M Lehujeur

S Chevrot

On the validity of eikonal equation for surface-wave phase-velocity tomography

Keywords: Seismology, Tomography, Surface waves and free oscillations

The phase velocity of surface waves can be directly determined from the amplitude and phase of the regional wavefield using the Helmholtz equation. However, the Helmholtz equation involves estimating the Laplacian of the amplitude field, a challenging operation to perform on noisy and sparsely sampled seismic data. For this reason, the amplitude information is often discarded. In that case, phase velocity maps are reconstructed with the eikonal equation, which relates the local phase slowness to the gradient of the phase.

Here, we derive analytical expression of the errors arising from neglecting the amplitude of the wavefield in eikonal tomography. In general, these errors are quite strong but they vary sinusoidally with the wave propagation direction. Consequently, if the azimuthal coverage is good, they will average out, and unbiased phase velocity maps can be obtained with eikonal tomography. We numerically validate these results with a synthetic tomography experiment.

INTRODUCTION

The Helmholtz equation relates the travel time and amplitude fields to the local phase velocity of surface waves, thereby providing a direct means to map lateral variations of phase velocities beneath dense regional arrays [START_REF] Friederich | Non-plane geometry of seismic surface wavefields and their implications for regional surface-wave tomography[END_REF][START_REF] Friederich | Interpretation of seismic surface waves in regional networks: joint estimation of wavefield geometry and local phase velocity. Methhod and numerical tests[END_REF]. The main advantage of Helmholtz tomography, apart from its implementation simplicity, is that it does not involve resolving an inverse problem, thus also avoiding degrading the resolution by introducing arbitrary damping and smoothing regularization constraints into the inversion. Accompanying the booming of dense regional seismic deployments, Helmholtz tomography rapidly developed and gained in popularity. Perhaps its most remarkable achievements came from the exploitation of the USArray Transportable Array (TA) data to image the shear velocity structure of the crust and uppermost mantle beneath USA with unprecedented resolution [START_REF] Pollitz | Observations and interpretation of fundamental mode Rayleigh wavefields recorded by the Transportable Array (USArray)[END_REF][START_REF] Lin | Eikonal tomography: Surface wave tomography by phase front tracking across a regional broad-band seismic array[END_REF][START_REF] Lin | Helmholtz surface wave tomography for isotropic and azimuthally anisotropic structure[END_REF][START_REF] Jin | Surface wave phase-velocity tomography based on multichannel crosscorrelation[END_REF][START_REF] Shen | Crustal and uppermost mantle structure beneath the United States[END_REF].

Helmholtz tomography involves estimating the Laplacian, i.e. second order spatial derivatives, of the amplitude field from its samples at the position of seismological stations. This is an ill-posed problem because differentiation is very sensitive to noise [START_REF] Scales | Prior information and uncertainty in inverse problems[END_REF][START_REF] Roy | On computing first and second order derivative spectra[END_REF]. In practice, resulting amplitude corrections are often found to be small compared to the uncertainty on phase velocity maps (e.g., [START_REF] Feng | A 3-D shear velocity model of the crust and uppermost mantle beneath Alaska including apparent radial anisotropy[END_REF] and are often ignored. Ignoring the amplitude information in the Helmholtz equation leads to the eikonal equation, which simply relates the gradient of the phase to the local slowness (Wielandt 1993;[START_REF] Lin | Eikonal tomography: Surface wave tomography by phase front tracking across a regional broad-band seismic array[END_REF]). Nevertheless, even though eikonal tomography leads to phase velocity maps with strong artefacts (e.g., Wielandt 1993;[START_REF] Friederich | Non-plane geometry of seismic surface wavefields and their implications for regional surface-wave tomography[END_REF][START_REF] Bodin | Resolution potential of surface wave phase velocity measurements at small arrays[END_REF] have shown that these artefacts can be strongly mitigated if the azimuthal coverage is good.

In this study, we characterize the errors introduced by neglecting the contribution of amplitude in the reconstruction of the velocity model, i.e. of using the eikonal equation instead of the complete Helmholtz equation. We discuss the limits of validity of eikonal tomography and suggest simple procedures to improve the quality and robustness of resulting phase velocity models. We then perform a synthetic eikonal tomography experiment to numerically validate our results.

THEORETICAL CONSIDERATIONS

The Helmholtz equation

In a smoothly varying Earth model, Love and Rayleigh wave displacements can be represented in terms of a 2-D potential χ that satisfy the Helmholtz equation [START_REF] Tanimoto | Modelling curved surface wave paths: membrane surface wave synthetics[END_REF][START_REF] Tromp | Variational principles for surface wave propagation on a laterally heterogeneous Earth -III. Potential representation[END_REF])

∆χ(r) + ω 2 c 2 χ(r) = 0, ( 1
)
where ∆ is the Laplacian operator, r the position vector, ω the angular frequency, and c the local Love or Rayleigh phase velocity.

On the validity of eikonal equation for surface-wave phase-velocity tomography 3

A simple high-frequency solution of (1) is given by

χ(r) = A(r)e iωT (r) , (2)
where A(r) and T (r) are respectively the amplitude and phase of potential χ at location r. Equation ( 2) is valid if A(r) and T (r) are both smooth functions of spatial coordinates. In that case, the Helmholtz equation ( 1) can be rewritten in the form (e.g., [START_REF] Cervený | Seismic Ray Theory[END_REF])

∆A -ω 2 A||∇T || 2 + ω 2 c 2 A + iω (2∇A • ∇T + A∆T ) = 0, (3)
where for the sake of conciseness we omitted the spatial dependences of A and T . Isolating the imaginary part of (3) gives the transport equation

2∇A • ∇T + A∆T = 0, (4)
whereas the real part leads to (Wielandt 1993)

1 c 2 = ||∇T || 2 - ∆A ω 2 A , (5)
which is often referred to as the Helmholtz equation in the literature (e.g., [START_REF] Lin | The local amplification of surface waves: A new observable to constrain elastic velocities, density, and anelastic attenuation[END_REF][START_REF] Jin | Surface wave phase-velocity tomography based on multichannel crosscorrelation[END_REF]. This equation provides a direct relationship between the local phase velocity and the phase and amplitude of a surface-wave front, two quantities that can be easily determined from seismic records.

The eikonal equation

The main limitation of Helmholtz tomography comes from the second term on the right side of (5). Its estimation involves computing the Laplacian of the amplitude field, i.e. computing its second derivatives, a challenging task because of noise measurement and irregular sparse station sampling. Ignoring the amplitude term leads to the eikonal equation [START_REF] Lin | Eikonal tomography: Surface wave tomography by phase front tracking across a regional broad-band seismic array[END_REF]):

1 c 2 = ||∇T || 2 . ( 6
)
In that case, the gradient of the traveltime field is directly related to the local phase slowness.

The validity of the eikonal equation is often justified by the ω -2 dependence of the amplitude term, thus considered as a finite-frequency correction that can be neglected at high frequency (e.g., [START_REF] Lin | Eikonal tomography: Surface wave tomography by phase front tracking across a regional broad-band seismic array[END_REF]. Numerical experiments have demonstrated that the amplitude field is controlled by the interference pattern between the incoming and scattered wavefields [START_REF] Friederich | A note on the interpretation of seismic surface waves over three-dimensional structures[END_REF][START_REF] Bodin | Resolution potential of surface wave phase velocity measurements at small arrays[END_REF]. These scattered waves, as well as the complexities of the incoming wavefield, can strongly bias phase velocity estimates relying purely on phase information (Wielandt 1993; Friederich We consider an incoming plane wave with wave vector k 0 . The scattered wavefield at r with wave vector k r depends on the scattering angle θ, the angle between wave vectors k 0 and k r , and on ρ, the distance from the scatterer.

of long-period surface waves (periods ≥ 20 s) made with large-aperture arrays have shown that the amplitude field is highly oscillating with magnitude of variations that can reach several hundreds percents (e.g., [START_REF] Friederich | Wave-theoretical inversion of teleseismic surface waves in a regional network: phasevelocity maps and a three-dimensional upper-mantle shear-wave-velocity model for southern Germany[END_REF][START_REF] Lin | Eikonal tomography: Surface wave tomography by phase front tracking across a regional broad-band seismic array[END_REF][START_REF] Pollitz | Observations and interpretation of fundamental mode Rayleigh wavefields recorded by the Transportable Array (USArray)[END_REF][START_REF] Jin | Surface wave phase-velocity tomography based on multichannel crosscorrelation[END_REF]. However, the estimations of the Laplacian of the amplitude field are usually small compared to the squared gradient of the phase, which often lead to drop the amplitude correction in practice (e.g., [START_REF] Feng | A 3-D shear velocity model of the crust and uppermost mantle beneath Alaska including apparent radial anisotropy[END_REF]. This short overview of the literature suggests that the limits of validity of the eikonal equation are still rather fuzzy. Our motivation is thus to provide new physical insights into the validity of the eikonal equation through a simple theoretical investigation of the problem, which is presented in the sequel.

ERROR DUE TO THE EIKONAL APPROXIMATION [Fig. 1]

In this section, we characterize the error that is made by dropping the amplitude term in (5), i.e.

by using the eikonal equation instead of the complete Helmholtz equation to derive phase velocities.

Following [START_REF] Bodin | Resolution potential of surface wave phase velocity measurements at small arrays[END_REF], we consider the simple case of an incident plane wave that propagates through a medium with background phase velocity c 0 . The wavefield u 0 at position r produced by this incident plane wave with wave vector k 0 = ω/c 0 is

u 0 (r) = exp (ik 0 .r) . (7)
On the validity of eikonal equation for surface-wave phase-velocity tomography 5

This wavefield interacts with a scatterer located at r s and produces the scattered wavefield

u s (r) = ε 2π||r -r s || exp (i [k 0 .r s + k r . (r -r s )]) , ( 8
)
where ε is the relative amplitude of the scattered wave field with respect to the incident wavefield, and k r the wave vector of the scattered wavefield. Figure 1 displays the geometry of this simple ansatz experiment. Assuming that ε 1 and using the Taylor series approximation (1 + x) α ≈ 1 + αx, the amplitude of the total wave field is to first order

A = ||u 0 + u s || 2 ≈ 1 + ε √ 2πρ cos(k s .ρ), (9)
where ρ = rr s and k s = k 0 -k r is the scattering vector.

