Working Principles, Materials, and Applications of Catalytic Converters and Three-Way Catalytic Converters(by Shandong Xiaoyuan Environmental Protection Technology Co., Ltd)

I. Overview: The catalytic converter is a key device for controlling vehicle exhaust emissions, used to reduce harmful substances in exhaust gas, such as: • Carbon monoxide (CO) • Unburned hydrocarbons (HC) • Nitrogen oxides (NOₓ). The three-way catalytic converter (TWC) is a common type of converter used in gasoline vehicles. It can simultaneously oxidize CO/HC and reduce NOₓ, hence the name "three-way." The core of a catalytic converter consists of three components: 1. Substrate: Support structure and airflow channels 2. Washcoat: Provides high surface area and metal adsorption sites 3. Precious metal catalyst layer: Active components such as Pt, Pd, and Rh

II. Working Principle 1. Working Mechanism of a Three-Way Catalytic Converter The core goal of a three-way catalytic converter is to simultaneously complete oxidation and reduction reactions: 1. CO oxidation reaction: CO + ½O₂ → CO₂ 2. HC oxidation reaction: CxHy + (x + y/4) O₂ → xCO₂ + y/2 H₂ O 3. NOₓ reduction reaction: 2NO → N₂ + O₂ or 2NO + 2CO → N₂ + 2CO₂ As gas flows through the honeycomb substrate inside the catalyst, the precious metal particles adsorb gas molecules and reduce the activation energy, enabling the reaction to proceed efficiently at low temperatures. Temperature control is crucial, with optimal conversion rates typically achieved between 200–800°C.

2. Oxygen Storage-Release Mechanism (OSC): To address fluctuations in the air/fuel ratio during different engine combustion conditions, the TWC utilizes a CeO₂–ZrO₂ oxygen storage system: • During oxygen-rich conditions, excess oxygen is stored. • During oxygen-lean conditions, the stored oxygen is released to ensure smooth CO/HC oxidation and NOₓ reduction reactions.

This allows the catalyst to maintain high conversion efficiency even under transient engine conditions.

III. Catalyst Materials 1. Substrate: Cordierite (low thermal expansion, strong thermal shock resistance). Gasoline engine TWC honeycomb matrix: Silicon carbide (SiC). High thermal conductivity, high strength. High-temperature diesel catalyst/industrial exhaust gas. The substrate structure typically features honeycomb or sheet-like corrugated channels to reduce backpressure and provide a large surface area.

2. Washcoat • Primarily composed of γ-Al₂O₃, CeO₂-ZrO₂, and TiO₂ • Functions: • Increases specific surface area (providing hundreds of square meters/g) • Immobilizes precious metals • Provides oxygen storage and release capabilities • Typically 10–30 μm thick, with a microporous structure ensuring adequate contact between gas and metal

3. Precious Metal Catalytic Layer • Platinum (Pt): Oxidizes CO and HC • Palladium (Pd): Oxidizes HC and CO • Rhodium (Rh): Reduces NOₓ Precious metals are formed into nanoparticles through impregnation, drying, and calcination, and then dispersed within the pores of the washcoat, ensuring high activity and durability.

IV. Application Scenarios 1. Automotive Exhaust Purification • Gasoline Vehicle TWC: Simultaneously oxidizes CO/HC and reduces NOₓ • Diesel Vehicle DOC + DPF Combination: Oxidizes unburned hydrocarbons and assists in particulate filter regeneration • SCR + DOC: Partial NOₓ is further degraded by a reducing agent (NH₃ or urea) 2. Industrial Waste Gas Treatment • VOC Catalytic Combustion • CO and NOₓ Treatment • Chemical Synthesis Reaction Optimization In industrial systems, the carrier is typically made of high-thermal-conductivity silicon carbide or alumina. Precious metals can be evenly distributed through the coating to ensure efficient conversion even at high exhaust temperatures.

V. Aging and Improvements • Aging Issues: Precious metal sintering, reduced coating porosity, and decreased oxygen storage capacity • Improvement Measures: • Stabilized coating (CeZr solid solution, La₂O₃ doping) • Nanodispersion technology to ensure high dispersion of precious metals • High thermal conductivity carrier materials to reduce local overheating

VI. Conclusion: Catalytic converters and three-way catalytic converters achieve efficient exhaust gas purification through honeycomb-structured carriers, optimized coatings, and precious metal dispersion technology. Their performance directly determines the effectiveness of emission control in automotive and industrial exhaust treatment.