Yi-Tao Cui

Single-atom catalysis of CO oxidation using Pt1/FeOx.

Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.

Aerobic oxidative coupling of alcohols and amines over Au–Pd/resin in water: Au/Pd molar ratios switch the reaction pathways to amides or imines

A facile switch of the reaction pathways of aerobic oxidative coupling of alcohols and amines from amidation to imination was realized for the first time by tuning the Au/Pd ratios in ion-exchange resin supported Au–Pd alloy catalysts (Au–Pd/resin). Amides were obtained with high yields on Au6Pd/resin while imines were obtained over AuPd4/resin. Various alcohols and amines underwent oxidative coupling smoothly in water to afford the desired products with good to excellent yields. Further investigation on the reaction mechanism suggested the synergistic effect between Au and Pd determined the adsorption strength of the aldehyde intermediate, which in turn dictated the reaction pathways. That is, on Au-rich alloys (e.g., Au6Pd) absorbed aldehyde species was formed, followed by further oxidation to yield amides, while on Pd-rich alloys (e.g., AuPd4), free aldehyde was generated, which then underwent condensation with amines to produce imines. The discovery might provide avenues to develop new efficient catalysts for the green synthesis of special chemicals.

Crop-Derived Polysaccharides as Binders for High-Capacity Silicon/ Graphite-Based Electrodes in Lithium-Ion Batteries

Rice to power: Amylopectin is a major component of agricultural products such as corn, potato, and rice. Silicon-graphite electrodes are prepared by using slurries of these polysaccharides as binders. Compared to the conventionally used binder PVdF, they exhibit drastically improved electrode performance in Li cells. The improved performance is coupled to the degree of branching.

Electronic structure of β-Ga2O3 single crystals investigated by hard X-ray photoelectron spectroscopy

By combination of hard X-ray photoelectron spectroscopy (HAXPES) and first-principles band structure calculations, the electronic states of β-Ga2O3 were investigated to deepen the understanding of bulk information for this compound. The valence band spectra of HAXPES presented the main contribution from Ga 4sp, which are well represented by photoionization cross section weighted partial density of states. The experimental data complemented with the theoretical study yield a realistic picture of the electronic structure for β-Ga2O3.

Measurement of the Ligand Field Spectra of Ferrous and Ferric Iron Chlorides Using 2p3d RIXS

Ligand field spectra provide direct information about the electronic structure of transition metal complexes. However, these spectra are difficult to measure by conventional optical techniques due to small cross sections for d-to-d transitions and instrumental limitations below 4000 cm-1. 2p3d resonant inelastic X-ray scattering (RIXS) is a second order process that utilizes dipole allowed 2p to 3d transitions to access d-d excited states. The measurement of ligand field excitation spectra by RIXS is demonstrated for a series of tetrahedral and octahedral Fe(II) and Fe(III) chlorides, which are denoted Fe(III)-Td, Fe(II)-Td, Fe(III)-Oh, and Fe(II)-Oh. The strong 2p spin-orbit coupling allows the measurement of spin forbidden transitions in RIXS spectroscopy. The Fe(III) spectra are dominated by transitions from the sextet ground state to quartet excited states, and the Fe(II) spectra contain transitions to triplet states in addition to the spin allowed 5Γ → 5Γ transition. Each experimental spectrum is simulated using a ligand field multiplet model to extract the ligand field splitting parameter 10Dq and the Racah parameters B and C. The 10Dq values for Fe(III)-Td, Fe(II)-Td, and Fe(III)-Oh are found to be -0.7, -0.32, and 1.47 eV, respectively. In the case of Fe(II)-Oh, a single 10Dq parameter cannot be assigned because Fe(II)-Oh is a coordination polymer exhibiting axially compressed Fe(II)Cl 6 units. The 5T → 5E transition is split by the axial compression resulting in features at 0.51 and 0.88 eV. The present study forms the foundation for future applications of 2p3d RIXS to molecular iron sites in more complex systems, including iron-based catalysts and enzymes.

