Methanol synthesis: Via CO2 hydrogenation over a Au/ZnO catalyst: An isotope labelling study on the role of CO in the reaction process

Article abstract of DOI:10.1039/c5cp06888f

Methanol synthesis for chemical energy storage, via hydrogenation of CO₂ with H₂ produced by renewable energies, is usually accompanied by the undesired formation of CO via the reverse water-gas shift reaction. Aiming at a better mechanistic understanding of methanol formation from CO₂/H₂ on highly selective supported Au/ZnO catalysts we have investigated the role of CO in the reaction process using isotope labelling experiments. Using ¹³C-labelled CO₂, we found for reaction at 5 bar and 240 °C that (i) the methanol formation rate is significantly higher in CO₂-containing gas mixtures than in a CO₂-free mixture and (ii) in mixtures containing both CO₂ and CO methanol formation from CO increases with the CO content up to 1% CO, and then remains at 20% of the total methanol formation up to a CO₂/CO ratio of 1/1, making CO₂ the preferred carbon source in these mixtures. A shift in the preferred carbon source for MeOH from CO₂ towards CO is observed with increasing reaction temperatures between 240 °C and 300 °C. At even higher temperatures CO is expected to become the dominant carbon source. The consequences of these findings for the application of Au/ZnO catalysts for chemical storage of renewable energies are discussed. the Owner Societies.

Full text of DOI:10.1039/c5cp06888f

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Methanol synthesis via CO hydrogenation over a
2
Au/ZnO catalyst: an isotope labelling study on the
Cite this:DOI: 10.1039/c5cp06888f
role of CO in the reaction process†
Yeusy Hartadi, Daniel Widmann and R. Jurgen Behm*
¨
2 2
Methanol synthesis for chemical energy storage, via hydrogenation of CO with H produced by
renewable energies, is usually accompanied by the undesired formation of CO via the reverse water–gas
shift reaction. Aiming at a better mechanistic understanding of methanol formation from CO /H on
highly selective supported Au/ZnO catalysts we have investigated the role of CO in the reaction process
2
2
1
3
using isotope labelling experiments. Using C-labelled CO
that (i) the methanol formation rate is significantly higher in CO
CO -free mixture and (ii) in mixtures containing both CO and CO methanol formation from CO
increases with the CO content up to 1% CO, and then remains at 20% of the total methanol formation
up to a CO /CO ratio of 1/1, making CO the preferred carbon source in these mixtures. A shift in the
preferred carbon source for MeOH from CO towards CO is observed with increasing reaction temperatures
2
, we found for reaction at 5 bar and 240 1C
2
-containing gas mixtures than in a
2
2
Received 10th November 2015,
Accepted 15th February 2016
2
2
2
between 240 1C and 300 1C. At even higher temperatures CO is expected to become the dominant carbon
source. The consequences of these findings for the application of Au/ZnO catalysts for chemical storage of
renewable energies are discussed.
DOI: 10.1039/c5cp06888f
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optimized to a very high performance level under industrial
relevant reaction conditions (at elevated temperatures and
1
. Introduction
A promising approach in renewable energy concepts to overcome pressures) because of the enormous importance of industrial
1
9–28
natural fluctuations in the supply of renewable energy involves the MeOH production.
Accordingly, one may assume that it is
conversion and storage of excess electrical energy in the form of also a reasonable, highly active candidate for CO hydrogenation
2
1
,2
chemical energy. Methanol (MeOH) as a storage molecule is of for the above described application. MeOH formation from CO2,
3,4
particular interest, since it can be easily stored and transported.
It can be used as a source of CO
however, is usually accompanied by the undesirable formation
2
-neutral synthetic fuels or as of CO via the reverse water-gas shift (RWGS) reaction.
feedstock for synthesis of a variety of basic materials. At present Thus, besides high MeOH formation rates, the selectivity
5
16,29–33
4,6
MeOH is mainly produced from syngas containing CO, CO
2
and towards MeOH formation represents another crucial factor that
6
H
2
, which is obtained from catalytic reforming of fossil fuels.
has to be considered. So far only a very limited number of
However, by replacing fossil fuels as the carbon source with catalytic systems is known, which show no or almost no activity
7
–11
anthropogenic CO from industrial exhaust or coal power plants,
for the CO formation in CO /H reaction gas mixtures for MeOH
2 2
2
12–17
34–36
for example, so-called ‘‘green MeOH’’ can be produced.
This way formation, for example, differently promoted Cu-based catalysts,
Technically this Au and Ag supported on 3ZnOꢀZrO , carbon nanotubes supported
7–17
35
also the overall CO emission can be diminished.
2
2
37 38
process was applied first in the ‘Emission-to-Liquid’ (ETL) technol- Pd–ZnO catalysts, or a pre-reduced LaCr0.5Cu0.5
ogy developed by Carbon Recycling International (CRI), which since Previous studies on commercial Cu-based catalysts have shown
011 has been operating the commercial George Olah Renewable that these are active for the reverse water-gas shift (RWGS)
Methanol Plant with a production capacity of 5 million liters per reaction between 230 and 260 1C, resulting in the formation of
3
O catalyst.
2
13,18
year.
rather large amounts of CO in addition to MeOH and, hence,
3
1,32,39–42
In the industrial MeOH production from CO -enriched syngas rather low selectivities towards MeOH.
typically Cu/ZnO + MeO catalysts are employed. These have been testing the activity and selectivity of two commercial Cu/ZnO/
For example,
2
6
x
Al O catalyst samples at total pressures between 5 and 50 bar
2
3
(
2 2
CO /H = 1/3), we recently obtained selectivities towards MeOH
Institute of Surface Chemistry and Catalysis, Ulm University,
31
of only 16% and 42%, respectively. Some supported Au catalysts,
Albert-Einstein-Allee 47, D-89081 Ulm, Germany. E-mail: juergen.behm@uni-ulm.de
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp06888f on the other hand, are significantly more selective compared to the
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commercial Cu-based catalysts under identical reaction condi- In that case CO does not represent a reaction intermediate in
3
1,32
31,32
tions.
This is particularly true for Au/ZnO.
