(779f) Effect of Reduced Adsorption Strength with Bimetallic Platinum Overlayer Catalysts

The growing demand for energy resources has stimulated extensive research on converting biomass into transportation fuels and valuable chemicals to alleviate the excessive consumption of fossil fuels. Platinum-based catalysts have shown excellent reactivity and stability for various biomass conversion reactions. However, literature studies have suggested that the strong adsorption of H₂ or CO reforming products would block platinum surface active sites and reduce the catalyst reactivity. One possible option to solve this problem is bimetallic overlayer catalysts. Bimetallic overlayer catalysts are widely studied recently due to their unique adsorption properties. For example, a Pt overlayer on top of Ni or Co was been demonstrated computationally and experimentally to display a reduced d-band center energy and thus weakened adsorption strength for H₂ and CO [1]. In this respect, overlayer catalysts with reduced binding strength of H₂ and CO are expected to show increased catalytic reactivity compared to pure Pt in reactions where strong H₂ or CO product binding are proposed to inhibit the reactivity. Moreover, Pt overlayer catalysts suggest possibility of taking advantages of catalytic properties of Pt metal, while increasing its reactivity through modifying its adsorption strength as desired.

In this research, 5 bimetallic platinum overlayer catalysts, Ni@Pt/SiO₂-Al₂O₃, Co@Pt/SiO₂-Al₂O₃, Ir@Pt/SiO₂-Al₂O₃, Ni@Pt/SiO₂, and Cu@Pt/SiO₂ (core@shell) have been synthesized. These combinations were chosen based on previous computational studies predicting the reduced adsorption strength of platinum overlayer over these bulk metals [3]. The goal of this work is to examine the catalytic reactivity of these platinum overlayer catalysts in two biomass reactions: aqueous phase glycerol hydrodeoxygenation and furfural hydrogenation. Both reactions were conducted with an excessive amount of hydrogen present, suggesting that the Pt site blockage by strong H₂ adsorption is likely to affect Pt reactivity. Therefore, Pt overlayer catalysts with reduced adsorption strength are expected to show higher catalytic reactivity in both reactions compared to pure Pt.

Monometallic catalysts and non-structured alloys were prepared by incipient wetness impregnation. The directed deposition synthesis technique was used to synthesize all overlayer catalysts by selectively depositing the overlayer (platinum) metal on the base metal without it depositing onto the support surface [2]. H₂ chemisorption and ethylene hydrogenation were used to studied the adsorption strength of all synthesized overlayer catalysts compared to pure Pt. Hydrogen isotherms at various temperatures were measured and hydrogen heats of adsorption were calculated using the Clausius-Clapeyron equation. All overlayer catalysts showed a lower hydrogen heat of adsorption compared to pure Pt. Ethylene hydrogenation reactivity, which is a strong function of H₂ surface coverage and H₂ adsorption strength, was also tested as a descriptor reaction. All platinum overlayer catalysts showed reduced turnover frequencies compared to pure platinum, implying lower hydrogen surface coverage and weaker hydrogen adsorption. To conclude the H₂ chemisorption and ethylene hydrogenation results, all overlayer catalysts showed decreased hydrogen adsorption strength, consistent with previous computation predictions [3].

Aqueous phase hydrodeoxygenation of glycerol using Ni@Pt, Co@Pt, and Ir@Pt supported on silica-alumina showed enhanced TOF in glycerol conversion and hydrocarbon production rate 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 exposed by reducing the adsorption strength of hydrogen or carbon monoxide. In terms of selectivity, Ni@Pt and Co@Pt showed higher selectivity toward C-C bond cleavage, while Ir@Pt showed higher selectivity toward C-O bond cleavage. In furfural hydrogenation, two silica-supported overlayer catalysts, Cu@Pt and Ni@Pt, showed higher TOF in furfural conversion compared to pure Pt and corresponding parent metals. With Cu@Pt, both high selectivity toward furfuryl alcohol and high reactivity in furfural conversion were achieved at the same time, indicating the advantage of overlayer catalysts in furfural hydrogenation reactions.

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. Biomass reaction results showed that the reactivity has been enhanced with overlayer catalysts, likely due to the weakened hydrogen or carbon monoxide adsorption and exposing more surface active sites. This demonstrated the viability of preparing overlayer catalysts aiming at efficient and effective biomass conversion.

[1] J.G. Chen, C.A. Menning, M.B. Zellner, Surface Science Reports 63 (2008) 201-254.

[2] 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.

[3] E. Christoffersen, P. Liu, A. Ruban, H.L. Skriver, J.K. Norskov, Journal of Catalysis 199 (2001) 123-131.