Syngas production via dry reforming of CH₄ over Co- and Cu-promoted Ni/Al₂O₃–ZrO₂ nanocatalysts synthesized via sequential impregnation and sol–gel methods

Highlights

• Sol–gel synthesis of uniform particles of Co- and Cu-promoted Ni/Al₂O₃–ZrO₂ nanocatalysts.

• Enhancement of surface alkaline property of nanocatalysts by sol–gel method.

• Uniformity and surface area were increased by Co and Cu promoter addition.

• Syngas production with H₂/CO very close to 1 at high temperature.

• Stability of Ni–Co/Al₂O₃–ZrO₂ nanocatalyst toward biogas reforming during 24 h.

Abstract

In this study, the effect of synthesis method (impregnation and so-gel) as well as Co and Cu addition on the catalytic and physicochemical properties of Ni/Al₂O₃–ZrO₂ nanocatalyst was evaluated in dry reforming of methane. The XRD, FESEM, PSD, BET and FTIR analysis were used to characterize the nanocatalysts. Highly dispersed Ni and Zr species and no spinel production in the sol–gel synthesized samples were confirmed by XRD results. The FESEM images showed small and uniform nanoparticles in the sol–gel synthesized catalyst. Also, after Cu added to Ni/Al₂O₃–ZrO₂, the particles were more compact than others. Moreover, the promoters addition especially Co to Ni/Al₂O₃–ZrO₂ nanocatalyst improved the particles size uniformity. Particle size distribution of sol–gel synthesized Ni–Co/Al₂O₃–ZrO₂ nanocatalyst represented that the majority of the particles (nearly 95%) lies between 10 and 40 nm with an average size of 27.4 nm. BET analysis showed higher surface area in the sol–gel samples, especially when it was coupled with Co and Cu addition. The results indicated the remarkable synergetic effect of sol–gel method and Co addition on the surface morphology and elemental dispersion. Activity of nanocatalysts were evaluated as a function of temperature from 550 to 850 °C at GHSV = 24 l/g h, P = 1 atm and CH₄/CO₂ = 1. The sol–gel synthesized nanocatalyst showed better catalytic performance. Moreover, it was observed that, Cu and Co addition improved feed conversion, products yield and better syngas ratio. The comparison between promoters revealed that activity of Co was better than Cu. During the 1440 min time on stream test at 850 °C, the sol–gel synthesized Ni–Co/Al₂O₃–ZrO₂ nanocatalyst exhibited the best activity compared to the other samples and produced syngas with approximately stoichiometry ratio.

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 AlMgO_x 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):

$${\text{CO}}_2+\text{C}-\text{M}\to 2\text{CO}+\text{M}(\text{M}=\text{active}\text{phase})$$

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).