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The decomposition of dimethyl methyl phosphonate (DMMP), a simulant for the nerve agent sarin, was investigated on Cu4/TiO2(110) and K/Cu4/TiO2(110) surfaces using a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory calculations (DFT). Mass-selected Cu4 clusters and potassium (K) atoms were deposited onto TiO2(110) as a metal catalyst and alkali promoter to improve the reactivity and recyclability of the TiO2 surface after exposure to DMMP. Surface reaction products resulting from decomposition of DMMP were probed by NAP-XPS measurements of phosphorus (P) 2p and carbon 1s core-level spectra. The Cu4/TiO2(110) surface is found to be very active for DMMP decomposition with highly reduced P-species observed even at room temperature (RT). The codeposition of K atoms and Cu4 clusters further improves the reactivity with no intact DMMP detectable. Temperature-dependent measurements show that the presence of K atoms promotes the removal of residual P-species at temperatures > 600 K. Detailed DFT calculations were performed to determine the surface structures and energetically accessible pathways for DMMP decomposition on Cu4/TiO2(110) and K/Cu4/TiO2(110) surfaces. The calculations show that DMMP and P-containing reaction products preferentially bind to the TiO2 surface, while the molecular fragments, i.e., methoxy and methyl, bind to both the Cu4 clusters and TiO2. The Cu4 clusters make the P-O, O-C, and P-C bond cleavages of DMMP markedly more exothermic. The Cu4 clusters are highly fluxional with atomic structures that depend on the configuration of fragments bound to them. Finally, the manifold of P 2p chemical shifts calculated for a large number of energetically favorable configurations of decomposition products is in good agreement with the observed XPS spectra and provides an alternative way of interpreting incompletely resolved core-level spectra using an ensemble of observed structures.

The three-dimensional (3D) structure of evaporated Cu atoms on a TiO2(110) surface premodified with o-anthranilic acid (o-AA) was examined using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) to elucidate the effect of the premodification on the Cu dispersion over the TiO2(110) surface. Cu was found to be atomically dispersed with the formation of N--Cu--O bonds (where the N is provided by the o-AA and the O is present in the TiO2 lattice) on the surface, and the factors for controlling the atomic dispersion are discussed. [ABSTRACT FROM AUTHOR]

Transition‐metal dichalcogenide (TMDC) islands and thin films are grown on TiO2(110) via physical vapor deposition of Mo and Se and are analyzed with electron diffraction, Auger spectroscopy, and scanning tunneling microscopy (STM). Surprisingly, large, crystalline TiSe2 patches develop on the TiO2 surface instead of the expected MoSe2 islands. They are identified by a hexagonal lattice with 3.4 Å periodicity, a unique (2 × 2) superstructure related to charge‐density waves, and an empty conductance doublet in STM spectroscopy. Density‐function‐theory calculations assign the imaging contrast and the conductance doublet to tunneling into the 3p orbitals of the topmost Se layer and Ti 3d‐related electronic states, respectively. Control experiments without Mo deposition confirm that the observed TMDC growth indeed results from a surface reaction between gas‐phase Se and Ti3+ atoms, segregating out of the reduced TiO2 crystal. From the unique shape and orientation of the TiSe2 islands in the low‐coverage regime, a distinct nucleation scheme is proposed, in which the Se(3 × 1) superstructure on TiO2 provides pinning centers for the TMDC. This article aims at increasing the awareness that Mo and Se co‐deposition does not automatically result in MoSe2 formation, but parasitic processes may trigger the growth of other TMDCs. [ABSTRACT FROM AUTHOR]

