Li Peng ,Limin Wang,Feng Zhu,Jinyun Liu,Wenfu Yan,Xuehong Gu,*

1 State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemical Engineering,Nanjing Tech University,5 Xinmofan Road,Nanjing 210009,China

2 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,College of Chemistry,Jilin University,Changchun 130012,China

Keywords:Zeolite membrane CO oxidation Gold catalyst Surface modification

ABSTRACT A hollow-fiber-supported stable Au/FAU catalytic membrane was successfully synthesized through a polydopamine coating modification-removal strategy and used as a flow-through catalytic membrane reactor for preferential oxidation of CO.Small Au nanoparticles can be efficiently isolated by dopamine and the dopamine-derived carbon shells.The interactions between Au nanoparticles and zeolite layer support are enhanced during annealing at high temperature under an inert atmosphere.A zeolite membrane supported Au nanoparticle catalyst was obtained after the removal of carbon shells,which showed high catalytic activity and stability for the removal of CO from hydrogen.

1.Introduction

Preferential oxidation of CO(PROX)has been regarded as one of the most straightforward and cost effective approaches to reduce the CO concentration in a H2-rich stream to an acceptable level(below 10-20 ppm)to meet the requirement for application in proton exchange membrane fuel cells(PEMFC)[1,2].Among various catalysts developed to achieve this goal,the supported Au catalyst is found to be highly active for oxidation of CO especially at low temperature[3-5].The CO-PROX performances of supported gold catalysts are influenced by the nature of the support or additives,the particle size of gold,and the method of catalyst preparation[6-9].

Zeolites are not the most effective supports for gold catalysts as compared with reducible oxides,such as TiO2[10,11],CeO2[12,13],ZrO2[14,15]and FeOX[16],because it cannot supply reactive oxygen for gold active sites[17].However,the unique structural property of zeolite,such as high surface area,ion exchange ability,and framework stabilizing function,makes it as a promising support for loading gold nanoparticles[18-20].Meanwhile,zeolites can be readily fabricated in the form of catalytic membrane reactors,which possess the advantages of controlled contact time,limited side-reactions and increased operating life[21-25].

FAU-type zeolite,with a spherical cage of diameter 1.3 nm and open aperture of 0.74 nm,has attracted much attention as a support to the preparation of Au/zeolite catalysts for CO oxidation.Au/FAU-type zeolite catalysts have been prepared using different gold precursors and prepare methods.For instance,Au2Cl6was used to incorporated Au(I)ions into NaY zeolite via solid-vapor reaction[26];[Au(en)2]Cl3was used to prepare Au/NaHY via a cation exchange method[27].Chloroauric acid solution,a relatively cheaper gold precursor,was the most frequently used one to prepare Au/FAU-type zeolite catalyst through simply ion-exchange treatment[28-30].Au nanoparticles with a diameter of 1 nm can be found to form in the supercage of Y zeolite,but more Au nanoparticles with larger sizes are formed on the external surface of the zeolite.These Au nanoparticles located outside supercages are thermodynamically unstable,and tend to minimize their surface energies by forming larger particles when exposed to elevated temperatures or upon prolonged storage.That is why the asprepared Au/Y showed high initial activity for low temperature CO oxidation but poor stability[28,31].The efficient stabilization of Au nanoparticles on support materials is therefore crucial for the practical application of Au catalysts.

Recently,a sacrificial coating strategy to stabilize Au nanoparticles on TiO2supports under high-temperature oxidation processes was successfully developed by Dai[32].Taking advantage of the unique coating chemistry of dopamine,the dopamine-derived carbon shells are constructed on the material surface to prevent Au nanoparticle sintering during annealing treatment.Inspired by this work,dopamine was used to form thin polydopamine films adhering on the surfaces of Au/FAU-type zeolite membrane obtained through ion-exchange treatment in chloroauric acid solution.The rigid carbon shells resulting from the following thermal annealing of polydopamine under an inert atmosphere,covered and isolated Au nanoparticles from agglomeration.Meanwhile,the interaction between Au nanoparticles and zeolite supports was enhanced after annealing at elevated temperatures[32,33].After post-thermal treatment in air to remove the carbon shells,the obtained Au/FAU catalytic membrane showed improved catalytic activity and stability for the CO-PROX reaction.

