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

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274 6 Photoelectron Spectroscopy of Catalytic Oxide Materials

(Figure 6.7 ) makes the V 2p core levels of VPOs analogous to binary oxides! Fur-

thermore, applying Coulston ’ s equation (Equation 6.7 ), calculated average valences

nicely agree with the result of the curve ﬁ tting. According to the electron attenua-

tion in the solid, the applied excitation energies correspond to approximately 1 and

3 nm information depth, respectively the notiﬁ cation “ surface ” and “ bulk ” . At

1254 eV excitation energy (which is typically also applied in laboratory experiments

with Mg K α excitation) the two samples are clearly different, mainly because of

the relatively high V

contribution in VPO

. However, surface - sensitive spectra

indicate many more similarities. Undoubtedly, VPO

is inhomogeneous in the

top 1 – 3 nm: some V

species (phase?) sit in sub - surface positions, while mainly

occupies the top surface position. As opposed to VPO

, VPO

is fairly homo-

geneous in XPS sampling depth. However, in line with the ﬁ tting results, the

opposite asymmetry of the peaks for surface - and bulk - sensitive modes clearly

indicates a small V

contribution at the surface. Whether or not VPO

also con-

tains V

in the top layer is not easily concluded simply on the basis of the spectra.

(Note that, theoretically, it is possible to answer this question, if a precise geo-

metrical model of the terminating few layers of VPO material is available. However,

the characteristics of the top few layers are widely disputed in the VPO community,

and furthermore the V

phase in the sub - surface position is not compatible with

the (VO)

structure. We thus decided not to model our depth distribution.)

Based on similar catalytic performance, however, it is likely that both materials

Figure 6.8 Vanadium 2p

3/2

region of two

VPO catalysts (a: VPO

, b: VPO

) in situ

under butane selective oxidation conditions

(T: 673 K) at two excitation energies.

(A) 1254 eV ( “ bulk ” sensitive), as typically

applied in laboratory XPS experiments.

(B) 730 eV ( “ surface ” sensitive). As the

spectral function of V

in VPO is not known,

formally three Gauss - Lorenz curves were

used to approximate the obviously visible

contribution in VPO

and to account

for the asymmetry seen on the low E

side

of the peaks. Although this latter peak,

according to its E

, might be assigned to V

its identiﬁ cation purely on this basis is not

straightforward. Its location however seems

to be more sub - surface than directly on the

surface.

6.3 Case Studies 275

contain small V

centers in a V

matrix at the surface. This result is in line with

numerous literature reports [79, 137, 140 – 143, 149, 157, 167] derived from con-

ventional XPS experiments, in which, however, precaution is necessary. It is

important to note that different conclusions would have been drawn on the sole

basis of the spectra at typical laboratory XPS excitation! The similarity of the two

VPO materials is further manifested in the surface - sensitive O 1s spectra (Figure

6.9 A), indicating nearly identical oxygen species. The main line at ∼ 530 eV is in

accordance with the lattice oxygen in binary vanadium oxide systems, while the

higher E

components might, as a ﬁ rst approximation, correspond to more elec-

trophilic surface oxygen species, for example OH groups or various oxygen species

bonded to phosphorus or carbon [149, 152, 166, 170] . (It is assumed in the litera-

ture that phosphate and pyrophosphate oxygen is more electrophilic and should

appear at signiﬁ cantly higher binding energy than oxygen bonding to vanadium.)

Comparison of these spectra with binary vanadium oxides has serious conse-

quences in data interpretation: in both cases the main line is found at the same

. Therefore, either (i) the surface consists mainly of VO

in VPO, and there is a

considerable excess of vanadium on the surface or (ii) phosphorus linkage does

not modify the binding energy of lattice oxygen (at least within the resolution of

our and previous XPS studies). In either case, most XPS interpretations and their

consequences related to mechanistic considerations in the literature would need

substantial revision! To be able to further comment on this issue, the surface P/V

ratio has to be taken into consideration.

