Abstract
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.
Highlights
Natural gas, an abundant energy resource with worldwide proven reserves of over 204.7 trillion m3 [1], is used primarily for electricity generation and heating
These data demonstrate that the adsorption/desorption of H2O from the Pd-Ni/Al2O3 catalyst (Pd) catalyst surface is temperature dependent and reaches equilibrium at temperatures above ~450 °C (723 K), even for H2O added in the gas phase
In another study of CH4 oxidation at low temperature (250–450 °C), a change in PdO dispersion was suggested as the main cause of deactivation of 0.5% Pd/Al2O3 and 0.5% Pd/SiO2 catalysts [50]
Summary
An abundant energy resource with worldwide proven reserves of over 204.7 trillion m3 [1], is used primarily for electricity generation and heating. A gasoline engine management system controls the A/F ratio or the exhaust gas composition (using an oxygen sensor connected to a secondary air supply) near the stoichiometric value. Under lean-burn conditions a gasoline engine may operate with sufficiently high A/F ratios so as to obtain a significant reduction in CO and NOx emissions and improved fuel efficiency. Lean-burn engines improve fuel efficiency, the exhaust gas temperature is significantly lower than from conventional gasoline powered engines, and catalysts with high oxidation activity at relatively low temperatures are needed for this application [5]. The high concentration of H2O in the NGV exhaust and the typically transient reaction conditions that result from cycling between oxidizing and reducing conditions in the NG engine [6,11] are known to significantly impact catalyst activity and stability. Interpretation of data from ideal catalyst studies allows direct links to be drawn between fundamental catalyst properties and catalyst performance for CH4 oxidation, whereas in real systems this may be more difficult to achieve
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