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

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204 5 Solid-State NMR of Oxidation Catalysts

Catalytically relevant applications of this technique are focused around the study

Al - containing systems [24, 32] . Figure 5.5 shows a single - pulse MAS

spectrum and the experimental and simulated off - resonance nutation spectra for

each individual coordination environment in AlPO

, a widely used catalyst support

[24] . From these spectra the quadrupolar coupling constants were determined to

be 3.4, 3.1 and 2.7 MHz for 4 - , 5 - and 6 - coordinated Al sites respectively. While

off - resonance nutation NMR lacks the resolution of MQMAS in discriminating

overlapping sites, it has proved itself more reliable in detecting all of the distinct

coordination environments present in a sample [32] .

5.2.1.5 Spin - Echo Mapping

While the techniques described above are widely applied in the study of metal

oxide catalysts, they are not necessary suitable for systems that contain paramag-

netic nuclei. Paramagnetic nuclei, as discussed in Section 5.3.1.2 , are not observ-

able by conventional NMR techniques and furthermore can increase the linewidths,

sometimes to beyond observable limits, of neighboring nuclei. This poses a

problem when studying catalytic metal oxides that contain paramagnetic centers.

Vanadium phosphorus oxide s ( VPO ), for instance, contain paramagnetic V

Spin - echo mapping allows the indirect observation of paramagnetic centers

by discerning their effect on neighboring nuclei. The technique was developed

Figure 5.5 (a)

Al MAS spectrum of amorphous AlPO

Resonances are Al(IV) (42 ppm), Al(V) (12 ppm), and Al(VI)

(− 11 ppm). The peak at 105 ppm is due to an Al impurity in the

rotor. For every coordination the experimental (top) and

simulated (bottom trace) off - resonance nutation spectrum is

given: (b) fourfold coordination, (c) ﬁ vefold coordination,

(d) sixfold coordination. Adapted from ref. [24] ; reprinted with

permission from the American Chemical Society.

5.2 NMR Techniques 205

through the work of Li and coworkers [33] , and its early application to VPO cata-

lysts has been reviewed by Tuel and coworkers [34] . In such systems, information

on the oxidation state of vanadium can be investigated through observing the

inﬂ uence of vanadium on the

P spectrum. For instance,

P nuclei bonded to V

nuclei are observed at 0 ppm, while those bonded to V

appear at ∼ 2500 ppm.

Figure 5.6 shows

P spin - echo mapping spectra of four distinct VPO catalyst

precursors VPO

, VPO

and VPO

prepared by two different methods

[35] . VPO

and VPO

are prepared from orthophosphoric acid and correspond

to samples before and after water reﬂ ux. VPO

and VPO

correspond to materials

synthesized from pyrophosphoric acid, again before and after water reﬂ ux.

Two different V environments are observed, V

in the VOHPO

· 0.5H

O structure

with a signal at 1725 – 1770 ppm and V

– V

dimers with a signal at around

900 ppm. The results show that the dimers formed by the latter route have a higher

stability in water. In addition to characterizing vanadium sites, spin - echo mapping

has also been used to provide similar information on paramagnetic cobalt

centers [36] .

Applied to VPO catalysts, spin - echo mapping not only provides information on

the oxidation state of different vanadium nuclei, but can also differentiate different

phases with the same oxidation state. The development of a NMR technique to

probe such materials has been extremely valuable, as their often poorly crystalline

nature prevents characterization through X - ray diffraction. Additionally, variable -

temperature spin - echo mapping has been shown capable of determining magnetic

characteristics of materials, such as their Weiss temperature [34, 37] .

Figure 5.6

P NMR spin - echo mapping spectra of VPO

VPO

, VPO

and VPO

catalyst precursors. V

species

appear at 1725 – 1770 ppm and V

– V

dimers at ∼ 900 ppm.

Adapted from ref. [35] ; reprinted with permission from the

Royal Society of Chemistry.

206 5 Solid-State NMR of Oxidation Catalysts

5.2.2

Surface Structure of Catalysts

While understanding the bulk structure of a catalyst is of obvious importance it

is often desirable to acquire information selectively about the nature of the catalyst

surface. Such information may include the nature of any acid sites present, or

differences in the coordination of atoms at the surface as compared to those in

the bulk. NMR is ideally suited to such studies owing to its inherent sensitivity to

the local environment of the nuclei under observation.