Transition‐metal dichalcogenide (TMDC) islands and thin films are grown on TiO2(110) via physical vapor deposition of Mo and Se and are analyzed with electron diffraction, Auger spectroscopy, and scanning tunneling microscopy (STM). Surprisingly, large, crystalline TiSe2 patches develop on the TiO2 surface instead of the expected MoSe2 islands. They are identified by a hexagonal lattice with 3.4 Å periodicity, a unique (2 × 2) superstructure related to charge‐density waves, and an empty conductance doublet in STM spectroscopy. Density‐function‐theory calculations assign the imaging contrast and the conductance doublet to tunneling into the 3p orbitals of the topmost Se layer and Ti 3d‐related electronic states, respectively. Control experiments without Mo deposition confirm that the observed TMDC growth indeed results from a surface reaction between gas‐phase Se and Ti3+ atoms, segregating out of the reduced TiO2 crystal. From the unique shape and orientation of the TiSe2 islands in the low‐coverage regime, a distinct nucleation scheme is proposed, in which the Se(3 × 1) superstructure on TiO2 provides pinning centers for the TMDC. This article aims at increasing the awareness that Mo and Se co‐deposition does not automatically result in MoSe2 formation, but parasitic processes may trigger the growth of other TMDCs. [ABSTRACT FROM AUTHOR]

The interaction of reactants with catalysts has always been an important subject for catalytic reactions. As a promising catalyst with versatile applications, titania has been intensively studied for decades. In this work we have investigated the role of bridge bonded oxygen vacancy (Ov) in methyl groups and carbon monoxide (CO) adsorption on rutile TiO2(110) (R-TiO2(110)) with the temperature programmed desorption technique. The results show a clear different tendency of the desorption of methyl groups adsorbed on bridge bonded oxygen (Ob), and CO molecules on the five coordinate Ti4+ sites (Ti5c) as the Ov concentration changes, suggesting that the surface defects may have crucial influence on the absorption of species on different sites of R-TiO2(110). [ABSTRACT FROM AUTHOR]

The decomposition of dimethyl methyl phosphonate (DMMP), a simulant for the nerve agent sarin, was investigated on Cu 4 /TiO 2 (110) and K/Cu 4 /TiO 2 (110) surfaces using a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory calculations (DFT). Mass-selected Cu 4 clusters and potassium (K) atoms were deposited onto TiO 2 (110) as a metal catalyst and alkali promoter to improve the reactivity and recyclability of the TiO 2 surface after exposure to DMMP. Surface reaction products resulting from decomposition of DMMP were probed by NAP-XPS measurements of phosphorus (P) 2p and carbon 1s core-level spectra. The Cu 4 /TiO 2 (110) surface is found to be very active for DMMP decomposition with highly reduced P-species observed even at room temperature (RT). The codeposition of K atoms and Cu 4 clusters further improves the reactivity with no intact DMMP detectable. Temperature-dependent measurements show that the presence of K atoms promotes the removal of residual P-species at temperatures > 600 K. Detailed DFT calculations were performed to determine the surface structures and energetically accessible pathways for DMMP decomposition on Cu 4 /TiO 2 (110) and K/Cu 4 /TiO 2 (110) surfaces. The calculations show that DMMP and P-containing reaction products preferentially bind to the TiO 2 surface, while the molecular fragments, i.e., methoxy and methyl, bind to both the Cu 4 clusters and TiO 2 . The Cu 4 clusters make the P-O, O-C, and P-C bond cleavages of DMMP markedly more exothermic. The Cu 4 clusters are highly fluxional with atomic structures that depend on the configuration of fragments bound to them. Finally, the manifold of P 2p chemical shifts calculated for a large number of energetically favorable configurations of decomposition products is in good agreement with the observed XPS spectra and provides an alternative way of interpreting incompletely resolved core-level spectra using an ensemble of observed structures.

Suppressing sintering of supported particles is of importance for the study and application of metal-TiO2 system. Theoretical study of Ostwald ripening of TiO2(110)-supported Pd particles would be helpful to extend the understanding of the sintering. In this paper, based on density functional theory (DFT), the surface energy of Pd and the total activation energy (the sum of formation energy and diffusion barrier) of TiO2-supported Pd were calculated. Since the total activation energy is mainly contributed from the formation energy, it is indicated that the ripening of Pd particles would be in the interface control limit. Subsequently, the calculated surface energy and total activation energy were used to simulate Ostwald ripening of TiO2(110)-supported Pd particles. As a result, in comparison with larger particles, smaller particles would worsen the performance of ripening-resistance according to its lower onset temperature and shorter half-life time. The differences on ripening-resistance among different size particles could be mitigated along with the increase of temperature. Moreover, it is verified that the monodispersity can improve ripening resistance especially for the smaller particles. However, the different performances of the ripening originating from difference of the relative standard deviation are more obvious at higher temperature than lower temperature. This temperature effect for the relative standard deviation is the inverse of that for the initial main particle size. It is indicated that the influence of dispersity of TiO2(110)-supported Pd particles on ripening may be more sensitive at higher temperature. In this contribution, we extend the first principle kinetics to elaborate the ripening of Pd on TiO2(110). It is expected that the information from first principle kinetics would be helpful to the study in experiments. [ABSTRACT FROM AUTHOR]

