SiO2 Catalyst for Hydrogen Production from Steam

Jul 2, 2010 - Catalytic steam reforming of ethanol has been regarded as a promising way to produce hydrogen. However, catalytic deactivation is a key ...
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Environ. Sci. Technol. 2010, 44, 5993–5998

A Novel Nano-Ni/SiO2 Catalyst for Hydrogen Production from Steam Reforming of Ethanol CHUNFEI WU AND PAUL T. WILLIAMS* Energy & Resources Research Institute, The University of Leeds, Leeds LS2 9JT, United Kingdom

Received March 31, 2010. Revised manuscript received June 14, 2010. Accepted June 15, 2010.

Catalytic steam reforming of ethanol has been regarded as a promising way to produce hydrogen. However, catalytic deactivation is a key problem in the process. In this paper, a novel nano-Ni/SiO2 catalyst was prepared by a simple sol-gel method and compared to catalysts prepared by an impregnation method in relation to the steam reforming ethanol process. Good Ni dispersion and high BET surface areas (>700 m2 g-1) were obtained for sol-gel catalysts, whereas only 1 m2 g-1 surface area was obtained for the Ni/SiO2 impregnation catalyst. The results of catalytic steam reforming of ethanol showed that about twice of the hydrogen production was produced with the Ni/SiO2 catalyst prepared by sol-gel (around 0.2 g h-1) compared with that prepared by impregnation (around 0.1 g h-1). The analysis of the used catalysts showed that 10Ni/SiO2-B and 20Ni/SiO2-B presented the highest stability, while other catalysts were fragmented into small pieces after the reforming process, especially the catalysts prepared by impregnation. A novel catalyst has been produced that has been shown to be effective in the production of hydrogen from the steam reforming of ethanol.

1. Introduction As a clean and efficient energy, hydrogen is predicted to play an important role in future energy systems (1, 2). However, about 96% of H2 is currently produced from fossil fuels, and alternative sustainable sources are being explored. Ethanol has been proposed to be a promising candidate (3, 4) because ethanol is of renewable origin and has a high content of H2. During the catalytic steam reforming of ethanol, the catalyst is believed to be determinative for maximum hydrogen production. Much research has been carried out to find a suitable catalyst with high stability and good selectivity to H2 (5, 6). Nickel-based catalysts have been ideally used and researched for the gasification process because of their good catalytic effect in relation to hydrogen production and comparatively low cost. Currently, the wet impregnation method has been extensively used for the preparation of Ni catalysts in the pyrolysis-gasification process due to its simple operation. However, the wet impregnation method normally produces a catalyst with a broad Ni size distribution. The poor dispersion of Ni metal in the catalyst would more readily result in catalyst deactivation. Therefore, the control of active sites of the Ni metal through dispersion and small Ni size is advantageous. Other metals such as Au (7) or Cu (8) have * Corresponding author phone: #44 1133432504; e-mail: [email protected]. 10.1021/es100912w

 2010 American Chemical Society

Published on Web 07/02/2010

been added to increase Ni dispersion, and in addition, different catalyst supports (9, 10) were also explored to improve the performance of Ni catalysts. The sol-gel method has been used to produce nanomaterials, where the metal in the catalyst could be dispersed at high homogeneity (11). Ni/SiO2 materials prepared by the sol-gel method have also been reported for the preparation of magnetic materials (12, 13). However, to our best knowledge, the Ni/SiO2 catalyst prepared through sol-gel chemistry has been rarely applied in the gasification process for hydrogen production. In this paper, Ni/SiO2 catalysts were prepared by sol-gel and wet impregnation methods. The advantage of Ni/SiO2 catalysts based on the sol-gel method was investigated in relation to the production of hydrogen from the ethanol reforming process.

