Nitrogen removal pathways in a steady-state NO + CO reaction on Pd(110)

Article abstract of DOI:10.1016/j.cplett.2004.02.090

Three removal processes of surface nitrogen, i.e., (i) the decomposition of the intermediate N₂O(a), (ii) its desorption without decomposition and (iii) the associative desorption of nitrogen atoms, were separately studied in a steady-state NO+CO reaction on Pd(110) through analysis of the angular and velocity distributions of desorbing products N₂ and N₂O. At temperatures below approximately 600 K, the processes (i) and (ii) prevailed, whereas at higher temperatures, the process (iii) contributed significantly. The branching was also sensitive to the CO/NO pressure ratio.

Full text of DOI:10.1016/j.cplett.2004.02.090

Chemical Physics Letters 388 (2004) 201–207
www.elsevier.com/locate/cplett
Nitrogen removal pathways in a steady-state
NO + CO reaction on Pd(1 1 0)
a
Yunsheng Ma , Izabela Rzeznicka , Tatsuo Matsushima
b
a,
*
a
Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
b
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
Received 20 January 2004; in final form 23 February 2004
Published online: 20 March 2004
Abstract
Three removal processes of surface nitrogen, i.e., (i) the decomposition of the intermediate N2O(a), (ii) its desorption without
decomposition and (iii) the associative desorption of nitrogen atoms, were separately studied in a steady-state NO + CO reaction on
Pd(1 1 0) through analysis of the angular and velocity distributions of desorbing products N2 and N2O. At temperatures below
approximately 600 K, the processes (i) and (ii) prevailed, whereas at higher temperatures, the process (iii) contributed significantly.
The branching was also sensitive to the CO/NO pressure ratio.
Ó 2004 Elsevier B.V. All rights reserved.
1
. Introduction
Ordinary kinetic measurements at the steady-state
NO + CO reaction are not informative for surface ni-
trogen-removal processes because the overall rate is
controlled by NO dissociation [5]. On the other hand,
the angular and velocity distributions of desorbing
products can provide information of the final de-
sorption process whenever any step becomes rate-
determining since these distributions do not directly
involve the reaction rate and are always related to the
product desorption [6]. These distributions were fre-
quently analyzed in the course of catalyzed NO de-
composition with several transient methods to avoid
the well-known large N2 formation on the reaction
chamber wall, such as modulated-molecular beams
(MMB) [7,8] and AR-temperature programmed de-
sorption (TPD) [2,9–11]. Steady-state conditions,
however, cannot be established for the reaction be-
cause of the periodical change of reactants in MBS.
Similar ambiguities were also induced in AR-TPD in
the presence of gaseous reactants. Neither angular nor
velocity distributions have been reported under steady-
state NO + CO reactions except for our earlier work
The decomposition of NO over metal catalysts
such as palladium and rhodium yields N2O, which has a
remarkable greenhouse effect, concomitantly with N2
[
1]. Reducing of N2O is highly desired on improved
catalysts. This Letter delivers the first analysis of
three surface nitrogen-removal processes, i.e., (i) the
decomposition of the intermediate, NO(a) + N(a) !
N O(a) ! N (g) + O(a), (ii) N O desorption without
2
2
2
decomposition and (iii) the associative desorption of
nitrogen atoms, 2N(a) ! N2(g), in
a steady-state
NO + CO reaction on Pd(1 1 0). This separation is pos-
sible in angle-resolved product desorption analysis (AR-
PDA) combined with cross-correlation time-of-flight
(TOF) techniques since the N O(a) decomposition emits
2
N in an inclined way, whereas the associative desorp-
2
tion of N(a) collimates along the surface normal [2–4].
The branching changed with the surface temperature
and the NO/CO pressure ratio.
[
12,13]. In the present work, AR-PDA was success-
fully performed for the steady-state NO + CO reaction
over a wide range of reactant pressures by using a gas
doser with a fine orifice.
*
Corresponding author. Fax: +81-011-706-9120.
E-mail address: tatmatsu@cat.hokudai.ac.jp (T. Matsushima).
0
009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.02.090

