Titania based catalysts for photoreduction of carbon dioxide: Role of modifiers

Abstract

<nbsp> Photocatalytic conversions on titania utilizing sunlight as the energy source have been studied extensively for a variety of processes/ synthesis, like removal of pollutants in air and liquid streams, self-cleaning, anti-fogging and anti-bacterial applications, splitting of water into hydrogen and oxygen and photoreduction of CO₂ by water to yield hydrocarbons. These processes are receiving global attention as an off-shoot of the frantic search for alternative energy sources. Though titania continues to be the preferred catalyst in view of its low toxicity, ability to resist photo-corrosion, versatility, and abundant availability at low cost, critical limitations do exist in terms of its inability to get activated with visible light and in achieving high conversion efficiency and quantum yield. Several techniques of modifying titania to improve its performance have evolved over the years resulting in correlations and concepts on structure-property-activity and the role of preparation methods. Such modifications have lead to changes in light absorption efficiency, electronic structure, energy levels, morphology, phase composition and other photophysical properties with moderate improvements in the performance. Efforts to understand the mode of action of the modifiers in terms of the first principles, i.e., rationalization of the activity in terms of electronic and structural properties and establishing theoretical basis for the photocatalytic action, have met with only partial success, due to conflicting observations/results.

<br> The objectives towards modifications, namely, extending the light absorption range, retarding charge carrier re-combination, facilitating their fast transport to the active sites on titania surface and incorporation of active elements suitable for redox reactions, have been achieved to a reasonable level. However, commensurate improvement in activity/CO₂ conversion has not been observed. Maximization of selectivity (to methane or methanol) and arresting catalyst deactivation are the two major issues yet to be understood in clear terms. An in-depth study to understand the surface transformations at molecular level under activation by light energy, is needed to achieve further improvements in the activity of the catalysts and the process. This review brings forth an account of the investigations on modified titania, capturing some significant and selected contributions out of the vast literature available, with an emphasis on application for photocatalytic reduction of CO₂ with water_.