Understanding the role of the surfaces and internal structure of the nanocrystal will help researchers develop more effective and efficient catalysts for many important catalytic processes. 

A catalyst is a substance that can be added to a reaction to increase the reaction rate without being consumed in the process. They typically speed up the reaction by reducing the energy needed to activate the reaction or by changing the mechanism by which the reaction occurs. Catalysis is one of the pillars of the chemical industry, so developing effective and efficient catalysts for a wide range of uses is crucial.

Prof. Kyriaki Polychronopoulou, Professor of Mechanical Engineering and Director of the Khalifa University Center of Catalysis and Separation (CeCaS), used an advanced form of imaging to analyze the structure and electronic properties of an engineered nanoparticle that is considered one of the emerging candidates for use in catalysis.

With Dr. Yasser Al Wahedi, Assistant Professor, Dr. Vijay Wadi, Research Scientist, Xinnan Lu, and Marios Katsiotis, all from Khalifa University, the findings were published in. The Khalifa University team collaborated with researchers from Stockholm University in Sweden, the Greek National Center for Scientific Research, and the Korean Electron Microscopy Research Center.

This research was also selected by the editors at Nature Communications to be featured in an Editors’ Highlights webpage of recent research called ‘Materials science and chemistry.’ The Editors’ Highlights pages showcase the 50 best papers recently published in a particular area of science.

“Nanocrystalline materials have been a hot research topic thanks to their use in many important applications from catalysis, to energy conversion and storage, and drug delivery,” Prof. Polychronopoulou said.

This is particularly useful in catalysis, as scaling down the particle size increases the number of sites available for the reactions to take place, but also modifies the material’s electronic properties. Additionally, catalytic reactivity and selectivity can be enhanced by modifying the arrangement of the surface atoms.

The crystals in these materials must be grown through chemical reactions to create the desired structures, with the atoms, molecules and ions assembling into a crystal structure one after another on the growth surface. Once created, these materials are approximately half crystal and half interface, ready for use in many applications. For Prof. Polychronopoulou, that use is catalysis.  

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However, the stability of Ni2P nanocrystals depends on their experimental synthesis conditions, and this dependence is not well understood. Prof. Polychronopoulou’s research found that when there is excess phosphorous during the synthesis, the nanoparticles come out with hexagonal rod-like shapes.

The underlying symmetry of the resulting crystals can be seen in the facets that appear on the surface. Facets are flat surfaces on geometric shapes: think of gemstones, which commonly have facets cut into them to improve their appearance by allowing them to reflect light. In grown crystals, the facets are a consequence of the material and the surface energy, as well as the general conditions under which the crystal formed.

An inherent challenge in using any material for catalysis is how to access the catalytic sites generally confined inside the structure. In producing Ni2P crystals, Prof. Polychronopoulou’s research combined chemistry with calculations to define the reaction parameters to grow the ideal crystal with the predicted facets and electronic structure.

“It is extremely difficult to manufacture nanostructured materials,” Prof. Polychronopoulou said. “Using advanced calculations, we predicted the structure of the crystals and experimentally verified the crystal facets and structure using nanocrystallography.”

Nanocrystallography is a technique used to analyze the diffraction patterns of a crystal targeted by a beam of electrons. After studying the nickel phosphide nanocrystals that Prof. Polychronopoulou grew using nanocrystallography, her team found that the nanocrystals comprised a variety of surfaces, with three primary facets exposed.

The team then used another advanced technique, known as solid-state nuclear magnetic resonance imaging, to probe the nanocrystal further and determine the distinct surface facets, while also experimentally proving that their calculations and predictions were correct.

Solid-state nuclear magnetic resonance spectroscopy is an atomic-level method to determine chemical structure, 3D structure, and dynamics of solids. It is sensitive to the structure and electronic environment at the atomic scale, and is able to distinguish between the surface facets and the interior of the nanoparticles.

