(779c) Influence of Sn and Bi on Pt-Catalyzed Aqueous Phase Oxidation of 1,6-Hexanediol

In this study, a series of carbon-supported PtM (M = Sn, Bi) bimetallic catalysts with various M/Pt ratios was prepared and evaluated in the oxidation of HDO by dioxygen in the aqueous phase. Extensive characterization of the catalysts before and after reaction as well as detailed kinetic measurements (including a kinetic isotope effect) were performed to enable correlation of catalyst performance to structure and composition.

To prepare the PtSn/C catalysts, the Pt and Sn precursors were reduced simultaneously by NaBH₄ in a slurry of activated carbon. The catalysts were then treated in H₂ at 673 K to induce the formation of PtSn alloy. The PtBi/C catalysts were prepared by selectively reducing Bi(NO₃)₃ in an aqueous solution onto supported Pt nanoparticles by H₂. The HDO oxidation reaction was performed under reaction conditions that eliminated artifacts from mass transfer limitations.

The catalysts collected after synthesis as well as those collected after pretreatment and reaction were extensively characterized by N₂-physisorption, H₂-chemisorption, X-ray diffraction, scanning electron microscopy, (scanning) transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma – atomic emission spectroscopy and X-ray absorption spectroscopy.

Based on the extensive characterization of the fresh and used catalysts, we propose a model for the structural evolution of PtSn and PtBi catalysts under pretreatment and reaction conditions. A PtSn/C catalyst prepared by NaBH₄ reduction was composed of SnO_x species that were well dispersed on the carbon support and the Pt nanoparticles, with negligible formation of Pt-Sn alloy. After the H₂ treatment at 673 K, Pt-Sn alloy particles were observed on the sample, which could be explained by the enhanced interaction between adjacent Pt and Sn species in H₂under high temperature. The oxidation of HDO in liquid water over the alloyed particles facilitated a phase segregation of Pt and Sn. A significant structural evolution of PtBi/C was also observed. During catalyst preparation, Bi was selectively deposited on Pt particles in the outer layer of the carbon support. However, under conditions of pretreatment and reaction, substantial migration of Bi species on the carbon support was observed.

The impact of adding Sn to Pt on the catalysis of HDO oxidation depended on the structure of the catalyst. The results suggest that the Pt-SnO_x interface modestly promotes the oxidation activity whereas alloying Pt with Sn inhibits activity under 1 MPa O₂. The impact of adding Bi to Pt depended on the dioxygen pressure used during HDO oxidation. Under high O₂ pressure, similar activity was observed on Pt/C and PtBi/C, whereas Bi significantly promoted the oxidation activity of Pt under low O₂ pressure. Indeed, the reaction order with respect to O₂ was zero over PtBi/C compared to 0.75 over Pt/C in the range of 0.02 – 0.2 MPa. Rate measurements in D₂O solvent revealed a moderate isotope effect (TOF_H/TOF_D = 1.4) on HDO oxidation over Pt/C under 0.02 MPa O₂ whereas a negligible isotope effect was observed on PtBi/C, indicating that the promotional effect of Bi was likely related to the formation of surface hydroxyl groups from reaction of O₂ and H₂O.

In summary, this work presents evidence for severe restructuring of PtSn and PtBi catalysts during pretreatment and alcohol oxidation in liquid water and demonstrates that the impact of added promoters on Pt depends on catalyst structure and reaction conditions.