Adolfo Iulianelli
email: a.iulianelli@itm.cnr.it

Matteo Manisco

Nicolas Bion

Anthony Le Valant

Florence Epron

Ozgur Colpan

Elisa Esposito

John C Jansen

Mario Gensini

Alessio Caravella

C O Colpan

Sustainable H2 generation via steam reforming of biogas in membrane reactors: H2S effects on membrane performance and catalytic activity

Keywords: Pd-Au/Al2O3 membrane, membrane reactor, hydrogen, Rh-based catalyst, biogas steam reforming

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Today, the general interest on ever more sustainable processes in strategic areas such as energy, environmental protection, etc. is involving a critical challenge represented by the transition from a fossil fuelbased to a sustainable and circular economy [START_REF] Kalmykova | Circular economy -From review of theories and practices to development of implementation tools[END_REF][START_REF] Handayani | From fossil fuels to renewables: An analysis of long-term scenarios considering technological learning[END_REF], adopting the principles of Process Intensification Strategy to drive the transition from conventional to alternative and environmentally friendly technologies [START_REF]Research Agenda for Process Intensification: Towards a Sustainable World of 2050[END_REF]. The concepts of this new strategy are well-matched by the membrane engineering, which is playing a crucial role in the substitution of a number of traditional devices used in the process engineering with membrane-based units such as the membrane reactors (MRs) [START_REF] Sirkar | Process intensification with selected membrane processes[END_REF][START_REF] Drioli | Process intensification strategies and membrane engineering[END_REF].

As a consequence of the growing interest toward the so-called hydrogen economy [START_REF] Biegler | The hydrogen economy[END_REF], in the last decades, hydrogen production and purification technologies have attracted considerable interest and, among them, MR technology received growing attention, because able to combine the exploitation of renewable sources [START_REF] Armaroli | The Hydrogen Issue[END_REF][START_REF]Research Agenda for Process Intensification: Towards a Sustainable World of 2050[END_REF][START_REF]Hydrogen production pathways[END_REF][10] with the simultaneous production and purification of hydrogen via reforming reactions [START_REF] Yan | Progress in catalytic membrane reactors for high purity hydrogen production[END_REF][START_REF] Iulianelli | New perspectives in hydrogen production, separation and utilization[END_REF][START_REF] Brunetti | Membrane reactors for low temperature applications: An overview[END_REF][START_REF] Meénendez | Inorganic membrane reactors for energy applications, Nanoporous Materials for Energy and the Environment[END_REF][START_REF] Iulianelli | The oncoming energy vector: hydrogen produced in Pd-composite membrane reactor via bioethanol reforming over Ni/CeO2 catalyst[END_REF]. In a MR, the presence of a hydrogen perm-selective membrane allows its selective removal from the reaction toward the permeate side, involving the so-called "shift-effect" on the reaction system, responsible for higher reaction products formation and further feed product conversion than an equivalent conventional reformer (CR) (for thermodynamically restricted reactions) [START_REF] Iulianelli | Development of membrane reactor technology for H2 production in reforming process for low-temperature fuel cells, Ch. 12 in Current Trends and Future Developments on (Bio-) Membranes: Co-generation systems and membrane technology[END_REF].

To date, most of the production of hydrogen at a larger scale comes from natural gas steam reforming reaction [START_REF] Muradov | From hydrocarbon to hydrogen-carbon to hydrogen economy[END_REF], which involves the exploitation of fossil fuels and the consequent formation of greenhouse gases (GHGs).

According to the standard ISO 14001:2015 Environmental Management, any change to the environment, whether adverse or beneficial, wholly or partially resulting from an organization's environmental aspects should be considered [START_REF]Environmental management systems -Requirements with guidance for use[END_REF]. Hence, a steam reforming plant, both classical and MR based, has an anthropogenic impact on the environment [START_REF]Environmental impact of hydrogen plants[END_REF].

Among the renewable feedstocks, biogas results to be a greener option than the natural gas for generating hydrogen by reforming processes, which may contributing to lower the emission of GHGs [START_REF] Brunetti | Process intensification for greenhouse gas separation from biogas: More efficient process schemes based on membrane-integrated systems[END_REF]. The composition of biogas may vary greatly, depending on the nature of the residual biomass (sewage treatment plants, animal waste, etc.) used during the anaerobic digestion process, even though it is mostly constituted of CH4 and CO2, besides N2, hydrogen, volatile organic compounds, H2S, etc. [START_REF] Lau | Biogas upgrade to syn-gas (H2-CO) via dry and oxidative reforming[END_REF][START_REF] Rasi | Trace compounds of biogas from different biogas production plants[END_REF]. However, the combination of biogas processing with MR technology was not largely studied in the literature, particularly because of the presence of H2S as main responsible for the membrane and catalyst poisoning and consequent performance loss. However, in the last years, there has been a growing attention toward the development of high performance reforming catalysts, showing high catalytic activity, high carbon deposition resistance, etc. According to the specialized literature, metal catalysts such as Rh, Pd or Ru, Pt, and Ni, showing high activity and selectivity, were used for hydrogen generation via steam reforming of biogas (SRB) [START_REF] Angeli | Catalyst development for steam reforming of methane and model biogas at low temperature[END_REF]. To date, most of the scientific literature about the SRB reaction in CRs is based on Ni catalysts, even though Rh presents a greater catalytic activity than Ni, associated to a lower coke formation tendency [START_REF] Araki | Durability of a Ni based monolithic catalyst in the autothermal reforming of biogas[END_REF][START_REF] Luneau | Deactivation mechanism of Ni supported on Mg-Al spinel during autothermal reforming of model biogas[END_REF][START_REF] Xu | Biogas reforming for hydrogen production over a Ni-Co bimetallic catalyst: Effect of operating conditions[END_REF][START_REF] Damyanova | Biogas reforming over bimetallic PdNi catalysts supported on phosphorus-modified alumina[END_REF][START_REF] Izquierdo | Ni and Rh single bond Ni catalysts supported on Zeolites L for hydrogen and syngas production by biogas reforming processes[END_REF]. The choice of the support is also of major importance since it plays an essential role in the stability of the catalyst.

