Photocatalytic hydrogen production by water splitting

By

AMR MOHAMED AWADALLA

A Dissertation submitted to the Faculty of Engineering, The British University in Egypt, inPartial Fulfilment of The Requirements for the Bachelor Degree in Chemical Engineering.

June, 2015

Photocatalytic hydrogen production by water splitting

Dissertation Approved:

Dr. Mostafa Soliman

Dissertation Adviser

Moderator

External Committee Member

Dr. Maguid Hassan

Dean of Engineering

Acknowledgments

This progress report could not have been completed without the effect and the assistance of somepeople. It is a great delight to thank them as they gave me instructions and made me gaincourage.

I would like to thank the researchers at the National Research Centre for their help.

Dr. Nahla IsmailDr. Hebba Ali

I would like initially to express my special acknowledgement and respect to my advisor who

helped me a lot and followed me step by step.Dr. Mostafa Soliman

Amr Moahmed AwadallaJune, 2015

Abstract

The photocatalytic and photo electrocatalytic hydrogen production were studied. In the photoelectrocatalytsis, Titanium dioxide (TiO2) nanotubes were prepared to be the workingphotoelectrode in the three electrode photo electrochemical cell (PEC), with KOH as anelectrolyte. The PEC was directed to Xenon lamp through a quartz glass for 1 hour. The H2 ratewas 0.32 ml/hr.

In the photocatlysis, two reactions were used to produce hydrogen, sulfide solutions wereprepared and also the photocatlaysts. They were held in a certain apparatus designed for theprocess, and directed for the sun light for 3 hours. There were more than one test done due to thevariety of the photocatalysts. Every photocatalyst had a result. There were negative results andpositive results. The maximum H2 rate of production was 0.2 ml/hr.

The photo electrocatalysis produced more H2 than The photocatalytsis production.

Table of ContentsAcknowledgments IAbstract IIList of Figures IIIList of Tables IVChapter 1 Introduction 8

1.1 About photocatalytic hydrogen production 91.2 project objectives 91.3 Equipment requirements 9

Chapter 2 Literature Review 102.1 About hydrogen 11

2.1.1 Physical properties of hydrogen 122.1.2 Chemical properties of hydrogen 122.1.3 Uses of hydrogen as source of energy 132.1.4 Safety 142.1.5 Methods of hydrogen production 14 2.1.6 Hydrogen production problems 15

2.2 Thermochemical hydrogen production Process 162.2.1 Concept 162.2.2 History 162.2.3 Principle 172.2.4 Problems 17

2.2.5 Examples 18 2.2.5.1 Sulfuriodine cycle 18 2.2.5.2 Cerium (IV) oxidecerium (III) oxidecycle 20 2.2.5.3 Iron oxide cycle 21 2.2.5.4 Zinczinc oxide cycle 22 2.2.5.5 Sulfur-ammonia cycle 23

2.2.5.6 Copper-chlorine cycle 24 2.2.5.7 Calcium-bromide cycle 25

2.2.6 Applications 26 2.2.6.1 Thermochemical hydrogen productionfrom a two-step solar-driven water-splitting cycle based oncerium oxides 26 2.2.6.2 Thermochemical hydrogen production bya redox system of ZrO2-supported Co(II)-ferrite 27

2.2.6.3 Hydrogen production by the CuClthermochemical cycle:

Investigation of the key step of hydrolysing CuCl2 to Cu2OCl2and HCl using a spray reactor 27 2.2.6.4 Exergy analysis of hydrogen productionby thermochemicalwater decomposition using the Ispra Mark-10 Cycle 282.3 Photocatalysis 29

2.3.1 Definition 292.3.2 Types of photocatalysis 292.3.3 Mechanism of homogeneous photocatalysis 292.3.4 Mechanism of heterogeneous photocatalysis 302.3.5 Valence band 32 2.3.5.1 Definition 32

2.3.6 Conduction band 32 2.3.6.1 Definition 32 2.3.6.2 Difference between valence band andconduction band 332.3.7 Band gap 33 2.3.7.1 Definition 33

2.3.7.2 List of band gaps 34

2.4 Photo catalytic hydrogen production 35 2.4.1 Definition 35 2.4.2 Concept 35 2.4.3 Principle 36 2.4.4 Photocatalyst systems 37

2.4.4.1 NaTaO3: La 37 2.4.4.2 K3Ta3B2O12 37

2.4.4.3 (Ga.82Zn.18)(N.82O.18) 37 2.4.4.4 Pt/TiO2 38

2.4.4.5 Cobalt based systems 38 2.4.4.6 Bismuth 38 2.4.5 Applications 39 2.4.5.1 Scale-up analysis and exergo economicassessment of large scale photo-catalytic hydrogenproduction plants 39 2.4.5.2 An Au/Cu2OTiO2 system for photo-catalytic hydrogen production.A pn-junction effect or asimple case of in situ reduction 39 2.4.5.3 Efficient photo catalytic hydrogenproduction from water over a CuO and carbon fiber comodified TiO2 nano composite photo catalyst 40 2.4.5.4 Photo catalytic hydrogen production And

Materials Science in Semiconductor Processing 41Chapter 3 Solar Simulators 10

3.1 Definition 29 3.2 Purpose 29 3.3 Uses 29 3.4 Types 29 3.5 Types of lamps 29 3.6 Photocatalysis lamps 29

Chapter 4 Photocatalysts Characteristics 10 4.1 List of Photocatalysts characteristics 29

Chapter 5 Methodology 10

5.1 Photo electrocatlysis 29 5.2 Photocatalysis 29

5.2.1 The first reaction 29 5.2.2 The second reaction 29

5.2.3 Description of the appratus 29Chapter 6 Experimental work 10

6.1 Experimental work at the National ResearchCentre 29 6.1.1 Materials used 29 6.1.2 Equipment used 29 6.1.3 Experimental procedures for thepreparation of pure (TiO2) nanotubes 29 6.1.4 Experimental procedures for the photoelectrochemical cell (Pec) process 29 6.2 Experimental work at the university 29 6.2.1 Materials used 29 6.2.2 Equipment used 29

6.2.3 The apparatus 29 6.2.4 The first reaction 29 6.2.4.1 Test 1: (cupric oxide(CuO)) 29 6.2.4.2 Test 2: (cuprous oxide(Cu2O)) 29 6.2.5 The second reaction 29 6.2.5.1 Preparation of Zinc sulphide

(ZnS) 29 6.2.5.2 Preparation of Cadmimumsulphide (CdS) 29 6.2.5.3 Test 1 29 6.2.5.4 Test 2 29 6.2.5.5 Test 3 29 6.2.5.6 Test 4 29 6.2.5.7 Test 5 29 6.2.5.8 Test 6 29

Chapter 7 Results and discussion 10 7.1 Results at the National Research Centre 29 7.2 Results at the university 29 7.2.1 Solubility results for the firstreaction 29 7.2.1.1 Test 1: (cupric oxide (CuO)) 29 7.2.1.2 Test 2: (cuprous oxide (Cu2O))

29 7.2.3 Results for the second reaction 29 7.2.3.1 Test 1(Cu2O) 29 7.2.3.2 Test 2 (CuO) 29 7.2.3.3 Test 3(TiO2) 29

7.2.3.4 Test 4(ZnS) 29 7.2.3.5 Test 5(CdS) 29 7.2.3.6 Test 6(CdS/ZnS) 29

Conclusion VBibliography V

LIST OF FIGURES

FIGURE 1 : HYDROGEN STRUCTURE 3FIGURE 2: USES OF HYDROGEN 5FIGURE 3: THERMOCHEMICAL HYDROGEN PRODUCTION CONCEPT 7FIGURE 4: SULFUR-IODINE CYCLE 9FIGURE 5: CERIUM (IV) OXIDE-CERIUM (III) OXIDE CYCLE 11FIGURE 6: IRON OXIDE CYCLE 12FIGURE 7: ZINC-ZINC OXIDE CYCLE 13FIGURE 8: CALCIUM-BROMIDE CYCLE 16FIGURE 9: VALENCE BAND 23FIGURE 10: GRAPH SHOWS BAND GAP 25FIGURE 11: PHOTOCATALYTIC HYDROGEN PRODUCTION CONCEPT 27FIGURE 12 : THE DESINED APPARTUS WITH DETAILS 3FIGURE 13: PREPARATION OF TIO

2 NANOTUBES 5

FIGURE 14: TIO2 NANOTUBES SHAPE 7

FIGURE 15: THREE ELECTRODE PHOTO ELECTROCHEMICAL CEL (PEC) 9FIGURE 16: THE APPARTUS 11FIGURE 17: PREPARATION OF CDS 12FIGURE 18: FILTRATION OF ZNS 13FIGURE 19: POTENTIAL VS PHOTOCURRENT DEISTY IN LIGHT ON 16FIGURE 20: POTENIAL VS PHOTOCURRENT DENISTY IN LIGHT OFF 23FIGURE 21: CUPROUS OXIDE TEST 25FIGURE 22: CDS TEST 23

FIGURE 23: CDS/ZNS TEST 25 LIST OF TABLES TABLE 1: PHYSICAL PROPERTIES OF HYDROGEN 5TABLE 2: CHEMICAL PROPERTIES OF HYDROGEN 5TABLE 3: LIST OF BAND GAPS 26TABLE 4: CHARACTERISTICS OF PHOTOCATALYSTS USED 5TABLE 5: LIGHT ON RESULTS 5TABLE 6: LIGHT OFF RESULTS 26TABLE 7: RESULTS OF PHOTO ELECTROCATALYSIS 26TABLE 8: RESULTS OF PHOTO CATALYSIS 26 Research background

Hydrogen production from water using photocatalytic semi-conductors is of great interestdue its possible application to convert solar energy to hydrogen which can be used as a fuel .

