Regeneration study of Ni/hydroxyapatite spent catalyst from dry reforming
Graphical abstract
Introduction
Carbon dioxide (CO2) reforming of methane (CH4) (Eq. (1)) has gained increasing attention due to the conversion of two main greenhouse gases into syngas (mixture of hydrogen and carbon monoxide), which can further be used in the production of energy and of high-value added chemicals [1], [2], [3]. Nickel-based catalysts have been extensively investigated for this process due to its high catalytic activity in this reaction. However, they undergo severe deactivation due to catalyst sintering under the high temperatures required for the reaction and mostly due to coke deposition over the active sites of the catalyst [4], [5]. In fact, many parallel reactions occur along with the dry reforming reaction (DRM) (Eq. (2)–(5)) and coke can originate from many of these reactions (Eq. (2)–(4)).
So, in the past few years, many studies have focused on the improvement of the physico-chemical properties of the catalysts and on the optimization of the reaction conditions in order to obtain high catalytic conversion and stability during long periods of time [6]. They include the use of basic promoters, of supports with oxygen storage capacity, the combination of different transitions metals, the optimization of preparation pathways of the catalysts etc [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Also, temperature, pressure, reactants ratio and type of reactor have been investigated [10], [20], [21].
Nevertheless, the capacity of regeneration of the catalysts is also an important parameter. The regeneration process allows cleaning the coke deposit from the surface and the pores of the catalyst without destroying or modifying the structure of the catalyst [7]. This aspect has economic and environmental impact. In fact, the costs of regeneration should remain lower than the costs of obtaining fresh catalysts [7]. Moreover, regenerating the catalyst instead of disposing it as solid waste is an environmentally friendly option since the spent catalyst can form toxic metal compounds in the environment [22]. So, regeneration studies of spent catalysts have also been conducted aiming to re-use the deactivated catalysts.
Vicerich et al. [23] investigated the regeneration with oxygen diluted in nitrogen at 450–550 °C of the PtReIn/Al2O3 catalyst used in cyclohexane dehydrogenation and cyclopentane hydrogenolysis. They showed that the regeneration process could fully recover the activity of the spent catalyst. Wu et al. [15] showed that the regeneration at 300 °C of the Rh0.1Ni10/BN catalyst could remove the carbon deposit and reactivate the catalyst, but a small decrease in the activity was observed. Simson et al. [24] investigated the regeneration of a Pt/Rh catalyst after steam reforming of an ethanol/gasoline mixture. They showed that thermal treatment under air could fully restore the performance of the catalyst. However, after regeneration, their catalyst deactivated more quickly than the fresh one. Similarly, Sanchez et al. [25] reported that the catalytic performance of their reforming catalyst could be recovered by air combustion of the coke deposits but its selectivity to H2 quickly decreased.
Most of the literature reports on catalyst regeneration are related to the combustion of coke with air due to its availability. Also, the mechanism of carbon species oxidation by O2 has been widely investigated. There are many references in the literature to carbon bulk diffusion through the metal particle, oxygen spillover and redox mechanism [26]. However, coke deposits can be eliminated not only by O2 but also by different gasifying agents, such as steam and CO2 [27]. There are only few studies that investigated the effect of other gasifying agents such as CO2 on the regeneration of catalysts [28] and the mechanism of carbon removal by these gases is not yet well known [26].
Hydroxyapatite has been investigated as catalyst support in many different reactions such as water gas shift [29], partial oxidation of methane [30], steam reforming of glycerol [31] etc. In fact, hydroxyapatite can undergo substitutions in its crystal lattice, which may favor the incorporation of an active phase to the catalyst [32], [33]. Also, this material present high chemical and thermal stability, it has very low solubility in water and it does not sinter at temperatures lower than 700 °C [34], [35]. Moreover, it presents basic sites that can reversibly adsorb carbon dioxide and help coke removal [36]. Despite all these advantages, only few works has been published using hydroxyapatite as catalyst for dry reforming of methane reaction. We have previously demonstrated that hydroxyapatite-based catalysts were performing for dry reforming of methane reaction [37], [38]. So, in the present work, Ni/hydroxyapatite catalyst was investigated in CO2 reforming of CH4 reaction, with a focus on the regeneration process of the deactivated catalyst. The effect of gasifying agent on the re-use of the catalyst was also investigated by using two different gasifying agents: air and carbon dioxide. CO2 was used in this work because it is one of the reactants of DRM and the reaction between CO2 and solid carbon (Boudouard reaction, Eq. (2)) produces CO, which is one of the products of DRM. Also, the basic properties of the hydroxyapatite could favor the CO2 adsorption and consequently, solid carbon (C(s)) gasification. The objective of this work was to evaluate the reproducibility of the performance of the Ni/hydroxyapatite catalyst under several reaction/regeneration cycles and under different regeneration atmospheres.
Section snippets
Catalyst preparation
The hydroxyapatite (Ca10(PO4)6(OH)2) support used in this work was provided by PRAYON. Besides coke deposition, one of the reasons for catalyst deactivation is the support sintering [7]. So, as in this work the DRM reaction is performed at mild temperature (700 °C), the hydroxyapatite support was previously sintered at 1000 °C for 5 h at 10 °C/min to ensure support stability during the catalytic tests and to avoid later catalyst deactivation by support sintering. The catalyst was prepared by
Physico-chemical properties of support and catalyst
Table 1 depicts the specific surface area (SBET), pore volume (Vp) and nickel amount of the Ca-HA1_S support and of the Ni/Ca-HA1_S catalyst. For the purpose of comparison, the SBET and the Vp of the hydroxyapatite support before thermal treatment (Ca-HA1) is also shown. The initial hydroxyapatite presented 7 m2/g of specific surface area and negligible pore volume. As expected, the hydroxyapatite support (Ca-HA1_S) and the Ni/Ca-HA1_S catalyst presented very low specific surface area and pore
Conclusions
The regeneration of a hydroxyapatite-based catalyst (Ni/Ca-HA1_S) was investigated during three successive cycles of DRM/regeneration. The in situ regeneration was carried out at 700 °C for 1h30 after a cycle of DRM at 700 °C for 30 h. The capacity of regeneration of the catalyst was tested under two different atmospheres: air and 21%CO2/N2. The catalyst presented good reproducibility between cycles for both atmospheres tested and only a small deactivation between cycles was observed. The
Acknowledgments
The authors gratefully acknowledge PRAYON for their financial support to this study. The technical help of colleagues at RAPSODEE research center is also acknowledged.
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