Influence of Hydrogen Sulfide and Redox Reactions on the Surface

Article Inﬂuence of Hydrogen Sulﬁde and Redox Reactions on the Surface Properties and Hydrogen Permeability of Pd Membranes

Wei Feng 1,2, Qingyuan Wang 1,2, Xiaodong Zhu 1,* ID , Qingquan Kong 1,2, Jiejie Wu 3 and Peipei Tu 1 1 Advanced Research Institute, Chengdu University, Chengdu 610106, China; [email protected] (W.F.); [email protected] (Q.W.); [email protected] (Q.K.); [email protected] (P.T.) 2 College of Architecture and Environment, Sichuan University, Chengdu 610065, China 3 School of Materials Science and Engineering, Beihang University, Beijing 100083, China; [email protected] * Correspondence: [email protected]; Tel.: +86-28-461-6823

Received: 28 March 2018; Accepted: 1 May 2018; Published: 3 May 2018

Abstract: Although hydrogen sulfide (H2S) was always a negative factor leading to the reduction of hydrogen permeability of palladium (Pd) membranes, its proper application could result in a ◦ positive effect. In this study, pure Pd membranes were firstly reacted with H2S at 23–450 C, and then treated by redox reactions. Afterwards, the hydrogen permeability was tested under different reaction conditions using a hydrogen penetrant testing device. Moreover, both products and morphology changes occurred on the Pd membrane surface were analyzed using XRD, XPS and SEM. The results ◦ showed that H2S was dissociated to produce sulfides at 23 C. With a rise of temperature, a regular change took place in the reaction products, morphology of the Pd membrane surface and hydrogen permeability. Adsorbed impurities such as sulfides and free carbon on the Pd membrane surface were removed by the redox treatment. The hydrogen permeability was improved by about 80% for the Pd membrane material subjected to the treatment method stated the above against the untreated one.

Keywords: Pd membrane; hydrogen; permeability; surface; redoxreaction

1. Introduction Hydrogen energy is a major clean energy source [1], widely used in new energy vehicles, aerospace and other fields [2]. The large-scale use of hydrogen energy is an effective way to solve the greenhouse effect caused by the current burning of traditional fossil fuels since the combustion of hydrogen does not release CO2 [3,4]. The development and application of new hydrogen production technology are expected to provide effective solutions for alleviating the global energy crisis and air pollution [5]. Hydrogen-rich syngas is commonly produced by steam-methanol reforming (SMR) in industry [6]. One of the most critical materials in the SMR process is the Pd membrane [7], which is used to generate hydrogen at low temperatures and greatly increase the conversion of the original gas [8]. However, the hydrogen permeation rate of Pd membrane was greatly affected in the effects on impurities as oil, sulfides and arsenides [9,10]. Especially, it showed the maximum fragility and sensibility in sulfides even if these sulfides were extremely low in content [11]. Though the sulfides could be desorbed using zinc oxide powder or high-temperature sulphur adsorbent [12,13], great damage was caused to the Pd membrane used in highly-pure hydrogen preparation [14]. Based on the development trend of the Pd membrane with sulfuration resistance, high hydrogen permeation and low cost [15–22], the study of the influence mechanisms of sulfides on the Pd membrane surface was necessary. Some valuable

Energies 2018, 11, 1127; doi:10.3390/en11051127 www.mdpi.com/journal/energies Energies 2018,, 11,, 1127x FOR PEER REVIEW 2 of 10 was necessary. Some valuable research results were obtained in the resistance of PdCu, PdCuAg and PdAuresearch to hydrogen results were sulfide obtained poisoning in the capacity resistance in rece of PdCu,nt years PdCuAg [20,23–27]. and For PdAu example, to hydrogen Peters [20] sulfide and poisoningZhao [28] et capacity al. found in recentthat the years activity [20,23 of– 27a PdAgCu]. For example, membrane Peters could [20] andbe restored Zhao [28 after] et al.the found source that of sulfidethe activity pollution of a PdAgCu was cut off membrane for 2.5 h couldby accurately be restored balancing after the Cu/Ag source contents. of sulfide However, pollution such was sulfides cut off asfor well 2.5 h as by reaction accurately principle balancing and Cu/Ag surface contents. structure However, change suchof Pd sulfides membrane as well should as reaction be of principleconcern [29,30].and surface structure change of Pd membrane should be of concern [29,30]. This paper presentspresents thethe detaileddetailed reaction reaction process process and and phenomena phenomena of of hydrogen hydrogen sulfide sulfide and and the the Pd Pdmembrane membrane surface, surface, and and further further demonstrates demonstrates an an approach approach that that the the sulfides sulfides absorbedabsorbed onon palladium surface can can be be removed removed by by the the redox redox method. method. It was It was found found by byhydrogen hydrogen permeation permeation testing testing that thatthis approachthis approach could could not notonly only restore restore the the activity activity of ofPd Pd membrane, membrane, but but also also improve improve the the hydrogen permeability of a Pd membrane to some extent. Th Thee research research methodology developed and the results obtained are of an importance for further optimizing high-purity hydrogenhydrogen preparationpreparation technology.technology.

