Thermal treatment in air of the Ru/ZnAl2O4 catalysts for the methane combustion

• Performance of the Ru/ZnAl2O4 catalysts for methane combustion has been studied.
• Thermal treatment in air at 300–600 °C leads to a decrease of the metal dispersion.
• Pretreatment procedure has a significant influence on activity of the Ru catalysts.
• Methane combustion on ruthenium catalysts depend on the crystallite size of RuO2.

The highly dispersed Ru/ZnAl2O4 catalysts (0.5 and 4.5 wt.% Ru) were treated in air at 300–600 °C and their catalytic performance for the methane combustion has been investigated. For comparison, catalytic activity of the impregnated, dried and calcined in air at 400 °C catalysts was also studied. Catalytic data show that pretreatment procedure has a significant influence on the activity of the Ru catalysts. The catalysts treated at 300 °C were more active than the calcined one and that treated at higher temperatures. For a given catalyst, the temperature of half conversion T50 was increased by 50 °C with treatment temperature. On the other hand, increase in Ru loading from 0.5% to 4.5 wt.% caused decreasing of T50 by about 100 °C. The best catalytic results were obtained over the high-loaded Ru catalyst treated at 300–400 °C, which was able to completely convert CH4 to CO2 at temperatures around 510 °C. XRD and TEM data show that small ruthenium nanoparticles (∼1.5 nm) undergoes oxidation during thermal treatment in air with the formation of larger RuO2 particles what led to a significant decrease of the ruthenium dispersion. For the 0.5% and 4.5% Ru catalysts treated at 300–600 °C, the mean RuO2 crystallite size increased from 18 to 31 nm and 16 to 33 nm, respectively. Large change of the crystallite size of the RuO2 oxide, in the catalyst at the same Ru content, cause that the exposed RuO2 surface reduces and this lead to a decrease of the specific reaction rate (μmol CH4 gRu−1 s−1) per gram of ruthenium. The activity of Ru/ZnAl2O4 catalysts was very stable when it was treated at 500 °C and above.

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