Competition between reverse water gas shift reaction and methanol synthesis from CO 2 : influence of copper particle size

Converting CO2 into value-added chemicals and fuels, such as methanol, is a promising approach to limit the environmental impact of human activities. Conventional methanol synthesis catalysts have shown limited efficiency and poor stability in a CO2/H2 mixture. To design improved catalysts, crucial for the effective utilization of CO2, an in-depth understanding of the active sites and reaction mechanism is desired. The catalytic performance of a series of carbon-supported Cu catalysts, with Cu particle sizes in the range of 5 to 20 nm, was evaluated under industrially relevant temperature and pressure, i.e. 260 °C and 40 bar(g). The CO2 hydrogenation reaction exhibited clear particle size effects up to 13 nm particles, with small nanoparticles having the lower activity, but higher methanol selectivity. MeOH and CO formation showed a different size-dependence. The TOFCO increased from 1.9 × 10−3 s−1 to 9.4 × 10−3 s−1 with Cu size increasing from 5 nm to 20 nm, while the TOFMeOH was size-independent (8.4 × 10−4 s−1 on average). The apparent activation energies for MeOH and CO formation were size-independent with values of 63 ± 7 kJ mol−1 and 118 ± 6 kJ mol−1, respectively. Hence the size dependence was ascribed to a decrease in the fraction of active sites suitable for CO formation with decreasing particle size. Theoretical models and DFT calculations showed that the origin of the particle size effect is most likely related to the differences in formate coverage for different Cu facets whose abundancy depends on particle size. Hence, the CO2 hydrogenation reaction is intrinsically sensitive to the Cu particle size

Cation Binding Properties of Benzo-type Crown Ethers. Effect of Ring Size upon Complexation Phenomena

The cation-binding by a series of benzo-type crown ethers with different ring size has been performed by the solvent extraction of monovalent metal picrates (Na^+, K^+, Rb^+, Cs^+, Ag^+, and TI^+) between water and dichlolomethane. The effect of ring size and symmetry of the crown ethers upon the cation binding ability is discussed. The results are compared with those of commercially available crown ethers (dibenzo-18-crown-6 and 18-crown-6) and it was found that the less-symmetrical benzo-crown ethers have lower cation binding ability while unique cation selectivity for thallium (I) has been observed

Cu-Co/ZnAl2O4 Catalysts for CO Conversion to Higher Alcohols Synthesized from Co-Precipitated Hydrotalcite Precursors

The role of Cu:Co composition in bi-metallic Cu-Co/ZnAl2O4 catalysts on higher alcohol synthesis (HAS) was investigated at H2:CO = 4. The addition of Cu strongly facilitated Co reduction upon catalyst activation and suppressed coke deposition during HAS. Formation of predominantly hydrocarbons and higher alcohols was observed on the bi-metallic catalysts. Co/ZnAl2O4 produced mainly CH4 and Cu/ZnAl2O4 mainly CH3OH, while at Cu:Co = 0.6 the best ethanol selectivity of 4.5 % was reached. The microstructure of the spent catalysts confirmed a close interaction of Cu and Co

Cation Binding Properties of Benzo-type Crown Ethers. 2. Effect of Large Ring Size upon Complexation Phenomena of Crown-7 and Crown-8 Derivatives

The cation-binding by two large ring-sized benzo-21-crown-7 and benzo-24-crown-8 has been performed by the solvent extraction of monovalent metal picrates (Na^+, K^+, Rb^+, Cs^+, Ag^+, and Tl^+) between water and dichlolomethane. The effect of ring size and benzo group upon the cation binding ability is discussed. The results are compared with benzo-18-crown-6 and simple unsubstituted crown ethers (18-crown-6, 21-crown-7 and 24-crown-8). It was found that large ring benzo-crown ethers have lower cation binding ability as the ring size increases compared with simple unsubstituted crown ethers while cation selectivity for large cations (Cs^+ and Tl^+) has been observed

Cation Binding Properties of Benzo-type Crown Ethers. 3. Effect of Large Ring Size upon Complexation Phenomena of Crown-9 and Crowm-10 Derivatives

The cation-binding by two large ring-sized benzo-27-crown-9 and benzo-30-crown-10 has been evaluated by a solvent extraction of monovalent metal picrates (Na^+, K^+, Rb^+, Cs^+, Ag^+, and Tl^+) between water and dichlolomethane. The effect of ring size and benzo group upon the cation binding ability is discussed. The results are compared with benzo-type crown-6-8 (benzo-18-crown-6, benzo-21-crown-7 and benzo-24-crown-8) and simple unsubstituted crown-6-8 ethers (18-crown-6, 21-crown-7 and 24-crown-8) The cation-binding ability and complex formation are discussed in view of the size of the crown ether ring and the basicity of oxygen atoms

