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 .