CO Selective Oxidation in Hydrogen-Rich Gas over Copper-Series ...

Journal of Natural Gas Chemistry 14(2005)29–34

CO Selective Oxidation in Hydrogen-Rich Gas over Copper-Series Catalysts Hanbo Zou1∗ ,

Xinfa Dong1 ,

Weiming Lin1,2

1. School of Chemical and Energy Engineering, South China University of Technology, Guangzhou 510640, China; 2. Department of Biological and Chemical Engineering, Guangzhou University, Guangzhou 510405, China [Manuscript received December 13, 2004; revised February 16, 2005]

Abstract: The performances of CO selective oxidation in hydrogen-rich gas over four catalytic systems of CuO/ZrO2 , CuO/MnO2 , CuO/CoO and CuO/CeO2 were compared. The reducibility of these catalysts and the effect of CuO and CeO2 molar ratio of CuO/CeO2 catalysts on the activity of selective CO oxidation are investigated by XRD and TPR methods. The results show that the catalysts with the exception of CuO/ZrO2 have the interactions between CuO and CoO, CeO2 or MnO2 , which result in a decrease in the reduction temperature. Among the catalysts studied, CuO/ZrO2 catalyst shows the lowest catalytic activity while CuO/CeO2 catalyst exhibits the best catalytic performance. The CuO(10%)/CeO2 catalyst attains the highest CO conversion and selectivity at 140 and 160 . The addition of 9% H 2 O in the reactant feed decreases the activity of CuO/CeO2 catalyst but increases its CO selectivity.

Key words: hydrogen-rich gas, copper-series catalyst, CuO/CeO2 catalyst, selective oxidation, carbon monoxide

1. Introduction Hydrogen fuelled polymer electrolyte membrane fuel cells (PEMFC) show considerable potential as a cleaner, more efficient system than the currently used compression engines in automobiles. In order to avoid problems associated with hydrogen distribution and storage, H2 can be produced on-board by steam reforming or autothermal reforming of hydrocarbon fuels, such as gasoline and methanol. Removal of about 1%CO remaining in hydrogen-rich gas to 100 µL/L is a critical technology. Selective oxidation of CO with oxygen appears to be the simplest and most effective method for removing CO. The catalysts proposed in the literatures for this process are noble metal-based catalysts, such as Au/γ-Al2 O3 [1], Pt/CeZrO2 [2], K-Rh/SiO2 [3]. Although precious catalysts

have desirable activity and long-term stability, the high cost may limit their applicability for transportation. Development of non-noble metal catalysts is attractive. It has been well documented that Cu and Au show superior low-temperature performance for selective oxidation of CO than Pt[4]. It is illustrated that CuO can produce a strong interaction with several 3d transition metal oxide or rare earth oxide and improves its catalytic activity[5–7]. In this study, four types of copper-series catalysts, namely CuO/ZrO2 , CuO/MnO2 , CuO/CoO and CuO/CeO2, are investigated on the CO selective oxidation with simulated reformate gas. The properties of these catalysts were characterized by TPR and XRD techniques. The effects of CuO and CeO2 molar ratio in CuO-CeO2 catalysts and the addition of H2 O in the feed on the catalytic activity and selectivity were also discussed.

Corresponding author. Tel: 020-87111884, E-mail: [email protected]; This work was financially supported by Guangdong Province Natural Science Foundation of China(000435), the Doctoral Program Foundation of the Ministry of Education (20010561003) and Guangzhou Municipal Science and Technology Project(2001J1C0211) ∗

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Hanbo Zou et al./ Journal of Natural Gas Chemistry Vol. 14 No. 1 2005

