The Effects of Exposed Specific Facets and Sulfation on the Surface Acidity of Cu2O Solids

Article abstract of DOI:10.1002/chem.201903231

Cuprous oxide microcrystals with {111}, {111}/{100}, and {100} exposed facets were synthesized. ³¹P MAS NMR using trimethylphosphine as the probe molecule was employed to study the acidic properties of samples. It was found that the total acidic density of samples increases evidently after sulfation compared with the pristine cuprous oxide microcrystals. During sulfation, new {100} facets are formed at the expense of {111} facets and lead to the generation of two Lewis acid sites due to the different binding states of SO₄ ^2? on {111} and {100} facets. Moreover, DFT calculation was used to illustrate the binding models of SO₄ ^2? on {111} and {100} facets. Also, a Pechmann condensation reaction was applied to study the acidic catalytic activity of these samples. It was found that the sulfated {111} facet has better activity due to its higher Lewis acid density compared with the sulfated {100} facet.

Full text of DOI:10.1002/chem.201903231

DOI: 10.1002/chem.201903231
Communication
&
Catalysis
The Effects of Exposed Specific Facets and Sulfation on the
Surface Acidity of Cu2O Solids
Yanan Wu,[a] Daofeng Huang,[a] Yingyi Fu,[a] Li Zhang,[a] Shixi Liu,[b] Gangfeng Tang,[a]
Yuanhang Ren,[a] Lin Ye,[a] Xueying Chen,[a] Bin Yue,*[a] and Heyong He*[a]
vestigate surface properties of specific Cu2O facets.[5] For Cu2O,
Abstract: Cuprous oxide microcrystals with {111},
{111}/{100}, and {100} exposed facets were synthesized.
31P MAS NMR using trimethylphosphine as the probe mol-
surface energies (g{100} <g{111} <g{110}) and the amount of Cu
dangling bonds ({100}<{111}<{110}) vary from different facets
and these intrinsic features determine electronic properties of
ecule was employed to study the acidic properties of sam-
Cu2O surfaces. Therefore, many methods have been developed
ples. It was found that the total acidic density of samples
to modify the facets, such as etching, template, and deposi-
increases evidently after sulfation compared with the pris-
tion.[2c]
tine cuprous oxide microcrystals. During sulfation, new
Sulfation of metal oxides is used to obtain solid acids.[6]
{100} facets are formed at the expense of {111} facets and
Therefore, sulfated metal oxides have been widely studied,
lead to the generation of two Lewis acid sites due to the
such as SO42À/ZrO2,[7] SO42À/TiO2,[8] SO42À/SnO2.[9] During the sul-
2À
different binding states of SO4 on {111} and {100} facets.
Moreover, DFT calculation was used to illustrate the bind-
2À
fation, SO4 tends to bond with metal atoms on the surface of
metal oxides. With the electron-withdrawing effect from S=O,
2À
ing models of SO4 on {111} and {100} facets. Also, a
2À
the electrons on metal atoms prefer to move towards SO4
,
Pechmann condensation reaction was applied to study
the acidic catalytic activity of these samples. It was found
that the sulfated {111} facet has better activity due to its
higher Lewis acid density compared with the sulfated
{100} facet.
which leads to electron deficiency on metal atoms and the for-
mation of stronger Lewis acid sites. Brønsted acid sites are
formed if hydroxyl groups bond with these Lewis acid sites.
However, it is rare to see the study on the acidic properties
after sulfation of metal oxides with specific exposed facets. In
the previous research we reported sulfated TiO2 solid super-
acids and found their acidic and catalytic properties are facet-
dependent.[10]
Design of catalysts with different morphologies is an important
topic due to atomic arrangement on different facets leading to
special surface and catalytic properties.[1] Cu2O is widely used
in gas sensors, solar energy transformations, lithium-ion batter-
ies and catalysis because of its small band gap (2.17 eV),
simple synthetic method, nontoxicity, scalability, and abun-
dance.[2] So, it is reasonable to realize that the synthesis of
Cu2O with certain exposed facets has aroused extensive atten-
tion in the past years.[2c,3] Some morphologies of Cu2O report-
ed include cubic exposed {100} facets, octahedra exposed {111}
facets, cuboctahedra exposed both {100} and {111} facets,
rhombic dodecahedron exposed {110} facets, etc.[4] These
pieces of research provide a number of desired samples to in-
In this work, three different Cu2O microcrystals with various
morphologies were prepared including octahedra with domi-
nant {111} facets, cuboctahedra with both {100} and {111}
facets, and cubes with dominant {100} facets. These Cu2O sam-
ples were sulfated and 31P MAS NMR was used to study their
acid properties with trimethylphosphine (TMP) as the probe
molecule. DFT calculations were applied to give the optimized
2À
models of SO4 bonded on {111} and {100} facets. Further-
more, the acid catalytic activity of all three solid acids was
tested in Pechmann condensation of 5,7-dihydroxy-4-methyl
coumarin.
