Overall photodecomposition of water on a layered niobiate catalyst

77

Catalysis Today, 8 (1990) 77-84 Elsevier Science Publishers B.V., Amsterdam

OVERALL

PHOTODECOMPOSITION

Dome",1

Kazunari

OF WATER

Akihiko

ON A LAYERED

Akira

Kudo.1

Tanaka'

NIOBIATE

and

Takaharu

'Research Laboratory of Resources Utilization, Tokyo 4259 Nagatsuta. Midori-ku. Yokohama 227 (Japan) 'Nikon

Co.,

1773

Asamizodai.

Sagamihara

228

CATALYST

Onishil

Institute

of Technology,

(Japan)

SUMMARY The photolysis of water into hydrogen and oxygen is a reaction involving the transformation of a high amount of energy. Recently, it was found that a photocatalyst having a layered structure gave not only a behaviour different from that observed with a conventional bulk-solid type semiconductor photocatalyst, but also that the layered photocatalyst gave a stable and high photocatalytic activity for water decomposition. A new type of photocatalyst, Ni-K4Nb6017, has ultrafine particles of nickel metal in the inter-layer spaces. It has a structure of what may be called a "two-dimensional photocatalyst," which decomposes intercalated water molecules. It is also interesting in its mechanism in which it works in the decomposition of water. In this paper, our recent results concerning this new type of photocatalyst are reviewed. INTRODUCTION The

decomposition

attracting

much

transformation The

final

of water

attention of

goal

photon

a system

at a ratio

of 2:l under

we

The level H20

still

--•

high H2 +

Therefore,

catalyst

of the

work

out

some

photosynthesis

can

(refs.

be efficiently

problems

to be

by energy

has been

to the

l-2). of water

would

and steadily

In order

irradiation.

difficult

light

decomposition

(I) is accompanied

0920-5861/90/$03.50

reaction

reverse

of Equation

to attain

solved

be

evolved at

such

in the

accumulation

simply

having

steadily, with

the

then

a site in the

we

for

a

future.

at a

reverse

In order

or the

0 1990 Elsevier Science Publishers B.V.

which water.

may

hope

for

work

intermediates

and

overall

to combine

absorbs We

type

reaction, for

will

as

steady

is a PtlTiO2

it is not sufficient

to decompose

products

the

system.

which

cannot

example

a semiconductor

a potential

to prevent

a substance

(I). A typical

Pt offers

to proceed

system

reaction,

be accumulated

catalysts

hf which device

in the

2). Wherein

of water

forming

e- and

light

using

kcal/mol.

for the

reaction

(ref.

decomposition

forms

to

oxygen

to its relations

(1) exists

of H2 and 02 cannot

and 02-

or

and

due

l/202

photocatalyst both

G=56.7

years

photocatalytic

some

of Equation as

hydrogen

H2 and 02

visible

to face

if there

an efficient progress

in which

have

reaction as

energy

to the overall

to provide

goal,

into

in recent

light need

H2and to

to undergo

reverse fine

reactions.

There

semiconductor

steadily

surfaces

quantities

was

interrupted

bulk

halfway of about

a higher

yield

type

Recently,

layered

the

type

no exposed

seems

having

metallic

gap

by using

such

discovered

from

those

of view.

as Ti02

a new

appearance This

system

which

and

may in

irradiation

catalysts

have

nm).

photocatalyst

and

SrTiO3.

a

To of those

K4Nb6017 system

having

photocatalysts

catalyst

is based

is interesting

will

are have

observed

the

used

new

was

(ca. 360

photocatalytic

5-8). The

particles

light

these

COMPOUND,

conventionally

(refs.

surface

02

of catalysts

no decrease when

containing

He and

type

catalyst

irradiation

USINGLAYERED

a mica-like

point

band

particles

have

these

catalysts

to be difficult

OF WATER

to produce

(ref. 4). However,

the

systems

of this

Actually.

on those

of reaction

catalyst

found

4). All

reaction.

formed

semiconductors

mechanism

(ref.

have

1% under

different

compound

reaction

examples

and

authors

greatly

bulk-solid

been

semiconductor

DECOMPOSITION

features

have Some

reverse

of Hz and 02

yield

solid

OVERALL

the

type

which

NiO-SrTiO3

of oxides,

promote

the

obtain

3) and

bulk-solid

ratio.

particles,

(ref.

effectively

quantum

those

in a stoichiometric

Rho,-SrTi03 outer

are

be described

of on a

from below

a in some

details.

