A PdCu nanoalloy catalyst for preferential CO oxidation in the presence of hydrogen

Article abstract of DOI:10.1039/d1nj00005e

Catalytic oxidation of carbon monoxide (CO) is one of the essential steps for several environment- and energy-related applications. Here, we report the synthesis of alloy PdCu nanoparticles supported on an Al₂O₃support as a catalyst for effective removal of CO from H₂-rich flue gases. The alloy phase between Pd and Cu was formed by deposition of organometallic Pd- and Cu-precursors on the Al₂O₃support followed by thermal reduction under H₂. The alloy PdCu nanoparticles (average size: 30 nm) are in a spherical shape and they are distributed in the range between 10 and 80 nm. The alloy PdCu catalyst showed improved CO oxidation performance both in the presence and absence of H₂compared to that of the reference Pd catalyst. It is also inferred that the alloy PdCu catalyst showed an onset temperature (T_o) of 50 °C in the presence of H₂compared to that of CO oxidation in the absence of H₂(T_o= 100 °C). In addition, the alloy PdCu catalyst showed selective CO oxidation performance below a temperature range of 300 °C. Therefore, the formation of the Pd-based alloy structures may have potential as the preferential CO oxidation catalyst for the catalytic removal of CO from various flue gases.

Full text of DOI:10.1039/d1nj00005e

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A PdCu nanoalloy catalyst for preferential CO
oxidation in the presence of hydrogen†
Cite this: New J. Chem., 2021,
a
a
a
45, 4246
Rohini Khobragade,
Pranali Dahake, Nitin Labhsetwar
and
b
Govindachetty Saravanan
*
Catalytic oxidation of carbon monoxide (CO) is one of the essential steps for several environment- and
energy-related applications. Here, we report the synthesis of alloy PdCu nanoparticles supported on an
Al
between Pd and Cu was formed by deposition of organometallic Pd- and Cu-precursors on the Al
support followed by thermal reduction under H . The alloy PdCu nanoparticles (average size: 30 nm) are
in a spherical shape and they are distributed in the range between 10 and 80 nm. The alloy PdCu
catalyst showed improved CO oxidation performance both in the presence and absence of H
compared to that of the reference Pd catalyst. It is also inferred that the alloy PdCu catalyst showed an
onset temperature (T ) of 50 1C in the presence of H compared to that of CO oxidation in the absence
of H (T = 100 1C). In addition, the alloy PdCu catalyst showed selective CO oxidation performance
2 3 2
O support as a catalyst for effective removal of CO from H -rich flue gases. The alloy phase
2 3
O
2
2
Received 1st January 2021,
Accepted 28th January 2021
o
2
2
o
below a temperature range of 300 1C. Therefore, the formation of the Pd-based alloy structures may
have potential as the preferential CO oxidation catalyst for the catalytic removal of CO from various
flue gases.
DOI: 10.1039/d1nj00005e
rsc.li/njc
continuously through polymer electrolyte membrane fuel cells
with minimum greenhouse gas emissions.
Transition metal-based catalysts in the form of metals,
Introduction
7
–10
Removal of toxic carbon monoxide (CO) from various flue
gases using catalysts, for instance automobile exhaust tailpipe mixed metals, oxides, mixed oxides, spinels, perovskites, etc.
1
1–13
emissions that contain significant amounts of CO, partial are considered for the removal of CO from the flue gases.
