Steering the methanol steam reforming performance of Cu/ZrO2 catalysts by modification of the Cu-ZrO2 interface dimensions resulting from Cu loading variation

Article abstract of DOI:10.1016/j.apcata.2021.118279

On Cu/ZrO₂ catalysts, variation of the Cu loading from 0.2 wt% to 80 wt% allows assessing the influence of the Cu-ZrO₂ interface on the methanol steam reforming (MSR) performance by steering Cu particle size and morphology, revealing the contribution of potential active sites at the interface and the intrinsic relative contributions of the support and Cu surface fraction. As ZrO₂ influences both CO₂-selective and selectivity-spoiling MSR reaction channels, disentangling support-specific effects from the special phase-boundary reactivity and the intrinsic Cu° reactivity is possible by our approach. By choosing a broad range of Cu loadings, a comparison of the most extreme cases of strong predominance of bulk-like Cu sites (80 wt% Cu) vs. highly dispersed Cu (0.2 wt%), dominated by interfacial and support reactivity, becomes accessible. Cu (80 wt%)/ZrO₂ evolves as the best MSR catalyst by avoiding adverse support effects and providing a high number of support-wetting bulk-like Cu sites.

Full text of DOI:10.1016/j.apcata.2021.118279

Applied Catalysis A, General 623 (2021) 118279
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Steering the methanol steam reforming performance of Cu/ZrO2 catalysts
by modification of the Cu-ZrO2 interface dimensions resulting from Cu
loading variation
Kevin Plonera, Parastoo Delir Kheyrollahi Nezhada, Maximilian Watschingera,
,
Lukas Schlickerb, Maged F. Bekheetb, Aleksander Gurlob, Albert Gilib c, Andrew Dorand,
e
e
e
f
¨
Sabine Schwarz , Michael Stoger-Pollach , Johannes Bernardi , Marc Armbrüster ,
a
a,
¨
Bernhard Klotzer , Simon Penner *
a Department of Physical Chemistry, University of Innsbruck, Innrain 52c, Innsbruck, 6020, Austria
b Chair of Advanced Ceramic Materials, Institut für Werkstoffwissenschaften und -technologien, Technical University Berlin, Hardenbergstr. 40, Berlin, 10623, Germany
c Institute of Chemistry, Technical University Berlin, Sekretariat TC 8, Straße des 17. Juni 124, Berlin, 10623, Germany
d Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA
e University Service Centre for Transmission Electron Microscopy, TU Wien, Wiedner Hauptstr. 8-10, Vienna, 1040, Austria
f Institute of Chemistry, Materials for Innovative Energy Concepts, Technical University Chemnitz, Straße der Nationen 62, Chemnitz, 09111, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
On Cu/ZrO2 catalysts, variation of the Cu loading from 0.2 wt% to 80 wt% allows assessing the influence of the
Cu-ZrO2 interface on the methanol steam reforming (MSR) performance by steering Cu particle size and
morphology, revealing the contribution of potential active sites at the interface and the intrinsic relative con-
tributions of the support and Cu surface fraction. As ZrO2 influences both CO2-selective and selectivity-spoiling
MSR reaction channels, disentangling support-specific effects from the special phase-boundary reactivity and the
intrinsic Cu◦ reactivity is possible by our approach. By choosing a broad range of Cu loadings, a comparison of
the most extreme cases of strong predominance of bulk-like Cu sites (80 wt% Cu) vs. highly dispersed Cu (0.2 wt
%), dominated by interfacial and support reactivity, becomes accessible. Cu (80 wt%)/ZrO2 evolves as the best
MSR catalyst by avoiding adverse support effects and providing a high number of support-wetting bulk-like Cu
sites.
Monoclinic zirconia
In situ X-ray diffraction
CO2 selectivity
Activity
Support
1. Introduction
is impaired by potential parasitic CO2-lowering side reactions such as
the reverse water-gas shift reaction, methanol dehydrogenation to CO
Methanol steam reforming (MSR) and methanol synthesis are two of
the most promising processes for reversible storage and release of
hydrogen [1]. Methanol as chemical hydrogen storage material exhibits
a number of advantages over higher alcohols and hydrocarbons. It is
and the subsequent partial methanation of intermediately formed CO
and CO2 [4,5]. Direct steering toward high CO2 selectivity is crucial to
obtain a high hydrogen yield and avoiding the potential catalyst poison
CO [2,6].
–
liquid at ambient conditions, the absence of a C C bond allows for a
The commercially available Cu/ZnO/Al2O3 methanol synthesis
catalyst exhibits a high CO2 selectivity (e.g. > 99 % at 90 % methanol
conversion and 306 ◦C [7]) in MSR, but produces too high levels of CO
for direct use of the reformate in e.g. polymer electrolyte membrane fuel
cells [8]. As MSR runs at higher temperatures than the synthesis reac-
tion, it suffers from rapid on-stream deactivation due to Cu particle
sintering [2]. Cu/ZrO2, on the other hand, has evolved as an alternative
CO2 selectivity close to 100 % and lowers the required reforming tem-
perature (e.g. by 100 ◦C–200 ◦C vs. ethanol) [2], it possesses the highest
hydrogen to carbon ratio and both a satisfactory volumetric and gravi-
metric energy density [3]. The CO2-selective methanol steam reforming
reaction (Eq. 1)
CH3OH(g) + H2O(g)⇄3 H2, (g) + CO2,(g) ΔHr0 = 49.6kJ m olꢀ 1
(1)
* Corresponding author.
E-mail address: simon.penner@uibk.ac.at (S. Penner).
https://doi.org/10.1016/j.apcata.2021.118279
Received 13 May 2021; Received in revised form 24 June 2021; Accepted 27 June 2021
Available online 1 July 2021
0926-860X/© 2021 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
catalyst material exhibiting excellent MSR performance [9,10].
The combined micro- and electronic structure control of metallic Cu
via the specific Cu/support interface and the oxygen content are dis-
cussed as the most important activity and CO2 selectivity steering pa-
rameters [2,11]. The exact operational role of ionic vs. metallic Cu
species is still subject to controversial discussion. Structurally ordered
and chemically pure metallic copper is inactive in MSR [12]. Hence, the
active form of copper is related to a non-equilibrium state of metallic Cu
[11]. The presence of ionic Cu species and purely geometrical strain
effects are offered as explanations for high activity and CO2 selectivity
and are related to synergistic Cu interactions with the support resulting
mainly from catalyst preparation [11].
a direct consequence of Cu loading) and its direct correlation to the Cu
bulk and surface oxidation state in combination with the influence of the
support chemistry to optimize the CO2 selectivity is the central goal of
the presented work. The variation of the Cu metal loading allows con-
trolling both Cu particle size and Cu morphology. The contribution of
potential active sites at the Cu-ZrO2 interface, as well as of the intrinsic
relative contributions of the support and Cu surface fraction, are equally
accessible. Based on recent studies [19], it is clear that ZrO2 is an active
support, influencing both the CO2-selective and selectivity-spoiling re-
action channels of the overall reaction mechanism. By using the same
monoclinic ZrO2 support it was possible to discriminate support-specific
effects from the special phase-boundary and the intrinsic reactivity of
metallic Cu. Choosing the full range of Cu loadings makes a comparison
of the most extreme cases possible: on the one hand, the strong pre-
dominance of the intrinsic Cu surface area contributions (that is, almost
all Cu sites are located within a bulk-like Cu environment and interfacial
and support surface site contributions are accordingly minimized) can
be assessed. On the other hand, the opposite scenario of almost
atomic-scale dispersion of Cu is accessible. Hence, the full compositional
range from 0.2 wt% to 80 wt% Cu is focused upon in the presented study
to provide a more comprehensive investigation of the influence of the Cu
loading in MSR. By using a phase-pure, structurally and chemically
invariant support, this strategy allows for unambiguous discrimination
of selectivity-promoting/spoiling contributions of the support inter- and
surface vs. the intrinsic Cu◦ contribution (which becomes predominant
at high loadings).
Beyond the exact structural and redox state of Cu, the Cu particle size
and loading significantly affect the catalytic performance. Attempts to
model and unravel the role of the Cu/ZrO2 interface by varying the
copper loading suffer from the poor definition of the initial crystal
structure and disparate preparation of ZrO2 [13–15]. Amorphous ZrO2
or mixtures of the monoclinic and the tetragonal ZrO2 (t-ZrO2) are all
employed. The variation and at the same time narrow compositional
range of the Cu loading utilizing the same synthesis routine and sample
treatments essentially yield different optimized Cu contents. A loading
of 80 wt% CuO on a structurally heterogeneous ZrO2 support yielded
superior MSR performance in terms of hydrogen production [13]. On the
contrary, Takezawa et al. obtained the highest H2 turnover frequency for
a loading of 7.78 wt% Cu on a similarly heterogeneous ZrO2 support
[14]. A methanol conversion increase with increasing Cu content up to
16 wt% Cu for Cu/t-ZrO2 catalysts containing traces of m-ZrO2 was also
reported [15]. Only Ritzkopf et al. explicitly account for the CO2
selectivity, observing an increased CO content in the product stream
with increasing Cu loading at a temperature of 300 ◦C [15].
2. Experimental
2.1. Catalyst synthesis
The chemical state of Cu during both the reforming and partial
oxidation of methanol (POM) has been discussed as a crucial parameter,
where especially ionic Cu+ species have been reported to be particularly
selective. Agrell et al. conducted partial oxidation of methanol on a Cu/
ZnO/ZrO2/Al2O3 catalyst and observed the formation of Cu+ by pseudo
in situ XPS measurements upon exposure to the reaction mixture (1:2).
Oxidized Cu, coexisting with metallic copper [7], is claimed to represent
the active Cu species under POM conditions. This interpretation is
challenged by NAP-XPS studies revealing subsurface oxygen as the
crucial Cu-activating species, which can only be observed in situ, rather
than inactive Cu oxides [16]. Investigations on a commercial Cu/Z-
nO/Al2O3 catalyst under different reaction conditions yielded a depen-
dence of the post-reaction oxidation state of copper on the feed gas
composition and the temperature. Substantial hydrogen production
upon the presence of either Cu◦ or Cu+ was displayed [17]. On Cu/ZrO2,
the formation of elemental Cu after prolonged MSR operation at 250 ◦C
is reported to depend on both Cu loading and methanol:water ratio [13,
18]. The best H2 yield was obtained using 80 wt% CuO on again struc-
turally poorly defined ZrO2. This ZrO2-supported catalyst almost
exclusively revealed the presence of Cu2O, which was suggested to
exhibit superior MSR properties [13,18]. We note that that the conclu-
sions were essentially inferred from ex situ characterization, which
naturally raises concerns about the stability of e.g. potential metallic
active sites upon exposure to air.
All catalysts were prepared following a similar procedure based on
standard aqueous impregnation. An aqueous solution (6 g lꢀ 1 for all
except 0.2 wt% with 0.6 g lꢀ 1) of Cu(II) acetate (Merck) is slowly added
to a stirred aqueous suspension (volume ratio solid/liquid of approxi-
mately 1:100) of monoclinic (m-)ZrO2 (Alfa Aesar, 99.978 %). Subse-
quent removal of the solvent at 60 ◦C, drying in vacuo and calcination in
air at 400 ◦C for 2 h results in the formation of the CuO/ZrO2 pre-
catalysts. Different ratios corresponding to 0.2 wt% to 80 wt% Cu◦
after pre-reduction were used to prepare the individual catalysts. For an
overview of the Cu loading of all samples, we refer to Table 1 in Section
3.
2.2. Catalytic testing
All catalytic tests were performed in a dedicated all-quartz recircu-
lating batch reactor. The gas phase composition is monitored via a
capillary leak to a quadrupole mass spectrometer (QMS, Balzers QMG
311) arranged in cross-beam geometry and equipped with a secondary
electron multiplier. The small reactor volume (13.8 mL) allows the
detection of trace by-products and the characterization of small sample
amounts (10 mg – 100 mg). Heating is performed using a Linn High
Table 1
Therefore, in general, in situ and operando methods must be utilized
to clarify the role of the active centers. In this manuscript, the recurrent
methodological theme using in situ X-ray diffraction (XRD) in combi-
nation with ex situ X-ray photoelectron spectroscopy (XPS) and electron
microscopy (TEM and SEM) is employed.
Overview of the studied samples with the nominal copper loading and the values
according to ICP-OES in the calcined state.
Sample
name
Nominal copper
loading (reduced
state, Cu0) / wt%
Nominal copper
loading (calcined
state, CuO)/ wt%
Copper loading from
ICP-OES (calcined
state) / wt%
As ZrO2 can act as a particularly suitable support for Cu to steer
methanol steam reforming toward optimum CO2 selectivity, we set out
to scrutinize the influence of the Cu-ZrO2 interface- and relative surface
dimensions on the MSR performance. To provide the most defined
catalyst starting state possible, well-ordered monoclinic zirconia (m-
ZrO2) was selected as the most promising support based on previous
studies with respect to high CO2 selectivity [19]. The Cu morphology (as
CmZ02
CmZ2
0.2
2.0
20
40
80
0.2
2.0
19
36
67
0.2
2.0
19
36
71
CmZ20
CmZ40
CmZ80
C = Cu; mZ = m-ZrO2, the number refers to the nominal copper loading after
pre-reduction.
2

