Thermal tuning of advanced Cu sol-gels for mixed oxidation state Cu/Cu xOy materials

Article abstract of DOI:10.1039/c3ta11957b

In this report, we describe new conditions for the formation of copper gels and aerogels via epoxide addition to CuBr₂ in dimethylformamide (DMF). These CuBr₂-derived sols undergo a rapid gel transition and result in materials that have enhanced mechanical properties when compared with the gels derived from CuCl₂. Additionally, upon air-annealing, they convert to nanoscopic CuO at temperatures much lower than the CuCl₂ analogues. Moreover, annealing under nitrogen results in copper species with reduced oxidization states including Cu₂O and metallic Cu. The ratio of copper oxidation states can be controlled by simple modification of the thermal program used to anneal the materials. The thermal reduction of the copper is attributed to retained DMF ligands in the as-prepared aerogels which was confirmed by FTIR spectroscopy. These aerogel materials were characterized by powder X-ray diffraction (PXRD), physisorption, thermogravimetric analysis (TGA), and temperature programmed reduction (TPR).

Full text of DOI:10.1039/c3ta11957b

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
PAPER
Thermal tuning of advanced Cu sol–gels for mixed
oxidation state Cu/CuxOy materials
Cite this: J. Mater. Chem. A, 2013, 1,
10898
*
Roya Baghi, Geneva R. Peterson and Louisa J. Hope-Weeks
In this report, we describe new conditions for the formation of copper gels and aerogels via epoxide
addition to CuBr2 in dimethylformamide (DMF). These CuBr2-derived sols undergo a rapid gel transition
and result in materials that have enhanced mechanical properties when compared with the gels derived
from CuCl2. Additionally, upon air-annealing, they convert to nanoscopic CuO at temperatures much
lower than the CuCl2 analogues. Moreover, annealing under nitrogen results in copper species with
reduced oxidization states including Cu2O and metallic Cu. The ratio of copper oxidation states can be
controlled by simple modification of the thermal program used to anneal the materials. The thermal
reduction of the copper is attributed to retained DMF ligands in the as-prepared aerogels which was
confirmed by FTIR spectroscopy. These aerogel materials were characterized by powder X-ray diffraction
(PXRD), physisorption, thermogravimetric analysis (TGA), and temperature programmed reduction (TPR).
Received 17th May 2013
Accepted 25th July 2013
DOI: 10.1039/c3ta11957b
www.rsc.org/MaterialsA
of oxidation states are present and the importance of physical
properties. As such, methods for the preparation of homoge-
Introduction
Copper-based heterogeneous catalysts are key materials for
environmentally-important reactions such as hydrogenation of
carbon dioxide to methanol,1 NOx selective reduction,2 CO
oxidation,3 and cross-coupling reactions.4 These materials are
usually supported on a substrate such as zinc oxide or
aluminium oxide in order to increase their active surface area,
improve their porosity characteristics, and inuence their
reactivity. Cutting-edge preparative methods in materials
science have recently opened the possibility of preparing
unsupported copper catalysts with high surface areas. For
example, sol–gel methods have recently been explored as a route
to unsupported porous copper materials,5 and Cu nanoparticles
have displayed excellent activity as heterogeneous catalysts for
the Sonogashira coupling reaction.6,7
Many studies have investigated the relative catalytic activities
of copper oxidation states for a given reaction. For example,
Jernigan and Somorjai studied CO oxidation by copper and
copper oxides8 and reported that the catalytic activity of the
copper species decreases in the order of Cu > Cu2O > CuO.
However, Huang indicated that Cu2O exhibits the most activity
for this reaction, but that non-equilibrium surface structures
are important.9 On the other hand, cupric oxide nanopowders
(CuO) have been reported recently as efficient catalysts for CO
oxidation whereas bulk CuO did not show any activity under
analogous conditions.10 The ambiguity of which is the “active
oxidation state” may point to cooperative effects when mixtures
nous copper nanomaterials with controlled ratios of the
oxidation states or of non-stoichiometric CuxOy nanomaterials
would provide interesting insights into the catalytic activity of
copper heterogeneous catalysts.
