Decomposition pathways of NO on carbide and oxycarbide-modified W(1 1 1) surfaces

Decomposition pathways of NO on carbide andoxycarbide-modified W(111) surfaces

M.H. Zhang a, H.H. Hwu a, M.T. Buelow a, J.G. Chen a,*, T.H. Ballinger b,P.J. Andersen b, D.R. Mullins c

a Department of Materials Science and Engineering, Center for Catalytic Science and Technology, University of Delaware,

Newark, DE 19716, USAb Johnson Matthey, Wayne, PA 19087, USA

c Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Received 11 June 2002; accepted for publication 13 September 2002

Abstract

The decomposition of NO has been studied on clean W(1 1 1), carbide-modified W(1 1 1), and oxycarbide-modified

W(1 1 1) surfaces. The decomposition pathways are investigated using a combination of temperature programmed

desorption, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, and soft X-ray photo-

electron spectroscopy. All these surfaces exhibit high activity toward the decomposition of NO, and the only N-con-

taining products are N2 and N2O. Furthermore, all three surfaces preferentially produce N2 over N2O from the

decomposition of NO. Oxygen atoms, produced from the decomposition of NO, react with carbide surfaces to produce

gas-phase CO at high temperatures. In addition, our results demonstrate that cycles of alternate NO/hydrocarbon

treatments can regenerate the carbide overlayer on W(1 1 1), and the regenerated C/W(1 1 1) surface remains active in

the decomposition of NO.

� 2002 Published by Elsevier Science B.V.

Keywords: Nitrogen oxides; Carbides; Tungsten; Electron energy loss spectroscopy (EELS); Thermal desorption; X-ray photoelectron

spectroscopy

1. Introduction

In the past few decades, Pt-group metals, such

as Rh, Pd and Pt, have been extensively utilized as

the main catalytic components in catalytic con-

verters to abate automobile emissions [1–4]. NOx

emissions include mainly harmful NO and NO2,

which provoke environmental problems related to

the acid rain and greenhouse effect. The growing

concerns for the environment have resulted in in-

creasingly stringent NOx emission standards [5]. It

is therefore imperative that the Pt-group metals

should be more effectively utilized to meet the

regulations. The high demand for Pt-group metals

has motivated a large number of studies examiningthe reaction of NO on well-defined surfaces of Pt-

group metals [6–14]. The main objectives of these

studies were to obtain a better understanding of

the interaction of NO with Pt-group metals, which

*Corresponding author. Tel.: +1-302-831-0642; fax: +1-302-

831-4545.

E-mail address: [email protected] (J.G. Chen).

0039-6028/02/$ - see front matter � 2002 Published by Elsevier Science B.V.

PII: S0039 -6028 (02 )02335 -X

Surface Science 522 (2003) 112–124

www.elsevier.com/locate/susc

mail to: [email protected]

should help the more efficient utilization of Pt-

group metals in catalytic converters.

The primary motivation of our work is to

search for less expensive and more abundant al-

ternative catalysts to replace Pt-group metals for

the dissociation of NO. The carbides of Groups 4–6 early transition metals often show catalytic ac-

tivities that are similar to Pt-group metals [15,16].

Our research group has investigated the reactivi-

ties of carbide-modified surfaces by using a variety

of chemical probe reactions [17]. For example, in

the dehydrogenation of cyclohexene to benzene,

we found that the benzene-yield on carbide-mod-

ified tungsten [18] and molybdenum [19] are verysimilar to that on Pt(1 1 1). Furthermore, in a

recent letter we have reported temperature pro-

grammed desorption (TPD) results of the struc-

ture-dependent activity and product selectivity

from the decomposition of NO on C/W(1 1 1), C/

W(1 1 0) and C/Mo surfaces [20].

The reaction of NO on tungsten carbides has

been investigated in several previous catalytic [21]and surface science [22,23] studies. For example,

Leclercq�s group has reported catalytic studies oftungsten carbide in synthetic exhaust gases [21]. In

addition, Freund�s group has performed detailedangular resolved UPS studies of the adsorption and

dissociation of NO on a single crystal WC(0 0 0 1)

surface. These authors have also reported the TPD

spectrum of mass 28 amu, which was contributedby the reaction products of CO and N2 [22,23].

The detection of N2 clearly indicates that the

tungsten carbide surface is active toward the dis-

sociation of NO and the subsequent production

of N2.

In this paper we attempt to determine the

decomposition pathways and product selectivity

of NO on the C/W(1 1 1) surface by using TPD,high-resolution electron energy loss spectroscopy

(HREELS) and soft X-ray photoelectron spec-

troscopy (SXPS). The utilization of the isotope15NO in TPD allowed us to determine the product

yield of CO and 15N2, as well as that of another

reaction product, 15N2O. And the spectroscopic

characterization using HREELS and SXPS en-

abled us to identify the reaction intermediates anddecomposition pathways of NO on the C/W(1 1 1)

surface. In addition, because of the presence of

oxygen on carbide is unavoidable under realistic

De-NOx reaction conditions, we have investigated

the NO decomposition on oxycarbide-modified

W(1 1 1) surface. Finally, we have evaluated the

possibility of regenerating the C/W(1 1 1) surface

to explore the potential catalytic application oftungsten carbides as De-NOx catalysts in the

presence of NO and hydrocarbons.

