High-Density Coordinatively Unsaturated Zn Catalyst for Efficient Alkane Dehydrogenation.

The exploration of non-noble metal catalysts for alkane dehydrogenation and their catalytic mechanisms is the priority in catalysis research. Here, we report a high-density coordinatively unsaturated Zn cation (Zncus) catalyst for the direct dehydrogenation (DDH) of ethylbenzene (EB) to styrene (ST). The catalyst demonstrated good catalytic performance (∼40% initial EB conversion rate and >98% ST selectivity) and excellent regeneration ability in the reaction, which is attributed to the high-density (HD) distribution and high-stability structure of Zncus active sites on the surface of zinc silicate (HD-Zncus@ZS). Density functional theory (DFT) calculations further illustrated the reaction pathway and intermediates, supporting that the Zncus sites can efficiently activate the C-H bond of ethyl on ethylbenzene. Developing the high-density Zncus catalyst and exploring the catalytic mechanism laid a good foundation for designing practical non-noble metal catalysts.

The Impact of Surface Chemistry and Synthesis Conditions on the Adsorption of Antibiotics onto MXene Membranes.

MXene, a two-dimensional (2D) nanomaterial with diverse applications, has gained significant attention due to its 2D lamellar structure, abundance of surface groups, and conductivity. Despite various established synthesis methods since its discovery in 2011, MXenes produced through different approaches exhibit variations in structural and physicochemical characteristics, impacting their suitability for environmental application. This study delves into the effect of synthesis conditions on MXene properties and its adsorption capabilities for four commonly prescribed antibiotics. We utilized material characterization techniques to differentiate MXenes synthesized using three prevalent etchants: hydrofluoric acid (HF), mixed acids (HCl/HF), and fluoride salts (LiF/HCl). Our investigation of adsorption performance included isotherm and kinetic analysis, complemented by density functional theory calculations. The results of this research pinpointed LiF/HCl as an efficient etchant, yielding MXene with favorable morphology and surface chemistry. Electrostatic interactions and hydrogen bonding between MXene surface terminations and ionizable moieties of the antibiotic molecules emerge as pivotal factors in adsorption. Specifically, a higher presence of oxygen terminations increases the binding affinities. These findings provide valuable guidance for etchant selection in environmental applications and underscore the potential to tailor MXenes through synthesis conditions to design membranes capable of selectively removing antibiotics and other targeted substances.

Low-Temperature Acetylene Semi-Hydrogenation over the Pd1-Cu1 Dual-Atom Catalyst.

The atomically dispersed metal catalyst or single-atom catalyst (SAC) with the utmost metal utilization efficiency shows excellent selectivity toward ethylene compared to the metal nanoparticles catalyst in the acetylene semi-hydrogenation reaction. However, these catalysts normally work at relatively high temperatures. Achieving low-temperature reactivity while preserving high selectivity remains a challenge. To improve the intrinsic reactivity of SACs, rationally tailoring the coordination environments of the first metal atom by coordinating it with a second neighboring metal atom affords an opportunity. Here, we report the fabrication of a dual-atom catalyst (DAC) that features a bonded Pd1-Cu1 atomic pair anchoring on nanodiamond graphene (ND@G). Compared to the single-atom Pd or Cu catalyst, it exhibits increased reactivity at a lower temperature, with 100% acetylene conversion and 92% ethylene selectivity at 110 °C. This work provides a strategy for designing DACs for low-temperature hydrogenation by manipulating the coordination environment of catalytic sites at the atomic level.

Fully-Exposed Pd Cluster Catalyst: An Excellent Catalytic Antibacterial Nanomaterial.

Exploring antibacterial nanomaterials with excellent catalytic antibacterial properties has always been a hot research topic. However, the construction of nanomaterials with robust antibacterial activity at the atomic level remains a great challenge. Here a fully-exposed Pd cluster atomically-dispersed on nanodiamond-graphene (Pdn /ND@G) with excellent catalytic antibacterial properties is reported. The fully-exposed Pd cluster nanozyme provides atomically-dispersed Pd cluster sites that facilitate the activation of oxygen. Notably, the oxidase-like catalytic performance of the fully-exposed Pd cluster nanozyme is much higher than that of Pd single-atom oxidase mimic, Pd nanoparticles oxidase mimic and even the previously reported palladium-based oxidase mimics. Under the density functional theory (DFT) calculations, the Pd cluster sites can efficiently catalyze the decomposition of oxygen to generate reactive oxygen species, resulting in strong antibacterial properties. This research provides a valuable insight to the design of novel oxidase mimic and antibacterial nanomaterial.