In the last years, it was experienced that catalysts constituted of Rh supported on -Al2O3 modified by cations such as Mg 2+ were efficient for steam reforming reactions [START_REF] Bion | Design of nanocatalysts for Green hydrogen production from bioethanol[END_REF][START_REF] Valant | Effect of higher alcohols on the performances of a 1%Rh/MgAl2O4/Al2O3 catalyst for hydrogen production by crude bioethanol steam reforming[END_REF]. On the other hand, only a few studies deal with the utilization of Pd-based MRs to carry out the steam reforming of a real or synthetic biogas stream. In these studies, both fluidized bed [START_REF] Durán | Pure hydrogen from biogas: Intensified methane dry reforming in a two-zone fluidized bed reactor using permselective membranes[END_REF][START_REF] Saebea | Enhancement of hydrogen production for steam reforming of biogas in fluidized bed membrane reactor[END_REF] and packed bed [START_REF] Vásquez Castillo | Effect of temperature and pressure on hydrogen production from steam reforming of biogas with Pd-Ag membrane reactor[END_REF][START_REF] Sato | Steam reforming of biogas mixtures with a palladium membrane reactor system[END_REF][START_REF] Raybold | Analyzing enhancement of CO2 reforming of CH4 in Pd membrane reactors[END_REF][START_REF] Di Marcoberardino | Life cycle assessment and economic analysis of an innovative biogas membrane reformer for hydrogen production[END_REF][START_REF] Iulianelli | Model biogas steam reforming in a thin Pd-supported membrane reactor to generate clean hydrogen for fuel cells[END_REF][START_REF] Silva | Evaluating hydrogen production in biogas reforming in a membrane reactor[END_REF][START_REF] Bagnato | Pure hydrogen production from steam reforming of bio-sources[END_REF] MR modalities were analyzed with the purpose of producing high grade hydrogen using synthetic H2S free biogas mixtures [START_REF] Durán | Pure hydrogen from biogas: Intensified methane dry reforming in a two-zone fluidized bed reactor using permselective membranes[END_REF][START_REF] Vásquez Castillo | Effect of temperature and pressure on hydrogen production from steam reforming of biogas with Pd-Ag membrane reactor[END_REF][START_REF] Sato | Steam reforming of biogas mixtures with a palladium membrane reactor system[END_REF][START_REF] Raybold | Analyzing enhancement of CO2 reforming of CH4 in Pd membrane reactors[END_REF], or considering a pre-treatment step useful to remove H2S prior to the reforming stage [START_REF] Alves | Overview of hydrogen production technologies from biogas and the applications in fuel cells[END_REF][START_REF] Khan | Biogas as a renewable energy fuel -A review of biogas upgrading, utilization and storage[END_REF][START_REF] Di Marcoberardino | Potentiality of a biogas membrane reformer for decentralized hydrogen production[END_REF]. In particular, Ru/Al2O3 catalyst and a thick self-supported Pd-Ag membrane (200 m) were used by Itoh's group [START_REF] Vásquez Castillo | Effect of temperature and pressure on hydrogen production from steam reforming of biogas with Pd-Ag membrane reactor[END_REF][START_REF] Sato | Steam reforming of biogas mixtures with a palladium membrane reactor system[END_REF], while Ni-based catalysts were packed in self-supported Pd-Ag [START_REF] Silva | Evaluating hydrogen production in biogas reforming in a membrane reactor[END_REF][START_REF] Bagnato | Pure hydrogen production from steam reforming of bio-sources[END_REF] and supported Pd (7 m layer)/Al2O3

MRs [START_REF] Iulianelli | Model biogas steam reforming in a thin Pd-supported membrane reactor to generate clean hydrogen for fuel cells[END_REF]. To the best of our knowledge, in only one case a commercial Rh-based catalyst was used in a fluidized bed MR housing a supported Pd-Ag (5 m layer)/Al2O3 membrane [START_REF] Durán | Pure hydrogen from biogas: Intensified methane dry reforming in a two-zone fluidized bed reactor using permselective membranes[END_REF].

According to what stated above, pure Pd membranes suffer a lot the H2S attack, which is responsible for the block of the H2 dissociation sites and, in some case, for the formation of the Pd4S in the membrane bulk, causing with the consequent their the performance loss due to the lower hydrogen permeability of Pd4S than pure Pd, or the membrane failure after long-term exposure [START_REF] Braun | Pd-based binary and ternary alloy membranes: Morphological and perm-selective characterization in the presence of H2S[END_REF][START_REF] Chen | The effect of H2S on the performance of Pd and Pd/Au composite membrane[END_REF][START_REF] Caravella | Quantitative contribution of non-ideal permeability under diffusion-controlled hydrogen permeation through Pd-membranes[END_REF][START_REF] Nam | Hydrogen separation by Pd alloy composite membranes: introduction of diffusion barrier[END_REF][START_REF] Wang | Review of alloy membranes/films for hydrogen separation or purification[END_REF][START_REF] Rahimpour | Palladium membranes applications in reaction systems for hydrogen separation and purification: A review[END_REF]. On the contrary, Pd-Au and Pd-Cu alloyed membranes result to be more H2S resistant than pure Pd ones [START_REF] Nielsen | Effect of temperature on the corrosion of Cu-Pd hydrogen separation membrane alloys in simulated syngas containing H2S[END_REF][START_REF] Acha | Thin PdCu membrane for hydrogen purification from insitu produced methane reforming complex mixtures containing H2S[END_REF][START_REF] Braun | PdAgAu alloy with high resistance to corrosion by H2S[END_REF][START_REF] Zhao | Tailoring palladium alloy membranes for hydrogen separation from sulfur contaminated gas streams[END_REF][START_REF] Lewis | PdAu and PdAuAg composite membranes for hydrogen separation from synthetic water-gas shift streams containing hydrogen sulfide[END_REF]. In particular, the growing interest in Pd-Au alloy membranes has been due to their superior hydrogen permeability performance and higher resistance to H2S poisoning with respect to Pd-Cu membranes [START_REF] Lewis | PdAu and PdAuAg composite membranes for hydrogen separation from synthetic water-gas shift streams containing hydrogen sulfide[END_REF]. For example, Chen and Ma [START_REF] Chen | The effect of H2S on the performance of Pd and Pd/Au composite membrane[END_REF] performed H2 permeation tests on a Pd-Au membrane in presence of ~ 55 ppm of H2S at 773 K and 200 kPa, founding that higher temperatures allowed quicker hydrogen recovery when H2S was removed, whereas longer H2S exposure determined longer hydrogen recovery times. Coulter et al. [START_REF] Coulter | Sulfur tolerant PdAu and PdAuPt alloy hydrogen separation membranes[END_REF] tested Pd-Au membrane foils under permeation tests of hydrogen-rich syngas streams containing from 20 to 50 ppm of H2S at 673 K and 1253 kPa, at different Au compositions (between 7 and 20 wt%), observing that higher Au content in the alloy determined a lower H2S inhibition effect on the H2 permeability. The most recent research focuses on Pd-ternary alloys (among Pd, Au, Pt, Ag etc.) membranes, although their fabrication and testing result to be at an infancy stage [START_REF] Braun | Pd-based binary and ternary alloy membranes: Morphological and perm-selective characterization in the presence of H2S[END_REF][START_REF] Coulter | Sulfur tolerant PdAu and PdAuPt alloy hydrogen separation membranes[END_REF][START_REF] Braun | PdAgAu alloy with high resistance to corrosion by H2S[END_REF][START_REF] Lewis | PdAu and PdAuAg composite membranes for hydrogen separation from synthetic water-gas shift streams containing hydrogen sulfide[END_REF]. In some case, Pd-ternary alloys membranes showed very good tolerance toward H2S (up to 1000 ppm) during hydrogen permeation tests, without a clear evidence of performance loss due to the Pd4S formation [START_REF] Braun | PdAgAu alloy with high resistance to corrosion by H2S[END_REF].