5.2.2.1 Double - Resonance Techniques

It is often the case that nuclei present at the surface of catalysts are also present

in the bulk, such as oxygen atoms in metal oxides. Therefore, a number of innova-

tive techniques have been developed in order to selectively probe surface species.

Frequently this entails the use of double - resonance methods, involving, for

example, polarization transfer between two nuclei.

Cross Polarization Cross - polarization ( CP ) NMR, normally combined with MAS

(CP - MAS NMR), provides selective information about nuclei at the surface of a

material. This technique is used to enhance the signal of surface nuclei (often the

metal nuclei, for example

Ga or

Al in the case of metal oxides such as Ga

or Al

) by magnetization transfer from the nuclei of a species present in, or

adsorbed onto, the surface. These nuclei are most commonly

H from surface

hydroxyls or adsorbed probe molecules, although examples of transferring mag-

netization from the metal nuclei in order to study the surface oxygen species also

exist [38] . In order to establish magnetization transfer, the rf pulses applied on the

two frequency channels must fulﬁ ll the Hartmann – Hahn condition [39] . The

surface - selective nature of this experiment arises from the fact that the cross -

polarization rate is dependent upon the magnitude of the dipolar interaction

between the two nuclei involved and hence on their internuclear distance. In cases

where the nucleus of interest is quadrupolar, for instance in

H -

Al CP - MAS

NMR, the central 1/2 – 1/2 transition is spin - locked and a modiﬁ ed Hartman – Hahn

condition applies [39] .

Figure 5.7 shows both

Si MAS NMR and

H →

Si CP - MAS NMR spectra of

(a) a silica - alumina support (ASA) and (b) a NiMo/ASA catalyst [40] . As can be

seen, the CP experiment selectively enhances the signals at − 100 and − 86 ppm.

These signals correspond to silicon atoms coupled to

H nuclei of OH groups.

Comparing the spectra of the supported catalyst with that of the pure support, a

strong decrease in intensity is observed at − 86 ppm and a slight decrease at

− 100 ppm. This indicates that Mo and Ni species interact with the silanol groups

and are adsorbed more easily on the geminal silanols present on the support.

Other Double Resonance Techniques Despite the fact that dipolar couplings add

to line broadening and are removed through techniques such as MAS, it is often

desirable to re - introduce them. This is because dipolar couplings contain informa-

5.2 NMR Techniques 207

tion about the environment of the nuclei, including interatomic distances. DOR

experiments are therefore often used to determine dipolar couplings between dif-

ferent nuclei. Examples of such techniques include Rotational Echo Double Reso-

nance ( REDOR ) [41, 42] , Rotational Echo Adiabatic Passage Double Resonance

( REAPDOR ) [43, 44] , Spin - Echo Double Resonance ( SEDOR ) [45] , Transferred -

Echo Double Resonance ( TEDOR ) [41, 46] and Transfer of Population in Double

Resonance ( TRAPDOR ) [47, 48] . TRAPDOR was developed by Grey and coworkers

[47, 48] and exploits the heteronuclear dipole coupling between quadrupolar and

spin - 1/2 nuclei, thus revealing connectivity and spatial interactions. It facilitates

the recoupling of quadrupole spins in Zeeman states other than ± 1/2. DOR experi-

ments have been employed by a number of workers in order to determine the

location of Br ø nsted acid sites on catalyst surfaces. For instance, Yang and cowork-

ers have used TRAPDOR to study the acid sites on a sulfated aluminum oxide

catalyst [49] . Figure 5.8 shows

H spin - echo spectra of (a) γ - Al

, (b) SO

/ γ - Al

(under TRAPDOR conditions) and (d) difference spectrum of (b)

and (c). The

H source is surface hydroxyl groups. Under TRAPDOR conditions

the

H signals at 0.3, 2.4 and 4.3 ppm are signiﬁ cantly suppressed, while the

intensity of the 11.2 ppm signal remains almost unchanged. This indicates that

Figure 5.7

Si MAS NMR and 1H →

Si CP/MAS NMR

spectra of (a) a silica - alumina support (ASA) and (b) NiMo/

ASA catalyst. Adapted from ref. [40] ; reprinted with permission

from Elsevier.

