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Mesoporous Graphitic Carbon Nitrides Supported Noble Metal Nanoparticles for Green Catalysis under Visible Light

Final Report Summary - NOMGCNP (Mesoporous Graphitic Carbon Nitrides Supported Noble Metal Nanoparticles for Green Catalysis under Visible Light)

The EU funded research project NOMGCNP was focused on the production of the chemical feedstocks by employing environmental-friendly and sustainable procedures. The improvements in the design of novel heterogeneous catalysts should both meet the increasing demands for chemicals from industry and simultaneously solve the incoming energy and environmental problems. The objective of this project is to develop highly efficient heterogeneous catalysts for catalyzing organic reactions.
Firstly, mesoporous carbon nitride (C3N4) composed of covalent bonding was used for supporting noble metal nanoparticles. Mesoporous C3N4 was synthesized by nanocasting mesoporous silica materials with cyanamide. Then, the noble metal nanoparticles supported on mesoporous C3N4 (M@m-C3N4, M = Au, Pd, or Au-Pd) were prepared by impregnating the as-prepared mesoporous C3N4 with the solutions of metal precursors, such as HAuCl4 and Pd(NO3)2. Because of large amount of Lewis and Brønsted basic sites existing in mesoporous C3N4, the strong acid-base interaction between acidic precursors of noble metals and basic sites on mesoporous C3N4 surfaces most likely drives the facile adsorption of noble metal precursors onto the surface of mesoporous C3N4, leading to noble metal nanocrystals formed by the reduction of NaBH4. The Au-Pd nanoparticles on mesoporous C3N4 demonstrating a medium activity for benzyl alcohol oxidation. However, it was found that C3N4 was not stable during the reaction at high temperature (120 °C) possibly due to the hydrolysis. After the reaction, it was difficult to separate the catalyst from the reaction mixture by high-speed centrifugation.
Thus, graphene oxide (GO) sheets, considering as surfactant molecules plenty of oxygen-containing groups, were found to be excellent inorganic stabilizer for immobilizing Au-Pd nanoparticles in place of general organic molecules such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP). We demonstrated the successful formation of highly accessible and well dispersed Au-Pd nanoparticles, stabilised with two-dimensional GO sheets, that itself is dispersed by intercalated titania particles to form ternary hybrid catalysts. The particle size distribution can be adjusted from 4.6 to 3.4 nm by varying the GO-to-metal mass ratio of 1 to 1/6 in the composite catalyst. The addition of titania efficiently hinders the stacking and agglomeration of the supported metal on GO sheets, facilitating diffusion of oxygen and reactants to the catalyst surface. This Au-Pd/GO/titania “ternary’ catalyst has been tested for the selective oxidation of a range of alcohols including benzyl alcohol, cinnamyl alcohol, 1-phenylethanol, cyclohexanol, and 1-octanol. The resulting optimised catalyst exhibits a comparable activity to a sol-immobilised derived catalyst where the metal nanoparticles are stabilized by PVA ligands, with the GO-stabilized hybrid catalyst on TiO2 having enhanced stability characteristics. CO oxidation measurements have provided an insight into the strong metal-GO interactions in this system. Also the existence of GO sheets that stabilized Au-Pd nanoparticles played important roles in preventing the catalytic sites from loss of activity.
Moreover, we also investigated the catalytic activity of GO-stabilized Au-Pd on TiO2 for direct synthesis of H2O2 from and H2 and O2. The reactions were performed at 2 °C using non-explosive dilute H2/O2 mixtures with CO2 as a diluent and using methanol/water as a solvent. The trend for the activities of the Au-Pd catalysts in H2O2 synthesis was observed similar to that in benzyl alcohol oxidation. The AuPd-PVA/TiO2 gave an activity of 91 mol H2O2 kgcat.-1 h-1 as determined after 30 minutes of reaction. The 4% GO stabilized Au-Pd NPs on TiO2 exhibited the better H2O2 productivity of 110 mol kgcat.-1 h-1, significantly higher than AuPd-PVA/TiO2. GO alone as support for the Au-Pd NPs (1%AuPd/GO) only afforded very poor yield of 22 mol H2O2 kgcat.-1 h-1 and pure GO is seldom active for this reaction.
To increase the catalytic activity of the supported Au-Pd nanoparticles, the more active site should be exposed. Thus, hierarchically meso- and microporous carbide-derived-carbons (CDCs) were prepared by the chlorination of the corresponding mesoporous silicon carbides to etch off the silicon element. The resulting CDCs have very high surface areas of over 2000 m2/g, and then were used as supports as PVA-stabilized Au-Pd nanoparticles which demonstrated very excellent activity for the oxidation of benzyl alcohol, more than 100% higher than the commercially available active-carbon-immobilized Au-Pd nanoparticles.
In conclusion, we have shown that Au-Pd alloy NPs can be stabilized on various supports including mesoporous C3N4, the GO/TiO2 composite, and high-surface-area CDCs. The high activity and stability of Au-Pd/GO/TiO2 makes it the ideal heterogeneous catalysts in the oxidation of a wide range of alcohols and direct synthesis of H2O2 from H2 and O2.