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Y. Ma et al. / Chemical Physics Letters 388 (2004) 201–207
2
. Experiments
The apparatus, which had three chambers separately
pumped, was described previously [14,15]. The reaction
chamber has reverse-view low-energy electron diffrac-
tion and X-ray photoelectron spectroscopy optics, an
ion gun, and a quadrupole mass spectrometer (QMS)
for angle-integrated (AI) measurements and monitoring
the partial pressures of reactants (PNO and PCO). The
chopper house, which had a pumping rate of about
7
m3 s for a high angle-resolution [16], has a narrow
ꢀ1
slit facing the reaction chamber and a cross-correlation
random chopper blade. The analyzer, connected
through a narrow tube, is equipped with another QMS
for AR-PDA and TOF analyses. The distance from the
ionizer to the chopper blade was 377 mm and the time
resolution was selected at 20 ls.
A gas mixture of 15NO and 13CO was introduced
through a doser with a small orifice (diameter; 0.1 mm)
about 2 cm from a sample crystal. For pressure-depen-
dence measurements, ordinary CO was additionally
backfilled. The products 15N2, 13CO and N O signals
15
2
2
were monitored in both AI and AR forms. Hereafter,
these are described as N , CO and N O in the text. The
2
2
2
desorption angle (polar angle; h) was scanned in the
plane along the [0 0 1] direction. The fragmentation of
N2O in both QMS was corrected.
3
. Results
3
.1. General features
Fig. 1. (a) TS-dependence of AR-signals of products with
a
(15
NO:13CO ¼ 1:4) mixture at 2.5 ꢁ 10
ꢀ5
Torr. (b) 15NO incidence
The AR signal was obtained by the analyzer QMS as
1
5
15
pressure dependence of the AR- N2 and N2O signals at h ¼ 0° at
the difference (DN2 for the N2 signal, for example) be-
tween the signal at the desired angle and the signal when
the crystal was away from the line-of-sight position. The
AR signals at h ¼ 0° for desorbing N , CO and N O,
15
fixed CO, and of the 15N2 signals at h ¼ 41°. The N2O fragmentation
was corrected for the 15N2 signals.
2
2
2
bution because of the Maxwellian velocity curves at the
surface temperature all over the angle [17]. The signal
intensity was corrected by this effect.
and the AR N signal at h ¼ 41° are displayed because of
2
their collimation angles as a function of the surface
temperature (TS) in Fig. 1a. All the signals in the steady-
state NO + CO reaction were negligible below 480 K. The
N2 signal at h ¼ 41° increased rapidly above 500 K with
increasing TS to a maximum at around 550 K and then
decreased at higher temperatures. The N2 signal at
h ¼ 0°, on the other hand, increased slowly and peaked at
3.2. Angular distribution
The CO2 desorption sharply collimated along the
surface normal in a cos13–16(h) form. On the other
hand, N2 desorption showed an inclined form colli-
mated at around 40° off the surface normal below
about 600 K. At higher temperatures, both the nor-
mally directed and cosine components were enhanced.
about 600 K. These differences in AR–N signals imply a
2
branching shift of N formation pathways with T . No
2
S
hysteresis was found in their signals with decreasing TS.
The above small orifice was effective to reduce N2
formation on the chamber wall, improving the AI-sig-
nal. Under this construction, the flux of incident reac-
tants towards the surface decreased proportionally to
cos(h) when the angle shifted from the normal direction.
This decrement was evaluated by monitoring the AR-
The N signal at 40° increased rapidly with P
(Fig.
NO
2
1b), whereas it showed a critical CO pressure in its
PCO-dependence (Fig. 2a). Thus, each desorption
component should be derived from angular and ve-
locity distribution analyses.
The AR–N signal is plotted against the desorption
2
angle in Fig. 2b. The total pressure with a mixture
signal of N O, which should follow the cosine distri-
2

