The Effects of Secondary Oxides on Copper-Based Catalysts for Green Methanol Synthesis

Article abstract of DOI:10.1002/cctc.201601692

Catalysts for methanol synthesis from CO₂ and H₂ have been produced by two main methods: co-precipitation and supercritical anti-solvent (SAS) precipitation. These two methods are compared, along with the behaviour of copper supported on Zn, Mg, Mn, and Ce oxides. Although the SAS method produces initially active material with high Cu specific surface area, they appear to be unstable during reaction losing significant amounts of surface area and hence activity. The CuZn catalysts prepared by co-precipitation, however, showed much greater thermal and reactive stability than the other materials. There appeared to be the usual near-linear dependence of activity upon Cu specific area, though the initial performance relationship was different from that post-reaction, after some loss of surface area. The formation of the malachite precursor, as reported before, is important for good activity and stability, whereas if copper oxides are formed during the synthesis and ageing process, then a detrimental effect on these properties is seen.

Full text of DOI:10.1002/cctc.201601692

DOI: 10.1002/cctc.201601692
Full Papers
The Effects of Secondary Oxides on Copper-Based
Catalysts for Green Methanol Synthesis
James S. Hayward,[a] Paul J. Smith,[a] Simon A. Kondrat,[a] Michael Bowker,[a, b] and
Graham J. Hutchings*[a]
Catalysts for methanol synthesis from CO2 and H2 have been
produced by two main methods: co-precipitation and super-
critical anti-solvent (SAS) precipitation. These two methods are
compared, along with the behaviour of copper supported on
Zn, Mg, Mn, and Ce oxides. Although the SAS method produ-
ces initially active material with high Cu specific surface area,
they appear to be unstable during reaction losing significant
amounts of surface area and hence activity. The CuZn catalysts
prepared by co-precipitation, however, showed much greater
thermal and reactive stability than the other materials. There
appeared to be the usual near-linear dependence of activity
upon Cu specific area, though the initial performance relation-
ship was different from that post-reaction, after some loss of
surface area. The formation of the malachite precursor, as re-
ported before, is important for good activity and stability,
whereas if copper oxides are formed during the synthesis and
ageing process, then a detrimental effect on these properties
is seen.
Introduction
In recent years the continuing rise in atmospheric anthropo-
genic carbon and the ensuing effects that this engenders on
the climate have led to increasing efforts towards capture, se-
questration, and utilisation of carbon dioxide. Given a general
societal trend towards greener energy and fuel sources, the
utilisation of CO2 as an abundant carbon source has become
more and more relevant. The hydrogenation of CO2 to produce
methanol for use as both a fuel and a chemical precursor is
one possible route to this goal. The concept of a cycle using
CO2 and methanol in such a manner is generally attributed to
Olah et al.,[1] and is referred to as the anthropogenic carbon
cycle. One of the attractive facets of this method is the possi-
bility of a truly green fuel; CO2 can be captured from sources
such as power stations and combined with hydrogen generat-
ed from a renewable source, such as electrolysis of water
where the electricity is supplied from solar power. The synthe-
sis of green methanol in this way can be regarded as a way of
storing H2, effectively for storing renewable energy chemically,
in addition to the production of a fuel.
elevated pressures and moderate temperatures. It has been
shown that carbon dioxide is the carbon source at the molecu-
lar scale, producing methanol and water. Carbon monoxide is
present to convert the water produced into CO2 and H2 via the
water-gas shift reaction. These reactions are represented by
Equations (1) and (2):
CO2 þ 3H2 ! CH3OH þ H2O
CO2 þ H2 ! H2O þ CO
ðmethanol synthesisÞ
ð1Þ
ð2Þ
ðreverse water-gas shiftÞ
The current industrial synthesis is from a syngas, deriving
from fossil fuels, which has a mix of CO and CO2, but the main
synthesis route is as in Equation (3), with little water produc-
tion:
CO þ 2H2 ! CH3OH
ð3Þ
However, the reaction studied herein differs from this
system in that there is no CO present, since we are testing the
possibility of using recaptured CO2 and renewable hydrogen,
as opposed to fossil fuel generated syngas. This means that
without CO being present to enable overall water-gas shift,
our system will contain a significantly higher proportion of
water vapour than a system running syngas.
