It has been reported that gold catalysts are active in the oxidation of VOC, however, as it occurs in other reactions, there are still discrepancies concerning the active sites and reaction mechanisms (1-5). In this work we demonstrate that changes in activity of Au/TiO2 catalysts having 0-2.7 Au wt. % occur during the oxidation of CH4 and C2H4. The catalytic activity during the oxidation of CH4 and C2H4 was measured by programmed temperature reaction cycles under different reaction conditions. Both reactions occurred at high temperature, 300oC in the case of C2H4 and 450oC for CH4 .
The activity increased with the Au content. In all cases the light-off temperature increased as the reaction cycles increased. We found also changes in used catalysts that involve modifications in both Au and support structure. These changes were temperature dependent and affected the catalytic activity. In the case of TiO2, XRD results indicate changes in the rutile to anatase ratio. During reaction the rutile to anatase ratio changed to 12 when we used only TiO2. The presence of Au nanoparticles inhibited the phase transition in TiO2, because in Au/TiO2 catalysts that ratio changed from 0.3 to almost 6 and was dependent on the Au content.
All catalysts had the characteristic Au surface Plasmon, detected by diffuse reflectance in the UV-Vis region. Its position and area was associated with the presence of metallic Au nanoparticles of different sizes that suffered sintering as well as the anatase to rutile phase change of TiO2. The position of the Au Plasmon (lSP) increased to high wavelength with the number of reaction cycles. Typically, lSP shifted from 545 to 600 nm. The activity depended also on the amount of rutile present in the support.
We discuss how the size of the nanoparticles, the oxidation state of gold and the interaction with the support can be related to further improve this important class of catalysts. References
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