Metal–Support Interaction in Pt/VOx and Pd/VOx Systems:A Comparative (HR)TEM Study

Simon Penner • Michael Stoger-Pollach •

Ramona Thalinger

Received: 24 July 2013 / Accepted: 28 August 2013 / Published online: 25 September 2013

� Springer Science+Business Media New York 2013

Abstract The manifestations of strong and reactive

metal–support interaction after reduction at elevated tem-

peratures in Pt/V2O3 and Pd/V2O3 systems were studied

using a dedicated thin film model system. Pt and Pd par-

ticles were prepared by electron-beam evaporation on

NaCl(001) growth templates and subsequently embedded

in a crystalline V2O3 matrix, prepared by thermal evapo-

ration of V metal in 10-4 mbar oxygen pressure. Template

temperatures of 600 K were used to induce the formation

of epitaxially-ordered metal–oxide systems. Engineering of

the metal–support interface by distinct annealing treat-

ments allows steering the extent and quality of metal–

support interaction. Whereas for Pt/V2O3 catalysts,

high-temperature reduction at 773 K in hydrogen causes

the epitaxial formation of a well-ordered body-centered

tetragonal Pt3V intermetallic phase, the Pd/V2O3 system is

mostly unaffected by similar treatments and remains in a

metal–oxide state. Nevertheless, oxidation at 773 K of both

catalysts prior to the hydrogen treatments lifts the epitaxial

relation between metal and oxide and in turn, subsequent

reduction at high-temperatures (T C 773 K) yields only

polycrystalline Pt3V and Pd3V intermetallic phases without

particular ordering with respect to the former growth sub-

strate. Along with this formation of intermetallic phases

goes a transformation of the support stoichiometry from

V2O3 to VO. Catalyst regeneration by partial oxidative

decomposition of the intermetallic state is only possible at

high-temperatures (T C 750 K), yielding mostly metal

particles and vanadium oxides with oxygen contents higher

than V:O = 1:2, in particular V3O7.

