Molecular imaging of reductive coupling reactions: interstitial-mediated coupling of benzaldehyde on reduced TiO2(110)

Conversion of Alcohols on Stoichiometric and Reduced Rutile TiO2 (110): Point Defects Meet Bifunctionality in Oxide (Photo-)Chemistry

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.

Graphical Abstract

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture

In this Perspective we present a comprehensive study of the multiple reaction products of metal-free porphyrins (2H-Ps) in contact with the rutile TiO₂(110) surface. In the absence of peripheral functionalization with specific linkers, the porphyrin adsorption is driven by the coordination of the two pyrrolic nitrogen atoms of the macrocycle to two consecutive oxygen atoms of the protruding O_br rows via hydrogen bonding. This chemical interaction favours the iminic nitrogen uptake of hydrogen from near surface layers at room temperature, thus yielding a stable acidic porphyrin (4H-P). In addition, a mild annealing (∼100 °C) triggers the incorporation of a Ti atom in the porphyrin macrocycle (self-metalation). We recently demonstrated that such a low temperature reaction is driven by a Lewis base iminic attack, which lowers the energy barriers for the outdiffusion of Ti interstitial atoms (Ti_int) [Kremer et al., Appl. Surf. Sci., 2021, 564, 150403]. In the monolayer (ML) range, the porphyrin adsorption site, corresponding to a TiO-TPP configuration, is extremely stable and tetraphenyl-porphyrins (TPPs) may even undergo conformational distortion (flattening) by partial cyclo-dehydrogenation, while remaining anchored to the O rows up to 450 °C [Lovat et al., Nanoscale, 2017, 9, 11694]. Here we show that, upon self-metalation, isolated molecules at low coverage may jump atop the rows of five-fold coordinated Ti atoms (Ti_5f). This configuration is associated with the formation of a new coordination complex, Ti-O-Ti_5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti-O-Ti_5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti_5f rows is, however, susceptible to the cross-talk between phenyls of adjacent molecules, which ultimately drives the TiO-TPP temperature evolution in the ML range along the same pathway followed by 2H-TPP.

Functionalization of silicene and silicane with benzaldehyde

Organic functionalization of nanomaterials offers exceptional flexibility in materials design, and applications in molecular sensors and molecular electronics are expected. However, more studies should be conducted to understand the interaction between nanomaterials and organic molecules. In this work, we studied the functionalization of silicene and silicane with benzaldehyde, performing nudged elastic band calculations within density functional theory. We calculated the structural changes of the adsorption process, electronic properties of the main states, and the energetics. In silicene, the adsorption of benzaldehyde on the top site was found to be the most stable, with an adsorption energy of -0.55 eV. For silicane, the functionalization proceeds through a self-propagating reaction on a highly reactive dangling bond generated by a hydrogen atom vacancy. Benzaldehyde adsorbed on this site depicts an adsorption energy of -1.39 eV, which is larger than in bare silicene. Upon attaching, the double C=O bond breaks down turning the molecule into a highly reactive radical, which in this case, abstracts a neighboring H atom of the sheet. This process is highly achievable since the energy barrier to abstract the H atoms is 0.81 eV, whereas the one needed to desorb the molecule is 1.39 eV. After H abstraction, a new dangling bond is generated at the substrate, making a chain reaction possible to potentially form benzaldehyde monolayers. Organic functionalization is an excellent tool to engineer properties of 2D systems, and having a deeper understanding of the adsorption processes is the first step toward the development of new generation devices. Graphical abstract Benzaldehyde adsorbed on silicene and silicane.

A DFT study on the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde in different solvents employing an OH-defected TiO2 cluster model

Density functional theory calculations are employed to study the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde (4-BBA) in acetonitrile and in ethanol solvents. A totally relaxed Ti₃O₉H₆ cluster model is proposed to represent titanium dioxide (TiO₂) surfaces. The reduction selectivity of an adsorbed 4-BBA molecule on Ti₃O₉H₆ has been investigated. Owing to the difference in the proton and H atom donating capabilities between explicit CH₃CN and C₂H₅OH solvent molecules, the photocatalytic reduction of 4-BBA is the debromination process in acetonitrile, whereas in ethanol it is the carbonyl reduction process. Therefore 4-BBA can be selectively reduced to benzaldehyde in acetonitrile and 4-bromobenzyl alcohol in ethanol, respectively. Our computational results have verified the reaction mechanism proposed by experiments and show that the debromination of 4-BBA would be efficient if both 4-BBA and Ti₃O₉H₆ have an extra photoelectron. The Ti₃O₉H₆ cluster, playing a role as a hydrogen source and a bridge to transfer photoelectrons from bulk TiO₂, would have potential to be an ideal molecular model for understanding photocatalytic reactions on the TiO₂ surface.

Chemistry at the square nanometer: reactivity at liquid/solid interfaces revealed with an STM

An overview is given of single molecule reactivity at a liquid/solid interface employing a scanning tunneling microscope.

