This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 15955–15959 15955

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 15955–15959

Polyoxometalate grafting onto silica: stability diagrams of H3PMo12O40

on {001}, {101}, and {111} b-cristobalite surfaces analyzed by DFTw

Xavier Rozanska,z*a Philippe Sautet,*aFrancoise Delbecq,

aFrederic Lefebvre,

b

Sergei Borshch,aHenri Chermette,

cJean-Marie Bassetyb and Eva Grinenval

b

Received 13th April 2011, Accepted 8th July 2011

DOI: 10.1039/c1cp21171d

The process of grafting H3PMo12O40 onto silica surfaces is studied using periodic density

functional theory methods. For surfaces with a high hydroxyl coverage, the hydroxyl groups are

consumed by the polyoxometalate protons, resulting in water formation and the creation of a

covalent bond between the polyoxometalate and the surface, and mostly no remaining acidic

proton on the polyoxometalate. When the surfaces are partially dehydroxylated and more

hydrophobic, after temperature pretreatment, less covalent and hydrogen bonds are formed and

the polyoxometalate tends to retain surface hydroxyl groups, while at least one acidic proton

remains. Hence the hydroxylation of the surface has a great impact on the chemical properties of

the grafted polyoxometalate. In return, the polyoxometalate species affects the compared stability

of the partially hydroxylated silica surfaces in comparison with the bare silica case.

1. Introduction

Polyoxometalates (POM) are molecular transition metal oxide

compounds with a structure built upon MOn and sometimes

with inclusion of XOm polyhedra, where M and X are a

transition metal and an heteroatom, respectively, and that

can present Brønsted acidity.1–4 Counter-ions are present to

balance the system charge in the solid state. The immobiliza-

tion of POM onto surfaces is of great interest from a chemical

point of view because it allows an additional control over their

physical and physico-chemical properties opening further use

in heterogeneous catalysis processes.1,2,5 Recently, Grinenval

et al.5 showed that it is possible to obtain well-defined mono-

dispersed H3PMo12O40 (H3POM) species onto silica surfaces,

allowing the accessibility to efficient heterogeneous catalysts.

Particularly, their grafting procedure permits us to get

H3POM units that retain seven times more protons than when

a classical impregnation procedure is employed.5

In this study, we focus on the grafting process of

H3PMo12O40 Keggin polyoxometalate onto silica surfaces.

This POM carries three protons and is approximately sphe-

rical with a diameter of ca. 1 nm (Fig. 1). The silica surface is

terminated with hydroxyl groups with a surface density that

depends on the temperature and water partial pressure.6–8 The

bonding between the POM molecule and the silica surface

occurs in two ways, namely via the formation of hydrogen

bonds between the acidic protons of H3POM and the surface

hydroxyl groups, be it accompanied or not by proton transfer

from H3POM to the surface hydroxyl groups, or via the

formation of covalent bond(s) between POM oxygen and

surface silicon(s), with release of water according to the

successive reactions:

RSi–OH�H3POM - H2O(g) + H2POM–SiR, (1a)

RSi–OH�H2POM–SiR - H2O(g) + HPOMQ(SiR)2,

(1b)

RSi–OH�HPOMQSiR - H2O(g) + POMR(SiR)3,

(1c)

where ‘SiR’ represents a surface Si atom bonded to three

oxygens of the silica surface, ‘–’, ‘Q’, and ‘R’ represent one,

two and three covalent bonds, respectively, and ‘�’ an hydrogen-

bonded complex. The study of this reaction is not trivial

because reaction (1) can enter in competition with the silica

surface dehydroxylation reaction:

HO–SiR + HO–SiR - H2O(g) + RSi–O–SiR, (2)

and reactions (1) and (2) can couple in:

aUniversite de Lyon, Institut de Chimie de Lyon, CNRS, EcoleNormale Superieure de Lyon, Laboratoire de Chimie, Laboratoire deChimie, Ecole Normale Superieure de Lyon, 46 Allee d’Italie, 69046Lyon, France. E-mail: xavier.rozanska@ens-lyon.fr,philippe.sautet@ens-lyon.fr

bUniversite de Lyon, Institut de Chimie de Lyon, C2P2 UMR 5265,CNRS–CPE–Universite de Lyon I, LCOMS, France

cUniversite de Lyon, Institut de Chimie de Lyon, CNRS UMR 5180Sciences Analytiques Chimie Physique Theorique–Universite de Lyon I,Francew Electronic supplementary information (ESI) available. See DOI:10.1039/c1cp21171dz Present address: Materials Design, 18 rue de Saisset, 92120Montrouge, France.y Present address: KAUST Catalysis Center (KCC), Thuwal,Kingdom of Saudi Arabia.

