Atomically decorating of MnOx on palladium nanoparticles towards selective oxidation of benzyl alcohol with high yield

Article abstract of DOI:10.1016/j.jcat.2020.03.029

Palladium nanoparticles are effective catalysts for selective oxidation of benzyl alcohol, which is a fundamental reaction in fine chemicals production. Herein, an atomic-scale oxide decoration catalytic structure is designed and implemented by atomic layer deposition (ALD). The MnO_x/Pd catalysts fabricated by ALD show enhanced conversion and selectivity simultaneously, thus a high yield of benzaldehyde is obtained. Aiming to the difficulty in controlling reaction pathways and byproducts distribution, Pd (1 1 1) facets are selectively passivated by MnO_x deposition. It is found that the byproduct toluene formation is completely suppressed. The proposed structure is also beneficial for inhibiting the decarbonylation reaction of benzaldehyde. On the other hand, the addition of MnO_x via ALD realizes the modification of Pd electronic structure. The MnO_x coated Pd nanoparticles forms reversed catalytic structure, which also creates large amount of highly active Pd-MnO_x interfacial perimeters, result in improved catalytic activity and reaction rate. The TOF of MnO_x decorated Pd catalyst reaches 31,561 h^?1, which is 8.7 times larger than that of the unmodified Pd catalyst. Meanwhile, the maximum conversion of benzyl alcohol and yield of benzaldehyde are improved to 84.7% and 76.5%, respectively.

Full text of DOI:10.1016/j.jcat.2020.03.029

Journal of Catalysis 386 (2020) 60–69
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Atomically decorating of MnO on palladium nanoparticles towards
x
selective oxidation of benzyl alcohol with high yield
Jianfeng Yang a,b, Kun Cao a, , Miao Gong , Bin Shan , Rong Chen
⇑
a
b
a,
⇑
a
State Key Laboratory of Digital of Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology,
037 Luoyu Road, Wuhan, Hubei 430074, PR China
State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037
1
b
Luoyu Road, Wuhan, Hubei 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium nanoparticles are effective catalysts for selective oxidation of benzyl alcohol, which is a funda-
mental reaction in fine chemicals production. Herein, an atomic-scale oxide decoration catalytic structure
is designed and implemented by atomic layer deposition (ALD). The MnO /Pd catalysts fabricated by ALD
x
show enhanced conversion and selectivity simultaneously, thus a high yield of benzaldehyde is obtained.
Aiming to the difficulty in controlling reaction pathways and byproducts distribution, Pd (1 1 1) facets are
Received 17 January 2020
Revised 20 March 2020
Accepted 24 March 2020
selectively passivated by MnO
suppressed. The proposed structure is also beneficial for inhibiting the decarbonylation reaction of ben-
zaldehyde. On the other hand, the addition of MnO via ALD realizes the modification of Pd electronic
structure. The MnO coated Pd nanoparticles forms reversed catalytic structure, which also creates large
amount of highly active Pd-MnO interfacial perimeters, result in improved catalytic activity and reaction
rate. The TOF of MnO decorated Pd catalyst reaches 31,561 h , which is 8.7 times larger than that of the
x
deposition. It is found that the byproduct toluene formation is completely
Keywords:
Selective oxidation of benzyl alcohol
Palladium nanoparticles
Atomically decorating
x
x
MnO
Atomic layer deposition
x
x
ꢀ1
x
unmodified Pd catalyst. Meanwhile, the maximum conversion of benzyl alcohol and yield of benzalde-
hyde are improved to 84.7% and 76.5%, respectively.
Ó 2020 Elsevier Inc. All rights reserved.
1
. Introduction
The selective oxidation of benzyl alcohol is considered to be one
prevent deactivation. For example, the Pd/Au nanocrystals show
enhanced selectivity in the oxidation of alcohols to aldehydes
due to the electronic modification of Au to Pd [14]. From previous
reports, compared with single MnO or CeO support, the catalytic
activity and selectivity for benzyl alcohol are significantly
improved by the MnCeO composite support through the synergis-
tic effect between Pd and MnO /CeO interfaces [19].
of the most fundamental reactions for fine chemicals production in
both laboratory and industrial scale [1–4]. In previous studies, var-
ious heterogeneous catalysts have been reported for this model
reaction [5,6]. Considering green chemistry principle, it is impor-
tant to improve the atomic utilization of the reactants, which could
also provide insights for other alcohols oxidation reactions. Sup-
ported palladium catalysts are very active for benzyl alcohol oxida-
tion [7–11]. However, during this process other side reactions also
take place and undesired byproducts are formed. Thus, the total
yield of target benzaldehyde product decreases. Methods are
developed to modify the active sites of the Pd catalysts by intro-
ducing promoters and nanotailoring of interfacial perimeter. Typi-
cally, metals like Bi [12], Pb [13], Au [14,15] as well as oxides such
x
2
x
x
2
Despite extensive studies on Pd catalysts, the description of Pd’s
active sites remains controversial. Based on previous researches,
metallic Pd0 species are supposed to be the active sites, and the
dehydrogenation step on the Pd catalysts during alcohol selective
oxidation is universal [20,21]. As discussed in detail, the activity
enhancement of PdO is attributed to its reduction by benzyl alco-
0
hol to Pd during the reaction [22,23]. The oxidative dehydrogena-
tion of the alcohol to aldehyde occurred on all exposed Pd surfaces
is the main reaction. Meanwhile the undesired decarbonylation
formation takes place preferentially at the hollow sites on Pd
(1 1 1) facets. Medlin et al. [24,25] proposed that on clean Pd
as FeO
x x
[16,17], MnO [18] etc., are introduced to construct com-
posite configurations to improve the catalytic performance and
(
1 1 1) facets, the reaction of benzyl alcohol was highly dependent
on its adsorption orientation. With low adsorption coverage, ben-
zyl alcohol presented a flat lying structure to produce CO and ben-
zene. While with high coverage, the benzyl alcohol presented an
⇑
Corresponding authors.