Many photocatalysts have been tried. These incude titania, CdS, ZnS, and copper salts.Sacrificial materials are used such as methanol, ethanol, sulfites and sulfides.

Chapter1: Introductio

n

1.1 About photocatalytic hydrogen production: Hydrogen fuel production has gained increased attention as oil and other nonrenewablefuels become increasingly depleted and expensive. Methods such as photocatalytic watersplitting are being investigated to produce hydrogen fuel, which burns cleanly and can be used ina hydrogen fuel cell. Water splitting holds particular interest since it utilizes water, aninexpensive renewable resource.

Photocatalytic water splitting is an artificial photosynthesis process with photocatalysis in aphotoelectrochemical cell used for the dissociation of water into its constituent parts, hydrogen(H2) and oxygen (O2), using either artificial or natural light. Theoretically, only solar energy,water, and a catalyst are needed.

1.2 research objectives: In this research, a study to be made on the use of titania, CdS, ZnS, cupric oxide , andcuprous oxide as photocatalysts for water splitting. And sulfites will be used as sacrificial agents.

Chapter 2:Literature

review 2.1About hydrogen: Hydrogen is a chemical element with chemical symbol H and atomic number 1. With anatomic weight of 1.00794 u, hydrogen is the lightest element on the periodic table. Its monatomicform (H) is the most abundant chemical substance in the universe, hydrogen is also a promisingsource of "clean" fuel on Earth. Named after the Greek words hydro for "water" and genes for"forming," hydrogen makes up more than 90 percent of all of the atoms, which equals threequarters of the mass of the universe. [1]

Figure 1 : Hydrogen structure

2.1.1Physical properties of hydrogen:Color Colorless

Phase

GasHydrogen changes from a gas to a liquid at a temperature of -252.77C(-422.99F)It changes from a liquid to a solid at a temperature of -259.2C (-434.6F)

Odor Hydrogen is an odorless gasTaste A tasteless gas

Density The lowest of any chemical element, 0.08999 grams per liter - the least dense ofall gasesSolubility Slightly soluble in water, alcohol and some other common liquids Table 1: Physical properties of hydrogen

2.1.2Chemical properties of hydrogen:

Chemical FormulaHHydrogen gas (H2)

Oxidation It burns in air or oxygen to produce waterH2 reacts with every oxidizing element

Reactivity with gasesCombining hydrogen and nitrogen at high pressure and temperatureproduces ammonia (NH3)Combined with carbon monoxide produces methanol (CH3OH)

Reactivity with non-metals

It combines readily with non-metals, such as sulfur and phosphorusIt combines readily with the halogens which include fluorine, chlorine,bromine, iodine, and astatine

Flammability Highly Flammable, a highly combustible diatomic gas

CombustionWhen mixed with air and with chlorine it can spontaneously explode byspark, heat or sunlight. Example: the destruction of the Hindenburgairship

Acid CompoundsCommon acids include hydrochloric acid (HCl), sulfuric acid (H2SO4),nitric acid (HNO3), acetic acid (HC2H3O2) and phosphoric acid (H3PO4)

Table 2: Chemical properties of hydrogen

2.1.3 Uses of hydrogen as source of energy: Uses for hydrogen varied widely. It is used in many different industries, includingpetroleum and chemical industries. It is also used in the food industry, in hydrogenating fats andoils, which permits to form margarine from vegetable oil. Hydrogen is also useful in producingmethanol. It is also used in reducing metal ores.Other uses for hydrogen may be found in welding, in power generators, and in cryogenicsresearch.In many recent studies, hydrogen is being pointed out as a source for clean fuel. There are manybenefits of using hydrogen instead of other sources of energy.For one, the use of hydrogen in power vehicles produces no greenhouse gasses that may affectMother Nature. If hydrogen is used as a fuel source, it will only need to be combined with

oxygen, whose byproducts are only heat and water. [1]

Figure 2: Uses of hydrogen

2.1.4 Safety: Like any other fuel or energy carrier hydrogen poses risks if not properly handled orcontrolled. The risk of hydrogen, therefore, must be considered relative to the common fuelssuch as gasoline, propane or natural gas. The specific physical characteristics of hydrogen arequite different from those common fuels. Some of those properties make hydrogen potentiallyless hazardous, while other hydrogen characteristics could theoretically make it more dangerousin certain situations. [2]

2.1.5 Methods of hydrogen production:From fossil fuels:-Steam reforming-Partial oxidationFrom water:-Electrolysis-Thermolysis-Thermochemical cycle-Ferrosilicon method-Photo biological water splitting-Photo catalytic water splitting [3]

2.1.6 Hydrogen production problems: The most daunting problem associated with current hydrogen production is the energyneeded to produce it and to provide for energy losses in the hydrogen-to-application chain. Usingexisting conventional technology, "hydrogen requires at least twice as much energy as electricitytwice the tonnage of coal, twice the number of nuclear plants, or twice the field of PV panels toperform an equivalent unit of work. Most of today's hydrogen is produced from natural gas,which is only an interim solution since it discards 30% of the energy in one valuable butdepletable fuel (natural gas) to obtain 70% of another (hydrogen). The challenge is to developmore appropriate methods based on sustainable energy sources, methods that do not employelectricity as an intermediate step. [4]

2.2 Thermochemical hydrogen production Process:

2.2.1 Concept:It requires only (i.e. mainly) water as a material input and mainly thermal energy, or heat,

as an energy input. The output of the process is hydrogen and oxygen and possibly (or probably)some waste heat. [5]

Figure 3: Thermochemical hydrogen production concept

2.2.2 History:This concept was first postulated by Funk and Reinstrom (1966) as a reflexion about the

most efficient way to produce fuels (e.g. hydrogen, ammonia) from stable and abundant species(e.g. water, nitrogen) and heat sources. Although fuel availability was scarcely considered beforethe oil crisis era, these researches were justified by niche markets. As an example, in the militarylogistics field, providing fuels for vehicles in remote battlefields is a key task. Hence, a mobileproduction system based on a portable heat source (a specific nuclear reactor was strikinglyconsidered) was being investigated with the uttermost interest. Following the crisis, manyprograms (Europe, Japan, and USA) were set up to design, test and qualify such processes formore peaceful purposes such as energy independence. High temperature (1000K) nuclearreactors were still considered as the heat sources. However, the optimistic expectations of thefirst thermodynamics studies were quickly moderated by more pragmatic analysis based on faircomparisons with standard technologies (thermodynamic cycles for electricity generation,coupled with the electrolysis of water) and by numerous practical issues (not high enoughtemperatures with nuclear reactors, slow reactivities, reactor corrosion, significant losses ofintermediate compounds with time...). Hence, the interest for this technology was fading awayduring the next decades, or at least some tradeoffs (hybrid versions) were being considered withthe use of electricity as a fractional energy input instead of only heat for the reactions (e.g.Hybrid sulfur cycle). A rebirth in the year 2000 can be explained by both new energy crisis andthe rapid pace of development of concentrated solar power technologies whose potentially veryhigh temperatures are ideal for thermochemical processes, while the environmentally friendlyside of these researches attracts funding in a period with the peak oil shadow. [5]

