(575aa) Structure-Property Relationships of Supported Pt-Ni Bimetallic Catalysts

            Hydrogen will be one of the most important fuels
in the future because it has the potential to be produced from renewable
resources and can provide a clean and efficient fuel for fuel cells.  Huber et
al. have proposed a method to renewably produce hydrogen via the aqueous
phase reforming (APR) of oxygenates.  They have identified Pt-Ni catalysts as
one of several bimetallic catalysts that display both high activity towards the
APR reaction and high selectivity to the desired H₂ product [1].  Identifying
and designing a more active and selective catalyst for the reforming of
oxygenates will make the production of H₂ from renewable resources
more efficient.

The reforming of oxygenates over
single crystal bimetallic surfaces has also been studied by Skoplyak et al.
in which the surface created by monolayer deposition of Ni on Pt(111),
designated Ni?Pt?Pt(111), was found to show superior reforming activity in
comparison to Pt(111), subsurface monolayer Pt?Ni?Pt(111), and a Ni(111) film
[2].  The stability of these monolayer surfaces was also studied by Menning et
al. in which it was found that the Ni monolayer in the Pt?Ni?Pt(111)
surface segregates to the surface and forms the Ni?Pt?Pt(111) surface under
oxidizing conditions [3].  Another unique property of Pt-Ni bimetallic systems
is that the Pt?Ni?Pt(111) surface binds hydrogen more weakly than either parent
metal allowing for novel low-temperature hydrogenation pathways [4].  The
objective of the current study is to extend previous investigations on single
crystal surfaces to supported catalysts in an attempt to understand
structure-property relationships and to bridge the materials gap.

For this study, Pt, Ni, and Pt-Ni
bimetallic catalysts supported on g-Al₂O₃
were synthesized via incipient wetness impregnation using both sequential
impregnation as well as co-impregnation for the bimetallic catalysts.  Previous
work by Shu et al. reported on the effect of impregnation sequence for
Pt-Ni bimetallic catalysts, and it was found that for Pt/Ni atomic ratios of
1/1 resulted in the formation of bimetallic catalysts using both sequences.  However
for Pt/Ni atomic ratios of 3/1, bimetallic catalysts were not formed for the Ni
first sequence [5].  Therefore in this work the Pt/Ni atomic ratios were chosen
to be 1/3 and 1/10 in order to determine the effect of metal loading on
catalytic activity and bimetallic formation.

            In the work presented here catalytic activity
was characterized both through gas phase batch and flow reactor studies.  In an
attempt to observe the unique activities displayed by the Pt-Ni bimetallic
systems in surface science experiments, both hydrogenation and oxygenate
reforming reactions were used as probe reactions to characterize the
catalysts.  In-situ extended x-ray absorption fine structure (EXAFS)
measurements were used to determine the extent of bimetallic formation and to
relate the catalytic activity to the catalyst structure.  Additional
characterization included CO chemisorption to quantify catalytic surface area
and transmission electron microscopy (TEM) to obtain average particle sizes.

[1]       G.
W. Huber, J. W. Shabaker, S. T. Evans, J. A. Dumesic, Aplied Catalysis B:
Environmental, 62 (2006) 226.

[2]       O.
Skoplyak, M. A. Barteau, J. G. Chen, Journal of Physical Chemistry B, 110
(2006) 1686.

[3]       C.
A. Menning, J. G. Chen, Journal of Chemical Physics, 128 (2008) 164703.

[4]       H.
H. Hwu, J. Eng Jr., J. G. Chen, Journal of American Chemical Society, 124
(2002) 702.

[5]       Y.
Shu, L. E. Murillo, J. P. Bosco, W. Huang, A. I. Frenkel, J. G. Chen, Applied
Catalysis A: General, 339 (2008) 169.