Energy Technology, Environment And Sustainability Reviews

Abstract

Our present dependence on fossil fuels means that, as our demand for energy inevitably increases, so do emissions of greenhouse gases, most notably carbon dioxide (CO2). It is evident that the concentration of CO2 and such greenhouse gases in the atmosphere must be stabilized to mitigate consequences on climate change. This review is focused on the methodologies, perspectives and future pathways for the conversion of CO2 to fuels. It touches on how carbon capture and conversion of CO2 can be used to produce sustainable, synthetic fuels.

Keywords: Carbon dioxide conversion, Fuel, Chemical, Electrocatalytic, Photocatalytic

Introduction

With increasing world population and development of economies, energy demand is quickly becoming one of the defining issues of this century. Till date, fossil fuels remain the main source of the world’s energy source, with oil, natural gas, and coal supplying up to nearly 90% of current energy needs. Having said that, the emission of CO2 and other greenhouse gases from the combustion of fossil fuels is one of the major contributors to climate change.

According to [1], there is a greater than 50% chance that global warming will exceed 3◦C by year 2100. With a large percentage of greenhouse gas emissions coming from the transportation sector, there is need to venture in research for cleaner fuels. Over the years, many efforts have been made to combat increasing CO2 emissions which include hydrogen fuels, biofuels, and battery-powered electric vehicles. A more recent approach is the conversion and reduction of CO2 into useful fuels for utilization. This encompasses the idea of using captured, anthropogenically produced CO2 to synthesize sustainable hydrocarbon and carbonaceous fuels (Arakawa et al. 2001; Olah 2005; Centi & Perathoner 2009 in [1]). Advancements in CO2 conversion has sparked the intriguing possibility of using primary energy from carbon-free sources such as wind, wave, solar or nuclear to convert CO2 into high-density vehicle fuels compatible with our current transportation infrastructure [1]. This is an attractive solution to provide effective energy sources taking into account factors such as cost, duration of implementation, and distribution of energy resources.

Literature Review

Molecular structure of CO2

CO2 is a linear triatomic molecule with a molecular weight of 44 Da. In the molecule, carbon (C) and oxygen (O) atoms are held together through bonds formed by sharing electrons and possess strong electrical affinities. O atoms are better in grabbing the electron pair than that of C atoms and therefore the electrons are pulled partially away from the C atom, resulting in the C atom being in a relatively low energy state. The bond energy of C=O in CO2 is much higher than that of O-H in H2O molecules. [2] 2. 2 Thermodynamics of CO2CO2 is one of the most common final products in any combustion reaction. CO2 conversion is difficult from a thermodynamic standpoint. From a thermodynamic standpoint as mentioned above, a reaction is spontaneous when its Gibbs free energy change is favorable (ΔG > 0). Nevertheless, some reactions may still be non-spontaneous due to high activation energies. Consequently, these CO2 conversion reactions re- quire a suitable catalyst. Catalysts can lower activation energies and allow or even accelerate the reaction at suitable temperatures. As mentioned above, CO2 is thermodynamically and kinetically stable, making its conversions to other chemicals/fuels difficult.

Heterogeneous

Catalysts CO2 conversion to useful chemicals by using heterogeneous and homogeneous catalysts. Under mild conditions, however, heterogeneous catalysts usually show lower catalytic selectivity and activity. Nevertheless, these heterogenous catalysts demonstrate a wide range of advantages which includes high efficiency in separation and recycling stages, reactor design, and stability. They are widely used in the industry and for the CO2 conversion process and include photocatalysts, photo-electro-catalysts, and electrocatalysts. Simple metal catalysts are used as electrodes in the electrochemical reduction of CO2. They can be divided into four main classes depending on the type of products formed. Among the types of metal catalysts used include Copper (Cu), Lead (Pb), Aurum (Au) and Nickel (Ni). The electrocatalysts work to provide decisive solutions to lower overpotentials and increase the selectivity of the CO2 conversion reaction. Research is still ongoing to develop a suitable catalyst that is stable, selective, and efficient.

