Syngas production via dry reforming of CH4 over Co- and Cu-promoted Ni/Al2O3–ZrO2 nanocatalysts synthesized via sequential impregnation and sol–gel methods
Graphical abstract
XRD results represented that sol-gel method prevents spinel production because of well dispersed ZrO2. A uniform morphology, high surface area and a high dispersion of active metals were observed in Ni–Co/Al2O3–ZrO2 nanocatalyst. It indicates that Ni ensemble size is controlled by using of sol-gel method and Co addition as promoter. FTIR analysis indicated sol–gel method increases surface basicity of catalysts due to high vibrating of –OH bounds. Activity of Ni based catalyst lightly increased by employing Cu as promoter. Activity tests showed Ni–Co/Al2O3–ZrO2 catalyst has the best activity and produces syngas with H2/CO molar ratio equal to 1.01. The activity of all catalysts remained stable at 850 °C during 1440 min which is a remarkable outcome in compression with similar last studies.
Introduction
Dry reforming of methane (DRM) is the main process for production of synthesis gas with H2/CO molar ratio of 1, which is the ideal rout for production of oxygenated compounds and Fischer–Tropsch synthesis (Zhang et al., 2007, Nagaraja et al., 2011, Zhang et al., 2008, San-Jose-Alonso et al., 2009, San Jose-Alonso et al., 2013, Rahemi et al., 2013a). The industrial application of this process is faced with several issues, including high cost of energy supply due to the endothermic property of the main reaction (Zhang et al., 2007, Nagaraja et al., 2011, Zhang et al., 2008, Therdthianwong et al., 2008). Providing a cheap and available feedstock as well as suitable catalyst with high activity and stability which will decrease the high expense of CO2 reforming of methane in industrial applications (Zhang et al., 2007, Zhang et al., 2008, San-Jose-Alonso et al., 2009, Aasberg-Petersen et al., 2011, Foo et al., 2011).
Biogas (BG) is a very cheap and available energy resource. Therefore, it could be an economical resource for this process (Zhang et al., 2008, San-Jose-Alonso et al., 2009, Therdthianwong et al., 2008, Zhang, 2008, Djinovic et al., 2012). Moreover, consumption of two important greenhouse gases (CH4 and CO2) and prevention of their emission are advantages of using biogas as a feedstock (San-Jose-Alonso et al., 2009, Li et al., 2010, Qian and Yan, 2003, Li and Wang, 2004, Xu et al., 2009, Wang and Ruckenstein, 2000).
Reported studies have shown that Ni/Al2O3 catalyst demonstrates higher activity and is more economical than the other catalysts in this process (Sajjadi et al., 2013, Rahemi et al., 2013b, Rahemi et al., 2013c, Aghamohammadi et al., 2013). However, it suffers from low stability due to coking, sintering and production of spinel composition (Therdthianwong et al., 2008, Pompeo et al., 2007, Pompeo et al., 2005). Different studies have been carried out in order to improve the activity and stability of Ni/Al2O3 catalyst by adding promoters and using effective synthesis methods (Nagaraja et al., 2011, Zhang et al., 2008, San-Jose-Alonso et al., 2009, Rahemi et al., 2013a, Foo et al., 2011, Djinovic et al., 2012, Li and Wang, 2004, Pompeo et al., 2007, Horvath et al., 2011, Amairia et al., 2010a, Amairia et al., 2010b, Abbasi et al., 2011). Certainly, the proper operating conditions such as CH4/CO2 = 1 and the optimized composition of promoters could affect the catalyst performance. In recent studies, all of these parameters have not been considered simultaneously (Zhang et al., 2007, Zhang et al., 2008, San-Jose-Alonso et al., 2009, San Jose-Alonso et al., 2013, Rahemi et al., 2013a, Foo et al., 2011, Li et al., 2010, Horvath et al., 2011).
Noble metals and other metals are two groups of active phase promoters which have been studied to improve the Ni based catalyst stability. For industrial applications, the second group is more appealing due to their lower cost and wider availability (Horvath et al., 2011). Zhang et al. used Mn, Cu, Co, Fe as promoters of Ni over AlMgOx for dry reforming of methane. Their results showed that the stability increased by addition of Co and Cu (less than 5 wt%). It has been proved that the Ni–Co catalyst exhibits the active phase with small size and excellent dispersion (Zhang et al., 2008, San-Jose-Alonso et al., 2009, San Jose-Alonso et al., 2013, Djinovic et al., 2012, Li et al., 2010, Roberts and Elbashir, 2003). The CO2 activation is enhanced over the small metal particles, which are a dam against the coke deposition. According to Le Chatelier's principle, the reverse Boudouard reaction will proceed when the CO2 adsorption increases (Zhang et al., 2008):
Additionally, it has been reported that the formation of Ni–Co improves catalyst stability by enhancing the resistance of active metal oxidation. It was revealed that the oxidation form of Ni(NiO) is inactive in methane activation. After combination of Ni and O2, the nickel ability to react will be decrease; therefore the reduction of NiO to Ni is essential before dry reforming process (Yang et al., 2002).
