Engineering highly selective CO2 electroreduction in Cu-based perovskites through A-site cation manipulation.

Perovskites exhibit considerable potential as catalysts for various applications, yet their performance modulation in the carbon dioxide reduction reaction (CO2RR) remains underexplored. In this study, we report a strategy to enhance the electrocatalytic carbon dioxide (CO2) reduction activity via Ce-doped La2CuO4 (LCCO) and Sr-doped La2CuO4 (LSCO) perovskite oxides. Specifically, compared to pure phase La2CuO4 (LCO), the Faraday efficiency (FE) for CH4 of LCCO at -1.4 V vs. RHE (reversible hydrogen electrode) is improved from 38.9% to 59.4%, and the FECO2RR of LSCO increased from 68.8% to 85.4%. In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy spectra results indicate that the doping of A-site ions promotes the formation of *CHO and *HCOO, which are key intermediates in the production of CH4, compared to the pristine La2CuO4. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and double-layer capacitance (Cdl) outcomes reveal that heteroatom-doped perovskites exhibit more oxygen vacancies and higher electrochemical active surface areas, leading to a significant improvement in the CO2RR performance of the catalysts. This study systematically investigates the effect of A-site ion doping on the catalytic activity center Cu and proposes a strategy to improve the catalytic performance of perovskite oxides.

Surface-sealing encapsulation of phosphotungstic acid in microporous UiO-66 as a bifunctional catalyst for transfer hydrogenation of levulinic acid to γ-valerolactone.

The efficient production of γ-valerolactone (GVL) from renewable lignocellulose that is synthesized in plants by photosynthesis to replace the declining fossil resources conforms to the principles of circular economy. Compared to direct hydrogenation by H2 molecules, catalytic transfer hydrogenation (CTH) of levulinic acid (LA) and/or its esters to GVL with organic alcohols as a hydrogen source is a much milder route. The synergistic catalysis between Lewis and Brønsted acids is indispensable in the CTH process. Considering that unsaturated coordinated Zr species could act as Lewis acid sites and phosphotungstic acid (PTA) could dissociate protons as Brønsted acid sites, UiO-66 (Zr) was thus "acidified" by encapsulating PTA in its channels to tune the ratio of Brønsted to Lewis acid sites as a bifunctional catalyst so as to better understand the catalytic structure-performance relationship in the CTH process. To address the dilemma of encapsulated PTA that is prone to leach, a rapid surface sealing strategy was adopted to establish a polyimide (PI) coating over the surface of UiO-66 introducing a space confinement effect via an anhydride-amine coupling reaction. The as-synthesized PTA/UiO-66@PI catalyst exhibited 100% of LA conversion, a 93.2% of GVL yield and high recyclability for at least five consecutive cycles. Moreover, a reaction pathway followed by esterification, hydrogenation and dealcoholization as well as a catalytic hydrogenation mechanism based on intermolecular hydride ß-H transfer were proposed. Current work not only provides a high-performance and high-stability catalytic system to selectively produce GVL from LA or its esters, but also sheds light on the catalytic mechanism of the CTH process at the molecular level.

Secondary Phosphine Oxide Functionalized Gold Clusters and Their Application in Photoelectrocatalytic Hydrogenation Reactions.

Ligands in ligand-protected metal clusters play a crucial role, not only because of their interaction with the metal core, but also because of the functionality they provide to the cluster. Here, we report the utilization of secondary phosphine oxide (SPO), as a new family of functional ligands, for the preparation of an undecagold cluster Au11-SPO. Different from the commonly used phosphine ligand (i.e., triphenylphosphine, TPP), the SPOs in Au11-SPO work as electron-withdrawing anionic ligands. While coordinating to gold via the phosphorus atom, the SPO ligand keeps its O atom available to act as a nucleophile. Upon photoexcitation, the clusters are found to inject holes into p-type semiconductors (here, bismuth oxide is used as a model), sensitizing the p-type semiconductor in a different way compared to the photosensitization of a n-type semiconductor. Furthermore, the Au11-SPO/Bi2O3 photocathode exhibits a much higher activity toward the hydrogenation of benzaldehyde than a TPP-protected Au11-sensitized Bi2O3 photocathode. Control experiments and density functional theory studies point to the crucial role of the cooperation between gold and the SPO ligands on the selectivity toward the hydrogenation of the C═O group in benzaldehyde.

