A.M. Banerjee

Photocatalytic H2 generation over In2TiO5, Ni substituted In2TiO5 and NiTiO3 – a combined theoretical and experimental study

We report here the role of Ni substitution in modifying the crystal structure, optical absorption properties and electronic properties of indium titanate, In2(1−x)Ni2xTiO5−δ (0.0 ≤ 2x ≤ 0.4) and its consequent effect on the photocatalytic properties for H2 generation. Rietveld refinement of observed XRD patterns of the titanates revealed that Ni2+ substitution has led to a decrease in lattice cell parameters and cell volume, contraction of InO6 octahedra and consequently improved charge carrier properties. Furthermore, the conduction band maximum (CBM) was found to be a hybrid state between Ni, Ti and In orbitals in 10% Ni-doped sample, which suggests that the photo-induced charges can be better transported in the substituted samples from zigzag chains of [·Ni–O–Ti⋯In–O–Ti----]. The UV-visible diffuse reflectance spectra exhibited that the band gap of the indium titanate phase decreased sequentially with an increase in the extent of Ni substitution. The underlying cause for band gap narrowing on Ni substitution was evaluated from plane wave based DFT calculations using the GGA + U approach. The decreasing order of photocatalytic activity (as a percentage of Ni substitution) for hydrogen generation from water–methanol mixture is as follows: 10% > 5% > indium titanate > 15% > 20%. The fall in activity below indium titanate coincided with the appearance of ilmenite NiTiO3 phase. Plane wave based DFT calculations performed on NiTiO3 revealed that strong intermixing of Ni-3d with O-2p orbitals occurred in the valence band of NiTiO3 and resulted in generation of a pseudo band gap of 0.3 eV at 1.4 eV below the Fermi level. This pseudo band gap might act as a hindrance and may contribute to weakening the intensity of the electronic transition due to Ni2+ → Ti4+ charge transfer. We propose here that an optimal concentration of 10% Ni substitution in indium titanate modifies the structural and electronic properties favorably leading to better photocatalytic activity by reducing the band gap, enhancing of the electron–hole separation and improving charge carrier properties.

Mechanism of CO + N2O Reaction via Transient CO32−Species over Crystalline Fe-Substituted Lanthanum Titanates

Some newer mechanistic aspects investigated by in situ Fourier transform infrared (FTIR) in conjunction with catalytic activity under similar conditions over crystalline lanthanum titanates as a function of Fe substitution at the B-site for the CO + N(2)O reaction are reported for the first time in the present communication. La(2)Ti(2(1-x))Fe(2x)O(7-delta) (0.0 < or = x < or = 1.0) was synthesized by gel combustion where Fe(3+) substitution effectively enhanced the conversion rates for N(2)O reduction as compared to the pristine La(2)Ti(2)O(7) (LTOGC). Among all samples, maximum conversion over La(2)Ti(0.8)Fe(1.2)O(7-delta) [LF(0.6)GC] catalyst was observed. In situ FTIR results reveal that substitution-induced anionic vacancies/defects provide additional sites on the surface of LF(0.6)GC for CO chemisorptions, whereas a perfect stoichiometric lattice like LTOGC is devoid of such sites. Surface-adsorbed CO reacts with surface lattice oxygen in the case of nonstoichiometric LF(0.6)GC to produce carbonates (M-CO(3)(2-)) at a much lower temperature. The reaction proceeds via carbonate formation, leaving the catalytic surface oxygen deficient in LF(0.6)GC, and therefore facilitates the reduction of preadsorbed, N(2)O [N(2)O(g) + * --> N(2) + *-O) by easily adsorbing the oxygen species (*-O) generated in N(2)O reduction, which is subsequently driven away by adsorbed/gas phase CO, whereas in the case of LTOGC, progress of the reaction was sluggish in the absence of transient carbonate species. Dissociative chemisorptions of N(2)O are not facilitated on stoichiometric oxygen excess titanate, as there is no vacancy in the surface to accommodate another oxygen atom. The redox mechanism via CO(3)(2-) species is proposed for CO + N(2)O reaction over La(2)Ti(2(1-x))Fe(2x)O(7-delta), as against the associative mechanism observed in the unsubstituted sample, La(2)Ti(2)O(7), as suggested by in situ FTIR in conjunction with catalytic activity results.

Fundamentals and Applications of the Photocatalytic Water Splitting Reaction

Catalysis under light irradiation, called photocatalysis, is attracting a great deal of attention from the viewpoint of fundamental science and for practical use. The topic of photocatalytic water splitting becomes increasingly important owing to its essential role in solving today’s environment- and energy-related problems. This chapter gives a comprehensive treatment to all important aspects of oxide materials for photocatalytic hydrogen production from water. First, we discuss the principles and processes involved in photocatalytic water splitting, followed by the experimental methods, and finally the different oxide semiconductor materials used as photocatalysts for these reactions and compare their performance.