zeolite catalysts

Applied Catalysis B: Environmental 46 (2003) 595–600

Hydrogen purification for fuel cells: selective oxidation of carbon monoxide on Pt–Fe/zeolite catalysts Masahiro Watanabe∗ , Hiroyuki Uchida, Kyoko Ohkubo, Hiroshi Igarashi Clean Energy Research Center, University of Yamanashi, Takeda 4, Kofu 400-8510, Japan Received 15 June 2003; received in revised form 30 June 2003; accepted 18 July 2003

Abstract A preferential oxidation of CO remaining in reformed H2 fuels (PROX) is one of the processes to avoid a serious poisoning of anode-catalysts in fuel cells. The crucial requirement for the PROX catalyst is high CO oxidation rate with high selectivity at low-temperatures of ≤200 ◦ C. We propose new Pt–Fe/mordenite catalysts for the PROX, which is extremely superior to conventional Pt/Al2 O3 catalysts, even to our Pt/mordenite. TEM observation showed that the most parts of metal catalysts were loaded in mordenite molecular pores. The performances were examined in comparison with Pt/mordenite at various experimental conditions, i.e. temperature ranges (80–300 ◦ C), gas-compositions including water vapor, gas hourly space velocities (GHSV: 20,000–80,000 h−1 ) or the compositions of metal-catalysts supported on mordenite, and found to exhibit a sufficient stability for the whole experimental period. On Pt–Fe/mordenite, we show a complete removal of CO with 100% selectivity by the addition of stoichiometric amount O2 from a simulated reformate, e.g. 1% CO, 25% CO2 , 20% H2 O and H2 balance, at 80–150 ◦ C with an extremely high GHSV (∼80,000 h−1 ). FTIR and TPD data showed that CO coverage and CO bond strength on Pt are extremely lowered by the combination with Fe. © 2003 Elsevier B.V. All rights reserved. Keywords: Preferential oxidation; Selective oxidation; Carbon monoxide; Zeolite catalyst; Hydrogen purification; Fuel cell; Pt–Fe alloy

1. Introduction Polymer-electrolyte fuel cells (PEFCs) are attracting enormous interest in the application to electricvehicles or residential power-generations. Purification of hydrogen reformed from hydrocarbon-fuels is the key-technology. Removal of CO ca. 1% remaining in the reformates to a trace-level is essential to avoid a serious poisoning of anode-catalysts in the cells [1]. The acceptable CO concentration is <10 ppm at Pt anode [2] and ≤100 ppm at CO-tolerant alloy ∗ Corresponding author. Tel.: +81-55-220-8620; fax: +81-55-254-0371. E-mail address: [email protected] (M. Watanabe).

anodes [3–6]. Some methods for the reduction of CO content in reformates have been tested. Among them, PROX is one of the best candidates, particularly to the mobile applications, due to the potential of size-reduction or high-responsibility. The crucial requirement for the PROX catalyst is high CO oxidation rate with high selectivity at low temperatures of ≤200 ◦ C, from the viewpoints of compactness of the system, simplicity of the control system or high fuel efficiency. Noble metal catalysts supported on some oxides, reported so far [7–11], were not so selective, e.g. conventional Al2 O3 supported Pt (Pt/Al2 O3 ) required ≥2% O2 addition to oxidize 1% CO in H2 -rich gas [7], although the O2 quantity stoichiometrically required is only 0.5% (CO + 1/2O2 → CO2 ).

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The remainder of O2 causes H2 combustion loss ≥3%. We have proposed Pt catalysts supported in the cages of A-type zeolite for the removal of 1% CO from H2 -rich gas by the PROX, taking advantage of its “chemical and/or physical molecular sieve effect” to make CO react with O2 [12]. The selectivity was found to be much superior to the conventional Pt/Al2 O3 and be affected by the types of supports used, in the order, A-type zeolite > mordenite > X-type zeolite > Al2 O3 . Pt/mordenite showed the highest conversion from CO to CO2 with almost similar selectivity to that of Pt/A-type zeolite among the catalysts examined [13]. Among various metals (Pt, Ru, Pd, Co, Pt–Ru) loaded on mordenite, Pt–Ru/mordenite exhibited fairly high conversion with a high selectivity of ca. 90% over a wide fuel-flow rate condition even at relatively low temperature of 150 ◦ C [14,15]. Here, we report a new catalyst, i.e. Pt–Fe/mordenite, that shows a complete removal of CO with the stoichiometric O2 addition in a simulated reformate (CO, CO2 , H2 O, and H2 ) at 80–150 ◦ C with 100% selectivity at a practically wide gas-flow condition (to ca. 80,000 gas hourly space velocity (GHSV)). Such an extraordinary PROX property has never been reported.