Aerogels are a diverse class of porous materials prepared by
the non-destructive removal of solvent from the pores of a gel,
usually by critical point drying.11 Here, we report a signicantly
improved facile method for the synthesis of unsupported
copper-based sol–gels and aerogels which exhibit enhanced
mechanical and physical properties. The resulting copper-based
aerogels convert to unsupported, nanocrystalline CuO at rela-
tively low temperatures. Additionally, residual organic material
in the as-prepared aerogel can be utilized as a latent reductant
during anaerobic annealing of this material to afford unsup-
ported mixed oxidation state copper nanomaterials. The ratio of
copper phases is easily tuned by adjustment of the thermal
program.
Results and discussion
1
Synthesis and physical properties of Cu aerogels
The formation of copper(II) aerogels from copper(II) chloride in
2-propanol using propylene oxide as a proton scavenger and
gelling agent has been reported previously.5 However, these gels
are fragile and crumble from their monolithic form easily.
Moreover, sols from CuCl2 undergo the sol–gel transition slowly
and incompletely. The “epoxide addition” synthetic route to
transition metal sol–gels is now well established.12 In this
approach, a metal salt solution is mixed with an epoxide which
gradually basies the solution leading to oxolation and olation
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX,
79409, USA. E-mail: louisa.hope-weeks@ttu.edu; Fax: +1 806 742 1289; Tel: +1 806
742 4487
10898 | J. Mater. Chem. A, 2013, 1, 10898–10902
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry A
reactions at the metal centre.12c Under favourable conditions,
these polycondensation steps create a continuous gel network.
For the metal centres in the nal gel, the exact nature and
number of the inner sphere ligands (such as water, hydroxide,
counterion, or solvent) are variable. The choice of precursor
epoxide,12c solvent,12b and salt12a are all known to inuence the
gelation process as well as the chemical and physical properties
of the resulting gels. The best combination of precursors is
usually empirically determined. Consequently, we examined
different solvents, epoxides, and copper salts to achieve a quick
and facile synthesis of copper aerogels with improved
mechanical properties. Among a variety of conditions, the best
result was obtained by using copper(II) bromide in DMF and
epichlorohydrin as the epoxide. In this system, the reactants
undergo the sol–gel transition within 10 minutes of mixing at
room temperature to afford a robust, opaque, turquoise-col-
oured gel. Aer washing the gel of residual reactants and critical
point drying with carbon dioxide, a similar coloured monolith
was isolated. The PXRD pattern of this as-prepared copper
aerogel indicates that it is weakly crystalline and exhibits
diffraction peaks which correspond to the Cu2(OH)3Br phase
(PDF no. 00-045-1309) (Fig. 1a). The analogous Cu2(OH)3Cl
phase was identied in the as-prepared CuCl2-derived gels.5
The BET surface area of the as-prepared CuBr2-derived aer-
ogel was 232 m2 gÀ1 compared with 313 m2 gÀ1 for the CuCl2-
derived aerogel, which indicates that some loss of surface area
accompanies the improved mechanical properties for the
CuBr2-derived aerogels. Additionally, the SEM images (Fig. 2) of
the as-prepared CuBr2-derived aerogel demonstrate a coral-like
morphology with a homogenous network of pores in a variety of
sizes, in strong contrast to the morphology of the CuCl2-derived
aerogels, which display clusters of joined spheres with large
inter-particle voids.
Fig. 2 SEM image of the as-prepared CuBr2 derived aerogel.
aerogel materials. Thermogravimetric analysis of copper basic
halides has been previously described13 and indicates that the
Cu2(OH)3Br phase undergoes phase transitions at lower
temperature than the analogous Cu2(OH)3Cl phase. Specically,
the thermal decomposition of the copper basic halides proceeds
through three steps. The rst two steps are dehydroxylation with
elimination of water and redox reaction to form Cu2X2 and X2.
