Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

References 385

In defects on transition metal oxides DFT again fails, giving structures that show

unlikely relaxations and tending to delocalize electrons associated with the defect

into conduction band states. Hybrid functionals and DFT + U have also been used

to correct the models in these cases, giving a localized picture of surface reduction.

These methods are now able to give useful descriptions of reactions at these defect

sites, including the transfer of electrons between surface and adsorbate required

in redox chemistry.

The DFT + U method is a pragmatic correction for the on - site Coulomb and

exchange interactions at metal centers. Its parameterized nature does not sit well

with “ ﬁ rst principles ” approaches, as the value of U must be set empirically.

However hybrid DFT also involves parameterization, with the amount of exact

exchange to add tailored to some physical property (such as the band gap or spin

density). It does have the advantage that it is applied even - handedly to the electron

density, whereas DFT + U is only applied to high angular momentum states (d

and f functions). As more examples come to the fore the advantages and disad-

vantages of these two methods will become clearer.

All the examples used in this chapter have involved either surface properties

alone or strong adsorbate/surface interactions. The weak adsorption of molecules

to surfaces depends on van der Waals interactions. At the electronic structure level

these require long - range correlation effects to be accounted for and so neither HF,

DFT or the admixtures discussed so far give a good description. A brief mention

of MP2 was given in the introduction and this extension of HF is able to treat the

weak interactions of alkanes with zeolite frameworks [8] . However, this is likely

to remain a computationally expensive option in the solid state. New functionals

are appearing in this area speciﬁ cally aimed at the long - range correlation problem

and promise to advance the ﬁ eld further [135] .

Acknowledgments

The projects from which our own illustrative calculations were drawn for this

chapter made use of the Mott2 and HPCx facilities through the Materials Consor-

tium. We are also indebted to Prof Graeme Watson and Dr Mark Bankhead for

use of some of their data and for useful discussions on all the topics presented.

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Thermal Analysis and Calorimetric Methods

Simona Bennici and Aline Auroux

391

9

9.1

Introduction

Thermal analysis is a group of techniques that study the changes in the properties

of a sample occurring while the sample is subjected to a controlled temperature

program. Calorimetric methods measure heat changes that occur during a process.

Important changes on a molecular level occur in a material as it is heated, and

their study may enable the analyst to draw valuable conclusions about the sample,

its previous history, preparation, chemical nature and the likely behavior during

its proposed use. The temperature at which a particular event takes place, or the

temperature range over which a reaction happens, is often characteristic of the

nature and history of a sample [1] . Calorimetry encompasses a very large variety

of techniques, including titration, ﬂ ow, reaction or sorption calorimetry. The most

frequently used thermal analysis techniques in catalysis and the properties they

measure are shown in Table 9.1 , together with the names most usually employed

for them.

During investigation of a new material it is unlikely that any single thermal

analysis technique will provide all the information required to understand its

behavior. Complementary information is usually needed, which may be from

another simultaneous thermal technique such as thermogravimetric – differential

scanning calorimetric – mass spectrometry ( TG - DSC - MS ), gas chromatography

(TG - GC, or DSC - GC), or spectroscopic methods such as IR spectroscopy or X - ray

photoelectron spectroscopy ( XPS ).

All these techniques, whether used by themselves or coupled to other tech-

niques, are fully described in extended reviews [1 – 3] .

Static or ﬂ ow adsorption microcalorimetry, less extensively used, is a very

powerful method for describing surface reactions and merits a more detailed

overview.

In heterogeneous gas – solid systems, a catalytic reaction proceeds according to

the following stages: adsorption of the reactant on the surface of the catalyst, reac-

tion on the surface, desorption of the products from the surface. All these steps

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves

ISBN: 978-3-527-31815-5

392 9 Thermal Analysis and Calorimetric Methods

involve the surface active sites, whose redox or acid – base properties are important

factors which determine their industrial application.

Many industrial processes are based on acid – base - or redox - catalyzed reactions.

The majority of these processes use heterogeneous catalysts, particularly mixed or

supported oxides possessing both hydroxyl groups (Br ø nsted acid or basic sites)

and coordinatively unsaturated metallic cations (Lewis acid sites) and anions

(Lewis basic sites such as O

2 −

ions). A good description and knowledge of the

surface active sites is essential for improved understanding of the relationships

between their physico - chemical properties and their catalytic behavior (activity,

selectivity, stability, etc.) The heterogeneity of oxides in terms of composition

(particularly between the bulk and the surface), nature (Br ø nsted, Lewis), strength,

concentration and distribution of sites renders the characterization more complex.