2.2.3 Principle:

Thermochemical production of hydrogen involves chemical splitting of water attemperatures lower than those needed for thermolysis, through a series of cyclical chemicalreactions that ultimately release hydrogen. Some of the more thoroughly investigated

thermochemical process cycles include the following: sulfuric acid -iodine cycle, hybrid sulfuricacid cycle, hybrid sulfuric acid -hydrogen bromide cycle, calcium bromide -iron oxide cycle ,and iron chlorine cycle. [2]

2.2.4 Problems: Depending on the temperatures at which these processes are occurring, relatively highefficiencies are achievable (40-50 per cent). However the problems related to movement of largemass of materials in chemical reactions, toxicity of some of the chemicals involved, andcorrosion at high temperatures remain to be solved in order for these methods to becomepractical. [2]

2.2.5 Examples:2.2.5.1 Sulfuriodine cycle: The sulfuriodine cycle (SI cycle) is a three-step thermochemical cycle used to producehydrogen. The SI cycle consists of three chemical reactions whose net reactant is water andwhose net products are hydrogen and oxygen. All other chemicals are recycled. The SI processrequires an efficient source of heat. [6]

Figure 4: Sulfur-iodine cycle

Process:The three reactions that produce hydrogen are as follows:

1. I2 + SO2 + 2 H2O 2 HI + H2SO4 (120 C)o The HI is then separated by distillation or liquid/liquid gravitic separation.

2. 2 H2SO4 2 SO2 + 2 H2O + O2 (830 C)o The water, SO2 and residual H2SO4 must be separated from the oxygen byproduct

by condensation.

3. 2 HI I2 + H2 (450 C)

o Iodine and any accompanying water or SO2 are separated by condensation, and thehydrogen product remains as a gas.

Net reaction: 2 H2O 2 H2 + O2The sulfur and iodine compounds are recovered and reused, hence the consideration of theprocess as a cycle. This SI process is a chemical heat engine. Heat enters the cycle in high-temperature endothermic chemical reactions 2 and 3, and heat exits the cycle in the low-temperature exothermic reaction 1. The difference between the heat entering and leaving thecycle exits the cycle in the form of the heat of combustion of the hydrogen produced. [6]

The characteristics of the SI process can be described as follows:

All fluid (liquids, gases) process, therefore well suited forcontinuous operation;

High utilization of heat predicted (about 50%), but very hightemperatures required (at least 850 C);

Completely closed system without byproducts or effluents(besides hydrogen and oxygen);

Corrosive reagents used as intermediaries (iodine, sulfurdioxide, hydriodic acid, sulfuric acid); therefore, advancedmaterials needed for construction of process apparatus;

Suitable for application with solar, nuclear, and hybrid (e.g.,solar-fossil) sources of heat;

More developed than competitive thermochemical processes(but still requiring significant development to be feasible onlarge scale). [7]

The sulfur-iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-basedeconomy. With an efficiency of around 50% it is more efficient than electrolysis, and it does notrequire hydrocarbons like current methods of steam reforming but requires heat fromcombustion, nuclear reactions, or solar heat concentrators. [7]

2.2.5.2 Cerium (IV) oxidecerium (III) oxide cycle:

The cerium(IV) oxidecerium(III) oxide cycle orCeO2/Ce2O3 cycle is a two-step thermochemical process that

employs cerium(IV) oxide and cerium(III) oxide for hydrogenproduction. The cerium-based cycle allows the separation of H2and O2 in two steps, making high-temperature gas separationredundant. [8]

Figure 5: Cerium (IV) oxide-cerium (III) oxide cycle

Process:

The thermochemical two-step water splitting process (thermochemical cycle) uses redoxsystems:

Dissociation: 2CeO2 Ce2O3 + 0.5 O2 Hydrolysis: Ce2O3 + H2O 2CeO2 + H2

For the first endothermic step, cerium (IV) oxide is thermally dissociated in an inert gasatmosphere at 2,000 C (3,630 F) and 100-200 mbar into cerium(III) oxide and oxygen. In thesecond exothermic step cerium (III) oxide reacts at 400 C (752 F)600 C (1,112 F) in a fixedbed reactor with water and produces hydrogen and cerium(IV) oxide. [8]

2.2.5.3 Iron oxide cycle: The iron oxide cycle (Fe3O4/FeO) is the original two-step thermochemical cycleproposed for use for hydrogen production. It is based on the reduction and subsequent oxidationof iron ions, particularly the reduction and oxidation from Fe3+ to Fe2+. The ferrites, or iron oxide,begins in the form of a spinel and depending on the reaction conditions, dopant metals andsupport material forms either Wstites or different spinels.

Figure 6: Iron oxide cycle

Process:The thermochemical two-step water splitting process uses two redox steps. The steps of solarhydrogen production by iron based two-step cycle are:(1) M(II)Fe2(III)O4 M(II)O + 2Fe(II)O + O2 (Reduction)

(2) M(II)O + 2Fe(II)O + H2O M(II)Fe2(III)O4 + H2 (Oxidation)

Where M can by any number of metals, often Fe itself, Co, Ni, Mn, Zn or mixtures thereof.

The endothermic reduction step (1) is carried out at high temperatures greater than 1400 oC,though the "Hercynite cycle" is capable of temperatures as low as 1200 oC. The oxidative watersplitting step (2) occurs at a lower ~1000 oC temperature which produces the original ferritematerial in addition to hydrogen gas. [9]2.2.5.4 Zinczinc oxide cycle: The zinczinc oxide cycle or ZnZnO cycle is a two-step thermochemical cycle basedon zinc and zinc oxide for hydrogen production with a typical efficiency around 40%.

Figure 7: Zinc-zinc oxide cycle

.

Process:The thermochemical two-step water splitting process uses redox systems:

Dissociation: ZnO Zn + 1/2 O2 Hydrolysis: Zn + H2O ZnO + H2

For the first endothermic step concentrating solar power is used in which zinc oxide is thermallydissociated at 1,900 C (3,450 F) into zinc and oxygen. In the second non-solar exothermic stepzinc reacts at 427 C (801 F) with water and produces hydrogen and zinc oxide. [10]

2.2.5.5 Sulfur-ammonia cycle: The sulfur-ammonia cycle or (S-A) cycle is a two-step thermochemical cycle based onsulfur and ammonia for hydrogen production.

Process:

SO2(g) + 2NH3(g) + H2O(l) (NH4)2SO3(aq)(Chemical absorption, 25 co) (NH4)2SO3(aq) + H2O(l) (NH4)2SO4(aq) + H2(g)(Solar photocatalytic,

Process:1-Hydrogen production step temperature2Cu(s) + 2HCl(g) 2CuCl(l) + H2(g) 450 2-Electrolysis step 4CuCl(aq) 2CuCl2(aq) + 2Cu(s) Ambient(electrolysis)(In HCl solution)3-Drying step 702CuCl2 (aq) 2CuCl2(s)2CuCl2 (slurry) 2CuCl2(s)2.2H2O (l) 2.2H2O (g)0.1HCl (aq) 0.1 HCl (g)

4-Hydrolysis step 3752CuCl2(s) + H2O(g) CuO.CuCl2(s) + 2HCl(g)

5-Oxygen production step 530CuO.CuCl2(s) 2CuCl (l) + 1/2O2 (g) [12]

2.2.5.7 Calcium-bromide cycle:

The calciumbromide-based cycles also have the potential of high efficiencies but withlower temperature requirements (?750 C) than the sulfur-based cycles. The common step inthese cycles is the conversion of CaO and Br2 to CaBr2 and O2at approximately 550 C, and theconversion of CaBr2 back to CaO and HBr at 730 C. The second recycle step, converting HBrto Br2 and generating hydrogen, can be done thermally in a solid-gas, fixed bed reactor of ironoxide. [13]

Figure 8: Calcium-bromide cycle

2.2.6 Applications: 2.2.6.1 Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides: The feasibility of a new thermochemical two-step cycle which has beenexperimentally demonstrated at lab scale and which is based on cerium oxides. This processproduces hydrogen from water. The solar energy supply (heat input) is converted and stored intoa sustainable energy carrier. The solar activation of Ce (IV) oxide was performed underconcentrated solar irradiation that permits it to reach the fusion of the material over 2000C andits thermal reduction by releasing O2 at reduced pressure. Then, the activated Ce (III) oxidereacted completely with water to produce hydrogen. The synthesis of a chemical intermediate(reduced cerium oxide) reactive with water also allows solving safety problems associated withhydrogen storage and transportation. The solid material stable and storable at ambienttemperature (its reactivity with water is not altered with time) could be used as hydrogen tank.Heating moderately the solid oxide in presence of water could release rapidly hydrogen on

demand, which makes on-board hydrogen production possible with such a system. In somecases, this chemical intermediate synthesis could permit to counteract the important problem ofhydrogen storage/transport. This study is the first demonstration of the CeO2/Ce2O3 cycle sinceit has never been studied before. Optimization of the high temperature solar thermal step anddesign of solar chemical reactor able to achieve high conversion rates are now needed for a pilot-scale process implementation. New thermo chemical cycle for H2production based on

CeO2/Ce2O3oxides which has been successfully demonstrated. It consists of two chemical steps: (1) reduction,

2CeO2 Ce2O3+ 0.5O2;

(2) Hydrolysis,

Ce2O3+H2O 2CeO2+H2.

