298 research outputs found
Role of steps and terrace width in gas-surface interaction: O2 /Ag(410)
No full textWe demonstrate by vibrational spectroscopy that open steps are the active site for O-2 dissociation on Ag(410), and that the barrier to adsorption at defects can be measured by energy and angle resolved investigation of the dynamics of the gas-surface interaction. We identify a molecular adsorption channel with considerably reduced activation barrier and a nonactivated dissociative pathway. The O-2 sticking probability is increased at steps and strongly reduced at terraces with respect to Ag(100), implying that reactivity is influenced by terrace width
Interaction of ethylene and oxygen with stepped Ag surfaces
No full textThe active role of defects in some catalytic reactions was predicted in the very early days of surface science. Most of the studies on gas adsorption at solid surfaces dealt, however, so far with nearly perfect low Miller index surfaces, which are rather unlike the active powders employed as catalysts in industrial reactors. The structure gap between the systems studied by surface scientists and the surface structure of real catalyst powders was therefore often indicated as the reason for the failure in reproducing some chemical reactions, which occur readily in industrial reactors, also in controlled conditions. Overcoming this limit without losing control over the experiment at the nanoscopic level is therefore an issue of pivotal importance, which could be addressed only now that the understanding of adsorption processes at flat surfaces is reasonably established. In this paper we shall review our most recent results on O-2 and C2H4 interaction with Ag(410) and Ag(210), which are vicinal surfaces of Ag(100) characterised by open (110)-like steps and narrow (100) terraces. The gases were dosed with a supersonic molecular beam, allowing to perform experiments at selected and well defined angles of incidence of the gas-phase particles. For both gases we find that the open steps affect gas-surface interaction considerably, changing the energy barriers to adsorption as well as the final chemisorption state
From adsorption at the surface to incorporation into subsurface sites: the role of steps for O/Ag
No full textMany reactions like the ethylene epoxidation which work readily in industrial reactors can not be reproduced under controlled laboratory conditions. This failure is ascribed to so-called "pressure" and "structure" gaps. The latter is possibly determined by an active role of defects, since surface science experiments are performed on nearly perfect surfaces, quite unlike those of active catalytic powders. One of the mostly investigated, but so far unresolved systems, is O/Ag which shows a unique selectivity for the partial oxidation of ethylene. Subsurface oxygen was claimed to play a pivotal role in activating oxygen adatoms, but the mechanism of O incorporation remained mysterious. The study of O-2 adsorption at stepped surfaces and by supersonic molecular beams allowed for considerable progresses, showing that open steps favour dissociation and constitute a gateway through which subsurface sites can be accessed
Dynamics of the interaction of O2 with stepped and damaged Ag surfaces
No full textThe role of defects in catalytic reactions, especially those involving low rates and high degrees of selectivity, has been debated ever since the very early days of surface science. However, most studies on gas-surface interaction and chemisorption performed under controlled laboratory conditions have dealt so far with nearly perfect low-Miller-index surfaces, which are rather unlike the active powders employed as catalysts in industrial reactors. The failure in reproducing some chemical reactions, which occur readily under industrial conditions, was therefore often ascribed to the so-called structure gap separating the surface science approach from the real industrial conditions. Overcoming this limit without losing control over the experiment at the nanoscopic scale is therefore an issue of pivotal importance. It can be attacked by studying adsorption on either single-crystal surfaces damaged by ion bombardment or on surfaces aligned along high-Miller-index planes. In this paper we shall discuss O-2 interaction with Ag(410), a vicinal surface of Ag(100) characterized by open (110)-like steps and by (100) terraces. The use of a supersonic molecular beam to dose O-2 allowed us to gain information on the interaction of the gas-phase molecules with steps and terraces separately, by selecting the angle of incidence and the impact energy. The open steps turned out to be active sites for dissociation, while flat Ag(100) planes are unreactive. For molecular adsorption a reduction in the activation barrier is observed at the steps, while the Ag(100) nanoterraces are much less reactive than wide (100) planes. The main results are confirmed by the preliminary investigation of O-2/Ag(210)
Direct Access to Subsurface Sites in Gas-Surface O2/Ag(210) interaction using supersonic molecular beams
No full textWe show with supersonic molecular beams and surface vibrational spectroscopy that, contrary to the case of Ag(100) and Ag(110), O-2 undergoes total dissociation on Ag(210) at 105 K. Moreover, metastable subsurface sites can be accessed either directly or indirectly. For the direct channel, the final configuration of the oxygen atoms depends on the angle and the energy with which the gas-phase molecules collide with the surface, being largest for normal incidence on the (100) nanofacets. Access into the subsurface site is enabled only in the presence of preadsorbed oxygen adatoms
Effect of surface interband transitions on surface plasmon dispersion: O/Ag(001)
No full textThe effect of surface interband transitions (SIT) on surface plasmon dispersion was investigated by high-resolution electron-energy-loss spectroscopy for Ag(001). We demonstrate that the anomalous linear dispersion on this face, which is at variance with the parabolic form present for the other low Miller index surfaces, is due to a SIT between Shockley states that nearly matches the surface plasmon frequency. The anomaly is eliminated when the substrate undergoes a missing-row reconstruction induced by oxygen adsorption that removes the SIT