Moreover, the hydrophilic or hydrophobic character of the surface is an important

parameter that should be known.

The main purpose of this chapter is to review the applications of thermal analy-

sis and calorimetric techniques to the study of catalysts such as oxides, heteropoly-

anions, hydrotalcites, layered silicates and microporous or mesoporous molecular

sieves. A brief summary of studies that made use of calorimetry to characterize

crystalline or amorphous oxides and related materials is also presented.

From the literature it is apparent that microcalorimetry is very useful in provid-

ing information on the strength and distribution of acidic and basic sites of cata-

lysts. The technique for determining the acid site distribution is quite well

developed, especially if ammonia is used as the basic probe molecule. Moreover,

the energetics of surface reactions, including oxidation and reduction of metal

oxides, oxidation of adsorbed hydrocarbons or hydrogen and decomposition reac-

tions can be determined directly by calorimetry [4] .

Table 9.1 Thermal analysis techniques in catalysis.

Technique Abbreviation Property Uses

Thermogravimetry or TG mass decomposition,

oxidation, reduction

Thermogravimetric analysis TGA mass

Differential thermogravimetry DTG d m /d t

Differential thermal analysis DTA temperature difference reactions, phase

changes

Differential scanning

calorimetry

DSC power difference or

heat ﬂ ow rate

heat capacity,

reaction, adsorption,

desorption

Heat ﬂ ow calorimetry heat ﬂ ow

9.2

Techniques and Procedures

9.2.1

Experimental Techniques

The thermal techniques most commonly used to investigate the acid – base or redox

character of solid surfaces are differential thermal analysis ( DTA ), thermogravi-

metric ( TG ) and differential thermogravimetric ( DTG ) methods, differential scan-

ning calorimetry ( DSC ) and calorimetry. These techniques can be used either

by themselves, or in conjunction with other techniques (for instance, TG - DSC,

calorimetry - volumetry, DSC - chromatography, etc.) [5, 6] .

Techniques such as DTA, TG or DSC study the thermal behavior of a catalyst

as it undergoes heating at a constant rate. For example, DTA relies on the mea-

surement, as function of time, of the difference between the temperature of the

sample ( T

s

) and that of a reference material ( T

r

), upon heating both materials in

a furnace. ∆ T = T

s

− T

r

is related to the heat evolved (or absorbed) by the sample

at a time when the sample undergoes an endothermic or exothermic reaction. In

the TG and DTG methods, the variation with time of the sample mass (d m or

d m /d t ) is measured as a function of temperature while the sample is heated at a

constant rate [7] .

Finally, in the case of DSC, the measured quantity is the heat ﬂ ow evolved per

unit time by the sample as it is being heated at a constant rate [8] . For the deter-

mination of the heat of a given process using DSC, it is necessary not only to

measure the DSC curve, but also to construct carefully the baseline [9] . These

techniques are preferentially used for studying the redox character of catalyst sur-

faces. In contrast, in the adsorption microcalorimetry technique, the sample is

kept at a constant temperature while a probe molecule adsorbs onto its surface,

and a heat ﬂ ow detector emits a signal proportional to the amount of heat trans-

ferred per unit time. Adsorption microcalorimetry is a very powerful tool for the

energetic characterization of solid surfaces, as well as for the thermodynamic

description of solid – gas interface phenomena. Several review articles describing

this technique in detail and the operating conditions are available [2, 4, 8,

10 – 17] .

The deﬁ nition of the various heats of adsorption/desorption and their relation-

ship with thermodynamic quantities directly obtainable by calorimetry are given

in Table 9.2 [2] .

Adsorption calorimetry consists of the coupling of a heat ﬂ ow calorimeter with

a system able to monitor the adsorption of a probe molecule by determining the

amount of probe gas that has reacted with the solid under study. It is probably the

most direct method for describing in detail both the quantitative and energetic

features of surface sites. The adsorption of a probe molecule is an exothermic

phenomenon ( ∆ H

ads

< 0), while desorption processes are associated with end-

othermic peaks ( ∆ H

des

> 0). Heats of reduction are generally associated with an

9.2 Techniques and Procedures 393

394 9 Thermal Analysis and Calorimetric Methods

endothermic peak and a weight loss, while heats of oxidation are associated with

an exothermic peak and a weight gain.