208 5 Solid-State NMR of Oxidation Catalysts

the

H species giving rise to the peak at 11.2 ppm are not in close proximity to an

Al nucleus and may instead be bound to a sulfur atom.

5.2.2.2 Spin - Echo Techniques

As discussed above, one of the key requirements in obtaining high - resolution

NMR spectra in the solid state is to overcome the various line - broadening mecha-

nisms that exist. Inhomogeneous broadening occurs because of the variation of

the macroscopic magnetic ﬁ eld over the sample, during the NMR experiment. This

may be caused by susceptibility effects or by imperfections in the instrumentation.

Spin - echo methods reverse the inhomogeneous part of the signal decay through

the application of a second rf pulse during the experiment. A spectrum is then

generated through the Fourier transformation of the echo - train, yielding informa-

tion on, for example, coupling constants. Such processes are discussed in detail

in a number of texts [50, 51] .

Spikelet - Echo The spikelet - echo experiment involves the digitization of an entire

train of spin - echoes [52] . The spectra produced by this method consist a series of

sharp “ spikelets ” (see Figure 5.9 ) which reveal the presence of more than one

species through inconsistencies in the spikelet spacings and linewidths, as these

are dependent upon the spin – spin relaxation time, T

, and hence species sensitive.

Figure 5.9 shows a

O MAS NMR spikelet spectrum of a partially metamict zircon

(ZrSiO

) as measured by Ashbrook and Farnan [53] . Two peaks are observed in

the spectrum. The peak at ∼ 150 ppm is typical of oxygen in a non - bridging environ-

Figure 5.8

H spin - echo spectra of (a) γ - Al

(without Al

irradiation), (b) SO

/ γ - Al

(without Al irradiation), (c) SO

/ γ -

(with Al irradiation), (d) difference spectrum of (b) and

(c). Asterisks denote spinning sidebands. Adapted from

ref. [49] ; reprinted with permission from the Royal Society

of Chemistry.

ment such as those found in the isolated SiO

tetrahedra, while that at ∼ 65 ppm

is typical of oxygen species that bridge SiO

tetrahedral units.

5.2.2.3 CRAMPS

Combined Rotation and Multiple Pulse Spectroscopy ( CRAMPS ) is a technique in

which the dipolar interaction is averaged through a multiple - pulse sequence [54,

55] . The simultaneous spinning around the magic angle, as in MAS NMR, aver-

ages the chemical shift anisotropy. Under appropriate conditions, CRAMP spectra

can be of greater resolution than MAS NMR spectra. While CRAMPS is not exclu-

sively a surface - sensitive technique, the majority of catalytic applications have

focused on the study of adsorbed species, and the information on surface structure

that can be extracted from their spectra.

Fitzgerald and coworkers have employed CRAMPS to identify and monitor the

H species of surface Al

−

OH groups and physisorbed water associated with a high

surface area alumina [56] . Figure 5.10 shows the

H CRAMPS spectra of four

alumina samples dehydrated under a variety of conditions. The data show that

heating at 25 ° C and 110 ° C at ambient pressure results in the partial removal of

physisorbed water (4.0 ppm). The other peaks in the spectra correspond to octahe-

dral AlOH (3.0 ppm) and Al

OH (8.2 ppm) sites.

5.3

Structure of Bulk Oxides

Metal oxide - based materials are widely employed as catalysts for a wide number

of applications, particularly in processes such as dehydrogenation and oxidation,

where redox chemistry is important. The structure of metal oxides facilitates these

reactions through the transfer of oxygen, or the removal of hydrogen. In order to

fully understand the structural dependence of these processes, and hence to reﬁ ne

existing catalysts and catalytic processes and to develop new active materials, it is

Figure 5.9

O spikelet - echo MAS NMR spectra of a partially

metamict ZrSiO

. Adapted from ref. [53] ; reprinted with

permission from Elsevier.

5.3 Structure of Bulk Oxides 209

210 5 Solid-State NMR of Oxidation Catalysts

necessary to understand the structure of the oxides themselves and the active sites

within them. Solid - state NMR techniques have proven to be an invaluable tool in

this area. Such techniques and their applications are outlined in the coming

sections.

