Chapter IV
Structural evolution and surface characteristics
of sol-gel derived Mn2+
ion doped alumina
Abstract
Effect of Mn2+
ion doping in sol-gel derived alumina from boehmite was studied
at various calcination temperatures and dopant concentrations to investigate the
structural evolution of various Al2O3 phases and also the formation of MnAl2O4 spinel
and its facilitation to generate α-Al2O3 at low temperatures. The chemical
transformations were monitored at various levels using TGA, FTIR, diffuse reflectance
and EPR spectra and also through detailed PXRD measurements. The in situ generated
MnAl2O4 spinel is found to be significantly facilitating the formation of α-Al2O3 at very
low temperature by nucleation from 1200ºC in the undoped xerogel to around 900ºC in
the Mn doped system. Even though the oxidation of Mn2+
to Mn3+
is highly favorable at
the study condition, traces of Mn2+
is seen to be getting trapped in the transition
aluminas which is sufficient enough to nucleate and facilitate the formation of α-Al2O3
at comparatively very low temperatures. The Mn3+
formed is seen to be existing in
highly dispersed form without forming any crystalline phase characteristic of Mn and is
leachable completely by mineral acids from samples calcined at higher temperatures,
while the Mn2+
ion present is found to be getting transformed into MnAl2O4 spinel
which cannot be leached by even concentrated HCl. Useful data could be obtained from
spectroscopic measurements to account for both chemical and structural evolution of
Mn doped system at various dopant concentration and calcination temperatures.
Surface characteristics such as surface acidity and surface area of the mixed oxides are
also monitored and significant differences are seen in Mn doped system compared to
undoped alumina.
124 Chapter IV
4.1. Introduction
Just as Cu2+
doped Al2O3 system MnOx/Al2O3 system is also of active
interest due to its versatile applications including that as heterogeneous catalysts1-9
.
In fact both high chemical stability and tunable redox behavior, which permit
the stabilization of various intermediate Mn oxidation states as in MnO2,
Mn5O8, Mn2O5, Mn3O4 and MnO and access to several crystalline phases
make them a highly potential system for various kinds of catalytic reactions.
Consequently such Mn-Al mixed oxides have been used as effective catalyst
systems in reactions such as selective catalytic reduction (SCR) of NOx3, 4
,
CO oxidation5 and oxidation of organic compounds
6-9. The use of transition
metal oxide based catalysts is an alternative to noble metals for the
abatement of volatile organic compounds; in particular manganese oxide is a
very active phase for the complete oxidation of oxygenated volatile organic
compounds9. The general strategies employed for generating various phases
of the MnOx/Al2O3 system have been discussed briefly in Chapter I.
Despite the application of MnOx/Al2O3 system to a wide variety of
important catalytic reactions, the nature of the active phases present and the
influence of these phases on the surface properties such as surface acidity
and surface area are still to be investigated in detail to identify the active
constituents and their phases in them. Numerous factors simultaneously
affect the physico-chemical properties and the catalytic abilities of
MnOx/Al2O3 system including preparation method, level of doping of MnOx,
extent of dispersion and calcination temperature. In most of the studies
reported so far either co-precipitation or incipient wetness technique was
used for the preparation of this mixed oxide system. These methods have
inherent weakness with respect to chemical composition and homogeneity
since atomic level mixing of components does not occur. Among various
Structural evolution and surface characteristics … 125
methods the sol-gel technique for the preparation of mixed oxides has
attracted the attention of solid-state chemists for the fact that products with
high purity and homogeneity can be obtained as the mixing of constituent
cations take place on an atomic scale in the precursor. Even though the early
formation of α-Al2O3 in Al2O3-MnOx system was reported earlier, no
systematic study was reported to investigate on these aspects so far. In this
chapter, we report the formation characteristics of MnAl2O4 spinel and the
effect of this in situ formed spinel on the structural evolution of α-Al2O3 on
calcination of the mixed xerogels. This chapter also discusses our attempt to
tune surface characteristics of sol-gel derived alumina by doping varying
extents of Mn2+
ions and monitoring their features in a systematic manner.
4.2.Experimental
4.2.1. Preparation of MnOx/Al2O3 mixed oxides
Mn2+
ion doped alumina mixed oxides were prepared by sol-gel
technique using boehmite as the source for alumina and Mn(NO3)24H2O as
the source for Mn2+
. The details regarding the preparation have been given in
Chapter II. Considering the possibility of formation of various oxidation
states and crystalline phases of Mn, the calcination temperatures selected
were 400, 500, 600, 700, 800, 900 and 1000ºC. The levels of doping of Mn2+
ions were restricted to 2, 4 and 8 mol. % (in terms of MnO). The details of
the samples prepared and the codes used are given in Table 4.1. Estimation
of soluble Mn in the calcined samples was carried out by reacting them with
conc. HCl and by employing atomic absorption spectroscopy. Acid washed
samples were named by labeling them with ‘w’ in their original name codes.
Thus 8MnAl10w means the residue obtained after washing the 8MnAl10
with conc. HCl.
126 Chapter IV
4.2.2. Analytical techniques
The crystalline phases, obtained for each heat treatment were
analyzed by powder X-ray diffraction (PXRD) methods using a Bruker make
instrument operating with Cu-K radiation; the patterns were recorded in the
2 ranges from 20 to 80º. Thermo gravimetry (TG) and differential thermal
analysis (DTA) of the xerogels were performed using a Perkin–Elmer
thermal analyzer. FTIR spectra of the doped and undoped alumina were
obtained on a Shimadzu-8400S spectrometer as KBr pellets. The Shimadzu-
2450 spectrophotometer was used to measure the diffused reflectance spectra
of the solids in the range 240-800nm. Atomic absorption spectral
measurements were carried out on an Avanta-GBC, Scientific Equipment
Company Ltd, Australia make instrument. The surface acidity of various
samples calcined at different temperatures was investigated by FTIR,
elemental analysis and TG/DTA method using pyridine as a probe molecule.
