The light driven electrochemical reduction of CO2 can be accomplished in a monolithic device composed of a semiconductor cathode coupled to anode forming a photoelectrochemical cell, or in a multiunit system in which a carbon dioxide electrolyzer is coupled to a photovoltaic array. The photoelectrochemical cell employs a semiconductor to absorb light energy, generate charge, and carryout the interfacial charge transfer leading to CO2 reduction. As such, the semiconductor electrode has both optical and heterogeneous catalytic roles. In a solar driven electrolyzer system, metal electrodes are employed and thus, the electrode processes are limited electrocatalysis. In both systems, the impingent light energy must overcome both the positive ΔGrxn associated with the multielectron reduction of CO2, and the free energy of activation, ΔG‡. This latter quantity, when converted to an electrochemical potential is referred to as the reaction “overpotential”. The properties of semiconductor and metal cathodes are considered in light of the energetic and kinetic parameters noted here. Attention is directed toward semiconductor materials that exhibit cathodic properties under illumination (i.e. p-type semiconductors), along with the physics of semiconductor-electrolyte interfaces, and molecular species that can electrocatalyze CO2 reduction to CO, formate, and methanol. Areas of limitation and chemical advances that will enhance the efficiency of CO2 reduction are identified.

Integration of the capture and utilisation processes potentially can offer value through synergetic benefits due to energy, material, and time savings. This chapter first briefly describes possible routes through which CO2 can be made available to the utilisation processes. Then it analyses two promising concepts of integrated carbon capture and utilisation, namely mineralisation and tri-forming. Also included are discussions on the commercial relevance of these processes including a rough assessment of their readiness to market, and problems that will need to be overcome for their widespread deployment..

This introductory chapter introduces the structural, physical and spectroscopic properties of carbon dioxide and shows how these are linked to its role in global warming. The phase behaviour of carbon dioxide is introduced including the accessibility of a supercritical phase. The kinetics and thermodynamics of reactions involving carbon dioxide are introduced to provide a theoretical basis for understanding the reactions of carbon dioxide and the limitations as to what catalysis can achieve. Finally, the commercially important chemical reactions of carbon dioxide are surveyed within this kinetic and thermodynamic framework.

This review examines the concept of transforming CO2 ultimately into high hydrocarbon fuels as a means of closing the carbon cycle. It is noted that this in effect is two challenges and must address not only the environmental impact, but also the need for clean renewable energy. It describes the two potential routes; Indirect routes which covers the generation of syngas via the Dry Methane Reforming process and the subsequent conversion of syngas into hydrocarbons via the Fischer-Tropsch process. Examination of the concept of methanol to hydrocarbons is also dealt with; while this is a more convoluted process compared to Fischer-Tropsch it allows production of gasoline range hydrocarbons with octane number enhancement properties. Direct routes are also described briefly and examine the concept of the chemical transformation of CO2 into fuels, in this section the limitations of such a process of demonstrated, with specific emphasis on the reduction of catalyst choice, due to reduced activity towards CO2 compared to CO. As such to overcome these limitations specific additives may be employed. The chapter concludes with a critical examination of the future perspectives for such processes and with specific emphasis upon potential areas of research to improve the viability of widespread application. In particular it is commented that perhaps the biggest and most difficult challenge for closing the carbon cycle arises from the need for a clean, cheap and most importantly renewable source of H2.

Four classes of CO2-based solvents are described, with their distinctive properties and applications. Supercritical CO2, meaning CO2 above its critical temperature and pressure, requires high pressure but is nontoxic, has excellent mass transfer rates and is easy to remove from products. Liquid CO2 has many of the same benefits but at a lower temperature and pressure, although it is a particularly weak solvent. CO2-expanded liquids are organic liquids into which a large amount of CO2 has been dissolved under pressure; the CO2 content modifies and tunes many of the liquid properties. Switchable solvents can reversibly change their properties whenever needed, so that they can, for example, dissolve some material and then later release it upon application of a trigger; CO2 at atmospheric pressure is the trigger of choice.

