Probing gas phase catalysis by atomic metal cations with flow tube mass spectrometry

Abstract

The evolution and applications of flow tube mass spectrometry in the study of catalysis promoted by atomic metal ions are tracked from the pioneering days in Boulder, Colorado, to the construction and application of the ICP/SIFT/QqQ and ESI/qQ/SIFT/QqQ instruments at York University and the VISTA-SIFT instrument at the Air Force Research Laboratory. The physical separation of various sources of atomic metal ions from the flow tube in the latter instruments facilitates the spatial resolution of redox reactions and allows the separate measurement of the kinetics of both legs of a two-step catalytic cycle, while also allowing a view of the catalytic cycle in progress downstream in the reaction region of the flow tube. We focus on measurements on O-atom transfer and bond activation catalysis as first identified in Boulder and emphasize fundamental aspects such as the thermodynamic window of opportunity for catalysis, catalytic efficiency, and computed energy landscapes for atomic metal cation catalysis. Gas-phase applications include: the catalytic oxidation of CO to CO₂, of H₂ to H₂O, and of C₂H₄ to CH₃CHO all with N₂O as the source of oxygen; the catalytic oxidation of CH₄ to CH₃OH with O₃; the catalytic oxidation of C₆H₆ with O₂. We also address the environmentally important catalytic reduction of NO₂ and NO to N₂ with CO and H₂ by catalytic coupling of two-step catalytic cycles in a multistep cycle. Overall, the power of atomic metal cations in catalysis, and the use of flow tube mass spectrometry in revealing this power, is clearly demonstrated.