Palladium-platinum powder catalysts manufactured by colloid synthesis: I. Preparation and characterization

Article abstract of DOI:10.1016/j.molcata.2005.04.064

Palladium, platinum and PdPt bimetallic colloidal particles were prepared in atomic ratios of 4:1, 1:1 and 1:4 by the reduction of K₂PdCl ₄ and/or K₂PtCl₄ in the presence of cationic surfactant, tetradecyltrimethylammonium bromide (C₁₄TABr). The nanoparticles in the as prepared state were characterized by TEM, EDS and XPS, UPS, permitting to determine their mean particle size, bulk and surface composition. The bulk composition of the bimetallic samples determined by EDS was close to their nominal value, while XPS indicated Pt enrichment near to the surface. All samples in the as received state contained surface oxide impurity and contained carbon impurities. The monometallic samples contained considerably more C as the bimetallic ones. A quasi-in situ H ₂ treatment at 473 K in the electron spectrometer resulted in cleaner metals, containing less carbon and oxygen, as shown by XPS and UPS.

Full text of DOI:10.1016/j.molcata.2005.04.064

Journal of Molecular Catalysis A: Chemical 238 (2005) 56–62
Palladium-platinum powder catalysts manufactured by colloid synthesis
I. Preparation and characterization
B. Veisza,b, L. Toth , D. Teschner , Z. Paal , N. Gyorffy , U. Wild , R. Schlogl
b
c,d
c,∗
c
d
d
´
´
˝
¨
a
Department of Colloid Chemistry, University of Szeged, Szeged H-6720, Hungary
b
Research Institute for Technical Physics and Material Science, Hungarian Academy of Sciences, P.O. Box 49, Budapest H-1525, Hungary
c
Institute of Isotopes, CRC, Hungarian Academy of Sciences, P.O. Box 77, Budapest H-1525, Hungary
d
Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany
Received 11 March 2005; accepted 26 April 2005
Available online 15 June 2005
Abstract
Palladium, platinum and PdPt bimetallic colloidal particles were prepared in atomic ratios of 4:1, 1:1 and 1:4 by the reduction of K2PdCl4
and/orK2PtCl4 inthepresenceofcationicsurfactant, tetradecyltrimethylammoniumbromide(C14TABr). Thenanoparticlesinthe“asprepared”
state were characterized by TEM, EDS and XPS, UPS, permitting to determine their mean particle size, bulk and surface composition. The
bulk composition of the bimetallic samples determined by EDS was close to their nominal value, while XPS indicated Pt enrichment near
to the surface. All samples in the “as received” state contained surface oxide impurity and contained carbon impurities. The monometallic
samples contained considerably more C as the bimetallic ones. A quasi-in situ H2 treatment at 473 K in the electron spectrometer resulted in
cleaner metals, containing less carbon and oxygen, as shown by XPS and UPS.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Palladium–platinum; Powder catalysts; Colloidal particles
1. Introduction
opening of small (C3 or C4) cycles [12–14]. The study of
their alloy has represented earlier mainly theoretical interest
[2]. They have been successfully used, however, for specific
purposes, such as sulfur resistant hydrogenation catalysts
[15].
Bimetallic catalysts have been widely used for various cat-
alytic processes [1]. The full understanding of their behaviour
needs interdisciplinary knowledge, solid-state physics, alloy
theory, chemisorption properties, as summarized concisely
in the excellent book by Ponec and Bond [2]. “Electronic”
and “ensemble” effects can arise between the components
[2,3]. Most of the bimetallic catalysts used in practical
applications employ components with rather different cat-
alytic properties [1], like Pt containing less active Sn [4]
or more active iridium [5]. Platinum is the best catalyst
for “skeletal” reactions of hydrocarbons [2] while palla-
dium is the most widely used catalyst for hydrogenation
of unsaturated organic bonds [6–8]. The properties of Pd
and Pt in hydrocarbon reactions are otherwise rather close
to each other [9–11]. Both are active catalysts, e.g., for
Preparation of metal nanoparticles (with a diameter of a
fewnanometres)hasbeensuccessfullyattemptedusingmeth-
ods of colloid chemistry [16]. This is based on reduction of
metal salts in the presence of surfactants which then form
a surface adlayer preventing the coalescence of small parti-
cles, as opposed to “traditional” preparation of metal blacks
producing usually larger particles or their aggregates [7,17].
Monometallic Pt and Pd [18,19] as well as PdPt particles
[20–22] were synthesized and tested in catalytic reactions
[23]. Catalytic tests could be carried out on the nanoparti-
cles alone or deposited on oxide supports, the latter being
more suited for practical purposes. These behaved as conven-
tional impregnated catalysts [18,20]. Superior samples were
obtained when the synthesis was carried out in the presence
of the destined support [23–26].
∗
Corresponding author. Tel.: +36 1 392 2531; fax: +36 1 392 2533.
E-mail address: paal@iserv.iki.kfki.hu (Z. Paa´l).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.04.064