2
While their the CO hydrogenation, but a byproduct in the direct hydro-
noble metal mass-normalized activity for MeOH is comparable genation of CO , and both, CO hydrogenation and CO hydro-
2
2
to that of Cu catalysts, their selectivity is much higher, about genation, proceed parallel to each other under the reaction
3
1,32
5
0% at 5 bar and almost 70% at 50 bar.
comparison of the catalytic performance (activities and selectivities) CO
over commercial Cu- and Au-based catalysts is given in ref. 31 and mixtures. A definite proof for this proposal, however, was not
2. At present, the much higher price of gold compared to that of possible in that study, since the individual contributions of CO
Cu renders CO hydrogenation to MeOH on supported Au catalysts and CO to the MeOH formation from a CO /CO/H mixture
economically not competitive, but these previous findings clearly could not be determined.
demonstrated their promising potential for certain applications, This is the topic of the present study, in which we explored
e.g., for small scale operation under dynamic operation conditions, the activities of a Au/ZnO catalyst for CO and CO hydrogena-
A more detailed conditions used in that study. Furthermore, it seems likely that
2
is the main carbon source for MeOH in CO /CO/H
2
2
3
2
2
2
2
2
3
1
as expected in renewable energy concepts. They are also very tion in a gas mixture containing both components, CO and
2
interesting from a scientific point of view, to understand the CO, together with H
origin of the high activity and selectivity of Au/ZnO catalysts for well-defined amounts of CO (between 0.5 and 15%) to a
2
by isotope labelling techniques. By adding
1
2
1
3
2 2 2 2
CO hydrogenation to MeOH. Here it is worth mentioning that reaction gas mixture containing 15% CO (CO /H = 1/3)
for both Au- and Cu-based catalysts the presence of Zn species and following the corresponding changes in the amounts of
1
3
12
seems to be mandatory in order to achieve high MeOH formation
rates as well as selectivities.
CH
3 3
OH and CH OH formation, we could quantify the
1
5,31,43
13
12
This suggests that Au/ZnO individual contributions of CO and CO to the total amount
2
and Cu/ZnO benefit from the same effect, namely the partial of MeOH formed. Besides, one can also derive insights into the
reduction of ZnO under reaction conditions in the highly reaction pathway for CO2 and CO hydrogenation from such
reductive atmosphere and subsequent migration of ZnO surface studies, in particular whether CO is directly hydrogenated to
x 2
species on the Au or Cu nanoparticles, resulting either in the MeOH or whether it is first converted to CO, which is subsequently
formation of a AuZn or CuZn (surface) alloy, or the formation of hydrogenated.
1
5,43–47
a ZnO
x
shell.
While for Cu-based catalysts the formation
In contrast to Cu-based catalysts, where a number of pre-
of such species and their contribution in the reaction mechanism vious studies presented ample evidence that the main carbon
of CO hydrogenation and CO hydrogenation to MeOH was already source for MeOH from synthesis gas containing both CO and
2
2
1
5,43–47
48,49
demonstrated,
similar information on Au/ZnO catalysts is CO is CO , e.g., by kinetic investigations,
scarce so far. Nevertheless, it may be envisioned that such effects measurements
isotope labelling
and theoretical calculations, direct
2
3
1
45,48,50–52
29
are also present on Au surfaces, which can thus explain the information to resolve this question for Au catalysts is not yet
superior catalytic performance of Au/ZnO for the MeOH formation available.
from CO
2
and H
2
as compared to other metal oxide supported Au
catalysts. Interestingly, Martin et al. just recently demonstrated physical properties of the commercial Au/ZnO catalyst used in
the promotion of Cu–ZnO–Al catalysts for the MeOH formation this study (Section 3.1), present the results of the kinetic
by addition of gold, which they explained by a stabilization of isotope labelling measurements using C labelled CO2 and
In the following, we will, after a short summary of the
3
1
2 3
O
1
3
0
36
12
metallic Cu species in close vicinity to ZnO .
CO (Section 3.2). The influence of the reaction temperature,
x
Although Au/ZnO catalysts predominantly produce MeOH, there upon increasing it from 240 1C to 300 1C for a fixed ratio of
1
3
12
will always be significant amounts of CO present in the reaction
2
CO / CO, is elucidated and discussed in Section 3.3. In
atmosphere because of its formation via the reverse water-gas shift addition to mechanistic insights we finally discuss the consequences
reaction. In particular this is the case at the end of the catalyst bed in of these findings for potential applications of Au/ZnO catalysts
practical applications, which aim at high CO conversions. Hence, for the chemical storage of renewable energies.
2
there will be an increasing CO partial pressure along the catalyst bed.
In order to minimize any detrimental effects of CO on the main
reaction, a detailed understanding of its influence is essential.
Recently we reported on the influence of CO on the MeOH formation
characteristics in the presence of low and high CO partial pressures.
2
. Experimental
2.1. Preparation and characterization of Au/ZnO
Based on the significantly higher activities for MeOH formation from For all measurements we used a commercially available Au/ZnO
pure CO /H mixtures than obtained upon addition of CO to the catalyst prepared by deposition precipitation (DP) from STREM
reaction gas feed, we proposed that CO is not converted to CO first, Chemicals. Prior to the kinetic measurements, the catalyst was
via the RWGS reaction (eqn (1)), which is subsequently hydrogenated calcined in situ in a flow of 20 Nml min of 1% O
2
2
2
ꢁ
1
2
in Ar at
32
to MeOH (eqn (2)), but is directly hydrogenated to MeOH (eqn (3)).
400 1C and atmospheric pressure for 1 h (denoted O400).
Subsequently, the catalyst was cooled to the reaction temperature
CO + H - CO + H O
(1)
(2)
(3)
2
2
2
(240 1C) in a flow of Ar. The Au loading, the Au particle size and the
surface area of the Au/ZnO catalyst after O400 were measured by
inductively coupled plasma optical emission spectroscopy (ICP-OES,
Horiba Jovin Yvon Ultima 2), transmission electron microscopy
CO + 2H
2
- CH
3
OH
CO
2
+ 3H - CH OH + H O
2
3 2
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TEM, JEOL 1400) and low temperature N
Paper
(
2
adsorption (BET, the Au dispersion (DAu, obtained from TEM imaging) following
Quantachrome Quadrasorb SI), respectively. Based on the Au eqn (5). Please note that these are normalized to the absolute
particle size distribution obtained from the TEM images, the amount of Au surface atoms rather than to the active site(s) for
volume–area mean diameter as well as the Au dispersion were MeOH formation, which are not known for Au/ZnO catalysts so
calculated. For calculation of the latter we assumed hemispherical far. The selectivity for MeOH formation (SMeOH) is defined as
1
5
ꢁ2
Au nanoparticles with 1.15 ꢂ 10 cm Au surface atoms.
.2. Kinetic measurements
the ratio of the MeOH formation rate to the overall CO
hydrogenation rate (to MeOH and CO, see eqn (6)).
