Sustainable removal of N2O by mediated electrocatalytic reduction at ambient temperature electro-scrubbing using electrogenerated Ni(I) electron mediator
Introduction
Nitrous oxide (N2O) is stable in the atmosphere for prolonged periods, and is an approximately 310 times more potent greenhouse gas than carbon dioxide (CO2) [1]. Therefore, N2O has been categorized as the greatest contributor to stratospheric ozone depletion, and is regarded as the third most significant anthropogenic greenhouse gas [[2], [3], [4], [5], [6]]. The projected growth in N2O emissions is estimated to reach 14.49 Mt/y by 2020. The industrial sector is regarded as the most significant emission source after agriculture, where N2O is produced as a by-product during the manufacture of adipic acid and nitric acid, or as an intermediate in the biological nitrification of wastewater [7,8].
A range of abatement solutions can be used to control N2O emissions, such as thermal decomposition, adsorption, and direct catalysis [9]. Catalysis is generally favored, where N2O is reduced to nitrogen and oxygen. Although numerous catalysts have been trialed, high operating temperatures coupled with interference from the presence of other gases [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]], and catalyst deactivation (or fouling), increase the process complexity, which can lead to technology failure, resulting in the need for frequent replenishment of the catalyst [6]. N2O is highly soluble because of the dipole-dipole interactions with water [23], which has led to the study of N2O absorption (or scrubbing) using packed column technology [24]. Whilst effective for gas phase separation, the absorbed N2O is concentrated in the liquid phase but not transformed. Synergy between technologies has been investigated to provide process intensification, in which absorption is first employed to separate and concentrate N2O from the gas phase, after which N2O is then desorbed and exposed to a catalytic treatment to facilitate N2O conversion (a three-stage process). Weißbach et al. [23] used desorption technology to separate and concentrate liquid phase N2O emissions from wastewater, after which N2O was blended with biogas (rich in methane) and combusted within a combined heat and power engine. This enables the control of N2O emissions, whilst also increasing power generation by 37% through the exothermal release of energy from N2O (82 kJ mol−1). This method avoids the limitations attributed to catalysis, and proposes a new value to the final gaseous product. On the other hand, its application is limited explicitly to applications in which N2O emissions and biogas generation facilities are co-located (i.e. large centralized wastewater treatment facilities).
Besides catalytic removal operated at high temperature, electrochemical technique provided room temperature degradation of many greenhouse gases by green catalyst ‘electron’, though the works done by fundamental cyclic voltammetry (CV) technique. At first step development to minimize the potential (or energy), electron mediators or catalyst were started to use because of many pollutants contain high oxidation/reduction potential such as N2O has thermodynamic potential of +1.77 V (vs SHE) [25]. The transition or noble metals as catalyst, here as electrode, have reduce the reduction potential of N2O to N2 to -0.8 V (vs Ag/AgCl) [26]. On the other side, a solution phase electron mediators have used to reduce the N2O to N2 reduction potential to -1.15 V (vs NHE) [27] with the help of Ni2+ complex of [I5 or 14]aneN4 {[I5 or 14]aneN4 = 1,4,8,12(or 11)-tetra-azacyclopenta(or tetra)decane} in aqueous solution. Similar way, Ni or Pt catalyst modified gas diffusion electrode and Co(II)/Co(I) redox process of tetraaminophthalocyaninatocobalt(II) (Co(II)TAPc) adsorbed on a graphite electrode were applied on N2O degradation to N2 [28,29]. Note that the published works so far on electrochemical reduction of N2O done by basic CV method to understand the fundamental electron transfer and ended up N2 as a product. In recent investigations, the electrochemical technique stepped towards industrial practice for air pollutants removal by adopting paired electrolysis with wet scrubber column. Through the combination of paired electrolysis and wet scrubbing (electroscrubbing), many gas pollutants, such as benzene [30] and odorous gases [31] have been removed using electrogenerated oxidative mediators by MEO (mediated electrochemical oxidation). In addition, homogeneous [Ni(I)(CN)4]3− was generated electrochemically on a Cu metal electrode at cathodic half-cell by constant current electrolysis for the first time and used to remove gaseous CCl4 by MER (mediated electrochemical reduction) at electroscrubbing [32]. In a very recent study on carbon tetrafluoride (CF4) degradation, electrochemical production of mediator Cu1+ facilitated a regenerative chemical reaction at room temperature, resulting in product transformation to trifluoroethane and ethanol without HF using Cu1+[Ni2+(CN)4]1- [33]. Hence, electroscrubbing enables process intensification through the provision of separation and product transformation within a single stage, simultaneously generating a product of commercial value that can improve the return on investment.
