A catalyst pore is an important performance-defining characteristic that affects the available surface for reactions to occur. Used to refer to the internal structure or internal surface area of catalysts, the size and structure of this pore influences permeability—the ease with which gases and fluids can travel through a solid. Finer pores give rise to low permeability, resulting in higher resistance to fluid flow, used to limit unwanted reactions. Only molecules of desired sizes can enter and leave, creating a selective catalyst that will produce the desired product.

What are the primary types of catalysis? 

Catalysts fall into one of two groups; as homogeneous catalysts and heterogeneous catalysts. Homogenous catalysis involves only one phase and usually occurs in the liquid phase. In contrast, heterogeneous catalysis occurs at or near an interface and can be found in all three phases of matter: solid phase, liquid phase, and gas phase.

Homogeneous catalyst 

Homogeneous catalysts offer several advantages. Being in the liquid phase results in a high reactivity and selectivity of reactions under low-temperature conditions (less than 250◦C). They usually have many active sites on the surface binding reactants, so the reaction progresses in those active sites. However, the recovery of homogeneous catalysts is comparatively difficult and expensive since the catalyst is in the same phase as the reaction mixture, making catalyst separation difficult. The thermal stability of homogeneous catalysts is also lacking. Transition metals and organometallic complexes are examples of homogeneous catalysts. 

Heterogeneous catalyst 

For heterogeneous catalysis, catalytic recovery is easy and inexpensive since the catalyst is in a different phase from that of the reaction mixture. Catalysis efficiently acts in high-temperature conditions, around 250 – 500◦C, however active sites of heterogeneous catalysts are not well-defined, which reduces selectivity of reactants. Typical examples of heterogeneous catalysts are metals, metal oxides, and the like.

How are pores measured?

Practical techniques used to investigate porosity include pycnometry, gas adsorption, mercury porosimetry, and porometry.

Gas pycnometry 

This is the standard technique for determining true, absolute, skeletal, and apparent volume and density. Being non-destructive, it uses inert gases, such as helium or nitrogen as the displacement medium to measure volume and calculate density.

Physical Gas Adsorption and Chemisorption

These experiments are standard techniques for the characterisation of porous solids. Chemisorption, for example, is used to quantitatively measure the number of surface-active sites vital to promoting a specified catalytic reaction. Interpreting critical parameters such as the active element, metal dispersion, and surface acidity are essential to produce meaningful data.

Mercury Porosimetry Analysis

Intruding mercury into a porous structure under stringently controlled pressures, this technique offers speed, accuracy, and a wide measurement range. Mercury porosimetry enables the calculation of numerous sample properties such as pore size distributions, total pore volume, total pore surface area, median pore diameter, and sample densities (bulk and skeletal).

Porometry

Porometry displaces a wetting liquid from the sample pores by applying a gas at increasing pressure. This process is used to measure minimum, maximum (or first bubble point), and mean flow pore sizes, and pore size distribution of the through pores in membranes nonwovens, paper, filtration and ultrafiltration media, hollow fibres, ceramics, and so on.

What is the difference between porosimetry and permeability?

Porosity is the measurement of void spaces within solids. The term porosimetry often includes the measurements of pore size, volume, distribution, density, and other porosity-related characteristics of a material. Permeability is the measurement that indicates how easily a fluid can flow in between these spaces or pores. Permeability provides insight as to the ease with which molecules of different geometries can access an active site, which can help to determine the selectivity of a catalyst. 

Porosity also defines the performance of a catalyst and is crucial in understanding the formation, structure, and potential use. The porosity of a material affects its physical properties and, subsequently, its behaviour in its surrounding environment. The adsorption and permeability, strength, density, and other factors influenced by a substance’s porosity determine appropriate uses.

Porous materials such as activated carbons, zeolites, or metal-organic frameworks largely owe their industrial value to the ability to control the rate of transport of different molecules.

ASTM D4404 – 18 is the standard test method intended for use in determining the volume and the volume distribution of pores in soil and rock concerning the apparent diameter of the entrances of the pores. In general, both the size and volume of the pores affects the performance of soil and rock. The pore volume distribution is useful in understanding its relationship to water infiltration, permeability, and water-holding capacity, which enables better soil-management practices.  

What are the best ways to measure the various types of catalysis?

Researchers use a wide range of techniques to assess catalyst performance.

Mercury porosimetry analysis enables you to calculate numerous sample properties such as pore size distributions, total pore volume, total pore surface area, median pore diameter, and sample densities (bulk and skeletal).

The AutoPore V Series Mercury Porosimeters can determine a broader pore-size distribution more quickly and accurately than other methods. This instrument also features enhanced safety features and offers new data analysis options that provide more information about pore geometry and fluid transport characteristics.

Following a reaction, it is desirable to re-quantify defining characteristics such as the number of active catalysts sites to observe any change, which typically requires the removal of the catalyst from the reactor and transferring it to a chemisorption system. There is widespread recognition this process undermines the integrity of the resulting characterisation data. The Micromeritics in-situ Catalyst Characterization System (ICCS) offers a solution—it is an advanced catalyst characterisation tool that enables the user to study the impact of a reaction on critical parameters such as the number of active sites under precisely controlled, process-representative conditions.

With a large selection of pore-size analysers that leverage a variety of measurement techniques—including gas sorption and mercury intrusion—Micromeritics can fulfil almost any need for detailed porosity characterisation.

Micromeritics AutoPore Mercury Porosimeters quickly and accurately determine a material’s pore size, diameter, volume, surface area, bulk, and absolute densities. MicroActive software significantly improves functionality, convenience, diagnostics, and data interpretation—establishing a new standard for high-performance results in mercury porosimetry. To learn more about porosimetry, speak to ATA Scientific today.