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

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416 9 Thermal Analysis and Calorimetric Methods

coworkers using calorimetry [39] . Such systems have been studied in an attempt

to enhance the redox properties of ceria by incorporating foreign cations in its

lattice. Besides the redox properties, the acid – base features could be expected to

change as well, since it is well known that the acid – base properties of a metal oxide

can be signiﬁ cantly modiﬁ ed by chemical mixing with another oxide.

It has been shown that the total concentration of the acid sites is the lowest for

pure ceria, and markedly increases upon addition of 20 mol% of zirconia, then

decreases as the ZrO

content is further increased up to 80 mol% and ﬁ nally attains

the highest value on pure zirconia. The heat evolved upon NH

adsorption increases

with increasing ZrO

content in the mixed oxides, while the basic properties are

attenuated as the zirconia content is increased up to 80 mol% and then grow again

for pure zirconia. Hence the inclusion of increasingly high contents of zirconium

into the ceria lattice has a complex inﬂ uence on both the acidity and basicity of

the pure parent oxide.

The co - precipitated ceria – lanthana samples were found to present initial Q

diff

values for ammonia adsorption ranging from 105 to 130 kJ mol

− 1

, the highest value

corresponding to CeO

and the lowest one to the La

sample. For all the samples,

an increase in the ammonia uptake is accompanied by a rapid decrease on the

differential heat to a low value ( ≈ 35 kJ mol

− 1

). Concerning basicity, the heterogene-

ity revealed by the Q

diff

versus uptake proﬁ les cannot be ascribed to sites of differ-

ent chemical nature (virtually the only basic sites on the ceria surface are

coordinatively unsaturated O

2 −

ions). The formation of bicarbonates either very

strongly or very weakly held could explain the occurrence of some very high and

very low CO

adsorption heats.

One - step partial oxidation of propane to acrylic acid (an essential chemical

widely used for the production of esters, polyesters, amides, anilides, etc.) has been

investigated so far on three types of catalysts, namely, vanadium phosphorus

oxides, heteropolycompounds and, more successfully, on mixed metal oxides. The

active catalysts generally consist of Mo and V elements, which are also found in

catalysts used for the oxidation of propene to acrolein and that of acrolein to acrylic

acid.

Mo – V – Te and Mo – V – Te – Nb mixed - metal oxide catalysts have been character-

ized by means of C

- TPR and NH

adsorption calorimetry. All samples were

strongly heterogeneous, with initial adsorption heats of ≈ 100 – 80 kJ mol

− 1

for the

Mo – V – Te samples. Introducing an Nb component into the catalysts slightly

decreased the initial adsorption heats to ≈ 60 kJ mol

− 1

but drastically increased the

surface density of weak acid sites ( < 30 kJ mol

− 1

) [83] .

The oxidative dehydrogenation ( ODH ) of lower alkanes is an attractive process

for the formation of alkenes. The ODH of propane to produce propene has been

particularly studied, given its high demand for the production of polypropene,

acrylonitrile and propene oxide. There is a combined inﬂ uence of the redox and

acid – base properties of the surface of the oxides used for propane ODH. Interme-

diate reducibility, weak Lewis acid centers and oxygen mobility represent the

essential requirements for selective ODH, as they are consistent with the trends

in ODH rates observed in VO

, MoO

and WO

based catalysts.

DSC has been used to determine the heat effects associated with propene

adsorption [84] on Mo/Si : Ti catalysts with different K : Mo molar ratios. The

heats associated with reversible −×

()

−−

3 193 10

.mJm

sample

and irreversible

−×

()

−−

0 126 10

.mJm

sample

adsorption of propene were found to decrease with the

addition of potassium.

