NOx reduction at near ambient temperatures and under lean-burn conditions on modified Pd surfaces

Article abstract of DOI:10.1002/cctc.201301033

Palladium surfaces that are modified with O atoms in the subsurface broaden the NO reduction temperature regime up to 325 K with O₂-rich NO+H₂+O₂ compositions. Compared to virgin Pd surfaces, up to 150 % higher deNO_x catalytic activity was observed with modified Pd surfaces at the reaction maximum. Molecular beam instrument and ambient-pressure photoelectron spectroscopy were employed to follow the kinetic and surface changes. These results open up a possibility to realize the cold-start reduction of NO_x (deNO_x). DeNO_x activity reported in the literature with supported Pd catalysts after a simple calcination in air compares well with our present observations. Surface modification is likely to demonstrate a high potential for other catalytic reactions at relatively low temperatures. Positive Pd to DeNOx! Pd(1 1 1) surfaces modified with O atoms in the subsurface broaden the NO reduction temperature up to 325 K with O₂-rich NO+H₂+O₂ compositions and open up a possibility to realize the cold-start reduction of NO_x (deNO_x). The 150 % higher deNO_x catalytic activity observed with modified Pd surfaces demonstrates its high potential for other catalytic reactions. Copyright

Full text of DOI:10.1002/cctc.201301033

CHEMCATCHEM
FULL PAPERS
DOI: 10.1002/cctc.201301033
NO Reduction at Near Ambient Temperatures and Under
x
Lean-Burn Conditions on Modified Pd Surfaces
[a, b]
[a, b]
Kanak Roy
and Chinnakonda S. Gopinath*
Palladium surfaces that are modified with O atoms in the sub-
surface broaden the NO reduction temperature regime up to
and surface changes. These results open up a possibility to re-
alize the cold-start reduction of NO (deNO ). DeNO activity re-
x
x
x
3
25 K with O -rich NO+H +O compositions. Compared to
ported in the literature with supported Pd catalysts after
a simple calcination in air compares well with our present ob-
servations. Surface modification is likely to demonstrate a high
potential for other catalytic reactions at relatively low tempera-
tures.
2
2
2
virgin Pd surfaces, up to 150% higher deNO catalytic activity
x
was observed with modified Pd surfaces at the reaction maxi-
mum. Molecular beam instrument and ambient-pressure pho-
toelectron spectroscopy were employed to follow the kinetic
Introduction
[6,7]
[8]
The aim of the three-way catalytic (TWC) converter used in au-
catalyst.
Dhainaut et al. demonstrate that in the absence
tomobiles is to convert pollutants (NO , CO, hydrocarbon (HC))
of O , the dissociation of NO species is assisted by chemisor-
x
2
ads
to benign gases, such as N and H O. Modern internal combus-
bed H atoms on Pd/Al O ; however, NO decomposition is not
2 3
2
2
tion engines works at a high air/fuel ratio, which is significantly
favored in the presence of O as H atoms are preferentially
2
[
1,2]
higher than the stoichiometric value (14.6).
The predomi-
consumed by O atoms. Between the two reaction pathways,
[9]
nant challenge is the reduction of NO (deNO ) under lean-
NO+H2 and H +O , the latter being faster, the effect of
x
x
2
2
burn (O -rich or fuel-lean) conditions. Recently, H -selective cat-
oxygen is detrimental on the overall NO+H +O reaction. Pd/
2 2
2
2
alytic reduction (SCR) has become of interest for deNO . H -
W/ZrO shows a high efficiency with 96% N selectivity but
2 2
x
2
[13]
[11]
SCR is a green method, as it generates only N and H O, and it
only between 393–453 K. However, Wen has shown that
2
2
has technological potential, as H is available in many post-
the inhibition of O on NO reduction at 373 K on Pd/MFI is not
2
2
combustion methods. H can be generated in diesel engines
significant, and shows activity onset from 350 K. Apparently,
2
[
3]
by the autothermal reforming of diesel. The above points en-
courage H -SCR for on-board deNO in automotive engines
there are two NO conversion maxima observed over Pd/TiO at
2
[9,10]
390 and 510 K.
