Identifying an O2 Supply Pathway in CO Oxidation on Au/TiO2(110): A Density Functional Theory Study on the Intrinsic Role of Water

The support effect on the size and catalytic activity of thiolated Au₂₅ nanoclusters as precatalysts

In this study, 6-mercaptohexanoic (MHA) protected Au25(MHA)18 nanoclusters (or thiolated Au NCs) deposited on various inorganic supports, including hydroxyapatite (HAP), TiO2 (Degussa P25), activated carbon (AC), pyrolyzed graphene oxide (PGO), and fumed SiO2 were prepared via a conventional impregnation method. Following that, calcination under a N2 stream was conducted to produce surface ligand free, highly dispersed Au NCs catalysts. The effects of supports on the size and catalytic activity of Au NCs were systematically investigated. No obvious size growth was observed for Au NCs on HAP and P25 after thermally induced ligand removal, due to the strong interaction between the metal and the supports. However, severe aggregations of Au NCs were seen after thermal treatment on three other supports, including AC, PGO, and SiO2. The removal of surface thiol ligands from the Au NCs is crucial to catalyze nitrobenzene hydrogenation, where only calcined Au/HAP and Au/P25 exhibited good catalytic activity. On the other hand, all the supported Au NCs were active for the styrene oxidation, where Au/HAP exhibited the best catalytic performance. Altogether, both the size effect and metal-support interaction are crucial for the design of supported Au NCs as efficient catalysts for targeted reactions.

Thermally activated surface oxygen defects at the perimeter of Au/TiO2: a DFT+U study

Density functional theory calculations were performed to examine the formation of oxygen atom vacancies on three model surfaces namely, clean anatase TiO₂(001) and, Au₃ and Au₁₀ clusters supported on anatase TiO₂(001).

Surface-mediated selective photocatalytic aerobic oxidation reactions on TiO2 nanofibres

The interaction between photocatalyst surface and the reactants may outweigh its light absorption ability in photocatalytic activities.

In situ synthesis of TiO2/SnOx–Au ternary heterostructures effectively promoting visible-light photocatalysis

A facile method based on in situ reduction is described for constructing TiO₂/SnO_x–Au ternary heterostructures with enhanced photocatalytic performance.

A theoretical study on the catalytic role of water in methanol steam reforming on PdZn(111)

The catalytic role of water in the methanol steam reforming process on the PdZn(111) surface is explored theoretically.

Catalytic oxidation of cellobiose over TiO2 supported gold-based bimetallic nanoparticles

A series of TiO₂ supported Au–M (M = Cu, Co, Ru and Pd) bimetallic catalysts were tested for cellobiose oxidation. Cu–Au and Ru–Au provided excellent gluconic acid selectivity’s, although via contrasting mechanism.

NO adsorption and diffusion on hydroxylated rutile TiO2(110)

Surface hydroxyls can favor NO adsorption at rutile TiO₂(110) and its roll-over diffusion that gives the characteristic bright-dark-bright STM features.

Tuning the charge state of Ag and Au atoms and clusters deposited on oxide surfaces by doping: a DFT study of the adsorption properties of nitrogen- and niobium-doped TiO2 and ZrO2

Defects (O vacancies) and dopants (nitrogen and niobium impurities) in titania and zirconia affect the properties of adsorbed Ag and Au clusters.

Facile synthesis of Co₃O₄@CNT with high catalytic activity for CO oxidation under moisture-rich conditions

The catalytic oxidation reaction of CO has recently attracted much attention because of its potential applications in the treatment of air pollutants. The development of inexpensive transition metal oxide catalysts that exhibit high catalytic activities for CO oxidation is in high demand. However, these metal oxide catalysts are susceptible to moisture, as they can be quickly deactivated in the presence of trace amounts of moisture. This article reports a facile synthesis of highly active Co3O4@CNT catalysts for CO oxidation under moisture-rich conditions. Our synthetic routes are based on the in situ growth of ultrafine Co3O4 nanoparticles (NPs) (∼2.5 nm) on pristine multiwalled CNTs in the presence of polymer surfactant. Using a 1% CO and 2% O2 balanced in N2 (normal) feed gas (3-10 ppm moisture), a 100% CO conversion with Co3O4@CNT catalysts was achieved at various temperatures ranging from 25 to 200 °C at a low O2 concentration. The modulation of surface hydrophobicity of CNT substrates, other than direct surface modification on the Co3O4 catalytic centers, is an efficient method to enhance the moisture resistance of metal oxide catalysts for CO oxidation. After introducing fluorinated alkyl chains on CNT surfaces, the superhydrophobic Co3O4@CNT exhibited outstanding activity and durability at 150 °C in the presence of moisture-saturated feed gas. These materials may ultimately present new opportunities to improve the moisture resistance of metal oxide catalysts for CO oxidation.

