Scar functions in the Bunimovich Stadium billiard

In the context of the semiclassical theory of short periodic orbits, scar functions play a crucial role. These wavefunctions live in the neighbourhood of the trajectories, resembling the hyperbolic structure of the phase space in their immediate vicinity. This property makes them extremely suitable for investigating chaotic eigenfunctions. On the other hand, for all practical purposes reductions to Poincare sections become essential. Here we give a detailed explanation of resonances and scar functions construction in the Bunimovich stadium billiard and the corresponding reduction to the boundary. Moreover, we develop a method that takes into account the departure of the unstable and stable manifolds from the linear regime. This new feature extends the validity of the expressions.Comment: 21 pages, 10 figure

Highly Active and Stable Ni/La-Doped Ceria Material for Catalytic CO2Reduction by Reverse Water-Gas Shift Reaction

[EN] The design of an active, effective, and economically viable catalyst for CO2 conversion into value-added products is crucial in the fight against global warming and energy demand. We have developed very efficient catalysts for reverse water-gas shift (rWGS) reaction. Specific conditions of the synthesis by combustion allow the obtention of macroporous materials based on nanosized Ni particles supported on a mixed oxide of high purity and crystallinity. Here, we show that Ni/La-doped CeO2 catalysts─with the "right"Ni and La proportions─have an unprecedented catalytic performance per unit mass of catalyst for the rWGS reaction as the first step toward CO2 valorization. Correlations between physicochemical properties and catalytic activity, obtained using a combination of different techniques such as X-ray and neutron powder diffraction, Raman spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, electron microscopy, and catalytic testing, point out to optimum values for the Ni loading and the La proportion. Density functional theory calculations of elementary steps of the reaction on model Ni/ceria catalysts aid toward the microscopic understanding of the nature of the active sites. This finding offers a fundamental basis for developing economical catalysts that can be effectively used for CO2 reduction with hydrogen. A catalyst based on Ni0.07/(Ce0.9La0.1Ox)0.93 shows a CO production of 58 × 10-5 molCO·gcat-1·s-1 (700 °C, H2/CO2 = 2; selectivity to CO > 99.5), being stable for 100 h under continuous reaction.We acknowledge the financial support of the Spanish Ministry of Science and Innovation (PID2021-123287OB-I00, PID2021-122477-OB-I00, PID2021-128915NB-I00, and RTI2018-101604-B-I00) and of the CSIC through the i-LINK 2021 program (LINKA20408). Financial support has also been received from AEI-MINECO/FEDER (Nympha Project, PID2019-106315RB-I00), “Comunidad de Madrid” regional government, and the European Structural Funds (FotoArt-CM project, S2018/NMT-4367). Authors also acknowledge financial support from the grant PLEC2021-007906 funded by MCIN/AEI/10.13039/501100011033 and the “European Union NextGenerationEU/PRTR”. We are grateful to ILL (France) for making all facilities available. This project also received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 832121. Computer time provided by the RES (Red Española de Supercomputación) resources at the MareNostrum 4 (BSC, Barcelona) node and the DECI resources at the BEM cluster of the WCSS based in Poland with the support from PRACE aislb is acknowledged

Insights into the nature of formate species in the decomposition and reaction of methanol over cerium oxide surfaces: a combined infrared spectroscopy and density functional theory study

Trabajo presentado en el 6th Workshop on Novel Methods for Electronic Structure Calculations, celebrado en La Plata (Argentina) del 12 al 14 de noviembre de 2015.Formation of formate species on oxide surfaces plays a role in reactions for hydrogen production such as the water-gas shift, and the steam reforming of alcohols. It has been suggested that bridge formates are the most common and stable configuration on metal oxides. Ceria-based catalysts are important for these reactions where ceria is a “non-innocent” support. In this work, the nature of the formate species that are formed during decomposition and reaction of methanol on ceria surfaces have been studied using a combination of infrared temperature programmed surface reaction (TPSR-IR) on a real powder catalyst support, and density functional theory (DFT) together with staistical thermodynamics for model CeO2 (111) surfaces. The influence of surface oxygen vacancies, hydroxyl groups, and water has been considered. Three different formate species have been identified (450 − 550 K). Initially, formates are adsorbed on the oxidized surface that is gradually hydroxylated by the release of hydrogen from methoxy groups (> 500 K), which leads to a partially reduced surface. On the former one kind of species is observed, whereas on the latter the other two kinds appeared. We provide computational evidence that the bonding is only initially of the bridge type, but becomes of the monodentate type, as the surface concentration of hydroxyl groups rises. The calculated frequencies of the O−C−O symmetric and asymmetric stretching modes for the three structures are in good agreement with those experimentally observed. The existence of monodentate species is discussed in terms of a stabilizing effect of hydrogen bonds. The combined experimental and theoretical results on real and model systems, respectively, thus provide important insights on the reaction of methanol on ceria surfaces.Peer Reviewe

