1263-1283 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.
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.</sub