# Platinum carbonyl Chini clusters as catalysts for photocatalytic H2 generation

Aleksander Senderowski, Ana Andrea Méndez-Medrano, Isabelle Lampre, Hynd Remita, Dorota Rutkowska-Zbik

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Aleksander Senderowski, Ana Andrea Méndez-Medrano, Isabelle Lampre, Hynd Remita, Dorota Rutkowska-Zbik. Platinum carbonyl Chini clusters as catalysts for photocatalytic H2 generation. Journal of Physical Chemistry C, 2025, 129 (17), pp.8011-8020. 10.1021/acs.jpcc.5c00212. hal-05362702

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Submitted on 13 Nov 2025

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Platinum carbonyl Chini clusters as catalysts for photocatalytic H₂ generation

Aleksander Senderowski¹, Ana Andrea Méndez-Medrano², Isabelle Lampre², Hynd Remita*²,
Dorota Rutkowska-Zbik*³

¹Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw, ul. Adolfa Pawińskiego 5A, 02-106 Warsaw, Poland

²Institut de Chimie Physique, UMR 8000 CNRS, Université Paris-Saclay, 91405 Orsay, France

³Jerzy Haber Institute of Catalysis and Surface Chemistry PAS, ul. Niezapominajek 8, 30-239 Kraków, Poland

e-mail: hynd.remita@cnrs.fr; ORCID: 0000-0003-3698-9327

e-mail: dorota.rutkowska-zbik@ikifp.edu.pl; ORCID: 0000-0001-9323-1710

**Abstract**

Platinum Chini clusters of a general formula [Pt₃(CO)₆]n²⁻ are formed by n stacked Pt₃ units. They have fascinating electronic and optical properties that can be tuned with n. In the current manuscript, the electronic, photochemical, and charge transport properties of the platinum Chini clusters are studied with the Density Functional theory (PBE and CAM-B3LYP/6-31G(d, p) + LANL2DZ) as a function of their nuclearity n. Our theoretical predictions are followed by experimental proof of concept, in which synthesized Chini clusters are deposited as cocatalysts on TiO₂ for photocatalytic hydrogen generation. We demonstrate that the smaller clusters (n = 4) are more effective than the larger ones (n = 7-8) and that the composites having lower Pt content perform better.

**Keywords:**

hydrogen generation, photocatalysts, Chini clusters, density functional theory (DFT), platinum nanostructures
# 1. Introduction

The synthesis of platinum carbonyl clusters of a $[Pt_3(CO)_6]_n^{2-}$ formula, known as Chini clusters, was proposed in the 1960s [1]. They can be synthesized by chemical reduction by CO in alkaline alcoholic solution. They can also be synthesized by radiolysis in alcohol solution under CO atmosphere, the nucleation n can be tuned by the irradiation dose [2, 3]. They consist of multiple stacking units of $Pt_3$ triangles stabilized by CO ligands bound in two different ways: terminal (connected to one Pt atom) and bridging (connected to two Pt atoms) – see Figure 1. Their physicochemical properties and their optical properties depend on the nuclearity number n. Their redox properties have been studied by pulse radiolysis [3]. They serve as precursors for Pt nanoparticles and nanowires, substrates to obtain metal-organic frameworks [4], or even alloy nanoclusters if mixed with other metals [2, 5-9]. The Chini clusters can be deposited on different supports to be used as composite materials in several electrocatalytic application [10, 11] and photo- and thermo-catalytic processes [12-16].

