preferential oxidation of co in a hydrogen-rich gas through au/nay catalytic membranes

TSINGHUA SCIENCE AND TECHNOLOGY ISSNll1007-0214ll05/16llpp397-403 Volume 15, Number 4, August 2010

Preferential Oxidation of CO in a Hydrogen-Rich Gas Through Au/NaY Catalytic Membranes∗

ZHU Ziping (朱自萍), LIU Yanmei (刘艳梅), YANG Zhanzhao (杨占照), GU Xuehong (顾学红)**, XU Nanping (徐南平)

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering,

Nanjing University of Technology, Nanjing 210009, China

Abstract: This paper describes novel Au/NaY catalytic membranes for preferential oxidation of CO

(CO-PROX) in an H2-rich gas. NaY zeolite membranes with a high CO2/N2 separation factor were loaded

with nanosized Au particles using an ion-exchanged method. X-ray diffraction analyses showed that the

structure of the NaY zeolite was not damaged by the ion exchange process. CO-PROX experiments showed

that the catalytic membranes had excellent catalytic performance for selective oxidation of CO. The CO/H2

molar ratio on the permeate side decreased with increasing operating temperature in the range of 80-200℃.

At 200℃, almost no CO was detected from the permeate stream of a catalytic membrane with the feed con-

taining 0.67% CO, 1.33% O2, 32.67% H2, and He in balance. Thus, these Au/NaY catalytic membranes

show a promise for CO removal from hydrogen fuels.

Key words: NaY zeolite membrane; membrane reactor; CO oxidation; nano-sized gold

Introduction

The proton exchange membrane fuel cell (PEMFC) has received much attention due to its low operating tem-perature and high energy efficiency. However, the Pt anodes in a PEMFC are easily poisoned by CO in the hydrogen fuel even at a very low CO concentration[1]. Thus, residual CO must be eliminated from the H2-rich gas stream before being fed to the PEMFC. Among CO removal methods, preferential oxidation of CO (CO- PROX) is considered as perhaps the simplest and most

economical method for practical applications in fuel cell vehicles for the near future[2].

Several catalysts have been investigated for the PROX reaction, such as Pt, Ru, and Pd[2] based metal-lic catalysts. Over the past few years, the striking cata-lytic activity of nanosized gold towards CO selective oxidation has motivated many researchers to work on new Au catalysts[3-8]. The catalytic performance of gold catalysts has been found to be dependent on the Au particle size, the support properties, and the distri-bution of the Au particles over the supports[9]. Various metal oxides have been used as supports for preparing the gold catalyst, such as Fe2O3

[4], ZnO[4], CeO2[5,6],

La2O3[6], MnOx[7], ZrO2

[8,9], and TiO2[10]. Zeolites were

recently considered for use as supports because of their well-defined microporous structure, large surface area and outstanding ion exchange behavior. Chi et al.[11] used surface-functionalized mesoporous silica to pre-pare highly dispersed Au nanoparticles. The loaded gold exhibited high activity towards CO oxidation with

Received: 2010-05-19; revised: 2010-06-11

* Supported by the National Key Basic Research and Development(973) Program of China (No. 2009CB623403), the National NaturalScience Foundation of China (Nos. 20706030 and U0834004), Science & Technology Support Program (Industry) of JiangsuProvince of China (No. BE2008141), and the Natural ScienceFoundation of the Jiangsu Higher Education Institutions (No.09KJA530002)

** To whom correspondence should be addressed. E-mail: [email protected]; Tel/Fax: 86-25-83172268

Tsinghua Science and Technology, August 2010, 15(4): 397-403

398

the catalytic activity found to increase with decreasing Au particle size. Chen et al.[12] loaded nano-gold into different types of zeolite (Y, β, and mordenite). They found that the gold loading in the zeolite depended strongly on the aluminum content in the zeolite, and the zeolite shape, and pore structure. An Au/NaY cata-lyst with more aluminum loading (12.4 wt.%) had greater CO oxidation activity. Jafari et al.[13] compared gold nanoparticles deposited on NaY and HY zeolites. They observed that the Au/NaY catalyst had better activity than the Au/HY catalyst. Although the nanosized gold particles had higher activity at low temperatures, the produced CO2 from the CO oxidation was apt to stay in the support micropores and react with the gold to form a carbonate resulting in activity decay. The deactivated catalyst could be recovered at high temperature with CO2 emission[8].