We now introduce the new variable a = log A which, using the Taylor series approximation

log(1 + x) ≈ x, is simply given by a = log A ≈ ε √ 2πρ cos(k s .ρ). ( 10
)
Using a, the amplitude correction term in the Helmholtz equation becomes

∆A A = ||∇a|| 2 + ∆a. (11)
The Laplacian of a is

∆a ≈ -ε √ 2πρ k 2 s cos(k s .ρ), (12)
and is thus of order ε. In contrast, because the gradient of a is

∇a ≈ -ε √ 2πρ k s sin(k s .ρ), (13)
the squared gradient is of order ε 2 with respect to the incident wave field:

||∇a|| 2 ≈ ε 2 2πρ k 2 s sin 2 (k s .ρ). (14)
Its contribution to (11) can thus be neglected to first order. We thus obtain the first-order approximation of the amplitude correction term in the Helmholtz equation:

∆A A ≈ ∆a ≈ -ε √ 2πρ k 2 s cos(k s .ρ). (15)
Introducing (15) into the Helmholtz equation ( 5) gives that

||∇T || -1 ≈ c 0 1 + ∆a k 2 0 -1 2 , (16)
which, using the same Taylor series approximation as above, leads to the eikonal phase velocity

c = ||∇T || -1 ≈ c 0 1 + 1 2 ε √ 2πρ k 2 s k 2 0 cos(k s .ρ) . (17)
Introducing the angle θ between k 0 and k r (Fig. 1), we have

k 2 s = 2k 2 0 (1 -cos θ), (18)
and

k s .ρ = k 0 ρ(cos θ -1). ( 19
)
Inserting these terms in (17) we thus finally obtain

||∇T || -1 ≈ c 0 1 + ε √ 2πρ (1 -cos θ) cos(k 0 ρ(1 -cos θ)) . ( 20
)
The second term in the right-hand side of (20) approximates the bias that comes from the amplitude term in (5), which results from the contribution of singly scattered waves. It thus approximates the error that is made when the eikonal equation is used to estimate phase velocities.

MITIGATING ERRORS IN EIKONAL TOMOGRAPHY

Figure 2a shows the apparent eikonal phase velocity model obtained with Eq. ( 20) considering an incident plane wave with a wavelength of 15 km (period 5 s) and a back azimuth of 270 • propagating in a homogeneous model (c 0 = 3 km/s) with a scatterer located at the center of the 2-D model. The relative amplitude of the diffracted wave field ε is 5 %. The apparent eikonal velocity model obtained is in excellent agreement with the results of [START_REF] Bodin | Resolution potential of surface wave phase velocity measurements at small arrays[END_REF] (their figure 2). In particular, it reproduces the strong errors resulting from the interference between the incident and back-scattered wave fields, which show oscillations with a spatial periodicity of half the incoming wavelength.

The error in (20) has an oscillatory pattern as a function of the azimuth of the incoming wavefront, which suggests that it can be reduced by azimuthal averaging, as pointed out in [START_REF] Bodin | Resolution potential of surface wave phase velocity measurements at small arrays[END_REF].

The phase velocity map obtained by azimuthal averaging, i.e. integrating (20) over the scattering angle θ, is given by

1 2π π -π ||∇T || -1 dθ ≈ 1 2π π -π c 0 1 + ε √ 2πρ (1 -cos θ) cos(k 0 ρ(1 -cos θ)) dθ. ( 21
)
Let us define

I(x) = 1 2π π -π (1 -cosθ)e ix(1-cosθ) dθ, (22)
which, after changing θ to π -θ, can be written

I(x) = 1 2π 2π 0 (1 + cosθ)e ix(1+cosθ) dθ = e ix 2π 2π 0 e ixcosθ dθ + 2π 0 cosθe ixcosθ dθ = e ix J 0 (x) -iJ 0 (x) , ( 23
)
where J 0 is the 0 th -order Bessel function of the first kind. Inserting the real part of I(k 0 ρ) in ( 21), we obtain 1 2π

2π 0 ||∇T || -1 dθ = c 0 1 + δc c 0 , ( 24
)
where

δc c 0 ≈ ε √ 2πρ cos(ρk 0 )J 0 (ρk 0 ) + sin(ρk 0 )J 0 (ρk 0 ) . ( 25
)
Equations (24,25) show that after azimuthal averaging, the error on phase velocity oscillates around the true velocity, with a radial pattern centered on the diffracting point (Fig. 2c).

At large distances from the scatterer (ρ λ):

J 0 (k 0 ρ)-→ 1 √ πk 0 ρ [cos(k 0 ρ) + sin(k 0 ρ)] J 0 (k 0 ρ)-→ 1 √ πk 0 ρ [cos(k 0 ρ) -sin(k 0 ρ)] , (26)
and the correction term in (25) becomes simply

δc c 0 ≈ ε √ 2k 0 πρ [sin(2k 0 ρ) + cos(2k 0 ρ)] . ( 27
)
Therefore, the biases in azimuthally averaged eikonal phase velocity maps decrease as a function of ρ, the distance from scatterer (Fig. 2c). Moreover, since this error term is dominated by a λ/2 periodic pattern, smoothing (or low-pass spatial filtering) is very efficient to attenuate it.

Fig. 2b illustrates the smoothed version of the apparent eikonal velocity model from Fig. 2a, obtained after applying a 2D Gaussian filter of width λ/2. Combining lateral and azimuthal averaging of the apparent eikonal phase velocity model strongly attenuates the contribution of the scattered wave field. We obtain a finite-extent blurred anomaly located at the position of the scatterer (Fig. 2d). Note that since these two operations are linear, they can be performed in any order.

[Fig. 2]

NUMERICAL EXPERIMENT

We now validate numerically our analytical developments by comparing the results of Helmholtz and eikonal tomography in a synthetic experiment. We consider incident plane surface waves at 5 s period propagating in a model with an average phase velocity of 3.0 km/s and velocity perturbations around 5 % (Fig. 3a). We solve the scalar wave equation with a finite difference method in a regular Cartesian grid. At the boundaries, we impose the displacement produced by an incoming plane wave with a phase velocity of 3.0 km/s (wavelength of 15 km). We implement Stacey boundary conditions to absorb the outgoing scattered wave field [START_REF] Stacey | Improved transparent boundary formulations for the elastic wave equation[END_REF]). The problem is solved for different azimuths of the incident wave.

Fig. 3c,d show the phase and amplitude fields modeled for an incoming azimuth of 280 • (white arrow). By unwrapping the phase of the complex wave field, we obtain the phase travel times relative to the center of the model (Fig. 3c). The amplitude field is characterized by strong fluctuations (Fig. 3d) caused by focusing/defocusing of the wave front. The location and scale of the amplitude anomalies depend on the azimuth of the incoming plane wave (not shown). Using the two terms of the Helmholtz equation ( 5) leads to the structural phase velocity model in Fig. 3b, which is very close to the true model (Fig. 3a). [Fig. 3] Dropping the amplitude correction term of the Helmholtz equation and using only the gradient of the travel time field, we obtain the eikonal phase velocity map shown in Fig. 4a, which differs from the true structural model (Fig. 3a), as emphasized by Wielandt (1993). The biases in the phase velocity map result from the interference between the incident and scattered wave fields. Errors up to 10 % are observed locally (Fig. 4b). Figure 4c shows the eikonal phase velocity model after application of a Gaussian filter which smooths out anomalies smaller than half the propagating wavelength (7.5 km).

Smoothing the phase velocity model reduces the error due to the eikonal approximation, which drops from about 10 % to about 3 % (Figure 4d). The smoothed eikonal model still differs from the expected structural model especially in the regions where the amplitude anomalies are large (for example near the eastern boundary of the model in Fig. 4c).

[Fig. 4]

In Figure 5, we compare the structural phase velocity model (Fig. 5a), the eikonal model obtained from a single azimuth (280 • , Fig. 5b), the eikonal model obtained after averaging all azimuths with an increment of 2.5 • (Fig. 5c), and the model obtained after both azimuthal averaging and Gaussian filtering (Fig. 5d). This test confirms the benefit of azimuthal averaging to minimize the errors due to the eikonal approximation, which drop to less than 2 % after azimuthal averaging (Fig. 5c) and 1.1 % after Gaussian smoothing (Fig. 5d).

[Fig. 5]

CONCLUSIONS

Our results reconcile the sometimes contradictory results that can be found in the literature regarding the reliability of eikonal tomography. Indeed, neglecting the amplitude term in the Helmholtz equation leads to velocity models with strong artefacts (Wielandt 1993). However, when measurements obtained on waves coming from different azimuths are combined, which is typically the case in most tomographic studies, the errors are dramatically reduced. Applying a low-pass Gaussian filter can further reduce the errors in phase velocity models. This additional spatial filtering does not degrade the resolution since the cut-off can be set at half the dominant propagating wavelength. The resolution

Figure 1 .

 1 Figure1. Eikonal tomography in the case of a point scatterer located at r s inside a homogeneous background medium. We consider an incoming plane wave with wave vector k 0 . The scattered wavefield at r with wave

Figure 2 .

 2 Figure 2. Effects of azimuthal averaging and smoothing on eikonal tomography. (a) Apparent eikonal phase velocity model obtained for a plane wave arriving from back azimuth 270 • . The white arrows show the incident scattered wave fields. The arrow length equates one wavelength. (b) Eikonal model obtained after applying a λ/2 Gaussian filter. (c) Azimuthal average of the eikonal models. (d) Azimuthal average of the filtered eikonal models.

Figure 3 .

 3 Figure 3. Synthetic numerical Helmholtz tomography experiment. (a) Structural phase velocity model (in km/s). (b) Phase velocity model obtained with the Helmholtz equation using the phase and amplitude fields shown respectively in (c) and (d). (c) Phase travel time of the simulated wave field. Isovalues are indicated with black lines. (d) Amplitude of the simulated wave field in arbitrary units. The white arrows indicate the azimuth of the incident wave (N280 • ).

Figure 4 .

