Investigations of Cu/Zn Oxalates from Aqueous Solution: Single-Phase Precursors and Beyond

Article abstract of DOI:10.1002/chem.201803124

The existence of a limited solid-solution series in the Cu/Zn binary metal oxalate system is reported. Coprecipitation was applied for the preparation of a comprehensive set of mixed Cu/Zn oxalates. Rietveld refinement of the XRD data revealed the formation of mixed-metal oxalate single phases at the compositional peripheries. Accordingly, the isomorphous substitution of Zn^II into Cu^II oxalate takes place at Zn contents of ≤6.6 and ≥79.1 atom %. Zn incorporation leads to a pronounced unit-cell contraction accompanied by Vegard-type trends for the lattice parameters. Morphologically, both solid solutions show close resemblance to the corresponding pure single-metal oxalates, and thus distinct differences are identified (SEM). The successful formation of solid solutions was further evidenced by thermal analysis. The decomposition temperature of the oxalate was taken as an approximation for Zn^II incorporation into the Cu^II oxalate structure. Single decomposition events are observed within the stated compositional boundaries and shift to higher temperature with increasing Zn content, whereas multiple events are present near Cu/Zn parity. Moreover, these findings are supported by IR and Raman spectroscopic investigations. This study on the Cu/Zn mixed-metal oxalate system sheds light on the important prerequisites for solid-solution formation and identifies the structural limitations that predefine its application as catalyst precursor.

Full text of DOI:10.1002/chem.201803124

A Journal of
Accepted Article
Title: Investigations of Cu/Zn Oxalates from Aqueous Solution: Single
Phase Precursors and Beyond
Authors: Leon Zwiener, Frank Girgsdies, Robert Schlögl, and Elias Frei
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To be cited as: Chem. Eur. J. 10.1002/chem.201803124
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Supported by

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Investigations of Cu/Zn Oxalates from Aqueous Solution: Single
Phase Precursors and Beyond
Leon Zwiener , Frank Girgsdies , Robert Schlögl[a,b], and Elias Frei*[a]
[
a]
[a]
Abstract: The existence of a limited solid solution series in the
Cu/Zn binary metal oxalate system is reported. Co-precipitation was
applied for the preparation of a comprehensive set of mixed Cu/Zn
oxalate samples. Rietveld refinement of the diffraction data (XRD)
revealed the formation of mixed metal oxalate single phases at the
compositional peripheries. Accordingly, the isomorphous substitution
oxalates such as of copper and zinc are being widely
[
5]
investigated as precursors in the synthesis of nanomaterials
[
6]
and catalysts . The fast and easy process of precipitation from
aqueous solutions and the subsequent emission of volatile CO
and CO
oxides[7] identifies the metal oxalates as ideal precursor phase.
joint cationic sublattice allows high level of Cu/Zn
2
during its thermal decomposition to metal or metal
II
II
of Zn into Cu oxalate takes place at a Zn content ≤6.6 and ≥79.1
at.-%. The Zn incorporation leads to pronounced unit-cell
A
a
a
substitution and therefore a nano-scale intermixing as a central
prerequisite for highly dispersed Cu/ZnO particles in the final
contraction accompanied by Vegard-type trends for the lattice
parameters. Morphologically both solid solutions share close
resemblance to the corresponding pure single metal oxalates and
thus distinct differences are identified (SEM). The successful
formation of solid solutions is further evidenced by thermal analysis
catalyst.[
8]
II
O (0 ≤ n ≤1[3-4, 9]) holds a special
Cu oxalate CuC
2
O
4
× n H
2
position within the group of first-row transition metal oxalates.
Unlike the others, it is not obtained as a dihydrate of the type
II
[10]
(
STA/EGA). The decomposition temperature of the oxalate is taken
M C
2
O
4
× 2 H
2
O (M: Mn, Fe, Co, Ni, Zn).
