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Hydrogenation of CO2 over nickel catalysts on rice husk ash-alumina prepared by incipient wetness impregnation
Applied Catalysis A: General 247 (2003) 309–320
Hydrogenation of CO2 over nickel catalysts on rice husk ash-alumina prepared by incipient wetness impregnation Feg-Wen Chang∗ , Maw-Suey Kuo, Ming-Tseh Tsay, Ming-Chung Hsieh Department of Chemical and Materials Engineering, National Central University, Chungli 32054, Taiwan Received 12 June 2002; received in revised form 6 November 2002; accepted 30 January 2003
Abstract Nickel catalysts supported on rice husk ash-alumina (Ni/RHA-Al2 O3 ) were prepared by the incipient wetness impregnation method. Characterizations were investigated by TPR, XPS, XRD, SEM, and BET techniques. The catalytic activities of nickel catalysts were tested by CO2 hydrogenation with H2 /CO2 ratio of 4:1 for temperatures between 400 and 800 ◦ C. The XPS analysis of Ni/RHA-Al2 O3 demonstrated the presence of spinel. The TPR analysis indicated that the interaction between nickel and support was strong and difficult to reduce with more than one nickel oxide compound, such as bulk NiO and two NiAl2 O4 -like species. The results of N2 -adsorption by BET indicated that the maximum surface area was found at 15 wt.% nickel loading with mesopores. The XRD analysis of samples suggested that the crystallite sizes of supported NiO increased with the increase in nickel loading. The CO2 conversion and CH4 yield for CO2 hydrogenation were found to depend on the nickel loading. The reaction temperature of 500 ◦ C might be the optimum temperature for CO2 hydrogenation to give the maximum yield and selectivity of CH4 . Furthermore, the hydrogenation tests showed that the performance of Ni/RHA-Al2 O3 is better than that of Ni/SiO2 -Al2 O3 . © 2003 Elsevier Science B.V. All rights reserved. Keywords: Rice husk ash-alumina support; Nickel catalyst; CO2 hydrogenation; Spinel; Incipient wetness impregnation
1. Introduction Supported nickel catalysts are widely applied in many important reactions, such as hydrogenation of aromatic hydrocarbon [1,2], methanation of CO2 [3–8], CO2 reforming of CH4 to syngas [9], ethylation and dealkylation [10], and methanation of coal synthesis. High surface area supports, usually oxides, are used extensively in industry for the preparation of metal catalysts including silica, alumina, silica-alumina, and zeolites. These catalysts are generally prepared by incipient wetness impregnation, ∗ Corresponding author. Tel.: +886-3-4227151x4202; fax: +886-3-4252296. E-mail address: [email protected] (F.-W. Chang).
ion exchange, and precipitation-deposition techniques with different precipitation agents [6–8,11,12]. In general, ion exchange technique is limited to catalysts with relatively low nickel loading, while incipient wetness impregnation technique is the simplest preparation procedure. Though the incipient wetness impregnation method does not result in the best dispersion of nickel on the support, easier control of loading and no liquid waste generation make this process the most preferred. In the chemical processing industry, a catalyst with high activity and selectivity is essential. To meet these requirements, the catalyst support of inert carrier should provide sufficient surface area for the metal to disperse. It has emerged that the support plays a very active role in the interaction between the nickel
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and the support. The nickel compounds on different support surfaces result in different extents of what are generally called “metal-support effects”. This implies that catalysts with different characteristics exhibit different performances toward activity and selectivity for a given process. Rice husk is a milling by-product of rice and also a major waste product of the agriculture industry. Nevertheless, it is a unique crop residue with uniform size and high content of ash (14–25 wt.%) [13]. The silica content of ash can be as high as 90–97% [13,14]. Amorphous silica, commonly referred to as rice husk ash, is obtained from rice husk by acid leaching, pyrolysis, and carbon-removing processes. The purity of silica in the product can reach as high as 99.9% with high specific surface area and high porosity [15–18]. These properties make the ash a valuable raw material for many uses. Of course, it can also be used for the support of catalysts. In our previous work [6–8,19], we have found that the rice husk ash-supported nickel catalyst used for hydrogenation of CO2 exhibits high selectivity. We concluded that the hydrogenation activity of nickel catalysts using rice husk ash as a support is better than that of those prepared by silica gel. In this study, nickel catalysts supported on rice husk ash-alumina were prepared by the incipient wetness impregnation method. The aim of the present work is to provide a basis for understanding the interrelationship between the physical and chemical properties of the nickel catalysts on rice husk ash-alumina and their reactivities on CO2 hydrogenation. A commercial silica-alumina was also chosen as another support for comparison.
2. Experimental 2.1. Raw material Rice husk obtained from a rice mill was first washed thoroughly with demineralized water to remove the adhering soil and dust and then dried at 110 ◦ C in an air oven. The dried rice husk was refluxed with 3N HCl in a round-bottomed glass flask at its boiling point, about 104 ◦ C, for 1 h. It was then washed 10 times with hot demineralized water and dried at 110 ◦ C for 24 h. The pyrolysis was carried out under a nitrogen atmosphere at 700 ◦ C for 1 h by placing the treated rice
husk in a quartz tubular reactor. After pyrolysis, the husk was further heated in an air furnace at 700 ◦ C for 1 h; the ash of husk obtained was more than 99% amorphous silica. This silica was used as one of the support materials for nickel catalysts and is designated hereafter as RHA. Aluminum nitrate (Merck, 99%) and nickel nitrate (Merck, 99%) were used for the preparation of the ash-Al2 O3 support and the catalyst, respectively. 2.2. Sample preparation Rice husk ash (designated as RHA; BET surface area: 222 m2 /g, average pore diameter: 57 Å, total pore volume: 0.35 cm3 /g) and aluminum nitrate were used in preparation of rice husk ash-alumina support (designated as RHA-Al2 O3 ). Following the simple impregnation to incipient wetness of the RHA with aqueous aluminum nitrate solution for 24 h, the sample was dried in an air oven at 110 ◦ C. The impregnated, dried sample was calcined in air at 350 ◦ C for 5 h at a heating rate of 2 ◦ C/min. This sample was then used as a support material for nickel catalysts. Nickel nitrate was used in preparation of supported nickel catalysts (designated as Ni/RHA-Al2 O3 ). The supported nickel catalysts were also prepared by simple impregnation to incipient wetness technique and dried as mentioned above. The dried samples were calcined in air at 500 ◦ C for 4 h at a heating rate of 2 ◦ C/min. Finally, the catalyst precursors were activated after being placed in the tubular reactor of reducing atmosphere in a H2 /Ar (5/95) stream at 800 ◦ C for 3 h at a heating rate of 10 ◦ C/min. The commercial SiO2 -Al2 O3 support (supplied by Aldrich Chemical Co., support grade, BET surface area: 539 m2 /g, average pore diameter: 42 Å, total pore volume: 0.76 cm3 /g) was used to make other samples by the same procedure for comparison (designated as Ni/SiO2 -Al2 O3 ). 2.3. Characterization The metallic impurities of rice husk ash were determined with an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Leeman Labs Model Leeman SP1000). Elemental analysis was carried out with an element analyzer (Carlo Erba, Model EA1108).