B. Veisz et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 56–62
57
Pt and Pd forms a solid solution in the whole composi-
2.3. XPS and UPS
tion range [2]. Theoretical calculations and analysis indicated
more or less inhomogeneous in-depth distribution of the com-
ponents. Different models applied for their description has
been summarized by Ponec and Bond [2]. The theory of metal
alloys predicts surface Pd enrichment and was confirmed
by Auger electron spectroscopy (AES) of films and pow-
ders [27]. This was also observed by Soft X-Ray Emission
Spectroscopy of two PdPt foils (containing 15 or 30% palla-
dium) [28]. Pd enrichment was also reported for a disperse
PtPd catalyst supported on ␤-zeolite [29]: its hydrogena-
tion activity increased parallel to the Pd content. Toshima
et al. [30], in turn, suggested surface Pt enrichment in PtPd
nanoparticles. There are indications of inhomogeneous dis-
tribution of Pt and Pd within the small particles [2]: isolated
Pt clusters on Pd surface [15] affected the X-ray photoelec-
tron spectroscopic (XPS) signal and could be regarded as
indications of Pt enrichment. Adsorption of CO [27] and
hydrogen [15] influenced the inhomogeneity. Electron spec-
troscopy and microscopy showed that the composition for
each particle was roughly consistent with that of the parent
solution of the Pt and Pd salt [21,22]. The catalytic behaviour
in alkane reactions [20], in turn, indicated surface Pd enrich-
ment with some, more active small Pt clusters on the surface.
This model was also described by Ponec and Bond [2] and
was confirmed by extended X-ray absorption fine structure
(EXAFS) analysis, too [31].
Surface composition was determined by X-ray photo-
electron spectroscopy (XPS) and ultraviolet photoelectron
spectroscopy (UPS). These measurements used a Leybold
LHS 12 MCD instrument as described earlier [33,34]; UPS
used He II excitation (40.8 eV), pass energy (PE) = 12 eV.
A Mg K␣ anode was used for XPS (PE = 48 eV). Atomic
compositions were determined from peak areas after Shirley
(and, in the case of Pd 3d, Bezier type) background sub-
traction using literature sensitivity factors [35]. This, of
course, could not give information on any microscopic
inhomogeneity.
3. Results and discussion
3.1. Catalyst preparation
Nanosized Pd, Pt and PdPt bimetallic particles were
prepared by the reduction of K2PdCl4 (0–3.8 mM) and/or
K2PtCl4 (0–2.8 mM) in the presence of cationic surfactant,
C14TABr (19.5 mM). A series of aqueous solutions of the
K2PdCl4, K2PtCl4 andtheC14TABrsurfactantwereprepared
separately and then mixed to give the desired concentra-
tions. The interaction between the precursor metal anions
(PdCl42−, PtCl4 ) and the C14TA+ surfactant cation in aque-
2
The present paper deals with the preparation of PdPt pow-
ders of different composition (precipitation in the presence
of surfactants), their characterization by photoelectron spec-
troscopy (XPS, UPS) and electron microscopy (TEM and
HRTEM) in the “as prepared state”, as well as after reduc-
tion. The effect of oxidation–reduction cycles on crystallite
sizes, composition as well as their catalytic behaviour in the
ring opening reaction of methylethylcyclopropane is going
to be reported in a subsequent paper [32].
ous solutions results in the formation of organic metal salt
[36]. The reaction system was subjected to vigorous stirring
when the reducing agent, an aqueous solution of NaBH4, was
introduced into the vessel in a 20-fold excess over the pre-
cursor metal(s). The reduction was conducted at 323 K. To
use the particles as catalysts, the Pd, Pt and PdPt particles
were first concentrated via precipitation by the addition of
2-propanol to the parent hydrosol. Washing of the precipitate
with ethanol only produced a strongly pyrophoric material.
This indicated the extremely high sensitivity of the clusters to
atmospheric oxygen and that the nanoscopic character of the
primary particles in the precipitate was preserved. The wash-
ing was continued by mixing increasing amount of distilled
water to the ethanol and finally the precipitate was washed
with distilled water only, filtered and stored in closed vessels
in air.
2. Experimental
2.1. Materials
Potassium tetrachloropalladate, K2PdCl4 (98%, Aldrich),
potassium tetrachloroplatinate, K2PtCl4 (99.9+%, Aldrich),
tetradecyltrimethylammonium bromide, C14TABr (99%,
Aldrich), sodium borohydride, NaBH4 (99%, Aldrich) and
2-propanol (p.a., reanal) were used as received.
The nominal composition of the powders prepared were
100% Pt, 100% Pd and PdPt in the atomic ratio 4:1, 1:1 and
1:4.
3.2. Electron microscopy
2.2. Electron microscopy
Typical TEM images of the catalyst powders are shown
in Figs. 1–3. Narrow size distribution and a good control
of particle size in the range of 6–8 nm in diameter (10 nm
average, and up to 15 nm for pure Pt) could be achieved by a
systematic variation of the experimental conditions. Most of
the particles imaged on the micrographs (Figs. 1–3) exhibited
a more or less hexagonal shape projection although some
The size distribution of the particles were determined by
transmission electron microscopy in a Philips CM20 analyti-
cal TEM. Some samples were studied also in a high resolution
microscope, using a JEOL 3010 TEM with a resolving power
of 0.17 nm. The bulk composition of the samples were deter-
mined by energy dispersive X-ray spectrometry (EDS).