Keywords Pt3V � Pd3V � Epitaxy � High-

temperature reduction � SMSI � Catalyst regeneration �Intermetallic compounds

1 Introduction

Vanadia-promoted and supported noble metals are well-

known for their pronounced activity and selectivity in a

variety of chemical reactions like CO hydrogenation, the

reforming of CO2 with CH4 toward synthesis gas or the

reduction of NO with CO [1–9]. It is suspected, that

important steps of the above reactions are favoured at the

metal–support interface [3]. Subsequently, vanadia-pro-

moted catalysts are prone to enter a state of strong metal–

support interaction similarly as e.g. their archetypical

TiO2 counterparts, although some differences, especially

regarding the solubility in water and the associated loss of

active metal area by partial encapsulation during prepara-

tion, are noticeable [10, 11]. Intimately connected with

metal–support interaction in general is one particular

drawback of using vanadium oxides as supports, promoters

or even catalysts: vanadium oxides are prone to adopt a

wide range of (sub-) stoichiometric structures upon oxi-

dation and reduction, with eventual differences in the

extent of metal–support interaction and catalytic behavior

[12, 13]. In turn, mixtures of different vanadium oxides are

usually present, severely interfering the correlation of the

observed structural changes with the associated changes in

catalytic activity. One of the best studied metal–vanadia

S. Penner (&) � R. Thalinger

Institute of Physical Chemistry, University of Innsbruck,

Innrain 52a, 6020 Innsbruck, Austria

e-mail: simon.penner@uibk.ac.at

M. Stoger-Pollach

University Service Center for Transmission Electron Microscopy

(USTEM), Vienna University of Technology, Wiedner

Hauptstrasse 8-10, 1040 Vienna, Austria

123

Catal Lett (2014) 144:87–96

DOI 10.1007/s10562-013-1095-2

system is Rh/VOx, which might therefore serve as a ref-

erence system for structural, morphological and chemical

changes. Studies are abundant, ranging from purely surface

science-related investigations of ultra-thin metal–oxide

films on single crystals and metal foils [14–19] to nano-

particulate systems [20–23]. Especially on the latter, a

range of structural and catalytic effects have been observed

upon high-temperature annealing, potentially fostered by

the metal–support contact area. This includes loss of metal

area due to encapsulation phenomena at low reduction

temperatures and more pronounced metal–support inter-

action effects, such as formation of intermetallic Rh-V

compounds at higher temperatures [21–24]. The latter most

extreme form of metal–support interaction was recently

termed ‘‘reactive metal–support interaction’’ [25]. Along

with these structural alterations go e.g. activity and selec-

tivity shifts in CO hydrogenation [18, 19, 23]. Also on the

corresponding Pt–V and Pd–V systems, previous studies

suggest that the metal–oxide contact area (including the

effects of strong metal–support interaction) plays a crucial

role in governing the catalytic performance of the catalytic

entity [9, 20].

Due to the importance of the metal–oxide interface,

several methods have been applied to increase the metal–

support contact area and to subsequently facilitate the

formation of key reaction intermediates in catalytic reac-

tions. Suitable and widely used methods—at least for the

Rh–VOx system include reduction at elevated temperatures

and the preparation and re-dispersion of ternary oxide

phases (e.g. RhVO4) [17]. However, for Pt–VOx and Pd–

VOx systems, despite their importance in a variety of

oxidation reactions, such as the heterogeneous Wacker

process [26] or the oxidation of benzene to phenol [20],

these methods have yet to prove their abilities in steering

the extent of the metal–oxide contact area. A particular

promising alternative method is based on the use of

NaCl(001) single crystal facets as growth templates. By

exploiting the structural similarities between noble metal,

oxide and NaCl, epitaxially grown and morphologically

well-defined metal–oxide thin film model systems with

particularly large and easy manipulable contact areas are

easily obtained. This in turn favours the studies of any kind

of metal–oxide phase boundary effects [20–23].

The aim of the present study therefore is to investigate

the strong and reactive metal–support interaction effects

observed in the Pt/VOx and Pd/VOx thin film model sys-

tems by systematic variation of the reduction temperature

and to identify their individual structural and electronic

manifestations. As in previous experiments on pure van-

adia samples a significant dependence of the oxide mor-

phology and oxidation state on the reduction temperature

has been observed [21], the extent of the metal–oxide

contact area could be additionally triggered by simple

oxidative treatments before reduction. This eventually will

show how the strong metal–oxide interaction in general,

and intermetallic formation in particular, are affected by

the extent of this contact area. Particular emphasis will also

be given to a comparison to the corresponding Rh-based

systems to eventually highlight the similarities and differ-

ences in metal–support interaction. Monitoring of the

structural and electronic changes upon reduction will

be performed by (high-resolution) electron microscopy

((HR)TEM) and related techniques, especially selected-

area electron diffraction (SAED) and electron-energy loss

spectroscopy (EELS).

2 Experimental

For preparation of the model catalyst systems, a dedicated

high-vacuum setup (base pressure 10-6 mbar) was used.

This basically consists of a Duran glass cross, to which the

deposition equipment could be mounted. For details we

refer to Ref. [22]. Pt and Pd particles were prepared by

electron-beam evaporation of Pt and Pd-wire wrapped

around a tungsten rod and deposition onto the freshly-

cleaved NaCl(001) template at 600 K. Nominal metal

thicknesses, as determined by quartz-crystal microbalance,

amounted to 5 nm. Subsequently, the metal particles were

covered by layers of crystalline V2O3, prepared by thermal

evaporation of vanadium metal in 10-4 mbar O2 also at

673 K. The resulting overall film thickness amounted to

*25 nm. The self-supporting films were in turn floated

into distilled water, dried and mounted on gold grids for

electron microscopy.

Reductive and oxidative treatments in hydrogen and

oxygen were carried out in a micro-reactor setup, allowing

for easy mounting and enabling treatments at temperatures

up to 973 K.

Structural characterization was performed using a

ZEISS EM 10C (overview imaging and SAED) and a

200 kV FEI Tecnai F20 S-TWIN (S)TEM electron

microscope equipped with a Gatan GIF Tridiem 2001

(high-resolution electron imaging (HRTEM) and EELS)).

