(392b) Structural Changes in Au Nanoparticles and in Tio2 during the Oxidation of CH4 and C2H4 on Au/Tio2 Catalysts

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, 300^oC in
the case of C₂H₄ and 450^oC 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 (l_SP)
increased to high wavelength with the number of reaction cycles. Typically, l_SP 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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[4]   F.E. Wagner, M. Karger,
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[5]   J. Ciric, US
Patent 3 972 983 (1976), to Mobil Oil Corporation.