Dry reforming of methane over La₂O₂CO₃-modified Ni/Al₂O₃ catalysts with moderate metal support interaction

Highlights

• La₂O₂CO₃ modified Ni/Al₂O₃ was synthesized by calcination under a CO₂ atmosphere.

• The interaction between Ni and Al₂O₃ was adjusted by La₂O₂CO₃.

• La₂O₂CO₃ modified Ni/Al₂O₃ exhibited superior activity and stability in DRM.

• The addition of La increases the alkalinity and promotes the activation of CO₂.

Abstract

The modulation of the interaction between the active metal and the underlying support material plays a key role and remains a challenge in many important catalytic reactions. This paper describes a new method for the modification of Ni/Al₂O₃ by La₂O₂CO₃, which is responsible for modulating the interaction between Ni and the Al₂O₃ support. When La₂O₂CO₃ is loaded on the Al₂O₃ support as a La₂O₃ precursor, combined with XRD, H₂-TPR, H₂-pluse chemisorption, Raman spectroscopy and TEM characterization, it is found that La₂O₂CO₃ can prevent Ni from entering the alumina support and forming NiAl₂O₄. This strategy allows more Ni to be reduced and exposed as active sites. Consequently, the La₂O₂CO₃ modified Ni/Al₂O₃ exhibited superior activity with respect to the reference La₂O₃ modified Ni/Al₂O₃ and original Ni/Al₂O₃ in dry reforming of methane (DRM). At the same time, La can cover the acidic sites of alumina and increase the alkalinity of the catalyst, thereby promoting carbon dioxide adsorption and activation, and inhibiting carbon deposition. In addition, the introduction of La also inhibits the sintering of Ni under harsh reaction conditions by enhancing the interaction between Ni metal and the support. This modification method can simultaneously improve the reactivity and stability of the catalyst by altering the interaction between the metal and the support.

Introduction

Al₂O₃ is a common and important support in industrial catalysts, with the advantages of large specific surface area and good thermal stability [[1], [2], [3]]. Al₂O₃-supported catalysts have a wide range of applications, from petrochemical to automotive tail gas treatment. Among them, Ni/Al₂O₃ has been widely studied in fuel reforming [[4], [5], [6]]. It has the advantages of large specific surface area, strong metal support interaction, high metal dispersion and high catalytic activity. However, as an acidic support, Al₂O₃ cannot provide the site for CO₂ activation in reactions such as dry reforming of methane (DRM). In the absence of adsorbed CO₂, Ni/Al₂O₃ suffers from deactivation due to severe carbon deposition from CH₄ during the DRM reaction [4].

In order to improve the anti-coking performance of Ni/Al₂O₃, alkali additives are usually introduced to promote the activation of CO₂ [7,8]. Among the common additives, rare earth elements such as La and Ce are more effective [[9], [10], [11], [12]]. However, La and Ce have completely different interactions with Al₂O₃. Fornasiero et al. studied the effect of La₂O₃ and CeO₂ addition to Pt/Al₂O₃ [13]. They found that La₂O₃ exhibits monolayer dispersion state on the surface of the catalyst, while CeO₂ tends to exist in the form of crystals. This means that there is a strong interaction between La₂O₃ and Al₂O₃. In addition, Bueno et al. demonstrated that La₂O₃ has stronger ability to suppress carbon deposit than CeO₂ [14]. Based on our previous work, although La₂O₃ has good performance for inhibiting carbon deposition, the low surface area (about 10 m²/g) strongly restricts its applications [[15], [16], [17]]. Loading La₂O₃ onto Al₂O₃ is one of the possible effective strategies to combine the beneficial effects of the alkalinity of La₂O₃ and large specific surface area of Al₂O₃.

There has been a number of research work on La₂O₃ modified Al₂O₃ catalysts [[18], [19], [20]]. Bernal et al. investigated that the influence of calcination temperatures on La₂O₃/Al₂O₃ sample, with the La₂O₃ loading close to monolayer [21]. They concluded that the calcination temperatures have a significant effect on the acid-base properties of the sample. However, so far, few studies have focused on the performance of La₂O₃ modified Ni/Al₂O₃ in DRM when the loading of La₂O₃ is close to monolayer. Our recent research studied that La-modified Ni/Al₂O₃ in ethanol steam reforming (ESR), and the results show that the optimal loading of La is much lower than that of monolayer [17]. This is because Ni can penetrate the La₂O₃ layers and form a strong interaction between Ni and Al₂O₃, when the amount of La is excessive, resulting in a decrease of Ni surface area. This phenomenon has also been confirmed by many other studies [6,8,9,22,23]. For example, Verykios et al. found that there was an induction process for Ni on La₂O₃ support before its catalytic activity reached maximum because the Ni elements covered by La₂O₃ were gradually reduced in the initial stage of the reaction [24]. Therefore, it would be more beneficial to prevent the migration of Ni from the surface to the bulk of the support.

In this paper, we designed a new synthesis method for La-modified Ni/Al₂O₃, in which the calcination of La₂O₃ on Al₂O₃ is under a CO₂ atmosphere to form La₂O₂CO₃ instead of La₂O₃ and thus prevent Ni from entering the bulk of Al₂O₃ and being covered with La₂O₃. The physical-chemical properties of the catalysts were analyzed by N₂-physisorption, H₂-temperature programmed reduction (H₂-TPR), H₂-pluse chemisorption, X-ray diffraction (XRD), Raman spectra and transmission electron microscopy (TEM). With the above analysis, the relationship between catalytic activity and structure was established. In addition, CO₂-temperature programmed desorption (CO₂-TPD) and thermogravimetric analysis (TGA) are applied to examine the basicity and deposited coke of samples during the DRM reaction.