E-mail addresses: kuncao@mail.hust.edu.cn (K. Cao), rongchen@mail.hust.edu.cn
(
R. Chen).
https://doi.org/10.1016/j.jcat.2020.03.029
021-9517/Ó 2020 Elsevier Inc. All rights reserved.
0

J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
61
upright structure and produced deoxygenation products such as
toluene and H O. Meanwhile, the aerobic oxidation of alcohols is
affected by various coupled factors, including conversion tempera-
ture, reaction time length etc. As a result, it is extremely difficult to
achieve high selectivity, conversion and reaction rate simultane-
ously. Generally, the over-oxidation of the reactants leads to the
200 s purge with N
2
flow was introduced between the reactants
2
doses to remove the physically adsorbed precursors. The MnO
coated Pd catalyst was labeled as MnO /Pd/Al . Meanwhile,
MnO /Pd/Al (co-impregnation) catalyst prepared by co-
x
x
2 3
O
x
2 3
O
impregnation method was used as a reference sample, the Mn
loading for this sample is 0.2 wt%.
decreasing of target product selectivity [26]. For Au/U
3 8
O catalyst
[
27], the selectivity of target product at initial conversion stage
2.2. Characterization of catalysts
was high. However, when increasing the reaction temperature
and prolonging the reaction time, the conversion of benzyl alcohol
could be increased, but the selectivity of benzaldehyde decreased
drastically.
X-ray fluorescence (XRF) analyses were performed using an
EAGLE III(EDAX Inc.) Explorer spectrometer to determine the com-
position of the catalysts. X-ray diffraction (XRD) were examined
with 2h values, utilizing X-ray Powder diffractometer (Empyrean,
In this work, an atomically manganese oxide decorated Pd
nanoparticles structure is designed and fabricated via atomic layer
deposition (ALD). In the ALD process, two gaseous precursors are
dosed alternately on the substrates. The self-limiting nature pro-
vides the advantages of depositing materials with atomic-scale
control. Catalysts prepared by ALD show enhanced conversion
and selectivity simultaneously, thus a high yield of target product
benzaldehyde is obtained. The method enables an effective way to
investigate the composite nanoparticles’ structure-property rela-
tionship. The reaction pathways are changed on ALD decorated
PANalytical B.V.), automatic divergence slits and Cu Ka1/2 1/a2
radiation (40 kV, 40 mA). Scan mode: continuous; step range:
10–80° 2h; step size: 0.0131° and counting time: 27.8 s/step. The
phase composition of the catalysts was estimated by thermo-
gravimetric analysis (TG-DTA) in nitrogen atmosphere using Dia-
mond TG/DTA produced by PerkinElmer Instruments. For this anal-
ysis, 5 mg of fresh catalyst was heated from room temperature to
ꢀ1
800 °C with a heating rate of 10 °Cꢁmin . Field-emission transmis-
sion electron microscopy (FTEM) (Tecnai G2 F30) was performed to
characterize the morphology and crystal structure of the catalysts.
UV–vis diffuse reflectance spectra were collected on a Lambda 35
UV–vis spectrophotometer equipped with a diffuse reflectance
accessory. The spectra were recorded in the range of 200–
1000 nm at room temperature with air as reference. The Raman
spectra were obtained for the fresh catalysts from a LabRAM
HR800 spectrometer (Horiba JobinYvon) at room temperature
(RT) equipped with a 532 nm Ar-ion laser beam. The composition
and binding energies were examined by X-ray photoelectron spec-
troscopy (XPS) on an AXIS-ULTRA DLD-600 W photoelectron spec-
Pd catalysts. Pd (1 1 1) facets are selectively passivated by MnO
x
deposition to eliminate the byproduct toluene formation. Toluene
products are not observed for modified catalysts. On the other
hand, the reversed oxides coating structure (compared with the
oxides supported catalysts) enriches the active metal-oxide inter-
x
faces. Meanwhile, the addition of MnO can modify the electronic
structure of Pd which increases the surface concentration of metal-
0
lic Pd species. Therefore, catalytic activity and reaction rate are
also improved. The TOF of MnO
x
coated Pd catalyst reaches
ꢀ1
3
1,561 h , which is 8.7 times larger than that of bare Pd catalyst.
