(779c) Influence of Sn and Bi on Pt-Catalyzed Aqueous Phase Oxidation of 1,6-Hexanediol
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 SnOx 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-SnOx 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 (TOFH/TOFD = 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.