Single atom catalysts by atomic diffusion strategy

The depletion of energy and increasing environmental pressure have become one of the main challenges in the world today. Synthetic high-efficiency catalysts bring hope for efficient conversion of energy and effective treatment of pollutants, especially, single-atom catalysts (SACs) are promising candidates. Herein, we comprehensively summarizes the atomic diffusion strategy, which is considered as an effective method to prepare a series of SACs. According to the different diffusion forms of the precursors, we review the synthesis pathways of SACs from three aspects: gas diffusion, solid diffusion and liquid diffusion. The gaseous diffusion method mainly discusses atomic layer deposition (ALD) and chemical vapor deposition (CVD), both of which carry out gas phase mass transfer at high temperatures. The solid-state diffusion method can be divided into nanoparticle transformation into single atoms and solid atom migration. Liquid diffusion mainly describes the electrochemical method and the molten salt method. We hope this review can trigger the rational design of SACs.

     

Visualization of hydrogen diffusion in steels by high sensitivity hydrogen microprint technique

Hydrogen diffusion in steels was examined by both a high sensitivity hydrogen microprint technique (HMT) and an electrochemical hydrogen permeation method. The main diffusion path in an extremely low carbon steel was lattice within grains; grain boundaries were not accelerated diffusion paths. In the case of a hypo-eutectoid steel, hydrogen diffused through proeutectoid ferrite and ferrite in pearlite under steady-state of hydrogen diffusion. The diffusion paths, however, were carbide/ferrite interfaces when hydrogen charging was interrupted before achievement of the steady state. This is probably ascribable to the reversible trapping effect of the interface. The detection efficiency of the high sensitivity HMT was 75% for the low carbon steel and 40% for the hypo-eutectoid steel.     

Visualization of hydrogen diffusion in steels by high sensitivity hydrogen microprint technique

Hydrogen diffusion in steels was examined by both a high sensitivity hydrogen microprint technique (HMT) and an electrochemical hydrogen permeation method. The main diffusion path in an extremely low carbon steel was lattice within grains; grain boundaries were not accelerated diffusion paths. In the case of a hypo-eutectoid steel, hydrogen diffused through proeutectoid ferrite and ferrite in pearlite under steady-state of hydrogen diffusion. The diffusion paths, however, were carbide/ferrite interfaces when hydrogen charging was interrupted before achievement of the steady state. This is probably ascribable to the reversible trapping effect of the interface. The detection efficiency of the high sensitivity HMT was 75% for the low carbon steel and 40% for the hypo-eutectoid steel.     

Quantum diffusion of hydrogen

A brief overview is given on low temperature diffusive behaviour of H in Nb(OH) _x , and measurement of diffusion, primarily by neutron scattering. We go in some detail into recent work on calculations of neutron structure factor as well as the diffusion coefficient of hydrogen, influenced by two-level excitations. Both the electronic and phononic contributions are covered. The importance of similar two-level excitations in insulating and metallic glasses is also pointed out.     

Investigation of the adsorption and diffusion interaction of atomic hydrogen with low-index surfaces of crystal aluminum nanoplates

By the method of density functional in the local density and generalized gradient approximation, the dependences of the potential energy of interaction of atomic hydrogen with low-index surfaces of aluminum nanoplates have been calculated. The spatial configurations of hydrogen atoms and surface layers of the plate corresponding to the stable structures formed by adsorption and diffusion have been determined. It has been shown that atomic hydrogen is adsorbed irreversibly on the Al surface, releasing energy of 2.8–3.1 eV whose value is determined by the atomic structure of the surface. On the atomic planes (100) and (110), the bridge form of chemisorption has the minimum energy. For the (111) surface, the energy-stable state is realized in the threefold coordinated position of the hydrogen atom. Diffusion of hydrogen is an activated process in which energy of 0.5–0.8 eV is absorbed depending on the atomic structure of the plate. Atomic hydrogen moves through interatomic voids sequentially occupying octa- and tetrahedral positions. It has been established that the transient state between stable O_h and T_d positions in the bulk of the plate is the geometrical configuration of the hydrogen atom having third-order symmetry.     

