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

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396 9 Thermal Analysis and Calorimetric Methods

the sample) and a secondary isotherm obtained after desorption under vacuum

and re - adsorption of the gaseous probe at the same temperature. This difference

can be roughly interpreted as the amount of strong sites (often associated with

adsorption heats of the order of at least 100 – 120 kJ mol

− 1

, depending on the probe

molecule and the experimental conditions).

For each dose or expansion, the equilibrium data such as the pressure P

and

the integral evolved heat ∆ Q

int, i

, are measured. Kinetic results for each dose are

also measured, such as the heat ﬂ ow and the evolution of the gas pressure as a

function of time. (Figure 9.2 ).

From these results we can also obtain the equilibrium pressure P , the adsorbed

amount up to the dose i ( Σ ∆ n

a, i

= n

), and the corresponding evolved heat ( Σ ∆ Q

int, i

= Q

int

). These data can be expressed in ﬁ ve ways, as illustrated in Figure 9.3 .

1. The volumetric isotherms ( n

, P ) for a cycle consisting of adsorption I,

desorption, and re - adsorption II (Figure 9.3 a)

2. The corresponding calorimetric isotherms ( Q

int

, P ) (Figure 9.3 b).

From these two types of isotherm (calorimetric and volumetric) the internal

energy and the molar entropy of adsorption

=+ +

∫

int

can be evaluated as functions of the degree of coverage, using the pairs of values

( n

, Q

int

) measured at the same equilibrium pressure.

When there are several possible adsorption mechanisms (e.g. reversible and

irreversible adsorption), these isotherms can be used to evaluate the sequence

and the importance (number of active sites and bond energy) of the different

processes.

3. Another representation of calorimetric data is that of integral heats as a function

of the adsorbed quantities ( Q

int

= f( n

)) (Figure 9.3 c). This representation leads

to the detection of coverage ranges with constant heat of adsorption, for which

the evolved heat is a linear function of the coverage.

4. Complementary information is given by the variation of the differential heats

diff

as a function of n

(Figure 9.3 d).

Figure 9.2 Recorded data (pressure and heat ﬂ ow signals) for

each dose of probe molecule as a function of time.

The variation of the differential heat of adsorption ( Q

diff

= ∆ Q

int, i

/ ∆ n

a, i

) as a

function of coverage is an indication of the homogeneity or heterogeneity of the

adsorption sites.

The ratio of the amount of heat evolved for each increment to the number of

moles adsorbed is equal to the average value of the differential enthalpy of

adsorption for the adsorbed quantity considered. The curve showing the

differential heat variation as a function of the adsorbed amount is traditionally

represented by histograms. However, for simpliﬁ cation, the histogram steps

are often replaced by a continuous curve connecting the midpoints of the

histograms.

A decrease in the differential heat values as a function of coverage is

generally attributed to heterogeneity of the adsorption sites on the surface of the

solids.

A classical calorimetric curve of differential heat of adsorption against probe

uptake is presented in Figure 9.4 .

This curve can present:

(a) An initial region of high adsorption heat, mainly ascribed to adsorption on

Lewis sites, which falls abruptly.

Figure 9.3 Data obtained from volumetric – calorimetric adsorption experiments.

9.2 Techniques and Procedures 397

398 9 Thermal Analysis and Calorimetric Methods

(b) One or more plateaux of intermediate strength sites with a nearly constant

heat, attributed preferentially to homogeneous Br ø nsted sites.

heterogeneity of the sites (probably Lewis sites).

A bump in the curve can also be observed sometimes, indicating interactions

between adsorbed molecules.

(d) A reversible adsorption domain, characteristic of physisorption of the probe,

or hydrogen bonding between the probe and the sample, or very weak Lewis

acidity. The curve reaches a value close to that of the heat of condensation

of the adsorbate in the liquid phase [14] .

Such a heat (enthalpy) diagram can be used to assess the uniformity – non - uni-

formity of the surface of the adsorbent with respect to energy, the energy of the

lateral (adsorbate – adsorbate) interactions, and the structural or textural changes

that the adsorbent often undergoes as a result of interaction with the adsorbate

[17] .

5. It is possible to obtain a distribution of the energies of the adsorption sites by

plotting d n

/d Q

diff

as a function of Q

diff

. The area under the curve is representative

of the number of molecules that are adsorbed with a given evolved heat and

thus the number of sites corresponding to a given strength (Figure 9.3 e).

However, this type of representation is somewhat less accurate than the

previous one.

