The Effects of ≡Ti–OH Site Distortion and Product Adsorption on the Mechanism and Kinetics of Cyclohexene Epoxidation over Ti/SiO2

Energy & Environmental Science

Integrated solar fuels devices for CO2 reduction (CO2R) are a promising technology class towards reducing carbon emissions. Designing integrated CO2R solar fuels devices requires careful co-design of electrochemical and photovoltaic components as well as consideration of the diurnal and seasonal effects of solar irradiance, temperature, and other meteorological factors expected for ‘on-sun’ deployment. Using a photovoltaic-electrochemical (PV-EC) platform, we developed a temperature and potential-dependent diurnal and annual model using experimentally-determined CO2R performance of Cu-based electrocatalysts, local meteorological data from the National Solar Radiation Database (NSRD), and modeled performance of commercial c-Si PVs. We simulated gaseous diurnal product outputs with and without the effects of ambient temperature. From these outputs, we observed seasonal variation in …

A scientific biography is presented as an introduction to a collection of papers by his colleagues and co-workers honouring the scientific contributions and legacy of the late Dr. Timothy J. Lee (1959–2022). Tim Lee performed highly regarded research on the methods and applications of computational quantum chemistry, particularly coupled cluster theory, and computational spectroscopy, with impacts in interstellar chemistry, aiding NASA missions, atmospheric chemistry, as well as fundamental science. His presence is deeply missed.

ACS Catalysis

Recent research has found that dealuminated zeolite BEA (DeAlBEA) is an attractive support for the dispersion of Pt and PtSn species that serve as catalysts for propane dehydrogenation (PDH). In this study, we report the preparation, structural characterization, and PDH activities of Pt-Sn-DeAlBEA catalysts as a function of the Pt/Al ratio (here Al represents the amount of Al present in the parent zeolite H-BEA). The support Sn-DeAlBEA was prepared by introduction of Sn to DeAlBEA. Characterization of this material by X-ray absorption spectroscopy (XAS) and UV–vis spectroscopy revealed that the Sn incorporated into the BEA framework as Sn(IV) cations. Pt-Sn-DeAlBEA catalysts were prepared with Pt/Al ratios (0.001–0.026) and were characterized with infrared (IR) spectroscopy of adsorbed probe molecules and XAS to understand the effect of changing Pt loading on the structure of Pt in Pt-Sn-DeAlBEA. Pt …

Journal of Chemical Theory and Computation

Theoretical predictions of NMR chemical shifts from first-principles can greatly facilitate experimental interpretation and structure identification of molecules in gas, solution, and solid-state phases. However, accurate prediction of chemical shifts using the gold-standard coupled cluster with singles, doubles, and perturbative triple excitations [CCSD(T)] method with a complete basis set (CBS) can be prohibitively expensive. By contrast, machine learning (ML) methods offer inexpensive alternatives for chemical shift predictions but are hampered by generalization to molecules outside the original training set. Here, we propose several new ideas in ML of the chemical shift prediction for H, C, N, and O that first introduce a novel feature representation, based on the atomic chemical shielding tensors within a molecular environment using an inexpensive quantum mechanics (QM) method, and train it to predict NMR …

Bulletin of the American Physical Society

K39. 00006: Prize Talk: Justin Jankunas Doctoral Dissertation Award Finalists: Developing a Quantum Chemical Toolbox for Accurate Modeling of K-edges and Beyond

Bipolar membranes (BPMs) enable control of ion concentrations and fluxes in electrochemical cells suitable for a wide range of applications. Here we present the multi-scale physics of BPMs in an electrochemical engineering context and articulate design principles to drive the development of advanced BPMs. The chemistry, structure, and physics of BPMs are illustrated and related to the thermodynamics, transport phenomena, and chemical kinetics that dictate ion and species fluxes and selectivity. These interactions give rise to emergent structure–property–performance relationships that yield design criteria for BPMs that achieve high permselectivity, durability, and voltaic efficiency. The resulting performance trade-offs for BPMs are presented in the context of emerging applications in energy conversion or storage, and environmental remediation. By connecting the fundamental physical phenomena in BPMs to …

Journal of the American Chemical Society

High or enriched-purity O2 is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O2-selective air separations, including the use of metal–organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O2 over N2 via electron transfer. However, most of these materials exhibit appreciable and/or reversible O2 uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework CuI-MFU-4l (CuxZn5–xCl4–x(btdd)3; H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin), which binds O2 reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal CuI …

