Working Principle and Main Materials of Ceramic Catalyst Carriers

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Working Principle and Main Materials of Ceramic Catalyst Carriers

—Applications of Coated Precious Metals in the Automotive and Industrial Fields ⸻

Introduction Ceramic catalyst carriers are one of the most fundamental components in automotive exhaust purification and industrial waste gas treatment systems. They do not directly participate in chemical reactions, but rather provide a supporting structure, high specific surface area, and thermally stable environment for catalysts (especially precious metal catalysts), enabling reactant gases to fully contact and react efficiently with the active metal. In a typical catalytic system, the catalyst consists of three parts: Ceramic carrier → Washcoat → Precious metal active component. The ceramic carrier provides mechanical and thermal stability, the washcoat provides a porous adsorption interface, and the precious metal layer is the active center for the actual oxidation and reduction reactions. These three components together determine the catalyst's activity, lifespan, and anti-aging properties.

II. Working Principle of Ceramic Catalyst Supports 1. Honeycomb Structure and Gas Distribution Ceramic supports in automotive and industrial catalytic systems mostly adopt a honeycomb structure (monolith), with hundreds to thousands of parallel channels (typically 200–600 CPSI). This structure has the following characteristics: • Uniform gas flow: ensures low pressure drop and stable flow rate when exhaust gas passes through the catalyst layer; • Large specific surface area: provides sufficient contact interface for the reaction; • High mechanical strength: can withstand the high temperature and vibration environment of engines or industrial equipment. When pollutant-containing gases (such as CO, HC, NOₓ, or VOCs) pass through the channels of the catalyst support, the reactant gas molecules diffuse to the coating and noble metal surface, are adsorbed and participate in the reaction, and are ultimately converted into harmless products such as CO₂, H₂O, and N₂. ⸻2. Overview of Working Mechanism The ceramic support primarily functions in the following ways: • Support and Distribution: Ensuring uniform distribution of the coating and precious metals, providing stable geometric support; • Thermal Stability and Insulation: Ensuring the catalyst does not melt or deform at 800–1100 °C; • Chemical Inertness: Preventing side reactions with reactant gases or precious metals;  Heat Transfer and Diffusion Properties: Helping to maintain temperature equilibrium in the reaction zone.

⸻III. Main Materials and Their Characteristics The materials used in the ceramic support must possess characteristics such as low thermal expansion, high mechanical strength, and good chemical stability. Current mainstream carrier materials include: Cordierite (gasoline engine TWC): low thermal expansion, resistant to thermal shock, and low cost; Silicon carbide (SiC): diesel DPF, high-temperature industrial applications: high thermal conductivity, high temperature resistance, and anti-clogging; Alumina (Al₂O₃): industrial catalysis: high specific surface area, can participate in carrier bonding; Mullite: high-temperature chemical reactions: high strength and corrosion resistance; Zirconia/Cerium oxide composite (ZrO₂–CeO₂): advanced TWC, VOC catalysts: strong oxygen storage/release capacity, improves the utilization rate of precious metals. These materials undergo high-temperature extrusion, sintering, and surface modification to form a stable honeycomb structure, allowing for long-term use in harsh environments. ⸻

IV. Coating and Noble Metal Coating Principles 1. Coating (Washcoat) • Provides a porous structure to immobilize noble metals, increasing specific surface area. • Commonly used materials: γ-Al₂O₃, CeO₂-ZrO₂, TiO₂, SiO₂, La₂O₃ • Forms a microporous network, providing adsorption sites for noble metals. 2. Noble Metal Coating and Mechanism of Action • Coating method: Impregnation with noble metal salt solution, drying, and calcination to form nanoparticles (2–10 nm). • Chemical and electronic interactions: Noble metals combine with the support, enhancing oxygen storage and release capacity. • Catalytic reactions: • CO oxidation: CO + ½O₂ → CO₂ • HC oxidation: HC + O₂ → CO₂ + H₂O • NOₓ reduction: 2NO → N₂ + O₂ ⸻V. Application Areas 1. Automotive Field • Three-way catalytic converter (TWC): Pt + Pd + Rh, oxidizes CO/HC, reduces NOₓ • Diesel oxidation catalytic converter (DOC)/particulate filter (DPF): Pt, oxidizes carbon soot, reduces regeneration temperature • SCR pre-oxidation layer: Pt, increases NO₂ generation, optimizes denitrification efficiency 2. Industrial applications • VOC catalytic combustion: Pt/Pd, oxidizes organic waste gas at 250–400 °C • Chemical tail gas purification: Rh/Pt, treats CO and NOₓ • Petrochemical hydrogenation reaction: Pd/Pt carrier used for hydrogenation and desulfurization

VI. Aging and Technological Improvement Aging problems: High temperatures and complex atmospheres cause precious metal particles to sinter, coating porosity to decrease, and metal loss. Improvement Measures: • Doping to stabilize the system (La₂O₃–Al₂O₃, CeZr solid solution) • Using a high thermal conductivity carrier (SiC) to reduce the thermal gradient • Atomic layer deposition (ALD) and nano-dispersion technology to control metal particle size and distribution ⸻VII. Conclusion Ceramic catalyst supports, through honeycomb structure design, coating optimization, and noble metal dispersion coating, form a highly efficient and durable catalytic system. In the treatment of automotive exhaust and industrial waste gas, its performance determines whether the system can meet emission standards.