Imran Majeed

Principles and mechanisms of photocatalytic dye degradation on TiO2 based photocatalysts: a comparative overview

The total annual production of synthetic dye is more than 7 × 105 tons. Annually, through only textile waste effluents, around one thousand tons of non-biodegradable textile dyes are discharged into natural streams and water bodies. Therefore, with growing environmental concerns and environmental awareness there is a need for the removal of dyes from local and industrial water effluents with a cost effective technology. In general, these dyes have been found to be resistant to biological as well as physical treatment technologies. In this regard, heterogeneous advanced oxidation processes (AOPs), involving photo-catalyzed degradation of dyes using semiconductor nanoparticles is considered as an efficient cure for dye pollution. In the last two decades TiO2 has received considerable interest because of its high potential as a photocatalyst to degrade a wide range of organic material including dyes. This review starts with (i) a brief overview on dye pollution, dye classification and dye decolourization/degradation strategies; (ii) focuses on the mechanisms involved in comparatively well understood TiO2 photocatalysts and (iii) discusses recent advancements to enhance TiO2 photocatalytic efficiency by (a) doping with metals, non-metals, transition metals, noble metals and lanthanide ions, (b) structural modifications of TiO2 and (c) immobilization of TiO2 by using various supports to make it a flexible and cost-effective commercial dye treatment technology.

Study of ethanol reactions on H2 reduced Au/TiO2 anatase and rutile: effect of metal loading on reaction selectivity

Abstract The effect of Au particle size and loading (over TiO2 anatase and rutile) on the reaction selectivity and conversion of ethanol has been studied using temperature programmed desorption. The addition of Au onto TiO2 had three main effects on the reaction. First, a gradual decrease is observed in the reaction selectivity of the dehydration (to ethylene) in favor of dehydrogenation (to acetaldehyde) with increasing Au loading on both polymorphs of TiO2. Second, a gradual decrease is seen in the desorption temperature of the main reaction products also with increasing Au loading. Third, secondary reaction products [mainly C4 (crotonaldehyde, butene, furan) and C6 (benzene) hydrocarbons] increased considerably with increasing Au loading reaching about 60% for benzene for the 8 wt-%Au/TiO2 anatase. An inverse relationship between the interface lengths of Au particles on TiO2 and desorption temperatures of reaction products is found.

Titania supported MOF-199 derived Cu–Cu2O nanoparticles: highly efficient non-noble metal photocatalysts for hydrogen production from alcohol–water mixtures

Fabrication of cheap and efficient photocatalysts is pivotal for practical applications of solar energy devices. Here, we illustrate a novel trend in generating Cu–Cu2O nanoparticles over TiO2 for water splitting systems. Titanium(IV) isopropoxide was hydrolysed in the presence of various wt% of MOF-199 ([Cu3(BTC)2(H2O)3]n) to obtain TiO2–MOF-199 composite materials. These composite materials are then calcined at various temperatures in air to produce highly dispersed Cu–Cu2O nanoparticles over TiO2; these nanoparticles were tested as photocatalyts for hydrogen generation from alcohol–water mixtures. The photocatalyst 1 wt% Cu/TiO2-400 was found to exhibit a hydrogen production rate some 2.5 times higher than that of CuO prepared by conventional precipitation methods. The calcination temperature of the TiO2–MOF composite was found to affect the oxidation state of Cu and the photocatalytic activity, with an optimum performance achieved at 400 °C. Calcination beyond this temperature led to oxidation and agglomeration of Cu–Cu2O nanoparticles into larger CuO deposits, which reduce the H2 production activity (ca. 80%).

