Three-way catalytic converter reactions aspects at near-ambient temperatures on modified Pd-surfaces

Article abstract of DOI:10.1016/j.crci.2015.11.019

The dissolution of oxygen in palladium plays an important role in palladium catalysis. The present study shows that the surface modification (SM) due to the dissolution of atomic oxygen into the subsurfaces of palladium can be used as a control to tune its catalytic activity. CO oxidation and NO?+?H₂?+?O₂ reaction was separately carried out on metallic Pd and on surface modified Pd using a molecular beam instrument and the results were compared. The metallic Pd does not show activity below 400?K for both reactions, whereas the SM-Pd shows activity at near-ambient temperatures. The electronic change due to SM was investigated using ambient pressure photoelectron spectroscopy, and the investigation clearly shows the effect of subsurface oxygen in the ambient temperature activity of palladium.

Full text of DOI:10.1016/j.crci.2015.11.019

C. R. Chimie xxx (2016) 1e7
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Full paper/M eꢀ moire
Three-way catalytic converter reactions aspects at near-
ambient temperatures on modified Pd-surfaces
a
a
a
a
Kanak Roy , Ruchi Jain , Manoj Kumar Ghosalya , Kasala Prabhakar Reddy ,
a, b, *
Chinnakonda S. Gopinath
Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
a
b
Centre of Excellence on Surface Science, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The dissolution of oxygen in palladium plays an important role in palladium catalysis. The
present study shows that the surface modification (SM) due to the dissolution of atomic
oxygen into the subsurfaces of palladium can be used as a control to tune its catalytic
Received 29 June 2015
Accepted 26 November 2015
Available online xxxx
activity. CO oxidation and NO þ H
2
þ O
2
reaction was separately carried out on metallic Pd
and on surface modified Pd using a molecular beam instrument and the results were
compared. The metallic Pd does not show activity below 400 K for both reactions, whereas
the SM-Pd shows activity at near-ambient temperatures. The electronic change due to SM
was investigated using ambient pressure photoelectron spectroscopy, and the investiga-
tion clearly shows the effect of subsurface oxygen in the ambient temperature activity of
palladium.
Keywords:
Heterogeneous catalysis
Environmental catalysis
Palladium
Surface modification
Molecular beam
Photoelectron spectroscopy
© 2016 Acad eꢀ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
without addressing the particular reaction system. The
reason is the role of reactants imparting influence in the
Any heterogeneous catalyst works within a certain
range of pressure and temperature, beyond which the
catalyst fails to show efficient activity. The most important
part of the catalyst development, therefore, is to design a
catalyst which shows activity in a broad range of pressure
and temperature. Modifications of the catalyst structure
and morphology, change of supports, change of active
metals are basic parameters which are employed and
investigated to broaden the catalysis regime of a catalyst.
However, these changes could bring into expensive modi-
fications to catalyst synthesis as well as in the reactor re-
quirements. Without changing its material elements, can
we alter the boundary conditions of a heterogeneous
catalyst in a simple manner. This seemingly elementary
question however turns out to be difficult to answer
catalyst surface modification (SM). SM of a heterogeneous
catalyst can influence its catalytic activity to shift the
boundary conditions and this can be used as a handle to
tune the activity regime of a catalyst. The elementary steps
of heterogeneous surface reactions, such as adsorption and
desorption, are simultaneous bond making and bond
breaking processes, and they depend on the electronic
changes of a surface. If the SM of a solid catalyst can induce
electronic changes in the catalyst surface, we believe, that it
can also change the activity of the catalyst. The present
manuscript shows the shift in activity regime of Pd due to
2
SM in the CO oxidation and reduction of NO by H .
CO oxidation and NO reduction are main reactions
which take place in a three-way catalytic converter (TWC),
and Pd is well-known metal used in it. However, like other
noble metals, Pd also is ineffective for CO oxidation below
4
00 K because of strong chemisorption of CO to Pd, and this
*
Corresponding author. Catalysis Division, National Chemical Labora-
phenomenon is known as “CO poisoning”. The other aspect
is high dissoluting nature of Pd for small species like C, H,
tory, Dr. Homi Bhabha Road, Pune 411 008, India.