The specific surface areas of selected samples were determined using BET
surface area analyzer. The Scanning Electron Micrographs (SEM) were
obtained employing a JEOL JSM-6390 microscope.
Structural evolution and surface characteristics … 127
Table 4.1. Details of Mn2+
ion doped alumina mixed oxides and the sample
codes used
Sl.No Amount of MnO
(mol.%)
Amount of Al2O3
(mol.%)
Calcination
temp (ºC) Sample code
*
1 2 98 Xerogel 2MnAlG
2 2 98 400 2MnAl4
3 2 98 500 2MnAl5
4 2 98 600 2MnAl6
5 2 98 700 2MnAl7
6 2 98 800 2MnAl8
7 2 98 900 2MnAl9
8 2 98 1000 2MnAl10
9 4 96 Xerogel 4MnAlG
10 4 96 400 4MnAl4
11 4 96 500 4MnAl5
12 4 96 600 4MnAl6
13 4 96 700 4MnAl7
14 4 96 800 4MnAl8
15 4 96 900 4MnAl9
16 4 96 1000 4MnAl10
17 8 92 Xerogel 8MnAlG
18 8 92 400 8MnAl4
19 8 92 500 8MnAl5
20 8 92 600 8MnAl6
21 8 92 700 8MnAl7
22 8 92 800 8MnAl8
23 8 92 900 8MnAl9
24 8 92 1000 8MnAl10
* For example, 2MnAlG and 2MnAl4 indicate 2% Mn doped boehmite xerogel and 2%
Mn doped xerogel calcined at 400ºC
128 Chapter IV
4.3. Results and discussion
Just as in the case of Cu2+
/Al2O3 system we have attempted to
generate Mn2+
ion doped system with the metal ion content varying from 2 to
8 mol.% to investigate on the dopant influenced structural evolution and
surface characteristics of aluminas. We have selected a calcination
temperature at the lower side at 400ºC to get rid of volatile components
including nitrates and also to effect the complete dehydroxylation and poly-
condensation. Similarly we have opted an upper limit of 1000ºC for
calcination since we have found that this temperature was sufficient enough
for the formation of all the possible thermodynamically stable phases.
4.3.1.Thermal analysis
Thermo gravimetric (TG) and differential thermal analysis (DTA) were
performed to study the weight loss and thermal behavior of the precursor
xerogels in a Perkin-Elmer thermal analyzer; the samples were heated from
room temperature to 1200ºC at 5ºC min-1
in N2 atmosphere. Fig. 4.1 shows the
weight loss and thermal behavior of 8 mol.% Mn2+
ion doped boehmite xerogel,
8MnAlG. The TG curve shows three steps of weight losses, which are
associated with elimination of physically, adsorbed water, decomposition of
nitrate anions and dehydroxylation of boehmite respectively. These changes are
also reflected in the DTA curve centered at 75, 201 and 430ºC respectively. The
removal of physically adsorbed water and dehydroxylation of boehmite xerogel
are endothermic processes and the decomposition of nitrate anion is exothermic.
The DTA curve of the mixed xerogel contains another exothermic peak centered
at 1000ºC without any loss of weight and is associated with the solid-state
reaction resulting in the formation of corundum phase. We expected the
MnAl2O4 spinel specific peak around this temperature in the DTA (as in the
Structural evolution and surface characteristics … 129
case of Cu2+
/Al2O3 system) but only a broad peak was visible in the Mn2+
/Al2O3
system. We did observe spinel formation in our PXRD measurements
(discussed section 4.3.5.) and therefore we believe that the DTA peak
corresponding to the spinel formation gets merged with the broad peak around
1000ºC associated with α-Al2O3 formation. It may be noted that α-Al2O3 related
DTA peak in the undoped sample is much higher (about 1195ºC; Fig. 3.2). Thus
it is evident that the incorporation of Mn2+
ions into the alumina matrix
significantly lowers the phase formation temperature of α-Al2O3.
Fig. 4.1. (a) TGA and (b) DTA curves of 8MnAlG
4.3.2. FTIR spectra
FTIR spectroscopic investigation was carried out to monitor the
structural changes in the sol-gel derived Mn2+
ion doped alumina system as a
function of calcination temperature. The formation of stable α-Al2O3 phase
from boehmite takes place by dehdroxylation and condensation followed by
130 Chapter IV
phase evolution through γ-, δ- and θ- forms on calcination. The boehmite and
α-Al2O3 consists of Al atoms in the octahedral coordination (AlO6 units)
while the intermediate transition aluminas formed have both octahedral and
tetrahedral Al units. The structural evolution taking place within the xerogel
during heat treatment can be understood by monitoring the Al-O related
vibrations in their FTIR spectra. Most of the M-O vibrations occur in the
region 1200-400cm-1
. In AlO6 moieties Al-O stretching modes appear in the
region 750-450cm-1
whereas in tetrahedrally coordinated AlO4 units the ν Al-
O peaks are seen in 950-750cm-1
region10-12
.
Fig. 4.2. FTIR spectra of Mn2+
ion doped boehmite calcined at 900C.
The samples are (a) 2MnAl9, (b) 4MnAl9 and (c) 8MnAl9
The FTIR spectra of the mixed oxides generated in the present study by
the calcination of the mixed xerogels at 400, 500, 600, 700 and 800ºC,
containing various levels of MnOx are seen to be identical to that of the
corresponding undoped samples showing Al-O bands in the region 950-
Structural evolution and surface characteristics … 131
450cm-1
characteristic of both tetrahedrally coordinated Al (950-750cm-1
) and
octahedrally coordinated Al atoms (750-450cm-1
). We have noticed a very
weak absorption peak around 1080cm-1
(presumably due to symmetric
bending mode of (Al-O-H) 13
indicating that traces of non-dehydroxylated
boehmite still remains even after three hours of calcination. However, on
calcination of the xerogels at 900ºC perceptible changes are seen in the
samples, depending on the level of Mn doping (Fig. 4.2). The 8% Mn doped
sample 8MnAl9 showed an intense band at 445 cm-1
characteristic of α-
Al2O314,15
and no bands in the Al-O stretching region of 950-750cm-1
,
indicating the predominance of AlO6 polyhedra in the system. However, for
4MnAl9 and 2MnAl9 no evidence for the existence of α-Al2O3 is seen and the
spectra are identical to that of the undoped sample calcined at 900ºC,
containing both AlO4 and AlO6 polyhedra.