combustion of biomasses from unorganized sectors, H -rich However, the performance for instances catalytic activity at a
2
flue gases as a result of reforming followed by water–gas shift relatively low onset temperature, long-term catalytic performance,
reactions, etc. is taking wide attention due to not only its toxic etc., needs to be improved further than that of the currently
behavior but also the need of continuous power generation for existing catalysts. Platinum group metals (PGM) showed much
decentralized applications in particular transportation, telecom better catalytic performance compared to that of the oxide-based
1
–6
tower, hospital and military applications, etc. The flash point catalysts due to their surface characteristics, however, their cost
of CO (i.e., a temperature that oxidizes CO completely) is and scarce minerals pose persistent challenges to their use as
14
generally above 600 1C in the absence of any catalysts, which catalysts for the target applications at a larger scale. In addition,
is a cost- as well as energy-intensive processes. On the other the stability of the existing catalyst structures (metals or metal
hand, it can be reduced significantly upon the use of catalysts oxides) under the severe CO oxidation conditions is another
under the given CO oxidation conditions due to their proven serious concern among others, which has to be improved
2
surface characteristics. The removal of CO from the H -rich significantly. Therefore, catalysts with improved catalytic
gases via preferential CO oxidation (CO-PROX) using the performance and ameliorated stability at a lower cost have to be
catalysts is one of the vital steps to generate cleaner energy developed for the catalytic removal of CO from flue gases. This
can be readily achieved by the development of PGM-based
intermetallic- or alloy-catalyst structures with minimum possible
a
Energy and Resource Management Division, CSIR-National Environmental
Engineering Research Institute (CSIR-NEERI), Nehru Marg, Nagpur 440 020, India
Chennai Zonal Centre, CSIR-National Environmental Engineering Research
Institute (CSIR-NEERI), CSIR-Madras Complex, Taramani, Chennai 600 113,
India. E-mail: g_saravanan@neeri.res.in
amounts of PGM. Alteration in the d-band centers and electronic
structures, bi-functional characteristics, etc. are the major
attributes to the improved performance of the intermetallic or alloy
b
15–17
catalysts compared to that of their individual counterparts.
The ever-developed alloy or intermetallic catalysts show improved
performance along with ameliorated stability compared to that of
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00005e
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their individual counterparts at the same or less-PGM were used to examine the formation of alloy phases between Pd
1
8–22
loading.
Although Pt-based alloy or intermetallic catalysts and Cu. HR-TEM analysis was used to observe the morphology
have shown superior CO-PROX performance compared with the and particle size distribution of the catalysts. ICP-OES was used
reference Pt catalysts, investigations are still required to find an to estimate the concentration of Pd and Cu present in the
alternative to the-state-of-the-art Pt catalyst (e.g., Pd and its catalysts. Similarly, Al O supported Pd nanoparticles were also
2
3
alloys) for most of the catalysis reactions due to its cost and synthesized and characterized as a reference catalyst to the
1
4,23–25
scarce minerals.
Pd-based intermetallic or alloy catalysts were developed for Pd catalysts was examined under CO and CO-PROX conditions.
various catalytic applications. Pd Cu alloy nanocrystals were
alloy PdCu catalyst. The catalytic activity of the alloy PdCu and
3
synthesized through a polyol reduction without any shape
directing agents that showed catalytic formic acid oxidation
performance for direct fuel cells. An intermetallic PdCu Materials
Experimental
2
6
catalyst supported on ZnAl O was developed for the selective
2
4
Organometallic salts of palladium(II) 2,4-pentanedionate
Pd; Pd assay 34.7%) and copper(II) 2,4-
pentanedionate (C H CuO , 98%) were purchased from Alfa
production of H
methanol. It was observed that the intermetallic phase between
2
through a steam reforming reaction in
(C
10
H
14
O
4
=
27
1
0
14
4
28
Pd and Cu was formed through a spillover mechanism. Pd- or
Rh-based intermetallic catalysts were prepared for hydrogenation
of p-nitrostyrene and cyclohexane derivatives or methanol as the
Aesar. Nano g-Al O powder was procured from SRL Pvt. Ltd.
2
3
Analytical reagent grade HCl (37%) and HNO (69%) and HPLC
3
grade methanol were purchased from Merck Ltd. O
and He (99.998%) and the physical gaseous mixture of CO (2%)
and H (70%) balanced with He, H (5%) gas as a reducing
agent balanced with N , and CO (5%) balanced with He were
supplied by Alchemie gases and chemicals Pvt. Ltd.
