Electron spin-echo studies of radical interaction and orientation on catalytic surfaces. CH2OH radicals in A-, Y-, and X-type zeolites

CH2OH radicals have been generated by γ-radiolysis at 77 K from methanol adsorbed on Na-A, Na-Y, Na-X, and K-X type zeolites. The electron spin resonance spectra of the radicals on the different zeolites are similar. However, electron spin-echo modulation patterns associated with aluminum nuclei in the zeolites are observed at 4 K and show differences between the different zeolites. These spin-echo results have been simulated in order to obtain structural information about the adsorbed radical site. Each radical interacts with one Si or Al nucleus at 3.4 A in Na+-containing zeolites and at 3.6 A in K+-containing zeolites. From these data and from structural data on the zeolites the CH2OH radical is suggested to be located in an α cage with its molecular dipole oriented toward a cation in the center of the square (X and Y zeolite) or hexagonal (A zeolite) faces of the α cage. The COH plane is aligned along a diagonal of the square face so that the radical site is oriented toward one Al or Si nucleus. The utility of electron spin-echo modulation analysis to probe the orientation of adsorbed species relative to the surface is emphasized