Reducing the CO₂ levels in Earth’s atmosphere is key to stop further environmental degradation. CO₂ conversion to methane (green fuel) using renewable hydrogen is considered as one of the best options with great potential for simultaneously resolving energy and environmental challenges, although the production of hydrogen from renewable resources also needs to be economically viable. Unfortunately, this process needs an expensive metal or complex organometallics and most of them suffer from instability and poor selectivity toward methane. In this work, using the defect engineering approach, we develop metal-free–ligand-free nanocatalysts, which convert CO₂ to methane at the significant rates, scales, and stabilities

Defect-containing nanosilica was found to be an alternative catalyst to expensive noble metals as well as complex organometallic-based catalysts. In this work, we observed that by generating and tuning the defects (type, concentration, and proximity) in nanosilica, CO₂ can be transformed into green fuel (methane) with good productivity and selectivity without the use of any metal nanoparticles. The optimum concentration of E′ centers, ODC, and NBOHC defect sites were needed, which allowed the defects to work synergistically to activate CO₂ and dissociate hydrogen, thus converting CO₂ to methane.

Unlike metal catalysts, whose activity decreases significantly with time, the loss of activity in the defect-containing silica catalysts was less significant. Notably, the regeneration of the defect-containing silica only required air (rather than hydrogen gas required for metal catalysts). Surprisingly, the catalytic activity of DNS-25 for methane production increased significantly after every regeneration cycle, reaching more than double the methane production rate (9,569 µmol g⁻¹⋅h⁻¹) after eight regeneration cycles as compared to the initial catalyst performance (3,810 µmol g⁻¹⋅h⁻¹). Also, defect-containing silica DNS-25 showed 6.6× more activity as compared to parent DFNS material, indicating the good potential of our defect-engineering approach. This activated catalyst remained stable for more than 200 h with a good formation rate and selectivity.

Thus, this magnesiothermic defect-engineering protocol may allow the development of metal-free nanocatalysts for CO₂ conversion at the significant rates, scales, and stabilities required to make the process economically competitive. This metal-free approach could also have a multidisciplinary impact and may facilitate the rational design of catalysts for various other catalytic processes apart from CO₂ conversion.

This work is one more tiny step towards the utilization of 30 thousand tons of CO2 in the earth's environment, and the process is far away from real use (seems unrealistic currently). However learning from this work and recent black-gold work, we hope that one day we will develop a commercially viable process by urgently required fundamental breakthrough in the field of catalysis.

Reference: Polshettiwar et al. Proc. Natl. Acad. Sci. (PNAS USA), 2020 (doi.org/10.1073/pnas.1917237117) 

More information about this project can be found on the Nanocat webpage