2. Experimental Section 2.1. Catalyst Preparation. Ni/SiO2 catalysts with different Ni contents of 10, 20, and 50 wt % were prepared by wet impregnation. During catalyst preparation, different amounts of Ni(NO3)2•6H2O (Sigma-Aldrich) was dissolved in deionized water and mixed with SiO2 (quartz) (Sigma-Aldrich). The precursor was dried at 105 °C overnight and calcined at 500 °C in an air atmosphere for 3 h. The three Ni/SiO2 wet impregnation catalysts were assigned as 10Ni/SiO2-A, 20Ni/ SiO2-A, and 50Ni/SiO2-A. Three Ni/SiO2 catalysts with Ni contents of 10, 20, and 50 wt % were also prepared by a simple sol-gel method adapted from the literature (14). Ni(NO3)2•6H2O, citric acid, deionized water, absolute ethanol, and tetraethyl silicate (TEOS) were used as raw materials. A total of 0.01 mol of Ni(NO3)2•6H2O and 0.015 mol of citric acid were first dissolved into 100 mL absolute ethanol and stirred at 60 °C for 4 h. Then, 60 mL of absolute ethanol was added together with deionized water (water/TEOS molar ratio, 3:1). A different amount of TEOS was dropped into the solution to obtain the required Si:Ni ratio and different Ni content in each catalyst. After drying at 80 °C overnight, the precursor was calcined at 450 °C in an air atmosphere for 3 h. The three Ni catalysts prepared by the sol-gel method were assigned as 10Ni/SiO2-B, 20Ni/ SiO2-B, and 50Ni/SiO2-B. All the catalysts used in this paper were crushed and sieved to granules at a size between 0.065 and 0.212 mm. All the catalysts used were reduced in 5 vol % H2 (balanced with N2) gases for 1.5 h at a temperature of 600 °C and further for 1.5 h at a temperature of 700 °C. 2.2. Catalyst Characterization. The BET surface area and porosity of the prepared catalysts were determined using a Micromeritics TriStar 3000 apparatus. The temperature-programmed oxidation (TPO) of used catalysts (around 100 mg) was carried out using a thermogravimetric analyzer (TGA) in an atmosphere of air heated at 15 °C min-1 to a final temperature of 800 °C, with a dwell time of 10 min. The differential thermogravimetry (DTG) results from the experiment of TPO were also obtained. High-resolution scanning electron microscopy (SEM, LEO 1530) coupled to an energy dispersive X-ray spectrometer (EDXS) and transmission electron microscopy (TEM) (Philips CM200) was used to characterize and examine the characteristics of the carbon deposited on the used catalysts. 2.3. Catalyst Testing for Steam Reforming of Ethanol. The prepared catalysts were tested for hydrogen production from the steam reforming of ethanol with a two-stage reaction system. The mixture of water and ethanol was injected with a syringe pump to the first reactor, where the feedstock was VOL. 44, NO. 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. N2 adsorption and desorption isotherms for the selected prepared catalysts.

TABLE 1. Surface Properties of Prepared Catalysts catalyst

Ni content (wt%)

BET surface Area (m2 g-1)

micropore volume (cm3 g-1)

total volume (cm3 g-1)

average pore size (nm)

0Ni/SiO2-A 10Ni/SiO2-A 20Ni/SiO2-A 50Ni/SiO2-A 10Ni/SiO2-B 20Ni/SiO2-B 50Ni/SiO2-B

0 10 20 50 10 20 50

0.8 0.6 1.6 729.4 667.7 357.7

0.0004 0.0004 0.001 0.380 0.350 0.250

0.002 0.002 0.003 0.430 0.420 0.500

50.8 10.5 12.9 10.4 3.1 3.4 6.5

preheated. The derived vapors were catalytically reformed in the second reactor where the catalyst was placed. The derived gaseous products passed through an air condenser and a dry ice condenser. The noncondensed gases were collected with a Tedlar gas sample bag. During each experiment, around 30 g of a mixture of water and ethanol (molar ratio of 4) started to be injected with a flow rate of 4.94 g h-1 to the reaction system, when the temperature of the first reactor and the second reactor stabilized at 190 and 600 °C, respectively. About 0.3 g of catalyst was used in the second reactor. The gas sample was collected at five different reaction times for each experiment. N2 gas was used as carrier gas with a flow rate of 80 mL min-1. Weight hourly space velocity, calculated by the ratio of the feedstock feed per hour divided by the weight of catalyst, was about 16.5 h-1 for each experiment. The gases collected in the sample bag were analyzed offline by packed column gas chromatography (GC) as described in our previous work (13). The liquids condensed in the condensers were regarded as nonconverted products. The liquid product yield was used to predict the catalytic activity of the catalyst for the steam reforming of ethanol. The liquid yield was calculated by the mass of condensed products divided by the weight of injected sample.

