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 CH4 and CO2 , two abundant greenhouse gases, mediated by Ta1,4 + ions to form larger oxygenated hydrocarbons. Coherent with proposed elementary steps, we expose products of CH4 dehydrogenation [Ta1,4 CH2 ]+ to CO2 in a ring electrode ion trap. Product analysis and reaction kinetics indicate a predisposition of the tetramers for C-O coupling with a conversion to products of CH2 O, whereas atomic cations enable C-C coupling yielding CH2 CO. Selected experimental findings are supported by thermodynamic computations, connecting structure, electronic properties, and catalyst function. Moreover, the study of bare Ta1,4 + compounds indicates that methane dehydrogenation is a significant initial step in the direct coupling reaction, enabling new, yet unknown reaction pathways.

CO2-Activation by size-selected tantalum cluster cations (Ta1-16+): 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 CO2 molecules are observed. This reveals two different pathways, namely oxide formation and the pickup of an entire molecule. Supported by calculations of the thermochemistry of TanO+ formation upon reaction with CO2, changes in the branching ratios at a particular cluster size are related to heat effects due to the vibrational heat capacity of the clusters and the exothermicity of the reaction.

Room-Temperature Methane Activation Mediated by Free Tantalum Cluster Cations: Size-by-Size Reactivity.

The energetics of small cationic tantalum clusters and their gas-phase adsorption and dehydrogenation reaction pathways with methane are investigated with ion-trap experiments and spin-density-functional-theory calculations. Tan+ clusters are exposed to methane under multicollision conditions in a cryogenic ring electrode ion-trap. The cluster size affects the reaction efficiency and the number of consecutively dehydrogenated methane molecules. Small clusters (n = 1-4) dehydrogenate CH4 and concurrently eliminate H2, while larger clusters (n > 4) demonstrate only molecular adsorption of methane. Unique behavior is found for the Ta+ cation, which dehydrogenates consecutively up to four CH4 molecules and is predicted theoretically to promote formation of a [Ta(CH2-CH2-CH2)(CH2)]+ product, exhibiting C-C coupled groups. Underlying mechanisms, including reaction-enhancing couplings between potential energy surfaces of different spin-multiplicities, are uncovered.

Catalytic Non-Oxidative Coupling of Methane on Ta8O2.

Mass-selected Ta8O2+ cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H2, 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. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study.

Regulating Photochemical Selectivity with Temperature: Isobutanol on TiO2(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 TiO2(110). The strong temperature dependence is attributed to competition between thermal desorption of the primary photoproduct and secondary photochemical steps. The aldehyde, isobutanal, is the primary photoproduct of isobutanol. At room temperature, isobutanal is obtained selectively from photo-oxidation because of rapid thermal desorption. In contrast, secondary photo-oxidation of isobutanal to propane dominates at lower temperature (240 K) due to the persistence of isobutanal on the surface after it is formed. The byproduct of isobutanal photo-oxidation is CO, which is evolved at higher temperature as a consequence of thermal decomposition of an intermediate, such as formate. The photo-oxidation to isobutanal proceeds after thermally induced isobutoxy formation. These results have strong implications for controlling the selectivity of photochemical processes more generally, in that, selectivity is governed by competition of desorption vs secondary photoreaction of products. This competition can be exploited to design photocatalytic processes to favor specific chemical transformations of organic molecules.

Carbide Dihydrides: Carbonaceous Species Identified in Ta4 + -Mediated Methane Dehydrogenation.

The products of methane dehydrogenation by gas-phase Ta4 + 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 H2 Ta4 C+ carbide dihydride structure over those indicative for a HTa4 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 MCH2 + carbene or HMCH+ carbyne hydride structures, the observation of a H2 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.

Why co-catalyst-loaded rutile facilitates photocatalytic hydrogen evolution.

As the conduction band edge of rutile is close to the reduction potential of hydrogen, there is a long-lasting discussion on whether molecular hydrogen can be evolved from this semiconductor. Our study on methanol photoreforming in the ultra-high vacuum reveals that photocatalysts comprising a TiO2(110) single crystal decorated with platinum clusters indeed enable the evolution of H2. This is attributed to a new type of mechanism, in which the co-catalyst acts as a recombination center for hydrogen and not as a reduction site of a photoreaction. This mechanism is an alternative pathway to the commonly used mechanism derived from photoelectrochemistry and must particularly be considered for systems, in which reducible semiconductors enable the surface diffusion of hydrogen species.

Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO2 (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. Surprisingly, bonds to methyl groups are not cleaved when the alcohol exhibits longer alkyl chains in the α-position to the hydroxyl group. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long-chain alkane resulting from recombination of two alkyl moieties. This work demonstrates that new synthetic routes may become possible by introducing photocatalytic reaction steps in which the co-catalysts may also play a decisive role.