The reactions producing either linear or cyclic carbonates from CO2 are presented. Wherever possible, the mechanisms of the reactions are highlighted. The thermodynamically favourable reaction of CO2 with epoxides is briefly covered, before the challenges associated with the endogenic, direct reaction of CO2 with alcohols and diols are addressed. The equilibrium yields of these reactions are on the order of a few percent. The synthesis of linear dimethyl carbonate from CO2 and methanol is discussed primarily using the examples of nBu2Sn(OMe)2 as a homogeneous catalyst and ZrO2 as a heterogeneous catalyst. The reactions of diols and CO2 to give cyclic carbonates are then covered, with the CeO2·ZrO2 and zinc acetate/acetonitrile (MeCN) systems as the most relevant examples. The effect of water traps for changing the equilibrium position of these reactions is presented, before speculation on the future industrial relevance of these reactions.

This chapter introduces the concept of carbon mineralization that involves the conversion of CO2 into solid inorganic carbonates. It starts by presenting an overall scheme of CO2 reactions with alkaline-rich materials such as minerals and industrial wastes containing calcium and magnesium. The engineered weathering of silicate minerals including olivine ((Mg,Fe)2SiO4), serpentine ((Mg,Fe)3(OH)4(Si3O5)) and wollastonite (CaSiO3) are discussed as one of the most safe and permanent carbon storage solutions. The method of using CO2 to treat industrial wastes such as fly ash, cement kiln dust, stainless steel slag and red mud is also presented in detail. The carbonated materials can be readily landfilled without the concern of contaminant leaching, or utilized in various applications such as in construction. Environmental implications and potential benefits of CO2 utilization via various carbon mineralization schemes are summarized.

Carbon dioxide capture allows separation of CO2 from gas streams, allowing it to be purified for subsequent use. Methods of sorption are considered from a physical and chemical perspective. The state-of-the art sorption agents and technologies are considered and their short-comings discussed. Novel sorption agents are introduced and technologies to apply them to separation processes are discussed. What becomes clear is that the development of new materials will require a compromise between cost and performance but will lead to more environmentally friendly processes as environmental emissions are reduced together with operational costs over extended periods of use.

High temperature electrolysis of carbon dioxide, or co-electrolysis of carbon dioxide and steam, has a great potential for carbon dioxide utilisation. A solid oxide electrolysis cell (SOEC), operating between 500 and 900°C, is used to reduce carbon dioxide to carbon monoxide. If steam is also input to the cell then hydrogen is produced giving syngas. This syngas can then be further reacted to form hydrocarbon fuels and chemicals. Operating at high temperature gives much higher efficiencies than can be achieved with low temperature electrolysis. Current state of the art SOECs utilise a dense electrolyte, commonly yttria-stabilised-zirconia (YSZ), with porous fuel and oxygen side electrodes. The electrodes must be both electron and oxide ion conducting, and maximising the active surface area is essential for efficient operation. For the fuel electrode a cermet of nickel and YSZ is often used, whereas a lanthanum strontium manganite – YSZ mix is utilised for the oxygen electrode. Long term durability and performance are key for commercialisation of SOEC technology. To date, experimental tests of 1000h on electrolysis stacks operated at low current density have shown little or no degradation when inlet gas cleaning is employed; however, operation at higher current density leads to cell degradation, which still needs to be overcome. Advances in materials and morphology are needed to further decrease cell degradation.

We presently know very little about public perceptions of Carbon Dioxide Utilisation (CDU) technology. The importance of understanding public opinion should not be underestimated, as the ‘public face’ of technologies can govern investment and siting decisions. This chapter offers a perspective on the complexity underlying the simple question: ‘What will the public think of CDU?’ Discussion centres on the importance of questioning what we should mean by ‘the public’ and outlines some of the challenges faced when attempting to accurately assess opinions of new and unfamiliar technologies. The results of a small qualitative study designed to elucidate opinions of CDU in a group of nonexperts are also presented.