Operando Synchrotron X-ray Powder Diffraction and Modulated-Excitation Infrared Spectroscopy Elucidate the CO2Promotion on a Commercial Methanol Synthesis Catalyst

Article abstract of DOI:10.1002/anie.201603204

Optimal amounts of CO₂are added to syngas to boost the methanol synthesis rate on Cu-ZnO-Al₂O₃in the industrial process. The reason for CO₂promotion is not sufficiently understood at the particle level due to the catalyst complexity and the high demands of characterization under true reaction conditions. Herein, we applied operando synchrotron X-ray powder diffraction and modulated-excitation infrared spectroscopy on a commercial catalyst to gain insights into its morphology and surface chemistry. These studies unveiled that Cu and ZnO agglomerate and ZnO particles flatten under CO/H₂and/or CO₂/H₂. Under the optimal CO/CO₂/H₂mixture, sintering is prevented and ZnO crystals adopt an elongated shape due to the minimal presence of the H₂O byproduct, enhancing the water-gas shift activity and thus the methanol production. Our results provide a rationale to the CO₂promotion emphasizing the importance of advanced analytical methods to establish structure–performance relations in heterogeneous catalysis.

Full text of DOI:10.1002/anie.201603204

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603204
German Edition: DOI: 10.1002/ange.201603204
Heterogeneous Catalysis
Operando Synchrotron X-ray Powder Diffraction and Modulated-
Excitation Infrared Spectroscopy Elucidate the CO Promotion on
2
a Commercial Methanol Synthesis Catalyst
Oliver Martin, Cecilia Mondelli, Antonio Cervellino, Davide Ferri, Daniel Curulla-FerrØ, and
Javier PØrez-Ramírez*
[3a]
Abstract: Optimal amounts of CO are added to syngas to
volcano plot shown in Figure 1 (described later). Klier et al.
2
boost the methanol synthesis rate on Cu-ZnO-Al O in the
speculated that an optimal oxidation state of Cu, i.e., partially
oxidized, correlates with the top of the curve. This hypothesis
has never been confirmed for commercial catalysts under
reaction conditions and neglects the essential role of ZnO
2
3
industrial process. The reason for CO2 promotion is not
sufficiently understood at the particle level due to the catalyst
complexity and the high demands of characterization under
true reaction conditions. Herein, we applied operando syn-
chrotron X-ray powder diffraction and modulated-excitation
infrared spectroscopy on a commercial catalyst to gain insights
into its morphology and surface chemistry. These studies
unveiled that Cu and ZnO agglomerate and ZnO particles
flatten under CO/H and/or CO /H . Under the optimal CO/
[3b]
uncovered in the CO promotion.
Multiple studies have
2
revealed that zinc oxide strongly influences the CO hydro-
2
genation, acting as a structural and electronic promoter for
[
5]
Cu. As indicated recently, this reaction appears to be
favored over the reverse water-gas shift (RWGS) on a com-
posite obtained by mechanical mixing of large particles of Cu
and Al O with ZnO in a plate-like shape with respect to the
2
2
2
CO /H mixture, sintering is prevented and ZnO crystals adopt
2
2
2
3
[
5d]
an elongated shape due to the minimal presence of the H O
case of a mixture containing rod-like ZnO crystallites,
2
byproduct, enhancing the water-gas shift activity and thus the
methanol production. Our results provide a rationale to the
CO2 promotion emphasizing the importance of advanced
analytical methods to establish structure–performance rela-
tions in heterogeneous catalysis.
