Preferential cleavage of C-C bonds over C-N bonds at interfacial CuO-Cu2O sites

Article abstract of DOI:10.1016/j.jcat.2015.08.001

Creation of substrate-accessible interfacial defect sites will bring about new catalytic discoveries because substrate binding and activation on these sites are pivotal for controlling reaction intermediate and product selectivity. The partial oxidation of pristine Cu₂O can lead to an excellent selective oxidation catalyst (CuO/Cu₂O). The CuO/Cu₂O, containing embedded CuO nanodomains on the surface and possessing abundant coordinatively unsaturated copper sites at the CuO-Cu₂O interface, shows very high activity toward C-C bond cleavage and excellent selectivity toward formamides in trialkylamines oxidation. This result is exceptional because the previous works mainly offer dealkylated amines via C-N bond cleavage. The unusual catalysis by CuO/Cu₂O is attributed to the co-activation of oxygen and amines in close proximity at the CuO-Cu₂O interface. The present study contributes a new concept of delicate controlling substrate-accessible interfacial active sites on pristine oxide surfaces, and also offers a novel formamide synthesis method by trialkylamine oxidation.

Full text of DOI:10.1016/j.jcat.2015.08.001

Journal of Catalysis 330 (2015) 458–464
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Preferential cleavage of CAC bonds over CAN bonds at interfacial
CuOACu O sites
2
⇑
Min Wang, Xiang-Kui Gu, Hai-Yan Su, Jian-Min Lu, Ji-Ping Ma, Miao Yu, Zhe Zhang, Feng Wang
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Creation of substrate-accessible interfacial defect sites will bring about new catalytic discoveries because
substrate binding and activation on these sites are pivotal for controlling reaction intermediate and pro-
Received 12 June 2015
Revised 2 August 2015
Accepted 3 August 2015
duct selectivity. The partial oxidation of pristine Cu
CuO/Cu O). The CuO/Cu O, containing embedded CuO nanodomains on the surface and possessing abun-
dant coordinatively unsaturated copper sites at the CuOACu O interface, shows very high activity toward
2
O can lead to an excellent selective oxidation catalyst
(
2
2
2
CAC bond cleavage and excellent selectivity toward formamides in trialkylamines oxidation. This result is
exceptional because the previous works mainly offer dealkylated amines via CAN bond cleavage. The
Keywords:
Heterogeneous catalysis
Defect sites
CAC bond activation
Molecular oxygen
Cuprous oxide
2
unusual catalysis by CuO/Cu O is attributed to the co-activation of oxygen and amines in close proximity
at the CuOACu O interface. The present study contributes a new concept of delicate controlling
2
substrate-accessible interfacial active sites on pristine oxide surfaces, and also offers a novel formamide
synthesis method by trialkylamine oxidation.
Ó 2015 Elsevier Inc. All rights reserved.
1
. Introduction
2
[26]. For example, Huang et al. have revealed that the initial Cu O
surface is oxidized to CuO, which has been proved to be the cat-
alytically active phase [27]. However, the underlying mechanism
of redox treatment on the oxide surface structure still remains
elusive.
In this work, we report the selective cleavage of (NA)CAC over
2
CAN bonds in trialkylamine oxidation over Cu O:
Nanostructured solid Cu-based catalysts have recently gener-
ated considerable research interests [1–8]. Unveiling the local
structure of active sites localized in interfacial phases and estab-
lishing a close relationship with catalytic activity have remained
crucial issues [9–13]. Stacchiola et al. have reported that the incor-
2 2
porated structure of TiO in Cu O leads to a highly active CO oxida-
tion catalyst [14]. Behrens et al. have reported that the CuOAZnO
This work: C-C bond cleavage
boundary phase plays a vital role in methanol synthesis [15].
O
Rodriguez has shown that the CuOACeO
adsorption and activation of CO [16]. Despite extensive studies
on complex metal oxide catalysts, little attention has been paid
to pristine Cu O, which is easily oxidized to CuO or reduced to
Cu, leading to the deactivated catalysts. Nevertheless, the compre-
hension of the well-defined Cu O model catalyst will be greatly
2
interface facilitates the
CuO/Cu O
2
N
2
N
O2
2
Previous works: C-N bond cleavage
2
helpful for understanding more complex oxide catalysts. On the
other hand, despite numerous advances in synthesizing nanoscale
Cu O particles, only a few examples are known to be catalytically
2
H
N
N
[oxidant]
active in organic synthesis [17–22].
