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C. Rameshan et al. / Journal of Catalysis 295 (2012) 186–194
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bimetallic bi-functional active sites for water activation and reac-
tion of methanol to CO2 [1,16]. Specifically for the ‘‘isolated’’ (i.e.,
unsupported) bimetallic Pd–Zn system, careful tuning of the inter-
metallic composition especially in surface-near regions turned out
to be a prerequisite for the formation of these bifunctional active
sites. In this respect, only a multilayer Pd–Zn surface alloy with a
Pd/Zn = 1:1 composition exhibited a ‘‘Zn-up/Pd-down’’ corrugation
affiliated with Pd1Zn1 surface entities being active for water split-
ting and exhibiting the formaldehyde-promoting ‘‘Cu-like’’ low-
ered density of states close to the Fermi edge [1,14]. The PdZn
results already suggest to extend these PdZn inverse model studies
to the corresponding intermetallic Pd–In inverse catalyst system,
with the objective to extract the role of the single purely interme-
tallic PdIn surface regarding its specific catalytic properties with-
out superimposed, potentially promoting metal-support interface
effects. The related, highly CO2-selective supported PdxIny/In2O3
catalyst [19] might indeed be strongly promoted by the ‘‘isolated’’
properties of the pure supporting oxide In2O3. Both supported cat-
alyst and pure oxide have already been shown in recent contribu-
tions by some of the present authors, focusing both on the Pd–
In2O3 interaction upon reduction of small In2O3-supported Pd par-
ticles in hydrogen [19] and the catalytic and reductive behavior of
pure In2O3 [20–22]. In short, pure In2O3 is very susceptible to lose
lattice oxygen upon annealing in hydrogen or CO [22] and is thus
prone to strong metal-support interaction effects [19]. Most
importantly, the reduced state of In2O3 is capable of activating
water, but almost completely inactive in the reaction of CO2 with
oxygen defects to CO [22]. Hence, it does not catalyze the inverse
water–gas shift reaction, which can spoil the CO2-selectivity in
methanol steam reforming. Moreover, pure In2O3 is, although
being not very active, a rather selective methanol steam reforming
catalyst with a CO2 selectivity >95% at ꢀ673 K reaction tempera-
ture [20]. This, however, sets In2O3 apart from ZnO [23,24] and
Ga2O3 [25], which are both water–gas shift active and thus
considerably less CO2-selective, especially at elevated reaction
temperatures.
Our primary aim therefore is to correlate the catalytic selectiv-
ity of In-metal and In2O3-modified Pd toward CO and CO2 with
in situ XPS and mass spectrometry under realistic MSR conditions.
These studies are a further step toward the thorough understand-
ing of the peculiar common catalytic properties of the pool of Pd-
based intermetallic phases featuring CO2-selective methanol steam
reforming. The present studies again reveal the universal validity
of the important concept of improved water activation by the dop-
ant. In combination with the previously assumed electronic struc-
ture explanation for suppression of total dehydrogenation of
methanol toward CO, and consequently enhanced formaldehyde
formation, via the Cu-like electronic structure of PdIn [7,13–18],
this provides a reliable basis for explanation of the pronounced
CO2-selectivity.
To correlate with the structure-insensitive total oxidation of
methanol with O2 toward CO2 and water at low temperatures on
the PdGa NSIP [2], 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 rang-
ing from partial methanol oxidation (CH3OH + 1/2 O2 ? CO2 + 2H2)
to total oxidation (CH3OH + 3/2 O2 ? CO2 + 2H2O). The interest in
comparing MSR and OSR is basically derived from the fact that
admission of a defined oxygen partial pressure to a methanol–
water mixture is common to additionally suppress CO formation
in the product stream by further CO oxidation and to compensate
for the endothermicity of the pure methanol steam reforming
reaction.
2. Experimental
2.1. Innsbruck experimental setup
The UHV system with attached all-glass high-pressure reaction
cell [26] 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
reaction products and even minor intermediates with high sensi-
tivity, either by discontinuous sample injection into the gas chro-
matography–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 cham-
ber 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 chamber is equipped with an XPS/Auger/ISS spec-
trometer (Thermo Electron Alpha 110) and a standard double
Mg/Al anode X-ray gun (XR 50, SPECS), an electron beam heater,
an ion sputter gun, and a mass spectrometer (Balzers).
For controlled In deposition, a home-built In evaporator was at-
tached, which consists of a small boron nitride crucible filled with
In metal (99.999%, Goodfellow) and heated by electron bombard-
ment. A water-cooled quartz-crystal microbalance monitored the
amount of deposited In.
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 catalytic mea-
surement (thermocouple mechanically contacted at the outside).
Accordingly, background (blind) activity of the reaction cell is rou-
tinely checked and no conversion was observed for all test reactions.
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 to 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 exter-
nally calibrated and corrected for fragmentation (i.e., CO and
CH2O fragments for methanol and CO fragment for CO2).
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 (surface carbon) were de-
tected by AES and XPS. Details of the preparation of the PdIn mul-
tilayer intermetallic phase will be given in Section 3.1. Methanol
and methanol/water mixtures were degassed by repeated freeze-
and-thaw cycles. All MSR reactions were conducted with metha-
nol/water mixtures of a 1:10 composition of the liquid phase. This
corresponds to a room temperature partial pressure ratio of meth-
anol/water = 1:2, as verified by mass spectrometry.