On the other hand, the addition of Cu as a promoter to Ni/Al2O3, increases methane decomposition and could modify the Ni ensemble environment by the Cu–Ni formation to suppress the coke deposition. Reduction improvement (Li et al., 2009, Lopez et al., 2012) and low content of coke deposition are the benefits of Ni–Cu bimetallic catalyst application for methane decomposition reaction (Reshetenko et al., 2003, Bonura et al., 2012). Lee et al. (Lee et al., 2004) showed that the carbon deposition was enhanced over Cu–Ni/Al2O3 catalyst when more than 5 wt% Cu was added. Therefore, in this study, cobalt and copper were chosen as promoter components to meet the discussed properties and evaluate their effects, comparatively.
Among the support promoters used for Ni/Al2O3 catalyst in methane reforming processes, ZrO2 has created especial physicochemical properties for this catalyst. It acts as a barrier against the entrance of active phase into alumina structure and formation of spinel. These components decrease the activity of active phases due to the fact that the spinels reduction is very hard (Rahemi et al., 2013a, Therdthianwong et al., 2008, Qian and Yan, 2003, Pompeo et al., 2007, Amairia et al., 2010b, Sosa Vazquez et al., 2005, Han et al., 2013). ZrO2 is known as an acid-base bi-functional catalyst support. According to the reactant species in the process environment, zirconia could be an acid or base catalyst (Tanabe and Yamaguchi, 1994). CO2 is an acid gas, so it needs base sites for adsorption on the surface catalyst. Therefore, ZrO2 could act as a base catalyst in dry reforming (Rahemi et al., 2013a, Therdthianwong et al., 2008, Pompeo et al., 2007, Therdthianwong, 2008). Rahemi et al. (Rahemi et al., 2013a) investigated Ni/Al2O3 and Ni/Al2O3–ZrO2 nanocatalysts synthesized via impregnation and treated with non-thermal plasma in dry reforming of methane. They observed that the catalytic activity and stability of plasma-treated Ni/Al2O3–ZrO2 nanocatalysts were higher than that of conventional catalyst. Unfortunately, there are a number of current challenges from practical point of view, preventing non-thermal plasma from being widely implemented. Using the sol–gel method as an effective nanomaterial synthesis method can facilitate the uniform dispersion of active component on the catalyst. Unique morphology, small particle size and sequentially high surface area and metal support interaction are the some other advantages of this method (Qian and Yan, 2003, Horvath et al., 2011, Tanabe and Yamaguchi, 1994, Seo et al., 2009, Al-Fatesh and Fakeeha, 2012).
Therefore, it seems that by using the best composition of suitable promoters and implementing proper preparation method, Ni/Al2O3–ZrO2 nanocatalyst could be a promising nanocatalyst for dry reforming process. To this aim, the effect of sol–gel method as well as Co and Cu addition as active phase promoters were assessed on the properties and performance of Ni/Al2O3–ZrO2 nanocatalyst. For comparison and evaluation of preparation method, Ni/Al2O3–ZrO2 nanocatalyst was also synthesized by sequential impregnation method. The synthesized nanocatalysts were characterized by XRD, FESEM, PSD, BET and FTIR techniques. Biogas was simulated with CH4 and carbon dioxide in a molar ratio of 1 without dilution gas. Nanocatalysts performance were evaluated as a function of reaction temperature from 550 to 850 °C at atmospheric pressure and 40 ml/min flow rate (GHSV = 24 l/g h).
Section snippets
Materials
For preparation of the nanocatalysts using sol–gel method, respective metal nitrates and citric acid as gelling agent were applied. In the impregnation method, γ-Al2O3 powder was used. The other precursors of impregnated sample were the same as the sol–gel synthesized nanocatalysts. All of the precursors were supplied from Merck Co. except Zr(NO3)4·5H2O which was provided from Aldrich Co.
Nanocatalyst preparation and procedures
Fig. 1 displays schematic flowchart for the preparation steps of the nanocatalysts synthesized by
XRD analysis
The X-ray diffraction patterns of γ-Al2O3 (a), NAZ-I (b), NAZ-SG (c), NCuAZ-SG (d) and NCoAZ-SG (e) nanocatalysts are illustrated in Fig. 3. The diffraction peaks of NAZ-I nanocatalyst at about 2θ = 37.4, 39.7, 42.8, 45.8 and 67.3° (JCPDS 00-004-0880) are assigned to γ-Al2O3. Moreover, respective XRD pattern (Fig. 3(b)) shows the presence of NiO cubic structure (JCPDS 01-073-1519) as indicated by diffraction peaks at 2θ = 37.3, 43.4, 63.0, 75.6 and 79.6°. It can be identified that XRD data
Conclusions
The results of this study showed that the sol–gel method had excellent effects on physicochemical properties of Ni/Al2O3–ZrO2 nanocatalyst in comparison to impregnation method. Furthermore, it is shown that activity and stability of this nanocatalyst were improved by Co addition in comparison to Cu in dry reforming. The syngas production with ratio of nearly unity was the one of the particular conclusions compared to last studies. In summary, Ni–Co/Al2O3–ZrO2 had the excellent activity and
Acknowledgements
The authors gratefully acknowledge Sahand University of Technology for the financial support of the research as well as Iran Nanotechnology Initiative Council for complementary financial support.
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