ð1Þ
2
O is easily reformed with nano-
Ex situ or in situ redox treatment is frequently used to activate
pristine oxide catalysts [23–25]. These treatments reconstruct the
oxide surface and create active sites for catalytic reactions to occur
We found that the surface of Cu
sized CuO domains in the topmost surface (denoted as CuO/Cu
2
O
hereafter) under oxidative atmospheres. The CuO/Cu O exposes
2
trialkylamine- and oxygen-accessible interfacial active sites for
the oxidation of trialkylamines to formamides, and accounts for
the preferential oxidative cleavage of the strong (NA)CAC
⇑
Corresponding author.
E-mail address: wangfeng@dicp.ac.cn (F. Wang).
http://dx.doi.org/10.1016/j.jcat.2015.08.001
021-9517/Ó 2015 Elsevier Inc. All rights reserved.
0

M. Wang et al. / Journal of Catalysis 330 (2015) 458–464
459
JEM-2100). Electron paramagnetic resonance (EPR) tests were per-
formed on a Bruker spectrometer in the X-band at room tempera-
ture with a field modulation of 100 kHz. The microwave frequency
was maintained at 9.401 GHz. X-ray photoelectron spectroscopy
(
2
XPS) measurements were performed on a Thermo ESCALAB
50Xi spectrometer equipped with a monochromated AlK X-ray
source (h = 1486.6 eV, 15 kV, 10.8 mA). The samples were dried
under vacuum at 120 °C for 12 h. A charge neutralizer system
a
m
ꢀ8
was used for all of the analyses. The base pressure was 1 ꢁ 10 Pa.
High-resolution spectra were recorded with pass energy 20 eV.
The pass energies corresponded to a Ag3d5/2 full width at half max-
imum of 0.65 eV. The data were acquired with 0.05 eV steps. The
binding energy (BE) was calibrated to the C1s signal (284.6 eV) as
a reference. The curve fitting procedure was performed using an
approximation based on a combination of the Gaussian and Lorent-
zian functions with subtraction of a Shirley-type background. The
accurate charge-to-mass ratio was measured using LTQ-Orbitrap-
Elite mass spectroscopy.
Fig. 1. XPS spectra. (a) CuO/Cu
treating CuO/Cu O with tributylamine at 120 °C for 3 h in N
CuO/Cu
O at 500 °C. h: satellite peak of Cu(II)2p3/2; 4: satellite peak of Cu(II)2p1/2
2
O, (b) reduction of CuO/Cu
2
O by H
, and (d) calcination of
2
at 250 °C, (c)
2
2
2
.
2.3. Density functional theory calculations
523.53 kJ molꢀ1) bond over the weak CAN bond (465.08 kJ molꢀ1).
This result is remarkably different from those in previous work,
which employs homogeneous metal complexes and enzymes,
and primarily gives dealkylated products via CAN bond cleavage
The density functional theory (DFT) calculations were per-
(
formed using the Vienna ab initio simulation package (VASP)
36] with project augmented wave (PAW) potential and the Per
dew–Burke–Ernzerhof (PBE) functional.[37] A plane wave cutoff
of 400 eV was used. The optimized lattice constant of bulk Cu
was 4.279 Å. The Cu
O (111) surface was modeled as a (2 ꢁ 2) slab
[
2
O
[
28–35]. The present study shows that trialkylamine oxidation is
structure-sensitive reaction, and the delicate control of
2
a
with eight layers of O and four layers of Cu. To prevent artificial
interactions between the repeated slabs in the z-direction, a 12 Å
vacuum was introduced with a correction for the dipole moment.
A (2 ꢁ 2 ꢁ 1) k-point mesh was used to sample the Brillouin zone.
The bottom five layers of O and two layers of Cu of the slab were
fixed, while the remaining atoms and adsorbates were relaxed
until the residual forces were less than 0.02 eV/Å. Because of on-
site Coulombic repulsion between d-electrons, it is difficult to
describe the electrons of transition metal oxides using the current
exchange–correlation functional. Therefore, the DFT + U correction
substrate-accessible interfacial active sites will be greatly helpful
in designing new catalysts.
2
. Experimental
2
.1. Creation of a CuOACu O interface
2
Controlled oxidation of commercially available powdered Cu
was used to create a CuOACu O interface. To avoid the formation
of a CuO layer completely coating on the Cu O surface, the temper-
ature needed to be lower than 160 °C. The Cu O was oxidized at
20 °C for 48 h in air to form CuO nanodomains on the surface of
Cu O.