Photocatalytic selectivity switch to C-C scission: α-methyl ejection of tert-butanol on TiO2(110).

The thermal and photochemical mechanistic pathways for tertiary alcohols on the rutile TiO2(110)-surface are studied with the example of tert-butanol. While the thermal reaction is known to yield isobutene, the photochemical ejection of a methyl radical is observed at 100 K. The C-C scission, which is accompanied by the formation of acetone, is the only photochemical reaction pathway at this temperature and can be attributed to the reaction of photoholes that are created upon UV-light illumination at the surface of the n-type semiconductor. At 293 K the selectivity of the reaction changes, as isobutene is additionally formed photochemically. A comparison of the kinetics of the different reactions reveals further insights. Together with the quantitative evaluation of the reaction products at low temperatures and the comparison of the reaction pathways at different temperatures it is demonstrated how thermal effects can influence the selectivity of the reactions in photocatalysis.

Communication: Water activation and splitting by single metal-atom anions.

We report experimental and computational results pertaining to the activation and splitting of single water molecules by single atomic platinum anions. The anion photoelectron spectra of [Pt(H2O)]-, formed under different conditions, exhibit spectral features that are due to the anion-molecule complex, Pt-(H2O), and to the reaction intermediates, HPtOH- and H2PtO-, in which one and two O-H bonds have been broken, respectively. Additionally, the observations of PtO- and H2 + in mass spectra strongly imply that water splitting via the reaction Pt- + H2O → PtO- + H2 has occurred. Extending these studies to nickel and palladium shows that they too are able to activate single water molecules, as evidenced by the formation of the reaction intermediates, HNiOH- and HPdOH-. Computations at the coupled cluster singles and doubles with perturbatively connected triples level of theory provide structures and vertical detachment energies (VDEs) for both HMOH- and H2MO- intermediates. The calculated and measured VDE values are in good agreement and thus support their identification.

Ethanol surface chemistry on MBE-grown GaN(0001), GaOx/GaN(0001), and Ga2O3(2¯01).

In this work, ethanol is used as a chemical probe to study the passivation of molecular beam epitaxy-grown GaN(0001) by surface oxidation. With a high degree of oxidation, no reaction from ethanol to acetaldehyde in temperature-programmed desorption experiments is observed. The acetaldehyde formation is attributed to a mechanism based on α-H abstraction from the dissociatively bound alcohol molecule. The reactivity is related to negatively charged surface states, which are removed upon oxidation of the GaN(0001) surface. This is compared with the Ga2O3(2¯01) single crystal surface, which is found to be inert for the acetaldehyde production. These results offer a toolbox to explore the surface chemistry of nitrides and oxynitrides on an atomic scale and relate their intrinsic activity to systems under ambient atmosphere.

Isomer-Selective Detection of Aromatic Molecules in Temperature-Programmed Desorption for Model Catalysis.

Based on three different molecules dosed on a Pt(111) single crystal the selectivity and sensitivity of REMPI-TPD in UHV is investigated for a potential application in heterogeneous catalysis. It is shown that the two structural isomers ethylbenzene and p-xylene can be discriminated by REMPI in a standard TPD experiment. The latter is not possible for the ionization with electrons in a Q-MS. It is further demonstrated by benzene TPD studies that the sensitivity of the REMPI-TOF-MS is comparable to commercial EI-Q-MS solutions and enables the detection of less than 0.6% molecules of a monolayer.

Ethanol photocatalysis on rutile TiO2(110): the role of defects and water.

In this work we present a stoichiometric reaction mechanism for the photocatalytic ethanol oxidation on TiO2(110). The reaction products are analyzed either under reaction conditions or after irradiation at lower temperatures. Water is identified as a quantitative by-product, which resides in a defect site. These water molecules cause a blocking of the defect sites which results in poisoning of the catalyst. By different preparation techniques of the TiO2(110) surface, the role of surface defects is further elucidated and the role of molecular oxygen is investigated. Based on the investigation, a complete photochemical reaction mechanism is given, which provides insights into general photon driven oxidation mechanisms on TiO2.

Cluster size effects in the photocatalytic hydrogen evolution reaction.

The photocatalytic water reduction reaction on CdS nanorods was studied as function of Pt cluster size. Maximum H2 production is found for Pt46. This effect is attributed to the size dependent electronic properties (e.g., LUMO) of the clusters with respect to the band edges of the semiconductor. This observation may be applicable for the study and interpretation of other systems and reactions, e.g. H2O oxidation or CO2 reduction.

Infra-red spectroscopy of size selected Au25, Au38 and Au144 ligand protected gold clusters.

