There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

Colloidal nanoparticles are being intensely pursued in current nanoscience research. Nanochemists are often frustrated by the well-known fact that no two nanoparticles are the same, which precludes the deep understanding of many fundamental properties of colloidal nanoparticles in which the total structures (core plus surface) must be known. Therefore, controlling nanoparticles with atomic precision and solving their total structures have long been major dreams for nanochemists. Recently, these goals are partially fulfilled in the case of gold nanoparticles, at least in the ultrasmall size regime (1–3 nm in diameter, often called nanoclusters). This review summarizes the major progress in the field, including the principles that permit atomically precise synthesis, new types of atomic structures, and unique physical and chemical properties of atomically precise nanoparticles, as well as exciting opportunities for nanochemists to understand very fundamental science of colloidal nanoparticles (such as the s...

Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities

The electron-induced dissociation of CO2 adsorbed at the oxygen vacancy defect on the TiO2(110) surface has been investigated at the single-molecular level using scanning tunneling microscopy (STM). Electron injection from the STM tip into the adsorbed CO2 induces the dissociation of CO2. The oxygen vacancy defect is found to be healed by the oxygen atom released during the dissociation process. Statistical analysis shows that the dissociation of CO2 is one-electron process. The bias-dependent dissociation yield reveals that the threshold energy for electron-induced dissociation of CO2 is 1.4 eV above the conduction-band minimum of TiO2. The formation of a transient negative ion by the injected electron is considered to be the key process in CO2 dissociation.

Electron-induced dissociation of CO2 on TiO2(110).

Active sites and structure–activity relationships for methanol synthesis from a stoichiometric mixture of CO2 and H2 were investigated for a series of coprecipitated Cu-based catalysts with temperature-programmed reduction (TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2O decomposition. Experiments in a reaction chamber attached to an XPS instrument show that metallic Cu exists on the surface of both reduced and spent catalysts and there is no evidence of monovalent Cu+ species. This finding provides reassurance regarding the active oxidation state of Cu in methanol synthesis catalysts because it is observed with 6 compositions possessing different metal oxide additives, Cu particle sizes, and varying degrees of ZnO crystallinity. Smaller Cu particles demonstrate larger turnover frequencies (TOF) for methanol formation, confirming the structure sensitivity of this reaction. No correlation between TOF and lattice strain in Cu crystallite...

Active Sites and Structure−Activity Relationships of Copper-Based Catalysts for Carbon Dioxide Hydrogenation to Methanol

The chemical nature of copper and copper oxide (Cu2O) surfaces in the presence of CO2 and H2O at room temperature was investigated using ambient pressure X-ray photoelectron spectroscopy. The studies reveal that in the presence of 0.1 torr CO2 several species form on the initially clean Cu, including carbonate CO32−, CO2δ− and C0, while no modifications occur on an oxidized surface. The addition of 0.1 ML Zn to the Cu results in the complete conversion of CO2δ− to carbonate. In a mixture of 0.1 torr H2O and 0.1 torr CO2, new species are formed, including hydroxyl, formate and methoxy, with H2O providing the hydrogen needed for the formation of hydrogenated species.

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Surface chemistry of Cu in the presence of CO2 and H2O.

The initial stages of water condensation, approximately 6 molecular layers, on two oxide surfaces, Cu2O and Al2O3, have been investigated by using ambient pressure X-ray photoelectron spectroscopy at relative humidity values (RH) from 0 to >90%. Water adsorbs first dissociatively on oxygen vacancies producing adsorbed hydroxyl groups in a stoichiometric reaction: Olattic + vacancies + H2O = 2OH. The reaction is completed at ∼1% RH and is followed by adsorption of molecular water. The thickness of the water film grows with increasing RH. The first monolayer is completed at ∼15% RH on both oxides and is followed by a second layer at 35−40% RH. At 90% RH, about 6 layers of H2O film have been formed on Al2O3.

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Adsorption of Water on Cu2O and Al2O3 Thin Films

The stoichiometric single- and bilayer ZnO(0001) have been prepared by reactive deposition of Zn on Au(111) and studied in detail with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. Both single- and bilayer ZnO(0001) adopt a planar, graphite-like structure similar to freestanding ZnO(0001) due to the weak van der Waals interactions dominating their adhesion with the Au(111) substrate. At higher temperature, the single-layer ZnO(0001) converts gradually to bilayer ZnO(0001) due to the twice stronger interaction between two ZnO layers than the interfacial adhesion of ZnO with Au substrate. It is found that Cu atoms on the surface of bilayer ZnO(0001) are mobile with a diffusion barrier of 0.31 eV and likely to agglomerate and form nanosized particles at low coverages; while Cu atoms tend to penetrate a single layer of ZnO(0001) with a barrier of 0.10 eV, resulting in a Cu free surface.

Xingyi Deng

Papers

Electron-induced dissociation of CO2 on TiO2(110).

Active Sites and Structure−Activity Relationships of Copper-Based Catalysts for Carbon Dioxide Hydrogenation to Methanol

Surface chemistry of Cu in the presence of CO2 and H2O.

Adsorption of Water on Cu2O and Al2O3 Thin Films

Growth of Single- and Bilayer ZnO on Au(111) and Interaction with Copper