Platinum is known as very active cocatalyst for hydrogen generation ascribable to its higher work function, extremely low required overpotential, and optimal hydrogen adsorption-free energy [17-19]. Most of the studies in this topic report on the use of Pt-based nanoparticles for photocatalytic hydrogen formation, however articles on clusters of controlled size as cocatalysts are scarce. Here we aim to check the applicability of carbonyl Pt clusters in photocatalytic hydrogen generation by determining their electronic, photochemical, and charge transport properties as a function of their nuclearity. By assuming that the reaction involves the formation of $H_2$ via the recombination of two hydrogen atoms, we focus on the binding of an atomic hydrogen and a $H_2$ molecule by the Chini clusters of n = 1-8. Our theoretical predictions are followed by experimental proof of concept, in which synthesized Chini clusters are deposited on $TiO_2$ and such a composite is tested in photocatalytic hydrogen generation.
<figure><img src="image_2.png" /><figcaption>Figure 1. Geometries of the Chini clusters for n=1 and n=2 with specific bonds marked. The illustration highlights the intra-triangle and inter-triangle Pt-Pt distances, and two types of CO ligand binding (terminal and bridging).</figcaption></figure>

## 2. Methodology

### 2.1 Theoretical methods

To determine the geometry and electronic properties of the Chini clusters quantum-chemical calculations within the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were done.

Geometries were optimized using PBE functional [20-22], which proved the right choice for this class of compounds [23], while the investigation of excited states was done with CAM-B3LYP hybrid functional [24]. The latter better describes long-range charge transfer states and overcomes issues with convergence and computation time. The 6-31G(d, p) basis sets for C, O and H [25, 26] as well as LANL2DZ pseudopotentials for Pt were implemented [27-29]. The calculations have been done in vacuum using the Gaussian 16 software [30].

Hole ($\lambda_h$) and electron ($\lambda_e$) reorganization energies were calculated using the following formulae:

$$ \lambda_h = (E_+^0 - E_0) + (E_0^+ - E_+) \quad (1) $$

$$ \lambda_e = (E_-^0 - E_0) + (E_0^- - E_-) \quad (2) $$

where $E_0$, $E_-$ and $E_+$ refer to energies of relaxed molecules: neutral ($E_0$) or with an additional electron ($E_-$) or with an additional hole ($E_+$); $E_0^-$ and $E_0^+$ refer to energies of molecules with an additional electron or hole in a geometry of a neutral state, respectively; $E_+^0$ and $E_-^0$ refer
to energies of neutral molecules in a geometry of a state with an additional electron or hole. A visual representation of these energy levels is shown in Figure S1 in the supplementary information file (SI).

The binding energies (BE), enthalpies (ΔH) and Gibbs free energies (ΔG) for the adsorption of the hydrogen species (H, H₂) on [Pt₃(CO)₆]ₙ²⁻ were calculated using the following reactions (for the H atom and the H₂ molecule, respectively):

$$[Pt_3(CO)_6]_n^{2-} + \frac{1}{2}H_2 \rightarrow [[Pt_3(CO)_6]_n - H]^{2-}$$

$$[Pt_3(CO)_6]_n^{2-} + H_2 \rightarrow [[Pt_3(CO)_6]_n - H_2]^{2-}$$

as a difference between the enthalpies/Gibbs free energies of the product and substrates. The calculations were done using the PBE functional with the Grimme's dispersion correction with Becke-Johnson damping [31], and the 6-31G(d, p) basis sets for C, O, and H and the LANL2DZ pseudopotentials for Pt, as described above. The enthalpy/Gibbs free energy corrections to the electron energies were computed at 298.15 K, as described in [32].

## 2.2 Synthesis of Pt carbonyl clusters and their deposition on TiO₂

All the chemicals were used as received: platinum(II) bis(acetylacetonate) (Pt(acac)₂, Aldrich, 99% purity), TiO₂ (P25, Evonik), ethanol, HCl, NaOH. Argon (Ar) and carbon monoxide (CO) gases were obtained from Air Liquide. Deionised water was generated using a MilliQ system (18.2 MΩ cm).

A platinum precursor Pt(acac)₂ was dissolved in ethanol to give a solution of 10⁻³ M. The pH was measured and controlled by adding droplets of aqueous solutions of NaOH and HCl since the pH control is essential to achieve the desired target molecules [3]. It is known that a strong basic environment (pH ~ 13) results in smaller clusters (n = 4, whereas a neutral pH (pH = 7) favours the formation of larger clusters (n = 7 or 8). The obtained solutions were deaerated with argon gas for 20 min and subsequently saturated with CO gas (1 atm). Next, the solutions were irradiated with a ⁶⁰Co panoramic gamma source for 20-40 min (the total dose 835 Gy, low-dose irradiation). UV-Vis absorption spectra were recorded using a single-beam Hewlett-Packard 8453 spectrophotometer and 1 cm optical path cuvettes.