This work describes a new catalytic membrane for CO-PROX, in which a polycrystalline NaY (FAU-type) zeolite membrane supported on a porous ceramic sub-strate is loaded with nanosized Au particles. When a H2-rich gas containing a small amount of CO diffuses through the catalytic membrane, the CO is preferen-tially oxidized due to the selective catalytic activity of the Au particles and the relatively low diffusion rate for CO molecules. This catalytic configuration is very efficient because the reactive components reach the active sites. Test results with Au/NaY catalytic mem-branes are presented in this paper.

1 Experimental

1.1 Catalytic membrane synthesis and characterization

NaY zeolite membranes were hydrothermally synthesized on disc-shaped porous α-Al2O3 substrates

using the secondary growth method. The substrates had diameters of 27 mm, thicknesses of 2 mm, and average pore sizes of ~100 nm with porosities of ~30%. The membrane synthesis solutions had a molar composition of 10.7SiO2:1Al2O3:18.8Na2O:850H2O. Hydrothermal crystallization was carried out at 100℃ twice with synthesis times of 4 h and 6 h. The detailed synthesis procedure was reported earlier[14]. The synthesized membranes were washed and dried at 50℃ for 12 h before use.

Au/NaY catalytic membranes were prepared by an ion-exchange method[15]. An aqueous HAuCl4 solution (2.0 mmol∙L−1) adjusted to PH=6 using 1 mol∙L−1 NaOH solution was used for the ion exchange. NaY zeolite membranes were immersed into the solution at 80℃ for 12 h with stirring. The as-prepared Au/NaY zeolite membranes were washed with D.I. water and dried in an oven at 50℃ for 12 h. The catalytic mem-branes were then fired at 150℃ for 2 h before testing of catalytic activity. For comparison, 2 μm NaY zeolite particles were also loaded with Au using the same synthesis method. The crystal phases of the prepared samples were characterized by X-ray diffraction (XRD; Bruker, D8-Advance) with a Cu-Kα radiation source. The morphologies were observed with a cold field emission scanning electron microscope (FESEM; HI-TACHI, S-4800) and a high-resolution transmission electron microscopy (HRTEM, JEM2010).

1.2 Membrane separation and reaction

Figure 1 shows the schematic diagram of the experi-mental apparatus used for the membrane reactions. The Au/NaY catalytic membranes were sealed by graphite rings and mounted in a stainless steel cell. A gas stream containing 0.67% CO, 1.33% O2, 32.67% H2 , and He as balance with a total flow rate of 75 mL∙min−1 or as

Fig. 1 Schematic diagram of the experimental apparatus used for membrane reaction

ZHU Ziping (朱自萍) et al.：Preferential Oxidation of CO in Hydrogen-Rich Gas …

399

otherwise indicated in the paper was fed into the membrane side (feed side). The substrate side of the zeolite membrane (permeate side) was swept by a he-lium stream. The products obtained from the permeate side were analyzed by an on-line gas chromatograph (GC2014, Shimadzu) equipped with two thermal con-ductivity detectors (TCD) using a GDX 101 packed column and a 5A molecular sieve. The separation per-formance of the NaY zeolite membrane was deter-mined using an equimolar CO2/N2 mixture (30 mL∙min−1) introduced into one side of the mem-brane while helium gas (30 mL∙min−1) was used to sweep away the separation products. The total pres-sures on both sides for separation of the reaction were maintained at atmospheric pressure.

The membrane separation factor or selectivity (α) for component i over component j is defined as

,

//

i ji j

i j

y yx x

α = (1)

where xi and xj are the mole compositions of compo-nents i and j in the feed stream and yi and yj are the mole compositions of i and j in the permeate stream.

The oxygen excess factor (λ) was defined as 2 in

in

2[O ][CO]

λ = (2)

The CO conversion (XCO) and O2 selectivity to CO oxidation (SO2) for the fixed-bed reaction were calcu-lated as:

in outCO

in

[CO] [CO][CO]

X −= (3)

2

in outO

2 in 2 out

0.5([CO] [CO] )[O ] [O ]

S−

=−

(4)

2 Results and Discussion 2.1 Sample characterization

Figure 2 shows the FESEM images of the surface and a cross section of a NaY zeolite membrane. A con-tinuous zeolite layer was formed on the porous α-Al2O3 substrate with well-intergrown zeolite crystals. The zeolite layers were about 5-10 μm thick. The qual-ity of the as-made NaY zeolite membranes was further confirmed by the separation of the equimolar CO2/N2 mixtures. Figure 3 shows the separation results for a fresh NaY membrane. The CO2 permeance through the membrane increased with temperature before 80℃

and then decreased after that. The CO2/N2 separation factor decreased with temperature for the tested tem-perature range of 30-150℃. The components permeat-ing through the NaY zeolite membrane were domi-nated by the adsorption-diffusion mechanism. A high separation factor of 27.0 for CO2 over N2 was achieved at 30℃ with a CO2 permeance of 2.65×10−8 mol∙m−2∙Pa−1∙s−1, indicating that the as-made membrane was very effective. Membranes with sepa-ration factors larger than 15 were used for the catalytic membranes.