Fig. 4.3. FTIR spectra of Mn2+
ion doped boehmite calcined at 1000C.
The samples are (a) 2MnAl10, (b) 4MnAl10 and (c) 8MnAl10
132 Chapter IV
Similarly the 8% doped samples calcined at 1000ºC, 8MnAl10,
showed the peak characteristic of α-Al2O3 and also bands in the 750-500cm-1
regions indicating the presence of Al atoms in octahedral coordination (Fig.
4.3). At this temperature the 4% doped sample 4MnAl10 also showed broad
bands in the 850-500cm-1
regions besides the peaks characteristic of α-Al2O3
indicating the presence of both transition aluminas and α-Al2O3 in the
system. However, the 2% doped sample 2MnAl10 gave spectra identical to
that of the undoped sample calcined at the same temperature, containing
transition aluminas only. No characteristic peaks corresponding to Mn-O
vibrations are observed in any of our samples since our samples contain very
low percentage of Mn and also due to the fact that Mn-O vibrations occur in
the same region as in the alumina16
. Thus the FTIR spectral analyses show
that the incorporation of Mn2+
ions in alumina matrix significantly lowers the
phase formation temperature of α-Al2O3.
4.3.3. Electronic spectra (diffuse reflectance)
We have also measured diffuse reflectance spectra of all the Mn doped
samples to find out the constituents present in the xerogel and calcined samples
and to monitor the structural evolution as a function of temperature in them. In
natural environment the most common oxidation states of Mn are Mn(II),
Mn(III) and Mn(IV) although other oxidation states are also known to exist.
Equally interesting aspect of Mn system is the nature of the metal ion to coexist
in several mixed oxidation states. The presence of partially occupied 3d orbitals,
variable electronic occupancy in them, easy accessibility for mixed oxidation
states and abilities to have diverse Mn-O and Mn-Mn interactions are known to
make the electronic nature of MnOx-Al2O3 system a complex one and the
interpretation of their electronic spectra rather difficult. However, the nature of
crystal field arrangement including the number, coordination geometry and
Structural evolution and surface characteristics … 133
distance at which O atoms get arranged around the metal ion can lead to
variations in electronic transitions which in turn can be used as useful
parameters for probing the structural modifications that happen around the metal
ion. Presented in Table 4.2 is the electronic spectral data for various samples
calcined at different temperatures.
5D
5Eg
5T2g
5Eg
5B2g
5A1g
5B1g
Free ion Oh field Tetragonal
distortion
Fig. 4.4. d-orbital splitting of Oh Mn (III)
The absorption spectrum of Mn(II) (d5) in octahedral coordination is
well characterized and consists of bands in the 450-600nm region 17,18
. All
the mixed xerogel samples with varying amount of Mn2+
ions are pink in
colour and show broad absorptions around 480nm and another strong
absorption at 250nm. The absorption at 480nm is characteristic of the
spin forbidden transition 6A1g→
4T1g of Mn
2+ in octahedral environment
and the strong band observed on the high-energy side is due to the M-O charge
134 Chapter IV
Table 4.2. Diffuse reflectance spectral data of Mn2+
ion doped xerogels
and calcined samples.
Sample Colour Absorption maxima (nm) Assignment Geometry about Mn ions
2MnAlG Pink
480b
250
6A1g→4 T1g
O2- → Mn2+ LMCT
Mn2+ Octahedron
2MnAl4,2MnAl5
2MnAl6,2MnAl7
2MnAl8,2MnAl9
and 2MnAl10
Dark brown
710
470b
250
5B1g → 5A1g 5B1g → 5B2g 5B1g → 5E g
O2- → Mn2+ LMCT
Mn3+ Distorted octahedron
4MnAlG Pink 480b
250
6A1g→4 T1g
O2- → Mn2+ LMCT
Mn2+ Octahedron
4MnAl4,4MnAl5
4MnAl6,4MnAl7
4MnAl8 and
4MnAl9
Dark brown
710
470b
250
5B1g → 5A1g 5B1g → 5B2g 5B1g → 5E g
O2- → Mn2+ LMCT
Mn3+- Distorted octahedron
4MnAl10
Dark brown
715
500b
360
250
5B1g → 5A1g 5B1g → 5B2g 5B1g → 5E g
O2- → Mn3+ LMCT
O2- → Mn2+ LMCT
Mn3+ Distorted octahedron
8MnAlG
Pink
480b
250
6A1g→4 T1g
O2- → Mn2+ LMCT
Mn2+-
Octahedron
8MnAl4,8MnAl5
8MnAl6,8MnAl7
and 8MnAl8
Dark brown
710
470b
250
5B1g → 5A1g 5B1g → 5B2g 5B1g → 5E g
O2- → Mn2+ LMCT
Mn3+- Distorted octahedron
8MnAl9
Dark brown
715
500b
360
250
5B1g → 5A1g 5B1g → 5B2g 5B1g → 5E g
O2- → Mn3+ LMCT
O2- → Mn2+ LMCT
Mn3+ Distorted octahedron
8MnAl10 Dark
brown
715
500b
360
250
5B1g → 5A1g 5B1g → 5B2g 5B1g → 5E g
O2- → Mn3+ LMCT
O2- → Mn2+ LMCT
Mn3+ Distorted octahedron
Structural evolution and surface characteristics … 135
transfer. The samples, containing varying amounts of Mn2+
ions, calcined at
400, 500, 600, 700 and 800ºC are deep brown in colour and have broad
absorptions around 710, 470nm and a sharp one at 250nm. A closer look at
these spectra indicates the predominant presence of Mn3+
ion in all these
calcined samples. It is known that Mn3+
ion (d4) in highly symmetric
octahedral geometry can give absorption around 500nm due to a single spin
allowed transition. However any distortion from the octahedral geometry can
give rise to uneven splitting of d levels leading to multiple electronic
transitions (Fig. 4.4).
