Manipulating Photoadsorption Kinetics:  NO on Cl-Treated Fe₂O₃

Abstract

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.

New Approach to Manipulating and Characterizing Powdered Photoadsorbents: NO on Cl-Treated Fe2O3

“…We observed saturation of the TPD peaks at an exposure of about 100 Torr s. The saturation density for NO desorbing below 430 K corresponded to 5 × 10 17 molec/g, or 5 × 10 13 molec/cm 2 . This amount composed approximately 10% of the total NO adsorbed as determined by previously measured adsorption rates . Hence, the saturation density for all NO (reversible plus irreversible) corresponds to 5 × 10 14 molec/cm 2 , or about 0.7 monolayer assuming a van der Waals packing radius of 2 Å …”

“…Indeed, X-ray photoelectron spectroscopy of our surfaces before adsorption shows Fe that is 90% in the +3 state and 10% in the +2 state. After adsorption, all the Fe converts to +3 . Since the saturation density of NO corresponds roughly to the density of surface Fe atoms, it is tempting to infer a correspondence between the 10% Fe 2+ and the 10% NO that adsorbs reversibly.…”

“…Rather than manipulating bulk properties, we influence the adsorption process by treating only the surface. We have shown that chlorinating the surface of iron oxide results in an active and robust photoadsorbent for NO . Our previous work has focused on uptake measurements; here we probe the nature of the photoadsorbed species via temperature-programmed desorption.…”