E-mail address: cs.gopinath@ncl.res.in (C.S. Gopinath).
http://dx.doi.org/10.1016/j.crci.2015.11.019
1
631-0748/© 2016 Acad eꢀ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: K. Roy, et al., Three-way catalytic converter reactions aspects at near-ambient temperatures on
modified Pd-surfaces, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.11.019

2
K. Roy et al. / C. R. Chimie xxx (2016) 1e7
and O [1e3]. Especially when oxygen is involved in the
reaction, the situation is intricate. The atomic oxygen dif-
fuses into Pd to subsurfaces and/or bulk regions depending
on the severity of the reaction conditions. Atomic O diffu-
sion into Pd subsurfaces was observed at and above 500 K
by many groups [4e11], and the extent of O diffusion in-
ray source (VG Scienta, MX 650), a dual anode (Al K
a, Mg
K
a) source, and a helium UV discharge lamp. The reactant
gases are introduced into the analysis chamber through a
variable leak valve. The samples can be cleaned by ion
sputtering in the preparation chamber, which is connected
to the main analysis chamber, and is separated by using a
manual gate valve.
2
creases linearly up to 900 K, though surface O desorption
reaches a maximum at approximately 750 K for Pd(111)
surfaces. Lundgren et al. [12] recently reported CO
adsorption patterns on different Pd surfaces and the nature
of Pd during the CO oxidation reaction under in situ con-
ditions. Kinetic and operando studies on NO reduction are
also reported by Granger et al. [13]. However, the role of
dissolved O in the catalytic activity of Pd has not been
explored. Herein we report the role of subsurface oxygen in
the catalytic activity of Pd in CO oxidation and selective
3
. Results and discussion
3.1. Surface modification
The clean Pd(111) surface was modified with oxygen in
the subsurface layers by direct dosing of O
2
beam for
ꢁ1
10e110 min at 900 K with an oxygen flux of 0.5 ML s . CO
titration was carried out at 600 K to remove any surface
oxygen, after the above surface modification. A significant
subsurface population starts from a dosing period of 10 min
and then gradually improves until a threshold/saturation
coverage of 0.45 ML in subsurface layers [11]. Pd surfaces
with 50 min of O dosing shows a significantly higher ac-
2
tivity compared to the virgin Pd surfaces. The results re-
ported in the present communication is after surface
modification by dosing O
catalytic reduction of NO by H
2
.
2
. Experimental section
The kinetic experiments were carried out in a home
built molecular beam instrument (base pressure
ꢁ10
2
ꢀ 10
Torr). The details about the system and molecular
beam generation set up can be found elsewhere [14, 15]. In
brief, the molecular beam instrument is a cylindrical
chamber equipped with a turbo molecular drag pump, a
doser assembly with a microcapillary, a four axis manipu-
lator for sample manipulation, an argon sputter-ion gun,
and a quadrupole mass spectrometer (QMS). The QMS was
placed out of site of the beam to avoid angular effects from
the single crystal as well as direct interaction with the re-
actants. The effusive molecular beam doser consists of a
ꢁ1
2
beam (flux ¼ 0. ML s ) at 900 K
1
3 mm disk multichannel array made up of microcapillary
glass tubes of 1 mm in length and 10 m in diameter each
Collimated Holes Inc), which helps to maintain a very high
m
(
beam profile. The gaseous reactants and products were
detected using the QMS. Pd(111) single crystal (8 mm
diameter and 1 mm thick, MaTeck, Germany) and Pd foils
(
MaTeck) were spot welded on a 0.5 mm thick Ta wire. The
samples were resistively heated and the heating was
controlled by using a homemade temperature controller
fitted with a PID control unit (eurotherm). The temperature
was measured using a CreAl (K type) thermocouple welded
on the periphery of the Pd sample. Pd surfaces were
þ
cleaned by Ar sputtering in an oxygen environment (total
ꢁ
6
pressure 1.5 ꢀ 10 Torr) at 900 K followed by flashing at
1100 K. All gases were used with high purity and without
any further filtration. In this set up the reactants are mixed
first in the gas manifold and the pressure is monitored by
using a baratron gauge. The reaction mixture is introduced
as
a molecular beam through the doser assembly,
controlled by a variable leak valve and the leak rate is al-
ways kept identical for the whole set of reactions.
