Chapter -3 Experimental - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/27786/8/08_chapter_3.pdf · Chapter-3 Experimental 55 was dissolved in 139ml 0.5 M HCl at 40°C

Chapter -3 Experimental

Chapter-3 Experimental

53

3.1. Introduction

This chapter illustrates the methodologies adopted for the preparation,

characterization, and activity studies of different catalysts. The basic principles and

experimental procedure details of different characterization techniques such as, N2

adsorption (BET surface area), X–ray diffraction (XRD), Temperature Programmed

Reduction (TPR), Fourier Transform-Infrared Spectroscopy (FT-IR), Pyridine adsorbed

FT-IR, Temperature Programmed Desorption of Ammonia (TPDA), X-ray photo electron

spectroscopy(XPS), Scanning Electron microscopy (SEM), Transmission Electron

microscopy (TEM), H2 pulse chemisorption used to characterize the catalysts have been

explained.

3.2. Reagents and catalysts

All chemicals and solvents used were of A.R Grade (99.9 %). Nickel nitrate, and

magnesium nitrate were supplied by M/s. S. D. Fine-Chem Ltd. The supports, TiO2, ZrO2,

ZnO, MgO (M/s. S. D. Fine-Chem Ltd, India), Al2O3 (M/s. Sud Chemie, India), SiO2 (M/s.

Aldrich Chemicals, USA) and zeolite such as HZSM-5 (M/s. Sud-Chemie India Pvt. Ltd.)

were obtained from commercial sources. Mesoporous silica’s (SBA-15, KIT-6, COK-12

and SBA-16) were prepared in the laboratory itself following the methods reported in the

literature.

3.2.1. Preparation of mesoporous supports

3.2.1.1. Preparation of SBA-15

SBA-15 has been synthesized in accordance with the procedure described elsewhere

[1]. In a typical experiment, 20 g of triblock copolymer (P 123, M/s. Aldrich Chemicals

,USA) was dispersed in a mixture of 465 g of distilled water and 137.5 g of 35%

hydrochloric acid (M/s. Loba Chemie, India). Then 44g of tetraethyl orthosilicate (TEOS,

M/s. Aldrich Chemicals ,USA) was added under constant stirring at 40 °C and the mixture

was subjected to the hydrothermal treatment at 100 °C for 24 h. The resultant slurry was

filtered, dried in air at 110 °C for 12 h and then calcined in air at 500 °C for 8 h.


54

3.2.1.2. Preparation of KIT-6

KIT-6 mesoporous silica was synthesized following the procedures available in the

literature [2]. In a typical synthesis batch, a homogeneous solution was obtained by

dissolving 32.0 g of P123 and 63.0 g of 35.0 wt% hydrochloric acid in 1150 g of deionised

water under stirring for 8 hr at 35 °C. 32 g of 1-butanol was added to this solution. After

stirring for 1 hr at 35 °C, 68.8 g of TEOS was added at once to the P123-butanol-HCl

solution. This mixture was continuously stirred at 35°C for 24hr, to get the KIT-6 silica

phase. The mixture containing KIT-6 was then heated to 130°C for 24hr, under static

conditions in a Teflon-lined autoclave. This temperature aging process was applied to

enlarge the silica pores. The KIT-6 product after 24hr was filtered, and dried at 100 °C in

air without washing. The product was washed with an HCl ethanol mixture to extract as

much P123 as possible, and was subsequently calcined at 550°C for 5hr in air.

3.2.1.3. Preparation of COK-12

The ordered mesoporous SiO2 such as COK-12 has been prepared by self assembly

method using long chain ionic surfactant i.e., P123 as template and sodium silicate as SiO2

source as reported in the literature [3] and was used as support for Ni catalysts. In a typical

synthesis, 4g of the triblock copolymer pluronic P123 (M/s. Sigma Aldirich Chemicals,

USA) was dissolved in 107.5ml of water. To this solution, 3.684g citric acid monohydrate

(M/s. S. D. Fine-Chem Ltd, India) and 2.54g trisodium citrate (M/s. S. D. Fine-Chem Ltd,

India) were added. The resulting surfactant solution was stirred for 24hr. 10.4g sodium

silicate solution (10% NaOH,27% SiO2, M/s. Merk, Germeny) was diluted with 30g of H2O

and added to the surfactant solution. The PH was measured prior to and after sodium silicate

addition. The solution was stirred for 5min at 175rpm with a mechanical stirrer and kept at

room temperature (20 ºC) without agitation for 24hr. The as-synthesized material was

filtered, washed and dried at 60ºC in air for 12h. Finally the material was calcined in air in

two steps, 8h at 300ºC and 8h at 500ºC with 1ºC min-1 ramping.

3.2.1.4. Preparation of SBA-16

SBA-16 was prepared by using a triblock copolymer surfactant (EO106PO70EO106,

F127) as a structure directing agent, tetraethylorthosilicate (TEOS) as silica source at low

acid concentration without using NaCl salt [4]. In a typical synthesis batch, 3.72g of F127


55

was dissolved in 139ml 0.5 M HCl at 40°C. After 2-3 hrs stirring 18 ml TEOS was added

drop wise. The following molar composition 1 TEOS: 0.00367 F127: 0.864 HCl: 100.231

H2O was stirred for 20 hr at 40°C, and subsequently transferred to Teflon bottle and aged at

autogeneous pressure for 24 hr at 100°C. The material was filtered washed with water and

dried at 80°C in air, and then calcined at 550°C for 4 h in air with a heating rate of 3°C per

min.

3.3. Preparation of catalysts (General Description)

The catalysts used in this thesis work were prepared by adopting two methods

namely impregnation, reductive deposition method. The salient features of these methods

are described below.

