Catalyst Characterization

Why pore structure matters in catalysis

In a heterogeneous catalyst the reaction happens on internal surfaces, so activity scales with the accessible surface area, while selectivity and effectiveness depend on how fast reactants and products diffuse through the pores. Industrial catalysts are often hierarchical: micropores or small mesopores carry most of the surface area, and larger mesopores and macropores in the pellet or extrudate act as transport highways.

Because no single technique covers that whole range, catalyst characterization almost always combines gas adsorption (micro- and mesopores, surface area) with mercury intrusion (meso- and macropores in shaped bodies), plus chemisorption for the metal surface area.

Method notes and common pitfalls

• BET on microporous solids. For zeolites and MOFs the BET number is an apparent "BET area", not a true geometric surface area. Choose the pressure range with the Rouquerol criteria (as ISO 9277:2022 recommends) and report the range used.
• Adsorbate choice. IUPAC (2015) recommends argon at 87 K for micropore size analysis because, unlike N₂, it has no quadrupole moment that interacts with polar surface sites and cations. See nitrogen vs. argon.
• BJH limits. BJH is based on the Kelvin equation and underestimates pore sizes below roughly 10 nm; NLDFT/QSDFT kernels matched to the material give more reliable distributions. Check whether the desorption branch is affected by cavitation (forced closure near p/p₀ ≈ 0.42 for N₂ at 77 K) before using it.
• Pellets vs. powders. Crushing a pellet removes the macropore network you may want to measure; analyse whole pellets by MIP and powders by gas adsorption when both scales matter.
• Degassing. Zeolites are usually outgassed at high temperature (often around 300–350 °C with a slow ramp) to remove water; temperature-sensitive supports, organics, and some MOFs need much milder conditions.

Tracking deactivation

• Sintering of supports and metal particles shows up as BET surface-area loss and a shift of the pore size distribution to larger sizes; chemisorption reveals the loss of metal dispersion.
• Coking fills or blocks micropores and pore mouths: micropore volume drops sharply, often more than the total BET area.
• Steaming / dealumination of zeolites removes framework aluminium and creates mesopores (the basis of ultrastable Y), visible as a new hysteresis loop and increased mesopore volume.