The ambient pressure experiments were carried out in a
custom built near-ambient pressure photoelectron spec-
trometer (NAPPES) (Prevac, Poland). In this set up, XPS can
be performed up to 1 mbar and UVPES up to 0.3 mbar with
an analyzer cone aperture of 0.8 mm. Full details about the
system is available in Refs. [16,17]. The analysis chamber
equips a differentially pumped high pressure electron
Fig. 1. Surface modification of Pd(111) by dosing labelled 18O
in a molecular
2
O was dosed at 900 K for 20 min, followed by CO titration at
2
18
beam setup.
00 K to remove any surface adsorbed oxygen. A 1:1 CO þ
beam was dosed on SM-Pd to show that subsurface oxygen does not diffuse
1
6
6
2
O molecular
18
18
to form C
2 2
O or CO O. Labelled CO traces are vertically shifted to avoid the
analyzer (VG Scienta, R3000 HP), a monochromatic Al Ka X-
overlap with other traces.
Please cite this article in press as: K. Roy, et al., Three-way catalytic converter reactions aspects at near-ambient temperatures on
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K. Roy et al. / C. R. Chimie xxx (2016) 1e7
3
for 50 min. Fig. 1 shows a typical surface modification
(1:1:1) from 325 K to 700 K. All the measurements were
carried out in an identical manner mentioned in our earlier
publications [19, 20]. The sample was kept at an initial
experiment in the molecular beam set up for 20 min of
18
dosing with labelled oxygen (
2
O ). All relevant mass
numbers (12, 18, 32, 36, 28, 44 and 46 amu) were followed
during the course of the reaction. The CO titration, followed
by oxygen dosing, does not produce any significant isotopic
temperature and then a beam of reactants NO þ H
2
þ O
2
was turned on; the reaction reaches to the steady state (SS)
within a very short period (about 60s), and the steady state
rate of formation was determined by deliberately closing
and opening the shutter (i.e. blocking and releasing the
beam to interact with the sample). Once the SS rate of the
reaction has been measured for a particular temperature,
the temperature of the sample was increased by 50 K. This
process was repeated up to 700 K. Fig. 2 shows the com-
18 16
CO
CO þ O
exclusively produces CO
perature programmed desorption (TPD) was carried out
2
(C
O
O) as evidenced from the 46 amu signal.
reaction carried out, after the above titration,
. After the above reaction, tem-
2
2
18
with a heating rate of 5 K/sec (not shown).
2
O desorption
was seen at higher temperatures around 1000e1100 K. A
similar observation was recorded by Leisenberger et al. [18]
from TPD spectroscopy, where they found chemisorbed
oxygen desorbs around 750 K, whereas the subsurface
oxygen desorbs at higher temperatures (>1000 K). Two
main points can be summarized from the experiment: (a)
the subsurface oxygen does not diffuse out to the surface or
is consumed during the CO titration, which emphasizes its
non-participation in the reaction. (b) The subsurface oxy-
gen is stable atleast up to 900 K. Any reaction carried out on
the above surface below 900 K is unlikely to affect the
subsurface oxygen population.
parison of the rate of formation of all the products N
2 2
, N O,
NH and H O for 1:1:1 NO:H :O beam composition on the
3
2
2
2
virgin Pd(111) surface and on SM-Pd(111) at different
temperatures. The result shows a higher rate of formation
of N for SM-Pd(111) compared to virgin Pd(111) surfaces.
2
Not only product enhancement, it is also worth mentioning
that the onset temperature for SM-Pd(111) shifts to 325 K.
Whereas the reaction starts at 450 K on virgin surfaces and
reaches the maximum at 550e600 K (Fig. 2). Indeed the
decomposition temperature of NO on Pd(111) surfaces is
known to occur above 300 K, but nitrogen desorption oc-
curs above 400 K [14,21].
Fig. 3 shows the rate of formation of all the products at
various temperatures for the 1:1:1 reaction on SM-Pd(111).