3.3.1. Impregnation

In this procedure a certain volume of aqueous solution containing known amount of

the metal (eg. nickel) precursor of the active phase is contacted with a previously weighed

solid support. The amount of metal ions that interactively remains bound to the support

surface depends on the adsorption capacity of the support, the period of impregnation and

temperature of impregnation. Two methods of contacting may be distinguished depending

on the volume of solution, namely wet impregnation and incipient wetness impregnation. In

wet impregnation an excess of solution is used. After a certain time the solid is separated

and the excess of solvent is removed by drying. In the incipient wetness impregnation the

volume of solution of appropriate concentration is equal to or slightly higher than the pore

volume of the support.

3.3.2. Reductive Deposition method

In this method, a support (like that used in impregnation); the active component and

additional reducing agent like hydrazine are used. After dissolving the metal salt in water

and followed by addition of base (NaOH) to this solution,hydrazine solution added. The

added base facilitates the reducing agent to decompose into N2 and H2. The evolved H2

pramotes the reduction of Ni+2 to Ni0.


56

3.4. Methodology of catalyst preparation adopted in the present work

In the present work two different methods of preparation have been adopted to

prepare supported Nickel catalysts.

3.4.1. Preparation of supported nickel catalysts by impregnation

ZrO2, TiO2, ZnO, HZSM-5, MgO, Al2O3, SiO2, SBA-15 , KIT-6, COK-12 and

SBA-16 supported Ni catalysts were prepared by conventional impregnation (IM) method

with variable amounts of ( 5-50 wt% ) Ni with respect to the support. In the conventional

IM method, calculated amounts of aqueous metal precursor solution was added to supports

and after allowing overnight adsorption, the excess solution was first evaporated to near

dryness on a water bath and then the partially dried material was dried in air oven at 120 °C

for 12 h. The catalysts were reduced in H2 flow at 500 °C for 4 h before its use for

nitrobenzene and levulinic acid hydrogenation reactions. The numerical numbers in the

catalyst code indicate the weight percentage of Ni on the support.

After impregnation, the catalyst mass is normally subjected to the following treatments

(i) Drying and (ii) Calcination

3.4.2. Drying

Drying involves elimination of solvent (usually water) from the pores of a solid.

This is a routine procedure for crystalline solids but becomes critical for flocculates and

even more so for hydrogels that contains large amounts of water. In these cases the removal

of water can result in a collapse of the texture and therefore drying has to be properly

controlled if high porosity is desired. For supports with relatively higher adsorption

capacity (high porosity), the condition of drying does not affect the uniform dispersion of

the active component. For carriers with a low adsorption capacity (low porosity) these

parameters influence the texture as well as the properties of the resulting catalysts. The rate

of evaporation should be slow and reversible to allow even redistribution of the active

components on the surface of support.


57

3.4.3. Calcination

Calcination means heating without the formation of a liquid phase and is a further

heat treatment beyond drying. It is carried out in air at temperatures higher than those used

in the catalytic reaction. In the process of calcination, several chemical and physical

transformations such as, decomposition of the impregnated metal salt into its oxide,

interaction between active component and support, sintering of the support and

condensation of hydroxyl groups of the support take place. During calcination, the catalyst

also solidifies into a final form, for example, amorphous into crystalline, therefore the

surface and mechanical properties of the catalyst are derived mainly in this process.

3.4.4. Preparation supported nickel catalysts by reductive Deposition method

In the reductive Deposition method the support (ZrO2, an TiO2) was first dispersed

into aqueous solution of Ni(NO3)2. 6H2O containing 5wt % Ni with respect to the support.

1 M NaOH was slowly added to the solution containing the Nickel precursor and the

support toget a PH value of the mixture to 10. Then N2H4 solution was added. The

suspension was then maintained at the same pH for 1h. The resultant solid was filtered and

washed with de-ionized water several times until no sodium ion was detected. The solid

thus obtained was oven dried at 120 °C for 12 h. The catalysts were reduced in H2 flow at

500 °C for 4 h before its use for hydrogenation reaction. The numerical numbers in the

catalyst code indicate the weight percentage of Ni on the support.

3.5. Catalyst Characterization

In the present investigation the following spectroscopic and non-spectroscopic

techniques were employed to characterize various composite oxide catalysts:

1. N2 sorption analysis

2. X-ray Diffraction Studies (XRD)

3. Temperature Programmed Reduction (TPR)

4. Temperature Programmed Desorption of Ammonia (TPDA)

5. Scanning Electron Microscopy (SEM)

6. Transmission Electron Microscopy (TEM)

7. X-ray Photoelectron Spectroscopy (XPS)


58

8. Chemisorption (H2 or CO)

9. CHNS analysis

Characterization of the catalysts is an important aspect in order to gain information

about molecular nature and structure of the active component which helps in optimizing

industrial catalytic processes. No single technique is available to get the complete

information about the surface structure of the active component. Supported nickel catalysts

are often used for investigation of fundamental properties of catalysts. In many cases,

however, the catalysts utilized in chemical industry consist of two or more components. It

is therefore, of practical as well as theoretical interest to study the influence of catalysts

characteristics of conventional as well as recently trend materials like mesoporous silica

supports etc. With this background the supported nickel catalysts have been prepared on

various supports and characterized by several techniques described in this chapter.

3.5.1. N2 sorption analysis

N2 gas sorption is a technique used for characterizing the catalyst materials. By

means of this method, material’s specific surface area, pore volume and pore size

distribution can be determined.

3.5.1.1. Physisorption isotherms

The physisorption data is presented in sorption isotherms with the amount of gas

adsorbed on the solid on Y-axis plotted versus the relative pressure on X-axis. The

isotherms can be classified into six types [5] which are shown in Fig.3.1.

Type I isotherms:

This type is characteristic for microporous materials. In micropores there is an increased

adsorbent-adsorbate interaction. The nearly horizontal plateau is reached at low relative

pressure which indicates a small external surface area.