The commercial TiO₂-P25 was used as a support to obtain Chini-TiO₂ composites having 0.1 and 0.02 wt% of clusters via an impregnation method. Adjusted volumes of previously
irradiated cluster solution (1.0 and 0.2 mL, 10⁻³ M Pt concentration) were added to the weighed TiO₂-P25 (~ 195 mg), and diluted with ethanol to give 8 mL suspensions. The ethanol suspensions of the Chini clusters and TiO₂ were covered with aluminium foil and stirred continuously overnight. Then, the obtained photocatalysts were centrifuged, washed with ethanol, and dried in an oven (60°C, 24 hours) to obtain dry powders.

## 2.3 Photocatalytic activity for hydrogen generation

The resulting powders were tested for the photocatalytic H₂ generation. The photocatalyst (20 mg) was put into a photoreactor. 15 mL of water and 5 mL of methanol (which plays a role of a hole scavenger) were added to form a heterogeneous mixture. The prepared suspensions were degassed in the dark for 20 min under an Ar atmosphere with a continuous stirring. Subsequently, they were irradiated using 300 W xenon lamp, generating a collimated beam with a continuous spectrum from UV to near-infrared (250-2000 nm). The amount of H₂ produced in time from the water-methanol solution was measured using a gas chromatography (GC) with an INFICON Micro GC Fusion Gas Analyzer.

# 3. Results and Discussion

## 3.1 Geometry

The geometries of the Chini clusters for n=1-8 were determined – see Table 1 and Figure 1 for the explanation of the notion used within, explained on the examples of [Pt₃(CO)₆]²⁻ and [Pt₃(CO)₆]₂²⁻. Figure S2 in SI illustrates the remaining Chini clusters for n=3-8. While different conformers (e.g. “wires”, “zigzags”, and others) are possible [7], “wires” are found to be energetically favourable, in agreement with the previous studies [5, 9]. As stated in the introduction, in the Chini clusters two different types of the CO ligands are present: terminal and bridging. The length of C-O bonds varies between 1.163 and 1.204 Å, depending on the cluster size and the type of the carbonyl ligand. For the terminal ones, both C-O and Pt-C bonds are slightly shorter (by around 0.02 Å) compared to the bridging ones. The Pt-C bond length ranges from 1.864 to 2.109 Å. The Pt-Pt bond lengths and angles slightly differ, primarily between internal and external Pt₃-units. Subsequent Pt₃-units are rotated by approximately 16-33°. The intra-triangle Pt-Pt distance ranges from 2.752 to 2.770 Å, while the inter-triangle (Δ-Δ) Pt-Pt distance varies between 3.136 and 3.200 Å. The obtained geometry parameters are in good agreement with the X-ray structures and previous theoretical calculations [33].
We noticed that with a growing cluster size, the Pt-C bond lengths tend to increase while the C-O bond lengths tend to decrease for both types of ligands (terminal and bridging). The intra-triangle Pt-Pt distance increases, whereas the inter-triangle Pt-Pt distance does not exhibit a consistent trend.

