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Lateral interactions between coadsorbed hydrocarbon species play an important role in their chemical transformations on catalytic metal surfaces. In this report, we present a mechanistic study on m...

We present a mechanistic study addressing formation, adsorption configuration, and stability of acetophenone oligomers on a catalytically active Pt(111) surface by a combination of scanning tunneli...

A mechanistic study on interaction of a chiral modifier - (R)-(+)-1-(1-naphthylethylamine) (R-NEA) - with a single crystalline Pt(111) surface is reported. The details of the adsorption geometry of individual R-NEA molecules and their intermolecular interactions are addressed by combination of infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM). The spectroscopic observations suggest that the molecules are tilted with respect to the underlying metal substrate with the long axis of the naphthyl ring being parallel and the short axis tilted with respect to the surface. In the medium coverage range, formation of directed 3-5 membered chains was observed by STM for the first time, which points to intermolecular bonding between individual molecules and might account for an unusual tilted adsorption geometry deduced from the IR spectra. Based on the STM images revealing the atomic structure of the Pt grid close to the R-NEA chains, we propose the adsorption configuration of NEA fitting both the IRAS and STM data. The obtained results suggest that this strong intermolecular interaction energetically stabilizes the tilted adsorption geometry of the naphthyl ring, which otherwise would be expected to lie flat on the metal to maximize the dispersive interactions. At the coverage close to saturation, R-NEA builds a self-assembled overlayer with hexagonal symmetry, exhibiting intermolecular distances larger than in the directed chains.

A microscopic-level understanding of the interaction of hydrocarbons with transition metal surfaces is an important prerequisite for rational design of new materials with improved catalytic propert...

Keto–enol tautomerization of carbonyl compounds to their enol form is theoretically predicted to enable a low-barrier pathway for hydrogenation of normally very stable C═O bond. In the scope of this anticipated mechanism, the reaction can proceed via two consecutive steps, including the formation of enol followed by an H insertion into the enolic C═C bond, and exhibits a lower activation barrier than the direct H insertion into the carbonyl group. Here, we present an experimental study on atomistic level details of hydrogenation of a simple carbonyl compound acetophenone over Pt(111) providing experimental evidence that keto–enol tautomerization plays a crucial role in this reaction. By employing a combination of spectroscopic and imaging techniques, we show that acetophenone forms ketone–enol dimers, in which the normally unstable form of enol is stabilized by H-bonding with the carbonyl group of the neighboring acetophenone molecule. These ketone–enol dimers can attach an H atom to form a reaction inter...

Lateral interactions between coadsorbed molecules on surfaces might trigger specific reaction pathways and stabilize otherwise energetically unstable reaction intermediates. These phenomena can crucially affect the overall activity and selectivity of catalytic processes and can be employed for purposeful functionalization of surfaces to improve their catalytic efficiency. In this report, we present a study on the atomistic-level details of intermolecular interactions between acetophenone adsorbates on Pt. We show that these interactions trigger an important reaction of keto–enol tautomerization producing an enol reaction intermediate, which is envisaged to enable low-barrier hydrogenation of simple carbonyl compounds. By combining scanning tunneling microscopy and infrared reflection–absorption spectroscopy, we found that several types of hydrogen-bonded oligomer species can be formed, including ketone–enol dimers and ketone–enol–enol trimers. Their evolution exhibits a strong temperature dependence, sugg...

Tautomerisation of simple carbonyl compounds to their enol counterparts on metal surfaces is envisaged to enable an easier route for hydrogenation of the C=O bond in heterogeneously catalyzed reactions. To understand the mechanisms of enol formation and stabilization over catalytically active metal surfaces, we performed a mechanistic study on keto-enol tautomerization of a monocarbonyl compound acetophenon over Pt(111) surface. By employing infrared reflection adsorption spectroscopy in combination with scanning tunneling microscopy, we found that enol can be formed by building a ketone-enol dimer, in which one molecule in the enol form is stabilized through hydrogen bonding to the carbonyl group of the second ketone molecule. Based on the investigations of the co-adsorption behavior of acetophenone and hydrogen, we conclude that keto-enol tautomerization occurs in the intramolecular process and does not involve hydrogen transfer through the surface hypothesized previously.

