Effect of second metal component on the reduction property and catalytic performance of NiO-MOx/Nb2O5-TiO2 for direct synthesis of 2-propylheptanol from n-valeraldehyde

Article abstract of DOI:10.1016/j.catcom.2020.106209

In order to improve the catalytic performance of NiO/Nb₂O₅-TiO₂, several kinds of the second metal oxide component MO_x (M = Pd, Co, Ir or Rh) were separately introduced and their effects on the reduction propert

Full text of DOI:10.1016/j.catcom.2020.106209

Catalysis Communications 149 (2021) 106209
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of second metal component on the reduction property and catalytic
performance of NiO-MOx/Nb2O5-TiO2 for direct synthesis of 2-propylhep-
tanol from n-valeraldehyde
,
,
,
Lili Zhaoa b, Hualiang Ana *, Xinqiang Zhaoa *, Yanji Wanga
a Hebei Provincial Key Lab of Green Chemical Technology and Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology,
Tianjin 300130, China
b College of Science and Technology, Hebei Agricultural University, Huanghua, Hebei 061100, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
In order to improve the catalytic performance of NiO/Nb2O5-TiO2, several kinds of the second metal oxide
component MOx (M = Pd, Co, Ir or Rh) were separately introduced and their effects on the reduction property
and catalytic performance were evaluated. The results showed that the reduction temperature of NiO decreased
and NiO-Co3O4/Nb2O5-TiO2 exhibited the best catalytic performance. The reason for the enhancement of NiO
reducibility may stem from the interaction of M with Ni and the hydrogen spillover as confirmed by various
characterization techniques including XRD, H2-TPR, H2-TPD and XPS.
n-Valeraldehyde
2-Propylheptanol
Direct synthesis
NiO-Co3O4/Nb2O5-TiO2
1. Introduction
acidity and basicity on the Ni/TiO2 surface and NiO/Nb2O5-TiO2 cata-
lyst could delay the formation of metal sites so as to suppress the direct
2-Propylheptanol (2-PH) is an important plasticizer alcohol. By
comparison with the traditional dioctyl phthalate (DOP) plasticizer
produced by 2-ethylhexanol, 2-PH-derived plasticizers such as 2-propyl-
heptyl phthalate (DPHP) are more environmentally friendly due to their
temperature resistance, lower volatility, and being nontoxic and so on
[1]. The industrial production of 2-PH mainly includes three reaction
steps: hydroformylation of butene, self-condensation of n-valer-
aldehyde, and hydrogenation of 2-propyl-2-heptenal. If n-valeraldehyde
self-condensation and 2-propyl-2-heptenal hydrogenation can be inte-
grated, that is, direct synthesis of 2-PH from n-valeraldehyde, the pro-
cess will be simplified and the energy consumption will be reduced, thus
the cost of production being lowered. Therefore, the investigation on
this reaction integration is of a crucial significance for both academic
research and industrial production of 2-PH.
hydrogenation of n-valeraldehyde. However, the formation of a majority
of incompletely hydrogenated products suggested that NiO was not
completely reduced in reaction. To facilitate the complete reduction of
NiO in reaction, a second-metal component was introduced into NiO/
Nb2O5-TiO2 catalyst to lower the reduction temperature of NiO.
The effect of a second-metal component on the reduction property
and the catalytic performance of Ni-based catalysts has been reported.