Three types of Cu2O microcrystals with dominant {111}
facets, both {100} and {111} facets and dominant {100} facets
were prepared according to the published method[4b] and are
denoted as C111, C100/111, and C100, respectively. In a typical
process, 0.171 g of CuCl2·2H2O, a certain amount of PVP (Mw:
29000) (3.333 g for C111, 1.111 g for C100/111, and 0 g for
C100) and 100 mL of deionized water were mixed in a 250 mL
round-bottomed flask with stirring in a 558C water bath. Then,
10 mL of NaOH aqueous solution (2 molLÀ1) was added drop-
wise. After 30 min, 10 mL of ascorbic acid aqueous solution
(0.6 molLÀ1) was added dropwise into the system. The mixture
was aged for 3 hours. The resulting precipitate was collected
with centrifuging and washed with water and absolute ethanol
three times each. The obtained Cu2O microcrystals were dried
[a] Y. Wu, D. Huang, Dr. Y. Fu, Dr. L. Zhang, G. Tang, Dr. Y. Ren, Dr. L. Ye,
Prof. X. Chen, Prof. B. Yue, Prof. H. He
Department of Chemistry and
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials
Collaborative Innovation Center of Chemistry for Energy Materials
Fudan University, Shanghai 200433 (P. R. China)
E-mail: yuebin@fudan.edu.cn
heyonghe@fudan.edu.cn
[b] Prof. S. Liu
School of Chemical Science and Technology
Yunnan University, Kunming, Yunnan 650091 (P. R. China)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201903231.
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at 608C in a vacuum oven over night. PVP was not observed
on the surface of three Cu2O pristine samples (Figure S1 and
S2, Supporting Information). In the sulfation process, 10 mL of
(NH4)2SO4 aqueous solution (1 molLÀ1) was added into a 50 mL
three-necked round bottom flask. Then N2 (80 mLminÀ1) was
purged continuously into the system to remove air in the flask.
After 30 min, 0.25 g of Cu2O was added, followed by stirring
for 8 hours with continuous N2 flow. The resulting precipitate
was centrifuged and washed with absolute ethanol, then dried
in a vacuum oven at 608C over night. The product was cal-
cined in N2 at 2508C for 2 hours and further at 3008C for 1 h
to remove residual (NH4)2SO4. The resulting sulfated Cu2O sam-
ples were denoted as SC111, SC100/111 and SC100 accordingly.
The powder XRD patterns of sulfated Cu2O are presented in
Figure 1a. SC111, SC100/111, and SC100 are indexed to the
crystal structure of Cu2O (JCPDS 05-0667, Figure S3). A trace
amount of CuSO4 (JCPDS 15-0775) is found in all sulfated sam-
ples, which is caused by oxidative corrosion on the surface of
Cu2O in (NH4)2SO4 solution. The SEM images of SC111, SC100/
111, and SC100 are shown in Figure 1b–d. Compared to their
pristine samples (Figure S4, Supporting Information), their mor-
phologies remain unchanged after sulfation. To further confirm
the surface component of sulfated samples, XPS was conduct-
ed and the results are shown in Figure 1e–f and Figure S5,
Supporting Information. Before sulfation, there are only two
peaks at 952.4 and 932.4 eV which are indexed to the charac-
teristic peaks of CuI 2p1/2 and CuI 2p3/2. After sulfation, the
peaks of CuI are still found. In addition, two weak peaks
appear around 955.0 and 935.0 eV belonging to CuII 2p1/2 and
CuII 2p3/2. Meanwhile, the broad satellite peaks around 963.1
and 943.1 eV are observed, which are the characteristic peaks
of CuII with the d9 configuration in the ground state.[11]
Solid-state NMR has been proven to be a powerful tech-
nique to study the surface properties of solid catalysts.[12] TMP
is often used as a probe molecule to characterize detailed
acidic properties, including acid-type, density, and strength.
