Preparation of Catalysts VII, Proceedings ofthe 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts

906 Pages·1998·24.71 MB·1-987\906

by  B. Delmon, P.A. Jacobs, R. Maggi, J.A. Martens, P. Grange and G. Poncelet (Eds.)

XV
FOREWORD
These are the Proceedings of the 7th Symposium on the cifitneicS Bases for eht noitaraperP of
suoenegoreteH ,stsylataC initiated in 1975 and on a regular basis jointly organized by the Unit6
de Catalyse et Chimie des Matrriaux Divisrs (UCL) and the Center of Surface Chemistry and
Catalysis (KULeuven). Apart from the first symposium, the location of all these events has
always been the UCL campus.
It is relevant to be reminded of the General Remarks formulated at the occasion of the first
symposium:
"At the moment the Organizing Committee has decided on the subject, the hope was that this
symposium could constitute a discussion of various scientific problems which are involved in the
manufacture of real, industrially used, heterogeneous catalysts. Catalysts are solid materials or
solid chemical products, possessing a high market value on a weight basis. The intention was to
discuss in an international meeting the scientific domains on which the activity of an important,
well distinct branch, of industry rests. The hope was that as little catalysis as possible would be
mentioned. Indeed, the preparation of a solid material involves mainly solid state chemistry and
adhesion phenomena. Since such a solid material has a complicated texture and a well developed
surface area, manufacture also implicates colloid chemistry and various interface phenomena in
addition to adhesion, diffusion and mobility processes in the solids or at their surface. The
technology involved is related to other fields, principally to the manufacture of ceramics, powder
technology, surface treatments, technical realization of adhesive joints between metal and/or
ceramics, materials technology, and cognate areas. The hope was that the symposium would help
to define better the fundamental phenomena involved and the technical similarities. People in
charge of the manufacture of catalysts are often catalysis-minded or at least are working in such
an environment. Some extra-disciplinary contributions, as well as multi-disciplinary approaches,
are thus necessary."
With minor changes, these remarks might serve as well as Preface of the present Symposium
Proceedings. Indeed, emphasis in all Symposia has been on the scientific aspects of the
preparation of new and industrial catalysts, or on new methods of preparation, rather than on the
catalytic reactions in which such solids are ultimately used. In the present context, the catalytic
event itself has only been considered as another, though often decisive, method of catalyst
characterization.
The series of Symposia have been backed up by Scientific Committees of which the majority of
the members were holding an industrial appointment. These Committees have always been able
during a joint meeting, to select those papers that best fitted the scope and specific topics of the
Symposium. This has allowed to select papers dealing with preparation aspects of real catalytic
systems, sometimes at the expense of excellent contributions on less timely systems. Industry has
contributed on a regular basis by presenting communications, although the Organizing Committee
sometimes had wished to see a higher number of them. Apparently, the share of Academia in the
establishment of the scientific bases for the preparation is (and has been) very significant indeed.
xvi
The scientific topics of the 7th Symposium are in line with the general scope of this series of
events. Emphasis is on what industry considers as being very timely at the end of the 2nd
millennium. On the other hand, the editors have decided to make the Proceedings available before
the scientific event itself. For obvious reasons, the sponsoring Companies and Agencies this time
cannot be acknowledged properly by citing them in the Proceedings. The same holds true for all
those who have contributed to the success of the meeting, such as secretaries staff, students,
postdocs, and the Lodging Service of UCL.
Fortunately, the organizers are in position to express their appreciation towards the Rector of
UCL, Professor M. Crochet, for allowing the event to be patronized again by the University, and
Professor B. Delmon, now enjoying retirement, by being scientifically more active than ever.
Bernard, you have been at the origin (amongst many others) of this initiative; you have been
continuously inspiring the local organizers and the Scientific Committees. As a tribute to your
contribution to catalysis, and more specifically to this series of Symposia, the editors of the
Proceedings of the ht7 Symposium dare to dedicate this volume ot you.