Moreover, this structure is also beneficial for inhibiting the decar-
bonylation reaction of benzaldehyde, further improving the yield
of target product. The maximum conversion of benzyl alcohol
and yield of benzaldehyde are improved to 84.7% and 76.5%
respectively.
trometer using Al Ka radiation. Accurate binding energies (±0.1 eV)
were calibrated with respect to the position of the adventitious C
1s peak at 285 eV. The in situ diffuse-reflectance infrared Fourier
transform spectroscopy (DRIFTS) of CO adsorption on catalysts
were collected by
MCT-A) detector, which was deployed on a Nicolet iS50 Fourier
transform infrared (FTIR) spectrometer. The -temperature-
programmed desorption (TPD) was also carried out by the AMI-
a wide-band mercury–cadmium–telluride
(
O
2
2
. Experiments
3
00, where the catalysts were first treated at 200 °C under He
ꢀ1
2.1. Catalysts fabrication
atmosphere with 10% of O
O
2
(30 mlꢁmin ). When the baseline of
2
signal in TCD maintained stable, the temperature was pro-
Al
impregnation method. 18.09 wt% palladium nitrate dihydrate
was impregnated into Al (99.99%, metals basic). The loading
2 3
O supported palladium catalysts were prepared by wet
grammed to increase from 40 °C to 800 °C with a ramping rate of
10 °Cꢁmin and TCD signal was recorded at the same time.
ꢀ1
2 3
O
of palladium was fixed to 2 wt%. The sample was placed at room
temperature for 24 h and dried at 80 °C overnight in a vacuum
oven. Then the sample was ground and calcined at 500 °C in the
air for 3 h, and naturally cooled to room temperature. The bare
2.3. Catalytic performance tests
The solvent-free selective oxidation of benzyl alcohol was per-
formed under atmosphere pressure. 100 mg catalyst was dispersed
in 5 ml benzyl alcohol in a three-necked batch reactor with a reflux
condenser under stirring. The suspension was kept at 393 K with
oxygen bubbled in at a flow-rate 20 ml/min. After the reaction,
the catalyst was separated from the solution by centrifugation
and the products were analyzed and identified by gas chromatog-
raphy (GC, Agilent Technologies 7890B with a 0.32 mm ꢂ 30 m HP-
5 capillary column, He carrier). GC–MS analysis was performed on
an Agilent Technologies 7890A GC system equipped with an Agi-
lent Technologies 5975C MSD Mass Spectrometer. The external
standard method was employed to calculate benzyl alcohol con-
version and benzaldehyde selectivity. The conversion of benzyl
alcohol and the selectivity toward benzaldehyde were defined as
follows:
Pd catalyst was denoted as Pd/Al
A rotary ALD reactor coupled with fluidization was utilized to
deposit MnO on the powder Pd/Al catalysts. The cylindrical
sample holder enables dynamic fluidization of the catalysts by dos-
ing constant N flow into the reactor. The rotation of sample holder
2 3
O .
x
2 3
O
2
is beneficial to improve the uniformity and stable fluidization of
particles. Manganese oxide ALD approach was performed with
Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)manganese(III)
(
manganese source, Strem Chemicals, 99%) and ozone (oxygen
source) as precursors. O was produced by high-purity oxygen
99.999%) through an ozone generator (11% vol. of O in O ). The
deposition temperature was 150 °C, and the manganese source
was heated at 133 °C and carried by N with mass flow of 35 sccm
dosing into the reactor. In one ALD cycle, the MnO ALD sequence
consisted of a 200 s dosing of Tris(2,2,6,6-tetramethyl-3,5-heptane
dionato)manganese(III) (Mn(thd) ) and a 200 s dosing of ozone.
3
(
3
2
2
x
moles of reactant converted
conversionð%Þ¼ 100% ꢂ
moles of reactant in feed
3

6
2
J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
moles of product formed
moles of reactant converted
2 3 3 4
Mn O to Mn O , respectively [30]. The results reveal the coexis-
tence of Mn and Mn species in ALD-coated samples. However,
selectivityð%Þ¼ 100% ꢂ
3
+
4+
the endothermic peaks of the 0.2 wt%MnO
not clear, probably due to the low loading of MnO
the detection limit.
x
/Pd/Al
2
O
3
catalyst are
x
that below
3
. Results and discussions
2 3 x 2 3
The FTEM images of Pd/Al O , 0.2 wt%MnO /Pd/Al O and
0
.6 wt%MnO /Pd/Al are shown in Fig. 3 (a)(b)(c), the Pd species
x
2 3
O
3.1. Phase and dispersion of catalysts
on the support cannot be observed clearly due to low magnifica-
tion and contrast. Meanwhile, Pd nanoparticles with large diame-
ter are not observed, indicating the highly dispersion of Pd
nanoparticles. HAADF-STEM measurements are performed to
The MnO
ALD cycles, respectively. With XRF calibration, the coating thick-
ness (ALD cycles) is converted to MnO loading of 0.1, 0.2, 0.4,
.6 wt%, respectively. The X-ray diffraction (XRD) patterns of
Al , bare Pd nanoparticles and MnO -decorated Pd nanocatalysts
are presented in Fig. 1. Typical diffraction peaks correspond to
Al support (JCPDS 29-1486, 2h = 37.8, 45.7 and 67.1) are
observed in all Pd/Al samples. Additionally, for bare Pd catalyst,
x x
coating structures are fabricated with 1, 2, 4, 6 MnO
x
investigate the MnO
shown in Fig. 3 (d) (e) (f), the Pd nanoparticles are uniformly dis-
persed on the Al support, and the average size is 3.62 ± 0.72 n
m for uncoated Pd sample. The particle sizes for 0.2 wt%MnO /Pd/
Al (3.62 ± 0.32 nm), 0.6 wt%MnO /Pd/Al (3.69 ± 0.30 nm)
x 2 3
/Pd/Al O samples in dark field mode. As
0
2
O
3
x
2 3
O
2 3
O
x
2
O
3
x
2 3
O
2 3
O
and the bare Pd catalyst are very similar, indicating that the ALD
process has minimal influence on the Pd particles size. The HRTEM
images of the catalysts are shown in Fig. 3(g)(h)(i). The Pd
nanoparticles are crystalline with a lattice fringe distance of
the well-defined diffraction peaks centered at 2h = 33.8 and 54.7
reveal the presence of Pd oxide species (JCPDS 41-1107). Mean-
while, there are no obvious diffraction peaks of metallic Pd phase
(
JCPDS46-1043, 2h = 40.2, 46.8 and 68.3) for the bare Pd/Al
alyst. On the other hand, for the MnO /Pd/Al catalysts, no
remarkable diffraction peaks of MnO are observed, which may
2 3
O cat-
2
.64 Å for PdO phases and 2.25 Å for metallic Pd phases. From sta-
tistical analysis, the amount of metallic Pd phases for MnO /Pd/
2 3
Al catalysts is larger than that of the unmodified Pd/Al O
x
2 3
O
x
x
2
O
3
be explained by the fact that the MnO
x
is highly dispersed with
x
loading, the intensity of
catalysts.