Lock et al. [4] investigated the effect of Pd addition on the catalytic
performance of Ni/Al2O3 for methane decomposition and found that
adding Pd into Ni/Al2O3 catalyst improved the catalytic activity and
inhibited carbon deposition. The H2-TPR characterization result showed
that the reduction temperature of NiO-PdO/Al2O3 catalyst shifted to a
lower temperature, indicating that the addition of Pd promoted the
reduction of NiO. Lin et al. [5] studied the effect of Ir addition on the
catalytic performance of Ni/TiO2 for hydrogenation of cinnamaldehyde
and found that adding Ir into Ni/TiO2 improved the catalytic perfor-
mance and stability. The results of H2-TPR, H2-TPD and XPS analyses
showed that the interaction between Ni and Ir could lower the reduction
temperature of NiO. Andonova et al. [6] compared the catalytic per-
formance of Ni-Co/Al2O3 with Ni/Al2O3 for hydrogen production from
ethanol steam reforming and found that Ni-Co/Al2O3 exhibited better
stability and hydrogen selectivity. Compared with the single metal
At present, a few studies about the direct synthesis of 2-PH from n-
valeraldehyde have been reported. Sharma and Jasra [2] employed Ru-
HT (ruthenium hydrotalcite) bifunctional catalyst for single pot syn-
thesis of 2-PH from n-valeraldehyde and obtained a 2-PH selectivity of
only 48%. In our previous work [3], the catalytic performances of Ni/
TiO2, Ni/Nb2O5-TiO2, and NiO/Nb2O5-TiO2 catalysts for the direct
synthesis of 2-PH from n-valeraldehyde were separately evaluated. The
results showed that the introduction of Nb2O5 could effectively tune the
* Corresponding authors.
E-mail addresses: anhl@hebut.edu.cn (H. An), zhaoxq@hebut.edu.cn (X. Zhao).
https://doi.org/10.1016/j.catcom.2020.106209
Received 11 August 2020; Received in revised form 29 September 2020; Accepted 21 October 2020
Available online 24 October 2020
1566-7367/© 2020 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

L. Zhao et al.
Catalysis Communications 149 (2021) 106209
system, the reduction temperature of NiO decreased and furthermore
the reducibility of NiO increased with the increase of Co content. Hou
et al. [7] found that Ni/Al2O3 catalyst had good catalytic activity in
methane reforming but the catalyst suffered from the problem of deac-
tivation. After introducing a small amount of Rh, the catalytic stability of
Ni/Al2O3 catalyst was improved. The characterization result showed
that Rh improved the dispersion of Ni and decreased the reduction
temperature of NiO. In conclusion, adding a second-metal component
into Ni-based catalysts can not only improve the catalytic activity and
stability, but also reduce the reduction temperature of NiO.
Table 1
Effect of MOx on catalytic performance of NiO/Nb2O5-TiO2.
Catalyst
XV/
SPO
/
S2-
S2-
S2-
S2-
Period of
%
%
/
/
/
/
instantaneous
hydrogen flow
time/min
PHA
PHEA
PH
PH
%
%
%
SPO
NiO/
93.1
100
3.2
39.6
2.3
56.0
0
0
0
None
Nb2O5-
a
TiO2
NiO-
23.0
21.8
28.9
33.9
20.6
61.3
2.7
0–45.4
PdO/
Consequently, a second-metal oxide component MOx (PdO, Co3O4,
IrO2 or Rh2O3) was introduced into NiO/Nb2O5-TiO2 catalyst to assist
NiO reduction in reaction in this work. We aimed to clarify the reason
why adding second-metal component could promote the reduction of
NiO in the reaction process. So the effect of the addition of a second
metal component on the catalytic performance of NiO/Nb2O5-TiO2 was
evaluated first and then the effect on the reduction property was
analyzed by a series of characterization techniques.
Nb2O5-
TiO2
NiO-
100
100
100
100
0
0
77.1
64.8
61.7
13.0
3.5
2.2
1.8
0.6
52.5–118.8
9–106.3
0–94
Co3O4/
Nb2O5-
TiO2
NiO-
0
0
IrO2/
Nb2O5-
TiO2
NiO-
0
0
2. Experimental
Rh2O3/
Nb2O5-
TiO2
2.1. Catalyst preparation and characterization
Co3O4/
Nb2O5-
24.1
23.1
None
b
NiO-MOx/Nb2O5-TiO2 catalysts were prepared by co-impregnation
method. The as-prepared samples were characterized by several tech-
niques, namely, XRD, H2-TPR, H2-TPD and XPS. More details about the
synthesis procedure and characterization can be found in the Supple-
mentary material.
TiO2
V: n-valeraldehyde; PO: n-pentanol; 2-PHA: 2-propylheptanal; 2-PHEA: 2-pro-
pyl-2-heptenal;
2-PH: 2-propylheptanol. X: conversion; S: selectivity.
