# Operando X-ray Absorption Study of TiO2-supported Au, Cu, and Au–Cu Catalysts for Butadiene Selective Hydrogenation

Abdallah Nassereddine, Laurent Delannoy, Xavier Carrier, Catherine Louis, Camille La Fontaine, Anthony Beauvois, Jaysen Nelayah, Axel Wilson

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Abdallah Nassereddine, Laurent Delannoy, Xavier Carrier, Catherine Louis, Camille La Fontaine, et al. Operando X-ray Absorption Study of TiO2-supported Au, Cu, and Au–Cu Catalysts for Butadiene Selective Hydrogenation. ChemCatChem, 2025, 17 (13), pp.e202500379. 10.1002/cctc.202500379. hal-05068934

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# Operando X-ray Absorption Study of TiO₂-Supported Au, Cu, and Au-Cu Catalysts for Butadiene Selective Hydrogenation

Abdallah Nassereddine,*[a, b] Laurent Delannoy,[c] Xavier Carrier,[c] Catherine Louis,[c] Camille La Fontaine,[d] Anthony Beauvois,[d] Jaysen Nelayah,[b] and Axel Wilson*[c]

The selective hydrogenation of unsaturated hydrocarbons is essential for the production of alkenes used in polymerization processes. Au-based catalysts offer greater selectivity than Pd but are limited by poor hydrogen activation. Alloying Au with Cu offers a promising approach to enhance catalytic performance by improving activity while retaining excellent selectivity. The structural dynamics of TiO₂-supported monometallic (Au, Cu) and bimetallic (AuCu, AuCu₃) catalysts were investigated during the reduction step where metal nanoparticles are structured, and the selective hydrogenation of butadiene, using operando X-ray absorption spectroscopy (XAS) and ex situ scanning transmission electron microscopy (STEM). XANES investiga-

tion shows that the temperature at which both metal precursors are reduced during the reduction step is strongly influenced by the alloy composition. This is explained by a Au⁰ assisted reduction of Cu²⁺ species mediated by H₂ spillover. EXAFS shows that Au maintains its metallic state during the catalytic reaction, while Cu shows minor electronic changes without forming segregated CuO phases. EXAFS also indicate a synergistic effect in AuCu/TiO₂ catalyst. STEM-HAADF investigation reveals that the crystalline structure of AuCu nanoparticles after reaction is more robust against size-dependent structural changes compared to pure Au, suggesting that alloying Cu with Au plays a role in the nanoparticle's stability.

## 1. Introduction

Selective hydrogenation of unsaturated hydrocarbons, particularly polyenes, alkadienes, and alkynes, plays a key role in the chemical industry. For instance, polymerization and hydroformylation reactions require high-purity alkenes (monomers).[1] However, catalytic cracking of petroleum at high temperatures induces the formation of impurities such as alkynes and alkadienes.[2] For example, C₄-cut hydrocarbons obtained from steam cracking can contain between 0.3% and 6% butadiene impurities.[3-5] Even in low proportions, these butadiene impurities have the potential to poison catalysts employed in alkene polymerization reactions. The selective hydrogenation

of butadiene is crucial for two reasons. Firstly, it purifies C₄ cuts, and secondly, it produces high-purity butenes, which are important raw materials for polymer production and alkylation processes.[6-9] The choice of the catalyst for the selective hydrogenation reaction of alkynes/dienes must take into consideration several factors: activity, selectivity and stability during reaction. Historically, nickel was the first metal used for selective hydrogenation.[10] Currently, palladium is chosen in the industry as a supported catalyst on oxide for the selective hydrogenation of unsaturated hydrocarbons to alkenes.[11] The selection of Pd arises from its high activity attributed to the low hydrogen dissociation barrier (Pd can dissociate H₂ at room temperature), enabling the use of catalysts with very low Pd content.[12-15] However, the main drawback of Pd is the production of alkanes (total hydrogenation) at high butadiene conversion, thereby limiting selectivity toward butenes.[16,17] In this context, the development of more selective catalysts that allow complete conversion of alkynes or dienes, while avoiding the conversion of alkenes to alkanes is desirable. Supported gold nanoparticles (Au NPs) have been known to be remarkably selective in hydrogenation reactions of various alkynes and dienes.[18-20] However, Au NPs active sites, generally attributed to low coordination sites, such as corners and edges, can only dissociate H₂ at high temperatures, limiting the reaction rate, which results in supported Au catalysts being significantly less active compared to Pd catalysts.[21-24]

Alloying Au with another metal possessing superior H₂ activation properties has been explored to enhance the activity of the catalysts.[11] Pd, Pt, Ni, Ag, and Cu have been combined with Au in various selective hydrogenation reactions, leading to improved activity while maintaining good selectivity.[25-29] In 2013, Lee et al. compared the performance of 6.7 wt% Au/SBA-15 and 3 wt% Cu/SBA-15 monometallic catalysts, with bimetallic

[a] A. Nassereddine
Institut Néel, UPR 2940 CNRS- Université Grenoble Alpes, Grenoble 38042,
France
E-mail: abdallah.nassereddine@esrf.fr

[b] A. Nassereddine, J. Nelayah
Université Paris Cité, CNRS, Laboratoire Matériaux et Phénomènes
Quantiques, Paris F-75013, France

[c] L. Delannoy, X. Carrier, C. Louis, A. Wilson
Sorbonne Université, Paris F-75005, France
E-mail: axel.wilson@sorbonne-universite.fr

[d] C. La Fontaine, A. Beauvois
Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin BP 48, Gif-sur-Yvette
91192 CEDEX, France

Laurent Delannoy deceased May 12, 2024

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Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202500379

