XPS In Situ Reaction Methane Oxidation

We observed during the stability test of the Pd/Al203 and Pd/Zr02 catalysts for the total oxidation of methane a significant induction period. The activity of the Pd/Zr02 increased suddenly 10 times after 3 h and then remained constant for 25 h. Unlikely was the behavior of the Pd/Al203, as shown in Fig. 11.10 [16]. 

The question is why These results suggested searching for an explanation of the surface properties and the effects of the metal—support interaction, the affecting of the induction periods, and stability of the Pd/Zirconia catalysts. 

The only way to explain it was to find out possible modifications occurring at the surface, using XPS equipped with a pre-chamber. The reaction was performed in the pre-chamber with T = 400 °C and O2/CH4 = 2 and with time on stream. After vacuum it was transferred in the ultrahigh vacuum chamber for analysis. For palladium (Pd 3d), for example, 30-200 scans will be held in order to verify the occurrence of Pd photoreduction by prolonged exposure of the samples to X-rays. For the connection of energy, correction was used as the reference peak C Is at 284.6 eV. The calculation of the atomic ratio (e.g., Pd/Al, 0/Al, and 0/Ce) was 

The induction time is approximately 3-5 h, which corroborates with the hypothesis of rearrangement of the PdO in a higher oxidation state and stabilization of this oxidation state at the surface and due to the interaction of PdOv/ZrOy interface. 

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