<table><caption>Table 1. Interatomic distances (in Å) and dihedral angles (°) for the Chini clusters (n=1-8) computed at PBE+D3/6-31G(d, p) for C, O, H and LANL2DZ for Pt.</caption><thead><tr><th rowspan="3">n</th><th colspan="6">Bond length [Å]</th><th rowspan="3">Dihedral angle [°]</th></tr><tr><th colspan="2">Pt-Pt</th><th colspan="2">Pt-C</th><th colspan="2">C-O</th></tr><tr><th>intra Δ</th><th>Δ - Δ</th><th>terminal</th><th>bridging</th><th>terminal</th><th>bridging</th><th>Pt-Pt-Pt-Pt</th></tr></thead><tbody><tr><td>1</td><td>2.752</td><td>-</td><td>1.864</td><td>2.079</td><td>1.187</td><td>1.204</td><td>-</td></tr><tr><td>2</td><td>2.761 – 2.763</td><td>3.137</td><td>1.874</td><td>2.080 – 2.097</td><td>1.175</td><td>1.193</td><td>32.8</td></tr><tr><td>3</td><td>2.748 – 2.767</td><td>3.136 – 3.140</td><td>1.878 – 1.885</td><td>2.085 – 2.103</td><td>1.169 – 1.171</td><td>1.186 – 1.189</td><td>20.2 – 20.2</td></tr><tr><td>4</td><td>2.752 – 2.770</td><td>3.142 – 3.155</td><td>1.881 – 1.889</td><td>2.088 – 2.105</td><td>1.167 – 1.169</td><td>1.184 – 1.187</td><td>16.7 – 22.8</td></tr><tr><td>5</td><td>2.754 – 2.770</td><td>3.153 – 3.172</td><td>1.883 – 1.893</td><td>2.090 – 2.108</td><td>1.165 – 1.167</td><td>1.183 – 1.186</td><td>16.7 – 21.2</td></tr><tr><td>6</td><td>2.755 – 2.769</td><td>3.163 – 3.181</td><td>1.884 – 1.894</td><td>2.091 – 2.108</td><td>1.164 – 1.167</td><td>1.182 – 1.185</td><td>16.5 – 21.4</td></tr><tr><td>7</td><td>2.755 – 2.769</td><td>3.172 – 3.192</td><td>1.885 – 1.896</td><td>2.092 – 2.108</td><td>1.164 – 1.166</td><td>1.181 – 1.185</td><td>16.8 – 21.7</td></tr><tr><td>8</td><td>2.756 – 2.768</td><td>3.175 – 3.200</td><td>1.886 – 1.897</td><td>2.093 – 2.109</td><td>1.163 – 1.165</td><td>1.181 – 1.184</td><td>17.2 – 22.0</td></tr><tr><td>n↑</td><td>?</td><td>↑</td><td>↑</td><td>↑</td><td>↓</td><td>↓</td><td>?</td></tr></tbody></table>

Further studies, including the TD-DFT calculations, were performed on the described set of geometries.

## 3.2 Frontier orbitals

Next, the HOMO and LUMO energy levels for the Chini clusters (n=1-8) were determined - see Figure 2. For the clusters having n=1-3, the HOMO energy level is above 0 eV, indicating that
the small Chini clusters may be more reactive than the larger ones and/or unstable. This reactivity can be attributed to the higher energy of the HOMO, making electron donation towards a reactant more likely. In contrast, as the cluster size increases, for n=4-8, the HOMO energy levels drop below 0 eV, indicating increased stability and/or decreased reactivity.

The HOMO-LUMO energy gap decreases for n=3-8, which is consistent with a redshift observed for calculated UV-Vis absorption spectra (see section 3.3). The decreasing energy gap in larger clusters also points towards enhanced electronic delocalization, which could affect the optical and electronic properties of these materials.

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

Figure 2. HOMO and LUMO energy levels for Chini clusters (n=1-8). HOMO-LUMO energy gaps, indicated by the dashed lines, with specific annotated values show a decreasing trend with an increasing cluster size n.

The observed trends in the HOMO and LUMO energy levels and energy gaps highlight that the electronic properties of the Chini clusters can be tuned by varying the cluster size. This feature is crucial for synthesizing clusters for specific applications, ranging from catalysis to electronic and optical devices.

## 3.3 Theoretical UV-Vis spectra
The colours of the solutions containing Pt Chini clusters depend on their size (or nucleation number). The theoretical UV-Vis absorption spectra for the Chini clusters (n=1-8) were calculated - see Figure 3 for the simulated spectra.

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

Figure 3. Simulated UV-Vis absorption spectra of the Chini clusters for n=1–8 calculated with CAM-B3LYP hybrid functional in a vacuum. FWHM is set as 0.25 eV to imitate experimental broadness. Absorbance was normalized in a range from 0 to 1. Numbers above spectral lines indicate the nuclearity n.

