Strain-Induced Self-Assembly at Interface of Two-Dimensional Heterostructures Boosts CO2 Reduction to Methanol by H2O.

CO2 conversion with pure H2O into CH3OH and O2 driven by solar energy can supply fuels and life-essential substances for extraterrestrial exploration. However, the effective production of CH3OH is significantly challenging. Here we report an organozinc complex/MoS2 heterostructure linked by well-defined zinc-sulfur covalent bonds derived by the structural deformation and intensive coupling of dx2 - y2(Zn)-p(S) orbitals at the interface, resulting in distinctive charge transfer behaviors and excellent redox capabilities as revealed by experimental characterizations and first-principle calculations. The synthesis strategy is further generalized to more organometallic compounds, achieving various heterostructures for CO2 photoreduction. The optimal catalyst delivers a promising CH3OH yield of 2.57 mmol gcat-1 h-1 and selectivity of more than 99.5%. The reverse water gas shift mechanism is identified for methanol formation. Meanwhile, energy-unfavorable adsorption of methanol on MoS2, where the photogenerated holes accumulate, ensures the selective oxidation of water over methanol.

Revealing the anti-sintering phenomenon on silica-supported nickel catalysts during CO2 hydrogenation.

The CO2 catalytic hydrogenation represents a promising approach for gas-phase CO2 utilization in a direct manner. Due to its excellent hydrogenation ability, nickel has been widely studied and has shown good activities in CO2 hydrogenation reactions, in addition to its high availability and low price. However, Ni-based catalysts are prone to sintering under elevated temperatures, leading to unstable catalytic performance. In the present study, various characterization techniques were employed to study the structural evolution of Ni/SiO2 during CO2 hydrogenation. An anti-sintering phenomenon is observed for both 9% Ni/SiO2 and 1% Ni/SiO2 during CO2 hydrogenation at 400°C. Results revealed that Ni species were re-dispersed into smaller-sized nanoparticles and formed Ni0 active species. While interestingly, this anti-sintering phenomenon leads to distinct outcomes for two catalysts, with a gradual increase in both reactivity and CH4 selectivity for 9% Ni/SiO2 presumably due to the formation of abundant surface Ni° from redispersion, while an apparent decreasing trend of CH4 selectivity for 1% Ni/SiO2 sample, presumably due to the formation of ultra-small nanoparticles that diffuse and partially filled the mesoporous pores of the silica support over time. Finally, the redispersion phenomenon was found relevant to the H2 gas in the reaction environment and enhanced as the H2 concentration increased. This finding is believed to provide in-depth insights into the structural evolution of Ni-based catalysts and product selectivity control in CO2 hydrogenation reactions.

Revealing the Nature of Active Oxygen Species and Reaction Mechanism of Ethylene Epoxidation by Supported Ag/α-Al2O3 Catalysts.

The oxygen species on Ag catalysts and reaction mechanisms for ethylene epoxidation and ethylene combustion continue to be debated in the literature despite decades of investigation. Fundamental details of ethylene oxidation by supported Ag/α-Al2O3 catalysts were revealed with the application of high-angle annular dark-field-scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS), in situ techniques (Raman, UV-vis, X-ray diffraction (XRD), HS-LEIS), chemical probes (C2H4-TPSR and C2H4 + O2-TPSR), and steady-state ethylene oxidation and SSITKA (16O2 → 18O2 switch) studies. The Ag nanoparticles are found to carry a considerable amount of oxygen after the reaction. Density functional theory (DFT) calculations indicate the oxidative reconstructed p(4 × 4)-O-Ag(111) surface is stable relative to metallic Ag(111) under the relevant reaction environment. Multiple configurations of reactive oxygen species are present, and their relevant concentrations depend on treatment conditions. Selective ethylene oxidation to EO proceeds with surface Ag4-O2* species (dioxygen species occupying an oxygen site on a p(4 × 4)-O-Ag(111) surface) only present after strong oxidation of Ag. These experimental findings are strongly supported by the associated DFT calculations. Ethylene epoxidation proceeds via a Langmuir-Hinshelwood mechanism, and ethylene combustion proceeds via combined Langmuir-Hinshelwood (predominant) and Mars-van Krevelen (minor) mechanisms.

Dual Atom Catalysts for Energy and Environmental Applications.

The pursuit of high metal utilization in heterogeneous catalysis has triggered the burgeoning interest of various atomically dispersed catalysts. Our aim in this review is to assess key recent findings in the synthesis, characterization, structure-property relationship and computational studies of dual-atom catalysts (DACs), which cover the full spectrum of applications in thermocatalysis, electrocatalysis and photocatalysis. In particular, combination of qualitative and quantitative characterization with cooperation with DFT insights, synergies and superiorities of DACs compare to counterparts, high-throughput catalyst exploration and screening with machine-learning algorithms are highlighted. Undoubtably, it would be wise to expect more fascinating developments in the field of DACs as tunable catalysts.