Reaction conditions: a weight percentage of catalyst =15%, T = 200 ◦C, P = 3
MPa, t = 6 h.
Nb2O5 loading was 5 wt% of TiO2, NiO loading was 13 wt% of Nb2O5-TiO2 and
MOx loading was 1 wt% of Nb2O5-TiO2.
2.2. Catalytic performance evaluation
a
NiO loading was 14 wt% of Nb2O5-TiO2.
The catalytic tests were carried out in a 100 mL stainless steel
autoclave charged with 3.6 g catalyst and 30 mL (24 g) of n-valer-
aldehyde. Prior to experiments, the air inside was replaced with
hydrogen. The mixture was heated and the reaction was carried out at
200 ◦C for 6 h under the H2 pressure of 3.0 MPa with stirring. After the
completion of reaction, the mixture was cooled to room temperature and
was separated by centrifugation. The liquid was quantitatively analyzed
by a SP-2100 gas chromatograph (Beijing Beifen-Ruili Analytical In-
strument Co., Ltd) equipped with a flame ionization detector (FID)
operated at 250 ◦C according to the literature [1].
b
Co3O4 loading was 14 wt% of Nb2O5-TiO2.
reaction is more competitive than n-valeraldehyde direct hydrogenation
reaction. In this work, the ratio of S2-PH to SPO was greater than 1 over
NiO-MOx/Nb2O5-TiO2 catalysts. Furthermore, the ratio of S2-PH to SPO
was the largest, up to 3.5 over NiO-Co3O4/Nb2O5-TiO2 catalyst; the
selectivity of 2-PH reached 77.1% while the selectivity of n-pentanol was
only 21.8%. Therefore, NiO-Co3O4/Nb2O5-TiO2 catalyst was chosen as
the suitable bimetallic catalyst for further research.
In order to analyze the reason why the NiO-Co3O4/Nb2O5-TiO2
catalyst has excellent catalytic performance, the catalytic performance
of Co3O4/Nb2O5-TiO2 was evaluated under the same conditions as NiO-
Co3O4/Nb2O5-TiO2 and the results were also listed in Table 1. The
condensation product was not completely hydrogenated, indicating that
a lower hydrogenation activity over Co3O4/Nb2O5-TiO2 catalyst. It also
showed that there was interaction between Ni and Co, which affected
the catalytic performance of NiO-Co3O4/Nb2O5-TiO2.
3. Results and discussion
3.1. Effect of MOx on catalytic performance of NiO/Nb2O5-TiO2
The effect of second-metal component on the catalytic performance
of NiO/Nb2O5-TiO2 was evaluated and the results are shown in Table 1.
It can be seen that n-valeraldehyde was incompletely converted and the
target product 2-PH was not formed over NiO/Nb2O5-TiO2 catalyst.
Some intermediates (2-propyl-2-heptenal and 2-propylheptanal) and a
small amount of direct hydrogenation product n-pentanol were formed
instead, indicating that NiO failed to be completely reduced in reaction.
However, n-valeraldehyde was completely converted and 2-PH became
the main product over NiO-MOx/Nb2O5-TiO2 catalysts, suggesting that
NiO could be reduced in reaction. It can also be seen that the ratio of S2-
The above results showed that adding second-metal component
could promote the reduction of NiO in the reaction process. To analyze
the promotion effect of a second-metal component on NiO reduction and
on the catalytic performance of NiO/Nb2O5-TiO2, NiO-MOx/Nb2O5-TiO2
catalysts were characterized by means of XRD, H2-TPR, H2-TPD and XPS
techniques.
