Catalysis carbon dioxide reduction

Catalysis of carbon dioxide reduction thus appears as a chemical catalysis process in which the most important step is stabilization of the catalyst-substrate adduct rather than its decomposition, which closes the catalytic loop. With divalent cations, Scheme 4.8 applies. 

Water photolysis is the simplest photochemical solar energy conversion system for which proton reduction catalysis is essential, but carbon dioxide reduction is still an attractive research subject as a synthetic model for CO2 reduction in photosynthesis. There has been much work on chemical and photochemical CO2 reduction [24], but it is not the aim of this section to review those projects that mainly involve low molecular weight compounds. 

Photochemical reduction systems (Figure 5.11) require efficient light harvesting, usually by a so-called dye or sensitizer, and efficient charge separation and energy utilization. Transition metal complexes, particularly tris(2,2 -bipyridine)ruthenium(ll), serve as sensitizers. The overall reaction carried out must be a useful one. That is, in addition to carbon dioxide reduction, the complementary oxidation process (which provides the electrons) should be a desirable one. Both reduction and oxidation processes generally require catalysis. For carbon dioxide reduction, a number of the catalysts used in electrochemical systems are also effective in photochemical systems, as outlined below. 

Homogeneous Chemical Catalysis of the Reduction of Carbon Dioxide. Synergistic Effect of Bronsted and Lewis Acids... 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

Rare earth containing catalysts are useful in stationary pollution control devices as for example lanthanum titanate catalysis in the reduction of sulphur dioxide with carbon monoxide to yield carbon dioxide and elemental sulphur. The disposal of elemental sulphur is far less hazardous than that of effluent from an SO2 scrubber. 

De I squale reported that a series of coordina lively unsaturated nickel complexes, such as Ni(PPli3)2 or Ni(PCy3)2, act as excellent catalysis. A mechanism is proposed consisting of a sequence of oxidative addition, insertion and reductive elimination steps which involve an oxometallocyclobutane intermediate [225], The decisive step is Uie insertion of carbon dioxide mto a metal -oxygen bond. 

Sponsored by the European Community, a Ct chemistry course was organized at Aachen by Prof. Keim. Dr Bchr and Dr Roper of the Technical University of Aachen, Prof, Teyssie and Prof. Hubert of the University of Liege and Prof. L go of the University of. Milan. The three-day course devoted to the application of predominantly liom( cneous transition metal based catalysis in C molecules formed the skeleton for this botrk. In nine chapters the following topics are covered the reduction of CO and reactions with CO. the chemistry of methanol, activation of carbon dioxide, hydrocyanation. methane chemistry and carbene chemistry. 

The electrochemical incorporation of CO2 into perfluoroalkyl derivatives has been explored in the case of (perfluoroalkyl)alkyl iodides and (perfluoroalkyl)alkenes, with an electrochemical system based on the use of consumable anodes combined with organometallic catalysis by nickel complexes. Iodide derivatives have been functionalized to the corresponding carboxylic acids by reductive carboxylation. Interesting and new results have been obtained from the fixation of CO2 into perfluoroalkyl olefins. Good yields of carboxylic acids could be reached by a carefull control of the reaction conditions and of the nature of the catalytic system. The main carboxylic acids are derived from the incorporation of carbon dioxide with a double bond migration and loss of one fluorine atom from the CF2 in a position of the double bond. 

Catalytic activity in the homogeneous gas-phase conversion of nitrogen oxides and carbon monoxide to nitrogen and carbon dioxide was observed to be most effective for the atomic ions Fe+, Os+, and Ir+ out of the investigation of 29 different transition metal cations M+ [462]. The overall catalytic scheme that was established in this study consists of the three catalytic cycles shown in Fig. 1.94. The catalysis occurs in two steps in which NO is first reduced to N2O. An analogous three-step catalytic reduction of NO2, in which NO2 is first reduced to NO, was also discovered. The three cycles in Fig. 1.94 were characterized with laboratory measurements of reactions of each of the three nitrogen oxides NO2, NO, and N2O with the different transition metal ions in an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer [462]. 

Hammouche M, Lexa D, Momenteau M, Saveant JM (1991) Chemical catalysis of electrochemical reactions—homogeneous catalysis of the electrochemical reduction of carbon dioxide by iron(O) porphyrins—role of the addition of magnesium cations. J Am Chem Soc... 

The solubility of carbon dioxide in aqueous and non-aqueous solutions depends on its partial pressure (via Henry s law), on temperature (according to its enthalpy of solution) and on acid-base reactions within the solution. In aqueous solutions, the equilibria forming HCO3 and CO3 depend on pH and ionic strength the presence of metal ions which form insoluble carbonates may also be a factor. Some speculation is made about reactions in nonaqueous solutions, and how thermodynamic data may be applied to reduction of CO2 to formic acid, formaldehyde, or methanol by heterogenous catalysis, photoreduction, or electrochemical reduction. 

Heterogeneous catalysis is activated when the catalyst slides against itself or other materials, e.g. ceramics. Oxidation reactions of hydrogen, carbon monoxide and methane were demonstrated as being enhanced by rubbing platinum, palladium and silver, respectively [29-31], and the reduction of carbon dioxide is enhanced by the rubbing of iron oxide [32],... 

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

---