plain the formation mechanisms of the N-complexes

experimentally observed.

2. Experimental

2.1. Reaction conditions and sample characterization by

XPS

The char used in the experiments described here wasprepared from a subbituminous coal (mesh 100/200)

collected from the Amag�a mine in Colombia. The coal

sample was pyrolyzed at 700 �C for 2 h under N2

atmosphere. The NO reduction reaction was carried out

in a tubular quartz reactor using 0.4 g of char. The

temperature was monitored with the aid of a thermo-

couple attached to the reactor external wall, completely

imbibed in the sample bed. Before each experiment, thesample was heat treated at 700 �C under a N2 flow of

620 mL/min for 30 min to partially clean the surface

oxides formed by exposure of the char to air during

handling. The reactor was then cooled down to the

reaction temperature, 270 �C; and the N2 was switched

to the reaction mixture: NO, NO/H2O, NO/O2 or NO/

H2O/O2, maintaining the same flow of 620 mL/min. The

gas concentrations used in the experiments were: NO,2000 ppm; O2, 5%; balance N2. For the reactions with

H2O, the gas mixture was passed through a water bub-

bler held at 60 �C (PVH2O ¼ 150 mmHg). The conduc-

tion line from the bubbler until the reactor entrance was

kept at 80 �C. At the reactor exit, a vapor trap was

placed to collect the nitric acid formed during the

reaction, the concentration was followed by the varia-

tion of the pH of the solution thus formed. After leavingthis trap, the product gases passed through a second

cold trap kept at )5 �C to condense water which could

interfere with the gas detectors, Fisher–Rosemoun

NDIR/UV gas analysers, BINOS-100 for CO and CO2,

BINOS-1004 for NO and NO2 and BINOS 1001 for O2.

After 2 h of reaction, the reactor was cooled under N2

flow to room temperature. After this, the quartz tube

with the sample under N2 was taken to the XPS foranalysis. The apparatus used was a VG-Microtech

Multilab Electron spectrometer with a Mg Ka (15 kV,

20 mA). The C 1s 284.6 eV value was taken as reference

for the binding energy.

In order to obtain a reference point for comparison of

the results, the unreacted char was heat treated at 700 �Cfor 30 min in N2 and then the surface complexes were

P. Garc�ıa et al. / Carbon 42 (2004) 1507–1515 1509

characterized by XPS as described above. The formation

of oxygen complexes was also followed by temperature

programmed desorption (TPD). For this purpose, each

reaction was repeated and then the sample thus pro-duced was heat treated under N2 atmosphere at 10�/min

up to 900 �C. The evolved gases (NO, NO2, CO and

CO2) were monitored with the gas detectors.

2.2. Computational details

A molecular modeling study of the reactions which

could explain the N-complexes formation under our

reaction conditions was carried out. Calculations were

done in order to obtain thermodynamic information

that fits the experimental data. A finite cluster of a single

graphene layer, shown in Fig. 2, represented our charmodel. The edge atoms on the upper side of the model

are unsaturated to simulate active sites while the other

edge carbon atoms are terminated with hydrogen atoms.

It has been shown that the use of H to terminate the

boundaries of finite graphite models is a good choice

[22]. A five six-member rings model was selected since

previous research has shown that carbon chemisorption

properties depended more strongly on the local structureof the active site than on the size of the graphene layer

used for the simulation [22–24]. In order to study the

proposed reactions, each structure of the reactants,

intermediates and products was fully optimized. All

models were optimized in their electronic ground state.

This was achieved by performing single-point energy

calculations at the same level of theory for several

electronic states of a given species; the ground state ta-ken as the one with the lowest energy. Furthermore,

frequency calculations were done in order to confirm

their stability. All calculations (energies, optimizations,

and frequencies) were carried out at B3LYP density

functional theory level (DFT), using the 6-31G(d) basis

set. In a previous study, it was shown that with this level

of theory spin contamination is small for carbonaceous

models [25]. All calculations were done using theGaussian 98 program [26].

H

H

H

HH

H

H

H

Fig. 2. Char model used to simulate the NO adsorption. Carbon

atoms without hydrogen represent active sites.