Finally, the systematic determination of the time constants (or thermokinetic

parameters) of the calorimetric signal peaks gives information on the diffusion

Figure 9.4 Regions in a typical curve of differential heats of

adsorption versus adsorbed amount. All regions (a, b, c, d)

can be observed for zeolite samples presenting both Lewis

and Br ø nsted acid sites, as probed by ammonia adsorption.

For oxides presenting only Lewis acid sites, the regions a, c

and d are observed.

rate of the probe molecule on the solid framework. The change of gas pressure is

related to the kinetics of the adsorption process; however, concerning the calori-

metric curve, the kinetic data necessary to detect reaction mechanisms can some-

times be distorted by the inertia of the calorimeter. The limitations and advantages

of the technique are fully described in Ref. [15] .

Micro - calorimetric adsorption measurements require a proper in situ vacuum

activation at a higher temperature than the adsorption process. A ﬁ rst pre - treat-

ment under oxygen is performed in the calorimetric cell in order to eliminate the

impurities present on the sample (essentially carbonates, nitrates, carbonaceous

residues and water present from the preparation, calcination and exposure to

atmosphere) and to avoid the partial reduction of the surface of an oxide that is

easily reduced under vacuum.

9.2.1.2 Gas Flow Methods

An alternative method is ﬂ ow adsorption microcalorimetry, which involves the use

of a carrier gas passing continuously through the adsorption cell. The catalyst is

placed on a glass frit in a gas circulation cell in the calorimeter. In order to deter-

mine the amounts of gas adsorbed, ﬂ ow calorimetry must be used in combination

with another technique, most frequently TG, MS or GC [8, 18] .

When the system is used in pulse mode, it allows the measurement of heats of

adsorption of a gaseous reactant on a solid or interaction heats between a gaseous

reactant and pre - adsorbed species. When used as a ﬂ ow reactor, it allows the

kinetic study of catalytic reactions as well as the study of the activation or the aging

of the catalyst. This is also a suitable system to perform calorimetric temperature

programmed reduction ( TPR ), temperature programmed oxidation ( TPO ) or tem-

perature programmed desorption ( TPD ) experiments. In addition to calorimetry,

temperature programmed desorption ( TPD ) of adsorbed probe molecules can in

principle also be used to estimate heats of adsorption [19] .

A comparison between pulsed ﬂ ow and conventional pulsed static calorimetry

techniques for characterizing surface acidity using base probe molecule adsorp-

tion has been performed by Brown and coworkers [20, 21] . In a ﬂ ow experiment,

both reversible and irreversible probe adsorption occurring for each dose can be

measured, and the composition of the gas ﬂ ow gas can be easily modiﬁ ed. The

∆ H

ads

versus coverage proﬁ les obtained from the two techniques were found to

be comparable. The results were interpreted in terms of the extent to which NH

adsorption on the catalyst surface is under thermodynamic control in the two

methods.

Thermal desorption techniques have gained great popularity owing to their

comparatively easy operation. Various different mathematical analyses of the ther-

modesorption proﬁ les have been proposed by different authors to provide kinetic

information about the distribution of the surface acid strength [22] . A simple

approach is based on the analysis of thermodesorption curves collected at different

heating rates ( β ). The shift of the temperature of the maximum rate of desorption

( T

max

) as a function of β can be exploited to derive activation parameters (i.e. E

)

9.2 Techniques and Procedures 399

400 9 Thermal Analysis and Calorimetric Methods

of the desorption reaction by means of different model equations. When hetero-

geneous solids are concerned, in order to derive the activation energy distribution

of the solid surface, the shift of T

max

with β has to be considered separately for

each peak. However, for this approach to provide a reliable determination of the

activation parameters, the thermodesorption curves have to be well resolved, with

clearly detectable T

max

values [23] . It has been shown that the results obtained by

this method are in good agreement with those derived from ammonia adsorption

calorimetry when the comparison is conﬁ ned to the strong fraction of acid sites.

In general, when the experiments are carefully conducted and the samples are

pre - treated identically, similar values of numbers of acid sites and heats of adsorp-

tion can be obtained from both calorimetric and TPD measurements when using

as probe. However, the results for pyridine adsorption can be quite different

for the two techniques [24] . The difference has been attributed to the fact that

desorption of pyridine can be severely limited by restricted diffusion, for example

within zeolite crystallites. The limitations and advantages of the TPD techniques

are fully described in Ref. [19] .

