636
X. Yang et al. / Journal of Catalysis 377 (2019) 629–637
excessively reduced under the same conditions for 6 h,
LMN decomposes to La2O3, MnO and dispersed Ni nano
particles.
(2) Catalytic performance of all the three catalysts increases
with the increase of testing temperature from 625 to
750 °C. R-LMN out-performances LMN due to the exsolution
of Ni nano particles on the perovskite substrate in R-LMN;
and ER-LMN exhibits the lowest performance because of
the decomposition of LMN. The catalytic activity of LMN is
probably associated with the presence of Mn4+ ions and oxy-
gen vacancies.
(3) Catalytic performance of R-LMN at 750 °C is essentially
stable without carbon deposition, and C2H6 conversion and
C2H4 selectivity are approximately on the level of 43% and
98%, respectively. The slow growth of Ni nano particles
caused by further exsolution during the test slightly
increases the conversion and decreases the selectivity,
which suggests that, to develop a high efficiency C2H6 dehy-
drogenation catalyst, it is necessary to optimize of Ni con-
tent in the doped LMN to balance of the amount of Ni that
is exsolved on the substrate against the amount of Ni that
remains in the substrate.
Fig. 11. Schematic diagram of C2H6 dehydrogenation process catalyzed by metallic
Ni.
in Fig. 11, which is similar with the catalytic process of Pt [32].
With the catalytic assistance of the Ni nano particles, the adsorbed
C2H6 with a single carbon-carbon bond will readily release H2 and
form C2H4 with a double carbon-carbon bond. It is because of the
high catalytic activity of the Ni nano particles that the performance
of R-LMN was higher than that of LMN for C2H6 dehydrogenation
(Fig. 5). In the ER-LMN, the original substrate LMN was completely
decomposed to La2O3, MnO and Ni. La2O3 and MnO are not as cat-
alytically active as perovskite LMN, and some of the Ni particles
was covered by La2O3 and MnO and not exposed to C2H6 gas.
Therefore, the performance of ER-LMN for C2H6 dehydrogenation
was the lowest among the three catalysts (Fig. 5).
As mentioned earlier, in the 50 h dehydrogenation test of R-
LMN, C2H6 conversion increased slightly and C2H4 selectivity
decreased somewhat after 30 h of testing. On the basis of the dis-
cussion above, this phenomenon can be understood. The R-LMN
was obtained by reducing the LMN at 800 °C in 5% H2-N2 atmo-
sphere for 1 h. Only part of the doped Ni was exsolved in the
pre-reduction process. During the long-term test, the R-LMN was
exposed constantly to a reducing (low oxygen partial pressure)
atmosphere consisting mostly of the unconverted C2H6 and the
formed C2H4 and H2. Under this circumstance, the Ni remaining
in the substrate would be exsolved gradually with time, resulting
in the continuous growth of the Ni nano particles rather than the
formation of ‘‘new” Ni nano particles (Fig. 9). Thus the surface area
of the exsolved Ni nano particles would increase with time, pro-
moting the process of C2H6 conversion. Since the growth of Ni nano
particles was slow, the increase of C2H6 conversion was observed
only after 30 h of testing, which was accompanied with insignifi-
cant decrease of C2H4 selectivity possibly due to the formation of
some CH4 as the Ni catalyst is very active for breaking the
carbon-carbon bonds. On the basis of these analyses, it becomes
clear that for the development of highly efficient LMN catalyst
for C2H6 dehydrogenation, the amount of Ni doping needs to be
optimized to balance the amount of Ni that remains in the per-
ovskite substrate against the amount of Ni that is exsolved on
the perovskite substrate in the form of Ni nano particles.
Acknowledgements
This research was financially supported by the National Key
Research & Development Project-International Cooperation Pro-
gram (2016YFE0126900), National Natural Science Foundation of
China (U1601207, 51672095). SEM, XRD and Raman spectrometry
characterizations were assisted by the Analytical and Testing Cen-
ter of Huazhong University of Science and Technology and the
State Key Laboratory of Material Processing and Die & Mold Tech-
nology. The authors acknowledge Prof. Zhidong Xiang of Wuhan
University of Science and Technology for proofreading the
manuscript.
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