Ion exchange dramatically improves the performance of the CO₂-reducing catalyst

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To produce carbon dioxide (CO2) in methanol (CH3OH), copper (shown in yellow) on a hydride-substituted support accelerates reactions mediated by hydrides and catalyzed by hydrogen atoms (shown in black) from surface-adsorbed formate, HCOO*. Credit: Yang He/ORNL, US Department of Energy

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To produce carbon dioxide (CO2) in methanol (CH3OH), copper (shown in yellow) on a hydride-substituted support accelerates reactions mediated by hydrides and catalyzed by hydrogen atoms (shown in black) from surface-adsorbed formate, HCOO*. Credit: Yang He/ORNL, US Department of Energy

A team of scientists led by the Department of Energy’s Oak Ridge National Laboratory has found an unconventional way to improve catalysts made from more than one material. The solution demonstrates a path to designing catalysts with greater activity, selectivity and stability.

A catalyst normally uses a carrier to stabilize nanometer-sized metal particles that accelerate important chemical reactions. The support, through interactions with the metal particles, also helps create a unique interface with sites that can dramatically improve reaction speed and selectivity. To improve catalytic efficiency, researchers typically try different combinations of metals and supports. The ORNL team instead focused on implanting specific elements right next to metal nanoparticles at their interface with the support to increase catalytic efficiency.

The researchers studied a catalyst that hydrogenates carbon dioxide to make methanol. The copper nanoparticles are supported by barium titanate. In the crystalline carrier, two positively charged ions, or cations, pair with negatively charged ions, or anions. When the team partially extracted oxygen anions from the carrier and implanted hydrogen anions, this ion exchange changed the reaction kinetics and mechanisms and resulted in a threefold yield of methanol.

“Tuneming the anion site of the catalyst support can have a major impact on the metal-support interface, leading to enhanced conversion of waste carbon dioxide into valuable fuels and other chemicals,” said project lead Zili Wu, leader of ORNL’s Surface Chemistry and Catalysis -group.

The research, published in Angewandte Chemie International Edition, it says on the back of the magazine. The findings point to a unique role that hydrogen anions, or hydrides, could play in improving the performance of catalysts that turn carbon dioxide into methanol. Wu’s team was the first to use anion substitution for this purpose. Such catalysts could join the portfolio of technologies aimed at achieving global net-zero carbon dioxide emissions by 2050.

When designing the catalyst, the team chose the support perovskite-barium titanate. It is one of the few materials in which hydrogen anions, which are highly reactive with air or water, can be incorporated to form a stable oxyhydride. Furthermore, the scientists hypothesized that the incorporated hydrogen anions could influence the electronic properties of adjacent copper atoms and participate in the hydrogenation reaction.

“With a perovskite, you can tune not only the cations almost across the periodic table, but also the anion sites,” Wu said. “You have a lot of tuning knobs to understand its structure and catalytic performance.”

The hydrogenation of carbon dioxide to make methanol requires high pressure: more than several dozen times the pressure of the Earth’s atmosphere at sea level. Investigating the catalyst under resting (“in situ”) versus operating (“operando”) conditions required expertise and equipment that is difficult to find outside national laboratories. This reaction has been studied for decades, but its active catalytic sites and mechanisms have remained unclear due to the lack of in situ/operando studies.

“I am very proud that we worked together from different teams to illuminate the underlying mechanism,” said Wu.

“We combined multiple in situ and operando techniques to characterize copper’s structure, support, and interface under reaction conditions,” said ORNL co-author Yuanyuan Li. She uses spectroscopy to reveal the dynamic atomic, chemical and electronic structure of materials under synthesis and reaction conditions. “Copper can change quickly after exposure to air or other environments. It was therefore very important for us to reveal the structure of the catalyst under real working conditions and then correlate that with its performance.”

To reveal the catalyst’s structure under operating conditions, Li and former ORNL postdoctoral fellow Yang He went to the Stanford Synchrotron Radiation Lightsource at the SLAC National Accelerator Laboratory. With Jorge Perez-Aguilar from SLAC in Simon Bare’s laboratory, they used in situ X-ray absorption spectroscopy to reveal the structure of the copper nanoparticles under high-pressure reaction conditions. The researchers collaborated through the Consortium for Operando and Advanced Catalyst Characterization via Electronic Spectroscopy and Structure, or Co-ACCESS.

Back at ORNL’s Center for Nanophase Materials Sciences, a DOE Office of Science user facility, ORNL Corporate Fellow Miaofang Chi and ORNL Postdoctoral Fellow Hwangsun “Sunny” Kim performed scanning transmission electron microscopy to compare the copper structure before and after the chemical reaction.

In addition, ORNL staff scientists Luke Daemen and Yongqiang Cheng performed in situ high-pressure inelastic neutron scattering on the VISION beamline of the Spallation Neutron Source, a DOE Office of Science user facility, to determine the structure of the hydride in the oxyhydride support characterize. Since neutrons are sensitive to lightweight elements, they were used to monitor the hydride structure after reaction at high pressure. It remained stable.

At Vanderbilt University, postdoctoral researcher Ming Lei, together with Professor De-en Jiang, used density functional theory to calculate the electronic structure of the material. The theory-based calculations and experimental results together showed that hydrides on the support directly participated in the hydrogenation of carbon dioxide to make methanol and changed the electronic state of copper to enhance methanol-producing reactions at the interface.

To learn more about the kinetics and mechanism of the chemical reaction, he and ORNL collaborator Felipe Polo-Garzon adapted a technique called steady-state isotopic transient kinetic analysis, or SSITKA, for use under high pressure. They coupled it with an operando high-pressure technique called diffuse reflectance infrared spectroscopy, or DRIFTS.

“We developed the method under real reaction conditions to understand both the reaction kinetics and the mechanisms,” said He, now at DOE’s Pacific Northwest National Laboratory. “That will contribute to the field by filling the gap between environmental and higher pressure studies.”

SSITKA suggested that the hydride-rich perovskite had a higher density of sites that were more active and selective for methanol production. The addition of DRIFTS revealed that a chemical species called formate – carbon dioxide with a hydrogen atom attached – was the key reaction intermediate. DRIFTS-SSITKA also showed that subsequent steps to hydrogenate formate to methanol limit the reaction rate.

Wu and colleagues will then change the reactivity of the hydride in the carrier by changing the composition of the perovskite.

“Then you may be able to further improve the performance of your catalyst,” Wu said. “This approach to anion tuning of catalysts provides a new paradigm for controlling chemical reactions.”

More information:
Yang He et al., Significant roles of surface hydrides in improving the performance of Cu/BaTiO2.8H0.2 catalyst for CO2 hydrogenation to methanol, Angewandte Chemie International Edition (2023). DOI: 10.1002/anie.202313389

Magazine information:
Angewandte Chemie International Edition

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