Global methanol production is in the region of 80 Mt per
annum.[2] Industrially, methanol is produced from a mixture of
carbon monoxide, carbon dioxide, and hydrogen (syngas) at
[a] Dr. J. S. Hayward, Dr. P. J. Smith, Dr. S. A. Kondrat, Prof. M. Bowker,
Prof. G. J. Hutchings
The catalysts used for this reaction are composed of copper,
zinc, and alumina, and are based on the catalysts originally de-
signed by ICI during the 1960s.[3,4] The optimisation of these
catalysts was performed long before modern techniques made
it possible to understand the fundamentals of the active sites
and reaction mechanism. In recent years there have been in-
creasing numbers of studies into these fundamental aspects of
this catalysis,[5,6] with the focus generally tending towards the
simpler binary system of Cu/ZnO. From this has risen the con-
Cardiff Catalysis Institute
Main Building, Cardiff University Park Place,
Cardiff CF10 3AT (UK)
E-mail: hutch@cf.ac.uk
[b] Prof. M. Bowker
Research Complex at Harwell, Rutherford Appleton Lab
Harwell Oxford, , Oxford OX11 0FA (UK)
This publication is part of a Special Issue on “Catalysis for New Energy
Technology”.
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sensus that the methanol productivity is strongly correlated
with the specific copper surface area of a catalyst,[7,8] and that
other factors such as the oxidation state of the copper,[9] and
the copper–zinc interaction,[10,11] also have an effect.
Results and Discussion
This study was conducted using Cu/MxOy catalysts, where M=
Mg, Zn, Mn, or Ce. Based on the results of previous studies
a molar ratio of 70:30 between copper and the secondary
oxide was chosen. This amount has been shown to be close to
the limit of incorporation of zinc into a malachite structure,[20]
and as such was used as a standard to which the other cata-
lysts were held.
The optimised industrial catalyst is granted excellent copper
surface area by way of the structure of the precursor material
from which it is derived. Such catalysts are synthesised by co-
precipitation of metal nitrate salts with sodium carbonate to
produce a hydroxycarbonate precursor phase, which is then
calcined to form CuO and ZnO. It is known that the most
active catalysts are formed from a precursor consisting pre-
dominantly of zinc-substituted malachite phases. When prop-
erly prepared, the specific structure of the final catalyst is de-
fined by this precursor phase, leading to a material with the
desired high copper surface area and good copper–zinc inter-
action.[12]
Co-precipitated catalysts
The catalysts were prepared as described in the Experimental
Section. The surface areas of the materials produced are
shown in Table 1.
Whilst copper comprises the active metal in the catalysts,
the role of the zinc is less clear,[13] and as such there have been
many investigations into copper-based catalysts with alterna-
tive secondary metal oxides such as magnesia,[14] ceria,[15,16]
and zirconia.[17,18] Of particular note are studies that show Cu/
MgO catalysts as having higher copper surface areas, and yet
having lower methanol activity,[14] which runs counter to the
accepted stance of copper surface area being directly linked to
such activity. The issue seems to stem from the nature of the
standard catalyst, the synthesis of which has been highly opti-
mised in terms of pH, temperature and ageing times. Such cat-
alysts are generally precipitated in the range of pH 6–7, and
have been shown to lose activity if precipitated at higher pH
ranges.[12] However, the precipitation of magnesium nitrate re-
quires a pH in the region of 9. Thus it is difficult to deconvo-
lute whether the negative effect on activity is an artefact of
the substituted oxide or the pH of synthesis.
Table 1. Co-precipitated catalyst details.
Surface areas [m2 gꢀ1
]
Catalyst
Synthesis pH Precursor colour
Precursor
Calcined
CuZn-CP-1
CuZn-CP-2
CuZn-CP-3
CuMn-CP
CuMg-CP
CuCe-CP
6.5
9
10
6.5
9
10
blue–green
blue–green
blue–green
green–brown
dark blue
121
120
105
116
128
85
119
117
102
109
125
77
dark brown
X-ray diffraction (XRD) of the precursor phases revealed pat-
terns consistent with those of malachite[21] in the cases of all
CuZn-CP catalysts, as shown in Figure 1. The other precursors
showed broader diffraction peaks. CuMg-CP and CuCe-CP
show two major diffraction peaks consistent with CuO, with
CuMg-CP having low, broad peaks consistent with malachite.
