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Understanding the structures of catalysts under realistic conditions with atomic precision is crucial to design better materials for challenging transformations. Under reducing conditions, certain reducible supports migrate onto supported metallic particles and create strong metal-support states that drastically change the reactivity of the systems. The details of this process are still unclear and preclude its thorough exploitation. Here, we report an atomic description of a palladium/titania (Pd/TiO2) system by combining state-of-the-art in situ transmission electron microscopy and density functional theory (DFT) calculations with structurally defined materials, in which we visualize the formation of the overlayers at the atomic scale under atmospheric pressure and high temperature. We show that an amorphous reduced titania layer is formed at low temperatures, and that crystallization of the layer into either mono- or bilayer structures is dictated by the reaction environment and predicted by theory. Furthermore, it occurs in combination with a dramatic reshaping of the metallic surface facets. [ABSTRACT FROM AUTHOR]

N-phosphino-functionalizedN-heterocyclic carbene (NHCP) ligandshave been evaluated as potential supporting ligands in ruthenium-catalyzedolefin metathesis. Initial density functional theory (DFT) calculationssuggested that these NHCP ligands may allow access to neutral 14 valenceelectron (VE) speciesequivalents of the active 14 VE speciesformed by phosphine dissociation from Grubbs II precatalystsviafacile decoordination of the NHCP phosphino donor of the strainedfour-membered [RuPNC] chelate systems. Their attempted synthesis fromNHCPs and Grubbs-type Ru carbenes revealed addition of an NHCP donoratom (P or C) to the alkylidene fragment, forming a new C–Por C–C bond in five-membered chelate structures. DFT investigationsshowed that these reactions are controlled kinetically and must notbe neglected as important possible deactivation routes in olefin metathesis. [ABSTRACT FROM AUTHOR]

The mechanistic course of the amination of alcohols with ammonia catalyzed by a structurally modified congener of Milstein's well-defined acridine-based PNP-pincer Ru complex has been investigated both experimentally and by DFT calculations. Several key Ru intermediates have been isolated and characterized. The detailed analysis of a series of possible catalytic pathways (e.g., with and without metal-ligand cooperation, inner-and outer-sphere mechanisms) leads us to conclude that the most favorable pathway for this catalyst does not require metal-ligand cooperation. [ABSTRACT FROM AUTHOR]

Methyl iodide induces the stoichiometriccleavage of nickelalactones, which are key intermediates in the nickel-mediatedreaction of CO2and alkenes to acrylates. Herein, we proposea modified and extended mechanism for this reaction on the basis oftheoretical and experimental investigations for the bidentate P ligand1,2-bis(di-tert-butylphosphino)ethane (dtbpe). Thecalculated elementary steps agree well with experimental findings:reaction barriers are reasonable and explain the facile liberationof acrylate from a nickelalactone by methyl iodide. We were able toisolate reactive intermediates and to verify the existence of proposedreaction pathways. Additionally, we have identified unproductive pathwaysleading to byproducts (e.g., propionates and catalytically inactiveorganometallic species). Although those side reactions can be suppressedto a certain extent, the strong binding of acrylate to nickel preventsa catalytic reaction, at least for the chosen ligand. [ABSTRACT FROM AUTHOR]

For more than three decades the catalytic synthesis of acrylates from the cheap and abundantly available C1 building block carbon dioxide and alkenes has been an unsolved problem in catalysis research, both in academia and industry. Herein, we describe a homogeneous catalyst based on nickel that permits the catalytic synthesis of the industrially highly relevant acrylate sodium acrylate from CO2, ethylene, and a base, as demonstrated, at this stage, by a turnover number of greater than 10 with respect to the metal. [ABSTRACT FROM AUTHOR]

The diffusion barriers of organic molecules through the pores of the zeolite H-SSZ-13 are investigated using periodic DFT calculations at the PBE-D3 level of theory. We calculate the diffusion barriers relative to the most stable adsorbed state on the acid site of the zeolite and find that barriers range from ~70 kJ/mol for smaller molecules such as ethene and propene up to 350 kJ/mol for durene, the largest molecule investigated here. For larger molecules, the diffusion barriers can be correlated with the vdW-radius of the diffusing molecules. For smaller molecules, which generally have smaller diffusion barriers, there is no clear correlation between the barrier and the vdW-radius. Here, the strength of adsorption at the acid site is also extremely important in determining the diffusion barrier. [ABSTRACT FROM AUTHOR]

