Ensemble Effect Research Articles

Author Correction: Ensemble effect for single-atom, small cluster and nanoparticle catalysts

Author Correction: Ensemble effect for single-atom, small cluster and nanoparticle catalysts

The Ways for Bi on Pt to Enhance Formic Acid Oxidation

This work presents a correlation between the behavior of formic acid oxidation (FAO) on various Bi-modified Pt(poly) disk electrodes and their morphologies observed on Bi-modified Pt(111) disk electrodes using electrochemical scanning tunneling microscopy (EC-STM) to understand the effects of Bi on Pt. To distinguish the FAO activities of Bi on Pt and plain Pt around Bi, additional Pt was intentionally deposited using two different routes: direct route and iodine route. In direct route, Pt was directly deposited on Bi islands and plain Pt sites around Bi islands, while in iodine route, Pt was exclusively deposited on Bi islands by protecting plain Pt sites with adsorbed iodine. Thus, a comparison of FAO performances on the two Bi-modified Pt electrodes with additional Pt (deposited in the different ways) disclosed a difference in FAO performances on plain Pt sites and Bi islands. When Bi coverage was ~0.04, the Bi deposits were scattered Bi islands enhancing FAO on Pt(poly). The additional Pt deposits using direct route increased FAO efficiency, while the ones using iodine route slightly decreased FAO current. The EC-STM observations indicated that Pt deposits around Bi islands, not on Bi islands, were responsible for the FAO current increase on Bi-modified Pt(poly). The FAO efficiency on Bi-modified Pt(poly) with a Bi coverage of ~0.25 increased by a factor of 2. However, the additional Pt deposits using the two Pt deposition routes notably decreased the FAO current. The dependency of FAO on Bi coverage was discussed in terms of electronic effect and ensemble effect.

Pt Island Decorated Au Anode for Formic Acid Oxidation Reaction

For direct formic acid fuel cell (DFAFC), Pt-based catalyst is widely used as an anode. The Pt catalyst shows high activity and stability for formic acid oxidation reaction (FAOR) in acidic condition. For FAOR, it is generally accepted that Pt follows dual pathway including direct and indirect pathway. During the indirect pathway the Pt surface is vulnerable to CO poisoning, which cause the activity degradation. To enhance the CO tolerance, various Pt-based bi/ternary catalysts have been investigated. In recent, Pt–Au alloy catalysts have reported higher current density for direct FAOR pathway compared with pure Pt catalyst. The enhanced FAOR activity is originated from lower CO adsorption energy. However, the alloy structure has low catalyst utilization, which is not applicable to practical device.In this study, we report Pt islands decorated porous Au electrode for anode of the DFAFC. The porous Au transport layer was fabricated on carbon fiber by galvanic displacement and subsequent electrochemical etching process. Using Au transport layer as a substrate, Pt self-terminated electrodeposition (SED) was performed. By adjusting the SED condition, the Pt loading and coverage on Au surface cab be easily controlled. The FAOR activity test of Pt–Au electrodes was performed by cyclic voltammetry (CV). It was revealed that the current density for direct FAOR is linearly increased as the Pt coverage on Au surface is lower. By increasing the CV cycles, the current density for direct FAOR is gradually increased owing to enhanced ensemble effect. Furthermore, the cycled electrode was applicated into practical DFAFC single cell. The single cell demonstrates more than two-order higher Pt mass activity compared with previously reported single cell using Pt–Au anode.

A new adaptive parallel resonance system based on cascaded feedback model of vibrational resonance and stochastic resonance and its application in fault detection of rolling bearings

Accurate extraction of weak feature information in strong background noise is a key to detect and identify rolling bearing faults. Stochastic resonance (SR) and vibrational resonance (VR) have received extensive attention and research in weak signal detection by reason of their advantages of utilizing additional inputs (i.e. noise or high frequency harmonic signals) to enhance weak signals. Considering the advantages and disadvantages of SR and VR in weak signal detection, this paper combines the two to construct a cascaded feedback model of VR and SR, and utilize it to form a parallel resonance system, which improves the detection performance of weak signals through the ensemble average effect. Furthermore, a multi-parameter optimization strategy based on the improved whale optimization algorithm (WOA) is proposed for the parameter selection of the parallel resonance system. It uses the constructed measurement index independent of the prior knowledge as the fitness function to realize automatic adjustment of multi-parameter, and obtains the final output by weighted summation of the optimal results obtained by multiple iterations. Finally, the suggested method is analyzed by numerical simulation signal and experimental data of rolling bearings, and the effectiveness and superiority of the proposed method in the detection of weak fault features are verified.

