Using XPS and x-ray-excited Auger electron spectroscopy (XAES), we have studied the variation in surface composition of CuO and Cu2O with a variety of high-vacuum treatments, including vacuum annealing, oxidation and hydrogen reduction. Prolonged annealing of CuO results in the formation of a thick layer of Cu2O at the surface whilst vacuum annealing of Cu2O produces a thin (possibly one monolayer) film of Cu metal. Both bulk Cu2O and the thick Cu2O film generated from vacuum-annealed CuO were oxidized to CuO by heating at 800 K in 1×10-4 mbar O2, the original surface being regenerated with vacuum annealing at the same temperature. Both CuO and Cu2O are reduced to metal at the surface by heating in 1×10-4 mbar hydrogen at 400 K. In the case of CuO, the extent of reduction varies with the thermal history of the sample, with prolonged vacuum annealing producing a more reducible surface. Hydrogen-reduced CuO and Cu2O were both reoxidized on vacuum annealing, demonstrating the diffusion of lattice oxygen to the surface.

Surface Oxidation and Reduction of CuO and Cu2O Studied Using XPS and XAES

FT-IR and Raman spectra for polycrystalline powders of silver (I, III) oxide, AgO, and silver (I) oxide, Ag2O, are reported. The vibrational spectra for each oxide are discussed in relation to its crystal structure, and were found to be consistent with factor group analysis predictions. Infrared and Raman spectroscopy, in conjunction with powder XRD, were also used to follow the thermal decomposition of AgO powder in air. Supplementary studies employing differential scanning calorimetry (DSC) and temperature programmed reaction (TPR), provided additional information relevant to the decomposition process. In agreement with mechanisms previously reported, AgO was thermally reduced to metallic silver ia two non-reversible steps, with the intermediate formation of Ag2O. The transformation of AgO to Ag2O occurred with heating in the 373–473 K region, while the product of
this reaction remained stable to temperatures in excess of 623 K. Complete thermal decomposition of the Ag2O intermediate to Ag and O2 occurred at 673 K.

The thermal decomposition of silver (I, III) oxide: A combined XRD, FT-IR and Raman spectroscopic study

Silver-alumina catalysts prepared by sol–gel method (Ag-Al2O3) and impregnation method (Ag/Al2O3) were studied for the selective catalytic reduction (SCR) of NO by higher alkanes (n-hexane and n-octane). UV–VIS and Ag K-edge XAFS results established the structure of the catalysts; below 2 wt.%, highly dispersed Ag+ ions are predominant Ag species, while at higher Ag loading, Agn clusters are predominant. A relationship between the structure of Ag species and their catalytic function for SCR by n-octane was clarified. Ag+ ions are responsible for the selective reduction of NO to N2, while the Agn clusters are responsible for the hydrocarbon combustion and N2O formation. The mechanism of SCR by n-hexane on Ag-Al2O3, which mainly consists of Ag+ ions, were investigated by in situ FTIR spectroscopy. During NO+n-hexane+O2 reaction, the acetate produced via the partial oxidation of n-hexane and the nitrates produced via the oxidation of NO were main adspecies in the steady-state condition at 473–623 K. The acetate, which was stable in O2 or NO, was reactive in NO+O2. Nitrates, which were relatively stable in n-hexane, were quite reactive toward n-hexane+O2. The rate of nitrates reaction in n-hexane+O2 was close to the steady-state rate of NO reduction over wide range of temperature, indicating that the nitrate is a possible intermediate in the SCR. A proposed mechanism, suggesting the reaction of nitrates with partially oxidized hydrocarbon species as a crucial step, explains the steady-state kinetic results. At relatively high NO concentration or at low temperature, nitrates should inhibit the reaction by strongly adsorbing on the catalyst surface.

Silver-alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism

CuBi2O4 is a multinary p-type semiconductor that has recently been identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. It has an optimal bandgap energy (∼1.8 eV) and an exceptionally positive photocurrent onset potential (>1 V vs RHE), making it an ideal candidate for the top absorber in a dual absorber PEC device. However, photocathodes made from CuBi2O4 have not yet demonstrated high photoconversion efficiencies, and the factors that limit the efficiency have not yet been fully identified. In this work we characterize CuBi2O4 photocathodes synthesized by a straightforward drop-casting procedure and for the first time report many of the quintessential material properties that are relevant to PEC water splitting. Our results provide important insights into the limitations of CuBi2O4 in regards to optical absorption, charge carrier transport, reaction kinetics, and stability. This information will be valuable in future work to optimize CuBi2O4 as a PEC material. ...

Comprehensive Evaluation of CuBi2O4 as a Photocathode Material for Photoelectrochemical Water Splitting

Oxidation of a polycrystalline silver foil by reaction with ozone

Low-temperature structures of CuO and AgO and their relationships to those of MgO and PdO

The results of an investigation of ${\mathrm{Bi}}_{2}$${\mathrm{CuO}}_{4}$ using x-ray and neutron powder diffraction, dc magnetometry, and electron-spin resonance are presented. Simultaneous refinement of the room-temperature x-ray and neutron-diffraction data was used to obtain accurate cell parameters and atomic positions. Neutron-diffraction data at 13 and 300 K show that the appropriate space group is P4/ncc at both temperatures and reveal the appearance at the lower temperature of two magnetic peaks, which can be indexed as (100) and (210) reflections. While they are clearly indicative of long-range antiferromagnetic order, on the basis of these powder data alone one cannot determine the moment direction. However, on the assumption that the moments lie along the c axis, the copper magnetic moment is (0.56\ifmmode\pm\else\textpm\fi{}0.04)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. dc magnetometry was performed at temperatures from 1.66 to 400 K and fields ranging from 0.5 to 50 kOe. The magnetization showed no field saturation even at 1.66 K and 45 kOe. The susceptibility showed a maximum near 50.4 K with a Curie tail observed at low temperatures. Antiferromagnetic interactions dominated at all temperatures. The magnetic behavior is like that of a three-dimensional antiferromagnetic system. ESR experiments were done over the temperature range 4.3--300 K. For temperatures above 50 K, one broad line with g=2.09 was observed. The resonance field shifted to higher values for T37 K, with an eventual splitting of the line below 15 K. These ESR signals can be associated with antiferromagnetic resonance modes, consistent with an ordered antiferromagnetic phase.

B.L. Ramakrishna

Papers

Low-temperature structures of CuO and AgO and their relationships to those of MgO and PdO

Long-range antiferromagnetic ordering in Bi2CuO4.

Electronic Factors Influencing the Local and Cooperative Jahn-Teller Interactions in Spinels with Copper(II) and Nickel(II)

Manifestation of the Cooperative Jahn-Teller Effect in the Raman Spectra of Bu2CuxZn1−xWO6Mixed Crystals