Improving control over active-site reactivity is a grand challenge in catalysis. Single-atom alloys (SAAs) consisting of a reactive component doped as single atoms into a more inert host metal feature localized and well-defined active sites, but fine tuning their properties is challenging. Here, we develop a framework for tuning single-atom site reactivity by alloying in an additional inert metal, which we term an alloy-host SAA. Specifically, we created about 5% Pd single-atom sites in a Pd₃₃Ag₆₇(111) single-crystal surface, and then identified Sn based on computational screening as a suitable third metal to introduce. Subsequent experimental studies show that introducing Sn indeed modifies the electronic structure and chemical reactivity (measured by CO desorption energies) of the Pd sites. The modifications to both the electronic structure and the CO adsorption energies are in close agreement with our calculations. These results indicate that the use of an alloy host environment to modify the reactivity of single-atom sites could allow fine-tuning of catalytic performance and boost resistance against strong-binding adsorbates such as CO.

The composition and distribution of Pd atoms on the surface were controlled by thermal treatments following sputtering of the surface of PdAg single crystal of composition Pd₃₃Ag₆₇ at 100 K. The Pd distribution at the (111) surface was varied from small ensembles to single atoms by thermal annealing between 400 and 820 K. On Pd ensembles, D₂ dissociates readily, populating the surface with two types of atomic D/H. D₂ (H₂) desorbs around 295 K (β-state) from pure Pd sites, while it desorbs from bimetallic or pure silver sites desorbs around 185–230 K (γ-state). In contrast, single Pd sites did not dissociate D₂ or H₂ under the conditions used herein. In the annealing process rapid segregation of Pd from the topmost surface into the near subsurface occurs with annealing within minutes, and a subsequent, much slower diffusion into the bulk occurs on the time scale of hours. Moreover, subsurface Pd can be employed for fine adjustment of the chemical bonding of adsorbates to these surfaces. This study establishes a method for tuning the compositional and configurational surface properties of well-defined substitutional bimetallic bulk alloys.

Oxidic (photo-)catalysts have the potential to play an important role to efficiently implement sustainable feedstocks and green energy sources into future energy technologies. They may be used not only for solar energy harvesting, but also for hydrogen production or being essential for the fabrication of fine chemicals. Therefore, it is crucial to develop a detailed understanding of how the atomistic environment of the catalyst can be designed in order to promote distinct reaction pathways to influence the final product distribution of chemical reactions. In this perspective article, we survey the surface (photo-)chemistry of methanol on rutile TiO₂ surfaces and hybrid catalysts based thereon. Especially the role of the surface bifunctionality by Lewis acidic and basic sites combined with the strong impact of point defects such as reduced titanium sites (mainly Ti³⁺ interstitials) shall be illuminated. It is shown how the selective activation of either O–H, C–H or C–O bonds in the methanol molecule can be used to tune not only the overall conversion, but to switch between oxidative and reductive routes in favor of either deoxygenation, partial oxidation or C–C coupling reactions. Especially the latter ones are of particular interest to introduce methanol from green sources such as biomass as a sustainable feedstock into already existing petrochemical technologies.

Quaternary ammonium halides are an important class of compounds because they are not only used as structural templates ororganic surface modifiers for inorganic minerals and other aluminosilicates such as zeolites, but are also well-known and widespread as phase-transfer catalysts and stabilizing agents for colloidal metal nanoparticle synthesis. Moreover, quaternary ammonium ions are frequently exploited as weakly coordinating cations in various ionic liquids. Thus, it is important to monitor, understand, and utilize thermal effects and limits of application. Herein, we report the thermal conversion of didodecyldimethylammonium bromide (DDAB) into organic amines on the surface of gold nanoparticles. In contrast to the pure compound, in which this decomposition appears as an endothermic process, the on-particle reaction is exothermic. This in situ modification of the ligand shell not only involves the essential step of halide removal through the gas phase but also turns out to be a valuable tool to produce uncovered metal sites and thus impacts the fabrication of nanostructured metal catalysts for ligand-directed chemistry.

