Non-thermal plasma (NTP) is used to activate CO2 molecules for hydrogenation into alternative fuels at low temperatures, also allowing the conversion of renewable electricity into chemical energy. Tokyo Tech researchers combined experimental and computational methods to study the CO hydrogenation pathway promoted by NTP2 on the surface of Pd2Ga/SiO2 catalysts. The mechanistic insights from their study can help improve the efficiency of catalytic hydrogenation of CO2 and allows engineers to design new catalyst concepts.
Climate change accelerated by excess CO2 emissions has been a major concern in recent years. To address this problem, technologies that can not only reduce and remove excess CO2 emissions but also transforming them into value-added chemicals are being developed. One of these methods is the hydrogenation of CO2 use renewable hydrogen to produce alternative fuels.
Over the years, different strategies have been developed to improve CO2 hydrogenation in the presence of metal catalysts. The most promising of these is non-thermal plasma (NTP). It promotes the hydrogenation of CO2 beyond the thermodynamic limit even at low temperatures without deactivating the metal catalysts, which are vulnerable at higher temperatures. Despite the growing popularity of this technique, the interactions between NTP-activated species and metal catalysts are still not well understood.
Fortunately, a team of researchers from the Tokyo Institute of Technology (Tokyo Tech), Japan, led by Professor Tomohiro Nozaki, designed a study to fill this gap in understanding. In their recent breakthrough, published in the Journal of the American Chemical Society, researchers revealed reaction dynamics for NTP-assisted CO2 hydrogenation at the surface of Pd2Ga/SiO2 alloy catalysts which lead to the formation of formate. Professor Nozaki explains that “Reaction mechanisms like Eley-Rideal or the ER pathway have been proposed to explain the efficiency of CO2 conversion at lower temperatures and the activation energy of this reaction decreases considerably. In addition, NTP produces an abundant amount of vibration-activated CO2 which is the key to improving CO2 conversion beyond thermal equilibrium.
The team studied the reactions between CO activated by NTP2 and Pd2Ga/SiO2 alloy catalysts in a fluidized bed dielectric barrier discharge reactor and compared them to conventional thermal catalysis. The results revealed that CO2 conversion to formate was more than doubled in the case of NTP-assisted hydrogenation compared to thermal conversion. To better establish the mechanisms of the mentioned conversion, scientists have adopted on the spot spectroscopic analysis and density functional theory (DFT) calculations.
The results revealed that the activation of NTP gave rise to vibrationally excited CO2 molecules that react directly with the hydrogen atoms adsorbed by the Pd sites on the catalyst via the ER pathway. One of the reacted O atoms of the species then adsorbed to the nearby Ga site, resulting in the formation of formate monodentate or m-HCOO. DFT calculations also inferred a decay pathway for the same m-HCOO species.
This experimental-theoretical study showed that NTP can promote CO2 hydrogenation to limits that conventional thermal methods can hardly reach. It also provided mechanistic insights into NTP-activated CO2 and catalyst interaction, which can be used to develop better catalysts and improve the hydrogenation process. “Through our research, we wanted to accelerate the waste-to-wealth initiative. Capturing CO2 and using it as a raw material for the synthesis of valuable fuels and chemicals will not only help us deal with the climate problem, but also slow down the depletion of fossil fuels to some extent,” concludes Professor Nozaki.