However, early
II
II
[9a]
as an approximation for Zn incorporation into the Cu oxalate
structure. Single decomposition events are observed within the
stated compositional boundaries shifting to higher temperature with
increasing Zn-content, while multiple events are present near the Cu
and Zn parity. Moreover, these findings are supported by IR- and
Raman spectroscopic investigations. This study on the mixed Cu/Zn
metal oxalate system sheds light on the important prerequisite of
solid solutions formation and identifies the structural limitations
which predefine its application as catalyst precursor.
investigations of Schmitler
already indicated that the small
is not an integral part of
the crystal lattice.[4, 7c] The dehydrated sample was obtained
after thermal treatment at 508 K in H
atmosphere.[9a] No single
2 4
proportion of water contained in CuC O
2
crystal structure determination has been reported on either
Moolooite or its synthetic analogue due to the lack of suitable
crystal quality.[9a, 9b, 11] Instead, structural information were
deduced mainly from consistent powder X-ray diffraction[9a, 9b]
and EXAFS[12] investigations. In this respect, a combined
neutron and X-ray powder diffraction study has been recently
published.[11] Accordingly, CuII oxalate crystallizes in the
monoclinic space group P2
ribbon-like motif formed by planar −Cu−C
1
/c. The structure consists of a
−Cu− bands along
Introduction
2
O
4
the c-axis (Figure 1a). Within in the ribbon, the divalent cation is
coordinated by four oxygen atoms of two chelating, bridging
oxalate ligands, while the two axial positions of the distorted
octahedral surrounding are occupied by oxygen atoms of the
adjacent ribbons in approximately perpendicular orientation.
Thus, a three-dimensional structure is formed by the interlinkage.
In addition, stacking faults representing slips of the ribbons
against each other are present along the b-axis, leading to a
Oxalic acid, the simplest dicarboxylic acid, is widely distributed
in nature including fungi, lichens, plants and animals.[1] It readily
forms metal oxalates leading to a broad diversity of different
salts and complexes comprising alkali, alkaline earth, transition
metal including copper and zinc and rare-earth elements.[2]
Among them, the majority of simple oxalates are insoluble in
water.[2] Hence, natural occurring oxalates constitute the largest
group of carboxylate minerals and represent the most common
group of organic minerals[3] including Moolooite, the naturally
occurring hydrated copper oxalate, first reported in the mid-
II
9
disordered phase. The Cu ion with the d electron configuration
shows the Jahn-Teller effect leading to distorted coordination
geometries.[13]
1
980s by Clarke and Wiliams.[4] In addition, the synthetic metal
2 4 2
The dihydrate α-ZnC O × 2 H O crystallizes in the
monoclinic space group C2/c. Here, the divalent cation is
coordinated by four oxygen atoms within the ribbon-like motif
while the octahedral coordination is completed with the oxygen
atoms of two neighboring water molecules (Figure 1b).[10b] Thus,
the ribbons are only interconnected indirectly via hydrogen
bonds. Dehydration of this compound gives rise to the
[
[
a]
L. Zwiener, Dr. F. Girgsdies, Prof. Dr. R. Schlögl, Dr. E. Frei
Department of Inorganic Chemistry
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin, (Germany)
E-mail: efrei@fhi-berlin.mpg.de
b]
Prof. Dr. R. Schlögl
disordered α-ZnC
belongs to monoclinic space group P2
× n H O shares close structural resemblance with the latter
2
O
4
(Pnnm) and the ordered β-ZnC
2 4
O which
Department of Heterogeneous Reactions
Max-Planck-Institut für Chemische Energiekonversion
Stiftstrasse 34 – 36, Mülheim an der Ruhr, 45470 (Germany)
/n (Figure 1c).[14] CuC
1
2 4
O
2
2 4
polymorph (Figure 1), although the β-ZnC O structure does not
include stacking faults.[11, 14]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.2018xxxxx.
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II
II
The successful isomorphous substitution by Zn in Cu oxalate
has been rarely reported in literature. In the thermal
compositions derived by XRF (Table 1) are in agreement with
the nominal values for all samples close and above the
equimolar range. However, there are characteristic deviations
present in the compositional periphery. The samples up to a
nominal value of 40 at.-% Zn (Zn_40) show a pronounced lack
of Zn in the precipitate. A similar observation has been reported
decomposition of
a co-precipitated Cu/Zn oxalate sample
[
15]
(
46.2 % Cu), Dalvi and Chavan
observed an anomalous
single decomposition step which they addressed to the
formation of a meta-stable solid solution. Applying the oxalate
gel-co-precipitation method to the ternary Cu/ZnO/Al
2
O
3
system,
in the case of Cu-rich mixed Cu/Zn basic formats
Deng et al.[ indicated the formation of the binary mixed metal
oxalate phase from ethanolic solution. It was noted that the ZnII
oxalate phase is present at low Cu content (Cu/Zn < 1/8)
respectively vice versa (Cu/Zn > 2). So far, a comprehensive
study on the substitution of Cu- and Zn-oxalates in aqueous
solutions has not yet been reported in the literature.