The small peak at 338 in CuCe-CP may be residual malachite,
and this material also shows broad peaks at angles consistent
with CeO2. Based on these observations it would seem that
the hydroxycarbonate phase is formed initially in all cases, as
Supercritical anti-solvent (SAS) precipitation presents an in-
teresting way to approach this problem, as the procedure rap-
idly precipitates material without the need for a base. It has
been shown that this method can produce copper–zinc cata-
lysts with high copper surface area, and that these catalysts
are active for methanol synthesis and water-gas shift reac-
tions.[19] A wide range of materials can be precipitated in this
manner, in all cases without the requirement of a specific pre-
cipitating agent. This allows us to sidestep the need for specif-
ic pH ranges found in co-precipitation, allowing us to remove
it as a factor.
In this study we report the changes in methanol synthesis
activity for copper catalysts synthesised with various secondary
oxides using both co-precipitation and SAS techniques. The
changes were monitored through reactivity measurements,
and through assessment of the copper surface area and parti-
cle size both before and after exposure to reaction conditions.
Through this we hope to discover what factors result in activity
loss in the co-precipitated catalysts, and to investigate which
metal oxides are capable of producing active, stable catalysts
when the negative effects of high-pH co-precipitation are re-
moved.
Figure 1. XRD patterns of co-precipitated catalyst precursors.
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evidenced by the blue coloured material often reported in
such cases. The colour change towards green in the CuZn and
CuMn catalysts can be attributed to the formation of the mala-
chite phase, whereas the darkening in colour of the CuMg and
CuCe catalysts can be attributed to the formation of copper
oxide phases. This cannot solely be attributed to the effects of
the pH, as the CuZn catalysts prepared at higher pH do not
show these phases. Therefore, this would seem to be an effect
of the oxide, although possibly this is in combination with the
elevated pH.
The work of Fujita et al.[22,23] showed that a calcination tem-
perature of 330 to 3508C is sufficient to form the final cata-
lysts. Based on this, all catalysts were calcined at 3308C for 3 h
in flowing air, with a thermal ramp rate of 58Cminꢀ1. The cata-
lysts were uniformly brown after this calcination step, with the
exception of CuCe-CP, which presented a slightly grey hue.
XRD of the calcined catalysts gave similar patterns for all the
materials except for CuCe-CP (Figure 2). All had peaks consis-
Table 2. Catalytic activity and copper surface areas of co-precipitated cat-
alysts.
CO2 conversion
[%]
Selectivity
MeOH [%] CO [%]
1 h 8 h Pre Post
Cu SSA
[m2 gꢀ1
]
Catalyst
1 h
8 h
1 h
8 h
CuZn-CP-1
CuZn-CP-2
CuZn-CP-3
CuMn-CP
CuMg-CP
CuCe-CP
4.5
4.1
3.5
2.8
3.9
2.3
4.3
3.7
3.2
3.5
1.1
0.8
55
53
32
38
69
38
53
51
30
54
40
61
45
47
68
62
31
62
47
49
70
46
6
21
20
18
18
24
15
15
12
10
5
7
3
39
copper–zinc catalyst was also evident; increasing pH lowered
CO2 conversion and methanol selectivity, while those at lower
pH preparation tended to show a lower degree of deactiva-
tion. CuZn 6.5-CP lost only 5% CO2 conversion from 1–8 h,
compared with 7% for CuZn 9-CP and 10% for CuZn 10-CP.
CuMn-CP and CuMg-CP had similar, if not higher, copper sur-
face areas than the CuZn-CP catalysts before reaction, but
were not as active. CuMg-CP gave good CO2 conversion and
excellent methanol selectivity, but continued to deactivate
after 3 h, stabilising after 6 h. CuMn-CP appears to have gained
activity over time, but a full time on-line reaction study
showed a slightly more complicated effect. CuMn-CP started
with the low activity shown above, and appeared to immedi-
ately start deactivating. However, after about 2 h, it began to
show a marked increase in activity over the next hour, with
both CO2 conversion and methanol selectivity rising rapidly to
approximately 6.3% conversion and 63% selectivity. It main-
tained this activity for about 2 h before undergoing a rapid de-
activation. Of particular interest is that the overall CO produc-
tion rate changed only slightly during this time, implying that
the increased CO2 conversion was primarily driven by a large
increase in methanol selectivity, and that the deactivation oc-
curred in the reverse manner. This would seem to indicate that
species or active sites are briefly formed on CuMn catalysts
that are highly active, but highly unstable. A repeat of this test
over a longer time period (16 h) showed that the deactivation
continued beyond 8 h, with the material having apparently sta-
bilised after about 11 h, at which time it displayed CO2 conver-
sion of <1%. CuCe-CP deactivated steadily, stabilising only in
the final hour of testing. Whilst it was the only catalyst to in-
crease methanol selectivity steadily, it does not seem to be
a viable catalyst due to high deactivation and low activity.
