Facile fabrication of porous La doped ZnO granular nanocrystallites and their catalytic evaluation towards thermal decomposition of ammonium perchlorate

Article abstract of DOI:10.1016/j.jssc.2019.05.001

The present article expresses synthesis, characterization and catalytic activity of pure and La doped ZnO (La-ZnO)towards thermal decomposition of Ammonium Perchlorate (AP). The catalytic materials (La-ZnO)with 0–1.00% of doped La were synthesized by simple co-precipitation method and thoroughly characterized by several spectroscopic techniques like FT-IR, XRD, UV–Visible, PL, SEM-EDS and BET analysis. The XRD pattern of synthesized materials displayed wurtzite ZnO structure. The SEM images of the materials showed granular morphology where gradual increase in particle size and crystallite size was observed to be increased with increase in doped La concentration. The BET surface area of the materials was 28.203 to 15.830 m²/g and exhibited mesoporous nature. In catalytic studies for thermal decomposition of AP, the pure ZnO catalyst offered single stage decomposition of AP along with lowering high-thermal decomposition (HTD)temperature of AP by 105 ^OC. However, La doped ZnO showed slightly lower activity due to promotion of NO formation in oxidation of ammonia generated on initial decomposition of AP and decrease in BET surface area.

Full text of DOI:10.1016/j.jssc.2019.05.001

Accepted Manuscript
Facile fabrication of porous La doped ZnO granular nanocrystallites and their catalytic
evaluation towards thermal decomposition of ammonium perchlorate
R.M. Jagtap, D.R. Kshirsagar, V.H. Khire, S.K. Pardeshi
PII:
S0022-4596(19)30220-8
https://doi.org/10.1016/j.jssc.2019.05.001
YJSSC 20744
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received Date: 7 February 2019
Revised Date: 17 April 2019
Accepted Date: 3 May 2019
Please cite this article as: R.M. Jagtap, D.R. Kshirsagar, V.H. Khire, S.K. Pardeshi, Facile fabrication
of porous La doped ZnO granular nanocrystallites and their catalytic evaluation towards thermal
decomposition of ammonium perchlorate, Journal of Solid State Chemistry (2019), doi: https://
doi.org/10.1016/j.jssc.2019.05.001.
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ACCEPTED MANUSCRIPT
(REVISED MANUSCRIPT, Ms. No.: JSSC-19-187)
Facile fabrication of porous La doped ZnO granular nanocrystallites and their catalytic
evaluation towards thermal decomposition of ammonium perchlorate
R. M. Jagtap1, D. R. Kshirsagar2, V. H. Khire2, S. K. Pardeshi1*
1Department of Chemistry, Savitribai Phule Pune University (formerly University of Pune),
Ganeshkhind, Pune, 411007, India
2High Energy Materials Research Laboratory, Sutarwadi, Pune, 411021, India
*Corresponding authors E-mail: skpar@chem.unipune.ac.in
Table of Contents
The present article encompasses the synthesis of pure and La doped ZnO for thermal catalytic
decomposition of Ammonium Perchlorate (AP). In the studied mesoporous materials, the
lowering of Higher Temperature Decomposition (HTD) of AP was observed by 105°C in
presence of ZnO. Targeted further lowering of HTD in presence of dopant La was insignificant
due to reduction of BET surface area of the catalysts.

