Prussian blue-based nanostructured materials: Catalytic applications for environmental remediation and energy conversion

Highlights

• Prussian blue nanomaterials are discussed for their role in environment and energy.

• The principles of Prussian blue analogues assisted processes are elucidated.

• The catalytic applications of Prussian blue nanomaterials are covered.

Abstract

Environmental issues pertaining to global warming and pollution have become a global concern and solutions are, therefore, being urgently sought with a wide ranging application of suitable entity to minimize the energy consumption while curbing associated economic costs. Prussian blue (PB) (${\text{Fe}}_4^{3+}{\left[\mathrm{F}{\mathrm{e}}^{2+}{\left(\text{CN}\right)}_6\right]}_3$) and its PB analogous (PBA) (${\mathrm{A}}_2\mathrm{T}\left[\mathrm{M}{\left(\text{CN}\right)}_6\right]$, A=Li, K, Na; T=Fe, Co, Ni, Mn, Cu, etc.; M=Fe, Mn, Co, etc.) are metal organic frameworks that have numerous appliances and have garnered major attention in recent years. However, there are only handful of reviews discussing the fundamental catalytic properties of PB-based materials and hence this effort to comprehensively review and systematically discuss the critical role of PB-based materials in broader catalytic context embracing environment and the energy aspects. Initially, the fundamentals and principles of underlying processes are elucidated followed by a detailed discussion on the catalytic performance and applications of PBA-based catalysts in advanced oxidation processes, reduction reactions, water splitting, and miscellaneous applications, among others. This comprehensive overview features details of the broad catalytic activities of PBA-based materials in diverse applications and hopefully would be a reference on PBA-centered catalysts in an array of applications including industrial applications.

Introduction

Due to the rapid growth of industrialization around the world, environmental issues such as global warming, air, soil, and water pollution have become an overwhelming problem and have attracted a global attention. Thus, several studies have been undertaken to overcome these global issues such as specific focus on renewable energy technologies like solar power, wind power, and biogas production to decrease the effect of global warming. In addition, advanced oxidation treatments have been developed to treat water, soil and air contamination leading to the fabrication and utilization of many materials and chemicals to conduct these methods. However, due to energy consumption involved and the economic constraints, the search for such ideal multipurpose materials with a broad range of applications is imperative; metal organic frameworks (MOFs) are such class of materials with various applications [1,2].

MOFs, with three dimensional crystalline and porous networks comprising metal ions and organic ligands, have been used in several industrial applications due to their unique structure and properties; most common appliances being in gas storage and broad-based catalytic activities [2]. The prominent characteristic of MOFs is porosity which is reminiscent of features represented by zeolites although their advantages outweigh zeolites [3]. Moreover, MOFs have displayed great persistence in microporosity after solvent evacuation [4].

Prussian blue (PB), termed ferric ferrocyanide, is a polynuclear complex including transition metal (Fe) and cyanide group (CN) and as a MOF have had industrial applications since 18^th century, being considered as the first synthetic pigment discovered in textile industry [5] with initial structural forms discovered in 1936 by Keggin and Miles [6]. Their proposed form showed that PB comprise iron ions including ferrous (Fe²⁺) and ferric (Fe³⁺) located at the corners of a cube which is linked by cyanide ligands with the general formula of ${\text{Fe}}_4^{3+}{\left[\mathrm{F}{\mathrm{e}}^{2+}{\left(\text{CN}\right)}_6\right]}_3$; their assembly can be achieved by mixing ferric or ferrous with hexacyanoferrate ions and different oxidation states of iron [7]. Since the advent of PB, extensive attempts have been made to synthesize other compounds by substituting iron with other transition metals like Ni, Cu, Co, and Mn. This has led to creation of PB analogues (PBA) with general formula of ${\mathrm{A}}_{\mathrm{x}}\mathrm{T}\left[\mathrm{M}{\left(\text{CN}\right)}_6\right].\mathrm{n}{\mathrm{H}}_2\mathrm{O}$ where A represents an alkaline ion (Li, Na, K), T represents transition metals (Fe, Co, Ni, Mn, Zn, Cu, Mg), and M also represents transition metals (Fe, Mn, Co) [1]. These larger varieties of assorted PBA's have made them useful materials with a wide-ranging application in diverse fields such as energy storage, sensors, medicine, and catalysis, among others [8], [9], [10], [11], [12], [13]. It is worth mentioning that the alkaline ions can be extracted or intercalated making PBA a good cathode material for ion batteries; they can be deployed for hydrogen storage [1]. The presence of transition metals has made PBA a suitable class of catalysts in varied applications namely oxygen evolution and hydrogen evolution reactions [14], [15], [16].

Although PB and PBA have a long history in industrial applications, new state-of-the-art applications have recently been emerged, as they have been used in environmental applications for decontamination purposes. Fig. 1 depicts the broad catalytic application of PB-based nanomaterials in different disciplines based on the publications since 1986. In addition, several synthesis methods have been documented including coprecipitation, hydrothermal methods, and electrodeposition. As shown in Fig. 2, the scientific community have embraced PB-based catalysts enthusiastically in recent years as illustrated by dramatic increase in the number of published papers since 2017. However, the number of review papers on the subject matter, that can be a keystone for future studies, is rather limited and is only 2.8% of the papers published on PB and its analogous (Fig. 3); to the best of our knowledge there is not a review paper comprehensively discussing the catalytic applications of PB and its analogues. Therefore, this review aims to provide an extensive overview of catalytic applications of PB-based materials. In this regard, first, chemical and structural properties are discussed followed by their catalytic applications in diverse fields and finally, the future perspective and challenges are deliberated.