Heterogeneous catalyst preparation in ionic liquids: Titania supported gold nanoparticles

• Use of ionic liquids as solvent for the synthesis of Au/TiO2 catalysts through a colloidal route.
• Control of Au NPs size and strength of metal support interaction with different ionic liquids.
• Surface decoration of Au NPs highly depending on ionic liquids composition.
• Selective hydrogenation is controlled by the surface composition.

This work reports the use of ionic liquids (ILs) as solvent for the synthesis of Au/TiO2 heterogeneous catalysts. It is shown that the versatility of the physico-chemical properties of ILs makes them interesting solvents for a broad control of supported metal nanoparticles (NPs) size and for a fine-tuning of metal–support interaction. Synthesis and deposition of Au NPs on TiO2 are carried out through a colloidal route in imidazolium-based ILs (1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) and trifluoromethanesulfonate (BMIMOTf)) and in a Deep Eutectic Solvent (DES, choline chloride and urea). The stabilization of the dispersed NPs and their deposition on an oxide carrier is investigated in pure ILs and in the presence of extra stabilizing agents (1-methylimidazole or polyvinylpyrrolidone). In the absence of TiO2, the NPs stability varies with the composition of the IL (BMIMOTf > BMIMPF6 > DES) and is critically depending on the addition of an extra stabilizing agent (SA). Conversely, when gold NPs are synthesized in the presence of TiO2 in imidazolium ILs, the addition of a SA is no longer required since the oxide support acts as a stabilizer for the gold colloids and prevents aggregation of NPs. A detailed XPS investigation of the surface composition of the TiO2-supported gold catalysts shows that the cationic part of the imidazolium ILs on the one hand and urea in DES on the other hand are co-adsorbed with gold NPs upon deposition on TiO2, probably decorating and stabilizing the Au NPs. The fate of the anionic part of the ILs (PF6−, OTf− and Cl−) is highly depending on the type of IL. Limited surface adsorption is reported for triflate (BMIMOTf) and chloride anions (DES), the latter being fully removed upon calcination, while PF6− (BMIMPF6) remains adsorbed on the surface during Au NPs deposition and leads to POxδ− and F− surface species upon calcination. The surface composition controls the intrinsic catalytic activity (TOF) in the selective hydrogenation of butadiene. It is shown that the surface doping of supported Au NPs with P and F anionic deposits (synthesized in BMIMPF6) has a promoting effect for the intrinsic catalytic activity.

Figure optionsDownload high-quality image (153 K)Download as PowerPoint slide