6a]
x
)
2
3
2
[16]
(Cu1-xZn
(OH)
HCO
.
Despite thorough washing of the
precipitate, minor potassium impurities are present in the
samples. This has been previously observed when potassium
oxalate was used as precipitation agent.[
17]
Table 1. Overview of prepared oxalate samples
including metal composition (XRF) and specific surface
area determined by N physisorption (SABET).
2
-
1
Sample
Exp. [at.-%]
SABET [m²g ]
Cu
Zn
Zn_0
Zn_5
100
99.0
93.4
87.3
74.0
68.7
56.1
50.0
44.8
43.4
30.5
20.9
18.2
6.8
0
10
10
9
1.0
Figure 1. Metal coordination and ribbon-like motif present in the structure of
Zn_15
Zn_20
Zn_30
Zn_40
Zn_45
Zn_50
Zn_55
Zn_60
Zn_70
Zn_80
Zn_85
Zn_95
Zn_100
6.6
CuC
ICSD card no: 109665). Structure model of Christensen et al.
visualization of CuC , stacking faults along b axis not shown.
2
O
4
(a,), α-ZnC
2
O
4
× 2 H
2
O (b, ICSD card no: 56466) and β-ZnC
2
O
4
(c,
[
11]
used for
12.7
26.0
31.3
43.9
50.0
55.2
56.6
69.5
79.1
81.8
93.2
100
10
10
10
13
14
15
14
13
16
14
14
11
2
O
4
In this work, we present a study on the successful Zn-
II
incorporation in Cu oxalate, ranging from the pure single metal
oxalates to equimolar ratios, also addressing the existing
structural limitations. Under a constant pH-value, co-precipitation
from aqueous solution is used for the preparation of a complete
set of mixed Cu/Zn oxalate samples. The obtained precipitates
were thoroughly characterized including XRD, XRF, STA/EGA,
IR- and Raman vibrational spectroscopy. Our findings provide
valuable information on the boundaries of the limited solid
solution series of Cu/Zn oxalates and identifying the regimes of
perfectly dispersed Cu- and Zn-moieties.
Results and Discussion
0
Synthesis, Composition and Structural Analysis
The powder X-ray diffraction patterns of the prepared sample
series are presented in Figure S2. The two samples obtained
from the single metal solutions show the diffraction pattern of
crystalline Cu oxalate (PDF2 card no. 21-297) in case of Zn_0,
respectively coincides with PDF2 card no. 25-1029 for the
Co-precipitation is a standard synthesis technique which was
applied to study the formation mixed Cu/Zn oxalates. Initial
precipitation titration experiments (see Supporting Information
II
(
SI), Figure S1) were conducted to find suitable reaction
conditions for their co-precipitation. By raising the pH value of
the acidified mixed metal solution (pH 0.1), CuII oxalate
2 4 2
Zn_100 sample. Thus, α-ZnC O × 2 H O rather than the
structurally related β-ZnC is formed under the given reaction
2
O
4
precipitates first, followed by the presence of α-ZnC
2 4 2
O × 2 H O
conditions. The patterns of samples containing up to nominal
below pH 1.0. The formation of unwanted K Cu(C O )2-× 2 H O
2
2 4 2
II
values of 95 at.-% Zn share close resemblance to the Cu
was observed above pH 1.8. Hence, co-precipitation from
aqueous solution was performed under a fixed pH of 1.0 at
oxalate pattern. Within this series, the incremental shift of
particular reflections and changes in their peak shape can be
observed caused by the increased Zn content (χZn) which
3
03 K (see experimental section). In order to investigate the
II
II
limits of Zn incorporation in the Cu oxalate structure, the Zn-
content was incrementally increased from 0 to 100 % within a
sample series. All samples are named according to their nominal
Zn-contents as presented in Table 1. The relative metal
II
II
indicates Zn incorporation in the Cu oxalate structure shown in
Figure 2. This is further evidenced by the shift of the strong 110
reflection from 22.81° (Zn_0) to 23.44° 2θ (Zn_95). The peak
shift is small in the Cu-rich region and pronounced for the Zn-
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rich samples. In the intermediate regime, the main peak exhibits
a more complex structure with shoulder or double maxima,
indicating the presence of two overlapping peaks with variable
ratios.
