Reinventing carbon dioxide into useful materials
Identifying promising materials for capturing and converting CO2 into useful products should help reduce the world’s reliance on fossil fuels, as Professor Karl Johnson and Jingyun Ye explain.
The atmospheric concentration of carbon dioxide has significantly increased over the last several decades, caused by the earth’s natural carbon cycle being unable to keep up with the rate of CO2 released as a result of burning fossil fuels. In 2012, the combined total of CO2 released from combustion of coal, natural gas and petroleum was around 33 billion tonnes – about equal to the mass of 99,000 Empire State Buildings.
Development of renewable energy sources may ultimately obviate the need for fossil fuels in the long run, but it is likely that the world will continue to rely on combustion of fossil fuels to produce a large fraction of our energy needs in the coming decades. A potential way to mitigate the increase of CO2 in the atmosphere is articulated by Nobel laureate, George Olah, in his description of an anthropogenic carbon cycle where CO2 is captured and combined with hydrogen obtained from renewable and nuclear energy sources to produce liquid fuels such as methanol. The idea is that renewable power sources, such as wind and solar produce power intermittently and, given the difficulty of efficiently storing electricity, this excess power could produce H2 from water splitting. The H2 would then be used as the reducing agent to produce liquid fuels from CO2.
Success with this anthropogenic carbon cycle requires the development of cost and energy efficient methods for capturing CO2 from point sources, such as power plants, and its catalytic conversion into fuels. Development of a single material or process that combines the capture and conversion steps could greatly improve the economics through process intensification. There is a critical need to develop materials that can both capture CO2 from flue gas and catalytically convert it to fuels.
The key to this is designing porous materials that selectively adsorb CO2 from CO2/N2 mixtures and hydrogenate it to formic acid and then methanol. The materials are designed using a computational quantum mechanical density functional theory (DFT) to represent each of their potential. The accuracy of the DFT approach is sufficient to justify experimental efforts in making materials identified from modelling more promising.
Finding the right material
The chemical reduction of CO2 is a challenging catalytic problem – because of its thermodynamic and kinetic stability, having strong double bonds between the carbon and the oxygens requires significant energy to break them. It has been shown that reduction of CO2 is most efficient if carried out on two electrons at a time. The addition of a single electron or a single hydrogen atom to CO2 creates a high-energy intermediate that is kinetically and energetically disfavoured. Adding two hydrogen atoms at the same time, equivalent to adding two electrons and two protons or one hydride and one proton, creates a high-energy intermediate. Thus, the ideal pathway from CO2 to formic acid and finally to methanol is through a series of two hydrogen transfers. Previous studies have shown that instead of directly adding hydrogen atoms to CO2, it is more efficient to add a hydride to the carbon of CO2 at the same time a proton is added to one of the oxygens.
By devising novel catalytic functional groups that will heterolytically split H2 into hydridic and protic species, these can be subsequently transferred to CO2.
These functional groups are composed of Lewis acid sites and Lewis base sites (Lewis pairs) that are geometrically placed to facilitate H2 dissociation, with the Lewis acid site binding the hydridic hydrogen and the base site binding the protic hydrogen. The partial negative charge on one of the oxygens in CO2 will readily accept positively charged protic hydrogen, and the positively charged carbon will accept the hydridic hydrogen. These functional groups are chemically bound to the linkers of porous materials – metal organic frameworks (MOFs).
Choosing to study a chemically and thermally stable MOF known as UiO-66 has significant advantages. It will selectively adsorb CO2 over N2 from a mixture, making it a candidate material for combined capture and conversion. Furthermore, the selectivity can be tuned by functionalising the pore.
The reaction barrier is the key property one seeks to control by the use of catalysts. In the reaction of CO2 with H2 to produce formic acid, two reaction barriers need to be controlled. The first of these is the barrier for dissociation of H2 onto the Lewis pair to produce the hydridic and protic moieties needed to reduce CO2. The second barrier in the process corresponds to the step where the charged hydrogen atoms are transferred in a concerted fashion to the carbon and one oxygen of CO2. Computational methods can then be used to screen different functional groups and help identify the optimal catalyst.
Following a series of calculations on different functional groups helps to identify trends relating the chemical nature of the groups and to the barriers of the two reaction steps. The relationships between the electronic properties of these functional groups to the reaction barriers allowed for the construction of a model to identify properties with the fastest reaction rates – a process called Sabatier analysis.
The next stages
The analysis draws on the overall ideal reaction rate depending on two simple properties of the catalytic functional groups – the chemical hardness of the Lewis pair radicals and the adsorption energy of H2 on the Lewis pair sites. The chemical hardness is defined in terms of the difference between the ionisation potential and the electron affinity of a given chemical group.
Thus, the materials that exhibit chemical hardness values of around 3.8eV and higher, and binding hydrogen with energies between -0.45eV to -0.1eV means these are likely to be increasingly active for hydrogenation of CO2. The ability to predict the activity of a catalyst based on relatively simple model calculations is a tremendous advantage because of the heavy computational cost of computing transition states and reaction barriers, especially for large systems.
Computational design of catalysts using DFT has identified promising materials for both capturing and converting CO2 to useful products, although there are still many design challenges ahead before practical materials can be realised. Future work in this field will focus on designing materials that reject water, in addition to N2, have higher CO2/N2 selectivities, improved resistance to poisoning, and are engineered to release product molecules easily.
Successful development of materials that can capture and also convert CO2 into useful fuels have the potential to significantly reduce net CO2 emissions, while at the same time providing a viable alternative to the use of fossil fuels for transportation. Carbon capture and reuse is a major step toward a more sustainable future.
Karl Johnson is a Professor in Chemical and Petroleum Engineering at the University of Pittsburgh, USA. In 1985–1987, he gained a BSc and MSc in Chemical Engineering at Brigham Young University, USA, and went on to study a PhD in Chemical Engineering at Cornell University, USA, in 1992. Johnson specialises in molecular modeling using statistical mechanical and quantum mechanical methods.
Jingyun Ye is a Postdoc researcher in Chemical and Petroleum Engineering at University of Pittsburgh, USA. Her current work has been studying the design of stratified functional nanoporous materials for CO2 capture and conversion.