Development of UV-Visible Light Active Photocatalysts for Hydrogen Production from Renewable

Abstract

The development of the “hydrogen economy” requires new technologies for H2 generation, of which photocatalytic water splitting by using visible light has been viewed as most promising pathway. In present study, the photocatalysts ranging from simple metal oxides and metal sulfides to more complicated systems have been studied to achieve suitable H2 production rates from water splitting reaction The hydrogen production from water in the presence of other renewable (ethanol, glycerol, triethanolamine and ethanol water-electrolyte mixtures) were studied over three different semiconductor (TiO2, CdS and an emerging g-C3N4) supports based photocatalysts. All the synthesized catalysts were characterized by various analytical techniques such as Powder X-ray Diffraction (PXRD) for determination of crystal phase composition and purity, X-ray Photoelectron Spectroscopy (XPS) for surface elemental composition, Diffused Reflectance UV-visible Spectroscopy (DRS) for optical properties, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy for particle size distribution and morphology, Photoluminescence (PL) for electron dynamics, Energy dispersive spectroscopy (EDS), Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) and X-ray Fluorescence (XRF) for elemental composition, Brunauer Emmett Teller (BET) for surface area calculation and Thermogravimetric analysis (TGA) to study changes in physical and chemical properties of materials as a function of temperature. Hydrogen production for each photoreaction was measured by Gas Chromatograph (GC) equipped with molecular sieve capillary column and TCD detector. In the first section, a series of novel non-noble metal supported semiconductor photocatalysts; Cu(OH)2– Ni(OH)2/P25 (P25=80% anatase +20% rutile) and Cu(OH)2–Ni(OH)2/TNR (TNR = Titania nanorods) were prepared by co-deposition–precipitation method (total metal loading ca. 1.0 wt%) and their performance was evaluated for H2 production. Among this series, the 0.8Cu(OH)2–0.2Ni(OH)2/P25 photocatalyst showed H2 production rate of 10 and 22 mmol h‒1g‒1, in 20 vol% ethanol-water and 5 vol% glycerol-water mixtures, respectively. The 0.8Cu(OH)2–0.2Ni(OH)2/TNR photocatalyst demonstrated very high H2 production rates of 26.6 and 35.1 mmol h‒1g‒1 in 20 vol% ethanol-water and 5 vol% glycerol-water mixtures, respectively. The mechanism for high hydrogen production rate over bimetallic hydroxide supported TiO2 is investigated and established with various experimental evidences. Followed by this, a new strategy was developed to produce highly dispersed Cu and Cu2O nanoparticles over TiO2 by using MOF-199 [Cu3(BTC)2(H2O)3]n as a source of copper nanoparticles. The photocatalyst 1 wt% Cu/TiO2-400 showed a hydrogen production rate vii some 2.5 times higher than that of CuO deposited over TiO2 by conventional precipitation methods. In the second section, highly crystalline and photocatalytically active hexagonal CdS nano-support was synthesized by sol-gel method and subsequently calcination in an inert atmosphere of nitrogen. Au nanoparticles were deposited over this hexagonal CdS by a novel reductive deposition KI method involving reduction of Au3+ ions with iodide ions and used as a model to test the effect of metal particle size as well as the reaction medium on hydrogen production activity. The photocatalyst 3 wt.% Au/CdS showed the highest performance (ca. 1 molecule of H2/AuatomS−1) under visible light irradiation from water electrolyte medium (0.1M Na2S–0.02 Na2SO3; pH 13) (92%)—ethanol (8%). The validity of this new Au loading method was established by comparing it with three other conventional methods including; deposition precipitation (DP), incipient wet impregnation (WI) and photo-deposition (PD). TEM studies of fresh and used catalysts showed that Au particle size increases (almost 5 fold) with increasing photo-irradiation time due to photo-agglomeration effect and yet no sign of deactivation was observed. A mechanism for hydrogen production from ethanol waterelectrolyte mixture is presented and discussed by evaluating some intermediate formed. It is found that Au/CdS photocatalyst showing higher plasmonic effect did not necessarily produced more hydrogen in visible light range. This work also supports the electron transfer mechanism from semiconductor to metal which may further be facilitated by metal to semiconductor energy transfer mechanism due to Au surface plasmon resonance. Finally, in third section, g-C3N4 was synthesized by thermal condensation of melamine at various temperatures to get close packing and strong interlayers binding of g-C3N4. Pd and Ag bimetallic as well as monometallic nanoparticles were deposited to cope with two inherent drawbacks of g-C3N4; low visible light absorption and high recombination rate of photogenerated charge carriers. High activity of Pd-Ag/g-C3N4 photocatalyst was attributed to inherent property of palladium metal to quench photogenerated electrons by schottky barrier formation mechanism and strong silver absorption in visible range by surface plasmon resonance mechanism (SPR).