The thermal reduction of Ce (IV) to Ce(III) (endothermic step) is performed in a solar reactorfeaturing a controlled inert atmosphere. The feasibility of this first step has been demonstratedand the operating conditions have been defined (T= 2000C, P= 100200 mbar). The hydrogengeneration step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and thereaction is complete with a fast kinetic in the studied temperature range 400600C. Therecovered Ce(IV) oxide is then recycled in first step. In this process, water is the only materialinput and heat is the only energy input. The only outputs are hydrogen and oxygen, and thesetwo gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbonproducts like CO, CO2), thus it can be used directly in fuel cells. The results have shown that thecerium oxide two-step thermochemical cycle is a promising process for hydrogen production.[14]

2.2.6.2 Thermochemical hydrogen production by a redox system of ZrO2-supportedCo(II)-ferrite:

The thermochemical two-step water splitting examined on ZrO2-supported Co(II)-ferrites below 1400 C, for purpose of converting solar high-temperature heat to clean hydrogenenergy as storage and transport of solar energy. The ferrite on the ZrO2-support was thermallydecomposed to the reduced phase of wustite at 1400C under an inert atmosphere. The reducedphase was reoxidized with steam on the ZrO2-support to generate hydrogen below 1000C in aseparate step. The ZrO2-supporting alleviated the high temperature sintering of iron oxide. As theresults, the ZrO2-supported ferrite realized a greater reactivity and a better repeatability of thecyclic water splitting than the conventional unsupported ferrites. The CoxFe3-xO4/ZrO2 with the xvalue of around 0.40.7 was found to be the promising working material for the two-step watersplitting when thermally reduced at 1400C under an inert atmosphere. [15]

2.2.6.3 Hydrogen production by the CuCl thermochemical cycle:Investigation of the key step of hydrolysing CuCl2 to Cu2OCl2 and HCl using aspray reactor:

A spray reactor for the hydrolysis of CuCl2to Cu2OCl2and HCl was designed andtested. Analyses by XRD, SEM, and wet chemistry methods were performed on the solidproducts. Two types of atomizers were investigated. With a pneumatic nebulizer, the counter-current flow reactor design leads to a significantly higher yield of Cu2OCl2 compared to the cocurrent flow design due to enhanced mass-transfer. An increase in the gas flow-to-liquid flow

ratio in the nebulizer increased the formation of Cu2OCl2, as a result of better atomization andsmaller droplets. However, some CuCl2 remained in all cases. The ultrasonic nozzle providedbetter results and was easier to use (no clogging). Higher yields of Cu2OCl2were obtainedwithout the use of an inert gas. The enhanced atomization provided by the ultrasonic nozzle ledto smaller Cu2OCl2 particles and a narrower particle size distribution. Lower velocity of the

reactants at the tip of the ultrasonic nozzle may also explain the larger conversion to Cu2OCl2.High yields of Cu2OCl2, up to 95%, with a small amount of CuCl could be achieved using asteam-to-copper ratio of 24 with the ultrasonic nozzle and co-current flow. Tests conducted atCEA with a steam-to-copper ratio of 15 showed that only the desired HCl was formed attemperatures390C. The formation of Cl2 was observed only at temperatures above 400C. Thus,the formation of a small amount of CuCl observed during the hydrolysis of CuCl2in the sprayreactor at

O3 + h O2 + O(1D) (? O3 "-" h O2 + O(1D) ?)

O(1D) + H2O OH + OH

O(1D) + H2O H2O2

H2O2 + h OH + OH

Similarly, the Fenton system produces hydroxyl radicals by the following mechanism

Fe2+ + H2O2 HO + Fe3+ + OH

Fe3+ + H2O2 Fe2+ + HO2 + H+

Fe2+ + HO Fe3+ + OH

In photo-Fenton type processes, additional sources of OH radicals should be considered: throughphotolysis of H2O2, and through reduction of Fe

3+ ions under UV light:

H2O2 + h HO + HO

Fe3+ + H2O + h Fe2+ + HO + H+

The efficiency of Fenton type processes is influenced by several operating parameters likeconcentration of hydrogen peroxide, pH and intensity of UV. The main advantage of this processis the ability of using sunlight with light sensitivity up to 450 nm, thus avoiding the high costs ofUV lamps and electrical energy. These reactions have been proven more efficient than the otherphotocatalysis but the disadvantages of the process are the low pH values which are required,since iron precipitates at higher pH values and the fact that iron has to be removed aftertreatment. [18]

2.3.4 Mechanism of heterogeneous photocatalysis: Heterogeneous catalysis has the catalyst in a different phase from the reactants.Heterogeneous photocatalysis is a discipline which includes a large variety of reactions: mild ortotal oxidations, dehydrogenation, hydrogen transfer, 18O2

16O2 and deuterium-alkane isotopicexchange, metal deposition, water detoxification, gaseous pollutant removal.Most common heterogeneous photocatalyts are transition metal oxides and semiconductors,which have unique characteristics. Unlike the metals which have a continuum of electronicstates, semiconductors possess a void energy region where no energy levels are available topromote recombination of an electron and hole produced by photoactivation in the solid. Thevoid region, which extends from the top of the filled valence band to the bottom of the vacantconduction band, is called the band gap. When a photon with energy equal to or greater than thematerials band gap is absorbed by the semiconductor, an electron is excited from the valenceband to the conduction band, generating a positive hole in the valence band. The excited electronand hole can recombine and release the energy gained from the excitation of the electron as heat.Recombination is undesirable and leads to an inefficient photocatalyst. The ultimate goal of theprocess is to have a reaction between the excited electrons with an oxidant to produce a reducedproduct, and also a reaction between the generated holes with a reductant to produce an oxidizedproduct. Due to the generation of positive holes and electrons, oxidation-reduction reactions takeplace at the surface of semiconductors. In the oxidative reaction, the positive holes react with themoisture present on the surface and produce a hydroxyl radical.

Oxidative reactions due to photocatalytic effect:

UV + MO MO (h + e)

Here MO stands for metal oxide ---

h+ + H2O H+ + OH

2 h+ + 2 H2O 2 H+ + H2O2

H2O2 HO + OH

The reductive reaction due to photocatalytic effect:

e + O2 O2

O2 + HO2 + H+ H2O2 + O2

HOOH HO + OH

Ultimately, the hydroxyl radicals are generated in both the reactions. These hydroxyl radicals arevery oxidative in nature and non-selective with redox potential of (E0 = +3.06 V). [19]

2.3.5 Valence band:2.3.5.1 Definition: In solids, the valence band is the highest range of electron energies in whichelectrons are normally present at absolute zero temperature.The valence electrons are bound to individual atoms, as opposed to conduction electrons (foundin conductors and semiconductors), which can move freely within the atomic lattice of thematerial. On a graph of the electronic band structure of a material, the valence band is locatedbelow the conduction band, separated from it in insulators and semiconductors by a band gap. Inmetals, the conduction band has no energy gap separating it from the valence band. [20]

Figure 9: Valence band

2.3.6 Conduction band:2.3.6.1 Definition: The conduction band quantifies the range of energy required to free an electronfrom its bond to an atom. Once freed from this bond, the electron becomes a 'delocalizedelectron', moving freely within the atomic lattice of the material to which the atom belongs.

Various materials may be classified by their band gap: this is defined as the difference betweenthe valence and conduction bands.Electrons within the conduction band are mobile charge carriers in solids, responsible forconduction of electric currents in metals and other good electrical conductors.The concept has wide applications in the solid-state physics field of semiconductors andinsulators. [20]

2.3.5.6.2 Difference between valence band and conduction band:

In insulators, the conduction band is much higher in energythan the valence band and it takes large energies todelocalize their valence electrons. Insulating materials havewide band gaps.

In semiconductors, the band gap is small. This explains whyit takes a little energy (in the form of heat or light) to makesemiconductors' electrons delocalize and conductelectricity, hence the name, semiconductor.