ultralow loading. With increasing MnO
0
the Pd reflection band located at 2h = 40.2 also increases as shown
in Fig. 1b. Furthermore, the diffraction peaks of Pd oxide slightly
x
3.2. Decoration configuration of MnO on Pd
decrease as MnO
that the concentration of metallic Pd phases increases after the
MnO ALD process.
In order to analyze the phase of the catalysts before and after
MnO decoration, DTA-TG tests are performed from room temper-
ature to 800 °C for Pd/Al and MnO /Pd/Al catalysts in nitro-
gen. As shown in Fig. 2a, the weight loss before 400 °C with the
endothermic peak located at ~140 °C can be attributed to the
removal of physiosorbed water and chemically bonded water
x
coating amount increases (Fig. 1c), indicating
0
x
In order to reveal the decoration structure of MnO on Pd, infra-
x
red spectroscopy using carbon monoxide (CO) chemisorption as
probe is performed. FTIR spectroscopy of CO chemisorption on Pd
is very sensitive to the exposed palladium terraces and step edges.
Therefore, it provides us with direct information about the avail-
able palladium active sites in the catalytic reaction. Fig. 4(a) shows
x
2
O
3
x
2 3
O
the FTIR spectra of CO saturated adsorption on Pd/Al
Pd/Al at room temperature. For the Pd/Al catalysts, a strong
band at 2090 cm and four weak bands at 2170, 2120, 1990 and
2 3 x
O and MnO /
2
O
3
2 3
O
ꢀ1
[
28]. Removal of residual nitrates is reflected in the TG/DTA in
higher temperature range of above 400 °C for Pd/Al [29]. Detail
weight loss in TG curves indicates the thermal decomposition of
unreacted metal organic Mn(thd) precursor occurs at 250–
00 °C for MnO /Pd/Al . Particularly, two broad endothermic
peaks in the 400–600 °C and 600–700 °C are observed for 0.6 wt
MnO /Pd/Al sample (Fig. 2(b)), which can be attributed to
the decomposition of MnO to Mn and the transformation of
ꢀ1
ꢀ1
2
O
3
1930 cm
can be observed. The band at 2090 cm
can be
assigned to atop-bonded CO on the Pd (1 1 1) defects, while the
ꢀ1
3
band at 1930 cm can be assigned to bridge-bonded CO on Pd
ꢀ
1
4
x
2
O
3
(1 1 1). Additionally, the band at 1990 cm
is attributed to
ꢀ1
bridge-bonded CO on Pd (1 0 0) [31–36]. The band at 2170 cm
is assigned to linear-bonded CO on ionic Pd
2120 cm is related to linear-bonded CO on ionic Pd [31]. From
2
+
%
x
2
O
3
and band at
ꢀ1
+
2
O
2 3
Fig. 1. XRD patterns of Pd/Al
2
O
3
, MnO
x
/Pd/Al
2
O
3
catalysts with 0.1, 0.2, 0.4, 0.6 wt% Mn loading.

J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
63
2 3 x 2 3
Fig. 2. TG and DTA curves of bare Pd/Al O and MnO /Pd/Al O catalysts with 0.2, 0.6 wt% Mn loading.
2 3 x 2 3 x 2 3
Fig. 3. FTEM, HAADF-STEM and HRTEM images of the (a)(d)(g) Pd/Al O , (b)(e)(h) 0.2 wt%MnO /Pd/Al O and (c)(f)(i) 0.6 wt%MnO /Pd/Al O catalysts.
the discussions of previous XRD tests, Pd oxide species can be
observed in Pd/Al catalyst. While partial Pd oxide species can
Pd particles are 100%, 8%, 3%, 2%, and 1% for Pd/Al
MnO /Pd/Al , 0.2 wt%MnO /Pd/Al , 0.4 wt%MnO
and 0.6 wt%MnO /Pd/Al , respectively. The percentages of
adsorbed CO at the {1 0 0} facets of Pd particles are 11%, 100%,
85%, 41%, and 28% for Pd/Al , 0.1 wt%MnO /Pd/Al , 0.2 wt%
MnO /Pd/Al , 0.4 wt%MnO /Pd/Al , and 0.6 wt%MnO /Pd/
Al ALD cycles, the band
2
O
3
, 0.1 wt%
2
O
3
x
2
O
3
x
2
O
3
x
2 3
/Pd/Al O ,
0
+
be reduced to Pd and Pd during the CO adsorption process [31].