Ensemble effect for single-atom, small cluster and nanoparticle catalysts

Ensemble effect for single-atom, small cluster and nanoparticle catalysts

Resolving the amine-promoted hydrolysis mechanism of N2O5 under tropospheric conditions

Hydrolysis of N2O5 under tropospheric conditions plays a critical role in assessing the fate of O3, OH, and NOx in the atmosphere. However, its removal mechanism has not been fully understood, and little is known about the role of entropy. Herein, we propose a removal path of N2O5 on the water clusters/droplet with the existence of amine, which entails a low free-energy barrier of 4.46 and 3.76 kcal/mol on a water trimer and droplet, respectively, at room temperature. The free-energy barrier exhibits strong temperature dependence; a barrierless hydrolysis process of N2O5 at low temperature (≤150 K) is observed. By coupling constrained ab initio molecular dynamics (constrained AIMD) simulations with thermodynamic integration methods, we quantitively evaluated the entropic contributions to the free energy and compared NH3-, methylamine (MA)-, and dimethylamine (DMA)-promoted hydrolysis of N2O5 on water clusters and droplet. Our results demonstrate that methylation of NH3 stabilizes the product state and promotes hydrolysis of N2O5 by reducing the free-energy barriers. Furthermore, a quantitative analysis of the internal coordinate distribution of the reaction center and the relative position of surrounding species reveals that the significant entropic contribution primarily results from the ensemble effect of configurations observed in the AIMD simulations. Such an ensemble effect becomes more significant with more water molecules included. Lowering the temperature effectively minimizes the entropic contribution, making the hydrolysis more exothermic and barrierless. This study sheds light on the importance of the promoting effect of amines and the entropic effect on gas-phase hydrolysis reactions, which may have far-reaching implications in atmospheric chemistry.

Charles Darwin as a statistical thinker

Recently historians and philosophers of science have been interested in the role of statistics and probability in investigating population variation. The focus is typically on investigators applying statistics and probability to explain large scale phenomenon that arise out of the collective behavior of numerous and varied individuals. The case studies that inform this work come mostly from molecular physics and 20th century genetical versions of evolutionary theory. Charles Darwin's work on evolution is rarely mentioned in this context except to point out his shortcomings—he made evolutionary theory “ripe” for statistical investigations, but he was not a statistical thinker. But this is a mistake, Darwin was a statistical thinker. In this essay I describe two instances where Darwin utilized statistical methods to investigate evolution. In the light of these cases, we ought to revise our views about Darwin's scientific methodology, in particular, how he came to develop his ideas about evolution and about the nature of his “population thinking”. Furthermore, Darwin's cases provide us with an expanded view about what constitutes “statistical thinking” in the biological sciences. In the examples we will find Darwin using statistical measures of type frequencies to detect large scale ensemble effects, confirm hypotheses by comparing between expected and observed averages, and applying the astronomer's law of error to explain evolutionary trends.

Recent Advances in Anode Electrocatalysts for Direct Formic Acid Fuel Cell-II-Platinum-Based Catalysts.

Platinum-based catalysts have a long history of application in formic acid oxidation (FAO). The single metal Pt is active in FAO but expensive, scarce, and rapidly deactivates. Understanding the mechanism of FAO over Pt important for the rational design of catalysts. Pt nanomaterials rapidly deactivate because of the CO poisoning of Pt active sites via the dehydration pathway. Alloying with another transition metal improves the performance of Pt-based catalysts through bifunctional, ensemble, and steric effects. Supporting Pt catalysts on a high-surface-area support material is another technique to improve their overall catalytic activity. This review summarizes recent findings on the mechanism of FAO over Pt and Pt-based alloy catalysts. It also summarizes and analyzes binary and ternary Pt-based catalysts to understand their catalytic activity and structure relationship.

