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.
While chemical surface modification is commonly employed to improve the performance of solid catalysts,
this approach has been largely neglected in optimizing photoadsorbents. Here we show that powdered Fe2O3
treated with aqueous NH4Cl and subsequently calcined near 300 °C exhibits appreciable activity for the
photoadsorption of NO, whereas the pure iron oxide does not. We employ a uniquely configured reactor to
develop well-characterized photoadsorption kinetics without shadowing or diffusion effects. Kinetics obey
simple Langmuir-type expressions for nondissociative adsorption. However, the adsorption process requires
the simultaneous presence of adsorbed chlorine and H2O, and X-ray photoelectron spectroscopy shows that
both the +2 and +3 oxidation states of iron play a role. This complexity mirrors corresponding complexity
in the bonding of NO to Fe cations in the analogous aqueous-phase coordination chemistry. Interestingly, the
photoadsorbent is not poisoned by exposure to SO2 or CO2.
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