isotropic size broadening and anisotropic strain broadening
according to the Stephens model [18]. For the fit, the atomic
coordinates were kept constant as published by Christensen et
al.[11], but the occupancy ratio between the two disordered sets
of coordinates was refined. The isotropic displacement
parameters were arbitrarily set to B = 1 Ų, the lattice
parameters were refined. The same structure model and
refinement procedure were then successfully applied to all other
apparently single-phase diffraction patterns (except Zn_100),
supporting our initial hypothesis that the moolooite structure type
2 4
persists up to very high Zn contents. The β-ZnC O structure
model of Kondrashev et al.[14] was also tested In comparison but
performed less well, even when the stacking disorder was added
to the model. In the two-phase regime, however, the extensive
peak overlap within and between the two phases in conjunction
with the anisotropic peak broadening did not allow the extraction
of reproducible lattice parameters using two freely refined
phases of the moolooite type because of a too large number of
refinable parameters. Thus, the fit model was restricted based
on chemical reasoning.
The two-phase regime in a miscibility gap is typically
characterized by the coexistence of two phases, which have
constant properties (e.g. stoichiometry, lattice parameters) but
vary continuously in their mixing ratio. Hence, all eight diffraction
patterns of this range were fitted simultaneously in a single
refinement. The lattice parameters were coupled across the
whole bi-phasic regime, i.e. they were restrained to have
identical values for all data sets, for both the copper-rich and the
zinc-rich phase. In contrast, the peak shape parameters,
disorder parameters and scale factors (phase amounts) were
still refined individually. As a result, all included patterns were
fitted sufficiently well to corroborate our model, yielding not only
stable lattice parameters but additionally approximate phase
amounts (Figure 3).
Figure 2. Shift of the 110 reflection as a function of χZn in the Rietveld fits of
selected Cu/Zn mixed oxalate samples. Experimental pattern (green circles),
calculated curve (grey line) and difference curve (bottom line) are given, while
the contribution of the Cu and Zn rich phases are shown in red and blue,
respectively.
We tentatively interpreted this effect as being caused by two
similar, co-existing phases resulting from a miscibility gap. The
diffraction pattern of the pure Cu sample was simulated in a
Rietveld fit using the disordered structure model of Christensen
et al.[ . Their stacking fault model consists of two alternative
sets of atomic coordinates, which are identical except for a shift
of the z coordinates by 0.5. Due to the stacking disorder, the
peak widths exhibit a pronounced anisotropy (hkl dependence),
which could be described satisfactorily using a combination of
Figure 3. Diagram of state showing the proportion of Cu-rich (red) and Zn-rich
(blue) mixed Cu/Zn oxalate phases as a function of the nominal Zn content as
derived from the corresponding Rietveld analysis.
11]
It has to be pointed out that the electron density of Cu and Zn is
too similar to be distinguished by XRD under such conditions.
Furthermore, only the integral composition of the samples, but
not of the individual phases in the two-phase region is known
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from elemental analysis. Thus, all fits were performed with only
Cu on the metal sites of the structure model, leading to small
systematic deviations in the XRD-based determination of the
phase amounts. All in all, single-phase synthetic mixed metal
oxalate samples are obtained up to a Zn content of 6.6 at.-%
The evolution of the lattice parameters a, b and c, the monoclinic
angle β and the unit-cell volume V as a function of the Zn
content is given in Figure 4 (data in Table S2). Both the Cu-rich
and Zn-rich phases show the same qualitative trends within their
respective single-phase regimes, which are also represented in
the comparison of the two phases with each other: with
increasing Zn content, the lattice parameters a and b decrease
while c and β increase. Overall, this results in a reduction of the
unit-cell volume with increasing Zn substitution.
(
Zn_15). When the composition exceeds this value a second
phase is found and with a further increase of the Zn content up
to 69.5 at.-% (Zn_70), its proportion rises (Figure 2 and
Figure 3) reaching again the monophasic state at compositions
of ≥ 79.1 at.-% Zn (Zn_80). The graphical representation of all
Rietveld fits is given in Figure S3 and Figure S4 for the Cu-rich
and Zn-rich samples, respectively. The corresponding lattice
parameters are summarized and compared to selected literature
references in Table S2.
SEM analysis of selected samples was performed in order
to investigate morphological changes, as expected to be present
in the proposed limited solid solution series of Cu/Zn oxalates.