Type II isotherms:

These isotherms are typical for non-porous, microporous and macroporous

materials. Here there is a monolayer-multilayer adsorption on an open and stable surface.

The knee-point at B indicates the completion of monolayer adsorption and the beginning of

multilayer adsorption. B indicates the material’s monolayer capacity which is defined as the


59

amount of adsorbate required to cover the unit mass of solid surface with completely

packed single layer of adsorbate molecules.

Figure 3.1.The six main types of adsorption isotherms according to IUPAC classification

[5].

Type III isotherms:

These isotherms are very uncommon and are characteristic for materials with very

weak adsorbate-adsorbent interactions.

Type IV isotherms:

This type of isotherm is typical for porous materials. At the beginning, the isotherm

is similar to the type II isotherms. The knee-point at B indicates here, as well as for type II

isotherms, the monolayer capacity of the material. At higher pressures, there is a hysteresis

loop (for different types of hysteresis loops see Fig.3.2), which is characteristic for type IV

isotherms.

Type V isotherms:


60

The type V isotherms are distinctive for porous materials with weak adsorbate-

adsorbent interactions. Initially they are similar to the type III isotherms but at higher

pressures there is a hysteresis loop.

Type VI isotherms:

These isotherms are due to layer-by-layer adsorption on a non-highly uniform

surface. The steps are formed by separate layers adsorbing onto each other.

Figure 3.2. The four main types of hysteresis loops according to IUPAC classification [5].

The four main types of hysteresis loops are divided according to IUPAC

classification [5]. The four types of hysteresis loops, seen in Fig.3.2, indicate how the pores

are formed and if there are any inclusions or plugs in them.


61

H1:

This type of hysteresis loop has steep parallel adsorption and desorption isotherms.

At these steps all pores are filled (adsorption) and emptied (desorption). It is typical for

mesoporous materials with uniform pores.

H2:

The H2 hysteresis loop has a smoother adsorption step and a sharp desorption step.

It is typical for materials with non-uniform pore shapes and/or sizes, e.g. silica gel or other

metal oxides.

H3:

This type of hysteresis loop is associated with slit-shaped pores. These often rise

from agglomerates of plate-like particles.

H4:

The H4 hysteresis loop is similar to H3 but has a more horizontal plateau which

indicates the presence of microporosity in the material.

3.5.1.2. Specific surface area determination, the BET method

The most frequently used procedure to determine the surface area of a porous

material is the Brunauer-Emmet-Teller (BET) method [6]. The method can be regarded as

the extension of the Langmuir theory [7] with multilayer corrections. It is assumed that

1. the adsorbent surface is uniform and all adsorption sites are equivalent

2. adsorbed molecules do not interact

3. all adsorption occurs through the same mechanism

4. at the maximum adsorption, only a monolayer is formed: molecules of adsorbate

do not deposit on other adsorbed molecules of adsorbate, only the free surface of

the adsorbent.

To calculate the BET surface area the monolayer capacity, nm, of the material is

determined from the BET-plot. This is the best linear fits of the adsorption isotherms that

includes the B point, see Fig.3.1, and is derived by the linear BET equation.


62

P n(P − P) =

1n C +

C − 1n C

PP (3.1)

where P/P0 is the relative pressure, n the amount adsorbed and nm , the maximum amount

adsorbed in a monolayer i.e. the monolayer capacity and C, a system dependent constant.

A sharp point, B is indicative of a high value of C and thereby a high adsorbent-adsorbate

interaction.

The BET specific surface area is then calculated by

a =Am =

n N σm (3.2)

where aS is the specific surface area, AS the total surface area, m the mass of the sample, NA,

Avogadro’s number and σ, the molecular cross-sectional area occupied by the adsorbate

molecule in the complete monolayer [5, 8]. If nm is in cm3 at STP then it is necessary to

convert it into number of moles. This can be done by dividing nm with molar volume i.e.,

22414 cm3/mole.

3.5.1.3. Micropore volume and external surface area

Each adsorbate-adsorbent system yields a unique isotherm due to variations in the

interaction between the species. Therefore each system needs a standard isotherm to

estimate the micropore volume, internal and external surface area. The standard isotherm

can also be used as a reference for adsorbed layer thickness. This isotherm is measured for

a nonporous sample of the same material as the specimen analyzed. For silica, standard

nitrogen adsorption data for LiChrospher Si-4000 silica is available for the P/P0 = 5.55-

0.988 [9] as a standard isotherm.

In this work, the micropore volume was estimated by using a t-plot [10], see

(Fig.3.3). The thickness of the adsorbed layer is determined by

t = tn

n (3.3)

where tm is the thickness of a monolayer, for nitrogen tm =3.54 A. In order to relate the layer

thickness to the relative pressure, several methods can be used, e.g. the Halsey [11],

Harkins and Jura [12] or Broekhoff-de Boer methods, or the reference isotherm.

When using the KJS method, the Harkins-Jura equation


63

t = 0.1 .

.

.

(3.4)

is used to determine the thickness.

To determine the micropore volume and external surface area, the volume adsorbed

is plotted against the t. The micropore volume is found as the intercept of the extrapolated

first linear region of the t-plot and the y-axis and the external surface area (the surface area

from meso- and macropores and the true external surface) is determined as the slope of the

second linear region, see Fig.3.3.

Figure 3.3.t-plot for mesoporous silica.

3.5.1.4. Mesopore size analysis

There are several methods, such as the [13], BdB [14-17] or KJS [18, 19] methods,

alternatively NLDFT [20-22], used to determine the pore size distribution (PSD) from

nitrogen sorption isotherms. Different methods are suitable for different pore shapes and

sizes. In this work, only the BJH -method has been used.

