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Functionalized carbon-supported Au Nanoparticles for Selective Oxidation Catalysis

Periodic Reporting for period 1 - FANSOC (Functionalized carbon-supported Au Nanoparticles for Selective Oxidation Catalysis)

Reporting period: 2016-03-01 to 2018-02-28

Selective oxidation of organic compounds, such as alcohols and aldehydes, is a highly important transformation traditionally performed in industry using either stoichiometric oxidants, such as Mn- and Cr-containing salts, or, less frequently, homogeneous catalysts. The stoichiometric oxidants have a strong environmental impact by producing a large amount of waste, often toxic/carcinogenic, while homogeneous catalysts have a recyclability issue due to the difficulty of their separation from the product mixture. The importance of selective oxidation is expected to grow further in the coming years due to emerging applications, such as conversion of renewable sources (biomass) to chemicals and materials.
A more environmentally friendly and economical alternative is oxidations using O2 as an oxidant and heterogeneous catalysts. Specifically, this action investigates gold nanoparticles supported on carbon materials with tailored surface properties. The main objectives include gaining an insight into how the surface characteristics of carbon support materials affect activity, selectivity and stability of supported metal nanoparticles in liquid-phase oxidations of alcohols and aldehydes, as well as understanding whether surface basic groups on carbon support can replace homogeneous base used as a co-catalyst. Better understanding of these relationships can advance targeted design of heterogeneous catalysts which can make oxidation reactions possible under milder conditions with improved reusability of the catalyst and better selectivity. All these can reduce the negative environmental and economic impact of selective oxidation processes.
We started the project with investigating the preparation of gold catalysts using gold colloids and removal or stabilizing polymer (polyvinylpyrrolidone, PVP) from the supported metal nanoparticles. Several ways to efficiently remove PVP from the supported gold nanoparticles without particle size change have been established, such as treatment with NaBH4 in combination with H2O. Furthermore, we obtained some insights into catalyst structure before and after treatments, such as the presence of free PVP on the surface of a catalyst support together with PVP-capped nanoparticles. The free PVP remains adsorbed on the support after treatments and has no effect on the optical and catalytic properties of gold nanoparticles. This work has been reported at the Netherlands' catalysis and chemistry conference in 2017 (oral presentation), and published in ChemCatChem (10.1002/cctc.201701760). The work has also been featured on a journal cover (10.1002/cctc.201800271).
We further investigated surface functionalization of carbon support materials. A range of carbons with different surface chemistries have been prepared and used as supports for gold nanoparticles. Colloidal gold particles have been used to ensure that particles are similar on all supports. We have found that basic surface groups cannot efficiently replace homogeneous base, however the surface properties of carbon support have a profound effect on the catalyst activity, selectivity and stability. As a result of this work we demonstrated that activity and selectivity of gold nanoparticles supported on carbon in the oxidation of 5-hydroxymethylfurfural (HMF) can be controlled by tuning the surface properties of the support. The results of this project have been published in ACS Catalysis (10.1021/acscatal.7b00829) and reported at ACS national meeting in 2016 (oral presentation), North American meeting on catalysis in 2017 (poster presentation), and further expected to be reported at several meetings in 2018.
We have further proceeded with investigating gold catalysts supported on other materials in the oxidation of HMF. In particular, catalysts supported on silica materials with different morphologies have been employed to understand the mechanism of Au particle growth under reaction conditions, and get an insight into the possible ways to combat it since it leads to catalyst deactivation. The results strongly indicated that particle diffusion and coalescence is the dominant mechanism for Au particle growth under reaction conditions, and that supports with narrow pores and ordered structure can be used to prevent particle growth leading to catalyst deactivation. Paper describing results of this work has been submitted to Applied Catalysis A.
Finally, we studied bimetallic AuAg catalysts in HMF oxidation to understand how metal alloying affects catalyst performance. Monometallic Ag is not active in HMF oxidation, whereas Au shows fairly good activity. Addition of Ag to gold has a detrimental effect on the first reaction step but strong beneficial effect on the second reaction step leading to strongly increased yield of FDCA. The observed synergistic effect was attributed to the improved adsorption of the oxygen-rich substrate and intermediates on the surface of bimetallic AuAg nanoparticles. These findings have been reported at EuropaCat 2017 (oral presentation) and the World congress on oxidation catalysis in 2017 (oral presentation). The manuscript on this work is currently in preparation.
The outcomes of the project progress our understanding between catalyst structure and composition and its performance. Novel insights obtained in this project, such as removal of protecting agents from supported metal nanoparticles, the role of the support surface chemistry and morphology, as well as metal nanoparticle composition on the performance of the catalysts in the oxidation reactions, would allow targeted design of improved catalysts. Implementation of improved catalysts will be highly advantageous for both economy and environment. Furthermore, application of improved catalysts to the conversion of renewables into valuable chemicals, such as oxidation of HMF into FDCA studied in this project, will provide a basis for creating green and cost-effective technologies for converting renewable resources into valuable chemicals in the longer term perspective.
Removal of PVP from supported gold nanoparticles
Possible scenarios of how surface groups on support can affect the adsorption of charged reactants