9.2.1.1 A Static Method: Coupling Calorimetry and Volumetry

The simultaneous assessment of the adsorbed quantity of a suitable probe mole-

cule and the heats evolved at increasing coverage makes it possible to picture the

heterogeneity of the surface (structural and/or chemical) by processing the overall

set of combined volumetric and calorimetric data.

A system that links microcalorimetry to the volumetric measurement of quanti-

ties of adsorbed reactants makes it possible to study gas – solid interactions and

catalytic reactions. This system works under static vacuum. The admission of

gases into the calorimeter can be performed either in a discontinuous way (by

successive doses) by means of a valve, or in a continuous manner by means

of a capillary. The classical technique of adsorption calorimetry by doses is the

most appropriate way to measure the energy of interaction between the adsorbed

species and the catalyst. If the surface can be a priori considered as heterogeneous,

the heat of adsorption, the amount adsorbed and the kinetics of adsorption must

be measured for very small successive doses of the adsorbate so as to obtain accu-

Table 9.2 Deﬁ nition of the various heats of adsorption/desorption.

Name Symbol Unit Deﬁ nition

Integral heat of adsorption Q

int

J Quantity of heat evolved when

n

a

moles are adsorbed at

constant temperature on an

adsorbent initially in vacuo ,

without a change in the volume

of the cell: Q

int

= n

a

( U

a

− U

g

)

Molar integral heat

− Q

int

= Q

int

/ n

a

J mol

− 1

Integral heat per mole adsorbed

Differential heat of adsorption

or adsorption enthalpy

Q

diff

= − ∆ H

ads

J mol

− 1

Deﬁ ned from integral heat by:

Q

diff

= ( ∂ Q

int

/ ∂ n

a

)

T,A

Isosteric heat Q

st

J mol

− 1

Heat of adsorption calculated

from isotherms of adsorption:

Q

st

= ( RT

2

( ∂ l n P / ∂ T )

n a

(Clapeyron)

Desorption energy

E

d

= ∆ H

des

J mol

− 1

Heat of adsorption derived from

TPD: E

d

= k

B

T

max

[ln( ν T

max

/ β ) −

const] (Redhead equation)

a, the adsorbed phase; A , surface area of the adsorbent; T , temperature of the gaseous phase;

n , number of moles; U , internal energy; P , pressure; k

B

, Boltzmann constant; T

max

, temperature

of the maximum desorption rate; ν , pre - exponential frequency factor; β , heating rate dT / dt .

rate information on the variation of these quantities as a function of the

coverage.

The volumetric determination of the adsorbed amount of gas is performed in a

constant - volume vessel linked to a vacuum pump. The apparatus consists of two

parts (Figure 9.1 ): the measuring element equipped with a capacitance manometer

and the cell section, which includes the cells placed in the calorimeter (a sample

cell in which the adsorbent solid is placed, and an empty reference cell). The

volume of this vessel is determined by the expansion of a known quantity of gas,

contained in the measuring part of the assembly, into the previously evacuated

cell section.

This calibration must be made with the same gas and at the same temperature

as the proposed study. The experiment consists of sending successive doses of the

reactant gas onto the initially out - gassed catalyst and waiting for thermal equilib-

rium after each increment. The heat ﬂ ow signal and the concomitant pressure

change are continuously recorded. The aim is to minimize the volume of each

dose in order to be able to observe any change as a function of the coverage as

closely as possible. The adsorption is considered complete when, for a signiﬁ cant

increase in pressure, there is no detectable heat evolution or gas adsorption. The

adsorption temperature is maintained at a constant value (usually of the order of

353 – 423 K, in order to ensure that there are no diffusional restrictions and to

ensure that adsorption is almost exclusively chemisorption). The irreversibly che-

misorbed amount ( V

irr

) can be evaluated from the difference between the primary

adsorption isotherm (adsorbed volume as a function of equilibrium pressure over

9.2 Techniques and Procedures 395

Figure 9.1 Schematic diagram of the volumetric - calorimetric line.

Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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