3. Results and discussion

The dissociative chemisorption of NO is an important

step during its reduction to N2 on a carbon surface [4–6].Therefore, the identification of nitrogen complexes

formed during the reaction can help understand the

reduction mechanism. The amount of active sites capa-

ble of forming N-complexes can be increased if the

gasification of the solid takes place [4,8,27–29]. If gasi-

fication does not occur, some of the active sites remain

blocked by oxygen complexes [8,30]. In order to select

the reaction temperature, preliminary experiments underisothermal conditions between 200 and 400 �C were

carried out. At 270 �C we observed gasification of the

char during reaction with NO/O2. However, we did not

observe any gasification in the reaction with NO alone.

Such a difference was the principal criterion to select 270

�C as the reaction temperature.

Cleaning of the surface by the in situ heat treatment

of the sample generates active sites that facilitate theformation of surface complexes during the NO–char

reaction. This procedure was particularly important in

the case of the reactions with NO and NO/H2O where

no gasification takes place at 270 �C.

3.1. Nitrogen complexes formation

Fig. 3 shows the XPS N1s spectra of the char after

partial cleaning of surface oxides at 700 �C in N2

atmosphere (Fig. 3(A)), and after reaction with NO

(Fig. 3(B)), NO/O2 (Fig. 3(C)), NO/H2O (Fig. 3(D)),

and NO/H2O/O2 (Fig. 3(E)). The y-axis is the same for

all figures. In general, all the spectra show similarcomplexes but in different proportions depending on the

reaction mixture. In order to make them comparable,

the signal intensities should be normalized with regard

to the corresponding area below the C 1s signal. C 1s

spectra are not presented here. The area ratios are cor-

rected by the atomic sensitivity factor [15].

The samples were introduced into the XPS chamber

under a high flow of He to partially prevent oxidation byair. The oxygen to carbon ratio observed in the cleaned

char was O/C¼ 670 · 10�4 which is lower than the one

present in the untreated char (O/C¼ 1450 · 10�4).

Therefore, since before each reaction all the samples

were thermally treated under similar conditions, the

char identified in Fig. 3(A) was named initial char and

was taken as reference to establish the relative changes

in nitrogen and oxygen complexes.Fig. 3(B)–(E) show substantial differences when

compared to the spectrum of the initial char (Fig. 3(A)).

The effects of the reaction mixtures on the nitrogen

complexes are presented as their comparative incre-

ments in Fig. 4. The values of the y-axis (in arbitrary

units) correspond to the subtraction of the N-complex/C

ratio of the initial char from the corresponding ratio of

395 400 405 410

N-Q

N-6

N-5

N-X

E

395 400 405 410

A

N-6

N-5

N-Q

N-X

Initial char

395 400 405 410

BN-6

N-5

N-X

N-Q

-NO2

-NO2

-NO2

-NO2

N-5

395 400 405 410

C

N-6

N-XN-Q

N-5

395 400 405 410

DN-6

N-X

N-Q

N-5

Fig. 3. Deconvoluted XPS data of nitrogen complexes on char. (A) Surfaces complexes remaining after thermal treatment at 700 �C. Surfacecomplexes formed on the char surface after reaction at 270 �C with (B) NO, (C) NO/O2, (D) NO/H2O, (E) NO/H2O/O2.

N-Q -NO2 N-5 N-X N-6

(arb

itrar

y un

its)

NONO/H2ONO/O2NO/H2O/O2

[N-c

ompl

exes

/C]

∆

pyridones-No

Fig. 4. Variation of the surface nitrogen complexes obtained from the

XPS data referred to the initial char.

1510 P. Garc�ıa et al. / Carbon 42 (2004) 1507–1515

the char after reaction. The x-axis shows the complexes

which experienced changes in their concentrations.