The results were compared with virgin Pd surfaces. It is
3
.2. NO þ H
2
þ O
2
reactions on SM-Pd
We carried out the NO þ H
2
þ O
2
reaction on surface
very appealing for lean de-NO
large amount of oxygen content (33%) in the beam. The
formation of N and H O can be seen up to 325 K, while no
x
at colder conditions with a
modified Pd (represented as SM-Pd) and the results were
compared with that of virgin Pd surfaces (Fig. 2). The ki-
netic experiments were measured with NO þ H
2
2
2
þ O
2
Fig. 2. A comparison of NO þ H
2
þ O
2 2
reactions performed on (a) virgin Pd(111), and (b) surface-modified Pd(111) between 700 and 325 K. All the products (N ,
H
2
O, NH , and N O) formation and response to beam oscillation are shown. The value of the y-axis range remains the same on both panels.
3
2
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4
K. Roy et al. / C. R. Chimie xxx (2016) 1e7
2
of CO:O . This observation demonstrates the capability of
SM-Pd in ambient temperature CO oxidation aspect. For
virgin surfaces the activity maxima can be seen toward
high temperatures (around 475e550 K). With increase in
SM, the onset of the reaction shifted toward lower tem-
perature. Oxygen rich beams show higher shifts in the rate
maxima. The 1:1 composition on SM-Pd shows maxima at
400e450 K, whereas the 1:2 beams shows the maxima
around 350e400 K. However, with oxygen increment, the
overall rate of the reaction decreases due to the decrease in
the overall CO content. It is also to be noted that the 1:1
beam composition shows a significantly higher rate of CO
2
production than 1:2; we attribute this deviation from the
LangmuireHinshelwood mechanism to significantly higher
sticking coefficient for CO than O [11,22].
2
To investigate the sustainability of the activity due to
SM, the reaction was carried out for a long period (1 h) at
isothermal conditions, T ¼ 325 K. Fig. 5 shows the SS pro-
duction of CO
2
at 325 K with CO: O
2
(1:x, x ¼ 2, 4, 7) beam
compositions on SM-Pd(111) for a period of 1 h. During the
course of the reaction, multiple shutter operation was
performed to determine the SS rate of the reaction at 325 K.
A clear CO and O
underscores the fact that the reaction is sustainable at
25 K. If the effect is transient and the subsurface oxygen
would have participated in the reaction, we believe that
2 2
uptake (not shown) and CO formation
3
this continuous sustainable CO
been observed. The rate of CO
given in Fig. 4 for all CO:O
2
formation would not have
formation is the same as
compositions.
2 2
Fig. 3. Steady state rate of the formation of (top panel) N , and H O, and
2
(
bottom panel) N
2 3
O, and NH . Solid and open symbols for the rates obtained
2
on SM and virgin Pd(111) surfaces, respectively.
3.4. Ambient pressure photoelectron spectroscopic study of
SM-Pd
NH
3
could be observed below 450 K. A shift in rate
maximum towards low temperatures, at least by 50 K, was
observed for all the products on SM-Pd(111), compared to
virgin surfaces. Further the rate maximum also broadens to
large temperature range highlighting the influence of sur-
face modification on the fundamental aspects of adsorp-
tion, dissociation of reactants and product formation. These
experiments are repeated multiple times to confirm the
low-temperature activity, due to the fact that subsurface
oxygen does not diffuse out or take part in the reaction.
Virgin Pd surfaces do not show activity for NO reduction
below 400 K, even though NO dissociation occurs but ni-
trogen desorption requires 400 K or above [14,21]. On the
other hand, for CO oxidation reactions activity below 400 K
is hindered because of the “CO poisoning” of Pd metal. To
investigate the role of SM-Pd in low-temperature catalytic
activity, photoelectron spectroscopy studies were carried
out under near-ambient pressure conditions. Before
2
exploring the CO þ O reaction system, we tried to under-
Otherwise, a sustainable and stable product (N
formation would not have been observed.
