In response to the depletion of petroleum reserves, the effective use of natural gas is being extensively studied. Among the methods of natural gas use, catalytic CO2 reforming of methane (CRM) to produce syngas has attracted attention; it is environmentally friendly because it converts two greenhouse gases (CO2 and CH4) to syngas and finally high-value-added products [1, 2, 3, 4].
Catalyst development is the bottleneck in the industrialization of CRM. Transitional metals are usually used. Noble metals such as Rh, Ru, Ir, and Pd show good catalytic performance in CRM, but their high prices limit their use. Ni-based catalysts are most widely used because of their low costs and high initial catalytic activity in CRM [5, 6, 7, 8]. However, Ni-based catalysts suffer from carbon deposition. It has been reported that the size of Ni particles greatly influences the amounts of carbon deposited, and catalysts with smaller NiO particles have stronger ability to resist carbon deposition [9].
There are several ways to prepare well-dispersed Ni-based catalysts. Catalysts with hydrotalcite [10, 11] and perovskite [12, 13, 14] structures can disperse Ni particles to some extent. However, these structures are apt to collapse at the high reaction temperatures (600-900 °C). Catalysts with small Ni particles can be obtained by loading Ni precursors onto supports with high specific surface areas and ordered mesoporous structures, such as MCM-41 and SBA-15, because of the confinement effects of the supports [15, 16, 17]. However, the Ni particles are sintered during thermal treatment.
In our previous study, we found that a modified impregnation method using β-cyclodextrin (βCD) was effective for preparing well-dispersed Ni-based catalysts and resisting Ni sintering during heat treatment and the CRM reaction [18]. In this study, the use of other CDs such as αCD and γCD as promoting additives for improving the dispersion of Ni particles on the surface of Ni/SBA-15 was examined, and the properties and performance of αCD- and γCD-modified catalysts were studied and compared with those of the unmodified one.
Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol) (P123) of molecular mass 5800 was obtained from Aldrich. Tetraethyl orthosilicate (TEOS) of purity 99.5% was purchased from Acros. Hydrochloric acid (36%-38%) was purchased from Beijing Chemical Company. Analytical- grade Ni(NO3)2·6H2O was provided by Shantou Xilong Chemical Company. αCD was purchased from Aladdin and γCD was purchased from TCI Company. All reagents were used as received.
The SBA-15 support (815 m2/g) was prepared with P123 as a template and TEOS as the precursor [19]. The catalysts prepared using αCD and γCD are denoted by Ni/SBA-15-αCD and Ni/SBA-15-γCD, respectively. The molar ratio n(CD)/n(Ni) was 1/50. Ni/SBA-15 prepared using the conventional impregnation method (without CDs), denoted by Ni/SBA-15-im, was used as a reference. The detailed preparation procedures are described in the literature [18]; the theoretical loadings of Ni for all the catalysts were 4.8 wt%.
The specific surface areas of the fresh catalysts were measured by the N2 adsorption-desorption method using a Micromeritics ASAP 2010 C analyzer. Before the measurements, the catalysts were degassed at 200 °C for 2 h.
X-ray diffraction (XRD) was used to investigate the crystalline phases of the Ni species in the catalysts. The XRD patterns were obtained with a Rigaku X-ray diffractometer using Ni-filtered Cu Kαradiation at 40 kV and 200 mA. The crystal sizes of the NiO particles were determined by the X-ray line- broadening method using the Scherrer equation.
The actual loadings of Ni on the fresh catalysts were determined using X-ray fluorescence (XRF; XRF-1800).
The catalyst morphology and NiO particle size were examined by transmission electronic microscopy (TEM) using a Tecnai G2 20 instrument. Before measurements, the samples were crushed and dispersed on copper meshes with ethanol as the solvent.
The reduction behavior of the catalysts was characterized using temperature-programmed H2 reduction (H2-TPR), and the TPR profiles were recorded with a Quantachrome adsorption instrument; details of the procedure are available in the literature [18].
The mass spectra of aqueous solutions of the promoter (αCD or γCD) and Ni(NO3)2 were obtained using a mass spectrometric detector (Turbomass) to identify the interactions between the promoters and Ni(NO3)2.
