Query: Conversion Efficiencies for the Photocatalytic Production of Hydrogen via TiO2 Semiconductor Electrodes

Candidate papers:

abstract: Overall solar energy conversion efficiencies of at least 0.8 percent, somewhat greater than those previously reported, have been obtained in the photocatalytic production of hydrogen using TiO/sub 2/ semiconductor electrodes prepared by heat treatment of titanium metal foils. A shift in the threshold frequency for hydrogen evolution toward energies slightly lower than the 3.02 eV bandgap of TiO/sub 2/ was observed for some of these electrodes. This result supports the possibility of utilizing a greater portion of the solar spectrum, thereby increasing overall conversion efficiency even further. A tentative explanation of this shift involves the presence of mixed phases of the titanium oxygen system (Magneli phases) in the semiconducting film.
author_names: John F Houlihan

David P Madacsi

E J Jun Walsh

L N Mulay
title: Improved solar energy conversion efficiencies for the photocatalytic production of hydrogen via TiO2 semiconductor electrodes
year: 1976

abstract: Abstract Hydrogen (H2) production via photocatalytic water splitting is one of the most promising technologies for clean solar energy conversion to emerge in recent decades. The achievement of energy production from water splitting would mean that we could use water as a fuel for future energy need. Among the various photocatalytic materials, titanium dioxide (TiO2) is the dominant and most widely studied because of its exceptional physico chemical characteristics. Surface decoration of metal/non metal on TiO2 nanoparticles is an outstanding technique to revamp its electronic properties and enrich the H2 production efficiency. Metal dopants play a vital role in separation of electron hole pairs on the TiO2 surface during UV/visible/simulated solar light irradiation. In this paper, the basic principles, photocatalytic reactor design, kinetics, key findings, and the mechanism of metal doped TiO2 are comprehensively reviewed. We found that Langmuir Hinshelwood kinetic model is commonly employed by the researchers to demonstrate the rate of H2 production. Copper (Cu) gold (Au) and platinum (Pt) are the most widely studied dopants for TiO2, owing to their superior work function. The metal dopants can amplify the H2 production efficiency of TiO2 through Schottky barrier formation, surface plasmon resonance (SPR) generation of gap states by interaction with TiO2 VB states. The recent advances and important consequences of 2D materials, perovskites, and other novel photocatalysts for H2 generation have also been reviewed.
author_names: Vignesh Kumaravel

Snehamol Mathew

John BartlettJ Bartlett

Suresh C Pillai
title: Photocatalytic hydrogen production using metal doped TiO2: A review of recent advances
venue: Applied Catalysis B: Environmental
year: 2019

abstract: Photocatalysis is a promising methodology for hydrogen production. It involves the use of an unlimited source of energy, sunlight, and a semiconductor material to split the water with zero carbon emissions. This Master's thesis reports high efficiencies in terms of hydrogen production via water splitting using 2.0v/v% ethanol as scavenger, platinum and palladium as noble metals on TiO2 photocatalysts at three metal loadings (1.0, 2.5 and 5.0wt% Both near UV and visible light were used in the studies developed. These photocatalysts were synthetized using a sol gel method whose morphological properties were enhanced due to the presence of soft template precursors. Experiments were carried out in the Photo CREC Water reactor II. This novel unit offers symmetrical irradiation allowing precise irradiation measurements for macroscopic energy balances. In all cases hydrogen production followed a zero order reaction. Optimal quantum efficiencies were achieved under near UV light, reaching a significant 30.8% The doped TiO2 photocatalysts were also studied under visible light obtaining a valuable 1.2% of quantum efficiency. In order to achieve this, the prepared semiconductors were photoreduced and activated with near UV light.
author_names: Bianca Rusinque
title: Hydrogen Production by Photocatalytic Water Splitting Under Near UV and Visible Light Using Doped Pt and Pd TiO2
year: 2018

