In situ XPS study of methanol reforming on PdGa near-surface intermetallic phases

Article abstract of DOI:10.1016/j.jcat.2012.03.009

In situ X-ray photoelectron spectroscopy and low-energy ion scattering were used to study the preparation, (thermo)chemical and catalytic properties of 1:1 PdGa intermetallic near-surface phases. Deposition of several multilayers of Ga metal and subsequent annealing to 503-523 K led to the formation of a multi-layered 1:1 PdGa near-surface state without desorption of excess Ga to the gas phase. In general, the composition of the PdGa model system is much more variable than that of its PdZn counterpart, which results in gradual changes of the near-surface composition with increasing annealing or reaction temperature. In contrast to near-surface PdZn, in methanol steam reforming, no temperature region with pronounced CO₂ selectivity was observed, which is due to the inability of purely intermetallic PdGa to efficiently activate water. This allows to pinpoint the water-activating role of the intermetallic/support interface and/or of the oxide support in the related supported Pd _xGa/Ga₂O₃ systems, which exhibit high CO ₂ selectivity in a broad temperature range. In contrast, corresponding experiments starting on the purely bimetallic model surface in oxidative methanol reforming yielded high CO₂ selectivity already at low temperatures (～460 K), which is due to efficient O₂ activation on PdGa. In situ detected partial and reversible oxidative Ga segregation on intermetallic PdGa is associated with total oxidation of intermediate C₁ oxygenates to CO₂.

Full text of DOI:10.1016/j.jcat.2012.03.009

Journal of Catalysis 290 (2012) 126–137
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
In situ XPS study of methanol reforming on PdGa near-surface intermetallic phases
Christoph Rameshan a,b, Werner Stadlmayr a, Simon Penner a, Harald Lorenz a, Lukas Mayr a,
Michael Hävecker b, Raoul Blume b, Tulio Rocha b, Detre Teschner b, Axel Knop-Gericke b, Robert Schlögl b,
Dmitry Zemlyanov c, Norbert Memmel a, Bernhard Klötzer a,
⇑
a Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria
b Department of Inorganic Chemistry, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4–6, D-14195 Berlin, Germany
c Purdue University, Birck Nanotechnology Center, 1205 West State Street, West Lafayette, IN 47907-2057, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
In situ X-ray photoelectron spectroscopy and low-energy ion scattering were used to study the prepara-
tion, (thermo)chemical and catalytic properties of 1:1 PdGa intermetallic near-surface phases. Deposition
of several multilayers of Ga metal and subsequent annealing to 503–523 K led to the formation of a
multi-layered 1:1 PdGa near-surface state without desorption of excess Ga to the gas phase. In general,
the composition of the PdGa model system is much more variable than that of its PdZn counterpart,
which results in gradual changes of the near-surface composition with increasing annealing or reaction
temperature.
In contrast to near-surface PdZn, in methanol steam reforming, no temperature region with pro-
nounced CO2 selectivity was observed, which is due to the inability of purely intermetallic PdGa to
efficiently activate water. This allows to pinpoint the water-activating role of the intermetallic/support
interface and/or of the oxide support in the related supported PdxGa/Ga2O3 systems, which exhibit high
CO2 selectivity in a broad temperature range. In contrast, corresponding experiments starting on the
purely bimetallic model surface in oxidative methanol reforming yielded high CO2 selectivity already
at low temperatures (ꢀ460 K), which is due to efficient O2 activation on PdGa. In situ detected partial
and reversible oxidative Ga segregation on intermetallic PdGa is associated with total oxidation of inter-
mediate C1 oxygenates to CO2.
Received 25 October 2011
Revised 2 March 2012
Accepted 11 March 2012
Available online 12 April 2012
Keywords:
PdGa near-surface intermetallic phase
Methanol dehydrogenation
Methanol steam reforming
Oxidative steam reforming
Water activation
Ambient-pressure XPS
In situ spectroscopy
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
still far from clear. Structure-wise, this refers in the first place to
the question, if also a 1:1 PdGa near-surface intermetallic phase
In recent years, a group of Pd-based catalysts has been studied
in search of possible alternatives to selective but sinter-unstable
Cu/ZnO catalysts for use in efficient hydrogen production by meth-
anol steam reforming [1–3]. It is widely accepted that the active
and CO2-selective state of these catalysts at least requires the pres-
ence of a single bimetallic (Pd–Zn, Pd–Ga or Pd–In), induced by
reductive activation in the temperature region 523–773 K [1–3].
By far, the most studied of these catalysts is the PdZn/ZnO system,
which is already scrutinized in many structural, electronic and cat-
alytic aspects [1–10]. A 1:1 PdZn intermetallic phase has been pro-
ven to be essential, as has been the role of the oxidized Zn centers
and their interaction with the intermetallic surface for efficient
water activation [9,10]. Despite the striking catalytic similarities
between the three systems (for all of the supported bimetallic
systems a CO2-selective temperature region has been verified),
key parameters of the Pd–Ga (and the related Pd–In) system are
(in the following abbreviated as ‘‘NSIP’’) can be prepared in a
similar way as PdZn and if this isolated bimetallic exhibits not only
comparable catalytic properties, but also comparable thermal and
thermochemical stability. This will moreover connect to previous
structural and sintering studies on oxide-supported bimetallic
PdxGa particles, where an – in comparison with PdZn – less-
pronounced thermal stability and generally a higher amount of sin-
tering have been observed [11,12]. For a direct comparison with
Pd1Zn1, the Pd–Ga phase diagram reveals that the Pd1Ga1 interme-
tallic is one of the stable, but not the thermodynamically most
stable compound, which is clearly the case for Pd1Zn1 [13]. The
Pd1Ga1 bulk intermetallic compound is, however, stable over a
wide range of temperatures and exhibits a FeSi crystal structure.
It can be regarded as a distorted NaCl structure with increased
coordination number from 6 to 7. Each Pd atom is therefore sur-
rounded by 7 Ga atoms (3 + 3 + 1) [14]. Thermodynamically, the
formation of stable intermetallic compounds can be explained on
the basis of considerable enthalpies of formation [15]. The energet-
ically favorable Pd–Ga interaction can be understood on the basis
⇑
Corresponding author. Fax: +43 512 507 2925.
E-mail address: Bernhard.Kloetzer@uibk.ac.at (B. Klötzer).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.03.009