The SEM micrographs of the samples with Zn content of Zn_5,
II
Zn_50 and Zn_95 are given in Figure 5. At a low level of Zn
2 4
incorporation into the CuC O lattice (Figure 5a) the obtained
rounded, spherical-like particles of varying µm-size feature a
porous texture and share close similarity to the pure Cu-oxalate
material (Figure S5). The Zn_95 sample (Figure 5c) drastically
differs. It comprises of prismatic stacked to tabular shaped
aggregates with a rough texture obviously identical to the
Zn_100 sample (Figure S6). The sample at the parity of Cu and
Zn (Figure 5b) consists of both aforementioned morphological
motifs. This is in agreement with the two mixed metal oxalate
phases present within the proposed miscibility gap. With
increasing Zn content, the specific BET area slightly increases
from around 10 m²g-1 to 14 m²g .
-1
Figure 4. Trends of lattice parameters a, b and c, the monoclinic angle β and
the unit-cell volume V in the oxalate sample series as a function of Zn content
determined by XRF. Error bars of 3ESD (estimated standard deviations) are
given. Single phase (light green) and binary phase (dark green) regions
highlighted. The constancy of lattice parameters in the two-phase regime is a
result of the restrictive coupled fit model.
Figure 5. SEM micrographs of the Cu: Zn-samples Zn_5 (a), Zn_50 (b) and
Zn_95 (c).
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Thermal Analysis
Besides the interpretation of the reaction scheme in air
atmosphere is debated in literature.[7c, 21] However, the detailed
investigation of the reaction path of pure Cu oxalate is beyond
II
The thermal behavior of all samples was investigated by
the scope of this paper.
2
STA/EGA techniques in 21 % O /Ar up to 770 K. The
simultaneous thermal analysis results for the Cu-rich samples
are given in Figure S9, respectively Figure S10 for the Zn-rich
samples. For all samples the presence of the corresponding
metal(II) oxides is verified by XRD analysis after the thermal
treatment (Figure 6b and Figure S12).
II
In the case of Zn oxalate (Zn_100) the decomposition step is
shifted to higher temperature and is present as an endothermal
event at 635 K coinciding with the release of CO and CO
2
(
Figure S10h). Independent from the applied atmosphere, it
[
7a, 20, 22]
follows the endothermal decomposition scheme (2).
While the thermal decomposition of CuII oxalate is always
exothermic[23]
containing atmosphere when the secondary exothermic
oxidation of CO to CO
takes place.[24] Besides, the distinct
,
it only applies for ZnII oxalate in oxygen
2
endothermal loss of crystallization water of the dihydrate is
observed at 340-470 K. The discussed results for both single
[
7a, 7c, 19a, 20, 22a,
metal oxalates agree very well with the literature.
2
2b, 25]
Thus, the oxalate anion decomposition temperature is
taken as a valuable approximation of the Zn incorporation into
II
2 4
the CuC O lattice for samples exhibiting a single decomposition
step between 534 – 635 K (in contrast to two separated ones).
Figure 6. Exemplary STA and EGA results of Zn_40 sample (a) and XRD
analysis after thermal analysis (b). Mass loss (black line), DSC curve (orange
line) and mass spectrometer signals of H
2
O (m/z 18, blue line), CO (m/z 28,
magenta line) and CO (m/z 44, green line) are given.
2
Figure 7. Effect of Zn content on the decomposition temperature of Cu-rich
(red) and Zn-rich (blue) mixed Cu/Zn oxalate phases and their relative
contribution to the observed mass loss. Nominal Zn content given. Peak
temperatures of DSC events and detected mass loss during oxalate anion
decomposition according to Table S3.
According to Dollimore et al., the decomposition of the two pure
II
II
oxalates of Cu and Zn was classified in inert atmosphere
according to the given reactions[19]
:
The observation of single oxalate decomposition events at the
peripheries of the mixed Cu/Zn oxalate systems is in excellent
agreement with the presented XRD analysis and further
evidences the formation of a limited solid solution series. As
expected, with increasing Zn-content the characteristic
exothermic peak is shifted to higher on-set temperatures
(Figure S11), while the decomposition completes at a distinct
lower temperature compared to the pure ZnII oxalate. This
clearly differs in case of a physical mixture of both oxalates,
were no tremendous effect on the thermal behavior of both
components has been reported.[26] The complexity of this system
is further indicated by the anomalous decrease in the
decomposition temperatures of the two separate mixed metal
II
The product of the thermal breakdown of Cu oxalate strongly
depends on the chosen atmosphere.[20] In 21 % O
2
II
/Ar the
thermal decomposition of the oxalate anion of pure Cu oxalate
Zn_0) takes place as an exothermal event located at a peak
temperature of 534 K (Figure S9a). As the final solid product
CuIIO is obtained (Figure S12). The formation of CuI
observed, but its presence as an intermediate towards Cu O
cannot be ruled out. Thus, the generalized scheme (3) is given.