• Preparation of a more realistic model catalyst for hydrodesulfurization. • Characterization of CoMoS/TiO 2 (110) using scanning tunneling microscopy and X-ray photoelectron spectroscopy. • The presence of Co during the sulfidation process enhances the yield of Co-promoted MoS 2 clusters by increasing the diffusion of S atoms while limiting the formation of other Mo sulfide structures formed due to the diffusion limitations of Mo and S without Co. • Atomic-level insights into the 2D material-metal oxide support interactions which are very useful for obtaining a fundamental understanding of the physical and chemical properties of this system that are useful for a wide range of applications in catalysis and energy storage and harvesting. Single-layer mixed metal transition metal dichalcodenides (TMDCs), such as Co-promoted MoS 2 supported on metal oxide supports like TiO 2 , have a wide range of applications in heterogeneous, electro- and photocatalysis, energy harvesting, and energy storage. In this work, we present atomic-level insights into the interaction between single-layer Co-promoted MoS 2 clusters and a model oxide rutile TiO 2 (110) support. We have used physical vapor deposition (PVD) to synthesize a precursor containing bimetallic Co-Mo nanoparticles supported on (1 × 1) TiO 2 (110). We sulfide this precursor using an H 2 S background of 1 × 10−3 mbar at 650 K to synthesize the Co-promoted MoS 2 clusters. We use a combination of scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to analyze the Co-promoted MoS 2 clusters. We compare our results with that of pristine MoS 2 clusters supported on TiO 2 (110), synthesized by an identical procedure, but without the Co atoms. We demonstrate that Co addition enhances the yield of (Co-promoted) MoS 2 clusters, in comparison to sulfidation of Mo nanoparticles to form MoS 2 clusters. Additionally, we also find that the Co-promoted MoS 2 clusters have predominantly high-index edge terminations which likely are stabilized by Co-S-Ti and Mo-S-Ti linkages to an (3 × 1) S-TiO 2 structure formed during the sulfidation. This causes an increase in the local density of states (LDOS) at the BRIM sites, thereby increasing their contrast in the STM images at the linkage sites. We have used our experimental results to propose a candidate atomic model for the Co-promoted MoS 2 clusters supported on TiO 2 (110). [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights • Exposure of SO 2 on rutile TiO 2 (110) at 133 K results in an SO 3 -like adsorbate configuration. • Water stabilized surface sulfite is formed upon increasing the temperature above 150 K. • A more strongly bound surface sulfite can be created in a reaction between SO 2 and a surface oxygen vacancy. • Oxidation of the vacancy bonded sulfite species is predicted to form stable surface sulfate species upon reactions at elevated temperatures. Graphical abstract Abstract The adsorption of SO 2 on single crystalline TiO 2 (110) has been investigated by means of polarized infrared reflection-absorption spectroscopy (IRRAS) experiments and density functional theory (DFT) calculations. IR absorption bands were detected at 1324 cm − 1 and 985 cm − 1 with p-polarized light incident along both the [1 1 ¯ 0] and [001] crystallographic directions at 123 K. When the temperature was increased to 153 K, the peak at 1324 cm − 1 disappears, while a new, weak band appears at 995 cm − 1. Simultaneously, a band at 995 cm − 1 also emerges with s-polarized light along the [1 1 ¯ 0] direction. Based on the symmetry properties of the IRRAS spectra and accompanying ab initio simulations of the spectra employing a three layer model (vacuum-adsorbate-substrate), it is shown that the low temperature absorption IRRAS bands can be attributed to an SO 3 -like adsorbate structure. This is also the most stable adsorption structure (E ad = −0.58 eV) on the stoichiometric surface. The combined IRRAS and DFT results show that the band appearing at 995 cm − 1 is associated with a surface sulfite specie which is stabilized by residual surface water. The DFT calculations also revealed that a stable adsorption structure exists on a reduced TiO 2 surface, where SO 2 binds strongly to an oxygen vacancy site. It is suggested that this is an intermediate that form surface sulfate upon further reactions with water, although it was not observed on the stoichiometric surface studied in this work. [ABSTRACT FROM AUTHOR]