Understanding the interplay between thermodynamics and kinetics is of high importance for the optimization of catalytic reactions involving the adsorption of CO2 on CeO2 (ceria). The present study explores the interaction of CO2 with ceria powder in near-realistic conditions by correlating adsorption and thermal desorption analyses. Activation energies for desorption, Ea, and kinetic parameters (adsorption time constants, τ, and sticking coefficients, s0) are determined using a new methodology based on surface science models. The sticking coefficients obtained for CO2 on ceria powder are significantly lower than those observed for CO2 on flat surfaces. CO2 is found to adsorb most rapidly on sites attributed to surface defects. CO2 adsorption is slower on nondefective active sites, leading to the formation of various carbonate species. The desorption analysis indicates that each peak in the CO2-TPD profiles is composed of several subpeaks, resulting from various binding sites for CO2 on the polycrystalline...

New lanthanide containing ionic liquids (ILs) based on thiocyanate complexes of the type [BMIM]x-3[Ln(NCS)x(H2O)y] (BMIM = 1-Butyl-3-Methylimidazolium; x = 6,7,8; y = 0–2, x + y The present work includes a thermal decomposition study of the anhydrous ionic liquids of this type with Samarium cation as the lanthanide. The thermal decomposition of [BMIM][NCS], [BMIM]3[Sm(NCS)6], [BMIM]4[Sm(NCS)7] and [BMIM]5[Sm(NCS)8] was investigated by employing a combination of thermo-gravimetric analysis (TGA), and temperature programmed desorption (TPD) coupled with mass spectrometry. The decomposition temperature of the samarium-containing ILs were found to be higher than that measured for [BMIM][NCS], suggesting stabilization of the ILs by complexation to the samarium cation. Thermal gravimetric analysis of [BMIM]x-3[Sm(NCS)x] confirms that the thermal decomposition occurs in a stepwise fashion to give Sm(NCS)3 via a [BMIM]3[Sm(NCS)6] intermediate, whereby [BMIM][NCS] units are decomposed to give gaseous products. Additional experimental evidence obtained with TPD-MS supporting this decomposition route and reveal that each [BMIM][NCS] unit decomposes by the rapid nucleophilic attack of the thiocyanate anion at the C-N bond of the methyl- and butyl- imidazole groups, and their proposed reaction mechanism are introduced here.

