PNA-SCR Catalyst Article for Cold Start NOx Reduction
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Solution Overview
Problem
Current SCR systems are inefficient during the cold start period of diesel engines due to low operating temperatures, which hampers the reduction of NOx emissions, necessitating a solution that enhances NOx conversion and selectivity at lower temperatures.
Innovation Solution
A catalyst article with a siliceous support, incorporating a blend of platinum group metals and SCR catalysts, including a passive NOx adsorber, is designed to improve NOx reduction by optimizing the zeolite-based structure and placement of catalysts to enhance NOx conversion and selectivity across varying temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional SCR catalysts are used during cold start period, then the system structure is simple, but NOx conversion efficiency is low due to low operating temperatures
Solution Approach 1:
The catalyst system is divided into multiple functional zones along the exhaust flow path: a first zone containing PNA and SCR catalyst, a second zone with oxidation catalyst, and a third zone with SCR catalyst. Each zone operates at different temperature ranges and performs specific functions, allowing the system to maintain high NOx conversion efficiency across the full temperature spectrum including cold start conditions.
Solution Approach 2:
The patent employs catalysts with different temperature characteristics and chemical compositions in each zone. The PNA in the first zone stores NOx at low temperatures, while the SCR catalysts and oxidation catalyst are formulated to activate at progressively higher temperatures, enabling the system to adapt to varying thermal conditions during cold start and steady-state operation.
2Productivity
If multiple catalyst zones are added to improve cold start performance, then NOx conversion efficiency improves, but device complexity increases
Solution Approach 1:
The patent combines multiple catalytic functions (PNA, SCR, and oxidation) into a single integrated catalyst article with zones arranged along the exhaust flow path. This merging approach achieves high NOx conversion efficiency across all temperature ranges while avoiding the complexity of multiple separate catalyst components and injection systems required by conventional cold start solutions.
Solution Approach 2:
The catalyst article is designed to perform multiple functions simultaneously: the first zone handles both PNA and SCR functions, the second zone provides oxidation capability, and the third zone delivers additional SCR activity. This multi-functional design allows a single device to address both cold start and steady-state emissions control needs without requiring separate systems.
3Productivity
If PNA and SCR catalysts are combined in the first zone, then low temperature NOx storage is improved, but ammonia slip increases
Solution Approach 1:
The catalyst article segments the ammonia management function across different zones: the first zone stores NOx and performs SCR reduction, the second zone oxidizes excess ammonia to prevent slip, and the third zone provides additional SCR capacity. This spatial segmentation allows the system to achieve effective NOx storage while controlling ammonia emissions through distributed functional zones.
Solution Approach 2:
The oxidation catalyst in the second zone acts as an intermediary that converts excess ammonia into nitrogen and water through oxidation reactions. This intermediary function prevents ammonia from passing through the system unreacted, thereby reducing ammonia slip while maintaining the beneficial low-temperature NOx storage capability of the PNA.
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 article effectively reduces NOx emissions during the cold start period by leveraging thermal swings and improved catalyst placement, ensuring compliance with stringent emission regulations while maintaining fuel economy and engine performance.
Implementation Method 1
a passive NOx adsorber (PNA) comprising a platinum group metal and a base metal, both on a molecular sieve
Implementation Method 2
The reduction of NOx to N2 is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR).
Implementation Method 3
an ammonia slip catalyst (ASC) comprising an oxidation catalyst comprising a platinum group metal on a support and a first SCR catalyst
Data Source
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Figure 5~6c
AI summary
A catalyst article including a substrate with an inlet side and an outlet side, a first zone and a second zone, where the first zone comprises a passive NOx adsorber (PNA) comprising a platinum group metal and a base metal, both on a molecular sieve, and an ammonia slip catalyst (ASC) comprising an oxidation catalyst comprising a platinum group metal on a support, and a first SCR catalyst; where the second zone comprises a catalyst selected from the group consisting of a diesel oxidation catalyst (DOC) and a diesel exotherm catalyst (DEC); and where the first zone is located upstream of the second zone. The first zone may include a bottom layer including a blend of: (1) the oxidation catalyst and (2) the first SCR catalyst; and a top layer including a second SCR catalyst, the top layer located over the bottom layer.