# Elucidating Cobalt Promotion in Supported Re Sulfide
Catalysts in HDS/HER

Daria Ryaboshapka, Pascal Bargiela, Mimoun Aouine, Valérie Briois, Laurent

Piccolo, Pavel Afanasiev

► To cite this version:

Daria Ryaboshapka, Pascal Bargiela, Mimoun Aouine, Valérie Briois, Laurent Piccolo, et al.. Elucidating Cobalt Promotion in Supported Re Sulfide Catalysts in HDS/HER. ACS Catalysis, 2025, 15 (12), pp.10210-10226. 10.1021/acscatal.5c02236. hal-05370455

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# Elucidating cobalt promotion in supported Re sulfide catalysts in HDS/HER reactions.

Daria Ryaboshapka*, Pascal Bargiela, Mimoun Aouine, Valérie Briois, Laurent Piccolo, Pavel Afanasiev*

Univ. Claude Bernard Lyon 1, CNRS, UMR5256, IRCELYON, F-69626, Villeurbanne, France.

SOLEIL synchrotron, UR1-CNRS, L'Orme des Merisiers, Route Départementale 128, 91190 Saint Aubin, France

## Abstract

Rhenium sulfide catalysts supported on carbon or alumina and modified with cobalt (Co) were investigated to understand the formation of the active sulfide phase and the effect of cobalt. These Co-Re-S materials were evaluated as catalysts for hydrodesulfurization (HDS) and the electrocatalytic hydrogen evolution reaction (HER). For the first time, *operando* X-ray absorption spectroscopy (XAS) at the Co K and Re L edges was employed to characterize these Re-based catalysts, while chemometric tools (MCR-ALS) were used to study their sulfidation behavior.

The sulfidation of initial Re oxide species proceeds through an oxysulfidic intermediate, leading to the formation of $ReS_2$ in both sulfidation atmospheres ($H_2S/N_2$ or $H_2S/H_2$). In contrast, the sulfidation of initial cobalt species gives $CoS_2$ in $H_2S/N_2$ and $Co_9S_8$ in $H_2S/H_2$. However, in the presence of Re, a mixed $CoReS$ phase is formed. The structure of the active $CoReS$ sites was proposed based on a combination of characterization techniques and computational methods, including DFT optimization of mixed metal sulfide structures, XANES simulations, and linear combination fitting. The impact of activation conditions was also studied, revealing that sulfidation in $H_2S/N_2$ produces more active catalysts than sulfidation in $H_2S/H_2$, with $ReS_2$ forming in both cases. The addition of cobalt to $ReS_2$ significantly enhances catalytic activity for both HDS and HER, indicating the formation of highly active $CoReS$ sites.

## Introduction
Layered transition metal sulfides are important materials in heterogeneous catalysis, photocatalysis and electrocatalysis [1, 2, 3, 4]. While MoS₂-based systems are widely studied, ReS₂-based catalysts are often overlooked despite their superior catalytic performance. In conventional hydrodesulfurization (HDS) of thiophene, both bulk and supported ReS₂ catalysts are known to be several times more active than their Mo-based counterparts [5, 6, 7]. These systems also demonstrate significant catalytic activity in other processes involved in petroleum transformation such as hydrodeoxygenation [8], hydrodenitrogenation [9], hydrogenation [10], which are also relevant to biomass valorization [11]. Additionally, ReS₂ demonstrates higher activity and stability in oxidizing environments in electrocatalysis. For instance, in the hydrogen evolution reaction (HER), ReS₂ exhibits a lower onset potential and Tafel slope, indicating faster reaction kinetics [12, 13]. Furthermore, ReS₂ is considered a promising co-catalyst in photocatalytic reactions [14, 15, 16].

MoS₂-based catalysts are often modified with Co to create CoMoS sites with enhanced catalytic activity [17, 18, 19]. Similarly, ReS₂ systems can be promoted with Co. In [9] the specific activity per g of the catalyst of unsupported Co-promoted ReS₂ in HDS of gas oil was up to two times higher than for sole ReS₂, for a Co/(Co + Re) ratio of 0.5 at 375 °C. For supported ReS₂/γ-Al₂O₃, the HDS promotion factor reached 4 with the same Co/(Co + Re) ratio, in the same reaction under industrial conditions [20]. Notably, this promotion effect has also been observed in the electrochemical HER [21]. The application of such Co-promoted systems can be extended to other reactions, including de-/hydrogenation, hydrodeoxygenation, and hydrogen evolution.

The nature of promotion in CoReS catalysts remains unresolved. Two hypotheses have been proposed: the first suggests hydrogen spillover from sulfidic cobalt phases to ReS₂, while the second attributes the enhanced activity to the Co-Re-S structure itself [9]. The formation of a CoReS phase, analogous to the CoMoS phase in Co-promoted Mo catalysts, has never been conclusively demonstrated. Conventional techniques like XRD and XPS have been unable to confirm its existence [9]. For instance, cobalt decorating the edges of ReS₂ particles does not form a bulk phase detectable by XRD, and the XPS signals of Co²⁺ in Co-Re-S and in pure Co₉S₈ are often difficult to distinguish. Advanced characterization techniques, such as *operando* X-ray absorption spectroscopy (XAS), could provide insights into the local coordination environment of cobalt. For example, L.Plais and colleagues successfully determined the structure of the CoMoS phase by recording XAS during the sulfidation of CoMo/Al₂O₃ catalysts [22]. However, no such studies have been conducted for (Co)Re sulfide systems, nor has the structure of a potential CoReS phase been proposed. Identifying the CoReS phase and elucidating its structure is a compelling challenge that could significantly advance the understanding of Re-based catalysts.

Beyond the unresolved questions regarding the nature of promotion, there is ongoing controversy in the literature concerning the state of Re species as a function of activation conditions and the support used. Under similar activation conditions (H₂S/H₂, 350–400 °C), some authors have reported the formation of layered ReS₂ particles [23], while others have observed spherical nanoparticles lacking a
lamellar structure [⁵,¹⁰]. Escalona et al. investigated the effect of sulfidation atmosphere and found that catalysts sulfided in H₂S/N₂ exhibit higher activity than those sulfided in H₂S/H₂. They attributed this difference to the formation of distinct products, particularly metallic Re in the presence of H₂ [²⁴]. The formation of sulfur-rich sulfides, such as Re₂S₇, under hydrogen-free conditions has also been proposed [²⁵], although the stability of such sulfur-rich species at the temperatures used during sulfidation remains questionable. Comparing these studies, we conclude that the reasons for the observed activity differences and the nature of the "spherical" phase seen in TEM remain unclear. *Operando* X-ray absorption spectroscopy (XAS) could provide critical insights into these issues.

In this work, we compared the catalytic performance of Co-Re sulfide catalysts with varying loadings (low and high), supports (alumina and carbon), and sulfidation environments (H₂S/H₂ and H₂S/N₂). We conducted *operando* XAS studies at the Co K-edge and Re L-edges during sulfidation. Our goal was to isolate the spectrum of the CoReS phase, compare it with simulated spectra, and propose its structure.

## Experimental

### Synthesis of supported catalysts

**Re/G Catalysts:** Carbon-supported Re catalysts were prepared using an impregnation technique. Mesoporous carbon consisting of multilayer graphite flakes, hereafter referred to as G or graphene was used as the support without pretreatment (S<sub>BET</sub> = 750 m²/g, V<sub>pore</sub> = 1.0 cm³/g). The support was impregnated with an aqueous solution of ammonium perrhenate (NH₄ReO₄) and dried at 100 °C for 12 h. Two systems were prepared: **10Re/G** and **0.6Re/G**, nominally containing 10 wt.% and 0.6 wt.% of Re, respectively.

**Co/G** catalysts were synthesized using cobalt acetylacetonate (Co(acac)₂). Specifically, 1 g of graphene was added to a solution of 32 mg Co(acac)₂ in 20 mL of methanol and stirred at 65 °C for 4 h. The solid was separated via centrifugation and dried overnight under argon at room temperature, yielding **Co/G** with 0.7 wt.% Co.

**Co-Re/G** Catalysts: The Re/G sulfide samples were promoted with cobalt using the acetylacetonate technique described above. Instead of graphene, 1 g of presulfided Re/G (10Re/G or 0.6Re/G) was used. The sulfidation was performed in either H₂S/N₂ or H₂S/H₂ flow (1:5 ratio) at 350 °C for 1 hour with a heating rate of 5 °C/min. After drying under argon, two samples were obtained: **0.3Co0.6Re/G** (0.3 wt.% Co, 0.6 wt.% Re) and **1Co10Re/G** (1 wt.% Co, 10 wt.% Re). The acetylacetonate technique, previously developed for CoMoS catalysts, enables the selective grafting of Co(acac)₂ onto the edges of sulfide slabs, resulting in highly active promoted systems [²⁶].

For comparison, a reference catalyst supported on γ-Al₂O₃ was prepared using a conventional co-impregnation technique. A mixture of Co(NO₃)₂·6H₂O and NH₄ReO₄ aqueous solutions was used to
impregnate 1 g of γ-Al₂O₃ support (S<sub>BET</sub> = 250 m²/g, V<sub>pore</sub> = 0.5 cm³/g). The sample was dried at 110 °C overnight under air, yielding **1Co10Re/Al₂O₃** (1 wt.% Co, 10 wt.% Re).

All catalytic tests and physicochemical characterizations (except for *operando* XAS) were performed on samples sulfided in either H₂S/H₂ or H₂S/N₂ flow (1:5 ratio) at 350 °C for 1 h with a heating rate of 5 °C/min. The sulfidation atmosphere is indicated in the sample names, e.g., 1Co10Re/G H₂S/H₂ or 0.3Co0.6Re/G H₂S/N₂. The details on the precursors (supplier, purity, etc.) are provided in Table S1.

## Characterizations

The cobalt (Co) and rhenium (Re) content in the samples was determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) on a Horiba Jobin Yvon Activa instrument. For carbon-supported samples, a mixture of H₂SO₄ and HNO₃ was used to dissolve the material, followed by heating to 300 °C. For alumina-supported catalysts, a mixture of H₂SO₄, HNO₃, and HF was employed. The textural properties of the catalysts were analyzed using low-temperature N₂ adsorption on a Micromeritics ASAP 2010 device. Powder XRD patterns were obtained using a Bruker D8 Advance A25 diffractometer with Cu Kα radiation. The phases were identified using the ICDD database and analyzed with X'Pert software. The size of the crystallites was calculated according to Sherrer formula:

$$D = \frac{K \times \lambda}{\beta \times \cos \theta}, \qquad (1)$$

where K is Sherrer constant (0.9), λ is the wavelength of the source (0.15406 nm), β is FWHM, rad, θ is Bragg angle, rad.

Raman spectra were collected on a LabRam HR spectrometer (Horiba-Jobin Yvon) equipped with a CCD detector. The spectrometer was calibrated using a silicon reference before measurements. An Ar laser (λ = 532 nm) was used as the excitation source, with laser power ranging from 0.25% to 3.2% (3.2% corresponding to 1 mW). The LabSpec software was used for data acquisition and analysis.

For Transmission Electron Microscopy (TEM) analysis, the catalysts were crushed in an agate mortar with a small amount of ethanol, and the resulting suspension was ultrasonicated for several minutes. A droplet of the suspension was placed on a holey carbon-coated copper grid and dried. TEM imaging was performed on a JEOL 2010 FEG instrument at an accelerating voltage of 200 kV. STEM-ADF images were acquired using a FEI Titan G2 aberration-corrected environmental ETEM operating at 300 kV. The images were analyzed using the Digital Micrograph Gatan software package.

