278676 Bimetallic Gold-Platinum Nanocatalysts in WATER-GAS Shift
The water gas shift (WGS) reaction plays an important role for hydrogen production from fossil and renewable resources. As an exothermal reaction, it is limited by thermodynamic equilibrium at high temperature and by slow kinetics at low temperatures. In order to achieve a high-equilibrium conversion and overcome slow kinetics, multifunctional catalysts for WGS have been widely studied. Au-based catalysts in particular have recently emerged as promising for low-temperature WGS, but the limited stability of Au is a major concern for technical application of these catalysts.
We have recently demonstrated that alloying of metal particles can result in highly active nanocatalysts with exceptional high-temperature stability. In those studies, metals with essentially complete miscibility across the entire range of compositions were utilized. In the present study, we evaluated the extension of this principle onto catalysts with a wide miscibility gap by alloying Au nanoparticles with Pt. Both metals are known to show good WGS activity, but Pt has a much higher melting point and hence better thermal stability than Au.
AuPt bimetallic nanoparticles (NPs) were prepared by reducing polyvinylpyrrolidone (PVP) protected Au and Pt precursors with sodium borohydride in aqueous solution. Despite the wide miscibility gap between the two (bulk) metals, we were able to form well-controlled, homogeneous bimetallic NPs over a broad range of Pt:Au ratios (1:1, 1:3, 1:9). The as-synthesized NPs were then deposited onto ceria and silica as supports to give catalysts with ~1wt% metal. The materials were characterized using a range of techniques, including XRD, TEM, HRTEM, and UV-Vis, and then evaluated with regard to thermal stability during calcination in air. We found that the bimetallic NPs segregated into two separate groups of NPs during heat treatment on both supports. However, while AuPt/SiO2 phase-separates into the thermodynamically stable Au-rich and Pt-rich phases already at T>500oC, the bimetallic NPs deposited onto ceria completely phase-separated into pure-metal NPs during deposition, and only fused to form the thermodynamically stable phases at T ~900oC. Clearly, metal-support interactions strongly dominate the behavior of this bimetallic system. Fixed-bed reactor experiments further demonstrate that the WGS activity of these catalysts correlates closely with the phase stability of the bimetallic nanoparticles. The WGS activity of the catalyst generally increased with increasing Pt content and the stability reflects the trends observed in the thermal stability studies.
Overall, our results demonstrate that, while our previously suggested principle of catalyst stabilization via alloying with a higher melting-point component may hold even for systems with a miscibility gap, metal-support interactions are critical in the consideration of materials stability and can even dictate catalyst stability.