2. Experimental

The experiments were conducted using three

separate ultra high vacuum (UHV) systems. Thetypical base pressure was in the range of 2� 10�10–

8� 10�10 Torr. The first two systems are located at

the University of Delaware and both contain fa-

cilities of Auger electron spectroscopy (AES),

TPD, low energy electron diffraction (LEED), and

ion sputtering. One of these systems is also

equipped with an LK ELS3000 high resolution

electron energy loss spectrometer. The third sys-tem is located at the U12 Beamline at the National

Synchrotron Light Source at Brookhaven Na-

tional Laboratory, which is equipped with capa-

bilities for soft X-ray photoemission spectroscopy

(SXPS) measurements.

The tungsten (1 1 1) single crystal was mounted

on sample manipulators by spot welding to two Ta

wires. The surface can be cooled by contact with aliquid nitrogen reservoir and heated resistively

through the Ta mounting wires. Sample prepara-

tion first entailed cleaning by several cycles of

sputtering with 2 keV Neþ bombardment at 300 K

followed by annealing in vacuum to 1200 K.

Carbon that cannot be removed by sputtering was

cleaned by exposing to oxygen at 1000 K followed

by annealing at 1200 K. This process was contin-ued until there were no detectable amounts of

carbon and oxygen impurities by AES. The C/

W(1 1 1) thin film was made by several cycles of

exposing W(1 1 1) to 3 L (1 L ¼ 1� 10�6 Torr�s)cyclohexene at about 120 K followed by annealing

to 1200 K in vacuum. Cyclohexene decomposes to

produce gas-phase hydrogen and atomic carbon

on the surface [18]. The surface prepared in thisfashion has a C/W atomic ratio of approximately

0.55 and exhibits affiffiffi3

p�

ffiffiffi3

pR30� LEED pattern.

M.H. Zhang et al. / Surface Science 522 (2003) 112–124 113

Cyclohexene was used as the chemical agent for

making carbides due to prior knowledge and

characterization of C/W(1 1 1) prepared in a simi-

lar fashion [18]. The O/C/W(1 1 1) surface was

generated by exposing the C/W(1 1 1) to 0.5 L of

oxygen at 900 K, which generates an oxygen-modified C/W(1 1 1) with an O/W atomic ratio 0.12

and a C/W atomic ratio 0.44 [18].

The TPD experiments involved exposing the

surfaces to 1 or 10 L of 15NO (Cambridge Isotopes,

98% isotopically pure) at 100 K. The surfaces were

then heated to 1200 K at a constant rate of 3 K/s

and the desorption products were measured by a

mass spectrometer. Several desorption productswere measured by monitoring m=q ¼ 31 (15NO), 30

(15N2), 28 (CO), 46 (15N2O), 47 (

15NO2), 18 (H2O or15NH3), 32 (O2), and 2 (H2). The only species to

desorb appreciably from all surfaces were 15NO,15N2, CO and

15N2O. The15NO isotope was used in

the TPD measurements to differentiate the reaction

products of 15N2 and CO.

The HREEL spectra were all acquired afterexposing the surface to 14NO at 90 K and then

heating to the specified temperatures. All HREELS

spectra were recorded at 90 K. The intensity of the

elastic peak was typically 1� 105 counts per sec-

ond (cps) with a resolution of 40–50 cm�1. The time

to collect a spectrum was approximately 30 min.

All spectra were recorded in the specular scattering

geometry.

The SXPS spectra were recorded using incidentenergy of 450 eV for N1s and of 600 eV for O 1s.

The energy of the each spectrum was calibrated

using the Fermi energy of the valence state. The

end-station contained a VSW EA125 electrosta-

tic analyzer which was operated in constant pass

energy mode for photoemission measurements.

The instrument resolution was better than 0.5 eV.

3. Results and interpretation

3.1. TPD results

3.1.1. NO on clean W(111)

TPD spectra acquired following the exposure

to 1 and 10 L 15NO at 100 K on clean W(1 1 1) areshown in Fig. 1(a) and (b), respectively. The de-

composition of 15NO on W(1 1 1) is evident by the

observation of the 15N2 and 15N2O desorption

Fig. 1. TPD spectra after exposing W(1 1 1) to 1 L (a) and 10 L(b) 15NO at 100 K.