2 2
or H O)
stand the Pd-oxygen (gas-solid) interaction at different
pressure regimes. Palladium is known to form mainly three
5 4
kinds of oxides- surface oxides, Pd O which are meta-
3
.3. CO þ O
2
reactions on SM-Pd
stable, subsurface oxide, and bulk oxide. To sort out the
stability aspects of subsurface oxide, we chose a wide
pressure and temperature regime for SM. We also matched
molecular beam SM conditions by keeping a similar oxygen
partial pressure and Pd surface temperature in the ambient
pressure XPS set up to confirm subsurface oxide formation.
Our earlier reports on CO oxidation [10, 11] highlighted
that subsurface diffusion of oxygen in palladium led to an
altered catalytic activity toward CO oxidation, and the
modified surfaces show CO oxidation activity even at 900 K,
whereas virgin Pd(111) shows no significant activity at and
above 700 K [22]. We investigated the impact of SM- Pd
toward low-temperature CO oxidation. The results ob-
tained are very promising. The SS rate of CO oxidation is
2
O interaction with Pd(111) surfaces are presented in Fig. 6
under molecular beam conditions as well as under near-
ambient pressure (0.07 mbar) conditions. The UV photo-
electron spectrum at ultrahigh vacuum (UHV) conditions
shows a Pd 4d doublet with strong Fermi level intensity,
which is a characteristic of the clean Pd-metal. Upon
introduction of oxygen, a feature at around 5.7e6 eV
(dashed line in Fig. 6) appears when heated up to
2
presented in Fig. 4 for the CO:O ratio of 7:1 to 1:7 from
temperature 300e525 K for virgin and SM-Pd(111) sur-
faces. CO oxidation activity can be observed from 400 K and
above on virgin Pd(111) surfaces, which is in good corre-
lation with literature reports [11,22]; whereas CO oxidation
activity can be observed from 300 K and above for any ratio
ꢁ
5
650e900 K at 1 ꢀ10 mbar pressure. Fermi level intensity
decreases and a clear broadening of the Pd 4d levels could
Please cite this article in press as: K. Roy, et al., Three-way catalytic converter reactions aspects at near-ambient temperatures on
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K. Roy et al. / C. R. Chimie xxx (2016) 1e7
5
Fig. 4. The SS rate of the formation of CO
2
2
on (a) Pd(111), and on (b) SM-Pd(111) surfaces. The CO:O composition varied between 7:1 to 1:7.
be observed. The work function increases from 6 eV for
virgin Pd(111) surfaces to 6.4 eV for SM-Pd surfaces
demonstrating the change in surface nature. On prolonged
exposure of oxygen under this condition, the feature
remains the same (not shown). The system was evacuated
after oxygen exposure and further spectra were recorded.
The feature at 5.8 eV remains stable on evacuation even up
to 900 K heating, and the valence band features are
Fig. 6. UVPES spectra recorded for PdeO
tures and oxygen pressures are shown. Dashed and dotted lines indicate the
formation of subsurface oxide upon Pd-O interaction. A sharp molecular
vibration feature from CO can be seen at 8.62 eV.
2
interaction at various tempera-
Fig. 5. CO þ O
2
reaction with different compositions, (a) 1:2, (b) 1:4, and (c)
:7) carried out on SM-Pd(111) at 325 K for 60 min.
2
1
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6
K. Roy et al. / C. R. Chimie xxx (2016) 1e7
restored back similar to the metallic state. This experiment
clearly indicates that the feature cannot be due to meta-
stable surface oxides or bulk oxides. The increase in oxygen
pressure broadens the feature at 5.7e6 eV, and a prolonged
exposure (150 min) at 0.07 mbar and 675 K develops a
feature at 6.4 eV (dotted line in Fig. 6), which we assign to
PdO-like surface oxide. XPS spectra also confirm a distinct
peak at 336.5 eV (see Supplementary data, Fig. S1). We
carried out CO dosing to check the stability of the feature
between 5.7 and 6.5 eV. After oxygen dosing at 0.07 mbar,
the system was evacuated and 0.07 mbar CO was intro-
duced at 675 K. Interestingly, the feature at 6 eV shows no
sign of disappearance. Neither heating in the presence of
CO nor heating under UHV conditions up to 900 K changes
the intensity of the above feature. We attribute this feature
to be subsurface oxide, and hence Pd surfaces with this
kind of attribute were represented as SM-Pd and they
showed ambient temperature catalytic activities for CO
composition was employed to explore the CO-poisoning
aspects on virgin and SM-Pd(111) surfaces. The unique
aspect of photoelectron spectroscopy under ambient con-
ditions is that the gasesolid interactions can be appreciably
understood from the gas phase vibrational features. On SM-
Pd surfaces, gas-phase O
appear at 6.4e7.4 and 8.35 eV, respectively. CO
features are distinctly observed at 12.4e12.9 eV during the
CO þ O reaction on SM-Pd. To verify this feature reference
CO gas phase spectra were also recorded and it is in good
2
and CO vibrational features
2
gas phase
2
2
correspondence with the spectra observed above. The core-
level C 1s spectrum recorded under the same NAP reaction
conditions exhibits gas-phase CO
CO at 286 eV (Fig. 7, inset) [7]; this observation unequivo-
cally demonstrates CO production at 350 K on SM-Pd(111).
