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.