Structure and properties of KNi–hexacyanoferrate Prussian Blue Analogues for efficient CO2 capture: Host–guest interaction chemistry and dynamics of CO2 adsorption

https://doi.org/10.1016/j.jcou.2021.101593Get rights and content

Highlights

  • KNiFe-PBAs with trace amounts of K+offer an excellent platform for CO2 capture.

  • The porosity and the Qstof CO2adsorption can be optimized by varying the Ni:K ratio.

  • The first FTIR spectroscopic observation of H2O–induced expansion of the PB framework.

  • The presence of H2O hardly affects the CO2 adsorption over the studied materials.

Abstract

Potassium Nickel hexacyanoferrate Prussian Blue Analogues (K-NiFe-PBAs) offer an excellent platform for efficient CO2 capture due to their porous nature and accessible channels. Herein, the effect of Ni:K atomic ratio on the structure and the CO2 storage capacity was studied by employing K-NiFe-PBAs with Ni:K ratio of ca. 2.5 and 12. The porosity and the isosteric heat of CO2 adsorption can be modulated and optimized by varying the Ni:K atomic ratio in the PB framework and thus, covering the thermodynamic criterion for easy CO2capture and release with acceptable energy costs. The synthesized K-NiFe-PBAs containing only trace amounts of K+ ions (with Ni:K = 12) shows an adsorption capacity (∼3.0 mmol g–1 CO2 at 273 K and 100 kPa) comparable to other well established CO2 adsorbents. In situ FTIR spectroscopy was further employed to elucidate the host–guest interaction chemistry and the dynamics of K-NiFe-PBAs within CO2 and H2O. The analysis enabled, to the best of our knowledge, is the first FTIR spectroscopic observation of the high sensitivity of the material to structural distortions induced by small changes under water vapor pressure. It was found that H2O hardly affects CO2 adsorption and the materials are perspective for CO2 capture in the presence of water.

Introduction

The steady increasing levels of anthropogenic emissions of carbon dioxide (CO2) into the atmosphere are one of the most significant environmental problems for our society. CO2 is the most critical greenhouse gas affecting climate change [[1], [2], [3]] and also represents an essential future carbon feedstock. Thus, the control of CO2 amount is considered a challenging research topic. Many governments are establishing joint efforts [4,5] to encourage the development of new technologies for more efficient CO2 capture.

Current CO2 capture technologies comprise the use of porous solid–state materials that have exceptionally high chemical stability, increased CO2 adsorption capabilities, and fast sorption kinetics to desorb CO2 under mild conditions. In this context, various porous materials with appropriate structural and chemical properties including zeolites [[6], [7], [8]], carbon [[9], [10], [11]], silica–based materials [[12], [13], [14]], and metal organic frameworks (MOFs) [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24]] have been widely investigated for CO2 capture.

Prussian blue (PB) is a simple coordination polymer. The iron metal centers in the PB framework can be replaced by various metal ions to form Prussian blue analogues (PBAs). This opens up large possibilities for adjusting their properties. PBAs represent a large family of materials with a general formula A2xM(3-x)[M(CN)6]2·nH2O(A = Li, K, Na, Cs, Rb; M = Cr3+, Mn2+,Fe2+/3+, Co2+/3+, Ni2+, Cu2+, Zn2+, and M= Fe2+/3+, Co2+/3+, Cr3+, Mn2+). In a three–dimensional (3D) face–centered cubic PB network, the metal ions are connected through cyanide bridging ligands to afford a structure with cavities filled with zeolitic water molecules and alkali cations. These materials recently have drawn growing attention due to their superior performance in various applications [25,26], in alkaline ion batteries [27,28], multivalent ion batteries [29], hydrogen storage [27,30,31], optics [32], magnets [33,34], and electrocatalysis [[35], [36], [37], [38]].

In particular, PB and PBAs have shown great potential as efficient adsorbents for CO2 capture [[39], [40], [41]] due to relatively low regeneration energies, efficient physical adsorption profile, and low–cost precursors. However, a very limited number of articles were published on CO2 adsorption and separation application that utilizes PBAs. There exist only several reports [39,40,[42], [43], [44], [45], [46]], which mostly focus on the performance aspect rather than providing a detailed spectroscopic account on the nature of the surface species and elucidation of the particular function of various structural components on the CO2 adsorption mechanism. The high CO2 adsorption capacity of PBAs has mainly been associated with their high surface area and uniform porosity, which provide easy access to the active sites.

