Table Of ContentQUANTUM THEORY OF CHEMICAL REACTIONS
QUANTUM THEORY
OF
CHEMICAL REACTIONS
III. Chemisorption, Catalysis, Biochemical Reactions
Edited by
RAYMOND DAUDEL
CNRS, Centre de Mecanique Ondulatoire Appliquee, Paris, France
and
ALBERTE PULLMAN
CNRS, Institut de Bioiogie Physico-Chimique, Paris, France
LIONEL SALEM
CNRS, Laboratoire de Chimie Theorique, Universite de Paris Sud, Orsay. France
ALAIN VEILLARD
CNRS, Universite Louis Pasteur, Strasbourg, France
SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.
Library of Congress Cataloging in Publication Data (Revised)
Main entry under title:
Quantum theory of chemical reactions.
Includes bibliographical references and index.
CONTENTS: v. 1. Collision theory, reaction path, static indices.-v. 2
Solvent effect, reaction mechanisms, photochemical processes-v. 3. Chemi
sorption, catalysis, biochemical reactions.
1. Quantum chemistry-Addresses, essays, lectures. I. Daudel,
Raymond.
QD462.5.Q38 541.2'8 79-22914
ISBN 978-94-015-6920-0 ISBN 978-94-015-6918-7 (eBook)
DOI 10.1007/978-94-015-6918-7
All Righ ts Reserved
Copyright © 1982 by Springer Science+Business Media Dordrecht
Originally published by D. Reidel Publishing Company, Dordrecht, Holland in 1982
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical
including photocopying, recording or by any informational storage and
retrieval system, without written permission from the copyright owner
TABLE OF CONTENTS
Preface V11
J. E. GERMAIN / Theoretical Background of Heterogeneous
Catalysis
E. J. BAERENDS and D. POST / Analysis of CO-Metal Cluster
Interaction Energies by the Hartree-Fock-Slater
Method 15
F. CYROT-LACKMANN / Chemisorption Properties of Transition
Metal 'Clusters 35
G. BERTHOLON / Gas, Organic Solid State Reactions and Their
Applications 55
I. G. CSIZMADIA / Some Theoretical Questions Concerning the
Mechanism of Fischer-Tropsch Synthesis 77
P. Th. VAN DUIJNEN and B. T. THOLE / Environmental Effects on
Proton Transfer. Ab Initio Calculations on Systems 1n
a Semi-classical, Polarizable Environment 85
O. TAPIA, C.-I. BRANDEN, and A.-M. ARMBRUSTER / Recent
Quantum/Statistical Mechanical Studies on Enzyme
Activity Serine Proteases and Alcohol Dehydrogenases 97
w.
G. RICHARDS / Applications of Quantum Chemistry to
Pharmacology 125
F. PERADEJORDI and E. L. DA SILVA / On the Pharmacophore
and Mode of Action of Some Schistosomicidal Agents.
Conformational Aspect 135
P. CLAVERIE / Intermolecular Interactions and Solvent Effects:
Simplified Theoretical Methods 151
Index 177
PREFACE
The third and last volume of this treatise IS concerned with important
applications of the quantum~theory of chemical reactions to chemisorption,
catalysis and biochemical reactions.
The book begins with an important paper devoted to the
theoretical background of heterogeneous catalysis. It is followed by
two papers showing typical applications of wave mechanics to the analysis
of chemisorption. Catalysed gas-solid reactions are chosen to illustrate
gas, organic solid state reaction and some aspects of the mechanism of
the FISCHER-TROPSCH synthesis are presented.
The second part of the book is devoted to biochemical
applications of quantum chemistry. Two papers are concerned with the
quantum theory of enzyme activity. Two others present recent progress
of quantum pharmacology.
Finally an important contribution to the theory of intermolecular
forces is made in the view of possible applications to biochemical
problems.
vii
R. Daudel, A. Pullman, L. Salem, and A. Viellard reds.), Quantum Theory o/Chemical Reactions, Volume III, vii.
Copyright © 1982 by D. Reidel Publishing Company.