Low-Temperature Adsorption and Diffusion of Methanol in ZIF-8 Nanoparticle Films

The adsorption of methanol by a zeolitic imidazolate framework-8 (ZIF-8) nanoparticle thin film was studied in situ using temperature-programmed desorption and X-ray photoelectron spectroscopy under low-temperature, low-pressure conditions. Partial pore penetration was observed at 90 K, but upon increasing the exposure temperature of the film to 130 K pore penetration was significantly enhanced. Although many studies exist involving bulk powders, this is the first work to our knowledge that demonstrates the ability to control and monitor the entry of a molecule into a metal organic framework (MOF) film in situ using temperature. In this case, nanoparticle films of ZIF-8 were prepared and studied in ultrahigh vacuum. The ability to control and monitor surface adsorption versus pore adsorption in situ is key to future fundamental study of MOFs, for example, in the identification of active sites in reaction mechanisms.

Elementary photocatalytic chemistry on TiO2surfaces

In this article, we review the recent advances in the photoreactions of small molecules with model TiO₂surfaces, and propose a photocatalytical model based on nonadiabatic dynamics and ground state surface reactions.

On-surface reductive coupling of aldehydes on Au(111)

A reductive process allows to covalently couple aldehydes on a Au(111) surface. While oxygen desorbs completely during the reaction, a strongly interlinked polyphenylene vinylene derivative is formed, remaining weakly adsorbed on the surface.

Perspectives on heterogeneous photochemistry

Heterogeneous photochemistry has a potentially important role in production of energy, in environmental remediation and in sustainable production of chemicals. Photochemical efficiency depends on both materials properties and the desired chemical reaction that is promoted through creation of an excited state. A detailed understanding of the interplay between materials properties and reactivity requires a molecular-scale approach that determines the elementary steps in the overall process. This personal account summarizes the role of defects in determining the photochemical and thermal reactions on rutile titania, a model for semiconductor metal oxide photocatalysts that defects, e.g., Ti interstitials present in the subsurface region, and O adatoms on the surface, have a substantial impact on the efficiency for photochemical conversion through modification of molecular binding and also through likely modification of charge carrier dynamics. Design of materials must include engineering of the optical and electronic properties of the semiconductor photocatalyst, and understanding of the key photochemical steps involved in specific processes to ensure proper alignment of their electronic states with the band structure of the material. Thus, fundamental surface science studies and development of time-dependent theoretical methods that map out the reaction mechanism for photochemical processes on materials with controlled composition and structure are critical.

Defects in surface chemistry--reductive coupling of benzaldehyde on rutile TiO₂(110)

The surface chemistry of oxygen and oxygenates on Rutile TiO2(110) is of great interest for various applications such as heterogeneous catalysis and photo catalysis. Though it is generally accepted that surface defects are active sites, the role of subsurface defects is under debate. We have therefore investigated the influence of the bulk defect density on the reductive coupling of benzaldehyde to stilbene as a model system. Using IRRAS we identify stilbene diolate as a reduction intermediate. The concentration of this intermediate is proportional to the bulk defect density, whereas adsorption of benzaldehyde at lower temperatures is not affected, which indicates a dominant role of Ti interstitials at temperatures above 400 K.

Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group's recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange.

Acetone-Assisted Oxygen Vacancy Diffusion on TiO2(110)

We have studied the dynamic relationship between acetone and bridge-bonded oxygen (Ob) vacancy (VO) defect sites on the TiO2(110)-1 × 1 surface using scanning tunneling microscopy (STM) and density function theory (DFT) calculations. We report an adsorbate-assisted VO diffusion mechanism. The STM images taken at 300 K show that acetone preferably adsorbs on the VO site and is mobile. The sequential isothermal STM images directly show that the mobile acetone effectively migrates the position of VO by a combination of two acetone diffusion channels: one is the diffusion along the Ob row and moving as an alkyl group, which heals the initial VO; another is the diffusion from the Ob row to the five-coordinated Ti(4+) row and then moving along the Ti(4+) row as an acetone, which leaves a VO behind. The calculated acetone diffusion barriers for the two channels are comparable and agree with experimental results.

Interaction of petroleum-relevant organosulfur compounds with TiO2(110)

The interaction of two sets of structurally related molecules, thiophenol/thioanisole, and thiophene/tetrahydrothiophene, with vacuum-annealed and ion-bombarded TiO(2)(110) surfaces has been studied using a combination of temperature-programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). All thioethers studied were observed to adsorb and desorb from both surfaces without producing reaction products, while thiophenol, the only species studied containing a S-H bond, reacted with both surfaces. Approximately 25% of surface bound thiophenol decomposed over the vacuum-annealed surface. On the bombarded surface, thiophenol both decomposed into surface-bound C(x)H(y)/S fragments, and reacted to form benzene, which desorbed from the surface at 400 K. We propose that phenylthiolate formation on the bombarded surface leads to the observed production of benzene. These results highlight the importance of defects in the reactivity of titania, and lay the foundation for the study of larger, refractory sulfur compounds present in fuel.