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© 2025 The Author(s). ChemCatChem published by Wiley-VCH GmbH. This is
an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
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Au-Cu/SBA-15 catalysts having different Cu/Au atomic ratios (4, 3, and 0.33), during the selective hydrogenation of acetylene.[30] The catalytic results indicated that Cu is more active than Au (40% conversion for Cu compared to 5% for Au at 200 °C), but less selective toward ethylene (<5% for Cu compared to 45% for Au at 200 °C). For bimetallic systems, Au₃Cu/SBA-15 catalyst exhibited both higher conversion and better selectivity than Au and Cu catalysts, between 160 and 200 °C.[30] Based on DRIFT spectroscopy analyses, the authors suggested that the addition of Au makes Cu more electron-deficient than monometallic Cu/SBA-15, thereby enhancing the activity of the bimetallic catalyst. Moreover, the substantial presence of Au in the Au₃Cu/SBA-15 sample dilute the proportion of Cu⁺ species (which is known to strongly adsorb alkynes even at low temperature), minimizing C-C coupling reactions (oligomer formations) and promoting ethylene desorption, thereby improving selectivity.[30]

By studying the stability of Au–Cu/TiO₂ catalysts during 20 h at 75 °C, Delannoy et al.[25] showed that the conversion of butadiene to butenes increased gradually during the initial hours of the reaction. This activation phase was more pronounced for the AuCu₃ bimetallic catalyst with the highest copper content. It was proposed that this activation of Au-Cu/TiO₂ catalysts could be attributed to a progressive segregation of copper (the most active element in this reaction) on the surface of bimetallic NPs during the reaction, as experimentally observed in certain oxidation reactions, notably the CO oxidation reaction.[31-35] As far as we know, there is no experimental study in the literature confirming hydrogen-induced segregation processes in a reducing environment in Au–Cu bimetallic systems, and a clear and precise picture at the level of structure and chemical ordering in the presence of adsorbates is still lacking. This is crucial for a fundamental understanding of the intrinsic catalytic properties of Au–Cu/TiO₂ catalysts in the selective hydrogenation of butadiene.

In a recent work, by combining aberration-corrected in situ environmental transmission electron microscopy (ETEM) and ab initio molecular dynamic (AIMD) simulations, we reported the atomic-scale structure and dynamics of TiO₂ supported sub-4 nm Au and AuCu NPs in H₂ gas environment under heating (200–400 °C), at atmospheric pressure.[36,37] Our results showed that Au NPs lose their face-centered cubic (fcc) symmetry to transform to icosahedral-like or other non-fcc structures, due to the high mobility of gold atoms induced by strong Au-H interactions in this critical size regime.[37] Conversely, AuCu NPs are found to keep their fcc symmetry in the same environment conditions (H₂ gas, 200–400 °C) and the same size ranges (2–5 nm).[36] The stability of the fcc structure observed in AuCu NPs has been attributed to a partial segregation of Cu atoms from the sub-surface to the upper surface layer, according to the results of AIMD simulations. This phenomenon appears to play a significant role in breaking the strong chains of H-Au-H-Au, consequently reducing the fluxionality of AuCu NPs in H₂, as revealed by AIMD simulations.[36]

The main objective of this work is to complement the understanding of the hydrogen-induced restructuration of Au-

Cu bimetallic systems. We present a comprehensive characterization of anatase TiO₂ supported Au–Cu (Au:Cu atomic ratios of 1:1, 1:3) bimetallic catalysts, prepared by deposition-precipitation with urea (DPU) technique, throughout the entire catalytic cycle of selective hydrogenation of butadiene, including: (i) monitoring the gold and copper species evolution during the activation of the freshly prepared samples under hydrogen at 300 °C, and (ii) tracking the active phases of the catalysts under the reaction mixture of the selective hydrogenation of butadiene in excess of propene. Here, we specifically use an excess of propene as the main alkene, rather than butenes, following the approach of Delannoy et al.[25] and Hugon et al.[38] This choice simplifies gas chromatographic detection during laboratory-catalytic tests, making it easier to distinguish the reaction products (1-butene, trans-2-butene, cis-2-butene) from the fully hydrogenated product (butane). Notably, Hugon et al. demonstrated that propene and butene exhibit comparable reactivity in selective hydrogenation, so substituting propene does not alter the fundamental catalytic behavior while enabling more accurate product analysis.

To reveal the active species of individual gold and copper atoms and their local and electronic structures under working conditions, operando X-ray absorption spectroscopy (XAS) experiments, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were conducted and complemented by ex situ scanning transmission electron microscopy (STEM). For the sake of comparison, monometallic Au/TiO₂ and Cu/TiO₂ catalysts are also investigated under the same conditions.

## 2. Experimental Section

### 2.1. Sample Preparation

Titania-supported monometallic gold (Au/TiO₂), copper (Cu/TiO₂), and bimetallic gold–copper (Au-Cu/TiO₂) nanoparticles were synthesized by the deposition-precipitation with urea technique (DPU), by employing a procedure detailed in these studies.[25,39,40] Commercial TiO₂ (100% anatase, US research nanomaterials, surface area 50 m² g⁻¹, purity = 99.98%) was used as a support for NPs. Briefly, 2 g of TiO₂ was mixed in a double-wall reactor with 150 ml of distilled water and an appropriate amount of HAuCl₄·3H₂O (Sigma-Aldrich) gold and/or Cu(NO₃)₂·3H₂O (Sigma-Aldrich) copper precursors, to achieve the desired nominal loading of 6 wt.% Au and 2 wt.% Cu for monometallic NPs, and the desired Au:Cu atomic ratio (1:1 for AuCu, 1:3 for AuCu₃) for bimetallic NPs. The solution was stirred and heated at 80 °C, followed by the addition of an excess amount of the urea precipitating agent CO(NH₃)₂ (Sigma-Aldrich) to achieve a molar ratio of [urea] = 100 × [metal]. The solution mixture was then stirred at 80 °C for 20 h in the dark to avoid uncontrolled reduction of the HAuCl₄·3H₂O precursor in Au/TiO₂ and Au-Cu/TiO₂ samples. The obtained precipitate was separated from the liquid solution by centrifugation and then washed three times with distilled water to eliminate the chlorine residues (chlorides). The samples were further dried at room temperature overnight in dark and under vacuum. The samples were then activated by heat treatment at 300 °C for 2 h under 100 mL min⁻¹ of H₂ at atmospheric pressure to reduce the metal precursors into metal nanoparticles, and then
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transferred to air and loaded on standard TEM sample holder for ex situ STEM characterization. For operando XAS measurements, the samples were directly activated in situ during experiments, by using the same protocol.

