The dynamic character of the active centers has made it difficult to unravel the reaction path for NH3-assisted selective catalytic reduction (SCR) of nitrogen oxides over Cu-CHA. Herein, we use ...density functional theory calculations to suggest a complete reaction mechanism for low-temperature NH3-SCR. The reaction is found to proceed in a multisite fashion over ammonia-solvated Cu cations Cu(NH3)2 + and Brønsted acid sites. The activation of oxygen and the formation of the key intermediates HONO and H2NNO occur on the Cu sites, whereas the Brønsted acid sites facilitate the decomposition of HONO and H2NNO to N2 and H2O. The activation and reaction of NO is found to proceed via the formation of nitrosonium (NO+) or nitrite (NO2 –) intermediates. These low-temperature mechanisms take the dynamic character of Cu sites into account where oxygen activation requires pairs of Cu(NH3)2 + complexes, whereas HO–NO and H3N–NO coupling may occur on single complexes. The formation and separation of Cu pairs is assisted by NH3 solvation. The complete reaction mechanism is consistent with measured kinetic data and provides a solid basis for future improvements of the low-temperature NH3-SCR reaction.
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Bioinspired structures are promising as improved catalysts for various redox reactions. One example is metal hangman-porphyrines (MHP), which recently have been suggested for oxygen ...reduction/evolution reaction (ORR/OER). The unique properties of the MHPs are attributed to both the hangman scaffold and the C6F5 side groups. Herein, the OER/ORR over various transition metal MHPs is investigated by density functional theory calculations within an electrochemical framework. A comparison of the reaction landscape for MHP, metal porphyrine (MP) and metaltetrafluorophenyloporphyrines (MTFPP), allow for a disentanglement of the different roles of the hangman motif and the side groups. In agreement with experimental studies, it is found that Fe and Co are the best MHP metal centers to catalyze these reactions. We find that the addition of the three-dimensional moiety in the form of hangman scaffold does not break the apparently universal energy relation between *OH and *OOH intermediates. However, the hangman motif is found to stabilize the oxygen intermediate, whereas addition of C6F5 groups reduces the binding energy of all reaction intermediates. Our results indicate that the combination of these two effects allow new design possibilities for macromolecular systems with enhanced catalytic OER/ORR activity.
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Carbon at metal nanoparticles (NPs) plays a fundamental role in heterogeneous catalysis. However, as experimental detection of small amounts of carbon is difficult, in particular when occupying ...subsurface sites, reaction mechanisms involving absorbed carbon are highly debated. Here, we show that the work function (WF) of metal NPs can be used as a measure of carbon adsorption and absorption, which we demonstrate by Kelvin probe force microscopy and density functional theory calculations for (111)-faceted palladium NPs (PdNPs) on graphite. Growth of PdNPs between 150 and 480 °C leads to carbon etching of the graphite steps and carbon absorption into the first subsurface layer below the NP’s facets. This strongly reduces the WF of Pd(111) by up to −1 eV. During a 1 h long postannealing at 650 °C, more carbon is etched from the graphite steps, leading to a carbon precursor structure adsorbed on the NP’s facets, as verified by scanning tunneling microscopy. The carbonaceous structures are replaced by graphene upon further annealing (1 to 2 h), followed by a decrease in the WF by ∼−1.4 eV. Similar phenomena are observed after short-time ethylene decomposition at PdNPs at 650 °C. Apart from subsurface carbon, we suggest that the large WF shifts observed experimentally could be attributed to structural defects on NP’s facets.
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Graphene-encapsulated metal nanoparticles (G@NPs) offer a possibility to observe confined reactions in the nanocontainer formed by the NP’s facets and graphene. However, direct experimental detection ...of adsorbed atomic and molecular species under the graphene cover is still challenging, and the mechanisms of intercalation and adsorption are not well understood. Here, we show that Kelvin probe force microscopy can largely contribute to the understanding of adsorption and desorption at the single NP level, which we exemplify by comparing oxygen adsorption experiments obtained at as-prepared PdNPs and G@PdNPs, both supported on highly oriented pyrolytic graphite and studied under ultrahigh vacuum (UHV) conditions. We show that oxygen adsorption at room temperature occurs at a much higher partial oxygen pressure on G@PdNPs compared to as-prepared PdNPs. Similarly, the removal of oxygen via a reaction with the residual gas of the UHV is slower on the G@PdNPs compared to as-prepared PdNPs. The differences can be explained by a limited facility for reactant and product molecules to enter and desorb from the nanocontainer via the defects of the graphene. Experimental observations are supported by assisting density functional theory calculations.
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Steps Control the Dissociation of CO2 on Cu(100) Hagman, Benjamin; Posada-Borbón, Alvaro; Schaefer, Andreas ...
Journal of the American Chemical Society,
10/2018, Volume:
140, Issue:
40
Journal Article
Peer reviewed
Open access
CO2 reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron ...spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO2 on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO2 dissociation provides an understanding that can be used in the design of future CO2 reduction catalysts.
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One approach to study reaction kinetics over metal nanoparticles is to combine linear scaling relations with kinetic Monte Carlo simulations. This methodology is based on the observation that ...adsorption energies commonly scale linearly with descriptors such as the generalized coordination number and that reaction barriers are related to the adsorption energies via the Brionsted-Evans-Polanyi relation. In this work, the sensitivity of the reaction kinetics on the slopes of the scaling relations is investigated for CO oxidation over Pt-nanoparticles. The obtained trends between the slope and the turnover frequency suggest a modest dependency and that a flat energy landscape with energies corresponding to edge-sites yields a high catalytic activity. We also explore the sensitivity of the O-2 sticking probability on the turnover frequency. This parameter is found to have a minor influence on the kinetics of the studied reaction.
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Density-functional theory computations on a cluster Au144(SR)60 with an icosahedral Au114 core with 30 RS−Au−SR units protecting its surface yield an excellent fit of the structure factor to the ...experimental X-ray scattering structure factor measured earlier for 29 kDa thiolate-protected gold clusters. This cluster has a special combination of atomic and electronic structure that provides explanations for the observed stability and capacitive charging properties with several available oxidation states in electrochemistry and optical absorption extending well into the infrared region.
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The mechanism for N2O formation over CHA and Cu-CHA zeolite catalysts during NH3-SCR is investigated using density functional theory calculations. Direct NH4NO3 decomposition, which is commonly ...regarded as the main source of N2O, is found to be associated with high barriers in the absence of Brønsted acid sites. Although Brønsted acid sites promote NH4NO3 decomposition, it is still a highly activated process. Low-temperature N2O formation is instead found to be connected with an NO + NH3 reaction over Cu-sites. In particular, N2O can be formed from H2NNO with a low barrier over Cu-OOH-Cu complexes, which are proposed intermediates in the catalytic cycle for NH3-SCR over Cu-CHA. This finding provides an explanation for the experimentally observed low-temperature N2O formation and the relation between Cu loading and N2O formation. The proposed mechanisms open up strategies to enhance the selectivity to N2 during NH3-SCR.
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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.
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