It is clear that the chemical and electronic
2
x
and for static applications.
nature of the active catalyst together with the nature of the
support decide the reaction pattern/mechanism. A careful look
Rh is better known than Pd for NO reduction to N ; however,
2
[8–13]
Rh forms irreversible oxides under net oxidizing conditions,
at the literature
reveals that, in general, two different pre-
whereas Pd shows more O tolerance. This promotes Pd-based
treatments were performed. Calcination in air at 773 K, as pre-
treatment, leads to two maxima for NO reduction at 390 and
2
[
4]
TWC converters over Rh/Pt-based versions. Indeed, the kinet-
ics and mechanism of NO reduction reactions on Pd single
crystals have been investigated by a surface science ap-
[9,10]
510 K on Pd/TiO
or activity onset from 350 K on Pd/
2
[11,12]
Al O ,
whereas pre-reduced catalysts show an activity
2
3
[
5–7]
[8]
proach,
and typical heterogeneous catalysis on supported
onset around 380 K. In particular, it is essential to isolate the
role of the active Pd species to understand the fundamental
aspects and to develop a better catalyst. To the best of our
knowledge, there is no report available on the NO+H +O re-
[
8–13]
Pd catalysts lead to better deNOx.
The most critical step in NO reduction is the dissociation of
NO. Molecular NO adsorption was observed up to 400 K,
above which NO dissociates into N and O atoms on Pd-based
2
2
action on Pd(111) surfaces. We address NO reduction in the
presence of H +O under net oxidizing conditions on Pd(111)
2
2
0
[
a] K. Roy, Prof. C. S. Gopinath
and modified Pd(111) surfaces, which represent Pd and mildly
Centre of Excellence on Surface Science
CSIR - National Chemical Laboratory
Dr. Homi Bhabha Road, Pune 411 008 (India)
E-mail: cs.gopinath@ncl.res.in
d+
oxidized Pd (Pd ), respectively, to simulate the pretreatment
conditions employed for supported catalysts by many re-
[8–13]
searchers.
Homepage: http://nclwebapps.ncl.res.in/csgopinath/
[b] K. Roy, Prof. C. S. Gopinath
Catalysis Division
CSIR - National Chemical Laboratory
Dr. Homi Bhabha Road, Pune 411 008 (India)
Results and Discussion
All kinetic experiments reported here for NO+H +O /Pd(111)
2
2
surfaces were performed in a manner identical to that reported
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201301033.
[14,15]
in our earlier publications.
Kinetic measurements were per-
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 531 – 537 531

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
formed between 325 and 700 K on virgin and O -pretreated
the reactants (products) partial pressure occurs, which gives
the rate of reaction after calibration with pure gas compo-
2
Pd(111) surfaces by using a home-built molecular beam instru-
ment (MBI). Control experiments were measured with a combi-
[14,15]
nents.
A marginal increase in product formation at 450 K
15
18
nation of labeled reactants ( NO, O , D ) to measure the con-
indicates the reaction onset, and no activity was observed
below 450 K. Details on the MBI and APPES are available in the
Supporting Information, and the experimental procedure and
rate calculation are given in the Experimental Section.
2
2
tribution from different overlapping mass species, such as NH
3
15
15
(
ND , ND , NH , and NH ). Nevertheless, all the reactions are
3 3 3 3
mentioned as NO+H +O , and the reactant ratio is denoted
2
2
as x:y:z in the same order. Ambient-pressure photoelectron
The interaction of O with Pd(111) under conditions that
2
[
16]
spectroscopy (APPES) was employed to show the changes in
varied widely suggested the formation of different phases of
oxide (oxide on the surface and subsurface, metastable oxides,
the surface characteristics as a result of O dosing on Pd surfa-
2
[17–20]
[19]
ces.
and bulk oxides).
Earlier reports from our group have
shown kinetic evidence of sub-
surface oxygen in Pd(111),
which influences CO oxidation.
The effect of subsurface oxygen
is interesting because it changes
the electronic nature of the sur-
face, which in turn changes the
charge-transfer features and
hence the adsorption character-
[17–20]
istics.
A simple heat treat-
ment to 1200 K desorbs the sub-
surface oxygen to bring back
[18,19]
the original surface.
To un-
derstand the effect of subsurface
oxygen, we performed the
NO+H +O reaction on Pd(111)
2
2
after populating oxygen in the
Pd(111) subsurface (referred to
as M-Pd(111)). The Pd(111) sub-
surface was populated with
18
oxygen by dosing O at 900 K
2
for 1200 s (Figure 1b). This was
followed by H titration at 525 K,
2
16
and then the H + O (1:1) reac-
2
2
tion was measured at 500 K.