2. Experimental Procedures

2.1. Experimental Process In the experiment, Pd membrane (99.95%, purity), which which was was 0.1 0.1 mm mm thick and prepared by China NorthwestNorthwest Institute Institute for for Non-ferrous Non-ferrous Metal Metal Research, Research, was usedwas asused a raw as material.a raw material. It was polished It was × polishedand burnished and burnished first, and first, then and cut then into cut square into pieces square (each pieces 15 (each mm 15 15mm mm) × 15 and mm) washed and washed under underultrasound ultrasound conditions conditions three times three with times acetone with acetone (99.95%, (99.95%, purity, Chengdupurity, Chengdu Kelong ChemicalKelong Chemical Reagent ReagentCo. Ltd., Chengdu,Co. Ltd., China),Chengdu, absolute China), ethyl absolute alcohol (99.95%,ethyl alcohol purity, (99.95%, Chengdu purity, Kelong Chengdu Chemical ReagentKelong ChemicalCo. Ltd.) and Reagent secondary Co. Ltd.) deionized and secondary water. Finally deionized it was water. dried inFinally the vacuum it was withdried ambient in the vacuum temperature. with Theambient surface temperature. reaction experiments The surface werereaction undertaken experiment in as quartzwere undertaken pipe device in [ 31a quartz], which pipe could device be filled [31], with one of flowing gases of Ar, H S, O and H (99.999%, purity, Chengdu Taiyu Gas Co. Ltd., which could be filled with one of flowing2 gases2 of Ar,2 H2S, O2 and H2(99.999%, purity, Chengdu Taiyu GasChengdu, Co. Ltd., China). Chengdu, In fact, China). these In gases fact, were these added gases successivelywere added accordingsuccessively to theaccording experimental to the experimentalrequirements. requirements. They were then They reacted were with then Pd reacte membraned with at Pd different membrane temperatures. at different The temperatures. experimental ± ◦ Thetemperature experimental fluctuation temperature was controlled fluctuation within was controll0.3 C,ed the within gas-filling ±0.3 °C, in the the gas-filling range of 10–15in the mL/s,range ofand 10–15 the reactionmL/s, and duration the reaction was 5 duration min. was 5 min. Figure1 1 shows shows thethe experimentalexperimental apparatusapparatus usedused inin thethe gasgas reactionreaction experiment,experiment, AA smallsmall squaresquare × section of the original Pd membrane (15 mm × 1515 mm) mm) was was first first placed into a silica tube (16.00 mm ID), which was filled filled with Ar to purge the air from the inside. The oven temperature was adjusted to the reaction temperature. The flow controller was adjusted to fill the tube with H S, O ,H and the Ar the reaction temperature. The flow controller was adjusted to fill the tube with H22S, O22, H22 and the Ar flowflow controllercontroller waswas shut shut off off at at the the same same time. time. The Th Are Ar flow flow controller controller was was opened opened to refill to refill the tube the withtube Ar after Pd membrane reacting with H S, O and H , while each of the gasflow controller was shut off with Ar after Pd membrane reacting with2 H2 2S, O2 2and H2, while each of the gasflow controller was shutgradually. off gradually. The silica The tube silica was tube removed was fromremoved the ovenfrom andthe cooledoven and in thecooled air. Afterin the the air. tube After had the cooled tube hadto room cooled temperature, to room temperature, the Pd membrane the Pd wasmembrane removed was from removed the oven. from the oven.

Figure 1. Schematic of the apparatus used for the gas reactions.

Energies 2018, 11, 1127 3 of 10

Energies 2018, 11, x FOR PEER REVIEW 3 of 10 2.2. Characterization 2.2.The Characterization sample surface micro-morphology was analyzed using the field emission scanning electron microscopeThe sample (FESEM) surface (Quanta450 micro-morphology FEG, Thermo was analyzed Fisher, Hillsboro, using the field OR, emission USA). The scanning matter electron phase of themicroscope sample was (FESEM) examined (Quanta450 and analyzed FEG, Thermo using Fish aner, X-ray Hillsboro, diffractometer OR, USA). (XRD,The matter DX2500, phase Haoyuan of the Scientificsample Inc.,was examined DanDong, and China). analyzed The using chemical an X-ray state diffractometer of the sample (XRD, surface DX2500, was Haoyuan analyzed Scientific using an X-rayInc., photoelectron DanDong, China). spectrometer The chemical with EX05 state ion ofsputtering the sample gun surface (Model was 250, analyzed ESCALAB, using Thermo an X-ray Fisher, Hillsboro,photoelectron OR, USA). spectrometer The hydrogen with EX05 permeability ion sputte afterring thegun sample (Model surface250, ESCALAB, was treated Thermo was evaluatedFisher, byHillsboro, the hydrogen OR, USA). penetration The hydrogen rate testing permeability device af madeter the by sample China surface Academy was treated of Engineering was evaluated Physics (CAEP,by the MianYang, hydrogen China)penetration [31]. rate testing device made by China Academy of Engineering Physics (CAEP,Figure MianYang,2 shows the China) hydrogen [31]. permeation experimental apparatus. Pd membranes were enclosed in a stainlessFigure steel2 shows tube the (12 hydrogen mmID) permeation by laser welding. experime Inntal the apparatus. first step, Pd the membranes high vacuum were enclosed pump and in a stainless steel tube (12 mmID) by laser welding. In the first step, the high vacuum pump and the the valves No. 2, 3, 4, 5, 6, 7, and 8 were opened sequentially, before the system was pumped to valves No. 2, 3, 4, 5, 6, 7, and 8 were opened sequentially, before the system was pumped to 3 × 10−4 3 × 10−4 Pa and heated to 350 ◦C. In the second step, valves No. 2 and 3 were shut off sequentially, Pa and heated to 350 °C. In the second step, valves No. 2 and 3 were shut off sequentially, while valve while valve no.1 was opened, allowing hydrogen gas to fill the chamber on the left side of the Pd no.1 was opened, allowing hydrogen gas to fill the chamber on the left side of the Pd membrane. membrane.Pressure Pressureloss data lossfor the data chamber for the chamberwere recorded were recorded using a usingpressure a pressure sensor and sensor transmitted and transmitted to a to acomputer. computer. The The duration, duration, temperature temperature and andthe pressure the pressure of the ofhydrogen the hydrogen permeation permeation experiment experiment were were600 600 s, 623 s, 623 K and K and 0.12 0.12 Mpa. Mpa.

Figure 2. General scheme for the hydrogen permeation experimental apparatus. Figure 2. General scheme for the hydrogen permeation experimental apparatus. 2.3. Calculation of Stable Permeation Flux 2.3. Calculation of Stable Permeation Flux In order to examine the effect of Pd membrane hydrogen permeability after and before the surfaceIn order treatment, to examine hydrog theen effect permeability of Pd membrane tests were hydrogen carried permeabilityout for the Pd after membrane and before material the surface in treatment,different hydrogenreaction conditions. permeability Here, tests it was were assumed carried that out the for stable the Pdpermeation membrane flux material(J) of hydrogen in different in reactionPd membrane conditions. was of Here, the hydrogen it was assumed permeation that effi theciency stable of permeation such membrane ﬂux [32]. (J) of The hydrogen term J was in Pd membraneequal to the was permeation of the hydrogen rate (Π) permeationtimes the driving efﬁciency force ( ofD). such As the membrane Π means the [32 hydrogen]. The term permeating J was equal to theability permeation through Pd rate membrane (Π) times per the unit driving area and force time, (D it). is As mainly the Πdeterminedmeans the by hydrogen a unit driving permeating force ability(hydrogen through pressure Pd membrane at permeating per unit side). area Thus, and the time, J can it be is mainlyexpressed determined by the following by a unit relationship: driving force

