3.2 Photocatalytic hydrogen evolution activity of the catalyst
The photocatalytic hydrogen production reaction of the catalyst was
performed in a reaction system of 15 vt% triethanolamine as a
sacrificial reagent. The use of EY sensitizer enhances the catalyst’s
ability to absorb light. In the dark state (without light), no hydrogen
was generated in all samples. The activity ratio experiments of
composite photocatalysts with different ratios and degrees of
phosphatization were performed in TEOA solution at pH = 10. It can be
clearly seen from Figure 6(a) that Cu-P and Co-P produced by complete
phosphation of Cu-MOFs and ZIF-9(Co) show a certain photocatalytic
hydrogen release activity, and the activity of Co-P is better than that
of Cu-P. The photocatalytic activity of Cu-Co-xP-2 (x = 1, 2, 3, 4 .)
composite photocatalysts was obtained from Cu-MOFs@ZIF-9(Co) composites
with different degrees of phosphating treatment, as shown in Figure
6(b). Under EY sensitized conditions, the in-situ phosphation was used
to control the phosphorization of the Cu-MOFs@ZIF-9(Co) core-shell
structure. The results show that the photocatalytic activity of the
composite catalyst constructed by introducing a certain amount of P on
the surface of Cu-MOFs@ZIF-9(Co) has been significantly improved. The
activity of Cu-Co-1P-2 and Cu-Co-2P-2 samples was significantly improved
during slight phosphating. However, after further phosphating, the
activity of Cu-Co-3P-2 and Cu-Co-4P-2 composite photocatalysts began to
decrease. Combined with XRD (Figure 2(c)), it can be seen that the
production of CoP on the surface of Cu-MOFs@ZIF-9(Co) and the exposure
of unphosphorylated Cu-MoFs on the inner layer are the main reasons for
the change in the activity of the composite photocatalyst. In order to
adjust the internal and external phosphating ratio of Cu-MOFs@ZIF-9(Co)
core-shell structure, Cu-Co-x (x = 1, 2, 3, 4.) materials with different
Cu-Co ratios were constructed by adjusting the amount of Cu-MOFs.
Cu-Co-2P-x (x = 1, 2, 3, 4.) photocatalyst was obtained under the same
phosphating treatment. As shown in Figure 6(c), the hydrogen-producing
activity of Cu-Co-2P-x (x = 1, 2, 3, 4.) composite catalysts with
different contents of Cu-MOFs was explored. The highest
hydrogen-producing activity was the Cu-Co-2P-2 photocatalyst. With the
increase of Cu-MOFs content, the phosphatization degree of the whole
sample also increased, so there is almost no Cu-MOFs peak in Cu-Co-2P-4.
It can be seen that the excellent photocatalytic activity of Cu-Co-2P-2
composite catalyst can be derived from two aspects, on the one hand, the
synergistic effect of CoP and Cu3P,which were derived
from MOFs phosphation, play an important role. On the other hand,
because the nucleated Cu-MOFs is not completely phosphated, it retains
the three-dimensional structure as a reaction carrier. We further
explored the effect of pH of system on the catalytic activity of
Cu-Co-2P-2 composite photocatalyst. It can be seen from Figure 6(d) that
the Cu-Co-2P-2 composite photocatalyst exhibits different selectivity to
the reaction environment and has the best activity at pH = 10. This
shows that a mild alkaline environment is the most favorable factor for
a hydrogen production system [29]. The system with extra acid orbase
is not conducive to theprocess of the hydrogen precipitation reaction
[30]. This is because the neutral solution contains a large amount
of H+, which may cause the protonation of the
triethanolamine solution and serious decrease of the electron donor
effectiveness [38, 39]. When the reaction conditions become strongly
basic, the thermodynamic driving force for hydrogen evolution is
obviously insufficient due to the lack of H+ [40].
In addition, the adsorption of EY molecules on the photocatalyst is also
affected by the solution pH of the reaction system.