Catalytic Transformation of PET and CO2 into High-Value Chemicals.

Polyethylene terephthalate (PET) and CO2 , two chemical wastes that urgently need to be transformed in the environment, are converted simultaneously in a one-pot catalytic process through the synergistic coupling of three reactions: CO2 hydrogenation, PET methanolysis and dimethyl terephthalate (DMT) hydrogenation. More interestingly, the chemical equilibria of both reactions were shifted forward due to a revealed dual-promotion effect, leading to significantly enhanced PET depolymerization. The overall methanol yield from CO2 hydrogenation exceeded the original thermodynamic equilibrium limit since the methanol was in situ consumed in the PET methanolysis. The degradation of PET by a stoichiometric ratio of methanol was significantly enhanced because the primary product, DMT was hydrogenated to dimethyl cyclohexanedicarboxylate (DMCD) or p-xylene (PX). This synergistic catalytic process provides an effective way to simultaneously recycle two wastes, polyesters and CO2 , for producing high-value chemicals.

Catalytic Synthesis of Formamides by Integrating CO2 Capture and Morpholine Formylation on Supported Iridium Catalyst.

Herein we report an efficient and recyclable catalytic system for tandem CO2 capture and N-formylation to value-added chemicals. CO2 is apt to be captured by morpholine solution, while a highly efficient heterogeneous catalyst, isolated iridium atoms supported over nanadiamond/graphene, is discovered to be highly reactive for the formylation of morpholine, leading to the formation of N-formylmorpholine with excellent productivity (with a turnover number of 5 120 000 in a single batch reaction) and selectivity (>99 %). In addition, the CO2 captured by morpholine under atmospheric conditions can be converted to N-formylmorpholine with decent conversion (51 %), which realizes the integration of CO2 capture and conversion to value-added chemicals.

Selective Catalytic Oxidation of Methane to Methanol in Aqueous Medium over Copper Cations Promoted by Atomically Dispersed Rhodium on TiO2.

Direct conversion of methane into value-added chemicals, such as methanol under mild conditions, is a promising route for industrial applications. In this work, atomically dispersed Rh on TiO2 suspended in an aqueous solution was used for the oxidation of methane to methanol. Promoted by copper cations (as co-catalyst) in solution, the catalysts exhibited high activity and selectivity for the production of methanol using molecular oxygen with the presence of carbon monoxide at 150 °C with a reaction pressure of 31 bar. Millimole level yields of methanol were reached with the selectivity higher than 99 % using the Rh/TiO2 catalysts with the promotion of the copper cation. CO was the reductive agent to generate H2 from H2 O, which led to the formation of H2 O2 through the reaction of H2 and O2 . Atomically dispersed Rh activated the C-H bond in CH4 and catalyzed the oxidation using H2 O2 . Copper cations maintained the low-valence state of Rh. Moreover, copper acted as a scavenger for suppressing the overoxidation, thus leading to the high selectivity of methanol.

Catalytic Urea Synthesis from Ammonium Carbamate Using a Copper(II) Complex: A Combined Experimental and Theoretical Study.

The synthesis of urea fertilizer is currently the largest CO2 conversion process by volume in the industry. In this process, ammonium carbamate is an intermediate en route to urea formation. We determined that the tetraammineaquacopper(II) sulfate complex, [Cu(NH3)4(OH2)]SO4, catalyzed the formation of urea from ammonium carbamate in an aqueous solution. A urea yield of up to 18 ± 6% was obtained at 120 °C after 15 h and in a high-pressure metal reactor. No significant urea formed without the catalyst. The urea product was characterized by Fourier transform infrared (FT-IR), powder X-ray diffraction (PXRD), and quantitative 1H{13C} NMR analyses. The [Cu(NH3)4(OH2)]SO4 catalyst was then recovered at the end of the reaction in a 29% recovery yield, as verified by FT-IR, PXRD, and quantitative UV-vis spectroscopy. A precipitation method using CO2 was developed to recover and reuse 66 ± 3% of Cu(II). The catalysis mechanism was investigated by the density functional theory at the B3LYP/6-31G** level with an SMD continuum solvent model. We determined that the [Cu(NH3)4]2+ complex is likely an effective catalyst structure. The study of the catalysis mechanism suggests that the coordinated carbamate with [Cu(NH3)4]2+ is likely the starting point of the catalyzed reaction, and carbamic acid can be involved as a transient intermediate that facilitates the removal of an OH group. Our work has paved the way for the rational design of catalysts for urea synthesis from the greenhouse gas CO2.

