Table 4.1: Differences between Anatase and Rutile forms

Anatase Rutile

It is optically negative It is optically positive

Soft/less hard with 5.5-6vs Mohs Firm/ more hard 6-6.5vs Mohs Refractive index is 2.5-3 Refractive index is 3.87

Band gap energy for anatase is 3.23 eV Rutile has band gap energy 3.02eV It only absorbs the UV light It does absorb some visible light Less dense with specific gravity 3.9 More dense with specific gravity 4.2 It is an excellent photocatalyst It is a not good photocatalyst

Anatase can be converted to rutile at higher temperature

Rutile has no any transformational property It remains stable

Anatase has higher Fermi level Fermi level of rutile is less Mobility of electrons is higher, so that

In addition reduction is associated with the reductive TiO2 surface sites in the presence of UV light resulting in the formation of higher reactive charge transfer excited state i.e. (Ti³⁺ - O^-) of the tetrahedral coordinated titanium oxide species on the catalyst surface [137]. Photocatalytic reaction takes place on the surface of the catalyst; in such a way, that surface phase of TiO2 play a key role in photoactivity which should be more crystalline to enhance the photoactivity.

Moreover, phase transformation from anatase to rutile is effected by particle size of the catalyst [138]. However, specific surface area is indirectly proportional to the particle size. If TiO2

nanoparticles size decreased from 29-14nm, which led to the higher production of CH4 and CH3OH, it is due to greater absorption of photonic energy by smaller particles [137]. Similarly, TiO2 powder prepared by sol-gel method having the particle size ranges from 8-15nm gives the

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64 higher photocatalytic activity as compared to greater particle size about 150nm of commercial TiO2 [139]. An optimum particle size might be exist between 25-40nm, having all other properties such as greater specific surface area, generation of efficient charge carrier and good absorption/scattering of light, which is challenging effect for CO₂ reduction [140]. By further decreasing the particle size less than 5nm CO₂ photocatalytic activity decreased harshly because of change in optional and electronics properties of the catalyst inhibit the CO₂ reduction process.

Overall, TiO₂ has been one of the most attractive and convenient candidates for photocatalytic application from last few decades and still are at the top position from all another types of photocatalysts amongst oxides and non-oxides due to its superior characteristics. For instance, its commercial availability, nontoxicity, photochemical stability, good oxidation power, strong resistance to chemicals, low cost, a low operational temperature, appropriate electronic and optical potentials, low energy consumption as well as an environmentally friendly material all of which have led to relevant applications for fuel production. In addition, various TiO2 advance technologies have been adapted such as TiO2 nanoparticles. Modified TiO2 nanoprticles with metals [19] titanium doped with Pt metal, Pt/TiO2 nanotubes [141] and hetero structured TiO2

[142] as well as Cu/TiO2 nanorods [100]. Dispersed nanoparticles on mesoporous silica support materials [10]. Among all these nanostructures materials 3 dimensional (gyroid cubic Ia3d) KIT-6 materials demonstrate the superior photocatalytic activity than 1 or 2dimentional SBA-15S or MCM-41 photocatalysts, it might be because of not only its higher surface area, but large pore structure and pore size, with multi pore channels provides the enough space for dispersion of materials on the support, as well as better light dispersion which creates a sufficient number of a e^-/h⁺ pairs to promote the photocatalytic activity for CO₂ reduction.

Consequently, titanium dioxide semiconductors are considered the paramount photocatalysts for CO₂ reduction because under the UV light illumination, photogenerated electrons in the bottom of the conduction (CB) band can have sufficient negative redox potential to motivation of CO₂ reduction, while on the other hand photogenerated holes in the top most level of the valance band (VB) can be more positive to oxidize the H2O towards the O2 formation and superoxide (−0.2V) simultaneously. TiO2 exhibits almost all of the required properties for an efficient photocatalyst must have for photocatalysis process, but only with the limitation of not absorbing visible light.

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65

4.2.3 Photocatalytic reduction of CO

on TiO

based materials and the application as semiconductor photocatalysts

Titanium dioxide (TiO₂) has drawn much research attention in recent years due to its potential applications specifically, photoreduction of CO₂ over TiO₂ based materials from last few years and this trend is dramatically increasing every year owing to its unique properties and easily availability. However, TiO2 can be used in its pure form (bare), doped with different metals and non-metals as well as highly dispersed form within mesoporous or zeolites materials to enhance its photoactivity as shown in Table 4.2. Moreover, the surface phase of TiO2 should be responsible for its photoactivity because the photocatalytic reaction takes place on the surface of catalyst though; the surface phase of TiO2, particularly during the phase transformation has not been well investigated. Additionally, the phase transformation from anatase to rutile is influenced by particle size [138]. UV Raman spectroscopy is found to be more sensitive to the surface phase of a solid sample when the sample absorbs UV light [143]. This outcome signs use to study the crystalline phase in the surface region of TiO2 by UV Raman spectroscopy as TiO2

strongly absorbs UV light and further try to correlate the surface phase of TiO2 and its photoactivity. TiO2 has very wide band gap (3.2eV for anatase) and it is only active in the UV range spectra below than 400nm. However, during CO₂ reduction, photo-generated electrons in the bottom of the conduction band can have sufficient negative redox potential to drive CO2

reduction, while the photo-generated holes in the valence band can be sufficiently energetic (positive holes) to act as acceptors and oxidize water to O₂. But luckily, in our present context TiO₂ was used under the UV region and satisfactory results were obtained from CO₂ reduction with H₂O vapor by using various nanostructured photocatalytic titania materials.

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66

Table 4.2: Literature survey of photocatalytic reduction of CO

on the bare