2
2
n_ MeOH;out
RMeOH ¼
(4)
(5)
(6)
Kinetic measurements were performed at 5 bar total pressure
and at a reaction temperature of 240 1C under differential
reaction conditions in a glass-lined stainless steel tube micro
reactor (inner diameter 4 mm). The lower pressure of 5 bar
mAu
RMeOH ꢂ MAu
DAu
TOFMeOH ¼
3
2
compared to previous kinetic measurements at 50 bar was
1
3
chosen to keep the consumption of expensive CO
tolerable level. Note that the total conversions of CO and CO
1% at the most in all kinetic experiments) were significantly
below the maximum conversion given by the thermodynamic
equilibrium, which is at approximately 15% and 8% for CO
to both MeOH and CO) and CO (to MeOH) hydrogenation,
respectively, under present reaction conditions. For all mea-
surements we used about 200 mg of pure Au/ZnO catalyst
powder, which was fixed in the center of the micro reactor
using quartz wool plugs on either side. This results in a catalyst
bed length of about 2 cm. High purity reaction gases were
2
at a
RMeOH
RMeOH
SMeOH ¼
¼
2
RCO2
RMeOH þ RCO
(
Calibration on an absolute scale was realized by comparison
with the signal of a test gas containing a well-defined amount
2
(
partial pressure) of MeOH, and secondly by a direct comparison
between GC and MS measurements under various reaction condi-
tions (with different MeOH formation rates) during the CO hydro-
genation on Au/ZnO. Additionally, an isotopic exchange between
(
2
13
12
2
CO and CO was separately determined over the Au/ZnO catalyst
under identical reaction conditions (same temperature, pressure and
space velocity as during the reaction), and was found to be negligible
under present reaction conditions.
2
supplied by Westfalen (H 5.0, Ar 5.0 and CO 4.7) and Campro
Scientific ( CO , 99 atom% C). The gases were mixed by mass
2
1
3
13
flow controllers (Brooks Instrument SLA5850) in order to
realize the desired reaction gas composition. A backpressure
controller (Tescom ER5000) was used in order to regulate the
pressure in the reactor. After calcination and subsequent cooling
3
. Results and discussion
3.1. Catalyst characterization
down to the reaction temperature of 240 1C in Ar at atmospheric The Au loading of the Au/ZnO catalyst was determined by
pressure (see Section 2.1), the pressure in the reactor was inductively coupled plasma optical emission spectroscopy
increased to the reaction pressure (5 bar) in a flow of the reaction (ICP-OES) and amounts to 1.0 wt%. TEM images of the sample
gas mixture.
after O400 pretreatment revealed an even distribution of the Au
In the kinetic measurements, the freshly calcined catalyst nanoparticles on the ZnO support. A representative TEM image
1
3
sample was first exposed to a CO /H mixture until a steady-state of the Au/ZnO catalyst after O400 pre-treatment as well as the
2
2
13
ꢁ1
was reached (15% CO , 45% H , balance Ar; 30 Nml min ). This corresponding Au particle size distribution, which is obtained
2
2
usually took approximately 100 minutes. Afterwards, different from the evaluation of more than 400 Au particles, are shown in
12
amounts of CO (between 0.5 and 15%) were stepwise added to Fig. 1. From the Au particle size distribution the volume–area
1
3
the CO
2
/H
/H
CO/balance Ar). Finally, the activity was measured once again in area of the Au/ZnO catalyst after calcination was 42 m
2
gas mixture by gradually replacing Ar while keeping mean diameter of the Au particles was determined to be 2.4 ꢃ
1
3
13
the CO
2
2 2 2
ratio constant at 1/3 (15% CO /45% H
/0.5–15% 0.4 nm, corresponding to a Au dispersion of 48.5%. The surface
12
2
ꢁ1
g
13
the presence of only CO in order to check for possible changes of (measured by low temperature N
adsorption). A summary of
2
2
the catalyst performance upon reaction in CO containing reaction the physical properties of the freshly calcined Au/ZnO catalyst is
atmospheres.
given in Table 1.
The influent and effluent gases were analyzed by mass spectro-
Note that we demonstrated already in a previous study that
metry (Pfeiffer Vacuum HiQuad QMG 700). In all measurements the changes in the average Au particle size and size distribution
masses 18, 28, 29, 31, 32, 44 and 45 were monitored. MeOH was as well as the surface area of the Au/ZnO catalyst during its
3
2
identified via masses 31 and 32, which represent the strongest exposure to reaction conditions are negligible.
12
13
signals for MeOH containing C and C, respectively. Based on
1
3
12
3
2
.2. MeOH formation from CO and CO
the molar flow rate of MeOH formed under differential reaction
:
conditions (nMeOH,out/CO2 and CO conversions r1%) and the We will first focus on the overall MeOH formation rates during
absolute amount of Au metal (m ), the Au mass-normalized MeOH the CO and CO hydrogenation in CO/H and CO /H reaction
Au
2
2
2
2
2 2
formation rates (RMeOH) were calculated according to eqn (4). gas mixtures, respectively, at 5 bar and 240 1C. For the CO /H
Turnover frequencies (TOFs) were calculated based on these Au reaction gas mixture we also additionally measured the influence
mass-normalized reaction rates, the molar mass of Au (MAu), and of CO addition on the overall MeOH formation rate. Such data
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Fig. 2 Time dependent MeOH formation activity (TOF) of the Au/ZnO
catalyst (STREM Chemicals) during CO
/balance Ar) and CO hydrogenation ( , 15% CO/45% H
bar and 240 1C after calcination in 1% O /Ar at 20 Nml min and 400 1C
for 1 h (O400). For CO hydrogenation the activity (TOF) ( ) and selectivity
m) for MeOH formation are also included.
2
hydrogenation ( , 15% CO
2
/45%
H
2
2
/balance Ar) at 5
ꢁ1
2
2
~
(
almost no more changes in activity are observed. The initial
increase in the MeOH formation rate is also reflected by the
selectivity towards MeOH formation, which increases over the
first 100 min, from B25 to B40%. For the remaining time, the
selectivity continues to increase slowly, to about 45% after 1000 min
on stream. These results agree closely with previous findings under
32
Fig. 1 (a) Representative TEM image of the Au/ZnO catalyst after calcination similar reaction conditions, but differ somewhat from the reaction
2
in 1% O /Ar at 400 1C and at atmospheric pressure for 1 h (O400) and (b) the
corresponding particle size distribution.
characteristics of Au/ZnO during CO hydrogenation at 50 bar. For
2
the latter case we recently reported that the catalyst is immediately in
its most active state, and that there is hardly any deactivation with
32
time on stream.