## 2.2. Scanning Transmission Electron Microscopy

High-resolution high-angle annular dark field (HAADF-STEM) observations of Au and Au-Cu/TiO₂ catalysts were carried out using a double aberration-corrected JEOL ARM 200F microscope equipped with a cold field emission gun electron source.[41] The microscope was operated in STEM mode at an accelerating voltage of 200 kV. For HAADF-STEM imaging, the probe setting was 8C and the convergence semi-angle was set to 20.5 mrad. The corresponding inner and outer collection semi-angles on the HAADF detector were 68 and 280 mrad, respectively. High-resolution HAADF-STEM images (2048 × 2048 pixels) were acquired at magnifications of × 10 M and × 20 M which corresponds to a pixel area of 8.97 × 10⁻³ Å² and 2.24 × 10⁻³ Å², respectively. For ex situ observation, after reduction and after reaction, the samples were prepared by dispersing the reduced Au and Au-Cu NPs in distilled water and depositing around 10 drops of 2 µL of the resulting solution onto standard carbon-coated copper TEM grid (Agar Scientific).

## 2.3. Operando X-ray Absorption Spectroscopy

To monitor the local and electronic structure of Au, Cu, and Au-Cu NPs during the selective hydrogenation of butadiene in excess of propene, operando XAS experiments coupled with mass spectrometry were performed on the ROCK beamline[42,43] at the SOLEIL Synchrotron facility. Cu K (8979 eV) and Au L₃ (11,919 eV) edges were successively studied in transmission mode, using a single-crystal monochromator Si (111). The monochromator was energy calibrated measuring the Cu K and Au L₃ edges of Cu/Au metal foil. Approximately 3 mg of powdered samples were loaded in 1.2/1.5 mm diameter capillaries placed inside a capillary furnace available on ROCK beamline. The cell was connected to an automated gas distribution system that controls the various gas mixture at atmospheric pressure. Prior to each experiment, the cell was purged with 5 ml min⁻¹ of He at room temperature. The samples were then reduced by introducing a flow of 5 ml min⁻¹ of H₂ during heat treatment from room temperature to 300 °C (ramp of 5 °C min⁻¹), followed by a plateau (300 °C, approx. 2 h). Quick-XAS spectra were recorded during this reduction process to observe the changes in metal oxidation states. The temperature was then lowered under the same flow of H₂ to room temperature in order to record EXAFS data of the reduced sample. Afterward, Au, Cu, and Au-Cu/TiO₂ catalysts were exposed to 5 mL min⁻¹ flow of the gas mixture for selective hydrogenation of butadiene (0.3% C₄H₆/ 20%H₂/ 30% C₃H₆/ 49.7% He) during heating up to 100 °C. After 4 h, the temperature was successively raised to 130 °C for 30 min and 150 °C for 30 min with a ramp of 5 °C min⁻¹. The gas composition at the outlet of the catalytic cell was measured throughout the experiment using an MKS cirrus 2 quadrupole mass spectrometer. The XAS data were energy calibrated and normalized using a Python GUI provided by the beamline.[44] EXAFS fitting was performed using Artemis from the Demeter suite.[45] Multivariate curve resolution-alternating least squares (MCR-ALS) of the XAS data was performed using Fastosh software[46] based on Jaumot et al. MCR-ALS toolbox.[47] Singular value decomposition revealed that either 2 or 3 components could be selected for the fits. The initial estimation was done with an evolving factor analysis on the one matrix of each fit. Non-negativity and closure constraints were applied to all fit species.

# 3. Results and Discussion

## 3.1. In Situ XAS Investigations of Metal Precursors Reduction and NPs Formation

The reduction step performed prior to the reaction serves a dual purpose: it activates metal precursors to their reactive metallic state and initiates the nucleation and subsequent growth of the NPs involved in catalytic reactions. This reduction-nucleation-growth mechanism demonstrates clear substrate dependency,[48] making in situ investigation a valuable approach for its comprehension. In situ XAS analysis was performed to shed light on the state of Au, Cu, and Au-Cu species in the freshly prepared catalysts (i.e., after DPU and room-temperature drying) and how they behave during the reduction step.

The evolution of the metal oxidation state of Au and Cu species in the monometallic and bimetallic catalysts was verified by XANES analysis during the reduction step under H₂. The normalized Au L₃ and Cu K-edges XANES spectra for the freshly prepared and reduced (300 °C) Au/TiO₂ and Cu/TiO₂ samples are shown in Figure 1, with comparison to reference spectra from pure Au and pure Cu foils. For the gold sample (Figure 1A), the high amplitude of the white line in the freshly prepared catalyst confirms the presence of an oxidized phase,[49] likely in the form of Au³⁺. This observation aligns with various literature studies employing the DPU preparation method for gold samples.[39,50–52] The presence of an oxidic species is also confirmed by the Fourier transform of the EXAFS (FT-EXAFS) spectra of the freshly prepared sample (Figure S1A in Supporting Information), revealing a peak around 1.5 Å corresponding to the Au–O bond. Consequently, a large part of the gold species in the freshly prepared Au/TiO₂ sample is present as Au³⁺ cations with oxygen surrounding.

