Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters.

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Pilot-scale investigation of performance and microbial community in a novel system combining fixed and suspended activated sludge.

The conventional activated sludge (CAS) process is a widely used method for wastewater treatment due to its effectiveness and affordability. However, it can be prone to sludge abnormalities such as sludge bulking/foaming and sludge loss, which can lead to a decrease in treatment efficiency. To address these issues, a novel bag-based fixed activated sludge (BBFAS) system utilizing mesh bags to contain the sludge was developed for low carbon/nitrogen ratio wastewater treatment. Pilot-scale experiments demonstrated that the BBFAS system could successfully avoid the sludge abnormalities. Moreover, it was not affected by mass transfer resistance and exhibited significantly higher nitrogen removal efficiency, surpassing that of the CAS system by up to 78%. Additionally, the BBFAS system demonstrated comparable organic matter removal efficiency to CAS system. 16S rRNA gene high-throughput sequencing revealed that the bacterial community structure within the BBFAS system was significantly different from that of the CAS system. The bacteria associated with ammonium removal were more abundant in the BBFAS system than in the CAS system. The abundance of Nitrospira in the BBFAS could reach up to 6% and significantly higher than that in the CAS system, and they were likely responsible for both ammonia-oxidizing and nitrite-oxidizing functions. Clear stratification of microbial communities was observed from the outer to inner layers of the bag components due to the gradients of dissolved oxygen and other substrates. Overall, this study presents a promising approach for avoiding activated sludge abnormalities while maintaining high pollutant removal performance.

Proton tunneling in the dissociation of H2+ and its asymmetric isotopologues driven by circularly polarized THz laser pulses.

Light-induced deprotonation of molecules is an important process in photochemical reactions. Here, we theoretically investigate the tunneling deprotonation of H2+ and its asymmetric isotopologues driven by circularly polarized THz laser pulses. The quasi-static picture shows that the field-dressed potential barrier is significantly lowered for the deprotonation channel when the mass asymmetry of the diatomic molecule increases. Our numerical simulations demonstrate that when the mass symmetry breaks, the tunneling deprotonation is significantly enhanced and the proton tunneling becomes the dominant dissociation channel in the THz driving fields. In addition, the simulated nuclear momentum distributions show that the emission of the proton is directed by the effective vector potential for the deprotonation channel and, meanwhile, the angular distribution of the emitting proton is affected by the alignment and rotation of the molecule induced by the rotating field.

Heterogeneous Hydrogenation with Hydrogen Spillover Enabled by Nitrogen Vacancies on Boron Nitride-Supported Pd Nanoparticles.

Heterogeneous hydrogenation with hydrogen spillover has been demonstrated as an effective route to achieve high selectivity towards target products. More effort should be paid to understand the complicated correlation between the nature of supports and hydrogenation involving hydrogen spillover. Herein, we report the development of the hydrogenation system of hexagonal boron nitride (h-BN)-supported Pd nanoparticles for the hydrogenation of aldehydes/ketones to alcohols with hydrogen spillover. Nitrogen vacancies in h-BN determine the feasibility of hydrogen spillover from Pd to h-BN. The hydrogenation of aldehydes/ketones with hydrogen spillover from Pd proceeds on nitrogen vacancies on h-BN. The weak adsorption of alcohols to h-BN inhibits the deep hydrogenation of aldehydes/ketones, thus leading to high catalytic selectivity to alcohols. Moreover, the hydrogen spillover-based hydrogenation mechanism makes the catalyst system exhibit a high tolerance to CO poisoning.

Surface Coordination Chemistry of Atomically Dispersed Metal Catalysts.