Insights into the Nature of Formate Species in the Decomposition and Reaction of Methanol over Cerium Oxide Surfaces: A Combined Infrared Spectroscopy and Density Functional Theory Study

Formation of formate species on oxide surfaces plays a role in reactions for hydrogen production such as the water–gas shift and the steam-reforming of alcohols. It has been suggested that bridge formates are the most common and stable configuration on metal oxides. Ceria-based catalysts are important for these reactions where ceria is a “non-innocent” support. In this work, the nature of the formate species that are formed during decomposition and reaction of methanol on ceria surfaces have been studied using a combination of infrared temperature-programmed surface reaction (TPSR-IR) on a real powder catalyst support and density functional theory (DFT) together with statistical thermodynamics for model CeO2(111) surfaces. The influence of surface oxygen vacancies, hydroxyl groups, and water has been considered. Three different formate species have been identified (450–550 K). Initially, formates are adsorbed on the oxidized surface that is gradually hydroxylated by the release of hydrogen from methoxy groups (>500 K), which leads to a partially reduced surface. On the former, one kind of species is observed, whereas on the latter, the other two kinds appeared. We provide computational evidence that the bonding is only initially of the bridge type but becomes of the monodentate type as the surface concentration of hydroxyl groups rises. The calculated frequencies of the O–C–O symmetric and asymmetric stretching modes for the three structures are in good agreement with those experimentally observed. The existence of monodentate species is discussed in terms of a stabilizing effect of hydrogen bonds. The combined experimental and theoretical results on real and model systems, respectively, thus provide important insights on the reaction of methanol on ceria surfaces.M.V.B. and A.L.B. are grateful for the financial suppport of the National Agency for the Promotion of Science and Technology (ANPCyT, grant PICT 2012-1280) and the Universidad Nacional del Litoral (grant CAI+D 2011 UNL, PI 501 20110100311). M.V.G.-P. thanks MINECO (CTQ2012-32928) for financial support and BIFI-ZCAM (Universidad de Zaragoza) for computer resources, technical expertise, and assistance. P.G.L. and H.F.B. acknowledge CONICET, (Project PIP 0667) and UNR (Project PID ING235) for financial support and the CCT-Rosario Computational Center, member of the High Performance Computing National System (SNCAD, MincyT-Argentina) for allocation of computer time. The COST action CM1104 is gratefully acknowledged.Peer Reviewe

Monoxide carbon and formate species frequency shift on fully oxidized and partially reduced TiO2 and CeO2 surfaces

Trabajo presentado en el 5th Workshop on Novel Methods for Electronic Structure Calculations, celebrado en Montevideo (Uruguay) del 3 al 5 de diciembre de 2013.Understanding the surface chemistry of oxide materials holds great promise for impacting countless technologies that will be critical for our energy and environmental future. Oxide surface chemistry is also crucial for making and using catalysts for the manufacture of chemicals and for pollution cleanup, and for the production and use of fuel cells(1-2), solar fuel photocatalysts(3-4) among others. In this presentation we show the studies of monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces and the stability of the formate species (HCOO-) in the surface of CeO2 (111) partially hydroxilated. In the first case the adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. For example, at low CO coverages we find, in good agreement with experimental results an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm−1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. On the other hand, formate species are considered as potential intermediary molecules in the reaction of water gas shift (WGS) as well as the decomposition of formic acid and methanol, among others. One of the important aspects in the elucidation of these reactions is the identification of the structure of the formate imtermediate: unidentate, bidentate or bridge in the presence of partially hydroxylated surface.Peer Reviewe

Study of formate species in the CH3OH/CeO2 reaction: combining IR spectroscopy and statistical thermodynamics techniques