Molybdenum Carbide: Controlling the Geometric and Electronic Structure of Noble Metals for the Activation of O-H and C-H Bonds.

In the field of heterogeneous catalysis, transition metal carbides (TMCs) have attracted growing and extensive attention as a group of important catalytic materials for a variety of energy-related reactions. Due to the incorporation of carbon atoms at the interstitial sites, TMCs possess much higher density of states near the Fermi level, endowing the material with noble-metal-like electron configuration and catalytic behaviors. Crystal structure, site occupancies, surface termination, and metal/carbon defects in the bulk phase or at the surface are the structural factors that influence the behavior of the TMCs in catalytic reactions. In the early studies of heterogeneous catalytic applications of TMCs, the carbide itself was used individually as the catalytically active site, which exhibited unique catalytic performance comparable to precious metal catalysts toward hydrogenation, dehydrogenation, isomerization, and hydrodeoxygenation. To promote the catalytic performance, the doping of secondary transition metals into the carbide lattice to form bimetallic carbides was extensively studied. As a recent development, the utilization of TMCs as functionalized catalyst supports has achieved a series of significant breakthroughs in low-temperature catalytic applications, including the reforming of alcohols, water-gas shift reactions, and the hydrogenation of functional groups for chemical production and biomass conversion. Generally, the excellence of TMCs as supports is attributed to three factors: the modulation of geometric and electronic structures of the supported metal centers, the special reactivity of TMC supports that accelerates certain elementary step and influences the surface coverage of intermediates, and the special interfacial properties at the metal-carbide interface that enhance the synergistic effect. In this Account, we will review recent discoveries from our group and other researchers on the special catalytic properties of face-centered cubic MoC (α-MoC) as both a special catalyst and a functional support that enables highly efficient low-temperature O-H bond activation for several important energy-related catalytic applications, including hydrogen evolution from aqueous phase methanol reforming, ultralow temperature water-gas shift reaction, and biomass conversion. In particular, α-MoC has been demonstrated to exhibit unprecedented strong interaction with the supported metals compared with other TMCs, which not only stabilizes the under-coordinated metal species (single atoms and layered clusters) under strong thermal perturbation and harsh reaction conditions but also tunes the charge density at the metal sites and modifies their catalytic behavior in C-H activation and CO chemisorption. We will discuss how to exploit the metal/α-MoC interaction and interfacial properties to construct CO-tolerant selective hydrogenation catalysts for nitroarene derivatives. Several examples of constructing bifunctional tandem catalytic systems using molybdenum carbides that enable hydrogen extraction and utilization in one-pot conversion of biomass substrates and Fischer-Tropsch synthesis are also highlighted.

Low Temperature Oxidation of Ethane to Oxygenates by Oxygen over Iridium-Cluster Catalysts.

Direct selective oxidation of light alkanes, such as ethane, into value-added chemical products under mild reaction conditions remains a challenge in both industry and academia. Herein, the iridium cluster and atomically dispersed iridium catalysts have been successfully fabricated using nanodiamond as support. The obtained iridium cluster catalyst shows remarkable performance for selective oxidation of ethane under oxygen at 100 °C, with an initial activity as high as 7.5 mol/mol/h and a selectivity to acetic acid higher than 70% after five in situ recycles. The presence of CO in the reaction feed is pivotal for the excellent reaction performance. On the basis of X-ray photoelectron spectroscopy (XPS) analysis, the critical role of CO was revealed, which is to maintain the metallic state of reactive Ir species during the oxidation cycles.

