Selective Reduction Catalyst for Lean-Burn Engine NOx Purification
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Solution Overview
Problem
Conventional catalysts and control methods fail to effectively purify nitrogen oxides from lean-burn engine exhaust gases, especially under high space velocity conditions, leading to inefficiencies and potential secondary pollution from ammonia slip.
Innovation Solution
A selective reduction-type catalyst comprising specific zeolite and composite oxides such as silica, tungsten oxide, ceria, and zirconia, along with titania, is used to purify nitrogen oxides using urea or ammonia, maintaining efficiency across a wide temperature range and high space velocities while reducing pressure loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional catalysts are used for nitrogen oxide purification in lean-burn engines, then the catalyst structure is simple, but the purification efficiency is insufficient especially under high space velocity conditions
Solution Approach 1:
The patent employs a composite catalyst system comprising zeolite (A) with iron element, composite oxide (B) with specific composition ratios of silica, tungsten oxide, ceria, and zirconia, and optionally composite oxide (C) for urea hydrolysis. This composite structure integrates multiple functional components that work synergistically to achieve high purification efficiency across wide temperature ranges and high space velocity conditions while maintaining structural stability.
Solution Approach 2:
The catalyst design assigns specific functions to different components: zeolite (A) provides nitrogen oxide adsorption and reaction sites, composite oxide (B) enhances thermal stability and catalytic activity, and composite oxide (C) promotes urea decomposition. Each component is optimized with specific composition ratios and particle size distributions to perform its designated function efficiently within the overall catalyst system.
2Productivity
If ammonia or urea is supplied for selective reduction, then nitrogen oxide purification is achieved, but ammonia slip occurs causing secondary pollution
Solution Approach 1:
The catalyst is designed to optimize the reaction parameters including temperature range (150-580°C), space velocity (up to 60,000 hr⁻¹), and ammonia-to-nitrogen oxide molar ratio. By controlling these parameters through the specific catalyst composition and structure, the system achieves complete ammonia consumption for nitrogen oxide reduction, preventing ammonia slip while maintaining high purification efficiency.
Solution Approach 2:
The catalyst system incorporates feedback mechanisms where the reaction conditions are continuously optimized based on the interaction between ammonia, nitrogen oxides, and oxygen in the exhaust gas. The presence of oxygen storage materials and the specific zeolite structure enable the system to self-regulate the reduction reactions, ensuring complete utilization of supplied ammonia and preventing harmful emissions.
3Productivity
If high space velocity conditions are handled, then exhaust gas flow rate increases, but pressure loss increases and purification efficiency decreases
Solution Approach 1:
The catalyst utilizes porous zeolite structures with controlled pore sizes and distributions that provide high surface area for reactions while maintaining low flow resistance. The porous structure allows efficient gas diffusion and contact between reactants and active sites even at high space velocities, enabling high exhaust gas processing capacity without significant pressure loss increases.
Solution Approach 2:
The catalyst design transitions from traditional planar or pellet structures to three-dimensional monolithic substrates with honeycomb configurations. This dimensional change provides numerous parallel flow paths that reduce flow resistance and pressure drop while maintaining high catalytic activity through the distributed active sites on the substrate walls and in the porous coating layers.
4Productivity
If catalyst activity is maintained at high temperatures, then purification efficiency is improved, but catalyst heat resistance becomes critical
Solution Approach 1:
The catalyst employs composite oxide (B) consisting of silica, tungsten oxide, ceria, and zirconia in specific ratios, along with zeolite (A), to achieve both high-temperature catalytic activity and thermal stability. The composite structure provides synergistic effects where each component contributes to heat resistance and maintains structural integrity at elevated temperatures up to 650°C, preventing catalyst degradation while sustaining purification efficiency.
Solution Approach 2:
The catalyst formulation incorporates materials with matched thermal expansion coefficients, particularly zirconia and ceria in the composite oxide structure, to minimize thermal stress and prevent cracking during repeated heating and cooling cycles. This thermal compatibility ensures long-term reliability and maintains catalytic activity after exposure to high-temperature exhaust gas conditions.
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 high-efficiency nitrogen oxide purification from 150°C to 580°C, maintains activity after heat treatment, and reduces pressure loss, addressing the challenges of ammonia slip and catalyst mounting space in automotive applications.
Implementation Method 1
a selective reduction method, or Selective Catalytic Reduction (hereafter may be called SCR)... NOx is finally reduced to N2 mainly by the following reaction equations
Implementation Method 2
reductive denitration by making exhaust gas comprising NOx contacted with a selective reduction catalyst having vanadium oxide, zeolite or the like, as a main component, under the presence of an ammonia (NH3) component, as a selective reduction method
Implementation Method 3
generating NH3 by pyrolysis or hydrolysis... NH2-CO-NH2 → NH3 + HCNO
Implementation Method 4
HCNO + H2O → NH3 + CO2... NH2-CO-NH2 + H2O → 2NH3 + CO2
Implementation Method 5
coating a catalyst layer comprising zeolite (A) comprising at least an iron element, and a composite oxide (B) consisting of silica, tungsten oxide, ceria and zirconia, as denitration components, at the surface of a monolithic structure-type substrate
Data Source
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AI summary
The invention discloses a selective reduction-type catalyst which is capable of effectively purifying nitrogen oxides contained in exhaust gas from a lean-burn engine such as a boiler, a gas turbine or a lean-burn engine such as a lean-burn-type gasoline engine, a diesel engine, even under high SV, as well as having small pressure loss, by supplying by spraying urea water or ammonia water, as a reducing component, to the selective reduction-type catalyst; and an exhaust gas purification apparatus along with an exhaust gas purification method using the same. The selective reduction-type catalyst for selectively reducing a nitrogen oxide by adding urea or ammonia as a reducing agent of the nitrogen oxide to exhaust gas discharged from a lean-burn engine, characterized by coating a catalyst layer comprising zeolite (A) comprising at least an iron element, and a composite oxide (B) consisting of silica, tungsten oxide, ceria and zirconia, as denitration components, at the surface of a monolithic structure-type substrate, wherein composition of the composite oxide (B) is silica: 20% by weight or less, tungsten oxide: 1 to 50% by weight, ceria: 1 to 60% by weight, and zirconia: 30 to 90% by weight, or the like.