The surface structure and reactivity of TiO2(110) with 1-15% of admixed vanadium were studied using thermal desorption spectroscopy, scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy with the methanol partial oxidation as the reactivity test reaction. Prior to the experiments the sample was oxidized at elevated temperatures with O2 of different pressures up to 10−6 mbar. As shown in a preceding publication (Song et al. in Surf Sci 653:181-186, 2016), vanadium leads to an increased reducibility of the mixed oxide, which resulted in the simultaneous presence of reduced and oxidized structural elements at the surface. At low vanadium concentrations (a few percent) small vanadia clusters with V4+ form above the fivefold Ti rows at the surface together with short vanadium decorated strands along [001]. These types of structure promote the partial oxidation of methanol towards formaldehyde at 550 K. At higher vanadium concentrations the vanadia aggregates at the surface contain V5+ in addition to V4+. They produce formaldehyde already at 480 K and below. The oxidized TiO2(110) layer with admixed vanadium releases QMS-detectable amounts of O2 already at a temperature of about 450 K, which is about 80 K below the corresponding temperature for TiO2(110) without vanadium. This is attributed to the increased reducibility of rutile with admixed vanadium. [ABSTRACT FROM AUTHOR]

CO adsorption and oxidation over supported Pt14 with different CO coverage on TiO2(110) surface were investigated using density functional theory (DFT) calculations and thermodynamic analysis. According to the phase diagram, Pt14/TiO2(110) and 11CO@Pt14/TiO2(110) were chosen to represent the low and high CO coverage of Pt clusters, respectively. Our study shows that the high coverage of CO can induce the structural change of supported Pt clusters and weaken the interaction between Pt clusters and TiO2 support. The CO adsorption and oxidation mechanism depends on the CO coverage, which is determined by the experimental reactant composition, pressure, and temperature. At low CO coverage, the dissociated oxygen is active specie to form CO2 by reacting with CO. At high coverage, the molecular oxygen can directly react with CO via the formation of OOCO intermediate. Our proposed mechanisms provide useful information for understanding the CO oxidation over Pt clusters with different CO coverage. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Various types of linked structures between added rows on a rutile TiO2(110) surface have been investigated by scanning tunneling microscopy (STM). A new type of linked structure, cycle-link structure comprised of eight oxygen atoms and six titanium atoms, has been observed for the first time on TiO2(110)-(1 × 2) surface. High resolution images for asymmetrical and cycle-link structures directly demonstrate that these link structures are only constructed by Ti3O6 added row model, suggesting that Ti3O6 is the suitable basis unit of the added rows on the TiO2(110) surface. [ABSTRACT FROM AUTHOR]

Abstract: One of the technologically most important requirements for the application of oxide-supported metal nanoparticles (NPs) in the fields of molecular electronics, plasmonics, and catalysis is the achievement of thermally stable systems. For this purpose, a thorough understanding of the different pathways underlying thermally-driven coarsening phenomena, and the effect of the nanoparticle synthesis method, support morphology, and degree of support reduction on NP sintering is needed. In this study, the sintering of supported metal NPs has been monitored via scanning tunneling microscopy combined with simulations following the Ostwald ripening and diffusion-coalescence models. Modifications were introduced to the diffusion-coalescence model to incorporate the correct temperature dependence and energetics. Such methods were applied to describe coarsening phenomena of physical-vapor deposited (PVD) and micellar Pt NPs supported on TiO2(110). The TiO2(110) substrates were exposed to different pre-treatments, leading to reduced, oxidized and polymer-modified TiO2 surfaces. Such pre-treatments were found to affect the coarsening behavior of the NPs. No coarsening was observed for the micellar Pt NPs, maintaining their as-prepared size of ~3nm after annealing in UHV at 1060°C. Regardless of the initial substrate pre-treatment, the average size of the PVD-grown NPs was found to increase after identical thermal cycles, namely, from 0.5±0.2nm to 1.0±0.3nm for pristine TiO2, and from 0.8±0.3nm to 1.3±0.6nm for polymer-coated TiO2 after identical thermal treatments. Although no direct real-time in situ microscopic evidence is available to determine the dominant coarsening mechanism of the PVD NPs unequivocally, our simulations following the diffusion-coalescence coarsening route were in significantly better agreement with the experimental data as compared to those based on the Ostwald-ripening model. The enhanced thermal stability of the micellar NPs as compared to the PVD clusters might be related to their initial larger NP size, narrower size distribution, and larger interparticle distances. [Copyright &y& Elsevier]

