Effect of various precipitants on activity and thermal stability of CuFe2O4 water-gas shift catalysts

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 42, Issue 9, Sep 2014 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2014, 42(9), 10871092

RESEARCH PAPER

Effect of various precipitants on activity and thermal stability of CuFe2O4 water-gas shift catalysts LIN Xing-yi*, ZHANG Yong, YIN Ling National Engineering Research Center of Chemical Fertilizer Catalysts, Fuzhou University, Fuzhou 350002, China

Abstract:

Three kinds of CuFe2O4 catalysts were synthesized by co-precipitation method using potassium hydroxide (A), sodium

carbonate (B) and sodium bicarbonate (C) as the precipitants. Their catalytic activity and thermal stability were evaluated in water-gas shift reaction (WGSR). The microstructure and surface property of as-prepared catalysts was investigated by X-ray diffraction (XRD), N2-physisorption, H2-temperature programmed reduction (H2-TPR), CO2-temperature programmed desorption (CO2-TPD) and cyclic voltammetry (CV). The results show that the catalyst prepared with potassium hydroxide as precipitant exhibits excellent WGSR activity. Potassium hydroxide plays an important role in promoting the generation of CuFe2O4, restraining growth of crystalline CuO and CuFe2O4, resulting in much better dispersion of CuO on the surface of catalysts, enhancing the reducibility of catalysts, and increasing the amount of weak basic sites. These factors remarkably improve the activity and thermal stability of catalysts. Key words: water-gas shift reaction; CuFe2O4 catalysts; reducibility; weak basic sites; thermal stability

Recently, hydrogen has been considered to be one of the promising new energy due to its unique property[1]. Fuel cells can produce electricity directly from chemical energy by consuming fuel (generally hydrogen) and oxygen[2]. Nearly 95% of hydrogen production was mainly based on the steam reforming of crude oil, coal, natural gas, organic wastes, and biomass[3]. However, the reformed fuel contains 1%–10% of CO, which may degrade the performance of Pt electrode in fuel cell systems. Thus, it is indispensable to purify hydrogen to satisfy the requirement of fuel cell systems. Generally, WGSR and CO oxidation are considered as a crucial process to reduce the CO concentration[3]. The WGSR is usually carried out via two stages: a high-temperature stage (HTS) operated at 350–450°C and a low- temperature stage (LTS) operated at 200–250°C. The CO concentration is reduced to 3%–5% using a Fe-Cr catalyst for HTS[4] and to less than 1% using Cu/ZnO/Al2O3 catalysts for LTS[5]. Copper is known as a highly active metal for H2O dissociation as well as CO oxidation[6]. The copper-based catalysts, such as Cu/ZnO[7], Cu/ZnO/Al2O3[8], and Cu/CeO2[9,10], have aroused great attention of scientists. In principle, the combination of two metals in an oxide matrix can achieve a superior performance activity[11]. The ferrites with the formula MFe2O4 have received much attention because of their unique properties and multiple applications. It is well known that CuFe2O4 belongs to a membership of “inverse spinel”

group. Copper atoms sit predominantly on octahedral sites and iron atoms split between the two[12]. It has been reported that CuFe2O4 shows good activity and stability in WGSR[12], DME reforming[11], CO oxidation[13], and removal of NOx[14]. Tsai et al[15] reported that an Al-Cu-Fe quasicrystal showed excellent performance for SEM. It was attributed to the immiscible interaction between Cu and Fe or Fe oxide. Yang et al[16] have successfully obtained nano-sized copper (~3.6 nm), which derived from the thermal hydrogen reduction of CuFe2O4 nano-crystals. It was approximately close to 100% conversion of methanol in steam reforming at 240°C. Kameoka et al[17] studied the tetragonal CuFe2O4 reduced by H2 flow, which exhibited high catalytic performance and thermal stability resulting from the immiscible interaction between copper and iron (or iron oxides). However, the investigation upon the catalytic activity and stability of CuFe2O4 catalysts in WGSR has almost been limited. In this work, a series of CuFe2O4 catalysts were prepared via co-precipitation method using various precipitants including potassium hydroxide (A), sodium carbonate (B) and sodium bicarbonate (C). Their physico-chemical and reducibility properties, which strongly affect the activity and stability of catalysts in WGSR, were investigated.

