Horn, RaimundRaimundHornIhmann, K.K.IhmannIhmann, J.J.IhmannJentoft, Freiderike C.Freiderike C.JentoftGeske, MichaelMichaelGeskeTaha, A.A.TahaPelzer, KatrinKatrinPelzerSchlögl, RobertRobertSchlögl2024-02-272024-02-272006-05-01Review of Scientific Instruments 77 (5): 054102 (2006)https://hdl.handle.net/11420/46050A newly developed apparatus combining a molecular beam mass spectrometer and a catalytic wall reactor is described. The setup has been developed for in situ studies of high temperature catalytic reactions (>1000 °C), which involve besides surface reactions also gas phase reactions in their mechanism. The goal is to identify gas phase radicals by threshold ionization. A tubular reactor, made from the catalytic material, is positioned in a vacuum chamber. Expansion of the gas through a 100 μm sampling orifice in the reactor wall into differentially pumped nozzle, skimmer, and collimator chambers leads to the formation of a molecular beam. A quadrupole mass spectrometer with electron impact ion source designed for molecular beam inlet and threshold ionization measurements is used as the analyzer. The sampling time from nozzle to detector is estimated to be less than 10 ms. A detection time resolution of up to 20 ms can be reached. The temperature of the reactor is measured by pyrometry. Besides a detailed description of the setup components and the physical background of the method, this article presents measurements showing the performance of the apparatus. After deriving the shape and width of the energy spread of the ionizing electrons from measurements on N2 and He we estimated the detection limit in threshold ionization measurements using binary mixtures of CO in N2 to be in the range of several hundreds of ppm. Mass spectra and threshold ionization measurements recorded during catalytic partial oxidation of methane at 1250 °C on a Pt catalyst are presented. The detection of C H3 radicals is successfully demonstrated. © 2006 American Institute of Physics.en1089-762320065American Institut of PhysicsChemistryJournal Article10.1063/1.2200872Journal Article