Under inert atmosphere, Cu2X2 volatilizes, whereas under air
the Cu2X2 is oxidized to CuO. The Cu2(OH)3Br system
undergoes the redox reaction to Cu2Br2 more readily than the
chloride analogue, so the rst two steps of the decomposition
process take place simultaneously:
2Cu2(OH)3Br / 3CuO + ½Cu2Br2 + ½Br2 + 3H2O
For the CuBr2-derived aerogel under investigation here, ther-
mogravimetric analysis under nitrogen generally follows this
literature pathway for Cu2(OH)3Br (Fig. 3). The rst mass loss
2
Thermal degradation & aerobic formation of nano CuO
ꢀ
In addition to the faster sol–gel transition and improved
mechanical properties, the use of CuBr2 in lieu of CuCl2 pres-
ents advantages in the thermal processing of the as-prepared
observed from 130 to 265 C can be attributed to the dehydrox-
ylation and the redox reaction to liberate Br2, and the second
mass loss from 300 to 400 ꢀC corresponds to volatilization of
Fig. 1 PXRD pattern of (a) as-prepared Cu aerogel derived from CuBr2 in DMF
with epichlorohydrin as a gelling-agent and (b) CuO obtained by annealing this
specimen at 250 ꢀC for 8 hours.
Fig. 3 Thermogravimetric profile of the as-prepared CuBr2-derived aerogel
under nitrogen flow.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 10898–10902 | 10899

View Article Online
Journal of Materials Chemistry A
Paper
Cu2Br2. The observed weight losses are 22% and 20%, respec- an intermediate by the literature thermal decomposition
tively, which are both lower than the calculated mass losses of route.13 This result suggests alternative decomposition path-
26% and 28%. Both of these calculated mass losses depend upon ways for the Cu-aerogel materials compared with the pure
the stoichiometric quantity of bromide in the material. If the Cu- Cu2(OH)3X phases. It is known that aerogel materials from the
aerogels are bromide-decient relative to the Cu2(OH)3Br epoxide addition method possess inexact chemical formulas
formula, then the mass losses would be lower, as observed here. and even a substantial amount of residual organic material.12a
This situation is consistent with the somewhat amorphous Leventis et al. have recently explored the reduction of copper
nature of the CuBr2-derived aerogel which likely possesses and iron derived aerogels by using a co-gelled organic compo-
inexact stoichiometry relative to the pure Cu2(OH)3Br phase.
The as-prepared aerogel was annealed under static air to
nent to mimic smelting reactions under inert atmosphere.15
In order to probe the capability of the CuBr2-derived aerogel
afford CuO, as shown in Fig. 1b (PDF no. 00-044-0706). The to likewise afford reduced copper intermediates as observed for
phase was completely changed to CuO aer annealing for 8 the CuCl2-derived aerogels, a series of aerobic annealing
hours at only 250 ꢀC compared with 450 ꢀC required for conditions were investigated (Table 1). However, the only
complete conversion of the CuCl2-derived aerogels. This mild decomposition products observed from aerobic annealing are
annealing resulted in a surface area of 47 m2 gÀ1 and an average Cu2Br2 and Cu(II) oxide. Based upon this information, anneal-
particle size of 11 nm, as calculated from the (111) reection of ing of the CuBr2-derived aerogels was conducted under nitrogen
the PXRD pattern (Fig. 1b). Overall, these results indicate that atmosphere in order to investigate the possibility of Leventis-
the as-prepared CuBr2-derived aerogel generally possesses style nano-smelting (Table 2).
similar chemical properties to pure Cu2(OH)3Br including its
aerobic thermal decomposition pathway.