Water influences critically the kinetics of the autocatalytic conversion of methanol to hydrocarbons in acid zeolites. At very low conversions but otherwise typical reaction conditions, the initiation of the reaction is delayed in presence of H2O. In absence of hydrocarbons, the main reactions are the methanol and dimethyl ether (DME) interconversion and the formation of a C1 reactive mixture--which in turn initiates the formation of first hydrocarbons in the zeolite pores. We conclude that the dominant reactions for the formation of a reactive C1 pool at this stage involve hydrogen transfer from both MeOH and DME to surface methoxy groups, leading to methane and formaldehyde in a 1:1 stoichiometry. While formaldehyde reacts further to other C1 intermediates and initiates the formation of first C-C bonds, CH4 is not reacting. The hydride transfer to methoxy groups is the rate-determining step in the initiation of the conversion of methanol and DME to hydrocarbons. Thus, CH4 formation rates at very low conversions, i.e., in the initiation stage before autocatalysis starts, are used to gauge the formation rates of first hydrocarbons. Kinetics, in good agreement with theoretical calculations, show surprisingly that hydrogen transfer from DME to methoxy species is 10 times faster than hydrogen transfer from methanol. This difference in reactivity causes the observed faster formation of hydrocarbons in dry feeds, when the concentration of methanol is lower than in presence of water. Importantly, the kinetic analysis of CH4 formation rates provides a unique quantitative parameter to characterize the activity of catalysts in the methanol-to-hydrocarbon process. [ABSTRACT FROM AUTHOR]

This study evaluates the finite size effect on the oxygen adsorption energy of coinage metal (Cu, Ag and Au) cuboctahedral nanoparticles in the size range of 13 to 1415 atoms (0.7–3.5 nm in diameter). Trends in particle size effects are well described with single point calculations, in which the metal atoms are frozen in their bulk position and the oxygen atom is added in a location determined from periodic surface calculations. This is shown explicitly for Cu nanoparticles, for which full geometry optimization only leads to a constant offset between relaxed and unrelaxed adsorption energies that is independent of particle size. With increasing cluster size, the adsorption energy converges systematically to the limit of the (211) extended surface. The 55-atomic cluster is an outlier for all of the coinage metals and all three materials show similar behavior with respect to particle size. [ABSTRACT FROM AUTHOR]

We studied the adsorption of CO on Pd/Pt nanoparticles (NPs) with varying compositions using polarization-dependent Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) and theoretical calculations (DFT). We prepared PtPd alloy NPs via physical vapor codeposition on α-Al 2 O 3 (0001) supports. Our morphological and structural characterization by scanning electron microscopy and grazing incidence X-ray diffraction revealed well-defined, epitaxial NPs. We used CO as a probe molecule to identify the particles' surface active sites. Polarization-dependent FT-IRRAS enabled us to distinguish CO adsorption on top and side facets of the NPs. The role of the Pd/Pt alloy ratio on CO adsorption was investigated by comparing the experimental CO stretching band frequency for different alloy arrangements to the results for pure Pd and Pt NPs. Moreover, we studied the influence of hydrogen adsorption on the NP surface composition. We determined the dependence of the IR bands on the local atomic arrangement via DFT calculations, revealing that both bulk alloy composition and neighboring atoms influence the wavenumber of the bands.

