# Effective Atomic Layer Deposition of palladium coatings for hydrogen separation membranes

Lucie Badouric, Martin Drobek, Jim Cartier, Christophe Charmette, Bertrand
Rebière, Eddy Petit, Maximilien Coronas, J Cambedouzou, Karim Aissou,
Anne Julbe, et al.

► To cite this version:

Lucie Badouric, Martin Drobek, Jim Cartier, Christophe Charmette, Bertrand Rebière, et al. Effective Atomic Layer Deposition of palladium coatings for hydrogen separation membranes. Surfaces and Interfaces, 2025, 62, pp.106181. 10.1016/j.surfin.2025.106181. hal-05081380

HAL Id: hal-05081380

https://hal.science/hal-05081380v1

Submitted on 23 May 2025

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# Effective Atomic Layer Deposition of palladium coatings for hydrogen separation membranes

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Lucie Badouric¹, Martin Drobek¹, Jim Cartier¹, Christophe Charmette¹, Bertrand Rebière², Eddy

Petit¹,², Maximilien Coronas¹, Julien Cambedouzou¹, Karim Aissou¹, Anne Julbe¹, Mikhael

Bechelany¹,³*

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¹ Institut Européen des Membranes, IEM, UMR-5635, Univ Montpellier, CNRS, ENSCM, Montpellier, France

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² UAR 2041, Plateforme d'Analyses et de Caractérisation (PAC) Chimie Balard Montpellier, Univ Montpellier, CNRS, ENSCM, 1919 route de Mende - 34293 Montpellier cedex 5, France

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³ Functional Materials Group, Gulf University for Science and Technology, Mubarak Al-Abdullah, 32093, Kuwait

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Corresponding author: Mikhael.bechelany@umontpellier.fr

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Keywords: Palladium, Atomic Layer Deposition, Nucleation, Membrane, Hydrogen separation

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## Graphical abstract

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## Abstract

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As global energy demand continues to rise, hydrogen purification plays a pivotal role in advancing the “hydrogen economy”, ensuring the required purity levels for its direct use or storage. This work presents the synthesis and functional characterization of palladium-based hydrogen-selective composite membranes applicable for gas separation below 300°C. These membranes were prepared
via Atomic Layer Deposition (ALD) of palladium (Pd) onto asymmetric ceramic tubular supports, using palladium hexafluoroacetylacetonate Pd(hfac)₂ and formalin at 220°C. Prior to Pd deposition, the γ-Al₂O₃ top-layer of the support was modified with an ultra-thin (~1 nm) amorphous alumina layer also deposited by ALD. This strategy enhances the number of nucleation sites for Pd, resulting in the formation of thin, uniform and compact membranes. It also substantially reduces Pd precursor consumption compared to conventional methods. The gas separation factors at 286°C were 86 for H₂/N₂, 79 for H₂/CH₄ and 67 for H₂/CO₂. Additionally, the composite membranes demonstrated resilience to repeated temperature cycling and were able to restore their performance after exposure to trace amounts of CO.

### Highlights:

* An original strategy for the synthesis of Pd-based composite membranes was applied using Atomic Layer Deposition (ALD).

* The membrane supports were pre-coated with amorphous alumina by ALD before the deposition of Pd.

* The highest selectivity to H₂ was obtained at temperatures around 286°C.

* The membranes exhibit high stability during temperature cycling and good regenerability after CO exposure.

## Introduction

Hydrogen is increasingly considered as a crucial solution to the current energy crisis due to its higher energy density compared to most conventional energy sources [1]. This specific characteristic has made hydrogen large-scale application one of the most researched areas, with significant efforts focused on developing efficient production methods. The most used processes to produce hydrogen are steam methane reforming (SMR) and other methods based on water dissociation, biomass or fossil fuels transformation [2]. However, before hydrogen storage and subsequent use [3], [4] it must be efficiently purified to remove undesirable contaminants and by-products such as carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), sulfur (H₂S, SOₓ) or nitrogen (NOₓ, NH₃) compounds. Among the available gas separation methods, membrane processes show great potential, offering a less energy-intensive alternative to other separation techniques such as Pressure Swing Adsorption (PSA) [5] or cryogenic distillation [6].