Significantly Enhanced Overall Water Splitting Performance by Partial Oxidation of Ir through Au Modification in Core-Shell Alloy Structure.

Developing efficient bifunctional electrocatalysts for overall water splitting in acidic conditions is the essential step for proton exchange membrane water electrolyzers (PEMWEs). We first report the synthesis of core-shell structure nanoparticles (NPs) with an Au core and an AuIr2 alloy shell (Au@AuIr2). Au@AuIr2 displayed 4.6 (5.6) times higher intrinsic (mass) activity toward the oxygen evolution reaction (OER) than a commercial Ir catalyst. Furthermore, it showed hydrogen evolution reaction (HER) catalytic properties comparable to those of commercial Pt/C. Significantly, when Au@AuIr2 was used as both the anode and cathode catalyst, the overall water splitting cell achieved 10 mA/cm2 with a low cell voltage of 1.55 V and maintained this activity for more than 40 h, which greatly outperformed the commercial couples (Ir/C||Pt/C, 1.63 V, activity decreased within minutes) and is among the most efficient bifunctional catalysts reported. Theoretical calculations coupled with X-ray-based structural analyses suggest that partially oxidized surfaces originating from the electronic interaction between Au and Ir provide a balance for different intermediates binding and realize significantly enhanced OER performance.

Highly Efficient and Selective N-Formylation of Amines with CO2 and H2 Catalyzed by Porous Organometallic Polymers.

The valorization of carbon dioxide (CO2 ) to fine chemicals is one of the most promising approaches for CO2 capture and utilization. Herein we demonstrated a series of porous organometallic polymers could be employed as highly efficient and recyclable catalysts for this purpose. Synergetic effects of specific surface area, iridium content, and CO2 adsorption capability are crucial to achieve excellent selectivity and yields towards N-formylation of diverse amines with CO2 and H2 under mild reaction conditions even at 20 ppm catalyst loading. Density functional theory calculations revealed not only a redox-neutral catalytic pathway but also a new plausible mechanism with the incorporation of the key intermediate formic acid via a proton-relay process. Remarkably, a record turnover number (TON=1.58×106 ) was achieved in the synthesis of N,N-dimethylformamide (DMF), and the solid catalysts can be reused up to 12 runs, highlighting their practical potential in industry.

Theoretical Insight into the Structural Nonplanarity in Aromatic Fused-Ring Metallabenzenes.

Metalla-aromatics have attracted considerable attention due to their fascinating structural and reactive properties as well as their potential as prospective functional materials. Metallabenzenes and their fused-ring counterparts are significant members of metalla-aromatics, while their crystal structures often display seemly counterintuitive nonplanar geometry. The geometric bending of metallabenzenes has been attributed to the unfavorable antibonding interactions in the σ-space orbitals rather than the general opinion regarding the π-space orbitals of an aromatic compound. However, the origin of the geometric bending in fused-ring metallabenzenes remains elusive. In this work, we elucidated that such a "σ-control mechanism" still holds for fused-ring metallabenzenes by performing systematic calculations for a plethora of metallabenzenes and fused-ring metallabenzenes. Furthermore, we found that a more bent geometry can be achieved for fused-ring metallabenzenes than their corresponding metallabenzenes by fusing the aromatic rings at the ortho-position of a metal center to induce extra repulsion. The more significant bending in fused-ring metallabenzenes also favors the aromaticity enhancement. These findings not only provide mechanistic insight into the unexpected geometric distortion in both metallabenzenes and fused-ring metallabenzenes but also pave the way to design and develop bent metalla-aromatics with enhanced metalla-aromaticity, which hold great potential as aromatic functional materials.