1
8
There is a clear uptake of O at
2
900 K indicated by a decrease in
the partial pressure at the initial
Figure 1. a) Kinetic data for NO+2H
2 2
+O /Pd(111) as a function of temperature and b) time evolution of different
shutter opening at t=15 s. After
1
8
mass signals (28, 32, 36, 44, 46, and 48 amu) after dosing
O
2
on Pd(111) surfaces at 900 K for 20 min followed by
(1:1) reaction at 500 K. Shutter close and open operations are shown
by upward and downward arrows, respectively. TPD was performed after the reaction was completed. The N and
O, and NH and N O signals in panel a were multiplied by two and four, respectively. c) The surface modification
procedure is depicted schematically to show the diffusion of O (red solid circles) in subsurface layers; virgin and
18
1
6
the O dosage, no H O produc-
2 2
H
2
titration at 525 K and then the H
2
+ O
2
tion was observed during H ti-
2
2
H
2
3
2
tration, which rules out the pos-
sibility of any surface-chemisor-
1
8
0
d+
surface modification is indicated by yellow and dark grey to denote the Pd and oxidized Pd , respectively. H
2
ox-
O exclusively without any O; this demonstrates that the subsur-
face oxygen is fully retained in the subsurface layers but influences the electronic nature of the surface layers.
1
8
1
6
18
bed
O. In the subsequent
reaction on the
M-Pd(111) surface, only H O
idation was performed with
2 2
O to produce H
16
H + O
2
2
2
(
18 amu) was observed, whereas
18
As an example of the results obtained, Figure 1a shows the
the H2 O (20 amu) signal showed no change in intensity. This
directly demonstrates that the subsurface oxygen does not dif-
fuse out during the reaction, at least below 900 K. However,
raw data for the NO+H +O (1:2:1) beam composition reac-
2
2
tion on a Pd(111) surface. The initial reaction temperature was
set at 400 K, and the reaction rate was measured in the steady
state, which is normally reached in 60 s. Subsequently, the
temperature was increased by 50 K, and the rate was mea-
sured; this step was repeated up to 700 K. The steady-state
rate was measured by closing/opening the shutter deliberately.
As a result of this oscillation, a definite increase (decrease) in
18
18
O desorbs at 1050–1200 K, and a small amount of O-con-
2
18
18
taining species (H2 O and C O (not shown)) desorb between
900 and 1000 K in the temperature-programmed desorption
(TPD) experiment. A minor amount of CO is a result of the in-
teraction of carbon impurities that segregate from the bulk to
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 531 – 537 532

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
surface, and some hydrogen diffusion into the subsurface de-
sorbs as water.
enhanced activity. N and N O were observed under steady-
2
2
state conditions at and above 400 K. Only a marginal amount
The surface modification changes observed in Figure 1b are
represented schematically in Figure 1c. Oxygen in the subsur-
face is retained within and does not participate directly in the
reaction. However, it influences the electronic state of the sur-
of NH was observed. A small amount of water formation was
3
observed under steady-state conditions up to 325 K. The en-
hancement in the overall rate of reaction is evident from the
formation of a large quantity of products. However, the reac-
tion maximum has shifted to 500–550 K (Figure 2b) from 550–
600 K on virgin Pd(111) surfaces (Figure 2a). 3) The 1:1:2 and
d+
face layers to give partially oxidized Pd from the initial zero-
0
valent Pd state. The chemisorption of various reactants is in-
fluenced significantly by the partially oxidized surfaces. These
1:1:3 ratios on M-Pd(111) surfaces show the maximum N for-
2
modified surfaces have been utilized to perform deNO reac-
mation at 450 and 375–425 K, respectively. Further N and H O
x
2
2
tions to demonstrate the influence of surface modification.
Below 1000 K, subsurface oxygen stays within the subsurface
layers and influences the electronic nature of the surface with-
out taking part in the catalysis reaction. Notably, the surface-
production is extended up to 325 K in the latter case. In spite
of a decreasing flux of NO (F ) from 33.3 (1:1:1) to 20% (1:1:3)
NO
with a concurrent increase in FO2 to 60% at a 1:1:3 composi-
tion, a sustainable NO reduction observed close to ambient
temperatures is appealing. A concurrent increase in the partial
pressure of H O and N well above the steady-state pressure
chemisorbed oxygen desorbs at around 750 K, and O uptake
2
at 900 K testifies the diffusion of atomic oxygen into subsur-
2
2
[
18–20]
face layers of Pd.
