Behind the Science: Reusable Gold Nanocomposite Catalyst

Dr. Arlette Itken-Fuder, Associate Editor for European Journal of Inorganic Chemistry, talks to Professor Jong Hyeon Lee, The Catholic University of Korea, about his recently published article on a reusable nanocomposite film catalyst based on gold and metal hydroxide nanoparticles.

During the preparation of the catalyst, gold nanoparticles are preferentially deposited on the surface of layered double hydroxide (LDH) nanocrystals that densely cover a glass substrate. One example of these hydroxides is [Mg₄Al₂(OH)₁₂]CO₃·n H₂O. The LDH surface protects the growing gold nanoparticles from agglomeration. The resulting hybrid film is a highly efficient and reusable catalyst for the reduction of p-nitrophenol with NaBH₄ to produce p-aminophenol.

Could you please briefly explain the background and findings of your article?

Gold nanoparticles embedded on solid supports have attracted interest as new heterogeneous catalysts for various organic reactions. To achieve enhanced catalytic activity, supporting materials are essential. They prevent aggregation of the gold nanoparticles during the catalytic process, enable easy separation from the reaction mixture, and thus improve reusability of the catalyst.

Recently, LDH materials have been introduced as promising nanoscopic supports for gold nanoparticles. In the article, we propose oriented thin films of LDH nanocrystals on glass substrates as a catalyst support. Our nanocomposite exhibits enhanced catalytic activity compared to those of other metal oxide supports, because of the size-selective synthesis and strong binding of metal nanoparticles onto the LDH surface.

Consequently, the film catalyst showed superior efficiency and durability without noticeable degradation in its original catalytic activity. Fabricating the LDH-supported films in the form of high-quality functional coatings on cost-effective glass substrates could further improve their usefulness.

What is the main significance of your results?

Several characteristics lead to the excellent catalytic activity and stability of the metal nanoparticles. We induce an electrostatic attraction to form a monolayer of gold nanoparticles on a LDH array without further chemical modifications. The surface features of LDHs are beneficial for interacting with metal complexes and metal nanoparticles. As a result of the strong electrostatic interaction between the opposite surface potentials of the gold and LDH nanoparticles, the gold nanoparticles are tightly anchored on the surfaces of the LDHs.

Why was your attention focused on layered double hydroxide films for the catalyst support?

Immobilization of metal nanoparticles on solid supports requires specific organic stabilizers or toxic chemical linkers, which inevitably reduce their catalytic activity. We used LDH films to overcome this problem. The hydroxide groups and positive surface charges of LDHs provide strong binding affinity to negatively charged metal nanoparticles as well as some solid substrates without chemical modifications.

Using powdery LDH as a supporting material in heterogeneous catalysis is not ideal, as it can induce fast aggregation in solution and partial removal of metal catalysts during catalytic reactions. This causes a significant loss in their catalytic activity. So, we decided to focus on the design and fabrication of this new type of thin film catalyst combining metal nanoparticles and LDH nanocrystals. The film catalyst can be separated easily and reused several times after simply washing with water, which makes it suitable for use in the industry.

How long did this investigation take?

Overall, I have been working together with my coauthor – she was an undergraduate student in the chemistry department and is currently studying in my group – for about one year on the development of the gold/LDH nanocomposite films and the investigation of their catalytic activity. The optimization of the synthesis of the uniform colloidal gold on the LDH film took about eight months, and the in situ investigation of the catalytic performance by UV/Vis spectroscopy took about four months.

Do you have any plans for future work extending from this study?

First of all, the growth mechanism of the gold nanoparticles and the thermodynamic control of their deposition need to be understood in more detail. This will allow the morphological control of gold nanostructures on nanoscopic supporting materials and help to improve their colloidal stability under various reaction conditions.

We are also working to fabricate the gold/LDH film on conducting substrates in order to develop more challenging catalytic systems, for example, electrochemically active redox catalysts. This can be useful in biological catalysis.