9.2.1.3 Calorimetry in Liquid Phase

Finally, it should be noted that calorimetric measurements can also be used to

monitor adsorption phenomena at the solid – liquid interface (in a solvent). This

method has been used to measure the adsorption heats evolved upon injection of

dilute solutions of pyridine in alkanes ( n - hexane, cyclohexane) onto an acidic solid

itself in a slurry with n - hexane. The amount of free base in solution is measured

separately with a UV - Vis spectrometer, leading to an adsorption isotherm that is

measured over the range of base addition used in the calorimetric titrations. The

combined data from the calorimetric titration and adsorption measurements are

analyzed simultaneously to determine equilibrium constants, quantities of sites

per gram and acid site strengths for different acid sites on the solid.

The measurements are performed in a non - interacting hydrocarbon solvent (e.g.

cyclohexane) whose molecular mass is close to that of the donor (e.g. pyridine) in

order to cancel out contributions from a dispersion component to the measured

enthalpy [25] . As an example, the acid strength of tungsten oxide supported on a

silica gel has been determined by this method [26] .

A similar technique has been used to determine the acidic character of niobium

oxide and niobyl phosphate catalysts in different solvents (decane, cyclohexane,

toluene, methanol and isopropanol) using aniline and 2 - phenyl - ethylamine as

probe molecules [27, 28] . The heat evolved from the adsorption reaction derives

from two different contributions: the exothermic enthalpy of adsorption and the

endothermic enthalpy of displacement of the solvent, while the enthalpy effects

describing dilution and mixing phenomena can be neglected owing to the differ-

ential design and pre - heating of the probe solution.

The titration of acid sites in liquids of different polarities and proticities (decane,

cyclohexane, toluene, methanol and isopropanol) makes it possible to discriminate

the acid site strength distribution more accurately than from the more conven-

tional gas – solid phase titration with ammonia.

9.2.2

Temperature Dependence of Adsorption – Desorption Heats

In adsorption microcalorimetry, surface equilibration depends not only on the

chosen probe molecule but also on the adsorption temperature. It is worth men-

tioning that the literature contains some controversial articles on this subject

[29] .

Heats of adsorption of probe molecules have frequently been measured at room

temperature [8] ; however, the results obtained from such measurements can

sometimes be of questionable accuracy as a result of non - equilibrium conditions

and non - speciﬁ c adsorption.

In fact, the adsorption temperature should not be too low, in order to allow

the detection of differences among the sites; otherwise under certain circum-

stances the measured evolved heat can be just an average value. Another impor-

tant issue is that one must ensure that chemisorption predominates over

physisorption.

Cardona - Martinez and Dumesic [30] have analyzed the problem of surface

mobility of the adsorbates for the particular case of adsorption of basic probe

molecules on acid sites of oxides. Without equilibration of adsorbate with surface

sites, the measured differential heat would only be an average value of the sites

that the molecules adsorb on, and differences among sites would not be detected.

Thus, ideally, measurements should be made at as a high a temperature as feasible

without desorbing or decomposing the adsorbate.

In addition to isothermal measurements, temperature programmed calorimetry

experiments can also be performed using DSC. For such measurements, the

sample is equilibrated with the adsorbate at low temperatures. The temperature

is then increased in a controlled manner, and the heat absorbed (or evolved) by

the sample over a certain temperature region is related to the heat of the event

occurring at that point.

The data obtained from the temperature programmed method must be inter-

preted with care in order to properly determine the distribution of surface sites of

different heats of adsorption. If adsorption is carried out at room or slightly ele-

vated temperatures at the beginning of the experiment, sufﬁ cient time must be

provided to ensure coverage of all the sites. Otherwise non - discriminating adsorp-

tion would occur. It has been shown that, for porous samples, re - adsorption along

the pore as a desorbed molecule diffuses out of a pore is practically unavoidable.

Therefore, although a temperature programmed experiment might appear to be

simpler to perform, there are more potential artifacts that complicate quantitative

interpretation of the results than in the case of isothermal experiments [4, 19] .

9.2.3

Probe Molecules

Appropriate probe molecules to be selected for adsorption microcalorimetry

should be stable with time and with temperature. Furthermore, in the case of

9.2 Techniques and Procedures 401

402 9 Thermal Analysis and Calorimetric Methods

zeolites and other microporous materials, they should be small enough to readily

penetrate into the intra - crystalline space. The adsorbed probe at a given tempera-

ture should also have sufﬁ cient mobility to equilibrate with active sites. As

calorimetry gives the total number of adsorption sites, and possibly catalytically

active centers, the values obtained depend on the nature and size of the probe

molecule [14] .

9.2.3.1 Probing Surface Basicity

The number of acidic probes able to cover a wide range of strength is rather small.

