Oxidation of lean methane over cobalt catalysts supported on ceria/alumina
Graphical abstract
Introduction
Cobalt-based catalysts have been largely used as oxidation catalysts in numerous environmental applications including VOC abatement and combustion of soot [1,2]. In this context, the control of emissions from natural gas vehicles (unburned methane) over this class of metal oxides has been also explored [[3], [4], [5]]. The use of cobalt catalysts with Co3O4 as the main active phase is frequently considered as a cheaper viable alternative to expensive noble-metal based catalysts with a proven higher specific activity [6]. The notable activity of this metal oxide is related to its good redox properties, and in particular, the easiness that the cobalt ions possess to switch between its two oxidation states (+3 and +2), which provides oxygen species in the lattice with a high mobility [7]. Since the oxidation of methane by these type of catalysts is deemed to occur via a Mars-van Krevelen mechanism, where the oxygen species from the spinel lattice are responsible for the oxidation reaction [8,9], these redox properties are crucial for the activity of Co3O4-based catalysts, to the point that even catalysts with poor textural and structural properties can exhibit a relatively good performance [10,11]. Consequently, when proposing an efficient strategy for designing highly active Co3O4 catalysts it is extremely important to maintain the structure and redox properties of the spinel as intact as possible.
The most commonly applied enhancing strategy is to support the cobalt oxide over the surface of a porous media, in order to disperse it and increase the amount of the available active surface area. Generally, this option produces catalysts with high specific surface areas and a small crystallite size of Co3O4, but it presents a major drawback in the form of a strong cobalt-support interaction that often negatively affects the redox properties of the cobalt oxide [12,13]. More specifically, when the support is alumina, this cobalt-support interaction usually provokes a partial reduction of Co3+ ions to Co2+ and their subsequent fixation in the alumina lattice, which leads to the formation of cobalt aluminate (CoAl2O4) [14]. The cobalt ions fixed in this phase lose almost all their mobility and cannot revert to the +3 oxidation state at moderate temperatures (<600 °C), and therefore are rendered inactive for the oxidation reaction [15].
A possible solution to this problem is to modify the alumina support with a metallic promoter, prior to the deposition of cobalt species, in order to increase its stability and reduce its propensity to interact with the cobalt oxide deposited over it. This promoter can be added during the synthesis of the alumina support itself. Thus, Liotta et al. [16] found that adding Ba during the synthesis of Al2O3 by a sol-gel method inhibited solid state diffusion of the Co2+ ions into the alumina after Co deposition. On the other hand, Cheng et al. [17] reported that the incorporation of a fourth element during the synthesis of an alumina supported copper-cobalt catalyst improved the reducibility of both metal cations, especially when that fourth element was either Mn or Fe. Alternatively, the promoter can be deposited over the surface of a as-synthesised alumina before the deposition of cobalt. In this sense, Park et al. [18] and Park et al. [19], on different studies, observed that the addition of P to Al2O3 resulted in the partial formation of AlPO4, which suppressed the formation of CoAl2O4. This inhibition effect was also found for other metallic promoters such as Mg or Zr [20,21]. In all cases, the deposition of the promoter over the alumina favoured a cobalt-promoter interaction at the cost of a cobalt-alumina interaction. However, this interaction does not always work in favour of the redox properties of the cobalt oxide. In this sense, previous investigations on the effect of surface protection of Al2O3 with Mg revealed that the resulting Co-Mg interaction led to the formation of a low reducibility CoO-MgO solid solution, as reported by Ulla et al. [22] and Ji et al. [23].
Considering all the above-mentioned background, and based on our previous results on the beneficial effect of cerium doping of Co3O4 bulk catalysts [24], this work proposes the design of highly active Co/Al2O3 catalysts obtained by a previous incorporation of ceria onto the support prior to cobalt precipitation. The premise supporting this hypothesis is that the deposited CeO2 could have a twofold function as a physical barrier between cobalt and alumina, thus inhibiting the formation of CoAl2O4, and as a redox promoter for Co3O4, thereby enhancing the intrinsic activity of the resulting catalyst. The specific objective of the study will be to determine the amount of cerium (5–30 %wt.) to be loaded on the alumina support for the optimal performance of a catalyst with a 30 %wt.Co loading in the oxidation of methane in trace amounts (<1 %) under both dry and humid conditions.
Section snippets
Synthesis of the ceria-alumina supports and cobalt supported catalysts
The ceria-modified alumina supports were prepared by a basic precipitation route of cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O, Sigma Aldrich) on a commercial γ-Al2O3 (Saint Gobain) thermally stabilized at 850 °C for 8 h. For each support, 5 g of γ-Al2O3 were mixed with 100 ml of the cerium precursor with adjusted concentrations of Ce and then a solution of Na2CO3 1.2 M was added dropwise until the pH reached 9.5. The temperature was kept constant at 80 °C. The selected cerium loadings
Characterisation of xCe-Al supports
The composition and textural and structural properties of the ceria-modified alumina supports were characterised by WDXRF, N2-physisorption, XRD and Raman spectroscopy. Firstly, it should be pointed out that the chemical analysis revealed that the amount of cerium species deposited on the alumina support was very close to the nominal loading, namely 5, 10, 15, 20 and 30 %wt.Ce (Table 1).
The specific surface area, total pore volume and pore size distribution maxima are summarised in Table 1. The
Conclusions
The effect of cerium deposition on alumina-supported cobalt oxide catalysts for the complete oxidation of lean methane was studied. The addition of cerium (5–30 %wt.) to the alumina was carried out prior to the deposition of cobalt (30 %wt.) by a precipitation synthesis route. The textural properties of the modified supports were hardly modified with respect to those of bare alumina, due to the good dispersion of ceria over the surface of the alumina. This dispersion also was responsible for a
CRediT authorship contribution statement
Andoni Choya: Investigation, Writing - original draft. Beatriz de Rivas: Methodology, Validation. Juan Ramón González-Velasco: Project administration, Funding acquisition. Jose Ignacio Gutiérrez-Ortiz: Formal analysis. Rubén López-Fonseca: Conceptualization, Writing - review & editing, Supervision.
Acknowledgements
The author wish to thank the financial support provided by the Ministry of Economy and Competitiveness (CTQ2016-80253-R AEI/FEDER, UE), Basque Government (IT1297-19) and the University of the Basque Country UPV/EHU (PIF15/335), and the technical and human support provided by SGIker (UPV/EHU).
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2022, Journal of Molecular StructureCitation Excerpt :In all cases, the presence of gamma-alumina was detected due to reflections at 2θ = ca 37°, 45°, and 67° [1-2,5,10,15-16]. The impregnation of cobalt into alumina via classical impregnation (Fig. 1A) resulted in the strengthening of the reflexes at 2θ = ca. 31°, 33°, 37°, 59° and the appearance of the shoulder at 65°, that suggests the occurrence of cobalt phase in the form of either Co3O4 or cobalt spinel [2,5,16]. According to Fayaz et al. [36], lover 2θ at 31° and 37° indicated the presence of cobalt as Co3O4, while the signal at 59° and the shoulder at 65° can be attributed to CoAl2O4.