There are mainly three interaction modes of TMP adsorbed on
the surface of solid acid sites. TMP molecules chemisorbed on
Lewis acid sites span over a wide chemical shift range from
À20 to À60 ppm. The protonation of TMP molecules on
Brønsted acid sites results in the formation of TMPH+ ionic
complexes, giving rise to 31P resonances in the range of 0 to
À5 ppm. Also, the chemical shift at around À62 ppm indicates
the physisorption of TMP molecules on the surface. Moreover,
TMP molecules may be oxidized to TMPO by the surface of
metal oxides.[13] 31P MAS NMR is applied here to study acid
properties of sulfated Cu2O and the results are shown in Fig-
ure 2a. The peaks at À63.9 ppm are assigned to physisorption
of TMP on sulfated Cu2O samples, and the peaks around 41.6
+
and 25.7 ppm are assigned to physisorped TMPO and P(CH3)4
+ [13]
formed from P(CH3)3 and HP(CH3)3
,
respectively.
According to the 2nI+1 rule (n is the number of atoms con-
nected with 31P, I is the nuclear spin number of atoms connect-
ed with 31P), coupling of a 31P nucleus to a Cu nucleus with
spin 3/2 will split the 31P signal into a quartet in NMR spectra.
Therefore, Lewis acid peaks in Figure 2a between À20 and
À50 ppm are quartets. After deconvolution, two groups of
Figure 2. (a) 31P MAS NMR spectra of TMP adsorbed on SC111, SC100/111
and SC100. (b,c and d) The spectral deconvolution results of Lewis acid
areas of SC111, SC100/111 and SC100 with red lines indicating type 1 Lewis
acid peaks and blue lines for type 2 Lewis acid peaks.
Figure 1. (a) XRD patterns of SC111, SC100/111 and SC100. SEM images of
(b) SC111, (c) SC100/111 and (d) SC100. (e) XPS spectra of C111, C100/111 and
C100. (f) XPS spectra of SC111, SC100/111 and SC100.
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quartets in these three samples can be observed (Figure 2b–
d). This indicates that all samples have two types of Lewis acid
sites on the surface. In addition, the central chemical shift of a
quartet represents acid intensity of corresponding Lewis acid
sites. The Lewis acids with chemical shift around À39 and
À33 ppm are denoted as type 1 and 2 Lewis acids, respective-
ly, and type 2 Lewis acid sites have stronger interactions with
TMP molecules and stronger acidity due to their downfield
chemical shift. To illustrate both types of Lewis acids sites, the
surface changes on {100} and {111} facets in (NH4)2SO4 solution
(pH 4.3) need to be analyzed. Morphological evolution of Cu2O
in acidic solution was discussed by Hua and co-workers.[5c] In
acidic solution, {100} facets maintain, while {111} facets tend to
be corroded from the edge along [110] to form {100} facets
(Figure S6, Supporting Information). Combined with acid densi-
ty results in Table 1, SC111 has the highest percentage of
type 1 Lewis acid while SC100 has the highest percentage of
type 2 Lewis acid among these three samples, it is reasonable
to attribute type 1 Lewis acid sites to metal atoms on {111}
facets, and type 2 Lewis acid sites to metal atoms on {100}
facets. Brønsted acids are not found on all sulfated Cu2O sam-
ples. After sulfation, total acid density increases evidently com-
pared to pristine Cu2O samples (Table S1, Supporting Informa-
tion). The acid density is in the order of SC111>SC100/111>
SC100. This may be owing to the sawtooth-like structures gen-
erated on {111} facets which results in more exposed binding
sites seldom observed on {100} facets (Figure 3).[5c]
tion, we believe that O atoms on the first layer of Cu2O are re-
moved by oxidative corrosion, making Cu atoms in second
layers exposed. Then CuSO4 is generated if oxidative corrosion
2À
occurred, or SO4 binds with exposed Cu atoms to form Lewis
acid sites. In order to find the most optimized binding models
2À
of SO4 on {111} and {100} planes, DFT calculations were ap-
plied (Figures S7–S8 and Table S2, Supporting Information). As
2À
shown in Figure 4, on {111} facets, two O (Os) atoms of SO4
2À
Figure 4. (a) Most optimized model of SO4 bonded on Cu2O {111}. (b) Most
optimized model of SO42À bonded on Cu2O {100} with removal of one O
atom on the surface.
are preferred to bind with two CuCUS atoms. This mode does
not need to remove O atoms on the first layer. On {100} facets,
2À
two OS atoms of SO4 are preferred to bind with two CuCSA
atoms which are linked with each other through an O atom
before sulfation in the first layer. The bond length of Cu–OS is
calculated to be 2.1 and 1.8 ꢁ on {111} and {100} facets, respec-
tively. Cu and OS are closer on {100} facets than on {111} facets,
which means the electron-withdrawing effect of S=O to affect
Cu atoms on {100} is stronger than that on {111} facets. There-
fore, Lewis acid sites on {100} are expected to be stronger
than those on {111} facets. In 31P MAS NMR spectra, chemical
shift of type 1 Lewis acid sites is around 38.5 ppm, while
type 2 is around 32.5 ppm. Thus type 1 Lewis acid is assigned
to Cu atoms on {111} facets, while type 2 is from {100} facets.