X-ray Photoelectron Spectroscopy (XPS) measurements were conducted on a Kratos Axis Ultra DLD spectrometer using monochromatic Al Kα radiation (1486.6 eV) as the X-ray source, operating at 180 W (15 kV, 12 mA). Before measurement, the sulfided samples were placed on conductive carbon tape under an argon atmosphere. Survey spectra were recorded in the range of 1200–0 eV with a pass
energy of 160 eV, while high-resolution spectra were acquired with a pass energy of 40 eV. The spectra were processed using CasaXPS software or the free XPSpeak41 software. The C 1s photoemission peak of graphene (sp² carbon) at 284.1 eV was used for calibration

Operando XAS measurements

The evolution of coordination environment of Re and Co was studied by means of operando Quick-XAS in the transmission mode at the SOLEIL synchrotron facility, ROCK beamline [27]. The spectra were recorded using a Si(111) channel-cut monochromator oscillating at 2Hz over angular oscillation amplitude set at 2° for the Co K edge (7709 eV) and at 2.4° for the Re L edges (all three edges L₁ - 12527 eV, L₂ - 11959 eV, L₃ - 10535 eV were observed). The Co K-edge and Re L-edge spectra were alternately recorded during the activation of bimetallic catalysts using the edge jump capability available at the ROCK beamline [28]. A set of three ionization chambers filled with a 10:90 Ar:N₂ mixture was used to measure the sample transmission and the transmission of a reference metallic foil for internal absolute energy calibration. For Co, a metallic Co foil was used, while for Re, a Pt foil with an L₃ energy at 11564 eV was applied. The switching of optics and ionization chambers between the Co K-edge and Re L-edge measurements was completed in 35 seconds. To improve the signal-to-noise ratio, 110 Quick-XAS spectra were recorded at the Co K edge over 60 seconds and 50 spectra recorded at the Re L-edges over 30 seconds.

Quartz capillaries with diameters of 1.5 mm or 2.5 mm and a wall thickness of 20 µm were used as *operando* microreactors. These capillaries were fixed on a rigid U-shaped metallic sample holder. High-loaded samples were placed in capillaries with smaller diameters, while low-loaded samples were placed in capillaries with larger diameters. The sample powder was loaded into the capillary between two layers of quartz wool, and the capillary was then inserted into the U-shaped holder. A two-component epoxy resin was applied to the ends of the capillary and the walls of the holder to ensure airtight fixation. After the epoxy resin hardened, the sample holder was positioned above the nozzle of a hot air gas blower (FMB Oxford) for the experiments. The gas and temperature treatment procedure consisted of the following steps: (i) Room temperature (RT) under a flow of N₂; (ii) Introduction of H₂S/N₂ (or H₂S/H₂) at RT, followed by linear heating to 350 °C at a rate of 5 °C/min; (iii) Maintaining the sample in the H₂S/N₂ (or H₂S/H₂) mixture for 20 minutes; (iv) Cooling to RT in a flow of N₂ at a rate of 10 °C/min; (v) Room temperature (RT) under a flow of N₂.

Preliminary XAS data treatment was performed using the ATHENA software [29]. Fitting of the EXAFS spectra was carried out with VIPER program both in *k* (BFT, Back Fourier Transformation) and *R* spaces [30]. The *R*-space fitting was performed in the (Im+Module) mode. During the fitting procedure the values of *R* (distance), *CN* (coordination number), σ² (Debye-Waller factor) and Δ*E*₀ (energy difference) were varied. Depending on the case, some variables were constrained to avoid overfitting. The goodness of the fit was determined by the value of *R*-factor.
To analyze the intermediate species appearing during sulfidation and reaction we applied a set of chemometric data analysis tools [²²]. All procedures were performed with Matlab R2021a software. First, we constructed a matrix of normalized XAS spectra recorded during the sulfidation of Re-containing catalysts with a merge over 15 s of the data recorded successively during the heating ramp, and merge over 300 s for the data recorded during isothermal steps. For the bimetallic samples the merge during the heating ramp was performed over 60 s at Co K edge and over 30 s at Re L edges. The normalization of the raw data was performed using the “normal GUI” interface available at the beamline [³¹]. The normalized data were assembled into a matrix using a homemade XAS-SVD Matlab GUI toolbox which allows for a preliminary PCA (Principal Component Analysis) estimation of the number of components from the scree plot of singular values [³²]. Afterwards, MCR-ALS (Multivariate Curve Resolution – Alternating Least Square) was applied on the matrix obtained in XAS-SVD, to extract the spectra of the pure components and their concentrations [²²].

### DFT optimization

The clusters were constructed using Avogadro software and pre-optimized with Molecular Mechanics tool available in the program. The atomic coordinates provided by Avogadro were then used for DFT calculations in ORCA ver. 5.0.2 [³³]. Generalized gradient BP86 functional (Becke '88 exchange and Perdew '86 correlation) was used [³⁴], generally recommended for studying transition metal compounds. The basis sets developed by Karlsruhe group were applied in combination with the functional: def2-SVP and def2-TZVP [³⁵]. An auxiliary basis set for Coulomb fitting was also applied – def2/J. To achieve convergence, a two-step scheme was applied, including first geometry optimization with the def2-SVP basis and loose convergence criteria, followed by the second optimization using def2-TZVP basis, fine grid, tight convergence tolerance, and the SlowConv or VerySlowConv options (high mixing degree). The optimized coordinates were then transferred to Avogadro for visualization and used for FDMNES/FeFF simulations of X-ray absorption spectra.

### FDMNES and FEFF calculations

The DFT optimized atomic coordinates in a cluster were used as input for Finite Difference Method Near Edge Structure (FDMNES) DFT simulations of XAS spectra at the Co K edge [³⁶,³⁷]. The cluster radius was chosen at 6 Å and the core level width at 1.33 eV [³⁸]. Self-consistent calculations (SCF) were performed. The simulation of the EXAFS spectra of the optimized CoReS structures was performed using FeFF module, implemented in the Artemis program [³⁹].

### Thiophene HDS test

A weighted amount of a freshly sulfided sample was placed in a flow fixed-bed Pyrex microreactor. A 3 L·h⁻¹ H₂ flow passed through a saturator containing liquid thiophene at 0 °C. The reaction was
performed at atmospheric pressure at three different temperatures: 320, 330, 340 °C. Steady-state conversions were estimated after at least 16 hours on-stream. The reaction products were analyzed by gas chromatography on an Agilent 7820A device equipped with a flame ionization detector. The HDS rates per unit of catalyst mass were estimated according to the equation (2),

$$r = -\left(\frac{F_{thio}}{m}\right) \times \ln(1 - X), \qquad (2)$$

where r is the specific HDS rate, $F_{thio}$ is thiophene molar flow (mol/s), m is mass of the catalyst (g) and X is thiophene conversion. Selectivities can be determined via the equation (3):

$$Selectivity = 100\% \times \frac{A_x}{A_p}, \qquad (3)$$

where $A_x$ is the area of the reaction product, $A_p$ is the area of all products.

To estimate the activation energies $E_A$ we used Arrhenius equation (4):

$$lnk = -\left(\frac{E_A}{R \times T}\right) + \ln A, \qquad (4)$$

Where k is reaction rate constant, R is the universal gas constant equal to 8.314 J/mol/K, T is the reaction temperature and A – a pre-exponential factor.

### Electrochemical HER test

Hydrogen evolution reaction (HER) was performed in a three-electrode cell (catalyst spread over the glassy carbon - working electrode, graphite rod – counter electrode, saturated calomel electrode – reference) in Ar-saturated 0.5M $H_2SO_4$ (pH = 0.5) at room temperature. 10 µl of catalytic ink prepared with 10 mg of the sample, 400 µl of Nafion and 800 µl of ethanol was spread over the surface of the working electrode. Linear Sweep Voltammetry (LSV) curves were recorded in – 600 to 200 mV potential range (vs RHE, Reversible Hydrogen Electrode) with 10 mV/s rate, then Tafel slopes were calculated from these curves. Chronoamperometry curve was recorded at 400 mV vs RHE.

## Results and discussion

This work aims to elucidate the nature of cobalt promotion in Re sulfide catalysts in HDS/HER reactions by examining its dependence on support material and activation conditions, while also exploring the potential formation, properties, and structure of the CoReS phase.

We used conventional preparation techniques to obtain CoRe catalysts: a simple co-impregnation of alumina with $NH_4ReO_4$ and $Co(NO_3)_2$; impregnation of mesoporous graphene with $NH_4ReO_4$ followed by adsorption of $Co(acac)_2$. The quantities of Co and Re determined by chemical analysis are reported in Table S2. The subsequent sulfidation was performed in two different atmospheres $H_2S/N_2$ and $H_2S/H_2$ in order to clarify previously reported results, diverging on the effect of hydrogen on the morphology of $ReS_2$ particles and potential reduction of $ReS_2$ to metallic rhenium [40].
Previously, we observed that few-atom CoMoS clusters form in Mo/G catalysts at low Mo loadings. These clusters, distinct from MoS₂ slabs due to their lack of "edges," exhibit high specific activity in hydrodesulfurization (HDS) [⁴¹, ⁴²]. In parallel with the primary objectives of this work, we investigated whether similar ultradispersed CoReS clusters could form in low-loaded Re/G systems. Low-loaded systems are particularly responsive to chemical environment variations in *operando* XAS experiments, prompting us to study two Re loadings: a low loading (around 1 wt.%, akin to previously studied ultradispersed catalysts) and a high loading (10 wt.%, representative of conventional catalysts; see Table S2). For the high-loading catalysts (1 wt.% Co, 10 wt.% Re), we used an excess of Re sulfide to increase the likelihood of CoReS phase formation and facilitate its detection. For the low-loading system (0.3 wt.% Co, 0.6 wt.% Re), we aimed to test the scalability of our approach and assess whether the CoReS phase could form with smaller, less ordered ReS₂ crystallites or clusters. To achieve these goals, we employed a comprehensive suite of characterization techniques, including Raman spectroscopy, TEM/STEM ADF, XPS, *operando* XAS at the Co K-edge and Re L-edges, combined with MCR-ALS analysis, DFT/FDMNES simulations, and linear combination fitting (LCF).

## X-ray diffraction

XRD technique is not competent to detect crystalline phases in the low-loaded samples [³⁹,⁴⁰], so we studied only the high-loaded ones. In the XRD patterns of 1Co10Re/Al₂O₃ H₂S/N₂ and 1Co10Re/G H₂S/N₂, aside from the expected graphite (00-008-0415) and γ-Al₂O₃ (00-001-1303) lines, there are very broad reflections corresponding to ReS₂ phase (Fig. S1). The most intensive peak of (001) plane of ReS₂ (00-052-0818) at 2θ of 14° is absent for the samples supported on graphene and is very weak for the sample on alumina, while the lines of (2-20) and (02-1) planes at 32-34° are visible for both supports. The crystallite size determined for 1Co10Re/Al₂O₃ H₂S/N₂ from Scherrer formula is around 2.7 nm. The lines of (00l) planes are often absent or weak for layered transition metal sulfides when there is no stacking of slabs. The XRD study attests that Re sulfide is well dispersed in our systems despite the relatively high loading.

## Raman spectroscopy
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Raman spectra of sulfided CoRe samples supported on alumina.</figcaption></figure>

Raman spectra of samples supported on graphene showed no features related to ReS₂ or sulfidic Co species expected in 100 – 600 cm⁻¹ region, perhaps due to high absorption of green laser light by graphene and thermal background (Fig. S2). However, for the 1Co10Re/Al₂O₃ samples sulfided in different atmospheres the sulfide signals become well resolved (Fig. 1). The bands below 250 cm⁻¹ correspond to vibrations of Re – Re bonds while those at higher frequencies likely belong to the vibrations of Re – S bonds [⁴³]. Notably, the spectra are similar: no shifts in the positions of the bands were observed, indicating the presence of the same ReS₂ phase in both samples. The only difference lies in the presence of the band characteristic of disulfide bridges at 512 cm⁻¹ for H₂S/N₂. As the amount of Co is low, no bands related to Co – Co, Co – Re or Co – S vibrations could be spotted.

