Calcination temperature effects on Pd/alumina catalysts: Particle size, surface species and activity in methane combustion

The effects of calcination temperature on Pd/γ-alumina catalysts were studied using a variety of techniques to measure particle sizes, surface species, and activity in methane combustion. Pd/Al₂O₃ catalysts were synthesized using a new impregnation-vortexing method. Three catalysts containing 3.3 wt.% Pd/Al₂O₃ were calcined at 150 °C, 250 °C, and 500 °C. The light-off temperature for methane combustion was 250-255 °C for all catalysts and 100% methane combustion was achieved at 275 °C in 20 min with the 3Pd/Al₂O₃ 250 °C catalyst under lean methane conditions. X-ray photoelectron spectroscopy showed PdO_x and PdO on the catalysts surface, and conversion of some PdO_x to PdO and Pd⁰ after reaction. Particle sizes and dispersions were different in the three fresh and spent catalysts as shown by scanning transmission electron microscopy (STEM) and CO pulse chemisorption. In situ and operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that after high temperature calcination, dormant monodentate carbonates are formed at the interface of Pd and alumina on the 3 Pd/Al₂O₃ 500 °C catalyst, which are absent on the 3Pd/Al₂O₃ 250 °C catalyst during reaction. Temperature-responsive CO species were observed for both 3Pd/Al₂O₃ catalysts, evidencing restructuring of the 3Pd/Al₂O₃ 250 °C catalyst upon heating in accordance with the STEM results. The results indicate a varying degree of strong metal-support interactions, resulting in different adsorbed species over the catalysts during methane oxidation reaction. We suggest that, over the 3Pd/Al₂O₃ 250 °C catalyst, the desorption of carbonates frees OH groups at the Pd-Al₂O₃ interface, which can react (with other OH or H) and desorb as water, a step which can be rate limiting on methane oxidation over Pd/Al₂O₃ at low temperatures. This research study highlights the possibility of altering catalytic properties and surfaces through calcination at different temperatures.