Identification of Intermediates in the Reaction Pathway of SO2 on the CaO Surface: From Physisorption to Sulfite to Sulfate

The interaction of CaO and Ca(OH)₂ with solvated or gaseous SO₂ plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO₂ from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO₂ with a CaO(001) single crystal. First, the surface chemistry of SO₂ was investigated at different temperatures using polarization-resolved IR reflection absorption spectroscopy. Three species were identified, and an in-depth density functional theory study was carried out, which allowed deriving a consistent picture. Unexpectedly, low temperature exposure to SO₂ solely yields a physisorbed species. Only above 100 K, the transformation of this weakly bound adsorbate first to a chemisorbed sulfite and then to a sulfate occurs, effectively passivatating the surface. Our results provide the basis for more efficient strategies in corrosion protection of urban infrastructure and in lime-based desulfurization of flue gas.

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

Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence from Infrared Spectroscopy

Charge transfer at organic/inorganic interfaces critically influences the properties of molecular adlayers. Although for metals such charge transfers are well documented by experimental and theoretical results, in the case of semiconductors, clear and direct evidence for a transfer of electrons or holes from oxides with their typically high ionization energy is missing. Here, we present data from infrared reflection-absorption spectroscopy demonstrating that despite a high ionization energy, electrons are transferred from ZnO into a prototype strong molecular electron acceptor, hexafluoro-tetracyano-naphthoquinodimethane (F₆-TCNNQ). Because there are no previous studies of this type, the interpretation of the pronounced vibrational red shifts observed in the experiment was aided by a thorough theoretical analysis using density functional theory. The calculations reveal that two mechanisms govern the pronounced vibrational band shifts of the adsorbed molecules: electron transfer into unoccupied molecular levels of the organic acceptor and also the bonding between the surface Zn atoms and the peripheral cyano groups. These combined experimental data and the theoretical analysis provide the so-far missing evidence of interfacial electron transfer from high ionization energy inorganic semiconductors to molecular acceptors and indicates that n-doping of ZnO plays a crucial role.

Structural Evolution of Water on ZnO(10 1 ‾ 0): From Isolated Monomers via Anisotropic H-Bonded 2D and 3D Structures to Isotropic Multilayers

The surface chemistry of water on zinc oxides is an important topic in catalysis and photocatalysis. Interaction of D₂O with anisotropic ZnO(101‾ 0) surfaces was studied by IR reflection absorption spectroscopy using s‐ and p‐polarized light incident along different directions. Interpretation of the experimental data is aided using isotopologues and DFT calculations. The presence of numerous species is revealed: intact monomers, a mixed 2D D₂O/OD adlayer, an anisotropic bilayer, and H‐bonded 3D structures. The isolated water monomers are identified unambiguously at low temperatures. The thermally induced diffusion of water monomers occurs at elevated temperatures, forming dimers that undergo autocatalytic dissociation via proton transfer. Polarization‐ and azimuth‐resolved IR data provide information on the orientation and strength of H‐bonds within the 2D and 3D structures. Ab initio molecular dynamics simulations reveal strong anharmonic couplings within the H‐bond network.

Three-Dimensional Functionalized Boron Nitride Nanosheets/ZnO Superstructures for CO2 Capture

Without any extra directing agents or surfactants, three-dimensional (3D) hierarchically cubic and spherical morphologies of functionalized boron nitride nanosheets (FBNNSs)/ZnO superstructures have been controlled successfully via the evaporation-induced solvothermal synthesis. As-resulted spherical FBNNSs/ZnO superstructures not only exhibit a high capture capacity of CO₂ around 63.4 cm³/g (124.5 mg/g) from 0 to 1 bar at 273 K but also show a good reusability of 10 cycles with an average removal ability up to 58.9 cm³/g (115.7 mg/g). The excellent adsorption property can be further explained by the chemisorption, van der Waals interaction, and H bonds from the surface of ZnO and the in-plane and edged amino groups of FBNNS. Therefore, the preparation of 3D FBNNSs/ZnO superstructures provides a new and promising material for CO₂ adsorption with tunable morphologies.

