Aged DOC is a Net Consumer of NO2: Analyses of Vehicle, Engine-dynamometer and Reactor Data

A typical diesel After-Treatment (AT) system to meet 2010 North American regulations consists of a Diesel Oxidation Catalyst (DOC) for oxidizing Hydrocarbons (HCs) and Carbon Monoxide (CO), followed by a Selective Catalytic Reduction (SCR) catalyst for NOx reduction, and a diesel particulate filter. DOCs are also used to oxidize NO in the exhaust to NO₂ that substantially improves low-temperature SCR NOx reduction performance. Vehicle and engine dynamometer data under typical operating conditions show (1) engine-out NO₂/NOx can be as high as 40%, (2) DOC can reduce NO₂ to NO in the presence of reductants, and (3) an aged DOC with a low CO/HC conversion can be a net consumer of NO₂ over a wide range of temperatures and space velocities. Flow reactor experiments simulating diesel exhaust conditions show that DOC-out NO₂ is independent of the inlet NO₂/NOx in the feed. NO₂ in the feed appears to be reduced to NO prior to the oxidation of NO to NO₂ which takes place when most of the reductants have been consumed. Flow reactor data also show that (1) NO₂ passes through the DOC unreacted in the absence of reductants, (2) NO₂ reduction may happen at the leading edge of the DOC, and hence may not be as space velocity (catalyst length) dependent as the subsequent NO oxidation, and, (3) both CO and HC individually are capable of reducing NO₂; however, HCs start reducing NO₂ at a higher temperature. Thus, AT strategies (like an upstream HC trap) that limit only one of the reductant entering the DOC may not be able to minimize NO₂ loss across the DOC, and engine operating strategies that generate NO₂ to aid SCR NOx reduction may not be useful as long as the exhaust passes through a DOC upstream of an SCR in the presence of reductants.