2
O
2
2
[38] was used to describe the Cu O with a value of U–J = 6 eV [39]
2
2
for the Cu3d orbitals. The adsorption energies of oxygen were
1
calculated by Eads(0) = Etotal ꢀ E
ꢀ Emol(g), where Etotal, Eslab, and
slab
2
Emol(g) are the total energies of the optimized adsorbate/substrate
system, the clean substrate, and the molecule in the gas phase,
respectively.
2.2. Characterization
X-ray powder diffraction (XRD) patterns were obtained using a
Rigaku D/Max 2500/PC powder diffractometer with CuK
2.4. Reaction procedures
a
radiation (k = 0.15418 nm). The samples were observed using
high-resolution transmission electron microscopy (HRTEM) (JEOL
The catalytic reactions were performed in a 10 mL autoclave
reactor with an internal Teflon insert. Typically, 0.5 mmol of the
2
Fig. 2. SAED pattern (a) of CuO/Cu O and HRTEM image (b) with the FFT pattern inset. The CuO{110} plane has a lattice distance of 0.27 nm.

4
60
M. Wang et al. / Journal of Catalysis 330 (2015) 458–464
Fig. 6. The catalytic performance of CuO/Cu
Cu OAH ), and pristine CuO. Reaction conditions: 0.5 mmol tributylamine, 10 mg
catalyst, 0.6 MPa O , 2 mL DMF.
2 2
O, CuO/Cu O after reduction at 250 °C
2 2
Fig. 3. XRD of (a) CuO/Cu O and (b) CuO/Cu O calcined at 500 °C.
(
2
2
2
Fig. 4. H
2
TPR-MS profiles of CuO/Cu
2
O and pristine CuO. Experimental conditions:
ꢀ
1
carrier gas 5%
H
2
+ 25% Ar; flow speed 30 mL min ; sample heating rate
ꢀ
1
5
°C min .
Fig. 7. EPR spectra of CuO/Cu
2
O before (a) and after (b) treatment with tributy-
lamine at 120 °C for 3 h under N
2
.
identified and quantified using gas chromatography–mass spec-
trometry (GC–MS) and an Agilent 7890A/5975C instrument
equipped with an HP-5 MS column (30 m in length, 0.25 mm in
diameter). n-Dodecane was used as the internal standard.
3
. Results and discussion
3
.1. Characterizations of CuO/Cu O
2
3.1.1. X-ray photoelectron spectroscopy characterizations
Cu(I) is prone to be oxidized to Cu(II) in air [40–42]; this is
Fig. 5. The reaction profile of tributylamine oxidation over CuO/Cu
2
O. Reaction
unavoidable under oxidative atmospheres at high temperature
[27]. We predicted that nanosized CuO domains can be formed
by virtue of partial oxidation under relatively mild reaction condi-
conditions: 0.5 mmol tributylamine, 10 mg catalyst, 0.6 MPa O , 2 mL DMF.
2
2
tion. Here we exposed powder Cu O to air at 120 °C for 48 h. XPS
substrate, 10 mg of Cu
reactor. Then the reactor was charged with 0.6 MPa O
to 120 °C under magnetic stirring. When the reaction reached com-
pletion, the reaction mixture was diluted with 4 mL ethanol, and
the catalyst was separated via centrifugation. The products were
2
O, and 2 mL of solvent were added to the
and heated
characterization shows that in addition to the main Cu(I)2p3/2
and Cu(I)2p1/2 peaks at 932.25 and 952.30 eV, respectively, Cu(II)
2p3/2 and Cu(II)2p1/2 peaks at 934.40 and 954.25 eV also appear,
along with the shake-up satellite peaks (Fig. 1a), confirming the
2
2
coexistence of Cu(II) and Cu(I) on the CuO/Cu O surface [43–45].

M. Wang et al. / Journal of Catalysis 330 (2015) 458–464
461
system with cell = 4.684 ꢁ 3.423 ꢁ 5.129 Å and b = 99.54°, and
Cu O is a cubic system with a = 4.270 Å [46]. The lattice mismatch
between CuO and Cu O phases readily generates defect sites,
which function as catalytically active sites. The H TPR–mass spec-
troscopy profile (H TPR-MS, Fig. 4) shows that Cu(II) in CuO/Cu
2
2
2
2
2
O
starts to be reduced at 167 °C. But for bulk CuO crystals, reduction
starts at 350 °C. This large temperature difference implies that the
embedded CuO domains are very active.