Promoting effects of ceria on the catalytic performance of gold supported on TiO2 for low-temperature CO oxidation

Doping with Ce enhanced the Au–support synergy and modified the active sites. The effortless decomposition of carbonates and quick recovery of oxygen vacancies on the Au/CeO₂–TiO₂ surface may be responsible for its high stability.

The role of reducible oxide-metal cluster charge transfer in catalytic processes: new insights on the catalytic mechanism of CO oxidation on Au/TiO2 from ab initio molecular dynamics

To probe metal particle/reducible oxide interactions density functional theory based ab initio molecular dynamics studies were performed on a prototypical metal cluster (Au20) supported on reducible oxides (rutile TiO2(110)) to implicitly account for finite temperature effects and the role of excess surface charge in the metal oxide. It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge. In the context of the well-known CO oxidation reaction, charge transfer facilitates the plasticization of Au20, which allows for a strong adsorbate induced surface reconstruction upon addition of CO leading to the formation of mobile Au-CO species on the surface. The charging/discharging of the cluster during the catalytic cycle of CO oxidation enhances and controls the amount of O2 adsorbed at oxide/cluster interface and strongly influences the energetics of all redox steps in catalytic conversions. A detailed comparison of the current findings with previous studies is presented, and generalities about the role of surface-adsorbate charge transfer for metal cluster/reducible oxide interactions are discussed.

Effect of hydrogen on O2 adsorption and dissociation on a TiO2 anatase (001) surface

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO2 anatase (001) surface is studied by first-principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O2 molecule is high enough to break the O=O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O=O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.

Defect-Electron Spreading on the TiO2(110) Semiconductor Surface by Water Adsorption

The dissociative adsorption of water at oxygen-vacancy defect sites on the TiO2(110) surface spatially redistributes the defect electron density originally present at subsurface sites near the defect sites. This redistribution of defect-electrons makes them more accessible to Ti(4+) ions surrounding the defects. The redistribution of electron density decreases the O(+) desorption yield from surface lattice O(2-) ions in TiO2, as excited by electron-stimulated desorption (ESD). A model in which OH formation on defect sites redistributes defect electrons to neighboring Ti(4+) sites is proposed. This switches off the Knotek-Feibelman mechanism for ESD of O(+) ions from lattice sites. Conversely, enhanced O(+) reneutralization could also be induced by redistribution of defect electrons. The redistribution of surface electrons by adsorption is further verified by the use of donor and acceptor molecules that add or remove electron density.

CO oxidation at the perimeters of an FeO/Pt(111) interface and how water promotes the activity: a first-principles study

The catalytic role of the Pt--Fe cation ensemble presented at the perimeters of the FeO film supported on Pt(111) for low-temperature CO oxidation and the promotion of water on activity were studied by using DFT calculations. We found that the perimeter sites along the edge of the FeO islands on Pt provided a favorable ensemble that consisted of coordinatively unsaturated ferrous species and nearby Pt atoms for O(2) and H(2) O activation free from CO poison. A dissociative oxygen atom at the Pt--Fe cation ensemble reacts easily with CO adsorbed on nearby Pt. The OH group from water dissociation not only facilitates activation of the oxygen molecule, more importantly it opens a facile reaction channel for CO oxidation through the formation of the carboxyl intermediate. The presence of the OH group on the FeO film strengthens interfacial interactions between FeO and Pt(111), which would make the FeO film more resistant to further oxidation. The importance of the Pt--Fe cation ensemble and the role of water as a cocatalyst for low-temperature CO oxidation is highlighted.

Hydrogen reactivity on highly-hydroxylated TiO2(110) surfaces prepared via carboxylic acid adsorption and photolysis

Combined scanning tunneling microscopy, temperature programmed desorption, photo stimulated desorption, and density functional theory studies have probed the formation and reactivity of highly-hydroxylated rutile TiO(2)(110) surfaces, which were prepared via a novel, photochemical route using trimethyl acetic acid (TMAA) dissociative adsorption and subsequent photolysis at 300 K. Deprotonation of TMAA molecules upon adsorption produces both surface bridging hydroxyls (OH(b)) and bidentate trimethyl acetate (TMA) species with a saturation coverage of nearly 0.5 monolayers (ML). Ultra-violet light irradiation selectively removes TMA species, producing a highly-hydroxylated surface with up to ~0.5 ML OH(b) coverage. At high coverages, the OH(b) species typically occupy second-nearest neighbor sites along the bridging oxygen row locally forming linear (2 × 1) structures of different lengths, although the surface is less ordered on a long scale. The annealing of the highly-hydroxylated surface leads to hydroxyl recombination and H(2)O desorption with ~100% yield, thus ruling out the diffusion of H into the bulk that has been suggested in the literature. In agreement with experimental data, theoretical results show that the recombinative H(2)O desorption is preferred over both H bulk diffusion and H(2) desorption processes.

Chemistry of O- and H-containing species on the (001) surface of anatase TiO2: a DFT study

The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO(2) (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H(2)O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO(2) surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O(2) adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O(2). In the adsorbed OOH, the O-O bond is considerably lengthened and thus weakened.