based on the stronger electronic interactions between ZnO
and Cu in the former catalyst. However, the relevance of the
structure–function relationships derived in these works is
limited by the use of model materials and/or ex situ character-
ization. Indeed, it has conclusively been demonstrated that
only in situ and operando methods enable the detection of
dynamic changes in the structure of heterogeneous catalysts
at work which may be linked to the reaction mechanism. As
a first step in this direction, operando X-ray absorption
spectroscopy (XAS), synchrotron X-ray powder diffraction
(SXRPD), and in situ transmission electron microscopy
(TEM) at (sub)ambient pressure have been applied over Cu
supported on ZnO, evidencing that copper forms disc-like
particles wetted by Zn atoms under strongly reducing gas
W
hile methanol (MeOH) synthesis via CO hydrogenation
2
[6]
might become practically relevant in the future thanks to
[
1]
advances in catalyst design, this basic chemical is currently
industrially produced from syngas (CO + 2H QCH OH) over
2
3
a
1
Cu-ZnO-Al O catalyst at elevated pressures (5.0–
0.0 MPa) and temperatures (473–573 K). Small amounts
2 3
[
2]
of CO (ca. 3 vol.%) are added to the syngas leading to a 3–6
2
[3]
times enhancement of the MeOH formation rate. It is
widely accepted that the so-called “CO promotion” is related
mixtures (CO/H ) and spherical particles in the presence of
H O and/or CO . Similar results were also obtained under
2 2
2
2
[
7]
to the transformation of CO into CO via the water-gas shift
2
(
WGS) reaction (CO + H OQCO + H ) and its subsequent
WGS conditions while the effect of the ZnO morphology has
not been assessed even though interactions between the Cu
and ZnO phases were verified. In addition, key intermedi-
2
2
2
[4]
hydrogenation to MeOH (CO + 3H QCH OH + H O).
2
2
3
2
[
8]
The latter is much faster than the hydrogenation of CO but
strongly limited by thermodynamics and product inhibition by
H O at high CO concentrations resulting in the typical
ates of CO hydrogenation could be detected on an industrial
2
Cu-ZnO-Al O catalyst using in situ diffuse reflectance infra-
2
2
2
3
red Fourier transform spectroscopy (DRIFTS) at 3.0 MPa
[
9]
[
*] Dr. O. Martin, Dr. C. Mondelli, Prof. J. PØrez-Ramírez
ETH Zurich, Department of Chemistry and Applied Biosciences,
Institute for Chemical and Bioengineering
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jpr@chem.ethz.ch
and 373–473 K. Still, these analyses have not assessed
structural and mechanistic aspects under industrially relevant
pressures and temperatures to explain the CO promotion for
a multicomponent commercial catalyst.
Consequently, we investigated for the first time a com-
mercial Cu-ZnO-Al O material by operando SXRPD during
hydrogenation of CO, CO , and CO/CO to MeOH at 543 K
2
Dr. A. Cervellino, Dr. D. Ferri
Paul Scherrer Institute
2
3
2
2
5
232 Villigen (Switzerland)
and 5.0 MPa, using the composition leading to the maximal
MeOH formation rate for the mixed syngas feed, to correlate
changes of the particle morphology with the reaction rates.
We derived mass distributions of the Cu and ZnO particles on
the basis of their dimensions applying a size- and shape-
Dr. D. Curulla-FerrØ
Total Research & Technology Feluy
Zone Industrielle Feluy C, 7181 Seneffe (Belgium)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603204.
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[10]
sensitive Debye function analysis (DFA). This investigation
uncovered morphological alterations occurring under CO/H2
and CO /H , which differ from previous results gathered
2
2
under conditions far from the industrial process. The distinct
MeOH yields observed in the CO -promoted regime over the
2
catalyst pretreated in CO/H or CO /H were correlated with
2
2
2
its WGS activity evidenced by operando modulated-excita-
tion DRIFTS (ME-DRIFTS). Under CO/CO /H , specific
2
2
morphologies of Cu and ZnO were detected that facilitate the
WGS reaction, thus identifying the structural reasons for the
CO promotion.
2
The Cu-ZnO-Al O catalyst herein studied was supplied
2
3
in pelletized form. The basic characterization of this material
is summarized in Tables S1 and S2 in the Supporting
Information. The fresh sample features a molar bulk ratio
of Cu:Zn:Al = 6.0:2.6:1.3 (Cu:Zn:Al = 50.6:22.5:4.6 wt.%,
Figure 1. Methanol space–time yield (STY) as a function of the feed
CO concentration (R) over Cu-ZnO-Al O upon step-wise increasing
2
2
3
Figure S1),
a
molar surface composition of Cu:Z-
the CO concentration in a CO/H feed (1, solid circles) and
2
2
n:Al:Mg:Ca = 6.0:5.1:1.3:1.1:0.3 (Figure S2), a Brunauer–
subsequently increasing the CO concentration in a CO /H feed (2,
2
2
2
À1
Emmett–Teller (BET) surface area of 118 m g
(Fig-
cat
open circles), displaying the typical volcano-type and hysteresis
behavior. For the sake of clarity, the error bar is shown for a single
2
À1
ure S3), a copper surface area of 24.0 m gcat , crystalline Cu
and ZnO particles exhibiting an average size of 4.9 and
data point. Conditions: T=543 K, P=5.0 MPa, molar H
:CO =4:1,
x
2
À1
and gas hourly space velocity (GHSV)=15000 h
.
