Biogas dry reforming over Ni/LnO_x-type catalysts (Ln = La, Ce, Sm or Pr)

Highlights

• Ni/LnO_x catalysts (Ln = La, Ce, Sm or Pr) were tested for biogas dry reforming.

• LaNiO₃ was formed for La and segregated NiO-LnO_x for the other lanthanides.

• Decomposition of LaNiO₃ perovskite led to smaller Ni nanoparticles.

• LNO presented improved activity but high activation energy and reduced stability.

• Deactivation observed due to amorphous and crystalline coke formation.

Abstract

Ni/LnO_x-type catalysts (Ln = La, Ce, Sm or Pr, denoted as LNO, CNO, SNO and PNO, respectively) were prepared via a citrate sol-gel method, characterized, and evaluated for the dry reforming of biogas. For the calcined catalysts, the formation of LaNiO₃ perovskite crystallites with high purity was observed in the case of La, whereas NiO-LnO_x mixed oxides were obtained for the other lanthanides. The reduction treatment led to the formation of medium-sized (∼15 nm) and highly dispersed Ni nanoparticles in LNO following the decomposition of the LaNiO₃ perovskite, in contrast to the other catalysts, where bigger Ni crystallites were formed (∼30 nm). As a result, LNO was shown to possess a higher catalytic activity in comparison to the other materials. Regarding the catalytic stability, LNO displayed a considerable activity loss followed by a high pressure drop due to reactor blockage, meaning that the use of Sm (Ni/Sm₂O₃) can be considered as an alternative strategy to restrict catalyst deactivation. As evidenced by the characterization of the spent catalysts, the deactivation for the most part can be attributed to the extensive coke deposition over the catalysts. The coke deposited was found to be both in the form of more disordered/amorphous carbon, as well as in the form of highly crystalline and multi-walled carbon nanotubes.

Introduction

The Glasgow Pact, resulting from COP26, expressed alarm at the fact that around 1.1 °C of warming has been caused by human activities, and at the rapid depletion of the Earth's remaining carbon budget consistent with 1.5 °C. Worryingly, despite the efforts undertaken over the past few decades to develop renewable energy resources, fossil fuels still provide around 80% of the world's energy supply [1,2]. Our reliance on fossil-based energy, apart from threatening the survival of our species, is also responsible for more immediate concerns related to the accessibility and affordability of these resources (and ultimately equity), especially since the continuity of supply is subject to the political will of only a handful of countries, which are often undemocratic or disregard the rule-based international order, as so vividly made clear by the recent events unfolding in Eastern Europe.

The use of biomass, as well as the utilization of CO₂, have long been identified as key pillars on which the future energy and chemical industry can be based [[3], [4], [5], [6], [7]]. Biogas in particular, a product of the anaerobic digestion of organic wastes, not only constitutes a promising renewable energy resource, but it can simultaneously provide an efficient way to manage the ever-increasing amounts of municipal, industrial and agricultural wastes [8,9]. Although the composition of biogas depends heavily on the type of biomass from which its being produced, it mainly consists of CH₄ (50–75%) and CO₂ (25–45%), i.e., the main greenhouse gases, water vapor (2–8%) and trace amounts of O₂, N₂, NH₃, H₂, H₂S [9,10], siloxanes, chlorides, and volatile organic compounds [11]. Biogas has a lower energy density than natural gas, which hinders application in combustion engines, however, its high CO₂ content makes it an ideal candidate for dry reforming applications (Eq. (1)) [[11], [12], [13], [14]].

$$\mathrm{D}\mathrm{R}\mathrm{M}:C{H}_4+C{O}_2\to 2CO+2H_2,\Delta H_{298K}=247kJmo{l}^{–1}$$

The dry reforming of biogas, in essence the dry reforming of methane (DRM), (Eq. (1)), is highly endothermic and produces syngas with a theoretical 1:1 H₂:CO molar ratio [15,16]. However, the process also poses the formidable challenge of controlling catalyst deactivation due to coke deposition, and the sintering of the metallic particles (e.g., Ni), that constitute the active phase. The former is induced mainly via the decomposition of CH₄ (Eq. (2)) and the disproportionation of CO (Boudouard reaction) (Eq. (3)) [17,18]. The latter is the result of the high temperature required to overcome the Gibbs free energy barrier of the reverse Boudouard reaction (700 °C) and the reverse water gas shift (RWGS) reaction (815 °C) (Eq. (4)) [19,20].

$$\mathrm{C}{\mathrm{H}}_4\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:C{H}_4\to C_{ads}+2H_2,\Delta H_{298K}=75kJmo{l}^{–1}$$

$$\mathrm{B}\mathrm{o}\mathrm{u}\mathrm{d}\mathrm{o}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{d}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:2CO\leftrightarrow C_{ads}+C{O}_2,\Delta H_{298K}=-172kJmo{l}^{-1}$$

$$\mathrm{R}\mathrm{W}\mathrm{G}\mathrm{S}:C{O}_2+H_2\leftrightarrow CO+H_{2}O,\Delta H_{298K}=41kJmo{l}^{-1}$$

The product of the DRM reaction, syngas, a mixture of CO and H₂, can be converted via Fischer-Tropsch synthesis to a wide range of hydrocarbons [21] or via the water gas shift (WGS) reaction to H₂. Although the interest in hydrogen energy has waxed and waned over the past few decades, technological advancements in its scaled-up production and storage mean that the sector is in the cusp of a much-anticipated boom [22,23]. Hydrogen has numerous advantages in comparison to other energy carriers, as its combustion does not emit CO₂ or CO and can be used for both electricity production and transportation [23,24]. At present, 98% of H₂ is being produced using naphtha, methane and coal [25], which means that efforts to produce it in a sustainable way should be intensified.