x
2 3
O
Therefore, for bare Pd catalysts, metallic Pd0 species are observed
after CO chemisorption. With increasing MnO
x
loading, the peaks
assigned to Pd and Pd gradually decrease for MnO decorated
Pd samples. As the loading of MnO increases, the band intensity
at 2090 cm (atop-bonded CO on the Pd (1 1 1) defects) sharply
2
O
3
x
2 3
O
2
+
+
x
x
2
O
3
x
2
O
3
x
x
2
O
3
, respectively. With increasing MnO
x
ꢀ1
ꢀ1
intensity at 2090 cm (atop-bonded CO on the Pd (1 1 1) defects)
disappears due to the covering of Pd (1 1 1) surface, while the band
ꢀ1
decreases, whereas the relative band intensity at 1990 cm
bridge-bonded CO on Pd (1 0 0)) increases with less cycles (1–2
ꢀ1
(
at 1990 cm (bridge-bonded CO on Pd (1 0 0)) still exists (Fig. 4
(b)). The results indicate that MnO deposition rate on the Pd
cycles). And the adsorption peak intensity of CO on the Pd (1 1 1)
and Pd (1 0 0) facets is quantified and normalized. As summarized
in Table S1, the percentages of adsorbed CO on the {1 1 1} facets of
x
(1 1 1) facets is faster, while leaves Pd (1 0 0) facets exposed.
The facet selectivity growth is realized through different binding

6
4
J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
Fig. 4. DRIFTS spectra of CO adsorbed at RT on Pd/Al
2 3 x 2 3
O , MnO /Pd/Al O catalysts with 0.1, 0.2, 0.4, 0.6 wt% Mn loading: (a) CO adsorption spectra; (b) CO chemisorption
spectra after 50 sccm N purge to remove physically adsorbed CO.
2
energy of Mn precursor chemisorbed on different Pd surface sites.
Similar facet-selective ALD approach is also reported for CeO (ex-
ploiting Ce(thd) and O as precursors) growth on Pt nanoparticles.
During the initial deposition stage, CeO is also highly preferred to
nucleate on the Pt (1 1 1) facet [37]. At the same time, the results
show that the MnO modified Pd prepared by the co-impregnation
method as a reference sample has no selective coating properties,
in which MnO randomly blocks the Pd’s surface sites that reduce
ultraviolet regions, and the center of the main band is located at
x
~295 nm [41]. The absorption between 240 nm and 305 nm can
2ꢀ
2+
2ꢀ
3+
4
3
be attributed to O ? Mn and O ? Mn charge transfer tran-
x
sitions, respectively. The continuous absorption of the visible
3
+
region is related to the d ? d transition on the Mn cation [41].
It reveals that after MnO deposition on Pd, the absorption strength
x
x
of the Pd species decreases, but the absorption band at 300 nm is
broadened. These facts are probably related to the electronic inter-
x
the CO chemisorption intensity homogeneously. The comparison
demonstrates the unique advantages of ALD fabrication method,
which enables directional and selective modification of Pd’s sur-
face active sites and facets.
action between Pd species and MnO
The Raman spectra of the Pd/Al
are plotted in Fig. 5(b). In the Pd/Al
x
.
2
O
3
and MnO
catalyst, the main Raman
band appears at 644 cm , which is attributed to the B1g mode of
x 2 3
/Pd/Al O catalysts
2 3
O
ꢀ1
ꢀ1
PdO [42]. And the band at ~280 cm
(LA) and 2X (TA) mode of PdO [43]. After MnO
the band of PdO gradually decreases as the loading of MnO
is probably assigned to
2
X
8
6
x
deposition,
3.3. Surface chemical state of the catalysts
x
increases. It is noted that the intensity of the band assigned to
the PdO species is progressively decreases along with a red shift
The UV–Vis spectra provide additional information about the
electronic state of Pd and MnO in the catalysts. The ionic state
x
ꢀ1
ꢀ1
from 644 cm to about 630 cm . In particular, the peak centered
of Pd can be determined in the ultraviolet–visible spectrum [38].
ꢀ1
0
at approximately 630 cm is instead the Raman contribution of
the MnO species [44]. The Raman spectra of MnO deposited on
Al and silicon wafer are shown in Fig. S1. The position of the
Raman peaks is consistent with the literature reports. The band
Yet metallic Pd is characterized by absorption without a defined
x
x
structure in all range of the spectrum [38]. As shown in Fig. 5(a),
differences have been observed in diffuse reflectance UV–Vis spec-
2 3
O
tra between the Pd/Al
results, two absorption bands at 237 nm and 415 nm are observed
for the Pd/Al , and these two bands can be attributed to the d
charge transfer transition between Pd and O and the d–d tran-
2 3 x 2 3
O and the MnO /Pd/Al O . According to the
ꢀ1
between 630 and 644 cm is the result of the superposition of
two species. However, the MnO /Pd/Al prepared by the co-
x
2 3
O
2
O
3
p-
impregnation method has no obvious electronic interaction com-
pared to the samples fabricated by ALD, which indicates that the
electronic regulation of Pd by MnO occurs in the ALD process. This
x
pp
sition of small PdO particles, respectively [39,40]. Moreover, the
elemental Mn exhibits a broad absorption peak in the visible and
2 3 x 2 3 x 2 3
Fig. 5. (a) Diffuse reflectance UV–Vis spectra and (b) Raman spectra of Pd/Al O , MnO /Pd/Al O fabricated with co-impregnation method, and MnO /Pd/Al O catalysts
fabricated with ALD method with 0.2, 0.6 wt% Mn loading.