(
2
O was not
II
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position at 1319 cm-1 (+2 cm ) The weakly ν(CC) at 923 cm
-1
-1
oxalate phases within the biphasic regime in the case of Zn_50
and Zn_55. Furthermore, Cu-rich samples exhibit sharper
signals in the DSC curve and a minor initial slow mass loss due
to dehydration (< 5%) while some Zn-rich samples (i.e. Zn_80)
show a distinct dehydration event around 375 K and overall
broader decomposition signals. The theoretical and
experimental weight losses upon thermal decomposition are
summarized in Table S3 and are in excellent agreement.
-1
and strong δ(OCO) at 819 cm move both to slightly lower
energies (-5 cm ). Besides, Zn_100 and Zn_80 show in
agreement with their thermal analysis a broad feature assigned
-1
to the vibrational mode of ρ(H
3360 cm-1 (Figure S15). It is assumed that the δ(H
2
O) at 743 cm-1 and the ν(OH) at
O)
2
overlapped with the distinct νas(CO). The observed signals in the
range from 650-450 cm-1 are assigned to metal-oxygen
stretching vibrations and deformation modes of both, the oxalate
ions and the metal-oxygen moieties[27a, 27b]. With higher Zn-
content, the signals get sharper and the intensity varies. Two
distinct modes are visible, one at 486 cm-1 which loses intensity
with increasing Zn-content, and another at 529 cm-1 which
moves slightly to lower wavenumbers with decreasing Zn-
content. The second band is visible as a shoulder of the main
Vibrational Spectroscopic Analysis
ATR-IR and Raman spectra were recorded to investigate
changes of the local –C
the incremental Zn incorporation into the CuC
spectra are given in Figure 8 and Figure 9 respectively. For both
CuC (Zn_0) and α-ZnC × 2 H O (Zn_100) the obtained IR
2
O
4
–M–C
2
O
4
– environment induced by
-1
2
O
4
structure. The
signal at 486 cm , at the parity ratios of Cu and Zn. Generally,
all spectroscopic features are related to CuC O and β-ZnC O
4
(for the samples Zn_0 until Zn_95) or α-ZnC O × 2 H O (for
2 4 2
2
4
2
2
O
4
2
O
4
2
and Raman spectral pattern (Figure S13 and Figure S14) are in
excellent agreement with the literature.[9c, 10c, 27] Since all
samples up to a nominal value of 95 at.-% Zn share close
resemblance to Cu oxalate (see XRD analysis), it serves as the
basis for the assignment [9c, 27] of the observed vibrational modes
Zn_100), which is absolutely in line with the results from powder
XRD and thermal analysis.
Complementary to the IR analysis, Raman measurements
are conducted. The vibrational motifs of all samples are
identified and qualitatively comparable to the IR data. The
spectra are given in Figure 9. The Raman spectra of the CO
stretching region feature two low intensity bands located at
1666 cm-1 and 1616 cm , both hardly a function of Zn-content.
Besides, the strong νas(CO) at 1518 cm-1 and the band at
II
(
Table S4 and Table S5). To the best of the authors knowledge
vibrational data of β-ZnC
O [14], structurally closely related to
CuC , has not been reported in the literature so far.
2
4
-1
2
O
4
1
489 cm-1 (ν
s
(CO) +ν(CC)) are strongly affected by the
incremental increase in Zn-content leading to pronounced
relative intensity changes. The asymmetric envelope is finally
-1
located at 1496 cm . The weak ν(CC))/ δ(OCO) at 924 and
33 cm-1 are shifted to slightly lower energy (–5 cm ) within the
-1
8
sample series. In the deformation mode region on the lower
energy side of the spectrum three bands are observed initially
-1
located at 613, 588 and 561 cm . With higher Zn-content, the
signals become broadened and their relative intensity change.
Besides, a minor red shift is noted especially for the signal at
61 cm-1 which gets relocated to 545 cm .
