Theory and Simulation of Solid‐Electrolyte Wide‐Range Sensors for Combustion‐Gas Streams

Abstract

Clarification of the complex chemical processes governing polarized wide-range air-to-fuel-ratio sensors should enable one to determine characteristics of the system geometry and physical properties necessary for successful sensor operation. To develop a mathematical description of these sensors, we have combined well-established theories for electron, electron-hole, and oxygen-vacancy transport within yttria-stabilized zirconia with transport equations that apply to gaseous diffusion within the dense, porous layer that covers the solid electrolyte. Along the interface between the solid electrolyte and the porous layer, a thin platinum electrode catalyzes various chemical reactions and an electrochemical charge-transfer reaction. The model equations have been simplified using an asymptotic analysis which is valid over a broad range of conditions of practical interest; in particular, it is valid for all simulations presented in this work. Model calculations are shown to compare well with experimental data, and acceptable ranges for the four dimensionless groups that dictate sensor performance are considered.lnfroduction The need to improve fuel economy and reduce unwanted emissions associated with internal combustion engines has promoted research into sensors that can determine airto-fuel ratios over a wide range of operating conditions, which can lead to an improved closed-loop control system.' It is possible that a wide-range sensor (WRS) will eventually replace the present potentiometric exhaustoxygen sensor (EOS),2-4 which provides only a binary response that indicates whether the engine is operating lean or rich.58 Before a WRS is to become used widely, it is likely that an economical catalyst system capable of reducing nitrogen oxides in a lean exhaust stream will be required, so that advantages such as fuel savings and reductions in hydrocarbon emissions obtained by operating an engine in the lean condition can be realized without excessive nitrogen oxide emissions.9 Conversely, when high power is demanded from an engine, a feed stream rich in fuel is often required; under such conditions, a WRS could determine quantitatively the degree to which the exhaust is rich in combustibles, and the resulting airfuel control system could reduce hydrocarbon emissions.A schematic illustration of a WRS based on an oxygentransporting solid electrolyte is shown in Fig. 1. The importance of these sensors has led to a large number of publications and patents over the past 20 years. The patent by Bhagat and Howarth'2 and those by Wang et al.13 provide a substantial listing of early United States patents issued on this topic area. References 14-27 describe sensors constructed with materials and geometries close to or analogous to that which is addressed in this communication, and references cited in these documents can be consulted for related studies. In addition, a number of relatively recent review articles discuss the construction and operation of zirconia-based wide-range sensors,2831 and the reviews in Ref. 32-35 presen...

Kinetics and Mechanism of the Heterogeneous Decomposition of Nitric Oxide on Metal Oxides in the Presence of Hydrocarbons

“…In this context it is interesting to point out that Lunsford et al recently suggested that the release of oxygen at moderate temperatures may be a common feature of all active oxide catalysts for this process 9b. Since the formation of surface O-vacancies can be promoted by O 2- migration in electrically conducting oxides, − our results suggest that the role of hydrocarbons in SCR over oxides is neither specific or essential. As a chemical sink for oxygen, hydrocarbon oxidation can be conceivable segregated from nitric oxide decomposition.…”