Atomically dispersed metal catalysts (ADCs), as an emerging class of heterogeneous catalysts, have been widely investigated during the past two decades. The atomic dispersion nature of the catalytic metal centers makes them an ideal system for bridging homogeneous and heterogeneous metal catalysts. The recent rapid development of new synthetic strategies has led to the explosive growth of ADCs with a wide spectrum of metal atoms dispersed on supports of different chemical compositions and natures. The availability of diverse ADCs creates a powerful materials platform for investigating mechanisms of complicated heterogeneous catalysis at the atomic levels. Considering most dispersed metal atoms on ADCs are coordinated by the donors from supports, this review will demonstrate how the surface coordination chemistry plays an important role in determining the catalytic performance of ADCs. This review will start from the link between coordination chemistry and heterogeneous catalysis. After the brief description on the advantages and limitations of common structure characterization methods in determining the coordination structure of ADCs, the surface coordination chemistry of ADCs on different types of supports will be discussed. We will mainly illustrate how the local and vicinal coordination species on different support systems act together with the dispersed catalytic metal center to determine the catalytic activity, selectivity, and stability of ADCs. The dynamic coordination structure change of ADCs in catalysis will be highlighted. At the end of the review, personal perspectives on the further development of the field of ADCs will be provided.

Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation.

Heterogeneous metal catalysts are distinguished by their structure inhomogeneity and complexity. The chameleonic nature of heterogeneous metal catalysts have prevented us from deeply understanding their catalytic mechanisms at the molecular level and thus developing industrial catalysts with perfect catalytic selectivity toward desired products. This Perspective aims to summarize recent research advances in deciphering complicated interfacial effects in heterogeneous hydrogenation metal nanocatalysts toward the design of practical heterogeneous catalysts with clear catalytic mechanism and thus nearly perfect selectivity. The molecular insights on how the three key components (i.e., catalytic metal, support, and ligand modifier) of a heterogeneous metal nanocatalyst induce effective interfaces determining the hydrogenation activity and selectivity are provided. The interfaces influence not only the H2 activation pathway but also the interaction of substrates to be hydrogenated with catalytic metal surface and thus the hydrogen transfer process. As for alloy nanocatalysts, together with the electronic and geometric ensemble effects, spillover hydrogenation occurring on catalytically "inert" metal by utilizing hydrogen atom spillover from active metal is highlighted. The metal-support interface effects are then discussed with emphasis on the molecular involvement of ligands located at the metal-support interface as well as cationic species from the support in hydrogenation. The mechanisms of how organic modifiers, with the ability to induce both 3D steric and electronic effects, on metal nanocatalysts manipulate the hydrogenation pathways are demonstrated. A brief summary is finally provided together with a perspective on the development of enzyme-like heterogeneous hydrogenation metal catalysts.

Nondipole effects on the double-slit interference in molecular ionization by xuv pulses.

The double-slit interference in single-photon ionization of the diatomic molecular ion H2 + is theoretically studied beyond the dipole approximation. Via simulating and comparing the interactions of the prealigned H2 + and the hydrogen atom with the xuv pulses propagating in different directions, we illustrate two kinds of effects that are encoded in the interference patterns of the photoelectrons from H2 +: the single-atom nondipole effect and the two-center-interference one, both associated with the finite speed of light. While the two effects could modify the maxima of the interference fringes, we show that the former one hardly affects the interference minima. Our results and analysis show that the interference minima rule out the influences of the photon-momentum transfer and, potentially, the multielectron effect, thus performing a better role in decoding the zeptosecond time delay for the pulse hitting one and the other atomic centers of the molecule.

Detecting and Characterizing the Nonadiabaticity of Laser-Induced Quantum Tunneling.

