Activation of CH, NH, and N by Tantalum Ions, Clusters and Their Oxides: What Can Be Learnt from Studies of Ions in the Gas Phase.

The emission control of harmful compounds and greenhouse gases and the development of alternative, sustainable fuel sources is a major focus in current research. A solution for this problem lies in the development of efficient catalytic materials. Here, gas phase model systems represent prominent examples for obtaining fundamental insights on reaction properties of prospective catalytic systems.

PtH : A Cuboctahedral Platinum Hydride Cluster Cage.

The platinum hydride cluster PtH is studied in gas phase by a combination of trapped ion electron diffraction and density functional theory computations. We find a cuboctahedral platinum cage with bridge bound hydrogen atoms. This unusual structure is stabilized by Pt-H-Pt multicenter bonds and shows characteristics of spherical aromaticity.

Same size, same support, same spectator? Selective acetylene hydrogenation on supported Pd nanoparticles.

The selective hydrogenation of acetylene catalyzed by Pd nanoparticles is industrially used to increase the purity of ethylene. Despite the implementation of Pd based catalysts on an industrial scale, little is known about metal-support interactions on a fundamental level due to the complexity of these systems. In this study, the influence of metal-support interactions between Pd nanoparticles and two electronically modified a-SiO thin films on acetylene hydrogenation is investigated under ultra-high vacuum (UHV) conditions.

On-Surface Carbon Nitride Growth from Polymerization of 2,5,8-Triazido--heptazine.

Carbon nitrides have recently come into focus for photo- and thermal catalysis, both as support materials for metal nanoparticles as well as photocatalysts themselves. While many approaches for the synthesis of three-dimensional carbon nitride materials are available, only top-down approaches by exfoliation of powders lead to thin-film flakes of this inherently two-dimensional material. Here, we describe an in situ on-surface synthesis of monolayer 2D carbon nitride films as a first step toward precise combination with other 2D materials.

Toward a Comprehensive Understanding of Photocatalysis: What Systematic Studies and Alcohol Surface Chemistry on TiO(110) Have to Offer for Future Developments.

Heterogeneous photocatalytic systems are usually described based on electrochemistry, which the vast majority of interpretations and strategies for optimizing photocatalysts rely on. Charge carrier dynamics are usually in the spotlight, whereas the surface chemistry of the photocatalyst is neglected. This is unjustified, because studies on alcohol photoreforming on metal-decorated rutile single crystals revealed that the electrochemical reaction model is not generally applicable.

Does Cluster Encapsulation Inhibit Sintering? Stabilization of Size-Selected Pt Clusters on FeO(001) by SMSI.

The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal-support interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for subnanometer clusters, where concomitant sintering and alloying might play a significant role.

A re-useable microreactor for dynamic and sensitive photocatalytic measurements: Exemplified by the photoconversion of ethanol on Pt-loaded titania P25.

Despite numerous advancements in synthesizing photoactive materials, the evaluation of their catalytic performance remains challenging since their fabrication often involves tedious strategies, yielding only low quantities in the μ-gram scale. In addition, these model catalysts exhibit different forms, such as powders or film(-like) structures grown on various supporting materials. Herein, we present a versatile gas phase μ-photoreactor, compatible with different catalyst morphologies, which is, in contrast to existing systems, re-openable and -useable, allowing not only post-characterization of the photocatalytic material but also enabling catalyst screening studies in short experimental time intervals.

The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters.

Size-selected iron oxide nanoclusters are outstanding candidates for technological-oriented applications due to their high efficiency-to-cost ratio. However, despite many theoretical studies, experimental works on their oxidation mechanism are still limited to gas-phase clusters. Herein we investigate the oxidation of graphene-supported size-selected Fe clusters by means of high-resolution X-ray Photoelectron Spectroscopy.

Direct Coupling of Methane and Carbon Dioxide on Tantalum Cluster Cations.

Understanding molecular-scale reaction mechanisms is crucial for the design of modern catalysts with industrial prospect. Through joint experimental and computational studies, we investigate the direct coupling reaction of CH and CO , two abundant greenhouse gases, mediated by Ta ions to form larger oxygenated hydrocarbons. Coherent with proposed elementary steps, we expose products of CH dehydrogenation [Ta CH ] to CO in a ring electrode ion trap.

CO-Activation by size-selected tantalum cluster cations (Ta): thermalization governing reaction selectivity.

The reactions of tantalum cluster cations of different sizes toward carbon dioxide are studied in an ion trap under multi-collisional conditions. For all sizes studied, consecutive reactions with several CO molecules are observed. This reveals two different pathways, namely oxide formation and the pickup of an entire molecule.

Tunable induced circular dichroism in gels.