Trabajo presentado en el Meeting COST Action CM1104-Reducible oxide chemistry, structure and functions, celebrado en Barcelona (España) del 12 al 14 de noviembre de 2014.Formate species (HCOO-) have been suggested as intermediates or spectators in chemical reactions of industrial relevance, such as the water-gas-shift and the methanol steam reforming reaction [1]. Several catalysts, some of them based on ceria, have been proposed to improve the performance of these reactions. One of the main reasons of choosing ceria has been its intrinsic redox properties, but only few works describe the formate formation under the oxidizing/reducing environment, usually modulated by the reaction itself. In this work, we have studied the stability of formate groups in the CH3OH/CeO2 system. In situ transmission infrared spectroscopy was used to study the temperature-programmed surface reaction (TPSR-IR) of adsorbed methanol. Three types of formates were observed based on the OCO stretching frequencies [(OCO)] (Figure 1). Type III formate (as = 1550 cm-1) was the first one obtained over 450 K, where no Ce3+ was detected. However, after increasing the temperature, types I and II (as = 1580 and 1561 cm-1) species emerged accompanied by the appearance of surface Ce3+ species. The phase diagram of formate structures in contact with a gas environment of O2 and H2, to simulate oxidizing/reducing conditions, was calculated using density-functional theory and statistical calculations. The formate binding structure was found to crucially depend on temperature and partial pressures of the reactants in the gas phase. In the absence of methanol the CeO2, CeO2-x, O2/CeO2 and several OH-coverages phases coexist. As methanol pressure increases, we found a bridge formate with two next neighbors hydroxyls adsorbed (NN-OHads) and no Ce3+, named Brg-A (1535 cm-1). At higher methanol chemical potential (Figure 2), a monodentate formate with three NN-OHads and a bridge state with one NN-OHads, named Mono-B and Brg-B, respectively, appeared together with the formation of Ce3+ species (1564 and 1540 cm-1, respectively). We conclude that the experimentally observed formate species correspond to those monodentate and brigde types.Peer Reviewe

Stability of formates species on CeO2(111): Influence of the oxidizing or reducing environment

Trabajo presentado en el VI Encuentro de Física y Química de Superficies, celebrado en Río Cuarto (Argentina) del 08 al 10 de septiembre de 2014.Ceria-based materials have a wide variety of applications including ceramics, gas sensors, solid state electrolytes, automotive exhaust catalysts, among others [1]. Particularly, ceria is used in the field heterogeneous catalysis as support or promoter to improve the activity, selectivity and/or stability of the catalysts. One of the crucial properties of a ceria-based material is its oxygen storage capacity by means of redox shift between Ce3+ and Ce4+ under oxidizing and reducing environments, which reduces the carbon formation and/or facilitate the oxidation of pollutants in three way catalysts (exhaust catalysts). Since then, other industrial reactions have been studied on such materials, for instance the water-gas-shift reaction (WGSR, CO + H2O → CO 2 + H2) and the steam reforming of methanol (SRM, CH3OH + H2O → CO2 + 3 H2). Numerous reaction mechanisms have been proposed to determine the limiting step of each reaction [2], where the possible reaction pathways involve formate species (HCOO −) formation as either spectator or intermediate [2]. The formates species can be coordinated differently on oxide surfaces, classifying the species according to the number of bondings with the Ce atom (Monodentate, Bidentate, Bridge). IR measurements performed under various experimental conditions have shown three types of formate species for the WGS and SRM reactions. From the theory, various efforts were made to assign the IR peaks to different formates species but in any case the experimental conditions consistently considered [3]. The aim of this work is to study the influence of oxidizing/reducing environment on the stability of various formate species. For this we include in our calculations H, OH, O2 and H2O co-adsorbed in the vicinity of formate, being calculated the Gibbs free energy from data obtained by the DFT + U and the results are summarized in phase diagrams depending on the hydrogen and oxygen chemical potentialsPeer Reviewe

Enhanced Methanol Production over Non-promoted Cu-MgO-Al2O3Materials with Ex-solved 2 nm Cu Particles: Insights from an Operando Spectroscopic Study