C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
127
of strong, localized and covalent Pd–Ga bonds [16]. The alteration
of the Pd coordination in Pd1Ga1 (as well as in Pd3Ga7 and Pd2Ga)
thus also leads to a strong modification of the electronic structure
around the Fermi level in comparison with elemental Pd. The XPS
spectra of, for example, Pd1Ga1(110) are dominated by features
between 2 and 5 eV below EF, attributed to localized Pd-4d states
[17]. This electronic modification, in combination with the struc-
tural isolation of active Pd sites, gives rise to the excellent catalytic
ethene semihydrogenation properties described, for example, in
[18].
With respect to a potential methanol reforming application, the
most CO2-selective and CO2-active oxide-supported bimetallic spe-
cies is the Pd2Ga phase-supported on b-Ga2O3 [1,11], which is com-
parably CO2 active and selective as PdZn/ZnO. The Ga-richer 1:1
phase, also supported on b-Ga2O3, has been previously determined
to be more affected by SMSI effects and oxidative segregation
under reaction conditions and thus to be less selective and active
in methanol steam reforming. Nevertheless, Pd1Ga1/b-Ga2O3 has
been shown to be still more CO2 active and selective than pure
b-Ga2O3 [11].
chamber is equipped with an XPS/Auger/ISS spectrometer (Thermo
Electron Alpha 110) and a standard double Mg/Al anode X-ray gun
(XR 50, SPECS), an Omicron ISE 100 ion gun to provide the focused
1 keV He+ ions for ISS, an electron beam heater, an ion sputter gun
and a mass spectrometer (Balzers). All ISS experiments were per-
formed at an angle of beam incidence
W = 45° and a scattering an-
gle of # = 90°, and after correction for the different cross-sections,
intensity normalization of the Pd and Ga signals was performed rel-
ative to the total backscattering yield, that is, IGa(normalized) = IGa
/
(IPd + IGa) and IPd(normalized) = IPd/(IPd + IGa). The Pd and Ga scatter-
ing cross-sections valid for our specific setup were determined by
measurement of the clean Pd foil and a sufficiently thick pure Ga
metal surface layer covering all Pd under identical experimental
conditions. For controlled Ga deposition, a home-built Ga evapora-
tor was attached, which consists of a small boron nitride crucible
filled with Ga metal (99.999%, Goodfellow) and heated by electron
bombardment. A water-cooled quartz crystal microbalance moni-
tored the amount of deposited Ga.
The UHV-prepared samples are thereafter transferred by means
of a magnetically coupled transfer rod from the UHV sample holder
to a Pyrex glass sample holder used inside the all-glass reaction
cell. With this all-glass setup of the ambient-pressure reaction cell,
no wires or thermocouples are connected to the sample during cat-
alytic measurement (thermocouple mechanically contacted at the
outside). Accordingly, background (blind) activity of the reaction
cell is routinely checked and was found to be negligible for all tests.
A detailed graphic representation of the ambient-pressure reaction
cell setup is provided in the Supplementary material (Fig. S1).
The main chamber is pumped by a turbomolecular pump, an ion
getter pump and a titanium sublimation pump to a base pressure
in the low 10À10 mbar range. High purity gases (H2, O2, Ar: 5.0)
were used as supplied from Messer–Griesheim and dosed via
UHV leak valves. The high-pressure cell is evacuated sequentially
by a rotary pump (via LN2 cooled zeolite trap) and then via the
main chamber down to UHV base pressure and can be heated from
outside to 723 K with an oven covering the cell. For better mixing
of the reactants, the high-pressure cell is operated in circulating
batch mode. By using an uncoated GC capillary attached to the
high-pressure cell, the reaction mixture in the close vicinity of
the sample is analyzed continuously by the electron ionization
detector (EID) of the GC/MS system. For quantitative measurement
of H2, we used (in parallel with the EID) an additional Balzers QMA
125 detector specifically tuned for optimum H2 detection. EID and
QMS signals of methanol, CO2, CO, H2 and CH2O were externally
calibrated and corrected for fragmentation (that is, CO and CH2O
fragments for methanol, CO fragment for CO2).
Secondly, the selectivity spoiling role of the inverse water–gas
shift reaction on the b-Ga2O3 support, based on the capability of
oxygen vacancy sites to activate water, has already been assessed
[11,19]. Although some similarities between ZnO and Ga2O3 espe-
cially regarding water activation exist [19], it is not per se clear
how the – for a high CO2 selectivity essential – water activation
works on the supported PdxGa/b-Ga2O3 systems.
Partly fueled by previous experiments on the structure and the
catalytic properties of supported Pd1Ga1 and Pd2Ga bimetallic par-
ticles, which revealed temperature regimes with structurally stable
Pd–Ga intermetallic compounds and, for the latter, a particularly
high CO2 selectivity, the present contribution aims at a thorough
characterization of the ‘‘isolated’’ Pd–Ga intermetallic surface
without contact to the b-Ga2O3 support, either to assess a potential
analogy to the bi-functional activity of the multi-layered Pd1Zn1
NSIP [9,10] or to reveal a possible bi-functional synergism of the
Pd–Ga intermetallic surface and the oxide support. In this context,
we were particularly interested in a potential role of the oxide’s de-
fect chemistry in activating water for the full methanol reforming
process toward CO2. Moreover, an exclusive role of a specific bime-
tallic surface composition (as already proven for the related Pd1Zn1
multilayer NSIP case [9,10]) remained to be verified or disproved.
Two types of reforming reactions were studied in situ, namely
‘‘water-only’’ methanol steam reforming (MSR), corresponding to
the ‘‘ideal’’ reaction CH3OH + H2O ? CO2 + 3H2, and oxidative
steam reforming (OSR), whereby a certain added amount of O2
may give rise to H2-formation stoichiometries ranging from partial
methanol oxidation (CH3OH + 1/2 O2 ? CO2 + 2H2) to total oxida-
tion (CH3OH + 3/2 O2 ? CO2 + 2H2O).
A polycrystalline palladium foil (Goodfellow, purity 99.999%,
0.125 mm thick, size 3.5 cm2) was cleaned on both sides by succes-
sive cycles of Ar+ ion bombardment (6.0 Â 10À5 mbar Ar, 503 eV,
1
l
A sample current), oxidation (5.0 Â 10À7 mbar O2, T = 1000 K),
and annealing in hydrogen (5.0 Â 10À7 mbar H2, T = 700 K) and in
vacuum (T = 1000 K) until no impurities were detected by AES
and XPS. Details of the preparation of the PdGa multilayer interme-
tallic phase will be given in Section 3.1. Methanol and methanol/
water mixtures were degassed by repeated freeze-and-thaw cy-
cles. All MSR reactions were conducted with methanol/water mix-
tures of a 1:10 composition of the liquid phase. This corresponds to
a room temperature partial pressure ratio of methanol/water = 1:2,
as verified by mass spectrometry.
2. Experimental
2.1. Innsbruck experimental setup
The UHV system with attached all-glass high-pressure reaction
cell [20] is designed for catalytic studies up to 1 bar on a larger piece
of 1.8 cm  2 cm polycrystalline Pd foil, allowing us to detect reac-
tion products and even minor intermediates with high sensitivity,
either by discontinuous sample injection into the gas chromatogra-
phy–mass spectrometry (GC–MS) setup (HP G1800A) or by direct
online MS analysis of the reaction mixture via a capillary leak into
the GC/MS detector. The system consists of an UHV chamber with
a long-travel Z-manipulator and a small-volume Pyrex glass reactor
(52 ml, no hot metal components) attached to the outside of the
UHV chamber and accessible via a sample transfer port. The UHV
The catalytic experiments were performed in a temperature-
programmed manner, that is, the reaction cell was heated at a con-
stant linear rate of ꢀ8 K/min to the final temperature of 623 K, and
then kept isothermal at this temperature for ꢀ20 min. Experimen-
tal details will be given in context with the individual reaction
runs. The advantage of the TPR (temperature-programmed reac-
tion) runs is that pronounced selectivity changes can be monitored

128
C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
via the partial pressure changes as a function of the reaction tem-
perature, yielding useful qualitative information about changes of
the reaction mechanism and the catalyst state. From the product
partial pressures vs. time plots the reaction rates were obtained
by differentiation and are usually given in partial pressure change
per minute [mbar/min], but whenever desired, the turnover fre-
catalytic measurements. To reproducibly prepare a Pd1Ga1 NSIP,
calibration of a monolayer equivalent (MLE) deposited amount of
Ga metal is necessary. As Zn and Ga exhibit similar atomic masses
(65.4 vs. 69.7 amu), our previous calibration of the quartz micro-
balance for Zn deposition [9,10] could be easily adapted to Ga
deposition. Accordingly, as all other experimental deposition
parameters were equivalent, deposition of 22.5 Hz corresponds to
1 MLE of Ga, based on a Pd atom density of 1.4 Â 1015 surface
Pd-atoms/cm2 (the average of evenly distributed (111) and
(100) facets). To back up this calibration, Ga metal was stepwise
(in 10 Hz steps) deposited with constant rate on the clean Pd foil
at 300 K. Thereafter, LEIS using He+ ions (1 keV) was performed
and complemented by a second series of LEIS measurements with
intermediate annealing to 523 K after each Ga deposition.
Fig. 1 shows the corresponding effects. Briefly anticipating the
298 K XPS spectra of Fig. 3, a strong Ga–Pd interaction (intermix-
ing) already after room temperature Ga deposition is evident.
The Pd3d shift clearly indicates partial NSIP formation, as evi-
denced by the high Pd3d BE of 336.15 eV and the ‘‘Cu-like’’ valence
band region. For comparison, the BE of Pd3d in clean Pd foil is
found at ꢀ335.1 eV, and a high ‘‘d-band’’ induced density of states
at the Fermi edge is to be expected on clean Pd.
In Fig. 1, major surface composition changes occur between 0
and 1 MLE (22.5 Hz) Ga (irrespective of the temperature); thereaf-
ter, only a slow increase in the Ga surface fraction from ꢀ66% up to
86% is visible at 298 K. In contrast, after 523 K annealing a constant
1:1 ratio is obtained at all depositions beyond 1 MLE, independent
of the extra Ga amount beyond 1 MLE. Taken together, a similar sit-
uation to the Pd1Zn1 NSIP growth process described in [8–10] is
likely: at 298 K incomplete alloying leads to a particularly high
Ga coordination of the top Pd layers by a combination of partial
Pd–Ga intermixing within the surface layer(s) but also extra Ga ad-
sorbed on top of this state, leading to a maximum Pd3d high BE
shift in the uppermost region in combination with surface Ga
enrichment relative to Pd in LEIS. This can explain the
ꢀ66:34 Ga/Pd ratio monitored by LEIS after deposition of 1 MLE
Ga at 298 K, which can be converted to a 50:50 ratio by thermal
annealing at ꢀ523 K, likely leading to a ‘‘bilayered’’ 1:1 NSIP, in
close analogy to bilayered PdZn [8]. We conclude that multilayer
Ga depositions exceeding 1 MLE Ga, followed by annealing at
ꢀ503–523 K, lead to the respective multi-layered states of the
quencies (TOF’s) given in molecules per site and second [siteÀ1 sÀ1
]
can be calculated by multiplication of the partial pressure change
with a factor f = 2.4, for example, a reaction rate of 1 mbar/min cor-
responds to a TOF of 2.4 siteÀ1 sÀ1 (one site corresponds to a single
Pd surface atom on the respective foil substrate; see below). The
conversion factor is based on the partial pressures of the reaction
products already corrected for the temperature change in the reac-
tion cell during the TPR run and for the steady removal of a fraction
of the reaction mixture through the capillary leak. The correction
has been achieved by adding 30 mbar Ar inert gas at the beginning
of the reaction run and monitoring the m/z = 40 Ar intensity
throughout the whole experiment. The Ar intensity over time then
was used to recalculate the changes of the molar amounts of all
products and reactants as referred to the initial state (before TPR
start, reactor volume 60.6 ml and 300 K in the whole re-circulating
batch system). The total number of potential catalytic surface sites
for the clean Pd foil Ns = 1 Â 1016 was determined from the Pd sur-
face (area 7 cm2) being composed of equally distributed (111) and
(100) facets. To derive TOF numbers on a sample with Pd:Ga = 1:1
surface composition, this number was divided by 2.
2.2. HZB/BESSY II experimental setup
The HZB/BESSY II system [21] (at beamline ISISS-PGM) allowed
us to perform in situ photoelectron spectroscopy up to 1 mbar total
reactant pressures. It is equipped with differentially pumped elec-
trostatic lenses and a SPECS hemispherical analyzer. The sample
is positioned inside the high-pressure/analysis chamber ꢀ2 mm
away from a 1 mm aperture, which is the entrance to the lens sys-
tem separating gas molecules from photoelectrons. Binding ener-
gies (BEs) were generally referred to the Fermi edge recorded
after each core level measurement. Samples were mounted on a
transferable sapphire holder. The temperature was measured by a
K-type Ni/NiCr thermocouple spot-welded to the side of the sam-
ple, and temperature-programmed heating was done by an IR laser
from the rear. Sample cleaning procedures consisted of repeated cy-
cles of Ar+ sputtering at room and elevated temperatures, annealing
up to 950 K in UHV, and exposure to O2, followed by flashing at
950 K for 60 s in UHV. The cleanliness of the Pd foil substrate was
checked by XPS. The sensitivity of the simultaneous MS detection
of the reaction products at HZB/BESSY II was not sufficient to ex-
tract reliable reaction rate and selectivity data for H2/CO/CH2O/
CO2, mainly because of an unfavorable ratio of the large total reac-
tant flow through the XPS high-pressure cell (which is generally
operated in constant flow mode) relative to the minor amounts of
products formed on the low surface area catalyst (only 0.5 cm2
PdGa intermetallic compound phase on Pd foil or Pd(111)). How-
ever, ‘‘connecting’’ experiments performed in the Innsbruck setup
using the same conditions with respect to initial reactant pressures,
PdGa NSIP preparation and reaction temperature range, allowed to
assess a possible ‘‘pressure gap’’ effect and provided a reliable con-
nection between the data obtained in either experimental setup.
Fig. 1. LEIS-determined ratio of Pd to Ga in the surface using 1 keV He+ ions plotted
vs. the resonant frequency of the quartz crystal in the film thickness monitor.
Measurement series with step-wise Ga deposition at 298 K (solid lines) and second
series with intermediate tempering at 523 K (dotted lines) prior to analysis. Vertical
dashed line: 22.5 Hz Ga metal deposition corresponding to 1 MLE of Ga, based on a
Pd surface atom density of 1.4 Â 1015 Pd atoms per cm2. The intersection of the
tangential to the 298 K uptake curve with the X-axis provides an additional hint for
the correctness of the coverage calibration.
3. Results and discussion
3.1. Preparation and characterization of the Pd1Ga1 NSIP
Similarly to the PdZn system, a thoroughly characterized and
reproducible initial bimetallic state is a prerequisite for meaningful

