Gold has long been known for its chemical inertness. However, when gold nanoparticles are combined with titanium dioxide (TiO2), they will have their attributes altered and promote a variety of catalytic reactions. It is widely accepted that the interface between gold (Au) and a TiO2 plays a critical role in heterogeneous catalysis. Little is known about the dynamic change of the Au-TiO2 interface in real time, thereby rendering it challenging to tune its intrinsic microstructure with atomic precision during catalytic reactions.

It has been a time-honored dream of the scientific community to open the “black box” of catalytic reactions and see how catalysis takes place. In the past five years, Prof. WANG Yong from the Zhejiang University Center of Electron Microscopy has conducted collaborative research in this field with GAO Yi from the CAS Shanghai Advanced Research Institute, and Prof. Wagner and Dr. Hansen from the Technical University of Denmark. They first observed that Au NPs strongly anchored on TiO2 (001) surfaces rotate epitaxially during CO oxidation using environmental transmission electron microscopy (ETEM). Furthermore, through control of the interfacial O2 by adjusting the reaction environment, they achieved in situ manipulation of the active Au-TiO2 interface. Their research findings were published in a research article entitled “In situ manipulation of the active Au-TiO2 interface with atomic precision during CO oxidation” in the journal Science.

Rotation of the catalyst

Gold particles loaded on the surface of TiO2 are an important catalyst for converting CO to CO2 and a common combination in research into industrial catalysis.

The team from Zhejiang University did research into catalytic reactions with their expertise in in situ environmental electron microscopy and clearly observed the whole catalytic process at the atomic level. What does this nanocatalyst look like? “Gold nanoparticles can be anchored onto TiO2 substrates like a magnet,” WANG Yong said.

Researchers discovered two major phenomena for the first time. First, they observed that gold nanoparticles on the surface of TiO2 underwent in-plane (epitaxial) rotation (about 9.5o) during the catalytic oxidation of CO, which was the first visual evidence of the Au-TiO2 interface as an active center through visualization experiments. Second, they observed that gold nanoparticles returned to their original position magically when CO was removed.

What is the challenging part of seeing through the “black box”? “Experiments are affected by a suite of factors, say, the preparation of high-quality samples, the choice of observation angles, and the interference of the electron beam,” said YUAN Wentao, lead author of the paper.

“To perform these experiments, we need to fabricate smooth Au-TiO2 interfaces at an atomic level,” said WANG Yong, “In this way, we can achieve the maneuverable rotation of gold nanoparticles. In addition, it is of immense importance to find the right observation angle, which can enable us to see the lattice arrangement clearly and describe the phenomenon in great detail.”

The rotation of the catalyst observed by the Zhejiang University research team was once thought to be impossible. As one of the reviewers mentioned, “the rotation of a whole particle is incredible.”

This is because gold nanoparticles and titanium dioxide are chemically bonded, thus becoming so firmly “welded” (with epitaxial relationships) that they will stay put even when bombarded by high-energy electron beams.

“The existence of a particular object calls for the minimum state of energy. The minimum amount of energy required for an object also differs in varying environments. Take boiling water for example. Water will be boiled at 100°C at low altitudes whereas it can be boiled at 90°C at high altitudes,” explained ZHANG Ze, a member of the Chinese Academy of Sciences.

How to break the “fixed force” of chemical bond

“Using ‘brutal force’ is far from feasible,” said WANG Yong, “This requires ingenuity. When oxygen is adsorbed at the Au-TiO2 interface, gold nanoparticles will be ‘held up’. We have been working in close partnership with the research team led by GAO Yi. After a series of theoretical calculations, we discovered that when CO was injected into the system to trigger a catalytic reaction with O2, some of the interface O2 would be consumed, thus making the support unsteady and gold particles rotate easily. When CO was not injected, the interface O2 would be replenished and gold particles would return to the original position.”

“This discovery is very intriguing. Catalytic particles stayed in the same position before and after the reaction, but rotated at a certain angle during the reaction. It would be impossible to find this phenomenon but for atomic-level in situ experiments,” said ZHANG Ze.

Frontier research and its application

In experiments, researchers also found that when the temperature reached 500°C, gold particles could reversibly rotate between two angles and thus form two interfacial configurations. If the temperature is lowered to room temperature when the structure displays remarkable catalytic performance, this interfacial structure can be ‘locked’ and display outstanding catalytic efficiency at low temperatures. “This discovery provides new ideas and opens up new horizons for the design of future catalysts,” said WANG Yong, “Moreover, this perspective can be adopted if researchers want to regulate other materials or reactions.”

Meanwhile, the Au-TiO2 catalyst has a positive effect on CO removal, poisoning prevention and environmental protection. It will thus open a new window for the development of more affordable, efficient, secure and stable catalysts.

“In doing scientific research, we should not only focus on the frontiers of the world, find new phenomena and discover new laws, but also make contributions to the development of national economy. I hope that our scientific discovery will promote the development of efficient and stable catalysts for the well-being of mankind,” said ZHANG Ze.