On the Condition Number of the Shifted Real Ginibre Ensemble

We derive an accurate lower tail estimate on the lowest singular value $\sigma_1(X-z)$ of a real Gaussian (Ginibre) random matrix $X$ shifted by a complex parameter $z$. Such shift effectively changes the upper tail behaviour of the condition number $\kappa(X-z)$ from the slower $\mathbf{P}(\kappa(X-z)\ge t)\lesssim 1/t$ decay typical for real Ginibre matrices to the faster $1/t^2$ decay seen for complex Ginibre matrices as long as $z$ is away from the real axis. This sharpens and resolves a recent conjecture in [arXiv:2005.08930] on the regularizing effect of the real Ginibre ensemble with a genuinely complex shift. As a consequence we obtain an improved upper bound on the eigenvalue condition numbers (known also as the eigenvector overlaps) for real Ginibre matrices. The main technical tool is a rigorous supersymmetric analysis from our earlier work [arXiv:1908.01653].

Investigating the effects of ensemble and weight optimization approaches on neural networks’ performance to estimate the dynamic modulus of asphalt concrete

This study hybridized the ensemble and weight optimization approaches with an artificial neural network (ANN) algorithm to forecast the dynamic modulus (E*) of asphalt concrete. For input selection, this study employed the random forest technique and tested various techniques, including evolutionary, backward, forward, and brute force, to be hybridized with random forest. The evolutionary-random forest technique was used as the best input-selection method. The ρ200, Vbeff, binder G* (dynamic shear modulus), and binder φ (phase angle) were selected as the most important variables for developing the ANN models. The findings of this research indicated that the artificial neural network-particle swarm optimisation (ANNPSO) model performed better than the other models. Also, the weight optimization techniques were more efficient than the ensemble techniques to improve the predictive power of ANN to forecast the dynamic modulus of asphalt concrete.

Atomic Pt clusters on Au dendrite for formic acid oxidation

The minimization of Pt loadings on Au via facile fabrication methods remains a grand challenge in the development of commercially viable electrodes for direct formic acid fuel cells. Herein, we report a simple electrochemical strategy to prepare atomic Pt clusters on Au dendrites supported by carbon paper (CP) as a gas diffusion electrode. At highly negative deposition potentials, Pt deposition is autonomously quenched by immediate adsorption of discharged proton (H) on Pt surface, while highly roughened Au dendrites lead to resistance–capacitance delay for the transient cathodic current. As a result, the suppressed Pt nucleation and self-terminated Pt growth allows for the formation of isolated Pt sites on Au dendrites. The Pt/Au/CP with an ultra-low Pt loading of 5.0 μgPt/cmgeo2 demonstrates high selectivity for direct pathway of formic acid oxidation reaction (FAOR), owing to the ensemble effect. A gradual increase of FAOR activity is observed during cycling test and then, the 20 cycled Pt/Au/CP shows remarkable FAOR activity in half-cell test, exceeding state-of-the-art Pt-Au catalysts. Theoretical calculation indicates that the stabilized intermediate on Pt clusters accelerates the direct FAOR pathway. CO chemisorption analysis reveals that the isolated Pt sites remain stable throughout the reaction. Single cell test for direct formic acid fuel cell with 20 cycled Pt/Au/CP anode demonstrate two-order higher Pt mass activity and mass power density, compared with values reported in recent literature.

Unraveling the Structure and Surface Chemistry of the Phosphosulfide Phase Formed on Ni2P under Hydrodesulfurization Reaction Conditions: A DFT Study