 4 Figure 4. (a) Eikonal phase velocity model obtained using only the gradient of the phase field shown in Fig. 3c. (b) Error of the eikonal phase velocity model shown in (a), expressed in percents. (c) Eikonal phase velocity after applying a λ/2 Gaussian filter. (d) Error of the filtered eikonal model.

Figure 5 .

 5 Figure 5. (a) Structural phase velocity model (in km/s). (b) Eikonal tomography model obtained using a single incoming plane wave from back azimuth 280 • (c) Phase velocity model obtained after azimuthal averaging of the eikonal tomographic models. (d) Eikonal model obtained after azimuthal averaging and Gaussian filtering.

[START_REF] Friederich | Complete synthetic seismograms for a spherically symmetrical Earth by a numerical computation of the Green's function in the frequency-domain[END_REF][START_REF] Friederich | Interpretation of seismic surface waves in regional networks: joint estimation of wavefield geometry and local phase velocity. Methhod and numerical tests[END_REF][START_REF] Pollitz | Observations and interpretation of fundamental mode Rayleigh wavefields recorded by the Transportable Array (USArray)[END_REF][START_REF] Pollitz | Rayleigh-wave phase velocity maps and three-dimensional shear velocity structure of the western US from local non-plane surface wave geometry[END_REF]. Observations

ACKNOWLEDGMENTS

We thank Emmanuel Kaestle and Malcolm White for their thoughtful and constructive reviews. This work is part of the OROGEN research project, a tripartite partnership between the CNRS, TOTAL and BRGM.

On the validity of eikonal equation for surface-wave phase-velocity tomography 13 Virieux, J. & Operto, S., 2009. An overview of full-waveform inversion in exploration geophysics, Geophysics, 74, WCC1-WCC26.

Wielandt, E., 1993. Propagation and structural interpretation of non-plane waves, Geophys. J. Int.,113,[45][46][47][48][49][50][51][52][53] This paper has been produced using the Blackwell Scientific Publications GJI L A T E X2e class file.


Gajeon Chon

Minhee Suk

Frédéric Jaouen

Min Wook Chung

Chang Hyuck Choi
email: chchoi@gist.ac.kr

M W Chung

Deactivation of Fe-N-C catalysts during catalyst ink preparation process

Keywords: Oxygen reduction reaction, Non-precious metal catalysts, Proton-exchange membrane fuel cells, Fe-N-C catalysts, Ink preparation, Sonication

The membrane electrode assembly (MEA) is a core component of low-temperature fuel cells. The first step of MEA manufacturing is the preparation of a catalyst ink suspension in which the catalyst powder is homogeneously dispersed in a liquid solvent through mechanical or sonic agitation. In this work, we have studied the effects of catalyst dispersion in water or alcohol solutions and subsequent drying processes on the physicochemical properties of Fe-N-C catalysts and their electrocatalytic oxygen reduction activities. We find that dispersing the model Fe-N-C catalyst comprising only FeN x C y moieties in water and subsequent drying treatment change neither its bulk structure nor surface composition, as indicated by various spectroscopic measurements before and after treatment. However, zeta potential measurements, which are very sensitive to the chemistry of functionalities present on the carbon surface, reveal that the Fe-N-C catalyst becomes slightly more acidic, and that the change in their acido-basicity is magnified with a) increasing treatment temperature and b) repetitions of a same wetting/drying treatment. This small change in the surface acidobasicity of the Fe-N-C catalyst results in a measurable and reproducible decrease in its electrocatalytic activity, which shows a positive correlation with the zeta potential changes measured at pH = 1. Observed on the Fe-N-C catalyst but not on Pt/C, it is surmised that the electrocatalytic activities of the oxygen-reducing FeN x C y moieties are influenced by the surface chemistry of the carbonaceous support. Since catalyst wetting and drying processes are essential for MEA fabrication for fuel cells, these results suggest that careful attention should be paid to the conditions employed to prepare and dry catalytic inks for the family of Fe-N-C catalysts in order to obtain their highest possible ORR activity.

Introduction

Proton-exchange membrane fuel cells (PEMFCs) are electrochemical devices that convert the chemical energy of an externally supplied fuel (e.g., hydrogen) directly into electricity.

Due to their high energy efficiency and specific power density at low operating temperatures (ca. < 80 ℃), PEMFCs have rapidly been adopted for stationary and automotive applications [1,2]. The core of the PEMFC system is the MEA, which consists of a proton-exchange membrane, catalyst layers and gas diffusion layers (GDLs). High proton and/or electron conductivity in the membrane and catalyst layers, easy transport of the gas into the catalyst layers and effective water management (e.g., for sufficient membrane hydration while avoiding catalyst layer flooding) are prerequisites for achieving high PEMFC performance [3][4][5]. Hence, many studies have focused on MEA fabrication processes in order to maximize PEMFC performance [6][7][8].

In general, the membrane and GDL components of the MEA are fabricated separately, and then assembled in a single MEA using hot-pressing methods [9,10]. The catalyst layers are usually either fabricated directly on the membrane via spray or painting methods (Catalystcoated membrane, CCM) or on rigid support such as the GDL itself (catalyst coated layer, CCL) or a polymer sheet. In the latter case, the catalyst layer is then subsequently transferred onto the membrane via a decal transfer [11][12][13]. Regardless of the particular method employed to fabricate the catalyst layer, the manufacturing begins with the preparation of an ink which is a suspension containing the catalyst powder, an ionomer and liquid solvents (e.g., water, ethanol, isopropyl alcohol, etc.). The quality of the ink suspension is of great importance for the quality of the final catalyst layers in the MEA. Dispersion of aggregated catalyst particles and of ionomer is typically achieved by a long-time wetting of the powder with the solvents under external powers of mechanical stirring or, more commonly, sonication (e.g., ultrasonic bath or tip-probe).

The principle of sonication leans on sound energy with an ultrasonic frequency higher than 20 kHz. As the frequency increases, the agitation of particles is intensified. However, the ultrasonic sound concurrently induces local temperature increase in the medium due to vibration and acoustic cavitation [14][15][16][17][18]. Consequently, sonication can also overcome energy barriers for chemical bond formation and cleavage [19][20][21][22]. Mild oxidation of carbon surfaces after sonication has for instance been repeatedly reported [23,24]. In spite of its potential to chemically modify substances, sonication (or mechanical stirring) processes have frequently been used to homogenize catalyst inks, without serious consideration of possible modifications to the surface chemistry of the catalysts during the energetic treatment.

In this study, we characterized changes in the surface chemistry of a model Fe-N-C catalyst (comprising Fe solely as FeN x C y moieties) before and after its dispersion in water in a temperature range of 25-75 ℃ and by means of either a) sonication or b) mechanical stirring.

Fe(or Co)-N-C catalysts, in which Me-N x active moieties are imbedded in a carbonaceous support, are a promising class of non-precious metal catalysts (NPMCs), with prospects to replace Pt/C catalysts for oxygen reduction reaction (ORR) at the cathode of PEMFCs [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. We find that both the sonication and stirring treatments induced subtle chemical modifications of the carbonaceous supports in the Fe-N-C catalyst, which led to a nonnegligible decay of its electrocatalytic activity (from ca. -15 to -65 % @0.8 V RHE ) in acidic medium. The extent of surface modification increased with increasing temperature, duration and number of repetition of the treatments. Since these modifications were observed on the model Fe-N-C catalyst and also on a commercial Fe-N-C catalyst (comprising both FeN x C y moieties and Fe particles) but not on a benchmark Pt/C catalyst, careful attention should be paid to the preparation of MEAs with cathodes based on Fe-N-C.

Experimental methods

Catalyst synthesis and wetting procedures

In order to explore the effect of wetting and dispersion conditions on the structure and activity of Fe-N-C catalysts, we selected Fe-N-C catalyst that is composed solely of organometallic single-metal-atom FeN x C y moieties and without detectable content of Fe aggregates (e.g., metallic Fe, carbide, oxide, etc.). This catalyst, prepared by us, has been characterized in several previous studies exploring various deactivation and degradation mechanisms of Fe-N-C catalysts [41][42][43][44]. It was synthesized by the pyrolysis of Fe II acetate, 1,10-phenanthroline (phen) and a zeolite imidazolate framework (ZIF-8, Basolite Z1200 from Sigma). The catalyst precursor (1 g), which contained Fe/phen/ZIF-8 in a mass ratio of 0.5/20/80, was ball-milled in a ZrO 2 crucible with 100 ZrO 2 balls (5 mm diameter) in a planetary ball-miller for four cycles of 30 min each at a rotation rate of 400 rpm. The homogenized powder was then pyrolyzed at 1050 ℃ for 1 h under an Ar atmosphere. The catalyst is henceforth labelled 'Fe 0.5 -NC'. Selected experiments were also performed on commercially available Pt/C (46 wt%, TEC10E50E, Tanaka) and Fe-N-C catalysts (Pajarito Powder; labelled 'Fe-N-C PP '

). The general synthetic approach of the latter catalyst is via silica-templating. A mesoporous silica powder is infiltrated with Fe, N and C precursors, pyrolyzed in inert gas and the silica template is then removed by acid treatment. This Fe-N-C catalyst comprises both FeN x C y moieties and Fe aggregates (e.g., metallic Fe, carbide, etc.; Fig. S1).

Deionized (DI) water was selected as the wetting solvent since it is present in most ink suspensions used for preparing catalyst inks for PEMFC MEAs. For the wetting treatment, the Fe-N-C catalyst (30 mg) was added to DI water (100 mL, Arium® mini Water System, Sartorius), and dispersed by only sonication or only mechanical agitation, at a fixed temperature. The range of temperature from 25 to 75 °C was explored. Sonication was carried out at an ultrasonic frequency of 40 kHz in an ultrasonic bath (UCP-10, Jeio Tech) with catalyst-containing solvent placed in a round bottom flask of 250 mL. Mechanical agitation was performed at a rotation speed of 1,000 rpm using a magnetic stirrer (diameter = 8 mm, length = 25 mm). After the treatment, the wetted catalyst was first dried in a forced convection oven (OF-12GW, Jeio Tech) at 70 ℃ for 3 h and then in a vacuum oven (OV-11, Jeio Tech) at 70 ℃ overnight. When the catalyst was wetted several times, the same number of drying step was followed after each wetting treatment. Depending on the conditions applied during the treatment, the samples are referred to as 'S x (yh-z)' or 'M x (yh-z)', where S and M denote sonication and mechanical agitation treatment, respectively. The x, y and z scalars correspond to the temperature (in ℃) during treatment, duration (in hour) and number of the treatments, respectively. To maintain the temperature of the ultrasonic bath close to room temperature, cooling tap-water (25 ℃) was continuously flowed through a copper tube heat exchanger in the bath, resulting after 10 minutes of sonication in a steady-state temperature of 29 ℃ (Fig. S2). To raise the temperature to a specific temperature between 35 and 75 ℃ during mechanical stirring, the catalyst suspension was heated by a heating mantle equipped with a reflux condenser to prevent liquid evaporation.