The SAED patterns were internally calibrated to the (111),

(200) and (220) spots of the face-centered cubic structures

of Pt and Pd. EELS spectra are background-corrected and

corrected for plural scattering.

3 Results and Discussion

3.1 The Initial Pt/VOx and Pd/VOx Systems

Figure 1 shows comparative TEM images, diffraction

patterns and particle statistics of Pt and Pd particles

88 S. Penner et al.

123

embedded in crystalline VOx. In Fig. 1a, the Pt particles

are easily recognized as mostly black and grey spots

exhibiting predominantly square and rectangular outlines.

On the basis of previous experiments, these outlines are

associated with truncated half-octahedral morphologies

[27]. The SAED pattern confirm the well-ordering of the

particles, due to epitaxial growth on the NaCl(001) tem-

plates, subsequently yielding predominantly [001] and

[011] particle zones axes. Table 1; inset in Fig. 1a, c

highlight important particle statistical parameters, indicat-

ing a mean particle diameter of *10 nm. The epitaxial

ordering of the Pt particles is also shown in the respective

HRTEM image (alongside its Fast Fourier Transform pat-

tern) of Fig. 2, exhibiting two sets of perpendicular (200)

lattice planes measured on a single Pt particle. As already

discussed in previous work [21], formation of a single

crystalline vanadium oxide support structure requires

careful tuning of the deposition parameters, most impor-

tantly oxygen partial pressure and deposition rate. In the

present case, the SAED patterns indicate the presence of a

well-ordered rhombohedral V2O3 structure [28] with par-

tial epitaxial ordering to both the fcc Pt and NaCl struc-

tures. Important reflections of both Pt and V2O3 have been

marked in the SAED pattern. The presence of V2O3 is

further confirmed by electron-energy loss spectra taken

before and after the reductive treatment (Fig. 3). In the

initial state (taken on a V2O3 particle), the spectra of the

V–L and O–K edges exhibit the typical features of V2O3,

namely a small, broad O–K peak and two sharp V–L peaks

with the characteristic V L3–V L2 ratio [21].

Pd particles embedded in V2O3 in principle exhibit

similar features as their Pt counterparts (see Fig. 1 c, d),

with the one notable exception of exhibiting a more

rounded appearance in overview TEM images due to the

presence of an increased number of [110] limiting particle

facets [27]. SAED patterns (Fig. 1d) reveal both the epi-

taxial ordering and the presence of Pd and V2O3. Table 1, 2

give an overview of the particle statistics, particle

Fig. 1 TEM overview images of Pt and Pd deposited at 600 K embedded in crystalline V2O3 in the as-grown states (a, c) alongside the

corresponding SAED patterns (b, d). The insets show representative particle size histograms

Metal–Support Interaction in Pt/VOx and Pd/VOx Systems 89

123

morphologies, phases and structures present, as well as an

assignment of the measured lattice distances to distinct

theoretical lattice planes.

Figure 4 in turn shows a compact overview of the mean

particle diameter and the associated particle density (in

particles cm-2) in the as-grown states and after different

reductive and oxidative treatments. In essence, both

parameters are more or less similar for Pt and Pd particles,

indicating almost identical starting conditions for moni-

toring metal–support interaction effects upon high-tem-

perature reduction.

3.2 Structure of the Pt/VOx and Pd/VOx Systems

after High-Temperature Reduction

Below reduction temperatures of 773 K, no structural,

compositional or morphological changes of neither the Pt

or Pd particles nor the V2O3 support were encountered,

despite beginning reduction of a part of V2O3 to VO and

minute sintering of the metal particles (see Table 1).

Nevertheless, reduction at 773 K not only induces con-

siderable metal–support interaction effects, but clear dif-

ferences in the reduction behavior of Pt and Pd particles are

notable.