114 M.H. Zhang et al. / Surface Science 522 (2003) 112–124

peaks. After an exposure of 1 L, the majority of15N2 desorbs at around 1007 K from the clean

W(1 1 1) surface, as indicated in Fig. 1(a). In ad-

dition, a relatively weak 15N2 desorption peak also

occurs at 175 K, which coincides with the de-

sorption of 15N2O at the same temperature. Themolecular desorption of 15NO at �111 K is at-

tributed to the desorption from the heating wires

upon initial heating. In addition to the products

shown in Fig. 1, 15NO2 and15NH3 were also mea-

sured, but desorption of these compounds was

not detected in the TPD measurements. The TPD

results after 10 L of 15NO exposure (Fig. 1(b))

are very similar to the 1 L 15NO TPD, suggestingthat 1 L of 15NO corresponds to a near-saturation

coverage of the W(1 1 1) surface. More about the

effect of 15NO exposure will be discussed later.

3.1.2. NO on C/W(111) and O/C/W(111)

Fig. 2(a) presents the TPD spectra acquired

after exposing the C/W(1 1 1) surface to 1 L of15NO at 100 K. The decomposition of 15NO isclearly demonstrated by the formation of the gas-

phase 15N2, CO and 15N2O products. The decom-

position of 15NO occurs at relatively low temper-

ature, as evident from the desorption of the 15N2O

product around 175 K. The O atom produced

from the decomposition of 15NO combines with

the C atom of the C/W(1 1 1) surface to produce

gas-phase CO at 909 K. As a result, the formationof CO reduces the carbon concentration of C/

W(1 1 1), as confirmed by AES measurements after

TPD experiments. The 15N atom also recombines

and desorbs as 15N2 at the same temperature.

Similar to that discussed for clean W(1 1 1), the15NO desorption peak at 114 K is attributed to

initial desorption from heating wires.

TPD spectra obtained following the adsorptionof 1 L 15NO at 100 K on O/C/W(1 1 1) are pre-

sented in Fig. 2(b). The O/C/W(1 1 1) is similarly

active to the decomposition of 15NO, as evident by

the formation of gas-phase 15N2 and15N2O. The

majority of 15N2 desorbs at 963 K and a weak 15N2

desorption peak appears at around 175 K. Again,

the low temperature 15N2 desorption peak coin-

cides with the desorption of 15N2O at 175 K.Carbon can be partially removed from O/C/

W(1 1 1) via the formation of CO. The desorption

Fig. 2. (a) TPD spectra after exposing C/W(1 1 1) to 1 L 15NO at 100 K and (b) TPD spectra after exposing O/C/W(1 1 1) to 1 L 15NO

at 100 K.


temperature of CO occurs at 941 K on O/C/

W(1 1 1), which is slightly higher than the tem-

perature of 909 K on C/W(1 1 1). The desorp-

tion peak of 15NO at �115 K from O/C/W(1 1 1)is again contributed to desorption from heating

wires.

We have also performed TPD measurements of

H2 following the adsorption of NO on W(1 1 1)

and C/W(1 1 1) surfaces; the reasons for such TPD

measurements will become clear after the descrip-

tion of the HREELS results below. Fig. 3 shows

the H2 TPD measurements following the decom-position of 1 L NO on C/W(1 1 1) at 100 K. For

comparison, the H2 TPD from the decomposition

of 2.2 L cyclohexene on C/W(1 1 1) is also shown

in Fig. 3, which corresponds to the production of

0.22 H2 per W atom [18]. The comparison in Fig. 3

indicates that the amount of H2 production from

the NO/C/W(1 1 1) surface is negligible.

3.2. HREELS results

To facilitate the understanding of the NO de-

composition pathways, we obtained HREEL

spectra of NO on clean W(1 1 1), C/W(1 1 1), and

O/C/W(1 1 1) surfaces, which are shown in Figs. 4–

6, respectively. The exposures of NO were made

with the surface temperature at 90 K; the adsorbed

layers were then heated to the indicated tempera-

tures, and allowed to cool before the HREELspectra were recorded. The height of the elastic

peaks in all spectra has been normalized to unity,

and the expansion factor for each individual spec-

trum represents the multiplication factor relative

to the elastic peak. We have chosen 10 L NO ex-

posure instead of 1 L for the HREELS measure-

ments in order to assure saturation coverage, which

should help minimize the adsorption from theresidue impurity gases during the HREELS mea-

surements (about 30 min per spectrum).

Fig. 3. TPD spectra of H2 after exposing C/W(1 1 1) to 10 L15NO at 100 K and to 2.2 L c-C6H10 at 120 K.

Fig. 4. HREEL spectra after exposing W(1 1 1) to 10 L NO at

90 K and heating to indicated temperatures.


3.2.1. NO on W(111)

Fig. 4 shows the HREEL spectra obtained afterexposing 10 L NO on the clean W(1 1 1) surface.

At 90 K, a relatively sharp peak is detected at 1772

cm�1, with unresolved shoulders at 1691 and 1847

cm�1. These features are attributed to the m(NO)modes of molecular NO [24]; the different fre-

quencies are likely due to the different adsorption

sites/environment of NO on the W(1 1 1) surface.