5.3.1

Vanadium Oxide Catalysts

Vanadium catalysts are among the most signiﬁ cant metal oxide catalysts. For

instance, considering only supported catalysts, between 1967 and 2000, 28% of all

published papers were concerned with vanadium oxide - based materials [57] . This

represents a greater fraction than for any other metal or metal oxide. One reason

for this is the wide range of applications in which vanadium oxide catalysts may

be employed, examples of which are outlined below.

5.3.1.1 Supported Vanadium Oxide Catalysts

Supported vanadium catalysts, whereby vanadium oxide is dispersed on a support

such as alumina or titania are of particular importance in, for instance, the oxida-

tive dehydrogenation of alkanes [58 – 64] . Such materials have attracted consider-

able interest in the direct dehydrogenation of butane, where a key driver is to

identify the relationship between catalytic activity and structural properties [5, 6,

65 – 68] . In the pure (solid) metal oxides the coordination of vanadium is well

deﬁ ned. However, this is not necessarily true in the case of supported catalysts.

Vanadium may be present on the support surface as: isolated vanadium ions;

dimeric or polymeric species; one - and two - dimensional chains of vanadium ions;

Figure 5.10

H CRAMP spectra (360 MHz) of Al

· 2.05H

materials prepared following various dehydration procedures:

(a) desiccation over Drierite at ambient pressure (725 Torr)

for 72 h, (b) heating at 110 ° C (in ambient pressure) for 5 h,

evacuation at 6.5 mTorr for 24 h, and (d) heating at 110 ° C

under vacuum (8 mTorr) for 24 h. Adapted from ref. [56] ;

reprinted with permission from the American Chemical Society.

amorphous or crystalline oxides such as V

; or as a mixed metal oxide phase

with the support, such as AlVO

in the case of an alumina support [69] . Examples

of these are shown in Figure 5.11 . A

V MAS NMR spectrum of pure V

shown in Figure 5.2 .

The complex nature of vanadium species in catalytic materials means that no

single technique is ideally suited to their study.

V NMR techniques can, however,

provide information on the local environment of vanadium nuclei, including the

geometry and coordination number of V

species, the number of non - equivalent

Figure 5.11 Possible molecular conﬁ gurations for supported

vanadium oxides (S represents the support): (a) isolated

vanadium oxide species; (b) dimeric vanadium oxide species;

crystals.

Adapted from ref. [69] ; reprinted with permission from Elsevier.

Figure 5.12

V MAS NMR spectra of three different VO

/TiO

catalysts. Vanadia loadings are: (a) 1 wt% V

(b) 3 wt% V

and (c) 6 wt% V

. Adapted from ref. [89] ; reprinted with

permission from the American Chemical Society.

5.3 Structure of Bulk Oxides 211

212 5 Solid-State NMR of Oxidation Catalysts

sites, the nature of atoms in the ﬁ rst coordination sphere and the distortion

of this sphere. The quadrupolar (spin = 7/2)

V isotope has a natural abundance

of 99.76%, and a relative NMR intensity, with respect to

H, of 0.38. These char-

acteristics make vanadium in the 5+ oxidation state an attractive nucleus to study

by NMR techniques.

Much of the pioneering work in this area was conducted by Mastikhin and

coworkers employing both static and MAS techniques [27, 70 – 87] . These studies

probed the development of different vanadate sites on the catalyst surface as a

function of vanadium loading. For instance, for Al

- supported vanadium it was

observed that at low loadings ( ∼ 1 wt% V) isolated VO

tetrahedra were present as

indicated by a species with a chemical shift of − 750 ppm [73] . On increasing the

catalyst loading, a second species, assigned to polymeric vanadate species in which

vanadium exists in a distorted octahedral environment ( − 350 ppm), is evident in

the spectra. At higher loadings, evidence of vanadium in a V

- like environment

is observed. V

has an isotropic chemical shift of − 612 ppm. When high loadings

are coupled with high calcination temperature the formation of AlVO

is observed.

The spectrum of AlVO

is discussed in Section 5.3.1.3 . In addition to Al

sup-

ports [70, 71, 73, 74, 77, 79, 86] this group have also studied vanadium supported

on SiO

[70, 74, 79] , SnO

[72, 74, 75, 79, 86] , TiO

[74, 79, 81, 86, 87] , ZrO

[74] ,

[75] , AlPO

[74, 79, 82] , MgO [79] and a number of promoted or mixed - oxide

supports [27, 74, 76, 78 – 80, 82, 85 – 87] .