-The

Structure As

form

shown

of GNb6017 in Fig. 1. K4Nb6017

a two-dimensional

layers

have

negative

layers

so as to make

layered charges,

consists

and

a balance

of octahedral

structure

via

positively

with

the

oxygen charged

negative

units atoms

K+ ions

charges

of Nb06.

(ref.

exist

of the

which

9). These between

layers.

These

Y

0:K

at

q :Nbo6

x=f , l :K

at

x=$-

at X=+ , q : Nbo6 Fig.

1. The

at

structure

x=+,

:

q Nbo6 at

of K4Nb6017

x=+

79 K+

ions

are

in which exists of

ion-exchangeable

Rb+

or Cs+

in the

space

interlayer

alternately. interlayer

space

other

hand,

under

a highly

the

photolysis catalyst. space

Water

the

from

hydrated

space

condition

or those

spaces each

niobic

acid

interlayer

space

II,

in air

This are

sheet

indicates

easily

Using

various

Variously

Modified

photocatalysts

based

Photodecomposition oxides.

of water

over

K4Nb6017

Amount

none Cr203 Mn304 ;:2;3 Ni a4 cue Ptoc RuO~~ Rh203C

The

but

that

between

as though

a tetra-

in the

into

fine

particles

loaded

of evolved

with

metal

gas

(umol)

Pretreatmentb Hz

02

R773-0473 R773-0473

63 27

7 0

R773-0473 R873-0473 R873-0473 R773-0473

; 32 630

: 0 310

12 19 12 77 9 la 3

: 0 16

R773-0473 untreated R773-0473 untreated R773-0473 untreated R773-0473

: 0

Catalyst: 1 g. H 0; 300 ml, light source: high pressure cell: inner irradiation mercury lamp (45 8 w), reaction reaction cell, reaction time: 10 h aThe represented metal oxides are the probable forms under the reaction condition, but not confirmed. bR773-0473 means reduction by H2 at 773 K and then oxidation by02 at473 K. '0.5 wt% loading.

On

the

is hydrated

the

case

of of

the the

(s 1 pm)

or

inside

interlayer sheet

&4Nb60,7

on the

ion types

appear

to form

1

Metal oxidea (1 wt%)

metal

are two

K4Nb6017'3tl20.

taken

it looks

structures

properties.

in air.

solution

is interlaid

II. so that

1, there

to form

hydrated

known

an alkali

which

in their

in an aqueous degrees.

are

in Fig.

I and

other

even

molecules

the

shown

II is not

or

of higher

the water

As

There

K+ ions. Such

a back.

Photolysis

Using

TABLE

interlayer

cations. for

layers.

different

In a sense,

and

two

I is easily

humid

various

substituted

i.e., interlayer

are

of water,

I and

surface

with

are

between

spaces, They

penta-hydrate

ions

of

has

a

80 K4Nb&7,

we

results under that and

attempted

expressed

the

as the

a 450-watt

high-pressure

ratio

although

of 21.

if it had

the

not

been

which

further

increasing

thus

with

various

transition

case

of Ru02,

a slight

K4Nb6017

was

modified

activity.

What

the time

course

a 450-watt is found been

roughly

rate

after

torr

of 02 This

ratio

of Hz and been

catalyst

do not

This

150 torr

place.

H2 i

1/202

with

about

is

was

the

activity

we

0.1 wt%

such by not

(2)

0

of the

in Fig. 2 was

30

40

60

t/h

Fig. 2. Photocatalytic decomposition of distilled water over the R773-0473 NiO-

nm).

catalyst

improved, obtained

as high

20

system and as an

as about

ccl?, 70%

inner

irradiation

reaction

with

NiO. It

3

(3)

yield

shown

further

a result,

irradiation

interesting

that

3.5% (at 330

Recently,

IO

high

phase

---f Hz0

quantum

reaction

0

supported

to

Fig. 2 shows

the

02 + e- ---* 02The

K4Nb6017

the

of Hp aim

In the

Hz and

gas

(2) and(3)do

take

an exceptionally

under

at a even

when

about

as expressed

Equations

over

decreased.

of 21.

