Ultra-thin silicate films on metals

Silica is one of the key materials in many modern technological applications. 'Surface science' approach for understanding surface chemistry on silica-based materials, on the one hand, and further miniaturization of new generation electronic devices, on the other, all these face the necessity of rational design of the ultrathin silica films on electrically conductive substrates. The review updates recent studies in this field. Despite the structural complexity and diversity of silica, substantial progress has recently been achieved in understanding of the atomic structure of truly 2D silicates

Exploring Zeolite Chemistry with the Tools of Surface Science: Challenges, Opportunities, and Limitations

The complexity of catalysts that the surface science community has been able to address has increased substantially in a systematic manner, starting with metal and oxide single crystal surfaces and evolving to an atomistic description of clusters and nanoparticles on well-defined, planar supports. The next step in adding complexity is now to address surfaces of porous oxide materials, in particular of zeolites, which are the most extensively used catalysts in the industry. The recently reported successful fabrication of well-ordered thin films, consisting of planar arrangement of aluminosilicate polygonal prisms on a metal substrate counting with highly acidic bridging hydroxyl groups on the surface, represents the limiting case of infinitely large pore and cages in zeolites. This model system allows one to study reactions catalyzed by zeolites using the toolkit of surface science. In this Perspective, we describe the zeolitic model system, with its virtues and limitations, as well as the challenges, opportunities and expectations for the future in modelling porous catalysts by a surface science approach

Two-dimensional silica: Crystalline and vitreous

Two-dimensional SiO₂ films may be grown on metal single crystal surfaces. It is possible to grow crystalline and vitreous (glassy) films and study their structural, vibrational, and electronic properties. In particular, the structures of a crystalline and a vitreous film may be imaged with atomic resolution side by side which opens avenues to study long standing problems of real space imaging of a crystal to glass transition

Ultrathin silicatene/silicon-carbide hybrid film on a metal substrate

Layered graphene/silica heterostructures may become interesting materials in nanotechnology with yet unknown properties. We have attempted here to intercalate graphene into a silicatene/Ru(0001) interface. The experimental results obtained by x-ray photoelectron spectroscopy, low energy electron diffraction, infrared reflection–absorption spectroscopy, and scanning tunneling microscopy suggest the formation of a well-ordered hybrid structure consisting of a single-layer silicatene on top of a silicon carbide monolayer adsorbed on a metal substrate

Acetylene and ethylene hydrogenation on alumina supported Pd–Ag model catalysts

Adsorption and co-adsorption of ethylene, acetylene and hydrogen on Pd-Ag particles, supported on thin alumina films, have been studied by temperature programmed desorption (TPD). The TPD results show that adding of Ag to Pd suppresses overall hydrogenation activity but increases selectivity towards ethylene, i.e. similar to that observed on real catalysts. The results are rationalized on the basis of a complex interplay between surface and subsurface hydrogen species available in the system, whereby the latter species are the most critical for total hydrogenation of acetylene to ethane

Methanol Reactivity on Silica-Supported Ceria Nanoparticles

Ceria (CeO2) has been used in a number of catalytic processes, either as a support or promoter. For a better understanding of the factors that control the reactivity of ceria, we have used well-ordered CeO2(111) films and ceria nanoparticles supported on an ordered SiO2 film, as model catalysts. The systems were examined in the dehydrogenation of methanol to formaldehyde as a test reaction by using the techniques of infrared spectroscopy and temperature programmed desorption. The results revealed low-temperature reactivity (below 450 K) for supported ceria particles that is not present on ordered films, which show reactivity at 565 K. The results indicate that low-coordinated sites play an important role in the methanol reactivity on ceri

Structural Evolution of Ga-Cu Model Catalysts for CO₂ Hydrogenation Reactions

We studied the initial stages of Ga interaction with the Cu(001) surface and environment-induced surface transformations in an attempt to elucidate the surface chemistry of the Cu–Ga catalysts recently proposed for CO2 hydrogenation to methanol. The results show that Ga readily intermixes with Cu upon deposition in vacuum. However, even traces of oxygen in the gas ambient cause Ga oxidation and the formation of two-dimensional (“monolayer”) Ga oxide islands uniformly covering the Cu surface. The surface morphology and the oxidized state of Ga remain in H2 as well as in a CO2 + H2 reaction mixture at elevated pressures and temperatures (0.2 mbar, 700 K). The results indicate that the Ga-doped Cu surface under reaction conditions exposes a variety of structures including GaOx/Cu interfacial sites, which must be taken into account for elucidating the reaction mechanism

Water and Carbon Dioxide Adsorption on CaO(001) Studied via Single Crystal Adsorption Calorimetry

A new method to analyze microcalorimetry data was employed to study the adsorption energies and sticking probabilities of D2O and CO2 on CaO(001) at several temperatures. This method deconvolutes the line shapes of the heat detector response into an instrument response function and exponential decay functions, which correspond to the desorption of distinct surface species. This allows for a thorough analysis of the adsorption, dissociation, and desorption processes that occur during our microcalorimetry experiments. Our microcalorimetry results, show that D2O adsorbs initially with an adsorption energy of 85–90 kJ/mol at temperatures ranging from 120 to 300 K, consistent with prior spectroscopic studies that indicate dissociation. This adsorption energy decreases with increasing coverage until either D2O multilayers are formed at low temperatures (120 K) or the surface is saturated (150 K). Artificially producing defects on the surface by sputtering prior to dosing D2O sharply increases this adsorption energy, but these defects may be healed after annealing the surface to 1300 K. CO2 adsorbs on CaO(001) with an initial adsorption energy of ~ 125 kJ/mol, and decreases until the saturation coverage is reached, which is a function of surface temperature. The results showed that pre-adsorbed water blocks adsorption sites, lowers the saturation coverage, and lowers the measured adsorption energy of CO2. The calorimetry data further adds to our understanding of D2O and CO2 adsorption on oxide surfaces

Planar model system of the Phillips (Cr/SiO₂) catalyst based on a well-defined thin silicate film

The Phillips catalyst (Cr/SiO2) is successfully used in the large-scale production of polyethylene and has attracted a great interest in catalytic community over the last sixty years. However, the atomic structure of the active site(s) and the reaction mechanism remain controversial, in particular due to the structural complexity and surface heterogeneity of the amorphous silica. In this work, we used a well-defined, atomically flat silicate bilayer film grown on Ru(0001) as a support offering the opportunity to investigate mechanistic aspects at the atomic scale. To fabricate a planar Cr/SiO2 model system suitable for surface science studies, chromium was deposited using physical vapor deposition onto the hydroxylated silica film surface. Structural characterization and adsorption studies were performed by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Hydroxyls groups seem to serve as anchoring cites to Cr ad-atoms. As monitored by IRAS, hydroxyls consumption correlated with the appearance of the new band at ~1007 cm-1 typical for Cr=O vibrations. In addition, CO titration experiments suggested also the presence of "naked" Cr, which transforms into mono- and di-oxo chromyl species and their aggregation upon oxidation treatments. TPD experiments of ethylene adsorption at low temperatures under UHV conditions showed the formation of butane as one of the main products. The resultant surfaces are thermally stable, at least, up to 400 K which allows to investigate ethylene polymerization further under more realistic conditions