PGM-Embedded Small Pore Molecular Sieve Catalyst for NOx Reduction
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Solution Overview
Problem
Conventional Platinum Group Metal (PGM)-based catalysts for NOx reduction in diesel exhaust gases face challenges such as low N2 selectivity at low temperatures and ammonia slip at high temperatures, due to poor incorporation of PGMs into molecular sieve structures, limiting their practical application.
Innovation Solution
Incorporating PGMs into the porous network of small pore molecular sieves using techniques like in situ synthesis and solid state ion exchange, ensuring at least 75% of the PGM is embedded within the molecular sieve, enhancing N2 selectivity and ammonia oxidation capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If PGM-based catalysts are used for NOx reduction at low temperatures, then NOx conversion activity is improved, but N2 selectivity deteriorates (less than 50% selectivity with significant N2O formation)
Solution Approach 1:
The patent utilizes small pore molecular sieves (zeolites with 8-ring pores such as chabazite, Rhombohedral, and Erionite structures) as the catalyst support. The small pore size provides shape selectivity that confines the reaction within the pores, enabling high N2 selectivity (>50%) while maintaining NOx conversion activity at low temperatures (150-250°C). This resolves the contradiction by using the porous structure to simultaneously achieve both conversion and selectivity.
Solution Approach 2:
The patent creates composite catalysts by incorporating Platinum Group Metals (PGMs such as Pt, Pd, Rh, Ru) into the small pore molecular sieve structure through incipient wetness impregnation. The composite structure combines the high catalytic activity of PGMs with the shape-selective properties of small pore zeolites, achieving both high NOx conversion and high N2 selectivity at low temperatures, thereby resolving the contradiction between activity and selectivity.
2Productivity
If PGM-based catalysts are used for NOx reduction at high temperatures, then reaction rate is improved, but ammonia slip deteriorates (oxidation of NH3 to NOx occurs)
Solution Approach 1:
The small pore molecular sieve structure provides shape selectivity that restricts the oxidation of ammonia to NOx at high temperatures while still allowing the reduction reactions to proceed. The pore geometry favors the formation of N2 over oxidized products, thereby maintaining high reaction rates while reducing ammonia slip and harmful byproduct formation.
Solution Approach 2:
The patent changes the operational temperature range and ammonia to NOx ratio parameters to optimize catalyst performance. By operating within specific temperature ranges and controlling the ammonia dosage, the catalyst achieves high conversion rates while minimizing ammonia slip and unwanted oxidation reactions, resolving the contradiction between reaction rate and ammonia slip.
3Ease of manufacture
If conventional incipient wetness methods are used to incorporate PGM into molecular sieves, then ease of manufacture is improved, but PGM exchange efficiency deteriorates (PGM deposits on surface rather than incorporating into walls)
Solution Approach 1:
The patent uses incipient wetness impregnation with a slight excess of PGM precursor solution to ensure thorough saturation of the molecular sieve pores. This partial/excessive impregnation approach, combined with controlled drying and calcination, enables PGM to penetrate and incorporate into the molecular sieve walls rather than just depositing on the external surface, achieving high PGM exchange efficiency while maintaining manufacturing simplicity.
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 PGM-embedded catalyst achieves high N2 selectivity (>50%) at low temperatures and effective ammonia oxidation at high temperatures, providing a dual functionality that is rare in existing catalysts, improving NOx reduction and reducing ammonia slip across a broad operational temperature range.
Implementation Method 1
The reduction of NOx to N2 in a lean burn exhaust gas, such as that created by diesel engines, is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. NOx can be reduced in a diesel exhaust gas, however, by a heterogenic catalysis process commonly known as Selective Catalytic Reduction (SCR).
Implementation Method 2
at high temperatures, e.g. greater than about 350°C, the low selectivity correlates to the oxidation of NH3 (the desired reductant) to NOx
Implementation Method 3
The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate.
Data Source
Figure 1~2
Figure 3
AI summary
Provided are catalysts comprising a small pore molecular sieve embedded with platinum group metal (PGM) and methods for treating lean burn exhaust gas using the same.