Thermal analysis studies provide insight into the stability of the catalyst and its

chemical evolution. DSC studies can be used to determine not only the tempera-

ture of the transformation if any, but also the corresponding heat evolved. For

example, El Jamal and coworkers [85] studied the stability of the β - phase of

bismuth molybdate (Bi

) by DSC in order to check whether this phase is

stable at the high temperatures required for propene oxidation. It was shown that

the β - phase is metastable at room temperature but stable enough to be used in

catalytic tests. It was also conﬁ rmed that the β - phase is different from an equi-

molar mixture of the α - and γ - phases of bismuth molybdate (Bi

and

MoO

respectively). The enthalpy of transformation of the α + γ mixture to β at

833 K was found to be 6330 J mol

− 1

. The endothermic nature of the observed peak

conﬁ rms the evolution of the system to a more organized one, according to the

reaction

Bi Mo O Bi MoO Bi Mo O

2312 2 6 229

2+→

Numerous examples of applications of DSC to the study of the stability of binary,

ternary and quaternary metal oxides can be found in the literature, and we will

not focus on this type of systematic characterization. Fewer studies have been

performed to determine the acid – base character of mixed oxides using adsorption

calorimetry of probe molecules.

Depending on the preparation method, on the SiO

/Al

ratio and on the

micro - or mesoporous structure, very different acidic properties have been observed

for silica – alumina mixtures. Some samples contain both Br ø nsted and Lewis sites,

while for others only acidity of the Lewis type was observed [86, 87] .

Thermogravimetric analysis ( TGA ) has been used to collect thermodesorption

curves of 2 - phenylethylamine ( PEA ) from acid surfaces of mixed oxides prepared

by the sol – gel method, with the aim of determining the amount and distribution

of the acid sites of the samples [23] . Thermodesorption curves from silica – alumina,

silica – zirconia and silica – titania samples ( ≈ 87 wt% SiO

) were collected at different

heating rates (5 ≤ β (K min

− 1

) ≤ 30) in an inert atmosphere. The activation energies

of PEA desorption from the acid sites were calculated from the dependence upon

the heating rate β of the displacements of the observed desorption peaks ( T

max

) as

determined from the derivative of the TGA proﬁ les. The numbers of acid sites, as

well as the activation energies, determined by this method were in the order

SiO

– TiO

< SiO

– ZrO

< SiO

– Al

However, this order is strongly dependent on the preparation method and

respective amounts of the two oxides. This fact is illustrated in Figure 9.11 , which

shows strengths and numbers of acid sites varying in a different order as deter-

mined by ammonia adsorption calorimetry for sol – gel - prepared silica – alumina,

9.3 Surface Properties of Oxides 417

418 9 Thermal Analysis and Calorimetric Methods

silica – titania and silica – zirconia with SiO

/Al

, SiO

/TiO

and SiO

/ZrO

ratios

of 31, 26 and 26 respectively.

Binary and ternary systems containing copper, zinc oxide and aluminum are

widely used for industrial methanol synthesis. The adsorption of CO on a fully

reduced and clean Cu surface is fully reversible at room temperature, with heats

of adsorption ranging between 70 kJ mol

− 1

at low coverage and 45 kJ mol

− 1

at high

coverage [88] .

Silica – alumina has been used as support for dispersing CuO, Ga

and SnO

oxide phases, and the corresponding CuO – Ga

and CuO – SnO

binary systems

[89] have been used in NO

reduction or methane combustion reactions. The

uptakes and interaction energies of CO adsorbed on the different samples were

determined by adsorption calorimetry at 303 K (Figure 9.12 ). As complementarily

shown by IR CO adsorption measurements, only the copper phase was able to

adsorb CO, so the scale of CO uptake was related to the amounts of dispersed

copper species (Cu

and Cu

δ +

) available at the surface. The rather high values of

the initial heats (around 105 – 115 kJ mol

− 1

) conﬁ rmed the abundant presence of

species, strongly interacting with the support.

Calorimetric and spectroscopic studies of NH

adsorption have been performed

on the same samples to study the surface acidities of the bare silica – alumina

support and of the CuO – Ga

/SiO

– Al

and CuO – SnO

/SiO

– Al

catalysts

(Figure 9.13 ) from the qualitative (nature of acid sites) and quantitative (number,

strength and strength distribution of acid sites) points of view [90] . The CuO

species increased the capacity to adsorb NH

(in particular on medium and weak

acid sites, 100 kJ mol

− 1

< Q

diff

< 150 kJ mol

− 1

), while the strongest fraction of acid

sites ( Q

diff

> 150 kJ mol

− 1

) was enhanced by the addition of Ga

and SnO

. In

these papers [89, 90] the authors demonstrated, by means of microcalorimetry

measurements, that it is possible to modulate the acid strength of metal oxide

surfaces by suitable addition of a second oxide component.