There is experimental evidence that the actual active phase of the Ni2P catalyst under hydrodesulfurization (HDS) reaction conditions is a nickel phosphosulfide phase formed on the catalyst surface. Combining DFT calculations and an atomistic thermodynamic approach, we investigated the possibility of sulfur adsorption as well as the replacement of surface phosphorus by sulfur on the (0001) and (1010) surfaces of the Ni2P catalyst. Our DFT calculations showed that sulfur could replace up to 100% of the surface phosphorus under hydrodesulfurization (HDS) reaction conditions. We identify that possible sulfur adsorption sites on both surface terminations are arrangements of Ni trimers (Ni3 sites). We found that only the phosphosulfide phase formed on the (1010)-AB_Ni2P surface termination has coordinatively unsaturated Ni atoms available for the adsorption of organic sulfur-containing compounds. Our results suggest that the phosphosulfide phase formed on the (1010)-AB_Ni2P termination and not on the (0001)-Ni3P2 termination could be responsible for the high HDS activity of the Ni2P catalyst. Upon the replacement of P by S, the Ni3 sites undergo tensile strain, and their reactivity toward S adsorption is modified by the imposed strain. We found that the Ni–Ni bond distance is an important parameter in describing the HDS activity of the Ni2P catalyst. By analyzing the relationship among the ensemble, ligand, and strain effects, we were able to provide a better understanding of the promoting effect introduced by the formation of the nickel phosphosulfide phase.

Pd/Cu2O/CuO as Active Sites on the Cyclometalated Pd(II)/Cu(II) Nanosheet: Active Centre Formation, Synergistic and Catalytic Mechanism

AbstractA key to understand relationships between structure and activity is help for rational designing high active catalytic films. In this research, a new bi‐cyclometallic ferrocene imine Pd(II)/Cu(II) catalytic monolayer supported on graphene oxide (GO@Fcl−Pd/Cu) was fabricated, which exhibited a higher activity (TOF=1.48×106 h−1) in Suzuki coupling reaction. Enhanced higher activity was related to the in situ formed Pd/ Cu2O (CuO) active centre on the surface during catalysis. Fe in ferrocene had the redox ability for making Pd0 more stability, meaning that Fe3+ was electron withdraw or donor during catalytic process, which could tune the electronic properties of the Pd and Cu in one complex and improve their electronic transferring to boost the activity. CuO/Cu2O could make Pd0 more negative and stability. Ensemble synergistic effect, including the metals, metal oxide, ligand and GO could be described as that the electron transferred from GO via ferrocene to Pd or via Cu(II) to Pd made more negative, improving the oxidative addition. Meanwhile, aryl boronic acid adsorbed on the vicinal Cu(II), Cu2O and CuO around Pd0 synergetic reaction easily with the Pd intermediate to form next intermediate.

Tuning crystal-phase of bimetallic single-nanoparticle for catalytic hydrogenation

Bimetallic nanoparticles afford geometric variation and electron redistribution via strong metal-metal interactions that substantially promote the activity and selectivity in catalysis. Quantitatively describing the atomic configuration of the catalytically active sites, however, is experimentally challenged by the averaging ensemble effect that is caused by the interplay between particle size and crystal-phase at elevated temperatures and under reactive gases. Here, we report that the intrinsic activity of the body-centered cubic PdCu nanoparticle, for acetylene hydrogenation, is one order of magnitude greater than that of the face-centered cubic one. This finding is based on precisely identifying the atomic structures of the active sites over the same-sized but crystal-phase-varied single-particles. The densely-populated Pd-Cu bond on the chemically ordered nanoparticle possesses isolated Pd site with a lower coordination number and a high-lying valence d-band center, and thus greatly expedites the dissociation of H2 over Pd atom and efficiently accommodates the activated H atoms on the particle top/subsurfaces.

A novel locally-constrained GAN-based ensemble to synthesize arterial spin labeling images

Arterial spin labeling (ASL) images that are capable to quantitatively measure the cerebral blood flow receive increasing research attention in recent dementia diagnosis studies. However, this important imaging modality is unfortunately not commonly seen in many well-established image-based dementia datasets. Hence, synthesizing ASL images to supplement the important modality in these datasets for further improving the accuracy of dementia diseases diagnosis is quite important and valuable. In this study, a novel locally-constrained generative adversarial networks (GAN)-based ensemble is introduced to fulfill the ASL image synthesis task for improving the dementia diseases diagnosis performance. Technically, new attention-based feature pyramid-GAN models are designed as local models of the novel ensemble. Also, multi-Gaussian-distributed noise is generated from a new flow-based generative model and utilized in medical image synthesis, for the first time. Experiments have been conducted to reveal the effectiveness of the novel GAN ensemble. Comparisons between the novel GAN ensemble and many other state-of-the-art methods in medical image synthesis have been carried out. Statistical analyses have suggested that, accuracies of dementia diseases diagnosis can be significantly improved with the help of the novel GAN ensemble, which brings about 41.62% performance improvement based on a 355-demented-patient dataset and approximately 25% performance improvement from the well-known ADNI-1 dataset.