Fig. 4.5. Diffuse reflectance spectra of Mn2+
ion doped boehmite calcined
at 1000ºC. The samples are (a) 2MnAl10, (b) 4MnAl10 and
(c) 8MnAl10
Thus the spectrum of Mn3+
ion in distorted octahedral environment can
have three characteristic bands around 370, 485 and 740nm19-21
. The
absorptions seen around 710 and 470nm for all our samples referred above
can, therefore, be assigned to 5B1g →
5A1g and superimposed
5B1g →
5B2g and
136 Chapter IV
5B1g →
5Eg crystal field d-d transitions respectively
19, 21. The strong absorption
seen in these samples at 250nm is characteristic of O2-
→ Mn2+
charge
transfer22
. The d-d transitions in the range 450-600nm characteristic of Mn2+
ions are not clearly discernible in the samples since Mn2+
ion has d5
configuration and all transitions are spin forbidden and therefore of low
intensity23
. However, the Mn3+
ion is a d4 system and therefore in Oh symmetry
it exhibits strong spin allowed transitions, which mask the Mn2+
spin forbidden
transitions in the same range. When the samples are calcined at 900ºC, the 2
and 4% samples show features exactly similar to that calcined at 800ºC.
However, for 8MnAl9 we observed a new clearly distinguishable broad peak
around 360nm characteristic of O2-
→ Mn3+
charge transfer and broad peaks
around 715 and 500nm. When the samples are calcined at 1000ºC the 2 %
doped 2MnAl10 sample shows peaks at 710, 470 and 250nm (Fig. 4.5).
Shown in the figure are also the spectra of 4MnAl10 and 8MnAl10, which
show peaks at 715, 500, 360 and 250nm. This indicates that the environment
around Mn3+
ion is identical in samples 4MnAl10, 8MnAl9 and 8MnAl10 and
is marginally different from other samples.
The diffuse reflectance spectral data of all the samples after treating
with con.HCl are given in Table 4.3. All the acid washed samples except
8MnAl9w, 4MnAl10w and 8MnAl10w are brown in colour and show diffuse
reflectance spectra almost similar to that of the corresponding unwashed
samples with less intense bands indicating the presence of Mn3+
ions in
alumina matrix which are non-leachable with conc. HCl. This shows that the
incorporation of Mn into the alumina matrix using sol-gel boehmite at various
calcination temperatures result in the formation of amorphous phases of
alumina contain some amount of Mn and the binding between Mn and alumina
phases are so strong that we could not leach the entire Mn with con.HCl.
Structural evolution and surface characteristics … 137
Table 4.3. Diffuse reflectance spectral data of the calcined samples after
treating with concentrated acid
Sample Colour Absorption
Maxima nm Assignment
Geometry around
Mn ions
2MnAl4w,2MnAl5w
2MnAl6w,2MnAl7w
2MnAl8w,2MnAl9w
and 2MnAl10w
Dark
brown
710
480b
250
5B1g →
5A1g
5B1g →
5B2g
5B1g →
5E g
O2-
→ Mn2+
LMCT
Mn3+
Distorted
octahedron
4MnAl4w,4MnAl5w
4MnAl6w,4MnAl7w
4MnAl8w and
4MnAl9w
Dark
brown
710
480b
250
5B1g →
5A1g
5B1g →
5B2g
5B1g →
5E g
O2-
→ Mn2+
LMCT
Mn3+
Distorted
octahedron
4MnAl10w
Pink
710
535
490
250
5B1g →
5A1g
6A1g→
4 T1g
6A1g→
4 T1g
O2-
→ Mn2+
LMCT
Mn3+
octahedral
Mn2+
tetrahedron
Mn2+
octahedron
8MnAl4w,
8MnAl5w
8MnAl6w,
8MnAl7w
and 8MnAl8w
Dark
brown
710
480b
250
5B1g →
5A1g
5B1g →
5B2g
5B1g →
5E g
O2-
→ Mn2+
LMCT
Mn3+
Distorted
octahedron
8MnAl9w
Pink
535
490
250
6A1g→
4 T1g
6A1g→
4 T1g
O2-
→ Mn2+
LMCT
Mn2+
tetrahedron
Mn2+
octahedron
8MnAl10w
Pink
535
490
250
6A1g→
4 T1g
6A1g→
4 T1g
O2-
→ Mn2+
LMCT
Mn2+
tetrahedron
Mn2+
octahedron
138 Chapter IV
Fig. 4.6. Diffuse reflectance spectra of (a) 8MnAl10w and (b) 8MnAl10
The acid washed samples 8MnAl9w and 8MnAl10w are pink in
colour and show no absorptions characteristic of Mn3+
ions but give peaks
around 535, 490 and 250nm (Fig. 4.6), the former two characteristic of the
6A1g →
4T1g forbidden d-d transition of Mn
2+ ions in tetrahedral and
octahedral coordination respectively23
and the latter peak corresponding to its
CT transition. This indicates that the Mn3+
ions in 8MnAl9 and 8MnAl10
exist in extractable form with conc. HCl and also suggests the formation of
MnAl2O4 in these samples. The colour characteristics of 8MnAl9 and
8MnAl9w are shown in Fig. 4.7. The HCl washed sample 4MnAl10w shows
peaks around 535, 490, 250 nm and also a shoulder around 710 nm, which
indicate the presence of Mn3+
ions (Fig. 4.8). The PXRD results revealed that
8MnAl9 and 8MnAl10 are biphasic and contain both α-Al2O3 and MnAl2O4
spinel, whereas 4MnAl10w contains transition aluminas also. This clearly
Structural evolution and surface characteristics … 139
suggests that in mixed oxides, Mn ions are present in the transition alumina
matrix and during crystallization of α-Al2O3 from transition aluminas the
dispersed Mn3+
ions get thrown out.