he photochemical conversion of organic compounds on tailored transition metal oxide surfaces by UV irradiation has found wide applications ranging from the production of chemicals to the degradation of organic pollutants e.g. in waste water treatment. Here, we present a systematic surface science-based study of the UV photoconversion of methanol on a rutile TiO₂(110) surface. Under the used conditions, the dominant photoreaction is the photo-oxidation forming formaldehyde, that is drastically boosted by the presence of adsorbed oxygen as well as (sub-)surface defects such as oxygen vacancies and Ti³⁺ interstitials. Moreover, a photostimulated and Ti³⁺ mediated C–C coupling was observed leading to the production of ethene. We have further deposited tungsten oxide clusters on the rutile surface and examined the impact on the methanol photochemistry. In this case, the C–C coupling can be suppressed. Surprisingly, especially for high Ti³⁺ contents the population of the photochemical pathway is quenched in favor of the population of the thermal reaction yielding more methane from the deoxygenation reaction. So, the common concept that long time charge separation is efficient by combining two photocatalysts with similar band gaps, but different work functions in order to enhance photochemical yields is apparently too naive for certain systems. We attribute the loss of photoproducts with tungsten oxide coadsorption to the “pinning” of Ti³⁺ centers and a related enhancement of electron density near the oxide clusters which makes a concomitant recombination of the photochemical relevant holes with the excess surface electrons more likely.

Tungsten oxide clusters deposited on rutile TiO₂ (110) single crystals were used as a model system for heterogenous oxide-oxide bifunctional catalysts. The population of different thermal reaction routes in methanol conversion in the presence of preadsorbed oxygen was probed under UHV conditions. By temperature programmed reaction spectroscopy, we have identified three thermal reaction channels, namely the deoxygenation under formation of methane, the partial oxidation forming formaldehyde and the condensation route under desorption of ethane and dimethyl ether. The specific local reaction environment at the oxidic surface was found to be key for the population of the different reaction channels as exhibited by the introduction of Lewis acidic and basic sites (especially (WO₃)_n clusters) and available charge carriers such as Ti³⁺. Especially the amount of bulk Ti³⁺ interstitials, that can partially transfer charge towards the tungsten oxide clusters at the TiO₂ surface, was found to be a key parameter that enables a relatively high methanol conversion in thermal reactions. It turned out that the deoxygenation is by far the most dominant reaction followed by the partial oxidation. The condensation is observed only in low amounts under special conditions, but is an interesting example for reactivity at defect sites.

Aiming for a comprehensive expertise of the adsorption and activation of small organic molecules on transition metal oxides, the chemistry of methylamine on rutile TiO₂ (110) is examined combining surface science experiments with first-principles calculations. Besides the physisorption of a methylamine multilayer, three types of methylamine adsorbates were ascertained: adsorption via hydrogen bonding toward bridging oxygen atoms, adsorption of methylamine at one individual Ti_5c site, and an adsorbate occupying two Ti_5c sites in an unprecedented multidentate bonding geometry. In all three adsorbate classes, multiple interactions with the TiO₂ substrate as well as intermolecular dispersion forces are identified to be crucial to explain the molecule–surface interaction and concomitant adsorption geometries. While the Ti–N bond surely makes the largest contribution to the molecules’ adsorption energy, smaller forces such as hydrogen bonds between NH–O and CH–O moieties are shown to be the central key for understanding the chemistry of methylamine on titania surfaces. Comparing different adsorption geometries, these multidentate adsorption motifs can boost the thermal stability in relation to ordinary adsorption, in the case of the adsorbate occupying two Ti_5c sites by more than 100 K or several tens of kJ per mol being thermally stable up to more than 500 K. Surprisingly, the multidentate adsorption is remarkably sensitive to the adsorption conditions and especially does not occur for elevated adsorption temperatures. Curiously enough, low adsorption temperatures thus yield high-temperature desorption species. Our findings are particularly important for the understanding of (photo-)chemical reactions, which usually requires an extensive knowledge of adsorbate-substrate interactions and the bonding situation toward the catalysts surface.