Tunning the Activity of Silver Alloy Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

A bottleneck to achieve efficient and stable electrocatalysts for Anion Exchange Membrane Fuel Cells (AEMFCs) is the relatively slow kinetics of the oxygen reduction reaction (ORR) taking place at the cathode. The rather mild alkaline conditions in the AEMFC enable the utilization of various PGM-free catalysts, which brings up new possibilities of finding abundant and inexpensive electrocatalysts without compromising the power density. From theory and previous experiments, it is known that Au and Ag bind oxygen weakly, whereas other transition metals (e.g., Pd, Cu, Fe, Ni) bind strongly to oxygenated species1. Therefore, a suitable geometric arrangement between Ag, an abundant metal with relatively high specific activity, and other metals may give rise to optimum binding of oxygen and thus a high activity. A good strategy to improve the activity of Ag-based catalysts could be a rational design of alloys that can enhance the intrinsic activity of Ag by both ensemble and electronic effects2–4. The former is based on the combination of active sites that facilitate the initial oxygen binding together with Ag active sites that help desorb hydroxide anions, whereas the latter is induced by the alloying metal (which binds oxygenated species strongly) on the electronic structure of the overlaying Ag that tunes its binding to oxygen5. Herein, we report the fabrication and characterization of Ag thin-films alloyed with Pd, Ni, Cu and Mo as a tool to investigate the ORR reaction in terms of ensemble and electronic effects and thus relate alloy composition and geometric arrangement with ORR activity. Ag thin-film model catalysts are fabricated by means of physical vapor deposition methods (PVD) with high control over the amount of material deposited. By varying alloy composition and evaluating electrochemical activity and stability, together with detailed physical characterization, the degree and extent of electronic effects are elucidated. In the same way, sputtering of the same amount of Ag on thin-films of different metals evidences the role of the alloying metal on tuning the activity of Ag through an adjustment of its d-band that changes the binding of oxygenated species, as well as through an ensemble effect that provides active sites for the first elementary steps of the reaction. These results provide insights for more tailored design of electrocatalysts in alkaline media by shedding new light on the mechanisms through which the ORR kinetics are improved in alkaline media. Nørskov, J. K. et al. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 108, 17886–17892 (2004).Betancourt, L. E. et al. Enhancing ORR Performance of Bimetallic PdAg Electrocatalysts by Designing Interactions between Pd and Ag. ACS Appl. Energy Mater. 3, 2342–2349 (2020).Slanac, D. A., Hardin, W. G., Johnston, K. P. & Stevenson, K. J. Atomic ensemble and electronic effects in Ag-rich AgPd nanoalloy catalysts for oxygen reduction in alkaline media. J. Am. Chem. Soc. 134, 9812–9819 (2012).Zamora Zeledón, J. A. et al. Tuning the electronic structure of Ag-Pd alloys to enhance performance for alkaline oxygen reduction. Nat. Commun. 12, 1–9 (2021).Stamenkovic, V. et al. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chemie 118, 2963–2967 (2006).

The rationalized correlation between selectivity and the surface ensemble site of the bimetallic catalyst

In this issue of Chem Catalysis , Shi et al. applied a combination of element-specific and surface-specific characterization techniques to conclude the minimum Ni domain fraction required for furan hydrogenation. As a glimpse of the “ensemble effect,” it provides an excellent example for studying the surface segregation of metallic catalysts. In this issue of Chem Catalysis , Shi et al. applied a combination of element-specific and surface-specific characterization techniques to conclude the minimum Ni domain fraction required for furan hydrogenation. As a glimpse of the “ensemble effect,” it provides an excellent example for studying the surface segregation of metallic catalysts.

