First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment

In this paper we investigate by means of first-principles density functional theory calculations the (111) surface of the Ag-Cu alloy under varying conditions of pressure of the surrounding oxygen atmosphere and temperature. This alloy has been recently proposed as a catalyst with improved selectivity for ethylene epoxidation with respect to pure silver, the catalyst commonly used in industrial applications. Here we show that the presence of oxygen leads to copper segregation to the surface. Considering the surface free energy as a function of the surface composition, we construct the convex hull to investigate the stability of various surface structures. By including the dependence of the free surface energy on the oxygen chemical potential, we are able compute the phase diagram of the alloy as a function of temperature, pressure and surface composition. We find that, at temperature and pressure typically used in ethylene epoxidation, a number of structures can be present on the surface of the alloy, including clean Ag(111), thin layers of copper oxide and thick oxide-like structures. These results are consistent with, and help explain, recent experimental results.Comment: 10 pages, 6 figure

Ag-Cu alloy surfaces in an oxidizing environment: a first-principles study

Recent experiments on model catalysts have shown that Ag-Cu alloys have improved selectivity with respect to pure silver for ethylene epoxidation. In this paper we review our first-principles investigations on the (111) surface of this alloy and present new findings on other low index surfaces. We find that, for every surface orientation, the presence of oxygen leads to copper segregation to the surface. Considering the alloy to be in equilibrium with an oxygen atmosphere and accounting for the effect of temperature and pressure, we compute the surface free energy and study the stability of several surface structures. Investigating the dependence of the surface free energy on the surface composition, we construct the phase diagram of the alloy for every surface orientation. Around the temperature, pressure and composition of interest for practical applications, we find that a limited number of structures can be present, including a thin layer of copper oxide on top of the silver surface and copper-free structures. Different surface orientations show a very similar behavior and in particular a single layer with CuO stoichiometry, significantly distorted when compared to a layer of bulk CuO, has a wide range of stability for all orientations. Our results are consistent with, and help explain, recent experimental measurements

Chapter Digi Skills Bsc – Revising Graphic Literacy in Bsc Architectural Design Education through a Software-Based Pedagogic Approach. A Shared Pilot Experience at the Politecnico di Milano

The 43rd UID conference, held in Genova, takes up the theme of ‘Dialogues’ as practice and debate on many fundamental topics in our social life, especially in these complex and not yet resolved times. The city of Genova offers the opportunity to ponder on the value of comparison and on the possibilities for the community, naturally focused on the aspects that concern us, as professors, researchers, disseminators of knowledge, or on all the possibile meanings of the discipline of representation and its dialogue with ‘others’, which we have broadly catalogued in three macro areas: History, Semiotics, Science / Technology. Therefore, “dialogue” as a profitable exchange based on a common language, without which it is impossible to comprehend and understand one another; and the graphic sign that connotes the conference is the precise transcription of this concept: the title ‘translated’ into signs, derived from the visual alphabet designed for the visual identity of the UID since 2017. There are many topics which refer to three macro sessions: - Witnessing (signs and history) - Communicating (signs and semiotics) - Experimenting (signs and sciences) Thanks to the different points of view, an exceptional resource of our disciplinary area, we want to try to outline the prevailing theoretical-operational synergies, the collaborative lines of an instrumental nature, the recent updates of the repertoires of images that attest and nourish the relations among representation, history, semiotics, sciences

Nanocatalysts Unravel the Selective State of Ag

In the present work, we report on a comparative study of model catalysts during ethylene epoxidation reaction under industrially relevant conditions. The catalysts consist of Ag nanoparticles <6 nm and a reference sample ∼100 nm. Combining catalytic data with transmission electron microscopy, thermal desorption spectroscopy, and density functional theory allows us to show that catalytic performance is linked to the oxygen concentration in/on the Ag particles. Isotope experiments using 18O2 and C18O2 are conducted to gain insight into the nature and location of oxygen in/on the Ag nanoparticles. The oxygen species responsible for the CO2 formation and inhibition of the overall catalytic activity are identified, and the abundance of those species is shown to depend strongly on the pre‐treatment and reaction conditions, showing both are critical for effective oxygen management. By comparison with a conventional Ag/α‐Al2O3 catalyst, we demonstrate a low concentration of oxygen in/on Ag leads to the highest selectivity regardless of particle size. However, particle size dependent oxophilicity leads to significantly lower TOFs for the Ag nanoparticles. This study provides fundamental understanding of the performance of supported Ag particles in ethylene epoxidation and offers new strategies to improve performance under industrially relevant conditions

Water Oxidation by Ru-Polyoxometalate Catalysts: Overpotential Dependency on the Number and Charge of the Metal Centers

Water oxidation is efficiently catalyzed by several Ru-based polyoxometalate (POM) molecular catalysts differing in the number, local atomistic environment and oxidation state of the Ru sites. We employ density functional theory calculations to rationalize the dependency of the reaction overpotential on the main structural and electronic molecular properties. In particular, we compare the thermodynamics of the water oxidation cycle for single-site Ru-POM and multiple-site Ru4-POM complexes. For the Ru-POM case, we also investigate the reaction free energy as a function of the Ru oxidation state. We find that the overpotential of these molecular catalysts is primarily determined by the oxidation state of the metal center and is minimum for Ru(IV). In solution, the number of active sites is shown to play a minor role on the reaction energetics. The results are rationalized and discussed in terms of the local structure around the active sites and of the electrostatic screening due to the molecular structure or the solvent

Mechanistic insights into CO2 activation via reverse water - Gas shift on metal surfaces

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Water Oxidation by Ru-Polyoxometalate Catalysts: Overpotential Dependency on the Number and Charge of the Metal Centers