J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
65
may be due to strong binding between the Mn precursors and the
peak (200–500 °C) can be assigned to the oxygen generated by per-
Pd particles thus increasing the interaction between Pd and MnO
decoration layer.
x
oxide of Pd during dissociating at high temperature [51,52]. For the
MnO /Pd/Al O samples, both the low temperature desorption
x 2 3
The surface chemical states of Pd and MnO
gated with XPS measurements. The XPS core level of Pd 3d and Mn
p are scanned and the deconvoluted XPS spectra are shown in
x
species are investi-
peak and the high temperature desorption peak shift to lower tem-
perature, and the intensity increases significantly compared to the
2
2 3
bare Pd/Al O sample. Due to the different surface morphology of
Fig. 6. The surface compositions and amounts are summarized in
Table 1. As shown in Fig. 6(a), before benzyl alcohol oxidation reac-
tion, the peaks at 335.9, 337.2, 341.2 and 342.5 eV can be observed
Pd/Al , the palladium superoxides and peroxides formed on
2 3
O
the surface are different, which will also cause the difference of
the high temperature oxygen desorption peak. Therefore, the
change in the peak shape of the high temperature oxygen desorp-
tion peak indicates that the surface morphology has changed. The
structure of the supported palladium catalyst has been fine-tuned.
Consequently, the modification of MnO improves the oxygen
x
migration ability of the catalysts, which is another reason for the
enhanced catalytic activity [53].
for MnO
x 2 3
/Pd/Al O , respectively. The peaks at 335.9 and 341.15 eV
0
are assigned to 3d5/2 and 3d3/2 of Pd , while the peaks at 337.2 and
+
3
42.5 eV are assigned to 3d5/2 and 3d3/2 of Pd2 species [45–47].
2
+
Only Pd is presented for the Pd/Al
2
O
3
catalyst. While Pd 3d peaks
/Pd/Al
can be deconvoluted to two oxidation states for MnO
x
2
O
3
0
2+
catalysts, indicating the mixture of metallic Pd state and Pd state
in MnO
-ALD decorated catalysts. Notably, the fraction of Pd0
increases with ALD cycles of MnO . The Mn(thd) precursor itself
is an active reducing agent due to its unsaturated ligands. Electron
transfer between intimately contacted Pd and Mn(thd) precursor
x
x
3
3.5. Catalytic performance
3
The catalytic performance of the Pd/Al
Pd are shown in Table 2 and Fig. 8(a), a MnO
2
O
3
and MnO decorated
2 3
/Pd/Al O catalyst is
x
occurs. The chemisorbed -thd ligands react with the surface oxy-
x
gen species on Pd nanoparticles and reduce Pd species to metallic
prepared with co-impregnation method and tested as a reference
sample. The unmodified Pd/Al catalyst shows the lowest con-
0
Pd . During ozone exposure to the reactor, the O
3
also reacts with
2 3
O
the residual -thd groups on Pd and oxidize the -thd groups to pro-
duce gaseous reactants such as CO, CO , H O etc. and purged out of
the reactor. Thus, the metallic Pd species are obtained during ALD.
Meanwhile, it is also observed that a very small amount of oxidized
version of benzyl alcohol (72.03%) at 120 °C. Meanwhile, the con-
version of benzyl alcohol increases first and then decreases as
the loading of MnO increases for the MnO decorated Pd catalyst
2
2
0
x
x
prepared by ALD. As the loading of MnO
x
is less than 0.2 wt%, the
Pd species exist for the MnO
x
coated catalysts. However, the results
activity is significantly improved. The main reason is that the con-
centration of Pd0 is increased. The enhanced oxygen migration
ability also promotes the conversion of benzyl alcohol, which is
provide clear evidence that Pd is reduced during the ALD process,
and the content of metallic Pd0 is much higher compared with
the uncoated Pd and MnO
x
/Pd catalysts prepared with co-
2
confirmed by the O -TPD tests. Under the same experimental con-
impregnation method. On the other hand, although the presence
ditions, the reference catalyst prepared by impregnation method
has lower catalytic activity and selectivity compared with ALD
method. According to the Raman measurements, MnOx has no
obvious electronic interaction with Pd for the sample prepared
by co-impregnation approach. Meanwhile, for the catalysts pre-
pared with ALD method, it is important to precisely control the
coating configurations, as the loading of MnOx further increases,
more Pd sites on the catalyst surface are covered. Hence, the activ-
ity of the reaction and the selectivity of benzaldehyde decrease
again.
3
+
4+
of mixed MnO
XRD, non-stoichiometric MnO
In Fig. 6b, the Mn 2p3/2 peak can be decomposed into Mn
x
(Mn , Mn ) species cannot be detected with
x
exhibits several oxidation states.