*Electron microscopy*

TEM microscopy images are presented on Fig. 2, Fig. S3–S4. For the low-loaded samples sulfided in H₂S/N₂ or H₂S/H₂ only graphite layers were detected, while for high-loaded samples some slabs and spherical nanoparticles were spotted. ReS₂ slabs are very small and barely visible on the graphite surface which is also layered (interlayer distance of 0.23 – 0.30 nm, Fig. S3). Most importantly, during the TEM acquisition we observed rapid changes occurring in the structure of ReS₂ particles which transforms the slabs into the round-shaped particles 1.6 – 1.8 nm in size, independently on the sulfidation atmosphere used for the sample preparation. The degradation of 1Co10Re/G H₂S/N₂ under the beam is so rapid that in most cases it prevents acquiring exploitable images. In [⁴⁰] for ReS₂ supported on alumina the influence of sulfidation atmosphere was suggested with sulfidation in H₂S/N₂ or H₂S/H₂ leading to different ratio of stacked ReS₂ slabs and of round particles. At the same time in [⁷] the formation of such round-shaped nanoparticles was observed for carbon/silica supports. We suppose that the reason of controversy is the poor stability of ReS₂ slabs under the beam. This confirms the
conclusions of the previous study where such degradation was observed for ReS<sub>2</sub>/γ-Al<sub>2</sub>O<sub>3</sub>: after 15 min of beam exposure, the slabs transformed into spherical particles, possibly, metallic Re [⁴⁴].

<figure><img src="image_3.png" /><figcaption>**Figure 2.** TEM images of 1Co10Re/Al<sub>2</sub>O<sub>3</sub> H<sub>2</sub>S/N<sub>2</sub>. The interlayer distances of ReS<sub>2</sub> slabs are marked with yellow lines.</figcaption></figure>

For the alumina supported ReS<sub>2</sub> the particles are larger and the layers could be observed by TEM (Fig. 2) with the interlayer distance of 0.62 – 0.66 nm, typical for ReS<sub>2</sub> [⁴⁵]. Apparently, large ReS<sub>2</sub> slabs are more stable under the beam. According to EDS analysis Re, Co and S are uniformly distributed over the samples (Fig. S5). For the samples supported on graphene and sulfided in H<sub>2</sub>S/N<sub>2</sub> we have performed STEM-ADF, for which the beam damage is much lesser (Fig. 3). Numerous small slabs were detected in low-loaded 0.3Co0.6Re/G H<sub>2</sub>S/N<sub>2</sub>: 2 – 4 nm slabs were detected alongside the agglomerates of several atoms and single atoms. As compared with earlier reported Mo catalysts which did not show any MoS<sub>2</sub> slabs [³⁹,⁴⁰], here we that some ReS<sub>2</sub> slabs are formed already at 0.6 wt.% loading.
<figure><img src="image_4.png" /><figcaption>Figure 3. STEM images of (a) 0.3Co0.6Re/G H₂S/N₂ and (b) 1Co10Re/G H₂S/N₂. Ultradispersed species are marked with arrows, some of ReS₂ slabs – with circles.</figcaption></figure>

Well-dispersed ReS₂ slabs along with some ultradispersed species were observed in 1Co10Re/G H₂S/N₂. Note that due to different symmetry (low-symmetry triclinic -P1) of ReS₂ there are no stacking polymorphs, no triangular or hexagonal 2D shapes or distinct M- and S-edges for ReS₂ as is the case of widely studied MoS₂ counterpart. Instead, ReS₂ particles tend to have elongated 2D shapes and show characteristic two-atom stipes along the (100) direction, so the corresponding preferentially exposed edges correspond to cleavage perpendicular to *a* axis (Fig. S6).

### XPS study

The oxidation states and coordination environments of both Co and Re were studied by means of XPS for the following samples: 0.3Co0.6Re/G, 1Co10Re/G sulfided in H₂S/N₂ and 1Co10Re/G sulfided in H₂S/H₂ and also for Co-containing references Co/G sulfided in H₂S/H₂ or in H₂S/N₂.
S 2p: For all samples from three to four sulfur species could be distinguished in the S 2p region: the doublet at 161.7 eV and 162.9 eV corresponds to S²⁻; another at 163.4 eV and 164.6 eV to (S₂)²⁻; and very broad doublets at 166.5 and 167.7 eV, 167.9 eV and 169.1 eV attributed to oxidized sulfur SOₓ (Fig. 4c, S9, S10) [⁴⁶]. Interestingly, the major sulfur species for the samples sulfided in H₂S/H₂ is S²⁻ typically observed for sulfides, while for the samples sulfided in H₂S/N₂ it is (S₂)²⁻. In [⁴⁷], the presence of (S₂)²⁻ in XPS spectra of ReS₂ supported on silica/alumina was attributed to formation of Re oxysulfide species. However, in our case the oxysulfidic Re in the Re 4f spectra appears only for the samples sulfided in H₂S/H₂, where the amount of (S₂)²⁻ is low (see below). Probably, the (S₂)²⁻ groups in the sample sulfided in H₂S/N₂ appear due to a contribution of (S₂)²⁻ groups on the surface of carbon.

<figure><img src="image_5.png" /><figcaption>Figure 4. XPS of 1Co10Re/G H₂S/H₂, (a) Co 2p, (b) Re 4f, (c) S 2p.</figcaption></figure>

Re 4f: only the signal of sulfided Re(IV) species as in ReS₂ is observed, with a sole doublet 4f₇/₂ at 41.4 eV and 4f₅/₂ at 43.8 eV for the samples sulfided in H₂S/N₂ (Fig. S8, S10) [⁴⁸, ⁴⁹]. If the sulfidation is performed in H₂S/H₂ the peaks at 44.9 and 47.3 eV were separated, typical for Re oxidic or oxysulfidic species (Re⁶⁺, Fig. 4b) [⁵⁰].

Co 2p H₂S/N₂: For Co/G H₂S/N₂ the main Co 2p peaks are at 778.7 eV and 793.8 eV, while for 1Co10Re/G H₂S/N₂ they are at 778.9 and 794 eV (Fig. S9), corresponding to CoS₂ phase [⁵¹]. The shift
is very small (0.2 eV), suggesting that Co species are similar in samples with and without Re (Fig. S9). The other two peaks observed at 780.8 and 795.8 eV (visible for Co/G H₂S/N₂) might arise due to the multiplet splitting, observed for transition metals with several unpaired 3d electrons, like Co(II) [⁵²].

Co 2p H₂S/H₂: For Co/G H₂S/H₂ one main doublet was separated at 778.4 eV and 793.4 eV (Fig. S9a), while the XPS spectrum of 1Co10Re/G H₂S/H₂ was best fitted with two contributions at 779 eV and 794.4 eV, and at 778.4 and 793.6 eV (Fig. 4a). We attribute the doublet at lower BE to the Co₉S₈ phase and the doublet at higher BE to the supposed CoReS [⁵³]. An analogy could be drawn to the Co 2p XPS evidence of CoMoS phase. In [⁵⁴] for the Co₉S₈ phase, the main peaks of Co 2p₃/₂ and 2p₁/₂ were observed at 778.09 and 793 eV, while the peaks for the CoMoS phase were at 778.64 and 793.55 eV. We observe a similar energy shift by around 0.6 eV (Fig. 5a). Note that according to literature the sulfidation of Co species in H₂S/N₂ results in CoS₂, whereas in H₂S/H₂ results in Co₉S₈ [⁵⁵]. The Co 2p₃/₂ contribution at 779.0 eV is close to that of CoS₂ in the Co/G H₂S/N₂ and 1Co10Re/G H₂S/N₂ samples (778.7 and 778.9 eV), but CoS₂ should not exist after sulfidation in H₂S/H₂. Other peaks at higher BE (2p₃/₂ at 781 and 785 eV) correspond to multiplet splitting and to the shake-up satellites [⁵⁶].

<figure><img src="image_6.png" /><figcaption>Figure 5. Comparison of XPS spectra of sole Co and CoRe samples sulfided in (a) H₂S/H₂ or (b) H₂S/N₂. The dashed lines represent the positions of the peaks.</figcaption></figure>

The main conclusion from the XPS analysis is therefore: while in Co/G and 1Co10Re/G sulfided in H₂S/N₂ the Co species have the same XPS signature, sulfidation of 1Co10Re/G in H₂S/H₂ leads to Co₉S₈ and another species which likely corresponds to promoted CoReS phase.

### *Operando X-ray absorption spectroscopy*

The behavior of Re catalysts under sulfidation conditions and the influence of these conditions on the nature of the synergy between Co and Re remain unexplored. To address this, we investigated the states of Co and Re species in the catalysts as a function of sulfidation treatment. Below, we first analyze the spectra of reference samples containing only Re or Co, followed by those containing both Co and Re.
<table><caption>Table 1. EXAFS fitting results at Re L$_{3}$ edge for 10Re/G during sulfidation in H$_{2}$S/N$_{2}$ (MCR-ALS components).</caption><thead><tr><th>Scatterer<br>atom</th><th>CN<sup>a</sup></th><th>R (Å)<sup>b</sup></th><th>σ<sup>2</sup> (Å<sup>2</sup>)<sup>c</sup></th><th>ΔE<sub>0</sub> (eV)<sup>d</sup></th></tr></thead><tbody><tr><td colspan="5" style="text-align: center; font-weight: bold;">1 component, R = 0.16</td></tr><tr><td>O</td><td>4.0(1)</td><td>1.72(1)</td><td>0.0020(1)</td><td>4.9(1)</td></tr><tr><td>O</td><td>1.0(3)</td><td>2.06(1)</td><td>0.0020(1)</td><td>4.9(1)</td></tr><tr><td colspan="5" style="text-align: center; font-weight: bold;">2 component, R = 0.18</td></tr><tr><td>O</td><td>3.2(2)</td><td>1.71(1)</td><td>0.0020(1)</td><td>4.6(3)</td></tr><tr><td>O</td><td>1.5(3)</td><td>2.03(1)</td><td>0.0043(2)</td><td>4.6(3)</td></tr><tr><td>S</td><td>0.4(1)</td><td>2.59(1)</td><td>0.0024(1)</td><td>-4.9(2)</td></tr><tr><td>Re</td><td>0.2(1)</td><td>2.87(1)</td><td>0.0034(9)</td><td>-4.4(1)</td></tr><tr><td colspan="5" style="text-align: center; font-weight: bold;">3 component, R = 0.18</td></tr><tr><td>O</td><td>1.4(1)</td><td>1.70(1)</td><td>0.0025(2)</td><td>-0.4(1)</td></tr><tr><td>O</td><td>1.2(1)</td><td>1.99(1)</td><td>0.0020(5)</td><td>-0.4(1)</td></tr><tr><td>S</td><td>0.9(1)</td><td>2.33(1)</td><td>0.0020(4)</td><td>4.9(1)</td></tr><tr><td>Re</td><td>0.4(1)</td><td>2.73(1)</td><td>0.0028(5)</td><td>-4.8(1)</td></tr><tr><td>Re</td><td>0.4(2)</td><td>3.59(2)</td><td>0.0025(1)</td><td>-4.8(1)</td></tr><tr><td colspan="5" style="text-align: center; font-weight: bold;">4 component, R = 0.10</td></tr><tr><td>S</td><td>3.7(1)</td><td>2.34(1)</td><td>0.0072(3)</td><td>4.8(1)</td></tr><tr><td>Re</td><td>2.3(1)</td><td>2.73(1)</td><td>0.0075(2)</td><td>4.2(1)</td></tr></tbody></table>

Experimental Re L$_{3}$ edge XANES spectra are presented in Fig. S11. At room temperature under N$_{2}$ flow the spectra of 10Re/G and 0.6Re/G are similar to that of NH$_{4}$ReO$_{4}$ precursor. However, at Re L$_{1}$ edge the shoulder just above the edge is less prominent, indicating that the structure of supported Re species might be slightly modified (Fig. S11). At the end of sulfidation independently of sulfidation atmosphere the XANES spectra resemble those of ReS$_{2}$ as described in literature (Fig. S11) [⁵⁷].

We have next performed PCA and MCR-ALS analysis for a set of data recorded for 10Re/G, 0.6Re/G at Re L$_{3}$ edge and at L$_{1}$ edge during sulfidation (Fig. 6, S12 – S14). Four components were isolated by PCA for sulfidation in H$_{2}$S/N$_{2}$ (Fig. 6, S11, S14) and three – for sulfidation in H$_{2}$S/H$_{2}$ (Fig. S15). The initial specie is oxidic Re – the 1st component, the last component is the fully sulfidic one ReS$_{2}$-like species. One or two components in between are sulfidation intermediates. The 1st component corresponds to the experimental spectrum recorded at room temperature in N$_{2}$, while the last component – to the one recorded at 350 °C after sulfidation in H$_{2}$S/N$_{2}$ or in H$_{2}$S/H$_{2}$.