This deduction is in good accordance with our analysis above.
Pechmann condensation of 5,7-dihydroxy-4-methyl coumar-
in was carried to study catalytic activity of Cu2O samples. After
sulfation, the catalytic activity of Cu2O samples is improved
compared with pristine Cu2O samples (Figure S9, Supporting
Information). The changes of yield vs. reaction time of these
three sulfated samples are presented in Figure 5. The yield of
5,7-dihydroxy-4-methyl coumarin is in the order of SC111>
SC100/111>SC100, which indicates that sulfated {111} facets
have higher catalytic activity than {100} facets. In Pechmann
condensation, the reaction begins with electrophilic substitu-
On {111} facets, the first layer contains 3-coordinated O
atoms and the second layer is composed of coordinatively sa-
turated Cu (CuCSA) atoms and coordinatively unsaturated Cu
(CuCUS) atoms. On {100} facets, the first layer contains 2-coordi-
nated O atoms, and the second layer is composed of CuCSA
atoms (Figure S7, Supporting Information). In (NH4)2SO4 solu-
Figure 3. HRTEM images of (a) SC111 and (b) SC100 in the corresponding
crystal area (red circles in the insets).
Table 1. Acid properties of sulfated Cu2O.
Sample
Total acid density (mmolgÀ1
)
Type 1 Lewis acid
Type 2 Lewis acid
d [ppm][a]
Acid density [mmolgÀ1
]
[%]
d [ppm][a]
Acid density [mmolgÀ1]
[%]
SC111
SC100/111
SC100
1018
632
450
À40.0
À39.0
À38.3
383
218
83.7
37.6
34.5
18.6
À32.5
À33.0
À32.5
635
414
366
62.4
65.5
81.4
[a] For quartets, the central chemical shift of all four peaks represents acid intensity of the corresponding Lewis acid.
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Keywords: acidity
·
cuprous oxide
·
facets
·
NMR
spectroscopy · sulfation
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dehydration, and both Brønsted acid and Lewis acid can cata-
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only Lewis acid sites with no obvious difference in acid
strength, so the acid density of these three samples is the deci-
sive factor for catalytic activity. From Table 1, it is found that
total acid density, acid density of type 1 and type 2 Lewis acid
are all in the order of SC111>SC100/111>SC100, which is in
consistent with catalytic activity results. The result shows that
{111} facets with higher Lewis acid density exhibit better cata-
lytic activity than {100} facets.
In conclusion, sulfated Cu2O microcrystals are successfully
obtained from pristine Cu2O with different ratio of {100}/{111}
facets. Two types of Lewis acid sites are found on all three sul-
fated samples. It is evidenced by NMR and DFT calculation that
type 1 and type 2 Lewis acid sites come from Cu atoms on
{111} and {100} facets after binding with SO42À, respectively.
SC111 has the highest catalytic activity towards Pechmann con-
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (91645201, 21673046 and 21473036) and
the Ministry of Science and Technology (2017YFB0602204).
Manuscript received: July 16, 2019
Conflict of interest
Revised manuscript received: September 16, 2019
Accepted manuscript online: September 17, 2019
The authors declare no conflict of interest.
Version of record online: && &&, 0000
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COMMUNICATION
Cuprous oxide microcrystals with {111},
{111}/{100}, and {100} exposed facets
were synthesized. 31P MAS NMR using
trimethylphosphine as the probe mole-
cule was employed to study the acidic
properties of the samples. It was found
&
Catalysis
Y. Wu, D. Huang, Y. Fu, L. Zhang, S. Liu,
G. Tang, Y. Ren, L. Ye, X. Chen, B. Yue,*
H. He*
&& – &&
2À
that the models of SO4 bonded on
The Effects of Exposed Specific Facets
and Sulfation on the Surface Acidity
of Cu2O Solids
Cu2O and surface acidity of sulfated
Cu2O varies from different facets (see
figure).
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