I guess Pd catalyst ZnO reduction

Under the applied vacuum annealing conditions, a PdZn alloy may be more thermodynamically more stable than the Pd@ZnO core-shell structure. Thus PdZn alloy formation will occur as it lowers the total surface energy of the system. Alternatively, as Wang Yunjian noted, metallic Pd may also catalyze the reduction of ZnO to Zn metal + 1/2 O2.