The relationship of the copper surface areas to activity is in-
teresting (Figure 3 and Table 2). Initial copper surface areas
appear to match trends in the activity quite well, with the no-
table exception of CuMg-CP. This catalyst possesses higher
copper surface area than any of the others, and yet has lower
activity. However, if one considers the post-reaction copper
surface areas, there is a more evident trend. Here, the higher
surface areas correspond to higher activities with the excep-
tion of the CuMn-CP sample. However, the CuMn-CP catalyst
was observed to be deactivating rapidly at the termination of
the reaction, and the depressurising and cooling steps before
recovery of the catalyst take approximately an hour. It may be
Figure 2. XRD patterns of co-precipitated catalysts after calcination.
tent with copper oxide, but with the CuCe-CP material having
additional peaks consistent with CeO2. With the exception of
the appearance of a small, sharp peak at 368, the precursor
and calcined versions of CuCe-CP are very similar. The precur-
sor and calcined versions of CuMg-CP are also highly similar,
with the calcined version losing the small, broad peaks associ-
ated with malachite. This is consistent with copper oxide al-
ready being formed during drying in these materials. The sur-
face areas of the materials after calcination decreased by less
than 10% from the values found in the precursors.
The catalyst samples were tested for methanol synthesis as
described in the Experimental Section, and the only significant
products seen were CO and methanol. The CuZn catalysts gen-
erally appeared to undergo a strong initial deactivation, but
were stable after 3 h. The other secondary oxides displayed
varying behaviour, which will be discussed below. Of the co-
precipitated catalysts, copper–zinc showed the highest activity
towards methanol production, and also showed the lowest
amount of deactivation (Table 2).
A trend amongst the
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Figure 4. XRD patterns for SAS catalyst precursors.
Figure 3. The dependence of catalyst activity on the Cu metal surface area
for co-precipitated catalysts. The lines are a guide for the eye. The red data
points are for initial conversion and pre-reaction metal area, whereas the
blue data points relate the final conversion and metal surface area measured
post-reaction.
thesis of supercritically prepared georgeite[19,24] It is possible
that the other oxides form similar amorphous materials as an
effect of the extremely rapid precipitation step found in SAS
precipitations.
XRD analysis on the calcined SAS materials (Figure 5)
showed a number of similarities to the CP materials. CuCe
shows a small diffraction peak in the region of copper(II) oxide,
and shows broad reflections consistent with the presence of
CeO2. The other materials all appear to be copper oxide, as
was observed in the CP materials. However, whereas the CP
materials all displayed a distinctive double peak, the CuZn-
and CuMg-SAS show a single, broader reflection. This is indica-
tive of smaller crystal domains in the material, which is likely
to be an effect of the highly amorphous precursor being
unable to generate long-range order upon calcination. Unlike
the co-precipitated catalysts, the SAS catalysts displayed a far
more significant loss of surface area upon calcination, with all
that the CuMn-CP catalyst continued to deactivate through
this time. It appears that, although conversion has generally di-
minished, the intrinsic per site activity has increased after 8 h
running, evident in the data of Figure 3. It is likely that this is
due to a morphology change of the Cu particles, perhaps such
that the CuꢀZnO interaction is not lost as much as the Cu sur-
face area.
Thus, the value of the copper–zinc catalysts would appear
to be a combination of high initial copper surface area and
their ability to better retain this surface area during reaction
conditions.
Supercritical anti-solvent (SAS) precipitation catalysts
Supercritical anti-solvent precipitations were carried out as de-
scribed below and produced very fine powder, which was
either blue or green depending on the secondary oxide
(Table 3).
XRD analysis on the precursor phases showed them to be
highly amorphous/nanoparticulate (Figure 4). It is difficult to
thus draw any conclusions about the materials formed, but
these observations are in line with those reported for the syn-
Table 3. Supercritical anti-solvent precipitation catalyst details.
Surface areas [m2 gꢀ1
]
Catalyst[a]
Precursor colour
Precursor
Calcined
CuZn-SAS
CuMn-SAS
CuMg-SAS
CuCe-SAS
light blue
light green
light blue
light green
157
100
150
112
110
72
144
69
Figure 5. XRD patterns of SAS catalysts after calcination.
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losses being in the region of 30%. A notable exception to this
is CuMg-SAS, which lost less than 5%.
