Why size matters for gold as a catalyst

September 6, 2018, Environmental Molecular Sciences Laboratory
Why size matters for gold as a catalyst
Dynamic structural changes of various gold nanostructures supported on CeO2(111) upon exposure to the reactant gases. (A–C) Model (A) and HRTEM images (B and C) of a 4-nm Au NP with ∼1,000 atoms; the arrow in C indicates the Au(100) surface reconstruction. (D–F) The <2-nm Au NP with 100∼200 atoms; the insets show corresponding fast Fourier transform of the Au NP. (G–I) The ∼2-nm SL with ∼40 atoms. (Scale bars: B, E, and H, 1 nm; also apply to C, F, and I, respectively). Credit: Environmental Molecular Sciences Laboratory

Gold is the noblest metal—the most resistant to oxidation. However, nano-size gold has a unique ability to perform as a catalyst, even at low temperatures. The underlying mechanism for this size-dependent change in properties has puzzled scientists since the phenomenon was discovered in the late 1980s.

A team of researchers, including Yingge Du, Chongmin Wang, and Jun Li of Pacific Northwest National Laboratory, set out to address this question using state-of-the-art aberration-corrected environmental . Their work has revealed new insights about the exceptional catalytic properties of ultrasmall gold particles when they are exposed to reactant gas. Details of their findings have been published in a Proceedings of the National Academy of Sciences paper titled "Size-dependent dynamic structures of supported in CO oxidation reaction condition."

In situ studies of ultrasmall gold clusters in a reactive environment have been lacking, making it difficult to verify the origin of the size effect in catalysis. This study used in situ transmission electron microscopy to reveal that gold nanoparticles, when isolated down to a critical size, undergo dynamic structural changes under the catalytic working conditions, and all gold atoms in a cluster can be activated to promote the catalytic reactions. Evidence of the transformation at this ultrasmall scale can only be obtained through in situ and operando characterization.

This finding challenges the classic thinking that gold as a retains the same structure between static and catalytic conditions. In fact, these otherwise stable ultrasmall gold clusters can transform to a metastable phase. The results also suggest that the nanocatalysts may work as dynamically generated single-atom catalysts, a concept that has aroused remarkable recent interest in the catalysis community.

"The detailed structure-stability-property relationships established here can lead to a paradigm shift in designing atom-efficient catalysts," Wang says.

Ultrasmall gold clusters supported on single crystalline ceria [CeO2(111)] thin films were exposed to reactant carbon monoxide and oxygen (CO + O2) gas using in situ environmental electron microscopy combined with computational modeling and ab initio molecular-dynamics simulations. The researchers observed different structural responses to the reactant gas depending on the size of the nanoparticle. In its ultrasmall (tens of atoms) form, a gold nanoparticle displayed dynamic structural changes under the catalytic working condition; the intrinsic structure was lost and the clusters became disordered, while dynamic low-coordinated atoms formed at the surface. The ab initio molecular-dynamics simulations confirmed these observations and further revealed that the generation of dynamic low-coordinated atoms via gold-carbonyl species could act as dynamic active centers for CO oxidation.

For slightly larger nanoparticles (up to a few hundred atoms), the otherwise stable face-centered cubic structure transformed to a disordered structure under CO and O2 exposure, which became liquid-like and simultaneously formed low-coordinated gold atoms.

In contrast, the larger nanoparticles maintained their structure while undergoing localized surface reconstructions.

The size-dependent structural response to reactant gas, especially generation of dynamic low-coordinated Au atoms in ultrasmall gold particles, can effectively boost the reaction through facile transport of the CO to reaction sites, hence delineating a root cause for why small size matters for gold catalysis and why larger gold nanoparticles tend to become inert.

The team believes more work should be done to explore how the transform from their ordered structure to a disordered , and to understand whether this size effect also exists in other catalytic systems. They are seeking additional funding and resources to support expanded research in this area.

Explore further: How do very small particles behave at very high temperatures?

More information: Yang He et al. Size-dependent dynamic structures of supported gold nanoparticles in CO oxidation reaction condition, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073/pnas.1800262115

Related Stories

Scientists' gold discovery sheds light on catalysis

August 13, 2012

(Phys.org) -- A physicist at the University of York has played a key role in international research which has made an important advance in establishing the catalytic properties of gold at a nano level.

Scientists uncover secret to gold's catalytic powers

September 27, 2016

Settling a decades-long debate, new research conclusively shows that a hierarchy of active species exists in gold on iron oxide catalysis designed for low temperature carbon monoxide oxidation; Nanoparticles, sub-nanometer ...

Scientists create gold nanoparticles in water

April 19, 2018

An experiment that, by design, was not supposed to turn up anything of note instead produced a "bewildering" surprise, according to the Stanford scientists who made the discovery: a new way of creating gold nanoparticles ...

Recommended for you

Matter waves and quantum splinters

March 25, 2019

Physicists in the United States, Austria and Brazil have shown that shaking ultracold Bose-Einstein condensates (BECs) can cause them to either divide into uniform segments or shatter into unpredictable splinters, depending ...

How tree diversity regulates invading forest pests

March 25, 2019

A national-scale study of U.S. forests found strong relationships between the diversity of native tree species and the number of nonnative pests that pose economic and ecological threats to the nation's forests.

Study suggests trees are crucial to the future of our cities

March 25, 2019

The shade of a single tree can provide welcome relief from the hot summer sun. But when that single tree is part of a small forest, it creates a profound cooling effect. According to a study published today in the Proceedings ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.