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

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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.