1

Experimental

Received: 21-May-2014; Revised: 18-July-2014. * Corresponding author. LIN Xing-yi, E-mail: [email protected]; Tel: +86-591-83731234. Foundation item: Supported by National Engineering Research Center of Chemical Fertilizer Catalysts, Fuzhou University. Copyright  2014, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

LIN Xing-yi et al. / Journal of Fuel Chemistry and Technology, 2014, 42(9): 10871092

Fig. 1 Activity and thermal stability test in water-gas shift reaction activity test of all the CuFe2O4 catalysts (a); thermal stability evaluation of CF-A (b) reaction conditions: catalyst 2.5 g, feed gas (10% CO, 60% H2, 12% CO2, balance N2 ), the ratio of steam to feed gas is maintain at 1:1, GHSV = 4750 h–1 (a): : equilibirium; : CF-A; : CF-B; : CF-C; : CuO; : Fe2O3; : commercial Fe-Cr

Fig. 2 XRD patterns of all the fresh CuFe2O4 catalysts

Fig. 3 XRD patterns of all the CuFe2O4 catalysts after reduction

a: CF-A; a'': CF-A''; b: CF-B; c: CF-C; d: CuO; e: Fe2O3

with the reactant gas at 200°C for 1 h a: CF-A; b: CF-B; c: CF-C; d: CuO; e: Fe2O3

1.1

Preparation

Three CuFe2O4 catalysts were prepared via co-precipitation method with various precipitants including potassium hydroxide (A), sodium carbonate (B) and sodium bicarbonate (C). A constant atom ratio value of copper and iron was fixed as 1:4. The precursor salts, Cu(NO3)2·3H2O and Fe(NO3)3·9H2O, were dissolved in de-ionized water to obtain the composited salt solution. Subsequently, the composited salt solution and an appropriate amount of precipitant solution were simultaneously dropped into the volumetric flask via a peristaltic pump under vigorous stirring. In this process, we maintained the pH value and temperature of the solution at 10±1 and 80°C, respectively. Then the precipitate was aged for another 4 h at the same temperature. The resulting precipitate was centrifuged to absolutely remove remaining alkali ions. The hydroxide precursor was dried at 110°C for 12 h in an oven, and then calcined at 650°C for 4 h at 5°C/min in air. Finally, the roasted catalysts were crushed and sieved.

The above as-prepared catalysts were marked as CF-A, CF-B, CF-C, respectively. For comparison, CuO and Fe2O3 were also synthesized by the same method with potassium hydroxide as a precipitant. 1.2

Characterization

The BET surface area and pore volume were measured by using nitrogen as adsorption gas at 77 K on a Micrometrics ASAP 2020 instrument. Pore volumes were obtained using Barret-Joiner-Halenda (BJH) method. XRD patterns were recorded by a PANalytical X’pert Pro diffractometer equipped with Co K ( = 0.1789 nm) radiation operating at 40 KV and 40 mV with 2 rang from 10 to 80. The crystalline size was calculated by Scherrer equation. For H2-TPR experiments, 50 mg of each sample was purged with high purity helium gas at 200°C for 1 h, cooled down to room temperature, and then switched to 10% H2/Ar (30 mL/min) heated at 5°C/min from room temperature to 300°C. The outlet gas was

LIN Xing-yi et al. / Journal of Fuel Chemistry and Technology, 2014, 42(9): 10871092

monitored by a Pfeiffer Omnistar mass spectrometer. For CO2-TPD, 50 mg of each sample was pre-reduced at 250°C for 1 h with a gas flow of 10% H2/Ar (50 mL/min), then CO2 was introduced into the reactor for 1 h after cooling down to 50°C. Finally, the samples were heated at 10°C/min from 50 to 420°C after removing physically adsorbed CO2 under high purity helium gas. The cyclic voltammetry (CV) spectra were recorded as described previously by our group[6]. About 5 mg of catalysts and 80 L Nafion solution were dispersed in 1 mL ethanol-water solution (volume ratio of water and ethanol was 4:1) with sonication to obtain a homogenous ink. Then 5 L catalyst ink was transferred with an injector to clean conductive glass electrode. After ethanol volatilization, the electrode was heated by infrared lamp for 10 min. Generally, conductive glass electrode coated with catalysts was used as the working electrode, and Pt foil and Ag/AgCl were used as the counter and reference electrode, respectively. Sodium sulfate (0.2 mol/L) was used as the electrolyte solution. Prior to the measurement, the electrolyte solution was degassed by bubbling with high pure argon. The electrodes were cycled from +1 to –1 V at a scan rate of 0.1 V/s. 1.3

