CO2 reduction

Chemical -Ippm Formula (1 ppm) Alkalinity Increase (1 ppm) Free CO2 Reduction (1 ppm) Hardness as CaCOj Increases... 

CO2 reduction Very good Very good Excellent... 

To diminish the influence of the concurrent hydrogen evolution, CO2 reduction is usually performed under conditions when the rate of hydrogen evolution is small. This can be achieved, for example, in neutral solutions with good bulfer properties (pH interval from 5 to 8). Often, 0.1 to 1 M KHCO3 solutions with pH about 8 are used. Because of the low solubility of COj in water (0.036 M at 20°C) and many non-aqueous solutions, continuous bubbling of CO2 through the solution is needed. 

The synthesis of catalytic photocathodes for H2 evolution provides evidence that deliberate surface modification can significantly improve the overall efficiency. However, the synthesis of rugged, very active catalytic surfaces remains a challenge. The results so far establish that it is possible, by rational means, to synthesize a desired photosensitive interface and to prove the gross structure. Continued improvements in photoelectrochemical H2 evolution efficiently can be expected, while new surface catalysts are needed for N2 and CO2 reduction processes. 

In contrast, the activity of supported rhodium catalysts is determined principally by the concentration of accessible surface Rh atoms, which catalyze methane decomposition, followed by CO2 reduction (186). As a result, the support plays a minimal role in the rhodium-containing catalysts. 

The information obtained can be used to give interesting information upon the CO2 reduction mechanism. Because the radical anion increases in concentration in the negative direction, it cannot be in equilibrium with the electrode. The increase in anion concentration at cathodic potentials may, however, be explained if CO2 is formed as an intermediate radical. Thus from equations 5-7... 

The focus in this chapter is on CO2 reduction and feedstock availability, rather than on fuel quality and composition and related local emissions. 

This leaves hydrogen s contribution to CO2 reduction to be assessed. This is arguably a most complicated assessment, and one that is very much interwoven with policy choices and stakeholder preferences. The present chapter aims to support the wider community of stakeholders in the Hydrogen Economy in articulating preferences and formulating policy. The main elements we will bring to bear on the topic are ... 

When CCS is practiced in association with central production it also contributes in a very significant way to CO2 reduction from the transport sector. This clean hydrogen could have well-to-wheel emissions of 2.5-kg C02 per kg H2, a level at which the transport-sector emissions would be reduced by 85%, when hydrogen fuel cell vehicles replace hybrid ICE vehicles. We have shown that this emission level can be achieved both for gaseous and liquid distribution of hydrogen, and that the total costs for both distribution modes are similar, offering the possibility to adapt flexibly to local retail preferences and industrial opportunities. 

It is also necessary to study the whole value chain as well as the biorefinery value chain for optimization of costs, CO2 reduction, and energy usage. 

Ru(bpy)2(CO)2]. The former is reduced to provide HC02 (Scheme 127). Product-selective electroreduction of CO2 to either CO or formate in an MeCN-Bu4NPF6-(Pt) system has been shown to occur using precursor complexes such as [Ru(trpy)(dppe)Cl]+ or [cis-Rh(bpy)2(TFMS)2] (trpy = 2,2 2"-tripyridine TFMS = trifluoromethane-sulfonyl anion) [338]. The Ru(II) complex is found to be a good CO2 reduction catalyst at a potential of —1.4 V (SCE) and the electrolysis results in the exclusive formation of CO (Scheme 128a). In contrast, the electroreduction of CO2... 

The electroreduction of CO2 using a Cu cathode leads preferentially to methane and ethane [584, 585]. The selectivity is dependent on the cationic species as well as on the HCOs" concentration [586]. Hydrogen evolution prevails over CO2 reduction in a Li+ electrolyte, whereas CO2 reduction proceeds favorably in Na+, K+, and Cs+ solutions [587-589]. 

CO2, selected Ci -C2 compounds Acetate fermentation CH3COOH CH4 + CO2 CO2 reduction C02 + 4H2 CH4 + 2H20... 

Methanogenesis H2 C02 OO2, possibly formate, acetate 4H2+ CO2 CH4 + 2H2O (CO2 reduction) CH3COOH CH4 + CO2 (acetate fermentation) Mesophilic to hyperthermophilic archaea at vents and seeps... 

A particularly intriguing potential application of this reaction is the upgrading of the products of Fischer-Tropsch catalysis to increase the eventual yield of desirable n-aUcane chain lengths (typically C9-C19). FT catalysis, already practiced on a large scale commercially, may prove to be a key route to the future utilization of coal, biomass, or other nonpetroleum carbon sources (including CO2 reduction powered by solar, wind, or nuclear energy) [51, 52]. 

During microbial action, kinetic isotope fractionations on the organic material by methanogenic bacteria result in methane that is highly depleted in typically with 5 C-values between -110 and -50%c (Schoell 1984, 1988 Rice and Claypool 1981 Whiticar et al. 1986). In marine sediments, methane formed by CO2 reduction is often more depleted in than methane formed by acetate fermentation in freshwater sediments. Thus, typical ranges for marine sediments are between -110 and -60%c, while those for methane from freshwater sediments are from -65 to -50%c (Whiticar et al. 1986 Whiticar 1999). 

Whiticar MJ, Faber E, SchoeU M (1986) Biogenic methane formation in marine and freshwater environments CO2 reduction vs. acetate fermentation-isotopic evidence. Geochim Cosmochim Acta 50 693-709... 

Several important energy-related applications, including hydrogen production, fuel cells, and CO2 reduction, have thrust electrocatalysis into the forefront of catalysis research recently. Electrocatalysis involves several physiochemical environmental dfects, which poses substantial challenges for the theoreticians. First, there is the electric potential which can aifect the thermodynamics of the system and the kinetics of the electron transfer reactions. The electrolyte, which is usually aqueous, contains water and ions that can interact directly with a surface and charged/polar adsorbates, and indirectly with the charge in the electrode to form the electrochemical double layer, which sets up an electric field at the interface that further affects interfacial reactivity. 

The German COORETEC CO2 Reduction Technologies) concept favours coal gasification with precombustion capture to introduce COj capture into coal fired power plants. The same capture process is suited to produce hydrogen from coal in an environmentally-friendly way. This technical option is outlined in the recently drawn up national vision on hydrogen technologies. The first projects in this area are to be funded in the near future. 

The German research concept COORETEC (CO2 Reduction Technologies) investigates options for the generation of electricity from fossil fuels with CO2 capture and storage which opens access to CO2 free production of hydrogen. 

The mechanism of electrocatalytic CO2 reduction has been studied in some detail [57-60] but is not fully understood. A proposed mechanism from an early investigation ]55] is shown in Fig. 7. The initial step in the reaction is the reversible reduction of c-Re(bpy)(CO)3Cl (E° = — 1.1 V in CH3CN), which is assigned to the bipyridyl-centered electron transfer (Eq. 5). This is followed by a second step (Eq. 6) at ca —1.5 V, which is irreversible at room temperature and may be accompanied by loss of Cl ... 

Since this review has focused on photoelectrochemical conversions of organic compounds, it has neglected the redox reactions of simple inorganic materials like nitrogen, water, and carbon dioxide, species which have a rich photoelectrochemical history. Recent progress made with photoelectrochemical CO2 reduction signals the possibility that in the future organic feedstocks may derive from aldehydes and alcohols produced by photoelectrochemical reductions. 

---