Recently, extensive research efforts have been focusing on developing new energy resources, due to the depleting of fossil fuels and increasing energy demands. Aqueous phase hydrodeoxygenation (APH) provides a feasible catalytic solution for converting biomass-derived feedstock (lignin, sugars, bio-oils) into fuels and value added chemicals. However, many aqueous phase reforming and fuel cell studies argued that the strong adsorption of H2 or CO would block the platinum surface active site and reduce the catalyst reactivity. On the other hand, bimetallic overlayer catalysts are widely studied recently due to their unique adsorption properties. Pt overlayer on top of Ni or Co have been demonstrated computationally and experimentally to display a reduced d-band center energy and thus weakened adsorption strength for H2 and CO . Accordingly, platinum overlayer catalysts with reduced binding strength of H2 and CO are expected to show increased reactivity in aqueous phase hydrodeoxygenation reaction.
In this research, two bimetallic overlayer catalysts, Ni@Pt and Co@Pt (core@shell) supported on solid acid (silica-alumina) have been synthesized. The goal of this work is to examine the effect of the platinum overlayer catalyst upon the hydrodeoxygenation of biomass using glycerol as a model compound. Silica-alumina supported nickel, cobalt and platinum monometallic catalysts and non-structured alloys (Ni-Pt and Co-Pt) were prepared by incipient wetness impregnation. The directed deposition technique was used to selectively deposit the overlayer (platinum) metal on the base metal without it depositing onto the support surface . The glycerol APH tests were performed in a 9.5 mm diameter stainless steel tube. The APH reaction conditions were 150 mg catalyst, 400 psi, 0.1 mL/min 10 wt % glycerol aqueous solution. Reaction temperatures were varied from 250ºC to 300ºC.
ICP analysis indicated that for Ni@Pt about 28% of nickel surface is covered with platinum overlayer while about 2% of cobalt particle surface is covered with platinum overlayer in Co@Pt assuming the platinum dispersion of 100%. Hydrogen isotherms at various temperatures are measured and hydrogen heats of adsorption are calculated using Clausius-Clapeyron equation. Ni@Pt and Co@Pt overlayer catalysts exhibited decreased hydrogen heats of adsorption from pure platinum, indicating successful modifications of reduced hydrogen adsorption strength on platinum surface. Non-structured bimetallic alloy catalysts showed different heat of H2 adsorption compared to the overlayer catalysts, likely due to the structural difference between overlayer catalysts and bimetallic catalysts. Ethylene hydrogenation reactivity, which is a strong function of H2 surface coverage and H2 adsorption strength, is also tested as a descriptor reaction. Both platinum overlayer catalysts, Ni@Pt and Co@Pt showed reduced turnover frequencies compared to pure platinum, implying a weaker hydrogen adsorption. To conclude the H2 chemisorption and ethylene hydrogenation results, overlayer catalysts showed decreased hydrogen adsorption strength, consistent with previous computation predictions .
Aqueous phase hydrodeoxygenation of glycerol using overlayer catalysts showed enhanced TOF in hydrocarbon production compared to their monometallic counterparts and non-structured bimetallic alloys. This improvement in TOF could be correlated with chemisorption and ethylene hydrogenation results, as more surface active sites are available by reducing the adsorption strength of hydrogen and carbon monoxide. Also, the distinct difference in selectivity and reactivity between overlayer catalysts and bimetallic alloys implies that a designed overlayer structure has been accomplished rather than creating a non-structure bimetallic alloy.
In conclusion, several bimetallic overlayer catalysts have been synthesized using the directed deposition technique. Through the characterization of hydrogen chemisorption and ethylene hydrogenation, it is demonstrated that the hydrogen adsorption on the surface was significantly reduced, which is consistent with computational predictions and previous research observations. Glycerol APH reaction results showed that the TOF toward hydrocarbons has been enhanced with overlayer catalysts, likely due to the weakened hydrogen and carbon monoxide adsorption and exposing more surface active sites. This demonstrated the viability of preparing overlayer catalysts aiming at efficient and effective biomass conversion.
 J.G. Chen, C.A. Menning, M.B. Zellner, Surface Science Reports 63 (2008) 201-254.
 M.D. Skoglund, C.L. Jackson, K.J. McKim, H.J. Olsen, S. Sabirzyanov, J.H. Holles, Applied Catalysis A: General 467 (2013) 355-362.
 E. Christoffersen, P. Liu, A. Ruban, H.L. Skriver, J.K. Norskov, Journal of Catalysis 199 (2001) 123-131.
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