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Term furnace capable of operating up to 1100 ◦C, where the temperature
is monitored via a K-type thermocouple (NiCr-Ni) near the sample.
To prevent condensation, the MSR reaction mixture is introduced at
100 ◦C by expansion of the gas phase over a 1:10 liquid methanol/water
mixture (resulting in a methanol:water ratio of 1:2 in the gas phase at
room temperature). The liquid mixture is degassed by several freeze-
and-thaw cycles. Standardized oxidative cleaning procedures in 1 bar
pure oxygen at 400 ◦C for 1 h (termed “O400”) and a pre-reduction
2.4. Determination of the specific surface area
Measurements of the specific surface area based on the BET method
were performed with a NOVA 2000e Surface Area & Pore Size Analyzer
(Quantachrome Instruments) using the software Quantachrome Nova-
Win. The samples were heated to 200 ◦C in vacuo for 30 min before the
adsorption of N2 at ꢀ 196 ◦C at five points from 0.05 p/p0 to 0.3 p/p0 was
conducted.
◦
treatment in 1 bar pure hydrogen at 300 C for 1 h (termed “H300”)
are performed before the MSR reaction mixture (≈ 28 mbar) is intro-
duced. Ar (≈ 6 mbar) for time-dependent QMS intensity correction (to
compensate the steady gas withdrawal through the leak into the mass
spectrometer) and He as carrier gas to improve the recirculation effi-
ciency and the heat transfer are subsequently added. During MSR, the
gas phase composition is continuously quantified by mass spectrometry.
The Ar m/z = 40 corrected intensity vs. time data, in combination with
external calibration of the relevant gases, including their relative frag-
mentation patterns, yield the partial pressure changes vs. time of the
reactants and products. Normalization of the reaction rates to the Cu
surface sites has been performed to calculate turnover frequency
numbers. We outlined the calculation of the turnover frequency (TOF)
values in Section A in the Supporting Information.
2.5. In situ thermogravimetric analysis (in situ TGA)
The in situ TG measurements were performed with a NETZSCH STA
449F3 Jupiter TGA/DSC setup equipped with Ar, O2 and H2 (all gases
99.999 % from Linde). The differential TG (DTG) curves were obtained
by differentiating the mass change with respect to the temperature. The
samples were pre-oxidized in 5% O2 in Ar (total flow 100 mL minꢀ 1) by
heating to 600 ◦C with 5 ◦C minꢀ 1, followed by an isothermal period of
30 min to ensure quantitative oxidation to CuO. The reduction behavior
was investigated by treating the samples in 10 % H2 in Ar (total flow 100
mL minꢀ 1) starting at 25 ◦C at a heating ramp of 5 ◦C minꢀ 1 up to a final
temperature of 300 ◦C. An isothermal period ensures full reduction and
enables deducing the Cu loading from the mass loss. An additional
oxidation step after reduction up to 600 ◦C in 5% O2 in Ar (total flow 100
mL minꢀ 1) with a ramp of 5 ◦C minꢀ 1 indicates intermediary Cu2O
formation before full oxidation to CuO. The mass increase in this mea-
surement is utilized to corroborate the value of the copper loading by
ICP-OES.
2.3. Powder X-ray diffraction (PXRD)
Ex situ powder X-ray diffraction measurements were carried out on a
Stadi P diffractometer (STOE & Cie GmbH, Darmstadt, Germany) in
transmission geometry. A MYTHEN2 DCS6 detector system (DECTRIS
Ltd., Switzerland) and a Mo X-ray tube (GE Sensing & Inspection
Technologies GmbH, Ahrensburg, Germany) with 40 mA and 50 kV
were used. Mo-Kα1 radiation with a wavelength of 0.7093 Å was selected
using a curved Ge(111) crystal. Evaluation of the data was performed
using the WinXPOW software [20]. Phase analysis was based on refer-
ences from the ICDD database [21]. Synchrotron-based in situ XRD in-
vestigations for the characterization of the copper bulk phase evolution
in MSR were conducted at the beamline 12.2.2 at the Advanced Light
Source (ALS) at the Lawrence Berkeley National Laboratory (LBNL),
California. The measurements were performed with a Perkin Elmer
image plate detector collecting one pattern per 180 s in transmission
2.6. Inductively coupled plasma optical emission spectroscopy (ICP-OES)
The Cu loading of the catalyst was determined by ICP-OES employ-
ing a Varian Vista RL instrument. As the solvent of the copper phase,
freshly prepared aqua regia (hydrochloric acid, 37 %, nitric acid, 68 %,
VWR chemicals AnalaR NORMAPUR) was utilized and diluted with
water to a concentration of 5% prior to analysis. The calibration was
performed for a concentration range from 20 mg lꢀ 1 to 100 mg lꢀ 1 by
dilution of a copper standard solution.
mode with a monochromatic 25 keV beam of 30 μm spot size. A LaB6
NIST standard was used for calibration of the sample-to-detector dis-
tance, the detector tilt and the exact wavelength (0.4982 Å) in the
Dioptas software [22]. Additionally, the two-dimensional detector im-
ages were integrated to obtain powder diffraction patterns with the
same software.
2.7. Determination of the specific Cu surface area via dissociative N2O
adsorption
The dissociative adsorption of N2O on metallic Cu is used for the
quantitative determination of the accessible surface area by selective
oxidation of the topmost layer to Cu2O [27–29]. The H2 consumption
during the reduction following this procedure is utilized for quantifi-
cation. The samples are loaded into an all-quartz reactor and its effective
volume is calibrated with the expansion of He (5.0, Messer) at the
reduction temperature of 300 ◦C. An initially degassed zeolite trap in the
cold zone of the reactor removes the reaction-formed water. Initially, all
samples are pre-reduced in flowing H2 (5.0, Messer) for 30 min at 300
◦C, which yields metallic Cu on all catalysts. After evacuation, the
reactor is purged with He and re-evacuated. Following cooling down in
vacuo to 70 ◦C, a defined pressure of N2O (2.0, Messer) monitored by a
Baratron pressure transducer (MKS Instruments) is provided in a
manifold. The consumption of N2O, as well as the evolution of N2, is
monitored by a quadrupole mass spectrometer (QMS) from Balzers
(QMA125; QME 125-9) for 30 min after opening the valve to the sample.
The samples were placed in capillaries (inner diameter =700 μm)
and heated with two infrared lamps illuminating a SiC sleeve [23]. The
gases for pre-oxidation (pure O2, 10 mL minꢀ 1, 400 C, 60 min) and
◦
pre-reduction (pure H2, 10 mL minꢀ 1, 300 C, 60 min) were supplied
◦
with the setup described by Schlicker et al. in [24]. For the in situ MSR
measurements, a liquid mixture of methanol:water = 1:10 was heated to
its boiling point in an Erlenmeyer flask. This provides a gas phase
composition of methanol:water = 1:2, which is seeded in He carrier gas
(5 mL minꢀ 1) and passed to the sample through a heated tube to prevent
condensation. In this atmosphere, the temperature was increased from
120 ◦C to 350 ◦C with a rate of 10 ◦C minꢀ 1, followed by an isothermal
period of 20 min. Pure He (15 mL minꢀ 1) for cooling was provided
through a different line to avoid further introduction of the MSR mixture
into the capillary. The Rietveld refinement (RR) of the XRD data was
conducted utilizing the FullProf software [25]. The profile function 7
(Thompson-Cox-Hastings pseudo-Voigt convoluted with an axial diver-
gence asymmetry function) [26] was used in all refinements. The reso-
lution function of the diffractometers was obtained from the structure
refinement of the LaB6 NIST 660b standard.
◦
The temperature is subsequently increased to 300 C after evacuation
and flushing with He. A defined amount of H2 is introduced into the
reactor and the pressure decrease resulting from reduction of the newly
formed surface Cu2O is observed for 35 min. The calculation of the
specific Cu surface area from N2O adsorption is discussed in detail in
Section A in the Supporting Information.
3

