Chemical Physics Letters Volume 110 Issue 1 1984 [Doi 10.1016_0009-2614(84)80146-3] Maki Kawai; Tomoji Kawai; Shuichi Naito; Kenzi Tamaru -- The Mechanism of Photocatalytic Reaction

8/10/2019 Chemical Physics Letters Volume 110 Issue 1 1984 [Doi 10.1016_0009-2614(84)80146-3] Maki Kawai; Tomoji Kawai; Shuichi Naito; Kenzi Tamaru -- The Mec

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Votr~n~c I IO. number 1

CHEMICAL PHYSICS LETTERS

THE MECHANIShl OF PHOTOCATALYTIC REACTION OVER Pt/l?02:

PRODUCTION OF Hz AND ALDEHYDE FROhf GASEOUS ALCOHOL AND WATER

Maki KAWAI *:, Tornoji KAWAI **, Sbuichi NAITO and Kenzi TAMARU

_

.

I _ Introduction

The pbotocatalytic reaction using smiconductor

powder as a catalyst 113sbeen one of tbc

most

impor-

tant systc~ns in the field of catalysis and solar-energy

conversion. In particular, hydrogen production from

alcoltofs and water over plafinized TX& is known to

be a very effective system for converting UV light to

energy in the form of hydrogen

[ 1.2 J. III

fact. the

quantum yield of Hz production from CH,OH and

If20 over PtfTi02 exceeds 40 for ~~~o~~ocI~ro~~~atIc

380 nnl iigllt 1I ] .

hi spite of tllc importance of tllis reaction, tile

mcchnnism of tlxse photocatalytic reactions has not

been studied in detail. ~urtIlerr~~orc~ tbc detcmining

fxtors for tbc activity and selectivity of this reaction

have not been elucidated. WC bavc rccentiy studied

tlx

11vx1m11isn1 of the

photocatalytic reaction between

gaseous n~clbanol and water over Pt/TiO, [3]. From

observation of IR spectra lt~ld from volumetric studies,

wc have

show~i that

Ihcrc arc time reaction paths co-

existing in t&s system f3 1. Tbc reaction mec1m~ism.s

arc shown below.

(A) Direct reaction of pbysisorbcd CI1~OH with a

* ircscnt sddrcss: Ruscarch Center, Osaka Gas Co. Ltd., 6-

19-9 Torishima, ~onoII~lniI-~U, O&GI 554. Japan.

** Institute crIS&ntific 2nd Industri;~I Rcscorclr. Osnkn

Uni

versity, Ibarnki. Osaka 567, J apan

l~ole produced

on the

semiconductor surfxe:

pistol-l(g) --j. ~I~~~H~d~

,

CH30H(ad) + 3p+ HCI-IO -I- 2H+

,

3-I+ + 2Z.e l-i-/

_-

(B) CO, production process_ Reaction between

Cl-l,O(ad) and -OH produced from H,O:

II,0 + p -+ *OH -I- H+

,

CH,OH( -+

CH,O(ad) f H&d)

.

CH30(ad) I- 5 -01-I + CO2 + 4H,O ,

ii+ 3-c

--f I *

(C)

Reaction bctwcen CH,OI-I and *OH:

H20+p--OH+H+,

In

this paper, we have focused our attention on

the relat~ol~s~lip between the reactivity of various alco-

bols and the electronic state of tlte adsorbed species,

and found that the valence band levelsof the adsorbed

species are very important in determining the reactivi-

ty and selectivity of these reactions. This is because

tlte hol produced by irradiation of the catalyst read-

58

0 0092614/84/S 03.00 0 Elsevier Science Publisiiers B.V.

(Norti~-HoIIand Physics Puhlisbing Division)


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Volun~e 110. nunlber 1

ClIEXlICAL PIIYSLCS LETTERS

ily reacted with the species with a shallower energy

mlSTP

Icvel.

0.3-

2. Experimental

The Pt/TiO, catalysts were prepared as described

previously [3]. Platinum was supported on TiO-,

(Aerosil P 25, n-type) by photodeposition from aque-

ous K,PtCI, solution. The catalyst, Pt/TiO, (2 wt ),

thus prepared was oxidized by 100 Torr of 0, at

400C for 20 h followed by evacuation at 458C

for

1 11 n a reactor before use. The reaction was carried

out using a closed glass circulation system and the

products were analyzed by gas chromatography. The

light source for the photocatalytic reaction was a 500

W Xe lamp (Ushio Electric Co.) or a 200 IV Xe lamp

giving ody wavelengths between 350 and 400 nm

through a colored glass filter. IR spectra were observed

by using an FT-IR (Nicolct Co.) spectrometer.

0.2-

0.1 -

0

.

0

.co2

I

0

3. Results and dis ussion

The photocatalytic reactions of a series of alcol~ols

and water were examined by the volumetric method

and by IR measurement. Compared with the reaction

of methanol, which was previously reported [3], the

reaction of ethanol showed quite different behaviour.

In fig. 1, the changes in the amount of reaction prod-

ucts when 2.8 cm Hg of C?H,OH and 0.1 cm Hg of

Hz0 vapor were introduced aver Pt/TiO, under irra-

diation are shown. As is shown in fig. 1)

Hz

2nd

CHJCHO were produced at identical rates. No other

products, such as CO, or CH,, were detected in the

gas phase. This wx e&rely different from the reaction

of CH,OH and water, where CO, was produced in

addition to HCHO and H,. When gaseous ethanol and

water were trapped by liquid N1, the reaction immcdi-

ately stopped, wlicll suggests that the physisorbed

species is active in this photocatalytic reaction.

