Heterogeneous catalysis is an enabling technology that utilises transition metal nanoparticles (NPs) supported on oxides to promote chemical reactions. Structural mismatch at the NP-support interface ...generates lattice strain that could affect catalytic properties. However, detailed knowledge about strain in supported NPs remains elusive. We experimentally measure the strain at interfaces, surfaces and defects in Pt NPs supported on alumina and ceria with atomic resolution using high-precision scanning transmission electron microscopy. The largest strains are observed at the interfaces and are predominantly compressive. Atomic models of Pt NPs with experimentally measured strain distributions are used for first-principles kinetic Monte Carlo simulations of the CO oxidation reaction. The presence of only a fraction of strained surface atoms is found to affect the turnover frequency. These results provide a quantitative understanding of the relationship between strain and catalytic function and demonstrate that strain engineering can potentially be used for catalyst design.
Density functional theory is used to investigate CO oxidation over an ultrathin MgO film supported on Ag(100). O2 is found to be activated on MgO/Ag(100) whereas CO is only weakly bonded to the ...surface. These adsorption properties together with a low activation barrier render the MgO/Ag system an efficient catalyst for CO oxidation at low temperatures. As the predicted mechanism is general in nature, the result is suggested to have implications for a wide range of oxidation reactions.
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•A phase diagram for NH3 solvated Cu-ions is presented.•Direct O2 dissociation is possible on pairs of Cu(NH3)2+ but not on single Cu(NH3)2+.•NO promotes O2 dissociation.•Nitrates may ...be a part of the NH3-SCR reaction.•Entropy changes during reactions in CHA are small.
Cu-CHA materials are efficient catalysts for NH3–SCR of NOx in oxygen excess. A crucial step in the reaction is oxygen (O2) activation, which still is not well understood. Density functional theory calculations in combination with ab initio thermodynamics and molecular dynamics are here used to study O2 dissociation on Cu(NH3)2+ species, which are present under NH3–SCR conditions. Direct dissociation of O2 is found to be facile over a pair of Cu(NH3)2+ complexes whereas dissociation on a single Cu(NH3)2+ species is unlikely due to a high activation energy. The presence of NO promotes oxygen dissociation on both single and pairs of Cu(NH3)2+ complexes. Nitrites and nitrates are easily formed as O2 dissociates, and NO adsorption over nitrates leads to facile formation of NO2. The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH3–SCR.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Density functional theory calculations are used to investigate the catalytic hydrogenation of ethylene to ethane over a wide range of transition metal (TM) surfaces. Assuming the Horuiti–Polanyi ...mechanism, the enthalpies of adsorption, surface diffusion, and hydrogenation barriers are examined over close-packed surfaces of Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Special attention is given to the effects of ethylene and hydrogen coverage on the reaction pathway and activation energies. The previously suggested importance of the balance between di-σ and π adsorption modes is reinvestigated, and most metals are found to exhibit a preference for the π state. Hydrogen coverage is found to control the reactant stability and promote a surface distortion which facilitates the hydrogen addition reaction. For all TMs, the calculated activation energies are low and span a narrow range.
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Alkanethiolate monolayers on gold are important both for applications in nanoscience as well as fundamental studies of adsorption and self-assembly at metal surfaces. While considerable experimental ...effort has been put into understanding the phase diagram of these systems, theoretical work based on density functional theory (DFT) has long been hampered by the inability of conventional exchange-correlation functionals to describe dispersive interactions. In this work, we combine dispersion-corrected DFT calculations using the new vdW-DF-CX functional with the ab initio thermodynamics method to study the stability of dense standing-up and low-coverage lying-down phases on Au(111). We demonstrate that the lying-down phase has a thermodynamic region of stability starting from thiolates with alkyl chains consisting of n ≈ 3 methylene units. This phase emerges as a consequence of a competition between dispersive chain–chain and chain–substrate interactions, where the strength of the latter varies more strongly with n. A phase diagram is derived under ultrahigh-vacuum conditions, detailing the phase transition temperatures of the system as a function of the chain length. The present work illustrates that accurate ab initio modeling of dispersive interactions is both feasible and essential for describing self-assembled monolayers.