The main principles for calculating pore sizes are based on the concept of capillary

condensation which is governed by the Kelvin equation

lnPP = −

2γVRT

cosθr (3.5)


64

where γ is the surface tension of the adsorptive liquid, VL the molar volume of the liquid, θ

the contact angle between the solid and the condensed phase and rK the mean radius of the

liquid meniscus. When a critical pressure is reached, the adsorptive will condensate in the

pores. Hence, the pore radius will determine if condensation can occur at a given pressure

as illustrated in Fig.3.4. This is seen as the hysteresis loop from the physisorption data.

Figure 3.4. Capillary condensation at a given pressure is determined by the pore radius

.The pore size is given by 2 (rk+t)

3.5.1.5. The BJH method

The commonly employed method is based on the Barrett-Joyner-Halenda (BJH)

method. Here, it is assumed that all pores have a cylindrical shape so that the simple Kelvin

equation (eqn. (3.6)) is applicable, the meniscus is hemispherical with θ=0 and that the

correction for multilayers is valid.

For capillary condensation in cylindrical pores, the Kelvin radius as a function of

relative pressure can be written as

rPP =

2γV

RTln PP

(3.6)

The pore size, rP, is then obtained by adding adsorbed layer thickness, t, to rK [9], so the

pore width is

r = 2 r + t (3.7)

For each step in the isotherm, the difference in amount of adsorptive represents the core

volume filled or emptied in that step. The thickness of the adsorbed layer remaining on the


65

pore walls is calculated with some method, this will be discussed in following paragraph.

Using eqns. (3.6) and (3.7) the pore size can now be calculated

To decide the amount of pores with this size, the shape of the pores is assumed to be

homogenous for all pores, e.g. cylindrical. Using the difference in core volume and the

volume of a cylinder with the radius rP, the total length of pores with this radius can be

calculated. From this, the area of these pores can be calculated. By performing these

calculations for all steps in the isotherms, the total PSD can be obtained.

3.5.1.6. The KJS method

This method was developed in the late 90’s to improve the PSD from the BJH method. It

is based on the BJH method and uses the Kelvin equation and Harkins-Jura thickness

equation. Furthermore, the sum in eqn. (3.7) underestimates the pore size of ≤ 0.3 nm.

Hence, the final expression for the pore size [nm] is found out according to the KJS-method

rPP =

2γV

RTln PP

+ 0.160.65

0.03071− log PP

.

+ 0.3 (3.8)

The method was first developed and calibrated for MCM-41 with maximum 6.5 nm pores.

This led to an overestimation of 1-2 nm in pore size when measuring on SBA-15 [23] and

therefore a correction for larger pore sizes was made to the method [19]. The pore size is

now calculated as

rPP =

1.15

log 0.875 PP

+ 0.260.65

0.03− log PP

.

+ 0.27 (3.9)

3.5.1.7. Porosity

The porosity of a material is determined by the total pore volume divided with the

volume of the material. The total pore volume, vp, is taken as the liquid volume adsorbed at

a given pressure e.g. P/P0 = 0.99. Since the amount adsorbed by the material when P/P0 →

1 depends on the magnitude of the external area and the upper limit of the pore size


66

distribution this method is not always yielding satisfactory results [24]. In the case of a type

IV isotherm there is often a horizontal plateau after filling of the mesopores. It is generally

assumed that the amount adsorbed at this plateau is a measure of the adsorption capacity.

3.5.2. Powder X-ray diffraction analysis

X-ray diffraction is one of the most widely used and versatile techniques for the

qualitative and quantitative analysis of solid phases and can provide the information about

the crystallite size of specific components. It can also be used to identify the structure of

substance, its allotropic transformation, transition to different phases, purity of the

substance, lattice constants and presence of foreign atoms in the crystal lattice of an active

component.

The wavelengths of X-rays are of the same order of magnitude as the distances

between atoms or ions in a molecule or crystal (A, 10-10 m). A crystal diffracts an X-ray

beam passing through it to produce beams at specific angles depending on the X-ray

wavelength, the crystal orientation, and the structure of the crystal. X-rays are

predominantly diffracted by electron density and analysis of the diffraction angles produces

an electron density map of the crystal. Powders of crystalline materials diffract X-rays. A

beam of X-rays passing through a sample of randomly oriented micro crystals produces a

pattern of rings on a distant screen. Powder X-ray diffraction is useful for confirming the

identity of a solid material and determining crystallinity and phase purity.

Modern powder X-ray diffractometers consist of an X-ray source, a movable sample

platform, an X-ray detector, and associated computer-controlled electronics. The sample is

either packed into a shallow cup-shaped holder or deposited as slurry onto a quartz

substrate, and the sample holder spins slowly during the experiment to reduce sample

heating. X-ray tubes generate X-rays by bombarding a metal target with high-energy (10 -

100 KeV) electrons that knock out core electrons. An electron in an outer shell fills the hole

in the inner shell and emits X-ray photon. Two common targets are Mo and Cu, which have

strong K(alpha) X-ray emission at 0.71073 and 1.5418 A, respectively. X-rays can also be

generated by decelerating electrons in a target or a synchrotron ring. These sources produce

a continuous spectrum of X-rays and require a crystal monochromatic to select a single

wavelength. The X-ray beam is fixed and the sample platform rotates with respect to the


67

beam by an angle theta. The detector rotates at twice the rate of the sample and is at an

angle of 2 theta with respect to the incoming X-ray beam.

The inter planar distances or d-spacing are calculated from the values of the peaks

observed from the Bragg’s equation.

n = 2dSin (3.10)

Where n = order of reflection and the values are 1, 2, 3 etc.

Fig. 3.5 Diffraction from an ordered arrangement of atoms

With the d-values tabulated in decreasing order and the relative intensities recorded

on a scale of 100 for the strongest line, the identification of the diffracting phases in the

sample can be made. Designating the XRD peaks as the strongest, 2nd strongest, 3rd

strongest etc. d-values of the pattern are termed as d1, d2, d3 etc.