In metals, the Fermi level is inside at least one band. TheseFermi-level-crossing bands may be called conduction band,valence band, or something else depending oncircumstance. [20]

2.3.7 Band gap:2.3.7.1 Definition: In solid-state physics, a band gap, also called an energy gap or bandgap, is an energyrange in a solid where no electron states can exist. In graphs of the electronic band structure ofsolids, the band gap generally refers to the energy difference (in electron volts) between the topof the valence band and the bottom of the conduction band in insulators and semiconductors.This is equivalent to the energy required to free an outer shell electron from its orbit about thenucleus to become a mobile charge carrier, able to move freely within the solid material, so theband gap is a major factor determining the electrical conductivity of a solid. Substances withlarge band gaps are generally insulators, those with smaller band gaps are semiconductors, whileconductors either have very small band gaps or none, because the valence and conduction bandsoverlap. [20]2.3.7.2 List of band gaps: Below are band gap values for some selected materials. For a comprehensive list ofband gaps in semiconductors, see List of semiconductor materials. [20]

Group Material Symbol Band gap (eV) @ 302K

IV Diamond C 5.5

IV Silicon Si 1.11

IV Germanium Ge 0.67

IIIV Gallium(III) nitride GaN 3.4

IIIV Gallium(III) phosphide GaP 2.26

IIIV Gallium(III) arsenide GaAs 1.43

IVV Silicon nitride Si3N4 5

IVVI Lead(II) sulfide PbS 0.37

IVVI Silicon dioxide SiO2 9

Copper(I) oxide Cu2O 2.1

Table 3: list of band gaps

Figure 10: Graph shows band gap

2.4 Photo catalytic hydrogen production:

2.4.1 Definition: Photocatalytic water splitting is an artificial photosynthesis process with photocatalysis ina photoelectrochemical cell used for the dissociation of water into its constituent parts, hydrogen(H2) and oxygen (O2), using either artificial or natural light. Theoretically, only solar energy(photons), water, and a catalyst are needed. [21]

2.4.2 Concept: When H2O is split into O2 and H2, the stoichiometric ratio of its products is 2:1:

The process of water-splitting is a highly endothermic process (H > 0). Water splitting occursnaturally in photosynthesis when photon energy is absorbed and converted into the chemicalenergy through a complex biological pathway. However, production of hydrogen from waterrequires large amounts of input energy, making it incompatible with existing energy generation.

[21]

Figure 11: Photocatalytic hydrogen production concept

The efficient storage of solar energy in chemical fuels, such as hydrogen, is essential for thelarge-scale utilization of solar energy systems. Recent advances in the photo catalytic productionof H2 are highlighted. Two general approaches for the photo catalytic hydrogen generation byhomogeneous catalysts are considered: HX (X = Cl, Br) splitting involving both proton reductionand halide oxidation via an inner-sphere mechanism with a single-component catalyst; andsensitized H2 production, employing sacrificial electron donors to regenerate the active catalyst.Future directions and challenges in photo catalytic H2 generation are enumerated. [22]

2.4.3 Principle: Photocatalysts must conform to several key principles in order to be considered effectiveat water splitting. A key principle is that H2 and O2 evolution should occur in a stoichiometric2:1 ratio; significant deviation could be due to a flaw in the experimental setup and/or a sidereaction, both of which do not indicate a reliable photocatalyst for water splitting. The primemeasure of photocatalyst effectiveness is quantum yield (QY), which is:

QY (%) = (Photochemical reaction rate) / (Photonabsorption rate) 100%

This quantity is a reliable determination of how effective a photocatalyst is; however, it can bemisleading due to varying experimental conditions. To assist in comparison, the rate of gasevolution can also be used; this method is more problematic on its own because it is notnormalized, but it can be useful for a rough comparison and is consistently reported in theliterature. Overall, the best photocatalyst has a high quantum yield and gives a high rate of gasevolution.The other important factor for a photocatalyst is the range of light absorbed; though UV-basedphotocatalysts will perform better per photon than visible light-based photocatalysts due to thehigher photon energy, far more visible light reaches the Earth's surface than UV light. Thus, aless efficient photocatalyst that absorbs visible light may ultimately be more useful than a moreefficient photocatalyst absorbing solely light with smaller wavelengths. [23]

2.4.4 Photocatalyst systems: 2.4.4.1 NaTaO3: La NaTaO3:La yields the highest water splitting rate of photocatalysts without using sacrificialreagents. This UV-based photocatalyst was shown to be highly effective with water splitting ratesof 9.7 mmol/h and a quantum yield of 56%. The nanostep structure of the material promotes

water splitting as edges functioned as H2 production sites and the grooves functioned as O2production sites. Addition of NiO particles as cocatalysts assisted in H2 production; this step wasdone by using an impregnation method with an aqueous solution of Ni(NO3)26H2O andevaporating the solution in the presence of the photocatalyst. NaTaO3 has a conduction bandhigher than that of NiO, so photogenerated electrons are more easily transferred to theconduction band of NiO for H2 evolution. [23]2.4.4.2 K3Ta3B2O12 K3Ta3B2O12, another catalyst activated by solely UV light and above, does not havethe performance or quantum yield of NaTaO3:La. However, it does have the ability to split waterwithout the assistance of cocatalysts and gives a quantum yield of 6.5% along with a watersplitting rate of 1.21 mmol/h. This ability is due to the pillared structure of the photocatalyst,which involves TaO6 pillars connected by BO3 triangle units. Loading with NiO did not assistthe photocatalyst due to the highly active H2 evolution sites. [24]2.4.4.3 (Ga.82Zn.18)(N.82O.18) (Ga.82Zn.18)(N.82O.18) has the highest quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents as of October 2008. The photocatalystgives a quantum yield of 5.9% along with a water splitting rate of 0.4 mmol/h. Tuning thecatalyst was done by increasing calcination temperatures for the final step in synthesizing thecatalyst. Temperatures up to 600 C helped to reduce the number of defects, though temperaturesabove 700 C destroyed the local structure around zinc atoms and was thus undesirable. Thetreatment ultimately reduced the amount of surface Zn and O defects, which normally functionas recombination sites, thus limiting photocatalytic activity. The catalyst was then loaded withRh2-yCryO3 at a rate of 2.5 wt % Rh and 2 wt% Cr to yield the best performance. [25]2.4.4.4 Pt/TiO2 TiO2 is a very efficient photocatalyst, as it yields both a high quantum number and a highrate of H2 gas evolution. For example, Pt/TiO2 (anatase phase) is a catalyst used in watersplitting. These photocatalysts combine with a thin NaOH aqueous layer to make a solution thatcan split water into H2 and O2. TiO2 absorbs only ultraviolet light due to its large bandgap(>3.0ev), but outperforms most visible light photocatalysts because it does not photocorrodeas easily. Most ceramic materials have large band gaps and thus have stronger covalent bondsthan other semiconductors with lower band gaps. [26]2.4.4.5 Cobalt based systems Photocatalysts based on cobalt have been reported. Members are tris(bipyridine)cobalt(II), compounds of cobalt ligated to certain cyclic polyamines, and certain cobaloximes.In 2014 researchers announced an approach that connected a chromophore to part of a largerorganic ring that surrounded a cobalt atom. The process is less efficient than using a platinumcatalyst, cobalt is less expensive, potentially reducing total costs. The process uses one of twosupramolecular assemblies based on Co (II)-templated coordination of Ru(bpy)32+ (bpy = 2,2-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle. TheCo(II) centres of both assemblies are high spin, in contrast to most previously describedcobaloximes. Transient absorption optical spectroscopies include that charge recombinationoccurs through multiple ligand states present within the photosensitizer modules. [26]

2.4.4.6 BismuthBismuth based systems have been demonstrated to have an efficiency of 5% with the advantageof a very simple and cheap catalyst. [27]

2.4.5 Applications:

2.4.5.1 Scale-up analysis and exergo economic assessment of large scale photo-catalytichydrogen production plants:

A scaled up analysis on photo-catalytic reactor for continuous operation under large-scale

process conditions with respect to concentrated sunlight and UVvisible lamp irradiation wasprovided .The hydrogen production rate of scaled-up photo-reactors can be enhanced byincreasing the light intensity. Scale-up methodologies based on a length-to-diameter ratioconstraint leads to lower enhancement in productivity at higher light intensities compared withlength or diameter constraints.The hydrogen production rate is upgraded fivefold by a 10 times increase of light intensity as theresult of light concentration or multi-directional illumination without changing the geometry anddimensions of the photo-reactor. Utilization of the parallel arrangement of small scale photo-reactors versus a large scale unit has to be determined through an economic feasibility study,since the light concentration has a significant impact on capital cost.The exergo-economic analysis of photo-catalytic water splitting plants at different operationalconditions and production scales indicates the following results:

The hydrogen production capacity has a strong dependency on exergetic losses. The hydrogen prices for medium production capacity and high capacity are very

close, although the capital costs are about 10 times less for lower productioncapacity. [28]

2.4.5.2 An Au/Cu2OTiO2 system for photo-catalytic hydrogen production.A pn-junctioneffect or a simple case of in situ reduction:

Photo-catalytic H2 production from water has been studied over AuCu2O nano particledeposited on TiO2 (anatase) in order to probe into both the plasmon resonance effect (Au nanoparticles) and the pn junction at the Cu2OTiO2 interface. The AuCu2O composite is in theform of ~10 nm Au nano particles grown on_475 nm Cu2O octahedral nano crystals with (111)facets by partial galvanic replacement. X-ray Photoelectron Spectroscopy (XPS) Cu2p andAuger L3M4,5M4,5 lines indicate that the surface of Cu2O is mainly composed of Cu+. The ratefor H2 production (from 95 water/5 ethylene glycol; vol.%) over 2 wt.% (Au/Cu2O)TiO2 isfound to be ~10 times faster than that on 2 wt.% AuTiO2 alone. Raman spectroscopy beforeand after reaction showed the disappearance of Cu+ lines (2Eu) at 220 cm-1. These observationscoupled with the induction time observed for the reaction rate suggest that in situ reduction fromCu+ to Cu0 occurs upon photo-excitation. The reduction requires the presence of TiO2 (electrontransfer).The prolonged activity of the reaction (with no signs of deactivation) despite the reduction toCu0 indicates that the latter takes part in the reaction by providing additional sites for thereaction, most likely as recombination centers for hydrogen atoms to form molecular hydrogen.This phenomenon provides an additional route for enhancing the efficiency and lifetime ofCu2OTiO2 photo catalytic systems, beyond the usually ascribed pn-junction effect. [29] 2.4.5.3 Efficient photo catalytic hydrogen production from water over a CuO and carbonfiber co modified TiO2 nano composite photo catalyst:

CuO and CF co modified TiO2 photocatalyst was prepared via a wet impregnation

method with commercial TiO2 (P25) as the support. Compared with TiO2 and CuO/TiO2, thepresent catalyst is more efficient for hydrogen evolution from an aqueous solution of ethanol. Inthe CuO/CF/TiO2 photo catalyst, a part of the CuO and TiO2 particles are combined with CF.Due to the excellent electronic conductivity of CF, this architecture decreases the resistance ofcharge transfer from TiO2 to CuO and leads to better solar light harvesting in the visible light

region. The experimental results suggest that the H2 evolution activity of CuO/CF/TiO2 is muchbetter than the commercial TiO2 and CuO/TiO2 catalyst and the composite photo catalyst is verystable. CF content in the photo catalysis is crucial for the photo catalytic activity, and a highhydrogen evolution rate of 2000 mmol h_1 g_1 is obtained at an optimum CF content of 1.0wt%, which is up to 45 times higher than the commercial TiO2, and is about 2 times higher thanthat of the sample CuO/TiO2. The existence of the optimum CF content means that the excessiveCF absorbs and scatters the photons, resulting in a decrease of the effective photons utilizationby TiO2 and CuO and the drop of the H2 evolution rate. [30] 2.4.5.4 Photo catalytic hydrogen production And Materials Science in SemiconductorProcessing:

The quantum efficiency for wide band gap oxide catalysts is worse in the UV range than

in the visible range for catalysts with a low band gap. Thus, it is necessary to narrow the bandgap of photo catalysts to harvest visible light of longer wave lengths and enhance photogenerated charge separation in photo catalysis. High- efficiency and cost-effectiveH2S splittingsystems based on these photo- catalysts should also be constructed. Photo catalysts free of noblemetals are highly preferable considering that they can be readily used in a more economic way. Itis anticipated that this demonstration of solar photo catalytic H2 production will attract attentionfor further studies in a promising direction. From the literature reviewed so far, it is evident thatliquid-phase recovery yields greater H2 production than gas-phase recovery. The current lack ofindustrial applications of this technology can mainly be attributed to two reasons: the low photocatalytic efficiency due to large-size photo catalysts and the resulting lack of agreement on howto quantify this efficiency, in particular for nano sized photo- catalysts and reactorconfigurations. Therefore, for nano- sized catalysts, reactor design and cost reductions for large-scale applications have to be given special priority. The other challenge is successful scale-up oflaboratory- scale photo catalyst is to an industrially relevant scale. [31]

Chapter3:

SolarSimulators

3.1 Definition:

The solar simulators are devices that give illumination approximating to the sun light. And it isknown as (the artificial sun)

3.2 Purpose:

Solar simulators purpose is to provide a controllable indoor test facility under certain laboratoryconditions.

3.3 Uses:

They are used in solar cells testing, sun screen plastics, for some materials and devices, and forphotocatalysis.

3.4 Types:

They could be divided to three types:

1-continuos: in this type the illumination is continuous in time.

2-flashed: this type is similar to the flash photography and use flash tubes.

3-pulsed: the pulsed simulator uses a shutter to block or unblock light from continuous source.

3.5 Types of lamps:

There are several types of lamps used as light source, which provide UV (Ultra Violet) light.

Xenon arc lamp: it is the most popular type of lamps used in solar simulation, and it could becontinuous or flashed simulators. They give unfiltered spectrum and high intensities which reallymatch the sunlight.

Metal Halide arc lamp: used in filming and television lighting.QTH (Quartz Tungsten halogen lamp): this lamp offer spectra that is close to black bodyradiation.LED (Light-Emitting Diodes): it is used mainly in laboratories to help in constructing solarsimulators.

Mercury lamp: it is a gas discharge lamp that is used as a source of light. [32]

3.6 Photocatalysis lamps:In photocatlysis, Xenon and Led and Mercury lamps could be used as a source of UV light tohelp the process, as an alternative for sun light.

Chapter4: Photocatlystscharacteristics

4.1 Here is a list of the photocatalysts and their characteristics that were used in producing H2:

photocatlsyt formula Characteristics

Cuprous oxide Cu2O Is one of the most studied andexperimented semiconductor,that have band gap of 2.137electron volt (eV).

Cupric oxide CuO A semiconductor, that havenarrow band gap of 1.2 eV.

Titania TiO2 Is a good photocatalyst undersunlight or UV light, whichhave much narrower band gapof 2.85 eV.

Zinc sulphide ZnS Is a semiconductor that can becubic or hexagonal form, whichhave band gap of 3.54 eV at300 kelvin.

Cadmium sulphide CdS Is One of the firstsemiconductors used and also isa direct band gapsemiconductor 2.42 eV

Table 4: Characteristics of photocatlysts used [33]

Chapter5: Methodology

There were two ways used for producing H2: photo electrocatalysis and photocatalysis.

5.1 Photo electrocatalysis:

The methods used in photo electrocatalysis are simple. It is to construct a photo electrochemicalcell that have three electrodes :( working, counter, and reference electrode). The working

electrode is the important electrode because it will be the photoelectorde that the whole processdepends on it to produce H2, and it will be pure (TiO2) nanotube. And it is used because its

unique properties, such as high photocatalytic activity, long term photo-stability, and also its lowcost.

After constructing the cell, it will be directed to UV light for 1 hour through a quartz glass due toits thermal properties and optical properties that are superior to any other type of glass, because

of its purity, which make it the best for UV transmission.

The hydrogen that will be produced, will be collected in a reverted burette.

5.2 Photocatlysis:

The methods used in photocatalysis are very simple. It is about producing hydrogen from asulfite solution with the help of photocatalyst. The reactions that will be used are:

5.2.1 The first reaction:

SO2 +2NH3+H2O (NH4)2 SO3 (1)

(NH4)2 SO3+Cu2O+ H2O Cu2 SO4+H2+2NH3 (2)

This reaction is in two steps:

1-First the ammonium sulfite [(NH4)2 SO3] is prepared from Sulphur dioxide (SO2), ammonia(2NH3), and distilled water (H2O).

2- Then the ammonium sulfite [(NH4)2 SO3] is to be added to a photocatalyst which will be inthis reaction whether (Cuprous oxide (Cu2O) or Cupric oxide (CuO)), with distilled water (H2O).

To give H2 as a separate product.

