G Model
MCAT-387; No. of Pages9
ARTICLE IN PRESS
2
A.T. Miah, P. Saikia / Molecular Catalysis xxx (2017) xxx–xxx
Introduction of metal dopants into the CeO2 lattice generates oxy-
gen vacancy defects and increases the acidity or basicity of CeO2
[4,10,15,16]. Finding an appropriate metal for doping is greatly
important and a lot of attention should be focused on this issue.
By making the use of acid-base as well as the redox properties,
genation and dimerization of alcohols, condensation of aldehydes,
ketonization of acids, hydrogenation of olefins, esterification of
cursors, ammonium cerium(IV) nitrate (Himedia) and terbium(IV)
nitrate (Himedia) in requisite quantities were dissolved sepa-
rately in double distilled water under mild stirring conditions and
mixed together. Upon complete mixing, required quantity of col-
loidal silica (Ludox 40 wt.%, Aldrich, AR grade) was added to the
abovementioned mixture solution under vigorous stirring. Dilute
aqueous ammonia solution was subsequently added dropwise until
the precipitation was complete (pH = ∼8.5). The resulting precip-
itate was carefully filtered off and washed with distilled water
until it became free from anion impurities. The accumulated paste
was left for overnight drying in a hood. Subsequently, the solid
product was dried in an oven at 393 K for 12 h, and crushed it
using an agate mortar to obtain fine powders. Finally, the dried
sample was calcined at 773 K for 5 h in air atmosphere. Some por-
tions of this sample were again heated at 873, 973, and 1073 K,
respectively for 5 h in air atmosphere for investigating the thermal
stability of the samples. A heating rate of 5 K/min was maintained
during all the calcination processes. For comparison, unsupported
CeO2-TbO2 (CT; 80:20 mol% based on oxides) was also prepared by
same method, annealed at different temperatures, characterized
and evaluated for catalytic usefulness.
is used as the starting material for manufacturing thermoplas-
tic polymers [21]. However, the dehydration reaction often leads
trace amounts of C6-alkenes [4,22]. Besides, the dehydration reac-
tion may be simultaneously accompanied by dehydrogenation
which produces 4-methylpentan-2-one along with higher ketones
in negligible amounts [4,22]. Hence, product selectivity remains a
fundamental issue particularly in alcohol dehydration and tremen-
dous effort has been directed to achieve satisfactory selectivity. It is
believed that the strength of acid- and base-sites govern the com-
petition among products of 4-methylpentan-2-ol dehydration [17].
Pure ceria predominantly produces 2-alkene as the alcohol dehy-
dration product. However, binary ceria-based mixed oxide solid
solutions reveal good catalytic activity as well as high selectiv-
It is reported that dispersion of ceria-based nano-oxide catalysts,
namely, CexZr1-xO2, Ce0.8Hf0.2O2, and CeO2-La2O3 over silica amaz-
ingly increase their catalytic activity in selective dehydration of
4-methylpentan-2-ol [4,24,25]. Reddy and co-workers reported
the efficiency of alumina supported ceria-terbia solid solution in
comparison to unsupported ceria-terbia toward enhancement of
OSC and CO oxidation ability [3]. In view of this, investigation on
ceria-terbia solid solutions supported on silica is vital for making a
rational comparison to the previously reported work. To the best of
our knowledge, very little research endeavor has been dedicated
toward investigation of acid-base characteristics of ceria-based
nanostructures correlating their catalytic activity, and particularly,
no report could be found on SiO2 supported CeO2-TbO2 composite
oxides so far.
Against the abovementioned background, we have undertaken
the present investigation to understand the dispersion effects of
CeO2-TbO2 composite oxide over the surface of silica obtained
from colloidal dispersion because of its certain advantages. First,
the colloidal dispersion is much less reactive towards the catalytic
material, and therefore, solid-state reactions are slightly less likely
to occur with the colloidal materials than with the co-precipitated
materials from soluble salts. Second, the particles of the colloid
are larger than the particles of the co-precipitated salt. This has
the feature of making larger pores and a more open structure
for the final catalyst. In this work, we therefore aim to evaluate
the physicochemical characteristics, nanostructural evolution, and
catalytic activity of CeO2-TbO2/SiO2 ternary oxide catalyst where
ceria-terbia mixed oxide acts as a promoter and silica forms part of
the substrate.
The thermogravimetric measurements were carried out on
a Mettler-Toledo TG-SDTA instrument. The catalyst sample was
heated from ambient to 1273 K under nitrogen flow at the heating
rate of 10 ◦C per minute. The BET surface areas were determined
by N2 physisorption at liquid N2 temperature on a Micromerit-
ics Gemini 2360 instrument using a thermal conductivity detector
(TCD). Prior to analysis, the samples were oven dried at 393 K for
12 h to remove the surface adsorbed moisture contents and finally
flushed with argon gas for 2 h. The powder X-ray diffraction (XRD)
patterns were recorded on a Rigaku Multiflex instrument using
nickel-filtered CuK␣ (0.15418 nm) radiation source and a scintil-
lation counter detector. The intensity data were collected over
a 2ꢀ range of 10–80◦ with a step size of 0.02◦ and a collection
time of 2 s per step. Average particle sizes were calculated using
Scherrer equation and the cell parameter ‘a’ of various catalysts
was determined by a standard cubic indexation method using the
most intense (111) peak. The Raman spectra were recorded on
a DILOR XY spectrometer equipped with a confocal microscope
and liquid N2 cooled charge-coupled device (CCD) detector. Sam-
ples were excited with the emission line at 325 nm from He-Cd
laser (Melles Griot Laser) under the microscope with the diam-
eter of the analyzed spot being ∼1 m. The wavenumber values
obtained from the spectra are precise to within 2 cm−1. UV–vis
diffuse reflectance spectra were collected on a UV–vis spectropho-
tometer, Model U-4100 spectrophotometer (solid). BaSO4 was used
as the reference material and the spectra were recorded in the
wavelength range from 200 to 800 nm. XPS investigations were
carried out on a Shimadzu (ESCA 3400) Spectrometer by using
MgK␣ (1253.6 eV) radiation as the excitation source. The sam-
ples were dried and evacuated at high vacuum before analysis and
then introduced into the analysis chamber. The recorded XPS spec-
tra were charge corrected with respect to the binding energy of
C 1s peak at 284.6 eV. Quantitative analysis of atomic ratios was
accomplished by determining the elemental peak areas as reported
in our previous literature [3]. Transmission electron microscopic
(TEM) investigations were made on a JEM-2100 (JEOL) instrument
equipped with a slow scan CCD camera and at an accelerating volt-
age of 200 kV. Samples were prepared by ultrasonic dispersion in
ethanol and deposited onto carbon-coated copper grids. The TPR-
TPO measurements were performed using a micro-reactor coupled
to a TCD. Oxygen storage capacity (OSC) was measured from the
oxygen release characteristics of the sample in the temperature
2. Experimental
2.1. Preparation of catalysts
The silica supported ceria-terbia (i.e., CeO2-TbO2/SiO2) cata-
lyst (CTS; CeO2:TbO2:SiO2 = 80:20:100 mol% based on oxides) was
synthesized by a deposition co-precipitation method. Nitrate pre-
Please cite this article in press as: A.T. Miah, P. Saikia, Dispersion of nanosized ceria-terbia solid solutions over silica surface: Evaluation