Fig.4.7. Colour characteristics of (a) 8MnAl9 and (b) 8MnAl9w
Fig. 4.8. Diffuse reflectance spectra of (a) 4MnAl10w and (b) 4MnAl10
4.3.4. EPR spectra
EPR study, because of its high sensitivity, is considered as a useful
and powerful tool for probing the local environment of some of the
paramagnetic dopant ions, even if the dopant is present in very low
140 Chapter IV
concentrations. Since the doped Mn2+
ions in the present study are seen
getting oxidized on calcination in air we have tried to monitor whether the
divalent Mn2+
ions are still present in any appreciable quantity after each
calcination process. The presence of Mn2+
is vital for the formation of the
MnAl2O4 spinel, which in fact is known to catalyze and facilitate the
formation of α-Al2O3 at considerably low temperature in our present study.
Fig. 4.9. EPR spectra of (a) 2MnAl6, (b) 4MnAl6 and (c) 8MnAl6
We have also tried to monitor the evolution of MnAl2O4 spinel and the
environment around the Mn2+
ions as a function of temperature with the help of
relevant spin-Hamiltonian parameters of the Mn2+
ions present after each
calcination step. It is observed that all the calcined samples, including the case of
Structural evolution and surface characteristics … 141
2% doped sample, are EPR active and give, in most cases, the characteristic 6-line
or broad spectra indicating the presence of Mn2+
ions in various coordination
atmospheres in these oxides on calcination at different temperatures.
Fig. 4.10. EPR spectra of (a) 2MnAl8, (b) 4MnAl8 and (c) 8MnAl8
Given in Figs. 4.9, 4.10 and 4.11 are the EPR spectra of 2, 4 and 8
mol.% samples calcined at 600, 800 and1000ºC respectively. The 2% sample
at all calcination temperatures studied (400-1000ºC), 4% sample up to a
calcination temperature of 800ºC and 8% sample up to 600ºC show a six-line
hyperfine pattern centered around g = 2.0, which is characteristic of Mn2+
ions having a nuclear spin I = 5/2 24-26
. These well-resolved hyperfine
structures indicate that Mn2+
ions are in more or less octahedral sites and are
isolated or significantly separated from each other so that there is no spin-
spin interaction.
142 Chapter IV
Among the various spin-Hamiltonian parameters, the hyperfine
constant A (which is a measure of the extent of electron spin interacting with
the Mn2+
ion nucleus with I = 5/2) is appreciably sensitive to the chemical
environment around the ion. This is based on the Fermi-contact interaction,
which is given by the equation,
A = (8π/3)gnμngeμe|ψ(0)|2 (1)
where gn and ge are the g-factors for nucleus and electron respectively, μn and
μe are the nuclear- and electron-Bohr magnetons and the |ψ(0)|2 is the
unpaired electron density felt at the Mn2+
nucleus27,28
.
Fig. 4.11. EPR spectra of (a) 2MnAl10, (b) 4MnAl10 and (c) 8MnAl10
Structural evolution and surface characteristics … 143
With this idea we have evaluated the hyperfine tensor A for
various samples calcined at different temperatures. The values are
tabulated along with other spin-Hamiltonian parameters in Table 4.4.
Given in Fig 4.10 are the EPR traces of the samples calcined at 1000ºC
which show a resolved spectra for 2% doped sample while the samples
containing higher percentage of Mn ions show considerable dipolar
broadening in the spectra29
. It is interesting to note that the A values
observed in our cases which are around the expected values for Mn2+
ion
in an octahedral environment of O atoms are progressively decreasing
with the increasing calcination temperatures. The trend is seen to be the
same for all the samples containing varying amounts of the dopant
concentrations. We could explain this trend based on the structural
evolution happening around the Mn2+
ion depending on the calcination
temperature. As evident from the Fermi-contact interaction defined by the
equation (1) the Mn hyperfine constant A should decrease when the
probability of the unpaired electron density at the nucleus decreases. We
find that in all the samples the A values are getting decreased when the
calcination temperature increase.
This clearly explains that as the temperature increase the O
environment around the Mn ions get more distinctly well defined forming
better MnO6 or MnO4 polyhedra with stronger bonding between Mn and O
atoms. Such a strong Mn-O bond would make the unpaired electron to get
more delocalized on to O atoms making the electron density less on the Mn
ion. The systematically decreasing trend seen in the A values on increasing
the calcination temperature suggest that the O environment around the
Mn2+
ions progressively leads to a better bond with the metal ion. This is in
144 Chapter IV
Table 4.4. EPR spin Hamiltonian parameters of Mn2+
ion doped xerogels
and calcined samples
Sample A (mT) g
2MnAlG 8.36 2.0145
2MnAl4 8.22 2.0210
2MnAl5 8.16 2.0228
2MnAl6 8.01 2.0231
2MnAl7 7.97 2.0252
2MnAl8 7.90 2.0258
2MnAl9 7.88 2.0276
2MnAl10 7.82 2.0284
4MnAlG 8.34 2.0182
4MnAl4 8.22 2.0215
4MnAl5 8.15 2.0230
4MnAl6 8.01 2.0235
4MnAl7 7.97 2.0255
4MnAl8 7.90 2.0279
8MnAlG 8.34 2.0188
8MnAl4 8.21 2.0221
8MnAl5 8.13 2.0232
8MnAl6 8.00 2.0234
Structural evolution and surface characteristics … 145
agreement with our FTIR and PXRD results. The trend observed in the g
values is also commensurate with the formation of stronger Mn-O bonds, the
g values increasing systematically with the increase in calcination
temperature (Table 4.4). As evident from the Table 4.4 the A values for the
2% doped sample the hf constant decreases from 8.36 mT for the xerogel
(2MnAlG) to 7.82 mT when the sample is calcined at 1000ºC (2MnAl10)
indicating considerable structural adjustment for forming well-defined
MnAl2O4 spinel.