The nonadiabaticity of quantum tunneling through an evolving barrier is relevant to resolving laser-driven dynamics of atoms and molecules at an attosecond timescale. Here, we propose and demonstrate a novel scheme to detect the nonadiabatic behavior of tunnel ionization studied in an attoclock configuration, without counting on the laser intensity calibration or the modeling of the Coulomb effect. In our scheme, the degree of nonadiabaticity for tunneling scenarios in elliptically polarized laser fields can be steered continuously simply with the pulse ellipticity, while the critical instantaneous vector potentials remain identical. We observe the characteristic feature of the measured photoelectron momentum distributions, which matches the distinctive prediction of nonadiabatic theories. In particular, our experiments demonstrate that the nonadiabatic initial transverse momentum at the tunnel exit is approximately proportional to the instantaneous effective Keldysh parameters in the tunneling regime, as predicted theoretically by Ohmi, Tolstikhin, and Morishita [Phys. Rev. A 92, 043402 (2015)PLRAAN1050-294710.1103/PhysRevA.92.043402]. Our study clarifies a long-standing controversy over the validation of the adiabatic approximation and will substantially advance studies of laser-induced ultrafast dynamics in experiments.

Deformation of Atomic p_{±} Orbitals in Strong Elliptically Polarized Laser Fields: Ionization Time Drifts and Spatial Photoelectron Separation.

We theoretically investigate the deformation of atomic p_{±} orbitals driven by strong elliptically polarized (EP) laser fields and the role it plays in tunnel ionization. Our study reveals that different Stark effects induced by orthogonal components of the EP field give rise to subcycle rearrangement of the bound electron density, rendering the initial p_{+} and p_{-} orbitals deformed and polarized along distinctively tilted angles with respect to the polarization ellipse of the EP field. As a consequence, the instantaneous tunneling rates change such that for few-cycle EP laser pulses the bound electron initially counterrotating (corotating) with the electric field is most likely released before (after) the peak of the electric field. We demonstrate that with a sequential-pulse setup one can exploit this effect to spatially separate the photoelectrons detached from p_{+} and p_{-} orbitals, paving the way towards robust control of spin-resolved photoemission in laser-matter interactions.

Identifying the Tunneling Site in Strong-Field Ionization of H_{2}

The tunneling site of the electron in a molecule exposed to a strong laser field determines the initial position of the ionizing electron and, as a result, has a large impact on the subsequent ultrafast electron dynamics on the polyatomic Coulomb potential. Here, the tunneling site of the electron of H_{2}^{+} ionized by a strong circularly polarized (CP) laser pulse is studied by numerically solving the time-dependent Schrödinger equation. We show that the electron removed from the down-field site is directly driven away by the CP field and the lateral photoelectron momentum distribution (LPMD) exhibits a Gaussian-like distribution, whereas the corresponding LPMD of the electron removed from the up-field site differs from the Gaussian shape due to the Coulomb focusing and scattering by the down-field core. Our current study presents the direct evidence clarifying a long-standing controversy over the tunneling site in H_{2}^{+} and raises the important role of the tunneling site in strong-field molecular ionization.

Helicity sensitive enhancement of strong-field ionization in circularly polarized laser fields.

We investigate the strong-field ionization from p± orbitals driven by circularly polarized laser fields by solving the two-dimensional time-dependent Schrödinger equation in polar coordinates with the Lagrange mesh technique. Enhancement of ionization is found in the deep multiphoton ionization regime when the helicity of the laser field is opposite to that of the p electron, while this enhancement is suppressed when the helicities are the same. It is found that the enhancement of ionization is attributed to the multiphoton resonant excitation. The helicity sensitivity of the resonant enhancement is related to the different excitation-ionization channels in left and right circularly polarized laser fields.

Anomalous isotopic effect on electron-directed reactivity by a 3-µm midinfrared pulse.

We have theoretically studied the effect of nuclear mass on electron localization in dissociating H2⁺ and its isotopes subjected to a few-cycle 3-µm pulse. Our results reveal an anomalous isotopic effect in which the degree of electron-directed reactivity can be even higher for heavier isotopes in the intense midinfrared field. We show, for the first time, the pronounced electron localization can be established through the interferences among the multi-photon coupling channels. Due to the relative enhancement of higher-order coupling channels with growing mass, the interference maxima at different kinetic energy of the spectra gradually become in phase, ultimately resulting in the larger dissociation asymmetries of heavier isotopes.

