MODEL STUDIES OF HYDROGEN REACTIVITY AND PRODUCTION ON METAL SURFACES

2003/2004L'idrogeno è considerato un buon candidato per limitare l'impatto ambientale causato dal sempre crescente fabbisogno energetico. Può essere utilizzato come vettore pulito di energia per alimentare celle a combustibile e può essere ottenuto da fonti rinnovabili (come, ad esempio, le bio-masse). Gli studi sperimentali nell'ambito della scienza delle superfici sono di importanza chiave: permettono infatti di ottenere una conoscenza approfondita ed una buona descrizione dei fenomeni chimici e dei meccanismi cinetici coinvolti nelle reazioni catalitiche. Gli atomi di idrogeno assorbiti su una superficie hanno deboli proprietà di scattering, presentano grandi rates di diffusione e generalmente sono difficilmente studiabili con le tecniche convenzionali della fisica delle superfici. Tutto ciò rende gli esperimenti mirati allo studio dell'interazione dell'idrogeno con i metalli un compito particolarmente difficile. Ad esempio, persino la determinazione del sito di assorbimento di un atomo di idrogeno sulla superficie orientata di un cristallo metallico non è banale. Come parte di questa tesi, è stato studiato il sistema H/Rh(lOO) ed è stato dimostrato che in linea di principio il sito di assorbimento può essere individuato mediante l'impiego della spettroscopia di fotoemissione ad alta risoluzione dei livelli di core del metallo con luce di sincrotrone. Un limite ben noto delle tecniche della scienza delle superfici in ultra-alto vuoto (UHV) è ciò che viene definito "pressure gap": il comportamento chimico dei sistemi catalitici può infatti cambiare sostanzialmente in base alle diverse condizioni di pressione. Nel caso dell'idrogeno, sorgenti di idrogeno atomico permettono di guadagnare l'energia di dissociazione della molecola: in questo modo, è disponibile più energia per la reazione, simulando quindi condizioni di alte pressioni. Per questi motivi, come parte di questo progetto di tesi è stata costruita e caratterizzata una sorgente di idrogeno atomico del tipo Bisch Bertel. Usando questo strumento, sono state studiate delle reazioni modello di idrogenazione sulle superfici (110) del nichel e (101 O) del rutenio. La parte finale di questa tesi riporta uno studio sperimentale sulla dissociazione dell'etanolo sul Rh(111). L'etanolo può essere ottenuto facilmente dalla fermentazione della bio-massa ed è considerato una promettente fonte di idrogeno. E' stata dimostrata l'elevata selettività del rodio nei confronti della rottura del legame intramolecolare C-C in condizioni di ultra-alto vuoto. Inoltre, la reattività delle superfici modello (100) e (111) del rodio è confrontata qualitativamente con quella di un catalizzatore supportato in condizioni di lavoro reali.Hydrogen is considered as a possible solution for limiting the environmental impact caused by the growing energy demand. It can be used as a clean energy vector for feeding fuel cells and can be obtained also from renewable energy sources, like for example biomasses. Experimental studies within the framework of surface science techniques are of core importance for a deep understanding and a proper description of the chemical and kinetic mechanisms involved in catalytic reactions. Hydrogen atoms adsorbed on a surface have weak scattering properties, high diffusion rates and generally show small cross sections to conventional surface science probes. This makes experiments on hydrogen-metal systems particularly hard to be carried out. For example, even the simple determination of the adsorption site for an H atom on a single crystal metal surface is nota straightforward task. As part of this thesis, the H/Rh(lOO) system was studied and it was shown that adsorption site identification can be obtained with the aid of high energy resolution Synchrotron Radiation (SR) X-Ray Photoelectron Spectroscopy (XPS) of the metal core levels. A well known limit of UHV surface science techniques is the pressure gap: the chemical behaviour of catalytic systems can change substantially under different pressure regimes. In the case of hydrogen, atomic sources allow to gain the H2 molecule dissociation energy: in this way more energy is available for surface reactions upon adsorption and absorption, thus simulating higher pressure conditions. For these reasons, a Bischler and Berte! atomic hydrogen source has been built and fully characterized within this PhD project. Using this source, model hydrogenation reactions by gas phase atomic hydrogen have been carried out on nickel (110) and ruthenium (10 l O) single crystal surfaces. The final part of the present thesis reports about an experimental study of ethanol decomposition on Rh(111). Ethanol can be easily obtained by waste bio-mass fermentation and is considered as a promising clean hydrogen source. High rhodium selectivity towards the intramolecular C-C bond cleavage under UHV conditions is reported and the overall metal reactivity is qualitatively compared to the real catalytic conditions.XVII Ciclo1976Versione digitalizzata della tesi di dottorato cartacea