Y. Ma et al. / Chemical Physics Letters 388 (2004) 201–207
203
15
15
Fig. 2. (a) PCO-dependence of the AR- N2 signals at h ¼ 0° and 40° at a fixed NO pressure and TS ¼ 550 K. (b,c) Angular distributions of de-
1
5
sorbing N2 at different PCO.
(
NO:CO ¼ 3:1) was fixed at 5.7 ꢁ 10ꢀ6 Torr and
consistent with the normally directed desorption. The
translational temperature of N2 was very high, more
than twice that of CO2. Typical velocity curves of N2 at
550 K are shown in Fig. 4. The apparent translational
temperature calculated from the average kinetic energy
TS ¼ 550 K. The N2 distribution was in an inclined way
2
8
approximated as cos ðh ꢀ 40Þ. The signals at 40° and 0°
are plotted against PCO in logarithm scales, where only
1
5
12
NO was dosed through the orifice, and CO was
backfilled (Fig. 2a). With increasing PCO, the signal at
0° increased steeply with a slope of 1.8 and reached a
(hEi) as T ¼ hEi=2k is shown in hi in the figure, where
hEi
4
k is the Boltzmann constant. The value was maximized
steady level above the ratio of PCO=PNO ¼ 0:7. The ratio
of the signal at 0° to that at 40° was kept under 0.05 over
a wide CO pressure range. On the other hand, at 640 K,
the signal ratio increased to about 0.7 with increasing
PCO up to PCO=PNO ¼ 4, where it showed a constant
value (Fig. 3a). Below this ratio, the angular distribution
changed, i.e., at low PCO, the N2 desorption was fairly in
the inclined way (Fig. 3b). At high PCO, the signal at
h ¼ 0° was enhanced, i. e., the desorption including the
normally directed and cosine components became
dominant (Fig. 3c and d).
to 3700 K at T ¼ 550 K at around the collimation angle
S
and to a similar value at T ¼ 640 K (Fig. 5). It de-
S
creased quickly with an increasing shift from the colli-
mation angle, consistently with the inclined desorption.
The velocity distributions of desorbing N2 at h ¼ 0°
commonly showed a significant thermalized component
to the surface temperature. Its intensity should be equal
to the cosine component involved in the angular distri-
butions in Figs. 2 and 3. In velocity curve analysis, its
contribution was first subtracted from the observed
curves. The resultant distributions were still broad,
2
2
yielding the speed ratio (SR) defined as (hv i=hvi
1
=2
1=2
3
.3. Velocity distribution
ꢀ1Þ =ð32=9p ꢀ 1Þ of 1.1 at 640 K even at the colli-
mation position. Here, v is the velocity of N2, hvi is the
2
The translational temperature of desorbing CO2 was
about 3 times higher than the surface temperature at
h ¼ 0° and decreased quickly with increasing h, being
mean velocity, and hv i is the mean square velocity. This
suggests that the resultant velocity curve still involves
two or more components. To deconvolute into two fast

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Y. Ma et al. / Chemical Physics Letters 388 (2004) 201–207
15
Fig. 3. (a) PCO-dependence of the AR- N2 signals at h ¼ 0° and 40° at fixed 15NO. (b–d) Angular distributions of desorbing 15N2 at different PCO.
The intensity was normalized to the maximum in (d). Typical deconvolutions are drawn by broken curves.
components, the modified Maxwellian distribution,
2
4. Discussion
3
2
f ðvÞ ¼ v expfꢀðv ꢀ v0Þ =a g, was fitted to each com-
ponent, where f ðvÞ is the modified Maxwellian distri-
4.1. Desorption components
bution function, v is the stream velocity and a is the
0
width parameter.
The above three pathways (i), (ii) and (iii) are oper-
ative. The quantity of [CO ]/{[N ] + [N O]} was kept at
It is difficult to determine each modified Maxwellian
uniquely because each form involves two parameters. In
the present study, only three parameters were deter-
mined by curve fittings at high- and low-velocity sides.
To omit the fourth parameter, we simply assumed a
common width parameter. In this treatment, the energy
of the faster component would be somewhat underesti-
mated, and that of the other should be overestimated
because the faster component is likely to yield a smaller
a value. The resultant deconvolutions are shown by
broken curves in Figs. 4 and 5. The fastest component
yielded a translational temperature of 5500–6400 K at
h ¼ 40–50° and an SR of 0.7. The second component
showed values of around 2000–3000 K and SR ¼ 0.94.
There are significant differences in the velocity curves
from the previous work [12,13]. The observation of
2
2
2
unity over the conditions studied, confirming no other
paths removing surface nitrogen or oxygen. [CO2], [N2]
and [N2O] designate the amount of produced CO2, N2
and N2O estimated from the AI-signals. According to
these cooperative pathways, the product ratio must be
[CO ]:[N ]:[N O] ¼ 2x + y:x:y, where x and y represent
2
2
2
the progress of N formation in the processes (i) and
2
(iii), and that of N O in the process (ii). The x/y ratio
2
represents the branching of N2/N2O formation. The
observed ratio steeply increased with increasing tem-
perature or CO pressure.
The total amount of desorbed species can also be es-
timated from the AR-signal at the collimation angle, the
sharpness of the angular distribution and the mass sen-
sitivity correction due to velocity [18]. The observed
signal at the collimation angle is proportional to the re-
action rate under the condition that the angular and
velocity distributions remain invariant. In other words,
ꢀ
1
high-velocity signals in the range of 2.5–4 km s might
be obscured in the high background in the previous
work because of low reactant pressures.