Water oxidation is efficiently catalyzed by several Ru-based polyoxometalate (POM) molecular catalysts differing in the number, local atomistic environment and oxidation state of the Ru sites. We employ density functional theory calculations to rationalize the dependency of the reaction overpotential on the main structural and electronic molecular properties. In particular, we compare the thermodynamics of the water oxidation cycle for single-site Ru-POM and multiple-site Ru4-POM complexes. For the Ru-POM case, we also investigate the reaction free energy as a function of the Ru oxidation state. We find that the overpotential of these molecular catalysts is primarily determined by the oxidation state of the metal center and is minimum for Ru(IV). In solution, the number of active sites is shown to play a minor role on the reaction energetics. The results are rationalized and discussed in terms of the local structure around the active sites and of the electrostatic screening due to the molecular structure or the solvent

A first principles study of water oxidation catalyzed by a tetraruthenium-oxo core embedded in polyoxometalate ligands

We present a computational study addressing the catalytic cycle of a recently-synthesized all-inorganic homogeneous catalyst capable to promote water oxidation with low overpotential and high turnover frequency [Sartorel et al., J. Am. Chem. Soc., 2008, 130, 5006; Geletii et al., Angew. Chem., Int. Ed., 2008, 47, 3896]. This catalyst consists of a tetraruthenium-oxo core [Ru(4)O(4)(OH)(2)center dot(H(2)O)(4)](6+) capped by two polyoxometalate [SiW(10)O(36)](8-) units. The reaction mechanism underpinning its efficiency is currently under debate. We study a reaction cycle involving four consecutive proton-coupled electron transfer (PCET) processes that successively oxidize the four Ru(IV)-H(2)O units of the initial state (S(0)) to the four Ru(V)-OH centers of the activated intermediate (S(4)). The energetics of these electrochemical processes as well as the structural and electronic properties of the reaction intermediates are studied with ab initio Density Functional Theory (DFT) calculations. After characterizing these reaction intermediates in the gas phase, we show that the solvated tetraruthenate core undergoes a solvent-induced structural distortion that brings the predicted molecular geometry to excellent agreement with the experimental X-ray diffraction data. The calculated electronic properties of the catalyst are instead weakly dependent on the presence of the solvent. The frontier orbitals of the initial state as well as the electronic states involved in the PCET steps are shown to be localized on the tetraruthenium-oxo core. The reaction thermodynamics predicted for the intermediate reaction steps is in good agreement with the available cyclic voltammetry measurements up to S(3), but the calculated free energy difference between the initial and the activated state (S(0)/S(4)) turns out to be significantly lower than the thermodynamic limit for water oxidation. Since the oxidizing power of the S(0)/S(4) couple is not sufficient to split water, we suggest that promoting this reaction would require cycling between higher oxidation states.We present a computational study addressing the catalytic cycle of a recently-synthesized all-inorganic homogeneous catalyst capable to promote water oxidation with low overpotential and high turnover frequency [Sartorel et al., J. Am. Chem. Soc., 2008, 130, 5006; Geletii et al., Angew. Chem., Int. Ed., 2008, 47, 3896]. This catalyst consists of a tetraruthenium-oxo core [Ru 4O4(OH)2\ub7(H2O) 4]6+capped by two polyoxometalate [SiW10O 36]8- units. The reaction mechanism underpinning its efficiency is currently under debate. We study a reaction cycle involving four consecutive proton-coupled electron transfer (PCET) processes that successively oxidize the four RuIV-H2O units of the initial state (S0) to the four RuV-OH centers of the activated intermediate (S4). The energetics of these electrochemical processes as well as the structural and electronic properties of the reaction intermediates are studied with ab initio Density Functional Theory (DFT) calculations. After characterizing these reaction intermediates in the gas phase, we show that the solvated tetraruthenate core undergoes a solvent-induced structural distortion that brings the predicted molecular geometry to excellent agreement with the experimental X-ray diffraction data. The calculated electronic properties of the catalyst are instead weakly dependent on the presence of the solvent. The frontier orbitals of the initial state as well as the electronic states involved in the PCET steps are shown to be localized on the tetraruthenium-oxo core. The reaction thermodynamics predicted for the intermediate reaction steps is in good agreement with the available cyclic voltammetry measurements up to S3, but the calculated free energy difference between the initial and the activated state (S0/S 4) turns out to be significantly lower than the thermodynamic limit for water oxidation. Since the oxidizing power of the S0/S 4 couple is not sufficient to split water, we suggest that promoting this reaction would require cycling between higher oxidation states. \ua9 2011 the Owner Societies

Rationalizing the Relation between Adlayer Structure and Observed Kinetics in Catalysis

Relating the kinetic behavior of catalytic reactions with adsorbate overlayer structure is a long-standing challenge in catalysis. Even for simple systems such as CO oxidation on Pd(111), recent studies have observed rich behavior. In particular, titration experiments by Kondoh and co-workers on this system (<i>J. Chem. Phys.</i> <b>2006</b>, <i>124</i>, 224712), demonstrated first-order reaction kinetics with respect to oxygen coverage at 190 K. Yet, the observed kinetics appeared as half-order at 320 K, a phenomenon that was attributed to island formation. We apply a theoretical approach to rationalize these observations, using first-principles kinetic Monte Carlo simulations. We employ an analysis that decomposes the overall order into two contributions: one pertaining to the adlayer structure and one quantifying coverage effects on the elementary events’ rates. It is shown that at both temperatures, the layer appears as almost well-mixed, whereas coverage effects due to adsorbate lateral interactions result in the distinct reaction orders