values at the point of shutter opening at 350 and 325 K
(shown in the dashed box) demonstrates sustainable NO disso-
ciation. Notably, the high-temperature activity at 700 K is re-
duced significantly in both cases compared to virgin Pd(111)
surfaces (Figure 2a). In spite of
We explored NO reduction as a function of temperature and
the NO+H +O ratio on Pd(111) and M-Pd(111) surfaces, and
2
2
the results are shown in Figure 2 for a virgin Pd(111) surface
the decreasing F , N O forma-
NO
2
tion was observed, but NH pro-
3
duction remains at marginal
levels. In general, the selectivity
of N , N O, and NH is 76Æ5,
2
2
3
1
6Æ5, and 5Æ5%, respectively,
at the reaction maximum. N
2
was
produced
exclusively
ꢀ
375 K on M-Pd(111) with an
O -rich mixture. These catalytic
2
runs were repeated at least fifty
times to demonstrate the true
influence of the modified Pd sur-
face for NO decomposition at
ambient temperatures. These
observations also support our
conclusions that subsurface
oxygen does not participate in
the reaction directly, which
would otherwise make the low-
temperature activity disappear.
The steady-state rate values
reported in Figure 3 follow the
above trend for different compo-
sitions. Rate values, especially,
those observed at low tempera-
tures (Figure 3), are very relevant
2 2
Figure 2. A comparison of NO+H +O reactions performed on a) virgin Pd(111) and b–d) surface-modified
Pd(111) between 700 and 325 K. Surface modification was performed by the first part of the oxygen-dosing ex-
periment shown in Figure 1b, which induces low-temperature activity closer to ambient temperatures. Reactions
were preformed from low to high temperature and vice versa, and no significant difference or hysteresis was ob-
served between them. Indeed, the results shown in panel a were obtained by performing the modification from
low to high temperature, but the plot is reversed for comparison. The y axis is the same in all panels.
with 1:1:1 composition and on M-Pd(111) surfaces with 1:1:1,
:1:2, and 1:1:3 ratios. The following points summarize the im-
for cold deNO conditions. Highlights of the results are as fol-
x
1
lows: (a) Although the steady-state rate of N production is
2
portant findings from these results: 1) virgin Pd(111) surfaces
show a NO decomposition onset at 450 K (as shown in Fig-
low ꢀ375 K, highly selective N formation is observed on M-
2
Pd(111) surfaces. This observation is interesting as it demon-
strates the NO dissociation closer to ambient temperatures
ure 1a) and N and N-containing product formation activity at
2
and above 500 K. A small amount of water production was ob-
served under steady-state conditions above 400 K (Figure 2a).
and without any NH production in the presence of excess
3
oxygen. It is very likely that NO molecules compete more
strongly for adsorption sites than oxygen, and the surface may
be effectively dominated by NO+H2 (vide infra, Figure 5).
2) A 1:1:1 ratio on M-Pd(111) surfaces shows an extended ac-
tivity with a characteristic change in the product pattern and
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 531 – 537 533

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
production shows a similar trend
as that of N . It is also known
2
that supported catalysts show
a decrease in NO dissociation ac-
tivity if CO or water is added to
2
[2,21]
the reactants.
We simulated
this condition by adding 2%
CO , and the results show hardly
2
any effect on NO dissociation
(
Figure S1). It is likely that the
support and metal–support in-
teraction play a significant role
to influence the NO dissociation
activity in the presence of CO2/
H O; however, this aspect is
2
beyond the scope of this study.
To investigate the influence of
precalcination or preoxidation
treatments of Pd, the oxidation
state of Pd was measured by
Figure 3. Steady-state rate measured for N-containing products for a) N
2
, b) NH
3
, and c) N
2
O. The rate value for
NO decomposition is equal to 2RN2+2RN O+RNH3 as the formation of each N
2
and N
2
O molecule requires the disso-
2
ciation of two NO molecules.
Figure 4. Pd3d core-level photoelectron spectra recorded at different partial
pressures of O by using the APPES system. The intensity of the Pd3d5/2 core
2
level is normalized to that of the clean Pd surface. The inset shows the de-
0
convolution that shows the presence of Pd , Pd with subsurface oxygen
(Pd
x y
O ), and PdO. Peak fitting parameters are given in the Supporting Infor-
mation.
Figure 5. Reactant uptake under steady-state reaction conditions measured
for the reaction shown in Figure 2c with a 1:1:2 composition between 500
and 325 K. The shutter close and open operations are shown by dashed and
(
b) M-Pd(111) shows a higher N production rate values than
dotted lines. Sustainable NO and H adsorption was observed at lower tem-
2
2
2
peratures, whereas no O adsorption was observed.
