Characterization Techniques For Heterogeneous Catalysts

Catalyst Characterization by Physisorption

Most catalysts are porous solids of extremely high surface area. Surface area correlates with the number of available sites for the reaction. If the “active site density” needs to be high then so does the surface area. But, not all reactions benefit from higher surface area due to the increased chance of side reactions and unwanted polymerization as products find additional active sites rather than leaving the catalyst. Therefore, for any given catalyst / reaction combination there exists an optimum surface area. The method of choice for determining the accessible area is gas physisorption. Typically nitrogen or argon is adsorbed at liquid nitrogen temperature (77) or liquid argon temperature (87K) , respectively as a function of increasing pressure (typically around 0.1 to 0.3 atmospheres). Very low surface areas are best measured by low pressure krypton adsorption at liquid nitrogen temperature . The surface area value is calculated using the BET (Brunauer, Emmett, Teller) model.

It is accepted industry practice to classify pores according to their sizes of:

 • Macropores-Pores with widths exceeding 50 nm

 • Mesopores-Pores between 2 nm and 5 nm

 • Micropore-Pores with widths not exceeding 2 nm

By analyzing the physisorption results with suitable data reduction models it is possible to obtain the following material information:

• total BET area of the solid

• total micro- and , external surface area

 • micro-, meso- and total pore volume.

 • micro- and mesopore size distribution structure of pore network, pore geometry

For an overview of Quantachrome’s data reduction capabilities contained in our software please click here

Quantachrome Instruments for Physisorption are the AutoFlow BET+, Autosorb 6iSA, Quadrasorb evo, NOVAtouch, and Autosorb iQ .

Catalyst Characterization by Chemisorption

Many heterogeneous catalysts are actually a combination of an active phase (normally a zero-valent metal) spread on an inert support (typically a refractory oxide or carbon). Therefore, the total area of the sample does not represent the catalytically active area. The latter is also determined by gas sorption, but adsorption of a reactive gas (at or around room temperature) not an inert gas. The gases most commonly used are hydrogen and carbon monoxide, and this technique is known as chemisorption. The ratio of surface metal atoms to metals atoms within the bulk of the metal nanoparticles is called dispersion. The better the dispersion the more efficient the use of (expensive) metal.

Chemisorption Catalyst Characterization using TPR / TPD / TPO
Temperature programmed oxidation is a popular means to characterize carbons and redox catalysts. Related temperature programmed (i.e. non-isothermal) methods such as TPR (temperature programmed reduction), TPD (temperature programmed desorption) and TPO (temperature programmed oxidation) are used to determine relative ease of reduction (of oxides), oxidation (of low valency species, particularly carbons) and desorption (of ammonia from acid sites for example). Activation energy for a given chemical process can be gleaned by employing different heating rates.

For more detailed information on these techniques please see TPR / TPD

Quantachrome Instruments for TPR / TPD / TPO: ChemBET Pulsar, Autosorb iQ-C, ChemStar TPX

Pore Structure Characterization of Catalysts

The pore geometry of the majority of catalysts consists of an interconnected three dimensional network of pores, capillaries, and interparticle spaces. Distribution is usually distributed over the catalyst in an irregular fashion. The pore structure of a heterogeneous catalyst will affect such characteristics as transport phenomena, and the selectivity in catalyzed reactions.





Pore Size
Reducing particle size in order to generate more external surface area can only be partially successful. Fine powders present unacceptably high pressure drop, and all heterogeneous catalysts, apart from oil hydrogenation and fluid bed catalysts, are formed into granules, extruded into cylinders or pressed into pellets. Therefore, surface areas over just a few m²/g can only be generated as the internal surface of pores. Optimizing the pore size minimizes steric hindrance and promotes rapid diffusion to and from the active sites which, at the far end of a tortuous pore system, might be extremely remote from the exterior and the bulk flow of reactants and products.

Gas sorption readily measures pore sizes from as small as the gas molecule, to a few hundred nanometers in diameter. Larger pores, such as those formed by the assembly of particles in a formed catalyst piece are particularly important for rapid diffusional access to the higher surface area small pores. The large-pore network can be very rapidly characterized by mercury intrusion porosimetry.

For a cylindrical pore the size is defined as the diameter of the pore. For a slit pore the pore size is defined as the distance between the sides.



Pore Size Distribution
Pore size distribution is the distribution of pore volume compared to pore size. It is an important performance characteristic of catalysts since it controls the diffusion of reactants and products. Pore size distributions can also affect the selectivity of catalysts. The size distribution of catalysts is often important to ensure efficient transport of reactants and products to and from the active surface. In some catalyst systems, smaller pore sizes are used to limit unwanted reactions. Reaction by-products and feedstock impurities can result in loss of loss of activity through pore blockage.




Total Pore Volume
Also known as specific pore volume is the total internal volume per unit mass of a catalyst. Depending upon how much of the pore volume is exposed will determine how much space is accessible to molecules participating in the catalytic reaction. A decrease in pore volume can represent a decrease in the volume of the impregnated active phase of the catalyst, or micropore blocking. It can also be an indicator whether the active phase is impregnated in the micropores, or non-micropore layer of a catalyst. Total micropore volume can also be an indicator of the adsorptive capacity and regeneration capacity of various catalyst adsorbent substrates such as zeolites.



Quantachrome Instruments for Catalyst Pore Size Determination: PoreMaster, Autosorb 6iSA, Quadrasorb evo, NOVAtouch, and Autosorb iQ.