2.Experimental

2.1.Sample preparation

Al2O3hollow fibers with an average pore size～0.65 μm and a porosity of about 48%were fabricated by a dry-wet spinning technique[34].The supports have an outer diameter of 1.8 mm and an inner diameter of ca.0.9 mm(about 5 cm in length),and are used as substrate to synthesize FAU zeolite membranes by the secondary growth method[35].The Au/FAU catalytic membranes were prepared by a liquid-state ion-exchange method in 2.0 mmol·L-1tetrachloroauric(III)acid tetrahydrate solution at 80°C for 12 h.The pH values of HAuCl4solutions were adjusted to 6.0 using 1 mol·L-1NaOH solution before ionexchange.The as-synthesized Au/FAU membranes were washed by D.I.water and dried at 60°C overnight.Then the Au/FAU membrane,with both ends sealed by Teflon tape was contacted with the dopaminecontaining(1 mg·ml-1)tris-buffer solution(50 ml,10 mmol·L-1;pH 8.5)for 24 h.Then the polydopamine covered membrane was rinsed and dried at 40°C in the vacuum.The resulting membrane was named as Au/FAU-PDA.Au/FAU-PDA was subjected to annealing treatment at 700 °C in N2flow for 2 h at a heating rate of 5 °C·min-1to get Au/FAU-carbon.Finally,Au/FAU-d500 was obtained after calcined in air atmosphere at 500°C for 2 h at a heating rate of 5°C·min-1.For comparison,Au/FAU was calcined in air atmosphere at 500°C for 2 h at a heating rate of 5°C·min-1to get Au/FAU-500.

2.2.Catalytic reaction tests and kinetic measurements

The schematic diagram of the experimental apparatus used for COPROX reactions is shown in Fig.1.The membranes were sealed in a stainless steel cell with an inner diameter of 6 mm.A gas stream containing 0.67%CO,1.33%O2,32.67%H2and He as balance with a total flow rate of 75 ml·min-1was fed into the membrane side(feed side).The reaction was conducted at temperature ranging from 25°C to 120°C.The products obtained from the permeate side were analyzed by an on-line gas chromatograph(GC7820,Agilent)equipped with two thermal conductivity detectors(TCD)using a GDX 101 packed column and a 5A molecular sieve.The GC injections for gas analysis for every data point used in this study were repeated three times to obtain more reliable results.

Fig.1.Schematic diagram of experimental apparatus for catalytic membrane reaction tests.

The CO conversion is calculated by:

The selectivity of O2is defined as the ratio of O2that reacted with CO to the total consumption:

2.3.Characterizations

The crystal phases were characterized by X-ray diffraction(XRD,Rigaku MiniFlex 600)with a Cu-Kα radiation source in the 2θ range of 5°-50°.The morphologies of the membranes were observed by scanning electron microscopy(SEM,Hitachi S-4800).Transmission electron microscopy(TEM)images were obtained with a Tecnai G2 F30 S-Twin transmission electron microscope operating at an accelerating voltage of 200 kV.X-ray photoelectron spectroscopy(XPS)measurements were performed with an ESCALAB 250X spectrometer equipped with a monochromatic Al X-ray source(1486.6 eV)(ThermoElectron Co.,America).The base pressure in the analysis chamber was 1×10-8Pa.The binding energy scale was corrected using the C1s signal located at 284.8 eV.