Y. Ma et al. / Chemical Physics Letters 388 (2004) 201–207
205
Fig. 4. Velocity distributions of 15N2 at TS ¼ 550 K, at h ¼ 0° and 41°, and at different 15NO/13CO ratios at a fixed 15NO pressure of 5 ꢁ 10 Torr.
ꢀ6
The average kinetic energy is indicated in hi. Typical deconvolutions are drawn by broken curves. The solid line indicates their summation.
the kinetics must be separately treated for each compo-
nent with different distributions. The observed angular
PNO or PCO because of increasing NO(a) or vacant sites.
The latter is created by removal of O(a). In fact, N2
inclined desorption showed near 2nd order in CO below
the critical PCO value (Fig. 2a). At high PCO, the surface
oxygen is mostly removed by CO and then the rate be-
comes insensitive to PCO. On the other hand, at high
temperatures, the associative process (iii) fairly removes
the surface nitrogen because it has a higher activation
energy [2] and the processes (i) and (ii) are suppressed by
decreasing amounts of NO(a). At high temperatures
above 600 K, the NO(a) density is low at low PNO, where
the process (iii) prevails (Fig. 3a), but the processes (i)
and (ii) are dominated at high PNO (Fig. 1b). In fact, at
28
distribution was approximated as 0:92fcos ðh þ 41Þ
2
8
5
þ cos ðꢀ41Þg þ 0:06 cos ðhÞ þ 0:08 cosðhÞ at T ¼ 550
S
K with the ratio of NO:CO ¼ 1:4 (Fig. 2c). The pre-fac-
tors indicate the relative intensity. At 640 K, the distri-
28
28
bution became 0:57fcos ðh þ 41Þ þ cos ðh ꢀ 41Þg þ
5
0
:28 cos ðhÞ þ 0:47 cosðhÞ (Fig. 3d), where the desorbed
amount in the inclined way was about 25% of the total
N2 formation [18]. At higher temperatures, the inclined
desorption was mostly suppressed, yielding a shift in the
reaction pathways.
4
.2. Kinetics of each process
640–680 K, the N desorbed at h ¼ 0° is highly ther-
2
malized. The amount of the thermalized component
after the distribution corrections reached about 40% of
the N2 total formation at low PCO [18]. With increasing
PCO, it increased linearly as well as the normally directed
fast N2, whereas the signal at h ¼ 40° remained invari-
ant or decreased. The thermalized component is likely to
be formed via the associative process.
Around the optimum temperature or below it, the
surface nitrogen is removed either through the inclined
desorption in (i) or as N2O in (ii). The former shares
about 80% with a mixture (NO:CO ¼ 1:1). Both pro-
cesses need the preceding process of N(a) + NO(a) !
N O(a), which may be enhanced with increasing either
2

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Y. Ma et al. / Chemical Physics Letters 388 (2004) 201–207
Fig. 5. Velocity distributions of 15N2 at 640 K, at h ¼ 0° and 41°, and at different 15NO/13CO ratios at a fixed 15NO pressure of 5 ꢁ 10ꢀ6 Torr. The
average kinetic energy is indicated in hi. Typical deconvolutions are drawn by broken curves.
4
.3. Comparison with other conditions
three surface nitrogen-removal processes involved in a
steady-state NO + CO reaction on Pd(1 1 0). The path-
way through the intermediate N O(a) prevails below
The inclined N desorption was first found by Ikai
2
2
and Tanaka [9], who performed AR-TPD measurements
of the product N2 on Pd(1 1 0) in the presence of gaseous
NO and H2 or CO. They proposed ‘the desorption-
mediated reaction without the intermediate N2O(a)’ for
this off-normal desorption because only the heating
procedure yielded the peculiar desorption and not in
their subsequent cooling [2,9]. However, this is not
surprising because in their heating, the surface was ini-
tially covered by NO, whereas in the subsequent cooling,
the absence of inclined desorption may be due to in-
sufficient NO(a). In addition, the intermediate N(a)
would be mostly removed as NH3 at their conditions of
NO ꢂ H2. In fact, the N2O formation in the process (i)
is highly sensitive to the amount of NO(a) and N(a) as
described above.
600 K and at lower PCO, whereas, at high temperatures
and high PCO, the associative desorption of nitrogen
atoms dominates the removal.
Acknowledgements
Izabela Rzeznicka is indebted to the Ministry of
Education, Science, Sports and Culture of Japan for a
2
000–2004 scholarship.
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