Atomic Insights into the Cu Species Supported on Zeolite for Direct Oxidation of Methane to Methanol via Low-Damage HAADF-STEM.

Precise determination of the structure-property relationship of zeolite-based metal catalysts is critical for the development toward practical applications. However, the scarcity of real-space imaging of zeolite-based low-atomic-number (LAN) metal materials due to the electron-beam sensitivity of zeolites has led to continuous debates regarding the exact LAN metal configurations. Here, a low-damage high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging technique is employed for direct visualization and determination of LAN metal (Cu) species in ZSM-5 zeolite frameworks. The structures of the Cu species are revealed based on the microscopy evidence and also proved by the complementary spectroscopy results. The correlation between the characteristic Cu size in Cu/ZSM-5 catalysts and their direct oxidation of methane to methanol reaction properties is unveiled. As a result, the mono-Cu species stably anchored by Al pairs inside the zeolite channels are identified as the key structure for higher C1 oxygenates yield and methanol selectivity for direct oxidation of methane. Meanwhile, the local topological flexibility of the rigid zeolite frameworks induced by the Cu agglomeration in the channels is also revealed. This work exemplifies the combination of microscopy imaging and spectroscopy characterization serves as a complete arsenal for revealing structure-property relationships of the supported metal-zeolite catalysts.

Engineering Heterogeneous Catalysis with Strong Metal-Support Interactions: Characterization, Theory and Manipulation.

Strong metal-support interactions (SMSI) represent a classic yet fast-growing area in catalysis research. The SMSI phenomenon results in the encapsulation and stabilization of metal nanoparticles (NPs) with the support material that significantly impacts the catalytic performance through regulation of the interfacial interactions. Engineering SMSI provides a promising approach to steer catalytic performance in various chemical processes, which serves as an effective tool to tackle energy and environmental challenges. Our Minireview covers characterization, theory, catalytic activity, dependence on the catalytic structure and inducing environment of SMSI phenomena. By providing an overview and outlook on the cutting-edge techniques in this multidisciplinary research field, we not only want to provide insights into the further exploitation of SMSI in catalysis, but we also hope to inspire rational designs and characterization in the broad field of material science and physical chemistry.

Rational Design of Precious-Metal Single-Atom Catalysts for Methane Combustion.

Supported precious-metal single-atom catalysts (PM SACs) have emerged as a new frontier of high-performance catalytic material with 100% atom utilization efficiency. However, the rational design of such material with guidance from fundamental understandings of the structure-activity relationship remains challenging. Here, we report the synthesis, characterizations, and mechanistic investigation of various PM SACs supported on nanoceria for CH4 combustion. Using density functional theory, two descriptors as the d-band center of PMs and oxygen vacancy formation energy are established, which jointly govern the reactivity for CH4 combustion. These descriptors are thus applied to predict a dual SAC consisting of proximate Pd and Rh sites, demonstrating a remarkable improvement versus Pd or Rh catalyst, respectively. Our results reveal the general strategy of integrating experimental and computational efforts for investigation of various PM SACs in methane combustion, thus paving the way for the next generation of advanced catalytic materials.

Oxo dicopper anchored on carbon nitride for selective oxidation of methane.

Selective conversion of methane (CH4) into value-added chemicals represents a grand challenge for the efficient utilization of rising hydrocarbon sources. We report here dimeric copper centers supported on graphitic carbon nitride (denoted as Cu2@C3N4) as advanced catalysts for CH4 partial oxidation. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photocatalytic reaction conditions, with hydrogen peroxide (H2O2) and oxygen (O2) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH4 with O2 achieves >10% conversion, and >98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol g Cu-1h-1. Mechanistic studies reveal that the high reactivity of Cu2@C3N4 can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C3N4 substrate, which do not only facilitate the heterolytic scission of C-H bond, but also promotes H2O2 and O2 activation in thermo- and photocatalysis, respectively.

Carbothermal shock synthesis of high-entropy-alloy nanoparticles.

The controllable incorporation of multiple immiscible elements into a single nanoparticle merits untold scientific and technological potential, yet remains a challenge using conventional synthetic techniques. We present a general route for alloying up to eight dissimilar elements into single-phase solid-solution nanoparticles, referred to as high-entropy-alloy nanoparticles (HEA-NPs), by thermally shocking precursor metal salt mixtures loaded onto carbon supports [temperature ~2000 kelvin (K), 55-millisecond duration, rate of ~105 K per second]. We synthesized a wide range of multicomponent nanoparticles with a desired chemistry (composition), size, and phase (solid solution, phase-separated) by controlling the carbothermal shock (CTS) parameters (substrate, temperature, shock duration, and heating/cooling rate). To prove utility, we synthesized quinary HEA-NPs as ammonia oxidation catalysts with ~100% conversion and >99% nitrogen oxide selectivity over prolonged operations.

