Ion exchange significantly improves CO₂ removing catalyst performance

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 multiple materials. The solution demonstrates a path toward designing catalysts with higher activity, selectivity, and stability.

A catalyst normally uses a support to stabilize nanometer-sized metal particles that accelerate important chemical reactions. The medium, through its interactions with the metal particles, also helps create a unique interface with the sites that can significantly 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 the metal nanoparticles at their interface, with the support needed to increase catalytic efficiency.

Researchers studied a catalyst that hydrogenates carbon dioxide to produce methanol. Its copper nanoparticles are supported by barium titanate. In the crystalline support, two positively charged ions, or cations, associate with negatively charged ions, or anions. When the team extracted partial oxygen anions from the support and implanted hydrogen anions, this ion exchange changed the reaction kinetics and mechanisms and allowed a tripling of the methanol yield.

“Tuning the anion site of the catalytic support can have a significant impact on the metal-support interface, leading to enhanced conversion of waste carbon dioxide into valuable fuels and other chemicals,” said Zili Wu, head project manager and leader of the surface chemistry and catalysis group at ORNL.

The research, published in International Edition of Applied Chemistry, appears on the back cover of the magazine. The results highlight the unique role that hydrogen anions, or hydrides, could play in improving the performance of catalysts transforming 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 net zero carbon dioxide emissions globally by 2050.

When designing the catalyst, the team chose barium titanate perovskite for the support. 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. Additionally, scientists hypothesized that the incorporated hydrogen anions could affect the electronic properties of neighboring copper atoms and participate in the hydrogenation reaction.

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

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

“I’m really proud that we brought in diverse teams to illuminate the underlying mechanism,” Wu said.

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

To reveal the structure of the catalyst under working conditions, Li and Yang He, a former ORNL postdoctoral fellow, traveled to the Stanford Synchrotron Radiation Light Source at the SLAC National Accelerator Laboratory. Along with SLAC’s Jorge Perez-Aguilar in Simon Bare’s lab, they used in situ X-ray absorption spectroscopy to reveal the structure of 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 Science, a DOE Office of Science user facility, ORNL business scientist Miaofang Chi and ORNL postdoctoral researcher Hwangsun “Sunny” performed scanning transmission electron microscopy to compare the structure of copper before and after the chemical reaction.

Additionally, ORNL 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 characterize the structure of the hydride in the oxyhydride support. Since neutrons are sensitive to light elements, they have been used to monitor the structure of the hydride after reaction at high pressure. He remained stable.

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

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

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

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

Next, Wu and his colleagues will change the hydride reactivity of the support by changing the composition of the perovskite.

“You can then potentially further increase the performance of your catalyst,” Wu said. “This approach to anionic tuning of catalysts provides a new paradigm for controlling chemical reactions.”