# Cu-based MOFs/TiO 2 Chitosan Beads for Green Photocatalytic H 2 Generation

Marie Le Pivert, Alisha Khan, Mireille Benoît, Christophe Colbeau-Justin, Johnny Deschamps, Hynd Remita

► To cite this version:

Marie Le Pivert, Alisha Khan, Mireille Benoît, Christophe Colbeau-Justin, Johnny Deschamps, et al. Cu-based MOFs/TiO 2 Chitosan Beads for Green Photocatalytic H 2 Generation. ACS Applied Energy Materials, 2025, 10.1021/acsaem.5c02179. [https://doi.org/10.1021/acsaem.5c02179]

HAL Id: hal-05362588

https://cnrs.hal.science/hal-05362588v1

Submitted on 13 Nov 2025

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# Cu-based MOFs/TiO₂ Chitosan Beads for Green
Photocatalytic H₂ Generation

Marie Le Pivert<sup>1,2</sup>, Alisha Khan<sup>1</sup>, Mireille Benoît<sup>1</sup>, Christophe Colbeau-Justin<sup>1</sup>, Johnny Deschamps<sup>2*</sup>, and Hynd Remita<sup>1*</sup>

**KEYWORDS:** Hydrogen, Photocatalysis, Titanium Dioxide (TiO<sub>2</sub>), Metal Organic Frameworks (MOFs), Composites, Chitosan (CS) beads.

**ABSTRACT:** Designing supported photocatalysts for photocatalytic hydrogen production is essential to overcome the time consuming or expensive photocatalyst post-collection additional step. Herein, a copper-based Metal Organic Framework (MOF)/TiO<sub>2</sub> (HKUST-1/TiO<sub>2</sub>) composite photocatalyst, very promising for hydrogen generation, was encapsulated in Chitosan (CS) spherical beads. These millimeter-sized photocatalytic beads were synthesized via a three-step method and diffuse reflectance as well as Fourier-transform infrared spectroscopies assert that HKUST-1/TiO<sub>2</sub> is not altered by the synthesis process. Moreover, time resolved microwave conductivity (TRMC) characterization proves that the charge carriers' dynamics is not altered by

<footnote n="1"><sup>1</sup> Institut de Chimie Physique, UMR 8000 CNRS, Université Paris-Saclay, 91405 Orsay, France</footnote>

<footnote n="2"><sup>2</sup> Unité Chimie et Procédés (UCP), ENSTA Paris, Institut Polytechnique de Paris, 91120 Palaiseau, France</footnote>
the encapsulation of HKUST-1/TiO₂. The beads possess a good photocatalytic activity for
hydrogen generation under UV-visible light. The number of beads and their stability with cycling
were also investigated. A significantly higher hydrogen generation with hydrated beads (459
µmol/g/h) was measured compared to the one with dried beads (45 µmol/g/h) according to better
water and hydrogen diffusion in the hydrated beads. The highest production rate reaches 854
µmol/g/h with 4 HKUST-1/TiO₂ CS beads loaded with 0.36 mg of photocatalyst per bead.
Recyclability tests reveal good durability without significant loss in efficiency.

INTRODUCTION

To address the issues of energy shortage and environmental pollution caused by the consumption
of fossil fuels, the use of alternative carbon-free energy sources is gaining increasing attention. In
this context, the conversion of renewable solar energy into hydrogen (H₂) via photocatalysis has
received significant attention due to its low cost and scalability, making it an attractive method for
producing green fuels with net-zero emissions¹.

H₂ generation by photocatalysis relies on the light activation of a photocatalyst, typically a wide bandgap semiconductor. This photo-activation results in the formation of electron/hole (e⁻/h⁺) pairs in the conduction and valence bands, respectively. The photo-generated electrons then react with H⁺ ions from water to produce H₂, while the photo-generated holes react with a scavenger, usually an alcohol such as methanol, added to the water to prevent water oxidation and to reduce the recombination rate². However, the production yields associated to this method remain low compared to other methods, such as electrolysis, due to the fast e⁻/h⁺ recombination rate and the low activity under sunlight.
A key factor in achieving high-performance $H_2$ production is the development of new efficient photocatalysts through the design of photocatalytic hybrid systems that enable visible light absorption and fast charge carriers' separation and transfer, leading to a better activation under solar light with a longer lifetime of the excited charge carriers$^{3-5}$. Consequently, the development of multiphase heterostructured materials, based on Metal Organic Frameworks (MOFs) materials, has become a focal point of strategy$^{5-7}$. MOFs are self-assembled hybrid (organic/inorganic) materials with 2D- or 3D-porous structures of metal ions and organic ligands, offering high surface area, high porosity and potentially behaving similarly to semi-conductors$^8$.

In our previous work, HKUST-1, a copper-based MOF, in which $Cu^{2+}$ ions serve as the central metal and benzene-1,3,5-tricarboxylate as coordination ligands, was coupled with $TiO_2$ (commercial P25) and demonstrated excellent performances for $H_2$ generation by glycerol photo-reforming$^2$ and water photoreduction$^9$. A promising $H_2$ production rate of 5.11 mmol/g/h was achieved with the composite HKUST-1/$TiO_2$ (1:20 optimized mass ratio) and compared to the production rate of 7.97 mmol/g/h reached in presence of $TiO_2$/Pt (1 % mass fraction) under the same conditions (20 mg in water / methanol mixture (3:1) irradiated by a 300 W Xenon lamp)$^9$. Additionally, HKUST-1/$TiO_2$ exhibited good stability with cycling (~25 h of use). The main limitation is its use as unsupported nanopowder in water, which leads to significant mass loss between the photocatalytic cycles (50 % mass loss after 6 cycles). This is a general issue in photocatalysis with powder in suspension involving photocatalyst additional steps of separation and mass loss after its collection.

To address the limitations of unsupported nanopowders, the immobilization of photocatalysts has been extensively studied for applications in air and water treatment, offering advantages such as improved technical implementation feasibility, extended catalyst life, and reduced operational
costs¹⁰. As a result, numerous materials have been developed, including functionalized foams¹¹, civil engineering materials¹², and polyester fabric¹³. Another approach was the encapsulation of photocatalysts in bio-polymer beads, such as chitosan or alginate, which have shown good efficiency in the water treatment fields¹⁰,¹⁴,¹⁵. Block copolymers have also been recently used to encapsulate organic donor-acceptor-donor (D-A-D) photocatalysts¹⁶.