Ensemble effects on the reconstruction of attosecond pulses and photoemission time delays

A crucial prerequisite for a detailed interpretation of the experimental results obtained with the most common attosecond spectroscopic techniques is a careful characterization of the attosecond extreme-ultraviolet (XUV) and femtosecond infrared (IR) pulses used in the measurements. A commonly adopted approach is based on the measurement of the spectra of the photoelectrons produced by the interaction of the attosecond pulses with a noble gas in the presence of a delayed IR pulse. Feeding the resulting spectrogram to reconstruction algorithms, it is then possible to retrieve the temporal properties of the XUV and IR pulses. To date, all reconstruction techniques are based on the assumption that the spectrogram is produced by the interaction of a single atom with a two-color (XUV-IR) field. In this work, we numerically investigate the effect of the actual XUV and IR beam spatial distributions, and we analyze their impact on the retrieval of the temporal characteristics of the XUV and IR pulses and on the determination of the photoemission time delays. We show that the impact of the ensemble effects can be severe, leading to notable variation of the photoelectron spectrograms, depending on the ratio between the XUV and IR beam spot sizes and on the IR peak intensity. We demonstrate that the photoemission time delay can be retrieved with great accuracy even in the presence of large deformations of the photoelectron spectrograms by employing suitable reconstruction procedures.

MIA-Former: Efficient and Robust Vision Transformers via Multi-Grained Input-Adaptation

Vision transformers have recently demonstrated great success in various computer vision tasks, motivating a tremendously increased interest in their deployment into many real-world IoT applications. However, powerful ViTs are often too computationally expensive to be fitted onto real-world resource-constrained platforms, due to (1) their quadratically increased complexity with the number of input tokens and (2) their overparameterized self-attention heads and model depth. In parallel, different images are of varied complexity and their different regions can contain various levels of visual information, e.g., a sky background is not as informative as a foreground object in object classification tasks, indicating that treating those regions equally in terms of model complexity is unnecessary while such opportunities for trimming down ViTs' complexity have not been fully exploited. To this end, we propose a Multi-grained Input-Adaptive Vision Transformer framework dubbed MIA-Former that can input-adaptively adjust the structure of ViTs at three coarse-to-fine-grained granularities (i.e., model depth and the number of model heads/tokens). In particular, our MIA-Former adopts a low-cost network trained with a hybrid supervised and reinforcement learning method to skip the unnecessary layers, heads, and tokens in an input adaptive manner, reducing the overall computational cost. Furthermore, an interesting side effect of our MIA-Former is that its resulting ViTs are naturally equipped with improved robustness against adversarial attacks over their static counterparts, because MIA-Former's multi-grained dynamic control improves the model diversity similar to the effect of ensemble and thus increases the difficulty of adversarial attacks against all its sub-models. Extensive experiments and ablation studies validate that the proposed MIA-Former framework can (1) effectively allocate adaptive computation budgets to the difficulty of input images, achieving state-of-the-art (SOTA) accuracy-efficiency trade-offs, e.g., up to 16.5\% computation savings with the same or even a higher accuracy compared with the SOTA dynamic transformer models, and (2) boost ViTs' robustness accuracy under various adversarial attacks over their vanilla counterparts by 2.4\% and 3.0\%, respectively. Our code is available at https://github.com/RICE-EIC/MIA-Former.

Electromagnetically induced transparency and absorption cross-over with a four-level Rydberg system

Electromagnetically induced transparency (EIT) and absorption (EIA) are quantum coherence phenomena which result from the interference of excitation pathways. Combining these with Rydberg atoms have opened up many possibilities for various applications. We introduce a theoretical model to study Rydberg-EIT and Rydberg-EIA effects in cold Cs and Rb atomic ensembles in a four-level ladder type scheme taking into account van der Waals type interactions between the atoms. The proposed many-body method for analysis of such systems involves a self-consistent mean field approach and it produces results which display a very good agreement with recent experiments. Our calculations also successfully demonstrate experimentally observed EIT-EIA cross-over in the Rb case. Being able to simulate the interaction effects in such systems has significant importance, especially for controlling the optical response of these.