We synthesized a copper rubeanate metal organic framework (CR-MOF) which has the potential to improve the catalytic activity of electrochemical reduction of CO 2 due to its characteristics of electronic conductivity, proton conductivity, dispersed reaction sites, and nanopores. Synthesized CR-MOF particles were dropped on carbon paper (CP) to form a working electrode. The onset potential for CO 2 reduction of a CR-MOF electrode was about 0.2 V more positive than that observed on a Cu metal electrode in an aqueous electrolyte solution. Our analysis of the reduction products during potentiostatic electrolysis showed formic acid (HCOOH) to be virtually the only CO 2 reduction product on a CR-MOF electrode, whereas a Cu metal electrode generates a range of products. The quantity of products from the CR-MOF electrode was markedly greater (13-fold at −1.2 V vs. SHE) than that of a Cu metal electrode. Its stability was also confirmed. The electrochemical reduction of carbon dioxide (CO 2 ) into useful products at ambient pressures and temperatures has been a focus of research interest. A huge number of studies have been carried out in the past several decades especially for metal electrodes in aqueous media.1-4 Most metals have little catalytic activity for CO 2 reduction and mainly evolve hydrogen due to electrolysis of water, a competing reaction. Some metals produce carbon monoxide (CO) and formic acid (HCOOH), and, especially for HCOOH formation, a high applied potential of below −1.5 V vs. SHE is generally needed to obtain sufficient current for the process. Copper (Cu) has the unique property of producing hydrocarbons 3,5,6 and would appear to be the most hopeful candidate as a catalyst for CO 2 electrochemical reduction; however, Cu generates a range of reaction products and the selectivity for each product tends to be low. To improve its selectivity, there have been several explorations with metal alloys. 7 The basic idea is to disperse and specify the reaction site and to change the concentration of protons that are needed for electrochemical reduction of CO 2 . These protons are adsorbed by the atoms adjacent to each Cu site, where they induce a change in local reaction conditions and alter the reduction products of CO 2 . 8 Metal Organic Frameworks (MOFs) which have backbones constructed from metals and organic ligands have also been studied intensively. MOFs are structured and porous materials with nano-scale pores, and have been tested for applications including gas storage, 9,10 gas separation, 9,10 and heterogeneous catalysis. 9,11,12 Interestingly, some MOFs, such as copper rubeanate and its derivatives, show proton conductivity 13 that results from the adsorption of a proton to a nitrogen atom in the ligand, accompanied by electron transfer from Cu(II) ions.14 Those MOFs have been applied as electrochemical catalysts in ethanol fuel cell systems and have demonstrated selective formation of acetaldehyde as an oxidant of ethanol. 15 In spite of the interest shown in this area, there have until now been n...

Photoelectrochemical reduction of carbon dioxide (CO2) on p-type silicon (p-Si) electrodes modified with small metal (Cu, Ag, or Au) particles has been studied. The electrodes in CO2-saturated aqueous electrolyte under illumination produce methane, ethylene, carbon monoxide, etc., similar to the metal (Cu, Ag, or Au) electrodes, but at ca. 0.5 V more positive potentials than the corresponding metal electrodes, contrary to continuous-metal-coated p-Si electrodes. The results clearly show that the metal-particle-coated p-Si electrodes not only have high catalytic activity for electrode reactions but also generate high photovoltages and thus work as an ideal type semiconductor electrode. It is discussed that the CO2 photoreduction proceeds with an upward shift of the surface band energies of p-Si in order to get energy level matching between the semiconductor and solution reactants, though hydrogen photoevolution occurs without such an upward shift. It is also discussed that the control of surface structure on a nanometer-sized level, as well as on an atomic scale, is important for getting higher efficiencies.

A widely used approach to obtain smooth oxide-free and (partially) H-terminated silicon (Si) surfaces is to immerse Si wafers into CP4A (a mixture of H2O, HNO3, CH3COOH and HF in a volume ratio of 22:5:3:3) and/or HF solutions of varying concentrations. It is usually assumed that such treatments result in a dramatic reduction of the surface density of states and that, therefore, no surface band bending can occur. In our experiments we investigated the electronic surface structure of a number of CP4A/HF treated n- and p-Si wafers with varying doping densities by x-ray photoelectron spectroscopy (XPS). XPS allows a straightforward detection of surface stoichiometry as well as one of band bending and surface photovoltages (SPV) on semiconductor materials because the positions of the core level peaks directly depend on the position of the Fermi level within the band gap at the surface. Our experiments show that on all surfaces investigated Fermi level pinning still exists after the samples were immersed in the CP4A/HF solutions and that the pinning states are located close to the conduction band. Most of the samples also showed SPV when measured under illumination. The measurements also show that up to 36.6% of the surfaces are covered by F atoms depending on the treatment and the doping density. From the amount of band bending we estimated the density of surface states present on the various samples.

Understanding the oxygen evolution reaction (OER) is crucial for improving the performance of water electrolysis. Copper delafossite oxides (CuBO 2 , B = transition metal) were investigated for their potential as OER catalysts using density functional theory (DFT) calculations. To identify an appropriate descriptor for OER activity, we examined the relationships between the calculated e g or t 2g occupancy of the B site and the experimentally determined OER activity. The calculated t 2g occupancy was found to be approximately linearly related to OER activity. We therefore propose that t 2g occupancy can be employed as an appropriate descriptor of the OER activity of delafossite oxide catalysts. The delectron occupancy of active sites, estimated using theoretical calculations, can be used to search efficiently for transition metal oxide catalysts with high OER activity.

CO2 reduction with water and light illumination is realized using a gallium nitride (GaN) photoelectrode in which excited electrons induce CO2 conversion at the counter electrode. For the counter electrode, a copper (Cu) plate was chosen. The low affinity and wide gap of the nitride semiconductor enable us to create an electron–hole pair which has a sufficient energy for both CO2 reduction and water oxidation, in spite of the fact that a high energy for CO2 reduction is required. Within this system, the generation of formic acid (HCOOH) with 3% Faradic efficiency was confirmed by light illumination alone.