3. Results and Discussion 3.1. Characterization of Fresh Catalysts. 3.1.1. Surface Analysis of Fresh Catalysts. Surface properties of the prepared catalysts are shown in Table 1. From Table 1, surface areas and pore volumes for the catalysts obtained by the sol-gel method were much higher than those prepared by wet impregnation. For example, more than 700 m2 g-1 of BET surface area was obtained for the 10Ni/SiO2-B catalyst; 5994

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however, only 0.8 m2 g-1 of BET surface area was shown for the 10Ni/SiO2-A catalyst. In addition, Table 1 shows that the surface area was reduced with an increase of Ni content in the catalysts prepared by the sol-gel method. N2 absorption and desorption isotherms for selected catalysts are shown in Figure 1. The absorption type for the 20Ni/SiO2-B catalyst seems to be a combination of type Ι and IV, IUPAC classification (15). However, 20Ni/SiO2-A was similar to a type III isotherm, in which adsorbent-adsorbate interactions are very weak (16). In addition, combined with the result from adsorption cumulative pore area (not shown here), a quite narrow pore distribution was obtained for the Ni/SiO2-B catalyst; thus, we might conclude that a mesostructured catalyst was prepared by the sol-gel method. Because of the good catalytic performance of high surface area mesostructured catalysts (17), the catalyst prepared by the sol-gel method in this paper was expected to have a high catalytic activity and stability for the steam reforming of ethanol. 3.1.2. SEM and TEM Analysis for Fresh Reduced Catalysts. SEM micrographs of the prepared Ni catalysts indicated a comparatively smooth surface for the Ni/SiO2-B catalyst, while nonuniform dispersion of Ni particles on the surface of the Ni/SiO2-A catalyst was found. Therefore, it is suggested that poor Ni dispersion was obtained for the catalyst prepared by the wet impregnation method compared to that of the sol-gel method. Further details of the fresh catalyst surface were investigated by TEM analysis, and the results are shown in Figure 2. Nonuniform dispersion of Ni particles was obtained for the catalyst prepared by the wet impregnation method, whereas good dispersion of Ni particles was observed for the catalysts prepared by the sol-gel method. Furthermore, the Ni particle size seemed increase with an increase in Ni content

FIGURE 2. TEM results of selected reduced catalysts.

TABLE 2. Gas and Hydrogen Yields from Steam Reforming of Ethanol with Ni/SiO2-A Catalysts Catalyst 10Ni/SiO2-A

20Ni/SiO2-A

50Ni/SiO2-A

reaction time

gas yield (g h-1)

H2 yield (g h-1)

gas yield (g h-1)

H2 yield (g h-1)

gas yield (g h-1)

H2 yield (g h-1)

10 35 95 155 215 total liquid production

0.74 0.86 0.54 0.23 0.17

0.08 0.09 0.06 0.02 0.02

0.94 0.86 0.77 0.70 0.67

0.10 0.09 0.08 0.07 0.07

1.06 1.18 1.20 1.15 1.13

0.11 0.12 0.13 0.12 0.11

84.9

77.1

67.6

TABLE 3. Gas and Hydrogen Yields from Steam Reforming of Ethanol with Ni/SiO2-B catalysts Catalyst 10Ni/SiO2-B

20Ni/SiO2-B

50Ni/SiO2-B

reaction time

gas yield (g h-1)

H2 yield (g h-1)

gas yield (g h-1)

H2 yield (g h-1)

gas yield (g h-1)

H2 yield (g h-1)

10 35 95 155 215 total liquid production

1.48 1.69 1.55 1.52 1.61

0.18 0.18 0.16 0.15 0.16

2.21 2.25 2.37 2.00 2.04

0.23 0.23 0.24 0.24 0.20

2.03 1.96 1.99 2.08 2.12

0.22 0.22 0.22 0.22 0.22

56.5

in the catalyst prepared by the sol-gel method. In addition, the dispersion of Ni/NiO seems to not be influenced after the reduction of the catalysts prepared by sol-gel because particles size of NiO in the nonreduced 20Ni/SiO2-B was close to the Ni size in the reduced 20Ni/SiO2-B catalyst (Figure 2 and Figure S5 of the Supporting Information).

41.6

47.0

3.2. Catalytic Test for Steam Reforming of Ethanol. 3.2.1. Gas and Hydrogen Yields from Catalytic Steam Reforming of Ethanol. Gas and hydrogen yields at various reaction times are presented in Tables 2 and Table 3. In addition, total liquid production is also presented in Tables 2 and 3, in order to evaluate the conversion of feedstock. VOL. 44, NO. 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Gas concentrations for the steam reforming of ethanol with the 20Ni/SiO2-A, 20Ni/SiO2-B, and 50Ni/SiO2-B.