Photoinduced Carrier Dynamics at the Interface of Black Phosphorus and Bismuth Vanadate.

Two-dimensional (2D) heterostructures of black phosphorus (BP)/bismuth vanadate (BVO) have attracted much attention due to their potential uses in photocatalytic water splitting. However, the interfacial photoinduced electron- and hole-transfer dynamics are not explored computationally. Herein, we have used density functional theory (DFT) calculations and DFT-based fewest-switches surface-hopping dynamics simulations to investigate the light-driven electron and hole dynamics taking place at the interface of BP and the BVO(010) surface. Our results show that the BP monolayer is adsorbed on BVO(010) via van der Waals interaction. Upon irradiation, the electron transfer takes place from BP to BVO(010) within 500 fs but with two distinct processes. In the first phase, the electron transfer proceeds adiabatically and is mainly driven by atomic motions. In the second phase, the electron transfer decays very slowly. The hole-transfer dynamics from BVO(010) to BP exhibits a similar ultrafast decay in the first stage followed by a slow decay; however, there is a comparable amount of hole trapped in a BP state due to a large energy gap from its higher state. These insights may be useful for the design of novel photocatalytic water-splitting materials.

Atomically Dispersed Pd on Nanodiamond/Graphene Hybrid for Selective Hydrogenation of Acetylene.

We reported here a strategy to use a defective nanodiamond-graphene (ND@G) to prepare an atomically dispersed metal catalyst, i.e., in the current case atomically dispersed palladium catalyst which is used for selective hydrogenation of acetylene in the presence of abundant ethylene. The catalyst exhibits remarkable performance for the selective conversion of acetylene to ethylene: high conversion (100%), ethylene selectivity (90%), and good stability. The unique structure of the catalyst (i.e., atomically dispersion of Pd atoms on graphene through Pd-C bond anchoring) blocks the formation of unselective subsurface hydrogen species and ensures the facile desorption of ethylene against the overhydrogenation to undesired ethane, which is the key for the outstanding selectivity of the catalyst.

Low-Temperature Transformation of Methane to Methanol on Pd1 O4 Single Sites Anchored on the Internal Surface of Microporous Silicate.

Direct conversion of methane to chemical feedstocks such as methanol under mild conditions is a challenging but ideal solution for utilization of methane. Pd1 O4 single-sites anchored on the internal surface of micropores of a microporous silicate exhibit high selectivity and activity in transforming CH4 to CH3 OH at 50-95 °C in aqueous phase through partial oxidation of CH4 with H2 O2 . The selectivity for methanol production remains at 86.4 %, while the activity for methanol production at 95 °C is about 2.78 molecules per Pd1 O4 site per second when 2.0 wt % CuO is used as a co-catalyst with the Pd1 O4 @ZSM-5. Thermodynamic calculations suggest that the reaction toward methanol production is highly favorable compared to formation of a byproduct, methyl peroxide.

Correlating photoacidity to hydrogen-bond structure by using the local O-H stretching probe in hydrogen-bonded complexes of aromatic alcohols.

To assess the potential use of O-H stretching modes of aromatic alcohols as ultrafast local probes of transient structures and photoacidity, we analyze the response of the O-H stretching mode in the 2-naphthol-acetonitrile (2N-CH3CN) 1:1 complex after UV photoexcitation. We combine femtosecond UV-infrared pump-probe spectroscopy and a theoretical treatment of vibrational solvatochromic effects based on the Pullin perturbative approach, parametrized at the density functional theory (DFT) level. We analyze the effect of hydrogen bonding on the vibrational properties of the photoacid-base complex in the S0 state, as compared to O-H stretching vibrations in a wide range of substituted phenols and naphthols covering the 3000-3650 cm(-1) frequency range. Ground state vibrational properties of these phenols and naphthols with various substituent functional groups are analyzed in solvents of different polarity and compared to the vibrational frequency shift of 2N induced by UV photoexcitation to the (1)Lb electronic excited state. We find that the O-H stretching frequency shifts follow a linear relationship with the solvent polarity function F0 = (2ε0 - 2)/(2ε0 + 1), where ε0 is the static dielectric constant of the solvent. These changes are directly correlated with photoacidity trends determined by reported pKa values and with structural changes in the O···N and O-H hydrogen-bond distances induced by solvation or photoexcitation of the hydrogen-bonded complexes.