2. Experimental 2.1. Catalyst preparation Four catalytic systems of CuO/ZrO2, CuO/ MnO2 , CuO/CoO and CuO/CeO2 were prepared by the alkaline coprecipitation method using Na2 CO3 solution as a precipitant. Under stirring, Na2 CO3 solution and mixed nitrate solution containing the desired ratio of cations were slowly dropped to a beaker simultaneously and pH of the solution was kept to 10 at 70 . The precipitate were aged for 2 h at the same temperature, then filtered and washed with hot distilled water several times for removing residual sodium. They were dried in air at 110 overnight and calcined at 500 for 4 h. 2.2. Catalytic reaction test Catalytic activity test were carried out in a continuous fixed-bed reactor at atmospheric pressure. The micro-reactor was a quartz tube with 7 mm of internal diameter. The bed was made of 0.3 g catalyst sieved to 40–60 mesh. Prior to all catalytic experiments, the copper-series catalysts were heated at 300 under a flowing 20%O2 /He mixture (30 ml/min) for 1 h. A thermocouple was inserted into the catalyst bed to monitor the reaction temperature. The feed stream contained 65%H2 , 25%CO2 , 1%CO (He as the balance gas) at a typical flow rate of 100 ml/min. An excess of oxygen was used for the selective CO oxidation experiments (λ=2[O2 ]/[CO]=3). The effect of H2 O was tested by the addition of 9%H2 O in the feed. Quantitative analysis of outlet gas was performed by gas chromatography (HP 4890) with TDX-01 column. When CO content is below the detection level, the infrared CO/CO2 analyzer with the resolution of 1 µL/L was used. The catalytic activity was evaluated by CO conversion (XCO ), CO selectivity (SCO ), and they are given as follows: XCO = SCO =

of 50 mg. The sample was heated in nitrogen at 300 for 0.5 h, and cooled to room temperature under nitrogen flow. Then H2 -TPR was performed in a mixture of 10% hydrogen in nitrogen with the flow rate of 30 ml/min. The temperature was raised at a constant rate of 15 /min from room temperature to around 600 . The water produced by the reduction was trapped on 5A molecule sieve. The crystal structure of the catalysts was determined on a Philips XPERPPRO diffractrometer using Cu Kα radiation, with the accelerated voltage of 40 kV, the filament current of 40 mA, and the scanning frequency of 5o /min in 2θ range of 20o –80o. 3. Results and discussion 3.1. Comparison of catalytic activity over dif ferent copper catalysts CO conversion and selectivity of the four catalysts with the best performance in their types are given in Figure 1.

nCO,in − nCO,out × 100% nCO,in

0.5(nCO,in − nCO,out ) × 100% nO2 ,in − nO2 ,out

2.3. Catalyst characterization The temperature-programmed reduction (TPR) of different catalysts was carried out using TP5000 adsorption instrument (made in China) with a sample

Figure 1. Comparison of CO selective oxidation over dif ferent catalysts

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It shows that CuO/ZrO2 has almost no activity below 120 , the activity improves with the increase of temperature. When the temperature reaches 240 , the CO conversion is even lower than 40%. The selectivity of CuO/ZrO2 decreases progressively from 100% at 120 30% at 240 . CuO/MnO2 performs the best catalytic activity to CO oxidation at lower temperature (120 ), but it also benefits to hydrogen oxidation, which is in agreement with the experimental observation that its CO selectivity was lower than those of other catalysts at the same reaction temperature. In the temperature region studied, the activity of CuO/CoO is lower than that of CuO/MnO2 untill 140 and higher. Furthermore when the temperature is higher than 160 , methanation can be detected by gas chromatography over the CuO/CoO catalyst. The CuO(10%, molar ratio)/CeO2 catalyst has the highest activity among all catalysts. At 140 and 160 , it attains 99% CO conversion with 77% and 36%CO selectivity respectively. Hydrogen oxidation becomes predominant at higher temperature and the selectivity decreases dramatically. With a wider temperature range for a 99% CO conversion and higher selectivity, CuO/CeO2 is the most suitable catalyst to the selective CO oxidation compared to other catalysts.

It indicates that the oxidative activity of either CuO or CeO2 is relatively low, but when suitable CuO and CeO2 are mixed the activity of the catalyst can increase to some extent. CuO/CeO2 catalyst with 10%CuO molar ratio gives the highest activity and is nearly constant with the increase of CuO molar ratio up to 30%. However, the activity decreases with atomic ratio of copper over 30%, due to the appearing and the increasing of crystalline CuO in the CuO/CeO2 catalyst. XRD analysis shows that no CuO phase is detected in CuO(10%)/CeO2 catalyst, which suggested that the catalyst had high CuO dispersion, CuO has doped into the CeO2 matrix and shows the strong interaction between CuO and CeO2 . It is in agreement with the report that the catalyst activity was promoted greatly due to the synergistic effect between CuO and CeO2 [8]. 3.3. Ef fect of H2 O on selective CO oxidation The results obtained for selective CO oxidation over CuO(10%)/CeO2 catalyst, with or without 9vol%H2 O in the reactant feed, are given in Figure 3.

