Methane Combustion over Pd/Al2O3 Catalysts in the Presence of Water: Effects of Pd Particle Size and Alumina Crystalline Phase

The effects of the Pd particle size and the Al2O3 crystalline phase of Pd/Al(2)O(3 )catalysts on the CH4 combustion in the presence of H2O were investigated. According to X-ray absorption fine structure (XAFS) measurements, it was revealed that, during the CH4 combustion, the Pd nanoparticles existed on the Al2O3 support as a PdO phase, while the crystallinity of PdO depended on the Pd particle size. On the basis of X-ray diffraction XRD), amorphous PdO particles with a size of <7 nm exhibited low activity for CH4 combustion. In contrast, as the Pd particle size increased, larger crystalline PdO particles (>7 nm) were formed, which were highly active for CH4 combustion. Comparing the effects of the different Al(2)O(3 )crystalline phases, Pd/alpha-Al2O3 was proved to be more resistant to deactivation by H2O than Pd/gamma-Al(2)O(3 )and Pd/theta-Al2O3. Moreover, according to H2O temperature-programmed desorption (TPD) and infrared (IR) measurements, since alpha-Al2O3 was relatively hydrophobic compared to gamma-Al2O3 and theta-Al2O3, a faster and reversible adsorption/desorption of OH/H2O species was achieved, while the H2O poisoning on PdO species in the vicinity was limited.