The stability of hydroxylated terminations of the 0001 surface of α-Fe 2 O 3 (hematite) is investigated computationally using PBE + U calculations with dispersion corrections. Hydroxylated surfaces with low OH concentrations are found to be most stable in a range of the chemical potential of water of -0.95 eV > μ H 2 O > -2.22 eV. These surfaces can be described as isolated Fe(OH) 3 groups adsorbed on the dry hematite surface and are predicted to be the exposed termination of the 0001 surface in a wide range of relevant experimental conditions. Most investigated reduced surfaces, containing Fe in oxidation state +2, are only stable in a range of the chemical potential of oxygen μ O < -2.44 eV, where bulk hematite is less than magnetite. The only reduced surface stable at a higher μ O is derived from the most stable nonreduced hydroxylated surfaces by removing a single OH group per unit cell., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Stable catalysts are essential to address energy and environmental challenges, especially for applications in harsh environments (for example, high temperature, oxidizing atmosphere and steam). In such conditions, supported metal catalysts deactivate due to sintering-a process where initially small nanoparticles grow into larger ones with reduced active surface area-but strategies to stabilize them can lead to decreased performance. Here we report stable catalysts prepared through the encapsulation of platinum nanoparticles inside an alumina framework, which was formed by depositing an alumina precursor within a separately prepared porous organic framework impregnated with platinum nanoparticles. These catalysts do not sinter at 800 °C in the presence of oxygen and steam, conditions in which conventional catalysts sinter to a large extent, while showing similar reaction rates. Extending this approach to Pd-Pt bimetallic catalysts led to the small particle size being maintained at temperatures as high as 1,100 °C in air and 10% steam. This strategy can be broadly applied to other metal and metal oxides for applications where sintering is a major cause of material deactivation., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Converting CO 2 into value-added chemicals and fuels, such as methanol, is a promising approach to limit the environmental impact of human activities. Conventional methanol synthesis catalysts have shown limited efficiency and poor stability in a CO 2 /H 2 mixture. To design improved catalysts, crucial for the effective utilization of CO 2 , an in-depth understanding of the active sites and reaction mechanism is desired. The catalytic performance of a series of carbon-supported Cu catalysts, with Cu particle sizes in the range of 5 to 20 nm, was evaluated under industrially relevant temperature and pressure, i.e. 260 °C and 40 bar(g). The CO 2 hydrogenation reaction exhibited clear particle size effects up to 13 nm particles, with small nanoparticles having the lower activity, but higher methanol selectivity. MeOH and CO formation showed a different size-dependence. The TOF CO increased from 1.9 × 10 -3 s -1 to 9.4 × 10 -3 s -1 with Cu size increasing from 5 nm to 20 nm, while the TOF MeOH was size-independent (8.4 × 10 -4 s -1 on average). The apparent activation energies for MeOH and CO formation were size-independent with values of 63 ± 7 kJ mol -1 and 118 ± 6 kJ mol -1 , respectively. Hence the size dependence was ascribed to a decrease in the fraction of active sites suitable for CO formation with decreasing particle size. Theoretical models and DFT calculations showed that the origin of the particle size effect is most likely related to the differences in formate coverage for different Cu facets whose abundancy depends on particle size. Hence, the CO 2 hydrogenation reaction is intrinsically sensitive to the Cu particle size.

Adsorption processes are often governed by weak interactions for which the estimation of entropy contributions by means of the harmonic approximation is prone to be inaccurate. Thermodynamic integration (TI) from the harmonic to the fully interacting system (λ-path integration) can be used to compute anharmonic corrections. Here, we combine TI with (curvilinear) internal coordinates in periodic systems to make the formalism available in computational studies. Our implementation of ab initio molecular dynamics in VASP is independent of the reaction path and can be thus applied to study adsorption processes relative to the gas phase and does hence provide a useful tool for computational catalysis. We discuss the application of the approach on three model systems for which exact semianalytical solutions exist and illustrate and quantify the importance of anharmonic vibrations, hindered rotations, and hindered translations (dissociation). Eventually, we apply the method to study the adsorption of small adsorbates in a zeolite (H-SSZ-13).

In the high-temperature environments needed to perform catalytic processes, supported precious metal catalysts severely lose their activity over time. Even brief exposure to high temperatures can lead to significant losses in activity, which forces manufacturers to use large amounts of noble metals to ensure effective catalyst function for a required lifetime. Generally, loss of catalytic activity is attributed to nanoparticle sintering, or processes by which larger particles grow at the expense of smaller ones. Here, by independently controlling particle size and particle loading using colloidal nanocrystals, we reveal the opposite process as a novel deactivation mechanism: nanoparticles rapidly lose activity by high-temperature nanoparticle decomposition into inactive single atoms. This deactivation route is remarkably fast, leading to severe loss of activity in as little as ten minutes. Importantly, this deactivation pathway is strongly dependent on particle density and concentration of support defect sites. A quantitative statistical model explains how for certain reactions, higher particle densities can lead to more stable catalysts., Competing Interests: Competing Interests. The authors declare no competing interests.

For more than three decades the catalytic synthesis of acrylates from the cheap and abundantly available C(1) building block carbon dioxide and alkenes has been an unsolved problem in catalysis research, both in academia and industry. Herein, we describe a homogeneous catalyst based on nickel that permits the catalytic synthesis of the industrially highly relevant acrylate sodium acrylate from CO(2), ethylene, and a base, as demonstrated, at this stage, by a turnover number of greater than 10 with respect to the metal., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)