In the field of metal-based membranes for hydrogen (H₂) purification, special attention is paid to palladium-based membrane materials, which have already shown attractive results for hydrogen purification [7]. However, the high cost of palladium (Pd) and issues related to membrane stability in the presence of poisoning species limit the range of applications and large-scale commercialization. To overcome this drawback, numerous studies have been devoted to the preparation of different Pd-based composites [8] and alloys [9] to increase membrane stability and at the same time to decrease the overall manufacturing costs compared to pure self-supported Pd membranes. Although the solution-diffusion transport mechanism of hydrogen in metallic Pd membranes leads to virtually infinite selectivity [10], it often results in low H₂ permeance due to the large membrane thickness. Hence, a potential solution to address this issue is to deposit a very thin layer of palladium on a porous membrane support [11]. Indeed, porous asymmetric ceramic substrates with a mesoporous internal layer can be advantageously used to deposit a very small amount of Pd while forming a continuous and compact gas-selective membrane barrier. Additionally, the porous ceramic substrate provides mechanical strength to the membrane, especially at the high temperatures required to induce selective hydrogen transport across the membrane layer.
Since alumina supports are rather inexpensive and offer good thermal and mechanical properties, much work has been devoted since the 90's to the fabrication of hydrogen-selective palladium-alumina composite membranes [12], [13]. Chemical Vapor Deposition (CVD) and metal sputtering are widely studied synthesis techniques for the preparation of such membrane materials. However, they often suffer from a lack of sufficient deposition conformality and relatively thick Pd layers are required for a uniform coverage of the support. Electroless Plating (ELP) offers better uniformity and can be advantageously applied on tubular ceramic supports [14], [15], but also leads to relatively thick membrane layers. Atomic Layer Deposition (ALD) is a more adapted strategy to obtain ultrathin Pd coatings as hydrogen-selective membranes. The method requires minimal amounts of Pd precursors and yield highly conformal coatings on/in the membrane support porous structure. Indeed, ALD is based on the self-limiting reaction between gaseous precursors, allowing precise control of the thickness of the deposited layer. Despite the extensive use of ALD/PEALD for the deposition of Pd nanoparticles as catalytic species [16], [17], [18], [19], [20], [21], [22], only few works have been reported on Pd-based hydrogen-selective membranes deposited by ALD [23]. For example, Weber et al. [24] successfully synthesized Pd-alumina composite membranes by ALD, with promising performance for hydrogen separation and relatively high H₂ permeance.

In order to improve further the membrane performance, we propose in this work an alternative synthesis protocol based on a modification by ALD of the pristine γ-Al₂O₃ support by an amorphous alumina layer before the deposition of Pd particles. This strategy allows to significantly increasing the number of nucleation sites for the formation of Pd particles deposited by ALD, and thus a better control of the growth of the membrane layer on the ceramic support.

Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to highlight the morphology of the deposits, while X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) as well as Energy Dispersive X-Ray spectroscopy (EDX) were used to determine the crystalline structure and the chemical composition of the material, respectively. The functional characterization of the membrane was first evaluated by single gas permeation measurements before conducting separation experiments with specific gas mixtures containing hydrogen. Temperature cycling and the presence of poisoning gas (e.g. CO) were also studied to assess the stability of membranes and their potential regeneration.

It was demonstrated that the Pd-based composite membrane prepared on commercial γ-Al₂O₃ supports pre-coated by ALD of amorphous alumina layer has great potential for hydrogen separation while requiring a very small amount of Pd precursor. The results obtained in this work pave the way for more efficient, stable and affordable hydrogen-selective membranes.

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## 102 Experimental section

### 1. Membrane supports and chemicals

The ceramic membrane supports with asymmetric structure and tubular geometry (Øout/in =10/7 mm) were supplied by Pall-Exekia (France). They are composed (from the outside to the inside) of three macroporous layers of α-alumina with pore sizes 10 µm, 800 nm, 200 nm, respectively and a mesoporous γ-alumina top-layer with a pore size of ~8 nm. The tubular supports were cut into 5 cm long pieces and sealed with a commercial enamel at both ends to avoid gas leakage during permeation measurements. Silicon wafers <100> from Neyco Vacuum & Materials, and flat porous ceramic supports (Ø= 30 mm) with a γ-alumina top-layer (provided by the Fraunhofer Institute of Ceramic Technologies and Systems IKTS in Germany) were used as supports to perform AFM, SEM, EDX, XRD and XPS characterizations. The ceramic disks were treated at the same temperature as the tubular
supports during the enamel thermal treatment. The chemicals used for the ALD of palladium were Pd(II) hexafluoroacetate (Pd(hfac)₂) (Strem Chemicals, CAS 64916-48-9) and formaldehyde, 37% in aqueous solution, stabilized with 10-15% methanol (Thermoscientific, CAS 50-00-0) [22]. Although the use of ozone, H₂, or O₂ plasma has proven to be highly effective for the deposition of high-purity Pd film by ALD [17], [25] formalin was selected as the co-reactant due to safety and technical concerns with the experimental set-up in our laboratory. The precursors applied for ALD of alumina were trimethylaluminum (TMA) (Sigma-Aldrich, CAS Number 75-24-1, 97%) and distilled water. All chemicals were used as received without any further purification. Each precursor was contained in stainless steel bubblers connected to the ALD reactor. Pd(hfac)₂ was heated to 70°C to ensure sufficient vapor pressure for dosing into the ALD chamber. Formalin, TMA and distilled water were kept at room temperature without further heating. Argon (Linde Gas, ≥ 99.9999 %) was applied to purge the ALD chamber during deposition cycles.

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## 2. Membrane material synthesis

The modification of the γ-Al₂O₃ support (top-layer) with ALD alumina was conducted in a home-built ALD reactor controlled by custom-programmed Labview software. A defined number of pulse-exposure-purge ALD sequences was applied as follows: 0.4s pulse of TMA + 40s exposure and 60s purge, followed by a 2s pulse of water + 40s exposure and 60s purge. The ALD device consists of a hot-wall chamber heated to 100°C, bubblers connected to an inlet line heated to 100°C, and an exhaust heated to 90°C and connected to a vacuum pump. The entire system is maintained under vacuum (2.10⁻² to 5.10⁻² mbar) and the injection of precursors into the chamber is controlled via the feed line by the programmed opening of automatic valves. During the purge step, argon is continuously introduced while being pumped to ensure complete removal of all by-products and unreacted species. Further details on ALD devices could be found elsewhere [26], [27], [28]. Based on a theoretical growth rate of ~0.2 nm per cycle for alumina [29], 5 ALD cycles are expected to deposit ~1 nm of Al₂O₃ on the pore walls of the support.

