Nickel and Iron Poisoning of Copper Based Methanol Synthesis Catalysts
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Discharged methanol synthesis catalysts are known to contain various amounts of nickel and iron. The purpose of this master thesis was to study nickel and iron poisoning of commercial copper based methanol synthesis catalysts and hence try to establish a relationship between the amount of deposited nickel and iron and the following decrease in methanol synthesis activity. Fresh commercial Cu/ZnO/Al2O3 catalysts were doped with nickel and iron to simulate nickel and iron poisoning from an industrial gas stream. To best represent nickel and iron levels found on discharged catalyst plant samples, the catalysts were doped with amounts in the range 0-5000 ppm. The resulting nickel and iron distribution was studied in a SEM and established to be different, i.e. nickel was more evenly distributed on the catalyst, while iron fully enriched the catalyst surface in concentrated areas. Nickel was found to have no convincing effect on the specific surface area determined by the BET method, while for iron a significant decrease was observed. By using a nitrous oxide (N2O) chemisorption method a decrease in the copper surface area was observed as a result from iron poisoning. Although the copper surface area was constant for nickel, the methanol synthesis activity was found to decrease with increasing amount of nickel on the catalyst, i.e. from catalysts impregnated with 1400 ppm, 2800 ppm and 4400 ppm nickel, a 36 %, 43 % and 51 % decrease in the methanol synthesis activity was observed, respectively. In correlation with the measured decrease in copper surface area, an even more significant decrease in activity was observed in the case of iron, i.e. catalysts poisoned with 670 ppm and 1300 ppm iron was observed to have a decrease in methanol synthesis activity of 54 % and 57 %, respectively suggesting that iron is more poisonous than nickel. During methanol synthesis the catalysts experienced some thermal sintering, which was observed from a decrease in specific surface area and an increase in the pore diameter (measured after synthesis). Iron was found to have an effect on the catalyst selectivity, as an increase in by-products like methane and wax were observed as a function of increasing amount of iron on the catalyst. No effect of nickel on the selectivity and by-product formation was found in the studied nickel concentration range.