Activity and Selectivity of Solid Catalysts — Revision Notes

⚡ 30-Second Revision

Activity: — Catalyst's ability to increase reaction rate.

- Factors: Surface area, active sites, optimal adsorption strength (Sabatier principle). - Example: Fe in Haber process.

Selectivity: — Catalyst's ability to direct reaction to specific product.

- Factors: Pore structure (shape selectivity), active site geometry. - Example: ZSM-5 zeolite (alcohol to gasoline), $CO + H_2$ to $CH_3OH$ (Cu/ZnO-$Cr_2O_3$).

Sabatier Principle: — Optimal adsorption strength (not too weak, not too strong) for maximum activity.
Shape Selectivity: — Molecular sieving effect by catalysts with specific pore sizes (e.g., zeolites).
Catalysts — lower $E_a$, do NOT change $Delta H$ or $K_{eq}$.

2-Minute Revision

Solid catalysts are crucial for speeding up reactions (activity) and guiding them to specific products (selectivity). Activity is how fast a catalyst makes a reaction go, primarily by lowering the activation energy.

Key factors for activity include a large surface area, the presence of specific active sites, and critically, the optimal strength of reactant adsorption – known as the Sabatier principle. If reactants bind too weakly, they don't react; if too strongly, products get stuck, blocking sites.

Examples include iron in the Haber process and nickel in hydrogenation. Selectivity is the catalyst's ability to choose a particular product when multiple are possible. This is vital for industrial efficiency.

Factors influencing selectivity include the catalyst's pore structure, leading to 'shape selectivity' (e.g., zeolites like ZSM-5, which act as molecular sieves), and the precise geometry of active sites.

The conversion of synthesis gas ($CO + H_2$) into different products (methanol, methane, hydrocarbons) using different catalysts is a prime example of selectivity. Remember, catalysts affect reaction rates but not equilibrium positions.

5-Minute Revision

Solid catalysts are materials that accelerate chemical reactions without being consumed, exhibiting two primary characteristics: activity and selectivity. Activity refers to the catalyst's efficiency in increasing the reaction rate.

This is achieved by providing an alternative reaction pathway with a lower activation energy ($E_a$). Factors governing activity include: 1) Surface Area: More surface area means more active sites.

2) Nature of Active Sites: Specific atomic arrangements or electronic properties (e.g., partially filled d-orbitals in transition metals) facilitate bond breaking and formation. 3) Adsorption Strength (Sabatier Principle): Reactants must adsorb with an optimal strength – strong enough to react, but weak enough for products to desorb.

Too weak, no reaction; too strong, active sites are blocked. For example, iron in the Haber process for ammonia synthesis is highly active.

Selectivity is the catalyst's ability to favor the formation of a specific product out of several possible ones. This is crucial for minimizing by-products and maximizing desired product yield. Key factors for selectivity include: 1) Geometric Factors (Shape Selectivity): Catalysts with specific pore structures, like zeolites (e.

g., ZSM-5), act as molecular sieves, allowing only molecules of a certain size and shape to react. This is widely used in the petrochemical industry to convert alcohols to gasoline. 2) Active Site Geometry and Electronic Properties: The precise arrangement of atoms on the surface can dictate which bonds are activated and which intermediates are stabilized, thereby directing the reaction pathway.

A classic example is the conversion of synthesis gas ($CO + H_2$) into methanol (using $Cu/ZnO-Cr_2O_3$), methane (using Ni), or higher hydrocarbons (using Fe/Co), where the catalyst dictates the product.

Remember, catalysts only affect the kinetics (rate) of a reaction, not its thermodynamics (equilibrium constant or $Delta H$).

Prelims Revision Notes

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Catalyst Definition: — Substance that alters reaction rate without being consumed. Lowers activation energy ($E_a$). Does NOT change $Delta H$ or equilibrium constant ($K_{eq}$).
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Activity: — Measure of catalyst's ability to increase reaction rate.

* Factors affecting Activity: * Surface Area: Larger surface area $ightarrow$ more active sites $ightarrow$ higher activity. (e.g., finely divided metals). * Active Sites: Specific locations on catalyst surface where reaction occurs.

* Sabatier Principle: Optimal adsorption strength is crucial. * Too weak adsorption: Reactants don't bind. * Too strong adsorption: Products don't desorb, block sites (poisoning). * 'Just right' adsorption: Efficient reaction and desorption.

* Electronic Configuration: Transition metals (partially filled d-orbitals) are highly active due to ability to form temporary bonds and variable oxidation states. * Promoters: Substances enhancing catalyst activity (e.

g., Mo with Fe in Haber process). * Poisons: Substances reducing/destroying activity (e.g., CO poisoning of Pt). * Examples: Fe in Haber process ($N_2 + 3H_2 xrightarrow{Fe} 2NH_3$), Ni/Pt/Pd in hydrogenation of oils.

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Selectivity: — Catalyst's ability to direct a reaction towards a specific product.

* Factors affecting Selectivity: * Shape Selectivity: Unique to catalysts with specific pore structures (e.g., Zeolites like ZSM-5). Act as molecular sieves, allowing only specific size/shape molecules to react.

* Active Site Geometry: Specific arrangement of atoms on surface favors certain reaction pathways. * Electronic Properties: Influence orientation and stability of intermediates. * Examples: * Synthesis gas ($CO + H_2$) conversion: * $CO + 2H_2 xrightarrow{Cu/ZnO-Cr_2O_3} CH_3OH$ (Methanol) * $CO + 3H_2 xrightarrow{Ni} CH_4 + H_2O$ (Methane) * $nCO + (2n+1)H_2 xrightarrow{Fe/Co} C_nH_{2n+2} + nH_2O$ (Hydrocarbons) * ZSM-5 converting alcohols to gasoline.

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Key Distinction: — Activity = how fast; Selectivity = which product. Both are vital for industrial processes.

Vyyuha Quick Recall

To remember factors for Activity and Selectivity:

Active Sites Often Promote Transition Metals (for Activity)

Active Sites
Optimal Adsorption (Sabatier Principle)
Promoters
Transition Metals

Selectivity Shapes Products Geometrically (for Selectivity)

Shape Selectivity (Zeolites)
Pore structure
Geometry of active sites