(hydrogen pressure at permeating side). Thus, the J can be expressed by the following relationship: == = (1) V V ∆Px In Equation (1), V is the gasJ Xvolume= ΠD at= the Pd membrane∆P = air inlet end, which is expressed by the (1) RST∆t RST ∆tx following formula: In Equation (1), V is the gas volume at the Pd membrane air inlet end, which is expressed by the V = VP + Vt = 0.1 L + 4.9 L = 5 L (2) following formula: here VP is the gas volume in pipeline, Vt is the volume of gas holder. R is a constant 8.31 J·mol−1·K−1, S V = VP + Vt = 0.1 L + 4.9 L = 5 L (2) is the area of Pd membrane 1.13 cm2. d is the inner diameter of pipeline (1.20 cm), T is the temperature −1 −1 herein VhydrogenP is the gas permeation volume in test pipeline, which isV t623is the K, volumeΔP is the of reduction gas holder. amountR is a of constant intensity 8.31 of Jhydrogen·mol ·K , S ispressure the area at of Pd Pd air membrane inlet end 1.13per unit cm2 .permeatingd is the inner time, diameter and Δt ofis pipelineunit permeating (1.20 cm), time.T is the temperature in hydrogen permeation test which is 623 K, ∆P is the reduction amount of intensity of hydrogen pressure at Pd air inlet end per unit permeating time, and ∆t is unit permeating time.

Energies 2018, 11, 1127 4 of 10

3. ResultsEnergies 2018 and, 11 Discussion, x FOR PEER REVIEW 4 of 10

3.1.3. Effect Results of Hydrogen and Discussion Sulfide (H2S) on Composition and Morphology of Pd Membrane Surface Figure3 shows the XRD testing results of Pd membrane surface under different reaction conditions. 3.1. Effect of Hydrogen Sulfide (H2S) on Composition and Morphology of Pd Membrane Surface It can be seen from the figure that the two phases Pd16S7 and Pd4S are visible in the XRD diffraction peak at 150Figure◦C. Their3 shows intensity the XRD was testing enhanced results gradually of Pd frommembrane 150 ◦C surface to 350 ◦ Cunder first, different and then reaction went down rapidlyconditions. at 450 It◦C. can Though be seen afrom characteristic the figure that PdS the diffraction two phases peak Pd occurred16S7 and Pd at4S 350are ◦visibleC or so, in itsthe intensityXRD wasdiffraction not significant. peak at However, 150 °C. Their it was intensity enhanced was rapidlyenhanced at gradually 450 ◦C, so from that 150 the °C characteristic to 350 °C first, diffraction and then went down rapidly at 450 °C. Though a characteristic PdS diffraction peak occurred at 350 °C or peaks of Pd, Pd S and Pd S were barely detected. so, its intensity16 7was not4 significant. However, it was enhanced rapidly at 450 °C, so that the The main products were produced by Pd membrane reacted with H2S were Pd16S7 and Pd4S characteristic diffraction peaks of Pd, Pd16S7 and Pd4S were barely detected. between 150 ◦C and 350 ◦C, however the product was PdS at 450 ◦C. With the rise of temperature, The main products were produced by Pd membrane reacted with H2S were Pd16S7 and Pd4S H2Sbetween was reacted 150 °C with and Pd350 membrane°C, however and the directlyproduct was produced PdS at PdS.450 °C. But With in view the rise of rapidof temperature, reduction of ◦ ◦ ◦ Pd16HS27S andwas Pdreacted4S at 450with C,Pd bothmembrane Pd16S7 andand directly Pd4S were prod firmlyuced PdS. changed But in into view PdS of atrapid 450 reductionC. As 450 ofC is thePd working16S7 and temperature Pd4S at 450 °C, of both Pd membrane, Pd16S7 and Pd the4S followingwere firmly reactions changed likely into PdS coexist at 450 at °C. this As temperature: 450 °C is the working temperature of Pd membrane, the following reactions likely coexist at this temperature: Pd + H2S(g) → PdS + H2(g); Pd16S7 + 9H2S(g) → 16PdS + 9H2(g); Pd4S + 3H2S(g) → 4PdS + 3H2(g). Pd + H2S(g) → PdS + H2(g); Pd16S7 + 9H2S(g) → 16PdS + 9H2(g); Pd4S + 3H2S(g) → 4PdS +

At the temperatures lower than 150 ◦C, the characteristic3H2(g). peak of sulfides could hardly be detected by XRD.At However, the temperatures the sulfide lower product than 150 might°C, the becharacteri lesser orstic quite peak thin,of sulfides or could could be hardly below be thedetected amount of theby XRD. matter However, phase requiredthe sulfide for product XRD might detection. be lesser For or this quite reason, thin, or the could XPS be wasbelow used the amount to study of the Pd membranematter phase surface required reacting for atXRD 23 ◦detection.C and 100 For◦C this in orderreason, to the understand XPS was used the reactionto study resultsthe Pd membrane below 150 ◦C in thissurface experiment. reacting at It 23 can °C be and found 100 °C from in order Figure to4 aunde thatrstand no other the reaction element results existed below but a150 little °C adsorbedin this carbonexperiment. on the PdIt can membrane be found from before Figure the reaction.4a that no However, other element there existed are significant but a little adsorbed S characteristic carbon on peaks the Pd membrane before the reaction. However, there are significant S characteristic peaks in Figure 4b,c, in Figure4b,c, where the relative content of elemental S reaches 67.88% and 82.73%, respectively, where the relative content of elemental S reaches 67.88% and 82.73%, respectively, by qualitative analysis by qualitative analysis and quantitative analysis of XPS. This indicates that at temperatures below and quantitative analysis of XPS. This indicates that at temperatures below 150 °C, hydrogen sulfide was 150 ◦C, hydrogen sulfide was also decomposed into S2− on the Pd surface and reacted with the Pd also decomposed into S2− on the Pd surface and reacted with the Pd surface, and the amount of S2− also 2− surface,increased and with the amountthe rise of of the S reactionalso increased temperature. with the rise of the reaction temperature.