On the Synergism between Cu and Ni for Photocatalytic Hydrogen Production and their Potential as Substitutes of Noble Metals

A series of Cu(OH)2–Ni(OH)2/P25 photocatalysts was prepared by co‐deposition–precipitation (total metal loading ≈1 wt %) and their performance was evaluated for H2 production. Among this series, the 0.8 Cu(OH)2–0.2 Ni(OH)2/P25 photocatalyst demonstrated very high H2 production rates in 20 vol % ethanol/water and 5 vol % glycerol/water mixtures (10 and 22 mmol h−1 g−1, respectively). Detailed analyses based on reaction kinetics, photoluminescence, X‐ray photoelectron spectroscopy (XPS), and charge carrier scavenging suggest that both working catalysts are composed of Cu and Ni metals in their active phases. Cu0 is produced directly by the transfer of electrons from the conduction band of TiO2 to surface Cu(OH)2 nanoclusters, whereas Ni0 is formed indirectly through a process of gradual dissolution of Ni(OH)2 to yield aqueous Ni2+ owing to the acidic environment of the medium, followed by Ni2+ reduction by electrons from the TiO2 conduction band. The high rates of H2 production that match those obtained with noble metals can be explained owing to a considerably less negative ΔGo of Cu oxide formation when compared with that of Ni oxide formation and higher work function of Ni than that of Cu.

Pd–Ag decorated g-C3N4 as an efficient photocatalyst for hydrogen production from water under direct solar light irradiation

A low visible light absorption efficiency and high recombination rate of photogenerated charge carriers are two major problems encountered in graphitic carbon nitride (g-C3N4) based photocatalysts for water splitting applications. In this work, Pd–Ag bimetallic and monometallic nanoparticles were decorated on graphitic carbon nitride by a simple chemical reduction method and evaluated for their ability to produce H2 during water splitting reactions. The physical and photophysical characteristics of the as-prepared Pd–Ag/g-C3N4 photocatalysts were studied by powder X-ray diffraction (PXRD), UV-visible diffuse reflection spectroscopy (DRS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and steady state photoluminescence (PL). The Pd0.7–Ag0.3/g-C3N4 photocatalyst with an overall metal loading of 1 wt% showed a very high H2 generation rate of 1250 μmol h−1 g−1, which is 1.5 and 5.7 times higher than those of the Pd/g-C3N4 and Ag/g-C3N4 photocatalysts, respectively. The high activity of the Pd–Ag/g-C3N4 photocatalyst was attributed to the inherent property of palladium metal to quench photogenerated electrons by the Schottky barrier formation mechanism and strong visible light absorption due to the characteristic surface plasmon resonance (SPR) of silver nanoparticles along with the absorption of g-C3N4.

Novel hetero-bimetallic coordination polymer as a single source of highly dispersed Cu/Ni nanoparticles for efficient photocatalytic water splitting

A new strategy for depositing highly dispersed Cu and Ni nanoparticles on the surface of TiO2 from a single source is demonstrated for photocatalytic hydrogen production. We used a newly synthesized cyanide-bridged hetero-bimetallic coordination polymer [{CuII(4,4′-dipy)2}{Ni(CN)4}]n·0.7(C2H6O2).1.6(H2O) (CP-1) (4,4′-dipy = 1,3-di(4-pyridyl)propane) as a single-source precursor of Cu–Ni nanoparticles. The structure of CP-1 was established by single-crystal X-ray diffraction analysis; CP-1 crystallizes in the monoclinic space group C2/c with β = 111.67(3)°. CP-1/TiO2 composites containing different weight percentages of CP-1 were achieved by hydrolyzing titanium isopropoxide in the presence of CP-1. Highly dispersed Cu and Ni nanoparticles were deposited on TiO2 by the calcination of the CP-1/TiO2 composites at different temperatures (420 °C, 470 °C and 520 °C) in air followed by reduction in H2/Ar atmosphere at 470 °C for 2 h. XRD, DRS/UV–Vis, TEM, Cu/Ni 2p XPS, and photoluminescence spectroscopy demonstrated the presence of CuO/Cu0 and NiO/Ni0 as active co-catalysts on the surface of TiO2. The 1 wt% Cu–Ni/TiO2-470 photocatalyst showed the maximum H2 production activity of 8.5 mmol h−1 g−1 in a glycerol/water mixture (20 vol%). The results are anticipated to direct the future development of efficient, low-cost and noble metal-free semiconductor photocatalysts for solar H2 production.