3.2. Catalytic performance
2
The catalytic performance of CuO/Cu O was tested in the oxida-
tion of tributylamine in dimethylformamide (DMF) at 120 °C for
h under 0.6 MPa oxygen. The conversion of tributylamine
increases with reaction time (Fig. 5). N,N-Dibutylformamide
DBF) is obtained with 93% yield at 8 h (NMR spectra, Figs. S1
and S2). No reaction occurs under N . The eliminated C chain is
converted into C AC oxygenates and CO (GC spectra, Fig. S3).
The catalyst can be reused three times with comparable results
Fig. S4). Cu(II) and Cu(I) still coexist in the surface layer of used
8
(
Fig. 8. The effect of oxygen pressure on the reaction performance. Reaction
conditions: 0.5 mmol tributylamine, 10 mg Cu O, 2 mL DMF, 120 °C, 8 h.
2
3
2
2
3
2
(
3
.1.2. Transmission electron microscopy characterizations
The CuO/Cu O was then examined with selected area electron
diffraction (SAED) and HRTEM. In the SAED pattern (Fig. 2), beside
the strong Cu O{200}, {111}, and {110} electron diffraction rings,
catalysts (Fig. S5).
2
We then studied the role of the embedded CuO phase. As indi-
cated by the results of H
Cu O with H at 250 °C (denoted as Cu
nate the CuO phase and accordingly the CuOACu
XPS results also show that most of the surface Cu(II) disappeared
after reduction (Fig. 1b). Catalytic results show that Cu OAH is
initially less active than CuO/Cu O, but eventually approaches it
Fig. 6). This implies that the initial pristine Cu O phase is activated
along the reaction course, which is very possibly caused by partial
oxidation of topmost Cu O layers to CuO domains. Calcination of
CuO/Cu O at 500 °C for 3 h gives a pristine CuO catalyst (Fig. 3b).
XPS demonstrates that no Cu(I) is present in the surface of CuO
Fig. 1d). The as-prepared CuO catalyst is much less active than
CuO/Cu O. As indicated by XPS (Fig. 1c) and EPR results (Fig. 7),
2
TPR-MS (Fig. 4), the reduction of CuO/
OAH ) can largely elimi-
O interface.
2
2
2
2
2
some diffraction spots between the {111} and {110} diffraction
rings are observed. The distance between two symmetrical diffrac-
2
ꢀ1
tion spots is 7.3 nm , which is in good accordance with CuO{110}
diffraction spots. Moreover, nanosized CuO domains (ca. 5–8 nm,
Fig. 2b) are clearly observed in the HRTEM image with a CuO
2
2
2
(
2
2
{110} lattice distance of 0.27 nm and are surrounded by a Cu O
phase. The appearance of the corresponding CuO{110} diffraction
spots in the fast Fourier transform (FFT) pattern further confirms
2
2
that CuO domains are present on Cu
2
O after oxidation treatment.
(
3.1.3. X-ray diffraction and H
2
temperature-programmed reduction
2
characterizations
Cu(II) in the CuO phase could be reduced to Cu(I) by the amines.
Some Cu(II) in the surface of pristine CuO may be reduced to Cu
(I) by amines during the reaction, which accounts for the consider-
able catalytic activity of CuO. Here we find that the existence of the
In principle, XRD can detect such large crystallites (5–8 nm), but
in this case, XRD did not (Fig. 3), which implies that the CuO phase
does not grow into the deep interior of the Cu O crystal but only
2
exists in the surface layers. This is reasonable, since only mild oxi-
dation conditions are adopted. The CuO crystal has a monoclinic
2 2
CuOACu O interface on the Cu O surface is indispensable for excel-
lent results in selective oxidation of CAC bonds.
(
a)
(b)
bridge
adsorption
linear
adsorption
-1
Eads (kJ mol ):
dO−O (Å):
-47.26
1.32
-45.33
1.28
ET (e):
0.50
0.23
Fig. 9. Optimized structures of O
2
adsorption on the CuOACu
2
O interface: (a) bridge adsorption of Cu(II)ꢀOꢀOꢀCu(I) and (b) linear adsorption of Cu(I)ꢀOꢀO. The red, brick
red (darker red), and blue balls represent oxygen, Cu(I), and Cu(II) atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)

4
62
M. Wang et al. / Journal of Catalysis 330 (2015) 458–464
to the EPR-silent Cu(I), which is in good accordance with XPS
results, reflected by the decrease of Cu(II)2p peak intensity (Fig. 1).