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15956 Phys. Chem. Chem. Phys., 2011, 13, 15955–15959 This journal is c the Owner Societies 2011

RSi–OH�H3POM + RSi–O–SiR

- H2POM–SiR + HO–SiR + HO–SiR, (3a)

RSi–OH�H2POM–SiR + RSi–O–SiR

- HPOMQ(SiR)2 + HO–SiR + HO–SiR, (3b)

RSi–OH�HPOMQSiR + RSi–O–SiR

- POMR(SiR)3 + HO–SiR + HO–SiR, (3c)

which implies that the silica surface reconstruction could be

affected by the presence of the POM. Hence, POM grafting

onto silica surfaces cannot be studied independently of the

hydroxylation/dehydroxylation processes of the silica surfaces.

Theoretical chemistry is of great help for an atomistic scale

understanding of the structure and stability of metal

complexes grafted on oxide supports.9 After the presentation

of the computational methods, we explore the stability

diagrams as a function of temperature and water partial

pressure for the grafting of H3PMo12O40 on three different

surfaces of b-cristobalite and we compare them with the

corresponding diagrams obtained on bare silica.

2. Methods

All calculations are performed in the framework of density

functional theory (DFT) for periodic systems, as implemented in

VASP.10–13 We use the PBE exchange–correlation functional14,15

with the following calculation parameters: (i) Van der Bilt

ultra-soft pseudopotentials16 for all atoms, (ii) an associated

energy cut-off for the plane wave basis set of 400 eV, (iii) a

K-point sampling restricted to the G-point owing to the

relatively large unit cells. The electronic wave-functions

Fig. 1 Geometry of the PMo12O403� polyoxometalate: the core of the

Keggin complex is a PO43 tetrahedron that is sheltered inside an

Mo12O36 shell that is constituted of twelve OQMoO4/2 distorted

pyramids, where O1/2 designates an oxygen atom that is shared by

two pyramids. Three protons are required to balance the negative

charge: they are located on bridging oxygen atoms between two Mo

for an isolated POM3,4 but migrate to maximize the number of strong

hydrogen bonds when in contact with the silica surface.5 These

protons show strong acidic properties.