Abstract: In order to understand the promoting mechanism of alkali additives, we have studied potassium overlayers on TiO2(110). The surface was prepared by thermal segregation of K from the bulk to the surface above 750–800K. The bulk diffusion of the Ti/O ions is required for the migration of large K ions inside the lattice. STM and LEIS revealed that segregated potassium forms small clusters of 1–2nm on the surface, containing also oxygen. These clusters are located preferentially on the one-dimensional defect sites (Ti2O3 strings) of the (1×1) rutile surface and on the Ti2O3 rows of the (1× n) reconstructed surfaces. According to XPS, the potassium on the surface after segregation at 1000K is only partially ionized and the Ti 2p region is dominated by the Ti4+ component. XPS and LEIS provided evidence of a very clear preference for the Rh clusters to grow near 300K not on the potassium structures but on the potassium-free parts of titania surface. This finding may imply the absence of a direct contact between Rh and K at low Rh coverages. Nevertheless, evaporation of Rh on K/TiO2(110) results in more cationic K sites, due to an electron transfer from K to Rh through titania. The decoration and encapsulation of Rh nanoparticles by TiO x proceed also in the presence of potassium. The capping layer does not contain potassium. At large Rh cluster sizes, the wheel-like structure of the cover layer could be identified with a structure found on top of Rh crystallites formed on the K-free titania surface. The presence of potassium stabilized CO on Rh nanoparticles, which is attributed to the indirect charge transfer from potassium structures to rhodium (long range effect). [Copyright &y& Elsevier]

Abstract: The deposition of potassium onto TiO2(110) surface at 330K and the effects of post-annealing are investigated by scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS) and Auger-electron spectroscopy (AES). At lower K coverages (a few percentage of a monolayer), 3–4nm long and 1–2nm wide islands appear which can be identified with K covered regions. At higher K coverages, the surface exhibits disordered structures. Depending on the initial K coverage, the annealing above 700K in UHV results in ordering of the surface. For app. 1/3 monolayer K and annealing at around 900K, the entire surface reconstructs into a (1×2) phase accompanied by the appearance of pits with an average diameter of 20–30nm. This morphology is characteristic up to 1000K. Above this temperature, the recovery of the (1×1) phase was observed. At low K coverages (<0.2ML) the annealing at 1000K resulted in the formation of protruding islands of approximately 2×2nm2 which were identified with a strongly bonded surface compound containing K. [Copyright &y& Elsevier]

Abstract: We calculated the Ti M2,3-VV Auger-photoelectron coincidence spectroscopy spectrum of TiO2(110) by a many-body theory. The spectral main line is governed by the DOS of the two O 2p holes living longer than the Ti ∣cd2 L 2〉→∣L 2〉 super Coster-Kronig (sCK) decay. The two O 2p holes are created by the charge transfer core–hole screening at the Ti atomic site. Here c and L are the Ti 3p hole and the ligand O 2p hole, respectively. Analysis of the spectrum shows that the two (or three) CT O 2p holes in the π bonding states are localized, whereas those in the σ bonding states are delocalized. The three localized CT O 2p (π) holes in ∣cd3 L 3〉 in Ti M2,3 main line (or satellite) of TiO2(110) live longer than the Ti ∣cd3 L 3〉→∣d1 L 3〉 sCK decay so that the Coulomb repulsion from the surrounding Ti ions gives the O+ ion desorption from the surface. [Copyright &y& Elsevier]