Figure 6.9 (A) Oxygen 1s region of two VPO catalysts

(a: VPO

, b: VPO

) in situ under butane selective oxidation

condition (T: 673 K) recorded at 730 eV ( “ surface ” sensitive)

excitation energy. (B) Phosphorus 2p region of two VPO

catalysts (a: VPO

, b: VPO

) in situ under butane selective

oxidation conditions (T: 400 ° C) recorded at 854 eV

( “ bulk ” sensitive) excitation energy.

276 6 Photoelectron Spectroscopy of Catalytic Oxide Materials

The second motivation for doing an XPS analysis of materials is to learn about

the elemental stoichiometry of the near - surface region, which brings us to the

widely disputed issue of the P/V ratio of VPOs. The P/V ratio is often considered

to be an important factor in the catalytic properties of VPO [126, 128, 140, 171,

172] . This is mainly based on conventional XPS experiments, in which phosphorus

enrichment in the top few nanometers was found [140 – 142, 150, 156, 157, 159 –

162, 164, 168, 172, 173] . According to a literature survey, the P/V ratio varied

mainly in the range of 0.8 – 3 (Table 6.3 ). Owing to the different sampling depth

of V 2p and P 2p in typical ESCA experiments (as a result of the different kinetic

energies of the photoelectrons), surface impurities (e.g. carbon deposits) will

attenuate the lower KE electrons from vanadium more strongly and hence slightly

overestimate the P/V ratios. Furthermore, the calculation relies on the often -

questioned accuracy of photoemission cross - sections or sensitivity factors. To cir-

cumvent this problem, several attempts were made to apply reference substances

in recalibrating possible incorrect sensitivity factors [158, 165, 168, 174] . Okuhara

and coworkers [165, 174] prepared VPO glasses with different stoichiometry, and

found that the Scoﬁ eld [175] sensitivity factors overestimate the P/V ratio by a

factor of 4.1/2.75 (i.e. by approximately +50%), decreasing the P/V ratio of their

VPO samples to values close to the nominal bulk composition. In the same vein,

using VPO glass Richter and coworkers [168] found a much smaller discrepancy

of only ∼ 14%, and their estimation gave a surface P/V ratio of 1.3 – 1.5. Considering

the sampling depth as 3 nm and assuming a proﬁ le of P segregation, the phos-

phorus concentration in the top layer was assumed to be even higher. Ion - scatter-

ing spectroscopy experiments, showing a P/V ratio of 2 – 3, conﬁ rmed this

hypothesis. Coulston and coworkers [158] used organometallic complexes contain-

ing vanadium and phosphorus for XPS calibration. The observed deviation was as

high as ∼ +75%, overestimating the theoretically calculated composition of the

phosphorus content. In this way, the surface composition of different VPO phases

nicely approached the nominal bulk values. Interestingly, the Scoﬁ eld sensitivity

factors gave a fairly reasonable V

5.18

stoichiometry for vanadium pentoxide,

indicating that the problem of high P/V ratios is manifested in the phosphorus

calculation. As Richter [168] pointed out, the VPOs (even VPO glasses) are sensi-

tive to water/moisture, giving rise to hydrolysis of V

−

P bonds and hydration

of phosphate groups (see below). Therefore, if moisture - driven segregation of P

occurs in both VPOs and VPO glasses, the recalibration of cross - sections will

simply cancel out the P segregation, resulting in a stoichiometry resembling that

of the bulk. Unfortunately, even for organometallic complexes it has not been

proven that the ligands do not undergo X - ray - induced damage and are stable under

UHV conditions, which may hamper determination of the real P/V ratio. Hence

the calculated P/V ratios (whether recalibrated or not), should be approached, in

most cases, with caution.

