Study published in the Journal of the American Chemical Society could help lessen  GHG emissions into the atmosphere by transforming CO₂ into fuels and useful chemical products

A study performed by researchers from the University of São Paulo (USP) resulted in a protocol for improving the efficiency of a crucial stage in the conversion of carbon dioxide (CO₂), which is one of the main greenhouse gases (GHG), into high-added-value products. The CO₂ is transformed into carbon monoxide (CO), after going through a chemical-hydrogenation reaction via the use of a previously treated nickel catalyst. The results of the study, coordinated by Liane Rossi, a full professor of USP’s Chemical Institute (IQ-USP), within the scope of the FAPESP Shell Research Centre for Gas Innovation (RCGI), were published in the prestigious Journal of the American Chemical Society (JACS), on March 4, 2021.

“We were able to achieve the best selectivity level of the catalyst exclusively for producing CO and for finding the reason why a nickel catalyst is more efficient after high-temperature treatment,” Rossi states, explaining that selectivity is important in this case, to avoid the production of methane (CH₄), which is another GHG, during the process.

The conversion of CO₂ into high-added-value molecules has been an enormous challenge for scientists who see the commercial use of products derived from CO₂ as a means of lessening the emissions of this gas, which is shown to be one of the main causes of the greenhouse effect and climate change.

Eliminating the unwanted – Different from CO₂ and CH₄, CO is more reactive and an important agent for producing nearly everything that is obtained from petroleum, such as fuels and polymer precursors with the same properties as petrochemicals, as well as oxygenated molecules used by other branches of the chemical industry. One of the possible products is aviation gas (kerosene).

The problem is that normally, during the CO₂ hydrogenation process_, a mixture of carbon monoxide and methane is produced. The proportion changes according to the catalyst used and to the temperature and pressure conditions of the process. One of the most widely used catalysts is nickel, which is cheap and abundant, and produces a greater proportion of methane. The simultaneous formation of methane is unwanted, when the focus is on producing longer-chain hydrocarbons, because it reduces the yield of the desired results.

The researchers discovered that after being subjected to temperatures of 800 °C, at an atmosphere of CO₂/H₂ or of methane or propane, the catalyst experiences a subtle structural change, which makes it possible to produce only carbon monoxide, thus avoiding creating methane, even at an excellent temperature for its formation.

According to Rossi, raising the temperature causes the formation of nickel carbide (Ni₃C) on the surface of the catalyst, and that makes all the difference in the hydrogenation reaction. “There is a change in the interaction of the CO with the surface of the catalyst, such that, with the carbide, it is as though the CO formed was released from that surface without being converted into methane by the reaction with the hydrogens that are also being adsorbed on the surface of the catalyst – the hydrides, as we call them,” she explains.

Rossi also says that the formation of nickel carbide is a crucial step for producing carbon monoxide from CO₂, without going on to being transformed into methane. This hypothesis was formed based on the experimental results obtained and confirmed by theoretical calculations. The researchers were able to study the surface of the catalyst under conditions similar to in operando conditions at the National Synchrotron Light Laboratory (LNLS) and at the National Nanotechnology Laboratory (LNNano), in Campinas, São Paulo, and at the Pacific Northwest National Laboratory (PNNL), in the United States.

The study involved the contributions of members of Professor Rossi’s group and of researchers from USP’s Institute of Physics in São Paulo and São Carlos, as well as the PNNL. The authors of this work see its publication in the JACS, which is one of the biggest and best chemical journals in the world – founded by the American Chemical Society in 1879 – as recognition of the study’s importance. “It is rare that Brazilian researchers have their work published in the journal; this will give visibility to what has been done and to Brazilian science,” the IQ-USP says.

Next step – Professor Rossi states that the study will now move ahead with analyses of whether other factors – the pressure of the gas within the reactor, for example – interfere in the process, in order to establish what are the best conditions under which the nickel catalysts should function. From that point, it will go on to the possible development of an integrated process that, first, transforms CO₂ into CO and, then, into added-value chemical products. “From now on, we will put the pieces together, starting from CO₂, to arrive at commercially important products.”

The researchers are working towards adding value to CO₂, showing that it can be more beneficial to use it as a raw material than to simply release it into the atmosphere, thus serving as incentive for mitigating its emissions.

A summary of the article Optimizing Active Sites for High CO Selectivity during CO₂ Hydrogenation over Supported Nickel Catalysts may be read at  https://pubs.acs.org/doi/10.1021/jacs.0c12689.