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
2.8. Transmission and scanning electron microscopy (TEM and SEM)
experiment. The BET surface area of all catalysts, which was determined
in the calcined state, is in the same range. A similar trend is found for the
specific copper surface area: only a factor of 2 separates CmZ02 and
CmZ80, whereas the Cu loading of these catalysts varies by a factor of
400. These observations translate to drastically different values for the
copper dispersion and average particle size. While Cu is highly dispersed
and exhibits particles with only a few nanometers in diameter in CmZ02,
the former decreases and the latter increases continuously with
increasing Cu loading. CmZ80 displays a very low dispersion of copper
and the assumption of embedded hemispherical particles used for the
estimation of the average particle size is not valid anymore (see TEM/
SEM characterization in Fig. 1). Due to the extreme variation of the
dispersion, the estimation of TOF values is essential for the comparison
of the catalytic performance of the five samples. We specifically note
that none of the N2 adsorption isotherms indicated the presence of
highly porous materials.
Structural and chemical transmission electron microscopic (TEM)
analysis was performed using a FEI TECNAI F20 field emission TEM
operated at 200 kV, equipped with a high angle annular dark-field STEM
detector (HAADF), an Apollo XLTW SDD X-ray detector and a GATAN
GIF Tridiem image filter. The spatial resolution of the EDX maps is about
1 nm. For SEM experiments, a FEI Quanta 250 field emission SEM was
used. Prior to SEM imaging, the samples were coated with 10 nm Au/Pd
to improve their conductance.
2.9. X-ray photoelectron spectroscopy (XPS)
A Thermo Scientific MultiLab 2000 spectrometer with an Alpha 110
hemispherical sector analyzer and a monochromatic Al Kα X-ray gun is
employed for the XPS investigations. A flood gun supplies electrons with
a kinetic energy of 6 eV for charge compensation and the base pressure is
kept in the low 10ꢀ 9 mbar range. Detailed scans of the most relevant
regions were recorded with a pass energy of 20 eV and an energy step
size of 0.05 eV. For background correction, a Shirley-type function is
utilized. The quantitative determination of the surface composition is
based on high-resolution Zr 3d and Cu 2p3/2 spectra using the relative
sensitivity factors (RSFs) [30] as well as the different inelastic mean free
paths using the predictive G1 formula according to Gries [31]. Reference
samples of various Cu species were measured using the same instrument
for qualitative assignment of the Cu state, including metallic Cu (sput-
ter-cleaned Cu foil, Goodfellow, ≥ 99.99 %), Cu2O (Sigma-Aldrich,
anhydrous, ≥ 99.99 % trace metals basis), CuO (Sigma-Aldrich, 99.999
% trace metals basis) and Cu(OH)2 (synthesized by precipitation with
CuSO4 ∙ 5 H2O (Merck, for analysis, 99.7 %) and NaOH (Roth, ≥ 99 %)).
In summary, while the specific surface area of copper increases
slightly with higher Cu loading, the copper dispersion displays a
continuous decrease from 0.2 wt% to 80 wt% Cu. This trend translates to
different initial Cu states of the samples for MSR in terms of the average
copper particle size.
3.2. Structural characterization of the calcined Cu/ZrO2 catalysts by
electron microscopy and X-ray diffraction
The SEM and TEM characterization of the Cu/m-ZrO2 catalysts with
different Cu loadings ranging from 0.2 wt% to 80 wt% in their calcined
state is highlighted in Fig. 1. To show the variation of the Cu distribution
as a function of loading, we rely on EDX mapping of representative
catalyst regions at different representative scales. As expected, the Cu
distribution and morphology transform from essentially uniform and
highly dispersed (uppermost panel, CmZ02) over Cu nanoparticles
(second panel from the top, CmZ2) to Cu wetting and covering large
areas of the individual m-ZrO2 grains (third, fourth and bottommost
panel, CmZ20, CmZ40 and CmZ80). For higher Cu loadings, Cu also acts
as a “glue” between the grains of the m-ZrO2 support.
3. Results and discussion
To ensure optimized comparability, in addition to the identical
synthesis route, exactly the same pre-treatments and MSR conditions
were applied to all systems. A complete sample list is provided in
Table 1. The deviation of the loading of CmZ80 determined by ICP-OES
in the calcined state is a consequence of its phase composition, which is
not exclusively CuO (as demonstrated by XRD, see SI Table S1). The lack
of oxygen will therefore result in an apparently higher Cu content in this
catalyst. The same is observed for CmZ40 and CmZ20 (see Table S1).
The higher the amount of Cu2O, the larger the deviation from the
nominal copper loading. As the deviation is not observed in the reduced
state, the correlation to incomplete Cu oxidation is evident.
Some isolated Cu particles are also observed for the lowest Cu
loadings (Fig. 2, upper panel). The lower panel of Fig. 2 features an
exemplary evaluation of the Cu bulk oxidation state for the CmZ80 via
EELS mapping experiments. Correlation of the featured total Cu-L in-
tensity (using electrons with a characteristic energy loss) with the in-
tensity of those Cu electrons with a defined energy loss characteristic for
CuO and Cu2O reveals at least partial oxidation of the ZrO2-wetting and
-encapsulating metallic Cu islands. The TEM data are in full accordance
with the XRD data and the obtained dispersion.
The microscopy results corroborate the trends observed in the copper
dispersion, where a transition from highly dispersed Cu at 0.2 wt% over
nanoparticles (CmZ2) and formation of copper islands (CmZ20 and
CmZ40) to encapsulation and considerable wetting of the zirconia sup-
port by Cu is observed. This variation in the copper morphology with the
loading provides a means to tune the catalyst properties regarding the
Cu particle size as well as the fraction of exposed support, which is re-
flected in the different behavior in MSR (see Section 3.3). Additionally,
changes to the Cu morphology after MSR were recorded by TEM (Fig. S1,
see SI), in direct accordance with Fig. 1. No alterations in all the samples,
except for CmZ02, were recognized, directly confirming their structural
and morphological stability. The TEM image of Fig. 2 reveals the pres-
ence of some isolated Cu nanoparticles already in the initial state of
CmZ02. As revealed by the EDX experiments on the post-MSR state, a
transformation of mostly highly dispersed Cu into agglomerated Cu
nanoparticles is observed for CmZ02, resembling the CmZ2 catalyst after
MSR (see Fig. 3). This directly explains the almost similar identical MSR
profiles in the second cycles of CmZ02 and CmZ2 (see Fig. 5).
3.1. (Copper) surface area, dispersion and average particle size
The results of the characterization of the total and Cu surface area,
including its dispersion and its average particle size, are listed in Table 2.
As Cu in its metallic state is required for this method, the catalysts were
pre-reduced in H2 at 300 ◦C prior to contact with N2O (see Section 2.7),
where no intermediate exposure to air occurred at any stage of the
Table 2
BET surface area, specific copper surface area, copper dispersion (DCu) and
average copper particle size (dCu) of the investigated Cu/ZrO2 catalysts.
Sample
BET surface area*1/ m2
SACu*2 / m2
DCu*2 / %
dCu*2 / nm
ꢀ 1
ꢀ 1
g
g
Cat
Cat
CmZ02
CmZ2
3
4
4
2
1
0.18
0.23
0.24
0.35
0.41
18
3
1.8
30
CmZ20
CmZ40
CmZ80
0.19
0.15
0.09
270
350
580
*1
Apart from the electron microscopic assessment of the calcined cat-
alysts, the detailed structural evolution of metallic Cu and ZrO2 as a
function of the Cu/ZrO2 interface dimension deserve particular
determined after calcination of the catalysts.
*2
presence of metallic Cu is required, so the catalysts were pre-reduced in H2
at 300 ◦C; BET surface area of pure m-ZrO2: 1 m2 gꢀ 1
.
4