ACCEPTED MANUSCRIPT
J. Solid State Chemistry (Revised Manuscript, Ms. No.: JSSC-19-187)
Facile fabrication of porous La doped ZnO granular nanocrystallites and their catalytic
evaluation towards thermal decomposition of ammonium perchlorate
R. M. Jagtap1, D. R. Kshirsagar2, V. H. Khire2, S. K. Pardeshi1*
1Department of Chemistry, Savitribai Phule Pune University (formerly University of Pune),
Ganeshkhind, Pune, 411007, India
2High Energy Materials Research Laboratory, Sutarwadi, Pune, 411021, India
*Corresponding authors E-mail: skpar@chem.unipune.ac.in
Abstract:
The present article expresses synthesis, characterization and catalytic activity of pure and La
doped ZnO (La-ZnO) towards thermal decomposition of Ammonium Perchlorate (AP). The
catalytic materials (La-ZnO) with 0 to1.00 % of doped La were synthesized by simple co-
precipitation method and thoroughly characterized by several spectroscopic techniques like FT-
IR, XRD, UV-Visible, PL, SEM-EDS and BET analysis. The XRD pattern of synthesized
materials displayed wurtzite ZnO structure
and the SEM images of the materials showed granular morphology where gradual increase in
particle size and crystallite size was observed to be increased with increase in doped La
concentration. The BET surface area of the materials was 28.203 to 15.830 m2/g and exhibited
mesoporous nature. In catalytic studies for thermal decomposition of AP, the pure ZnO catalyst
offered single stage decomposition of AP along with lowering high-thermal decomposition
O
(HTD) temperature of AP by 105 C. However, La doped ZnO showed slightly lower activity
due to promotion of NO formation in oxidation of ammonia generated on initial decomposition
of AP and decrease in BET surface area.
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Keywords: La doped ZnO, Porous granular nanocrystallites, Thermal decomposition,
Ammonium perchlorate
1. Introduction:
Ammonium perchlorate (AP) is a widely used oxidizer in composite solid propellants (CSPs)
and contributes more than 60% of the total weight of a CSP. The combustion behavior of the AP-
based CSP can be optimized by controlling parameters like particle size and thermal behavior of
AP. [1-5] The enhancement of burning rate of propellant can be achieved by either reducing the
particle size of AP or decreasing its thermal decomposition temperature (TDT). In order to
enhance thermal decomposition properties of AP by reducing its higher decomposition
temperature (HDT), a broad spectrum of transition metal oxide (TMO) catalysts like Co3O4,
NiO, Fe2O3, TiO2, CuO and CoO [6-12] have been employed by researchers. The lower the
HTD temperature, the shorter is the propellant ignition delay time and the higher is the burning
rate. [13] The two proposed mechanisms to reduce HDT of a CSP by added TMO catalyst in a
minute amount (up to 5 % of wt of AP) includes formation of easily melting eutectics between
metal oxides and AP [12]; and generation of Vo-related defects to release oxygen for enhanced
burning rate.[14]
The excellence of ZnO as a promising TMO catalyst, even at industrial scale, is attributed to its
exceptional reactivity, non-toxicity, low cost, versatile formation and thermal stability. [15-16]
Recently, various nanostructures of ZnO have been focused as catalysts for thermal
decomposition of AP and found to be very effective in lowering HTD of AP. Various modified
synthetic methods are found to be effective to get superior physical and catalytic properties of
ZnO. In current literature, Fazli et al. have reported bioactive egg shell membrane aided
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synthesis of ZnO [17], whereas, Zeng et al. have demonstrated the exposed (2-1-1-0), (001), (0-
0-0-1) facets of ZnO for lowering HTD and activation energy for AP combustion. [18-20] In
addition to this, Wu et al.have coated ZnO on carbon black [21] to further reduce the HTD of
AP. In another distinguished effort, Zeng et al. [22] have also reported ZnO/AP shell-core
nanocomposites prepared by liquid deposition method to overcome the agglomeration of metal
oxide (MOX) nanocatalysts on mechanically mixing with AP. The sodium carboxymethyl
cellulose as a crystal growth modifier was utilized by Feng Gao et al. [23] and Yin et al. [24] to
afford hollow and broccoli-like superstructures of ZnO respectively with advanced catalytic
activity. Hydrothermally processed Twin-Cones [14] and gas-growth method synthesized
nanospheres of ZnO are also reported in literature. Apart from pure ZnO as a catalyst,
researchers have also studied N-doped ZnO [25-26] and NiO-ZnO composites [27-28] for
synchronized catalytic effect of two transition metal ion centers. Also, there are few reports
where ZnO nanoparticles are used as additives in AP-HTPB based CSPs to evaluate their effect
on thermal and ballistic properties of propellants. [29-30]
The noble metal incorporated ZnO nanohybrids are equally noteworthy as it is found to enhance
photocatalytic efficiency of ZnO. M. Zahouily et al. have synthesized mesoporous Ag/ZnO
quasi-spherical nanohybrid material using simple and green route via sodium alginate media.