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Temperature programmed reduction (TPR) measurements were carried out in a HP5860A GC apparatus, equipped with a thermal conductivity detector (TCD). About 50 mg of catalysts were placed in the quartz reactor in a tubular furnace and heated from room temperature to 900 ◦ C at a heating rate of 10 ◦ C/min under a 40 ml/min H2 /Ar (5/95) flow stream. The consumption of H2 was detected and recorded. For X-ray photoelectron spectroscopy (XPS) analysis, the powder samples were put into a disk-shaped holder and lightly pressed to give a compact smooth surface. VG McroLab 310-D photoelectron spectrometer was used for the analysis, which scans of the Ni 2p spectral region. Primary excitation was achieved with Mg K␣ X-rays (1253.6 eV) and a multi-channel detector was used to detect the emitted photoelectrons. The spectrometer ran in the constant analyzer energy (CAE) mode. Data processing was performed using the Eclipse V2.0 data system running on an OS/2 Windows environment. The X-ray diffraction (XRD) patterns of samples were recorded over a Philips PW1820 goniometer with PW1710 processor diffractometer using graphite monochromater Cu K␣ radiation (λ1 = 1.5406 Å, λ2 = 1.5444 Å). Samples were scanned between 10 and 80◦ (2θ) at a scan step size of 0.02◦ and scan time constant of 2 s per step. The crystallite size (dXRD ) of the nickel oxide was determined by X-ray line broadening analysis (XLBA) for NiO (0 1 2) peaks according to the Scherrer formula: κλ dXRD = (1) β cos θ where κ, the particle shape factor, was taken as 0.94, and the wavelength λ was taken as 1.5418 Å. The measured half width ‘B’ was corrected for the instrumental line broadening ‘b’ using β = B − b. The instrumental line broadening ‘b’ was determined to be 0.055◦ using the silicon reference supplied by Rigaku. The scanning electron micrographs (SEM) were obtained by a HITACH S-800 scanning electron microscope. The specific surface area (BET, N2 ), pore volume, and pore size were measured by nitrogen adsorption– desorption with an analyzer (Model ASAP2010 mi-
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crometrics, USA). Prior to measurement, each sample was degassed at 200 ◦ C. 2.4. Hydrogenation of CO2 The gas-phase hydrogenation of CO2 to methane was carried out in a micro fixed-bed operation under atmospheric pressure. The design of the reactor and related apparatus has been described previously [6,7]. For each run, about 50 mg of catalysts were loaded into a quartz reactor of 4 mm i.d. and reduced in situ under a continuous flow of H2 /Ar (5/95) at the rate of 40 ml/min. After reduction, the reactor was adjusted to the desired reaction temperature under the same gas flow. When the temperature became stabilized, the flow rate of H2 /CO2 (4:1) mixture was adjusted to 25 ml/min and fed into the reaction system. All product gas concentrations were measured chromatographically using a thermal conductivity detector (TCD).
3. Results and discussion In order to avoid any effect of impurities on the chemical reaction, the rice husk was washed with hot demineralized water after acid treatment, and kept for several minutes between each wash to assure complete leaching of the impurities from the pores of rice husks. The analyses of metallic impurities and organic elements in the rice husk ashes are shown in Table 1. As can be seen, most of the metallic impurities and organic elements were removed after pretreatment, so the purity of RHA used in this work was >99.9%. We believe that the activity of catalysts will not be affected by the contaminants of RHA. Table 1 ICP-AES analysis and element analysis of rice husk ash ICP-AES analysis
Element analysis
Impurity
Concentration (ppm)
Impurity
Concentration (wt.%)
Na K Ca Fe Mg Mn
142 <400 142 180 84 48
C H
<0.01 0.11
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Fig. 1. TPR curves of Ni/RHA-Al2 O3 catalyst precursors with various nickel loading and unsupported NiO: (a) 2.5 wt.%; (b) 5.0 wt.%; (c) 10.0 wt.%; (d) 15.0 wt.%; (e) 25.0 wt.%; (f) unsupported NiO (ramp rate, 10 ◦ C/min; calcination temperature, 500 ◦ C).
Temperature-programmed reduction (TPR) was used to characterize the Ni/RHA-Al2 O3 catalyst precursors with respect to the degree of interaction between nickel oxide and support and to understand the effect of loading on the reducibility. For the supports used in this work the molar ratio of Al2 O3 /SiO2 is 13/87, thus, the RHA may be considered to be almost completely covered by alumina. Fig. 1 summarizes the TPR results obtained for 2.5–25 wt.% nickel loading in the Ni/RHA-Al2 O3 catalysts. The reduction profile of the unsupported NiO sample obtained by calcination of nickel nitrate at 500 ◦ C for 4 h is also shown. The single sharp reduction peak of unsupported NiO can be used to identify the bulk NiO on Ni/RHA-Al2 O3 catalyst samples. The TPR profiles of all Ni/RHA-Al2 O3 catalysts present a broad peak. This peak corresponded to the reduction of NiO. Obviously, the nickel oxide obtained in the absence of support is more easily reduced than the supported oxide. The profiles of 2.5 and 5.0 wt.% Ni/RHA-Al2 O3 cat-
alysts show that the reduction occurred at higher temperature and that there was only one broad reduction peak at 660 ◦ C. These results mean that NiO particles are well dispersed on these supports and that stronger interactions occurred between NiO and supports. At high nickel loading (10, 15 and 25 wt.%), the TPR patterns exhibit multi-peaks: one low temperature peak (<400 ◦ C) and two high temperature peaks. The low temperature peak is attributed to the property of bulk NiO in which the support acts only as a dispersing agent; the high temperature peaks are attributed to different degrees of the strong metal support interaction between nickel oxide and supports. These may be due to solid solution and surface compound formation or to NiAl2 O4 -like species formation. In the Ni/Al2 O3 impregnation system, Richardson and Twigg [20] concluded that the impregnation of Al2 O3 with Ni(NO3 )2 solution proceeded so that nickel oxide crystals deposited during drying and decomposed in the early phase of calcination to give a very acidic environment
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that dissolves Al3+ ions from the Al2 O3 surface. These ions then become associated with the NiO, either by incorporation into the crystallite or by accumulation in the immediate vicinity, and are responsible for the decrease in reducibility. Wu and Hercules [21] and Li et al. [22] have reported that nickel impregnated on alumina after calcination was observed in three chemical states: namely surface NiO and two NiAl2 O4 -like species. One of the NiAl2 O4 -like species appears to be unreducible at 500 ◦ C (attributed to Ni2+ in tetrahedral sites in alumina); the other is reducible (associated with Ni2+ in octahedral sites in alumina). In general, several reduction peaks are observed in the TPR profiles. The low temperature peak is generally attributed to the reduction of larger NiO particles which are similar in nature to bulk NiO, while the high temperature peaks are attributed to the reduction of NiO in intimate contact with the oxide support [23]. From these TPR profiles, it is clear that the fraction of bulk NiO and NiAl2 O4 -like species increases with increasing Ni loading. These phenomena also suggest
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that the NiO dispersion decreases with increase in Ni loading. XPS has been employed to characterize nickeldeposited alumina catalysts and has contributed to the understanding of the role of nickel in terms of interaction between nickel and the active component of catalysts. We found that the difference in binding energy (B.E.) between Ni 2p3/2 and Ni 2p1/2 is 17.6 eV (Fig. 2) for all the catalyst precursors after calcination, even at only 500 ◦ C. From the literature, nickel oxide shows little interaction with silica but strong interaction with alumina in the impregnated catalysts after calcination [21]. NiO reacts with Al2 O3 in air at 450–600 ◦ C and results in surface spinel, NiAl2 O4 [24]. The difference in B.E. between Ni 2p3/2 and Ni 2p1/2 in NiAl2 O4 compound is 17.6 eV [25]. Thus, we believe that NiAl2 O4 is present in the catalyst precursors. Though the spinel peak was not detected by XRD measurement, this could be explained by the poor crystallinity and the small amount of spinel at low calcination temperatures. These XRD patterns
Fig. 2. XPS spectra of Ni 2p in catalyst precursors with different calcination temperatures: (a) 500 ◦ C; (b) 700 ◦ C; (c) 900 ◦ C.