However, despite these advances, the use of encapsulated photocatalysts as an alternative to free
catalysts remains underexplored for hydrogen production, and few studies have reported results
with this approach. Zhang and al. described the H₂ generation by methanolysis of NaBH₄ with
metal nanoparticles supported on polyacrylamide water beads¹⁷. Mandal and al. reported the
development of alginate-encapsulated rGO-CdS milli-spheres for H₂ photocatalytic generation
from water without the use of a sacrificial agent¹⁸. Zhang and al. recorded a photocatalytic activity
of 11.88 mmol/h/g with 18.42 mg of CdS-0.02NiSx confined in 2.5 g of chitosan beads immersed
in a 300 mL sacrificial agent mixture (a mixed solution of 0.35 mol/L Na₂S and 0.25 mol/L
Na₂SO₃) and exposed to visible light (> 420 nm)¹⁹. Nevertheless, even if Cd-based catalysts reach
good photocatalytic activity, it is important to underline that their use can present a dangerous
source of pollution considering the toxicity of cadmium and its leaching in water.

Hydrogel beads not only facilitate future upscaling of H₂ generation, but can also operate as
miniature reactors to enhance the photocatalytic H₂ production rate. Furthermore, photocatalytic
hydrogel beads can inhibit the recombination of photogenerated charge carriers due to lower
charge resistance¹⁸,²⁰ and a promoting directional migration of photogenerated holes towards the
electron-rich microenvironment induced by -NH₂ and -OH groups of the polymer¹⁹,²¹. These holes
can then be rapidly consumed by hole scavengers. Chitosan (CS) has been widely used as the
second most abundant polysaccharide natural biopolymer for developing hydrogel beads to embed
or support catalysts for water remediation because of its unique properties including nontoxicity, biocompatibility, and stabilization agent for metal particles<sup>14, 22, 23</sup>. Moreover, this heteropolymer of chitin extracted from the exoskeleton of shellfish and crustacean has also the advantage to be shaped into beads, sheets, or composites<sup>14</sup>.

Despite their potential application in photocatalysis, the use of hydrogel beads remains underexplored. Moreover, it is important to underline that the majority of the bio-polymer beads used for H₂ generation are previously dried, sometimes even rehydrated after drying, instead of being used directly as their hydrated form as synthesized. In this regard, the objective of this work was to determine the feasibility and the relevance of as synthesized hydrated chitosan beads to embed HKUST-1/TiO₂ photocatalyst for high H₂ generation and easy photocatalyst collection. To the best of our knowledge, it is the first time that a MOF-based composite embedded in hydrated beads (as synthesized) was used for photocatalytic hydrogen generation. HKUST-1/TiO₂ chitosan beads were thoroughly characterized by Scanning Electron Microscopy (SEM), Diffuse Reflectance Spectroscopy (DRS), Time-resolved microwave conductivity (TRMC), and Fourier-transform infrared spectroscopy (FTIR). These photocatalytic beads were then tested for H₂ generation by photocatalysis under a Mercury lamp. The H₂ production rate was studied as a function of the concentration of photocatalyst per bead, the influence of the beads' hydration, and the number of used beads.

# EXPERIMENTAL SECTION

This section describes the experimental methods and procedures used to develop photocatalytic beads and investigate its efficiency for hydrogen production.

**Photocatalyst preparation and characterization.** The synthesis of HKUST-1/TiO₂ chitosan (CS) beads involves three steps: (1) the synthesis of HKUST-1/TiO₂ composite; (2) the
homogeneous dispersion of HKUST-1/TiO₂ into the CS gel; (3) the bead formation. Briefly, 0.95 mg of HKUST-1 (Sigma-Aldrich) and 19.05 mg of TiO₂-P25 (Evonik), corresponding to a mass ratio of 1:20, were added to 5.0 mL of methanol (MeOH) and sonicated for 3 min, followed by degassing with nitrogen for 7 minutes and stirring overnight. HKUST-1/TiO₂ composite has been developed by our team for H₂ generation and the HKUST-1:TiO₂ (1:20) mass ratio corresponds to the optimal composition for photocatalytic hydrogen generation, as determined in our previous study⁹. Subsequently, the composite was dried at 70°C. The dried powder of HKUST-1/TiO₂ (1:20) was then introduced to 2.5 mL of 2 % CS gel (mass ratio) in a 1 % acetic acid solution (volume ratio, pH ~ 2.7). The mixture was stirred overnight to achieve a homogeneous dispersion. Finally, the gel was dropped into a 1 M NaOH solution (pH ~ 14) and kept in the NaOH solution for 24 hours to ensure stabilization and prevent cracking during the photocatalytic tests. It is important to note that a shorter stabilisation time in NaOH basic solution resulted in more fragile beads prone to cracking during the first photocatalytic test. The obtained beads were thoroughly washed with deionized water 5 times. Hydrated beads were either stored in a hermetic closed tube or air-dried at room temperature for 24 hours. For reference, pristine CS beads were synthesized using the same method without the addition of HKUST-1/TiO₂ composite.

Various gel/composite ratios were investigated, resulting in a bead composition of 0.20 mg, 0.36 mg, and 0.50 mg of photocatalyst composite per bead. Briefly, 10 mg, 20 mg or 30 mg of HKUST-1/TiO₂ were introduced to the 2.5 mL of 2 % CS gel leading to have 50, 55 or 60 photocatalytic beads per batch of synthesis, respectively. The optimized amount of HKUST-1/TiO₂ photocatalyst loaded per bead turned out to be equal to the ratio between the total mass of photocatalyst introduced in the CS gel and the number of synthesized beads. It is important to note that the number of beads obtained per batch was relatively constant, varying only by two or three units.
The morphology of the HKUST-1/TiO₂ CS beads was examined by Scanning Electron Microscopy (SEM, ZEISS Supra 55VP). The chemical stability and elemental interaction of the photocatalyst beads were investigated by Fourier-transform infrared spectroscopy (FTIR, Bruker Vertex 70). The optical properties of the embedded composite were explored using Diffuse Reflectance Spectroscopy (DRS, Cary 5000, Agilent). The charge carriers' dynamics, which is a key factor in photocatalysis, was investigated by time-resolved microwave conductivity (TRMC) under two excitation wavelengths, 380 nm and 550 nm, thanks to a tunable pulsed laser light source (Nd: YAG Laser) equipped with an optical parametric oscillator (OPO; EKSPLA, NT342B). A Gunn diode (30 GHz) was used to generate microwaves.