The steps generally followed to identify a single compound or a component of a mixture

is as follows:

i) The proper Hanawalt group for d1 is located in the numerical index.

ii) The values of d2 and d3 are next sought within the group.


68

iii) Then the match of d-values is found for d1, d2 and d3 and relative intensities are

compared.

iv) Agreement of the intensities as well as the d-values suggests the identity of phase and

confirmation is obtained by reference to the compounds data from ICDD cards. The

method is restricted to crystallites larger than 40A, which is the limit of X-ray

diffraction. XRD patterns of all the catalysts in this study were obtained on a Ultima-

IV Rigaku Miniflex X-ray diffractometer (M/S. Rigaku Corporation, Japan) using Ni

filtered Cu K radiation.

The XRD patterns of the catalysts in Calcined , reduced an used form were recorded

at a 2θ scan speed of 2ο/min. The crystallite size of Ni was calculated by X-ray line

broadening (XLB) method on the same instrument.

3.5.3. Temperature Programmed Reduction (TPR) studies

Temperature programmed reduction (TPR) is a versatile technique which provides

information about the reducibility of solids like oxides, chlorides, fluorides and carbides

etc. This technique is widely used to characterize the metal/metal oxide catalysts etc. The

extent of reduction of solid by a gas preferably H2 against the rise in at the same time the

temperature of the system (reactor) at a pre-programmed rate is monitored and the chemical

information is derived from a record of the analysis of the gaseous products. Generally

solid is reduced by flowing hydrogen gas diluted with inert gas (Ar/He/N2), the change in

concentration of hydrogen is monitored down stream of the reactor. The analysis record is

simply the change in hydrogen concentration as function of temperature of the reactor/

catalyst bed.

The following table gives the thermal conductivity values (relative to air) of some

important gases normally used in the TPR experiments [25].


69

Name Conductivity relative to Air

Air 1.00

Ammonia 0.92

Argon 0.68

Ethane 0.79

Helium 5.84

Hydrogen 7.07

Methane 1.29

Butane 0.68

TPR experiments provide very useful information to decide the proper reduction

conditions of the metal oxide precursor and to recognize the presence of different precursor

phases, their oxidation state, their interaction with the support and location of pareticles.

So, TPR patterns can be used to study and optimize the catalyst pretreatment. In the

industrial laboratories, TPR is used as a quality control device to determine the efficacy of

the preparation procedures. In case of bimetallic catalysts, TPR is very useful to

characterize the state of the metallic components, giving information on their mutual effect

and on the factors, which influence the formation of an alloy. In the present case, TPR

studies were carried out on a homemade system (Fig. 3.6) which consists of a quartz reactor

placed in a metal furnace equipped with a temperature programmer cum controller with a k-

type thermocouple and a TCD equipped GC connected to the outlet of the reactor.

The data station with standard GC software permits recording the profiles. About

50 mg of the catalyst sample (18/25 mesh sieved particles) was placed at the centre of the

quartz reactor and packed in between the quartz wool plugs and heated linearly at a ramp of

10 °C min-1 from ambient temperature to 800 °C and isothermal conditions are maintained

for 30 min at 800 °C, while allowing passing the reducing gas mixture (5% H2 balance

argon) to flow over the catalyst. In between the out let of the reactor and GC, a molecular

sieve trap was placed to remove the moisture. GC-17A with TCD, (M/S. Shimadzu

Instrument Corporation, Japan) was used for analyzing and recording the profiles.


70

Figure 3.6. Temperature Programmed Reduction (TPR) setup.

3.5.4. H2 pulse-chemisorption

H2 – Chemisorption using pulse (100µL) titration procedure was carried out at 40

°C on a AUTOSORB-iQ, automated gas sorption analyser (M/s. Quantachrome

Instruments, USA) to know the dispersion and metal particle size, metal surface area of the

catalyst. Prior to the experiment, the catalyst was reduced at 500 °C for 2 h followed by

evacuation for 2 h. The monolayer uptake of hydrogen, Active metal surface area, Metal

dispersion, Active crystallite size can be calculated as reported in literature [26-28].

Monolayer uptake of hydrogen in micromoles per gram was calculated by

Nm=44.61Vm (3.11)


71

Where Nm is in µmoles g-1 and Vm is in cm3 g-1 (STP). Assuming a stochiometry of 2

between Ni and H2, Active metal surface area (AMSA) was calculated by:

AMSA = NmSAm/166 (3.12)

Where S is adsorption coefficient and Am is cross sectional area occupied by each active

surface atom

Metal dispersion (D) was calculated by: D=NmSM/100L (3.13)

where M and L are molecular weight and percentage loading of supported metal catalyst.

Active crystallite size was obtained by: d= 100Lf/ (AMSA* Z) (3.14)

where Z is the density of metal and f is the particle shape correction factor (6 for spherical

particles)

3.5.5. Scanning electron microscopy (SEM)

The catalyst surface frequently consists of a complex chemical mixture of different

phases evolved with the incorporation of activators or promoters. The active component is

frequently in the form of very small crystals dispersed on a large surface area of a

supporting component of the catalyst. The behavior of catalyst depends upon the structure

and the morphology of its supporting medium. For understanding the catalytic behaviour, it

may be necessary to examine its structure at a range of magnification from a few hundreds

to few millions of times. This allows determination of the form and distribution of the

active component, the nature of porosity. To observe morphological changes and also to

know the shape of the catalyst particles SEM is a useful technique. The scanning electron

micrograms of the selected catalyst samples were recorded on a Hitachi S-520 SEM. For

obtaining the micrograms, the catalyst samples were mounted on a silver sample holder

with the help of an adhesive. To make the sample surface conductive, it was coated with

gold metal at 10mm Hg pressure. The microscope was operated at 20kV accelerating

voltage with different magnification.