The CuMn-SAS sample once again displayed a more com-
plex behaviour than is suggested. Initial results shown here are
at t=1 h, but data recorded before this point show CuMn-SAS
to be highly active, more so than CuZn-SAS. However, it imme-
diately deactivates, losing over 60% of its activity in the initial
hour. After 4 h it has almost completely deactivated and is pre-
dominantly selective towards the production of CO. The
copper surface area measurements show a significant loss of
surface area throughout the reaction, which is likely to be the
cause of this deactivation. It is of note, though, that the initial
activity is significantly higher than the pre-reaction Cu SSA of
the CuMn-SAS sample would suggest. It is possible that this is
a similar effect to that which was seen for CuMn-CP, but with-
out the induction period. The highly mixed and amorphous
structure of the CuMn-SAS could be very active initially, but
then undergoes severe deactivation for the same reasons as
before. This would seem to be borne out by the dramatic drop
in copper surface area.
Catalytic testing and copper surface area measurements
were carried out in an identical manner to those described for
the co-precipitated materials.
The reactivity behaviour of the SAS catalysts can be seen to
be significantly different from that of the co-precipitated mate-
rials, with the possible exception of the CuCe-SAS material,
which in both cases shows decreasing activity but increasing
methanol selectivity (Table 4). These catalysts all displayed
Table 4. Catalytic activity and copper surface areas of SAS catalysts.
CO2 conversion
[%]
Selectivity
MeOH [%] CO [%]
1 h 8 h Pre Post
Cu SSA
[m2 gꢀ1
]
Catalyst
1 h
8 h
1 h
8 h
CuZn-SAS
CuMn-SAS
CuMg-SAS
CuCe-SAS
5.7
2.3
4.6
3.5
1.8
0.8
3.6
1.8
71
77
66
42
46
18
64
56
29
23
34
58
54
82
36
44
31
21
29
26
8
3
12
8
CuMg-SAS displayed behaviour entirely contrary to the CP
equivalent, proving to be the most stable of the SAS catalysts
in terms of both CO2 conversion and selectivity. It does not
appear to display such rapid deactivation, nor the switching to
CO production of its CP counterpart, and after 8 h is compara-
ble in activity to CuZn 6.5-CP, which is due to its higher selec-
tivity. Whilst it is shown to lose a significant amount of copper
surface area, the loss is nowhere near as severe as in the cases
of the other SAS materials.
stronger initial deactivation than their co-precipitated counter-
parts, but were all stable after 5 h. The CuZn-SAS material is of
particular interest. In keeping with evidence that increased
copper surface area is directly linked to increased CO2 conver-
sion (Figure 6), it is the most active catalyst when based on
the results taken after 1 h. It remains active for approximately
4 h, but loses methanol selectivity as it does so. After this
point, it begins to rapidly lose activity and continues to lose
selectivity, stabilising after 5 h. Cu SSA measurements show
a significant drop as a result of the reaction.
Conclusions
A number of conclusions can be drawn from the results
herein. One of the initial questions was to what extent, in co-
precipitated catalysts, is he activity of copper-based catalysts
determined by the pH of precipitation and to what extent is it
affected by the secondary oxide. Based on the results shown,
we can say that both play a role in the activity of the catalyst.
CuZn-CP catalysts were more active than all of the other sec-
ondary metals at the equivalent pH. From the results of the
CuZn materials we see that increased pH leads to decreased
activity. A difference in precipitation behaviour was seen as
well; whereas the CuZn-CP catalysts form zincian malachite at
all three pH values, when the zinc is replaced with magnesium
or ceria at elevated pH it leads to the direct formation of
copper oxides during the ageing step of the synthesis, as con-
firmed by XRD. The better performance of the Zn materials is
probably due to the formation of this phase.
CuMn-CP showed interesting behaviour, in that there ap-
peared to be an induction period where the activity increased,
reaching a plateau for a time before rapidly deactivating. In
many ways this mirrors the findings of Helveg et al.,[25] who
showed that copper–zinc catalysts will display similar tenden-
cies depending on the oxidising or reducing nature of the at-
mosphere. Although the gas mixture is highly reducing due to
60% H2, the oxidising nature of the atmosphere increases with
increasing steam content.[26] As H2O is a by-product of both
the methanol and reverse water-gas shift reactions which
occur, it would seem that the catalyst generates an active
Figure 6. The dependence of catalyst activity on the Cu metal surface area.