Table 1 Au particle size, dispersion and catalyst surface area of the Au/
Moving on to the CO hydrogenation, this situation changes
significantly. Here the initial activity at 5 bar is close to zero and
increases steadily during the reaction, until it reaches its
ZnO catalyst after calcination in 1% O
2
/Ar at 400 1C for 1 h (O400)
After O400
a
ꢁ3 ꢁ1
Au particle size /nm
2.4 ꢃ 0.4
48.5
41.6
maximum value after about 200 min (TOF 0.8 ꢂ 10
s ).
b
Dispersion /%
Surface area /m g
c
2
catꢁ1
Subsequently it decreases continuously with time on stream,
ꢁ
3
ꢁ1
reaching a steady-state activity of 0.5 ꢂ 10
s
after 1000 min.
hydro-
a
b
Measured by TEM. Calculated assuming hemispherical Au nano-
The discrepancy between the initial activity for the CO
2
c
2
particles. Measured by low temperature N adsorption (BET).
genation to methanol and the CO hydrogenation (to methanol)
already indicated that under present reaction conditions
had been measured before, but only at a total pressure of methanol formation from CO
2
does not proceed via the for-
3
2
50 bar. Time dependent turnover frequencies (TOFs) of MeOH mation of CO (by the RWGS reaction) and its subsequent
formation during CO hydrogenation (15% CO, 45% H , balance hydrogenation to MeOH, as for the latter reaction the initial
2
Ar) and CO hydrogenation (15% CO , 45% H , balance Ar) on a activity is close to zero. The present findings for CO hydrogena-
2
2
2
freshly pre-treated Au/ZnO catalyst (O400) are presented in Fig. 2. tion at 5 bar and its temporal evolution also differ remarkably
Obviously, both the reaction characteristics and the MeOH from the behavior at 50 bar under otherwise identical condi-
3
2
formation rates differ significantly, despite the identical partial tions. In the latter case, we found that the activity is the
pressures of CO and CO and, hence, identical C/H ratios. The highest right at the start of the reaction, followed by a contin-
initial activity (TOF) of Au/ZnO for the (overall) CO hydrogenation is uous slight decrease with time on stream for at least 1000 min.
2
2
ꢁ
3
ꢁ1
B5 ꢂ 10
s
, and it then decreases steadily in an approximately
The difference in the initial phase of the reaction, where we
after observed an increase in the methanol formation rate for both
ꢁ3
ꢁ1
exponential way, until reaching a final activity of 3.4 ꢂ 10
s
1
000 min on stream. Focusing on methanol formation only, the CO hydrogenation and CO hydrogenation for the reaction at
2
ꢁ3
ꢁ1
situation is very different. It starts at 1.1 ꢂ 10
s
and increases 5 bar, while at 50 bar it was about constant, may well be due to
ꢁ
3
ꢁ1
during the initial 70 min to 1.8 ꢂ 10 s . Afterwards there is a an experimental artifact. While for the reaction at 5 bar the first
slight decrease in activity with time on stream (deactivation), which gas sample was taken at about 25 min after changing from
ꢁ
3
ꢁ1
finally results in a TOF of 1.5 ꢂ 10
s
after 1000 min, and when bypass to the reaction gas mixture, this delay time was much
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longer for the reaction at 50 bar, since it took longer to reach overall MeOH formation rate of Au/ZnO, the corresponding TOFs
steady-state conditions in the reactor gas composition. When obtained under steady state conditions are shown in Fig. 3.
the first gas sample was taken at 50 bar, the catalyst was Evidently, the addition of up to 15% CO has almost no effect on
exposed already for more than 1 h to a gas atmosphere which the MeOH formation rate under present reaction conditions.
was getting increasingly closer to the final reaction gas compo- Similar to the reaction in a CO /H mixture, the Au mass-
2
2
sition. In that case it is not surprising that the initial activation normalized MeOH formation rates in all different CO
2
/CO/H
Au , which corresponds
(Table 2). These MeOH formation
note that the long term performance of the Au/ZnO catalyst, rates are also comparable to the value reported previously for the
which is relevant for possible technical applications, is very CO hydrogenation on the same Au/ZnO catalyst under identical
promising. For the operation at 50 bar we did not find any reaction conditions (3.8 ꢂ 10 mol s gAu , 1.6 ꢂ 10
2
ꢁ
6
ꢁ1
ꢁ1
observed at 5 bar is not detected for the reaction at 50 bar, but mixtures are about 3.6 ꢂ 10 mol s
g
ꢁ
3
ꢁ1
occurs before the first probe is taken. Finally we would like to to a TOF of 1.5 ꢂ 10
s
2
ꢁ
6
ꢁ1
ꢁ1
ꢁ3 ꢁ1
s
) at
5 bar and 240 1C. Moreover, also the selectivity to MeOH in
The comparison of the MeOH formation activities from CO2 CO /H mixtures in both studies is almost identical (44% and
3
1
indication for deactivation over 50 hours on stream.
2
2
and CO under steady-state reaction conditions shows that this is 48%, respectively). Hence, in spite of the increasing total amount
significantly lower, by a factor of about 3, during the CO hydro- of carbon containing species (CO and CO) and the increased
C/H ratio, there is no significant increase in the rate of MeOH
hydrogenation and for CO hydrogenation, respectively, formation. This indicates that, similar to previous conclusions
2
ꢁ
6
ꢁ1
ꢁ1
ꢁ6
ꢁ1
ꢁ1
g
Au
genation (3.6 ꢂ 10 mol s
g
Au and 1.2 ꢂ 10 mol s
for CO
2
ꢁ
3
ꢁ1
ꢁ3 ꢁ1
32
corresponding to TOFs of 1.5 ꢂ 10
s
and 0.5 ꢂ 10
2
s ). This for the CO hydrogenation reaction at 50 bar, CO is not an
trend is consistent with previous results at 50 bar, demonstrating intermediate in the CO hydrogenation reaction (eqn (1)). If CO2
2
that at both 5 and 50 bar CO hydrogenation on Au/ZnO is faster is first converted to CO, via the RWGS reaction, and the resulting
2
than CO hydrogenation at 240 1C under steady-state conditions.
Next we determined the influence of adding various amounts expect an increase in the MeOH formation rate with increasing
of CO (0.5–15% CO) to the CO /H reaction gas mixture on the CO content, independent of whether the first reaction step
RWGS of CO ) or the second step (CO hydrogenation) is rate
CO is subsequently hydrogenated to MeOH (eqn (2)), one would
2
2
(
2
limiting. This is obviously not the case under present reaction
conditions. At 50 bar under otherwise identical reaction condi-
tions, the MeOH formation rate even decreased as CO was added
3
2
to a reaction gas mixture containing CO only. Hence, in both
2
cases hydrogenation of CO2 and CO seem to proceed via
different reaction pathways, which can occur in parallel.
Further information on the effect of CO on the hydrogena-
tion of CO
2
was obtained from isotope labelling experiments,
1
2
13
determining CH
3
OH and CH
mixtures. This provides direct information on
the carbon source for MeOH. The individual contributions of
3
OH product formation from
1
3
12
2 2
CO / CO/H
1
3
12
CO and CO to the total amount of MeOH formed at 5 bar
2
1
2
and 240 1C with increasing amounts of CO added to the
1
3
2 2
CO /H /Ar mixture are presented in Fig. 4a (see also Table 2).