FigureFigure 3. The3. The XRD XRD spectra spectra of of Pd Pd membranes membranes on: on: ( a(a)) TheThe surfacesurface after reacting with with hydrogen hydrogen sulfide sulfide at ◦at 23 °C; (b) The surface of Pd16S7 and Pd4S produced after reacting with hydrogen sulfide at 150 °C; ◦ 23 C; (b) The surface of Pd16S7 and Pd4S produced after reacting with hydrogen sulfide at 150 C; (c) The surface of Pd16S7 and Pd4S produced after reacting with hydrogen sulfide at 250 °C; ◦(d) The (c) The surface of Pd16S7 and Pd4S produced after reacting with hydrogen sulfide at 250 C; (d) The surface of Pd16S7, Pd4S and PdS produced after reacting with hydrogen sulfide at 350 °C; (e) The surface of Pd S , Pd S and PdS produced after reacting with hydrogen sulfide at 350 ◦C; (e) The surface surface of16 PdS7 produced4 after reacting with hydrogen sulfide at 450 °C; (f) The surface of PdO of PdS produced after reacting with hydrogen sulfide at 450 ◦C; (f) The surface of PdO produced after produced after sample (e) was oxidized at 500 °C; (g) The surface after PdO in sample (f) was reduced ◦ sampleby hydrogen (e) was oxidizedat 23 °C. at 500 C; (g) The surface after PdO in sample (f) was reduced by hydrogen at 23 ◦C.

Energies 2018, 11, 1127 5 of 10 Energies 2018, 11, x FOR PEER REVIEW 5 of 10

Figure 4. The XPSXPS spectraspectra of of samples samples on: on: (a )(a Pure) Pure Pd Pd membrane membrane surface; surface; (b) The(b) The chemical chemical element element state stateon the on Pd the membrane Pd membrane surface surface after reacting after reacting with hydrogen with hydrogen sulfide sulfide at 23 ◦C; at (c23) The°C; chemical(c) The chemical element elementstate on state the Pd on membrane the Pd membrane surface aftersurface reacting after withreacting hydrogen with hydrogen sulfide at sulfide 100 ◦C; at ( d100) The °C; chemical (d) The chemicalelement stateelement on the state Pd on membrane the Pd membrane surface after surface reacting after with reacting hydrogen with sulfide hydrogen at 450 sulfide◦C and at then450 °C by andredox then reaction. by redox reaction.

The phase transformation of palladium sulfide mainly depends on amount of the S element on The phase transformation of palladium sulﬁde mainly depends on amount of the S element on Pd surface and reaction rate between S and Pd, and both of them were determined by the reaction Pd surface and reaction rate between S and Pd, and both of them were determined by the reaction temperature. There for, any analysis of the reasons for morphology changes on the Pd membrane temperature. There for, any analysis of the reasons for morphology changes on the Pd membrane surface should focus on the reaction temperature. No significant morphology changes occurred on surface should focus on the reaction temperature. No signiﬁcant morphology changes occurred on the the Pd surface from 23 °C to 150 °C. However, when the temperature was above 150 °C, it can be seen Pd surface from 23 ◦C to 150 ◦C. However, when the temperature was above 150 ◦C, it can be seen from Figure 5a that the Pd surface clearly became roughened. This corresponds to the Pd4S and Pd16S7 from Figure5a that the Pd surface clearly became roughened. This corresponds to the Pd 4S and Pd16S7 morphology features detected by XRD. Both Pd4S and Pd16S7 crystal particles increased further in morphology features detected by XRD. Both Pd S and Pd S crystal particles increased further in volume and came into contact at 250 °C. Figure 5b4 shows that16 the7 produced crystal particles presented volume and came into contact at 250 ◦C. Figure5b shows that the produced crystal particles presented quite striking morphological features, such as a clear crystal boundary and around 0.2–0.4 μm in quite striking morphological features, such as a clear crystal boundary and around 0.2–0.4 µm in crystal particle size. According to Figure 5c, an expansion of the crystal boundary was produced in crystal particle size. According to Figure5c, an expansion of the crystal boundary was produced in Figure 5b at 350 °C, which looks like spider webs. This phenomenon indicates a possible occurrence Figure5b at 350 ◦C, which looks like spider webs. This phenomenon indicates a possible occurrence of Pd4S according to the XRD spectrum analysis, indicating Pd16S7 appeared and accumulated more of Pd4S according to the XRD spectrum analysis, indicating Pd16S7 appeared and accumulated more and more in the crystal locations of Pd4S and Pd16S7. Globular products occurred on the Pd surface and more in the crystal locations of Pd S and Pd S . Globular products occurred on the Pd surface with the rise of temperature above 4004 °C, which16 became7 more and more in quantity and volume with the rise of temperature above 400 ◦C, which became more and more in quantity and volume with the temperature rise. It can be seen from Figure 5d that the Pd membrane surface was nearly with the temperature rise. It can be seen from Figure5d that the Pd membrane surface was nearly covered with all these globular products at 450 °C. Based on XRD detection results, the globular covered with all these globular products at 450 ◦C. Based on XRD detection results, the globular product was PdS. After that, the composition and morphology of the Pd surface remained unchanged product was PdS. After that, the composition and morphology of the Pd surface remained unchanged with the continuous rise of reaction temperature. Based on the fact the working temperature of the with the continuous rise of reaction temperature. Based on the fact the working temperature of the Pd Pd membrane is around 450 °C, PdS may be produced over the Pd surface in its engineering membrane is around 450 ◦C, PdS may be produced over the Pd surface in its engineering applications. applications. Thus, in order to have a better understanding of the generation and removal effect of Thus, in order to have a better understanding of the generation and removal effect of such product, such product, we conducted a surface redox treatment based on Figure 3c,d. we conducted a surface redox treatment based on Figure3c,d.

