Bonamali Pal

Photocatalytic activity of transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions : Correlation with electron-hole recombination kinetics

Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (kr) of recombination of photoexcited electrons and positive holes determined by femtosecond pump–probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H2 , pipecolinic acid (PCA), and CO2) under UV (>300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorption at 620 nm arising from excitation by a 310 nm pulse (<100 fs). The second-order rate constant (kr) markedly increased with loading, even at a low level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized M-TiO2 for H2 and PCA production under deaerated conditions depended strongly on kr, but the relation between kr and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the rate of the latter. These different kr dependences of photoactivity on the reaction kinetics governed by e−–h+ recombination were attributed to the presence of O2 and Pt deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the effect of recombination kinetics on photoactivity is discussed.

Photocatalytic Organic Syntheses : Selective Cyclization of Amino Acids in Aqueous Suspensions

Photocatalytic reactions occurring at semiconductor particles/solution interfaces can be applied to organic syntheses. In this review article, examples of photocatalytic syntheses of cyclic amino acids by suspended semiconductor particles, e.g., titanium(IV) oxide or cadmium(II) sulfide are introduced and interpreted. Different from the photocatalytic decomposition of pollutants under aerobic conditions, selective conversion of organic compounds can be driven by the photocatalytic reactions under deaerated conditions.

Photocatalytic redox-combined synthesis of l-pipecolinic acid from l-lysine by suspended titania particles: effect of noble metal loading on the selectivity and optical purity of the product

Photocatalytic (>300 nm) conversion of l-(S)-lysine (l-Lys), in its neutralized aqueous solution, into l-pipecolinic acid (l-PCA) under deaerated conditions at 298 K was investigated in detail using suspended TiO2 powders (Degussa P-25, Ishihara ST-01, and HyCOM TiO2) loaded with platinum (Pt), rhodium (Rh), or palladium (Pd). A common feature of the results of experiments using a wide variety of metal-loaded TiO2 photocatalysts is that the rate of PCA formation (rPCA) was greatly reduced when higher optical purity of PCA (OPPCA), i.e., enantio excess of the l-isomer of PCA, was obtained; higher rPCA was achieved by the use of Pt-loaded TiO2 powders, while these powders gave relatively low OPPCA. Selectivity of PCA yield (SPCA), i.e., amount of PCA production based on l-Lys consumption, also tended to increase with decrease in OPPCA, giving a master curve in the plots of OPPCA versus SPCA. Among the TiO2 powders used in this study, HyCOM TiO2 showed relatively high OPPCA and SPCA but not optimum SPCA and OPPCA simultaneously. In order to interpret such relations, the mechanism of stereoselective synthesis of the l-isomer of PCA (l-PCA) was investigated using isotope-labeled α-15N-l-lysine with quantitative analysis of incorporation of 15N in PCA and ammonia (NH3), a by-product. It was observed for several photocatalysts that the 15N proportion (P15) in PCA was almost equal to OPPCA, suggesting that oxidative cleavage by photogenerated positive holes of the e-amino moiety of l-Lys gave optically pure l-PCA through retention of chirality at the α-carbon in the presumed intermediate, a cyclic Schiff base (α-CSB), which undergoes reduction by photoexcited electrons into PCA. From P15 in NH3 and PCA, the selectivity of oxidation between α and e-amino groups in l-Lys by photoexcited positive holes (h+) and the efficiency of reduction of α-CSB (produced via e-amino group oxidation to give optically pure PCA) and e-CSB (produced via α-amino group oxidation to give racemic PCA) by photoexcited electrons (e−) were calculated. The former was found to be independent of the kind of photocatalyst, especially the loaded metal, while the latter was influenced markedly only by the loaded metal. It was clarified that OPPCA and SPCA obtained for various TiO2 powders used in the present study were strongly governed by the reduction stage, i.e., the efficiency of reduction of two types of CSB. When SPCA was relatively low, photocatalysts, favoring the reduction of α-CSB rather than e-CSB, gave higher OPPCA but lower SPCA, since some e-CSB remained unreduced to give racemic PCA. In contrast, at higher SPCA, both CSBs were reduced nonselectively and OPPCA was found to be determined mainly by the selectivity in the oxidation stage. The relatively low yield of molecular hydrogen (H2) when higher SPCA was achieved is consistent with the mechanism in which H2 liberation occurs instead of the reduction of CSBs by e−. Thus, the general tendency of plots between OPPCA and SPCA could be explained by the above-described redox-combined mechanism of photocatalysis.