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Fig. 1. Overview of the TEM/SEM and EDX evaluation of the Cu/ZrO2 catalysts in their calcined state with increasing Cu content. From top to bottom, the Cu amount
increases. The respective columns feature a representative TEM overview, HAADF (for CmZ02, CmZ2 and CmZ20) and secondary electron (SE, for CmZ40 and
CmZ80) images and the characteristic Cu-L, Zr-L and O-K intensities following EDX mapping of different representative sample regions. The last column sche-
matically highlights the transformation of the Cu morphology as a function of the Cu loading. The scale of the EDX images is identical to the HAADF or SE images.
attention – especially in view of the suspected importance of strain ef-
fects in the metallic Cu phase for high CO2 selectivity [11]. We are able
to provide information on the change in lattice parameter, crystallite
size and microstrain as a function of Cu loading and, thus, the Cu-ZrO2
interface dimension as derived from Rietveld refinement of the ex situ
collected XRD patterns (Fig. 4). Previous work showed that the lattice
parameters and unit cell volume of ZrO2 decrease with Cu substitution in
the ZrO2 lattice [32]. However, there is no significant change in the
lattice parameters and unit cell volume with increasing Cu content in
our case (Panel A). We also calculated the crystallite size for all phases
with (open symbols) and without (closed symbols) considering micro-
strain (Panel B) as a function of Cu loading. As expected, the crystallite
size of all phases is slightly higher, when microstrain is considered. In
essence, the crystallite size of ZrO2 is no strong function of the Cu
content. As expected, the crystallite sizes of the Cu2O and CuO phases
increase with Cu content. The higher the Cu content, the higher the
probability of Cu oxide formation. No peak broadening is observed in
the XRD reflections of the metallic Cu phase, suggesting that no
nano-sized crystallites are present. In contrast to the crystallite size, the
determined microstrain is found to be independent of the Cu content for
all phases. CuO has the highest microstrain among all samples due to its
small crystallite sizes (< 20 nm), while ZrO2 exhibits the smallest
microstrain because it has the largest crystallite sizes.
dispersed Cu (0.2 wt%) to m-ZrO2, the MSR performance of CmZ02 and
pure m-ZrO2 are compared in Fig. 5. The profiles of the formation rate of
CO are very similar, whereas the H2 formation rates on CmZ02 are
higher by a factor of ≈ 40 at 330 ◦C. Pure m-ZrO2 features almost 100 %
selectivity towards CO. This support-related formation of CO (onset
temperature ≈ 290 ◦C) causes most of the carbon monoxide formed on
CmZ02. However, the onset temperature of CO is ≈ 260 ◦C on CmZ02,
suggesting the creation of an additional low-temperature reaction
pathway toward CO requiring the presence of Cu. This already indicates
that the suppression of this support-related selectivity-spoiling activity
toward CO is crucial for high CO2 selectivity.
The MSR performance of CmZ02 in the first two catalytic cycles is
visualized once more in Fig. 6, Panel A to enable direct comparison with
CmZ2 in Panel B. The first and the second MSR run differ considerably,
supporting the changes observed with electron microscopy (see Fig. 3).
During the 1st cycle, the H2 and CO2 formation rates exhibit an
extremely steep increase and cannot be fitted by an Arrhenius equation
utilizing physically meaningful parameters, suggesting ongoing activa-
tion of the catalyst at the MSR onset temperature. This effect is explained
by the incorporation of Cu into the ZrO2 lattice, which also accounts for
the invisibility of Cu in TGA and XRD. The observations are similar to
those attributed to formation of a ternary Cu-Zr-O oxide phase as
described by Tada et al. [33] for a Cu/amorphous ZrO2 system. The
maximum activity in this study is also lowered, hinting at deactivation
by Cu sintering as indicated by XPS and electron microscopy in-
vestigations after catalysis (see Table 4 and Fig. 3). The CO2 selectivity,
the methanol conversion and the CO-TOF essentially remain the same.
The MSR profile of CmZ2 (Fig. 6 Panel B) does not show the steep
3.3. Influence of the Cu-ZrO2 interfacial dimensions on the catalytic
methanol steam reforming performance
To emphasize the significant effect of the addition of highly
5