Energies 2018, 11, 1127 6 of 10 Energies 2018, 11, x FOR PEER REVIEW 6 of 10

Figure 5. The morphology of Pd membrane surface under different reaction conditions: (a) After the Figure 5. The morphology of Pd membrane surface under different reaction conditions: (a) After the Pd Pd membrane reacted with hydrogen sulfide at 150 °C; (b) After the Pd membrane reacted with membrane reacted with hydrogen sulfide at 150 ◦C; (b) After the Pd membrane reacted with hydrogen hydrogen sulfide at 250 °C; (c) After the Pd membrane reacted with hydrogen sulfide at 350 °C; (d) sulfide at 250 ◦C; (c) After the Pd membrane reacted with hydrogen sulfide at 350 ◦C; (d) After the Pd After the Pd membrane reacted with hydrogen sulfide at 450 °C; (e) The Pd membrane underwent membrane reacted with hydrogen sulfide at 450 ◦C; (e) The Pd membrane underwent redox treatment redox treatment after reacting with hydrogen sulfide at 350 °C; (f) The Pd membrane underwent after reacting with hydrogen sulfide at 350 ◦C; (f) The Pd membrane underwent redox treatment after redox treatment after reacting with hydrogen sulfide at 450 °C. reacting with hydrogen sulfide at 450 ◦C.

3.2. The Effect of Redox Reaction on the Pd Membrane Surface Affected by Hydrogen Sulfide 3.2. The Effect of Redox Reaction on the Pd Membrane Surface Affected by Hydrogen Sulfide In this experiment, an oxidizing reaction was carried out for the aforementioned two kinds of In this experiment, an oxidizing reaction was carried out for the aforementioned two kinds surfaces at 500 °C ◦using oxygen (O2). Then the reduction reaction was conducted in the 23 °C using◦ of surfaces at 500 C using oxygen (O2). Then the reduction reaction was conducted in the 23 C hydrogen (H2) with a reacting duration of 5 min [31]. Figures 3f and 1g show that all the characteristic using hydrogen (H2) with a reacting duration of 5 min [31]. Figures3f and1g show that all the sulfide diffraction peaks disappeared completely and the PdO characteristic diffraction peak characteristic sulfide diffraction peaks disappeared completely and the PdO characteristic diffraction appeared on the surface of Pd membrane that reacted with hydrogen sulfide at 450 °C and then was peak appeared on the surface of Pd membrane that reacted with hydrogen sulfide at 450 ◦C and then oxidized at 500 °C. ◦This indicates that the Pd sulfides such as Pd16S7 and PdS were oxidized was oxidized at 500 C. This indicates that the Pd sulfides such as Pd16S7 and PdS were oxidized completely. However, after the sample was reacted with hydrogen at the 23 °C, it can be seen from completely. However, after the sample was reacted with hydrogen at the 23 ◦C, it can be seen Figure 3f that only the characteristic Pd diffraction peak appeared on the Pd surface. Also, the peak from Figure3f that only the characteristic Pd diffraction peak appeared on the Pd surface. Also, was the same as the surface characteristic diffraction peak of the Pd membrane material prior to the peak was the same as the surface characteristic diffraction peak of the Pd membrane material reaction. This indicates that PdO was quite unstable in hydrogen, and it could be reduced to Pd prior to reaction. This indicates that PdO was quite unstable in hydrogen, and it could be reduced completely. At the same time, it can be seen from Figure 4d that no other elements were found other to Pd completely. At the same time, it can be seen from Figure4d that no other elements were than Pd element on the Pd membrane surface. In addition the free carbon adsorbed originally on Pd found other than Pd element on the Pd membrane surface. In addition the free carbon adsorbed surface was also removed. This tells us that the redox reaction not only removed the sulfides on the originally on Pd surface was also removed. This tells us that the redox reaction not only removed the Pd membrane surface, but also presented a good removal effect for other adsorbed impurities, sulfides on the Pd membrane surface, but also presented a good removal effect for other adsorbed especially the carbon-bearing ones. Figures 5e and 3f show the Pd surface morphology after the impurities, especially the carbon-bearing ones. Figures5e and3f show the Pd surface morphology reaction. By comparing Figures 5c and 3e, one can find that the originally prominent Pd16S7 after the reaction. By comparing Figures5c and3e, one can find that the originally prominent disappeared and a gap was subsequently formed, so that the Pd membrane surface presented a granulated surface as a whole. Similar phenomena and effects were found by comparison of Figures

Energies 2018, 11, 1127 7 of 10

Energies 2018, 11, x FOR PEER REVIEW 7 of 10 Pd16S7 disappeared and a gap was subsequently formed, so that the Pd membrane surface presented a granulated surface as a whole. Similar phenomena and effects were found by comparison of 5d and 3f. The original globular PdS disappeared, and instead, round holes which were close to the Figures5d and3f . The original globular PdS disappeared, and instead, round holes which were close original globular PdS in size appeared. Compared with the raw Pd membrane material, the Pd to the original globular PdS in size appeared. Compared with the raw Pd membrane material, the Pd membrane surface became more roughened after such a series of reactions. After comparing the membrane surface became more roughened after such a series of reactions. After comparing the quantity of samples before and after the reaction, it was found that their quantity did not change quantity of samples before and after the reaction, it was found that their quantity did not change significantly. This phenomenon indicates that the reaction to gases such as oxygen or hydrogen signiﬁcantly. This phenomenon indicates that the reaction to gases such as oxygen or hydrogen sulﬁde sulfide does not lead to any loss of palladium. However, the rough surface and the round holes might does not lead to any loss of palladium. However, the rough surface and the round holes might cause a cause a decrease in the actual thickness of the palladium membrane, which leads to the enhancement decrease in the actual thickness of the palladium membrane, which leads to the enhancement of the of the hydrogen permeation ability. hydrogen permeation ability.

3.3.3.3. Change Change of of Pd Pd Membrane Membrane Properties Properties TheThe stable stable permeation permeation flux ﬂux ( (JJ)) of of Pd Pd membrane membrane can can be be expressed expressed by by the the rate rate of of Pd Pd membrane membrane high-pressurehigh-pressure end, end, as as shown shown in in Figure Figure 66.. It can be found from the ﬁgurefigure that thethe PdPd membranemembrane pressurepressure at at the the high-pressure high-pressure end end decreases decreases in in an an a approximatepproximate straight straight line line with with the the time time extension, extension, ∆P ∆P withwith ∆Px being just a slope of thisthis line,line, andand thethe datadata ofof 1 ~~ 11 andand JJ1–J–J11 areare obtained obtained by by calculations calculations ∆tx ∆t1 ∆t11 1 11 asas shown shown in in Table Table 11..

FigureFigure 6. 6. ChangeChange of of the the Pd Pd membrane membrane pressure pressure at at the the high-pressure high-pressure end end with with the the time. time.