[31] In an another highlighting work, Y. Li et al. have successfully synthesized Au ZnO hybrid
nanoparticles with hexagonal pyramid-like structure exhibiting superior photocatalytic efficiency
than pure ZnO nanocrystals. [32]
Doping with rare earth elements provides enhanced optical properties of ZnO materials and
Lanthanum (La) doped ZnO (La-ZnO) materials shows excellent gas sensitivity and
photocatalytic activity. [33] Also, there are some distinguished reports on use of copper-rare
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earth metal oxide (REMO) composite catalysts for oxidation of NH3. [34-37] The mechanism of
thermolysis of AP consists formation of adsorbed and gaseous NH3 and HClO4 by proton
transfer which react by radical reactions producing N2, H2O along with oxides of chlorine and
nitrogen. [38] Moreover, Singh et al. [38] and Patil et al. [39] have exhibited significant
lowering of TDT of AP by using rare earth metal oxide (REMO) catalyst as a consequence of
ability of the REMO to further oxidize ammonia to N2.
Taking into consideration the merits of both ZnO and REMO in advanced AP degradation
applications, herein we report the synthesis and catalytic application of pure and lanthanum-
doped ZnO (La-ZnO) towards the thermal decomposition of the ammonium perchlorate with
reduced HTD value.
2.0 Experimental:
2.1 Materials:
.
Zn(OAc)2 2H2O and La(NO3)3.6H2O (Analytical Grade, Merck India Ltd.) were used as Zinc
and Lanthanum sources respectively. Pottasium hydroxide (KOH) was procured from Merck
India (Analytical Grade). The AP used without any further processing or purification in this
study and was provided by the High Energy Materials Research Laboratory, India. All solvents
were used for this work were of Analytical Grade, purchased from Thomas Baker, India.
2.2 Preparation of catalysts:
The simple co-precipitation method was used to prepare pure and La doped ZnO (La-ZnO)
materials from their hydroxide precipitates. The required volumes of 1M zinc acetate dihydrate
and 1M lanthanum nitrate hexahydrate were stirred in 250 ml beaker to get a 100 ml volume of
cationic solutions. Further, 100 ml 1M potassium hydroxide solution was added to these
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solutions to get a final volume 200 ml of thoroughly mixed solutions. The white precipitates of
hydroxide precursors were filtered under vacuum using Whatmann filter paper No.42 by giving
five to six washings of distilled water, dried and consequently ground to fine powders. The
precursors were finally calcined at 400°C (temperature obtained from thermal studies which are
not shown here) in a muffle furnace for 6 Hrs. The pure and La-ZnO nanocrystallites with
different mole % composition of La were obtained as shown in Table 1. The pure ZnO was
obtained as a white coloured material, whereas increasing % of La in doped materials led to
gradually enhanced pale yellow colour..
Table 1 Preparation of pure and La doped ZnO (La-ZnO) materials
Entry
1M
1M
1 M
Desired Composition
(Mol % of La in ZnO)
.
La(NO3)3.6H2O
Zn(OAc)2 2H2O KOH
ml
ml
ml
1.
2.
3.
4.
5.
6.
0
100
100
pure ZnO
0.10
0.25
0.50
0.75
1.00
99.90
99.75
99.50
99.25
99.00
100
100
100
100
100
0.10 % La-ZnO
0.25 % La-ZnO
0.50 % La-ZnO
0.75 % La-ZnO
1.00 % La-ZnO
2.3 Measurements and characterizations:
The pure ZnO and La-ZnO materials were characterized by FTIR spectroscopy using Shimadzu
FTIR-8400 spectrometer equipped with KBr beam splitter in the wavenumber range of 4000-400
cm-1 at a resolution of 4 cm-1. X-ray diffraction (XRD) measurements were conducted by Bruker
AXSD-8 Advance X-ray diffractometer with monochromatic CuKα radiation with λ=1.5406 A°.
