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.