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Fig. 2. Top panel: TEM and HRTEM image of a single Cu particle decorating a m-ZrO2 grain featuring atomically resolved Cu(111) lattice fringes on CmZ02 after
calcination. Bottom panel: HAADF and EELS mapping images of a representative region on CmZ80 after calcination. For the EEL spectrum images, the total Cu-L
intensity is shown alongside images using the fraction of electrons with a characteristic energy loss for electrons stemming from CuO and Cu2O.
increase in the H2 and CO2 formation rates in the first cycle and the MSR
reaction onset is observed already at ≈ 200 ◦C. The general character-
istics of the catalytic profiles of CmZ02 and CmZ2 in the second cycle are
almost identical except for small shifts in the onset temperatures. In
contrast to CmZ02, however, the TOFs of CmZ2 including the methanol
conversion are increased in the second cycle, suggesting further acti-
the activity trend between the catalysts with different Cu loadings.
The methanol steam reforming profiles of CmZ20 in Fig. 7 Panel A
depict different characteristics compared to CmZ2 and CmZ02.
Although the onset temperatures of H2 and CO2 formation are very
similar to the ones of CmZ2, the activation from the first and the second
cycle is much less pronounced. The onset temperature of CO, however, is
decreased from ≈ 260 ◦C (CmZ2) to ≈ 220 ◦C in CmZ20, which narrows
the temperature regime of > 90 % CO2 selectivity.
◦
◦
vation of the catalyst. Up to 270 C (250 C in the second cycle), the
catalyst exhibits almost 100 % CO2 selectivity, which is a common
feature of all investigated samples. A striking feature of both catalysts
with the lowest Cu loading is the pronounced activation between the
first and the second cycle. For CmZ02, we ascribe this to the decom-
position of the ternary Cu-O-Zr oxide in the first cycle (cf. Fig. 3) and the
appearance of small Cu particles. In the second MSR cycle, we tenta-
tively assign the further activation to an increased number of those
formed Cu nanoparticles. We hence assume that partial activation oc-
curs during both the first and the second cycle. For CmZ2, small Cu
particles were present even before the first cycle, explaining the earlier
onset of reaction. We ascribe the further activation from the first to the
second cycle to the slight agglomeration of Cu particles to an optimized
Cu//ZrO2 interface for MSR (which at the end of the first cycle/during
the second cycle apparently is similar to CmZ02 derived from the almost
perfect match of the catalytic MSR traces and the similar TEM data). A
similar reasoning applies for CmZ20, which also exhibits an increase in
activity form the first to the second MSR cycle.
The trend of lower CO2 onset temperatures continues with CmZ40 in
Fig. 7 Panel B, where H2 and CO2 start to be formed around 160 ◦C and
180 ◦C. Additionally, the onset of CO is lowered to around 200 ◦C. The
profiles of the two MSR cycles are almost identical and the overall
appearance is similar to CmZ20.
The two MSR cycles of CmZ80 match each other, indicating that no
additional activation occurs, and the onset temperatures of H2 and CO2
are located at ≈ 130 ◦C (see Fig. 8). Several features of this sample are
◦
distinct. The onset of CO formation increases once more to ≈ 220 C
compared to 190 ◦C on CmZ40. This expands the temperature regime of
> 90 % CO2 selectivity and the maximum TOFs of H2 and CO2 are
located around the onset of CO formation.
As the attentive reader will have noticed, the samples CmZ02, CmZ2,
CmZ20 and CmZ40 all display reproducible non-MSR related CO2 for-
mation in both catalytic cycles (except CmZ02 in the first cycle), re-
flected in the absence of concomitant H2 formation. Rather, this
particular CO2 formation starts at lower temperatures and is super-
imposed on the CO2 arising from MSR at its reaction onset. Since this
effect is neither visible on CmZ80 (see Fig. 8) nor pure m-ZrO2 (see
Fig. 5) and its extent decreases slightly with increasing Cu loading up to
40 wt%, it can be ascribed to a cooperative operation of Cu and ZrO2.
A short note on the use of TOF values for CmZ02 and CmZ2 should be
added. We deliberately used the number of accessible Cu atoms as
derived from N2O adsorption experiments as a measure for TOF calcu-
lation, although the catalysts activate between the first and the second
cycle. We take this as a reasonable approximation to internally compare
6

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Fig. 3. TEM EDX maps showing the Cu-L intensities of CmZ02 and CmZ2 before and in the post-MSR state to document the evolution of the Cu nanoparticles during
MSR for the catalysts with the lowest Cu loadings.
This is in good agreement with the TEM results, where wetting and
encapsulation of the support by Cu is visible on CmZ80 (see Fig. 1),
reducing the accessible Cu-ZrO2 interface sites. There are two possible
explanations for this non-MSR-related CO2 formation. The first one is
based on a redox reaction of Cu oxide species at the Cu-ZrO2 interface
and methanol, yielding CO2, water and metallic Cu. This phase bound-
ary oxide could be formed upon introduction of the MSR mixture at 100
◦C after the pre-reduction. This theory is supported by observations of
Werner et al. on a Cu foil that was pre-dosed with O2, where CO2 for-
mation from methanol was detected around 200 ◦C [34]. The other
potential explanation revolves around storing CO2 stemming from pre-
vious treatments/MSR cycles. Upon heating, the stored CO2 (presum-
ably present as (bi-) carbonates that decompose at elevated
temperatures) is re-released again as additional CO2 between 120 ◦C and
250 ◦C. The temperature range of carbonate decomposition fits the re-
ported values of Pokrovski et al., who observed the formation of ionic
and bidentate bicarbonates as well as mono- and bidentate carbonates
upon exposure of m-ZrO2 to CO2. Starting at 25 ◦C, desorption/de-
composition of bicarbonates with a maximum rate at around 130 ◦C and
further decomposition of carbonates between 180 ◦C and 380 ◦C occurs
[35].
passes through a minimum on CmZ40 and the difference between the
first and the second MSR cycle is generally less pronounced.
3.4. Development of the surface atomic Cu/Zr ratio as a function of the
Cu-ZrO2 interface: X-ray photoelectron spectroscopy
The spectra of the Cu 2p and Cu LMM regions are depicted in Fig. S2
in the SI. Since the measurements were conducted ex situ with inter-
mediate exposure to air, the spectra are not clearly representative of the
post-MSR state anymore. Hence, no conclusions regarding the influence
of the copper loading on the pre- and post-MSR surface oxidation state of
Cu could be drawn. However, the atomic ratio of Cu and Zr at the
catalyst surface was calculated (Table 4) and continuously increases
with increasing copper loading after calcination, confirming the trends
observed by microscopy and dissociative N2O adsorption. Another
consequence of this trend is the concomitant decrease in the exposed m-
ZrO2 surface area, which implies that the extent of support-related ef-
fects should decrease with increasing Cu loading in MSR. Furthermore,
the Cu/Zr ratio decreases after MSR for all samples but CmZ2, indicating
slight sintering of metallic Cu during MSR. Relating the XPS data to the
TEM investigations, we note that also the Cu/Zr ratios indicate consid-
erable wetting of ZrO2 by Cu, although not by 100 %. The situation for
the CmZ80 catalyst is reminiscent of a “Raney-Cu catalyst”, with resid-
ual patches of oxide. In fact, a broad patent literature for Raney-type Cu
catalysts is available, with the residual oxide component (typically
interfacial Al2O3) serving the purpose to keep the strongly predominant
In conclusion, with increasing surface-wetting (blocking) of the
support (reflected by the increase in Cu loading), several trends for MSR
operation are observed (visualized in Fig. 9 and summarized in Table 3).
Firstly, the onset temperatures of CO2 and H2 decrease. Secondly, the
◦
TOFs of CO2 and H2 at 220 C increase. The onset temperature of CO
7