C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
129
must not necessarily represent a major obstacle in the Pd1Ga1 NSIP
case. At least in view of electronic structure, as can be seen in Fig. 3,
changes of the valence band DOS of Pd by Ga are still strong and
the ‘‘Cu-like’’ situation already shown for the multi-layered Pd1Zn1
NSIP could be clearly verified also for the Pd1Ga1 NSIP case. By
comparison of the Pd3d binding energy position of 336.0 eV
reported for the bulk Pd1Ga1 compound [23] and of 335.6 eV for
the more Ga-depleted Pd2Ga compound [24] with the Pd3d bind-
ing energies between 335.9 eV and 335.75 eV determined after
thermal annealing of 4 MLE Ga deposited on Pd foil between
473 K and 523 K (see Fig. 3, Section 3.2), an intermediate electronic
state and a pronounced ‘‘intermetallic compound character’’ of the
fcc-type Pd1Ga1 NSIP between bulk Pd1Ga1 and bulk Pd2Ga1 can be
assumed.
1.0
0.8
0.6
0.4
0.2
0.0
60x103
50
AES Intensity of Ga L3M45M45(1G)
LEIS Ga/(Pd+Ga)
LEIS Pd/(Pd+Ga)
40
30
20
10
3.2. Thermal stability of the Pd1Ga1 NSIP
300
400
500
600
700
800
Temperature [K]
To gain additional insight into the temperature-dependent sur-
face composition and the associated thermal stability of the Pd1Ga1
NSIP, 4 MLE Ga were deposited at ꢀ300 K and step-wise annealed
to 800 K. The spectroscopic fingerprints were measured using AES
and LEIS and the results were corroborated by in situ XPS measure-
ments at HZB/Bessy II.
Fig. 2. Ga and Pd surface fractions starting from 4 MLE deposited Ga metal, as
determined by LEIS and via Peak-to-Peak intensity of the differentiated Ga–
L3M45M45 Auger signal, as a function of the annealing temperature.
Pd1Ga1 NSIP with variable thickness. This picture was further cor-
roborated by XPS-depth profiling, showing only little changes of
the relative Ga concentration between ꢀ5 and ꢀ13 Å information
depth after deposition of 4 MLE Ga and 523 K annealing.
Fig. 2 shows an immediate sharp decrease in the Ga–L3M45L45
signal up to about ꢀ500 K. This is obviously related to the in-
creased 1:1 NSIP formation with Ga progressing into deeper layers.
In contrast to experiments on PdZn, where Zn ‘‘on-top’’ multilayers
easily desorbed to the gas phase already at low temperatures
(ꢀ450 K), the system Pd–Ga shows a quantitative transition of all
deposited Ga into the NSIP state. This is inferred from the much
lower vapor pressure of Ga, which therefore cannot desorb to the
gas phase in the temperature region 300–800 K. If more than 1
MLE Ga is deposited on the Pd foil and subsequently tempered
to, for example, 523 K, the entire Ga transforms into the interme-
tallic state and forms a correspondingly thick 1:1 Pd–Ga layer.
The LEIS measurements correspondingly show a strong decrease
in the Ga signal between 300 and 500 K until the ꢀ1:1 surface
composition is finally reached at ꢀ503 K. Between 503 K and
ꢀ623 K, a small plateau region is visible in the Ga–L3M45L45 signal,
which seems to indicate a certain stability range of the Pd1Ga1
As the Pd1Ga1 NSIP was initially intended to be a model system
for bulk (FeSi-type) Pd1Ga1, we must address the question of poten-
tial structural and electronic differences. The 3-dimensional struc-
ture within the film has been studied recently by impact-collision
ion scattering (ICISS) after 523 K annealing of several MLE Ga
[22], revealing an fcc (Pd metal-like) coordination geometry of Pd
and Ga which is quite similar to that observed for the multi-layered
PdZn NSIP. Thus, a 1:1 coordination chemistry similar to the PdZn
NSIP (down to ꢀ13 Å information depth) could be established and
closely related MSR properties were initially suspected.
As the structural identity of NSIP and bulk phase was equally
not given in case of the Pd1Zn1, but nevertheless the Pd1Zn1-NSIP
was found to be a highly CO2-selective model system in a certain
range of reaction temperature [9,10], the structural differences
Pd 3d5/2
Ga 3d
VB
873 K
873 K
823 K
773 K
698 K
298 K
373 K
398 K
423 K
673 K
648 K
448 K
473 K
503 K
523 K
623 K
598 K
573 K
548 K
573 K
548 K
523 K
503 K
473 K
598 K
623 K
648 K
673 K
698 K
773 K
823 K
448 K
423 K
398 K
373 K
873 K
298 K
298 K
338 337 336 335 334
20
19
18
10
8
6
4
2
0
Binding Energy [eV]
Fig. 3. Temperature-programmed XPS measurements of the Pd 3d5/2 signal (measured at 470 eV photon energy) and the Ga 3d and valence band signals (measured at
120 eV). Note that the Ga 3d spectra are shown in inverted order as compared to Pd 3d due to the inverse intensity effects. Spectra have been taken in 25 K steps between 373
and 698 K. 4 MLE Ga were initially deposited. For comparison, the BE of Pd3d in clean Pd foil is reported at ꢀ335.1 eV, and a high ‘‘d-band’’ induced density of states at the
Fermi edge is to be expected.

