S. Li et al.
Catalysis Communications 149 (2021) 106250
Mn which is responsible for the high catalytic activity and stability.
Table 1
Catalytic properties of NiO-MnO2/Nb2O5-TiO2 with different mass ratios of Ni/
Mn.
2. Experimental
Mass ratio of Ni/Mn
XBA/%
SBO/%
S2E2H/%
S2EH/%
S2EHO/%
The catalyst preparation, characterization and catalytic performance
evaluation are enclosed in the Supporting Information.
11:0
10:1
9:2
100.0
100.0
100.0
100.0
100.0
100.0
16.6
11.4
8.8
0
0
77.9
88.5
0
0
0
37.8
80.6
67.9
70.4
43.6
8:3
10.3
12.7
2.1
0
0
0
0
3. Results and discussion
6:5
0
0:11
0
3.1. Effect of second-metal component on the catalytic performance of
NiO/Nb2O5-TiO2
Reaction conditions: a weight percentage of catalyst = 15%, T = 180 ◦C, P = 4
MPa, t = 6 h.
BA: butanal; BO: butanol; 2E2H: 2-ethyl-2-hexenal; 2EH: 2-ethylhexanal; 2EHO:
2-ethylhexanol. X: conversion; S: selectivity.
The effects of second-metal components on the performance of NiO/
Nb2O5-TiO2 were evaluated and the results are shown in Table S1 in the
supporting information. n-butyraldehyde was completely converted for
all the bimetallic catalysts, suggesting that the addition of the second-
metal components showed no significant effect on the catalytic activ-
ity. The selectivity of 2EHO decreased in the following order: Mn > Co
> blank>Fe > Ir > Rh > Ru > Pt > Pd, and n-butanol selectivity
decreased in the following order: Pt > Ru > Ir > Rh > Pd > Fe >
blank>Co > Mn. Among the bimetallic catalysts, NiO-MnO2/Nb2O5-
TiO2 showed the best catalytic performance; the selectivity of 2EHO
reached 88.5% whereas the selectivity of n-butanol was only 11.4%.
In order to further investigate the effect of second-metal component
on the reduction property of NiO, XRD tests of the bimetallic catalysts
were performed before and after reaction and the results are shown in
Fig. S1 in the supporting information. The diffraction peak of NiO at 2θ
= 43.26◦ can be detected in the fresh catalyst in addition to the
diffraction peaks of anatase TiO2 at 2θ = 25.18◦, 37.69◦, 47.91◦, 53.78◦,
54.93◦, 62.61◦, 68.65◦, 70.17◦, 74.94◦ and 82.56◦. The diffraction peaks
of metal Ni can be found at 2θ = 44.59◦, 51.72◦ and 76.33◦ in the
recovered catalysts, indicating that NiO was reduced during the reaction
process. However, weak characteristic peaks of NiO can be detected
either for the recovered bimetallic catalysts with Ru, Rh, Pd, and Pt as
the second-metal component, indicating that NiO was not completely
reduced and thus leading to a poor catalytic performance. No charac-
teristic peaks of the second-metal and its metal oxide were detected
before and after reaction due to its low concentration, small grain and
good dispersion.
3.2. Effect of preparation conditions of NiO-MnO2/Nb2O5-TiO2
3.2.1. Influence of Ni/Mn mass ratio
The influence of Ni/Mn mass ratio on the catalytic performance of
NiO-MnO2/Nb2O5-TiO2 was investigated and the results are shown in
Table 1. n-Butyraldehyde achieved a complete conversion within the
range of Ni/Mn mass ratio investigated, suggesting that Ni/Mn mass
ratio shows insignificant influence on the catalytic activity. As Ni/Mn
mass ratio decreases, the selectivity of 2EHO increases first, reaches its
optimum at a Ni/Mn mass ratio of 10: 1 and then decreases suddenly to
0. We suppose that excessive amount of MnO2 will ruin the hydroge-
nation activity of NiO-MnO2/Nb2O5-TiO2 catalyst by covering the sites
of metal Ni. The coverage of the sites of metal Ni by Mn will destroy the
hydrogenation activity of the catalyst definitely and cause the formation
–
–
–
–
–
–
of 2E2H (C C bond and C O bond are left) and 2EH (C O bond is left)
instead of 2EHO, as entry 3 of Table 1 showed. Therefore, the suitable
Ni/Mn mass ratio is determined to be 10: 1.
