Continuous CO2 abatement via integrated carbon capture and conversion over Ni-MgO-Al2O3 dual-functional materials
Graphical abstract
Introduction
Since the Industrial Revolution, rapid population growth and the burst of fossil energy consumption have led to massive CO2 emissions [1]. For the first time in over 800,000 years, CO2 concentration in the atmosphere rose above 400 ppm. As a result, CO2 reduction is recognized as an unavoidable task among the international community [2], and carbon neutrality is becoming a new benchmark to measure the global ambitions to combat climate change. In this context, CO2 capture, utilization and storage (CCUS) is widely accepted as an indispensable technical pathway to achieve carbon neutrality [3], [4].
The past few years witnessed great development of the CCUS technology, several large-scale demonstrations came on-line, and valuable engineering experiences are established. However, cost of the existing CCUS technology is still very high, this represents one of the major bottlenecks for their wide deployment [5], [6]. In order to further lowering the cost of CCUS, research efforts were devoted to the development of more efficient materials and processes. However, most of the previous practice was focused on single segment of the CCUS chain, e.g. discovering new adsorbents for CO2 capture, novel routes for catalytic CO2 conversion, etc. On the other hand, process integration within the CCUS framework has long been overlooked.
Recently, a new strategy of integrated carbon capture and conversion (ICCC) was proposed [7], [8], [9], [10]. Center to this idea is that CO2 in its captured form is directly converted, therefore the energy requirement for CO2 desorption and activation can be integrated on a molecular level. As a result, overall cost of CO2 mitigation by CCUS can be lowered considerably as the recovery of CO2 from a capture agent (e.g. amine-based absorbent) accounts for more than 60 % energy input in the entire CCUS chain.
Despite that development of ICCC is still in its early infancy, several approaches focusing on different products including methanol [8], [11], [12], [13], [14], methane (CH4) [2], [9], [15], [16], [17], [18], [19], [20], synthesis gas [21], [22], [23], [24], and others [25], [26], [27] have already been verified. Among them, ICCC to CH4 is particularly interesting, this is because that as compared to other CO2 conversion processes, the reaction conditions of methanation are significantly milder (ca. 300 °C and atmospheric pressure). Therefore, when it is applied for the decarbonization of industrial flue gas, less temperature and pressure alteration will be needed, resulting in lower energy consumption. Moreover, the methanation reaction is characterized by high selectivity and conversion rates, and the produced CH4 can be directly used in the existing energy infrastructure [10], [19].
The ICCC to CH4 process can be realized by several strategies [28], [29]. For example, Veselovskaya et al. designed a “capture-conversion tandem bed” set-up, namely a fixed-bed adsorber loaded with K2CO3/Al2O3 composites was directly connected with a reactor charged with Ru/Al2O3 methanation catalysts. Regeneration of the CO2 loaded adsorbents was carried out in H2, and the desorbed gas flow going straight from the adsorber outlet to the preheated catalytic reactor, where the CO2 can be converted to CH4 [30]. Alternatively, Miguel and co-workers showed that in a reactor with layered K-based adsorbents and nickel-based catalysts, the ICCC to CH4 process can be proceeded at 300–350 °C [15]. This adsorbent-catalyst mixing strategy was also adopted by Sun et al. [29]. Apart from the above, direct integration of adsorption and catalysis in one material, namely the development of dual-functional materials (DFMs) that are effective for both CO2 adsorption and conversion represents the research frontier for the ICCC to CH4 process [19]. Previously, oxides and carbonates of alkali metals (Na, K) and alkaline earth metals (Ca, Mg) have been widely employed as the adsorption sites in DFMs [9], [30], [31], and metals including Ni, Ru, and Rh were investigated as the catalytic functionalities [16], [32], [33]. For example, Farrauto’s group reported a range of DFMs for the ICCC to CH4 process [9], [31], [33], [34], [35], [36], [37], [38], and the influence of components, compositions, and reaction conditions was comprehensively investigated. It was found that compared with CaO, using K2CO3 and Na2CO3 as the adsorption site resulted in higher CH4 yield [35]. On the other hand, Ni is more vulnerable in oxidative atmosphere, and its activity and stability are slightly lower than that of Ru and Rh [32], [36], [37], [39], [40]. Nevertheless, due to its cost effectiveness, Ni has also received a lot of attention [16], [36]. In the previous work of our group, 2D-Layered Ni-MgO-Al2O3 nano-sheets were prepared by a coprecipitation method, and for the first time, continuous decarbonization of simulated flue gas by the ICCC strategy was demonstrated [41].
