G. Ma et al. / Journal of Catalysis 260 (2008) 134–140
135
found that H2S can be stoichiometrically converted to H2 and S in
ethanolamine solvent under visible light irradiation, and that the
quantum efficiency for the hydrogen production can be as high
as 30% for CdS-based semiconductor materials loaded with noble
metals as co-catalysts.
was dispersed in a Pyrex reaction cell containing 100 mL of H2S-
amine solutions. A thermostatted water jacket was set around the
reaction cell to control the reaction solution at a prespecified tem-
perature. A 300-W Xe lamp equipped with an optical cutoff filter
(λ > 420 nm) was used as the light source. A shutter window filled
with water was placed between the Xe lamp and the reaction cell
to remove infrared light illumination.
2. Experimental
2.1. Preparation and characterization of CdS
2.4. Raman spectra
Highly crystalline CdS nanoparticles were prepared by a precip-
itation-hydrothermal process. All of the reagents were analytical
grade and used without further purification. In a typical prepara-
Raman scattering spectra were recorded in back-scattering ge-
ometry on an Acton Raman spectrometer equipped with a liquid
nitrogen-cooled CCD detector at a resolution of 4 cm−1. A 532-
nm semiconductor laser was used as the excitation source, and the
laser power at the sample was set as 60 mW. All experiments were
performed with a quartz tube at room temperature.
tion procedure, Cd(CH3COO)2·2H2O was dissolved in water with
−1
a concentration of 0.15 mol L
(M) and precipitated by aqueous
Na2S solution. The yellow amorphous CdS was washed with co-
pious deionized water, followed by hydrothermal crystallization at
◦
200 C for 3 days in a Teflon-lined stainless steel autoclave. The
2.5. Electrochemical experiments
CdS powder was collected by filtration and washed several times
◦
with water and ethanol. After drying at 80 C in a vacuum, the fi-
A typical three-electrode electrochemical system was used for
linear sweep voltammetry measurements. A platinum plate was
used as the working electrode with surface area of around 6 cm2,
with another platinum plate with a much larger surface area used
as the counter-electrode and an SCE used as the reference elec-
trode. The desired solid Na2SO4 was added to the H2S–DEA or
Na2S–NaOH solution up to a concentration of 0.2 M to func-
tion as a supporting electrolyte. The scan rate was 50 mV/s, and
all measurements were carried out on an EG&G 2273A potentio-
stat/galvanostat at room temperature. The electrolyte was kept un-
der continuous magnetic stirring during the measurements.
nal yellow powder was obtained.
Transmission electron microscopy (TEM) (JEOL JEM-2000EX,
120 kV) images of the catalyst sample demonstrated a mean crys-
talline size of CdS particles of about 50 nm. X-ray diffraction (XRD)
(Rigaku D/max-2500 diffractometer, CuKα, 40 kV, 100 mA) and
high-resolution TEM (HRTEM) showed that the prepared CdS was
well crystallized with a pure hexagonal wurtzite structure.
2.2. Loading of M, Pt–M, MS, and Pt–MS on CdS
H2PtCl6·6H2O(99.99%) and RuCl3·xH2O were purchased from
China Medicine Shanghai Chemical Reagent Corp. PdCl2·2H2O and
RhCl3·xH2O were purchased from China Beijing Chemical Reagent
Corp. The aqueous solution of noble metal compound was pre-
pared from deionized water. All chemicals were used as received.
Noble metals (M and Pt–M) were loaded onto CdS as a co-
catalyst by in situ photochemical deposition [15–19]. Typically, the
prepared CdS powder (0.5 g) was dispersed in aqueous acetic so-
lution (150 ml 0.1 M), followed by the addition of appropriate
amount of noble metal compound solutions. Then the mixture was
irradiated under visible light (300 W Xe lamp, λ > 420 nm) in vac-
uum for 0.5 h to photoreduce the noble metal on the CdS surface.
The reacted solution was filtrated and washed with deionized wa-
ter to get an M/CdS (Pt–M/CdS) photocatalyst. Noble metal sulfide
(MS or Pt–MS) was loaded on CdS by precipitation method. Typ-
ically, the CdS (Pt/PdS) was dispersed in aqueous Na2S solution
(150 ml 0.3 M), followed by the slow addition of the corresponding
noble metal compound solution. Then, similar to the aforemen-
tioned process, the solution was irradiated in vacuum and filtrated
to produce MS/CdS (Pt–MS/CdS).
3. Results and discussion
3.1. Photocatalytic splitting of H2S in ethanolamines
Fig. 1 shows the photocatalytic H2 evolution from H2S split-
ting on Pt/CdS catalyst in the different ethanolamine solvents
used to absorb/dissolve H2S and also as the reaction media. These
ethanolamines—monoethanolamine (MEA), H2NCH2CH2OH, dieth-
anolamine (DEA), HN(CH2CH2OH)2, and triethanolamine (TEA),
N(CH2CH2OH)3—are frequently used as absorbents in industry for
H2S absorption [21]. The Pt/CdS photocatalyst showed high activ-
ity in hydrogen production under visible light irradiation for all
the three ethanolamine solvents dissolved with H2S. Among the
three solvents tested, the highest activity was observed for DEA
2.3. Photocatalytic reactions
Photocatalytic reaction solutions were prepared by dissolving
H2S gas (99.9%) in different amine solvents that had been pre-
viously dehydrated with molecular sieves (5-Å). The concentra-
tion of sulfide ions in solvent was determined by the titration
method [20]. The H2S-amine solution was mixed with an excess of
acidic I2–KI solution (0.05 M). The excess iodine was back-titrated
with Na2S2O3 solution (0.05 M), using starch as an indicator:
C + H2S = CI –KI × VI –KI/VH
.
2S
2
2
Fig. 1. Photocatalytic H2 production under visible light irradiation over Pt/CdS
(0.20 wt% Pt) in different solutions. DEA: diethanolamine, TEA: triethanolamine,
MEA: monoethanolamine. Reaction conditions: volume of solution, 100 ml; concen-
tration of H2S, 0.30 M; amount of catalyst, 0.025 g; reaction temperature, 30 C;
light source, 300-W Xe lamp with a cutoff filter (λ > 420 nm).
Photoactivities of the samples were examined in a closed gas
circulation and evacuation system [15–19]. The evolved amounts
of H2 were analyzed by online gas chromatography (TCD, molec-
ular sieve 5-Å column, and Ar carrier). The M/CdS photocatalyst
◦