React. Kinet. Gatal. Lett., Vol. 35, No8 1-2, $91-402 (1987)

SOME ASPECTS OF ELECTRONIC INTERACTIONS IN HETEROGENEOUS

CATALYSIS

D.M. Shopov

Institute of Kinetics and Catalysis, BulgarianAcademy

of Sciences, Sofia 1040, Bulgaria

Received February 13, 1987

It is now generally accepted that after the introduction

of the ideas on the existence of active sites, Balandin's theory

about the influence of geometrical and electronic factors on

the mechanism of the reactions is a substantial contribution

to the development of heterogeneous catalysis. In fact, this

was the first attempt to connect the mechanism of heterogeneous

catalytic reactions with the structure of the solid and the

reagents. This idea found numerous applications in further de-

velopment of the theory.

In one of the most recent versions of this idea, Sachtler

and Ponec suggested the therms "ligand" and "ensemble" to de-

scribe the effect of electronic and structural factors on the

adsorption and catalytic properties of bimetallic systems. In

the original version of their approach the authors assign a

secondary role to the ligand effect, which is connected to as-

scribing "independence" to the atoms in the ensemble. The present

ideas about solid state structure give no ground for such arti-

ficial division of electronic and structural factors.

One should note that in the last years the widespread

"local" approach in catalysis contains, or to be more correct,

allows to suggest the existence of "individual" behavior of

the structural elements of the solid body. However, one should

not forget that this is usually made for methodological reasons

with the aim of stressing the chemical nature of catalytic

phenomena. We would like to point out that neglecting the ex-

Akad6miai Kiad6, Budapest

isting collective interactions in the solid body might lead to

rather serious negative consequences.

In the Present lecture, we discuss the basis of the theo-

retical ideas about the nature of chemical bonding, a number of

experimental results concerning the structure and catalytic

properties of bimetallic clusters, the influence of the envi-

ronment on the formation and properties of metal particles, the

role of size effects on the catalytic properties, etc., with

the aim to clarify the role of the electronic and structural

factors.

Theoretical approach

Let us write the following equation for a bulk metal:

HeffT = QT (i)

where Hef f is an effective Hamiltonian with eigenvalue Q. The

latter corresponds to the heat of atomization of the bulk met-

al, defined as the difference between the total energy of the

bulk metal and the sum of the energies of the separate atoms.

The one electron approximation is applied here and the exist-

~nce of fine effects such as electron correlation and magnetic

properties are neglected.

Now let us have two interacting systems with Hamiltoni-

ans H 1 and H 2 and heats of atomization Q1 and Q2" The secular

matrix of the two systems is:

HI1 - E HI2 - ESI2

= O (2) H21 - ES21 H22 - E

The solution of eq.(2) gives the following eigenvalues:

- (2H12S12-Hll-H22) • 2-4 |1-S~2) |HllH22- ~ El, 2= 211_S~2)

One can notice that E 1 or E 2

the original interacting levels Q1

392

(3)

will be higher or lower than

and Q2' respectively, if the

2>I I term HI2 is HI2 HIIH22 .

Therefore, splittting of the levels Q1 and Q2 occurs

and the higher level goes higher, while the lower level shifts

down. This shift of the higher level means that after the in-

teraction the bond strength in the corresponding system is

weakened in general and indicates a tendency of the system to

split into smaller subsystems, e.g. to increase the dispersity.

The opposite is valid for the system with higher heat of atom-

ization, e.g. with the lower energy level. In this case the

effect of the interaction will be strengthening of the chemical

bonds of the system and a tendency to decrease the dispersity. 2

Therefore, if the relation HI2>IHIIH~I is valid, one can

use the heats of atomization to predict the effect of the second

component on the dispersity of the main component in bimetallic

catalysts. We should note that the experimental heats of atom-

ization are measured for bulk metals, where Q does not depend

on the number of atoms. For supported metal clusters, whose

size is often less than 50 A, the heat of atomization differs

from that of the bulk metal. If the functions Q=f(N) for the

two interacting systems do not cross each other, e.g. they do

not form an alloy, then the heats of atomization of the bulk

metal can be used to predict the effect of the second component

on the dispersity of the main, catalytically active component.