Abstract: Oxygen deficiency and excess of rutile titania (TiO2) surfaces are important factors for catalytic activities of metal nano-particles on the TiO2 supports. Medium energy ion scattering (MEIS; 80keV He+) coupled with elastic recoil detection analysis (ERD; 150keV Ne+) can determine the numbers of bridging O (Obr) vacancies (VO) and excess O atoms adsorbed on the 5-fold Ti rows of TiO2(110) surfaces. The amounts of VO and adsorbed O were derived by H2O and 18O2 exposure followed by ERD and MEIS analyses, respectively. The present analysis revealed that only about a half of VO are filled and a comparable amount of O atoms are adsorbed on the reduced TiO2(110) surface after exposure to O2 (1000L; 1L=1×10−6 Torrs) at room temperature (RT). We also detected the adsorbed O for the hydroxylated TiO2(110) after 18O2 exposure at RT. Finally, it is shown that the O adsorbed on the Ti rows reacts with CO probably to form CO2 at RT. Based on the results obtained here, we clarify the reason why only a half of VO are filled by exposing reduced surface to O2 at RT and what is the primary source of subsurface excess electronic charge, which acts as a leading part of the surface electrochemistry and gives the defect state in the band gap seen in the valence band spectra for reduced and hydroxylated TiO2(110) surfaces. [Copyright &y& Elsevier]

Abstract: The interaction between 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and the rutile TiO2(110)–(1×1) surface under ultrahigh vacuum (UHV) conditions was investigated using X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The NEXAFS results showed that HHTP molecules formed a submonolayer and a monolayer that aligned along the [001]-direction with, respectively, a more or less flat downward orientation and a more upright orientation to the TiO2 surface. The HHTP molecules that aligned along the [001]-direction were most likely grafted onto the TiO2(110) surface by a bidentate bridge between each of the oxygen atoms of one of the catechol units within the HHTP molecule and two adjacent Ti(5f)4+ ions on the TiO2(110) surface. The coordination is non-dissociative in the case of the submonolayer, but dissociative in the monolayer, according to the analysis of the C1s XPS, UPS, C1s NEXAFS data and complementary DFT calculations. [Copyright &y& Elsevier]

Abstract: The technological importance of TiO2 has led to a broad effort aimed at understanding the elementary steps that underlie catalytic and photocatalytic reactions. The most stable surface, rutile TiO2(110), in particular, has became a prototypical model for fundamental studies of TiO2. In this critical review we have selected oxygen, water, and alcohols to evaluate recent progress relevant for applications in the areas of water splitting and oxidation of organic contaminants. We first focus on the characterization of defects and the distribution of excess charge that results from their formation. The subsequent section concentrates on the role of individual surface sites and the effect of available charge in the adsorption processes. The discussion of adsorbate dynamics follows, providing models for intrinsic and extrinsic diffusion processes as well as rotational dynamics of anchored alkoxy species. The final section summarizes our current understanding of TiO2(110) catalyzed reactions between water, oxygen, and their dissociation products. [ABSTRACT FROM AUTHOR]

Abstract: The investigation of Rh, Mo and Rh–Mo nanosized clusters formed by physical vapor deposition on TiO2(110) single crystal was performed by X-ray Photoelectron Spectroscopy (XPS), Low Energy Ion Scattering (LEIS) and Auger Electron Spectroscopy (AES). There was no sign for site-exchange between Mo and Rh atoms during deposition of Mo onto Rh particles at 330K. Mixing between Ti and Mo ions was facilitated at the Mo particle–titania interface due to reaction at 550–700K. The redox process between titania and Mo deposit was hindered at 330K by forming predeposited rhodium layer (Θ Rh =2.0ML), but reached nearly the same extent as without Rh after moderate heating to 600K. The encapsulation of Rh by titania was complete by about 700K in the presence of 1.2ML Mo, in case of Mo-predeposition and Mo-postdeposition as well. Elevating the temperature of TiO2/Rh–Mo layers above 700K, these metals form alloy at the Mo–Rh interface irrespective of deposition sequences. [Copyright &y& Elsevier]

