Self‐Assembly and Visible‐Light Photocatalytic Properties of W/Nb Mixed‐Addendum Polyoxometalate and Transition‐Metal Cations

Abstract

Nowadays, energy issues are one of the most pressing problems faced by human beings. The conversion of solar energy into hydrogen as a clean and renewable fuel is a promising and attractive approach. Since Fujishima and Honda first reported photoinduced water splitting on titanium dioxide electrodes under UV irradiation in 1972, [1] large numbers of metal oxides and sulfides have been explored as heterogeneous photocatalysts for hydrogen production from splitting of water. [2] In this field, although significant progress has been made as hundreds of photocatalysts have been prepared and tested, great effort is needed to search for photocatalysts with high enough efficiencies and visible-light activity for practical applications. [3] Polyoxometalates (POMs) are an outstanding class of inorganic metal oxide clusters with abundant structural diversity and versatile physical and chemical properties that have potential applications in catalysis, electrical conductivity, photochemistry, medicine, and magnetism. [4] Similar to metal oxide semiconductors (such as TiO 2 , titanates, Ta 2 O 5 , and tantalates), POMs are mostly constructed from corner-and edge-shared MO 6 (M = Mo 6 + , W 6 + , V 5 + , Nb 5 + , Ta 5 + ) octahedra. The similarities in composition and structure endow POMs and metal oxide semiconductors with similar electronic and light-absorption characteristics in the photocatalytic system. Moreover, these clusters are soluble in water and could be viewed as the smallest nanoparticles with ordered structures at the atomic level. Therefore, besides the advantage of extra-large surface area allowing for full contact of photocatalysts with water, POMs provide an ideal opportunity to study the relationship between photocatalytic activity and inorganic cluster structures. [5] During the past few years, the photocatalytic H 2 evolution activity of polyoxotungstates has been studied. [6] However, the development of polyoxoniobate photocatalysts is still in its in-fancy, [7] and the reported polyoxoniobates are mostly active under UV irradiation. [8] Therefore, it is of great interest to develop novel polyoxoniobate photocatalysts with visible-light activity for more efficient use of solar energy. To achieve visiblelight H 2 evolution activity, the only solution is to increase the valence-band position of the photocatalysts. One effective strategy to achieve this is to dope transition-metal cations (Ni 2 + , Cr 3 + ) or to introduce main-group cations into the metal oxide photocatalysts to increase the valence-band position by hybridization of O 2p with d or s orbitals of metal cations. [5a, 9]

A Giant Mo/Ta/W Ternary Mixed-Addenda Polyoxometalate with Efficient Photocatalytic Activity for Primary Amine Coupling

“…Changing the type and ratios of addenda atoms can fine-tune the electronic, electrochemical, photochemical, , and catalytic properties of mixed-addenda POMs, making them attractive for the design of new materials with a variety of applications. A series of two-metal (V/W, V/Mo, V/Nb, Mo/W, Nb/W) mixed-addenda POMs with Lindqvist, Keggin, or Wells–Dawson structures have been reported so far. − For instance, the presence of one or more vanadium­(V) centers in the W­(VI) or Mo­(VI) POM skeleton has been shown to lead to greatly enhanced catalytic performance. [PV 2 MoO 40 ] 5– is an excellent robust oxidation catalyst for a series of organic reactions. , The [W 6 O 19 ] 2– anion is a relatively poor nucleophile.…”

“…[PV 2 MoO 40 ] 5– is an excellent robust oxidation catalyst for a series of organic reactions. , The [W 6 O 19 ] 2– anion is a relatively poor nucleophile. Substitution of the W­(VI) atom by Nb­(V) in [NbW 5 O 19 ] 3– and [Nb 2 W 4 O 19 ] 4– greatly increases the reactivity and allows POM-supported organometallic derivatives to be obtained. ,, The “orbital engineering” for Ta/W, Nb/W, , V/Mo, , and V/W mixed-addenda POMs to adjust the electronic structure, light absorption, and photocatalytic activity has also been reported in recent years. Design and synthesis of new mixed-addenda POMs species with novel structures and unprecedented performances remain an ongoing challenge and a significant task.…”

Toward Exploiting the Behavior of Niobium-Containing Mesoporous Silicates vs. Polyoxometalates in Catalysis

“…Su et al have employed TM cations (Cr 3+ and Fe 3+ ) to react with the [Si(NbO 2 ) 3 W 9 O 37 ] 7− anion, resulting in three TM-containing nanoclusters: [(Si 2 W 18 Nb 6 O 78 )Cr(H 2 O) 4 ] 7− , [(Si 2 W 18 Nb 6 O 78 )Cr 2 (H 2 O) 8 ] 4− , and [(Si 2 W 18 Nb 6 O 78 )FeCl 2 (H 2 O) 2 ] 9− . According to the same authors, these are the first W/Nb mixed-addendum POM photocatalysts with a visible-light-driven activity for the H 2 production (Huang et al, 2013 ).…”

Low-temperature microbial and direct conversion of lignocellulosic biomass to electricity: Advances and challenges

“…In addition, POM acts as both photo-catalyst and charge-carrier in the fuel cell. Transition metal addend POMs are well known to be excited by absorbing light extended to visible wavelength [149,150]. A large number of POMs with different metal components, different structures, or different redox properties can be synthesized and used for degradation of biomass in LCFC.…”