Palladium (Pd) deposition was carried out in another custom-built ALD reactor of similar construction. The hot-wall reactor chamber was heated to 220°C and the palladium deposition sequence was as follows: 5s pulse of Pd(hfac)₂ + 60s exposure and 10s purge, followed by a 1s pulse of formalin + 60s exposure and 60s purge. The 60s exposure process allows effective infiltration and chemisorption of the precursors into the porous structure of the support, ensuring adequate Pd deposition for the synthesis of hydrogen-selective membranes.

All membranes were stored in ambient atmosphere between the alumina and Pd deposition steps by ALD.

## 3. Physico-chemical characterizations

Scanning Electron Microscopy (SEM FEG, Zeiss Sigma 300) was used to characterize the morphology, thickness and homogeneity of the ALD coatings. An acceleration voltage of EHT=5kV has been used. X-ray Photoelectron Spectroscopy (XPS) was carried out using an ESCALAB 250 spectrometer (ThermoElectron) equipped with a monochromatic radiation source Al Mono (AlKα=1486.6 eV) operating at 150 W (15kV and 10mA). The spectra are calibrated in binding energy with respect to the C 1s energy of the C-C component at 284.8 eV. The charge is compensated by a low energy (-2 eV) electron beam. The samples were exposed to ambient atmosphere during their transfer from the ALD reactor to the XPS chamber. The X-ray diffraction measurements were conducted with the Malvern-Panalytical X'Pert PRO Multi-Purpose Diffractometer with a copper anticathode tube, a theta-theta goniometer and an X'Celerator linear sensor. The Bragg-Brentano geometry was applied and for measurements on silicon wafer, a 1° offset so that ω=(2θ/2)+offset was introduced to suppress the
diffraction peaks of the Si support. This offset in incidence angle $\omega$ allows to remove specific reflection of crystalline silicon substrate [30]. Atomic Force Microscopy (AFM Nano-Observer, CSInstruments) was used to characterize the surface morphology of ALD deposits on Si substrates using silicon cantilevers (PPP-NCH, Nanosensors) with a typical tip radius of ~5 nm and a resonance frequency of ~235 kHz. Data were processed using the WsXm software [31].

## 4. Gas permeation measurements

The membranes were first characterized by single gas permeation measurements with $N_2$ (99.995%), and He (99.999%), supplied by Linde Gas. The tubular membranes were equipped with silicone O-rings to ensure a tight seal between the membrane and the stainless steel permeation cell. The single gas flow rate through the membrane was measured with a bubble flowmeter connected to the atmosphere. Transmembrane pressures ($\Delta P$) in the range $\Delta P= 0.5-2$ bar were applied for each gas in order to obtain measurable permeance values and measurements were always made from the highest to the lowest $\Delta P$. The membranes were systematically outgassed at room temperature before the introduction of each new gas. A sufficiently long stabilization time was applied between each measurement to ensure the reliability of the permeance ($\Pi$) values calculated using the following equation:

$$ \Pi = \frac{D}{A \times \Delta P} \quad (1) $$

where $D$ is the gas flow rate measured experimentally (mol.s$^{-1}$), $A$ is the geometric surface area of the tubular membrane (m$^2$) available for gas permeation, and $\Delta P$ is the applied transmembrane pressure (Pa).

Gas separation experiments were performed with two different gas mixtures (M1 and M2) supplied by Linde Gas. M1 consisted of $N_2$ (70.04%), $CO_2$ (10.03%), $H_2$ (9.97%) and $CH_4$ (9.96%), while M2 contained $H_2$ (70.35%), $CH_4$ (9.97%), $CO$ (9.91%) and $CO_2$ (9.77%). The transmembrane pressure was set at $\Delta P=3$ bar and measurements were carried out in the temperature range from 25°C to 334°C. The gas mixtures were preheated before introduction into the membrane module and He was used as a sweep gas on the permeate side of the membrane. For measurements above 150°C, graphite O-rings were used to seal the membrane in the permeation cell. Gas concentrations in the permeate were determined using a TCD-GC (PerkinElmer-Clarus 400) equipped with a shincarbon ST 100/120 column and a sampling loop. All tested membranes were kept inside the module under $N_2$ flow between measurement series. The separation factors were calculated as follows:

$$ F_{i/j} = \frac{x_{i \text{ downstream}}}{x_{i \text{ upstream}}} \cdot \frac{x_{j \text{ upstream}}}{x_{j \text{ downstream}}} \quad (2) $$

where $F_{i/j}$ is the separation factor of gas "i" relative to gas "j", and $x_i$ and $x_j$ are the gas concentrations of "i" and "j" before and after passing through the membrane (upstream and downstream, respectively).