Abstract: The interaction between a semi-large aromatic hydrocarbon compound (perylene) and the TiO2(110)-(1×1) surface under ultra high vacuum conditions has been probed by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) methods. UPS measurements of the adsorbate system have been compared with an experimental UPS spectrum of perylene in the gas phase and a calculated spectrum obtained by means of density functional theory (DFT) methods. NEXAFS results of perylene molecules adsorbed on TiO2(110)-(1×1) were compared with data from an α-phase perylene single crystal. A novel analysis of the valence data has been employed to show that no strong chemical interaction takes place between perylene and the TiO2(110)-(1×1) surface. Furthermore, angle-dependent NEXAFS measurements and the growth curve results suggest that the perylene molecules are oriented flat down onto the TiO2 substrate due to weak van der Waals interactions. [Copyright &y& Elsevier]

• Porphyrins adsorb on TiO 2 (110)-(1×1) as free-base and protonated molecules. • Raising the temperature results in porphyrin self-metalation. • Higher coverages require increasing temperatures for self-metalation to take place. • Self-metalation is accompanied by formation of Ti3+ surface defects. • Surface reduction is due to the formation of O vacancies due to water desorption. Fundamental studies on the interaction of porphyrin molecules with oxide surfaces are important due to the relevance of these systems in emerging technologies. In this work, we used X-ray photoelectron spectroscopy (XPS) to study the interaction of solution-deposited monophosphonic acid tetraphenylporphyrin (MPTPP) molecules with well-defined rutile TiO 2 (110)-(1×1) surfaces as a function of coverage and temperature. We found that molecules adsorb mainly in their free-base form with some molecules protonating on the surface at room temperature. In agreement with the behavior of carboxylic acid functionalized porphyrin molecules, raising the temperature results in porphyrin self-metalation and the temperature onset for self-metalation shifts to higher values as the coverage is increased. Furthermore, we found that self-metalation is accompanied by surface reduction, and we propose a mechanism based on the formation of oxygen vacancies driven by water desorption. Our findings contribute towards the understanding of the rich surface chemistry that porphyrin molecules have on oxide surfaces. [Display omitted] [ABSTRACT FROM AUTHOR]

• Anchoring of benzohydroxamic acid on a rutile TiO 2 (110)-(1×1) surface. • Combined experimental and theoretical study allows for detailed conclusions. • Comparison of layers prepared by evaporation in ultrahigh vacuum with deposition from ethanol and acetonitrile solutions. • Behavior strongly depends on surface coverage. Hydroxamic acids are an interesting anchor group for organic molecules on oxide surfaces, but very few studies exist on their adsorption on well-defined single-crystal surfaces under well-defined conditions. In the present paper, we use X-ray photoelectron spectroscopy to follow the adsorption of benzohydroxamic acid on a rutile TiO 2 (110)-(1×1) single-crystal surface. We compare evaporation in ultrahigh vacuum with deposition from ethanol and acetonitrile solutions. Furthermore, we carried out density functional theory calculations to support the assignment of adsorbates. Several species appear on the surface, which are most evident in the N 1s region. The low-coverage species can be explained as originating from a hydroxamate, and decomposed species related to oxygen vacancies or defects. Identification of the high-coverage species, however, remains elusive, and several possibilities are discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The carboxylic acids are stably adsorbed on TiO2(110) surfaces at room temperature. To demonstrate the neutralization mechanism proposed by Ashima et al. [H. Ashima, W.-J. Chun, K. Asakura, Surf. Sci. 601 (2007) 1822.] that explains the stable adsorption of carboxylic acids, we studied the full-coverage adsorption structure of acetic anhydride on a TiO2(110) surface by STM (scanning tunneling microscopy). We directly observed three postulated species on the TiO2(110) surface; normal acetates (termed acetate A) forming a (2×1) ordered structure, a minor acetate species (termed acetate B) which was present between the bridging oxygen and the 5-fold Ti, and the oxygen vacancies. We determined the ratio of these three species. This ratio was in good agreement with the postulated conversion reaction of acetate B to A. [Copyright &y& Elsevier]