The XANES spectra of the corresponding MCR-ALS components at L$_{3}$ edge are presented in Fig. 6a and S11 – S12a. The spectrum of the 1st component is close in shape to that of perrhenate, thus confirming that the initial Re is oxidic. While the shape of the XANES spectrum of the 2d component (only visible in H$_{2}$S/N$_{2}$) resembles that of the 1st one, the position of the edge is shifted towards lower energies. For the 3d component in H$_{2}$S/N$_{2}$ (2d in H$_{2}$S/H$_{2}$) it shifts further, suggesting that the oxidation

<footnote><sup>a</sup> coordination number, using $S_0^2 = 1.0$; <sup>b</sup> radial distance to the neighboring atom, <sup>c</sup> distance variance (free variable between 0.002 and 0.009); <sup>d</sup> energy variance (free variable between -5 and 5 eV), R-factor – minimized function of the fitting parameters, characterizes the goodness of the fit.</footnote>
state of Re decreases and that sulfur appears in the coordination environment of Re. The spectrum of the 4th component in H₂S/N₂ (3d in H₂S/H₂) is very similar to the reported spectrum of ReS₂ [⁵⁸].

At the L₁ edge a pre-edge is clearly visible for the 1st component indicating that Re is present in oxidic non-centrosymmetric environment (Fig. S12, S15d). As the sulfidation starts, the pre-edge becomes less resolved, because apparently sulfur begins replacing oxygen. The pre-edge disappears completely in the 3d and 4th components in both atmospheres when more sulfur enters Re coordination environment.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** (a) L₃ Re XANES spectra of four MCR-ALS components for 10Re/G during sulfidation in H₂S/N₂; (b) Corresponding FT-EXAFS.</figcaption></figure>

The evolution of oxidic Re species in H₂S/N₂ begins already at room temperature as sulfur slowly starts to appear in the first coordination shell of Re, though its amount is still small (Fig. S13). Around 100 °C the contribution of sulfur further increases resulting in the formation of ReOₓSᵧ species. Fully sulfidic component begins to appear at around 135 °C and the sulfidation is complete at 260 °C. Interestingly the 4th component in case of high loaded sample appears at lower temperature of 90 °C. We have earlier observed similar evolution pattern for the Mo-based catalysts [⁴¹].

In H₂S/H₂ the appearance of oxysulfidic component is registered at room temperature (Fig. S15c). Sulfidic component appears at around 75 °C and the sulfidation is fully complete at 250 °C.

In order to estimate the S₀² (amplitude reduction factor), we have performed the fit for the NH₄ReO₄ reference. When S₀² is set to 1.0, the CN (O) equals 4.0, therefore, it was chosen for further fitting [⁵⁹,⁶⁰]. The FT-EXAFS of MCR-ALS components of 10Re/G during sulfidation in H₂S/N₂ or H₂S/H₂ (Table 1, Table S4) are as follows:

**1st component:** The 1st component corresponds to oxidic Re species. FT-EXAFS contains only one peak at low R, indicating that Re does not possess neighbors in second coordination shell. In the k region 4 – 13 Å⁻¹ it can be fitted with two contributions: the first one of a short oxygen neighbor with CN of 4 at 1.72 Å and the second one – with a longer Re-O distance near 2 Å and CN of 1. Interestingly, for 10Re/G and 0.6Re/G the fit is almost the same, indicating that even at loadings as high as 10 wt.%
Re species are monomeric. Unlike the supported molybdate species, oxidic Re does not agglomerate on the surface of graphene, but remains mononuclear [⁶¹].

Further evolution of Re species upon sulfidation occurs via slightly different pathways, depending on the gas atmosphere: there are two intermediates detected in H₂S/N₂, but only one intermediate in H₂S/H₂. However, as one intermediate state and the final state similar, we discuss them on a common basis, as follows.

**2<sup>d</sup> component (only observed in H₂S/N₂):** While the second shell (Re-Re) is still absent, sulfur enters the coordination environment, which can be inferred from appearance of the peak corresponding to Re – S distance. For 10Re/G the Re – S bond is long (around 2.55 Å) and the CN of S is around 0.4, while for 0.6Re/G the fitting is best performed with a shorter Re – S bond (2.37 Å) with CN of 0.3 (Table S4). Oxygen persists in the Re coordination environment, and the overall CN(O) is still between 4 and 5. This component can be characterized as oxysulfidic Re in non-stoichiometric ReOₓSᵧ [⁶²].

**3<sup>d</sup> component in H₂S/N₂; 2<sup>d</sup> in H₂S/H₂:** For this component CN of oxygen decreases, while CN of sulfur slightly increases, indicating that oxygen is further replaced by sulfur. The Re – S distance shortens and Re-Re distances (2.73 and 3.59 Å) appear. Similar distances were observed in Re₂S₇ (2.76 Å) and in ReS₂ (Re – Re at 2.80 Å and 3.56 Å) [⁵⁸]. Overall, this component can be considered as oxysulfide, however, containing more sulfur than previous one.

**4<sup>th</sup> component in H₂S/N₂, 3<sup>d</sup> in H₂S/H₂:** Further on, all oxygen in the coordination sphere of Re becomes replaced by sulfur. For bulk ReS₂ the fit is performed with 6 S neighbors and 3 Re (crystallographic data) at the average distances Re-S of 2.40 Å and Re-Re of 2.80 Å (ReS₂ structure has three not equivalent Re – S bonds between 2.31 and 2.55 Å, three Re-Re distances in the range from 2.69 to 2.90 Å, and the next three Re-Re at 3.56 – 3.77 Å [⁵⁸]). We did not observe distant Re-Re bonds above 3 Å for the 4<sup>th</sup> component. The Re – S and Re – Re distances are at 2.34 and 2.73 Å, respectively and the corresponding CNs for 10Re/G, 0.6Re/G samples are lower than in the bulk sulfide, being around 4.0 for S and 2.3 – 2.6 for Re. Probably, the mean distances are shortened and the coordination numbers are lower than in bulk material because of small particles size. This result agrees with the microscopy study (Fig. 3), where highly dispersed ReS₂ slabs and small clusters were observed.

In summary, EXAFS fitting of MCR-ALS pure components shows that the local coordination of Re in the low-loaded and high-loaded samples evolves in a similar manner and the nature of sulfidation intermediates does not change significantly with loading (Fig. S11). Also, the final state in H₂S/N₂ and H₂S/H₂ was similar. It was suggested in the literature that in H₂S/H₂, Re species might transform into metallic Re. However, we demonstrate that sulfidic ReS₂-like species are formed in H₂S/H₂ (Table S5).

As at 350 °C thermal disorder contributes significantly to the noise, we studied EXAFS spectra detected after sulfidation and cooling to room temperature, to obtain better fitting quality for the final state. In H₂S/N₂ the CN(S) is obviously higher than in H₂S/H₂, whereas Re species on graphene are easier sulfided than those on alumina (Fig. S17). The overall CN(S) is lower than 6 in bulk ReS₂ possibly because of high dispersion and abundance of sulfur deficient Re sites (Table S6). Alongside with Re
neighbors at around 2.70 Å that we observed at 350 °C (CN(Re) 1.2 – 2) now other Re neighbors at higher bond distances become observable as well at 3.95 Å and 4.05 Å. These distances are higher than those observed in bulk ReS₂ (3.56 – 3.77 Å) and might correspond to Re-S-Re fragments in the few-atom sulfidic clusters, which deviate from the ReS₂ structure. The CN(Re) of these distant neighbors is much lower than in ReS₂ (0.2 – 0.8).

## The state of cobalt in the monometallic sample

In the previous section we demonstrated that in both atmospheres Re sulfide slabs are formed and they coexist with smaller ReSₓ clusters. Further goal is to understand how they interact with Co. At first, we studied the evolution of sole Co/G reference solid during the sulfidation in H₂S/H₂ up to 350 °C. The XANES spectrum of Co/G recorded at ambient temperature in Ar resembles that of Co(acac)₂ precursor [⁶³], but with a less resolved shoulder (Fig. S18a). As Co/G was not calcined but only dried, Co(acac)₂ species were preserved on carbon surface. In [²²] similar shape of Co K spectrum was observed for dried Co/Al₂O₃ obtained from Co(NO₃)₂, where the authors suggested a mixture of Co species, including CoAl-layered double hydroxide and [Co(H₂O)₆]²⁺. Overall, the shape of the spectrum (white line, very small pre-edge and a typical “octrahedral” XANES profile) suggests the presence of oxidic Co²⁺ in the Oₓ coordination (Fig. S18a). We will further refer to these species as CoOₓ.

The usual product of sulfidation of Co oxide species in H₂S/H₂ flow for the conventional supported HDS catalysts is Co₉S₈. However, while the shape of the pre-edge of our XANES spectrum recorded after sulfidation in H₂S/H₂ resembles Co₉S₈, there are some differences after the edge (Fig. S18b). There is a possibility that the final spectrum is a mixture with intermediate sulfidic Co species. In [²²] for Co/Al₂O₃ it was demonstrated that a small amount of CoS₂ intermediate (10%) is still present even after reaching 400 °C. Such differences might also arise due to the high dispersion of this phase (invisible by XRD and TEM).

Next, we have performed MCR-ALS to estimate the nature of the sulfidation intermediates. The spectra obtained during sulfidation in H₂S/H₂ can be decomposed to three pure components: the initial oxidic Co, an intermediate compound and the final sulfidic Co species (Fig. S19). As shown the concentration profiles (Fig. S19c), the 2d component appears immediately after introduction of the H₂S/H₂ flow and remains present until the end of sulfidation. Around 90 °C, the 3d component appears, when the concentration of the 2d component remains constant. In [²²] subsequent transformation was proposed: CoOₓ → CoS₂ → Co₉S₈, with the transformation to CoS₂ achieved at 200 °C, and then CoS₂ mostly transformed into Co₉S₈ between 230 and 270 °C. For our Co/G sample, the percentage of the intermediate is almost constant from 100 to 230 °C and above 230 °C the concentration of the third component increased drastically. Overall, the process of sulfidation of Co/G seems to resemble the sulfidation of Co/Al₂O₃ described in [²²], though with some variations. The state of Co was analyzed via EXAFS fitting for the three MCR-ALS components. The S₀² factor was set at 0.7, as a result of fitting Co nitrate reference (Oh Co-O with six oxygen atoms).
**1st component:** The FT-EXAFS can be fitted with good accuracy with six oxygen neighbors (Fig. S19b, Table S7). Addition of the Co-Co neighbor was not justified. Therefore, monomeric CoOₓ species are formed on the surface of graphene. In [²²] these CoOₓ species were described as Co(II) octahedra, in which Co is surrounded by six O²⁻ and OH groups, and we suppose that similar species formed on the surface of graphene.

**2<sup>d</sup> component:** L. Plais and coll. proposed CoS₂ as the sulfidation intermediate [²²]. Crystalline CoS₂ possesses 6 sulfur neighbors in the first coordination shell at 2.27 Å and two types of Co – Co bonds, at 3.70 and 3.86 Å, respectively (Fig. S19b, Table S7). However, we believe that on the surface of graphene, CoS₂ is well dispersed and, therefore, the CNs should be lower than in bulk CoS₂. If the fit is performed with CNs of bulk CoS₂, the Debye-Waller factors are unusually high (0.02 or 0.04) for the same temperature and the goodness of the fit is low. If we keep the Debye-Waller factor values restricted to usually observed values (0.002 – 0.009), then instead of CNs around 7.2 and 12.0, we get lower values – 1.7 and 3.7, respectively. The corresponding Co-S and Co-Co distances are similar to those in CoS₂. This component resembles CoS₂, however, as it is dispersed over the surface of carbon, the CNs are lower.