## Results and discussion

As described in the introduction, this study aims to develop hydrogen-selective palladium-based composite membranes by optimizing ALD Pd coating on ceramic membrane supports. The strategy is based on a prior modification of the support by a layer of amorphous alumina deposited by ALD, in
order to increase the number of Pd nucleation sites and thus promote the formation of a continuous
and compact thin Pd membrane.

## 1. Growth mechanism of Pd coatings on alumina membrane supports

The ALD deposition process of Pd on the selected tubular alumina supports relies on a self-limited surface reaction governed by the alternating pulses of Pd(hfac)₂ and formalin. A major drawback of this synthesis strategy is the “island growth behavior” of Pd, which tends to form nanoparticles instead of a continuous palladium layer [32] with a very small growth rate of 0.2Å/cycle [33]. Although Pd initially grows in the form of nanoparticles, an increased number of ALD cycles results in the coalescence of particles and the subsequent formation of Pd thin films [34]. This phenomenon is caused by a slow nucleation step of Pd, due to surface poisoning by Al(hfac)* species, formed between the Al-OH* of the pristine alumina surface and the Pd(hfac)₂ precursor. This site blocking then prevents other Pd(hfac)₂ molecules from chemisorbing on the surface [35]. In order to overcome this limitation, we proposed in this work a modification of the γ-Al₂O₃ support with a very thin alumina layer to increase the number of nucleation sites for Pd growth and at the same time slightly reduce the pore size of the γ-Al₂O₃ support layer. The ~1 nm thick alumina layer deposited by ALD on the pore walls of the γ-Al₂O₃ layer reduces the pore diameter. The nucleation sites are represented by the hydroxyl groups and addition of an amorphous alumina layer is expected to increase their number and thus facilitate the formation of more compact Pd layers. To study the influence of the primer layer on the growth of Pd coatings, Pd membranes with and without ALD amorphous alumina (annotated 5Al1000Pd and 1000Pd) were synthesized and compared. For the former, before applying 1000 ALD cycles of Pd, the support was modified with 5 ALD cycles of alumina. Figure 1.a) shows the asymmetric structure of the pristine tubular membrane support consisting of four alumina layers with pore sizes ranging from 10 µm (macroporous α-Al₂O₃ support) to ~ 8 nm (γ-Al₂O₃ top-layer), while Figure 1.b) represents the Pd coating infiltrated in the γ-Al₂O₃ top-layer when no amorphous alumina pre-coating is deposited. The observation of the Pd deposit infiltrated inside the γ-Al₂O₃ porous structure is limited by the SEM resolution and the size of nanoparticles (a few nm), however, a homogeneous distribution of Pd across the support layer is clearly observed.

<figure><img src="image_8.png" /><figcaption>**Figure 1.** SEM observations of membrane cross-section: a) Pristine Al₂O₃ asymmetric support and b) composite Pd-Al₂O₃ membrane obtained by deposition of Pd on pristine γ-Al₂O₃ top-layer, without modification by ALD amorphous alumina.</figcaption></figure>

Indeed, the ALD technique allows transporting gaseous precursors into the pores of the alumina support, thus leading to a highly conformal deposition throughout the porous structure of the support.
This homogeneous growth of Pd nanoparticles is mainly ensured by a long exposure time (60s) of both Pd(hfac)₂ and formalin, which are able to diffuse in the pores and finally, the palladium nanoparticles are dispersed in the thickness of the support. This first result confirms the suitability of ALD to deposit palladium inside this type of support. However, despite the homogeneous growth of Pd deposits in the porous structure of the support, it remains challenging to form compact Pd membrane layers. Indeed, the tendency to grow relatively large nanoparticles leads to the formation of undesirable defects that impair membrane selectivity for efficient hydrogen separation.

In order to improve the membrane quality, 5 ALD cycles of amorphous alumina were performed before Pd deposition. As a result, the pore size of the γ-Al₂O₃ support layer was slightly reduced and the Pd deposit was more concentrated in its upper part (near the outer surface). EDX analysis (figure 2) clearly demonstrated that Pd was homogeneously dispersed through the cross-section of the support without any ALD alumina pre-coating, whereas it was almost exclusively present only in the upper part of the γ-Al₂O₃ support layer modified by ALD amorphous alumina. Nevertheless, the other porous layers of the support are coated with palladium in the same manner, regardless of whether an alumina pre-coating layer is present.

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 2.** EDX mapping of the membrane cross-sections for a) 1000Pd and b) 5Al1000Pd samples.</figcaption></figure>

To further investigate the morphology of the Pd deposit and address the influence of the pre-coated ALD amorphous alumina layer, AFM measurements were conducted on companion samples prepared on silicon wafers. Figure 3 presents 2D AFM topographic images alongside corresponding surface profiles for samples prepared via direct Pd deposition on a silicon wafer *with only the native oxide on its surface* and those pre-coated with ALD amorphous alumina.
<figure><img src="image_11.png" /><figcaption>**Figure 3.** 2D AFM topographic images at 1x1 µm² and corresponding line profiles for a) 1000Pd and b) 5Al1000Pd samples on Si wafers.</figcaption></figure>