Pd(111) surfaces and extended activity at lower temperatures
up to 375 K) with a 1:1:1 reactant composition. (c) 1:1:2 and
:1:3 compositions exhibit N production up to 350 and 325 K,
(
1
2
respectively. Moreover, the NO content decreases with increas-
X-ray photoelectron spectroscopy (XPS). APPES measurements
were performed on Pd(111) and polycrystalline Pd surfaces at
ing O content in the reaction mixture. A relatively steep de-
2
crease in N production at Tꢁ550 K with a 1:1:3 composition,
573 K in the presence of O at different pressures, and the re-
2
2
compared to that of other compositions, is likely because of
the onset of Pd O formation. (d) NH production shows a com-
sults are shown in Figure 4. There is evidence for changes in
the nature of Pd surfaces owing to their interaction with
oxygen. Our results on the Pd(111) surface are in good agree-
x
y
3
plex trend. A 1:1:3 (1:1:2) composition shows the maximum
minimum) NH production at temperatures >425 K; a 1:1:1
[17]
(
ment with those of Ketteler et al. and Teschner et al. and
hence will not be discussed further. APPES results obtained on
polycrystalline Pd surfaces are directly relevant to real-world
3
composition shows a similar trend on Pd(111) and M-Pd(111),
except for a shift to low temperatures with the latter. (e) N O
2
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 531 – 537 534

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Pd-based catalysts as both surfaces are similar in terms of the
variety of defects and multiple adsorption sites and hence the
results can be correlated directly rather than compared to
those of a single crystal.
on supported Pd catalysts. A small amount of Pd (ꢀ1 wt%) on
any support is likely to be in the nanoparticulate form with
a high defect density. Oxygen diffusion into the subsurfaces of
Pd particles is easily possible at moderate temperatures
around 573 K, which will modify the surface as a result of calci-
nation in air. However, the pre-reduced catalyst exhibited
a metallic Pd character, which is different from the catalysts
that are pre-calcined in air or pre-oxidized. When the reactions
were measured on surfaces with different oxidation states,
they show different activity because of changes in the elec-
tronic structure and hence the interaction with reactants. In
any case, chemisorption involves charge transfer and this is se-
verely influenced by the surface electronic structure and hence
the subsequent catalysis. A correlation between NO reduction
activity and surface electronic structure reveals that the M-
Pd(111) surface is partially oxidized, which decreases the sur-
face electron density. Oxygen atoms that are diffused into the
subsurface layers interact with Pd to form Pd O (x>y). A sig-
The spectrum obtained on clean polycrystalline Pd shows
a typical metallic Pd3d5/2 feature at 335.0 eV. In the presence
of O up to 0.2 mbar and 573 K, the Pd3d core level shows
2
5/2
a broadening at 335.7 eV attributed to Pd O . However, on in-
x
y
creasing the pressure to 0.3 mbar, a distinct shoulder appears
at 336.5 eV that then grows into a full peak, which is attributed
[
22]
to PdO. In addition to metallic Pd, both of the above fea-
[
17]
tures at 335.7 and 336.5 eV are present, which is evident
from the deconvolution shown in Figure 4 (inset). It is also
clear that PdO grows at the expense of metallic Pd. A further
increase in O pressure and/or temperature increases the sur-
2
face concentration of PdO. These results compare well with
[
17]
those of Ketteler et al. on Pd(111) at 0.35 Torr and 660 K.
The defects present on polycrystalline Pd surfaces enhance
oxygen diffusion and hence PdO was facilitated under less
x
y
nificant amount of electron density is transferred from Pd to
oxygen to form Pd O , which in turn decreases the overall elec-
[
19]
severe conditions.
Notably, the intensity of the valley be-
x
y
tween the metallic Pd and PdO features decreases with in-
tron density of surfaces. Electron-deficient M-Pd surfaces en-
hance NO dissociation at the cost of O2 adsorption below
400 K. This is further confirmed from the analysis of reactant
adsorption below 400 K. The adsorption of reactants under
steady-state conditions between 500 and 325 K for the results
reported in Figure 2c with a 1:1:2 composition are shown in
Figure 5. The adsorption of all reactants can be seen clearly be-
tween 375 and 500 K through an increase (decrease) in the
partial pressure for shutter-closed (open) operations. However,
below 375 K, no O2 adsorption could be observed, even
creasing O pressure, which demonstrates the coexistence of
2
three surface components (Pd, Pd O , and PdO) under the pres-
x
y
ent experimental conditions. However, NO reduction reactions
measured on PdO-dominated surfaces show a much lower NO
conversion, which indicates the inactive nature of the surface.