6
.4 nm, respectively, and an amorphous Al O phase. Scan-
2 3
ning electron microscopy-energy dispersive X-ray spectros-
copy (SEM-EDXS) uncovered copper-, zinc-, aluminum-, and
carbon-rich regions, indicating the heterogeneous composi-
tion of the material at the mm scale (Figure S4). The MeOH
space–time yield measured over this catalyst in a fixed-bed
similar reaction rates as previously measured for this cata-
[4d]
lyst.
The SXRPD patterns collected during the catalytic
runs (Figure S6) were matched through atomistic cluster
simulations of the standard crystalline phases of Cu, Zn, and
[4d]
[10a]
reactor set-up described elsewhere exhibits the character-
their oxides. Total scattering methods such as the DFA
istic volcano-like trend upon stepwise replacing CO by CO in
were applied as they enable a more effective detection of
subnanometer-sized particles and of their shapes with respect
2
the feed gas (Figure 1) with a maximum for 2.4 vol.% of CO2.
Aiming at explaining this intriguing and long-debated
behavior, we designed a home-made operando cell to conduct
synchrotron-based XRPD investigations in Debye–Scherrer
geometry under the same reaction
[10b,c,d]
to classical approaches like profile analysis,
which is
given in Table S3 for comparison. The diffractograms
unveiled that the copper particles did not change their
conditions in order to follow the
changes of the crystalline Cu and
ZnO phases at the extremes of the
volcano (CO/H2 and CO /H ) and
2
2
under the optimal gas mixture (CO/
CO /H ). The methodology of this
2
2
structural analysis is shown and
explained in Figure 2. Briefly,
a fused-silica capillary fixed-bed reac-
tor was housed in a heating block
enabling the detection of the dif-
fracted beams and connected to a gas
feeding system. The composition of
the gas at the reactor outlet was
analyzed by an online mass spectrom-
eter. The catalyst was initially exposed
to CO/H or CO /H (Figure 3a, top
2
2
2
and middle, respectively) at 5.0 MPa,
À1
5
43 K, and 15000 h . The final
Figure 2. Operando SXRPD experiment at the synchrotron beamline. Diffraction patterns (bottom
left) are collected by shining the X-ray beam onto a capillary fixed-bed reactor (top left) where the
reaction takes place over Cu-ZnO-Al O under industrial conditions of temperature, pressure, and
MeOH concentration upon CO
hydrogenation was ca. 50% of that
2
3
during CO hydrogenation (Figure 3a,
2
space velocity, while the reactor outlet gas mixture is analyzed online by a mass spectrometer
top right). The measured SXRPD data (blue) are processed (red) by applying a Debye function
top and middle, respectively). The
concentrations of the gas components
in the operando studies indicated
(
analysis to derive the mass distributions of the Cu and ZnO particles (bottom right) based on
their shape and size. A photograph of the set-up is shown in Figure S5.
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In spite of the unaltered morphol-
ogy, the average size of Cu particles
increased from 4.9 (Table S1) to 6.0
and 7.1 nm under CO/H and CO /H ,
2
2
2
respectively (Figure 3b, top and
middle). In the former case, sintering
was gradual (Figure S6b) and likely
favored by the segregation of Cu from
ZnO, indicated by energy-dispersive
X-ray spectroscopy (EDXS) (Figur-
es 3d, top, and S8), while it was rapid
(Figure S6d) and possibly induced by
the presence of the H O byproduct in
2
the latter environment (Figure 3a,
[
2]
middle). Since CO hydrogenation is
2
the main reaction pathway to MeOH
[4a–d]
from mixed syngas,
we evaluated
how the different sizes of the Cu
particles induced by CO /H and CO/
2
2
H affect the activity of this reaction
2
(Figures 4a and c, respectively).
MeOH was formed over the catalyst
pretreated in CO/H in a moderately
2
larger amount than over a sample
exposed to CO /H2 (2.55 vs.