3.5.6. Transmission electron microscopy (TEM)

Heterogeneous catalysts usually consist of highly divided solid phases that are

closely interconnected and thus difficult to characterize. Transmission electron microscopy

(TEM) offers the unique advantage of allowing the direct observation of catalyst


72

morphology with a resolution tuneable in the range 10−4−10−10 m and of obtaining

structural information by lattice imaging and micro diffraction techniques. The technique of

high-resolution electron microscopy (HREM) is performed with axial illumination using an

objective aperture, which allows several diffracted beams to be combined with the axial

transmitted beam to form the image. The HREM images can be directly related with the

atomic structure of the material. From images it is possible to obtain data on the shape and

size of particles belonging to supports as well as active phases and to unravel how they are

distributed with respect to each other. Structural information such as symmetry and unit cell

parameters of crystallites, crystal orientations (e.g. epitaxial relationship between support

and active phase), lattice defects can be obtained by electron diffraction and lattice imaging

techniques.

Specimen preparation is a critical step in electron microscopy because the image

quality is highly dependent on how the different solid phases are dispersed on the

microscope grid and on their thickness. The thickness of solid phases should be less than

50-100 nm to allow sufficient transmittance. Thinner the samples better is the resolution

and better contrast. Another important factor is the stability of the preparation. Specimens

have to be deposited on 2/3 mm diameter copper grids (100−400 mesh) covered with a thin

amorphous carbon film. The easiest way is to ultrasonically disperse a few milligram of the

powder in a few milliliter of ethanol (EtOH), take a drop of the suspension deposit it on a

carbon coated grid and let the liquid to evaporate. In this study, the morphological features

of the catalysts were monitored using a JEOL JEM 2000EXII transmission electron

microscope, operating between 160 and 180 kV. The specimens were prepared by

dispersing the samples in methanol using an ultrasonic bath and evaporating a drop of

resultant suspension onto the lacey carbon support grid. The sizes of the catalyst particles

were measured by digital micrograph software (version 3.6.5, Gatan Inc.)

3.5.7. Electron Spectroscopy for Chemical Analysis (ESCA)

X-ray photoelectron spectroscopy (XPS) is based on the photoelectric effect [29,

30]. An atom absorbs a photon of energy, h; and, a core or valence electron with binding

energy, Eb is ejected with kinetic energy, Ek:

Ek = h - Eb. (3.15)


73

Commonly used X-ray sources are Mg K (h = 1253.6eV) and Al K (h =

1486.3eV). XPS being a surface spectroscopic technique, it is preferably operated under

high vacuum (pressures in the 10-10 mbar range). In order to obtain meaningful results on

the intrinsic properties of a clean surface or of the surface with a certain adsorbed gas,

contamination by residual gases in the measurement chamber should be prevented. The

XPS spectrum is a plot of intensity of photoelectrons, N (E) as a function of their kinetic

energy Ek, or, more often, versus binding energy Eb. Photoelectron peaks are labeled

according to the quantum numbers of the level from which the electron originates. An

electron coming form an orbital with main quantum number n, orbital momentum l

(0,1,2,3,… indicated as s, p, d, f,..) and spin momentum s (+1/2 or –1/2) is indicated as nll+s.

Examples are Rh 3d5/2, Rh 3d3/2, O 1s, Fe 2p3/2, Pt 4f7/2. For every orbital momentum l > 0

there are two values of the total momentum: j = l + ½ and j = l – ½, each state filled with 2j

+ 1 electrons. Hence, most XPS peaks come in doublets and the intensity ratio of the

components is (l + 1)/l. In case the doublet splitting is too small to be observed (as in

practice with Si 2p, Al 2p, Cl 2p), the subscript l + s is omitted. Because a set of binding

energies is characteristic for an element, XPS can be used to analyze the surface

composition of samples. Almost all photoelectrons used in laboratory XPS have kinetic

energies in the range of 0.2 to 1.5 keV, and probe the outer layers of the catalyst. The mean

free path of electrons in elemental solids depends on the kinetic energy. Optimum surface

sensitivity is achieved with electrons at kinetic energies of 50-250eV, where about 50% of

the electrons come from the outermost layer. Binding energies are not only element

specific but contain chemical information as well: the energy levels of core electrons

depend on the chemical state of atom. Chemical shifts are typically in the range 0-3eV. In

general, the binding energy increases with growing oxidation state and, for a particular

oxidation state, with the electro negativity of the ligands. The binding energies measured

by XPS are not necessarily equal to the energies of the orbital from which the photoelectron

is emitted. The difference is caused by the reorganization of the remaining electrons when

an electron is removed form an inner shell. Thus the binding energy of the photoelectron

provides information on the state of the atom before photo ionization (the initial state) and

on the core-ionized atom left behind after the emission of an electron (the final state).


74

Although XPS is predominantly used for studying surface compositions and oxidation

states, the technique can also yield information on the dispersion of supported catalysts.

The ESCA spectra of selected samples were measured on a Kratos Axis 165 XPS

spectrometer equipped with an Al-K and Mg-Kα dual source and hemispherical analyzer

connected to a five-channel detector. During measurement the base pressure of the system

is maintained at around 5x10-10 mbar. Spectra were recorded with constant pass energy of

20 eV. Binding energies were determined by computer fitting of the measured spectra.

Samples were pressed in indium foil. Binding energy correction is performed using the C

1s peak at 284.6 eV as a reference.

3.5.8. Differential thermal analysis / Thermo gravimetric analysis

Thermal analysis includes a group of methods by which the physical and chemical

properties of a substance, a mixture and/or reaction mixtures are determined as a function

of temperature or time, while the sample is subjected to a controlled temperature program.