Scanning electron micrograph-electron dispersion spectroscope, SEM-EDS JEOL JSM-6360A
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was used for morphology and elemental analysis of the materials. Photoluminescence (PL)
spectra were recorded on Shimadzu RF-5301PC instrument and UV-Vis spectrophotometer (UV-
1601, Shimadzu) was used to study absorbance of the materials. The specific surface area and the
pore size distribution estimated from nitrogen physisorption isotherms at liquid nitrogen
temperature on a Quantachrome Version 3.01 BET instrument.
2.4 Catalytic activity towards thermal decomposition of AP:
The catalytic activity of the synthesized materials was studied towards thermal decomposition of
AP. In order to study catalytic roles of pure and La doped ZnO samples on AP decomposition,
the catalyst and AP were premixed in a mass ratio of 1:100 respectively for the TG-DTA
analysis using SDT Q600 V20.9 Build 20 instrument at HEMRL, Pune. The heating rate of
10°C/min in N2 atmosphere with flow of 100 ml/min was maintained within temperature range of
30 to 650°C.
3.0 Results and Discussion:
3.1 FTIR analysis:
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Fig. 1 FTIR spectra of pure and La doped ZnO
The FTIR spectra of pure and La doped ZnO are demonstrated in Fig. 1. In FTIR spectra,
frequency bands below 500 cm-1 are usually recognized as metal-oxygen stretching. The FTIR
spectra of synthesized materials exhibited two metal-oxygen stretching frequency bands at 424
cm-1 and 481 cm-1, which were clearly seen in 0.50, 0.75 and 1.00 % La-ZnO indicating presence
of Zn-O and La-O bonding. Along with these expected metal oxygen IR bands, the additional IR
bands were observed at 1354, 1526, 2335 and 3458 cm-1, which were attributed to C-O bending
2-
in acetate species, C-O bending of CO3 , symmetric-asymmetric stretching, C=O stretching
vibrations of acetate species and –OH stretching [40-41]. These additional IR bands revealed the
presence of adsorbed acetate, carbonate species and hydroxide ions on material surface.
3.2 XRD analysis:
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Fig. 2 XRD pattern of pure and La doped ZnO
The XRD pattern of synthesized pure and La doped ZnO are displayed in Fig. 2. All the two
theta (2θ) values of the peaks obtained were clearly matched with 100, 002, 101, 102, 110, 103,
200, 112, 201, 004 and 202 planes of the reference XRD-pattern of wurtzite ZnO structure
indexed as JCPDS No. 36-1451. The presence of doped La in ZnO was ascertained by Bragg’s
angle shifting of 101 plane towards the higher 2θ value by 0.2° from pure ZnO to 1.00 % La-
ZnO. These observations from XRD-patterns confirmed that the pure and doped ZnO was
obtained in wurtzite crystal structure. The average crystallite size of the diffracted materials was
determined using Debye Scherrer’s equation using the full-width at half-maximum data at the
diffraction peak of the wurtzite 101 plane. [42-43] The calculated average crystallite size for the
pure ZnO, 0.10, 0.25, 0.50, 0.75 and 1.00 % La-ZnO were found to be 30.13, 32.26, 34.95,
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39.72, 41.62 and 44.82 nm respectively. The average particle size was found to gradually
increase with incorporation of the La in pure ZnO material.
3.3 UV-Visible spectroscopy:
Fig. 3 UV-Visible spectra of pure and La doped ZnO
The UV-Visible spectra of the synthesized materials showed a bathochromic shift in absorption
maxima of pure ZnO with increase in mole % of La in La-ZnO materials as shown in Fig. 3. The
pure ZnO showed absorption maxima at 325 nm, whereas 0.10, 0.25, 0.50, 0.75 and 1.00 % La-
ZnO have showed the absorption maxima at 357, 359, 365, 367 and 370 nm respectively. This
gradual increase in absorption maxima was in correlation with increase in La % of composition
which illustrated the role of La as optical responsive centers in La-ZnO. The band gap energies
of the materials were calculated by formula 1250 nm / absorption maxima in nm. The obtained
band gap energies (Eg) for pure ZnO, 0.10, 0.25, 0.50, 0.75 and 1.00 % La-ZnO were found as
3.85, 3.57, 3.50, 3.42, 3.41 and 3.38 eV respectively.