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Fig. 3. XRD patterns of unsupported NiO and Ni/RHA-Al2 O3 catalyst precursors with various nickel loadings: (a) unsupported NiO; (b) 2.5 wt.%; (c) 5.0 wt.%; (d) 10.0 wt.%; (e) 15.0 wt.%; (f) 20.0 wt.%; (g) 25.0 wt.% (ramp rate, 10 ◦ C/min; calcination temperature, 500 ◦ C).
(Fig. 3) reveal the relation between Ni loading, dispersion, and the growth of NiO crystals in catalyst precursors with Ni loading from 2.5 to 25 wt.% that were calcined at 500 ◦ C. The broad peaks of the XRD patterns of catalyst precursors with Ni loading of less than 5.0 wt.% indicate that nickel oxide was present in an amorphous state or highly dispersed on supports. However, the peak intensity of NiO increases with increase in nickel loading and the peak became sharper up to 25 wt.% nickel loading. These results indicate that the crystallite size increases with increase in nickel loading. It is also clear that only a small NiO peak was detected for 10 wt.% nickel loading. This phenomenon reveals that a building up of NiO crystallites began and that the bulk NiO was formed when nickel loading exceeded 10 wt.%. This is consistent with the results of TPR examination. The surface structure of support and Ni/RHA-Al2 O3 catalyst precursors were examined by scanning elec-
tron microscopy (SEM). The SEM images of unsupported NiO, RHA-Al2 O3 support, and Ni/RHA-Al2 O3 catalyst precursors are presented in Fig. 4. The large crystallites of unsupported NiO can be clearly seen in Fig. 4a. The surface structure of the alumina dispersed on the surface of the rice hush ash is shown in Fig. 4b. As can be seen, the surface of the rice husk ash is covered with a layer of amorphous structure, which is different from the unsupported NiO. As shown in Fig. 4c and d, the catalyst precursors presented a quasi-meshy structure with mesh sizes of about 0.06–0.2 m. It is clear that the structure of nickel compounds changed when nickel was supported on RHA-Al2 O3 , which did not occur on the unsupported NiO and RHA-Al2 O3 supports before impregnation. It is also noted that the surface structure of Ni/RHA-Al2 O3 catalyst precursors remains the same before and after calcination. This meshy structure is also observed in the SEM images of nickel supported on alumina. However, it
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Fig. 4. Scanning electron micrographs of (a) unsupported NiO (10,000×); (b) RHA-Al2 O3 support (10,000×); (c) 15 wt.% Ni/RHA-Al2 O3 catalyst precursors after drying at 110 ◦ C (10,000×); (d) 25 wt.% Ni/RHA-Al2 O3 catalyst precursors after calcination at 500 ◦ C (10,000×).
was not present on Ni/RHA. In studying this interaction, one finds that alumina has a greater capacity for nickel ions than does silica during preparation by impregnation, and that alumina interacts more strongly with nickel ions than silica does [26]. Therefore, the quasi-meshy structure may be attributed to the nature
of the nickel supported on alumina by impregnation technique. This is because the strong interactions of nickel nitrate with alumina on the support surface form such a new structure. The physical characterization of supported nickel catalyst precursors by nitrogen adsorption is presented
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Table 2 Textural properties of unsupported NiO, supports (RHA-Al2 O3 ) and catalyst precursors (Ni/RHA-Al2 O3 ) Sample
Nickel loading (wt.%)
Calcination temperature (◦ C)
Nickel crystallite size (Å)b
BET surface area (m2 /g)c
Pore volume (cm3 /g)d
Pore size (Å)d
NiOa RHA-Al2 O3 Ni/RHA-Al2 O3 Ni/RHA-Al2 O3 Ni/RHA-Al2 O3 Ni/RHA-Al2 O3
– – 5 15 20 25
500 350 500 500 500 500
– – 28 47 60 106
8 94 165 200 199 184
0.05 0.16 0.22 0.26 0.25 0.25
202 51 42 41 40 41
a
Obtained Obtained c Obtained d Obtained b
from from from from
Ni(NO3 )2 ·6H2 O after calcination at 500 ◦ C. XRD analysis (Eq. (1)). BET method. BJH desorption average pore diameter and pore volume.
in Table 2. The nickel loading varied from 5 to 25 wt.%. Properties of unsupported NiO and RHA-Al2 O3 supports are also listed for comparison. Before impregnation, the BET surface areas of unsupported NiO and RHA-Al2 O3 were only 8 and 94 m2 /g, respectively. When 5% nickel was supported on the RHA-Al2 O3 , the BET surface area of catalyst precursor increased to 165 m2 /g, significantly larger than that of unsupported NiO and support. These findings may be explained by the mechanism of nickel oxide phase formation during preparation of Ni/RHA-Al2 O3 . NiO was well dispersed on supports, forming many mesopores during calcination. These phenomena give rise to a large pore volume and higher surface area. The BET surface area of catalyst precursors increased with nickel loading of catalyst at low nickel loading (5–15 wt.%) and then decreased with further deposit of catalyst (15–25 wt.%). This is because, at low nickel loading, the nickel oxides are present in a well-dispersed state and the small nickel crystallites do not agglomerate. Thus, a sufficiently porous surface compound of nickel forms with increasing surface area. The bulk-like nickel oxides are gradually formed with further deposit of NiO. At high nickel loading, an agglomeration appeared due to the formation of multi-layer nickel oxide which blocked the micropores of nickel compound. The interaction between nickel particles and supports then diminished gradually with further deposit and resulted in significant increases in nickel oxide density and crystallite size. Consequently, the nickel oxide dispersion and surface area were decreased. TPR and XRD also verify this point. The average pore size was evaluated by
the BJH desorption method. Table 2 shows that the variation in average pore size between RHA-Al2 O3 support and Ni/RHA-Al2 O3 catalyst precursors. It is clear that the pore size of Ni/RHA-Al2 O3 is smaller than that of RHA-Al2 O3 support and that the pore volume is larger than that of RHA-Al2 O3 support. This is explained by the covering of support surface with NiO structure as Ni loaded on supports and all the pores of the supports were replaced by NiO structure. It is also noted that the pore size and pore volume of Ni/RHA-Al2 O3 remained almost constant with further increase in nickel loading. These findings may be explained by the formation of meshy structures of nickel oxide phase as nickel loaded on supports. The nickel compounds grew along a fixed orientation, axially along the mesh wall of meshy structure for any further nickel loading, thus, the pores sizes were kept almost constant for further deposit. In general, the catalyst affects the chemical reaction mainly by its exposed active sites and its surface properties. According to the TPR behavior, the reduction temperature was selected to be 800 ◦ C. Although the number of active sites increases with increasing metal loading, the dispersion of metal will decrease with increasing loading. Fig. 5 displays the effect of nickel loading on CO2 conversion and CH4 yield for the hydrogenation of CO2 at 500 ◦ C. This figure indicates that CO2 conversion is almost independent of nickel loading. However, CH4 yield increased rapidly and linearly with nickel loading up to 20 wt.% and then decreased slightly with further nickel loading. This can be explained by the variation in the yield of CH4 with the total amount of active sites of nickel. It is well
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Fig. 5. Effect of nickel loading on CO2 conversion and CH4 yield for CO2 hydrogenation over Ni/RHA-Al2 O3 catalysts: (a) CO2 conversion (䊊); (b) CH4 yield (䊉) (calcination temperature, 500 ◦ C; reduction temperature, 800 ◦ C).