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Fig. 5. MSR profiles of CmZ02 and pure m-ZrO2 between 100 ◦C and 350 ◦C.
The traces are shown up to a maximum methanol conversion of 30 %. Upper
panel: orange – methanol conversion, blue ꢀ CO2 selectivity; lower panel:
Formation rates of brown – H2, green ꢀ CO2, black ꢀ CO, gray ꢀ CH4. Dashed
lines refer to CmZ02. The inset represents a zoomed region highlighting the
performance of pure m-ZrO2. Heating rate: 5 ◦C minꢀ 1. Sample mass: CmZ02 –
78.0 mg; m-ZrO2 – 82.2 mg.
To circumvent these deficiencies and to corroborate the values of the
SACu(N2O) obtained by dissociative N2O adsorption, the Cu/Zr atomic
surface ratio from XPS was utilized in combination with the BET-
determined specific catalyst surface area. Based on the Cu/Zr ratio,
the SACu(XPS) was estimated by multiplication of the BET surface area
with the near-surface atomic Cu percentage (based on the Cu 2p and Zr
3d regions) from XPS. A comparison of the Cu surface areas determined
from N2O dissociation and XPS, SACu(N2O) and SACu(XPS), respectively,
is given in Table 4. The general trend considering only CmZ02 and
CmZ80 is reflected in both values, while the continuity of the increase
present in SACu(N2O) is not visible in SACu(XPS). However, as disso-
ciative N2O adsorption requires the presence of Cu in its metallic form,
the catalysts have to be pre-reduced for these experiments, which might
involve unavoidable sintering of Cu particles. The BET surface area of
the catalysts was measured in the state after calcination, which is why
the estimation of SACu(XPS) was only conducted for the calcined state.
The BET surface area could be altered upon MSR and the involved pre-
treatments. Additionally, the Cu/Zr surface ratio after MSR and the
involved pre-treatments is lower for all samples except for CmZ2,
inferring that Cu particle sintering occurs. However, considering the Cu/
Zr surface ratio after MSR in combination with the BET surface area after
calcination yields more comparable values for SACu(N2O) and
SACu(XPS). Even more importantly, the trends in the continuous in-
crease of the TOFs of H2 and CO2 at 220 ◦C in MSR calculated with either
SACu(N2O) or SACu(XPS) are similar (see Table 3).
Fig. 4. Development of ZrO2 lattice parameters/unit cell volume (Panel A),
crystallite size (Panel B) and microstrain (Panel C) as a function of Cu loading
and, therefore, Cu/ZrO2 interface dimension, as derived from Rietveld refine-
ment of the ex situ collected XRD data. Lines drawn as a guide to the eye.
3.5. Redox behavior of the Cu/ZrO2 catalysts: in situ thermogravimetry
The redox behavior of each sample in an oxidation-reduction-
oxidation cycle was investigated to characterize the kinetics of oxida-
tion/reduction and to further confirm the copper loading. The pre-
oxidation as well as the re-oxidation are discussed in the SI Section D.
The number of signals observed in the reduction of Cu-based catalysts in
MSR has been reported to deliver information on the interaction of Cu
with the support [36]. Liu et al. [37] describe the reduction behavior of
Cu/ZrO2 catalysts with two signals, where they relate the
low-temperature feature to copper species strongly interacting with the
support and the high-temperature peak to bulk CuO-like copper species.
The reduction in hydrogen up to 300 ◦C does not show any change of
the mass for the m-ZrO2 reference sample (see Fig. 10). The TPR treat-
ment yields a single reduction signal for all catalysts, including the CuO
metal phase dispersed.
The determination of the specific Cu surface area via dissociative
N2O adsorption over a loading range from 0.2 wt% to 80 wt% Cu im-
poses certain necessary assumptions on the calculation of the TOF
values. The redox chemistry of highly dispersed Cu with a high metal-
support interface area (CmZ02) differs from the bulk-like support-wet-
ting Cu phase in CmZ80. Thus, the restriction of only surface oxidation
of metallic Cu to Cu2O cannot be upheld. The embedded hemispherical
particle approximation is also not valid anymore in CmZ80, which
would make the estimation of the average particle size less reliable.
8

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Fig. 6. MSR profiles of CmZ02 between 180 ◦C and 330 ◦C (Panel A) and CmZ2 between 130 ◦C and 340 ◦C (Panel B). The traces are shown up to a maximum
methanol conversion of 30 %. Upper panel: orange – methanol conversion, blue ꢀ CO2 selectivity; lower panel: TOF of brown – H2, green ꢀ CO2, black ꢀ CO, gray
ꢀ CH4. Dashed lines indicate traces of the second cycle. Heating rate: 5 ◦C minꢀ 1. Sample mass: CmZ02 – 78.0 mg; CmZ2 – 64.0 mg.
Fig. 7. MSR profiles of CmZ20 between 130 ◦C and 290 ◦C (Panel A) and CmZ40 between 120 ◦C and 250 ◦C (Panel B). The traces are shown up to a maximum
methanol conversion of 30 %. Upper panel: orange – methanol conversion, blue ꢀ CO2 selectivity; lower panel: TOF of brown – H2, green ꢀ CO2, black ꢀ CO, gray
ꢀ CH4. Dashed lines indicate traces of the second cycle. Heating rate: 5 ◦C minꢀ 1. Sample mass: CmZ20 – 65.0 mg; CmZ40 – 88.6 mg.
reference material. The exception is CmZ02, since the sensitivity of the
TGA measurements is too low to detect any significant mass loss upon
reduction in hydrogen as well as in the re-oxidation afterwards (see SI
Fig. S4). The signal temperatures, given in Table 5, are very similar, but
tend to increase with higher copper loading. The signal of CmZ80 is
shifted to lower values relative to CmZ40, suggesting a more pro-
nounced reducibility of CmZ80. The reduction onset temperature for a
CuO reference sample is slightly higher than for the catalysts supported
on zirconia, which suggests a synergistic effect of ZrO2 in the reduction
of the copper phase, potentially by supplying an active phase boundary
for hydrogen activation. In addition, the reduction signal areas serve as a
measure for the copper loading (see Table 5), which agrees well with the
results from XRD, as well as ICP-OES.
particle sizes of copper in the catalysts. The onset temperature of pure
CuO, however, is located at approximately 200 ◦C, suggesting the
occurrence of a synergistic effect of ZrO2 for the reduction of the copper
phase in all samples.
3.6. Dynamical response of the Cu bulk oxidation state during methanol
steam reforming: in situ X-ray diffraction
We investigated the bulk structural changes of the copper phase to
elucidate the role of oxidized bulk Cu species on the catalytic perfor-
mance in MSR. We also discuss the dynamical behavior of the bulk Cu
oxidation state during the oxidative and reductive pre-treatments in this
section (experiments not shown, except for CmZ02, see SI Fig. S6).
Due to the detection limit of XRD, no copper phase can be observed
in CmZ02 during the pre-reduction (see SI Fig. S6), even though the
loading was confirmed by ICP-OES (see Table 1). However, this does not
exclude the presence of copper in an amorphous form or the incorpo-
ration of Cu into the zirconia lattice resulting in a ternary oxide, as re-
ported for Cu/ZrO2 catalysts [33]. The in situ XRD patterns of CmZ2 and
¨
In contrast to the reports of Ritzkopf et al. [15], Koppel et al. [36]
and Liu et al. [37], only one broad signal is observed in the TPR profiles
of the catalysts in this study. The signal temperatures of the profiles
increase continuously with the exception being CmZ80, whereas the
◦
onset temperatures of all samples lie around 180 C. The shift of the
signal temperatures is induced by kinetic effects due to the different
9