Through the discovery of ligand protected metal clusters with cores of a precise number of atoms, the exploration of the third dimension of the periodic table for fundamental research and also for applications has become less remote. So far, the exact number of metal atoms in the core has been determined unambiguously only using mass spectrometry and single crystal X-ray diffraction. Gold clusters protected by 2-phenylethanethiol ligands, for instance, show distinct magic numbers that correspond to either electronic or geometric shell closings. For efficient control of their synthesis simple-to-use in situ spectroscopies are required. In the specific case of Au25(SCH2CH2Ph)18 clusters (1) we found a distinct shift of the aromatic C-H stretching band from 3030-3100 cm(-1) to below 3000 cm(-1) whose origin is discussed as an electronic interaction of the aromatic rings of the ligands with each other or with the gold core. This IR-feature is specific for Au25; the spectra of Au38(SCH2CH2Ph)24 (2) and Au144(SCH2CH2Ph)60 (3) clusters do not show this distinct shift and their IR-spectra in the C-H stretching regime are similar to that of the bare ligand. This significant change in the IR spectrum of Au25(SCH2CH2Ph)18 is not only of fundamental interest but also allows for in situ determination of the purity and monodispersity of the sample using FTIR spectroscopy during synthesis.

Rational design, characterization and catalytic application of metal clusters functionalized with hydrophilic, chiral ligands: a proof of principle study.

A proof of principle is presented for the rational design of metal clusters functionalized with hydrophilic, chiral ligands. A colloidal method is used to prepare "unprotected" metal clusters of well-defined size that are subsequently functionalized in a separate step with hydrophilic, chiral ligands. As clusters from the same batch are functionalized with different organic molecules while the cluster size is maintained, the approach allows for systematic investigations and the differences in the observed properties to be related to the influence of the functionalizing ligand. Within this work cysteine and two cysteine derivatives (glutathione and N-acetyl-cysteine) are used as functionalizing ligands for Pt clusters. The materials are characterized using various methods allowing for the determination of ligand coverage, binding mode and chiro-optical properties. Finally, 2-butanone hydrogenation is used as a simple model reaction to demonstrate that these systems exhibit the potential to be used as asymmetric, heterogeneous catalysts. The observed differences in selectivity and reactivity are discussed based on the knowledge gained from the characterization.

Communication: In search of four-atom chiral metal clusters.

A combined study utilizing anion photoelectron spectroscopy and density functional theory was conducted to search for four-atom, chiral, metal, and mostly metal clusters. The clusters considered were AuCoMnBi(-/0), AlAuMnO(-/0), AgMnOAl(-/0), and AuAlPtAg(-/0), where the superscripts, (-/0), refer to anionic and neutral cluster species, respectively. Based on the agreement of experimentally and theoretically determined values of both electron affinities and vertical detachment energies, the calculated cluster geometries were validated and examined for chirality. Among both anionic and neutral clusters, five structures were identified as being chiral.

Size-selected subnanometer cluster catalysts on semiconductor nanocrystal films for atomic scale insight into photocatalysis.

We introduce size-selected subnanometer cluster catalysts deposited on thin films of colloidal semiconductor nanocrystals as a novel platform to obtain atomic scale insight into photocatalytic generation of solar fuels. Using Pt-cluster-decorated CdS nanorod films for photocatalytic hydrogen generation as an example, we determine the minimum amount of catalyst necessary to obtain maximum quantum efficiency of hydrogen generation. Further, we provide evidence for tuning photocatalytic activities by precisely controlling the cluster catalyst size.

Toward a Comprehensive Understanding of Photocatalysis: What Systematic Studies and Alcohol Surface Chemistry on TiO2(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. Hence, many photocatalytic reactions may proceed in a different manner and the thermal chemistry needs to be accounted for. The new mechanism is particularly relevant for reactions in gaseous environments in the absence of solvated ionic species. Here, we compare both mechanisms and highlight their differences and consequences for photocatalysis. Based on alcohol photochemistry, we demonstrate the importance of thermal reactions in photocatalytic mechanisms and the relevance of systematic studies in different environments for a holistic understanding of photocatalysis.

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. Sensitive and time-resolved reaction monitoring at ambient pressure is realized by a lid-integrated capillary, transmitting the entire gas flow from the reactor chamber to a quadrupole mass spectrometer. Due to the microfabrication of the lid from borosilicate as base material, 88% of the geometrical area can be illuminated by a light source, further enhancing sensitivity. Gas dependent flow rates through the capillary were experimentally determined to be 1015-1016 molecules s-1, and in combination with a reactor volume of 10.5 µl, this results in residence times below 40 s. Furthermore, the reactor volume can easily be altered by adjusting the height of the polymeric sealing material. The successful operation of the reactor is demonstrated by selective ethanol oxidation over Pt-loaded TiO2 (P25), which serves to exemplify product analysis from dark-illumination difference spectra.