Growing borophene on metal substrates: a theoretical study of the role of oxygen on Al(111)

Charge transfer from a metal substrate stabilizes honeycomb borophene, whose electron deficit would otherwise spoil the hexagonal order of a $\pi$-bonded 2D atomic network. However, the coupling between the substrate and the boron overlayer may result in the formation of strong chemical bonds that would compromise the electronic properties of the overlayer. In this paper we present a theoretical study, based on state-of-the-art density-functional and genetic-optimization techniques, of the electronic and structural properties of borophene grown on Al(111), with emphasis on the impact of oxygen on the strength of the coupling between substrate and overlayer. While our results confirm the formation of Al-B bonds, they also predict that oxygen doping reduces charge transfer between aluminum and borophene, thus allowing modulation of their strength and paving the way to engineering the electronic properties of 2D-supported borophene sheets for industrial applications. Our study is completed by a thorough study of the thermodynamic stability of the oxygenated borophene-Al(111) interface

Carbon dioxide reduction on Ir(111): Stable hydrocarbon surface species at near-ambient pressure

Stable hydrocarbon surface species in the carbon dioxide hydrogenation reaction on Ir(111) were identified by means of infrared-visible sum-frequency generation vibrational spectroscopy and X-ray photoelectron spectroscopy under near-ambient pressure conditions (0.1 mbar). By introducing gas phase binary and ternary mixtures of CO2, CO, and H2 into the reaction chamber, stable ethylidyne and ethynyl species were found at the metal surface above 425 K, in remarkable analogy with that observed during the ethylene decomposition process yielding graphene. In addition, upon increasing temperature (up to 600 K depending on the reaction conditions), vibrational and electronic spectroscopic fingerprints appeared that could be attributed to the nucleation of aromatic hydrocarbons at the edge of metastable graphenic clusters interacting with the metal surface

Spectroscopic fingerprints of iron-coordinated cobalt and iron porphyrin layers on graphene

Achieving design capabilities of monolayer 2D functional catalysts represents a challenging perspective. Coordinated single metal atom sites can offer tailored electronic configuration, ligation geometries, chemical activity and selectivity, together with stability. We report spectroscopic evidence of the formation of a 2D metal-organic framework supported by a single graphene sheet in which coordination among Tetra-Pyridyl-Porphyrins (TPyPs) is spontaneously obtained by exploiting single iron atoms. The spectroscopic characterization, together with ab initio methods, reveals that metal inter-molecular coordination occurs via the terminal nitrogen atoms contained in the pyridinic residues of adjacent TPyPs. Interestingly, the peripheral coordination of metal atoms is found to affect the electronic configuration of the porphyrins core. Due to the chemical stability of the supporting graphene layer, its weak interaction with the metal-organic framework, and the known electrochemical activity of the latter, this system represents an optimal candidate for the design and engineering of prototype 2D electrocatalytic materials

CO on supported Cu nanoclusters: Coverage and finite size contributions to the formation of carbide via the boudouard process