This paper proposes to build upon the successful development of MER, and introduces Ni(I) as a new reductive electrochemical mediator, that can facilitate the effective abatement of N2O through chemical transformation into a comparatively benign final product. The subsequent integration of MER into an electroscrubber, to form a single stage process introduces considerable process intensification versus conventionally applied methods and since MER can be operated at ambient temperature, as well as being comparatively insensitive to fouling, this proposition overcomes the limitations commonly associated with gas phase catalysis. The nickel based complexes, mostly organic/aqueous mixture solvents or modified electrodes due to insolubility, are good in effective water splitting at reduced potential [34,35]. In similar way, high potential organic compounds are reduced at less potential of electrogenerated Ni(I) complexes [36]. In addition, a planar type orientation of electrogenerated Ni(I) complexes are more reactive either by nucleophilic vs radical type reactivity depending upon the ligand [37] and more specifically, a chemically reduced [Ni(I)(CN)4]3− used as hydrogenation catalysts in organic reactions [38]. Because of solubility restriction in aqueous medium, many nickel complexes used as modified electrode or dissolved in non-aqueous medium [[39], [40], [41], [42]]. In reverse, [Ni(II)(CN)4]2- is quite soluble in alkaline media [32] and yields a reduction potential of -900 mV(vs Ag/AgCl) [43]. Therefore, use of aqueous soluble nickel based complex such as [Ni(II)(CN)4]2- may open possibilities in generation of commodity products during reduction of N2O. Ammonia is a critical building block for many industrial and pharmaceutical chemicals, foods, and fertilizer formulations. Currently, ammonia (NH3) is manufactured primarily through the Haber-Bosch process, which utilizes 19.3 kW h kgN-1 and is believed to consume 7% of natural gas globally [44]. Significant focus has recently been placed on identifying new sustainable sources of ammonia, to generate high value products from waste streams, such as the production of single cell protein [45].
This study introduces and examines the reaction pathway of MER based [Ni(II)(CN)4]2−, specifically to identify the potential to transform N2O to NH3 as a high value end product that can improve both the return on investment and sustainability criteria of the process. The specific objectives of this study were as follows: (i) demonstrate N2O removal at room temperature in a single stage using electro-scrubbing; (ii) identify and quantify the N2O reduction products and propose a possible reaction pathway; (iii) determine the mass transfer coefficient to describe the separation and conversion rate; and (iv) evaluate a sustainable operation in the form of reduced electron mediator Ni(I) regeneration.
Section snippets
Experimental
The supporting information section outlines the following: complete experimental details for the preparation of nickel cyanide complex reported elsewhere [46]; electrolytic cell setup with a wet scrubber for the active electron mediator generation and removal of N2O gas (based on our experience [47,48] along with a schematic presentation (Fig. SI 1)); and the analysis type and conditions used in the present investigation.
Identification of N2O removal at the electroscrubber
The absorption of N2O into KOH was initially evaluated in the recycle from the catholyte tank but without activation of the electrochemical cell or inclusion of Ni(I) to observe only the physical separation of N2O (Fig. 1). Gondal [49] reported that the solubility of N2O was higher in KOH (7817 kPa m−3 kmol−1, 1.79 M KOH, 25 °C) than in water (4199 kPa m−3 kmol−1, 25 °C). In this study, despite the initial N2O separation of >99%, a significant decrease in removal efficiency was observed after
Conclusions
The continuous removal of N2O at room temperature was demonstrated by integrating a mediated electrocatalytic mediator (MER) Ni(I) into the alkaline absorption solvent of a packed column. Absorption using only an alkaline or electrolyzed alkaline KOH solution showed that N2O absorption was unsustainable, which indicated that electrogenerated Ni(I) successfully mediates N2O removal in this electroscubber MER process. An extensive evaluation of the reaction pathway showed that MER can facilitate
Acknowledgment
This study was supported by the National Research Foundation of Korea (NRF) funded by Ministry of Engineering Science and Technology (MEST) from the Korean government (Grant No. NRF-2017R1A2A1A05001484).
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Equally contributed to this work.