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Spraying or co-precipitation? What are the differences between the two methods of catalyst preparation?
In chemistry, a catalyst support is a solid material with a high surface area that is often used to immobilize catalysts. The activity of heterogeneous catalysts is primarily promoted by atoms on the accessible surface of the material. Therefore, chemists strive to maximize the specific surface area of the catalyst, and one popular approach is to evenly distribute the catalyst on the surface of the support. The support may be inert or may participate in the catalytic reaction. Common supports include activated carbon, bauxite, and silicon oxide.
The activity of a catalyst depends primarily on the surface it contacts, which makes the method of catalyst preparation crucial.
Apply catalyst to support
There are two main methods for preparing catalysts: impregnation and co-precipitation. In the impregnation method, a suspension of a solid support is treated with a solution of a precursor, and the resulting material is subsequently activated under specific conditions, converting the precursor (usually a metal salt) into a more active state, such as the metal itself. In this case, the catalyst support is usually in granular form. Another method is to prepare the catalyst from a homogeneous solution, which is called coprecipitation.
Using the co-precipitation method, the acidic solution of the aluminum salt and precursor is reacted with alkaloids to precipitate mixed hydroxyl groups, followed by calcination to obtain the catalyst.
Many precursors need to react with hydrogen flow at high temperatures to be activated. Such a process requires the support to have high thermal stability. In addition, catalysts may become contaminated after long-term use, so in some cases, the catalyst can be reactivated through redox cycles, which also require high-temperature conditions.
Overflow phenomenon
Supports are often viewed as inert materials: catalysis occurs on catalytic "islands" and the support exists only to provide a high surface area. However, experiments show that this model is often oversimplified. It is known that adsorbates such as hydrogen and oxygen, for example, are able to interact with the support and even migrate from one island to another without re-entering the gas phase. This process is called spillover.
Hydrogen can "spill out" through the support to form hydroxyl groups. This reveals the potential activity of the support in catalysis.
Catalyst leaching
In heterogeneous catalysis, a common problem is catalyst leaching, a form of deactivation in which active species are lost in the liquid phase. Leaching not only has a negative impact on the environment but also causes commercial losses. Therefore, this issue must be taken into consideration when using catalysts for long periods of time.
Strong metal-supports interaction
The phenomenon of strong metal-support interactions highlights the problem of the simplified model that heterogeneous catalysts are simply supported on inert materials. For example, studies have shown that platinum, when supported on titanium oxide, no longer binds to hydrogen in the same way as platinum alone, suggesting that the presence of the support affects the electronic behavior of the catalyst.
Original evidence confirms that catalyst performance is greatly affected by the support material, especially in the case of strong metal-support interactions.
Electrocatalytic support
In electrocatalysis, supports are often used to provide mechanical stability to catalyst nanoparticles or powders. These supports can reduce particle mobility, promote chemical stability, and act as solid coating agents. Recently, graphene has been regarded as one of the most promising support materials due to its excellent porosity, electronic properties, thermal stability, and active surface area.
Examples and prospects
Almost all major heterogeneous catalysts are support-based. The preparation and application of these catalysts takes into account the nature of the support, the activity of the catalyst and its stability. With the development of technology, the performance of catalysts is still improving, making catalytic reactions more efficient and environmentally friendly.
Catalyst design and preparation is an evolving field. How can these methods be further improved in the future?