4
+
3
+
(
643.8 eV) and Mn (641.9 eV) [48]. As the loading of MnO
x
4+
increases, the amount of Mn increases slightly. The electronic
interaction that occurs at the interfacial perimeter of Pd and MnO
tunes the electronic properties of catalysts.
x
x
3.4. Oxygen migration at the MnO -Pd interfaces
For the selective oxidation of benzyl alcohol reaction, the target
product is benzaldehyde, other by-products include toluene, ben-
zoic acid, benzyl benzoate and benzene. The products distribution
after reaction is shown in Table 2. Among the transition metal oxi-
Fig. 7 is the O
orption peaks of Pd/Al
temperature oxygen desorption peaks in the range of 50–200 °C
2
-TPD patterns of the catalysts. All the oxygen des-
can be divided into three regions, low
2 3
O
and high temperature oxygen desorption peaks in the range of
x
des, manganese oxide (MnO ) is reported to be effective for cat-
2
2
00–500 °C. Typically, the low temperature desorption peak (50–
00 °C) can be assigned to the oxygen adsorbed on the surface of
alytic oxidation, since it contains labile oxygen, which is
necessary to complete the catalytic redox cycle for benzylic oxida-
Pd nanoclusters [49–51]. The high temperature oxygen desorption
x
tion [54]. Therefore, the MnO added via both the co-impregnation
2 3 x 2 3 x 2 3.
Fig. 6. XPS spectra, (a) Pd 3d spectra before catalytic reaction; (b) Mn 2p spectra before catalytic reaction. (I) Pd/Al O , (II) 0.2 wt%MnO /Pd/Al O , (III) 0.6 wt%MnO /Pd/Al O

6
6
J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
Table 1
Surface composition with XPS analysis of as-synthesized catalystsa,b
.
Catalyst
Surface composition (%)
0
Pd2+/Pd
Mn3+/Mn
Mn4+/Mn
Pd /Pd
Pd/Al
2
O
3
0
35
51
100
65
49
None
45
41
None
55
59
0
0
.2MnO
.6MnO
x
x
/Pd/Al
/Pd/Al
2
O
O
3
3
2
a
The concentration of palladium species (Pd0 and Pd2+) was derived from the deconvoluted Pd 3d5/2 XPS peak.
The concentration of manganese species (Mn , Mn ) was derived from the deconvoluted Mn 2p3/2 XPS peak.
b
3+
4+
3.6. Mechanisms discussion
The product distribution is complex for selective oxidation of
benzyl alcohol under practical working conditions. The reactivity
and products selectivity are affected by various experimental
parameters such as alcohol concentration, oxygen concentration
and temperature [55,56]. The effects of reactants depletion with
temperature are further studied with kinetic insight, the conver-
sions of benzyl alcohol of the catalysts at different temperatures
are shown in Fig. S2. The TOF values (shown in Table S3) are also
n
mꢃt
obtained, TOF values are calculated with the formula TOF ¼
,
the n refers to the molar amount of reactants converted during
time t, and m is the number of active sites available on the surface.
The TOF values are measured after 1 h reaction, the conversions of
benzyl alcohol are all lower than 20% for the catalysts. The corre-
sponding conversion values of benzyl alcohol versus time are
exhibited in Fig. S3. The number of active sites available on the sur-
face are calculated based on the Pd nanoparticles dispersion for
uncoated Pd catalyst. For the MnO coated Pd catalysts, the expo-
x
sure percentage of the Pd surface is calculated based on FTIR mea-
surements with carbon monoxide (CO) chemisorption as probe
2 2 3 x 2 3
Fig. 7. O -TPD patterns of the Pd/Al O , MnO /Pd/Al O catalysts with 0.2, 0.6 wt%
Mn loading.
(
shown in Table S3). The number of exposed Pd sites (not covered
by MnO ) are considered as active sites for MnO /Pd/Al cata-
lysts. The TOF of 0.2 wt%MnO /Pd/Al catalyst at 120 °C is
1,561 h , which is 8.7 times larger than that of the unmodified
Pd catalyst. Here, DRIFTS CO chemisorption measurements indi-
cate that the number of Pd active sites reduces after MnO deposi-
tion. Therefore, the improvement of TOF can be attributed to the
formation of the highly active MnO -Pd interfaces, as indicated
by the surface chemical state analysis and H -TPR characteriza-
tions (Fig. S4). The higher content of the metallic Pd phase and
the improved oxygen migration ability are beneficial for the
enhancement of activity. Arrhenius plots of the TOF values give
the apparent activation energies (Ea) of 45.98 kJ/mol for 0.2 wt%
and ALD approaches in this work promotes the further oxidation of
toluene that reduces its formation. While the addition of MnO to
Pd via ALD effectively eliminates the conversion to toluene. No
toluene is observed for MnO coated Pd samples by ALD (Fig. 8b).
In addition to the effects of MnO decoration described above, it
might be attributed to the selective passivation of the Pd(1 1 1)
facets by MnO prepared by ALD, as explained by the DRIFTS
x
x
2 3
O
x
x
2 3
O
ꢀ
1
3
x
x
x
x
x
results. Since Pd (1 1 1) is an active facet for toluene formation.
A high conversion of benzyl alcohol can be achieved and improved
to 84.72% for the 0.2 wt%MnO /Pd/Al O catalyst. At the same time,
x 2 3
2
0
the maximum yield of benzaldehyde (76.54%) is achieved, which is
higher than a series of state-of-the-art researches (Table S2) on Pd-
based catalysts ever reported. Achieving high selectivity and con-
version simultaneously is extremely difficult. The catalytic selec-
tivity of a few catalysts from previous reports is higher than our
work (Table S2) due to the short reaction time or low reaction tem-
perature. Further increasing the conversion will decrease the selec-
tivity, which may be caused by the over-oxidation of the reactants.
As the temperature or the reaction time increases, benzaldehyde is
further converted into other by-products, thus the total yield
decreases.