**Hydrogen generation by photocatalysis.** The photocatalytic H₂ generation was conducted in a 37 mL cylindrical quartz reactor containing 20 mL of MeOH:H₂O (1:3) solution with photocatalyst beads and surrounded by an aluminium reflector to ensure uniform lateral illumination over its whole surface by a 150 W mercury lamp (Lamp TQ150 exclusive UVCP HERAEUS; 46.5 W/m² from 280 nm to 315 nm, 30.8 W/m² from 315 nm to 400 nm, and 70.0 W/m² from 400 to 700 nm; emission spectrum available in Figure S1). The experimental setup is shown in Figure S1. Prior to each photocatalytic test, the reactor was purged with an inert gas (Nitrogen, N₂) to remove oxygen and to prevent undesired and parasitic reactions. The photocatalytic tests were performed in the absence of magnetic stirring to avoid beads mechanical damage and a water-cooling system was used to maintain a constant temperature of 20°C. H₂ generation was then monitored every hour using micro-gas chromatography (Micro GC Fusion, Chemlys). The H₂ detected by micro-gas chromatography was then normalized by the mass of photocatalysts introduced in the reactor (mass of photocatalyst = (photocatalyst loading / beads) × number of beads). Various quantities of beads were placed in the reactor to vary the photocatalyst amount from 1.4 mg to 20 mg and to determine
the optimal conditions for photocatalytic tests. Each photocatalytic test was performed at least two
times with beads from different batch synthesis performed at different times, rather than a single
batch, in order to ensure both the reproducibility of the synthesis method and the reliability of H₂
production.

RESULTS AND DISCUSSION

**HKUST-1/TiO₂ composite.** In accordance with our previous work⁹, the HKUST-1/TiO₂ composite was successfully synthesized by physical mixing in methanol. High resolution TEM image of HKUST-1/TiO₂ composite revealed strong interactions at the junction of HKUST-1 and TiO₂, according to the observation of bright dot clusters (Figure 1(a)). These bright dots correspond to copper nanoclusters (Cu⁺ and Cu⁰) formed at the edges of TiO₂ nanoparticles. They were identified and deeply characterized by Transmission Electron Microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDX), X-Ray photoelectron spectroscopy (XPS) and their activity was investigated by electron paramagnetic resonance (EPR) studies and density functional theory (DFT) calculations in our previous work⁹. The strong interactions between TiO₂ and Cu coordination sites from HKUST-1 are facilitated by the configuration of HKUST-1, which is a MOF with open metal sites. In this structure, the Cu coordination sites are not blocked by the ligands, but are located on the walls of the structure and facing toward pores, making the Cu sites completely accessible²⁴.

The size of the copper nanoclusters ranges from 0 to 3 nm, with an average size of 1.6 nm ⁹. This
observation is in agreement with the existing literature, where HKUST-1 has been used as a
precursor to generate Cu nanoclusters measuring 0.8 – 2 nm on the (001) facet of TiO₂²⁵. In another
study, protective Cu(I) ions were generated by post-coordinative reduction of Cu(II) nodes of
HKUST-1 using hydroquinone as a reducing agent, leading to the reduction of 31 % of copper
sites in HKUST-1<sup>26</sup>. Additionally, Z. Lin et al. used methanol as a reducing solvent to synthesize
defective HKUST-1, resulting in the partial reduction of some Cu<sup>2+</sup> sites and leading to a mixture
of Cu<sup>+</sup> and Cu<sup>2+</sup>, along with the development of a hierarchical porous structure<sup>27</sup>. Our recent
findings, supported by EPR, XRD, DRS and FTIR studies, also indicate that the crystallinity and
the structure of the HKUST-1 in the HKUST-1/TiO<sub>2</sub> composite were not altered by the formation
of some Cu nanoclusters at the edges of the TiO<sub>2</sub><sup>9</sup>. We therefore conclude that the Cu nanoclusters
formation is likely minimal compared to the Cu(II) nodes, which explains the non-alteration of the
HKUST-1 structure. The lattice spacing of TiO<sub>2</sub> was 0.348 nm, corresponding to the (101) anatase
plane, suggesting also the non-alteration of TiO<sub>2</sub> structure by the formation of nanocluster at its
surface.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** TEM image of HKUST-1/TiO<sub>2</sub> composite (a), photography of hydrated (b) and dried (c) composite beads, SEM image of CS dried bead surface (d-e), SEM image of HKUST-1/TiO<sub>2</sub> (0.36 mg/bead) CS dried bead surface (f-g), and photography of hydrated beads after 3 months of</figcaption></figure>
storage in hermetic tube directly closed after the synthesis (at the left a top-view of beads and at the right the two top-views of a bead cut in its middle) (h).

**HKUST-1/TiO₂ chitosan beads.** Both the hydrated and dried forms of HKUST-1/TiO₂ CS beads demonstrated a good spherical appearance and uniform size. The diameters recorded were 3.8 ± 0.1 mm for hydrated beads (Figure 1 (b,h)) and 1.3 ± 0.1 mm for the dried ones (Figure 1 (c)). Moreover, no size modification was observed as a function of the photocatalyst mass loaded per bead, which ranged from 0.20 mg to 0.50 mg. The HKUST-1/TiO₂ CS beads are characterized by a white and light blue opaque appearance, whereas the CS beads without the photocatalyst are transparent and slightly yellow, attesting to the presence of the materials in the chitosan beads (Figure 1 (b-c)). Moreover, surface investigations on dried beads using SEM Figure 1 (d-g) revealed significant differences. The composite bead exhibited a more homogenous and rougher surface compared to the surface of the pure CS beads, which displayed minimal cavities and short fibrous structures. This study confirms the optimal integration of the photocatalysts into the beads, both within their structure and on their surface. A higher porosity is also induced by the internal microstructure of HKUST-1/TiO₂ CS beads and is confirmed by the microscopy images.

**Evaluation of HKUST-1/TiO₂ integrity in the chitosan beads.** To investigate the stability and the interactions of the composite during the embedding process, the beads were characterized by DRS. The absorption spectrum of the HKUST-1/TiO₂ CS beads revealed that the characteristic spectrum of the HKUST-1/TiO₂ composite remained unchanged. This spectrum illustrated predominant absorption in the UV range, from 200 to 390 nm, related to the band gap of TiO₂, and a second broad absorption band in the visible-light range from 480 to 800 nm, attributed to the d-d spin-allowed transition of Cu²⁺ from HKUST-1 framework (Figure 2 (a)). The second absorption band, linked to the Cu²⁺ ions, confirms the integrity of the MOFs in the composite. This result also
highlights the excellent stability of the HKUST-1/TiO₂ composite under acidic and basic conditions encountered during the embedding process. Moreover, it demonstrates the composite's long-term stability in aqueous solutions (over 38 hours), justifying its potential for application in photocatalysis.