Figure 5 clearly shows that reduction of the Pt particles

at 773 K leads to a pronounced reconstruction of the par-

ticle outlines, without increasing the mean particle diam-

eter considerably. Whereas the initial Pt particles exhibited

square and rectangular outlines, the particles now are ter-

minated by very sharp edges and facets (some of the par-

ticles have been marked by arrows). Some larger, more

irregular particles are also encountered.

Furthermore, the corresponding SAED patterns reveal

that these structural changes are accompanied by consid-

erable compositional changes, as a number of additional

spot and ring reflections appear in the patterns. Pt spots and

reflections of the initial V2O3 structure are still visible, but

pronounced fourfold split diffraction spots, clearly in a

pronounced azimuthal ordering to the Pt (200) and Pt (220)

spots, appear (marked in the SAED pattern). These spots

are found at a distance of 3.89 A and correspond to the

(002) reflections of a body-centered tetragonal (BCT) Pt3V

structure (dtheor(002) = 3.87 A; space group: I4/mmm,

lattice parameters a = 3.87 A, c = 7.82 A) [29]. All the

other reflections (for a complete listing and assignment to

the BCT Pt3V structure, see Table 2) can also be addressed

to this structure, with the one exception of a rather

strong ring reflection, being assignable to the reduced

vanadium oxide species V2O with monoclinic structure

(dexp = 2.98 A, dtheor(-255) = 3.03 A) [30]. It is worth

noting, that also the ring reflections have faint elongated

diffraction spots superimposed, also indicating ordered

structures. HRTEM images (Fig. 6) corroborate the pre-

sence of the intermetallic Pt3V phase. The images a and b

show two single Pt3V particles with (110) and (004) lattice

fringes, measured at 2.79 and 1.98 A. Figure 6c finally

Table 1 Summary of the structural and morphological characteristics of the studied Pt/VOx and Pd/VOx thin film catalyst systems in the as-

grown states and after different annealing treatments

Treatment Metal particle morphology Support morphology Metal particle Particle Phases

Diameter (nm) Density (cm-2)

Pt/VOx

As-grown Square and rectangular Rounded grains,

slightly porous

9.7 5.1 9 1011 Pt; V2O3

H 573 Square and rectangular Rounded grains,

slightly porous

9.3 4.8 9 1011 Pt; V2O3

H 773 Mostly sharp-edged square

and rectangular

Rounded grains,

slightly porous

9.7 4.4 9 1011 Pt; Pt3V BCT; V2O3; (VO,V2O)

H 773 Rounded, irregular outlines Platelet-like 11.2 3.7 9 1011 Pt; (Pt3V BCT); V3O7 BCT

O 750

O 723 Rounded, irregular outlines Porous 15.4 2.8 9 1011 Pt3V BCT (V2O3), VO

H 773

Pd/VOx

As-grown Rounded Rounded grains 9.9 3.1 9 1011 Pd; V2O3

H 573 Rounded Rounded grains 9.4 2.7 9 1011 Pd; V2O3

H 773 Rounded Rounded grains 10.9 2.4 9 1011 Pd; V2O3

O 723 Rounded Porous 14.5 1.4 9 1011 Pd3V BCT; VO

H 773

Minor components are given in brackets

BCT body-centered tetragonal

90 S. Penner et al.

123

shows a high-resolution detail of the reduced support

structure, with extended (-255) lattice fringes of the V2O

structure. To follow the changes in electronic structure

during formation of the Pt3V intermetallic phase, EEL

spectra were additionally collected. Figure 3 in turn

reveals, that a slight support reduction of V2O3 has taken

place (measured on a former V2O3 particle), as the inten-

sity ratio of V L3–V L2 peaks has moderately increased

[21].

A completely different behaviour upon reduction at high-

temperatures has been observed for Pd/V2O3. Figure 7

reveals that almost no metal–support interaction has taken

place after reduction at 773 K, again apart from moderate

Fig. 2 HRTEM image of a single Pt particle in the initial state,

exhibiting two sets of perpendicular (200) lattice planes. The

corresponding Fast Fourier Transform is shown as inset

Fig. 3 EEL spectra of Pt particles embedded in crystalline V2O3

taken before (grey) and after reduction at 773 K in hydrogen (black).

