Pulsed EPR Study of Orthophosphoric and Boric Acid Modified γ-Alumina

CW EPR, pulsed ENDOR, and one- and two-dimensional ESEEM techniques have been applied to study the influence of H 3 PO 4 and H 3 BO 3 acid modification of γ-Al 2 O 3 on the strong electron acceptor sites of γ-Al 2 O 3 using adsorbed anthraquinone and o -chloranil as probe molecules. After the adsorbtion, electron transfer produces free radicals at active sites on the activated γ-Al 2 O 3 . The observed spectra are the superposition of spectra from radicals simply adsorbed on the surface and from three different paramagnetic complexes of anthraquinone with coordinatively unsaturated aluminum. The isotropic hyperfine coupling of aluminum nuclei in these complexes is ∼2.5, 21, and 28 MHz. Treatment of γ-Al 2 O 3 with H 3 PO 4 decreases the relative amounts of adsorbed radical and the complex with 2.5 MHz hyperfine coupling. The H 3 BO 3 treatment, in contrast, gives only the complex with the 21 MHz hyperfine coupling, and its resolved EPR spectrum indicates two equivalent aluminums in this active site. The ESEEM spectra show only weak dipole−dipole interaction with 31 P and 11 B nuclei located around the paramagnetic complex. These results suggest that acid modification alters the catalytic properties by changing the number and type of catalytic sites on the surface without introducing new species.