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

Abstract

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.

Manipulating Photoadsorption Kinetics: NO on Cl-Treated Fe2O3

“…After reaching this minimum, the NO pressure then began to increase slowly. We attribute this increase to progressive site filling, consistent with other results we have obtained using temperature programmed desorption (TPD) . Interestingly, small amounts of H 2 O desorbed during NO adsorption.…”

“…Figures and indicate standard Langmuir-type adsorption kinetics for NO that are first order in empty site density. These results accord well with those we have obtained independently by temperature programmed desorption (TPD) in this system …”

“…The slope gives m = 0.8 with an uncertainty due to a random error of 0.1. Given the systematic uncertainty entailed by employing the fast-pumping approximation, and independent TPD data showing m = 1.0 ± 0.1 for states desorbing below 157 °C, we believe our present data are most consistent with a value of m = 1.

6 Dependence of the sticking probability S(θ) on unoccupied site density (1−θ).

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“…Interestingly, we found that increasing the water pressure above 300 mTorr resulted in a slight decrease in adsorption rate. We observed a similar effect in TPD . As with photoreaction, we suspect that excessive water simply blocks active sites for NO adsorption.…”

“…For example, oxygen photoadsorption rates on TiO 2 depend on the heating history. − We employ a much more forceful approach for rate enhancement by introducing new chemical elements to the photoadsorbent surface. The present work focuses on uptake measurements; related work probes the nature of the photoadsorbed species via temperature programmed desorption . To reproduce the form of the adsorbent as it might be used in actual applications, we examine the material in powder form.…”