Formation and evolution of orientation-specific CO2 chains on nonpolar ZnO(10͞10) surfaces

Clarifying the fundamental adsorption and diffusion process of CO₂ on single crystal ZnO surfaces is critical in understanding CO₂ activation and transformation over ZnO-based catalysts. By using ultrahigh vacuum-Fourier transform infrared spectroscopy (UHV-FTIRS), we observed the fine structures of CO₂ vibrational bands on ZnO(100) surfaces, which are the combinations of different vibrational frequencies, originated from CO₂ monomer, dimer, trimer and longer polymer chains along [0001] direction according to the density functional theory calculations. Such novel chain adsorption mode results from the relatively large attractive interaction between CO₂ and Zn_3c atoms in [0001] direction. Further experiments indicate that the short chains at low coverage evolve into long chains through Ostwald ripening by annealing. At higher CO₂ coverage (0.7 ML), the as-grown local (2 × 1) phase of chains first evolve into an unstable local (1 × 1) phase below 150 K, and then into a stable well-defined (2 × 1) phase above 150 K.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Adsorbate-induced lifting of substrate relaxation is a general mechanism governing titania surface chemistry

Under ambient conditions, almost all metals are coated by an oxide. These coatings, the result of a chemical reaction, are not passive. Many of them bind, activate and modify adsorbed molecules, processes that are exploited, for example, in heterogeneous catalysis and photochemistry. Here we report an effect of general importance that governs the bonding, structure formation and dissociation of molecules on oxidic substrates. For a specific example, methanol adsorbed on the rutile TiO₂(110) single crystal surface, we demonstrate by using a combination of experimental and theoretical techniques that strongly bonding adsorbates can lift surface relaxations beyond their adsorption site, which leads to a significant substrate-mediated interaction between adsorbates. The result is a complex superstructure consisting of pairs of methanol molecules and unoccupied adsorption sites. Infrared spectroscopy reveals that the paired methanol molecules remain intact and do not deprotonate on the defect-free terraces of the rutile TiO₂(110) surface.

Molecules on a metal surface may be modified by the presence of oxide layers, but further mechanistic understanding is still required. Here the authors show for methanol on rutile TiO₂(110) that strongly bonded adsorbates lift surface relaxations, leading to substrate-mediated interaction between adsorbates.

The effect of the morphology of supported subnanometer Pt clusters on the first and key step of CO2 photoreduction

Other than the chemistry of the Pt cluster, the cluster morphology also determines CO₂ binding, attributed to structural fluxionality and bonding competitions among Pt atoms and CO₂.

cis-to-trans isomerization of azobenzene investigated by using thin films of metal–organic frameworks

The energetic barrier for the cis-to-trans isomerization of azobenzene was experimentally investigated by using thin films of azobenzene-containing MOFs as well-defined model system.

Adsorption and interaction of CO2 on rutile TiO2(110) surfaces: a combined UHV-FTIRS and theoretical simulation study

P-polarized RAIR spectra of CO₂ adsorbed on reduced rutile TiO₂(110) surfaces as a function of CO₂ dosage and adsorption temperature. The incidence plane is along (a) the [001] azimuth and (b) the [11̄0] azimuth respectively.

O2 adsorption dependent photoluminescence emission from metal oxide nanoparticles

Optical properties of metal oxide nanoparticles are subject to synthesis related defects and adsorbates.

NO adsorption and reaction on single crystal rutile TiO2(110) surfaces studied using UHV–FTIRS

Schematic models of cis-(NO)₂ dimers (I and III) and N₂O molecules (II) on reduced TiO₂(110) surfaces. (I) Bidentate adsorption configuration on Ti_5c⁴⁺ sites and (III) monodentate adsorption configuration at Vo sites.

Chemical activity of oxygen vacancies on ceria: a combined experimental and theoretical study on CeO2(111)

The chemical activity of oxygen vacancies on well-defined, single-crystal CeO₂(111)-surfaces is investigated using CO as a probe molecule.