As already described above, the overall MeOH formation rate is
essentially the same for all gas mixtures except for CO only. The
13
Fig. 3 Steady-state MeOH formation rates (TOFs) during CO
genation (15% CO , 45% H , balance Ar) in the absence and presence of
.5–15% CO as well as during the CO hydrogenation (15% CO, 45% H
2
hydro-
activity for MeOH formation from CO
decreases upon the addition of 0.5% CO (CO
2
2
, however, slightly
/CO = 30/1), from
(Fig. 4b). Obviously, MeOH
2
2
12
12
0
2
,
ꢁ3
ꢁ1
ꢁ3 ꢁ1
balance Ar) on a Au/ZnO catalyst (STREM Chemicals) at 5 bar and 240 1C. 1.5 ꢂ 10
s
to 1.2 ꢂ 10
s
13
Table 2 Au mass-normalized MeOH formation rates, turnover frequencies (TOFs) and MeOH isotopic compositions during CO
2
hydrogenation
) in the absence and presence of CO (0.5–15%) in the reactant gas mixture as well as during pure CO hydrogenation at 5 bar and
40 1C under steady-state conditions over a Au/ZnO catalyst (STREM Chemicals)
12
12
(
2 2
15% CO , 45% H
2
MeOH formation
rate/10 mol s
MeOH formation
Fraction of MeOH
Fraction of MeOH
from CO
ꢁ6
ꢁ1 Auꢁ1
ꢁ3 ꢁ1
13
12
CO concentration/%
0
g
TOF/10
s
from CO
2
3.6
3.3
3.5
3.4
3.6
3.6
1.2
1.5
1.4
1.5
1.4
1.5
1.5
0.5
100
90
82
83
80
78
0
0
10
18
17
20
22
100
0
1
5
1
1
.5
0
5
Pure CO
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for CO concentrations of 1% and higher. Remarkably, there is also
no further inhibition of the CO hydrogenation upon CO addition,
2
clearly demonstrating that the main reaction pathway for this
reaction is not affected by the increasing presence of CO. This
situation, however, is different for the other products formed (MeOH
and water), which have been demonstrated to have an inhibiting
effect on the MeOH formation rate on Au/ZnO (similar to Cu-based
3
2
catalysts). This is further evidence that the dominant reaction
pathways of both reactions, CO and CO hydrogenation, proceed
2
parallel to and are independent of each other. This finding
agrees well with our previous findings, where we came to similar
conclusions regarding the influence of CO on the overall MeOH
3
2
formation rate for the reaction at 50 bar. In that study we had
also performed spectroscopic measurements in order to detect
differences in the reaction mechanism, e.g., different adsorption
sites for CO and CO
surface species during CO
tion under reaction conditions (240 1C, 5 bar) by diffuse reflec-
2
, following the formation of adsorbed
2
hydrogenation and CO hydrogena-
3
2
tance infrared Fourier transform spectroscopy (DRIFTS). From
these measurements it was, however, not possible to derive any
insights on the adsorption sites for CO or CO2.
It should be noted that although independent of the CO
2
/CO
ratio the majority of MeOH results from CO hydrogenation,
2
this does not mean that MeOH formation from CO is little
effective. This is illustrated best by the fact that even at a
composition of CO /CO = 15/0.5, about 10% of the resulting
2
MeOH comes from CO hydrogenation, which is significantly
more than expected from the composition of the reaction gas
2
Fig. 4 (a) Ratios of MeOH formation from CO and CO, respectively,
13
during CO
and presence of 0.5–15% CO as well as during the CO hydrogenation
15% CO, 45% H , balance Ar) and (b) the corresponding MeOH formation
activities (TOFs) on a Au/ZnO catalyst (STREM Chemicals) at 5 bar and
40 1C.
2 2 2
hydrogenation (15% CO , 45% H , balance Ar) in the absence
12
12
(
2
2
(assuming reaction orders of 1 for CO and CO). On the other
2
hand, considering the overall objective of our work to efficiently
convert CO
is somewhat inhibited by the presence of formation of CO does reduce the activity for CO
CO. At the same time, about 10% of the overall MeOH formed to methanol in general, which is hardly the case.
2
to methanol, it is most important whether the
formation from CO
2
2
hydrogenation
ꢁ
3
ꢁ1
originate from CO (TOF = 0.14 ꢂ 10
s
), although the partial
In this sequence of measurements we had not considered so
pressure of CO is much lower than that of CO . Accordingly, the far that the addition of CO to the CO /H mixture at constant
2
2
2
fraction of MeOH from CO decreases from 100% (for CO /H
CO and H partial pressures results in increasingly higher C/H
2 2
2
2
2
only) to 90% in the presence of 0.5% CO (Fig. 4a).
ratios than during the initial CO hydrogenation (CO /H = 1/3).
2 2 2
Upon increasing the CO concentration to 1% (CO
fraction of MeOH formed from CO decreases further to approxi- the hydrogenation of CO
mately 80%. Hence, up to 1% CO in the reaction gas mixture there is C/H ratio of 1/3 (with 45% H
2
/CO = 15/1) the Since this may hamper the understanding of the effect of CO on
, we also tested the reaction at a similar
), using a reaction gas mixture of
/7.5% CO/45% H (balance Ar). Comparable with
/CO, respectively. Increasing the CO concentration further to 5% the reaction in the presence of 15% CO and 15% CO, this again
and higher (up to 15%; CO /CO = 1/1), however, does not result in results in an equimolar CO /CO ratio, but this time with a C/H
2
2
2
1
3
12
an almost linear decrease/increase in the MeOH formation rate from 7.5% CO
CO
2
2
2
2
2
2
significant changes in the CO and CO contributions to the total ratio of 1/3, identical to the original gas mixture during CO2
2
MeOH production. For each of the reaction gas mixtures tested (15% hydrogenation only (15% CO , 45% H ). In that case the overall
2
2
CO
fractions of MeOH from CO
respectively, (see Table 2). Hence, even for identical partial pressures of MeOH formed from CO
of CO and CO the majority of MeOH formed originates from CO are almost identical to the reaction atmosphere with 15% CO
clearly indicating that over a wide range of CO /CO compositions, and 15% CO. Obviously, the ratio of MeOH formed from CO
2 2
, 45% H , +1%, +5%, +10%, and +15% CO, balance Ar), the MeOH formation rate is slightly lower compared to the carbon
ꢁ
3
ꢁ1
2
and CO are about 80% and 20%, rich reaction atmosphere (TOF 1.1 ꢂ 10
s ), but the fractions
2
and CO (77% and 23%, respectively)
2
2
,
2
2
2
CO is the main carbon source in MeOH formation under present and CO does not depend on the C/H ratio under present reaction
2
reaction conditions (240 1C, 5 bar). Moreover, there is no further conditions. This also means that the direct comparison between
increase in the MeOH formation rate from CO upon increasing the CO /H and CO /CO/H is possible despite the increasing
2
2
2
2
CO concentration to values higher than 1%. This can be most easily amount of carbon in the reaction gas feed.
explained by a saturation of CO adsorption sites under these
In another additional measurement we also tested for changes
conditions, which results in a reaction order for CO close to zero in the MeOH formation rate upon increasing the partial pressure of
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5
2
CO
2
, while keeping the partial pressure of H
2
constant. Hence, total pressures, were reported by Yang et al., who observed for
1
2
13
instead of adding CO we added CO
2
to the original reaction gas the reaction of a CO / CO/D reaction gas mixture (1: 1 : 6) on
2 2
mixture, which equally results in an increased C/H ratio, similar to a Cu/SiO catalyst under almost identical reaction conditions
2
the above described experiments (CO + CO). The corresponding (6 bar and 240 1C), that also at much lower pressures than in
2
results are shown in the ESI† (Fig. S1). In this case there is almost the studies cited above the majority (78%) of MeOH formed
2
no change in the MeOH formation rate, which stays almost originates from CO .
