Showing papers by "Ravikrishnan Vinu published in 2008"

Kinetics of Simultaneous Photocatalytic Degradation of Phenolic Compounds and Reduction of Metal Ions with Nano-TiO2

Abstract: The simultaneous photocatalytic degradation of phenol and 4-nitrophenol and reduction of metal ions like copper (Cu2+) and chromium (Cr6+) was studied with solution combustion synthesized nanoanatase TiO2 (CS TiO2) and commercial titania, Degussa P-25. The presence of metal ion reduces the rate of degradation of phenol and 4-nitrophenol. It was found that Cu2+ reduction to Cu+ is accelerated in the presence of phenol. In the case of Cr6+, CS TiO2 enhances the initial adsorption of Cr6+ and complete reduction is achieved within the first 10 min of UV irradiation. The presence of phenol or 4-nitrophenol also enhances the initial adsorption of Cr6+ and its reduction. The metal ion reduction in the presence of CS TiO2 is compared with that of Degussa P-25. The rate of reduction of metal ions in presence of Degussa P-25 is twice as slow as that of CS TiO2 in presence of both phenol and 4-nitrophenol. The presence of Cu2+ and Cr6+ also induces the formation of the intermediates which were not observed for the phenol-CS TiO2 system. The formation and consumption of the intermediates are modeled with a simple series reaction mechanism. A detailed dual-cycle, multistep reaction mechanism of TiO2 photocatalysis for the simultaneous degradation and reduction is proposed and the model is developed following the network reduction technique. The kinetic rate constants in the model are evaluated for the systems studied.

Synthesis and photoactivity of Pd substituted nano-TiO2

Abstract: Palladium substituted and palladium impregnated nanocrystalline anatase titania were prepared by solution combustion method. These catalysts were characterized by XRD, XPS, FT-IR, TGA, BET, UV–vis absorption, TEM and photoluminescence (PL) measurements. The catalysts have been used for the first time for liquid phase photocatalysis. The photocatalytic degradation of various dyes such as Alizarin Red S, Methylene Blue, Orange G and Rhodamine B and phenol and 4-chlorophenol was investigated under UV exposure. Though Pd-ion substituted titania has significantly higher photocatalytic activity compared to unsubstituted titania for NO reduction and NO decomposition, the initial rate of degradation for all the dyes, phenol and 4-chlorophenol was lesser in Pd substituted and impregnated titania compared to that of unsubstituted titania. This reduced reaction rate was attributed to a decrease in the surface area and PL intensity. This study shows that the photoluminescence is the key factor in determining the photocatalytic activity in liquid phase reactions.

Photocatalytic degradation of poly(acrylamide-co-acrylic acid).

Abstract: Poly(acrylamide-co-acrylic acid) copolymers of different compositions were synthesized and characterized. The copolymers were statistical with a relatively high percentage of acrylamide units, as determined by (13)C NMR. Reactivity ratios calculated by the Finemann-Ross and Kelen-Tudos methods showed that the copolymers were random with a reactivity ratio of r AM = 3.76 and r AA = 0.28. The photolytic and photocatalytic degradation of the copolymers and the homopolymers was conducted in the presence of combustion-synthesized nano anatase titania. The degradation of the copolymer in the presence of combustion-synthesized titania was significantly higher than that observed in the presence of commercial titania, Degussa P-25. The degradation was modeled by using continuous distribution kinetics by following the time evolution of molecular weight distribution. The degradation follows a two step mechanism, wherein the rapid first step comprises the scission of weak acrylic acid units along the chain which is followed by the breakage of relatively strong acrylamide units. The rate constants for the weak and strong links follow a linear trend with the percentage of acrylic acid and acrylamide in the copolymer, respectively. This linear variation can be correlated with a similar trend observed for the activation energies obtained for the pyrolytic degradation of the polymers.

Kinetics of photoconversion of cyclohexane and benzene by LnVO4 and LnMo0.15V0.85O4 (Ln = Ce, Pr and Nd)

Abstract: The orthovanadates LnVO4, and molybdovanadates LnMo0.15V0.85O4 (Ln = Ce, Pr and Nd), were synthesized via solid-state method. The crystal structure and the band gap of these compounds were determined by powder X-ray diffraction and UV–visible spectroscopy, respectively. The photocatalytic activities of these materials towards the oxidation of cyclohexane to cyclohexanol and cyclohexanone and towards hydroxylation of benzene to phenol were investigated. A detailed free radical mechanism for the oxidation of cyclohexane has been proposed and the reaction rate coefficients were determined. The hydroxylation of benzene to phenol was modeled using a simple series reaction mechanism. The rate coefficients show that CeVO4 exhibited superior photocatalytic activity and selectivity for the oxidation of cyclohexane to cyclohexanone compared to all other catalysts. For the hydroxylation of benzene, PrMo0.15V0.85O4 yielded high amounts of phenol. The observed trends in photoactivity are in good agreement with the band structure of these compounds.