The annealing of the as-prepared CuBr2-derived aerogel
under inert atmosphere resulted in the formation of a mixed
Temperature programmed reduction (TPR) analysis was oxidation state copper oxide material (Fig. 5). To further
performed on the CuO sample calcined at 250 ꢀC using a understand the effect of annealing conditions on the oxidation
mixture of 5% hydrogen and 95% nitrogen to investigate the state of the nal product, a series of conditions were investi-
reduction behaviour of this material (Fig. 4). The TPR curve gated to explore the inuence of time and temperature in these
exhibits a single peak indicating a one-step reduction mecha- phase transitions. The results are summarized in Table 2 and
nism, directly from CuO to metallic Cu without formation of the Fig. 5. All annealed phases were highly crystalline, as indicated
Cu2O intermediate. This thermal behavior is consistent with the by the at baselines of the PXRD patterns, which show no
report of Ahmadi et al.14 that indicates that reduction of nano- amorphous background. Examination of these patterns reveals
CuO particles follows a one-step mechanism whereas micro- that very subtle changes in the thermal treatment of this single
CuO produces two reduction peaks indicating a two-step aerogel precursor can produce dramatic changes in the nal
mechanism with formation of Cu2O as an intermediate. phase compositions. A mixture of species (CuO/Cu2O/CuBr) was
Consequently, both our physical (PXRD) and chemical (TPR) obtained aer brief annealing at 400 ꢀC for 2 hours. Increasing
analyses conrm the nanoscopic nature of this easily-prepared, the time and temperature led to products with lower oxidation
unsupported CuO.
states. Annealing at 450 ꢀC for 7 hours resulted in a mixture of
Cu metal and Cu2O as the major phases with a small amount of
residual CuO. This mixed oxidation state copper material
3
Anaerobic production of mixed oxidation state Cu
Interestingly, in the case of the CuCl2-derived aerogels, a
mixture of Cu2O and CuO phases were formed aer annealing
at 350 ꢀC, even though this Cu(I) oxide phase is not predicted as
Table 1 Static air annealing conditions, CuBr2-derived aerogel
Target temp.
(ꢀC)
Ramp rate
Dwell
time (h)
(ꢀC minÀ1
)
Observed phases
400
275
275
275
250
10
2
2
2
2
2
2
6
8
8
CuO
CuO/CuBr
CuO/CuBr
CuO
CuO
Table 2 Inert atmosphere annealing conditions, CuBr2-derived aerogel
Target temp.
(ꢀC)
Ramp rate
Dwell
time (h)
(ꢀC minÀ1
)
Observed phases
400
400
450
450
450
10
2
2
2
2
2
7
4
6
7
CuO/Cu2O/CuBr
CuO/Cu2O/Cu/CuBr
Cu2O/CuBr
Cu/Cu2O/CuBr
Cu2O/Cu/CuO (minor)
Fig. 4 Temperature programmed reduction (TPR) of the CuO aerogel in 5% H2
95% N2 stream at a constant thermal ramp of 5 ꢀC minÀ1
.
10900 | J. Mater. Chem. A, 2013, 1, 10898–10902
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 5 PXRD patterns of the Cu aerogel material after various annealing programs.
for the pure Cu2(OH)3Br, which has been reported previously.16
In addition, the as-prepared aerogel also displays bands near
1400 cmÀ1 which are attributable to coordinated DMF mole-
cules. Retention of DMF in the Cu sol–gel matrix is perhaps not
surprising in view of the good capability of DMF to act as a
ligand and the affinity of copper for nitrogenous bases.