The effect of the support on the dispersity can be treat-

ed on the same grounds. The oxide supports, generally used in

catalysis, always have higher heats of atomization than the

supported metals. Therefore, one can expect that the supports

will always favor the dispersity.

Role of the second component in bimetallic catalysts

Table 1 contains the heats of atomization of some metals

and the predicted effect of the second component on the disper-

sity of platinum [I ]. One can note the good agreement between

the predicted and experimental results. Indeed, metals such as

Re, Ir and Ru, which have higher heats of atomization than Pt,

increase its dispersity. And contrary to that, metals such as

393

Bi, Ti, Au, Pb and In, which have lower heats of atomization

than Pt, decrease the dispersity of Pt in full agreement with

the prediction.

For a more detailed understanding of this process we have

carried out quantum-chemical calculations using the extended

H~ckel method [I]. The calculations were carried out for two

model systems Pt-Re and Pt-Ti (Fig. 1). They were chosen be-

cause they represent the two typical cases describing the effect

of the second component on the dispersity of Pt.

Table 1

Effect of the second component on Pt dispersity

("yes" - increase, "nO" - decrease of dispersity)

System:

Main Metal Second Metal

Heat of atomization (kJ/g atom)

Prediction Experiment (Theory)

Pt-566.1 Re-781.6

Pt-566.1 Ir-665.3

Pt-566.1 Ru-648.9

Pt-566.1 Bi-205.7

Pt-566.1 Ti-180.1

Pt-566.1 Au-365.5

Pt-566.1 Pb-195.6

Pt-566.1 In-242.4

yes yes

y e s y e s

yes n o , y e s ?

no no

n o n o

n o n o

n o n o

n o n o i n f l u e n c e

Table 2

Bond populations for the Pt-Re system

System

4Pt atoms linear 4Pt-2Re

4Pt-4Re

Bond population between adjacent atoms, .... 1-2 2-3 3-4 4-5 5-6 13

0.0061 0.1169 0.0061 0.0257 -0.1039 0.0241 0.4032 0.8474 1-2 2-3 3-4 5-6 6-7

0.0768 -0.1144 0.0768 0.5773 0.3809 7-8

0.5778

394

0 0 0 0 0 0 1 2 3 /* 5 6

O - Pt

O - Re Ti

1 2 3 /,

0 0 0 0 0 0 0 0 5 6 7 8

B

Fig. i. Geometries used in the calculation

Table 3

Bond populations for the Pt-Ti system

System Bond population between adjacent atoms P. z3

1-2 2-3 3-4 4-5 5-6

4Pt-2Ti 0,295 0.303 0.162 0.334 0.186

1-2 2-3 3-4 5-6 6-8 7-8

4Pt-4Ti 0.267 0.269 0.267 0.241 0.298 0.241

The calculations indicate (Table 2), that in the presence

of Re the bond population between the Pt atoms is decreased,

while in the presence of Ti it is increased (Table 3). The

bond population values clearly indicate the opposite effects

of Re and Ti on the size of the Pt particles.

The analysis of the computational data allows to explain

this effect in more detail. Charge transfer from Re to Pt

occurs, thus increasing the number of electrons in the antibond,

ing orbitals of Pt. This contributes to the weakening of the

bonds between the Pt atoms and even to breaking of some bonds.

The charge transfer has also been observed experimentally [2].

39,~

The opposite effect is true in the Pt-Ti system. The

transfer of electrons from the antibonding orbitals of Pt to

the empty d-orbitals of Ti results in strengthening of the Pt-

Pt bonds.

Role of the support on the dispersity of supported metal cat-

al[sts

Let us accept as an evaluation of the effective energy of

one bond in the oxide support the value E = Do/n, where D o is

the experimental heat of atomization [3], and n is the number

of ligands surroinding the metal ion in the oxide lattice.

Baker et al. [4] have studied the structure and growth in

hydrogen of very small Pt particles under heating on thin films

of TiO2, AI203 and SiO 2. The following particle sizes (Table 4)

are found. One can notice the good agreement between the par-

ticle size and the E value.