MnO
22.96 kJ/mol for Pd/Al
the lowest compared with 83.06 kJ/mol for 0.6 wt%MnO
Al ). The conversion of benzyl alcohol and catalytic selectivity
x 2
/Pd/Al O
3
catalyst which is reduced more than half of
(Fig. S5, 0.2 wt%MnO /Pd/Al is also
/Pd/
1
2
O
3
x
2 3
O
x
2 3
O
to benzaldehyde are improved simultaneously, and a high reaction
rate (TOF) is achieved.
The selective oxidation of benzyl alcohol is considered as
surface-limited-reaction. The elimination of b-H on the surface is
Table 2
x 2 3
/Pd/Al O
catalystsa.
Catalytic performance of bare Pd and MnO
Selectivity (%)
Toluene
Catalyst
Conv. (%)
Benzaldehyde
Benzoic acid
Benzyl benzoate
Benzene
Yield(%)
Pd/Al
2
O
3
72.03
84.52
84.72
81.57
75.11
80.06
12.36
0
0
0
0
6.95
79.16
85.63
90.34
88.15
87.94
78.66
2.71
6.08
3.16
5.87
6.10
3.63
3.66
7.45
5.67
5.19
5.23
9.63
2.11
0.84
0.83
0.79
0.74
1.13
57.01
72.37
76.54
71.91
66.05
62.98
0
0
0
0
.1MnO
.2MnO
.4MnOx/Pd/Al
.6MnO /Pd/Al
/Pd/Al
x
x
/Pd/Al
/Pd/Al
2
O
O
O
3
3
3
2
2
x
2
O
3
MnO
x
2
O
3
(co-impregnation)
a
2
Reaction conditions: benzyl alcohol 5 ml; catalyst 100 mg; O 20 ml/min; temperature 120 °C; time 4 h.

J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
67
Fig. 8. (a) Catalytic performance of bare Pd and MnO
x
/Pd catalysts. (b) Comparison of toluene and benzene selectivity between bare Pd and ALD MnO
x
/Pd catalysts. Reaction
test conditions: benzyl alcohol 5 ml; catalyst 100 mg; O
2
20 ml/min; temperature 120 °C.
Fig. 9. Schematic diagrams of catalytic structure-property mechanisms.
the rate-determining step [57]. The large decrease of Ea may indi-
cate that the elimination of b-H and replenishment of reactive oxy-
gen species are much easier for 0.2 wt%MnO /Pd/Al . Based on
the above analysis, the mechanism of solvent-free aerobic oxida-
tion of benzyl alcohol over MnO decorated Pd catalysts is
explained as follows (Fig. 9): for bare Pd/Al catalyst, benzyl
4. Conclusions
x
2
O
3
In summary, an atomically MnOx decorated Pd nanoparticles
structure is designed and fabricated via ALD. The ALD prepared cat-
alysts show enhanced activity and selectivity simultaneously, thus
x
2
O
3
a high yield of benzaldehyde is obtained. The MnO coating struc-
x
alcohol is first adsorbed on the Pd, and a Pd atom is located
between the O-H of the alcohol to form a metal alkoxide and a
metal hydride (adsorbed on adjacent Pd atoms); then, a hydrogen
atom is removed by the surface Pd atom through the b-H elimina-
tion process to form benzaldehyde; finally, the adsorbed hydrogen
is oxidized by absorbed oxygen to regenerate the Pd active sites.
tures selectively passivate Pd (1 1 1) facets to prevent decarbony-
lation reaction of benzaldehyde and eliminate the formation of
toluene. Meanwhile, MnO fabricated via ALD forms strong binding
x
x
x
with the Pd particles, which increases the MnO -Pd interactions
and O migration at interfaces. The addition of MnO via ALD also
0
increases the active metallic Pd species concentration. Both effects
However, for the MnO
of MnO on catalysts surfaces, electron transfer between intimately
contacted Pd and Mn(thd) precursor occurs. In terms of this view,
x
modified catalysts, during the deposition
result in the enhancement of the catalytic activity and reaction
rate. The TOF of MnOx decorated Pd catalyst reaches 31,561 h ,
which is 8.7 times larger than that of the unmodified Pd catalyst.
Meanwhile, the maximum conversion of benzyl alcohol and yield
of benzaldehyde are improved to 84.7% and 76.5% respectively.
ꢀ1
x
3
the electrons inherently are donated to the electron-deficient Pd
0
sites. As a result, more Pd species are exposed, which is beneficial
for the adsorption of benzyl alcohol. At the same time, molecular
oxygen is adsorbed on MnO
of the metal alkoxide formation process and the H atom of the b-
H elimination process to form benzaldehyde and H O. The addition
of MnO enhances oxygen adsorption and activation and it is
reported that different oxygen adsorption properties significantly
affect catalytic performance [58]. Selective passivation of Pd
x
and activated to adsorb the H atom
Declaration of Competing Interest
2
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
x
(
1 1 1) facet by MnO
and eliminates the formation of toluene. While for MnO
catalyst prepared via co-impregnation method, MnO is randomly
x
inhibits the decarbonylation of benzaldehyde
Acknowledgements
x
coated Pd
x
This work is supported by the National Natural Science Founda-
tion of China (51702106, 51835005, and 51871103, 51911540476).
The authors acknowledge the Analytical and Testing Center, the
dispersed on Pd surface. Thus the toluene formation cannot be
avoided.

6
8
J. Yang et al. / Journal of Catalysis 386 (2020) 60–69
Flexible Electronics Research Center of Huazhong University of
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