Perspectives

Methodology

Carbon monoxide (CO) is formed when carbon undergoes incomplete combustion when there is not enough oxygen present. CO2, on the other hand, is formed when carbon undergoes complete combustion in the presence of enough oxygen. In order to convert CO2 to CO, the use of a reduction agent and / or a catalyst is required due to the thermodynamic and kinetic stability of the CO2 molecule. Several methods exist to facilitate this process of converting CO2 to fuels. Among the main processes include photocatalytic conversion, electrocatalytic conversion, and enzymatic conversion. CO2 must first be captured from emission point sources using carbon capture technologies (CCS). The CO2 is then undergone physio-chemical processes to be converted into fuels. The process starts with the activation of CO2 upon electron transfer from a catalyst surface. CO2 electroreduction is very similar to photosynthesis such that the catalyst converts electrons supplied by renewable energy into chemical energy that is stored in reduced products of CO2, just as a plant converts sunlight into chemical energy.

Electrochemical conversion / Electrocatalysis of CO2

The electrochemical conversion of CO2 to fuels is a chemical process that uses a CO2 reduction reaction (CO2RR) to convert the CO2 gas into useful fuels such as methanol, Carbon Monoxide, and Hydrocarbons using water and renewable electricity as the main source of energy. The electrochemical transformation of renewable energy into high energy density liquid fuels using captured CO2 offers the prospect of long-term, large-scale, seasonal energy storage. This allows for integration of renewable electricity into the transportation system and in chemical production.

Photocatalytic conversion

Photocatalysis, also known as “artificial photosynthesis” is a chemical process whereby a catalyst reduces CO2 using direct sunlight. In this process, solar energy directly from the Sun is converted into fuel by using semiconductor catalysts in gaseous phase. The integrated photochemical (PC) systems mimic natural photosynthesis, relying purely on solar radiation to produce fuels. [4]The role of the catalyst is crucial in governing the kinetics in a photocatalysis process. Three main categories of common photocatalyst materials are semiconductor materials (TiO2, ZnO, CdS and SiC, etc), graphene-based nano-catalyst, and organometallic complexes. The photocatalytic conversion process is a complex combination of photochemical and photophysicalProcesses.

The artificial photoconversion for CO2 utilization presents a challenge to the hydrogenation process which requires H2. In general, the photocatalysis process to reduce CO2 with H2O involve three main steps [3], [4](i) Exposure of catalysts (such as semiconductor) to photon irradiation from a light source excites the electrons. An energy greater than or equal to the band-gap causes the electrons to move from its valence band to a conduction band, generating electron-hole pairs. (ii) The photo-generated electrons hole pairs pairs are then separated from each other and transfer to active sites at the surface of the catalyst (semiconductor). (iii) A large fraction of electron-hole pairs will recombine together and energy is released in the form of photons. CO2 is reduced by the generated electrons into HCOOH, CH4, CH3OH, or CO. H2O will be oxidized by the holes to O2 in the third step. (iv) With the assistant of these photo-generated electrons, CO2 is reduced into various useful fuels (e. g. CO, CH4, CH3OH etc. ) with or without H2O at the catalyst surface.

Enzymatic conversion

Enzymatic conversion refers to the biological or biochemical processes that occur in organisms such as plants animals, or microorganisms. This is the most common method for CO2 conversion and can provide an efficient and more environmentally friendly solution compared to chemical CO2 conversion processes because of the specificity of the mechanism.

Discussion and Recommendations

Recent Advancements

The team is now working to uncover the subsequent reaction steps—to see how CO2 is further transformed—and to develop superior catalysts based on earth-abundant elements such as Cu (copper) and Sn (tin).

The study is titled "On the origin of the elusive first intermediate of CO2 electroreduction. "Vision for the Future of Fuels and Chemicals from CO2 for the Coming Decades

Despite the ongoing challenges of CO2RR to higher-carbon products, recent advancesin the field offer untapped potential for the realization of CO2 transformation.