2
(99.99%)
29
source of H2. It was observed that the intermetallic PdPb catalyst
offered a better chemo-selective hydrogenation performance due
to the higher electronegativity of Pb among other elements
used for the synthesis of Pd-based intermetallics. Intermetallic
PdGa compounds were developed for the selective, partial-
hydrogenation reaction for acetylene. It was observed that the
electronic structures at the Fermi level of Pd were modified in the
2
2
2
Synthesis of Pd and alloy PdCu nanoparticles on Al
Pd- or alloy PdCu-nanoparticles on the Al support as the
catalysts were prepared by a wet-impregnation method. 5 wt%
of metallic Pd-loading was maintained on the Al support for
2 3
O
30
O
2 3
PdGa intermetallics due to the presence of Ga atoms around Pd.
Bimetallic PdCu catalysts were developed for the synthesis of
2 3
O
methanol through hydrogenation of CO . It was observed that
2
both the catalysts. The organometallic salts of Pd (0.1433 g,
0.4697 mmol) and Cu (0.0689 g, 0.2632 mmol) were dissolved in
methanol (50 ml) at room temperature. Nano Al O powder was
2 3
the PdCu bimetallic catalyst possesses a synergistic effect for the
31
hydrogenation of CO
Pd-based intermetallic or alloy catalysts, these catalysts haven’t
been tested for the removal of CO in the presence of H , which
2
.
Despite several advantages of the
mixed with the methanolic solution of organometallic Pd- and
Cu-salts followed by stirring for 6 h. The composite solutions
were heated at 70 1C in order to deposit the organometallic salts
on the Al O powder. The resultant product was ground for
2
is attempted in this study, and is an important step for the
generation of continuous power with improved fuel efficiency
through fuel cells for decentralized applications.
2
3
1
5 min using a mortar and pestle followed by heating at 800 1C
under a H atmosphere for 8 h. Pd nanoparticle supported
Al as the reference catalyst was prepared similarly, however,
In this work, Al O supported alloy PdCu nanoparticles as a
2
3
2
catalyst were realized using an impregnation method followed
by thermal reduction under a H environment for the catalytic
removal of CO via an oxidation reaction both in the presence
CO-PROX) and absence of H (Fig. 1). XRD- and XPS-analyses
2 3
O
2
without the Cu-salts. It should be noted that all the experi-
mental conditions were the same to prepare these catalysts.
(
2
Characterization
pXRD analysis was performed by scanning (2y range) of the
Pd- and alloy PdCu-catalysts between 10 and 901 in order to
examine the formation of alloy phase between metallic Pd and
Cu. CuKa radiation (l = 0.15418 nm) was generated from a Cu
source using an X-ray diffractometer (Miniflex II-DD34863). The
shape, size and distributions of the Pd- and alloy PdCu-
nanoparticles on the Al O support were observed at an
2 3
acceleration voltage of 200 kV by HR-TEM analysis (JEM-2100F).
The Pd and alloy PdCu catalysts were sonicated in ethanol for
10 min followed by recovering them on a Cu grid (200 mesh) as
per standard TEM sample preparation. The concentration of Pd
and Cu present in the catalysts was quantified by ICP-OES
(Thermo Fischer, iCAP 6300 DUO). An aliquot amount of the Pd
Fig. 1 Pictorial representation of catalytic CO oxidation in the presence
and absence of H on the surface of a PdCu alloy system.