Surface characterization of silica glass substrates treated by atomic hydrogen

Silica glass substrates with very flat surfaces were exposed to atomic hydrogen at different temperatures and durations. An atomic force microscope was used to measure root-mean-square (RMS) roughness and two-dimensional power spectral density (PSD). In the treatment with atomic hydrogen up to 900 °C, there was no significant change in the surface. By the treatment at 1000 °C, the changes in the RMS roughness and the PSD curves were observed. It was suggested that these changes were caused by etching due to reactions of atomic hydrogen with surface silica. By analysis based on the k-correlation model, it was found that the spatial frequency of the asperities became higher with an increase of the treatment time. Furthermore, the data showed that atomic hydrogen can flatten silica glass surfaces by controlling heat-treatment conditions.     

Two-atom hydrogen clusters and diffusion of hydrogen in metal

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 25, No. 3, pp. 7–12, May–June, 1989.     

Possibilities of atomic hydrogen storage by carbon nanotubes or graphite materials

Atomic hydrogen storage by carbon nanotubes (CNTs) and highly oriented pyrolytic graphite (HOPG) has been studied using a flow catalytic reactor and an ultra-high vacuum surface science apparatus including scanning tunneling microscope (STM), respectively. Defect sites on CNTs as adsorption sites of atomic hydrogen are introduced by oxidation pretreatment using La catalyst. Pd catalysts are then deposited on CNT surfaces for dissociation of H₂ into atomic hydrogen, which spills over to the defect sites. In the best case, 1.5 wt% of hydrogen is stored in the defective CNT with Pd particles at 1 atm and 573 K. In temperature programmed desorption (TPD) experiments, H₂ starts to desorb at 700–900 K depending on the annealing temperatures of CNTs prior to hydrogen storage. On the HOPG surface, hot atomic hydrogen produced by dissociation of H₂ using tungsten wire desorbs from graphite terraces at 400–700 K, which is much lower than that on CNTs. It is possible that one can decrease the desorption temperature by changing the method of H₂ dissociation.     

Ultra-sensitive bolometers for atomic hydrogen detection

Two different types of bolometers have been developed to be used for spin-polarized atomic hydrogen detection. One of them uses a thin Cu shell as the absorber and a doped Ge chip as the thermometer element. The other uses a silver film as the absorber and a superconductor-insulator-normal metal junction to measure the temperature rise of the electrons in the absorber. These bolometers have a calculated noise equivalent power of 10^–15 WHz^–1/2 and 10^–17 WHz^–1/2, respectively, and can detect down to 6 × 10³ and 800 particles, respectively, at an operating temperature of 100 mK. If these detectors perform as predicted, the minimum number of detectable hydrogen atoms will be improved by several orders of magnitude.     

Ion-implanted resist removal using atomic hydrogen

We removed B-, P-, and As-ion-implanted positive-tone novolak resists with an implantation dose of 5 × 10¹² to 5 × 10¹⁵ atoms/cm² at 70 keV, using atomic hydrogen. Though the removal rate decreased with increase in the implantation dose, all of the ion-implanted resists were removed. The rates of thickness of the surface-hardened layer/all resist layer of B, P, and As implanted resists were 0.38, 0.26, and 0.16, respectively, by SEM observation. The removal rate decreased with increasing the rate of the surface-hardened layer. The energy supplied from the ions to the resist concentrated on the surface side in the increasing order of B-P-As.     

Fiber-optic cavity sensing of hydrogen diffusion

A novel type of fiber-optic cavity sensor for hydrogen diffusion into and out of fibers is presented. The sensor is an implementation of a cavity ringdown scheme in a silica-based single-mode fiber that has been exposed to gaseous hydrogen at normal pressure. The measured ringdown times during the H2 diffusion show good agreement with a theoretical diffusion model. This model allows the determination of the diffusion coefficient of hydrogen in silica, resulting in D = (3.02 +/- 0.07) x 10(-15) m2/s at 30 degrees C.     