The Editors
xvii
ORGANIZING COMMITTEE
President
Prof. .B DELMON, Universit6 Catholique de Louvain
Executive Chairmen
Prof. .P GRANGE, Universit6 Catholique de Louvain
Prof. P.A. JACOBS, Katholieke Universiteit Leuven
.rD .R MAGGI, Universit6 Catholique ed Louvain
Prof. .J MARTENS, Katholieke Universiteit Leuven
Dr .G PONCELET, Universit6 Catholique ed Louvain
SCIENTIFIC COMMITTEE
.rD .G BARON, VUB, Belgium
.rD .P COURTY, Institut Frangais du P6trole, France
Prof. .B DELMON, UCL, Belgium
Prof. .P GRANGE, UCL, Belgium
.rD .K HARTH, BASF, Germany
Prof. P.A. JACOBS, KUL, Belgium
.rD .K JOHANSEN, Haldor Topsae, Denmark
Prof. .F KING, ICI Katalco, United Kingdom
.rD .E KRUISSINK, DSM Research, The Netherlands
.rD E.G.M. KUIJPERS, Engelhard De Meern ,VB The Netherlands
.rD E.-L. ,A_AMOKAL Neste Oy, Finland
.rD J.-P. LANGE, Shell International, The Netherlands
.rD .F LUCK, G6n6rale sed Eaux, France
.rD .R MAGGI, UCL, Belgium
Prof. J.A. MARTENS, KUL, Belgium
.rD .G MATrHYS, Exxon Chem. Int., Belgium
.rD .M NEUBER, Clariant GmgH, Germany
.rD .G NEUENFELDT, Solvay Deutschland, Germany
.rD .R NOELS, Universit6 ed Liege, Belgium
.rD .K NOWECK, Condea Chemic GmbH, Germany
.rD .C PEREGO, Eniricerche S.p.A., Italy
.rD J.-P. PIRARD, Universit6 de Li6ge, Belgium
.rD .G PONCELET, UCL, Belgium
.rD .S ROSSINI, Snamprogetti, Italy
.rD .P RUIZ, UCL, Belgium
.rD .F SCHMIDT, duS Chemie AG, Germany
.rD J.-P. SCHOEBRECHTS, Solvay, Belgium
Prof. B.-L. ,US FUNDP, Belgium
.rD .M TOKARZ, Eka Nobel AB, Sweden
.rD .M TWIGG, Johnson Matthey, United Kingdom
.rD .A VAN GIJSEL, UCB, Belgium
Prof. .E VANSANT, Universiteit Antwerpen, Belgium
.rD .E ZIRNGIEBL, Bayer, Germany
(cid:14)9 8991 reiveslE ecneicS .V.B llA sthgir .devreser
noitaraperP fo stsylataC IIV
.B nomleD te ,.la .srotide
The quantitative representation of heterogeneity in supported
metal catalysts
A.S. McLeod, K.Y. Cheah and L.F. Gladden
University of Cambridge, Department of Chemical Engineering, Pembroke
Street, Cambridge, CB2 3RA, U.K.
A micro-reactor study of ethene hydrogenation, catalysed by a series of
silica supported platinum catalysts, has been undertaken and the data
obtained compared to numerical simulations of this reaction. Monte carlo
simulations have been used to account for the complex kinetic features of this
reaction and to relate the observed kinetic behaviour to the structure of each
of the catalysts studied.
.1 INTRODUCTION
Discrete lattice models of surface reactions, as exemplified by monte carlo
or cellular automata algorithms, are potentially a valuable computational
tool for the simulation of reactions occurring on heterogeneous surfaces. In
contrast to algebraic kinetic models, discrete statistical simulations allow for
the structural features of a catalyst surface to be incorporated into kinetic
models of heterogeneous catalytic reactions. The incorporation of this
information into reaction models is necessary if the kinetic characteristics of
heterogeneous catalysts are to be unambiguously related to the structure of
the catalyst, and consequently to the catalyst preparation route.
