Einführung in die Katalyse

Catalysts are substances that are added to chemical systems to enable chemical reactions to occur faster, using less energy.

The minimum amount of energy required to initiate a reaction is called the activation energy. Catalysts provide an alternate reaction pathway with a lower activation energy, allowing the reaction to take place under less extreme conditions. The activation energy is described by the Arrhenius equation.

Enzymes are biological molecules that behave as extremely specific catalysts. Enzymes are shape specific, and guide reactant molecules, called substrates, into the optimal configuration for reaction. Homogeneous catalysts are in the same phase as the reactants. Most frequently, the catalyst and reactants are both dissolved in the liquid phase. In heterogeneous catalysis, the catalyst and reactants are in different phases, separated by a phase boundary. Commonly, heterogeneous catalysts are solid and consist of a nano-scale catalytic entity, typically a metal nanoparticle, which is dispersed on a support material.

The support material, usually carbon, silica, or a metal oxide, is used to increase the surface area and impart stability against aggregation of the nanoparticles. Porous membranes and beads, mesh, and stacked sheets are some of the support geometries used in catalysis.

In heterogeneous catalysis, nanoparticles have active sites on the surface, where the reaction takes place. Depending on the reaction, these active sites could be planar faces or crystal edges on the surface of the particle. Typically, smaller nanoparticles have higher catalytic activity, due to the higher amount of surface atoms per mole of catalyst.

This video will highlight the basics of catalysis, and demonstrate how to perform a basic catalytic reaction in the laboratory.

There are several types of catalysts. At high temperature, molecules move faster and collide more frequently. Since the proportion of molecular collisions is higher, the reactants have enough energy to overcome the activation energy of the reaction. The catalyst provides an alternate reaction mechanism that increases the proportion of collisions at a lower temperature, thereby decreasing the amount of energy needed to complete the reaction. The catalyst may participate in multiple chemical transformations, however it is unchanged at the completion of the reaction and can be recycled and reused.

The reaction on the catalyst surface begins with adsorption of the reagents to the active site, followed by the reaction on the surface. The surface reaction can occur between one adsorbed species and one in the bulk, called the Eley-Rideal mechanism, or between two adsorbed species, called the Langmuir-Hinshelwood mechanism. The products then desorb from the surface into the bulk.

Now that you understand the basics of catalysis, let's look at the reduction of 4-nitrophenol to 4-aminophenol using a commercially available palladium catalyst supported on ground active carbon. The reaction progress will be measured using the color change that occurs during the reaction.

Before beginning the experiment, be sure to wear appropriate personal protective equipment, such as a lab coat, safety goggles, and gloves. To prepare the materials, first weigh 14 mg of 4-nitrophenol and dissolve it in 10 mL of deionized water in a glass vial to make a 10 mM solution. Next, weigh 57 mg of sodium borohydride and dissolve it in 15 mL of DI water to make a 100 mM solution. Mix the two, and stir at room temperature to form a uniform solution. The solution color should not change, as the sodium borohydride cannot fully reduce 4-nitrophenol without the catalyst. Weigh 10 mg of palladium on active carbon and 10 mg of active carbon without catalyst as a control sample.

Transfer the weighed catalysts into separate vials, and add 100 mL of deionized water to each. Sonicate the vials with an output power of 135 Watts until catalysts are well distributed in the water.

Now that the materials are prepared, the catalytic reduction of 4-nitrophenol can be performed. Measure 1.15 mL of the prepared 4-nitrophenol and sodium borohydride solution, and transfer to a 5-mL glass vial.

Observe and record the color of the solution in the vial. Add 1 mL of the prepared palladium on active carbon catalyst solution to the vial, and shake by hand to mix.

Observe the reaction for 20 min, and record when the solution color begins to change and then completely fades. When all of the color has faded, the reaction is complete.

Repeat the same procedure for the active carbon control solution. As the reaction progresses, the color changes from yellow to colorless, indicating the consumption of 4-nitrophenol. To quantify this change, measure UV-Vis absorbance of the sample at 400 nm.

Plot the natural log of absorbance versus time. The absorbance decreases over the course of the reaction, indicating the consumption of 4-nitrophenol. The control sample showed no catalytic activity.

Catalysts are of vital importance to a wide range of industrial and scientific fields.

In the presence of a palladium catalyst, carbon-carbon coupling reactions occur, known as the Heck Reaction. The Heck reaction is regarded as the first correct mechanism for transition metal-catalyzed coupling reactions. It is so valuable to modern catalysis that Richard F. Heck received the Nobel Prize in Chemistry for his discovery. The Heck Reaction can be performed using a palladium catalyst, as shown in this experiment. Here, the catalyst was synthesized at room temperature. After the reaction, the product was analyzed using nuclear magnetic resonance spectroscopy, or NMR.

In nature, enzymes are catalysts that enable a wide range of biological reactions. For example, acetate kinase is an enzyme found in microorganisms that facilitates the reversible conversion of acetate to acetyl phosphate.

The enzyme activity was measured using UV-Vis spectrophotometry, with a standard curve.

The amount of acetyl phosphate consumed was monitored throughout the reaction, and the enzyme kinetics plotted as a function of time.

Polymers are another field that can take advantage of catalysis. Here, star-shaped polymer particles were synthesized.

First, the catalyst was prepared and dried at room temperature. The polymer branches were then mixed with the catalyst, and then a cross-linker was added to form the particles.

The particle size was then analyzed using gel permeation chromatography. Polymeric nanoparticles, like the star polymers fabricated in this example, are used for a wide range of applications such as drug delivery and self-assembly.

You've just watched JoVE's Introduction to catalysis. After watching this video, you should understand the concept of catalysis and how to run a simple reaction in the laboratory.

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