Rate Inhibition by Impurities and Products

1 Inhibition by H2S and SO2 - Natural gas typically has a low sulfur content, of less than a few ppm. However, the presence of even trace amounts of sulfur, derived not only from the gas but also from lubricating oils, can severely poison a methane combustion catalyst. This poisoning is particularly critical for natural-gas vehicles (NGVs) because sulfur affects the methane combustion much more drastically than combustion of other hydrocarbons. As a result, NGVs equipped with Pd catalysts can meet the standards for non-methane hydrocarbon emissions, but fail the total hydrocarbon emission tests, which require methane elimination at lower temperatures. 

Depending on temperature, and whether the environment is oxidizing or reducing, sulfur species may exist in the form of sulfur dioxide, sulfate, or sulfide. Khairulin et a/. have shown that the presence of 100 ppm H2S raised the light-off curve for Pd/Al203 by 150 °C. When compared at the same temperature, the loss of activity in the poisoned catalyst was more than 90%. This activity loss was attributed to the irreversible adsorption of sulfate species on the metal. Adsorption of sulfate species on Pd surface has been demonstrated by XPS studies, which showed the presence of a S2p peak after the Pd/Al203 catalyst had been 

The PdS species formed after the H2 treatment can be converted back to PdS04 upon exposure to the CH4/O2 mixture, preventing the regeneration. 

2 Inhibition by Water and CO2 - The products of the methane oxidation reaction, water and carbon dioxide, may also act as rate inhibitors. Water is formed not only by the methane oxidation reaction, but it is already present at high concentrations (up to 15%) in typical exhausts. It is generally agreed that the inhibiting effect of water is more severe than that of CO2. The reaction order with respect to H2O has been found to be — 1, while that for CO2 depends upon concentration. Below 0.5 vol%, CO2 exhibited no effect, but at higher concentrations it strongly suppressed the rate with an order dependency of —2. However, other studies have found a weak correlation or no inhibition above a CO2 concentration of 10%.  

Cullis et were the first to report that, in the presence of 1.3% water, the rate suppression was independent of the added water. It was proposed that water interacts with the PdO surface, forming thermodynamically favored Pd-hydro-xide species according to the reaction. 

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