Photocatalytic carbon dioxide reduction

Conclusion There are so many hurdles associated invariably with the photocatalytic carbon dioxide reduction process which must be rectified in order to create an energy, sustainable society using direct sunlight as a primary energy source just as plants do. It could happen only by the collaborative research effort from various groups, irrespective of the implicit bias among the scientific community. 

Electrocatalysis at metal electrodes in aqueous (1.2) and non-aqueous ( ) solvents, phthalocyanine ( ) and ruthenium ( ) coated carbon, n-type semiconductors (6.7.8),and photocathodes (9,10) have been explored in an effort to develop effective catalysts for the synthesis of reduced products from carbon dioxide. The electrocatalytic and photocatalytic approaches have high faradaic efficiency of carbon dioxide reduction (1,6). but very low current densities. Hence the rate of product formation is low. Increasing current densities to provide meaningful amounts of product, substantially reduces carbon dioxide reduction in favor of hydrogen evolution. This reduction in current efficiency is a difficult problem to surmount in light of the probable electrostatic repulsion of carbon dioxide, or the aqueous bicarbonate ion, from a negatively charged cathode (11,12). 

Major limitation associated with carbon dioxide reduction is the accuracy of the analytical measurements employed. The photocatalytic process is a multielectron transfer process, hence the reaction leads to the formation of a variety of products like carbon monoxide, methane, higher hydrocarbons, alcohol, aldehydes, carboxylic acid etc., with some intermediates. The identification and quantification of the products are needed for the best selection of photocatalyst, comparison and elucidation of reaction mechanisms. Currently there is no standard analysis method that has been developed for product analysis of carbon dioxide reduction. Hence the results of these measurements also include the products derived from the carbon contamination invariably present in the reaction sys-... 

Photocatalytic reduction of carbon dioxide is an immediate need in the energy as well as environmental points of view. The amount of product formed during the reaction still suffered by lower yield say up to some micromoles. A complete revisit to each individual process associated with the carbon dioxide reduction is required for elucidating the reaction pathways and preparing a suitable mechanism. At this point, the scientist should deviate from the conventional TiCk route [167,172]. The mechanistic studies using TiCk are highly encouraged due... 

Hirano, K., Inoue, K., and Yatsu, T. (1992) Photocatalysed reduction of C02 in aqueous Ti02 suspension mixed with copper powder. Journal of Photochemistry and Photobiology A Chemistry, 64 (2), 255-258. Adachi, K., Ohta, K., and Mizuno, T. (1994) Photocatalytic reduction of carbon dioxide to hydrocarbon using copper-loaded titanium dioxide. Solar Energy,... 

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

Sayama, K., Arakawa, H. 1993. Photocatalytic decomposition of water and photocatalytic reduction of carbon-dioxide over ZrOj catalyst. J Phys Chem 97 531-533. 

Besides, in an interesting study Sing Tan et al. [63] reported the possibilities to use the photocatalytic reduction of carbon dioxide with water to produce hydrogen and methane. 

Ishitani O, Inoue C, Suzuki Y, Ibusuki T. Photocatalytic reduction of carbon dioxide to methane and acetic acid by an aqueous suspension of metal-deposited Ti02. J Photochem Photobiol A Chem 1993 72 269-71. 

Small carbon-containing molecules such as atmospheric CO2 are considered to be important renewable feedstocks (144,145). In the context of mankind s increasing demand for carbon-based materials, food, and liquid fuels, the photocatalytic reduction of carbon dioxide under solar light irradiation is an attractive option. Such types of artificial photosynthetic processes could greatly enlarge the possibilities of abiotic CO2 recycling. 

Effect of solvents on photocatalytic reduction of carbon dioxide using semiconductor photocatalysts... 

Photocatalytic reduction of carbon dioxide on TiO-, nanocrystals embedded in Si02 matrices (Q-Ti02/Si0,) and bulk CdS particles with and without surface modifieation by several thiol compounds was investigated in various kinds of solvents. Formate and carbon monoxide were obtained as the major reduction products and the ratio of the former to the latter was increased with an increase in the dielectric constant of the solvents used for the use of Q-Ti02/Si0, and bare CdS particles as photocatalysts. The surface modification of CdS particles with thiol compounds was effective in enhancing the ratio of formate to carbon monoxide. The observed selectivity of CO2 reduction products was explained well in terms of the stabilization of reaction intermediates on the photocatalyst surface. 

These processes occur in Ti02 suspensions in water. Zr02 and other oxides were also found to be active in the photocatalytic reduction of carbon dioxide. 

Photocatalytic Reduction of Carbon Dioxide Issues and Prospects... 

Methods We explored the content of more than 200 original research articles that are Hariprasad Narayanan relevant to the desired topic and extracted the current knowledge on the so called photocatalytic reduction of carbon dioxide. We have approached all the articles in a perspective way rather than a word to word recent advances in the field and found out the major limitations that have to be rectified. 

The main objective of this write up is to analyze the issues associated with the state-of-the-art photocatalytic reduction of carbon dioxide into chemicals and fuels. 

Reduction of carbon dioxide produces a number of products like alcohols, acids, aldehydes etc. Since the photocatalytic reduction is a multielectron process, the products formed in the reaction system consist of many compounds, both in liquid (alcohols, aldehydes, acids etc.) as well as gaseous state (CO, CH4, and unreacted CO2 etc.). The exact identification and quantification of these products are required for the analysis of the overall reaction efficiency and hence the design of a better photocatalyst. Normally used analytical techniques are GC, HPLC, LC-MS, IR, IEC, UV-Vis, or 13C NMR. 

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