Previous work on the kinetics of reactions occurring on homogeneous
catalyst surfaces has demonstrated that monte carlo simulation can provide
the necessary numerical framework for interpreting apparently anomalous
kinetic data 1,2. In this paper we extend previous work on the simulation of
reactions occurring on homogeneous surfaces by considering the simulation of
catalytic reactions occurring on well characterised heterogeneous surfaces.
Specifically, we consider the influence of the metal particle distribution and
surface heterogeneity on the kinetics of the ethene hydrogenation reaction.
Geometric disorder is accounted for by constructing Voronoi tesselations of
the plane to represent a particular distribution of particles dispersed on an
inert catalyst support.
To demonstrate the application these models to realistic catalytic
systems, we have undertaken a combined experimental and theoretical study
of the ethene hydrogenation reaction. A series of Pt/silica catalysts have beeja
prepared from inorganic and organic precursor compounds and characterised
by XRD, TEM and NMR techniques. The results of the eatalyst
characterisation are subsequently used to produce lattices representative of
each of the catalysts on which the reaction simulations are conducted. A
micro-reactor study of the ethene hydrogenation reaction is then presented
and the kinetic data obtained correlated with the results of the catalyst
characterisation. Monte carlo simulations of the ethene hydrogenation
reaction are then conducted and compared to the micro-reactor data.
2. SIMULATION DETAILS
In the monte carlo method the catalyst surface is considered to be
composed of a regular square lattice of discrete sites. Each single location of
the simulation lattice may represent either the catalyst support or the active
metal component. The model neglects the influence of metal-support
interactions and assumes the support material to be inert. The catalytically
active sites can either be vacant or be occupied by one of the hydrocarbon
species or a dissociated hydrogen atom. Once adsorbed onto the catalyst
surface, a reactant molecule may desorb back into the bulk gas phase or react
with a neighbouring molecule.
000 000
000 000
000 000
&
2+2H "~:2H
~"---*H+*5H2C C2H6 ~ rg r b r a
(~ )~( )~( er 000 cr 0 0 0
000 ----~4.-, @~0 .-~ 000
000 f'~ @@0 d'~ @@0
C2H4*+H*~ "5H2C C2H4+2"~ H2C 4.
Figure .1 The transformations of the lattice in a monte carlo simulation that
represent the steps a-g of the Horiuti-Polanyi mechanism. Empty circles
represent unoccupied lattice sites. Shaded circles represent O - ethene, O -
hydrogen and e- ethyl.
The hydrogenation of ethene is assumed to proceed by the sequential
hydrogenation of the hydrocarbon, the half hydrogenated ethyl species is
assumed to be the sole active intermediate 3. The reaction mechanism is
represented as a series of discrete time steps, each leading to a change in the
arrangement of the adsorbed species on the lattice. The steps of the
hydrogenation mechanism are represented by the transformations of a small
section of the lattice, as shown in Figure .1 Diffusion of the dissociated
hydrogen atoms across both the metal particles and the support (due to
hydrogen spillover) may also occur.
Adsorption of both ethene and hydrogen is assumed to require two
adjacent, unoccupied metallic sites. Adsorbed species that form nearest
neighbour pairs may react with each other with a rate constant, ra, where a
denotes one of the elementary steps of the reaction mechanism. As the
strength of the substrate-adsorbate bond is typically much greater than any
adsorbate-adsorbate interaction, the rate constants are assumed to be
dependent only on the location of the adsorbate on the catalyst surface.
An outline of the time-dependent monte carlo algorithm is now presented,
further details of the simulation algorithm are presented elsewhere 4. The
probability, ,}Lc{rP of a particular event, ,~c occurring with respect to the other
possible events is given by
Pr{a} - ~r." (1)
An event results in the transformation of the simulation lattice, and can
represent a diffusion, adsorption, diffusion, reaction or desorption step. The
rate of reaction, ra, is dependent on both the intrinsic rate constant and the
number of reactive groups on the surface. In general the reactant distribution
will be non-random, and therefore cannot be obtained by the simultaneous
solution of a set of mean-field site balance equations. Mlowing for both the
spatial variation in the rate constant and the non-random reactant
distribution the probability of an event occurring will be
,~-----n~kE
Pr{a} = (2)
where ka is the rate constant for reaction a and na the number of possible
reactions of type .zo If it is assumed that the events occur according to a
F -_. ;
:".,%
I'" 'i "|"
/ L t. 4
(a) (b)
Figure 2. Simulation lattices used to represent various dispersions of metal
particles (black regions) on a catalyst support (white regions) with the
fraction of polygons occupied by the metallic crystallites assigned as (a) 0.10
and (b) 0.25.