The aim of this experimental study is to perform the steam reforming of a synthetic biogas mixture containing H2S in two MRs: the first, housing a high hydrogen permeable and low hydrogen perm-selective supported Pd-Au/Al2O3 membrane; and the second, housing a low hydrogen permeable and high hydrogen permselective self-supported Pd-Ag membrane. Both the MRs are packed with a non-commercial Rh(1%)/MgAl2O4/Al2O catalyst. In our previous studies, Pd-Au membranes supported on ceramic [START_REF] Iulianelli | Hydrogen permeation and separation characteristics of a thin Pd-Au/Al2O3 membrane: The effect of the intermediate layer absence[END_REF] and porous stainless steel [START_REF] Iulianelli | Supported Pd-Au membrane reactor for hydrogen production: membrane preparation, characterization and testing[END_REF] supports were studied in terms of hydrogen permeation characteristics at different experimental conditions, analyzing the role of the presence/absence of an intermediate layer between the separative metallic layer and the support, without evaluating the effect of the contaminants presence such as H2S in the feed stream. Furthermore, a H2S-free synthetic biogas stream was used to produce high grade hydrogen via steam reforming reaction performed in a Pd/Al2O3 MR [START_REF] Iulianelli | Model biogas steam reforming in a thin Pd-supported membrane reactor to generate clean hydrogen for fuel cells[END_REF].

The novelty of this study consists in evaluating the effects of the H2S, presence in the model biogas stream, on the catalytic activity and the hydrogen perm-selectivity characteristics of the Pd-alloyed membranes. As stated above, the effect of H2S contained in hydrogen-rich feed streams during the hydrogen permeation through Pd-Au membranes was investigated by a number of scientific papers, but to our best knowledge no papers deal with the steam reforming of a H2S-containing biogas stream carried out in a Pd-Au based MR, where the effects of the simultaneous presence of H2S, CO2 and other reaction byproducts were evaluated in terms of hydrogen permeation membrane performance and catalytic activity as well. Furthermore, an equivalent H2S free biogas mixture is used for better highlighting the role of H2S during the SRB on the MRs performance, evaluated in terms of methane conversion, hydrogen recovery and purity.

Experimental

Catalyst preparation and characterization

The experimental campaign was performed using a Rh(1%)/MgAl2O4/Al2O catalyst. The catalyst support was constituted of -alumina beads (200 m 2 g -1 , 1-2 mm in diameter, provided by AXENS) impregnated with magnesium acetate (Mg(CH3COO)2•4H2O, Alpha Aesar) to achieve 5 wt.% of magnesium in the support [59].

The choice of the acetate was due to the better control of the acidic-base behaviors of the support. After impregnation, the support was subjected to various optimal thermal treatments, especially at high temperature (1273 K) in order to form the MgAl2O4 spinel structure at the outer shell of the alumina support [59]. Rh (1% wt), as metallic catalyst phase, was wet impregnated to the support by using RhCl3 (Alpha Aesar) for 4 h at room temperature. After impregnation procedure, the catalyst was placed under stirring at 318 K for 24 h and dried at 393 K for 15 h. Then, it was activated by calcination at 973 K for 4 h (heating ramp 295 K min -1 ).

The bare alumina beads, the alumina modified by the addition of magnesium and thermal treatment and Rh/MgAl2O4/Al2O3 catalyst were characterized by nitrogen adsorption (Micromeritics Tristar 3000) for the determination of the BET surface area, hydrogen chemisorption for the determination of the metal accessibility, ICP-OES (Perkin-Elmer Optima 2000 DV) for the metal content, X-Ray Diffraction (Siemens D5000) and Scanning Electron Microscopy (INCA 300 OXFORD).

Membranes characterization and testing

The non-commercial supported Pd-Au/α-Al2O3 membrane (Nanjing Tech. University & GaoQ Functional Materials Co., Ltd., China) is constituted of a thin Pd-Au layer (~ 8 μm) deposited on the porous substrate (-Al2O3) via electroless plating technique and the total membrane length is 7.5 cm, with 5.0 cm of active length, o.d. 13 mm and i.d. 8 mm, Figure 1(a). Chemical and morphological analyses were performed by scanning electron microscopy (Phenom Pro X desktop SEM, Phenom-World), which is equipped with an energy dispersive X-ray spectroscopy detector (EDX) allowing elemental analysis of membrane surface and cross-section before and after (post mortem analysis) the experimental campaign. The bench-scale Pd-Au/Al2O3 MR module is illustrated in Figure 1(b), in which the feed, sweep-gas, retentate and permeate streams are clearly shown. The MR module is 12 cm in total length and 3.0 cm of o.d. and the supported Pd-Au based membrane is housed inside by using two graphite gaskets useful to prevent the mixing within permeate and retentate streams. The Rh-based catalyst (1.5 g) was packed in the annulus of the MR, while the permeated products were collected in the lumen of the MR for successive gas chromatograph (GC) analyses.

The commercial self-supported Pd-Ag membrane (Johnson Matthey, UK) is constituted of a wall thickness of about 150 μm, o.d. 10 mm, 145 mm as total length, and 100 mm as active length, Figure 1(c). In this study, the Pd-Ag membrane is closed at one end and the catalyst is packed in the lumen side, Figure 1(d). Before the reaction tests, the Rh-based catalyst was reduced under pure hydrogen for 1 h. The MR was heated up by using an electrical heating tape connected to a temperature controller and a thermocouple. The experimental campaign started with single gas (H2, N2, CO2, CH4) permeation tests, performed between 573 and 823 K, by varying the feed pressure between 150 and 250 kPa (abs.). Afterwards, the SRB reaction was carried out in the MRs with the temperature varying between 673 and 823 K and the pressure between 150 kPa (abs.) and 250 kPa (abs.), by means of a back-pressure controller (Fantinelli, Italy) placed at the outlet side of the retentate stream. The H2O/CH4 feed molar ratio was set at 2/1 and 3/1, and two synthetic biogas mixtures (with and without H2S) flowed into the MRs. The composition of the gas mixture for both cases is given in Table 1. The weight hourly space velocity (WHSV) was varied during the reaction tests from 0.2 to where CH4-in and CH4-out are the inlet and outlet CH4 molar flow rates, respectively.

1.3 h -1 .

Hydrogen recovery (

%) = 100 ) H (H H retentate 2 permeate 2 permeate 2      (2)
where H2-permeate and H2-retentate are the outlet H2 molar flow rates in the permeate and retentate sides, respectively.