The interaction of carbon monoxide with an ordered array of copper nanoclusters was investigated under ultrahigh vacuum conditions by means of in situ X-ray photoelectron spectroscopy in combination with density functional theory calculations. The Cu clusters were supported on an alumina template grown on the Ni3Al(111) termination. Adsorption and dissociation of carbon monoxide occur at the copper clusters, yielding accumulation of carbidic carbon at the metal particles through the Boudouard process. The involved mechanisms are investigated at the atomic level, unveiling the effects of cluster finite size, reconstruction, support, and of local CO coverage. It is found that the high coverage of CO at the cluster surface, which considerably exceeds that achievable on single crystal surfaces, facilitates the metal restructuring and the reaction, yielding carbon incorporation into the bulk of the particles

On the concentration of large deviations for fat tailed distributions, with application to financial data

Large deviations for fat tailed distributions, i.e. those that decay slower than exponential, are not only relatively likely, but they also occur in a rather peculiar way where a finite fraction of the whole sample deviation is concentrated on a single variable. The regime of large deviations is separated from the regime of typical fluctuations by a phase transition where the symmetry between the points in the sample is spontaneously broken. For stochastic processes with a fat tailed microscopic noise, this implies that while typical realizations are well described by a diffusion process with continuous sample paths, large deviation paths are typically discontinuous. For eigenvalues of random matrices with fat tailed distributed elements, a large deviation where the trace of the matrix is anomalously large concentrates on just a single eigenvalue, whereas in the thin tailed world the large deviation affects the whole distribution. These results find a natural application to finance. Since the price dynamics of financial stocks is characterized by fat tailed increments, large fluctuations of stock prices are expected to be realized by discrete jumps. Interestingly, we find that large excursions of prices are more likely realized by continuous drifts rather than by discontinuous jumps. Indeed, auto-correlations suppress the concentration of large deviations. Financial covariance matrices also exhibit an anomalously large eigenvalue, the market mode, as compared to the prediction of random matrix theory. We show that this is explained by a large deviation with excess covariance rather than by one with excess volatility.Comment: 38 pages, 12 figure

Tetrapyrroles at near-ambient pressure: porphyrins and phthalocyanines beyond the pressure gap

Many complex mechanisms underlying the fascinating functionalities provided by tetrapyrrolic macrocycles in biochemistry have been already unraveled. Light harvesting, molecular transport, and catalytic conversion are some of the processes performed by tetrapyrrole-based centers embedded in protein pockets. The main function is determined by the single atom species that is caged in the macrocycle, while a finer tuning (band gap, chemical selectivity etc) is granted by the geometric and electronic structure of the tetrapyrrole, including its residues, and by the proximal and distal structures of the protein surroundings that exploit the molecular trans-effect and direct weak interactions, respectively. Hence, a scientific and technological challenge consists in the artificial replication of both structure and functionality of natural reaction centers in 2D ordered arrays at surfaces. Nano-architected 2D metalorganic frameworks can be indeed self-assembled under controlled conditions at supporting surfaces and, in the specific, porphyrin- and phthalocyaninebased systems have been widely investigated in ultra-high vacuum conditions by means of surface science approaches. Deep insight into the geometry, electronic structure, magnetic properties, ligand adsorption mechanisms, and light absorption has been obtained, with the strong experimental constraint of vacuum. Especially in the case of the interaction of tetrapyrroles with ligands, this limit represents a relevant gap with respect to both comparison with natural counterparts from the liquid environment and potential applicative views at both solid\u2013liquid and solid\u2013gas interfaces. Thus, a step forward in the direction of near-ambient pressure is strongly necessary, while maintaining the atomiclevel detail characterization accuracy. Nowadays this becomes feasible by exploiting state-of-the-art experimental techniques, in combination with computational simulations. This review focusses on the latest advances in this direction

X-rays and Extreme Ultraviolet Radiation: Principles and Applications

X-Rays and Extreme Ultraviolet Radiation: Principles and Applications, D. Attwood, A. Sakdinawat, 2nd ed., Cambridge University Press, 2016, ISBN-13:978-1107062894: Book revie