-1
5
Figure 8. Evolution of ATR-IR spectra as a function of nominal Zn content.
The extended spectrum up to 3850 cm-1 is presented in Figure S15.
The evolution of the ATR-IR spectra with increasing Zn content
is given in Figure 8. All vibrational patterns with their features
increasingly deviate from the single metal oxalate CuC
Zn_0) with increasing Zn content. The ATR-IR spectrum of the
CO stretching region show a broad asymmetric band around
605 cm-1 assigned to νas(CO) fluctuating of ±3 cm-1 within the
sample series up to a nominal Zn content of 95 at.-%.
2 4
O
(
1
Additionally, two intense vibration modes from ν
s
(CO) and
δ(OCO) centered at around 1300 cm-1 (1361 cm-1 and 1317 cm
for Zn_0) are broadened with increasing Zn-content (not affected
by ATR-correction, Figure S15). The former shifts to higher
wavenumbers of 1374 cm-1 while the latter merely alters its
-1
Figure 9. Evolution of Raman spectra as a function of nominal Zn content.
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It is generally well established, that all fundamental modes of the
oxalate ions are affected by coordination with the metal (M)
ion.[28] Furthermore, as the M-O bond strength decreases, the
corresponding C-O bond gets stronger, while the free C-O bond
gets weaker, thus leading to a decrease of the energetic gap of
Experimental Section
General: All reagents were purchased from commercial vendors and
used without further purification. For all experiments ultrapure water,
taken from a Milli-Q water treatment system (Merck Millipore), was used.
ν
s
and νas vibrational bands.[28a] The extent of covalency of the
M-O bond depends on the electronegativity (EN) of the
Synthesis: A 1.0 M aqueous solution containing the desired proportions
of dissolved Cu(NO
(> 99 %, Sigma-Aldrich) was prepared. The pH was adjusted to a value
of 0.5 by the addition of HNO (> 65 wt.-%, Carl Roth). The precipitating
agent was aqueous 1.2 M K × H O (> 98 %, Carl Roth). The
3 2 2 3 2 2
) × 3 H O (> 99 %, Carl Roth) and Zn(NO ) × 6 H O
corresponding metal. Zn (EN: 1.6) is less electronegative
compared to Cu (EN: 1.9).[29] Thus, by the incremental addition
3
of ZnII into the CuII oxalate lattice, a less covalent M-O
interaction will probably occur in the mixed oxalate than in the
C O
2 2 4
2
reaction vessel was filled with Millipore water (50 ml). Under strict pH-
controlled co-precipitation (pH=1.0) and temperature (303 K) control the
metal salt solution was added dropwise within 20 minutes under vigorous
stirring. In order to keep the pH constant at 1.0, appropriate amounts of
pure single metal CuC
energetic alignment of the νas(CO) and ν
Figure 9) indicate the successful substitution. The observed
band shifts are more pronounced on the compositional periphery
especially on the Zn-rich side). This could be due to the
2
O
4
.[26] In this light, the observed trend of
(CO) vibrational bands
s
(
2 2 4
K C O solutions was added during the co-precipitation step. The
precipitation was followed by an aging time of 5 min (303 K, pH=1.0).
Afterwards the precipitate was either filter-collected or separated by
centrifugation (Rotina 380, Hettich) and washed several times with
Millipore water until the conductivity of the washing medium was < 0.5
(
dampening effect of the envelope spectrum based on multiple
phases contributing to the overall measured spectrum around
the parity ratio of Cu and Zn. The presence of a miscibility gap
has already been indicated by both XRD and thermal analysis.
-1
mScm . The solid was dried at 353 K in air over night and stored in a
desiccator over silica gel orange.
Characterization: Powder X-ray diffraction (PXRD) data were recorded
on
a STOE STADI P transmission diffractometer using a primary
Conclusions
focusing Ge monochromator for Cu Kα1 radiation and a DECTRIS
MYTHEN 1K position sensitive solid-state detector. The finely ground
sample powders were fixed between two layers of thin polyacetate film
with small amounts of X-ray amorphous grease before mounting. The
diffraction patterns were analyzed by full pattern fitting according the
Rietveld method as implemented in the TOPAS software package
We report on the successful synthesis of a series of mixed
Cu/Zn oxalates by co-precipitation in aqueous solution at
constant pH value. Rietveld analysis of the powder XRD data
II
indicated the successful isomorphous substitution of Zn into the
II
Cu oxalate lattice at the compositional peripheries, namely ≤6.6
(TOPAS version 5.0, 1999-2014 Bruker AXS).