A simple reduction treatment of Pd surfaces in H at 0.1 mbar
2
and 573 K, exposed previously to 0.6 mbar O2 at 573 K
(Figure 4), reduces only the surface PdO feature to metallic Pd;
indeed, the spectrum recorded after the reduction treatment is
the same as that of the Pd surface treated with 0.2 mbar O at
though there is plenty of O available in the gas phase; where-
2
2
573 K. This highlights that the surface modification that results
as sustainable NO and H adsorption was observed even at
2
from subsurface oxygen is retained, even in the presence of
hydrogen, under the APPES and reaction conditions in the
MBI. Our results shown in Figure 1b are in full agreement with
the above results and increase the reliability of the correlation.
325 K. Indeed, this supports a cationic M-Pd(111) surface that
hinders oxygen adsorption as the electron-donating capacity
of the cationic surface decreases considerably. A simple com-
parison of reactants adsorption at a reaction maximum (500 K)
and <400 K demonstrates that only the NO+H2 reaction
occurs in the latter, even though oxygen makes up 50% of the
reactant content in the gas phase (Figure 5).
[
11]
It is worth noting an observation made by Wen: a preoxi-
dized Pd/MFI catalyst in O at 773 K was evaluated for NO re-
2
duction with H at 373 K and compared with that of the same
2
material after H2 reduction at 573 K. The reduced Pd/MFI
shows NO conversion from the beginning, whereas the NO
conversion gradually increases with the preoxidized catalyst
and shows a similar activity to that of reduced Pd/MFI after ap-
proximately 60 min. Although there is no mention of oxygen
diffusion into Pd subsurfaces in Ref. [11], this is inevitable
under atmospheric pressure and 773 K. Pre-reduction converts
Conclusions
NO conversion to innocuous N2 with increasing efficiency
under net oxidizing conditions will be attempted as long as
gasoline/diesel-driven vehicles and power-plants exist. A po-
tential remedy for this problem by the surface modification of
Pd is suggested by minimizing oxygen adsorption under net
oxidizing conditions. Indeed, fine tuning of the pretreatment
the surface PdO to Pd, which also happens under NO+H reac-
2
tion conditions on a preoxidized catalyst to result in a gradual
improvement in activity. However, we believe that the high
deNO activity observed at 373 K is likely because of modified
of Pd-based supported catalysts in air/O could show improved
x
2
surfaces, similar to the concept proposed in this report. Never-
theless, more work on supported catalyst systems is required
regarding surface modification and correlation with low-tem-
deNO activity in an O -rich environment under ambient condi-
x
2
tions. This aspect needs to be evaluated carefully. A simple
change in the surface modification that could broaden the cat-
alytic activity regime, especially towards lower temperatures,
has been demonstrated. Although it is demonstrated here for
NO reduction, similar surface modification is very likely to
perature deNO activity.
x
The results shown in Figures 2–4 explain the importance of
surface modification through oxygen diffusion as well as the
[
8–13]
[19]
contradictory results
observed for the NO+H +O reaction
broaden the catalytic activity regime, especially towards am-
2
2
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 531 – 537 535

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
bient temperatures and it is worth exploring for different cata-
lytic reactions.
Experimental Section
All kinetic measurements were performed by using a home-built
MBI using an effusive mixed molecular beam. The detailed descrip-
tion of the set up can be found in the Supporting Information and
[14–15]
in our earlier reports.
Isothermal kinetic experiments with
NO+H and NO+H +O were conducted on clean Pd(111) surfaces
2
2
2
between 400 and 700 K and on a modified (subsurface populated
with oxygen) Pd(111) surface between 325 and 700 K with
xNO:yH :zO ; hereafter x:y:z represents the composition of the re-
2
2
spective individual components varied from x=1, y=1–2, and z=
–3. Different steps involved in most of the experiments can be
0
elaborated with reference to Figure 1a: (1) First the temperature of
the crystal is set at 400 K. At t=10 s, a molecular beam of a mixture
of reactants was turned on with the shutter in the intercepting po-
sition. This leads to an immediate increase in the reactant partial
pressure. As the shutter is in the intercepting position the beam
cannot react directly with Pd(111), but some adsorption from the
background cannot be avoided at this stage. (2) The shutter was
removed at t=15 s. The reactant beam can then directly react
with Pd(111) kept at 400 K. A clear decrease in the partial pressure
of the reactants (NO, H , and O ) was observed, which indicates
À1
Figure 6. Steady-state rate [MLs ] calculation for the data collected at 600 K
shown in Figure 1a. The rate of NO adsorption is equal to the sum of all
N-containing products. Similarly, the rate of oxygen adsorption (from O
NO) is equal to that of its consumption through water formation.
