Dynamic Hydrogen–Deuterium Exchange to Determine Metallic Surface Areas of Catalysts

Abstract

The dynamic exchange of hydrogen and deuterium is used to determine metal surface areas of solid catalysts. The measurements are carried out in situ, i.e. no transfer of the catalyst from the catalytic reactor to a probing chemisorption device is necessary. This makes the method suitable for measurements at any instant of catalyst activation or kinetic testing. The principle of operation is to perform gas switches from H₂ to D₂ (deuterium) at various hydrogen partial pressures under dynamic flow conditions using Ar as balance. When switching the gas, D₂ reacts with the adsorbed H atoms and with H₂ remaining in the gas phase, producing HD molecules. The exchange is followed mass spectrometrically. A linear relationship between the number of detected HD molecules and the hydrogen partial pressure is observed. The extrapolation to ${\text{p}}_{{{\text{H}}_{2} }}$ = 0 gives access to the number of H atoms adsorbed on the catalyst surface before switching gases. Provided the hydrogen/metal adsorption stoichiometry is known the specific surface area of the metal can be determined. This approach is methodically new and has been validated by comparing the surface areas of pure metallic Co samples (without any support) and their BET surface areas from measurements in the same experimental set-up. We finally demonstrate the usefulness of the methodical approach to determine “active” surface areas of Co-based catalysts after various pretreatments and kinetic tests.

Graphical Abstract