Structural properties, redox behaviour and methane combustion activity of differently supported flame-made Pd catalysts

Catalysts made up of Pd on different supports (Al2O3, SiO2, ZrO2, TiO2, MgAl2O4, and CeO2) have been synthesized by single-step flame-spray pyrolysis. The as-prepared, non-porous, nano-sized particles showed high surface area and compared to a conventional alumina supported palladium catalyst a relatively high content of PdO. Another characteristic feature of the flame-made materials is that part of the Pd-constituent was stabilized in the supporting oxide matrix, and thus inaccessible, as pertinent dissolution tests indicated. Characterization of the oxidation state of the Pd-component in different atmospheres and under reaction conditions revealed a strong correlation between the amount of PdO and methane combustion activity, although under reaction conditions, metallic Pd was still present. The origin of the observed hysteresis behaviour in methane conversion in heating–cooling cycles was investigated by in situ thermogravimetry combined with mass spectrometry, and confirmed in fixed-bed reactor tests. A strong correlation between the light-off temperature during methane oxidation and the shift of the characteristic PdO Raman signal at ca. 650 cm−1 was observed. Samples with an increasing light-off temperature show a Raman shift shifting to that of bulk PdO, indicating an increase in light-off temperature with increasing particle size. Another property identified to be crucial for high activity is the thermal stability of the support, which was strongly correlated to the extent of PdO regeneration during cooling. Amongst the supports tested, the flame-made Al2O3 supported catalyst proved to possess the highest thermal stability and activity in methane combustion.