After treatment under H₂ at 300 °C, the XANES spectra of Au/TiO₂ catalyst is similar to that of the Au foil reference (Figure 1A), and the FT-EXAFS spectra shows a clear emergence of Au–Au bond peak around 2.3 Å (Figure S1A in Supporting Information), indicating that the Au³⁺ oxide species present in the freshly prepared sample are totally reduced to Au⁰ gold metallic phase after the reduction cycle. For the Cu K-edge, the pre-edge of the reference spectra generally shifts to higher energies with an increase in the copper oxidation state (8982 eV for Cu⁰ and 8985 eV for Cu²⁺), along with an increase in the white line intensity (Figure 1B). The freshly prepared catalyst (blue spectra in Figure 1B) exhibits an intense white line, indicating that copper is fully oxidized to Cu²⁺.[53] However, the absence of the pre-edge at 8985 eV is typical of amorphous CuO NPs on TiO₂, which feature a five-coordinated square pyramidal structure, in contrast to CuO with a four-coordinated square planar structure.[54] Following treatment under H₂ at 300 °C, a clear reduction of the initially present Cu²⁺ oxide species to metallic Cu⁰ is observed, evidenced by an XANES spectra closely similar to that of the Cu foil reference (Figure 1B), and by the FT-EXAFS spectra exhibiting a large Cu–Cu bond feature around 2.3 Å
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<figure><img src="image_9.png" /><figcaption>**Figure 1.** Au L₃-edge and Cu K-edge operando XANES spectra of monometallic A) Au/TiO₂ and B) Cu/TiO₂ catalysts before and after reduction cycle under 5 ml min⁻¹ of H₂ at 300 °C. The experimental XANES spectra of the two catalysts are compared to the Au foil, Cu foil, and CuO reference spectra.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 2.** Au L₃-edge and Cu K-edge operando XANES spectra of bimetallic A,C) AuCu/TiO₂ and B,D) AuCu₃/TiO₂ catalysts before and after reduction cycle under 5 ml.min⁻¹ of H₂ at 300 °C. The experimental XANES spectra of the two catalysts are compared to the Au foil, Cu foil, and CuO reference spectra.</figcaption></figure>

(Figure S1B in Supporting Information). The appearance of an intense pre-edge peak in the reduced sample (black spectra in Figure 1B) is due to the transition 1s to 4p in Cu atom.[55-57]

The same behavior was observed for Au and Cu species in bimetallic AuCu/TiO₂ and AuCu₃/TiO₂ catalysts (Figure 2). Indeed, after treatment under H₂ at 300 °C, a remarkable reduction in the intensity of the white line is observed for both freshly prepared catalyst at Au L₃-edge (Figure 2A,B) and Cu K-edge (Figure 2C,D), confirming the evolution of the Au³⁺ and Cu²⁺ oxi-dation states to a more reduced state. These observations were also confirmed by the FT-EXAFS spectra (Figure S2 in Supporting

Information) of both catalysts showing the metal-metal Au–Cu, Au–Au, and Cu–Cu bonds formation after reduction, indicating that the bimetallic Au–Cu catalysts were fully reduced.

It has to be noted that in the case of gold, the intensity and shape of the white line in the reduced bimetallic catalysts (Figure 2A,B) is slightly different from that observed in the monometallic Au/TiO₂ catalyst (Figure 1A). It is probably affected by the change in the electronic properties of the catalysts after alloying. Indeed, it is known that the white line peak in the Au L₃-edge is related to the dipole transitions from 2p₃/₂ to 5d states, and the intensity of the white line is directly related to the unoc-
<figure><img src="image_11.png" /></figure>

cupied d density of states.[58,59] Consequently, the increase in the intensity of the Au white line in the bimetallic AuCu/TiO₂ (black spectra in Figure 2A) and AuCu₃/TiO₂ (black spectra in Figure 2B) catalysts indicates that the gold present in these samples contains more unfilled d states compared to Au foil and Au/TiO₂ catalyst (black spectra in Figure 1A).[57,58] This modification in electronic properties could highly affect the catalytic performance of the bimetallic catalyst upon alloying.

## 3.2. Reduction Behavior of Precursors Under H₂ in Relation to Temperature

In addition to examining the initial and final states of Au, Cu, and Au-Cu/TiO₂ samples before and after reduction at 300 °C under H₂, the setup allowed to follow the time-resolved Au L₃ and Cu Kedges using quick-XAS throughout this reduction step. This allowed to precisely track the exact reduction temperatures for each metal precursor as a function of composition. Multivariate curve resolution with alternating least squares (MCR-ALS) fitting analysis has been implemented to gain insights into the transitions and reduction of the different gold and copper species present after DPU during the reduction step under H₂ at 300 °C, based on time-resolved XAS data. The MCR-ALS approach has emerged as a reliable mean for isolating pure spectra of intermediate species and has been successfully applied to various systems and reactions studied by time-resolved XAS.[60-62]

Operando XANES spectra of Au L₃- and Cu K-edges were collected during a temperature programmed reduction (TPR) of the Au, AuCu, AuCu₃, and Cu catalysts, from room temperature to 300 °C (heating ramp: 5 °C/min) under 5 mL/min of H₂ at 1 bar. Figure 3 illustrates the MCR-ALS analysis considering the whole set of time-resolved XANES spectra at the Au L₃- and Cu K-edges from room-temperature to 160 °C recorded with a time scale of 140 s. The Au L₃-edge is depicted in yellow, whereas the Cu K-edge is shown in red. The PCA analysis and theoretical XANES spectra generated using MCR-ALS are shown in Figures S3 and S4 for Au L3-edges and Cu K-edges, respectively. For Au, AuCu, and AuCu₃ the best MCR-ALS analysis was obtained with 2 components. The coloured dashed and solid lines represent the precursor and metallic components, respectively. For Cu the best MCR-ALS analysis was obtained with 3 components, solid line for precursor, dashed line for metallic, and dotted line for a 3rd intermediary specie. This discrepancy regarding the number of components is coherent with TPR analysis performed previously by Delannoy et al.[25] on similar samples. Only pure Cu displayed two peaks, attributed to the successive reduction of Cu²⁺ to Cu⁺ first then to Cu⁰. As per previous investigations,[25] it confirms that the presence of Au contributes to the Cu reduction, seemingly directly from Cu²⁺ to Cu⁰.

