Development of high surface area ceriaand titania modified mesoporous zirconia with tetragonal nanocrystalline framework for novel applications

Abstract

The present work is undertaken on multigram synthesis of high surface area
mesoporous pure ZrO2 as well as TiO2 and CeO2 modified mesoporous ZrO2 with a tetragonal
(t-) nanocrystalline framework using cheaper inorganic metal precursors and surfactants as
pore-directing agent via surfactant-assisted route. These materials would find potential
applications in practical industrial processes, such as adsorbent, transportation/storage of fluids
and gases, separation technology, solid oxide fuel cell, chemical sensor and catalysis because of
their special structural characteristics. Two methods are developed here in order to derive
mesostructured pure and Ti4+ or Ce4+ modified inorganic-organic hybrid precursors and in turn
their ceramic derivatives of pure ZrO2, Ti4+: ZrO2 and Ce4+: ZrO2 in different polymorphs by
heating in air at elevated temperatures. The reaction in water extends viability of the methods
for a mass scale production of them at an economic cost. The synthesis procedure developed
here is also suitable for synthesis of small particles of t-ZrO2 without addition of any surfactant.
The small quantity of surfactant used during synthesis is sufficient to induce a large quantity of
porosity in the samples. The work involves a systematic study and analysis of the structure,
microstructure, optical properties using several characterization techniques like
thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), small
angle X-ray scattering (SAXS), N2-sorption analysis, scanning electron microscopy (SEM),
high resolution transmission electron microscopy (HRTEM), Fourier transmission Infra-red
(FTIR) spectroscopy, UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and
photoluminescence (PL) studies.
Surfactants play important role in stabilization of t-ZrO2 phase, inducing mesoporosity,
and significantly enhancing surface area of mesoporous ZrO2. A high-quality template-free t- ZrO2 retaining mesoporous structure with crystalline pore wall can be obtained on calcination
at 500°C of the amorphous as-prepared inorganic organic hybrid synthesized using dodecyl
amine (DDA) or Brij 56 but not with sodium dodecyl sulphate (SDS). Pure ZrO2 synthesized
with cationic surfactant of DDA consisted of wormlike mesopores and exhibited a nearly
uniform and narrowly distributed pore size distribution with lower quantity of micropores, and
t-ZrO2 phase stability up to as high as 800C.
The crystallization behavior of mesoporous ZrO2 is affected with introduction of TiO2
and CeO2. The addition of TiO2 and CeO2 (2 to 20 mol%) into ZrO2 lattice led to the formation
of their respective solid solutions with effective t-ZrO2 phase stabilization up to as high as 800
and 1300°C, respectively. The as-prepared pure as well as TiO2 and CeO2 modified ZrO2
samples were mesoporous with high surface area in the range 465-234 m2g
-1
for ZrO2 samples
synthesized using different surfactants, and 365-255 m2
g
-1 and 275-143 m2
g
-1
for TiO2 and
CeO2 modified ZrO2 samples, respectively.
The mesostructure was well preserved up to 650°C in all pure and doped samples. The
mesostructure comprises of inter-connected wormlike channels with thermally stablex
nanocrystalline pore walls, as confirmed from HRTEM and SAXS analysis. Incorporation of
TiO2 decreased the band-gap of ZrO2, making it efficient for photocatalysis under direct
sunlight. Thus, the TiO2-modified samples could successfully degrade phenol (~ 72%) from
aqueous solution under exposure to direct sunlight. CeO2 doping demonstrated an absorption
shift to longer wavelengths in the visible region thereby making it proficient for visible light
photocatalysis. The mesoporous ZrO2 samples showed broad and intense visible PL spectra
centered around 430 nm ascribed to the presence of surface defects. Band positions and
intensity of PL emission vary as the functions of the (i) microstructure, (ii) dopant
concentration, and (iii) ZrO2 polymorphism. These mesoporous ZrO2-based materials having
strong photoluminescence in the visible region can find applications in optical devices. These
high surface area mesoporous adsorbents with high pore volume also exhibited remarkable
ability to remove heavy metal ion Cr(VI) at room temperature, and without any adjustment of
pH. These present studies show that the mesoporous zirconia could be considered as a potential
adsorbent for removing toxic Cr (VI) ions from aqueous solutions.