2. Experimental Pt–Fe (3:1, 2:1, and 1:1 in weight ratio) catalysts and Pt catalyst (as a reference) supported on mordenite [Na8 (Al8 Si40 O96 )·24H2 O], at 6 wt.% of total metals, were prepared by a conventional ion-exchange method [12–15]. The obtained powders were pelletized, crushed, and then sieved to 100–200 meshes. Before use, they were heated in a quartz tube reactor in an O2 flow at 500 ◦ C for 0.5 h and then in a H2 flow at 500 ◦ C for 1 h. They will be denoted as Pt–Fe/M and Pt/M, respectively. The catalytic oxidation of CO was carried out in a conventional tubular flow reactor (4 mm in inner diameter). The samples were pretreated until they exhibited the stable performances by repeating temperature cycles between 80 and 300 ◦ C in a flow of the practical reactants. Reaction temperature was measured with a thermocouple attached to an outer wall of the reactor tube. Unless otherwise stated, the reaction mix-

ture consisted of 1.0% CO, 0.5% O2 , and H2 balance. On-line gas chromatograph with a TCD detector was used to measure inlet- and outlet-gas compositions. Hydrogen was used as carrier gas to avoid any interference from a large amount of H2 in the quantification of O2 and CO in the sample gas. Lower limit of the detection for CO was 10 ppm. The CO conversion was calculated based on CO2 formation, and the O2 conversion based on O2 consumption. The selectivity index O2 (CO) is defined as the fraction of oxygen that is used for the oxidation is the inlet O of CO to CO2 , i.e. O2 (CO) = {0.5 [CO2 ]/([O2 ]0 − [O2 ])} × 100%, where [O2 ]0 is the inlet O2 concentration and [O2 ] is the outlet O2 concentration after the reaction. Total amount of carbon contained in CO and CO2 at the outlet agreed well with that in the reactant gas under all reaction conditions, i.e. any hydrocarbon was not formed (lower limit of the detection for CH4 was 10 ppm). Steady-state performances at each temperature or flow rate were collected after ca. 6 h, so that only stable data were used.

3. Results and discussion Fig. 1 shows scanning TEM (STEM) images observed at different magnifications on test pieces of Pt–Fe/M prepared by slicing with microtome. From a low magnification image (Fig. 1A), it is found that metallic particles with different sizes (1 nm in diameter) present on the surfaces of mordenite particles (≥100 nm), most of which are observed on the edges of sliced pieces (see also Fig. 1B). On the other hand, small metallic particles uniformly dispersed with almost the same size (ca. 1 nm in diameter) are clearly observed inside the periphery of the sliced pieces, as seen in image (Fig. 1B). Such small particles are also observed within lattice fringe images of ca. 1.1 nm spacing, which probably correspond to the repeating structure of mordenite cages observed in (0 0 1) projection, at any sliced mordenite particle (Fig. 1C). The STEM results strongly suggest that large parts of the supported catalysts exist inside mordenite cages, although some parts of particles exist outside them. Ratio of the total surface areas of the small metallic particles of inside to that of outside was roughly estimated to be more than 80%, based on the total numbers of two category particles and their average

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Conversion and Selectivity, %

100 80 60 40 20

CO conversion O2 conversion CO selectivity,O 2(CO)

(A) Pt-Fe/M

0 100 80 60 40 20

(B) Pt/M 0 50 100 150

200

250

300

350

o

Temperature / C Fig. 2. Variation of the CO conversion, the O2 conversion, and the selectivity index O2 (CO) on (A) Pt–Fe/M (Pt/Fe = 2:1) and (B) Pt/M as a function of temperature. The amount of catalyst was 0.100 g for Pt/M and 0.025 g for Pt–Fe/M. Gas flow (1.0% CO, 0.5% O2 , H2 balance) was 50 cm3 min−1 . Note that O2 (CO) on Pt/M may include an error at 150 ◦ C because of the small CO and O2 conversions.

Fig. 1. Scanning transmission electron microscope (STEM) images on Pt–Fe/M (Pt/Fe = 2:1): dark field-images (A), (B), and high resolution bright-field image (C).

particle sizes observed in representative STEM images. The observations agreed well with the fact that XRD patterns assigned to metallic catalyst particles were hardly detected.