The ideal probe molecule should be speciﬁ c to basic sites and should not be

amphoteric. For example CO

(p K

= 6.37) is commonly chosen to characterize the

basicity of solids, but it may be either adsorbed on cations or physisorbed, or may

react with hydroxyls and oxide ions to form carbonated species.

is a poor donor but a good electron acceptor. Owing to its acidic character,

it is frequently used to probe the basic properties of solid surfaces. IR evidence

concerning the formation of carbonate - like species of different conﬁ gurations has

been reported for metal oxides [31] , which accounts for the heterogeneity of the

surface revealed by micro - calorimetric measurements. The possibility that CO

could behave as a base and interact with Lewis acid sites should also be considered.

However, these sites would have to be very strong Lewis acid sites and this particu-

lar adsorption mode of the CO

molecule should be very weak and can usually be

neglected [32] .

The same problems may arise when using SO

as an acidic probe, despite the

fact that SO

(p K

= 1.89) is more acidic than CO

and, thus, more likely to probe

the total basicity of the surface.

9.2.3.2 Probing Surface Acidity

Ammonia (p K

= 9.24, proton afﬁ nity in gas phase = 857.7 kJ mol

− 1

) and pyridine

(p K

= 5.19, proton afﬁ nity in gas phase = 922.2 kJ mol

− 1

) are the favored molecules

for probing the overall solid acidity, since both Lewis and Br ø nsted acid sites retain

these molecules. However the use of IR spectroscopy or XPS is necessary to dis-

tinguish qualitatively and unambiguously between these two types of sites. In

addition to NH

and pyridine, trimethylamine and triethylamine have also been

used to probe the acidity of supported oxides. However, it has been mentioned

[33] that these two molecules might not be able to equilibrate completely with the

surface under typical experimental conditions. The use of substituted pyridines

(2,6 - dimethylpyridine) has also been considered in order to probe speciﬁ cally the

Br ø nsted sites [34] .

Ammonia is among the smallest strongly basic molecules, and its diffusion

is hardly affected by a porous structure. In the light of the different possible

modes of interaction of this molecule with the oxide surface, it has been found

that NH

mostly coordinates to Lewis sites, but in a few cases the results were

interpreted by considering the simultaneous occurrence (to a low extent) of

dissociative adsorption leading to NH

−

and OH

−

surface species. Dissociative

adsorption of ammonia is related to the presence of very strong Lewis - type acid

sites.

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes

formed by strong and weak bases with acid sites in zeolites. They examine the

adsorption enthalpies of a series of strongly basic molecules such as alkylamines,

pyridines and imines. These workers also performed studies of the adsorption

properties of weak bases, including water, alcohols, thiols, oleﬁ ns, aldehydes,

ketones and nitriles. They report a poor correlation between the differential heats

of adsorption on H - MFI zeolites and the enthalpies of protonation in aqueous

solutions, but a much better correlation with gas - phase proton afﬁ nities [37] .

Acetonitrile is also an interesting molecule for probing acid sites in catalysts.

It is a weak base, so no protons are abstracted and actual hydroxyl groups can

be observed. It also allows the investigation of both Lewis and Br ø nsted acidities

[15] .

NO and CO can also be employed as probes to identify Lewis acid sites and

characterize their density and strength [38] .

9.2.3.3 Probing Redox Properties

The heats of reduction of oxide samples can be determined by studying the adsorp-

tion of hydrogen, CO and various hydrocarbons on the fully oxidized catalysts. The

extent of reduction of the catalyst surface can be evaluated in particular using H

The measurement of hydrocarbon (e.g. propene, propane, acrolein, etc.) adsorp-

tion heats is complicated by the subsequent reaction of the adsorbed species or by

incomplete desorption of the products.

In the case of CO reduction, the catalyst – oxygen bond energy has to be calculated

by subtracting the heat of formation of CO

However, it is known that, in the absence of processes other than plain surface

coordination, CO acts as a weak Lewis base and can interact with the strongest

surface Lewis acid sites. NO can also be employed either as a probe to identify

Lewis acid sites (as mentioned in Section 9.2.3.2 ) or as a reducing agent. However,

NO may disproportionate into N

O and oxygen and it is also very likely to form

nitrosyl complexes in the presence of transition metal ions.

The heats of oxidation of the reduced oxides can be further measured using O

adsorption. Large variations of the re - oxidation heat can be sometimes observed

when any further oxidation is limited by the diffusion of oxygen into the reduced

portion of the particle.