It is widely acknowledged that the stability of MOFs in water represents a significant limitation, especially in the case of HKUST-1. It has been shown that massive hydration or repeated water exposure severely compromise both the structure and the performances of the materials. Therefore, the integration of HKUST-1 into the HKUST-1/TiO₂ composite seems to prevent its deterioration. This enhanced stability could be attributed to the presence of the Cu nanoclusters formation within the composite. Indeed, recent studies have suggested that the partial reduction of Cu(II) nodes in HKUST-1, without altering its porosity and structure, enhances the stability of the MOF framework against moisture exposure²⁶.

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>
**Figure 2.** UV-visible DRS spectra (a), Tauc-Lorentz plot (b) and Fourier-transform infrared (FTIR) spectra (c) of HKUST-1/TiO₂ chitosan beads, HKUST-1 and TiO₂.

Using the Tauc-Lorentz plot, the band gap values of TiO₂ in bare TiO₂ and HKUST-1/TiO₂ CS beads were estimated to be 3.3 eV and 3.2 eV, respectively (Figure 2 (b)). This slight shift, also observed in our previous work, suggests that the electronic band structure of TiO₂ is influenced by the presence of HKUST-1 and copper nanoclusters. Another possible explanation could be the overlap of the TiO₂ band with the band of HKUST-1 at 375 nm, associated to the ligand-to-metal charge transfer.

The integrity of the composite was further examined by FTIR (Figure 2 (c)). The most intense bands recorded in the FTIR spectrum of the HKUST-1/TiO₂ CS beads are assigned to the chitosan beads matrix. The higher intensity of these bands is explained by the difference in the mass ratio between chitosan and the loaded photocatalyst. The presence of N-acetyl groups from chitosan was confirmed by the bands at 1645 cm⁻¹, 1589 cm⁻¹, and 1325 cm⁻¹, corresponding to C=O stretching of amide I, the N-H bending of the primary amine NH₂, and C-N stretching of amide III, respectively²⁸. Shifts toward higher wavenumbers were observed in the position of amide I and II regions, which are around ~1622 cm⁻¹ and 1540 cm⁻¹ prior to NaOH treatment (Figure S2), due to the change in the conformation of amide moiety and deprotonation of the amine group after neutralization with NaOH, respectively²⁹. The CH₂ bending and CH₃ symmetric deformation from the chitosan polymer were observed at 1423 cm⁻¹ and 1375 cm⁻¹, respectively. The band at 1262 cm⁻¹ is attributed to the bending vibration of hydroxyl groups present in chitosan. The band at 1153 cm⁻¹ is assigned to asymmetric stretching of the C-O-C bridge³⁰. It is important to note that the absence of shift on the C-O-C bridge band, which is characteristic to the CS saccharide structure, suggests that the main structure of CS is not altered and only a modification of conformation
appeared<sup>29</sup>. The bands at 1066 cm<sup>-1</sup> and 1028 cm<sup>-1</sup> are due to C-O stretching vibrations in the
chitosan polymer. The band at 896 cm<sup>-1</sup> is associated with the C-H out-of-plane bending in the
ring structures of monosaccharides. Additional bands observed in the FTIR spectrum of the
HKUST-1/TiO<sub>2</sub> CS beads further confirm the optimal integration of the HKUST-1/TiO<sub>2</sub> composite
into the chitosan matrix. Notably, the onset of the characteristic TiO<sub>2</sub> broad band, centered around
600 cm<sup>-1</sup> and related to Ti-O bond vibrations in the TiO<sub>2</sub> structure, is noticed from 775 cm<sup>-1</sup> to 700
cm<sup>-1</sup>. This clearly indicates the presence of TiO<sub>2</sub> within CS beads. Some characteristic bands from
HKUST-1 are also detected between 1700 cm<sup>-1</sup> and 700 cm<sup>-1</sup>, as highlighted in the zoomed-in
section of the Figure 2 (c). It is important to note that the clear observation of the HKUST-1 bands
in the composite is hindered by the relatively low loading of HKUST-1 within the composite and
the chitosan matrix.

Despite the challenges in clearly observing all the characteristic bands of HKUST-1 due to the
low loading, several notable bands are still identified within the composite. The bands located at
1646 cm⁻¹ and 1447 cm⁻¹ are attributed to the stretching vibration of COO-Cu₂³¹. The third
characteristic band associated with this stretching vibration at 1373 cm⁻¹ is not clearly observed
due to its overlap with the CH₃ symmetric deformation of chitosan. The presence of benzene-1,3,5-
tricarboxylic acid (BTC) organic linkers in the MOF structure is also confirmed by the band at
1553 cm⁻¹. This band is assigned to the C=C stretching vibration of the aromatic rings and the
asymmetric stretching of -COO⁻ from BTC. Additionally, the band at 940 cm⁻¹ is attributed to the
C-CO₂ stretching³¹⁻³³. However, the bands at 727 cm⁻¹, indicating the Cu-O stretching vibration,
and at 1112 cm⁻¹, attributed to the C-O-Cu bond stretching vibration, are not observable in the
HKUST-1/TiO₂ CS beads due to the overlap with the bands of TiO₂ and chitosan, respectively.
### Photocatalytic hydrogen generation.

In the present work, controlled experiments with TiO₂ or HKUST-1 were not repeated, but the activity of the bare materials had already been thoroughly investigated in our previous study⁹. Briefly, after 4 h bare TiO₂ and HKUST-1 reached a H₂ production rate of 0.15 mmol/g/h and 0.002 mmol/g/h, respectively. However, HKUST-1/TiO₂ (1:20) composite reached a production rate of 5.11 mmol/g/h. Therefore, the present work focused exclusively on the composite beads' study.

The photocatalytic hydrogen generation by the HKUST-1/TiO₂ CS beads was investigated as a function of several parameters, such as hydration levels and catalyst loading within the CS beads. For reference, hydrogen generation tests were also carried out with hydrated CS beads as photocatalyst. The results demonstrated that only a negligible amount of H₂ was generated in the presence of CS beads after 4h under light irradiation (H₂ < 200 ppm), confirming that the photocatalytic activity exhibited by HKUST-1/TiO₂ CS beads is due to the HKUST-1/TiO₂ composite.