Recovery of ammonium from aqueous solutions using ZSM-5.

The demand of reactive nitrogen (N), such as ammonium (NH4+) and nitrate (NO3-), continues to increase for fertilizer applications as the population grows, but the Haber Bosch (HB) process currently employed for industrial N fixation is challenged by low efficiency and high energy consumption. Here we report on the investigation of ZSM-5 as a superior sorbent for the recovery of ammonium from aqueous solutions. Fast capture and release of ammonium (NH4+) have been achieved with >90% overall efficiency of recovery using synthetic solutions of NH4Cl and NaCl, respectively. The ZSM-5 sorbent has also been found to be recyclable and sustain high recovery efficiencies after multiple capture-release cycles. The capture of N has been further studied systematically in dependence of the dose of sorbent and reaction temperature, based on which the mechanism, thermodynamics and kinetics of ion exchange are discussed. Compared to other ion-exchange materials, the ZSM-5 sorbent exhibits superior selectivity for capturing ammonium in the presence of competing cations (NH4+ ¼ Ca2+ > Mg2+ > K+ > Na+) and demonstrates high efficiency of ammonium recovery from real wastewater streams.

Metal-ion induced ferromagnetic polarization in a mixed-spin system.

Three new 3d-3d heterometallic complexes, [CuMII(hmb)6(OH)]ClO4·H2O (M = Zn (1), Ni (2) and Co (3)) (Hhmb = 2-hydroxy-3-methoxy-benzaldehyde), have been synthesized. Structural analysis reveals that the three complexes are isostructural. Three CuII ions are in a perfect trigonal geometrical frustration and form a tetrahedral frustrated system with the introduced fourth ion ZnII/NiII/CoII. Magnetic studies indicate the antiferromagnetic coupling between metal ions in all 1, 2 and 3. Due to the geometrical frustration, spin-frustrated magnetism is also a typical feature of 1-3 and the magnetostructural correlation is far more complicated than their structures. The detailed magnetic investigation found that the ground state spin of 2 cannot be simply determined by the antiferromagnetically linear arrangement of spins, because the strong antiferromagnetic coupling between CuII and NiII ions quenches the spin frustration and ferromagnetically polarizes the spins of the [Cu] unit. Contrarily, the antiferromagnetic coupling between CuII and CoII ions is not strong enough in 3, so there is no similar behaviour compared to 2.

Water induced spin-crossover behaviour and magneto-structural correlation in octacyanotungstate(iv)-based iron(ii) complexes.

A series of octacyanotungstate(iv)-based iron(ii) complexes with the general formula Fe(L)8[WIV(CN)8]·nH2O [L = (3-pyridyl)methanol (1, 2), 3-methylpyridine (3), (4-pyridyl)methanol (4), and 4-methylpyridine (5); n = 4 for 1, and n = 0 for 2-5] have been synthesized and characterized. Single crystal X-ray diffraction analysis reveals that the FeII ions lie in the centre of the compressed [FeN6] octahedron in all complexes. FeII and WIV ions are alternately bridged by cyano groups forming a three-dimensional (3D) bimetallic framework. Magnetic investigation shows that 1 displays a gradual spin-crossover (SCO) phenomenon with a spin transition temperature (T1/2) of 200 K, and such SCO behaviour is obviously correlated with the lattice water content of the sample. The magnetic measurements of dehydrated samples show that the fractional conversion from the high-spin (HS) to the low-spin (LS) state is reduced with the increasing of dehydration temperature. Complexes 2-5 are in the HS state and do not exhibit SCO properties in the range of 2-300 K. Comparing the octahedral geometry of [FeN6] of five complexes, quantified by using continuous shape measures, the distortion of complex 1 is the highest as a result of the intermolecular hydrogen bonds, which shorten the Fe-N bond distances and thus increase the ligand field strength at the FeII sites. The analysis of correlations between the structural characteristics and magnetic behaviour of 1-5 suggests that the SCO is mainly tuned by the octahedral distortion of the [FeN6] core caused by intermolecular hydrogen bonds. There is an exact correlation between SCO behaviour and the amount of lattice water molecules existing in the crystal. The spin crossover behaviour of these complexes has been computationally studied using the DFT method. The results of the calculations are consistent with the experiments, which prove that complex 1 with severe distortion of the coordination sphere of FeII is prone to exhibit SCO in theory.