CRM to syngas was conducted in a fixed-bed quartz reactor with an inner diameter of 5 mm at atmospheric pressure. Before the reaction, a portion of the catalyst (0.20 g) was reduced with 20% H2/80% Ar at 600 °C for 2 h. The temperature was raised to the reaction temperature (700 °C), and then CH4 (99.9%) and CO2 (99.9%) in a molar ratio of 1/1 were introduced into the reactor at a total flow rate of 43 mL/min. The flow rate of the gaseous products was measured with a soap-flowmeter, and the components were analyzed using online gas chromatography. The relative amounts of gases in the products were calculated using the normalization method. The equations used for calculating CH4 conversion, CO2 conversion, and selectivity are shown below:
The textural properties of the SBA-15 support and the fresh catalysts (Ni/SBA-15-αCD, Ni/SBA-15-γCD, and Ni/SBA-15-im) were determined using N2 adsorption-desorption; the isotherms and pore size distributions are shown in Fig. 1. All the catalysts had type IV adsorption-desorption isotherms with H2 hysteresis loops (Fig. 1(a)). These are typical of the ordered mesoporous structure of SBA-15, i.e., Ni loading and modification with promoters did not destroy the SBA-15 structure. The average pore sizes of these catalysts were 4-6 nm and were similar to each other (Fig. 1(b)).
The specific surface areas of these catalysts are listed in Table 1. The data show that the αCD- and γCD-modified catalysts had similar specific surface areas, in the range of 529-580 m2/g, i.e., lower than that of the SBA-15 support (815 m2/g).
The Ni contents of these catalysts were analyzed using XRF; the results are listed in Table 1. The Ni contents of all the catalysts were around 4.8-5.5 wt%, close to the theoretical value.
The crystal structure of the fresh catalysts was determined using XRD; the results are shown in Fig. 2. In the small-angle XRD patterns (Fig. 2(a)), there were diffraction peaks at 1°-2°; this indicates that all the catalysts maintained the ordered mesoporous structure of SBA-15 even after calcination at 650 °C. In the wide-angle XRD patterns (Fig. 2(b)), there was a wide and broad peak at 10°-30°, ascribed to amorphous silica. Furthermore, similar to the results obtained for a βCD-modified catalyst (Ni/SBA-15-βCD) [18], in the wide-angle XRD patterns of the αCD- and γCD-modified catalysts, the NiO peaks (at 37.5°, 43.4°, and 63.0°) were too weak to be used to calculate the NiO particle sizes using the Scherrer equation. However, the NiO peaks in the profile of the unmodified Ni/SBA-15-im catalyst were sharp, and the average NiO size was 9.1 nm (from the Scherrer equation). These results show that αCD and γCD improved Ni dispersion on the modified catalysts.
The morphology and size of the NiO particles in these Ni/SBA-15 catalysts were studied using TEM; the results are shown in Fig. 3. In the TEM images of all the catalysts, the ordered mesoporous structure of SBA-15 can be observed, consistent with the Brunauer-Emmett-Teller (BET) results and XRD patterns. In addition, NiO particles of size 10-20 nm can be clearly observed on the unmodified Ni/SBA-15-im. The SBA-15 channel size is 4-6 nm [18], therefore it can be conjectured that in Ni/SBA-15-im the NiO particles are mostly distributed on the outer surfaces of SBA-15. In contrast, similar to the results for Ni/SBA-15-βCD in our previous study [18], no NiO particles could be seen in the TEM images of the αCD- or γCD-modified catalysts, although energy-dispersive X-ray spectroscopy showed the presence of Ni species in the two catalysts (figure not shown). These results show that as in the case of Ni/SBA-15-βCD, the NiO particles in Ni/SBA-15-αCD and Ni/SBA-15-γCD were too small to be observed by TEM. It can be speculated that the NiO particles are dispersed inside the SBA-15 channels.
The reduction behavior of NiO and the interactions between NiO particles and the supports was investigated using H2-TPR; the results are shown in Fig. 4. Only one wide NiO reduction peak, at 600 °C, can be observed for the Ni/SBA-15-im catalyst. The NiO reduction profiles of the Ni/SBA-15 catalysts modified with αCD or γCD were similar to each other; there were two NiO reduction peaks for each of the CD-modified catalysts, with one centered at 400-500 °C and the other centered at 700-800 °C. Generally, the positions of the NiO reduction peaks reflect the NiO particle sizes and the strength of the interactions between NiO and the supports. The lower the NiO reduction temperature on the catalyst, the larger the NiO particles and the weaker the interactions between NiO and the support [20]. It can therefore be speculated that modification with αCD or γCD greatly decreased the size of the NiO particles, and that the interactions between NiO and the support were much improved over the αCD- and γCD-modified catalysts.