The spectra were shifted with respect to each other on the intensity

axes for better visibility

Table 2 Interplanar distances d (hkl)/A measured on the Pt/VOx and

Pd/VOx thin film catalyst systems in the as-grown states and after

different annealing treatments and possible assignment to fcc Pt and

Pd, rhombohedral V2O3, cubic VO, monoclinic V2O, body-centered

tetragonal V3O7, body-centered tetragonal Pt3V and body-centered

tetragonal Pd3V

d (hkl)exp Lattice plane d (hkl)theor

Pt/VOx (As-grown)

2.72 V2O3(104) 2.71

2.48 V2O3(110) 2.48

2.27 Pt(111) 2.27

2.18 V2O3(113) 2.18

2.06 V2O3(202) 2.05

1.95 Pt(200) 1.96

1.68 V2O3(116) 1.69

1.44 V2O3(300) 1.43

1.37 Pt(220) 1.39

Pt/VOx (H 773)

3.89 Pt3V(002) 3.87

3.43 Pt3V(101) 3.46

2.98 V2O(-255) 3.03

2.75 Pt3V(110) 2.73

2.67 V2O3(104) 2.71

2.45 V2O3(222) 2.48

2.26 Pt(111) 2.27

2.12 Pt3V(103) 2.16

1.98 Pt(200) 1.96

Pt3V(004) 1.96

1.81 V2O3(024) 1.82

1.73 Pt3V(202) 1.73

1.66 Pt3V(211) 1.68

V2O3(116) 1.69

1.60 Pt3V(114) 1.59

1.44 Pt3V(213) 1.43

V2O3(300) 1.43

1.38 Pt(220) 1.39

Pt/VOx (H 773 O 750)

10.0 V3O7(110) 9.90

4.94 V3O7(220) 4.95

3.99 Pt3V(002) 3.87

3.54 V3O7(101) 3.59

3.30 V3O7(330) 3.30

3.15 V3O7(211) 3.19

2.98 V3O7(301) 2.92

2.79 V3O7(510) 2.75

2.70 V3O7(321) 2.68

2.43 V3O7(440) 2.47

2.29 Pt(111) 2.27

1.99 Pt(200) 1.96

1.84 V3O7(002) 1.86

1.73 V3O7(222) 1.74

Metal–Support Interaction in Pt/VOx and Pd/VOx Systems 91

123

sintering. Structure, morphology and ordering of both Pd

particles and V2O3 support are basically unchanged and no

signs of encapsulation effects or formation of intermetallic

phases are noticeable, as well as no pronounced reduction of

the V2O3 support.

To test the stability of the formed Pt3V intermetallic

compound, re-oxidation measurements have been subse-

quently carried out. Figure 8 shows, that after oxidation at

750 K, the structure of both (inter-)metallic particles and

support have again been drastically altered. Firstly, the sharp

particle outlines, obtained after reduction at 773 K, have

again vanished, indicating that formation of intermetallic

phases is a reversible process. This process is, however,

Fig. 4 Metal particle diameter and particle density of Pt and Pd

particles measured after different reductive and oxidative annealing

treatments

Fig. 5 Overview TEM image of the Pt/V2O3 thin film system after

reduction at 773 K and its corresponding SAED pattern (inset). Some

of the sharp-edged particles have been marked by arrows

Table 2 continued

d (hkl)exp Lattice plane d (hkl)theor

1.58 V3O7(651) 1.61

1.52 V3O7(831) 1.50

1.40 Pt(220) 1.39

Pt/VOx (O 723 H 773)

3.53 Pt3V(101) 3.46

3.07 V2O(-255) 3.03

2.72 Pt3V(110) 2.73

2.47 V2O3(222) 2.48

2.15 Pt3V(103) 2.16

2.05 VO(200) 2.05

1.83 V2O3(024) 1.82

1.74 Pt3V(202) 1.73

1.69 Pt3V(211) 1.68

1.62 Pt3V(114) 1.59

1.45 VO(220) 1.45

1.40 Pt3V(213) 1.43

1.36 Pt3V(204) 1.37

1.33 Pt3V(220) 1.36

1.16 Pt3V(301) 1.26

1.24 Pt3V(312) 1.16

PdVOx (As-grown)