After heating to 300 K, two broad peaks centeredat �3328 and 3606 cm�1 begin to appear, which

are characteristics for the m(NHx) and m(OH)modes, respectively. The observation of the

m(NHx) mode suggests that the reaction between

H2 in the UHV background with the surface N

atoms that are produced from the decomposition

of NO; the detection of the m(OH) mode indicatesthe reaction of H2O in the UHV background dur-

ing the HREELS experiments. In addition, several

new features, at 1258, 1481, and 1610 cm�1, appear

at 300 K as the NO mode at 1772 cm�1 diminishes.

The new features, at 1258–1610 cm�1, are in the

frequency range for the m(NO) modes of stronglychemisorbed NO on metal surfaces [24]. The onset

of two additional peaks, at �846 and 994 cm�1,

is also observed at 300 K. After heating to 400 K,all vibrational modes remain in the spectrum.

Further heating to 600 K results in the disap-

pearance of all features at frequencies above 1100

cm�1. The 846 and 994 cm�1 peaks are tentatively

assigned to the m(W–N) modes of atomic N, re-sulting from the decomposition of NO. These two

features disappear after heating to 1000 K, which

coincide with the desorption of N2 in the TPDmeasurements. The two remaining features at 453

and 629 cm�1 are assigned to the m(W–O) modes,mainly because AES measurements indicate that

Fig. 5. HREEL spectra after exposing C/W(1 1 1) to 10 L NO

at 90 K and heating to indicated temperatures.

Fig. 6. HREEL spectra after exposing O/C/W(1 1 1) to 10 L

NO at 90 K and heating to indicated temperatures.


oxygen is the only surface species after heating

NO/W(1 1 1) to 1000 K.

The inset in Fig. 4 provides a more detailed

comparison of the vibrational spectra after heating

the NO/W(1 1 1) layer to 150 and 200 K. As de-

scribed earlier, the N2O gas-phase product is de-tected at �175 K. The HREEL spectra in the insetindicate that N2O is not present as a surface in-

termediate, which should produce a m(N@NO)mode in the vicinity of the gas-phase value of 2224

cm�1 [25]. The combined HREELS and TPD re-

sults therefore suggest that the desorption of the

N2O product is a reaction-limited process.

3.2.2. NO on C/W(111) and O/C/W(111)

Fig. 5 shows the HREEL spectra obtained after

exposing 10 L NO on the C/W(1 1 1) surface. At 90

K, the spectrum exhibits a relatively intense mo-

lecular feature of m(NO) at 1792 cm�1. After the

adsorbed layer is heated to 300 K, additional

modes appear at 846, 1001, 1705, and 3336 cm�1.

As described for the clean W(1 1 1) surface, the3336 cm�1 feature is characteristic of the m(N–H)mode of NHx species, which are likely produced

via the reaction of atomic nitrogen with H2 in

the UHV background during the acquisition of

HREEL spectra. As shown earlier the control

TPD experiments in Fig. 3, H2 is not produced on

NO/C/W(1 1 1) in the TPD experiment, which was

performed as soon as the C/W(1 1 1) surface wasexposed to NO. Therefore, the formation of the

NHx is most likely an experimental artifact that

involves the reaction of H2 with the atomic N on

C/W(1 1 1) during the HREELS measurements.

The 1705 cm�1 is in the frequency range that is

typical for molecular NO [24]. The lower fre-

quency modes at 846 and 1001 cm�1 are again

assigned to the m(W–N) modes. At 400 K thespectrum generally resembles that at 300 K, but

with diminished intensities for features between

1500–1800 cm�1 and increased intensities of fea-

tures at 846, 1001, and 3336 cm�1. These results

indicate that more NO is decomposed to produce

surface N, NHx and surface O species. The 600 and

750 K spectra are nearly identical, which reveal

that most of the NHx species have dissociated bythe disappearance of the m(N–H) mode at �3336cm�1. After heating to 1000 K, the m(W–N) modes

at 846 and 1001 cm�1 disappear, which coincide

with the detection of N2 and CO in the TPD

measurements (Fig. 2(a)). The 622 cm�1 at the

1000 K spectrum is assigned to the m(W–O) modeas described after the decomposition of NO on

clean W(1 1 1) in Fig. 4. The 331 cm�1 feature isassigned to the m(W–C) mode based on our pre-vious study of C/W(1 1 1) [18]. The thermal be-

havior of NO on the O/C/W(1 1 1) surface (Fig. 6)

is nearly identical to that on the C/W(1 1 1) sur-

face. Similar to the 90 K spectrum on the C/

W(1 1 1) surface, the 90 K spectrum of NO on O/

C/W(1 1 1) also exhibits an intense molecular fea-

ture at 1779 cm�1. After heating to 300 K, addi-tional modes appear at 994, 1691, and 3315 cm�1.