Fig. 4 shows that after reaction the most significant

increments were in the N-6 complexes and the smallest

variations were in the N-Q complexes. The reaction

mixture had a noticeable effect on the magnitude of thechanges and this effect was observed in the nitrogen

complexes. In general, the reaction in NO/H2O gave the

smallest variations, suggesting that at 270 �C water can

compete for the same active sites on the carbon surface

where the NO can react. This can be seen as a partial

inhibition of the NO reaction as will be discussed below

in the analysis of the effect of the reaction mixture in the

formation of nitrogen complexes.–NO2 complexes formation. The presence of –NO2

complexes on the carbon surface (Fig. 3(B)–(E)) even in

experiments with mixtures without O2, indicates that

these complexes can be formed by NO chemisorption on

adjacent sites to oxygen complexes. It is important to

mention here that stable oxygen complexes like semi-

Table 1

Desorbed species up to 900 �C by TPD under N2 before reaction in

NO, NO/H2O, NO/O2 or NO/H2O/O2; heating rate: 10 �C/min

Reaction

mixture

Complexes desorbed by TPD (lmol/gC)

NO NO2 CO CO2

NO 5 – 300 165

NO/H2O – – 500 220

NO/O2 20 – 690 720

NO/H2O/O2 20 – 1150 890

P. Garc�ıa et al. / Carbon 42 (2004) 1507–1515 1511

quinone (complex a in R1) have been suggested to form

in the NO–char reaction, even at low temperatures [30–

32]. In experiments carried out at 100 �C [19], an

important increment was observed in –NO2 complexes.

These complexes, however, decreased by about 70%

after reaction at 270 �C showing that they are very la-

bile.

The thermal instability of –NO2 coincides with thedrastic reduction of reversible NO chemisorption when

the reaction temperature was increased to 270 �C. Forexample, while the reversible NO chemisorption was

between 80 and 170 lmol/gC after the reaction at 100

�C [19], the maximum amount at 270 �C was 20 lmol/

gC (see Table 1).

The formation of –NO2 (complex c) can be explained

by reaction R1. Our theoretical calculations show thatthe chemisorption of NO on an active site located be-

tween two semiquinone complexes is an exothermic

process. The energy released is sufficient to favor the

endothermic rearrangement to produce the –NO2

(complex c).

H

H

HHH

H

H

H

O O

H

H

HHH

H

H

H

O ONO

H

H

HHH

H

H

H

O ON

O

∆E = -145 kJ/mola b

NO

∆E = 98 kJ/mol c

ðR1Þ

Theoretical calculations done by Zhu et al. [9] show

similar tendency, although different energy values, per-

haps due to the use of different levels of theory in their

calculations. Zhu et al. [9] reported that a complex like c

does not lead to the cleavage of N–O bond of the

originally chemisorbed NO molecule. Rather these

complexes are able to desorb again as NO by heating, in

accordance with steps c, b and a in R1. They also foundthat the oxygen complexes, like semiquinones, make the

NO chemisorption in the Ndown position more favorable

than the molecular oxygen chemisorption on the same

structure [9]. Therefore, NO molecules can compete for

the char active sites in the presence of high concentra-

tions of molecular oxygen and therefore they can be

preferentially chemisorbed on oxidized surfaces.

Another possibility for the formation of the –NO2

complex is the reaction between NO2 formed in the

gaseous phase (reaction R2) with the carbon surface.

This reaction takes place at low temperatures and can becatalyzed by the char surface [9,33–35] as was observed

in some of our blank experiments.

2NOþO2 ! 2NO2 ðR2Þ

The reaction of NO2 with organic compounds has been

observed [36]. In the case of carbon surfaces, NO2 can

be chemisorbed forming surface complexes through the

sequence presented in R3. In fact, NO2 is reduced to NO

by carbon [37] probably after a chemisorption step.

Theoretical calculations show that reaction R3 is ther-

modynamically favorable with an exothermicity of 360kJ/mol. Experimentally, a large increase of –NO2 com-

plexes on char after reaction in mixtures of NO with

oxygen (Fig. 4) was observed. Reaction R3 shows one of

the mechanisms that could account for the –NO2 com-

plexes formation.