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Table 3
Summary of key catalytic parameters.
CmZ02
CmZ2
CmZ20
CmZ40
CmZ80
E
a CO2 / kJ
molꢀ 1
103a
110
96
92
85
102
101
93
91
85
Ea CO / kJ
108
121
107
103
107
105
130
130
220
220
130
130
molꢀ 1
115
113
105
103
Onset T(H2)
240
200
180
160
/
◦C
200
190
180
160
Onset T(CO)
260
270
220
190
/
◦C
260
250
220
190
Onset T
240
150b, 230
150b, 210
120b, 180
120b, 180
(CO2) / ◦C
200b, 230
150b, 200
150b, 200
120ꢀ 300c,
240ꢀ 335
150ꢀ 315c
200ꢀ 315d
150ꢀ 320c
230ꢀ 320d
150ꢀ 290c
210ꢀ 290d
150ꢀ 285c
200ꢀ 285d
130ꢀ 310e
e
180ꢀ 300d,
e
T(> 90 %
CO2) / ◦
C
120ꢀ 305c,
200ꢀ 325c
230ꢀ 325d
130ꢀ 310e
e
180ꢀ 305d,
e
TOF(N2O)
H2 at
0.15
0.37
0.39
0.87
2.6
7.0
8.1
15.9
16.0
220 ◦C /
4.12
10ꢀ 3 s-1
TOF(XPS)
H
2 at
0.049
0.12
0.059
0.13
0.32
0.49
2.3
2.7
7.6
7.7
220 ◦C /
10ꢀ 3 s-1
TOF(N2O)
CO2 at
Fig. 8. MSR profiles of CmZ80 between 110 ◦C and 230 ◦C. The traces are
shown up to a maximum methanol conversion of 30 %. Upper panel: orange –
methanol conversion, blue ꢀ CO2 selectivity; lower panel: TOF of brown – H2,
green ꢀ CO2, black ꢀ CO, gray ꢀ CH4. Dashed lines indicate traces of the second
cycle. Heating rate: 5 ◦C minꢀ 1. Sample mass: 86.6 mg.
0.023
0.57
0.48
0.48
0.97
1.4
2.2
2.5
6.0
6.1
220 ◦C /
10ꢀ 3 s-1
TOF(XPS)
CO2 at
0.0077
0.19
0.074
0.073
0.12
0.17
0.74
0.84
2.9
2.9
220 ◦C /
10ꢀ 3 s-1
Ea = apparent activation energy obtained by fitting the Arrhenius equation to
the experimental TOF values with the first and the second cycle given vertically
adjacent for comparability.
a
no Arrhenius behavior, therefore unreliable.
b
non-MSR-related CO2 formation starts at this temperature, onset tempera-
ture for catalytic CO2 formation is estimated below.
c
d
including the non-MSR-related CO2 formation.
neglecting the non-MSR-related CO2 formation.
e
upper temperature limit above of the depicted traces, but shown for
completeness of the parameters; TOF(N2O) = TOFs obtained with the SACu
determined via dissociative N2O adsorption; TOF(XPS) = TOFs obtained with the
SACu based on the surface atomic Cu/Zr ratio from XPS and the specific catalyst
surface area from BET.
CmZ02, where this value could be interpreted as the limit for Cu
incorporation into this specific ZrO2 phase accompanied by the presence
of crystalline Cu. During pre-reduction, Cu2O is transformed into
metallic Cu, leading to the formation of a stable amount of about 1.2 wt
% of metallic Cu during MSR. (SI Fig. S7).
Fig. 9. Visualization of the key catalytic parameters of all studied Cu/m-ZrO2
samples. The TOFs at 220 ◦C are compared on the left axis and the onset
temperatures are given on the right axis. The data of the first cycle is depicted
with filled markers and solid lines, whereas the information on the second cycle
is shown with hollow markers and dashed lines. The temperature at which the
CO2 selectivity decreases below 90 % is illustrated in dark blue and the
resulting temperature region of > 90 % CO2 selectivity is highlighted with a
semi-transparent red bar, where the congruency of the first and second cycle is
visible as a more saturated color. Lines have been drawn as a guide to the eye.
After calcination of CmZ20, metallic Cu next to Cu2O and CuO is
observed (SI Fig. S5. Several Cu oxide phases are obtained during the
pre-oxidation step after 10 min at 400 ◦C in O2. The final composition is
a mixture of 6.5 wt% Cu2O, 17.0 wt% CuO and 76.4 wt% m-ZrO2. Pre-
reduction quantitatively converts the Cu oxide phases to metallic Cu
(19.9 wt%), which remains stable during MSR (Fig. S8).
◦
For CmZ40, the calcination in air at 400 C for 2 h results in the
CmZ20 collected during MSR are also presented in the SI, since no in-
termediate formation of Cu2O or CuO was observed (see SI Figs. S7 and
S8, respectively).
formation of metallic Cu, Cu2O and CuO (SI Fig. S5). After pre-oxidation,
metallic Cu does not disappear completely and the final composition is
0.8 wt% Cu, 11.6 wt% Cu2O, 32.1 wt% CuO and 55.5 wt% m-ZrO2. Pre-
reduction yields metallic Cu as the predominant phase with a final
composition of 39.6 wt% Cu◦, 0.4 wt% Cu2O and 60.0 wt% m-ZrO2,
which is in excellent agreement with the nominal loading as well as with
the data from ICP-OES and TGA. During MSR, traces of Cu2O are already
present at the beginning at 120 ◦C (approximately 0.8 wt%, see Fig. 11
Panels A and B). Above 170 ◦C, Cu◦ is continuously oxidized to Cu2O,
In CmZ2, only 0.5 wt% of Cu2O is detected as the only copper phase
after calcination (see SI Table 1 and Fig. S5). The remaining fraction
must be present in an X-ray amorphous state or in phases below the
technique’s detection limit, since both ICP-OES and TGA measurements
confirmed the copper loading (2 wt%). The remaining Cu could be
present in the form of an amorphous ternary oxide similar to the one in
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K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Table 4
Atomic surface ratio of Cu and Zr according to XPS after calcination and after MSR and a comparison of differently obtained SACu values.
CmZ02
CmZ2
CmZ20
CmZ40
CmZ80
After calcination
After MSR350
0.2
0.6
1.0
1.1
6.2
Atomic Cu/Zr ratio
0.1
0.7
0.5
0.8
3.5
SACu(N2O)*1 / m2
SACu(XPS)*2 / m2
g
0.18
0.5
0.23
1.5
0.24
2.0
0.35
1.0
0.41
0.9
ꢀ 1
Cat
Comparison of Cu surface area values
*1
ꢀ 1
g
Cat
the presence of metallic copper is a prerequisite, hence, the samples were pre-reduced in H2 at 300 ◦C.
*2
the BET surface area and the Cu/Zr atomic surface ratio were obtained in the calcined state, which explains why they are consistently larger than the corre-
sponding SACu(N2O) values.
Table 5
Summary of the TGA results.
Sample
name
Copper
Reduction
signal
Re-oxidation
signal
Re-oxidation
signal area /
%*
loading from
TGA (reduced
state) / wt%
temperature /
temperature /
◦C
◦C*
a
a
a
a
CmZ02
CmZ2
-
-
-
-
1.8
19
38
225
245
265
160, 290
170, 245, 365
170, 225, 345,
435
44, 56
CmZ20
CmZ40
31, 25, 44
25, 15, 42,
18
CmZ80
CuO
79
99
255
300
180, 245, 360,
500
30, 3, 43, 24
235, 355, 430,
535
22, 22, 8, 48
a
*
not detectable with TGA.
Experiments discussed and shown in the SI, Section D.
transformed to the metallic state.
Similar to CmZ40, CmZ80 exhibits a mixture of 9.9 wt% Cu, 44.7 wt
% Cu2O, 27.6 wt% CuO and 17.8 wt% m-ZrO2. After pre-oxidation and
pre-reduction, the sample is composed of 80.5 wt% metallic Cu and 19.5
wt% m-ZrO2. Intermediate formation of Cu2O (up to 0.8 wt% at 190 ◦C,
Fig. 12 Panels A and B) occurs during MSR. Cu2O decomposes at 240 ◦C,
coinciding with the region of close to 100 % CO2 selectivity and metallic
Cu remains stable during the isothermal period (Fig. 12 Panels A and C).
Several points that distinguish the phase evolution of the samples
during MSR operation are important. No crystalline copper phases are
visible in CmZ02, although Cu is present in the sample as revealed by
other characterization techniques such as ICP, TEM-EDX and XPS. CmZ2
and CmZ20 behave similarly and do not feature formation of crystalline
Cu2O or CuO. A stable concentration of metallic copper is observable
during MSR. Both CmZ40 and CmZ80 display intermediary formation of
Cu2O during MSR operation. The selective intermediary oxidation of Cu◦
to Cu+ in both CmZ40 and CmZ80 has already been correlated with an
enhanced methanol reforming performance of Cu-based catalysts.
Oguchi et al. observed the formation of Cu2O in the MSR mixture at 250
◦C on a Cu/ZrO2 (80 wt% CuO) catalyst, displaying a superior perfor-
mance compared to an analogous Cu/CeO2 system [13,18]. Agrell et al.
observed the formation of a stable Cu+ species on a pre-reduced Cu/Z-
nO/ZrO2/Al2O3 during preferential oxidation of methanol [7]. Rai-
mondi et al. [17] inferred a temperature dependency of the Cu oxidation
state in POM. Cu+ species were detected after exposure to the reaction
Fig. 10. DTG profiles of the TPR treatment in 10 vol% H2 up to 300 ◦C. The five
samples, denoted by their nominal copper loading, including the two pure
reference materials, m-ZrO2 and CuO, are depicted. The inset in the CuO panel
shows the full profile including the isothermal and the cooling period, where
the two red lines separate the three regions of the temperature cycle (heating,
isothermal, cooling).
◦
◦
mixture below 237 C and re-formation of metallic Cu above 257 C.
This upper temperature limit resembles the re-reduction temperature to
the metallic Cu phase in CmZ80.
We also focused upon the associated structural changes involving
lattice parameter or microstrain variation during reduction and/or MSR
operation for all samples and the CmZ80 sample as the catalytically most
promising one in particular. After pre-reduction and at different condi-
tions during MSR operation, the lattice parameters of ZrO2 and metallic
Cu monitored by in situ XRD only feature the expected thermal lattice
expansion during heating, but no strong change in lattice parameter,
crystallite size or microstrain for any Cu loading is observed. We might
therefore infer that albeit the dimensions of the Cu/ZrO2 interface vary
reaching its maximum at the beginning of the isothermal period with a
weight fraction of 27 wt%. Only traces of Cu2O remain during the
isothermal period (≈ 0.3 wt%, Fig. 11 Panels A and C) due to the
increased reduction potential of the MSR reaction mixture at higher
methanol conversions at high temperatures. The rest of Cu is re-
11