Figure 9.11 Differential heats of ammonia adsorption versus

coverage for various mixed oxides.

Figure 9.12 Differential heats of carbon monoxide adsorption versus coverage.

Figure 9.13 Differential heats of ammonia adsorption versus coverage.

9.3 Surface Properties of Oxides 419

420 9 Thermal Analysis and Calorimetric Methods

Ceria - based catalysts are intensively used because of their high chemical and

physical stability, high oxygen mobility and high oxygen vacancy concentrations,

which are characteristic of ﬂ uorite - type oxides. The possibility of cycling easily

between reduced and oxidized states (Ce

↔ C e

) permits the reversible addition

and removal of O

from CeO

. All these features explain the applicability of these

materials as catalysts or catalytic supports in many oxidation reactions or as three -

way catalysts.

The combination of ceria with another oxide from a group III metal (boria,

alumina, gallia and india) using either a co - precipitation method or a sol – gel route

has led to mixed oxide samples with speciﬁ c acid – base and redox features, as evi-

denced by thermal techniques [91] .

The calorimetric study of the adsorption of NH

and SO

has shown that, among

all the investigated co - precipitated samples, only those containing boria had sig-

niﬁ cantly enhanced acidity, whereas the basicity has been found to depend on the

nature and amount of group III metal. The amphoteric character of ceria, already

known from the literature, has been conﬁ rmed. The Me

– CeO

mixed oxides

have also shown amphoteric behavior, in a manner dependent on the character of

the group III metal. Among the investigated systems, Al

– CeO

and In

– CeO

express somewhat more pronounced basic than acidic characters. Importantly,

– CeO

samples present an acidic character that becomes more evident with

an increase in the boron content.

In the case of sol – gel solids [92] , even if the boria – ceria sample had a higher

amount of boria compared to co - precipitated samples, it expressed only insigniﬁ -

cant acidity. The sol – gel method produces an unfavorable distribution of the guest

oxide particles in the mixed oxide, and the acid – base features of the investigated

mixed oxides are quite different from those of the samples obtained by

co - precipitation.

Ammonia adsorption microcalorimetry experiments have been conducted on

various acidic tungsten/zirconia catalysts prepared by different techniques [93] .

The results show that the co - precipitation method produced a tungsten/zirconia

catalyst with a greater number of acidic sites than impregnation of tungsten on

hydrous zirconia, and that the addition of small amounts of iron to the tungsten/

zirconia catalyst increased the acid site strength. The acid site strength of the

tungsten/zirconia materials was similar to or slightly higher than that found in

zeolites or sulfated zirconia, with initial heats ranging from 120 to 170 kJ mol

− 1

depending on the samples.

9.3.5

Hydrotalcites

Hydrotalcites are layered double hydroxides with general formula

MMOH A H

2−

−

()

()()

⋅

, where the divalent ion may be Mg

, Ca

, Zn

, the trivalent ion, Al

, Fe

, Cr

, and so on, and the charge compensating

anions, OH

−

, Cl

−

, NO

−

, CO

2 −

or SO

2 −

. The value of x can be between 0.25 and 0.33.

Thermal treatments induce dehydration, dehydroxylation and loss of the charge -

compensating anions, resulting in mixed oxides with the MgO - type structure.

Hydrotalcites are consequently a class of precursors useful for the preparation of

catalytically active oxides showing basic properties [94] . The acid – base properties

of Mg – Al mixed oxides are governed by the Mg : Al molar ratio, calcination tem-

perature and preparation conditions. The study of the inﬂ uence of the acid – base

properties and chemical composition on the catalytic performance of calcined

hydrotalcites is thus of interest.