In order to further analyze the influence of Ni/Mn mass ratio on the
catalytic performance of NiO-MnO2/Nb2O5-TiO2, the XPS analyses of Ni
2p and Mn 2p were carried out. The XPS spectra are shown in Fig. S10
and the binding energy data are listed in Table S4 and Table S5. Ac-
cording to literatures [10,14,15], the binding energies of Ni 2p3/2 of
Ni2+ are 853.1 eV and 856.1 eV; the binding energy of Ni 2p1/2 of Ni2+
are 863 eV and 873 eV; the binding energies of Mn 2p3/2 of Mn4+ is
642.01 eV; the binding energy of Mn 2p1/2 of Mn4+ is 653.7 eV; the
binding energies of Mn 2p3/2 of Mn2+ is 641.2 eV, and the satellite of
MnO is 646.4 eV; the binding energy of Mn 2p1/2 of Mn2+ is 652.8 eV;
the binding energy of pure metal Ni is 852.4 eV; and the binding energy
of pure metal Mn is 640.1 eV. It is demonstrated from the binding energy
data of Ni 2p and Mn 2p in Table S4 that the valence state of Ni and Mn
elements in the fresh catalysts separately is Ni2+ and Mn4+ and their
corresponding metal compounds are respectively NiO and MnO2. It is
found from the binding energy data in Table S5 that the Ni 2p3/2 binding
energy of metal Ni in the recovered catalysts is less than that of the pure
metal Ni while the Mn2p3/2 binding energy of metal Mn in the recovered
catalysts is greater than that of the pure metal Mn. So this demonstrates
For better analyzing the effect of the second-metal component on the
physical structure of NiO/Nb2O5-TiO2, N2 adsoprtion and desorption
tests were performed and the results are shown in Table S2 in the sup-
porting information.
For further investigating the effect of the second-metal component
Mn on the reduction temperature of NiO in NiO/Nb2O5-TiO2, H2-TPR
analyses of NiO/Nb2O5-TiO2 and NiO-MnO2/Nb2O5-TiO2 catalysts were
performed and the results are shown in Fig. S2 in the supporting infor-
mation. For comparison, the H2-TPR profiles of other supported bime-
tallic oxide catalysts are presented in Figs. S3-S9 and the H2-TPR data
are listed in Table S3. There are two reduction peaks in NiO/Nb2O5-
TiO2: the peak at 328.5 ◦C belongs to the reduction of large NiO grains
which do not interact with the support, and the peak at 429.5 ◦C is
ascribed to the reduction of NiO grains which interacts with the support
[11,12]. NiO-MnO2/Nb2O5-TiO2 also has two obvious reduction peaks:
–
that there is an interaction between Ni and Mn and thus a Ni Mn alloy
possibly forms. In addition, the characteristic peaks of NiO, MnO and
MnO2 can also be detected in the XPS spectra of the spent catalysts due
to the oxidation of the surface metals Ni and Mn in the air during the
analysis process.
the peak at 326.3 ◦C is attributed to both the reduction of MnO2
Mn2O3 and large NiO grains which do not interact with the support
while the peak at 403.2 ◦C belongs to the reduction of both Mn3O4
→
For a further verification of the existence of interaction between Ni
→
–
and Mn and a Ni Mn alloy, XRD pattern of NiO-MnO2/Nb2O5-TiO2
MnO and NiO which interacts with the support [13]. Compared with
NiO/Nb2O5-TiO2, the reduction peak of NiO shifts to a lower tempera-
ture and the hydrogen consumption of NiO decreases, possibly because
the interaction between Ni and Mn weakens the interaction between the
metals and the support. Additionally, it is concluded from Figs. S3-S9
and Table S3 that the addition of the second metal component facilitates
the reduction of NiO.
catalyst with a Ni/Mn mass ratio of 10:1 was recorded and the result is
showed in Fig. S11. It can be seen that an obvious diffraction peak can be
detected around 44.6◦, which is between 44.507◦of pure metal Ni (111)
crystal plane and 44.785◦of pure metal Mn (222) crystal plane, indi-
–
cating that Ni Mn alloy is formed. In order to determine the distribu-
tion of metal Ni and Mn, EDS mapping was measured and the results are
illustrated in Fig. S12. The EDS mapping showed that metal Ni and Mn
were well-dispersed within the catalyst grain. TEM-EDS line scan
2