abstract: Abstract Here we report a versatile strategy based on the use of BaO as an electron promoter to boost the photocatalytic activity of TiO2 based, Pd supported photocatalysts for hydrogen production from water splitting. Pd BaO nanoparticles were deposited on the TiO2 surface by simple chemical reduction method using NaBH4 as reductant. The as synthesized Pd BaO/TiO2 photocatalysts showed a remarkably high performance in hydrogen production, approximately 29.6 mmol g 1h 1 using an ethanol water mixture (5% ethanol and 95% water) which is two times higher than Pd/TiO2 photocatalyst, where the latter is already regarded as the most active metal based TiO2 photocatalysts for H2 production. This high hydrogen production efficiency was attributed to the inherent high catalytic activity of palladium nanoparticles and electronic promotional effect of BaO. The Ba (in the form of BaO) greatly enhanced the electron transfer from the semiconductor conduction band to Pd centers by raising the Fermi level of TiO2 during photo irradiation. Most importantly, Pd is present exclusively in zero oxidation state, which is ideal for proton reduction in the as prepared photocatalysts due to close proximity of highly electropositive BaO. The structural morphologies and mechanistic aspects of the Pd BaO/TiO2 photocatalysts are addressed by using UV Vis/DRS, XRD, TEM, and XPS characterization techniques.
author_names: Ejaz Hussain

Imran Majeed

Muhammad Amtiaz Nadeem

Azhar Iqbal

Yuxiang Chen

Mohammad Choucair

Rongchao Jin

Muhammad Arif Nadeem
title: Remarkable effect of BaO on photocatalytic H2 evolution from water splitting via TiO2 (P25) supported palladium nanoparticles
venue: Journal of Environmental Chemical Engineering
year: 2019

abstract: AbstractPhotocatalytic water splitting using particulate semiconductor materials has been studied as a simple means of hydrogen production. However, there are still many obstacles to the development of complete, practical and renewable solar hydrogen production processes. This review discusses particulate photocatalyst systems intended for large scale solar hydrogen production via water splitting, focusing on their current status and potential impact. The cost and efficiency targets for solar to fuel conversion on a practical scale are also reviewed, based on the maximum allowable cost of solar hydrogen production systems, which has been estimated to be US$102 m 2, at most. Particulate photocatalyst material design principles are discussed, using efficient oxide photocatalysts as examples. Approaches to constructing photocatalytic reactors extensible to large areas are also introduced. Finally, challenges related to the development of efficient and inexpensive photocatalyst systems and potentially useful analytical methods are outlined.Photocatalytic water splitting with particulate semiconductors represents a promising strategy for the generation of hydrogen as a solar fuel. This review covers recent advances in the development of reaction systems and photocatalysts towards the scale up of this technology, emphasizing at the same time the challenges to overcome.
author_names: Takashi Hisatomi

Kazunari Domen
title: Reaction systems for solar hydrogen production via water splitting with particulate semiconductor photocatalysts
venue: Nature Catalysis
year: 2019

abstract: The production of cleaner fuels from renewable and safer energy resources are highly demanding to mitigate energy crises and global warming. In this study, the use of cleaner photo technology for selective and enhanced CO2 reduction to fuels over nickel (Ni) modified titanium dioxide (TiO2) dispersed in structured montmorillonite (MMT) nanoclay for photocatalytic CO2 hydrogen conversion via reverse water gas shift (RWGS) reaction has been investigated. The catalyst samples, prepared by a single step sol gel method, were characterised by XRD, FTIR, FESEM and UV visible spectroscopy. XRD results revealed reduced in TiO2 crystallite size with Ni and MMT loading and produced anatase phase of TiO2. MMT is found efficient for the enhanced dispersion of TiO2 while Ni promoted efficient charges separation with hindered recombination rate over the structured MMT/TiO2 nanocomposite. The photoactivity of Ni/TiO2 MMT composite for CO2 reduction was conducted in a continuous flow photoreactor using hydrogen as the reducing agent. The main products detected were CO and CH4 with appreciable amounts of C2H4, C2H6 and C3H6 hydrocarbons. The maximum yield of CO produced as the main product over 3 wt% Ni 10 wt% MMT/TiO2 catalyst was 9,429umole/g cat, 209 fold higher than the amount of CO detected over the pure TiO2. Evidently, Ni promoted TiO2 photocatalytic activity, while MMT is favourable for improved dispersion of Ni/TiO2 catalyst. The dynamic and selective CO evolution was evidently due to efficient light distribution, enlarged active surface area and efficient charges separation with their hindered recombination rate by Ni and MMT. The stability of Ni/TiO2 dispersed over MMT sustained over the irradiation time. With the use of green nanocomposite catalyst, CO2 can be efficiently converted to cleaner fuels with all sustainable systems.
author_names: Tahir Beenish