Layered double hydroxides with the hydrotalcite structure were synthesized with

varying Mg : Al atomic ratios and with different contents of exchangeable Cl

−

and

2 −

anions. The CO

adsorption isotherms showed an increase of the uptake and

consequently of the basicity with initial CO

2 −

content and calcination temperature

up to 800 K. Increasing the Mg : Al ratio of the hydrotalcites from 2.33 to 3 resulted

in an increase of the total number of basic sites [94] .

In another study, Mg – Al hydrotalcite catalysts with different Mg : Al molar ratios

(0.6, 1.4, 2.2, 3.0) were characterized by microcalorimetry using CO

in the gas

phase and benzoic acid in toluene [95] . The calcined Al - rich sample (Mg : Al molar

ratio of 0.6) possesses Lewis acid sites similar in strength to those found on Al

but stronger than those found on the Mg - rich hydrotalcites. The liquid - phase

basicity microcalorimetry measurements with benzoic acid in toluene correlated

very well with the catalytic activity for Michael additions.

The strength and accessibility of the basic sites of hydrotalcites with an

Mg : Al ratio of 2, prepared via co - precipitation of the respective nitrates using

carbonate or oxalate as the compensating anions, were assessed by calorimetry of

adsorption. Two different methods were used to activate the Mg – Al hydro-

talcites and impart Br ø nsted basicity. The initial enthalpies of CO

adsorption at

303 K on the activated hydrotalcites presented very similar values of ≈ 108 kJ mol

− 1

[96] .

The acid – base properties of CuMgAl and NiCuMgAl mixed oxides with hydro-

talcite - like structures containing different proportions of Ni

, Cu

, Mg

and Al

cations have been investigated using adsorption microcalorimetry and XPS with

(for basicity) and NH

(for acidity) as probe molecules [97, 98] . For the

CuMgAl mixed oxides, the adsorption data indicated an optimum of the concen-

tration and strength of acid sites for the material with Cu : Mg : Al = 1 : 1 : 1, and

suggested that the basicity (number and strength) is the most important for

Mg - rich samples.

The basicity of the NiCuMgAl materials depends principally on the Ni/Cu

ratio and increases with the proportion of Ni. The basic sites are strong and par-

ticularly homogeneous (plateau around 150 – 170 kJ mol

− 1

depending on the

samples), and they are of both Br ø nsted and Lewis types (the former being pre-

dominant) as conﬁ rmed by XPS. Meanwhile, the acidity (of Lewis type only) is

rather weak, heterogeneous and dependent on the Ni : Cu ratio (as is basicity). The

observed heterogeneity of these acidic sites can be related to the heterogeneity of

the interaction between the nickel atoms and the other elements. The acidity

9.3 Surface Properties of Oxides 421

422 9 Thermal Analysis and Calorimetric Methods

and the basicity both increase with the Ni : Cu ratio, as shown in Figure 9.14 ,

whereas the basicity decreases with increasing Mg : Al ratio [98, 99] .

The decarbonation of carbonated layered double hydroxides containing Mg with

either Al, Fe or Cr trivalent cations or Al with Mg, Ni, Cu or Zn divalent cations

has been studied by thermal analysis. The enthalpy of adsorption of CO

on the

resulting calcined mixed oxides was measured by calorimetry, with initial heats of

adsorption close to those reported for MgO (about 100 kJ mol

− 1

) and a relatively

homogeneous strength distribution [100] .

9.3.6

Bulk and Supported Heteropolyacids

Keggin - type heteropoly compounds have attractive and important characteristics

in terms of catalysis. They consist of heteropolyanions and counter - cations such

as H

, Cs

or NH

. When the counter - cations are protons, they are called hetero-

polyacid s ( HPA ). An important characteristic of HPAs, such as 12 - tungstophos-

phoric acid (H

), is the presence of very strong Br ø nsted acid sites. But

the characteristics of HPAs strongly depend on temperature and relative humidity.

When they are used in heterogeneous catalysis, it is often necessary to support

them on high - surface - area oxides or activated carbons, in order to increase the

surface contact with the reactants.

These materials are used industrially mainly in the reaction of hydration of

alkanes (acid catalysis) and in the synthesis of methacrolein or isobutyric acid

(redox catalysis). Because of the considerable number of available polyanion struc-

tures, it is possible to vary their acidic and redox properties according to the needs

of a speciﬁ c application.

