(514a) Structure-Property Relationships of Supported Pt/Ni Bimetallic Catalysts

Bimetallic catalysts are of great interest because they often display properties that differ from either of their parent metals. Previous studies on the Ni/Pt(111) bimetallic system have shown that the location of Ni atoms in the Pt(111) surface has a strong influence on the electronic and chemical properites of the surface [1]. The bimetallic surface consisting of a monolayer of Ni on top of bulk Pt(111), designated Ni?Pt?Pt(111), binds hydrogen and alkenes much more strongly than either parent metal, resulting in decreased hydrogenation activity. In contrast the surface consisting of a monolayer of Ni atoms in the subsurface region designated as Pt?Ni?Pt(111), has been shown to weaken metal-hydrogen bonds in comparison to Ni?Pt?Pt(111) or either parent metal surface. The resulting abundance of weakly bound hydrogen and alkenes on the Pt?Ni?Pt(111) surface increases its activity for novel low temperature hydrogenation pathways [2, 3].

The objective of current study is to extend previous investigations on single crystal surfaces to supported catalysts in an attempt to bridge the materials gap. Both monometallic and bimetallic catalysts were synthesized on γ-Al₂O₃ via incipient wetness. Two series of bimetallic catalysts were synthesized in order to study the effects of Pt:Ni metal atomic ratio and impregnation sequence. Benzene and 1,3-butadiene hydrogenations were used as a probe reactions and it was found that for both hydrogenations the bimetallic catalysts were more active than either monometallic catalyst. Fourier transform infrared (FTIR) spectroscopy was used to characterize CO chemisorption, which showed that the bimetallic catalysts bound CO in a different manner than either monometallic catalyst. The results of both the hydrogenation and chemisorption experiments suggested that there was a bimetallic effect, which justified further physical characterization using extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM). EXAFS of the Pt L_III edge confirmed the presence of bimetallic Pt-Ni interactions, and the magnitude of these interactions was found to correlate with the observed trends in hydrogenation activities for the two series of bimetallic catalysts. TEM imaging was performed in high angle annular dark field (HAADF) mode, and the resulting images showed a majority of particles with diameters on the order of 1 to 2 nm.

[1] Kitchin, J., Khan, N., Barteau, M., Chen, J., Yakshinskiy, B., and Madey, T., Surf. Sci. 544, 295 (2003).

[2] Hwu, H., Eng, J., and Chen, J., J. Am. Chem. Soc. 124, 702 (2002).

[3] Chen, J., Menning, C., and Zellner, M., Surf. Sci. Rep. (2008).