H

H

HHH

H

H

H

OH

H

HHH

H

H

H

ON

OO

H

H

HHH

H

H

H

O ON

O

∆E = -279 kJ/mol

NO2

c∆E = -81 kJ/mol

d e

ðR3ÞN-5, pyridones or nitro complex formation. The N1s

bonding energy at 400.2 eV can be assigned to pyrrolic

(N-5 complexes) [7,13,15], pyridones [17] and nitrocomplexes (–NO) [17,38] (Fig. 1(B)–(D)). The most

important increment in this group of complexes was

observed after the reaction with NO/H2O/O2 (Fig. 4),

where the gasification tendency was the highest,

according to the amount of CO and CO2 evolved in the

TPD (Table 1). As the O2 concentration in both NO/O2

and in NO/H2O/O2 mixtures was the same (5%), an

equal degree of char oxidation can be expected. How-ever, this was not the case as can be seen in Table 1. The

largest char oxidation observed in the latter mixture can

be associated with the formation of HNO3 (R4 reac-

tion). The low pH value of the condensed vapor at the

exit of the reactor (a pH¼ 2 for the reaction with char

sample and a pH¼ 5 for a blank experiment using only

quartz wool) was evidence for the reaction in NO/H2O/

O2. Since the formation of the acid requires the previousformation of NO2 (R2 reaction), it is assumed that in

this mixture NO2 is also present and therefore the fol-

lowing reacting systems can take place.

4NO2 þ 2H2OþO2 ! 4HNO3 ðR4Þ

2HNO3 þ C ! H2OþNOþNO2 þ CO2 ðR5Þ

1512 P. Garc�ıa et al. / Carbon 42 (2004) 1507–1515

NO2 and the nitric acid decompose on the char surface

forming oxygenated complexes accompanied by simul-

taneous gasification of the solid (R5 reaction)

[7,37,39,40]. These reactions could explain the highestincrement of oxygenated complexes after the reaction in

NO/H2O/O2 (Table 1). Such results show the synergistic

effect when NO, O2 and H2O are present in the reaction

at low temperatures (R5 reaction).

Since char gasification was more pronounced in NO/

H2O/O2, we would have an increase in the possibility of

nitrogen complexes formation [8] such as that observed

in the increments of the N-5 group and N-6 (Fig. 4). Thehighest increment in the N-5 group in this mixture could

be associated with the pyridones formation, preceded by

N-6 (the N-6 formation will be presented later). Some of

the N-6 complexes could be oxidized forming pyridones,

considering that in the strongest oxidant mixture the

H2O could contribute with OH group to its formation.

Pyridine-N-oxide formation. Results in Figs. 3 and 4

show that there is a significant increment in the pro-duction of N-X complex particularly in the case of the

reaction with NO alone. When other gases are present in

the gas mixture, the N-X is drastically reduced. In

reactions where gasification was not observed, the

nitrogen incorporation on carbon structures can take

place on the active sites generated during the thermal

treatment of the char [8,41]. The work of Montoya et al.

[42,43] has shown that the energy barrier for COdesorption from complexes like f and for NO desorption

from complexes like h (reaction R6) are similar (�350

and �330 kJ/mol, respectively). It has been proposed

that these desorptions produce a five-member ring

(structure g in R6) [42,44,45]. The five-member ring

structure thus generated is a susceptible site where the

NO molecule can be chemisorbed at low temperatures to

give the N-oxide complex. Here we propose that thepyridine-N-oxide (N-X complex) formation at 270 �Coccurs in sites left by CO desorption during the thermal

treatment at 700 �C as presented in reaction R6. Addi-

tionally, the theoretical calculations results show that

this is a process with an exothermic heat of )143 kJ/mol,

which indicates that this reaction is thermodynamically

feasible. Therefore, reaction R6 can be used to explain

the N-oxide complexes formation with and withoutgasification during the carbon–NO reaction.

H

H

HHH

H

H

H

OH

H

HHH

H

H

H

NH

H

HHH

H

H

H

O

gf∆E = 151 kJ/mol

-CO700 °C

NOT>100 °C

∆E = -143 kJ/molh

ðR6ÞThe drastic reduction in the proportion of N-X (Fig.

4) when O2 and H2O are present in the reaction mixture

suggests the existence of possible competing reactions

for the same carbon active sites that could lead to the

formation of pyridine N-oxides complexes.