Fig. 2 FESEM images of NaY zeolite membrane

Fig. 3 Gas permeances and CO2/N2 separation factor of NaY membrane as functions of temperature

Figure 4 shows the XRD patterns for NaY and Au/NaY membranes. A FAU-type zeolite structure was


400

identified on the fresh membrane. After Au loading, the membrane still had the FAU structure but the intensity of the characteristic peaks decreased a little. The energy dispersive spectrometer (EDS) analysis on the catalytic membrane surface showed that the loaded gold con-centration was about 2.9 wt.%. However, no Au peaks were detectable by the XRD analysis. This result sug-gests that the loaded Au species was well dispersed in the zeolite structure of the membrane layer. The Au/Al molar ratio for the catalytic membrane was about 0.024. The achieved Au load and the Au/Al molar ratio for the catalytic membrane are comparable to high activity Au/NaY catalysts reported in the literature[12].

Fig. 4 XRD patterns of (a) NaY zeolite membrane and (b) Au/NaY catalytic membrane

Figure 5 shows FESEM images of an Au/NaY cata-lytic membrane. No cracks were found in the NaY membrane after the Au loading. However, there were apparent aggregations of Au particles on the catalytic membrane surface, which could be due to Au particle coagulation caused by heating during preparation or

stimulation by the high energy electron beam from the FESEM[12]. Au/NaY zeolite prepared using a similar preparation method to the catalytic membrane were further characterized to show the existence of nanosized Au particles in the zeolite structure. Figure 6 shows the HRTEM results for the as-made Au/NaY zeolite particles. Small nano-gold particles with sizes of about 1 nm were present, which should lodge in the supercages of the NaY zeolite. The observed larger gold particles were mainly located on the exterior sur-faces of the zeolite particles.

Fig. 5 FESEM images of Au/NaY catalytic membrane

Fig. 6 HRTEM images of as-prepared Au/NaY catalyst

2.2 Catalytic activity

The catalytic activity of fresh NaY membranes for CO oxidation was investigated using permeation tests with

a feed gas containing 0.67% CO, 1.33% O2, 32.67% H2, and He as balance. The feed flow rate was con-trolled at 75 mL∙min−1. He gas with the flow rate of 25 mL∙min−1 was used to sweep at the permeates. No


401

CO2 was detected at the exits of either membrane sides for the test temperature range of 100-150℃, indicating that the fresh NaY membrane did not have any CO oxidation catalytic function. Table 1 presents the per-meation results for the H2 and CO components. H2 permeance was found to increase with operating

temperature while the H2/CO separation factor was stable between 2.2 and 2.4. The results indicate that the NaY membrane showed H2 selectivity for the separa-tion of H2/CO mixtures, which would contribute to CO-PROX.

Table 1 The permeation results of NaY and Au/NaY membranes

CO permeance H2 permeance Membrane t / ℃ mol∙m−2∙Pa−1∙s−1 mol∙m−2∙Pa−1∙s−1

H2/CO selectivity

NaYa 100 2.28×10−8 5.45×10−8 2.39 NaYa 120 2.54×10−8 6.13×10−8 2.40 NaYa 150 4.12×10−8 9.28×10−8 2.25 Au/NaYb 150 6.05×10−8 19.2×10−8 3.17

Noted: a feed gas containing 0.67% CO, 1.33% O2, 32.67% H2, and He as balance; b feed gas containing 0.67% CO, 32.67% H2, and He as balance

The separation performance of an Au/NaY catalytic membrane was then measured using a feed gas con-taining 0.67% CO, 32.67% H2, and He as balance in-troduced into the feed side at a flow rate of 75 mL∙min−1. The flow rate of the sweep gas (He) was controlled at 25 mL∙min−1. The separation experiment was operated at 150℃. The catalytic membrane was found to have H2 permselectivity. The H2/CO separa-tion factor for the catalytic membrane was higher than that for the fresh membrane. Knudsen diffusion should have an important effect on the H2 selectivity for both the fresh NaY zeolite membrane and the catalytic membrane. Because of their different molecular weights, H2 molecules have higher diffusion rates than CO molecules through NaY zeolite channels. There-fore, the H2 molecules will have shorter residence times in the zeolite layer of the catalytic membrane, which will reduce the H2 oxidation[16]. The H2 per-meance through the catalytic membrane was about twice that through the fresh zeolite membrane without gold loading, perhaps due to the slight change in the zeolite structure indicated by the XRD results in Fig. 4.

