(728g) Structure Property Relationships of Supported Pt/Ni Bimetallic Catalysts: Correlating Pt-Ni Bimetallic Bond Formation to Catalytic Activity

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 g-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 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. 

These catalysts are also expected
to perform well for reforming chemistry based on previous surface science
studies in which the Ni?Pt?Pt(111) showed increased reforming activity over
either monometallic surface as well as the Pt?Ni?Pt(111) surface [4]. 
Experiments to test these catalysts are currently in progress.

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

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

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

[4]   Skoplyak,
O., Barteau, M. A., Chen, J.G, J. Phys. Chem. B 110, 1686 (2006)