3.4. Activation of molecular oxygen
Next we studied the process of oxygen activation, which is vital
for selective oxidation reactions [52–55]. Reaction of tributylamine
and 2.0 equivalents of CuO/Cu O or CuO under N yields no pro-
2
2
duct, which indicates that lattice oxygen cannot oxidize amines,
and the reaction is not via a Mars–van Krevelen mechanism [56].
The reaction rate depends on the oxygen pressure and the conver-
sion of tributylamines increases with increasing oxygen pressure
(
(
Fig. 8). DFT calculations shed more light on oxygen activation
Fig. 9). We consider six types of adsorption mode on the
2
CuOACu O interface (Fig. S6) [57]. The calculations show that
molecular oxygen is unstable on either multiple or single Cu(II)
sites. The parallel adsorption of two oxygens onto a Cu(I) site is
unstable and eventually transforms to the linear adsorption of Cu
(
I)AOAO. The calculations suggest that only two adsorption modes
are stable, the bridge adsorption of Cu(II)AOAOACu(I) on the Cu
II)ACu(I) dual site and the linear adsorption of Cu(I)AOAO. The
former mode is more stable (adsorption energies: 47.26 vs.
(
ꢀ1
Fig. 10. Detection of the iminium cation intermediate by LTQ-Orbitrap-Elite high-
resolution mass spectroscopy. (a) The reaction conditions. (b) The mass spectrum of
the iminium cation.
45.33 kJ mol ), with stronger interaction (electron transfer from
Cu to O : 0.5e vs. 0.2e) and greater OAO bond length (1.32 vs.
.28 Å). DFT calculations indicate that the Cu(II)ACu(I) dual site
at the CuOACu O interface is more favorable for the activation of
2
1
2
3.3. Amine adsorption on CuO/Cu
2
O
molecular oxygen.
We then investigated the coordination environment of the
copper site of CuO/Cu O by EPR characterization of tributylamine
3.5. Mechanism of CuO/Cu O-catalyzed CAC bond cleavage
2
2
adsorption (Fig. 7). In EPR spectra, besides the typical peak at
g = 2.0863 associated with Cu(II) in the distorted octahedral coor-
dination environment [47–50], a new peak at g = 3.4278 appears.
This is caused by a different coordination environment Cu(II)
induced by lattice mismatch. The line broadening along the loss
of the four-line hyperfine splitting is due to the dipolar coupling
that arises from the strong interaction of Cu(II) sites [51]. After
To gain further insight into the reaction mechanism, we tried to
detect reaction intermediates. LiBF is used to provide an anion
4
ꢀ
( BF ) to stabilize the iminium cation. The iminium cation was
4
detected using high-resolution mass spectroscopy (Fig. 10).
According to the EPR results, the cation is probably derived from
one electron transfer from N atoms to Cu(II), followed by hydrogen
abstraction by the neighboring activated oxygen. DFT calculations
of the bond energy of the iminium cation show that the CAN bond
2
CuO/Cu O interacts with tributylamine, the EPR intensity at
ꢀ
1
g = 3.4278 decreases, the linewidth narrows, and the four-line
hyperfine splitting appears. This implies that some Cu(II) in the
CuO domain is reduced, and some nonreduced Cu(II) remains as
isolated Cu(II) sites. EPR tests ensure that amine adsorption occurs
on Cu(II) and electron transfer from amines to CuO reduces Cu(II)
energy (465.08 kJ mol ) strengthens after the formation of a C@N
ꢀ1
bond (480.99 kJ mol ). At the same time, the (NA)CAC bond
ꢀ1
energy (523.53 kJ mol ) decreases with the formation of (N@)
ꢀ1
CAC bonds (383.39 kJ mol ), and therefore is more easily attacked
by the nearby activated oxygen, leading to CAC bond cleavage. This
C H
2
5
H
H
O* C H9
N
C H
4
9
O*
4
CuO
Interface
Cu O
2
C H
2
5
H
H
+
O2
C H
N
C H
4
C H
4
9
9
2
5
H
O* C H
N
C H
4
9
4
9
CuO
Interface
Cu O
CuO
Interface
Cu O
2
2
O
+
CxH Oz
y
C H
N
C H
4
4
9
9
2
Fig. 11. Proposed selective CAC bond cleavage reaction mechanism over CuO/Cu O in the oxidation of tributylamine to dibutylformamide.