A new custom-designed ultrahigh vacuum (UHV) apparatus combining molecular beam techniques and in situ surface spectroscopy for reactivity measurements on complex nanostructured model surfaces is described. It has been specifically designed to study the mechanisms, kinetics, and dynamics of heterogeneously catalyzed reactions over well-defined model catalysts consisting of metal nanoparticles supported on thin oxide films epitaxially grown on metal single crystals. The reactivity studies can be performed in a broad pressure range starting from UHV up to the ambient pressure conditions. The UHV system includes (i) a preparation chamber providing the experimental techniques required for the preparation and structural characterization of single-crystal based model catalysts such as oxide supported metal particles or ordered oxide surfaces and (ii) the reaction chamber containing three molecular beams—two effusive and one supersonic, which are crossed at the same point on the sample surface, infrared reflection-absorption spectroscopy for the detection of surface-adsorbed species, and quadrupole mass spectrometry for gas phase analysis. The supersonic beam is generated in a pulsed supersonic expansion and can be modulated via a variable duty-cycle chopper. The effusive beams are produced by newly developed compact differentially pumped sources based on multichannel glass capillary arrays. Both effusive sources can be modulated by a vacuum-motor driven chopper and are capable of providing high flux and high purity beams. The apparatus contains an ambient pressure cell, which is connected to the preparation chamber via an in situ sample transfer system and provides an experimental possibility to study the reactivity of well-defined nanostructured model catalysts in a broad range of pressure conditions—up to ambient pressure—with the gas phase analysis based on gas chromatography. Additionally, a dedicated deposition chamber is connected to the preparation chamber, which is employed for the in situ functionalization of model surfaces with large organic molecules serving as promoters or modifiers of chemical reactions. We present a general overview of the apparatus as well as a description of the individual components and their interplay. The results of the test measurements involving the most important components are presented and discussed.A new custom-designed ultrahigh vacuum (UHV) apparatus combining molecular beam techniques and in situ surface spectroscopy for reactivity measurements on complex nanostructured model surfaces is described. It has been specifically designed to study the mechanisms, kinetics, and dynamics of heterogeneously catalyzed reactions over well-defined model catalysts consisting of metal nanoparticles supported on thin oxide films epitaxially grown on metal single crystals. The reactivity studies can be performed in a broad pressure range starting from UHV up to the ambient pressure conditions. The UHV system includes (i) a preparation chamber providing the experimental techniques required for the preparation and structural characterization of single-crystal based model catalysts such as oxide supported metal particles or ordered oxide surfaces and (ii) the reaction chamber containing three molecular beams—two effusive and one supersonic, which are crossed at the same point on the sample surface, infrared reflecti...

We determined the oxidation mechanism of porous ST-198, which mainly comprises Zr2Fe. Oxidation kinetics depended on temperature, oxygen partial pressure, and oxidation extent. The passivation role of oxidation in hydrogen scavenging is probably due to the development of a surface oxide, independent of oxygen concentration. Zr2Fe would be a superior hydrogen getter in oxygen-contaminated environments at high temperatures, as most oxygen will be consumed at the outer shell by mass transfer limitations, protecting the bulk of the getter for hydrogen scavenging.

Ni(II)L(2), L(2) = 1-propyl-1,3,5,8,12-pentaazacyclotetradecane, was covalently bound to a silica support. This complex can be reversibly oxidized to the corresponding Ni(III) complex. The latter complex is relatively long lived. Therefore electron exchange columns based on this material can be prepared.

Identifying the surface processes governing the selectivity in hydrogenation of α,β-unsaturated carbonyl compounds on late transition metals is crucial for the rational design of catalytic material...

We determined the oxidation mechanism of porous ST-198, which mainly comprises Zr2Fe. Oxidation kinetics depended on temperature, oxygen partial pressure, and oxidation extent. The passivation role of oxidation in hydrogen scavenging is probably due to the development of a surface oxide, independent of oxygen concentration. Zr2Fe would be a superior hydrogen getter in oxygen-contaminated environments at high temperatures, as most oxygen will be consumed at the outer shell by mass transfer limitations, protecting the bulk of the getter for hydrogen scavenging.

Recently, the nature of the carbon radicals stabilized in various coals was characterized using Electron Paramagnetic Resonance (EPR) spectroscopy. It was demonstrated that introducing diamagnetic gases, such as He, CO2, or N2, under STP conditions to the coal surface induces the appearance of a new type of carbon surface radical. This interesting phenomenon was not observed for all coal types, which suggests that the use of EPR measurements can provide information on functional groups that exist on the carbon surface. In the current study coupling Nuclear Magnetic Resonance (NMR) with gas flow in situ EPR measurements significantly enhances the ability to characterize the nature of these radicals and the surface functional groups of coal samples. It was observed that the oxidative reaction with aliphatic groups leads to the increase in stable carbon centered radicals. In addition, there are some species of carbon centered radicals that show reversible binding to O2. This phenomena, however, is dependent on the coal rank, sample porosity and the degree of the coal sample to undergo structural changes under the LTO process. These findings shed new light onto the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.