2
and
2
2
the adsorption of the reactants on Pd(111). An increase in the par-
[
16]
APPES was employed to show the changes in surface character-
istics as a result of O2 dosing on Pd surfaces. A description of
APPES is available in the Supporting Information as well as in our
tial pressure of the products N2 (28 amu), H O (18 amu), NH3
2
(
17 amu), and N O (44 amu) indicates their evolution. The system
2
was allowed to evolve until a steady state was reached, which gen-
erally occurred within 60 s of unblocking the beam under the ex-
perimental conditions employed here. The period from the un-
blocking of the beam to when the steady state was reached is
termed as the transient state (TS). (3) In the steady state, the rate
of the reaction can be measured by shutter operation (deliberately
blocking the shutter for ꢂ30 s) for the particular temperature (t=
[16]
earlier publication.
Acknowledgements
K.R. thanks CSIR, New Delhi for a senior research fellowship.
Funding from two funding agencies are acknowledged; BRNS
project (2011/37C/36/BRNS), and DST-SERB, New Delhi (SR/S1/PC-
1
20–150 s for 400 K). (4) After the shutter operation for a given
temperature was performed, the crystal was quickly heated to the
next temperature, and the system was allowed to reach steady-
state conditions to measure the rate at the next temperature. This
procedure was repeated for several temperatures up to 700 K in
this example. (5) Finally the beam was turned off at t=900 s (Fig-
ure 1a). After the pressure of the ultra-high vacuum (UHV) cham-
ber reached the initial background level, the TPD of all the relevant
species was recorded.
1
6/2012).
Keywords: heterogeneous catalysis · nitrogen · oxygen ·
palladium · surface chemistry
[
1] P. G. Savva, C. N. Costa, Catal. Rev. Sci. Eng. 2011, 53, 91–151.
The partial pressures of the different gases were then converted
into reaction rates by following a calibration procedure described
[2] Z. Liu, J. Li, S. I. Woo, Energy Environ. Sci. 2012, 5, 8799–8814.
[
3] a) G. A. Deluga, J. R. Salge, L. D. Schmidt, X. E. Verykios, Science 2004,
303, 993–997; b) S. Velu, N. Satoh, C. S. Gopinath, K. Suzuki, Catal. Lett.
[14–19]
in our earlier publications.
As an example of these calculations,
2
002, 82, 145–152; c) V. Subramanian, E. S. Gnanakumar, D.-W. Jeong,
W.-B. Han, C. S. Gopinath, H. S. Roh, Chem. Commun. 2013, 49, 11257–
1259.
Figure 6 displays the temporal evolution of the (calibrated) reac-
tion rates for all the products and reactants during the steady-
1
state part of the experiment performed at 600 K with a 1:2:1 NO+
[
4] a) S. Tagliaferri, R. A. Koppel, A. Baiker, Stud. Surf. Sci. Catal. 1998, 116,
61–66; b) C. S. Gopinath, F. Zaera, J. Catal. 2001, 200, 270–287; c) S. Na-
garajan, C. S. Gopinath, J. Indian Inst. Sci. 2010, 90, 245–260.
À1
H +O beam composition. The rate of NO uptake (0.041 MLs ) is
2
2
almost equal to that of all N-containing products. Similarly, the rate
of oxygen adsorption and generation from NO is equal to its con-
sumption through water formation, within the experimental error
[5] a) Y. Ma, T. Matsushima, J. Phys. Chem. B 2005, 109, 1256–1261; b) C. A.
de Wolf, B. E. Nieuwenhuys, Surf. Sci. 2000, 469, 196–203; c) V. Bustos,
C. S. Gopinath, R. UÇac, F. Zaera, G. Zgrablich, J. Chem. Phys. 2001, 114,
limit. The formation of one N molecule requires the dissociation/
2
10927–10931; d) F. Zaera, S. Wehner, C. S. Gopinath, J. L. Sales, V. Gar-
conversion of two NO molecules, which also applies to N O; hence
2
guilo, G. Zgrablich, J. Phys. Chem. B 2001, 105, 7771–7774.
the rate values for N and N O were multiplied by two to account
2
2
[
[
6] H. Conrad, G. Ertl, J. Kꢁppers, E. E. Latta, Surf. Sci. 1977, 65, 245–260.
7] a) K. Thirunavukkarasu, K. Thirumoorthy, J. Libuda, C. S. Gopinath, J.