2
2
.40 vol.%, respectively). In both
tests, the Cu particles experienced
a further growth, but they remained
smaller (6.7 vs. 7.8 nm) in the sample
Figure 3. a) Temporal evolution of the CO, MeOH, and H O concentrations during the hydro-
2
genation of CO (top), CO (middle), and CO+CO (optimal mixture, bottom). For the sake of
2
2
pretreated in CO/H . These observa-
2
clarity, error bars are shown for a single data point in each graph. Mass distributions of b) Cu
particles of a given diameter (d) and of c) ZnO particles of a given base diameter (b) and prism
height (h) and d) EDXS mapping of Cu and Zn for the catalyst at the end of each catalytic run.
The insets in (b,c) depict models of the particles calculated based on the average dimensions.
tions are in line with the slightly higher
CO2 hydrogenation turnover fre-
quency over Cu/ZnO catalysts featur-
ing a higher Cu dispersion (d = 5–
[12]
cuboctahedral morphology upon exposure to the different gas
compositions. Note that, in view of the high sensitivity of the
synchrotron XRPD methodology, changes in the shape of the
crystallites would have resulted in clearly detectable aniso-
tropic broadening of the reflections. We further confirmed the
preservation of the cubic symmetry of the Cu phase by
conducting modulated-excitation SXRPD (ME-SXRPD)
experiments (Figure S7), i.e., alternating the feed between
CO/Ar and CO /Ar or CO/H and CO /H under the above-
36 nm, determined ex situ). A minor loss in crystallinity
of the Cu phase was detected in ME-SXRPD upon exchang-
ing CO by CO in H or Ar (Figure S7b and c, respectively),
2
2
pointing to the surface oxidation of Cu, which was also
observed by Auger electron spectroscopy (Figure S2c).
However, the reversible nature of this change does not
explain the different catalyst activity after the distinct
pretreatments discussed above.
In addition, the mass distributions of the ZnO particles
were calculated from the SXRPD patterns revealing a hex-
agonal prism-like morphology. The average size (6.4 nm) did
2
2
2
2
mentioned reaction conditions every 29 min. Such technique
produces difference patterns with a higher signal-to-noise
ratio, thus enabling the detection of subtle modifications of
the crystallites, which may include surface changes in spite of
the general bulk nature of XRPD. The retention of the shape
of the Cu particles is in striking contrast to previous reports on
an impregnated model Cu/ZnO catalyst, which claimed
a flattening of the Cu crystallites in CO-containing atmos-
not change under the CO/H atmosphere (Figure 3c, top), but
2
largely increased (9.4 nm) upon CO2 hydrogenation. A
preferential growth of the hexagonal base diameter was
determined, resulting in flatter ZnO crystallites (Figure 3c,
middle). Likely due to the poor contrast between Cu and
ZnO, such structural alterations were not visualized by
transmission electron microscopy (TEM) of the used materi-
[
6]
pheres. The origin of this discrepancy likely is the 10-times
lower Cu loading of that material with respect to our
commercial bulk system and the application of (sub)ambient
pressure during its analysis by SXRPD, XAS, and TEM. On
the other hand, density functional theory (DFT) calculations
exploring the impact of pressure on the morphology of
[5d]
als (Figure S9). Based on the work by Tsang et al.
on
mechanically-mixed Cu/ZnO/Al O catalysts, it was expected
2
3
that the different proportion between the base and the height
of the prisms of ZnO would impact the CO hydrogenation
2
rate since the selectivity to MeOH from CO /H was higher
2
2
[11]
supported metal nanoparticles
shape evidenced at 5.0 MPa.
corroborate the spherical
over plate-like rather than rod-like particles. That difference
was explained by an improved electronic interaction with Cu
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featured signals attributed to CO ad-
sorbed on low-index (e.g., (111), 2058
À1
and 2077 cm ) and high-index faces of
0
À1
Cu (e.g., (755), 2092 cm ), as well as
+
À1 [9]
on Cu sites (2129 cm ) irrespective
of the pretreatment. Since the RWGS
reaction predominantly occurs over the
low-index Cu planes, the Cu particle
size may not have a significant impact
on the reaction rate. Still, CO adsorbed
À1
on Cu sites next to ZnO (2058 cm )
was identified in both experiments,
which hints to the sensitivity of this
reaction to the interaction of Cu with
the ZnO particles and, in turn, the size
and shape of the latter. This evidence
correlates with the enhancement of the
CO hydrogenation and RWGS over
2
Cu29 clusters on polar ZnO surfaces
[14]
compared to pure Cu.