THEORETICAL BACKGROUND OF HETEROGENEOUS CATALYSIS
J.E.Germain
Laboratoire de Catalyse Appliquee et Cinetique Heterogene
L.A. 231 du Centre National de la Recherche Scientifique
Universite Claude Bernard Lyon I, E.S.C.I.L.
43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex.
Heterogeneous Catalysis is a surface Kinetic phenomenon
by which a chemical reaction between molecules of a fluid phase is
accelerated (activity) and oriented (selectivity) by contact with a
solid phase (catalysts, without change of the solid.
\,)"e shall mostly discuss the case of metal catalysts.
I. FUNDAHENTAL PROCESSES.
ferfectly described by I.Langmuir in the years 1920-30, they
include : adsorption, surface reaction, desorption.
a) Chemical adsorption (or Chemisorption).
The clean surface of a metal traps selectively gas molecules
with a large heat release (20-100 kcal/mole) around 300K. The number of
molecules thus adsorbed per unit surface area is limited (saturation)
in simple cases (CO, H2, 02), stoechiometric relationships are found
between this number and the atomic density of usual crystal faces
(N 10 15/ cm2) .
There facts are best explained by chemical reaction of these
molecules with unsaturated adsorption sites created by disruption of
the threedimensional crystal lattice ("free valencies"). By analogy
with classical chemistry, one predicts that unsaturated molecules
(CO, C2H4) may adsorb directly (associative adsorption) while saturated
molecules (H2, CH4) must break into radicals (dissociative adsorption).
This is generaly the case.
The following energy scheme shows that several adsorption
states (1,2,3 ..• ) are available; at a given temperature, the only
accessible states are the ones such that activation barriers (EX) are
not too high (Ex/RT factor) (Fig. 1).
The favored equilibrium state will be the lowest of these
accessible states (several such states of similar energies could exist).
R. Daudel, A. Pullman, L. Salem, and A. Viellard (eds.), Quantum Theory of Chemical Reactions, Volume III, 1-13.
Copyrigh t © 1982 by D. Reidel Publishing Company.
2 J. E. GERMAIN
Energie Fig .1
gas
adsorbed
states
3
Remarks - the first accessible state usualy corresponds to associa
tive adsorption.
- on metal surfaces, the first activation energic (Ex) is
frequently very low.
- the transition between 2 states is an elementary step in
the sense of kinetics.
The following examples illustrate particular cases (Fig. 2).
CO/Ni 300 K non activated associative adsorption
H2/Pt 300 K non activated dissociative adsorption
N2/Fe 500 K activated dissoci~tive adsorption
Fig.2
co
35 Kcal
2 H 44 Kcal
co
2 N
Dissociative adsorption may lead to an adsorbed speCles plus
a gaseous species :
N + H2
o + N2
As predicted by the general scheme (Fig. 1), the state of
adsorption may change when temperature is raised as the number of
accessible states increase.
e.g. (Fig. 3) : NH3/Fe
THEORETICAL BACKGROUND OF HETEROGENEOUS CATALYSIS 3
1/2 N + 3/2 H2
2
Fig.3
300K 450K 500K
44 Kcal
22 Kcal
Prediction of these energy schemes (potential surfaces) for
a given gas-solid system is not yet possible ; the specific states of
each catalyst is given by experience.
We shall point out the remarkable activity of transition
metals, capable to dissociate, at room temperature, very stable mole
cules (H2, 02, CH4, ••• ) and the high binding energies between atoms
and sites (qH-H = 60 Kcal/l1 on Ni).
At higher temperatures, adsorbed atoms may dissolve in the
°
metal (C, N in a-Fe, in Ni) or produce defined chemical compounds
(carbide, oxide).
b) Catalysis.