The lines are a guide for the eye, but indicate linear behaviour, though here
the number of data points is limited. The red data points are for initial con-
version and pre-reaction metal area, whereas the blue data points relate the
final conversion and metal surface area measured post-reaction.
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phase which is then adversely affected by the increased water
content that this improved activity engenders. This then leads
to the severe deactivation seen.
SAS has the lower initial surface area, but proves to be the
more active catalyst in the long run due to its stability. This
focus on stability appears to be a strength of the co-precipitat-
ed CuZn materials, which displayed the lowest amount of de-
activation.
Once the results of the SAS catalysts are factored in, more
conclusions can be drawn. Whilst for the CP materials in-
creased pH leads to lower activity, the SAS results show that
this is not the sole determinant. The activities still do not corre-
late exactly with the Cu SSA, as the CuMg has a higher area
than CuZn. This shows that there is indeed an additional effect
from the secondary oxide beyond the simple improvement of
the active metal surface area, and that the relatively lower ac-
tivity of the CuMg catalysts is not only a result of the higher
precipitation pH required in CP.
Thus, the stability of the materials, and their effectiveness as
catalysts, can be attributed to a number of factors beyond ini-
tial copper surface area. The formation of the malachite phase
seems to be especially important in coprecipitation; CuZn and
CuMn-CP catalysts form this phase, and were significantly
more active than their amorphous SAS counterparts. This
phase appears to grant a greater degree of stability to the re-
sulting catalysts. Where materials did not form this phase, they
were all found to be less stable. This effect cannot be attribut-
ed to the presence of zinc as the secondary oxide, as the
CuZn-SAS catalyst was highly unstable. By contrast, the forma-
tion of the CuO phase during precipitation was indicative of
a poor catalyst.
Further investigation of these effects are required to ascer-
tain which properties of the secondary oxides are affecting the
Cu. H2-TPR could be useful to investigate the reducibility of
the catalysts, and CO2-TPD can be used to assess changes in
the basicity of the catalysts.
The CuZn-SAS catalyst shows a similar deactivation to that
reported before, although it does not show the initial induc-
tion period. Interestingly, the CuMn catalyst appears to have
very similar behaviour, although it deactivates even more swift-
ly. Both CuZn-SAS and CuMn-SAS suffer a particularly pro-
nounced loss of Cu surface area during the reaction, with
CuMn-SAS falling to the lowest value of any tested catalyst.
Based on these overall results, it would seem that Mn and Zn
behave in a broadly similar manner when paired with Cu, but
that Zn is the better choice due to increased stability of the
supported Cu metal.
The results obtained using the SAS-prepared catalysts help
to back up the benefits of the malachite phase, but also show
that for some materials the pH is a significant factor. CuMg is
a good example of this; neither the CP nor the SAS catalyst
form the malachite phase, but the elevated pH led to the for-
mation of the undesirable CuO phase during co-precipitation.
Where this phase was not observed, in the SAS material, the
catalyst was far more effective. This was not the case for the
CuCe materials, which were less effective regardless of prepa-
ration method. This indicates that the choice of oxide is highly
relevant.
CuMg catalysts proved interesting, as they were the only in-
stance in which the SAS material was more stable than the CP
material. This is seen in both the activity data and the copper
surface area data, and could be down to a number of factors.
The CuMg-CP material showed evidence of CuO formation
during the initial precipitation, and whilst this material had
a high copper surface area it swiftly deactivated under reaction
conditions. This behaviour was not observed in the SAS materi-
al, implying that the formation of the CuO phase was not con-
ducive to retention of the high copper surface area even
though it generated a high initial value. The amorphous SAS
precursor, however, led to a material more stable than its CP
counterpart or any other SAS prepared material. This may be
due to the properties of MgO itself, which is not reported to
form strong interactions with Cu (unlike zinc) and does not
have a variety of possible oxidation states (unlike manganese
and ceria).
Overall, the results seem to show that when considering co-
precipitation, CuZn catalysts appear to be significantly better
due to a number of benefits granted by the precursor phase.
CuMn catalysts behave in a similar manner, but deactivate
more rapidly. When the materials were prepared by a method
which leads to a highly amorphous precursor, other oxides
become viable. CuMg seems in particular to be hampered by
the high pH needed for precipitation. Once this limitation was
removed, it proved to be an effective catalyst. This could po-
tentially be of use as other precipitation methods are
investigated.
Another important conclusion is the apparent confirmation
of the work of Hadden et al.,[27] who suggested that the corre-
lation between copper surface area and activity was only valid
between families of catalysts prepared with similar method.