Figure 3 highlights the temperature at which 50% of the XANES signal transitions from a precursor to a reduced metallic species under pure H₂ gas, thus providing insights into the conditions under which Au and Cu are activated for the hydrogenation reaction. For the pure Au and pure Cu samples, the temperature onsets are at 59.5 and 90 °C, respectively. In the

bimetallic samples, the temperature onset for the Au increases with the Cu content at 6 and 71 °C for 1:1 and 1:3 Au:Cu ratios, respectively. As for Cu, the temperature onset decreases with Au content, at 75 and 72.5 °C for 3:1 and 1:1 Au:Cu ratios, respectively. The interplay between Au and Cu species during the reduction step has been already explored using TPR for similar DPU prepared catalysts.[25] This interaction had been also reported in the literature on AuCu,[63] AuPd,[64] and PtCu[65] nanoalloys. Several mechanisms have been proposed to explain this interdependence between Au and Cu regarding the reduction temperature onset. The presence of Au may improve the dispersion of Cu leading to the nucleation of smaller particles, that are more easily reduced. The presence of Au in the vicinity of Cu precursor may weaken the Cu—O bond leading to lower reduction temperature. Finally, hydrogen spillover constitutes the most probable explanation to explain the manifest shift in reduction temperature.[66] In our context, hydrogen atoms generated by H₂ decomposition on Au⁰ sites may diffuses to an acceptor site, possibly involving the TiO₂ support,[67] leading to the reduction Cu²⁺ cations. It is important to note that each of those hypotheses place Au and Cu in close proximity leading to the nucleation and growth of bimetallic NPs.

Delannoy et al. reported[25] a similar trend of Au and Cu influencing each other's reduction temperature, but it occurred at notably higher temperatures, with reduction onsets at 119, 132, 147, and 175 °C for Au, AuCu, AuCu₃, and Cu, respectively. This corresponds to temperature differences ΔT of 60, 61, 74, and 85 °C for Au, AuCu, AuCu₃, and Cu respectively, when compared to the present in situ experiment. This significant discrepancy in reduction temperature between TPR[25] and operando XAS may be attributed to two main factors: i) the influence of the X-ray beam on the onset of the reduction of metal precursors and ii) the difference in geometries, flow rate between TPR and XAS reactors, for example, shape of the catalytic bed, placement of the thermocouple, characteristic of the gas flow. Zabilska et al. recently investigated beam damage during in situ XAS experiments[40] reporting an X-ray-induced photochemical reduction with VOₓ catalysts supported on TiO₂. Their findings highlighted that this reduction did not occur under inert atmospheres but notably occurred at moderate temperatures (30–100 °C) in a reductive environment. Newton et al. conducted a study monitoring the Cu K-edge of activated organometallic systems,[68] specifically Cu¹¹(TMEDA) deposited on Al₂O₃ and SiO₂, in the presence of methane. Through a comparison of XANES data acquired from low and high brilliance sources, the team underscored a distinct impact of source brilliance on the reduction kinetic from Cu¹¹ to Cu¹. Their experiments testing defocusing and filtering solutions proved effective in mitigating or eliminating beam-sample interactions. This aligns with other documented instances of beam damage during XAS experiments.

## 3.3. Detailed Understanding of Catalysts Structure After Reduction: EXAFS Analysis

To obtain more detailed information on the nature of the crystalline structure of the metal NPs and to quantitatively
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 3.** MCR-ALS analysis of the time-resolved XANES spectra obtained during reduction of Au, Cu, AuCu, and AuCu<sub>3</sub>/TiO<sub>2</sub> samples during heating from 25 to 300 °C under H<sub>2</sub> (temperature ramp: 5 °C/min, reducing conditions: 5 ml.min<sup>-1</sup> H<sub>2</sub>).</figcaption></figure>

study the local atomic arrangement of both Au and Cu in the Au, Cu and AuCu/TiO<sub>2</sub> catalysts, a non-linear curve-fitting analysis was performed on the Au L<sub>3</sub> and Cu K-edge EXAFS spectra for the reduced catalyst. The EXAFS analyses of the AuCu<sub>3</sub>/TiO<sub>2</sub> sample and the Cu K-edge of the AuCu/TiO<sub>2</sub> samples were not included in this study due to the presence of numerous glitches in the post-edge region. These artifacts significantly compromised the data quality, making it unsuitable for reliable interpretation and drawing robust conclusions. EXAFS fittings including k-space are shown in Figures S5 and S6 in Supporting Information. The fitting parameters, i.e., interatomic distances, atomic coordination numbers (CN) and Debye-Waller factors (σ<sup>2</sup>), are provided in Table 1. These parameters were fitted using a fixed amplitude value (S<sub>0</sub> = 0.8) obtained by fitting the spectra from the reference Au and Cu foils (Figure S5).