The number and strength of the acid centers of tungstic heteropolyacids have

been determined by ammonia adsorption calorimetry. Ammonia is irreversibly

absorbed, with the formation of the corresponding ammonium salts. An increase

in the number of protons in Keggin heteropolyanions decreases the acidic strength.

Figure 9.14 Differential heats of sulfur dioxide adsorption

versus coverage on NiCuMgAl mixed oxides.

The acidity varies between compounds, but always presents a plateau of sites of

the same strength. The initial heats vary in the order H

> H

SiW

> H

. Diffusion phenomena can be important for some of

the compounds, such as H

, depending on the structure [101] , as can be

observed in Figure 9.15 .

It is worth noting that differences in acid strength between anhydrous Keggin

heteropolyacids (H

> H

SiW

> H

PMo

> H

SiMo

) mea-

sured by ammonia adsorption calorimetry did not correlate simply with the cata-

lytic activity in the reaction of rapeseed oil transesteriﬁ cation with methanol and

ethanol [102] .

A study of the acidity of carbon - supported and unsupported heteropolyacid cata-

lysts by ammonia sorption calorimetry has been performed at 423 K [103] . The

acidity of the supported Keggin - type heteropolyacids is slightly reduced compared

to that of the corresponding bulk samples, while that of H

132

is enhanced

when supported on carbon. Heterogeneity in acid strength is greatly enhanced

when the HPAs are grafted on the carbon support.

The acidic properties of the alkaline earth salts of 12 - tungstophosphoric acid

have been investigated using ammonia adsorption microcalorimetry [104] . As a

result of the substitution of two protons with one alkaline earth cation, decreases

were observed in both the total number of acid sites and the number of the stron-

gest acid sites (characterized by differential heats higher than 150 kJ mol

− 1

), com-

pared to the values found for 12 - tungstophosphoric acid. In the case of Sr salts,

some steric effects were observed. The acidity of the investigated systems, even if

strong, is uniform only in the case of the parent tungstophosphoric acid. On the

contrary, energetic heterogeneity is evident for the other investigated solids, both

from differential heat versus ammonia uptake proﬁ les and from the results of TPD

of ammonia performed in a TG - DSC - MS experiment.

Figure 9.15 Differential heat of ammonia adsorption versus

ammonia uptake for various HPA samples.

9.3 Surface Properties of Oxides 423

424 9 Thermal Analysis and Calorimetric Methods

Changes in the acidic properties of Cs

2.5

0.5

upon heat treatment in

water have been investigated by Nakato and coworkers [105] . Ammonia adsorption

calorimetry showed that the water treatment reduced the acid site number, whereas

the acid strength was little changed, indicating that the decrease observed in cata-

lytic activity was due to that in the number of acid sites. The plateau of heat of

adsorption observed was only slightly affected by the water treatment: 165 kJ mol

− 1

before water treatment and 160 kJ mol

− 1

after treatment.

The acidity of a series of Cs

3 − x

samples has been studied by NH

ﬂ ow

calorimetry linked to mass spectrometry [18] . The incorporation of Cs lowers the

number of titratable acid sites, in quantitative agreement with the theoretical

degree of H

exchange. The average heat of NH

adsorption falls only slightly with

Cs doping up to x = 2.3, suggesting that the residual acid strength of the Cs - doped

HPW materials is not strongly perturbed during proton exchange. In contrast,

however, the acidity of the heavily substituted ( x > 2.4) samples is signiﬁ cantly

lower.

Ru – HPA metal – acid bifunctional catalysts on various supports (silica, graphite,

KL zeolite) have also been characterized by NH

adsorption calorimetry, which

revealed heterogeneous acid site strength distributions varying in the order HPA -

SiO

> HPA - graphite > HPA - KL [106] .

9.3.7

Pillared Clays and Layered Silicates

Pillared clay s ( PILC ) have been reported to show catalytic properties comparable

to zeolites in several reactions, and therefore to have comparable acidities [107] .

The preparation and properties of smectites pillared with Al hydroxy oligomers

have been extensively investigated. Most of the work has been performed on

montmorillonite and hectorite, which have been intercalated with a great variety

of pillaring agents and oligocations.