There might, however, be some exceptions. Once the calculated ratios exceed 2,

even using Coulston ’ s recalibration, the P/V ratio (2/1.75) will be high enough to

account for a surface terminated by pyrophosphate groups. Sometimes, values

even higher than 2 were reported (see Table 6.3 ), which would clearly indicate

6.3 Case Studies 277

phosphorus segregation to the surface. In other cases, P enrichment is strongly

indicated when, using the same calculations, a series of samples shows a clear

variation/tendency in the P/V ratio. Such a situation was described in [150, 160,

162, 164] . Phosphorus enrichment was further supported by low - energy ion scat-

tering ( LEIS ) experiments by two independent research groups [142, 176] . This

technique, as opposed to XPS, is sensitive to the topmost surface layer. Delich è re

and coworkers [142] observed an initial P/V ratio of 2.35, which slowly decreased

to below 1.2 by the end of the measurement owing to the sputtering nature of the

experiment. From the sputtering proﬁ le of the individual elements, they concluded

that, in fact, it is not an enrichment of phosphorus, but rather a deﬁ ciency of

vanadium, that is responsible for the observations. Jansen and coworkers [176]

detected a P/V ratio of 2 ± 0.2 and they argued that VPO catalysts could be termi-

nated by a distorted vanadyl pyrophosphate structure, where the excess phospho-

rus is positioned between the vanadyl units and the phosphate groups. To sum

up, there are many cases in which no conclusion on surface enrichment can be

drawn; however, in many others phosphorus does indeed seem to segregate to the

surface.

Phosphorus spectra were collected during in situ experiments as well; hence the

P/V ratios could be calculated. Applying theoretical cross - sections by Yeh and

Lindau [177] , the P/V ratios were as high as 2.5. When using measurements on

reference VOPO

phases, the ratio drops to ∼ 1.2 – 1.3, as calculated by Kleimenov

[100] . Repeated experiments on VPO

conﬁ rmed the small phosphorus excess in

the topmost layers ( ∼ 1.2) while with higher ( ∼ 3 nm) sampling depth the ratio

approached the nominal stoichiometry of 1. Note, however, that possible phos-

phate segregation in the reference material might give rise to underestimation of

the P/V ratio of our VPOs.

As opposed to the P/V ratios, the phosphorus core levels are rarely depicted in

the literature. In situ P 2p spectra are show in Figure 6.9 B. In general, phosphorus

in the non - elemental form is represented by an unresolved doublet with energy

separation of ∼ 0.85 eV and area ratio of 2 : 1. The shape of our P 2p spectra can be

best described by the presence of at least two doublets, indicating a minimum

number of two distinct phosphorus species. Two doublets were used in the curve

ﬁ tting procedure. The 3/2 spin orbit component of the more abundant species is

close to the maximum of the 2p envelope at 132.8 eV, which is again lower than

most of the reported energies. Morgan and coworkers [20] investigated alkali

phosphates and pyrophosphates, and cesium and potassium pyrophosphate would

match perfectly to this energy. All the other (Rb, Na, Li) pyrophosphates would

also ﬁ t, assuming small differential charging in Morgan ’ s work, relative to the

reference C1s line, as the separation of O 1s – P 2p was mainly independent of the

type of cation. Morgan observed the corresponding phosphate 2p values at 0.5 –

0.7 eV lower energy, which is not far away from the 0.8 – 0.9 eV in our case. There-

fore, we suppose the presence of mixed pyrophosphate/phosphate species in

the surface - near region. Recalling the V 2p spectra, VPO

contained signiﬁ cant

amounts of V

, which is most likely one of the phosphate phases/species,

while the other sample contained much less. The relative concentration of

278 6 Photoelectron Spectroscopy of Catalytic Oxide Materials

in both samples ﬁ ts quite well to the phosphate/pyrophosphate ratio,

indicating the correspondence of the two core levels. Obviously, owing to lack of

resolution in the 2p core level, the presence of additional P species cannot be

excluded.

