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

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

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

“Tuning the anion site of the catalyst support can greatly impact the metal-support interface, which leads to enhanced conversion of waste carbon dioxide to valuable fuels and other chemicals,” said project head Zili Wu, leader of ORNL’s Surface Chemistry and Catalysis group. 

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

In designing the catalyst, the team chose the perovskite barium titanate for the support. It is one of the few materials in which hydrogen anions, which are highly reactive to air or water, can be incorporated to form a stable oxyhydride. Moreover, the scientists hypothesized that the incorporated hydrogen anions might affect the electronic properties of neighboring copper atoms and participate in the hydrogenation reaction.

“A perovskite allows you to tune not only the cations almost across the periodic table, but also the anion sites,” said Wu. “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 tens of times the pressure of Earth’s atmosphere at sea level. Probing the catalyst under resting (“in situ”) versus working (“operando”) conditions took expertise and equipment that are hard to find outside national labs. This reaction has been studied for decades, but its active catalytic sites and mechanisms had remained unclear until now because of the dearth of in situ/operando studies. “I'm really proud that we pulled from diverse teams to illuminate the underlying mechanism,” Wu said.