Lean-Condition Exhaust Gas Catalysts for Passive NOx Reduction
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Solution Overview
Problem
Existing three-way conversion (TWC) catalysts for gasoline engines are ineffective in reducing NOx emissions under lean operating conditions due to excessive oxygen in the exhaust, and urea SCR systems introduce complexity and maintenance challenges.
Innovation Solution
A catalyst comprising a molecular sieve promoted with copper or iron, and a second material with oxides of Ni, Fe, or Co on a support, effectively reduces NOx, hydrocarbons, and carbon monoxide, with a configuration that maintains stability under lean/rich aging conditions, particularly at high temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional TWC catalysts are used, then they are effective at stoichiometric conditions, but they are ineffective under lean operating conditions due to excessive oxygen
Solution Approach 1:
The catalyst is divided into two distinct functional layers: a first layer containing copper-exchanged molecular sieve for NOx storage and SCR reduction, and a second layer containing platinum group metal for TWC function. This segmentation allows each layer to specialize in specific conditions, with the first layer handling lean conditions and the second layer handling stoichiometric/rich conditions, thereby resolving the contradiction between adaptability and reliability across different operating modes.
Solution Approach 2:
The invention changes the chemical composition and structure of the catalyst by introducing a copper-exchanged molecular sieve in the first layer, which has different catalytic properties compared to conventional TWC catalysts. This parameter change enables the catalyst to maintain effectiveness under lean conditions by providing a different reaction pathway for NOx reduction that is not inhibited by excess oxygen.
2Reliability
If urea SCR systems are used to reduce NOx, then NOx reduction is achieved, but system complexity and maintenance requirements increase
Solution Approach 1:
The catalyst system uses ammonia generated in-situ from the exhaust gas itself (through passive ammonia generation from fuel-rich conditions) rather than requiring external urea injection. The copper-exchanged molecular sieve automatically performs NOx storage and reduction using this self-generated ammonia, eliminating the need for urea tanks, injection systems, and associated control systems, thereby maintaining high NOx reduction performance while significantly reducing system complexity.
Solution Approach 2:
The first layer of the catalyst performs multiple functions: it stores NOx, generates ammonia passively under fuel-rich conditions, and reduces NOx under lean conditions. This multi-functionality replaces the need for separate urea storage, injection, and SCR catalyst systems, achieving reliable NOx reduction while minimizing device complexity.
3Reliability
If high copper loading is used in molecular sieve for SCR activity, then NOx conversion improves, but thermal stability and resistance to lean/rich aging deteriorate
Solution Approach 1:
The invention optimizes the copper loading parameter to a specific range (0.1% to 2% by weight on oxide basis) that balances SCR activity with thermal stability. This parameter change ensures sufficient NOx conversion efficiency while preventing excessive copper aggregation and maintaining structural integrity under lean/rich aging conditions. The molecular sieve framework and copper distribution are tuned to achieve this optimal balance.
Solution Approach 2:
The catalyst uses a composite structure combining copper-exchanged molecular sieve with a specific framework type (maximum ring size of eight tetrahedral atoms and double six-ring unit) and controlled copper loading. This composite material design provides both the necessary SCR activity for NOx conversion and the thermal stability required to resist lean/rich aging, resolving the contradiction between activity and stability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The catalyst achieves significant NOx conversion (at least 30%) under stoichiometric engine conditions, even at temperatures above 850°C, while minimizing ammonia leakage and system complexity.
Implementation Method 1
the first material is effective to catalyze selective catalytic reduction of nitrogen oxides in the presence of ammonia
Implementation Method 2
the second material is effective to abate hydrocarbons and carbon monoxide; the second material comprises at least one oxide of a metal selected from Ni, Fe, Co, and Cu
Implementation Method 3
in the presence of an excess of residual reductants and a deficiency of air, a fraction of NOx is over-reduced on the close-coupled TWC catalyst to generate ammonia passively
Data Source
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AI summary
Described are catalysts effective to abate NOx, hydrocarbons, and carbon monoxide from a gasoline engine exhaust gas. Such catalysts include a substrate having a first and second material disposed thereon, the first material effective to catalyze selective catalytic reduction of nitrogen oxides in the presence of ammonia and the second material effective to abate hydrocarbons and carbon monoxide, the first material comprising a molecular sieve promoted with copper and/or iron in a low loading, the second material comprising at least one oxide of Ni, Fe, Mn, Co, and Cu on a support selected from oxides of Ce, Ce-Zr, Zr, Mn, Pr and combinations thereof. Also described are gasoline engine exhaust gas treatment systems and methods of treating exhaust gas from a gasoline engine.