**3<sup>d</sup> component:** The last component bears similarity with Co<sub>9</sub>S<sub>8</sub> phase. In bulk Co<sub>9</sub>S<sub>8</sub> there are four S neighbors in the first coordination shell and two Co neighbors in the second. In our case the Co absorber has also four Co-S bonds at 2.19 Å (Fig. S19b, Table S7). In Co<sub>9</sub>S<sub>8</sub> both octahedral and tetrahedral Co sites are present [⁶⁴]. The Co-Co CN Co neighbor at 2.51 Å equals 1.2 which is lower than in bulk Co<sub>9</sub>S<sub>8</sub>. Overall, our results obtained during sulfidation of Co/G resemble those obtained by L. Plais et al.[²²]: the sulfidation proceeds via formation of a CoS₂-like species, from which the final Co<sub>9</sub>S<sub>8</sub>-like phase is formed.

## Search for CoReS phase: XANES

After discussing sole Co and sole Re species separately, we can investigate the mixed species in the bimetallic catalysts. The spectra were recorded at the Co K and Re L edges simultaneously thanks to the edge jumping capability available at the ROCK beamline, which allowed us to directly follow their evolution.

Firstly, it should be noted that the presence of Co does not influence the spectra of Re in any manner: both the spectra in the initial state and final state are the same as without Co (Fig. S20). In addition, a Co neighbor of Re will be very difficult to observe in EXAFS at Re L<sub>3</sub> edge as its contribution will be overlapping with stronger signal of other Re atoms in the ReS<sub>2</sub> structure, present in a large excess. Therefore, to detect CoReS phase we focus on the Co K edge XANES/EXAFS.
<figure><img src="image_8.png" /><figcaption>Figure 7. XAS (XANES) spectra recorded at the Co K edge for sole Co and CoRe samples sulfided in (a) H₂S/H₂ and (b) H₂S/N₂ atmosphere.</figcaption></figure>

We compared low-loaded 0.3Co0.6Re/G, high-loaded 1Co10Re/G and γ-alumina supported catalyst, 1Co10Re/Al₂O₃ prepared by co-impregnation. The initial oxidic Co species (CoOₓ) that are obtained after adsorption of Co(acac)₂ for 0.3Co0.6Re/G and 1Co10Re/G are similar to those for Co/G, discussed above. XANES of the initial species for the alumina-supported reference resembles the spectrum of the Co(NO₃)₂ × 6H₂O precursor (Fig. S21a) [²²].

After sulfidation in different atmospheres XANES of the Co species expectedly has similar shape for three samples, as all spectra correspond to Co in the sulfide environment (Fig. 7). However, there are still some small yet significant differences which can be better demonstrated via the comparison with Co/G. In H₂S/H₂ atmosphere the XANES of CoRe samples is slightly different from that of Co/G H₂S/H₂: the slope is less steep just above the edge and the pre-edge is not located at exactly the same position (Fig. 7a). In H₂S/N₂ atmosphere the changes observed are slightly more prominent (Fig. 7b). The pre-edge in the samples supposedly containing CoReS is higher and the shoulder just above the edge is smoother (Fig. 7). Such differences indicate that there might be another phase in the Co-Re samples in addition to those obtained in monometallic Co/G.

FDMNES simulations and LCF
<figure><img src="image_9.png" /><figcaption>Figure 8. Possible structures of the CoReS cluster simulated in ORCA.</figcaption></figure>

To provide further insight into the state of Co in Co-Re sulfide samples, we have built several structural models and optimized them using the ORCA code. By analogy with CoMoS species we constructed a $ReS_x$ cluster and placed Co atoms at the edges. One or two Co atoms were inserted into the $ReS_x$ cluster mimicking an [1 0 0] edge that is preferentially exposed according to STEM (Fig. 8). We have chosen plausible configurations where the Co atoms might occupy tetrahedral (Fig. 8 (a)), pyramidal (Fig. 8 (c) and (d)) and octahedral (Fig. 8 (b) and (e)) sites. After optimizing these structures with DFT, we simulated their XANES spectra using parallel version of the FDMNES code [³⁶, ³⁷] in the finite difference mode, using SCF and at least 6 Å radius. The simulated spectra of CoReS structures and those of $Co_9S_8$ and $CoS_2$ are presented in Fig. 9, S22 - S24.

To unravel the difference between bimetallic CoRe and monometallic Co sulfide clusters in the Co/G and 1Co10Re/G experimental spectra, we replaced all Re atoms by Co in the model CoReS structures, thus obtaining isostructural monometallic $Co_xS_y$ clusters. Then the Co atoms in the $Co_xS_y$ clusters corresponding to the CoReS counterparts were selected as scatterers for XANES simulations (Fig. S23).

The geometry variations of Co site result in drastic changes in the FDMNES-simulated spectra: distorted tetrahedral site and pyramidal site expectedly possess a higher pre-edge than octahedral one and are very different above the edge (Fig. 9). Interestingly, if Re atoms are located in the second coordination shell of Co site, a feature just above the edge appears as two peaks while for monometallic Co references ($Co_9S_8$, $CoS_2$, $Co_xS_y$ with octahedral Co atom as absorber) this feature is a single peak
(Fig. S23). In the CoₓSᵧ cluster with tetrahedral Co absorber, this feature appears as two peaks, but the intensities are distributed differently.

<figure><img src="image_10.png" /><figcaption>**Figure 9.** Changes in simulated XANES spectra upon addition of sulfur atoms to Co site. Yellow balls correspond to S atoms, dark blue to Re and pink to Co.</figcaption></figure>

The next step was to compare simulated spectra with the experimental ones. However, direct comparison is difficult as the experimental spectra exhibit less prominent features due to the instrumental broadening including the divergence of the X-ray source and the lifetime of the excited core level [⁶⁵]. Most importantly, the simulated species are well-resolved as they correspond to single well-defined site configurations, whereas the experimental spectra correspond most probably to the mixtures of different states. To cope with this problem, we attenuated natural broadening by deconvoluting the experimental spectra using the Lorentzian function with σ of 2.4 (energy to pass to Lorentzian line shape). Such σ was chosen empirically because at this point the reference spectra begin to resemble those simulated by FDMNES. Due to deconvolution, the XAS features both before and after the edge are better resolved (Fig. 10). In addition, the differences between Co and CoReS experimental spectra become more prominent.

Secondly, we performed LCF (Linear Combination Fitting) to simulate our experimental spectra using as components the FDMNES - simulated XANES spectra. Utilization only of the simulated spectra of CoReS structures for LCF does not give a good fit as there is probably a large contribution of monometallic Co species, not involved in the interaction with Re atoms. Therefore, we used the deconvolved experimental spectrum of Co/G H₂S/H₂ or H₂S/N₂ as an LCF component and added various CoReS simulated spectra to explain the differences. Among the CoReS samples we have chosen 1Co10Re/G as a representative.
<figure><img src="image_11.png" /><figcaption>Figure 10. Deconvolved XANES spectra of 1Co10Re/G and Co/G sulfided in (a) H₂S/H₂ and (b) H₂S/N₂.</figcaption></figure>

The deconvolved experimental spectrum of 1Co10Re/G H₂S/H₂ (Fig. 11a, Table 2) can be best described with the contributions of the deconvolved experimental spectrum of Co/G H₂S/H₂ (weight 0.47) and of the calculated XANES spectra of CoReS sites with octahedral (0.20) and distorted tetrahedral coordination (0.33). In H₂S/N₂ atmosphere the contribution of CoReS phases is lower by around 10% according to LCF and both distorted tetrahedral (0.11) and octahedral (0.29) CoReS sites are necessary for a good fit (Fig. 11b, Table 2). 1Co10Re/G H₂S/N₂ contains a greater number of octahedral CoReS sites than 1Co10Re/G H₂S/H₂, which is expected from chemical viewpoint (extra sulfur removed by hydrogen). Pyramidal configuration, with two octahedral Co sites in ReSₓ cluster, simulated bulk CoS₂/Co₉S₈ references and CoₓSᵧ clusters were rejected by the LCF minimization in both atmospheres.

<figure><img src="image_12.png" /><figcaption>Figure 11. LCF fitting performed for CoReS samples: (a) 1Co10Re/G H₂S/H₂, (b) 1Co10Re/G H₂S/N₂.</figcaption></figure>

Even if the amount of sole Co species is important, addition of CoReS contribution significantly improves the LCF fit and in combination with the XPS results allows us to suggest their presence.
<table><caption>Table 2. LCF results for 1Co10Re H₂S/H₂ and 1Co10Re/G H₂S/N₂.</caption><thead><tr><th>Structure</th><th>weight</th><th>error</th><th>E₀, eV</th><th>error</th><th>R<sub>f</sub>, χ<sup>2</sup></th></tr></thead><tbody><tr><td colspan="6">1Co10Re/G H₂S/H₂</td></tr><tr><td>Co/G H₂S/H₂</td><td>0.48</td><td>0.04</td><td>-0.0538</td><td>0.0994</td><td rowspan="3">0.0047<br>0.128</td></tr><tr><td>Oct CoReS</td><td>0.20</td><td>0.02</td><td>0.1021</td><td>0.2261</td></tr><tr><td>Two tetr CoReS</td><td>0.33</td><td>0.03</td><td>0.7603</td><td>0.1865</td></tr><tr><td colspan="6">1Co10Re/G H₂S/N₂</td></tr><tr><td>Co/G H₂S/N₂</td><td>0.60</td><td>0.04</td><td>-0.0540</td><td>0.0000</td><td rowspan="3">0.0051<br>0.144</td></tr><tr><td>Oct CoReS</td><td>0.29</td><td>0.03</td><td>0.1020</td><td>0.2743</td></tr><tr><td>Tetr CoReS</td><td>0.11</td><td>0.02</td><td>0.0000</td><td>0.0000</td></tr></tbody></table>

### Search for CoReS phase: EXAFS

The CoReS phase was never characterized by EXAFS, so the Co-Re distance in it is unknown. Some CoRe complexes (containing P) were reported, in which the Co – Re distance is around 2.79 Å [⁶⁶] and a Re₃Co sulfide cluster complex [⁶⁷] with Co-Re bond length around 2.60 Å.

To facilitate the fitting procedure, we have simulated the FT-EXAFS of our CoReS clusters utilized for FDMNES and LCF. The distances between the absorbing Co atom and the closest scattering Re atoms vary from 2.67 to 3.3 – 3.4 Å and their oscillations are relatively intense (Fig. S24 – S27). However, according to LCF, the CoReS moieties coexist with comparable amounts of the monometallic CoSₓ phases, so a strong overlap between Co-Re and Co – Co oscillations is expected.

<figure><img src="image_13.png" /><figcaption>Figure 12. EXAFS spectra for CoReS catalysts sulfided in (a) H₂S/H₂ and (b) H₂S/N₂.</figcaption></figure>

After sulfidation in H₂S/H₂ at 350 °C and cooling down to room temperature the fitting of the first coordination shell can be performed with around 4.1 – 4.6 S atoms, indicating that the structure of Co site is close to tetrahedral (Fig. 12). As with the second coordination shell, the contribution of Co atom located at around 2.50 Å is required by fit, as in Co₉S₈ phase (though the CN is much lower than in the bulk sulfide). The contribution of Re neighbors at around 2.7 – 2.8 Å (or at 3.00 Å) can be added to improve the quality of the fit, although CN(Re) is quite low and is close to 0.5. The distances correspond well to FeFF results obtained for tetrahedral CoReS site, where Re neighbors appear at 2.67, 2.97, 3.32
Å. Overall, according to the fit, Co sites have tetrahedral geometry and possess a small number of metallic Co/Re neighbors.

