Rajaram Mahalakshmy

Application of photo catalysis for mitigation of carbon dioxide

Photo catalytic reduction of carbon dioxide by water or artificial photosynthesis to yield hydrocarbons (methane and methanol, etc., termed “solar fuels”) is being studied extensively, with the twin objectives of developing an effective means of limiting atmospheric CO2 levels and evolving a sustainable alternative route for production of fuels and chemicals. This short review covers the origin and thermodynamic and kinetic features of the process, the basic photocatalytic principles involved, the rationale behind the choice of different catalysts and their performance, the effect of process conditions, the effect of the structural and photophysical properties of the different catalysts on their performance, mechanistic pathways, surface transformations, challenges involved in the practical application of the process, and future directions for research.

Selective hydrogenation of cinnamaldehyde on nickel nanoparticles supported on titania: role of catalyst preparation methods

With nickel acetate as the precursor, 15% (w/w) Ni/TiO2 (P-25) catalysts have been prepared by using four different methods, namely, direct impregnation (IM), deposition–precipitation (DP) with urea and chemical reduction using hydrazine hydrate (HH) and glucose (GL) as reducing agents. XRD, hydrogen chemisorption, TEM and TPR analyses reveal that smaller Ni crystallites, 8–12 nm in size, are obtained by adopting HH & GL methods compared to 15–20 nm crystallites obtained by using IM and DP methods. The nature of metal–support interactions (MSI) varies depending on the method of preparation. XPS studies reveal the presence of residual Ni2+ ions along with the Ni metal. All the catalysts exhibit good conversion of cinnamaldehyde (CAL) (in methanol, at 20 Kg cm−2 H2 pressure and 80–140 °C) and selectivity to cinnamyl alcohol (COL)/hydrocinnamaldehyde (HCAL) up to 1 h, beyond which further hydrogenation of COL to hydrocinnamyl alcohol (HCOL) occurs. The catalysts prepared by HH & GL display higher CAL conversion and selectivity to COL. Conversion of HCAL to HCOL proceeds at a slower rate compared to that of COL to HCOL. Introduction of HCAL along with CAL as a feed increases selectivity to HCOL, while introduction of COL in the same manner decreases selectivity to HCAL. Infrared spectra of CAL adsorbed on the catalysts reveal surface bonding through CC and CO groups. The steric hindrance due to adsorption of COL and the presence of small amounts of Ni2+ favour adsorption of CAL through the CO group leading to higher selectivity to COL.

Photocatalytic Reduction of Carbon Dioxide by Water: A Step towards Sustainable Fuels and Chemicals

Photo catalytic reduction of carbon dioxide or artificial photo synthesis to yield hydrogen and hydrocarbons like methane, methanol etc., has emerged as a subject/process of intensive study due to its potential applications towards abatement of atmospheric CO2 levels and conversion to fuels and chemicals. This Chapter provides a comprehensive picture of the process that has posed several scientific and technological challenges, like activation of most stable molecules-CO2 and water, extremely low conversion rates, complex reaction pathways involving multi electron transfer steps and short catalyst life. All the major aspects/developments on this process like, the salient features and technological aspects, thermodynamic and kinetic characteristics, various types of photo-active catalysts-, like, titania based catalysts and titania with various dopants and modifiers, various metal oxides/sulfides/nitrides/ layered titanates, binary and ternary oxides of Nb, Ta, Ga & In mixed oxide catalysts, metal complexes, and supra molecular catalysts-, sensitization by macro cylic ligands, influence of process parameters, catalyst structure-property-activity correlations, aspects of deactivation of catalysts, reaction mechanistic aspects and sequential surface reaction pathways, recent trends and future directions have been covered. Design and development of efficient catalyst systems and achieving higher yield of desired products (higher selectivity) and extending the catalyst life are the key issues being pursued by the researchers. The process is in nascent stage and further improvements are needed as CO2 conversion rates are extremely small, with products formed in terms of 1-10 micro moles/hr. One of the means of improving the process efficiency is to carry out electrochemical reduction of CO2 using solar electric power, with an integrated Photo electrochemical cell (PEC). Yet another option is to reduce CO2 to methanol with hydrogen produced using solar powered PEC.

Visible light driven reduction of carbon dioxide with water on modified Sr3Ti2O7 catalysts

Layered perovskite type Sr3Ti2O7 catalysts, doped with N, S and Fe have been prepared by modified polymer complex method, characterized and evaluated for photo reduction of CO2 in aqueous alkaline medium using UV-visible radiation. EDXA and XRD data reveal the incorporation of the dopants into the titanate matrix. The presence of N in substitutional and interstitial locations, S as S6+ and Fe as Fe3+ species are indicated by XPS analysis. DRS and photo luminescence studies show that the dopants form additional energy levels within the band gap, promoting visible light absorption and retarding recombination of the charge carriers. Morphological changes and smaller crystallites also minimize recombination. Layered structure of Sr3Ti2O7 facilitates easy transport of charge carriers and separation of oxidation/reduction reaction centres. Structural, morphological and photo physical characteristics of doped catalysts improve the activity significantly. Sr3Ti2O7 co-doped with N, S and Fe together, displays maximum apparent quantum yield for CO2 reduction products.