(292d) Controlling Catalysts’ Structure and Methane Oxidation Performance Using Flame Spray Pyrolysis

Unburned methane emissions are caused by incomplete combustion in natural gas engines at low engine temperatures. Novel catalysts are demanded to reduce at least 50% of unburned methane below 400 ºC since methane is a strong greenhouse gas. Hence, a series of 0.8wt% Pd/Ce_xZr_y (where x, y = 1,2) solid solution catalysts were synthesized at different element ratios, precursor molarities, and synthesis conditions (i.e., liquid feed rate, oxygen flow rate) via flame spray pyrolysis (FSP).

Oxygen mobility and Pd speciation are essential factors that affect methane oxidation activity. X-ray diffraction and Raman spectroscopy show that the formation of single-phase and oxygen vacancy amount in the support can be adjusted by Ce:Zr element ratio and FSP synthesis conditions. A low oxygen flow rate can promote oxygen vacancy formation in solid solution catalysts. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicates that the formation of Pd²⁺ species can be favored by the oxidizing environment in the flame induced by high oxygen flow rate and low amount of fuel in the precursor. Besides, the samples with high oxygen vacancy amount can cause a strong metal-support interaction and speed up the re-oxidation from Pd⁰ to Pd²⁺, which eventually promotes the Pd²⁺ species formation. Activity tests show that solid solution catalysts with a higher oxygen vacancy amount and more Pd²⁺ species could perform better. The T₅₀ of the best sample reaches 400 ºC. Our results indicate that catalysts’ structure, including oxygen vacancy amount and Pd speciation, can be adjusted by the FSP synthesis, speeding up the functional material discovery process.