Hydrogen permeate purity (%) = (H2-permeate/Fpermeate) x 100 [START_REF]Research Agenda for Process Intensification: Towards a Sustainable World of 2050[END_REF] where Fpermeate represents the outlet permeate molar flow rate. Furthermore, to analyze the permeation behaviours of the Pd-Au/Al2O3 and Pd-Ag membranes, single gas permeation tests were done according to the following general equation, regulating the H2 flux permeating through a generic membrane:

) p (p Pe J n -permeate H2 n -retentate H2 H 2   (4)
where JH2 indicates the H2 flux permeating through the membrane, Pe is the H2 permeance, pH2-retentate and pH2-permeate are the H2 partial pressures in the retentate and permeate sides, respectively, and n (variable in the range 0.5 -1) is the dependence factor of the H2 permeating flux on the H2 partial pressure;

i H2 i / 2 H J J α  (5)
where H2/i represents the "ideal selectivity", calculated as the ratio within the hydrogen permeating flux (JH2) over the permeating flux of another pure gas (Ji, with i = CO2, CH4, and N2). The correlation between the H2 membrane permeability (PH2) and the temperature was described by an Arrhenius-like equation (Eq. 6).

(

) 6
where PH2,0 represents the pre-exponential factor, R is the universal gas constant, EA is the apparent activation energy and T is the temperature. The main characteristics of these materials are reported in Table 2. The thermal treatment at high temperature and the addition of Mg 2+ induce a strong decrease of the BET surface area of the support from 207 m 2 g -1 for the bare alumina to 84 m 2 g -1 for while the addition of Rh does not modify the support. Rh and Mg contents are similar to the nominal values and the Rh dispersion of 40 wt% corresponding to Rh particle sizes of 2 nm considering hemispherical particles. Afterwards, single gas permeation tests were carried out on the Pd-Au/Al2O3 membrane in order to evaluate the H2 perm-selectivity characteristics and the resistance to the effect of H2S during the permeation test using the gaseous mixtures (Mixture 1 and Mixture 2) of Table 1. In particular, pure H2 permeation tests were conducted between 573 and 823 K, varying the transmembrane pressure between 0 and 100 kPa, Figure 5(a). As expected, the H2 flux permeating through the membrane increased by raising the transmembrane pressure (as higher the transmembrane pressure as higher the H2 permeation driving force) and the temperature. In particular, an increase of temperature allowed an enhancement of the H2 permeating flux; indeed, the H2 permeability depends on the temperature according to an Arrhenius-like equation, Figure 5(b)

Results and Discussion

Characteristics of the

The apparent activation energy (EA) was graphically estimated at 50 kPa of transmembrane pressure from the plot of Figure 5(b), which gives 12.1 kJ/mol with the correlation coefficient R 2 ~ 0.97, hence comparable to other literature data [START_REF] Ryi | Pd-Cu alloy membrane deposited on alumina modified porous nickel support (PNS) for hydrogen separation at high pressure[END_REF][START_REF] Iulianelli | A supported Pd-Cu/Al2O3 membrane from solvated metal atoms for hydrogen separation/purification[END_REF][START_REF] Patki | Apparent activation energy for hydrogen permeation and its relation to the composition of homogeneous PdAu alloy thin-film membranes[END_REF][START_REF] Islam | Characterization of Pd-Cu membranes fabricated by surfactant induced electroless plating (SIEP) for hydrogen separation[END_REF][START_REF] Brenner | Hydrogen purification with palladium and palladium alloys on porous stainless steel membranes[END_REF][START_REF] Wang | The effect of co-existing nitrogen on hydrogen permeation through thin Pd composite membranes[END_REF][START_REF] Liguori | Performance and long-term stability of Pd/PSS and Pd/Al2O3 for hydrogen separation[END_REF][START_REF] Chen | Polarization phenomena of hydrogen-rich gas in high-permeance Pd and Pd-Cu membrane tubes[END_REF][START_REF] Pomerantz | Novel method for producing high H2 permeability Pd membranes with a thin layer of the sulfur tolerant Pd/Cu fcc phase[END_REF], Table 3.

Furthermore, a graphical assessment of the H2 permeating flux plot as a function of the H2 permeation driving force at different "n-value" was carried out. The "n" exponent of the hydrogen partial pressure represents the dependence factor of the H2 flux permeating through the membrane on the H2 partial pressure, and the regression factor (R 2 ) indicates when it achieves the maximum value, the best experimental data fitting.



Commonly, for thin metallic layers (< 5 m) "n" may range between 0.5 and 1.0 and, when n = 0.5, the Eq. ( 4) describing the correlation between the H2 permeating flux and H2 permeation driving force becomes the Sieverts-Fick law, which means that the H2 permeation depends substantially on the diffusion of atomic H2 into the bulk [START_REF] Islam | Characterization of Pd-Cu membranes fabricated by surfactant induced electroless plating (SIEP) for hydrogen separation[END_REF]. The latter case results to be the rate-limiting step of the overall permeation process. On the other hand, when n = 1, Eq. ( 4) describes the H2 permeation controlled by the mass transport to or from the surface or by the dissociative adsorption or associative desorption. When "n-value" ranges from 0.5 to 0.8, the H2 permeation is regulated by different contributions such as diffusion, Knudsen, or viscous flow [START_REF] Yun | Correlations in palladium membranes for hydrogen separation: A review[END_REF].

In Table 4, the n-values are reported as a function of temperature and each value of them represents the best data fitting (maximum R 2 ) at a set temperature. According to what was demonstrated by Caravella et al. [START_REF] Caravella | Sieverts law empirical exponent for Pd-based membranes: critical analysis in pure H2 permeation[END_REF] for membrane thickness (Pd-based layer) lower than 20 m, the trend of n-value may show a maximum that, in this work, was reached at 623 K. Therefore, at a lower temperature, the H2 permeation through the Pd-Au/Al2O3 membrane is controlled by the diffusion through the Pd-Au bulk, showing the best n-value is equal to the Sieverts coefficient (0.5). Increasing the temperature, the maximum R 2 was achieved at higher n-values. In particular, at 623 K, the best n-value was equal to 1, highlighting that the H2 permeation is completely controlled by the surface mass transport. By increasing the temperature, the best n-value achieved a plateau around 0.8. This means that different mechanisms contribute to control the hydrogen permeation through this membrane. Afterwards, as a function of the operating time, the permeation of H2 through the membrane was studied as a component in H2-rich gas mixtures (a constant H2 stream, 474 mL/min, mixed with a gaseous stream constituted of CO2/CH4/N2 with and without H2S, Table 5) and compared to that as single gas. This allowed to evaluate the eventual depletion of the H2 permeating volume flow rate due to the effects of the concentration polarization, the adsorption of some components on the membrane surface, and -particularly -the harmful presence of H2S. Figure 6 sketches a constant trend of the permeating volume flow rate of H2 (around 380 mL/min as mean value) as single gas in a range time of 14 h under testing at 673 K and 100 kPa of H2 partial pressure difference across the membrane. To evaluate the effects on the H2 permeation of other compounds typically present in a SRB reformed streams (CO2, CH4, N2, and H2S), the two gaseous mixtures (with and without H2S) were mixed with a constant H2 stream and the H2 partial pressure difference across the membrane was set at 100 kPa.