The program may involve heating or cooling (dynamic), or holding the temperature

constant (isothermal), or any combination of these.

3.5.8.1. Differential thermal analysis (DTA)

DTA, (in analytical chemistry) is a technique for identifying and quantitatively

analyzing the chemical composition of substances by observing the thermal behavior of a

sample when it is heated. The technique is based on the fact that when a substance is

heated, it undergoes reactions and phase changes that involves absorption or emission of

heat. In DTA the temperature of the test material is measured relative to that of an adjacent

inert material. The temperature difference between a sample and an inert reference material

is monitored while both are subjected to a linearly increasing environmental temperature.

A thermocouple imbedded in the test piece and another in the inert material are connected

so that any differential temperatures generated during the heating cycle are graphically

recorded as a series of peaks on a moving chart. The reference material must be selected on

the basis of thermal stability, showing no phase changes or decomposition within the range

of temperatures to be covered. Alumina (Al2O3) in α-form is often used for this purpose. α-


75

phase of Al2O3 is a high temperature phase and cannot be converted into other phases.

Because of its stability during temperature rise, it can be used as a reference material.

With constant rate of heating, any transition or thermally induced reaction in the

sample will be recorded as a peak or dip and otherwise straight line will be yielded. An

endothermic process will cause the thermocouple junction in the sample to lag behind the

junction in the reference material and hence develop a voltage, whereas an exothermic

event will produce a voltage of opposite sign. It is customary to plot exotherms upward

and endotherms downward.

The amount of heat involved and temperature at which these changes take place are

characteristic of individual elements or compounds; identification of a substance, therefore,

is accomplished by comparing DTA curves obtained from the unknown with those of

known elements or compounds. Moreover, the amount of a substance present in a

composite sample will be related to the area under the peaks in the graph, and this amount

can be determined by comparing the area of a characteristic peak with areas from a series of

standard samples analyzed under identical conditions. The DTA technique is widely used

for identifying minerals and mineral mixtures.

3.5.8.2. Thermogravimetric analysis (TGA)

This is a technique by which the mass of the sample is monitored as a function of

temperature or time, while the sample is subjected to a controlled temperature program.

Thermogravimetric analysis is mainly directed in establishing optimum temperature ranges

for drying or igniting precipitates, however, has a much wider potential in estimating the

composition of moisture content, solvent content, additives, polymer content and filler

content. It has also been used in the identification of characteristic decomposition

temperatures, determination of thermal stability of the material and to note the rate of mass

change/decomposition, phase transitions in various dehydration, decarboxylation, oxidation

and decomposition reactions.

The DTA/TG profiles of selected catalyst samples were recorded on a Metler

Toledo 851E (Switzerland) instrument at a heating rate of 10 K min-1 in air.


76

3.5.9. CHNS analysis

CHNS-O Analyzer is an elemental analyzer dedicated to the simultaneous

determination of the amount of (%) of Carbon, Hydrogen, Nitrogen, Sulphur and Oxygen

contained in organic, inorganic and polymeric materials and in substances of different

nature and origin i.e. solid, liquid and gaseous samples. Technique used is based on

DYNAMIC FLASH COMBUSTION.

Elemental analyses of total nitrogen and carbon (and sulfur) is performed to provide

carbonate and organic carbon and to get some idea of the composition of the organic matter

(i.e., to distinguish between marine and terrigenous sources, based on total organic

carbon/total nitrogen [C/N] ratios).

Fig. 3.7. Block Diagram of CHNS – analyzer

The sample weighed in milligrams housed in a tin capsule is dropped into a quartz

tube kept at 1020°C temperaturewith constant helium flow (carrier gas). A few seconds

before the sample drops into the combustion tube, the stream is enriched with a measured

amount of high purity oxygen to achieve a strong oxidizing environment which guarantees

almost complete combustion/oxidation even of thermally resistant substances. The

combustion gas mixture is driven through an oxidation catalyst (WO3) zone, then through a

subsequent copper zone which reduces nitrogen oxides and sulphuric anhydride (SO3)


77

eventually formed during combustion on catalyst reduction to elemental nitrogen and

sulphurous anhydride (SO2) and retains the oxygen excess.

The resulting four components of the combustion mixture are detected by a Thermal

Conductivity detector (TCD) in the sequence N2, CO2, H2O and SO2. In case of oxygen,

which is analyzed separately, the sample undergoes immediate pyrolysis in a Helium

stream, which ensures quantitative conversion of organic oxygen into carbon monoxide

separated on a GC column packed with molecular sieves.

3.5.10. Fourier transform infrared (FT-IR) spectroscopy

The atoms in a molecule rotate and vibrate in different ways at certain quantized

energy levels. The infrared spectrum of a molecule results due to vibrations and rotations of

its atoms producing a change in permanent dipole moment of the molecule. The most

commonly used range of infrared spectrum is between 4000 cm-1 at high frequency end and

400 cm-1 at lower frequency end.

FTIR spectra provide valuable information about the basic characteristics of the

molecule, namely, the nature of atoms, their spatial arrangement and their chemical linkage

forces. Infrared spectroscopy has been extensively used for identifying the various

functional groups of the support and the active component. It can be used also to measure

the surface acidity of the catalysts. There are different ways of preparing the samples

depending on the goal of the study. The catalyst in powder form is generally prepared as a

thin pellet in order to be transparent to the infrared beam. In general for a verification of

vibrational and rotational spectra of catalyst samples with a KBr pellet is prepared through

grinding of about 2-mg catalyst with 200 mg KBr. In the present study the FTIR spectra of

fresh and used catalysts were recorded on a DIGILAB (USA) IR spectrometer by using the

KBr pellet method.

