Titanium dioxide photocatalytic oxidation of nitric oxide: Reaction parameters and kinetics.

摘要

Nitric Oxide is the major component of nitrogen oxides (NO_X), which are precursors of tropospheric ozone and particulate matter. NO _X also contribute to acid rain. Environmental regulations thus make NO_X emission control a top priority. Photocatalytic oxidation (PCO) using titanium dioxide (TiO₂) has gained much attention in air and water pollution control due to its ability to oxidize low concentration pollutants and ease of operation compared to other competing technologies (e.g., catalytic oxidation, incineration, carbon adsorption). In PCO, the chemical activation is provided by UV (380 nm) to generate electron-hole (e− - h+) pairs which in turn yield the highly oxidative hydroxyl radical (·OH).; In this study, nitric oxide, the major component of NO_X, was photocatalytically oxidized to nitric acid and nitrogen dioxide (NO₂ ). The latter can be collected as nitric acid in an adsorbent bed. The treatment is a nitrogen fixation process and converts a pollutant to a raw material of fertilizer. The thin-film photoreactor was irradiated with two 8W or 25W black lights with adjustable light intensity. NO and NO₂ were measured with Thermo Environmental Model 10S chemiluminescent NO-NO _X gas analyzer. The photocatalytic oxidation takes hours to reach a steady state and the time frame depends on inlet NO concentration, light intensity, and the catalyst weight. The amounts of NO₂− and NO₃− recovered from the catalyst were analyzed with an Alltech Universal Ion Chromatograph (IC); these amounts also change with time before a steady state is reached. A typical space time is ∼12 seconds.; The NO conversion decreases with temperature from of 74 to 195°F. The conversion increases with space time and decreases with inlet concentration. Two 25W black lights were used with a dimming electronic ballast to vary the light intensity. The light was measured with a radiometer in the range of 320–390 nm. Light intensity increases the conversion of NO and NO ₂ selectivity. Humidity increases the oxidation rate, which is consistent with the ·OH oxidation mechanism. The kinetic data at the steady state fit a Langmuir-Hinshelwood equation very well, The process involves a series of oxidation steps by the ·OH radical: NO → HNO₂ → NO₂ → HNO₃. The reaction essentially reaches equilibrium at a long space time (e.g., 12 seconds).; The photocatalytic oxidation mechanism was proposed. ·OH radical was a key oxidant in this reaction. HNO₂ was an intermediate species which would further oxidized to NO₂ and H₂O, HNO ₃, the major product deposited on the catalyst, was produced from the reaction between NO₂ and ·OH radical.