58
B. Veisz et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 56–62
is excluded from the imaging process, resulting in darker
contrast. If a crystal is far from a zone-axis orientation, almost
all of the transmitted intensity is within the direct beam, and
the crystal is apparently light. The bulk composition of the
Fig. 1. TEM images of (a) the pure Pd and (b) the pure Pt powders in the
“as prepared” state.
quadratic and triangular shapes were occasionally observed,
too. In bright-field images, some crystals appear dark and
other light due to diffraction contrast: the closer a crystal is to
a perfect zone-axis orientation, the more diffracted intensity
Fig. 3. HRTEM images of (a) the Pd:Pt 4:1, (b) the Pd:Pt 1:1 and (c) the Pd:Pt
1:4 bimetallic catalysts. The alloyed nanoparticles seem to have aggregated
and show more irregular, rounded shape than the pure Pd and Pt particles.
The amorphous area on the bottom of (a) is the carbon film supporting the
metal particles.
Fig. 2. HRTEM image of the pure Pt powder. The particles exhibited hexag-
onal shape projection and the crystals were perfectly ordered, except for
occasional twinning on (1 1 1) planes.

B. Veisz et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 56–62
59
Table 1
Comparison of the bulk and the surface Pd:Pt atomic ratio of the PdPt alloy catalysts in the “as received” state and after H2 at 473 K
Pd:Pt
1:0
4:1
1:1
1:4
0:1
14
0:100
0:100
0:100
0:100
Mean particle size, nm (TEM)
Nominal ratio
Measured in bulk: EDS
Measured at surface: XPS
Measured at surface: XPS, after H2 at 473 K
5.5
4.2
5.8
6.5
100:0
100:0
100:0
100:0
80:20
85:15
73:27
75:25
50:50
50:50
28:72
33:67
20:80
23:77
7:93
10:90
samples determined by EDS was found to be close to their
nominal value (Table 1).
(BE ∼ 530–533 eV) from the Pd 3p 3/2 line (BE = 532.5 eV),
therefore, the oxygen content was only be determined directly
from O1s, in the pure Pt sample. The O2psensitivity is solow
that its intensity was below detection limit. Comparing the
Pt 4f and Pd 3d lines of the “as received” bimetallic samples
with those measured after H2 treatment, a rough estimation
could be done concerning their oxidation.
Concentrating on the two main metal components, XPS
of the alloy samples indicated more Pt in the “information
depth”, i.e., near to the surface (Table 1). This finding may
be caused by the difference in the reduction rates of the Pd
and Pt precursor salts. Accordingly, Pt atoms might have
been formed at a slower rate during reduction process and
deposited on the surface of Pd particles already formed.
Fig. 4 shows the Pd 3d spectra of untreated catalysts. The
BE maximum of the Pd 3d line was at 335.3 eV, like the
value measured with another Pd black sample [37]. The peak
shapes indicated low amounts of oxidized component(s). The
Pt 4f 7/2 peaks show a BE of 71.1 eV, indicating metallic
HRTEM of bimetallic nanoparticles (Fig. 3) show more
aggregated particles, especially with PdPt 4:1 and 1:4. They
showed more irregular, rounded shape than the pure Pt and
Pd particles. Individual crystallites of PdPt 1:1 can still be
observed (Fig. 3b). Fourier-transform analysis of the local