Enhanced methanol production is obtained over a non-promoted Cu-MgO-Al2O3 mixed oxide catalyst derived from a Cu-Mg-Al hydrotalcite precursor (HT) containing narrowly distributed small Cu NPs (2 nm). Conversions close to the equilibrium (∼20%) with a methanol selectivity of 67% are achieved at 230 °C, 20 bar, and a space velocity of 571 mL·gcat-1·h-1. Based on operando spectroscopic studies, the striking activity of this Cu-based catalyst is ascribed to the stabilization of Cu+ ions favored under reaction conditions due to lattice reorganization associated with the "HT-memory effect"promoted by water. Temperature-resolved infrared-mass spectrometry experiments have enabled the discernment of monodentate formate species, stabilized on Cu+ as the intermediate in methanol synthesis, in line with the results of density functional theory calculations. These monodentate formate species are much more reactive than bridge formate species, the latter ones behaving as intermediates in methane and CO formation. Moreover, poisoning of the Cu0 surface by strongly adsorbed species behaving as spectators is observed under reaction conditions. This work presents a detailed spectroscopic study highlighting the influence of the reaction pressure on the stabilization of active surface sites, and the possibility of enhancing methanol production on usually less active non-promoted nano-sized copper catalysts, provided that the proper support is selected, allowing the stabilization of doped Cu+. Thus, a methanol formation rate of 2.6 × 10-3 molMeOH·gcat-1·h-1 at 230 °C, 20 bar, and WHSV = 28 500 mL·gcat-1·h-1 is obtained on the Cu-MgO-Al2O3 HT-derived catalyst with 71% methanol selectivity, compared to 2.2 × 10-4 molMeOH·gcat-1·h-1 with 54% methanol selectivity obtained on a reference Cu/(Al2O3/MgO) catalyst not derived from a HT structure.The research leading to these results received funding from the Spanish Ministry of Science, Innovation and Universities through “Severo Ochoa” Excellence Programme (SEV-2016-0683) and through the RTI2018-099668-B-C21 and RTI2018-101604-B-I00 projects. The authors also thank the Microscopy Service of UPV for kind help on measurements. J.C. thanks the Spanish Government (MINECO) for a “Severo Ochoa” grant (BES-2015-075748). J.M. thanks the MICINN (CTQ2015-67592) for his Ph.D. scholarship. M.E.D. thanks financial support by the Spanish Government (PGC2018-097277-B100). P.G.L. thanks the CSIC for the extension of the MSCA-IF Grant Agreement no. 832121. Computer time provided by the RES (Red Española de Supercomputación) resources at Mare Nostrum 4 (BSC, Barcelona) and La Palma (IAC, La Palma) nodes is acknowledged

Theoretical studies of the adsorption of CH4 on Ni-CeO2 systems: Evidence of a strong metal-support interaction

Trabajo presentado en el 6th Workshop on Novel Methods for Electronic Structure Calculations, celebrado en La Plata (Argentina) del 12 al 14 de noviembre de 2015.Natural gas is one of the cheapest sources of energy available on the planet. Methane can be used directly for the production of heat and/or electricity or it can be used for the production of syngas a fuel gas mixture consisting primarily of hydrogen and carbon monoxide via a reforming processes. The dry reforming of methane (DRM), CH4+CO2 2CO+2H2, represents a very interesting approach both to valorize a cheap source or carbon (CO2) as well as to reduce the overall carbon footprint of the increasing worldwide fossil-based methane consumption. Efficient and not too expensive catalysts for the dry reforming of methane (DRM) reaction are sought. Selective and stable conversion remains challenging due to the need to activate methane and CO2, unravel the mechanism that sustains good activity and selectivity, and mitigate deactivation through carbon deposition. In this context model Ni/CeOx(111) catalysts were created. Ceria’ ability to stabilize reduced states by accommodating electrons in the localized 4f states is well known. Yet, finding the minimum energy structures with respect to both the localization of the electrons and the character of the 4fstate associated to each Ce3+ ion still represents a challenge in the theoretical modeling of reduced ceria systems using DFT. We here employ the DFT+U approach. We find that strong interactions between Ni and the reducible ceria support and the presence of oxygen vacancies are at the basis of the modifications of the oxidation state of Ni from Ni2+ [1] to Ni for clean and heavily reduced support, respectively. The results are consistent with Ni 3p XPS data for corresponding experimental model catalysts. Moreover, we correlate these changes with bonding properties of adsorbed CH4 and co-adsorbed CH3 and H.Peer Reviewe