It can be clearly observed that Pd nanoparticles grown on Si wafer are larger ($\emptyset_{Pd} \sim 160$ nm) than those grown on Si wafer modified with ALD alumina ($\emptyset_{Pd} \sim 85$ nm). The latter form a more compact Pd layer, with a very small inter-particle space compared to the one in Figure 3.a). The average, Ra, and root mean square, Rms, surface roughness values of samples without ALD alumina coating are 7.7 nm and 9.7 nm, respectively, while they are found to be 5.3 nm and 6.7 nm for their pre-coated counterparts. These data support the hypothesis that ALD alumina increases the number of nucleation sites due to the presence of reactive hydroxyl groups on its surface. Thereby inducing a growth of smaller Pd nanoparticles and thus their better interconnection (coalescence) during the formation of compact and defect-free Pd thin membranes. The quality of these membranes is evaluated later in this work through single gas permeation tests, revealing virtually zero viscous flow contribution. These observations are consistent with the work of Lu et al. on the deposition of an ALD alumina atomic interlayer between each Pd ALD cycle [36]. Indeed, by comparing the deposition of Pd obtained on a pristine alumina substrate using a conventional AB-type thermal ALD process (alternating pulses of a reactant and a co-reactant to form an ALD cycle) with another involving a low-temperature ABC-type method (alternating pulses of three different ALD precursors to form an ALD cycle), they assumed that the deposition of ALD alumina interlayers allowed to significantly increasing the quantity of nucleation sites. In the present work, the same phenomenon should occur, but to a lesser extent due to a more limited modification of the support.

## 2. Crystalline structure and composition of the composite membrane

X-ray diffraction analysis was carried out on equivalent membrane layers deposited on silicon wafers to assess the crystalline structure of the ALD deposits. Figure 4 compares the XRD pattern of a sample
278 subjected to 1000 ALD cycles of Pd (blue) with that of its counterpart, whose support was modified by
279 ALD amorphous alumina before Pd deposition (red).

<figure><img src="image_12.png" /><figcaption>280
281 **Figure 4.** XRD patterns of equivalent Pd-ALD membrane layers deposited on pristine (1000Pd) and
282 ALD-Al₂O₃ modified (5Al1000Pd) silicon wafers.</figcaption></figure>

283 The characteristic peaks of the face-centered cubic (fcc) lattice of palladium can be identified for both
284 samples. The peak assignment is consistent with the literature [17], [37]. Note that no peak
285 corresponding to the alumina structure could be resolved in the 5Al1000Pd sample, which was
286 expected as alumina deposited by ALD at 100°C is known to be amorphous [38]. The sharp peak (111)
287 for the 1000Pd sample indicates a preferential (111) crystal orientation. The same Pd(111) peak is also
288 visible in the 5Al1000Pd sample. Since the intensity of peaks depends on the equipment and acquisition
289 parameters, only the FWHM was used to calculate the crystallite size. While the XRD analysis was not
290 performed on the final membrane substrate, the effect of Al₂O₃-ALD on Pd nucleation remains an area
291 for further exploration. In order to verify whether this profile change could be linked to the size of the
292 Pd nanoparticles, the average crystallite sizes were calculated using the Scherrer equation with K=0.86
293 [39] (more details in Supplementary Information, S1), giving D̄₁₀₀₀Pd = 216 nm and D̄₅Al₁₀₀₀Pd =
294 172 nm. As expected from AFM observations, the crystallite size of Pd deposited on a Si substrate
295 pretreated with ALD alumina was found to be smaller than that of Pd particles deposited on a pristine
296 Si wafer. Hence, by combining the data from AFM and XRD analyses, the two palladium layers could
297 be schematically represented as shown in Figure 5a. This hypothesis is supported by the 3D AFM
298 reconstruction presented in Figure 5b.
<figure><img src="image_13.png" /><figcaption>**Figure 5.** Surface morphology of equivalent 1000Pd and 5Al1000Pd samples: a) expected from XRD and 2D AFM topographies and b) obtained by 3D AFM reconstruction at 1x1 µm².</figcaption></figure>

X-ray Photoelectron Spectroscopy (XPS) measurements confirm the presence of both metallic (Pd⁰) and oxidized (PdO) palladium [40], [41] in the equivalent samples 5Al1000Pd and 1000Pd (Figure 6). “The presence of this oxide layer is expected, as samples were exposed to ambient atmosphere during their transfer from the ALD reactor to the XPS chamber. Consequently, the Pd in contact with the air undergoes oxidation, and the proportion of PdO in each sample may serve as an indicator of the Pd nanoparticle active area. Considering that PdO is more electronegative than Pd⁰, the absorption of the latter is shifted towards higher binding energies.

