An Electrochemically Driven and Electrochemically Regenerated NOx Trap

Abstract

Absorption traps are used in a wide variety of different applications to prevent potentially harmful chemicals from entering the environment and/or from coming into contact with sensitive downstream equipment. Many of the materials in use cannot be regenerated so that concerns relating to sustainable usage and environmentally acceptable disposal are important; in other cases, regeneration can only be accomplished with relative difficulty. Herein we describe a new principle that can be used to address both these issues. A key application where chemical traps have found widespread use is in the control of NOx emissions generated by fuel-efficient automotive engines which, by reducing the atmospheric CO2 burden, can significantly mitigate the impact of road transport on global warming. Unfortunately, such engines produce much higher concentrations of nitrogen oxides (NOx) than conventional gasoline engines [1] with attendant adverse impact on human health and the environment. As a consequence, conventional “three-way” catalytic converters are inadequate and a post-converter NOx trap is required. These traps consist of an alkaline-earth component (usually barium) that is able to store NOx species as various nitrates and nitrites under the oxygen-rich conditions typical of “lean” engine operation. Precious metals, usually platinum, are also used as they aid in the uptake of NOx by providing sites for the adsorption and catalytic oxidation of NO to NO2. To restore the trap the engine is momentarily switched to “rich operation”, generating a large concentration of reducing species that cause reduction of the adsorbed nitrate/nitrite species to nitrogen thus regenerating the active BaO/BaCO3 component. A disadvantage of this procedure is that relatively high temperatures ( 900 K) are required for complete regeneration. Although the current generation of NOx traps is very effective when run under these conditions, a number of significant problems remain, especially the need for regular high-temperature excursions, the requirement of periodic rich engine operation (which partially defeats the object of lean operation), and susceptibility to poisoning by sulfur compounds present in the fuel. Herein we report a novel electrochemically driven NOx trap that can operate effectively over a range of temperatures and can be regenerated as required in a controlled manner, without the requirement for temperature excursions or changes in gas-phase composition. Although NOx trapping is used herein to illustrate the technique, the general method employed could be applicable in a wide variety of different applications, including, for example, removal oxides of carbon and sulfur. The trap consists of a thin porous layer of a precious metal (Pt, Pd, or Rh) deposited onto the surface of a solid electrolyte wafer that is biased to deliver the active species to the metal surface where it encounters the adsorbed NOx. We used sodium and potassium ionic conductors although a variety of other solid electrolytes could be used, depending on the application. NOx-trapping and trap-regeneration measurements were performed in a well-mixed microreactor operated at atmospheric pressure and described in detail elsewhere. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a VG ADES 400 spectrometer system and in situ infrared spectroscopy measurements were performed with a Perkin Elmer GX2000 spectrometer utilizing a Harrick Refractor Reactor specular reflection accessory. The performance of the trap is illustrated in Figure 1 which shows the effects of 1) electrochemically trapping NOx