Clearly, synergy between the adsorption and catalysis sites in DFMs is crucial to achieve high ICCC efficiency. This is to say that except general factors such as composition, pore structure and crystallinity, considerations on concentration and activity balance between the adsorption and catalytic sites are also needed for the preparation of DFMs [42]. Currently, impregnation and coprecipitation are the major methods for the preparation of DFMs, both of which possesses its pros and cons. For example, DFMs prepared by impregnation of a catalytic component might result in its surface enrichment, but this also led to pore blockage and coverage of adsorption sites, and thus decreasing porosity and CO2 adsorption capacity [16]. At the same time, the DFMs prepared by impregnation are more prone to agglomeration and deactivation due to weak metal-support interaction [43]. On the other hand, coprecipitation favors the formation of a more homogeneous system with stronger metal-support interaction, but a portion of the catalytic sites may be embedded in the skeleton and thus became inaccessible to the reactants [44]. Therefore, preparation protocols to appropriately adjust the adsorption-catalysis synergy of DFMs, which is very important for the smooth operation of ICCC, are urgently needed.
Herein, Ni2+ was directly impregnated on an un-calcined Mg-Al layered double hydroxide (MgAl-LDH), the mixture was then heated to initiate simultaneous decomposition of the MgAl-LDH and Ni(NO3)2, such a protocol enabled active interaction between the pre-loaded species during calcination, leading to the formation of NiO-MgO-Al2O3 DFMs. Characterization results indicated that the obtained samples differ significantly from the sample prepared from either impregnation or coprecipitation, and more importantly, a better balance between adsorption and catalytic properties for ICCC can be achieved. By optimizing the Ni loading and calcination temperature, it was found that 20NiMgAl(5 0 0) has the best performance. In the continuous flue gas decarbonization mode similar to our previous report [41], excellent efficiency of 280 L·h−1·kg−1 can be achieved at 300 °C. Note this is a general requirement to ensure the overall CO2 capture ratio and efficiency are reasonable for industrial operation, while in most previous publications, unacceptable large amounts of CO2 were discharged during the ICCC process. In the current case, over 97 % CO2 in the feed can be captured with over 95 % conversion to CH4 (∼100 % selectivity) and negligible deactivation in 10 cycles. To the best of our knowledge, similar results were rarely reported before, particularly under the consecutive CO2 abatement manner. As such, this study may direct the development of the ICCC technology to practical applications, and inspire other integration approaches between CO2 capture and the following mitigation technologies.
Section snippets
Chemicals and preparation
Chemicals including NaOH (AR, ≥96.0 %), Na2CO3 (AR, ≥99.8 %), Mg(NO3)2·6H2O (AR, ≥99.0 %), Ni(NO3)2·6H2O (AR, ≥98.0 %), Al(NO3)3·9H2O (AR, ≥99.0 %), and ethanol (GR, ≥99.8 %) are purchased from Sinopharm, all of which are directly used without future purification.
The NiO-MgO-Al2O3 DFMs were prepared from a two-step procedure. Firstly, a mixed solution of Mg(NO3)2 and Al(NO3)3 (CMg = CAl = 0.2 M) was coprecipitated with another mixed solution of NaOH and Na2CO3 (CNaOH = CNa2CO3 = 1 M),
Preparation and characterization
Previously, we prepared 2D Ni-MgO-Al2O3 nano-sheets by a coprecipitation method, and continuous abatement of CO2 in simulated flue gas via ICCC was reported [41]. Interestingly, we found that the surface enrichment and relative abundance of the adsorption sites (MgO) and catalysis sites (Ni) dictated the ICCC efficiency. However, their adjustment during the coprecipitation process turned out to be unsuccessful. Therefore, this work aims to develop an alternative preparation method to enhance
Conclusions
Ni-MgO-Al2O3 DFMs for continuous CO2 abatement via integrated carbon capture and conversion (ICCC) were prepared by calcination of Ni-impregnated Mg-Al layered double hydroxide (MgAl-LDH). Systematic characterization indicate that the samples showed distinct structural properties such as higher porosity, medium Ni-Mg interactions, and so on. Taking together, a better balance between adsorption and catalytic behavior can be achieved, which can profoundly affect the ICCC efficiency of the DFMs.
CRediT authorship contribution statement
Xingbo Wang: Methodology, Investigation, Formal analysis, Data curation, Visualization, Writing – original draft. Deng Hu: Methodology, Formal analysis, Visualization, Writing – review & editing. Yingdong Hao: Formal analysis, Visualization. Lina Zhang: Formal analysis. Nannan Sun: Writing – review & editing, Conceptualization, Supervision. Wei Wei: Conceptualization, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key R&D Program of China (No. 2022YFE0208100), “CAS Photon Science Research Center for Carbon Dioxide” and Shanghai Science and Technology Committee (No. 21DZ1207000).
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