Table 4

Particle size of Pt depending on the carrier and the temperature

Carrier Pt particle size (nm)

825 K 975 K Exi0 -3

(kJ/K moi)

Ti407 0.51 0.56 1270

AI203 0.51 0.67 509.9

SiO 2 0.71 0.96 462.1

The effect of a number of oxides with n=6 o~ the disper-

sity of supported Ni catalysts is investigated in [5]. The

following order has been found:

AI203 > MgO > Cr203 > ReO 3 > CaO > SrO

Ex10 "3 (kJ/K mol) 509.9 166.8 445.2 194.2 176.9 166.9

Again the results are in good agreement with the corresponding

values of E, with the exception of MgO.

396

This is also confirmed in the work of Taghavi et al. [6]

concerning the dispersity of Cu (Table 5). One can again observe

the clear correlation between the particle size and the E value.

Table 5

Particle size of supported Cu depending on the support

S a m p 1 e Mean particle size Ex10 -3 �9 of Cu (nm) (kJ/k mol)

1.93% Cu/AI203 4.5 509.9

1.93% Cu/MgO 10.0 166.8

1.73% Cu/SiO 2 67.0 462.1

In a series of investigations Ioffe et al. [7] have

established that the prior deposition of W, Mo and Re oxides

on the surface of SiO 2 greatly increases the dispersity of

supported Pt. This modification of the surface leads to the

formation of a new solid system, containing W-O, Mo-O and Re-O

bonds of larger bond strength, which interact with the metal

in the above mentioned way.

Our experimental results [8] confirmed the favorable

effect of modifying the surface of silica and alumina by molyb-

denum oxide on the dispersity of supported Pd (Table 6).

Table 6

Disperslty of molybdenum modified palladium samples

Samples Pd Pd3%Mo Pd6%Mo Pd15%Mo Pd20%Mo on SiO 2

Dispersity 0.18 0.62 0.80 0.40 0.39

Samples Pd Pd3%Mo Pd6%Mo Pd15%Mo Pd20%Mo on A1203

Dispersity 0.26 0.49 0.60 0.62 0.55

397

The above examples show that the E value can be used as

a criterion for predicting the effect of the carrier on the

dispersity of supported metals.

Interesting results were also obtained [9] when investi-

gating the formation of palladium particles by reduction in

liquid phase of different polarity of PdCI 2 supported on alumina

or silica (Table 7).

Table 7

Dispersity of Pd particles formed after reduction of PdCI 2

liquid phase

in

Liquid phase dioxane chloroform acetone

SiO 2 0.18 0.27 0.36 Sample

AI203 0.26 0.47 0.84

It is obvious that the dispersity is increased with in-

creasing polarity of the solvent in the order:

acetone > chloroform > dioxane

In the process of formation of the metal particles, interaction

between them and the molecules of the liquid phase takes place.

One can conclude that the role of the liquid phase is similar

to that of the second component in the above discussed cases.

A number of experimental facts related to changes in the

properties of the metal particles caused by interaction with

adsorbed species can be treated on the basis of the above con-

siderations.

Effect of adsorbed species on the electronic structure of metal

clusters

Quantum-chemical calculations carried out by us [10] con-

cerning the effect of adsorbed electron acceptor elements such

as H, O, C1 and S on Pt(111) and Pt(100) clusters indicate that

the adsorption leads to a considerable decrease of the bond

strength between the metal atoms. It is this effect that causes

398

the experimentally observed increased mobility of the surface

metal atoms, the reconstruction of the surface planes or re-

dispersion of metal particles, for example, when treated in oxy-

gen. One cannot but indicate the similarity between the behavior

of these systems and the process taking place in the bimetallic

catalysts under the influence of the second component or the

effect of the support on the dispersity of metal particles.

However, the question arises what is the cause for this de-

stabilization of the metal clusters under the influence of the

above mentioned electron acceptor elements, having in mind e-

lectron transfer from the antibonding metal orbitals to the ad-

sorbate. This electron transfer should lead to stabilization of

the metal particles. However, this stabilizing effect is accom-

panied by shifting of the MO of the metal particles toward higher

energies as a result of interaction with the low lying orbitals

of the adsorbate. This destabilizing effect overcomes the stabi-

lizing effect due to charge transfer, thus resulting in de-

stabilization of the metal particles.

