Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters Gholami, Rahman; Alyani, Mina; Smith, Kevin J.
Effects of H₂O on the activity and deactivation of Pd catalysts used for the oxidation of unburned CH₄ present in the exhaust gas of natural-gas vehicles (NGVs) are reviewed. CH₄ oxidation in a catalytic converter is limited by low exhaust gas temperatures (500–550 °C) and low concentrations of CH₄ (400–1500 ppmv) that must be reacted in the presence of large quantities of H₂O (10–15%) and CO₂ (15%), under transient exhaust gas flows, temperatures, and compositions. Although Pd catalysts have the highest known activity for CH₄ oxidation, water-induced sintering and reaction inhibition by H₂O deactivate these catalysts. Recent studies have shown the reversible inhibition by H₂O adsorption causes a significant drop in catalyst activity at lower reaction temperatures (below 450 °C), but its effect decreases (water adsorption becomes more reversible) with increasing reaction temperature. Thus above 500 °C H₂O inhibition is negligible, while Pd sintering and occlusion by support species become more important. H₂O inhibition is postulated to occur by either formation of relatively stable Pd(OH)₂ and/or partial blocking by OH groups of the O exchange between the support and Pd active sites thereby suppressing catalytic activity. Evidence from FTIR and isotopic labeling favors the latter route. Pd catalyst design, including incorporation of a second noble metal (Rh or Pt) and supports high O mobility (e.g., CeO₂) are known to improve catalyst activity and stability. Kinetic studies of CH₄ oxidation at conditions relevant to natural gas vehicles have quantified the thermodynamics and kinetics of competitive H₂O adsorption and Pd(OH)₂ formation, but none have addressed effects of H₂O on O mobility.
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