Bridging hydroxyls (Si–OH–Al) in zeolites are catalytically active for a multitude of important reactions, including the catalytic cracking of crude oil, oligomerization of olefins, conversion of methanol to hydrocarbons, and the selective catalytic reduction of NO_x. The interaction of probe molecules with bridging hydroxyls was studied here on a novel two-dimensional zeolite model system consisting of an aluminosilicate forming a planar sheet of polygonal prisms, supported on a Ru(0001) surface. These bridging hydroxyls are strong Brönsted acid sites and can interact with both weak and strong bases. This interaction is studied here for two weak bases (CO and C₂H₄) and two strong bases (NH₃ and pyridine), by infrared reflection absorption spectroscopy, in comparison with density functional theory calculations. Additionally, ethene is the reactant in the simplest case of the olefin oligomerization reaction which is also catalyzed by bridging hydroxyls, making the study of this adsorbed precursor state particularly relevant. It is found that weak bases interact weakly with the proton without breaking the O–H bond, although they do strongly affect the O–H stretching vibration. On the other hand, the strong bases, NH₃ and pyridine, abstract the proton to produce ammonium and pyridinium ions. The comparison with the properties of three-dimensional zeolites shows that this two-dimensional zeolite model system counts with bridging hydroxyls with properties similar to those of the most catalytically active zeolites, and it provides critical tools to achieve a deeper understanding of structure–reactivity relations in zeolites

Water adsorption on a double-layer silicate film was studied by using infrared reflection-absorption spectroscopy, thermal desorption spectroscopy and scanning tunneling microscopy. Under vacuum conditions, small amounts of silanols (Si-OH) could only be formed upon deposition of an ice-like (amorphous solid water, ASW) film and subsequent heating to room temperature. Silanol coverage is considerably enhanced by low-energy electron irradiation of an ASW pre-covered silicate film. The degree of hydroxylation can be tuned by the irradiation parameters (beam energy, exposure) and the ASW film thickness. The results are consistent with a generally accepted picture that hydroxylation occurs through hydrolysis of siloxane (Si-O-Si) bonds in the silica network. Calculations using density functional theory show that this may happen on Si-O-Si bonds, which are either parallel (i.e., in the topmost silicate layer) or vertical to the film surface (i.e., connecting two silicate layers). In the latter case, the mechanism may additionally involve the reaction with a metal support underneath. The observed vibrational spectra are dominated by terminal silanol groups (ν(OD) band at 2763 cm(-1)) formed by hydrolysis of vertical Si-O-Si linkages. Film dehydroxylation fully occurs only upon heating to very high temperatures (∼ 1200 K) and is accompanied by substantial film restructuring, and even film dewetting upon cycling hydroxylation/dehydroxylation treatment.

Well-ordered, ultrathin silica films grown on metal substrates are composed of layers of corner-sharing [SiO4] tetrahedra (silicatene). Yet unrealized in practice as unsupported material, the double-layer silicatene could constitute the thinnest silica membrane ever fabricated. We addressed here the permeability of such a membrane by using a metal substrate as a gas detector. Permeation of CO and D2 was examined by infrared reflection absorption spectroscopy and temperature-programmed desorption. The results reveal a complex response of such systems upon gas exposures which involves gas transport through amorphous silica pores as well as chemisorption and diffusion across the metal surface underneath the silicatene. Such a hybrid system, which would combine a robust molecular-sieve membrane and a chemically active metal underneath, could become interesting materials for technological applications, in particular, in catalysis and sensors.

Adsorption of water on a metal-supported sheet-like silica film was studied by infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). As expected, the silica surface is essentially hydrophobic. Hydroxo species, primarily in the form of isolated silanols (Si–OH), were observed only upon water condensation at low temperatures and subsequent heating above 200 K. The amounts of silanol species account for less than a few percent of the surface Si atoms, and they are found to be thermally stable up to 900 K. Isotopic experiments showed that hydroxyls form almost exclusively from the adsorbed water molecules and do not undergo scrambling with the lattice oxygen atoms upon heating. Steps within the silica sheet, due to a terraced topography and/or the presence of “holes”, are proposed as the active sites for hydroxylation. The acidic properties of silanol species were studied with CO and NH₃ as probe molecules. In the case of ammonia, an H–D exchange reaction was observed between OD species and NH₃, and the same reaction was found to occur for OD(OH) and H₂O(D₂O), respectively. The results are compared with those reported in the literature for amorphous silica

