Dynamic hydrogen-deuterium exchange to determine metallic surface areas of catalysts

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 pH2 = 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. © Springer Science+Business Media New York 2013.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Fischer-tropsch synthesis: Differences observed in local atomic structure and selectivity with pd compared to typical promoters (Pt, Re, Ru) of Co/Al <inf>2</inf>O <inf>3</inf> catalysts

Pd was examined as a promoter for Fischer- Tropsch synthesis, and its effects on cobalt oxide reduction and product selectivities relative to commonly used promoters (i.e., Pt, Re, and Ru) at atomically equivalent levels were compared. Pd was identified to promote cobalt oxide reduction to even lower temperatures than Pt and Ru. However, Pd addition deleteriously affected product selectivity, and a clear shift to favor light products was observed. XANES analysis of an activated model catalyst revealed that Pd was in the reduced state. Local atomic structure was examined by EXAFS. Unlike Pt, Re, and Ru promoters, where previous investigations by groups such as Dr. Guczi's and ours have only observed coordination of the promoter with cobalt, Pd displayed both direct coordination to Co as well as other Pd atoms. The results suggest that this feature may be responsible for the measurably higher light gas selectivities observed. © Springer Science+Business Media, LLC 2012