Similar studies on other supported vanadium oxide catalysts have been carried

out by other workers with similar results [88 – 93] . Among the materials studied

are VO

/Ga

and VO

/MgAl

catalysts for the oxidative dehydrogenation of

propane [94, 95] and VO

/TiO

catalysts for the selective catalytic reduction of NO

Figure 5.12 shows

V MAS NMR spectra of three VO

/TiO

catalysts differentiated

only by their loading [89] . The changes occurring in the spectra as a function of

loading are indicative of the development of new surface vanadate species, with

domains of V

dominating at 6 wt% V

. A feature of these studies is that

crystalline V

is typically only observed in

V NMR spectra of supported catalysts

above the theoretical monolayer coverage. This is the loading at which the support

surface is entirely covered by a two - dimensional vanadate network. The value at

which this occurs varies with the nature of the support surface and the geometry

of the vanadate species present. For γ - Al

, TiO

and ZrO

monolayer coverage

is typically achieved at vanadium loadings of ∼ 3.5 to 7 wt% [57] .

The nature of vanadium species on different supports can vary signiﬁ cantly. For

instance, the polymeric vanadate species shown in Figure 5.11 and observed on

alumina supports, does not form on silica - supported catalysts [62, 96] . Sasikala

and coworkers have added metal ion modiﬁ ers in an attempt to increase the dis-

persion of vanadate species in VO

/SiO

catalysts and thereby improve catalytic

activity [97] . Figure 5.13 shows that, unlike that for VO

/SiO

, the

V MAS NMR

spectrum of aluminum - modiﬁ ed VO

/SiO

exhibits a side band pattern similar to

that of bulk - like V

superimposed over a broad peak characteristic of tetrahedral

species. From these results it can be inferred that the dispersion of vanadia is

indeed better on the modiﬁ ed support.

A knowledge of the nature of the vanadium species on the catalyst surface allows

links to be made between catalyst structure and catalytic action [98] . Steinfeldt and

coworkers have investigated the effect of both catalyst loading and calcination

temperature for VO

/Al

catalysts [99] . The effect of loading is similar to that

seen by other workers and discussed above. The effect of calcination temperature

is dependent upon loading. At high (up to 700 ° C) calcination temperatures AlVO

forms, but only at loadings of greater than 4.5 wt% V. At lower loadings the disper-

sion of more crystalline vanadium into polymeric, tetrahedrally coordinated vana-

dium species is observed. The link between speciation and activity was established

by testing the catalysts for the oxidative dehydrogenation of propane ( ODP ). Cor-

relating NMR and activity measurements it was concluded that isolated and polym-

erized tetrahedrally coordinated V

species are active in propane dehydrogenation

while AlVO

and bulk - like V

have only a low catalytic activity. In a similar study

we have investigated the direct dehydrogenation of butane over VO

/Al

cata-

lysts. Calcination of a catalyst with a vanadium loading of 3.5 wt% at 973 K resulted

in reduction in the intensity of the V

peak in the MAS NMR spectrum of 71%.

Simultaneously a broad peak, assigned to more dispersed vanadium in tetrahedral

environments, appears (Figure 5.14 ). Similarly to Steinfeldt ’ s observation, when

the catalyst loading was increased to 8 wt% V the formation of AlVO

was observed

after calcination at 700 ° C. Catalytic studies on the same catalysts have related

dehydrogenation activity to the presence of polymeric vanadate species [5] .

Recent developments have allowed for an improved understanding of the local

environment of vanadium in supported catalysts. Lapina and coworkers [27] have

studied phosphorus - modiﬁ ed VO

/TiO

catalysts by SATRAS and MQMAS tech-

niques. A key question in such materials is what inﬂ uence the phosphorus has

on the structure of the catalyst. This is not directly answerable by conventional

Figure 5.13

V static and MAS NMR spectra of V

/SiO

and VO

supported on aluminum - modiﬁ ed SiO

(Si

0.8

0.2

). Adapted from ref. [97] .

5.3 Structure of Bulk Oxides 213