Hz

of K4Nb~Ol7

However,

at a ratio water

the

accumulated.

in the

indicating

reactions

With

is largely

interfere

reaction. as

activity

observed.

gained

formed in pure

lamp

for the formation

features.

and

in that

existing

mercury

the

was

compound

were

decomposition

in a

characterized 02

the

Hz and 02

water

at the modification

cases,

of both

not precisely

at a

50 hours,

have

NiO.

In most

in activity

was

the

reaction

it is noted

formation

suitable

its structural

1 gives

10 hours

glance,

formed

for overall

attempted

of Hz and 02 evolution

stoichiometric that.

with

we

a first

thus

Table

after

simultaneous

catalysts

from

formed

H2 and O2 have

produced

constant

metals.

high-pressure that

a capacity

activity,

is more,

to the

by those

increase

lamp. At

of Hz and O2

derived

of water.

of Hz and 02

leads

had

modified

is probably the

decomposition

mercury

quantities

K4Nb5017

and 02.

300

alone

the use of K4Nb6017 Oz.

overall

quantities

cell.

81 using

Rb4Nb6017

Rb4Nb6017)

(ref. 8). Why,

catalyst

system

then,

can

completely

the

Ni-modified

decompose

water

K4Nb6017

at such

(or

a high

activity

level?

Structure This

of

aqueous then

nickel

calcining

decomposed the with

oxygen quantity

What

nitrate the

When nickel

throughout

the

nickel

been

nickel

shell

and

peak

they

form are

of nickel oxidized with

edge

changed

nitrate

at 5OO'C. having

structure

Fig.

the

to the after

Ni-Ni

Ni-Ni

had

into

remains

at the

metallic

at 200°C

in the

final

Ni

attached

these

metal,

state

step

EXAFS.

variously

the

in (c). even

of activation to the

outer

we

intensity

ratio

nickel

ratio R773

after

it has

treatment. surfaces

The

R773-0773

NiO(lwt%)-K Nb6017 NiO(lwt%)-KWbO3 NiNb206

KNb03 and NiNb206 are reference materials BET surface areas to that of K4Nb6017.

which

have

of Ni

similar

K

Peaks

from

1

NiO;

In the

case

exists

in

at 500°C,

been

most

re-

re-oxidation

of K4Nb6017,

of Ni2p3/2/Nb3d R773-0473

of how

in (b). Interestingly,

Catalyst untreated

most

reduced

of Ni2p3/2/Nb3d

Peak

that

Ni metal.

2

peak

it was

and

pretreated.

are

NiO.

examined

derived

that

particles

as shown

by XPS.

transformation

from

of

a

calcination

indicating

it is found

When

by such

respectively,

derived

calcination,

particles.

been

bonds,

bond

Fourier

Thus,

activity.

in terms

examined

the

2). Using

3 shows

catalyst

and

were

after

an and

is

state.

catalytic

0.1 wt%

re-oxidation.

K 4 Nb 6 0 17 (Table

Ni-0

nickel

surfaces

and

a high

with

it is oxidized

of catalyst

of the catalyst

on

powder

at this

and then

about

the

completely

the

nickel

time, activity

surfaces

after

fine

to dryness.

preferably

in the

existed.

(a) obtained

powder

catalyst

of reduction

to the

impregnated

exist

inside

inside

of NiO-like

02 oxidizes

TABLE XPS

taken

2 corresponds

of catalyst the

not

periods

absorption

3 correspond

did

the

is most

about

the outer

taken

and

added

K4Nb6017

little

once

mechanism

the

At that

has

hydrogen

to obtain

the

4OO'C.

catalyst

with

and the reaction --

impregnating

drying

at about The

is brought

found

had

NiO.

of nickel

treatment?

the

powder

at 200°C

photocatalyst,

by first

solution,

is reduced

change

that

K4Nb6017

is prepared

to form

catalyst

The

Ni-loaded

catalyst

but

82 does

not

inside.

oxidize In what

nickel

metal

exchange

space only

by

only: with

that

K+

comes

close

obtained

was

image

that

size

ultrafine

the

layered

Therefore,

the

active

in

4.