This is borne out in our results, as the higher surface area ma-
terials do not always prove to be the most active, and nor is
the copper surface area across the range of oxides always di-
rectly proportional to the activity. We can extend these conclu-
sions to account for the post-reaction surface area losses. It
seems that different preparation conditions, methods, and sec-
ondary oxides strongly influence the rate of initial deactivation
of the catalysts, which is a key factor in their activity after
stabilisation.
When taken as a whole, the results strongly imply that
whilst the initial copper surface area is important, the ability to
retain this surface area whilst under reaction conditions would
appear to be key. Further, the idea that copper surface area is
directly correlated to methanol activity may not be easily appli-
cable to materials using different secondary oxides. An excel-
lent example of this lies in the CuMg catalysts. CuMg-CP has
a higher initial copper surface area than its CuZn-CP equiva-
lent, but its rapid deactivation means that the post-reaction
area value shows a truer measure of its activity. The same is
true of CuMg-SAS and CuZn-SAS. In this instance the CuMg-
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gave a scCO2/mixed metal solution molar ratio of 22:1. The system
pressure and temperature were maintained and the preparation
conditions were carefully controlled. Leak checks were also periodi-
cally carried out throughout the procedure using snoop solution.
When all the mixed-metal solution had been processed, a drying
step was carried out. This was achieved by pumping pure ethanol
at 6.5 mLminꢀ1 co-currently with scCO2 for 30 min, before leaving
with just scCO2 to pump for a further 60 min. This was to wash the
vessel in case residual solvent condensed during depressurisation
and partly solubilised the prepared materials. When the drying
step was complete the scCO2 flow rate was stopped, the vessel
was depressurised to atmospheric pressure and the precipitate
was collected. Experiments were conducted for approximately 5 h
which resulted in the synthesis of ca. 2.5–3 g of solid.
Experimental Section
Materials
Copper(II) acetate monohydrate (puriss. p. a., ꢁ99.0%), zinc(II) ace-
tate dihydrate (puriss. p. a., ꢁ99.0%), manganese(II) acetate tetra-
hydrate (99+%) copper(II) nitrate hemipentahydrate, zinc(II) nitrate
hexahydrate, manganese(II) nitrate tetrahydrate, cerium nitrate,
magnesium(II) nitrate hexahydrate, sodium carbonate, and cerium
acetylacetonate hydrate were all purchased from Sigma–Aldrich.
Magnesium(II) acetate tetrahydrate (analytical) was obtained from
Amresco. Ethanol (absolute 99.8%, Certified AR) was purchased
from Fischer Scientific and CO2 (CP grade) was provided by BOC.
All purchased materials were used as received. Deionised water
was provided in-house.
XRD
Co-precipitated catalysts
Powder X-ray diffraction measurements were performed using
a PANalytical X’pert Pro diffractometer with Ni filtered CuKa radia-
tion source operating at 40 kV and 40 mA. Patterns were recorded
over the range of 10–808 2q using a step size of 0.0168. All pat-
terns were matched using the ICDD database.
The co-precipitated catalyst precursors were synthesised by co-pre-
cipitation of metal salts using a Toledo Metrohm autotitrator.
Copper nitrate hemipentahydrate (Cu(NO3)2·2.5H2O), zinc nitrate
hexahydrate (Zn(NO3)2·6H2O), and aluminium nitrate nonahydrate
(Al(NO3)2·9H2O) were dissolved in deionised water to create
a mixed-metal solution with a total molar concentration of 0.25 M.
Additionally, a base solution was created by dissolving sodium car-
bonate (Na2CO3) in deionised water to give a concentration of
1.5m Na2CO3.
Surface area measurements
Cu surface area analysis was carried out on a Quantachrome
ChemBET chemisorption analyser equipped with a thermal conduc-
tivity detector (TCD). Calcined samples (50 mg) were reduced to
catalysts using 10% H2/Ar (30 mLminꢀ1) with heating to 1408C at
108Cminꢀ1, and then to 2258C at 18Cminꢀ1. For Cu surface area
analysis, catalysts were cooled to 658C under He for N2O pulsing.
12 N2O pulses (113 ml each) were followed with 3 N2 pulses for cali-
bration. The amount of N2 emitted was assumed to amount to half
a monolayer coverage of oxygen and that the surface density of
A small aliquot (20 cm3) of the mixed metal solution was added to
the reaction vessel, which was stirred continuously. The amount of
liquid was chosen such that it was sufficient to cover the pH
probe. This initial aliquot was brought to pH 6.5 by the addition of
the base solution until the target pH was reached.