Bulletin of the American Physical Society

B40. 00004: Photodissociation of Fe (CO) 5: Insights from femtosecond core-level spectroscopy and theory*

ACS Catalysis

Zn Lewis acid centers were grafted onto the silanol nest created by dealumination of H-BEA zeolite (DeAlBEA). The resulting material was characterized and investigated for propane dehydrogenation to propene and n-butane dehydrogenation to 1,3-butadiene (1,3-BD). For Zn/Al molar ratios (Al is the molar amount in H-BEA) below 0.12, Zn sites are present as isolated (≡SiOZn–OH) species, but for Zn/Al ratios between 0.12 and 0.60, the ≡SiOZn–OH species form nests in which enhanced electron transfer between Zn and O atoms of the neighboring ≡SiOZn–OH group and H-bonding interaction between adjacent Zn–OH groups occur. The turnover frequency (TOF) for both propane and n-butane dehydrogenation is virtually identical for Zn-DeAlBEA for Zn/Al < 0.12 and then increases almost linearly with increasing Zn/Al ratio from 0.12 to 0.36, indicating the superior activity of Zn atoms in ≡SiOZn–OH nests. In …

The electrified aqueous/metal interface is critical in controlling the performance of energy conversion and storage devices, but an atomistic understanding of even basic interfacial electrochemical reactions challenges both experiment and computation. We report a combined simulation and experimental study of (reversible) ion-transfer reactions involved in anodic Ag corrosion, a model system for interfacial electrochemical processes generating or consuming ions. With the explicit modeling of the electrode potential and a hybrid implicit-explicit solvation model, the density functional theory calculations produce free-energy curves predicting thermodynamics, kinetics, partial charge profiles, and reaction trajectories. The calculated (equilibrium) free-energy barriers (0.2 eV), and their asymmetries, agree with experimental activation energies (0.4 eV) and transfer coefficients, which were extracted from temperature-dependent voltage-step experiments on Au-supported, Ag-nanocluster substrates. The use of Ag nanoclusters eliminates the convolution of the kinetics of Ag+(aq.) generation and transfer with those of nucleation or etch-pit formation. The results indicate that the barrier is controlled by the bias-dependent competition between partial solvation of the incipient ion, metal-metal bonding and electrostatic stabilization by image charge. Both the latter factors are weakened by more positive bias. We also report simulations of the bias-dependence of defect generation relevant to nucleating corrosion by removing an atom from a perfect Ag(100) surface, which is predicted to occur via a vacancy-adatom intermediate. Together, these experiments and …

Journal of Chemical Theory and Computation

We formulate a one-center nonorthogonal configuration interaction singles (1C-NOCIS) theory for the computation of core excited states of an initial singlet state with two unpaired electrons. This model, which we refer to as 1C-NOCIS two-electron open-shell (2eOS), is appropriate for computing the K-edge near-edge X-ray absorption spectra (NEXAS) of the valence excited states of closed-shell molecules relevant to pump-probe time-resolved (TR) NEXAS experiments. With the inclusion of core-hole relaxation effects and explicit spin adaptation, 1C-NOCIS 2eOS requires mild shifts to match experiment, is free of artifacts due to spin contamination, and can capture the high-energy region of the spectrum beyond the transitions into the singly occupied molecular orbitals (SOMOs). Calculations on water and thymine illustrate the different key features of excited-state NEXAS, namely, the core-to-SOMO transitions as …

arXiv preprint arXiv:2402.13538

In periodic systems, the Hartree-Fock (HF) exchange energy exhibits the slowest convergence of all HF energy components as the system size approaches the thermodynamic limit. We demonstrate that the recently proposed staggered mesh method for Fock exchange energy [Xing, Li, and Lin, Math. Comp., 2024], which is specifically designed to sidestep certain singularities in exchange energy evaluation, can expedite the finite-size convergence rate for the exact exchange energy across a range of insulators and semiconductors when compared to the regular and truncated Coulomb methods. This remains true even for two computationally cheaper versions of this new method, which we call Non-SCF and Split-SCF staggered mesh. Additionally, a sequence of numerical tests on simple solids showcases the staggered mesh method's ability to improve convergence towards the thermodynamic limit for band gaps, bulk moduli, equilibrium lattice dimensions, energies, and phonon force constants.