II
II
and ≥79.1 at.-% Zn. While both Cu and Zn have similar ionic
radii, which are well within the limits set by Goldschmidt
differ in their electron configuration. In this respect, Cu (d ) is
known to be affected by the Jahn-Teller effect leading to
distorted coordination geometries. In contrast to this, ZnII (d )
[
30]
they
Wavelength dispersive X-ray fluorescence (WDXRF) measurements
were performed in a S4 PIONEER (Bruker AXS) spectrometer on 40 mm
solidified molten glass sample discs. Quantification of the data is based
on a 10-point reference curve.
II
9
10
prefers
a symmetric coordination environment. Thus, the
SEM images were taken on a S-4800 (Hitachi) equipped with a field
emission gun (FEG) system. The sample was dispersed on a tape of
conductive carbon (Plano). The instrument was operated at low
accelerating voltage (1.5 kV) for increased resolution of the surface
features of the sample.
formation of a continuous solid solution series seems unlikely.
The formation of a Cu containing phase isostructural to
α-ZnC O × 2 H
2 4 2
O, as reported by Deng et al.[6a] for co-
precipitated samples from ethanolic solutions, was not verified.
SEM analysis visualized drastic morphological and textural
changes. The miscibility gap is distinguished by the rising
amount of the Zn-rich solid solution with increasing Zn content.
The addition of this second metal is reflected in a change of all
lattice parameters leading to a contraction of the unit-cell for the
Zn-rich solid solution. Thermal analysis showed single
decomposition events observed within the stated compositional
boundaries shifting to higher temperature with increasing Zn-
content, while multiple events were observed near the equal
ratio of Cu and Zn. In addition, consistent relative phase
contributions of both solid-solutions within in the miscibility gap
were obtained from XRD and thermal analysis. Moreover,
vibrational spectroscopy identified changes of the local
environment induced by the incremental Zn incorporation into
The Brunauer–Emmett–Teller (BET) surface area [31] was derived by N
2
physisorption at liquid nitrogen temperature (77 K) using an Autosorb-
6B2-KR (Quantachrome). Prior to measurement, the samples had been
degassed at 353 K for 4-17.5 h using an Autosorb Degasser setup
(Quantachrome).
STA measurements (TG/DSC) were conducted using a STA 449 C
Jupiter thermoanalyzer (Netzsch) under a controlled gas atmosphere
-
1
(
21 % O
spectrometer (QMS200 Omnistar, Balzers) for evolved gas analysis
EGA) via a quartz capillary heated to 313K. Each measurement was
2
/Ar, 100 mlmin ) and connected to a quadrupole mass
(
performed with approximately 10-20 mg sample in a temperature range
of 300 - 773 K (2 Kmin-1). Upon cooling to room temperature, the residual
powder sample was further analyzed by powder X-ray diffraction. All data
was analyzed using the NETZSCH Proteus Thermal Analysis software
package (Version 6.10). Partial smoothing was applied to the STA and
EGA data.
the CuC
2
O
4
structure. This established the compositional
boundaries of the limited solid solution series of mixed Cu/Zn
oxalate. The presented work serves as an essential contribution
for the successful synthesis of single phase Cu/Zn oxalate
precursors, potentially applied in heterogeneous catalysis.
IR spectra of the powder samples were recorded in a range of 450 –
-1
6
000 cm-1 (resolution: 1.0 cm ) at room temperature using a Varian 670
FTIR spectrometer equipped with a diamond ATR unit (GladiATR, Pike).
For this purpose, a 632.8 nm laser with a power of no more than 0.6 mW
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Chemistry - A European Journal
10.1002/chem.201803124
FULL PAPER
was used. Spectra were recorded using 256 scans for each sample after
performing a background measurement (air).
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FULL PAPER
Solid solutions: A wide-ranging set
of mixed Cu/Zn oxalates was
synthesized by constant pH co-
precipitation. The observed limited
degree of isomorphous substitution
is reflected in the presence of a
distinct miscibility gap (see Figure).
The compositional boundaries of the
limited solid solution series are
established.
Leon Zwiener, Frank Girgsdies, Robert
Schlögl, and Elias Frei*
Page No. – Page No.
Investigations of Cu/Zn Oxalates
from Aqueous Solution: Single
Phase Precursors and Beyond
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