130
C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
Ga3d
VB
Pd3d5/2
573K
548K
528K
523K
573K
548K
528K
573K
548K
528K
523K
503K
473K
423K
373K
298K
503K
473K
423K
373K
523K
503K
473K
423K
373K
298K
298K
338 337 336 335 334 333
22
20
18
12
8
4
0
Binding Energy [eV]
Fig. 4. In situ XPS spectra of the Ga2O3-covered Pd foil, prepared by deposition of 4 MLE Ga converted to Ga2O3 in 10À3 mbar O2. Subsequent annealing was carried out in
0.1 mbar H2. Photon energies were the same as in Fig. 4.
NSIP. However, close inspection of the in-parallel measured LEIS
spectra indicates changes in surface composition with increasing
temperature from ꢀ52:48 at 503 K to ꢀ46:54 Ga:Pd at 623 K.
The surface composition hence remains close to 1:1, but small
changes due to preferential LEIS-induced sputtering of Ga espe-
cially in the top-most layers are likely on the basis of control exper-
iments with variable scattering times. The Auger signal at 1068 eV
kinetic energy is less affected by compositional changes in the top-
most layers, thus the plateau region is more pronounced. Above
ꢀ623 K, the LEIS-derived change of the Pd:Ga ratio is less strong
than the related AES intensity drop, and changes almost linearly
up to 800 K. As the Ga–L3M45L45 Auger signal decreases much fas-
ter than the Ga-LEIS signal, we suspect a PdZn-analogous relative
stabilization of the topmost PdGa layer relative to subsurface Ga,
as described in [9,10,22]. According to this picture, at higher tem-
peratures rather subsurface Ga than ‘‘in-surface’’ Ga migrates into
deeper regions of the Pd bulk and the Pd1Ga1 NSIP should thus be-
come specifically diluted in the subsurface region. Accordingly, at
870 K, only the surface composition remains close to Pd2Ga, mean-
ing that the 870 K state is unfortunately not representative for a
Pd2Ga bulk alloy surface. In summary, the combined data show a
relatively stable 1:1 surface and near-surface composition at
ꢀ523 K.
collected with photon energies of 470 eV (Pd 3d5/2) and 120 eV
(Ga 3d, VB), respectively, to ensure almost identical kinetic ener-
gies and hence identical information depth of the electrons.
For the experiments shown in Fig. 3, 4 MLE Ga metal were
deposited and the Pd foil subsequently annealed to the indicated
temperatures. We note that a small O2 or H2O background pressure
cannot be avoided in the HZB/BESSY II setup; therefore, the depo-
sition was carried out in 10À6 mbar H2 background pressure to
avoid partial oxidation of Ga metal. The Pd 3d5/2 signal shifts to
lower binding energies as the temperature increases, that is, from
about 336.15 eV at 298 K to 335.25 eV at 873 K. In analogy to the
PdZn system, no distinct stability region is visible in XPS, and the
Pd–Ga coordination appears to change in a more or less gradual
manner [9]. The 503 K–annealing spectrum indicates the prepara-
tion and ‘‘initial catalyst’’ conditions for the subsequent catalytic
MSR and OSR experiments. The Pd 3d5/2 signal for this preparation
is typically found at ꢀ335.8 eV. The associated Ga 3d signal is
attenuated with increasing temperature due to progressive NSIP
formation with Pd (i.e., the coordination of Ga with Pd increases)
and shifts to lower binding energies. Between 298 and 448 K, some
remaining GaxOy is still visible as a broad background in the BE re-
gion ꢀ20.5 to ꢀ19.5 eV. This background vanishes above ꢀ500 K
due to hydrogen reduction of the oxide. For the catalytically rele-
vant Pd1Ga1 NSIP tempered at 503 K, the peak maxima of the dou-
blet are found at 18.65 eV and 19.1 eV. Regarding the valence band
spectra, we note that the NSIP prepared at 503 K exhibits a consid-
erably decreased ‘‘Cu-like’’ density of states (DOS) at the Fermi
edge, as compared to clean Pd. The valence band series therefore
reflects the expected trend of the DOS at the Fermi edge toward
clean Pd at high temperatures.
To connect to analogous experiments on PdZn, a standard prep-
aration routine for a Pd1Ga1 multilayer NSIP has been utilized. ꢀ1,
ꢀ2 and ꢀ4 MLE Ga metal were deposited onto the Pd foil and sub-
sequently annealed to 503 K. Ideally, these preparations represent
2, 4 and 8 perfect 1:1 PdGa layers of the Pd1Ga1 NSIP, respectively.
At this point, it should be noted that angle-resolved LEIS spectra
conducted on ꢀ4 MLE Ga on Pd(111) tempered at 503–523 K did
not show the analogous Ga-outward corrugation effects as
observed for the multi-layered Pd1Zn1 NSIP [9,10,22]. Potential cat-
alytic implications of these structural differences are discussed in
Section 3.4.
3.3. Ga2O3–PdGa NSIP transition by hydrogen reduction
The above-discussed measurements were in turn comple-
mented by additional in situ XPS experiments at HZB/BESSYII to
gain information about the composition of the NSIP vertical to
the surface. The experimental conditions were chosen as closely
as possible to the ones of the Innsbruck experimental setup (com-
pare Section 2.1). Multilayers of Ga metal (and also of Ga2O3, to
study the reducibility of the oxide in contact to Pd in hydrogen)
were deposited onto the Pd foil and stepwise annealed with in-par-
allel detection by in situ XPS. Fig. 3 shows the results of these
measurements in a three-panel overview, highlighting the Pd
3d5/2, Ga 3d and valence band (VB) regions. The signals were
To simulate the reductive activation of Pd/Ga2O3 supported cat-
alysts also on the corresponding inverse model catalysts, experi-
ments were also conducted with Ga2O3 as starting material. To
ensure reproducible preparation of the Ga2O3 overlayer and to
avoid the deposition of Ga metal or Ga suboxide species, a back-
ground pressure of 10À3 mbar O2 was chosen during the deposition
of 4 MLE Ga to produce Ga2O3. This preparation leads to fully oxi-
dized Ga2O3 layers, as deduced from spectral comparison with a
stoichiometric Ga2O3 thin film reference sample, and provides a re-
dox-defined starting point for the reduction experiments in
0.1 mbar hydrogen described in the following.