The entirely neglected, but extremely important, region of XPS investigations

of VPO material is the valence band. The high - lying levels correspond to bonding,

anti - bonding and delocalized orbitals, which hold essential information about the

catalytic properties of the material. As the valence region is more sensitive to the

chemical environment (temperature, presence of reactive gases; i.e. reaction condi-

tions) than the core levels, in situ instrumentation is highly beneﬁ cial, and is

expected to unravel important pieces in the puzzle of VPO chemistry. Fortunately,

recent advances in theoretical calculations can facilitate the interpretation of the

complex overlapping structures observed in the valence band. Considering ﬁ rst a

naive ionic view, VPOs with fully oxidized vanadium and phosphorus (V

, P

)

will show only an O 2p contribution in the valence band. From the more simple

system, it can be easily concluded that it is most likely incorrect, as all -

electron DFT calculations by Eyert and H ö ck [64] have clearly shown strong

hybridization of the crystal ﬁ eld split V 3d and O 2p states. Therefore in the valence

band of VPOs a strong mixing of V 3d, O 2p and P 3p/3s orbitals is expected.

Using cluster and DFT calculations, Hermann, Witko and coworkers [69, 70]

investigated the electronic structure of the perfect VPP surface and model clusters.

The calculations show that vanadyl pyrophosphate forms a material with mixed

ionic – covalent character. According to DFT results, the vanadium atoms are

described by atomic charges ∼ +1.0 and phosphorus atoms by ∼ +1.3 [69] . Major

covalent contributions participate in the V

−

O as well as in the P

−

O bonding.

Triply coordinated oxygen sites are found to be the most negatively charged

( − 0.74). Furthermore, the calculations indicated that doubly coordinated oxygen

sites and oxygen atoms singly coordinated to phosphorus are characterized by a

similar local susceptibility with respect to electrophilic attack. Figure 6.10 depicts

the valence band region of VPO

in situ (as shown previously for the low - lying

core levels). Since vanadium is mainly in the reduced formal V

state, its 3d orbital

contribution at ∼ 1 eV represents the highest occupied molecular orbital ( HOMO )

state of VPO. The 2 – 7 eV region is typically occupied by non - bonding O 2p and

mixed bonding orbitals in vanadium oxides, while the features between 8 and

12 eV are characteristic of P

−

O mixed orbitals [170] . XPS analysis by Sherwood

and coworkers [152 – 155] of different (pyro)phosphate materials indicate a non -

negligible contribution of the (pyro)phosphate orbitals in the higher lying 3 – 8 eV

region, as well. Thompson and coworkers [178, 179] calculated local and total

density of states ( DOS ) of vanadyl pyrophosphate. Their calculation included DOS

of bulk, relaxed and non - relaxed (100) surface, pyrophosphate termination and the

hydrated surface. Our data adequately reproduced the hydrated surface (the calcu-

lation is included in Figure 6.10 ), while it did not match the other DOS curves.

Some of the intensity marked by “

䊊

” might be described by additional contribu-

tions from phosphate orbitals [170] , and the arrow indicates a tiny contribution

6.3 Case Studies 279

from gas - phase O

. (Some tiny contribution of O

is blurred at ∼ 6.5, 11 and 12.5 eV,

as well.) Recalling the problem with the O 1s spectra, they seem to be the most

compatible with the interpretation of O 1s being composed of a main unresolved

peak of lattice oxygen of all types (V

−

O, P

−

O, V

−

PV, V

−

P; remember the

relatively small difference in their electron population [69] ) and a higher binding

energy hydroxyl peak (P

−

OH, V

−

OH). As most of the VPO material contains a

signiﬁ cant amount of carbon from the preparation steps, it will also give a small

contribution to the O 1s envelope. The weak photon energy dependence of the O

1s envelopes (not shown) and the relatively high information depth of the valence

band (1 – 2 nm) indicate that not only the top, but likely more layers (2 – 3) are

hydrolyzed and hydrated. The enrichment of phosphorus at the surface was clearly

linked to this process [168] . Nevertheless, the mismatch of the hydrated and all

the other surfaces calculated by Thompson clearly indicates that mechanistic con-

siderations have to start with a highly hydrated (and likely defect - rich [180, 181] )

surface, which will obviously modify the acid – base character of the material.