Iridium-Catalyzed Selective Cross-Coupling of Ethylene Glycol and Methanol to Lactic Acid.

An atom-economic approach that has an unprecedented high selectivity for the synthesis of lactic acid (LA) based on a catalytic dehydrogenative cross-coupling by using inexpensive bulk ethylene glycol and methanol is described. This method relies on the synthesis and utilization of a novel iridium catalyst bearing three N-heterocyclic carbenes derived from 1,3-dimethylbenzimidazolium salts, and exhibits outstanding activity in the production of LA [turnover frequency (TOF) up to 3660 h-1 ] owing to an elegant metal-ligand cooperation.

Selective Catalytic Dehydrogenative Oxidation of Bio-Polyols to Lactic Acid.

The global demand for lactic acid (LA) is increasing due to its successful application as monomer for the manufacture of bioplastics. Although N-heterocyclic carbene (NHC) iridium complexes are promising molecular catalysts for LA synthesis, their instabilities have hindered their utilization especially in commercial applications. Here, we report that a porous self-supported NHC-iridium coordination polymer can efficiently prevent the clusterization of corresponding NHC-Ir molecules and can function as a solid molecular recyclable catalyst for dehydrogenation of bio-polyols to form LA with excellent activity (97 %) and selectivity (>99 %). A turnover number of up to 5700 could be achieved in a single batch, due to the synergistic participation of the Ba2+ and hydroxide ions, as well as the blockage of unwanted pathways by adding methanol. Our findings demonstrate a potential route for the industrial production of LA from cheap and abundant bio-polyols, including sorbitol.

Enhanced QM/MM sampling for free energy calculation of chemical reactions: A case study of double proton transfer.

Free energy calculations for chemical reactions with a steep energy barrier require well defined reaction coordinates (RCs). However, when multiple parallel channels exist along selected RC, the application of conventional enhanced samplings is difficult to generate correct sampling within limited simulation time and thus cannot give correct prediction about the favorable pathways, the relative stability of multiple products or intermediates. Here, we implement the selective integrated tempering sampling (SITS) method with quantum mechanical and molecular mechanical (QM/MM) potential to investigate the chemical reactions in solution. The combined SITS-QM/MM scheme is used to identify possible reaction paths, intermediate and product states, and the free energy profiles for the different reaction paths. Two double proton transfer reactions were studied to validate the implemented method and simulation protocol, from which the independent and correlated proton transfer processes are identified in two representative systems, respectively. This protocol can be generalized to various kinds of chemical reactions for both academic studies and industry applications, such as in exploration and optimization of potential reactions in DNA encoded compound library and halogen or deuterium substitution of the hit discovery and lead optimization stages of drug design via providing a better understanding of the reaction mechanism along the designed chemical reaction pathways.

Ag10 Ti28 -Oxo Cluster Containing Single-Atom Silver Sites: Atomic Structure and Synergistic Electronic Properties.

Supported single-atom catalysts have been emerging as promising materials in a variety of energy catalysis applications. However, studying the role of metal-support interactions at the molecular level remains a major challenge, primarily due to the lack of precise atomic structures. In this work, by replacing the frequently used TiO2 support with its molecular analogue, titanium-oxo cluster (TOC), we successfully produced a new kind of Ti-O material doped with single silver sites. The as-obtained Ag10 Ti28 cluster, containing four exposed and six embedded Ag sites, is the largest noble-metal-doped Ti-O cluster reported to date. Density functional theory (DFT) calculations show that the Ag10 Ti28 core exhibits properties distinct from those of metallic Ag-based materials. This Ti-O material doped with single Ag sites presents a high ϵd and moderate CO binding capacity comparable to that of metallic Cu-based catalysts, suggesting that it might display different catalytic performance from the common Ag-based catalysts, for example, for CO2 reduction. These results prove that the synergism of active surface metal atoms and the Ti-O cluster support result in unique physical properties, which might open a new direction for single-atom-included catalysts.