Two distinct absorption peaks in the visible range of the spectrum are found. To assign the character of the electronic transitions responsible for such a spectrum, the character of the calculated transitions was analysed for the Chini cluster with n=3 – Figures S3 and S4 in SI. The broad, red-shifted band (lower energy peak) corresponds to inter-triangle transitions. They occur between different Pt₃ units, involving interactions across the cluster. The narrow, higher energy band is associated with intra-triangle transitions, where electronic excitations involve CO orbitals within the same Pt₃ unit. These transitions are more localized, higher-energy, and affect the transverse direction of the cluster.

Knowing the characteristics of the transitions and orbitals involved within, one can explain a significant redshift of the lower-energy absorption band (from 420 nm to 950 nm) observed for n=3-8. As the size of Chini clusters increases the excitation process primarily affects the electronic structure in the longitudinal (inter-triangle) direction, while the transverse (intra-triangle) one remains rather unaffected - the second absorption band shifts slightly from 350
nm to 370 nm. This anisotropic behaviour in electronic excitation can be attributed to the
spatial arrangement and interactions of Pt₃-units within each cluster.

The observed redshift is also consistent with the decrease in the HOMO-LUMO energy gap as the cluster size increases (see section 3.2), indicating that smaller energy is required for electronic transitions. Furthermore, the changes observed in the UV-Vis spectra are consistent with the surface plasmon resonance (SPR) effect, which is characteristic of metal nanoparticles. The SPR effect results from the collective oscillation of electrons in response to light, which is stronger for larger clusters due to increased electron delocalization and interactions between Pt₃ units. This effect contributes to the significant redshift and broadening of the absorption bands as the cluster size increases.

Our computational results are consistent with the experimental UV-Vis spectra and other
theoretical studies found in literature [3, 34], regarding the presence of the two distinct
absorption band and the size-dependence of the energy of the low-energy transition.

**3.4 Charge transport properties**

To describe charge transport properties of the Pt Chini clusters, which are an important
parameter of materials used in photocatalytic processes, hole and electron reorganization
energies were determined using equations (1) and (2) – see Figure 4. Charge transfer reactions
are associated with subtle or significant geometry modifications such as changes in bond
lengths and bond angles. Reorganization energies help in understanding these processes as
they represent energy losses associated with a structural change when an electron is donated
or removed. They consist of internal reorganization energy, which involves changes within a
molecule, and external reorganization energy, which involves changes in surrounding solvent
molecules. Here, the Chini clusters were modelled in a vacuum, so the external reorganization
energies were neglected.
<figure><img src="image_5.png" /><figcaption>Figure 4. Calculated hole and electron reorganization energies for the Chini clusters of different nuclearities. Dashed lines are employed for clarity and readability.</figcaption></figure>

Higher reorganization energies typically result in slower charge transfer rates because more energy is required to overcome the energetic barriers associated with reorganizing the surrounding environment. Lower reorganization energies facilitate faster charge transfer processes. Based on our results, we can assume that the Chini clusters preferably act as hole transport materials ($\lambda_h$ << $\lambda_e$). Notably, as the cluster size increases, electron transfer becomes more prevalent. Decreasing values of $\lambda_e$ for larger clusters suggest that delocalization processes stabilize structures, leading to lower energy costs of electron transfer. For $\lambda_h$ we observe fewer fluctuations in values compared to $\lambda_e$ – all values are within the range of 0.1-0.2 eV. Increasing the size of clusters seems not to have a significant impact on the hole transfer. $\lambda_h$ decreases for n = 2 – 7, but then increases for n = 8, showing an irregular behaviour.

The relatively small reorganisation energy accompanying the changes in the number of electrons, especially for the structures with n>4, also confirms that platinum Chini clusters can be employed as nanocapacitors or electron sinks in molecular and nano-electronic devices [5, 35].