Subsequently, the mixed metal solution was added to the vessel at
a rate of 5 cm3 minꢀ1 with continuous stirring. Concurrently, base
solution was added at a sufficient rate to ensure that the reaction
mixture maintained a constant pH of 8. Once all of the mixed
metal solution was added, the pH was monitored and controlled
for a further 10 min to ensure complete precipitation of the materi-
al. Thereafter, the precipitate was allowed to age in solution at
658C for 3 h.
Cu is 1.47ꢁ1019 atoms mꢀ2
.
Catalytic methanol synthesis
The catalytic performance of the catalysts for CO2 hydrogenation
was determined in a fixed-bed continuous-flow reactor. The cata-
lyst (0.2 g, 425–600 mm) was placed in a stainless steel tube reactor
with an internal diameter of 4.57 mm. Prior to the reaction, the cat-
alysts were prereduced in a flow of 5% H2/He (30 mLminꢀ1) for 1 h
at 2258C under atmospheric pressure. The reactor was then al-
lowed to cool to room temperature before gas flow was switched
to the reactant mixture (CO2:H2:N2 20:60:20 molar %). The pressure
was increased to 20 bar using a backpressure regulator before the
flow was set to 12.5 mLminꢀ1 to give a GHSV of 1000 hꢀ1. The reac-
tions were conducted at 2258C. All post-reactor lines and valves
were heated at 1108C to avoid product condensation. The gas
products were analysed via online gas chromatography using an
Agilent 7890 system with a flame ionisation detector (FID) and
TCD. Nitrogen was used as an internal standard. Samples were
taken every 15 min over the course of 8 h. CO2 conversion was cal-
culated by the change in moles of CO2 compared to calibration
runs. The selectivities of methanol and CO represent the respective
carbon molar % of the products. In all cases, methanol and CO
were the only products observed.
This precipitate was filtered under suction and washed with water
to remove excess sodium salts. This material was then dried at
110 8C for 16 h before being calcined at 3258C (thermal ramp rate
58Cminꢀ1) in static air for 3 h to produce 2.5–3 g of the catalyst.
Supercritical anti-solvent precipitation catalysts
A mixed solution of Cu(OAc)2·H2O (4.1561 mgmLꢀ1) with either
Zn(OAc)2·2H2O (1.9584 mgmLꢀ1), Mg(OAc)2·4H2O (1.9132 mgmLꢀ1),
Mn(OAc)2·4H2O
(2.1866 mgmLꢀ1),
or
Ce(acac)3·xH2O
(3.9026 mgmLꢀ1) was prepared in a 5 vol% H2O/ethanol mixture
(1000 mL) to give a nominal Cu:X molar ratio of 70:30. SAS prepa-
ration was performed using apparatus manufactured by Separex.
Liquefied CO2 was pumped to give a flow rate of 6.5 kghꢀ1 and
the whole system was pressurised to 110 bar and held at 408C. Ini-
tially, pure solvent (5 vol% H2O/ethanol) was pumped through the
fine capillary into the precipitation vessel, with a flow rate of
6.5 mLminꢀ1 for 15 min, in co-current mode with scCO2 in order to
obtain steady state conditions inside the vessel. After this initial
period, the flow of liquid solvent was stopped and the mixed
metal solution was delivered at a flow rate of 6.5 mLminꢀ1. This
After the reaction, the reactor was depressurised and left under
flowing helium (10 mLminꢀ1) until cool. The catalyst was then re-
covered from the tubes and subjected to another set of copper
surface area measurements as above. The reduction step was per-
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Accepted Article published: February 15, 2017
Final Article published: && &&, 0000
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Supercritical anti-solvent (SAS) precipi-
tation and co-precipitation methods
were used to make catalysts for metha-
nol synthesis from CO2 and H2, utilising
Cu supported on Zn, Mg, Mn, and Ce
oxides. The SAS method produces ini-
tially active materials with high Cu spe-
cific surface area, but they are unstable.
Co-precipitation led to much greater
stability materials. There was a near-
linear dependence of activity upon Cu
specific area (see picture), but the initial
performance was different from that
post-reaction, after loss of surface area.
J. S. Hayward, P. J. Smith, S. A. Kondrat,
M. Bowker, G. J. Hutchings*
&& – &&
The Effects of Secondary Oxides on
Copper-Based Catalysts for Green
Methanol Synthesis
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