Priority PAHs in orthodox black tea during manufacturing process

Orthodox black tea is obtained from fresh leaves followed by withering, rolling, fermentation and drying. The presence of 16 priority polycyclic aromatic hydrocarbons (PAHs) was studied in fresh leaves and at various stages of manufacturing. Benzo(a)pyrene (2A: probable human carcinogen) was found in dried tea leaves only whereas, naphthalene (2B: probable human carcinogen) was present during all the stages of manufacturing. Dry tea leaves showed higher content of total 16 PAHs (∑PAHs) about 3 and 211 times than present in withered and dried leaves, respectively. Chrysene, benzo[g,h,i]perylene, indendo[1,2,3-c,d]pyrene, dibenzo[a,h]pyrene and benzo[a]antracene were not found during manufacturing stages of tea.

Fine CuO anisotropic nanoparticles supported on mesoporous SBA-15 for selective hydrogenation of nitroaromatics.

SBA-15 modified with APTMS (3-aminopropyl trimethoxysilane) having pore diameter (∼8 nm) has been synthesized and impregnated with 1-10 wt.% Cu using Cu(NO3)2 as a metal source followed by calcination at 350 °C. As-prepared CuO/ap-SBA-15 powder showed changes in the color from white for bare SBA-15 to light green due to formation of anisotropic CuO nanoparticles that exhibited a characteristic plasmon absorption band at 359 and 747 nm. TEM studies showed a change in the morphology of CuO NPs as a function of increased Cu loading. Moreover, well dispersed CuO nanospheres (∼5-6 nm) and nanorods (aspect ratio ∼11-20 nm) having monoclinic crystal phase were observed within the mesoporous channels of SBA-15. Elemental mapping studies confirmed uniform distribution of CuO nanoparticles on the surface of SBA-15. An increase in surface area was also observed from 694 m(2) g(-1) for SBA-15 to 762 m(2) g(-1) for 10 wt.% Cu loading probably due to the deposition of excess of CuO nanoparticles on the outer siliceous surface. The catalytic activity also increased with Cu loading and 10 wt.% CuO/ap-SBA-15 catalyst displayed the highest catalytic activity for the reduction of m-chloronitrobenzene and m-nitrotoluene with 83% and 100% selectivity for m-chloroaniline and m-aminotoluene respectively.

Highly dispersed Au, Ag and Cu nanoparticles in mesoporous SBA-15 for highly selective catalytic reduction of nitroaromatics

This paper demonstrates a homogeneous dispersion of 4 wt% coinage metal nanoparticles (Au, Ag and Cu) of different morphologies in the pores (∼8 nm) of 3-aminopropyltriethoxysilane (APTES) modified mesoporous SBA-15 for selective catalytic reduction of m-dinitrobenzene to phenylenediamine. EDX, elemental mapping and HR-TEM analysis confirmed the uniform dispersal of metal nanoparticles within the mesoporous matrix having lattice fringes with a d-spacing of 0.232 nm for Au (111), 0.23 nm and 0.20 nm for Ag (111) and (200) and 0.25 nm for CuO (111) planes. XPS results illustrated the presence of Au and Ag in their metallic states whereas Cu was oxidized to CuO. XRD, TEM and surface area analysis revealed that formation of metal nanoparticles within the sieves led to a significant change in the surface structural and physicochemical properties. Metal nanospheres with increasing size i.e., ∼5 nm (Au) < ∼11 nm (Ag) < ∼13 nm (Cu) were formed within the channels of SBA-15, while small nanorods (aspect ratio ∼2–4 nm) were also formed in the case of Ag and Cu impregnation. The catalytic activity was found to depend on the nature, size and dispersion of metal nanoparticles relative to negligible reactivity of bare SBA-15. Au nanosphere (∼5 nm) impregnated SBA-15, having the lowest surface area (292 m2 g−1), exhibited the best catalytic activity for m-dinitrobenzene reduction (k = 1.765 × 10−1 min−1) with 89% selectivity to m-phenylenediamine.

The synthesis, structure, optical and photocatalytic properties of silica-coated cadmium sulfide nanocomposites of different shapes.