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Figure 4. Band gap energies (Eg) of pure and La-doped ZnO materials from TAUC plots
The band gap energies (Eg) of the materials were also evaluated by using Tauc equation by
plotting (αhυ)2 in (eV cm-1)2 vs. Energy (hυ) in eV as shown in Fig. 4. The Eg values were
estimated using Tauc’s plot by extrapolating the linear portion of the plot of (αhυ)2 vs. photon
energy. [42-43] The Eg values for pure ZnO, 0.10, 0.25, 0.50, 0.75 and 1.00 % La-ZnO were
found to be 3.84, 3.50, 3.48, 3.42, 3.40 and 3.37 eV, which are in good agreement with the
earlier calculated values. Thus, the higher % La in material composition led to reduced band gap
energy of the semiconducting material and hence possible photocatalytic activity in sunlight.
3.4 Photoluminescence spectroscopy:
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Fig. 5 Photoluminescence spectra of pure and La doped ZnO
The photoluminescence (PL) spectra for pure and La doped ZnO at excitation wavelength 300
nm are demonstrated in Fig. 5. At this excitation wavelength, the pure and La-doped ZnO
materials showed relatively weaker UV emission peaks at around 390 nm and 370 nm
respectively. These peaks were corroborated to the near band edge (NBE) emission originated by
recombination of the free excitons of ZnO. [44]
Along with these emission bands, all the materials have also showed the yellow green emission
bands at 509 nm which were obtained by deep level (DL) defect emission related to oxygen
vacancies in ZnO lattices. The green yellow emission band of 1.00 % La-ZnO was of the highest
intensity and also was found to be 1.7 times stronger than that of pure ZnO. The intensity of this
green yellow emission band was observed to be enhanced with increased La % in materials
which denotes enhancement in oxygen vacancy or defect in doped materials. [45]
3.5 SEM-EDS analysis:
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The SEM images of pure and La doped ZnO spotted homogeneous granular morphology of the
materials with substantial porosity, indicating the uniform doping of La in doped materials as
shown in Fig. 6.
Fig. 6 SEM images of pure ZnO (a), 0.10 %La-ZnO (b), 0.25 %La-ZnO (c), 0.50 %La
ZnO (d), 0.75 %La-ZnO (e) and 1.00 %La-ZnO (f)
The marginal agglomeration observed in 0.75 and 1.00 % La-ZnO (e-f) may be attributed to their
higher level defects arised due to higher La content. The crystallite size of all the studied
materials was observed in a range of 40 to 130 nm attributed to low temperature synthesis of
these materials.
Fig. 7 represents the size distribution histograms which were derived by extracting the size of
hundred particles from the SEM images of pure and La doped ZnO nanostructures using image J
processing software. Each histogram is fitted by a Gaussian curve. According to these
histograms the average particle size of ZnO nanoparticles in pure ZnO was 25.00 nm. The
particle size was also found to be enhanced on doping of La. The average particle size for 0.10,
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0.25, 0.50, 0.75 and 1.00 % La-ZnO were found to be 26.60, 30.30, 34.58, 38.17and 41.71nm
respectively. Indeed, the nanoparticle size is found to be bigger than the crystallite size,
suggesting that particles can be formed by one to several crystallites. [43]
Fig. 7 Particle size distribution histograms of pure and La-doped ZnO materials
The EDS elemental analysis of synthesized materials showed the presence of La in doped
materials. The observed La % in 0.10, 0.25, 0.50, 0.75 and 1.00 % La-ZnO were found to be
0.07, 0.14, 0.41, 0.56 and 0.71 % respectively. This increase in La % was in agreement with the
added La3+ concentrations while getting their hydroxide precursors. The observed differences in
La % from theoretical and observed values may be attributed to loss of La during washing of
hydroxide precursor and also heating these precursors during calcination stages.
