Composite SCR Catalyst for Transient NOx Control
Solution Overview
Problem
Current SCR catalysts for mobile source applications face challenges in managing transient NOx emissions due to varying driving conditions, including inadequate NH3 storage capacity and sensitivity to hydrocarbons, leading to sub-optimal NOx conversion and ammonia slip.
Innovation Solution
A heterogeneous catalyst comprising a combination of a large pore BEA framework molecular sieve with 0.5-5% iron and a small pore CHA framework molecular sieve with 0.5-5% copper, coated on a monolith substrate, which enhances NOx conversion and transient response by optimizing NH3 storage and utilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a single molecular sieve catalyst is used, then the catalyst structure is simple, but the NOx conversion activity is insufficient under transient conditions
Solution Approach 1:
The patent employs a composite catalyst system combining two distinct molecular sieves: Cu-SSZ-13 (small pore CHA framework) for high NH3 storage capacity and Fe-BEA (large pore BEA framework) for high catalytic activity. This composite material approach allows the catalyst to maintain high NOx conversion activity across varying operating conditions while managing ammonia slip effectively, resolving the contradiction between activity and structural simplicity.
2Reliability
If high NH3 storage capacity is increased, then ammonia slip is reduced, but the catalyst may slip NH3 to atmosphere under low load conditions
Solution Approach 1:
The patent applies local quality by assigning different functional roles to different catalyst components: Cu-SSZ-13 provides high NH3 storage capacity for transient high-load conditions, while Fe-BEA provides high catalytic activity for efficient NOx conversion. This functional differentiation allows the catalyst system to adapt to varying load conditions without excessive ammonia slip or emission, resolving the contradiction between reliability and harmful emissions.
Solution Approach 2:
The patent utilizes parameter changes by selecting molecular sieves with different pore sizes and chemical compositions (Cu-SSZ-13 vs. Fe-BEA) to achieve different functional characteristics. The small pore CHA framework of Cu-SSZ-13 provides high NH3 storage capacity, while the large pore BEA framework of Fe-BEA provides high catalytic activity. This parameter variation allows the catalyst to optimize performance across different operating conditions, balancing ammonia slip control and emission prevention.
3Temperature
If copper-based SCR catalyst is used for low temperature operation, then low temperature activity is improved, but tolerance to hydrocarbons is reduced
Solution Approach 1:
The patent uses a composite catalyst system where Cu-SSZ-13 provides low temperature SCR activity and Fe-BEA provides hydrocarbon tolerance. The Fe-BEA component is particularly effective at treating NOx under high temperature conditions and is more tolerant to hydrocarbons. This composite approach allows the catalyst to maintain low temperature activity while being tolerant to hydrocarbon poisoning, resolving the contradiction between temperature performance and hydrocarbon sensitivity.
4Temperature
If iron-based SCR catalyst is used for high temperature and hydrocarbon tolerance, then high temperature activity is improved, but low temperature activity is reduced
Solution Approach 1:
The patent employs a composite catalyst where Fe-BEA provides high temperature activity and hydrocarbon tolerance, while Cu-SSZ-13 provides low temperature SCR activity. The Cu-SSZ-13 component is particularly effective at low temperatures due to its high NH3 storage capacity and copper-based active sites. This composite approach allows the catalyst to maintain high activity across a wide temperature range, resolving the contradiction between high temperature and low temperature performance.
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 higher cumulative NOx conversion with lower ammonia slip compared to individual components, improving activity at lower NH3 exposures and maintaining excellent transient response and selectivity, especially in varying NO:NO2 ratios.
Implementation Method 1
SCR catalysts can adsorb (or store) nitrogenous reductant, thus providing a buffer to the appropriate supply of available reductant
Implementation Method 2
selective catalytic reduction catalyst comprising metal-promoted molecular sieves for converting oxides of nitrogen (NOx) present in exhaust gas
Implementation Method 3
As used herein, the term 'selective catalytic reduction' (SCR) defines the catalytic process of reducing oxides of nitrogen to dinitrogen (N2) using a nitrogenous reductant
Data Source
Figure 1~2
Figure 3
Figure 4a~4d
AI summary
A heterogeneous catalyst article having at least one combination of a first molecular sieve having a medium pore, large pore, or meso-pore crystal structure and optionally containing a first metal, and a second molecular sieve having a small pore crystal structure and optionally containing a second metal, and a monolith substrate onto or within which said catalytic component is incorporated, wherein the combination of the first and second molecular sieves is a blend, a plurality of layers, and/or a plurality of zones.