Test of catalytic performance

The activity and stability of the catalysts in WGSR were performed in a fixed-bed reactor under atmospheric pressure. 2.5 g of catalysts (20–40 mesh) was placed between two layers of quartz granules inside a stainless steel tube (12 mm). The reaction temperature was controlled by a thermocouple, which was located near the center of the catalyst bed. The experiment was performed with a feed gas (10% CO, 60% H2, 12% CO2, and balance N2) flowing at 78.2 mL/min. The ratio of vapor to feed gas was maintained at 1:1 (GHSV = 4750 h–1). The catalysts were pretreated by feed gas (20 mL/min) at 500°C for 10 h and then cooled down to 200°C prior to the activity test. The activity was evaluated at elevated temperatures from 200 to 400°C. The outlet gas passed through a condenser to remove residual water before entering a gas chromatography equipped with a thermal conductivity detector. The activity was guided by the conversion of CO, defined as: xCO(%) = (1–CO,out/CO,in) / (1 + CO,out) ×100%[18] (1)

Where, CO,in and CO,out are the inlet and outlet content of CO (dry base), respectively. In addition, the CF-A was maintained at 250°C for 40 h to further investigate thermal stability under the same experimental condition.

2 2.1

Results and discussion Activity and stability test of catalysts

Figure 1(a) shows the CO conversion versus reaction temperature over CuFe2O4 catalysts operated at 200–400°C and GHSV = 4750 h–1. In addition, the WGSR activity of CuO and Fe2O3 are investigated. For comparison, we also investigated the catalytic activity of commercial Fe-Cr catalyst under the same WGSR condition. CO conversion over CuO and Fe2O3 are quite low, indicating very low catalytic activity for WGSR. Meanwhile, the WGSR activity of commercial Fe-Cr is much lower than that of all the CuFe2O4 catalysts. For all the CuFe2O4 catalysts, the CO conversion increases with increasing reaction temperature. The sequence of the initial activity upon CuFe2O4 catalysts is CF-A > CF-B > CF-C. In addition, the CF-A was maintained at 250°C for 40 h to further investigate thermal stability under the same experimental condition. As shown in Figure 1(b), CO conversion nearly remains a certain constant value during the whole process. In summary, the CF-A has high initial activity and thermal stability in WGSR. 2.2

Structure of catalysts

Figure 2 shows the XRD patterns of the CuFe2O4 catalysts as well as those of CuO and Fe2O3 for comparison. The diffraction peaks ascribed to Fe2O3 (JCPDS 1-089-0596) as well as indexed to CuFe2O4 (JCPDS 0-034-0425) are observed. There are little diffraction peaks of crystalline CuO. This may suggest that most of Cu species, which incorporate into CuFe2O4 lattice or present an amorphous phase on the surface of catalysts, are beyond detection of XRD. The crystalline sizes of CuFe2O4 and Fe2O3 are calculated by Scherrer equation and the results are presented in Table 1. It is worth to note that the crystalline sizes of CuFe2O4 (27.4 nm) and Fe2O3 (49.6 nm) on CF-A are much smaller than those of the others.

Table 1 Physico-chemical property of all the catalysts

a

Catalyst

ABET/(m2·g–1)

vp/(cm3·g–1)

dp/nm

CF-A

28

0.18

CF-B

12

CF-C

9

Fe2O3 b

25

27.4

49.6

0.09

23

36.4

52.3

0.06

18

34.4

52.3

calculated by the half-height width of the diffraction peak at 21.2° in Figure 2;

b

Crystalline size d/nm CuFe2O4 a

calculated by the half-height width of the diffraction peak at 38.6° in Figure 2

LIN Xing-yi et al. / Journal of Fuel Chemistry and Technology, 2014, 42(9): 10871092

the CuFe2O4 catalysts. The CF-A sample presents much larger BET surface area (28 m2/g) and pore volume (0.18 cm3/g) compared with the others. It promotes the dispersion of active Cu species on the surface of catalysts. 2.4

Fig. 4 H2-TPR profiles of all the CuFe2O4 catalysts H2 MS signal: a: CF-A; b: CF-B; c: CF-C; H2O MS signal: a': CF-A; b': CF-B; c': CF-C

