Physical characterization of catalysts

Hie chemical composition is not the only factor determining the activity of catalysts. In many cases, the physical characteristics of the catalysts, such as the surface area, particle porosity, pore size, and pore size distribution influence their activity and selectivity for a specific reaction significantly. The importance of the catalyst pore structure becomes obvious when one considers the fact that it determines the transport of reactant and products from the outer catalyst surface to the catalytic surface inside the particle. 

Several experimental methods are available to characterize catalyst pore structure. Some of them, useful in quantifying mass transfer of reactant and product inside the porous particle, will be only briefly discussed here. More details concerning methods for the physical characterization of porous substances are given by various authors [5,8,9], 

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Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Christenn C, Steinhilber G, Schulze M, Friedrich KA (2007) Physical and electrochemical characterization of catalysts for oxygen reduction in fuel cells. J Appl Electrochem 37 1463-1474... 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

For more than five decades, the methods of surface physics and chemistry have provided some of the most incisive results advancing our understanding of the catalytic action of solids at the molecular scale. Characterizations by physical methods have demonstrated the dynamic nature of catalyst surfaces, showing that their structures, compositions, and reactivities may all be sensitive to temperature and the composition of the reactive environment. Thus, the most insightful catalyst characterizations are those of catalysts as they function. This volume of Advances in Catalysis is dedicated to the topic of physical characterization of solid catalysts in the functioning state. Because the literature of this topic has become so extensive, the representation will extend beyond the present volume to the subsequent two volumes of the Advances. 

The growing interest in physical characterization of solid catalysts as they function has stimulated a new series of congresses, the first held in Lunteren (The Netherlands) in 2003 and the second in Toledo in 2006. The subject has been documented in recent books (B. M. Weckhuysen, Ed., In situ Spectroscopy of Catalysts, American Scientific Publishers, 2004, and J. F. Haw, Ed., In situ Spectroscopy in Heterogeneous Catalysis, Wiley-VCH, 2002) and in topical issues of journals Top. Catal. 15 (2001) Phys. Chem. Chem. Phys. 5, issue 20 (2003) and Catal. Today 113 (2006). It is our intention that our set of volumes be more nearly comprehensive than these publications, as well as providing many newer results. 

The use of physical constants is, however, limited in the case of more complicated chemical processes the more complex the chemical change, the larger the number of physical properties necessary to investigate completely the chemical transformations. This especially holds when catalysts are involved in the reactions. When studying catalytic reactions we are dealing with catalysts as mixtures of a far more complicated nature than is the case, for example, in the structural analysis of hydrocarbon mixtures. The latter can be described, as was shown in the preceding sections, by means of a limited number of physical constants, from which either the chemical composition of the mixture or a series of other physical constants can easily be derived. For the characterization of catalysts completely different principles have to be applied even in simple cases, because in the case of a catalyst it is not chiefly its chemical composition that is important, but its chemical activity, which determines the result obtained by its chemical action. 

FIGURE 9.17 Physical characterization of a 30% Pt09Sn01/C catalyst with 30wt% metal loading prepared with the Bonnemann method (a) TEM image and (b) particle size distribution. 

Volumes 50 and 51 of the Advances, published in 2006 and 2007, respectively, were the first of a set of three focused on the physical characterization of solid catalysts in the functioning state. This volume completes the set. The six chapters presented here are largely focused on the determination of structures and electronic properties of components and surfaces of solid catalysts. The first chapter is devoted to photoluminescense spectroscopy it is followed by chapters on Raman spectroscopy ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy X-ray photoelectron spectroscopy X-ray diffraction and X-ray absorption spectroscopy. 

The electron microscope offers a unique approach for measuring individual nano-sized volumes which may be catalytically active as opposed to the averaging method employed by spectroscopic techniques. It is just this ability of being able to observe and measure directly small crystallites or nano-volumes of a catalyst support that sets the microscope apart from other analyses. There have been many studies reported in the literature over the past fifteen years which emphasize the use of analytical and transmission electron microscopy in the characterization of catalysts. Reviews (1-5) of these studies emphasize the relationship between the structure of the site and catalytic activity and selectivity. Most commercial catalysts do not readily permit such clear distinction of physical properties with performance. The importance of establishing the proximity of elements, elemental distribution and component particle size is often overlooked as vital information in the design and evaluation of catalysts. For example, this interactive approach was successfully used in the development of a Fischer-Tropsch catalyst (6). Although some measurements on commercial catalysts can be made routinely with a STEM, there are complex catalysts which require... 

Electron microscopy, with its high spatial resolution, plays an important role in the physical characterization of these catalysts. Scanning electron microscopy (SEM) is used to characterize the molecular sieve particle sizes and morphologies as a function of preparation conditions. Transmission electron microscopy (TEM) is used to follow the changes in the microstructure of the iron silicates caused by different growth conditions and subsequent thermal and hydrothermal treatments. 

There has been extensive study of tlic process of physical adsorption, and it has been of great importance in the characterization of catalyst surfaces, particularly in measurements of the surface area of catalysts via the measurement of the amount of adsorbate rccpiired to form a uni-molecular film. However, it is not likely that these weak Van der Waals forces are important in chemical catalysis. 