According to thermodynamics, the desorption (endothermal) is
possible at sufficiently high temperature, and sufficently low pressu
re (P < 1 Torr).
Thus, if Q = 20 Kcal/M at T = 400-500 K
Q = 100 Kcal/H at T = 2000-2500 K
But at these temperatures new gas phase states become also
accessible for the system, and the initial adsorbed gas molecules are
not necesarily restituted : desorption is not univocal.
ex. (Fig. 4) H2/Pt "reversible" adsorption
NO/Pt "irreversible" adsorption
2 H
NO/Pt Fig.4
NO
H /Pt
2
1/2 N + 1/2 O
2 2
134 Kcal
41 2 Kcal
18
N + 0
H 350 K NO NO
.~
400K 2 H ~ H2 450 K 2 N -4 N2
800 K 2 o O
~ 2
4 J. E. GERMAIN
The possibility of decomposition (or isomerization) catalysis
arises. We may replace the cycle: low temperature adsorption of M,
high temperature desorption, by the contact of M with the solid at a
temperature T such that desorption is possible in 2 directions. A
continuous decomposition (Fig. 5)
H+P+Q
will be observed, by maintaining a constant pressure of M, since P and
Q with desorb freely. --
/
"-
/
Fig.S
/ "
/ E \
/
\
/
"
M
"- p + Q
IE~
I
This reaction will be accelerated if the activation barrier E
of the homogeneous reaction 1S replaced by a series of lower barriers
(E 1x , E2x, E3X).
The reaction is also oriented since P and Q depend of the
nature of the adsorbed intermediate I, characteristic of the catalyst.
The above scheme has been simplified by grouping the formation
of products P and Q, which are in fact distinct processes.
ex. NO + N2 + ~ O2
(11) (P) (Q)
(p)
l
f22
NO+NO+N + 0
(Q)
(N) (If) (I)
The principle of microscopic reversibility indicates that by
changing the direction of each elementary step, we obtain the mechanism
of catalytic synthesis
P + Q + 11
In every ciase, we may distinguish the following steps
adsorption EX ::/: 0
surface reaction E~ = EX2
desorption EX
3
THEORETICAL BACKG ROUND OF HETEROGENEOUS CAT AL YSIS 5
The energetic scheme may be more simple or more complex than
above.
- Very simple case : 2HD t H2+D2
The only parameter is the adsorption heat Q = EX
We expect the rate to increase when Q decreases ; in fact if Q 1S too
small, the density of the adsorbed phase gets too low. These 2 oppo
site factors lead to a "volcano" shaped- curve v = f (Q).
This 1S a rather general principle in catalysis : an optimal adsorp
tion of the reactant is required.
- Complex cases. With heavy molecular reactants, the number of accessi
ble adsorbed states becomes rapidly very large.
Some of these states open catalytic reaction pathways, while other
are dead-ends.
Catalytic selectivity is high when one of these reaction pathways is
strongly favored.
A detailed kinetic analysis of these complex reaction networks, com
bined with isotopic tracing, lead to the structure of active interme
diates.
For ex. This analysis has been pushed very far for the systems hydro
gen-hydrocarbon, and the following adsorbed species have been proposed:
C H
n 2n+2
(-H) C H .I.
n 2n+1 a 'c" + H
" "
, -' C
(-2H) Cn H2 n ..... C" ..... C - C"/ "'C' .= C(... . "" C/ 'C"/
aa II as ay- ~ ,)
I
#I I
/ \ " C " C
(-3H) C-H "C - C --C /C ~ C/ , "C~ -'- C/
n 2n-1 " • / ""-
aaSJJ ~I
.' , J • 'IT
(-4H) C H "C - C/ C = C/ -C C-
n 2n-2 S~II hI .,/ '. 'Iii!
aa • •
The mechanism of olefinic double bond hydrogenation and
migration may be represented by scheme (Fig. 6)
On Ni, the rapid interconversion of a = a S species leads to
extensive migration ; this process is slow on Pt.
