Structural evolution and surface...

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.


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


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


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


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-


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


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


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


8MnAl10 Dark

brown

715

500b

360

250

5B1g → 5A1g 5B1g → 5B2g 5B1g → 5E g

O2- → Mn3+ LMCT

O2- → Mn2+ LMCT



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.


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


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


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.


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

.



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


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)


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.




(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.




(d) 8MnAl10w. Peaks characteristics of transition aluminas (T),

α-Al2O3 (α) and MnAl2O4 (s) are indicated


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.


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


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.


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.


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


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.


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