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

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

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

Particle Size Effects of Carbon Supported Nickel Nanoparticles for High Pressure CO2 Methanation

Supported nickel nanoparticles are promising catalysts for the methanation of CO2. The role of nickel particle size on activity and selectivity in this reaction is a matter of debate. We present a study of metal particle size effects on catalytic stability, activity and selectivity, using nickel on graphitic carbon catalysts. Increasing the Ni particle size from 4 to 8 nm led to a higher catalytic activity, both per gram of nickel and normalized surface area. However, the apparent activation energy remained the same (∼105 kJ mol−1). Comparing experiments at atmospheric to 30 bar pressure demonstrates the importance of testing under industrially relevant pressures; the highest selectivity is obtained at high CO2 conversions and pressures. Finally, the selectivity was particle size-dependent. The largest particles were not only most active but also most selective to methane. With this work we contribute to the ongoing debate about Ni particle size effects in CO2 methanation

Electronic excitations of α- Fe2 O3 heteroepitaxial films measured by resonant inelastic x-ray scattering at the Fe L edge

Resonant inelastic x-ray scattering (RIXS) spectra of hematite (α-Fe2O3) were measured at the Fe L3 edge for heteroepitaxial thin films which were undoped and doped with 1% Ti, Sn, or Zn, in the energy-loss range in excess of 1 eV to study electronic transitions. The spectra were measured for several momentum transfers q, conducted at both low temperature (T=14 K) and room temperature. While we cannot rule out dispersive features possibly owing to propagating excitations, the coarse envelopes of the general spectra did not appreciably change shape with q, implying that the bulk of the observed L-edge RIXS intensity originates from (mostly) nondispersive ligand field excitations. Summing the RIXS spectra over q and comparing the results at T=14 K to those at T=300 K revealed pronounced temperature effects, including an intensity change and energy shift of the ≈1.4 eV peak, a broadband intensity increase of the 3-4 eV range, and higher energy features. The q-summed spectra and their temperature dependencies are virtually identical for nearly all of the samples with different dopants, save for the temperature dependence of the Ti-doped sample's spectrum, which we attribute to being affected by a large number of free charge carriers. Comparing with magnetization measurements for different temperatures and dopings likewise did not show a clear correlation between the RIXS spectra and the magnetic ordering states. To clarify the excited states, we performed spin multiplet calculations which were in excellent agreement with the RIXS spectra over a wide energy range and provide detailed electronic descriptions of the excited states. The implications of these findings to the photoconversion efficiency of hematite photoanodes is discussed

Decoding reactive structures in dilute alloy catalysts

Rational catalyst design is crucial toward achieving more energy-efficient and sustainable catalytic processes. Understanding and modeling catalytic reaction pathways and kinetics require atomic level knowledge of the active sites. These structures often change dynamically during reactions and are difficult to decipher. A prototypical example is the hydrogen-deuterium exchange reaction catalyzed by dilute Pd-in-Au alloy nanoparticles. From a combination of catalytic activity measurements, machine learning-enabled spectroscopic analysis, and first-principles based kinetic modeling, we demonstrate that the active species are surface Pd ensembles containing only a few (from 1 to 3) Pd atoms. These species simultaneously explain the observed X-ray spectra and equate the experimental and theoretical values of the apparent activation energy. Remarkably, we find that the catalytic activity can be tuned on demand by controlling the size of the Pd ensembles through catalyst pretreatment. Our data-driven multimodal approach enables decoding of reactive structures in complex and dynamic alloy catalysts

Artificial Intelligence-Aided Mapping of the Structure-Composition-Conductivity Relationships of Glass-Ceramic Lithium Thiophosphate Electrolytes

Lithium thiophosphates (LPSs) with the composition (Li2S)x(P2S5)1-x are among the most promising prospective electrolyte materials for solid-state batteries (SSBs), owing to their superionic conductivity at room temperature (>10-3 S cm-1), soft mechanical properties, and low grain boundary resistance. Several glass-ceramic (gc) LPSs with different compositions and good Li conductivity have been previously reported, but the relationship among composition, atomic structure, stability, and Li conductivity remains unclear due to the challenges in characterizing noncrystalline phases in experiments or simulations. Here, we mapped the LPS phase diagram by combining first-principles and artificial intelligence (AI) methods, integrating density functional theory, artificial neural network potentials, genetic-algorithm sampling, and ab initio molecular dynamics simulations. By means of an unsupervised structure-similarity analysis, the glassy/ceramic phases were correlated with the local structural motifs in the known LPS crystal structures, showing that the energetically most favorable Li environment varies with the composition. Based on the discovered trends in the LPS phase diagram, we propose a candidate solid-state electrolyte composition, (Li2S)x(P2S5)1-x (x ∼0.725), that exhibits high ionic conductivity (>10-2 S cm-1) in our simulations, thereby demonstrating a general design strategy for amorphous or glassy/ceramic solid electrolytes with enhanced conductivity and stability