Phosphorus-modified cobalt single-atom catalysts loaded on crosslinked carbon nanosheets for efficient alkaline hydrogen evolution reaction.

Efficient and low-cost transition metal single-atom catalysts (TMSACs) for hydrogen evolution reaction (HER) have been recognized as research hotspots recently with advances in delivering good catalytic activity without noble metals. However, the high-cost complex preparation of TMSACs and insufficient stability limited their practical applications. Herein, a simple top-down pyrolysis approach to obtain P-modified Co SACs loaded on the crosslinked defect-rich carbon nanosheets was introduced for alkaline hydrogen evolution, where Co atoms are locally confined before pyrolysis to prevent aggregation. Thereby, the abundant defects and the unsaturated coordination formed during the pyrolysis significantly improved the stability of the monatomic structure and reduced the reaction barrier. Furthermore, the synergy between cobalt atoms and phosphorus atoms was established to optimize the decomposition process of water molecules, which delivers the key to promoting the slow reaction kinetics of alkaline HER. As the result, the cobalt SAC exhibited excellent catalytic activity and stability for alkaline HER, with overpotentials of 70 mV and 192 mV at current densities of -10 mA cm-2 and -100 mA cm-2, respectively.

Pyridine coordination enabled stepwise PT/ET N-H transfer and metal-independent C-C cleavage mechanism for Cu-mediated dehydroacylation of unstrained ketones.

A density functional theory study of copper-mediated dehydroacylation of 4-phenyl-2-butanone to the corresponding olefin reveals a flexible N-H transfer process and a metal-independent C-C cleavage mechanism. When N'-methylpicolinohydrazonamide (MPHA) acts as the activating reagent, N-H cleavage can easily take place via stepwise proton transfer/electron transfer (PT/ET) and the rate-determining step is C-C homolysis with a total free energy barrier of 22.6 kcal mol-1, which is consistent with experimental observation of no kinetic isotope effects (KIE) at ß-H. Besides, copper is found to have little influence on C-C cleavage, but is responsible for triggering single electron oxidation of the pre-aromatic intermediate (PAI). When replacing MPHA with picolinohydrazonamide (PHA), the second N-H transfer is 2.7 kcal mol-1 more favorable than C-C cleavage and dominates the pathway to aromatization, which explains there being no C-C cleavage product well. When N'-methylbenzohydrazonamide (MBHA) is adopted, the lack of pyridine coordination significantly reduces the stability of CuII and N-H transfer proceeds via a much more difficult proton coupled electron transfer (PCET) pathway, thus making N-H cleavage a rate-determining step with a total free energy barrier of up to 28.1 kcal mol-1.

Highly Active Si Sites Enabled by Negative Valent Ru for Electrocatalytic Hydrogen Evolution in LaRuSi.

The discovery and identification of novel active sites are paramount for deepening the understanding of the catalytic mechanism and driving the development of remarkable electrocatalysts. Here, we reveal that the genuine active sites for the hydrogen evolution reaction (HER) in LaRuSi are Si sites, not the usually assumed Ru sites. Ru in LaRuSi has a peculiar negative valence state, which leads to strong hydrogen binding to Ru sites. Surprisingly, the Si sites have a Gibbs free energy of hydrogen adsorption that is near zero (0.063 eV). The moderate adsorption of hydrogen on Si sites during the HER process is also validated by in situ Raman analysis. Based on it, LaRuSi exhibits an overpotential of 72 mV at 10 mA cm-2 in alkaline media, which is close to the benchmark of Pt/C. This work sheds light on the recognition of real active sites and the exploration of innovative silicide HER electrocatalysts.

Unexpected binuclear O-O cleavage and radical C-H activation mechanism for Cu-catalyzed desaturation of lactone.

A density functional theory study of Cu-catalyzed desaturation of δ-valerolactone into α,ß-unsaturated counterparts reveals an unexpected binuclear di-tert-butyl peroxide (DTBP) homolysis with spin-crossover and a radical α-C-H bond activation mechanism. The rate-determining step in the reaction catalyzed by CuIOAc-CyPPh2 is the homolysis of the O-O bond in DTBP with a total free energy barrier of 26.9 kcal mol-1, which is consistent with the observed first-order dependences on LCuI-PR3 and DTBP, as well as the pseudo-zeroth-order with lactone. The α- and ß-H transfer steps have 0.3 and 14.8 kcal mol-1 lower barriers than the O-O cleavage process, respectively. Such different barriers well explain the observed weak kinetic isotopic effect (KIE) at α-H and no KIE at ß-H. In addition, we found that the replacement of CyPPh2 for pyridine in the Cu complexes leads to much higher barriers for O-O bond cleavage and C-H bond activations with the formation of more stable binuclear Cu complexes.

Linearly excited indium fluorescence imaging for temporally resolved high-precision flame thermometry.