<figure><img src="image_14.png" /><figcaption>**Figure 6.** X-ray Photoelectron Spectroscopy analysis of a) 1000Pd, and b) 5Al1000Pd equivalent samples on porous flat disks, in the region Pd3d with spin-orbit components. The corresponding binding energy values are reported in Table 1.</figcaption></figure>

<table><caption>**Table 1.** Binding energy values obtained by X-ray Photoelectron Spectroscopy of the 3d orbital of Pd</caption><thead><tr><th rowspan="3"></th><th colspan="4">Binding energy (eV)</th></tr><tr><th colspan="2">PdO</th><th colspan="2">Pd⁰</th></tr><tr><th>Pd3d<sub>3/2</sub></th><th>Pd3d<sub>5/2</sub></th><th>Pd3d<sub>3/2</sub></th><th>Pd3d<sub>5/2</sub></th></tr></thead><tbody><tr><td>1000Pd</td><td>342.00</td><td>337.64</td><td>341.35</td><td>336.56</td></tr></tbody></table>
<table><tr><td>5Al1000Pd</td><td>342.70</td><td>337.34</td><td>340.95</td><td>335.70</td></tr></table>

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The relative intensity of PdO peaks compared to Pd⁰ ones is lower in Figure 6.b) than in Figure 6.a). The intensity ratio of PdO and Pd⁰ peaks (peak1/peak2 and peak3/peak4 ratios) reaches 0.8 and 1, respectively, for the 1000Pd sample, while it is 0.6 and 0.7 for 5Al1000Pd, respectively. Hence, the amount of PdO is decreased in the presence of the ALD alumina layer. Nevertheless, the presence of PdO species does not hamper the dissociation of H₂, since the thin oxide layer on top of the Pd⁰ nanoparticles is likely reduced by hydrogen at the elevated temperatures applied during gas permeation tests [42], [43].

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### 3. Evaluation of membrane performance

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#### 1.1. Single gas permeation measurements

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Single gas permeation measurements with N₂ and He were carried out at room temperature to check the quality of the prepared Pd/alumina composite membranes. Low permeance values (much lower than those of the pristine γ-Al₂O₃, which is typically ≥10⁻⁵ mol.m⁻².s⁻¹.Pa⁻¹ for N₂ at T=25°C and ΔP~0.5 bar) were expected to confirm successful coating/loading of the porous support with Pd. In comparison with the pristine membrane support, the drop of single gas permeances after only 5 ALD cycles of alumina (designated as 5Al in Table 2) also confirms the slight reduction in pore size of the pristine γ-Al₂O₃ support. The measured single gas permeance values of the Pd/alumina composite membranes prepared on pristine γ-Al₂O₃ supports and those modified by ALD alumina coating are compared in Table 2.

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<table><caption>**Table 2.** Comparison of He and N₂ permeance values (at 25°C) for Pd/alumina composite membranes obtained by ALD on/in the pristine γ-Al₂O₃ support (1000Pd) and on the support pre-modified by an amorphous alumina layer (5Al1000Pd).</caption><thead><tr><th rowspan="2">ALD deposition</th><th rowspan="2">ΔP (bar)</th><th colspan="2">Permeance Π (mol.m<sup>-2</sup>.s<sup>-1</sup>.Pa<sup>-1</sup>)</th></tr><tr><th>He</th><th>N<sub>2</sub></th></tr></thead><tbody><tr><td>Pristine support</td><td>0.5</td><td>Above the detection limit</td><td>~10<sup>-5</sup></td></tr><tr><td>1000Pd</td><td>0.5</td><td>1.10<sup>-5</sup></td><td>4.10<sup>-6</sup></td></tr><tr><td>5Al</td><td>1</td><td>8.10<sup>-6</sup></td><td>3.10<sup>-6</sup></td></tr><tr><td>5Al1000Pd</td><td>2</td><td>9.10<sup>-10</sup></td><td>3.10<sup>-10</sup></td></tr></tbody></table>

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Considering the measured permeance values, it can be concluded that the support pretreated with an ALD alumina layer allows the formation of more compact membrane barriers decreasing the permeance by a factor x10⁴ compared to the Pd composite membranes prepared on/in pristine γ-Al₂O₃ supports. In fact, for the latter, it can be assumed that the pores of the γ-Al₂O₃ support are not completely filled with Pd, thus leaving non-selective pathways that compromise the membrane separation efficiency for hydrogen. Hence, only 5Al1000Pd membranes were further tested with gas mixtures at different temperature ranging from 25°C to 334°C to assess their potential for hydrogen separation.

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#### 1.2. Gas mixture permeation measurements

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Figure 7 shows the evolution with temperature of the gas permeance values and separation factors for a quaternary gas mixture (M1 – see experimental section) containing H₂ as well as N₂, CO₂ and CH₄. The pink dotted line corresponds to measurements made by replacing the gas mixture M1 by M2 containing H₂, CO, CH₄ and CO₂.
a)

<figure><img src="image_15.png" /></figure>

b)

<figure><img src="image_16.png" /></figure>

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**Figure 7.** Results of gas permeation measurements with M1 and M2 gas mixtures (70%N₂, 10%CO₂, 10%H₂, 10%CH₄ for M1 and 70%H₂, 10%CH₄, 10%CO, 10%CO₂ for M2, respectively) in the temperature range 25-334°C and ΔP=3 bar: a) gas permeance values (with N₂, CH₄ and N₂ symbols overlapping because of their too close permeance values) and b) corresponding separation factors.