structure in PdPt particles is not possible, due to the very
close lattice parameters and identical fcc structure of Pd and
Pt.
3.3. XPS and UPS
Pt 4f, C 1s, Pd 3d (together with the neighboring Pt
4d), and whenever possible, also the O 1s lines were mon-
itored by XPS. Due to binding energy (BE) coincidence,
the Pd 3d spectrum should be separated from the Pt 3p
3/2 peak (done by considerate background subtraction). It
was, however, extremely difficult to separate the O 1s line
Fig. 5. C 1s XPS spectra for Pt, Pd and mixed PdPt samples in the “as
received” state. The carbon percentages are shown on the corresponding
spectra.
Fig. 4. Pd 3d region of the palladium containing catalysts in the “as received”
state.

60
B. Veisz et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 56–62
Table 2
Comparison of the surface composition of the catalysts measured by XPS in the “as is” state and after treatment in H2 at 473 Ka
Catalysts
As is (%)
Pd
Pretreated in H2 at 473 K (%)
Pt
C
Oa
Pd
Pt
C
Oa
Pd
72.1
62.7
21.7
5.9
0
0
27.9
14.2
21.8
11.1
37.2
30.5
75.5
67.1
28.1
8.9
0
0
24.5
11.1
15.8
9.1
31.1
27.7
PdPt 4:1
PdPt 1:1
PdPt 1:4
Pt
23.1
56.5
83.0
62.8
51.6
21.8
56.1
82
68.9
61.4
Ptb
0
17.9
0
10.9
a
The O 1s peak and the Pd 3p 3/2 peaks overlapped, therefore, the O 1s peak could not be determined in Pd containing samples. The composition of those
samples was normalized to Pd + Pt + C = 100%.
b
The composition for all components is shown in the last row.
Pt. The comparison of the C 1s peaks (present as another
unavoidable impurity in untreated samples [34,37]) indicates
a marked difference between pure metal and alloy parti-
cles (Fig. 5). Monometallic Pd and Pt retained more than
twice as much carbon as the composite powders. Of the three
latter ones, PdPt 4:1 and 1:4 contained conspicuously less
carbon. The maximum BE of the C 1s peak (∼284.5 eV)
showed that graphitic entities were among the main com-
ponents, but “CxHy” and “disordered C” were also present
[33,38,39].
cation process (O2 and H2 treatment [34]). Such results will
be reported in Part II [32]. Table 2 indicates that even the
present treatment brought the Pd:Pt ratio closer to the nom-
inal values. Up to ∼30% of the original carbon content was
removed.
About 60% of the original O 1s area remained on the
hydrogen treated Pt sample (Table 2). The degree of oxida-
tion could be judged in other cases from difference spectra
before and after H2 treatment. Fig. 6a shows difference spec-
tra for three Pd samples: small maxima appeared in the region
corresponding to Pd2+ and Pd4+ [40]. Similar Pt 4f difference
spectra are seen in Fig. 6b for Pt catalysts. The presence of
two oxidized Pt species can be seen: the “PtO(ads)” entity with
Treating the samples with H2 at 473 K represented a cus-
tomary cleaning process for Pt [34] and Pd [37], although
the cleanest metals were observed after a more severe purifi-
Fig. 6. XP spectra of the main components in the “as received” state and after H2 treatment at 473 K, together with the difference spectra (after normalization
for Pt). Monometallic catalysts, 1:1 samples and 4:1 samples (for the richer component) are shown. (a) Pd 3d and (b) Pt 4f.