3.Results and Discussion

3.1.Structural and morphological characterization of membranes

The XRD patterns of the FAU zeolite membranes obtained in each preparation steps are shown in Fig.2.All of the membranes have similar XRD patterns,which can be identified as the characteristic FAU type zeolite(JCPDS card 43-0168)and α-Al2O3(JCPDS card 46-1212).No distinct diffraction peaks attributable to Au nanoparticles(with characteristic peak at 38.19°and 44.39°)could be observed for all of the Aucontaining FAU membranes owing to the low Au loading amount.FAU zeolite membrane maintains good crystallinity after the introduction of Au and the following modification steps.

The digital pictures of Au-containing FAU zeolite membranes obtained in each preparation steps are shown in Fig.3a.The color of the Au/FAU membrane is mauve.After dopamine modification,the color is brown,indicating that the polydopamine layer has attached to the surface of the zeolite membrane.The color of the membrane changes into black after thermal treatment in N2,due to the transformation of polydopamine layer into carbon layer.With the removal of carbon,the color of the membrane changes back to mauve.SEM images of the cross-section of Al2O3support and zeolite membrane are shown in Fig.3b and c,respectively.The thickness of the zeolite membrane is about 6 μm,and the Au content in the membrane is about 0.62 wt%according to EDX results.The changes of the surface morphologies of membranes are shown in Fig.3d-g.A continuous,well-intergrown zeolite crystal layer can be found on the surface of the Au/FAU membrane.An obvious coating layer can be found on the surface of the zeolite crystal layer after modification by polydopamine and carbon.With the carbon removal,an intact zeolite crystal layer remains.

Fourier-transformed infrared(FT-IR)absorption analysis clearly shows that three peaks at 1275 cm-1(stretching vibration of phenolic CO),1499 cm-1(NH shearing vibration)and 3367 cm-1(the stretching vibration of phenolic OH and NH)can be observed on Au/FAU-PDA,proving the successful coating of polydopamine on zeolite membranes(Fig.4).

Fig.2.XRD patterns of the samples obtained in each preparation step.

The successful coating of polydopamine on zeolite membranes can be further confirmed by comparing the X-ray photoelectron spectroscopy(XPS)spectra of Au/FAU and Au/FAU-PDA,displayed in Fig.5.An obvious N1s peak appeared at 400 eV in Au/FAU-PDA and its C1s signal is significantly increased.The C1s spectrum of Au/FAU,can be fitted into three peaks centered at about 284.9,286.8,and 288.9 eV,corresponding to CC(or CH),CO and CO groups,originated from the glue used for the test.For Au/FAU-PDA,a new peak centered at about 286.3 eV corresponding to CN group can be observed.The N1s spectrum of Au/FAU-PDA can be fitted into three peaks centered at about 402.4,400.1,and 398.4 eV,corresponding to NH2,NH andNgroups,which originated from dopamine,rearrangement of dopamine and polydopamine(Fig.6).

The size evolution of Au nanoparticles on the surface of the FAU zeolite membranes during each preparation step is traced by TEM images,which is shown in Fig.7.Au nanoparticles can be seen as dark contrasts on the zeolite particles.For the Au/FAU,most of the Au nanoparticles sizes are above 20 nm,while for the polydopamine and carbon coated Au/FAU-PDA and Au/FAU-carbon,most particle sizes are below 10 nm.The possible reason could be the aggregation of Au nanoparticles for Au/FAU during the storage without the protecting layer.Most Au nanoparticles are between 10 and 20 nm for Au/FAUd500 membranes,which clearly demonstrates that Au nanoparticles can be stabilized by the sacrificial coating strategy.

Fig.3.Digital picture of four samples(a),SEM images of the cross-section of Al2O3support(b)and zeolite membrane(c)and SEM images of the surface area of Au/FAU(d);Au/FAU-PDA(e);Au/FAU-carbon(f);and Au/FAU-d500 membranes(g).

Fig.4.FTIR patterns of the samples obtained in each preparation step.