The FT-EXAFS spectra for the reference Au and Cu foil samples (Figure S5) are typical of a fcc structure, with the main peak corresponding to the shortest Au–Au (R<sub>Au–Au</sub> = 2.86 Å, CN = 12) and Cu–Cu distance (R<sub>Cu–Cu</sub> = 2.54 Å, CN = 12) (Table 1). The fitted coordination number of Au and Cu atoms in the Au/TiO<sub>2</sub> and Cu/TiO<sub>2</sub> monometallic catalysts are 8.7 ± 0.8 and 12.8 ± 3.9, respectively. As previously reported,<sup>[50,51–69]</sup> the decrease in the CN of Au/TiO<sub>2</sub> compared to Au foil may be explained by the dis-

tortions of the metal fcc structure and nano-size effects, induced by the formation of small NPs. Indeed, due to the large proportion of surface atoms in NPs, most of Au atoms are at or near the surface, thus reducing the average CN as compared to the bulk reference (Au foil). This decrease was recently observed by Zheng et al.,<sup>[70]</sup> who reported a CN of 9.8 for Au/TiO<sub>2</sub> NPs (with an average size of Au NPs of 3 nm) also prepared by DPU, justifying the fact that the metal bond length is shorter in NPs, leading to a smaller CN.<sup>[71]</sup>

For the AuCu/TiO₂ catalyst at the Au L₃-edge (FT-EXAFS in Figure S6), the fitting (Table 1) gives 3.8 ± 0.6 for the Cu CN, and 3.8 ± 0.7 for Au CN, which indicates that the central atom Au is surrounded by about 6 to 8 Au and Cu atoms in a form of fcc structure after reduction. As in the case of Au NPs, obtaining a smaller CN (6–8) in the first layer compared to CN = 12 for an Au–Cu fcc structure could be attributed to the small size of the NPs.<sup>[72,73]</sup> The similar CN values for both Au (3.8) and Cu (3.8) suggest that the Au and Cu atoms are well-mixed and uniformly distributed throughout the NPs, rather than forming a core-shell structure where one metal would be more centrally located than the other. The formation of the fcc structure was already confirmed in our previous study by environmental in situ TEM,<sup>[36]</sup> clearly showing that AuCu NPs smaller than 4 nm, obtained after DPU synthesis and reduction under H₂ at 300 °C, exhibit a
<figure><img src="image_14.png" /></figure>

<table><caption>**Table 1.** Curve fitting results of Au L<sub>3</sub>-edge and Cu K-edge for Au foil, Cu foil, monometallic Au/TiO<sub>2</sub>, Cu/TiO<sub>2</sub>, and Au L<sub>3</sub>-edge of bimetallic AuCu/TiO<sub>2</sub> catalysts after reduction. The underlined characters denote fixed parameters. EXAFS spectra were fitted in a Fourier-transform range of 3.3–12.1 Å<sup>-1</sup> (for Au foil), 3.6–11.6 Å<sup>-1</sup> (for Cu foil), 3.3–10.5 Å<sup>-1</sup> (for Au/TiO<sub>2</sub>), 4.5–10.6 Å<sup>-1</sup> (for Cu/TiO<sub>2</sub>), and 3.5–9.7 Å<sup>-1</sup> (for AuCu/TiO<sub>2</sub>) with a Hanning window applied between 1–3.4 Å (for Au foil), 1–3 Å (for Cu foil), 1–3.5 Å (Au/TiO<sub>2</sub>), 1.4–3 Å (for Cu/TiO<sub>2</sub>), and 1–3.7 Å (for AuCu/TiO<sub>2</sub>).</caption><thead><tr><th>Sample</th><th>Scattering Paths</th><th>CN</th><th>R (Å)</th><th>σ<sup>2</sup> (Å<sup>2</sup>)</th><th>ΔE (eV)</th><th>R-factor</th></tr></thead><tbody><tr><td>Au/TiO<sub>2</sub></td><td>Au-Au</td><td>8.7 ± 0.8</td><td>2.80</td><td>0.01 ± 0.0009</td><td>3.1 ± 0.7</td><td>0.016</td></tr><tr><td>Cu/TiO<sub>2</sub></td><td>Cu-Cu</td><td>12.8 ± 3.9</td><td>2.54</td><td>0.009 ± 0.002</td><td>5.8 ± 3.6</td><td>0.022</td></tr><tr><td rowspan="2">AuCu/TiO<sub>2</sub></td><td>Au-Cu</td><td>3.8 ± 0.6</td><td>2.61</td><td>0.01 ± 0.001</td><td>1.0 ± 1.5</td><td>0.002</td></tr><tr><td>Au-Au</td><td>3.8 ± 0.7</td><td>2.77</td><td>0.006 ± 0.001</td><td></td><td></td></tr><tr><td>Au foil (fcc model)</td><td>Au-Au</td><td><u>12.0</u></td><td>2.86</td><td>0.007 ± 0.0003</td><td>5.2 ± 0.3</td><td>0.004</td></tr><tr><td>Cu foil (fcc model)</td><td>Cu-Cu</td><td><u>12.0</u></td><td>2.54</td><td>0.008 ± 0.0007</td><td>4.8 ± 0.8</td><td>0.005</td></tr></tbody></table>

<figure><img src="image_15.png" /><figcaption>**Figure 4.** Atomic scale ex situ HAADF-STEM imaging of TiO<sub>2</sub> supported AuCu NPs after DPU synthesis and reduction under H<sub>2</sub> at 300 °C. (A,B) AuCu NPs with sizes below 4 nm, and C) AuCu NP with sizes larger than 4 nm. Regardless of size, these NPs exhibit an fcc structure after DPU synthesis and reduction under H<sub>2</sub> at 300 °C.</figcaption></figure>

truncated octahedral morphology with clear differentiation of the (111) and (100) facets. Moreover, ex situ atomic-scale observation of AuCu NPs by STEM-HAADF (Figure 4) in the same conditions confirms the formation of NPs with fcc structures. Indeed, AuCu NPs exhibit interatomic distances $d_{hkl}$ varying between 2.24 and 2.32 Å, which, according to Vegard's law,[74] correspond to the (111) crystallographic planes of the alloyed Au-Cu fcc structure ($d_{hkl}$ = 2.22 Å for AuCu).