The stable permeation flux (J) of Pd membranes. TableTable 1. 1. The stable permeation flux (J) of Pd membranes. Membrane 1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11# MembraneΔPx/Δtx 1#3.00 2#1.42 3#0.37 4#0.38 0.43 5# 0.53 6# 3.20 7# 3.30 8# 3.62 9# 5.00 10# 5.40 11# ∆Px/∆tJxx 3.002.56 1.42 1.22 0.370.32 0.380.32 0.430.37 0.45 0.53 2.73 3.20 2.82 3.30 3.09 3.62 4.27 5.00 4.62 5.40 Jx 2.56 1.22 0.32 0.32 0.37 0.45 2.73 2.82 3.09 4.27 4.62 Table 1 shows the changes of stable permeation fluxes (J) of Pd membrane under different reactionTable conditions.1 shows the It was changes found of stablefrom J permeation2–J6 that the fluxesstable (permeationJ) of Pd membrane flux of Pd under membrane different could reaction be decreased by around 52.34% even if the hydrogen sulfide was reacted with Pd membrane at 23 °C. conditions. It was found from J2–J6 that the stable permeation flux of Pd membrane could be decreased Also,by around the stable 52.34% permeation even if theflux hydrogen was more sulfide slashed was by 86.72% reacted or with so when Pd membrane the temperature at 23 ◦C. rose Also, to 150 the °C.stable If the permeation temperature flux continued was more to slashed rise, the by flux 86.72% could or sonot when change the greatly temperature any more, rose toindicating 150 ◦C. Ifthat the atemperature more compact continued sulfide layer to rise, came the into flux being could on not the changePd membrane greatly surface any more, above indicating 150 °C, which that a could more resultcompact in the sulfide loss layer of H came2S dissolution into being of on the the Pd Pd membrane membrane surfacesurface. aboveA small 150 quantity◦C, which of could hydrogen result permeation might be produced as micro-cracks occurred in sulfides on the Pd membrane surface in the loss of H2S dissolution of the Pd membrane surface. A small quantity of hydrogen permeation undermight hydrogen be produced pressure. as micro-cracks This led to occurred a part of in hydr sulfidesogen oncoming the Pd into membrane contact with surface the underPd surface hydrogen that was not poisoned by hydrogen sulfide. It was found from J7–J11 that after the Pd membrane that had reacted with hydrogen sulfide at different temperatures reacted with oxygen at 500 °C, its stable permeation flux was not restored but rather exceeded the stable permeation flux of the Pd membrane which was not reacted chemically. Moreover, the higher the temperature was for the Pd membrane

Energies 2018, 11, 1127 8 of 10 pressure. This led to a part of hydrogen coming into contact with the Pd surface that was not poisoned by hydrogen sulfide. It was found from J7–J11 that after the Pd membrane that had reacted with hydrogen sulfide at different temperatures reacted with oxygen at 500 ◦C, its stable permeation flux Energies 2018, 11, x FOR PEER REVIEW 8 of 10 was not restored but rather exceeded the stable permeation flux of the Pd membrane which was not reacted chemically. Moreover, the higher the temperature was for the Pd membrane reactions with reactions with hydrogen sulfide, the more a stable hydrogen permeation flux was obtained by redox hydrogen sulfide, the more a stable hydrogen permeation flux was obtained by redox reaction. The flux reaction. The flux increased by around 80% against the original Pd membrane when the temperature increased by around 80% against the original Pd membrane when the temperature in the reaction with in the reaction with hydrogen sulfide was up to 450 °C. hydrogen sulfide was up to 450 ◦C. It can be seen from Figure 7 that after reaction with hydrogen sulfide and redox reaction, no It can be seen from Figure7 that after reaction with hydrogen sulfide and redox reaction, no other other clear changes occurred on the Pd membrane surface, except for that the surface became highly clear changes occurred on the Pd membrane surface, except for that the surface became highly roughened due to a great number of holes, which are attribute to a different structure between Pd roughened due to a great number of holes, which are attribute to a different structure between Pd and PdO [33]. The carbon pollutants also were evidently removed, according to the XPS spectra in and PdO [33]. The carbon pollutants also were evidently removed, according to the XPS spectra in Figure 4d. The hydrogen permeating in Pd membrane mainly went through three processes, i.e., (1) Figure4d. The hydrogen permeating in Pd membrane mainly went through three processes, i.e., hydrogen molecules were adsorbed and dissolved on the Pd membrane surface; (2) the hydrogen (1) hydrogen molecules were adsorbed and dissolved on the Pd membrane surface; (2) the hydrogen atoms after dissolution were diffused; (3) the hydrogen atoms were combined with one side of the Pdatoms membrane after dissolution to generate were hydrogen diffused; molecules (3) the hydrogen which were atoms desorbed. were combined with one side of the Pd membrane to generate hydrogen molecules which were desorbed.

