Multi-Stage Catalyst for NOx Reduction in Lean-Burn Engines
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Solution Overview
Problem
Current catalytic NOx reduction technologies are ineffective for high-efficiency lean-burn natural gas engines, particularly in oxygen-rich environments, as three-way catalysts are inactive in such conditions and ammonia-based selective catalytic reduction systems are impractical due to size and cost, while existing NOx trap systems face issues with sintering and SO2 poisoning.
Innovation Solution
A catalyst system comprising an oxidation catalyst, such as cobalt on zirconia, to convert nitric oxide (NO) to nitrogen dioxide (NO2), and a reduction catalyst, such as palladium on sulfated or tungstated zirconia, to convert NO2 to nitrogen (N2), utilizing hydrocarbons present in the exhaust stream, thereby enhancing NOx removal efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If three-way catalysts are used for NOx reduction, then NOx removal efficiency is improved, but the catalyst becomes inactive in oxygen rich environments
Solution Approach 1:
The catalyst system is divided into two separate functional components: an oxidation catalyst (first metal on first inorganic oxide) that converts NO to NO2, and a reduction catalyst (second metal on second inorganic oxide) that converts NO2 to N2. This segmentation allows each catalyst to operate optimally in its specific function without the deactivation issues of three-way catalysts in oxygen-rich environments.
Solution Approach 2:
Nitrogen dioxide (NO2) serves as an intermediary species between NO and N2. The oxidation catalyst first converts NO to NO2, which then serves as the substrate for the reduction catalyst to produce N2. This intermediary approach allows the system to overcome the limitation of direct NO reduction in oxygen-rich environments.
2Productivity
If ammonia-based selective catalytic reduction is used, then NOx removal efficiency is improved, but system size and cost increase
Solution Approach 1:
The system uses hydrocarbons that are already present in the exhaust stream as the reducing agent, eliminating the need for external ammonia injection and storage systems. The exhaust stream itself provides the reducing agent needed for NO2 conversion to N2, simplifying the overall system architecture.
Solution Approach 2:
The catalyst system handles multiple functions: the oxidation catalyst addresses NO oxidation, the reduction catalyst addresses NO2 reduction, and the system simultaneously manages the hydrocarbon combustion issue by using the same hydrocarbons as reducing agents. This multi-functionality reduces the need for separate systems.
3Productivity
If NOx trap systems are used, then NOx removal capability is improved, but the materials become susceptible to sintering and SO2 poisoning
Solution Approach 1:
The system changes the chemical state of nitrogen oxides from NO to NO2 through oxidation, which then allows reduction to N2. This parameter change in the nitrogen oxide form enables the use of alternative catalyst materials that are resistant to sintering and SO2 poisoning, as the reduced NO2 form is less susceptible to these deactivation mechanisms.
4Ease of manufacture
If hydrocarbons are used as reducing agents in NO removal, then cost effectiveness is improved, but hydrocarbon combustion blocks NO reduction reactions
Solution Approach 1:
The oxidation catalyst performs preliminary conversion of NO to NO2 before the reduction step. This preliminary action changes the nitrogen oxide form to a state (NO2) that can be reduced by hydrocarbons without being blocked by hydrocarbon combustion, as the NO2 reduction pathway is distinct from direct NO reduction.
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 system achieves high conversion rates of NOx to N2, overcoming the limitations of existing technologies by oxidizing NO to NO2 and then reducing it in oxygen-rich environments where hydrocarbon combustion would otherwise block NO reduction reactions, reducing NOx emissions effectively without the need for additional hydrocarbon injection.
Implementation Method 1
an oxidation catalyst comprising a first metal supported on a first inorganic oxide for catalyzing the oxidation of NO to nitrogen dioxide (NO2)
Implementation Method 2
a reduction catalyst comprising a second metal supported on a second inorganic oxide for catalyzing the reduction of NO2 to nitrogen (N2)
Implementation Method 3
an oxidation catalyst comprising a first metal supported on a first inorganic oxide for catalyzing the oxidation of NO to nitrogen dioxide (NO2), and a reduction catalyst comprising a second metal supported on a second inorganic oxide for catalyzing the reduction of NO2 to nitrogen (N2)
Data Source
AI summary
Catalyst systems and methods provide benefits in reducing the content of nitrogen oxides in a gaseous stream containing nitric oxide (NO), hydrocarbons, carbon monoxide (CO), and oxygen (O2). The catalyst system comprises an oxidation catalyst comprising a first metal supported on a first inorganic oxide for catalyzing the oxidation of NO to nitrogen dioxide (NO2), and a reduction catalyst comprising a second metal supported on a second inorganic oxide for catalyzing the reduction of NO2 to nitrogen (N2).