Two-line atomic fluorescence (TLAF) is a promising technique for two-dimensional (2D) flame thermometry. However, it suffers either from a low signal-to-noise ratio (SNR) when excited in the linear regime or a quenching effect and nonlinear behavior in the nonlinear regime. This work aims to develop a new TLAF modality, which can overcome the aforementioned limitations based on a specifically designed laser source that can generate long pulses (∼400ns) with a moderate energy of ∼0.9µJ and operate at a repetition rate up to ∼22kHz. A proof-of-concept experiment was conducted and linearly excited fluorescence images with an SNR up to ∼14 were obtained within 1 ms acquisition time by synchronizing the laser with the microchannel plate (MCP) of a 10 Hz-rate intensified camera. The SNR achieved was comparable to that of a traditional nonlinear TLAF implementation and superior to a conventional linear TLAF approach. This approach offers a novel solution for recording linearly excited indium fluorescence images and is expected to make TLAF a temporally resolved and high-precision 2D thermometry for the first time.

Simple calibrated nonlinear excitation regime two-line atomic fluorescence thermometry.

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a promising two-dimensional (2D) thermometry technique for turbulent sooty flames. However, the complexity of calibrating three system parameters and expensive instruments restricts the application of the current NTLAF technique. Here we propose a simple and cheap NTLAF measurement approach based on a one-parameter model and tunable diode laser absorption spectroscopy (TDLAS) calibration. Using this methodology, only one system parameter, instead of three as in traditional NTLAF, is to be calibrated by path-averaged temperature acquired by the TDLAS technique. As a demonstration, instantaneous 2D thermometry data of a homemade burner were acquired using this approach, with measurement uncertainty of ∼4.5% and deviation from both reference TDLAS results and Raleigh scattering measurement results less than 50 K, typically within 20 K. This approach offers a novel simplified NTLAF solution for noncontact, in-suit, high-resolution 2D temperature measurement and is expected to greatly improve the compatibility of the NTLAF technique in scientific research and engineering applications.

Novel method for quantitative and real-time measurements on engine combustion at varying pressure based on the wavelength modulation spectroscopy.

The wavelength modulation spectroscopy (WMS) technique has been demonstrated as a powerful and indispensable tool for quantitative and real-time measurements on the combustion process of various industrial devices. However, the varying pressure occurred in the aero-engine combustor significantly affects the accuracy and efficiency of the WMS technique. To address this issue, this work reports a novel method named WMS pressure correction model, which can enable fast signal processing in the measurements at varying pressure. The method was first validated in a heated optical cell, and then applied to the pressure and temperature measurements in an aero-engine combustor. The results show that the new method can efficiently and accurately measure the pressure and temperature at the varying pressure conditions.

Aerobic oxidation of starch catalyzed by isopolyoxovanadate Na4Co(H2O)6V10O28.

The partial oxidation of starch was achieved in the presence of oxygen with Na4Co(H2O)6V10O28·18H2O (abbreviated as CoV10) as catalyst. The oxidation degree of starch was determined by FT-IR, XRD and SEM measurements, which indicated that the aerobic oxidation of starch was promoted by oxidative catalyst CoV10. The application of CoV10 could give a high oxidation degree (DO) of 1.35 COOH/100 GU and 2.07 CO/100 GU with 86 wt.% yield of solid starch under mild reaction conditions (pH=6; reaction time, 8 h; temperature, 50 °C; catalyst amount, 8 mg, when 1.5 g starch was used as substrate; atmospheric pressure). Among some vanadium compounds, CoV10 exhibited 4-fold activity higher than orthovanadate due to its coordination effect of cobalt and V10O28. Meanwhile, CoV10 could be recycled for six times with only a slight decrease in activity. Thus, CoV10/O2 is one of the most efficient systems for partial oxidation of starch reported so far.

[Emission spectrum temperature sensitivity of Mg4FGeO6 : mn induced by laser].

In order to develop a new sort of thermally sensitive phosphor coating, the emission spectrum thermally sensitivity of Mg4FGeO6 : Mn induced by laser was studied. The spectrum measurement system with heating function was set up, and the emission spectrum of Mg4FGeO6 : Mn at various temperatures were measured. Absorption spectrum was measured, and the mechanism of formation of the structure of double peak was analyzed with the perturbation theory of crystal lattice. The group of peaks around 630 nm is represented by the transitions 4F"2 to 4A2, whereas the group of peaks around 660 nm is due to the transitions 4F'2 to 4A2. The occupancy of both excited states 4F'2 and 4F"2 is in thermal equilibrium. Thus increasing temperature causes the intensity of the emission in the group around 630 nm to increase at the expense of the emission intensity of the group around 660 nm. The various spectral regions in emission differ with temperature, which could be used to support the intensity-ratio measurement method. The intensity-ratio change curve as a function of temperature was fitted, which shows that the range of temperature measurement is between room temperature and 800 K.