Intercalation of hydroxy silicoaluminum compounds increases the thermal sta-

bility and the acidity of the pillared clays.

The distributions of acid strengths of various pillared clays obtained by calorim-

etry of NH

adsorption showed that Al - PILC displays high acidity, with a small

number of sites as strong as those measured on zeolites. Acidity should be directly

related to the environment of the Al ions in the pillars, which could be modiﬁ ed

by the preparation. The adsorption of NH

on pillared beidellite showed the pres-

ence of many strong acid sites (160 kJ mol

− 1

) [108] .

A natural clay has been pillared with mixed solutions containing both Al and

Fe, Ti or Cr. The intercalation - generated solids ’ distribution of acid strengths

measured by calorimetric adsorption of ammonia is comparable to that of zeolites.

The surfaces appear as heterogeneous and show initial adsorption heats close to

150 – 160 kJ mol

− 1

if one excludes the ﬁ rst point of the differential heat versus cover-

age curves, which is much higher ( ≈ 190 kJ mol

− 1

) [109] .

The acidity of mesoporous expanded clay catalysts has been studied using

ammonia adsorption microcalorimetry [110] . The inclusion of SiO

– TiO

clusters

drastically increased the total (Br ø nsted + Lewis) acidity of the clay, in terms of

both the strength and the density of acid sites.

The acidity of pillared clays has been characterized by both microcalorimetric

measurements of the adsorption of aromatic molecules and pyridine and the cata-

lytic ethylbenzene test reaction [111] . The aromatic probe molecules used were a

reactant and a product of the catalytic reaction: ethylbenzene and m - diethylben-

zene, respectively. In this way, only the strongest of the accessible acid sites were

titrated. The heats of adsorption of these molecules indicate that a zirconium oxide

pillared clay had stronger acidity than an aluminum oxide pillared clay, whereas

the pyridine results were equal for both samples.

9.3.8

Zeolites

Aluminosilicates can be divided into two categories, amorphous silica – aluminas

(described in Section 9.3.4 ) and crystalline zeolites or molecular sieves.

In crystalline aluminosilicates, all aluminum and silicon atoms form tetrahedra

which are linked by shared oxygen atoms. These tetrahedra join to form secondary

building units, which can be interconnected to give numerous distinctive zeolite

structures. Each has a regular and well deﬁ ned pore structure together with inner

cavities. The precise control of pore size is one of the greatest distinctions between

zeolites and amorphous silica – aluminas. The morphology, free volume, pore

geometry and electric ﬁ eld gradients determine their acidity and selectivity.

Numerous reviews dealing with the adsorption capacities and acid – base proper-

ties of zeolites have been published [4, 8, 10 – 14, 17] so we will not give a detailed

description of these systems. Only some case studies will be given in order to

assess the possibilities of thermal techniques for characterizing such materials. In

general the total number of acid sites is greater in zeolites than in amorphous

silica – aluminas for a similar Si : Al ratio.

The acidity of high - silica zeolites produced either by direct synthesis or by

chemical dealumination of parent zeolites (either by steaming and acid leaching

or by SiCl

vapor treatment), which makes it possible in principle to extract the

aluminum from the network without the structure collapsing, was extensively

studied in the 1990s.

Together with ZSM - 5 zeolites, dealuminated HY (ultra - stable Y or USY) zeolites

are the most widely used in petrochemistry (ﬂ uid catalytic cracking processes)

[112] . The active centers are Br ø nsted acid sites carried by the zeolite framework.

USY is derived from synthetic Y faujasites dealuminated by chemical or, more

commonly, hydrothermal treatments. These treatments partially remove alumi-

num atoms from the crystal framework, improving the thermal stability, as well

as decreasing the number of Br ø nsted sites, which is a function of the concentra-

tion of Al atoms, but generating extra - framework aluminum ( EFAL ) species and

framework defects, which are generally associated with Lewis acidity. The Lewis

sites may have their own catalytic activity and also may interact with Br ø nsted

sites, increasing their strength [62] .

9.3 Surface Properties of Oxides 425