The CdS nanostructure undergoes photochemical dissolution, and hence, the photocatalytic activity deteriorates with light irradiation time. A thin layer of silica coating over CdS surface may prevent the photocorrosion and coalescence of quantum size CdS particles. Hence, we synthesized SiO(2)@CdS nanocomposites of different shapes and characterized them by XRD, HRTEM, EDX, SAED, BET surface area measurement and absorption and emission study. The dispersion of spherical CdS (Cd-2.62 at% and S-2.33 at%) nanoparticles of cubic crystal structure into thick amorphous SiO(2) (43.79 at%) matrix is demonstrated here. The fabrication of core (CdS)-shell (SiO(2)) structure (SiO(2)@CdS) consisting of CdS nanorod (Cd-19.79 at% and S-22.90 at%) core (length ~126 nm and width ~6 nm) having characteristic lattice fringes of hexagonal crystals and thin SiO(2) (12.81 at%) shell (thickness=1-1.4 nm) is successfully achieved for the first time. The surface area (21.2m(2)/g) of CdS nanorod (aspect ratio=21) is found to increase (42.3m(2)/g) after SiO(2) coating. The photoluminescence of CdS nanosphere (485 nm) and nanorod (501 nm) is highly quenched after SiO(2) layer formation. The superior photocatalytic activity of SiO(2)@CdS composites for the benzaldehyde oxidation under UV irradiation has been displayed.

Plasmonic coinage metal–TiO2 hybrid nanocatalysts for highly efficient photocatalytic oxidation under sunlight irradiation

The conventional methods of Au co-catalyst deposition onto TiO2 surfaces generally do not offer fine tuning of the metal–TiO2 interface for enhanced photoactivity because of the non-uniform distribution of the size and shape of metal nanodeposits. Hence, this study demonstrated the comparative co-catalysis activity imparted to TiO2 by as-prepared coinage metal (Au, Ag and Cu) quantum dot particles of similar sizes (3–5 nm) as a function of their plasmonic interactions with TiO2 under visible light irradiation. The physiochemical and interfacial properties of metal–TiO2 composites are studied by optical band gap, XRD, XPS, TEM, surface area, time resolved spectroscopy, current–voltage characteristics, GC and GC-MS analysis. It was revealed that the optical band gap is shifted to 2.9 eV from 3.2 eV of bare TiO2 and the specific surface area, 50 m2 g−1, of TiO2 is notably reduced to 20–33 m2 g−1 after metal nanoparticles impregnation (M–TiO2) that were found to exist in the Au0 and Ag0, and Cu+2 oxidation states. The average relaxation time ≈18 μs (bare TiO2) < 20 μs (Cu–TiO2) < 24 μs (Au–TiO2) < 27 μs (Ag–TiO2) of photoexcited charge species and the highest conductance value 1.65 × 10−7 S of Ag–TiO2 as revealed by current–voltage studies strongly established that Ag–TiO2 interface acts as a better electron sink to capture and store photogenerated electrons, thus displaying superior photocatalytic activity than Au/or Cu–TiO2 interfaces. Thus, Ag–TiO2 exhibited the highest rate constant k = 4 × 10−2 min−1 relative to k = 2.7 × 10−2 min−1 (Au–TiO2) and k = 1.93 × 10−2 min−1 (Cu–TiO2) for the oxidative degradation of benzaldehyde and nitrobenzaldehyde, respectively, to CO2 under direct sunlight (40–50 mW cm−2) exposure.

Photocatalytic syntheses of azoxybenzene by visible light irradiation of silica-coated cadmium sulfide nanocomposites

Photoirradiation (lambda= 436 nm) of a deaerated 2-propanol aqueous solution containing nitrobenzene and rhodium-loaded silica-coated cadmium sulfide nanoparticles produced azoxybenzene with relatively high selectivity (68%), the photocatalytic activity being enhanced with a decrease in the size of the semiconductor particle core.

Improved catalytic activity and surface electro-kinetics of bimetallic Au–Ag core–shell nanocomposites

This paper demonstrates the preparation of core–shell nanocomposites (NCs) of Aucore–Agshell (Au@Ag) and Agcore–Aushell (Ag@Au) for measuring their catalytic activity and electro-kinetic properties relative to their respective monometallic counterparts. A significant blue-shift (530 → 408 nm) and a red-shift (420 → 550 nm) of the surface plasmon band for Au@Ag and Ag@Au NCs, respectively, were observed due to increased size of binary composites depending on the nature of the core and shell material. The thickness of the deposited Ag shells varied from ∼3–10 nm on the Au core leading to the formation of Au@Ag NCs. On the other hand, the Ag core served as a sacrificial template, where Ag@Au NCs were converted to hollow Ag–Au alloy shells (∼15 nm) because of the galvanic reaction between them due to the difference in their redox potential. An increased zeta potential was found for resulting Au@Ag (+57.8 mV) and hollow Ag–Au alloy shell (−20.13 mV) NCs in comparison to monometallic Au (−6.13 mV) and Ag nanospheres (−5.74 mV) due to surface passivation with aqueous AgNO3 and AuCl4− solution, respectively. These bimetallic NCs exhibited ∼2 times higher catalytic activity than the monometallic nanoparticles depending on the shell thickness and the core of the respective metals for the nitrobenzene and 1,3-dinitrobenzene reduction.