Tahir Muhammad

Nor Aishah Saidina Amin
title: Photocatalytic co2 hydrogen conversion via rwgsr over ni/tio2 nanocatalyst dispersed in layered mmt nanoclay
year: 2018

abstract: The production of carbon nanotubes (CNTs) is usually carried out either via catalytic decomposition of methane or by recrystallization of bio char species obtained by pyrolysis of solid wastes. Both techniques are generally executed at high temperatures and consume long time. Thus, these two methodologies are not considered to be economically feasible. This research work reports a facile and cost effective new route for the generation of CNTs via photocatalytic conversion of water to hydrogen. Specifically, water splitting photochemical reaction using a metal organic framework (Cr MIL 101) and TiO2 nanoparticles was demonstrated. The coupling of the aforementioned semiconductors, as novel matrices, to improve the photocatalytic reactivity toward hydrogen production in the presence of methanol, as a hole scavenger, under visible light was performed. The results revealed the high yield evolution of H2 showing a maximum purity of 99.3% with the in situ formation of carbon nanotubes over the surface of the Cr MIL 101/TiO2 binary system that was obtained by sonication. The use of this binary photocatalyst, however, as a physical mix could produce nano carbon species while simultaneously producing hydrogen rich syngas. The production of H2 gas that was simultaneously stored was noted when Cr MIL 101 was used individually as a photocatalyst. These data provide new advances in the field of water splitting by introducing promising candidate materials for the production of pure hydrogen as a clean fuel.
author_names: Rasha S Mohamed

Amal A Al Kahlawy

Ahmed M A El Naggar

Heba M Gobara
title: Innovative approach for the production of carbon nanotubes (CNTs) and carbon nanosheets through highly efficient photocatalytic water splitting into hydrogen using metal organic framework (MOF) nano TiO2 matrices as novel catalysts
year: 2020

abstract: Abstract The photocatalytic hydrogen evolution on inorganic semiconductors has been studied for several decades. However, the conversion efficiency of solar energy is still poor caused by the recombination of photo generated electron hole pairs. Semiconductors metal organic frameworks (MOFs) hybrid photocatalysts are being regarded as promising candidates due to large surface area and porosity. Here we reported the coordinative integration of a composite material with efficient capacity of H2 evolution driven by solar light, namely TiO2@ZIF 8, made up of titanium dioxide hollow nanospheres (TiO2 HNPs) externally decorated with zeolitic imidazolate framework 8 (ZIF 8) via a facile sonochemical route. The resulting composite demonstrates the high dispersion of ZIF 8 on the surface of TiO2 HNPs and this kind of close connection makes for an efficient photocatalyst through the synergistic effect. Up to 50.89% of apparent quantum efficiency (AQE) the hybrid double shell HNPs exhibits 3.5 times higher H2 evolution rate (HER) than the bare TiO2 HNPs under solar light and shows good stability and recyclability. It is further proposed by photoluminescence spectra and optoelectronic measurement that the remarkably enhanced photocatalytic activity of TiO2@ZIF 8 is not only attributed to the efficient charge separation with electron injection from ZIF 8 to TiO2 HNPs, but also more active reaction sites provided by cavity structure of ZIF 8. Overall, this work exemplifies that surface engineering of semiconductors with MOFs is a great strategy to achieve advanced photocatalytic performance for solar energy conversion.
author_names: Ming Yang Zhang

