Goldhas long been known for its chemical inertness. However, when goldnanoparticles are combined with titanium dioxide (TiO2), they will have theirattributes altered and promote a variety of catalytic reactions. It is widelyaccepted that the interface between gold (Au) and a TiO2 plays a criticalrole in heterogeneous catalysis. Little is known about the dynamic change ofthe Au-TiO2 interface in real time, thereby rendering it challenging totune its intrinsic microstructure with atomic precision during catalyticreactions.

Ithas been a time-honored dream of the scientific community to open the “blackbox” of catalytic reactions and see how catalysis takes place. In the past fiveyears, Prof. WANG Yong from the Zhejiang University Center of ElectronMicroscopy has conducted collaborative research in this field with GAO Yi fromthe CAS Shanghai Advanced Research Institute, and Prof. Wagner and Dr. Hansenfrom the Technical University of Denmark. They first observed that Au NPsstrongly anchored on TiO2 (001) surfaces rotate epitaxially during COoxidation using environmental transmission electron microscopy (ETEM).Furthermore, through control of the interfacial O2 by adjusting thereaction 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 atomicprecision during CO oxidation” in the journal Science.

Rotationof the catalyst

Goldparticles loaded on the surface of TiO2 are an important catalyst forconverting CO to CO2 and a common combination in research into industrialcatalysis.

Theteam from Zhejiang University did research into catalytic reactions with theirexpertise in in situ environmental electron microscopy and clearly observed thewhole catalytic process at the atomic level. What does this nanocatalyst looklike? “Gold nanoparticles can be anchored onto TiO2 substrates like amagnet,” WANG Yong said.

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

Whatis the challenging part of seeing through the “black box”? “Experiments areaffected 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.

“Toperform these experiments, we need to fabricate smooth Au-TiO2 interfacesat an atomic level,” said WANG Yong, “In this way, we can achieve themaneuverable rotation of gold nanoparticles. In addition, it is of immenseimportance to find the right observation angle, which can enable us to see thelattice arrangement clearly and describe the phenomenon in great detail.”

Therotation of the catalyst observed by the Zhejiang University research team wasonce thought to be impossible. As one of the reviewers mentioned, “the rotationof a whole particle is incredible.”

Thisis because gold nanoparticles and titanium dioxide are chemically bonded, thusbecoming so firmly “welded” (with epitaxial relationships) that they will stayput even when bombarded by high-energy electron beams.

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

Howto 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 willbe ‘held up’. We have been working in close partnership with the research teamled by GAO Yi. After a series of theoretical calculations, we discovered thatwhen CO was injected into the system to trigger a catalytic reaction with O2,some of the interface O2 would be consumed, thus making the supportunsteady and gold particles rotate easily. When CO was not injected, theinterface O2 would be replenished and gold particles would return to theoriginal position.”

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

Frontierresearch and its application

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

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

“Indoing scientific research, we should not only focus on the frontiers of theworld, find new phenomena and discover new laws, but also make contributions tothe development of national economy. I hope that our scientific discovery willpromote the development of efficient and stable catalysts for the well-being ofmankind,” said ZHANG Ze.