known that increasing nickel loading can increase the concentration of active sites. A larger amount of active nickel implies a higher CH4 yield. However, at higher nickel loading, bulk NiO will form, thus decreasing the active sites gradually. The temperature dependences of CO2 conversion and CH4 yield over Ni/RHA-Al2 O3 are shown in Fig. 6. The CO2 conversion increases rapidly as the reaction temperature increases up to 500 ◦ C and remains constant between 500 and 600 ◦ C, followed by gradual increase in conversion with reaction temperature. The CH4 yield increases as the reaction temperature increases up to a maximum (500 ◦ C) and then decreases. This can be explained from the viewpoint of thermodynamics. The reaction of CO2 hydrogenation to CH4 is highly exothermic [27]. The excessive heat will result in inactivation of the catalyst and will affect the thermodynamic equilibrium. Therefore, when the temperature rises to such an extent, hydrogenation becomes limited due to the thermodynamic equilibrium. A higher reaction temperature promotes side reactions and implies a lower CH4 selectivity. Therefore,
reaction proceeding under a suitable temperature can promote the yield of methane. Obviously, a reaction temperature of 500 ◦ C is the optimum condition for hydrogenation of CO2 over Ni/RHA-Al2 O3 catalysts. Comparison of the catalytic behavior of catalysts between Ni/RHA-Al2 O3 and Ni/SiO2 -Al2 O3 are shown in Figs. 6 and 7. The yield and the selectivity of 15 wt.% Ni/RHA-Al2 O3 toward CH4 formation are higher than that of Ni/SiO2 -Al2 O3 with the same nickel loading during the reaction. This is due to the difference in the surface properties of these catalysts. As for the physical properties, the surface properties of these two types of catalysts and their supports are different, as shown in Table 3. The BET surface area of SiO2 -Al2 O3 support is 5.8 times that of RHA-Al2 O3 support. In general, the support with higher surface area implies a higher dispersion. However, the average pore size of SiO2 -Al2 O3 is smaller than that of RHA-Al2 O3 , while the pore volume of SiO2 -Al2 O3 is 5 times that of RHA-Al2 O3 , implying that the holes of SiO2 -Al2 O3 are deeper than those of RHA-Al2 O3 . During the impregnation, the
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Fig. 6. Comparison of CO2 conversion and CH4 yield for CO2 hydrogenation over 15 wt.% Ni/RHA-Al2 O3 and 15 wt.% Ni/SiO2 Al2 O3 catalysts: (a) CO2 conversion on Ni/RHA-Al2 O3 (䊊); (b) CH4 yield on Ni/RHA-Al2 O3 (䊉); (c) CO2 conversion on Ni/SiO2 -Al2 O3 (䉫); (d) CH4 yield on Ni/SiO2 -Al2 O3 (䉬) (calcination temperature, 500 ◦ C; reduction temperature, 800 ◦ C).
nickel nitrate could not diffuse easily into the small pores of SiO2 -Al2 O3 , causing a decrease in metal dispersion. Table 3 also shows that the average pore size of Ni/SiO2 -Al2 O3 catalyst is smaller than that of Ni/RHA-Al2 O3 catalyst, while the pore volume of Ni/SiO2 -Al2 O3 is larger than that of Ni/RHA-Al2 O3 ,
implying that the holes of Ni/SiO2 -Al2 O3 are deeper than those of Ni/RHA-Al2 O3 after reduction. Therefore, the diffusion of gas is affected, which in turn affects the rate of chemical reaction. It is clear that RHA-Al2 O3 is a good support for preparing supported nickel catalyst by incipient
Table 3 Comparison of surface properties of supports (RHA-Al2 O3 , SiO2 -Al2 O3 ) and catalyst precursors (Ni/RHA-Al2 O3 , Ni/SiO2 -Al2 O3 ) Sample
Nickel loading (wt.%)
BET surface area (m2 /g)a
Pore volume (cm3 /g)b
Average pore size (Å)b
Nickel crystalline size (Å)c
RHA-Al2 O3 SiO2 -Al2 O3 Ni/RHA-Al2 O3 Ni/SiO2 -Al2 O3
– – 15 15
94 539 200 396
0.16 0.76 0.26 0.50
51 42 41 39
– – 47 96
a
Obtained from BET method. Obtained from BJH desorption average pore diameter and pore volume. c Obtained from XRD analysis (Eq. (1)). b
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Fig. 7. Comparison of CH4 selectivity for CO2 hydrogenation over 15 wt.% Ni/RHA-Al2 O3 and 15 wt.% Ni/SiO2 -Al2 O3 catalysts on: (a) Ni/RHA-Al2 O3 (); (b) Ni/SiO2 -Al2 O3 (䉱) (calcination temperature, 500 ◦ C; reduction temperature, 800 ◦ C).
wetness impregnation method for hydrogenation of CO2 .
genation tests showed that Ni/RHA-Al2 O3 catalysts preform better than Ni/SiO2 -Al2 O3 ones.
4. Conclusions
Acknowledgements
RHA-Al2 O3 supports were prepared by impregnation of rice husk ash with an aluminum nitrate solution and were then used to prepare supported nickel catalysts (Ni/RHA-Al2 O3 ) by incipient wetness impregnation method. These catalysts with high surface area and mesopores structure are advantageous for chemical reactions. The interaction between nickel and support is very strong. Consequently, the small nickel oxide crystallites were formed with good dispersion and more than one species, such as NiO or NiAl2 O4 -like ones, were detected. Moreover, 500 ◦ C is suitable for hydrogenation of CO2 with maximum yield and selectivity of CH4 . Furthermore, the hydro-
The authors would like to express their thanks to the National Science Council of Taiwan for its financial support under Project NSC88-2214-E008-006.
