2.3 Enzymatic Production of H 2 Assisted by Photocatalysts
The combination of hydrogenases with electrochemical systems has been extensively researched, however, the cases for photocatalysts acting simultaneously as
scaffold and electron donor for hydrogenases are, most of them, more recent. Two
strategies have been regarded to achieve the photobiocatalyzed reduction of protons. The first one links the hydrogenase to an organometallic dye to form a
homogeneous catalytic system, thus, requiring a sacrificial electron donor. The
second strategy consists on a solid support such as a modified photoelectrode
loaded with hydrogenases, yielding heterogeneous catalytic processes.
2.3.1 Homogeneous Photobiocatalytic Production of H 2
Armstrong´s group developed a colloidal system comprising TiO 2 nanoparticles
modified with several hydrogenases and organometallic complexes to harvest visible light [44]. The most successful result by far was obtained using D. baculatum
NiFeSe-hydrogenase and a ruthenium organic complex, [Ru(bpy) 2 (H 4 dbpy)]
2+
[45]. Upon illumination and in presence of the electron donor triethanol amine the
ruthenium complex harvested the visible light and transferred the excited electrons
to the conduction band of TiO 2 nanoparticles, which were further ceded to the
hydrogenase to produce 50 mol H 2 Â s
−1 per mole of enzyme.
Besides TiO 2 , other light harvesting semiconductors were also explored as
candidates to provide high-energy electrons to hydrogenases for reduction of protons. The group of Paul King showed that CdTe is a suitable semiconductor [46].
2.5 nm CdTe particles modified with mercaptopropionic acid (MPA) served both as
energy harvester and scaffold for the FeFe-hydrogenase from C. acetobutylicum.
After immobilization of the enzyme and the enzyme activation, the samples were
illuminated with a 150 W halogen white light during 5 min, in presence of ascorbic
acid acting as sacrificial electron donor. After monitoring the activity at several pH
values, the authors found that maximum H 2 production was obtained at pH 4.75,
obtaining 1.94 ± 0.30 lmol  mg
−1
 min
−1 . Interestingly, at this pH value, the
hydrogenase is less than half active than at pH 7 measured using MV, meaning that
the interaction between the CdTe and the enzyme, and the stability of MPA on
CdTe play a major role in the electron transfer. After optimization of pH (4.75) and
other critical parameters such as the electron donor concentration ([Ascorbic
acid] = 0.5 M) and the ratio of CdTe:Hydrogenase (4:1), the system produced at a
turnover number of 25 mol H 2 per mol of hydrogenase. The efficiency of the hybrid
catalyst was determined as 9% when irradiated with monochromatic light or 1.8%
when irradiated with white light at 1.5 AM intensity [46]. These results using CdTe
were further studied spectroscopically by FTIR, in which the hybrid
CdTe-hydrogenase system was compared with that involving hydrogenase and the
photosensitizer Ru(bpy) 3
2+ [47]. The differences in the interfacial interactions
between either sensitizers and the hydrogenase were analysed. It was found that
CdTe performed better than Ru(bpy) 3
2+ due to its possibility to host multiple
photon excitation and multielectron pathways, which are not feasible for single
complex molecules.
254
M. Martins et al.
The combination of hydrogenases with electrochemical systems has been extensively researched, however, the cases for photocatalysts acting simultaneously as
scaffold and electron donor for hydrogenases are, most of them, more recent. Two
strategies have been regarded to achieve the photobiocatalyzed reduction of protons. The first one links the hydrogenase to an organometallic dye to form a
homogeneous catalytic system, thus, requiring a sacrificial electron donor. The
second strategy consists on a solid support such as a modified photoelectrode
loaded with hydrogenases, yielding heterogeneous catalytic processes.
2.3.1 Homogeneous Photobiocatalytic Production of H 2
Armstrong´s group developed a colloidal system comprising TiO 2 nanoparticles
modified with several hydrogenases and organometallic complexes to harvest visible light [44]. The most successful result by far was obtained using D. baculatum
NiFeSe-hydrogenase and a ruthenium organic complex, [Ru(bpy) 2 (H 4 dbpy)]
2+
[45]. Upon illumination and in presence of the electron donor triethanol amine the
ruthenium complex harvested the visible light and transferred the excited electrons
to the conduction band of TiO 2 nanoparticles, which were further ceded to the
hydrogenase to produce 50 mol H 2 Â s
−1 per mole of enzyme.
Besides TiO 2 , other light harvesting semiconductors were also explored as
candidates to provide high-energy electrons to hydrogenases for reduction of protons. The group of Paul King showed that CdTe is a suitable semiconductor [46].
2.5 nm CdTe particles modified with mercaptopropionic acid (MPA) served both as
energy harvester and scaffold for the FeFe-hydrogenase from C. acetobutylicum.
After immobilization of the enzyme and the enzyme activation, the samples were
illuminated with a 150 W halogen white light during 5 min, in presence of ascorbic
acid acting as sacrificial electron donor. After monitoring the activity at several pH
values, the authors found that maximum H 2 production was obtained at pH 4.75,
obtaining 1.94 ± 0.30 lmol  mg
−1
 min
−1 . Interestingly, at this pH value, the
hydrogenase is less than half active than at pH 7 measured using MV, meaning that
the interaction between the CdTe and the enzyme, and the stability of MPA on
CdTe play a major role in the electron transfer. After optimization of pH (4.75) and
other critical parameters such as the electron donor concentration ([Ascorbic
acid] = 0.5 M) and the ratio of CdTe:Hydrogenase (4:1), the system produced at a
turnover number of 25 mol H 2 per mol of hydrogenase. The efficiency of the hybrid
catalyst was determined as 9% when irradiated with monochromatic light or 1.8%
when irradiated with white light at 1.5 AM intensity [46]. These results using CdTe
were further studied spectroscopically by FTIR, in which the hybrid
CdTe-hydrogenase system was compared with that involving hydrogenase and the
photosensitizer Ru(bpy) 3
2+ [47]. The differences in the interfacial interactions
between either sensitizers and the hydrogenase were analysed. It was found that
CdTe performed better than Ru(bpy) 3
2+ due to its possibility to host multiple
photon excitation and multielectron pathways, which are not feasible for single
complex molecules.
254
M. Martins et al.