9.3

Surface Properties of Oxides

On one hand, both acidity and basicity of catalysts are known to be important

factors for partial oxidation reactions. Moreover, strong acidity can reduce the

selectivity by carbon – carbon bond breaking and by promoting the production of

9.3 Surface Properties of Oxides 403

404 9 Thermal Analysis and Calorimetric Methods

. Acid sites are cations that exhibit either a high oxidation state or an unsatu-

rated coordination. On the other hand, redox properties are also known to play an

important role and to be related to M

n +

→ M

( n − 1)+

equilibrium constant and to lattice

2 −

ion lability.

Since various supported and unsupported metal oxides are oxidation catalysts

for hydrocarbons, there has been interest in determining the relationship between

the reduction/re - oxidation properties of the oxides and their catalytic properties.

A common oxidation mechanism involves a redox cycle in which lattice oxygen is

removed in the reaction and then replenished by oxygen from the gas phase. Thus

there are studies to determine the energetics of removal of oxygen from the cata-

lysts and replenishment by various routes. These energetics have been determined

as a function of catalyst formulation or modiﬁ cation, and as a function of the oxi-

dation state of the catalyst [4] .

In order to try to clarify the different types of mechanism involving either redox

cycles and/or acid – base properties, a study of the surface chemistry of single,

doped and mixed oxides is of much interest. The calorimetric technique, by allow-

ing heat transfer measurements, can provide very informative data on the ther-

modynamics of solid – gas interactions and for the study of the surface and reactivity

of these metal oxides.

9.3.1

Bulk Oxide Catalysts

The surface acid – base properties of bulk oxides can be conveniently investigated

by studying the adsorption of suitably chosen basic – acidic probe molecules on

the solid. Acidic and basic sites are often present simultaneously on solid surfaces.

The two centers may work independently or in a concerted way, and the occur-

rence of bifunctional reaction pathways requiring a cooperative action of acidic

and basic centers has also received considerable attention [39] . The acid – base

properties of numerous amorphous metal oxides investigated by microcalorime-

try have been summarized in an extensive review by Cardona - Martinez and

Dumesic [11] .

The inﬂ uence of the pre - treatment temperature on the acidic properties is a very

important factor. For Br ø nsted sites, the differential heat is the difference between

the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation

of the probe molecule. For Lewis sites, the differential heat of adsorption repre-

sents the energy associated with the transfer of electron density towards an elec-

tron - deﬁ cient, coordinatively unsaturated site, and probably an energy term related

to the relaxation of the strained surface [40] .

Increasing the pre - treatment temperature modiﬁ es the surface acidity of the

solids. For γ - alumina, there are numerous surface models, and various acid sites

having different strengths are formed on the surface during dehydration. The

inﬂ uence of the pre - treatment temperature, between 573 and 1073 K, on the

surface acidity of a transition alumina has been studied by ammonia adsorption

microcalorimetry. The number and strength of the strong sites, which should be

mainly Lewis sites, have been found to increase when the temperature increases.

Moreover the pre - treatment temperature affects the whole spectrum of adsorption

heats at various coverages, and not only the initial heat [13] .

Niobium oxide surfaces are also very dependent on the dehydration tempera-

ture. An illustration is given in Figure 9.5 , which represents the differential heats

of NH

adsorption versus ammonia coverage for a niobium oxide from CBMM

pre - treated at 423, 523, 623, 723 and 823 K [41] .

As can be seen, a regular decrease in the adsorption heats occurs when the

evacuation temperature increases from 423 to 823 K. The number of strong sites

is less affected by dehydroxylation of the surface than that of weak sites, and tends

to a limit.

Figure 9.6 represents the differential heats of NH

and SO

adsorption as a

function of coverage for silica, magnesia and γ - alumina samples [15] .

Silica presents low heats of adsorption for both basic and acidic molecules,

indicating that the surface sites on silica are either weakly acidic or weakly basic.

The adsorption is mainly due to hydrogen bonding and van der Waals interaction.

The differential heats of adsorption of ammonia on silica show a decrease as the

adsorption temperature increases [12] . The differential heats of adsorption of

ammonia and sulfur dioxide on the surface of alumina show the presence of

strong acid and strong basic sites, respectively. The heats of adsorption for alumina

are typical of a strong acidic surface, but also indicate the amphotericity of the

surface. The initial heat increases and the adsorption capacity decreases with

increasing pre - treatment temperature [13] . By contrast, magnesia displays only

basic sites adsorbing SO

molecules preferentially, and very few acidic sites.

The differential heats of adsorption of NH

and CO

over 18 bulk oxides

have been determined [42, 43] . These oxides were classiﬁ ed, according to their

9.3 Surface Properties of Oxides 405

Figure 9.5 Variation with the activation temperature of the

differential heats of adsorption versus ammonia coverage.