to SPO was 0 and the selectivity of n-pentanol was only 3.2% over
3.2. Characterization of NiO-MOx/Nb2O5-TiO2 catalysts
PH
NiO/Nb2O5-TiO2 catalyst, indicating that n-valeraldehyde self-
condensation reaction took place predominately while hydrogenation
reaction hardly occurred. The ratio of S2-PH to SPO varied with different
bimetallic catalysts and decreased as following order: NiO-Co3O4/
Nb2O5-TiO2 > NiO-PdO/Nb2O5-TiO2 > NiO-IrO2/Nb2O5-TiO2 > NiO-
Rh2O3/Nb2O5-TiO2. If the ratio of S2-PH to SPO is 1, the competitiveness
of n-valeraldehyde self-condensation reaction is comparable with
respect to that of n-valeraldehyde direct hydrogenation reaction. If the
ratio of S2-PH to SPO is greater than 1, n-valeraldehyde self-condensation
3.2.1. Analysis of XRD
XRD patterns of the fresh and the recovered NiO/Nb2O5-TiO2 and
NiO-MOx/Nb2O5-TiO2 catalysts are shown in Fig. 1. NiO were detected
besides anatase TiO2 in the fresh NiO-MOx/Nb2O5-TiO2 catalysts.
However, the characteristic peaks of Nb2O5, PdO, IrO2, Rh2O3 and
Co3O4 were not observed due to their low loading, smaller grain size and
high dispersion on the supporter surface. For the recovered NiO/Nb2O5-
TiO2 catalyst, a weak diffraction peak of NiO was detected besides
2

L. Zhao et al.
Catalysis Communications 149 (2021) 106209
Fig. 1. XRD patterns of different catalysts before and after reaction 1: NiO/Nb2O5-TiO2; 2: NiO-PdO/Nb2O5-TiO2; 3: NiO-Co3O4/Nb2O5-TiO2; 4: NiO-IrO2/Nb2O5-
TiO2; 5: NiO-Rh2O3/Nb2O5-TiO2.
*: fresh; #: recovered. ●: NiO; ◆: Ni0.
metallic nickel, indicating that NiO was incompletely reduced in the
reaction process. However, NiO was not detected while an obvious
diffraction peak of Ni was detected in the recovered NiO-MOx/Nb2O5-
TiO2 catalyst, indicating that adding MOx could indeed promote the
reduction of NiO in reaction process, which was consistent with the
activity evaluation results.
be attributed to the reduction of both Co2+ to Co0 and the reduction of
Ni2+ strongly interacted with the Nb2O5-TiO2 surface [9]. NiO-IrO2/
Nb2O5-TiO2 catalyst had four hydrogen consumption peaks separately at
163.5 ◦C, 220.6 ◦C, 299.2 ◦C and 409.1 ◦C, being ascribed to the
reduction of IrO2 with large grains, IrO2-NiO, NiO strongly interacted
with IrO2 and the Nb2O5-TiO2 surface, and NiO interacted with Nb2O5-
TiO2 [5]. NiO-Rh2O3/Nb2O5-TiO2 catalyst had four hydrogen con-
sumption peaks at 81.5 ◦C, 254.6 ◦C, 288.8 ◦C and 401.2 ◦C, being
separately ascribed to the reduction peaks of Rh2O3 with large grains,
Rh2O3-NiO, NiO strongly interacted with Rh2O3 and the Nb2O5-TiO2
surface, and NiO interacted with the Nb2O5-TiO2 surface [10].