NH

H

HHH

H

H

H

C OO

O

NH

H

HHH

H

H

O

∆E = -364 kJ/mol

CO2

∆E = -14 kJ/mol

i j

NH

H

HHH

H

H

H

O

OONH

H

HHH

H

H

H

O

-CO2

DE = -14 kJ/mol kl

ðR7Þ

NH

H

HHH

H

H

H

O

NH

H

HHH

H

H

H

NH

H

HHH

H

H

H

∆E = -256 kJ/mol

NO

∆E = -10.5 kJ/mol

CO+ CO2

+ NO2m

n

n

ðR9Þ

N-pyridine formation. Pyridine ring, or N-6 com-

plexes, had the most important increment after reaction

at 270 �C in mixtures with O2 and water (Fig. 4). These

were 45% and 85% higher with NO/O2 and NO/H2O/O2

respectively, than the increments observed in the same

reaction system at 100 �C [19]. In the mixtures without

oxygen, the increments in N-6 were similar to the values

observed at lower temperatures [19]. This suggests thatthe small amount of char gasification promoted by O2 at

this temperature enhances the incorporation of pyridinic

nitrogen as observed by other researchers in reactions

with NO at high temperatures [7]. Here we propose three

reaction mechanisms, R7–R9, that can lead to the for-

mation of a pyridine ring. All of them have the pyridine

N-oxide as the starting complex.

ðR8Þ

0

50

100

150

200

250

NO NO+H2O NO+O2 NO+H2O+O2

∆(O

/C)x

103

Fig. 5. Variation of the surface oxygen concentration incremental as

determined by XPS after reaction with different gas mixtures at 270 �Creferred to the initial char.

P. Garc�ıa et al. / Carbon 42 (2004) 1507–1515 1513

Reaction R7 is based on the adsorption of a CO2

molecule, produced during gasification, in two active sites

next to a pyridine-N oxide complex to produce a lactone

group. Lactones have been reported to be produced byreadsorption of CO2 (complex j) [44,46]. The CO2

adsorption energy on the nitrogen containing model used

here has an exothermicity of )364 kJ/mol. The lactone in

complex j can be converted into a new lactone (complex k)

through an exothermic process of )14 kJ/mol. This lac-

tone group should be more labile and therefore should

desorb as CO2 in a process with an exothermicity of )14kJ/mol. Step j-k-l in R7 form a possible reaction path dueto the characteristics of the N–O bond, which is a single

bond with a negative charge on the oxygen atom. The net

reaction would appear as a 1–3 migration of the oxygen

atom of the N-oxide complex. Since reaction R8 requires

the presence of CO, it should be more likely to take place

at high temperatures where gasification is relevant and

the CO concentration is large. However, in the low tem-

perature regime, for the carbon–nitric oxide reaction,small amounts of CO have been reported to desorb at

temperatures as low as 250 �C [30]. The CO thus pro-

duced can promote the production of pyridine rings from

the pyridine N-oxide complexes. Reaction R9 is the other

plausible route for the production of the pyridine ring

which will give NO2 as one of the products.

Formation of quaternary nitrogen. The variations in

N-Q complexes (Fig. 4) were too small to establish anycorrelation with the reaction gas mixture. The slight

variations could be associated with weak interactions of

some N-pyridine with oxygen complexes [7,13,16].

3.2. Variations in the surface char oxygen complexes

Fig. 5 shows the oxygen increments according to the

XPS analysis. The data are presented as changes re-ferred to the initial char. The y-axis values correspond to

the subtraction between the O/C ratio of the initial char

and the corresponding ratio of the char after the reac-

tion with the different gas mixtures. In all cases, oxygen

increments were one to two orders of magnitude higher

than those of nitrogen. Note that in the case of the

reaction with only NO, this effect is due to the fact that

the NO molecule is dissociatively chemisorbed formingN2 and/or N2O. A similar effect was observed at 100 �C[19]. The scheme presented in R10 has been proposed to

explain the N2 formation [10].

