Research PaperVariability of particle configurations achievable by 2-nozzle flame syntheses of the Au-Pd-TiO2 system and their catalytic behaviors in the selective hydrogenation of acetylene

- Au-Pd-TiO2 system with various particle configurations were synthesized by 2-nozzle FSP for the first time.
- Composition, metal-metal and metal-support interaction, location of Pd (or AuPd) on TiO2 were varied.
- Fraction of bimetallic alloy particles on TiO2 was maximized by feeding Au and Pd precursors to the same nozzle and the Ti precursor to the other one.
- The enhanced hydrogenation activity and high selectivity to ethylene (>95%) is attributed to the presence of bimetallic AuPd alloy particles.

Catalysts with Au and Pd supported on TiO2 (Au:Pd 1:1 wt/wt%) were prepared by 1- and 2-nozzle flame spray pyrolysis (FSP). The 2-nozzle configuration allowed to synthesize various particle configurations by separate or co-feeding of the metal precursor solutions to the two nozzles. For the Au-Pd/TiO2 system, four different catalyst particle configurations were investigated: “TiO2 + AuPd”, “Pd/TiO2 +Au”, “Au/TiO2 +Pd”, and “Pd/TiO2 + Au/TiO2”, where + separates the corresponding precursor solutions fed to the two nozzles. There were no significant differences in the specific surface areas and the average TiO2 crystallite sizes of the catalysts (100 m2/g and 16-17 nm, respectively) with the exception of “Pd/TiO2 +Au/TiO2”, which exhibited larger surface area and smaller crystallite size (152 m2/g, 12 nm) due to halving of the Ti precursor concentration in each nozzle. As revealed by CO chemisorption, XPS, and STEM-EDX results, the catalyst properties varied largely in terms of bimetallic AuPd particle compositions, the interaction between metal-metal and metal-support, and the location of Pd (or AuPd) on the TiO2. Among the catalysts studied, “TiO2 + AuPd” prepared with the 2-nozzle system exhibited the highest conversion of acetylene (∼50%) at 40 °C with high selectivity to ethylene ( > 95%). Co-feeding the noble metal precursors together with the Ti precursor afforded less active catalysts due to the formation of Ti-O species partially covering the most active bimetallic AuPd particles. Compared to the commercially available acetylene hydrogenation catalyst and the AuPd/TiO2 prepared by conventional co-impregnation and deposition-precipitation, all the FSP-AuPd/TiO2 catalysts showed superior performances under the reaction conditions used.

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