3.2.2. Analysis of H2-TPR
H2-TPR analyses of NiO-MOx/Nb2O5-TiO2 (M = Pd, Ir, Rh or Co)
were made and their profiles are shown in Fig. 2 while the measurement
data is listed in Table S1 in the Supplementary material. As to NiO-PdO/
Nb2O5-TiO2 catalyst, there were four hydrogen consumption peaks
separately at 72.2 ◦C, 235.0 ◦C, 306.7 ◦C and 419.1 ◦C. The weak peak at
72.2 ◦C was ascribed to the reduction of PdO. The peak at 235.0 ◦C was
ascribed to the reduction of PdO-NiO, while the peak at 306.7 ◦C was
attributed to the reduction of NiO strongly interacted with PdO and the
Nb2O5-TiO2 surface [8]. The peak at 419.1 ◦C was ascribed to the
reduction peak of NiO interacted with the Nb2O5-TiO2 surface. NiO-
Co3O4/Nb2O5-TiO2 catalyst had four hydrogen consumption peaks
separately at 262.2 ◦C, 302.1 ◦C, 364.7 ◦C and 399.2 ◦C. The peak at
262.2 ◦C and 302.1 ◦C should be ascribed to the reduction of both Co3+
to Co2+ and Ni2+ to Ni0 while the peak at 364.7 ◦C and 399.2 ◦C should
In the case of NiO-MOx/Nb2O5-TiO2 (M = Pd, Ir, Rh or Co) catalysts,
the main reduction peaks appeared at lower temperatures compared
with that of NiO/Nb2O5-TiO2. More specifically, the reduction peak of
NiO shifted to lower temperatures after the addition of PdO, IrO2, Rh2O3
and Co3O4 into NiO/Nb2O5-TiO2 catalyst, indicating that the reduction
of NiO was promoted. This promoting effect is due to the hydrogen
spillover and the dissociated hydrogen species which can move from the
metals (M = Pd, Ir, Rh or Co) to the surface of NiO [9,11,12]. Moreover,
according to the work of Xiang et al. [11], the simultaneous reduction of
–
Ni and Co species would benefit the formation of Ni Co alloy phase and
–
that the formation of Ni Co alloy phase could facilitate the reduction
process. Therefore, we speculate the interaction between Ni and Co and
–
the formation of Ni Co alloy in the catalyst reduction process.
The above analyses showed that the interaction between Ni and M
(M = Pd, Co, Ir or Rh) was the main reason for the shift in the reduction
peak of NiO to lower temperatures in NiO-MOx/Nb2O5-TiO2 catalysts. In
addition, it can also be seen from Fig. 2 that the reduction peak tem-
perature and hydrogen uptake varied with the kind of the metals added.
The order of hydrogen uptake at higher temperature decreased as the
following order: NiO-Co3O4/Nb2O5-TiO2 > NiO-IrO2/Nb2O5-TiO2
>
NiO-PdO/Nb2O5-TiO2 > NiO-Rh2O3/Nb2O5-TiO2, in accordance with
the order of total yield of products (2-PH and 2-propylheptanal) in
Table 1. This indicates that the more the hydrogen uptake at higher
temperature, the more difficult it is to be reduced in reaction, the more
advantageous it is to improve the competitiveness of n-valeraldehyde
self-condensation and then the selectivity of 2-PH.
3.2.3. Analysis of H2-TPD
NiO-MOx/Nb2O5-TiO2 were reduced firstly and then characterized
by H2-TPD. The profiles are shown in Fig. S1 while the measurement
data is summarized in Table 2. It can be seen that Ni-M/Nb2O5-TiO2
catalysts had different desorption peak temperature and different
hydrogen desorption amount with respect to different M. The top
Fig. 2. H2-TPR curves of NiO-MOx/Nb2O5-TiO2 catalysts.
3

L. Zhao et al.
Catalysis Communications 149 (2021) 106209
Table 2
H2-TPD analysis data of different bimetallic catalysts.
Catalyst
H2 desorption peak at lower temperature
H2 desorption peak at higher temperature
Total H2 desorption amount/
μ
mol⋅gꢀ 1
Top temperature/◦C
H2 desorption amount/
Top temperature/◦C
H2 desorption amount/
μ
–
mol⋅gꢀ 1
μ
mol⋅gꢀ 1
Ni/Nb2O5-TiO2
–
426.7
18.4
18.4
61.7
31.5
71.3
84.9
Ni-Pd/Nb2O5-TiO2
Ni-Co/Nb2O5-TiO2
Ni-Ir/Nb2O5-TiO2
Ni-Rh/Nb2O5-TiO2
226.6
246.4
195.7
184.0
61.7
31.5
71.3
84.9
–
–
–
–
–
–
–
–
temperature of hydrogen desorption peak increased in the following
order: Ni-Rh/Nb2O5-TiO2 (184.0 ◦C) < Ni-Ir/Nb2O5-TiO2 (195.7 ◦C) <
Ni-Pd/Nb2O5-TiO2 (226.6 ◦C) < Ni-Co/Nb2O5-TiO2 (246.4 ◦C) < Ni/
Nb2O5-TiO2 (426.7 ◦C). The hydrogen desorption peak area decreased in
the following order: Ni-Rh/Nb2O5-TiO2 > Ni-Ir/Nb2O5-TiO2 > Ni-Pd/
Nb2O5-TiO2 > Ni-Co/Nb2O5-TiO2 > Ni/Nb2O5-TiO2. The top tempera-
ture of hydrogen desorption peak of Ni-M/Nb2O5-TiO2 catalysts was
lower than that of Ni/Nb2O5-TiO2 catalyst while the hydrogen desorp-
tion peak area of Ni-M/Nb2O5-TiO2 catalysts was larger than that of Ni/
Nb2O5-TiO2 catalyst, indicating that the interaction between metal sites
and hydrogen was weak and the number of metal sites was more over Ni-
M/Nb2O5-TiO2 catalysts. This suggested that adding the second metal
component increased the number of metal sites and promoted hydrogen
spillover. In addition, hydrogen spillover was beneficial to the migration
of hydrogen species adsorbed on the catalyst surface and to the decrease
of reduction temperature of NiO.
