Heterogeneous Catalysis by Supported Metals

1. Heterogeneous Catalysis by Supported Metals • By far the most widely used industrial catalysts are heterogeneous formulations of inorganic solids. Zeolites and clays are widely used in cracking processes, and similar materials are employed as supports for finely dispersed metals. • Catalytic converters use supported metals of the platinum group to promote both oxidation and reduction reactions of exhaust components. • Catalytic chemistry? Design of materials? Optimization?

2. General Classification of Bulk Solids as Catalysts

3. General Classification of Bulk Solids as Catalysts

4. Industrial Processes Catalyzed by Inorganic Solids

5. Industrial Processes Catalyzed by Inorganic Solids

6. Heterogeneous Catalysis: Topics of Study • Characterization of the Surface of Catalytic Inorganic Solids • Surface area, pore size • Structural analysis techniques • Kinetics of Heterogeneous Catalysis • Simple kinetic analysis of unimolecular and bimolecular reactions on dispersed metal catalysts • Control of Automobile Emissions • Structure and composition of catalytic converters • Legislated reactions and developments in catalyst technology • Cat. converter design considerations

7. Physical Forms of Catalytic Surfaces • Three broad forms of catalytic surface can be distinguished on the basis of general physical structure. • Non-porous, bulk-solid catalyst particle • Pt metal foil • Porous, bulk-solid catalyst particle • Zeolites, silica, Raney nickel • Supported catalyst with discontinuous coverage on the external surface of a support material. • Pt on ceramic (cat. converters), TiCl3 on MgCl2 (Ziegler-Natta polymerization) • The majority of heterogeneous reactions occur at the interface of the solid and a fluid, with limited activity from the bulk solid phase. Exposed surface area per unit mass is therefore a key design parameter.

8. Surface Area: N2 Adsorption • Knowledge of the volume of N2 required to establish an adsorbed monolayer on the solid (Vm) is sufficient to estimate the surface area of the solid. • cross-sectional area of an adsorbed N2 molecule is taken to be 0.162 nm2 • Vm is usually estimated using the BET isotherm • Areas derived from this procedure are useful indicators of available surface, but the activity of the surface for a particular reaction depends on the reactant which will have different adsorption characteristics.

9. Modeling of the Adsorption Behaviour of Solids

10. Typical Surface Areas of Catalysts and Supports

11. Pore Size Distribution • The effectiveness of a catalyst particle is governed by the balance of reaction rates and transport processes. While surface area can be a useful indicator of available reaction sites, pore size and pore size distribution are key parameters in transport equations. • Mercury penetration and N2 desorption measurements provide information on pore size distribution, as shown below for alumina (g-AI2O3). Curve 1: distribution of macropore sizes as determined by mercury penetration; Curve 2: complete size distribution determined by a combination of Hg porisometry and N2 adsorption.

12. Pore Size Distribution • Zeolites are crystalline aluminosilicates • that, depending on the Si/Al ratio, • template agents and preparation, • have unique micropore structures. • Virtually all acid catalyzed reactions can be conducted with acidic form of zeolites, provided the reactant is small enough to enter the pores. • The acidic sites in HZSM-5 are strong enough to protonate paraffins, leading to widespread use as an industrial cat. cracking catalyst.

13. High Surface Area Forms of Metals • Platinum blacks are finely divided powdered forms of the metal prepared by reduction of aqueous solutions of H2PtCl6. • Raney metals are “skeleton” forms prepared by leaching out of Al from a binary alloy. • Supported metals, as shown at • right, are highly dispersed on an • inorganic support. Once the • support is impregnated with a • suitable solution (often aqueous) • of the precursor and dried, • reduction (often with H2) of the • metal on the surface generates • metal crystallites. Electron micrograph of a supported metal catalyst, Rh/SiO2. The metal crystallites are present on the surfaces of primary particles of SiO2

14. High Surface Area Forms of Supported Metals • Plots generated by data obtained for the • chemisorption of hydrogen on a series of • rhodium metal catalysts supported on • silica at ambient temperature. • The horizontal axis indicates the • percentage of the solid material that • is rhodium (dispersed on the surface). • The left-hand vertical axis is the volume • (V) of hydrogen adsorbed at @ STP by • unit mass of solid. • The right-hand vertical axis (H2 / Metal) • is the number of hydrogen molecules • adsorbed per rhodium atom present • (surface and bulk), evaluated from the • corresponding value of V.

15. High Surface Area Forms of Supported Metals • The aggregation of dispersed metals, called sintering, can be an important high-temperature catalyst deactivation mechanism, since surface area is lost. • Here are scanning electron microscope images and calculated particle size distributions for 4 nm platinum dispersed on quartz, and the same sample after heating to 900°C for 24 hours.

16. Kinetic Treatment of Surface Reaction Data • Relatively few industrial processes (ammonia and methanol syntheses aside) are operated under conditions that make chemical reaction rates of practical importance. In most cases rates are large enough to reach a mass transfer limitation or essentially complete conversion. • Kinetics experiments have • yielded important insight • into plausible catalytic • mechanisms, under the • assumption that stage 3 • in the overall reaction is • rate determining.

17. Kinetic Control: Langmuir-Hinshelwood Models • Idealized plot of the initial rate of isomerization of cis-2-butene to trans-2-butene on a silica-alumina catalyst at 358K as a function of the pressure of cis-2-butene. • The heavy line corresponds to the pressure range accessible in the experimental study. • The dashed extensions to extrapolation on the basis of the Langmuir adsorption • isotherm. • The Langmuir-Hinshelwood treatment of KINETIC CONTROLLED heterogeneous reactions assumes that the rate is proportional to product of the fractional coverages (q) of the reactants, since these are proportional to the corresponding surface concentrations.

18. Kinetic Control: Langmuir-Hinshelwood Models • In a bimolecular reaction, the reactants must compete for equivalent surface sites if this treatment is to be applied. The reaction rate will be: • which, after applying the Langmuir isotherm as a model of surface adsorption, yields: • In some cases, reactants are adsorbed onto different sites. Although strictly outside the bounds of this simple treatment, it is often applied in the absence of alternate techniques/information.

19. Kinetic Control: Langmuir-Hinshelwood Models • Tabulated below are several examples of rate expressions for surface catalyzed reactions and their apparent interpretations according to the Langmuir-Hinshelwood scheme.