4.3.5. Powder X-ray diffraction (PXRD)
We have carried out powder X-ray diffraction analysis to determine
the phase composition of calcined samples at various temperatures for both
undoped and Mn doped samples. The PXRD patterns of both Mn2+
doped
and undoped oxides generated by the calcination of the xerogels at 400ºC
show similar patterns with peaks characteristic of boehmite and γ-Al2O3. No
characteristic peaks corresponding to any of the crystalline manganese oxide
phases were observed at all the doping levels. This clearly shows that the
doped Mn2+
ions are in a highly dispersed amorphous state at this
temperature. On calcination above 400ºC the boehmite in all the samples
were seen to be getting converted into transition aluminas (γ, δ and θ).
146 Chapter IV
Fig. 4.12. PXRD patterns of Mn2+
ion doped boehmite calcined at 800ºC.
The samples are (a) Al8, (b) 2MnAl8, (c) 4MnAl8 and
(d) 8MnAl8. Peaks characteristics of transition aluminas (T) are
indicated
However, we do not see any effect of Mn2+
ion in all the doped
samples calcined at 500, 600, 700 and 800ºC, as they show the same PXRD
patterns similar to that of the corresponding undoped samples. Further, no
evidence is seen for the presence of any crystalline MnOx phases in any of
these samples. This indicates that the MnOx species exist as well dispersed
amorphous state at all these temperatures. The PXRD patterns of the mixed
oxides generated at 800ºC are given in Fig. 4.12. However, on calcination at
900ºC, the PXRD pattern of the sample doped with 8 mol.% Mn2+
(8MnAl9)
Structural evolution and surface characteristics … 147
showed significant difference from that of the undoped and also 2% and 4%
doped samples calcined at the same temperature. The natures of the PXRD
patterns of these samples are presented in Fig. 4.13.
Fig. 4.13. PXRD patterns of Mn2+
ion doped boehmite calcined at 900ºC.
The samples are (a) Al9, (b) 4MnAl9, (c) 8MnAl9 and
(d) 8MnAl9w. Peaks characteristics of transition aluminas (T),
α-Al2O3 (α) and MnAl2O4 (s) are indicated
The sample 8MnAl9 showed no peaks characteristic of any of the
transition aluminas but only of α-Al2O3 and MnAl2O4 spinel. However, the
undoped sample (Al9) and also the doped samples containing 2 and 4 mol.%
Mn2+
ions calcined at the same temperature (2MnAl9 and 4MnAl9) showed no
evidence for the presence of α-Al2O3 or the Mn spinel but only of the
transition aluminas. On calcination at 1000ºC, 8MnAl10 showed the presence
148 Chapter IV
of α-Al2O3 and MnAl2O4 spinel only (Fig. 4.14). In the case of 4% doped
sample 4MnAl10 the formation of α-Al2O3 is seen at this temperature but we
also find peaks due to transition aluminas also. There is indication of
formation of MnAl2O4 spinel also but these peaks are seen getting merged
with those of transition aluminas. In contrast, 2MnAl10 showed the presence
of only the transition aluminas and no indication is seen for the presence of α-
Al2O3 and Mn2+
spinel, MnAl2O4. As mentioned in the discussion on
electronic spectral data, we could observe the presence of Mn3+
in appreciable
quantity in all the doped samples calcined at all the temperatures. However,
we could not see the presence of any phase involving Mn3+
in their PXRDs.
Fig. 4.14. PXRD patterns of Mn2+
ion doped boehmite calcined at 1000ºC.
The samples are (a) Al10, (b) 4MnAl10, (c) 8MnAl10 and
(d) 8MnAl10w. Peaks characteristics of transition aluminas (T),
α-Al2O3 (α) and MnAl2O4 (s) are indicated
Structural evolution and surface characteristics … 149
It has been reported earlier that even higher level doping of
manganese acetate in γ-alumina (by impregnation) followed by calcination at
higher temperature did not show any phase characteristic of MnOx30-32
. This
has been attributed to the nano-level dispersion (~4nm) of MnOx species in
the Al2O3 matrix32
. In our sol-gel preparation technique in controlled
condition there is much better chance for finer dispersion of MnOx in Al2O3
and hence the absence of any MnOx specific peaks in PXRD is fully justified
in our case.
We have also calculated the particle size of the α-Al2O3 (Table 4.5)
formed on calcination of the Mn ion doped samples at 1000ºC using Scherrer
equation and it was observed that the particle size were about 30nm.
However, in the case of the Cu2+
ion doped samples calcined at the same
temperature, the particle size of α-Al2O3 formed were in the 23-24 nm range
(see chapter III). This shows that the doped metal ion has influence on the
particle size of the α-Al2O3 formed from metal ion doped boehmite on
calcination.
Table 4.5. Particle size of α-Al2O3 formed at 1000ºC from various metal
ions doped boehmite xerogels.
Sample Particle size of α-Al2O3 nm
4MnAl10 29.44
8MnAl10 30.28
4CuAl10 24.43
8CuAl10 23.04
150 Chapter IV
4.3.6. Leaching experiment
As the electronic spectra of all the calcined samples of the Mn doped
system suggest the formation of Mn3+
species but do not give peaks of any
phase characteristic of the trivalent metal ion in their PXRDs we attempted
reacting the samples with conc. HCl to get the leachable form of the metal
ion and estimate the extracted Mn ions. Given in Fig. 4.15 are the amounts of
Mn that get leached out on washing with HCl for xerogels calcined at various
temperatures. It is seen that in all the cases some amount of Mn ions get
retained in the alumina matrix even after reacting with conc. HCl for
sufficiently long time. In the case of 2% doped sample the leachability of Mn
ion is seen to increase uniformly with increase in calcination temperature.
