What is Heterogenous Catalysis and How are Catalysts Synthesized?

Heterogeneous catalysis refers to when a liquid or gas phase reaction is executed over a solid catalyst. This concept is at the center of modern chemical and energy industries. Catalysts can be found everywhere.

Almost every object that surrounds an individual is reliant upon catalysts, including laundry detergent, Post-It notes, a pair of jeans, a car, and even beer.

Catalysts are required to break down paper pulp to create the glossy paper found in magazines. They also transform milk into yogurt and petroleum into bicycle helmets, and plastic milk jugs.

The unsung heroes of chemical reactions, catalysts offer solutions to some of the most challenging global issues and can assist in making each valuable atom count.

They are the foundation of a great number of industrial processes. They are also accountable for the degree of comfort that is enjoyed in the present day.

Catalyst fabrication as a process has not changed considerably over the last 50 years. Catalysts continue to be synthesized by wet chemical techniques that were made throughout the second world war.

Only gradual improvements have occurred since this time. To reach a new era where technological advancements are employed to decrease humanity’s footprint on the earth, the current fabrication methods for catalysts should be critically examined.

Catalyst Fabrication

A catalyst material comprises host material (porous support) and active sites in the design of nanoparticles. The porous support performs as a template where the particles are fixed.

A wide range of techniques has been established to create such materials over the years. Wet-chemical techniques and the latest gas-based techniques can be utilized for synthesis.

Wet chemical synthesis techniques are reputable, have various parameters to adjust the features of the catalyst, and are affordable to produce industrial quantities.

The difficulty is that it is hard to maintain consistency for all these parameters, which makes reproducibility and upscaling challenging in the preparation of catalysts.

From an academic point of view, it is also hard to conduct a performance comparison between the same catalyst that has been synthesized in various ways.

The ideal situation would be the ability to adjust the particle size, particle composition, and the support without having to adjust the recipe. This is where gas-based techniques can be beneficial.

Wet Chemical Methods

The most popular methods utilized in the industry are established on impregnation or precipitation. They share the same process that the support is uncovered to metal salt solutions, which introduces metal atoms within the material’s pores.

A step is then necessary where these metal atoms collect into nanoparticles. The added impurities as a result of the previous steps are removed in the final stage.

With precipitation methods, the induction of metal particle growth occurs by the supersaturation of the precursor solution, which produces growth and nucleation of metal particles.

This can occur at the same time as the production of the support (coprecipitation) or on existing support (deposition precipitation). The use of impregnation methods is an alternative.

Two primary impregnation techniques are observed. These are wet impregnation, where an excess amount of solution is employed, and pore volume impregnation, where an amount that almost fills the pore volume of the support is employed.

Current Limitations

The strategy for impregnation and precipitation appears to be simple when generally described, but it takes some effort to blend the desired loading, the nanoparticle size, distribution, and composition onto particular support.

The loading is reliant on the complete starting concentration that is available in the pores and is a weakness when impregnation techniques are used.

The composition is the creation of an intricate interaction between the desired nanoparticle material, the substrate, and the solvent, particularly when adsorption performs a crucial role.

The size of the nanoparticle relies on the driving force for growth and can be managed using temperature or pH. It is complex to achieve a similar temperature or pH profile across the sample over time.

This also means that sample reproduction is highly complex. To make matters even more complicated, the support can also be influenced in the process, particularly when the pH changes.

There are also remnants of the anionic deposits that must be removed as they impact the performance of catalysts and can even perform as poisons.

A calcination step is used to achieve this, which is a high-temperature treatment in air, resulting in oxidation. Once this step is complete, the material can be decreased when elemental metal nanoparticles are required.

Comparison with Gas-Based Methods

Gas-based synthesis methods such as spark ablation distinguish nanoparticle synthesis from deposition. In this method, the support does not affect the formation of nanoparticles and the features and loading can be adjusted independently.

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Compared to the techniques previously mentioned, the nanoparticles evolve outside of the support’s pores.

In this example, porous bulk supports such as zeolites cannot be utilized as the nanoparticles do not travel far into the pores, but thin porous membranes or finely dispersed powders can be employed.

Along with this, the composition can be changed separately, as the initial material is a solid bulk alloy.

The materials are converted into a gas state and are transported by an inert carrier gas. They evolve from atomic clusters into spherical particles of less than 5 nm. The polydisperse output can be changed to monodisperse through the use of filtering.

Adjustments are viable by combining various vapors to produce core-shell particles or using oxygen in the carrier gas to create oxides for example.

Deposition occurs through diffusion and the Van der Waals forces generate a weak, substrate independent interaction. An additional step post-treatment can further emphasize the nanoparticle-support interaction.

The Way Forward

To make significant advancements in the field of catalysis in the next few years, the parameter space of size, material, support, and loading should be examined for the most appropriate reactions.

This should be achieved in a scalable and reproducible manner. Contemporary techniques create samples that are hard to contrast and are not adaptable enough to attain this goal within a sensible time frame.

Gas-based techniques can have a considerable effect due to their reproducibility and flexibility. These techniques should be the way forward.

Acknowledgments

  • Produced from materials originally authored by Aaike van Vugt from VSPARTICLE. 

This information has been sourced, reviewed and adapted from materials provided by VSPARTICLE B.V.

For more information on this source, please visit VSPARTICLE B.V.

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