It can be seen from Table 1 that the hydrogenation activities of Ni-M/
Nb2O5-TiO2 catalysts (except for NiO-Co3O4/Nb2O5-TiO2) were signifi-
cantly improved and the instantaneous hydrogen flow could be observed
at the beginning of the reaction, resulting in higher yield of n-pentanol.
This result was ascribed to lower top temperature of hydrogen desorp-
tion peak and larger hydrogen desorption amount over Ni-M/Nb2O5-
TiO2 catalysts in H2-TPD measurement. However, as for Ni-Co/Nb2O5-
TiO2 catalyst, the hydrogen desorption amount was smaller and the top
temperature of hydrogen desorption peak was higher than other Ni-M/
Nb2O5-TiO2 catalysts, indicating that the metal active sites on Ni-Co/
Nb2O5-TiO2 catalyst were less and the interaction between hydrogen
and the metal active sites was stronger, so a higher temperature was
required for hydrogen desorption. This may be the main reason for a
relatively low initial hydrogenation activity of NiO-Co3O4/Nb2O5-TiO2
catalyst compared with other NiO-MOx/Nb2O5-TiO2 catalysts. Fortu-
nately, the low initial hydrogenation activity restrained n-valeraldehyde
direct hydrogenation and favored the n-valeraldehyde self-
condensation.
Fig. 3. Ni 2p XPS spectra of NiO/Nb2O5-TiO2 before and after reaction.
respectively 851.9 eV, 852.2 eV, 852.1 eV and 852.2 eV with respect to
different M (M = Pd, Co, Ir and Rh), respectively shifting 0.5 eV, 0.2 eV,
0.3 eV and 0.2 eV to a low binding energy in comparison with the
binding energy of Ni0 in the recovered NiO/Nb2O5-TiO2 catalyst. This
indicates a strong interaction between M (M = Pd, Co, Ir or Rh) and Ni
via the electron transfer from M to Ni, resulting in an electron-deficient
M and an electron-enriched Ni. This can be also verified by the shift of
metal M binding energy. In the Pd 3d spectra of the recovered NiO-PdO/
Nb2O5-TiO2 catalyst, the binding energies at 335.5 eV and 340.8 eV are
respectively ascribed to the Pd 3d5/2 and Pd 3d3/2 peaks of Pd0 and the
former increases 0.5 eV in comparison with the standard binding energy
of 335.0 eV [14]. In the Co 2p spectra of the recovered NiO-Co3O4/
Nb2O5-TiO2 catalyst, the binding energy at 778.1 eV is assigned to the
Co 2p3/2 of Co0, which is shifted 0.2 eV towards a higher binding energy
than the standard binding energy of 777.9 eV [9]. In the Ir 4f spectra of
the recovered NiO-IrO2/Nb2O5-TiO2 catalyst, the Ir 4f7/2 peak of Ir0 is
located at 61.2 eV, shifting 0.3 eV to a higher binding energy compared
to the standard binding energy of 60.9 eV [5]. In the Rh 3d spectra of the
recovered NiO-Rh2O3/Nb2O5-TiO2 catalyst, the Rh 3d5/2 and Rh 3d3/2
peaks of Rh0 are located respectively at 307.8 eV and 312.5 eV and the
former is shifted by 0.2 eV to a higher binding energy compared to the
standard binding energy of 307.6 eV [15]. The above analyses confirm
the existence of a strong interaction between Ni and M.