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Hydrogenation of CO2 over nickel catalysts on rice husk ash-alumina prepared by incipient wetness impregnation Feg-Wen Chang∗ , Maw-Suey Kuo, Ming-Tseh Tsay, Ming-Chung Hsieh Department of Chemical and Materials Engineering, National Central University, Chungli 32054, Taiwan Received 12 June 2002; received in revised form 6 November 2002; accepted 30 January 2003
Abstract Nickel catalysts supported on rice husk ash-alumina (Ni/RHA-Al2 O3 ) were prepared by the incipient wetness impregnation method. Characterizations were investigated by TPR, XPS, XRD, SEM, and BET techniques. The catalytic activities of nickel catalysts were tested by CO2 hydrogenation with H2 /CO2 ratio of 4:1 for temperatures between 400 and 800 ◦ C. The XPS analysis of Ni/RHA-Al2 O3 demonstrated the presence of spinel. The TPR analysis indicated that the interaction between nickel and support was strong and difficult to reduce with more than one nickel oxide compound, such as bulk NiO and two NiAl2 O4 -like species. The results of N2 -adsorption by BET indicated that the maximum surface area was found at 15 wt.% nickel loading with mesopores. The XRD analysis of samples suggested that the crystallite sizes of supported NiO increased with the increase in nickel loading. The CO2 conversion and CH4 yield for CO2 hydrogenation were found to depend on the nickel loading. The reaction temperature of 500 ◦ C might be the optimum temperature for CO2 hydrogenation to give the maximum yield and selectivity of CH4 . Furthermore, the hydrogenation tests showed that the performance of Ni/RHA-Al2 O3 is better than that of Ni/SiO2 -Al2 O3 . © 2003 Elsevier Science B.V. All rights reserved. Keywords: Rice husk ash-alumina support; Nickel catalyst; CO2 hydrogenation; Spinel; Incipient wetness impregnation
1. Introduction Supported nickel catalysts are widely applied in many important reactions, such as hydrogenation of aromatic hydrocarbon [1,2], methanation of CO2 [3–8], CO2 reforming of CH4 to syngas [9], ethylation and dealkylation [10], and methanation of coal synthesis. High surface area supports, usually oxides, are used extensively in industry for the preparation of metal catalysts including silica, alumina, silica-alumina, and zeolites. These catalysts are generally prepared by incipient wetness impregnation, ∗ Corresponding author. Tel.: +886-3-4227151x4202; fax: +886-3-4252296. E-mail address: [email protected] (F.-W. Chang).
ion exchange, and precipitation-deposition techniques with different precipitation agents [6–8,11,12]. In general, ion exchange technique is limited to catalysts with relatively low nickel loading, while incipient wetness impregnation technique is the simplest preparation procedure. Though the incipient wetness impregnation method does not result in the best dispersion of nickel on the support, easier control of loading and no liquid waste generation make this process the most preferred. In the chemical processing industry, a catalyst with high activity and selectivity is essential. To meet these requirements, the catalyst support of inert carrier should provide sufficient surface area for the metal to disperse. It has emerged that the support plays a very active role in the interaction between the nickel
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and the support. The nickel compounds on different support surfaces result in different extents of what are generally called “metal-support effects”. This implies that catalysts with different characteristics exhibit different performances toward activity and selectivity for a given process. Rice husk is a milling by-product of rice and also a major waste product of the agriculture industry. Nevertheless, it is a unique crop residue with uniform size and high content of ash (14–25 wt.%) [13]. The silica content of ash can be as high as 90–97% [13,14]. Amorphous silica, commonly referred to as rice husk ash, is obtained from rice husk by acid leaching, pyrolysis, and carbon-removing processes. The purity of silica in the product can reach as high as 99.9% with high specific surface area and high porosity [15–18]. These properties make the ash a valuable raw material for many uses. Of course, it can also be used for the support of catalysts. In our previous work [6–8,19], we have found that the rice husk ash-supported nickel catalyst used for hydrogenation of CO2 exhibits high selectivity. We concluded that the hydrogenation activity of nickel catalysts using rice husk ash as a support is better than that of those prepared by silica gel. In this study, nickel catalysts supported on rice husk ash-alumina were prepared by the incipient wetness impregnation method. The aim of the present work is to provide a basis for understanding the interrelationship between the physical and chemical properties of the nickel catalysts on rice husk ash-alumina and their reactivities on CO2 hydrogenation. A commercial silica-alumina was also chosen as another support for comparison.
2. Experimental 2.1. Raw material Rice husk obtained from a rice mill was first washed thoroughly with demineralized water to remove the adhering soil and dust and then dried at 110 ◦ C in an air oven. The dried rice husk was refluxed with 3N HCl in a round-bottomed glass flask at its boiling point, about 104 ◦ C, for 1 h. It was then washed 10 times with hot demineralized water and dried at 110 ◦ C for 24 h. The pyrolysis was carried out under a nitrogen atmosphere at 700 ◦ C for 1 h by placing the treated rice
husk in a quartz tubular reactor. After pyrolysis, the husk was further heated in an air furnace at 700 ◦ C for 1 h; the ash of husk obtained was more than 99% amorphous silica. This silica was used as one of the support materials for nickel catalysts and is designated hereafter as RHA. Aluminum nitrate (Merck, 99%) and nickel nitrate (Merck, 99%) were used for the preparation of the ash-Al2 O3 support and the catalyst, respectively. 2.2. Sample preparation Rice husk ash (designated as RHA; BET surface area: 222 m2 /g, average pore diameter: 57 Å, total pore volume: 0.35 cm3 /g) and aluminum nitrate were used in preparation of rice husk ash-alumina support (designated as RHA-Al2 O3 ). Following the simple impregnation to incipient wetness of the RHA with aqueous aluminum nitrate solution for 24 h, the sample was dried in an air oven at 110 ◦ C. The impregnated, dried sample was calcined in air at 350 ◦ C for 5 h at a heating rate of 2 ◦ C/min. This sample was then used as a support material for nickel catalysts. Nickel nitrate was used in preparation of supported nickel catalysts (designated as Ni/RHA-Al2 O3 ). The supported nickel catalysts were also prepared by simple impregnation to incipient wetness technique and dried as mentioned above. The dried samples were calcined in air at 500 ◦ C for 4 h at a heating rate of 2 ◦ C/min. Finally, the catalyst precursors were activated after being placed in the tubular reactor of reducing atmosphere in a H2 /Ar (5/95) stream at 800 ◦ C for 3 h at a heating rate of 10 ◦ C/min. The commercial SiO2 -Al2 O3 support (supplied by Aldrich Chemical Co., support grade, BET surface area: 539 m2 /g, average pore diameter: 42 Å, total pore volume: 0.76 cm3 /g) was used to make other samples by the same procedure for comparison (designated as Ni/SiO2 -Al2 O3 ). 2.3. Characterization The metallic impurities of rice husk ash were determined with an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Leeman Labs Model Leeman SP1000). Elemental analysis was carried out with an element analyzer (Carlo Erba, Model EA1108).