Furthermore, PBAs are highly sensitive to small structural distortions induced by external (mechanical) [47,48] or internal (chemical) pressure [[49], [50], [51]]. Thus, the structure of particular PBAs may undergo specific structural rearrangements, which involve alterations in the geometry of the framework through bending of the M′−C≡N−M coordination modes and/or tilting of the [M(CN)6] units around their crystallographic positions. Based on this, many of the unusual and intriguing properties of these materials were explained considering the slight structural distortions. Therefore, many efforts [47,49,[52], [53], [54]] have been devoted to their experimental detection and quantification. A direct evidence of such small structural distortions is, however, a challenging task, especially when the materials exhibit an intrinsic degree of disorder due to the presence of a variable amount of [M(CN)6] entities, alkali cations, and water molecules.

In this work, the effect of nickel and potassium content on the structure and the CO2 adsorption properties of KNi hexacyanoferrate PBAs (K-NiFe-PBAs) was studied by employing samples with different Ni:K atomic ratios of ca. 2.5 and 12, respectively. The samples were synthesized with substantially different porosity and composition containing high– or trace amounts of potassium, as a consequence of charge balance. To study their application as functional materials for efficient CO2 capture, we performed a series of measurements and analysis of the adsorption isotherms, the CO2 storage capacity and the isosteric heat of CO2 adsorption.

The samples were also studied by using a series of complementary characterization experiments including specific surface area, pore volume, pore size distribution, X–ray diffraction (XRD), thermogravimetric and differential thermal (TG–DTA) analysis, Mössbauer spectroscopy, and energy dispersion X–ray spectroscopy in conjunction with scanning electron microscopy (EDX–SEM) to establish a structure & adsorption relationship.

Finally, in situ FTIR spectroscopy was employed to elucidate the host–guest interaction chemistry and dynamics of K-NiFe-PBAs with CO2 and H2O. The study enabled, to the best of our knowledge, is the first FTIR spectroscopic observation of the high sensitivity of the material to structural distortions induced by small changes under water vapor pressure. Moreover, a drastic effect of potassium on the adsorption properties was established.

Section snippets

Synthesis

Two KNi–hexacyanoferrates PBAs with a general formula of KxNiy[Fe(CN)6]2 nH2O were synthesized via a co–precipitation method. K3[Fe(CN)6] and Ni(NO3)2.6H2O precursors used in the synthesis were purchased from Sigma Aldrich and were of analytical grade purity. In the synthetic protocol, 3 mmol of K3[Fe(CN)6] were first dissolved in 50 mL deionized water at room temperature. Then, an aqueous solution of Ni(NO3)2.6H2O (2 or 3 mmol in 50 mL water) were added drop-wise to the above solution. The

Chemical composition and structural analysis via EDX–SEM and XRD

The synthesis method, described in the experimental section, is commonly employed to prepare PBAs [39,56,57] with controlled compositions and morphologies. This can also be confirmed in the current study where the final composition of both prepared materials is almost identical to that pre–set in the synthesis with a Ni:Fe molar ratio, ranging from ∼1:1 to ∼3:2. As a consequence of charge balance, the synthesized K-NiFe-PBAs samples were obtained with a substantially different Ni:K atomic

Conclusions

Two different K‑containing nickel hexacyanoferrate Prussian Blue Analogues (K-NiFe-PBAs) with a cubic crystal structure were synthesized via a co–precipitation method. The composition analysis via EDX–SEM reveals that the as-synthesized compounds are characterized with substantially different Ni:K atomic ratios of ca. 2.5 and 12 and composition containing rich– or trace amounts of potassium due to the charge balance. The FTIR spectroscopy studies reveal the presence of two main coordination

Author statement

Stanislava Andonova: Conceptualization; Investigation; Writing- Original draft preparation; Sina Sadigh Akbari: Sample synthesis; Investigation; Ferdi Karadaş: Investigation; Writing- Reviewing and Editing; Ivanka Spassova: Investigation; Daniela Paneva: Investigation; Konstantin Hadjiivanov: Writing- Reviewing and Editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This work was supported by the Bulgarian Ministry of Education and Science (Contract No. DO1-214/28.11.2018) under the National Research Programme “Low-carbon Energy for the Transport and Domestic Use - EPLUS” approved by DCM #577/17.08.2018. S.S.A. thanks TUBITAK for support (Project No: 118Z277).

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