Additional
weak features were detected in both
ME-DRIFTS experiments (Figur-
es S10b and d). Bands at 1932 and
À1
1
910 cm have been associated with
CO adsorbed on CuZn alloys, which
x
some studies considered as the active
[5e,g,7]
sites in CO hydrogenation.
The
2
changes of the ZnO and Cu reflections
in the ME-SXRPD experiment (Fig-
ure S7b) could be compatible with the
reversible formation of such species in
line with their observation on the sur-
face of lab-prepared Cu-ZnO-Al O by
Figure 4. a,c) Temporal evolution of the CO and MeOH concentrations during hydrogenation of
CO and CO+CO (optimal mixture) over Cu-ZnO-Al O pretreated for 4 h in a) CO /H and c)
2
2
2
3
2
2
CO/H . For the sake of clarity, error bars are shown for a single data point in each graph. b,d)
2
Phase-resolved DRIFT spectra obtained from a modulated-excitation experiment in which pulses
(
t=20 min) of H and CO /H were alternatively admitted to the catalyst after a 4-h pretreatment
2 2 2
2
3
in b) CO /H and d) CO/H . Arrows in (b,d) mark the bands of gaseous CO and CO adsorbed on
[5g]
2
2
2
in situ Auger electron spectroscopy.
distinct surface sites. The corresponding MS signals are depicted in Figures S10e and f.
However, their contribution to the
RWGS reaction appears to be minor
in view of the much stronger intensity
Conditions for the DRIFTS experiments: T=373 K, P=5.0 MPa, molar H :CO =4:1, and
2
2
À1
GHSV=30000 h
.
in the former case, originating from an enhanced exhibition of
polar facets (e.g., (0001)). Although our selectivity patterns
of the infrared signals of CO adsorbed on the other Cu sites.
Bands of gaseous CO were absent for the sample pretreated
in CO /H , evidencing its hindered desorption when platelet-
upon CO2 hydrogenation after the CO /H2 and CO/H2
2
2
2
pretreatments (Figures 4a,c) agree well with Ref. [5d], the
difference in MeOH concentration was rather low in our
study (2.55 vs. 2.40 vol.%). Consequently, we propose that the
concomitant RWGS reaction is more sensitive to the ZnO
morphology, corroborating the strong inhibiting effect of
like structures of ZnO are formed (Figure 4b). In contrast,
prominent and modulating signals of gaseous CO were
detected in the measurement conducted after the CO/H2
pretreatment (Figure 4d), supporting the higher RWGS
activity of a sample in which Cu likely interacts more with
non-polar facets of ZnO (e.g., (1À100), Figure 4c). We
propose that, according to the principle of microscopic
[4d,13]
ZnO covering the Cu phase for this reaction
and the
much higher concentration of CO measured after the CO/H2
pretreatment (2.81 vs. 1.97 vol.%).
[15]
reversibility,
the WGS activity relates to the particle
In order to correlate the alterations at the particle level
with the surface chemistry, we conducted operando ME-
structure similarly to the RWGS activity. This emphasizes
the importance of the derived correlations for CO -promoted
2
DRIFTS investigations upon CO hydrogenation after pre-
MeOH synthesis explaining why Cu-ZnO-Al O3 catalysts
2
2
treatment of the catalyst in CO /H and CO/H (Figures 4b,d,
and S10), in analogy to the SXRPD experiments. The gas feed
with large zinc contents do not catalyze the WGS and, hence,
2
2
2
[4d]
do not feature CO promotion. In line with the catalytic
2
composition was exchanged every 20 min between CO /H2
data, the pretreatment does not affect the mechanism of CO2
hydrogenation (Figures S10a–d). In this respect, the phase-
resolved analysis (Figures S10b and d) uncovered two
important aspects. Firstly, the unperturbed intensity of
formate bands during the modulation suggests that such
species are spectators rather than reaction intermediates, in
2
and H at 373 K and 5.0 MPa. The lower reaction temperature
2
was selected to prevent an excessive CO production, since
intense bands of gaseous CO would mask the signals related
to adsorbed CO species (Figure S11). As described in greater
detail in the Supporting Information, the spectra obtained
1
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[
5e,9,16]
contrast to recent kinetic models
but in agreement with
observed under CO /H . Since CO hydrogenation was only
2 2 2
[17]
studies proposing a carboxyl (COOH*)-based route. Sec-
ondly, the detection of bands assigned to methoxy species on
Cu and ZnO sites emphasizes the necessity to consider both
catalyst components in future molecular studies.