2.70 V2O3(104) 2.71

2.46 V2O3(110) 2.48

2.26 Pd(111) 2.25

2.18 V2O3(113) 2.18

2.07 V2O3(202) 2.05

1.95 Pd(200) 1.95

1.68 V2O3(116) 1.69

1.62 V2O3(018) 1.62

1.48 V2O3(214) 1.47

1.42 V2O3(300) 1.43

1.36 Pd(220) 1.37

Pd/VOx (H 773)

2.74 V2O3(104) 2.71

2.47 V2O3(110) 2.48

2.32 V2O3(006) 2.33

2.23 Pd(111) 2.25

2.05 V2O3(202) 2.05

1.96 Pd(200) 1.95

1.66 V2O3(116) 1.69

1.44 V2O3(018) 1.62

1.37 Pd(220) 1.37

Pd/VOx (O 723 H 773)

2.37 VO(111) 2.37

2.19 Pd3V(112) 2.21

2.06 VO(200) 2.05

1.90 Pd3V(200) 1.91

1.46 VO(220) 1.45

92 S. Penner et al.

123

accompanied by pronounced sintering (cf. Table 1; Fig. 4).

Secondly, also the support structure has transformed into a

more platelet-like structure, with extended lattice fringes of

*9.9 A covering large areas of the sample (attributable to

the V3O7 (110) lattice fringe, see Table 2; inset in Fig. 8a).

The SAED patterns reveal the presence of ordered metallic

Pt, and a large array of Debye–Scherrer-type diffraction

rings. The latter exclusively arise from the presence of a

more oxidized support species, namely V3O7 (right inset and

Table 2; body-centered tetragonal, space group I4/mmm;

lattice constants a = 14.01 A, c = 3.72 A) [31]. The

observed support morphology is very typical of vanadium

oxides with V:O ratios with oxygen contents higher than 1:2,

particularly V2O5-containing systems.

To further shed light on the particular requirements for

formation of intermetallic compounds in the Pt/VOx and

Fig. 6 HRTEM images of single Pt3V particles and the reduced

support structure after reduction at 773 K. a Pt3V (004) lattice fringes,

b Pt3V (110) lattice fringes, c (-255) fringes of monoclinic V2O

Fig. 7 Overview TEM image of the Pd/V2O3 thin film system after

reduction at 773 K and its corresponding SAED pattern (inset)

Fig. 8 Overview TEM image of the Pt/V2O3 thin film system after

reduction at 773 K followed by oxidation at 750 K. The right inset

highlights the corresponding SAED pattern, the left inset a high-

resolution detail of extended (110) lattice fringes of the V3O7

structure measured at 9.9 A

Metal–Support Interaction in Pt/VOx and Pd/VOx Systems 93

123

Pd/VOx systems, pre-oxidation before high-temperatures

has been carried out. Previous experiments on the corre-

sponding Rh-containing systems showed, that this induces

two important changes: (i) V2O3 is converted into higher

oxidized vanadium oxide species (mostly V2O5) and (ii)

the epitaxy between metal and vanadium oxide is subse-

quently lifted [22, 23]. As this changes the extent of the

metal–support contact area, implications for formation of

intermetallic phases are therefore expected. Oxidation of

both systems at 723 K yields the corresponding Pt and Pd

particles embedded in vanadium oxides with V:O ratios

higher than 1:2 (particularly V2O5) as starting structures

[32] (not shown).

Figure 9 shows the overview TEM images alongside the

corresponding SAED patterns after an oxidation–reduction

cycle at 723 and 773 K, respectively. By comparison to

Fig. 5, 7, the differences are immediately obvious. Based

on the particle statistics analysis shown in Fig. 4, due to the

missing epitaxy, both initial metallic Pt and Pd particles are

now prone to considerable sintering. In the case of Pt, only

very large rounded and irregular aggregates, but no parti-

cles with sharp outlines are formed. The SAED patterns

correspondingly only show ring reflections, but no distinct

diffraction spots. According to the analysis of the diffrac-

tion patterns and in striking contrast to what was observed

if well-ordered V2O3 was used as the supporting starting

point, metallic Pt has vanished and the transformation into

BCT Pt3V is complete. Note that due to the missing epitaxy

in the starting structure, only a structurally less-ordered

Pt3V phase (with respect to the former growth template) is

formed and the support structure is reduced down to VO.