Satellites in the Ti 1s core level spectra of SrTiO3 and TiO2

Satellites in core level spectra of photoelectron spectroscopy (PES) can provide crucial information on the electronic structure and chemical bonding in materials, particularly in transition metal oxides. This paper explores satellites of the Ti 1s and 2p core level spectra of SrTiO3 and TiO2. Conventionally, soft x-ray PES (SXPS) probes the Ti 2p core level; however, it is not ideal to fully capture satellite features due to its inherent spin-orbit splitting (SOS). Here, hard x-ray PES (HAXPES) provides access to the Ti 1s spectrum instead, which allows us to study intrinsic charge responses upon core-hole creation without the complication from SOS and with favorable intrinsic linewidths. The experimental spectra are theoretically analyzed by two impurity models, including an Anderson impurity model (AIM) built on local density approximation (LDA) and dynamical mean-field theory (DMFT), and a conventional TiO6 cluster model. The theoretical results emphasize the importance of explicit inclusion of higher-order Ti-O charge-transfer processes beyond the nearest-neighboring Ti-O bond to simulate the core level spectra of SrTiO3 and TiO2. The AIM approach with continuous bath orbitals provided by LDA+DMFT represents the experimental spectra well. Crucially, with the aid of the LDA+DMFT method, this paper provides a robust prescription of how to use the computationally cheap cluster model in fitting analyses of core level spectra

Realizing Low-Temperature Charge-Transfer-Type Insulating Ground State in Strained V2O3Thin Film

Controlling the electronic properties of strongly correlated systems, observing electron-electron correlation-driven metal to insulator transition (MIT) is a key point for the next-generation solid-state Mottronic devices. Thus, the knowledge of the exact nature of the insulating state is an essential need to enhance the functionality of the material. Therefore, we have investigated the electronic nature of the insulating state of a classical Mott insulator V2O3 thin film (epitaxial) using low-temperature (LT) (120 K) resonant photoemission spectroscopy and X-ray absorption near-edge spectroscopy measurements. Temperature-dependent valence band spectra (VBS) reflect the transfer of spectral weight from the metallic coherent band (AM) near the Fermi level (EF) to the insulating Mott-Hubbard screened band (CI) at a binding energy of around 2.4 eV. Such a transfer of spectral weight upon MIT leads to vanishing of the density of states at EF and opens a band gap. The strong presence of the 3dnL final state is observed near the EF of LT VBS, confirming the presence of an O 2p band participating in low-energy charge fluctuation. This study further endorses the charge-transfer (CT)-type (U > Δ) insulating nature of a strained V2O3 thin film at LT, unlike its bulk counterpart, which is placed intermediate (U-Δ) between the CT and the Mott-Hubbard regime. Modifying the electronic ground state of V2O3 to the CT nature via the epitaxial strain in thin films provides a way to tailor the electronic energetics, with its implications to next-generation correlation-derived switching devices

Anisotropy of 4f states in 3d-4f single-molecule magnets

We have measured angular-dependent fluorescence-yield x-ray magnetic circular dichroism spectra on single crystals of the heterometallic 3d-4f 12-metallacrown-4 TbMn4 and DyMn4 complexes. Simulated spectra using crystal-field multiplet calculations reproduce the experimentally observed spectra. The orientation of the molecules causes linear dichroism spectra of the 4f absorption spectra. This natural linear dichroism shows the anisotropic charge distribution of the rare-earth 4f state in the tetragonal crystal field despite the small 4f crystal-field splitting. The magnetic moment of the molecule is dominated by the rare-earth moment revealing a considerably large contribution of orbital moment. From a sum-rule analysis of experimental and simulated x-ray magnetic circular dichroism, we determined corrected spin and orbital Dy moments at low temperature (14 K) within a magnetic field of 7 T. We find a significant angular dependence of the Dy magnetic moments, indicating the presence of fourth-order magnetic anisotropy

Direct and real-time observation of hole transport dynamics in anatase TiO2 using X-ray free-electron laser

Carrier dynamics affects photocatalytic systems, but direct and real-time observations in an element-specific and energy-level-specific manner are challenging. In this study, we demonstrate that the dynamics of photo-generated holes in metal oxides can be directly probed by using femtosecond X-ray absorption spectroscopy at an X-ray free-electron laser. We identify the energy level and life time of holes with a long life time (230 pico-seconds) in nano-crystal materials. We also observe that trapped holes show an energy distribution in the bandgap region with a formation time of 0.3 pico-seconds and a decay time of 8.0 pico-seconds at room temperature. We corroborate the dynamics of the electrons by using X-ray absorption spectroscopy at the metal L-edges in a consistent explanation with that of the holes