Electrochemical characterization

The electrochemical results were obtained using a modulated speed rotator (RRDE-3A, ALS) in a Teflon cell with a three-electrode system consisting of a graphite rod as the counter electrode and a saturated Ag/AgCl reference electrode (RE-1A, EC-Frontier). The electrolyte was 0.1 M HClO 4 , which was prepared from concentrated HClO 4 (70%, Aldrich). Prior to any electrochemical measurements, the Ag/AgCl reference electrode was calibrated against a Pt electrode immersed in H 2 -saturated 0.1 M HClO 4 electrolyte, acting as a reversible hydrogen electrode (RHE). For Fe-N-C catalysts (Fe 0.5 -NC and Fe-N-C PP ), the inks based on pristine or previously treated catalyst (sonication or mechanical agitation, as described in section 2.1) were prepared by dispersing 10 mg of catalyst in 804 μL of water and 80 μL of 5 wt% Nafion solution. Regardless of the catalyst nature, the ink suspension was then sonicated for 1 h in the ultrasonic bath at 29 ℃. Working electrodes were prepared by pipetting 5 L of the catalyst ink onto the glassy carbon disk (0.071 cm 2 ) of the rotating disk electrode (011169, ALS), resulting in a catalyst loading of 800 μg cm -2 for Fe-N-C materials. For the Pt/C catalyst, Pt loading on the working electrode was set to 20 μg Pt cm -2 . The catalyst loaded electrode was dried in a convection oven at 35 ℃ for 30 min. The ORR polarization curve was recorded at a 10 mV s -1 scan rate and a 900 rpm rotation speed in an O 2 -saturated electrolyte. For all catalysts, ORR polarization curve measured at a negative-going scan was selected. To remove the non-Faradaic capacitance current density, polarization curve measured using the same procedure but in an Ar-saturated electrolyte was subtracted from the result. The kinetic current density was estimated using the Koutecky-Levich equation; i.e., 1/j = 1/j k -1/j d , where j k and j d are the kinetic and diffusion current densities, respectively. Electrochemical reduction of the carbon surface was performed with a M 45 (1h-3) sample, which exhibited the highest activity decay among the water-treated samples. Cyclic voltammetry (CV) was applied in a potential range from 0 to -1.6 V Ag/AgCl for 100 cycles with a scan rate of 100 mV s -1 in an Ar-saturated 0.5 M NaCl electrolyte [45]. 57 Fe Mössbauer spectroscopy was used to characterize various iron species in Fe-N-C PP .

Physical characterization

The measurements were carried out with a γ-radiation source of 57 Co-Rh and a NaI scintillation detector at room temperature. X-ray photoelectron spectroscopy (XPS) was carried out using a K-Alpha (Thermo Scientific) instrument equipped with a micro-focused Al K α monochromator X-ray source and a 180° double-focusing hemispherical analyzer. Xray diffraction (XRD) patterns were acquired using an X'Pert PRO (Malvern Panalytical) with Cu K α X-ray radiation generated at 40 kV and 40 mA. Raman scattering was detected using a Nicolet Almega XR (Thermo Scientific) equipped with a CCD detector. A 532 nm laser beam with a ×10 magnification objective was used. The acido-basicity of the catalyst's surface was determined by dispersing 40 mg of a catalyst in 30 mL of a pH 6 aqueous solution (pH i ); the pH was adjusted using a mixture of 0.1 M H 2 SO 4 and 0.1 M KOH. The solutions were kept saturated with Ar throughout the experiment. The pH was recorded using a pH meter, and the final pH (pH f ) was recorded when the pH value became steady with time.

To compare the acido-basicity of the wetted samples relative to the pristine Fe-N-C catalyst, we used the expression ΔpH f = pH f (wetted Fe-N-C catalyst) -pH f (pristine Fe-N-C catalyst).

A negative ΔpH f value would thus indicate more acidic surface after treatment. The zeta potentials of the catalysts were recorded using a Zetasizer Nano ZS (Malvern) with a 4 mW-633 nm He-Ne laser. Before the measurements, 5 mg of the catalyst was dispersed in a 0.1 M HClO 4 solution (7.5 mL), and the suspensions were then ultra-sonicated to prevent the agglomeration of the catalyst particles. An aliquot (ca. 150 μL) of the suspension was placed into a glass cell. The zeta potential of each sample was recorded more than six times, and the average value of all the measurements was determined.

Results and discussion

ORR activity of the Fe-N-C catalyst after wetting treatments

We first investigated the effects of sonication of the Fe-N-C suspension on its ORR activity without using temperature control in the sonication bath (i.e., no heat exchanger tube). This uncontrolled condition, which is typically used in catalyst ink preparation, induced vibrational motion of the water molecules and increased the temperature of the sonication bath to 55 ℃ after 3 h of operation (Fig. S2). Figure 1 shows the ORR polarization curves of Fe 0.5 -NC before and after 2 h of sonication in DI water without temperature control. The j k value measured at 0.8 V RHE was ca. 3.2 mA cm -2 for the pristine Fe 0.5 -NC, but it decreased to ca. 1.6 mA cm -2 after the treatment (Fig. 1a). In spite of this non-negligible drop in the ORR activity, the rate-determining step (RDS) was unchanged after the treatment, as revealed by the similar Tafel slopes of ca. 63-66 mV dec -1 before and after treatment (Fig. 1b). These preliminary results represent that the typical agitation method used in catalyst ink preparation can negatively affect the intrinsic ORR activity of the Fe-N-C catalysts. Sonication treatment involves acoustic cavitation, and therefore generates vibration and heat energy. This could possibly lead to the dissociation of water molecules into hydrogen (•H) and hydroxyl radicals (•OH) [14,[46][47][48][49], which in turn can induce chemical reactions on the Fe-N-C surface. To determine whether the activity decay after the treatment was provoked by radical species or by simple wetting with DI water, the ORR activities of Fe-N-C catalysts treated using either sonication or stirring (i.e., mechanical agitations) in DI water for 1 h were compared. In both conditions, the suspensions were maintained at a temperature of 29 ℃, which was the equilibrium temperature of the sonication bath in the presence of the copper tube heat exchanger (Fig. S2), to exclude incidental thermal effects on the ORR activity. As can be seen in Fig. 2a, the j k values at 0.8 V RHE of the treated Fe 0.5 -NC, i.e., S 29 (1h-1) and M 29 (1h-1), were lower than that of the pristine Fe 0.5 -NC by ca. 1.2 mA cm -2 .

The decay in their ORR activity was almost identical for both treatment methods. This result indicates that wetting with water and subsequent drying (rather than radical formation) was the primary cause of the ORR activity loss of the Fe-N-C catalysts in these experimental conditions. We thus conducted further studies on the effects of wetting and drying on the ORR activity of Fe 0.5 -NC using mechanical stirring in water (Fig. S3). Fig. 2b shows the changes in the ORR activity after conducting the wetting treatments at various temperatures. The duration of the treatment was set to 1 h. As the temperature was increased, the extent of the decline in the activity (∆j k at 0.8 V RHE ) increased by ca. 1.4 mA cm -2 ; no further activity loss was found for a 1 h single treatment at temperatures higher than 45 ℃. The duration and number of treatments were also found to influence the ORR activity decay of Fe 0.5 -NC (Figs. 2c andd).

When the treatment duration and/or the number of treatments increased, the ORR activity of Fe 0.5 -NC decreased. Other typical solvents for ink preparations (e.g., ethanol and isopropyl alcohol) also resulted in performance drops similar to that caused by water (Fig. S4).

Furthermore, this deactivation was also observed for the commercial Fe-N-C PP (Fig. S5), that comprises not only FeN x moieties but also core-shell Fe and Fe 3 C particles surrounded by graphene shell (Fig. S1). This deactivation in conditions of ink preparation thus seems to be specific for the Fe-N-C catalysts, and was not observed on commercial carbon-supported Pt nanoparticles (Fig. S5a).

Physicochemical changes in the Fe-N-C catalyst after the wetting treatments

We utilized various physicochemical characterization techniques to understand why the wetting of Fe-N-C catalysts with DI water decreases their ORR kinetics. The series of M 25 (1h-z) and M 45 (1h-z) samples (z = 1, 2 and 3) was used for the characterizations due to the incremental changes in their ORR activity. The XRD patterns of the pristine and treated Fe 0.5 -NC all showed two broad peaks at ca. 24° and 44°, which corresponded to the (002) and (101) planes of graphite (Fig. 3a). The XRD signals of the catalysts did not exhibit significant changes, regardless of the wetting treatments applied. Similarly, their Raman spectra revealed almost identical D (at ca. 1350 cm -1 ) to G (at ca. 1590 cm -1 ) band intensity ratios (I D /I G = ca. 0.96 ± 0.04) for all the samples (Fig. 3b). Therefore, the XRD and Raman spectra indicated that the modifications in the bulk carbon structure of the catalysts induced by simple wetting treatments were negligible. Other possible chemical changes were further studied with XPS (Fig. 4), which is more sensitive to surface changes than the two techniques above. However, the changes to the N 1s and O 1s signals after the wetting treatments were also insignificant (Figs. 4a-d), and the spectra of the various signals were nearly identical. The nitrogen and oxygen contents normalized by carbon content (i.e., N/C and O/C at%) were within the range of ca. 6.5 ± 0.5 at% for N/C and 6.5 ± 0.6 at% for O/C (Fig. 4e). While the changes in the chemical composition near the catalyst surface seem to be negligible and within the experimental error range of ca. 1 at%, we could not fully rule out marginal modifications of the surface chemistry of the Fe-N-C catalysts. For instance, we observed the activity recovery after electrochemical reduction of M 45 (1h-3) sample (showing the highest activity loss among the water-treated samples, Fig. S6). This activity recovery was previously reported on Fe-N-C catalysts with abundant surface-oxygen functional groups introduced by acidic H 2 O 2 treatment [44]. Hence, we further measured the surface basicity of the treated Fe 0.5 -NC to clarify the changes in surface chemistry of Fe 0.5 -NC during wetting treatments. The surface basicity (i.e., ∆pH f ) of Fe 0.5 -NC, which has been suggested as a descriptor of their ORR kinetics [50], changed by ca. -0.5 after simple wetting treatment at 25 and 45 ℃ (Fig. 5). In our previous study, significant ORR activity decrease was observed for Fe 0.5 -NC and also for a similarly prepared catalyst (with twice higher Fe content) after oxidative H 2 O 2 treatment [44]. The performance drop was correlated to changes in the surface chemistry, namely, a substantial decrease in the basicity, i.e., a ∆pH f of ca.