The influence of the I-I,0 vapor pressure on this

reaction was examined. Fig. 2a shows the rate of pro-

duction of Hz and CH,CHO versus H20 pressure. Even

when the Hz0 pressure was changed by more than one

order of magnitude, the activity of H, and CH,CHO

production was independent of the H,O vapor

pressure. Moreover, these products were not affected

Fig. 2.

The cffcct of 1110 lxcssurc on the rate of pllotocata-

lytic reaction bctwccn aIco11olsand water owr It/TiO-. . (a)

Solid lint. Rates of 112 (0) and C11 310 (0) prodaction lion1

ethanol and water. 13llanol press~~cwas 1.9 cnl 11~. b)

Broken line. Ratcsol II; 0) and CO? (A X l/10) production

from alethanol and water. hlcthnol prcssurc was 2.9 cm II:: .

1.0

TIME

2.0 hr

11. 0,. . * , @ *

0.5

1.0

cmHg

H,O PRESSURE

59


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Ef

1

T

-iso-PrOH

- EtOH

Trrblc 1

m

t: 5.5

-

-0

. f

- MeOH

- H20

m

- n-PrOH

6.0

-I

Hz0 (fig. 4). the Role reacts selectively wit13 ethanol

m3d the I-l20 pressure effect is not observed, and 0131~

production of CH3CHO and Hz occurs.

On the basisof this consideration, the reactions be-

tween isopropanol and water, and iI-propanol and

water were examined. As shown in fig. 4, isopropanol

has a 1nuc11 shallower donor level and n-propanol has

a deeper level than H,O. A difference in tl3e Hz0 ef-

fect for these alcohols is espected. The results for tl3e

HZ0

pressure effect a33d tl3e

kind of products are

shown in table I_ As expected, the reaction between

isopropanol and H,O did not show an HZ0 vapor pres-

sure dependence, and the one between n-propanol

and

HZ0 was

strongly dependent 013 the

HZ0

pres-

sure. Furthernlore, for the alcohols that did not show

the I-I20 pressure effect, only the deliydrogenatcd

species and H, were obtained. Only CH,CHO (acetal-

dehyde) and Hz were produced in the case of ethanol

and water, and (CH&CO (acetone) and H2 for iso-

propanol and water. On the other hand, for alcollols

with the Hz0 pressure effect, lower carbon-number

species such as CO2 were obtained (table 1 . It is well

known that -OH and hi are produced when physi-

sorbed HZ0 is oxidized with a hole in Pt/TiO, [6]

_

These facts suggest that the HZ0 effect

on

the reacti-

vity and the breaking of the C-C bond of alcol~ols is

connected with the -OH.

Table 1

The reactiorl mechanisms for the different kinds of

alcohols we have discussed are sumniarized in table 2.

There arc two types of reacrion mechanism in the

photocatalytic reaction of alcohols and water over

Pt/TiO?. One is the direct reaction of a physisorbed

alcohol species with the hole . The other is the reac-

tion of the adsorbed species with the *OH which is

produced by the oxidation of adsorbed Hz0 by the

hole. In the direct reaction with a hole, aldehyde and

H, were produced. On the other hand, in the reaction

with *OH. further oxidized species such as CO, were

produced. These selectivities are controlled by the rel-

ative energy position of the adsorbed alcol~ols against

that of H,O.

4. Conclusion

In the photocatalytic reaction between alcollols

and water, the reaction is initiated by the oxidation

In the direct reactioli of physisorbed ~llcol~As 13d he ltolc

in table 1, the current doublii process wn be included.

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Volume 1 IO. number 1 CHEMICAL PHYSICS LETTERS

14 September 1984

of the physisorbed species (alcohols and water) by a

hole, and that hole selectively reacts with the physis-

orbed species which has a shallower donor level. Con-

sequently, for alcohols which have a shallower donor

level than H20, direct reaction of physisorbed alco-

hol species with the hole dominates, and for alcohols

with deeper donor levels, the hole reacts with Hz0 to

produce -OH. The adsorbed alcohol species then re-

acts with this radical to produce aldehydes. The selec-

tivity of these reactions is reasonably understood by

taking this phenomenon into consideration_ The -OH

also reacts with the chemisorbed species and produces

some smaller carbon-number products or CO?. In the

case of methanol, -OH reacted with methoxy and

produced CO2 and Hz. Here we have found that the

valence bar.3 energy level of the physisorbcd species

is one of the most important factors determining the

activity and selectivity of photocatalytic reactions be-

tween alcohols and water.

References

[ l] T. Ikwai and T. Sakata, J. Chem. Common. (1980) 694.

[2] P. Pichat, J.-M. Hcrrmann, J. Diddier, H. Courbon and

M.-N. hfozzanga, Now. J. Chim. 5 (1981) 627.

[ 31 31. Kawai, T. Kawai, S. Naito and K. Tamaru, Chem.

Phys. Letters 98 (1983) 377.

[4] M. Bowker and R.J. hladh, Surface Sci. 95 (1960) 190;

116 (1982) 549.

[5 ] C.B. Fisher and J.L. Gland, Surface Sci. 94 (1980) 446.

[6] C.D. Jaeger and A.J. Bard, 3. Phys. Chem. 83 (1979)

3146.

Documents

Chemical Physics Letters Volume 110 Issue 1 1984 [Doi 10.1016_0009-2614(84)80146-3] Maki Kawai; Tomoji Kawai; Shuichi Naito; Kenzi Tamaru -- The Mechanism of Photocatalytic Reaction