Catalytic Materials & Preparation Methods

• Catalytic materials and preparation methods are key to understanding catalysts.

Learning Outcomes

• Describe the different materials used for catalysts.
• Distinguish between supported and unsupported catalysts.
• Explain the primary processes for catalyst preparation.
• Describe the industrial processes for preparing supported and unsupported catalysts.
• Explain the post-treatment methods and their impacts on the final catalyst properties.
• Explain the requirements of a good catalyst.

Catalytic Materials

• Metals such as Pt, Rh, Fe, Cu and Co are employed in hydrogenation, Fischer-Tropsch (FT) processes, and ammonia synthesis.
• Non-stoichiometric oxides, like ZnO and TiO2, are used in partial oxidation and dehydrogenation.
• Insulating oxides such as Al2O3 and SiO2 with acidic sites are used in cracking reactions.
• Ceramics, including carbides and nitrides, are utilized.
• Rare-earth compounds like La2O3 and CeO2 are used as promoters.
• Zeolites are used for acid catalysis and cracking.
• Others (sulphides, carbon, etc.) such as CoMoS, carbon nanofibers, and activated carbon are also included.

Unsupported Catalysts

• The entire catalyst consists of the catalytically active substance.
• Examples include zeolites, metal oxides like SiO2 Al2O3, TiO2, ZnO, CuO, MgO, ZrO2, perovskite (CaTiO3), Raney Ni, Ag grids, Pt-Rh grids, and metal salts.

Supported Catalysts

• Active Phase: Contains sites where the reaction occurs; typically made of metals or oxides.
• Promoter: Textural, structural, or electronic promoters enhance catalyst performance.
• Support/Carrier: Increases surface area and dispersion, enhances mechanical strength, and can be inert or active.

Properties of Typical Supports

• Al2O3: The crystallographic phases are mostly α and γ with a surface area (SA) up to 400 m²/g; it is thermally stable and used in three-way catalysts.
• SiO2: Amorphous with SA up to 1000 m²/g; it is thermally stable and used in hydrogenation.
• Carbon: Amorphous with SA up to 1000 m²/g; unstable in oxidizing environments; used in hydrogenation.
• TiO2: Anatase and rutile with SA up to 150 m²/g; limited thermal stability; used in Selective Catalytic R catalysts.
• MgO: Has an fcc structure with SA up to 200 m²/g; rehydration may be problematic; used in steam reforming catalysts.
• Zeolites: Various forms (faujasites, ZSM-5) with a highly defined pore system; shape-selective and bifunctional.
• Silica/alumina: Amorphous with SA up to 800 m²/g with medium strong acid sites; used for dehydrogenation and bifunctional catalysts.

Preparation of Heterogeneous Catalysts

• Solid catalysts (bulk catalysts) and supports are the two types
• The solid catalysts proceed via metal salt solutions, mixing, precipitation(amorphous or crystalline solid), and post treatment to yielding bulk catalysts.
• Supported Catalysts (e.g. impregnated) proceed via impregnation of the support (solutions of the active component), evaporation, and post-treatment to supported metal catalyst.
• Post treatment often involves; Washing, drying, forming, calcination and activation.

Macroscopic Distribution of a Metal Within a Support

• Homogeneous Uniform Distribution occurs if precursors and competitors interact equally with the surface if the impregnating solution is concentrated and viscous and if there has been room temperature drying with weakly adsorbed precursors; suitable for catalysts with fair activity and no mass transfer resistance.
• Egg Shell Catalysts Active phase is located in the outer surface of the support; forms with a highly viscous solution and strong adsorption of precursors unto support and can also occur for slow drying regime for low concentration, low viscosity, and weak adsorption, suitable for fast reactions and for improving selectivity.
• Eggwhite Catalyst Active phase is included in the region between the outer-shell and the inner-core; suitable for reactions that range between those conducted with eggshells or egg yolk with mass transfer resistance, wear, and high possibility of poisoning.
• Egg yolk The active phase is present in the inner-core of the support; distribution occurs when competitor ions have stronger interaction with support; fast drying regime with predominant back diffusion. Suitable for use in fluidized or moving beds, where the outer layer can easily wear or be poisoned.

Post Treatment of Heterogeneous Catalysts

• Bulk catalyst post-treatment involves washing, drying, milling, forming, calcination, and activation.
• Supported catalyst post-treatment involves drying, decomposition (calcination), forming, and final activation (e.g., reduction).

Requirements of a Good Catalyst

• High activity per unit volume in the reactor.
• High selectivity towards the desired product at the conversion levels used, with minimal by-product formation.
• Sufficiently long lifetime with respect to deactivation.
• Possibility to regenerate, especially if deactivation is fast.
• Reproducible preparation.
• Sufficient thermal stability against deactivation (e.g., sintering, structural change, or volatilization).
• High mechanical strength against crushing.
• High attrition resistance.