National Science Review

The electrochemical reduction of the CO2 reaction (CO2RR) offers an attractive means for converting the carbon content of CO2, released from a stationary source or the atmosphere, into fuels and chemicals [1]. Utilization of CO2 captured from the atmosphere, and energy provided by electricity sourced from wind or solar energy, are particularly appealing because they enable sustainable production of carbon-containing products. The most attractive type of CO2 electrolyzer for commercial application is a membrane electrode assembly (MEA), as shown in Fig. 1 a [2, 3]. This device consists of an anion-exchange membrane (AEM) sandwiched between two gas diffusion electrodes (GDEs), each of which comprises a gas diffusion layer (GDL) and a catalyst layer (CL). The cathode CL contains metal nanoparticles, typically Ag or Cu, that promote the electrochemical reduction of CO2 using water vapor as the source of …

We investigate the early stages of cesium lead bromide perovskite formation through absorption spectroscopy of stopped flow reactions, high-throughput mapping and direct synthesis and titration of potential precursor species. Calorimetric and spectroscopic measurements of lead bromide complex titrations combined with theoretical calculations suggest that bromide complexes with higher coordination numbers than previously considered for non-polar systems can better explain observed behaviors. Synthesis mapping of binary lead halides reveals multiple lead bromide species with absorption peaks higher than 300 nm, including a previously observed species with a peak at 313 nm and two species with peaks at 345 and 370 nm that also appear as reaction intermediates during formation of lead bromide perovskites. Based on theoretical calculations of excitonic energies that match within 50 meV, we give a preliminary assignment of these species as two dimensional magic sized clusters with side lengths of 2, 3 and 4 unit cells. Kinetic measurements of the conversion of benzoyl bromide precursor are connected to stopped flow measurements of product formation and demonstrate that the formation of complexes and magic sized clusters (i.e. nucleation) is controlled by precursor decomposition, whereas the growth rate of 2D and 3D perovskites is significantly slower.

Journal of the American Chemical Society

The catalytic transformation of C–H to C–N bonds offers rapid access to fine chemicals and high-performance materials, but achieving high selectivity from undirected aminations of unactivated C(sp3)–H bonds remains an outstanding challenge. We report the origins of the reactivity and selectivity of a Cu-catalyzed C–H amidation of simple alkanes. Using a combination of experimental and computational mechanistic studies and energy decomposition techniques, we uncover a switch in mechanism from inner-sphere to outer-sphere coupling between alkyl radicals and the active Cu(II) catalyst with increasing substitution of the alkyl radical. The combination of computational predictions and detailed experimental validation shows that simultaneous minimization of both Cu–C covalency and alkyl radical size increases the rate of reductive elimination and that both strongly electron-donating and electron-withdrawing …

Chemical Science

The electrified aqueous/metal interface is critical in controlling the performance of energy conversion and storage devices, but an atomistic understanding of even basic interfacial electrochemical reactions challenges both experiment and computation. We report a combined simulation and experimental study of (reversible) ion-transfer reactions involved in anodic Ag corrosion/deposition, a model system for interfacial electrochemical processes generating or consuming ions. With the explicit modeling of the electrode potential and a hybrid implicit-explicit solvation model, the density functional theory calculations produce free-energy curves predicting thermodynamics, kinetics, partial charge profiles, and reaction trajectories. The calculated (equilibrium) free-energy barriers (0.2 eV), and their asymmetries, agree with experimental activation energies (0.4 eV) and transfer coefficients, which were extracted from …

Dalton Transactions

The synthesis, thermolysis, and surface organometallic chemistry of thermolytic molecular precursors based on a new germanosilicate ligand platform, –OGe[OSi(OtBu)3]3, is described. Use of this ligand is demonstrated with preparation of complexes containing the first-row transition metals Cr, Mn, and Fe. The thermolysis and grafting behavior of the synthesized complexes, Fe{OGe[OSi(OtBu)3]3}2 (FeGe), Mn{OGe[OSi(OtBu)3]3}2(THF)2 (MnGe) and Cr{OGe[OSi(OtBu)3]3}2(THF)2 (CrGe), was evaluated using a combination of thermogravimetric analysis; nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies; and single-crystal X-ray diffraction (XRD). Grafting of the precursors onto SBA-15 mesoporous silica and subsequent calcination in air led to substantial changes in transition metal coordination environments and oxidation states, the …