B. Veisz et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 56–62
61
Fig. 7. He II UPS of Pt, Pd and mixed Pt–Pd samples (a) in the “as received” state and (b) after H2 treatment at 473 K.
ꢀBE of ∼0.8 eV and the PtO species with ꢀBE of ∼2.4 eV
[41]. Fig. 6a and b depict also the Pd–Pt 1:1 sample. The Pd
3d peak is shown for Pd–Pt 4:1 and the Pt 4f for Pd–Pt 1:4.
The ionic components of Pt are practically the same in all
three cases, The Pd–Pt 4:1 sample shows ca. twice as much
oxide component than pure Pd.
not sufficient to suppress the high intensity at the FE in PdPt
samples. H2 treatment at 473 K increased the Fermi-edge
intensities.
4. Conclusions
The O 1s peak of the monometallic Pt sample indi-
cated the presence of four components. These were iden-
tified [38] as metal oxide (BE ∼ 530 eV); –OH groups
(BE ∼ 531.5 eV), “oxidized carbon” (BE ∼ 532 eV) and
adsorbed H2O (BE ∼ 534 eV). H2 treatment at 473 K
removed some O 1s components, the PtO component with
lowest BE disappearing entirely.
ThegeneralshapeoftheHeIIUPSspectraisseenin Fig. 7.
The Fermi-edge (FE) intensity of the untreated samples was
lowest in the monometallic samples, in agreement with their
large carbon impurity. Oxygen has also been detected on Pt,
although the Pt 4f spectrum showed minor amounts of “oxi-
dized Pt”, in agreement with the O 1s peak. More surface
OH and H2O can be assumed (broadening at ca. 6 eV and a
peak at >10 eV [37,38]). The He II UP spectrum of Pd-rich
samples showed a cleaner metal state after hydrogen treat-
ment (more intense Fermi-edge and the disappearance of the
O 2p binding orbitals at ∼2 and ∼4 eV). The carbon com-
ponent never formed a more or less compact surface layer,
since it would bring about a maximum at ca. 7.5 eV [34,39].
The results are in agreement with XPS: UPS with lower infor-
mation depth indicates with higher sensitivity the presence of
surface impurities (for example, the O 2p peak at ∼4 eV). The
less carbon and also the low amount of oxidized metals were
Nanosized Pd, Pt and PdPt bimetallic particles have been
successfully prepared by the reduction of K2PdCl4 and/or
K2PtCl4 in the presence of cationic surfactant, C14TABr.
They were characterized by different techniques. The bulk
composition and the mean particle size of the catalysts have
been determined by EDS and HRTEM. The size of most of
the particles was 4–6 nm, except for Pt, showing typical sizes
around 14 nm. The monometallic Pd and Pt samples exhib-
ited a more or less hexagonal shape projection. The bimetallic
nanoparticles, in turn, seem to have aggregated and exhibited
a more irregular, rounded shape. The bulk composition of the
bimetallic samples determined by EDS in the “as prepared”
state was close to their nominal value, while XPS indicated
Pt enrichment near to the surface. The Pt content decreased
slightly after treatment in H2 at 473 K. XPS detected car-
bon impurities in all samples, Pd and Pt retaining more than
twice as much carbon as the bimetallic (4:1 and 1:4) parti-
cles. The O 1s line (observable with Pt only) showed ca. 15%
oxygen, present mainly as surface PtO as well as adsorbed
OH/H2O. The comparison of the Pd 3d and Pt 4f lines of the
untreated samples with those measured after H2 treatment
pointed to the presence of small amounts of oxidized metals
in the “as received” state. H2 at 473 K removed most of the

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B. Veisz et al. / Journal of Molecular Catalysis A: Chemical 238 (2005) 56–62
oxygen impurity. The UPS spectra after H2 were closer to
those characteristic of pure metals.
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´
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