The 4% doped samples also show similar trend till 900ºC but on calcination
at higher temperature significant increase in the leachability of Mn ion is
seen. For 8mol.% sample the leachability is seen to be comparatively
moderate till 800ºC but at higher temperatures very significant amount of Mn
ion is seen to be getting leached out. These observations could be explained
by taking into consideration the substantial oxidation of Mn2+
ion to Mn3+
state on calcination, the reported ability of amorphous transition aluminas to
trap both Mn2+
and Mn3+
in its matrix, the tendency of transition alumina to
combine with Mn2+
ion to form the MnAl2O4 spinel and also the seeding
capacity of the in situ generated spinel in facilitating the formation of α-
Al2O3. The samples, which contain 2%, Mn does not seem to generate any
spinel or facilitate the formation of α-Al2O3 even at 1000ºC; the entire
alumina is seen to exist only as transition aluminas at this condition. This is
essentially due to the conversion of majority of Mn2+
ions to the Mn3+
form,
which was observed from our UV-visible spectral measurements.
Structural evolution and surface characteristics … 151
Fig. 4.15. Amount of leachable Mn at various calcination temperatures
In the case of 4% doped sample calcined at 1000ºC (4MnAl10), which
shows the presence of MnAl2O4 spinel, α-Al2O3 and traces of transition
aluminas show substantial leachabilty on reacting with conc. HCl. This could be
attributed to the solid state reaction involving the Mn2+
ion present in the system
with the transition aluminas which leads to the spinel (MnO + Al2O3 →
MnAl2O4), the seeding/nucleation effect of MnAl2O4 which converts the
transition aluminas to the highly crystallized α-Al2O3 and to the forced
expulsion (due to the phase incompatibility) of the dispersed Mn3+
and its easier
removal from the bulk matrix by conc. HCl. For the 8% doped samples
calcined at 900 and 1000ºC (8MnAl9 and 8MnAl10) we find complete reaction
of the entire Mn2+
ions present in the system to its MnAl2O4 spinel and
152 Chapter IV
quantitative conversion of all the transition aluminas into α-Al2O3. The
substantial leachability (of the dispersed Mn ions) observed for these samples
(Fig .4.15) agrees well with the factors mentioned above.
In Chapter III we have discussed in detail our studies on doping
effect of Cu2+
in alumina on structural evolution of CuAl2O4 spinel and α-
Al2O3. We could see the predominant formation of the spinel and establish
its seeding effect on the early formation of α-Al2O3. Quantitative conversion
of Cu2+
to its spinel CuAl2O4 is seen to occur for 4 and 8 mol.% dopant
concentrations at the calcination temperature of 1000ºC for the system. We
also find that 2% Cu2+
doped sample leads to the formation of traces of
CuAl2O4 spinel with concomitant conversion of the transition aluminas into
α-Al2O3 at 1000ºC. Significant differences are seen in Mn doped system both
in terms of the spinel formation and α-Al2O3 generation compared to Cu2+
ion doped alumina system. We find that even traces of Mn2+
in the system
can form MnAl2O4 spinel at comparatively low temperature (900ºC) and
the in situ generated MnAl2O4 spinel is found to be significantly facilitating
the formation of α-Al2O3 at very low temperature by nucleation (from
1200ºC in the undoped xerogel to around 900ºC in the Mn doped system),
which is again much better than the CuAl2O4 facilitation at 1000ºC.
4.3.7. Surface acidity
Since solid acid catalysts are used extensively in the chemical
industry, measuring the acidity of solids is of great interest and the methods
providing information on the number, type and strength of the acid sites are
of great importance for understanding their catalytic reactivity and behaviors.
The methods based on the adsorption of Lewis bases have been the most
widely used for the determination of surface acidities. We have investigated
Structural evolution and surface characteristics … 153
the surface acidities of the Mn2+
ion doped alumina mixed oxides by (a)
TGA method and (b) elemental analysis. In all method pyridine was used as
the probe molecule since it is a strong and stable Lewis base and a versatile
probe molecule to analyze the surface acidity of oxide systems. The samples
calcined at 900˚C and above are not included in the surface acidity studies
since the PXRD results show that they contain varying proportions of α-
Al2O3, which has very low surface acidity.
Fig 4.16. (a) TGA and (b) DTA curves of pyridine saturated 8MnAl5
In TGA method we have quantified the total surface acidities and
acid site distribution of Mn2+
ion doped alumina mixed oxides which is
based on the measurement of weight change as a function of temperature and
it has proven to be a simple and effective method for the investigation of
surface acidity of oxide materials. A typical TGA/DTA curve of a sample
that is exposed to pyridine is given in Fig. 4.16. The DTA shows three
endothermic peaks, at 55, 260 and 450ºC, which are associated with,
154 Chapter IV
desorption of pyridine from sites with different acid strengths. The data are
in given in Table 4.6.
Table 4.6. Surface acidity data of undoped and Mn2+
ion doped alumina
mixed oxides
Sample
Surface
acidity
mmol.g-1
Sample
Surface
acidity
mmol.g-1
Sample
Surface
acidity
mmol.g-1
Sample
Surface
acidity
mmol.g-1
Al4 0.4678 2MnAl4 0.5148 4MnAl4 0.5843 8MnAl4 0.6582
Al5 0.5436 2MnAl5 0.6232 4MnAl5 0.7182 8MnAl5 0.8356
Al6 0.5417 2MnAl6 0.6018 4MnAl6 0.6926 8MnAl6 0.7998
Al7 0.4528 2MnAl7 0.5806 4MnAl7 0.6274 8MnAl7 0.7228
Al8 0.4163 2MnAl8 0.4812 4MnAl8 0.5027 8MnAl8 0.5395
The surface acidity of mixed oxides has been generally considered to
result from an excess of negative or positive charges caused by the formation
of bridged hetero metal oxygen bonds (M-O-M’). In a mixed oxide
containing M and M’ metals, the degree of M-O-M’ bond formation depends
on the homogeneity of the mixed oxide. The mixed oxides derived from sol-
gel processing are more homogeneous than other conventional methods and
therefore they are more acidic. It is found that the surface acidities of Mn2+
ion doped alumina mixed oxides at all calcination temperatures studied
increases as the level of doping of Mn2+
ions increases (Fig.4.17). This is
attributed to the fact that the doped Mn2+
ions produce Mn-O-Al linkages,
which increases the surface acidity. As the concentration of Mn2+
ions
increases the number of hetero linkages increases which increases the surface
acidity. This is in accordance with the PXRD results which show that the
doped Mn2+
ions exist in a highly dispersed amorphous state.