Poisson process, then the time increment corresponding to each event, St, will
be given by
1
td = -~ln(y), (3)
~ naka
(2
where 7 is a uniform random number between 0 and 1. Simulations were
conducted on 128x128 lattices for approximately 107 time steps, this was
found to be a sufficient simulation time for steady state to be obtained. At
steady state the rate of reaction was obtained as a turnover number, R,
where
R_ (4)
tN~
In equation (4) Ng is the total number of reactions of type g that have
occurred up to time t and N is the total number of sites on the catalyst
surface, r is the fraction of sites occupied by the metallic component.
In order to determine the influence of metal particle size, the distribution
of the metallic crystallites over the catalyst support is represented by a series
of tesselations of the lattice, these tesselations partition the surface into a
number of separate regions. Each of these regions is either assigned as a
region of catalyst support, or a region representing a metallic crystallite.
Each polygon is then randomly assigned, with a specified probability, as
either an active catalytic region or as an inactive region of the support
material. Examples of simulation lattices are shown in Figure .2 By
assigning a greater number fraction, ,b~ of the polygons as active regions, the
average size of the regions increases due to the coalescence of the individual
polygons into larger single particles. The characteristic length of the metal
particles is determined and converted from arbitrary lattice units to "real"
units by assuming the side length of each adsorption site to be 0.39 nm, the
lattice constant for platinum.
3. PREPARATION AND CHARACTERISATION OF MATERIALS
3.1. Catalyst Preparation
Dispersion of platinum on the silica support was achieved by an
impregnation from an aqueous medium and by anchoring of an organic
precursor. For all the catalysts considered, the silica support was a porous
sol-gel silica (Grace, Type 254), used as supplied by the manufacturer. The
catalysts prepared by the aqueous route were prepared by impregnation of
hexachloroplatinic acid. The anchored catalysts were prepared by the
attachment of platinum acetylacetonate, Pt(acac)2, to the silica support. The
catalysts were then calcined in air and reduced at 300~ in a stream of 20%
v/v hydrogen in helium 5. The preparation procedures are summarised in
Table .1
Table .1 Summary of catalyst preparation conditions.
, , .,, ,,,,,,. ,m, ., ,, ,, ,, ....
Catalyst Metal Loading Calcination Temp Procedure
(wt )% (oc)
Pt-A 2.5 30 Aqueous impregnation
Pt-B 2.5 200 Aqueous impregnation
Pt-C 2.5 200 Organic anchoring
Pt-D 2.5 400 Organic anchoring
3.2. Metal particle size and geometry
Information on the geometry and size of the dispersed metal particles was
obtained by powder X-ray diffraction and electron microscopy. The average
particle diameter for each sample was obtained using a Phillips powder
diffactometer with a Cu Ka radiation source ~)( = 0.154 nm .) The particle
diameters and the corresponding metal dispersion for each material are
shown in Table 2. The dispersions of the catalysts prepared from the organic
precursor were found to be lower than those obtained by the aqueous
precursor. This result is contrary to what may be expected given that the
organic precursor forms a bond with the hydroxyl groups on the silica surface
6. Recent studies of the characteristics of platinum catalysts prepared from
organic anchoring are, however, in agreement with the results presented above
7.