This was done by varying the total feed pressure as a consequence of the H2 composition variation in the final H2-rich gas mixtures in order to be able to correctly compare the H2 permeation through the membrane at the same H2 partial pressure difference set for the case of H2 tested as a single gas. Considering the H2-rich gas mixture without H2S, it is evident that the lower the composition, the higher the of the H2 permeating flow rate as a consequence of the ever more pronounced concentration polarization effect. mL/min after 13 h of pure H2 flowed as single gas into the MR module. However, the depletion of the H2 permeating flow rate was more severe when H2S was present in the H2-rich gas mixture. A part from the aforementioned effects of the concentration polarization and CO2 inhibition, the presence of H2S lowered the H2 permeating flow rate, which passed from 365 mL/min at xH2 = 0.95 to 81 mL/min at xH2 = 0.42. Even in this case, the H2 permeation characteristics of the Pd-Au/Al2O3 membrane were reversibly recovered after flowing once again pure H2 to the membrane module. The capacity of fully restoring the hydrogen permeation characteristics shown by the non-commercial membrane results to be one of the most significative results of this work because it avoids the substitution of the membrane, hence resulting economically attractive for possible scale up of this technology.

experimental tests on the commercial self-supported Pd-Ag membrane: gas permeation analyses

The single gas permeation tests on the self-supported Pd-Ag membrane were performed at different temperatures ranging between 573 and 673 K, by varying the transmembrane pressure between 0 and 200 kPa, Figure 7(a). The maximum operating temperature for this commercial membrane is set at 673 K, consequently, all the experimental campaign considered the aforementioned temperature as the maximum operable condition. The experiments evidenced that only H2 may permeate through the membrane for all the tested temperatures. Therefore, the H2 permeation mechanism is regulated by the diffusion through the membrane bulk and the H2 permeating flux followed the Sieverts-Fick law (n-value equal to 0.5). As for the supported Pd-Au/Al2O3 membrane, also for the self-supported Pd-Ag membrane the H2 permeating flux increased as a consequence of both temperature and transmembrane H2 partial pressure difference raise. In this case, the apparent activation energy was equal to 14.8 kJ/mol.

SRB reaction tests

SRB reaction tests were carried out firstly in the Pd-Au/Al2O3 MR at 823 K and 150 kPa, by varying the H2O/CH4 molar ratio between 2/1 and 3/1, and the WHSV between 0.5 and 1.3 h -1 , analyzing the experimental results in term of CH4 conversion, Figure 8(a) and H2 recovery, Figure 8(b). As shown in Figure 8(a), at constant WHSV CH4 conversion increased at a higher H2O/CH4 molar ratio (from 18 to almost 25% at WHSV = 0.5 h -1 ) because, as expected, an excess of steam favored higher conversions from a thermodynamic point of view.

Nevertheless, no higher feed molar ratios were used because a larger excess of steam may determine a negative effect on the H2 permeation mechanism, acting a dilution of the products present in the reaction side, lowering the H2 partial pressure and, consequently, the H2 permeation driving force. In comparison with an equivalent CR operated at the same experimental conditions, the Pd-Au/Al2O3 MR showed a better conversion at lower WHSV and stoichiometric feed molar ratio, whereas it was comparable at higher WHSV, Figure 8(a). On the contrary, in the presence of an excess of water in the feed, CH4 conversion overcame the thermodynamic equilibrium conversion of the equivalent CR at both the WHSVs investigated in this work.

However, at each feed molar ratio, an increase of WHSV lowers the CH4 conversion due to the lower contact time between the reactants and the catalyst, Figure 8(a). This involves a reduction of H2 produced during the reaction and, consequently, a lower H2 permeation driving force, responsible for a lower H2 removal and, consequently, for a lower H2 recovery, Figure 8(b). On the contrary, an increase of feed molar ratio induces an enhancement of H2 production, involving a higher H2 permeation driving force with a consequent larger H2 removal from the reaction to the permeate side. This favors an increase of H2 collected in the latter side and, consequently, the increment of the H2 recovery (from 70 to almost 80% at WHSV = 0.5 h -1 ), Figure 8(b).

To confirm that coke was formed during the SRB reaction, after each experimental test, a H2/N2 mixture was fed to the reaction side of the MR in order to observe a possible formation of CH4 as proof of its presence.

No O2 was used because it could oxidize the metallic layer of the membrane, causing its failure. In all the experimental tests, the GC analyses confirmed the production of CH4, and the evolution of the depletion of the H2 stream fed during the H2/N2 mixture treatment was detected up to completely convert the coke

.5 h -1 1.3 h -1 (a)
present in the reaction zone. This objective was achieved when the H2 molar flow rate fed to the MR (by using the H2/N2 mixture) was equal to that present in the outlet stream, as a proof that no C was present in the reaction side. As expected, more coke was formed at lower feed molar ratio and higher WHSV. In order to be sure about the validity of the experimental tests, the carbon balance within the inlet and outlet streams was evaluated also taking into account the amount of coke formed during the H2/N2 mixture treatment (this was done by integrating the evolution of the CH4 molar flow rates detected during the H2/N2 treatment). The carbon balance was then closed in all the experimental tests with a maximum error equal to ±6%.

From a general point of view, although superior to the maximum conversions achievable in the equivalent CR at the same set conditions, the conversions of the Pd-Au/Al2O3 MR were limited by the relatively low H2 perm-selectivity (as a consequence of the non-full H2 perm-selectivity of this membrane), which did not allow to involve a stronger 'shift effect' of the SRB reaction system.

On the contrary, the experimental campaign using the self-supported Pd-Ag MR evidenced great performance in terms of conversion, although with limited H2 recovery. Figure 9(a) shows the CH4 conversion at 673 K as a function of reaction pressure and WHSV. The different reaction temperature at which the Pd-Ag MR was exercised with respect to the Pd-Au/Al2O3 MR was due to the maximum temperature limit imposed for avoiding the Pd-Ag membrane failure. As for the first MR, the conversion was improved by increasing the feed molar ratio and lowering the WHSV. As the best value, the Pd-Ag MR reached almost 85% of CH4 conversion at 673 K, feed molar ratio = 3/1 and WHSV = 0.2 h -1 . Being the Pd-Ag membrane fully H2 perm-selective, the shift effect operated by the membrane in removing H2 from the reaction side to the permeate side was more effective, determining enhanced conversions with respect to the previous MR investigated. In Table 6, the best performance of the supported Pd-Au/Al2O3 and unsupported Pd-Ag MRs are reported.