Figure 7. Morphology of the Pd membrane surface: (a) Original Pd membrane surface where no Figure 7. Morphology of the Pd membrane surface: (a) Original Pd membrane surface where reaction has occured; (b) Pd membrane surface reacted with 450 °C H2◦S and then treated by redox no reaction has occured; (b) Pd membrane surface reacted with 450 CH2S and then treated by reaction. redox reaction. There are three possible causes promoting a stable hydrogen permeation flux in Pd membrane material,There i.e., are (1) three the possiblerise of Pd causes membrane promoting surface a stableroughness hydrogen resulted permeation in an effective ﬂux in increase Pd membrane of the Pdmaterial, membrane i.e., (1) surf theace rise area of after Pd membrane H2S erosion surface and redox roughness reactions, resulted so that in there an effective were more increase active of sites the toPd dissolve membrane hydrogen surface areaand afteradsorb H 2moreS erosion hydrogen and redox molecules reactions, on sothe that Pd theremembrane were more surface. active As sites the result,to dissolve more hydrogenprotons could and be adsorb disso morelved and hydrogen diffused molecules on the Pd on membrane; the Pd membrane (2) the roughened surface. surface As the phase,result, morecompared protons to the could smooth be dissolved one, could and further diffused im onprove the the Pd membrane;catalytic activity (2) the of roughenedthe Pd membrane surface phase,and accelerate compared the to dissolution the smooth of one, hydrogen could further molecu improveles on the the Pd catalytic membrane activity surface; of the (3) Pd the membrane carbon pollutantsand accelerate were the removed dissolution from ofthe hydrogen Pd membrane molecules surface, on thewhich Pd membranemade the surface surface; become (3) the cleaner. carbon pollutantsFurthermore, were the removed dissolved from sites the that Pd membranewere occupied surface, originally which madeby carbon the surface atoms becomecould now cleaner. be availableFurthermore, for hydrogen the dissolved molecules. sites that As were the result, occupied the originallycatalytic activity by carbon of the atoms Pd membrane could now besurface available and dissolvingfor hydrogen capacity molecules. of the As Pd the membrane result, the surface catalytic could activity be further of the Pd improved. membrane surface and dissolving capacity of the Pd membrane surface could be further improved. 4. Conclusions 4. Conclusions Based on the research work presented, the following conclusions can be drawn: Based on the research work presented, the following conclusions can be drawn: (1) Hydrogen sulfide (H2S) could be dissolved on the Pd membrane surface at 23 °C and (1) Hydrogen sulﬁde (H S) could be dissolved on the Pd membrane surface at 23 ◦C and contaminate contaminate the surface.2 Pd16S7, Pd4S and PdS were generated on the Pd membrane surface successivelythe surface. with Pd16 Sthe7, Pdrise4S of and temperature. PdS were generatedThe globular on PdS the Pdproduct membrane came into surface being successively on the Pd surfacewith the atrise temperatures of temperature. over 350 The °C. globular Temperature PdS product plays an came important into being role onin the Pdchange surface of Pd at ◦ membranetemperatures surface over morphology 350 C. Temperature as well as plays the type an important and quantity role of in the the products change of generated Pd membrane after H2S was reacted with Pd membrane. (2) The sulfides and carbon impurities on the Pd membrane surface could be removed completely by redox reactions, and at the same time, the Pd membrane surface became highly roughened and porous. The Pd membrane surface morphology could be controlled by controlling the temperature for the H2S corrosion and redox reactions.

Energies 2018, 11, 1127 9 of 10

surface morphology as well as the type and quantity of the products generated after H2S was reacted with Pd membrane. (2) The sulﬁdes and carbon impurities on the Pd membrane surface could be removed completely by redox reactions, and at the same time, the Pd membrane surface became highly roughened and porous. The Pd membrane surface morphology could be controlled by controlling the temperature for the H2S corrosion and redox reactions. (3) After H2S corrosion and redox reactions, the hydrogen permeability of the Pd membrane could increase by around 80%compared to that of the pure Pd membrane before the reactions. The cause for such a permeability increase was that the catalytic and dissolving capacities of the Pd membrane were enhanced with the increase of Pd membrane surface roughness and reduction of carbon pollution.

Author Contributions: Conceptualization, W.F. and X.Z.; Methodology, Q.W.; Formal Analysis, W.F.; Investigation, W.F., Q.K. and X.Z.; Resources, W.F.; Data Curation, Q.K. and J.W.; Writing-Original Draft Preparation, W.F.; Writing-Review & Editing, X.Z.; Visualization, P.T.; Supervision, Q.W.; Project Administration, W.F.; Funding Acquisition, W.F. Acknowledgments: The authors acknowledge the Applied Basic Research Programs of Sichuan province (Grant No. 2018JY0062) and the Natural Science and Technology Research Projects of Chengdu (Grant No. 2015-HM01-00385-SF) and the National Natural Science Research Foundation of China (Grant No. 11572057) and Open Research Subjectof Key Laboratory of Special material and preparation technology (Grant No. szjj2017-062) and The National College Students’ innovation and entrepreneurship training programs(Grant No. 201711079001). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Sato, T.; Sato, S.; Itoh, N. Using a hydrogen-permeable palladium membrane electrode to produce hydrogen from water and hydrogenate toluene. Int. J. Hydrogen. Energy 2016, 41, 5419–5427. [CrossRef] 2. Franzitta, V.; Curto, D.; Rao, D.; Viola, A. Hydrogen Production from Sea Wave for Alternative Energy Vehicles for Public Transport in Trapani (Italy). Energies 2016, 9, 850. [CrossRef] 3. Koj, J.C.; Wulf, C.; Schreiber, A.; Zapp, P. Site-Dependent Environmental Impacts of Industrial Hydrogen Production by Alkaline Water Electrolysis. Energies 2017, 10, 860. [CrossRef] 4. Didaskalou, C.; Buyuktiryaki, S.; Kecili, R.; Fonte, C.P.; Szekely, G. Valorisation of agricultural waste with an adsorption/nanofiltration hybrid process: From materials to sustainable process design. Green Chem. 2017, 19, 3116–3125. [CrossRef] 5. Hafez, H.; Nakhla, G.; Naggar, H.E.I. Biological Hydrogen Production from Corn-Syrup Waste Usinga Novel System. Energies 2009, 2, 445–455. [CrossRef] 6. Chen, W.H.; Hsia, M.H.; Chi, Y.H.; Lin, Y.L.; Yang, C.C. Polarization phenomena of hydrogen-rich gas in high-permeance Pd and Pd–Cu membrane tubes. Appl. Energy 2014, 113, 41–50. [CrossRef] 7. Fernandez, E.; Helmi, A.; Medrano, J.A.; Coenen, K.; Arratibel, A.; Melendez, J.; de Nooijer, N.C.A.; Spallina, V.; Viviente, J.L.; Zuñiga, J.; et al. Palladium based membranes and membranereactors for hydrogen production and purification: An overview of research activities at Tecnalia and TU/e. Int. J. Hydrogen. Energy 2017, 42, 13763–13776. [CrossRef] 8. Ali, A.; Drioli, E.; Macedonio, F. Membrane Engineering for Sustainable Development: A Perspective. Appl. Sci. 2017, 7, 1026. [CrossRef] 9. Montesinos, H.; Julián, I.; Herguido, J.; Menéndez, M. Effect of the presence of light hydrocarbon mixtures on hydrogen permeance through Pd-Ag alloyed membranes. Int. J. Hydrogen. Energy 2015, 40, 3462–3471. [CrossRef] 10. Kurokawa, H.; Yakabe, H.; Yasuda, I.; Peters, T.; Bredesen, R. Inhibition effect of CO on hydrogen permeability of Pd-Ag membrane applied in a microchannel module configuration. Int. J. Hydrogen. Energy 2014, 39, 17201–17209. [CrossRef] 11. Gabitto, J.F.; Tsouris, C. Sulfur Poisoning of Metal Membranes for Hydrogen Separation. Int. Rev. Chem. Eng. 2009, 1, 394–411. 12. Mundschau, M.V.; Xie, X.; Evenson, C.R., IV; Sammells, A.F. Dense inorganic membranes for production of hydrogen from methane and coal with carbon dioxide sequestration. Catal. Today 2006, 118, 12–23. [CrossRef] Energies 2018, 11, 1127 10 of 10