• We envisage at least six potentially disruptive CO2 catalytic conversion technologies that are currently topics of intense research. The time line for realization on a large scalebetween 5 and 70+ years.
• An optimistic prediction of technology advancement in the future assuming that carbon dioxide conversion remains a topic of widespread interdisciplinary interest and global activity.
• The technologies based on electrochemical conversion of CO2 are closest to commercialization with startup and established companies such as Opus-12, Mitsui Chemicals, Carbon Recycling International, Dioxide Materials, and Carbon Electrocatalytic Recycling Toronto currently leading the pack tomonetize the technology.
• As the price of renewable energy continues to decrease, the electrochemical conversion of CO2 becomes more attractive as the electricity cost is the largest expense.
• Direct solar to fuel conversion using semiconductor catalysts in gas-phase CO2 reactors are another attractive technology that has seen great advances. These integrated photochemical (PC) systems mimic natural photosynthesis and hold an advantage of mobility afforded by being independent of an electricity source, relying purely on solar radiation to produce fuels.
• Biohybrid systems couple inorganic water-splitting catalysts with enzymes or genetically modified bacteria that convert CO2.
• These systems have potential to utilize natural enzymatic pathways to convert CO2 into a wide range of products.
• This bioelectrochemical approach has only started to be explored, but holds great promise if key issues such as long-term stability can be solved.
• Thermocatalysis in nanoporous materials for the conversion of hydrocarbons has been known for some time and has been industrially implemented.
• However, nanoporous materials have primarily been explored for capture of CO2 gas as solid sorbents or as supports for thermal catalysis; they have seldombeen explored as catalysts for electrochemical conversion of CO2.
• Two promising technologies that have not yet been realized for CO2 conversion include· polymerization chemistry such as chain insertion catalysts using activate CO2 molecular machines for dynamic CO2 catalysis.
• The ability to use CO2 as a monomeric unit directly would be transformative in the production of consumer goods, and allow for the sequestration of gaseous CO2 into solid products. Molecular machines are made up of rotating ring units around rigid struts in porous materials such as metal-organic frameworks; the ability to mechanically control the movement of molecules at the atomic level has the potential to unlock artificial molecular factories not dissimilar to enzymes.
• Development of new efficient catalysts will be essential for the development of the CO2 conversion process

Challenges and Future Pathways

The poor product selectivity and the high/low reactiontemperatures are considered to be the main barriers in theheterogeneous CO2 reduction process. For example, thesuitable catalysts are needed for the conversion of CO2 tomethanol at lower reaction temperatures. Further, many barriers exist in the CO2electrocatalysis reduction, in which the electrocatalysts areneeded to be used at lower over potentials with higherselectivity. Many different heterogeneous electrocatalystswhich are selective, fast and energy-efficient are known,but they are unstable. Photochemical processes can offeran attractive approach for CO2 reduction using solarenergy.

However, this method is not widely usedbecause it needs a critical condition to absorb therequired amount of solar energy. In general, some ofbarriers still exist and make the improvements of CO2utilization technologies slower, such as:

Compared to the other energy-related technologies

CO2 utilization technology is much less supported.

Fossil fuel plants still get benefits currently byenergy regulations

CO2 utilization processes that contribute to CO2reduction at commercial scale are yet to bedemonstrated

Therefore, more researches are necessary in order toimprove the effective CO2 utilization technologies, tomake more advances in heterogeneous catalysts for CO2reduction process and to apply various approaches in theeffective catalytic CO2 conversion.

Conclusion and Recommendations

The future of energy and carbon utilization hinges on the fundamental discovery of materials and catalysts that efficiently and selectively convert CO2. With the large number of hydrocarbon molecules that can be made from the CO2 building blocks, the question remains: which products should we pursue and which products should we forego? For energy storage needs, hydrogen, methane, and ethane are all excellent fuels. In addition, ethylene and ethanol are versatile CO2-derived chemicalfeedstocks. Is there a catalytic method yet to be realized that will efficiently, selectively, and consistently convert CO2 into complex molecules? [4] By doing so, the prospect of decarbonizing transport without the paradigm shift in infrastructure required by electrification of the vehicle fleet or by conversion to a hydrogen economy (Pearson et al. 2009).