Indirect identification and compensation of lateral scanner resonances in atomic force microscopes

Improving the imaging speed of atomic force microscopy (AFM) requires accurate nanopositioning at high speeds. However, high speed operation excites resonances in the AFM's mechanical scanner that can distort the image, and therefore typical users of commercial AFMs elect to operate microscopes at speeds below which scanner resonances are observed. Although traditional robust feedforward controllers and input shaping have proven effective at minimizing the influence of scanner distortions, the lack of direct measurement and use of model-based controllers have required disassembling the microscope to access lateral scanner motion with external sensors in order to perform a full system identification experiment, which places excessive demands on routine microscope operators. Further, since the lightly damped instrument dynamics often change from experiment to experiment, model-based controllers designed from offline system identification experiments must trade off high speed performance for robustness to modeling errors. This work represents a new way to automatically characterize the lateral scanner dynamics without addition of lateral sensors, and shape the commanded input signals in such a way that disturbing dynamics are not excited. Scanner coupling between the lateral and out-of-plane directions is exploited and used to build a minimal model of the scanner that is also sufficient to describe the nature of the distorting resonances. This model informs the design of an online input shaper used to suppress spectral components of the high speed command signals. The method presented is distinct from alternative approaches in that neither an information-complete system identification experiment nor microscope modification are required. Because the system identification is performed online immediately before imaging, no tradeoff of performance is required. This approach has enabled an increase in the scan rates of unmodified commercial AFMs from 1-4 lines s(-1) to over 40 lines s(-1).     

Reduction of oxide layer on various metal surfaces by atomic hydrogen treatment

The reduction of various metallic oxides was examined. Atomic hydrogen generated on a heated tungsten catalyzer was used for reduction. It was found that Cu, Ru, Nb, Mo, Rh, Pd, Ir and Pt oxides can be reduced by irradiation with atomic hydrogen. The activation energy for oxide removal was examined and it was found that the values were very small, 10^− 2 to 10^− 4 eV.     

ESR on adsorbed doubly spin-polarized atomic hydrogen

Studies of the ESR spectrum of spin-polarized atomic hydrogen adsorbed on a liquid helium film are presented. The absorption peaks associated with the surface atoms are displaced relative to that of the gaseous atoms due to the electronic dipole-dipole interaction, which does not average to zero for atoms moving in two dimensions. This phenomenon, first observed by Reynolds et al., allows the surface atom density to be measured directly and, through the lineshape, information on the dynamics of the 2-D gas can in principle be obtained. Here we present a more detailed study, with a better characterized substrate for the helium film.Using only the assumption that the ideal gas approximation is valid for the experimental conditions, we find that the binding energy of hydrogen atoms to the liquid helium surface is 1.03 (2) K. Although these measurements of the binding energy are not the most accurate, they are the most direct.The ESR lineshape of the absorption peak of the bulk atoms is determined by the inhomogeneity of the applied magnetic fields, whereas the resonance lineshape of the adsorbed atoms, which is very asymmetric and much broader than the main resonance, is clearly due to some other mechanism. In spite of a considerable effort to explain the observed lineshapes, we have not reached satisfactory conclusions.     

液氢泄漏扩散数值模拟研究

通过ALOHA软件进行了不同风速、环境温度、泄漏量扩散模拟,分析了不同风速、温度、泄漏量条件下的扩散规律。结果表明,泄漏扩散距离随着风速的增大而变小,风速越大,泄漏扩散范围越小;泄漏扩散距离随着环境温度的升高而变大,温度越高,泄漏扩散范围越大;泄漏扩散距离随着泄漏量的增加而变大,泄漏量越大,泄漏扩散范围越大。将液氢蒸发和氢气扩散试验的体积分数数据与ALOHA软件模拟数据进行对比,结果表明:ALOHA软件对氢的体积分数值模拟结果具有良好的精度,在液氢泄漏事故应急中具实用性。     

Anisotropic diffusion of hydrogen in nanoporous carbons

It is accepted that hydrogen transport capacity through carbons depends on the anisotropy of the empty spaces that constitute their porous structure. However, very little is known about this relationship. Computational simulation is an excellent tool to accomplish this kind of studies. Simulation requires digital representations of materials and a model describing the interaction potential among the gas molecules and the solids surfaces. In this work, it is proposed to use the analytical solutions of the truncated pore problem for modeling the potentials, and an immiscible lattice gas for obtaining the representations. The degree of anisotropy was quantified by using the mean intercept length method. The adsorption isotherms and the self-diffusion coefficients in the three orthogonal directions were found by the grand canonical and kinetic Monte Carlo methods, respectively. The results suggest the existence of a gas pressure at which a molecular saturation threshold (P _s) is reached. P _s determines if the degree of anisotropy is or not a representative variable of diffusive transport. For P ≤ P _s, the degree of anisotropy favors the molecular mobility. When P > P _s, the degree of anisotropy loses influence on mobility.