13. Slimane, R.B.; Abbasian, J. Copper-Based Sorbents for Coal Gas Desulfurization at Moderate Temperatures. Ind. Eng. Chem. Res. 2000, 39, 1338–1344. [CrossRef] 14. Cheah, S.; Carpenter, D.L.; Magrini-Bair, K.A. Review of Mid-to High-Temperature Sulfur Sorbents for Desulfurization of Biomass-and Coal-derived Syngas. Energy Fuel. 2009, 23, 5291–5307. [CrossRef] 15. Tarditi, A.M.; Imhoff, C.; Braun, F.; Miller, J.B.; Gellman, A.J.; Cornaglia, L. PdCuAu ternary alloy membranes:

Hydrogen permeation properties in the presence of H2S. J. Membr. Sci. 2015, 479, 246–255. [CrossRef] 16. Nayebossadri, S.; Speight, J.; Book, D. Novel Pd-Cu-Zr hydrogen separation membrane with a high tolerance to sulphur poisoning. Chem. Commun. 2015, 51, 15842–15845. [CrossRef][PubMed] 17. Lewis, A.E.; Zhao, H.B.; Syed, H.; Wolden, C.A.; Douglas Way, J. PdAu and PdAuAg composite membranes for hydrogen separation from synthetic water-gas shift streams containing hydrogen sulﬁde. J. Membr. Sci. 2014, 465, 167–176. [CrossRef] 18. Guerreiro, B.H.; Martin, M.H.; Roué, L.; Guay, D. Hydrogen solubility in PdCuAu alloy thin ﬁlms prepared by electrodeposition. Int. J. Hydrogen. Energy 2014, 39, 3487–3497. [CrossRef] 19. Braun, F.; Tarditi, A.M.; Miller, J.B.; Cornaglia, L.M. Pd-based binary and ternary alloy membranes:

Morphological and perm-selective characterization in the presence of H2S. J. Membr. Sci. 2014, 450, 299–307. [CrossRef] 20. Peters, T.A.; Kaleta, T.; Stange, M.; Bredesen, R. Development of ternary Pd-Ag-TM alloy membranes with improved sulphur tolerance. J. Membr. Sci. 2013, 429, 448–458. [CrossRef] 21. Coulter, K.E.; Way, J.D.; Gade, S.K.; Chaudhari, S.; Alptekin, G.O.; DeVoss, S.J.; Paglieri, S.N.; Pledger, B. Sulfur tolerant PdAu and PdAuPt alloy hydrogen separation membranes. J. Membr. Sci. 2012, 405–406, 11–19. [CrossRef] 22. Braun, F.; Miller, J.B.; Gellman, A.J.; Tarditi, A.M.; Fleutot, B.; Kondratyuk, P.; Cornaglia, L.M. PdAgAu alloy

with high resistance to corrosion by H2S. Int. J. Hydrogen. Energy 2012, 37, 18547–18555. [CrossRef] 23. Pati, S.; Jat, R.A.; Mukerjee, S.K.; Parida, S.C. Hydrogen Isotope Effect on Thermodynamic and Kinetics

of Hydrogen/Deuterium Absorption−Desorption in Pd0.77Ag0.10Cu0.13 Alloy. J. Phys. Chem. C. 2015, 119, 10314–10320. [CrossRef] 24. Zhao, L.; Goldbach, A.; Bao, C.; Xu, H. Structural and Permeation Kinetic Correlations in PdCuAg Membranes. ACS Appl. Mater. Interfaces 2014, 6, 22408–22416. [CrossRef][PubMed] 25. Tosques, J.; Martin, M.H.; Roué, L.; Guay, D. Hydrogen solubility in PdCuAg ternary alloy ﬁlms prepared by electro deposition. Int. J. Hydrogen. Energy 2014, 39, 15810–15818. [CrossRef] 26. Nayebossadri, S.; Speight, J.; Book, D. Effects of Low Ag Additions on the Hydrogen Permeability of Pd-Cu-Ag Hydrogen Separa-tion Membranes. J. Membr. Sci. 2013, 451, 216–225. [CrossRef] 27. Tarditi, A.M.; Braun, F.; Cornaglia, L.M. Novel PdAgCu ternary alloy: Hydrogen permeation and surface properties. Appl. Surf. Sci. 2011, 257, 6626–6635. [CrossRef] 28. Zhao, L.; Goldbach, A.; Xu, H. Tailoring palladium alloy membranes for hydrogen separation from sulfur contaminated gas streams. J. Membr. Sci. 2016, 507, 55–62. [CrossRef] 29. Rahman, M.A.; García-García, F.R.; Li, K. Development of a catalytic hollow ﬁbre membrane microreactor as a Microreformer unit for automotive application. J. Membr. Sci. 2011, 390–391, 68–75. [CrossRef] 30. Dijkstra, J.W.; Pieterse, J.A.; Hui, L.; Boon, J.; Delft, Y.C.; Raju, G.; Peppink, G.; Brink, R.W.; Jansen, D.

Development of membrane reactor technology for power production with pre-combustion CO2 capture. Energy Procedia 2011, 4, 715–722. [CrossRef] 31. Feng, W.; Liu, Y.; Lian, L.; Peng, L.; Li, J. Effect of surface oxidation on the surface condition and deuterium permeability of a palladium membrane. Appl. Surf. Sci. 2011, 257, 9852–9857. [CrossRef] 32. Andrew, P.L.; Peacock, A.T.; Pick, M.A. Interpretation of deuterium pumping by plasma-facing beryllium surfaces. J. Nucl. Mater. 1992, 4, 196–198. [CrossRef] 33. Yokosawa, T.; Tichelaar, F.D.; Zandbergen, H.W. In-situ TEM on epitaxial and non-epitaxial oxidation of Pd and reduction of PdO at P = 0.2–0.7 bar and T = 20–650 ◦C. Eur. J. Inorg. Chem. 2016, 2016, 3094–3102. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).