A. Shukla, et al.
MolecularCatalysis490(2020)110943
compounds are reduced to anilines in stepwise manner by forming ni-
troso and hydroxylamine intermediates. Meanwhile, when these inter-
mediates undergo condensation step, then formation of azoxy and azo
compounds takes place [12,56–58]. From last few years, Corma and co-
enes over various catalytic systems (mainly gold as active metal) and
emphasized the role of support for selective reduction processes. During
their studies, they investigated the effect of support by using Au/CeO2
and Au/TiO2 (with same Au particle size) catalysts and found that
support does not act as a simple carrier but also intervene in the re-
action during catalysis. Previously, we reported Pt-CeO2 [23] and Ni-
ZrO2 [24] catalyst for the selective reduction of nitroarenes to corre-
sponding amines using molecular hydrogen and found that reaction was
proceeding via direct route. But in present case, the reaction proceeded
through condensation route, where nitroso intermediate condensed
with hydroxylamine intermediate to form azoxy compound selectively.
The azobenzene and aniline were formed in very less amount. To in-
vestigate the reaction pathway, we subjected the reaction intermediates
(nitrosobenzene, phenylhydroxylamine, nitrosobenzene + phenylhy-
droxylamine, azoxybenzene, azobenzene) as reactants, under optimized
reaction condition (Table S4). During the study, it was found that when
we introduced nitrosobenzene as a reactant then good yield of azox-
ybenzene was formed within 8 h of reaction time. Moreover, when
mixture of nitrosobenzene and phenylhydroxylamine was subjected
then higher yield of azoxybenzene was obtained within 5 h of reaction
time. While phenylhydroxylamine, azoxybenzene and azobenzene re-
duced to aniline with a poor yield i.e. only trace amount of aniline was
detected, which indicated that further reduction of azoxybenzene to
Conclusions
We have developed a facile, cost-effective, selective and efficient Ni-
TiO2 catalyst system for the removal of organic pollutants. The catalyst
(2.6 % Ni-TiO2EG−W) synthesized by hydro-solvothermal method using
poly (diallyldimethylammonium chloride) and hydrazine hydrate was
highly active for hydrogenation process. It was observed that 2.6 % Ni-
EG-W-H
TiO2
catalyst exhibited very high activity (high TON and low
activation energy) for room temperature selective reduction of ni-
trobenzene to azoxybenzene, in aqueous medium using hydrazine hy-
drate as a reductant and conversion of nitrobenzene was found to be 89
EG-
% with 92 % selectivity of azoxybenzene. In addition, 2.6 % Ni-TiO2
W-H catalyst also showed good yield of azoxy compounds with different
substituted nitroarenes, under optimized reaction conditions. Probable
reason for excellent activity of 2.6 % Ni-TiO2EG-W-H catalyst was due to
presence of highly dispersed (∼6.8 nm) Ni nanoparticles over TiO2
nanocrystals, providing large number of active sites for catalysis and
strong metal-support interaction at the catalyst surface, which pre-
vented the leaching of active species during catalysis. Hence, 2.6 % Ni-
EG-W-H
TiO2
catalyst maintained heterogeneity throughout the reaction
and recycled up to several successive runs without significant loss in
reactivity. Performing number of experiments, we concluded that the
reduction of nitrobenzene to azoxybenzene occurs via condensation
route instead of direct route. During the study, it was also found that
the catalyst preparation methods and solvents used during catalyst
synthesis significantly altered the physicochemical characteristics as
well as reactivity of respective catalyst system.
azobenzene and azobenzene to aniline was not favourable with 2.6 %
Declaration of Competing interests
EG−W−H
Ni-TiO2
catalyst. In our case, strong metal-support interaction
(SMSI) favoured the rapid dissociation of hydrazine hydrate at the
metal support interfaces and the formation of active hydrogen species
take place. After that, nitrobenzene gets physisorbed over the active
surface of the catalyst and the subsequent conversion of nitrosobenzene
to phenylhydroxylamine occurs. As soon as phenylhydroxylamine form,
it immediately reacts with nitrosobenzene to form dihydroxy inter-
mediate [11], which get dehydrated to form azoxybenzene. Further
reduction of azoxybenezene to azobenzene and azobenzene to aniline
does not proceed under optimized reaction conditions, which might be
due to much higher hydrogenation barrier of these steps than previous
steps (up to azoxybenzene formation) [11]. Therefore, we can conclude
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
CRediT authorship contribution statement
Astha Shukla: Investigation, Formal analysis, Validation, Writing -
original draft. Rajib Kumar Singha: Visualization, Writing - review &
editing. Takehiko Sasaki: Formal analysis, Writing - review & editing.
Shubhadeep Adak: Formal analysis. Sonu Bhandari: Validation.
V.V.D.N. Prasad: Writing - review & editing. Ankur Bordoloi: Writing
- review & editing. Rajaram Bal: Conceptualization, Writing - review &
editing, Supervision.
that the reduction of nitrobenzene followed the condensation route
EG-W-H
over 2.6 % Ni-TiO2
catalyst.
Turnover number (TON) is an important factor to measure the ef-
EG−W−H
ficiency of catalytic system. The higher TON of 2.6 % Ni-TiO2
Acknowledgements
catalyst again confirmed the superiority of system than other prepared
catalysts (Tables 2 & S3) and reported non-noble metal based catalyst
systems (Table S5). It was found that with increasing metal dispersion
and decreasing Ni-species size of the catalyst, catalytic activity in-
creased. So, it is obvious and also supported by different literature re-
ports that active Ni-species sites at the catalyst’s surface are the active
centres for the reaction. Here, strong metal-support interaction at the
catalyst surface is generated by electron transfer between metal (Ni)
and support (TiO2) but not due to solid solution and/ or encapsulation
phenomena.
AS and SB thank to CSIR and UGC, New Delhi, India for their re-
spective fellowships. Director, CSIR-IIP is acknowledged for his help
and support. Analytical Science Division, CSIR-IIP is acknowledged for
providing analytical facilities. The XAFS measurements were performed
at KEK-IMSS-PF with the approval of the Photon Factory Advisory
Committee (project 2017G190).
Appendix A. Supplementary data
EG−W−H
To further elucidate the applicability of 2.6 % Ni-TiO2
Supplementary material related to this article can be found, in the
catalyst, the reaction was extended to different nitroarenes, under op-
EG−W−H
timized reaction condition. It was noted that 2.6 % Ni-TiO2
catalyst showed excellent yield of azoxy compounds with para-sub-
stituted (electron withdrawing or electron donating) nitro compounds.
2.6 % Ni-TiO2EG-W-H catalyst showed 78 %–89 % conversion of different
nitro compounds with the 84 %–92 % selectivity of corresponding
azoxy compounds (Table S6).
References
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