Preparation & Characterization of heterogeneous catalyst Lec.10

Introduction Industrial catalysts are generally shaped bodies of various forms, e. g., rings, spheres, tablets, pellets. The production of heterogeneous catalysts consists of numerous physical and chemical steps. The conditions in each step have a decisive influence on the catalyst properties. the main physical properties of a catalyst that are influenced by the production Conditions are: Active surface area; pore structure; mechanical strength.

I. Preparation of heterogeneous catalyst Components of a Typical Heterogeneous Catalyst

Bulk catalysts Bulk catalysts also known as precipitated catalysts are mainly produced when the active components are cheap. The preferred method of production is precipitation. One or more components in the form of aqueous solutions are mixed and then co precipitated as hydroxides or carbonates. An amorphous or crystalline precipitate or a gel is obtained, which is washed thoroughly until salt free. This is then followed by further steps: drying, shaping, calcination, and activation

Preparation of bulk catalyst

Impregnated catalyst ( support catalyst) One of the best known methods for producing catalysts is the impregnation of porous support materials with solutions of active components. Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a support with a solution of the metal salt. Especially catalysts with expensive active components such as noble metals are employed as supported catalysts. A widely used support is Al2O3, Activated carbon and silica gel. Industrial examples: Ethylene oxide catalysts in which a solution of a silver salt is applied to Al2O3 Catalysts in the primary reformer of ammonia synthesis, with 10–20 % Ni on Al2O3

impregnation The support is immersed in a solution of the active component under precisely defined conditions (concentration, mixing, temperature, time). Depending on the production conditions, selective adsorption of the active component occurs on the surface or in the interior of the support. The result is non uniform distribution.

Supported metal catalyst

The advantages of impregnated catalysts compared with precipitated catalysts Pore structure and surface of the catalyst can be controlled. More economic, since the content of expensive active components is often low. The distribution and crystallite size of the active components can generally be varied over a wide range. Multiple impregnation is possible.

II. Characterization of Heterogeneous Catalysts Physical properties: pore size, surface area, and morphology of the carrier; and the geometry and strength of the support Chemical properties: composition, structure, and nature of the carrier and the active catalytic components Changes during the catalysis process: deactivation

Physical Properties of Catalysts Surface Area and Pore Size of the Carrier Surface area Pore size: (Pore size distribution ,Pore structure, Pore volume) It is usually advantageous to have high surface area (large number of small pores) to maximize the dispersion of catalytic components. If the pore size is too small, diffusion resistance will becomes a problem.

Scanning electron Microscope (SEM) Scanning electron microscope (SEM) is equipped with an energy dispersive analyzer or wavelength dispersive analyzer. The bombardment of a sample with electrons generates X rays characteristic of the elements present. peanut shells

X-Ray Diffraction (XRD) Gives information about Elemental Composition, Catalyst Structure and Particle Size, Width of peaks reveals particle size Bragg ´s Law X-rays in X-rays out catalyst Can be used in situ

FTIR(Fourier Transform Infra Red) FT-IR stands for Fourier Transform Infra Red, the preferred method of infrared Spectroscopy. So, what information can FT-IR provide? • It can identify unknown materials • It can determine the quality or consistency of a sample • It can determine the amount of components in a mixture

Infrared spectroscopy IR out IR in catalyst IR through An in situ method