INTRODUCTION TO CHARACTERIZATION AND TESTING OF CATALYSTS
J.R. ANDERSON
INTRODUCTION TO CHARACTERIZATION AND TESTING OF CATALYSTS - 1ra.edición - AUSTRALIA ACADEMIC PRESS 1985 - 457 pg Ilustrado 18 cm x 24 cm
Introduction
1 Surface area measurement
Physical adsorption of gases
total surface area
Chemisorption of gases
Adsorption measurement methods
References
2 Particle size
Representation of particle size
Estimation of particle size
References
3 Pore structure
Total pore volume
Pore structure from physical adsorption
Pore structure from mercury porosimetry
Small angle X-ray scattering
Small angle neutron scattering
References
4 Bulk properties
Density
Mechanical properties
Thermal properties
References
5 Chemical characterization
Surface acidity
Surface basicity
Miscellaneous aspects of surface group functionality
Thermal analysis methods
Chemical characterization of bimetallic catalysts
References
6 Testing of catalyst activity
The objectives of catalyst testing
Rates and limitations in laboratory reactors
Types of reactors in the laboratory
Peripheral equipment
Automated reactors
Off-the-shelf catalyst test units
Standardization of catalyst activity testing methods
Laboratory safety
References
7 Physical instrumental methods
Electron microscopy
Electron spectroscopy
X-ray emission and absorption
lon beam methods
Mössbauer spectroscopy
Magnetic resonance spectroscopy
Vibrational spectroscopy
UV-visible diffuse reflectance spectroscopy
Photoacoustic spectroscopy (PAS)
References
Appendixes Index
There are many reasons for catalyst characterization. Two illustrations will suffice. At one extreme, characterization can be a matter of commercial necessity: a catalyst manufacturer needs to demonstrate that his product meets a given set of specifications. At another extreme, characterization is an integral part of any worthwile catalytic research and development program. To carry out catalytic research with an ill-characterized catalyst is about as sensible as studying a chemical reaction when one of the reactants is un-
known: it should not be done if it can possibly be avoided. The characterization of a catalyst provides information of three distinct but related sorts. These are: chemical composition and chemical structure, texture and mechanical properties, and catalytic activity.
By chemical composition and chemical structure, we refer to matters such as: elemental composition: the composition, structure, and proportions of individual phases which may be present; surface composition: the nature and proportions of functional groups which may be present on the surface.
The texture of a catalyst refers to its geometric structure and morphology. ranging from the grossest macroscale down to the finest microscale. This deals with. for instance, size and shape of individual catalyst units (for example, individual particles, pellets); pore structure; total surface area; the way in which individual phases are arranged relative to one another. The mechanical properties refer to those which are important to the integrity of the catalyst in an industrial application. This refers to matters such as abra-sion or attrition resistance, strength, and thermal shock resistance.
The characterization of a catalyst in terms of its activity is obviously a quantitative measure of the ability of a catalyst to carry out a particular chemical transformation under specified conditions. Basically this will speci-fy a quantity such as speed of reaction, or some quantity related to speed of reaction, per unit quantity of catalyst. Bearing in mind that many catalytic reactions are not specific for the formation of a particular molecular product, the specification of activity must also include product selectivity.
Ideally a catalyst would be characterized in terms of activity under exactly the same conditions as those under which it will be used in practice. This will often not be possible because the ultimate use may be in a very large-scale reactor, whereas for reasons of economy and convenience it is highly de-sirable, indeed often mandatory, that activity assessment be made on a small scale under reaction conditions that differ from those used on the large scale.
It will often not be possible to estimate accurately large-scale behaviour from activity assessments made on a small scale, unless recourse is had to empir-ical correlations between data obtained in the two regimes. In any case, in assessing catalytic activity it is essential that the behaviour of the catalytic reactor be understood, so that the significance of the data obtained with it can be properly gauged.
Under reaction conditions all catalysts suffer from progressive deacti-vation to some extent at least. In practice, deactivation limits the lifetime of the catalyst, and the effective lifetime is a parameter of considerable eco-nomic significance. In general, deactivation results either from a change in the chemical composition of the catalyst resulting from the addition or removal of matter, from a change in the texture and structure of the catalyst, or from a combination of these factors. If the reasons for deactivation are understood it may be possible to devise regeneration procedures, although in many cases this may not be possible, or be possible to only a limited extent. It will be clear that any study of deactivation and regeneration necessarily involves a study of the same characterization parameters as are used with a pristine catalyst.
In principle, an account of methods for catalyst characterization has a vast range of scientific and technical techniques to draw upon. It is possible to make the case that virtually every technique known to materials science is of some potential value, and in addition there are many types of measurement which are peculiar to catalytic science. Nevertheless, experience has shown that out of this vast range, a relatively restricted number of techniques and types of measurement are of dominant importance: we have attempted to confine this book to these accepted methods. Even so, the field is quite large. and in many situations there are alternative techniques available. In some of these cases we have recommended a preferred method: this does not imply that we are pointing to an accepted standard method, merely that the pre-ferred method has given, in the author's experience, the best results, or for which good grounds for selection are clear.