*Charge carriers' dynamic and mechanisms investigation.* It should be highlighted that in accordance with our previous work on HKUST-1/TiO₂²,⁹, a reversible color change of HKUST-1/TiO₂ CS beads was observed during the photocatalytic tests: The white HKUST-1/TiO₂ CS beads, with light blue highlights, turned purple under light irradiation, and then turned back to their initial color after the light was turned off and the exposure to air for collection (Figure 3 (a)). The incorporation of HKUST-1/TiO₂ into CS beads did not affect the structural integrity of the composite (MOF/TiO₂) itself, nor modify the charge carriers' separation, and thus, the photocatalytic mechanism (Figure 3 (b)).
TRMC is based on the measurement of the microwave power change reflected by a solid sample, correlated to the difference of its conductivity induced by the laser-pulsed illumination, as a function of time³⁴. Therefore, the main information provided by the TRMC can be summarized as follows (Figure 3): (a) the maximum number of charge carriers generated by the laser, observed approximately 10 ns after the pulse and linked to the maximum signal intensity (Imax); (b) the short decay of charge carriers, arbitrarily fixed up to 40 ns after the beginning of the pulse, and represented by the I40ns/Imax ratio; (c) a long-range decay, evaluated beyond 100 ns, which follows first-order kinetics and is associated with slow processes (Table S1)³⁴. Due to the low intensity of the signals beyond 100 ns and its noise, the long-range decay could not be studied in this case.

As observed in Figure 3 (c-f) and Table S1, the relatively similar TRMC signals for both bare HKUST-1/TiO₂ composite and HKUST-1/TiO₂ CS beads confirm that the charge carriers' separation and the photocatalytic mechanism remain unchanged. This mechanism involves electrons transfer from TiO₂-P25 conduction band to Cu entities (Cu²⁺ sites and Cu nanoclusters), leading to the partial or total reduction of the Cu sites, which is confirmed by the observed color change, the improved hydrogen production (Figure 3 (b)), and the value of I40ns/Imax ²,⁹. On the other hand, the holes in the valence band of TiO₂ react with the methanol scavenger. They could also react with the electron-rich microenvironment induced by -NH₂ and -OH groups of CS or some acid group of acetic acid present in the gel. However, the TRMC signals of the bare HKUST-TiO₂ photocatalyst or imbedded in chitosan are very similar. This indicates a minor effect of CS and acetic acid on holes' scavenging. The recovering of the initial color reveals the reversible character of Cu sites in the composite, thereby enhancing its sustainability over time.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Pictures of photocatalytic reactor containing HKUST-1/TiO₂ hydrated CS beads before and after exposition under Mercury lamp (a), Proposed photocatalytic mechanism of H₂ production over HKUST-1/TiO₂ composite (b), Time-resolved microwave conductivity (TRMC) transient signal and normalized transient signal of HKUST-1/TiO₂ chitosan beads, HKUST-1/TiO₂, HKUST-1 and TiO₂ after an excitation at 380 nm (c,d), and Time-resolved microwave conductivity (TRMC) transient signal and normalized transient signal of HKUST-1/TiO₂ chitosan beads, HKUST-1/TiO₂ after an excitation at 550 nm (e,f).</figcaption></figure>
*Influence of the hydration state of the photocatalytic beads.* S. Mandal et al.<sup>18</sup> demonstrated that the use of water pre-adsorption step in the dried organic beads network has a crucial impact on hydrogen photocatalytic generation. They assumed that interaction among the polymer, confined water molecules, and photocatalyst plays a key role in: (1) improving the hydrophilicity and adsorption capacity of the inner photocatalyst; (2) enhancing charge transfer; and (3) reducing the recombination rate due to the stabilization of the photocatalyst by water. Z. Jiang et al.<sup>20</sup> also demonstrated that a hydrogel 3D network structure, in which large volumes of water can freely diffuse through the network until the surface of the immobilized catalyst, is profitable for H<sub>2</sub> generation and acts as a miniature hydrogen production reactor. Therefore, the objective was to evaluate the relevance of using hydrated beads compared to dried ones (Figure 4). Instead of using rehydrated beads after the drying process, which is time-consuming, as synthesized hydrated beads were selected as photocatalysts.

Due to the millimeter-scale size of HKUST-1/TiO<sub>2</sub> CS beads photocatalyst, photocatalytic H<sub>2</sub> generation occurs not only at the surface of the beads, but may also take place inside the beads, thanks to the easy penetration of light and water diffusion into the porous channels (Figure 5 (b))<sup>19,21</sup>. Therefore, the H<sub>2</sub> evolution performance may be influenced by both the surface and the internal microstructure of photocatalyst<sup>19</sup>, explaining then the difference in the hydrogen production shown Figure 4 (a). A total production of 1.84 ± 0.15 mmol/g was achieved after 4 hours under a mercury lamp with HKUST-1/TiO<sub>2</sub> hydrated CS beads against 0.28 ± 0.14 mmol/g with HKUST-1/TiO<sub>2</sub> dried CS beads.

In heterogeneous photocatalysis, it is well known that the diffusion and adsorption of water molecules on active sites are fundamental processes determining the H<sub>2</sub> production rate. Therefore, the poor rate recorded with dried beads could be linked to the limited contact between the
photocatalyst and water, caused by the slow water diffusion and adsorption processes, which leaves a significant portion of the inactive photocatalyst in the interior of the beads. Indeed, it is important to note that the HKUST-1/TiO₂ CS dried beads exhibit relatively slow swelling kinetics. After 4 hours of photocatalytic test, the HKUST-1/TiO₂ CS dried beads expand from 1.3 mm to 1.8 mm, representing only ~47 % of their original size in the hydrated form (Figure 4 (b)).

Conversely, in the case of HKUST-1/TiO₂ CS hydrated beads, the high number of active photocatalyst sites surrounded by water immediately after light activation, without any delay due to water up-take, could explain the 6.6-fold increase in the H₂ production rate measured (459 µmol/g/h). Furthermore, the water inside the porous structure of the beads (illustrated by the microscopy images) creates an ideal environment for boosting the adsorption of nanoconfined water molecules on the active sites of the photocatalyst, compared to the bulk water system³⁵. The higher porous volume of the hydrated beads, in contrast to the dried ones (47 % of expanding between dried and hydrated form), facilitates the exchange between the interior of the beads, the surrounding solution, and the gas. This enhancement further justifies the significant improvement in the H₂ production rate observed with the use of hydrated beads rather than dried ones.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Hydrogen production as a function of irradiation time with hydrated or dried HKUST-1/TiO₂ CS beads loaded with 0.36 mg of HKUST-1/TiO₂ composite per bead (14 beads) (a)</figcaption></figure>
(Mercury lamp 150 W, 20 mL MeOH/H₂O (1:3)), and photography of dried HKUST-1/TiO₂ CS beads after 4 hours of use for photocatalytic hydrogen evolution (b).

