V. Jeyalakshmi et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 200–207
201
ies [11] have shown that La modified NaTaO3, loaded with various
2.2. Computational details
co-catalysts, like, Pt, Ag, Au, CuO, NiO and RuO2, displays signifi-
NaTaO3 being a typical wide band gap semi-conductor, absorbs
only <5% of solar energy (UV region). Several approaches to improve
its visible light activity by doping with cations and anions have
been explored [6,12–27]. Sato et al. [28,29] reported CO2 photo
reduction using N-doped Ta2O5 linked with Ru complexes, which
showed high activity and selectivity towards HCOOH. Sensitiza-
for the utilization of visible light by tantalates. Metallo porphyrins
are good candidates for such coupling, due to their ability to absorb
visible light and their structural similarity with the natural light
harvesting moieties [30]. Choice of metal complexes for sensitizing
large band gap semiconductor depends on certain characteristics
like:
All the calculations in the present study were carried out using
Gaussian 09 program package [39]. In order to calculate the reduc-
tion potential of the CoTPP complex, density functional theory
(DFT) was used. The geometry optimization of the complex was
performed at Becke’s three-parameter exchange Lee, Yang and Parr
correlation (B3LYP) hybrid functional with lanl2dz basis set [40].
The vibrational frequency analysis of the optimized geometry con-
firmed that the optimized geometry corresponded to minimum on
the potential energy surface as revealed from the real values for all
vibrational frequencies. Time-dependent density functional theory
(TDDFT) calculations were carried out to simulate the electronic
transitions at the same basis set. The optimized geometry of the
CoTPP is shown in Fig. 1.
2.3. Characterization of catalysts
Phase analyses of the catalysts was carried out by X-ray diffrac-
tometer (Rigaku-MiniFlex-II) using Cu K␣ radiation (=1.54056 Å)
with the scan range of 2 = 5–90◦ at a speed of 3◦/min. The crystal-
lite sizes were calculated by the Scherrer’s formula, t = K/ cos,
where t is the crystallite size, K is the constant dependent on crys-
tallite shape (0.9 for this case) and ꢀ = 1.54056 Å,  is the FWHM
(full width at half maximum) and is the Bragg’s angle.
Diffuse reflectance spectra of the catalysts in the UV–vis region
were recorded using a Thermo Scientific Evolution 600 spec-
trophotometer equipped with a Praying Mantis diffuse reflectance
accessory.
I.) suitable redox potential with respect to the conduction band
of the semi-conductor
II.) optical absorption, mainly in the visible solar region,
III.) strongly bound to the semiconductor surface,
IV.) photo conductivity and chemical stability.
Phorphyrin derivatives display the requisite characteristics and
sitize wide band gap semiconductors like TiO2 [30]. Porphyrins
possess suitable ground and excited state redox potentials with
respect to the semiconductor conduction band and a - electron
conjugate system which facilitates conductivity of the material and
light absorption in the solar visible region [31–33]. Besides, sensi-
tization results in multi electron reduction products compared to
other modifications, due to high conductivity and high absorption
co-efficient of such sensitizers within the solar region. Consider-
ing such advantages associated with the use of metallo porphyrins,
PCRC with water by using Cobalt (II) tetra phenyl porphyrin (CoTPP)
as sensitizer for wide band gap photo catalyst Na(1−x)LaxTaO(3+x)
has been investigated. To the best of our knowledge, this is the first
report covering sensitization of NaTaO3 by metallo porphyrins to
enable visible light utilization.
Fourier Transform Infrared Spectra were collected at room tem-
perature by using Perkin-Elmer FTIR spectrophotometer in the
range 4000–400 cm−1
.
Photoluminescence spectra were recorded under the excitation
with a 450W Xenon lamp and the spectra were collected using
JobinYvon Fluorolog-3-11 spectro fluorimeter.
Surface area and pore volume of the catalysts were determined
using Micromeritics ASAP 2020. Samples were degassed at 373 K for
2 h and at 423 K for 3 h. Pure nitrogen at liquid nitrogen temperature
(77 K) was used.
Transmission electron micrographs were recorded using JEOL
3010 model. Few milligrams of the samples (1–2 mg) were dis-
persed in few mL (1–2 ml) of ethanol by ultra-sonication for 15 min.
A drop of the dispersion was placed on a carbon coated copper grid
and allowed to dry in air at room temperature.
2. Experimental
Scanning electron micrographs were recorded using FEI, Quanta
200, equipped with EDXA attachment for elemental analysis. The
samples in the powder form were taken on the carbon tape and
mounted on the SEM sample holder.
2.1. Preparation of catalysts
5, 10, 15, 20-Tetra phenyl porphinato Cobalt(II) (CoTPP) was
synthesised using reported procedures [34,35]. NaTaO3 and 2.0%
(w/w) lanthanum promoted NaTaO3 catalysts were prepared by
hydrothermal route [36,37]. 0.6 g of NaOH dissolved in 20 ml of
water (0.75 M) and 0.442 g of Ta2O5 were added into a teflon
lined stainless steel autoclave. After hydrothermal treatment at
140 ◦C for 12 h, the precipitate was collected, washed with deion-
ized water and ethanol and finally several times with water and
dried at 80 ◦C for 5 h. La modified NaTaO3, (Na(1−x)LaxTaO(3+x) with
x = 0.00014 for 2.0% w/w of La) was prepared by the same pro-
cedure, by adding 0.0117 g of La2O3 along with NaOH and Ta2O5
for hydrothermal treatment. 1.0% w/w −CoTPP/Na(1−x)LaxTaO(3+x)
composite was prepared by adopting the method reported previ-
ously [38]. Briefly, 10 mg of CoTPP was dissolved in 20 ml of ethanol,
followed by 1 g of Na(1−x)LaxTaO(3+x) support with stirring. The
resulting solution was refluxed at 80 ◦C for 130 min. The solvent
was removed by evaporation under vacuum and the solid sam-
ple was dried at 60 ◦C for 6 h to obtain. 1.0% w/w −CoTPP/Na(1−x)
LaxTaO(3+x) (Actual CoTPP loading-0.99% w/w)
Activity of the catalysts in UV visible region (300–700 nm) was
evaluated in batch mode, using jacketed, all glass reactor (620 ml)
[41] fitted with quartz window (5 cm dia) and filled with 400 ml
of aqueous 0.2 N NaOH solution. Besides increasing the solubility
of CO2, alkaline water acts as hole scavenger. 0.4 g of catalyst was
dispersed in the alkaline solution with vigorous stirring (400 rpm).
Increasing the stirring rate beyond 400 rpm did not increase the
conversion, indicating that under the present experimental con-
ditions, the mass transfer limitations are overcome at this speed.
Aqueous alkaline solution (pH-13.0) was saturated with CO2 by
continuous bubbling for 30 min after which pH reduced to 8.0. Reac-
tor in-let and out-let valves were then closed and irradiation with
Hg lamp with 77W power (from WACOM HX-500 lamp house) was
started. Gas and liquid phase samples were taken out at periodic
intervals with gas-tight/liquid syringes and analysed by GC. Liq-