C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
131
As can be seen in Fig. 4, the Pd 3d5/2 signal is initially completely
screened by the multilayers of Ga2O3. Only above 503 K, a weak Pd
signal arises at ꢀ335.7 eV. By increasing the temperature up to
ꢀ523 K, the Pd signal gains some intensity, which is accompanied
by a slight shift to higher BE’s, reflecting the increased formation of
a Ga-enriched NSIP surface. Above ꢀ523 K, despite the strong
increase in Pd 3d intensity, this BE trend is reversed due to the
increasingly accelerated thermal dissolution of Ga into deeper Pd
bulk layers. Above 523 K, the formation of a PdxGay NSIP with a
peak maximum at ꢀ18.25 eV is evident from the Ga 3d spectral
region. This component increases up to 573 K and the oxide com-
ponent at ꢀ20.9 eV almost vanishes. The valence band spectra also
show the decrease in the Ga-oxide component at ꢀ5.5 eV starting
at 523 K, along with the increase in the (bi-)metallic density of
states at the Fermi edge. We emphasize, that the reduction/bime-
tallic formation temperature of ꢀ573 K is in very good agreement
with previous experiments on the reductive formation of sup-
ported Pd2Ga bimetallic particles [11,12].
3.4. Methanol steam reforming (MSR) on Pd1Ga1 NSIP
To connect to the MSR measurements on the multi-layered
Pd1Zn1 NSIP [9,10], catalytic experiments in MSR have also been
conducted on multi-layered Pd1Ga1 NSIPs. To ensure a direct com-
parison, also otherwise identical experimental parameters were
used. The reactor temperature was linearly ramped from 298 to
623 K (ꢀ9 K/min), and subsequently, the temperature was held
isothermally at 623 K. Compositional and electronic characteriza-
tion was carried out ex situ before and after the reaction by AES,
LEIS and XPS, and by in situ XPS at HZB/BESSY II, respectively.
The catalytic measurements were performed on model surfaces
with variable Ga content. The initial Ga depositions of 1, 2 and 4
MLE ideally correspond to ꢀ2, ꢀ4 and ꢀ8 layers of Pd1Ga1 NSIP.
Fig. 5 shows the MSR reaction rates (given in mbar/min) on the
Pd1Ga1 NSIP prepared with 1, 2 and 4 MLE Ga. CO formation starts
for all samples at roughly the same temperature (ꢀ520 K), the CO2
formation increases at somewhat higher temperatures but remains
at a very small rate in all cases (note the different scales used for
the CO- and CO2-rates in Fig. 5). An initial increase in the CO2 for-
mation rate at ꢀ400 K is observed for both the 1 and the 2 MLE Ga-
doped Pd1Ga1 NSIP, but is missing for the 4 MLE Ga-doped Pd1Ga1
NSIP. The reason for this feature was a minor leakage of O2 (air)
into the reactor leading to an undesired OSR low-temperature rate
contribution (compare also the ‘‘controlled’’ OSR rates of Fig. 8),
which could be finally suppressed experimentally in case of the 4
MLE experiment (lowest panel of Fig. 5). Considerable differences
are notable for the formaldehyde formation. For the model cata-
lysts with higher Ga loading, the formaldehyde increases parallel
with the CO formation and the formaldehyde formation rate is
higher than that of CO2. For 2 and 4 MLE Ga, the maximum forma-
tion rates of formaldehyde are 14 and 6 Â 10À3 mbar/min, also the
(much higher) maximum rates of CO formation are comparable for
2 and 4 MLE Ga at ꢀ0.20–0.25 mbar/min. In contrast, the formalde-
hyde formation is almost negligible for the 1 MLE Ga-prepared cat-
alyst, but the CO formation rate is again comparable to the others
at around 0.15 mbar/min. In the isothermal region all rates gener-
ally decrease due to progressive C deposition on the catalyst sur-
face and consummation of the reactants.
Fig. 5. Temperature-programmed MSR reaction on three different Pd1Ga1 NSIP’s.
The diagram shows, starting from the top, data of model catalysts resulting from 1,
2 and 4 MLE Ga deposition, each annealed at 503 K. Reaction conditions: 12 mbar
methanol + 24 mbar water, 1000 mbar He. Linear ramp (9 K/min) up to 623 K,
subsequent isothermal reaction at 623 K for 25 min (note the different scales used
for the CO- and CO2-rates). An initial increase in the CO2 formation rate at ꢀ400 K is
observed for both the 1 and the 2 MLE Ga-doped Pd1Ga1 NSIP, but is missing for the
4 MLE Ga-doped Pd1Ga1 NSIP. The reason for this feature was a minor leakage of O2
(air) into the reactor leading to the undesired OSR low-temperature rate contribu-
tion, which could be finally suppressed experimentally in case of the 4 MLE
experiment (lowest panel). The respective TOF and product/reactant partial
pressure data to Fig. 5 are shown in the Supplementary material, Figs. S2–S4.
8x10-3
465K
623K
0.10
0.08
0.06
0.04
0.02
linear Ramp
isothermal Reaction
653K annealed
6
4
2
0
0.00
0.25
523K annealed
Formaldehyde
8x10-3
CO
0.20
0.15
0.10
0.05
0.00
CO2
6
4
2
0
0
10
20
30
40
Reaction Time [min]
Fig. 6. Temperature-programmed MSR reaction on two differently tempered PdGa
NSIP states. The diagram shows PdGa intermetallic compounds prepared with 4
MLE Ga and subsequent tempering at 653 K (upper panel) and with 4 MLE Ga and
tempering at 523 K (lower panel). Reaction conditions: 12 mbar metha-
nol + 24 mbar water, 1000 mbar He. Linear ramp (9 K/min) up to 623 K, subsequent
isothermal reaction at 623 K for 25 min. The respective TOF and product/reactant
partial pressure data to Fig. 6 are shown in the Supplementary material, Figs. S5–S7.
The Pd1Ga1 NSIP with the lowest Ga loading therefore catalyti-
cally resembles a slightly modified Pd surface, since on clean Pd
also no intermediate formaldehyde is detectable due to the fast
dehydrogenation of HCHO to CO. Since CO, CO2 and formaldehyde
are formed in the same temperature region, but the CO2 formation
rate stays very low, all three Pd1Ga1 NSIP’s must be considered not
CO2 selective at all and almost not capable of water activation.
Depending on the Ga loading, the main benefit of the sufficiently
doped NSIP states therefore is a gradual promotion of more selec-
tive dehydrogenation of methanol to formaldehyde, but – despite
the desired ‘‘Cu-like’’ electronic valence band structure (Fig. 3, right
panel) – nevertheless mostly CO is formed. Furthermore, an
outward corrugation of the dopant, which was shown to be related

132
C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
Ga3d
VB
Pd3d5/2
572K
555K
530K
492K
572K
555K
530K
492K
466K
568K
542K
518K
466K
443K
423K
480K
455K
443K
423K
431K
423K
337
336 335
334
22
20
18
8
4
0
Binding Energy [eV]
Fig. 7. Pd 3d5/2 (left), Ga 3d (middle) and valence band (right) spectra, monitored during MSR reaction starting from the 503 K-annealed 4 MLE Pd1Ga1 NSIP initial state.
Photon energies as in Fig. 4. The spectroscopic assignment of the superimposed gas-phase valence band features is provided in the Supplementary material (Fig. S11).
0.30
373K
491K
623K
isothermal Reaction
linear Ramp
selectivity around 550 K. Thus, CO formation is suppressed relative
to CO2 both in comparison with the pure b-Ga2O3 support and the
isolated Pd1Ga1 NSIP, which hardly exhibits any CO2-activity, but a
lot of CO formation in the same temperature range. This strongly
points to metal-support interaction playing an important role in
low-temperature formaldehyde conversion, and a bi-functional
synergism of both the intermetallic and the (interface to the) oxide
on the supported catalyst system is quite likely.
12x10-3
0.25
0.20
0.15
0.10
0.05
0.00
MSR
OSR
8
4
0
1.2
Formaldehyde
0.5
0.4
0.3
0.2
0.1
0.0
CO
1.0
0.8
0.6
0.4
0.2
0.0
CO2
In this context, a quantitative comparison of the rates of CO2-
vs. CO formation on our multi-layered Pd1Ga1 NSIP’s (Fig. 5) with
literature TOF data (in the following consistently given in the
dimension of a reaction frequency sÀ1 per surface site) on model-
PdZn and oxide-supported PdZn and PdxGay catalysts is useful.
From the Supplementary material, the respective TOF values
(Fig. S2) as well as the reactant and product partial pressures
(Figs. S3 and S4) corresponding to the rate data of Fig. 5 can be read
out for a given reaction time, reaction temperature and reaction
conversion. Necessarily, TOF’s are based on the idealized picture
of a stable, static bimetallic surface exhibiting uniform Pd1Ga1 ac-
tive surface sites throughout the entire reaction run, which is an
artificial assumption for certain, surface-destabilizing reaction
conditions, as well as for a ‘‘structure-sensitive’’ reaction proceed-
ing preferentially at the bimetal-oxide phase boundary. In princi-
ple, the time- and temperature-resolved gas-phase partial
pressure data can be used to recalculate ‘‘true’’ chemical reaction
rates, provided that the respective individual reaction orders with
respect to products and reactants were known from a fully param-
eterized kinetic study.
At first, lacking water activation on the bimetallic surface itself
sets the Pd1Ga1 NSIP apart from the related multi-layered Pd1Zn1
NSIP, for which a CO2-TOF of 0.04 sÀ1 (without simultaneous CO
formation) was reported at ꢀ540 K (at maximum 0.144 sÀ1 at
570 K) on the (experimentally proven) basis that a known number
of surface PdZn = 1:1 ensemble sites is available, active and selec-
tive for the full MSR process [9,10]. Under otherwise identical con-
ditions (same reaction cell and operation mode, gas-phase
composition and temperature), the CO2-TOF is below 0.0013 sÀ1
at 540 K in the present PdGa case (4MLE Ga sample). Because the
‘‘uniform’’ bimetallic 1:1 PdGa surface sites are hardly MSR/CO2-
active, likely only some special active centers (of unknown number
and nature) contribute to the maximum CO2 formation rate of
0
10
20
30
40
50
Reaction Time [min]
Fig. 8. Comparison of temperature-programmed MSR and OSR reaction starting
from the 503 K-annealed 4 MLE Pd1Ga1 NSIP initial state. Reaction conditions: MSR:
12 mbar methanol + 24 mbar water, 1000 mbar He, OSR: 16 mbar metha-
nol + 24 mbar water + 11 mbar O2, 1000 mbar He. Linear ramp (9 K/min) up to
623 K, subsequent isothermal reaction at 623 K for 25 min. The respective TOF and
product/reactant partial pressure data to Fig. 8 are shown in the Supplementary
material, Figs. S8–S10.
to water activation and CO2-selective total oxidation of intermedi-
ate formaldehyde in the PdZn case [9,10], was not observed for
PdGa [22]. Assuming that only by a combination of appropriate
electronic and geometric surface structure of the bimetallic the fol-
lowing three conditions can be met, namely
– selective dehydrogenation of methanol to formaldehyde with-
out further reaction to CO,
– efficient water activation,
– fast and selective total oxidation of formaldehyde to CO2,
it is obvious that at least the second condition is hardly
matched by any tested Pd1Ga1 NSIP model system.
At this point we note that Ga2O3-supported bimetallic Pd1Ga1
particles are indeed rather CO2-selective [11] (though not as
active/selective as Pd2Ga/b-Ga2O3 [11,25]), showing
a
ꢀ75%

