The synthesis and characterization of a new ligand, tris((6-phenyl-2-pyridyl)methyl)amine (TPPA) and some of its copper complexes are described. The complexes [Cu(TPPA)]BPh (7) and [Cu(TPPA)(AN)](ClO& (8) (AN = acetonitrile) were prepared and their X-ray crystal structures and redox potentials were determined. The X-ray structure of 7 (triclinic space group, P1; a = 14.603(3), b = 15.137(3), c = 12.974(3) A, a = 91.76(3), p = 105.87(3), y = 117.00(3)", V = 2417.4( 12) A3, Z = 2) displays a copper(1) atom with a distorted trigonal pyramidal coordination sphere, but the X-ray structure of 8 (triclinic space group, Poi; a = 13.458(3), b = 13.586(3) c = 11.082(2) A, a = 113.00(3), p = 94.48(3), y = 90.53(3)", V = 1857.7(9) A3, Z = 2 ) indicates that the copper(I1) atom has the expected trigonal bipyramid geometry. The reduction potential of 8 in dimethylformamide, dimethylacetamide, acetonitrile, and isobutyronitrile is quite positive compared to [Cu(TPA)](ClO& (TPA = tris(2-pyridylmethy1)amine) under identical conditions. The various factors that may contribute to the difference in oxidation potential are discussed. The positive redox potential combined with steric factors accounts for the lack of reactivity of [Cu(TPPA)]PF6 with molecular 0 2 .

Electrochemical water splitting for hydrogen production, fuel cells, and metalair batteries are some of the promising approaches to tackle the environmental and energy problems. Central to these approaches involves oxygen evolution reaction (OER), which is a sluggish four-electron process. Finding a cost-effective OER catalyst with excellent activity and high stability will accelerate the wide spread employment of these techniques. [1] Owing to their low cost, nonprecious-metal-based electrocatalysts are in highly sought after for OER or other reactions. The first row transition metals such as Ni and Fe are promising candidates for designing OER electrocatalysts because their 3d electrons can be easily manipulated just like the precious metals, but their OER activity still needs to be improved. [2] Increasing the reaction active sites and optimizing the intrinsic activity of the catalysts are the two effective ways to improve their OER performance. Nanostructure engineering is widely used to enlarge the surface area in order to expose more active sites. Transition metal elements with high oxidation states are believed to have high intrinsic activity, especially for Ni. [3] Ni 2+ is the common ion in nickel salt used for the synthesis of Ni catalysts, and it is often difficult to oxidize Ni 2+ to Ni 3+ or even Ni 4+ and stabilize them under mild conditions. Instead, extreme conditions such as high pressure and oxidative environment are required. [3e] Ni in the ABO 3 perovskite structure with the high oxidation state of +3 was used for the synthesis of Ni-based catalysts, but the precursor is also the Ni 2+ salt, and the synthesis process involves multiple steps including adding organic additives and using high temperature. [3a,c] Therefore, there is a need to develop simpler and cost-effective methods for the preparation of high-valence Ni for electrocatalytic applications. Ceria (CeO 2) is a nonstoichiometric material. The facile shift between Ce 4+ and Ce 3+ should promote the electron transfer between CeO 2 and other components in Ce-containing hybrid catalysts, if they are made for this purpose. [4] In our previous work, [5] we showed that when CeO 2 is reduced to CeO 2−x , electron transfer is further enhanced. Herein, by using Transition metal elements such as Ni in high oxidation states can promote oxygen evolution reaction (OER) activity, but it is difficult to prepare dispersed Ni 3+ or even Ni 4+ species under mild conditions. Herein, a onestep synthesis of high-valence nickel-doped CeO 2−x covered with FeOOH nanosheets in the presence of Ni 2+ /Fe 3+ is reported. A series of ex situ and in situ experiments reveal the etching effect on ceria of the H + species from the hydrolysis of Fe 3+ , which induces substitutional doping of Ni 2+ ions into the etched sites and their further oxidation to the high-valance Ni 3+ /Ni 4+ by coupling to the Ce 4+ /Ce 3+ pair in the oxygen-vacancy-rich CeO 2−x. Concomitantly, Fe 3+ is deposited on the surface of ceria as FeOOH nanosheets. The dispersed high-valent Ni 3+ /N...