<table><caption>**Table 3. EXAFS fitting results for CoRe samples sulfided in H₂S/H₂. Spectra recorded at room temperature.**</caption><thead><tr><th>Scatterer<br>atom</th><th>CN<sup>a</sup></th><th>R (Å)<sup>b</sup></th><th>σ<sup>2</sup> (Å<sup>2</sup>)<sup>c</sup></th><th>ΔE<sub>0</sub> (eV)<sup>d</sup></th></tr></thead><tbody><tr><td colspan="5" style="text-align: center; font-weight: bold;">0.3Co0.6Re/G H<sub>2</sub>S/H<sub>2</sub>, R=0.07</td></tr><tr><td>S</td><td>4.1(1)</td><td>2.19(1)</td><td>0.0030(1)</td><td>-2.5(1)</td></tr><tr><td>Co</td><td>1.0(1)</td><td>2.51(1)</td><td>0.0037(3)</td><td>-4.8(1)</td></tr><tr><td>Re</td><td>0.5(2)</td><td>2.78(2)</td><td>0.0020(1)</td><td>-4.7(2)</td></tr><tr><td colspan="5" style="text-align: center; font-weight: bold;">1Co10Re/G H<sub>2</sub>S/H<sub>2</sub>, R=0.11</td></tr><tr><td>S</td><td>4.6(1)</td><td>2.21(1)</td><td>0.0046(4)</td><td>2.2(1)</td></tr><tr><td>Co</td><td>0.4(1)</td><td>2.55(1)</td><td>0.0020(1)</td><td>-4.8(1)</td></tr><tr><td>Re</td><td>0.3(1)</td><td>2.99(1)</td><td>0.0020(1)</td><td>-4.9(1)</td></tr><tr><td colspan="5" style="text-align: center; font-weight: bold;">1Co10Re/Al<sub>2</sub>O<sub>3</sub>, R=0.06</td></tr><tr><td>S</td><td>4.2(1)</td><td>2.21(1)</td><td>0.0054(1)</td><td>0.7(1)</td></tr><tr><td>Co</td><td>0.7(1)</td><td>2.53(1)</td><td>0.0024(1)</td><td>-4.9(1)</td></tr><tr><td>Re</td><td>0.5(3)</td><td>2.71(1)</td><td>0.0044(10)</td><td>-4.7(1)</td></tr></tbody></table>

After sulfidation in H₂S/N₂ and cooling to room temperature, the Co atoms are coordinated with around six sulfur atoms in the first shell (Table 4). In the second shell Co atoms are present at 3.72 Å and 3.84 Å; the contribution of Re neighbor can be added (at around 2.70 Å, 3.00 Å) to improve the fit (Fig. 12b). It should be emphasized that according to FeFF for octahedral CoReS site there are more distant Re neighbors (at around 3.60 Å) but they cannot be distinguished due to the presence of cobalt. Note that the only form of the catalyst stable in the conditions of catalysis is that formed in H₂S/H₂, because the catalytic reaction is carried out under excess of hydrogen.

<table><caption>**Table 4. EXAFS fitting results for CoRe samples sulfided in H₂S/N₂. Spectra recorded at room temperature.**</caption><thead><tr><th>Scatterer<br>atom</th><th>CN<sup>a</sup></th><th>R (Å)<sup>b</sup></th><th>σ<sup>2</sup> (Å<sup>2</sup>)<sup>c</sup></th><th>ΔE<sub>0</sub> (eV)<sup>d</sup></th></tr></thead><tbody><tr><td colspan="5" style="text-align: center; font-weight: bold;">0.3Co0.6Re/G H<sub>2</sub>S/N<sub>2</sub> R=0.10</td></tr><tr><td>S</td><td>6.2(3)</td><td>2.24(1)</td><td>0.0045(10)</td><td>1.1(3)</td></tr><tr><td>Re</td><td>0.7(2)</td><td>3.00(6)</td><td>0.0020(1)</td><td>4.4(5)</td></tr><tr><td>Co(Re)</td><td>1.2(1)</td><td>3.65(2)</td><td>0.0020(1)</td><td>-4.8(0.1)</td></tr><tr><td>Co</td><td>1.4(2)</td><td>3.81(1)</td><td>0.0020(1)</td><td>-4.8(0.1)</td></tr><tr><td colspan="5" style="text-align: center; font-weight: bold;">1Co10Re/G H<sub>2</sub>S/N<sub>2</sub> R=0.08</td></tr><tr><td>S</td><td>6.1(2)</td><td>2.24(1)</td><td>0.0059(6)</td><td>2.2(2)</td></tr><tr><td>Re</td><td>0.4(1)</td><td>2.74(1)</td><td>0.0036(16)</td><td>-3.3(2)</td></tr><tr><td>Co(Re)</td><td>0.8(1)</td><td>3.75(1)</td><td>0.0020(1)</td><td>-4.8(1)</td></tr><tr><td>Co</td><td>1.2(1)</td><td>3.85(4)</td><td>0.0031(9)</td><td>-4.8(1)</td></tr><tr><td colspan="5" style="text-align: center; font-weight: bold;">1Co10Re/Al<sub>2</sub>O<sub>3</sub> H<sub>2</sub>S/N<sub>2</sub> R=0.09</td></tr><tr><td>S</td><td>6.3(1)</td><td>2.25(1)</td><td>0.0054(5)</td><td>1.1(1)</td></tr><tr><td>Re</td><td>0.6(1)</td><td>3.01(1)</td><td>0.0020(1)</td><td>-2.8(1)</td></tr><tr><td>Co(Re)</td><td>0.7(1)</td><td>3.75(1)</td><td>0.0020(1)</td><td>-4.8(1)</td></tr></tbody></table>

<footnote>a coordination number, using $S_0^2 = 0.7$; b radial distance to the neighboring atom, c distance variance (free variable between 0.0020 and 0.0090); d energy variance (free variable between -5 and 5 eV), R-factor – minimized function of the fitting parameters, characterizes the goodness of the fit.</footnote>
<table><tr><td>Co</td><td>1.7(1)</td><td>3.84(1)</td><td>0.0080(8)</td><td>-4.8(1)</td></tr></table>

a coordination number, using $S_0^2 = 0.7$; b radial distance to the neighboring atom, c distance variance (free variable between 0.0020 and 0.0090); d energy variance (free variable between -5 and 5 eV), R-factor – minimized function of the fitting parameters, characterizes the goodness of the fit.

In conclusion, the combination of XPS and XAS supported by FDMNES/LCF suggests that alongside sulfidic $CoS_x$ phases a significant part of Co is involved in the interaction with $ReS_2$. In $H_2S/N_2$ the majority of Co sites embedded in $ReS_2$ appear to be in octahedral coordination, while in $H_2S/H_2$ the coordination is distorted tetrahedral one. We suppose that such simulated $CoReS$ structures can represent the real ones that are demonstrating high catalytic activity.

### Thiophene HDS

<figure><img src="image_14.png" /><figcaption>Figure 13. (a) Comparison of $CoRe$ and $Re$ catalysts supported on graphene and sulfided in $H_2S/N_2$, the insert demonstrates the reaction scheme, (b) Comparison of $CoRe$ catalysts with reference 0.7Co0.7Mo/G $H_2S/H_2$ and 3Co10Mo/Al$_2$O$_3$ $H_2S/H_2$.</figcaption></figure>

As the Co-Re synergy and the influence of activation atmosphere are in focus of this study, the activity of the systems at stake was evaluated in the model thiophene HDS reaction and compared with the benchmark references (Mo and $CoMo$ systems on the same “graphene” carbon support and on commercial alumina).

$CoRe$ sulfide catalysts were highly active in model thiophene HDS reaction (Fig. S28). Arrhenius plots show good linearity attesting the kinetic control of the reaction (Fig. S29). Steady state reaction rates are reported on Fig. 13, S28. The $Co(acac)_2$-based preparation technique results in $CoReS$ catalysts with HDS rate higher than $14 \times 10^{-3}$ mol/(mol of metal)/s at 320 °C, which is close to the best ultradispersed $CoMoS$ catalyst reported previously ($18 \times 10^{-8}$ mol/(mol of metal)/s) [42]. The promotion factor with respect to the catalyst without Co is around 1.8 – 2.0 both in $H_2S/N_2$ and $H_2S/H_2$ (Fig. S28) at 320 °C, which is lower than the best reported for the HDS of gas oil on unsupported catalysts (around 4 at 325 °C) [9]. However, our catalysts were not optimized with respect of the Co/Re ratio, because the main focus was on the detection of $CoReS$ phase, so an excess of Re was applied. Indeed, in our catalysts Co/Re atomic ratio is 0.28 while the optimal value reported was near 0.5, therefore they demonstrate
somewhat lower HDS activity. In any case a significant synergy was observed in the HDS process, in line with formation of Co-Re-S species.

The sulfidation atmosphere obviously influences the HDS activity: for all the samples it is higher for the solids activated under H₂S/N₂ mixture vs the counterparts activated in H₂S/H₂. The effect of atmosphere is moderate for the carbon-supported catalysts (both low- and high-loaded samples) and at some temperatures it goes barely beyond the accuracy of experiment. Sulfide phases containing only Co do not contribute significantly to the overall activity, therefore, their influence can be ruled out. As we clearly see that ReS₂ is the final product for both activation ways, the moderate differences of activity could be attributed to the morphology of the ReS₂. Among other factors such as dispersion, the activity difference can be attributed to the abundance of sulfur present in disulfide bridges that can be activated during the reaction. The same trend was observed before in [²⁴] for ReS₂/Al₂O₃, where the authors suggested that sulfidation in H₂S/N₂ creates a larger sulfidation degree and also that in H₂S/H₂ the reduction to metallic rhenium competes with sulfidation. By means of XPS/XAS we did not observe any metallic rhenium, only ReS₂, which means that this hypothesis can be ruled out.

The samples supported on carbon are several times more active vs the alumina-supported counterpart. The differences between CoRe/G and CoRe/Al₂O₃ can be explained not only by intrinsic (chemical) support effect, but also by the much higher dispersion of ReS₂ particles on graphene resulting in more abundant active sites. Indeed, on TEM images of the alumina-supported sample, ReS₂ particles are longer and have higher stacking degree.

On Fig.S30-F31 are shown the selectivities of the products calculated at 330 °C. A non-negligible amount of light products appears in the system for 0.3Co0.6Re/G along with typical HDS products: butenes and butane [⁶⁸]. The high-loaded samples are similar in terms of selectivity; however, 1Co10Re/G possesses higher hydrogenating ability than 1Co10Re/Al₂O₃, probably due to the presence of higher amount of disulfide bridges on the catalyst surface.

*Hydrogen evolution reaction*

<figure><img src="image_15.png" /><figcaption>**Figure 14.** (a) LSV curves: (a) 0.3Co0.6Re/G H₂S/N₂ catalyst; (b) Comparison of 0.3Co0.6Re/G H₂S/N₂ with monometallic catalysts.</figcaption></figure>
The goal of this brief study was to check whether the Co-Re synergy observed in HDS, persists in the electrochemical HER process. In line with HDS section, we have tested the low-loaded 0.3Co0.6Re/G H₂S/N₂ in HER in order to estimate if there is some promotion. For 0.6Re/G H₂S/N₂ the onset potential is around 210 mV, the current density of 10 mA/cm² (η₁₀) can be achieved at overpotential of 450 and the Tafel slope is around 186 mV/dec (Fig. S32a). Co/G H₂S/N₂ also showed non-negligible activity in HER with the onset of 306 mV, η₁₀ of 450 mV and surprisingly low Tafel slope of 99 mV/dec (Fig. S32b). However, it should be considered that the quantity of Co in Co/G H₂S/N₂ is higher than in 0.3Co0.6Re/G H₂S/N₂.

Upon modification of 0.6Re/G H₂S/N₂ with Co the catalytic activity is improved: the onset is similar to that for 0.6Re/G H₂S/N₂ (around 200 mV), the value of η₁₀ decreases to 403 mV and the Tafel slope to 125 mV/dec (Fig. 14a). The currents observed in the range of the applied potentials increase significantly, thus, at 550 mV a current of 50 mA/cm² can be obtained, two times higher than that of sole Re. According to chronoamperometric studies CoReS catalyst loses some of HER activity during the first minutes of the reaction and stabilizes at 2 mA/cm² over time (Fig. S33).

Therefore, the bimetallic system enhances HER performance compared to monometallic Re or Co, mirroring synergies observed in HDS. The improved Tafel slope and lower η₁₀ suggest Co modifies Re's HER pathway, possibly optimizing hydrogen adsorption/desorption steps. These results hint at a broader applicability of HDS-active bimetallic sulfides (like CoRe) in HER, which could guide the design of cost-effective, non-precious-metal HER catalysts. However, the stability issues and mechanistic details warrant deeper investigation.

