CO2 sequestration and conversion

Dongwha Lee

Reducing carbon dioxide (CO2) emission into the atmosphere is one of the most important challenges, society needs to address in this century. The development of new technologies that are both economically viable and technologically efficient is a key research goal in recent years. The question of how the CO2 emission could be controlled is closely connected to progresses in different technological avenues pursued by the scientific community and industry (e.g. geological sequestration and photocatalytic reduction, etc). Each approach has both advantages and disadvantages and a combination of multiple approaches might be required to address the CO2 emission problem as a whole. Our group has been using two different approaches to address this issue.

1. Biomimetic CO2 hydrolysis

Hydrolysis of carbon dioxide occurs rapidly by enzyme inside the human body. Diverse studies have proven that the CO2 hydrolysis can be mimicked into much small molecular system with a metal-porphyrin active site. In this study, we have implemented the active site into carbon nanotube (CNT). Using accurate first principles electronic structure calculations, we predict how the catalytic hydrolysis reaction in carbonic anhydrase (CA) can be mimicked in a metal-porphyrin CNT. The two-step catalytic process can be improved remarkably by controlling the porphyrin oxidation state via the nanotube charge state and by substituting the porphyrin metal atom. The oxidation state and the metal substitution both have profound effects on the reaction energetics for the initial hydration reaction step.  For the subsequent product-release reaction step, two different reaction mechanisms could take place. These mechanisms are distinctively sensitive to either the oxidation state change or the metal substitution but not to both. For the overall catalytic cycle, a significant dependence on the nanotube charge state at low pH and on the metal substitution at high pH is expected.

Figure: Schematic of the catalytic cycle (left) in carbonic anhydrase (CA) is shown. The zinc cation is surrounded by three imidazole ligands (Im). The hydroxyl group reacts with CO2 molecule at the transition metal site, to form zinc-bound bicarbonate. The bicarbonate then reacts with either a proton or a water molecule, releasing the molecule from the active site of CA and to regenerate the zinc hydroxide. The structure of zinc-porphyrin CNT (right top) is drawn with the electron rich (blue) and deficient (gray) region near the porphyrin under different oxidized condition. Potential energy profile (right down) along the normalized reaction coordinate for CO2 hydration reaction step is shown under different oxidized condition. The potential energy profiles in different charge states are shown with respect to the reference charge state [1].

[1] Donghwa Lee, and Yosuke Kanai, "Biomimetic Carbon Nanotube for Catalytic CO2 Hydrolysis: First-Principles Investigation on the Role of Oxidation State and Metal Substitution in Porphyrin", The Journal of Physical Chemistry Letters 3, 1369-1373 (2012)

2. Photo-catalytic CO2 to fuel conversion

Photo-catalytic reduction of carbon dioxide (CO2) into hydrocarbons is an attractive approach for mitigating CO2 emission and generating useful fuels at the same time. Titania (TiO2) is one of the most promising photo-catalysts for this purpose, and nano-structured TiO2 materials often lead to an increased efficiency for the photocatalytic reactions. However, the conversion efficiency remains still too low to apply for industrial applications and understanding of the conversion process is still lacking. Further improvement can be made by clarifying the reduction mechanism and obtaining physical insights into the enhanced reactivity associated with nano-materials. First-principles density functional theory (DFT) approaches have employed to understand the photocatalytic CO2 reduction process occurs at the TiO2 surface and what aspects of specific materials features are being responsible for the improved reactivity in the nano-materials for different reaction pathways. Especially, significant barrier reduction observed on the nano-cluster surface is discussed in terms of how the under-coordinated titanium atoms and quantum confinement influence CO2 reduction kinetics at surface. Our results show that the under-coordinated titanium atoms can facilitate the reactions for both formic acid and carbon monoxide formation and make CO2 anion state favorable at the surface, while the quantum confinement only reduce the reaction barrier for the formic acid formation.
Figure 2: Ternary energy diagram for the reaction pathways on anatase (101) bulk (top) surface and TiO2 QD surface (bottom). On the bulk surface, formic acid (HCOOH) formation is an exothermic reaction while carbon monoxide (CO) formation is an endothermic reaction. Both HCOOH and CO formation encounter a significant energy barrier. For TiO2 QD surface, both reactions are exothermic and the energy barrier is much smaller than that on the bulk surface. The right panels show the geometries along the reaction path for each system.