Excited-state intramolecular hydrogen transfer (ESIHT) of 1,8-dihydroxy-9,10-anthraquinone (DHAQ) characterized by ultrafast electronic and vibrational spectroscopy and computational modeling.

We combine ultrafast electronic and vibrational spectroscopy and computational modeling to investigate the photoinduced excited-state intramolecular hydrogen-transfer dynamics in 1,8-dihydroxy-9,10-anthraquinone (DHAQ) in tetrachloroethene, acetonitrile, dimethyl sulfoxide, and methanol. We analyze the electronic excited states of DHAQ with various possible hydrogen-bonding schemes and provide a general description of the electronic excited-state dynamics based on a systematic analysis of femtosecond UV/vis and UV/IR pump-probe spectroscopic data. Upon photoabsorption at 400 nm, the S2 electronic excited state is initially populated, followed by a rapid equilibration within 150 fs through population transfer to the S1 state where DHAQ exhibits ESIHT dynamics. In this equilibration process, the excited-state population is distributed between the 9,10-quinone (S2) and 1,10-quinone (S1) states while undergoing vibrational energy redistribution, vibrational cooling, and solvation dynamics on the 0.1-50 ps time scale. Transient UV/vis pump-probe data in methanol also suggest additional relaxation dynamics on the subnanosecond time scale, which we tentatively ascribe to hydrogen bond dynamics of DHAQ with the protic solvent, affecting the equilibrium population dynamics within the S2 and S1 electronic excited states. Ultimately, the two excited singlet states decay with a solvent-dependent time constant ranging from 139 to 210 ps. The concomitant electronic ground-state recovery is, however, only partial because a large fraction of the population relaxes to the first triplet state. From the similarity of the time scales involved, we conjecture that the solvent plays a crucial role in breaking the intramolecular hydrogen bond of DHAQ during the S2/S1 relaxation to either the ground or triplet state.

Fully exposed Pt clusters for efficient catalysis of multi-step hydrogenation reactions.

For di-nitroaromatics hydrogenation, it is a challenge to achieve the multi-step hydrogenation with high activity and selectivity due to the complexity of the process involving two nitro groups. Consequently, many precious metal catalysts suffer from low activity for this multi-step hydrogenation reaction. Herein, we employ a fully exposed Pt clusters catalyst consisting of an average of four Pt atoms on nanodiamond@graphene (Ptn/ND@G), demonstrating excellent catalytic performance for the multi-step hydrogenation of 2,4-dinitrotoluene. The TOF (40647 h-1) of Ptn/ND@G is significantly superior to that of single Pt atoms catalyst, Pt nanoparticles catalyst, and even all the known catalysts. Density functional theory calculations and absorption experiments reveal that the synergetic interaction between the multiple active sites of Ptn/ND@G facilitate the co-adsorption/activation of reactants and H2, as well as the desorption of intermediates/products, which is the key for the higher catalytic activity than single Pt atoms catalyst and Pt nanoparticles catalyst.

Membrane permeation induced by aggregates of human islet amyloid polypeptides.

Several neurodegenerative diseases such as Alzheimer's and Parkinson's diseases as well as nonneuropathic diseases such as type II diabetes and atrial amyloidosis are associated with aggregation of amyloid polypeptides into fibrillar structures, or plaques. In this study, we use molecular dynamics simulations to test the stability and orientation of membrane-embedded aggregates of the human islet amyloid polypeptide (hIAPP) implicated in type II diabetes. We find that in both monolayers and bilayers of dipalmitoylphosphatidylglycerol (DPPG) hIAPP trimers and tetramers remain inside the membranes and preserve their ß-sheet secondary structure. Lipid bilayer-inserted hIAPP trimers and tetramers orient inside DPPG at 60° relative to the membrane/water interface and lead to water permeation and Na(+) intrusion, consistent with ion-toxicity in islet ß-cells. In particular, hIAPP trimers form a water-filled ß-sandwich that induce water permeability comparable with channel-forming proteins, such as aquaporins and gramicidin-A. The predicted disruptive orientation is consistent with the amphiphilic properties of the hIAPP aggregates and could be probed by chiral sum frequency generation (SFG) spectroscopy, as predicted by the simulated SFG spectra.