arXiv preprint arXiv:2404.09078

We develop a static quantum embedding scheme, utilizing projection equations to solve coupled cluster (CC) amplitudes. To reduce the computational cost (for example, of a large basis set calculation), we solve the local fragment problem using a high-level coupled cluster method and address the environment problem with a lower-level M{\o}ller-Plesset (MP) perturbative method. This embedding approach is consistently formulated within the coupled cluster framework and will be called MP-CC. We demonstrate the effectiveness of our method through several prototypical molecular examples by analyzing a global quantity, that is, the total correlation energy of the system in the study of potential energy curves (PEC) and thermochemical reaction energies. We have shown that our method can achieve comparable accuracy both with a small and large basis set when the fragment Hilbert space size remains the same. Additionally, our results indicate that increasing the fragment size can systematically enhance the accuracy of observables, approaching the precision of the full coupled cluster solver.

ACS Catalysis

In the search for stable and active catalysts, density functional theory and machine learning-based models can accelerate the screening of materials. While stability is conveniently addressed on the bulk level of computation, the modeling of catalytic activity requires expensive surface simulations. In this work, we develop models for the surface adsorption energy of O and OH intermediates across a consistent and extensive data set of pure transition metal oxide surfaces. We show that adsorption energies across metal oxidation states of +2 to +6 are well captured from the metal–oxygen bond strength extracted from the bulk level calculation. Specifically, we calculate the integrated crystal orbital Hamiltonian population (ICOHP) of the metal–oxygen bond in the bulk oxide and employ a simple normalization scheme to obtain a strong correlation with the adsorption energetics. By combining our ICOHP descriptor with …

ACS Catalysis

The utilization of weak coordination in promoting site-selective C(sp3)–H functionalization is of immense importance. Herein, we report a Pd-catalyzed distal γ-C(sp3)–H arylation that harnesses the weak coordination affinity of keto groups with the native noncoordinating amide moiety. The current protocol overcomes one of the major challenges associated with the diversification of synthetic modular frameworks of quaternary centers: controlling the mono- vs difunctionalization of chemically equivalent C–H bonds. The developed condition overrides the interference of the acidic α-hydrogen for possible side reactions of amides and delivers the exclusive formation of the γ-monoarylated product. The association of 2-hydroxy pyridine ligands bearing electron-withdrawing substituents demonstrated the best partnership with the Pd–Ag hetero-bimetallic complex to achieve this distal γ-C(sp3)–H activation of a range of …

ACS Catalysis

H2-reduced phosphomolybdate (PMA) demonstrated promising aerobic selective methane oxidation activity at room temperature. However, this catalytic system remains limited by insufficient methane activation efficiency at the molybdenum (Mo) sites. Here, we introduce inorganic sulfite anions as a promoter to enhance the H2-treated heterogeneous palladium (Pd)-containing Pd/CsPMA catalyst. With the optimal sulfite addition, the methanol productivity was improved by more than 4 times without compromising high methanol selectivity. Spectroscopic techniques and theoretical calculation imply that sulfite serves as a modification ligand, binding to exposed Mo sites from the on-top or bridge locations. This alters the redox properties of PMA, facilitating the creation of more reactive oxygen species for hydrogen abstraction from methane.

ACS Catalysis

Strong synergy between proximate Bro̷nsted acid and metal sites over a Pt/amorphous silica–alumina (ASA) catalyst confers striking activity and selectivity enhancements for m-cresol hydrodeoxygenation to methylcyclohexane, a fuel additive and potential liquid organic hydrogen carrier for storing renewable energy. Molecular shuttling back and forth between cooperative acid and metal sites activates m-cresol for consecutive tautomerization, ring hydrogenation, and dehydration, ensuing eventual hydrogenation to methylcyclohexane with a fourfold yield enhancement compared to a physical mix of solid acid and Pt/SiO2.