Zeta potential measurements of the Fe-N-C catalyst after the wetting treatments

To evaluate the surface modifications more precisely, the zeta potential of the Fe-N-C catalyst after the wetting treatments was also measured. Zeta potential (ζ) is the electric potential at the solid-liquid interface, and is strongly affected by the composition of the outermost surface of the solid material as well as by the pH of the solution [51][52][53][54][55]. Therefore, this method has been widely used to measure the surface functionality of carbonaceous materials, such as their acidity and basicity [53,56,57]. Figure 6a shows the zeta potential of Fe 0.5 -NC before and after wetting treatment at 25 or 45 ℃. All the measurements were performed in a 0.1 M HClO 4 solution. The pristine Fe 0.5 -NC exhibited a positive zeta potential value of ca. 3 mV. After the wetting and drying steps, however, all the treated catalysts exhibited values lower than zero. The zeta potential shifted to more negative values as either the treatment temperature or the number of treatments was increased. Therefore, this suggests that the wetting treatment induced a negative charge on the surface of the Fe-N-C catalysts. In general, the introduction of oxygen-containing functional groups increases the acidity of carbon materials, while nitrogen functional groups increase their basicity [58][59][60][61]. However, the failure to observe modifications in the functional groups in the XPS-N 1s and O 1s spectra led us to surmise that the changes in the surface composition induced by the wetting treatments are very small. While the gradual modification of the surface charge after the different conditions applied during the wetting treatment could be clearly discerned in the zeta potential measurements, the variations (from 3 to -12 mV) were smaller compared to that observed for carbon materials functionalized with acidic groups via oxidative treatments such as HNO 3 treatments leading to |∆ζ | > 20-30 mV, as measured in acidic pHs [62,63].

However, this inconspicuous change in the surface basicity of the Fe-N-C catalysts resulted in non-negligible ORR activity losses (Fig. 2), and a positive correlation was observed between the ORR activity and the zeta potential at pH = 1 (Fig. 6b). This correlation suggested that the ORR activity of the active FeN x C y moieties could be modulated by the basicity/acidity of the surrounding carbon surface, as has previously been reported for NH 3or H 2 O 2 -treated Fe-N-C materials, increasing the basicity and acidity, respectively [44,50,64,65].

Conclusions

In conclusion, we showed that modifications of the surface occurring during wetting and subsequent drying process of Fe-N-C catalyst powders can lead to a non-negligible decrease in their ORR activity, depending on the sonication or mechanical agitation conditions. The conditions investigated here mimicked those typically employed during ink preparation for MEA fabrication. While ex situ XPS analyses did not reveal significant differences in the oxygen and nitrogen elemental content nor in the type of functional groups in Fe 0.5 -NC, which typically determine the surface acido-basicity of carbon materials, the modifications of Fe 0.5 -NC surface after the treatments could be measured by ∆pH f and more precisely by zeta potential analyses. The ORR activity drop was magnified with increasing temperature, duration and number of repetitions applied during the mechanical agitation or sonication treatments. Observed on Fe-N-C catalysts comprising FeN x C y moieties but not on a commercial Pt/C catalyst, we can surmise that the site-specific activity of FeN x C y active moieties is strongly influenced by the surface chemistry of the carbonaceous support in which they are embedded. These findings imply that careful attentions should be paid to the methods and experimental conditions used for preparing Fe-N-C inks for RDE and/or PEMFC measurements, in order to avoid premature aging and ORR activity discrepancy from laboratory to laboratory for a same sample.

Fig. 1 .

 1 Fig. 1. (a) ORR polarization curves and (b) Tafel plots of the Fe 0.5 -NC before and after 2 h of sonication in DI water without temperature control. After sonication, the catalyst powder was dried in an oven and re-dispersed to prepare the ink suspension. The electrochemical responses were measured in an O 2 -saturatred 0.1 M HClO 4 electrolyte with a scan rate of 10 mV s -1 and a rotation speed of 900 rpm.

Fig. 2 .

 2 Fig. 2. ORR kinetic current density measured at 0.8 V RHE before and after wetting treatment using different conditions for Fe 0.5 -NC: (a) different wetting methods (mechanical agitation, M vs. sonication, S); mechanical agitation (b) at temperatures from 25 to 75 ℃, (c) for different combinations of temperature (25 or 45 °C) and duration (1 h, 3 h or 5 h) and (d) for different combinations of temperature (25 or 45 °C) and numbers of treatments (1, 2 or 3).

Fig. 3 .

 3 Fig. 3. (a) XRD patterns and (b) Raman spectra of Fe 0.5 -NC before and after wetting treatment at 25 and 45 °C. The catalyst was wetted through mechanical agitation in water for 1 h, and the number of treatments was varied.

Fig. 4 .

 4 Fig. 4. High resolution XPS spectra of Fe 0.5 -NC before and after the treatments. Different numbers of wetting treatments were conducted at (a, b) 25 and (c, d) 45 °C. The catalyst was

  -1.5 to -2.5 depending on the H 2 O 2 treatment temperature. The much smaller changes in ∆pH f observed in the present study imply that the wetting treatment in pure DI water led to much subtle modification of the surface of the Fe-N-C catalysts compared to the case of H 2 O 2 treatment in acidic medium. It can thus be speculated that this small modification was difficult to be identified with XPS but was reflected in the change of ∆pH f values for the M 25 (1h-z) and M 45 (1h-z) samples (z = 1, 2 and 3), although the extent of the changes was not clearly differentiated.

Fig. 5 .

 5 Fig. 5. Surface basicity changes (∆pH f ) for Fe 0.5 -NC after mechanical agitation in DI water.

Fig. 6 .

 6 Fig. 6. (a) Zeta potential values of the pristine and water-treated Fe 0.5 -NC measured in 0.1 M HClO 4 solution and (b) the correlation between the ORR activity and zeta potential. The ORR activity was quantified using the kinetic current density j k at 0.8 V RHE .

Acknowledgments

This research was supported by Technology Development Program to Solve Climate Changes through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (2018M1A2A2063861). We thank Dr. Moulay-Tahar Sougrati for Mössbauer spectroscopy study.


Matthieu Weber

Martin Drobek

Bertrand Rebière

Christophe Charmette

Jim Cartier

Anne Julbe

Mikhael Bechelany
email: mikhael.bechelany@umontpellier.fr

Hydrogen selective palladium-alumina composite membranes prepared by Atomic Layer Deposition

Keywords: palladium, composite, ceramic membranes, hydrogen separation, Atomic Layer Deposition ToC

published or not. The documents may come

Introduction:

Hydrogen gas is crucial for upcoming transportation technologies and is key for many chemical processes. Moreover, hydrogen is considered as one of the best clean energy carriers which could contribute to tackle the major environmental issues our society is facing. Nowadays, the first world scale liquid hydrogen production unit dedicated to the hydrogen energy market is being built, [START_REF] Parkinson | Air Liquide to build $150m liquid hydrogen production plant in western US[END_REF] requiring cost-effective and efficient means to separate hydrogen gas from other species. The principal route to produce pure hydrogen is based on membrane separation/purification technologies. In this context, hydrogen selective membranes represent an attractive opportunity, and many membrane materials have been designed for selective hydrogen extraction, including pure inorganic or polymer membranes and their composites, hybrid membranes (silicon-based membranes, MOFs), metal alloys etc. [START_REF] Ockwig | Membranes for hydrogen separation[END_REF] In the area of metal-based membranes, palladium represents a particularly attractive material thanks to its high selectivity towards hydrogen enabling the production of pure H2. [START_REF] Ma | Thin composite palladium and palladium/alloy membranes for hydrogen separation[END_REF] Indeed, the mechanism of hydrogen purification through dense palladium membranes is based on the selective transport of atomic H through the dense metal layer, [START_REF] Julbe | About the role of adsorption in inorganic and composite membranes[END_REF] which blocks the transport of all other gas molecules. In addition, these membranes are highly resistant to oxidation [START_REF] Tamaoka | Oxidation and hydrogenation of Pd: suppression of oxidation by prolonged H 2 exposure[END_REF] and exhibit good thermal stability (up to ~550°C [START_REF] De Nooijer | Long-term stability of thin-film Pd-based supported membranes[END_REF]), allowing their applications at high operating temperatures. [START_REF] Rothenberger | High pressure hydrogen permeance of porous stainless steel coated with a thin palladium film via electroless plating[END_REF][START_REF] Quek | Palladium Membrane Technology for Hydrogen Production, Carbon Capture and Other Applications[END_REF] Considerable research work was carried out on Pd-based membranes with thick foils or bulk tubes, [START_REF] Ma | Thin composite palladium and palladium/alloy membranes for hydrogen separation[END_REF][START_REF] Eichenauer | The Palladium Hydrogen System[END_REF][START_REF] Koffler | Hydrogen permeation through alpha-palladium[END_REF] although these designs are not well adapted for large scale applications. In fact, selfsupported Pd membranes are costly, as palladium is nowadays even more expensive than gold and its resources are limited. [START_REF]Money Metals Exchange, Palladium Spot Price History & Current Prices[END_REF] In addition, the competitive adsorption of even small amounts of CO, CO2, steam or sulfur compounds (e.g. H2S, SOx), strongly reduces the H2 permeating flux. [START_REF] Kian | Performance of Pd-Based Membranes and Effects of Various Gas Mixtures on H2 Permeation[END_REF] As a result, many research efforts have been undertaken to fabricate more stable and economically viable Pd membranes. A relevant strategy to reduce the impact of poison species and which could also reduce the adsorption activation energy of H2, consists in coating Pd-based membrane surface with a protective film. [START_REF] Alique | Review of supported Pd-based membranes preparation by electroless plating for ultra-pure hydrogen production[END_REF][START_REF] Li | Recent progress in Pd-based composite membranes[END_REF] In order to prepare relatively cheap and highly permeable Pd-based membranes, thin dense Pd layers have to be deposited on top of porous supports. In addition to a reduced consumption of the precious metal, such supported membranes also permit to offer high hydrogen flux. [START_REF] Ma | Thin composite palladium and palladium/alloy membranes for hydrogen separation[END_REF] Porous alumina is an attractive support for the deposition of this type of membrane material, as alumina offers a good thermal stability enabling its utilization for hydrogen separation/purification up to 400-500°C.