2
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or alloy PdCu catalysts (10 mg) was carefully digested in aquaregia
3
(1 : 3, HNO and HCl) and their corresponding dilutions were
made as per standard ICP protocol. The oxidation states of Pd and
Cu of the catalysts were examined by XPS analysis (JEOL, JPS
9010MC) using Al Ka radiation. The narrow- and survey-scans of
the catalysts were recorded at the pass energies of 40 and 160 eV,
respectively. Conductive carbon was mixed with the catalysts
using a mortar and pestle in order to exclude the possible
2 3
charging effect due to the insulator behaviour of the Al O support
during the XPS analysis. The composition of the PdCu catalyst
was quantified further by integration of the corresponding
photoemission peaks.
2 3 2 3
Fig. 2 pXRD patterns for Pd/Al O and alloy PdCu/Al O . Simulated lines
Catalytic activity
for Pd are shown for reference. The XRD diffraction peaks for the Al
support are shown as asterisks.
2 3
O
The CO oxidation reaction of the catalysts under CO and
CO-PROX conditions was examined using a fixed bed catalytic
reactor that contains adequate mass flow controllers for various
set gases, a temperature programmable furnace, gas chromato-
graphy with a thermal conductivity detector, etc. 50 mg of the
Pd or alloy PdCu catalysts were placed between the quartz wool
inside the quartz reactor that is fixed in the bore of the furnace.
Stoichiometric CO or H /CO and O gases were subjected on the
Cu or oxides of Cu (CuO and Cu
calculated total metal loading (Pd and/or Cu) is relatively low
B5–6.7 wt%) in the synthesized catalysts (see ICP-results), the
characteristic diffraction peaks were very clearly observed due to
the adopted synthesis conditions that facilitate ordered alloy
structures between Pd and Cu, where the impregnated Pd- and
2
O) (Fig. S1, ESI†). Although the
(
2
2
2
Cu-precursors were heated at 800 1C under a H atmosphere. The
catalysts to evaluate their catalytic CO oxidation performance.
32
observed results are consistent with earlier studies. It should be
noted that both the catalysts showed the characteristic diffraction
peaks for the Al O support in the XRD patterns marked as
À1
Feed gases (flow rate: 50 ml min ) containing CO (2%) and O
2
(5%) for the CO oxidation and CO (2%) and H
2
(70%) balanced
2
3
by He and O
2
(1%) for the CO-PROX oxidation were subjected
asterisks.
Pd nanoparticles of the Pd/Al
on the catalysts to examine their performance. Both the cata-
lysts were pre-treated at 200 or 500 1C for 1 h under the feed gas
environment prior to the catalytic CO oxidation. After the pre-
treatment, the catalysts were heated from ambient temperature
to 300 or 500 1C in the presence of the desired gaseous
compositions. The concentration of the feed- and outlet-gases
after the catalysis reaction was analyzed using a carboxane
column (Supelco Analytical) at every set temperature. All of
the catalytic evaluations of the catalysts were performed at least
three times and the obtained values were averaged to examine
their catalytic performance.
2 3
O catalyst are in a spherical
shape that is finely dispersed on the support as shown in
Fig. 3a. The average particle size of the Pd nanoparticles is
3
(
0 nm and their size is in the range between 15 and 40 nm
Fig. 3b). Similarly, the alloy PdCu nanoparticles are also
dispersed on the Al support and are in a spherical shape
Fig. 3c), however, their size is slightly in a higher range
2 3
O
(
compared to that of the Pd nanoparticles, which are observed