The representative EDS spectra for pure ZnO and 1.00 % La-ZnO are shown in Fig. 8 and the
EDS spectra for all the materials are given in supplementary data (S1).
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Fig. 8 EDS spectra of pure and 1.00 % La-ZnO
The EDS-element mapping of Zn, O and La elements demonstrated uniform distribution of
doped element La in synthesized materials. Element mapping confirmed the homogeneity of the
materials obtained by simple co-precipitation process as shown in Fig. 9.
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Fig. 9 EDS-element mapping of Zn, O and La elements in pure ZnO (a), 0.10 %La-ZnO
(b), 0.25 %La-ZnO (c), 0.50 %La -ZnO (d), 0.75 %La-ZnO (e) and 1.00 %La-ZnO
(f). Inset: uniform distribution of La
3.6 BET surface area measurements:
The N2 sorption isotherms of pure ZnO and 0.10%, 0.25%, 0.50%, 0.75% and 1.00% La-ZnO
materials are as shown in Fig. 10.
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Fig. 10 Nitrogen adsorption (lower)-desorption (upper) isotherms of pure and La-ZnO
materials. Inset: the corresponding BJH pore size distribution
The Brunauer–Emmett–Teller (BET) method was used to evaluate the specific surface areas and
the pore size distribution curves were generated by the Barrett-Joyner-Halenda (BJH) model
from the desorption and adsorption branches of the isotherm. [46] For the synthesized pure ZnO,
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0.10, 0.25, 0.50, 0.75 and 1.00 % La-ZnO samples, the specific surface area values were found to
be 28.203, 26.600, 21.907, 20.499, 16.884 and 15.830 m2/g respectively. These observations are
in corroboration with the SEM images where the gradual agglomeration of the materials was
observed with increase in doped amount of La. However the pore size (radius) of all the
materials was observed to be 1.8 to 1.9 nm revealing mesoporous nature of these materials. All
the samples obeyed the isotherm type III with hysteresis loops of different sizes according to the
IUPAC classification. [47]
3.7 Catalytic activity of Pure and La doped ZnO towards thermal decomposition of AP:
The as-prepared porous ZnO and La-ZnO materials with different percentages of La were
explored as catalysts to study their effect on thermal decomposition of AP. The role of these
catalysts was evaluated in terms of TG-DTA analysis of premixed AP with 1 % mass basis of
catalysts.
At a heating rate of 10°/min in N2 atmosphere, the TG curve of bare AP showed two step
decomposition (Fig. 11), exhibiting low-temperature decomposition (LTD) with 29 % wt loss at
O
300 C and complete high-temperature decomposition (HTD) at 394°C. The observed LTD and
HTD were in well agreement with the values reported as 298°C and 397°C by Zeng et al.[20]
The LTD process is a heterogeneous process, proposed to include a proton transfer in the AP
subsurface to yield NH3 and HClO4. The further adsorption of NH3 and HClO4 in the porous
structure proceeds towards final decomposition of HClO4 to HCl and O2, and oxidation of NH3 to
N2 as shown in Scheme 1. However, the HTD process purely involves the oxidation of NH3 to
N2 by HClO4 in the closed sample pans.[14]
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Scheme 1. Schematic representation for thermal decomposition of AP
Fig. 11 TG curves of AP in absence and presence of pure and La doped ZnO catalysts
More interestingly, under identical conditions the added ZnO catalyst exhibited a single step
decomposition of AP in TG analysis with merged HTD and LTD value of 289°C. This outcome
clearly indicated that ZnO has a definite role to play to reduce the decomposition temperature by
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105°C. This promoted decomposition of AP may be attributed to the porous morphology of ZnO
nanocrystallites providing higher contact surface to facilitate the AP decomposition.