Experimental
Copper bromide (Alfa Aesar), epichlorohydrin (Acros) and
dimethylformamide (DMF, Fisher) were all used as received. All
syntheses were performed under ambient conditions. PXRD
analyses were performed on a Rigaku Ultima III diffractometer
using Cu Ka radiation. The BET surface area was determined
from N2 adsorption/desorption data collected at 77 K on a NOVA
4200e model surface area/pore size analyzer (Quantachrome
Instrument Corp.). Seven data points were selected in the rela-
tive pressure region of 0.05–0.3 P/P0. The morphology of the gel
Fig. 6 FTIR spectrum of the as-prepared CuBr2-derived aerogel, prepared as a
KBr pellet.
possessed a surface area of 10 m2 gÀ1 as a result of grain growth was studied by scanning electron microscopy (SEM) Hitachi S-
and densication during extended annealing. Importantly, 4300 scanning electron microscope. TPR was carried out with a
within a window of only 50 ꢀC and 5 hours annealing time, ChemBET TPR/TPD Analyzer (Quantachrome Instrument
substantial changes in the ratios of oxidation states can be Corp.). Thermogravimetric analyses were conducted with a
accessed.
Shimadzu TGA-50 Thermogravimetric Analyzer under N2 (50 ml
In order to substantiate the hypothesis that these reduced minÀ1). For TGA experiments, the thermal program raised the
copper species result from smelting-type reactions with specꢀimen to 100 ꢀC at a ramp rate of 10 ꢀC, held the sample at
retained organic material in the aerogel matrix, elemental 100 C for one hour to remove residual volatile materials, and
analysis was obtained for the as-prepared aerogel material then raised the specimen to 600 ꢀC at a ramp rate of 5 ꢀC.
which revealed the presence of 6.33% carbon, 1.79% hydrogen, Specimens for FTIR spectroscopy were prepared as KBr pellets
and 1.34% nitrogen. Additionally, an FTIR spectrum of the as- using ꢁ10 mg of as-prepared aerogel in ꢁ150 mg of KBr. FTIR
prepared aerogel was acquired which supports the presence of spectra were collected on a Hitachi FTIR-8400.
retained DMF molecules in the aerogel matrix (Fig. 6). The
Copper sol–gels were prepared by addition of epichlorohy-
spectra obtained for the as-prepared aerogel exhibits OH drin (2.75 ml, 0.03 mol) and H2O (0.24 ml, 0.013 mol) to a
vibrations near 3400 cmÀ1 which correspond to those observed solution of CuBr2 (0.7371 g, 0.0033 mol) dissolved in DMF (5
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 10898–10902 | 10901

View Article Online
Journal of Materials Chemistry A
Paper
ml) to yield a dark black-green solution. Following combina-
tion of all the reagents, the solution was stirred for a few
seconds and then poured into 6 dram shell vials as moulds
which were capped and le undisturbed to gel. Aer 10
minutes the gel started to form and the solution turned lighter
green and opaque. Sol–gels were allowed to age 24 h, and then
were washed with DMF and acetone (3 times each solvent, 24 h
each time). The gel samples were then dried in a SPI-DRY
model critical point dryer using carbon dioxide to generate
aerogels.
S. C. E. Tsang, Angew. Chem., Int. Ed., 2012, 51, 5832–5836;
(c) F. L. Liao, Y. Q. Huang, J. W. Ge, W. R. Zheng,
K. Tedsree, P. Collier, X. L. Hong and S. C. Tsang, Angew.
Chem., Int. Ed., 2011, 50, 2162–2165; (d) M. Behrens,
F. Studt, I. Kasatkin, S. Kuehl, M. Haevecker, F. Abild-
Pedersen, S. Zander, F. Girgsdies, P. Kurr, B.-L. Kniep,
M. Tovar, R. W. Fischer, J. K. Norskov and R. Schloegl,
Science, 2012, 336, 893–897.
2 N. Scotti, D. Monticelli and F. Zaccheria, Inorg. Chim. Acta,
2012, 380, 194–200.
3 U. R. Pillai and S. Deevi, Appl. Catal., B, 2006, 64, 146–151.
4 K. M. Pan, H. Ming, H. Yu, Y. Liu, Z. H. Kang, H. Zhang and
S. T. Lee, Cryst. Res. Technol., 2011, 46, 1167–1174.
5 C. N. Sisk and L. J. Hope-Weeks, J. Mater. Chem., 2008, 18,
2607–2610.
