Silver Bromide as a Photocatalyst for Hydrogen Generation from CH3OH/H2O Solution

Article abstract of DOI:10.1021/jp990287q

A AgBr/SiO2 catalyst prepared from Schumann emulsion has been used for photolysis of CH3OH/H2O solution. Under UV illumination, H2 generation was observed and hydrogen was continuously evolved for 200 h without destruction of AgBr although Ag(0) was detected by X-ray diffraction analysis after the reaction. It is presumed that the hydrogen production from methanol in distilled water occurs on the Ag(0) and the support plays important roles for the photocatalysis as well as TiO2/SiO2 (J. Phys. Chem. B 1997, 101, 2611).

Full text of DOI:10.1021/jp990287q

© Copyright 1999 by the American Chemical Society
VOLUME 103, NUMBER 29, JULY 22, 1999
LETTERS
Silver Bromide as a Photocatalyst for Hydrogen Generation from CH3OH/H2O Solution
Noriyoshi Kakuta,* Naoko Goto, Hironobu Ohkita, and Takanori Mizushima
Department of Materials Science, Toyohashi UniVersity of Technology, Tempaku, Toyohashi 441-8580, Japan
ReceiVed: January 25, 1999; In Final Form: April 28, 1999
A AgBr/SiO2 catalyst prepared from Schumann emulsion has been used for photolysis of CH3OH/H2O solution.
Under UV illumination, H2 generation was observed and hydrogen was continuously evolved for 200 h without
destruction of AgBr although Ag0 was detected by X-ray diffraction analysis after the reaction. It is presumed
that the hydrogen production from methanol in distilled water occurs on the Ag0 and the support plays important
roles for the photocatalysis as well as TiO2/SiO2 (J. Phys. Chem. B 1997, 101, 2611).
Introduction
supported-silver halide catalysts to investigate whether or not
the photosensitivity can be available for water decomposition.
Silver halides are well-known as photosensitive materials and
widely employed as source materials in photographic films. The
photographic process in silver halides comprises basically the
following: Absorption of a photon liberates an electron and a
positive hole. The electron will combine with a mobile interstitial
silver ion, leading to separation of a silver atom. Upon repeated
absorption of photons, ultimately a cluster of silver atoms can
be formed. This is the so-called latent-image speck. The critical
size, which was investigated by Fayet et al.,1 is four silver atoms
for the development. This means that silver halides are unstable
under irradiation of light, leading to the color change of silver
halide after the exposure to sunlight. From those properties,
silver halides have not been so much subjected to studies on
In this study, AgBr, which was prepared using a Schumann
emulsion5 described in the film technology, was dispersed on
silica support and CH3OH/H2O solution was employed to
examine the photocatalytic activity. The formation of H2 was
observed, and H2 was continuously evolved even after UV
illumination for 200 h.
Experimental Section
AgBr emulsion was prepared according to the preparation
method for a Schumann dry plate,5 which is widely used for an
UV-sensitive photograph, because UV light was employed in
all experiments. In brief, two solutions were prepared: Adequate
amounts of potassium bromide and gelatin were dissolved in
distilled water at 373 K (solution A) and an adequate amount
of AgNO3 was also dissolved in distilled water at 373 K
(solution B). Solution B was slowly poured into solution A under
vigorous stirring, and the mixed solution was kept at 373 K for
30 min under mild stirring. Next, the mixed solution was cooled
with ice for 2 h to form gel, and then the gel pieces were
wrapped with a cloth to wash with flowing water for 2 h. After
washing, the solid pieces were completely dissolved in distilled
water at 368 K. Finally, AgBr emulsion was prepared by
filtration using a funnel packed with glass wool.
the photolysis of water such as TiO2 and CdS.3
2
Recently, Calzaferri et al.4 reported that thin silver chloride
layers on SnO2-coated glass plates evolved O2 in the presence
of small excess Ag+ ions in aqueous solution under near-UV
illumination. The role of Ag+ ions was to supply AgCl species
to perform continuous photooxidation of water. This indicates
that silver halides have a potential for water splitting under
irradiation if we can discover a suitable condition to avoid the
photodecomposition of silver halides. We have thus employed
* Author to whom correspondence should be addressed.
10.1021/jp990287q CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/07/1999