Synthesizing MOF-derived NiNC catalyst via surfactant modified strategy for efficient electrocatalytic CO2 to CO.

Recently, Ni, N-doped carbon (NiNC) electrocatalysts synthesized using metal-organic frameworks (MOFs) as templates have demonstrated attractive catalytic performances in the CO2 reduction reaction (CO2RR). However, most of the reported preparations of MOFs-based precursors are carried out in organic solvents, and the resulting NiNC materials have relatively low metal loadings and mainly exhibit microporous structures, which is unfavorable for the mass transport. Herein, Ni, N-doped meso-microporous carbon electrocatalysts with a range of Ni loadings (M-NiNCx/CNTs) were prepared by the pyrolysis of MOFs-based precursors synthesized in aqueous solution using the surfactant cetyltrimethylammonium bromide (CTAB) as a modifier to promote the adsorption of Ni2+ ions and the formation of mesopores. Owing to the unique morphology, porous structure and high contents of Ni-Nx sites and pyrrolic-N, the optimal catalyst (M-NiNC2/CNTs) shows superior electrocatalytic activity for the CO2RR with a maximum CO Faradaic efficiency (FECO) of 98 % at -0.7 V vs. reversible hydrogen electrode (RHE), and the FECO can reach over 80 % in a wide potential range of -0.5 to -1.0 V vs. RHE. This work develops a facile and environmentally friendly strategy to obtain high-performance and low-cost transition metal-nitrogen-doped porous carbon electrocatalysts for the CO2RR.

An efficient approach to utilize copper smelting slag: Separating nonferrous metals and reducing iron oxide at high temperature.

Over 50 million tons of copper slag are produced worldwide annually. Stacking is currently the primary method used to treat copper slag, resulting in resource wastage and environmental issues. Using slag as a raw material in the steel industry is areasonablesolution. However, the presence of nonferrous metals degrades steel performance and corrodes smelting equipment, and the remaining slag poses environmental and sustainable challenges. Thus, this study focused on removal of nonferrous metals from copper slag and subsequent reduction of iron oxide. The experimental results showed that increasing the percentage of the chlorinating agent (calcium chloride), temperature, and duration, the removal percentage of copper initially increased linearly, then plateaued. The acidity coefficient had a marginal effect on copper removal percentage. The optimum chlorination roasting conditions were 13 % calcium chloride, 1373 K, and 0.5 h, resulting in removal percentages of 90.3 %, 81.9 %, and 82.7 % for copper, zinc, and lead, respectively. The appropriate oxygen partial pressure for chlorination roasting was 10-5-0.7 atm. The reduction percentage of iron oxide was approximately 82 % under roasting conditions of 1.5 h at 1373 K. Based on these results, a novel scheme for copper slag utilization is proposed that involves chlorination roasting for nonferrous metal removal, reduction roasting for iron oxide reduction, and melting to obtain molten iron and separate molten slag. The iron and slag are used for steel manufacturing and rock wool preparation, respectively. This scheme provides apromisingway to efficiently use copper slag, which will reduce primary resource consumption and pollution.

Upgrading heterogeneous Ni catalysts with thiol modification.

Precious metal catalysts are the cornerstone of many industrial processes. Replacing precious metal catalysts with earth-abundant metals is one of key challenges for the green and sustainable development of chemical industry. We report in this work a surprisingly effective strategy toward the development of cost-effective, air-stable, and efficient Ni catalysts by simple surface modification with thiols. The as-prepared catalysts exhibit unprecedentedly high activity and selectivity in the reductive amination of aldehydes/ketones. The thiol modification can not only prevent the deep oxidation of Ni surface to endow the catalyst with long shelf life in air but can also allow the reductive amination to proceed via a non-contact mechanism to selectively produce primary amines. The catalytic performance is far superior to that of precious and non-precious metal catalysts reported in the literature. The wide application scope and high catalytic performance of the developed Ni catalysts make them highly promising for the low-cost, green production of high-value amines in chemical industry.