3.6. Activity studies

The activity assessment of Ni based catalysts for the hydrogenation of nitrobenzene

to aniline and Levulinic acid to γ-valerolactone was carried out at atmospheric pressure.

The experimental setup for catalytic hydrogenation was as shown in the figure 3.8.

The reaction is carried out in a down flow fixed bed quartz reactor (300 mm long and 14


78

mm i. d.). About 1 g of the catalyst was sandwiched at the centre of the reactor between

two plugs of quartz wool mounted in an electrically heated furnace. The reaction was

carried out in a temperature range of starting from 225 °C to 300 °C in H2 flow

(Hydrogenation reaction) at atmospheric pressure. Nitrobenzene or levulinic acid was fed

on to the catalyst using syringe pump (Perfusor FT, M/s.B. Braun, Germany) at a rate of 1

cm3/h in each experiment. Product components were analysed by a FID equipped GC and

also confirmed using GC-MS (M/S. SHIMADZU Instruments, Japan, model: QP-5050)

with electron ionizer and quadruple mass analyzer.

Figure.3.8: Fixed bed reactor setup for the vapor phase hydrogenation of nitrobenzene

and levulinic acid.

The rate of conversion of the feed nitrobenzene and levulinic acid was calculated as

follows:

The conversion, yield and selectivity were calculated based on the GC results.

Rates were calculated using the following expressions.


79

(moles of reactant fed - moles of reactant remaining)

% Conversion = 100 x

moles of reactant fed

moles of Product formed

% Selctivity = 100 x

moles of reactant converted

(Feed flow rate (moles/sec) Fractional conversion)

Rate (moles sec-1 g-1) =

Weight of the catalyst (g)

3.7. Product Identification by GC - MS

The GC–MS (Model QP 5050) supplied by M/s. Shimadzu Instruments

Corporation, Japan was used for the identification of various components present in the

product mixture. Both gaseous as well as liquid products were injected into GC-MS for the

identification. This GC-MS has mass range of 10 – 900 amu, which is convenient to detect

all the components present in the product mixture. For the separation of components, DB-5

MS capillary column (0.32mm id, and 25 mt. long made up of silica) supplied by M/s.

J&W Scientific, USA, was used.

3.8. References [1]. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. Chmelka and G.

D. Stucky, Science, 1998, 279, 548.

[2]. T.W.Kim, F.Klitz, B.Paul, R.Ryoo, J.Am.Chem.Soc. 2005, 127, 7601.

[3]. J. Jammaer, A. Aerts, J. D’Haen,J. W. Seo, J. A. Martens, J. Mater. Chem., 2009, 19,

8290.

[4]. R.M.Grudzien, B.E.Graicka, M.Jaroniec. J.Mater. Chem., 2006, 16, 819.


80

[5]. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.

Siemieniewska, Pure & Appl. Chem. 1985, 57 , 603.

[6]. S. Brunauer, P.H. Emmett, T. Teller, J. Am. Chem. Soc. 1938, 60 , 309.

[7]. I. Langmuir, J. Am. Chem. Soc. 1916, 38, 2221.

[8]. J. Rouquerol , F. Rouquerol, K.S.W. Sing, Adsorption by Powders and Porous Solids -

Principles, Methodology and Applications, Academic Press, San Diego, 1999.

[9]. M. Jaroniec, M. Kruk, J.P. Olivier, Langmuir. 1999, 15, 5410.

[10]. B.C. Lippens, B.G. Linsen, J.H.d. Boer, Journal of Catalysis. 1964, 3, 32.

[11]. G. Halsey, J. Chem. Phys. 1948, 16, 931.

[12]. W.D. Harkins, G. Jura, J. Am. Chem. Soc. 1944, 66, 1366.

[13]. E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 1951, 73, 373.

[14]. J.C.P. Broekhoff, J.H. de Boer, Journal of Catalysis. 1967, 9, 8.

[15]. J.C.P. Broekhoff, J.H. De Boer, Journal of Catalysis. 1967, 9, 15.

[16]. J.C.P. Broekhoff, J.H. De Boer, Journal of Catalysis. 1968, 10 , 153.

[17]. W.W. Lukens, P. Schmidt-Winkel, D. Zhao, J. Feng, G.D. Stucky, Langmuir. 1999,

15 , 5403.

[18]. M. Kruk, M. Jaroniec, A. Sayari, Langmuir. 1997, 13, 6267.

[19]. M. Jaroniec, L.A. Solocyoc, Langmuir. 2006, 22, 6757.

[20]. P.I. Ravikovitch, A.V. Neimark, Colloids and Surfaces A: Physicochemical and

Engineering Aspects. 2001, 187-188 , 11.

[21]. A.V. Neimark, P.I. Ravikovitch, Microporous and Mesoporous Materials. 2001, 44-

45, 697.

[22]. P.I. Ravikovitch, A.V. Neimark, Langmuir. 2002, 18 , 1550.

[23]. S.S. Kim, A. Karkamkar, T.J. Pinnavaia, M. Kruk, M. Jaroniec, J. Phys. Chem. B.

2001,105 ,7663.

[24]. R.M.Grudzien, B.E.Graicka, M.Jaroniec. J.Mater. Chem., 2006, 16, 819.

[25]. C.D. Hodgman, R.C. Weast and S.M. Selby, Hand Book of Chemistry and Physics,

42nd Edition (1961).

[26]. I. Langmuir, J.Am.Chem.Soc, 1916,38,2221.



81


[29]. G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry, VCH,

Weinheim, 1985.

[30]. D. Briggs and M.P. Seah (Editors), Practical Surface Analysis by Auger and X-Ray

Photoelectron spectroscopy, Wiley, New York, 1983.

Documents

Chapter -3 Experimental - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/27786/8/08_chapter_3.pdf · Chapter-3 Experimental 55 was dissolved in 139ml 0.5 M HCl at 40°C