slightly improved over smaller Cu particles and was insensi-
tive to the morphology of ZnO, we deduced that this reaction
cannot become a limiting step for the promotional effect. In
the CO/CO /H mixture with the optimal CO concentration,
2
2
2
In order to unravel the structure of Cu and ZnO in the
promotional regime, the catalyst was exposed to mixed syngas
the low amount of H O formed prevented Cu and ZnO
2
sintering. Moreover, the ZnO prism-like crystallites prefer-
entially elongated along their hexagonal axis. ME-DRIFTS
during reaction evidenced a strong suppression of the RWGS
in catalyst samples containing platelet-like ZnO particles. The
higher amount of polar facets in these structures leads to
enhanced electronic interactions with Cu, which inhibit the
active sites on the latter. In contrast, the preferred elongation
of ZnO under the optimal gas mixture augmented the fraction
of non-polar facets exposed, boosting the WGS reaction and,
(
CO/CO /H ) with the optimal CO content (R = CO /(CO +
2 2 2 2
CO ) = 12%, 2.4 vol.% CO , Figure 1). The final MeOH
2
2
concentration in this experiment (Figure 3a, bottom) was ca.
.6-times larger than the sum of the MeOH concentrations
1
expected for the amounts of CO and CO in the feed, in line
2
with the activity enhancement observed in Figure 1. Under
this condition, the Cu and ZnO particles substantially
retained their original size (5.3 and 6.4 nm, respectively,
Figure 3b and c, bottom) and relative distribution (Figure 3d,
thus, the MeOH formation from CO . These findings ration-
2
bottom). It is worth noting that the H O produced via CO2
alize the promotional effect and indicate that the best catalyst
2
hydrogenation was almost fully consumed by the WGS
reaction (Figure 3a, bottom), which might account for the
minimized sintering of Cu and ZnO. Nevertheless, a slight
elongation of the ZnO crystallites along the (0001) axis
became evident. To shed light on the impact of the ZnO
morphology on the CO promotion, we applied CO/CO /H
structure for CO -promoted MeOH synthesis comprises small
Cu particles in contact with rod-like ZnO crystallites,
preferably stabilized by structural promoters.
2
2
2
2
2
Experimental Section
gas mixtures over samples pretreated in CO /H or CO/H
2
2
The Cu-ZnO-Al O catalyst was purchased in pelletized form
2
3
(
Figures 4a,c, respectively) and thus featuring different ZnO
particles, as described above. The extent of promotion was ca.
0% lower after the former treatment, in line with the
(5.5 mm in diameter and 3.6 mm in height) from Alfa Aesar (product
No. 45776). The bulk and surface composition, the copper surface
area, and the porosity and structure of the fresh catalyst were studied
by X-ray fluorescence spectroscopy, X-ray photoelectron spectrosco-
2
hysteresis behavior shown in Figure 1, i.e., the promotional
effect of CO is still present but less pronounced when starting
from a CO /H feed with respect to starting from a CO/H
2
feed. Ex situ characterization has excluded that this effect is
associated with an irreversible deactivation of the catalyst,
e.g., continuous sintering upon the cycle. Interestingly, the
CO hydrogenation rate was similar after the pretreatments in
CO /H or CO/H (Figure 4a,c, respectively), substantiating
that this reaction is not the limiting step under the promo-
py, temperature-programmed reduction with H after oxidation with
2
2
N O, N sorption at 77 K, and scanning (transmission) electron
2
2
2
2
microscopy-energy dispersive X-ray spectroscopy. Operando syn-
chrotron X-ray powder diffraction (SXRPD) was carried out at the
X04SA-MS beamline of the Swiss Light Source (SLS) synchrotron of
[3b]
[18]
the Paul Scherrer Institute (PSI), Switzerland.