A fundamental reaction in industries for producing aldehydes and ketones is the partial oxidation of alcohols. As a model reaction, we investigated the photo-oxidation of 2-propanol on rutile titania, which is a promising chemically nontoxic photocatalyst. Photochemical infrared reflection absorption spectroscopy (PC-IRRAS) was used to study the reaction on powder catalysts in the liquid phase (neat liquid and dissolved in dichloromethane). We compare these results with polarized Fourier transform (FT)-IRRAS and temperature-programmed desorption (TPD) experiments on rutile TiO₂(110) single crystals in ultrahigh vacuum (UHV). Our in situ liquid-phase experiments showed that 2-propanol converts into acetone on rutile powders, which is in accordance with previous ex situ studies. Mass transport limitations are the key to avoid total oxidation. However, the yield of acetone is limited. We identified water formed as a byproduct and suspected that water might block the active sites. To elucidate possible reaction mechanisms, further experiments were performed on rutile TiO₂(110) single crystals in the presence and absence of oxygen and UV irradiation under UHV conditions. Here, we obtained further insights into the elementary steps of the different 2-propanol reactions. We demonstrated that acetone desorbs from a diolate species, which forms in the presence of oxygen under UV irradiation at temperatures around 200 K. Furthermore, propane was identified for the first time as a new thermally activated deoxygenation product besides the simultaneously formed, formally reported, propene. Propene formation is quenched by UV irradiation. Active site blocking by water is confirmed by TPD and polarized FT-IRRAS measurements.

Heterogeneous (photo)catalysts are often complex mixtures of different nanostructured oxidic compounds. Chemical and electronic interactions within such combined materials may play a key role in improving the performance in technological applications but are difficult to investigate under technical conditions. This work presents a systematic study of the interactions between tungsten oxide clusters and the underlying rutile TiO₂ (110) surface in special consideration of point defects such as Ti³⁺ interstitials. Using electron beam evaporation from WO₃ powder, stoichiometric (WO₃)_n, and oxygen-deficient (WO_3–x)_n, tungsten oxide clusters are produced simultaneously. Based on cluster coverage- and temperature-dependent X-ray photoelectron spectroscopy studies, the formation of a surface layer of stoichiometric and oxygen-deficient tungsten oxide clusters is shown, and the formation of mixed oxides can be excluded. For the first layer up to 7.1 WO₃ nm^–2, stoichiometric clusters are dominant at the TiO₂ surface. The lack of W⁵⁺ indicates an electron transfer from the clusters toward the substrate under formation of Ti³⁺ interstitials. Furthermore, we found at elevated temperatures relevant for catalytic reactions that the tungsten oxide clusters are more stable on TiO₂ surfaces than on other substrates such as the silicon oxide layer of Si wafers. Up to 900 K, only slight changes were observed on titania. We observed an accumulation of Ti³⁺ at the TiO₂ surface between 500 and 800 K in the case of high bulk Ti³⁺ content in TiO₂. As the Ti³⁺ accumulation is accompanied by significant changes of the W 4f signals, we suggest an interaction between these sites under a possible generation of surface fields or an anionic [(WO₃)_n]^z−-like cluster by electron transfer from Ti³⁺.

To obtain a mechanistic understanding of occurring processes on oxide surfaces at the atomic level, systematic studies under ultra-high vacuum (UHV) conditions on single crystalline surfaces are commonly used. Usually, the sample preparation protocol for these surfaces includes argon-ion bombardment followed by annealing at elevated temperatures. For reduceable metal oxides, this leads to a significant reduction of the surface. Up to now, the particular role of the remaining argon in the subsequent formation of clean surfaces and the possible incorporation of argon into the crystal lattice as a dopant are typically neglected or remain unclear. This work presents combined, temperature-dependent X-ray photoelectron spectroscopy and a low-energy electron diffraction study under UHV conditions of the bulk-assisted reoxidation and restructuring of the rutile TiO₂(110) single crystal surface after argon-ion bombardment. The formation of an ordered and reoxidized (110)(1 × 1) surface is accompanied by a stepwise desorption of argon from the sample. Moreover, we present a systematic study of the incorporation of argon in the rutile crystal as well as the diffusion and desorption of argon from these samples. By following the temperature-dependent Ar 2p photoelectron spectra, the change of the electronic environment of embedded argon is elucidated, demonstrating the interaction with reduced Ti cations. Hence, residual argon (in case of Ar⁺) possibly acts as a strong oxidant or induces significant lattice distortions. Our results show that residual argon from the sample preparation is an important hidden dopant and needs to be considered in the evaluation of typical studies on oxide surfaces under UHV conditions in future work.