Volume 50 of Advances in Catalysis, published in 2006, was the hrst of a set of three focused on physical characterization of solid catalysts in the functioning state. This volume is the second in the set. The hrst four chapters are devoted to vibrational spectroscopies, including Fourier transform infrared (Lamberti et al.), ultraviolet Raman (Stair), inelastic neutron scattering (Albers and Parker), and infrared-visible sum frequency generation and polarization-modulation infrared rehection absorption (Rupprechter). Additional chapters deal with electron paramagnetic resonance (EPR) (Bruckner) and Mossbauer spectroscopies (Millet) and oscillating microbalance catalytic reactors (Chen et al.). 

A number of modern physical techniques are used to characterize heterogeneous catalysts. These methods range from techniques probing the interaction of catalysts with probe molecules, to in situ surface characterization techniques as well as structural elucidation under both in situ and ex situ conditions. In general, interaction of catalysts with probe molecules is followed using some spectroscopic property of the probe molecule itself and/or the changes induced by the heterogeneous catalyst. The spectroscopic techniques used include vibrational spectroscopies, NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry to name a few examples. Similarly, in situ techniques tend to use properties of probe molecules but also combined with structural techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). In recent years XAS has been widely used in the characterization of catalysts and catalyst surfaces. 

Physico-chemical techniques are widely used for characterization of catalysts and porous materials in general. Well-known methods based on physical adsorption of inert gases (N2 and CO2) and penetration of mercury at elevated pressures provide information on the total surface area, pore volume, and pore size distribution (PSD) of the sample [1,2]. Gas adsorption and mercury porosimetry are often compared since they generate data of similar nature in the pore size range 4 - 100 nm. 

In order to assess the viability of quantitative XPS as a tool in the physical characterization of coke deposits, we have studied a number of spent catalysts which contained considerable amounts of coke. The spent heavy-oll -processing catalysts were obtained from vacuum gas oil hydroconver ion experiments. The activity of fresh and spent catalysts was evaluated from thiophene hydrodesulfurization tests. 

Catalysts regenerated by methods (A) and (B) are evaluated on the basis of bench-scale activity tests, characterization of catalyst physical and chemical properties, and physical integrity tests, such as particle attrition resistance,... 

Abstract Immobilized metallic and bimetallic complexes and clusters on oxide or zeolite supports made from well-defined molecular organometaUic precursors have drawn wide attention because of their novel size-dependent properties and their potential applications for catalysis. It is speculated that nearly molecular supported catalysts may combine the high activity and selectivity of homogenous catalysts with the ease of separation and robustness of operation of heterogeneous catalysts. This chapter is a review of the synthesis and physical characterization of metaUic and bimetallic complexes and clusters supported on metal oxides and zeohtes prepared from organometaUic precursors of well-defined molecularity and stoichiometry. 

In this chapter we review studies, primarily from our laboratory, of Pt and Pt-bimetallic nanoparticle electrocatalysts for the oxygen reduction reaction (ORR) and the electrochemical oxidation of H2 (HOR) and H2/CO mixtures in aqueous electrolytes at 274—333 K. We focus on the study of both the structure sensitivity of the reactions as gleaned from studies of the bulk (bi) metallic surfaces and the resultant crystallite size effect expected or observed when the catalyst is of nanoscale dimension. Physical characterization of the nanoparticles by high-resolution transmission electron microscopy (HRTEM) techniques is shown to be an essential tool for these studies. Comparison with well-characterized model surfaces have revealed only a few nanoparticle anomalies, although the number of bimetallics... 

Catalysts employed in the oxidative production of PA are V205-based compositions which are generally of the monolayer type and are supported on titania (anatase). With such catalysts, selectivities in excess of 80 mole % PA are achieved at essentially complete conversion. The utilization of Sb-V-oxidc-based catalysts supported on anatase improves the PA selectivity. However, little is known about the intrinsic chemical or electronic effects of Sb203 in such catalytic systems, as well as the chemical and physical characterization of the supported Sb-oxide or supported Sb-V-mixed metal oxide [1]. 

The above scheme shows the importance of both adsorption and desorption processes. Adsorption of at least one of the reactant molecules is required for catalysis to occur. If the accelerated rate of reaction is simply due to the concentration of molecules at the surface, catalysis may result from physisorption of the reactants. On the other hand, chemisorption can be used primarily to quantitatively evaluate the number of surface active sites, which are likely to promote (catalyze) chemical reactions. Chemisorption analyses are applied to physically characterize a catalyst material, to determine a catalyst s relative efficiency in promoting a particular reaction, to study catalyst poisoning, and in monitoring the degradation of catalytic activity over time of use. 

The physical properties of pore volume, pore distribution, and BET surface area are nowadays routinely monitored in the production and use of industrial catalysts. In contrast, the chemical characterization of catalysts and microstructural investigations, especially of the catalyst surface, are far more laborious and are rarely carried out in industry. 

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