Ethylene adsorption on clean and oxygen covered flat and stepped Ag(001)
No full textIn the present paper we review our findings on ethylene adsorption on clean and oxygen covered Ag(001) surfaces investigated by dosing the gas with a Supersonic Molecular Beam and analysing the adsorption state either by High Resolution Electron Energy Loss Spectroscopy or by High Resolution X Rays Photoemission Spectroscopy. The final adsorption state depends on the translational and on the internal energy of the gas-phase molecules and on the presence of defects. At low translational energy ethylene either physisorbs or very weakly chemisorbs at flat terrace sites. The physisorption probability is thereby hindered by rotational excitation. A more strongly bound, pi bonded, state forms at higher translational energy, the activation barrier being related to the energy needed to form the relevant defect at which chemisorption takes place. A further even more strongly bound state forms only when dosing vibrationally excited molecules from the gas phase
“Bridging the structure gap: Surface Chemistry at well defined defects”
No full textOne of the main goals of surface science is the understanding of the elementary steps occurring in catalytic reactions in the heterogeneous phase, in order to identify promoters and rate limiting factors at the atomic scale with the ultimate scope of designing more efficient catalysts. To reduce the complexity of the problem and focus attention on individual steps of the reaction of interest, most experiments have been performed so
far under controlled, ultra-high vacuum conditions and on single crystal surfaces cut along low Miller index planes. On the other hand, catalytic reactions occurring industrially for the massive production of everyday life chemicals are far away from these well-defined conditions. Reactors work at high temperature and pressure, while the catalysts consist usually of supported powders exhibiting different atomic terminations and a
high concentration of low coordinated sites (steps, kinks, vacancies etc.). The structural difference between the ordered samples used in surface science and the real catalysts is known as structure gap and, in the understanding of catalytic processes, it can be as relevant as the more widely
invoked pressure gap, related to the difference in pressure between chemical reactors and ultra-high vacuum apparatuses. Although the importance of defect sites and the relevance of the structure gap have been evident for decades, the systematic study of defected surfaces began only recently,
after a reasonable understanding of the simpler systems was reached. The most promising approach to this topic is the use of single crystal surfaces cut along high Miller index planes, i.e. stepped surfaces showing a high density of one majority low coordination site which mimics a defect. This approach allows a shortcut between the need for ordered substrates and controlled conditions and the availability of particular atomic
configurations, a condition only partially mitigated with the advent of scanning probe microscopy with nanoscale resolution. Of course, only one defect-type at a time can be studied in this way. The present report summarizes the knowledge achieved so far for the gas–surface interaction in presence of well-defined defects and for simple reactions at such sites
Influence of Defects and Heteroatoms on the Chemical Properties of Supported Graphene Layers
No full textA large and growing number of theoretical papers report the possible role of defects and heteroatoms on the chemical properties of single-layer graphene. Indeed, they are expected to modify the electronic structure of the graphene film, allow for chemisorption of different species, and enable more effective functionalisation. Therefore, from theoretical studies, we get the suggestion that single and double vacancies, Stone–Wales defects and heteroatoms are suitable candidates to turn nearly chemically inert graphene into an active player in chemistry, catalysis, and sensoristics. Despite these encouraging premises, experimental proofs of an enhanced reactivity of defected/doped graphene are limited because experimental studies addressing adsorption on well-defined defects and heteroatoms in graphene layers are much less abundant than theoretical ones. In this paper, we review the state of the art of experimental findings on adsorption on graphene defects and heteroatoms, covering different topics such as the role of vacancies on adsorption of oxygen and carbon monoxide, the effect of the presence of N heteroatoms on adsorption and intercalation underneath graphene monolayers, and the role of defects in covalent functionalisation and defect-induced gas adsorption on graphene transistors
Coverage dependence of the sticking probability of ethylene on Ag(410)
No full textThe interaction of ethylene with Ag(410), a vicinal surface of Ag(100) characterised by a high density of open steps, is investigated. Energy and angle resolved measurements of the sticking coefficient are performed with the retarded reflector method of King and Wells using a supersonic molecular beam. We find that open steps remove the translational barrier for adsorption into the pi-bonded state present for Ag(100). Steering removes the angle dependence of the initial sticking probability. The coverage dependence of the adsorption probability indicates that a precursor mechanism is present and that adsorbate assisted adsorption is important at hyperthermal energies. With increasing coverage steering becomes ineffective and rotations inhibit adsorption. Admolecules with second nearest neighbours are destabilised and these sites can be populated only by gas-phase molecules impinging along trajectories for which the step heights are exposed
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