The latter exhibits a characteristic porous structure in TEM

images and elongated, broad diffraction rings in SAED

patterns [21], both of which are clearly visible in Fig. 9 a,

b. The same is in principle true for Pd particles. After

oxidation at 723 K followed by reduction at 773 K, con-

siderable sintering is also observed. Structure-wise,

metallic Pd is quantitatively transformed into a body-cen-

tered tetragonal Pd3V structure (space group I4/mmm,

lattice constants a = 3.84 A, c = 7.74 A) [32], alongside

deep reduction of the support to VO. The SAED pattern

shows two faint reflections of this Pd3V structure at 2.19

and 1.90 A, corresponding to the (112) and (200) reflec-

tions [33]. Hence, Pd particles now exhibit similar features

of metal–support interaction as their Pt counterparts.

To put these results into perspective of literature data

and previous results obtained on comparable thin film

systems, a short revision of the Pt–V and Pd–V phase

diagrams is useful. Both phase diagrams basically exhibit

the same overall shape with similar stable M3V, M2V and

MV3 structures with increasing V content (M = Pt, Pd)

[34, 35]. In the case of Pt–V, the situation is somewhat

more complex due to the presence of a PtV structure and

additionally due to the formation of a range of meta-stable

intermetallic compounds [33]. To understand the formation

of intermetallic compounds in these thin film systems,

especially in the case of Pt, it is worth to note that Pt is able

to form a range of Pt3M compounds (M = Si, Al, Ce) upon

reduction of small Pt particles embedded in the respective

oxide supports SiO2, Al2O3 and CeO2 [36]. In all cases, the

epitaxial formation of Pt3Si, Pt3Al, and Pt3Ce phases with

cubic Cu3Au structures has been observed [36]. As the

driving force for formation of these special intermetallic

compounds, the almost perfect crystallographic match of

the lattice constants of the respective metal component (Pt)

and the intermetallic compounds has been held responsible.

The lattice mismatch of these structures and the corre-

sponding fcc Pt lattice is between 1.5 and 5 %. In the case

of body-centered tetragonal Pt3V, the lattice mismatch is

Fig. 9 Overview TEM image of Pt/V2O3 (a) and Pd/V2O3 (b) thin

film system after a oxidation–reduction cycle at 723 and 773 K,

respectively. The corresponding SAED patterns are shown as insets

94 S. Penner et al.

123

1.8 %, for tetragonal Pd3V (referenced to fcc Pd) 1.1 %.

This explains structurally-wise the preferred (epitaxial)

formation of these intermetallic compounds. It should be

finally mentioned, that also a Pt3V intermetallic compound

adopting a Cu3Au structure exists, but this phase represents

only a meta-stable structure [35]. From a mechanistic

viewpoint, the formation of the intermetallic compound

was observed to include at least three different steps:

Starting with the (partial) reduction of the support and

proceeding via the diffusion of the reduced species to the

structure of the metallic component, the final step is the

epitaxial growth of the intermetallic phase on top of the

initial metallic structure. For Pt/SiO2, these individual steps

have been followed and identified by high-resolution

electron microscopy and EELS [37]. Exemplified in the

present case for Pt/V2O3, EEL spectra also show at least

partial reduction of the support. Additionally, reduction of

the support is most evident in the SAED patterns of Fig. 9,

which both show the formation of VO. However, the most

clear indicator for the formation of the Pt3M intermetallic

compounds (apart from SAED patterns), is the adoption of

a distinct sharp-edged particle morphology, only found if

these special intermetallic compounds are formed. Never-

theless, reconstruction of Pt particles is a quite common

feature, typically observed upon selective adsorption of

nitrogen, hydrogen, hydrogen sulphide or mixtures of the

latter two on different crystal facets of the particles, thus

leading to selective energetic stabilization of distinct lim-

iting crystal facets. Different morphologies could be

obtained, also including square particles by prolonged

adsorption of H2S/H2 or other shape-controlling techniques

[38–40]. In the present case, however, the reconstruction is

due to the epitaxial formation of Pt3V compounds. A

similar line of argumentation in principle also holds for the

respective formation of the Pd3V intermetallic phases,

although it appears that the structural reconstruction is a

sole property of the Pt particles. The special crystallo-

graphic requirements for the epitaxial formation of the

Pt3V compound are further corroborated by the fact that

lifting the ordering between metal, support and the NaCl

growth template by pre-oxidation, does induce only the

formation of the respective intermetallic phase, but without

pronounced ordering with respect to the former growth

template. Why the Pd3V compound is not formed by direct

reduction remains an open question at the moment, but

might tentatively be explained with an extra structural

stabilization of Pd and V2O3 in the initial state (which can

be clearly seen in the SAED pattern shown in Fig. 1d),

hampering the necessary reduction of V2O3 and the sub-

sequent diffusion of vanadium (oxides) to the Pd lattice.

Also for the corresponding Rh/V2O3 systems, formation

of epitaxially grown intermetallic compounds has been

observed, but in addition also encapsulation effects at

lower reduction temperatures, with pronounced implication

on CO hydrogenation [22, 23]. Such encapsulation effects

(and the associated loss of accessible metal area) at least

structurally have never been observed in the case of Pt or

Pd. This might be explained by a different reduction

behavior at the metal–support interface and/or by different

energetics of wetting between metal and reduced (sub-)

stoichiometric vanadium oxide. Rh3V intermetallic com-

pounds exist, but the Rh-V phase diagram does exhibit a

very different shape at Rh-rich compositions in comparison

to Pt–V and Pd–V, mainly consisting of peritectically

melting intermetallic compounds. Neverthless, the forma-

tion of Rh-V intermetallic phases was observed to be

strongly dependent on the V/Rh ratio in the initial state. On

purely VOx-supported Rh films, different epitaxial rela-

tions led to the formation of Rh5V3. Lower V/Rh ratios also

yield Rh3V phases above 723 K and RhV compounds at

even higher reduction temperatures [24].

4 Conclusions

Despite the structural similarities of the Pt/V2O3 and Pd/

V2O3 catalyst systems in the initial state, significant dif-

ferences in the reduction behaviour and in turn, in extent of

metal–support interaction, have been observed. Whereas

the Pt/V2O3 system upon direct reduction yields a well-

ordered Pt3V structure by epitaxial growth on the Pt

structure due to the excellent crystallographic matching of

Pt and Pt3V, no such intermetallic compound has been

observed upon repeatedly treating Pd/V2O3 under similar

experimental conditions. Exploiting the advantages of the

thin film model concept, it could be shown how the extent

of metal–support interaction could be triggered by altering

the metal/oxide interface, i.e. by lifting the epitaxial rela-

tionship between metal and oxide. This was achieved by

simple oxidation before inducing the strong and/or reactive

metal–support interaction by reduction, thereby yielding

deep reduction of the supporting oxide and facilitated

formation of similar, however epitaxially less-defined

intermetallic compounds. This in turn also highlights the

importance of the well-defined initial state in obtaining

epitaxially-ordered intermetallic phases. Subsequently a

convenient pathway is offered to not only metal–oxide

interface engineering and in turn to deliberate adjustment

of the extent of the metal–oxide contact area by thermal

treatments, but also to catalytic characterization of the

resulting intermetallic phases and catalytic studies of

adlineation effects using a well-defined metal–oxide con-

tact area.

Acknowledgments We thank the Austrain Science foundation

(FWF) for financial support under project F4503-N16, which is also

Metal–Support Interaction in Pt/VOx and Pd/VOx Systems 95

123

performed within the framework of the Forschungsplattform Mate-

rials- and Nanoscience.

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