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Temperature programmed reduction (TPR) measurements were carried out in a HP5860A GC apparatus, equipped with a thermal conductivity detector (TCD). About 50 mg of catalysts were placed in the quartz reactor in a tubular furnace and heated from room temperature to 900 ◦ C at a heating rate of 10 ◦ C/min under a 40 ml/min H2 /Ar (5/95) flow stream. The consumption of H2 was detected and recorded. For X-ray photoelectron spectroscopy (XPS) analysis, the powder samples were put into a disk-shaped holder and lightly pressed to give a compact smooth surface. VG McroLab 310-D photoelectron spectrometer was used for the analysis, which scans of the Ni 2p spectral region. Primary excitation was achieved with Mg K␣ X-rays (1253.6 eV) and a multi-channel detector was used to detect the emitted photoelectrons. The spectrometer ran in the constant analyzer energy (CAE) mode. Data processing was performed using the Eclipse V2.0 data system running on an OS/2 Windows environment. The X-ray diffraction (XRD) patterns of samples were recorded over a Philips PW1820 goniometer with PW1710 processor diffractometer using graphite monochromater Cu K␣ radiation (λ1 = 1.5406 Å, λ2 = 1.5444 Å). Samples were scanned between 10 and 80◦ (2θ) at a scan step size of 0.02◦ and scan time constant of 2 s per step. The crystallite size (dXRD ) of the nickel oxide was determined by X-ray line broadening analysis (XLBA) for NiO (0 1 2) peaks according to the Scherrer formula: κλ dXRD = (1) β cos θ where κ, the particle shape factor, was taken as 0.94, and the wavelength λ was taken as 1.5418 Å. The measured half width ‘B’ was corrected for the instrumental line broadening ‘b’ using β = B − b. The instrumental line broadening ‘b’ was determined to be 0.055◦ using the silicon reference supplied by Rigaku. The scanning electron micrographs (SEM) were obtained by a HITACH S-800 scanning electron microscope. The specific surface area (BET, N2 ), pore volume, and pore size were measured by nitrogen adsorption– desorption with an analyzer (Model ASAP2010 mi-
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crometrics, USA). Prior to measurement, each sample was degassed at 200 ◦ C. 2.4. Hydrogenation of CO2 The gas-phase hydrogenation of CO2 to methane was carried out in a micro fixed-bed operation under atmospheric pressure. The design of the reactor and related apparatus has been described previously [6,7]. For each run, about 50 mg of catalysts were loaded into a quartz reactor of 4 mm i.d. and reduced in situ under a continuous flow of H2 /Ar (5/95) at the rate of 40 ml/min. After reduction, the reactor was adjusted to the desired reaction temperature under the same gas flow. When the temperature became stabilized, the flow rate of H2 /CO2 (4:1) mixture was adjusted to 25 ml/min and fed into the reaction system. All product gas concentrations were measured chromatographically using a thermal conductivity detector (TCD).
3. Results and discussion In order to avoid any effect of impurities on the chemical reaction, the rice husk was washed with hot demineralized water after acid treatment, and kept for several minutes between each wash to assure complete leaching of the impurities from the pores of rice husks. The analyses of metallic impurities and organic elements in the rice husk ashes are shown in Table 1. As can be seen, most of the metallic impurities and organic elements were removed after pretreatment, so the purity of RHA used in this work was >99.9%. We believe that the activity of catalysts will not be affected by the contaminants of RHA. Table 1 ICP-AES analysis and element analysis of rice husk ash ICP-AES analysis
Element analysis
Impurity
Concentration (ppm)
Impurity
Concentration (wt.%)
Na K Ca Fe Mg Mn
142 <400 142 180 84 48
C H
<0.01 0.11
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Fig. 1. TPR curves of Ni/RHA-Al2 O3 catalyst precursors with various nickel loading and unsupported NiO: (a) 2.5 wt.%; (b) 5.0 wt.%; (c) 10.0 wt.%; (d) 15.0 wt.%; (e) 25.0 wt.%; (f) unsupported NiO (ramp rate, 10 ◦ C/min; calcination temperature, 500 ◦ C).
Temperature-programmed reduction (TPR) was used to characterize the Ni/RHA-Al2 O3 catalyst precursors with respect to the degree of interaction between nickel oxide and support and to understand the effect of loading on the reducibility. For the supports used in this work the molar ratio of Al2 O3 /SiO2 is 13/87, thus, the RHA may be considered to be almost completely covered by alumina. Fig. 1 summarizes the TPR results obtained for 2.5–25 wt.% nickel loading in the Ni/RHA-Al2 O3 catalysts. The reduction profile of the unsupported NiO sample obtained by calcination of nickel nitrate at 500 ◦ C for 4 h is also shown. The single sharp reduction peak of unsupported NiO can be used to identify the bulk NiO on Ni/RHA-Al2 O3 catalyst samples. The TPR profiles of all Ni/RHA-Al2 O3 catalysts present a broad peak. This peak corresponded to the reduction of NiO. Obviously, the nickel oxide obtained in the absence of support is more easily reduced than the supported oxide. The profiles of 2.5 and 5.0 wt.% Ni/RHA-Al2 O3 cat-
alysts show that the reduction occurred at higher temperature and that there was only one broad reduction peak at 660 ◦ C. These results mean that NiO particles are well dispersed on these supports and that stronger interactions occurred between NiO and supports. At high nickel loading (10, 15 and 25 wt.%), the TPR patterns exhibit multi-peaks: one low temperature peak (<400 ◦ C) and two high temperature peaks. The low temperature peak is attributed to the property of bulk NiO in which the support acts only as a dispersing agent; the high temperature peaks are attributed to different degrees of the strong metal support interaction between nickel oxide and supports. These may be due to solid solution and surface compound formation or to NiAl2 O4 -like species formation. In the Ni/Al2 O3 impregnation system, Richardson and Twigg [20] concluded that the impregnation of Al2 O3 with Ni(NO3 )2 solution proceeded so that nickel oxide crystals deposited during drying and decomposed in the early phase of calcination to give a very acidic environment
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that dissolves Al3+ ions from the Al2 O3 surface. These ions then become associated with the NiO, either by incorporation into the crystallite or by accumulation in the immediate vicinity, and are responsible for the decrease in reducibility. Wu and Hercules [21] and Li et al. [22] have reported that nickel impregnated on alumina after calcination was observed in three chemical states: namely surface NiO and two NiAl2 O4 -like species. One of the NiAl2 O4 -like species appears to be unreducible at 500 ◦ C (attributed to Ni2+ in tetrahedral sites in alumina); the other is reducible (associated with Ni2+ in octahedral sites in alumina). In general, several reduction peaks are observed in the TPR profiles. The low temperature peak is generally attributed to the reduction of larger NiO particles which are similar in nature to bulk NiO, while the high temperature peaks are attributed to the reduction of NiO in intimate contact with the oxide support [23]. From these TPR profiles, it is clear that the fraction of bulk NiO and NiAl2 O4 -like species increases with increasing Ni loading. These phenomena also suggest
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that the NiO dispersion decreases with increase in Ni loading. XPS has been employed to characterize nickeldeposited alumina catalysts and has contributed to the understanding of the role of nickel in terms of interaction between nickel and the active component of catalysts. We found that the difference in binding energy (B.E.) between Ni 2p3/2 and Ni 2p1/2 is 17.6 eV (Fig. 2) for all the catalyst precursors after calcination, even at only 500 ◦ C. From the literature, nickel oxide shows little interaction with silica but strong interaction with alumina in the impregnated catalysts after calcination [21]. NiO reacts with Al2 O3 in air at 450–600 ◦ C and results in surface spinel, NiAl2 O4 [24]. The difference in B.E. between Ni 2p3/2 and Ni 2p1/2 in NiAl2 O4 compound is 17.6 eV [25]. Thus, we believe that NiAl2 O4 is present in the catalyst precursors. Though the spinel peak was not detected by XRD measurement, this could be explained by the poor crystallinity and the small amount of spinel at low calcination temperatures. These XRD patterns
Fig. 2. XPS spectra of Ni 2p in catalyst precursors with different calcination temperatures: (a) 500 ◦ C; (b) 700 ◦ C; (c) 900 ◦ C.