3.2.4. Analysis of XPS
To further analyze the interaction between Ni and M (M = Pd, Ir, Co
or Rh), the chemical environment and the chemical states of Ni and M
were investigated by XPS analysis. Fig. 3 displays Ni 2p spectra of the
NiO/Nb2O5-TiO2 catalyst before and after reaction. It can be seen that
the binding energies of Ni 2p3/2 and Ni 2p1/2 in the fresh NiO/Nb2O5-
TiO2 catalyst were respectively 855.3 eV and 873.0 eV, which were
ascribed to NiO [12]. Besides the binding energy of NiO, the binding
energy of Ni0 was 852.4 eV in the recovered NiO/Nb2O5-TiO2 catalyst
[13], and the peak intensity was rather weak due to both the incomplete
reduction of NiO in the reaction process and the reoxidation of the
surface metallic nickel by contacting air in the analysis process.
Subsequently, Fig. 4 respectively displays the spectra of Ni 2p, Pd 3d,
Co 2p, Ir 4f and Rh 3d in the NiO-MOx/Nb2O5-TiO2 catalysts before and
after reaction. In the Ni 2p spectra of the fresh NiO-MOx/Nb2O5-TiO2
catalysts, the characteristic peaks of NiO can be observed besides that of
PdO, IrO2, Rh2O3 or Co3O4. In the Ni 2p spectra of the recovered NiO-
MOx/Nb2O5-TiO2 catalysts, the characteristic peaks of Ni0 can be obvi-
ously observed. Moreover, the binding energy of Ni 2p3/2 of Ni0 is
The above characterization results confirm the interaction between
Ni and M. The interaction causes the decrease of NiO reduction tem-
perature, being consistent with the results of the previous literatures
[4–7]. What is more, NiO-MOx/Nb2O5-TiO2 (M = Pd, Co, Ir or Rh)
catalysts show different reduction properties with respect to different M,
thus to further affect the catalytic performance.
4

L. Zhao et al.
Catalysis Communications 149 (2021) 106209
Fig. 4. XPS spectra of NiO-MOx/Nb2O5-TiO2 before and after reaction.
5

L. Zhao et al.
Catalysis Communications 149 (2021) 106209
[2] S.K. Sharma, R.V. Jasra, Synthesis of 2-propylheptanol from 1-pentanal in a single
pot using bi-functional ruthenium hydrotalcite catalyst in batch and flow reaction,
Indian J. Chem. A 54 (2005) 451–458.
4. Conclusions
The catalytic performance of NiO-MOx/Nb2O5-TiO2 (M = Pd, Co, Ir
or Rh) catalysts for direct synthesis of 2-PH from n-valeraldehyde were
investigated and the results showed that adding MOx could promote
reduction of NiO in the reaction process. NiO-Co3O4/Nb2O5-TiO2 cata-
lyst showed the best catalytic performance; the selectivity of 2-PH
reached 77.1% with a complete conversion of n-valeraldehyde. The
characterization of NiO-MOx/Nb2O5-TiO2 catalysts were performed
using H2-TPR, H2-TPD, XRD and XPS techniques and results indicated
that an interaction between M and Ni and a hydrogen spillover from M
site to Ni site played important roles in the reduction of NiO in reaction
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgements
This work was financially supported by National Natural Science
Foundation of China (Grant No. 21476058, 21506046, 21978066) and
Basic Research Program of Hebei Province for Natural Science Foun-
dation and Key Basic Research Project (18964308D), Natural Science
Foundation of Hebei Province (B2018202220, B2020204023), and the
Key Project for Natural Science Foundation of Hebei Province
(B2020202048). The authors are gratefully appreciative of their
contributions.
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Appendix A. Supplementary data
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