M. Wang et al. / Journal of Catalysis 330 (2015) 458–464
463
represents a very good example of how to steer reaction paths and
control product selectivity by creating suitable CuOACu O interfa-
2
cial structure.
[2] P.C. Dai, W. Li, J. Xie, Y.M. He, J. Thorne, G. McMahon, J.H. Zhan, D.W.
Wang, Forming buried junctions to enhance the photovoltage generated
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13493–13497.
Based on the above-mentioned results, we propose a reaction
mechanism as depicted in Fig. 11. Molecular oxygen is activated
on interfacial Cu(II)ACu(I) dual sites. Amine is adsorbed and acti-
vated on CuO domains, accompanied by electron transfer to Cu
[3] G. Ghadimkhani, N.R. de Tacconi, W. Chanmanee, C. Janaky, K. Rajeshwar,
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II). Further hydrogen abstraction of (NA)CAH by activated oxygen
1720.
forms the iminium cation, which contains strong C@N bonds and
weak CAC bonds. Finally, high selectivity for the CAC cleavage pro-
duct is generated. This embedded structure provides a geometri-
cally and structurally favorable microenvironment for co-
activation of oxygen and amine in close proximity. The present
study shows how to steer the reaction path and product selectivity
by creating a suitable catalyst structure.
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. Conclusions
[
[
Interfacial defect sites are crucial to catalysis. However, the dif-
ficulty lies in creating a large-molecule-accessible phase boundary.
Particularly for crystalline Cu O, the transient oxidation state
makes it likely to be oxidized or reduced, depending on ambient
conditions. We here establish a substrate-accessible CuOACu
9] Q. Hua, T. Cao, X.K. Gu, J.Q. Lu, Z.Q. Jiang, X.R. Pan, L.F. Luo, W.X. Li, W.X. Huang,
2
Crystal-plane-controlled selectivity of Cu
2
O catalysts in propylene oxidation
with molecular oxygen, Angew. Chem. Int. Ed. 53 (2014) 4856–4861.
2
O
[10] A.E. Baber, X.F. Yang, H.Y. Kim, K. Mudiyanselage, M. Soldemo, J.
Weissenrieder, S.D. Senanayake, A. Al-Mahboob, J.T. Sadowski, J. Evans, J.A.
Rodriguez, P. Liu, F.M. Hoffmann, J.G.G. Chen, D.J. Stacchiola, Stabilization of
catalytically active cu plus surface sites on titanium copper mixed-oxide films,
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phase boundary and achieve the preferential cleavage of CAC
bonds over CAN bonds in the oxidation of trialkylamines.
2
CuO nanodomains are formed on the surface layer of Cu O by
controlling the oxidation temperature (<160 °C). The CuO phase
is clearly observed by TEM and SAED. The lattice mismatch of
CuO and Cu O induces the disordered coordination of Cu ions
2
[11] K. Nagase, Y. Zheng, Y. Kodama, J. Kakuta, Dynamic study of the oxidation state
of copper in the course of carbon monoxide oxidation over powdered CuO and
2
Cu O, J. Catal. 187 (1999) 123–130.
[
12] L. Zhou, S. Gunther, D. Moszynski, R. Imbihl, Reactivity of oxidized copper
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and generates interfacial oxygen defect sites, providing bonding
sites for molecular oxygen. Amines selectively adsorb onto the
Cu(II) sites of the embedded CuO phase near the strained area.
Globally, two substrates are co-activated in close proximity at
the boundary phase. Moreover, in situ-generated iminium cations
strengthen the CAN bonds and weaken the (NA)CAC bond. Conse-
quently, the (NA)CAC bonds get cleaved by the activated oxygen,
even though they show higher bond energy in the initial state. This
embedded structure provides synergistic centers for the activation
of oxygen and amines. Because of the formation of iminium
cations, the reaction generates the CAC cleavage product, not the
CAN cleavage one, which is opposite to biological or homogeneous
catalytic processes. This approach not only provides a promising
strategy for the selectively oxidative cleavage of CAC bonds of tri-
alkylamines to tertiary formamides, but also shows that the deli-
cate control of the surface chemistry of pristine oxides can create
a suitable activation environment, which can steer reaction paths
and product selectivity. This understanding provides more insight
into the pretreatment and activation processes, which are required
for most catalysts.
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Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (21303183, 21273231) and Dalian municipal
science and technology plan project (2014J11JH126).
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