between 10 and 80 nm (Fig. 3d). It should be noted that the
average particle size of the alloy PdCu is, however, virtually the
2 3
same as that of the Pd nanoparticles dispersed on the Al O
support, observed at 30 nm. These results inferred that the
particle size range of Pd nanoparticles increased when Cu-salts
were used for the synthesis of the alloy PdCu system. The
observed results are consistent with our previous results, where
the particle size of intermetallic PtCu nanoparticles increased
Results and discussion
Catalyst characterization
Fig. 2 shows the XRD patterns of the as-synthesized Al O
2
3
3
3
supported Pd nanoparticles (hereafter denoted as Pd/Al O ) with increase of the Cu-amounts during the synthesis. The
2
3
and the alloy PdCu nanoparticles (hereafter denoted as PdCu/ surface of Pd- and alloy PdCu-nanoparticles was observed
Al ). The XRD pattern of Pd/Al showed characteristic further to see the facets present, which is very important to
peaks at 40.01, 46.61, 68.01, 82.11 and 86.51 for the planes of facilitate the catalysis reaction. The surface of the Pd nano-
111), (200), (220), (311), and (222), respectively (JCPDS #: particles is atomically ordered with a d-spacing value of
1-088-2335) (FCC-type structure, Fm3%m, a = 0.38 nm). The XRD 0.23 nm, assigned to the {111} plane (Fig. 3e). Similarly, the
pattern of the PdCu/Al O also showed similar characteristic surface of the alloy PdCu nanoparticles is also atomically
2
O
3
2 3
O
(
0
2
3
peaks, however, shifted to higher angles and observed at 40.61, ordered and their d-spacing value is calculated to be 0.22 nm,
47.31, 69.1, 83.41 and 88.01, corresponding again to FCC-type assigned again to the {111} plane, which is virtually the same as
structure (Fm3%m, a = 0.37 nm) due to the formation of alloy phase that of the Pd nanoparticles (Fig. 3f). These results inferred that
between Pd and Cu. It should be noted that this catalyst showed the surface of the Pd nanoparticles is not altered significantly
no other characteristic diffraction peaks corresponding to metallic upon the addition of Cu content in its lattice.
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2 3 2 3
Fig. 3 TEM image of Pd/Al O (a) and alloy PdCu/Al O (c) and their
particle size distribution is shown in the histograms (b and d). HR-TEM
image of Pd/Al (e) and alloy PdCu/Al (f).
2
O
3
2 3
O
Fig. 4 X-ray photoelectron spectral profiles of Pd/Al
Al in the regions of Pd 3d (a) and Cu 2p (b).
2
O
3
and alloy PdCu/
The oxidation states of Pd and Cu were examined by XPS
analysis. The XPS data of the Pd and alloy PdCu catalysts in the
regions of Pd 3d and Cu 2p are shown in Fig. 4. The charging
2 3
O
effect due to the non-conductive nature of the Al O support metallic Cu formed upon reduction of the organometallic
2
3
was eliminated by mixing of the catalysts with Vulcan carbon, Cu-salts under a H2 environment at 800 1C in the case of
3
2,34
where the binding energy of C 1s was observed at 284. 4 eV.
Pd/Al showed a characteristic doublet, spin–orbit photo- Al O catalyst in the region of Cu 2p. Two clear asymmetric peaks
PdCu/Al O . Fig. 4b shows the XPS profile of the alloy PdCu/
2 3
2 3
O
2
3
emission peaks at 340.2 and 335.1 eV, corresponding to 3d3/2 were observed in the regions of 930–940 eV and 950–960 eV
and 3d5/2, respectively, due to the presence of metallic Pd in the along with a broader peak at 943.8 eV, indicating that Cu exists
catalyst that is formed upon reduction of the organometallic in more than one oxidation state in the alloy PdCu catalysts.