6 T. Murata, M. Murai, Y. Ikeda, K. Miki and K. Ohe, Org. Lett.,
2012, 14, 2296–2299.
7 F. Bellina and M. Lessi, Synlett, 2012, 773–777.
8 G. G. Jernigan and G. A. Somorjai, J. Catal., 1994, 147, 567–
577.
9 T. J. Huang and D. H. Tsai, Catal. Lett., 2003, 87, 173–
178.
10 D. A. Svintsitskiy, A. P. Chupakhin, E. M. Slavinskaya,
O. A. Stonkus, A. I. Stadnichenko, S. V. Koscheev and
A. I. Boronin, J. Mol. Catal. A: Chem., 2013, 368, 95–
106.
Conclusions
In this study, unsupported Cu-based aerogels were successfully
prepared using the epoxide addition method from a copper
bromide salt. The counter-ion (Br rather than Cl), the solvent
(DMF rather than 2-propanol), and the epoxide (epichlorohy-
drin rather than propylene oxide) all contribute to a faster gel
time and greater mechanical strength.
The as-prepared CuBr2-derived aerogels produce PXRD
patterns consistent with poorly crystalline Cu2(OH)3Br and
share many properties with the pure Cu2(OH)3Br phase
including a low thermal threshold for the aerobic formation of
crystalline CuO. Both PXRD and TPR conrm the nanoscopic
nature of this unsupported CuO material.
Despite similarities to the Cu2(OH)3Br phase, the weakly-
crystalline as-prepared aerogel possesses important differences
in composition and structure, as illustrated by thermal anal-
yses, PXRD, FTIR, elemental analysis, and annealing studies.
These analyses indicate the presence of residual organic
material within the aerogel network structure, which is
attributed to the replacement of some bromide ligands by DMF
ligands. This organic component enables the formation of
mixed oxidation-state Cu materials by smelting-type reactions
under inert atmosphere. Subtle changes to the thermal
program allow control of the ratio of the oxidation states.
Altogether, this work illustrates the potential to design sol–gel
materials with latent reactivity that can be unlocked by
careful thermal processing and paves the way for studies
regarding the catalytic activity of different copper heteroge-
neous catalysts.
11 N. Husing and U. Schubert, Angew. Chem., Int. Ed., 1998, 37,
22–45.
12 (a) A. E. Gash, J. H. Satcher and R. L. Simpson, J. Non-Cryst.
Solids, 2004, 350, 145–151; (b) P. D. Brown, S. K. Gill and
L. J. Hope-Weeks, J. Mater. Chem., 2011, 21, 4204–4208; (c)
A. E. Gash, T. M. Tillotson, J. H. Satcher, J. F. Poco,
L. W. Hrubesh and R. L. Simpson, Chem. Mater., 2001, 13,
999–1007.
13 P. Ramamurthy and E. A. Secco, Can. J. Chem., 1969, 47,
2185–2190.
14 S. J. Ahmadi, M. Outokesh, M. Hosseinpour and
T. Mousavand, Particuology, 2011, 9, 480–485.
15 (a) N. Leventis, N. Chandrasekaran, A. G. Sadekar,
C. Sotiriou-Leventis and H. Lu, J. Am. Chem. Soc., 2009,
131, 4576–4577; (b) N. Leventis, N. Chandrasekaran,
C. Sotiriou-Leventis and A. Mumtaz, J. Mater. Chem., 2009,
19, 63–65.
Notes and references
16 X. D. Liu, M. Hagihala, X. G. Zheng and Q. X. Guo, Vib.
Spectrosc., 2011, 56, 177–183.
1 (a) F. C. Meunier, Angew. Chem., Int. Ed., 2011, 50, 4053–
4054; (b) F. Liao, Z. Zeng, C. Eley, Q. Lu, X. Hong and
10902 | J. Mater. Chem. A, 2013, 1, 10898–10902
This journal is ª The Royal Society of Chemistry 2013