ACS Catalysis

Exsolved Ni–Fe alloy perovskite catalysts exhibit remarkable coking resistance during C–H and C–O activation. However, metallic utilization is typically incomplete, resulting in relatively low catalytic activity. Herein, we investigated minimal doping with Rh to boost the catalytic activity in the dry reforming of methane by promoting exsolution and enlargement of the three-phase boundary between the alloy, support, and reactants. The Rh influences the formation of the Ni–Fe alloy, as revealed by X-ray diffraction, and promotes the individual and collective CH4 and CO2 conversions, as revealed by packed bed reactor runs, temperature-programmed surface reactions, and in situ infrared spectroscopy. A minimal 0.21 wt % Rh addition enlarges the three-phase boundary while improving oxygen mobility and storage. The oxygen mobility is responsible for promoting CH4 dissociation and dynamic removal of carbon …

ACS catalysis

Isolated platinum(II) ions anchored at acid sites in the pores of zeolite HZSM-5, initially introduced by aqueous ion exchange, were reduced to form platinum nanoparticles that are stably dispersed with a narrow size distribution (1.3 ± 0.4 nm in average diameter). The nanoparticles were confined in reservoirs within the porous zeolite particles, as shown by electron beam tomography and the shape-selective catalysis of alkene hydrogenation. When the nanoparticles were oxidatively fragmented in dry air at elevated temperature, platinum returned to its initial in-pore atomically dispersed state with a charge of +2, as shown previously by X-ray absorption spectroscopy. The results determine the conditions under which platinum is retained within the pores of HZSM-5 particles during redox cycles that are characteristic of the reductive conditions of catalyst operation and the oxidative conditions of catalyst regeneration.

ACS catalysis

We thank the Ministerio de Ciencia e Innovación (MICINN) and Fondo Europeo de Desarrollo Regional (FEDER, UE) for financial support (Agencia Estatal de Investigación/Projects PGC2018-098660-B-I00, PID2021-127655NB-I00, and TED2021-129970B-C22). JC thanks the Ministerio de Educación, Cultura y Deporte (MECD), for an FPU fellowship. This research was also funded by the European Union’s Horizon 2020 research and innovation program under European Research Council (ERC) through the HyMAP project, grant agreement No. 648319. Financial support was received from AEI-MICINN/FEDER, UE through the Nympha Project (PID2019-106315RB-I00) and NovaCO2 (PID2020-118593 RB-C22) funded by MCINN/AEI/10.13039/50110001103. The computations were enabled by resources provided by the National Academic Infrastructure for Supercomputing in Sweden (NAISS) and the Swedish …

ACS Catalysis

Biomass photorefining is a promising strategy to address the energy crisis and transition toward carbon carbon-neutral society. Here, we demonstrate the feasibility of direct cellulose photorefining into arabinose by a rationally designed oxygen-doped polymeric carbon nitride, which generates favorable oxidative species (e.g., O2–, •OH) for selective oxidative reactions at neutral conditions. In addition, we also illustrate the mechanism of the photocatalytic cellulose to arabinose conversion by density functional theory calculations. The oxygen insertion derived from oxidative radicals at the C1 position of glucose within cellulose leads to oxidative cleavage of β-1,4 glycosidic linkages, resulting in the subsequent gluconic acid formation. The following decarboxylation process of gluconic acid via C1–C2 α-scissions, triggered by surface oxygen-doped active sites, generates arabinose and formic acid, respectively. This …

ACS Catalysis

Unspecific peroxygenases (UPOs) are fungal enzymes that attract significant attention for their ability to perform versatile oxyfunctionalization reactions using H2O2. Unlike other oxygenases, UPOs do not require additional reductive equivalents or electron transfer chains that complicate basic and applied research. Nevertheless, UPOs generally exhibit low to no heterologous production levels and only four UPO structures have been determined to date by crystallography limiting their usefulness and obstructing research. To overcome this bottleneck, we implemented a workflow that applies PROSS stability design to AlphaFold2 model structures of 10 unique and diverse UPOs followed by a signal peptide shuffling to enable heterologous production. Nine UPOs were functionally produced in Pichia pastoris, including the recalcitrant CciUPO and three UPOs derived from oomycetes─the first nonfungal UPOs to be …

ACS Catalysis

H2-reduced phosphomolybdate (PMA) demonstrated promising aerobic selective methane oxidation activity at room temperature. However, this catalytic system remains limited by insufficient methane activation efficiency at the molybdenum (Mo) sites. Here, we introduce inorganic sulfite anions as a promoter to enhance the H2-treated heterogeneous palladium (Pd)-containing Pd/CsPMA catalyst. With the optimal sulfite addition, the methanol productivity was improved by more than 4 times without compromising high methanol selectivity. Spectroscopic techniques and theoretical calculation imply that sulfite serves as a modification ligand, binding to exposed Mo sites from the on-top or bridge locations. This alters the redox properties of PMA, facilitating the creation of more reactive oxygen species for hydrogen abstraction from methane.

