Multi-Layer TWC Formulation with Pd-Rh Nesting for NOx
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Solution Overview
Problem
Rhodium (Rh) sintering and migration in three-way catalysts (TWCs) lead to activity deterioration due to high temperatures, and blending palladium (Pd) with Rh can cause detrimental NOx reduction behavior, limiting TWC performance.
Innovation Solution
A catalyst configuration with specific weight ratios of Pd and Rh in adjacent layers, forming a 'Pd nest' to trap migrated Rh, preventing alloy formation and enhancing TWC activity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Rh is used as the main catalyst for NOx reduction, then NOx conversion efficiency is improved, but Rh sintering and migration occur at high temperatures leading to activity deterioration
Solution Approach 1:
The patent implements a nested structure where a Pd-containing washcoat layer is positioned between the Rh-containing washcoat layer and the substrate. This nested arrangement allows the Pd layer to trap migrated Rh atoms, preventing their loss while maintaining NOx reduction activity. The structure effectively creates a protective environment for Rh without directly alloying it.
Solution Approach 2:
The Pd-containing washcoat layer acts as an intermediary between the Rh catalyst and the substrate. It mediates the interaction by providing a trapping mechanism for migrated Rh, thereby protecting the Rh from complete deactivation while allowing the system to maintain high NOx conversion efficiency.
2Stability of the object's composition
If Pd is blended with Rh in the same washcoat layer, then Rh stability is improved through alloy formation, but detrimental over-oxidation of reductants occurs limiting TWC performance
Solution Approach 1:
The patent segments the catalyst into distinct functional layers: a Rh-containing washcoat layer for NOx reduction, a Pd-containing washcoat layer for trapping migrated Rh, and support layers. This segmentation prevents direct Pd-Rh alloying in the same washcoat, avoiding over-oxidation while maintaining Rh stability through the nested trapping mechanism.
Solution Approach 2:
The patent applies local quality by creating a specific Pd-rich zone (the nested washcoat layer) adjacent to but separate from the Rh-containing layer. This localized Pd distribution provides trapping functionality without causing the harmful over-oxidation effects that occur when Pd and Rh are uniformly blended throughout the same washcoat.
3Stability of the object's composition
If high Pd loading is used to trap migrated Rh, then Rh stability is improved, but PdRh alloy formation with Pd core-Rh shell structure occurs reducing TWC activity
Solution Approach 1:
The patent carefully controls the Pd loading parameter in the nested washcoat layer, optimizing it to provide sufficient Rh trapping capacity while preventing excessive Pd accumulation that would lead to Pd core-Rh shell alloy formation. This parameter optimization maintains both Rh stability and TWC activity.
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
Improves NOx, CO, and HC conversion rates, particularly in cold start conditions, by stabilizing Rh and optimizing Pd-Rh interactions.
Implementation Method 1
utilizes Rh migration phenomena by building a 'Pd nest' layer/region to trap the migrated Rh to form adjacent Pd-Rh nanoparticles
Implementation Method 2
the oxidation of CO and HCs to CO2 and steam (H2O) is mainly catalyzed by Pd
Implementation Method 3
the reduction of NOx to N2 is mainly catalyzed by Rh
Data Source
Figure 1a~1d
Figure 2a~2d
Figure 2e~2h
AI summary
A catalyst article for treating exhaust gas comprising: a substrate comprising an inlet end, an outlet end with an axial length L; a first catalytic region comprising a first palladium component; a second catalytic region comprising a second rhodium component; a third catalytic region comprising a third palladium component; wherein the first catalytic region is adjacent to the second catalytic region; and wherein the first palladium component and the second rhodium component have a weight ratio of from 3:1 to 19:1, based on element.