Pulse Chemisorption in Flow
The Pulse Chemisorption method (dynamic adsorption) is performed by injecting known doses of a suitable reactive probe gas in the reactor, where a catalyst has been previously activated to the chemical reaction. The advantages of using the Pulse technique are summarized here after:
- The analyses are very fast
- During the analysis the physical adsorption and weak chemisorption effects are minimized. Therefore the second adsorption run is not necessary (as occurs with static volumetric apparatus), obtaining immediately the data related to the strongly chemisorbed gas
- The technique allows an easier characterization of samples with a small dispersion or a very low percentage of active phase
- Manual operations are very easy
The sample preparation is similar to the ones applied before static adsorption: the catalyst must be cleaned and pre-treated adequately in order to perform a correct quantification of the active sites. The gas to be adsorbed is introduced as a pulse, by a special loop valve built-in the TPDRO, into the stream of the inert gas used as carrier. The injection system is placed before the sample reactor. The choice of the probe gas and of the carrier gas must assure the best answer of the detector if a TCD is used. In this case the two gases (probe gas and carrier) should have different thermal conductivity to have the best sensitivity of the detector. It is important also to use gases with the highest purity; both gases should flow through a water trap or a suitable filter to completely retain eventual vapors. The peaks obtained in the chromatogram represent the non-reacted gas and must be integrated. Therefore, the relation between the peak area and quantity of gas injected can be easily calculated.