Similar to the prior two surfaces, the decrease in

the intensity of 1779 cm�1 peak, coupled with the

appearance of the 839 and 994 cm�1 features, in-

dicate that NO starts to decompose to produce

atomic N on the surface. The HREEL spectrum

after heating to 400 K is essentially the same as

that of 300 K, with the exception of a decrease inthe intensity of m(NO) vibrations. Between 600 and750 K, most of the NHx species are decomposed,

leaving only surface C, N, and O. After heating to

1000 K, the HREEL spectrum is characteristic of

an oxygen-modified C/W(1 1 1) surface [18], con-

sistent with the TPD detection of N2 at 963 K (Fig.

2(b)).

3.3. SXPS results

The most important conclusion from the

HREELS study of NO on C/W(1 1 1) and O/C/

W(1 1 1) surfaces is that a significant fraction of

NO decomposes to atomic N and O, as revealed by

the appearance and then disappearance of the

m(W–N) modes at approximately 839–846 and at994–1001 cm�1. To further confirm these vibra-

tional modes are related to atomic N, instead of

any NxOy molecular intermediates, we have per-

formed SXPS measurements following the disso-

ciation of NO on the C/W(1 1 1) surface. Fig. 7

shows the N (1s) and O (1s) spectra after exposing

C/W(1 1 1) to 10 L NO at 90 K, followed by

heating to 300 and 600 K. As reported previouslyin the XPS study of NO on W and other surfaces

[26,27], molecularly adsorbed NO is typically


characterized by the N (1s) peak at �399.8 eV andO (1s) peak at �531.4 eV. As shown in Fig. 7, afterheating to 300 K, new N (1s) and O (1s) peaks are

shifted to 397.1 and 530.3 eV, respectively. The

detection of lower-energy N (1s) and O (1s) peaks

is consistent with the formation of atomic N and Oon the surfaces [26,27]. Finally, after heating to

600 K, the molecular NO peaks are completely

converted to those of atomic N and O. The SXPS

results in Fig. 7, in particular the N (1s) peak,

further reveal that a small fraction of NO under-

goes dissociation even at 90 K, as indicated by the

presence of the relatively weak N (1s) feature at

397.1 eV. Furthermore, based on the relative peakareas of the N (1s) peaks at 300 K, we estimated

that the ratio of molecular and dissociated NO is

approximately 25% after the adsorbed layer is

heated to 300 K.

3.4. Regeneration of the C/W(111) surface

Attempts were made to regenerate C/W(1 1 1)surfaces after the adsorption and reaction of NO,

and the subsequent consumption of a fraction of

carbon atoms by the formation of gas-phase CO.

The decomposition of 15NO over regenerated C/

W(1 1 1) was examined to determine if the regen-

erated C/W(1 1 1) remains active to this reaction.

In these experiments, the C/W(1 1 1) surface was

first exposed to 10 L 15NO at 100 K and then he-

ated to 1200 K. The C/W atomic ratio was reducedfrom 0.56 to 0.23 based on the Auger measure-

ment; the loss of carbon is due to the fact that O

atoms react with C from the C/W(1 1 1) to produce

CO at �909 K (e.g., Fig. 2(a)). The C/W(1 1 1)

surface was then regenerated by exposing to 3 L

cyclohexene at 120 K and flashing to 1200 K in

vacuum. The C/W(1 1 1) atomic ratio increased to

0.53 after regeneration. The TPD spectrum fol-lowing the exposure of 15NO over the regenerated

C/W(1 1 1) surface is shown in Fig. 8. The detec-

tion of 15N2 over the regenerated C/W(1 1 1) sur-

face indicates that the regenerated surface remains

active toward the decomposition of 15NO. In fact,

the C/W(1 1 1) surface remains active after multi-

ple cycles of NO/cyclohexene treatments. On the

other hand, if the C/W(1 1 1) surface was not treatedwith cyclohexene after successive NO adsorption

and TPD measurements, all carbon atoms were

eventually consumed and oxygen atoms were left

Fig. 7. SXPS measurements following the adsorption of 10 L NO at 90 K.


on the W(1 1 1) surface. For comparison, the TPD

spectrum following the adsorption of NO on

this O/W(1 1 1) surface is also included in Fig. 8.This surface is essentially inert toward the disso-

ciation of NO, as suggested by the absence of

the 15N2 peak and the relatively weak15N2O peak.

The comparison in Fig. 8 clearly demonstrates the

importance of regeneration, as well as the po-

tential feasibility of using tungsten carbides for

De-NOx via sequential injection of NO and hy-

drocarbons.