Assuming that one is dealing with a catalyst in some divided form (powder. pellets. and so on), it is necessary to say something about the problem of sampling. Many catalysts are materials which are produced commercially in large volumes, so that it is a matter of some importance for the sample which is to be used for a characterization procedure to be as representative as possible of the bulk material. In this situation, the sampling procedure will usually be related to the methods used for bulk handling at
the end of the catalyst manufacturing process. Two golden rules for sampling have been suggested:
1. The sample should be taken while the material is in motion
0120583208
CATALYSTS
QD501H / 3208
INTRODUCTION TO CHARACTERIZATION AND TESTING OF CATALYSTS - 1ra.edición - AUSTRALIA ACADEMIC PRESS 1985 - 457 pg Ilustrado 18 cm x 24 cm
Introduction
1 Surface area measurement
Physical adsorption of gases
total surface area
Chemisorption of gases
Adsorption measurement methods
References
2 Particle size
Representation of particle size
Estimation of particle size
References
3 Pore structure
Total pore volume
Pore structure from physical adsorption
Pore structure from mercury porosimetry
Small angle X-ray scattering
Small angle neutron scattering
References
4 Bulk properties
Density
Mechanical properties
Thermal properties
References
5 Chemical characterization
Surface acidity
Surface basicity
Miscellaneous aspects of surface group functionality
Thermal analysis methods
Chemical characterization of bimetallic catalysts
References
6 Testing of catalyst activity
The objectives of catalyst testing
Rates and limitations in laboratory reactors
Types of reactors in the laboratory
Peripheral equipment
Automated reactors
Off-the-shelf catalyst test units
Standardization of catalyst activity testing methods
Laboratory safety
References
7 Physical instrumental methods
Electron microscopy
Electron spectroscopy
X-ray emission and absorption
lon beam methods
Mössbauer spectroscopy
Magnetic resonance spectroscopy
Vibrational spectroscopy
UV-visible diffuse reflectance spectroscopy
Photoacoustic spectroscopy (PAS)
References
Appendixes Index
There are many reasons for catalyst characterization. Two illustrations will suffice. At one extreme, characterization can be a matter of commercial necessity: a catalyst manufacturer needs to demonstrate that his product meets a given set of specifications. At another extreme, characterization is an integral part of any worthwile catalytic research and development program. To carry out catalytic research with an ill-characterized catalyst is about as sensible as studying a chemical reaction when one of the reactants is un-
known: it should not be done if it can possibly be avoided. The characterization of a catalyst provides information of three distinct but related sorts. These are: chemical composition and chemical structure, texture and mechanical properties, and catalytic activity.
By chemical composition and chemical structure, we refer to matters such as: elemental composition: the composition, structure, and proportions of individual phases which may be present; surface composition: the nature and proportions of functional groups which may be present on the surface.
The texture of a catalyst refers to its geometric structure and morphology. ranging from the grossest macroscale down to the finest microscale. This deals with. for instance, size and shape of individual catalyst units (for example, individual particles, pellets); pore structure; total surface area; the way in which individual phases are arranged relative to one another. The mechanical properties refer to those which are important to the integrity of the catalyst in an industrial application. This refers to matters such as abra-sion or attrition resistance, strength, and thermal shock resistance.
The characterization of a catalyst in terms of its activity is obviously a quantitative measure of the ability of a catalyst to carry out a particular chemical transformation under specified conditions. Basically this will speci-fy a quantity such as speed of reaction, or some quantity related to speed of reaction, per unit quantity of catalyst. Bearing in mind that many catalytic reactions are not specific for the formation of a particular molecular product, the specification of activity must also include product selectivity.
Ideally a catalyst would be characterized in terms of activity under exactly the same conditions as those under which it will be used in practice. This will often not be possible because the ultimate use may be in a very large-scale reactor, whereas for reasons of economy and convenience it is highly de-sirable, indeed often mandatory, that activity assessment be made on a small scale under reaction conditions that differ from those used on the large scale.
It will often not be possible to estimate accurately large-scale behaviour from activity assessments made on a small scale, unless recourse is had to empir-ical correlations between data obtained in the two regimes. In any case, in assessing catalytic activity it is essential that the behaviour of the catalytic reactor be understood, so that the significance of the data obtained with it can be properly gauged.
Under reaction conditions all catalysts suffer from progressive deacti-vation to some extent at least. In practice, deactivation limits the lifetime of the catalyst, and the effective lifetime is a parameter of considerable eco-nomic significance. In general, deactivation results either from a change in the chemical composition of the catalyst resulting from the addition or removal of matter, from a change in the texture and structure of the catalyst, or from a combination of these factors. If the reasons for deactivation are understood it may be possible to devise regeneration procedures, although in many cases this may not be possible, or be possible to only a limited extent. It will be clear that any study of deactivation and regeneration necessarily involves a study of the same characterization parameters as are used with a pristine catalyst.
In principle, an account of methods for catalyst characterization has a vast range of scientific and technical techniques to draw upon. It is possible to make the case that virtually every technique known to materials science is of some potential value, and in addition there are many types of measurement which are peculiar to catalytic science. Nevertheless, experience has shown that out of this vast range, a relatively restricted number of techniques and types of measurement are of dominant importance: we have attempted to confine this book to these accepted methods. Even so, the field is quite large. and in many situations there are alternative techniques available. In some of these cases we have recommended a preferred method: this does not imply that we are pointing to an accepted standard method, merely that the pre-ferred method has given, in the author's experience, the best results, or for which good grounds for selection are clear.
Assuming that one is dealing with a catalyst in some divided form (powder. pellets. and so on), it is necessary to say something about the problem of sampling. Many catalysts are materials which are produced commercially in large volumes, so that it is a matter of some importance for the sample which is to be used for a characterization procedure to be as representative as possible of the bulk material. In this situation, the sampling procedure will usually be related to the methods used for bulk handling at
the end of the catalyst manufacturing process. Two golden rules for sampling have been suggested:
1. The sample should be taken while the material is in motion
0120583208
CATALYSTS
QD501H / 3208