### 3.5 Theoretical prediction of the Chini clusters activity in the H₂ generation
The adsorption of H₂ molecule and H atom was investigated to predict the activity of the Chini clusters in the H₂ generation. The results are shown in Table 2 and Figure S5 in SI.

<table><caption>Table 2. Pt-H bond lengths in [Å] and binding energies (BE), ethalpies (ΔH), and Gibbs free energies (ΔG) in [eV] of H atom and H₂ molecule adsorption on the Chini clusters with varying sizes (n=1-8). Energies are calculated at PBE+D3/6-31G(d, p) for C, O, H and LANL2DZ for Pt.</caption><thead><tr><th rowspan="2">n</th><th colspan="4">adsorption of H</th><th colspan="4">adsorption of H₂</th></tr><tr><th>Pt-H [Å]</th><th>BE [eV]</th><th>ΔH [eV]</th><th>ΔG [eV]</th><th>Pt-H [Å]</th><th>BE [eV]</th><th>ΔH [eV]</th><th>ΔG [eV]</th></tr></thead><tbody><tr><td>1</td><td>1.638</td><td>0.706</td><td>0.691</td><td>0.747</td><td>1.664 - 1.821</td><td>0.271</td><td>0.268</td><td>0.461</td></tr><tr><td>2</td><td>1.645</td><td>1.311</td><td>1.343</td><td>1.289</td><td>1.610 - 1.611</td><td>1.339</td><td>1.442</td><td>1.600</td></tr><tr><td>3</td><td>1.648</td><td>1.136</td><td>1.121</td><td>1.242</td><td>1.606 - 1.611</td><td>1.226</td><td>1.282</td><td>1.652</td></tr><tr><td>4</td><td>1.629</td><td>1.118</td><td>1.117</td><td>1.202</td><td>1.606 - 1.611</td><td>1.187</td><td>1.246</td><td>1.608</td></tr><tr><td>5</td><td>1.615</td><td>1.056</td><td>1.066</td><td>1.158</td><td>1.606 - 1.612</td><td>1.181</td><td>1.242</td><td>1.566</td></tr><tr><td>6</td><td>1.610</td><td>0.953</td><td>0.968</td><td>1.058</td><td>1.607 - 1.612</td><td>1.200</td><td>1.262</td><td>1.584</td></tr><tr><td>7</td><td>1.606</td><td>0.872</td><td>0.892</td><td>0.984</td><td>1.607 - 1.612</td><td>1.222</td><td>1.285</td><td>1.610</td></tr><tr><td>8</td><td>1.603</td><td>0.767</td><td>0.790</td><td>0.904</td><td>1.608 - 1.612</td><td>1.259</td><td>1.324</td><td>1.650</td></tr></tbody></table>

<table><thead><tr><th rowspan="2">n</th><th colspan="4">adsorption of H₂</th><th colspan="4">adsorption of H</th></tr><tr><th>Pt - H length / Å</th><th>BE / eV</th><th>ΔH / eV</th><th>ΔG / eV</th><th>Pt - H length / Å</th><th>BE / eV</th><th>ΔH / eV</th><th>ΔG / eV</th></tr></thead><tbody><tr><td>1</td><td>1.664 - 1.821</td><td>0.520</td><td>0.517</td><td>0.710</td><td>1.638</td><td>0.808</td><td>0.793</td><td>0.849</td></tr><tr><td>2</td><td>1.610 - 1.611</td><td>1.230</td><td>1.333</td><td>1.491</td><td>1.645</td><td>1.159</td><td>1.191</td><td>1.137</td></tr><tr><td>3</td><td>1.606 - 1.611</td><td>1.128</td><td>1.184</td><td>1.554</td><td>1.648</td><td>1.220</td><td>1.205</td><td>1.326</td></tr><tr><td>4</td><td>1.606 - 1.611</td><td>1.077</td><td>1.136</td><td>1.498</td><td>1.629</td><td>1.125</td><td>1.124</td><td>1.209</td></tr><tr><td>5</td><td>1.606 - 1.612</td><td>1.035</td><td>1.096</td><td>1.420</td><td>1.615</td><td>0.989</td><td>0.999</td><td>1.091</td></tr><tr><td>6</td><td>1.607 - 1.612</td><td>1.003</td><td>1.065</td><td>1.387</td><td>1.610</td><td>0.881</td><td>0.896</td><td>0.986</td></tr><tr><td>7</td><td>1.607 - 1.612</td><td>0.977</td><td>1.040</td><td>1.365</td><td>1.606</td><td>0.791</td><td>0.811</td><td>0.903</td></tr><tr><td>8</td><td>1.608 - 1.612</td><td>0.956</td><td>1.021</td><td>1.347</td><td>1.603</td><td>0.717</td><td>0.740</td><td>0.854</td></tr></tbody></table>