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As shown in figure 7a), the permeance of H₂ increases with temperature from 1.4x10⁻⁹ mol.Pa⁻¹.m⁻².s⁻¹ at 25°C to 3.1x10⁻⁸ mol.Pa⁻¹.m⁻².s⁻¹ at 218°C. In contrast, the permeances of N₂, CH₄ and CO₂ remain stable, not exceeding ~ 1.0x10⁻⁹ mol.Pa⁻¹.m⁻².s⁻¹ over the entire temperature range studied. The observed thermally activated transport of H₂ led at 190°C to separation factors of 40, 38 and 30 for the gas pairs H₂/N₂, H₂/CH₄ and H₂/CO₂, respectively, and further increased to 56, 55 and 34, respectively, at 218°C. It should be also underlined that the membrane exhibits high stability, as repetitive gas permeation tests performed with the membranes maintained at 218°C for 15 days revealed no decrease in membrane performance (virtually no change in gas permeance values or separation factors). Another important aspect to be addressed when developing Pd-based membranes is their sensitivity to poisoning species [44], [45]. To determine whether the presence of CO, a well-known poisoning species for Pd, influences the membrane performance, a single permeation experiment was carried out at 218°C with the M2 gas mixture (70%H₂, 10%CH₄, 10%CO and 10%CO₂). As shown in Figure 7.a), H₂ permeance dropped down in presence of CO by about 10 times at ~3x10⁻⁹ mol.Pa⁻¹.m⁻².s⁻¹, thus confirming a detrimental effect of CO on the membrane performance. However, the observed phenomenon was found to be reversible since the H₂ permeance was recovered by a simple introduction of a gas mixture without any poisoning elements (M1). Further research on the protection of the Pd surface is currently in progress, focusing on the passivation of Pd with various ultra-thin ALD coatings to avoid undesirable interactions with poisoning species such as CO or H₂S.

After the test in the presence of CO (M2 gas mixture), the temperature was increased until 334°C with the M1 gas mixture. The highest H₂ permeance value ~ 5.1x10⁻⁸ mol.Pa⁻¹.m⁻².s⁻¹ was obtained at 303°C. To compare these membranes with those reported in the literature, a comparative table (Table S3) highlighting the performance of Pb-based membranes is provided in the SI. On the other hand, the highest separation factors were obtained at 286°C, with values of 86, 79 and 67 for the gas pairs H₂/N₂, H₂/CH₄ and H₂/CO₂, respectively. Indeed, above 286°C the membrane performance starts to deteriorate as the separation factors decreased significantly (Figure 7.b). This decrease seems to be consistent with the critical temperature for hydrogen sorption by Pd at 298°C [46]. Below this point, hydrogen permeates through palladium by being absorbed into the Pd-fcc crystal lattice at its octahedral sites, forming palladium hydride [47]. This inevitably induces an expansion of the lattice parameters resulting in the coexistence of two different crystallographic phases: α and β (Nucleation
of the Pd hydride β-phase within the α-phase). Although both are fcc, the α phase is the low hydrogen phase with a unit cell lattice parameter of 0.3895 nm (0.3890 nm for hydrogen-free Pd) while the β-phase is the high hydrogen phase with a lattice parameter of 0.410 nm [48]. Thus, there is a miscibility gap between the α and β phase below 298°C, in which hydrogen is able to dissociate and pass through the Pd [10], [49]. The coexistence of these two phases occurs in a lattice volume expansion of 10% that generates internal stresses, which can lead to membrane failure after repeated sorption and desorption cycles [50], [51]. [10], [49][10], [48][10], [48][10], [47][10], [46][10], [45][10], [41][10], [40]. It is thus recommended to use these composite Pd/Al₂O₃ membranes below the critical temperature in order to maintain high permeability and separation performance.

The performance of the membrane developed in this work demonstrates a significant improvement over that of the Pd-based composite membranes prepared by Weber *et al.* using the ALD technique on the same type of porous tubular support [24]. Indeed, their protocols not only required the application of a significantly higher amount of Pd (2000 ALD cycles), but also the best separation factors reported were well beyond those obtained in this work. Therefore, the modification of the γ-Al₂O₃ support with ALD alumina clearly activates the growth of uniform and thin Pd-membranes with attractive performance for H₂ separation below 300°C. In this context, it is worth mentioning that alumina deposits could eventually be replaced by other materials. For instance, the deposition of a TiN layer by ALD prior to Pd, as studied in our previous work [52] showed that a spillover effect could occur upon heating, facilitating the transport of hydrogen through the TiOₓNᵧ-Pd membrane.

In order to check whether the studied Pd composite membrane can withstand thermal cycling even after overpassing the critical temperature, the membrane tested in Figure 7 was naturally cooled under N₂ gas and stored under ambient conditions for ~1 month and then submitted to another series of gas separation experiments with the M1 gas mixture (Figure 8). Figure 8.a provides a comparison of the permeance values measured for a fresh membrane (pale blue) and for the same (aged) membrane after a first thermal cycling up to 334°C (bright blue). The separation factors shown in Figure 8.b correspond to results obtained with the “aged membrane” (permeance values reported in Figure 8.a)

a)

<figure><img src="image_17.png" /></figure>

b)

<figure><img src="image_18.png" /><figcaption>**Figure 8.** Results of permeation measurements with the M1 gas mixture (70%N₂, 10%CO₂, 10%H₂ and 10%CH₄): a) comparison of gas permeance values between a freshly prepared membrane (bright colors), and the same membrane after one temperature cycling up to 334°C (pale colors) and b) separation factors obtained with the aged membrane.</figcaption></figure>