Qigao Shang

Yuqi Wan

Qingrong Cheng

Guiying Liao

Zhi-quan Pan
title: Self template synthesis of double shell TiO2@ZIF 8 hollow nanospheres via sonocrystallization with enhanced photocatalytic activities in hydrogen generation
venue: Applied Catalysis B: Environmental
year: 2019

abstract: Solar to chemical energy conversion is a challenging photochemical reaction for renewable energy storage. In recent decades, photocatalytic H2 evolution has been studied extensively. TiO2 is a well established semiconductor in the field of photocatalytic H2 production; however, its low efficiency for solar energy utilization, and high photocarrier recombination rate, restrict its photocatalytic efficiency. Here, a series of K intercalated g C3N4 modified TiO2 nanobelts (TCN Kx) with different dosages of K atoms were fabricated using a hydrothermal method followed by a calcination process. XRD, TEM and XPS tests indicate that a tight interfacial connection is formed between K g C3N4 and the TiO2 nanobelts. DFT calculations indicated that K dopants prefer to be at the interlayer sites of g C3N4, suggesting increased charge transfer efficiency. The H2 production efficiency of the TCN Kx composite materials from water splitting under visible light irradiation was clearly improved. Steady fluorescence spectroscopy and photocurrent measurements confirmed that the improvement in photocatalytic H2 production activity was due to the superior charge separation and electron transfer efficiency of TCN Kx composite materials.
author_names: Jian Ma

Wei Zhou

Xin Tan

Tao Yu
title: Potassium ions intercalated into g C3N4 modified TiO2 nanobelts for the enhancement of photocatalytic hydrogen evolution activity under visible light irradiation.
venue: Nanotechnology
year: 2018

