Preparation and characterization of niobia-containing solid acid
Author(s)
Advisor
Kantcheva, MargaritaDate
2010Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
The research in this work is directed towards (i) the development of effective
methods for synthesis of new solid acids based on zirconia and niobia resulting in high
concentration of acid sites and (ii) investigation of the potential of these materials as
catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with
hydrocarbons and alcohol transformations. A series of NbOx/ZrO2 and WOx/Nb2O5
samples have been obtained by the so called “peroxo route” using acidic H2O2 solutions
as a reaction medium. The impregnation of hydrated zirconia with solutions of
[Nb2(O2)3]
4+ ions allows the synthesis of Nb2O5-ZrO2 samples with niobium loading up
to 30 wt % without formation of a seperate Nb2O5 phase. The presence of a single phase of Zr6Nb2O17 has been observed for the sample containing 25 wt% of Nb. The formation
of Zr6Nb2O17 is favored by the partial solubility of hydrated zirconia in the H2O2 solution.
Among the fully crystallized Nb2O5-based samples, the solid WO3-Nb2O5 solution
containing 20 wt % of WO3 has the highest and strongest Brønsted acidity.
Catalytic activity tests for the C3H6-SCR of NOx have shown that Pd-free and Pdpromoted
niobia-based samples containing 20 wt% WO3 are inactive, whereas the
Zr6Nb2O17 and 0.1Pd/ Zr6Nb2O17 samples catalyze this reaction. The conversion of NOx
in the SCR of NOx with propene in excess oxygen over Zr6Nb2O17 passes through a
maximum at 493 K. The mixed oxide displays good water tolerance and resistance
toward SO2 poisoning. Based on the in situ FT-IR results, a reaction mechanism is
proposed with nitroacetone and NCO species as the key reaction intermediates. The
results of the investigation show that the catalytic properties of the new solid acid
Zr6Nb2O17 could be of interest regarding the development of sulfur- and water-tolerant,
low-temperature catalysts for the SCR of NOx with hydrocarbons.
The Zr6Nb2O17 and Nb2O5-containing 20 % WO3 samples have potential as lowtemperature
catalysts for oxidant-free or oxidative transformation of alcohols,
respectively.
Keywords
Nb2O5/ZrO2WO3/Nb2O5
Synthesis from H2O2 precursor solutions
HCSCR of NOx
Catalytic activity
In situ FT-IR spectroscopy
Reaction mechanism
Alcohol transformation