(778e) Ni@Pt and Co@Pt Overlayer Catalysts for Aqueous Phase Reforming

Ni@Pt and Co@Pt Overlayer Catalysts for Aqueous
Phase Reforming
Joseph H.
Holles, Michael D. Skoglund, and Allen Morris
Department of
Chemical and Petroleum Engineering, University of Wyoming

Creating new and sustainable fuels to meet our world's ever
growing energy demands is a national challenge. 
Aqueous phase reforming (APR) has emerged as a process capable of
turning renewable feedstocks into hydrogen gas or alkanes.  While most previous APR studies have used
feedstocks that are derived from biomass (methanol, ethylene glycol, glycerol,
sorbitol, and glucose), further studies using a direct biomass feed would be
beneficial.  Lactose, a sugar waste
product of the cheese and dairy industry, is one possible candidate.

Past APR studies have identified bimetallic Pt/Ni and Pt/Co
as the best catalysts for production of hydrogen through APR.  Literature computational work has also
predicted that a Pt overlayer on Ni or Co will decrease the heat of adsorption
of the H₂ and CO products.  Thus,
we have synthesized a series of Ni@Pt and Co@Pt pseudomorphic overlayer type bimetallic catalysts
which aim to increase the activity of the catalyst while reducing the necessary
loading of costly platinum. 

The directed deposition technique was used to selectively
deposit the overlayer (platinum) metal on the base metal without it depositing
onto the support surface.  Hydrogen chemisorption
and ethylene hydrogenation were used as characterization tools to identify catalysts
for subsequent APR studies.  Pt and Ni or
Co X-ray Absorption Spectroscopy (XAS) was performed on all catalysts at beamline 10-ID at Argonne National Laboratory to determine
the structure associated with the overlayer metal.  APR studies were performed using 3 w/o
lactose in water solutions at varying temperatures to determine activity and
selectivity to the desired hydrogen product.

As predicted computationally, the heat of adsorption for
hydrogen decreased for the overlayer compared to Pt.  The weaker hydrogen adsorption for both
overlayer catalysts when compared to monometallic Pt should aid in increasing
APR reaction rates because the hydrogen product will not be as tightly bound to
the metal surface; thus freeing up active sites for adsorption of reactants.  The deposition of the Pt overlayer resulted
in catalysts that were more active than the parent catalyst of pure Ni or Co,
as expected.  In addition, when compared
to pure Pt, both the Ni@Pt and Co@Pt
catalyst showed decreased activity.  These
results agree with computationally predicted d-band shifts from the literature that
would cause weaker hydrogen adsorption on the metal surface, decreased surface
coverage, and ultimately reduced activity for ethylene hydrogenation when
compared to platinum metal alone. 
Reaction order studies indicate H₂ surface coverage
consistent with this conclusion.  Thus, using
the chemisorption and reactivity descriptors, we have identified several
promising candidates for subsequent APR studies.

XAS studies of the Pt overlayer were conducted to determine
the structure of this overlayer metal. 
If the Pt is in an overlayer structure as desired, the interatomic
distance to Ni is expected to be measurably shorter than a Pt-Pt distance.  In addition, since the Pt is on the surface,
the coordination number should be lower than the bulk Ni or Co.  XAS data has been obtained and analysis is
currently underway and the results will be discussed.

Finally, APR studies of the Ni@Pt
and Co@Pt are currently being conducted.  Preliminary data has shown the catalysts
capable of converting all of the lactose. 
Additional studies will determine the reactivity and selectivity for all
overlayer catalysts as well as Co, Ni, and Pt only catalysts and Pt/Ni and
Pt/Co co-impregnated (non-overlayer) catalysts. 
All of the APR reactivity data will be correlated with the XAS, hydrogen
heat of adsorption, and ethylene hydrogenation results.