2
at 291 eV and adsorbed
2
We believe that oxygen diffusion into the subsurface layers
makes the Pd surface to be marginally oxidized. Due to the
oxidized character, there is no back-donation of electron on
CO adsorption. This makes the adsorbed CO to be mobile, in
comparison to virgin surfaces, and hence facilitates CO
oxidation at ambient temperatures. Nonetheless, further
studies, such as in-situ IR, are required to confirm this
aspect. The above APPES results demonstrate CO oxidation
on SM-Pd and are in resonance with the MBI results.
oxidation and NO þ H
2
þ O
2
reaction.
The CO oxidation reaction was followed in-situ by
APPES. Fig. 7 shows the He I UVPES spectra measured at
3
50 K (total pressure: 0.1 mbar) on clean Pd(111) and on
SM-Pd with a 2:1 ratio of CO:O . Deliberately the CO-rich
2
2
However, no CO feature was observed on virgin Pd(111)
surfaces under similar reaction conditions. It is also to be
noted that the intensity of the first Pd 4d feature at 1.2 eV
seems to be very susceptible to CO and O
2
and the same can
environment,
be seen in Figs. 6 and 7. Under the CO þ O
2
the first Pd 4d feature seems to be completely suppressed
and we do not know the reason for this. Another interesting
change is the shift in the binding energy of molecular vi-
bration features of CO and O
2
towards lower energy by
0.25e0.35 eV on SM-Pd(111) compared to the virgin
Pd(111) surface. The above shift in energy of vibrational
features is not simply due to work function changes. It is
also to be underscored that while CeO vibration energy
remains the same (200 meV) [23] on virgin and modified
surfaces, the electronic state is shifted to lower BE. Shift
towards lower binding energy underscores the higher
reactivity of the surface due to surface modification.
Further, it also reflects the change in surface potential by
0.25e0.35 eV, after surface modification. All these obser-
vations highlight the changes observed in catalysis and
spectral aspects due to surface modification are in good
correspondence. It is worth exploring this aspect for
different surfaces as well as different reactions.
4. Conclusion
This study, along with our previous reports [10,11,19,24],
highlights that subsurface population of oxygen definitely
plays an important role in the catalytic activity of Pd. The
reaction temperature extends to ambient regimes upon
SM. SM-Pd shows activity from 325 K onwards for
Fig. 7. UVPES spectra measured while the CO þ O
out on virgin Pd(111) and SM-Pd surfaces at 350 K at a total pressure of
.1 mbar. Molecular vibration features of CO directly observed on SM-Pd is
shown in the rectangular dotted box, which demonstrates CO production.
The inset shows the core-level C 1s spectra under the same conditions for
SM-Pd.
2
(2:1) reaction was carried
NO þ H
þ O
2
reaction and for CO oxidation. However, we
2
do not generalize this aspect that all of the reactions are
catalyzed by Pd, and moreover investigations are pending
to establish this aspect over ranges of Pd-based powder
catalysts under practical catalysis conditions.
0
2
2
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K. Roy et al. / C. R. Chimie xxx (2016) 1e7
7
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Please cite this article in press as: K. Roy, et al., Three-way catalytic converter reactions aspects at near-ambient temperatures on
modified Pd-surfaces, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.11.019