abstract: Increasing CO2 anthropogenic concentration in the atmosphere is surpassing sustainable levels and unambiguously jeopardising the global climate. The main share of CO2 emissions corresponds to the production of energy, currently 17 TW per year. By analysing historical trends on population growth and energy consumption, it is expected that the global energy demand will reach 30 TW by 2050, which in turn will aggravate the stress on the environment. In order to alleviate adverse climate change related consequences, it was agreed to reduce global CO2 emissions. This could be achieved by shifting the current energy system towards a carbon free energy vector, hydrogen. Hydrogen can be obtained from the hydrolysis of water using surplus electricity generated from renewables, without contributing to CO2 emissions. The main drawback to switching from a carbon to a hydrogen based energy system is that H2 is a gas and current technology has evolved around liquid fuels. In order to circumvent this energy transition, hydrogen can be further transformed to liquid fuels via its reaction with CO2. In a polymer electrolyte membrane (PEM) electrolyser, water is oxidised at the anode to oxygen and protons, protons migrate through the membrane to the cathode where they recombine with electrons to form hydrogen. Due to the slow kinetics and multiple reaction steps on the anode compared to the cathode, the oxidation of water to oxygen (or oxygen evolution reaction, OER) is responsible for high overpotentials. Additionally, the anode needs to be made of iridium based catalysts, and because of its low natural abundance, it is essential to develop materials with an efficient Ir usage and to optimise its catalytic activity and stability. This research is divided in two defined themes. The first (chapter 3 to chapter 6) focuses on the optimisation of IrO2 catalysts for the oxygen evolution reaction, necessary half reaction in a PEM water electrolyser for the production of hydrogen. The second (chapter 7) comprises of the optimisation of PdZn catalysts supported on TiO2 for the further transformation of H2, by its reaction with CO2, to solar fuels. In chapters 3 and 4, the effect of the base on the hydrothermal synthesis of unsupported and supported amorphous iridium oxo hydroxides is studied. The hydrothermal synthesis was chosen because it allowed the synthesis of amorphous IrOx materials without the need for heat treatment at high temperature, thus minimising the possible crystallisation and the concomitant decrease in activity towards OER. It was observed that the base plays an important role in tailoring the morphology, surface area and surface hydroxide concentration of IrOx catalysts, and thus it has a direct effect on the catalytic activity and stability. Specifically, the use of Li2CO3 as a base led to a catalyst with porous morphology, higher surface area and higher hydroxide concentration, which this translated to an improved activity and stability towards OER compared to the state of the art catalyst IrO2*2H2O (Premion, Alfa Aesar) In both chapters, heat treatment was proven to hinder the catalytic activity towards OER, presumably as a result of higher crystallinity, the loss of Ir(III) sites and the decrease in hydroxide concentration. In chapter 5, two different IrO2 crystalline structures (rutile and hollandite) were synthesised, characterised and compared as OER catalysts. In accordance with the literature, the transformation of amorphous iridium oxo hydroxide, containing Ir(III)/Ir(IV) sites, to crystalline rutile IrO2, made only of Ir(IV) sites, led to a decrease in catalytic activity and stability. However, the presence of Li2CO3 in the amorphous IrOx catalyst led to the formation of hollandite IrO2 instead of rutile IrO2, with lithium as the host cation within the hollandite channels. Apart from the difference in crystallinity, characterisation on hollandite IrO2 indicates that it was closer in nature to amorphous IrOx than to rutile IrO2. The presence of Ir(III) and Ir(IV) was confirmed by XPS, shorter Ir Ir bond distances and longer Ir O, compared to rutile IrO2, were observed by EXAFS, and comparable OER activity to IrO2 Li2CO3 was detected by LSV. Additionally, the conversion of amorphous IrO2 Li2CO3 to hollandite IrO2 led to improved stability under OER reaction conditions. In order to use iridium more efficiently and to reduce the iridium loading on the electrode, in chapter 6 IrO2 was diluted with a more abundant and economic metal oxide, nickel or copper oxide. Catalysts with a homogeneous metal distribution and with a core shell distribution, concentrating iridium at the surface and the non noble metal oxide at the core, were prepared following a modification of the hydrothermal synthesis method. The synthesis of mixed oxide catalysts with a homogeneous metal distribution led to a decrease in the catalytic activity and the stability of the catalyst, which was proven to be an ineffective synthetic route for considerably decreasing the iridium loading on the electrode. The observed decline in the catalytic performance was attributed to the dissolution of the non noble metal oxide in contact with the reaction media. However, through a core shell distribution, IrOx was concentrated on the surface of the catalyst, whilst the non noble metal remained protected against dissolution inside the nanoparticle core. Following the core shell synthetic approach, the iridium loading on the electrode was successfully halved without impairing the catalytic activity or stability, compared to pure IrO2 Li2CO3. The second part discussed in chapter 7, studied the optimisation of PdZn/TiO2 catalysts prepared by chemical vapour impregnation (CVI) for the CO2 hydrogenation (pre reduction at 400 degC, 1 h, reaction at 250 degC, 20 bar, 30 ml*min 1, 60 H2, 20 CO2, 20 N2) to methanol, as a stable alternative to copper catalysts. The first section of the chapter focused on the Pd to Zn molar ratio in the material, maintaining the palladium loading at 5 wt. Increasing the Pd:Zn molar ratio from (1:1) to (1:5) led to a greater formation of PdZn alloy, which improved CO2 conversion, but without considerably affecting methanol selectivity. The further addition of zinc, as observed for the catalyst with a Pd:Zn molar ratio of (1:10) led to a decrease in the CO2 conversion. This was presumably caused by zinc blocking active sites when in large excess. The atomic proportion of zinc in the PdZn alloy can vary from 40 at. to 60 at. Hence it could be hypothesised that increasing the pre reduction temperature could lead to a higher proportion of zinc within the alloy, which in turn can improve methanol selectivity. In general, increasing the pre reduction temperature from 400 degC to 650 degC led to an increase in the methanol productivity because of improved methanol selectivity, although lower CO2 conversion was observed as a result of particle sintering. However, more interestingly, the CH4 selectivity decreased by one order of magnitude after increasing the pre reduction treatment from 400 degC to 650 degC, simultaneously with the transformation of ZnO and TiO2 to rhombohedral ZnTiO3. This lead to the hypothesis that the PdZn TiO2 interphase is responsible for methane production. To prove this hypothesis, PdZn/ZnTiO3 and Pd/ZnTiO3 catalysts were prepared by CVI, after pre reduction at 400 degC. PdZn alloy formation was confirmed by XRD on both systems, indicating that Zn in the ZnTiO3 phase can migrate out of the lattice to form PdZn. Thus, the PdZn TiO2 interface was generated in Pd/ZnTiO3 but not in PdZn/ZnTiO3. When tested for CO2 hydrogenation to methanol, the formation of methane on the former catalyst and its absence on the latter corroborated the formulated hypothesis that PdZn TiO2 acts as the active site for CH4 formation.
author_names: Jonathan Ruiz Esquius
title: Catalysis to produce solar fuels from the production of hydrogen via water splitting, to hydrogen conversion to methanol by its reaction with CO2
year: 2019