3.2. Ef fect of CuO and CeO2 molar ratio on the catalytic performance For CuO/CeO2 catalysts, the effect of different CuO molar ratio on selective CO oxidation is shown in Figure 2.

Figure 2. Relationship between CO conversion and the composition of CuO/CeO2 catalysts (1) 140 , (2) 160

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Figure 3. Ef fect of H2 O on CO conversion and selectivity over CuO(10%)/CeO2 catalyst (1) Without H2 O, (2) With 9%H2 O

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Addition of H2 O to the feed stream decreases the catalytic activity for selective CO oxidation. The temperature at which the 99% conversion of CO is obtained, shifts to a higher temperature by 20 . When the temperature is lower than 120 , it is found that the catalytic activity decreased markedly with 9vol%H2 O. The selectivity is kept at a constant value at lower temperatures (180 ) with or without the addition of H2 O. But it increases obviously between 120 and 180 in the presence of H2 O. It is assumed that the decrease in CuO(10%)/CeO2 catalytic activity may be attributed to the blockage of catalytic active sites by the absorbed water, as well as to the formation of CO-H2 O surface complexes[9].

catalysts with the exception of CuO/ZrO2 have some interaction between CuO and the other component of CoO, CeO2 or MnO2 . While for CuO/ZrO2 catalyst, the higher temperature peak is in agreement with the experimental result that it is less active in the CO selective oxidation.

3.4. TPR results The reducibility of the catalysts was studied by H2 -TPR technique. The H2 -TPR profiles of different catalysts are presented in Figure 4.

Figure 5. TPR prof ile of CuO/CeO2 catalyst with dif ferent CuO molar ratio (1) CuO(10%)/CeO2 , (2) CuO(30%)/CeO2 , (3) CuO(50%)/CeO2 , (4) CuO(70%)/CeO2 , (5) CuO(90%)/CeO2 , (6) Pure CuO, (7) Pure CeO2

Figure 4. H2 -TPR prof iles of dif ferent copperseries catalysts (1) CuO(10%)/CeO2 , (2) CuO(10%)/ZrO2 , (3) CuO(20%)/CoO, (4) CuO(80%)/MnO2

CuO/CoO and CuO/CeO2 show two peaks. For the case of CuO/CoO catalyst, the first peak at 184.4 is not as distinct as for the case of CuO/CeO2 catalyst at 150.5 . While for CuO/MnO2 and CuO/ZrO2 catalysts only one distinct peak appears at higher temperature of 308.5 and 317.1 respectively. The reduction profile of pure CuO is characterized by a single peak at 310.8 (shown in Figure 5). Because of the lower temperature peak than that of CuO, the

TPR profiles of CuO/CeO2 with different CuO loading (CuO molar ratio) are shown in Figure 5. It is shown that the temperatures for the reduction peaks of pure CuO and CeO2 are 310.8 and 516.2 , respectively. For the sample of CuO(10%)/CeO2 catalyst, two reduction peaks with T max at 150.2 and 176.3 respectively can be detected clearly. In contrast, for the sample of CuO(30%)/CeO2, there are three peaks at 155.0, 198.9 and 242.4 in its TPR profile. According to Kundakovic et al.[10], when copper content is sufficiently low (2.4wt% of CuO per gram of catalysts), copper is well dispersed and is only present as isolated copper Cu2+ ions or highly dispersed clusters. In our case, CuO(10%)/CeO2 catalyst contains 4.885wt% of CuO per gram of catalyst. According to the literature data, for the CuO(10%)/CeO2 catalyst copper can exist as well dispersed copper and in cluster form, which is confirmed by XRD measurements shown in Figure 6. Thus the first peak at 150.2 is due to the reduction of cluster species and the peak centered at 176.3 is assigned to isolated