Table 1 Dimensions of the unit cells and energies of the systemsa

Label Dimensions of the unit cell Composition of the unit cell Energy

H2O 2 � 2 � 2 O, 2H �14.27b-cristobalite bulk 0.73 � 0.73 � 0.73 8Si, 16O �190.28001-1:POM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 136O, 35H �1625.67001-1–POM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 135O, 33H �1610.92001-2:POM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 132O, 27H �1566.92001-2–POM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 131O, 25H �1552.39001-2QPOM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 130O, 23H �1537.70001-3:POM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 132O, 27H �1566.44001-3–POM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 131O, 25H �1552.15001-3QPOM 1.47 � 1.47 � 3 12Mo, 1P, 40Si, 130O, 23H �1537.30001-6:POM 1.47 � 1.47 � 3 12Mo, 1P, 43Si, 136O, 23H �1602.68001-6–POM 1.47 � 1.47 � 3 12Mo, 1P, 43Si, 135O, 21H �1587.92001-6QPOM 1.47 � 1.47 � 3 12Mo, 1P, 43Si, 134O, 19H �1572.72101-1:POM 2.08 � 1.47 � 3 12Mo, 1P, 48Si, 152O, 35H �1813.00101-1–POM 2.08 � 1.47 � 3 12Mo, 1P, 48Si, 151O, 33H �1797.92101-1QPOM 2.08 � 1.47 � 3 12Mo, 1P, 48Si, 150O, 31H �1783.30101-1RPOM 2.08 � 1.47 � 3 12Mo, 1P, 48Si, 149O, 29H �1769.21101-2:POM 2.08 � 1.47 � 3 12Mo, 1P, 52Si, 156O, 27H �1844.32101-2-POM 2.08 � 1.47 � 3 12Mo, 1P, 52Si, 155O, 25H �1829.67101-2QPOM 2.08 � 1.47 � 3 12Mo, 1P, 52Si, 154O, 23H �1815.13101-3:POM 1.56 � 1.47 � 3 12Mo, 1P, 40Si, 128O, 19H �1497.98101-3–POM 1.56 � 1.47 � 3 12Mo, 1P, 40Si, 127O, 17H �1483.23101-3QPOM 1.56 � 1.47 � 3 12Mo, 1P, 40Si, 126O, 15H �1468.54111-1:POM 2.08 � 2.08 � 3 12Mo, 1P, 64Si, 184O, 35H �2194.16111-1–POM 2.08 � 2.08 � 3 12Mo, 1P, 64Si, 183O, 33H �2179.37111-1QPOM 2.08 � 2.08 � 3 12Mo, 1P, 64Si, 182O, 31H �2164.94111-1RPPOM 2.08 � 2.08 � 3 12Mo, 1P, 64Si, 181O, 29H �2150.24111-3:POM 2.08 � 2.08 � 3 12Mo, 1P, 68Si, 188O, 27H �2224.64111-3–POM 2.08 � 2.08 � 3 12Mo, 1P, 68Si, 187O, 25H �2210.19111-3QPOM 2.08 � 2.08 � 3 12Mo, 1P, 68Si, 186O, 23H �2195.30a The dimensions are in nm and the energies in eV. All unit cells possess orthorhombic symmetry (a = 901, b = 901, g = 901) except 111 systems,

which are hexagonal (a = 901, b = 901, g = 1201, b in the xy direction).

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are restricted to close-shell configurations in agreement with

the nature of the system. All atomic positions are allowed to

relax in the geometry optimization. The geometries are

considered optimized when forces on all atoms are below

0.03 eV A�1. The vectors and atomic formula of the unit cells

are reported in Table 1.

The Gibbs free energies are evaluated with the usual

thermochemical equations (see ref. 1 and 17 and electronic

supporting information) to analyze the stable POM-silica

surface structures as a function of T and PH2O. The surface

labels used here are the same as in ref. 17 for easier comparison.

The atomic coordinates of all silica surfaces can be found in the

supporting information of ref. 17.

3. Results and discussion

We analyzed previously the structures, reconstruction, and

stability of b-cristobalite surfaces during a dehydroxylation

process following reaction (2) and their comparison with

experimental amorphous silica surfaces.17 This permits us to

access silica surface models that cover a large range of hydro-

xyl group coverage, associated with more or less severe pre-

treatment conditions. In the current study, we chemisorb

H3PMo12O40 onto our b-cristobalite surface models and define

the subsequent systems by removing one to three H2O, when

possible, following reaction (1). In the hydrogen-bonded

POM�complex, the number of hydrogen bonds is not only

based on the number of protons in the POM but also on the

availability of neighboring surface hydroxyl groups to interact

with it. The formation of three covalent bonds (after dehydration)

between the POM and the surface is sometimes not permitted

due to topological inadequacy (Fig. 2 and 3). Our investigation

of the POM�silica surface systems is not limited to the

thermodynamically most stable bare silica surfaces in the

absence of the POM but also includes metastable terminations

that could be stabilized by the presence of the POM (Table 1,

Fig. 4).17 With this procedure, we consider all reactions from

(1) to (3). We describe now the POM�surface systems that are

the most stable. The determination of the Gibbs free energies

of the reactions5,17 allows the construction of the stability

diagrams as a function of water partial pressure and temperature.

In this diagram, only the most stable surface system is shown, for

given conditions of temperature and pressure. The comparison

of these diagrams in the presence and absence of the POM is

particularly revealing (Fig. 5). In Fig. 5, we also include the

hydroxyl coverage of the silica surface prior to grafting and,

between parentheses, the hydroxyl coverage after the POM

grafting reaction (1).