Abstract. Bilayer silicate films grown on metal substrates are weakly bound to the metal surfaces, which allows ambient gas molecules to intercalate the oxide/metal interface. In this work, we studied the interaction of oxygen with Ru(0001) supported ultrathin silicate and aluminosilicate films at elevated O 2 pressures (10 -5 -10 mbar) and temperatures (450 -923 K). The results show that the silicate films stay essentially intact under these conditions, and oxygen in the film does not readily exchange with oxygen in the ambient. O 2 molecules readily penetrate the film and dissociate on the underlying Ru surface underneath. The silicate layer does however strongly passivate the Ru surface towards RuO 2 (110) oxide formation that readily occurs on bare Ru(0001) under the same conditions. The results indicate considerable spatial effects for oxidation reactions on metal surfaces in the confined space at the interface. Moreover, the aluminosilicate films completely suppress the Ru oxidation, providing some rationale for using crystalline aluminosilicates in anti-corrosion coatings.

ABSTRACT:BaO x (0.5 MLE -10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NO x storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaO x /Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For θ BaOx < 1 MLE, (2 Â 2) or (1 Â 2) ordered overlayer structures were observed on Pt(111), whereas BaO(110) surface termination was detected for θ BaOx = 1.5 MLE. Thicker films (θ BaOx g 2.5 MLE) were found to be amorphous. Extensive NO 2 adsorption on BaO x (10 MLE)/Pt(111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaO x (10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO 2 formation at 650 K and (2) NO(g) + O 2 (g) evolution at 700 K. O 2 (g) treatment of the BaO x (10 MLE)/ Pt(111) surface at 873 K facilitates the BaO 2 formation and results in the agglomeration of BaO x domains leading to the generation of exposed Pt(111) surface sites. BaO 2 formed on BaO x (10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (θ BaOx = 2.5 MLE), BaO 2 partially decomposes into BaO, indicating that small BaO 2 clusters in close proximity of the exposed Pt(111) sites are prone to decomposition. Nitrate decomposition temperature decreases monotonically from 550 to 375 K with decreasing BaO x coverage within θ BaOx = 0.5 to 1.0 MLE. Nitrate decomposition occurs at a rather constant temperature range of 650À700 K for thicker BaO x overlayers (2.5 MLE < θ BaOx < 10 MLE). These two distinctly characteristic BaO x -coveragedependent nitrate decomposition regimes are in very good agreement with the observation of the so-called "surface" and "bulk" barium nitrates previously reported for realistic NSR catalysts, clearly demonstrating the strong dependence of the nitrate thermal stability on the NO x storage domain size.

The vibrational properties of well-defined, two-dimensional silica films grown on Ru(0001) are characterized by high-resolution electron energy loss spectroscopy (HREELS). It is an interesting model system because it can adopt both crystalline and vitreous states. A transformation between these states induced by thermal annealing does hardly change the vibrational spectrum despite the redistribution of ring sizes. This holds good for the two intense phonon modes as well as for a variety of weaker modes observed by HREELS. The HREELS spectra allow the characterization of the structural arrangement of the oxygen atoms on the Ru(0001) surface underneath the silica bilayer. The density of oxygen at the interface can be controlled by the oxygen partial pressure during annealing, resulting in a characteristic change of the corresponding signals, which can be assigned to different oxygen structures based on density functional theory calculations. By comparison with quantum mechanical calculations and spectroscopic results from the literature, we assign most of the remaining weak signals observed here to the dipole-inactive modes of the bilayer film, structural imperfections such as patches of monolayer structure, and additional silica particles on top of the bilayer film.

Cataloged from PDF version of article.In the current contribution, we provide a direct demonstration of the thermally induced surface structural transformations of an alkaline-earth oxide/transition metal oxide interface that is detrimental to the essential catalytic functionality of such mixed-oxide systems toward particular reactants. The BaO(x)/TiO(2)/Pt(111) surface was chosen as a model interfacial system where the enrichment of the surface elemental composition with Ti atoms and the facile diffusion of Ba atoms into the underlying TiO(2) matrix within 523-873 K leads to the formation of perovskite type surface species (BaTiO(3)/Ba(2)TiO(4)/Ba(x)Ti(y)O(z)). At elevated temperatures (T > 973 K), excessive surface segregation of Ti atoms results in an exclusively TiO(2)/TiO(x)-terminated surface which is almost free of Ba species. Although the freshly prepared BaO(x)/TiO(2)/Pt(111) surface can strongly adsorb ubiquitous catalytic adsorbates such as NO(2) and CO(2), a thermally deactivated surface at T > 973 K practically loses all of its NO(2)/CO(2) adsorption capacity due to the deficiency of surface BaO(x) domains