Fig. 2 shows the variation of the CO conversion, O2 conversion, and selectivity index O2 (CO) at Pt–Fe/M (Pt/Fe = 2:1) and Pt/M as a function of temperature, obtained just by the addition of the stoichiometric amount of O2 (0.5%) to the fuel stream. No scattering was seen in the performances of both catalysts, independent of various operating conditions of the experiment for a few weeks, indicating that they were very stable in the temperature range and the duration examined. In a high temperature region beyond 200 ◦ C, both catalysts show a reduction of the steady CO conversion and selectivity with increasing operating temperature, although O2 conversion was almost 100%. This indicates that a larger fraction of O2 was consumed for H2 oxidation reaction at a higher temperature; otherwise CO was re-generated by a reverse shift reaction (CO2 + H2 = CO + H2 O) after O2 was consumed completely for CO oxidation. In a temperature region of ≤200 ◦ C, it is clarified that Pt–Fe/M exhibits 100% CO conversion and selectivity just by the addition of the stoichiometric

Conversion & Selectivity, %

amount of O2 , although Pt/M exhibits a steep decrease in CO conversion in spite of its high catalyst loading compared with the former. The low CO conversion results from the slow PROX rate as indicated by the decrease of O2 conversion with decreasing temperature. Pt/M at the same 0.025 g loading as the Pt–Fe/M exhibited fairly low CO conversion, e.g. 16.1% even at 200 ◦ C [14]. The effect of Fe contents (Pt/Fe = 3:1, 2:1, 1:1) on the PROX was examined as a function of temperature in the fuel stream (1% CO, 0.5% O2 , 20% H2 O, and H2 balance, W/F = 0.03 g s cm−3 ). As shown in Fig. 3, the catalyst with Pt/Fe = 2:1 showed the best performance, i.e. 100% CO selectivity and 100% CO conversion in the temperature region between 80 and 200 ◦ C. It was also confirmed that no noticeable degradation of these performances occurs at any temperature between 80 and 200 ◦ C for 24 h in the presence of 20% H2 O, indicating that both of the supported metallic catalyst and the mordenite support are extremely stable in such a high humidity condition examined. It is important for a practical fuel processor in PEFCs to exhibit the high CO conversion and selectivity over a wide contact time (W/F) condition, where W and F are weight of the catalyst used and the total flow rate of the reactant gas, respectively. Fig. 4 shows dependencies of the performances of two catalysts on the W/F. The CO conversion on 100 80 60 40

Pt : Fe 3:1 2:1 1:1

20 0

150

200

250

300

o

Temperature / C Fig. 3. Variation of the CO conversion, the O2 conversion, and the selectivity index O2 (CO) on Pt–Fe/M (Pt/Fe = 3:1, 2:1, 1:1) as a function of temperatures. The amount of each catalyst was 0.025 g. The rate of gas flow (1.0% CO, 0.5% O2 , 20% H2 O, H2 balance) was 50 cm3 min−1 .

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M. Watanabe et al. / Applied Catalysis B: Environmental 46 (2003) 595–600

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100 80 60 40 20

(A)

0 100 80 60 40

Pt-Fe/M Pt/M (B)

20 0

0.05

0.10

/ g s cm-3

0.15

Fig. 4. Changes of (A) CO conversion and (B) selectivity O2 (CO) on Pt–Fe/M (Pt/Fe = 2:1) at 150 ◦ C (䊉) and Pt/M at 200 ◦ C (䊐) as a function of W/F. The amount of catalyst and the gas composition were the same as in Fig. 2.

Pt/M decreases with decreasing W/F (shorter contact time), reflecting the low catalytic activity for the CO oxidation even at 200 ◦ C, although the CO selectivity was kept at ca. 80% at a W/F range from 0.06 to 0.12 g s cm−3 . Pt–Fe/M (Pt/Fe = 2:1) demonstrates the extraordinary performances of 100% CO conversion and 100% selectivity in the wide W/F ranging from 0.03 to 0.12 g s cm−3 at 150 ◦ C. The same results were obtained at 100 and 200 ◦ C. The value of W/F = 0.03 g s cm−3 corresponds to a large enough GHSV of ca. 80,000 h−1 , which is four times larger than that of the minimum requirement for fuel cell vehicle applications and may contribute to the size and cost reductions for the system. Conventional reformed gases usually contain CO2 and H2 O besides H2 and CO, e.g. about 25% of CO2 and 20% of H2 O in a methanol reformate. So, the effects of CO2 and H2 O on the PROX performances were examined at Pt–Fe/M (Pt/Fe = 2:1) as a function of temperature. As shown in Fig. 5, both CO conversion and selectivity were 100% at the temperature between 80 and 150 ◦ C for the reactant of 1.0% CO, 0.5% O2 , 25% CO2 , 20% H2 O, H2 balance with W/F = 0.03 g s cm−3 . The performances do completely match