C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
133
0.012 sÀ1 measured at 623 K in parallel with strong CO formation.
3.5. In situ XPS measurements under MSR reaction conditions
Since CO, which has been recently shown to destabilize the inter-
metallic surface of Pd2Ga particles [25], is already present in the
gas phase in substantial amounts (ꢀ0.8 mbar), and also the ther-
mal decay of the NSIP is proceeding (see Fig. 3), it is even more
doubtful to assign this (rather small) CO2 activity to the total num-
ber of initially present PdGa surface ensembles.
To gain full insight into the redox changes on the Pd1Ga1 NSIP
under MSR conditions, the associated in situ XPS experiments have
been also conducted at HZB/BESSY II under comparable conditions
on the 4 MLE Ga-doped and 523 K-annealed Pd1Ga1 NSIP (cf. 523
K – spectra of Fig. 3 for the pre-reaction state).
Likely because of facilitated water activation both the sup-
ported PdZn and Pd2Ga catalysts exhibit a much broader tempera-
ture range with high CO2 selectivity and activity, due to improved
conversion of the intermediate formaldehyde toward CO2, and gen-
erally become active for methanol conversion at considerably low-
er temperatures (below 490 K) than the ‘‘isolated’’ bimetallics
(ꢀ520 K). CO2-TOF’s of 0.17 sÀ1 on a supported PdZn/ZnO and of
0.06 sÀ1 on a PdxGay/Ga2O3 catalyst were reported at 493 K reac-
tion temperature by Takezawa and Iwasa [1]. Ranganathan et al.
reported 0.8 sÀ1 at 503 K [26] and Conant et al. 0.39 sÀ1 at 523 K
[27], both for PdZn/ZnO.
Fig. 7 shows the related in situ XPS data in a reaction mixture of
0.12 mbar methanol + 0.24 mbar water. Hardly any spectral
changes can be seen in the observed temperature range ꢀ423–
570 K in all presented spectral regions. This especially applies to
the temperature-invariant oxide component at ꢀ20.7 eV, which
almost remains constant in intensity, showing that initially occur-
ring partial surface segregation of oxidized Ga species is basically
finished already at around 423 K, and no additional reaction-
caused oxide segregation is thereafter observed. The Pd 3d5/2 signal
even shows a small shift to higher BE’s between 431 and 518 K,
which again confirms the high thermochemical stability of the
Pd1Ga1 NSIP under MSR conditions. As a consequence, the Pd1Ga1
NSIP on the one hand represents a highly stable model system,
but on the other hand hardly is appropriate to model the MSR cat-
alytic properties of the oxide-supported nano-particulate bulk
intermetallic compound phases, at least in its purely bimetallic
state.
The above-mentioned study by Haghofer et al. [25] moreover
allows to compare our CO-TOF’s with those deduced for supported
Pd2Ga particles. These are reportedly affected by partial (surface-
near) Pd metal segregation, induced by the MSR reaction environ-
ment. A methanol conversion TOF of 0.08 sÀ1 is reported at 523 K,
corresponding to a CO-TOF of 0.008 sÀ1 (ꢀ10% CO selectivity). By
fitting our CO-TOF data (Fig. S2) with a simple Arrhenius rate func-
tion in the temperature region 480 K-580 K, the CO-TOF’s at 523 K
on the 1,2 and 4 MLE Pd1Ga1 NSIP’s can be recalculated also for the
3.6. Oxidative methanol steam reforming on Pd1Ga1 NSIP
low rate values at 523 K, yielding CO-TOF values around 0.015 sÀ1
,
respectively. This TOF comparison is somewhat more straightfor-
ward because the unwanted ‘‘full’’ methanol dehydrogenation to-
ward CO is likely ‘‘structure insensitive’’, that is, largely takes
place at the bimetallic surface itself, which may be comparably
‘‘gas-phase-disturbed’’ also under our conditions. As the CO-TOF’s
are similar, we may derive additional support for the blocking of
the low-temperature CO2-forming reaction channel on the ‘‘iso-
lated’’ intermetallic surface.
3.6.1. Catalytic experiments
Analogous experiments in oxidative steam reforming (OSR)
have been additionally performed. The reaction conditions are sim-
ilar to those of the MSR reaction, with the one exception of O2
being admitted before the reaction (ratio methanol/water/oxygen
ꢀ1:2:0.7). Adding a certain partial pressure of oxygen to a metha-
nol–water feed is common to match two requirements: firstly, to
compensate for the endothermicity of the ‘‘pure’’ MSR reaction
(‘‘autothermal reforming’’) and secondly, to additionally suppress
the CO concentration in the product stream via enhanced CO oxi-
dation. Fig. 8 (lower panel) highlights the OSR reaction rates (given
in mbar/min) measured on a 4 MLE Ga, 523 K-annealed Pd1Ga1
NSIP. For a better comparison, the upper panel shows the analo-
gous MSR reaction run in the absence of O2. From the Supplemen-
tary material (Figs. S8–S10), the respective reactant and product
partial pressures as well as the respective TOF values can be
deduced for a given reaction time, reaction temperature and reac-
tion conversion (the TOF’s are based on the idealized picture of a
stable bimetallic surface exhibiting uniform Pd1Ga1 active surface
sites throughout the entire reaction run).
The most notable difference to the MSR reaction without added
O2 is the very high CO2 formation rate setting in already at low
temperatures (ꢀ423 K). The maximum rate of CO2 formation is
achieved at around 491 K with 1.2 mbar/min, which is almost
three orders of magnitude higher than under MSR conditions
(4 Â 10À3 mbar/min), and even higher than the maximum CO for-
mation rate (ꢀ0.48 mbar/min) observed in the same experiment
at around 623 K. In parallel with CO2 formation, formaldehyde for-
mation sets in, but at a much smaller rate (ꢀ0.05 mbar/min at
maximum). Interestingly, a period of deactivation at ꢀ550 K is
evident after about 16 min of reaction, which is mainly due to
quantitative oxygen consumption from the reaction mixture. CO
formation starts at ꢀ530 K, and the maximum rate is reached at
623 K (ꢀ0.48 mbar/min). In the isothermal region, the CO forma-
tion decreases to less than 0.05 mbar/min due to almost quantita-
tive consumption of the initial methanol amount in the batch
reactor.
To corroborate the formaldehyde-stabilizing role of the multi-
layered Pd1Ga1 NSIP, Fig. 6 shows similar MSR experiments for a
4 MLE Ga-doped Pd1Ga1 NSIP tempered to 523 K and a 4 MLE
Pd1Ga1 NSIP sample tempered to much higher temperature, that
is, 653 K. Again, the TOF values corresponding to Fig. 6, as well as
the respective reactant- and product partial pressures can be read
out in the Supplementary material (Figs. S5–S7) for a given reac-
tion time, reaction temperature and reaction conversion. The
methanol conversion starts at ꢀ520 K and shows similar maxi-
mum rates for product formation on both model systems. The
dominant CO formation starts in the same temperature region.
However, the maximum rate of CO formation is almost doubled
on the 523 K-tempered sample, compared to the 653 K-annealed
NSIP. The main difference is noted in the formaldehyde formation
rates. On the multilayer Pd1Ga1 NSIP with a lower tempering tem-
perature, and therefore, a higher Ga content in the top-most layers,
the formation of formaldehyde starts in parallel with CO formation
and the maximum rate is higher than that of CO2, whereas formal-
dehyde is almost completely suppressed on the 653 K-tempered
sample. Again, the maximum rates of CO and CO2 formation are
found at around 623 K. In the isothermal region, once again
deactivation processes and reactant consumption attenuate the
reaction rates. In summary, in no case a PdZn-analogous CO2-selec-
tive low temperature region has been observed under MSR condi-
tions. We note that the measurements on the 653 K-tempered
sample and the PdGa sample with 1 MLE Ga (cf. Fig. 5) very much
resemble each other, pointing to the presence of a rather thin, and
thus more strongly Pd-coordinated and thus catalytically more
‘‘Pd-like’’, PdGa NSIP state in both cases.