ACS catalysis

Most cytochrome P450 (P450) oxidations are considered to occur with the active oxidant being a perferryl oxygen (FeO3+, Compound I). However, a ferric peroxide (FeO2̅, Compound 0) mechanism has been proposed, as well, particularly for aldehyde substrates. We investigated three of these systems, the oxidative deformylation of the model substrates citronellal, 2-phenylpropionaldehyde, and 2-methyl-2-phenylpropionaldehyde by rabbit P450 2B4, using 18O labeling. The formic acid product contained one 18O derived from 18O2, which is indicative of a dominant Compound 0 mechanism. The formic acid also contained only one 18O derived from H218O, which ruled out a Compound I mechanism. The possibility of a Baeyer–Villiger reaction was examined by using synthesized possible intermediates, but our data do not support its presence. Overall, these findings unambiguously demonstrate the role of the …

ACS Catalysis

Understanding the structure change of the electrocatalysts during the electrochemical CO2 reduction reaction (CO2RR) is of crucial importance to illustrate the structure–performance relationship. Here, the reconstruction of Bi–O–M (M = S, Se, or Cl) nanosheets induced by the in situ dissociated chalcogenide anions toward efficient CO2RR to formate is reported. The surface work function and potential of zero charge (PZC) of metallic Bi are reduced upon anions’ adsorption, facilitating the regeneration of active Bi–O structures during reduction. Moreover, a correlation between the pKb values of the anions and the local pH of the catalyst/electrolyte interface can be established. The anion with a smaller pKb (S2– < Se2– < Cl–) would induce a more alkaline environment and further promote the formation of Bi–O structures. Among them, Bi2O2S with in situ released S2– during reconstruction exhibits the best CO2RR-to …

ACS Catalysis

With the remarkable capability of encapsulating multifunctional active sites inside, carbon-based materials play vital roles in multifarious chemical transformations such as hydrogenation reactions, oxidation reactions, esterification reactions, etc. Herein, we propose an efficient and facile approach to construct a Cu/Co2P@C-NS catalyst, which consists of nitrogen–sulfur co-doped carbon (C-NS) encapsulated cobalt phosphide (Co2P) and copper double active sites. This approach focuses on the design and preparation of core–shell materials, that is, the ZIF-67 core is coated with an adhesive poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) polymer shell. The ZIF-67 core possesses a highly porous structure, coexistence of carbon and nitrogen elements, and uniform dispersion of Co species, making it an ideal template. The heteroatomic PZS polymer with decent coating ability makes it a promising …

ACS catalysis

The reduction of C═X (X = N, O) bonds is a cornerstone in both synthetic organic chemistry and biocatalysis. Conventional reduction mechanisms usually involve a hydride ion targeting the less electronegative carbon atom. In a departure from this paradigm, our investigation into Old Yellow Enzymes (OYEs) reveals a mechanism involving transfer of hydride to the formally more electronegative nitrogen atom within a C═N bond. Beyond their known ability to reduce electronically activated C═C double bonds, e.g., in α, β-unsaturated ketones, these enzymes have recently been shown to reduce α-oximo-β-ketoesters to the corresponding amines. It has been proposed that this transformation involves two successive reduction steps and proceeds via imine intermediates formed by the reductive dehydration of the oxime moieties. We employ advanced quantum mechanics/molecular mechanics (QM/MM) simulations …

ACS catalysis

A detailed multiscale study of the mechanism of CO2 hydrogenation on a well-defined Ni/CeO2 model catalyst is reported that couples periodic density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations. The study includes an analysis of the role of Eley–Rideal elementary steps for the water formation step, which are usually neglected on the overall picture of the mechanism, catalytic activity, and selectivity. The DFT calculations for the chosen model consisting of a Ni4 cluster supported on CeO2 (111) show large enough adsorption energies along with low energy barriers that suggest this catalyst to be a good option for high selective CO2 methanation. The kMC simulations results show a synergic effect between the two 3-fold hollow sites of the supported Ni4 cluster with some elementary reactions dominant in one site, while other reactions prefer the another, nearly equivalent site. This …