Further, the DOS calculations have shown that the lowest unoccupied molecular

orbital ( LUMO ) of VPP was composed of V 3d unoccupied orbitals, strongly sug-

gesting that both Lewis acid and base character reside in reduced vanadium

centers.

Armed with these results, in what follows we attempt to construct a model best

describing the observations and the essential literature knowledge on VPO chem-

istry. Vanadium phosphates are fascinating materials, capable of abstracting

hydrogen and transferring many electrons and oxygen during the selective oxida-

Figure 6.10 Synchrotron based valence band

spectrum of VPO

in situ under butane

selective oxidation conditions (T: 673 K). In

comparison, the total density of states

according to DFT cluster calculation [178] of

hydrated VPP is shown. The DFT result is

reproduced with kind permission of D.

Thompson. The theoretical curve is aligned

with the experiment to coincide with the band

gap transition at ∼ 1 eV. (The peak marked by

the arrow corresponds to gas phase signal,

while intensity in the position labeled with a

circle “ 䊊 ” is partly due to phosphate orbitals.)

280 6 Photoelectron Spectroscopy of Catalytic Oxide Materials

tion of butane. In spite of the proposed correlation between the unique bulk

structure of VPP and the oxidation process, the overwhelming evidence (effective-

ness of different bulk structures, amorphous VPO, signiﬁ cant P excess at the

surface) suggest that the active site of the reaction cannot be represented as a

simpliﬁ ed termination of the VPP phase. However, the surface seems to have

some similarities with VPP: it is a strongly hydrolyzed and hydrated VPP [182] ,

and likely containing additional minority of V

(and possibly V

sub - surface)

sites, as well. The high P/V ratio is apparently the consequence of the local struc-

ture decomposing, slowly releasing phosphoric acid [168] , which segregates to the

surface. This process is in line with the regenerative treatment of VPO catalysts

using phosphorus compounds and/or steam, according to patent literature (for

example refs [183, 184] ). The destruction of the top few layers is compatible with

the 1 nm non - crystalline adlayer found on equilibrated catalysts after back extrapo-

lation to zero time (to remove beam damage) in high - resolution transmission

electron microscopy ( TEM ) experiments [185] . The local nucleophilicity in the

surface is manifested through occupied vanadium 3d orbitals (see valence band),

but empty vanadium 3d levels predominate in low - lying conduction band regions,

indicating that vanadium is also the surface acid site [178, 186] . The kinetic isotope

effect in butane oxidation indicated that the rate - determining step is the activation

of a methylene C

−

H bond [146] . Nucleophilic activation of the methylene position

was deduced by comparing the HOMO – LUMO structure of butane [185] . Quantum

chemical calculations [186] of butane adsorption on VPP clusters have shown that

butane is adsorbed via nucleophilic attack of methylene carbon on vanadium,

donating electron density toward it. Concomitantly, transfer of electron density

from vanadium to methylene hydrogen occurs, with rupture of the C

−

H bond.

Since the vanadium sites, being the most nucleophilic centers, are not modiﬁ ed

upon hydration, the calculation gives a highly likely scenario on the decomposed

VPP surface as well. Furthermore, in the post - activation steps the participation of

mono - coordinated terminal P

−

O oxygen (as the most basic O) was suggested by

the calculations. However, hydration clearly changed that part of the surface DOS

[178] thus eliminating the possibility of this scenario. It was shown that butadiene,

and to a lesser extent furan, can be produced on reduced VPP and the formation

of MA was strongly hindered [144] . Lattice oxygen ions located in the top surface

