Chuong 6 san xuat hydro

Many photocatalytic systems have been reported to be active for “overall” water splitting i.e., simultaneous generation of both H2 and O2, most of them require ultraviolet UV light < 4

Chương 6

SẢN XUẤT HYDRO

One Advantage of using hydrogen

approximately 2.6 times the energy per unit mass as gasoline, and the disadvantage is that it needs about 4 times the volume for a given amount of energy

1 General Introduction

Current global hydrogen production

Primary Uses for Hydrogen Today

ammonia (NH 3 ) fertilizer

 2 The other half of current hydrogen

production is used to convert heavy petroleum sources into lighter fractions suitable for use as fuels

Hydrogen Production Processes

 Steam Methane Reforming

1 Steam Methane Reforming

bulk hydrogen

used in the industrial synthesis of ammonia

 It is the least expensive method

 High temperature process (700 – 1100 °C)

 Nickel based catalyst (Ni)

The Steam Methane Reforming Process

nickel based catalyst (Ni), steam reacts with methane to yield carbon monoxide and hydrogen

 CH4 + H2O → CO + 3 H2

 Additional hydrogen can be recovered by

a lower-temperature gas-shift reaction with the carbon monoxide produced The reaction is summarized by:

 CO + H2O → CO2 + H2

2 Photocatalytic water splitting

1 Introduction

1.1 Production of H2 from water using solar light

A.Fijishima and K.Honda Nature

1972, 238, 37

TiO 2 + 2 hv 2 e– + 2 h + (1) (at the TiO 2 electrode)

2 H + + 2 e – H 2 (2) (at the Pt electrode)

H 2 O + 2 h + 1/2 O 2 + 2 H + (3) (at the TiO 2 electrode)

H 2O + 2 hv 1/2 O2 + H 2 (4) (overall reaction)

Mechanism of photocatalytic water splitting

Many photocatalytic systems have been reported to be active for

“overall” water splitting (i.e., simultaneous generation of both H2 and

O2), most of them require ultraviolet (UV) light ( < 400 nm) due to the large bandgap of semiconductor materials

Since nearly half of the solar energy incident on the Earth’s surface lies in the visible region (400nm< < 800 nm) it is essential to use visible light efficiently to realize H2 production on a huge scale by photocatalytic water splitting

Fig 1 Solar spectrum and maximum solar light conversion efficiencies

for water splitting reaction with 100% of quantum efficiency

1.2 Difficulties in achieving water splitting under visible light using

heterogeneous semiconductor photocatalysts

Fig 2 Schematic illustration of water splitting over semiconductor photocatalyst

Fig 3 Band energy levels of various semiconductors

1.3 Two strategies for achieving water splitting using heterogeneous

photocatalysts under visible light

Fig 4 Schematic energy diagrams of photocatalytic water splitting systems:

(a) two-step photoexcitation system and (b) conventional one-step system

1.4 Difficulties in achieving water splitting using two-step photoexcitation mechanism

Fig 5 Forward and backward reactions in the two-step photoexcitation

system

1.5 Photoelectrochemical water splitting using semiconductor

photoelectrodes under visible light

Fig 6 Photoelectrochemical water splitting systems using n-type

semiconductor photoanode (a), p-type semiconductor photocathode (b)

2 Photocatalytic water splitting into H2 and O2 under visible light

through two-step photoexcitation between two different

photocatalysts (Z-scheme)

Fig 7 Overview of water splitting on Z-scheme photocatalysis system

with an iodate (IO3−) and iodide (I−) ion redox couple

2.1 Z-scheme water-splitting system that uses two different oxide

photocatalysts in the presence of an IO3−/I− shuttle redox mediator

Fig 8 Time course of photocatalytic O2 evolution over TiO2 photocatalysts suspended in aqueous solution (400 mL, pH 11 adjusted by NaOH) containing (a) 1mmol of NaIO3 and (b) 1mmol of NaIO3 and 40mmol of NaI The reactions were carried out using an inner irradiation type reactor, in which a light source (400W high-pressure Hg lamp, Riko Kagaku) was covered with a Pyrex glass-made cooling water jacket (cutoff < 300 nm) to keep the reactor temperature constant at 293 K

Fig 9 Time course of photocatalytic O2 evolution over Pt(0.5 wt%)/WO3 and Pt(0.5 wt%)/BiVO4 (inset) suspended in an aqueous solution (250 mL, pH 6.5 without adjustment) containing only NaIO3 (0.25 mmol) or containing both and NaIO3 (0.25 mmol) and NaI (10 mmol) The suspension was irradiated using a Xe lamp (300W) fitted with a cutoff filter (HOYA, L-42) and a water filter to eliminate the UV and infrared regions, respectively Visible light with a wavelength from 400 to 800nm was irradiated The temperature of reactant solution was maintained at 293K by a flow of cooling water during the reaction

Fig 10 Adsorption

properties of iodate (IO3−) and iodide (I−) anions on various photocatalyst

powders measured at 293 K

Fig 11 Schematic illustration of photocatalytic reactions with iodate

(IO3−) and iodide (I−) anions

Fig 12 Time courses of photocatalytic evolution of H2 and O2 using a mixture of Pt/TiO2-A1 and TiO2-R2 photocatalysts from 0.1 M-NaI

aqueous solution (pH 11, adjusted by NaOH) under UV light Triangles indicate H2 evolution using Pt/TiO2-A1 alone The reaction conditions are same as to those in Fig 8

Fig 13 Time course of photocatalytic evolution of H2 and O2 using a mixture of Pt(0.3 wt%)/SrTiO3 (Cr, Ta 4mol% doped) and Pt(0.5 wt%)/WO3 photocatalysts suspended in 5mMof NaI aqueous solution (pH 6.5 without adjustment) under visible light irradiation (> 420 nm) Triangles indicate H2 evolution using Pt/SrTiO3:Cr/Ta alone The reactions were carried out without cooling

2.2 Application of tantalum oxynitride photocatalysts to H2 evolution

part in Z-scheme system with IO3−/I− redox mediator

Fig 14 Time courses of gas evolution over a mixture of the photocatalyst

Pt/TaON (0.2 g, 0.3 wt% Pt) and Pt/WO3 (0.3 g, 0.5 wt% Pt) under visible light irradiation from an aqueous NaI solution (5mM, pH 6.5) The reactions were carried out without cooling

Fig 15 Crystal structure (a) and diffused reflectance spectra (b) of

TaON and ATaO2N (A= Ca, Sr, Ba)

2.3 Z-scheme water splitting under visible light using Fe3+/Fe2+ redox mediator

Fig 17 Overview of overall water splitting on Z-scheme

photocatalysis system with an iron ion redox couple