### 3.4. Evolution of Au, Cu, and AuCu Species During Selective Hydrogenation of Butadiene Reaction

To gain a clearer understanding on the evolution of the electronic structure of gold and copper species during the selective hydrogenation of butadiene, operando XAS spectra of Au and AuCu/TiO<sub>2</sub> catalysts were recorded at different reaction temperatures (100, 130, and 150 °C). The results were compared with those of the samples after H<sub>2</sub> reduction at 300 °C (Figure 5). The gaseous product distributions during operando XAS experiments of Au, Cu, and Au-Cu/TiO<sub>2</sub> catalysts were detected by mass spectrometry (Figure S7 in Supporting Information). Regardless of the catalyst studied, a decrease is observed in the partial pressure of butadiene (m/z = 54), accompanied by an increase in the

partial pressure of butenes (m/z = 56), when the temperature is raised to 130 and 150 °C. This indicates that the Au, Cu, and AuCu/TiO<sub>2</sub> catalysts effectively convert butadiene into butenes. Moreover, the partial pressure evolution of propene (m/z = 42), propane (m/z = 44), and butane (m/z = 58) (see Figure S7 in Supporting Information) remains constant throughout the reaction, indicating that no further hydrogenation of propene into propane or butene into butane occurs. This observation confirms that the process is selective for the hydrogenation of butadiene into butenes. Moreover, these MS results confirms that the operando XAS experiments reproduce the same catalytic behavior observed under real catalytic testing on the same catalysts.[25]

As observed in Figure 5A,C, the XANES spectra at the Au L<sub>3</sub>-edge of the Au/TiO<sub>2</sub> (A) and AuCu/TiO<sub>2</sub> (C) catalysts after in situ reduction under H<sub>2</sub> at 300 °C remain stable and show no modification during the reaction, regardless of the temperature. This observation indicates that gold maintains its metallic Au<sup>0</sup> specification during selective hydrogenation in both the monometallic and bimetallic catalysts. In contrast, upon examining the XANES spectra at the Cu K-edge under the same conditions, a slight but distinct modification in the intensity of the white line is noticeable in both Cu/TiO<sub>2</sub> (B) and AuCu/TiO<sub>2</sub> catalysts (D) (see inset of the 8990–8996 eV energy range in Figure 5B,D). Indeed, in
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 5.** Au L₃-edge and Cu K-edge operando XANES spectra of monometallic A) Au/TiO₂, B) Cu/TiO₂ and bimetallic C,D) AuCu/TiO₂ catalysts after reduction and during the selective hydrogenation of butadiene at different temperatures: 100, 130, and 150 °C.</figcaption></figure>

both catalysts, though more prominently in the monometallic one, a slight modification in the intensity of the white line is observed, especially when the reaction reaches temperatures of 130 and 150 °C. Subsequently, at 150 °C, no further modification is observed, and the XANES region remains stable. This slight modification may be due to a (very minor) change in the electronic state of copper during the reaction, while maintaining its metallic state. The presence of copper in Cu⁰ metallic form during the reaction cycles at 100, 130, and 150 °C (Figure 5D) eliminates any possibility of significant copper segregation in the form of CuO/Cu₂O on the surface of NPs. The formation of Au-CuO core-shell NPs, which was previously shown to enhance catalytic properties during the CO oxidation reaction,[³¹⁻³⁵] is thus ruled out during the selective hydrogenation reaction.

To have a clearer idea of the structure of the metal NPs, the radial distribution functions of the EXAFS spectra (FT-EXAFS) of each catalyst after in situ reduction under H₂ (300 °C) and during reaction at 100, 130, and 150 °C are shown in Figure 6 (the corresponding k² weighted EXAFS spectra are shown in Figure S8 in Supporting Information). For a more comprehensive comparison, the experimental FT-EXAFS spectra are compared to those of known reference spectra: Au foil, Cu foil, and CuO oxide. No alteration in the metallic state of gold and copper was observed during the selective hydrogenation of butadiene, which is consistent with the XANES analysis in Figure 5.

Indeed, at any temperature, both monometallic catalysts Au and Cu/TiO₂ retained their Au-Au and Cu-Cu bonds (peak around 2.5 Å in Figure 6A,B). At the Au L₃-edge for the AuCu/TiO₂ catalyst, the FT-EXAFS spectra (Figure 6C) reveal a prominent peak at 2.22 Å, that is, at a lower distance than for Au/TiO₂. This peak position (uncorrected for phase shift) falls between Au–Au (2.33 Å) and Cu–Cu (2.17 Å, Figure 6D). In contrast to Au/TiO₂, the first shell formed by the Au–Cu/Au bond in AuCu/TiO₂ shows a larger scattering amplitude and a shorter bond length compared to the first Au–Au shell in monometallic Au/TiO₂. This difference is attributed to the synergistic effect resulting from the alloying of Au and Cu atoms, as previously reported in the literature.[55,75] Indeed, this modification in the structural and electronic properties of atoms in bimetallic catalysts may serve as the electronic parameter enabling the improvement of catalytic properties in AuCu/TiO₂ catalysts during the selective hydrogenation of butadiene, in comparison to monometallic Au/TiO₂.

After the operando XAS experiments, the catalysts were collected and observed ex situ at the atomic scale by high-resolution STEM-HAADF to analyse the local structures of Au and AuCu NPs after the selective hydrogenation of butadiene. Figures 7 and 8 show atomic-scale HAADF-STEM images of Au and AuCu NPs, respectively. The analysis of the crystalline structure of individual Au NPs (Figure 7) in Au/TiO₂ catalyst,
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 6.** Au L₃-edge and Cu K-edge FT-EXAFS spectra for the A) Au/TiO₂, B) Cu/TiO₂ and C,D) AuCu/TiO₂ catalysts after reduction and during the selective hydrogenation of butadiene at different temperatures: 100, 130, and 150 °C.</figcaption></figure>

reveals a size-dependent crystalline structure. Specifically, for sizes larger than 2 nm, the images of Au NPs (Figure 7A–C) and their corresponding Fourier transforms (Figure 7D–F) indicate an fcc structure, which confirms the results obtained by operando XANES and EXAFS measurements. The structure of Au NPs below 2 nm recall the disordered structures of Au NPs in the same size range, observed in our recent study following in situ and ex situ exposure to pure H₂ between 400 and 200 °C.[37] In the later study, by combining in situ ETEM observations and ab initio molecular dynamics simulations, a size-dependent crystalline structure of Au NPs under pure H₂ conditions was revealed. It was highlighted that there is a critical size below which Au NPs lose their fcc structure due to their strong interaction with hydrogen.[37] Accordingly, we can propose that the loss of fcc structure in Au NPs with a size below 2 nm (Figure 7G–I), following the selective hydrogenation of butadiene, may be attributed to the strong interaction of these NPs with hydrogen, which is present in excess in the reaction mixture.