(and to a lesser extent in the sub - surface) layer are suggested to be responsible for

the oxidation of butane [146] . Hence it is reasonable to assume, as suggested in

many publications [137, 140 – 145, 187] , that V

sites are necessary in the last

oxygen insertion steps. This is compatible with the XPS observations. Although,

up to now VPO is the only relevant material for this process, MA can be formed

on supported binary V

, but with lower selectivity [188 – 190] . That said, the role

of phosphorus in VPO seems to increase MA selectivity by isolating individual

active ensembles in two dimensions, controlling the over - oxidation of the

vanadium ensemble [128, 191] (i.e. hampering the formation of V

). Conse-

quently, the total oxidation is rendered a minority path. A similar model was

constructed and proposed recently by Schl ö gl [185] .

6.3 Case Studies 281

6.3.3.2 Direct Oxidation of Propane to Acrylic Acid Over M o V - Based

Mixed Metal Oxides

In recent years, the direct (amm)oxidation of propane into acrylic acid (or to acry-

lonitrile) has received considerable interest from both an industrial [192 – 194] and

an academic point of view [127, 195, 196] , owing to its possible high economic

beneﬁ t in future technology. Although many different types of materials were

investigated for this process, by far the most promising is the orthorhombic phase

of a mixed Mo/V/(Te or Sb)/Nb oxide, called M1 [197] . Its structure, shown in

Figure 6.11 , was reﬁ ned by DeSanto and coworkers [198] and Murayama and

coworkers [199] , and consists of ﬁ ve - , six - and seven - membered rings of corner -

linking octahedra. The octahedra are proposed to be occupied by molybdenum

and vanadium, the pentagonal ring (pentagonal bipyramid) by niobium, and the

larger channels with tellurium (or antimony). Another phase often found in

MoVTeNb mixed oxides is the so - called M2 phase, which, however, was shown to

be inactive for propane activation, but revealed good activity and selectivity in the

subsequent reaction steps [197, 200, 201] . M1 crystallizes in the form of needles,

and its basal (001) plane, exposing the multi - membered rings, was suggested to

contain the active sites of the selective oxidation reaction [202 – 204] . In numerous

communications, Grasselli and coworkers [204 – 206] proposed a detailed reaction

mechanism at the molecular level assuming perfect termination of the basal plane

on the basis of “ chemical reasoning ” . The few XPS investigations (Table 6.4 ) on

the M1 phase, however, indicated that the surface is typically enriched by Te (or

Sb) and is deﬁ cient in V [202, 208 – 212] . The enrichment of Te was as high as

+233%, compared to the bulk composition. Phase - pure M1 samples prepared by

Celaya [218] were investigated by in situ XPS under propane oxidation conditions

in the sub - millibar pressure range. In line with the ex situ XPS experiments, the

top few layers were signiﬁ cantly different from the bulk (Figure 6.12 ), that is, Te

was strongly enriched while V and Nb were depleted. Even more importantly, the

surface seems to possess a certain ﬂ exibility, since the addition of water to the feed

Figure 6.11 Schematic representation of the structure of M1

phase along the (001) direction as determined in [198] .

282 6 Photoelectron Spectroscopy of Catalytic Oxide Materials

Ref V2p (3/2) Mo 3d (5/2) Nb 3d (5/2) Te 3d (5/2) [Sb 3d 5/2] Note

Table 6.4 XPS characteristics of the MoVTeNb related materials according to the literature survey.