Thus, it is urgent to develop catalytic systems with increased activity and durability at relatively low reaction temperatures. Although noble metals materials, such as Rh, Pt, Pd, and Ru have been shown to be highly active and more resistant to carbon deposition [26], Ni-based catalysts have attracted considerable research interest, as they too show good activity but are also much cheaper in comparison, advantages that make them an attractive choice for potential industrial applications. However, Ni-based catalysts present two major drawbacks, as they are prone to, firstly, the formation of coke on their surface (which covers the active sites and blocks the porous system), and secondly, to the aggregation of Ni nanoparticles (a result of the high reaction temperatures), a phenomenon that reduces the population of active sites on the catalysts surface [[27], [28], [29]]. As shown by a number of works in the literature [30,31], coke deposition can be minimized by supporting materials with oxygen vacancies, increased oxygen storage capacity, and surface basic sites, while smaller Ni nanoparticles in enhanced interaction with the support can restrict the agglomeration of Ni nanoparticles.

A plethora of metal oxides (e.g., Al₂O₃, ZrO₂, CeO₂, La₂O₃) and multi-composite oxides have been widely used as supporting materials in reforming reactions of biogas [12,32]. Dan et al. [33] investigating the catalytic performance of Ni/γ-Al₂O₃ and Ni/CeO₂/γ-Al₂O₃ catalysts (concentrations of Ce equal to 5 and 15 wt%) in the reaction of steam reforming of biogas argued that ceria-containing catalysts presented the highest CH₄ and CO₂ conversion values for the entire temperature range, due to the larger dimension of pores, the better reducibility, the improved intimate contact between Ni, alumina and cerium oxide, and the more stable Ni nanoparticles. Armbruster et al. [34], studying the effects of promoters (Gd, Sc, La) on the stability of Ni-based catalyst during biogas dry reforming, argued that the addition of Sc and of Gd caused the generation of stronger metal-support interaction (SMSI) reinforcing the stabilization of Ni nanoparticles during the reaction, which is very important to prevent catalyst deactivation. It has also been shown that La₂O₃, due to its basic characteristics, favors the adsorption and dissociation the gaseous CO₂, reinforcing the catalytic activity at relatively low temperature range [29,35]. Furthermore, the presence on La₂O₃ on the catalyst surface enhances the resistance of the catalyst against deactivation due to the formation of La₂O₂CO₃, which reinforces the removal of coke deposits from adjacent Ni nanoparticles [29,35].

To avoid sintering and coke formation, the idea of stabilizing active metals as ionic lattice sites in stable structured perovskite oxides has attracted considerable research attention. Perovskites can be described by the general formula ABO₃, where A can be an alkali or alkaline earth, or rare earth metal (e.g., La, Sr, Ca, Ce, Pr, Sm) and B can be a transition metal (e.g., Ni, Co, Fe, Mn) [36]. The catalytic activity of these materials depends on the nature of A and B elements, synthesis method used and their partial substitution. They have controllable structure, high thermal stability and high catalytic efficiency, which makes them ideal candidates for processes carried out at high temperatures, like biogas dry reforming [37]. Usually, in the DRM and other methane conversion reactions, the perovskite-derived catalysts are more active and stable compared to supported catalytic materials, due to the stronger metal-oxide interaction and the higher dispersion of the active sites [37,38]. In the case of Ni-based perovskites, gradual reduction of Ni occurs to form highly dispersed Ni nanoparticles with high activity and resistance to coke formation under DRM conditions, which is sometimes accompanied by the destruction of the initial perovskite matrix (e.g., in the case of LaNiO₃) [37]. Additionally, different methods have been proposed in the literature (i.e., Pechini [39], sol-gel [40], co-precipitation [41], auto-combustion [42]) to synthesize perovskites with increased surface area and small crystallite sizes.

The experimental research study presented herein investigates the structure, physicochemical properties, and catalytic performance of Ni/LnO_x (Ln = La, Ce, Sm, Pr) catalytic materials in synthesis gas production from biogas via the dry reforming process. The samples were prepared via the citrate sol-gel method and thoroughly characterized. The investigation of the catalytic activity was carried out in the temperature range of 600–750 °C, while short and long time-on-stream experiments were performed under different reaction conditions in order to investigate coke deposition and the degree of aggregation of the Ni active sites.