*Hydrogen trapping in the photocatalytic beads.* The higher porous volume of the hydrated system appears insufficient to permit the efficient diffusion of H₂ from the interior of the beads to the reaction medium, resulting in the H₂ storage within the composite beads. Notably, a portion of the H₂ produced within the beads contributes to their floatability over time, correlating with the increasing amount of H₂ generated and trapped (Figure 3 (a)). As a consequence, the reported H₂ production rate is underestimated, and could be further improved by tuning the porosity of CS beads, underscoring the importance of considering the internal microstructure of the system.

Moreover, the storage of H₂, attributed to the non-optimal diffusion through the 3D-network of chitosan, may be influenced by the high content of amino (-NH₂) and hydroxyl (-OH) groups present in this chitosan polysaccharide biopolymer, which can serve as adsorption sites. The functionalization of materials with these functional groups is a well-established strategy for increasing the H₂ storage capacity of in solid matrices³⁶.

To accurately assess the true H₂ production rate, after 4 hours of photocatalytic cycle, hydrated beads were fully smashed and then destroyed by strong magnetic stirring, directly in the sealed reactor, before recording H₂ production rate. After destruction of the beads, the production rate of H₂ measured in the gas phase corresponds to the total amount of H₂ produced including hydrogen initially trapped within the core of the beads (not detected before destruction of the beads). Following this process, the detected H₂ reached 4.47 mmol/g, corresponding to a production rate of 1.12 mmol/g/h. This rate is 2.5 times higher than the recorded one with the floatable non-smashed beads, indicating that nearly half of the H₂ produced was retained within the beads structure. It should be noted that this method does not detect the hydrogen trapped trough reactions
with the amino (-NH₂) and (-OH) groups present in the chitosan. To accurately quantify this
contribution to hydrogen storage and to better assess the hydrogen generation capacity,
temperature-dependent hydrogen detection would be required. Consequently, the current reported
H₂ production rate of 1.12 mmol/g/h can be considered as underestimated. In further experiments,
hydrated beads in their synthesized form were selected to conduct the study.

*Photocatalytic beads vs. photocatalyst in suspension.* To assess the impact of embedding HKUST-1/TiO₂ in chitosan beads on the H₂ photocatalytic production, 5.0 mg of bare HKUST-1/TiO₂ were tested in the same conditions as previously except that the photocatalyst was stirred during the experiment to keep it in suspension. The H₂ production rate noticeably increased from 1.12 mmol/g/h to 10.39 ± 1.99 mmol/g/h. This strong increase in activity for suspended photocatalyst was anticipated, as supported photocatalysts are known to be less efficient than those in suspension, likely due to a possible reduced effective surface area available for reaction. In addition, the internal hydrogen pressure within the beads may influence the reaction kinetics and, thus, the rate of hydrogen evolution. Adjusting the porosity of the beads could therefore represent an approach to narrowing the performance gap between suspended and encapsulated photocatalysts in the future. Finally, another contributing factor may be the use of stirring, highlighting the importance of developing a reactor design that enables efficient stirring of the beads while minimizing the risk of mechanical damage.

*Influence of photocatalyst HKUST-1/TiO₂ loading in chitosan beads.* The confinement effect and the porous volume induced by the microstructure of HKUST-1/TiO₂ CS beads demonstrated a crucial role in H₂ photocatalytic generation by affecting water diffusion, adsorption, and retention time within the CS beads. Therefore, the second focus of this study was to determine the optimal mass loading of photocatalysts per CS bead. Three different photocatalyst loading ratios were
examined: 0.20 mg/bead, 0.36 mg/bead, and 0.50 mg/bead. To maintain the total photocatalyst amount of 5.0 mg across all experiments, varying numbers of beads were used in the reactor: 25, 14, and 10.

The H₂ production rate noticeably increased as the photocatalyst loading per bead rose from 0.20 mg to 0.36 mg, with the rate improving from 339 µmol/g/h to 459 µmol/g/h (Figure 5 (a)). This enhancement can likely be attributed to the shorter diffusion path of water from the solution to the photocatalyst surface within the CS beads as the photocatalyst amount per bead increased (Figure 5 (b)), facilitating faster kinetics, and thus higher H₂ generation. Additionally, increasing the photocatalyst loading per bead may also enhance the water confinement effect, which is known to positively impact the H₂ generation. Literature reports that water molecules stabilized in the nanoscale confinement spaces behave differently from those on the bulk photocatalyst¹⁸,²⁰. Water molecules in the interior of the nanometer scale cavities exhibit modified electronic and chemical adsorption properties, which promote the interactions between water molecules and inner active sites³⁵.

Conversely, at higher photocatalyst loading per bead (0.50 mg/bead), the photocatalytic H₂ production rate drastically decreases at 171 µmol/g/h (Figure 5 (a)). This decrease can be due to multiple factors induced by the increase of the photocatalyst loading. Firstly, the increment of the opacity of the CS beads, due to higher photocatalyst concentration within the chitosan 3D network, limits the penetration of the photon flux, and, consequently, the activation of photocatalyst by light (Figure 5 (b))³⁷. The quantification of internal luminous flux would be highly valuable to justify and quantitatively assess the influence of this factor. Secondly, the aggregation of the photocatalyst nanostructures at higher loadings leads to a loss in reachable surface area and active sites, reducing the available activated surface for the photocatalytic reaction to occur³⁰. This aggregation may also
decrease the porosity of the CS beads or even block some pores, hindering the diffusion of water, methanol, and H₂ through the bead network, thereby decreasing the H₂ production rate. Additionally, the reduced contact between HKUST-1/TiO₂ and chitosan, caused by particle-particle interactions, may also contribute to the sharp decline in efficiency recorded with the bead loading set at 0.50 mg of HKUST-1/TiO₂ per bead. Chitosan, as a natural nitrogenous polysaccharide polymer, contains –NH₂ and –OH groups with lone-pair electrons that create an electron-rich environment. This electron-rich environment promotes the attraction of holes from the valence bands of TiO₂ and/or HKUST-1, which effectively enhances the charge carriers' separation and life time²¹. The holes are then consumed by the methanol used as scavenger.