134
C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
The time-resolved H2-mass balance of the OSR reaction run of
reaction. These experiments allowed a direct evaluation of the oxi-
dation potential of the gas phase on the dynamical redox state of
the catalyst surface.
Fig. 8 has been precisely determined by continuous measurement
of the H2-amount as a function of reaction time and –temperature
and is also represented graphically in the Supplementary material
(Fig. S10). During the oxygen-consuming reaction, which extends
up to a temperature of ꢀ550 K, about 87% of the converted meth-
anol is totally oxidized to CO2 and water, and only a minor 13%
contribution of partial oxidation toward CO2 and H2 is observed.
In conclusion, only this minor fraction of the methanol becomes
partially oxidized, according to the reaction stiochiometry CH3
OH + 1/2 O2 ? CO2 + 2H2. Nevertheless, this proves that a reaction
pathway for a hydrogen-producing reaction even in the simulta-
neous presence of O2 is possible and that total oxidation according
to the stoichiometry CH3OH + 3/2 O2 ? CO2 + 2H2O does not take
place exclusively. Moreover, the very high CO2-selectivity in the
temperature range around 500 K indicates that O2 addition could
indeed be helpful to optimize CO2 selectivity and to efficiently sup-
press the CO content of the reformate gas also under continuous
flow reaction conditions.
Fig. 9 depicts the spectroscopic results (Pd 3d5/2, Ga 3d and va-
lence band regions) of the temperature-programmed OSR reaction
on the 4 MLE Ga 503 K-annealed Pd1Ga1 NSIP. After tempering, but
before start of the OSR reaction, the Pd 3d5/2 peak maximum is
found at ꢀ335.9 eV and the Ga 3d signal only shows the Pd1Ga1
NSIP contribution, but no oxidic contribution. Also the valence
band shows only fingerprints of the multilayer Pd1Ga1 NSIP. After
admission of the O2-containing reaction mixture and slight in-
crease in the reaction temperature (ꢀ365 K), the peak maximum
of the Pd peak shifts to slightly lower BE’s, but still stays in the
range of a typical Pd1Ga1 NSIP (ꢀ335.8 eV; to be compared to
336.0 eV for bulk Pd1Ga1 [23] and to 335.6 eV for bulk Pd2Ga
[24]). The valence band region now shows also the gas-phase
signals of the reactant mixture (the spectroscopic assignment of
the individual gas-phase contributions is provided in the Supple-
mentary material (Fig. S11)). The Ga 3d signal shows the develop-
ment of an oxidized Ga component at ꢀ20.4 eV, which reaches its
maximum intensity at 464 K, that is, slightly below the tempera-
ture where the maximum of the CO2 formation rate is observed
(491 K, cf. Fig. 8).
By correlation of the relative intensity trend of the Ga(ox) com-
ponent with the catalytic selectivity pattern, this component is
apparently linked to highly selective CO2 formation. The nature
and potential catalytic function of the related oxidized Ga species
(active vs. spectator, etc.) is nevertheless unclear at present. Any-
thing in between Ga–O(ads) on intact Pd1Ga1 sites, resulting from
‘‘simple’’ dissociative O2 adsorption on the persisting intermetallic
surface, and local/partial oxidative segregation giving rise to more-
or less-extended two-dimensional Pd-ensembles and nearby Ga
(hydr)oxide islands is conceivable. According to the depth profiling
information of Fig. 10, the oxidized Ga species detected at ꢀ500 K is
rather ‘‘flat’’ (estimated thickness ꢀ0.5 nm, based on the relative
intensity change of bimetallic vs. oxidized Ga observed between
120 eV and 650 eV photon energy) and situated in close vicinity of
a still stable surrounding intermetallic environment (compare Pd
3d5/2 BE maximum at 335.8 eV at 460–500 K). Thus, extended oxi-
dative decomposition of the model surface into three-dimensional
In summary, a highly CO2 selective, combined total/partial oxi-
dation reaction was obtained under OSR conditions in the temper-
ature region ꢀ420–520 K, in strong contrast to MSR.
In conclusion, the Pd1Ga1 NSIP is capable of efficiently activat-
ing oxygen at low temperatures, while being inactive for CO forma-
tion at the same time. Methanol is converted to CO2 mainly via
total oxidation of intermediary formed C1-oxygenates at the
surface with some simultaneous hydrogen production. Since we
are not aware of methanol total or partial oxidation TOF’s mea-
sured on any of the literature-reported PdGa or PdZn systems, a
comparison of the OSR rates between model and real systems is
currently not possible.
3.6.2. In situ XPS experiments under OSR conditions and the influence
of O2 on the dynamical redox state of the catalyst
The in situ XPS experiments were performed in close experi-
mental analogy to the OSR batch reactor measurements of Section
3.6.1 to ensure a reliable correlation of the results. As the measure-
ments at HZB/BESSY II were performed under flowing conditions,
‘‘switching’’ experiments with defined admission and removal of
oxygen were additionally feasible without interrupting the
VB
Pd3d5/2
Ga3d
OSR
623K
612K
623K
623K
600K
557K
529K
499K
568K
539K
612K
568K
539K
511K
511K
464K
424K
384K
363K
464K
424K
459K
419K
379K
384K
363K
369K
UHV 298K
UHV 298K
UHV 298K
4
337 336
335 334
22
20
18
8
0
Binding Energy [eV]
Fig. 9. Pd 3d5/2 (left), Ga 3d (middle) and valence band (right) spectra, taken during OSR reaction (ratio methanol/water/oxygen = 1:2:0.5, total pressure 0.25 mbar), starting
from the 503 K-annealed 4 MLE Pd1Ga1 NSIP initial state. Photon energies were the same as in Fig. 4. The ‘‘initial state’’ spectra at 298 K were collected immediately before
admission of the gas mixture. The spectroscopic assignment of the superimposed gas-phase valence band features is provided in the Supplementary material (Fig. S11).

C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
135
650 eV Photon Energy
120 eV Photon Energy
Ga3d
464K
623K
424K
384K
363K
612K
568K
x2
539K
511K
UHV 298K
23
22
21
20
19
18
23
22
21
20
19
18
17
Binding Energy [eV]
Fig. 10. Direct comparison of the Ga 3d signals taken during OSR reaction (reaction conditions as in Fig. 10) and measured with 120 eV and 650 eV photon energies,
respectively. To visualize the rather small Ga2O3 intensity of the 298 K spectra, both were increased by a factor of 2.
Pd metal and Ga2O3 domains can be excluded at least at around
500 K.
in contrast to the clean MSR reaction, where a higher stability limit
has been verified. In the batch reactor OSR experiment of Fig. 8,
quantitative O2 consumption and degradation of the initial NSIP
state coincide at this limit, thus the pronounced transition from
CO2- to CO formation is observed.
The shift of the Pd 3d5/2 signal remains constant up to ꢀ529 K at
the value typical for the Pd1Ga1 NSIP. Only at higher temperatures
(557–623 K) a pronounced shift to 335.25 eV due to extended oxi-
dative decomposition of the NSIP toward three-dimensional Pd
metal is observed, which coincides with increasing CO formation
in Fig. 8 (lower panel). This trend is also reflected in the Ga 3d sig-
nal. From 539 to 612 K, the Ga bimetallic component at ꢀ18.4 eV
gains intensity relative to the oxidic component at ꢀ20.4 eV. This
could be explained by the rearrangement of the two-dimensional,
partially oxidized Ga layer to three-dimensional Ga-oxide islands,
accompanied by extended formation of areas of clean Pd, which
in turn progressively catalyze formation of CO. From 612 to
623 K the oxide component again gains very much intensity, which
can be explained by extra Ga2O3 segregation from deeper Ga lay-
ers, corresponding to more or less complete thermal/oxidative
destruction of the NSIP. This interpretation is also consistent with
the strong shift of the Pd 3d signal toward the Pd metal typical va-
lue of 335.25 eV. The related valence band spectra show increased
formation of oxidic components at 4–5 eV at 623 K and an increase
in the DOS at the Fermi edge typical for metallic Pd.
During the OSR reaction, we monitored not only the surface-
sensitive Ga 3d Signal at a photon energy of 120 eV, but also a
corresponding signal at 650 eV photon energy in order to gain
information about the vertical distribution of the different Ga spe-
cies. Fig. 10 shows that the CO2-active oxidized Ga species, which
has been formed during the reaction at <540 K, is found mainly
as a ‘‘wetting’’ species (average thickness ꢀ0.5 nm, see above) in
the topmost layer of the model catalyst. For all temperatures below
540 K, the change of the photon energy from 120 to 650 eV thus
results in a strong relative decrease in the oxide component com-
pared to the bimetallic component, for example, at 464 K from 3:2
to 1:3. In contrast, the oxide species segregated at temperatures
above 540 K appears to grow in thickness, as inferred from a less
depth-dependent ratio of the components at different photon ener-
gies (e.g., at 623 K 2:3 and 1:2 at 120 and 650 eV). This strongly
supports our interpretation of increased three-dimensional segre-
gation of unreactive gallium oxide on an already Pd-like metal
surface above 540 K.
In order to study the dynamic response of the catalyst surface to
the change of the gas-phase oxidation potential, we additionally
performed experiments with deliberate on/off switching of oxygen,
that is, between MSR and OSR conditions, again starting from the
503 K-annealed 4 MLE Pd1Ga1 NSIP. Fig. 11 shows these compara-
tive experiments at three selected reaction temperatures. The Pd
3d5/2 signal is found at ꢀ335.7 eV at 463 K (‘‘CO2-active’’ state in
OSR, compare Fig. 8). The change between MSR and OSR conditions
obviously has no measurable effects on the BE of Pd at around
463 K (NSIP largely stable). The Ga 3d signal, however, shows a
reversible decrease/increase in the oxidized Ga component
(ꢀ20.2 eV) upon switching between OSR and MSR. This means that
the ‘‘wetting’’ Ga(ox) species exhibits a fast ‘‘kinetic’’ response to
the gas phase and thus exhibits pronounced methanol-induced
reducibility in the absence of O2 (in parallel, the valence band spec-
tra exhibit a change in the gas-phase signals (e.g., at ꢀ8 eV) due to
the altered composition and ionization probability of the gas
phase, see Supplementary material Fig. S11). When the reaction
temperature is increased to 523 K under OSR conditions (close to
the ‘‘inactive state’’ in Fig. 8), the Pd 3d5/2 signal already exhibits
a shift to a Pd-like BE (ꢀ335.35 eV). In accordance, the DOS at
the Fermi edge is increased. Switching between MSR and OSR again
causes no significant changes in the Pd signal. Partially reversible
alterations in the Ga 3d signal (decrease in the oxidized Ga
component when switching off O2) are still observed. If the reac-
tion temperature is then raised under MSR conditions to 573 K
(‘‘CO-active’’ state), the oxidic component in the Ga 3d signal van-
ishes completely due to the increased reductive potential of the gas
phase. We note that complete reduction toward the intermetallic
state at this temperature is necessarily connected with fast Ga loss
into deeper bulk layers occurring in parallel with reduction. The Pd
3d5/2 signal again remains constant at 335.35 eV. Only upon
switching to OSR conditions, the Pd signal shifts to even lower
BE’s (ꢀ335.2 eV) due to additional Ga-depletion of Pd by oxidative
Ga segregation combined with simultaneous Ga bulk diffusion.
Again, the oxidic component in the Ga 3d signal appears in the
presence of O2. Taken together, this indicates almost complete
Summarizing, the results indicate a thermochemical stability
limit of the Pd1Ga1 NSIP at around 540 K under OSR conditions,