ꢁ
6
ꢁ1
ꢁ1
constant at about 3.8 ꢂ 10 mol s
gAu . Moreover, there are
also no significant changes in the selectivity for MeOH formation,
which is in between 44% and 41% for all reaction gas mixtures
3
.3. Influence of the reaction temperature on MeOH
formation from CO2
For the reaction gas mixture with 15% CO
we also investigated changes in the isotopic composition of the
product MeOH with increasing reaction temperatures at a
constant total pressure of 5 bar. After reaching a steady-state
situation at 240 1C, the reaction temperature was increased to
2 2
investigated (15–30% CO , 45% H , balance Ar). Hence, similar to
1
3
2
2
/15% CO/45% H ,
the above described findings for CO, all CO adsorption sites seem
2
to be blocked in a reaction gas mixture containing 15% CO , and
2
further addition of CO is not beneficial for the MeOH formation
2
rate (zero reaction order for CO
MeOH formation rates in the presence of 7.5% CO
2
). However, considering the lower
as described
2
2
70 1C and finally to 300 1C. The corresponding MeOH formation
rates and the isotopic composition of MeOH are plotted in Fig. 5
see also Table 3 and Table S3, ESI†). As expected for the
kinetically controlled regime (note that both CO and CO conver-
above (7.5% CO
the CO content lower than 15% CO
2
, 7.5% CO, 45% H
2
) one can assume that for
this is not true any longer,
2
2
(
and the reaction order for CO
partial pressures.
2 2
becomes positive at lower CO
2
sion were always well below the thermodynamic limit), the overall
rate of MeOH formation increases with increasing reaction tem-
Regarding the main source of carbon in the MeOH formation
on supported Au catalysts, our findings are also in agreement
with an earlier proposal from Sakurai et al., who also claimed
ꢁ
6
ꢁ1
ꢁ1
ꢁ3 ꢁ1
perature, from 3.6 ꢂ 10 mol s gAu (TOF 1.5 ꢂ 10
s ) to
ꢁ6
ꢁ1 Auꢁ1
ꢁ3 ꢁ1
8
1
.6 ꢂ 10
mol s
g
(3.6 ꢂ 10
(4.4 ꢂ 10
s
) and finally to
) at 240 1C, 270 1C,
5
3
that MeOH is mainly formed via the CO
conclusion was based on a comparison of the CO
2
hydrogenation. This
hydrogena-
ꢁ
6
ꢁ1
ꢁ1
ꢁ3 ꢁ1
0.4 ꢂ 10 mol s
g
Au
s
2
and 300 1C, respectively. Although the MeOH formation rate
tion and CO hydrogenation activities over Au catalysts and,
hence, neither in the presence of both reactants simultaneously
nor directly by isotope labelling experiments. On the other hand,
the results obtained for the Au/ZnO catalyst in this work are
similar to the results of previous tracer atom experiments on
Cu-based catalysts. First experiments to determine the main
2
from CO hydrogenation also increases with increasing tem-
perature, its fraction of the overall MeOH formation decreases
source of carbon in MeOH formation from CO
on a commercial copper-containing oxide catalyst and on a
Cu/ZnO/Al catalyst were reported by Rozovskii et al. and
Chinchen et al., respectively, using C-labelled species.
Under conditions similar to those employed industrially, i.e., at
0 bar and temperatures between 180 and 250 1C, both studies
arrived at the conclusion that MeOH was predominantly formed
2 2
/CO/H mixtures
2 3
O
1
4
48,51
5
5
1
from CO
or 10% CO
the ratio of the partial pressures of CO
found that at higher partial pressures of CO
MeOH was essentially formed from CO only, and that MeOH
formation from CO occurred only at extremely low levels of CO
2
, using reaction mixtures containing 20% CO
2
/1% CO
/10% CO. Chinchen et al. additionally also varied
and CO. In that case they
(CO : CO = 1 : 2)
4
8
2
2
2
2
Fig. 5 MeOH formation rates (TOFs, left axis) and the fraction of MeOH
2
13
formed from CO
2
(right axis) under steady-state conditions during
2
13
12
reaction in 15% CO
2 2 2
/15% CO/45% H (CO /CO = 1/1) at 5 bar on a
4
8
(
o100 ppm CO ). In another more recent isotope labelling
2
Au/ZnO catalyst (STREM Chemicals) at various temperatures between 240
experiment on Cu-based catalysts performed at 30 bar and and 300 1C.
1
3
2
30 1C with a gas mixture containing 8% CO
(balance inert gas), Studt et al. found that more than 90% of
the resulting MeOH originated from CO , both for Cu supported
on irreducible MgO, which exhibited a poor MeOH formation
rate, and also for Cu on ZnO, which exhibits a high turnover (balance Ar) at 5 bar under steady-state conditions over a Au/ZnO catalyst
2
/6% CO/59%
H
2
Table 3 Au mass-normalized MeOH formation rates, turnover frequen-
2
13
2
cies (TOFs), and fraction of MeOH formation from CO during hydro-
13
12
2 2
genation of a gas mixture containing 15% CO /15% CO/45% H
4
5
MeOH rate. Although the reaction conditions in these studies (STREM Chemicals) at different temperatures between 240 and 300 1C
were markedly different from ours, in particular the total pres-
sure during the reaction, there is nevertheless good agreement
Temperature/ MeOH formation rate/ MeOH TOF/ Fraction of MeOH
ꢁ6
ꢁ1
ꢁ1
ꢁ3 ꢁ1
13
1C
10 mol s
g
Au
10
s
from CO
2
on the dominant contribution of CO
which points to rather similar reaction characteristics on sup-
ported Cu and Au catalysts. Similar findings, yet for much lower 300
2
hydrogenation to MeOH,
2
2
40
70
3.6
8.6
10.4
1.5
3.6
4.4
78
73
68
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due to the more pronounced increase of the CO hydrogenation
with temperature, from about 78% at 240 1C via 73% at 270 1C
to finally 68% at 300 1C. Obviously, the temperature has a
pronounced influence on the ratio of the (parallel) reaction
pathways for CO2 and CO hydrogenation, with continuously
less contribution from CO
tendency had already been reported by Sakurai et al. when
comparing the CO and CO hydrogenation activities of Au/ZnO
2
with increasing temperatures. Such
5
3
2
catalysts at temperatures between 150 and 400 1C and at 8 bar
total pressure. They observed that up to 250 1C the MeOH yield
from CO (for CO /H = 3 and for CO /H = 2) was higher than
2
2
2
2
2
that from CO (CO/H = 2). At higher temperatures (Z300 1C),
2
however, the MeOH yield from the CO/H reaction gas mixture
2
became dominant. While their findings are in good agreement
with our present results, both with the reaction activities
measured separately in CO
40 1C, and with the temperature dependent trend in the
activities for CO hydrogenation in the CO /CO/H mixture
2 2 2
/H and CO/H gas mixtures at
2
x
2
2
described above, their proposal was somewhat tentative since
those authors did not perform measurements in the simulta-
neous presence of both CO and CO2.