Journal of Natural Gas Chemistry Vol. 14 No. 1 2005

Cu2+ ions. For the CuO(30%)/CeO2 catalyst, the existence of crystalline CuO in addition to the highly dispersed Cu2+ species causes the appearance of the third reduction peak at 242.4 . With the continuous increase in copper atomic ratio, copper is mainly present as larger CuO particles, which causes a weaker interaction with CeO2 and shows only one peak at the higher temperature. When combined with the activity data, it may be presumed that the well-dispersed CuO on the catalysts is benefital to the catalyst performance. 3.5. XRD characterization

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lyst. By collecting together the information obtained from TPR and activity test, one can conclude that the larger particle size is the plausible interpretation for the deactivation of the catalyst after reaction. It is apparent from Figure 7 that the fresh CuO(10%)/CeO2 possesses the spheroidal structure in a uniform size distribution. While on the surface of the deactivated CuO(10%)/CeO2 sample, there are clearly many irregular and massive structures, which demonstrates that the surface morphological structures of the fresh and deactivated CuO(10%)/CeO2 are quite different and this may be a factor for the reduction of the catalytic performance.

Figure 6 shows the XRD patterns of the fresh and deactivated CuO(10%)/CeO2 catalyst, and their SEM results are shown in Figure 7.

Figure 6. XRD patterns of fresh and deactivated CuO(10%)/CeO2 catalyst (1) Fresh, (2) Deactivated CuO(10%)/CeO2 catalyst

The deactivated CuO(10%)/CeO2 catalyst can not reach 90% for CO conversion at the optimal temperature. According to the XRD patterns, only the face-centred cubic cerianite phase is revealed for the CuO(10%)/CeO2 catalyst and the CuO peak can not be observed. It is because that the Cu particles are too small or finely dispersed on the catalyst surface and thus not shown on their XRD patterns. The diffraction peaks of cerianite of the deactive sample are weak and sharp, which means that the cerianite has formed relatively large crystals. The average crystallite size of deactivated catalyst in the direction normal to the (111) plane of cerianite was calculated to be 76.3 nm by Sherrer equation, which is larger than the crystallite size 24.2 nm of the fresh cata-

Figure 7. SEM photograph of (a) fresh and (b) deactivated CuO(10%)/CeO2 catalysts magnified by 2000

4. Conclusions (1) In the four catalytic systems of CuO/ZrO2 , CuO/MnO2 , CuO/CoO and CuO/CeO2, CuO/ZrO2 shows the lowest catalytic activity while CuO/CeO2 exhibits the highest catalytic performance. The methanation reaction occurs between hydrogen and CO over the CuO/CoO catalyst. There are strong interactions between CuO and MnO2 , CoO and CeO2 which result in the decrease in the reduction temper-

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ature of the catalysts. (2) Comparing to the CuO/CeO2 catalysts with different CuO atomic ratio, CuO(10%)/CeO2 catalyst exhibits catalytic performance due to the synergistic effect between CuO and CeO2 , which is in agreement with the results of TPR profiles. (3) CO conversion of CuO(10%)/CeO2 catalyst decreases in the reactant feed with 9%H2 O, while the CO selectivity of the catalyst increases in the presence of 9%H2 O at the same temperature than the case when no H2 O is present. The temperature with 99%CO conversion is 20 higher than that of without H2 O in the feed. References [1] Bethke G K, Kung H H. Appl Catal A, 2000, (194195): 43

[2] Roh H S, Pordar H S, Jun K W et al. Catal Lett, 2004(93): 203 [3] Tanaka H, Ito S I, Kameoka S et al. Catal Commun, 2003, 4(1): 1 [4] Kandoi S, Gokhale A A, Grabow L C et al. Catal Lett, 2004(93): 93 [5] Wang J B, Lin S C, Huang T J. Appl Catal A, 2002, 232: 107 [6] Avgouropoulos G, Ioannides T, Matralis H K et al. Catal Lett, 2001, 73(1): 33 [7] Yu W G, Bai X, Liu Y et al. Proceeding of 12th National Catalyst Science. Beijing: Petroleum & Chemical Industry Committee of China Chemical Industry and Engineering Society, 2004: 374 [8] Papavasiliou J, Avgouropoulos G, Ioannides T. Catal Commun, 2004, 5(5): 231 [9] Snytnikov P V, Sobyanin V A, Belyaev V D et al. Appl Catal A, 2003, 239(1-2): 149 [10] Kundakovic L J, Flytzani-Stephanopoulos M. Appl Catal A, 1998, 171(1): 13