Despite a decrease in the overall H₂ permeance values measured for the aged membrane, the H₂/N₂, H₂/CH₄ and H₂/CO₂ separation factors obtained at 218°C and 273°C remained similar to those obtained
for a fresh membrane in the same temperature range. In fact, the permeance of other gases also decreased, thus compensating the H₂/(other gas) permeance ratio (Figure 8.b). Regardless of the permeance change, the membranes could be claimed to present operational stability. In fact, the Pd confined in the pores of the γ-Al₂O₃ layer is certainly constrained, limiting the expansion of the Pd lattice and thus its dislocation during temperature cycling. Weber et al. [24] also proposed this hypothesis, suggesting that the confinement of Pd in the rigid ceramic support mesopores represents an attractive option for limiting the embrittlement of the membrane. Moreover, cooling under controlled atmosphere (here N₂) should contribute to maintain the integrity of the membrane, since no hydrogen atom could induce a lattice distortion during the cooling step.
428 **Conclusion**

429 Pd-based composite membranes were prepared by ALD of Pd coatings on porous ceramic tubular
430 supports. Compared to similar membrane materials previously described in the literature, Pd
431 nucleation/growth on the membrane support was improved by ALD of thin amorphous alumina layers
432 cast prior to Pd deposition. In addition, the number of ALD cycles was decreased, which reduced the
433 consumption of Pd precursor and thus the manufacturing costs. The optimized membranes exhibit
434 homogeneous and compact Pd thin layers well anchored on/in the top-surface of the porous support.
435 Such membrane architecture leads to attractive hydrogen selectivities at temperatures above 200°C
436 and remarkable long-term stability when used below 300°C. The gas separation factors at 286°C were
437 86 for H₂/N₂, 79 for H₂/CH₄ and 67 for H₂/CO₂, with H₂ permeance value up to ~4.0x10⁻⁸ mol.Pa⁻¹.m⁻².s⁻¹.
438 It was also demonstrated that the membranes poisoning after CO exposure is reversible since the
439 initial hydrogen permeance can be easily recovered. We believe that the as-prepared membranes with
440 optimized morphology, structure, chemical composition and performance for hydrogen separation
441 applications represent a real asset for the further development of highly selective Pd-based
442 membranes with minimized amounts of Pd. Nevertheless, further work is required to improve the
443 membrane characteristics, including exploring methods to reduce chemical contamination during the
444 ALD deposition of Pd. Specifically, the use of other co-reactants such as O₃, O₂, or H₂ is recognized to
445 limit fluorine contamination [53] and will be discussed in our next communication.
446

446

447 **Acknowledgments**

448 Authors thank the French National Research Agency (ANR, program ALD4MEM-ANR-20-CE09-0008-01) and the Occitanie Region (FEDER React-Eu Défi-clé Hydrogène Vert, MEMbyALD) for funding this research.
449
450

451

452 **Data availability statement**

453 The authors confirm that the data supporting the findings of this study are available within the article
454 and its supplementary materials.

455

456 **CRediT author statement**

457 Lucie Badouric: Methodology, Formal analysis, Investigation, Data Curation, Writing - Original Draft,
458 Visualization; Martin Drobek : Methodology, Validation, Writing - Review & Editing, Supervision; Jim
459 Cartier: Methodology, Validation, Formal analysis, Data Curation, Writing - Review & Editing;
460 Christophe Charmette: Conceptualization, Methodology, Validation, Formal analysis, Writing -
461 Review & Editing, Supervision. Bertrand Rebière : Methodology, Validation, Formal analysis, Data
462 Curation, Writing - Review & Editing; Eddy Petit: Methodology, Validation, Formal analysis, Data
463 Curation, Writing - Review & Editing; Maximilien Coronas: Methodology, Validation, Formal analysis,
464 Data Curation, Writing - Review & Editing; Julien Cambedouzou: Validation, Formal analysis, Writing -
465 Review & Editing; Karim Aissou : Validation, Formal analysis, Writing - Review & Editing; Anne Julbe:
466 Conceptualization, Methodology, Validation, Formal analysis, Writing - Review & Editing,
467 Visualization, Supervision; Mikhael Bechelany: Conceptualization, Methodology, Validation,
468 Resources, Writing - Review & Editing, Supervision, Project Administration, Funding Acquisition.
469

469
470

471 ORCID

472 Lucie Badouric : 0009-0001-1357-544X

473 Martin Drobek : 0000-0003-2679-282X

474 Jim Cartier : 0000-0002-7677-1683

475 Christophe Charmette : 0000-0003-2352-9823

476 Bertrand Rebière : 0000-0002-1767-6547

477 Eddy Petit : 0000-0002-0999-4107

478 Maximilien Coronas : 0000-0002-4448-1956

479 Julien Cambedouzou : 0000-0002-8811-5667

480 Karim Aissou : 0000-0002-9236-464X

481 Anne Julbe : 0000-0001-7195-5355

482 Mikhael Bechelany : 0000-0002-2913-2846
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