2
Pd-salts under the H environment at 800 1C (Fig. 4a). PdCu/ Therefore, each asymmetric peak was deconvoluted further to
Al O also showed the characteristic doublet, spin–orbit photo- identify the oxidation states of Cu present in the catalysts by
2
3
emission peaks corresponding to metallic Pd, however, they are fitting the experimental data with the Voigt function. The Cu 2p3/2
shifted to a slightly higher binding energy level by B0.3 eV. As peak was deconvoluted into two characteristic peaks having the
can be seen in more detail, the doublet photoemission peaks binding energies of 931.9 and 934.5 eV. Similarly, a Cu 2p1/2
for Pd 3d3/2 and 3d5/2 of the alloy PdCu/Al O catalyst were peak was deconvoluted into two characteristic peaks and
2 3
observed at 340.5 and 335.4 eV, respectively. Although metallic observed at 952.1 and 954.6 eV. The doublet, spin–orbit photo-
Pd was formed upon reduction, the observed higher binding emission peaks observed at 931.9 and 952.1 eV, correspond to
2 3
energy shift for Pd of the alloy PdCu/Al O catalyst is due to the metallic Cu, whereas the doublet, spin–orbit photoemission
formation of alloy phase with Cu, which alters slightly the peaks observed at 934.5 and 954.6 eV correspond to CuO. It
electronic structure of metallic Pd in the alloy PdCu catalyst. should be noted that the observed binding energies of metallic
It should be noted that the spin–orbit splitting (d) value was Cu are slightly lower than that of the pristine metallic Cu surface
35
observed to be B5.1 eV for both the catalysts, which is (2p1/2 = 932.6 eV; 2p3/2 = 952.6 eV) due to the formation of alloy
3
2
consistent with the reported results. From this result, it can phase between Pd and Cu. It may be recalled that the Pd of the
be understood that the metallic Pd was formed upon reduction alloy PdCu catalyst showed a higher binding energy shift (B0.3 eV)
of the organometallic Pd-salts under a H
2
environment, which compared with metallic Pd, which seems to be consistent in the
is most likely involved in the formation of alloy phase with case of alloy or intermetallic systems reported earlier by us and
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32,34
other research groups.
It should be noted that the spin–orbit
splitting value (d) for Cu of both metallic Cu and CuO is B20.2 eV,
36
consistent with the reported results. The alloy PdCu/Al O
2
3
catalyst also showed the characteristic satellite peak at 943.8 eV
corresponding to Cu(II). These results indicated clearly that
metallic Cu, formed upon reduction of the organometallic
Cu-salts under the H
formation of alloy phases between Pd and Cu. In addition to this,
CuO was also present with the PdCu/Al catalyst, which was
2
environment, was involved with the
2 3
O
formed most likely by aerial oxidation of the metallic Cu after the
synthesis, which is not involved in the alloy formation between Pd
and Cu. The amounts of Cu and CuO in the PdCu alloy catalyst
were quantified further by integration of individual photo-
emission peaks of Cu 2p. It was observed that 58%
(B0.1527 mmolCu) and 42% (B0.1105 mmolCuO) of Cu and
CuO was present in the catalyst, respectively. However, the
presence of CuO was not observed in the XRD pattern likely due
to the amorphous nature of CuO.
Fig. 5 Catalytic CO oxidation of Pd/Al
function of temperature. Inset shows the comparison of the onset (T
maximum conversion temperature (T ) of these catalysts.
2
O
3
(a) and alloy PdCu/Al
2
O
3
(b) as a
o
) and
m
The total amounts of Pd and Cu present in the Pd/Al O and
2
3
PdCu/Al O catalysts were quantified by ICP-OES analysis. The
2
3
estimated amount of Pd present in the Pd/Al O and PdCu/ electronic structure compared with pristine Pd as ensured by
2
3
2 3
Al O catalysts is 0.508 and 0.492 mg, respectively, closer to the the XPS analysis (see Fig. 4 and related discussions). Despite
calculated Pd amounts (5 wt%). Similarly, the amount of Cu the same Pd-loading and unfavorable particle size and its
present in the alloy PdCu catalyst was estimated to be 0.165 mg, distribution in the case of the PdCu catalyst compared with
which is again closer to the initially calculated Cu amount the reference Pd catalyst, the electronic structure of the alloy
(1.67%). It should be noted that both the catalysts were PdCu catalyst may enhance most likely the CO-PROX perfor-
prepared by the deposition of the organometallic Pd- and Cu- mance, consistent with the other reported alloy/intermetallic
3
2,34
salts followed by heating under a H environment at 800 1C and catalysts for several catalysis reactions.