## Conclusions

In this work, for the first time, we have provided evidence of the existence of a CoReS phase, that is highly active in reactions involving hydrogen. A considerable boost of catalytic activity in gas-phase HDS and in liquid-phase HER is observed if Re sulfide catalysts are modified by Co. The sulfidation atmosphere (H₂S/N₂ or H₂S/H₂) does not influence the state of Re, as in both cases ReS₂ is formed. However, activation atmosphere has a great impact on the state of Co. In both cases, the sulfidation of supported oxidic Re species proceeds via oxysulfidic ReOₓSᵧ intermediate and results in formation of well-dispersed ReS₂ and ReSₓ clusters, while oxidic Co transforms into highly dispersed CoS₂-like species in H₂S/N₂ and Co₉S₈-like species in H₂S/H₂. In presence of Re, in addition to phases containing only Co, the contribution of the CoReS phase arises: according to LCF distorted tetrahedral or octahedral Co sites surrounded with Re atoms are formed. These sites are likely contributing to the increased HDS/HER activity.

Beside evidencing the CoReS species, we resolved some controversies concerning the Re sulfide systems. First, *operando* XAS clearly shows that sulfidation leads to the same ReS₂ phase whatever the sulfidation atmosphere. Indeed, sulfidation in H₂S/N₂ leads to somewhat more active
systems, but this occurs probably due to the morphological advantage and the presence of the abundant disulfide bridges. Such effects are well known and were observed previously for the related $MoS_2$ – based catalysts. Second, the “spherical particles”, reported in several earlier papers, are not a primary sulfidation product, but a product of $ReS_2$ degradation by the microscope electron beam, as we directly observed during the TEM experiments. This easy degradation is probably related to easier reducibility of $ReS_2$ vs. other lamellar sulfides such as $MoS_2$ or $WS_2$.

References

<footnote n="1">¹ J. Parsons, M. Alotaibi, The Application of Transition Metal Sulfide Nanomaterials and Their Composite Nanomaterials in the Electrocatalytic Reduction of $CO_2$: A Review, Appl. Sci. 13(5) (2023) 3023. https://doi.org/10.3390/app13053023</footnote>

<footnote n="2">² M. Wang, L. Zhang, Y. He, H. Zhu, Recent advances in transition-metal-sulfide-based bifunctional electrocatalysts for overall water splitting, J. Mater. Chem. A. 9 (2021) 5320-5363. https://doi.org/10.1039/D0TA12152E</footnote>

<footnote n="3">³ T. Weber, R. Prins, R. A. van Santen, Transition Metal Sulphides Chemistry and Catalysis, first ed., Springer Dordrecht, 1997 https://doi.org/10.1007/978-94-017-3577-3</footnote>

<footnote n="4">⁴ Q. Zhu, Q. Xu, M. Du, M. Du, X. Zeng, G. Zhong, B. Qiu, J. Zhang, Recent Progress of Metal Sulfide Photocatalysts for Solar Energy Conversion, Adv. Mater. 34 (2022) 2202929. https://doi.org/10.1002/adma.202202929</footnote>

<footnote n="5">⁵ T.A. Pecoraro, R.R. Chianelli, Hydrodesulfurization Catalysis by Transition Metal Sulfides, J. Catal. 67 (1981) 430-445. https://doi.org/10.1016/0021-9517(81)90303-1</footnote>

<footnote n="6">⁶ J. P. R. Vissers, C. K. Groot, E. M. Van Oers, V. H. J. De Beer, R. Prins, Carbon-Supported Transition Metal Sulfides, Bull. Soc. Chim. Belg. 93 (1984) 813-822. https://doi.org/10.1002/bscb.19840930822</footnote>

<footnote n="7">⁷ M. Ledoux, The influence of sulfide structures on the hydrodesulfurization activity of carbon-supported catalysts, J. Catal. 102(2) (1986) 275-288. doi:10.1016/0021-9517(86)90164-8</footnote>

<footnote n="8">⁸ C. Sepúlveda, R. García, P. Reyes, I. T. Ghampson, J. L. G. Fierro, D. Laurenti, M. Vrinat, N. Escalona, Hydrodeoxygenation of guaiacol over $ReS_2$/activated carbon catalysts. Support and Re-loading effect, Appl. Catal. A: Gen. 475 (2014) 427-437. https://doi.org/10.1016/j.apcata.2014.01.057</footnote>

<footnote n="9">⁹ N. Escalona, J. Ojeda, P. Baeza, R. García, J. M. Palacios, J. L. G. Fierro, A. Lopez Agudo, F. J. Gil Llambías, Synergism between unsupported Re and Co or Ni sulfide catalysts in the HDS and HDN of gas oil, Appl. Catal. B. 287 (2005) 47-53. https://doi.org/10.1016/j.apcata.2005.03.019</footnote>

<footnote n="10">¹⁰ J. Quartararo, S. Mignard, S. Kasztelan, Hydrodesulfurization and hydrogenation activities of alumina-supported transition metal sulfides, J. Catal. 192(2) (2000) 307-315. doi:10.1006/jcat.2000.2825</footnote>

<footnote n="11">¹¹ P. Sirous-Rezaei, D. Creaser, L. Olsson, Reductive liquefaction of lignin to monocyclic hydrocarbons: $ReS_2/Al_2O_3$ as efficient char inhibitor and hydrodeoxygenation catalyst, Appl. Catal. B: Environ. 297 (2021) 120449. https://doi.org/10.1016/j.apcatb.2021.120449</footnote>

<footnote n="12">¹² Y. Zhou, E. Song, J. Zhou, J. Lin, R. Ma, Y. Wang, W. Qiu, R. Shen, K. Suenaga, Q. Liu, J. Wang, Z. Liu, J. Liu, Auto-optimizing Hydrogen Evolution Catalytic Activity of $ReS_2$ through Intrinsic Charge Engineering, ACS Nano. 12 (2018) 4486-4493. https://doi.org/10.1021/acsnano.8b00693</footnote>

<footnote n="13">¹³ Q. Sun, B. Zhang, L. Diao, B. Chen, K. Song, L. Ma, F. He, Engineering the electronic structure of $1T'-ReS_2$ through nitrogen implantation for enhanced alkaline hydrogen evolution, J. Mater. Chem. A. 8 (2020) 11607-11616. https://doi.org/10.1039/D0TA02683B</footnote>

<footnote n="14">¹⁴ F. Wang, J. Hu, R. Liang, W. Lei, Z. Lou, X. Pan, B. Lu, Z. Ye, Novel $ReS_2$/g-$C_3N_4$ heterojunction photocatalyst formed by electrostatic self-assembly with increased $H_2$ production, Int. J. Hydrog. Energy. 47(68) (2022) 29284-29294. https://doi.org/10.1016/j.ijhydene.2022.06.251</footnote>

<footnote n="15">¹⁵ J. Xu, W. Zhong, F. Chen, X. Wang, H. Yu, In situ cascade growth-induced strong coupling effect toward efficient photocatalytic hydrogen evolution of $ReS_2$/ZnIn₂S₄, Appl. Catal. B: Environ. 328 (2023) 122493. https://doi.org/10.1016/j.apcatb.2023.122493</footnote>
¹⁶ M. Rahman, K. Davey, S. - Z. Qiao, Advent of 2D Rhenium Disulfide (ReS₂): Fundamentals to Applications, Adv. Funct. Mater. 27(10) (2017) 1606129. doi:10.1002/adfm.201606129

¹⁷ H. Topsøe, B. Hinnemann, J.K. Nørskov, J. V. Lauritsen, F. Besenbacher, P.L. Hansen, G. Hytoft, R.G. Egeberg, K.G. Knudsen, The role of reaction pathways and support interactions in the development of high activity hydrotreating catalysts, Catal. Today, 107 – 108 (2005) 12 – 22. https://doi.org/10.1016/j.cattod.2005.07.165

¹⁸ P.G. Moses, B. Hinnemann, H. Topsøe, J.K. Nørskov, The effect of Co-promotion on MoS₂ catalysts for hydrodesulfurization of thiophene: A density functional study, J. Catal. 268 (2009) 201–208. https://doi.org/10.1016/j.jcat.2009.09.016

¹⁹ J. V. Lauritsen, J. Kibsgaard, G.H. Olesen, P.G. Moses, B. Hinnemann, S. Helveg, J.K. Nørskov, B.S. Clausen, H. Topsøe, E. Lægsgaard, F. Besenbacher, Location and coordination of promoter atoms in Co- and Ni-promoted MoS₂-based hydrotreating catalysts, J. Catal. 249 (2007) 220–233. https://doi.org/10.1016/j.jcat.2007.04.013

²⁰ N. Escalona, Ph. Doctor Thesis, Universidad de Santiago de Chile (2002)

²¹ R. Chen, Y. Ao, C. Wang, P. Wang, The surface engineering of ReS₂ with cobalt for efficient performance in hydrogen evolution under both acid and alkaline conditions, Chem. Comm. 56 (2020) 8472–8475. https://doi.org/10.1039/d0cc01300e

²² L. Plais, C. Lancelot, C. Lamonier, E. Payen, V. Briois, First in situ temperature quantification of CoMoS species upon gas sulfidation enabled by new insight on cobalt sulfide formation, Catal. Today. 377 (2021) 114–126. https://doi.org/10.1016/j.cattod.2020.06.065.

²³ A. N. Startsev, V. N. Rodin, V.I. Zaikovskii, A. V. Kalinkin, V. V. Kriventsov, D. I. Kochubei, Silica-supported Sulfide Catalysts: IX. Synthesis and Properties of Structural Analogs of the Active Component of Hydrodesulfurization Catalysts, Kinetics and Catalysis. 38(4) (1997) 598-606.

²⁴ N. Escalona, F.J. Gil Llambias, M. Vrinat, T.S. Nguyen, D. Laurenti, A. López Agudo, Highly active ReS₂/γ-Al₂O₃ catalysts: Effect of calcination and activation over thiophene hydrodesulfurization, Catalysis Communications. 8 (2007) 285–288. https://doi.org/10.1016/j.catcom.2006.05.053.

²⁵ G. A. Seisenbaeva, S. Gohil, K. Jansson, K. Herbst, M. Brorson, V.G. Kessler, Solution interaction of O-donor ligand metal complexes with thiocarbonyl compounds—a new general route to metal sulfide materials, New J. Chem. 27 (2003) 1059–1064. https://doi.org/10.1039/B300398C.

²⁶ I. Bezverkhyy, P. Afanasiev, M. Lacroix, Promotion of highly loaded MoS₂/Al₂O₃ hydrodesulfurization catalysts prepared in aqueous solution, J. Catal. 230 (2005) 133–139. https://doi.org/10.1016/j.jcat.2004.12.009

²⁷ V. Briois, C. La Fontaine, S. Belin, L. Barthe, T. Moreno, V. Pinty, A. Carcy, R. Girardot, E. Fonda, ROCK: the new Quick-EXAFS beamline at SOLEIL, J. Phys.: Conf. Ser. 712 (2016) art.n° 012149. 10.1088/1742-6596/712/1/012149

²⁸ E. Fonda, A. Rochet, M. Ribbens, L. Barthe, S. Belin, V. Briois, The SAMBA quick-EXAFS monochromator: XAS with edge jumping, J. Synchrotron Rad., 19(3) (2012) 417-424. https://doi.org/10.1107/S0909049512009703

²⁹ A. L. Ankudinov, B. Ravel, J. J. Rehr, S. D. Conradson, Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure, Phys. Rev. B. 58 (1998) 7565–7576. https://doi.org/10.1103/PhysRevB.58.7565

³⁰ K.V. Klementev, Extraction of the fine structure from x-ray absorption spectra, J. Phys. D: Appl. Phys. 34 (2000) 209–217. 10.1088/0022-3727/34/2/309

³¹ C. Lesage, E. Devers, C. Legens, G. Fernandes, O. Roudenko, V. Briois, High pressure cell for edge Jumping X-ray absorption spectroscopy: Applications to industrial liquid sulfidation of hydrotreatment catalysts, Catal. Today. 336 (2019) 63 – 73. https://doi.org/10.1016/j.cattod.2019.01.081

³² L. Plais, Catalyseurs d'hydrotraitement Mo/Al₂O₃ doublement promus prepares à partir d'hétéropolyanions d'Anderson: Suivi de la sulfuration par Quick-EXAFS, PhD thesis, University of Lille, 2017