Rutile TiO₂ is an important model system for understanding the adsorption and conversion of molecules on transition metal oxide catalysts. In the last decades, point defects, such as oxygen vacancies and Ti³⁺ interstitials, exhibited an important influence on the reaction of oxygen and oxygen-containing molecules on titania surfaces. In brief, partially reduced TiO₂ containing a significant amount of Ti³⁺ is often more active for the conversion of such molecules. In this study, we investigate an even higher reduced surface prepared by argon ion bombardment of a rutile TiO₂ (110) single crystal. By X-ray photoelectron spectroscopy we show that, besides Ti⁴⁺, this surface is almost equally dominated by Ti³⁺ and Ti²⁺. To probe the reactivity of these highly reduced surfaces, we have adsorbed two different classes of oxygen-containing molecules and utilized temperature programmed reaction spectroscopy to investigate the conversion. While alcohols (in this case methanol) already show a defect-dependent partial conversion in a deoxygenation reaction on the (stochiometric or slightly reduced) rutile TiO₂ (110) surface, ketones (e.g. acetone) are usually not converted on the rutile TiO₂ (110) surface independent on the bulk defect density. Here, we present a nearly full conversion for both molecules via deoxygenation reactions and reductive C–C coupling, forming different hydrocarbons at different temperatures between 375 K and 640 K on the sputtered Ti²⁺ rich surface.

Aluminum (Al)-ion batteries can be an attractive alternative to lithium-ion batteries because of low costs, high volumetric capacities and dendrite-free formation when Al is used as anode. However, there are limited cathode materials for Al-ion batteries that can deliver satisfactory electrochemical performance, especially cycling stability. The major reason for that is the sluggish kinetics of ion intercalation/deintercalation, resulting from large coulombic attraction between Al³⁺ and cathodes. Herein, a concept of hybrid Al−Li-ion batteries is proposed to circumvent the poor Al³⁺ ions insertion/extraction kinetics in Al-ion batteries, and maintain the dendrite-free characteristics of Al-ion batteries. The high volumetric capacity (32.5 mAh/cm³ at 100 mA/cm³, based on the total volume of cathode), enhanced rate capability (21.5 mAh/cm³ at 1000 mA/cm³) and excellent cycling performance (70.1 % retention after 3000 cycles) have been achieved in the hybrid Al−Li-ion battery composed of vanadium oxide nanosheets on carbon fibers as cathode and Al as anode in a mixed [EMIM][Cl]/AlCl₃/LiCl electrolyte. Combining with the good flexibility of cathode and anode, the hybrid Al−Li-ion battery maintains structural and capacity stability under different bending angles. This study unveils a safe, cost-effective and flexible hybrid Al−Li-ion battery that presents highly competitive advantages among various energy storage devices.

Titanium dioxide (TiO₂) is among the most studied model (photo)catalyst materials. The influence of surface point defects, like oxygen vacancies and particularly bulk defects such as Ti³⁺ interstitials, is usually underestimated or even ignored. We present a systematic study under well-defined UHV conditions illustrating the importance of such defects for the thermal reaction of methanol at the rutile TiO₂ (110) single crystal surface by using temperature-programmed reaction spectroscopy (TPRS) and Fourier-transform infrared reflection–absorption spectroscopy (FT-IRRAS). It will be shown that the population of different reaction pathways, namely, the partial oxidation of methanol to formaldehyde and the deoxygenation forming hydrocarbons, especially methane, depends on the bulk defect density and the presence or absence of oxygen adsorbates. While at elevated temperatures molecular desorption is pronounced for the less defective substrates, for higher reduction grades the high-temperature deoxygenation channel via methoxy intermediates is favored. In addition, preadsorption of oxygen enables low- and high-temperature partial oxidation forming formaldehyde, likely from a dioxomethylene-like adsorbate.