The
effects of the Pd particle size and the Al2O3 crystalline phase of Pd/Al2O3 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 Al2O3 crystalline phases, Pd/α-Al2O3 was proved to be more resistant to deactivation
by H2O than Pd/γ-Al2O3 and
Pd/θ-Al2O3. Moreover, according to H2O temperature-programmed desorption (TPD) and infrared (IR)
measurements, since α-Al2O3 was relatively
hydrophobic compared to γ-Al2O3 and θ-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.