These results further demonstrate the crucial role of both external and internal structures of the photocatalyst in this miniature system. The optimum concentration of HKUST-1/TiO₂ composite per CS bead was 0.36 mg/bead with a production rate of 459 µmol/g/h. Hence, this mass loading was used in subsequent experiments. It is important to consider that the H₂ production rate of 459 µmol/g/h is underestimated due to the trapping of some of the H₂ molecules produced within the CS beads, leading to their floatability over time.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Hydrogen production as a function of the loading of HKUST/TiO₂ in the hydrated CS beads (a) (Mercury lamp 150 W, 20 mL MeOH/H₂O (1:3), total mass of the HKUST-1/TiO₂ used during photocatalytic tests = 5,0 mg equivalent to 25, 14 and 10 beads for 0.20 mg/bead, 0.36 mg/bead, and 0.50 mg/bead, respectively), Schematic illustration of hydrogen production mechanism on and in the hydrated photocatalytic beads as function in the photocatalyst mass per bead loading (b), and hydrogen production as a function of irradiation time with hydrated HKUST-1/TiO₂ CS beads used directly after the synthesis or after two months of storage, in laboratory, in an Eppendorf tube directly closed, after the synthesis, without any precaution (c) (0.36 mg/bead, total mass of the HKUST-1/TiO₂ used during photocatalytic tests = 5,0 mg equivalent to 14 beads).</figcaption></figure>

*Stability of the beads over time.* Photocatalyst stability is a critical issue for efficient H₂ production over time. Therefore, a stability study of the photocatalytic beads was performed over the span of two months of storage without any precautions in an Eppendorf tube. After 60 days of
storage, a hydrogen production of 1.8 mmol/g was achieved in the presence of 14 photocatalytic beads with a mass loading of HKUST-1/TiO₂ equal at 0.36 mg/bead (Figure 5 (c)). This value is comparable to the 1.9 ± 0.2 mmol/g reached under the same conditions, but with fresh beads directly after their synthesis. This result confirms the good stability of the HKUST-1/TiO₂ composite, closely linked to the partial reduction of HKUST-1 in this composite. Furthermore, this finding suggests that HKUST-1/TiO₂ CS beads are not only suitable for hydrogen production in water with easy recovery of the photocatalyst and easy scalability, but also for photocatalyst conservation and transportation. Indeed, no significant modification of the hydrated beads, in term of diameter, was observed (Figure 1 (h)).

*Influence of the number of photocatalytic beads in the reactor.* In a simple heterogeneous photocatalytic system, it is well-known that the evolution of the H₂ production rate follows two trends as a function of the photocatalyst concentration typically ³⁸,³⁹:

(1) a first phase, from no photocatalyst loading to an optimum limit dose, characterized by a positive slope where the increasing production rate is attributed to the presence of a larger number of active sites for proceeding H₂ generation, thereby boosting the reaction;

(2) a second phase, beyond the limit dose, is characterized by a negative slope where the decreasing of the production rate is assigned to an increasing aggregation effect and an increasing opacity in the reactor. These effects hinder the light penetration and reduce the activated surface for proceeding H₂ generation.

In this research work, the aim was therefore projected on the optimization of the H₂ generation by determining the optimal number of photocatalytic beads that would provide a high active surface area while avoiding side effects from an excessive number of beads. H₂ generation tests were carried out with varying the number of beads in the 37.5 mL reactor (diameter ~ 2.30 cm and
Height ~ 13.9 cm), ranging from 4 beads (1.4 mg of HKUST-1/TiO₂) to 56 beads (20 mg of HKUST-1/TiO₂).

Contrary to expectations, the evolution of the H₂ production rate as a function of the beads number, shown in Figure 6 (a), did not follow the simple two-phases model described earlier. The highest production rate, 859 µmol/g/h, was achieved with only 4 HKUST-1/TiO₂ CS beads. This was closely followed by a rate of 794 µmol/g/h reached in the presence of 7 beads. A sharp decline to 459 µmol/g/h occurred with 14 beads, followed by a subsequent increase to 546 µmol/g/h with 28 beads. However, further increases in the number of beads resulted in a significant drop to 319 µmol/g/h with 42 beads and 214 µmol/g/h with 56 beads. This fluctuating pattern seems to be related to the formation of different bead layers in the photoreactor as the number of beads increased. Using 4 to 14 beads resulted in a single layer, while 28 beads formed two layers, and 56 beads resulted in three layers (Figure 6 (b)). Thus, there seems to be an optimal number of beads for each layer configuration (in our reactor described previously).

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Hydrogen production rate as a function of the number of HKUST/TiO₂ hydrated CS beads (0.36 mg/beads) placed in the reactor (a) (Mercury lamp 150 W, 20 mL MeOH/H₂O (1:3)), and schematic illustration of the layers of the photocatalytic beads as function in the photocatalyst beads number (b).</figcaption></figure>
It is also important to note that the production rate of H₂ decreases as the number of beads layers increases. Two hypotheses could explain this observation: (1) A stronger shading effect caused by the additional bead layers, reducing the amount of surface exposed to light, thereby activated; (2) The potential capture of H₂ produced by the lower layer of beads by the upper layers, leading to an underestimation of the current H₂ production.

These results emphasize the importance of not only considering the structure and properties of the photocatalyst, but also the design of the reactor itself<sup>40,41</sup>. Reactor material and geometry can indeed significantly impact the number of photons reaching the photocatalyst due to light absorbance, transmission, scattering processes, or reflection<sup>42</sup>. The reactor design can also impact the diffusion and mass transfer pathways of solution and hydrogen produced. Moreover, several other parameters like the volume of the reactor, its shape, the volume of the solution, the head space volume, or even the stirring must be also considered and optimized. Nonetheless, these results highlight the feasibility of using HKUST-1/TiO<sub>2</sub> CS beads, and underscore the need for reactor designs tailored to these newly supported photocatalysts.

*Reusability and stability.* The reusability of a photocatalyst is essential for sustainable H<sub>2</sub> production. To evaluate, the relevance of HKUST-1/TiO<sub>2</sub> CS beads for H<sub>2</sub> photocatalytic generation, their reusability was tested over several cycles, with only a simple rinsing by deionized water (DI) between each cycle. As shown in Figure 7, which represents the 20 hours of photocatalytic hydrogen generation with the same seven HKUST-1/TiO<sub>2</sub> CS beads, the H<sub>2</sub> production rate remains relatively stable across five cycles, proving the suitability of the HKUST-1/TiO<sub>2</sub> CS beads.