Fig. 5 CO2-TPD profiles of all the CuFe2O4 catalysts

To further figure out the reduction temperature of different phases, the especial experiments are designed as below. The CF-A is reduced by feed gas at 165°C for 1 h and marked as CF-A" in Figure 2. Compared with CF-A, the diffraction peak of CuFe2O4 (101) at 21.4 begins to disappear. Meanwhile, the negligible diffraction peak of Cu (111) at 50.8 is observed. It could be inferred that CuFe2O4 may be initially reduced to Cu at 165°C. Figure 3 shows XRD patterns of the CuFe2O4 catalysts as well as those of CuO and Fe2O3 for comparison after reduction with the reactant gas at 200°C for 1 h. Under the present conditions, two diffraction peaks at 50.4 and 59.3 are attributed to the transition of CuO to metallic Cu (JCPDS 01-003-1015), respectively. Meanwhile, Fe2O3 is reduced to Fe3O4 (JCPDS 01-075-1609). For all the CuFe2O4 catalysts, the peaks indexed to CuFe2O4 and Fe2O3 disappear after exposure to the reactant gas, and these ascribed to metallic Cu and Fe3O4 occurs imultaneously. 2.3

N2-physisorption

Table 1 summarizes the results of N2-physisorption upon

H2-TPR

The reducibility of catalysts was investigated by H2-TPR. As described in Figure 4, both the consumption of H2 and production of H2O are monitored simultaneously by online mass spectrometry. According to the report[19], the monoclinic CuO shows one board reduction peak at 280°C, which is ascribed to the reduction of CuO to Cu. While Fe2O3 shows one small peak at 420°C, which is assigned to the transition of Fe2O3 to Fe3O4[20]. For all the CuFe2O4 catalysts, a mild H2 consumption peak at 145°C and an overlapping peak at 165°C are observed. In addition, a negligible H2 consumption peak at 220°C also occurs. Yang et al[16] reported that CuFe2O4 presented one reduction peak at 187C, which is related to the reduction of CuFe2O4 to Cu and Fe2O3, then the hematite Fe2O3 was rapidly reduced to magnetite Fe3O4[11]. According to XRD analysis, the overlapping peak at 165°C can be responsible for the reduction of CuFe2O4. As is known, the highly dispersed CuO can easily be reduced at low temperature. Thus, the mild peak at 145°C can be assigned to the reduction of highly dispersed CuO. Particularly, CF-A shows the lowest reduction temperature among all the catalysts due to its higher BET surface area and smaller crystalline size. In addition, CF-A shows the highest amount of H2 consumption i.e., the largest amount of reductive Cu species. This may suggest that much more active CuFe2O4 is generated on CF-A, resulting in a good interaction between copper and iron oxides on CF-A and improving the reducibility. 2.5

CO2-TPD

Fig. 6 Cyclic voltammograms of all the CuFe2O4 catalysts in 0.2 mol/L Na2SO4 electrolyte at a scan rate of 0.1 V/s a: CF-A; b: CF-B; c: CF-C

LIN Xing-yi et al. / Journal of Fuel Chemistry and Technology, 2014, 42(9): 10871092

It is known that basicity has an effect on the catalytic performance in WGSR based on associative mechanism[19]. In this work, CO2-TPD measurement is employed to investigate the basicity of catalysts and the results are exhibited in Figure 5. All the catalysts show a main peak at low temperature (95°C) and a board peak at high temperature (325°C). Li et al[10] attributed the low-temperature peak to the CO2 desorption of monodentate carbonate formed on weak basic sites, while the high-temperature peak might be assigned to the CO2 desorption of bidentate carbonate formed on strong basic sites. The bidentate carbonate presented a higher thermal stability than the monodentate carbonate because of the strong binding between carbonate and oxide surface[19]. The intensity of low-temperature CO2 desorption peak is much larger for CF-A than that of the others. It is indicated that larger amount of weak basic sites were formed on CF-A. Generally, the bidentate carbonate is considered as occupying active sites, while the monodentate carbonate can decompose to regenerate active sites to promote H2O dissociation and CO activation in the WGS reaction[21]. Thus, CF-A presents the highest activity in WGSR.

lowest reduction temperature, and the largest amount of weak basic sites. These factors remarkably improved the activity and thermal stability of the catalyst.

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2.6

Cyclic voltammetry (CV)

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3

Conclusions

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Three CuFe2O4 catalysts were prepared by co-precipitation method with various precipitants. Their microstructure, physico-chemical property, and reducibility were investigated by XRD, N2-physisorption, H2-TPR, CO2-TPD and CV measurements. The WGSR activity of commercial Fe-Cr was much lower than that of all the CuFe2O4 catalysts under the same experimental conditions. In addition, the catalyst prepared with potassium hydroxide as a precipitant exhibited excellent WGSR activity. To take all above involved factors into consideration, we could draw a conclusion that CF-A had the largest BET surface and pore volume, the smallest crystalline size, the largest amount of copper species, the

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