Conversion and selectivity, %

M. Watanabe et al. / Applied Catalysis B: Environmental 46 (2003) 595–600

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100 95 90 85 80 50

CO conversion O2 conversion CO selectivity, O2(CO)

100 150 200 o Temperature / C

250

Fig. 5. Variation of the CO conversion, the O2 conversion, and the selectivity index O2 (CO) on Pt–Fe/M (Pt/Fe = 2:1) as a function of temperature in a methanol-reformate composition: gas flow (1.0% CO, 0.5% O2 , 25% CO2 , 20% H2 O, H2 balance) rate was 50 cm3 min−1 , the amount of catalyst was 0.025 g, i.e. W/F = 0.03 g s cm−3 .

the requirement for the PROX systems to be operated rather a lower temperature than 150 ◦ C. The performances slightly decreased to 99.1% at 200 ◦ C due to a reverse shift reaction by the presence of CO2 , but the remaining CO level of 90 ppm is still acceptable for CO-tolerant anodes in PEFCs. If still necessary, the remaining CO can be eliminated easily by a slight excess addition of O2 . Therefore, the Pt–Fe/M PROX catalyst does not require a precise control of the temperature to achieve 100% CO conversion and selectivity as long as the temperature is kept within such a range, which is installed in practical systems between the reformer-outlet (≤200 ◦ C) and the PEFC stack (ca. 80 ◦ C). It must be noted again that the 100% selectivity at the Pt–Fe/M promises no H2 loss (the highest fuel efficiency), whereas 2% O2 addition to oxidize 1% CO (25% selectivity) for the state-of-the-art Pt/Al2 O3 catalyst results in 3% H2 loss, which is a large deficit of the conversion efficiency in the PEFCs. We have not yet clarified the role of Fe in enhancing the PROX performances at Pt–Fe/M. However, we have observed the following experimental results that make us image about the distinctive catalysis mechanism. At a temperature programmed desorption (TPD) experiment shown in Fig. 6, CO was pre-adsorbed on the catalysts at room temperature. The CO desorption in helium carrier gas commenced on Pt–Fe/M just after raising temperature and finished at 150 ◦ C, whereas it commenced on Pt/M at 130 ◦ C and finished at 210 ◦ C.

Fig. 6. Temperature programmed desorption (TPD) behaviors of pre-adsorbed CO on Pt and Pt–Fe/M (Pt/Fe = 2:1). CO was pre-adsorbed on the sample (0.3 g) at room temperature by repetitive pulse injection, followed by heating at 10 ◦ C/min under helium carrier gas flow (80 cm3 min−1 ). The detector was quadrupole mass analyzer.

The TPD peak area for Pt–Fe/M was ca. 1/3 that of Pt/M. At Fourier transform infrared spectroscopy (FTIR) measurement (Fig. 7), negligible CO adsorption was seen on the Pt–Fe/M in a flow of reactant-gas mixtures (1% CO, 0.5% O2 and H2 balance) at 30 ◦ C, although CO may adsorb preferentially on Pt sites at

Fig. 7. Fourier transform infrared spectroscopy (FTIR) on various catalysts in a flow of reactant-gas mixtures (1% CO, 0.5% O2 and H2 balance) at 30 or 150 ◦ C.

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such a low temperature. In contrast, strong IR bands were observed at Pt/M even at 200 ◦ C in the same gas flow condition. Based on the lowered and weakened CO adsorption properties at Pt–Fe/M observed above, we assume that the superior performances for the PROX are brought by neighboring of Pt and Fe sites in cages of mordenite, resulting in lowered CO adsorption on Pt sites and enhanced adsorption of O2 on the neighboring sites free from CO, maybe on Fe sites. The results described above demonstrate that Pt–Fe/M is an extremely promising catalyst for CO cleaning in reformates and gives tremendous impacts to the practical application of PEFCs to electric vehicles or residential co-generation systems, e.g. the highest fuel efficiency, wide operational conditions such as temperature and GHSV, etc. Details of the catalysis by Pt–Fe/M will be discussed elsewhere.

Acknowledgements This work was supported by Leading Project of the Administration of Education, Science, Culture and Sport on “Research and Development of Materials for the Next Generation Fuel Cells”.

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