The computed geometries of the adducts revealed specific sites on the clusters where hydrogen atoms prefer to bind. For the H atoms we observe them bound to a single Pt centre. The Pt-H distance ranges from 1.603 to 1.648 Å, consistent with the known values for Pt-H complexes [36]. The H atom is positioned symmetrically slightly moving to the centre of the triangle as the cluster increases, giving Pt-Pt-H angles that are similar in value, generally ca.
85-95°. The example of such a structure formed with [Pt₃(CO)₆]₄²⁻ and the H atom is shown in Figure S6 in the supplementary information file. Geometries of clusters bound with H atoms show slight distortions, particularly in the smaller clusters. When the cluster size increases, the presence of more Pt₃ units helps to stabilize structures minimizing the impact of H on their deformations.

Similarly, for the H₂ molecules both H atoms are coordinated to a single Pt centre. Several attempts to obtain a cluster geometry with two H atoms on neighbouring Pt centres were unsuccessful. Both Pt-H distances are similar in value, ranging from 1.607 to 1.821 Å and resulting in the H-H distance from 1.53 to 1.76 Å (the H-H distance in an isolated H₂ molecule is 0.74 Å). The H₂ ligand binding geometry indicates that its σ bonding orbital is involved in the coordination with the Pt centre, in agreement with previous studies on H₂ interaction with Pt surfaces [37]. Again, the example of such an interaction is illustrated in Figure S5, taking [Pt₃(CO)₆-H₂]₄²⁻ as an example. Notably, for smaller clusters, significant structural deformations or even decomposition are observed during the H₂ bonding (the upper Pt₃ unit is moving further apart from the rest of the cluster). The presence of more Pt₃ units stabilizes the structures. However, such deformations could limit effectiveness of these clusters and stability as catalysts in the hydrogen generation processes.

In addition, the energetic parameters accompanying hydrogen species binding (BE, ΔH, and ΔG) were analysed. The observed trends indicate that the adsorption energies vary with the cluster size, with a maximum of all energetic parameters for clusters with n= 2-3 and showing a general decrease in all energetic parameters as the cluster size increases further. The positive values of ΔG indicate that the adsorption process is not thermodynamically favourable for the H₂ molecules and the H atoms. The Chini clusters do not readily attract and retain these species on their surfaces. The desorption process, or the release of the H₂ molecules from the Chini clusters, is thermodynamically favourable. This suggests that H₂ is likely to desorb spontaneously when it is formed on the surface of the cluster. That is the property which is desired from the good candidates for the H₂ generation.

### 3.6 Experimental verification

To verify theoretical prediction, the Chini clusters were synthesised with different pH conditions and deposited on TiO₂, to check their activity in photocatalytic hydrogen
generation. The nuclearity of the obtained systems was assigned using UV-Vis spectroscopy: the samples synthesised at pH = 13 contain [Pt₃(CO)₆-H₂]₄²⁻, while those synthesised at pH = 7 contain [Pt₃(CO)₆-H₂]₇²⁻ / [Pt₃(CO)₆-H₂]₈²⁻ - see Figure S7 in SI. Worth to mention, we have not obtained smaller clusters than n = 4, in agreement with the instability assumption made by the positive HOMO energy.