2
no washing steps were involved in the synthesis of these
catalysts. Therefore, it is quite reasonable that the estimated further using both the catalysts as shown in Fig. 6. The catalytic
amounts of Pd and Cu present in the catalysts are closer to their CO oxidation in the presence of H is significantly different in
compared to that of the catalytic CO oxidation
performance in the absence of H . It should be noted that both
The catalytic CO-PROX performance was also examined
2
initially calculated amounts taken for the synthesis.
particular T
o
2
the catalysts showed solely less than 50% activity up to 300 1C
in the presence of H , whereas they showed 100% CO oxidation
catalytic activity at temperatures above 200 1C in the absence of
2
Catalytic activity
Catalytic CO oxidation of the Pd/Al O and alloy PdCu/Al O
catalysts is shown in Fig. 5. Both the catalysts showed CO oxidation in the presence of H
typical catalytic performance trends, where they didn’t show alloy PdCu/Al
H
2
(see Fig. 5). For more details, Pd/Al
up to 150 1C. Whereas the
catalyst showed significant CO oxidation
any catalytic activity up to 100 1C, however, they showed activity compared to that of the Pd/Al catalyst. T of the
significant CO oxidation activity above the temperature of alloy PdCu and Pd catalysts is 50 and 150 1C, respectively, as
00 1C. For more details, Pd/Al showed an onset temperature shown in Fig. 6 (inset). Interestingly, T of the alloy PdCu/Al
To) (i.e., 1% CO oxidation) at 150 1C and the maximum CO catalyst is improved for the CO oxidation in the presence of H2
oxidation temperature (T ) of Pd/Al O at 250 1C. Whereas the compared with the CO oxidation performance in the absence of H2.
2 3
O showed virtually no
2
3
2 3
2
2 3
O
2
O
3
o
2
2 3
O
o
2 3
O
(
m
2 3
alloy PdCu/Al O catalyst showed improved catalytic CO oxida- The improved CO-PROX catalytic activity of the PdCu catalyst
2
3
tion activity, where both T
compared to that of the Pd/Al
PdCu/Al catalyst were observed at 100 and 200 1C, respec- ment (i.e., reduction environment) that can aid retention of the
tively. Although the Pd loadings (5 wt%) in both the catalysts catalyst in the form of metallic state under CO-PROX condi-
2 3
are the same, the average particle size of the alloy PdCu tions. In addition, T of both Pd/Al and alloy PdCu/Al O
o
and T
m
were decreased by 50 1C which showed lower onset temperature compared with that
2
O
3
catalyst. T
o
and T of the alloy of the Pd catalyst is due to the presence of a H rich environ-
m
2
2 3
O
m
2
O
3
nanoparticles is virtually the same as that of the Pd nano- catalysts is significantly reduced in the presence of H which
2
particles with wide distribution of the PdCu nanoparticles was limited to B45%. Both the catalysts showed no significant
(
10–80 nm) compared to that of the Pd nanoparticles (15 and difference in the CO oxidation performance between 250 and
0 nm); the alloy PdCu/Al catalyst, however, exhibited 300 1C in the presence of H . It should be noted that both the
improved catalytic activity compared with the Pd/Al catalyst catalysts showed no significant H conversion up to 300 1C,
due to the alloy structure of PdCu, which has different which is one of the essential prerequisites for the CO-PROX
4
2
O
3
2
2
O
3
2
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Fig. 6 Catalytic CO oxidation in the presence of H
2
of Pd/Al
conversion of these
catalysts is shown as dotted lines. Inset shows the comparison of the onset
) temperature of these catalysts.
2
O
3
(a) and
Fig. 7 CH
oxidation in the presence of H
4
formation as a function of temperature during catalytic CO
alloy PdCu/Al
2
O
3
(b) as a function of temperature. H
2
2
of Pd/Al
2 3 2 3
O (a) and alloy PdCu/Al O (b).
