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 material performance by chemical surface modification. Here we show that powdered Fe₂O₃ treated with aqueous NH₄Cl exhibits appreciable activity for the photoadsorption of NO, whereas the pure iron oxide does not. Through the use of a specially designed apparatus for performing temperature-programmed desorption with illuminated powders, we demonstrate the existence of a distribution of binding states, only some of which exhibit reversibility. Analytical methods developed recently in this laboratory permit extraction of the sticking probability, desorption order, preexponential factor, and energy distribution for the reversible states. The results suggest nondissociative chemisorption with an average binding energy near 25 kcal/mol. We rationalize the unusually low prefactor for desorption (2 × 10¹¹ s^-1) in terms of a new physical picture invoking adsorbate ionization on a semiconductor surface.