4. Discussion

4.1. Surface reactivity and product selectivity

4.1.1. Surface reactivity and product selectivity over

W(111)

The selectivity of the two 15N-containing

products, 15N2 and15N2O, can be estimated based

on combined AES and TPD measurements. AES

spectra were performed by collecting after expos-

ing the W(1 1 1) surface to 10 L 15NO at 100 K,

and after a newly prepared NO/W(1 1 1) surface

was heated to 300 K; these two surfaces corre-

sponded to the surfaces before and after the de-sorption of 15N2O, respectively. From the AES

measurements, we estimated that the atomic ratio

of N/O was 1.0 at 100 K and 0.96 at 300 K. In

addition, the O/W atomic ratio at 300 K was es-

timated from AES to be 0.76, using standard

sensitivity factors for the W(182 eV) and O(KLL)

transitions [28]. Eqs. (1) and (2) describe the mass

balance over the W(1 1 1) surface at 300 K:

a 15NO! a215N2OðgÞ þ

a2O ð1Þ

b 15NO! xb 15NOþ ð1� xÞb 15Nþ ð1� xÞbOð2Þ

Symbol a represents the number of 15NO mole-

cules leading to the production of 15N2O, and

symbol b represents the number of 15NO molecules

remaining on the W(1 1 1) surface, either in the

molecular or decomposed form, after heated to

300 K. Therefore, we obtain the following rela-

tionships at 300 K:

Fig. 8. Comparison of TPD spectra after the reaction of NO on regenerated C/W(1 1 1) and oxidized W(1 1 1) surfaces.


N

O¼ b

a2

�.þ b

�¼ 0:96 ð3Þ

O

W¼ a

2

�þ b

�¼ 0:76 ð4Þ

The values for a and b can be solved as 0.06 and0.73, respectively, on the per W atom basis from

Eqs. (3) and (4). The surface reactivity of W(1 1 1)

in the decomposition of 15NO, A11 1, can then be

defined as the total number of 15NO molecules

ða=2þ bÞ that undergo decomposition on the perW atom basis, which corresponds to a value of0.76 15NO/W on clean W(1 1 1).

The selectivity of 15N2O from W(1 1 1) can be

estimated from the values of a and b as follows,with the symbol S15N2O Wð1 1 1Þ representing the

15N2O

selectivity over W(1 1 1):

S15N2O Wð1 1 1Þ ¼a

aþ b¼ 0:06

0:06þ 0:73

� �¼ 8%

Accordingly, the 15N2 selectivity is about 92% for

the 10 L 15NO/W(1 1 1) overlayer.

The activity and selectivity of W(1 1 1) surfaceafter 1 L 15NO exposure can be estimated by

comparing the 15N2 and15N2O TPD peak areas at

1 and 10 L of 15NO exposures. The mass balance

on the W(1 1 1) surface at 300 K can be described

by Eqs. (5) and (6) after exposing to 1 L 15NO and

heated to 300 K:

a0 15NO! a0

215N2OðgÞ þ

a0

2O ð5Þ

b0 15NO! xb0 15NOþ ð1� xÞb0 15Nþ ð1� xÞb0Oð6Þ

Symbol a0 represents the number of 15NO mole-cules that are involved in the production of 15N2O,

and symbol b0 represents the number of 15NO

molecules that are remaining on the W(1 1 1) sur-

face, either in the molecular or decomposed form,

after the surface was heated to 300 K. The overall

TPD peak area of 15N2 from C/W(1 1 1) is the sum

of the low-temperature (175 K) and high-temper-

ature (1007 K) peaks; the 175 K peak area is

normalized by subtracting the 11% contribution

from the cracking fragment [29] of 15N2O at thesame temperature. By comparing TPD peak areas

of 15N2O and 15N2 over the W(1 1 1) surface after 1

L 15NO exposure with those over W(1 1 1) after 10

L 15NO exposure, seen in Table 1, the values of a0

and b0 can be estimated on the per W atom basis as

follows:

a0 ¼ 5:08� 106

1:21� 107� a ¼ 0:025

b0 ¼ 1:06� 108

9:63� 107� b ¼ 0:80

The sum of a0=2 and b0 provides the surface reac-tivity in a value of 0.81 15NO/W. From the values

of a0 and b0, the selectivity to 15N2O and 15N2 is

determined to be 3% and 97%, respectively.

4.1.2. Surface reactivity and product selectivity over

C/W(111) and O/C/W(111)

Similarly, the mass balance on the C/W(1 1 1)

surface at 300 K can be described by Eqs. (7) and

(8) after exposing to 1 L 15NO and heated to 300 K,

c 15NO! c215N2OðgÞ þ

c2O ð7Þ

d 15NO! xd 15NOþ ð1� xÞd 15Nþ ð1� xÞdOð8Þ

Symbol c represents the number of 15NO mole-

cules decomposing to produce 15N2O. Sym-

bol d represents the number of 15NO molecules

Table 1

Comparison of surface reactivity and product selectivity

Surface Dosage of 15NO (L) 15N2 peak area15N2O peak area Surface reactivity

(# 15NO/W)

Selectivity of 15N2 (%)

W(1 1 1) 10 9:63� 107 1:21� 107 0.76 92

W(1 1 1) 1 1:06� 108 5:08� 106 0.81 97

C/W(1 1 1) 1 6:7� 107 4:39� 106 0.52 96

O/C/W(1 1 1) 1 6:49� 107 4:68� 106 0.50 96


remaining on the W(1 1 1) surface, either in the

molecular or decomposed form, after the surface

was heated to 300 K.

