3.6 Photocatalytic activity and stability
Fig. 6 shown the
photocatalytic H2evolution performances of diverse catalysts with
Na2S/Na2SO3(0.35M/0.25M) solution as a sacrificial agent under visible light
irradiation (λ> 400 nm). InFig. 6a , the hydrogen
production activity for bare
NiSe2 was not detected, which manifested that it could
not generate charge carries and had no photocatalytic activity. The MCS
reveals poor photocatalytic activity (95.74 μmol) when the irradiation
time lasted for 5 h. The H2 evolution rate of MCNS-x
first increased and then decreased with the added amount of loaded
NiSe2 from 3 to 20 wt%. Obviously,
the maximal hydrogen production
amount obtained of 713.78 μmol when the
NiSe2 content was
10%, increased by a factor of around 7.5 in comparison to pure
MCS.
However, the H2production activity start to decrease when the content of
NiSe2 exceed 10%,
which may be that the shielding
effect of the mixed catalyst and the lack of hydrogen production
activity of NiSe2 [33]. Besides, apparent quantum
efficiency (AQE) and hydrogen evolution stability of MCNS-10 were
important indicators to measure the practical application of catalysts.Fig. 6b shown the AQE value of MCNS-10 under different
monochromatic light wavelengths. Th AQE and the intensity of UV
absorption were basically the same, confirming that the activity of
hydrogen production was related to the wavelength of incident light
[44].
The cyclic tests of H2 production of MCS and MCNS-10
were performed in Fig. 6c . Compared with the composite catalyst
MCNS-10, the total hydrogen production of MCS was basically unchanged,
indicating that MCS has good stability. The hydrogen production activity
of the composite catalyst MCS decreased significantly, the reason for
this phenomenon may be increased in reaction time, which would
exacerbate consumption of
Na2S/Na2SO3(0.35M/0.25M) solution [4]. Secondly, this was probably due to
partial detachment of MCS and NiSe2 during the
reaction. Fig. 6d compares
the XRD patterns of composite catalyst MCNS-10 original and recycled
hydrogen production. The position of diffraction peak was no significant
change, but the intensities was reduced by half, which indicated that
the crystallinity of the catalyst became worse.