In a recent study on MeOH formation on the Au/ZnO catalyst
at pressures between 20 and 40 bar and at a temperature of
3
00 1C, Strunk et al. compared the activity of the catalyst for
MeOH formation in CO -containing and CO -free reaction gas
). They found that the partial replacement of
CO by CO at 300 1C (switching from 15% CO/85% H to 6%
2
2
5
4
mixtures (CO/H
2
2
2
5
4
CO/8% CO /64% H ) resulted in lower MeOH formation rates.
2
2
(
Note that this involved also a change in the C/H ratio.) They
explained this decrease of the MeOH formation rate in the
presence of CO by either blocking oxygen vacancies on ZnO, CO
Fig. 6 (a) Steady-state MeOH formation rates (TOFs) during CO
2
(15%
2
2
, 45% H , balance Ar) and CO (15% CO, 45% H , balance Ar) hydro-
2
2
genation at 5 bar and temperatures between 220 and 270 1C and (b)
Arrhenius plots of the MeOH formation rates obtained in the above
measurements. For comparison we also show the activities for MeOH
which they had proposed as active sites for CO hydrogenation,
by stable adsorbed formate species, or by the annihilation of
these sites by the extra oxygen present in CO . At first glance the
2
formation from CO
2 2
and CO in a mixture containing 15% CO , 15% CO,
higher MeOH formation rate in CO/H compared to that in a
2
45% H , balance Ar.
2
CO -containing reaction gas mixture (CO/CO /H ) reported in
2
2
2
their study seems to be in contrast to our previous findings at
0 bar and 240 1C on Au/ZnO, where (i) the addition of 5–15% and 270 1C in the CO
CO to a 15% CO /45% H mixture resulted in a decrease in the obtained are presented in Fig. 6a. They show that for both
MeOH formation rate and (ii) the MeOH formation rates in the reactions the TOF for MeOH formation continuously increases
presence of CO (in the absence and presence of CO) were as temperature increases. Consistent with the results presented
always significantly higher than those obtained in a reaction above, the MeOH formation from CO is always higher than
5
2 2 2
/H and CO/H mixtures. The results
2
2
2
2
3
2
gas mixture containing only CO/H2. However, taking into that from CO in the temperature range investigated (up to
account the continuously higher contributions from CO to 270 1C). The apparent activation energies for MeOH formation
the overall MeOH formation with increasing temperatures, this calculated from the temperature dependence of the rates are
ꢁ
1
apparent discrepancy can easily be rationalized. Obviously, CO 66 ꢃ 5 and 92 ꢃ 5 kJ mol for the CO
2
hydrogenation and CO
hydrogenation becomes increasingly more important for hydrogenation on Au/ZnO at 5 bar, respectively, (see Fig. 6b).
MeOH formation upon increasing the reaction temperature For comparison, the activities for MeOH formation from CO
from 240 to 300 1C. and CO measured in the simultaneous presence of CO and CO
The more pronounced increase in the MeOH formation (15% CO /15% CO/45% H
2
2
2
2
) at 240 1C, 270 1C, and 300 1C are
from CO with increasing temperatures points directly to a also included in this figure. For 240 1C and 270 1C the MeOH
higher apparent activation energy for the CO hydrogenation formation rates from CO in the CO /CO mixture and in pure
2
2
as compared to the CO hydrogenation. To derive more insights CO
2
are rather close to each other and show an almost identical
into the apparent activation barriers in the MeOH formation via temperature dependency. Only at 300 1C the MeOH formation
the CO hydrogenation and the CO hydrogenation, we mea- from CO is somewhat lower than expected, but this may easily
sured their activities in a temperature range between 220 1C be rationalized by a decreasing selectivity for MeOH formation
2
2
2
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3
2
with increasing reaction temperature. Similar to that, also for with the formation of MeOH. The present study provided clear
MeOH formation from CO the activities are slightly higher for evidence that the presence of up to 15% CO in the reaction
CO only than obtained in the presence of CO , and the atmosphere has almost no influence on the overall MeOH for-
2
temperature dependence up to 270 1C seems comparable. mation under present reaction conditions (5 bar, 240 1C). Although
Overall it is obvious from these measurements that the apparent MeOH formation from CO is slightly diminished in the presence
2
activation energy for CO hydrogenation is higher compared to of CO, this is counterbalanced by additional MeOH formation from
that for MeOH formation from CO
as well as for reaction gas mixtures containing CO
provides additional evidence that the CO hydrogenation to different from the present findings for reaction at 5 bar the MeOH
MeOH proceeds not via the RWGS reaction (eqn (1), see above). formation from CO was found to decrease slightly in the presence
2
, for the individual reactions CO hydrogenation. The origin of the resulting MeOH formation,
2
and CO. This CO or CO hydrogenation, was unraveled. It should be noted that
2
2
2
32
Otherwise, CO hydrogenation would be part of the MeOH of CO concentrations 41% for reaction at 50 bar. This has to be
formation from CO , which accordingly should have at least considered for practical applications, where the reaction will most
2
the same activation barrier as CO hydrogenation. Hence, these likely be performed at rather high reaction pressures, due to higher
results also indicate that CO
under present reaction conditions. Since neither for the CO
2
is directly hydrogenated to MeOH MeOH yields at higher pressure.
After the principal potential of Au/ZnO catalysts for CO
2
2
hydrogenation nor for the CO hydrogenation the nature and hydrogenation to methanol has been clearly demonstrated,
number of active sites or the local coverages of adspecies and future work shall focus on (i) improving the activity and thus
reaction intermediates are known, the physical origin of different lower the catalyst costs and (ii) addressing mechanistic aspects
apparent activation energies and, hence, the increasing contribu- by using operando spectroscopy. In this context the specific role
tion from CO to the overall MeOH formation with increasing of Au/ZnO is of particular interest.
temperatures needs to be addressed in future studies.