<figure><img src="image_6.png" /><figcaption>Figure 5. Photocatalytic H₂ generation rates over time (left) and average H₂ generation rates (right) for the Chini clusters impregnated on TiO₂-P25. Red tones indicate samples with higher cluster content (0.1 wt%), while blue tones represent lower content (0.02 wt%). Darker tones and dashed lines correspond to higher pH (thus, lower nuclearity, assigned to n = 4), whereas lighter tones and dotted lines indicate lower pH, associated with larger clusters (n = 7–8).</figcaption></figure>

Time Resolved Microwave Conductivity (TRMC) studies have shown that Pt Chini clusters scavenge very efficiently electrons from the CB of TiO₂ decreasing the charge carriers' recombinations, which is beneficial for photocatalytic applications.¹⁴ The results of the photocatalytic experiments are shown in Figure 5. A substantial rise in the H₂ generation activity is observed when the Chini-TiO₂ composites are used instead of bare TiO₂. This points out the activity of the Chini clusters in the H₂ generation reaction. The analysis of the obtained data reveals two trends.

First, using photocatalysts with the Chini clusters obtained at higher pH conditions (pH = 13) leads to a higher rate of H₂ generation. The Chini clusters synthesized in higher pH conditions
(pH = 13) are smaller, with a size of n = 4, while those synthesized at neutral pH (pH = 7) are larger, with a size of n = 7-8. It confirms that smaller clusters formed at higher pH are more effective in the investigated catalytic processes.

The positive values of ΔH and ΔG for adsorption of H₂ suggest the adsorption process is not thermodynamically favourable, implying that H₂ is more likely to desorb spontaneously once formed on the surface of the cluster. Furthermore, higher values of ΔH and ΔG observed for smaller Chini clusters (n = 4) compared to the larger ones (n = 7 - 8) indicate that the desorption process is more likely to occur faster due to their higher reactivity. It is desired for potential photocatalysts and may impact the H₂ generation rates observed experimentally.

Similarly, differences between calculated electron and hole reorganization energies show that smaller Chini clusters can enhance charge separation more than larger ones. For the Chini clusters of n = 4 the difference is ca. 0.6 eV, while those of n = 8 show lower difference (less than 0.1 eV). Charge separation is one of critical factors impacting photocatalytic properties.

Second, a higher amount of the Chini clusters leads to an increased H₂ generation. However, when the Pt amount in the samples is compared, the ones having lower Pt content show better performance. It is likely that the clusters are more effectively utilized, maximizing the available active sites on the TiO₂ surface. Higher concentrations may lead to aggregation and other processes reducing the overall catalytic efficiency per Pt atom.

The reported results are just proof of the DFT results presented above. The optimisation of their performance, including a long-term stability of the Chini – TiO₂ composites in the investigated process, requires further in-depth study.

## 4. Conclusions

The electronic structure analysis provides insights into the stability, reactivity, and potential applications of the Chini clusters, with size-dependent properties playing a leading role. Not only the photochemical properties, such as the position of the electron absorption band, but also electron and hole transport properties change in line with their nuclearity number n.

The reactivity of the Chini clusters towards the H and H₂ species was also probed. Our calculations indicate that these systems may be considered as components of the catalytic materials used for hydrogen generation. These assumptions were confirmed by testing the Chini-TiO₂ composites in the photocatalytic H₂ generation from water – methanol mixture.
The electronic, optical and photocatalytic properties can be tuned by controlling the size of
the metal cluster. Higher photocatalytic activity for H₂ generation is obtained with the lowest
nuclearity (n=4 for experimental tests) in agreement with DFT calculations.

**Acknowledgements:** AS acknowledges Erasmus+ mobility program. DRZ acknowledges the National Science Centre, Poland within OPUS project no 2021/43/B/ST4/02969. This publication is based upon work from COST Action CA 18234 "Computational materials sciences for efficient water splitting with nanocrystals from abundant elements", supported by COST (European Cooperation in Science and Technology). This research was carried out with the support of the Interdisciplinary Centre for Mathematical and Computational Modelling University of Warsaw (ICM UW) under computational allocation no G88-1225 and G95-1740.

**Conflict of interest:** Authors declare no conflict of interest.

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