"Our work goes against the general thinking that noble metal nanoparticles are usually immobilized on polymer, carbon and molecular sieves," says Ge Wang, a researcher at the University of Science and Technology Beijing. She explains that previous efforts to immobilize catalytic nanoparticles have not effectively protected the nanoparticles from leaching nor enhanced the movement of reactants and products to improve reaction rates.

The multicomponent structure developed by Ge and her team deals with several issues that have plagued catalysis in one versatile design. "Our catalyst has a large surface area and an adjustable shell layer and it is tunable, so we can use it for other reactions," says Ge.

One design many functions

The researchers set out to design a catalyst that met a wide range of criteria: huge surface area; permeable structure to allow exposure to the reactants; flexibility and robustness; reusability; and less aggregation. They began by synthesizing Fe3O4 nanoparticles for the structure core so that the catalyst could be easily collected using an external magnetic field. The core was then coated with a layer of silica (SiO2) and a layer of titania (TiO2).

Hydrothermal treatment removed the SiO2 layer forming double-shelled urchin-shaped structures with large surface areas for supporting 5 nm catalytic gold nanoparticles, both on the surface as well as inside the structure. The structure also allows transport of the reactants while preventing coalescence of the catalytic nanoparticles.

"Our structure can be thought of as a ball of wool, where you can load lots of nanoparticles on the fibre while the whole structure is still at the nanoscale," says Ge. "Both the size of the cavity and the length of the fibre are adjustable."

Gaining a practical advantage

While developing the initial design was far from trivial, there were additional challenges in practically implementing it. The preparation conditions in general need to be carefully controlled as they can determine the level of loading and dispersion of the gold nanoparticles on the structures. They also ensure the magnetic properties of the Fe3O4 core are maintained.

"Now we can repeat the process again and again because we spent a lot of time making sure of all the parameters required," says Ge. "Subtle changes in many of the parameters can easily convert it to something else."

Vivek Polshettiwar, who was not involved in the current research, described the work as "another good example of the use of magnetically recoverable catalysts to make the process sustainable". Polshettiwar works on similar research as professor in the Nano-Catalysis Laboratory at the Tata Institute of Fundamental Research in India.

"Use of an urchin-like support structure to activate the catalytic sites is a novel concept,” says Polshettiwar. “I also feel that due to the presence of anatase TiO2 in this catalyst system, it may show interesting photocatalytic activity."

Next the researchers plan to optimize the integrated functions of the catalyst for potential industrial applications.

"We love this support and want to immobilize two or three kinds of catalytic nanoparticles or an alloy at a time so that we can achieve one-pot reactions with it," Ge adds.

Full details of this research are reported in Nanotechnology 26 095601.