In our previous studies we already compared Au catalysts
with different oxide supports and different Au particle sizes (for
4
. Conclusions
Au/ZnO) in order to unravel the active site(s). Moreover, we also
performed operando IR spectroscopic measurements during From detailed kinetic isotope labelling measurements on
both CO hydrogenation and CO hydrogenation on Au/ZnO to MeOH formation from CO in CO containing syngas mixtures
gain further insight into the nature of the adspecies formed ( CO /CO/H ) on highly selective Au/ZnO catalysts, performed
2
2
1
3
2
2
during the reaction. So far, however, these measurements did at 5 bar total pressure and in the temperature range of
not allow us to draw firm conclusions on the nature of the 240–300 1C, we arrived at the following conclusions:
active site or the reaction intermediates. This is the topic of
ongoing studies in our laboratory, which aim at gaining more ZnO catalyst for CO
mechanistic insight, for example, by using Au/ZnO catalysts higher than that for CO hydrogenation in CO/H
with different Au loadings. over, for the reaction in CO /CO/H mixtures, up to equimolar
amounts of CO and CO (15% CO and 15% CO), CO is the
preferred carbon source of MeOH.
. During the reaction in CO /CO/H mixtures the fraction of
1. Under present reaction conditions the activity of the Au/
hydrogenation in CO /H is significantly
syngas. More-
2
2
2
2
2
2
2
2
2
3
.4. Consequences of our results on application in renewable
energy storage
The main aim of the present study was to further elucidate the MeOH formed from CO increases linearly from 10% to approxi-
role of CO in the hydrogenation of CO to MeOH. The latter mately 20% for the CO content up to 1%. At higher CO content,
reaction may be part of concepts for renewable energy storage, however, the changes in the CO contribution to the final MeOH
if the H required for this reaction is produced by renewable product are negligible. This is tentatively explained by saturation of
energies. It may help to overcome natural fluctuations in the the active CO adsorption sites under these conditions.
supply of renewable energy as well as to diminish the overall 3. The presence of CO in the reaction gas mixture has little
CO emission. This, however, is only feasible if (i) the catalysts effect on the activity of the Au/ZnO catalyst for CO hydrogena-
2
2
2
2
2
2
2
used for CO hydrogenation are highly selective towards MeOH tion to MeOH, as indicated by the very small decay in the TOF
2
formation, (ii) the CO formed via the parallel RWGS reaction for MeOH formation from CO . Furthermore, the addition of
2
has no or only little detrimental effect on the MeOH formation CO (0.5–15% CO) to a gas mixture containing only CO
activity from CO , and (iii) the CO resulting from the RWGS almost no effect on the overall MeOH formation rate of Au/ZnO.
reaction can also be efficiently hydrogenated to MeOH. We have 4. From the fact that the main reaction pathway for CO
already demonstrated previously that Au/ZnO catalysts, to hydrogenation is little affected by the presence of CO we
the best of our knowledge, have higher selectivities towards conclude that CO and CO hydrogenation reactions proceed
MeOH formation in this process in comparison to commercial in parallel reaction pathways, independent of each other.
2 2
/H has
2
2
2
3
2
Cu-based systems. Although there are also catalysts with even Further evidence for this suggestion comes from (i) the time
higher selectivities (up to 100%) under comparable reaction dependence of the reaction rates in CO /H and CO/H , where
2
2
2
3
4–38
conditions,
always some CO present in the reaction gas atmosphere, whose initial phase of the reaction is in contrast to the much higher
partial pressure within the catalyst bed will increase together initial activity for methanol formation from CO hydrogenation
2
during CO hydrogenation on Au/ZnO there is the very low activity observed for CO hydrogenation in the
2
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and (ii) the significantly higher apparent activation energy of
Au/ZnO for MeOH formation from CO compared to CO .
2
R. W. Fischer, J. K. Nørskov and R. Schl o¨ gl, Science, 2012,
336, 893.
5
. The relative activities for CO and CO2 hydrogenation 16 M. Behrens, Angew. Chem., Int. Ed., 2014, 53, 12022.
depend strongly on the temperature. For reaction with equimolar 17 A. Goeppert, M. Czaun, J. P. Jones, G. S. Prakash and G. A.
amounts of CO and CO the main carbon source for MeOH at
Olah, Chem. Soc. Rev., 2014, 43, 7995.
18 The George Olah Renewable Methanol Plant, 2009, www.
carbonrecycling.is.
2
reaction temperatures between 240 1C and 300 1C is always CO
2
,
but with an increasing tendency towards CO hydrogenation with
increasing temperatures. Hence, at even higher temperatures, 19 H. H. Kung, Catal. Rev.: Sci. Eng., 1980, 22, 235.
CO is expected to eventually dominate the MeOH formation 20 K. Klier, Adv. Catal., 1982, 31, 243.
under otherwise identical conditions.
21 G. Ghiotti and F. Boccuzzi, Catal. Rev.: Sci. Eng., 1987, 29, 151.
Considering (i) the similar trends when comparing the 22 J. C. J. Bart and R. P. A. Sneeden, Catal. Today, 1987, 2, 1.
activities of CO hydrogenation and CO hydrogenation as well 23 G. C. Chinchen, P. J. Denny, J. R. Jennings, M. S. Spencer
2
as (ii) the qualitatively identical influence of CO addition to a
CO /H
gas mixture on the overall MeOH formation rate at 24 A. Kiennemann and J. P. Hindermann, Stud. Surf. Sci. Catal.,
bar and at 50 bar, we are confident that the present findings
1988, 35, 181.
and K. C. Waugh, Appl. Catal., 1988, 36, 1.
2
2
5
are valid also at elevated pressures (at 50 bar), which are more 25 R. G. Herman, Stud. Surf. Sci. Catal., 1991, 64, 265.
relevant for practical applications. Overall, the results further 26 K. C. Waugh, Catal. Today, 1992, 15, 51.
underline the remarkable potential of Au/ZnO catalysts for 27 X.-M. Liu, G. Q. Lu, Z.-F. Yan and J. Beltramini, Ind. Eng.
application in the hydrogenation of CO to ‘‘green MeOH’’ as
Chem. Res., 2003, 42, 6518.
2
an energy storage molecule.
28 K. C. Waugh, Catal. Lett., 2012, 142, 1153.
2
3
9 L. C. Grabow and M. Mavrikakis, ACS Catal., 2011, 1, 365.
0 J. Graciani, K. Mudiyanselage, F. Xu, A. E. Baber, J. Evans, S. D.
Senanayake, D. J. Stacchiola, P. Liu, J. Hrbek and J. F. Sanz,
Science, 2014, 345, 546.
Acknowledgements
We gratefully acknowledge extended discussions with S. Dahl
3
3
3
1 Y. Hartadi, D. Widmann and R. J. Behm, ChemSusChem,
(Haldor Topsøe A/S).
2
015, 8, 456.
2 Y. Hartadi, D. Widmann and R. J. Behm, J. Catal., 2015,
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