Much better CH4 conversion was achieved in the Pd-Ag MR due to a large effect of the SRB reaction shift towards the products, as a consequence of the selective hydrogen permeation through the membrane. The former MR achieved great performance even though it was exercised at a lower temperature (673 K) and this constituted a further advantage over the Pd-Au/Al2O3 MR in terms of energy saving. However, due to the low hydrogen permeability characteristics of the unsupported Pd-Ag membrane related to its thick wall (150 m) the hydrogen recovery was not higher than 40% although its purity was around 100% with a CO content ≤ 20 ppm, making this stream useful for low-temperature (LT) PEM fuel cell supplying. Nevertheless, the composition of hydrogen of the retentate stream is equal to a bit more than 50% at the best conditions Figure 10(a), decreasing to around 40% at lower H2O/CH4 molar ratio, Figure 10(b). Hence, the retentate stream would need to be treated in a second stage process to further transform the residual CH4 into H2 and CO2 and recover as much as possible H2, implying additional operation costs due to a higher number of stages processing. On the other hand, the Pd-Au/Al2O3 MR allowed higher hydrogen recovery (80% at the best conditions as reported in Table 6) with a purity not adequate to low-temperature PEM fuel cells supplying (CO content at the best operation conditions investigated in this work equal to 1300 ppm), but useful for feeding hightemperature PEM or other kinds of fuel cells. comparable with other few data about supported Pd-based MRs present in literature working at similar temperatures (Table 6), although the experimental data of this work dealt with a model biogas stream containing H2S.

To evaluate the negative effect on the catalytic performance due to the presence of H2S in the feed stream, a H2S free model biogas stream (Table 5) was adopted to carry out the SRB reaction in both the MRs and the results were reported in Table 6. As shown, the Pd-Au/Al2O3 MR reached better CH4 conversion (45%) at 823 K, 150 kPa of pressure and 3/1 as feed molar ratio than the case of supplying a model biogas stream containing H2S (~ 25%). Meanwhile, both H2 recovery (82%) purity (55%) were slightly higher, probably due to the larger amount of H2 present in the reaction side that globally enhanced the H2 permeation driving force. Also in the case of the Pd-Ag MR, at 673 K, 200 kPa, and 3/1 as feed molar ratio, CH4 conversion improved up to 97% as well as the H2 recovered in the permeate side (45%). What reported above demonstrated that the presence of H2S in the model biogas feed stream affected negatively the catalyst and membrane performance in both the MRs. This was also recently confirmed by Yin et al. [START_REF] Ryi | Pd-Cu alloy membrane deposited on alumina modified porous nickel support (PNS) for hydrogen separation at high pressure[END_REF], who carried out the SRB in a CR at 973 K, 120 kPa, and 3/1 as feed molar ratio, observing the CH4 conversion decrease from 85% (in the case of a model H2S free biogas stream supplying) to around 20% (using a model H2S containing biogas stream), Table 6. In addition, Papurello et al. [START_REF] Brenner | Hydrogen purification with palladium and palladium alloys on porous stainless steel membranes[END_REF] observed lower catalytic performance during SRB in a CR exercised at 1073 K, 100 kPa and stoichiometric feed ratio owing to the negative effect of H2S present in the model biogas feed stream, registering the depletion of CH4 conversion from 99 to 22%.

To check the performance (in terms of CH4 conversion) during SRB in both of the MRs under longer-term reaction tests, the operating conditions were set at 823 K, 150 kPa and feed molar ratio = 3/1 for the Pd-Au/Al2O3 MR and at 673K, 200 kPa and feed molar ratio = 3/1 for the Pd-Ag MR, supplying the synthetic biogas stream containing H2S. As shown in Figure 11, in the first 50 minutes transient phenomena occurred, in which CH4 conversion increased up to around 25-30% for the Pd-Au/Al2O3 MR and 85% for the Pd-Ag MR.

Afterwards, the conversion remained stable in the first MR up to around 400 min; then, it slightly decreased up to less than 10% after 1000 min under testing. For the second MR, the conversion remained stable up to around 200 min; after that, it decreased quickly up to reach around 10% after 1000 min under testing. 2).

Although qualitatively the Pd-Au/Al2O3 MR lower conversions than the Pd-Ag MR, it is possible to observe that the first MR contrasted better the exposure to the H2S containing biogas feed stream, doubling the time in which a performance decrease was observed in the second MR. This could be due to the higher H2S resistance of the Pd-Au alloy than the Pd-Ag one [START_REF] Wang | The effect of co-existing nitrogen on hydrogen permeation through thin Pd composite membranes[END_REF], making stable the performance of the Pd-Au/Al2O3 MR for a longer operation time.

Post mortem analyses

After the experimental campaign, a post mortem analysis was carried out on the supported Pd-Au/Al2O3

Conclusion

The steam reforming of a H2S containing biogas mixture was carried out over a non-commercial Rh(1%)/MgAl2O4/Al2O catalyst in two Pd-based MRs to evaluate the effects of the H2S presence during the SRB reaction on the performance in of hydrogen permeation characteristics and catalytic activity as the first time (to our best knowledge).

A high hydrogen permeable and low hydrogen perm-selective supported Pd-Au/Al2O3 membrane housed in a MR showed limited CH4 conversion, around 25% (the best value), reached at 823 K and 150 kPa, but relatively high hydrogen recovery, 80%, reached at the same operating conditions. For comparison, feeding a H2S free biogas mixture the conversion and hydrogen recovery were superior to the previous case (45% and 82%, respectively, at 823 K and 150 kPa), pointing out how the H2S induced a depletion of the Pd- Au/Al2O3 MR performance. A depletion of catalytic activity with time was also observed as there was a progressive decrease of CH4 conversion, from 25-30% to less than 10%, under H2S containing biogas stream conditions. As confirmed by the hydrogen-rich gas mixture (H2S containing) permeation tests, the Pd-Au alloy seems to be more resistant to the H2S attack than the Pd-Ag, even though the defects in the metallic layer of the supported Pd-Au/Al2O3 membrane limited the performance in terms of hydrogen perm-selectivity.

Furthermore, as evidenced by the EDX tests in the post mortem analyses, the presence of sulfur was detected in the Pd-Au metallic layer. Nevertheless, the hydrogen permeation characteristics of the Pd-Au/Al2O3 membrane were recovered by feeding pure hydrogen allowing its utilization in other experimental campaigns.

The utilization of a low permeable but fully hydrogen perm-selective Pd-Ag membrane (commercial) in a MR allowed better performance than the previous MR in terms of conversion, 85%, reached at milder operating temperature (673 K). It was due to the larger shift effect realized by the fully hydrogen perm-selective Pd-Ag membrane on the SRB reaction system with the further advantage of recovering a pure hydrogen stream. On the contrary, owing to the thicker metallic layer, the hydrogen permeability was relatively low and the hydrogen recovered was not higher than 40% at 673 K and 200 kPa.

Under long-term SRB reaction tests, the Pd-Ag MR showed a faster decrease of the CH4 conversion (from 85 to around 10%) than the Pd-Au/Al2O3 MR due to the effect of H2S present in the biogas feed mixture because -as also confirmed in the specialized literature ( [START_REF] Chen | The effect of H2S on the performance of Pd and Pd/Au composite membrane[END_REF][START_REF] Caravella | Quantitative contribution of non-ideal permeability under diffusion-controlled hydrogen permeation through Pd-membranes[END_REF][START_REF] Nam | Hydrogen separation by Pd alloy composite membranes: introduction of diffusion barrier[END_REF]) -the Pd-Au alloy seems to be more H2S resistant than other Pd-alloys. Furthermore, the negative effect of H2S was permanent in the Pd-Ag membrane, determining its failure after a prolonged exposure, which was not recovered by supplying pure hydrogen as in the case of the supported Pd-Au membrane of this work.

