Climate change is one of humanity’s greatest challenges, and one primary way to address it is by reducing greenhouse gas emissions. One of the main greenhouse gases is carbon dioxide (CO₂), which is produced through activities like burning fossil fuels. According to NASA, human activities have increased atmospheric carbon dioxide by 50%. Reducing the amount of carbon dioxide we put into our atmosphere is the primary goal of the Sustainable Development Goals, eventually reaching net-zero carbon emissions by 2050.
In a research study that connects directly to this goal, a team of scientists from Birla Institute of Technology and Science (BITS) Pilani and Institut Jean le Rond d’Alembert, Sorbonne Université examined how to convert carbon dioxide into methane, a main ingredient in natural gas, which can be used as a cleaner fuel. They have developed a novel catalyst made of nickel and cobalt that has shown incredible efficiency at converting carbon dioxide into methane.
The team found that a combination of nickel (Ni) and cobalt oxide (Co₃O₄) can catalyse, or speed up, the reaction that turns CO₂ into methane. Catalysts are materials that help a chemical reaction happen faster or at a lower temperature without themselves being used up.
The scientists created a family of catalysts by adding different amounts of nickel into cobalt oxide. They used a method called solution combustion, where a homogeneous solution of metal precursors and a fuel (like urea or glycine) is heated, yielding solid oxide products. This method allowed the scientists to make the catalysts in a simple, one-step process. Then, they tested each catalyst to see how it performed in the methane-producing reaction.
They noted that increasing the amount of nickel up to a certain point improved how well the catalyst changed CO₂ into methane. They also found that reduction, which is a heating step in the presence of hydrogen, made the catalyst even better. One catalyst in particular, where 20% of the cobalt atoms were replaced by nickel, had the best performance. It converted up to 58% of the CO₂, with almost 90% of the product being methane rather than other unwanted chemicals.
To figure out why these catalysts worked so well, the researchers used several techniques to look at their structure and chemistry. They used X-ray diffraction (XRD) to identify the crystal structure of catalysts; X-ray photoelectron spectroscopy (XPS) to measure what forms of nickel, cobalt, and oxygen are present on the surface; Temperature-programmed reduction (TPR) and desorption (TPD) to see how easily these catalysts took up hydrogen or released certain gases; and In situ Fourier-transform infrared (FTIR) spectroscopy to watch how chemical steps happened on the catalyst surface in real-time.
The tests helped the team confirm that adding nickel to cobalt oxide created extra oxygen vacancies. An oxygen vacancy is a missing oxygen atom in the crystal structure. CO₂ is a relatively stable molecule and won’t break apart easily. However, on a surface where certain oxygen atoms are missing, the CO₂ molecule can interact more strongly with the material. This is because the vacant spots or ‘holes’ on the surface can act like docking stations for CO₂ molecules. Once CO₂ is anchored, it can be more easily combined with hydrogen to form methane and, in turn, helps the reaction proceed more rapidly to create methane.
Additionally, nickel atoms themselves play an important role, as they help split hydrogen molecules into individual hydrogen atoms. These freed-up hydrogen atoms then attach to the CO₂, step by step, to make methane (CH₄). Cobalt oxide helps store and deliver oxygen and pull CO₂ molecules close, but the nickel adds extra power to break the hydrogen molecules apart. The balance between nickel and cobalt oxide, along with the right number of oxygen vacancies, turns out to be key for better performance.
The researchers then used a mathematical tool known as Density Functional Theory (DFT), a computer-based approach to model atoms and molecules at the quantum scale to estimate how the reaction processes might happen on the catalyst surface. The DFT results lined up with the experimental findings, showing that oxygen vacancies reduce energy barriers, making it easier for CO₂ to convert into methane.
Turning CO₂ into something valuable rather than letting it escape to the atmosphere could help slow down climate change. Though there are still challenges to ensure the catalyst remains stable and economical over time, the study nevertheless offers important insights that could help shape new technologies. As efforts continue around the globe to reduce our carbon footprint, these catalysts might become part of a larger strategy for capturing CO₂ and creating a cleaner, more circular energy economy, helping to close the carbon loop.
This research article was written with the help of generative AI and edited by an editor at Research Matters.