Ammonia Formation over Pd/Rh Three-Way Catalysts during Lean-to-Rich Fluctuations: The Effect of the Catalyst Aging, Exhaust Temperature, Lambda, and Duration in Rich Conditions

Abstract

The formation of ammonia (NH 3 ) as a byproduct during the operation of a three-way catalyst (TWC) in a simulated exhaust stream was investigated using a commercially available Pd/Rh TWC under steady-state and lean/rich cycling conditions. Ion molecular reaction-mass spectrometry was applied to determine NO, NO 2 , and NH 3 concentrations at a time resolution of 0.6 s. Catalyst aging was shown to result in a significant increase in the amount of NH 3 formed, which has received limited attention in the literature to date. The selectivity toward NH 3 formation has been shown to increase with the decrease in the oxygen storage capacity (OSC) of a TWC induced by thermal aging. NH 3 has been shown to mainly form within the exhaust temperature range of 250−550 °C. Typical lambda and rich operational condition duration periods found in vehicle test procedures were also employed to investigate their effects on NH 3 formation. The results suggest that a decrease in the lambda and/or an increase in the duration of rich operating conditions will lead to an increase in the selectivity toward NH 3 formation. Improving the OSC of TWCs and effectively controlling the lambda near to 1.0 with limited duration in rich operating conditions are therefore significant factors in the reduction of NH 3 emissions.

Effect of Extreme Temperatures and Driving Conditions on Gaseous Pollutants of a Euro 6d-Temp Gasoline Vehicle

“…The cold start contribution was around 50% for NH 3 . NH 3 has been shown to primarily form within the exhaust temperature range of 250-550 • C [57]. NH 3 formation over the catalyst is enhanced at low air/fuel (lambda) ratios (rich operating conditions).…”

Source Apportionment of Urban Ammonia and its Contribution to Secondary Particle Formation in a Mid-size European City

“…The NH3 emission factor (EF) of vehicles in the U.S. increased from 1.3 ± 3.5 mg km -1 in 1981 (Pierson and Brachaczek 1983) to 60 mg km -1 in 1993 (Fraser and Cass 1998;Perrino et al 2002). Ammonia is emitted as a byproduct during the operation of TWCs (Wang et al 2019). A decreasing oxygen storage capacity of the catalyst caused by thermal aging in the TWC leads to overly reducing conditions in the catalyst and thereby increases the amount of produced NH3 in the exhaust gas stream (Bielaczyc et al 2012).…”

“…5 to convert NOx in the exhaust stream to N2 and water at temperatures between 150 and °C (formula (1)) (Wang et al 2019;Boyce 2012).…”

Can NOx reduction by CO react over carbon‐based single‐atom catalysts at low temperatures? A theoretical study

“…Therefore, using CO instead of NH 3 to reduce NO x has been considered as an attractive amino‐free strategy to remove both harmful NO x and another important pollutant CO. In addition, it has been proven that NO reduction by CO is the primary pathway for N 2 formation over three‐way catalysts (TWCs) used in catalytic convertors on gasoline burning vehicles, 8,9 which provides scientific rationality and value rationality for reducing NO x by CO in the industrial exhaust gas. Another advantage of using CO as a reducing agent is the avoidance of carbon deposition and corrosion of equipment 10–12 .…”