As a future work, a better deposition of the metallic layers (Pd and Au) on the Al2O3 support will constitute the future development of this kind of supported membrane in order to improve its hydrogen permselectivity and, consequently, the conversion and hydrogen permeation performance during the steam reforming of biogas mixtures (in natura).

Figure 1 .

 1 Figure 1. Photos of the (a) supported Pd-Au/α-Al2O3 membrane before the experimental tests; (b) bench-scale tubular MR module; (c) photo of the self-supported Pd-Ag membrane; (d) scheme of the Pd-Ag MR.

  setup, deionized water for the SRB reaction, which was vaporized in a preheating zone and, then, mixed with the synthetic biogas mixture, was fed by a P680 HPLC pump (Dionex). The outlet streams were cooled by a cold-trap to condensate the unreacted steam and, hence, analyzed by a temperature-programmed HP 6890 GC, equipped with two thermal conductivity detectors at 523 K and using Ar as a carrier gas. The GC was equipped by three packed columns: Porapack R 50/80 (8 ft  1/8 inch) and Carboxen TM 1000 (15 ft  1/8 inch) connected in series, and a Molecular Sieve 5 Ǻ (6 ft 1/8 inch).Furthermore, a constant flow rate of N2 (~ 25 mL/min) was used as an internal standard gas to analyze the outlet compositions (dry state) of both permeate and retentate streams. Both single gases (CH4, CO2, N2, H2) and mixtures were supplied by means of Brooks Instruments 5850S mass-flow controllers, driven by the Lira (Italy) software. Each experimental reaction data of this work represents an average value of -at least -5 experimental tests taken in 150 min at the steady-state condition with an error variation lower than 2% for each experimental point reported in this work. The performance of the MRs during SRB reaction was evaluated by means of the following definitions:

  Rh/MgAl2O4/Al2O3 catalyst for reforming reaction X-Ray diffractograms of the bare alumina support, of the alumina modified by Mg 2+ and of the Rh/MgAl2O4/Al2O3 catalyst are displayed in Figure 2(a). It can be seen that the impregnation of the magnesium precursor salt followed by a thermal treatment allows the incorporation of the Mg 2+ cations into 𝑃 𝐻2 = 𝑃 𝐻2,0 𝑒𝑥𝑝 (-𝐸 𝐴 𝑅𝑇 ) the alumina support forming a magnesium aluminate phase, MgAl2O4, coexisting with an α-Al2O3 phase, with no trace of isolated Mg species, in accordance with the results of Aupretre et al. [59]. The SEM image of an alumina bead after modification by Mg 2+ shows a core-shell structure with MgAl2O4 in the shell (thickness in the range of 190-210 m) and a core of Al2O3. When rhodium is added, the diffractogram is similar to that of MgAl2O4/Al2O3 with no diffraction lines corresponding to Rh species indicating a good dispersion of Rh species on the support, Figure 2(b).

Figure 2 .

 2 Figure 2. (a) X-Ray diffractograms of the alumina beads (Al2O3), the alumina beads modified by addition of Mg 2+ (MgAl2O4/Al2O3) and the final catalyst (Rh/ MgAl2O4/Al2O3) (• : MgAl2O4, ○ : α-Al2O3 , ж : γ-Al2O3) and (b) SEM image of one bead of MgAl2O4/Al2O3.

Figures 3 (Figure 3 .

 33 Figures 3(a) and (b) show the Pd-Au/Al2O3 membrane, evidencing the characteristic cauliflower morphology

Figure 4 .

 4 Figure 4. EDX analysis on the Pd-Au/Al2O3 membrane before the gas permeation and reaction tests.

Figure 5 .

 5 Figure 5. (a) H2 flux permeating through the Pd-Au/Al2O3 membrane vs transmembrane pressure at different operating temperatures; (b) H2 permeability vs 1/T for the Pd-Au/Al2O3 membrane at 50 kPa of transmembrane pressure.

Figure 6 .

 6 Figure 6. Volume flow rate permeating through the Pd-Au/Al2O3 membrane of H2 as single gas and as component in H2-rich gaseous mixtures (with and without H2S) vs time at 673 K and 100 kPa of H2 partial pressure difference across the membrane.

Furthermore, as demonstrated

  by Mejdell et al.[START_REF] Mejdell | Effects of CO and CO2 on hydrogen permeation through a ∼3 μm Pd/Ag 23 wt.% membrane employed in a microchannel membrane configuration[END_REF], the presence of a growing amount of CO2 in the gaseous mixture is a further reason of the depletion of the H2 permeating flow rate, since it may be adsorbed on the membrane surface, inducing an inhibitive effect on the H2 permeation. Hence, the H2 flow rate permeating through the Pd-Au/Al2O3 membrane ranged from 340 mL/min at xH2 = 0.95 to 140 mL/min at xH2 = 0.42. The initial H2 permeation membrane performances were recovered reaching the permeating flow rate of 380

Figure 7 .

 7 Figure 7. (a) H2 flux permeating through the self-supported Pd-Ag membrane vs transmembrane pressure at different operating temperature; (b) H2 permeability vs 1/T for the self-supported Pd-Ag membrane at 50 kPa of transmembrane pressure.

Figure 7 (

 7 Figure 7(b) shows the plot for the Pd-Ag membrane at transmembrane pressure equal to 50 kPa.

Figure 8 .

 8 Figure 8. (a) CH4 conversion vs feed molar ratio at different WHSV, T = 823 K and p = 150 kPa during SRB reaction carried out in the Pd-Au/Al2O3 MR in comparison with the equilibrium conversion at the set conditions in an equivalent CR; (b) H2 recovery vs feed molar ratio at different WHSV, T = 823 K and p = 150 kPa during SRB reaction carried out in the Pd-Au/Al2O3 MR.

Figure 9 (

 9 b) sketches the H2 recovery against the feed molar ratio at 673 K, 100 kPa of transmembrane pressure and different WHSVs. Being the Pd-Ag membrane less permeable than the Pd-Au/Al2O3 membrane, the H2 recovery during SRB reaction in the Pd-Ag MR was macroscopically lower than that achieved in the Pd-Au/Al2O3 MR. The best value, equal to almost 40%, was reached at feed molar ratio = 3/1 and WHSV = 0.2 h -1 .

Figure 9 .

 9 Figure 9. (a) CH4 conversion vs feed molar ratio at T = 673 K and different reaction pressure and WHSV during SRB reaction carried out in the Pd-Ag MR; (b) H2 recovery vs feed molar ratio at different WHSV, T = 673 K and p = 200 kPa during SRB reaction carried out in the Pd-Ag MR.

Figure 10 .

 10 Figure 10. Molar retentate composition (dry) at WHSV = 0.2 h -1 , T = 673 K and p = 200 kPa during SRB reaction carried out in the Pd-Ag MR: (a) feed molar ratio = 3/1; (b) feed molar ratio = 2/1.

Figure 11 .