Hydrogen energy has become one of the most attractive candidates to replace traditional fossil fuels because of its lack of pollution and its high energy density. Electrocatalytic water splitting is a “green” and sustainable way to produce hydrogen but is still not sufficiently efficient at this stage. In recent years, Ce-based materials have become very popular as the electrocatalysts for water splitting primarily because of the multivalence state of Ce and easily formed oxygen vacancies readily formed in CeO2. However, until now, this interesting subject has seldom been reviewed, especially for electrocatalysts for alkaline water splitting. Herein, we outline and discuss recent progress on the active sites of Ce-based electrocatalysts for hydrogen evolution and oxygen evolution. Oxygen vacancies and interfaces between CeO2 and mixed metal components could provide optimized binding of hydrogen evolution reaction (HER) intermediates, thus promoting HER performance. For the oxygen evolution reaction (OER), Ce3+/Ce4+ redox, oxygen vacancies, and exogenous transition metals could optimize the binding of OER intermediates toward top catalytic activities. The aim of this review is to seek an overall understanding about the reaction sites in Ce-based electrocatalysts for water splitting, which may provide a guide for the future development of HER and OER Ce-based electrocatalysts toward industrial applications.

Cu is commonly modified with sulfur to obtain high selectivity for formate since S can promote the formation of the key *OCHO intermediate along the formate pathway. In the present work, we demonstrate that Cu-vacancies on copper sulfide can surprisingly switch the formate pathway to the CO pathway, and the concentration of Cu vacancies can deterministically regulate the CO faradaic efficiency and partial current density. The J CO of SNC@Cu1.96S (Cu1.96S coated sulfur, nitrogen-co-doped carbon) can reach 37.2 mA cm–2 in an H cell, which is the highest among the Cu-based catalysts and comparable to other top CO production catalysts. According to DFT calculations, the Cu vacancies formed in copper sulfide change the electronic structures of the S sites in such a way that the H* takes a large Gibbs free energy, which in turn suppresses the formation of formate. However, the resulting fewer surface Cu cations and more surface S anions weakens the adsorbate–metal interaction, synergizing the adsorption structural transition of the surface intermediates from *OCHO (two O–Cu bonds) to *COOH (one C–Cu bond) in favor of CO production.

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Ceria-based catalysts for the oxygen evolution reaction (OER) have received keen interest in recent years owing to their potential excellent cost performance. However, despite a flurry of research activities, the mechanism on how ceria activates those hybrid catalysts is still puzzling. Herein, by controllably modifying the oxidation state of Ce in ceria, it was revealed that creating Ce 3 + species, which are redox-coupled to Ce 4 + under OER conditions, could enhance the conductivity and optimize the OH* binding, leading to greatly improved OER activity of the catalysts. More importantly, the ceria-based hybrid catalysts also exhibited excellent long-term stability even when operating at a high current density of 50 mA cm À 2 in the strong alkaline electrolyte of 6 m KOH for more than 50 h. This work unveils the underlying role of ceria in improving the activity/ stability of ceria-based catalysts and opens the way to design and fabricate ceria-based electrocatalysts for water splitting.

For electrochemical CO2 reduction to HCOOH, an ongoing challenge is to design energy efficient electrocatalysts that can deliver a high HCOOH current density (JHCOOH) at a low overpotential. Indium oxide is good HCOOH production catalyst but with low conductivity. In this work, we report a unique corn design of In2O3-x@C nanocatalyst, wherein In2O3-x nanocube as the fine grains dispersed uniformly on the carbon nanorod cob, resulting in the enhanced conductivity. Excellent performance is achieved with 84% Faradaic efficiency (FE) and 11 mA cm−2JHCOOH at a low potential of − 0.4 V versus RHE. At the current density of 100 mA cm−2, the applied potential remained stable for more than 120 h with the FE above 90%. Density functional theory calculations reveal that the abundant oxygen vacancy in In2O3-x has exposed more In3+ sites with activated electroactivity, which facilitates the formation of HCOO* intermediate. Operando X-ray absorption spectroscopy also confirms In3+ as the active site and the key intermediate of HCOO* during the process of CO2 reduction to HCOOH.