5918 J. Phys. Chem. B, Vol. 103, No. 29, 1999
Letters
Figure 2. Effect of UV illumination for hydrogen evolution on AgBr/
SiO2 catalyst from CH3OH/H2O.
Figure 1. Hydrogen evolution on AgBr/SiO2 catalyst from CH3OH/
H2O.
AgBr/SiO2 catalyst was also prepared as follows: An
adequate amount of AgBr emulsion and distilled water were
added to SiO2 (Aerosil 300, Nippon Aerosil Co.), and then the
mixture was stirred until AgBr was dispersed homogeneously.
AgBr/SiO2 powder obtained by vacuum evaporation was dried
at 383 K for 24 h. Finally, the powder was calcined at 773 K
for 3 h. BET surface area and Ag content were 125 m2 g-1 and
10 wt %, respectively. The catalyst prepared was wrapped with
an aluminum film to avoid exposure to light because AgBr was
a photosensitive material.
The photocatalytic activity was examined using an inner-
irradiation type quartz cell with a 100 W high-pressure Hg lamp-
(USHIO Inc.). An amount of 0.2 g of AgBr/SiO2 catalyst was
suspended with 300 mL of methanol/distilled water(volume
ratio, 1:1), followed by thorough deaeration by bubbling with
Ar for 1 h. The suspension, where the solution temperature was
kept at room temperature using a water bath, was stirred
continuously under Ar flow with 20 mL min-1 and irradiated
through a water filter for removing IR radiation. Hydrogen was
analyzed by gas chromatography (Shimazu GC-8) using a
column packed with Molecular Sieves 5A. Ag/SiO2 catalyst was
also characterized by X-ray diffraction (Rigaku, RINT 1000)
and UV-vis diffuse reflectance spectroscopy (Shimazu, UV-
2400PC).
Figure 3. Behavior of hydrogen evolution on AgBr/SiO2 catalyst from
CH3OH/H2O for 200 h.
catalyst.9 The formation rate of H2 was estimated to be 0.65
mmol h-1 g-1. The total amount of H2 per 1 mol of AgBr was
beyond 1.0 even after 2 h, indicating that the reaction seems to
occur catalytically and the photodegradation of AgBr seems to
be inhibited. It is noted that the activity of H2 production was
poor when distilled water only was used as a trial. This suggests
that CH3OH acts effectively for hydrogen formation. The effect
of UV illumination was also investigated. As shown in Figure
2, H2 evolution stopped in the absence of UV illumination.
However, after the UV light was used for irradiation again, H2
evolution recommenced with the same formation rate, showing
that the reaction is induced by the absorption of UV light. The
durability of AgBr/SiO2 catalyst is given in Figure 3. The H2
production continued even after 200 h, although the formation
rate somewhat decreased. Total volume of evolved H2 was
attained to 445 mmol per g of AgBr, whose value was
corresponded to 84 mol per mol of AgBr. This result contains
important evidence because it is generally accepted in the
photographic field that silver halides are easily decomposed by
light absorption to form silver metals as the latent image center.
Results and Discussion
An XRD spectrum of the AgBr/SiO2 catalyst showed sharp
diffraction peaks assigned to AgBr6 and a broad peak corre-
sponding to amorphous SiO2. The UV-vis spectrum also
implied that there was a strong increase in absorbance at 450
nm corresponding to the indirect band gap as well as a strong
absorption peak around 290 nm assigned to the direct band gap
of AgBr.7 Those indicate that the dispersed AgBr crystallites
on SiO2 possess almost the same photoproperty reported in the
literature.8
The catalyst was subjected to the photolysis of the CH3OH/
H2O solution. The result is shown in Figure 1. H2 was
continuously evolved with the illumination time. On the other
hand, we presume that CO2 might be formed as the oxidation
product according to a previous report using RuO2/TiO2/Pt

Letters
J. Phys. Chem. B, Vol. 103, No. 29, 1999 5919
AgBr/SiO2 catalyst. The Ag0 species is initially generated as
described in the photographic theory, but the photodecompo-
sition of AgBr is, in fact, inhibited even after UV illumination
for 200 h, suggesting that the behavior of AgBr in the CH3-
OH/H2O solution might be somewhat different from the latent
image formation. One explanation is that the formed Ag0 species
might act as the sites for the H2 formation such as Pt on Pt/
TiO2 catalyst, and then further destruction of AgBr did not take
place. The role of Ag species as the site for H2 formation was
reported by Sclafani et al.13 using Ag/TiO2, although the activity
was less than that of Pt/TiO2. In addition, alcohol radicals are
known to generate hydrogen in the presence of silver colloids.14
Consequently, although there is no evidence about whether the
charge separation is smoothly performed in the presence of Ag
species or Ag species catalyze the H2 production from alcohol
radicals formed by photoinduced hole, the high activity of AgBr/
SiO2 catalyst might be intimately related to the photogenerated
Ag species. We also presume that the roles of SiO2 support
might assist the photocatalysis by the enhanced adsorption of
methanol as proposed by Anderson and Bard15 and might
promote the high dispersion of AgBr crystallites. The stabiliza-
tion mechanism of AgBr under UV illumination is not clear.
Further experiments are necessary to understand the photocata-
lytic activity of AgBr on SiO2 and behavior of methanol under
UV irradiation. The results will be published elsewhere.
Figure 4. XRD spectra of AgBr/SiO2 catalyst before (a) and after (b)
UV illumination.
Previously, Chandrasekaran and Thomas10 carried out the
photolysis of water containing excess silver nitrate using the
AgCl colloid solution. They found the formation of O2 and the
rapid breakdown of the colloid turned black as silver metals.
This reaction is not a catalytic process because the colloids are
eventually destroyed. Furthermore, the strong decrease in the
O2 yield was observed by the addition of a small amount of
CH3OH. It is noted that H2 was not detected in their experiments.
We also observed a similar effect in H2 evolution. The H2
evolution was lowered by the addition of Ag+ ions to the CH3-
OH/H2O solution. Calzaferri et al.11 measured the O2 evolution
over Ag+ zeolite (not AgCl) from water containing Ag+ ions
under illumination, where Ag+ ion was used for a scavenger of
photoinduced electrons.
To elucidate the activity for H2 formation, the catalyst was
characterized by XRD after the photolysis of the CH3OH/H2O
solution. Figure 4 shows XRD patterns before and after the
photolysis. Initially, all sharp peaks were assigned to AgBr as
described above, whereas after the reaction a new XRD peak
corresponding to Ag12 appeared together with the XRD peaks
of AgBr and amorphous SiO2. This means that the Ag formation
occurred on AgBr at the early stage of the reaction, but AgBr
was not destroyed under successive UV illumination. We
consider the reason for the photoassisted H2 production over
References and Notes
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