 11 Figure 11. CH4 conversion vs time on stream during SRB reaction in (a) Pd-Au/Al2O3 MR, (b) Pd-Ag MR. Experimental conditions: (a) T = 823 K, p = 150 kPa, feed molar ratio = 3/1; (b) T = 673, p = 200 kPa, feed molar ratio = 3/1. A synthetic H2S containing biogas mixture was used as feed stream (Table2).

(Figure 12 .

 12 Figure 12. Post mortem analysis: (a) SEM images of the Pd-Au/Al2O3 membrane (outer) surface at different magnification and an accelerating voltage of 15 kV; (b) EDX analysis on the Pd-Au/Al2O3 membrane after the experimental campaign.

Table 1 .

 1 Synthetic biogas mixtures used during the experimental reaction tests (molar percentages).

	Compound Mixture 1 (with H2S) [%] Mixture 2 (H2S free) [%]
	CO2	35.03	35.00
	CH4	59.94	60.00

Table 2 .

 2 Main characteristics of the studied catalyst.

	Catalyst	BET surface area	Rh content	Mg content	Rh dispersion a
		(m 2 g -1 )	(wt.%)	(wt.%)	(%)
	-Al2O3	207	-	-	-
	MgAl2O4/Al2O3	84	-	4.6	-
	Rh(1%)/MgAl2O4/Al2O3	84	0.93	4.6	40
	a determined from H2 chemisorption measurements considering a stoichiometry H/Rh of 1
	3.2 Experimental tests on the Pd-Au/Al2O3 membrane: gas permeation, SEM and EDX analyses

The first part of the whole experimental campaign of this work was devoted to analyzing the chemicalphysical characteristics of the Pd-Au/Al2O3 membrane before the permeation and reaction tests.

Table 3 .

 3 Comparison of the activation energy (EA) of the Pd-Au/Al2O3 membrane with other literature data.

	Metallic layer	Layer thickness (µm)	Substrate	Permeation T (K)	p (kPa) EA (kJ/mol)	Ref.
	Pd-Cu	7	Al2O3-PNS a	813	2000	19.0	[60]
	Pd-Cu	20	Al2O3	673	50	18.5	[61]
	Pd-Au	5	YSZ b	623-823	100-700	7.5	[62]
	Pd-Cu	17	PSS c	823	70	9.4	[63]
	Pd-Ag	48	PSS c	623-823	180-250	9.0	[64]
	Pd	15	Al2O3	623-923	30	10.0	[65]
	Pd	7	Al2O3	623-723	100	11.4	[66]
	Pd-Cu	10	ZrO2/PSS c	713	-	15.4	[67]
	Pd-HF	5	Al2O3	873	200	21.3	[68]
	Pd-Au	8	Al2O3	673	50	12.1	This work

a Porous nickel support  b Yttria stabilized zirconia  c Porous stainless steel

Table 4 .

 4 H2 partial pressure exponent (n-value) at the maximum R 2 as a function of temperature for the Pd-Au/Al2O3 membrane.

	T [K]	n-value
	573	0.5
	623	1.0
	673	0.8
	823	0.8

Table 5 .

 5 H2-rich gas mixtures composition constituted of a constant H2 stream, 474 mL/min, mixed with variable volume flow rates of gaseous streams constituted of CO2/CH4/N2, with and without H2S.

QTOT-mix gases (*) [mL/min] QH2 [mL/min] QTOT H2-rich mix [mL/min] xH2-rich mix

				[%]
		474	500.4	95
	54.6	474	528.6	90
	11.8	474	586.8	81
	173.4	474	647.4	73
	294	474	768	62
	666.6	474	1140.6	42
	(*) Gaseous mixtures composition (without H2);
	a) with H2S: CO2(35.03%)/CH4(59.94%)/N2(5.01%)/H2S(0.02%);
	b) without H2S: CO2(35%)/CH4(60%)/N2(5%).

Table 6 .

 6 reaction carried out in MRs and CRs. Qualitative comparison among the experimental results of this work and literature data.Nevertheless, the low hydrogen perm-selectivity characteristics of the supported Pd-Au/Al2O3 membrane affected at lot the SRB reaction performance in terms of CH4 conversion even though it was qualitatively

	Reactor typology	Membrane typology	Selective dense layer [m]	Catalyst	T [K]	p [kPa]	H 2 O/CH 4 feed ratio	CH 4 conversion [%]	H 2 recovery [%]	H 2 purity [%]	Ref.
	MR a	Pd-Au/Al 2 O 3	~ 8	Rh/MgAl 2 O 4 /Al 2 O 3	823	150	3/1	25	~ 80	~ 50	This work
	MR b	Pd-Au/Al 2 O 3	~ 8	Rh/MgAl 2 O 4 /Al 2 O 3	823	150	3/1	45	82	55	This work
	MR a	Pd-Ag	150	Rh/MgAl 2 O 4 /Al 2 O 3	673	200	3/1	85	~ 40	~ 100	This work
	MR b	Pd-Ag	150	Rh/MgAl 2 O 4 /Al 2 O 3	673	200	3/1	97	45	~ 100	This work
	MR b	Pd-Ag	200	Ru/Al 2 O 3	723	500	3/1	-	~ 10	~ 100	[32]
	MR b	Pd-Ag	200	Ru/Al 2 O 3	723	100	3/1	-	50	~ 100	[33]
	MR b	Pd/Al 2 O 3	7	Ni/Al 2 O 3	723	350	3/1	34	70	70	[36]
	MR b	Pd-Ag	~ 80	Ni/Al 2 O 3	823	100	-	15	-	~ 100	[38]
	CR b	-	-	Ni-Rh/Al 2 O 3	973	120	3/1	85	-	~ 77 c	[72]
	CR a	-	-	Ni-Rh/Al 2 O 3	973	120	3/1	~ 20	-	~ 20 c	[72]
	CR b	-	-	Ni/Al 2 O 3	964	-	2/1	~ 85	-	58	[73]
	CR b	-	-	Ni/CeO 2	873	-	1/1	58	-	59	[74]
	CR b	-	-	Rh/CeO 2	1173		3/1	100	-	63	[75]
	CR b	-	-	Ni/Al 2 O 3	1073	100	2/1	99	-	45	[76]
	CR a	-	-	Ni/Al 2 O 3	1073	100	2/1	22	-	-	[76]
	a H2S in the feed
	b H 2 S free feed
	c Calculated

/CH 4 = 2/1 Pd-Ag MR Retentate composition [%]

		(b)
	CO	CH 4
		4 % 4 %
		26 % 26 %
		41 % 41 %
	H 2	29 % 29 %
		CO 2
	p = 200 kPa
	T = 673 K
	H 2 O

/CH 4 = 3/1 Pd-Ag MR Retentate composition [%]

		(a)
	2 %
	15 %
	52 %	31 %

Acknowledgments

Nanjing Tech. University & GaoQ Functional Materials Co., Ltd. (China) are particularly acknowledged for providing the Pd-Au/α-Al2O3 membrane used in this work.

Acronyms

CR