As mentioned earlier, reactions (1) and (2) are in competi-

tion because they both lead to dehydroxylation and H2O

release from the surface. Considering the stability diagrams

at low temperature and high water partial pressure in Fig. 5,

the presence of the POM has a rather equivalent effect on the

{001}, {101}, and {111} surfaces: it lowers the temperature or

increases the water partial pressure for the first water release in

comparison to surfaces without POM. The easier water for-

mation in presence of the POM could be related to the more

acidic character of the POM in comparison to the surface

hydroxyl groups. The extent of this effect, which results from

the POM grafting through reaction (1), is different for the

three surfaces. For the {111} surface, three hydroxyl groups

are consumed at PH2O/P1 = 1 and T = 300 K, and H3POM

has lost its three protons, forming three covalent bonds with

the surface in return. In the same conditions two hydroxyl

groups are consumed and the POM forms two covalent bonds

on the {101} surface. The case of the {001} surface is slightly

different. Without the POM, this surface has already a

different behavior from the two other ones: it undergoes a

first dehydration according to reaction (2) in ambient conditions

of pressure and temperature, whereas {101} and {111} surfaces

require harder conditions. When the POM is present on {001},

the dehydrated form is still favored but the shift in pressure

and temperature for dehydration is much less pronounced

than for the other surfaces. The grafting of the POM occurs

through two covalent bonds.

Furthermore, as suggested by reaction (3), the POM pre-

sence alters the very local topology of the silica surface while

Fig. 2 Front and side views of POM grafted with two covalent bonds

onto partially dehydroxylated {001} silica surface (product of reaction

1b, 001-2QPOM in Fig. 5). The Si–O–Mo angles usually do not have

much flexibility: they are close in Fig. 2 and 3. The value for all systems

(Fig. 5) is 155+21/�71. The unavailability of hydroxyl to anchor the

POM while conserving this Si–O–Mo angle prevents here the forma-

tion of a third covalent bond, although both POM’s proton and

surface hydroxyl groups are present and interact via hydrogen bonds.

Fig. 3 View of tricovalently grafted POM onto {111} surface

(111-1RPOM in Fig. 5). The formation of three covalent bonds

between the POM and the surface is favorable in ambient conditions

of temperature and water partial pressure when the topologies of the

accessible silica surface hydroxyl groups permit it.

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conserving the hydroxyl coverage. In the case of the {001}

surface for an hydroxyl coverage of 3.71 nm�2, the honeycomb

arrangement of the hydroxyl is ca. 20 kJ mol�1 nH2O�1 less

stable than a row-pattern.17 After the POM grafting, the

stability becomes reversed. This is explained by the most

favorable fitting of H3PMo12O40 in the honeycomb nests of

the surface, allowing a larger number of hydrogen bonds

between POM and surface.

Similarly, the maximum number of covalent bonds that can

be formed between the POM and the silica surface depends on

the local disposition of the surface hydroxyl groups. Only two

covalent bonds can be formed in the case of the {001}

honeycomb pattern surface, whereas three can be formed with

{101} and {111} surfaces over a large range of water partial

pressure and temperature.

When silica surface dehydroxylation is considered at higher

temperature and/or lower water partial pressure than

previously, it appears in contrast that the POM presence slowsFig. 4 Definition of the silica surfaces used in this work and their

labels (silicon: light grey, oxygen: dark grey, hydrogen: white).

Fig. 5 Stability diagrams of {001} (top), {101} (middle), and {111}

(bottom) b-cristobalite surfaces in absence (left) and presence (right) of

H3PMo12O40 as a function of the water partial pressure and tempera-

ture. The bonding between the POM and the surface is labeled as

follow: ‘:’, ‘–’, ‘Q’, and ‘R’ stand for hydrogen-bonds, one, two, and

three covalent bonds between the POM and the surface (reaction (1)),

respectively. The numbers are hydroxyl coverage in nm�2. For the

POM�surfaces (right), two values are given when relevant: the first

corresponds to the hydroxyl coverage of the corresponding silica

surface in the absence of POM, whereas the value in parentheses

is calculated by subtracting the surface hydroxyl groups that are

consumed in reaction (1).