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Fig. 11. Panel A: In situ X-ray diffraction (XRD)
measurement of CmZ40 in the MSR mixture
from 120 ◦C - 350 ◦C at 25 keV beam energy (λ
= 0.4982 Å). The temperature evolution is
color-coded with the heating period as
a
gradient from blue to red with the isothermal
period in black. The references at the bottom of
all depicted in situ XRD graphs were taken from
ICDD database [21] entries and relate the re-
flections to the different structures, namely
m-ZrO2 (PDF card #00-037-1484) [38], Cu (PDF
card #00-004-0836) [39], CuO (PDF card
#00-045-0937) [40] and Cu2O (PDF card
#00-005-0667) [41]. Panels B and C: Composi-
tion of the sample during the heating period and
the isothermal phase according to RR.
strongly as a function of Cu loading, the interface itself exhibits a high
structural stability and a more or less unstrained state irrespective of Cu
loading. As these features remain stable even during reduction or MSR
operation, a predominant and single direct influence on CO2 selectivity
appears unlikely.
microscopy, where the Cu distribution varies from highly dispersed
(CmZ02) over formation of larger particles (CmZ2) and islands (CmZ20,
CmZ40) to wetting and encapsulation of ZrO2 (CmZ80). A decrease of
the Cu/Zr ratio atomic surface ratio after MSR indicates sintering of Cu.
However, a clear correlation of the oxidation state of Cu or the degree of
the decrease of the Cu/Zr ratio at the catalyst surface is not obvious.
TGA investigations indicate a similar reduction behavior for all
catalysts with only slight shifts of the reduction peak temperatures. The
presence of only one reduction peak points to the presence of merely one
Cu (oxide) species, as additional peaks can be ascribed to a separation of
bulk-like Cu and Cu strongly interacting with the support [15,36]. In situ
XRD studies reveal a distinct intermediary bulk oxidation of the Cu
phase during MSR in CmZ40 and CmZ80, while only metallic Cu can be
observed in CmZ2 and CmZ20 (Cu in CmZ02 not visible). The amount of
bulk Cu2O formed is considerably different for the two catalysts, where
up to 27 wt% are visible in CmZ40 in contrast to only 0.8 wt% in CmZ80.
Since CmZ40 and especially CmZ80 exhibit the best performance in
3.7. Discussion of the results
The variation of the copper loading in Cu/m-ZrO2 catalysts from 0.2
wt% to 80 wt% results in a drastically different dispersion of the Cu
phase accompanied by a distinct catalyst morphology. Even though
dissociative N2O adsorption reveals a similar specific copper surface
area for all catalysts, the values for the Cu dispersion range from 18 %
(CmZ02) to 0.09 % (CmZ80). The general validity of these values is
confirmed by estimation of the SACu utilizing the Cu/Zr atomic surface
ratio determined by XPS and the specific BET catalyst surface area. This
directly reflects the morphology as demonstrated by electron
12

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
Fig. 12. Panel A: In situ XRD measurement of CmZ80 in the MSR mixture from 120 ◦C-350 ◦C at 25 keV beam energy (λ = 0.4982 Å). The temperature evolution is
color-coded with the heating period as a gradient from blue to red with the isothermal period in black. The reflections of m-ZrO2, Cu, Cu2O and CuO are marked
below. Panels B and C: Composition of the sample during the heating period and the isothermal phase according to RR.
MSR, the occurrence of Cu2O is correlated with beneficial properties in
MSR as an indicator of a temporary super-saturation of Cu with oxygen,
providing a non-equilibrium state of Cu [11], especially as direct
structural reasons, such as microstrain, can be excluded as discussed
above.
and suppressing selectivity-spoiling support-related effects, leads to
beneficial properties in MSR.
4. Conclusions
The catalytic onset temperatures and the apparent activation en-
ergies reveal significant differences in the performance of the catalysts
during MSR. The onset temperatures of H2 and CO2 continuously
decrease with increasing Cu content accompanied by a continuous in-
crease in the TOFs of H2 and CO2 at 220 ◦C. A similar trend for the onset
temperature can be observed for CO, whereas it increases once more
from 40 wt% to 80 wt% Cu. Combined with the lowest onset and highest
TOF at 220 ◦C for H2 and CO2 and the largest extent of the region of > 90
% CO2 selectivity, CmZ80 is identified as the catalyst with the best
performance. This implies that the morphology of bulk-like Cu covering
and wetting the m-ZrO2 support, providing both an active copper phase
As ZrO2 is a particularly suitable support for Cu to steer the methanol
steam reforming reaction toward almost 100 % CO2 selectivity, we
scrutinized the influence of the Cu-ZrO2 interface- and relative surface
dimensions (resulting from Cu loading variation) both on methanol
steam reforming activity and selectivity and to identify the associated
steering parameters towards high CO2 selectivity.
The variation of Cu metal loading allows steering not only the Cu
particle size and the Cu morphology, but also the contribution of po-
tential active sites at the Cu-ZrO2 interface as well as of the intrinsic
relative contributions of the support and Cu surface fraction. Based on
recent studies, it is clear that ZrO2 is by no means an inactive support
13

K. Ploner et al.
Applied Catalysis A, General 623 (2021) 118279
and can therefore influence both the CO2-selective and selectivity-
spoiling reaction channels of the overall reaction mechanism. Addi-
tionally, the specific ZrO2 surface chemistry is highly preparation-
dependent, rendering it difficult to disentangle largely support-specific
effects from the special phase-boundary reactivity and the intrinsic
Cu◦ reactivity. By choosing the broad range of Cu loadings, a comparison
of the most extreme cases becomes accessible, namely strong predomi-
nance of the intrinsic Cu surface area contributions (almost all Cu sites
within a bulk-like Cu environment, interfacial and support surface site
contributions minimized) vs. almost atomic-scale dispersion of Cu,
representing the opposite scenario. The combination of low onset tem-
peratures and high TOFs of H2 and CO2 as well as a wide temperature
range of CO2 selectivity > 90 % distinguish the Cu/m-ZrO2 catalyst with
the highest Cu loading of 80 wt% Cu from all other catalysts with cor-
responding lower Cu loading. It fulfils the proposed requirements of the
presence of support-wetting bulk-like Cu avoiding adverse support ef-
fects to directionally enhance the CO2 selectivity. This even holds for a
support exhibiting an already relatively low selectivity-spoiling contri-
bution. Hence, we propose that loading variation can be important for
any support to distinguish between selectivity-spoiling and -promoting
cases, which is also important in view of catalyst design toward CO-
depleted reformate gas. As a particular important outcome, we could
exclude a predominant dependence of structural effects (change in lat-
tice parameter or microstrain) on the Cu-ZrO2 interfacial dimension and,
accordingly, the CO2 selectivity. After focussing on the fundamental
issues of the steering parameters for explaining the interfacial Cu-ZrO2
reactivity, future work should be directed at assessing the long-term
stability of the Cu-ZrO2 interfaces to infer potential stabilizing factors
for structural stability. Furthermore, analysis of the XANES and EXAFS
of the samples could provide additional information on the interaction
of Cu and ZrO2 [42,43].
05CH11231, where in situ powder XRD measurements were conducted
at the beamline 12.2.2 in the framework of the AP program ALS-08865.
We particularly thank the group of H. Huppertz from the Department of
General, Inorganic and Theoretical Chemistry of the University of
Innsbruck for providing the possibility of using their X-ray diffractom-
eter for ex situ powder XRD measurements.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118279.
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