Structural evolution and surface characteristics … 155
Fig. 4.17. Surface acidities of undoped and Mn2+
ion doped alumina at
various doping levels and calcination temperatures.
Total surface acidities of selected samples of Mn2+
ion doped alumina
mixed oxides were also investigated by elemental analysis of the samples
exposed to pyridine. The total surface acidity values were calculated from
the percentage of carbon present in the samples exposed to pyridine and the
data are given in Table 4.7. Thus the surface acidity data show that the sol-
gel incorporation of Mn2+
ions into alumina matrix produces an increase in
surface acidity and as the level of doping increases the surface acidity also
increases. The surface acidity of the sample containing 8 mol.% Mn2+
ions
and calcined at 500˚C (8MnAl5) is about 54% higher than that of the
undoped sample calcined at the same temperature.
156 Chapter IV
Table 4.7. The elemental (C,N) analysis of pyridine sorbed samples and their
surface acidity data
Sample C (%) N (%) Mass of
pyridine mgg-1
Surface acidity
Mmol.g-1
Al5 3.017 0.723 41.38 0.5239
2MnAl5 3.457 0.827 47.69 0.6037
4MnAl5 3.972 0.942 55.19 0.6986
8MnAl4 3.843 0.916 53.30 0.6747
8MnAl5 4.778 1.214 67.19 0.8506
8MnAl6 4.264 1.104 59.55 0.7538
8MnAl7 4.164 0.991 58.02 0.7344
8MnAl8 3.131 0.751 43.01 0.5444
4.3.8. Surface area
To study the influence of Mn loading on the specific surface area of
mixed oxides we have measured the specific area of all the samples
generated at 500ºC. The selection of calcination temperature (500ºC) is
based on the fact that at all doping levels the samples calcined at this
temperature had shown maximum surface acidity. The observed surface
areas are 235.15 (Al5), 241.38 (2MnAl5), 246.24 (4MnAl5) and 248.68m2g
-1
(8MnAl5). The Fig. 4.18 shows the variation of the specific surface area with
level of doping of Mn. It is observed that the specific surface area increases
with level of doping of Mn and may be due to the presence of highly
dispersed MnOx species in these mixed oxides as revealed from the PXRD
measurements.
Structural evolution and surface characteristics … 157
Fig. 4.18. Surface area of Mn2+
doped alumina at various doping levels at
500ºC.
4.3.9. Scanning electron microscopy (SEM)
Scanning electron microscopy was used to investigate the morphological
changes produced by the doped Mn in the samples calcined at various
temperatures. The SEM micrographs of all the samples, which contain transition
aluminas, are characteristic of amorphous solids. However, the samples, which
contain appreciable amount of α-Al2O3, show clear difference in morphology.
Scanning electron micrographs of undoped, 4 and 8% Mn doped boehmite
calcined at 900ºC are shown in Fig. 4.19. The SEM micrographs of Al9 and
4MnAl9 are almost identical and characteristic of amorphous solids, indicating
that the doping 4% Mn in alumina matrix at this temperature has no influence on
the morphology of the resulting mixed oxide. However, the sample 8MnAl9
shows appreciable morphological changes from Al9 and 4MnAl9, indicating the
strong structural changes taking place in the sample in this temperature regime.
The PXRDs also clearly revealed that the sample 8MnAl9 is biphasic and
contain thermodynamically stable and structurally compact α-Al2O3 and
158 Chapter IV
MnAl2O4 spinel, while the samples Al9 and 4MnAl9 consist of amorphous
transition aluminas. The particle coarsening, a well-known phenomenon
associated with the structural transformation from transition aluminas to α-
Al2O3 is clearly visible in the SEM images.
Fig. 4.19. SEM images of (A) Al9, (B) 4MnAl9 and (C) 8MnAl9
Structural evolution and surface characteristics … 159
4.4. Conclusion
The structural and surface characteristics of Mn2+
ion doped sol-gel
derived alumina from boehmite were studied at various calcination
temperatures and dopant concentrations. The chemical transformations were
monitored at various levels using TGA, FTIR, diffuse reflectance and EPR
spectra and also through detailed PXRD measurements. Significant
differences are seen in Mn doped system both in terms of the structural
evolution of α-Al2O3 and surface characteristics such as surface acidity and
surface area compared to undoped alumina. While the α-Al2O3 formation
happens only around 1200ºC, in undoped alumina, Mn ion doping is seen to
facilitate the formation of this stable phase as early as 900ºC. It is seen that
the in situ generated MnAl2O4 spinel which also gets formed early at this
temperature nucleates and facilitates the formation of α-Al2O3 quantitatively.
Even though there is substantial oxidation of doped Mn2+
ion to Mn3+
, we
find that all the calcined samples are EPR active indicating the presence of
Mn2+
ion in these samples, even traces of Mn2+
in the system, can form
MnAl2O4 spinel at comparatively low temperature which nucleates and
facilitates the very early formation of α-Al2O3 (at 900ºC) from the system.
While the residual Mn2+
ions present get converted to MnAl2O4 spinel at
higher calcinations temperatures, the oxidized Mn3+
ion present in substantial
quantity is seen to be existing in highly dispersed and impregnated form
without forming any crystalline MnOx phase. The existence some amount of
non-leachable Mn (both Mn2+
and Mn3+
) in the amorphous alumina matrix
indicate the strong binding achieved between the matrix and dopant Mn in
the sol-gel derived samples. The surface acidity data show that the sol-gel
incorporation of Mn2+
ions into alumina matrix produces an increase in
surface acidity and as the level of doping increases the surface acidity also
160 Chapter IV
increases. The surface acidity of the sample containing 8 mol.% Mn2+
ions
and calcined at 500˚C (8MnAl5) is about 54% higher than that of the
undoped sample calcined at the same temperature. It was also observed that
the incorporation of Mn into alumina matrix produces an increase in specific
surface area at relatively lower calcination temperatures.
Structural evolution and surface characteristics … 161
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