ABSTRACT
In this article we report preparation of 5 wt% HoVO4 doped ZnO via template-free hydrothermal process and investigated its photocatalytic activity against azo dyes Rhodamine-B (Rh-B), Trypan Blue (TB) and Acid Black 1 (AB 1) in solar light irradiation. The as prepared HoVO4 doped ZnO was characterised by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Field emission Transmission electron microscopy (FE-TEM), Brunauer-Emmett-Teller (BET) surface area measurements, X-ray photoelectron spectroscopy (XPS), Diffused reflectance (DRS) and Photoluminescence (PL) spectroscopy. The SEM images clearly indicate the formation of nanoparticles in the range of 20-50 nm. BET surface area of the HoVO4-ZnO is 2 times that of ZnO. Higher activity of HoVO4-ZnO in natural sunlight may be due to higher visible light absorption of HoVO4- ZnO when compared to undoped ZnO. The results suggested that HoVO4 doping on ZnO has great influence on the photocatalytic activity. The prepared photocatalyst HoVO4-ZnO possesses high stability and reusability without appreciable loss of catalytic activity up to four runs. Significant hydrophobicity of HoVO4-ZnO reveals its self cleaning property.
ABSTRACT
Rate constants were measured for the oxidative chlorodehydrogenation of (R,S)-2-phenoxypropanoic acid and nine ortho-, ten para- and five meta-substituted derivatives using (R,S)-1-chloro-3-methyl-2,6-diphenylpiperidin-4-one (NCP) as chlorinating agent. The kinetics was run in 50% (v/v) aqueous acetic acid acidified with perchloric acid under pseudo-first-order conditions with respect to NCP at temperature intervals of 5 K between 298 and 318 K, except at the highest temperature for the meta derivatives. The dependence of rate constants on temperature was analyzed in terms of the isokinetic relationship (IKR). For the 20 reactions studied at five different temperatures, the isokinetic temperature was estimated to be 382 K, which suggests the preferential involvement of water molecules in the rate-determining step. The dependence of rate constants on meta and para substitution was analyzed using the tetralinear extension of the Hammett equation. The parameter lambda for the para/meta ratio of polar substituent effects was estimated to be 0.926, and its electrostatic modeling suggests the formation of an activated complex bearing an electric charge near the oxygen atom belonging to the phenoxy group. A new approach is introduced for examining the effect of ortho substituents on reaction rates. Using IKR-determined values of activation enthalpies for a set of nine pairs of substrates with a given substituent, a linear correlation is found between activation enthalpies of ortho and para derivatives. The correlation is interpreted in terms of the selectivity of the reactant toward para- or ortho-monosubstituted substrates, the slope of which being related to the ortho effect. This slope is thought to be approximated by the ratio of polar substituent effects from ortho and para positions in benzene derivatives. Using the electrostatic theory of through-space interactions and a dipole length of 0.153 nm, this ratio was calculated at various positions of a charged reaction center along the benzene C1-C4 axis, being about 2.5 near the ring and decreasing steeply with increasing distance until reaching a minimum value of -0.565 at 1.3 nm beyond the aromatic ring. Activation enthalpies and entropies were estimated for substrates bearing the isoselective substituent in either ortho and para positions, being demonstrated that they are much different from the values for the parent substrate. The electrophilic attack on the phenolic oxygen atom by the protonated chlorinating agent is proposed as the rate-determining step, this step being followed by the fast rearrangement of the intermediate thus formed, leading to products containing chlorine in the aromatic ring.