Phys. Chem. B 2005, 109, 13272–13282; b) K. Thirunavukkarasu, K. Thir-
umoorthy, J. Libuda, C. S. Gopinath, J. Phys. Chem. B 2005, 109, 13283–
13290; c) S. Nagarajan, K. Thirunavukkarasu, C. S. Gopinath, J. Counsel,
L. Gilbert, M. Bowker, J. Phys. Chem. C 2009, 113, 9814–9819; d) S. Na-
for the NO conversion and, similarly, the H and O adsorption. The
2
2
proportions expected from the stoichiometry of the overall reac-
tion are maintained within the experimental error limits. This stoi-
chiometry was found to hold for most of the reaction conditions
used in our experiments, especially below 600 K.
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 531 – 537 536

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
garajan, K. Thirunavukkarasu, C. S. Gopinath, J. Phys. Chem. C 2011, 115,
[16] K. Roy, C. P. Vinod, C. S. Gopinath, J. Phys. Chem. C 2013, 117, 4717–
4726.
21299–21310; e) S. Nagarajan, K. Thirunavukkarasu, C. S. Gopinath, S. D.
Prasad, J. Phys. Chem. C 2011, 115, 15487–15495; f) V. Johꢂnek, S.
Schaꢁrmann, M. Laurin, C. S. Gopinath, J. Libuda, H. J. Freund, J. Phys.
Chem. B 2004, 108, 14244–14254.
8] a) F. Dhainaut, S. Pietrzyk, P. Granger, Top. Catal. 2007, 42–43, 135–141;
b) C. Dujardin, I. Twagirashema, P. Granger, J. Phys. Chem. C 2008, 112,
[17] a) G. Ketteler, D. F. Ogletree, H. Bluhm, H. Liu, E. L. D. Hebenstreit, M. Sal-
meron, J. Am. Chem. Soc. 2005, 127, 18269–18273; b) D. Teschner, A.
Pestryakov, E. Kleimenov, M. Hꢃvecker, H. Bluhm, H. Sauer, A. Knop-Ger-
icke, R. Schlçgl, J. Catal. 2005, 230, 186–194.
[18] F. P. Leisenberger, G. Koller, M. Sock, S. Surnev, M. G. Ramsey, F. P. Netzer,
B. Klçtzer, K. Hayek, Surf. Sci. 2000, 445, 380–393.
[19] a) S. Nagarajan, K. Thirunavukkarasu, C. S. Gopinath, J. Phys. Chem. C
2009, 113, 7385–7397; b) C. S. Gopinath, K. Thirunavukkarasu, S. Nagar-
ajan, Chem. Asian J. 2009, 4, 74–80; c) K. Thirunavukkarasu, S. Nagara-
jan, C. S. Gopinath, Appl. Surf. Sci. 2009, 256, 443–448.
[20] A. F. Lee, J. N. Naughton, Z. Liu, K. Wilson, ACS Catal. 2012, 2, 2235–
2241.
[
[
1
7183–17192.
9] A. Ueda, T. Nakao, M. Azuma, T. Kobayashi, Catal. Today 1998, 45, 135–
38.
1
[
10] a) N. Macleod, R. Cropley, R. M. Lambert, Catal. Lett. 2003, 86, 69–75;
b) N. Macleod, R. Cropley, J. M. Keel, R. M. Lambert, J. Catal. 2004, 221,
20–31.
[
[
[
11] B. Wen, Fuel 2002, 81, 1841–1846.
12] J. B. Yang, O. Z. Fu, D. Y. Wu, S. D. Wang, Appl. Catal. B 2004, 49, 61–65.
13] M. Leicht, F. J. P. Schott, M. Bruns, S. Kureti, Appl. Catal. B 2012, 117 –
[21] G. C. Mondragꢄn Rodrꢅguez, B. Saruhan, Appl. Catal. B 2010, 93, 304–
313.
1
18, 275–282.
[22] C. Murali, M. S. Shashidhar, C. S. Gopinath, Tetrahedron 2007, 63, 4149–
4155.
[
14] a) K. Thirunavukkarasu, C. S. Gopinath, Catal. Lett. 2007, 119, 50–58;
b) F. Zaera, C. S. Gopinath, J. Chem. Phys. 1999, 111, 8088–8097.
15] a) C. S. Gopinath, F. Zaera, J. Catal. 1999, 186, 387–404; b) F. Zaera, C. S.
Gopinath, J. Mol. Catal. A 2001, 167, 23–31; c) C. S. Gopinath, F. Zaera,
J. Phys. Chem. B 2000, 104, 3194–3203.
[
Received: December 5, 2013
Published online on January 22, 2014
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 531 – 537 537