By comparing TPD peak areas of 15N2O and15N2 from the C/W(1 1 1) surface after 1 L 15NOexposure with those over the W(1 1 1) surface after

10 L 15NO exposure, the values of c and d can beestimated in the unit of per W atom as follows:

c ¼ 4:39� 106

1:21� 107� a ¼ 0:02

d ¼ 6:7� 107

9:63� 107� b ¼ 0:51

The surface reactivity of the decomposition of15NO on C/W(1 1 1) can be derived from the sum

of c=2 and d, which corresponds to a value of 0.5215NO per W atom. From the values of c and d, theselectivity to 15N2O and 15N2 is determined to be

4% and 96%, respectively.

Repeating the same procedure of TPD peak

area analysis, the surface reactivity for the de-composition of NO on O/C/W(1 1 1) is estimated

to be 0.50 NO per W atom. The selectivity for15N2O and 15N2 on O/C/W(1 1 1) surface is deter-

mined to be 4% and 96%, respectively. The values

for the selectivity and surface reactivity over all

surfaces are summarized in Table 1.

It should be pointed out that the 1 L NO/

W(1 1 1) and 10 L NO/W(1 1 1) surfaces show aslight difference in the overall surface reactivity

(0.81 vs. 0.76) and N2 selectivity (97% vs. 92%).

Such differences are likely due to the uncertainty in

the TPD measurements and TPD peak area anal-

ysis. They could also be partially attributed to the

assumption that surface with higher NO coverages

should slightly favor the formation of N2O, which

should lead to a reduction in the overall surfacereactivity and in the selectivity to N2. More de-

tailed studies are necessary to determine the effect

of NO coverage on the product selectivity.

4.2. Decomposition pathways

The decomposition pathways over C/W(1 1 1)

and O/C/W(1 1 1), as investigated by the combi-

nation of HREELS, TPD, AES and SXPS, are

summarized in the equations below:

NO! NðaÞ þOðaÞ ð9Þ

NðaÞ þNOðaÞ ��!175 KN2O " or

2NOðaÞ ��!175 KN2O " þOðaÞ ð10Þ

NðaÞ þNðaÞ ��!>900 KN2 " ð11Þ

OðaÞ þ C=Wð111Þ��!>900 KCO " ð12Þ

Eqs. (9)–(11) are the only reaction pathways in-

volved in the decomposition of 15NO over W(1 1 1)

as CO is not produced from clean W(1 1 1) surface.

A fraction of NO molecules dissociate into ad-

sorbed N and O atoms at low temperatures over

all surfaces, and the decomposition is completed

after heating to 600 K, as confirmed by theHREELS results. Some of the N atoms combine

with adsorbed NO to desorb as N2O at 175 K; the

production of N2O could also result from the

disproportion reaction of two adjacent NO mole-

cules at 175 K. In addition, HREELS results (inset

of Fig. 4) suggest that the desorption of N2O is a

reaction-limited process. The remaining N atoms

stay on the surface and recombine to desorb as N2

at T > 900 K, with the O atoms staying on the

clean W(1 1 1) surface after heating to 1000 K.

Similar decomposition pathways also occur on

the C/W(1 1 1) and O/C/W(1 1 1) surfaces, except

that oxygen atoms from the decomposition of NO

react with the C atoms from the carbides and de-

sorb as CO at T > 900 K. Our results indicate that

the decomposition of NO over C/W(1 1 1) and O/C/W(1 1 1) is very facile, as evident by the pro-

duction of N2. As summarized in Table 1, the two

surfaces show similar activity for the decomposi-

tion of NO. The observation that CO desorbs at a

slightly higher temperature from the O/C/W(1 1 1)

surface (at 941 K), as compared with the C/

W(1 1 1) surface (at 909 K), suggests that the

presence of O in C/W(1 1 1) slightly enhances thestability of the carbide surface. A fraction of car-

bon atoms are stripped away from both C/W(1 1 1)

and O/C/W(1 1 1) due to the formation of CO.

However, the TPD results in Fig. 8 indicate that

NO decomposes over C/W(1 1 1) and regenerated

C/W(1 1 1) surfaces in a almost identical manner,


which suggests that the C/W(1 1 1) surface can be

regenerated by reacting with hydrocarbons, such

as cyclohexene.

4.3. Brief comparison of C/W(111), O/C/W(111)

and Pt-group metal surfaces

In this section we will briefly compare the sur-

face reactivity of NO on tungsten carbides with

previous UHV studies on the surfaces of the cur-

rent commercial De-NOx catalysts, Pt and Rh.