Already in the 1990's, different research groups successfully used magnetron sputtering and chemical vapor deposition (CVD) to synthetize dense Pd-films on top of y-Al2O3 and α-Al2O3 membrane substrates, aiming to prepare H2-selective membranes. [START_REF] Xomeritakis | Fabrication of thin metallic membranes by MOCVD and sputtering[END_REF][START_REF] Uemiya | Preparation of highly permeable membranes for hydrogen separation using a CVD technique[END_REF][START_REF] Xomeritakis | Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition[END_REF][START_REF] Xomeritakis | CVD synthesis and gas permeation properties of thin palladium/alumina membranes[END_REF] In 1994, Yan et al. were the first to show the feasibility of a dense (5 microns thick) Pd/Al2O3 composite membrane prepared by deposition of Pd inside the macropores of an alumina support using low-pressure metallorganic CVD. [START_REF] Yan | Thin palladium membrane formed in support pores by metal-organic chemical vapor deposition method and application to hydrogen separation[END_REF] Employing Pd-acetylacetonate as a metallic precursor, Uemiya et al. reported the synthesis of a Pd membrane by deposition of Pd inside the pores of α-Al2O3 using atmospheric pressure CVD. [START_REF] Uemiya | Preparation of highly permeable membranes for hydrogen separation using a CVD technique[END_REF] Xomeritakis et al. reported the deposition of Pd inside the pores of mesoporous alumina supports using a CVD process with both PdCl2 and H2. Despite being confirmed as an attractive approach for the preparation of supported dense (composite) membranes on/in inorganic supports, neither CVD nor sputtering could achieve the high conformality required to coat uniformly the pores within alumina substrates. Finally, CVD limits its applications only to small disk membrane supports. [START_REF] Xomeritakis | Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition[END_REF][START_REF] Xomeritakis | CVD synthesis and gas permeation properties of thin palladium/alumina membranes[END_REF][START_REF] Jiang | Sub-10 nm thick microporous membranes made by plasma-defined atomic layer deposition of a bridged silsesquioxane precursor[END_REF] Indeed, for industrial applications with tubular membrane designs, a Pd deposition technique offering both a good uniformity and excellent conformality of the films is required. In the last decade, electroless plating (ELP) has been considered as a promising technique for this purpose, since it provides higher uniformity of the films when compared to their CVD and PVD counterparts. Using this method, thin dense Pd/Al2O3 composite membranes with fine and uniform microstructures [START_REF] Rothenberger | High pressure hydrogen permeance of porous stainless steel coated with a thin palladium film via electroless plating[END_REF][START_REF] Zhang | A modified electroless plating technique for thin dense palladium composite membranes with enhanced stability[END_REF][START_REF] Anzelmo | Fabrication & performance study of a palladium on alumina supported membrane reactor: Natural gas steam reforming, a case study[END_REF] were prepared by different research groups for H2 separation applications. Composite membranes containing Pd particles prepared by ELP were developed by Tanaka et al. [START_REF] Pacheco Tanaka | Fabrication of hydrogen-permeable composite membranes packed with palladium nanoparticles[END_REF] However, these membranes are generally quite thick (several microns) and the synthesis procedure requires at least two steps (support seeding, ELP...). Hence, a simple and direct deposition technique enabling the preparation of thin, uniform and conformal Pd nanomaterials on commercial porous substrates with complex geometry is desired.

In this respect, Atomic Layer Deposition (ALD) is a very attractive technique. It is a CVD derived route allowing the direct preparation of thin films of a large range of materials on high aspect ratio substrates with precise thickness control at the nanometer scale, high uniformity and excellent conformality. [START_REF] George | Atomic layer deposition: An overview[END_REF][START_REF] Leskelä | Atomic layer deposition (ALD): From precursors to thin film structures[END_REF][START_REF] Parsons | Progress and future directions for atomic layer deposition and ALD-based chemistry[END_REF] The technique is based on sequential pulses of precursors and co-reactants in the gas phase, leading to self-limiting chemical reactions at the substrate surface. [START_REF] Leskelä | Atomic layer deposition (ALD): From precursors to thin film structures[END_REF][START_REF] Puurunen | Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process[END_REF] The nanomaterials typically synthesized by ALD are metal oxides, [START_REF] Puurunen | Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process[END_REF] although metals [START_REF] Leick | In situ spectroscopic ellipsometry during atomic layer deposition of Pt, Ru and Pd[END_REF][START_REF] Weber | Sub-nanometer dimensions control of core/shell nanoparticles prepared by atomic layer deposition[END_REF][START_REF] Weber | Enhanced Catalytic Glycerol Oxidation Activity Enabled by Activated-Carbon-Supported Palladium Catalysts Prepared through Atomic Layer Deposition[END_REF][START_REF] Mackus | Atomic layer deposition of Pd and Pt nanoparticles for catalysis: on the mechanisms of nanoparticle formation[END_REF], nitrides [START_REF] Weber | Novel and Facile Route for the Synthesis of Tunable Boron Nitride Nanotubes Combining Atomic Layer Deposition and Annealing Processes for Water Purification[END_REF][START_REF] Kim | Atomic layer deposition of metal and nitride thin films: Current research efforts and applications for semiconductor device processing[END_REF][START_REF] Weber | Mechanical properties of boron nitride thin films prepared by atomic layer deposition[END_REF] and other more complex materials could be prepared as well. Because ALD enables subnanometer control of the film thickness, this technique became a key technology for the deposition of thin films in numerous applications, ranging from microelectronics [START_REF] Raaijmakers | Current and Future Applications of ALD in Micro-electronics[END_REF] to photovoltaics, [START_REF] Van Delft | Atomic layer deposition for photovoltaics: Applications and prospects for solar cell manufacturing[END_REF] and from biosensing [START_REF] Graniel | Atomic layer deposition for biosensing applications[END_REF] to membranes. [START_REF] Weber | Atomic Layer Deposition for Membranes: Basics, challenges and opportunities[END_REF][START_REF] Weber | Atomic layer deposition (ALD) on inorganic or polymeric membranes[END_REF] Due to the excellent conformality of the ALD films, this route is particularly suited for both pore tailoring and modification of membrane support microstructure. [START_REF] Weber | Atomic Layer Deposition for Membranes: Basics, challenges and opportunities[END_REF][START_REF] Weber | Atomic layer deposition (ALD) on inorganic or polymeric membranes[END_REF][START_REF] Grigoras | Coating of Nanoporous Membranes: Atomic Layer Deposition versus Sputtering[END_REF][START_REF] Yang | Atomic layer deposition for membrane interface engineering[END_REF][START_REF] Yang | Crude-Oil-Repellent Membranes by Atomic Layer Deposition: Oxide Interface Engineering[END_REF] Despite the great potential of this synthesis route, there are very few studies reporting ALD of metals for gas selective membrane preparation. [START_REF] Narayan | Atomic layer deposition-based functionalization of materials for medical and environmental health applications[END_REF][START_REF] Daub | Ferromagnetic nanotubes by atomic layer deposition in anodic alumina membranes[END_REF] To the best of our knowledge, ALD has not been applied yet for the synthesis of Pd-based hydrogen selective composite membrane materials on/in tubular ceramic supports. ALD of metals is challenging but has been already achieved in the past. According to literature reports, the best results have been obtained when using palladium hexafluoroacetylacetonate (Pd(hfac)2) as the metallic precursor, which is also the most classically adopted precursor for Pd ALD. [START_REF] Mackus | Atomic layer deposition of Pd and Pt nanoparticles for catalysis: on the mechanisms of nanoparticle formation[END_REF][START_REF] Elam | Atomic layer deposition of palladium films on Al2O3 surfaces[END_REF][START_REF] Weber | Atomic layer deposition of high-purity palladium films from Pd(hfac)2 and H2/O2 plasmas[END_REF][START_REF] Weber | Enhanced Catalytic Glycerol Oxidation Activity Enabled by Activated-Carbon-Supported Palladium Catalysts Prepared through Atomic Layer Deposition[END_REF][START_REF] Feng | Palladium catalysts synthesized by atomic layer deposition for methanol decomposition[END_REF] Starting from this metalorganic precursor, our research group recently synthesized a large variety of Pd-based nanomaterials for both catalysis and sensing applications. [START_REF] Weber | Boron Nitride as a Novel Support for Highly Stable Palladium Nanocatalysts by Atomic Layer Deposition[END_REF][START_REF] Weber | High Performance Nanowires Hydrogen Sensors by Exploiting the Synergistic Effect of Pd Nanoparticles and MOF Membranes[END_REF][START_REF] Weber | Highly Efficient Hydrogen Sensors Based on Pd Nanoparticles Supported on Boron Nitride Coated ZnO Nanowires[END_REF] The applied ALD protocol was based on both Pd(hfac)2 and formalin as a co-reactant, as recommended by Elam's group. [START_REF] Elam | Atomic layer deposition of palladium films on Al2O3 surfaces[END_REF] In the present work, we report for the first time a proof of concept for the preparation of hydrogen selective membranes by ALD of Pd within the pores of a γ-Al2O3 top-layer covering the inner side of a tubular ceramic support. The ability of ALD to coat high aspect ratio substrates with high uniformity and excellent conformality has been advantageously applied to embed the Pd material within the pores of tubular supports having higher S/V ratio compared to classical disc supports. The physical and chemical properties of the prepared Pd/Al2O3 composite membranes were determined together with their gas transport properties and separation performance for hydrogen purification.

