Elsevier

Catalysis Today

Volume 357, 1 November 2020, Pages 214-220
Catalysis Today

CO2 methanation under dynamic operational mode using nickel nanoparticles decorated carbon felt (Ni/OCF) combined with inductive heating

https://doi.org/10.1016/j.cattod.2019.02.050Get rights and content

Highlights

  • Three-dimensional Ni/OCF composites of tuneable shape and dimension.

  • Robust catalysts for CO2 methanation under induction heating.

  • CO2 methanation under dynamic operational modes.

  • Ni/OCF exhibits high catalytic activity and stability under moderate conditions.

Abstract

Carbon dioxide (CO2) hydrogenation to methane (CH4) (Sabatier reaction) is a fundamental process that meets with several key challenges of our modern society. Besides representing a convenient way to the metal-mediated conversion of a natural and abundant “waste” into a fuel of added value, its combination with H2 from renewable resources (REs) represents a challenging technology for the REs storage. In addition, its practical exploitation can give a concrete answer to many critical societal and environmental issues largely related to the steadily increase of CO2 concentration in the Earth’s atmosphere caused by the main anthropic activities. Although many fundamental achievements have also been reached since its discovery at the beginning of the twentieth century, alternative and conceptually new protocols for the process can provide valuable solutions to the optimization of the catalyst performance, process energetics and catalyst life-time on stream.

This contribution describes the synthesis of an efficient and robust catalyst for the CO2 methanation, based on Nickel nanoparticles (Ni-NPs) grown on electrically conductive and macroscopically shaped oxidized carbon-felt disks (OCF), heated at the target reaction temperature by electromagnetic induction. At odds with the more classical external heat sources (based on contact heat conduction), induction heating allows the electromagnetic energy to be directly absorbed by the susceptor (OCF) who converts it into heat to be transferred to the catalyst active sites (Ni NPs). Inductive heating (IH) of Ni/OCF gives CO2 conversion (XCO2) up to 74% and CH4 selectivity (SCH4) close to 97% already at 320 °C, showing an excellent control of the catalyst stability under forced dynamic operational conditions.

Introduction

The restless development of renewable energy sources (REs), i.e. solar and wind, calls for the development of more efficient energy storage systems capable to cope with problems related to the extra and intermittent renewable energy supply. Indeed, the fluctuating nature of REs has to be taken into account while managing electricity grids in order to reduce problems linked to security and stability of power networks and ensure a constant supply of electricity on demand [[1], [2], [3]]. Apart from electrochemical storage devices, i.e. supercapacitors and batteries, chemical energy storage is a fast growing technology that allows the conversion of exceeding electricity from REs into H2 (by water electrolysis) [[4], [5], [6]]. The latter is supposed to be used in turn as an energy vector or an intermediate for the production of chemicals and commodities. On this regard, CO2 hydrogenation [[7], [8], [9], [10]] (Sabatier reaction) using H2 from RE sources [11,12] is a highly promising approach that meets with several key challenges of our modern society. Besides representing a sustainable method towards the production of chemicals and fuels [[13], [14], [15], [16], [17], [18], [19], [20]] from REs it also copes with the main environmental and climate urgencies directly linked to steadily state increase of CO2 concentration in the atmosphere. Catalytic hydrogenation of CO2 with RE H2 is nowadays at the forefront of many chemical technologies devoted to the production of low molecular weight olefins [21], hydrocarbons [22], formic acid [23], and alcohols [[24], [25], [26]]. It also represents the core business of the “power to gas’’ (PtG) technology [5,6,[27], [28], [29]]. Indeed, it provides a powerful approach to the production of synthetic natural gas (SNG) as fuel (CO2 methanation) that can be injected in the existing natural gas pipelines thus creating a virtuous link between existing power networks and natural gas grids [30]. CO2 methanation is a well-established transformation promoted by a relatively large number of metal active phases [i.e. Ni, Ru, Rh, Co, Pd nanoparticles (NPs)] deposited on various oxide supports (i.e. TiO2, SiO2, Al2O3, CeO2, MgO and ZrO2) [[31], [32], [33], [34], [35], [36], [37], [38], [39], [40]]. It is a highly exothermic process (ΔH = −165 kJ mol−1) and thus, thermal conductive supports (carbon nanotubes, graphene, and silicon carbide just to mention a few) [[41], [42], [43], [44], [45]] are generally recommended in order to prevent as much as possible the generation of local temperature gradients (hot spots) inside the catalyst bed during the process. Indeed, the generation of “hot spots” inside the catalyst can be largely detrimental both in terms of catalyst life-time and performance of the methanation process.

