Jorge Anibal Boscoboinik

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO(4)] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO(4)] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO(4)]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm(-1) for a monolayer film, ∼1300 cm(-1) for the bilayer structures, and ∼1250 cm(-1) for the bulk-like vitreous silica.

Modellierung von Zeolithen durch zweidimensionale Aluminosilicatfilme auf Metallunterlagen

Zeolites are one of the most widely used materials in heterogeneous catalysis. However, the current understanding of the relation between structure and reactivity of these complex and highly porous materials mostly comes from studies employing bulk-sensitive techniques and from theoretical calculations based on educated assumptions about the inner surface within the pores present in the framework. Zeolite frameworks are formed by ordered arrangements of [SiO4/2] and [AlO4/2 ] tetrahedra, conferring the characteristic negative charge to the system, which is typically compensated by extra-framework metal cations M or H. Modeling such materials under controlled conditions, and taking advantage of the analytical tools commonly used in surface science, would provide a new playground for exploring structures and chemical reactions on zeolites. The preparation of welldefined aluminosilicate thin films was first reported using a Mo(112) substrate. It was shown that this film consists of a single layer network of corner-sharing [SiO4/2] tetrahedra and [AlO3/2] units, and the film is strongly bound to the Mo(112) surface by Si-O-Mo linkages (Figure 1a). Certainly, for those monolayer films the metal support has to be explicitly included in the proper description of the system. Furthermore, this film lacks the negative framework charge present in zeolites, which is responsible for the presence of acidic OH groups. To create a more adequate model system, herein we present the preparation of aluminosilicate films that a) are constituted of tetrahedral [SiO4] and [AlO4 ] building blocks, b) are weakly bound to the underlying metal support, and c) expose highly acidic OH species. Our results open up an avenue for experimental and theoretical modeling of zeolite surfaces that is aimed at a fundamental understanding of structure–reactivity relationships in those materials. As a starting point for the preparation of the aluminosilicate films, we used the recently reported preparation of a silica bilayer film weakly bound to a metal, in this case, Ru(0001) (see Figure 1b). The structure allows oxygen atoms to reversibly adsorb directly on the metal surface underneath the silica film, which can be grown either in the crystalline or vitreous state. 6] We will refer to all these films as silica films. For aluminosilicate films, reported here (see Experimental Section), the sum of the molar amounts of Si and Al was equal to the amounts of Si necessary to prepare the bilayer silica film. The structural characterization was performed by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and infrared reflection–absorption spectroscopy (IRAS), in combination with density functional theory (DFT) calculations. Using a silica film as a reference sample, the XPS results show that both Si and Al are in the highest oxidation states. For the O1s core level, a signal at about 530.7 eV develops as a shoulder to the main peak at 531.7 eV that originates from the O atoms surrounded solely by Si in the silica films (Supporting Information, Figure S1). This shoulder becomes more prominent at increasing Al/Si ratios and it has previously been assigned to the Al-O-Si linkage. The fact that the integral O1s signal intensity remains practically constant upon Al doping is consistent with Al substituting Si in the [SiO4] tetrahedra, giving an AlxSi(1 x)O2 composition, where x is the Al molar fraction. At low Al/Si ratios, the resulting aluminosilicate surfaces show only (2 2) LEED patterns and are nearly atomically flat. STM images of an Al0.12Si0.88O2 film, revealed irregularly shaped areas (marked A in Figure 2) with a slightly different Figure 1. Structural models of a) an AlSi7O19 film on Mo(112); b) a HAlSi7O16 film on O(2 1)/Ru(0001); and c) chabasite (H-CHA) with the proton on O1. Top and cross views are shown in (a) and (b), adsorbed CO are shown in (b) and (c). One of the surface O atoms on Ru(0001) underneath the film is not seen in the top view. Si yellow, O red, Al dark gray, C black, H white.

Low Pressure CO2 Hydrogenation to Methanol over Gold Nanoparticles Activated on a CeOx/TiO2 Interface

Capture and recycling of CO2 into valuable chemicals such as alcohols could help mitigate its emissions into the atmosphere. Due to its inert nature, the activation of CO2 is a critical step in improving the overall reaction kinetics during its chemical conversion. Although pure gold is an inert noble metal and cannot catalyze hydrogenation reactions, it can be activated when deposited as nanoparticles on the appropriate oxide support. In this combined experimental and theoretical study, it is shown that an electronic polarization at the metal-oxide interface of Au nanoparticles anchored and stabilized on a CeO(x)/TiO2 substrate generates active centers for CO2 adsorption and its low pressure hydrogenation, leading to a higher selectivity toward methanol. This study illustrates the importance of localized electronic properties and structure in catalysis for achieving higher alcohol selectivity from CO2 hydrogenation.