The inset shows CO and H
of temperature.
2
conversion of both the catalysts as a function
(T
o
CO-PROX conditions up to 500 1C. Although the intensities of
the characteristic diffraction peaks of the PdCu alloy were
decreased, their peak positions were not altered significantly
(Fig. S2, ESI†). These results implied that the alloy phase
between Pd and Cu is stable even after the harsh CO-PROX
reactions, which can facilitate the catalytic CO-PROX
performance.
catalyst. However, Pt-based catalysts showed more than 85%
superior CO-PROX performance (T = 50 1C, T = 150 1C) as
reported earlier, compared with the tested Pd-based catalysts in
this study. It is worth mentioning here that the selected catalyst
is a state-of-the-art Pt-free catalyst and realization of an alloy
structure between Pd and Cu can enhance the CO-PROX perfor-
mance compared with its counterpart Pd catalyst. It is anticipated
that reduction in the size of PdCu alloy nanoparticles (average
o
m
14
particle size: 30 nm) can improve the CO-PROX performance Conclusions
further, where the size of the-state-of-the art Pt catalytic centres
14
Al O -supported alloy PdCu nanoparticles were synthesized by
2 3
impregnation of the organometallic Pd- and Cu-salts followed by
is generally lesser than 5 nm.
Selectivity towards the formation of CO
concentration of H during the CO-PROX reaction is another
2
without altering the
thermal reduction under a H atmosphere. A higher diffraction
2
2
shift was observed in the case of alloy PdCu catalysts due to the
formation of alloy structures of PdCu. The alloy PdCu nano-
particles are in a spherical shape with an average particle size of
important criterion for the CO-PROX catalysts. Therefore, both
the inlet- and outlet-gases were carefully examined further
during the CO-PROX reaction. The inlet reactant gases (CO,
30 nm. The electronic structure of Pd in the alloy PdCu catalysts
2 2 2 4
H and O ) and CO and the other possible products (e.g., CH )
was altered significantly as its 3d photoemission peak was
shifted to a higher binding energy compared to that of the pure
Pd catalyst. The Pd-loading of both alloy PdCu and Pd catalysts
was the same, which was estimated to be 5 wt.%, however, the
alloy PdCu catalyst showed much improved CO oxidation per-
formance compared to that of the reference Pd catalyst both in
the presence and absence of H . Realization of the Pd-based
2
alloy catalyst structures using copper may have the potential for
various catalysis-related applications.
due to the undesired reactions from the outlet of the reactor
after the CO-PROX reaction were carefully monitored in order
to understand the selectivity of the catalysts. As explained, both
the catalysts showed selective CO oxidation without changing
the H concentration up to 300 1C (inset). Although the catalytic
2
CO oxidation performance of these catalysts increased with
increasing temperature under the CO-PROX conditions, it was
observed that CH formed above 300 1C was steeply increased
4
with increasing temperature, and reached almost 100% at the
temperature of 500 1C as shown in Fig. 7. It should be noted
2
that the conversion of H also increased with increasing the
temperature above 350 1C as shown in Fig. 7 (inset). These
Conflicts of interest
There are no conflicts to declare.
results implied that the alloy PdCu/Al O catalyst can oxidize
2
3
CO selectively in the presence of H without influencing the H
2
2
concentration up to the temperature range of 300 1C.
Acknowledgements
The PdCu catalyst was tested structurally further to examine
its stability. It should be noted that the alloy phase of the PdCu The authors gratefully thank the Science and Engineering
catalyst is retained when it was exposed under the harsh Research Board (SERB), Department of Science & Technology,
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Paper
Govt. of India for financial support (EMR/20l6/006063). The
authors thank the Director, CSIR-NEERI for the guidance and
J. Rossmeisl, I. Chorkendorff and J. K. Nørskov, Nat. Chem.,
2009, 1, 552–556.
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2020/MAY/CZC-ERMD/1.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2021