Rational designing of 8-hydroxyquinolin-imidazolinone-based fluorescent protein mutants with dramatically red shifted emission: A computational study.

Engineering fluorescent proteins to be the customized in vivo labels for monitoring cellular dynamic events is critical in biochemical and biomedical studies. The design and development of novel red fluorescent proteins is one of the most important fronts in this field due to their potential of imaging the entire organism. A recent fluorescent protein mutant eqFP650-67-HqAla with the 8-hydroxyquinolin-imidazolinone (HQI) chromophore has the plausible bathochromic shift of ~30 nm in its emission spectrum wavelength comparing to the parent fluorescent protein eqFP650. However, molecular mechanism of this significant shift remains somewhat obscure. In this study, we carefully benchmarked our computational methods and performed extensive calculations to investigate various structural components' effect on the chromophore's emission energy and decipher the molecular origin of the spectral shift. The influences of conjugation size, substituent group, substituent site as well as the number of substituents have been examined by elaborately designed chromophore derivatives. Accordingly, we proposed several chromophore mutants with dramatic bathochromic shift of up to ~60 nm in their emission spectra. We further evaluated their structural stability in the protein using molecular dynamics simulations. Present theoretical study connects the structural feature of chromophore derivatives in red fluorescent proteins with their splendid performances in shifting the emission frequency and offer the molecular insight. The computational protocol and successive examination procedure to extract the structural effect utilized herein can also be widely applied to other fluorescent proteins in general. © 2018 Wiley Periodicals, Inc.

Understanding the Nonplanarity in Aromatic Metallabenzenes: A σ-Control Mechanism.

Metallabenzenes, the organometallic counterparts of benzene with one of the C atoms being replaced by a metal atom, expand the family of aromatics and further create prospective candidates for novel applications as functional materials. One intriguing feature of these complexes is that their MC5 rings do not always constrict into a planar configuration as in the C6 ring of benzene. Such a deviation has often been attributed to the unfavorable antibonding interactions between an occupied metal d orbital and the π orbitals of the C5 moiety. We herein scrutinize the frontier orbital interactions in both σ and π spaces in a plethora of metallabenzene complexes using extensive density functional theory calculations. Unexpectedly, the nonplanarity in metallabenzenes is found to be hardly related to the π orbitals. It is the antibonding interaction between an occupied metal d orbital and the σ orbitals of the C5 moiety that dominates the observed distortion. Such a σ-control mechanism not only provides an explanation for the commonly observed nonplanarity in metallabenzenes but also points out a novel direction toward the rational design of functional materials with enhanced metalla-aromaticity.

Rational design of model Pd(ii)-catalysts for C-H activation involving ligands with charge-shift bonding characteristics.

In this work, five new palladium(ii) complexes have been designed as the model catalysts for methane to methyl trifluoroacetate conversion. All these compounds are analogues of the well-established (bis-NHC)PdBr2 complex (NHC, N-heterocyclic carbenes), derived by complexing the palladium(ii) metal ion with the derivatives of bis-2-borabicyclo[1.1.0]but-1(3)-ene (bis-2BB) ligands using the sp2 carbons. Our density functional theory calculation results suggest that the (bis-2BB)PdBr2 catalysts outperform the popular (bis-NHC)PdBr2 complex in the desired catalytic process, and further reveal that the charge-shift bonding in the bis-2BB ligands contributes to the improved catalytic performance. These findings may spark new ideas for experimental design of more efficient organometallic catalysts for C-H bond activation and functionalization.

Efficient free energy calculations by combining two complementary tempering sampling methods.