“This is the first time that facet analysis of a transition metal nano-sized catalyst and the relevant electronic changes were experimentally verified, demonstrating that solid-state nuclear magnetic resonance nanocrystallography is an emerging tool in the study of metal nanocatalysts,” Prof. Polychronopoulou said. 

Jade Sterling
Science Writer
23 September 2021

The post Looking Inside Nanocrystals with Advanced Imaging Techniques to Create New Catalysts appeared first on Khalifa University.

A team of researchers from Khalifa University has discovered an easy, low-cost and sustainable way to make catalysts that can split oxygen molecules from water, and in turn, produce hydrogen for energy storage and clean fuel applications.

The new catalyst, which is made of electrodeposited metallic elements cerium, nickel and iron, can split oxygen from water (called the Oxygen Evolution Reaction, or OER) at a rate that is two times more efficient than conventional catalysts, which are primarily made from noble metal oxides.

The collaborative team includes KU Post-Doctoral Researcher Dr. Ranjith Bose, KU Professor of Chemical Engineering Dr. Akram Alfantazi, Dr. Dhinesh Babu Velusamy from KAUST in Saudi Arabia, and Prof. Hyun-Seok Kim and Dr. K. Karuppasamy, both from Dongguk University in Seoul. The results of the team’s work was published in in August 2019.

Traditional catalysts are made via a solution-based process, which can pose serious issues related to the catalyst efficacy. With a solution-based process, engineers cannot control the size of nanomaterials that are used, or control the growth of the crystals used to make the catalyst. The solution-based process is not recommended for industrial applications due to the complicated synthesis procedures, thus limiting the use of catalysts made this way to small-scale applications.

The researchers developed an alternative method to synthesizing catalysts using an electrodeposition technique. Electrodeposition is the process of coating an ultrathin layer of one metal on top of a different metal to modify its surface properties. It is a simple process that can be easily scaled up for industrial applications.

“Electrodeposition is low cost and offers high controllability, as well as compatibility with nano-scale features. Furthermore, it can be performed at room-temperature,” said Dr. Ranjith.

However, electrodeposition for synthesizing metallic catalysts also has limitations. Previous studies have reported that a flat substrate, such as the ultrathin metallic layer coating, results in limited available active sites – or places where the catalytic “action” happens. This is because only the outermost electrons of the catalyst substrate are in contact with the electrolyte – the electrically conducting solution that interfaces with the catalyst to complete the reaction. More robust reactivity depends on a larger surface area, and flat 2D structures don’t offer high surface area.

To overcome these obstacles, Prof. Akram’s team layered the ultrathin metallic composites on a 3D foam structure, giving their catalysts a larger surface area, which translated into more active sites and better catalytic performance.

“We developed a catalyst made of a 3D nickel foam core, coated with an ultrathin layer of cerium oxide and nickel-iron hydroxide. This unique design combines the features of cerium oxide and nickel-iron hydroxide, which have outstanding mass-transfer properties, enhanced active sites, and energetics for OER, with the mechanical robustness of the 3D nickel foam core,” Dr. Ranjith explained.

The cerium oxide and nickel-iron hydroxide was synthesized by a two-step process that started with the preparation of nickel-iron oxide by electrodeposition, followed by anodic electrolysis – a technique that uses an electric current – to introduce the cerium oxide into the nickel-iron coated film.

The resulting composite catalyst exhibited excellent OER activity with a lower overpotential – the difference between the applied and thermodynamic potentials of a given electrochemical reaction – and higher electrocatalytic activity.

The research is an important contribution to the selection, production and optimization of electrocatalytic materials that can be leveraged to improve the efficiency of hydrogen electrolytic production.

“The results achieved by our catalysts undoubtedly represent an important milestone toward the development of efficient catalysts that use electricity to break water into hydrogen and oxygen to further reduce the operational costs of hydrogen production,” Dr. Ranjith said.

The work being done through this project and others reflects KU’s commitment to supporting the UAE’s clean energy transformation.

Erica Solomon
Senior Editor
3 November 2019

The post A New Approach to Synthesizing Catalysts appeared first on Khalifa University.