H

H

H

HH

H

H

NO

H

H

H

HH

H

H

NO ON

H

H

H

HH

H

H

O O

NO - N2

o p q

ðR10Þ

The N–O bond of the first chemisorbed molecule splits

and facilitates the chemisorption of other NO molecule

forming the complex p. Then, the complex p dissociates

to give N2 [10]. An alternative mechanism was proposed

by Teng and Suuberg [30] to explain the large incre-

ments of C(O) over C(N) complexes and the evolution

of N2 at low temperatures. This mechanism involves theformation of NO dimers, complex r. Reaction R11 im-

plies the simultaneous formation of molecular nitrogen

and formation of oxygen bonds on the carbon surface

[30,31].

OO N N OO N2

r s

ðR11Þ

The population of CO and CO2 precursors on the car-

bon surface after the NO/H2O reaction is higher than

that obtained in the reaction with just NO as can be seen

in the TPD data (Table 1). This result implies that the

H2O molecules may be able to oxidize a wider number

of active sites. Nevertheless, the XPS analyses show

oxygen increments higher after reaction in NO than inthe NO/H2O reaction (Fig. 5). The apparent discrepancy

between these results is related to the characteristics of

these analyses. The XPS can analyze only the most

external surface while the TPD analysis involves data

from the external surface as well as from the inner pore

surface. As the largest area is inside the pores, the TPD

data suggest that water might be reacting preferentially

inside the pore network of the char particle. ReactionR12 [46] can explain the fact that the oxygen increment

observed by XPS (Fig. 5) in the reaction with NO/H2O

was lower than that in the NO reaction. The decom-

position of the chemisorbed H2O can neutralize active

sites with hydrogen atoms. Such an effect partly explains

the inhibition observed in the nitrogen complexes

increments after reaction in NO/H2O (Fig. 4).

1514 P. Garc�ıa et al. / Carbon 42 (2004) 1507–1515

H

H

H

HH

H

H

O

H

H OH

H

H

H

HH

H

H

H O

H

H

H

HH

H

H

H H

t u v

ðR12ÞThe surface oxygen increase observed by XPS after the

reaction with NO/O2 (Fig. 5) is an unexpected result for

which we do not have a clear explanation. In the inves-tigation of the nature of carbon-oxygen complexes by

Hall et al. [47] using different oxidizing agents, it was

found that the O2–carbon reaction takes place mainly

inside the pore structure of the carbon particle while the

NO–carbon reaction is restricted to the external surface

area. These results can partly explain our findings.

However, the information collected here is not sufficient

to explain the reason for the apparent inhibition of thesurface NO–carbon reaction in the reaction of NO and O2

with carbon surface, as can be observed in Fig. 5. In the

case of the reaction inNO/H2O/O2 the large proportion of

surface oxygen is probably due to the effect of the nitric

acid, a strong oxidizing agent, formed during the reaction.

4. Conclusions

The reactions of coal char with NO at 270 �C forms

stable nitrogen and oxygen complexes on the char sur-face. The presence of O2 and H2O in the reacting mix-

ture affects the formation of these complexes. Water

inhibits the formation of nitrogen complexes through

neutralization of active sites with hydrogen atoms. In

the mixtures with O2, there is gasification of the solid

carbon and this facilitates the nitrogen incorporation

into the solid matrix, mainly as N-pyridinic. The dif-

ference in the gasification effect by NO/O2 and NO/H2O/O2 mixtures is associated with NO2 formation in the first

case, and additionally, HNO3 formation in the latter

case. Theoretical calculations show that five member

ring structures are susceptible to chemisorb NO giving

pyridine-N-oxide at temperatures where gasification is

not relevant. These complexes can be transformed into

N-pyridine in a reaction that can be assisted by CO, CO2

or NO. –NO2 complexes can be formed by NO chemi-sorption on adjacent sites to oxygen complexes or by

chemisorption of NO2 present in the gas mixtures that

contain oxygen. These complexes are responsible for the

NO reversible desorption.

Acknowledgements

The authors would like to acknowledge the financial

support from Colciencias and the University of Antio-

quia, project 1115-05-11504, the University of Antio-

quia, project IN-5216CE, and the University of Alicante

project PPQ2002-01025. P. Garc�ıa wishes to thank the

University of Antioquia and the University of Alicantefor her visiting scholarship.

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