Although energy barriers can be efficiently crossed in the reaction coordinate (RC) guided sampling, this type of method suffers from identification of the correct RCs or requirements of high dimensionality of the defined RCs for a given system. If only the approximate RCs with significant barriers are used in the simulations, hidden energy barriers with small to medium height would exist in other degrees of freedom (DOFs) relevant to the target process and consequently cause the problem of insufficient sampling. To address the sampling in this so-called hidden barrier situation, here we propose an effective approach to combine temperature accelerated molecular dynamics (TAMD), an efficient RC-guided sampling method, with the integrated tempering sampling (ITS), a generalized ensemble sampling method. In this combined ITS-TAMD method, the sampling along the major RCs with high energy barriers is guided by TAMD and the sampling of the rest of the DOFs with lower but not negligible barriers is enhanced by ITS. The performance of ITS-TAMD to three systems in the processes with hidden barriers has been examined. In comparison to the standalone TAMD or ITS approach, the present hybrid method shows three main improvements. (1) Sampling efficiency can be improved at least five times even if in the presence of hidden energy barriers. (2) The canonical distribution can be more accurately recovered, from which the thermodynamic properties along other collective variables can be computed correctly. (3) The robustness of the selection of major RCs suggests that the dimensionality of necessary RCs can be reduced. Our work shows more potential applications of the ITS-TAMD method as the efficient and powerful tool for the investigation of a broad range of interesting cases.

Theoretical study on the mechanism of aqueous synthesis of formic acid catalyzed by [Ru3+]-EDTA complex.

Because formic acid can be effectively decomposed by catalysis into very pure hydrogen gas, the synthesis of formic acid, especially using CO and H2O as an intermediate of the water gas shift reaction (WGSR), bears important application significance in industrial hydrogen gas production. Here we report a theoretical study on the mechanism of efficient preparation of formic acid using CO and H2O catalyzed by a water-soluble [Ru(3+)]-EDTA complex. To determine the feasibility of using the [Ru(3+)]-EDTA catalyst to produce CO-free hydrogen gas in WGSR, two probable reaction paths have been examined: one synthesizes formic acid, while the other converts the reactants directly into CO2 and H2, the final products of WGSR. Our calculation results provide a detailed mechanistic rationalization for the experimentally observed selective synthesis of HCOOH by the [Ru(3+)]-EDTA catalyst. The results support the applicability of using the [Ru(3+)]-EDTA catalyst to efficiently synthesize formic acid for hydrogen production. Careful analyses of the electronic structure and interactions of different reaction complexes suggest that the selectivity of the reaction processes is achieved through the proper charge/valence state of the metal center of the [Ru(3+)]-EDTA complex. With the catalytic roles of the ruthenium center and the EDTA ligand being carefully understood, the detailed mechanistic information obtained in this study will help to design more efficient catalysts for the preparation of formic acid and further to produce CO-free H2 at ambient temperature.

Metal-organic framework threaded with aminated polymer formed in situ for fast and reversible ion exchange.

A porous metal-organic framework composite with flexible anion-exchange polymers threaded within the host cavity demonstrates very fast and reversible ion-exchange activity. Polyvinyl benzyl trimethylammonium hydroxide (PVBTAH) caged in ZIF-8 is synthesized in steps of chloro-monomer impregnation, in situ polymerization, amination, and alkaline ion exchange. The synthesized non-cross-linked PVBTAH∼ZIF-8 material exhibits superior ion-exchange kinetics compared to conventional ion-exchange resins.

Sub-10-nm-sized Au@AuxIr1-x metal-core/alloy-shell nanoparticles as highly durable catalysts for acidic water splitting.

The absence of efficient and durable catalysts for oxygen evolution reaction (OER) is the main obstacle to hydrogen production through water splitting in an acidic electrolyte. Here, we report a controllable synthesis method of surface IrOx with changing Au/Ir compositions by constructing a range of sub-10-nm-sized core-shell nanocatalysts composed of an Au core and AuxIr1-x alloy shell. In particular, Au@Au0.43Ir0.57 exhibits 4.5 times higher intrinsic OER activity than that of the commercial Ir/C. Synchrotron X-ray-based spectroscopies, electron microscopy and density functional theory calculations revealed a balanced binding of reaction intermediates with enhanced activity. The water-splitting cell using a load of 0.02 mgIr/cm2 of Au@Au0.43Ir0.57 as both anode and cathode can reach 10 mA/cm2 at 1.52 V and maintain activity for at least 194 h, which is better than the cell using the commercial couple Ir/C‖Pt/C (1.63 V, 0.2 h).