³³ F. Neese, Software update: The ORCA program system—Version 5.0, Wiley Interdiscip. Rev: Comput. Mol. Sci. 12(5) (2022) e1606

³⁴ A. D. Becke, Density-functional exchange-energy approximation with correct asymptotic behavior, Phys Rev A. 38 (1988) 3098–3100. doi: 10.1103/PhysRevA.38.3098

³⁵ A. Hellweg, D. Rappoport, Development of new auxiliary basis functions of the Karlsruhe segmented contracted basis sets including diffuse basis functions (def2-SVPD, def2-TZVPPD, and def2-QVPPD)
for RI-MP2 and RI-CC calculations, Phys. Chem. Chem. Phys. 17 (2015) 1010-1017.
https://doi.org/10.1039/C4CP04286G

³⁶ https://fdmnes.neel.cnrs.fr/

³⁷ Y. Joly, O. Bunău, J. E. Lorenzo, R. M. Galéra, S. Grenier and B. Thompson, Self-consistency, spin-orbit and other advances in the FDMNES code to simulate XANES and RXD experiments, J. Phys.: Conf. Ser. 190 (2009) 012007. 10.1088/1742-6596/190/1/012007

³⁸ M. O. Krause, J. H. Oliver, Natural widths of atomic K and L levels, Kα X-ray lines and several KLL Auger lines J. Phys. Chem. Ref. Data 8 (1979) 329 – 338. https://doi.org/10.1063/1.555595

³⁹ B. Ravel, M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT, J. of Synchrotron Radiat. 12 (2005) 537 – 541.
doi:10.1107/S0909049505012719

⁴⁰ N. Escalona, M. Vrinat, D. Laurenti, F. J. Gil Llambías, Rhenium sulfide in hydrotreating. Appl. Catal. A: Gen. 322 (2007) 113 – 120. doi:10.1016/j.apcata.2007.01.019

⁴¹ D. Ryaboshapka, T. Len, P. Bargiela, M. Aouine, C. Geantet, V. Briois, L. Piccolo, P. Afanasiev, Genesis of the active phase and support effect in ultradispersed Mo sulfide catalysts, ACS Catal. 2023, 13, 15, 10511 – 10526. https://doi.org/10.1021/acscatal.3c01672

⁴² D. Ryaboshapka, L. Piccolo, M. Aouine, P. Bargiela, V. Briois, P. Afanasiev, Ultradispersed (Co)Mo catalysts with high hydrodesulfurization activity, Appl. Catal. B: Environ. 302 (2022) 120831.
https://doi.org/10.1016/j.apcatb.2021.120831

⁴³ Y. Feng, W. Zhou, Y. Wang, J. Zhou, E. Liu, Y. Fu, Z. Ni, X. Wu, H. Yuan, F. Miao, W. 1+, D. Xing, Raman vibrational spectra of bulk to monolayer ReS₂ with lower symmetry Phys. Rev. B. 92 (2015) 054110. https://doi.org/10.1103/PhysRevB.92.054110

⁴⁴ D. Laurenti, K. T. N. Thi, N. Escalona, L. Massin, M. Vrinat, F. J. G. Llambías, Support effect with rhenium sulfide catalysts, Catal. Today. 130(1) (2008) 50 – 55. 10.1016/j.cattod.2007.07.022

⁴⁵ J. Wang, J. He, G. O. Odunmbaku, S. Zhao, Q. Gou, G. Han, C. Xu, T. Frauenheim, M. Li, Regulating the electronic structure of ReS₂ by Mo doping for electrocatalysis and lithium storage, Chem. Eng. J. 414 (2021) 128811. https://doi.org/10.1016/j.cej.2021.128811

⁴⁶ P. Afanasiev, C. Lorentz, Oxidation of Nanodispersed MoS₂ in Ambient Air: The Products and the Mechanistic Steps, J. Phys. Chem. C. 123(12) (2019) 7486 – 7494. doi:10.1021/acs.jpcc.9b01682

⁴⁷ C. Sepulveda, R. Garcia, N. Escalona, D. Laurenti, L. Massin, M. Vrinat, Unexpected Support Effect in Hydrotreating: Evidence of a Metallic Character for ReS₂/Al₂O₃ and ReS₂/SiO₂ Catalysts, Catal. Lett. 141 (2011) 987 – 995. https://doi.org/10.1007/s10562-011-0578-2

⁴⁸ S. M. Davis, Photoemission studies of rhenium disulfide oxidation: Altered core-level structure and reactivity of defect sites. Catal. Lett. 2(1) (1989) 1 – 7. doi:10.1007/bf00765324

⁴⁹ J. A. Aliaga, T. Zepeda, J. F. Araya, F. Paraguay-Delgado, E. Benavente, G. Alonso-Núñez, S. Fuentes, G. González, Low-Dimensional ReS₂/C Composite as Effective Hydrodesulfurization Catalyst, Catalysts. 7 (2017) 377. https://doi.org/10.3390/catal7120377

⁵⁰ R. Bassm, M. Villarroel, F. J. Gil Llambías, P. Baeza, J. L. García-Fierro, N. Martinez, P. Olivera, K. Leiva, N. Escalona, Support effect on conversion on conversion of quinoline over ReS₂ catalyst. J. Chil. Chem. Soc. 61(4) (2016) 3170 – 3176. doi:10.4067/s0717-97072016000400004

⁵¹ F. Chen, F. Yang, S. Che, H. Liu, N. Chen, Z. Wu, Y. Li, Titanium Carbide Composite Hollow Cobalt Sulfide Heterojunction with Function of Promoting Electron Migration for Efficiency Photo-Assisted Electro-Fenton Cathode, Catalysts. 13(2) (2023) 253. https://doi.org/10.3390/catal13020253

⁵² D. Briggs, V. A. Gibson, Direct observation of multiplet splitting in 2P photoelectron peaks of cobalt complexes. Chem. Phys. Lett. 25(4) (1974) 493 – 496. doi:10.1016/0009-2614(74)85350-9

⁵³ I. Alstrup, Mössbauer emission spectroscopy study of the state of cobalt in sulfided, supported, and unsupported Co Mo catalysts, J. Catal. 77(2) (1982) 397–409. doi:10.1016/0021-9517(82)90181-6

⁵⁴ A. D. Gandubert, C. Legens, D. Guillaume, S. Rebours, E. Payen, X-ray Photoelectron Spectroscopy Surface Quantification of Sulfided CoMoP Catalysts-Relation Between Activity and Promoted Sites-Part I: Influence of the Co/Mo Ratio X-ray Photoelectron Spectroscopy Surface Quantification of Sulfided CoMoP Catalysts Relation Between Activity and Promoted Sites Part I: Influence of the Co/Mo Ratio, Oil & Gas Sci. Technol.-Rev. IFP. 62 (2007) 79 – 89.
https://doi.org/10.2516/ogst:2007007i
<footnote n="55">⁵⁵ I. Bezverkhyy, P. Afanasiev, M. Danot, Preparation of Highly Dispersed Pentlandites (M, M')₉S₈ (M, M' = Fe, Co, Ni) and Their Catalytic Properties in Hydrodesulfurization, J. Phys. Chem. B. 108(23) (2004) 7709–7715. https://doi.org/10.1021/jp049692u</footnote>

<footnote n="56">⁵⁶ G. H. Major, N. Farley, P. M. A. Sherwood, M. R. Linford, J. Terry, V. Fernandez, K. Artyushkova, Practical guide for curve fitting in x-ray photoelectron spectroscopy, J. Vac. Sci. Technol. A. 38 (2020) 061203. https://doi.org/10.1116/6.0000377</footnote>

<footnote n="57">⁵⁷ E. Butanovs, A. Kuzmin, S. Piskunov, K. Smits, A. Kalinko, B. Polyakov, Synthesis and characterization of GaN/ReS₂, ZnS/ReS₂ and ZnO/ReS₂ core/shell nanowire heterostructures, Appl. Surf. Sci. 536 (2021) 147841. https://doi.org/10.1016/j.apsusc.2020.147841</footnote>

<footnote n="58">⁵⁸ D. E. Schwarz, A. I. Frenkel, R. G. Nuzzo, T. B. Rauchfuss, A. Vairavamurthy, Electrosynthesis of ReS₄. XAS Analysis of ReS₂, Re₂S₇, and ReS₄, Chem Mater. 16(1) (2004) 151 – 158. https://doi.org/10.1021/cm034467v</footnote>

<footnote n="59">⁵⁹ A. Tougerti, S. Cristol, E. Berrier, V. Briois, C. La Fontaine, F. Villain, Y. Joly, XANES study of rhenium oxide compounds at the L₁ and L₃ absorption edges, Phys. Rev. B. 85 (1998) 125136. https://doi.org/10.1103/PhysRevB.85.125136i</footnote>

<footnote n="60">⁶⁰ W. W. Lukens, D. A. McKeown, A. C. Buechele, I. S. Muller, D. K. Shun, I. L. Pegg, Dissimilar behavior of technetium and rhenium in borosilicate waste glass as determined by X-ray absorption spectroscopy, Chem. Mat. 19 (2007) 559–566. https://doi.org/10.1021/cm0622001</footnote>

<footnote n="61">⁶¹ S. R. Bare, S. D. Kelly, F. D. vila, E. Boldingh, E. Karapetrova, J. Kas, G. E. Mickelson, F. S. Modica, N. Yang, J. J. Rehr, Experimental (XAS, STEM, TPR, and XPS) and theoretical (DFT) characterization of supported rhenium catalysts, J. Phys. Chem. C. 115 (2011) 5740–5755. https://doi.org/10.1021/jp1105218</footnote>

<footnote n="62">⁶² P. Arnoldy, J.A.M. Van Den Heijkant, V.H.J. De Beerb, A. Moulijn, Temperature-programmed sulfiding of Re₂O₇/Al₂O₃ catalysts, Appl. Catal. 23 (1986) 81 – 99. https://doi.org/10.1016/S0166-9834(00)81454-2</footnote>

<footnote n="63">⁶³ A. A. Danopoulos, P. Braunstein, K. Yu. Monakhov, J. van Leusen, P. Kögerler, M. Clémancey, J.-M. Latour, A. Benayad, M. Tromp, E. Rezabal, G. Frison, Heteroleptic, two-coordinate [M(NHC){N(SiMe₃)₂}] (M = Co, Fe) complexes: synthesis, reactivity and magnetism rationalized by an unexpected metal oxidation state, Dalton Trans. 46 (2017) 1163 – 1171. https://doi.org/10.1039/C6DT03565E</footnote>

<footnote n="64">⁶⁴ M. Fronzi, M. Hussein, N. Assadi, M. J. Ford, Ab Initio Investigation of Water Adsorption and Hydrogen Evolution on Co₉S₈ and Co₃S₄ Low-Index Surfaces, ACS Omega. 3(9) (2018) 12215–12228. https://doi.org/10.1021/acsomega.8b00989</footnote>

<footnote n="65">⁶⁵ https://xraypy.github.io/xraylarch/xafs_preedge.html</footnote>

<footnote n="66">⁶⁶ U. Flörke, H.-J. Haupt, The First Dinuclear Cobalt-Rhenium Cluster Compound: Hexacarbonylbis(~dicyclohexylphosphanido) cobalt rhenium, Acta Cryst. C49 (1993) 1906 – 1908. https://doi.org/10.1107/S0108270193005013</footnote>

<footnote n="67">⁶⁷ N. Sakai, T. Saito, Synthesis and structures of tetranuclear rhenium–cobalt mixed metal sulfide clusters [Re₃CoS₄Cl₆(PMe₂Ph)₄]. Polyhedron. 23(17) (2004) 2611 – 2614. doi:10.1016/j.poly.2004.06.028</footnote>

<footnote n="68">⁶⁸ P. G. Moses, B. Hinnemann, H. Topsøe, J. K. Nørskov, The hydrogenation and direct desulfurization reaction pathway in thiophene hydrodesulfurization over MoS₂ catalysts at realistic conditions: A density functional study, J. Catal. 248 (2007) 188–203. https://doi.org/10.1016/j.jcat.2007.02.028</footnote>