Imprints of the molecular-orbital geometry on the high-harmonic ellipticity.

The influence of the orbital symmetry on the ellipticity of the high-order harmonics is investigated. It is found that the ellipticity maps have distinct shapes for the molecular orbitals with different symmetry. Our analysis shows that the feature of the harmonic ellipticity map is essentially determined by the nodal structure of the nonsymmetric orbital. The results indicate that the molecular-orbital geometry is imprinted on the ellipticity of the high-order harmonics, which invites the use of ellipticity measurements as a probe of the orbital structure for polar molecules.

Dramatic cutoff extension and broadband supercontinuum generation in multi-cycle two color pulses.

We propose a method to markedly increase the electron-recollision kinetic energy, using the combination of a 0.8 µm/13 fs driving pulse with a much weaker multi-cycle mid-infrared pulse at 10.4 µm. The results show that the synthesized field effectively lengthens the accelerated distance of electron wave packet and the harmonic cutoff is significantly extended to IP+26UP, which is covered with the water-window spectral region. In addition, only one single quantum path contributes to harmonics, and those higher than IP+15UP become continuous. This supercontinuum can support the generation of an isolated sub-100 as pulse with tunable central wavelength and also the pulse with the duration below one atomic unit of time (24 as). Moreover, our scheme can further extend to more longer driving pulses, which provides a dramatic approach for cutoff extension and broadband supercontinuum generation with multi-cycle pump pulses.

Atomic overlayer of permeable microporous cuprous oxide on palladium promotes hydrogenation catalysis.

The interfacial sites of metal-support interface have been considered to be limited to the atomic region of metal/support perimeter, despite their high importance in catalysis. By using single-crystal surface and nanocrystal as model catalysts, we now demonstrate that the overgrowth of atomic-thick Cu2O on metal readily creates a two-dimensional (2D) microporous interface with Pd to enhance the hydrogenation catalysis. With the hydrogenation confined within the 2D Cu2O/Pd interface, the catalyst exhibits outstanding activity and selectivity in the semi-hydrogenation of alkynes. Alloying Cu(0) with Pd under the overlayer is the major contributor to the enhanced activity due to the electronic modulation to weaken the H adsorption. Moreover, the boundary or defective sites on the Cu2O overlayer can be passivated by terminal alkynes, reinforcing the chemical stability of Cu2O and thus the catalytic stability toward hydrogenation. The deep understanding allows us to extend the interfacial sites far beyond the metal/support perimeter and provide new vectors for catalyst optimization through 2D interface interaction.

Phase dependence of electron localization in HeH²âº dissociation with an intense few-cycle laser pulse.

The electron localization in the dissociation of the asymmetric charged molecular ion HeH²âº exposed to an intense few-cycle laser pulse is studied by solving numerically the 3D time-dependent Schrödinger equation. By varying the carrier-envelope phase (CEP) and the intensity of the pulse, the upward shift of the localization probability and the suppression of the dissociation channel He²âº+H are observed. Our analysis shows that the phenomenon is attributed to the asymmetric structure of the molecule as well as the recollision-assistant field-induced ionization of the electron wave packets localized on H⁺ in the trailing of the pulse.

Wavelength dependence of electron localization in the laser-driven dissociation of H2(+).

We theoretically investigate the laser wavelength dependence of asymmetric dissociation of H2(+). It is found that the electron localization in molecular dissociation is significantly manipulated by varying the wavelength of the driving field. Through creating a strong nuclear vibration in the laser-molecular interaction, our simulations demonstrate that the few-cycle mid-infrared pulse can effectively localize the electron at one of the dissociating nuclei with weak ionization. Moreover, we show that the observed phase-shift of the dissociation asymmetry is attributed to the different population transfers by the remaining fields after the internuclear distances reach the one-photon coupling point.