EXPERIMENTAL SECTION

Membrane support

The tubular membrane supports used for Pd deposition were purchased from Pall-Exekia, France.

The supports present an asymmetric structure composed of a mesoporous γ-Al2O3 inner top-layer with mean pore sizes in the range 5-8 nm, deposited on a macroporous support (1.5 mm thick) consisting of 3 layers with mean pore sizes of ~ 0.2 µm, 0.8 µm and 10 µm from the inner to the outer side of the tubes. The support dimensions were as follows: OD/ID=10/7 mm with a total length of 50 mm. In order to avoid any gas leakage during permeation tests, both tube extremities (1 cm length) were sealed with a commercial enamel at a temperature of ~900°C.

ALD of Palladium

The ALD of palladium process allowing for the synthesis of the selective composite membrane was achieved by using a low-pressure (home-built) hot-wall ALD reactor. The precursors Pd(hfac)2 (Sigma-Aldrich, CAS 64916-48-9) and formalin (Sigma-Aldrich, 37 wt. % in H2O, containing 10-15% methanol as stabilizer, CAS Number 50-00-0) were used as received without any further purification. The bubbler containing the Pd(hfac)2 precursor was heated at 70 °C to provide sufficient vapor pressure, and the formalin container was kept at room temperature. The deposition chamber was set at the temperature of 220 °C, and the feed lines were heated at 100 °C to avoid any condensation. A typical ALD cycle consisted of 5 s pulse of Pd(hfac)2, 15 s exposure, and 10 s purge, followed by a 1 s pulse of formalin, 15 s exposure and 60 s purge with Ar. Further details about both this deposition protocol and the associated ALD reactor can be found elsewhere. [START_REF] Weber | Boron Nitride as a Novel Support for Highly Stable Palladium Nanocatalysts by Atomic Layer Deposition[END_REF][START_REF] Weber | Novel and Facile Route for the Synthesis of Tunable Boron Nitride Nanotubes Combining Atomic Layer Deposition and Annealing Processes for Water Purification[END_REF]

Physico-chemical characterization and permeation measurements

The morphology, thickness and homogeneity of the composite membrane materials were studied at different locations on/in the supports using a scanning electron microscope (SEM, Zeiss Evo hd15) with LaB6 source at 1.5 keV. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250 spectrometer (Thermo Electron, monochromatic radiation source Al Kα = 1486.6 eV). The highresolution spectra were recorded at a step of 0.1 eV (transition energy: 20 eV). The binding energy (BE) measurement was corrected on the basis of the energy of C1s at 284.4 eV, and the quantification was carried out from the corresponding XPS peak area after correction with a suitable sensitivity factor. X-ray diffraction (XRD) patterns were recorded using a PANAlytical Xpert-PRO diffractometer equipped with a X'celerator detector using Ni-filtered Cu-radiation. Energy dispersive X-ray spectroscopy (EDX) (Silicon Drift Detector (SDD), X-MaxN, Oxford Instrument) coupled to Zeiss EVO HD15 SEM analyzer was used to determine the location and degree of Pd infiltration into the mesoporous γ-Al2O3 inner top-layer.

Gas permeation and separation experiments

The permeance and selectivity of the prepared membranes have been measured both for different single gas series (He, H2, N2, CO2 and CH4) and a selected gas mixture containing H2, N2, CO2. The H2 (99.9992%) and CO2 (99.995%) gases were provided by Air Products while N2, He and CH4 with purities 99.995%, 99.999% and 99.995%, respectively were provided by LINDE.

Membranes were equipped with silicon o-rings (Sephat), placed in a stainless steel module and outgassed under vacuum.

Single gas flow through the membrane was evaluated by a bubble flowmeter connected to the atmosphere enabling a rapid screening of the membrane permeance values. Additional 30 minutes outgassing treatment in vacuum was operated after each change of gas. Ideal selectivities were calculated as the ratio of single gas permeances. Single gas permeance values were measured in the temperature range 35 °C -188 °C, at transmembrane pressures in the range ΔP = 1-3 bar. The stabilization time before each gas flow measurement was typically fixed at 30 min and the gas permeation was always carried out from the highest to the lowest transmembrane pressure. For all the measured single gas permeance values, the estimated error was ~12%.

Gas separation experiments were performed with a gas mixture containing 80.8% N2, 15.4% H2 and 3.8% CO2, at ΔP = 3 bar and temperatures of 103 and 188°C. Helium was used as a sweep gas and the gas concentration in the permeate was measured using TCD-GC (PerkinElmer-Clarus 400) equipped with one column (Shincarbon ST,100/120 mesh,2m ,1 mm ID micropacked). Gas mixture compositions were analyzed by continuous dosing of the permeate diluted with He (sweep gas) and transported to the sampling loop of the gas chromatograph. The transmembrane flux was measured with an electronic flow-meter (PE 1000 Perkin Elmer). Before each measurement, the membranes were outgassed under vacuum.

RESULTS AND DISCUSSION

The strategy developed in this work aimed to the fabrication of hydrogen selective Pd/Al2O3 composite membranes with Pd-clusters confined within the pores of a γ-Al2O3 top-layer covering the inner side of a tubular ceramic support. This novel type of membrane material was prepared by Atomic layer Deposition (ALD) and the characteristics of the as-obtained membranes are described hereafter in relation with their morphology, structure, composition, single gas permeation properties and gas mixture separation performance.

Fabrication and physico-chemical characterization of the Pd/Al2O3 composite membranes

A porous alumina tube presenting an asymmetric structure with a γ-Al2O3 inner top-layer (pore size ~ 5-8 nm) has been used as a support for Pd deposition. It should be noted that the pore sizes (5-8 nm) of the γ-Al2O3 layer in the sample tubes are slightly larger than those (3-5 nm) in the original support, due to the thermal treatment at 920°C required for sealing the tubes extremities with enamel. The heating protocol leads to slight pore opening (resulting in more open porous structure) but does not alter the integrity of the alumina top-layer. The tubular alumina support has been considered as more attractive membrane geometry for industrial applications compared to the simple disc supports classically used for Pd composite membrane preparation by PVD or CVD methods. Indeed, tubular supports offer higher S/V ratio and enhanced mechanical robustness, facilitating their handling and ensuring membrane stability under demanding operation conditions (temperature and pressure).

The ALD of Pd has been carried out applying a process based on alternate pulses of Pd(hfac)2 and formalin, separated by argon (Ar) purges (cf. Experimental section). Typically 2000 ALD cycles have been applied to obtain a sufficient amount of Pd forming a selective membrane, and being confined within the pores of the mesoporous γ-Al2O3 layer. Figure 1 Obviously, due to the conformality of ALD coatings, Pd metal was deposited not only at the surface but also within the porous volume of the asymmetric support structure. In order to gain further information on the deposit morphology and degree of Pd infiltration, both SEM observations and EDX measurements have been performed on the obtained Pd/Al2O3 composite membranes. Both FESEM observations and EDX analysis (Figure 2a) evidenced a uniform composite membrane design, with fully confined metallic Pd in the γ-Al2O3 mesoporous layer. As observed on both the FESEM image (retro-diffusion mode) and the corresponding Pd-mapping, the preferential location of Pd was in the upper part of the γ-Al2O3, reaching depth of ~700 nm. The notable conformality feature of ALD allows the coatings to be uniformly deposited in the whole porous support, however because gradual pore closing progressively hinders the diffusion of reactants (Pd(hfac)2 and formalin), lower amount of confined Pd nanoclusters were found in deeper locations of the γ-Al2O3 layer (i.e. ˃ 700 nm from the membrane surface). EDX analysis of the cross-section revealed that when taking into account the mass % of Pd, Al and O elements and the density of Pd and γ-Al2O3 (11.9 and 3.7 g/cm 3 , respectively), the total volume of Pd occupying the space in the γ-Al2O3 layer represents about ~40 % of the total layer volume (700 nm thick infiltrated area). This result nicely fits the theoretical porosity of the γ-Al2O3 mesoporous layer which is expected to be in the range 35-40 %. Hence, this observation supports the presumption of well infiltrated Pd clusters filling up the majority of the γ-Al2O3 layer porosity to a depth of 700 nm. Beneath this compact composite layer (beyond a depth of 700 nm), the volume of Pd represents only about 20% of the total membrane layer volume, suggesting that only half of the available γ-Al2O3 pore volume is filled up with Pd clusters.

Figure 2b shows that Pd also penetrated the tubular support through the outer (macroporous) side, confirming the conformality of the ALD route. Advantageously, the ALD precursors can penetrate from the support side across its whole thickness and infiltrate the γ-Al2O3 membrane layer at its interface with the macropores. This has been confirmed by EDX measurements revealing higher Pd concentrations at the interface between the γ-Al2O3 membrane layer and the 200 nm pore size intermediate support layer. This phenomenon could reveal useful for the synthesis of double-side membranes able to limit concentration-polarization phenomena. [START_REF] Peters | Hollow fibre microporous silica membranes for gas separation and pervaporation: synthesis, performance and stability[END_REF] In fact, the advantages of double-sided membranes have been already evidenced elsewhere, [START_REF] Gardner | Enhanced flux through double-sided zeolite membranes[END_REF][START_REF] Sebastian | Synthesis of capillary titanosilicalite TS-1 ceramic membranes by MW-assisted hydrothermal heating for pervaporation application[END_REF] suggesting that, under specific conditions, the flux through membranes can be significantly higher when the selective films are located on both sides of the porous support rather than on a single side. Consequently, for separations in which selectivity increases with membrane thickness, double-sided membranes could yield fluxes and selectivities that are higher than those of single supported films. Currently, we are studying the possibility of a controlled selective layer deposition to obtain double-side membranes of the same thickness in order to confirm the benefit of this kind of composite membranes. Such novel membrane design will be subject of future communications.