Wetting Induced Oxidation of Pt-based Nano Catalysts Revealed by In Situ High Energy Resolution X-ray Absorption Spectroscopy

In situ high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) was used to systematically evaluate interactions of H2O and O2 adsorbed on Pt and Pt3Co nanoparticle catalysts in different particle sizes. The systematic increase in oxidation due to adsorption of different species (H2O adsorption <O2 adsorption <O2 + H2O coadsorption) suggests that cooperative behavior between O2 and H2O adsorptions is responsible for the overpotential induced by hydrated species in fuel cells. From the alloying and particle size effects, it is found that both strength of O2/H2O adsorption and their cooperative effect upon coadsorption are responsible for the specific activity of Pt catalysts.

Thickness and structure of thin films determined by background analysis in hard X-ray photoelectron spectroscopy

We report on the analysis of inelastic backgrounds associated with photoelectron peaks from thin films of Ru on Si using hard X-ray photoelectron spectroscopy (HAXPES) with an X-ray energy of 7939 eV. To extract information on the thickness and morphology of the Ru films, the Tougaard-background-analysis method was used. Consistent results from the analysis of the Si 1s peaks as well as the Ru 2p, 2s peaks to the thicknesses determined with X-ray reflectivity were found. Good agreement was also found for surface topography (the Ru forms islands on the Si surface for film thicknesses <12 nm and covers the complete surface for larger thicknesses) determined by our fitting results and scanning electron microscopy. It is demonstrated that with this method it is possible to obtain information on films up to 150 nm thickness, which corresponds to ∼20 times the inelastic mean free paths (IMFPs). This is larger than the previously reported ∼10 times the IMFP for X-ray photoelectron spectroscopy with conventional ...

Electronic Spectra of Iron–Sulfur Complexes Measured by 2p3d RIXS Spectroscopy

Iron sulfur (FeS) proteins perform a wide range of biological functions including electron transfer and catalysis. Understanding the complex reactivity of these systems requires a detailed understanding of their electronic properties, which are encoded in the low-energy d-d excited states. Here we demonstrate that iron L-edge 2p3d resonant inelastic X-ray scattering (RIXS) can measure d-d excitation spectra in a series of monomeric, dimeric, and tetrameric FeS model complexes. RIXS provides advantages over traditional optical spectroscopies, because it is capable of measuring low-energy electronic excitations (0-10 000 cm-1) and spin-flip transitions. RIXS reveals the dense manifold of d-d excited states in dimeric [2Fe-2S] and tetrameric [MFe3S4]2+ (M = V or Mo) complexes resulting from covalency and exchange coupling. These results support recent ab initio theoretical predictions that FeS clusters possess a much greater number of low-lying excited states than predicted by model Hamiltonians.

Catalytic cascade conversion of furfural to 1,4-pentanediol in a single reactor

The synthesis of bio-based linear diols is the subject of many research studies. However, one of the main obstacles in industrial development is the difficulty in controlling product selectivity. Here, we report the catalytic conversion of furfural to 1,4-pentanediol (PD) in the presence of Ru supported on an ordered mesoporous carbon (CMK-3) under pressure of H2 and CO2 in water. In contrast to previous catalytic pathways, this work is distinct in that it yields 1,4-PD as an exclusive product, instead of a mixture of 1,2- and 1,5-PD as usual. Under optimized conditions, 1,4-PD was obtained in 90% yield, and in a one-pot reaction, directly from furfural. We disclose that the conversion of furfural to 1,4-PD followed an unusual catalytic route. It implies a bifunctional catalytic pathway based on sequential catalytic hydrogenation reactions and an acid-catalyzed Piancatellis rearrangement.

Evolution of optical properties and band structure from amorphous to crystalline Ga2O3 films

The optical properties and band structure evolution from amorphous to crystalline Ga2O3 films was investigated in this work. Amorphous and crystalline Ga2O3 films were obtained by changing the growth substrate temperatures of pulsed laser deposition and the crystallinity increase with the rising of substrate temperature. The bandgap value and ultraviolet emission intensity of the films increase with the rising of crystallinity as observed by means of spectrophotometer and cathodoluminescence spectroscopy. Abrupt bandgap value and CL emission variations were observed when amorphous to crystalline transition took place. X-ray photoelectron spectroscopy core level spectra reveal that more oxygen vacancies and disorders exist in amorphous Ga2O3 film grown at lower substrate temperature. The valence band spectra of hard X-ray photoelectron spectroscopy present the main contribution from Ga 4sp for crystalline film deposited at substrate temperature of 500 oC, while extra subgap states has been observed in amor...