5.2.2 The second reaction:

Na2 SO3+H2O H2+Na2 SO4

In this reaction sodium sulphite Na2 SO3 is added to distilled water (H2O) to make a sulfitesolution with a photocatalyst. To produce H2.

An apparatus is needed to put in it the sulfites solutions and the photocatlyst.

Figure12: The designed apparatus with details

5.2.3 Description of the apparatus:

This apparatus is made of glass, the sulfite solution and the photocatalyst will be in it and thenleft for 3 hours at a top of building or something like that to be directed exactly to sunligt for 3

hours. If the reaction produced H2, it will push the water up through the glass tube in thegraduated cylinder. So the amount of water that pushed up is the amount of H2 that is produced.

Chapter6: Experimental

work

The experimental work that has been done. Was done in two places, in the National Research

Centre and in the university. And that is because at first the required equipment to complete theresearch wasnt available at the university. Which are (Xenon lamp, the apparatus that is required

to have the photocatalyst and the solution in it to react and produce H2).So as an alternative the work was done at National Research Centre at first, because the requiredequipment was there. After the work and results were finished there, an apparatus was designedthat can hold this reaction and used the sun light as an alternative for Xenon lamp. So the workalso was completed at the university.

6.1 Experimental work at the National Research Centre:

This work was done as an alternative way to produce H2, it was done at National ResearchCentre in Dokki. And it is about constructing a photo electrochemical cell to produce H2 by water

splitting directed to UV light from xenon lamp, which the national research Centre had. Bypreparation of pure TiO2 nanotubes as a photoelectrocatalyst, and KOH as an electrolyte.

6.1.1 Materials used:

-Titanium foil

-Ethylene glycol

- Ammonium fluoride (NH4F)

- Hydrogen peroxide (H2O2)

-Platinum rod

-Deionized water

-Ethanol

-TiO2 nanotubes

-KOH

6.1.2 Equipment used:

-Dc power supply

-Three-electrode photo electrochemical cell (PEC)

-Potentiostat

-Xenon lamp (Zolix LSP-X150)

-Quartz glass

-Burettes

6.1.3 Experimental procedures for the preparation of pure (TiO2) nanotube:

Titanium (Ti) foil with thickness 0.128 (99.6% purity) was used, Then was cut into (50 mmx 10 mm) dimensions. Then was putted in ethylene glycol with 5 wt% ammonium fluoride(NH4F) and with 5wt% also of hydrogen peroxide (H2O2), then anodization was made withconstant potential 30v using dc power supply for 4 hours, the Ti foil was the anode and theplatinum rod was the cathode, after the 4 hours were done. Washing with deionized water

and ethanol for 10 seconds, then sintering it at 400co for 2 hours, the pure (TiO2) nanotubeswas formed.

30v

Pt Ti

Figure13: Preparation of (TiO2) nanotubes

Figure14: TiO2 nanotubes shape

6.1.4 Experimental procedures for the photo electrochemical cell (PEC) process:

After preparation of pure (TiO2) nanotube, a three-electrode photo electrochemical cell (PEC)need to constructed, the TiO2 nanotube as the working photoelectrode, the platinum rod ascounter electrode, and the reference electrode was saturated calomel electrode (SCE). The

solution that was used as an electrolyte was 1 M KOH with 1wt% ethylene glycol. Then the threeelectrodes were connected to a potentistat to measure the voltage and current. And to produce auniform and continuous light spectrum, a xenon lamp (Zolix LSP-X150) was used with 150 W

and with intensity of 800 W/m2 as source of UV light.

Pt

-

-

Ti

+

Figure15: Three electrode photo electrochemical cell (PEC)

The light was transmitted by quartz glass 100%. The measurement of hydrogen evolution was

done under constant voltage 0.6 V versus SCE for 1 hour. The hydrogen that was going to evolvewas going to be collected using a reverted burette. A potential was swept from -1 to 1 V at scan

rate of 5 m V/s, the photocurrent versus the potential was to be plotted with light on and light off.Then the photocurrent density was to be calculated by using this equation: [Light current darkcurrent (at a given potential)]. After calculating photo current density [mA/cm], the photo

conversion efficiency () to be calculated by using this equation:

Also the solar to hydrogen efficiency (STH) to be calculated by this equation:

After that a table to be constructed with the calculated photo current density [mA/cm], the photoconversion efficiency (), the solar to hydrogen efficiency (STH), and the amount of hydrogenproduced.

6.2 Experimental work at the university:

This is the work that was done in the British university, and it involves two reactions with thehelp of a photocatalyst to produce H2, directed to the sunlight or UV light. The second reaction is

the important one, because it is the one that all the tests done on it by using a certain apparatusthat was made for this tests. There was no source of UV light because it demands a xenon lamp,

which is too hard to get due to its very high cost, so I have worked in the sunlight.

6.2.1 Materials used:

-ammonia solution

- Cuprous oxide (Cu2O)

- Cupric oxide (CuO)

- Titania (TiO) 99% purity-Zinc sulphate (ZnSO4)

-Cadmium sulphate (CdSO)-Sodium sulphide (NaS)

- Zinc sulphide (ZnS)

- Cadmium sulphide(CdS)

- Sodium metabisulfite

-distilled water

6.2.2 Equipment used:

-beakers

-flasks

-filter papers

-funnels

6.2.3 The apparatus:

Figure16: The appartus

6.2.4 The first reaction:

SO2 +2NH3+H2O (NH4)2 SO3 (1)

(NH4)2 SO3+Cu2O+ H2O Cu2 SO4+H2+2NH3 (2)

This was the first reaction with using certain photocatalysts: (cupric oxide and cuprous oxide)

So first solubility tests were done for cupric oxide and cuprous oxide in ammonia solution beforebeginning the reaction.

Two tests were done to know the solubility of cupric and cuprous oxide in ammonia solution32%.

6.2.4.1 Test 1: (cupric oxide (CuO)):

1 g of CuO with 15 ml of ammonia solution

6.2.4.2 Test 2: (cuprous oxide (Cu2O)):

1 g of Cu2O with 15 ml of ammonia solution

Then after the solubility tests were done, everything is ready to begin the work, but there weresome difficulties as: to make this reaction Sulphur dioxide cylinder is needed, and it is verydangerous. Also another thing this reaction will take a lot of working time and a lot of tests

needed to be done.

6.2.5 The second reaction:

So to save more time and also to be safer, this reaction was used and also it was easy to beprepared: Na2 SO3+H2O H2+Na2 SO4

In presence of a photocatalyst

The photocatalysts that were used:

- Cuprous oxide (Cu2O)

- Cupric oxide (CuO)

- Titania (TiO) 99% purity

- Zinc sulphide (ZnS)

- Cadmium sulphide(CdS)

The Cuprous oxide (Cu2O), Cupric oxide (CuO), Titania (TiO) 99% purity were bought andready to use. But the Zinc sulphide (ZnS) and Cadmium sulphide(CdS), couldnt find, so they

need to be prepared first.

6.2.5.1 Preparation of Zinc sulphide (ZnS):

15 gm of zinc sulphate (ZnSO4) with 15 gm of sodium sulphide (NaS)

6.2.5.2 Preparation of Cadmium sulphide (CdS):

15 gm of cadmium sulphate (CdSO) with 15 gm of sodium sulphide (NaS)

Figure 17: Preparation of CdS

Then after the preparation of zinc sulpide and cadmium suphide, left it for 1 day and then make

filtration to get the zinc and cadmium sulphide.

Figure18: Filtration of ZnS

After that, the tests could start. But Before making any test, the solution must be in a dark place

for one day, then put it in the apparatus for 3 hours or more in the sunlight, then the amount ofhydrogen that could be generated could be measured by measuring the water that H2 pushed it

up.

6.2.5.3 Test 1:

5 g of cuprous oxide (Cu2O) was added with 5 g of sodium metabisulfite with 600 ml distilledwater in the bottle of the apparatus.

6.2.5.4 Test 2:

5 g of cupric oxide (CuO) added with 5 g of sodium metabisulfite with 600 ml distilled water inthe bottle of the apparatus.

6.2.5.5 Test 3:

5 g of titania (TiO) 99% purity was added with 5 g of sodium metabisulfite with 600 ml distilledwater in the bottle of the apparatus

6.2.5.6 Test 4:

5 g of prepared zinc sulphide (ZnS) was added with 5g of sodium metabisulfite with 600 mldistilled water in the bottle of the apparatus.

6.2.5.7 Test 5:

5 g of prepared cadmium sulphide(CdS) was added with 5 g of sodium metabisulfite with 600 mldistilled water in the bottle of the apparatus.