Support effects on the atomic structure of ultrathin silica films on metals

We studied the atomic structure of ultrathin silica films on Pt(111) in comparison with the previously studied films on Mo(112) and Ru(0001). The results obtained by scanning tunneling microscopy, photoelectron spectroscopy, and infrared reflection absorption spectroscopy suggest that the metal-oxygen bond strength plays the decisive role in the atomic structure of the silica overlayers on metal substrates. Metals with high oxygen adsorption energy favor the formation of the crystalline monolayer SiO2.5 films, whereas noble metals form primarily vitreous SiO2 bilayer films. The metals with intermediate energies may form either of the structures or both coexisting. In the systems studied, the lattice mismatch plays only a minor role.

One-dimensional supramolecular surface structures: 1,4-diisocyanobenzene on Au(111) surfaces

One-dimensional supramolecular structures formed by adsorbing low coverages of 1,4-diisocyanobenzene on Au(111) at room temperature are obtained and imaged by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. The structures originate from step edges or surface defects and arrange predominantly in a straight fashion on the substrate terraces along the <110> directions. They are proposed to consist of alternating units of 1,4-diisocyanobenzene molecules and gold atoms with a unit cell in registry with the substrate corresponding to four times the lattice interatomic distance. Their long 1-D chains and high thermal stability offer the potential to use them as conductors in nanoelectronic applications.

Patterned Defect Structures Predicted for Graphene Are Observed on Single-Layer Silica Films

Topological defects in two-dimensional materials such as graphene are considered as a tool for tailoring their physical properties. Here, we studied defect structures on a single-layer silica (silicatene) supported on Ru(0001) using a low energy electron diffraction, scanning tunneling microscopy, infrared reflection-absorption spectroscopy, and photoelectron spectroscopy. The results revealed easy formation of periodic defect structures, which were previously predicted for graphene on a theoretical ground, yet experimentally unrealized. The structural similarities between single-layer materials (graphene, silicene, silicatene) open a new playground for deeper understanding and tailoring structural, electronic, and chemical properties of the truly two-dimensional systems.

Ultrathin Silica Films: The Atomic Structure of Two‐Dimensional Crystals and Glasses

For the last 15 years, we have been studying the preparation and characterization of ordered silica films on metal supports. We review the efforts so far, and then discuss the specific case of a silica bilayer, which exists in a crystalline and a vitreous variety, and puts us into a position to investigate, for the first time, the real space structure (AFM/STM) of a two-dimensional glass and its properties. We show that pair correlation functions determined from the images of this two-dimensional glass are similar to those determined by X-ray and neutron scattering from three-dimensional glasses, if the appropriate sensitivity factors are taken into account. We are in a position, to verify, for the first time, a model of the vitreous silica structure proposed by William Zachariasen in 1932. Beyond this, the possibility to prepare the crystalline and the glassy structure on the same support allows us to study the crystal-glass phase transition in real space. We, finally, discuss possibilities to use silica films to start investigating related systems such as zeolites and clay films. We also mention hydroxylation of the silica films in order to adsorb metal atoms modeling heterogenized homogeneous catalysts.

Atomic Structure of an Ultrathin Fe-Silicate Film Grown on a Metal: A Monolayer of Clay?

Ultrathin Fe-doped silicate films were prepared on a Ru(0001) surface and, as a function of the Fe/Si ratio, structurally characterized by low-energy electron diffraction, X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, and scanning tunneling microscopy. Density functional theory (DFT) was used to identify the atomic structure. The results show that uniform substitution of Si by Fe in the silicate bilayer frame is thermodynamically unfavorable: the film segregates into a pure silicate and an Fe-silicate phase. The DFT calculations reveal that the Fe-silicate film with an Fe/Si = 1:1 ratio consists of a monolayer of [SiO4] tetrahedra on top of an iron oxide monolayer. As such, it closely resembles the structure of the clay mineral nontronite, a representative of the Fe-rich smectites. Furthermore, the DFT calculations predict formation of bridging Fe-O-Ru bonds between the Fe-silicate film and the Ru substrate accompanied by charge transfer from the metal substrate to the film, so that iron is in the oxidation state +III as in nontronite.