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Fig. 3. XRD patterns of unsupported NiO and Ni/RHA-Al2 O3 catalyst precursors with various nickel loadings: (a) unsupported NiO; (b) 2.5 wt.%; (c) 5.0 wt.%; (d) 10.0 wt.%; (e) 15.0 wt.%; (f) 20.0 wt.%; (g) 25.0 wt.% (ramp rate, 10 ◦ C/min; calcination temperature, 500 ◦ C).
(Fig. 3) reveal the relation between Ni loading, dispersion, and the growth of NiO crystals in catalyst precursors with Ni loading from 2.5 to 25 wt.% that were calcined at 500 ◦ C. The broad peaks of the XRD patterns of catalyst precursors with Ni loading of less than 5.0 wt.% indicate that nickel oxide was present in an amorphous state or highly dispersed on supports. However, the peak intensity of NiO increases with increase in nickel loading and the peak became sharper up to 25 wt.% nickel loading. These results indicate that the crystallite size increases with increase in nickel loading. It is also clear that only a small NiO peak was detected for 10 wt.% nickel loading. This phenomenon reveals that a building up of NiO crystallites began and that the bulk NiO was formed when nickel loading exceeded 10 wt.%. This is consistent with the results of TPR examination. The surface structure of support and Ni/RHA-Al2 O3 catalyst precursors were examined by scanning elec-
tron microscopy (SEM). The SEM images of unsupported NiO, RHA-Al2 O3 support, and Ni/RHA-Al2 O3 catalyst precursors are presented in Fig. 4. The large crystallites of unsupported NiO can be clearly seen in Fig. 4a. The surface structure of the alumina dispersed on the surface of the rice hush ash is shown in Fig. 4b. As can be seen, the surface of the rice husk ash is covered with a layer of amorphous structure, which is different from the unsupported NiO. As shown in Fig. 4c and d, the catalyst precursors presented a quasi-meshy structure with mesh sizes of about 0.06–0.2 m. It is clear that the structure of nickel compounds changed when nickel was supported on RHA-Al2 O3 , which did not occur on the unsupported NiO and RHA-Al2 O3 supports before impregnation. It is also noted that the surface structure of Ni/RHA-Al2 O3 catalyst precursors remains the same before and after calcination. This meshy structure is also observed in the SEM images of nickel supported on alumina. However, it
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Fig. 4. Scanning electron micrographs of (a) unsupported NiO (10,000×); (b) RHA-Al2 O3 support (10,000×); (c) 15 wt.% Ni/RHA-Al2 O3 catalyst precursors after drying at 110 ◦ C (10,000×); (d) 25 wt.% Ni/RHA-Al2 O3 catalyst precursors after calcination at 500 ◦ C (10,000×).
was not present on Ni/RHA. In studying this interaction, one finds that alumina has a greater capacity for nickel ions than does silica during preparation by impregnation, and that alumina interacts more strongly with nickel ions than silica does [26]. Therefore, the quasi-meshy structure may be attributed to the nature
of the nickel supported on alumina by impregnation technique. This is because the strong interactions of nickel nitrate with alumina on the support surface form such a new structure. The physical characterization of supported nickel catalyst precursors by nitrogen adsorption is presented
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Table 2 Textural properties of unsupported NiO, supports (RHA-Al2 O3 ) and catalyst precursors (Ni/RHA-Al2 O3 ) Sample
Nickel loading (wt.%)
Calcination temperature (◦ C)
Nickel crystallite size (Å)b
BET surface area (m2 /g)c
Pore volume (cm3 /g)d
Pore size (Å)d
NiOa RHA-Al2 O3 Ni/RHA-Al2 O3 Ni/RHA-Al2 O3 Ni/RHA-Al2 O3 Ni/RHA-Al2 O3
– – 5 15 20 25
500 350 500 500 500 500
– – 28 47 60 106
8 94 165 200 199 184
0.05 0.16 0.22 0.26 0.25 0.25
202 51 42 41 40 41
a
Obtained Obtained c Obtained d Obtained b
from from from from
Ni(NO3 )2 ·6H2 O after calcination at 500 ◦ C. XRD analysis (Eq. (1)). BET method. BJH desorption average pore diameter and pore volume.
in Table 2. The nickel loading varied from 5 to 25 wt.%. Properties of unsupported NiO and RHA-Al2 O3 supports are also listed for comparison. Before impregnation, the BET surface areas of unsupported NiO and RHA-Al2 O3 were only 8 and 94 m2 /g, respectively. When 5% nickel was supported on the RHA-Al2 O3 , the BET surface area of catalyst precursor increased to 165 m2 /g, significantly larger than that of unsupported NiO and support. These findings may be explained by the mechanism of nickel oxide phase formation during preparation of Ni/RHA-Al2 O3 . NiO was well dispersed on supports, forming many mesopores during calcination. These phenomena give rise to a large pore volume and higher surface area. The BET surface area of catalyst precursors increased with nickel loading of catalyst at low nickel loading (5–15 wt.%) and then decreased with further deposit of catalyst (15–25 wt.%). This is because, at low nickel loading, the nickel oxides are present in a well-dispersed state and the small nickel crystallites do not agglomerate. Thus, a sufficiently porous surface compound of nickel forms with increasing surface area. The bulk-like nickel oxides are gradually formed with further deposit of NiO. At high nickel loading, an agglomeration appeared due to the formation of multi-layer nickel oxide which blocked the micropores of nickel compound. The interaction between nickel particles and supports then diminished gradually with further deposit and resulted in significant increases in nickel oxide density and crystallite size. Consequently, the nickel oxide dispersion and surface area were decreased. TPR and XRD also verify this point. The average pore size was evaluated by
the BJH desorption method. Table 2 shows that the variation in average pore size between RHA-Al2 O3 support and Ni/RHA-Al2 O3 catalyst precursors. It is clear that the pore size of Ni/RHA-Al2 O3 is smaller than that of RHA-Al2 O3 support and that the pore volume is larger than that of RHA-Al2 O3 support. This is explained by the covering of support surface with NiO structure as Ni loaded on supports and all the pores of the supports were replaced by NiO structure. It is also noted that the pore size and pore volume of Ni/RHA-Al2 O3 remained almost constant with further increase in nickel loading. These findings may be explained by the formation of meshy structures of nickel oxide phase as nickel loaded on supports. The nickel compounds grew along a fixed orientation, axially along the mesh wall of meshy structure for any further nickel loading, thus, the pores sizes were kept almost constant for further deposit. In general, the catalyst affects the chemical reaction mainly by its exposed active sites and its surface properties. According to the TPR behavior, the reduction temperature was selected to be 800 ◦ C. Although the number of active sites increases with increasing metal loading, the dispersion of metal will decrease with increasing loading. Fig. 5 displays the effect of nickel loading on CO2 conversion and CH4 yield for the hydrogenation of CO2 at 500 ◦ C. This figure indicates that CO2 conversion is almost independent of nickel loading. However, CH4 yield increased rapidly and linearly with nickel loading up to 20 wt.% and then decreased slightly with further nickel loading. This can be explained by the variation in the yield of CH4 with the total amount of active sites of nickel. It is well
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Fig. 5. Effect of nickel loading on CO2 conversion and CH4 yield for CO2 hydrogenation over Ni/RHA-Al2 O3 catalysts: (a) CO2 conversion (䊊); (b) CH4 yield (䊉) (calcination temperature, 500 ◦ C; reduction temperature, 800 ◦ C).
