The increase in atmospheric carbon dioxide due to human activities is the primary cause of global warming. Many Energy Frontier Research Centers (EFRCs) around the country are working to discover new ways to reduce emissions of this greenhouse gas. These efforts generally fall into three categories: capture, sequestration, and conversion. In this article, we focus on the latest in capture research and showcase two new conversion possibilities. While no single strategy alone appears likely to reduce atmospheric carbon dioxide concentrations, the employment of all strategies creates an assembly line of sorts, where scientists can dismantle the grand carbon dioxide challenge piece by piece.
Carbon dioxide capture: Increasing porosity to increase capture. Carbon dioxide forms incredibly easily. It is a stable byproduct of burning fuel at power plants or in cars, cement manufacturing, and creating commodity chemicals such as methanol and adipic acid, both of which play a large part of the chemical industry. A key strategy for reducing carbon dioxide requires its capture and subsequent conversion to useful chemicals. One of the most difficult parts of capturing carbon dioxide, however, is that it is a fundamentally inert molecule. It does not like to interact with other molecules or materials that could capture it.
One technique is to use liquids similar to ammonia as an absorption sink for carbon dioxide, but these solutions are very corrosive, highly volatile, and expensive. A potential alternative to these solutions is porous carbon with amine functionality. These solid materials work similarly to the corrosive liquids they replace without many of the hazards associated with them. Unfortunately, solid materials typically do not work as effectively.
To create a more efficient carbon-capture agent, researchers treated carbon dotted with pores (white) and then heated it to extremes. The result? Tiny channels that more than doubled the amount of very small pores in the carbon.
Researchers at the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME) are increasing the efficiency of solid porous carbon for use in carbon dioxide capture. By mixing the carbon with sodium amide and heating it to extreme temperatures, they more than doubled the amount of microporosity, or very small pores, in the carbon. These pores act as traps for the carbon dioxide to stick. After doubling the microporosity in the material, they discovered that it was capable of holding three times as much carbon dioxide at atmospheric pressure. Research like this is an important step to make carbon capture feasible on a large scale. After the carbon is captured, something needs to be done with it to keep it from escaping back to the atmosphere. One possibility is conversion.
Post capture: Using carbon dioxide for a beneficial purpose. Carbon dioxide conversion is the process by which it is transformed into a chemical feedstock for the creation of useful chemicals such as carbon monoxide, methanol, and hydrocarbons that can be used as energy sources. However, a central challenge in doing so is carbon dioxide’s resilience to transformation due to its remarkable stability. To use carbon dioxide as a reactant, employment and design of catalytic materials—materials that can effectively activate carbon dioxide for its subsequent conversion—are required.
Scientists at the Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC) EFRC are building instrumentation to interrogate reactions and surfaces under relevant catalytic conditions. Through clever use of X-ray-based spectroscopic techniques, they have shown that nickel materials have promise for promoting chemical transformation from carbon dioxide to valuable chemicals such as carbon monoxide and hydrocarbons. In doing so, scientists identified key intermediates, such as carbonate, nickel oxide, and atomic carbon, along the chemical pathways to these commercially in-demand products.
Nickel surfaces contain sites where carbon dioxide can easily bind and react when exposed to hydrogen. Once carbon dioxide binds, it dissociates into carbon monoxide and an oxygen atom, thereby forming nickel oxide as a result. In the absence of hydrogen, adsorbed carbon dioxide can react with the newly formed nickel oxide to give rise to carbonate formation, which scientists at IMASC observed in their spectroscopic data. However, they noticed that increasing temperature and hydrogen pressure led to a loss of these oxygen-containing species (nickel oxide and carbonate) on the surface to form nickel-bound atomic carbon. By adding more and more hydrogen, the atomic carbon undergoes further reduction to form hydrocarbons, which are valuable chemicals for fuels and commercial chemical processes (see figure). Such investigations show immense promise in employing novel catalysts to utilize carbon dioxide as a feedstock using cheap materials as catalysts.
Schematic of carbon dioxide’s transformation over nickel (Ni) surfaces in the presence of hydrogen (H2). Once collected, carbon dioxide can bind to Ni, and dissociate into one carbon monoxide (CO) molecule and one oxygen (O) molecule. Carbon monoxide can then be released and utilized in many commercial chemical processes or fully reduced to eventually synthesize hydrocarbons.
Such catalytic transformations usually require high temperatures, meaning that the reaction demands large amounts of energy. To overcome these challenges, biological routes to carbon dioxide transformation may hold promise as well, allowing biological cycles, with assistance from abundant sunlight, to drive the production of electrons and subsequently activate carbon dioxide.
Biological capture: Getting help from bacteria. Scientists at Biological Electron Transfer & Catalysis (BETCy) have illustrated this concept by genetically remodeling an enzyme known as the nitrogenase in a bacterium called Rhodopseudomonas palustri to convert carbon dioxide to methane. This study is particularly interesting because the nitrogenase enzyme typically facilitates the conversion of nitrogen to more biologically useful forms such as ammonia molecules, which are used for a variety of biological processes, rather than to reduce carbon dioxide to useful chemicals. However, through clever genetic manipulation, BETCy scientists tuned the enzyme’s functionality. Researchers discovered that by selectively substituting two critical amino acids on the enzyme, it could, in fact, activate and convert carbon dioxide. As a result, they found that methane formation rose with increasing light intensity without the need for excessively high temperatures or large energy demands.
(a) Nitrogenase turns atomic nitrogen (N2) into ammonia (NH3) when powered by sunlight, energy, protons, and electrons. However, once remodeled by genetic modification under similar conditions, this same enzyme can turn carbon dioxide into methane and hydrogen! (b) Increasing light intensity increases total methane (CH4) yield after 12 hours of reaction.
Scaling up of such biological processes, however, remains a challenge. Large bacterial farms require a large surface area that can be exposed to sunlight. Due to the large quantities of carbon dioxide in the atmosphere, it is unlikely that biological processes alone can lower atmospheric concentration.
There is no silver bullet for reducing levels of atmospheric carbon dioxide. In addition to decreasing net carbon dioxide production in commercial applications, it is critical to employ a variety of techniques including capture and chemical transformations to remove excess carbon dioxide from our planet.