The strong inhibiting effect of sulfur in a number of cat-

alytic reactions can be treated on analogous grounds. For example,

the addition of Cu or Ag to Pt/AI203 catalysts promotes the re-

action of oxidation of CO and C2H 4, while very small amounts of

sulfur strongly inhibit this reaction. The nature of these pro-

moting and inhibiting interactions was studied quantum-chemi-

cally by us [11] and is brought to the above described shifting

of the metal MO up or down under the influence of the inhibitor

or the prmmoter.

All phenomena discussed here have a common feature -- all

of them are the result of electronic interactions, we indicated

that for all systems mentioned the interaction leads to the

formation of a new system, characterized as a whole by a new

local energy minimum and new energy states of the components

of the system. We should note that the changes in the electron

properties sometimes might also cause structural changes, such

as changes in the particle size, in their habitus, the appear-

ance of defects, etc. However, in all cases, the electronic

399

(energetic) factor is the one that dominates and determines the

physical properties of the system.

Effect of metal particle size on the catalytic activity

We have established [12] that the catalytic oxidative cou-

pling of styrene on palladium strongly depends on the particle

size. The catalytic reaction takes place at dispersities above

0.15. A more detailed study shows that catalytic activity is

manifested by particles having sizes less than 1.5 nm. With the

aim of clarifying the influence of the change of the metal sur-

face area we have studied by means of IR spectroscopy the ad-

sorption of CO on metal particles of different size [13]. Only

the bond at 2110 cm -1, characteristic of linearly adsorbed CO

(Fig. 2), was observed for metal particles less than 1.5 nm in

size. For larger particles in polydisperse samples both the

linear (above 2000 cm -I) and the bridged form (below 2000 cm "1 )

o ~ ~i -21~176

I I I I

1900 2100 cm-1

Fig. 2. IR spectra of CO adsorbed on samples of different

dispersities: a - 0.18; b - 0.10

400

of adsorption of CO on the Pd(100) and Pd(111) faces was ob-

served. Correlation between catalytic activity and the integral

intensity of the linear form was found. It could be suggested

that on the large size particles different active sites (in the

sense of geometrical ensembles of particular size) exist. For

smaller particles the surface is unified geometrically; transi-

tion, for example, from cubic lattice into cubooctahedral, or

finally into spherical. These changes in the geometry are caused

by changes in the electronic interactions between the separate

atoms in the metal particle. The essence of these changes is

an increased localization of the wave functions.

An interesting example for the influence of the electronic

factor and of the dispersity is observed in the presence of oxy-

gen. The presence of strongly bound subsurface oxygen in addi-

tion to surface species was established for group VIII metals

such as Pt, Pd, Ir, Ni, etc.

We have studied by quantum-chemical calculations [14] the

changes in the electronic structure of Pt(111), Pd(111), Ir(111)

and Ni(111) clusters that occur in the presence of subsurface

oxygen and its influence on the dissociative chemisorption of

02 . One of the interesting effects of subsurface oxygen is that

it decreases the work function of the above metals, which is

confirmed experimentally [15].

The subsurface oxygen also influences the dissociation of

02 over different size clusters. For small size clusters the

subsurface oxygen enhances the dissociative chemisorption of 02

However, for clusters having more than 18 metal atoms in (111)

configuration, the dissociative chemisorption of 02 is impeded.

For a particular size of the cluster, the destabilizing effect

of subsurface oxygen overcomes the stabilizing effect connected

with the electron transfer from the metal particle to the sub-

surface oxygen. Larger particles are more sensitive to shifting

up of the MO of the cluster. As a result, dissociative chemi-

sorption is impeded.

401

Concluding remarks

The examples we have discussed indicate that it is possible,

on the basis of quantum-chemical description of the interaction

between energy levels, to explain many processes in heteroge-

neous catalysis.

The division into electronic and geometrical factors ap-

peared historically, as an analysis of the processes, with the

aim to simplify their understanding. The present development

of the theory of catalysis shows that achieving of the basic

goal, e.g. explanation and prediction of the properties of cat-

alytic systems necessitate a synthesis between the components

and the mechanism of their action. Undoubtedly, this synthesis

can be carried out successfully only upon recognition of the

decisive role of electron factor.

Acknowledgment. I wish to express my gratitude to Prof. A.

Andreev and Dr. T. Halachev for fruitful discussions.

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