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down silica surface dehydroxylation and reconstruction in

comparison to the situation in the absence of the POM. Hence,

deep dehydration following reaction (2) is inhibited albeit the

initial dehydration following reaction (1) is favored by the

POM. For instance, the reconstructed {101} surface with an

hydroxyl coverage of 2.62 nm�2 is reached at a temperature of

ca. 600 and 725 K at PH2O = 10�5 bar in absence and presence

of the POM, respectively. Similar results are found with all

surfaces.

Overall, the size of the surface stability domains is different

in the presence or absence of the POM. With high hydroxyl

coverage surfaces, the surface hydroxyl groups are consumed

by the acid protons of the POM, forming water and a covalent

bond linkage. The resulting hydroxyl coverage is then lower

than in the absence of the POM. When the surfaces become

partially dehydroxylated and more hydrophobic, less covalent

and hydrogen bonds are formed between the POM and the

surface and reaction (3) comes into play to permit the POM to

retain surface hydroxyl groups in comparison to the bare silica

surfaces.

Further dehydroxylation, which we do not show here

because this goes beyond the experimental hydroxyl coverage

of ca. 1.5 nm�2,5 would lead to surface hydroxyl rarefaction

which eventually prevents the forming of more than one

covalent bond between the POM and surfaces. However,

one should keep in mind that hydroxyl group distribution

on the silica surfaces is not uniform,5,8 which can lead to the

possibility of multiple covalent bond formation even at relatively

low hydroxyl group coverage. For totally dehydroxylated

silica surfaces, a covalent bond could still be formed

between the POM and the silica surface following a reaction

equivalent to (3). Fully dehydroxylated silica surfaces are

experimentally obtained below PH2O = 105 Torr and above

T = 1473 K.6,7

The stability diagrams are insightful in interpreting the

experimental data.5 Under thermochemical equilibrium and

for moderate dehydration pretreatment conditions, e.g. 105 bar

and 500 K, the number of covalent bonds between the

POM and the silica surfaces should be two or three depending

on the surface topology. The POM should hence retain only a

very small number of protons (one or zero). This situation

corresponds to the ‘classical impregnation’ in ref. 5. The

temperature that is employed in this impregnation procedure

permits surface and POM covalent grafting to respect the

thermochemistry of the system (Fig. 5).

In the surface organometallic chemistry (SOMC) strategy,

the silica surface is pretreated at high temperature (T = 773 K)

under high vacuum (P = 10�5 Torr) before POM introduction.

The surfaces show hydroxyl coverage of ca. 1.4 nm�2 under

these conditions. The POM is then grafted at 298 K

in anhydrous conditions, which prevents silica surface

reconstruction and rehydration. Calculations suggest that only

dehydrated surfaces like 001-6, 101-2, and 111-4 exist under

these conditions (Fig. 5, left). The number of covalent bonds

between the POM and these surfaces is at most two (Fig. 5,

right), hence preserving at least one acidic proton on the POM.

The calculations hence clearly explain the influence of the

silica preparation conditions and surface hydroxyl density on

the final Brønsted acidity of the grafted POM.

4. Conclusions

We studied the grafting of H3PMo12O40 onto surfaces of

b-cristobalite by means of DFT, which permits us to construct

the stability diagrams as a function of temperature and water

partial pressure. The comparison with the diagrams of the bare

surface is particularly revealing and shows the full complexity

of the system. Indeed, silica dehydroxylation couples with the

covalent bonding of the acidic POM onto the surface, which

leads to a modification of the stability diagram of silica. The

dehydration following the POM grafting occurs at lower

temperature and at higher water partial pressure than the first

dehydration of the bare surface. In contrast, the second

dehydration is less easy on POM grafted surface than on bare

surface. Our results are compared with experimental data,

which greatly helps in their interpretation and comprehension.

In particular, our results explain the number of remaining

POM protons after grafting, and the superior density of

protons obtained by the SOMC approach.

Acknowledgements

This research was supported by the Agence Nationale de la

Recherche, research project METHANOX. CINES, IDRIS

(CNRS), and PSMN (ENS de Lyon) are greatly acknowledged

for the granted CPU time allocations.

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