3.2.Catalytic performance

The catalytic performance of Au/FAU and Au/FAU-d500 in CO-PROX is presented in Fig.8.To investigate the effect of thermal treatment on the activity of catalytic membranes,Au/FAU was calcined in air atmosphere at 500°C for 2 h at a heating rate of 5°C·min-1to get Au/FAU-500,and its catalytic performance is also given.The reaction temperature that led to the best catalytic performance over all samples is 100-120 °C.Among them,Au/FAU-d500 exhibited the best catalytic performance of 71%CO conversion at 100°C,and Au/FAU-500 exhibited the worst catalytic performance of only 18%CO conversion at 120°C.The O2selectivity over these membranes follows the order of Au/FAUd500＞Au/FAU＞Au/FAU-500 within the test temperature range,and decreases with increasing temperature.It is quite easy to understand the reason for the low activity of Au/FAU-500 in CO-PROX.Au nanoparticles tend to aggregate even at room temperature,the thermal treatment at temperature as high as 500 °C would lead to the further aggregation of Au nanoparticles,causing a decline of catalytic activity.However,with the protection of polydopamine and carbon coating,the growth of Au nanoparticles can be effectively restrained during the storage and even the thermal treatment process.

Fig.5.XPS analysis of Au/FAU membranes before and after dopamine coating:(a)overall scan;(b)C1s;and(c)N1s.

Fig.6.Dopamine polymerization process.

Fresh fabricated Au/FAU,Au/FAU-500 and Au/FAU-d500 membranes were adopted to evaluate the long-term stability(Fig.9).The stability was tested at 80 °C,the operating temperature of a PEMFC.The stability over these membranes follows the order of Au/FAU-d500＞Au/FAU-500＞Au/FAU.The CO conversion of Au/FAU-d500 decreased by approximately 2%,but still maintained at approximately 65%after 100 h.For Au/FAU-500,a stable CO conversion can be achieved during the first 35 h,then decreased gradually.After 100 h testing,the CO conversion decreased from 15%to 7%.The CO conversion of Au/FAU membrane dropped quickly from 25% to 6%in 4 h,then gradually decreased to 2% in 55 h.The stability test clearly shows that,the catalytic stability of the Au-containing FAU zeolite membranes can be greatly improved by the thermal treatment,especially after high-temperature annealing.The thermally induced enhancement interaction between Au nanoparticles and zeolite support can explain the observed phenomena[32].

For the practical application in PEMFCs,the operation process usually comprises repeated heating and cooling,so the thermal cycling performance of catalytic membranes is important.The thermal cycling performance of Au/FAU-d500 catalytic membrane is presented in Fig.10.Six test temperature points(25,40,60,80,100,and 120°C)were held for 100 min to stabilize the CO-PROX reaction.For five testing cycles,the CO conversion varied no more than 2%at the same temperature.

Fig.7.TEM images of(a)Au/FAU;(b)Au/FAU-PDA;(c)Au/FAU-carbon;and(d)Au/FAU-d500 membranes.

Fig.8.(a)CO conversion and(b)O2selectivity over Au/FAU,Au/FAU-500 and Au/FAU-d500 catalytic membranes.

Fig.9.Comparison of the long-term stability over Au/FAU,Au/FAU-500 and Au/FAU-d500 catalytic membranes.

4.Conclusions

Hollow-fiber-supported Au/FAU catalytic membranes were prepared for the CO preferential oxidation(CO-PROX)in H2-rich gas.The catalytic membranes were prepared by the modification of FAU zeolite membranes with HAuCl4by ion-exchange,and then a sacrificial coating strategy was adopted to stabilize Au on FAU zeolite membrane.Au nanoparticles can be effectively isolated by dopamine coating and dopamine-derived carbon coating during high temperature annealing.The obtained catalytic membranes showed excellent long-term stability and good thermal cycling performance.With the advantage of high packing density of hollow fibers in a membrane reactor,the as-developed catalytic membrane is a promising candidate for the removal of CO from H2to meet the requirement for application in proton exchange membrane fuel cells.

Fig.10.Thermal cycling stability of Au/FAU-d500 catalytic membrane between 25°C and 120°C.