FIGURE 4. DTG-TPO results for the used catalysts. The catalytic activity of Ni/SiO2-A catalyst increased with increasing Ni content (Table 2). However, increasing the Ni content from 10 to 50 wt % in the sol-gel Ni/SiO2-B catalyst resulted in the highest catalytic activity at 20 wt % of Ni content (Table 3). From Table 2, an obvious reduction of gas and hydrogen yields was obtained with an increase in reaction time for the 10Ni/SiO2-A and 20Ni/SiO2-A catalysts, respectively. It is suggested that these two catalysts (10Ni/SiO2-A and 20Ni/ SiO2-A) might be easily deactivated. However, the 50Ni/SiO2-A catalyst showed a stable catalytic activity according to the gas and hydrogen yields. From Table 3, the Ni/SiO2-B catalyst prepared by the sol-gel method seems to present a stable catalytic activity during the steam reforming of ethanol. 3.2.2. Gas Concentrations from Catalytic Steam Reforming of Ethanol. Gas concentrations for CO, H2, CO2, CH4, and for C2-C4 hydrocarbon gases at different reaction times are presented in Figure 3. The results show that the Ni/SiO2-B catalyst generated more stable gas concentrations compared 5996

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to the wet impregnation prepared Ni/SiO2-A catalyst. In addition, a higher concentration of H2 (around 70 vol %) was obtained for the catalyst prepared by sol-gel compared with the catalyst prepared by wet impregnation (around 60 vol %). 3.3. Characterization of Coked Catalysts. 3.3.1. TPO Results of Coked Catalysts. Coke deposition on the used catalysts was investigated by TPO experimentation. DTGTPO results are shown in Figure 4. According to our previous research (10, 18), TPO experiments of coked catalyst might include three main stages: water vaporization (at around 100 °C), Ni phase oxidation (at around 350 °C), and carbon combustion (after 400 °C). As shown in Figure 4, the three stages of oxidation seemed to be applicable in the used catalysts discussed in this paper. The amount of coke deposited on the catalyst is one of the most important parameters to evaluate the catalyst stability (19, 20), although the type of deposited carbon might play a more important role for catalytic performance. The

FIGURE 5. TEM results for the selected coked catalysts. amount of coke deposition was calculated from the weight loss of coked catalyst after 400 °C divided by the used sample weight after the TPO experiment. The results showed that the amount of coke deposited on the reacted wet impregnation 10Ni/SiO2-A, 20Ni/SiO2-A, and 50Ni/SiO2-A catalysts was 27.4, 68.3, and >76.2 wt %, respectively. By comparison, the amount of coke on the reacted sol-gel prepared Ni/SiO2-B catalysts was 2.0, 3.4, and 110.9 wt % for the 10Ni/SiO2-B, 20Ni/SiO2-B, and 50Ni/SiO2-B, catalysts, respectively (Figure 4). It is suggested that the sol-gel prepared 10Ni/SiO2-B and 20Ni/SiO2-B catalysts showed good stability for about 220 min of reaction time for the steam reforming of ethanol. About 5.5 mg of carbon were deposited on 1 g of 10Ni/SiO2-B catalyst per hour in this paper; the carbon deposition rate is lower than reported work (21), where a nanocompound Ni-Mg-Al catalyst was applied for the steam reforming of ethanol. Contrary to the performance of the 10 and 20 wt % Ni/ SiO2-B sol-gel catalysts, the catalyst prepared by the sol-gel method with a Ni content of 50 wt % showed poor stability (high coke deposition). This might be due to the comparatively low surface area (Table 1) and the poor Ni dispersion (irregular Ni particles are observed in Figure 2) compared with the 10 and 20 wt % of Ni catalysts. DTG-TPO results showed that there might be at least two types of carbons deposited on the used catalyst after steam reforming of ethanol due to the several oxidation peaks observed in Figure 4. It is suggested that two types of carbons, layered and filamentous, might be deposited on the catalyst during the gasification process (10, 19). Oxidation of layered carbon was suggested to start at around 500 °C, while filamentous carbon oxidation might start from around 600 °C. From Figure 4, coke oxidation was observed after 500 °C. However, the main oxidation peak was found after 600 °C. Therefore, it is suggested that most of the coke deposited on

