New approach to manipulating and characterizing powdered photoadsorbents: NO on Cl-treated Fe2O3

Photon-enhanced catalysis and adsorption offer significant advantages over thermal processes in many environmental protection and cleanup applications . However, up to now relatively little work has focused on improving materi al performance by chemical surface modification. Here we show that powdered Fe2O3 treated with aqueous NH4Cl exhibits appreciable activity for the pho toadsorption of NO, whereas the pure iron oxide does not. Through the use o f a specially designed apparatus for performing temperature-programmed deso rption with illuminated powders, we demonstrate the existence of a distribu tion of binding states, only some of which exhibit reversibility. Analytica l methods developed recently in this laboratory permit extraction of the st icking probability, desorption order, preexponential factor, and energy dis tribution for the reversible states. The results suggest nondissociative ch emisorption with an average binding energy near 25 kcal/mol. We rationalize the unusually low prefactor for desorption (2 x 10(11) s(-1)) in terms of a new physical picture invoking adsorbate ionization on a semiconductor sur face.