Figure 7 compares the catalytic performance of CO-PROX as a function of temperature for the as-made Au/NaY membrane and Au/NaY particles. The amount of NaY zeolite on the membrane was es-timated to be about 50 mg by subtracting substrate weight from that of the as-made membrane. Therefore, 50 mg of Au/NaY catalyst was loaded in a fixed-bed reactor to test the catalytic performance. The same feed condition of a feed gas containing 0.67% CO, 1.33% O2, 32.67% H2, and He as balance was used for the fixed-bed reaction. In the fixed-bed reactor, the CO/H2

molar ratio obtained at the exit was about 1.75% while the oxygen concentration in the products was about 1.27%. The CO/H2 molar ratio decreased to 0.91% as the temperature increased to 150℃ due to the in-creased CO oxidation activity of the nanosized gold. However, when the temperature was further increased above 150℃, the CO/H2 molar ratio did not continu-ously decrease with even a slight increase.

Fig. 7 CO/H2 molar ratio and O2 concentration of products as functions of reaction temperature: (●) mem-brane reactor (MR) and (▲) fix-bed reactor (FBR)


402

Figure 8 shows the CO conversion and O2 selectiv-ity over the Au/NaY catalyst in the fixed-bed reactor. The CO conversion increased with temperature below 150℃ but decreased after that. A high CO conversion of about 55.8% was achieved at 150℃. The O2 selec-tivity decreased gradually from 74.9% to 26.5% as the temperature increased from 80 to 200℃. The results indicate that the nanosized Au exhibited more H2 oxi-dation activity at the elevated temperature (>150℃), which increased the CO/H2 ratio. For the catalytic membrane, however, the CO/H2 molar ratio vs. oper-ating temperature showed a different trend for tem-peratures larger than 150℃, as shown in Fig. 7. At 80℃, a relatively low CO/H2 molar ratio of 0.95% was measured at the catalytic membrane permeate side exit compared with the fixed-bed reactor, which was as-sumed to be partially due to the H2 permselectivity of the membrane. The CO/H2 molar ratio continuously decreased with increasing temperature. At 200℃, the CO concentration in the products dropped to below the GC detection limit. Thus, the preferential oxidation of CO over the nanosized Au particles significantly im-proved the CO removal. The catalytic membrane had much higher CO oxidation selectivity at 200℃ than the fixed-bed reactor, perhaps due to the relatively low diffusion rate for CO over H2 through the zeolite channels.

Fig. 8 CO conversion and O2 selectivity over Au/NaY catalyst as functions of temperature in a fixed-bed reactor

The oxygen concentrations at the exit of both reac-tors are shown in Fig. 7. For the fixed-bed reactor, the oxygen concentration decreased quickly with tem-perature in the fixed-bed reactor, mainly due to the enhanced H2 oxidation. The oxygen concentration from the permeate side of the catalytic membrane re-mained very low due to the dilution of the sweep gas. A slight increase in oxygen concentration was

observed between 80-120℃, which was mainly attrib-uted to the increased oxygen permeation through the membrane at elevated temperatures. The oxygen con-centration decreased for temperatures >120℃. The enhanced oxidation reaction which increased the oxy-gen consumption could play an important role at the higher temperatures. Others have noted that excessive oxygen in the feed has an important effect on the cata-lytic results for CO-PROX[17]. Figure 9 shows the re-action results as a function of oxygen excess factor (λ) for the Au/NaY catalytic membrane at 150℃. The CO/H2 mixture flow rate was fixed at 25 mL∙min−1. The flow rate of 2% O2 in He was adjusted to obtain λ. As shown in Fig. 9, the CO/H2 molar ratio decreased from 0.69% to 0.41% as the oxygen excess factor was increased from 1 to 6. The results suggest that in-creased oxygen concentration in the feed improved the CO oxidation. However, a very high oxygen concen-tration could increase the H2 consumption.

Fig. 9 CO/H2 molar ratio of products as a function of oxygen excess factor (λ) at 150℃

3 Conclusions

Au/NaY zeolite catalytic membranes were used for preferential oxidation of CO. This membrane reactor increased the reaction efficiency for CO-PROX due to the excellent contact between Au and CO and the rela-tively slow diffusion rate for CO molecules through the zeolite channels. Almost no CO was detected from the permeate side of the catalytic membrane for CO- PROX at 200℃. Thus, the catalytic membrane shows promise for CO removal from hydrogen.

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preferential oxidation of co in a hydrogen-rich gas through au/nay catalytic membranes

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