Performance of Pd/Al2O3 composite membranes for single gas permeation

With the optimized Pd/Al2O3 composite membranes, different permeation tests have been performed in order to assess their potential for hydrogen separation. Initially, the prepared membranes were tested in single gas permeation measurements to ensure their good quality in terms of absence of any macro-defects (viscous flow contribution) in the membrane structure. In order to select the best membranes, helium permeance has been first measured at room temperature on membrane series prepared with different numbers of ALD cycles. The increasing of ALD cycles led to gradual decrease of He permeance, although the He permeance after 500 -1000 cycles of Pd ALD still remains in the range of the virgin support permeance (~ 10 -5 mol.s -1 .m -2 .Pa -1 or ~10 5 GPU). Such high permeance values reveal that the pores of mesoporous γ-Al2O3 are only partially filled with Pd-clusters and the permeating gas can easily flow through the remaining open pathways (non-selective pores). After applying 1500 ALD cycles, the He permeance was found to decrease significantly (more than 100x) reaching the values ~1 x 10 -7 mol.s -1 .m -2 .Pa -1 (ПHe ~1000 GPU) confirming more important (but still partial) closing of the membrane pores. Further increase of the number of ALD cycles (~ 2000 cycles) led to a decrease of permeance by more than 1000x (ПHe ~ 100 GPU) with Pd-clusters fully confined within the γ-Al2O3 pores. Such composite membranes were considered having an optimum design as any further increase of the number of ALD deposition cycles was found to lead to a gradual growth of a pure Pd top-layer on the γ-Al2O3 surface. Such a Pd toplayer was not desired for the targeted composite membrane design where Pd-clusters have to remain confined within the γ-Al2O3 mesopores. The fact that the as-prepared composite membrane was not completely gas-tight was attributed to the presence of remaining small pathways between the Pd-clusters and the γ-Al2O3 pore walls. However the very low helium flux obtained after 2000 ALD cycles confirmed the formation of an extremely compact composite membrane layer inside the tubular ceramic support. Hence, for any further tests, single gas permeation measurements and gas mixture separation experiments have been carried out on the membranes prepared by applying 2000 ALD cycles of Pd.

It must be noted that virtually zero contribution of viscous flow was detected for all the measured gases, as the membrane permeances were never found to increase with the transmembrane pressure. This result confirms no damage of the γ-Al2O3 mesoporous layer during both the tube sealing treatment and the ALD deposition process.

Single gas permeation measurements with H2, He, N2 and CO2 (table 1) were used to evaluate the membranes reproducibility and their performance in terms of Pd-clusters contribution to the selective transport of hydrogen. At 188°C, the order of gas permeances (i) for He, N2 and CO2 clearly follows the order of their kinetic diameters: He (2.60 Å) > CO2 (3.30 Å) > N2 (3.64 Å). This effect is still observed at lower temperature although the permeance of N2 is the only one which remains stable in the whole studied temperature range. This result should reveal that N2 is able to diffuse through mesopores, although He (and CO2, with lesser extent) is able to access smaller pores (micropores) more and more easily when the temperature increases. This results in the activated transport of He beyond room temperature, thus leading to increasing permselectivity values α*He/N2 (from = 1.9 at 35°C, up to 3.9 at 188°C).

Interestingly, a strongly marked activated transport was evidenced for H2. Considering the kinetic diameter of H2 (2.89 Å) which is larger than He (2.60 Å), the higher permeances of the former could only be explained by the active contribution of Pd species which have a strong affinity with H2 and might form PdHx species facilitating hydrogen transport. Hydrogen transport through the composite Pd/Al2O3 membrane results from a complex contribution of transport through remaining meso/micropores coupled with the chemical diffusion of (atomic) hydrogen through the metallic Pd network. The evolution of membrane permselectivity for selected gas pairs (Figure 3) clearly evidences an exponential increase of the membrane selectivity values towards H2 which are an order of magnitude higher than membrane selectivity values towards He.

Figure 3. Evolution with temperature of the Pd/Al2O3 membrane permselectivity for selected gas pairs.

In order to estimate the potential interest of such Pd-alumina composite membranes, their performance has been reported on a Robeson plot.

Evaluation of membrane performance for gas mixture separation

Finally, the performance of the developed Pd/Al2O3 membranes has been measured for the separation of the components in a gas mixture containing 80.8% N2, 15.4% H2 and 3.8% CO2. Due to detection limitation, gas permeance and separation factors have only been measured for sufficiently high H2 permeance values, i.e. above 100°C.

As shown in table 2, the permeance values with the gas mixture are similar to the values measured with single gases, yielding promising separation efficiency for both H2/N2, H2/CO2 gas pairs. The typical separation factors are in the range 10-15 and the H2 permeance is ~1000 GPU at 188°C. It must be recognized that such performance is below those typically reported in the literature for Pd membranes. However, it should be underlined that the present membranes exhibit promising separation performance in a temperature window lower than those typically reported in the literature (˃300°C). In addition, the originality of this membrane bears also on its preparation method enabling the deposition of much lower quantity of expensive metallic Pd and thus decreasing significantly the membrane cost. Higher membrane stability could also be expected upon thermal cycling of these composite membranes in which the Pd material is confined within a ceramic matrix.

Numerous parameters still need to be investigated in order to fully evaluate the potential of this new concept for the preparation of nanocomposite membranes as an alternative to classical supported palladium gas selective barriers. For example, further improvement of the membrane performance in terms of higher permeance should derive from a judicious adjustment of the membrane architecture, e.g. decreasing the thickness of the mesoporous support layers and consequently the depth and location of the infiltrated Pd clusters. The latter parameters could be achieved by optimizing the ALD process parameters for ensuring effective pore plugging only at the pore entrance and thus improving the membrane performance in terms of gas permeance/selectivity ratio. The presented proof of concept, reporting the fabrication of low-cost hydrogen selective Pd/Al2O3 composite membranes using ALD is a considerable improvement over similar membrane materials reported in the literature in which gas-selective layers prepared typically by CVD or sputtering techniques were much thicker (in the micron range) and deposited on flat disk supports. This work showed that the conformality benefit of ALD can be applied to tune the gas selective properties of membrane materials, thus opening prospects for the separation of different gas mixtures. In fact, this strategy could be further extended to the design of other gas selective or catalytic membranes composed of various composite materials conformably deposited on/in ceramic supports with high surface to volume ratio.

  compares a bare and Pd-coated alumina membrane support by ALD.

Figure 1 :

 1 Figure 1: Images of (a) bare and (b) Pd-coated alumina membranes by ALD.

Figure 2 :

 2 Figure 2: (a) High-resolution XPS spectrum of the Pd 3d region, depicting the Pd 3d3/2 and Pd 3d5/2 peaks, and (b) XRD pattern of the Pd-coated alumina membrane (the characteristic Pd peaks are annotated*).

Figure 2 :

 2 Figure 2: SEM images and corresponding EDX mapping of a) inner part of the tubular membrane support depicting Pd infiltrated in both γ-Al2O3 top-layer (Øpore = 5-8 nm) and α-Al2O3 (Øpore = 200 nm) intermediate layer, b) outer part of the tubular membrane support (Øpore = 10 µm) also coated with Pd.

  [START_REF] Robeson | The upper bound revisited[END_REF] A membrane thickness of ~1 µm (overevaluated) has been considered for converting H2 permeances to permeability values (underevaluated) expressed in Barrers. As shown in Figure4, the membrane performance is just on the upper bound for α*H2/N2 permselectivity vs. H2 permeability and well above upper bound for α*H2/CO2 permselectivity vs. H2 permeability. Promising performance could thus expected for such composite membrane materials for both.

Figure 4 :

 4 Figure 4: Representation of the performance (red dot) at T=188°C, ΔP=3 bars of the Pd/Al2O3 composite membrane (1 µm thick selective layer), on the Robeson plot and its upper bound[56]: (a) α*H2/N2 permselectivity vs. H2 permeability, (b) α*H2/CO2 permselectivity vs. H2 permeability.

CONCLUSIONS

  In this work, we reported for the first time the synthesis of hydrogen selective Pd/Al2O3 composite membranes by ALD. The Pd ALD process was based on the reaction of Pd(hfac)2 and formalin at 220°C. The as-prepared Pd clusters were confined within the pores of a γ-Al2O3 membrane layer, thus demonstrating the successful use of this ALD nanomanufacturing route for the fabrication of Pdbased composite membrane materials in tubular ceramic supports. The gas permeation measurements showed promising permselectivities (α* H2/N2 ~ 23 and α* H2/CO2 ~ 13) together with attractive separation factors (F H2/N2 ~ 16 and F H2/CO2 ~ 9) and reached values above 1000 GPU for the H2 flux at 188°C.

Table 1 :

 1 Gas permeance and permselectivity values of Pd/Al2O3 composite membrane, for different temperatures (ΔP=1 bar).

	Temperature	Permeance (GPU)		Permselectivity α*
	(°C)	He	H2	N2	CO2	He/N2 He/CO2	H2/N2	H2/CO2
	35	84 136 45	40	1.9	2.1	3.0	3.4
	103	129 453 46	87	2.8	1.5	10.0	5.2
	188	171 991 44	79	3.9	2.3	23.0	12.5
	It can be observed in table 1 that the H2 permeation is strongly favored through the composite Pd-
	alumina membrane, in comparison with other gases. This effect is further enhanced when the
	temperature increases.

Table 2 :

 2 Gas permeance and separation factors of Pd/Al2O3 composite membrane for a gas mixture containing 80.8%, N2, 15.4% H2 and 3.8% CO2 (ΔP=3 bar).

	Temperature		Separation factor			Permeance GPU
	(°C)	H2/N2	H2/CO2	N2/CO2	N2	H2	CO2
	103°C	6	5	1	37	251	38
	188°C	16	9	1	41	1124	68

ACKNOWLEDGMENTS

The authors would like to thank Bruno Navarra for his valuable technical assistance. This research was funded by the French national research agency (ANR, program MeNiNA -ANR-17-CE09-0049).

The support of the COST Action MP1402 "HERALD", a European cooperation program, is also acknowledged.