What

mechanism

suggest

We

particles the

active

sites

is, the

acid

layer

move

to the

particles, is then

does

of nickel

That

where

02 formed?

structure.

this

metal

K4Nb6017

0123456

as shown

structure

the ultrafine existing

I would

hydrogen

light

sites

to

I without

decomposition

e- formed

by

Such be able

space

that

space for

of nickel

illustratively

believe

interlayer

dotted

Ni-loaded

for the water

reaction?

7).

of

be considered

a structure

Fig.

(ref.

was

5A.

destroying

can

a high-

inside

would

interlayer

photocatalyst

with

detectable

of about

particles

in the

having

reduction

particles

a barely

in

active

the

of K4Nb6017

the

only

re-oxidation

the

ultrafine

having

exist

I. The

after

used

that

exist

microscope

lattice

many

of NiO

by using

observed

particle

we

electron

It was

metal

ions

described

examined

resolution

10).

interlayer

condition

and the

1 23

interlayer

it exchanges

space

hydrogen

with

(ref.

et al, it is likely

interlayer

oxygen

al

impregnation

particles

catalyst

to an ion-

in one

in the

the

inner

K4Nb5017

et

is.

to the

by Kinomura ultrafine

the

using

ions

I. Since

with

does

existing

According

to exist

space

the

form

Kinomura

found

IS

nickel

exist?

experiment

conducted Ni2+

the

act

evolution.

in the

irradiation of ultrafine

water

niobic would Ni

is reduced.

Three

in as

Where

possibilities

R/A

Fig. 3. Fourier transforms of Nj K edge EXAFS functions K X(K) of NiO(1 wt%)-K4Nb6017 catalysts. Phase shift was not corrected. (a) untreated, (b) R773, (c) R773-0473. (d) P7730773. Interatomic distance 1 corresponds to Ni-0 bond of NiO, 2 corresponds to Ni-Ni bond of Ni metal and 3 corresponds to Ni-Ni of NiO.

33 are

conceivable

edges

of

more

closely

places

-- the

layers. Among associated

and for which

catalyst. completely

In such

it possible

taking

place. would

inhibiting be regarded

from

diffuse

into

overall

explain

any

once the

some

02

as a "two-dimensional

Nb6017 unit,

place

of water

characteristic

sites

products

by the

have

thus

photocatalyst"

0*, -‘J:‘.’

This

been

explains

found which

: Ni metal

into

that makes

gas

the can

be

are other

of this are

sheets,

on the nickel

desorbed

spaces.

It has

than

sites

acid

and

which

behaviours

niobic

reaction

II,

space

II is the

in Fig. 4, Hz-forming

reverse

interlayer

interlayer

space

decomposition

as shown

02-forming

of gaseous

I. the

interlayer

the

to prevent

Furthermore,

effect

with

we can

space

the

a mechanism

separated

making

them

interlayer them,

metal phases,

the this

from few

of

less catalyst

effective

particle

Fig. 4. Schematic structure of the active Ni-loaded K4Nb 0 $17 phtocatalyst and the reaction mechanism of Hz0 decompose ion Hp and 02.

thus

into

can

use of

84

the

surfaces

and

intercalated The

between

compounds

activities al. found and

the

cases

to the based

possible on

these

been

of these were

of our

characteristic

has

such

not yet some

11). One

different

from

layered

development

those

water

molecules

for

most too,

to the

the

high

the

systems

There

Recently,

the

photocatalytic

reaction

having

the

are

many

photocatalytic

in the visible

compounds.

has

semiconductor

activity.

of them.

light

other

of catalyst

but

had

is to modify

of these

structure

of conventional

structures,

studied

goals

a layered

photocatalytic

compounds,

structures

to decompose

having

a high

responsive

future

sheets

layers.

on K4Nb6017

clearly

having

have that

in a few

(ref.

and

acid

opposite

based

quite

photocatalysts

of niobium

two

photocatalyst

characteristics

other

backs

author

light

region

systems

We are higher

utilizing

looking level

et

acitvities

forward

structures

compounds.

REFERENCES

1 2 3

4 5

6

7

8

9 10 11

M. A. Gratzel, Academic Press

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Inc.,

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through

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2.