Previous studies of NO decomposition over Pt-

group metals have indicated its structure sensitive

feature. Pt and Rh have the face-centered cubicstructure, and the (1 1 1) plane is the most closely

packed surface. In the previous study of NO de-

composition on Pt single crystal surfaces, negligi-

ble NO dissociation was reported on the (1 1 1)

plane [7,30], while a surface reactivity �0.25 NOper Pt atom was found on the more open-struc-

tured (1 0 0) plane [7,13,31,32]. Previous extensive

studies of NO adsorption and dissociation onsingle crystal Rh surfaces revealed similar general

trend that the more open and stepped Rh surfaces

show higher activities toward the decomposition

of NO [33]. Similar or lower surface reactivity was

found from Rh single crystal surfaces compared to

tungsten carbide surfaces, for instance, �0.37 [13]or �0.44 [34] NO per Rh atom for Rh(1 1 1) sur-

face, and 0.34 NO per Rh atom for Rh(1 0 0)surface [35]. However, the reconstruction of Rh

surfaces upon NO exposure results some compli-

cation in the comparison. Generally, NO to N2O is

not considered as an important reaction channel in

the decomposition of NO on the single crystal

surfaces of Pt and Rh. For instance, either unde-

tectable or negligible amount of N2O was reported

from Pt(1 1 1) [7,30], Pt(1 0 0) [7,8], Pt(1 1 0) [7],Rh(1 1 1) [13], Rh(1 1 0) [14] and Rh(1 0 0) [35]

surfaces under the UHV conditions that were

similar to the current study.

Overall, the decomposition of NO on C/W(1 1 1)

and O/C/W(1 1 1) show general similarities to those

observed on the Pt-group metal surfaces. Our re-

sults revealed that W(1 1 1), C/W(1 1 1), and O/C/

W(1 1 1) surfaces were highly active toward thedecomposition of NO. In addition, N2 and N2O

were the only N-containing products and no O2

was detected as the gas-phase products. It should

be pointed out that W has the body-centered cubic

structure. The relatively more open-structured

W(1 1 1), C/W(1 1 1) and O/C/W(1 1 1) surfaces all

exhibit relatively high activity in the decomposi-

tion of NO, and all surfaces preferentially producesignificant amount of N2 > 92%. The selectivity

of N-containing products is nearly the same over

all three surfaces, although clean W(1 1 1) has a

higher activity compared with the other two sur-

faces.

Our comparison of the reactivity of C/W(1 1 1)

with Pt-group metals are based on the number of

NO molecules undergoing decomposition on a permetal atom basis. However, another important

parameter regarding the decomposition of NO on

different surfaces is the desorption temperature of

the N2 product. As shown in our TPD results, only

a fraction of the N2 product desorb from the C/

W(1 1 1) surface at 175 K, while the majority de-

sorb at 909 K, Fig. 2(a). For comparison, the

recombinative desorption of N2 from the Pt-groupmetal surfaces typically occur in the temperature

range of 400–800 K [6–14]. Therefore, from the

perspective of N2 desorption, the Pt-group metals

should be considered as more active than the C/

W(1 1 1) surface toward the catalytic decomposi-

tion of NO.

5. Conclusions

Two competing reaction pathways are detected

in the decomposition of 15NO over W(1 1 1), C/

W(1 1 1) and O/C/W(1 1 1), which produce 15N2

and 15N2O, respectively. The activity of NO de-

composition on the per W atom basis is in the

order of Wð111Þ > C=Wð111Þ � O=C=Wð111Þ.All three surfaces produce almost entirely gas-

phase 15N2, and the selectivity of 15N2 is nearly

identical over the three surfaces (>92%). The Oatoms from the decomposition of 15NO strip away

a fraction of C atoms on C/W(1 1 1) and O/C/

W(1 1 1) and desorb in the form of gas-phase CO.

Finally, the TPD and AES results clearly indicate

that the C/W(1 1 1) surface can be regenerated viaa post-treatment of the surface by exposing to

cyclohexene at 120 K and flashing to 1200 K in


vacuum, and that the regenerated C/W(1 1 1) sur-

face remains active toward the decomposition of

NO. Such finding suggests the potential feasibility

of using tungsten carbides for De-NOx via se-

quential injection of NO and hydrocarbons.

Acknowledgements

We acknowledge partial support from the En-

vironmental Protection Agency (Grant No. EPA

STAR 82962401). The authors would like to thank

Johnson Matthey for partial financial support of

this work. We also like to thank H.-Y. Chen andP. Shady of Johnson Matthey for helpful discus-

sion. DRM is supported by the US Department of

Energy, under contract DE-AC05-00OR22725

with Oak Ridge National Laboratory, managed

and operated by UT-Battelle, LLC.

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Documents

Decomposition pathways of NO on carbide and oxycarbide-modified W(1 1 1) surfaces