Abstract: We have investigated the adsorption and thermal decomposition of copper hexafluoroacetylacetonate (CuII(hfac)2) on single crystal rutile TiO2(110). Low energy electron diffraction shows that room temperature saturation coverage of the CuII(hfac)2 adsorbate forms an ordered (2×1) over-layer. X-ray and ultra-violet photoemission spectroscopy of the saturated surface were recorded as the sample was annealed in a sequential manner to reveal decomposition pathways. The results show that the molecule dissociatively adsorbs by detachment of one of the two ligands to form hfac and CuI(hfac) which chemisorb to the substrate at 298K. These ligands only begin to decompose once the surface temperature exceeds 473K where Cu core level shifts indicate metallisation. This reduction from Cu(I) to Cu(0) takes place in the absence of an external reducing agent and without disproportionation and is accompanied by the onset of decomposition of the hfac ligands. Finally, C K-edge near edge X-ray absorption fine structure experiments indicate that both the ligands adsorb aligned in the 〈001〉 direction and we propose a model in which the hfac ligands adsorb on the 5-fold coordinated Ti atoms and the CuI(hfac) moiety attaches to the bridging O atoms in a square planar geometry. The calculated tilt angle for these combined geometries is approximately 10° to the surface normal. [Copyright &y& Elsevier]

Abstract: On 100K rutile TiO2(110) surfaces with 3.5±0.5% surface bridge-bonded oxygen vacancies (BBOV’s), propene formation by dehydration of 2-propanol was monitored by temperature programmed desorption (TPD). The BBOV’s were either filled or unfilled when the 2-propanol was dosed. Propene desorption rates exhibit two local maxima, nominally at 350 (LT) and 570K (HT). The former is not altered by filling BBOV’s while the latter is reduced by factors of 2.5 and 5 when the vacancies are pre-filled with water (H–OH) and alcohol (R–OH), respectively. The HT process is attributed to a reaction of 2-propoxy groups located on surface BBO’s. To account for much of the HT C3H6 yield when BBOV’s are filled before (CH3)2CHOH dosing, we propose a model whereby, during TPD, vacancies form and are filled with 2-propoxy. The factor of two difference between titrating BBOV’s with H2O and alcohols is attributed to stoichiometry; twice as many OH groups form on surface BBO rows when H2O is used so twice as many vacancies are created when OH recombines. [Copyright &y& Elsevier]

The activation energies for alkene formation via dehydration of alcohols on bridge-bonded oxygen (BBO) vacancy sites of TiO2(110) is found to correlate with the inductive electron donating effect of alcohol alkyl groups as measured by the Taft parameter. Based on this correlation we conclude that the reaction involves a single transition state that undergoes concerted rupture of the C–O bond of the alkoxide and a C–H bond of the alkyl group attached to the β-carbon. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the adsorption structure of acetic anhydride on a TiO2(110) surface using XPS (X-ray photoelectron spectroscopy), LEED (low energy electron diffraction) and HREELS (high resolution electron energy loss spectroscopy) to determine the origins of the unique adsorption properties of carboxylic acids on a TiO2(110) surface. The C 1s XPS data indicated that the saturation carbon amount of adsorbed acetic anhydride was 12±3% larger than that of the adsorbed acetic acid. LEED showed p(2×1) weak spots for the acetic anhydride adsorbed surface. The HREELS spectra revealed the dissociative adsorption of acetic anhydride. Based on these findings, we concluded that the neutralization of the bridging oxygen atoms associated with the dissociative adsorption is necessary for the stable adsorption of carboxylates on the 5-fold Ti sites. [Copyright &y& Elsevier]

Abstract: A procedure is described for preparing a novel model early transition metal oxide system for catalysis studies—direct sublimation of tungsten trioxide on TiO2(110). Isolated monodispersed cyclic trimers, i.e., (WO3)3, can be formed on TiO2(110) that are thermally stable up to at least 750K. Although not readily generalizable to monodispersed (WO3) x clusters other than cyclic trimers, this protocol provides an ideal nanocluster platform for carrying out model system catalysis studies over a wide temperature range. [Copyright &y& Elsevier]