In a similar study, Y. Zhang and al.<sup>21</sup> confirmed the long-term photostability of CdS chitosan beads for H<sub>2</sub> evolution over 55 hours under light irradiation in the presence of a sacrificial agent.
The researchers attributed this stability to the molecular structure of chitosan, which contains - NH₂ and -OH groups. These functional groups promote the separation of charge carriers and favor the capture and consumption of photogenerated holes by the sacrificial agent, which plays a dual role as an electron donor and protective agent preventing chitosan oxidation. However, it is important to remind that during the photocatalytic tests with hydrated beads, a part of the H₂ produced is trapped within the HKUST-1/TiO₂ CS beads, causing the beads floatability. We believe that this floatability increases the liquid-gas interfacial area, leading to efficient hydrogen desorption from the catalyst surface, and thus, explaining the slight increase in the second cycle³⁸. Over prolonged use of the HKUST-1/TiO₂ CS beads for hydrogen generation, some cracks were observed on the beads, likely due to the internal pressure resulting from the accumulation of trapped H₂ (Figure 7(b)). Consequently, the H₂ production rate evolves to the initial value similar to the one of the first cycle.

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Hydrogen production rate as a function of the cycle of use of 7 HKUST-1/TiO₂ hydrated CS beads (0.36 mg/beads) (Mercury lamp 150 W, 20 mL MeOH/H₂O (1:3)) (a) and photography of a cracked bead used for several photocatalytic tests (b).</figcaption></figure>

Despite the comparison of photocatalytic activity in literature reveals that different conditions have noticeable effect on photocatalytic activity, our results were compared with some literature data
(Table S2). Beside the good photocatalytic activity of the imbedded composite and the good stability of the HKUST-1/TiO₂ CS beads, the comparison reveals that this hydrogen production rate could be optimized and a prospect to achieve this, is the management of the porosity of the beads and the temperature of the solution, to manage the in-situ storage of hydrogen in the CS beads during the photocatalytic process.

CONCLUSION

Millimeter-sized HKUST-1/TiO₂ chitosan beads were successfully synthesized in three steps and characterized to confirm their uniform spherical appearance and stability. The hydrated beads had an average diameter of 3.8 ± 0.1 mm, while the dried beads measured 1.3 ± 0.1 mm. The size of the photocatalytic beads remained constant, regardless of the amount of composite (from 10 mg to 30 mg) used in the 2.5 mL of 2 % chitosan gel. SEM analysis showed the presence of photocatalysts both inside the beads and on their surface. The HKUST-1/TiO₂ composite's stability was confirmed by UV-Vis and FTIR spectroscopy, demonstrating resilience under both acidic and basic conditions and prolonged immersion in water. TRMC studies show that charge carriers' dynamics is similar for HKUST-1/TiO₂ free nanopowder and HKUST/TiO₂ CS beads.

TiO₂/HKUST-1 chitosan beads present a good photocatalytic activity for hydrogen generation and are stable with cycling. Various parameters, including bead hydration state, photocatalyst loading, bead quantity, and recyclability, were investigated. The optimal concentration for bead preparation was determined to be 0.36 mg of photocatalyst per bead, balancing photocatalyst loading and water nanoconfinement while preventing aggregation and pore blocking. Hydrated beads achieved a significantly much higher H₂ production rate (459 µmol/g/h) compared to dried
beads (45 µmol/g/h) due to larger porous volume without water up-take delay. The study also highlighted the importance of reactor design, with the optimal bead quantity correlating with bed layer formation in the reactor caused by the additional light screen effect between the beads. The highest H₂ production rate in this study was 854 µmol/g/h with 1.4 mg of photocatalyst (4 beads), comparable to that of the HKUST-1/TiO₂ 20 mg free powder (906 µmol/g/h). Recyclability tests demonstrated good sustainability of the photocatalytic system, though prolonged use led to surface cracking, indicating also a need for further optimization of bead porosity and reactor design. Despite these challenges, the findings highlight the potential of organic beads to serve as eco-friendly and reusable hosts for the next-generation of photocatalyst systems for H₂ generation by photocatalysis. Moreover, it is also important to note that the H₂ production rate reported in this study may be underestimated due to hydrogen trapping within the beads, which caused their progressive floatability in the reactor. The results also emphasize the importance of controlling the porosity of the beads to either avoid or take advantage of this trapping effect.

Finally, active and durable composite photocatalysts without noble metals were embedded in chitosan for photocatalytic applications. This finding demonstrates that chitosan beads can serve as effective stabilizers for photocatalysts, specifically for hydrogen generation. This strategy of encapsulation could be adapted to other photocatalysts and provides a robust platform for the synthesis of stable eco-friendly photocatalyst materials easy to collect.

## ASSOCIATED CONTENT

### Supporting Information.

Photography of the photocatalytic experimental set-up (a) and emission spectrum of the Mercury lamp (b); Fourier-transform infrared (FTIR) spectra of commercial chitosan powder and a dried film of 2% CS gel (mass ratio) in a 1% acetic acid solution (volume ratio); TRMC parameters for
bare HKUST-1 and TiO₂, HKUST-1/TiO₂ (1:20) and HKUST-1/TiO₂ CS beads samples extracted
from the Figure 3(c,e); Comparison of hydrogen generation under UV-visible light of the different
photocatalysts.

AUTHOR INFORMATION

**Corresponding Authors**

hynd.remita@universite-paris-saclay.fr

johnny.deschamps@ensta-paris.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.

Funding Sources

This work was supported by the ANR ASTRID-Energy PhotoStor2 project (ANR-22-ASEN-
0005) supported by Direction Générale de l'Armement (DGA/AID).

ACKNOWLEDGMENT

The authors are very grateful to Daniel Bahena-Uribe for his valuable contribution to the TEM
image acquisition. The authors acknowledge Viet Dung Duong and Faiz Md Alam for their help
in FTIR characterization measurements. The authors are grateful to Dorota Rutkowska-Zbik and
Eunice Lopez for helpful discussions.
ABBREVIATIONS

CS Chitosan, DRS Diffuse Reflectance Spectroscopy, FTIR Fourier-Transform Infrared Spectroscopy, MOFs Metal Organic Frameworks, TiO₂ Titanium Dioxide, SEM Scanning Electron Microscopy, TRMC Time Resolved Microwave Conductivity, EDX Energy-dispersive X-ray Spectroscopy, XPS X-Ray Photoelectron Spectroscopy (XPS), EPR Electron Paramagnetic Resonance, DFT Density Functional Theory.

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HKUST-1/TiO₂ chitosan beads are developed for green hydrogen generation, which is studied as function of operational parameters for better optimization.