known that increasing nickel loading can increase the concentration of active sites. A larger amount of active nickel implies a higher CH4 yield. However, at higher nickel loading, bulk NiO will form, thus decreasing the active sites gradually. The temperature dependences of CO2 conversion and CH4 yield over Ni/RHA-Al2 O3 are shown in Fig. 6. The CO2 conversion increases rapidly as the reaction temperature increases up to 500 ◦ C and remains constant between 500 and 600 ◦ C, followed by gradual increase in conversion with reaction temperature. The CH4 yield increases as the reaction temperature increases up to a maximum (500 ◦ C) and then decreases. This can be explained from the viewpoint of thermodynamics. The reaction of CO2 hydrogenation to CH4 is highly exothermic [27]. The excessive heat will result in inactivation of the catalyst and will affect the thermodynamic equilibrium. Therefore, when the temperature rises to such an extent, hydrogenation becomes limited due to the thermodynamic equilibrium. A higher reaction temperature promotes side reactions and implies a lower CH4 selectivity. Therefore,
reaction proceeding under a suitable temperature can promote the yield of methane. Obviously, a reaction temperature of 500 ◦ C is the optimum condition for hydrogenation of CO2 over Ni/RHA-Al2 O3 catalysts. Comparison of the catalytic behavior of catalysts between Ni/RHA-Al2 O3 and Ni/SiO2 -Al2 O3 are shown in Figs. 6 and 7. The yield and the selectivity of 15 wt.% Ni/RHA-Al2 O3 toward CH4 formation are higher than that of Ni/SiO2 -Al2 O3 with the same nickel loading during the reaction. This is due to the difference in the surface properties of these catalysts. As for the physical properties, the surface properties of these two types of catalysts and their supports are different, as shown in Table 3. The BET surface area of SiO2 -Al2 O3 support is 5.8 times that of RHA-Al2 O3 support. In general, the support with higher surface area implies a higher dispersion. However, the average pore size of SiO2 -Al2 O3 is smaller than that of RHA-Al2 O3 , while the pore volume of SiO2 -Al2 O3 is 5 times that of RHA-Al2 O3 , implying that the holes of SiO2 -Al2 O3 are deeper than those of RHA-Al2 O3 . During the impregnation, the
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Fig. 6. Comparison of CO2 conversion and CH4 yield for CO2 hydrogenation over 15 wt.% Ni/RHA-Al2 O3 and 15 wt.% Ni/SiO2 Al2 O3 catalysts: (a) CO2 conversion on Ni/RHA-Al2 O3 (䊊); (b) CH4 yield on Ni/RHA-Al2 O3 (䊉); (c) CO2 conversion on Ni/SiO2 -Al2 O3 (䉫); (d) CH4 yield on Ni/SiO2 -Al2 O3 (䉬) (calcination temperature, 500 ◦ C; reduction temperature, 800 ◦ C).
nickel nitrate could not diffuse easily into the small pores of SiO2 -Al2 O3 , causing a decrease in metal dispersion. Table 3 also shows that the average pore size of Ni/SiO2 -Al2 O3 catalyst is smaller than that of Ni/RHA-Al2 O3 catalyst, while the pore volume of Ni/SiO2 -Al2 O3 is larger than that of Ni/RHA-Al2 O3 ,
implying that the holes of Ni/SiO2 -Al2 O3 are deeper than those of Ni/RHA-Al2 O3 after reduction. Therefore, the diffusion of gas is affected, which in turn affects the rate of chemical reaction. It is clear that RHA-Al2 O3 is a good support for preparing supported nickel catalyst by incipient
Table 3 Comparison of surface properties of supports (RHA-Al2 O3 , SiO2 -Al2 O3 ) and catalyst precursors (Ni/RHA-Al2 O3 , Ni/SiO2 -Al2 O3 ) Sample
Nickel loading (wt.%)
BET surface area (m2 /g)a
Pore volume (cm3 /g)b
Average pore size (Å)b
Nickel crystalline size (Å)c
RHA-Al2 O3 SiO2 -Al2 O3 Ni/RHA-Al2 O3 Ni/SiO2 -Al2 O3
– – 15 15
94 539 200 396
0.16 0.76 0.26 0.50
51 42 41 39
– – 47 96
a
Obtained from BET method. Obtained from BJH desorption average pore diameter and pore volume. c Obtained from XRD analysis (Eq. (1)). b
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Fig. 7. Comparison of CH4 selectivity for CO2 hydrogenation over 15 wt.% Ni/RHA-Al2 O3 and 15 wt.% Ni/SiO2 -Al2 O3 catalysts on: (a) Ni/RHA-Al2 O3 (); (b) Ni/SiO2 -Al2 O3 (䉱) (calcination temperature, 500 ◦ C; reduction temperature, 800 ◦ C).
wetness impregnation method for hydrogenation of CO2 .
genation tests showed that Ni/RHA-Al2 O3 catalysts preform better than Ni/SiO2 -Al2 O3 ones.
4. Conclusions
Acknowledgements
RHA-Al2 O3 supports were prepared by impregnation of rice husk ash with an aluminum nitrate solution and were then used to prepare supported nickel catalysts (Ni/RHA-Al2 O3 ) by incipient wetness impregnation method. These catalysts with high surface area and mesopores structure are advantageous for chemical reactions. The interaction between nickel and support is very strong. Consequently, the small nickel oxide crystallites were formed with good dispersion and more than one species, such as NiO or NiAl2 O4 -like ones, were detected. Moreover, 500 ◦ C is suitable for hydrogenation of CO2 with maximum yield and selectivity of CH4 . Furthermore, the hydro-
The authors would like to express their thanks to the National Science Council of Taiwan for its financial support under Project NSC88-2214-E008-006.
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