In Situ EPR Study of Chemical Reactions in Q-Band at Higher Temperatures: A Challenge for Elucidating Structure−Reactivity Relationships in Catalysis

Abstract

For the first time, heterogeneous catalytic reactions have been monitored by in situ EPR spectroscopy in the Q-band using a homemade heatable probe head equipped with a flow reactor. The reactions of Al(2)O(3)-supported TEMPO with NO and H(2) as well as of SiO(2)/Al(2)O(3)-supported H(4)PVMo(11)O(40) with methanol and formaldehyde were studied up to 400 degrees C. TEMPO radicals are immobilized on the support in positions which impose a different reactivity to NO and H(2). This may be due to different accessibility, which changes during thermal treatment. By combined evaluation of anisotropic X- and Q-band spectra with a complex hyperfine structure (e.g., from VO(2+)), spin Hamiltonian parameters can be derived with higher precision, since limits of the specific resolution in both frequency bands are compensated for. In addition to VO(2+), Mo(5+) is formed above 180 degrees C depending on the O(2) content of the feed, which is easily discriminated in the Q-band but not in the X-band.

Defects of thin CaO(001) on Mo(001): an EPR spectroscopic perspective

“…Among the different paramagnetic states of Mo-ions, EPR signals of Mo( v ) centers were reported for several Mo-doped oxides characterized by g -values below g = 2. 55–57 In particular, such centers were found after annealing of MgO(001)-films on Mo(001) by EPR spectroscopy exhibiting g -values around 1.927. 42 While these results do not allow for an assignment of signals with g > 2, they allow assigning the signal at g = 1.974 observed after annealing of the film prepared under low Ca flux conditions.…”

Unraveling Structure and Dynamics in Porous Frameworks via Advanced In Situ Characterization Techniques

“…So far the vast majority of in situ EPR cells for investigations of catalytic reactions over metal oxide surfaces in a continuous gas flow68c,75 and of adsorption processes in MOFs under a static pressure72a,c,d were constructed for X‐band frequencies with one notable exemption working at Q‐band frequencies (34 GHz) . Today state of the art X‐band EPR spectrometers provide a realistic sensitivity limit of about 10 13 spins cm −3 even in case of TMI species in polycrystalline samples which seems to be fully sufficient for most EPR applications in MOF research.…”

Electron Spin Resonance. Part two: A Diagnostic Method in the Environmental Sciences

“…The TEMPO radicals were immobilised on the support in positions which impose a different reactivity to NO and H 2 , as may be due to different accessibility, which changes during thermal treatment. Mo 5 þ is formed above 180 C, In addition to VO 2 þ , depending on the O 2 content of the feed, which is easily resolved Q-band but not at X-band 116 . Highly photoactive, tetrahedral Ti 4 þ sites can be created in mixed-phase TiO 2 nanocomposites, and which are shown to be an intermediate formed during the thermally driven phase-transformation from anatase to rutile 117 .…”