Fig. 12 DTA curves of AP in absence and presence of pure and La doped ZnO catalysts
The DTA curves of pure AP in absence of catalyst and in presence of 1 % mass basis of different
catalysts (pure and La doped ZnO) at a heating rate of 10°C/min are shown in Fig. 12. The pure
AP on thermal decomposition showed an endothermic peak at 241°C, which were attributed to
the transformation of AP crystal from orthorhombic to cubic phase. [30] The two more
exothermic peaks were observed in pure AP at 300 and 394°C corresponding to its low-
temperature decomposition (LTD) and high-temperature decomposition (HTD) respectively. The
thermal decomposition of AP in presence of 1% mass basis of pure ZnO and La-ZnO catalysts
exhibited the similar endothermic peaks as that of bare AP at 240-245°C for transformation of
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AP crystal from orthorhombic to cubic phase. However, La doped ZnO catalysts showed slight
increase in decomposition temperature of AP where the HTD values for 0.10, 0.25, 0.50, 0.75
and 1.00 % La-ZnO were found to be 290°C, 290°C, 292 °C, 293 °C and 294°C respectively.
The observed increase in HTD was also evidenced in DSC plots of AP decomposition as shown
in Fig. 13. The pure AP showed two stage decomposition with LTD 294 °C and HTD of 398 °C.
The addition of 1% catalysts showed the similar HTD values as that of DTA plot for ZnO, 0.10,
0.25, 0.50, 0.75 and 1.00 % La-ZnO as 287°C, 290°C, 291 °C, 294 °C and 294°C respectively.
Fig.13 DSC curves of AP in absence and presence of pure and La doped ZnO catalysts
This slight increase in HTD values on increasing La content indicates that La has played a
retarding role to decrease the catalytic activity of pure ZnO by promoting the formation of NO
instead of N2 in further oxidation of NH3. These outcomes were in agreement with the results
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obtained by Hung et al. [33] during selective catalytic oxidation of ammonia over La doped
CuO. The selectivity for the NO by-product was reported up to 41% than N2 production (up to
30%) with 97% NH3 conversion. The slight higher HTD for La doped ZnO may be the
consequence of the further energy consumption for final oxidation of NO to N2. [48] Another
important factor which played a comprehensive role to this increase in HTD is decrease in
surface area of the catalysts from 28.203 to15.830 m2/g for ZnO to 1.00 % La-ZnO catalyst.
However, there is a need of further comprehensive studies to identify the precise reason for
observed negative catalytic effect of La in La-ZnO on the basis of actual estimation of gaseous
decomposition products.
Conclusions:
The La doped ZnO materials were synthesized by simple co-precipitation method. The IR
spectra of pure and doped ZnO showed remarkable bands at 424 cm-1 and 481 cm-1 for metal-
oxygen bonding and also ascertained the presence of adsorbed acetate, carbonate and hydroxide
species on catalyst surface. The XRD pattern of ZnO and La-ZnO revealed the wrutzite crystal
structure and the presence of La doped ZnO was ascertained by shifting of Bragg’s angle
towards higher 2θ value by 0.20°. The calculated average crystallite size for the pure ZnO to 1.00
% La-ZnO were found to be 30.13 to 44.82 nm respectively. The UV-Visible spectra of the
materials confirmed that the La acts as UV-absorption centers leading to bathochromic shift of
absorption maxima from 325 nm to 370 nm from pure ZnO to 1.00 % La-ZnO respectively. The
band gap energy (Eg) values for pure ZnO, 0.10, 0.25, 0.50, 0.75 and 1.00 % La-ZnO were found
to be 3.84, 3.50, 3.48, 3.42, 3.40 and 3.37 eV. The PL spectra of the doped materials showed
enhanced DL defects and oxygen vacancy which was reflected from intensified yellow green
emission bands. The SEM images of the materials showed the porous granular nature and
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particle size of about 40-150 nm. The % La observed from EDS analysis was in well agreement
with the expected values in respective compositions. The EDS-element mapping demonstrated
uniform distribution of doped element La in synthesized materials. The BET specific surface
area values were found to be decrease on La incorporation from 28.203 to15.830 m2/g for pure
ZnO and 1.00 % La-ZnO respectively. In catalytic application, when pure ZnO was added in 1%
amount, reduced the HTD of AP by 105°C. In La doped ZnO, the marginal increase in HTD than
pure ZnO catalyst from 289°C to 294°C was observed which is due to promotion of NO
formation than N2 while final oxidation of NH3.
Declarations of interest: none
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