Table 2. Mean metal particle diameter and the corresponding dispersion of
the Pt/silica catalysts. The particle dispersion has been calculated using the
approximate method of Bond 8.
i i i i
Catalyst Particle diameter Dispersion
(s
Pt-A 50 0.20
Pt-B 63 0.16
Pt-C 163 0.061
Pt-D 187 0.053
i
3.3. Deuterium NMR
A series of deuterium NMR studies have been conducted to determine the
mobility and the extent of interaction of an adsorbed hydrocarbon molecule
with each catalyst surface. This information on molecular mobility will be
used in the monte carlo model to determine the possible influence of
molecular surface diffusion on the reaction kinetics. Deuterated benzene was
used as a probe molecule in this study as the relaxation behaviour of this
molecule on metal oxide surfaces has been considered by a number of previous
workers 9. Fully deuterated benzene (99.5% C6D6 HPLC grade, Alltech)
was adsorbed on the catalyst sample subsequent to reduction at 400oc,
under vacuum, for 2 hours.
Following the model introduced Boddenberg and Beerwerth 9 for the
dynamics of sorbate motion on catalyst surfaces, the mobility of the adsorbed
molecule is considered to be described by three separate motional correlation
times. In addition to the isotropic translational motion of the molecule,
interpreted as corresponding to the diffusion of the molecule across the
catalyst surface, the simultaneous occurrence of librational and rotational
motions are also allowed for. As each separate motion is assumed to be a
thermally activated process, pre-exponential factors and activation energies
are required to describe the temperature dependence of each motional
correlation time. For the translational, rotational, and librational motions
the pre-exponential factors and activation energies are denoted by tin, tp, ts
and by Era, Ep, Es respectively. The parameter S is an order parameter and
is, therefore, bounded by 0 and .1 The librational order parameter quantifie~
the correlation of successive librational motions, higher values of S indicate a
lesser degree of librational freedom.
By considering the rotational activation energies for each of the catalysts,
it can be concluded that Pt-A and Pt-B differ from Pt-C and Pt-D. In the
former pair, the activation energy for the rotational motion is significantly
higher than for the latter pair while the librational energy, where significant,
is lower. This result suggests a greater sorbate-substrate interaction for
catalysts Pt-A and Pt-B than for Pt-C or Pt-D. The rotational pre-exponential
factors obtained for Pt-A and Pt-B are significantly higher those obtained for
Pt-C and Pt-D. This is indicative of a stronger sorbate-substrate interaction
for catalysts Pt-A and Pt-B. It is reasonable to conclude that the preparation
route is responsible for the differences between these two pairs of catalysts.
Table .3 ALS model parameters obtained for the dynamics of benzene
adsorbed on silica. Activation energies are given in kJ mo1-1 and pre-
exponential factors in s-1. Parameters not reliably determined are denoted by
an asterisk.
,= , .,,, ... ,..,, , , ...., ,,. ,
Sample Em Ep Es tm tp ts S
(kJ 1-1om ) (xl0-1 ls-1)(xl0-14s-1)(xl0-13s )1-
)2+_( (+2) (+5) (+10%) (+10%) (+50%) (+0.05)
Pt-A 17.7 11.1 8.0 3.0 289 1.0 0.32
Pt-B 19.6 12.2 -*- 1.9 204 -*- 0.32
Pt-C 16.9 7.0 12.0 0.77 17.7 1.9 0.28
Pt-D 17.4 6.5 11.5 9.8 3.0 2.4 0.30
i ii i ill , ii ,i I I II I II II ll;
3.4. 1H MAS and CRAMPS NMR
The extent to which the surface chemistry of the catalyst support was
influenced by the preparation procedure was studied by 1H MAS and
CRAMPS NMR. Data were collected using a Bruker MSL-200 spectrometer
operating at 200MHz (1H resonance). MAS spectra were obtained by rotation
of the sample at 4.5 KHz, CRAMPS spectra were obtained at a spinning
frequency of 2.3 KHz.
Proton MAS spectra of two catalyst samples prepared from inorganic and
organic precursors are shown in Figure 5. The organic and inorganic precursor
samples appear nominally identical, with a single broad resonance at 3ppm.
The spectra obtained for the supported metal catalysts were also found to be
identical to that obtained for a sample of the silica support. A reduction in