Can Lead-Free Double Halide Perovskites Serve as Proper Photovoltaic Absorber?

The emerging Pb-free double perovskites (DPs) are acknowledged as the most potential nontoxic alternatives to lead halide perovskites for thin-film photovoltaics, yet their photophysical properties significantly lag behind expectations. To tackle this issue, it is imperative to conduct a systematic investigation of the structure and optoelectronic properties and to sift through vast chemical space to extract new types of Pb-free DPs with exceptional optoelectronic characteristics and thermal stability. Through high-throughput first-principal calculations, we demonstrate that apart from a select few Pb-free DPs (e.g., Cs2InSbCl6 and Cs2TlBiBr6), other categories, even with suitable direct electronic bandgaps, exhibit inferior optical absorption due to the inversion symmetry-induced parity-forbidden transitions. The mismatch between the electronic and optical bandgap, thence, casts doubt on the reliability of the electronic bandgap as a criterion for Pb-free DPs in various optoelectronics. The assessed limited thermostability under operational conditions, however, hinders any Pb-free DPs from effectively serving as photovoltaic absorbers. Alongside the compositional engineering discussed above, the prospect of manipulating local-site symmetry and disrupting the parity forbidden transitions in stabilized Pb-free DPs through materials engineering should be recognized as a pivotal and rational avenue toward achieving high performance.

Theoretical studies on Grignard reagent formation: radical mechanism versus non-radical mechanism.

Here we present a systematic theoretical investigation on the mechanisms of Grignard reagent formation (GRF) for CH(3)Cl reacting with Mg atom, Mg(2) and a series of Mg clusters (Mg(4)-Mg(20)). Our calculations reveal that the ground state Mg atom is inactive under matrix condition, whereas it is active under metal vapor synthesis (MVS) conditions. On the other hand, the excited state Mg ((3)P) atom, as produced by laser-ablation, can react with CH(3)Cl barrierlessly, and hence is active under matrix condition. We predict that the bimagnesium Grignard reagent, though often proposed, can barely be observed experimentally, due to its high reactivity towards additional CH(3)Cl to produce more stable Grignard reagent dimer, and that the cluster Grignard reagent RMg(4)X possesses a flat Mg(4) unit rather than a tetrahedral geometry. Our calculations further reveal that the radical pathway (T4) is prevalent on Mg, Mg(2) and Mg(n) clusters of small size, while the no-radical pathway (T2), which starts at Mg(4), becomes competitive with T4 as the cluster size increases. A structure-reactivity relationship between barrier heights and ionization potentials of Mg(n) is established. These findings not only resolve controversy in experiment and theory, but also provide insights which can be used in the design of effective synthesis approaches for the preparation of chiral Grignard reagents.

Microenvironment Regulation of the Ti3C2Tx MXene Surface for Enhanced Electrochemical Nitrogen Reduction.

The overwhelmingly competitive hydrogen evolution reaction (HER) is a bottleneck challenge in the electrocatalytic nitrogen reduction reaction (eNRR) process. Herein, we develop a general and effective strategy to suppress the HER via covalent surface functionalization to modulate the local microenvironment of the electrocatalyst. A hydrophobic molecular layer with tunable coverage density was coated on the surface of Ti3C2Tx MXene, and the one with appropriate coverage density significantly improved the eNRR efficiency with an excellent faradaic efficiency (FE) of 38.01% at -0.35 V and a high NH3 yield rate of 17.81 µg h-1mgcat-1 at -0.55 V (vs RHE) in a Na2SO4 solution, which were 3.5-fold in FE and 6.5-fold in NH3 yield rate higher than those of the pristine Ti3C2Tx. Experimental results combined with molecular dynamics (MD) simulations reveal that the hydrophobic molecular layer on the surface greatly limits the proton transfer and benefits higher exposure of active sites with enhanced N2 chemisorption ability, which cumulatively contribute to the boosted eNRR efficiency.