the used catalysts might be filamentous carbons. In addition, very small amounts of filamentous carbons could be deposited on the reacted 10Ni/SiO2-B and 20NiSiO2-B catalysts due to their low carbon oxidation peaks shown in Figure 4. 3.3.2. SEM and TEM Results of Used Catalysts. SEM and TEM analysis of the used catalysts derived from the catalytic steam reforming of ethanol was investigated, and the TEM results are shown in Figure 5. Low magnification SEM micrographs (Figure S3 of the Supporting Information) of the used catalysts showed that the wet impregnation Ni/ SiO2-A catalysts were reduced to small fragments after steam reforming; however, the Ni/SiO2-B catalyst prepared by the sol-gel method showed a higher stability according to the SEM results with less fragmentation. The low magnification SEM micrographs (Figure S3 of the Supporting Information) showed that numerous small fragmented particles were found for the reacted higher Ni content 50Ni/SiO2-B catalyst, while almost no fragmented particles were observed from the lower Ni content reacted 10Ni/SiO2-B and 20NiSiO2-B catalysts. Thus, a lower catalytic activity (Table 3) and lower stability (higher coke deposition, Figure 4) of the 50Ni/SiO2-B catalyst was confirmed by the SEM analysis. A formation mechanism for coke deposited on Ni catalysts during gasification has been reported by Wu and Williams (22). They suggested that the fragmentation of a catalyst during the gasification process might be originated from reactions inside the catalyst where carbons are initially formed and developed. As shown in Figure 5, Ni crystals and filamentous carbon could be observed for all the investigated catalysts. Different sizes of Ni crystals were also found throughout the micrograph (Figure 5). A larger than 100 nm size of filamentous carbon was observed on the used wet impregnation 10Ni/ SiO2-A catalyst, whereas a uniform size (about 14 nm) of filamentous carbons was observed on the used sol-gel 10Ni/ SiO2-B catalyst. VOL. 44, NO. 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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It is known that catalyst deactivation is mostly controlled by coke deposition and sintering (21). From the TEM analysis, sintering seemed to be a problem for the investigated catalysts especially those prepared by wet impregnation. As shown in Figure 5, several large particles could be found on the TEM micrograph of the 20Ni/SiO2-B catalyst, while more nonuniform Ni particles were observed for the higher Ni content 50Ni/SiO2-B catalyst. The 10Ni/SiO2-B catalyst is suggested to be more stable due to its good dispersion of Ni. However, the size of the Ni particles in the 10Ni/SiO2-B catalyst seemed to be slightly increased after the ethanol reforming process. The size of the Ni particles in the original 10Ni/SiO2-B catalyst was about 8 nm and changed to around 14 nm after the reforming process (Figure 2 and Figure 5). Therefore, the dispersion of Ni was suggested to be reduced after the steam reforming process for the catalysts prepared by the sol-gel method. In this paper, catalysts prepared by the sol-gel method have shown better performance (higher hydrogen production and better stability) than those prepared by a wet impregnation method. Among the catalysts prepared by the sol-gel method, the higher Ni content 50Ni/SiO2-B catalyst showed the worst catalytic activity according to its stability (catalyst fraction, catalyst sintering, and coke deposition), although it gave high gas and hydrogen yields in the steam reforming of the ethanol process. The 20Ni/SiO2-B sol-gel prepared catalyst produced higher gas and hydrogen yields than the 10Ni/SiO2-B catalyst due to its higher Ni content; however, the sintering problem appeared to be more serious for the 20Ni/SiO2-B catalyst compared to the 10Ni/SiO2-B catalyst. Our results have shown significant improvement in hydrogen production of Ni-based catalysts prepared by the sol-gel method (10Ni/SiO2-B and 20Ni/SiO2-B) in relation to the steam reforming of ethanol. The results suggest that the modification of metal content, size, and dispersion of active sites such as Ni and the structure of the catalyst can be manipulated to optimize the production of hydrogen from ethanol. Other additives such as Cu, Mg, Ca, etc. could also be used in the sol-gel process to improve the catalyst performance.

Acknowledgments The authors are grateful for ORS (UK), IRS (Leeds) and EPSRC (EP/D053110/01) support.

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(14) (15)

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Supporting Information Available Schematic diagram of the reaction system and additional SEM, TEM, TGA-TPO results. This information is available free of charge via the Internet at http://pubs.acs.org.

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