On the other hand, for bimetallic Au-Cu/TiO₂ catalysts, no size dependence of the crystalline structure was observed (Figure 8). Similarly, the atomic composition of Cu has no effect on the crystalline structure of Au-Cu NPs. Indeed, the 2D projection of AuCu (Figure 8A–C) reveals irregular and rounded morphologies, concurrently exhibiting a disordered structure with deformed particles. FCC structures were also observed in this case (Figure 8C).

## 4. Conclusion

This study provides a detailed investigation of the hydrogen-induced restructuring and catalytic behaviour of Au–Cu bimetallic systems supported on anatase TiO₂, with Au atomic ratios of 1:1 and 1:3. By employing operando XAS techniques, we closely monitored the evolution of Au and Cu species during activation and under reaction conditions. For comparison, the behaviour of monometallic Au/TiO₂ and Cu/TiO₂ catalysts was also studied under identical conditions.

Our findings reveal that during the reduction step at 300 °C under H₂, Au, and Cu species in both monometallic and bimetallic catalysts underwent significant transformation, with the reduction of Au³⁺ and Cu²⁺ oxide species to their metallic states. The EXAFS analysis confirmed the formation of Au–Au and Au–Cu bonds with similar coordination numbers in the bimetallic catalysts, indicating that both metals form well-mixed, uniformly distributed NPs with an fcc structure. Notably, the bimetallic catalysts exhibited slight differences in electronic properties compared to the monometallic ones, as reflected by the increased intensity of the Au L₃ white line, which corresponds to a greater number of unfilled d-states in the bimetallic samples. This electronic modification likely influences the catalytic performance of the bimetallic catalysts.

Additionally, operando quick-XAS allowed precise tracking of the temperature-dependent reduction behaviour of Au and
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 7.** Atomic scale ex situ HAADF-STEM imaging of TiO₂ supported Au NPs after selective hydrogenation of butadiene in excess of propene conducted during operando XAS experiments. (A–C) Au NPs with size larger than 2 nm. (D–F) FFT of the HAADF-STEM images a, b and c showing the FCC crystalline structure of Au NPs. (G–I) Au NPs with size below 2 nm.</figcaption></figure>

Cu species. The results indicate that the presence of Au in Au-
Cu bimetallic catalysts facilitates the reduction of Cu at lower
temperatures, possibly due to mechanisms such as hydrogen
spillover from Au⁰ sites to Cu species, with a possible influence
of the X-ray beam. These interactions lead to the earlier reduc-
tion of Cu²⁺ and Cu¹⁺ species, contributing to the formation of
smaller, more reactive nanoparticles.

During the selective hydrogenation of butadiene and
under hard X-ray illumination, both the Au and Cu species
retained their metallic states, with minimal changes observed

in their electronic structures. The bimetallic AuCu/TiO2 cata-
lysts, however, displayed improved catalytic performance over
monometallic counterparts. This enhancement is attributed
to the structural and electronic interplay between Au and Cu
atoms in the bimetallic nanoparticles, as suggested by EXAFS
analysis showing shorter bond lengths and greater scattering
amplitudes for Au-Cu interactions compared to Au-Au.

Finally, post-reaction high-resolution HAADF-STEM imaging
confirmed the structural integrity of the catalysts, revealing
that Au-Cu nanoparticles maintained their fcc structure with
<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>**Figure 8.** Atomic scale ex situ HAADF-STEM imaging of TiO₂ supported AuCu NPs after selective hydrogenation of butadiene in excess of propene conducted during operando XAS experiments. (A–C) AuCu NPs with size below than 2 nm.</figcaption></figure>

uniform alloy distribution. For Au/TiO₂ catalyst, Au nanoparticles smaller than 2 nm showed disordered structures, possibly due to strong hydrogen interactions, whereas the bimetallic AuCu/TiO₂ catalysts displayed no size-dependent structural changes.

## Acknowledgements

This work was supported by the French National Research Agency through the TOTEM project, Grant Number ANR-17-CE07-0031. A.N. was supported by a Ph.D. fellowship granted through the same project. The authors are grateful to Region Ile de France for convention SESAME E1845 for the support of the JEOL electron microscope installed at Université de Paris. The authors thank Dr. Clément Guibert for his assistance on ROCK beamline. The authors thank the SSHADE spectroscopic database (DOI:10.26302/SSHADE/FAME) for providing the spectral data of Au foil (doi:10.26302/SSHADE/EXPERIMENT_DT_20171221_001) and Cu foil (doi.org/10.26302/SSHADE/EXPERIMENT_DT_20180117_002).

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are openly available in https://entrepot.recherche.data.gouv.fr/dataverse/sorbonne-univ at https://doi.org/10.57745/EQL6HT.

**Keywords:** Atomic probe STEM · Au-Cu catalysts · Heterogeneous catalysis · Operando quick XAS · Selective hydrogenation of butadiene

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Manuscript received: March 4, 2025

Revised manuscript received: April 10, 2025

Accepted manuscript online: April 24, 2025

Version of record online: ■■, ■■