The undiluted
catalyst (Wcat = 50 mg, particle size = 75–150 mm) was loaded into
2
a custom-made capillary cell connected to a gas feeding system and
2
2
2
3
a mass spectrometer, activated in a gas flow (F = 16.5 cm STP
total
À1
À1
min ) of 5 vol.% H in Ar at 5.0 MPa and 543 K (2 Kmin ) for
2
tional regime. Additionally, it is evident that CO promotion
2
30 min, and then exposed to a gas feed with a molar composition of
was suppressed to a comparable extent to the RWGS reaction
in samples with platelet-like ZnO crystallites (Figure 4a).
Previous DFT studies on a Cu surface did not indicate optimal
turnover frequencies nor coverages of intermediates under
H :CO:Ar (coded “CO/H ”) or H :CO :Ar (coded “CO /H ”) =
2
2
2
2
2
2
4:1:1.5 or H :CO:CO :Ar (coded “CO/CO /H ”) = 4:0.88:0.12:1.5
2
2
2
2
for 4 h. In certain experiments, the catalyst was additionally contacted
with CO /H or CO/CO /H for 4 h after activation and exposure to
2
2
2
2
[16]
CO/H or CO /H . In modulated-excitation SXRPD (ME-SXRPD)
2 2 2
mixed syngas conditions. We therefore explain the top of
the cycle (Figure 1) mainly as a result of the depleted contacts
between Cu particles and polar ZnO facets due to the
elongation of the latter along the hexagonal prism axis (0001)
under the optimal CO/CO /H mixture boosting the WGS and
studies, the activated catalyst was subjected to 10 cycles of 57 min
during which the gas feed was alternated between two compositions.
The beam energy was set at 22 keV for optimal penetration.
Diffraction patterns were collected in the 2–1208 2q range over 43.5
(ME-SXRPD) or 60 s (SXRPD) with a resolution of 0.00368 2q with
the aid of the position-sensitive single-photon counting MYTHEN II
2
2
consequently the CO promotion.
2
[19]
detector.
Measured diffraction patterns were background sub-
In conclusion, our study established correlations between
the Cu and ZnO particle size and morphology, the reaction
[9]
tracted and fitted by the Debye function analysis method deriving
mass fractions on the basis of the dimensions of each crystalline
phase. The patterns of the known structures of Cu, Zn, and their
oxides were sufficient to match all diffractograms acquired in this
study, since the residual patterns were flat and stochastic (except for
texture artefacts). Operando modulated-excitation diffuse reflec-
tance infrared Fourier transform spectroscopy (ME-DRIFTS) inves-
tigations were performed using the same reactor set-up employed for
the SXRPD analysis but replacing the capillary cell by a high-pressure
diffuse reflectance reaction chamber (Harrick) placed into a Bruker
mechanism, and the activity of a commercial Cu-ZnO-Al O3
2
catalyst upon methanol synthesis under various gas compo-
sitions and industrial conditions. Operando SXRPD eluci-
dated that Cu particles retained their cuboctahedral shape but
sintered under CO/H and CO /H . This phenomenon was
2
2
2
associated with the segregation of Cu from ZnO upon CO
hydrogenation and with the presence of H O upon CO2
2
hydrogenation. With respect to the hexagonal ZnO particles,
Equinox 55 spectrometer equipped with a liquid-N -cooled MCT
2
a CO/H atmosphere did not provoke significant alterations
2
detector. The activated catalyst (Wcat = 25 mg, 1:1 dilution with Si
(Acros Organics, 99%), particle size = 75–150 mm) was pretreated in
but growth and formation of platelet-like morphologies were
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CO/H or CO /H at 543 K and 5.0 MPa for 4 h and then cooled to
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2
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73 K in an Ar flow. After complete purging, the gas flow was
alternated between molar H :Ar= 4:2.5 (1200 s) and H :CO :Ar=
2
2
2
4
6
:1:1.5 (1200 s) 10 times. Time-resolved spectra were acquired every
0 s accumulating 200 scans in the 4000–600 cm range (4 cm
À1
À1
resolution) and processed by a reported procedure to derive phase-
[20]
resolved spectra. Further details to these procedures are provided
in the Supporting Information.
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Received: April 1, 2016
Revised: June 22, 2016
Published online: July 7, 2016
1
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