Differentially pumped molecular beam extraction system to probe catalytic chemistry at ambient pressure conditions: A CFD-aided design

We report a design and finite element fluid-dynamics modeling of a differentially pumped molecular beam extraction system. The system relies on two stages of pumping and consists of three sections: a catalytic reactor, a differential chamber, and an analysis chamber. The design is specifically tailored for integration with the analysis chamber housing a Hemispherical Deflection Electron Analyzer and a Time-of-Flight Mass Spectrometer (HDA-TOFMS) at the Nano and Molecular Systems Research Unit, University of Oulu, as well as a similar setup at the FinEstBeAMS beamline of the MAX IV Laboratory synchrotron radiation facility (Lund, Sweden). The differential chambers’ inlet samples gas from the effluent stream of a catalytic tubular (packed bed) reactor operating at 1 bar and delivers a well-defined molecular beam of the sampled gas into the analysis chamber, where the pressure is maintained in the range of 1 × 10−6 mbar for safe operation of the HDA-TOFMS. With this upgrade, we anticipate significant enhancements in the capability of the HDA-TOFMS based gas phase methodologies to investigate heterogeneous catalytic processes under industrially relevant conditions. The simulation studies for optimizing the design parameters were performed using COMSOL Multiphysics software. The free molecular flow module was utilized to simulate the pressure distribution within the model geometry. The simulation results indicated the formation of a narrow molecular beam feature in the analysis chamber. Under steady-state conditions, the simulated average pressures inside the differential and analysis vacuum chambers were in close agreement with calculations made using the analytical expression for conductance through a thin aperture.