136
C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
Pd3d5/2
Ga3d
VB
573K
523K
OSR 573K
MSR 573K
573K
573K
MSR 523K
OSR 523K
523K
MSR 463K
OSR 463K
463K
463K
337 336 335 334
22
20
18
12
8
4
0
Binding Energy [eV]
Fig. 11. Pd 3d5/2 (left), Ga 3d (middle) and valence band (right) spectra, taken at 463 K, 523 K and 573 K during switching from MSR (ratio methanol/water 1:2, total pressure
0.26 mbar) to OSR conditions (ratio methanol/water/oxygen 1:2:0.5, total pressure 0.31 mbar). Initial catalyst state: 503 K-annealed 4 MLE Pd1Ga1 NSIP. Photon energies as in
Fig. 3. The spectroscopic assignment of the superimposed gas-phase valence band features is provided in the Supplementary material (Fig. S11).
top-most surface layers. The oxide component of the Ga 3d signal
is attenuated in deeper layers (ratio of oxidized Ga to bimetallic Ga
Ga3d
C1s
3:7 vs. 1:4 at 120 eV vs. 650 eV photon energy). At the highest
temperatures (573 K), the decomposition of the multilayer Pd1Ga1
NSIP to three-dimensional oxide segregation is increasingly evi-
dent (ratio of oxidized to bimetallic Ga 3:7 vs. 3:7 at 120 vs.
650 eV photon energy). The in-parallel recorded C1s signal shows
that at 463 K and 523 K reaction temperature almost the same
amount of carbon is present on the surface under both MSR and
OSR conditions. At 573 K a strong decrease in the C 1s signal indi-
cates reactive removal of carbon by O2 in course of full NSIP
decomposition toward the Pd-like surface.
OSR 573K
MSR 573K
MSR/OSR 573K
MSR/OSR 523K
MSR 523K
OSR 523K
MSR 463K
4. Conclusions
We have shown that in close analogy to the Pd1Zn1 NSIP system,
preparation by deposition of several MLE Ga and thermal annealing
at ꢀ523 K leads to analogous multi-layered Pd1Ga1 NSIP states.
In contrast to MSR measurements on supported Pd1Ga1/b-
Ga2O3, Pd2Ga/b-Ga2O3, and on the related Pd1Zn1 NSIP, which all
showed increased CO2 selectivity at T 6 550 K, the multi-layered
Pd1Ga1 NSIP is a poor, unselective catalyst in MSR, as evidenced
by simultaneous, unselective production of mainly CO, little CO2
and some HCHO in parallel in the same temperature region. Never-
theless, it is highly CO2 selective and active in the presence of O2.
The most likely explanation is that water activation on the NSIP
is not working, but O2 activation works well, so in MSR, the re-
quired ‘‘source of oxygen’’ for total oxidation of C1 intermediates
is missing but becomes available under OSR conditions. The H2-
mass balance of the OSR around 500 K, moreover, implies that once
a ‘‘source of oxygen’’ is available, also some (partial) reforming
ability of the intermetallic surface toward CO2 and H2 is estab-
lished already at low temperatures. For the (relative to the Pd1Ga1
NSIP) considerably more CO2-active supported Pd1Ga1 system [11]
we thus rather suspect a water-splitting role of the bimetal-oxide
phase boundary and/or its oxidic catchment area than an intrinsic
ability of the intermetallic surface itself to perform full reforming
including water splitting. This distinguishes the 1:1 PdGa NSIP also
from the 1:1 PdZn NSIP, for which a bi-functional, water-activating
role of ‘‘isolated’’ bimetallic surface ensembles was verified in Refs.
[9,10].
OSR 463K
MSR/OSR 463K
21
20
19
18
288
286
284
282
Binding Energy [eV]
Fig. 12. Direct comparison of the Ga 3d signals taken during OSR and MSR reaction
at three different reaction temperatures (463 K, 523 K and 573 K) with variable
photon energy at 120 eV (solid line) and 650 eV (dotted line). The right panel shows
the C 1s spectra taken at 410 eV photon energy (dotted – OSR, solid – MSR). Initial
catalyst state: 503 K-annealed 4 MLE Pd1Ga1 NSIP.
oxidative decomposition of the multilayer Pd1Ga1 NSIP at ꢀ573 K.
The DOS at the Fermi level is also strongly altered and now exhibits
the shape of clean Pd.
A reversible dynamic response is therefore proven for all three
reaction temperatures, with the strongest response at 573 K. At
this temperature, Fig. 4 already indicated the reductive formation
of an intermetallic surface layer from pre-formed stoichiometric
Ga2O3. In presence of a variable oxygen pressure this process can
be driven almost fully reversible. However, the so-prepared PdxGay
NSIP surfaces strongly favor methanol dehydrogenation toward CO
due to their electronic similarity to clean Pd and the dominant
three-dimensional oxide segregation.
Fig. 12 finally shows that the changes observed during the
switching from MSR to OSR around 463 K mainly affect the

C. Rameshan et al. / Journal of Catalysis 290 (2012) 126–137
137
Obviously, for the supported PdGa system it is important to
consider the role of the support and the formation of suitable me-
tal-oxide interfacial sites capable of water activation. A bi-func-
tional synergism therefore rather applies to the bimetallic/oxide
interface than to a regular, ordered array of active sites on the
bimetallic surface itself. This is also corroborated by in situ XPS
measurements under MSR reaction conditions, which revealed that
the NSIP did neither structurally nor chemically change during the
MSR runs. Especially, no signs of water-induced surface oxidation
of the NSIP were observed, in contrast to the analogous Pd1Zn1
NSIP.
Society. Support for the measurements at HZB/BESSY II was
granted through EU Program RII3-CT-2004-506008, Proposal No.
2008_2_80336. The authors thank the BESSY staff for their support
of the in situ XPS measurements.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.03.009.
Only under in situ OSR conditions, ‘‘wetting’’ Ga(ox) species are
reversibly formed/segregated and this segregation process is obvi-
ously important for the combined total/partial oxidation of C1-oxy-
genate intermediates (HCHO, CO, etc.) to CO2. Mechanistically, it
remains unclear whether the formation of (even very small/thin)
Pd-ensembles favors full dehydrogenation of methanol toward
CO(ads), followed by oxidation of CO by O(ads) toward CO2, or
whether the total oxidation/CO2-formation propensity of the
Pd1Ga1 NSIP around 500 K represents an intrinsic property of the
intact Pd1Ga1 surface. If, for example, minor partial oxidative seg-
regation was limited to the top layer of the original NSIP, changes
to the mean Ga coordination of top-layer Pd atoms should be min-
or, too, which could somehow explain the rather stable 1:1 NSIP
binding energy position of the Pd3d signal around 500 K during
OSR. Partial, near-surface oxidative decomposition in the presence
of either CO or O2 already at low temperatures has been recently
shown in an in situ PM-IRAS and XPS study of ambient-pressure
gas-phase effects on Ga2O3-supported Pd2Ga particles [25]. Some
shortcomings of the NSIP approach have to be admitted: the 1:1
PdGa NSIP, although multi-layered, does not represent an unam-
biguous model for the surface of Pd1Ga1 nanoparticles or bulk com-
pounds, although the XPS-observed VB electronic structure after
ꢀ503 K annealing is representative for the expected ‘‘Cu-like’’ elec-
tronic situation favoring selective methanol dehydrogenation
toward HCHO. So far, no direct structural analogies between the
ꢀ4 and 8 layers thick model NSIP (Pd-like, substitutional fcc struc-
ture) and the related 1:1 PdGa bulk intermetallic compound (FeSi-
structure) could be verified.
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This work was financially supported by the Austrian Science
Fund through Grants P20892-N19 and SFB F4503-N16. Ch. Rame-
shan acknowledges a PhD scholarship granted by the Max-Planck