The catalytic performance of the Ni/SBA-15 catalysts in CRM to syngas under the reaction conditions of atmospheric pressure, 700 °C, and gas hourly space velocity (GHSV) 1.28 × 104 mL/(g h) was evaluated. The results are shown in Fig. 5. Ni/SBA-15-im showed the lowest CH4 and CO2 conversions (the initial CO2 conversion was 85% and the initial CH4 conversion was 71%). The CH4 conversion increased to 85% and the CO2 conversion increased to about 95% over the CD-modified catalysts, i.e., modification with CDs improved the catalytic activity.
Long-term stability tests showed that the conversion of CH4 and CO2 decreased gradually over Ni/SBA-15-im during 50 h, whereas the modified catalysts showed excellent stability. As in the case of modification with βCD, modification with αCD or γCD not only improved the activity but also enhanced the stability of the Ni/SBA-15 catalyst.
The crystal structure of the spent Ni/SBA-15 catalysts was examined using XRD; the results are shown in Fig. 6. The Ni and C diffraction peaks for the unmodified Ni/SBA-15-im catalyst are much sharper than the corresponding peaks for the CD-modified catalysts. The sharper Ni peak indicates that aggregation was severe and much larger Ni particles were formed on Ni/SBA-15-im during CRM, and the sharper peak for C implies that larger amounts of C were deposited on Ni/SBA-15-im.
The amounts of C deposited were determined using thermogravimetric analysis (TGA); the results are shown in Fig. 7. The mass loss at temperatures lower than 200 °C was ascribed to removal of adsorbed water, whereas the mass loss at about 600 °C arose from oxidation of deposited carbon. The amount of deposited carbon was calculated from the mass loss at about 600 °C; the results are listed in Table 1. The amounts of C deposited on the CD-modified samples (3.8%-4.9%) were much lower than those deposited on Ni/SBA-15-im (28.9%), showing that modification with CDs improved the anti-carbon- deposition properties of the Ni-based catalysts.
As stated above, compared with the Ni/SBA-15-im catalyst prepared by the traditional impregnation method, Ni/SBA-15 catalysts prepared by modified impregnation methods using αCD or γCD had smaller NiO particles and exhibited stronger ability to resist C deposition in CRM to syngas. In a previous study [18], the mechanism of the improvement of Ni particle dispersion by βCD was investigated. In this study, the reasons for the improvement in NiO particle dispersion by αCD or γCD during catalyst preparation processes were examined and confirmed. Aqueous solutions of Ni(NO3)2 and the promoter (αCD or γCD) were prepared and analyzed using mass spectrometry. For both solutions, peaks attributed to complexes formed between Ni2+ and αCD or γCD were observed in the MS positive scan (αCD + 2Ni2+, αCD + 5Ni2+, or γCD + 2Ni2+), and peaks attributed to complexes formed between NO3− and αCD or γCD were observed in the MS negative scan (αCD + NO3−, αCD + 4NO3−, or γCD + 2NO3−). These results show that interactions occurred between Ni(NO3)2 and the promoter (αCD or γCD). It has been reported that αCD and γCD can encapsulate metal cations [21]. It can therefore be speculated that when αCD- or γCD-modified impregnation methods are used to prepare the catalysts, Ni2+ species are encapsulated within the CD cavities, and NO3− ions connect with the CD hydroxyl groups through hydrogen bonds to achieve charge balance (Fig. 8). Ni2+ then migrates into the SBA-15 channels together with the CDs, in the form shown in Fig. 8. These Ni species do not aggregate easily because of the presence of the CDs, which separate the Ni species and protect them from sintering. However, in the preparation of Ni/SBA-15-im by the conventional impregnation process, Ni2+ migrates into the SBA-15 channels under the action of concentration gradient, and the Ni species are sintered during thermal treatment and cannot be well dispersed.
αCD and γCD were suitable promoters for improving the dispersion of Ni over Ni/SBA-15 catalysts. αCD or γCD exerted effects similar to that of βCD. Compared with the unmodified Ni/SBA-15-im catalyst, the Ni/SBA-15-αCD and Ni/SBA-15-γCD catalysts had much smaller Ni particles and gave better catalytic performance in CRM to syngas. A mechanistic study showed that interactions occurred between Ni(NO3)2 and the CDs, and these changed the states of the Ni species and affected NiO dispersion.
2015, Vol. 36 Issue (3): 283-289
PDF (935 KB)
2015, Vol. 36 Issue (3): 283-289
PDF (935 KB)