PtG chain urgently needs of novel concepts and breakthroughs to optimize performance of existing plants and to cope as much as possible with dynamic operational conditions that often occur within the reactor. One main dynamic parameter to be controlled is represented by the intermittent nature of reactants supply from REs. Such dynamic conditions can be responsible for the occurrence of severe temperature fluctuations within the reactor, not easy to be controlled even in apparatus equipped with external cooling systems [46].

Induction heating (IH) represents a valid alternative to the more traditional Joule heating (JH) approach, with the former listing a series of key advantages compared to the latter in the heating of electrically conductive materials. Induction heating is likely a cleaner, efficient, cost-effective, precise and repeatable method for providing heat necessary for chemical processes to occur. Heating takes place on electrically conductive objects when they are placed in a varying magnetic field and it occurs precisely where heat is needed and not to the whole volume of the medium and the gaseous reactants; it is also defined as a noncontact heating method. Such a heating scheme allows for rapid up/down temperature ramps (i.e. hundred degrees per minute), with great potentials for large scale and rapid manufacturing processes and improved energy efficiencies [47].

IH is expected to be a technique of choice for operating heterogeneous catalysis and in particular for those processes where high heat targeting along with fast heating/cooling rate provide key control on the reaction course and catalyst stability on stream, i.e. rapid temperature gradient control during exothermic or endothermic processes also caused by variable reagents loads. Most importantly, the temperature control at the catalyst sites can be directly monitored by a laser pyrometer that maps the catalyst temperature in real time (and not that of the reactor) thus allowing an extremely fast response to any temperature deviation occurring at the catalyst bed. By this way, the catalyst operates under almost isothermal conditions.

IH has already been used with success for running liquid-phase catalytic reactions [48,49] and recently for the gas-phase CO2 methanation reaction. On the latter process, Bordet et al. [50] have elegantly demonstrated the use of the induction heating with Ni-coated iron-carbide NPs (ICNPs@Ni) or ICNPs supported on Ru-doped Silica-alumina hydrate oxides (SIRALox) as catalysts for continuous-flow reactors working under atmospheric pressure. For both their systems, the authors idea was to take advantage of ICNPs heating capacities and transfer the heat released from that NPs to activate either their Ni-coating or the neighbouring Ru NPs, thus catalysing the Sabatier process.

In this work we describe the straightforward synthesis of Ni(0) NPs on a highly defective carbon felt (Ni/OCF) support and the use of the composite as a stable and effective heterogeneous catalyst for CO2 methanation, using an induction heating (IH) setup. We took advantage from the high and rapid heat response (heating/cooling) of OCF to magnetic stimuli of the inductive heater for a rapid temperature control at the heart of the catalytic system. Such a configuration provides an effective way to control the catalyst temperature from sudden deviations especially occurring during the start/stop of the catalytic runs where gradients of reactants loads can be formed.

Section snippets

Materials and methods

Commercial soft polyacrylonitrile (PAN)-based carbon felt (CF) was purchased from Carbone Lorraine Ltd. [RVC 4002 Carbon felt (d = 100 kg m−3; 96% carbon content; Specific Surface Area (SSA): 2.0 ± 1 m2 g-1] in the form of a black carpet with a thickness of 12 mm. It can be easily adapted into various macroscopic shapes including cylinders, disks or others as a function of its downstream application. The as-received CF sample was cut into disks of external diameter of 22 mm before being

Ni/OCF catalyst synthesis and characterization

The acid treatment of CF is used to confer a hydrophilic character to the pristine hydrophobic material as to get a more wettable surface for the impregnation step with an aqueous solution of [Ni(NO3)2·6H2O]. Although such an acid treatment deeply contributes to the modulation of the chemico-physical properties (vide infra) of the CF surface, it negligibly affects the sample’s macroscopic shape. Accordingly, the catalyst shape can be fixed at priori on the pristine CF as a function of its final

Conclusions

In summary, an efficient and robust catalyst for the CO2 methanation, based on Nickel nanoparticles (Ni-NPs, 8.4 wt.%) grown on electrically conductive and macroscopically shaped oxidized carbon-felt disks (OCF), has been prepared and heated at the target reaction temperature by electromagnetic induction. Inductive heating (IH) is proved to have largely beneficial effects on the catalyst performance compared to the more classical Joule heating methods. Indeed, the electromagnetic energy

Acknowledgements

G. G. and C. P.-H. thank the TRAINER project (Catalysts for Transition to Renewable Energy Future) of the “Make our Planet Great Again” program (Ref. ANR-17-MPGA-0017) for support. The Italian team would also like to thank the Italian MIUR through the PRIN 2015 Project SMARTNESS (2015K7FZLH) “Solar driven chemistry: new materials for photo- and electrocatalysis” for financial support to this work. W.W. and Z.X. would like to thank the China Scholarship Council (CSC) for financial support during

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