6.2.5.8 Test 6:

In this test I have made a CdS and ZnS solution with ratio 2:1 with respect to the molecularweight and number of moles. So first a calculation needed to be done to know the exact amount

of CdS with respect to ZnS.

The calculation:

First the molecular weights are:

Zn=65

Cd=112

S=32

O=32

H=2

Then

ZnSO4= 161

CdSO4=208

But the formula of the ZnSO4 wasnt like it was ZnSO4.7H2O. And also the CdSO4 wasCdSO4.8H2O.

So ZnSO4.7H2O= 287

CdSO4.8H2O= 352

So if 1 mole of ZnSO4.7H2O is added then CdSO4.8H2O is 2 moles.

Then mass= number of moles X molecular weight.

Then mass of ZnSO4.7H2O = 287X1=287 g.

CdSO4.8H2O =352X2=704 g.

So if 5 g of ZnSO4.7H2O is added, it will need 12.26 g of CdSO4.8H2O with respect to the ratio.

Then add the 5 g of ZnSO4.7H2O and the 12.26 g of CdSO4.8H2O with 10 g of sodium sulphide(Na2S). To form the (CdS/ZnS) solution.

After calculating the exact amount, the test could begin.

The test:

5 g of (CdS/ZnS) solution was added with 5 g of sodium metabisulfite with 600 ml distilledwater in the bottle of the apparatus.

Chapter7:Results anddiscussion

There are two results done in two different places, one at the National Research Centre, and the

other at the university.

7.1 Results at the National Research Centre:The voltage-photocurrent density was plotted under 800 W/m2 illumination to know their

performances, and to calculate the photocurrent density.

Light on:

Potential [V] Photocurrent density[mA/cm]

-1 0.08477

-0.8 0.52328

-0.6 0.72657

-0.4 0.82298

-0.2 0.85331

0 0.87504

0.2 0.88432

0.4 0.89539

0.6 0.90619

0.8 0.91652

1 0.93107

Table5: Light on results

Figure19: Potential VS photocurrent density in light on

According to this results, it appears that the potential [V] is directly proportional with thephotocurrent density in light on.

Light off:

Potential [V] Photocurrent density

[mA/cm]

-1

-0.13509

-0.8 -0.02494

-0.6 0.00954

-0.4 0.00205

-0.2 1.64E-04

0 -0.0013

0.2 -0.001

0.4 2.37E-04

0.6 0.00171

0.8 0.00569

1 0.01522

Table6: Light off results

Figure20: Potential VS photocurrent density in light off

This results gives the effects of the TiO2 on the PEC performances in light and dark.

To calculate the photocurrent that generated H2:

Light current dark current (at a given potential)

0.93107 0.01522 = 0.91585[mA/cm] (at 1 V)

Then photoconverison efficiency is to be calculated ():

Where Jp is the photocurrent density [mA/cm]

E0rev is the standard reversible potential for water splitting (1.23 V)

Eapp is the applied potential

I0 is the power density of light (mW/cm2)

Jp= 0.91585[mA/cm]

E0rev = 1.23 V

Eapp = 1 V

I0 = 800 W/m2 = 80 mW/cm2

Then (%) =

Also the solar to hydrogen efficiency (STH) is to be calculated:

Where Jp is the photocurrent density [mA/cm]

I0 is the power density of light (mW/cm2)

Jp = 0.91585[mA/cm]

I0 = 800 W/m2 = 80 mW/cm2

Then STH (%) =

The H2 that was evolved was at rate of 0.32 ml/hr.

But it is needed to be converted to (mol h-1cm-2).

So 1 mole occupies 22.4 liters at NTP

Then [(0.32 X 10-3) 22.4] X 106 = 14.28 mol h-1cm-2

Photocurrent density

(mA/cm)

Photoconversionefficiency () (%)

Solar to hydrogen(STH) efficiency (%)

Hydrogen productionrate (mol h-1cm-2)

0.91585 0.26 1.40 14.28

Table 7: Results of photo electrocatalysis

This table gives the photocurrent density with its photoconversion efficiency and solar tohydrogen efficiency and it hydrogen production rate in mol h-1cm-2.

According to this results, it appears that if the photocurrent density is increased and the powerdensity of light decreased, the Photoconversion efficiency (), and the solar to hydrogen

efficiency (STH) will increase also according to the equations, and this will affect the hydrogenproduction rate for guarantee.

7.2 Results at the university:

7.2.1 Solubility results for the first reaction:

7.2.1.1 Test 1: (cupric oxide (CuO)):

Observations:

When it added together it gave dark blue color.

Results:

The 1 g of CuO was soluble with 15 ml of ammonia solution.

7.2.2 Test 2: (cuprous oxide (Cu2O)):

Observations:

When it added together it gave brown color.

Results:

The 1 g of Cu2O was soluble with 15 ml of ammonia solution.

7.2.2 Results for the second reaction:

7.2.2.1 Test 1(Cu2O):

Observations:

The solution that was formed was brown.

Figure21: Cuprous oxide test

Results:

After leaving the apparatus for 3 hours at the roof of the building directed to the sun light, therewas no water pushed up, therefore no hydrogen production. So this gives ve results.

7.2.2.2 Test 2(CuO):

Observations:

The solution that was formed was black

Results:

After leaving the apparatus for 3 hours at the roof of the building directed to the sun light therewas no water pushed up, therefore no hydrogen production. So this gives ve results.

7.2.2.3 Test 3(TiO2):

Observations:

The solution that was formed was white.

Results:

After leaving the apparatus for 3 hours at the roof of the building directed to the sun light, therewas no water pushed up, therefore no hydrogen production. So this gives ve results.

7.2.2.4 Test 4(ZnS):

Observations:

The solution that was formed was light grey.

Results:

After leaving the apparatus for 3 hours at the roof of the building directed to the sun light, therewas no water pushed up, therefore no hydrogen production. So this gives ve results

7.2.2.5 Test 5(CdS):

Observations:

The solution that was formed was yellow.

Figure22: CdS test

Results:

After leaving the apparatus for 1.5 hours at the roof of the building directed to the sun light, therewas a 0.5 ml of water pushed up, after the whole 3 hours there was 2 ml, therefore there is

hydrogen production. So this gives +ve results.

7.2.2.6 Test 6(CdS/ZnS):

Observations:

The solution that was formed was yellow.

Figure23: Cd/ZnS test

Results:

After leaving the apparatus for 3 hours at the roof of the building directed to the sun light, therewas a 3 ml of water pushed up, therefore there is hydrogen production. So this gives +ve results.

(H2 rate(ml h

-1 g-1 Weight of the photocatalyst Photocatlyst

ve- g 5 Cu2O

ve- g 5 CuO

ve- g 5 TiO

ve- g 5 ZnS

0.133333333 g 5 CdS

0.2 g 5 CdS/ZnS

Table 8: Results of photocatalysis

This table views all the photocatalyst used with their weights and their H2 rate of production.

But the H2 rate here is by ml h-1 g-1, so the amount of milliliters was divided by the number of

hours and the photo catalyst weight in grams.

According to this results, Cu2O, CuO, ZnS, and TiO didnt produce H2, but CdS, and CdS/ZnSproduced 0.133333333 ml h-1 g-1 and 0.2 ml h-1 g-1 respectively. The amounts that was produced

is very little and this may be due to the probability of a leakage happened in the apparatus andalso some days the weather is cloudy and the sun light cant reach clearly from the clouds to the

top of the building and those could be a source of errors. Maybe if the apparatus was sealed goodenough or a continuous source of UV light was there. Cu2O, CuO, ZnS, and TiO could produce

H2, also CdS, and CdS/ZnS could

Conclusion

The two methods produced H2 with small rate. The photo electrocatlysis method was betterbecause it produced 0.32 ml/h, which is more than the photocatlaysis method 0.2 ml/h. in thephoto electrocatlysis, maybe if the potential [V] is increased the hydrogen rate will increase dueto the increase of the photocurrent density which in return increase the efficiency. And in thephotocatlysis, may be by designing better apparatus to hold the reaction could produce morehydrogen, because may be the leakage affect the rate, and also directing it to a UV source insteadof the sun light that is not a continuous source due to clouds.

Also there are another photocatalyst that havent been tried in this research, maybe it also couldimprove the hydrogen production. And another solutions than the sulfite solutions maybe havemore effect.

The two methods were cheap in cost but not very efficient due to their small amounts they haveproduced. So hydrogen still is not used as a mainly source of fuel, so there is a plenty of time todiscover a way to produce H2 with a big amount and a cheap cost.

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