Ref. V2p (3/2) Mo 3d (5/2) Nb 3d (5/2) Te 3d (5/2) [Sb 3d 5/2] Note

(eV)

[identiﬁ ed as]

Relative

change to

the bulk (%)

(eV)

[identiﬁ ed as]

Relative change

to the bulk (%)

(eV)

[identiﬁ ed

as]

Relative

change to

the bulk (%)

(eV)

[identiﬁ ed

as]

Relative

change to

the bulk (%)

[207] 516.7 – 516.5

[5+ and 4+]

232.5 – 232.8

[6+ and 5+]

206.3 – 206.6

[mainly 5+]

576.2 – 576.3

[4+]

MoVTeNb

[208] 516.7

[4+]

− 39%

235.7/234.6

( 3/2! ) [6+/5+]

norm. to Mo

206.8

[5+]

− 7%

576.7

[4+ and 6+]

+233% M1 (Te)

before ammoxidation

[208] 516.7

[4+]

− 50%

235.5 ( 3/2! )

[6+]

norm. to Mo 206.7 +33% 576.7

[4+ and 6+]

+167% M1 (Te)

after ammoxidation

[208] 516.3

[4+]

− 50%

235.9/234.8

( 3/2! ) [6+/5+]

norm. to Mo 206.3

[5+]

+33% 540.1

[4+ and 6+]

+123% M1 (Sb)

before ammoxidation

[208] 516.1

[4+]

− 43%

235.9/234.8

( 3/2! ) [6+/5+]

norm. to Mo 206.3

[5+]

+27% 540.1

[4+ and 6+]

+92% M1 (Sb)

after ammoxidation

[209] 516.2/517.3

[82% 4+]

0% 232.7/231.7

[56% 6+]

norm. to Mo ?

[5+]

+46% 576.2

[4+]

+32% T6 - 2 sample (M1)

[210] 516.2/517.3

[84% 4+]

0% 232.7/231.7

[57% 6+]

norm. to Mo ?

[5+]

− 9%

576.2

[4+]

+91% NC - 5 (M1)

6.3 Case Studies 283

Ref

V2p

(3/2)

(5/2)

[Sb

5/2]

Note

(eV)

[identiﬁ ed as]

Relative

change to

the bulk (%)

(eV)

[identiﬁ ed as]

Relative change

to the bulk (%)

(eV)

[identiﬁ ed

as]

Relative

change to

the bulk (%)

(eV)

[identiﬁ ed

as]

Relative

change to

the bulk (%)

[211] 516.7

− 45%

232.8/231.9

[81% 6+]

norm. to Mo 207.2

[5+]

+27% 576.7

[6+]

+64% M1 before ammoxidation

[211] 516.3

− 55%

232.8/231.9

[72% 6+]

norm. to Mo 207.2

[5+]

+18% 576.4

[6+]

+73% M1 after ammoxidation

[202] 516.1

[4+]

− 49%

232.4

[6+]

+22% – – 575.9

[4+]

+13% Mo

(M1)

(Air/N2 grinding)

[212] ?

[4+]

− 28%

[93% 6+]

norm. to Mo ?

[5+]

+42% +73% M1

[213] ?

− 21%

? norm. to Mo ?

− 41%

? 0% HT - 5 (MoVTeNb)

[214] 516.4/517.5

[85% 4+]

− 14

232.8/231.8

[57% 6+]

norm. to Mo 206.5

[5+]

+12 576.3

[4+]

+12 MoVTeNb - 5

[215] 517.0

[5+]

+130 232.0

[6+]

– – 576.5

[4+]

− 20

MoV

0.2

0.1

/SiO

(I)

(Not M1)

[216] 516.7 - 517.1

[5+]

− 48%

232.8/231.5

[mainly 6+]

− 6%

207.3

[5+]

− 14%

576.3 - 576.8

[4+ and 6+]

+57% MN - 30: after ammoxidation

(M2 phase)

[205] 516.9 - 515.6

[5+ and 4+]

232.7/231.6

[6+ and 5+]

207.1

[5+]

576.1/572.9

[4+ and 0]

M 1

[217] 516.3/517.3

[mainly 4+]

− 20% ∼ 232.6

[mainly 6+]

norm. to Mo ?

− 12%

? +83% #678 (M1andM2)

a Changes in surface concentration relative to bulk were normalized to Mo concentration.

b Binding energy is not given in the publication.