Chiral sum frequency generation for in situ probing proton exchange in antiparallel ß-sheets at interfaces.

Studying hydrogen/deuterium (H/D) exchange in proteins can provide valuable insight on protein structure and dynamics. Several techniques are available for probing H/D exchange in the bulk solution, including NMR, mass spectroscopy, and Fourier transform infrared spectroscopy. However, probing H/D exchange at interfaces is challenging because it requires surface-selective methods. Here, we introduce the combination of in situ chiral sum frequency generation (cSFG) spectroscopy and ab initio simulations of cSFG spectra as a powerful methodology to probe the dynamics of H/D exchange at interfaces. This method is applied to characterize H/D exchange in the antiparallel ß-sheet peptide LK7ß. We report here for the first time that the rate of D-to-H exchange is about 1 order of magnitude faster than H-to-D exchange in the antiparallel structure at the air/water interface, which is consistent with the existing knowledge that O-H/D dissociation in water is the rate-limiting step, and breaking the O-D bond is slower than breaking the O-H bond. The reported analysis also provides fundamental understanding of several vibrational modes and their couplings in peptide backbones that have been difficult to characterize by conventional methods, including Fermi resonances of various combinations of peptide vibrational modes such as amide I and amide II, C-N stretch, and N-H/N-D bending. These results demonstrate cSFG as a sensitive technique for probing the kinetics of H/D exchange in proteins at interfaces, with high signal-to-noise N-H/N-D stretch bands that are free of background from the water O-H/O-D stretch.

Selective Depolymerization of Lignin Towards Isolated Phenolic Acids Under Mild Conditions.

The selective transformation of lignin to value-added biochemicals (e. g., phenolic acids) in high yields is incredibly challenging due to its structural complexity and many possible reaction pathways. Phenolic acids (PA) are key building blocks for various aromatic polymers, but the isolation of PAs from lignin is below 5 wt.% and requires harsh reaction conditions. Herein, we demonstrate an effective route to selectively convert lignin extracted from sweet sorghum and poplar into isolated PA in a high yield (up to 20 wt.% of lignin) using a low-cost graphene oxide-urea hydrogen peroxide (GO-UHP) catalyst under mild conditions (<120 °C). The lignin conversion yield is up to 95 %, and the remaining low molecular weight organic oils are ready for aviation fuel production to complete lignin utilization. Mechanistic studies demonstrate that pre-acetylation allows the selective depolymerization of lignin to aromatic aldehydes with a decent yield by GO through the Cα activation of ß-O-4 cleavage. A urea-hydrogen peroxide (UHP) oxidative process is followed to transform aldehydes in the depolymerized product to PAs by avoiding the undesired Dakin side reaction due to the electron-withdrawing effect of the acetyl group. This study opens a new way to selectively cleave lignin side chains to isolated biochemicals under mild conditions.

Structure-dependence and metal-dependence on atomically dispersed Ir catalysts for efficient n-butane dehydrogenation.

Single-site pincer-ligated iridium complexes exhibit the ability for C-H activation in homogeneous catalysis. However, instability and difficulty in catalyst recycling are inherent disadvantages of the homogeneous catalyst, limiting its development. Here, we report an atomically dispersed Ir catalyst as the bridge between homogeneous and heterogeneous catalysis, which displays an outstanding catalytic performance for n-butane dehydrogenation, with a remarkable n-butane reaction rate (8.8 mol·gIr-1·h-1) and high butene selectivity (95.6%) at low temperature (450 °C). Significantly, we correlate the BDH activity with the Ir species from nanoscale to sub-nanoscale, to reveal the nature of structure-dependence of catalyst. Moreover, we compare Ir single atoms with Pt single atoms and Pd single atoms for in-depth understanding the nature of metal-dependence at the atomic level. From experimental and theoretical calculations results, the isolated Ir site is suitable for both reactant adsorption/activation and product desorption. Its remarkable dehydrogenation capacity and moderate adsorption behavior are the key to the outstanding catalytic activity and selectivity.