ACS catalysis

Wacker oxidations are ubiquitous in the direct synthesis of carbonyl compounds from alkenes. While the reaction mechanism has been widely studied under aerobic conditions, much less is known about such processes promoted with peroxides. Here, we report an exhaustive mechanistic investigation of the Wacker oxidation of styrene using hydrogen peroxide (H2O2) and tert-butyl hydroperoxide (TBHP) as oxidants by combining density functional theory and microkinetic modeling. Our results with H2O2 uncover a previously unreported reaction pathway that involves an intermolecular proton transfer assisted by the counterion [OTf]− present in the reaction media. Furthermore, we show that when TBHP is used as an oxidant instead of H2O2, the reaction mechanism switches to an intramolecular protonation sourced by the HOtBu moiety generated in situ. Importantly, these two mechanisms are predicted to …

ACS Catalysis

We provide experimental evidence that is inconsistent with often proposed Langmuir−Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au–Fe2O3. Passing CO and H2O, but no O2, over Au-γ-Fe2O3 at 25 °C, we observe significant CO2 production, inconsistent with LH mechanistic hypotheses. Experiments with H218O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe2O3. Guided by density functional theory, we instead postulate a water-promoted Mars–van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO2 production in the absence of O2 and with CO oxidation in the presence of H218O and 16O2. In contrast, for Au-TiO2, our data is consistent with previous LH mechanistic hypotheses.

ACS Catalysis

We investigate an approach to tune the d-band center and enhance the oxygen reduction reaction (ORR) activity of Pt material without relying on foreign metals or the process of alloying/dealloying. It is known that Pt exhibits suboptimal ORR catalytic activity due to its strong binding to oxygen, therefore requiring a downshift in the d-band center by approximately 0.2 eV to weaken the Pt-O binding energy and boost ORR kinetics. We found that the d-band center can be tuned by inducing microstrain in the Pt electrodeposit, simply achieved by introducing polymer into the electrodeposition bath. Pt electrodes (Pt-P1 and Pt-PLA) prepared with the addition of poly-N-(6-aminohexyl)acrylamide (P1) or poly-l-arginine (PLA) exhibit improved ORR activity compared to Pt electrodeposited without polymer addition (Pt-alone) in both acidic and basic environments, with the order of activity being Pt-P1 > Pt-PLA > Pt-alone. Pt-P1 …

ACS catalysis

Machine learning (ML), when used synergistically with atomistic simulations, has recently emerged as a powerful tool for accelerated catalyst discovery. However, the application of these techniques has been limited by the lack of interpretable and transferable ML models. In this work, we propose a curriculum-based training (CBT) philosophy to systematically develop reactive machine learning potentials (rMLPs) for high-throughput screening of zeolite catalysts. Our CBT approach combines several different types of calculations to gradually teach the ML model about the relevant regions of the reactive potential energy surface. The resulting rMLPs are accurate, transferable, and interpretable. We further demonstrate the effectiveness of this approach by exhaustively screening thousands of [CuOCu]2+ sites across hundreds of Cu-zeolites for the industrially relevant methane activation reaction. Specifically, this large …

ACS Catalysis

Supported gold nanoclusters of the formula [Au13(L)5Cl2]3+ where L = N-heterocyclic carbene (NHC) or phosphine ligands are examined in the electrocatalytic CO2 reduction reaction (eCO2RR) in a membrane electrode assembly cell configuration. Gold nanoclusters bearing bisNHC ligands are shown to exhibit improved catalytic performance compared with diphosphine-stabilized nanoclusters after activation at the optimum treatment temperatures. The thermal properties of the nanoclusters are shown to have a significant impact on their catalytic activity. Thermogravimetric analysis, UV–vis absorption spectroscopy, and X-ray photoelectron spectroscopy revealed that thermal treatment of [Au13(diphosphine)5Cl2]3+ nanoclusters results in complete loss of diphosphine ligands while [Au13(bisNHC)5Cl2]3+ nanoclusters show stepwise and partial removal of bisNHC ligands. We propose that the partial removal of …