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Time-resolved in situ, energy-dispersive X-ray absorption spectroscopy and mass spectrometry are used to correlate changes in the chemical state of alumina- and ceria-supported palladium ...nanoparticles with changes in activity and selectivity for methane oxidation. Specifically, modulation excitation spectroscopy experiments are carried out by periodically cycling between net-reducing and net-oxidizing reaction conditions. The XANES and EXAFS data show that the palladium nanoparticles are readily bulk-oxidized when exposed to oxygen, forming a PdO-like phase, and reduced back to a reduced (metal) phase when oxygen is removed from the feed. The difference between the two support materials is most noticeable at the switches between net-oxidizing and net-reducing reaction conditions. Here, a brief reduction in conversion is observed for the alumina-supported catalyst, but for the ceria-supported catalyst, this reduction in conversion is minor or not observed at all. This difference is attributed to differences in the oxidation kinetics and the oxygen storage capability of ceria.
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Methane dissociation and subsequent formation of water and carbon-dioxide over Pd and Pt are investigated with density functional theory calculations and microkinetic modeling. Adsorption energies ...for reaction intermediates and activation barriers for CH4 dissociation and water formation are calculated for the (111), (100), (211) and (321) facets. The dissociative adsorption of methane is found to be the rate determining step on all considered facets. The results show that Pt has higher catalytic activity than Pd and that the (100) surface is the most active facet at moderate temperatures for both Pd and Pt. At low temperatures, the reaction is limited, in particular on Pd(100), by poisoning of OH-groups.
•Complete methane oxidation is studied with DFT and microkinetic modeling.•Dissociative adsorption of methane is the rate determining step on all studied facets.•Pt has higher catalytic activity than Pd.•Step sites are O and OH poisoned at typical reaction conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A first-principles microkinetic model is developed to investigate the low-temperature ammonia-assisted selective catalytic reduction (NH3-SCR) of NO over Cu-chabazite (Cu-CHA). The reaction proceeds ...over NH3-solvated Cu sites by the formation of H2NNO and HONO, which decompose to N2 and H2O over Brønsted acid sites. Nonselective N2O formation is considered by H2NNO decomposition over the Cu sites. The adsorption of NH3 at oxidized Cu sites is found to inhibit the reaction at low temperatures by hindering NO adsorption. For the reactions, we find positive reaction orders with respect to NO and O2, whereas the reaction order with respect to NH3 is negative. The reaction orders and the obtained apparent activation energy are in good agreement with experimental data. A degree of rate control analysis shows that NH3-SCR over a pair of Cu(NH3)2 + is mainly controlled by NO adsorption below 200 °C, whereas the formation of HONO and H2NNO becomes controlling at higher temperatures. The successful formulation of a first-principles microkinetic model for NH3-SCR rationalizes previous phenomenological models and links the kinetic behavior with materials properties, which results in unprecedented insights into the function of Cu-CHA catalysts for NH3-SCR.
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A general challenge in density functional theory calculations is to simultaneously account for different types of bonds. One such example is reactions in zeolites where both van der Waals and ...chemical bonds should be described accurately. Here, we use different exchange-correlation functionals to explore O2 dissociation over pairs of Cu(NH3)2+ complexes in Cu-Chabazite. This is an important part of selective catalytic reduction of NOx using NH3 as a reducing agent. The investigated functionals are PBE, PBE+U, PBE+D, PBE+U+D, PBE-cx, BEEF and HSE06+D. We find that the potential energy landscape for O2 activation and dissociation depends critically on the choice of functional. However, the van der Waals contributions are similarly described by the functionals accounting for this interaction. The discrepancies in the potential energy surface are instead related to different descriptions of the Cu-O chemical bond. By investigating the electronic, structural and energetic properties of reference systems including bulk copper oxides and (Cu2O2)2+ enzymatic crystals, we find that the PBE+U approach together with van der Waals corrections provides a reasonable simultaneous accuracy of the different bonds in the systems.
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