The ICD phenomenon has drawn a lot of attention in recent years in applicable fields such as chiral sensing and chiroptical devices. In this work, we first gaze at the issues of thin spin-coated films not being able to deliver consistent ICD signals. A hypothesis of the underlying problem is proposed through a brief elucidation of the spin-coating process.

Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study.

The possibility of depositing precisely mass-selected Ag clusters (Ag, Ag, and Ag) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement.

Water-Assisted Electron-Induced Chemistry of the Nanofabrication Precursor Iron Pentacarbonyl.

Focused electron beam deposition often requires the use of purification techniques to increase the metal content of the respective deposit. One of the promising methods is adding HO vapor as a reactive agent during the electron irradiation. However, various contrary effects of such addition have been reported depending on the experimental condition.

Understanding laser desorption with circularly polarized light.

We present aspects of emerging optical activity in thin racemic 1,1'-Bi-2-naphthol films upon irradiation with circularly polarized light and subsequent resonant two-photon absorption in the sample. Thorough analysis of the sample morphology is conducted by means of (polarization-resolved) optical microscopy and scanning electron microscopy (SEM). The influence of crystallization on the nonlinear probing technique (second harmonic generation circular dichroism [SHG-CD]) is investigated.

Carbide Dihydrides: Carbonaceous Species Identified in Ta -Mediated Methane Dehydrogenation.

The products of methane dehydrogenation by gas-phase Ta clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H] reveal a dominance of vibrational bands of a H Ta C carbide dihydride structure over those indicative for a HTa CH carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M typically leads to the formation of either MCH carbene or HMCH carbyne hydride structures, the observation of a H MC carbide dihydride structure implies that it is imperative to consider this often-neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.

Regulating Photochemical Selectivity with Temperature: Isobutanol on TiO(110).

Selective photocatalytic transformations of chemicals derived from biomass, such as isobutanol, have been long envisioned for a sustainable chemical production. A strong temperature dependence in the reaction selectivity is found for isobutanol photo-oxidation on rutile TiO(110). The strong temperature dependence is attributed to competition between thermal desorption of the primary photoproduct and secondary photochemical steps.

Catalytic Non-Oxidative Coupling of Methane on TaO.

Mass-selected TaO cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C-C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane.

Hydrogenation of small hydrocarbons on MgO supported Pd nanoparticles: The A-E-model expanded.

The hydrogenation of ethylene and acetylene was studied on a Pd/MgO/Mo(100) model system containing palladium particles with a narrow size distribution around Pd (Pd to Pd). Reactivity measurements were carried out in an ultrahigh vacuum chamber under isothermal conditions in the presence of deuterium. The catalyst system can readily hydrogenate both of these small molecules, and for acetylene, an alternative reaction network exists, in which it is trimerized to benzene.

Thermal C-O coupling reactions of Ta methylene clusters [TaCH] (n = 1, 4) with O.

Cationic tantalum carbenes [TaCH] and [TaCH], products of thermal methane dehydrogenation, are collected and stored in a ring-electrode ion trap. In there, potential C-O coupling reactions are probed by exposing the tantalum carbenes to dioxygen in the presence of helium buffer gas, thereby facilitating reactions under-well defined multi-collision conditions at 300 K. Time-of-flight mass spectrometry determines the time-dependent reaction products and a kinetic analysis illustrates significantly different selectivities.

Enantiospecific Desorption Triggered by Circularly Polarized Light.

The interest in enantioseparation and enantiopurification of chiral molecules has been drastically increasing over the past decades, since these are important steps in various disciplines such as pharmaceutical industry, asymmetric catalysis, and chiral sensing. By exposing racemic samples of BINOL (1,1'-bi-2-naphthol) coated onto achiral glass substrates to circularly polarized light, we unambiguously demonstrate that by controlling the handedness of circularly polarized light, preferential desorption of enantiomers can be achieved. There are currently no mechanisms known that would describe this phenomenon.

Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO (110).

According to textbooks, tertiary alcohols are inert towards oxidation. The photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal reveals unexpected and new reactions, which can be described as disproportionation into an alkane and the respective ketone. In contrast to primary and secondary alcohols, in tertiary alcohols the absence of an α-H leads to a C-C-bond cleavage instead of the common abstraction of hydrogen.

Introducing catalysis in photocatalysis: What can be understood from surface science studies of alcohol photoreforming on TiO.

Mechanisms in heterogeneous photocatalysis have traditionally been interpreted by the band-structure model and analogously to electrochemistry. This has led to the establishment of 'band-engineering' as a leading principle for the discovery of more efficient photocatalysts. In such a picture, mainly thermodynamic aspects are taken into account, while kinetics are often ignored.